ELECTROLYTIC COPPER EXTRACTION PROCESS

Abstract

The present invention relates to an electrolytic copper extraction process suitable for producing high-quality cathodes from highly contaminated electrolytes. The process is carried out in electrolytic extraction cells comprising a plurality of anodes and cathodes, equipped with gas bubbling elements at their lower portion. It includes the step of bubbling gas through the cathodes and is characterized by the solution containing more than 100 mg / L of arsenic. The invention provides an alternative solution to the problem of cathode quality when dealing with highly contaminated electrolytes, particularly those containing high concentrations of arsenic.

Description

ELECTROLYTIC COPPER EXTRACTION PROCESS The present invention relates to an electrolytic copper extraction process suitable for the production of enhanced quality cathodes from highly contaminated electrolytes. Smelting processes applied to primary or secondary copper-containing materials typically result in a copper-based metal alloy. This alloy is most often sulfide-based, and is then called matte. Depending on the materials fed into the smelter, significant quantities of other elements, such as precious metals and impurities like arsenic, antimony, bismuth, lead, tellurium, and selenium, may also be collected at this stage. The copper-based phase then undergoes further processing stages to recover precious metals quickly and efficiently. Producing the copper itself is also crucial. According to established processes, copper-based alloys or mattes are finely crushed and then leached in sulfuric acid under oxidizing conditions. The precious metals remain in a residue, which is separated by decantation and / or filtration. This leachate contains copper sulfate and is called the electrolyte, given the subsequent stage of the electrolytic extraction process where copper is recovered in the form of cathodes. This electrolyte will also contain many of the impurities present in the alloy or matte. During electrolytic extraction, sulfuric acid is regenerated at the anode. The highly acidic, copper-depleted spent electrolyte is recirculated to the leaching stage. Due to this closed loop, impurities gradually accumulate in the electrolyte. This accumulation must be mitigated, typically by diverting a fraction of the total electrolyte stream and subjecting it to dedicated purification stages. The diverted flow, also known as purging, is compensated for by adding fresh acidic solution. Generally, the amount of purge is to be limited, as dedicated purification stages are complex and expensive. To achieve this, relatively high concentrations of impurities in the electrolyte must be tolerated. However, the presence of impurities in the electrolyte has a direct impact on the purity of copper cathodes. In fact, impurities can be incorporated into cathodes through various mechanisms. They can be deposited together with the copper by electroplating (e.g., silver and bismuth) or embedded in the cathodes as precipitates (arsenic, antimony, bismuth) or as particles (lead). The commercial value of the cathodes is directly affected by these impurities. This problem is further exacerbated when current densities above 250 A / m² are applied. The level of impurities in cathodes depends on the impurities in the primary or secondary copper-containing materials being treated. Arsenic is frequently the most significant impurity, followed by bismuth. ASTM B115-10 (2016) specifies the limiting amounts of impurities in Grade 1 electrolytic copper cathodes. According to this standard, arsenic is permitted up to 5 ppm and bismuth up to 1 ppm. While producing Grade 1 cathodes is certainly desirable, it is not mandatory. The problem of cathode purity when dealing with highly contaminated electrolytes—those containing high concentrations of impurities—is frequently addressed by incorporating a copper solvent extraction process into the electrolyte loop. The electrolytic extraction stage is then performed in a nearly pure copper sulfate solution, ensuring the highest possible cathode quality. However, the addition of solvent extraction involves considerable drawbacks, such as the capital costs of the installation and the operational challenges of working with flammable solvents. The object of the present invention is to provide an alternative solution to the problem of cathode quality when dealing with highly contaminated electrolytes, particularly those containing high concentrations of arsenic or bismuth. Gas bubbling is used at the bottom of the electrolytic extraction cells. Air bubbling systems in copper electrolytic extraction cells are known from, for example, US patent 3,959,112(A). These systems have been recognized as enhancing the surface smoothness of cathodes. This can be important for suppressing dendrite formation, which can lead to short circuits between anodes and cathodes. However, the use of bubbling in combination with highly contaminated electrolytes is not disclosed. Little effort has been made to avoid the inclusion of arsenic or bismuth, since most electrolytic extraction plants work with solvent extraction between the leaching and electrolytic extraction operations to remove impurities or do not contain these elements in the raw materials before leaching. The present invention relates to a process for the electrolytic extraction of copper from an acidic copper sulfate solution, wherein the process is carried out in electrolytic extraction cells comprising a plurality of anodes and cathodes, equipped with gas bubbling elements, and comprising the step of bubbling gas, preferably uniformly across the cathodes, and is characterized in that the solution contains more than 100 mg / L of arsenic. The bubbling effect is particularly beneficial when the solution contains more than 500 mg / L of arsenic and even more so when the solution contains more than 2 g / L of arsenic. Suitable solutions may contain from 20 to 60 g / L of copper and from 80 to 220 g / L of free acid; these concentrations are those typically found in the electrolytic extraction of copper. It is observed that, in an electrolytic extraction, the anodes are inert anodes, in other words, anodes that do not dissolve significantly in the electrolyte under the processing conditions used. In electrolytic copper extraction, the anodes themselves are free of copper. The gas bubble elements are preferably placed below the lowest edge of the cathodes. Gas bubbling elements are preferably placed at the bottom of the electrolytic extraction cells. Bubbling can be achieved by injecting gas into the bottom of the electrolytic extraction cells through tubes installed along the cell's length. These tubes can be positioned perpendicular to the cathodes. The tubes can be microporous or contain millimeter-sized holes along their entire length, thus achieving a uniform distribution of gas across the cathodes. Arsenic concentrations well below 100 mg / L are less of a concern, as the amounts embedded in the cathodes remain tolerable, even when using high current densities. Qyfrnini ζηζ / Ε / γίΛΐ of 250 A / m2or more. The process is also effective in reducing bismuth contamination of cathodes, particularly when the solution contains more than 1 mg / L of bismuth. Bubbling remains useful when dealing with a solution containing more bismuth, such as 10 mg / L or more. In fact, the bubbling technology according to the invention provides a significant reduction of, among others, arsenic and bismuth in the cathodes. The quality of the cathodes remains acceptable, or even compatible with Quality 1, for solutions comprising up to 5 g / L of arsenic and / or up to 200 mg / L of bismuth. Solutions containing even higher impurities can still be advantageously processed according to the invention, although the cathodes are expected to be of lower quality. The above maximum concentrations of arsenic or bismuth will rarely be achieved in practical situations, as other impurities, such as silver, will determine the purging level required to ensure lower concentrations. In a preferred embodiment, the process is a process for the electrolytic extraction of copper that has, at most, 15 ppm of As. In a preferred embodiment, the process is a process for the electrolytic extraction of copper that has a maximum of 3 ppm of Bi. Both limits are consistent with the upper limit allowed for 'Quality 2' copper according to ASTM B115-10 (2016). The bubbling gas can be any non-reactive gas, such as nitrogen, but it can also contain oxygen. Air is preferred. A gas flow rate between 0.02 and 0.5 normal m³ / h per m³ of solution is preferred. Lower rates may be insufficient to ensure a clear effect on cathode quality, while higher rates may produce a prohibitive amount of acid mist when bubbling through the electrolyte. The designation m3normales is defined in ISO 2533:1975 and indicates a volume of gas expressed at a pressure of 101.3 kPa (1.013 mbar) and a temperature of 15 °C. In engineering, the symbol Nm3 is used for this. From an economic perspective, it is advantageous to perform the electrolytic extraction process with a current density of more than 250 A / m2. The invention also relates to the use of electrolytic extraction cells comprising a plurality of anodes and cathodes, equipped with elements of Q / frnini ζπζέ / υιλι gas bubbling for gas bubbling, preferably uniformly through the cathodes, for the recovery of copper from an acidic copper sulfate solution which also comprises 100 mg / l to 5 g / l of arsenic. Preferably, the gas bubbling elements are placed at the bottom of the electrolytic extraction cells. This above use is preferred for solutions that also comprise 1 to 200 mg / l of bismuth. The invention also relates to a process for copper production, wherein an acidic copper sulfate solution is produced by dissolving one or more raw materials in aqueous sulfuric acid, and wherein the acidic copper sulfate solution is subsequently treated in a process for the electrolytic extraction of copper according to the invention. Preferably, the acidic copper sulfate solution is produced by non-electrolytic dissolution and / or in a reactor that is separate from the electrolytic extraction cells. Several mechanisms are believed to lead to the incorporation of impurities, such as arsenic and bismuth: (i) the inclusion of solid particles containing arsenic and bismuth, (ii) the reduction of arsenic and subsequent co-deposition of copper arsenides, (iii) bismuth coating, and (iv) electrolyte inclusion. These mechanisms are more prevalent at higher current densities and when copper nucleation begins. At higher current densities, mixed potentials are obtained in the starting sheets, resulting in very high current densities locally. These high current densities result in highly porous copper deposits, leading to the inclusion of electrolytes and particles and copper depletion at the surface, which in turn leads to the reduction of bismuth and arsenic, with the subsequent plating of metallic bismuth and copper arsenide.Therefore, work in the electrolytes mentioned above is normally limited to a relatively low and uneconomical current density of less than 200 A / m2. According to the invention, the impurity encapsulation described above can be mitigated or avoided by bubbling. Bubbling is assumed to ensure better mixing on the cathode surface, resulting in a decrease in boundary layer thickness. Copper depletion, which occurs especially when the current is increased locally, can be avoided in this way. For example, the current density increases significantly during cathode collection and the re-feeding of blanks. Another reason for the higher current densities locally, up to 1,000 A / m², is the difference in the passivation layer thickness of the stainless steel blanks. The co-plating of silver and bismuth and the formation of copper arsenide occur particularly under these conditions of higher current densities.The improved mixing process, which delivers sufficient copper ions to the cathode, results in reduced plating of other elements. The reduced boundary thickness also leads to better copper nucleation on the steel surface and a denser copper structure. This prevents the inclusion of arsenic and bismuth precipitates. Examples 1 and 2 illustrate the invention using synthetic solutions containing, respectively, As and Bi. Example 3 is implemented using actual tank room solutions. The bismuth content of these solutions varies considerably, depending on the materials processed by the smelter. In these three examples, the electrolytic extraction is performed using laboratory-scale equipment. Example 4 is performed in a real tank room. The results obtained with and without bubbling are compared. In all examples, lead-based anodes were used. Example 1 Copper sulfate crystals, sulfuric acid, and arsenic (as H3As2O5) were added to water to form an aqueous solution containing 40 g / L of copper, 2.5 g / L of arsenic, and 180 g / L of hydrogen sulfide (H2SO4). Approximately 0.270 liters of this electrolyte were transferred to two individual Hull cells, each with an anodic surface area of ​​30 cm² and a cathodic surface area of ​​46 cm². A current of 2 A was applied using a rectifier, resulting in a cathodic current density between 75 and 2,070 A / m². In one Hull cell, the electrolyte was bubbled through microporous tubing, while in the other cell, no air was supplied. Oxygen evolution was the primary reaction at the anode, and copper reduction was the primary reaction at the cathode. After 3 hours, the experiment is stopped and the chemical quality of the deposited copper is determined for different zones with varying current densities. With the current density relevant to the In most electrolytic extraction installations (250 to 500 A / m2), the arsenic concentration at the cathode in the air bubbling experiment reaches between 1 and 2 ppm, while the As concentration in the experiment without bubbling reaches between 1,700 and 5,800 ppm. This is clearly visible in the physical appearance of the cathodes, as the black deposits suggest the formation of copper arsenide and, therefore, the presence of As. As, at a concentration of 2.5 g / l, it is strongly suppressed, therefore, by bubbling, to a level that may be compatible with Quality 1 cathodes. Example 2 Copper sulfate crystals, sulfuric acid, and Bi (as B₂SO₄) were added to water to form an aqueous solution containing 40 g / L of Cu, 200 mg / L of Bi, and 180 g / L of H₂SO₄. Approximately 0.270 liters of this electrolyte were transferred to two individual Hull cells, each with an anodic area of ​​30 cm² and a cathodic area of ​​46 cm². A current of 2 A was applied using a rectifier, resulting in a cathodic current density between 75 and 2,070 A / m². In one Hull cell, the electrolyte was bubbled through microporous tubing, while in the other cell, no air was supplied. After 3 hours, the experiment was stopped, and the chemical quality of the deposited copper was determined for different zones with varying current densities.With the current density relevant to most electrolytic extraction installations (250 to 500 A / m2), the bismuth concentration at the cathode of the air bubbling experiment rises to between 50 and 1,100 ppm, while the concentration of Bi in the experiment without bubbling rises to between 3,000 and 5,000 ppm. B¡, at a concentration of 200 mg / l, is remarkably well suppressed, therefore, by bubbling, although the desirable compatibility with Quality 1 criteria is not always achieved. Example 3 In this experiment, an electrolyte from a copper electrolytic extraction tank room was used, containing 37 to 50 g / L of Cu, 1.5 to 3 g / L of As, 10 to 200 mg / L of Bi, and 160 to 200 g / L of H₂SO₄. Approximately 0.270 liters of this electrolyte were transferred to two individual Hull cells, each with an anodic surface of 30 cm² and a cathodic surface of 46 cm². A current of 2 A was applied with a rectifier, resulting in a cathodic current density between 75 and 2,070 A / m². In one Hull cell, the electrolyte was bubbled through microporous tubing, while in the other cell, no air was supplied. After 3 hours, the experiment is stopped and the chemical quality of the deposited copper is determined for different zones with varying current densities.With the current density relevant to most electrodeposition installations (250 to 500 A / m2), the impurity concentration on the cathode of the air bubbling experiment rose to between 1 and 2 ppm of As and between 1 and 10 ppm of Bi, while the impurity concentration in the experiment without bubbling was 20 to 1,000 ppm of As and 180 to 650 ppm of Bi. As and B1, at concentrations up to 3 g / l and 200 mg / l respectively, are well suppressed by bubbling, to a level that may be compatible with As Quality 1 cathodes. Example 4 In this experiment, two commercially available electrolytic extraction cells were used, each with a separate recirculation tank but a common rectifier. Each cell contained 40 anodes and 39 cathodes, each with a surface area of ​​0.84 m². One cell was operated with air bubble tubes at the bottom, while the other cell did not. During the experiments, the current density was varied between 275 A / m² and 425 A / m². Typical electrolyte compositions, ranging from 37 to 50 g / L of Cu, 1.5 to 5 g / L of As, 10 to 20 mg / L of Bi, and 160 to 200 g / L of H₂SO₄, were used. The cathodes were allowed to develop for approximately 7 days and were harvested when the thickness reached 6 to 10 mm. After collection and separation, 50 kg of sample were collected by perforating copper on the diagonal of the cathode.The sample was melted in an induction furnace and the impurity concentration was determined by spark emission optical spectroscopy. The impurity concentration is shown in Table 1. Qjfrnini ζπζέ / υιλι Table 1: Concentration (ppm) of impurities in the cathodes Bubbling Current Density (A / dm2) As (ppm) Bi (ppm) No 310 5 2 Yes 310 1 1 No 370 4 3 Yes 370 1 1 a / frnini ζπζέ / υιλι As and Bi, at concentrations up to 5 g / l and 20 mg / l respectively, are remarkably well suppressed by bubbling, to a level that meets the 5 criteria for Quality 1 As and Bi cathodes.

Claims

1. Process for the electrolytic extraction of copper from an acidic copper sulfate solution, wherein the process is carried out in electrolytic extraction cells that include a plurality of anodes and cathodes, equipped with gas bubbling elements, comprising the step of bubbling gas through the cathodes and characterized in that the solution comprises more than 100 mg / l of arsenic.

2. Process according to claim 1, wherein the solution also comprises more than 1 mg / l of B1.

3. Process according to claim 1 or 2, wherein the solution comprises up to 5 g / l of As and / or up to 200 mg / l of Bi.

4. Process according to any one of claims 1 to 3, wherein the bubbling gas is air.

5. Process according to any one of claims 1 to 4, wherein the bubbling gas flow rate is between 0.02 and 0.5 normal m3 / h per m3 of solution.

6. Process according to any one of claims 1 to 5, wherein the electrolytic extraction process is carried out at a current density of more than 250 A / m2.

7. Process according to any one of claims 1 to 6, wherein the process is a process for the electrolytic extraction of copper having, at most, 15 ppm of As.

8. A process according to any one of claims 1 to 7, wherein the process is a process for the electrolytic extraction of copper having, at most, 3 ppm of Bi.

9. Process for the production of copper, wherein an acidic copper sulfate solution is produced by dissolving one or more raw materials in aqueous sulfuric acid, wherein the acidic copper sulfate solution is subsequently treated in a process according to any one of claims 1 to 8.

10. Process for the production of copper according to claim 9, wherein the acidic copper sulfate solution is produced by non-electrolytic dissolution.

11. Process for the production of copper according to claim 9 or 10, wherein the acidic copper sulfate solution is produced in a reactor that is separate from the electrolytic extraction cells.

12. Use of electrolytic extraction cells including a plurality of anodes and cathodes, equipped with gas bubbling elements for bubbling gas, preferably uniformly through the cathodes, for the recovery of copper from an acidic copper sulfate solution also comprising 100 mg / l to 5 g / l of arsenic.

13. Use according to claim 12, wherein the solution comprises from 1 to 200 mg / l of bismuth.