A process for converting a feed containing sodium thioarsenate

Abstract

The invention relates to a process for converting a feed containing sodium thioarsenate comprising the steps: step a) providing an aqueous solution comprising sodium thi- oarsenate, wherein the solution has an pH value above 10, step b) adding lime to the solution of step a) to form in an aqueous slurry, step c) pressure oxidizing the slurry of step b) with oxygen, wherein oxygen is provided with an oxygen pressure of at least 6 bar, wherein the slurry is heated to a temperature of at least 100°C, and wherein a pH value above 10 is maintained during the pressure oxidation, to generate johnbaumite.

Description

[0001]Aurubis AG, 20539 Hamburg, DE A process for converting a feed containing sodium thioarsenate The invention relates to a process for converting a feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), into a product that is amenable to conversion to a non-hazardous form of disposal. Arsenic is an intrinsic part of some ores, in particular cop- per sulfide ores like enargite or tennantite. The copper can be extracted from copper sulfide ores by alkaline leaching, known from US 3,911,078, for example. After the extraction of the precious metals like copper the arsenic remains in the tailings, resulting in a feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), which is a hazardous material with pentavalent arsenic. Various processes are known that allow for non-hazardous disposal of the arsenic in the form of scorodite or via vitrification. Although these known processes allow the conversion from so- dium thioarsenate to a non-hazardous form and therefore a safe disposal of arsenic, known processes often use vast amounts of acids and often generate unwanted by-products like acid and in particular hydrogen sulfide. Therefore, the known processes to treat hazardous arsenic produce various hazardous substances themselves that need to be taken care of. Besides environmen- tal risks this also increases costs. Accordingly, it is the objective of the invention to provide an improved process to convert a feed containing sodium thi- oarsenate, for example trisodium tetrathioarsenate (Na3AsS4), into a product that is amenable to conversion to a non-hazard- ous form of disposal, in particular allowing for separation of arsenic from soluble sodium-based ions and also for the later disposal of arsenic based compounds as scorodite or glass en- capsulated products. The objective of the invention is solved with the features of the independent claims. A process for converting a feed containing sodium thioarse- nate, for example trisodium tetrathioarsenate (Na3AsS4), is proposed, comprising the steps: step a) providing an aqueous solution comprising sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), preferably from a feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), wherein the solution has a pH value above 10 step b) adding lime (Ca(OH)2) to the solution of step a) to form a slurry, step c) pressure oxidizing the slurry of step b) with oxygen (O2), wherein - oxygen (O2) is provided with an oxygen pressure of at least 6 bar, wherein - the slurry is, preferably autothermically, heated to a temperature of at least 100°C, and wherein - a pH value above 10 is maintained during the pres- sure oxidation, to generate johnbaumite (Ca5(AsO4)3OH), and preferably gypsum (CaSO4) and preferably a solution of sodium sulfate (Na2SO4). In johnbaumite, the arsenic is in the arsenic(V) state and in a calcium-based arsenate product so that the hazardous sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), is converted by the proposed process to a mineral that is stable and amenable for further cost-efficient pro- cessing and / or conversion to a non-hazardous form such as scorodite or glass encapsulated products. The prefix “thio” in sodium thioarsenate refers to the fact that at least one oxygen atom in the compound has been re- placed by sulfur. Trisodium tetrathioarsenate (Na3AsS4) may al- ready be oxidized with oxygen in part, for example. Accord- ingly, sodium thioarsenate in the meaning of this application refers to a compound of sodium and monothioarsenate, dithi- oarsenate, trithioarsenate and / or tetrathioarsenate. Prefera- bly, the sodium thioarsenate in the aqueous solution of step a) is trisodium tetrathioarsenate (Na3AsS4). The proposed process allows the efficient use of a feed from an alkaline leach system of copper ores, for example known from US 3,911,078. In such alkaline leach systems, the feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), has a high pH value after the ex- traction of copper. As a result, the aqueous solution compris- ing sodium thioarsenate, for example trisodium tetrathioarse- nate (Na3AsS4), having a pH value above 10, preferably above 11, is readily available as tailings from alkaline leaching of copper ores, like enargite or tennantite. The aqueous solution comprising sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), of step a) may also contain en- trained sodium sulfide (Na2S) as well as a background concen- tration of NaOH. In general, the aqueous solution of step a) may also contain other materials in solution or precipitated form. This is because the feed is preferably based on the tailings from ore extraction. The proposed process has found that the conversion to johnbau- mite is possible in a one-step process in an oxygen rich auto- clave process by adding lime to the aqueous solution compris- ing sodium thioarsenate, for example trisodium tetrathioarse- nate (Na3AsS4), to form a slurry in step b), wherein the solu- tion of step a) and the slurry of step b) have a pH value above 10, for example 12. The process allows for a simultane- ous exothermic oxidation of the sulfide component in the thi- oarsenate to arsenate (AsO43-) and fixing the arsenate (AsO43-), in particular under hydrothermal conditions, as Johnbaumite (Ca5(AsO4)3OH). Herein, the addition of lime has the surprising effect of an efficient conversion to johnbaumite and limiting the generation of hydrogen sulfide (H2S) as unwanted by-prod- uct, while generating only moderate amounts of gypsum (Ca[SO4]·2H2O), so that not all the sulfate is precipitated as gypsum. It is common knowledge that lime is less reactive at high pH values associated with NaOH, so that it is not obvious to the skilled person to utilize lime in an alkaline system. Lime is known to be useful only at a pH value below 7 due to reduced reactivity. The lower reactivity of lime at higher pH is due to a sharp drop in solubility of calcium hydroxide, wherein the solubility of Ca2+ions drops from above 0.6 g / l to below 0.05 g / l at 25oC, for example. The pH value above 10, preferably above 11, for example 12, during step c) suppresses the production of hydrogen sulfide (H2S). Moreover, no additional acid is required as a high pH value of the feed can be maintained throughout the process. Therefore, an environmentally friendly and cost-efficient pro- cessing can be achieved. The slurry of step b) comprising sodium thioarsenate, for ex- ample trisodium tetrathioarsenate (Na3AsS4), and lime (Ca(OH)2) in water at a pH value above 10, preferably above 11, for ex- ample 12, is processed in a pressure vessel or autoclave in a step c). In step c), the oxygen rich pressurized atmosphere and elevated temperature with the high pH value enable multi- ple reactions in a one-step process to generate johnbaumite (Ca5(AsO4)3OH) with pentavalent arsenic. Since the proposed pro- cess is a one-step process the process only requires a single pressure vessel or autoclave. Advantageously, in step c) the sodium thioarsenate, for exam- ple trisodium tetrathioarsenate (Na3AsS4), is oxidized to so- dium arsenate (Na3AsO4) as an intermediate product, which fur- ther reacts with the lime (Ca(OH)2) to johnbaumite (Ca5(AsO4)3OH). These two reactions preferably occur simultane- ously during step c). The oxidation of sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to sodium arsenate (Na3AsO4) is exothermic, reducing the energy consump- tion during step c). In other words, the sulfur content in so- dium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), is leveraged to exothermically support the high tem- perature and pressure (hydrothermal) conversion of sodium ar- senate (Na3AsO4) to johnbaumite (Ca5(AsO4)3OH). The total pres- sure during step c) is preferably above 14 bar, for example 15 bar. Preferably, the present lime (Ca(OH)2) can directly react with generated sodium arsenate (Na3AsO4) to johnbaumite (Ca5(AsO4)3OH). Thereby, the conversion of sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to johnbau- mite (Ca5(AsO4)3OH) is complete. It is further preferred that no sulfur is added to the solution of step a), the slurry of step b) and during step c). In step c) of the proposed process, the oxidation of sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to sodium arsenate (Na3AsO4) in step c) may generate also sulfuric acid (H2SO4) in some embodiments. The sulfuric acid (H2SO4) is generally an unwanted by-product. Further in step c) of the proposed process, the conversion of sodium arsenate (Na3AsO4) with the lime (Ca(OH)2) preferably generates also caustic soda (NaOH). The caustic soda (NaOH) generated by the johnbaumite preferably offsets the acid, in particular the sulfuric acid (H2SO4), generation of oxidizing the sodium thioarsenate, for example trisodium tetrathioarse- nate (Na3AsS4), to sodium arsenate (Na3AsO4). In a preferred embodiment, in a step b1) caustic soda (NaOH) is added to the solution of step a) and / or slurry of step b), preferably before step c). The additional caustic soda (NaOH) is beneficial to maintain a pH value above 10, preferably above 11, during step c) and is in particular beneficial to depress the generation of gypsum (Ca[SO4]·2H2O) during step c). The generation of gypsum (Ca[SO4]·2H2O) during step c) is an unwanted side reaction. In a preferred embodiment, the sulfuric acid (H2SO4) is neu- tralized with caustic soda (NaOH) in step c). Since the reac- tion to sodium arsenate (Na3AsO4) and further to johnbaumite (Ca5(AsO4)3OH) is performed simultaneously during step c) of the proposed process, generated sulfuric acid (H2SO4) can be neutralized or offset directly so an acidic built-up and a drop of the pH value below 10, in particular below 11, can be avoided. It is further preferred that the sulfuric acid (H2SO4) gener- ated by the oxidation of sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to sodium arsenate (Na3AsO4) in step c) is neutralized with lime (Ca(OH)2). The neutralization with lime (Ca(OH)2) is preferably additional to the neutralization with caustic soda (NaOH) in step c). In another preferred embodiment, no additional caustic other than lime is added to maintain the pH value above 10, prefera- bly 11, during step c). Preferably, in step b) lime (Ca(OH)2) is added in the ratio of 2 to 5 grams of lime (Ca(OH)2) per gram of arsenic (As) con- tained in the solution of step a). Preferably, the temperature during step c) is below 200°C. It is possible to perform the proposed process below 200°C, which enables an energy efficient processing of the feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4). This is in particular more energy efficient than other processes for converting a feed containing sodium thi- oarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to a product in which calcium arsenate is produced at a higher processing temperature. In a preferred embodiment, the temperature during step c) is above 170°C, for example 180°C. This allows for a good yield producing johnbaumite as well as good energy efficient pro- cessing. Conducting this conversion above 170oC yields commercially acceptable residence times of under 2 hours. The high temperature (above the atmospheric boiling point of water – hence requiring higher pressures to suppress boiling) con- version of a reaction in an aqueous environment is commonly termed hydrothermal as it reflects conditions deep in the earth’s crust often used to describe geothermal reactions. Further on, the pH value in step a), step b) and step c) is above 11 in preferred embodiments. A pH level above 11 is ben- eficial as most alkaline sulfide leaching requires a pH above 11 to prevent the formation of toxic H2S from Na2S – to this effect most alkali sulfide leach liquors contain between 0.25 molar (10 g / l) and 6 molar (240 g / l) sodium hydroxide. Thus, a pH value above 11 further suppresses the production of hydro- gen sulfide (H2S) which is beneficial for an environmentally friendly and cost-efficient processing. Step c) is preferably maintained for at maximum 140 minutes, for example 120 min, and further preferably maintained for at least 60 min. Further on, the slurry is preferably stirred during step c). Further preferably, oxygen (O2) is provided with an oxygen pressure in the range of 6.5 bar to 8.5 bar. The total pres- sure may be in advantageous embodiments above 14 bar, for ex- ample 15 bar. In a preferred embodiment of the proposed process, the pro- cessing of step c) is autothermic. Thus, the temperature dur- ing step c) in the pressure vessel is preferably self-sustain- ing by the process without additional heat transfer to the slurry. Further preferably, the slurry is heated autothermi- cally. Preferably, in a step d) the slurry is filtered after step c), wherein the residue of step d) comprises johnbaumite (Ca5(AsO4)3OH) and gypsum (Ca[SO4]·H2O). The filtrate of step d) preferably comprises a solution containing sodium sulfate (Na2SO4). Further substances may also be in the residue, de- pending on the composition of the incoming feed. Further preferably, the residue of step d) has a ratio of johnbaumite (Ca5(AsO4)3OH) to gypsum (Ca[SO4]·2H2O) of at least 50:50, preferably at least 60:40. The high ratio of johnbau- mite in the residue of step d) allows a cost-efficient han- dling of the arsenic. In a preferred embodiment, the johnbaumite (Ca5(AsO4)3OH) of step d) is dissolved with sulfuric acid to solubilize the ar- senic as arsenic acid (H3AsO4) and gypsum. The gypsum is fil- tered and disposed while the arsenic acid is then contacted with the ferric ions to form scorodite. In a further preferred embodiment, the johnbaumite (Ca5(AsO4)3OH) is further processed by vitrification. Both these preferred embodiments would allow for non-hazardous depositing of pentavalent arsenic. In the following the invention shall be illustrated on the ba- sis of preferred embodiments with reference to the accompany- ing drawings. Fig. 1 shows Ca2+solubility at 25 °C regarding NaOH [g / L] concentration. In a first advantageous embodiment of the invention, a feed is provided from an alkaline leaching of enargite as a copper sulfide ore. The copper has been extracted from the enargite and the hazardous tailings comprise the arsenic that comes with the enargite ore. In this example, the crystals obtained from an enargite leach solution have the following composi- tion: 8.8 wt.% As; 14.7 wt.% Na. Balancing the arsenic and assuming, that the crystals only consist of Na3AsS4*8H2O and Na2S*9H2O gives the following compo- sition of the crystals in this embodiment: 49.1% Na3AsS4*8H2O; 50.9% Na2S*9H2O. Dissolving these crystals gives the start solution as an aque- ous solution in a step a) for the process for converting the feed containing sodium thioarsenate, in particular trisodium tetrathioarsenate (Na3AsS4), to johnbaumite (Ca5(AsO4)3OH) in the proposed process. The aqueous solution of step a) has the fol- lowing example composition in this embodiment: 42.9 g / L As; 84.3 Na g / L. Further on, the aqueous solution of step a) comprising sodium thioarsenate, in particular trisodium tetrathioarsenate (Na3AsS4), has a pH value of 12. In step b) lime (Ca(OH)2) is added to the solution of step a) and a aqueous slurry is formed, at an addition of 5.6 g of Ca(OH)2 per gram of As contained in the start solution of step a). Figure 1 shows the lower reactivity of lime (Ca(OH)2) at higher pH is due to a sharp drop in solubility of calcium hydroxide. Figure 1 shows that the solubility of Ca2+ions dropping from above 0.6 g / l to below 0.05 g / l at 25oC. Therefore, the effi- ciency of lime in the proposed process is surprisingly high. Step c) is carried out in an autoclave for 2 hours. The total pressure is set to 15 bar and the temperature is set to 170°C. The oxygen pressure is provided with at least 6 bar. The so- dium thioarsenate, in particular trisodium tetrathioarsenate (Na3AsS4), is oxidized to sodium arsenate (Na3AsO4) as an inter- mediate product, which further reacts with the lime (Ca(OH)2) to johnbaumite (Ca5(AsO4)3OH). These two reactions preferably occur simultaneously during step c). The lime (Ca(OH)2) can di- rectly react with generated sodium arsenate (Na3AsO4) to johnbaumite (Ca5(AsO4)3OH). Further in step c), the conversion of sodium arsenate (Na3AsO4) with the lime (Ca(OH)2) generates also caustic soda (NaOH). The caustic soda (NaOH) generated by the johnbaumite offsets the sulfuric acid (H2SO4) generation of oxidizing the sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to sodium arsenate (Na3AsO4). Once the autoclave step c) is finished the output solution has the following composition: 0.062 g / L As; 77.9 g / L Na. In a step d) the slurry of step c) is filtered after step c), wherein the residue of step d) comprises the solids. The out- put solids have the following composition: 9.4 wt.% As; 2.9 wt.% S; 32.5.wt.% Ca. Through XRD-analysis arsenic could be only detected as johnbaumite (Ca5(AsO4)3OH), with the solids having the following composition: Portlandite Johnbaumite Mirabilite Thenardite (Ca(OH)2) % (Ca5(AsO4)3OH) % (Na2SO4*10H2O) % (Na2SO4) % 56.5 26.4 5.8 11.3 Accordingly, the arsenic is bound in johnbaumite and amendable to further processing to scorodite or a glass encapsuled prod- uct. Moreover, the built-up of gypsum can be reduced to a min- imum in this embodiment. In an advantageous embodiment, the johnbaumite (Ca5(AsO4)3OH) of step d) is dissolved with sulfuric acid to solubilize the ar- senic as arsenic acid (H3AsO4). The arsenic acid is then con- tacted with the ferric ions to form scorodite. In another embodiment, the crystals obtained from an enargite leach solution deviates from the first embodiment and have the following composition: 13.9 wt.% As; 17.6 wt.% Na. Balancing the arsenic and assuming, that the crystals only consist of Na3AsS4*8H2O and Na2S*9H2O gives the following compo- sition of the crystals in this embodiment: 77.0% Na3AsS4*8H2O; 23.0% Na2S*9H2O. Dissolving these crystals gives the start solution as an aque- ous solution in a step a) for the process for converting the feed containing sodium thioarsenate, for example trisodium tetrathioarsenate (Na3AsS4), to johnbaumite (Ca5(AsO4)3OH) in the proposed process. The aqueous solution of step a) has the fol- lowing example composition in this embodiment: 28.7 g / L As; 49.0 Na g / L. In step b) lime (Ca(OH)2) is added to the solution of step a), at an addition of 3.8 g of Ca(OH)2 per gram of As contained in the start solution. Step c) is carried out in an autoclave. The total pressure is set to 15 bar and the temperature set to 170°C. The oxygen pressure is provided with at least 6 bar. The sodium thioarse- nate, for example trisodium tetrathioarsenate (Na3AsS4), is ox- idized to sodium arsenate (Na3AsO4) as an intermediate product, which further reacts with the lime (Ca(OH)2) to johnbaumite (Ca5(AsO4)3OH). These two reactions preferably occur simultane- ously during step c). The lime (Ca(OH)2) can directly react with generated sodium arsenate (Na3AsO4) to johnbaumite (Ca5(AsO4)3OH). Further in step c), the conversion of sodium ar- senate (Na3AsO4) with the lime (Ca(OH)2) generates also caustic soda (NaOH). The caustic soda (NaOH) generated by the johnbau- mite offsets the sulfuric acid (H2SO4) generation of oxidizing the sodium thioarsenate, for example trisodium tetrathioarse- nate (Na3AsS4), to sodium arsenate (Na3AsO4). Once the autoclave step c) is finished the output solution has the following composition: 0.03 g / L As; 45.0 g / L Na. In a step d) the slurry of step c) is filtered after step c), wherein the residue of step d) comprises the solids. The out- put solids have the following composition: 13.6 wt.% As; 13.5 wt.% S; 25.2 wt.% Ca, 2.8 wt.% Na. Calculating elements back into species from the XRD-measure- ment of the first example gives the following composition: Gypsum Johnbaumite Mirabilite Thenardite Hemihydrite (Ca5(AsO4)3OH % (Na2SO4*10H2O) (Na2SO4) % ((CaSO4)2*H2O) % % 51.6 37.9 3.6 6.9 In this embodiment, the johnbaumite (Ca5(AsO4)3OH) of step d) is dissolved with sulfuric acid to solubilize the arsenic as ar- senic acid (H3AsO4) and gypsum. The gypsum is filtered and dis- posed while the arsenic acid is then contacted with the ferric ions to form scorodite. In another preferred embodiment, additional caustic soda (NaOH) is added to the solution of step a) or slurry of step b) of the preceding embodiments.

Claims

Claims:

1. A process for converting a feed containing sodium thi- oarsenate comprising the steps: step a) providing an aqueous solution comprising sodium thioarsenate, wherein the solution has a pH value above 10, step b) adding lime (Ca(OH)2) to the solution of step a) to form in an aqueous slurry, step c) pressure oxidizing the slurry of step b) with ox- ygen (O2), wherein - oxygen (O2) is provided with an oxygen pressure of at least 6 bar, wherein - the slurry is heated to a temperature of at least 100°C, and wherein - a pH value above 10 is maintained during the pressure oxidation, to generate johnbaumite (Ca5(AsO4)3OH).

2. The process according to claim 1, wherein the sodium thi- oarsenate in the aqueous solution of step a) is trisodium tetrathioarsenate (Na3AsS4).

3. The process according to claim 1 or 2, wherein in step c) the sodium thioarsenate is oxidized to sodium arsenate (Na3AsO4) as an intermediate product, which further reacts with the lime (Ca(OH)2) to johnbaumite (Ca5(AsO4)3OH).

4. The process according to claim 3, wherein the oxidation of sodium thioarsenate to sodium arsenate (Na3AsO4) in step c) generates sulfuric acid (H2SO4).

5. The process according to claims 3 or 4, wherein the con- version of sodium arsenate (Na3AsO4) with the lime (Ca(OH)2) generates caustic soda (NaOH).

6. The process according to any one of the preceding claims, wherein in a step b1) caustic soda (NaOH) is added to the solution of step a) and / or slurry of step b).

7. The process according to claim 4 and claim 5 or 6, wherein in step c) the sulfuric acid (H2SO4) is neutralized with caustic soda (NaOH).

8. The process according to any one of the preceding claims, wherein the oxidation of sodium thioarsenate to sodium arsenate (Na3AsO4) in step c) generates sulfuric acid (H2SO4) which is neutralized with lime (Ca(OH)2).

9. The process according to any one of the preceding claims, wherein in step b) lime (Ca(OH)2) is added in the ratio of 2 to 5 grams of lime (Ca(OH)2) per gram of arsenic (As) contained in the solution of step a).

10. The process according to any one of the preceding claims, wherein the temperature during step c) is below 200°C.

11. The process according to any one of the preceding claims, wherein the temperature during step c) is above 170°C.

12. The process according to any one of the preceding claims, wherein the pH value in step a) and step c) is above 11.

13. The process according to any one of the preceding claims, wherein step c) is maintained for at maximum 140 minutes.

14. The process according to any one of the preceding claims, wherein the processing of step c) is autothermic.

15. The process according to any one of the preceding claims, wherein in a step d) the slurry is filtered after step c), wherein the residue of step d) comprises johnbaumite (Ca5(AsO4)3OH) and gypsum (Ca(SO4)·H2O).

16. The process according to claim 15, wherein the johnbaumite (Ca5(AsO4)3OH) of step d) is dissolved with sulfuric acid to solubilize the arsenic as arsenic acid (H3AsO4) and gypsum, wherein the gypsum is filtered and disposed while the arsenic acid (H3AsO4) is contacted with ferric ions to form scorodite.

17. The process according to any one of the preceding claims, wherein the johnbaumite (Ca5(AsO4)3OH) is further processed by vitrification.

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