Copper electrowinning method

Abstract

The present invention relates to a copper electrowinning process suitable for producing improved quality cathodes from highly contaminated electrolyte solutions. The process is carried out in an electrowinning cell containing a plurality of anodes and cathodes, with a gas sparging element at their bottoms. The process includes sparging gas over the cathodes, and is characterized in that the solution contains more than 100 mg / L of arsenic. The present invention provides an alternative solution to the problem of cathode quality when dealing with highly contaminated electrolyte solutions, especially those containing high concentrations of arsenic.

Description

[Technical field]

[0001] The present invention relates to a copper electrowinning process suitable for producing improved quality cathodes from highly contaminated electrolytes. [Background technology]

[0002] Refining processes applied to copper-bearing primary or secondary materials typically end up producing an alloy of metals with copper as the main component, most often of a sulfidic nature, which is therefore called "matte". Depending on the material fed to the smelting furnace, significant amounts of other elements may also be collected at this stage, such as precious metals and impurity groups such as arsenic, antimony, bismuth, lead, tellurium, and selenium.

[0003] The copper-based phase is then subjected to further process steps to recover the precious metals rapidly and in high yield. It is also essential to extract the copper. According to known methods, the copper-based alloy or matte is finely ground and then leached under oxidizing conditions in sulfuric acid. The precious metals remain in the residue, which are separated by decantation and / or filtration. The leachate contains copper sulfate and is named "electrolyte" in view of the subsequent process steps of electrowinning, where copper is recovered in the form of cathode. It will also contain many of the impurities contained in the alloy or matte.

[0004] During electrowinning, sulfuric acid is regenerated at the anode. The spent electrolyte, which is highly acidic and copper-depleted, is recycled to the leaching process. Due to this closed loop, the electrolyte gradually accumulates impurities. This accumulation must be mitigated, which is usually done by diverting a portion of the total electrolyte stream and subjecting it to a dedicated purification process. The diverted stream, also known as the "bleed", is replenished by the addition of fresh acid solution.

[0005] Because dedicated purification processes are complex and expensive, it is generally desirable to limit the amount of bleed, and to this end, relatively high concentrations of impurities in the electrolyte may be tolerated.

[0006] However, the presence of impurities in the electrolyte has a direct impact on the purity of the copper cathode. Indeed, impurities can be included in the cathode by different mechanisms. They can either be co-deposited with the copper by electroplating (e.g. silver and bismuth) or embedded in the cathode as precipitates (arsenic, antimony, bismuth) or as particles (lead). The commercial price of the cathode is directly affected by these impurities. This problem is particularly evident in the 250 A / m 2 This is even worse when current densities above 1000 .mu.m are applied.

[0007] The level of impurities in the cathode depends on the impurities in the copper-containing primary or secondary material being processed. Arsenic is often the most significant element, followed by bismuth. ASTM B115-10 (2016) specifies the limits of impurities in electrolytic copper "Grade 1" cathodes. This standard allows up to 5 ppm arsenic and 1 ppm bismuth. The production of Grade 1 cathodes is certainly desirable, but not required.

[0008] The problem of cathode purity when dealing with highly contaminated electrolytes, meaning that they contain high concentrations of impurities, is often addressed by supplementing the electrolyte loop with a copper solvent extraction process. In that case, the electrowinning step is carried out with an almost pure copper sulfate solution to ensure the highest cathode quality. However, the addition of solvent extraction entails significant disadvantages, such as the capital cost of the equipment and the operational difficulties of working with a flammable solvent. Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide an alternative solution to the problem of cathode quality when dealing with highly contaminated electrolytes, especially when they contain high concentrations of arsenic or bismuth. Gas sparging at the bottom of the electrowinning cell is used.

[0010] Air sparging systems in electrolytic cells for electrowinning copper are known, for example, from US 3,959,112 (A). These systems are recognized to enhance the smoothness of the cathode surface, which may be important to suppress the formation of dendrites that may result in a short circuit between the anode and the cathode. However, the use of sparging in combination with highly polluted electrolytes is not disclosed.

[0011] Little effort has been made to avoid arsenic or bismuth contamination, since most electrowinning plants either engage in solvent extraction between the leaching and electrowinning operations to remove impurities or do not include these elements in the raw material prior to leaching. [Means for solving the problem]

[0012] The present invention relates to a method for electrowinning copper from an acidic copper sulfate solution, the method being carried out in an electrowinning cell containing a plurality of anodes and cathodes and equipped with a gas sparging element, preferably including the step of sparging gas uniformly across the cathodes, the method being characterized in that the solution contains more than 100 mg / L of arsenic. The effect of sparging is particularly beneficial when the solution contains more than 500 mg / L of arsenic, and even more beneficial when the solution contains more than 2 g / L of arsenic. A suitable solution may contain 20 to 60 g / L of copper and 80 to 220 g / L of free acid, concentrations that are typically encountered in copper electrowinning.

[0013] It is noted that in electrowinning, the anode is an inert anode, in other words an anode that does not dissolve significantly in the electrolyte under the process conditions used.

[0014] In copper electrowinning, the anode does not itself contain copper.

[0015] The gas sparging element is preferably located below the lowest edge of the cathode.

[0016] The gas sparging element is preferably placed at the bottom of the electrowinning cell.

[0017] Sparging can be performed by gas injection at the bottom of the electrowinning cell through tubes installed along the length of the cell. They may be positioned perpendicular to the cathode. The tubes may be either microporous or contain millimeter-sized openings along their entire length, thereby achieving uniform distribution of gas across the cathode. Arsenic concentrations well below 100 mg / L are then tolerated, since the amount embedded in the cathode is then an acceptable residual of 250 A / m 2 This is not a significant problem even when using current densities above this.

[0018] The method is also effective for reducing contamination of the cathode with bismuth, especially when the solution contains more than 1 mg / L bismuth. Sparging is also useful when processing solutions containing more Bi, such as 10 mg / L or more.

[0019] The sparging technique according to the present invention does indeed result in a significant reduction in, among other things, arsenic and bismuth in the cathode.

[0020] The quality of the cathode is still acceptable or even meets grade 1 for solutions containing up to 5 g / L arsenic and / or up to 200 mg / L bismuth. Solutions containing even more impurities can still be advantageously treated according to the invention, even if a lower quality cathode is then expected. The above maximum values ​​for arsenic or bismuth will rarely be reached in practical situations, since other impurities such as silver will determine bleed levels that guarantee lower concentrations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In a preferred embodiment, the method of the present invention is a method for electrowinning copper having up to 15 ppm As.

[0022] In a preferred embodiment, the method of the present invention is a method for electrowinning copper having up to 3 ppm Bi.

[0023] Both of these limits are consistent with the upper limits permitted for “Grade 2 Copper” according to ASTM B115-10 (2016).

[0024] The sparging gas can be any non-reactive gas, such as nitrogen, but may also contain oxygen. Air is preferred. 3 of solution at standard conditions from 0.02 to 0.5 m 3 A gas flow rate of 1000 s / hour is preferred, as lower rates may be insufficient to ensure a significant effect on cathode quality, while higher rates may produce prohibitive amounts of acid mist when bubbled through the electrolyte.

[0025] Standard state m 3 The designation Nm is defined in ISO 2533:1975 to indicate the expression of gas volume at a pressure of 1013 millibars and a temperature of 15°C. In engineering, Nm 3 The symbol is used for this purpose.

[0026] From an economical point of view, 250A / m 2 It is advantageous to carry out the electrowinning process at a current density above .

[0027] The present invention also relates to the use of an electrowinning cell comprising a plurality of anodes and cathodes, preferably equipped with a gas sparging element for sparging gas uniformly across the cathodes, for recovering copper from an acidic copper sulfate solution also containing 100 mg / L to 5 g / L of arsenic.

[0028] Preferably, the gas sparging element is located at the bottom of the electrowinning cell.

[0029] This above mentioned use is also preferred for solutions containing 1 to 200 mg / L of bismuth.

[0030] The present invention also relates to a method for producing copper, in which an acidic copper sulphate solution is prepared by dissolving one or more raw materials in an aqueous sulphuric acid solution, which acidic copper sulphate solution is then treated with the method for electrowinning copper according to the invention. Preferably, the acidic copper sulphate solution is prepared by non-electrolytic dissolution and / or in a reactor separate from the electrowinning cell.

[0031] It is believed that various mechanisms can lead to the incorporation of impurities such as arsenic and bismuth: (i) incorporation of solid particles containing arsenic and bismuth, (ii) reduction of arsenic and the resulting codeposition of copper arsenides, (iii) bismuth plating, and (iv) electrolyte contamination. These mechanisms become more evident when working at high current densities and when copper nucleation begins. When working at higher current densities, mixed potentials are obtained at the seed plate, resulting in locally very high current densities. The latter results in very porous copper deposits, which lead to electrolyte and particle contamination and copper depletion at the surface, which results in the reduction of bismuth and arsenic with the plating of metallic bismuth and copper arsenides. Therefore, operation in the above electrolyte is possible at currents of up to 200 A / m 2 However, current densities below 1000 s are usually limited to relatively low and uneconomical current densities.

[0032] According to the invention, the encapsulation of the above-mentioned impurities can be reduced or avoided by sparging. Sparging is believed to ensure a better mixing at the cathode surface, which results in a reduction in the thickness of the boundary layer. Copper depletion, which occurs especially when the current is locally increased, can thus be avoided. For example, the current density increases greatly during the harvesting of the cathode and re-entry into the cavity. 2Another reason for the locally increased current density up to 10 ...

[0033] Examples 1 and 2 illustrate the invention based on synthesis solutions containing As and Bi, respectively.

[0034] Example 3 is carried out using actual tank house solutions. The bismuth content of these solutions varies considerably depending on the material being processed by the smelter. In these three examples, electrowinning is carried out using laboratory scale equipment.

[0035] Example 4 is carried out in a real tank house. The results obtained with and without sparging are compared.

[0036] In all cases, a lead-based anode was used. EXAMPLES

[0037] Copper sulfate crystals, sulfuric acid and As(H 3 As 2 O 5 (as a 100% aqueous solution) was added to the water to obtain 40 g / L Cu, 2.5 g / L As and 180 g / L H. 2 SO 4 Approximately 0.270 liters of this electrolyte was added to each of 30 cm 2 anode surface of 46 cm 2 The electrolytic solutions are then transferred to two individual Hull electrolytic cells with a cathode surface of 1000 nm. A current of 2 A is applied using a rectifier, with a range of 75 to 2070 A / m 2In one Hull cell the electrolyte is sparged with a microporous tube, while in the other cell no air is supplied. Oxygen evolution is the main reaction at the anode and copper reduction is the main reaction at the cathode. After 3 hours the experiment is stopped and the chemical quality of the deposited copper is determined for the different zones of varying current density. The current densities associated with most electrowinning devices (250 to 500 A / m 2 ), the arsenic concentration in the cathode from the air sparged experiment is 1 to 2 ppm compared to 1700 to 5800 ppm As in the non-sparged experiment. This can be clearly seen in the physical appearance of the cathode, as black deposits indicate the formation of copper arsenide and therefore the presence of As.

[0038] Thus, As, at a concentration of 2.5 g / L, is strongly suppressed by sparging, down to a level compatible with a grade 1 cathode. EXAMPLES

[0039] Copper sulfate crystals, sulfuric acid and Bi(BiSO 4 (as a 100 mg / L salt) was added to the water to obtain 40 g / L Cu, 200 mg / L Bi and 180 g / L H. 2 SO 4 Approximately 0.270 liters of this electrolyte was added to each of 30 cm 2 anode surface of 46 cm 2 The electrolytic solution is transferred to two individual Hull cells with a cathode surface of 75 to 2070 A / m. A current of 2 A is applied using a rectifier, resulting in a current of 75 to 2070 A / m. 2 In one Hull cell the electrolyte is sparged through a microporous tube, whereas in the other cell no air is supplied. After 3 hours the experiment is stopped and the chemical quality of the deposited copper is determined for the different zones of varying current density. The current densities most relevant to electrowinning devices (250 to 500 A / m 2), the concentration of bismuth at the cathode from the air sparged experiments ranges from 50 to 1100 ppm, compared to 3000 to 5000 ppm for the Bi concentrations in the non-sparged experiments.

[0040] Thus, at a concentration of 200 mg / L, Bi is sufficiently suppressed by sparging to be significantly suppressed, even if the desired compliance with Grade 1 criteria is not always obtained. EXAMPLES

[0041] 37 to 50 g / L Cu, 1.5 to 3 g / L As, 10 to 200 mg / L Bi, and 160 to 200 g / L H 2 SO 4 The electrolyte from a copper electrowinning tank house containing 2 anode surface of 46 cm 2 The electrolytic solution is transferred to two individual Hull cells with a cathode surface of 75 to 2070 A / m. A current of 2 A is applied using a rectifier, resulting in a current of 75 to 2070 A / m. 2 In one Hull cell the electrolyte is sparged through a microporous tube, whereas in the other cell no air is supplied. After 3 hours the experiment is stopped and the chemical quality of the deposited copper is determined for the different zones of varying current density. The current densities most relevant for electrowinning (250 to 500 A / m 2 ), impurity concentrations in the cathode from the air sparged experiments are 1 to 2 ppm As and 1 to 10 ppm Bi, compared to 20 to 1000 ppm As and 180 to 650 ppm Bi in the non-sparged experiments.

[0042] As and Bi are sufficiently suppressed by sparging at concentrations up to 3 g / L and 200 mg / L, respectively, to levels compatible with a Class 1 cathode for As. EXAMPLES

[0043] In this experiment, two commercially available electrowinning cells were used, each with a separate recirculation cell but a common rectifier. Each cell had a 0.84 m 2 The electrolytic cells contained 40 anodes and 39 cathodes with a surface area of ​​1000 nm. One cell was operated with an air sparging tube at the bottom of the cell, whereas the other cell did not provide air sparging. During the experiment, the current density was 275 A / m 2 and 425A / m 2 In this experiment, the concentrations of Cu, As, Bi, and H were varied between 37 and 50 g / L, 1.5 and 5 g / L, 10 and 20 mg / L, and 160 and 200 g / L, respectively. 2 SO 4 A typical electrolyte composition was used resulting in a cathode with a thickness of between 6 and 10 mm. The cathodes were grown for approximately 7 days and harvested when they were between 6 and 10 mm thick. After harvesting and stripping, a 50 kg sample was collected by tapping copper at the diagonal of the cathode. The samples were refined in an induction heater and the impurity concentrations were determined by optical emission spectroscopy. The impurity concentrations are reported in Table 1.

[0044] [Table 1]

[0045] As and Bi are suppressed significantly enough by sparging at concentrations up to 5 g / L and 20 mg / L, respectively, to levels that meet the criteria for Class 1 cathodes for As and Bi.

Claims

1. 1. A method for electrowinning copper from an acidic copper sulfate solution, the method being carried out in an electrowinning cell containing a plurality of anodes and cathodes and equipped with a gas sparging element, comprising sparging gas across the cathodes, the solution containing greater than 100 mg / L and up to 5 g / L arsenic and greater than 1 mg / L and up to 200 mg / L Bi; the flow rate of the sparging gas is between 0.02 and 0.5 m3 / h standard per m3 of solution; 13. A method according to claim 12, wherein the electrowinning process is carried out at a current density of greater than 250 A / m 2 .

2. The method of claim 1 , wherein the sparging gas is air.

3. 3. The method according to claim 1 or 2, which is a method for electrowinning copper having a maximum of 15 ppm As.

4. 4. The method according to claim 1, which is a method for electrowinning copper having a maximum of 3 ppm Bi.

5. 5. A method for producing copper, wherein an acidic copper sulphate solution is produced by dissolving one or more raw materials in an aqueous solution of sulphuric acid, said acidic copper sulphate solution being subsequently treated with the method according to any one of claims 1 to 4.

6. The method for producing copper according to claim 5, wherein the acidic copper sulfate solution is produced by non-electrolytic dissolution.

7. 7. The method for producing copper according to claim 5 or 6, wherein the acidic copper sulfate solution is produced in a reactor separate from the electrowinning cell.

8. 1. Use of an electrowinning cell comprising a plurality of anodes and cathodes, with a gas sparging element for sparging gas uniformly across said cathodes, for recovering copper from an acidic copper sulfate solution also containing 100 mg / L to 5 g / L of arsenic and 1 to 200 mg / L of bismuth; the flow rate of the sparging gas is between 0.02 and 0.5 m3 / h standard per m3 of solution; Use wherein the electrowinning is carried out at a current density of more than 250 A / m 2 .

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