Method for removing arsenic from materials containing same

Abstract

Disclosed is a method for the removal of arsenic from materials containing a high arsenic content, or materials containing a high content of arsenic and selenium.

Claims

1. A process for the removal of arsenic from copper concentrates and/or copper cement with an arsenic content higher than 0.5% by dry weight or copper concentrates and/or copper cement with a high content of arsenic and selenium, comprising: adding the copper concentrates and/or copper cement to a pressure reactor; adding an alkaline leaching solution of a strong base selected from sodium hydroxide and potassium hydroxide dissolved in water to the pressure reactor; adding an oxidizing gas to the pressure reactor; mixing the above components in the pressure reactor to obtain a homogenous pulp and subjecting it to a pressure lixiviation that is selective for arsenic, wherein operation conditions of the pressure lixiviation are: temperature between 100 C. and 220 C., residence time of the pulp within the pressure reactor between 30 and 150 minutes, quantity of the leaching solution in the case of NaOH between 1.87 and 45.0 kg NaOH/kg As contained in the pulp, and oxidizing gas overpressure between 0 and 100 psig, wherein the dissolution of copper present in the copper concentrates and/or copper cement during said pressure lixiviation is less than 0.05% of the total copper, the dissolution of gold present in the copper concentrates and/or copper cement is less than 4% of the total gold and the dissolution of silver present in the copper concentrates and/or copper cement is less than 0.4% of the total silver; subjecting the pulp obtained from the pressure lixiviation to a first solid-liquid separation step thereby separating a solid with low arsenic content and a liquor containing dissolved arsenic in its +5 oxidation state in the form of arsenate; subjecting the liquor with dissolved arsenic to a precipitation of the arsenic with a precipitating agent that provides cations selected from Ce.sup.3+, Fe.sup.3+, Mg.sup.2+, or a combination of Fe.sup.3+ and Ca.sup.2+, said precipitating agent being selected from cerium chloride, ferric sulfate, magnesium sulfate, or ferric sulfate with addition of lime milk; subjecting the product of the arsenic precipitation step to a second solid-liquid separation step, thereby obtaining a solid arsenic-containing product and an alkaline liquor free of arsenic.

2. The process for the removal of arsenic according to claim 1, further comprising: subjecting the alkaline liquor free of arsenic to a sodium sulfate (Na.sub.2SO.sub.4) crystallization step, thereby obtaining a pulp composed of Na.sub.2SO.sub.4 crystals and an alkaline liquor free of Na.sub.2SO.sub.4; and subjecting the product of the Na.sub.2SO.sub.4 crystallization step to a third solid-liquid separation step, thereby obtaining a solid comprising Na.sub.2SO.sub.4 crystals and an alkaline liquor.

3. The process for the removal of arsenic according to claim 2, wherein the alkaline liquor free of arsenic from the third solid-liquid separation step is partially or totally recirculated as part of the leaching solution of the pressure lixiviation, and the liquor that is not recirculated being subjected to a secondary arsenic elimination step of adsorption or ion exchange, or used as process water.

4. The process for the removal of arsenic according to claim 2, wherein a fraction of the liquor with dissolved arsenic from the first solid-liquid separation step is recirculated to the pressure lixiviation while the other fraction is sent to the arsenic precipitation step, and the alkaline liquor free of arsenic from the third solid-liquid separation step is used as process water for recirculation.

5. The process for the removal of arsenic according to claim 1, wherein a fraction of the alkaline liquor free of arsenic from the second solid-liquid separation step is recirculated to the pressure lixiviation while the other fraction is sent to an Na2SO4 crystallization step or sent to tailings.

6. The process for the removal of arsenic according to claim 2, wherein the Na.sub.2SO.sub.4 crystallization step comprises a technique selected from the group consisting of continuous evaporation at constant volume, semi-continuous evaporation at constant volume, cooling, total evaporation of solvent, and evaporation in a solar pond.

7. The process for the removal of arsenic according to claim 1, wherein the copper concentrates and/or copper cement to be treated also contains a high selenium content and the pressure lixiviation is selective for the dissolution of arsenic and selenium.

8. The process for the removal of arsenic according to claim 1, wherein the oxidizing gas is selected from pure oxygen, enriched air, or air.

9. The process for the removal of arsenic according to claim 1, wherein the pressure reactor is an autoclave, horizontal or vertical, with one or more stirrers, with one or more compartments separated by baffles, and with submerged or overhead injection of gas or both.

10. The process for the removal of arsenic according to claim 1, wherein the mixing step is carried out by a repulping of the copper concentrates and/or copper cement with the alkaline leaching solution, and homogenizing the pulp to keep the percentage of solid within a range of 10-40% by weight.

11. The process for the removal of arsenic according to claim 1, wherein when the oxidizing gas is air, the overpressure in the pressure reactor is 10-40 psig.

12. The process for the removal of arsenic according to claim 1, wherein the pulp resulting from the pressure lixiviation has a pH of 10-14 and a redox potential greater than 0.5 V vs. SHE.

13. The process for the removal of arsenic according to claim 1, wherein the solid-liquid separation steps are carried out by filtration, sedimentation, clarification, thickening, centrifugation, dewatering or decantation.

14. The process for the removal of arsenic according to claim 1, wherein the solid with low arsenic content obtained from the first solid-liquid separation step is further subjected to a wash, where said wash liquor is sent to the arsenic precipitation step together with the mother liquor, and the washed solid is stored, or sent to a process of recovery of the remaining valuable components therein.

15. The process for the removal of arsenic according to claim 1, wherein when the precipitating agent is CeCl.sub.3, the dose of the agent is 1.80-7.50 kg Ce.sup.3+/kg As and the precipitation is carried out at a pH of 6-12, and the pH is adjusted with H.sub.2SO.sub.4.

16. The process for the removal of arsenic according to claim 1, wherein when the precipitating agent is Fe.sub.2(SO.sub.4).sub.3, the dose of the agent is 0.70-8.0 kg Fe.sup.3+/kg As and the precipitation is carried out at a pH of 6-10, and the pH is adjusted with H.sub.2SO.sub.4.

17. The process for the removal of arsenic according to claim 16, wherein the Fe.sub.2(SO.sub.4).sub.3 is added directly, or is previously prepared from iron (II and III) oxide with H.sub.2SO.sub.4 or from ferrous sulfate (FeSO.sub.4) with H.sub.2O.sub.2, H.sub.2SO.sub.4 and hot water.

18. The process for the removal of arsenic according to claim 16, wherein additionally lime milk is added to the Fe.sub.2(SO.sub.4).sub.3 precipitating agent at a dose of 0.50-2.5 kg Ca.sup.2+/kg As.

19. The process for the removal of arsenic according to claim 1, wherein when the precipitating agent is MgSO.sub.4, the dose of the agent is 0.45-1.50 kg Mg.sup.2+/kg As and the precipitation is carried out at a pH of 7-14, and the pH is adjusted with H.sub.2SO.sub.4.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1: A schematic flow diagram of the process of the present application, in which the arsenic compounds are selectively solubilized for the removal of contained arsenic by lixiviation under pressure through the action of a fresh alkaline solution and a recycled solution coming from the sodium sulfate filtration step.

(2) FIG. 2: A schematic flow diagram of an alternative, equally satisfactory, configuration of the process of the present application in which the liquors with solubilized arsenic are recirculated to be used in the alkaline lixiviation step and a purge is set up for treatment to remove the arsenic and crystallize Na.sub.2SO.sub.4. The final solution is process water to be recirculated to the plant.

(3) FIG. 3: A schematic flow diagram of an alternative, equally satisfactory, configuration of the present application in which the liquors with solubilized arsenic are treated to remove the arsenic and are then recirculated to be used in the alkaline lixiviation step. A purge is set up to remove the excess Na.sub.2SO.sub.4.

(4) The diagrams shown in FIGS. 1, 2 and 3 are equally valid for materials that contain arsenic and/or selenium. Selenium follows the same route as the arsenic insofar as it is present in the solutions. The removal methods leave it in the same precipitates as the arsenic.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present application refers to a process for the selective removal of arsenic from copper concentrates and other materials with high arsenic content. The present application also refers to the selective removal of arsenic and/or selenium from materials with high arsenic and/or selenium content. The present application also comprises the precipitation of arsenic and/or selenium from the resulting alkaline solutions for its safe and environmentally sustainable disposal. The complete process (removal and precipitation) is based on experimental results on laboratory and pilot scale, and also considers technical aspects and industrial criteria for its scale-up.

(6) Three block diagrams and a detailed description of each step of the process are presented. The first figure shows the general process of selective removal and precipitation of arsenic; FIGS. 2 and 3 show equally satisfactory alternatives for performing the process. Numerical references are included during the description of the process of the invention, as applicable. The same numerical references will be used to indicate the same steps or flows in the figures.

(7) The present application proposes a process for the selective removal of arsenic from copper concentrates and other materials with high arsenic content, and from other materials that contain said element. The present application also refers to the selective removal of arsenic and/or selenium from materials with high arsenic and/or selenium content. This process also describes steps to treat the arsenic and/or selenium removed from the starting material in such a way as to obtain two main products: a solid material with a low level of arsenic and/or selenium and another solid material with a high percentage of arsenic and/or selenium that forms part of a compound that is stable from the environmental point of view, allowing its safe disposal in duly authorized sites.

(8) The preferred material to be treated is a copper concentrate, without prejudice to the fact that the process is also applicable to copper cement and smelting and/or toasting filter powders (sulfides, oxides, metal or other) with high arsenic content that contain arsenic in concentrations higher than 0.5% by dry weight.

(9) For the case of copper concentrates, the arsenic compounds are preferably enargite (Cu.sub.3AsS.sub.4) and tennantite (Cu.sub.12As.sub.4S.sub.13). In addition to copper sulfides and arsenic compounds, the copper concentrate may contain iron sulfides, silica, alumina, feldspars and similar compounds.

(10) The process of the present application comprises a lixiviation step under pressure in a pressure reactor (4), which involves the contact of the material to be lixiviated (1) with an alkaline lixiviating solution of NaOH (2) in an oxidizing atmosphere (3), which dissolves the arsenic from the material to produce a pulp (5) that contains the liquor with dissolved arsenic and the solid material with low arsenic content.

(11) The reagents are fed into the lixiviation step (4) by re-pulping the copper concentrate or other material that contains a high arsenic content (1) with the alkaline lixiviating solution of NaOH (2). It must be ensured that the pulp generated is as homogeneous as possible to maintain the specific percentage of solid for the lixiviation step (4), preferably 10-40% by weight.

(12) The lixiviation step (4) of the material (1) comprises the dissolution of arsenic preferably in the form of arsenate (AsO.sub.4.sup.3) as a soluble anion in the pulp (5) obtained in the lixiviation step.

(13) In the case of treatment of copper concentrates that contain enargite and/or tennantite, the chemical reactions that describe the phenomenon that occurs in the lixiviation step (4) are the following:
Cu.sub.3AsS.sub.4+8NaOH+5O.sub.2=1.5Cu.sub.2S+Na.sub.3AsO.sub.4+2.5Na.sub.2SO.sub.4+4H.sub.2O(I)
Cu.sub.3AsS.sub.4+5NaOH+2.75O.sub.2=3CuS+Na.sub.3AsO.sub.4+Na.sub.2SO.sub.4+2.5H.sub.2O(II)
Cu.sub.12As.sub.4S.sub.13+26NaOH+15.5O.sub.2(g)=6Cu.sub.2S+4Na.sub.3AsO.sub.4+7Na.sub.2SO.sub.4+13H.sub.2O(III)
Cu.sub.12As.sub.4S.sub.13+14NaOH+6.5O.sub.2(g)=12CuS+4Na.sub.3AsO.sub.4+Na.sub.2SO.sub.4+7H.sub.2O(IV)

(14) The above chemical reactions are based on the formation of arsenate (AsO.sub.4.sup.3) and copper sulfides. Nevertheless, there are chemical reactions that will also show formation of AsO.sub.4.sup.3 while forming copper oxide (CuO) instead of copper sulfides. As the thermodynamic data for the compounds taking part in these chemical reactions (enargite, tennantite and sodium arsenate) are not known, the occurrence of reactions forming arsenate and copper sulfides could only be confirmed experimentally. The mechanism by which CuO would be formed instead of CuS is the following:
Cu.sub.3AsS.sub.4+11NaOH+8.75O.sub.2=3CuO+Na.sub.3AsO.sub.4+4Na.sub.2SO.sub.4+5.5H.sub.2O(V)
Cu.sub.12As.sub.4S.sub.13+38NaOH+30.5O.sub.2(g)=12CuO+4Na.sub.3AsO.sub.4+13Na.sub.2SO.sub.4+19H.sub.2O(VI)

(15) The caustic soda used in the process also dissolves the gangue from the concentrate, as is shown in the following reactions:
SiO.sub.2+2NaOH=Na.sub.2SiO.sub.3+H.sub.2O(VII)
KAlSi.sub.3O.sub.8+6NaOH=KOH+Al(OH).sub.3+3Na.sub.2SiO.sub.3+H.sub.2O(VIII)

(16) The process is also applicable to other materials (1) that contain arsenic in the form of sulfides or oxides, such as copper cements with high arsenic sulfide content and filter powders from smelting or toasting containing sulfides and oxides of arsenic.

(17) The conversion of these chemical reactions will depend on factors such as the residence time of the pulp within the equipment, the temperature, pressure and quantity of reagent.

(18) The present invention comprises a step subsequent to the lixiviation (4) which is considered to be a first solid-liquid separation step (7) of the liquor containing dissolved arsenic (9) from the solid with low arsenic content (8).

(19) In accordance with Alternative 1, presented in FIG. 1, the process comprises a step (11) that involves the precipitation of the arsenic dissolved in the liquors (mother liquor and wash liquor, if applicable) using a precipitating agent (10), forming a precipitate (12) that is a stable compound for disposal.

(20) In accordance with Alternative 1, the process may include a second solid-liquid separation step (13), separating the precipitated solid arsenic compound (14) from the alkaline liquors (15). This step leads to a solid arsenic compound that is stable for disposal (14).

(21) In the process described above, silica dissolved as sodium silicate co-precipitates to form part of the solid arsenic precipitate.

(22) Following the second solid-liquid separation step (7), Alternative 1 comprises a step consisting of an Na.sub.2SO.sub.4 crystallization process (16) from the alkaline liquors and a third solid-liquid separation step (18) of the product from the crystallization step (17). In this third solid-liquid crystallization step (18), a solid formed of Na.sub.2SO.sub.4 crystals (19) and an alkaline liquor (20) are obtained. The latter may in certain cases be used in part (20a) or in its entirety as a feed for the lixiviation step as a recycled solution.

(23) The lixiviation step (4) may be supplied with a recycled solution (20a) or with fresh alkaline solution (2). As the recycled solution may be used in part (20a) or in its entirety (20) to feed into the lixiviation step, any part that is not recirculated to the lixiviation step can be used as process water (20b).

(24) The lixiviating liquor in the present invention is based on sodium hydroxide as the main alkaline component. Nevertheless, other alkaline compounds can also be used, such as, for example, potassium hydroxide.

(25) The sodium hydroxide content in the lixiviating liquor (2) depends on the arsenic content of the material to be lixiviated (1). In this way, the dose of NaOH to perform the lixiviation (4) corresponds to a value of 1.87-45 kg NaOH/kg As contained in the material.

(26) The temperature used in the lixiviation step (4) is in the range of 100-220 C. For this reason, the lixiviation step (4) must be carried out in equipment suitable for such an operation, e.g., an autoclave. The operative basis of the autoclave(s) in this invention may be batch or continuous. The autoclave in itself may have various designs, e.g., horizontal or vertical; regardless, in all these designs the autoclave may have one or more stirrers, with one or more compartments separated by baffles, with submerged or overhead injection of gas or both.

(27) Furthermore, the lixiviation step (4) must be carried out with an oxidizing gas (3) feed. The oxidizing gas (3) may be pure oxygen, enriched air or air. In the case of this invention, it is been found that the oxidizing gas (3) is preferably air, as this allows a better control of the reduction potential of the solution so that the dissolved arsenic remains in the domain of stability for arsenate. This facilitates its removal as a stable compound, and also allows the dissolution of arsenic to be increased while reducing the solubilization of copper, gold and silver.

(28) The overpressure of oxidizing gas (3) depends on the objectives of the process, which are: the removal of arsenic from the solid to a final concentration of less than or equal to 0.5%; the maintenance of the arsenic in the liquors in the form of arsenate (As.sup.5+); and the non-dissolution of copper, gold, silver and/or other valuable metals. For the case of the correct operation of the lixiviation step (4) of the present invention, the overpressure must be in the range of 0-100 psig (0-689.5 kPa). If air is used, the overpressure is preferentially in the range of 10-40 psig (68.95-275.8 kPa), more preferentially around 20 psig (137.9 kPa).

(29) The pulp (5) formed by the lixiviating liquor (2) and the solid material (1) in the lixiviation step (4) must preferably have a solid content in the range of 10-40% by weight, this solid-liquid ratio being available as the result of the combination of available technology and know-how.

(30) The residence time of the pulp within the reactor must be sufficient for the chemical reactions to occur correctly. It has been found that good arsenic lixiviation results are obtained with residence times in the range of 30-150 minutes. With longer residence times within the range mentioned above, the product obtained has levels of arsenic lower than 0.5%. This enables mixtures to be made with materials with high arsenic levels, thereby obtaining a new material with an arsenic level acceptable for subsequent industrial processes.

(31) The process of the present application can be used to treat copper concentrates and any type of material with a high arsenic content (1). This includes materials such as ores, concentrates, copper cements, filter powders from smelting and/or roasting and/or similar materials. The process of the present invention gives good results for arsenic removal from these materials with high arsenic content.

(32) In this document, good results for arsenic removal and arsenic levels acceptable for subsequent industrial processes mean that the solid obtained from the process of the invention contains at most 0.5% arsenic by dry weight.

(33) Depending on the operational values used in the lixiviation step (4) mentioned above, formation of the arsenate ion (AsO.sub.4.sup.3) is possible. This is dissolved in the alkaline solution (2), mainly due to the conditions of pH and potential of the liquor that allow this. The pH of the pulp (5) resulting from the lixiviation step (4) is in a range of 10-14, while the redox potential of this alkaline solution is higher than 0.5 V with respect to the SHE.

(34) The process of the present invention is effective in the removal of arsenic and can also dissolve other elements such as selenium and silicon, but not elements of interest such as copper, silver and gold.

(35) With respect to the first solid-liquid separation step (7), any solid-liquid separation process can be used for the separation of the solid product with low arsenic content (8) from the alkaline liquor with high arsenic content (9). Commonly used techniques include: filtration, sedimentation, clarification, thickening, centrifugation, dewatering and decantation. The selection of the solid-liquid separation technique is not critical for the success of the present invention.

(36) Once the solid product with low arsenic content (8) has been separated from the mother liquor with high arsenic content (10), an optional washing of the solid product with washing water (7) can be carried out to remove the impregnated mother liquor therein. Finally the solid product obtained (9) can be stored or conveyed to another process for recovery of its valuable components.

(37) The mother liquor and the wash liquor (10) obtained from the first solid-liquid separation step (8) must be treated to remove their arsenic content. This removal is carried out through an arsenic precipitation step (12). The means of precipitating the arsenic contained in the liquors, which is preferably in the form of arsenate (AsO.sub.4.sup.3), is to add reagents (11) for the precipitation thereof and then to separate it in a second solid-liquid separation step (14). The precipitation agents (11) used in the arsenic precipitation step (12) are Ce.sup.3+, Fe.sup.3+ and Mg.sup.2+, and the combination of Fe.sup.3+ and Ca.sup.2+. There are also other reagents, such as Al.sup.3+, that can also fulfill the function of precipitating the arsenic.

(38) When the precipitating agent (11) is Ce.sup.3+, the reagent used can be cerium chloride (CeCl.sub.3). The chemical reaction that explains this precipitation is the following:
Na.sub.3AsO.sub.4+CeCl.sub.3=CeAsO.sub.4+3NaCl(IX)

(39) The dose of CeCl.sub.3 in the precipitation solution corresponds to a value of 1.80-7.50 kg Ce/kg As. The conditions for carrying out this precipitation are preferably a pH of 6-12, more preferably 8-10. The pH value may be preferably adjusted with H.sub.2SO.sub.4. The results show a precipitation of arsenic greater than 99.16%.

(40) When the precipitating agent (11) is Fe.sup.3+, the reagent used can be ferric sulfate (Fe.sub.2(SO.sub.4).sub.3). The chemical reaction that explains this precipitation is the following:
2Na.sub.3AsO.sub.4+Fe.sub.2(SO.sub.4).sub.3=2FeAsO.sub.4+3Na.sub.2SO.sub.4(X)

(41) The dose of Fe.sub.2(SO.sub.4).sub.3 in the precipitation solution corresponds to a value of 0.70-8.0 kg Fe.sup.3+/kg As. The conditions for carrying out this precipitation are preferably a pH of 6-10, more preferably 7-8. The pH value may be preferably adjusted with H.sub.2SO.sub.4. The results show a precipitation of arsenic greater than 99.31%.

(42) When using ferric sulfate, there is a possibility of adding it directly or preparing it in advance using iron(II,III) oxide and sulfuric acid in accordance with the following chemical reaction:
Fe.sub.3O.sub.4+4H.sub.2SO.sub.4=Fe.sub.2(SO.sub.4).sub.3+FeSO.sub.4+4H.sub.2O(XI)

(43) Additionally, the ferric sulfate can be prepared from ferrous sulfate by mixing it with H.sub.2O.sub.2 or other oxidant, sulfuric acid and hot water.
2FeSO.sub.4+H.sub.2O.sub.2+H.sub.2SO.sub.4Fe.sub.2(SO.sub.4).sub.3+2H.sub.2O(XII)

(44) Furthermore, milk of lime may be added to the system formed by the ferric solution and the arsenate to obtain a mixed FeCaAs salt. If the option of arsenic precipitation with iron and calcium is used, the doses are 0.70-8.0 kg Fe.sup.3+/kg As and 0.5-2.5 kg Ca.sup.2+/kg As. The conditions for carrying out this precipitation are preferably a pH of 6-10, more preferably 7-8. The pH value may be preferably adjusted with H.sub.2SO.sub.4. The results show a precipitation of arsenic greater than 99.09%.

(45) When the precipitating agent (11) is Mg.sup.2+, the reagent used can be magnesium sulfate (MgSO.sub.4). The chemical reaction that explains this precipitation is the following:
3MgSO.sub.4+2Na.sub.3AsO.sub.4=3Na.sub.2SO.sub.4+Mg.sub.3(AsO.sub.4).sub.2(XIII)

(46) The dose of MgSO.sub.4 in the precipitation solution corresponds to a value of 0.45-1.50 kg Mg.sup.2+/kg As. The conditions for carrying out this precipitation are a pH in the range of 7-14, preferably a pH in the range of 8-12 and more preferably a pH of around 10: the pH value may preferably be adjusted with H.sub.2SO.sub.4. The results show a maximum precipitation of arsenic of 71.39%.

(47) In this way, in the second solid-liquid separation step (14), the solid arsenic compound (15) must be separated from the alkaline liquor (16) that is already free of arsenic. This will be carried out by a conventional solid-liquid separation technique, such as those already mentioned for the first solid-liquid separation step.

(48) Once the filtrate has been obtained from the second solid-liquid separation step (14), which corresponds to an alkaline liquor free of arsenic (16), a crystallization step (17) to crystallize the Na.sub.2SO.sub.4 dissolved in this alkaline liquor is carried out. The process to crystallize Na.sub.2SO.sub.4 from this alkaline liquor is not critical to the success of the present invention and conventional methods can be used such as constant-volume evaporation (either continuous or semi-continuous), batch evaporation (crystallization by cooling or the total evaporation of solvent) or evaporation in a solar pond.

(49) Once the pulp composed of Na.sub.2SO.sub.4 crystals (18) and an alkaline liquor free of Na.sub.2SO.sub.4 have formed, a third solid-liquid separation step (19) of the pulp (18) formed in the crystallization step (17) is carried out. In this third solid-liquid crystallization step (19), a solid formed of Na.sub.2SO.sub.4 crystals (20) and an alkaline liquor (21) are obtained. The latter may be reused in part (21a) as the lixiviating solution for the lixiviation (5) of materials with high arsenic levels (1).

(50) Up to 100% of the alkaline lixiviating solution free of arsenic (21) is recycled to be used in the lixiviation step (5). In accordance with the above, the lixiviation step (5) can be configured to work as an open or closed circuit, the latter involving the recirculation of alkaline lixiviating liquor (21).

(51) It should be taken into account that the liquor (21b) that is not recirculated to the lixiviation step (5) may have its arsenic level further reduced through a secondary step such as adsorption or ion exchange.

(52) In another, equally satisfactory configuration of the process, defined as Alternative 2 and shown in FIG. 2, the alkaline lixiviation pulp (6) undergoes an initial solid-liquid separation step (8), and a fraction of the filtrate (10a) is recirculated to the alkaline lixiviation (5) to use the contained sodium hydroxide. The other fraction (10b) (purge) is sent to the arsenic precipitation process (12) and a second solid-liquid separation step (14). The new filtrate (16) undergoes a sodium sulfate recovery process through crystallization (17) or another similar process. The pulp (18) formed in the crystallization step (17) undergoes a third solid-liquid separation step (19); the filtrate from this last step (21) is used as process water for recirculation in the plant.

(53) The criterion for scheduling the purge is based on the control of the sodium sulfate saturation to prevent its crystallization in the alkaline lixiviation reactor, whether the process is carried out in batch or continuous mode.

(54) In another, equally satisfactory configuration of the process, defined as Alternative 3 and shown in FIG. 3, the alkaline lixiviation pulp (6) undergoes an initial solid-liquid separation step (8) and the filtrate (10) is sent to the arsenic precipitation process (12), then to a second solid-liquid separation step (14). A fraction (16a) of the new filtrate is recirculated to the alkaline lixiviation (5) and the other fraction (16b) (purge) undergoes a process of sodium sulfate recovery by crystallization or other similar process, or is discarded.

(55) The criterion for scheduling the purge is based on the control of the sodium sulfate saturation to prevent its crystallization in the alkaline lixiviation reactor, whether the process is carried out in batch or continuous mode.

(56) These process descriptions are also applicable to materials that contain arsenic and/or selenium. If selenium is present, it follows the same route as the arsenic insofar as it is present in the solutions. The removal methods leave it in the same precipitates as the arsenic.

EXAMPLES

Example 1. Lixiviation with Pure Oxygen. Study of the NaOH Dose and the Liquid-Solid Ratio

(57) In this example are shown the experimental trials carried out to define the NaOH dose necessary for the lixiviation step for a copper concentrate with 31.6% copper and an arsenic content of 2.75% as enargite. Once the dose necessary for the lixiviation of arsenic was obtained, the influence of the percentage of solid in the pulp on the efficiency of arsenic extraction was studied. The temperature, residence time, and oxygen overpressure were kept constant throughout these trials.

(58) TABLE-US-00001 Trials Units 1 2 3 4 5 6 7 Variables Liquid-solid ratio mL/g 2 2 2 3 4 6 10 Lixiviating reagent *** NaOH NaOH NaOH NaOH NaOH NaOH NaOH Dose of lixiviating kg/kg As 22.2 19.05 7.61 22.2 22.2 22.2 22.2 reagent Lixiviation temperature C. 160 160 160 160 160 160 160 Oxidizing gas *** O.sub.2 O.sub.2 O.sub.2 O.sub.2 O.sub.2 O.sub.2 O.sub.2 Overpressure of oxidizing psig 80 80 80 80 80 80 80 gas kPa 551.6 551.6 551.6 551.6 551.6 551.6 551.6 Results Arsenic removal % 98.7 80.7 53.3 96.4 82.8 74.1 47.6

(59) It is concluded from this example that the optimum dose of NaOH is 22.2 kg NaOH/kg As contained in the copper concentrate. The liquid-solid ratio that gives the best results in this example is between 2/1 and 4/1.

Example 2. Lixiviation with Pure Oxygen. Study of the Process Kinetics

(60) This example shows the experimental trials carried out with the aim of studying the arsenic dissolution kinetics from the same copper concentrate as in example 1. The temperature, solid-liquid ratio of the pulp, and oxygen overpressure were kept constant throughout these trials.

(61) TABLE-US-00002 Trials Units 8 9 10 11 12 Variables Arsenic in initial solid % 2.8 2.8 2.8 2.8 2.8 Lixiviating reagent *** NaOH NaOH NaOH NaOH NaOH Lixiviation temperature C. 160 160 160 160 160 Lixiviation time Minutes 30 60 120 150 180 Oxidizing gas *** O.sub.2 O.sub.2 O.sub.2 O.sub.2 O.sub.2 Overpressure of oxidizing psig 80 80 80 80 80 gas kPa 551.6 551.6 551.6 551.6 551.6 Results Arsenic in final solid % 0.7 0.5 0.5 0.3 0.2 Arsenic removal % 75.5 81.5 82.8 88.6 92.6

(62) It is concluded from this example that good results are achieved with a lixiviation time of 60-180 minutes.

Example 3. Recycling Study

(63) In these trials, the effect of the use of liquors generated in previous trials (Trials 11 and 12 respectively) for the dissolution of arsenic from a copper concentrate (in Trials 13 and 14 respectively) was studied. The temperature, the residence time, the solid-liquid ratio of the pulp, and the oxygen overpressure were kept constant throughout these trials, and the concentration of sodium hydroxide was fixed by Trials 11 and 12.

(64) TABLE-US-00003 Trials Units 13 14 Variables Arsenic in initial solid % 2.8 2.8 Liquid-solid ratio mL/g 4 4 Volume of recycled mother liquor % 67 18 Volume of recycled wash liquor % 16 7 Volume of fresh lixiviating solution % 17 75 Lixiviation temperature C. 160 160 Oxidizing gas *** O.sub.2 O.sub.2 Overpressure of oxidizing gas psig 80 80 kPa 551.6 551.6 Results Arsenic in final solid % 0.4 0.4 Arsenic removal % 86.8 85.2

(65) This example shows that a recycled solution can be used efficiently.

Example 4. Process Study with Copper Concentrate of Different Mineralogy and with a Higher Arsenic Content

(66) In this example, the experimental trials carried out to verify the efficiency of arsenic dissolution in the process are shown. The material is a copper concentrate with 19.7% copper and 6.11% arsenic as tennantite. The oxygen overpressure was kept constant throughout these trials.

(67) TABLE-US-00004 Trials Units 15 16 17 Variables Arsenic in initial solid % 6.1 6.1 6.1 Liquid-solid ratio mL/g 5 4 4 Lixiviating reagent *** NaOH NaOH NaOH Lixiviation temperature C. 160 160 220 Lixiviation time Minutes 150 240 240 Oxidizing gas *** O.sub.2 O.sub.2 O.sub.2 Overpressure of oxidizing gas psig 80 80 80 kPa 551.6 551.6 551.6 Results Arsenic in final solid % 3.4 2.8 0.5 Arsenic removal % 48.8 60.0 92.3

(68) This example shows that the process is also efficient for a material that contains arsenic in the form of tennantite.

Example 5. Lixiviation of Copper Concentrates with Pure Oxygen. Study of the Dissolution of Copper, Gold and Silver

(69) The trial in this example was carried out under non-optimum conditions for arsenic removal and the dissolution of copper, gold and silver; it shows the selectivity of the process and the low values for dissolution of copper, gold and silver that can be obtained.

(70) TABLE-US-00005 Trial Units 18 Variables Arsenic in initial solid % 2.05 Liquid-solid ratio mL/g 4 Lixiviating reagent *** NaOH Dose of lixiviating reagent kg/kg As 22.2 Lixiviation temperature C. 160 Lixiviation time min 150 Oxidizing gas *** O.sub.2 Overpressure of oxidizing gas psig 40 kPa 275.8 Results Arsenic removal % 82.3 Copper removal % 0.05 Gold removal % 3.99 Silver removal % 0.31

(71) This example shows that the dissolution of copper is insignificant and that the dissolution of gold and silver is very low.

Example 6. Lixiviation with Air. Study of the Effect of the Working Pressure

(72) In this example, the use of air instead of pure oxygen as the oxidizing agent was studied. The use of pure oxygen at an industrial level presents a series of difficulties that make the process and the investment more expensive, such as complex plants and a finer control of the operation.

(73) In this example are shown the experimental trials on a copper concentrate with 27.6% copper and an arsenic content of 2.1% as enargite.

(74) It was carried out to verify the efficiency of arsenic dissolution when the working overpressure is varied. The temperature, residence time, and solid-liquid ratio in the pulp were kept constant in this study.

(75) TABLE-US-00006 Trials Units 19 20 21 22 Variables Arsenic in initial solid % 2.1 2.1 2.1 2.1 Liquid-solid ratio mL/g 4 4 4 4 Lixiviating reagent *** NaOH NaOH NaOH NaOH Lixiviation temperature C. 160 160 160 160 Oxidizing gas *** Air Air Air Air Overpressure of oxidizing psig 80 40 20 10 gas kPa 551.6 275.8 137.9 68.9 Results Arsenic in final solid % 0.4 0.3 0.2 0.3 Arsenic removal % 82.8 87.8 91.1 86.0

(76) This example shows that the process operates satisfactorily over the entire overpressure range studied.

Example 7. Lixiviation with Air. Study of the Process Kinetics

(77) This example shows the experimental trials carried out with the aim of studying the arsenic dissolution kinetics using air as the oxidizing gas and the same copper concentrate as in example 5. The temperature, solid-liquid ratio of the pulp, and air overpressure were kept constant in these trials.

(78) TABLE-US-00007 Trials Units 23 24 25 26 27 28 Variables Arsenic in initial solid % 2.1 2.1 2.1 2.1 2.1 2.1 Liquid-solid ratio mL/g 4 4 4 4 4 4 Lixiviating reagent *** NaOH NaOH NaOH NaOH NaOH NaOH Lixiviation temperature C. 160 160 160 160 160 160 Lixiviation time Minutes 30 60 90 120 150 180 Oxidizing gas *** Air Air Air Air Air Air Overpressure of oxidizing psig 20 20 20 20 20 20 gas kPa 137.9 137.9 137.9 137.9 137.9 137.9 Results Arsenic in final solid % 0.9 0.5 0.3 0.2 0.2 0.1 Arsenic removal % 60.4 78.3 84.4 91.2 92.4 95.7

(79) It is concluded from this example that good results are achieved with a lixiviation time of 60-180 minutes using air as the oxidizing gas.

Example 8. Copper Cement

(80) This example shows a trial of arsenic dissolution from a copper cement containing 62% Cu, 0.63% Se and 2.40% As as arsenic sulfide (initial solid). The objective of this trial was to verify the effectiveness of the process for a material other than copper concentrate and with an additional contaminant (Se). As can be seen, the trial was carried out according to the following parameters:

(81) TABLE-US-00008 Trial Units 29 Variables Arsenic in initial solid % 2.4 Se in initial solid % 0.6 Cu in initial solid % 62 Liquid-solid ratio mL/g 4 Lixiviating reagent *** NaOH Lixiviation temperature C. 160 Oxidizing gas *** Air Overpressure of oxidizing gas psig 20 kPa 137.9 Results Arsenic in final solid % 0.1 Arsenic removal % 95.4 Se in final solid % 0.05 Selenium removal % 93.7 Cu in final solid % 72.1 Copper removal % 0.05

(82) In this example, in which the final solid corresponds to the initial solid already treated by the process of the present invention, it is shown that the process effectively removes both arsenic and selenium from the copper cement and that the dissolution of copper is insignificant with respect to the selective lixiviation of arsenic and selenium.

Example 9. Smelting Filter Powders

(83) This example shows a trial of arsenic dissolution from a filter powder from smelting of copper concentrate containing 25.4% Cu and 7.3% As. The objective of this trial was to verify the effectiveness of the process for a material other than copper concentrate in which the arsenic is mainly present as its oxide. As can be seen, the trial was carried out according to the following parameters:

(84) TABLE-US-00009 Trial Units 30 Variables Arsenic in initial solid % 7.3 Liquid-solid ratio mL/g 4 Lixiviating reagent *** NaOH Lixiviation temperature C. 160 Oxidizing gas *** Air Overpressure of oxidizing gas psig 20 kPa 137.9 Results Arsenic in final solid % 0.3 Arsenic removal % 94.2

(85) This example shows that the process is also satisfactory for the removal of arsenic from smelting filter powders.

Example 10. Precipitation of Arsenic from Liquors Arising from the Removal of Arsenic from Materials

(86) To precipitate the arsenic from an alkaline solution arising from the removal of arsenic from a material, the variables to be monitored are: the precipitating reagent, its dose and the pH. The regulation of pH is carried out with NaOH or H.sub.2SO.sub.4. No temperature control was carried out during the process.

(87) TABLE-US-00010 Trial Variables Units 31 32 33 34 As in initial solution g/L 2.35 2.35 2.35 2.35 Precipitating reagent *** Ce.sup.3+ Fe.sup.3+ Fe.sup.3+ and Mg.sup.2+ Ca.sup.2+ Initial temperature C. 25 25 25 25 Results pH Arsenic removal 12 52.89% 36.31% 51.89% 44.20% 11 81.28% 64.15% 95.60% 53.18% 10 99.16% 77.06% 91.13% 71.39% 9 97.73% 97.37% 93.20% 54.09% 8 99.73% 99.31% 99.09% 44.47% 7 91.32% 99.99% 99.99% 32.19%

(88) This example shows that it is possible to efficiently precipitate arsenic from alkaline liquors using various precipitating agents.

ADVANTAGES OF THE INVENTION

(89) The present invention shows a complete process that allows: 1. The selective elimination of arsenic contained in copper concentrates and other materials that contain arsenic, with insignificant dissolution of copper (less than 0.1%) and also with a very low dissolution of gold and silver, leaving the concentrates and other materials in a condition to be used without violating current environmental regulations. 2. The dissolution of other contaminants such as selenium. 3. Relatively rapid kinetics (0.5-2.5 hours) compared to other processes described in the literature (4-8 hours). 4. The efficient precipitation of arsenic from liquors arising from alkaline lixiviation (with an efficiency greater than 99%), in the form of a stable compound that can be disposed of safely in authorized sites, in the form of scorodite or mixed salts of As.sup.5+, Fe.sup.3+ and Ca.sup.2+.