TW202440953A - Method and process for electrochemical oxidation - Google Patents

Abstract

The invention relates to a method and system for generation of an oxidant solution including an electrochemical cell having an anode and a cathode, the method comprising the steps of; (i) supplying a feedstock electrolyte to a reaction area between the anode and cathode, the feedstock electrolyte consisting of sulphate ions (SO4<SP>2-</SP>) and ferrous ions (Fe<SP>2+</SP>); (ii) in an operational cycle, electrolysing the feedstock electrolyte to produce an oxidised acid solution comprising peroxydisulphate (S2O8<SP>2-</SP>) and ferric ions (Fe<SP>3+</SP>); and (iii) supplying said oxidised acid solution. The oxidant solution is preferably used for leaching metals from waste, including e-waste and metal containing ores.

Description

Electrochemical oxidation method and process

The present invention relates to the field of electronic chemistry.

In one form, the invention relates to the production of an oxidant solution using an electrochemical cell.

In one particular aspect, the invention is applicable to metal extraction, such as metal extraction from solid or solid particulate waste such as mining or electronic waste (e-waste) sources.

In another specific aspect, the present invention is applicable to a metal extraction process, or alternatively an electrosmelting process.

The following will conveniently be disclosed with respect to electronic waste, particularly electronic waste comprising copper, however, it should be understood that the invention is not limited to its use, but may be applied to other feed or waste streams comprising one or more other metals or metal ions, such as those generated by the mining industry.

It should be understood that any discussion of files, devices, actions or knowledge in this specification is included to explain the context of the invention. Furthermore, the discussion throughout this specification is due to the inventor's realization and/or identification of certain related field problems by the inventor. In addition, to the knowledge and experience of the inventor, the discussion of materials such as files, devices, actions or knowledge in this specification is included to explain the context of the invention, and accordingly, any such discussion will not be regarded as an admission that any material formed part of the basis of the prior art or was common knowledge in the relevant field in Australia or other regions on or before the priority date of the present disclosure and the scope of the patent claimed herein.

A variety of feed and waste streams include at least one metal compound. These include ores, landfill residues, sludges, tailings, slags, ashes, filter dust from incinerators, blanks or electronic waste, and wafers from electronic circuits. Electronic waste is defined as waste generated from a wide range of electronic devices such as computers, mobile phones, televisions and household appliances.

The problem of electronic waste is growing as the number of electronic products increases due to rapid technological development and the growing consumer demand for electronic products. Many countries have introduced legislation and policies to manage electronic waste. For example, in 2011, Australia implemented the National Television and Computer Recycling Scheme (NTCRS). The NTCRS aims to achieve 80% recycling of electronic waste by 2030 and provides industry-funded electronic waste recycling for households and small businesses to dispose of obsolete electronic equipment. The implementation of this type of recycling initiative provides opportunities to extract and recycle valuable metals.

Metals can be recovered from waste streams by electrochemical means using oxidized acids. In this process, the oxidized acid can be produced by oxidizing a starting acid at the anode of an electrochemical cell, followed by rinsing or soaking the starting material with the oxidized acid to dissolve the metal or metal compound, and finally depositing the dissolved metal at the cathode of the electrochemical cell.

Electrochemistry uses the flow of electrons to drive oxidation and reduction reactions. An electrochemical battery typically consists of two half cells, one associated with the anode (positive electrode) and the other associated with the cathode (negative electrode), with an electrolyte in between to facilitate the reaction and movement of ions.

In electrochemical cells, oxidation of metals occurs at the anode and reduction of metals occurs at the cathode in so-called "redox" reactions. Redox reactions occur in the presence of an oxidizing agent, usually in an electrolyte, to oxidize another substance by absorbing electrons and being reduced. Oxidizing acids are particularly useful in electrolytes used in electrochemical reactions because they can often oxidize metals that have a lower oxidizing power than other acids.

For example, German Patent No. 10 2015 110 179 (DE 10 2015 110 179) discloses the use of oxidized acids and diamond-doped cathodes in electrochemical cells to extract metals from solid feeds. The method has been successfully tested on solutions containing Zn, Cu, Fe, Ni and Sn slag, Chilean copper slag (CuFe), brass ore (CuFeS2) and copper ore.

According to DE 10 2015 110 179, the use of oxidizing acids such as peroxodisulfate provides a substantial increase in the metal brought into solution compared to the use of other acids, in many cases at least doubling the amount of the metal in solution. DE 10 2015 110 179 also states that neither the process parameters nor the concentration of the oxidizing acid are critical, and that a concentration of oxidizing acid of at least 0.1 mol/l, preferably at least 0.5 mol/l and even more preferably at least 1 mol/l should be present. Concentrations beyond this are not critical.

The use of oxidized acids such as peroxodisulfate is also discussed in U.S. Patent Nos. 10,259,727 and 10,046,898 to Advanced Diamond Technologies for use in electrochemical systems for water disinfection and removal of organic matter. The use of diamond anodes (and optionally diamond cathodes) operating at high current densities is advocated to provide high current efficiency, extended life operation, and improved cost efficiency.

The object of the present invention is to enable a more efficient method to be used for the electrochemical generation of anodic electrolyte solutions.

Another object of the present invention is to improve the economics of the process of extracting metals using an anodic electrolyte solution, including improving metal extraction and reducing the energy consumption of the process.

Another object of the present invention is to provide a more efficient anodic electrolyte solution for extracting metals or metal ions from feed or waste streams.

Another object of the present invention is to provide an electrochemical process that can regenerate and reuse key electrolyte ions.

Another object of the present invention is to alleviate at least one disadvantage associated with the related art.

The purpose of the embodiments disclosed herein is to overcome or alleviate at least one of the above-mentioned disadvantages of the related art system or to provide at least a useful alternative to the related art system.

The present invention generally relates to a method for electrochemical regeneration of an anolyte solution comprising peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ) from an electrolyte feed comprising sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ).

In the first embodiment disclosed herein, a method for generating an oxidant solution using an electrochemical cell having an anode and a cathode is provided, the method comprising the following steps:

(i) supplying a feed electrolyte to the reaction zone between the anode and the cathode, the feed electrolyte consisting of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ );

(ii) electrolyzing the feed electrolyte to produce an oxidized acid solution comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and iron ions (Fe ³⁺ ) during an operation cycle; and

(iii) supplying the oxidized acid solution.

In the second embodiment disclosed herein, a method for generating an oxidant solution using an electrochemical cell having an anode half-cell and a cathode half-cell is provided, the method comprising the following steps:

(i) supplying a feed electrolyte to the anode half-cell to enter and leave the fluid path formed between the anode and the cathode, the feed electrolyte consisting of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ );

(ii) during an operation cycle, electrolyzing the feed electrolyte to produce an anodic electrolyte solution comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and iron ions (Fe ³⁺ ); and

(iii) supplying the anodic solution from the anodic half-cell.

Typically, the anode half-cell and the cathode half-cell are separated by a separator, such as a porous film or an ion exchange membrane. In a preferred embodiment, the ion exchange membrane is a cation exchange membrane or an anion exchange membrane, more preferably, a monovalent selective anion membrane. The use of a separator preferably allows the oxidation efficiency of the acid to be increased. When used herein, the term "divided cell" configuration refers to a configuration in which a separator is inserted to separate the anode and the cathode.

In another embodiment, the anode half-cell and the cathode half-cell can be combined, such as by removing the separator to form a single electrochemical cell, so that the anode electrolyte solution is supplied as the electrolyte of the electrochemical cell. As used herein, the term "undivided cell" configuration refers to a configuration in which there is no separator or other barrier between the anode and cathode.

The process cycle of the method may include a single pass or it may operate in a cyclic mode. The process cycle may include electrolyzing the electrolyte in a single batch or a continuous flow system. The circulation of the electrolyte and the electrolysis batch volume may be used to increase the concentration of the oxidant converted from the feed.

In one embodiment, the anodic electrolyte solution from the chemical cell can be provided to a downstream process such as an electrowinning cell. In another embodiment, the anodic electrolyte solution can be generated and supplied on-site and fed directly into a process for wet metallurgy and metal extraction from deposits, ores, mines, mining pumps, and landfills. For example, the anodic electrolyte can be generated at a mine site and pumped into a mine shaft, forced along a long mine shaft, and then returned to the surface where separation of the metal from the anodic electrolyte can occur. In another embodiment, the anodic electrolyte solution can be generated on-site and supplied to replace the tapping solution in conventional wet metallurgical processes, such as heap tapping and tank and tank tapping.

The use of aqueous chemical methods to recover metals from ores, concentrates and recycled or residual materials is known as wet metallurgy. Wet metallurgy is a more economical and environmentally friendly alternative to metallurgical processes. It has shown its advantages in the treatment of low-grade ores and mainly includes the steps of dewatering, in-situ dewatering and tank dewatering. However, wet metallurgical processes have not been widely used in the treatment of some sulfide ores, such as brass ores, due to their slow dissolution rate in acid, which is mainly due to the formation of passivation layers of polysulfides ( ^Sn2- ) and elemental sulfur ( _S0 ). Therefore, researchers have improved the extraction efficiency by adding various oxidants, including Fe ³⁺ , O ₂ , H ₂ O ₂ , Cr ₂ O ₇ ^2- , ClO ₄ ^- , O ₃ , MnO ₄ ^- and S ₂ O ₈ ^- . It has been concluded in the prior art that the dissolution kinetics of brassite when reacting with non-radical oxidants such as H ₂ O ₂ mainly follows the shrinking core model, in which the surface chemical reaction is the rate-determining step. Therefore, it is necessary to add a large amount of oxidant to the reaction system during stack extraction to maintain high extraction efficiency and compensate for the consumption and deactivation of the oxidant.

In another embodiment, the anodic electrolyte solution from the chemical battery can be used in a process for extracting and selectively recovering metals from electronic waste, including common metals such as Cu, Ni, Fe; precious metals such as Au, Ag; platinum-based metals such as Pt, Pd, Rh, Ir and Ru; rare metals such as Te, Ga, Se, Ta and Ge; and harmful metals such as Pb, Cd, In, Sb. In a particularly preferred embodiment, the metals extracted from electronic waste are Cu, Ni, Zn and Al.

Typically, the concentration of sulfate ions (SO ₄ ^2- ) in the feed electrolyte is between 0.1 and 5 volume molars.

Typically, sulfate ions (SO ₄ ^2- ) would be provided from a sulfuric acid (H ₂ SO ₄ ^2- ) feed. The present invention uses low acid concentrations compared to equivalent processes of the prior art which use 15 to 20 volume moles of acid. Lower acid concentrations reduce feed costs and improve process safety.

Typically, the concentration of ferrous ions (Fe ²⁺ ) is between 0.1 and 0.5 volume mol.

Typically, the aqueous feed electrolyte is electrolyzed at a current density of about 50 to 200 mA/cm ^-2 .

Preferably, the ratio of SO ₄ ^2- :Fe ²⁺ is between 1:0.05 and 1:0.5, more preferably between 1:0.05 and 1:0.1.

Typically, the anode is a doped diamond electrode, such as a boron-doped diamond electrode. However, other electrode materials such as carbon composites, stainless steel, copper or titanium are also suitable.

Typically, the cathode is a doped diamond electrode. However, other electrode materials such as carbon composites, stainless steel, copper or titanium are also suitable.

The above method may also include regenerating and reusing key electrolyte ions. For example, in one process cycle, a feed electrolyte composed of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ) may be used to produce an anodic electrolyte solution comprising peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ). The ferric ions (Fe ³⁺ ) are used to extract metals from the waste and, as a result, are converted to ferrous ions (Fe ²⁺ ). Ferrous ions (Fe ²⁺ ) react with peroxodisulfate (S ₂ O ₈ ^2- ), which are converted into sulfate ions (SO ₄ ^2- ) and ferric ions (Fe ³⁺ ), thus completing the regeneration of ions from the feed electrolyte.

In a third aspect of the embodiment disclosed herein, a method for regenerating the aforementioned oxidant solution is provided, the method comprising the following further steps:

(iii) supplying the oxidized acid solution to the metal-containing waste so that iron ions (Fe ³⁺ ) are reduced to ferrous ions (Fe ²⁺ );

(iv) bringing peroxodisulfate (S ₂ O ₈ ^2- ) into contact with the ferrous ions (Fe ²⁺ ) to regenerate ferric ions (Fe ³⁺ ) and sulfate ions (SO ₄ ^2- ).

Furthermore, regeneration can be performed with or without a separator between the anode half-cell and the cathode half-cell.

In a fourth aspect of the embodiment disclosed herein, a system for generating an oxidant solution is provided, the system comprising:

(a) Anode and cathode, which define the reaction area of the electrochemical cell,

(b) an inlet and flow controller for passing an aqueous feed electrolyte between the electrodes, the feed electrolyte consisting of sulfate ions (SO ₄ ²⁻ ) and ferrous ions (Fe ²⁺ ) selected from the electrochemical generation of an oxidant comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and ferric ions (Fe ³⁺ );

(c) current means for supplying current to electrolyze the aqueous feed electrolyte to produce an oxidant solution containing the oxidant in the reaction zone; and

(d) an outlet for supplying the oxidant solution from the electrochemical cell.

The oxidant solution may be suitable, for example, for extracting metals from electronic waste and minerals, including waste streams associated with mineral processing.

In a fifth aspect of the embodiment disclosed herein, a system for generating an oxidant solution on-site for metal extraction is provided, the system comprising:

(a) Anode half-cell and cathode half-cell;

(b) inlet and flow control means for passing an aqueous feed electrolyte through the anodic half-cell, the feed electrolyte consisting of sulfate ions (SO ₄ ²⁻ ) and ferrous ions (Fe ²⁺ ) selected for the electrochemical generation of an oxidant comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and ferric ions (Fe ³⁺ );

(c) current means for supplying current to electrolyze the aqueous feed electrolyte to produce an oxidant solution containing the oxidant in the anode half-cell; and

(d) an outlet for supplying the oxidant solution from the anode half-cell of the electrochemical cell.

Oxidant solutions generated on site may be useful, for example, in the extraction of metals from electronic waste or minerals, including waste streams associated with mineral processing.

In another aspect of the embodiment disclosed herein, a method for extracting metal from metal-containing waste is provided, the method comprising the following steps: providing an oxidant solution of the present invention, and bringing the oxidant solution into contact with the metal-containing waste.

Metals extracted from metal-bearing waste typically include copper.

Other aspects and preferred forms are disclosed in the specification and/or defined in the attached patent application scope, forming part of the specification of the present invention.

In essence, embodiments of the present invention arise from the recognition that the presence of ferrous ions (Fe ²⁺ ) can substantially improve the generation of oxidant solutions using electrochemical cells. Furthermore, it has been recognized that ferrous ions (Fe ²⁺ ) in combination with sulfate ions (SO ₄ ^2- ) for the electrochemical generation of an oxidant comprising peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ) provide an excellent oxidant for the extraction of metals. It has also been recognized that it is also possible, preferably, to regenerate the oxidant and acid (peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ )) during multiple operating cycles.

The advantages provided by the present invention include the following:

˙Improved electrochemical cell efficiency in terms of power consumption;

˙Improved electrochemical battery efficiency in economic terms;

˙The oxidant/acid can be regenerated during multiple operating cycles of the electrochemical cell, thereby reducing operating costs and waste;

˙Compared to previous technologies, lower acid concentrations can be used, thereby reducing feed costs and improving process safety;

˙Complete extraction of certain metals can be achieved in a relatively short period of time.

The further scope of the applicability of the embodiments of the present invention will be apparent from the specific embodiments given below. However, it should be understood that the specific embodiments and specific embodiments, although indicating the preferred aspects of the present invention, are given only by way of example, because various changes and modifications within the spirit and scope disclosed herein are obvious to those with ordinary knowledge in the art from this detailed description.

Parts List

The illustrations and instructions in this article should be read with reference to the following component symbols appearing in the illustrations and text:

1: Anode

2: cathode

3: Membrane

4: Anode electrolyte

5: Cathode electrolyte

6:DC power supply

7: Electrolyzer

8: Peristaltic pump

9: Electro-oxidation anode battery

10: Electroreduction cathode battery

11: Ring battery

12: Battery waste

13:Battery Feed

14: Electrolyte pump

15: Circulation tank

16: Electro-oxidation

17: Metal extraction

18:Metal recycling

19: Recycled metals

20: Used solution

21: Granular electronic waste

22:Metal deposits

23: Metal-rich solution

23: Electronic waste bed

24: Hydrogen generation

25: Oxidant

26: Solution application

27: Filter solution

28: Ore

29: Crushing and reunion

30: Accumulation

31: Cleaning liquid pool

32: Fresh solids

33: Concentrated extract

34: Used solids

35: Interphase solution

36: Fresh solvent spray

37: Solid feed

38: Solid waste

39: Extraction

40: Pump for solvent/solution spraying

41: Solvent/solution in sprayer

42: Solid

43: Electronic waste bed

Other disclosures, purposes, advantages and aspects of the preferred and other embodiments of this application can be understood by those with ordinary knowledge in the art by referring to the following descriptions of aspects and in conjunction with the accompanying drawings, which are given only by way of example and therefore are not intended to limit the contents disclosed herein, and in which:

Figure 1 illustrates a typical electrochemical cell;

FIG2 illustrates a reactor apparatus including an electrochemical cell having a membrane separating an anode half-cell from a cathode half-cell;

FIG. 3A illustrates an electrochemical cell of the type used in the present invention;

FIG3 is a plot of peroxodisulfate ions (S ₂ O ₈ ^2- ) versus time, using the electrochemical cell shown in FIG3A ;

FIG. 4A illustrates an electrochemical cell of the type used in the present invention;

FIG. 4B is a graph showing the conversion of ferrous ions (Fe ²⁺ ) to ferric ions (Fe ³⁺ ) versus time in the cell of FIG. 4A ;

FIG5 illustrates an electrochemical cell for in-situ copper extraction according to the present invention;

6A and 6B are graphs illustrating the degree of copper conversion in the electrochemical cell of FIG. 5 ;

FIG. 7 is a graph showing copper conversion versus reaction time for the electrochemical cell of FIG. 5 ;

FIG8 is a graph showing the iron composition (Fe ²⁺ /Fe ³⁺ ) measured using the electrochemical cell of FIG5 versus reaction time;

FIG. 9 is a graph showing power consumption measured using the electrochemical cell of FIG. 5 versus different compositions of the extraction medium;

FIG. 10 is a graph of power consumption versus reaction time, measured using the electrochemical cell of FIG. 5 ;

FIG11 is a schematic diagram of an electrolyzer for draining electronic waste;

FIG12 is a schematic diagram of a factory-scale electrolyzer with a block-based battery configuration according to the present invention;

FIG13 is a schematic diagram of an electro-deposition reactor apparatus for metal recovery according to the present invention;

FIG14 is a flow chart illustrating the general concept of process development for metal extraction;

FIG. 15 is a schematic representation of the in-situ removal of oxidant generation (Fe ³⁺ and/or peroxodisulfate) and electronic waste in an undivided cell;

FIG16 is a schematic representation of oxidant generation (Fe ³⁺ and peroxodisulfate) and stripping in a single step in a split cell;

FIG. 17 is a graph showing the peroxodisulfate concentration curves at different initial sulfate concentrations and under the same reaction conditions (i.e., current density = 150 mA cm-2, and flow rate = 50 mL min-1);

18A and 18B are curves of iron ions and peroxodisulfate ions as a function of reaction time in a split cell with an anion exchange membrane, with (A) 0.5 M Fe ²⁺ and (B) 0.25 M Fe ²⁺ and different initial sulfate concentrations and under the same reaction conditions (i.e., current density = 150 mA cm ^-2 and flow rate = 50 mL min ^-1 );

Figures 19A and 19B are graphs showing (A) the extracted copper (wt%) and (B) the ^molar fraction of iron ions as a function of reaction time at different SO ₄ ^2- /Fe ²⁺ ratios under the reaction conditions of current density = 150 mA cm -2 and flow rate = 50 mL min ^-1

FIG. 20 is a plot of the concentration of metals extracted in a laboratory-scale electrolyzer as a function of reaction time under reaction conditions of SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 150 mA cm ^-2 and flow rate = 50 mL min ^-1 ;

Figure 21 is a plot of metals extracted as a function of reaction time in a plant-scale electrolyzer. Reaction conditions: SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 150 mA cm ^-2 and flow rate = 1 L min ^-1 ;

FIG22 is a plot of the concentration of metals extracted in a plant-scale electrolyzer as a function of reaction time for three batches of electronic waste at an initial sulfate concentration of 2.5 M, current density = 150 mA cm ^-2 and flow rate = ¹ L min-1;

Figure 23 shows the proposed design of a commercial electrolyzer;

FIG24 is a plot of copper recovery as a function of reaction time;

FIG. 25 is a plot of oxidation reduction potential (ORP) as a function of reaction time for laboratory and plant scale electrolyzers under reaction conditions of SO ₄ ² -/Fe ²⁺ =1/0.1 and current density = 150 mA cm ^-2 ;

FIG. 26 is a plot of ORP as a function of reaction time for three batches of electronic waste for a plant-scale electrolyzer at an initial sulfate concentration of 2.5 M, current density = 150 mA cm ^-2 and flow rate = 1 L min ^-1 ;

FIG. 27 is a plot of copper recovery from brass ore as a function of time and temperature for a laboratory scale reactor under reaction conditions of SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 100 mA cm ^-2 and 700 RPM;

FIG. 28 is a graph of copper recovery from brass ore as a function of time under the conditions of SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 100 mA cm ^-2 and 700 RPM, and depicts the effect of mixing at 700 rpm with and without ultrasound;

FIG. 29 is a graph showing the effect of stirring rate on copper recovery from brass ore under the conditions of SO ₄ ^2- /Fe ²⁺ =1/0.1 and current density = 100 mA cm ^-2 ;

FIG. 30 is a plot of the concentration of extracted metals as a function of reaction time for a batch of nickel ore 1 in a laboratory scale electrolyzer at an initial sulfate concentration of 2.5 M, SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 150 mA cm ^-2 and flow rate = 50 mL min ^-1 ;

FIG31 is a plot of the concentration of extracted metals as a function of reaction time for batch 2 of nickel ore in a laboratory scale electrolyzer at an initial sulfate concentration of 2.5 M, SO ₄ ^2- /Fe ²⁺ =1/0.1, current density = 150 mA cm ^-2 and flow rate = 50 mL min ^-1 ;

Figure 32 shows the improvement in metal recovery after 2 hours of treatment with the anodic electrolyte solution in the electrolyzer relative to the standard acid stripping with 2.5MH ₂ SO ₄ ; the electrolyzer conditions were: 2.5 M initial sulfate concentration, SO ₄ ^2- /Fe ²⁺ = 1/0.1, current density = 150 mA cm ^-2 and flow rate = 50 mL min ^-1 ;

FIG33 is a simplified flow chart describing a standard pile draining process;

Figure 34 is a simplified diagram of the Shanks system;

FIG. 35 is a front view ( FIG. 35A ) and a top view ( FIG. 35B ) of the Rotocel extractor.

For purposes of this specification, the terms "upper", "lower", "right", "left", "rear", "front", "vertical", "horizontal", "inner", "outer" and their derivatives shall relate to the present invention as oriented as shown in FIG. 5. However, it shall be understood that the present invention may assume various alternative orientations unless expressly specified to the contrary. It shall also be understood that the specific devices and processes illustrated in the accompanying drawings and disclosed in the following specification are simply exemplary aspects of the concepts of the present invention as defined in the appended claims. Therefore, the specific dimensions and other physical characteristics disclosed herein are not to be considered limiting unless otherwise specified in the claims. Furthermore, unless otherwise specified, it should be understood that discussion of a particular feature of a component or the like extending in a given direction or along a given direction does not imply that the feature or component follows a straight line or axis in that direction or that it extends only in that direction or in that plane without other directional components or deviations, unless otherwise specified.

Figure 1 illustrates a typical electrochemical cell, which includes two half-cells and an electrolyte therebetween, one half-cell being associated with an anode (1) (positive electrode) and the other half-cell being associated with a cathode (2) (negative electrode). In the relevant redox reaction, a metal is oxidized at the anode and the oxidized metal is reduced at the cathode. When an oxidant is present, typically in the electrolyte, a redox reaction occurs to oxidize another substance by absorbing electrons and being reduced.

Figure 2 illustrates an electrochemical cell comprising a membrane (3) that allows only negatively charged ions to migrate from the anode (1) to the cathode (2). Positively charged ions remain on the anolyte side of the membrane.

Oxidizing acids are often included in the electrolyte of electrochemical cells because they can oxidize some metals that have a low reactivity with other acids. The present invention is concerned with the electrochemical regeneration of an anodic electrolyte solution comprising peroxodisulfate (S ₂ O 8 2- ) and ferric ions (Fe ³⁺ ) from an electrolyte feed comprising sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ). Peroxodisulfate ions (S ₂ O ₈ ^{2- )} have a strong oxidizing _power due to their ability to generate sulfate radicals (SO ₄ ^．- ). Chemical dissolution of metals by peroxodisulfate ions is also known as oxidative dissolution or dewatering of metals and involves the process of dissolving metals into solution by utilizing peroxodisulfate salts as oxidizing agents. As a result of the dissolution, the dissolved metals will be involved in activating the peroxodisulfate ions to generate sulfate radicals. This provides an environmentally friendly alternative to traditional dewatering agents such as hydrogen and sulfuric acid, which have significant environmental risks.

Preferably, the oxidant solution is generated using an electrochemical cell having an anodic half-cell and a cathodic half-cell. A feed electrolyte passes between the anodic half-cell and the cathodic half-cell. If a porous membrane is used, the feed electrolyte passes through the anodic half-cell. The feed electrolyte, which is composed of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ), is oxidized during the operating cycle of the electrochemical cell to produce an oxidized acid solution containing peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ).

The oxidized acid solution from the electrochemical cell may be formed in the reaction zone between the anode or cathode. Alternatively, the oxidized acid solution may be formed in the anodic half-cell when the anodic half-cell and the cathodic half-cell are separated by a separator, such as a porous membrane or an ion exchange membrane. A reservoir in fluid communication with the anodic half-cell may be used to circulate or store the anodic electrolyte solution.

The operation cycle of an electrochemical cell may involve recycling a batch volume of electrolyte and electrolysis to increase the concentration of oxidant converted from the feed.

Waste containing metal

The operating cycle of an electrochemical cell may involve electrolyzing metal-containing waste in a single pass or continuous flow system. Dissolved metals may be separated by precipitation at the cathode, and the acid may be regenerated by reoxidation at the anode, thus allowing the process to operate in a single pass. Alternatively, if drained waste material can be removed from the reaction zone and new waste material can be fed in, the process may operate continuously.

The process can be used to extract metals from liquids, solutions or solid materials such as circuit boards or wafers and deposit them at the cathode. The metal deposited on the cathode can be removed from the cathode by a variety of known physical methods. Alternatively, electrochemical means can be used to bring the deposited metal back into solution and redeposit the metal in pure form on another cathode.

For example, oxidized acid solutions from chemical batteries can be provided to downstream processes for the extraction of valuable metals from electronic waste. Electronic waste typically consists of 40% metals, 30% ceramics (silicon oxide, aluminum oxide, alkaline earth metal oxides) and 30% plastic materials (polyethylene, polypropylene, polyvinyl chloride, polystyrene, peroxide resins, nylon, etc.). These metals can be further classified into several categories, including common metals (Cu, Ni, Fe), precious metals (Au, Ag), platinum-based metals (Pt, Pd, Rh, Ir and Ru), rare metals (Te, Ga, Se, Ta and Ge) and harmful metals (Pb, Cd, In, Sb).

More specifically, the most abundant element in electronic waste is copper (Cu) (about 32wt%), while the concentrations of other elements such as iron (Fe), aluminum (Al), tin (Sn), and zinc (Zn) are 13wt%, 5.7wt%, 1.9wt%, and 1.7wt%, respectively.

The general concept of metal recovery from electronic waste is depicted in Figure 14. The present invention aims to integrate the generation of oxidants with the extraction and selective recovery of metals from electronic waste, followed by the regeneration of the oxidants from the spent solution. The different steps of this process can be unified in a single operation or separated in a series of unit operations.

In another process, the oxidized acid solution from the chemical cell can be provided to a downstream process such as an electrosmelting cell. In another embodiment, the oxidized acid solution can be generated and supplied on-site and fed directly into a process to extract metals from mineral deposits, ores, mines, mining pumps, and landfills. For example, the oxidized acid solution can be generated at a mine site and pumped into a mine shaft, forced down a long mine shaft, and then returned to the surface where separation of metals from the anodic electrolyte can occur.

As renewable energy technologies drive down the cost of electricity, the use of electrochemical oxidation (EO) to generate oxidants in situ within a reaction system becomes a promising approach for sustainable development. Indirect oxidation processes are widely considered to be the main mechanism for electrolysis of ores. Therefore, the generation of oxidants in a reaction system is a key step for electrooxidation of ores.

OER is a competing reaction in the prior art slurry electrolysis process, with the side effect of reducing the current efficiency. Obviously, electrodes with low OER overpotential (high OER catalytic activity), such as _IrO2 , _RuO2 and platinum, produce strongly absorbed free radical species, which subsequently lead to _O2 formation and reduce the oxidative capacity of ROS. In contrast, electrodes with high OER overpotential (low OER catalytic activity), such as _SnO2 , _PbO2 and BDD, are beneficial in suppressing the generation of _O2 . At the same time, weakly adsorbed ROS show stronger oxidative capacity, making them ideal for use in electrolytic slurries. BDD anodes are gaining more and more attention in electrochemical oxidation processes due to their unique properties, such as extremely wide potential window, strong corrosion stability, high energy efficiency, inert surface with low adsorption characteristics and wide working pH range.

Embodiment

The present invention will be further illustrated with reference to the following non-limiting embodiments:

Example 1: FIG. 3A illustrates an electrochemical cell of the type used in the following examples. The electrochemical cell was 80 mL working volume. In the left-hand half-cell, in the anodic electrolyte (4), sulfate ions (SO ₄ ^2- ) from sulfuric acid (H ₂ SO ₄ ^2- ) are oxidized to form peroxodisulfate ions (S ₂ O ₈ ^2- ). In the right-hand half-cell, the cathodic electrolyte (5) includes copper ions, which are reduced at the cathode and deposited as copper metal (Cu).

Example 2: FIG. 3B is a plot of the concentration of peroxodisulfate ions (S ₂ O ₈ ^2- ) versus time, using the electrochemical cell shown in FIG. 3A . Four different concentrations of sulfate ions (SO ₄ ^2- ) were used: 0.615 M, 1.3 M, 2.3 M, and 5.0 M. The higher the sulfuric acid concentration, the greater the amount of peroxodisulfate ions (S ₂ O ₈ ^2- ) formed in the analyte, thereby increasing the oxidizing power of the electrolyte to extract copper.

Example 3: FIG. 4A illustrates an electrochemical cell of 80 mL working volume, in which in the left-hand half-cell, sulfate ions (SO ₄ ^2- ) from 4.5 M sulfuric acid (H ₂ SO ₄ ^2- ) and 0.5 M ferrous sulfate (Fe ₂ SO ₄ ^2- ) in the anodic electrolyte (4) are oxidized to form peroxodisulfate ions (S ₂ O ₈ ^2- ) and iron ions (Fe ³⁺ ). A separator in the form of an ion exchange membrane promotes an increase in the oxidizing efficiency of the generated acid. In the right-hand half-cell, the cathodic electrolyte includes copper ions, which are reduced at the cathode and deposited as copper metal (Cu). The current density used is 50 mA cm ² .

This arrangement is analogous to providing an anolyte solution from an electrochemical cell to a process for metal extraction on site. This would involve, for example, pumping the anolyte into a mine shaft, forcing it along a long mine shaft, and then returning to the surface where separation of the metal from the anolyte can occur.

FIG4B is a plot of the molar fraction of ferrous ions (Fe ²⁺ ) and ferric ions (Fe ³⁺ ) versus time. The plot illustrates that peroxodisulfate ions (S ₂ O ₈ ^2- ) promote oxidation of ferrous ions (Fe ²⁺ ) and ferric ions (Fe ³⁺ ), and the porous membrane prevents the plasma from migrating to the right-hand half-cell, where the ions will be reduced at the cathode. The plot also indicates that an oxidizing solution for metal extraction can be obtained after a reaction time of 3 hours.

Example 4: FIG5 illustrates an electrochemical cell for in situ copper extraction. The illustrated cell is the same as the cell shown in FIG4A and includes an ion exchange membrane (3) as a separator between the anodic half-cell and the cathodic half-cell. In this aspect of the invention, cuprous ions (Cu ²⁺ ) are introduced into the anodic electrolyte solution (4) and reduced to copper (Cu), which is deposited in the analyte reservoir.

FIG5 also illustrates a reaction that facilitates the regeneration and reuse of key electrolyte ions. As shown, in one operating cycle, a feed electrolyte composed of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ) can be used to produce an anodic electrolyte solution containing peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ). The ferric ions (Fe ³⁺ ) are used to decant cuprous ions (Cu ²⁺ ) and, as a result, are converted to ferrous ions (Fe ²⁺ ). Ferrous ions (Fe ²⁺ ) react with peroxodisulfate (S ₂ O ₈ ^2- ), which are converted into sulfate ions (SO ₄ ^2- ) and ferric ions (Fe ³⁺ ), thus completing the regeneration of ions from the feed electrolyte.

In this reaction, the reaction variables related to copper extraction can be expressed as follows:

X _cu = copper conversion

W _cu (0) = initial mass of copper

W _cu (t) = final mass of copper

In addition, the response variable regarding power consumption can be expressed as follows:

V = average battery voltage (volts)

P = power consumption (kWh/kg Cu)

I = Applied current (amperes)

t = reaction time (h)

Example 5: Example 5 illustrates the effect of current on oxidant in copper conversion. Example 5 also illustrates the superior copper conversion achieved by using both peroxodisulfate ions (S ₂ O ₈ ^2- ) and iron ions (Fe ³⁺ ) in the anodic electrolyte by the process of the present invention, compared to anodic electrolytes typically used in the prior art that do not contain both such electrolytes.

Figures 6A and 6B are graphs illustrating the degree of copper conversion under different conditions for the electrochemical cell with a working volume of 80 mL of Figure 5. In all cases, the sulfate ion (SO ₄ ^2- ) concentration was 2.5 M, the current density was 150 mA cm ^-2 , the reaction time was 1 hour, and the initial copper concentration was 3.0 g.

As a control, no current was initially passed through the cell to show no copper conversion. Application of current prompted the formation of oxidants and initiated copper extraction. FIG. 6A illustrates small copper extraction in an anolyte comprising peroxodisulfate (S ₂ O ₈ ^2- ) alone. FIG. 6B illustrates significantly better copper extraction in an anolyte comprising both peroxodisulfate ions (S ₂ O ₈ ^2- ) and iron ions (Fe ³⁺ ).

A ratio of 1:0.2 (sulfate ions (SO ₄ ^2- ) to ferrous ions (Fe ²⁺ )) provides better results than a ratio of 1:0.6.

Example 6: Example 6 explores the effect of reaction time on the degree of copper conversion.

FIG7 is a plot of copper conversion versus reaction time for the electrochemical cell of FIG5. The reaction included a sulfate ion ( _SO42- ) concentration of ^2.5 M, a current density of 150 mA cm ^-2 , a working volume of 80 mL, and 3.0 g of initial copper. The plot illustrates how copper conversion increases with time, reaching a maximum conversion concentration (about 80%) after 3 hours of reaction time.

Example 7: Example 7 is about the effect of reaction time on the degree of iron ion (Fe ³⁺ ) generation.

FIG8 is a plot of the iron composition (Fe ²⁺ /Fe ³⁺ ) versus reaction time measured using the electrochemical cell of FIG5 ; the reaction included a sulfate ion (SO ₄ ^2- ) concentration of 2.5 M, a current density of 150 mA cm ^-2 , a working volume of 80 mL, and 3.0 g of initial copper. Complete conversion of ferrous ions (Fe ²⁺ ) to ferric ions (Fe ³⁺ ) was achieved after 2 hours of reaction time. After 2 hours, copper extraction was driven by the presence of ferric ions (Fe ³⁺ ) and peroxodisulfate ions (S ₂ O ₈ ^2- ).

Example 8: Example 8 explores the effects of current and oxidant on the power consumption of an electrochemical cell.

FIG9 is a plot of power consumption versus time (t) for different compositions of the stripping medium measured using the electrochemical cell of FIG5 . The reaction included a sulfate ion (SO ₄ ^2- ) concentration of 2.5 M, a current density of 150 mA cm ^-2 , a working volume of 80 mL, and 3.0 g of initial copper. The power consumption of the anolyte comprising only 2.5 M sulfuric acid (H ₂ SO ₄ ), a mixture of 1:0.2 (sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ )), and a mixture of 1:0.6 (sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ )) was measured.

The plot illustrates that it is important to optimize the ratio of sulfate ions (SO ₄ ^2- ) to ferrous ions (Fe ²⁺ ).

Example 9: Example 9 explores the effect of reaction time on the power consumption of an electrochemical cell.

Figure 10 is a plot of power consumption versus reaction time, measured using the electrochemical cell of Figure 5. The reaction involved a sulfate ion (SO ₄ ^2- ) concentration of 2.5 M, a current density of 150 mA cm ^-2 , a working volume of 80 mL, and 3.0 g of initial copper.

Electronic waste disposal

Example 10: In this example, electronic waste is collected from used computer units, mainly from CPU motherboards and desktop monitors. The electronic waste material is first crushed into small particles and sieved to a particle size of 2.7±1.2mm (<4mm), and then mixed with aqua regia (a mixture of _HNO3 and HCl, 3:1 ratio). For a typical analysis, 0.5g of crushed electronic waste is mixed with 12mL of aqua regia. The mixture of electronic waste and aqua regia is then heated to 180°C in a microwave system for 30 minutes. Under these conditions, most metals dissolve in the solution and their concentrations are measured.

Electrolyzer apparatus for extracting metals from electronic waste

Electrolyzers for leaching metals from electronic waste were built in two different scales, namely, lab-scale and pilot-scale. The electrolyzer consisted of a working electrode (anode) and a counter electrode (cathode). The anode was made of BDD material, while a stainless steel plate was used as the cathode. Experiments for leaching metals from electronic waste were conducted at a constant current density of 150 mA cm ^-2 from a DC power source.

FIG11 is a schematic diagram of an electrolyzer according to the invention in a divided configuration. Two reservoirs (4, 5) (preloaded with 100 mL of solution) are used to circulate the solution independently through the anodic and cathodic compartments. Both reservoirs are loaded with oxidant precursor solution (0.5 to 2.5 M H ₂ SO ₄ ) and the electronic waste sample is loaded in the anodic electrolyte reservoir (4). A constant flow rate of 50 mL min ^-1 is delivered by a peristaltic pump (8). The surface area of each electrode is 10 cm ² .

The plant-scale electrolyser (7) shown in Figure 12 was used to evaluate metal extraction at larger capacities and followed a similar configuration to the laboratory-scale electrolyser. The working electrode comprised BDD and the counter electrode comprised stainless steel, each electrode having a surface area of 100 ^cm2 .

For a typical run, 50 g of electronic waste is loaded into a container (represented as a solid bed) as shown in Figure 12. The anolyte and catholyte chambers are loaded with 1 L of oxidant precursor solution (e.g., sulfuric acid and ferrous ions). The outlet of the anolyte stream containing the oxidant is connected to the bottom of the solid bed, passes through the solid bed, and then circulates back to the anolyte reservoir.

The solution containing the extracted metals from the plant scale electrolyser is treated again by an electro-deposition process to recover the metals. The apparatus for metal recovery is shown in Figure 13. The solution is circulated through the electrolyser. The dissolved metals are electro-deposited in a ring cell (11) comprising an anode rod mounted in the center of the electrolyser and a cathode sheet with a surface area of 400 ^cm2 mounted in the inner wall. The experiments were carried out at a constant current density and flow rate, i.e. 50 mA cm ^-2 and 5.5 L/min.

Example 11 : Peroxodisulfate and metal concentrations, oxidation strength and pH obtained during metal recovery from electronic waste were measured and disclosed in the following paragraphs. Peroxodisulfate concentration was measured using a ThermoFisher ion chromatography. Dissolved metals were measured using inductively coupled plasma (ICP). Oxidation strength and pH were measured using a Mettler Toledo multi-parameter sensor probe.

Method 1: Electrooxidation in an undivided cell for in-situ extraction of metals

In the first approach, the electrooxidation step is performed in a laboratory-scale electrolyzer with a split configuration. In-situ stripping is defined as the generation of peroxodisulfate oxidant from sulfate oxidation, which is immediately consumed to oxidize Fe2 ⁺ and a pre-loaded solid metal sample (e.g., electronic waste) in the anolyte reservoir. The main reactions that may occur in the electrolytic cell using this approach are shown in Figure 15.

Method 1 was tested in the absence of a solid metal sample to observe the rate of peroxodisulfate production from sulfate oxidation in the BDD anode. After 2 hours of operation, only a small concentration of peroxodisulfate was measured in the solution (about 7% yield). Without being bound by theory, the low production of peroxodisulfate in this configuration suggests that there are competing reactions that prevent peroxodisulfate from accumulating in solution. In the unblocked electrolyzer configuration, peroxodisulfate ions migrate to the cathode and are reduced back to sulfate ions.

A copper wire (representing an electronic waste sample) was then introduced into the system to observe the copper extraction rate in an undivided battery configuration. After 3 hours of operation, approximately 30% copper extraction was achieved. Most of the extracted copper was deposited on the cathode side, indicating that the dissolved copper was electrodeposited at the cathode.

Method 2: Electrooxidation in a block cell for in-situ extraction of metals

In the second method, the electrooxidation step is carried out in an electrolyzer having anodic and cathodic compartments separated by a monovalent selective anion exchange membrane. The membrane allows only monovalent ions such as ^Cl- and ^Br- to pass through the membrane. Subsequently, the generated oxidants (i.e., peroxodisulfate and Fe3 ⁺ ) are isolated in the anodic electrolyte compartment and are therefore not reduced. At the end of the process, the anodic electrolyte solution contains dissolved metals, which can be further recovered in a separate working unit. The main reactions that will occur in an electrolytic cell using this method are shown in Figure 16.

The use of the membrane provided a significant difference in the generation of peroxodisulphate. In the absence of solid metal sample, a large amount of peroxodisulphate was measured in the anodic electrolyte solution (about 48% yield); indicating that the membrane eliminated any competing reactions that might oppose the evolution of peroxodisulphate. The membrane potentially prevented the reduction of peroxodisulphate on the cathodic side. When a copper wire was introduced into the system, about 50% copper extraction was achieved after 3 hours of operation time. At the end of the process, a transparent blue anodic electrolyte and a colorless cathodic electrolyte solution were observed by naked eye, indicating that the dissolved copper was isolated in the anodic electrolyte solution.

Based on these two methods, the block battery configuration shows better performance in terms of oxidant generation and copper extraction. The following results were collected from the experiments with the block battery configuration.

result

FIG17 shows the curves of peroxydisulphate concentration and yield as a function of reaction time. The production of peroxydisulphate was performed in a microfluidic electrolysis cell at different initial sulfate concentrations (i.e., 0.5, 1, and 2.5 M) using a current density and flow rate of 150 mA cm ^-2 and 50 mL min ^-1 , respectively. As can be seen in FIG17, the peroxydisulphate concentration shows an increasing trend as the reaction time increases from 0.25 h to 1 h for all the initial sulfates studied. The same trend can be observed at different initial sulfate concentrations at each reaction time; wherein higher initial sulfate concentrations produce larger amounts of peroxydisulphate. This trend highlights the importance of initial sulfate concentration in influencing peroxodisulfate production and may have significant implications for optimizing electrochemical processes for metal extraction.

Peroxodisulphate generation was also tested using both anion and cation exchange membranes. There was no significant difference in peroxodisulphate generation, indicating that both membranes could prevent peroxodisulphate from migrating to the cathodic side.

Figure 18 shows the curves of ferric and peroxodisulfate concentrations as a function of reaction time at different initial sulfate and ferrous iron concentrations. As shown in Figure 18, at a constant initial ferrous iron concentration of 0.5 M, the generation rate of ferric ions is independent of the initial sulfate concentration, i.e., complete conversion of ferrous iron to ferric ions is achieved within 2 hours. The evolution of peroxodisulfate was observed after 1 hour of reaction time, and gradually increased to 0.05 to 0.35 M at 3 hours of reaction time.

Without wishing to be bound by theory, the peroxodisulfate generation mechanism is speculated to be initiated by electrooxidation of sulfate ions to produce peroxodisulfate (shown in equation (3)).

Peroxodisulfate is a stable oxidizing agent that subsequently oxidizes ferrous ions to ferric ions. The reaction of peroxodisulfate with ferrous ions proceeds at a stoichiometric ratio of 1, i.e., 1 mole of peroxodisulfate reacts with 1 mole of ferrous to produce 1 mole of ferric ions, as shown in equation (4).

Based on Figure 18, using initial ferrous iron concentrations of 0.25M and 0.5M, complete conversion of ferrous iron to ferric iron was achieved after 0.5 hour and 1 hour of reaction time, respectively. Compare this result with the observations made with reference to Figure 13, where about 0.3M and about 0.6M peroxodisulfate were produced after 0.5 hour and 1 hour of reaction time. Peroxodisulfate is consumed to promote the oxidation of ferrous iron to ferric iron. Therefore, the evolution of peroxodisulfate in solution only continues when conversion of ferrous iron to ferric iron has been achieved, which occurs after 1 hour of reaction time.

At lower initial ferrous iron concentrations (e.g., 0.25 M), the ferric ion generation rate was significantly faster, with complete conversion of ferrous ions to ferric ions achieved after 45 minutes of reaction time (as shown in Figure 19B). Initial sulfate concentration did not affect the ferric ion generation rate, consistent with what they observed at an initial ferrous iron concentration of 0.5 M.

FIG. 19 illustrates the copper extraction and ferrous ion concentration as a function of reaction time at different SO ₄ ^2- /Fe ²⁺ ratios. As shown in FIG. 19A , at all SO ₄ ^2- /Fe ²⁺ ratios, the amount of copper extracted shows a steady increase as a function of reaction time. The _{SO 4} ^2- /Fe ²⁺ ratio has no significant effect on copper extraction, with only a relatively small amount of copper (about 15%) being extracted after 1 hour of reaction time. In the absence of ferrous ions, about 50% of the copper is extracted after 3 hours of reaction time. Adding ferrous ions to the decanting process, i.e., a SO ₄ ^2- /Fe ²⁺ ratio of 1/0.1, increased the copper extraction rate to about 75%; while further increasing the SO ₄ ^2- /Fe ²⁺ ratio to 1/0.5 resulted in about 57% copper extraction.

The curves of ferrous ion generation as a function of reaction time at different SO ₄ ^2- /Fe ²⁺ ratios are shown in FIG. 19B . It is clear that the initial concentration of ferrous ions affects the rate of ferrous ion generation. At low concentrations of ferrous ions (e.g., SO ₄ ^2- /Fe ²⁺ =1/0.05 and 1/0.1), complete conversion of ferrous ions to ferric ions occurs within 1 hour. Further increasing the SO ₄ ^2- /Fe ²⁺ ratio to 1/0.5 results in a slower conversion of ferrous ions to ferric ions, i.e., about 70%-80% conversion of ferrous ions is achieved after 3 hours.

At low ferrous iron concentrations (SO ₄ ^2- /Fe ²⁺ ratio = 1/0.05 and 1/0.1), peroxodisulfate was consumed to oxidize ferrous iron and extract the copper wire over 1 hour, resulting in a small amount of extracted copper (about 15%). Once complete conversion of ferrous iron was achieved (after 1 hour), copper extraction was assisted by iron ions and peroxodisulfate, resulting in a significant increase in copper extraction, i.e., as the reaction time increased from 1 hour to 3 hours, the copper extraction increased from about 10% to 75%. When a higher concentration of ferrous ions is introduced (i.e., SO ₄ ^2- /Fe ²⁺ =1/0.4 and 1/0.5), peroxodisulfate is consumed to oxidize ferrous ions to ferric ions. Under this condition, it is speculated that copper extraction is assisted only by ferric ions. Compared with the absence of ferrous ions (SO ₄ ^2- /Fe ²⁺ =1/0), i.e., 50% copper extraction is achieved by a single oxidant after 3 hours of reaction time, the amount of copper extracted under SO ₄ ^2- /Fe ²⁺ =1/0.5 is relatively good. The trend shown in Figure 19 indicates. The ratio of SO ₄ ^2- /Fe ²⁺ =1/0.1 is the optimal oxidant ratio at which copper extraction is maximized. This confirms the results presented in Figures 13 and 14, where the evolution of the oxidant is initiated by the generation of peroxodisulfate and can subsequently oxidize any species in solution.

Laboratory-scale extraction of metals from electronic waste

Figure 20 shows the concentration of metals in the solution after extraction as a function of reaction time. Extraction was performed in a laboratory scale electrolyzer at a current density of 150 mA cm ^-2 , SO ₄ ^2- /Fe ²⁺ = 1/0.1 and a flow rate = 50 mL min ^-1 . Elements measured in the solution included Cu, Al, Fe, Zn, Ni and Pb.

As can be seen in Figure 20, the concentrations of all metals increased within 1 hour before the reaction, decreased after 2 hours, and then remained stable until the 4-hour reaction time. Cu and Fe were the most abundant elements measured in the solution after dewatering (reaching about 10 g/L and about 7 g/L, respectively, after 4 hours of dewatering). The concentrations of other elements (including Zn, Ni, Al, and Ni) ranged from 0.1 to 0.5 g/L, and the concentration of Pb in the solution was <0.005 g/L.

Considering that Cu and Fe are the most abundant elements in electronic waste, it is understandable that the concentrations of Cu and Fe in the solution after extraction are relatively higher than other elements. However, in the comparison of Zn extraction and Al extraction, the Zn concentration is higher than Al (0.8 vs. 0.2 g/L), while the Zn content in electronic waste is lower than Al (1.7% vs. 5.7%). This trend may indicate that the extraction rate of metals in electronic waste is not uniform, that is, Zn shows a higher extraction rate than Al.

Plant-scale extraction of metals from electronic waste

The concentrations of metals in the anolyte solution collected from the plant-scale electrolyzer are shown in Figure 21. As shown, the concentrations of all metals increased during the first hour of reaction time, declined after 2 hours, and then remained stable until 4 hours of reaction time. Cu and Fe were the most abundant elements measured in the solution after draining (reaching approximately 15 g/L and approximately 3 g/L, respectively, after 4 hours of draining). The concentrations of other elements (including Zn, Al, and Ni) ranged from 0.3 to 2.2 g/L, while the concentration of Pb was <0.01 g/L. In general, the elements measured in the stripped solutions from the plant-scale electrolyzers follow the same distribution as observed in the corresponding laboratory-scale electrolyzers.

ORP is a key parameter in the stripping process because it indicates whether the solution is oxidizing or reducing. Positive ORP values indicate an oxidizing solution, while negative values indicate a reducing solution. Figure 25 shows ORP as a function of reaction time for stripping processes performed in laboratory-scale and plant-scale electrolyzers. In both cases, ORP is shown to increase from approximately 400mV to 1000mV over a 4-hour reaction period, indicating that the electrolyzer produces an oxidizing solution for metal stripping.

Figure 22 shows the curves of metal concentrations in the solution after decanting for three batches of electronic waste. The operation time and electronic waste load for each batch were 4 hours and 50 g, respectively, and 2.5 M sulfuric acid was used as the oxidant precursor for batch 1. At the end of each batch, the remaining solids were collected and the solution was reused for metal extraction in batches 2 and 3.

Figure 24 shows the copper recovery curve as a function of reaction time. Complete copper recovery (about 99%) was achieved in about 90 minutes of operation time.

Cu was the most abundant element extracted in all batches, and approximately 15 g/L of copper was extracted in each batch. The other metals extracted, including Fe, Al, Fe, Zn, Ni, and Pb, all followed the same trend as Cu. Approximately 1.3 g/L of Fe and Zn, approximately 0.35 g/L of Al, and approximately 0.16 g/L of Ni were extracted in each batch. The concentration curves of the metals presented in FIG. 22 show that the electrolyzer according to the present invention maintains the same performance in extracting metals for a variety of electronic waste loadings. It further shows that the stripping solution can be recycled multiple times with the same oxidizing strength. FIG. 26 shows the curve of the ORP of the stripping process under the conditions presented in FIG. 19. In Batch 1, the ORP showed an increase from about 400 mV to 1000 mV after 4 hours of reaction time, and this value remained stable at about 1000 mV when the solution was used to extract metals from two subsequent electronic waste loads. The ORP curves shown in Figure 26 clearly demonstrate the ability of the electrolyzer according to the present invention to regenerate the oxidant for metal extraction, thus providing a promising green process for extracting metals from electronic waste.

Figure 23 shows the design of a commercial electrolyzer for use in the method of the present invention. Multiple e-waste beds (43) can be installed in parallel at the outlet stream of the anolyte (4). The extraction of e-waste can be continued by passing the anolyte stream through the interior of one e-waste bed for a period of time until most of the metals have been extracted. Once this is achieved, the anolyte outlet stream can be switched to a second e-waste bed for further metal extraction. While the electrolyzer is extracting metals from the second e-waste bed, the retentate from the e-waste bed can be collected for further processing. This step can be repeated to extract metals from the e-waste beds. Following this approach, the electrolyzer can be used for continuous metal extraction using the same oxidant solution.

Mineral extraction

Example 12 : In this example, brass ore was obtained and ground to D90 45 μm. The ore phase of the brass ore was measured by an X-ray diffraction meter (XRD) manufactured from Rigaku Smartlab. The target was CuKα, and the tube current was 40 mA. The tube voltage was 40 kV, and the scanning range 2θ was 15-80°. The sample composition was characterized by using X-ray fluorescence (XRF) generated from Rigaku Supermini200 ^TM .

An analysis was performed to understand the effect of using different temperatures on copper recovery from brass ores. A range of temperatures was studied: 25°C, 35°C and 45°C. The effect of ultrasound and stirring speed on copper recovery was also studied.

Electrolyzer apparatus for extracting metal from brass ores

An electrolyzer for dredging metals from ores was set up on a laboratory scale. The electrolyzer consists of a working electrode (anode) and a counter electrode (cathode), with a cathodic membrane separating the electrode compartments. The anode is made of BDD material, while a platinum plate serves as the cathode. Experiments for dredging metals from brass ore were conducted at a current density of 25 to 100 mA cm ^-2 from a DC power source.

Two reservoirs (pre-loaded with 100 mL of solution) were used to circulate the solution through the anode and cathode compartments independently. Both reservoirs were loaded with an oxidant precursor solution with an initial sulfate concentration of 2.5 M, SO ₄ ^2- /Fe ²⁺ =1/0.1, and in a typical run, 2 g of chalcopyrite ore sample was loaded into the anodic electrolyte reservoir. A mechanical stirrer was used to increase mass transfer at a stirring speed of 400-1000 r/min. The BDD anode size was limited to 2.25 cm ² and the platinum cathode was 1.5 cm ² .

The metal concentration in the digestion solution was measured using an Aquaculture Photometer. A 3M KCl electrode was used as a reference electrode. The electrolytic cell was placed in an ultrasonic water bath (Unisonics-FXP10 ^™ ) with a power of 500 W and a frequency of 40 KHz.

Method 1: Temperature Effects on Electrooxidation in a Bulk Cell for In-Situ Extraction of Metals

In the first approach, a combination of electrooxidation steps at different temperatures was performed in a laboratory-scale electrolyzer with a split configuration to explore the effect of temperature on copper conversion. In-situ stripping is defined as the generation of peroxodisulfate oxidant from sulfate oxidation, which is immediately consumed to oxidize Fe2 ⁺ and a solid metal sample (e.g., brass ore) preloaded in the anodic electrolyte reservoir.

Method 1 tests at a range of temperatures (25°C, 35°C and 45°C) to observe the efficiency of copper conversion as a result of temperature. After 48 hours of operation, the efficiency of copper conversion during the electrolysis process was significantly affected by temperature. Figure 27 shows that the increase in temperature significantly improves the copper conversion efficiency over time. At 45°C, the copper conversion efficiency reaches about 84.75% after 48 hours; at 25°C and 35°C, the conversion efficiency is 25.79% and 35.69%, respectively.

From the perspective of surface reaction rate, higher temperature helps to accelerate the reaction of reactive oxygen species (ROS) and peroxydisulfate (PDS) with brass ore.

)之生成。加入更多自由基氧化劑對於加速瀝取過程至關重要。 Regarding the generation of oxidants, higher temperature conditions favor the generation of oxidants such as hydroxyl radicals (‧OH) and sulfate radicals (

) formation. Adding more free radical oxidants is crucial to accelerate the extraction process.

Method 2: Mixing effects on electrooxidation in a bulk cell for in situ extraction of metals

In the second approach, electrooxidation steps with different combinations of stirring rates were performed in a laboratory-scale electrolyzer with a block configuration to explore the effect of stirring rate on copper conversion.

Method 2 is to test a certain range of stirring speeds (400 to 1000r/min) to observe the efficiency of copper conversion as a result of stirring speed. After 48 hours of operation, the efficiency of copper conversion during the electrolysis process is significantly affected by the stirring speed. Figure 28 shows that the increase in stirring speed significantly improves the copper conversion efficiency over time. Compared to 24.90% when no stirring is performed at all, at 700r/min, the conversion rate increases to 52.50% after 48 hours.

After incorporating stirring into the reaction system, the mass transfer of the ore was significantly enhanced, allowing the ore to reach the ROS reaction zone on the BDD. This facilitated the participation of ROS in the extraction process. The significant increase in the extraction rate was attributed to the participation of ROS, as reflected in the data.

Furthermore, Figure 29 demonstrates that varying the stirring rate between 400-1000 RPM does not significantly affect the tapping conversion, suggesting that stirring rates above 400 RPM are sufficient to eliminate the mass transfer limitation between the ore particles and the oxidant of the free radical oxidant. This indicates that once a certain stirring rate is integrated into the system, a critical shift in the limiting step from mass transfer to the generation of ROS occurs.

Method 3: UV Effects on Electrooxidation in a Block Cell for In-Situ Extraction of Metals

In the third approach, the electrooxidation step combined with the use of UV was performed in a laboratory-scale electrolyzer with a block configuration to explore the effect of UV on copper conversion.

Method 3 was tested using UV to observe the efficiency of copper conversion as a result of placing the electrolytic cell in an ultrasonic water bath with a power of 500W and a frequency of 40KHz. After 48 hours of operation, the efficiency of copper conversion during the electrolysis process was significantly affected.

The efficiency of copper conversion from brass ore in an electro-extraction system was significantly amplified by ultrasound, which focused on enhancing mass transfer within the system. Figure 28 summarizes the significant improvement in conversion when this mechanism was employed. At 24 hours, the conversion increased to 80.73%, compared to 24.90% when using electro-oxidation alone.

Example 13: Analyses were performed to understand the recovery of metals tapped from two high grade nickel ores (Nickel Ores 1 and 2, both with a particle size of <3.35 mm). The tapped metals studied included nickel, iron, copper, cobalt, manganese and chromium.

Electrolyzer apparatus for extracting metal from nickel ore

An electrolyzer for the extraction of metals from ores was set up on a laboratory scale. The electrolyzer consists of a working electrode (anode) and a counter electrode (cathode), with a cathodic membrane separating the electrode compartments. The anode is made of BDD material, while a stainless steel plate is used as the cathode. Experiments for the extraction of metals from nickel ores were conducted at a current density of 150 mA cm ^-2 from a DC power source.

Two reservoirs (pre-loaded with 120 mL of solution) were used to circulate the solution through the anode and cathode compartments independently. Both reservoirs were loaded with an oxidant precursor solution with an initial sulfate concentration of 2.5 M, SO ₄ ^2- /Fe ²⁺ = 1/0.1. In a typical run, 5 g of nickel ore sample was loaded into the anode electrolyte reservoir.

Laboratory-scale extraction of metals from nickel ore

Figures 30 and 31 show the concentration of metals in the solution after extraction as a function of reaction time for nickel ores 1 and 2, respectively. Extraction was performed in a laboratory scale electrolyzer at a current density of 150 mA cm ^-2 , SO ₄ ^2- /Fe ²⁺ =1/0.1 and a flow rate = 50 mL min ^-1 . Elements measured in the solution included Ni, Cu, Co, Fe, Mn and Cr.

As can be seen in Figure 30, the concentrations of all metals from nickel ore 1 increased during the 10-hour reaction. Ni and Fe were the most abundant elements measured in the solution after dewatering (reaching approximately 140 mg/L and approximately 1050 mg/L, respectively, after 4 hours of dewatering). The concentrations of other elements (including Co, Co, Mn, and Cr) ranged from 2 to 20 mg/L.

Figure 31 shows that after 10 hours of reaction, all metal concentrations from nickel ore 2 increased; however, once again, Ni and Fe were the most abundant elements in the decanted solution. In both cases, this is expected since Ni and Fe are the most abundant elements in the original ore.

Example 14: Analytical analysis performed to understand the recovery of metals extracted from vanadium containing ores. The metals extracted were studied, including vanadium, iron, chromium, nickel, cobalt and manganese. Characterization of the ore was performed by microwave digestion of aqua regia at 180°C and 3 bar for 30 minutes.

Electrolyzer apparatus for extracting metals from vanadium ores

An electrolyzer for the extraction of metals from ores was set up on a laboratory scale. The electrolyzer consists of a working electrode (anode) and a counter electrode (cathode), with a cathodic membrane separating the electrode compartments. The anode is made of BDD material, while a stainless steel plate is used as the cathode. Experiments for the extraction of metals from vanadium ores were conducted at a current density of 150 mA cm ^-2 from a DC power source.

Two reservoirs (pre-loaded with 120 mL of solution) were used to circulate the solution independently through the anode and cathode compartments. Both reservoirs were loaded with an oxidant precursor solution with an initial sulfate concentration of 2.5 M, SO ₄ ^2- /Fe ²⁺ = 1/0.1. A solid concentration of 10% was used. Elements measured in the decanted solution included V, Fe, Cr, Co, Mn, Ni.

Laboratory-scale extraction of metals from vanadium ores

Figure 32 shows the improvement in metal recovery after 2 hours of treatment in the electrolyzer using the anodic electrolyte solution relative to a standard acid stripping with 2.5 MH ₂ SO ₄ alone. There is evidence of significant improvement in metal recovery, primarily for V, Fe, and Cr (1000% recovery improvement for V; and 820% for each of Fe and Cr).

Example 15: The above discovery can be extended to achieve improved metal recovery on an industrial scale from ores, landfill residues, sludge, tailings, slag, ash, slag from incinerators, billets, electronic waste or other metal-containing wastes. Specific examples are used to recover metals from low-grade ores or tailings in mining. These hydrometallurgical processes mainly include heap leaching, in-situ leaching and tank leaching.

Method 1: Accelerated recovery of metals in heap extraction using analyte solution

In this embodiment, an anodic electrolyte solution may be generated at the mining site and supplied on-site to replace reagents such as acidic sulfate or alkaline carbonate in heap draining of ore and tailings. In heap draining, mined ore (such as precious metals, copper, nickel and uranium) is crushed and placed on a drain pad that is impermeable to a plastic or clay liner. The heap is flooded with a drain solution to dissolve the metals in the ore, which are then recovered. The anodic electrolyte solution may be used as a drain solution in place of standard acidic and alkaline reagents to flood tailings or low-grade ores, thereby draining the metals into a pregnant solution. The wash liquor containing the dissolved metals is subsequently treated to recover the metals by standard methods. FIG. 33 illustrates a standard process for heap extraction of copper from low-grade ores. An improvement over the standard process is to increase the efficiency of metal recovery by using an anodic electrolyte solution instead of, for example, acidic sulfate, as described in FIG. 28 for copper and in FIG. 32 for V, Fe, Cr, Co, Mn, Ni.

Method 2: Accelerated recovery of metals from tank or trough extraction using analyte solutions

In this embodiment, the anodic electrolyte solution replaces the standard extraction reagent (e.g., sulfuric acid) in the tank or tank extraction process. Tank and tank extraction involves placing the ore or other metal-bearing solids, typically after size reduction and classification, in large tanks and then flooding them with the extraction solution (in the case of tank extraction) or grinding and mixing with water to form a slurry, and then adding the extraction reagent (in the case of tank extraction).

In some cases, the tank is equipped with a stirrer and stirring paddles to keep the solids suspended and thus increase the efficiency of metal extraction. Stirring vessels are vertical or horizontal sealed cylindrical containers with power-driven paddles or stirrers on vertical or horizontal main shafts. They are provided with means at the bottom for pumping out the extraction solution at the end of the operation. In some designs, a horizontal drum is the extraction vessel and the solids and liquids are tumbled inside by the rotation of the drum on rollers. They operate on a batch basis and each is a single extraction stage. They can also be connected in series for multi-stage operations.

For the extraction of finely divided solids, Pachuca tanks are used. This has been extended to the metallurgical industry. These tanks are constructed of wood, metal or cement and lined with a suitable material, depending on the nature of the liquid to be extracted. Agitation is applied by means of an air lift. Air bubbles rising through the central tube cause an upward flow of the liquid and the solids suspended in the tube, and thus cause circulation of the mixture. Conventional mechanical agitators are also used for this purpose.

Once the desired extraction is achieved, agitation is stopped, the solids are allowed to settle, and the clarified supernatant liquid is decanted by siphoning at the top of the tank or by discharging through a drain pipe placed at an appropriate height on the side of the tank. The solids settle to form a compressible sludge, the retained solution will be more, and in such cases, the last traces of solute are generally recovered in a counter-current manner. In this new aspect, the anodic electrolyte solution will act as an electrolyte, increasing the efficiency of metal recovery compared to the use of, for example, acidic sulfate, as depicted in the case of copper in Figure 28 or in the case of V, Fe, Cr, Co, Mn, Ni in Figure 32. Further improvements were achieved through optimization including temperature, stirring speed and use of UV, as shown in Figures 27 and 28.

Method 3: Accelerated recovery of metals from countercurrent separations using analyte solutions

Countercurrent extraction is used to more efficiently extract metals from materials such as ores, concentrates, slags, scrap and metallic wastes (and other terms used in the industry to explain wastes containing metals) in a continuous flow of tank and tank processes. There are many variables using this method, but the principles of solvent strength, contact time, surface area, temperature and flow rate will remain key components of the process.

Important factors in CCE extraction include what the target metal is that is being extracted and what material it is contained within. In addition, how the target is prepared will also be a factor in the success of many CCE processes. For example, lithium will need to be roasted and crushed before the extraction process begins, and ores such as copper and nickel will need to be crushed to a uniform size for extraction.

Although there are many forms of CCE, from a very simple pumping process in a tank containing ore to a very complex agitation system and tank arrangement commonly referred to as a shanks system (Figure 34), the concept of CCE remains the same. As illustrated in Figure 34, the solid or slurry moves countercurrently to the liquid stream (independently, in the direction of the arrows from (a) to (h)), with the emphasis on the choice of an effective solvent.

Sometimes the pressure drop of the liquid flow due to gravity is very high, or the solvent is highly volatile. In such cases, the liquid is pumped through a bed of solids in a container called a diffuser. The main advantage of these units is that they prevent evaporation losses of the solvent as they operate above its boiling point. The use of percolation tanks is compatible with this operation, but they are not suitable if the ore or target is in very fine form. In such cases, the solids can be filtered and drained in a filter press by pumping the solvent through a press cake.

The Rotocel extractor depicted in Figure 33 is a modification of the Shanks system in which the draw tank is continuously moved, allowing the continuous introduction and discharge of solids. It consists of an annular housing divided into a number of cells, each cell being equipped with an articulated screen bottom for supporting the solids. This housing rotates slowly on a fixed compartment tank. As the rotor rotates, each cell passes in turn under a prepared solids feeder and then through a series of sprayers, by means of which the contents of each cell are periodically wetted with solvent for draw-off. By completing one revolution, when the expected draining is complete, the solids drained by each unit are automatically dumped into one of the lower fixed compartments from which the solids are continuously transported. The solvent sprayed on each unit filled with solids percolates downward through the solids and the supporting screen into the appropriate compartment of the lower tank from which the solvent is pumped for the next spraying. The draining is countercurrent and the strongest solution comes from the unit filled with fresh solids. The improvement over the standard process is the increase in the efficiency of metal recovery by using an anodic electrolyte solution. The improvement with respect to sulfuric acid is depicted in the case of copper in Figure 30.

Although the present invention has been disclosed in conjunction with its specific aspects, it should be understood that it is capable of further modification. This application is intended to cover any changes, uses or modifications of the present invention, which generally follow the principles of the present invention and include deviations from the present disclosure, which are known or customary practices in the field to which the present invention belongs and can be applied to the basic characteristics described above.

Since the present invention can be exemplified in several forms without departing from the spirit of the essential characteristics of the present invention, it should be understood that, unless otherwise specified, the above-mentioned aspects are not limiting of the present invention, but should be broadly interpreted within the spirit and scope of the present invention as defined in the attached patent application scope. The disclosed aspects are considered in all aspects to be illustrative only and not restrictive.

Various modifications and equivalent arrangements are intended to be included in the spirit and scope of the present invention and the attached patent applications. Therefore, the specific aspects should be understood as being exemplified in a variety of ways, and the principles of the present invention can be implemented through these specific aspects.

"Include" and "including" are used in this specification to specify the existence of the cited features, wholes, steps or components, but do not exclude the existence or addition of one or more other features, wholes, steps or components. Therefore, throughout the specification and the scope of the patent application, unless the context clearly requires otherwise, the words "include", "including", etc. are constructed in an inclusive sense, as opposed to an exclusive or exclusive sense; in other words, it means "including but not limited to".

When a Markush group or other grouping is used herein, all individual members of the group and all possible combinations and subcombinations of such group members are intended to be individually included in the present disclosure. Every combination of components disclosed or exemplified herein can be used to practice the present invention unless otherwise specified.

Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, and all individual values included in the given range are intended to be included in the present disclosure. It should be understood that any subrange or individual value within a range or subrange included in the specification herein may be excluded from the scope of the patent application herein.

Those of ordinary skill in the art will recognize that materials and methods other than those specifically exemplified may be used in the practice of the present invention without undue experimentation. All functional equivalents of any such materials and methods known in the art are intended to be included in the present invention. The terms and expressions employed are used as descriptive rather than limiting terms, and no equivalents of the features shown and disclosed are intended to be excluded in the use of such terms and expressions, but it should be recognized that various modifications are possible within the scope of the present invention. Therefore, it should be understood that although the present invention has been specifically disclosed through embodiments, preferred embodiments and optional features, modifications and changes of the concepts disclosed herein may be adopted by those having ordinary knowledge in the art, and such modifications and changes are considered to be within the scope of the present invention as defined by the appended patent claims.

1: Anode

2: cathode

3: Membrane

Claims (15)

A method for generating an oxidant solution using an electrochemical cell having an anode and a cathode, the method comprising the following steps: (i) supplying a feed electrolyte to the reaction zone between the anode and the cathode, the feed electrolyte consisting of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ); (ii) electrolyzing the feed electrolyte to produce an oxidized acid solution comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and iron ions (Fe ³⁺ ) during an operation cycle; and (iii) supplying the oxidized acid solution. The method of claim 1, wherein the concentration of sulfate ions (SO ₄ ^2- ) in the feed electrolyte is between 0.1 volume mol and 5 volume mol. The method of claim 1 or claim 2, wherein the concentration of ferrous ions (Fe ²⁺ ) in the feed electrolyte is between 0.1 vol mol and 0.5 vol mol. The method of any one of claims 1 to 3, wherein the ratio of SO ₄ ^2- :Fe ²⁺ is between 1:0.05 and 1:0.5. The method as claimed in claim 1, wherein the anode comprises boron-doped diamond. A method for generating an oxidant solution using an electrochemical cell according to any one of claims 1 to 5, wherein the electrochemical cell has an anode half-cell including the anode and a cathode half-cell including the cathode, the method comprising the following steps: (i) supplying the anode half-cell with a feed electrolyte, the feed electrolyte entering and leaving the fluid path formed between the anode and the cathode, the feed electrolyte consisting of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ); (ii) during an operation cycle, electrolyzing the feed electrolyte to produce an anodic electrolyte solution comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and iron ions (Fe ³⁺ ); and (iii) supplying the anode electrolyte solution from the anode half-cell. A method for generating an oxidant solution as described in claim 6, wherein a separator is positioned between the anode half-cell and the cathode half-cell. A method for generating an oxidant solution using an electrochemical cell according to any one of claims 1 to 7, comprising the further steps of regenerating the oxidant solution by: (iii) supplying the oxidized acid solution to the metal-containing waste so that the iron ions (Fe ³⁺ ) are reduced to ferrous ions (Fe ²⁺ ); and (iv) bringing peroxodisulfate (S ₂ O ₈ ^2- ) into contact with the ferrous ions (Fe ²⁺ ) to regenerate ferric ions (Fe ³⁺ ) and sulfate ions (SO ₄ ^2- ). A system for generating an oxidant solution according to any one of claims 1 to 8, comprising: (a) Anode and cathode, which define the reaction area of the electrochemical cell, (b) an inlet and flow controller for passing an aqueous feed electrolyte between the electrodes, the feed electrolyte consisting of sulfate ions (SO ₄ ^2- ) and ferrous ions (Fe ²⁺ ) selected for the electrochemical generation of an oxidant comprising peroxodisulfate (S ₂ O ₈ ^2- ) and ferric ions (Fe ³⁺ ); (c) current means for supplying current to electrolyze the aqueous feed electrolyte to produce an oxidant solution containing the oxidant in the reaction zone; and (d) an outlet for supplying the oxidant solution from the electrochemical cell. A system for on-site generation of an oxidant solution as described in any one of claims 1 to 8, comprising: (a) an anode half-cell and a cathode half-cell, (b) inlet and flow control means for passing an aqueous feed electrolyte through the anodic half-cell, the feed electrolyte consisting of sulfate ions (SO ₄ ²⁻ ) and ferrous ions (Fe ²⁺ ) selected for the electrochemical generation of an oxidant comprising peroxodisulfate (S ₂ O ₈ ²⁻ ) and ferric ions (Fe ³⁺ ); (c) current means for supplying current to electrolyze the aqueous feed electrolyte to produce an oxidant solution containing the oxidant in the anode half-cell; and (d) an outlet for supplying the oxidant solution from the anode half-cell of the electrochemical cell. A system for generating an oxidant solution as described in claim 10, wherein a separator is positioned between the anode half-cell and the cathode half-cell. A system as described in any one of claims 9 to 11, wherein the oxidant solution is used for metal extraction, preferably extraction of one or more metals selected from the group consisting of Cu, Co, Ni, V, Cr, Mn, Fe, Au, Ag, Pt, Pd, Rh, Ir, Ru, Te, Ga, Se, Ta, Ge Pb, Cd, In and Sb. The system of claim 12, wherein the metal is extracted from electronic waste or minerals. A method for extracting metal from metal-containing waste, the method comprising the steps of: supplying an oxidant solution produced by the method described in any one of claims 1 to 7, and bringing the oxidant solution into contact with the metal-containing waste. A method for extracting metal as described in claim 14, wherein the metal to be extracted is selected from the group consisting of Cu, Co, Ni, V, Cr, Mn and Fe.