THREE-COMPARTMENT ELECTROWINNING CELL AND METHODS

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 63/725,887 filed on Nov. 27, 2024, which is incorporated herein by reference in its entirety.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to methods for electrowinning metals.

BACKGROUND OF THE INVENTION

Critical materials are non-fuel minerals, elements, substances, or materials which have a high risk of supply chain disruption or serve an essential function in energy technologies. Many critical metals are currently produced by carbothermic reduction of their oxides. The conventional carbothermic reduction processes utilize large quantities of process carbon, such as metallurgical coke, and generate copious amounts of metallurgical slag. Carbothermic reduction has two large sources of emissions: the creation of metallurgical coke, and use of the coke to reduce the metal oxide. Electrowinning the metal will eliminate both process carbon sources.

Zinc is an important metal used in a variety of alloys and products such as brass, galvanized steel, sacrificial anodes, and die castings. Zinc is listed as a critical material by the United States Geological Survey (USGS) for its high risk of supply chain disruption. The primary reason for this is the lack of excess zinc reserves and production capacity. Of the 6,600,000 tons of identified zinc reserves in the United States, 750,000 tons were mined in 2023 according to the USGS. If the United States continues to mine and produce zinc at that rate, there will be less than 9 years before there are no zinc reserves remaining. Coupling this with approximately 25% of worldwide zinc production relying on pyrometallurgical carbothermic reduction, ZnO(s)+½ O₂(g)+C(s)→Zn(s)+CO₂(g), a net chemical reaction that results in the release of one mol of CO₂ for every mol of zinc, presents an inherent sustainability issue that must be addressed.

The United States has one primary (ore) and one secondary zinc carbothermic reduction smelter; the secondary sources are recovered from galvanizing residues and crude zinc oxide from electric arc furnace dust. The lack of prevalence and diversity within domestic zinc production creates a supply chain risk and this proposal outlines a new, decarbonized method which works with primary and secondary zinc sources.

One way of avoiding process carbon for critical metals is electrowinning, in which metal ions are electrochemically reduced to the metal. Conventional methods for electrowinning zinc metal from aqueous solutions can be broken into zinc sulfate and zinc chloride processes. The zinc sulfate process first oxidizes zinc sulfide ore to zinc oxide or directly uses zinc oxide from secondary sources. The zinc oxide is leached in sulfuric acid to form zinc sulfate, which is conventionally electrowon in a single compartment cell to produce zinc metal and oxygen in a closed loop cycle. Sulfuric acid concentration increases over time due to oxygen generation, and the increased acidity decreases current efficiency (from 97.4% to 94%) at 400 A/m². The lower current efficiency is due to parasitic hydrogen production at the cathode, although the higher acid concentration lowers the cell voltage due to increased electrolyte conductivity. Zinc electrowinning cells employ additives which decrease hydrogen production, increase current efficiency, and prevent dendrites, thereby increasing overall energy efficiency. Overall, the zinc sulfate electrowinning process has a specific energy consumption of 2780 kWh/ton. Although higher temperatures (50° C.) can improve specific energy consumption to 2700 kWh/ton, there are no readily available additives applicable to hotter operating points, which leads to formation of impure, dendritic deposits.

The existing chloride process is similar to the sulfate process, except that chlorine gas is generated at the anode. The chlorine gas may be used to chlorinate oxide, make HCl with hydrogen or natural gas, or can be sold as a commodity; however, it does require special handling equipment, which adds cost to the process. All avenues for chlorine handling add cost compared to the sulfate process and therefore the chloride process is seldom used industrially. The chloride process uses current densities (323 A/m²) at similar current efficiencies (>96%) as the sulfate process, however the chloride process allows for even higher current densities without producing impure, dendritic deposits.

The primary cause for the loss of current efficiency and limit of current density in both chloride and sulfate-based zinc electrowinning is the parasitic production of hydrogen. The hydrogen evolution reaction is propagated by the formation of dendrites which simultaneously decrease the purity through the trapping of salt and serve as sites for hydrogen evolution. In the sulfate process this is combatted by lowering the current density, however doing so limits the rate of production. In the chloride process it has been shown that the introduction of additives promotes planar growth thereby reducing sites for hydrogen evolution and increasing the purity of deposits, and effectively increasing the current efficiency at current densities beyond the limit of the sulfate process.

In addition to critical material production, non-critical metals, such as iron, have considerable undesirable environmental impacts due to the use of process carbon in the metal production process. Carbothermic iron production utilizes enormous quantities of process carbon, producing a large quantity of carbon dioxide emissions and metallurgical slag. Attempts at conventional electrowinning of iron from iron(+3) chloride result in both chlorine production and significant and undesirable iron dendrite formation.

There is an ongoing need for methods of metal production that avoid or eliminate process carbon from metal production. The methods described herein address this ongoing need.

SUMMARY OF THE INVENTION

Room-temperature three-compartment aqueous electrowinning and leaching apparatus and methods described herein, which use a closed loop process to isolate critical metals such as zinc, cobalt, tin, bismuth, nickel, iron, lead, copper, or manganese without any process carbon are described herein. The methods described herein use environmentally friendly electrowinning in a closed loop system to isolate the metals from a metal chloride solution without electrochemical generation of undesired chlorine gas. Electrowinning methods use electricity rather than process carbon to reduce metal ions to the metallic state, enabling decarbonization of metal production, with concomitant elimination of greenhouse gas emissions. The methods can be conveniently illustrated for zinc and iron production.

The methods described herein are an improvement over conventional chloride-based and sulfate-based electrowinning by breaking down the electrochemical reactions into three compartments, instead of the one compartment of conventional electrowinning cells. The methods described herein operate with high current density, high current efficiency, and closed loop capability.

The three-compartment electrowinning cells described herein comprise a first compartment including a cathode and a catholyte; a second compartment adjacent the first compartment and separated therefrom by an anion exchange membrane (AEM); and a third compartment comprising an anolyte and an anode in contact with the anolyte; the third compartment being adjacent the second compartment and separated therefrom by a cation exchange membrane (CEM). The cathode comprises a metal suitable for depositing the electrowon metal onto, such as brass 260, aluminum, or zinc. The anode comprises and inert metal such as platinum or platinized titanium. In some embodiments, the catholyte comprises aqueous hydrochloric acid and a metal chloride of a metal to be electrodeposited on the cathode, the anolyte comprises aqueous sodium hydroxide, and the second compartment comprises aqueous sodium chloride. In some other embodiments, the catholyte comprises aqueous hydrochloric acid and a metal chloride of a metal to be electrodeposited on the cathode, the second compartment comprises aqueous hydrochloric acid and a salt, and the anolyte comprises aqueous sulfuric acid and a metal sulfate. Optionally, the salt in the second compartment can be the same as the metal chloride in the catholyte.

In some embodiments, the catholyte comprises an aqueous solvent that includes an additive which can coordinate with the metal ion to be electrodeposited on the cathode in preference to water (e.g., an additive having a higher Gutman donor number than water), such as a DMSO, which excludes water from the solvation sheath, reducing parasitic water reduction, and thus undesirable hydrogen production. In some embodiments, the catholyte comprises a DMSO/H₂O solution of the metal chloride (e.g., 25% dimethylsulfoxide (DMSO)—75% H₂O). The DMSO/H₂O electrolyte also provides additional benefits, such as solid-electrolyte interface (SEI) formation. The electrochemical reduction of DMSO forms a dense and self-repairing SEI on the cathode surface. SEI formation prevents dendrites from forming during metal deposition at the cathode via a protective layer allowing metals such as Zn(0) to electrodeposit onto the cathode while simultaneously reducing parasitic water electrolysis and oxidation of the cathode. This lowers the specific energy consumption (in kWh/ton) of the process compared to conventional electrowinning. Separating the electrowinning process into three cells separated by an anion exchange membrane (AEM) and a cation exchange membrane (CEM) lowers operational costs by enabling higher current densities without sacrificing current efficiency. SEI formation also helps control the morphology of the deposited metal, suppressing dendrite formation. Other additives that are useful in this system include, but are not limited to, 1-Butyl-3-methylimidazolium chloride (BMIM-C1) dimethylformamide (DMF), gelatin, glues, gums, polysaccharides, ionic liquids, glycine, indium chloride, tin chloride, various organic solvents miscible with water, and various other metal chlorides.

In some embodiments, the electrowinning is performed in a single three-compartment cell described above. One example of this is electrowinning metallic zinc from a zinc chloride solution. This process can also be applied to other metals, such as cobalt, tin, bismuth, nickel, iron, lead, copper, or manganese, all of which have reportedly been recovered by traditional aqueous electrowinning processes in the past.

In other embodiments, the methods described herein can be performed in multiple three-compartment electrowinning cells in cases where a higher valent cation (+3 or greater) may benefit from step-wise reductions to the metallic state. One example of this is reduction of Fe⁺³ for iron production, in which iron ore is leached with hydrochloric acid at or near room temperature, and the resulting FeCl₃ solution is purified to remove insoluble materials (e.g., by filtration) and soluble impurities (e.g., by ion-elective separation processes), and then electrowinning the resulting FeCl₃ solution in two separate electrochemical steps. It is beneficial to break down the reduction into two different steps in order to avoid the iron dendrite formation observed in single cell electrowinning of iron directly from iron(+3). In this two-step process, Fe⁺³ chloride solution is electrochemically converted to Fe⁺² chloride in a first three-compartment cell, and then Fe⁰ is electrowon from the Fe⁺² chloride in a second three-compartment electrowinning cell. Electrowinning from Fe⁺² avoids the dendrite formation found in conventional iron electrowinning directly from Fe⁺³. Other metals that can be electrowon in two or more isolated process steps, like iron, include, but are not limited to tin (2⁺ and 4⁺) or other metals which have two stable oxidation states.

The electrowinning cells can tolerate intermittent energy supplies as modular electrolysis cells can easily be throttled. The method eliminates process carbon, such as metallurgical coke, and creates a potential process carbon emissions savings of 52 million tons of CO₂ if industrially adopted for iron production.

The following non-limiting embodiments are set forth below to highlight certain features and aspects of the membranes described herein.

Embodiment 1 is a method for electrowinning a metal, the process comprising the steps of:

• • (a) providing a three-compartment electrowinning cell comprising a cathode in contact with an aqueous catholyte in a cathode compartment; an aqueous anolyte in contact with an anode in an anode compartment; and an aqueous solution of a salt in a middle compartment between the cathode compartment and the anode compartment; wherein the catholyte in the cathode compartment is separated from the aqueous solution in the middle compartment by an anion exchange membrane (AEM), and the anolyte in the anode compartment is separated from the aqueous solution in the middle compartment by a cation exchange membrane (CEM); the catholyte comprises a metal chloride dissolved in aqueous hydrochloric acid; optionally, the anolyte comprises a cations that differ from the cations of the catholyte;
  • (b) applying an electric potential across the anode and cathode for a period of time sufficient to reduce the cations of the first metal to their metallic state and thereby deposit the first metal onto the cathode; and
  • (c) recovering the first metal from the cathode;
  • wherein when the electric potential is applied across the anode and the cathode, chloride ions from the catholyte migrate through the AEM into the aqueous solution in the second compartment; cations from the anolyte migrate through the CEM into the aqueous solution in the second compartment, and oxygen is generated at the anode.

Embodiment 2 is the method of embodiment 1, wherein the first metal is selected from the group consisting of Zn, Sn, Ni, Co, Bi, Mn, Fe, Pb, Cr, Cu and other transition metals which can be electrodeposited in an aqueous environment.

Embodiment 3 is the method of embodiment 1 or embodiment 2, wherein the catholyte further comprises an additive having a Gutman number greater than that of water.

Embodiment 4 is the method of embodiment 3, wherein the additive comprises at least one material selected from the group consisting of an organic solvent miscible with water (e.g., dimethylsulfoxide, dimethylformamide), gelatin, BMIM-Cl, glues, gums, polysaccharides, ionic liquids, glycine, and metal chlorides (e.g., indium chloride, tin chloride).

Embodiment 5 is the method of embodiment 4, wherein the additive is an organic solvent that is miscible with water and is present in the catholyte at a concentration in the range of about 5% to about 50% by weight (e.g., about 25% by weight); or the additive is selected from the group consisting of gelatin, 1-butyl-3-methylimidazolium chloride (BMIM-Cl), glues, gums, polysaccharides, ionic liquids, glycine, and metal chlorides (e.g., indium chloride, tin chloride), and is present in the catholyte at a concentration in the range of about 1 mg/L to about 100 mg/L (e.g., about 15 mg/L).

Embodiment 6 is the method of embodiment 4, wherein the additive comprises dimethylsulfoxide.

Embodiment 7 is the method of embodiment 6, wherein the dimethylsulfoxide is present in the catholyte at a concentration in the range of about 5% to about 50% by weight.

Embodiment 8 is the method of embodiment 6, wherein the dimethylsulfoxide is present in the catholyte at a concentration in the range of about 10 to about 30% by weight.

Embodiment 9 is the method of any one of embodiments 1 to 8, wherein the cations of the first metal are present in the catholyte at a concentration in the range of 0.1 M to about 5 M.

Embodiment 10 is the method of any one of embodiments 1 to 9, wherein the salt is present in the aqueous solution in the middle compartment at a concentration in the range of about 0.1 M to about 5 M.

Embodiment 11 is the method of any one of embodiments 1 to 10, wherein the anolyte comprises aqueous sulfuric acid and/or a metal sulfate, and the aqueous solution in the middle compartment further comprises aqueous hydrochloric acid in addition to the salt.

Embodiment 12 is the method of embodiment 11, wherein the metal sulfate comprises magnesium sulfate.

Embodiment 13 is the method of any one of embodiments 1 to 12, wherein the salt in the aqueous solution in the middle compartment is a metal chloride.

Embodiment 14 is the method of any one of embodiments 1 to 10, wherein the anolyte comprises aqueous sodium hydroxide, and the salt of the aqueous solution in the middle compartment comprises sodium chloride.

Embodiment 15 is the method of any one of embodiments 1 to 10, wherein the first metal is Zn, the additive comprises dimethyl sulfoxide and 1-butyl-3-methylimidazolium chloride, and the surface of the zinc that is deposited on the cathode is etched with HCl to remove surface dendrites therefrom.

Embodiment 16 is the method of embodiment 15, wherein the concentration of 1-butyl-3-methylimidazolium chloride is about 10 to 20 mg/L (e.g., about 15 mg/L), and the concentration of dimethyl sulfoxide is about 10 to 40 wt % (e.g., about 25 wt %) in water.

Embodiment 17 is a three-compartment electrowinning cell comprising:

• • a catholyte compartment containing a cathode;
  • an anolyte compartment spaced from the catholyte compartment and containing an anode; and
  • a middle compartment between the catholyte compartment and the anolyte compartment;
  • wherein the catholyte compartment is defined by a first portion of a housing and an anion exchange membrane, the anolyte compartment is defined by a second portion of the housing and a cation exchange membrane, and the anion exchange membrane and the cation exchange membrane are spaced from each other to define the middle compartment in combination with a third portion of the housing; and wherein the anode and cathode are adapted for connection to a power source.

Embodiment 18 is the electrowinning cell of embodiment 17, wherein each compartment includes an inlet and an outlet configured to allow: a catholyte to be pumped through the catholyte compartment, an anolyte to be pumped through the anolyte compartment, and a salt solution to be pumped through the middle compartment.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate certain features and aspects of the methods, apparatus and systems described herein and are not meant to be limiting.

FIG. 1 provides a schematic illustration of a three-compartment electrowinning cell as described herein.

FIG. 2 provides a schematic illustration of an experimental setup for electrowinning Zn. from ZnCl₂.

FIG. 3 provides scanning electron micrographs of zinc electrodeposited in 1M ZnCl₂+0.1M HCl+100% water electrolyte.

FIG. 4 provides a scanning electron micrographs of zinc electrodeposited in various electrolytes: 1M ZnCl₂+15 mg/L gelatin+100% water, with (a1) and without (a2) 0.1M HCl; 1M ZnCl₂+15 mg/L BMIM[Cl]+100% water, with (b1, b3), and without (b2, b4) 0.1M HCl; 1M ZnCl₂+25% DMSO 75% water at 20 mA/cm², with (c1) and without (c2) 0.1M HCl.

FIG. 5 provides scanning electron micrographs of zinc electrodeposited in 1M ZnCl₂+15 mg/L BMIM[Cl]+25% DMSO+75% water electrolyte prior to HCl etch (a and b), and post HCl etch (c and d).

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A three-compartment electrowinning cell comprises a cathode in contact with a catholyte comprising a metal chloride dissolved in aqueous hydrochloric acid, optionally containing an additive, such as DMSO, and an anode in contact with an aqueous anolyte. The catholyte and anolyte are in separate compartments separated from each other by an intermediate compartment defined by an anion exchange membrane (AEM) on the catholyte side, and a cation exchange membrane (CEM) on the anolyte side. The intermediate compartment comprises an aqueous solution of a salt (e.g., NaCl or another metal salt). The salt solution provides a compartment to maintain charge neutrality and provides a clean solution for subsequent electrodialysis, which closes the process loop. The electrowinning products are a metal (e.g., a critical metal such as Zn or a non-critical metal such as iron), which deposits on the cathode, and oxygen gas which is generated at the anode.

During the process, chlorine anions cross the AEM (e.g., SELEMION AMV anion exchange membrane) from the catholyte and metal cations (e.g., Na+) or hydronium (H⁺) cations cross the CEM (e.g., NAFION 115 cation exchange membrane) from the anolyte, forming a chloride salt or hydrochloric acid in the intermediate compartment. When the salt is NaCl, electrodialysis can be used to regenerate HCl for use in leaching metal ores to form the catholyte, and NaOH for use as the anolyte.

The methods described herein use metal oxides from primary (ore based) or secondary (recycled) sources. Secondary oxides can be sourced from the aforementioned industrial streams. The Zn reduction described above can be adapted to other critical metals, such as cobalt, tin, bismuth, nickel, or manganese, as well as non-critical metals, such as iron, Pb, Cu, and Cr. The metal oxides are leached via hydrochloric acid in an aqueous medium, such as DMSO/H₂O (e.g., 25% DMSO—75% H₂O). The leaching step can comprise a closed loop HCl cycle where HCl is consumed during leaching and regenerated via electrodialysis of NaCl. The leached metal chloride then moves to a filtration and separation step where deleterious cations are removed via a variety of processes, e.g., ion exchange, chemical precipitation, flocculation, crystallization, distillation, evaporation, and the like. Finally, the metal chloride is electrowon in a three-compartment electrowinning cell, operating, e.g., at about 10 A, which leverages discrete aspects of the zinc chloride and zinc sulfate processes described above.

The aqueous anolyte can comprise either a basic or an acidic medium. In some embodiments, the anolyte comprises an aqueous base, such as sodium hydroxide or potassium hydroxide. In other embodiments, the anolyte comprises an acidic solution such as aqueous sulfuric acid, or a solution of a metal salt, such as magnesium sulfate, iron sulfate, aluminum sulfate, and other polyvalent metal sulfates and the like, dissolved in aqueous sulfuric acid. When included in the anolyte, the base (e.g., NaOH) is present at a concentration of about 0.01 M to about 5 M (preferably 0.05 M to about 0.2 M). When included in the anolyte, the acid (e.g., sulfuric acid) is present at a concentration of about 0.01 M to about 5 M (preferably 0.05 M to about 0.2 M). When included in the anolyte, the metal sulfate (e.g., magnesium sulfate) is present at a concentration of about 0.1 M to about 5 M (preferably 0.5 M to about 2 M).

The aqueous catholyte comprises a metal chloride of the metal to be electrowon at a concentration of about 0.1 M to about 5 M (preferably 0.5 M to about 2 M in hydrochloric acid at a concentration of about 0.01 M to about 1 M (preferably 0.05 M to about 0.2 M.

The middle or intermediate compartment of the three-compartment electrowinning cell described herein comprises a salt dissolved in an aqueous medium. The salt (e.g., sodium chloride, or another metal chloride) is present in the aqueous medium at a concentration of about 0.1 M to about 5 M (preferably 0.5 M to about 2 M). Optionally, the salt can be the same as the metal chloride in in the catholyte.

The cathode and the anode can be in the form of a plate, rod, or of any other geometrical configuration. The cathode is formed from a metal suitable for the electrodeposition of the metal being electrowon, such as aluminum, zinc, copper, brass, nickel, iron, titanium, the metal being electrowon, or other cathode materials which provide good bonding. The anode comprises a chemically inert metal surface, such as platinum, platinized titanium, nickel or iridium.

The electrowon metal can be recovered from the cathode by physically separating the electrowon metal by peeling, melting the electrowon material, or in the case of the cathode comprising the electrowon metal melting the whole cathode and electrodeposited metal together.

Any AEM and CEM can be used in the electrowinning cell. AEMs and CEMs are well known to those of ordinary skill in the ion exchange art.

FIG. 1 shows a three-compartment electrowinning cell as described herein. Cell 100 comprises a catholyte compartment 102 at one end thereof, an anolyte compartment 104 at another end thereof, and an intermediate or middle compartment 106 between catholyte compartment 102 and anolyte compartment 104. A cathode 108 is disposed within catholyte compartment 102, while anode 110 is disposed within anolyte compartment 110. AEM 112 forms a partition between catholyte compartment 102 and middle compartment 106. CEM 114 is spaced from AEM 112 and forms a partition between middle compartment 106 and anolyte compartment 104. Housing 120 surrounds compartments 102, 104, and 106. Housing 120 defines inlets 122, 126, and 127 to compartments 102, 106, and 104, respectively. Housing 120 also defines outlets 124, 128, and 132 to compartments 102, 106, and 104, respectively. Inlet 122 and outlet 124 are arranged so that a catholyte solution can flow through catholyte compartment 102 during use. Inlet 126 and outlet 128 are arranged so that a salt solution can flow through middle compartment 106 during use. Inlet 130 and outlet 132 are arranged so that an anolyte solution can flow through anolyte compartment 104 during use. When the catholyte, anolyte and salt solution are present in their respective compartments, applying an electric potential between the cathode and anode initiates electrowinning of a metal chloride present in the catholyte to deposit the metal of the metal chloride onto cathode 108 and generate oxygen at anode 110. Once electrowinning is complete, cathode 108 can be removed and the metal can be recovered from the cathode. Materials of construction for the cell can be any material that will not react with the components of the anolyte, catholyte and salt solution during use (e.g., a chemical resistant plastic).

FIG. 2 illustrates an experimental setup for electrowinning zinc metal from a zinc chloride catholyte. When cathode 208 and anode 210 of apparatus 200 are placed in circuit with a power source, chloride ions from the catholyte compartment 202 migrate through AEM 221 into the middle compartment 206, while protons migrate through CEM 214 from the anolyte in anolyte compartment 204 into middle compartment 206. Zn metal deposits on cathode 208 and oxygen is generated at anode 210.

Screening of Additives for Zn Electrowinning.

Various additives were evaluated for electrowinning Zn from ZnCl₂. Table 1, below reports EDS-derived elemental analysis results (in weight percent of Zn, Cl, and O) for the Zn metal deposited with each additive system for ZnCl₂ electrowinning, the additive systems being (a) HCl, (b) HCl plus gelatin, (c) HCl plus BMIM-Cl, (d) HCl plus BMIM-Cl in 25% aqueous DMSO, (e) BMIM-Cl in 25% aqueous DMSO, (f) 25% aqueous DMSO with HCl etching of the deposited Zn surface, and (g) 25% aqueous DMSO plus BMIM-Cl with HCl etching of the deposited Zn surface. The deposited Zn was also examined by SEM.

The screening results indicate that 15 mg/L BMIM-Cl+25% DMSO prevents dendrites on deposited zinc through the formation of an SEI-like surface and the replacement of water throughout the solvation shell. A concentration of 15 mg/L gelatin with and without HCl promotes planar deposits where the gelatin is adhered but over the experiment time it was not well distributed over the surface. Concentrations of 15 mg/L BMIM-Cl+0.1M HCl in 25% DMSO, and 15 mg/L BMIM-Cl in 25% DMSO promote planar deposits and are well distributed on the zinc surface with an hour deposition at the present flow rate. The addition of the 0.1M HCl etch to the BMIM-Cl and DMSO no-HCl depositions smoothens the finished surface revealing planar deposits in the SEM.

The (1M ZnCl₂+0.1M HCl+100% water) sample's microstructure can be seen in FIG. 3. The sample was deposited to simulate zinc electrodeposition from ZnCl₂ assuming left over HCl leachant without the addition of additives as a control. The bulk sample displayed a heavily porous surface with pores extending to the brass substrate. Though investigation of the microstructure indicates planar regions throughout the surface (FIG. 6 a, b, c), the surface is riddled with sites of hydrogen nucleation represented as pores throughout the surface that greatly decrease current efficiency and deposit purity.

Electrowinning Zn from 1M ZnCl₂+15 mg/L gelatin in water with and without 0.1M HCl was evaluated to assess the effect of gelatin on zinc electrodeposition from ZnCl₂. With 0.1M HCl, the gelatin zinc deposit appeared to be planar under the sections at which the gelatin is adsorbed but dendritic in the areas where it was not absorbed, with a poor distribution over the surface in these experimental conditions (FIG. 4, panel a1). This follows, but to a lesser degree, in the gelatin zinc deposit without 0.1M HCl as the surface was considerably less planar underneath the gelatin and the gelatin was less adhered (FIG. 4, panel a2).

Electrowinning Zn from 1M ZnCl₂+25% DMSO+0.1M HCl 75% water at 20 mA/cm²) and from 1M ZnCl₂+25% DMSO+75% water was evaluated to assess the effect of DMSO on zinc electrodeposition from ZnCl₂ with and without assuming left over HCl leachant. With 0.1M HCl, the zinc deposit appears planar with crystallites of planar steps in random orientation (FIG. 4, panel c1). Without 0.1M HCl, there appears to be a similar deposit, but the surface dendrites prevented investigation (FIG. 4, panel c2).

The microstructure of the additive combination of BMIM[Cl] and DMSO was also investigated. Electrowinning Zn from 1M ZnCl₂+15 mg/L BMIM[Cl]+25% DMSO+75% water, prior to and post-10 minute 0.1M HCl etch was tested to assess the synergetic effect of BMIM[Cl] and DMSO. Prior to the HCl etch the sample appears similar to the DMSO samples but with smaller crystallites (FIG. 5, panels a and b). As in the previous samples without HCl, the surface is littered with surface dendrites, although there are isolated regions of visibly planar deposits (FIG. 5, panels b1 and b2).

This pattern of dendrites on the surface of samples without HCl was investigated in for the 1M ZnCl₂+15 mg/L BMIM[Cl]+25% DMSO+75% water conditions through a 10 minute etch in 0.1M HCl to simulate the post-experiment processing. In areas where the sample was etched the surface dendrites were eliminated revealing a planar deposit underneath without any sites of hydrogen nucleation displaying the synergetic effect of BMIM[Cl] and DMSO (FIG. 5, panels c and d).

Chemical Composition of the Deposits by Additive

Energy-dispersive x-ray spectrometry (EDS) elemental maps were taken of samples from several additive series with their elemental atomic wt % computed to assess the impact gelatin, BMIM[Cl], DMSO, and BMIM[Cl]+DMSO have on the purity of zinc deposits with and without the presence of 0.1M HCl. The results are shown in Table 2. SEM images were also used to investigate the nature of the resulting Zn deposits.

Connecting SEM images to the EDS results of Table 2, the samples with more surface dendrites have readings of higher concentrations of chlorine. This is to be expected as dendrites can accumulate precipitated ZnCl₂ as trapped salt beneath or between the dendrites. The varying O atomic wt % seen on each sample can be attributed to differences in the equipment and sensitivity to surface oxidation as DMSO (HCl) post-etch, and DMSO+BMIM(HCl) post-etch were captured on the JEOL IT800HL SEM, while all other samples were captured on the Hitachi S-4700-II SEM. As seen in Table 2, the inclusion of HCl+BMIM[Cl], DMSO, and BMIM[Cl]+DMSO greatly reduces Cl atomic wt % creating high purity zinc deposits.

In summary, the effects of gelatin, BMIM[Cl], DMSO, and HCl additives on the surface morphology and chemical composition of zinc electrodeposited from a ZnCl₂ at 40 mA/cm² were studied. Electrowinning Zn from ZnCl₂ with 15 mg/L BMIM[Cl]+25% DMSO prevents dendrites on deposited zinc through the formation of an SEI-like surface and the replacement of water throughout the solvation shell. Electrowinning Zn from ZnCl₂ with 15 mg/L gelatin with and without 0.1M HCl promotes planar deposits where the gelatin is adhered but over the experiment time gelatin was not well distributed over the surface. Using 15 mg/L BMIM[Cl]+0.1M HCl, 25% DMSO, and 15 mg/L BMIM[Cl]+25% DMSO promotes planar and high purity deposits and the BMIM[Cl] was well distributed on the zinc surface with an hour deposition at the flow rate used in the experiments. finally, the addition of the 0.1M HCl etch to the BMIM[Cl] and DMSO no-HCl depositions smooths out the finished surface revealing planar deposits beneath.

Example 1. Electrowinning Procedure

A three-compartment cell as shown in FIG. 1 is used to electrowin a desired metal from a metal chloride solution in aqueous hydrochloric acid. Typically, a catholyte comprising a 1 M solution of the metal chloride in 0.1 M hydrochloric acid is pumped through of catholyte compartment 102 via inlet 122 and outlet 124. At the same time, an aqueous salt solution (e.g., 1 M salt in 0.1 M HCl) is pumped through middle compartment 106 via inlet 126 and outlet 128, and an aqueous anolyte (e.g., 1 M MgSO₄ in 0.1 M H₂SO₄) is pumped through anolyte compartment 104 via inlet 130 and outlet 132. Optionally, the cell can be run in a static condition where the compartments are filled with their respective solutions, but are not flowing through the cell, or just the catholyte may be flowing through the cell, while the other compartments are filled with their respective solutions, but not flowing. A peristaltic pump can be conveniently used to pump the solutions through the compartments. An electric potential is applied to anode 110 and cathode 108 to initiate electrochemical reduction of the metal chloride to the metal in the catholyte compartment. And the metal deposits on cathode 108. At the same time, oxygen is generated at anode 110 and the oxygen can be separated from the anolyte and collected.

Additives such as DMSO, 1-butyl-3-methyl-imidazolium chloride (BMIM Cl), gelatin, dimethylformamide (DMF) and the like) are included in the catholyte to improve the physical properties of the deposited metal. The metal chloride can be any metal chloride from which a metal is desired to be electrowon (e.g., ZnCl₂, FeCl₂, SnCl₂, and the like). Typically, the cell is operated at a current density of about 40 mA/cm². The anolyte compartment typically utilizes a platinized titanium anode, NAFION NR212 cation exchange membrane, and an anolyte of 1 M MgSO₄, 0.1 M H₂SO₄ in tap water. The catholyte compartment typically utilizes a brass 260 cathode, a PIPER-ION 20 micron anion exchange membrane, and a catholyte comprising 1 M metal chloride in 0.1 M HCl in tap water, with the various additives added. The middle compartment between the AEM and CEM typically utilizes a 1 M solution of the metal chloride and 0.1 M HCl in tap water. Note, the metal chloride in the middle compartment is acting as a charge carrier only, and is not electrochemically reduced or oxidized.

A preferred additive for Zn electrowinning is 25% (by weight) solution of DMSO in water with 15 mg/L BMIM-Cl. For metal ions that will reduce DMSO (e.g., Fe⁺² or Sn⁺²) an additive such as dimethylformamide (DMF) can be used.

Metal purity can be measured by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). EDS estimates the metal purity by exposing the sample to an electron beam and detecting the Xray signatures from the elements within the sample. The main drawback of EDS is that it only looks at the surface level of a sample, not the purity throughout the sample. Inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS), and other known techniques can be utilized for bulk sample purity analysis.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing materials or methods (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “consisting of” and “consists of” are to be construed as closed terms, which limit any compositions or methods to the specified components or steps, respectively, that are listed in a given claim or portion of the specification. In addition, and because of its open nature, the term “comprising” broadly encompasses compositions and methods that “consist essentially of” or “consist of” specified components or steps, in addition to compositions and methods that include other components or steps beyond those listed in the given claim or portion of the specification. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All numerical values obtained by measurement (e.g., weight, concentration, physical dimensions, removal rates, flow rates, and the like) are not to be construed as absolutely precise numbers, and should be considered to encompass values within the known limits of the measurement techniques commonly used in the art, regardless of whether or not the term “about” is explicitly stated. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate certain aspects of the materials or methods described herein and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the claims.

Preferred embodiments are described herein, including the best mode known to the inventors for carrying out the claimed invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the claimed invention to be practiced otherwise than as specifically described herein. Accordingly, the claimed invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the claimed invention unless otherwise indicated herein or otherwise clearly contradicted by context.