A METHOD AND APPARATUS FOR RECOVERING COPPER FROM ORES CONTAINING COPPER AND IRON SULPHIDE

Abstract: Method for recovering copper from sulphidic copper and iron bearing ores in which the copper is precipitated onto a cathode (6) in an electrochemical diaphragm cell (1) divided into a cathode chamber (8) and an anode chamber (9), and in which the iron is precipitated as an oxidized readily filtered precipitate. The invention is characterized by mixing the ore with an electrolyte containing an effective quantity of chloride-ions to form a suspension, whereafter the suspension is caused to flow over anode surfaces (15) present in the anode chamber (9) in order to ensure supply of suspension to the anode surfaces (15) and to increase the contact between suspension and anode surfaces (15). The anodic current density is selected as high as possible in dependence on the rate at which suspension is caused to flow over the anode surfaces (15) without liberating chlorine from the suspension. The invention also relates to an electrolytic cell for carrying out the method.

Description: A METHOD AND APPARATUS FOR RECOVERING COPPER FROM ORES CONTAINING COPPER AND IRON SULPHIDE The present invention relates to a method for recovering copper from sulphide ores containing copper and iron, by precipitating the copper onto a cathode incorporated in an electrochemical diaphragm cell divided into a cathode cham¬ ber and an anode chamber, and by oxidizing the iron to form a readily filtered precipitate. The invention also relates to a preferred cell for carrying out the method. A method of this kind is described generally in US-A-4,061,552 (Dextec). According to this method a suspension of the ore and a chloride containing elec¬ trolyte is maintained in the anodic chamber of an electrochemical cell, while thoroughly mixing the suspension, or slurry, with air or some other oxygen-con- taining gas at the same time. It is reported in the specification of this patent that the iron which passes into solution is precipitated, through oxidation, as iron(III)oxide, whereas the sulphide sulphur is converted to elementary sulphur. The copper content passes into solution as Cu(D- and Cu(II)-ions, depending on the duration of the process and on the conditions under which the process takes place. Copper is precipitated from the solution in the cathode chamber. A significant drawback with this known method, however, resides in the relative¬ ly limited anodic current density which can be used without the risk that chloride will be oxidized anodically to chlorine gas, since the more copper to be dissolved the greater the chloride content required. The departure of chlorine is unaccept¬ able for a number of reasons, although the problems relating to environmental contamination and corrosion must be considered paramount. This is probably a major contributing factor to why hydrometallurgical processes of the afore- described kind, leaching and electro-winning, have not yet been found sufficient- ly attractive for economical application. It has now been found possible to increase considerably the anodic current densi¬ ty when winning copper in an effective and simple manner, by leaching and elec¬ tro-winning, in electrochemical diaphragm cells without needing the risk of the departure of chlorine from the cell. The invention is characterized by the features set forth in the following claims. Thus, the copper-iron-sulphide concentrate charged to the process is slurried in the electrolyte to form a suspension. This suspension: is caused to flow in the electrolytic cell against or in the close proximity of the surface of the anodes present in the cell. By "surface" is meant here and in the following an electro- chemically active surface of the anode. The highest possible anodic current den¬ sity is maintained during the winning process. By "highest possible" is meant an anodic current density which will enable the highest possible quantity of lead to be recovered without resulting in the troublesome emission of chlorine-gas from the cell. In this respect there is used in the majority of cases an anodic current density greater than about 200 A/m 2 . The current density is limited upwards by the speed at which oxidizable suspension can be supplied to the anode surfaces. It has been found totally impossible to reach current densities as high as 500 A/m 2 , at least with the apparatus available at present. There is maintained in the electrolyte an active chloride-ion strength greater than 2M. The best results are obtained with a chloride-ion strength of about 4-7 M. Higher contents impair the mass transport at the anode, due to increased viscosity, and the electrical resistance of the bath is also increased, which results in higher energy losses. Leaching and winning of copper takes place in one and the same vessel, the elec¬ trolytic cell, which may in principle have any suitable construction. However, the cell is preferably one in which electrodes are placed around a centrally located anode chamber. A diaphragm separates the anode chamber from the cathode chamber in a known manner. When carrying out the invention it has been found particularly effective to use a cylindrical cell in which electrodes are arranged to extend radially. The use of such a cell in conjunction with the method according to the invention will be discussed hereinafter. In order to ensure an effective supply of concentrate suspension to the anodes and also effective contact between the suspension and the anode surfaces, there is arranged in the centre of the cell an agitating means, which transports the suspension vertically in the centre of the cell. The flow of suspension is caused to flow' outwardly from the cell edge at the top or the bottom of the cell, depending on the direction of suspension flow, and from there obliquely down- wards and outwards into contact with the vertical anode surfaces or along the same. Air for oxidizing the suspension, inter alia for forming the iron precipitate, is conveniently charged while agitating the suspension so as to be finely dispersed therein, for example similar to the principle applied in a flotation cell. It has been found that the red iron precipitate formed with prevailing conditions has the formula FeOOH(iron(III)oxyhydrate). The flow of concentrate onto or along the anode surface is controlled in a man¬ ner to maintain a sufficiently high rate of flow over all parts of the anode, sur- faces present in the cell. This enables the generation of chlorine gas to be coun¬ teracted to a significant degree. The presence of gaseous chlorine has the draw- -back of, inter alia, causing corrosion of the anode. The oxidation of Cu(I)ions and the oxidation of Fe(II)ions present in the electro- lyte takes place on or in the close vicinity of the anode surface. When chlorine- -gas or oxygen-gas is generated in limited quantities during the anode reaction, the gas generated is absorbed effectively in the flowing suspension and does not leave the cell, at least not to any significant extent. The maximum anodic current density is dependent on the mass transfer at the anode surface. By forcing the concentrate into contact with the anode surface, as in the case with the present invention, good mass transfer is also obtained as a result of the high flow of suspension over the anode. The flow is uniform over the whole surface of the anode. As before mentioned, the method can be applied with any suitable type of elec¬ tro-winning cell in which a flow of suspended concentrate can be maintained in the immediate proximity of the anode or anodes present. However, there is preferably used an electrolytic cell of cylindrical construction having a conical bottom in which the electrodes are placed alternatively in a star-shaped forma- tion. The cell incorporates means for guiding the suspension towards and against the anode surfaces, and also an agitating means placed centrally in the cell. The cell is suitably of a construction in which the cathode chambers extend down to the bottom of the cell and through said bottom, beneath which a conical, further bottom is arranged. All copper recovered is collected in this further bottom in the form of powdered copper, the copper first forming on the surfaces of the cathode and then falling down gravitationally into said bottom. The copper powder can be removed through the cell bottom. Thus, in relation to the prior art, the inventive method entails ensuring that an intensive flow of suspension is obtained over the anode surfaces, which enables (and requires) the application of higher anodic current densities, and thus higher productivity. The surprising effect afforded by the invention is prob¬ ably due to the fact that Cud ons are constantly present in the vicinity of the anode surface throughout the whole of the process, therewith preventing the occurrence of other, undesirable, oxidation processes, for example processes in which chlorine gas is formed from the chloride present in the suspension. The invention will now be described in more detail with reference to a preferred embodiment of an electrolytic cell illustrated in the accompanying drawing, and also with reference to a working example. The drawing illustrates a preferred embodiment of an electrolytic cell according to the invention. Figure 1 illustrates the cell in a vertical section. Figure 2 is a top plan view of the cell. The cell 1 comprises a vessel 2 having a first conical bottom 3 and a second conical bottom 4 located beneath said first bottom 3. Extending radially in the cell are electrodes having the form of anodes 5 and cathodes 6. A diaphragm 7 is arranged between the electrodes. The cathode chamber 8 is connected directly with the second bottom 4, whereas the anode chamber 9 is connected with the first bottom 3. The electrolyte present in the anode chamber 9 is referred to as anolyte, and the electrolyte present in the cathode chamber as catholyte. The agitator 10 causes the suspension of concen¬ trates and anolyte to flow into and up through a conduit 11, as shown by arrows 13,14, and then outwardly and upwardly onto the surfaces 15 of the anodes 5, as shown by the arrows 12. Air for oxidizing suspension to form a readi- ly filtered iron precipitate is supplied through a hollow agitator shaft 17, as shown by arrow 18. The air is mixed thoroughly with the suspension in the vicini¬ ty of the agitator 10, as shown by the arrow 19. The copper ions formed during the anodic reaction precipitate out onto the cathodes 6, subsequent to passing the diaphragm 7, and metallic copper powder falls progressively from the cathodes down into the conical second bottom 4, from where the powder is removed through a bottom-emptying device 16.

Claims: CLAIMS 1. A method for recovering copper from sulphidic copper and iron bearing ores in which the copper is precipitated onto a cathode in an electrochemical diaphragm cell divided into a cathode chamber and an anode chamber, and in which the iron is precipitated as an oxidized readily filtered precipitate, charac¬ terized by mixing the ore with an electrolyte containing an -effective quantity of chloride-ions to form a suspension; causing the suspension to pass over anode surfaces present in the anode chamber in order to ensure supply of suspension to the anode surfaces and to increase the contact between suspension and anode surfaces; and by selecting an anodic current density of the highest possible value in dependence on the rate at which suspension is caused to flow over the anode surfaces without liberating chlorine from the suspension. t 2. A method according to claim 1, characterized by maintaining a current density which at least exceeds about 200 A/m 2 . 3. An electrolytic cell for carrying out the method according to claim 1- and claim 2, characterized by a cylindrical vessel; an agitating means in the centre of the vessel; electrodes arranged in star formation and comprising alter¬ nately anodes and cathodes arranged respectively in a cathode and an anode chamber, said chamber being separated by a diaphragm and having respective bottoms; and means for guiding the flow of suspension towards the anodes and for supplying an oxidizing agent to the suspension. 4. An electrolytic cell according to claim 3, characterized in that the agitat¬ ing means is incorporated in a hollow cylindrical conduit which is open at both ends. 5. An electroytic cell according to claim 3 and claim 4, characterized by means for discharging formed copper from the bottom. 6. An electrolytic cell according to any of claims 3-5, characterized in that the cylindrical vessel has a conical bottom. 7. An electrolytic cell according to claim 1, characterized in that the agi¬ tator means is arranged to introduce an oxidizing agent into the suspension and to finely disperse the agent therein.

Inventors: HEDSTROEM LARS ARTUR, LINDSTROEM NILS FOLKE RUNE, SJOEBERG KENNETH AOKE SOEREN, HEDSTROEM, LARS, ARTUR, LINDSTROEM, NILS, FOLKE, RUNE, SJOEBERG, KENNETH, AOKE, SOEREN
Applicants: BOLIDEN AB, BOLIDEN AKTIEBOLAG
Application year: 1986
Main classification: C25C1
IPC codes: C25C1, C25C5, C25C7