US 3865744 A
Copper is processed via solutions of copper salts, both cupric and cuprous, in acidified aqueous solutions containing organic nitriles. Methods of producting solutions of cuprous salts include reduction of cupric salts and oxidation of copper and copper sulphides. Solutions of cuprous salts are thermally or electrochemically disproportionated to produce copper and solutions of cupric salts.
Description (OCR text may contain errors)
Unite States Patent 1 Parker et a1.
[ 1 Feb. 11, 1975 METHOD OF PRODUCING COPPER AND COMPOSITION THEREFOR Inventors: Alan James Parker; Winfield Earle Waghorne; Dion Ewing Giles; John Howard Sharp; Robert Alexander; David Michael Muir, all of c/o The Research School of Chemistry, The Austrialian University, Canberra A.C.T., Australia 2600 Filed: Mar. 22, 1972 Appl. No.: 237,061
Foreign Application Priority Data Apr. 2, 1971 Apr. 2, 1971 Apr. 2, 1971 Apr. 28, 1971 June 4, 1971 Sept. 14, 1971 Sept. 24, 1971 Oct. 20, 1971 Oct. 26, 1971 Dec. 14, 1971 Jan. 26, 1972 U.S. Cl
Australia 4500/71 Australia 4501/71 Australia 4502/71 Australia 4735/71 Australia 5110/71 Australia... 6281/71 Australia... 6411/71 Australia 6718/71 Australia 6798/71 Australia... 7383/71 Australia 7776/72  Int. C1..C22b 15/00, C2215 15/08, C22b 15/12  Field of: Search 75/.5 A, 117, 108; 203/31, 203/32, 34, 35, 38, 57; 423/27, 29, 23, 41,
OTHER PUBLICATIONS Technique of Inorg. Chem, Vol. I, John Wiley, NY. (1963), P. 42.
Primary ExaminerHerbert T. Carter Attorney, Agent, or FirmFleit & Jacobson [5 7] ABSTRACT Copper is processed via solutions of copper salts, both cupric and cuprous, in acidified aqueous solutions containing organic nitriles. Methods of producting solutions of cuprous salts include reduction of cupric salts and oxidation of copper and copper sulphides. Solutions of cuprous salts are thermally or electrochemically disproportionated to produce copper and solutions of cupric salts.
25 Claims, 7 Drawing Figures pmgmgmzal 11975 '1. 865,744
SHEET 1 0F 6 Figure 2 Figure I PATENTEU W31 5 SHEET 5 OF 6 R Enm METHOD OF PRODUCING COPPER AND COMPOSITION THEREFOR This invention relates to methods of separating and recovering copper from materials containing copper and of preparing purer forms of copper, via solutions of certain cuprous salts in acidified mixtures of water and organic nitriles. The invention includes methods for preparing solutions of copper salts of compositions which are useful for copper recovery or copper purification. Further details of organic nitriles and cuprous salts are given later in this application.
BACKGROUND OF THE INVENTION Conventional methods of extracting and refining copper are described by A. Butts in Copper the Metal its Alloys and Compounds Reinhold Publishing Corp., N.Y., I954. The most common method is pyrometallurgy through flotation of crushed ore, then smelting in an oxidising atmosphere to blister and anode copper, and finally electrorefining of anodic to cathodic copper, via acidic cupric sulphate solutions. This method has the disadvantages of pollution by sulphur dioxide, high capital and operating costs and losses of valuable materials in fumes and slags. Another method is that of hydrometallurgy, i.e., leaching copper oxides or sulphides with sulphuric acid, sometimes in the presence of ferric sulphate, to give cupric sulphate solutions. Copper itself can be dissolved in hot sulphuric acid under oxidising conditions to give cupric sulphate solutions. In these methods the cupric sulphate solutions are then stripped of their copper by electrowinning or cementation. More recent procedures include treating the ore with an oxidising or reducing roast, the TORCO segregation process, bacterial leaching, and leaching with ferric chloride or ammoniacal solutions. All these processes, require a final step of electrorefining or electrowinning via aqueous acidified cupric sulphate solutions, if one is to produce copper of sufficient purity for many commercial processes. The electrorefining step is a slow process. typical tankhouse cycles being ll-l4 days. A number of tankhouse staff are needed, handling of anodes, treatment of slimes, attention to short circuits and cathode quality, all create problems. The electrowinning process is also slow and consumes considerable quantities of power, but costs associated with anode handling are reduced, compared to electrorefin- SUMMARY OF THE INVENTION TION It is a main object of this invention to provide a composition of matter suitable for processing copper via cuprous ions and which comprises a composition of matter comprising at least by volume water, an acid selected from the group consisting of sulphuric, sulphurous and nitric acid, at least 4% by volume of an organic nitrile and a copper salt of the corresponding acid.
Various other objects, advantages, methods of using the compositions and explanation of terms will become apparent from the detailed description and discussion of preferred embodiments which appear hereinafter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Some preferred aspects of the invention are described with reference to the drawings. However, it is to be understood that the drawings are diagrammatic only and are not intended to be limitative of the invention. In the drawings:
FIG. 1 illustrates the layout of a system including an electrochemical disproportionation cell from which anolyte is removed and after treatment returned to the cell. This system is suitable for copper powder electrorefining.
FIG. 2 illustrates a modification of FIG. 1 and relates to copper powder electrorefining.
FIG. 3 illustrates the layout of apparatus for use in electrochemical disproportionation and in particular a design of the cells, plus a circuit illustrating the method of cuprous electrowinning.
FIG. 4 illustrates the layout of a system for cuprous winning.
FIG. 5 illustrates the layout of a system suitable for continuous copper powder refining.
FIG. 6 summarizes and illustrates methods of generating cuprous salt solutions, their purification, methods for their disproportionation to cupric salt solutions and copper and methods of converting the cupric salt solutions to cuprous salt solutions.
FIG. 7 is a flowsheet illustrating the separation of copper and silver from materials containing those metals.
The overall field of this invention is illustrated in FIG. 6 (explained in detail hereinafter) and involves methods of producing solutions of certain cuprous salts in acidified mixtures of sufficient of suitable organic ni triles and water. This is followed by two methods of disproportionating the cuprous salt to copper and cupric salt either electrochemically or by thermally removing the nitrile from the solution. The cupric salt which is produced by disproportionation is then converted to cuprous salt by reduction with copper or reaction with other suitable substances and the process continues. Reduction of Cu by copper is a method of copper refining, reduction by other reducing agents is a method of copper winning from cuprous solutions and reaction with cuprous or cupric sulphide is a method of leaching by oxidising the sulphide.
The reactions which produce cuprous salts are with one exception reactions between solids and reagents in solution, and as such have the usual advantages when performed with well agitated, finely divided solids at elevated temperatures if possible. The use of volatile hydrolyzable nitriles in aqueous acidified solutions and of oxidisable cuprous ions imposes limitations and it is desirable to operate whenever possible in sealed systems in the absence of significant amounts of oxygen.
Some of the methods described are named for the purposes of identification in this application as cuprous electrowinning, cuprous winning, copper powder electrorefining and copper powder refining, but it will be apparent from FIG. 6 that the four processes are very closely linked. Thus a cupric solution from the anolyte of copper powder electrorefining could be used to oxidise copper produced by thermal disproportionation and thus convert the powder to cathodic copper.
The field of this application can be further summarized as:
a. Methods and compositions for producing cuprous salt solutions from slightly soluble cuprous or cupric salts.
b. Methods and compositions for leaching copper,
from materials containing copper and insoluble impurities,.as cuprous salt solutions.
0. Methods and compositions for reducing solutions of cupric salts to solutions of cuprous salts.
d. Methods of electrochemically disproportionating cuprous salt solutions to copper and cupric salt solutions in the presence of soluble impurities.
e. Methods of thermally disproportionating cuprous salt solutions to copper and cupric salt solutions in the presence of soluble impurities.
f. Linking these concepts in a variety of ways, as
sketched in FIG. 6, to give new processes for producing purer forms of copper metal from materials containing copper, cupric ions, or cuprous ions. Some of these ways are described e.g. as cuprous electrowinning, copper powder refining, but we do not exclude other methods of processing copper e.g. leaching of cuprous sulphide by Ca, which link together the five embodiments (a) (e), and are shown in FIG. 6 but are not given a new defined name. I
For example, a solution of cuprous sulphide could be treated with cupric sulphate to give a cuprous solution. This is then thermally disproportionated to copper and cupric sulphate. The copper is then dissolved by an anolyte from a cell in which electrochemical disproportionation is taking place and deposited as cathodic copper in that cell. The cupric sulphate from the thermal disproportionation is reacted with atomised blister copper to give a cuprous sulphate solution and this is again thermally disproportionated, but this time the cupric sulphate solution is not recombined with the nitrile distillate, rather a higher boiling nitrile is added, the CuSO, is reduced with iron to cuprous sulphate and the solution is then electrochemically disproportionated.
The methods are all different from the current methods for extracting copper or recovering purer copper, from materials containing copper or from solutions containing copper salts. Specific details, such as compositions of solvents, methods of preparing solutions, nature of nitriles and reductants, and notional design of equipment, are presented later in the application, together with examples illustrating some of the applications of the methods.
Processes Based on Electrochemical Disproportionation Copper Powder Electrorefining and Cuprous Electrowinning In one sense the division of methods is artificial because in both, cuprous sulphate or bisulphate is disproportionated electrochemically to give cathodic copper using an inert anode in a more or less conventional way, with an electrolyte composed of cuprous sulphate or CuHSO, in water, an organic nitrile and sulphuric acid, plus additives, such as glue and sodium alkylaryl sulphonates, designed to improve the quality of the cathodic deposit. The anolyte, containing cupric sulphate, is removed continuously and is replaced by fresh cuprous sulphate electrolyte. The division of methods is based on what is done with the anolyte containing cupric sulphate.
In the preferred form of copper powder electrorefining, the anolyte is reacted with copper, or materials containing copper, preferably in a finely divided state and preferably containing a high 99%) proportion of copper, when reaction 1 of FIG. 6 occurs. Other possible reactions (3, 8 and 10 of FIG. 6) with suitable materials may occur if these are also present in the copper, all of which produce cuprous salt solutions. The species M of reaction 8 are reductants which are often present in the copper containing material, e.g. iron or nickel. Such species usually pass into the electrolyte as soluble oxidation products (e.g. salts of Fe and Ni). Inert components of the copper containing materials, such as gold, silver, lead, silica, bauxite and carbonaceous material form a slime. Basic impurities react with acid in the electrolyte. The dissolution of Cu O by H to form extra cuprous sulphate and eventually cupric sulphate as well as air oxidation of Cu", help to balance losses of Cu 80., due to reducing agents in the copper. The cuprous sulphate solution produced by these reactions replaces the anolyte in the electrochemical cell, after any necessary treatment, such as filtration, removal of soluble impurities and adjustment of acid and cuprous levels.
In the preferred form of cuprous electrowinning, the solution of cuprous sulphate which replaces the anolyte containing cupric sulphate is produced by any suitable method, but the anolyte containing cupric sulphate is not reduced with copper or materials containing copper, although it may be reduced by other reducing agents such as iron (reaction 8 of FIG. 6).
The distinction between the two methods is used because in copper powder electrorefining the process is effectively transferring copper from powdered" materials containing copper to a purer form on a cathode, whereas in cuprous electrowinning, only 50% of the copper initially is being electrowon from a solution of cuprous sulphate, but subsequent reductions of CuSO, then effectively increase this recovery of cathodic copper.
As noted, the electrolytic part of the process is very like that for conventional electrowinning of copper from aqueous cupric sulphate solutions (cf. Butts loc. cit) except that the electrolyte is different and anolyte must be removed and replaced by fresh cuprous electrolyte. Anodes coated with lead oxide are not inert to cuprous solutions and so are not suitable inert anodes. Lead lined vessels are suitable containers, but precautions may have to be taken to contain volatile nitriles and minimise oxidation by air. With most inert anodes, e.g. carbon, it has been found that there are substantial advantages of lower operating voltage in having good circulation of electrolyte past the anode, but with other, more expensive anodes, e.g. platinum, we find that the degree of circulation has less effect on the operating voltage. Cathode quality is improved at I lower operating temperatures, but the operating voltage is higher, so that a balance between these factors is needed. An operating temperature of between 20-45C is preferred. High temperatures (e.g. 50C) encourage loss of nitrile by hydrolysis.
The electrode reactions are believed to be:
Cu e Cu at the cathode Cu Cu e at the anode The cupric sulphate, which is produced at the anode in increasing quantities as reaction proceeds, reduces the efficiency of the process, because it dissolves copper from the cathode to a limited extent and more seriously, because of the electrode process Cu e Cu at the cathode. Thus, cupric sulphate should be prevented from reaching the cathode in significant amounts. As noted, this is best done by continuously replacing anolyte with fresh cuprous sulphate electrolyte.
Copper powder electrorefining offers a new way of converting impure particulate copper to purer cathodic copper without casting anodes. Anode handling is simplified and being a one electron process, power consumption per pound of copper is low, although slightly higher than cupric refining of copper anodes.
Without removal of CuSO the electrochemical disproportionation proceeds satisfactorily for only about recovery of copper. After this, the efficiency drops rapidly.
Precautions should be taken to minimise oxidation and to contain volatile, toxic and inflammable nitriles and there are advantages in using a high boiling water soluble nitrile, such as Z-hydroxycyanoethane under an inert atmosphere although voltages are higher than with an equivalent mole proportion of acetonitrile.
Two of the methods for semi-continuous operation of copper powder electrorefining are outlined in FIGS. 1 and 2. The method of FIG. 1 is preferred. In FIGS. 1 and 2, representing two possible methods of copper powder refining the numerals represent: 1 an inert anode e.g. of carbon, 2 a copper cathode, 3 a power supply, 4 a cuprous sulphate, sulphuric acid, water, organic nitrile electrolyte, with or without additives, 5 an electrolysis vessel, preferably covered with the electrolyte under a non-oxidising atmosphere, 6 a pipe for transferring anolyte containing cupric sulphate, 7 a reaction vessel, 8 a system for removing insoluble slimes, 9 a system for adding copper containing materials, preferably of high copper content, 10 a system for removing solids from the electrolyte, 11 a pump for circulating electrolyte, 12 a system for bleeding off electrolyte for purification and/or adjustment of composition, 13 a system for electrolyte purification, 14 a system for removing impurities e.g. ferrous sulphate, extracted from the electrolyte in 13, 15 a pipeline for returning the cuprous sulphate, sulphuric acid, water and organic nitrile electrolyte with or without additives to the electrolysis vessel. The preferred requirement for such a method is that, in conjunction with a more or less conventional electrowinning cell (5), in which copper is being electrowon from the specified well agitated cuprous sulphate solutions (4), by more or less conventional methods, the anolyte containing cupric sulphate be continously removed (6) from the vicinity of the electrodes (1) and replaced by cuprous sulphate solution (15), which has been generated as in reaction 1 of FIG. 6 by passing the anolyte over suitable finely divided material containing copper in reaction vessel (7), at such a speed that the copper dissolves as cuprous sulphate and the concentration of cupric sulphate in the cell is kept preferably below 10% of the total copper in solution. There should be provision (8) for extracting slirnes, for excluding insolubles (10) from the electrolytic part of the cell and provision for removal of soluble impurities (l2, 13, 14) (e.g. FeSO NiSO from the electrolyte by diversion of the electrolyte.
The electrochemical process, irrespective of the nature of the replacement of anolyte byCu SO solution, operates preferably at a current density of 5-30 amps/sq.ft, more preferably 5-20, but for the method outlined in FIG. 1, the current density, the total anode area, the rate of flow of anolyte, and the rate of reaction between Cu and CuSO, should be coordinated to keep the concentration of CuSO, in the cell at an acceptable level. Coordination can also be influenced by varying the temperature of operation, preferably between 2055C and the size of the copper containing material.
In cuprous electrowinning one form of which is illustrated by FIG. 3 it is necessary to include a reducing cycle to reduce the anolyte and return it to the cell as cuprous sulphate, with if necessary, prior removal of the oxidation products of the reductant. With iron for example as reductant, the process is very closely related to copper powder electrorefining because the reactions in the reducing circuit are thought to be as follows:
Initially, with excess iron Cu Fe Cu Fe With more Cu Cu Cu 2 Cu Overall 2Cu Fe 2 Cu Fe Such a cuprous solution can replace the anolyte, preferably after removal of some of the ferrous sulphate. Other methods of reduction or of converting cupric'sulphate to cuprous sulphate are outlined in FIG. 6 and are described in detail later.
Compared with the conventional electrowinning of cupric solutions in water, the electrochemical disproportionations just described proceed at much lower voltages, gases are not evolved at the inert anode, and being a one electron process, power consumption is less per pound of copper treated. A disadvantage is the need for covered very well stirred electrolyte.
Copper powder electrorefining offers a convenient way of converting copper powder to purer cathodes without the necessity for melting the powder and electrorefining a cast anode. The use of an inert anode avoids the trouble associated with the use of soluble copper anodes.
A convenient cell arrangement, allowing efficient circulation, removal of anolyte and replacement by fresh electrolyte, is shown in FIG. 3. In FIG. 3 a form of cuprous electrowinning is illustrated but the main feature is a design which is a preferred form of the electrolytic cell for electrochemical disproportionation. The electrodes are connected to a power supply, which is not shown. The numerals represent: I an inert anode, e.g. of carbon, 2 a copper cathode, 3- inlet pipes for admitting at high speed an electrolyte of cuprous sulphate in acidified water containing an organic nitrile, with good agitation past the electrodes, 4 outlet pipes for removing anolyte containing cupric sulphate, 5 high speed circulating pumps, 6 storage vessel for cuprous sulphate solutions, 7 electrolysis vessel, preferably covered with the electrolyte under a nonoxidising atmosphere, containing the cuprous sulphate solution, 8 storage for cupric sulphate solutions, 9 reaction vessel for reduction of cupric sulphate solutions, 10 system for removing insoluble impurities (slimes) 11 system for adding reductant, l2 a system for removing solids from the electrolyte, 13 a system for bleeding off the electrolyte, l4 a system for purifying the electrolyte and/or adjusting its composition, 15 a system for removing impurities removed from the electrolyte in 14, 16 a system for conveying the electrolyte containing cuprous sulphate.
Thermal disproportionation of Cu".
Copper can be recovered from aqueous solutions of cupric salts by reduction methods other than the electrowinning process. With solid reducing agents such as iron, nickel and zinc, the process is known as cementation. With reducing agents, such as sulphur dioxide, hydrogen and carbon monoxide, it is known as precipitation.
Cementation suffers from the disadvantage that the cupreous powder is very finely divided, well oxidised and badly contaminated by the solid reducing agent and other species which are reduced concurrently. The process is inefficient because the reducing agent becomes coated with copper and is not exposed to more cupric salt solution.
Precipitation is usually from ammoniacial solutions and, h gh temp a ures a p e su es a e nee Hy:
drogen is the preferred reducing agent and sulphur di' oxide gives incomplete reduction at high temperature and pressure.
Cement copper must be purified by the pyrometallurgical route and losses of copper are a problem.
Thermal disproportionation of Cu involves precipitation of copper from acidified solutions of cuprous salts in mixtures of volatile organic nitriles and water by thermal removal of the nitrile e.g. by distillation. The solution is free of solids prior to the winning and if necessary certain soluble impurities have been removed prior to the winning step. Thus a pure copper, whose quality as a powder can be controlled by additives, seeds and the conditions of precipitation, is produced. This powder is a more desirable product than cupreous powder produced by cementation.
The process is applied to solutions of cuprous salts, prepared for example as outlined in FIG. 6. Control of pH prevents precipitation of salts, e.g. of Fe and of SO and precipitation of Cu O or CuOH. The concentration of acid influences the nature of the precipitated copper, high acidities H 50 by volume) tend to give less discrete particles. The organic nitrile is removed in whole or in part from the cuprous solution, usually by efficient distillation, preferably under a reduced pressure. This allows lower temperatures for boiling and thus minimises hydrolysis of the nitrile. The distillation is preferably carried out in the absence of significant amounts of oxygen and recovery of copper is enhanced by using a reducing or non-oxidising atmosphere (e.g. of N The degree and nature of stirring, during removal of the nitrile, the speed at which nitrile is removed, and the presence of additives and seeds all influence the nature of the precipitated copper. We prefer to remove the nitrile slowly from a well agitated tumbling solution containing glue and other additives such as sodium alkylarylsulphonates or ammonium lignin sulphonates. The nitrile usually comes off as an azeotrope with water.
Once the proportion of nitrile to cuprous ion in solution is below 2-3 moles to one of cuprous ion, the cuprous ion disproportionates to give copper and a cupric salt.
Removal of the nitrile is continued until the desired amount of copper powder is obtained. This copper is removed by a conventional solid-liquid separation (e.g. centrifuged), is washed and dried. This portion of the process uses conventional technology for handling copper powders, separating them from liquids and for avoiding their oxidation (cf. Butts). A preferred modification is in provision to recover the valuable nitriles by distillation from the wet copper and the washings. A preferred method of washing is to use the acidified distillate of the nitrile-water and return this to the cir- Cult. The copper is removed from the circuit and is washed with hot acidified water, then with hot water in a non-oxidising atmosphere.
The process of thermal disproportionation may be tionation in at least two steps, because the first solids which are precipitated will contain high proportions of the more easily reduced material.
An alternative procedure for removing reducible species, especially in the case of solutions containing significant amounts of silver salts, is to cement the reducible species with an excess of copper to produce a colourless solution, prior to thermal disproportionation. Reaction 13 of FIG. 6 takes place readily with finely divided copper and solutions of silver salts in acidified nitrile-water mixtures in the absence of Cu This method of thermal disproportionation usually enables one to recover a maximum of 50% of the copper from a cuprous salt solution, the other 50% is cupric salt. A highly desirable addition to the basic invention is to recycle the nitrile-depleted cupric salt solution with the distilled azeotrope containing the nitrile and the washings, then to regenerate cuprous salts from cupric by any of the methods outlined in FIG. 6 or by any other acceptable method. This is known as cuprous winning, copper powder refining or cuprous leaching, depending on the method of regeneration.
Cuprous Winning This is illustrated in a preferred form in FIG. 4.
In FIG. 4 the numerals represent: 1 a distillation vessel for disproportionation of cuprous salt solutions, 2 a reaction vessel for reduction of cupric salt solutions, 3 a system for removing, washing and drying copper powder, 4 removal of volatile nitrile and its azeotrope and condensation, 5 vessel for storing distilled nitrile and its azeotrope, 6 system for splitting the distillate into roughly equal proportions, 7 a system for effecting separation of solids and liquids, 8 a system for adding reducing agent to the reaction vessel, 9 a system for adding nitrile and its azeotrope to the reaction vessel, 10 a solution containing some cupric salt solution in a solution depleted of nitrile by distillation, l1 a solution of cuprous salt in acidified water containing a volatile organic nitrile, with or without additives and copper seeds, 12 copper depleted solution containing oxidised reductant.
For example, the cupric salt in the nitrile-depleted solvent (10), prepared by thermal disproportionation of a cuprous salt solution (11), is recombined (9), with the nitrile-water azeotrope distillate and washings and is reduced by the addition of a suitable reducing agent (8), such as iron or S0 in proportions which satisfy the stoichiometry of equations 8 or 9 of FIG. 6.
(where Red is the reductant and Ox is its oxidised form) The reduced solution of cuprous salt is filtered if necessary and the cycles of nitrile removal, copper precipitation and cupric salt reduction are repeated until the desired recovery of copper is achieved.
Ideally, the overall material balance is represented y.
Cu 2Red Cu 20x but this would require an infinite number of cycles, so a choice must be made, and the remaining small amount of copper in solution is recovered in other ways, e.g. cupric cementation by iron.
It will be appreciated that the feed for a thermal disproportionation followed by a cuprous winning process may be a solution of cupric sulphate, to which sufficient suitable nitrile has been added, followed by reduction with the same suitable reducing agent, e.g. iron or S chosen for the later steps of the cuprous winning process.
Precautions against loss of the valuable, volatile, toxic and inflammable nitrile are normally taken, e.g. reactions in sealed vessels or under condensers. The process is best suited to an enclosed system with oxygen excluded as much as possible.
Recovery or use of the oxidised form of the reductant, of traces of undistilled nitrile, and of traces of copper in the spent solution is an economically desirable part of the process. Where S0 is the reductant, the acid produced may be used for leaching of oxidised copper to give cupric sulphate solutions, which may be reduced to cuprous sulphate or sulphite solutions as outlined and thus a continuous cuprous winning is achieved.
Copper Powder Refining This principle is illustrated in a preferred form of a continuous process in FIG. 5.
In FIG. the numerals refer to: I a reaction vessel for converting copper and cupric salts to cuprous salts, 2 removal of insoluble impurities (slimes), 3 system for adding particulate materials containing copper to the reaction vessel, 4 system for removing solids from the cuprous salt solution, 5 Distillation vessel for disproportionation of cuprous salt solutions, 6 removal of distillate containing a volatile nitrile or its azeotrope, 7 condensed nitrile or its azeotrope, 8 system for dividing a liquid into two portions, 9 removal of copper powder and its washing with condensate, l0 system for further washing and drying of copper powder, 11 system for separating solids and liquids, l2 solution containing cupric salt in a nitrile depleted solution, 13 solution of a cuprous salt in acidified water containing a volatile organic nitrile with or without additives, 14 system for purifying solution 12 and adjusting its composition, 15 system for removing soluble impurities.
Conventional methods of purifying copper powders (cf. Butts) involve either pyrometallurgical treatment followed by cupric electrorefining of copper anodes, for low grade powders 98% copper) or melting and casting to anodes, followed by cupric electrorefining. These methods involve significant losses of copper, are slow and require considerable handling of the copper. Cupreous powder and the TORCO concentrate, for example, require the complete smelting and refining operation to produce pure copper.
Our method of copper powder refining avoids pyrometallurgy with its high capital cost, and also avoids the slow cupric electrorefining step. It is a cheaper and faster method of preparing purer copper from any finely divided form of copper-containing material, than is any existing method known to us. Capital costs are relatively low compared to pyrometallurgy.
The method is based on thermal disproportionation, except that in the preferred procedure one starts with a solution of a suitable cupric salt in a mixture of sufficient of a suitable volatile organic: nitrile and acidified water and reduces this with impure copper as reducing agent (cf. reaction 1 of FIG. 6). The resulting cuprous solution is then transferred to a distillation vessel (5) and distilled to remove nitrile (6),. precipitate the copper (9) and regenerate cupric salt. The cupric salt solution (12) and distillate (7) are combined and are transferred to a reaction vessel (1) in which reduction with copper (3) is repeated. Thus ideally, the copper concentration of the electrolyte is unchanged, and crude particulate copper (3) is converted to purer particulate copper (10). The thermal disproportionation step in which nitrile is distilled to precipitate copper from the cuprous solution is as already described and the method of performing reaction 1 is described in detail later. The method is preferably continuous, with several cycles of copper leaching and copper precipitation taking place in reaction and precipitation vessels respectively. The two vessels are separated by conventional systems for removing solids from the electrolyte.
Suitable materials containing finely divided copper metal are efficiently agitated by conventional solidliquid leaching procedures (cf. Butts) in vessels sealed from the atmosphere, with a solution of a suitable cupric salt (preferably cupric sulphate) in a solvent which is kept acidic with a suitable acid throughout the process and which contains water and sufficient of a suitable organic nitrile, preferably acetonitrile. The preferred temperature of leaching is 4065C, but this is not crucial, although leaching is faster at elevated temperatures. The copper and some other oxidisable and basic materials (e.g. iron, tin, copper oxide, arsenic, nickel and to some extent bismuth) dissolve, to give cuprous solutions, plus soluble impurities. Other impurities, e.g. gold, silica, alumina, carbonaceous material, lead, and to a large extent antimony and silver, do not dissolve, they form slimes. These are filtered off at appropriate intervals. The cuprous solutions are separated from any solid material e.g. by filtration, settling, or centrifuging, and the organic nitrile is separated from the cuprous solution. Removal of volatile nitrile is continued until the desired amount of copper is obtained and separated, as outlined. The cupric salt is recycled to the leaching tank, after combining with the nitrile-water distillate, to dissolve more copper from the copper containing material and to generate more cuprous solution by reaction 1. The process is preferably continuous. Portions of the nitrile-deficient electrolyte are treated by conventional methods (cf. Butts loc. cit.) from time to time to keep soluble impurities, e.g. OX at an acceptable level, consistent with the desired purity of the precipitated copper.
Copper powder refining can of course be a batch process, using a reservoir of Cu* salt, as shown in FIG. 6, with removal of cupric solution and eventual reduction with copper, using the distillate or some other source of nitrile. Suitable heat exchangers are desirable, since in principle there is very little consumption of energy.-
Oxygen should be excluded as much as possible from the process, because both cuprous solutions and the wet copper powder are oxidised to some extent by oxygen. A reducing or non-oxidisingatmosphere is desirable (e.g. N Precautions against loss of the volatile, toxic and inflammable nitriles should be taken and the process is best suited to an enclosed system. The solutions are not particularly corrosive, except in the leaching tank, which should be resistant to the oxidising power of Cu in the presence of nitriles. Lead, glass and PVC are suitable materials, but stainless steel is not inert over long periods.
Cuprous Leaching This is closely related to copper powder refining, except that in the preferred method cuprous salt is regenerated from the cupric salt solution plus the nitrile azeotrope produced by thermal disproportionation, by reacting Cu with cuprous sulphide, or materials containing cuprous and sulphide ions or with cupric sulphide, or materials containing cupric and sulphide ions, in a leaching vessel under the conditions of reaction 3 or or of the combined reaction of 3 and 5 of FIG. 6 respectively. The resulting cuprous salt solution is filtered, freed of sulphur, purified and is thermally disproportionated.
The net result is either Cu S Cu +CuS CU S CuS Cu S so that effectively copper sulphides are converted to copper and sulphur via cuprous ions. The reactions are equilibra so that continuous removal of cuprous salt and sulphur and replacement with fresh cupric sulphate solutions is a desirable feature.
Cuprous leaching has the advantage that, unlike smelting of copper sulphides, sulphur dioxide is not produced thus minimising pollution and the cuprous salt solution produced by the leach is readily converted to copper. In principle no chemicals are consumed. The method is simpler than ferric sulphate leaching followed by electrowinning in aqueous solution.
As noted later, reactions 3 and 5 and the combined reaction require quite high concentrations of acid and of volatile nitrile. Chalcopyrite is not leached effectively by this procedure but if it is preheated with sulphur to more than about 400C a leachable form of copper sulphide is produced.
Much the same scheme as is illustrated in FIG. 5 is preferably used for continuous cuprous leaching with the reaction vessel suitably designed for operation of reactions 3 or 5 or the combined reactions.
Methods of Preparing Solutions of Cuprous Salts The processes of electrochemical and thermal disproportionation depend on having suitable solutions of cuprous salts, preferably of what we believe to be cuprous sulphate or CuI-ISO, in the first process and Cu S0 CuHSO CuNO or CuHSO in the second.
Cuprous salts, such as cuprous sulphate, are not stable in aqueous solutions at useful concentrations and cuprous copper only exists in completely aqueous solution at a useful concentration when complexed with suitable bases, as in CuClf, Cu(NH and Cu(CN) The chemistry of monovalent copper is outlined in Butts (loc.cit.). We believe that none of the processes described here which are based on cuprous chemistry in nitrile-water mixtures are possible in purely aqueous solvents.
We have found that in the presence of an organic nitrile, solutions of cuprous salts, e.g. those believed to be Cu SO CuClO CuNO CuHSO and Cul-ISO are stable, even in highly aqueous solutions. In general,
more than 2.5 moles of the nitrile per mole of stabilized cuprous ions, are needed for stability. The role of the nitrile is believed to be that of an uncharged complexing agent which strongly solvates cuprous ion and stabilizes it with respect to its disproportionation into copper and cupric cation, even in acid solution. Suitable organic nitriles (e.g. acetonitrile) appear to solvate cuprous ion more powerfully than does water, even in highly aqueous solutions.
We have found that the following reactions are practicable in solvents containing a suitable organic nitrile and usually water,- but sometimes in anhydrous nitriles, to give stable solutions of what we believe to be cuprous bisulphate or sulphate, or in some cases certain other cuprous salts. A requirement is that the anion accompanying the cuprous cation (e.g. sulphate, perchlorate, bisulphite, BF,) generally be inert". That is, it should not interfere with the general chemistry of cuprous ion and copper as outlined in this application, and as required by the processes, (e.g. concentrated nitric acid dissolves copper and oxidises Cu", and CuI-I- SO is unstable in strongly acidic solutions, so nitrates and sulphites are generally unsuitable in processes e.g. electrochemical disproportionation, preferably requiring strongly acid conditions, but restricted forms of thermal disproportionation, preferably at relatively high pH, are possible in the presence of these anions). Relevant anions are described later in this application. Sulphates or bisulphates are preferred over all other salts for the reactions shown, partly for economic reasons, but in some cases the greater solubility of copper nitrate salts is an advantage.
The reactions which produce solutions of cuprous salts are believed to proceed as shown by the following equations but the complete stoichiometry has not been established in all cases. In many cases the solutions should be filtered, their proportions adjusted to a suitable composition and by-products of the reactions partly removed before transfer to the central storage" of cuprous salt solution, which is shown in FIG. 6 and which feeds the thermal or electrochemical disproportionation systems.
In FIG. 6 the numerals refer to the following:
1 supplies of appropriate materials containing copper or copper salts including copper sulphides and ores, 2 reaction vessesl in which reactions 1 13 take place, to produce solutions of cuprous salts in acid solutions of water and organic nitriles, 3 systems for separating solids from these solutions, 4 systems for adjusting electrolyte composition and purifying it as needed, 5 removal of insolubles from the reactions 1 13, 6 removal of soluble impurities form reactions 1 l3, separated as in 4, 7 vessel for storing cupric salt solutions, 8 vessel for storing cuprous salt solutions, 9 cell for electrochemical disproportionation of cuprous sulphate solutions, 10 vessel for thermal disproportionation of cuprous salts in acidified aqueous solutions containing volatile organic nitriles including source of heat or method of reducing the pressure, 11 removal of volatile nitrile as distillate and condensation, 12 solution of a cupric salt in acidified water containing an organic nitrile, 13 solution of a cuprous salt in an acidified aqueous solution containing an organic nitrile with or without additives, 14 an inert anode attached to a power supply, 15 a copper cathode starting sheet attached to the power supply, 16 system for washing and drying copper, 17
removed copper cathodes and 18 copper powder. Methods of producing solutions of cuprous salts in nitrile water mixtures.
The following equations are believed to represent the stoichiometry and other products in reactions which do provide methods of preparing cuprous solutions, when the reactants shown are mixed in solutions of water and organic nitriles. Species shown above the reaction arrow are believed to be catalysts. The equilibria shown depend among other things on the proportion of organic nitrile present.
2. 2Cu /20, 2n+ c uf+ 2cu H 3. Cu S cu 512cm CuS 4. 2Cu S 2H /20 2 cm H O ZCuS 5. CuS 01 gfz cu +,s
Z H? V1. 2. QETZQW +111 2 8. M 2Cu 2Cu M (M Fe, Ni, Zn, Cd,
ll. Cu S 2H* H 2Cu in anhydrous MeCN) l2. 3CuO 350 H O CuSO 2CuHSO (or CuCO Cu(OH) for CuO).
Reaction l Cu +Cu 2Cu To our knowledge, cuprous sulphate solutions have not been prepared from materials containing copper in aqueous solution. However equivalent reactions do proceed in water in the presence of certain complexing anions (e.g. CN', Cl). However the resulting cuprous complexes are not suitable for thermal disproportionation. The reaction of dry cupric perchlorate with pure copper to give CuClOi solutions in anhydrous acetonitfii'i's"a'kfififiactidn," but such solutions are not satisfactory for thermal or electrochemical disproportionation. Until our discovery, there was no reason to believe that this reaction could be extended to reactions of other cupric salts, especially of cupric sulphate, with impure copper, in acidified solvents containing a variety of nitriles and very significant proportions of water. Indeed, before our discovery we had expected that water would be the active solvent and would disproportionate cuprous salts, when added for example to anhydrous solutions of cuprous perchlorate in acetonitrile, prepared as described in the literature. We were also unable to extrapolate to the behaviour of other salts in water-nitrile mixtures from the behaviour of perchlorates in anhydrous acetonitrile, partly because for example cupric sulphate will not satisfactorily generate solutions containing significant amounts of cuprous sulphate when reacted with copper in anhydrous acetonitrile.
Before this work, the behaviour of impurities commonly present in impure copper (e.g., iron, nickel, sil- 6O mony. Previously copper was leached as cupric salts using oxidising acids.
The following description of the preferred method of preparing solutions of what we believe to be cuprous sulphate or CuHS Q, substantially free of cupric sulphate illustrates the method.
Mix equimolar or less than equimolar proportions of cupric sulphate, and finely divided copper (minus 16 plus mesh) in a reaction vessel with a solvent containing about twelve moles of a suitable organic nitrile per mole of cupricions, plus sufficient water to dissolve the resulting cuprous sulphate or bisulphate. The solution is kept acidic with a proportion of sulphuric acid which is appropriate to the particular process, preferably between 0.5 and 10% by volume H2SO4. It is well agitated, e.g. by magnetic stirring or tumbling, preferably at a temperature of between 40 and 60C in the absence of oxygen. Higher temperatures encourage loss of nitrile by hydrolysis, lower temperatures give slower dissolution of copper. The blue cupric sulphate solution changes to colourless as the copper dissolves. As expected, the rate of copper dissolution is enhanced by elevated temperatures, by increasingly finely divided copper and by the increased efficiency of mixing the solid and liquid phases. If necessary, the solution is filtered to remove insoluble material such as lead, gold, antimony, silver, silica, carbon, unreacted copper and alumina. If the solution develops a significan blue colour on standing in a stoppered vessel, then it is being oxidised, or lacks sufficient organic nitrile and more should be added together with a little copper to remove the blue colour. A careful monitoring to assure acidity is desirable for solutions originally low in acid (ca. pH It is desirable to exclude oxygen as much as possible, because it oxidises cuprous salts and generates base and cupricion. This, together with the fact that many of the nitriles are volatile, toxic and inflammable, makes it desirable to operate in sealed reaction vessels whenever possible. Reaction vessels should be resistant to the solutions. Lead, glass, and PVC are some of the suitable materials for reaction vessels.
If finely divided materials containing copper plus impurities are used to prepare the cuprous solutions by this method, we have found that readily oxidisable materials, such as iron, nickel, tin, arsenic, cadmium and zinc, which are commonly present in impure copper, also reduce the cupric sulphate under the reaction con ditions and themselves dissolve, presumably as sulphates. Other materials, such as gold, lead, silica, carbonaceous material, and to a large extent antimony, bismuth and silver, which are commonly found in impure copper, were found not to dissolve in the electrolyte under the reaction conditions. To a certain extent, cuprous oxide and copper sulphides associated with the copper, dissolve in some compositions to give cuprous sulphate solutions as outlined for following reactions.
A process of forming cuprous sulphate solutions from materials containing copper has been described. It is now obvious to those skilled in chemistry that we are dealing with the chemistry of Cu, Cu and Cu, so that some other cupric salts of inert non-complexing anions, e.g. CU(Cl 4)g (hazardous) and (."u(NO,-,) (at controlled pH), other than cupric sulphate, could be used, even if only in a restricted way, to generate certain cuprous salt solutions from copper in the nitrile-water mixtures. However we believe that sulphates are the most suitable salts for the process, particularly for economic reasons. The question of suitable salts is considered in more detail below, but nitrates in solutions which are less than 0.3, more preferably less than 0.2 molar in l-l are next preferred to sulphates for reaction 1.
A modification of reaction .1 is reaction 2 in which less than stoichiometric amounts of cupric salt are used as reagent, but cupric salt is regenerated by the action of oxygen on the product cuprous salt in acidified solution. The regenerated cupric salt converts more copper to cuprous salt. The net result is a reaction which appe ars to be the reaction of copper with sulphuric acid in the presence of oxygen, with cupric salt as catalyst:
The rate of supply and the pressure of oxygen or air is preferably controlled so that excess oxygen does not oxidise more cuprous to cupric sulphate than is required by the stoichiometry of the reaction shown. Thus cuprous solutions are generated.
The method of dissolving copper is as before, but preferably at lower temperature, (2030C) with provision for good mixing of air with the composition and solids, and for admission of air and acid, as oxygen and acid are consumed. The composition contains less, preferably l-20% of the concentration of cupric sulphate required by the stoichiometry of reaction 1.
The oxidising method has the disadvantage that acid is consumed and it is slow except when the oxygen or air is under pressure, but it is a very useful way of initiating both reaction 1 and subsequent copper powder refining or copper powder electrorefining processes, especially if supplies of cupric sulphate are not available for starting the process. if operated to a limited extent it helps to control copper concentrations in copper powder refining and copper powder electrorefining. Methods of producing solutions of cupro us from cupric salts using reducing agents (reactions 5-9 and 12).
Cupric salts are conventionally reduced (cf. Butts) to copper in aqueous solution by processes known as cementation (e.g. with iron) or precipitation (e.g. with hydrogen). Reduction with $0 in water is inefficient and requires extreme conditions of temperature and pressure. Cement copper is usually a very crude material, heavily contaminated with iron and well oxidised. Cementation with nickel or cadmium for example is used to purify solutions of their metal salts which are contaminated with cupric salts.
Our new methods of reducing cupric to cuprous solutions in acidic mixtures of water with organic nitriles have no direct precedent, to our knowledge. They have the advantage that they produce solutions which are suitable for thermal or electrochemical disproportionation. In some cases they also provide new leaching methods.
Reactions 7 and 8 2Cu 2CU+ 'l' l 2H 7 2 Cu M 2Cu M (8 Stable solutions of cuprous salts may be prepared by the following method in which cupric salts are reduced with the appropriate proportion of iron, zinc, nickel, tin or cadmium, or with sulphur dioxide or sodium metabisulphite, in aqueous solutions containing an organic nitrile. Iron and $0 at normal temperature and pressure are the preferred reductants, but in the case of S02,
the "solid reductant. Reduction with nickel is p'articu larly effective.
The reactions are illustrated by the best method known to us for preparing cuprous from cupric sulphate solutions by reduction, on a laboratory scale.
To an aqueous solution of cupric sulphate containing a little sulphuric acid is added sufficient of a suitable organic nitrile. If cupric sulphate precipitates on addition of the nitrile it will redissolve as reduction proceeds and cuprous sulphate solution is formed. The solution is stirred or shaken with a suitable reducing agent in the proportion of one equivalent of reducing agent per mole of cupric sulphate for those reducing agents (e.g., iron, zinc, nickel or cadmium) which would produce cement copper if present in higher proportions. For less powerful reducing agents, e.g. S0 at atmospheric pressure and temperature, which will reduce cupric sulphate to cuprous salts but not cuprous salts to copper, an excess of reducing agent maybe used. It is desirable to remove the acid, e.g. by having sufficient of a weak base (e. g. CaCO or ammonium acetate) prese n tto complete the reduction, when S0 is the reductant. Alternatively, reduction of cupric sulphate to cuprous salts by S0; proceeds tolerably at moderate pressures (e.g. 5 atmospheres) and temperatures (of 50l0OC) in the absence of bases, at an initial pH of between 2-6, in the nitrile-water mixtures.
The reduction is complete when the blue colour of cupric sulphate has disappeared, consistent with any colour of the reducing agent or its oxidation product. e.g. cuprous bisulphite is yellow in the composition and NiSO is green.
Rates of reduction increase with increasing temperature, but an upper limit is imposed by hydrolysis of the nitrile (significant at C) and the volatility of the nitrile (e.g. acetonitrile-water azeotrope at 78C).
If the reducing agent (e.g. iron, zinc, cadmium or nickel) is a solid, the reduction is faster the more finely divided the solid and the more efficient the stirring or shaking. Solids of minus 16 mesh are preferred in well stirred solutions. However even with bulky iron (e.g. iron nails) the reduction is much faster than is aqueous cementation with the same reductant. If the reducing agent (e.g.SOZ) is a gas, reaction is faster and more efficient under increased pressure and lower pH values are possible, but the advantage of simply bubbling the reductant through a buffered solution at atmospheric pressure, makes this the preferred method, on a laboratory scale, when the reductant is 50;.
Precautions against volatile, toxic and inflammable nitriles are desirable, so that equipment should be sealed wherever possible, or should include condens-- ers. The oxidation products of the reductants, e.g. FeSO H250 NiSO4, build up in the solution and may have to be removed from the cuprous solution,
depending on the subsequent application of the solution. Removal can be done by known and conventional methods, e.g. of precipitation, neutralization, ion-exchange, etc., to the extent demanded by the subsequent process and the purity of copper required.
The processes are illustrated in detail by the examples given later.
The reduction reactions 8 can be utilized in the following new methods for producing copper from cupric sulphate and for leaching nickel, cadmium or cobalt values from materials containing one or more of these metals.
The conventional process most closely related to the new methods is cementation of aqueous cupric sulphate solutions with nickel, cadmium or cobalt to give copper and solutions of NiSOh CdSOi or C0804. This cementation is difficult with many forms of nickel and requires specially precipitated or sponge nickel. It also suffers from the disadvantage that the cementing agent becomes coated with copper powder, so that cementation is slowed and reduced in efficiency. The new methods provide a much faster method of dissolving up the metals and of reducing the cupric solution.
The method is based on a combination of the reduction reactions 8 and the processes of thermal disproportionation of cuprous sulphate solutions. The preferred form of the process is summarized in the following reaction sequence and if more than 20% by volume of organic nitrile is present, is particularly suitable for the treatment of high grade mattes containing nickel and copper which are of low sulphur content. In such cases those parts of the matte which are metal sulphides are oxidised to sulphur and the cupric sulphate is reduced to cuprous sulphate.
An excess of cupric sulphate dissolved in weakly acidified water-sulphuric acid, plus sufficient of a suitable volatile organic nitrile, preferably acetonitrile, to stabilize the eventual proportion of Cu SO is agitated with preferably finely divided (-l6+240 mesh) material containing nickel and/or cobalt and/or cadmium (M). Sufficient cupric sulphate is present to maintain some cupric ion in solution at completion of reaction. The leaching and subsequent steps, as far as possible,
Leaching reaction Solid liquid separation are preferably carried out in sealed vessels at elevated temperatures (ca. 40-65 C) in the absence of oxygen. The cuprous salt solution is separated from solids and is thermally disproportionated, as herein described, to give copper, and solutions of cupric sulphate and the other metal sulphates. The solutions are reconstituted, preferably with distilled nitrile, and more cupric sulphate is added. This solution is recycled with fresh material containing nickel, and/or cobalt, and/or cadmium until a stage is reached after n cycles, where the nitriledepleted solution from the thermal disproportionation contains nMSO, CuSO A preferable procedure then is to add sufficient powdered active metal M (e.g. sponge nickel) to the nitrile depleted solution of nMSO, CuSO, to cement the copper, as in conventional aqueous cementation of cupric sulphate by cadmium, nickel or cobalt. The resulting solution of (n+1 )MS(), in an almost aqueous medium is suitable for processing to recover M. Alternatively, cupric sulphide can be precipitated by treating the solution with H 8 for M Ni or C0, or Cd in strong acid solutions to remove copper.
As in reaction 2, which is a modification of reaction 1, an alternative but slower preferred procedure is to carry out the leaching step in the presence of less than the stoichiometric two moles of cupric sulphate, per mole of metal M, but to admit sufficient oxygen and sulphuric acid to the composition to regenerate cupric from cuprous sulphate, so that the cupric sulphate is effectively a catalyst and the reaction becomes M 2H /2O Q3 M +H2O 14 A concentration of between 0.5 4% cupric ion is preferred, but if copper metal is with M, then less cupric sulphate can be used. At appropriate stages of the leach, the solution contains M and Cu sulphates, the oxygen supply is cut off and more M is added to reduce the Cu to cuprous sulphate. This is thermally disproportionated to give copper and cupric sulphate. The copper is filtered off. The solutions are reconstituted and the leach continues, with the cupric concentration halved. Thus control of the copper content during leaching can be achieved.
X MSOr, Cu SOL,
Thermal disproportionation cu uso, Ms0..
Add CuSO.,, reconstitute electrolyte if necessary Recycle with more X M After n cycles Cement with powdered "active" M (or treat with H 8 to form CuS in place of Cu) solid-liquid separation Ni, Co, Cd
= insoluble impurities X ZMSOr, Cu SOI,
nMSOr, CuSO from thermal disproportionation to hydrometallurgical treatment (The reaction sequence of obtaining copper from cupric sulphate and for leaching nickel, cobalt, sulphates (cadmium) from the metals in a non-oxidising atmosphere illustrates a preferred method, using compositions and reactions already described).
A modification of the reduction of cupric salts to cuprous salts by S provides a new method for extracting cuprous salts from basic oxidised copper, e.g. CuO, Tenorite, Malachite, Azurite, and Chrysocolla, using reaction 12, with sulphur dioxide in water containing sufficient of a suitable organic nitrile. conventionally (cf. Butts) oxidised ores are leached with sulphuric acid and copper is cemented with iron or is electrowon from the cupric solution. Such methods are costly and inefficient. In the new process, cheap S0 acts as a reducing agent for Cu and produces acid, which is consumed by leaching the basic copper, so that the process continues. The reactions take place at atmospheric pressure and at normal temperatures and are believed to be represented by the following equations, which use CuO as an example of basic oxidized copper,
3CuO+ 3SO H O ZCuHSO CuSO, (12).
The method consists of mixing the basic oxidised copper in an atmosphere of sulphur dioxide with a composition composed of sufficient of a suitable organic nitrile and water, preferably in a leaching vessel, which is sealed, and leaching by conventional leaching methods for extracting copper values from materials containing copper with solid, liquid and gaseous reactants.
The solution of CuHSO and CuSO, is separated from solids, then copper is recovered for example by thermal disproportionation. Under some conditions, cuprous sulphite is precipitated with the copper, but if washed with dilute aqueous sulphuric acid, this disproportionates to copper and cupric salt solutions so that copper can be separated. Sulphur dioxide reduction is also possible to give a homogenous solution if the copper ammonium salt solutions from an ammoniacal leach, e.g. of copper sulphides, are mixed with an organic nitrile and the solution is saturated with sulphur dioxide until the blue colour disappears.
Another method of producing cuprous solutions and of leaching copper values from cupric sulphide or materials containing cupric sulphide uses reaction 5, or 6, in which cupric sulphate is thought to be reduced by sulphide ion, or hydrogen sulphide, which is in equilibrium with cupric sulphide in acid solutions containing water and sufficient of a suitable organic nitrile. The reaction is similar to the reduction of ferric salts in aqueous solution by cupric sulphide, with production of sulphur, except that here, cupric salts are reduced. The reactions proceed at useful rates at elevated temperatures and are believed to be represented by the following equilibria, although cuprous ions may react with sulphur to give cuprous sulphide, not cupric sulphide in the reverse reac ions Both cuprous salt solutions and sulphur were isolated, but other oxidation products of sulphur and cupric ions may also be formed, especially if oxygen is used under pressure in reaction 6.
A preferred method for preparing cuprous solutions by reaction uses conventional solid-liquid leaching procedures in sealed systems at elevated temperatures,
with finely divided (l00 mesh) cupric sulphide, and an excess of CuSO, in 5 to 10% by volume of H 80 water and between 35 60% by volume of a suitable nitrile at 50 C in a non-oxidising atmosphere. Too much CuSO, leads to separation of nitrile and water, so it is sometimes useful to add CuSO, as reaction proceeds. Reaction is continued until the copper concentration reaches a steady value. The cuprous sulphate solution is separated from sulphur and other solids, including unleached material, and is stored in a sealed vessel and the leach is continued with fresh cupric sulphate solution. Preferred nitriles are acetonitrile and 2-hydroxycyanoethane.
A modification of this method is reaction 6, where the preferred procedure is much the same, except that less than the stoichiometric amount of cupric sulphate is used, and oxygen is admitted to regenerate cupric ions from cuprous ions. Provision is made to add more acid and oxygen as reaction proceeds, but the supply of oxygen is preferably controlled so that the stoichiometry of equation 6 is preserved and a final solution of cuprous sulphate, rather than cupric sulphate, is formed. As in reaction 2, the cupric sulphate is a catalyst and preferably between i 20% of that required by the stoichiometry of equation 5 is used. Good mixing of the solid, liquid and gas phase is highly desirable. It is believed that reaction 6 may not be the only representation of reactions giving cooper in solution by this method. Some oxidation of sulphide by oxygen to other sulphur species and of Cu to Cu may take place, especially with oxygen under pressure.
The oxidising method has the disadvantage that acid is consumed, but is useful when adequate supplies of cupric sulphate are not available and equilibrium can be forced further to the right as cuprous ions are oxidised. Further notes on reactions 5 and 6 are included with the discussion of reaction 3 and 4.
Reduction with silver provides a method both for leaching silver from materials containing silver and for reducing cupric to cuprous salt solutions. The silver can readily be recovered as illustrated later. The method uses reaction 9. The equilibrium requires a relatively high proportion of organic nitrile in the water if it is to proceed to a useful extent to the right. A mixture of 0.05M cuprous and silver sulphates in solutions low in acetonitrile (ca. 20% by volume) is rapidly converted to cupric sulphate and silver by the reverse reaction.
In the preferred method, finely divided silver (l6 mesh) or finely divided materials containing silver, such as dore metal, lead-silver mixtures and cement silver, are shaken with an excess of cupric sulphate or cupric nitrate in a solution preferably 35 to 60% by volume acetonitrile, and acidified water. In the case of cupric nitrate, the acidity should be kept low 0.2M in H) to prevent nitric acid oxidising cuprous ion. Shaking is continued until the silver content of the solution reaches a constant value or is done in more than one extraction with fresh cupric salt, if optimum extraction is desired. The resulting solution is filtered to remove solids to give a mixture of silver, cuprous and cupric' salts. As with most solid-liquid reactions, reaction 9 isfaster at elevated temperatures, with more finely divided material containing silver and with more effective agitation. Reduction of the cupric solution is effective at room temperature in well stirred solutions, with most silver-containing materials, between minus 16 plus 240 mesh.
Oxidation by silver cations (reaction 13) Particulate copper is stirred with an equimolar proportion of a suitable silver salt, preferably silver sulphate, in sufficient of a suitable organic nitrile, and acidified water. Silver is cemented and a colourless cuprous salt solution is produced.
Acid-base reactions (reaction Cuprous oxide dissolves in aqueous sulphuric acid to give cupric sulphate, but an equal amount of copper is also formed, presumably because any cuprous sulphate formed, immediately disproportionates in water. We believe that stable cuprous sulphate solutions have not been prepared by this method in purely aqueous solvents.
Cuprous sulphate or bisulphate solutions, suitable for cuprous winning, cuprous electrowinning, powder electrorefining, and cuprous electrorefining may be prepared by a new method, a preferred form of which follows. The reaction provides an excellent method of pickling i.e. of removing cuprous oxide films from case copper or hot rolled or copper rod, to give a material which is free of copper powder and thus more suitable for wire drawing, unlike copper pickled in aqueous sulphuric acid. The solvent is prepared by mixing water with at least sufficient of a suitable organic nitrile to stabilize the eventual concentration of cuprous salt.
Sufficient sulphuric acid to satisfy the stoichiometry of the above equation and to provide at least a slight excess of acid, is added carefully with stirring, and the euprous oxide, is stirred or shaken with the acidic solvent, preferably in a non-oxidising atmosphere in a reaction vessel. On a large scale, the conventional leaching methods for reaction of sulphuric acid with oxidised copper (cf. Butts loc. cit.) could be used, except that steps should be taken to contain volatile, toxic and inflammable nitriles and to minimise oxidation, so that sealed leaching vessels are preferred. The solution is separated from any solids and concentrated or diluted if necessary, to provide desired compositions. It is stored in sealed vessels, in a non-oxidising atmosphere.
In cuprous pickling the oxidised copper is washed in the acidified water containing the organic nitrile and is then preferably washed in a conventional aqueous pickling bath.
The resulting solution should be colourless or very pale blue, but if it is significantly blue it is preferably decolourised by addition of a little copper powder. If the blue colour persists or returns, more organic nitrile can be added. The rate of dissolution of the cuprous oxide is enhanced by efficient stirring and by having finely divided solids. It is greater at higher temperatures, but an economic upper limit is imposed by bydrolysis of the nitrile in the highly acidic solution. A preferred temperature range is 30- 50C.
The reaction has been extended to ores such as cuprite and materials containing basic cuprous salts other than cuprous oxide, e.g. CuOH, Cu CO and basic cuprous sulphate.
Dissolution of cuprous sulphide (reaction 11) Cuprous sulphide is insoluble in aqueous sulphuric acid but is soluble in freshly acidified dry acetonitrile. This reaction has more limited application than the dissolution of cuprous oxide, because it is carried out in anhydrous acetonitrile rather than in water-nitrile mixtures and the acetonitrile is sometimes polymerised by the anhydrous acid. It is best suited to small scale reactrons.
Differential precipitation exchange of cupric and cuprous sulphides (reactions 3 and 4) The reaction initially is thought to be as in reaction 3 and provides a new method of leaching copper values from materials containing cuprous and sulphide ions and of preparing cuprous solutions.
Solutions of cuprous sulphate, suitable for thermal or electrochemical disproportionation, may be prepared by this reaction. The reaction occurs also to varying extents with copper mattes, chalcocite, bornite and related materials containing cuprous ions and sulphide ions. It is very slow and inefficient with chalcopyrite and pretreatment of chalcopyrite, e.g. with sulphur, is necessary for a satisfactory leach.
Cuprous sulphide has a much lower solubility product than cupric sulphide in acidified water, so that the equilibrium shown lies to the left in water. Thus cupric sulphate will not generate solutions containing significant amounts of cuprous sulphate when added to cuprous sulphide suspended in acidified water.
We have found that in water containing sufficient (preferably 30 60% by volume) of suitable organic nitriles in moderately strong sulphuric acid, equilibrium is rapidly established even at room temperature, such that significant amounts of cuprous sulphate or bisulphate are produced. Indeed more copper ions are generated in solution than is expected from the stoichiometry of the above equation when cuprous sulphide is heated at 50 with excess cupric sulphate containing 10% by volume sulphuric acid in 50% by volume acetonitrile and water. It is thought that the aforementioned redox reaction (5) to produce sulphur, also takes place in the acidified aqueous nitrile solution during the reac tion and that this increases the concentration of cuprous ions above that expected from the stoichiometry of the primary reaction. Thus cuprous sulphide and related materials can be oxidised to sulphur and solutions of cuprous sulphate by the combined equilibria 3 and 5.
Reaction rates are enhanced by having finely divided material (e.g. 100 mesh) containing cuprous sulphide, by having high concentrations of sulphuric acid, by having well stirred solutions and elevated temperatures and by having increased proportions of nitrile. Hydrolysis of the nitrile imposes economic limitations on the strength of acid and the temperature chosen. A preferred method of carrying out reaction 3, followed in part by reaction 5, is as follows:
Finely divided cuprous sulphide (about l00 mesh) or materials containing cuprous ions and sulphide ions, such as copper matte, bornite, chalcocite, djurleite and digenite are well stirred or shaken, preferably at 50 C in a sealed vessel in the absence of oxygen with an excess of cupric sulphate in a solvent consisting preferably of 30 60% by volume but not less than 20% by volume of a suitable nitrile (preferably acetonitrile or Z-hydroxy cyanoethane), water, and sufficient sulphuric acid to give an acceptable rate of formation of cuprous solution, but not so much that hydrolysis of the nitrile at the necessarily elevated temperatures is economically unacceptable. We prefer between 5 and 10% sulphuric acid by volume. The process is continued until the copper in solution approaches a steady value (i.e. complete leaching or equilibrium). The total volume of solvent should be sufficient to dissolve the final concentration of cuprous sulphate or bisulphate. We find, for example, that a solution containing gms of Cu /litre, presumably as sulphate or bisulphate, in acidified 50% acetonitrile-water at 50C is homogeneous. It is not essential that all the cupric sulphate be dissolved during the initial stages of the exchange reaction but if too much is present, the solution separates into two layers, so addition of CuSO, as reaction proceeds is preferred. After completion of reaction, the solution is filtered free of cupric sulphide, sulphur and other solids and is stored in a sealed vessel in a non-oxidising atmosphere. Higher yields are obtained if the solids are repeatedly leached with fresh cupric sulphate solution, because reaction 3 is an equilibrium.
An alternative procedure is to carry out reaction 3 as specified, but in the presence of oxygen or air at l atmosphere or above, and preferably less than the stoichiometric amount of cup'ric sulphate, with provision for the addition of more acid as required. The oxygen slowly oxidises the product cuprous sulphate to cupric sulphate so' that the cupric sulphate is effectively a catalyst and reaction 3 becomes 4. The amount of oxygen or air preferably is controlled at various stages of the leach so that cuprous, rather than cupric sulphate is formed, in accord with the stoichiometry of reaction 4.
The preferable amount of cupric sulphate used is such that it is more than 0.5% by weight- Cu for most of the leaching process, because lower concentrations lead to slower reactions. Reaction is preferably continued until the concentration of copper in solution approaches a steady value. The solution is filtered fre of cupric sulphide and other solids and is stored in a sealed vessel in a non-oxidising atmosphere.
It is desirable to have good mixing between the solid, liquid and gas phases and the cuprous sulphide materials should be finely divided, preferably l mesh.
As noted, reaction and 6 are often consecutive with reactions 3 and 4, to consume CuS and generate more copper in solution, and further notes on the combined equilibria, which is effectively the redox reaction, Cu S 2Cu 4Cu S, follow.
Further notes on Equilibria 3 and S and'the combined reactions The following reactions represent equilibria which are strongly influenced by the proportion of organic nitrile present in partly aqueous acidified solutions.
Cu S Cu CuS 2Cu (3) CuS Cu F 2Cu S (5) Overall: Cu S 2Cu F 4Cu S (3) (5) ln purely aqueous solution the equilibria lie so far to the left that the reactions are of no value for leaching copper values from materials containing copper and sulphur ions, but in the presence of sufficient ofa suitable organic nitrile in acidified solution, the reactions proceed at a convenient rate and lie sufficiently to the right so that copper values can be extracted from many materials containing copper and sulphur ions, such as bornite, chalcocite, copper matter, cupric sulphide and digenite.
The relation between the equilibrium constants for the overall reaction and the proportion of nitrile is such that a proportion of between 30-60% by volume of acetonitrile is a preferred part of the composition from the thermodynamic viewpoint. Acid appears to lead to a faster reaction, as do elevated temperatures. In low proportions of acetonitrile, equilibrium is established less rapidly, so kinetic aspects are important. More copper is extracted if cuprous ion is removed as it is formed, e.g. by thermal or electrochemical disproportionation of the cuprous solution, or by its oxidation to becomes effectively a leach of copper sulphides by sul- V V 7 Z 7 phuric acid and oxygen, catalysed by cupric ions as shown in the equations 2+ e.g. Cu S 4I-l 0 i" 2Cu S ZH O (4) In these circumstances the reaction proceeds to completion.
In practice, reactions 4 and 6 are slower than reactions 3 or 5 respectively, so that some cupric sulphide is formed and the first 50% of copper available from cuprous sulphide is leached more rapidly than the final 50%.
The combined reactions are not suitable for efficient leaching of chalcopyrite, but if the chalcopyrite is pretreated, for example by heating it with sulphur in a sealed vessel for several hours at 400500C, then a leachable form of copper sulphide and a virtually non leachable iron sulphide is formed.
Processing of silver via cuprous salts.
Preferred methods for separating silver and copper from materials containing silver and copper and for extracting and purifying silver materials containing silver, but no copper, are summarized in FIG. 7. The methods follow directly from the inventive concepts of reactions 1, 9 and 13 and of thermal disproportionation of cuprous salts, in nitrile-water mixture, as covered by this application. The methods are well suited to treatment of Dor metal, silver-lead-copper mixtures, and cement silver e.g. as obtained from the refining and smelting of non-ferrous base metals.
Silver at present is extracted and purified in a number of ways, including cyanidation, roasting and leaching, by cupellation, by melting with zinc or by electrorefining. These methods either consume chemicals or require equipment of relatively high capital cost. The new method is cheap and simple and has the advantage that extraction and formation of copper powder is a concurrent process. The preferred method follows from FIG. 7 which merely illustrates the sequence of steps and involves a combination of reactions whose procedure is discussed elsewhere in this application, several solid-liquid separations and two thermal disproportionation steps, as herein described, thus a volatile nitrile is necessary and we illustrate this by discussing acetonitrile.
FIG. 7 is a form of flow sheet in which the numerals refer to the following: 1 a source of particulate material containing copper, silver and impurities e.g. lead, gold and iron, 2 a solution of cupric sulphate, water, sulphuric acid and less than 35% by volume, preferably l5-25% by volume of a volatile organic nitrile, 3 a leaching vessel for reaction l, 4 separation of solids and liquids, 5 a solution of cuprous sulphate plus soluble impurities, 6 solid materials containing silver, 7 a solution of cupric sulphate, water, sulphuric acid and more than 35% by volume of a volatile organic nitrile, 8 a solution of cuprous sulphate and silver sulphate, 9 insoluble impurities e.g. lead, silica, l system for cementing silver with particulate copper, l1 metallic silver, 12 a solution of cuprous sulphate, 13 an apparatus for thermal disproportionation of cuprous sulphate solutions, 14 removal of nitrile distillate and condensation, l cupric sulphate in a nitrile depleted solution, 16 copper powder, 17 system for purifying electrolyte solution and adjusting its composition, 18 removal of soluble impurities removed from the electgrolyte in 17, 19 system for washing silver with part of the solutin numbered as 2 using solution bled off from 2.
The key feature of the preferred method is that cop per in finely divided materials containing copper, l in FIG. 7 is oxidised by certain cupric salts in acidified mixtures, 2, of water containing less than 35% acetonitrile by volume (i.e. reaction 1), whereas a rapidly decreasing proportion of silver is oxidised as the proportion of acetonitrile in the composition decrases (i.e. reaction 9). Thus a composition, preferably saturated, but not excluding suspensins of certain cupric salts, acidified water, preferably to by volume and necessarily less than by volume acetonitrile extracts copper, but very little silver, from a mixture containing copper and silver. ln compositions containing more than 35% by volume acetonitrile, acidified water and preferably saturated with, but not excluding suspensions of, cupric salt, an increasproprotion of silver is oxidised by Cu with increasing proportions of acetonitrile, once all the copper has been dissolved. Thus, in the preferred procedure, once copper has been leached froma copper-silver mixture with a composition containing ca. 15 to 25% acetonitrile by volume, as described and shown to point 6 in FIG. 7, silver is leached separately with a composition, 7, of excess cupric salt, acidified water, and preferably 35-a% by vol ume acetonitrile. The esulting Cu Cu, Ag solution is cemented, 10, with the appropriate proportion of copper powder to give silver. The cement silver is washed free of copper, step 19, with a dilute solution of cupric salt preferably in 15 to 25% acetonitrileacidified water. The thermal disproportionation steps shown as in 13 in FIG. 7 are as described previously.
Preferred salts are cupric sulphate in solutions acidified with sulphuric acid preferably at least 0.5% and more, preferably 2 to 5% by volume, or cupric nitrate, acidified with preferably 0.2 molar nitric acid. Higher concentrations of nitric acid cause oxidation of copper. Nitrates tend to be more soluble than sulphates, but sulphates are cheaper. Lead sulphates are insoluble whereas lead nitrate is much more soluble, which is a disadvantage for nitrates.
A preferred method for materials containing silver,
but no copper, is to leach directly with a composition of excess cupric salt, acidified waer and more than 35%, most preferably 45 to 605 by volume acetonitrile and continue the process as shown following Ag X, in FIG. 7. ln estimating the amount of cupric salt required for this process, it should be remembered that many other reducing agents, Fe, Ni, Cd, Zn, but not lead, in the presence of sulphates, reduce Cu more readily than does silver in nitrile-water mixtures. The presence of such species in the material containing silver and copper requires an appropriate proportion of cupric salt. Suitable and preferred materials for thermal and electrochemical disproportionation, and for reactions l-l 3.
The following expressions have been used in this application and are now explained. Suitable Organic Nitriles Acetonitrile, 2-hydroxy-cyanoethane and acrylonitrile are suitable organic nitriles and are preferred with the reservations as to their volatility, stability and homogeneity of compositions noted for certain processes. Throughout the description and claims of this application we have referred to organic nitriles, acrylonitrile and propionitrile while recognising that many organic nitriles, including acrylonitrile and propionitrile, are not miscible in all proportions with water. Most of our compositions and methods require homogenous solutions, so that it is to be understood that where necessary for homogeneity, the terms organic nitrile, propionitrile, and acrylonitrile include mixtures of these species with sufficient of a solubilizing agent, such as ethanol, to ensure homogeneity. The terms organic nitrile and volatile organic nitrile" include mixtures of organic nitriles and of volatile organic nitriles respectively.
Whether a nitrile other than acetonitrile, 2-hydroxycyanoethane, or acrylonitrile with solubilizing agent, is suitable or not for the purposes of this invention is decided by the following tests. Our use of the terms or ganic nitrile, nitriles, volatile organic nitrile and so on in the application refers only to those species of the general formula RCN which pass the following tests and to acetonitrile, acrylonitrile, propionitrile General tests for nitriles 1. (5 X MWt)/l3 gm of the nitrile under test is stirred with 65 gm of water containing 5 ml of sulphuric acid. The stoppered solution should remain homogeneous over a period of one hour at 50C.
la. If the solution is not homogeneous, up to 30 ml of ethanol is added. The stoppered solution should remain homogeneous for one hour at 50C. lf homogeneous solutions cannot be prepared by these procedures then the nitrile is deemed unsuitable.
2. To the homogeneous solutions, prepared either as in l or la, there is added 4 gm. of CuSO... 5H Oand this is stirred with 2 gm of copper powder (minus 100 mesh) for one hour at 50C. If the solution is still homogeneous, except for undissolved copper powder and has become colourless, very pale blue, or coloured with due regard for any coloured nitriles, then the nitrile has passed this test.
3. The successful solutions from test 2 are flushed with nitrogen, stoppered and warmed at 50 for 24 hr. If the solution is still homogeneous and colourless or very pale blue or coloured with due regard for any coloured nitriles and contains more than 1 l0 ppm of copper by atomic absorption, then the nitrile has passed this test.
Specific tests for nitriles.
Whether the nitrile is suitable for thermal disproportionation, e.g. as in winning or copper powder refining is further decided by its volatility.
4. The solutions from successful tests 3 are slowly distilled through an efficient distillation column. In some cases the distillation is of a water-nitrile, in others, e.g. propionitrile, of an ethanol-nitrile azeotrope. If more than of the nitrile can be distilled off below C, as measured for example by vpc of the distillate, and if some copper powder is precipitated on complete removal of the nitrile, then the nitrile is deemed suitable for thermal disproportionation.
Nitriles which pass tests 1-3, but which cannot be distilled below 90C in test 4, are deemed suitable only