US3755111A - Elimination of floating slime during electrolytic refining of copper - Google Patents

Abstract

A method for eliminating floating slime in the electrolytic refinement of copper. The method uses a novel electrolyte which contains specific quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony; the arsenic contents being obtained and maintained by direct addition of the substances to the solution, or by increasing the arsenic content in the anode.

Description

United States Patent [1 1 Lindstrom I ELIMINATION OF FLOATING SLIME DURING ELECTROLYTIC REFINING OF COPPER Inventor: Nils Folke Rune Lindstrom,

Skelleftehamn,-Sweden Assignee: Bollden Aktiebolag, Stockholm,

Sweden Filed: May 25, 1971 Appl. No.: 146,850

Foreign Application Priority Data May 28, 1970 Sweden 7380/70 US. Cl. 204/108, 204/293 Int. Cl C22d 1/16, BOlk 3/06 Field of Search 204/106, 108, 293

11] 3,755,111 [451 Aug.-28, 1973 References Cited 7 A Primary Examiner-John H. Mack Assistant Examiner-R. L.- Andrews An0rneyStevens, Davis, Miller and Mosher [57} ABSTRACT A method for eliminating floating slime in the electrolytic refinement of copper. The method uses a novel electrolyte which contains specific quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony; the arsenic contents being obtained and maintained by direct addition of the substances to the solution, or by increasing the arsenic content in the anode.

6 Claims, No Drawings 1 ELIMINATION OF FLOATING SLIME DURING ELECTROLYTIC REFINING OF COPPER The present invention relates to a method for eliminating floating slime when refining copper electrolytically with copper anodes which are contaminated with antimony and/or bismuth.

Copper refining by electrolysis is effected between an anode and a cathode in an electrolyte comprising an aqueous solution of copper sulphate, the copper content normally being 35-50 g/l, and in thevpresence of approximately 150-250 g/l sulphuric acid, which increases the electrical conductivity of the electrolyte. It is known that arsenic together with antimony and bismuth in solution forms so-called floating slime, which is deleterious to the current efficiency and the cathode quality.

The electrolysis is normally carried out at temperatures between 55 and 65C. At temperatures higher than 65C the amount of water evaporated from the electrolyte is extremely high, which ultimately results in higher heating costs. Moreover, at such high temperatures the cathode structure and therewith the quality of the cathode copper is impaired. At temperatures below 55C there is a risk of anode passivation.

The anode normally contains from 98 to 99.5% copper.

When the anode is dissolved electrolytically, metals. of more noble character than copper, such as silver, gold and the platina metals are insoluble and together with other insoluble impurities in the copper, such as selenides and tellurides, form a grayish-black to blackslime which is initially built up as a 0.5-1 cm thick coating on the anode surface but which gradually loosens and sinks to the bottom of the electrolytic cell. The layer of slime on the anode, so called anode slime, also contains finely divided, copper powder due to the fact that the anode contains oxygen. The oxygen in the anode copper is present, iner alia, as copper(l)oxide, Cu O. Copper(l)oxide is dissolved in the electrolyte according to the reaction Cu O H 80 CuSO H O Cu because at equilibrium Cu Cu 2Cu the reaction is considerably moved to the left. The copper powder formed is incorporated in the anode slime. Copper powder is also formed in the anode slime owing to the fact that the reaction Cu Cu 2e" is slower than the reaction Cu Cu +e. In this way a surplus of Ca ions is formed adjacent the anode surface, which ions, in accordance with the above equilibrium, are later disproportioned to Cu -ions and finely divided copper.

The lead present in the system presses primarily into solutions as Pb -ions, but is immediately precipitated as PbSO, and follows the anode slime.

The tin present in the system primarily passes into solution as Sn-ions, but is then precipitated as tin(lV)- hydroxide in gel form and also follows the anode slime.

The nickel and arsenic present in the system pass practically completely into solution.

The Ni -ions are not precipitated and the concentration of Ni* -ions in the electrolyte would increase if the electrolyte were not drained OE and replaced with freshly produced nickel free electrolyte. By removing a quantity of saturated electrolyte and replenishing with fresh electrolyte, the contamination of nickel in the cathode can be maintained at an acceptably low level, as this contamination normally being solely in the form of electrolyte inclusions. The drained electrolyte can be evaporated and the nickel sulphate and the copper sulphate can be recovered. Residual copper in the drained electrolyte is. then recovered by electrolysis using insoluble anodes. The presence of nickel-ions in the solution also reduces the solubility of the copper sulphate and lowers the electrical conductivity of the electrolyte.

The amount of arsenic contained by the anode normally exceeds the quantity required stoichiometrically for precipitating antimony and bismuth, and hence draining off the electrolyte also assists in regulating the arsenic content in the solution. Draining of the electrolyte also serves to maintain a constant content of copper in the electrolyte. For example, if no electrolyte were drained off, the copper content of the electrolyte would increase as a result of the aforementioned reaction and because oxygen dissolved in the electrolyte oxidizes the finely divided copper powder contained in the anode slime and the electrodes which consist of copper, whereupon copper(il)ions pass into solution. The fresh electrolyte which is used to replace the drained electrolyte should therefore be free from nickel and arsenic and have a low copper content. It is often an advantage to use an electrolyte which comprises solely diluted sulphuric acid. The copper content in the electrolyte can also be kept at a constant level by introducing insoluble anodes, for instance lead, in one or more cells. Thus, it is possible to reduce the copper content without increasing the drainage. In this way it is possible to keep a high arsenic content in the electrolyte without taking further measures. Soluble oxygen in the electrolyte also oxidizes the three valent arsenic and three valent antimony present to the'five valent state. It is presumed that Cu(l)ions catalize this oxidation.

Practically all the bismuth present passes into solution in trivalent form and is not oxidized by atmospheric oxygen dissolved in the electrolyte.

During the electrolytic process a drop in voltage is obtained owing to an ohmic resistance in the busbars, electrical contacts and the ohmic resistance in the electrolyte, together with so called activation polarisation, caused by the resistance transforming copper ions in the electrolyte to copper atoms in the metal lattice and vice versa, and so called concentration polarisation, caused by differences in ion concentration adjacent the electrodes in relation to the concentration in the main body of the electrolyte. Movement of the copper ions between the anode and the cathode takes place mainly through the process of convection and diffusion, while current transport ismainly effected with hydrogen ions. Thus, a thin layer of electrolyte is formed around the surfaces of the electrodes having a composition different to the main body of the electrolyte. Circulation in a conventional electrolytic cell cannot affect the composition of these films to any appreciable extent. The cathode flm becomes deplete of copper ions and slightly enriched with sulphuric acid, and consequently the density becomes lower than the density of the main body of the electrolyte, whereby the depleted electrolyte flows upwards along the cathode. In turn, the anode film is enriched with copper ions and slightly depleted of sulphuric acid and thereby obtains a greater density than that of the main body of electrolyte, and hence the anode film enriched with copper sulphate flows downwards along the anode. This behavior of the films causes an enrichment of copper sulphate in the lower parts of the electrolytic cell. To avoid an uneven result, this must be counteracted by some form of vertical circulation of the electrolyte through the cell. A vertical circulation can be achieved by removing the electrolyte either from the upper part of the cell and admitting electrolyte to the lower part or vice versa, at the same time as a certain horizontal circulation is effected by admitting the electrolyte to one end of the cell and removing it from the other. The differences in the concentration of the copper sulphate between the upper part and the lower part of the cell is more efl'ectively equilized with increasing circulation rate. On the other hand, at high circulation rates there is a risk of the anode slime swirling. A normal circulation rate is -20 l/min through an electrolysis tank containing 5,000 l.

The amount of energy consumed per unit of precipitated copper increases with increasing current density. The fact that with an increased current density it is possible to increase the production rate and obtain a more rapid run of material is naturally an economic advantage. In general, the consumption of energy has small economical significance in comparison with the capital costs, and therefore most plants operate with the highest possible current density, normally 200-270 A/m.

In order to obtain a high quality of the cathode copper, it is necessary to use different organic additives. Glue, thiocarbamides, goulac are normally used in this respect.

With technical electrolysis the size of the cathode and anode is normally selected on the basis of a compromise made between different economical and technical view points, and is generally approximately 1 X l m. In certain instances the electrolysis process is carried out with each electrode in series, whereby one side of the electrode serves as a cathode and the other side as an anode. As a rule, however, all cathodes and anodes of each cell are connected in parallel. The distance between the centre lines of the anodes is normally 9-13 cm and the cathodes are placed centrally therebetween.

With increasing distance between the electrodes the risk of falling slime from the anodes contaminating the cathodes decreases and, at the same time, a better cathode structure is obtained. On the other hand, plant costs increase with increased electrode spacing.

When producing electrolytic copper, a floating slime is normally formed which consists of an oxide complex of bismuth, arsenic and antimony in proportions which are dependent on the concentration of these substances in the electrolyte. This concentration depends primarily on the contents of said elements in the anodes. Floating slime shows very small tendency to settle, remains suspended in the electrolyte and impurities the cathode copper and causes growths to form on the cathode, which can result in short circuiting between the anode and the cathode and thereby reduce the current effeciency. Moreover, growths on the cathode act as settling surfaces for the anode slime.

Despite the fact that a large number of works have been published in which attempts have been made to establish the reasons why floating slime is formed, no one has hitherto been able to find the causal connection. Among others, Livshits and Pazukin (J. Applied Chemistry of the USSR, 27 (1954), page 283-292) have carried out a comprehensive examination and reached the conclusion that the only way in which floating slime can be avoided is to reduce the antimony content in the anodes to an extend such that the content of antimony in the electrolyte lies beneath 0.5 g/l. G. Graf and A. Lange also discuss in Neue Hutte 10 (I965 page 2 1 6220, the reasons for the formation of floating slime and report that this depends on the formation of antimony arsenate in amorphous form. Graf and Lange also recommend a restricted amount of antimony in the anode. None of these publications indicate or show a method for producing high grade cathode copper from anodes with high content of antimony.

It has been assumed in the aforementioned publications that the floating slime derives from the precipitation of trivalent antimony with pentavalent arsenic, the latter being oxidized by oxygen which is dissolved in the electrolyte. According to Graf and his colleagues, trivalent antimony is oxidized to pentavalent antimony and with sufficiently high contents is also precipitated as Sb,O

The oxidation of antimony (III) and arsenic (III) to pentavalent ions in the absence of a catalyst is extremely slow, but, as previously mentioned, the presence of monovalent copper is considered to catalyse the oxidation by forming 0, according to the reaction Cu+ 0 9 Cu 0;.

When the antimony content of the anode is greater than approximately 400 g/t and/or the bismuth content of the anode is greater than approximately 200 g/t it has not previously been possible to effectuate a copper electrolysis obtaining a high grade cathode copper. As the quality and purity demands placed on the cathode copper is continually increasing the contaminants which deleteriously affect the quality properties of the copper must be maintained as low as possible. Antimony and/or bismuth strongly impair the quality of cathode copper. Normally, the quality of the cathode is valued by deterimining the soft annealing temperature (recrystallisation temperature). Even relatively small contents of antimony and/or bismuth will result in an excessively high soft annealing temperature,

which renders the product less suitable for important fields of use, particularly such fields as the manufacture of fine wire filaments etc. It is true that the major content of the antimony can be removed during the processing of the copper raw materials to anode copper but it is still very difficult and expensive to attain the desired low antimony content. It is still more difficult to remove bismuth during these treatment stages. Consequently, it has hitherto been impossible to utilize raw copper material with high antimony and/or bismuth content for producing high grade electrolytic copper.

DESCRIPTION OF THE INVENTION The present invention is concerned with a method for electrolytically refining copper anodes which contain more than 200 grams of antimony per ton and/or more than grams of bismuth per ton without getting floating slime, and is characterized by the steps of regulating the quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony in the electrolyte so that in the steady state the content of As(II) exceeds 1 g/l, the content of As(V) exceeds 2 g/l and the quantity of Sb(V) is less than 0.5 g/l.

Thus, in accordance with the invention trivalent arsenic can be introduced to the electrolyte in quantities at which the content of trivalent arsenic at the steady state is maintained above 1 g/l, preferably between 2 and 5 g/l. The trivalent arsenic present in the system will absorb oxygen dissolved in the electrolyte, the arsenic being, at the same time oxidized to pentavalent arsenic. At the same time, the electrolyte shall contain at the steady state at least 2 g of As(V)/l preferably 7-l5 g/l, causing crystalline arsenates of trivalent antimony and bismuth to precipitate. in the case of anodes which do not contain antimony but which do contain bismuth, the content of pentavalent arsenic is of greater significance than the content of trivalent arse- The increase of As(lll )-content of the electrolyte can be caused by dissolving a suitable As(lll) compound, for example AS203, in the electrolyte or by adding an aqueous solution of As 0 thereto. It is also possible to admit arsenic to the electrolyte via the anodes, from which it passes into solution as trivalent arsenic.

The present invention thus includes a method for completely avoiding the formation of flotation slime in the case of anodes which contain high contents of antimony, and it has been discovered that anodes having, for example, up to 2,500 grams of antimony per ton can be used successfully. Furthennore, the present invention solves the problems which occur when the anodes contain large quantities of bismuth, for example, up to 2,500 g/t. Theoretically, anodes which have higher contents of antimony and particularly bismuth can be used since in respect of the formation of floating slime no upper limit with regard to the antimony and bismuth contents has been observed.

The mechanism by which floating slime is formed has now been established, and above all the key role played by pentavalent antimony. It has namely been discovered that antimony and arsenic pass into solution in trivalent form and that further oxidation to pentavalent form is primarily catalysed by elementary copper. In the conventional electrolysis of copper which contains antimony a certain amount of pentavalent antimony is always formed as a result of oxidation of the antimony with atmospheric oxygen which to some extent is dissolved in the electrolyte. This amount is normally of the order of 0.05-0.20 g/l, while in the presence of antimony(V) non-settling amorphous floating slime precipitates are obtained. These precipitates are chemically non-defined compounds existing between pentavalent antimony, trivalent antimony, trivalent bismuth (when present), pentavalent arsenic, oxygen and water. Trivalent arsenic is not included in the floating slime. Floating slime gives no defraction lines when subjected to X-ray defraction analysis. The composition of the compounds defining floating slime varies with the concentration of the components in the electrolyte, although the atomic relationship between arsenic and the sum of antimony and bismuth and normally lies between 1:15 and 1:2. It is probable that the presence of elementary copper is necessary for the oxidation of arsenic(IlI) and antimony(lll), since peroxides are formed on the surface of the copper, which in turn oxidize arsenic(lll) and antimony(lll). It has been discovered that the rate of oxidation is higher with the oxidation of arsenic(IIl) than with the oxidation of antimony(lll), and hence arsenic(lll) primarily absorbs the oxygen dissolved in the electrolyte. It has been established that pentavalent antimony is deleterious to the copper electrolysis not only because it contributes to the formation of floating slime but also because it retards the precipitation of antimony and/or bismuth arsenates. A conventional electrolyte thus becomes highly oversaturated with respect to bismuth and antimony.

It has also established that if no pentavalent antimony is present or if only present in quantities below approximately 0.05 g/l electrolyte, only crystalline precipitates of antimony arsenate and/or bismuth arsenate are formed and no floating slime. X-ray defraction analysis of the precipitates clearly shows lines of well-formed crystalline phases. The atomic relationship between arsenic and the sum of antimony and bismuth is always equal to one. Antimony arsenate is a defined chemical compound which has been found to have monoclinic crystal modification with the edge length of a=5.3l8 Angstrom, M917 Angstrom and c==4.8l4 Angstrom and the monoclinic angle 3 9375. The composition is probably SbOH,AsO Similarly, bismuth arsenate is a defined compound which occurs in two crystal modifications, tetragonal (the stable) of monoclinic, with the composition BiAsO The crystalline precipitates settle readily and behave principally in the same manner as, for example, PbSO and sink to the bottom of the tank together with other anode slime.

The method of the present invention thus prevents the formation of pentavalent antimony to an extent such that the concentration in the solution is maintained below 0.05 g/l, preferably below 0.02 g/l. In this way, formation of the previously described floating slime is rendered impossible and crystalline antimony arsenate and bismuth arsenate percipitate onto graft crystals present in the anode slime. If minor quantities of pentavalent antimony are formed these are removed by co-crystallisation. The crystallisation also causes the total contents of antimony and bismuth in the electrolyte to be greatly reduced.

In the addition to the use of trivalent arsenic, as a mean of preventing the formation of pentavalent antimony, oxidation of trivalent antimony by ambient air can be prevented by excluding contact of the electrolyte and atmospheric air by means of air tight pumps and an air tight circulation system, for example. The content of pentavalent antimony can be maintained in this way below 0.05 g/l. It is also possible to reduce the quantity of oxygen dissolved in the solution by using an inert gas to protect the circulation system. The electrolytic cells can be protected against atmospheric oxygen by known expedients devisedv for the purpose of reducing evaporation of the electrolyte surface, for example expedients as covering the electrolyte with buoyant plastic bodies, plastic cloths or an oil layer.

EXAMPLE The experiment was carried out on factory scale and the electrolyte partially protected against oxidation by ambient air by using an air tight circulation system. As- (Ill) was supplied to the system as a reduction agent.

A conventional electrolyte comprising 40 g/l Cu, 170 g/l H 4 g/l As, of which 0.5 g/l was As(Hl), 0.43 g/l Sb of which 0.10 g/l Sb(V) and 0.35 g/l Bi was used during the initial stages of the experiment. This represents a conventional electrolyte at the steady state.

, Trivalent arsenic was added in the form of an aqueous solution containing 30 g/l A3 0 (saturated at room temperature). During the first two weeks, an aqueous solution containing 60% As 0 was also added. The anodes comprised and anode copper having 98% Cu,

0.40% Ni, 040% Ag, 0.11% As, 0.035% Sb and The experiment was conducted in 28 conventional copper electrolytic cells in commercial operation. The steady state was reached after three weeks, the contents of the electrolyte being approximately 4 g/l As- (Ill) and approximately 11 g/l As(V). The total anti mony content stabilized at least than 0.02 g/l of which was Sb(V) less than 0.02 g/l, and the bismuth content at less than 0.15 g/l. During the running-in time, cathodes of normal quality were initially obtained. The quality of the cathodes was progressively improved, however, owing to the reduced impurities of antimony and bismuth in the cathodes.

Subsequent to reaching the steady state, the original anodes, which had an antimony content of 350 g/t, were replaced with anodes having an antimony content of 800 g/t. The good quality of the cathodes was unaffected by the high antimony content of the anodes owing to the fact that no floating slime was formed.

As opposed to conventional electrolysis, no deposits were obtained in the conduit sytems.

Anodes having an antimony content of 1,100 g/t also gave the same good result. The total antimony content stabilized at 0.2 g/l, of which Sb(V)-constituted less than 0.02 g/l and the bismuth content at less than 0.1 g/l. Analysis carried out on wirebars manufactured from cathodes obtained by electrolysing anodes con taining 1100 g/t antimony gave the following results calculated in g/t:

Ag Fe Ni Pb Bi Sb As Te Se S 7 1.5 1.5 0.l 0.3 0.5 (H 0.4 7

Analyses made on wirebars manufactured from cathodes obtained from a conventional electrolyses process with only 350 g/t antimony in anodes gave the following result:

Se S 0.4 7

Ag Fe Ni 10 7 1.5

What I claim is:

1. A method for electrolytically producing cathode copper by the electrolysis of copper anodes containing at least one of the substances antimony and bismuth in quantities exceeding 400 grams per ton with respect to antimony and 200 grams per ton with respect to bis muth, characterized by supplying arsenic ions to the electrolyte in such quantities that in the steadystate the content of trivalent arsenic exceeds 1 g/l and the content of pentavalent arsenic exceeds 2 g/l, and by ensuring that the content of pentavalent antimony in the steady state does not exceed 0.05 g/l.

2. A method according to claim 1, characterized by adding trivalent arsenic to the electrolyte in the form of arsenic trioxide.

3. A method according to claim 1, characterized by adding pentavalent arsenic to the electrolyte in the form of arsenic pentoxide.

4. A method according to claim 3, characterized in that the arsenic pentoxide is added to the electrolyte in the form of an aqueous solution.

5. A method according to claim 1, characterized in that trivalent arsenic is added to the electrolyte by means of arsenic alloyed in the anodes.

6. A method according to claim 1, characterized by withdrawing electrolyte from the main circulating flow thereof, supplying arsenic ions to the withdrawn electrolyte and recirculating said electrolyte with the main flow thereof.

Claims (5)

1. 2. A method according to claim 1, characterized by adding trivalent arsenic to the electrolyte in the form of arsenic trioxide.
2. 3. A method according to claim 1, characterized by adding pentavalent arsenic to the electrolyte in the form of arsenic pentoxide.
3. 4. A method according to claim 3, characterized in that the arsenic pentoxide is added to the electrolyte in the form of an aqueous solution.
4. 5. A method according to claim 1, characterized in that trivalent arsenic is added to the electrolyte by means of arsenic alloyed in the anodes.
5. 6. A method according to claim 1, characterized by withdrawing electrolyte from the main circulating flow thereof, supplying arsenic ions to the withdrawn electrolyte and recirculating said electrolyte with the main flow thereof.