TOTAL CARBONACEOUS MATERIAL CHARACTERIZATION (TCM) METHODOLOGIES

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefits of U.S. Provisional Application Ser. No. 63/699,593, filed Sep. 26, 2024, and 63/747,760, filed Jan. 21, 2025, having the same title, each of which is incorporated herein by this reference in its entirety.

FIELD

The disclosure relates generally to precious metal extractive metallurgy and particularly to precious metal hydrometallurgy of refractory and double refractory carbonaceous ores, concentrates, and tailings comprising preg-robbing carbonaceous materials.

BACKGROUND

The principal technology used to recover precious metals is cyanide leaching, in which gold is leached from the ore by treatment with a solution of cyanide. For lower grade ores, run of mine (ROM) dump leach only simply excavates the ore and places the excavated ore onto the heap or dump leaching pad. For higher grade and refractory (sulfide) ores, the first step is comminution (grinding) to increase surface area and expose the gold to the extracting solution. The extraction is conducted by dump or heap leaching processes using sodium cyanide. The crude ore is washed with a solution of cyanide in air, often repeatedly, and the aqueous extract is collected and refined further. Recovery from the pregnant leach solution typically involves adsorption on activated carbon in a carbon in pulp (CIP), carbon-in-leach (CIL), or carbon in column (CIC) process.

A “refractory” gold ore is an ore that has ultra-fine gold particles disseminated throughout its gold occluded minerals. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment for cyanidation to be effective in recovery of the gold. A refractory ore generally contains sulfide minerals (with arsenopyrite and pyrite being the primary gold carriers), preg-robbing carbon or both. Carbonaceous material (CM) is a carbon-rich material that typically contains organic matter (e.g., humic acid and hydrocarbons) and/or graphite (e.g., elemental carbon). Carbonaceous ores can be classified as mildly carbonaceous (less than 1% organic and graphitic carbon) or highly carbonaceous (more than 1% carbon).

Gold in refractory sulfide ores are often not recoverable by cyanidation as the gold can be in solid solution with the sulfide minerals, encapsulated in ultra-fine or sub-microscopic particles within the sulfide matrix, and/or chemically bound to the sulfide minerals. Sulfide minerals are impermeable to cyanide leaching and thus can occlude gold particles, making it difficult for the leach solution to form a complex with the gold.

Carbonaceous material in the form of preg-robbing carbon can likewise directly or indirectly interfere with lixiviation. Direct interference with lixiviation is generally due to either occlusion of the gold within the carbonaceous material or formation of a stable gold-carbon complex similar to a chelate. Preg-robbing carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon. This process is known as “preg-robbing” because the carbonaceous matter “robs” the gold cyanide complex from the pregnant leach solution. This so-called “preg-robbing” carbon is typically lost because it is significantly finer than the carbon recovery screens typically used to recover activated carbon, thereby reducing gold recovery.

While preg-robbing is most frequently associated with cyanidation processes, the preg-robbing phenomenon is also known to occur with other gold-lixiviant complexes such as gold-chloride when using a chloride media and gold-thiourea complexes when using a thiourea lixiviant.

Treatments of carbonaceous ores with chemical surfactants as blinding agents to limit preg-robbing have shown promising results. The blinding or “blanking” of the preg-robbing carbon are selectively adsorbed by and passivate the exposed surface of the preg-robbing carbon until it is saturated and unable to adsorb the gold-lixiviant complexes in solution. Diesel oils, kerosene, fuel oil, natural oils, cationic, and anionic surfactants and RV-2 (para-nitrobenzolazo salicyclic acid) have been used as blinding agents with varying degrees of success.

Depending on the sulfide content, some gold ores may require oxidation processes to enable effective gold extraction. Oxidation pre-treatment options for refractory ores include roasting, bio-oxidation (e.g., bacterial oxidation), and pressure oxidation. Roasting is used to oxidize both the sulfur and preg-robbing carbon at high temperatures using air and/or oxygen. Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation is an aqueous process for sulfur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. Oxidation roasting is a popular pre-treatment technique that converts CM to carbon dioxide (CO2). The refractory ore treatment processes may be preceded by concentration (usually sulfide flotation).

Refractory gold ores, like carbonaceous gold ores, are becoming increasingly important as easily leachable gold ore reserves deplete.

SUMMARY

These and other needs are addressed by the various embodiments and configurations of the present disclosure.

In an embodiment, a process can include the steps of:

• • (a) receiving a refractory precious metal-containing material comprising a preg-robbing material;
  • (b) determining a preg-robbing strength of the preg-robbing material relative to a sorbent, and one or more of: a preg-robbing capacity either before or after (typically after) pressure oxidation of the precious metal-containing material and a preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent; and
  • (c) based on the preg-robbing strength of the preg-robbing material relative to a sorbent and the one or more of the preg-robbing capacity before or after pressure oxidation of the precious metal-containing material and the preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent, directing the refractory precious metal-containing material to a selected one of a plurality of unit processing operations for recovery of the precious metal in the refractory precious metal-containing material.

In an embodiment, a process can include the steps of:

• • (a) receiving a refractory precious metal-containing material comprising preg-robbing carbon;
  • (b) determining a preg-robbing strength of the preg-robbing carbon relative to one of activated carbon and a precious metal-selective ion exchange resin and one or more of: a preg-robbing capacity before or after pressure oxidation of the precious metal-containing material and a preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent; and
  • (c) based on the preg-robbing strength of the preg-robbing carbon relative to the one of activated carbon and a precious metal-selective ion exchange resin and the one or more of the preg-robbing capacity before or after pressure oxidation of the precious metal-containing material and the preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent, directing the refractory precious metal-containing material to a selected one of a plurality of unit processing operations for recovery of the precious metal in the refractory precious metal-containing material.

In an embodiment, a process can include the steps of:

• • (a) receiving a refractory precious metal-containing material comprising preg-robbing carbon;
  • (b) determining a preg-robbing strength of the preg-robbing carbon relative to one of activated carbon and a precious metal-selective ion exchange resin and a preg-robbing capacity after pressure oxidation of the precious metal-containing material and a preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent; and
  • (c) based on the preg-robbing strength of the preg-robbing carbon relative to the one of activated carbon and a precious metal-selective ion exchange resin and the preg-robbing capacity after pressure oxidation of the precious metal-containing material and the preg-robbing capacity of the refractory precious metal-containing material in the presence of a blanking agent, directing the refractory precious metal-containing material to a selected one of a plurality of unit processing operations for recovery of the precious metal in the refractory precious metal-containing material.

The present disclosure can provide a number of advantages depending on the particular configuration. As will be appreciated, the advantages of preg-robbing carbon characterization cannot be underestimated. Properly characterizing key properties of preg-robbing carbon in a given ore sample can determine not only whether the precious metal can be extracted economically but also which hydrometallurgical process may be employed to best extract the precious metal from the ore. Understanding the adsorption and adherence of dissolved gold by carbonaceous minerals during cyanide leaching can also provide accurate geo-metallurgical characterization and appropriate ore routing. Effective ore routing can be important for maximizing economic returns from variable ore bodies, particularly when multiple processing options are available. The various tests disclosed in the disclosure enable not only a determination of which processing option is optimal for the tested batch of precious metal-containing material but also approximates the portion of the refractory activity caused by sulfide sulfur occlusion of gold (e.g., locking of gold in sulfide minerals which would be unaffected by the addition of a sorbent or blanking agent but would be affected by pressure oxidation)) compared to preg-robbing carbon gold adsorption. The subject matter of the present disclosure is not limited to cyanide lixiviants but also applies to other gold lixiviants, such as chloride media, thiosulfate, and thiourea.

These and other advantages will be apparent from the disclosure of the disclosure contained herein.

As used herein, “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C”, “A, B, and/or C”, and “A, B, or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. When each one of A, B, and C in the above expressions refers to an element, such as X, Y, and Z, or class of elements, such as X₁-X_n, Y₁-Y_m, and Z₁-Z_o, the phrase is intended to refer to a single element selected from X, Y, and Z, a combination of elements selected from the same class (e.g., X₁ and X₂) as well as a combination of elements selected from two or more classes (e.g., Y₁ and Z_o).

It is to be noted that the term “a” or “an” entity refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably.

The term “disseminated carbonaceous material” or (DCM) refers to carbonaceous material finely disseminated in otherwise non-carbonaceous particles, such as particles of quartz or other gangue particles.

The term “inorganic carbon” refers primarily to the metallic carbonates, such as calcium carbonate and sodium carbonate, and to binary compounds of carbon such as carbon oxides, carbides, carbon disulfides, etc., ternary compounds, such as metallic cyanides, metallic carbonyls, carbonyl sulfides, etc.

The term “means” as used herein shall be given its broadest possible interpretation in accordance with 35 U.S.C., Section 112(f) and/or Section 112, Paragraph 6. Accordingly, a claim incorporating the term “means” shall cover all structures, materials, or acts set forth herein, and all of the equivalents thereof. Further, the structures, materials or acts and the equivalents thereof shall include all those described in the summary of the disclosure, brief description of the drawings, detailed description, abstract, and claims themselves.

A “mill” refers to any facility or set of facilities that process a metal-containing material, typically by recovering, or substantially isolating, a metal or metal-containing mineral from a feed material. Generally, the mill includes an open or closed comminution circuit, which includes crushers or autogenous, semi-autogenous, or non-autogenous grinding mills.

The term “precious metal” refers to gold and silver and the platinum group metals (i.e., ruthenium, rhodium, palladium, osmium, iridium, and platinum), with gold and silver being more common, and gold even more common.

“Preg-robbing carbon” refers to “Total Carbonaceous Matter” or “TCM” and Disseminated Carbonaceous Matter” or “DCM”. TCM particles commonly consist of almost 100% carbon. DCM particles are disseminated in minerals, such as quartz or other gangue minerals with different degrees of finely disseminated carbonaceous matter. The distribution of the carbonaceous material on these grains is discontinuous and shows very high variability from grain to grain or from ore to ore. In some ores, the preg-robbing carbon includes one or more of (1) an activated carbon component capable of adsorbing gold-chloride complexes and gold-cyanide complexes from solution, (2) a mixture of high molecular weight hydrocarbons usually associated with the activated carbon components; (3) an organic acid, similar to humic acid containing functional groups capable of interacting with gold complexes to form organic gold compounds; and (4) graphitic carbon.

“Preg-robbing material” refers to preg-robbing carbon and other substances, such as clay materials (e.g., smectite, montmorillonite, illite, montmorillonite, and kaolinite that adsorb the precious metal complex (such as the gold-cyanide complex) from a pregnant leach solution.

A “sorbent” is a material that sorbs another substance; that is, the material has the capacity or tendency to take it up by sorption.

“Sorb” means to take up a liquid or a gas by sorption.

“Sorption” refers to adsorption and absorption, while desorption is the reverse of adsorption.

Unless otherwise noted, an expression that a substance or material, regardless of phase, comprises or includes a particular element in the Periodic Table of Elements, such as aluminum, boron, calcium, europium. gold, hydrogen, iodine, lanthanum, molybdenum, niobium, oxygen, potassium, rhodium, samarium, tungsten, uranium, vanadium, and yttrium, is to be construed to include the element not only in its elemental form but also any other form of the element, including ions of the element and compounds and other molecules comprising the element.

Unless otherwise noted, all component or composition levels are in reference to the active portion of that component or composition and are exclusive of impurities, for example, residual solvents or by-products, which may be present in commercially available sources of such components or compositions.

All percentages and ratios are calculated by total composition weight, unless indicated otherwise.

It should be understood that every maximum numerical limitation given throughout this disclosure is deemed to include each and every lower numerical limitation as an alternative, as if such lower numerical limitations were expressly written herein. Every minimum numerical limitation given throughout this disclosure is deemed to include each and every higher numerical limitation as an alternative, as if such higher numerical limitations were expressly written herein. Every numerical range given throughout this disclosure is deemed to include each and every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written herein. By way of example, the phrase from about 2 to about 4 includes the whole number and/or integer ranges from about 2 to about 3, from about 3 to about 4 and each possible range based on real (e.g., irrational and/or rational) numbers, such as from about 2.1 to about 4.9, from about 2.1 to about 3.4, and so on.

The preceding is a simplified summary of the disclosure to provide an understanding of some aspects of the disclosure. This summary is neither an extensive nor exhaustive overview of the disclosure and its various embodiments. It is intended neither to identify key or critical elements of the disclosure nor to delineate the scope of the disclosure but to present selected concepts of the disclosure in a simplified form as an introduction to the more detailed description presented below. As will be appreciated, other embodiments of the disclosure are possible. Also while the disclosure is presented in terms of exemplary embodiments, it should be appreciated that individual aspects of the disclosure can be separately claimed. or in combination, one or more of the features set forth above or described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow schematic of a process according to an embodiment of the present disclosure;

FIG. 2A is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon (scanned at the wavelength of 488 nm laser using the Renishaw in Via microscope and deconvoluted with the Renishaw WiRE software for G, D, amorphous, T, and D′ bands);

FIG. 2B is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2C is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2D is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2E is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2F is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2G is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2H is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2I is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 2J is a plot of intensity (a.u.) (vertical axis) against wave number (cm⁻¹) (horizontal axis) in connection with Ramam spectra of TCM particles in selected samples and model carbon as described above;

FIG. 3 is a plot of relative width of D (vertical axis) against relative height of D (horizontal axis) for five classes of TCM and model carbons identified by deconvoluted D-band.

FIG. 4A is a plot of preg-rob factor (PRF) (% gold) (vertical axis) versus blinding agent:TCM ratio (horizontal axis) in connection with the effect of the blinding agent on PRF;

FIG. 4B is a plot of PRF (% gold) (vertical axis) versus blinding agent:TCM ratio (horizontal axis) in connection with the effect of the blinding agent on PRF;

FIG. 4C is a plot of PRF (% gold) (vertical axis) versus blinding agent:TCM ratio (horizontal axis) in connection with the effect of the blinding agent on PRF;

FIG. 4D is a plot of PRF (% gold) (vertical axis) versus blinding agent:TCM ratio (horizontal axis) in connection with the effect of the blinding agent on PRF;

FIG. 5A is a plot of preg-robbing gold (oz gold/ton TCM) (vertical axis) versus final solution grade (oz gold/ton);

FIG. 5B is a plot of preg-robbing gold (oz gold/ton TCM) (vertical axis) versus final solution grade (oz gold/ton);

FIG. 6A is a plot of preg-robbing gold (oz gold/ton TCM) (vertical axis) versus final solution grade (oz gold/ton);

FIG. 6B is a plot of preg-robbing gold (oz gold/ton TCM) (vertical axis) versus final solution grade (oz gold/ton);

FIG. 7 is a plot of PRF (% gold) (vertical axis) versus Raman TCM group identity (horizontal axis) in connection with PRF, standard preg-rob (PR) analysis, spiked carbon-in-leach (CIL), and spike CIL test results;

FIG. 8 is a plot of change in PRF (standard-standard CIL or standard carbon-in-pulp (CIP)) (% gold) (vertical axis) against standard PRF (% gold) in connection with decrease in PRF by adding granular activated carbon (GAC) vs PRF, spiked CIL, and spiked CIL and sorted by Raman TCM grouping;

FIG. 9 is a plot of BLT spike recovery (% gold) (vertical axis) against preg-rob factor (% gold) (horizontal axis) sorted by Raman TCM grouping; and

FIG. 10 is a flow schematic of a process according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

To facilitate proper ore routing and design of a hydrometallurgical process to recover precious metals, particularly gold, from a refractory preg-robbing carbon-containing precious metal-bearing material, a preg-robbing carbon characterization method is provided. The method performs various tests on the material to quantify a number of relevant characteristics of the material. These characteristics can include a plurality of untreated preg-robbing capacity (e.g., as determined using preg-rob capacity tests to determine how much gold in the cyanide leach solution will the preg-robbing carbon preg rob), preg-robbing strength (e.g., as determined using spiked carbon-in-leach (CIL), carbon-in-pulp (CIP), resin-in-leach (RIL), and resin-in-pulp (RIP) process tests to determine how tightly will the preg-robbing carbon hold onto the preg robbed gold in the presence of a sorbent such as activated carbon or a gold-selective ion exchange resin), blanking agent preg-robbing capacity to determine responsiveness to blinding agents (e.g., as determined by chemical tests such as preg-robbing carbon masking tests using a blinding agent), POX preg-robbing capacity to determine the level of neutralization of the preg-robbing carbon as a result of pressure oxidation (e.g., as determined by pressure oxidation bench tests and partial chemical oxidation using bleach), optionally material surface characteristics (e.g., as determined using a surface scanning technique such as Raman spectroscopy or X-Ray diffraction analysis to obtain semi-quantification information concerning textural mineralogy of the samples), correlation between sulfide sulfur and carbon speciation (e.g., as determined by sulfur speciation analyses using conventional Leco and and/or Eltra carbon and sulfur speciation analyzers), and/or thermal stability of the preg-robbing carbon (e.g., as determined using thermogravimetric analysis).

A preliminary set of analyses to be performed is to determine the precious metal (particularly gold) content and mineralogy of the precious metal-containing material. As will be appreciated, the material may be refractory due to precious metal entrapment by sulfide minerals and/or preg-robbing due to preg-robbing carbon. To assess potential causes of refractory behavior, the material is typically analyzed for sulfide sulfur content and the sulfide minerals hosting the precious metals and total carbon content (both total carbon and non-carbonate carbon). As will be appreciated, non-carbonate carbon is typically representative of all the preg-robbing carbon in the material. Carbonate level can be determined using the difference between total carbon and non-carbonate carbon assays. This information can be determined by any suitable test(s), such as head analysis for gold by fire assay or atomic absorption spectrometry, Leco carbon specification testing, Eltra carbon specification testing, and the like.

As will be appreciated, head analysis for gold by fire assay is a standard test comprising melting a sample with fluxes like lead oxide at high temperatures (over 1,000° C.) to separate gold from impurities to assay gold content. The molten gold, along with lead, form a button that is then placed in a porous cupel, where the lead and other base metals oxidize and are absorbed. The remaining gold (prill), which is primarily gold, is then weighed and further analyzed to determine the exact gold content of the sample.

The LECO carbon testing method uses combustion analysis to determine the concentration of carbon in a sample by burning it in an ultra-high purity oxygen atmosphere at high temperatures. The carbon in the sample is converted into carbon dioxide (CO₂), which is then measured using infrared absorption. This allows for precise quantification of the carbon content to compare against established specifications, ensuring material quality and composition. The Leco carbon specification testing method typically comprises sample preparation, sample combustion at high temperatures (e.g., 1350° C.) in the presence of ultra-high purity oxygen, gas conversion to convert carbon in the sample to carbon dioxide, gas separation and cleaning using filters and traps to remove water and other interfering elements, ensuring that only CO₂, nitrogen, and other relevant gases reach the detector, detection using a non-dispersive infrared cell in which carbon dioxide absorbs infrared light at a specific wavelength, and quantification of the amount of carbon in the original sample to determine the final carbon content. In some applications, a hydrochloric acid or sodium carbonate digestion residual method is used to determine carbon speciation. Preg-robbing carbon is reported as residual carbon content after hydrochloric acid digestion. Carbonate carbon is reported as the difference between total carbon and preg-robbing carbon (carbon lost during HCL digestion). Sulfide sulfur content is reported from the HCL digestion analyses as the residual sulfur (after digestion). The primary sulfur speciation analysis can be conducted using a standard sodium carbonate digestion method, with the residual (after digestion) carbon reported as sulfide sulfur.

The Eltra carbon specification testing method is a form of high-temperature combustion analysis to determine the carbon content in a sample. As will be appreciated, an Eltra analyzer is similar to the LECO analysis—the difference is that the ELTRA analysis can use a resistance furnace as well as an induction furnace. The resistance furnace offers the possibility of lower controlled temperatures for combustions. The test typically comprises preparation of the sample, combusting the sample in a furnace over a range of ignition temperatures in a pure oxygen atmosphere, reacting the carbon in the sample with oxygen to form carbon dioxide and carbon monoxide, purifying the combustion gases to remove moisture and particulates, measuring by infrared detection the concentration of carbon dioxide gas (as noted the amount of carbon in the sample is directly proportional to the amount of carbon dioxide detected), and quantification of the amount of carbon in the original sample to determine the final carbon content.

The sulfide sulfur content and mineralogy types can be determined by LECO analytical techniques or diagnostic leaching tests using a mineral acid to dissolve specific sulfide minerals, optical microscopy, energy dispersive X-Ray diffraction and scanning electron microscopy, atomic absorption spectrometry, Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis, Inductively Coupled Optical Emission Spectrometry (ICP-OES), and other known techniques.

The material is further tested for refractory behavior by suitable tests, such as cyanide solubility (e.g., the cyanide shake test) and the like. Cyanide solubility typically refers to the standard cyanide bottle roll or shake test for determining the leachability of precious metals (particularly gold) from the material. The test also assesses the material's tendency to consume cyanide. This method measures how effectively a cyanide solution can dissolve target precious metals from a ground ore sample. The test typically comprises sample preparation, slurry mixing, pH adjustment by an alkaline reagent, cyanide addition, agitation, and sampling and analysis. The test provides information regarding precious metal recovery, leach kinetics, cyanide consumption, and process parameter optimization information.

In some applications, the untreated preg-robbing capacity of the preg-robbing carbon in the precious metal-containing material is quantified. Preg-robbing capacity indicates a relative fraction of the dissolved gold in a cyanide leach solution that can be adsorbed by the preg-robbing carbon. While any suitable test may be employed, a preg rob assay (also called a preg-robbing test or preg-robbing index test) is typically used to determine how much precious metal (particularly gold), during cyanidation, will be “preg-robbed,” or adsorbed, by carbonaceous matter in a refractory gold ore. The test may employ the Preg-robbing Recovery Standard or PRS test protocol. The test typically involves exposing a first split of a sample of ore to a cyanide solution under specific conditions (typically a cyanide concentration of about 2 g NaCN/L to about 8 g NaCN/L @30%-33% solids (@125 to about 250 mesh (Tyler) pulverized) and a pH ranging from about pH 10 to about pH 12) and measuring the naturally leachable gold content and exposing a second split of the sample to a gold spiked cyanide solution (spiked with a known concentration of gold) and otherwise having the same specific conditions as the solution in the first test and measuring the amount of gold remaining in the solution. The tests typically comprise contacting the spiked or unspiked cyanide solution with the sample, mixing the slurry (e.g., by stirring or rolling), separating the solid sample from the solution such as by centrifugation or filtration, analyzing the gold concentration in the solution (e.g., by inductively coupled plasma ICP analysis or other technique), analyzing the gold concentration in the leached residues by fire assay or other suitable techniques, and calculating the preg-robbing capacity. The sulfide sulfur content can be removed from the sample prior to the test to remove any refractory behavior due to the sulfide sulfur. The unspiked leach test determines the amount of cyanide soluble gold in the material while the gold spiked leach test determines the capacity of the material to adsorb gold(I) cyanide from solution. From these results, an index or value can be determined based on the gold concentration remaining in each solution at the end of the test. While there are many variations for determining the index or value (hereinafter referred to as the untreated preg-robbing capacity), one methodology determines the index based on the concentration of gold absorbed by the material (with an index of zero indicating no preg-robbing activity, a value between zero and 1 indicating minimal preg-robbing activity, between 1 and 2.5 indicating moderate preg-robbing activity, and more than 2.5 indicating that the material is highly preg-robbing) while another determines the index based on the percentage change in gold concentration (with a value of 100% indicating that all gold from solution has been adsorbed by the material and a value of 0% indicating that no gold from solution has been adsorbed by the material). Other formulations are possible depending on the application. The gold concentration in the spiked solution after leaching (after removal of the naturally leachable gold content) is compared to the initial gold concentration of the spiked solution before leaching. In accordance with the latter methodology, an untreated preg-robbing capacity of the sample can be determined by the equation below:

Untreated ⁢ Preg - Rob ⁢ Capacity =  [ ( Spike + AuCN - AuCN_Spike ) / ( Spike + AuCN ) ] * 100 ⁢ where : Spike = Au ⁢ added ⁢ during ⁢ cyanide ⁢ shake ⁢ with ⁢ spike ⁢ ( e . g . , 0.1 ozAu / ton ⁢ ore ) ; AuCN = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ ( unspiked ⁢ cyanide ⁢ shake ⁢ analysis ⁢ results ⁢ in ⁢ oz / ton ⁢ ore ) ⁢ and AuCN_Spike = cumulative ⁢ cyanide ⁢ ⁢ soluble ⁢ Au ⁢ content ⁢ recovered ⁢ after ⁢ leach ⁢ ( oz / ton ⁢ ore ) .

In another formulation, the untreated preg-rob capacity can be determined for gold loaded on preg-robbing carbon by mass balancing the gold lost from the spiked gold-containing solution divided by the mass of preg-robbing carbon in the sample (e.g., calculated as oz gold/ton preg-robbing carbon). The preg-rob index or factor is determined by dividing the gold loaded on the preg-robbing carbon by the sum of the gold added in the spike solution and naturally leachable gold content.

Other techniques, such as ToF-SIMS analysis, may be used to determine the preg-robbing capacity of the sample.

Variations of the above technique may be employed.

In one variation, the untreated preg-robbing capacity of the preg-robbing carbon in the precious metal-containing material is quantified by determining an untreated preg-robbing capacity by varying the spiked leach solution mass to ore mass ratio (first modified test methodology) and variation in spiked leach solution grade (second modified test methodology). While any suitable test may be employed, a modified version of the preg-robbing test or preg-robbing index test discussed above is typically used as the basis for both the first and second modified test methodologies. The test typically involves exposing a sample of ore to a gold-spiked cyanide solution under specific conditions (typically a cyanide concentration of about 2 g NaCN/L to about 8 g NaCN/L @30%-33% solids (@125 to about 250 mesh (Tyler) pulverized) and a pH ranging from about pH 10 to about pH 12) and measuring the amount of gold remaining in the solution. In the first modified test methodology, the solution:ore ratio is varied in a series of tests using different splits of the sample, typically from about 1:1 to about 30:1. In the second modified test methodology, the solution ore ratio is maintained constant but the spiked solution grade is varied in a series of tests using different splits of the sample typically from about 0.05 to about 1 oz gold/ton solution.) The slurries are filtered to separate solids from liquids, final solution volumes measured and sampled for pH, free cyanide and gold and leached residues washed with fresh water, dried, weighed and fire assayed directly to determine gold content. From these results, an index or value or untreated preg-rob capacity can be determined as noted above based on the gold concentration remaining in each solution at the end of the test.

In response to the presence of preg-robbing carbon, the preg-robbing strength of the preg-robbing carbon in the precious metal-containing material can be quantified by determining a competitive preg-robbing strength. Competitive preg-robbing strength indicates a relative gold absorption strength of the preg-robbing carbon when compared to that of a gold recovery sorbent, such as granular activated carbon used in carbon-in-leach or CIL, or carbon-in-pulp or CIP (using activated carbon as the sorbent) or ion exchange resin used in resin-in-leach or RIL, or resin-in-pulp or RIP (e.g., using a gold-selective ion-exchange resin as the sorbent). The strength can be determined using unspiked or spiked CIL, CIP, RIL, or RIP tests to determine how strongly will the preg-robbing carbon hold onto the preg robbed gold in the presence of a selected sorbent, such as activated carbon or a resin. In the competitive preg-robbing strength tests, baseline agitated cyanidation tests with a sample of the precious metal-containing material can be conducted in the presence of a selected sorbent (e.g., granular activated carbon or an ion exchange resin) and in the absence of the selected sorbent being present. The tests can be conducted on a selected sample size (typically one pound pulverized sample as noted above) for a selected leach cycle (such as ranging from about 2 to about 24 hours) and a selected cyanide concentration (e.g., the same composition noted above with the exception of inclusion of the sorbent-typically a cyanide concentration of about 2 g NaCN/L to about 8 g NaCN/L @30%-33% solids and a pH ranging from about pH 10 to about pH 12). Each pulverized sample is typically mixed with fresh water in a bottle to achieve 30-33% solids (wt/wt). Hydrated lime can be added to adjust the pH to about pH 11 to about pH 12 while mixing. Sodium cyanide, equivalent to the selected concentration, can be added to the alkaline pulp. For the CIP, CIC, or CIL tests, pretreated granular activated carbon (GAC) of from about 10 to about 30 20 g/L slurry can be added to the sodium cyanide. The GAC can be pretreated by attritting to remove fines and soaking in barren cyanide solution at the same cyanide concentration used for the leaching tests for a predetermined period of time (e.g., 6 hours) before adding to the test. The cyanide can be optionally spiked with gold using the same gold concentration as the tests noted above. After leaching, the slurries are weighed and filtered to separate solids and liquids. The slurries are screened to recover the loaded carbon, before filtering. Final solution volumes are measured and sampled for pH, free cyanide, and gold (by inductively coupled plasma analysis or ICP). Final solids are washed, dried, weighed and fire assayed directly to determine residual gold content. Loaded carbon samples from the tests were dried, weighed, and fire assayed to determine gold recovery.

The competitive preg-robbing strength can be determined by any number of methodologies. In one methodology, the strength is determined by the following equation:

Pre - Rob ⁢ Strength = ( Baseline - ( Gold ⁢ Spike ⁢ or ⁢ Unspiked ) ⁢ Sorbent ) / ( Total ⁢ Au ) * 100 ⁢ where : Baseline = Au ⁢ remaining ⁢ ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ ( exclusive ⁢ of ⁢ the ⁢ sorbent ) ⁢ after ⁢ the ⁢ baseline ⁢ leach , Sorbent = Au ⁢ remaining ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ ( exclusive ⁢ of ⁢ the ⁢ sorbent ) ⁢ after ⁢ sorbent ⁢ leach ⁢ ⁢ in ⁢ which ⁢ a ⁢ selected ⁢ sorbent , such ⁢ as ⁢ activated ⁢ carbon , is ⁢ present ; and Total ⁢ Au = total ⁢ fire ⁢ assayed ⁢ gold ⁢ content ⁢ of ⁢ sample ⁢ before ⁢ leaching .

In another methodology, the strength is determined by the following equation:

Preg - Rob ⁢ Strength =  [ ( Spike + AuCN - AuCN_Spike ) / ( Spike + AuCN ) ] * 100 ⁢ where : Spike = Au ⁢ added ⁢ during ⁢ cyanide ⁢ shake ⁢ with ⁢ spike ⁢ ( e . g . , 0.1 ozAu / ton ⁢ ore ) ; AuCN = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ ( unspiked ⁢ cyanide ⁢ shake ⁢ analysis ⁢ results ⁢ ⁢ in ⁢ oz / ton ⁢ ore ) ⁢ and AuCN_Spike = cumulative ⁢ cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ recovered ⁢ by ⁢ sorbent ⁢ after ⁢ leach ⁢ ( oz / ton ⁢ ore ) .

As noted above, the strength can also be determined based on the concentration of gold absorbed by the final solids (exclusive of sorbent) or sorbent (in the former case an index of zero indicates no preg-rob strength, a value between zero and 1 indicates minimal preg-rob strength, between 1 and 2.5 indicates moderate preg-rob strength, and more than 2.5 indicates that the material has a high preg-rob strength).

In another formulation, the preg-rob capacity can be determined for gold loaded on preg-robbing carbon or the sorbent by mass balancing the gold lost from the spiked gold-containing solution divided by the mass of preg-robbing carbon in the sample (e.g., calculated as oz gold/ton preg-robbing carbon) or the amount of added sorbent. The competitive preg-rob index can be determined by dividing the gold loaded on the preg-robbing carbon by the sum of the gold added in the spike solution and naturally leachable gold content or by dividing the gold loaded on the sorbent by the sum of the gold added in the spike solution and naturally leachable gold content.

As will be appreciated, other mathematical formulations measuring the competitive preg-robbing strength can be employed.

In response to the presence of preg-robbing carbon, the post-pressure oxidation or POX preg-robbing capacity of the preg-robbing carbon after pressure oxidation can be determined. While any test can be employed (such as bench scale pressure oxidation tests), one such test to determine the reduced preg-robbing capacity is a bleach test, which is intended to replicate the impact on the preg-robbing carbon by acidic pressure oxidation. While a bleach solution is referenced, it is to be appreciated that other chemical oxidants may be employed in addition to or in lieu of bleach. As will be appreciated, the bleach will replicate pressure oxidation of the preg-robbing carbon by partially chemically oxidizing the preg-robbing carbon. The bleach test contacts a sample with a bleach solution (e.g., a 10% NaOCl solution @30%-33% pulverized solids) for a determined period of time (e.g., 30, 45, or 60 minutes). The bleach solution is removed and the solid sample rinsed with fresh water and contacted with an unspiked or gold spiked cyanide solution having the same composition as that noted above (e.g., a cyanide solution comprising from about 2 g NaCN/L to about 8 g NaCN/L @30%-33% solids pulverized and a pH ranging from about pH 10 to about pH 12), the slurry mixed (e.g., by stirring or rolling such as using a cyanide shake test), and the solid sample separated from the solution such as by centrifugation or filtration. Final solution volumes are measured and sampled for pH, free cyanide, and gold (e.g., by ICP). Final solids are washed, dried, weighed and fire assayed directly to determine residual gold content.

The POX preg-robbing capacity can be determined by any number of methodologies. In one methodology, the capacity is determined by the following equation:

POX ⁢ Preg - Rob ⁢ Capacity = ( Baseline - ( Gold ⁢ Spiked ⁢ or ⁢ Unspiked ) ⁢ Residue ) / ( Total ⁢ Au ) * 100 ⁢ where : Baseline = Au ⁢ remaining ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ after ⁢ the ⁢ baseline ⁢ leach ; Residue = Au ⁢ remaining ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ after ⁢ bleach ⁢ test ; and Total ⁢ Au = total ⁢ fire ⁢ assayed ⁢ gold ⁢ content ⁢ of ⁢ sample ⁢ before ⁢ leaching .

In another methodology, the POX preg-robbing capacity is determined by the following equation:

POX ⁢ Preg - Rob ⁢ Capacity =  [ ( Spike + AuCN - AuCN_Spike ) / ( Spike + AuCN ) ] * 100 ⁢ where : Spike = Au ⁢ added ⁢ during ⁢ bleach ⁢ test ⁢ ( if ⁢ any ) ⁢ with ⁢ spike ⁢ ( e . g . , 0.1 ozAu / ton ⁢ ore ) ; AuCN = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ ( unspiked ⁢ bleach ⁢ leach ⁢ test ) ; and AuCN_Spike = cumulative ⁢ cyanide ⁢ ⁢ soluble ⁢ Au ⁢ content ⁢ recovered ⁢ after ⁢ leach ⁢ ( oz / ton ⁢ ore ) .

As noted above, the POX preg-rob capacity can also be determined based on the concentration of gold absorbed by the final solids (an index of zero indicates no POX preg-rob capacity, a value between zero and 1 indicates minimal POX preg-rob capacity, between 1 and 2.5 indicates moderate POX preg-rob capacity, and more than 2.5 indicates that the material has a high POX preg-rob capacity).

In another formulation, the POX preg-rob capacity is determined for gold loaded on preg-robbing carbon by mass balancing the gold lost from the spiked gold-containing solution divided by the mass of preg-robbing carbon in the sample (e.g., calculated as oz gold/ton preg-robbing carbon). The POX preg-rob index is determined by dividing the gold loaded on the preg-robbing carbon by the sum of the gold added in the spike solution and naturally leachable gold content.

In another formulation, a gold spike recovery index can be calculated to determine the increased gold recovery after partial preg-robbing carbon neutralization after POX. The index can be determined by dividing the cyanide soluble gold content resulting from the preg-rob test by the gold spike and adjusting to account for the spike solution dilution. The dilution results from entrainment of wash solution in the decanted solids fed to the spiked preg-rob test.

In response to the presence of preg-robbing carbon, the blanking agent preg-robbing capacity of the preg-robbing carbon in response to application of a blinding or blanking agent can be determined. The preg-robbing blanking agent responsiveness indicates a level of impact on the preg-robbing capacity due to the use of a selected blanking agent. As will be appreciated, any of a number of blanking agents may be employed, including one or more of diesel oils, kerosene, fuel oil, natural oils, cationic surfactants, anionic surfactants, RV-2 (para-nitrobenzolazo salicyclic acid), and methylphenol and phenolate salt thereof. Methylphenols, particularly methylphenols in the form of 2-isopropyl-5-methylphenol or thymol and carvacrol, are typically used as they can be much more effective than diesel and fuel oils and kerosene and require much lower concentrations to be added, thereby limiting blinding agent carryover to activated carbon in CIL, CIC, or CIP circuits. Methylphenols can be stable in heap leaching applications and capable of blinding or blanking preg-robbing carbon from preg-robbing gold from the pregnant leach solution.

The responsiveness can be determined by any suitable chemical test, such as a preg-robbing carbon masking test using a blinding agent. In one type of test, a sample is contacted with a selected blanking agent in a gold spiked cyanide solution having the same cyanide content, solids content, and pH as the solutions above with the exception of inclusion of the blanking agent (e.g., a cyanide solution comprising from about 2 g NaCN/L to about 8 g NaCN/L from about 0.1 to about 25 g/L of blanking agent @ 30%-33% solids pulverized and a pH ranging from about pH 10 to about pH 12) for a determined period of time (e.g., t<30, 30, 45, 60, or t>60 minutes). The cyanide solution can be optionally spiked with gold using the same gold concentration as the tests noted above. The slurried unspiked or gold spiked cyanide solution containing the blanking agent is mixed (e.g., by stirring or rolling such as using a cyanide shake test), and the solid sample separated from the solution such as by centrifugation or filtration. Final solution volumes are measured and sampled for pH, free cyanide, and gold (e.g., by ICP). Final solids are washed, dried, weighed and fire assayed directly to determine residual gold content.

The blanking agent preg-rob capacity can be determined by any number of methodologies. In one methodology, the capacity is determined by the following equation:

Blanking ⁢ Agent ⁢ Preg - Rob ⁢ Capacity = ( Baseline - ( Gold ⁢ Spiked ⁢ or ⁢ Unspiked ) ⁢ Residue ) / ( Total ⁢ Au ) * 100 ⁢ where : Baseline = Au ⁢ remaining ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ after ⁢ the ⁢ baseline ⁢ leach ; Residue = Au ⁢ remaining ⁢ in ⁢ the ⁢ final ⁢ solids ⁢ after ⁢ responsiveness ⁢ test ; and Total ⁢ Au = total ⁢ fire ⁢ assayed ⁢ gold ⁢ content ⁢ of ⁢ sample ⁢ before ⁢ leaching .

In another methodology, the blanking agent preg-rob capacity is determined by the following equation:

Blanking ⁢ Agent ⁢ Preg - Rob ⁢ Capacity =  [ ( Spike + AuCN_Spike ) / ( Spike + AuCN ) ] * 100 ⁢ where : Spike = Au ⁢ added ⁢ during ⁢ bleach ⁢ ⁢ test ⁢ ( if ⁢ any ) ⁢ with ⁢ spike ⁢ ( e . g . , 0.1 ozAu / ton ⁢ ore ) ; AuCN = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ ( unspiked ⁢ responsiveness ⁢ leach ⁢ test ; and AuCN_Spike = cumulative ⁢ cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ recovered ⁢ after ⁢ leach ⁢ ( oz / ton ⁢ ore ) .

As noted above, the blanking agent preg-rob capacity can also be determined based on the concentration of gold absorbed by the final solids (an index of zero indicating no blanking agent preg-rob capacity, a value between zero and 1 indicating minimal blanking agent preg-rob capacity, between 1 and 2.5 indicating moderate blanking agent preg-rob capacity, and more than 2.5 indicating that the material has a high blanking agent preg-rob capacity).

In another formulation, the blanking agent preg-rob capacity is determined for gold loaded on preg-robbing carbon by mass balancing the gold lost from the spiked gold-containing solution divided by the mass of preg-robbing carbon in the sample (e.g., calculated as oz gold/ton preg-robbing carbon). The blanking agent preg-rob index is determined by dividing the gold loaded on the preg-robbing carbon by the sum of the gold added in the spike solution and naturally leachable gold content.

In another formulation, a gold spike recovery index can be calculated to determine the increased gold recovery due to the presence of the blinding agent. The index can be determined by dividing the cyanide soluble gold content resulting from the preg-rob test by the gold spike and adjusting to account for the spike solution dilution. The dilution results from entrainment of wash solution in the decanted solids fed to the spiked preg-rob test.

In some applications, spectrographic textural minerology analysis is performed, such as using X-Ray diffraction (XRD), infrared spectroscopy (e.g., Fourier-Transform Infrared Spectroscopy or FTIR), Raman spectroscopy, and Scanning Transmission Electron Microscope (STEM) to determine the presence and content of refractory minerals such as calcite, quartz, kaolinite, dolomite, and mica/illite and refractory preg-robbing carbon as well as the spectral characteristics of the preg-robbing carbon. By way of illustration, preg-robbing carbon spectral characteristics (e.g., relative height of D (X axis, indicating the abundance of disordered structures), relative width of D (Y axis, indicating the variety of disorder), and relative height of G (Z axis, indicating the order of graphitic structure)) have been determined to indicate a preg-robbing capacity and strength of the preg-robbing carbon. Preg-robbing carbon spectral characteristics indicate five preg-robbing carbon types: namely (a) activated carbon-like preg-robbing carbon having a high preg-robbing capacity and strength, (b) coal-like preg-robbing carbon having a high preg-robbing capacity but significantly lower preg-robbing strength, (c) medium preg-robbing preg-robbing carbon having a medium preg-robbing capacity and strength, (d) high preg-robbing ordered preg-robbing carbon having a high preg-robbing capacity and strength, and (e) graphite-like preg-robbing carbon having a low preg-robbing capacity and strength. As will be appreciated, the various types of preg-robbing carbon are typically present at varying levels on a preg-robbing carbon-containing material. The relative concentrations of each type will determine the overall preg-robbing characteristics of the material.

In some applications, thermal gravimetric analysis or TGA is performed. As will be appreciated, TGA analyzes weight losses and off-gas composition over a range of burn temperatures from ambient to 1,000° C. to determine the onset of preg-robbing carbon combustion for roasting as a processing option. The optimal onset temperature for preg-robbing carbon combustion typically ranges from about 350 to about 750° C. and more typically from about 450 to about 650° C.

FIG. 1 depicts the use of the above indices to route precious metal-containing material to appropriate unit processing operations (e.g., roasting, oxide leaching, and pressure oxidation) for substantially maximized precious metal recovery. As will be appreciated, the levels of preg-robbing carbon and sulfides and relative mix of preg-robbing carbon types can vary spatially in different portions of the deposit being mined and processed.

With reference to FIG. 1, the precious metal-containing material 100 can be in the form of a sulfidic or oxidized precious metal-containing material.

The sulfidic precious metal-containing material typically comprises from about 1 to about 100 g/tonne gold, from about 2 to about 60 wt. % sulfides, and from about 0.05 to about 10 wt. % preg-robbing carbon. Commonly, the sulfide minerals are predominantly pyrite, realgar, orpiment, chalcopyrite and arsenopyrite, with minor amounts of enargite, pyrrhotite, sphalerite, galena, stibnite, cinnabar, covellite, chalcocite and other commonly occurring sulfide minerals. The precious metal-containing material can be in any form, such as a flotation concentrate, raw ore, flotation tailings, and the like.

The oxidized precious metal-containing material can be oxidized by environmental exposure in which other metallic elements and sulfides are gradually leached away, leaving behind gold and insoluble oxide minerals as surface deposits. Typically, it comprises from about 1 to about 100 g/tonne gold, no more than about 5 wt. % sulfides, from about 2 to about 60 wt. % oxide minerals, and from about 0.05 to about 10 wt. % preg-robbing carbon or residual carbonaceous matter. Commonly, the sulphide minerals are predominantly pyrite, realgar, orpiment, chalcopyrite and arsenopyrite, with minor amounts of enargite, pyrrhotite, sphalerite, galena, stibnite, cinnabar, covellite, chalcocite and other commonly occurring sulfide minerals. The precious metal-containing material can be in any form, such as a flotation concentrate, raw ore, flotation tailings, and the like.

The precious metal-containing material 100 is analyzed in step 110 using the various indices noted above to determine which set of unit operations will produce the highest precious metal recovery and the precious metal-containing material 110 is routed accordingly. The routing decision will be discussed with reference to FIG. 10.

Referring to FIG. 10, an embodiment of the routing logic will be discussed with reference to a mining operation comprising three sets of unit processing operations, namely roasting or heap and/or dump leaching, oxide milling and pressure oxidation. It is to be understood that many mining operations will have only two sets of unit processing operations, such as roasting and oxide milling, oxide milling and pressure oxidation, and the like. Other mining operations may have other unit processing operations, depending on the valuable metal content and mineralogy of the material. Examples include heap and dump leaching. While FIG. 10 discusses material routing based solely on the various indices noted above, it is to be understood that the capital and operating costs and capacities of each set of unit processing operations and the recoverable precious metal content in the material by each set of unit processing operations are also important considerations.

In step 1000, the untreated preg-robbing capacity, preg-robbing strength, POX preg-robbing capacity, and blanking agent preg-robbing capacity of a selected lot of precious metal-containing material are determined. As noted, the preg-robbing carbon content and preg-robbing carbon type will spatially vary throughout the ore deposit.

In decision diamond 1004, it is determined, based on the untreated preg-robbing capacity, whether the selected lot of precious metal-containing material is refractory. As noted, the material is deemed to be refractory when the untreated preg-robbing capacity exceeds a first selected threshold. As will be appreciated, the first selected threshold can be based on the value of any preg-robbed precious metals compared to the capital and operating costs associated with additional unit processing operations required to recover such preg-robbed precious metals.

When the selected lot of precious metal-containing material is determined to be refractory, it is determined in decision diamond 1008 whether the POX preg-robbing strength is less than a second selected threshold. As will be appreciated, lower values of POX preg-robbing capacity indicate that the pressure oxidation effectively reduces the refractory behavior of the material, whether such behavior is due to sulfide occlusion or preg-robbing carbon. When the POX preg-robbing capacity is within a range of about 5-15% of an unacceptably high untreated preg-robbing capacity, using less expensive unit processing operations may be desirable.

When the POX preg-robbing capacity is less than the second selected threshold, it is determined in decision diamond 1012 whether the preg-robbing strength is less than a third selected threshold. As will be appreciated, even though the POX preg-robbing capacity may be a moderate or intermediate, the competitive strength of the sorbent can be further reduced by pressure oxidation to make the preg-robbing carbon less competitive when compared to the sorbent. When the selected precious metal-containing material is not refractory or the POX preg-robbing capacity is less than the second selected threshold and preg-robbing strength is less than the third selected threshold, the selected lot of material is routed in step 1016 to the autoclaving set of unit operations. In some embodiments, the third selected threshold is no more than the POX preg-robbing capacity to avoid POX causing the preg-robbing carbon from becoming more competitive compared to the selected sorbent.

When the POX preg-robbing capacity is not less than the second selected threshold or when the preg-robbing strength is not less than the third selected threshold, it is determined in decision diamond 1020 whether the blanking agent preg-robbing capacity is less than a fourth selected threshold. In other words, the blanking agent may neutralize the preg-robbing carbon in the CIL CIC, CIP, RIL, RIC, or RIP recovery of the precious metal following the selected set of processing operations. The fourth selected threshold can be related to thresholds applied to one or more of the other preg-robbing indices. For example, the fourth selected threshold can be within 5-15% of the untreated preg-robbing capacity, the POX preg-robbing capacity, or the preg-robbing strength.

When the blanking agent preg-robbing capacity is less than the fourth selected threshold, the selected lot of precious metal-containing material is routed in step 1024 to the oxide milling set of unit operations.

When the blanking agent preg-robbing capacity is more than the fourth selected threshold, the selected lot of material is routed in step 1028 to the roasting or heap or dump leaching set of unit operations.

In some embodiments, the blanking agent preg-robbing capacity is used to determine whether or not to perform cyanide leaching in the presence of a blanking agent regardless of the set of unit processing operations selected. In other words, the other indices are used to select the set of unit processing operations with the use of a blanking agent in cyanide leaching following the set of unit processing operations being based on the comparative value of the blanking agent preg-robbing capacity compared to the fourth selected threshold.

While the above example is based on the preg-robbing indices being higher for more preg-robbing materials, it is to be understood that the preg-robbing indices can be determined in an opposite manner such that a lower index indicates a higher level of preg-robbing behavior. In that event, the comparative results (e.g., greater than or less than) and routing behaviors would be reversed.

Returning to FIG. 1, the precious metal-containing material 100 in step 104 is comminuted, such as by wet or dry crushing or grinding to a desired size range. The comminution circuit typically includes the steps of crushing/grinding in comminution step 104, and thickening 112 to produce a comminuted precious metal-containing material 108, that is typically from about 30 to about 60 wt. % solids. The overflow from the thickening circuit 112 (which is primarily water) can be recycled back to the grinding step for reuse. Additional water is typically added to the grinding device (which is typically a Semi-Autogencous or SAG, ball mill, high pressure grinding roll or HPGR, or rod mill, or combination of thereof) as needed to provide the desired liquid fraction to the slurry outputted by the grinding step. As will be appreciated, there are a large number of other comminution circuit designs and/or components that can be used in the process. The size range depends on the particular process to be employed. Typically for non-heap leaching applications, the comminuted feed material 108 has a Pro size ranging from about 600 to about 100 mesh (Tyler) and for heap or dump leaching applications, the comminuted feed material, before agglomeration, has a Pso size ranging from about 10 mesh (Tyler) to about 5 inches.

When the comminuted material 108 is routed to roasting as the subsequent process and the precious metal-containing material is sulfidic precious metal-containing material, the thickened comminuted feed material is dried 116, roasted 120, and the calcine quenched 124. Modern roasters use a fluidized bed construction and conventional fuel source to heat the ores at start-up. Roasters are typically autogenous and use the fuel energy within the ore itself to maintain operating temperatures. Conventional fuel sources such as propane or natural gas are generally used upon start-up as a pre-heat step until the bed ignites. Roasting temperatures are usually between about 500° and 700° C. After roasting, the calcine is separated from dust and off-gasses and then quenched.

For any particular ore composition, roasting plants operate in a narrow range of tolerances. Below optimum temperature the carbon in the ore is not oxidized and remains actively preg-robbing. Above the optimum temperature, the gold in the ore becomes increasingly less amenable to cyanidation or other extraction techniques. Because of the degrading gold recovery with higher temperatures, many roasters are operated toward the lower side of the range. The blanking agent is added to passivate any unroasted carbonaceous matter. Accordingly, roaster efficiency in a plant environment tends to vary widely with variation in feed stock and operating conditions.

Following quenching, the oxidized ore or calcine can be pretreated 126 and processed using traditional cyanide extraction techniques. Pretreatment 126 typically comprises forming the calcine into a slurry and pH adjusting the slurry with acid consumers to about pH 10.5.

When the comminuted material 108 is routed to oxide milling as the subsequent process and the precious metal-containing material is oxidized precious metal-containing material, thickening 112 and pretreatment 128 are performed before precious metal leaching. The slurried precious metal-containing material is adjusted by adding water and acid consumers, such as carbonate containing flotation tailing, limestone and lime. The pH value of the slurry is adjusted in the process of cyanide treatment, to neutralize the acidity natural to the comminuted material, and carbonated acid produced in the process of cyanide, and avoid the action of H and cyanide to produce HCN (hydrogen cyanide). In addition, it can reduce the destruction effect of iron minerals on CN⁻ (cyanide ion). Generally, the liquid-solid weight ratio of the pulp is 2-4:1, which can ensure that the activated carbon is suspended in the pulp, so as to facilitate the subsequent adsorption operation. The equipment required in this stage is mainly one or more agitation tanks. The pre-cyanidation treatment of oxidized precious metal-containing slurry is in a stirred reactor, typically one or more pressure kettles, with the addition of sodium cyanide (reaction under the pressure of about 0.6 MPa˜1.0 MPa). The blanking or blinding agent can be added during pre-cyanidation treatment. The general cyanide pretreatment time is 0.5-2 hours to ensure that the pre-cyanide and blanking reactions are fully conducted. The stirred tank reactor stirring speed is in the range of about 600-1200 r/min, which can promote the diffusion of CN-and oxygen in the pulp and improve the pretreatment efficiency.

When the comminuted material 108 is routed to pressure oxidation as the subsequent process and the precious metal-containing material is sulfidic precious metal-containing material, slurry from the comminution circuit, at approximately 35% solids and 80-85% passing 135 mm is pumped to one or more thickeners. Thickener underflow, at approximately 54% w/w solids, is pumped to a train of four acidulation tanks in acidulation step 114. Sulfuric acid is added to the slurry to destroy sufficient carbonate prior to entering the pressure oxidation circuit. Process air is also injected into the acidulation tanks to aid in carbon dioxide removal. Carbonate levels are typically reduced to <2% in the acidulation tanks.

The slurry is advanced from the acidulation tanks through a series of direct contact heater (“splash”) vessels in preheating step 136 before feeding into the autoclave. The slurry is preheated to a temperature of 165-175° C. as it passes through the splash vessels. The heat source in the splash vessels is the flashed steam, which is released from the slurry discharging from the autoclave as it passes through the pressure let down (“flash”) vessels. The slurry enters the splash vessels at the top and cascades down the internal baffles, while the flashed steam enters the lower section of the vessel and rises, contacting the slurry for direct heat transfer. The bottom of each vessel serves as a pump box for the interstage feed pumps. The discharge from the high-pressure splash vessel is pumped into the autoclave.

The preheated and pretreated material is introduced into the first compartment of the autoclave. The autoclave will typically have at least six compartments to minimize short circuiting of the feed slurry to the pressure oxidized slurry as can occur in autoclaves with fewer compartments. Short circuiting reduces the degree of completion of the pressure oxidation reactions. Excess gas, including components such as carbon dioxide, oxygen, nitrogen, and argon, is vented through a vent. As will be appreciated, the autoclave atmosphere typically contains at least about 80% steam, 10% molecular oxygen, and 10% inert gases.

Each compartment includes one or more agitators and sparge tubes for introducing molecular oxygen. As will be appreciated, the autoclave can have any number of compartments and be of any suitable design, including a stacked or vertical autoclave design. Cooling water (not shown) can be added to the various compartments to maintain desired slurry temperatures. Preferably, no more than about 1% of the precious metal in the slurry is solubilized into the liquid phase of the pressure oxidized slurry during pressure oxidation.

The autoclave is preferably operated under conditions to promote hematite formation in the first one and/or two compartments of the multi-compartment autoclave. Desirably, hematite formation is promoted by maintaining the sulfuric acid concentration in the first compartment at a relatively low level. Once formed, hematite provides a favorable nucleation site for further hematite formation and suppresses formation and precipitation of basic iron sulphate and jarosite in downstream compartments of the autoclave. The average total residence time in the autoclave typically ranges from about 0.75 to about 2 hours.

The slurry discharges from the autoclave and passes through a series of pressure let down stages 140 called flash vessels. Pressure and temperature are gradually let down to atmospheric pressure and 96° C., after passing through the flash vessel circuit. The steam released by the instantaneous reduction in pressure through the flash vessel is ducted to the corresponding splash vessel. Slurry leaving the pressure let down circuit is then cooled from 96 to 48° C. by a series of six to eight shell and tube heat exchangers. The cooling water is on the shell side of the heat exchanger and the slurry passes through the tubes. In summary, the flash/splash system is a heat-recovery system, which minimizes the use of direct steam with inherent operating cost benefits.

After the slurry passes through the slurry coolers it is pumped to two parallel trains of neutralization tanks in neutralization 144, where the pH value is elevated from pH 1-2 to about pH 10.5. The neutralization step 144 can be performed in two stages. In the first stage, which can have multiple reactors, free flotation tailings or inexpensive limestone is contacted with the dissolved ferric sulphate and free sulfuric acid to form ferric hydroxide and gypsum. In a second stage to achieve a higher pH, typically at least about 90% of the dissolved ferric sulphate is precipitated. In the second stage which can also have multiple reactors, lime is contacted with the slurry discharged from the first stage of neutralization to reach the final pH.

In some configurations, a hot cure step 148 is used to convert the (solid) basic ferric sulphates in the discharged slurry to dissolved ferric sulphate. Preferably, the discharge slurry is held in the hot cure step 148 long enough for most of the basic ferric sulphates to be converted into the dissolved ferric sulphate. Dissolved ferric sulphate can be separated readily from the solid phase in a solid/liquid separation circuit. Moreover, the dissolved ferric sulphate in the separated liquid phase will be readily reacted with limestone during the subsequent neutralization to produce ferric hydroxide. Typically, the slurry 127 is held in the hot cure step 130 for a time ranging from about 1 to about 24 hours. The hot cure step 148 is preferably carried out in one or more stirred tank reactors at atmospheric pressure.

As noted, other processes can be used to oxidize the sulfidic precious metal-containing material instead of roasting or pressure oxidation. These techniques include chemical oxidation, such as using chlorine oxidation and oxidation using other oxidants, and bacterial leaching. In chemical oxidation using chlorine, the precious metal-containing material is ground and mixed with water to form a slurry. Bacterial leaching uses bacteria to biologically degrade sulfide minerals and liberate precious metal values so that they can be recovered by conventional technologies. The most widely used and studied bacteria for this process is Thiobacillus ferrooxidans. Chemical and bioleaching can be performed in a heap of particulates or in a continuous stirred tank reactor containing a slurry. The material may be agglomerated for heap leaching applications.

Suitable leaching techniques may also be utilized. In some embodiments, lower grade ores and other precious metal-containing materials are treated by run of mine (ROM) dump leaching techniques in which the material is excavated and placed onto the heap or dump leaching pad. In some embodiments, lower grade ores and other precious metal-containing materials are comminuted, optionally agglomerated, and heap or tank leached to oxidize sulfides. In either ROM dump or heap or tank leaching, the precious metal-containing residue is treated by precious metal leaching techniques noted above to dissolve and recover precious metals.

Regardless of the process employed, the oxidized precious metal-containing slurry is optionally contacted with the blinding agent in step 152. The blinding agent may be added in any step of the process provided that it remains effective in downstream processing steps. As shown by the multiple occurrences of step 152, the blinding agent can also or alternatively be added during comminution 104 and/or thickening 112 depending on the stability of the blinding agent in subsequent process steps. The concentration of the blinding agent in the liquid component in contact with the precious metal-containing material and/or in the slurry containing the precious metal-containing material typically ranges from about 0.1 to about 25 g/L, more typically from about 0.25 to about 15 g/L, and even more typically from about 0.5 to about 10 g/L. The weight ratio of blinding agent to the carbonaceous material (e.g., preg-robbing carbon) typically ranges from about 0.001:1 to about 10:1, more typically from about 0.005:1 to about 5:1, and more typically from about 0.05:1 to about 2.5:1.

The precious metal is dissolved by leaching the oxidized precious metal-containing material in the pretreated/neutralized slurry in the precious metal leach step 156. The leaching agent or lixiviant is typically alkali- or acid-based, with exemplary lixiviants being cyanide, halides (iodide, bromide, chloride), ammonium, calcium, or sodium thiosulfate, and thiourea. In one configuration, the leach step is performed at atmospheric pressure and under alkaline conditions (at or above a pH of about pH 7) to produce a pregnant leach solution containing (at least) most of the precious metal content of the oxidized precious metal-containing material. The precious metal leach step 156 may be done by any suitable technique including using cyanide leaching and Carbon-In-Pulp or CIP techniques, Carbon-In-Column or CIC techniques, Carbon-In-Leach or CIL techniques, cementation techniques, Resin-In-Pulp or RIP techniques, Resin-In-Column or CIC techniques, Resin-In-Leach or RIL techniques, or by circulating a pregnant leach solution and/or slurry through one or more gold sorbent columns. In the CIL, CIC, CIP, RIP, RIC, RIL, and other sorbent-based techniques, a sorbent, such as activated carbon or an ion exchange resin, sorbs the precious metal dissolved in the lixiviant. The blinding agent inhibits precious metal sorption on the preg-robbing carbon, but typically does not carry over to the target sorbent, thereby providing a high gold recovery. The sorbed precious metal is stripped from the sorbent in precious metal recovery step 160 by an acidic or alkaline eluant to form a barren sorbent for recycle to the leach step 156 with and/or without regeneration, and a pregnant eluate containing most of the precious metal sorbed on the sorbent.

In the precious metal recovery step 160, the precious metal is recovered from the pregnant leach solution (or pregnant eluate) by suitable techniques, such as electrowinning or cementation followed by smelting, to form a precious metal product. When required, the barren residue from the leaching step is subjected to cyanide detoxification or destruction and discarded as tailings.

EXPERIMENTAL

The following examples are provided to illustrate certain embodiments of the disclosure and are not to be construed as limitations on the disclosure, as set forth in the appended claims. All parts and percentages are by weight unless otherwise specified.

Despite numerous studies to date, the large variability and unpredictability of the preg-robbing phenomenon are not yet fully understood, leading to challenges in ore routing. Years of historical gold ore processing have led to numerous observations of varied plant gold recovery responses with seemingly similar organic carbon constituents. Two samples of the same preg-robbing carbon content can easily give different recoveries under the same processing or test conditions. For example, one sample having about 40% preg-robbing carbon factor may go through the plant without incident, achieving high gold recovery, while another one with similar preg-robbing carbon content gives a much lower gold recovery. This suggests the need to enhance the capability of differentiating between more subtle preg-robbing carbon characteristics for improved gold recovery response in the operating plants.

This experimental work aimed to evaluate, quantify, and differentiate the characteristics of naturally occurring carbonaceous matter for loading and retaining gold during dissolution and recovery processes. Ore samples underwent extensive chemical analyses, including controlled leaching and carbon adsorption tests, and spectrographic analysis. These tests identified gold ores containing carbonaceous materials with varying levels of preg-robbing capacity and strength. The adsorption kinetics and equilibrium between gold in solution and various carbonaceous materials revealed distinct patterns relevant to gold retention and the recovery process. Carbonaceous materials were analyzed, identified, and categorized into five primary classes according to their spectroscopic features, which were subsequently correlated with the results of chemical tests.

The preg-robbing potential of an ore can be assessed through several chemical procedures that involve the use of gold spikes, oxidants, and passivation reagents. These procedures quantitatively measure how much gold from the leaching solution is lost to adsorption or other reactions. The primary tests include: carbon adsorption test, carbon activity index, spiked leach test (carbon-in-pulp (CIP) or carbon-in-leach (CIL) Simulation), roasting test, diagnostic leaching, organic carbon content analysis and kerosene passivation test.

More advanced spectroscopic techniques, such as X-ray diffraction (XRD) and infrared spectroscopy (IR), can also be utilized to analyze the structural characteristics of carbonaceous matter in its various forms. These methods provided insights into the molecular structure, bonding, and composition of the carbonaceous materials, helping to elucidate their behavior during mineral processing and their impact on preg-robbing in gold recovery processes.

The objective of this study extends beyond enhancing the characterization of carbonaceous matter; it also seeks to address key questions related to gold bond strength and the effectiveness of diagnostic methods. To achieve this, chemically and spectroscopically diverse composites and individual samples derived from refractory ores were analyzed. Seventeen samples were subjected to chemical testing to evaluate their metallurgical behavior, complemented by Raman spectroscopic analysis of approximately seven hundred individual carbonaceous particles. These combined approaches aimed to provide a detailed understanding of the carbonaceous materials' composition, structure, and impact on mineral processing techniques.

Traditional analytical methods included normal head analyses for gold by fire assay, cyanide solubility (cyanide shake test), preg-rob assay, LECO determination of total sulfur and sulfide sulfur, along with total and organic (TCM) carbon. Sulfide sulfur speciation was completed using a sodium carbonate digest followed by LECO determination of the residual S, while carbon speciation was completed using a hydrochloric acid (HCl) digestion followed by LECO determination of the residual carbon (TCM). A cyanide soluble gold to head assay ratio (CN/FA) was also reported for each sample.

The conventional preg-rob test series consisted of the cyanide shake analysis, as well as a cyanide shake analysis conducted using the same procedures, but with a “gold spike” added. The gold spike that was added to the leach solution used was equivalent to 0.10 ozAu/ton ore (0.03 g Au/ton ore). A comparison of results from the two tests was used to calculate a percent preg-rob factor, defined as:

Preg - Rob ⁢ Factor =  [ ( Spike + AuCN - AuCN_Spike ) / ( Spike + AuCN ) * 100 ⁢ where : Spike = Au ⁢ added ⁢ during ⁢ cyanide ⁢ shake ⁢ with ⁢ spike ⁢ ( 0.1 ozAu / ton ⁢ ore ) AuCN = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ ( cyanide ⁢ shake ⁢ analysis ⁢ results ⁢ in ⁢ oz / ton ⁢ ore ) AuCN_Spike = cyanide ⁢ soluble ⁢ Au ⁢ content ⁢ resulting ⁢ ( oz / ton ⁢ ore ⁢ from ⁢ spiked ⁢ cyanide ⁢ shake ⁢ analysis )

Non-traditional chemical testing included a bleach leach test, TCM masking tests, two types of preg-rob capacity testing, and both parallel and sequential CIP and CIL testing of the loaded ore samples. Some of these tests were either developed or modified from their original protocol as the project evolved and results from an earlier stage were evaluated to provide more clarity around the preg-robbing carbon characteristic being investigated.

In the bleach leach test, a sample of ore is pretreated with a weak bleach solution and then subjected to a spiked cyanide solubility test. The percent spike recovery provided guidance on how to route the material-higher values meant the ore could be routed to the autoclave, while lower values required routing to the roaster. A mid-range value utilized the preg-rob value of the untreated ore sample to guide where the material could be processed: at the autoclave versus the roaster. This test, therefore attempted to evaluate the preg-robbing strength of the ore samples.

The bleach leach test procedure was conducted in two stages. During the first stage, a 10-gram aliquot of sample was leached using a 10% bleach solution at a slurry density of 33.3% solids for 1 hour. After leaching, the bleach solution was removed by centrifuging and decanting, the solids are rinsed with deionized (DI) water and the rinsed solids were subjected to a gold spiked cyanide shake analysis, using the procedure used for the preg-rob analysis described above. The initial leach with bleach solution was expected to passivate the weaker preg-robbing TCM and thereby gauge the preg-robbing strength of the sample, allowing for differentiation between TCM of different preg-robbing strengths. A gold spike recovery was calculated for each test by dividing the cyanide soluble gold content resulting from the spiked preg-rob test (after bleach leaching) by the gold spike and adjusting to account for the spike solution dilution. The dilution was expected to be the result from entrainment of wash solution in the decanted solids fed to the spiked preg-rob test.

Some additional testing of the bleach leach procedure included varying the amount of bleach used in the pre-leach step in an attempt to normalize the bleach to preg-robbing carbon ratio for samples with varying preg-robbing carbon content. Testing of higher ratios of bleach to preg-robbing carbon were also included.

Novel preg-robbing carbon masking tests were conducted using a blinding agent. Pretreatment of the samples using the blinding agent generally was conducted using reagent solution containing the blinding agent at a 10 gram per liter concentration in sufficient volume to achieve blinding agent:TCM mass ratios of between 0.05:1 and 1:1. Pretreatment generally was conducted by agitating the sample slurry (33.3% solids) with the blinding agent, for a period of 24 hours.

Two series of custom preg-rob capacity tests were conducted on each sample to characterize the preg-rob potential of the samples:varying the spiked leach solution mass to ore mass ratio (designated “Method 1”—volume basis) and varying the spiked leach solution grade (designated “Method 2”—concentration basis). A total of six tests were conducted on each sample, including three Method 1 tests and three Method 2 tests. The test procedure consisted of contacting a split of pulverized ore with a known volume of synthetic gold bearing pregnant (“gold spike”) solution for a 2-hour period and analyzing the resulting solution and leach solids for gold content, to assess preg-robbing by TCM contained in the samples. Varied response to test conditions (either solution: ore ratio or solution grade) offers a useful means for distinguishing preg-robbing capacity differences.

Carbon-in-leach (CIL) and carbon-in-pulp (CIP) cyanidation bottle roll tests were conducted with a gold spike added to further assess the preg-robbing characteristics of the ore samples. In the case of the spiked CIL tests, the purpose was to evaluate how the native-contained TCM would compete with added granular activated carbon (GAC) in the recovery of dissolved gold from leach slurries. In the case of the spiked CIP tests, the GAC was added after an initial spiked preg-rob test to evaluate how strongly the native TCM held onto the pre-adsorbed gold. Both tests were conducted to evaluate how strongly the preg-robbed gold was held by the native TCM.

Products from each test, including solutions, leached solids and loaded granular activated carbon were assayed to determine gold content. A preg-rob factor was calculated for each test, based on an assessment of the gold loaded onto the native TCM, compared to either gold recovery by traditional CIL test (for the spiked CIL test) or gold recovery by cyanidation with GAC added after spiked gold adsorption (for the spiked CIP test).

		Sample	Slurry	NaCN	GAC	Au Spike	Test
		Mass,	Density,	Concentration	Added,	Concentration,	Duration,
Analysis	Description	grams	% solids	g/L	g/L slurry	ozAu/ton ore	hr
Fire Assays	Conventional fire	30	N/A	N/A	N/A	N/A	N/A
	assay fusion.
CN Shake	Conventional	10	33	5.0	N/A	N/A	1.0
Analyses	cyanide solubility
	analysis.
Preg-Rob	Conventional	10	33	5.0	N/A	0.1	1.0
Assays	cyanide solubility
	analysis with a
	“gold spike” added
	to the solution.
Bleach	2 stage test: Leach	10	33	5.0	N/A	0.1	1.0
Leach Tests	with bleach solution
	followed by preg-
	rob assay.

A summary of the analytical methods and conditions of the chemical tests conducted is provided in Table 1.

TABLE 1
Summary of Analyses Conducted and Tests Conducted with Test Conditions

Carbon	Leco methods with	0.2	N/A	N/A	N/A	N/A	N/A
Speciation	and without HCl
	digestion.
Sulfur	Leco methods with	0.2	N/A	N/A	N/A	N/A	N/A
Speciation	and without sodium
	carbonate digestion.
TCM Masking	Pretreatment with	10	33	5.0	N/A	0.1	1.0
Tests	blinding agent
	solution followed by
	cyanide shake and
	preg-rob assay.
CN Tests	Bottle roll leach test	~500	33	2.0	N/A	N/A	24.0
CIL Tests	Carbon-in-leach	~500	33	2.0	20.0	N/A	24.0
	bottle roll test, with
	activated carbon
	(GAC) added.
PR Capacity	“Gold spike” preg-	~100	33	5.0	N/A	0.1	24.0
Tests,	rob bottle roll test
Method 1	series (3 tests),
	using constant spike
	concentration and
	varied solution:ore
	ratios.
PR Capacity	“Gold spike” preg-	~100	33	5.0	N/A	Varied	2.0
Tests,	rob bottle roll test					(~0.2-0.8)
Method 2	series (3 tests),
	using constant
	solution:ore ratio
	and varied spike
	concentration.
Spiked CIL	“Gold spike” preg-	~100	33	5.0	N/A	0.1	2.0
Tests	rob bottle roll test
	with GAC added
	during spike
	solution contact
	time.
Spiked CIP	2- stage bottle roll	~100	33	5.0	20	0.1	2.0
Tests	test, with initial gold				(during	(during
	spike ore solution				2nd stage	1st stage
	contact period (no				only)	only)
	GAC added)
	followed by
	solid/liquid
	separation and
	contact of solids
	with barren cyanide
	solution for 2 more
	hrs with GAC
	added.

Spectroscopic examination of the seventeen ore samples included sample prep followed by Raman analysis of each sample. Approximately 5 g of ore particles were embedded in an epoxy section and the surface was polished with 600 and 1200 grit abrasives, followed by 6 μm, 3 μm, and 6 μm diamond suspensions, using a Metkon automated polisher.

Raman analysis was conducted using a Renishaw inVia Qontor Raman microscope. TCM particles were identified by reflected optical light through a 100× objective of the Renishaw confocal microscope. A 488 nm blue laser was selected for Raman spectra acquisition, due to its consistency between different ore samples. The laser energy and exposure time were carefully controlled to obtain enough counts without burning the TCM. The target grains on the surface of the polished section were first identified by optical microscopy and then confirmed by Raman.

The Raman spectra have been processed using Renishaw WiRE software for baseline correction and deconvolution. The deconvoluted Raman bands selected for deconvolution are summarized in Table 2. This signal treatment reveals the individual bands corresponding to symmetric vibration of various carbon atoms within the TCM molecular structure. Analyzing the correlation between specific carbon vibrational bands for each TCM particle provides a more detailed understanding of TCM composition and structure.

TABLE 2
Raman Bands Deconvoluted from Spectra by the Renishaw WiRE Software.

Type	Name and Centre (cm−1)

Normal	G	D	G’ or 2D	T	D + D’	D + D”	Amorph	2D’
	1600	1354	2715	1250	2930	2500	1530	3180
Graphite	G	D	G’ or 2D	D’	D + D’	D + D”	2D’	C—H
								Aromatic
	1600	1354	2715	1613	2930	2500	3240	3060

In addition to the TCM particles from ore samples, model carbon, such as graphite, activated carbon, and coal, were analyzed by chemical tests and Raman spectra in this study. The comparison between model carbon samples and the TCM particles would validate the structural characteristics of carbon and help classify the TCM groups.

Standard Chemical Testing

A summary of the results from the routine head analyses and tests is presented in Table 3 and discussed in detail below. Fire assay showed that the samples tested ranged in gold grade from 002 to 0.837 and averaged 0.337 oz/ton.

Leco carbon speciation analyses showed that the samples contained between 0.05% and 2.77% (0.79% avg) total carbonaceous matter (TCM), based on residual carbon content after hydrochloric acid digestion. Carbonate carbon content represented an additional 0.02% to 5.15% C, or 0.10% to 25.7% CO₃. The samples also contained elevated sulfide sulfur levels of 0.86% to 4.36% S.

Cyanide shake analysis results showed that all the samples tested were refractory to cyanide leaching and that none of the samples had a cyanide soluble gold to head assay ratio (CN/FA) that exceeded 20%. Interpretation of cyanide leach data was complicated by the expected “double-refractory” nature of the samples. Most of the samples tested were expected to be in part refractory because of locking of a portion of the contained gold in sulfide minerals, as well as because of preg-robbing by naturally occurring carbonaceous matter. Preg-rob factors for the sample set ranged from 2.9 to 100%.

As shown in Table 3 and in FIG. 9, the preg-rob factors of the samples tested were not well correlated to TCM content. The larger circles representing higher TCM content were spread across the x-axis (Preg Rob or PR Factor or PRF), as were the smaller circles.

TABLE 3
Analyses and Traditional Leach Test Results

Leco Analysis

Head Assay

CN Shake

Preg-Rob

Bleach

Agitated Cyanidation

TCM,

Sulfide,

oz/ton ore

CN/FA,

Factor,

Leach Spike

Gold Recovery, %

Sample	% C	% S	Au	Ag	% Au	% Au	Recovery	no GAC	CIL

2	1.12	2.20	0.127	0.14	1.1	82.0	13.8	<0.8	14.4
3	0.31	2.09	0.649	0.27	10.2	44.7	189.0	14.5	35.7
4	0.66	1.62	0.266	0.08	0.9	96.5	135.9	0.4	34.5
5	2.77	1.15	0.363	0.02	0.1	100	36.3	<0.3	2.4
6	0.63	1.33	0.519	0.02	5.0	62.4	179.3	0.6	21.2
7	0.23	2.25	0.724	0.02	12.5	2.9	266.8	11.3	12.3
8	0.54	0.86	0.002	0.02	<0.01	100	45.9	<14.3	<12.5
9	0.55	2.37	0.083	0.44	0.4	81.9	74.7	<1.3	8.5
10	0.77	1.59	0.107	0.09	1.3	87.4	79.7	<1.0	22.5
11	1.13	2.87	0.278	0.05	19.5	5.6	126.3	17.9	23.3
12	1.14	2.54	0.332	0.04	4.2	18.3	81.1	4.4	6.5
13	1.38	3.23	0.837	0.04	2.0	68.6	48.7	0.5	23.2
14	0.51	4.36	0.241	0.01	0.1	98.0	7.8	0.4	10.9
15	0.05	3.33	0.227	0.00	4.4	9.3	86.4	5.3	6.3
16	0.05	4.20	0.289	0.00	12.5	13.4	125.9	12.4	16.4
17	0.75	3.39	0.432	0.02	2.2	50.5	39.8	0.5	7.4
18	0.83	3.33	0.255	0.01	4.7	35.2	29.2	0.9	9.0

Bleach leach tests (BLTs) on all seventeen samples showed that the test procedure gave variable results, particularly for samples containing high levels of TCM. Additional testing (not presented here) was conducted to evaluate the effects of varied bleach to TCM mass ratios, achieved by varying solids density and bleach leach solution strength (NaOCl concentration). Gold dissolution during bleach leaching also was observed to be a significant potential source of variable error under certain conditions. Further extensive testing would be required to assess the reliability of this testing procedure for ore characterization purposes. Furthermore, any changes that may occur during bleach leaching to the gold-sulfide mineral locking may impact results in a manner not related to the preg-robbing character of the ore.

The relationships between TCM content, preg-rob factor and bleach leach test results are shown graphically in FIG. 9. Neither preg-rob factor nor bleach leach test results were well correlated with TCM content, demonstrating the complex nature of the TCM mineralization. Neither was there a strong correlation between the bleach leach test result and the percent preg-rob factor, although nearly all of this sample set was defined as double-refractory in nature.

FIG. 9 refers to Raman groupings (1 through 5) for the various samples and is shown as a function of both BLT and PR Factor results. Raman groupings are discussed below and provide additional support to the tie between chemical testing results and the spectroscopic interpretations, highlighting some natural groupings.

TCM Groupings by Raman Spectroscopy

After deconvolution, the absolute height (highest point of band) and width (width of band at half height) of each band were acquired. The relative height and width of each band were then calculated by normalizing their absolute values to the G-band, enabling consistent comparisons of different TCM samples using the same baseline. Five groups of TCM were classified by the Raman spectroscopic characteristics, most importantly by the relative height of D-band, the relative width of D-band, and the relative height of G′ or 2D-band.

Model carbons, including graphite, activated carbon and coal, were also analyzed using the same procedures for their Raman spectra features. Comparing the model carbons with the native ore TCM gives a more broad and clear vision of the correlation between the carbon molecular structures and their adsorption and adherence characterization of dissolved gold.

The relative height of D-band (indicating the abundance of disorder) and the relative width of D-band (Y axis, indicating the variety of disorder) have been considered to classify the TCM particles analyzed by Raman spectroscopy as five TCM groups:

Group 1. Graphitic, Low-PR: As shown in FIGS. 2A and B, this class of TCM has a narrow and short D-band (red color) as well as a tall G′-band (2D-band, light green color), indicating a very ordered graphitic structure. Ores within Group 1 TCM have PRF <20%.

Group 2. Ordered, Variable-PR: As shown in FIGS. 2C and D, this class of TCM has a narrow but very tall D-band compared to G-band (height ratio over 1.70). For example, Group 2's D-band is much taller than Group 3, see FIGS. 3 E and F. This suggests an even more ordered graphitic structure over the medium-PR class. Samples 3, 13, 16, 17, and 18, containing about 10 to 20% of this TCM type, have a lower PRF of 13 to 52%. However, sample 14 (62.5% TCM particles identified by Raman microscope belonging to this class) has a strong PRF of 98%.

Group 3. Mix-Ordered, Medium-PR: As shown in FIGS. 2E and F, this TCM class has a relatively narrow and slightly taller D-band compared to the G-band (height ratio of 1.2˜1.6). Its G′-band is taller than the activated carbon or coal-like TCMs. Its D and G′ bands indicate some level of ordered graphitic structure, which translates to less disorder and nanopores for gold cyanide adsorption. The samples within this class of TCM exhibited a PRF ranging from 40 to70% and, therefore was designated as “Medium-PR”.

Group 4. Activated Carbon, High-PR: As shown in FIGS. 2G and H, this class of TCM has a slightly wider but about same-height D-band (red color) compared to G-band (green color). This is constant with the optimal size and abundance of nanopores in the activated carbon structure, for a maximized gold cyanide adsorption. Its G′-band is short. Samples having TCM particles belonging to this group exhibit a PRF >90%, even with a TCM composition as low as 0.1%.

Group 5. Coal-Like, High-PR: As shown in FIGS. 2I and J, this TCM class has a much wider D-band (red color), compared to activated carbon-like TCM. This wider D-band indicates a larger variety of disorders/defects in the graphitic structure. The nanopores in this TCM class probably have a greater diameter than activated carbon, which accounts for high preg-robbing capacity (abundance of nanopores) but less preg-robbing strength (size of nanopores). Chemical tests proved coal is not preg-robbing, but we suspect the volatiles in mined coal samples may have blocked the nanopores for gold cyanide adsorption, whereas the dry environment in Nevada possibly released most volatiles in TCM. This class is named “coal-like” for now. Ores with mostly coal-like TCM have a PRF between 80% and 90%.

FIG. 3 shows the shapes where the five groups of TCM particles classified by Raman spectra fall in the 2D coordinates using relative height of D-band for X axis and relative width of D-band for Y axis. A total of 700 Raman spectra were scanned at the wavelength of 488 nm laser using the Renishaw in Via microscope to generate this graph. Table 4 lists the approximate ranges of the relative height and the relative width of D-band deconvoluted from Raman spectra, for each TCM group.

TABLE 4
Value Ranges of Relative Height and Width of
the Deconvoluted D-Band for Five TCM Classes,
as well as the Corresponding PRF Ranges.

	Relative Height	Relative Width
TCM Class	of D-Band	of D-Band	PRF
1. Graphite-Like,	<0.7	1.0~2.0	<20%
Low-PR
2. Ordered,	1.7~2.0	0.8~1.1	13~52%
Variable-PR			(samples 3,
			13, 16, 17,
			18) but 98%
			(sample 14)
3. Mix-Ordered,	1.2~1.7	0.9~1.5	40~70%
Medium-PR
4. Activated	0.9~1.2	1.4~2.4	>90%
Carbon,
High-PR
5. Coal-Like,	0.6~1.0	2.4~3.2	80~90%
High-PR

Table 5 lists all TCM samples analyzed by Raman microscope. TCM particles in each sample have been categorized into five groups according to their Raman spectra. The percentage of TCM classes in each sample can be correlated to the PRF from the conventional chemical tests.

TABLE 5
TCM Particles in Each Sample Categorized into Five Classes.

		No. of TCM			Group 3.	Group 4.	Group 5.
		Particles	Group 1.	Group 2.	Mix-	Activated	Coal-
Ore		Analyzed	Graphitic.	Ordered.	Ordered,	Carbon.	Like.
Sample	PRF	by Raman	Low-PR	Variable-PR	Medium-PR	High-PR	High-PR

2	79%	30	—	—	—	—	100.0%
3	37%	40	5.0%	15.0%	78.0%	—	2.0%
4	96%	40	2.5%	—	7.5%	37.5%	52.5%
5	100%	30	—	—	—	100.0%	—
6	62%	30	3.3%	—	20.0%	76.7%	—
7	3%	30	100.0%	—	—	—	—
8	100%	30	3.3%	—	3.3%	93.3%	—
9	81%	30	—	—	—	—	100.0%
10	89%	30	—	—	—	—	100.0%
11	6%	40	70.0%	—	30.0%	—	—
12	18%	30	70.0%	—	30.0%	—	—
13	69%	30	—	6.7%	93.3%	—	—
14	98%	40	20.0%	62.5%	17.5%	—	—
15	9%	30	83.3%	6.7%	10.0%	—	—
16	13%	30	86.6%	10.0%	3.3%	—	—
17	52%	30	—	16.7%	83.3%	—	—
18	36%	30	—	23.3%	76.7%	—	—

Some samples contained TCM particles either wholly or at least predominantly within a single grouping, while others had a spread of particles across several groupings. Not surprisingly, the two composites tested showed the largest variety of groupings.

Pretreatment with the blinding agent was effective in substantially decreasing the preg-rob factor for each of the samples tested. Results were fairly consistent for Raman Groups 2, 3 and 5 dominant. The preg rob factor for those samples generally were decreased to below 20% using a blinding agent dose equivalent to a mass ratio of 0.5:1 or lower. Behavior of the Raman Group 4 dominant samples was notably different. For the two samples in this group without significant secondary Raman Group TCM particles present (samples 5 and 8), masking of the TCM with the blinding agent was less effective, particularly at the lower dose tested (0.05:1). The other Raman Group 4 sample (number 6), which contained significant, but lesser amounts of Raman Group 3 TCM, responded to the blinding agent pretreatment in a similar manner to the Raman Groups 2, 3 and 5 samples.

Results from the two series of custom preg-rob capacity tests designated “Method 1”—volume basis and “Method 2”—concentration basis are shown in FIGS. 5A and B and 6A and B, respectively.

Representative test data, presented as gold loading onto TCM versus final solution grade, are shown graphically for the Group 4 in the FIGS. 5A and 6A series graphs, while Group 5 is shown in the FIGS. 5B and 6B series graphs.

In the case of the Method 1 (constant solution grade, varied solution: ore ratio), preg-robbed gold is expected to increase with increasing solution to ore ratio, until an excess of available gold is provided at a given solution grade. From this ratio upward, the plots are expected to show essentially constant TCM loading (e.g. samples 2, 6 and 10). In cases where equilibrium loading capacity was not reached, the loading vs. final solution grade curve is expected to continue increasing (e.g. 4, 5 and 8).

In the case of Method 2 (constant solution to ore mass ratio, varied solution grade), the same Raman Group 4 samples that were less affected by masking with a blinding agent (samples 5 and 8) displayed a much steeper slope for the curves relating gold loaded onto preg-robbing TCM versus the final solution grade, compared to the other Raman Group 4 sample and the Raman Group 5 samples.

Customized Chemical Testing

Novel TCM masking tests showed that a proprietary blinding agent was effective in masking the preg-robbing strength of TCM for cyanide leaching. While masking TCM using this blinding agent has been shown to be generally effective, the relationship between preg-rob potential can vary with the sample mineralogy.

The relationship between the blinding agent pretreatment dose and the resulting preg-rob factor are shown for Raman dominant groups 2 through 5, as shown in FIG. 4A through D, respectively. Results from Raman Group 1 dominant samples are not shown as those samples generally displayed negligible preg-robbing.

Summary results showing preg-rob factors for the various test types (preg-rob assay, spiked CIL and spiked CIP) are shown by Raman group in FIG. 7. A plot showing the difference between the standard preg-rob factor and those generated during the spiked CIL and spiked CIP tests are shown versus the standard preg-rob factor in FIG. 8.

As expected, there was a general tendency for lower dissolved gold losses to TCM when carbon was added during ore contact with the spiked pregnant solution, due to the direct competition of the GAC with the TCM for adsorption of the dissolved gold. There was also a tendency for some of the gold lost to TCM to be recovered when the residue from the spiked preg-rob test was contacted with GAC in barren cyanide solution after leaching (spiked CIP). Somewhat surprisingly, there were some cases where the apparent preg-rob factor was significantly lower for the spiked CIP test, compared to the corresponding CIL test. This was observed with samples 6 (Raman Group 4) and 13 (Raman Group 3). The cause for this anomaly was not well understood.

As shown in FIG. 8, there was a general association of the preg-rob factor with the general Raman spectroscopic groupings. Not surprisingly, less active carbon character showed a smaller change in PRFs. With the addition of GAC, the change of PRFs tended to show an increase with increasing preg-rob factor, for samples with PRFs lower than about 70% to 80%. For samples with high PRFs (>80%), the improvement in PRF decreased with increasing PRF, suggesting that adding GAC during leaching would be less effective for gold recovery, because of the TCM outcompeting the GAC for gold recovery from these ore types. Raman groups 1, 2, 3 and 5 were represented in this subset of samples.

CONCLUSIONS

Chemical testing provides information on a bulk sample basis, as compared to Raman spectroscopy that shows individual particle information, with the collection providing the bulk sample distribution. TCM was found to have varying degrees of gold adsorption capacity, based on both the amount of gold and the concentration of gold dissolved in the solution in contact with the TCM bearing material. Granular activated carbon can be an effective competitor for dissolved gold and varies based on the characteristics of the TCM. A blinding agent can improve gold stability in solution to a degree that was related to the nature of the TCM. The addition of this simple test to the current preg-rob suite helps to categorize the type and nature of TCM. Stronger TCM characteristics required a higher blinding agent ratio to native TCM.

Raman spectroscopic studies have identified five groupings of TCM characteristics within the current sample set. Modal carbons are aligned with three of the five groupings, including graphitic, activated carbon, and coal-like classifications. Samples from a single location can contain varied types of TCM characteristics. Operating parameters in the plant can be tailored to specific TCM characteristics in order to maximize gold recovery in the presence of TCM.

Future work direction includes recovery of specific data associated with the different types of TCM, increasing the sample variability and volume in the study, identification of modal carbons associated with the other 2 groupings of TCM character, and correlation of the varied TCM characteristics with specific geology, leading toward implementation of TCM character modeling and improved ore routing.

A number of variations and modifications of the disclosure can be used. It would be possible to provide for some features of the disclosure without providing others. The present disclosure, in various embodiments, configurations, or aspects, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, configurations, aspects, subcombinations, and subsets thereof. Those of skill in the art will understand from the foregoing how to make and use the subject matter of the present disclosure. The present disclosure, in various embodiments, configurations, and aspects, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments, configurations, or aspects hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving case and/or reducing cost of implementation.

The foregoing discussion of the disclosure has been presented for purposes of illustration and description. The foregoing is not intended to limit the disclosure to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the disclosure are grouped together in one or more embodiments, configurations, or aspects for the purpose of streamlining the disclosure. The features of the embodiments, configurations, or aspects of the disclosure may be combined in alternate embodiments, configurations, or aspects other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the claimed disclosure requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment, configuration, or aspect. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the disclosure.

Moreover, though the description of the disclosure has included description of one or more embodiments, configurations, or aspects and certain variations and modifications, other variations, combinations, and modifications are within the scope of the disclosure, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments, configurations, or aspects to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.