EP3074545A1

EP3074545A1 - Method and arrangement of separating arsenic from starting materials

Info

Publication number: EP3074545A1
Application number: EP14809444.4A
Authority: EP
Inventors: Akusti JAATINEN; Kari Hietala; Madeleine SCHEIDEMA; Tuukka KOTIRANTA
Original assignee: Outotec Finland Oy
Current assignee: Outotec Finland Oy
Priority date: 2013-11-29
Filing date: 2014-11-27
Publication date: 2016-10-05
Also published as: FI126884B; KR20160075688A; AR098554A1; KR101763549B1; WO2015079116A1; CL2016001252A1; FI20136200A; CN105765090A

Abstract

A method and arrangement of separating arsenic and optionally antimony from a starting material, comprising -a low acid leaching step for leaching a first part of iron, arsenic, copper and optionally antimony, into the first leaching solution, -a first solid-liquid separation step, wherein a first solid leach residue is separated from the first leach solution, -a high acid leaching step for leaching a second part of iron, arsenic, copper and optionally antimony into the second leach solution, -a second solid-liquid separation step for obtaining a second solid leach residue and the second leach solution is recycled back to the low acid leaching step, -a precipitation step for obtaining a precipitate comprising ferric arsenate and optionally antimony compounds, and -a third solid-liquid separation step, wherein the solution comprising copper is separated from the obtained precipitate.

Description

METHOD AND ARRANGEMENT OF SEPARATING ARSENIC FROM STARTING MATERIALS

FIELD OF THE INVENTION

The present invention relates to a method and arrangement of sepa- rating arsenic from copper-containing starting materials and more particularly to a method and arrangement of separating arsenic and optionally antimony and other heavy metals from copper-containing starting materials.

BACKGROUND OF THE INVENTION

Arsenic and antimony are two of the most relevant impurities, whose contents in an anode copper must be controlled to be within a reasonable level in a copper smelting process. As the copper ore grades decline the amount of impurities to be removed in the smelting process tends to increase. The problems caused by these impurities are even more complicated in a direct to blister -process compared to a traditional two-stage smelter comprising smelting and converting.

The use of impure concentrates leads to problems with the product anode copper quality. The anode copper is usually further refined by electrolysis, in which the presence of impurities such as arsenic (As) and antimony (Sb) cause major problems.

The last chance for preventing arsenic, antimony and other impurities from gathering in the anode copper is in the anode furnace. It is well known in the art that high removal of arsenic and antimony can be achieved by fluxing the anode furnace with a mixture of soda (Na₂CO₃) and lime (CaO, CaCO₃). The slag thus formed contains large amounts of arsenic and antimo- ny, which are desired to be removed from the process. However, discarding the slag is not an option due to its significant copper content. Some parts of the slag are also highly soluble in water and thus it would be environmentally hazardous waste.

In a conventional two-stage smelting process, the obtained so- da/lime slag is returned to an earlier process stage in the smelter, i.e. either to a reverbatory smelting furnace or to the Peirce-Smith converters. In these unit processes most of the arsenic and antimony will volatilize and arrive in the gas cleaning section or perhaps even in the atmosphere.

In a direct-to-blister process or in a Double Flash process eeding of the soda/lime anode furnace slag to flash furnaces requires grinding of the slag and volatilization of arsenic, antimony and other heavy metals is challenging in the Flash furnaces. This results in the accumulation of arsenic, antimony and other heavy metals in high levels in the circulation of the process. Furthermore, if electric slag cleaning furnaces were used, almost all of the arsenic and anti- mony would result in the metal phase.

Another output from the copper process from which it is desired to recover all the product metals is the flue dust. In current practice flue dust produced during flash smelting is either recycled directly back into the furnace or an impurity removal process is used prior to returning the dust to the furnace. In a conventional flash smelting process substances or compounds having high partial pressures are partially removed to gas phase during smelting and converting. However, a substantial proportion of these is ultimately contained in anode copper, which, as stated above, is then usually further refined by electrolysis, in which the presence of impurities such as arsenic (As) and anti- mony (Sb) cause major problems.

Directly recycling the flue dust will not solve the impurity problem as there is no outlet for the impurities and thus will even cause build-up of impurity concentrations in the process. The dust, however, needs to be recycled due to its high copper content in order to reduce the copper loss in the process.

In BG62291 B1 the method is used for the treatment of waste slags from the copper production, in particular for the preparation of finely ground slag waste in the flotation treatment of furnace and converter slags. It includes sulphur-acid extraction of copper and iron from the ground slag waste by waste acids with H₂SO concentrations from 5 to 25 g/l where the process occurs at intensive agitation and aeration of the suspension having solid matter content from 20 to 45 wt.% until the neutralization of H₂SO (pH about 2) and subsequent dewatering. The solid matter residue having relative share up to 70% is used in mixtures for mine pit infilling, and the copper is extracted successively by cementation with FeO, and arsenic - as a ferriarsenate - by the iron extract- ed from the slag by the addition of lime cream.

US 5762891 A discloses a method to remove arsenic from arsenic- containing materials, such as an ore or concentrate, by roasting the arsenic- containing material to convert arsenic sulfides into arsenic oxides. The arsenic oxides are contained in the roasted arsenic-containing material. The roasted arsenic-containing material is contacted with a lixiviant to solubilize the arsenic in the oxide in a pregnant leach solution. Ferric arsenate, an environmentally stable compound, is formed in the lixiviant. The ferric arsenate can be removed to provide a treated solution.

US 2009022639 A1 discloses a method for the treatment of material containing at least one valuable metal and arsenic to form a valuable metal- depleted scorodite sediment and a pure aqueous solution to be discharged from the process. The valuable metals are first removed from the material to be treated and then arsenic precipitation from the solution is performed in two stages. By means of the method, the aim is to obtain as low a valuable metal content as possible in the scorodite sediment that will be formed. Likewise, the arsenic and valuable metal content of the aqueous solution that is formed during arsenic precipitation also remains so low that the water can be released into the environment.

Various methods of precipitating arsenic from starting materials are known. However, none of these solve the problem of recovering all the availa- ble copper in the same process. For example, publication JP 2008143741 discloses a method of synthesizing a compact ferric-arsenic compound by treating an arsenic-containing liquid by using an inexpensive oxidizing agent. Publication US 2009/0078584 discloses a process for producing a crystalline scorodite from an acidic aqueous solution containing pentavalent As and trivalent Fe. Publication US 2008/0075644 discloses a method of producing an iro- arsenic compound by adding an oxidizing agent to an aqueous solution containing arsenic ions and bivalent iron and allowing an iron-arsenic compound precipitation reaction proceed.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a method and an arrangement for implementing the method so as to alleviate the above disadvantages. The objects of the invention are achieved by a method and an arrangement, which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the de- pendent claims.

The invention is based on the idea of separating arsenic and optionally antimony from a starting material comprising copper, arsenic and optionally iron and antimony, which method comprises a low acid leaching step for leaching a first part of copper, arsenic and if present iron and antimony into the first leaching solution and a high acid leaching step for leaching a second part of copper, arsenic and if present iron and antimony into the second leach solution and a precipitation step for obtaining from the first leaching solution a precipitate comprising ferric arsenate and optionally antimony compounds. The method also produces a leach solution comprising copper and optionally other metals, which can be recovered by known methods.

An advantage of the method and arrangement of the invention is that arsenic and antimony and other heavy metals contents in an anode copper are controlled to be within desired level in a copper process. A further advantage of the present invention is that the method and arrangement provide a cost effective way of handling the problems caused by the presence of arsenic, antimony and possibly other heavy metals in the direct-to-blister and/or Double Flash processes, wherein no convenient point for recycling an anode furnace soda/lime slag as such exists. A further advantage of the present invention is that the arsenic is collected as stable ferric arsenate product from the process instead of volatilizing the arsenic. The risk of some of the arsenic resulting in the atmosphere is thereby significantly reduced. A further advantage of the present method and arrangement is also that the arsenic and antimony separated from the starting material are removed from the process in most concentrated form.

A further advantage of the method and arrangement of the present invention is that copper can be recovered with high yield. The present method and arrangement are especially suitable to be used in connection with a direct- to-blister or double flash processes. Especially, the quality and quantity of anode copper is greatly improved, when applying the present method and ar- rangement. Recovery of copper is at highest possible level due to the two-step leaching in the method and the re-circulation of the intermediate products, i.e. the leaching residue comprising unleached copper, after the high acid leaching step, back to the smelter process and thereby enabling recovering the unleached copper in useful form.

An advantage of the present invention is that by combining the hy- drometallurgical process steps with the precipitation step the co-precipitation of copper in the precipitation step can be minimized and the copper can be recovered in a suitable form by using the hydrometallurgical process steps. Thereby, the precipitate, such as ferric arsenate, does not comprise significant amounts of valuable metals and can be removed as waste from the method and a very high yield of copper, typically over 99 % of all copper present in the starting material, is recovered as a copper product, such as copper sulphate solution. An advantage of the present method is that the only possible copper losses, typically not more than 0.5 to 2% of the total copper, occur in the precipitation step. Copper is recoverable form all streams, such as the leaching residue, which is typically recycled back to smelting process.

BRIEF DESCRIPTION OF THE DRAWING

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawing, in which

Figure 1 shows an example embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of separating arsenic and optionally antimony from a starting material comprising copper, arsenic, and optionally iron and antimony, wherein the method comprises

- a low acid leaching step, wherein the starting material is contacted under atmospheric pressure with a first leach solution comprising sulphuric acid for leaching a first part of copper, arsenic, and if present iron and antimony into the first leaching solution,

- a first solid-liquid separation step, wherein a first solid leach resi- due is separated from the first leach solution,

- a high acid leaching step, wherein the first solid leach residue is contacted under atmospheric pressure and oxidizing conditions with a second leach solution comprising sulphuric acid for leaching a second part of copper, arsenic and if present iron and antimony into the second leach solution,

- a second solid-liquid separation step, wherein a second solid leach residue is separated from the second leach solution and the second leach solution is recycled back to the low acid leaching step,

- a precipitation step, wherein the first leach solution comprising copper, arsenic and if present iron and antimony obtained from the first solid- liquid separation step is contacted with a precipitating agent for obtaining a precipitate comprising ferric arsenate and optionally antimony compounds,

- a third solid-liquid separation step, wherein the solution comprising copper is separated from the obtained precipitate.

The starting material may be any material comprising copper, iron, arsenic, antimony and optionally other heavy metals. According to an embodi- ment of the invention the starting material is anode furnace slag, flue dust or mixture thereof. The anode furnace slag is typically slag obtained from anode furnaces of direct-to-blister process and/or double flash process. Flue dust is typically obtained from a smelter or metallurgical furnace. Flue dust is typically used as starting material in the method together with anode furnace slag. Thereby the amount of the leached material can be optimized to be small and the size of the process equipment is not unnecessarily increased.

The anode furnace soda/lime slag typically comprises oxides and carbonates of sodium and calcium, arsenic and antimony in compounds con- taining also sodium, calcium and oxygen, and other impurities. Soda/lime slag contains copper as dissolved oxides in the slag matrix as well as metallic copper droplets mechanically entrained in the slag.

The flue dust typically comprises small particles from the smelting process feed materials that become oxidized and sulphatized in the process. More volatile species of the feed materials will become concentrated in the flue dust, as they can become volatilized in the smelting process, but solidify again during cooling. Mostly the dust comprises copper and iron as oxides and sulphates. Of impurities, particularly zinc, lead, arsenic and bismuth are concentrated in the flue dust.

The starting material, such as anode furnace slag, flue dust or mixture thereof is typically cooled to a suitable temperature before feeding to the low acid leaching step. In connection with the cooling some metallic fractions, such as a copper fraction may be recovered mechanically.

When both anode furnace slag and flue dust are used as starting material they can be fed to the low acid leaching step either simultaneously or in turns, i.e. alternating between feeding anode furnace slag and flue dust.

If needed, the method of the present invention optionally comprises crushing and/or grinding of the starting material to a suitable particle size before feeding to a low acid leaching step. The suitable particle size is typically in the range of 0 - 100 μιτι. The crushing and/or grinding may be performed with any methods of crushing or grinding known in the art.

The method comprises a low acid leaching step, wherein the starting material, optionally grinded, is contacted under atmospheric pressure, and optionally in oxidizing conditions, with a first leach solution comprising sul- phuric acid for leaching a first part of copper and arsenic, and if present iron and antimony into the first leaching solution. The first leaching step of the pre- sent method is a neutralizing low acid leaching step, wherein the acidic intermediate leach solution, i.e. the second leach solution obtained from the high acid leaching step, is contacted with the starting material.

The optionally applied oxidizing conditions are typically obtained by feeding oxygen-containing gas, such oxygen, air and/or air enriched with oxygen into the low acid leaching step. The oxidizing conditions are applied for obtaining better leaching yield at this stage.

The first leach solution comprises sulphuric acid, typically in the range of 10 to 50 g/l, more typically in the range of 15 to 45 g/l, even more typ- ically in the range of 15 to 30 g/l.

The temperature in the low acid leaching step is typically in the range of 20 to 100°C, more typically in the range of 60 to 90°C.

In the low acid leaching step a first part of copper, arsenic and if present iron and antimony are leached into the first leaching solution. Optional- ly other heavy metals, such as Cd, Te, Se, may also be leached into the first leaching solution. Typically, only unavoidable amounts of these are left un- leached in the first solid leach residue. However, if leaching yields of these elements are not sufficient in the low acid leaching step, the first solid leach residue will be fed to a high acid leaching step to be leached in a significantly higher acid concentration. The first part of copper, typically copper in easily leachable form, such as Cu₂O and/or CuO, which is leached in the low acid leaching step, is typically in the range of 40 to 60% of the total copper present in the starting material. However, this is only a typical example and the amount of metals leached in the low acid leaching step depend on the composition and content of the starting material and on the conditions used in the low acid leaching step, such as the presence of oxidation. The method of the present invention is able to handle a variety of different starting materials with only very small changes.

After the low acid leaching step the present method comprises a first solid-liquid separation step, wherein a first solid leach residue is separated from the first leach solution. The first solid-liquid separation step may be performed by any method known in the art, such as a thickener, centrifuge, filter or any combination thereof. The first solid leach residue typically comprises the unleached metals from the low acid leaching step. The composition of the first solid leach residue depends on the starting material used and the low acid leaching conditions applied. The method of the present invention comprises a high acid leaching step, wherein under atmospheric pressure and oxidizing conditions the first solid leach residue is contacted with a second leach solution comprising sulphuric acid for leaching a second part of copper, arsenic and if present iron and antimony into the second leach solution.

Here too, the oxidizing conditions are typically obtained by feeding oxygen-containing gas, such oxygen, air and/or air enriched with oxygen into the high acid leaching step. The oxidising conditions are applied for leaching the metallic copper in the high acid leaching step.

The second leach solution comprises sulphuric acid, typically in the range of 50 to 150 g/l, more typically in the range of 60 to 1 10 g/l, even more typically in the range of 65 to 100 g/l.

The temperature in the high acid leaching step is typically in the range of 40 to 100°C, more typically in the range of 85 to 98°C.

In the high acid leaching step a second part of copper, arsenic, and if present iron and antimony, typically copper in metal form, is leached into the second leaching solution. Optionally also a second part of other heavy metals, such as Cd, Te, Se, i.e. metals not leached during the low acid leaching step, may be leached during the high acid leaching step. The second part of copper, which is leached in the high acid leaching step, can be as high as 99% of the copper present in the first solid leach residue. However, this naturally depends on the composition of the starting material and in which form the copper is present in the starting material.

The low acid leaching step and the high acid leaching step are typi- cally operated in a co-current manner.

After the high acid leaching step the method comprises a second solid-liquid separation step, wherein a second solid leach residue is separated from the second leach solution and the second leach solution is recycled back to the low acid leaching step. The second solid-liquid separation step may be performed by any method known in the art, such as a thickener, centrifuge, filter or any combination thereof. The second leach solution, typically comprising sulphuric acid in the range of 50 to 150 g/l, more typically in the range of 60 to 1 10 g/l, even more typically in the range of 65 to 100 g/l and copper in leached form typically in the range of 40 to 70 g/l, is returned in its entirety to the low acid leaching step. Thereby it is enabled that all the copper leached in the high acid leaching step will result in the first leach solution and is removed from the low acid leaching step as a solution to be fed to the precipitation step. Here again, the amount of leached copper depend on the starting material used and the conditions, such as oxidation, applied during leaching steps.

The second solid leach residue, typically comprising copper 0.5 - 2% and small amounts of all metals contained in the feed material, such as Pb and Ag, which are not leachable in a sulphate environment, may be fed back to a previous process step, such as smelting furnace or slag concentrator. If necessary, the second solid leach residue may be dried before feeding to the smeltering furnace or slag concentrator. If the second solid leach residue does not contain significant amount of copper desired to be recovered, it may be safely discarded.

After the first solid-liquid separation step the method of the invention comprises a precipitation step, wherein the first leach solution comprising copper, arsenic and if present iron and antimony separated in the first solid-liquid separation step is contacted with a precipitating agent, typically under oxidizing conditions, for obtaining a precipitate comprising ferric arsenate and antimony compounds and unavoidable amounts of copper. The ferric arsenate product is an outlet for arsenic and antimony, and optionally other heavy metals from the process of the present invention.

The precipitating agent is typically selected from the group consisting of calcium, sodium or ammonia based precipitation agents such as NaOH, Na₂CO₃, ammonia gas, ammonia water, CaO, CaCO₃, Ca(OH)₂ or mixtures thereof. Typically the precipitating agent is CaCO₃ or Ca(OH)₂, .When calcium- based precipitating agents are used for precipitating As also gypsum is formed.

Often the first leach solution obtained from the first solid-liquid separation step and fed to the precipitation step contains enough iron for precipitating the arsenic and if present antimony as ferric arsenate and antimony compounds. However, if this is not the case, iron may be added to the precipitation step in the form of ferric sulphate (Fe₂(SO₄)₃) and/or ferrous sulphate FeSO₄. Also fajalitic waste slag can be used as an iron source. If needed the fajalitic waste slag is added to the low acid leaching step. By ensuring that iron is present in an adequate amount in the precipitation step, the co-precipitation of copper, and thus copper loss, is minimized. Typically the Fe:As ratio is between 4:1 - 1 :1 , more typically approximately 2:1 .

The precipitation step is typically performed under oxidizing conditions, which are obtained by supplying oxygen, oxygen-containing gas, such as air or air enriched with oxygen into the precipitation step or by oxidizing the solution in the previous process step (low acid leaching). According to an embodiment of the invention the presence of copper and the oxidizing conditions optionally applied ensure that arsenic is as As⁵⁺ in the solution to be precipitat- ed. This has the effect that the arsenic is precipitateable in its most stable form.

The precipitation step is performed typically in a pH in the range of 1 .0 to 3.5, more typically in the range of 1 .5 to 2.5. The precipitate thus formed is scorodite and the co-precipitation of copper can be minimized.

After the precipitation step the obtained solution comprises a significant amount of copper and therefore the present method comprises a third solid-liquid separation step, wherein the solution comprising copper is separated from the obtained precipitate. The third solid-liquid separation step may be performed by any method known in the art, such as a thickener, centrifuge, filter or any combination thereof.

From the copper-containing solution obtained from the third solid- liquid separation step the copper can optionally be recovered as copper sulphate crystals or the obtained copper-containing solution can be routed to an existing solvent extraction, electrorefining or electrowinning plant for recovering the copper. The obtained copper sulphate crystals can be sold as a product as such or recycled to the smelting process to be recovered in a more suitable form. After the copper sulphate crystals have been separated a bleed is obtained, from which it is possible to recover remaining copper and optionally some other metals by precipitating them as hydroxides or sulphides. The ob- tained precipitate may be further refined or be recycled back to the smelter. The solution from copper sulphate precipitation may be partly recycled back to the low acid leaching step.

From the copper-containing solution obtained from the third solid- liquid separation step the copper can alternatively be recovered as sulphides or hydroxides by precipitation. The obtained precipitate can be further refined or routed back to the smelter. The obtained solution is mainly sodium sulphate and may be discarded if permitted by environmental laws. Alternatively, all metals left in the solution may be crystallized and the sodium may be recycled for fluxing an anode furnace.

The residence time in each process step is adjusted to a suitable level in which the desired effect is accomplished. The present invention relates also to an arrangement for implementing the method of the present invention. The arrangement of separating arsenic and optionally antimony from a starting material comprising copper, arsenic and optionally iron, and antimony, wherein the arrangement comprises

- a low acid leaching unit adapted for contacting the starting material under atmospheric pressure, and optionally under oxidizing conditions, with a first leach solution comprising sulphuric acid for leaching a first part of copper, arsenic, and if present iron and antimony, into the first leaching solution,

- a first solid-liquid separation unit adapted for separating a first sol- id leach residue from the first leach solution,

- a high acid leaching unit adapted for contacting the first solid leach residue under atmospheric pressure and oxidizing conditions with a second leach solution comprising sulphuric acid for leaching a second part of copper, arsenic and if present iron and antimony into the second leach solution,

- a second solid-liquid separation unit adapted for separating a second solid leach residue from the second leach solution and means adapted for recycling the second leach solution back to the low acid leaching unit,

- a precipitation unit adapted for contacting the first leach solution obtained from the first solid-liquid separation unit with a precipitating agent, optionally under oxidizing conditions for obtaining a precipitate comprising ferric arsenate and optionally antimony compounds, and

- a third solid-liquid separation unit, adapted for separating the solution comprising copper from the obtained precipitate.

In an embodiment of the arrangement the low acid leaching unit and the high acid leaching unit are adapted to function in a co-current manner.

In an embodiment of the arrangement the first, second or third solid- liquid separation unit comprises a thickener, a filter, a centrifuge or any combination thereof.

Figure 1 is an example embodiment of the present invention. The method comprises an optional grinding 6 of anode furnace slag 2 before feeding to low acid leaching step 8. Flue dust 4 is optionally fed to the low acid leaching step 8. Also optionally water and sulphuric acid or sulphuric acid containing solution and recycled solution from the copper recovery step or part of stream 36 solution may be fed to the low acid leaching. The anode furnace slag 2 and the flue dust can be fed to the process in campaigns (only slag or only dust) or they can be fed simultaneously. Oxygen 10 is optionally fed to the low acid leaching step 8, wherein the starting material 2 and optionally 4 are leached in the presence of sulphuric acid in the concentration of 15 to 30 g/l. Temperature in the low acid leaching step is 80°C and the pressure is atmospheric pressure. A first part of copper, iron, arsenic, antimony, possibly other heavy metals present are leached in the low acid leaching step 8. From the low acid leaching step 8 the obtained first leaching solution 21 is fed to a first solid- liquid separation step 12, wherein the solid matter is separated from the first leach solution by a thickener and a filter. The obtained solid matter 23 is fed to a high acid leaching step 14, wherein the sulphuric acid concentration is kept at 80 g/l and the temperature at 95°C. Sulphuric acid 1 1 and optionally oxygen 13 are fed to the high acid leaching step 14. The pressure is kept at atmospheric pressure. In the high acid leaching step 14 the second part of iron, arsenic, antimony and copper is leached. The second leach solution 25 obtained from the high acid leaching is fed to a second solid-liquid separation step 16, wherein the solid leach residue is separated from the solution 20 containing copper in leached form. The solution 20 is recycled back to the low acid leaching step 8. The leach residue 18 may be recycled back to a smelter (not shown in the Figure).

From the first solid-liquid separation step 12 the obtained solution 22 is fed to a precipitation step 24. Precipitating agent 26 comprising Ca(OH)₂ and optionally oxygen 28 are fed to the precipitation step 24. The pH is kept in the range of 1 .5 to 2.5. Arsenic and iron are precipitated as ferric arsenate compound, such as scorodite. The obtained slurry 30 is fed from the precipitation step 24 to a third solid-liquid separation step 32, wherein the ferric arse- nate precipitate 34 is separated from the copper sulphate -containing solution 36. The copper sulphate -solution may be fed to further treating and recovering units (not shown in the Figure).

EXAMPLE

The feed material for low acid leaching was grinded copper material and flue dust. The combined feed material analysis was Cu 39%, Fe 4.7%, As 4.8% and Pb 18%. The total solid material fed to the leaching was 1 kg. The particle size of D80 for the copper material was 50 μιτι and 41 % of the copper in copper material was metallic copper. The batch size for low acid leaching test was 5 liters and for high acid leaching and arsenic precipitation test was 2.5 liters. The residence time was 3 hours for low acid leaching, 6 hours for high acid leaching and 10 hours for arsenic precipitation test. The tests were done in an agitated reactor. The acid used in leaching tests was concentrated sulphuric acid (about 97%) and the neutralization agent used in arsenic precipitation was limestone slurry with 250 g/l solid concentration.

The material was leached first at low acid leaching in conditions of acid concentration 35 - 45 g/l and oxygen was not fed to the reactor. The yields for elements were: Cu 55%, Fe 81 % and As 67%. The solids were then separated from the leach solution and solids were fed to the high acid leaching test. The solids were not washed. The solids contained 29% Cu, 1 .5% Fe, 2.6% As and 30% Pb. The solution contained 38 g/l Cu, 6.7 g/l Fe and 6.7 g/l As.

The conditions in high acid leaching test were acid concentration of 70 - 1 10 g/l and oxygen was fed to the leaching. The yields for elements were: Cu 99%, Fe 40% and As 55% in the high acid leaching stage. The total yield in both leaching stages was: Cu 99%, Fe 89% and As 85%. The leaching residue contained 1 .2% Cu, 1 .3% Fe, 1 .7% As and 43% Pb. All lead fed to the low acid leaching reported to this leaching residue. The solution contained 61 g/l Cu, 1 .9 g/l Fe and 2.9 g/l As.

The arsenic precipitation test was done for solution containing: 74 g/l Cu, 7.3 g/l Fe and 7.7 g/l As. The acid concentration was kept at 15 g/l for first three hours, at 10 g/l for next four hours and at 5 g/l for last three hours. At the end of the test 75% of iron and 88% of arsenic was precipitated. Also 0.5% of copper was co-precipitated. Since the neutralizing agent was limestone the precipitate contained mainly gypsum. It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be imple- mented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

LIST OF REFERENCE NUMBERS

2 anode furnace slag

4 flue dust

6 grinding

8 low acid leaching step

10 oxygen

1 1 sulphuric acid

12 a first solid-liquid separation step

13 oxygen 14 a high acid leaching step

16 a second solid-liquid separation step

18 leach residue

20 solution containing copper in leached form

21 first leaching solution

22 solution obtained from the first solid-liquid separation step 12

24 a precipitation step

23 solid matter

25 a second leaching solution

26 precipitating agent

28 oxygen

30 slurry

32 a third solid-liquid separation step

34 ferric arsenate precipitate

36 copper sulphate -containing solution

Claims

1 . A method of separating arsenic and optionally antimony from a starting material comprising copper, arsenic, and optionally iron and antimony, wherein the method comprises

2. The method according to claim 1 , wherein the starting material is anode furnace slag, flue dust or mixture thereof.

3. The method according to claim 2, wherein the anode furnace slag is grinded to particle size in the range of 0-10Όμηη before feeding to the low acid leaching step.

4. The method according to any one of the preceding claims, where- in the first leaching solution in the low acid leaching step comprises sulphuric acid 10 - 50 g/l, typically 15 - 45 g/l, more typically 15 - 30 g/l.

5. The method according to any one of the preceding claims, wherein the temperature in the low acid leaching step is in the range of 20 - 100°C, typically in the range of 60 - 90°C.

6. The method according to any one of the preceding claims, wherein the second leaching solution in the high acid leaching step comprises sulphuric acid in the range of 50 - 150 g/l, typically 60 - 1 10 g/l, more typically in the range of 65 - 100 g/l.

7. The method according to any one of the preceding claims, wherein the temperature in the high acid leaching step is in the range of 40 - 100°C, typically in the range of 85 - 98°C.

8. The method according to any one of the preceding claims, wherein all of the second leach solution obtained from the second solid-liquid separa- tion step is returned to the low acid leaching step.

9. The method according to any one of the preceding claims, wherein the precipitating agent is selected from the group consisting of NaOH, Na₂CO₃, ammonia gas, ammonia water, CaO, CaCO₃, Ca(OH)₂ or mixtures thereof.

10. The method according to any one of the preceding claims, wherein the precipitation step is performed in a pH in the range of 1 .0 - 3.5, typically in the range of 1 .5 - 2.5.

1 1 . The method according to any one of the preceding claims, wherein ferric sulphate (Fe₂(SO₄)₃) and/or ferrous sulphate FeSO₄ is added to the precipitation step.

12. The method according to any one of the preceding claims, wherein fajalitic waste slag is added to the low acid leaching step.

13. The method according to any one of the preceding claims, wherein Fe:As ratio is between 4:1 - 1 :1 , typically 2:1 .

14. The method according to any one of the preceding claims, wherein the second leach residue is routed to a smelting furnace or slag concentrator.

15. The method according to any one of the preceding claims, wherein the low acid leaching step is performed under oxidizing conditions.

16. The method according to any one of the preceding claims, wherein the precipitation step is performed under oxidizing conditions.

17. An arrangement of separating arsenic and optionally antimony from a starting material comprising copper, arsenic and optionally iron and antimony, wherein the arrangement comprises

- a low acid leaching unit adapted for contacting the starting material under atmospheric pressure and optionally oxidizing conditions with a first leach solution comprising sulphuric acid for leaching a first part of copper, arsenic and if present iron and antimony into the first leaching solution,

- a first solid-liquid separation unit adapted for separating a first solid leach residue from the first leach solution,

- a high acid leaching unit adapted for contacting the first solid leach residue under atmospheric pressure and oxidizing conditions with a second leach solution comprising sulphuric acid for leaching a second part of copper, arsenic, and if present iron and antimony into the second leach solution,

- a second solid-liquid separation unit adapted for separating a sec- ond solid leach residue from the second leach solution and means adapted for recycling the second leach solution back to the low acid leaching unit,

- a precipitation unit adapted for contacting the first leach solution obtained from the first solid-liquid separation unit with a precipitating agent, optionally under oxidizing conditions for obtaining a precipitate comprising fer- ric arsenate and optionally antimony compounds, and

- a third solid-liquid separation unit, adapted for separating solution comprising copper from the obtained precipitate.

18. The arrangement according to claim 17, wherein the low acid leaching unit and the high acid leaching unit are adapted to function in a co- current manner.

19. The arrangement according to claim 17 or 18, wherein the first, second or third solid-liquid separation unit comprises a thickener, a filter, a centrifuge or any combination thereof.