Precious metal leaching method based on electrochemical oxidation and thiosulfate salt

Abstract

Disclosed in the present invention is a precious metal leaching method based on electrochemical oxidation and a thiosulfate salt, comprising: by using a thiosulfate salt-electrolyte-alkali liquor system as an electrolyte solution, adding a precious metal material into an anode chamber of an electrolytic cell, and connecting a cathode and an anode provided in the electrolytic cell to an external power supply, to leach a precious metal by electrochemical oxidation, wherein the electrolyte is a salt other than a thiosulfate salt. In the present invention, precious metals are extracted by an electrochemical thiosulfate salt method, the leaching effect is superior to that of existing copper-ammonia technology, and efficient and green extraction of precious metals (such as gold and silver) can be achieved; the present invention avoids the addition of large amounts of oxidants such as copper, and significantly reduces the consumption of thiosulfate while improving the leaching rate of precious metals, thereby reducing the cost of leaching agents, promoting green and sustainable development of the precious metal industry, and facilitating industrial application.

Description

A method for electrochemical oxidation leaching of precious metals using thiosulfate Technical Field

[0001] The invention relates to the technical field of hydrometallurgy, and in particular to a method for electrochemically oxidizing and leaching precious metals using thiosulfate. Background Art

[0002] The cyanidation method is currently the most important method for gold extraction in the world due to its advantages such as low cost and mature technology. It uses an aqueous solution of alkali metal cyanide (KCN, NaCN) as a solvent to leach gold from gold mines in the presence of oxygen. However, cyanide is highly toxic, and its large-scale use will seriously harm the environment and human health, which is contrary to the current concept of green development. Thiosulfate is widely favored in precious metal extraction because of its advantages such as being green and harmless, good selectivity, no corrosion to equipment, and mild leaching conditions. It is considered to be the most promising method for cyanide-free extraction of precious metals. At present, thiosulfate leaching of gold is mainly based on the copper ammonia-thiosulfate system. In this system, O2 is used as an oxidant by adding Cu2 to thiosulfate in an alkaline environment. 2+ and ammonia, can increase the leaching rate of rare and precious metals. In this process, copper ions and ammonia form a relatively stable copper-ammonia complex ion Cu(NH3)4 2+ The complex ion can act as an oxidation catalyst to promote Au(NH3)2 + The formation of , significantly increased the gold dissolution rate (18-20 times), strengthening the thiosulfate leaching of gold. However, due to the E θ (Cu 2+ / Cu) and E θ (Cu(NH3)4 2+ / Cu(S2O3)3 5- )>E θ (S4O6 2- / S2O3 2- )>E θ (Au(NH3)2 + / Au 0 ), and (Cu(NH3)4 2+ and Cu 2+ The diffusion coefficient of thiosulfate is large and it is highly dispersed in the solution, resulting in continuous oxidation and consumption of thiosulfate, with the consumption reaching more than 50kg / t, which seriously restricts the industrial application of thiosulfate gold extraction technology.

[0003] In response to the above issues, researchers have conducted extensive research in recent years on the high consumption of thiosulfate leaching reagents. Currently, the main approaches to reducing thiosulfate consumption include controlling the concentration of basic reagents, adding stabilizers, and developing non-copper ion leaching systems. For example, researchers have developed methods to form several different thiosulfate systems by adding additives (sodium chloride, ethylenediaminetetraacetic acid, carboxymethyl cellulose, sodium sulfite, etc.) or changing individual leaching reagents in the system (such as adding metallic iron, nickel, and cobalt to form nickel ammonia, cobalt ammonia, and iron oxalate systems). In addition, some researchers believe that the main reason for the high consumption of thiosulfate leaching reagents is that most gold ores contain copper minerals. During the leaching process, copper ions are also continuously dissolved, resulting in a high concentration of copper-ammonia complex ions in the solution. Therefore, it is proposed to add chelating agents such as disodium ethylenediaminetetraacetic acid (EDTA), amino acids, and carboxymethyl cellulose (CMC) to the leaching system to control the concentration of copper-ammonia complex ions in the solution, thereby reducing reagent consumption.

[0004] In the above method, when the cobalt ammonia-thiosulfate and nickel ammonia-thiosulfate systems are used to leach precious metals, the results show that when Co(NH3)6 3+ and Ni(NH3)6 2+ As a catalyst, the degree of thiosulfate oxidation and decomposition is significantly reduced, and the reagent consumption can be as low as 1.2 kg / t. However, due to the variety of complex ions formed by cobalt and nickel ions and ammonia molecules (Co(NH3)4 2+ 、Co(NH3)5 2+ 、Ni(NH3)2 2+ 、Ni(NH3)3 2+ 、Ni(NH3)4 2+ 、Ni(NH3)5 2+ etc.), the electrode potential of different complex ions is different, resulting in unstable indicators for leaching precious metals. At the same time, the leaching mechanism of cobalt ammonia / nickel ammonia-thiosulfate is still unclear, and its industrial application is difficult to promote in a short period of time. 3+ The thiosulfate leaching of gold was carried out using Fe(C2O4) as oxidant and oxalate as complexing agent. The results showed that the consumption of reagents in the leaching system of precious metals was low. 2- The complex stability is poor. When the solution pH is higher than 6.4, Fe(C2O4) 2-It is converted into Fe(OH)3 precipitation, which reduces the gold leaching efficiency. Under low pH conditions, thiosulfate will spontaneously decompose, and the reagent consumption will increase. In addition, the use of too many additives will increase the viscosity of the solution, causing the mineral powder to not be fully dissolved in water, reducing the chance of precious metals contacting the reaction reagents, and thus resulting in poor metal leaching effect. In summary, the three methods of controlling the concentration of leaching reagents, adding stabilizers, and developing non-copper ion gold leaching systems can all reduce the reagent consumption of thiosulfate gold leaching to a certain extent, but the three methods have strong limitations and cannot effectively take into account the dissolution and leaching efficiency of precious metals.

[0005] Therefore, in order to promote the replacement of green and environmentally friendly thiosulfate extraction technology for cyanide extraction of precious metals, it is urgent to innovate thiosulfate extraction technology for precious metals, reduce the cost of leaching agents, and thus promote the green and sustainable development of the precious metals industry.

[0006] Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention provides a method for leaching precious metals using electrochemical thiosulfate oxidation. This method replaces the traditional copper ammonia oxidation method with an electrochemical oxidation method, addressing the current difficulty in balancing reagent consumption and leaching efficiency in thiosulfate leaching of precious metals.

[0008] To achieve the above object, the present invention is implemented through the following technical solutions:

[0009] A method for leaching precious metals by electrochemical oxidation with thiosulfate comprises: arranging a cation exchange membrane in an electrolytic cell, the cation exchange membrane dividing the electrolytic cell into an anode chamber and a cathode chamber; using a thiosulfate-electrolyte-alkaline solution system as an electrolyte, adding precious metal materials into the anode chamber of the electrolytic cell, and connecting the cathode and anode arranged in the electrolytic cell to an external power supply to leach the precious metals through electrochemical oxidation; the electrolyte is an electrolyte that does not contain thiosulfate.

[0010] The proposed thiosulfate electrochemical oxidation method for extracting precious metals differs significantly from existing leaching technologies in that it does not use any metal catalysts or complexing agents. Instead, it utilizes only thiosulfate-alkaline solution, with potassium chloride as the primary electrolyte, to leach the precious metals through electrochemical anodic oxidation under power conditions. This method not only achieves excellent leaching results, but also saves costs and allows for the reuse of the leachate, significantly reducing chemical usage and the generation of secondary waste.

[0011] Preferably, the pH of the electrolyte is 7-13, more preferably pH=10.

[0012] Preferably, the concentration ratio of thiosulfate to electrolyte in the electrolyte is (0.1-0.5):(0.1-0.5).

[0013] Preferably, the thiosulfate is one or more of sodium thiosulfate, ammonium thiosulfate, and potassium thiosulfate.

[0014] Preferably, the alkali solution is one or more of ammonia water, sodium hydroxide and potassium hydroxide.

[0015] The alkali solution is further preferably ammonia water. When the alkali solution is ammonia water, the concentration ratio of thiosulfate, electrolyte, and ammonia water in the electrolyte is (0.1-0.5): (0.1-0.5): (0.3-2.0).

[0016] Preferably, the electrolyte is one or more of chloride, sulfate, and carbonate.

[0017] Preferably, the chloride salt is one or both of potassium chloride and sodium chloride.

[0018] Preferably, a cation exchange membrane is provided in the electrolytic cell, and the cation exchange membrane separates the electrolytic cell into an anode chamber and a cathode chamber.

[0019] Preferably, the electrochemical condition is: the voltage is 0.3V to 3V.

[0020] More preferably, the electrochemical condition is: the voltage is 0.6V.

[0021] Preferably, the anode is platinum, titanium, copper, lead, glassy carbon, silicon carbide, stainless steel, graphite electrode or graphite felt, and the cathode is titanium, copper, stainless steel or graphite electrode.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present invention adopts the electrochemical thiosulfate method to extract precious metals, and the leaching effect is better than the existing copper ammonia technology, which can achieve efficient and green extraction of precious metals (such as gold and silver).

[0024] 2. The present invention is simple to operate and easy to control, does not introduce other impurity ions, avoids the generation of waste liquid and waste gas, and reduces recovery costs.

[0025] 3. The present invention not only avoids the large-scale consumption of oxidants such as copper, but also significantly reduces the consumption of thiosulfate while improving the leaching rate of precious metals, reduces the cost of leaching agents, and thus promotes the green and sustainable development of the precious metal industry, and is easy to realize industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a diagram showing the gold immersion effects of Example 1 and Comparative Example 1;

[0027] FIG2 is a diagram showing the effects of immersion silvering in Example 2 and Comparative Example 2;

[0028] Figure 3 shows the consumption of thiosulfate after 24 hours of gold immersion in Example 1, Comparative Example 1 and Comparative Example 3;

[0029] Figure 4 is a diagram of the gold immersion effect under different systems;

[0030] FIG5 is a diagram showing the gold immersion effect under different voltage conditions of Example 3. DETAILED DESCRIPTION

[0031] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] Electrochemical leaching is an emerging technology, which is favored by many scholars because of its green, efficient and avoidance of secondary pollution. Based on this, the present invention proposes a novel thiosulfate electrochemical oxidation method for extracting precious metals, comprising a thiosulfate-electrolyte-alkali solution system as an electrolyte, wherein thiosulfate includes but is not limited to one or more of sodium thiosulfate, ammonium thiosulfate, and potassium thiosulfate, and the electrolyte may be one or two of chlorides, sulfates, and carbonates, preferably potassium chloride. The alkali solution may be one or more of ammonia water, sodium hydroxide, and potassium hydroxide, preferably ammonia water. In some embodiments, the concentration ratio of thiosulfate, electrolyte, and ammonia water in the electrolyte is (0.1-0.5): (0.1-0.5): (0.3-2.0). The pH of the electrolyte is 7-13, more preferably pH=10.

[0033] The electrolytic cell is separated into an anode chamber and a cathode chamber by a cation exchange membrane, and precious metal materials are added to the anode chamber of the electrolytic cell. Platinum, titanium, copper, lead, glassy carbon, silicon carbide, stainless steel, graphite electrode or graphite felt are selected as anodes, and titanium, copper, stainless steel or graphite electrodes are selected as cathodes. They are placed in the anode chamber and cathode chamber respectively and connected to an external power supply. The precious metals are leached by electrochemical oxidation at a voltage of 0.3V to 3V.

[0034] The present invention provides a novel thiosulfate electrochemical oxidation leaching system. This method not only causes an electrochemical reaction on the surface of mineral particles, but also utilizes the anodic reaction to leach precious metals from the minerals. During the leaching process, the metal mineral collides with the anode, loses electrons, and undergoes electrochemical oxidation and direct dissolution (electrochemical corrosion). The reaction formula is as follows: Au-e - →Au + Ag-e - →Ag +

[0035] Thiosulfate and metal ions (Au + 、Ag + ) to form a complex Au+ +2S2O3 2- =Au(S2O3)2 3- Ag + +2S2O3 2- =Ag(S2O3)2 3-

[0036] The presence of ammonia in the system can prevent the insoluble products (such as elemental sulfur) generated by thiosulfate during the dissolution of precious metals from being deposited on the mineral surface. At the same time, the complex Au(S2O3)(NH3) - 、Ag(S2O3)(NH3) - The formation of can prevent the oxidation / disproportionation of unstable thiosulfate, so the alkali solution of the present invention is preferably ammonia water.

[0037] Therefore, in the present invention, no additional chemical oxidants and complexing agents need to be added, making the leaching process safer and more cost-effective. This method allows the leachate to be reused, significantly reducing the use of chemicals and the generation of secondary waste.

[0038] In the technical solution provided by the present invention, an electrochemical reaction occurs on the surface of mineral particles, and the anodic reaction is utilized to leach metals from the mineral. The liquid phase in the electrolytic cell serves as both a leaching agent for the mineral raw material and an electrolyte. Leaching of metals from the mineral raw material is a dual effect of chemical leaching of the mineral raw material by the leaching agent and electrochemical leaching by the anodic reaction. The cation exchange membrane serves to isolate the cathode and anode regions, preventing non-metallic particles from the anode region from entering the cathode region while also preventing gold / silver thiosulfate complex ions from entering the cathode region, which could affect the determination of the leached amount.

[0039] The present invention is described in detail below with reference to specific embodiments.

[0040] The cation exchange membrane used in the electrolysis system of the following examples was purchased from Shandong Tianwei, model: EDCIS.

[0041] Example 1

[0042] S2O3 2- -The electrolyte composition of the KCl-NH3 system is: 0.3M Na2S2O3, 0.2M KCl, 0.5M ammonia water, and the initial pH is adjusted to 10 with saturated NaOH.

[0043] The electrolytic cell is separated into two chambers, the positive and negative, by a cation exchange membrane to prevent gold complex anions from entering the cathode and thus affecting the determination of the leaching amount. The prepared electrolyte is poured into the electrolytic cell, keeping the liquid levels of the positive and negative chambers level. The gold powder concentration in the anode chamber is 333 mg / L. A pre-moistened graphite felt is used as the anode, and a polished titanium plate is used as the cathode. After that, a power supply is connected, the voltage is set to 0.6 V, and the speed is adjusted to 900 rpm for gold leaching. Samples are taken at different times, and the concentration of precious metal gold leached at different time points is detected by atomic absorption spectrometry. The concentration of thiosulfate in the leachate (i.e., electrolyte) after 24 hours of leaching is titrated by iodine titration to calculate its consumption.

[0044] Comparative Example 1

[0045] Prepare S2O3 in the same manner as in Example 1 2- -KCl-NH3 system reaction electrolyte, and then use a cation exchange membrane to separate the electrolytic cell into anode and cathode chambers to prevent gold complex anions from entering the cathode and affecting the determination of the leaching amount. Pour the prepared electrolyte into the electrolytic cell, keeping the liquid levels of the cathode and cathode chambers level. The gold powder concentration in the anode chamber is 333 mg / L. Without power, the speed is adjusted to 900 rpm for gold leaching, and samples are taken at different times. The concentration of precious metal gold leached at different time points is detected by atomic absorption spectrometry. The concentration of thiosulfate in the leachate (i.e., electrolyte) after leaching for 24 hours is titrated by iodine titration to calculate its consumption.

[0046] Figure 1 shows the S2O3 of Example 1 and Comparative Example 1 2- -KCl-NH3 system electrolytic and non-electrolytic gold immersion comparison chart, it can be seen that S2O3 2- The gold leaching concentration measured under the conditions of power addition in the KCl-NH3 system after 24 hours was 80.9 mg / L, while the gold leaching concentration without power addition was 0.315 mg / L after 24 hours, which is equivalent to no leaching. This shows that power addition effectively promotes gold leaching.

[0047] Example 2

[0048] Prepare S2O3 in the same manner as in Example 1 2- -KCl-NH3 system reaction electrolyte, and then use a cation exchange membrane to separate the electrolytic cell into anode and cathode chambers to prevent silver complex anions from entering the cathode and affecting the determination of the leaching amount.

[0049] The gold powder concentration in the anode chamber was 333 mg / L. The pre-moistened graphite felt was used as the anode, and the polished titanium plate was used as the cathode. The power supply was then connected, the voltage was set to 0.6 V, and the speed was adjusted to 900 rpm for silver leaching. Samples were taken at different times, and the concentration of precious metal silver leaching in the samples at different time points was detected by atomic absorption spectrometry.

[0050] Comparative Example 2

[0051] Precious metal silver was leached according to a method substantially the same as that in Example 2, except that no power was applied during the silver leaching process, and samples were taken at different times, and the concentration of precious metal silver leached from the samples at different time points was detected by atomic absorption spectroscopy.

[0052] Figure 2 shows the S2O3 of Example 2 and Comparative Example 2. 2- -KCl-NH3 system electrolytic and non-electrolytic silver immersion effect comparison chart, it can be seen that S2O3 2- -KCl-NH3 system under the condition of electricity leaching for 24 hours, the concentration of silver leaching for 24 hours was 58.5 mg / L, and the concentration of silver leaching for 24 hours without electricity was 20.1 mg / L, indicating that electricity can greatly promote the leaching of silver.

[0053] Comparative Example 3

[0054] S2O3 2- -Cu 2+ The leachate composition of the -NH3 system is: 0.3M Na2S2O3, 1mM CuSO4, 0.5M ammonia water, and the initial pH is adjusted to 10 with saturated NaOH.

[0055] The electrolytic cell is separated into two compartments, the cathode and the cation exchange membrane, to prevent the gold complex anions from entering the cathode and thus affecting the determination of the leaching amount. The prepared leachate is poured into the electrolytic cell, keeping the liquid levels of the cathode and the cation chambers level. The gold powder concentration in the anode chamber is 333mg / L. Without power, the speed is adjusted to 900rpm to leach gold. Samples are taken after 24 hours, and the leached concentration of precious metal gold is detected by atomic absorption spectrometry. The concentration of thiosulfate in the leachate (i.e., the electrolyte) after 24 hours of leaching is titrated by iodine titration to calculate its consumption.

[0056] Figure 3 shows the consumption of thiosulfate after 24 hours of gold immersion in Example 1, Comparative Example 1 and Comparative Example 3. It can be seen that compared with Comparative Example 1, S2O3 2- The consumption is not much different, while the traditional copper ammonia oxidation method S2O3 in Example 3 2- The consumption increased significantly, indicating that the electrochemical oxidation method provided by the present invention can reduce the consumption of reagents while immersing gold.

[0057] Comparative Example 4

[0058] S2O3 2- -Cu 2+ The leachate composition of the -KCl-NH3 system was: 0.3 M Na2S2O3, 0.2 M KCl, 1 mM CuSO4·5H2O, 0.5 M ammonia water, and the initial pH was adjusted to 10 with saturated NaOH.

[0059] The electrolytic cell is separated into two compartments, the cathode and the cation exchange membrane, to prevent gold-complexed anions from entering the cathode and affecting the leaching yield. The prepared leachate is poured into the electrolytic cell, maintaining the liquid levels in the cathode and cation compartments at equal levels. The gold powder concentration in the anode compartment is set at 333 mg / L. Without power, the leaching process is performed at a speed of 900 rpm. Samples are taken after 24 hours, and the leached concentration of the precious metal gold is determined using atomic absorption spectroscopy.

[0060] Figure 4 shows the gold immersion effect under different systems. It can be seen that in Example 1, S2O3 2- -KCl-NH3 system under the condition of electricity leaching 24h gold concentration is 80.9mg / L, comparative example 3S2O3 2- -Cu 2+ -NH3 system (copper ammonia system) gold leaching concentration of 24h is 27.999mg / L, comparative example 4S2O3 2- -Cu 2+ The gold leaching concentration of the -KCl-NH3 system after 24 hours was 36.693 mg / L. The results showed that the copper-ammonia system did not have as good a leaching effect as the electrochemical system of the present invention.

[0061] Example 3

[0062] Prepare S2O3 in the same manner as in Example 1 2- -KCl-NH3 system reaction electrolyte, and then use a cation exchange membrane to separate the electrolytic cell into anode and cathode chambers to prevent gold complex anions from entering the cathode and affecting the determination of the leaching amount.

[0063] The gold powder concentration in the anode chamber was 200 mg / L. The pre-moistened graphite felt was used as the anode, and the polished titanium plate was used as the cathode. The power supply was then connected, and the voltages were set to 0V, 0.3V, 0.6V, 0.9V, and 1.2V for five parallel experiments. The rotation speed was adjusted to 900 rpm, and the leaching concentration of precious metal gold after 24 hours was detected by atomic absorption spectrometry.

[0064] Figure 5 shows S2O3 2- -KCl-NH3 system gold leaching effect comparison under different voltage conditions. It can be seen that with the increase of voltage, the leaching gold concentration first increases, then decreases, and then increases again. Among them, the leaching effect is best when the applied voltage is 0.6V, and the thiosulfate consumption is also less at this time.

[0065] In summary, the present invention uses an electrochemical oxidation method to replace the traditional copper ammonia oxidation method, and oxidizes and dissolves precious metals by electrochemical anodic oxidation at low voltage without the need to add other oxidants or complexing agents (such as Cu 2+), making the leaching process safer and more cost-effective. This method allows for the reuse of leachate, significantly reducing chemical usage and secondary waste generation. This enables green and efficient leaching of precious metals while reducing reagent consumption, resolving the current issue of thiosulfate gold leaching technology, which struggles to balance reagent consumption and leaching efficiency.

[0066] It should be noted that the above embodiments all belong to the same inventive concept, and the description of each embodiment has its own focus. For any details not described in individual embodiments, reference may be made to the description in other embodiments.

[0067] The above-described embodiments merely illustrate the implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for electrochemically oxidizing and leaching precious metals with thiosulfate, characterized in that, The method includes the following steps: A cation exchange membrane is arranged in an electrolytic cell, and the cation exchange membrane divides the electrolytic cell into an anode chamber and a cathode chamber; a thiosulfate - electrolyte - alkali solution system is used as the electrolyte solution, a precious metal material is added to the anode chamber of the electrolytic cell, and the cathode and anode arranged in the electrolytic cell are connected to an external power source, and precious metals are leached through electrochemical oxidation; the electrolyte is a salt that does not contain thiosulfate.

2. The method for electrochemically oxidizing and leaching noble metals with thiosulfate according to claim 1, characterized in that, The pH of the electrolyte solution is 7 - 13.

3. The method for electrochemically oxidizing and leaching precious metals with thiosulfate according to claim 1, characterized in that, The concentration ratio of thiosulfate to electrolyte in the electrolyte solution is (0.1 - 0.5):(0.1 - 0.5).

4. The method for electrochemically oxidizing and leaching precious metals with thiosulfate according to claim 1, characterized in that, The thiosulfate is one or more of sodium thiosulfate, ammonium thiosulfate, potassium thiosulfate; the alkali solution is one or more of ammonia water, sodium hydroxide, potassium hydroxide; the electrolyte is one or more of chloride salts, sulfate salts, carbonate salts.

5. The method for electrochemically oxidizing and leaching precious metals with thiosulfate according to claim 4, wherein The alkali solution is ammonia water, and the concentration ratio of thiosulfate, electrolyte, and ammonia water in the electrolyte solution is (0.1 - 0.5):(0.1 - 0.5):(0.3 - 2.0).

6. The method for electrochemically oxidizing and leaching precious metals with thiosulfate according to claim 5, wherein, The chloride salt is one or two of potassium chloride and sodium chloride.

7. The method for electrochemically oxidizing and leaching precious metals with thiosulfate according to claim 1, characterized in that, The conditions for electrochemical oxidation are: voltage 0.3V - 3V.

8. The method for electrochemically oxidizing and leaching noble metals with thiosulfate according to claim 7, wherein The conditions for electrochemical oxidation are: voltage 0.6V.

9. The method for electrochemically oxidizing and leaching noble metals with thiosulfate according to claim 1, characterized in that The anode is a platinum, titanium, copper, lead, glassy carbon, silicon carbide, stainless steel, graphite electrode or graphite felt, and the cathode is a titanium, copper, stainless steel or graphite electrode.

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