WO2007077290A1

WO2007077290A1 - Method for improving sulphidic concentrate leaching

Info

Publication number: WO2007077290A1
Application number: PCT/FI2006/000425
Authority: WO
Inventors: Mikko Ruonala; Heikki Takala; Marko Lahtinen; Ilkka Turunen
Original assignee: Outotec Oyj.
Priority date: 2006-01-04
Filing date: 2006-12-29
Publication date: 2007-07-12
Also published as: PE20071049A1; FI118225B; FI20060011A0; FI20060011A

Abstract

The invention relates to a method for improving the leaching of a sulphidic concentrate that contains valuable metals in a hydrometallurgical fabrication process of said valuable metal. The purpose is to accelerate the oxidation of sulphidic sulphur during leaching and thus the dissolving of the valuable metal. In order to achieve this, at least some of the solution formed in leaching containing divalent iron is oxidised in a separate, tube-like oxidation reactor equipped with static mixers to form trivalent iron. The invention is particularly suitable for the hydrometallurgical fabrication method of zinc, in which the raw material is zinc concentrate and the zinc concentrate is leached directly without roasting.

Description

METHOD FOR IMPROVING SULPHIDIC CONCENTRATE LEACHING

FIELD OF THE INVENTION

The invention relates to a method for improving the leaching of a sulphidic concentrate that contains a valuable metal in a hydrometallurgical metal fabrication process. The purpose is to accelerate the oxidation of sulphidic sulphur with trivalent iron during leaching and thus the dissolving of the valuable metal. In order to achieve this, at least some of the solution formed in leaching that contains divalent iron is oxidised in a separate, tube-like oxidation reactor equipped with static mixers to form trivalent iron. The invention is particularly suitable for the hydrometallurgical fabrication method of zinc, in which the raw material is zinc concentrate and the zinc concentrate is leached directly without roasting.

BACKGROUND OF THE INVENTION

The atmospheric direct leaching technology for zinc concentrate is described in several patents such as US 6340450 and Fl 109457. On the other hand, the method according to this invention can also be used in the hydrometallurgical production of other metals, such as copper, if the raw material is a concentrate containing sulphidic copper compounds.

It is already known that the valuable metal of concentrates containing a sulphidic valuable metal, such as zinc, copper or nickel, can be leached from the concentrate minerals in soluble form by utilising the oxidation power of iron in trivalent form. The main reactions of the dissolution are presented below, and show for example that as zinc sulphide dissolves, ferric iron oxidises the sulphide into elemental sulphur and at the same time the ferric iron is reduced into ferrous iron. For the dissolving reaction to continue, the ferrous iron must be oxidised back into ferric iron. Usually the oxidant used is the purest oxygen gas possible. In iron oxidation the sulphuric acid is neutralised in accordance with reaction formula (2). ZnS + Fe₂(SO₄)₃ ^■» ZnSO₄ + 2 FeSO₄ + S⁰ (1 )

2 FeSO₄ + ¹/2 O₂ + H₂SO₄ ^■* Fe₂(SO₄)₃ + H₂O (2)

The sulphate solution containing a valuable metal that is formed in leaching, such as zinc sulphate solution, is routed to liquid-solids separation then to solution purification and on to the electrolytic recovery of the metal. In order to attain a sufficiently high dissolution rate there has to be sufficient iron in the solution. Iron is obtained for the solution from the raw material i.e. from the zinc concentrate or on the other hand possibly from a calcine, if calcine leaching is also included in the overall process. Depending on the iron content of the raw material and the proportion of soluble iron, various technical process solutions can be used, such as internal iron residue precipitation and recirculation, to maintain a sufficient iron content in the leaching process. The sulphur that is left in the solids can be separated for instance by flotation and the iron precipitated in the desired form, for example as jarosite, goethite or hematite.

In the concentrate leaching process the concentrate is fed first into a concentrate elutriation reactor, into which is also fed trivalent iron and an aqueous solution containing free sulphuric acid. The slurry from the elutriation reactor proceeds to the actual leaching reactors, which are connected several in series. At the beginning of the leaching process, dissolution is intense and the greater part occurs in the first and second reactors. The rest of the reactors act thus as reactors to extend the duration of the leaching process, in which the remainder of the soluble zinc is dissolved. Conventionally the leaching time of concentrate in an atmospheric leaching process is around 20 - 24 h.

It has been observed in atmospheric direct leaching processes for zinc concentrate that especially at the beginning of the leaching process, i.e. in the first reactor of the reactor series, the ferric iron content of the solution falls very low as a result of the intensity of the dissolution reaction. In other words, the majority of the iron in the solution is in ferrous form. This arises from the fact that the oxidation of ferrous iron back to ferric iron generally is not sufficiently fast and effective, even if surplus oxygen is fed into the reactors, and because iron that is oxidised into trivalent reacts immediately with the surplus metal sulphide of the concentrate that is in the slurry. It naturally follows that effective concentrate leaching is prevented and the time needed for leaching and the size of the reactors required also increases.

An attempt to solve the problems of the slow oxidation of iron and the slow leaching of concentrate was made for example by means of the concentrate leaching process described in EP publication 1 245 686. The essential feature of the method is that a solution containing free sulphuric acid and ferric iron to be used in concentrate leaching is fed into the grinding stage of the concentrate. So that the ferrous iron can be oxidised into ferric iron, at least part of the process is performed in a pressurised space. According to the method, the leaching of the entire concentrate can be carried out in an autoclave with raised oxygen pressure. In that case the oxidation and dissolution of the iron is fast. The problem in pressurised leaching processes lies however with the well-known problems of autoclave processes, such as high investment and operating costs as well as complicated and difficult operation due to the high pressure. Dispersion of oxygen into the solution is not efficient in autoclaves and for this reason relatively high pressures (0.7 - 1.0 MPa) have to be used in order to achieve effective iron oxidation and rapid concentrate dissolution.

EP publication 1 245 686 also presents a method for combining atmospheric and pressurised leaching. In addition to charging the solution containing acid and ferric iron into the grinding stage, one alternative of the method includes combining a pressurised reactor with the atmospheric leaching stage. The purpose of the pressurised reactor is to act as the process section in which ferrous iron is oxidised into ferric iron. According to the publication, the pressurised reactor is only either a directly pressurised tube or alternatively a more complex autoclave arrangement. In the simple pressurised tube, however, the oxygen dissolves poorly, because its dispersion into the solution is not effective due to a lack of mixing.

The solution or slurry containing ferric iron from the pressurised reactor is returned back to grinding and from there on to the actual atmospheric leaching reactor. According to the publication, the autoclave arrangement, which is divided into several sections, can be used for the effective oxidation of the iron and then the slurry or solution containing ferric iron is routed onwards to atmospheric leaching. In other words, this is a question of a combination of atmospheric and pressurised leaching equipment, which still includes the problems of an autoclave arrangement and process.

The method according to EP publication 1 245 686 contains one essential feature, which is the routing of the solution to be oxidised to the concentrate grinding stage. Evidently the aim is to improve the dissolution of the concentrate by means of grinding. However, implementing a leaching arrangement that includes concentrate grinding on industrial scale is difficult and expensive. On the other hand, there has been a trend particularly in the zinc concentrate markets over the last 20 years for a decrease in the particle size of concentrates, so that the share of concentrates with a small particle size suitable for the direct concentrate leaching process has been growing constantly.

PURPOSE OF THE INVENTION

The purpose of the method according to the invention presented here is to offer a more effective leaching method for metal sulphides, in which the oxygen dispersion and iron oxidation stages in particular have been improved. Thus the purpose is to improve the leaching of a concentrate containing a sulphidic valuable metal by oxidising at least some of the solution containing ferrous iron that is used in leaching into ferric iron in oxidation apparatus external to the leaching stage, which is preferably equipped with static mixers, achieving improved mixing. According to the method, the effective leaching of the concentrate is therefore achieved without feeding the entire amount of the concentrate into an autoclave or grinding the concentrate during leaching.

SUMMARY OF THE INVENTION

The invention relates to a method for improving the leaching of the valuable metal of a sulphidic concentrate containing at least one valuable metal in a hydrometallurgical metal fabrication process. The purpose of the method is to speed up the oxidation of the sulphidic sulphur that occurs during the leaching stage and thus the dissolving of the valuable metal, and in order to achieve this at least some of the solution containing divalent iron formed in leaching is oxidised in a separate tube-like oxidation reactor equipped with static mixers, and the solution containing oxidised ferric iron is returned to the concentrate leaching stage.

It is preferable to carry out oxidation by means of the purest oxygen possible. The concentrate leaching stage is performed at atmospheric pressure and temperature. The temperature of the oxidation stage that takes place in the oxidation reactor is regulated to be below the melting point of sulphur. The oxidation reactor operates either at atmospheric pressure or preferably at a low overpressure, around 0.5 MPa at the most.

In particular the invention is connected to the hydrometallurgical fabrication method of zinc, in which the raw material is zinc concentrate and where said zinc concentrate is leached directly without roasting.

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

LIST OF DRAWINGS

Figure 1 presents a flow chart of one embodiment of the method, in which zinc concentrate leaching is performed as sulphate-based leaching, and Figure 2 is a graphical representation of a comparison between a reactor containing a static mixer and an arrangement including just a tube reactor in zinc concentrate leaching, presented by means of leaching yield curves.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the iron reduced to divalent during sulphidic concentrate leaching is oxidised back to ferric iron at least partially in additional equipment external to the actual leaching stage. Naturally the oxygen used in oxidation can also be fed into the leaching reactors of the actual leaching stage. In this case the above-mentioned additional equipment is used only for instance in addition to the oxygen feed to the reactors, in connection with the first reactor in the reactor series, in which dissolution is fastest of all and in which the iron oxidation effect is usually insufficient for effective dissolution. At small ferric iron concentrations even a minimal increase in ferric iron concentration has been found to accelerate concentrate leaching significantly.

To improve and accelerate oxygen dispersion and iron oxidation in the method according to the invention, a tube-like reactor equipped with static mixers is preferably used as the oxidation reactor. With static mixers the size of the oxygen gas bubbles can be reduced and thus the mass transfer surface increased. In addition, the gas-liquid mass transfer coefficient can be improved. As a result, the dissolution of oxygen gas and furthermore the oxidation of iron will be accelerated.

Static mixers are generally mixers attached to tube reactors, which do not contain moving parts. Instead, their mixing efficacy is based on elements shaped in different ways inside the tube. The liquid and gas to be fed into the tube have to change direction continuously and at the same time divide into several sub-streams, which then combine and divide again into random sub- streams. When the tube is equipped with a static mixer, for instance dispersing a gas into a liquid is improved many times over in relation to just a tube. In addition, mixing with a static mixer is even so that the concentration profile regarding the cross-section of the tube is uniform and the axial dispersion is minor. For this reason concentrations and other conditions in the reactor are uniform, which facilitates process control. Conditions in the tube reactor thus have an effect in that the flow rate and pressure in the tube improve tube reactor operation.

The sulphidic concentrate leaching method is described with reference to appended flow sheet 1. Concentrate leaching takes place in concentrate leaching reactor 1 , into which concentrate slurry 5 is fed. Please note that here the whole leaching stage is depicted with one reactor, although leaching normally consists of several reactors. The concentrate and process solution containing ferric iron and free sulphuric acid are mixed in a separate slurry reactor (not shown in detail in the drawing), whereupon any carbonate compounds dissolve and the carbon dioxide that is formed is released. During concentrate dissolution the ferric iron in the solution is reduced to ferrous iron and the oxidation strength of the solution weakens. For this reason, some of the slurry is pumped through tube reactor 3 with pump 2. Oxygen 4 is fed into the slurry ahead of tube reactor 3. The oxygen fed into the slurry is dispersed in the tube reactor by means of the static mixers and possibly by means of slightly raised pressure (overpressure of 1 - 5 bar = 0.1 - 0.5 MPa) and the ferrous iron in the solution is made to oxidise back into ferric iron effectively. The slurry in which all the iron is ferric iron is returned to leaching reactor 1. In addition, it should be noted that oxygen can also be fed directly into the first reactor, whereupon the tube reactor is used as additional process equipment to improve oxygen dispersion and the oxidation of iron. In continuous processes, a quantity of slurry 6, equivalent to the amount fed into the leaching reactor, is taken out of the reactor.

When a tube reactor is equipped with static mixers, the dispersion of oxygen into the solution to be used in concentrate leaching is improved considerably. At the same time the oxygen utilisation efficiency increases and oxygen consumption can be reduced. In fact the greatest benefit of the tube reactor is achieved when the oxygen feed amount is as close as possible to the required amount, in other words, when no large surplus of oxygen is used. Additionally, the benefits obtained with a tube reactor are further increased on industrial scale, when the wall effects due to the small scale of the equipment are reduced.

In the method according to our invention no other equipment is required to improve the concentrate dissolution rate apart from a tube-like reactor equipped with static mixers outside the leaching stage, preferably operating at a low overpressure, into which is fed at least some of the oxygen needed for the concentrate leaching reactors. The solution according to the invention is easy to connect to existing process equipment. In this case, the actual process conditions do not need to be changed. Instead, the temperature of the concentrate leaching stage, the composition of the solution, the atmospheric conditions and solids content of the slurry can be kept as they were before.

The invention is described further by means of the following examples.

EXAMPLES:

Example 1

An arrangement was used in the tests, which consisted of a 200-litre mixing tank and a three-metre-long tube reactor (tube size DN 32). The mixing tank illustrates the leaching stage of the method. The total amount of solution used in the tests was 60 litres. Before starting the test, the composition of the solution was as follows: 40 g/l of sulphuric acid, 10 g/l of ferrous sulphate, 1 g/l of copper sulphate and 120 g/l of zinc concentrate. The copper concentration of the solution was adjusted with copper sulphate to correspond roughly to a normal process solution, because copper has a catalytic effect on the oxidation of ferrous iron. On the basis of analysis the composition of the concentrate was as follows: 51.8 % zinc, 35.1 % sulphur, 3.08 % iron, 2.96 % lead and 0.052 % copper. During the test run, the sulphuric acid concentration of the solution was kept more or less constant. This took place by adding, in several batches during the leaching test, the amount of sulphuric acid that was estimated to be consumed in dissolving the concentrate and by monitoring the sulphuric acid concentration of the slurry from solution samples.

At the start of the test the solution was fed into the mixing tank and heated to a temperature slightly below 100 ⁰C. Then the concentrate and copper sulphate were added into the tank. The concentrate feed time was considered to be the starting point of the test. Testing was performed at a temperature of 105 ⁰C and at a pressure of 3 bar (abs) i.e. 0.2 MPa overpressure.

37.5 l/min of solution was pumped out of the mixing tank into the tube reactor by centrifugal pump. Pure oxygen was fed into the solution at the front end of the reactor. The tube reactor was empty, i.e. there were no static mixers improving mass and heat transfer in the reaction mixture.

The solution was returned from the tube reactor back to the mixing tank, in which the undissolved gas was separated from the solution phase and routed out of the tank. The exhaust gases were cooled, whereupon it was possible to separate the water bound to the gas. The water was not returned to the reactor, but its mass was taken into account when calculating the mass balances.

Slurry samples were taken while the test was underway. The sample was filtered, so that the solution phase and solid phase separated from each other. The amount of dissolved zinc was analysed from the solution sample to determine the zinc yield and the sulphuric acid content. The solid samples were washed and some of the solid samples were analysed for the amount of zinc that remained. This was done with the aim of assuring the validity of the solution samples. On the basis of the analyses, the calculated zinc yields in the tests at different points of time are presented in Table 1. Table 1 Zinc leaching yields in example test 1.

Example 2

Apparatus was used in the tests, which consisted of a 200-litre mixing tank and a three-metre-long tube reactor (tube size DN 32). The mixing tank illustrates the leaching stage of the method. The total amount of solution used in the tests was 60 litres. Before starting the test, the composition of the solution was as follows: 40 g/l H₂SO₄, 10 g/l of ferrous sulphate, 1 g/l of copper sulphate and 120 g/l of zinc concentrate. The copper concentration of the solution was adjusted with copper sulphate to correspond roughly to a normal process solution, because copper has a catalytic effect on the oxidation of ferrous iron. On the basis of analysis the composition of the concentrate was as follows: 51.8 % zinc, 35.1 % sulphur, 3.08 % iron, 2.96 % lead and 0.052 % copper. During the test run, the sulphuric acid concentration of the solution was kept more or less constant. This took place by adding, in several batches during the leaching test, the amount of sulphuric acid that was estimated to be consumed in dissolving the concentrate and by monitoring the sulphuric acid concentration of the slurry from solution samples. At the start of the test the solution was fed into the mixing tank and heated to a temperature slightly below 100 ⁰C. Then the concentrate and copper sulphate were added into the tank. The concentrate feed time was considered to be the starting point of the test. Testing was performed at a temperature of 105 ⁰C and at a pressure of 3 bar (abs) i.e. 0.2 MPa overpressure.

37.5 l/min of solution was pumped out of the mixing tank into the tube reactor by centrifugal pump. Pure oxygen was fed into the solution at the front end of the reactor. A tube reactor, which was equipped with Sulzer SMX-type static mixers, was used to improve the mass and heat transfer in the reaction mixture.

Slurry samples were taken while the test was underway. The sample was filtered, so that the solution phase and solid phase separated from each other. The amount of dissolved zinc was analysed from the solution sample to determine the zinc yield and the sulphuric acid content. The solid samples were washed and part of the solid samples were analysed for the amount of zinc that remained. This was done with the aim of assuring the validity of the solution samples. On the basis of the analyses, the calculated zinc yields in the tests at different points of time are presented in Table 2. Table 2 Zinc leaching yields in example test 2.

The differences in zinc leaching yields are depicted in Figure 1 , which presents the test results from examples 1 and 2. The leaching yield curves show that using a static mixer enables faster zinc concentrate leaching. The same graph presents the percentage difference between the leaching yields of these two tests. This graph shows unambiguously that especially at the early stage of the leaching process, when dissolving is fastest and when there is a lack of ferric iron in the leaching reactor, the greatest increase in dissolution rate is achieved with the use of a tube reactor containing a static mixer. For just this purpose the combination of separate oxygen dispersion and iron oxidation equipment containing a static mixer in addition to an actual leaching reactor offers a significant improvement to current concentrate leaching equipment.

Figure 2 shows that for example to achieve an 85% leaching yield only 120 minutes are required when using a static mixer, whereas without static mixers about 200 minutes are required to reach the same leaching yield. It was stated in the description of the prior art that for instance leaching time for conventional zinc concentrate leaching is 20 - 24 h. According to the example described above, a yield of over 95 % was reached within 4 - 6 h, which is absolutely sufficient when the other stages of the process are taken into account. When, in accordance with the invention, at least some of the leaching stage slurry is fed into the oxidation stage in the tube-like reactor, in which oxygen is dispersed into the slurry, the length of the leaching stage is reduced to at least a quarter. It is advantageous to the method that oxidation is carried out at a low overpressure, whereupon the demands placed on the reactor are considerably less than in reactors requiring higher pressure. On the other hand, using a higher pressure in a tube reactor is also more beneficial than it is for example in an autoclave arrangement. Effective oxidation is achieved at a low overpressure or even in atmospheric conditions, when the reactor is equipped with a static mixing element.

Claims

PATENT CLAIMS

1. A method for improving a valuable metal leaching of a sulphidic concentrate that contains at least one valuable metal in a hydrometallurgical metal fabrication process, whereupon the concentrate is leached by means of an aqueous solution containing trivalent iron and free sulphuric acid, characterised in that the divalent iron formed during the oxidation of the sulphur and the dissolution of the valuable metal in the concentrate is oxidised to trivalent at least partially in a separate, tube-like oxydation reactor equipped with static mixers, and the oxidised solution containing ferric iron is returned to the leaching reactor of the concentrate leaching stage.

2. A method according to claim 1 , characterised in that the oxidation reactor operates at atmospheric pressure.

3. A method according to claim 1 , characterised in that the oxidation reactor operates at a low overpressure.

4. A method according to claim 1 or 3, characterised in that the overpressure of the oxidation reactor is a maximum of around 0.5 MPa.

5. A method according to any of previous claims 1 - 4, characterised in that the leaching stage of the concentrate is carried out at atmospheric pressure and temperature.

6. A method according to any of previous claims 1 - 5, characterised in that the temperature of the oxidation stage occurring in the oxidation reactor is regulated to be below the melting point of sulphur.

7. A method according to any of previous claims 1 - 6, characterised in that the sulphidic concentrate is a zinc concentrate.

8. A method according to any of previous claims 1 - 6, characterised in that the sulphidic concentrate is a copper concentrate.

9. A method according to any of previous claims 1 - 8, characterised in that part of the oxygen to be used in the oxidation of the divalent iron is fed into the leaching stage.

10. A method according to any of previous claims 1 - 9, characterised in that the concentrate leaching stage comprises several reactors, whereupon the solution oxidised in the oxidation reactors is returned to the first reactor of the stage.