Patent Publication Number: US-3880733-A

Title: Preconditioning of anodes for the electrowinning of copper from electrolytes having a high free acid content

Description:
United States Patent [191 Eggett et al.  
 [ Apr. 29, 1975 PRECONDITIONING OF ANODES FOR THE ELECTROWINNING OF COPPER FROM ELECTROLYTES HAVING A HIGH FREE ACID CONTENT [75] Inventors: Geoffrey Eggett, Stockton-on-Tees;  
 Wayne Robert Hopkins, lngleby Greenhow, both of England [73] Assignee: Davy Powergas Limited, London,  
 England 22 Filed: Nov. 20, 1973 211 App]. No.: 417,483  
 [30] Foreign Application Priority Data May 7, I973 United Kingdom 21649/73 [52] US. Cl. 204/108; 204/140; 204/293; 75/101 BE [51] Int. Cl. C22d 1/16; B0lk 3/06; C22b 3/00 [58] Field of Search 204/l06, 108, 292, 293, 204/140, 57; 75/101 BE [56] References Cited UNITED STATES PATENTS 3,392,094 7/1968 Gauncc et al.... 204/292 3.755,] l2  
 8/1973 Fountain et al 204/l08 Primary ExaminerR. L. Andrews Attorney, Agent, or FirmMorton, Bernard, Brown, Roberts &amp; Sutherland 5 7] ABSTRACT Electrolytic production of copper is effected by electrolysis of copper containing electrolytes which include at least 50 grams per litre of free sulphuric acid with the use of a lead or lead alloy anode which has been preconditioned. Preconditioning is effected by a separate preliminary electrolysis of a sulphuric acid electrolyte which is substantially free from copper. The lead or lead alloy anode is used as the anode in the preliminary electrolysis. Fluoride ions may be present in the preliminary electrolysis to an extent of up to 80 grams per litre of electrolyte and may be provided as alkali metal fluoride. The electrolysis of the copper electrolyte preferably constitutes the final stage in the recovery of copper by extraction of copper values by an organic material such as a mixture of a hydroxy substituted aliphatic oximes and substituted phenoximes, and subsequent acidification of the organic extract. The acidification provides an aqueous copper containing electrolyte of high acidity electrolysis of which by the present method nevertheless results in low lead contamination of the copper so produced.  
 11 Claims, No Drawings PRECONDITIONING OF ANODES FOR THE ELECTROWINNING OF COPPER FROM ELECTROLYTES HAVING A HIGH FREE ACID CONTENT The present invention relates to the electrolytic production of copper and more especially to the electrolysis of copper containing electrolytes having a high proportion of free acid present in the electrolyte.  
 Electrowinning of copper is normally carried out with the use of aqueous electrolytes obtained from the treatment of copper ores with acid, usually sulphuric acid, and the free acid content of the copper salt solution finally subjected to electrolysis, after separation of other impurities, has been low and of the order of 10 to grams per litre of, for example, sulphuric acid. With low acid content of the electrolyte the use of lead or lead alloys as the anode in an electrolytic cell for the deposition of metallic copper does not result in a metallic copper product that is unduly contaminated with lead as an impurity. In the case of electrolytes having a high acid content of for example 50 or 100 grams per litre or more of sulphuric acid, electrolysis with the use of an anode of lead or lead alloys results in a measurable lead impurity in the metallic copper produced.  
  According to the present invention there is provided a process for the electrolytic production of copper in which the copper containing electrolyte includes 50 grams per litre or more of free sulphuric acid and in organic which a lead or lead alloy anode is employed which has itself been conditioned by separate preliminary electrolysis as anode in a sulphuric acid electrolyte which is substantially free from copper.  
  The invention may be advantageously adopted for the electrolysis of copper containing electrolytes including 100 grams per litre or more of free sulphuric acid such as those electrolytes containing 125 grams per litre or more of free sulphuric acid. In the treatment of the anode before its use in the electrolytic production of copper, the sulphuric acid electrolyte consists of essentially free acid and preferably contains fluoride ions provided by the addition of an alkali metal fluoride to the acid. Up to 80 grams per litre of fluoride ion may be added but an addition of about 40 grams per litre can be found to give good results. In addition to sulphuric acid and sodium fluoride the pretreatment electrolyte may contain such elements as-cobalt, silver, tellurium, thallium and tin either singly or in combination. The anodes may be of lead or of binary, ternary or even quaternary alloys of lead with for example, combinations taken from the following elements, antimony, arsenic, calcium, cobalt, silver, tellurium, thallium and tin.  
  The present invention is of particular value in providing an improved method for the production of metallic copper from copper containing materials by an ion exchange process using a liquid or solid exchange medium with a high affinity for copper followed by aqueous acid treatment of the copper loaded ion exchange Cu S0 R Cu 2H aqueous organic aqueous material and by electrolysis of the separated aqueous acid solution in the manner indicated above. The separated aqueous acid solution is high in free acid content and normally contains from 50 to 400 grams per litre of sulphuric acid.  
  An organic ion exchange material may be used in a liquid organic phase and may be one specific for the extraction of copper so that the uptake of ferric iron which causes particular corrosion problems in subsequent electrolysis, is at very low levels. Such an organic ion exchange material may be a mixture of a-hydroxy substituted aliphatic oximes and substituted phenoximes. A specific example of a material suitable for the extraction of copper is that designated LIX64N manufactured by General Mills Inc., of USA.  
  The liquid ion exchange process is conducted by first contacting an aqueous liquor containing copper values with a solution of the organic reagent and allowing the aqueous and organic phases to separate. The organic phase will then contain part of the copper values previously contained in the feed liquor. By successive counter-current extractions over of the copper content of the feed liquor can be transferred to the organic phase and an aqueous feedstock with only a moderate or low copper content can be effectively treated in this way. The organic reagent is understood to operate in effect as a liquid ion exchange material and the process of extraction may be represented by the following equation:  
 The copper contained in the finally separated organic phase is reconverted to aqueous solution by contactwith a strongly acidic aqueous solution thereby reversing the reaction represented in the above equation. The proportion of copper transferred to the new aqueous phase will depend on the number of counter-current extracting stages and on the acid content of the aqueous phase. A final aqueous solution having a copper content suitable for electrolysis has a very low, ferric iron content and other metallic ions are not present in any important amount. The organic phase is returned to the extraction of copper from the original feed liquor. The aqueous copper containing solution obtained by such a process may contain from 20 to 60 grams per litre of copper, from 50 to 400 grams per litre of sulphuric acid, up to 2 grams per litre of iron, and may contain trace quantities of one or more metal ions known to enhance the stability of the lead oxide formed on the lead or lead alloy anode during electrolysis under the stated conditions. Such metal ions may be,  
 for example,&#39;cobalt, silver, tellurium, thallium or tin which are referred to above as elements which may be present in the preliminary anode conditioning electrolyte. Alternatively, these ions may be deliberately added to the electrolyte to enhance the anode stability.  
  Electrolysis of a copper solution with a high acid content carried out with a lead or lead alloy anode produces copper having a lead content as will be shown by the following comparative example.  
 Run No. Cell elec- H SO Average Current Average lead trolyte (free) voltage efficiency content of Cu grams grams cathode per litre per copper ppm litre COMPARATIVE EXAMPLE An aqueous cell electrolyte containing 23 grams per litre of copper, 155.9 grams per litre of free sulphuric acid and 2 grams per litre of total iron was electrolysed at C with a current density of 40 amps per square foot of immersed cathode area. Electrolyte flow rate was 0.06 US. gal. per minute per square foot of active immersed cathode area, and anode cathode spacing was two inches. Current efficiency was 98.6% and a voltage 2.2 volts. The cell was operated for 500 hours with a lead/silver anode having 1% by weight silver and 99% lead. The cathode was a thin copper starter sheet of one-sixteenth inch thickness.  
  The average lead content of the resulting cathode copper was 73 parts per million by weight after 106 hours, 66 parts per million by weight after 300 hours operation and 80 parts per million by weight after 500 hours. These results parallel the results of zinc electrolysis with a lead anode described in Example 1 of US. Pat. No. 3,392,094 which also describes a pretreatment of the lead anode serving to promote a protective coating on the lead anode during the subsequent zinc electrolysis. This protective coating has on investigation by other workers in the field been found to be a hard glassy layer of manganese dioxide (anode preconditioning and other changes in Comincos electrolytic zinc operations by R. H. Farmer in Electrometallurgy editor H. Baker, 1969), manganese always being present as an impurity in zinc electrolytes (and in all the examples in US. Pat. No. 3,392,094) but not in copper electrolytes and not in the Examples 1 to 13 following.  
  In the following examples 1 to 13 similar lead/silver anodes to those employed in the comparative example were first electrolysed, as anodes, in a solution of 56 grams per litre of free sulphuric acid and 41 grams per litre of fluoride ion added as sodium fluoride. Electrolysis was conducted at 21C for 12 hours at a current density of amps per square foot of immersed anode. Cathodes were copper starter sheets.  
  Lead/silver anodes first treated in this way yielded the following results when employed as anodes in the electrolysis of aqueous copper containing solutions of high acid content as quoted but not containing manganese ions. All other conditions of the electrolysis were the same as those obtaining in the comparative example given above.  
  The average lead content of the cathode copper is based upon the analysis of copper produced during successive weeks operation.  
  Results of runs 8 to 11 were obtained using anodes which after the preliminary conditioning treatment were stored (unpolarised) in water for 400 hours.  
  Results of runs 12 and 13 were obtained using anodes which after the preliminary conditioning treatment were stored (unpolarised) in air for 760 hours.  
  It will be seen that the conditioned anodes may accordingly be used with advantage either immediately after the treatment or after storage. The cathode in the electrolysis of the copper containing electrolyte may be stainless steel or titanium blanks in place of the copper starter sheet, the electrolytically deposited copper being subsequently stripped from the stainless steel or titanium.  
  It will also be seen that a protective coating is formed on the lead anode even in the absence of manganese ions which serve to form a protective coating in the case of the electrolysis of zinc electrolytes. Accordingly in Examples 1 to 13 given above in which electrolysis of solutions containing copper sulphate and sulphuric acid is effected following the anode pre-treatment, a  
 stable insoluble surface coating of the lead anode is formed of which the predominent constituent must be a lead compound.  
  Furthermore it has been found that stabilisation of the oxide coating on the lead anode effected in a conditioned anode gives less particulate matter in the electrolyte resulting in the reduction of nodulation of the cathode copper product. Nodule formation of the cathode not only results in a risk of short-circuiting in the cell but also results in an increased lead content which is higher in the nodules than in other parts of the cathode copper.  
 What we claim is:  
  l. A process for the electrowinning of copper in which the copper-containing electrolyte includes 50 grams per liter or more of free sulphuric acid, and in which a lead or lead alloy anode is employed wherein said anode has been conditioned by electrolysis as the anode in a fluoride ion-containing, sulfuric acid electrolyte which is substantially free from copper to provide a stable, insoluble surface coating on said anode.  
  2. A process for the electrowinning of copper in which the copper-containing electrolyte includes 50 grams per liter or more of free sulfuric acid, said electrolyte having a substantial absence of manganese, and in which a lead or lead alloy anode is employed wherein said anode has been conditioned by electrolysis as the anode in a fluoride ion-containing, sulfuric acid electrolyte which is substantially free from copper to stabi-.  
 lize said anode for use in the electrolytic production of copper.  
  3. The process of claim 1 wherein the coppercontaining electrolyte includes more than 100 grams per liter of sulfuric acid.  
  4. The process of claim 1 wherein the coppercontaining electrolyte includes more than 150 grams per liter of sulfuric acid.  
  5. The process of claim 1 wherein the fluoride ions in the anode-conditioning, sulfuric acid electrolyte are provided by the addition of alkali metal fluoride to the electrolyte.  
  6. The process of claim 1 in which the fluoride ions are present in the anode-conditioning, sulfuric acid electrolyte in an amount up to 80 grams per liter.  
  7. The process of claim 1 wherein one or more of the elements cobalt, silver, tellurium, thallium, or tin are present in at least one of the copper-containing electrolyte or anode-conditioning, sulfuric acid electrolyte.  
  8. The process of claim 1 wherein the conditioned anode is stored in water or air before use for the electrolytic production of copper.  
  9. The process of claim 1 wherein the coppercontaining electrolyte is obtained by acidic aqueous treatment of an organic liquid, ion-exchange material employed for the extraction of copper from a source material.  
  10. The process of claim 1 wherein the anode contains silver.  
  11. The process of claim 10 wherein the anode comprises about 1 percent silver and 99 percent lead by weight.