SYSTEM AND PROCESS FOR ELECTROWINNING

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Application No. 63/717,354, filed on Nov. 7, 2024, which is hereby incorporated by reference in its entirety.

FIELD

This application relates generally to systems and processes for electrowinning, and more particularly to closed loop precious metal recovery.

SUMMARY

In some embodiments, methods, systems, and apparatus, include receiving from a leaching unit into a chamber body, a concentrated metal salt solution comprising a base metal. An electropotential is applied to the concentrated metal salt solution that reduces base metals from the concentrated metal salt solution thereby creating a diluted metal salt solution. Base metals are captured onto a solid cathode and gaseous oxygen is released by an anode. The diluted metal salt solution is transferred from the chamber body for subsequent processing. Captured base metals are transferred from the chamber body.

In some embodiments, methods, systems, and apparatus, include receiving from a leaching unit into a first portion of a chamber body, a concentrated metal salt solution comprising a base metal. The chamber body is divided by a membrane forming the first portion of the chamber body and a second portion of the chamber body. An electropotential is applied to the concentrated metal salt solution that reduces base metals from the concentrated metal salt solution thereby creating a diluted metal salt solution. Base metals are captured onto a solid cathode and gaseous oxygen is released by an anode. The diluted metal salt solution is transferred from the chamber body for subsequent processing. Captured base metals are transferred from the chamber body. A solution of one or more ligand species are received into the second portion of the chamber body, where the one or more ligand species flow through the membrane to the first portion of the chamber body.

The appended claims may serve as a summary of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a process flow chart illustrating an exemplary method 100 that may be performed in some embodiments.

FIG. 2 is a diagram illustrating equations that are referenced in the specification.

FIG. 3 is a diagram illustrating equations for other metals including platinum, palladium iridium and osmium.

FIG. 4 is a diagram illustrating an exemplary metal recycling process and system according to an embodiment.

FIG. 5 is a diagram illustrating an exemplary electrowinning cell unit according to an embodiment.

FIG. 6 is a diagram illustrating an exemplary pH equilibration unit according to an embodiment.

FIG. 7 a process flow chart illustrating an exemplary method 700 that may be performed in some embodiments.

FIG. 8 is a diagram illustrating an exemplary closed loop metal recycling process and system according to an embodiment

FIG. 9 is a diagram illustrating an exemplary electrowinning cell unit according to an embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In this specification, reference is made in detail to specific embodiments of the invention. Some of the embodiments or their aspects are illustrated in the drawings.

For clarity in explanation, the invention has been described with reference to specific embodiments, however it should be understood that the invention is not limited to the described embodiments. On the contrary, the invention covers alternatives, modifications, and their equivalents as may be included within its scope as defined by any patent claims. The following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations on, the claimed invention. In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In addition, well-known features may not have been described in detail to avoid unnecessarily obscuring the invention.

FIG. 1 a process flow chart illustrating an exemplary method 100 that may be performed in some embodiments.

In step 110, a tank is provided that has a chamber with a chamber body, a liquid stream inlet, a liquid stream outlet, a gas outlet and solid metal outlet. A porous anode is positioned in an upper interior portion of the chamber. A solid cathode is positioned in a lower interior portion of the chamber.

In step 120, the chamber receives from a leaching unit a concentrated metal salt solution comprising a base metal. The chamber is in fluid communication with the leaching unit, and receives the concentrated metal solution, directly or indirectly, from the leaching unit.

In step 130, an electropotential is applied that reduces base metals from the concentrated metal salt solution thereby creating a diluted metal salt solution, where the diluted metal salt solution has less metal in the solution than the concentrated metal salt solution.

In step 140, based metals are captured onto or by the solid cathode.

In step 150, gaseous oxygen is released by the anode and exits the chamber via the gas outlet.

In step 160, the diluted metal salt solution is transferred from the chamber body via the liquid stream outlet.

In step 170, the captured base metals are transferred from the chamber via the solid metal outlet.

Further details of the system and process 100 are described herein. Precious metal recycling requires three major units: 1) a leaching stage that converts metals into water soluble metal salts. 2) a purifying stage that concentrates the target metal salt from all other competing salts. 3) a recovery stage that converts the concentrated target metal salt back to its reduced metallic form—solid metal. When the process of precious metals recycling is viewed overall, the leaching stage consumes an oxidizer and ligands to produce a metal salt species, while simultaneously, the ligand species are shed off in the recovery stage alongside the production of a byproduct oxidizing group. Due to the challenge of separation and inexpensive nature of many oxidants and ligands for metals recycling, current dogma in industry is to simply destroy or dispose of spent ligand and repurchase.

There have been significant advances in the purification stage of precious metals recycling, with the aim of minimizing energy and materials consumption for this important industrial process. For example, Dr. Stephen Cotty's work on electrified liquid-liquid extraction cleverly utilizes a gold selective redox extractant to capture, purify, and concentrate gold and other precious metals using only electricity. In addition, many solvent extraction and biological approaches have been shown, however due to the circularity of these approaches—utilizing closed chemical loops—accumulation of byproduct chemicals and stabilization of pH is a major hinderance, particularly in the target metal accumulation loop. Leaching systems that utilize oxygen for example typically result in an increase in pH and consumption of H2O, requiring infusion of an acidic water solution to rectify the imbalance. For complete closed loop metal recycling, utilizing current purification technologies where no chemicals are consumed, accumulation of by-products must be mitigated in all three process units: 1) leaching, 2) purifying, and 3) recovery.

Here, a novel method for invasively recovering precious metals from a purified metal salt solution is demonstrated that recycles the ligand and oxidant species for reuse in the leaching stage, while maintaining proper water and pH balance with the 1) leaching and 2) purifying stages. The result is a system that is truly closed loop—recycling critical metals without the consumption of chemicals or production of waste byproducts streams.

The following text describes two similar yet different Embodiments for achieving a fully closed loop metal recycling process by synergistically combining the leaching and electrowinning stages to minimize waste byproducts and efficiently reuse all chemical components. Process Embodiment A achieves this by regenerating and gasifying the oxidant and ligands in the electrowinning cell allowing simple separation and reuse in the leaching reaction. Embodiment A focus is on cyanidation reactions. Process Embodiment B utilizes a divided membrane electrowinning cell allowing for complete isolation of the typically parasitic anode reaction, to directly generate the leaching oxidant in the leaching loop.

FIG. 4 is a diagram illustrating an exemplary metal recycling process and system according to an embodiment. The process in FIG. 4 begins with precious metal rich material (such as gold laden electronic circuit boards) entering the leaching unit along with oxidant and ligand species. For gold leaching, oxygen is the oxidant, and the cyanide anion is the ligand species, and the leaching proceeds according to EQN 1 (see FIG. 2 in reference to referenced equations; FIG. 3 describe equations reactions for the recovery of other metals that may be used with the systems and processes ad described herein). Unreactive solid material exits the leach reactor from the bottom. The exiting leach stream is made highly alkaline from the leach reaction and consists of the target gold salt stabilized with ligand species alongside a significant excess of competing metals species. This leach stream then enters a purification stage using one of numerous available technologies (not covered by this patent) where the target metal-ligand complex is selectively transferred to a new water stream called the reduction loop. The target metal-baren leach loop then flows by a pH equilibration membrane where any water or pH imbalance from leaching is rectified. For gold cyanidation, water and H⁺ is transferred to the leach loop from the reduction loop.

The leach loop then flows to the competing metal electrowinning cell, where an electrical potential is applied that reduces base metals, capturing them onto the cathode. The anode undergoes an oxygen evolution reaction releasing gaseous oxygen, which is captured and fed to the leach tank. The aqueous leach solution then flows back into the leach reactor to complete the leaching loop.

Returning to the reducing loop, the gold-ligand complex rich stream enters the gold electrowinning cell where metallic gold is captured on the cathode following EQN 2, liberating cyanide ligand species. The anode of the gold electrowinning cell undergoes hydrolysis (EQN 3) liberating gaseous O₂ and decreasing the solution pH. The decrease of solution pH is buffered by the reaction shown in EQN 4, where the anionic cyanide ligand species is converted to gaseous

hydrogen cyanide. The gold electrowinning cell is sealed to capture all liberated O₂ and HCN gas, and the gases flow to the leach reactor vessel from the natural pressure gradient formed.

The reduction loop stream flows from the gold electrowinning cell to the pH equilibration membrane (as discussed earlier) where water and pH is equalized between the leach loop and reduction loop. From there, reduction loop stream exits the equilibration membrane and enters the selective gold purification process to complete the reduction loop. In summary, crude gold enters the process as e-waste and the unreacted ewaste exits. The gold is plated out and all leaching agents are regenerated and returned to be reused. All other metals (namely copper) are plated out and all leaching agents are also regenerated and returned to be reused. Overall, the system only consumes electrical power, and minimal power is required for pumping the solution loops.

Embodiment A Electrowinning Cell Design

For Embodiment A, there are two electrochemical flow cells of similar design - one for the deposition of target metal located in the reducing loop and one for the deposition of competing metals located in the leach loop of FIG. 4. The electrochemical cell (see FIG. 5) is designed to make complete use of both electrode reactions to mitigate side reactions and byproduct formation enabling true closed loop metal recycling.

The core function of the electrowinning cell is to electrodeposit the target metal species by electrochemically reducing the oxidized metal salt to its metallic zero-valent form (EQN 2 for example). Electrodeposition is accomplished by exposing the metal salt solution to an electrically conductive cathode charged with a reducing potential (typically <0.5V vs Ag/AgCl) which loosely plates the cathode with target metal in the solid phase that is easily physically separated from the liquid solution. The cathode can either be removed and replaced to harvest the target metal species or the metal can be removed continuously by allowing it to fall to the bottom of the electrowinning cell.

The electrowinning cell takes full advantage of the counter electrode reaction occurring at the anode, enabling a dual purpose of the cell—to regenerate and recycle the oxidizer and ligand species needed for leaching. For gold cyanidation (EQN 1), oxygen is the oxidizer and CN—is the ligand species. The anode of the electrowinning cell is a porous and electrically conductive allowing for the hydrolysis of water to evolve oxygen gas (EQN 3). Hydroxide ions are consumed at the anode in reaction 3, resulting in the acidification of the liquid stream, which in turn results in the reaction of CN—ligand species that was released at the cathode (EQN 2) to evolve the gaseous HCN ligand. Therefore, the cathode reaction results in the gasification of oxidizer and ligand which is captured in the sealed electrowinning cell, exiting the top chimney where these essential reactants are recycled back to the leaching reactor. The gases flow due to the natural pressure differential created between the electrowinning cell(s) and the leaching reactor, minimizing complexity and utility costs.

The electrochemical cell can be operated in a constant current or constant potential mode; however, it will likely be operated at constant current, allowing measurement of the overall cell potential to be used as a diagnostic. The electrochemical cell can be operated with a third reference electrode for added control/monitoring of the reduction potential, but it is not necessary for operation.

The electrode materials must be electrically conductive. The anode can be constructed of any metal or graphite but is typically made of either porous graphite or a titanium MMO electrode. The cathode can be constructed of any metal but is typically made of titanium, copper, or stainless steel (316SS).

pH Equilibrium Membrane Unit

For gold cyanidation, the gold leaching reaction (EQN 1) consumes water (H2O) and produces hydroxide anions (OH−), and the combined gold electrowinning and oxygen evolution reaction in the electrowinning cell results in the opposite—the production of water and consumption of hydroxide. The fundamental purpose of the two liquid loops (the leach and reduction loops) is for each to remain separate, allowing the target metal—gold—to be isolated to the reduction loop. Therefore, exchange of water and equilibration of pH between the two isolated process loops is a non-trivial and essential to combat runaway chemical accumulation within each loop. The purpose of the pH equilibrium unit is twofold: 1) reduce the pH gradient between isolated liquid loops by allowing the free passage of H+ and OH−, and 2) allow free passage of H2O molecules.

The pH equilibrium unit of the Embodiment A process, shown in detail in FIG. 6, comprises two liquid flow paths separated by a membrane that only allows H2O and H+/OH− ions to passively flow between flow paths. The first flow path is the leach loop stream, and the second flow path is the reducing loop stream. The membrane material must allow H2O and H+/OH− to flow with mild restriction. A mild pH differential between flow paths is acceptable (delta pH <4). The membrane material may be an anion exchange membrane, cation exchange membrane, proton exchange membrane, microfiltration membrane, nanofiltration membrane, or RO (reverse osmosis) membrane. The membrane material is likely either a proton exchange membrane or nanofiltration membrane.

Embodiment B Process Description

FIG. 7 is a process flow chart illustrating an exemplary method 700 that may be performed in some embodiments.

In step 710, a tank is provided that has a chamber comprising a chamber body, a reducing loop inlet, a reducing loop outlet, a leach loop inlet and a leach loop outlet. A membrane is positioned in the chamber body forming a first interior chamber portion and a second interior chamber portion. The first interior chamber portion is in fluid communication with the leach loop inlet and the leach loop outlet. The second interior chamber portion is in fluid communication with the reducing loop inlet and the reducing loop outlet and a solid metal outlet. An anode is positioned in the first interior portion of the chamber. A cathode is positioned in the second interior portion of the chamber.

In step 720, the chamber receives into the first interior chamber portion from a leaching unit, a concentrated metal salt solution comprising a base metal. The first interior chamber portion is in fluid communication with the leaching unit, and receives the concentrated metal solution, directly or indirectly, from the leaching unit.

In step 730, an electropotential is applied that reduces base metals from the concentrated metal salt solution thereby creating a diluted metal salt solution, where the diluted metal salt solution has less metal in the solution than the concentrated metal salt solution.

In step 740, based metals are captured onto or by the solid cathode.

In step 750, the chamber receives into the second interior chamber portion a solution comprising one or more ligand species.

In step 760, the diluted metal salt solution is transferred from the first interior chamber portion of the chamber body via the leach loop outlet.

In step 770, the captured base metals are transferred from the chamber via the solid metal outlet.

Further details of this process are described with regard to FIGS. 8 and 9. Overall, the process in FIG. 8 comprises two closed liquid loops, the leach loop (yellow) and the reduction loop (green). Covered in this patent is a leaching unit, copper electrowinning unit, and an isolated electrode electrowinning unit. Not covered here is a selective salt capture and release unit that could be any technology. The process in FIG. 8 begins with the inlet of precious metal rich material (such as gold laden electronic circuit boards) entering the leaching unit along with oxidant and ligand species in the leaching loop. The typical material feed consists of <1% precious target metal and the remaining mass including competing metals and inert solids. The leaching reaction that occurs in the leaching unit follows either EQN 1 or EQN 5 for the example of gold as the target species, where metallic Au(0) is oxidized to an ionic salt with an oxidizer (O2, I2, etc) and the ionic gold salt is stabilized with ligands (CN—, I—, etc). Target and competing metal salts leave the leaching unit as the leaching loop stream, and inert solid materials leave the leaching unit through a separate solid stream.

The exiting leaching unit stream containing target and competing metal salts proceeds to a target metal selective separation unit (not covered in this patent), where the target metal salt is removed. The remaining competing metals in the leach stream then enter the anode side of the divided flow electrowinning cell (will be discussed later), where spent oxidizer (iodide, I— for example) is regenerated (following EQN 3 or EQN 6 for example). The membrane within the divided flow electrowinning cell allows pH, water and ligand species to equilibrate between the isolated leaching loop and reducing loop.

Following the divided electrowinning cell, the leach loop stream enters the competing metal electrowinning cell, where competing metals are removed and captured onto the cathode via electro-reduction, liberating the ligand species. On the anode side of the electrowinning cell, spent oxidizer is regenerated. The leach stream exits and flows back to the leaching unit to complete the leach stream loop.

The target metal salt removed from the leach stream loop (previously mentioned) is then released into the reduction stream, where the target metal salt is isolated from competing metal salts and allowed to concentrate. The concentrated target salt in the reduction stream then enters the cathode side of the divided flow electrowinning cell, where the target metal species is electrochemically reduced to a solid zero-valent (metallic) state (EQN 8 for gold). The target metal roughly coats the electrode and can be allowed to separate with gravity to the bottom of the cell for continuous removal or the target coated cathode can be periodically replaced resulting in >99% purity target metal.

The stabilizing ligand species are released to the reduction loop stream where the naturally flow through the membrane divider of the electrowinning cell into the leaching loop stream, where ligand species can be reused.

Embodiment B Divided Electrowinning Cell

The purpose of this electrowinning cell is to 1) separate the target metal species from the reduction loop stream by electrochemically reducing the aqueous metal salt to purified solid metal, 2) enable free ligand species liberated from electrodeposition (EQN 2 and EQN 8) to transfer back to the leaching loop through the selectively permeable membrane, and 3) isolating the anode reaction to the leaching loop where oxidant species can be regenerated for the leach reaction instead of byproduct/waste stream formation. To summarize, the divided electrowinning cell electrochemically removes target metal while recycling the ligand and regenerating the oxidizer for closed loop leaching.

The divided electrowinning cell (FIG. 9) comprises two electrodes divided by a selectively permeable membrane. The membrane segregates the cell into two separate flow paths or channels, each with its own electrode. The cathode side has the reducing loop stream flowing through, where concentrated target metal salt is electrodeposited onto the electrode surface and removed. The cathode must be electrically conductive and can be made of graphite or metal, likely titanium, copper, or stainless steel. A material is chosen that minimizes the overpotential of metal electrodeposition and allows facile removal of the target electrodeposited metal.

The anode side (+) of the cell has the leaching loop stream flowing through, where spent oxidizer is regenerated to be reused in the leach reactor following EQN 3 and 6 for example. The anode electrode must be electrically conductive and is likely made of graphite, MMO (mixed metal oxide) on titanium, gold, or platinum. A material is chosen that minimizes the overpotential of oxidizer regeneration and remains electrochemically inert within the leach loop stream.

The dividing membrane of the cell must allow counterions, ligand species, water and H+/OH− to flow between the Leach loop and reduction loop. The membrane materials shown to work are ion exchange membranes, microfiltration membranes, nanofiltration membranes, and RO membranes.

The electrochemical cell can be operated in a constant current or constant potential mode; however, it will likely be operated at constant current, allowing measurement of the overall cell potential to be used as a diagnostic. The electrochemical cell can be operated with a third reference electrode for added control/monitoring of the reduction potential, but it is not necessary for operation.

Electrode Size Range, Operational Parameters, and Material Composition

The typical electrode size ranges from 10 cm2 to 2000 cm2 for both Embodiment A and B; typical range for a production of 1000 grams of gold per day would be around 700 cm2.

The operational parameter for the electrochemical cells is within a range of 100 to 1000 A/m2 , typically near 100 A/m2 . The overall cell potential ranges from 0.5 volts to 10 volts, typically 3V for Embodiment A and 2V for Embodiment B.

The composition of all electrochemical cells is similar and can be made of any inert material such as glass, polypropylene, acrylic, PET, Teflon, epoxy, silicone, viton rubber, or UV cured resin for example. The anode can be made of graphite, stainless steel, platinum, or MMO (mixed metal oxide) coated titanium. The cathode can be made of gold, copper, stainless steel, or graphite.

It will be appreciated that the present disclosure may include any one and up to all of the following examples.

Example 1. An electrowinning process comprising, the operations of: providing a tank, the tank comprising: a chamber, the chamber comprising: a chamber body; a liquid stream inlet; a liquid stream outlet; a gas outlet; and a solid metal outlet; a porous anode positioned in an upper interior portion of the chamber; and a solid cathode positioned in a lower interior portion of the chamber; receiving from a leaching unit, a concentrated metal salt solution comprising a base metal; applying an electropotential that reduces base metals from the concentrated metal salt solution thereby creating a diluted metal salt solution, where the diluted metal salt solution has less metal in the solution than the concentrated metal salt solution; capturing base metals onto the solid cathode; releasing gaseous oxygen by the anode and exiting the chamber via the gas outlet; transferring the diluted metal salt solution from the chamber body via the liquid stream outlet; and transferring the captured base metals from the chamber via the solid metal outlet.

Example 2. The electrowinning process of Example 1, wherein the base metal is metallic gold, and the metallic gold is captured on the solid cathode via a reaction according to the equation:

Example 3. The electrowinning process of claim 1, wherein the porous anode undergoes a reaction according to the equation:

Example 4. The electrowinning process of any one of Examples 1-3, further comprising the operations of: buffering the metal salt solution, wherein an anionic cyanide ligand species is converted to gaseous hydrogen cyanide via a reaction according to the equation:

Example 5. The electrowinning process of any one of Examples 1-4, further comprising: performing via the leaching unit a reaction process according to the equation:

Example 6. The electrowinning process of any one of Examples 1-5, further comprising: providing a pH equilibration unit, comprising a chamber having a proton exchange membrane; and transferring the metal salt solution from chamber via liquid stream outlet to the pH equilibration unit.

Example 7. An electrowinning process comprising the operations of: providing a tank, the tank comprising: a chamber, the chamber comprising: a chamber body; a reducing loop inlet; a reducing loop outlet; a leach loop inlet; a leach loop outlet; a membrane positioned in the chamber body forming a first interior chamber portion and a second interior chamber portion, the first interior chamber in fluid communication with the leach loop inlet and the leach loop outlet, the second interior chamber portion in fluid communication with the reducing loop inlet and the reducing loop outlet; a solid metal outlet; an anode positioned in the first interior portion of the chamber; and a cathode positioned in the second interior portion of the chamber; receiving from a leaching unit, a concentrated metal salt solution comprising a base metal; applying an electropotential that reduces the base metal from the concentrated metal salt solution thereby creating a diluted metal salt solution, where the diluted metal salt solution has less metal in the solution than the concentrated metal salt solution; capturing base metals onto the cathode; and transferring the captured base metals from the second interior chamber portion via the solid metal outlet.

Example 8. The electrowinning process of Example 7, wherein the base metal is metallic gold, and the metallic gold is captured on the cathode via a reaction according to the equation:

Example 9. The electrowinning process of any one of Examples 7-8, wherein the base metal is metallic gold, and the metallic gold is captured on the cathode via a reaction according to the equation:

Example 10. The electrowinning process of any one of Examples 7-9, wherein the porous anode undergoes a reaction according to the equation:

Example 11. The electrowinning process any one of Examples 7-10, further comprising the operations of: regenerating spent oxidizer via a reaction according to the equation:

Example 12. The electrowinning process of any one of Examples 7-11, further comprising the operations of: regenerating spent oxidizer via a reaction according to the equation:

Example 13. The electrowinning process of any one of Examples 7-12, further comprising the operations of: receiving into the second interior chamber portion a solution comprising one or more stabilizing ligan species via the reducing loop inlet, wherein the one or more ligand species flow through the membrane to the first interior chamber portion.

In the foregoing disclosure, implementations of the disclosure have been described with reference to specific example implementations thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of implementations of the disclosure as set forth in the following claims. The disclosure and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.