METHOD FOR UPGRADING COPPER CONCENTRATE

FIELD OF THE INVENTION

The invention relates to a method for the preparation of a copper concentrate suitable for pyrometallurgical treatment from a concentrate in which zinc is present as an impurity. In accordance with the method, zinc is leached from the concentrate with a solution containing sulphuric acid in oxidising conditions at atmospheric temperature and pressure. At the end of the leach, the conditions are regulated to be such that any copper that may have dissolved will be precipitated.

BACKGROUND OF THE INVENTION

Complex copper sulphide concentrates contain other non-ferrous metals besides copper, such as zinc. When sulphide concentrate intended for pyrometallurgical recovery of copper contains zinc, for example, it complicates the process. In a pyrometallurgical process, the zinc contained in the concentrate evaporates and enters the fine dust as zinc oxide, from which it should be recovered separately. Current smelters charge a penalty for impurities in concentrate, for instance the zinc content of copper concentrate should be below 3%.

Many ways of separating zinc from copper concentrate are known in the prior art. Separation using selective flotation is described in many patent publications such as e.g. US 4,279,867, JP 59 160558 and RU 2135298. However, from time to time the mineralogy of the ore is such that good results cannot be achieved even with selective flotation.

Another method is to treat the concentrate first at such a temperature that the zinc vaporises and is recovered as zinc oxide, after which the copper concentrate can be processed further in the desired way. This kind of method is described in e.g. patent CN 1288966. If the further processing of the concentrate takes place pyrometallurgically, the method requires the

construction of two furnace units. Additionally, one disadvantage is the formation of sulphur dioxide, which necessitates its own processing equipment.

US patent publication 4,260,588 describes a way to handle the upgrading of a complex copper sulphide concentrate. There the concentrate is leached with a copper-containing chloride solution at an elevated temperature and pressure at a pH value of about 3. As a result of leaching, a sulphidic concentrate containing copper and iron is obtained and the other non-ferrous metals contained in the complex concentrate such as zinc, nickel and cobalt are recovered from the leach. The method appears fairly complicated and costly. Another fact that could be regarded as a drawback is that the chlorides need to be removed from the concentrate meticulously, so that they do not cause problems in further processing of the concentrate.

PURPOSE OF THE INVENTION

The purpose of the invention is to achieve a method for upgrading copper sulphide concentrate containing zinc by removing the zinc from the concentrate, so that the concentrate is suitable for the pyrometallurgical recovery of copper.

SUMMARY OF THE INVENTION

The invention relates to a method for upgrading copper sulphide concentrate containing zinc by removing the zinc from the concentrate, so that the concentrate is suitable for the pyrometallurgical recovery of copper. It is typical of the method that zinc is removed from the concentrate by leaching the concentrate with a solution containing sulphuric acid at atmospheric pressure and temperature, firstly in oxidising conditions and that before the end of the leach, a reducing agent is fed into the leaching stage to precipitate out the dissolved copper.

It is typical of the method accordant with the invention that the sulphuric acid concentration at the end of the leaching stage is regulated to the region of 0- 100 g/l, preferably 5 - 25 g/l.

It is also typical of the method accordant with the invention that an oxidising agent is fed into the leaching stage.

According to one embodiment of the invention, the oxidising agent used is a gas, which is at least one of the following: air, oxygen-enriched air and oxygen.

According to another embodiment of the invention, the oxidising agent used is at least one of the following: hydrogen peroxide, manganese dioxide and potassium permanganate.

It is also typical of the method accordant with the invention that the redox potential is adjusted so that there is a minimum of 10 mg/l of copper at the start of the leach and that at the end of the leach there is no copper in the solution.

According to one embodiment of the invention, the reducing agent to be fed into the end of the leaching stage is a reducing gas. The gas is typically hydrogen sulphide.

According to another embodiment of the invention, the reducing agent to be fed into the end of the leaching stage is sodium hydrosulphide or sodium sulphide.

According to yet another embodiment of the invention, the reducing agent to be fed into the end of the leaching stage is copper sulphide concentrate.

It is typical of the method accordant with the invention that there is a minimum of 0.1 g/l of iron and at least 10 mg/l of copper in the solution at the start of the leach.

The essential features of the invention will be made apparent in the attached claims.

DETAILED DESCRIPTION OF THE INVENTION

In the method accordant with the invention, a copper sulphide concentrate containing zinc is routed to leaching, with the aim of leaching such an amount of zinc from the concentrate that the concentrate is suitable for pyrometallurgical processing. Leaching is sulphate-based and occurs in atmospheric conditions. Atmospheric conditions means leaching that takes place in unpressurised reactors and at a temperature that is at the most 105 ⁰ C. The copper sulphide concentrate is mainly a primary copper sulphide such as chalcopyrite and does not substantially contain secondary sulphides. Chalcopyrite does not dissolve substantially in conditions where zinc is leached from the concentrate.

Zinc-containing copper sulphide concentrate is fed into a solution containing sulphuric acid, in which the acid concentration is regulated to be in the range of 0 - 100 g/l at the end of the leach and preferably 5 - 25 g/l. At the beginning of the leach an oxidising agent is fed into the solution, said agent being for example an oxidising gas such as air, oxygen-enriched air or oxygen. The oxidising agent may also be something other than gas, such as hydrogen peroxide, potassium permanganate or manganese oxide.

It is also advantageous for leaching that the acid-containing solution contains a small amount of dissolved iron and copper. A sufficient amount of iron allows the zinc to dissolve at a sufficient rate i.e. it improves the dissolution kinetics. Preferably there is at least 0.1 g/l of iron, typically over 5 g/l. There is at least 10 mg/l of copper in the solution. The copper in the solution

catalyses the oxidation of the iron. The oxidised iron participates in the dissolution of zinc according to reaction (2) below;

2FeSO ₄ + H ₂ SO ₄ + 0.5O ₂ -» Fe ₂ (SO ₄ ) ₃ + H ₂ O (1) ZnS + Fe ₂ (SO ₄ )S -» ZnSO ₄ + 2FeSO ₄ + S (2)

It has been found in the tests carried out that during the time that the zinc dissolves, the dissolution of copper is, however, fairly minor. Nevertheless, copper dissolution is not a problem, because it is precipitated back at the end of the leach.

The redox potential of the leach is adjusted so that at the beginning of the leach there is at least 10 mg/l of copper in the solution and at the end of the leach there is no copper in the solution.

It is characteristic of the selective leaching method accordant with the invention that the progress of the leach is monitored so that the acid concentration of the solution is regulated to be within the desired region at the end of the leach. At the same time the amount of oxidising agent required to dissolve the zinc is determined, and is fed into the leaching reactor at the start of the leach. The leaching reactor is preferably an agitated reactor. When the required amount of oxidising agent has been fed, the reactions continue further, even though the feed of oxidant has finished. When the zinc content of the concentrate has fallen to the desired level, part of the copper may also have dissolved. Dissolved copper is precipitated at the end of the leach by means of some reducing agent. The reducing agent may be solid, liquid or gaseous such as hydrogen sulphide. Other reducing agents are for instance sodium hydrosulphide NaHS, sodium sulphide Na ₂ S or copper sulphide concentrate itself, which acts as a slow reductant. The result obtained from leaching is copper concentrate, with a zinc content that is suitably low for the pyrometallurgical processing of concentrate. The zinc

sulphate solution is processed in the desired way in order to fabricate a commercial product,

The aim is to keep the total time and costs of leaching in process conditions at the most economically viable level. Thus the long leaching time demanded by the slowly-reducing reductant and on the other hand the possibly higher cost of a quick-acting reductant and the reagent consumption are taken into account (i.e. time vs. reagent consumption).

EXAMPLES

Example 1

Copper sulphide concentrate was leached in a reactor that had a volume of

10 L. The concentrate was composed of the following minerals: CuFeS ₂ ,

FeS ₂ , ZnS. The grain size of the concentrate was d _o .s = 10 μm and d _o . ₉ = 35 μm. The composition of the concentrate was as follows:

Table 1

Leaching was performed at atmospheric pressure and a temperature of 95 ⁰ C and the leaching time was 9 h. The solution contained 9.6 g/l of ferrous iron and 1 g/l of copper. The solids content of the solution was 200 g/l and at the beginning of the leach the H ₂ SO ₄ concentration of the solution was 50.8 g/l. The oxygen-containing gas used was oxygen, which was fed into the reactor for 3 h after which time the feed was stopped. Fresh concentrate was used for reduction, and 1.1 kg was fed into the reactor when 6 - 7 hours had passed since the start of the test. As shown in Table 2 below, the zinc content of the concentrate had fallen to a value of 1.4 % and the copper content had risen to 21.4%. According to the balance calculation, the amount of copper lost in leaching was below 10%.

The results show that the concentrate acts as a slow reductant, and if leaching had continued, the copper would have gradually been precipitated back. Concentrate is without doubt the most cost-efficient reducing agent, if it is possible to perform reduction for a long period of time. The redox potential was measured with Pt electrodes vs. Ag/AgCI electrode.

Table 2

Example 2

In this test the composition of the leached concentrate was the same as in example 1 , likewise the leaching reactor, leaching pressure and temperature. Leaching time was 11 h. The solution contained 52.5 g/l of sulphuric acid, 9.8 g/l of ferrous iron and 1.1 g/l of copper. Oxidation of the solution was stopped after 1 h and hydrogen sulphide was fed as the reducing gas between 10 - 11 h. The zinc content of the concentrate had fallen to a value of 1.0% and the copper content had risen to a value of 20.6%. The reduction precipitated all the dissolved copper, so no copper losses were generated. The progress of leaching is shown in Table 3.

Table 3 shows that, when oxidation ended, the redox potential fell quickly from a level of 406 mV to a level of 390 mV. Before the feed of reductant the value was 330 mV and fell quickly once the reductant feed began to a value of 86 mV. The final value of 207 mV indicates the probable oxygen introduced with the make-up water. The table also indicates that a smaller

feed of reduction gas would have been sufficient, because the dissolved copper was precipitated as soon as the feed of reduction gas was begun.

Table 3

Example 3

The composition of the concentrate leached in the test was as given in Table

4.

Table 4

The leaching reactor, leaching pressure and temperature were the same as in examples 1 and 2. Leaching time was 12 h. At the start the solution contained 51.4 g/l of sulphuric acid, 9.1 g/l of ferrous iron and 0.9 g/l of copper. The oxidation of the solution was stopped after 4h and hydrogen sulphide was fed as the reduction gas between 11.5 - 12 h. The zinc content of the concentrate had fallen to a level of 1.4% and the copper content had risen to a level of 21.7%. Reduction precipitated all of the dissolved copper

so no copper losses were generated. The progress of leaching is shown in Table 5:

Table 5