Production of scorodite from solutions containing arsenic and sulfuric acid

Abstract

The invention relates to a method for producing scorodite without using neutralisers from solutions comprising arsenic and sulfuric acid, which comprises the steps of: i. subjecting a first arsenic solution comprising arsenic in mixtures of arsenite ion and arsenate ion and sulfuric acid to an oxidation process to oxidise the arsenite ion into arsenate ion in order to obtain a second arsenic solution; ii. adding to the second arsenic solution a first solution containing ferrous ion to generate a third arsenic solution; iii. adding to the third arsenic solution a first magnetite portion to adjust the molar ratio of ferric ion to arsenate ion to between 0.3 and 1 in order to generate a first arsenic pulp; iv. adding to the first arsenic pulp a part of a scorodite pulp, recirculated from step viii, as a base for the nucleation of scorodite particles to obtain a first scorodite pulp in the range of 2-15% w / w; v. heating the first scorodite pulp to 80-90°C; vi. maintaining the first scorodite pulp at 80-90°C for 5-48 hours to obtain a second scorodite pulp; vii. sending the second scorodite pulp to a solid–liquid separation step to obtain a third scorodite pulp and a treated solution; viii. recirculating a first part of the third scorodite pulp to step iv; and ix. filtering a second part of the third scorodite pulp.

Description

[0001] Obtaining scorodite from solutions with arsenic and sulfuric acid

[0002] Field of Invention

[0003] The invention relates to a process for generating scorodite (hydrated ferric arsenate) in which the use of neutralizers to adjust the pH for its generation is not necessary, from solutions with a high concentration of arsenic, which can come from a plurality of mining waste treatment processes of different types, from which arsenic is obtained as one of the main impurities that require a long-term stabilization method. As a result of this process, arsenical waste is obtained consisting essentially of scorodite and an arsenic-depleted solution.

[0004] State of the art

[0005] Application WO2020237361 (D1) discloses a process for producing crystalline scorodite substantially free of gypsum, which comprises adding an iron oxyhydroxide compound (FeOOH) to a pentavalent arsenic solution to cause the formation of said scorodite. In D1, by adding iron oxyhydroxide, the iron source implicitly contains base, which serves as an acidity regulator, generating a neutralization of the arsenical solution. D1 mentions that to produce iron oxyhydroxide, it is necessary to add a base such as magnesium oxide, which shows a notable difference with the present patent application where the non-use of bases is explicit. Furthermore, D1 does not mention that the addition of ferrous sulfate is required for the catalysis of the precipitation reaction as claimed in the present application.

[0006] Invention Patent CL No. 50423 (D2) aims at the abatement in a hydrometallurgical treatment plant of liquid waste and effluents with a high content of contaminants, such as arsenic and others, for the production of scorodite. D2 discloses the steps of oxidation, adjustment of the Fe(lll) / As(V) molar ratio to at least 1.0 by adding a magnetite leaching solution and pH adjustment by adding neutralizer. D2, by using calcium-based neutralizers, co-precipitates gypsum in the arsenical residue. D2 does not mention the use of scorodite seed recirculation to improve precipitation kinetics.Patent CL No. 66875 (Application CL 202101684) D3 discloses a process for obtaining a mining or industrial waste, comprising scorodite with a high arsenic content greater than 15% from highly acidic solutions, higher in acid concentration than 45 g / L, comprising copper, arsenic and, optionally, iron, antimony and / or bismuth. In its steps, D3 discloses that the process requires the steps of acidity adjustment generating a first gypsum residue, two stages of As(lll) oxidation, adjustment of the Fe(lll) / As(V) ratio by adding a solution containing ferric ion, recirculation of a portion of scorodite pulp, acidity adjustment and heating. D3 differs from the present application in that the present patent application does not require an acidity adjustment step, and adding solid magnetite for the adjustment of the Fe(l I l) / As(V) molar ratio and not a ferric solution.

[0007] Document CA 2066905 (D4) discloses a process for reducing arsenic levels in a solution comprising sulfuric acid, water, and arsenic acid, comprising the steps of reacting the solution with copper or a copper-containing compound in a sufficient amount and at a sufficient temperature to produce copper arsenate; adding an alkaline solution to precipitate the copper arsenate; and adding a ferric ion compound to precipitate ferric arsenate. Document D4 does not interfere with the present patent application since it only discloses reducing arsenic levels in solutions containing them.

[0008] Technical Problem

[0009] The problem with the technique is that there is currently no process for producing an arsenical residue comprising scorodite with an As content above 25% w / w, in which the addition of a neutralizer is not required for the treatment of solutions with a sulfuric acid concentration between 10 and 80 g / L.

[0010] The advantages of the present invention lie in the ability to generate an arsenical residue with an arsenic content above 25% w / w without requiring the use of neutralizers. Additionally, the process claimed in this patent application allows for the oxidation of part of the arsenous ion and ferrous ion present in the solution. The acid remaining in the arsenic-depleted solutions can be used as an oxidizing agent for other resources such as copper oxides.

[0011] The oxidation of arsenous acid and ferrous sulfate occurs in the presence of oxygen naturally dissolved in the arsenical solution, through the generation of free radicals on the surface of the magnetite. The precipitation reaction of scorodite from magnetite occurs as follows:

[0012] Eq 1. 0.5Fe3O4+ 2H2S04-> 0.5Fe2(SO4)3+ 0.5FeSO4+ 2H2O

[0013] Eq 2. H3ASO4+ 0.5Fe2(SO4)3+ 2H2O -> FeAsO4* 2H2O + 1.5H2SO4

[0014] Consequently, the overall reaction is as follows:

[0015] Eq. 3. H3ASO4+ 0.5Fe3O4+ 0.5H2SO4-> FeAsO4* 2H2O + 0.5FeSO4

[0016] As a result, the overall reaction consumes sulfuric acid, so adding a base to perform neutralization is not necessary. Adding a base is complicated because sodium-based neutralizers generate sodium sulfate, a salt that is difficult to remove and thus affects the reuse of solutions in contexts where water resources are scarce. Additionally, the presence of sodium favors the formation of natrojarosite, which can exchange arsenate ions and generate unstable arsenical compounds. Furthermore, the use of calcium-based neutralizers generates gypsum, which has disadvantages due to the coprecipitation of gypsum with scorodite, which increases the amount of arsenical waste generated, increasing disposal costs.

[0017] Scorodite precipitation by magnetite has other benefits associated with controlling ferric ion supersaturation. Since the iron source is in a solid state, the magnetite slowly dissolves to generate ferrous sulfate and ferric sulfate, where the latter rapidly reacts in the boundary layer between the magnetite and the liquid to generate ferric arsenate. This ferric arsenate evolves into crystalline ferric arsenate, i.e., scorodite. In this way, the ferric ion concentration remains low, with a constant gradient between the arsenate and ferric ion concentrations, which favors the formation of scorodite, as documented in the literature.This saturation control represents important differences with respect to the addition of soluble ferric ion salts, which require a chain precipitator system to control ferric ion supersaturation and thus maintain the gradient of arsenate ion and ferric ion relatively high.

[0018] The addition of iron oxyhydroxides generated from the dissolution of ferric ion sources such as magnetite and / or ferric sulfate salts requires, in the case of the former, leaching the magnetite with sulfuric acid into ferrous sulfate and ferric sulfate, and second, adding a sodium and / or magnesium-based neutralizer that allows generating an iron oxyhydroxide without the formation of gypsum, given the absence of calcium, and solubilized salts of sodium and / or magnesium sulfate that make the reuse of process water difficult. Iron oxyhydroxides are easily soluble in high temperature environments and sulfuric acid concentrations above 10 g / L, which generates an immediate drop in the pH of the solution.

[0019] The precipitation reaction of scorodite from iron oxyhydroxides occurs as follows:

[0020] Eq 4. 0.5Fe3O4+ 2H2S04-> 0.5Fe2(SO4)3+ 0.5FeSO4+ 2H2O

[0021] Eq 5. 0.5Fe2(SO4)3+ 3NaOH -> FeOOH + l,5Na2S04+ H2O

[0022] Eq. 6 FeOOH + 1.5H2SO4-> 0.5Fe2(SO4)3+ 2H2O

[0023] Eq. 7. H3AsO4+ 0.5Fe2(SO4)3+ 2H2O -> FeAsO4* 2H2O + 1.5H2SO4

[0024] Consequently, the overall reaction is as follows:

[0025] Eq. 8. H3ASO4+ 0.5Fe304+ 2H2S04+ 3NaOH -» FeAsO4* 2H2O + 0.5FeS04+ l,5A?a2SO4+ 3H2O

[0026] From the above it becomes evident that the overall relationships for the production of individualized scorodite in equations Eq. 3 and Eq. 8 differ in the consumption of neutralizer that equation Eq. 8 carries out, in contrast to equation Eq. 3 which does not require the addition of neutralizer.

[0027] In summary, the novel and inventive aspects of this application lie in that:

[0028] A. Precipitation of scorodite at high sulfuric acid concentrations, above 35 g / L. This procedure produces a stabilized arsenical residue with an As content greater than 25% w / w and essentially no gypsum formation.

[0029] B. Precipitation of scorodite without the use of neutralizer to adjust the sulfuric acid concentration, within the sulfuric acid concentration range between 35 and 80 g / L.

[0030] C. Arsenite ion oxidation by precipitating arsenic in the form of scorodite with magnetite, which reduces the requirement for an oxidizing agent necessary for the oxidation of arsenite ion to arsenate ion. Arsenite ion oxidation yields range from 20 to 30% w / w.

[0031] D. Use of ferrous ion as a catalyst for the scorodite precipitation reaction, allowing the precipitation system to operate at Fe(lll) / As(V) molar ratios lower than 1.0. Figure Description

[0032] Figure 1. Shows the kinetic effect of varying the percentage of solids on the precipitation of arsenic(V) in the form of scorodite, with an arsenical solution with a concentration of 10 g / L, a sulfuric acid concentration of 70 g / L and a ferrous ion concentration of 20 g / L.

[0033] Figure 2. Shows the effect of sulfuric acid concentration between 5 and 25 g / L and ferrous ion concentration on the kinetics of scorodite precipitation in sulfuric acid plant effluents - Fe(lll) molar ratio Mag / As(V) = 1 , 0. Graphs at the same concentration of sulfuric acid 5 and 25 g / L varying ferrous ion concentration.

[0034] Figure 3. Shows the effect of sulfuric acid concentration between 40 and 70 g / L and ferrous ion concentration on the kinetics of scorodite precipitation in sulfuric acid plant effluents - Fe(lll) molar ratio Mag / As(V) = 1 ,0. Graphs at the same concentration of sulfuric acid varying concentration of ferrous ion.

[0035] Figure 4. Shows the effect of ferrous ion concentration 0.5, 2 and 5 g / L on the kinetics of scorodite precipitation in sulfuric acid plant effluents - Fe(lll) molar ratio Mag / As(V) = 1 ,0. Graphs at equal ferrous ion concentration varying concentration of H2SO45, 25, 40 and 70 g / L

[0036] Figure 5. Shows the effect of ferrous ion concentration on the concentration of residual sulfuric acid in EPAS solution (Sulfuric Acid Plant Effluent) - Fe(lll) molar ratio Mag / As(V) = 1 ,0

[0037] Figure 6. Shows the effect of sulfuric acid concentration between 5 and 25 g / L and ferrous ion concentration on As concentration in TCLP (Toxicity Characteristic Leaching Procedure) test of arsenical residue (ReAs) - Fe(lll) molar ratio Mag / As(V) = 1 ,0

[0038] Figure 7. Shows the effect of sulfuric acid concentration between 40 and 70 g / L and ferrous ion concentration on As concentration in TCLP (Toxicity Characteristic Leaching Procedure) test of arsenical residue (ReAs) - Fe(lll) molar ratio Mag / As(V) = 1 ,0

[0039] Figure 8. Shows the effect of sulfuric acid concentration between 5 and 25 g / L and ferrous ion concentration on the As content in the arsenical residue ReAs-Fe(lll) molar ratio. Mag / As(V) = 1 ,0 Figure 9. Shows the effect of sulfuric acid concentration between 40 and 70 g / L and ferrous ion concentration on the As content in the arsenical residue ReAs-Fe(lll) molar ratio Mag / As(V) = 1 ,0

[0040] Figure 10. Shows the effect of sulfuric acid concentration between 5 and 25 g / L and ferrous ion concentration on the Fe content in the arsenical residue ReAs-Fe(lll) molar ratio. Mag / As(V) = 1 ,0

[0041] Figure 11. Shows the effect of sulfuric acid concentration between 40 and 70 g / L and ferrous ion concentration on the Fe content in the arsenical residue ReAs-Fe(lll) molar ratio. Mag / As(V) = 1 ,0

[0042] Figure 12. Shows the effect of sulfuric acid and ferrous ion concentration at high ferrous ion concentrations in the range between 5 and 20 g / L on the kinetics of scorodite precipitation in EPAS solution - Fe(lll) molar ratioMag / As(V) = 1 ,0

[0043] Figure 13. Shows the effect of high sulfuric acid concentration and ferrous ion concentration on the As and Fe content of the arsenical residue ReAs - Fe(lll)Mag / As(V) molar ratio = 1.0

[0044] Figure 14. Shows the effect of sulfuric acid concentration between pH 2 (0 g / L) and 10 g / L on the concentration of As in the effluent solution of a reduced sulfuric acid plant - ferrous concentration 5 g / L and Fe(lll) molar ratio Mag / As(V) = 0.5

[0045] Figure 15. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and ferrous ion concentration in the As concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L

[0046] Figure 16. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and ferrous ion concentration in the As(lll) concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L

[0047] Figure 17. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and ferrous ion concentration in the Fe concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L

[0048] Figure 18. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and ferrous ion concentration on Fe(ll) concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L Figure 19. Shows the effect of the decrease in the Fe(lll) molar ratio Mag / As(V) and ferrous ion concentration in the As and Fe content of the arsenical residue ReAs - sulfuric acid concentration 5.0 g / L

[0049] Figure 20. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) from 0.5 to 0.3 in the concentration of As and As(lll) in the treated EPAS solution - ferrous concentration 5.0 g / L and sulfuric acid concentration 5 g / L.

[0050] Figure 21. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) from 0.5 to 0.3 in the concentration of Fe and Fe(ll) in the treated EPAS solution - ferrous concentration 5.0 g / L and sulfuric acid concentration 5 g / L.

[0051] Figure 22. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) from 0.5 to 0.3 in the concentration of As and Cd released in the TCLP test (Toxicity Characteristic leaching procedure) of scorodite waste - ferrous concentration 5.0 g / L and sulfuric acid concentration 5 g / L.

[0052] Figure 23. Shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) from 0.5 to 0.3 in the concentration of As and Fe of the scorodite waste - ferrous concentration 5.0 g / L and sulfuric acid concentration 5 g / L.

[0053] Figure 24. Shows the effect of lowering the initial concentration of As(lll) on the concentration of As and As(lll) in the treated EPAS solution - Fe(lll) molar ratio Mag / As(V) 0.5 - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L

[0054] Figure 25. Shows the effect of increasing the initial concentration of As(lll) on the concentration of As and As(lll) in the treated EPAS solution - Fe(lll) molar ratio Mag / As(V) 1 ,0 - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L

[0055] Figure 26. Shows the effect of increasing the initial concentration of As(lll) on the concentration of Fe and Fe(ll) in the treated EPAS solution - Fe(lll) molar ratio Mag / As(V) 0.5

[0056] - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L

[0057] Figure 27. Shows the effect of lowering the initial concentration of As(lll) on the concentration of Fe and Fe(ll) in the treated EPAS solution - Fe(lll) molar ratio Mag / As(V) 1 ,0

[0058] - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L

[0059] Figure 28. Shows the effect of the decrease in the initial concentration of Fe(ll) on the concentration of As and As(lll) - Fe(lll) molar ratio Mag / As(V) 1 ,0 - sulfuric acid concentration 25-75 g / L - ferrous ion 0.5 g / L Figure 29. Shows the effect of the decrease in the initial concentration of Fe(ll) on the concentration of As and As(lll) - Fe(lll) molar ratio Mag / As(V) 1 ,0 - sulfuric acid concentration 5 g / L - ferrous ion 1 ,0-2.0 g / L

[0060] Figure 30. Shows the effect of the initial concentration of Fe(ll) in the range 3-4 g / L on the concentration of As and As(lll) - sulfuric acid concentration 5.0 - Fe(lll) ratio Mag / As(V) 0.50 mol / mol (equivalent to FeTMag / As(V) 0.50 mol / mol)

[0061] Figure 31. Shows the arsenic concentration in scorodite generation tests adding only magnetite and only ferrous sulfate as iron sources.

[0062] Figure 32. Shows a diffractogram of arsenical residues ReAs generated in 24 h of precipitation with molar ratio Fe(lll) Mag / As(V) 1 ,0 and ferrous ion concentration 5 g / L and sulfuric acid concentration of 5 g / L.

[0063] Figure 33. Shows a scanning electron microscopy image of arsenical residues ReAs generated in 24 h of precipitation with Fe(lll) molar ratio Mag / As(V) 1 ,0 and ferrous ion concentration 5 g / L and sulfuric acid concentration of 5 g / L.

[0064] Figure 34. Shows the particle size distribution of arsenical residues ReAs generated in 48 h of precipitation with Fe(lll) molar ratio Mag / As(V) 0.5 and ferrous ion concentration 4 g / L and sulfuric acid concentration of 5 g / L.

[0065] Figure 35. Shows a scanning electron microscopy image of magnetite used in scorodite precipitation tests.

[0066] Definition of the invention

[0067] In the field of stabilization of hazardous elements, one of the defined routes for the generation of stable arsenical waste is related to the optimization of concentrated scorodite generation, which the applicant of this patent application has been working on.

[0068] One of the ways that has been proposed to carry out these optimizations is to use magnetite as a direct input for the precipitation of scorodite using solutions from the EcoMetales Industrial Plant, in order to consume the sulfuric acid present in the PLS ("pregnant leach solution"), and at the same time reduce the consumption of limestone by adjusting the acidity of the process. A more efficient consumption of sulfuric acid helps in the reduction of gypsum generation, and consequently reduce the mass of arsenical residue produced while maintaining and even increasing the arsenic precipitation yield in the abatement process.As a result of the lessons learned in the design of the arsenic abatement process for the PLS processed by EcoMetales, the alternative of evaluating this technology for the abatement of arsenic from sulfuric acid plant effluents generated in smelting and / or testing processes has been proposed.

[0069] Specifically, this application discloses a process for obtaining scorodite without the use of neutralizers from solutions comprising arsenic and sulfuric acid, which comprises the following steps: i. subjecting a first arsenical solution comprising arsenic in mixtures of arsenite ion and arsenate ion and sulfuric acid, to an oxidation process in order to oxidize the arsenite ion into arsenate ion, to obtain a second arsenical solution;

[0070] i. adding to the second arsenical solution a first solution comprising ferrous ion, to generate a third arsenical solution; iii. adding to the third arsenical solution a first portion of magnetite to adjust the molar ratio of ferric ion and arsenate ion, to generate a first arsenical pulp; iv. adding to the first arsenical pulp a portion of a scorodite pulp, recycled from step (viii), as a base for nucleating scorodite particles, to obtain a first scorodite pulp; v. heating the first scorodite pulp to a temperature between 80 and 90°C; vi. maintaining the first scorodite pulp at a temperature between 80 and 90°C for a time between 5 and 48 h with constant stirring to obtain a second scorodite pulp; vii. sending the second scorodite pulp to a solid-liquid separation stage, to obtain a third scorodite pulp and a treated solution; viii.recirculating a first portion of the third scorodite pulp to step (iv); and ix. filtering a second portion of the third scorodite pulp to obtain a scorodite cake that is sent for final disposal. In a preferred option, the first arsenical solution has an arsenic concentration ranging from 1 to 15 g / L.

[0071] In another preferred option! The first arsenical solution has a sulfuric acid concentration ranging from 5 to 80 g / L.

[0072] In another preferred option! the oxidation process in step (i) consists of the addition of one of hydrogen peroxide or sodium chlorite, or a photooxidation process.

[0073] In another preferred option! the first solution of step (i) comprises ferrous sulphate.

[0074] In another preferred option! in stage (i) the ferrous ion concentration is adjusted between 0.5 and 25 g / L.

[0075] In another preferred option!, magnetite is added in such a way that the molar ratio of ferric ion: arsenate ion in step (iii) is adjusted between 0.3 and 1.

[0076] In another preferred option! In the portion of magnetite added in stage (iii), 80% of the particles have a size less than 75 microns.

[0077] In another preferred option! the portion of scorodite pulp added in step (iv) is added so that the solids content of the first scorodite pulp is within the range of 2 to 15% w / w.

[0078] In another preferred option! the temperature of step (v) is 85°C.

[0079] Application examples

[0080] The following examples should be considered as embodiments of the present invention, and in no case as limitations thereof, since the different adaptations that can be made of it will be covered within the object claimed by this invention.

[0081] Example 1

[0082] Variation in percentage of solids

[0083] 3800 g of an arsenical solution were placed in a 5000 mL reactor. The concentration of sulfuric acid in the arsenical solution was 70 g / L. Magnetite with a purity of 96% was added to the reactor such that the Fe(lll) ratio Mag / As(V) (expressed as moles of ferric ion in magnetite) was equal to 1 and was maintained with constant mechanical stirring at 600 RPM and a temperature of 85°C with constant reflux using a condenser for 48 h. A defined amount of seed solids was added as scorodite to adjust the percentage of solids in the test between 4-15%. At different times, pulp samples were taken for analysis of the concentration of As, As(lll), Fe, Fe(ll) and H2SO4 in the liquid phase, and of the solid at the end of the test, the results of which are shown in Figure 1 .

[0084] The results show that there is a clear improvement when adding a greater quantity of seed solids such as scorodite, allowing the kinetics of arsenic(V) precipitation to be improved, with the best results being obtained using a percentage of recirculated scorodite solids of 15% w / w.

[0085] Example 2

[0086] Scorodite precipitation from sulfuric acid plant effluent solutions (EPAS)

[0087] 3800 g of an arsenical solution were placed in a 5000 mL reactor. The concentration of sulfuric acid in the arsenical solution varied between 0 g / L (pH 2) and 5 g / L. The arsenical solution had a concentration that vaporized between 5 and 12 g / L of As. The As(lll) was oxidized with hydrogen peroxide, such that between 100-300 mg / L of As(lll) remained in the solution. In application examples where the As(lll) concentration was higher than this range, arsenic trioxide was added to increase the starting concentration. Magnetite with a purity of 96% was added to the reactor such that the molar ratio (expressed as RM in the figures) Fe(lll) Mag / As(V) (expressed as moles of ferric ion in magnetite) varies between 0.3 and 1.2. The molar ratio was also expressed as FeT Mag / As(V), a value that is multiplied by 1.5 times the value of the Fe(lll) molar ratio Mag / As(V) because the magnetite mineral used in the experiments contains two moles of ferrous ion for every two moles of ferric ion (chemical formula magnetite FeOFe2O3). The ferrous ion concentration was varied between 0.5 and 20 g / L, supplying ferrous sulfate heptahydrate for concentration adjustment. The initial scorodite seed solids percentage was 15% w / w, supplied as pre-precipitated magnetite. The pulp was maintained under constant mechanical stirring at 600 RPM and a temperature of 85°C with constant reflux using a condenser for between 24 and 72 h. At different times, pulp samples were taken for analysis of the concentration of As, As(lll), Fe, Fe(ll) and H2SO4 in the liquid phase, and of the solid at the end of the test, the results of which are shown in figures 2 to 35. Effect of the concentration of H2SO4 and the concentration of ferrous ion

[0088] Figures 2, 3 and 4 show the evolution of arsenic concentration during scorodite precipitation, using different concentrations of sulfuric acid and ferrous ion with a magnetite molar ratio of Fe(lll) Mag / As(V) of 1.0. The results confirm that as the acidity increases, the kinetics of arsenic precipitation, measured in particular by the As concentration at 12 h of precipitation, becomes slower. For the case of a sulfuric acid concentration of 100 g / L over 48 h, very slow abatement kinetics are observed. The results show that the ferrous ion concentration has a beneficial effect on the arsenic abatement kinetics, since after 12 h of precipitation for all tests between 5 and 40 g / L of H2SO4, lower As concentrations are observed when using a ferrous ion concentration of 5 g / L compared to concentrations of 0.5 and 2 g / L.

[0089] Figure 5 shows the evolution of the concentration of sulfuric acid and ferrous ion during the course of scorodite precipitation, at the initial concentration of 5 g / L of sulfuric acid and an initial concentration of ferrous ion between 0.5 and 5 g / L with a magnetite molar ratio of Fe(lll) Mag / As(V) of 1.0. The results show that there is little variation in the concentration of sulfuric acid (except for some analytical differences), which indicates that there is a balance between the consumption of acid attributable to the leaching of magnetite and / or oxidation of ferrous sulfate with the generation of acid produced by the depletion of As(V) in the form of scorodite.

[0090] Figure 6 and 7 show the concentration of As released in TCLP assay for arsenic abatement tests with Fe(lll) molar ratio Mag / As(V) = 1.0 at different sulfuric acid concentrations and in the presence and absence of ferrous ion. The results show that for all solids generated, the As released in the TCLP test was below 1 mg / L, indicating that the waste was stable.

[0091] Figures 8, 9, 10 and 11 show the effect of high sulfuric acid concentration and high ferrous ion concentration on the As and Fe contents of the arsenical residue (ReAs) - Fe(lll)Mag / As(V) molar ratio = 1 , 0. The results show that the As grade of the arsenical residue (ReAs) varies around 26-29%, while the Fe grade varies between 21 -24%. Although an improvement in terms of reduction of Fe content in the arsenical residue is observed, which is associated with a lower magnetite content in the sample, the results indicate that it is not possible to obtain a complete conversion of magnetite in the presence of high ferrous ion concentrations. As a consequence, using high ferrous ion concentrations accelerates the kinetics, but some of the magnetite may not react, which would result as an impurity, reducing the As grade.On the other hand, lower ferrous ion concentrations generate slower precipitation kinetics, but favor a complete conversion of magnetite, which results in arsenical residue more enriched in arsenic.

[0092] Effect of increasing ferrous ion concentration to 20 g / L

[0093] Figure 12 shows the effect of sulfuric acid and ferrous ion concentration at high ferrous ion concentrations in the range of 5 to 20 g / L. Increasing the ferrous ion concentration to 20 g / L shows a positive effect in improving the abatement kinetics, however, this effect is more limited with respect to changes in sulfuric acid concentration as presented in the previous sections, and is also more limited with respect to the ferrous ion concentration range of 0.5 to 5.0 as observed in the previous section.

[0094] Figure 13 shows the effect of high sulfuric acid concentration and high ferrous ion concentration on the As and Fe content of the arsenical residue ReAs - Fe(lll) molar ratio. Mag / As(V) = 1 ,0 The results show that the As content of the arsenical residue ReAs varies around 26-29%, while the Fe content varies between 24-26%. Although an improvement in terms of reduction of the Fe content in the arsenical residue is observed, which is associated with a lower magnetite content in the sample, the results indicate that it is not possible to obtain a complete conversion of magnetite in the presence of high concentrations of ferrous ion, so there is a compromise in improving the precipitation kinetics in the presence of ferrous ion and magnetite and the As content of the arsenical residue.

[0095] Effect of the decrease in the Fe(lll) molar ratio Mag / As(V) from 1.0 to 0.5 in pH range 2 and 10 g / L of sulfuric acid

[0096] Figure 14 shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and the ferrous ion concentration on the As concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L. The results show that the most relevant effect is the reduction in the ferrous ion concentration to 2.5 g / L, which does not reduce all the arsenic present in the solution. In kinetic terms, similar behaviors are observed, but the curves with ferrous ion concentration at 2.5 g / L show a stabilization of the total arsenic concentration between 300-700 mg / L. Effect of the decrease in the Fe(lll) molar ratio Mag / As(V) from 1.0 to 0.5 and the decrease in ferrous ion concentration from 5 to 2.5 g / L

[0097] Figure 15 shows the comparison between Fe(ll) concentration of 2.5 and 5.0 g / L and Fe(lll) molar ratio Mag / As(V) of 0.5 and 1.0, showing that the most significant kinetic effect of arsenic abatement is associated with the concentration of ferrous ion, without observing major kinetic differences between the molar ratios 0.5 and 1.0.

[0098] Figure 16 in particular shows a very relevant effect of this technology related to the decrease in the concentration of As(lll) from 130 mg / L to values ​​below 20 mg / L, which shows that the system is capable of oxidizing a fraction of the As(lll) present in solution.

[0099] Figures 17 and 18 show the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and the ferrous ion concentration on the Fe and Fe(II) concentration in the treated EPAS solution - sulfuric acid concentration 5.0 g / L. The results show that when operating at an Fe(II) concentration of 2.5 g / L the system operates with a ferrous deficit, being consumed within the first 12 h. This results in relatively slower kinetics, and eventually in incomplete abatement due to the lack of iron in the system, as shown in Figure 15. All tests show a reduction in the FeT concentration indicating that at least part of the ferrous is consumed to precipitate arsenic, so the system operates by ferrous ion oxidation reactions for the precipitation of arsenic in the form of scorodite.

[0100] Figure 19 shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) and ferrous ion concentration on the As and Fe content of the arsenical residue ReAs - sulfuric acid concentration 5.0 g / L. The results show that even at Fe(lll) / As(V) molar ratios lower than 1.0 it is possible to generate precipitates with high As concentrations, and that compared to the Fe content the residues show Fe / As molar ratios close to 1.0 which is consistent with the chemical formula of scorodite. Effect of the low Fe(lll) molar ratio Mag / As(V) from 0.5 to 0.3 at a sulfuric acid concentration of 0.5 g / L and ferrous ion concentration of 0.5 g / L

[0101] Figure 20 shows the effect of lowering the Fe(lll) molar ratio. Mag / As(V) from 0.5 to 0.3. The results show that there is an effect on the As(lll) oxidation yield, but given that the starting As(lll) concentration was low at around 130 mg / L, this slowdown did not have a considerable effect on the arsenic precipitation yield.

[0102] Figure 21 shows the concentration of Fe, Fe(ll) and Fe(lll) in the treated EPAS solution. For both molar ratios Fe(lll) Mag / As(V) similar behaviors are observed in the reduction of Fe concentration in solution and the reduction in ferrous ion concentration. These results demonstrate that the precipitation process can be carried out at Fe(lll) / As(V) molar ratios lower than 1 , where the required Fe(lll) is generated by the oxidation of the ferrous ion and by the contribution of magnetite leaching. The Fe(lll) concentration is always observed to be low, which supports the fact that the system operates under conditions where the concentration gradient between As(V) and Fe(lll) is high enough to trigger the generation of nuclei for scorodite precipitation.Consequently, it is not necessary to operate the scorodite precipitation operation in a series of reactors in series in order to operate the precipitation at low doses of ferric ion, since there is a continuous contribution of ferric to the system by magnetite and the oxidation of the ferrous ion, which is immediately consumed by the As(V) in solution to generate scorodite.

[0103] Figure 22 shows the concentration of As and Cd released in TCLP (Toxicity Characteristic Leaching Procedure) tests, obtaining values ​​below the limits that determine the hazard of this type of waste. In particular, it is observed that the precipitated solids with a molar ratio of Fe(lll) Mag / As(V) 0.3 have higher values ​​of As and Cd released in TCLP assay than the Fe(lll) molar ratio Mag / As(V) 0.5, which may be due to a lower degree of crystallinity at lower molar ratios because the reaction is more dependent on the precipitation of As(V) resulting from the oxidation of ferrous ion.

[0104] Figure 23 shows the As and Fe contents of the arsenical residues generated from the precipitation of scorodite in molar ratios of Fe(lll) Mag / As(V) 0.3 and 0.5. The results show Fe / As ratios close to 1, attributed to scorodite precipitation. Effect of the initial As(lll) concentration

[0105] Figures 24 and 25 present the effect of increasing the initial concentration of As(lll) on the concentration of As and As(lll) in the treated EPAS solution - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L. The results show that with a magnetite dose of Fe(lll) Mag / As(V) of both 0.5 and 1.0 mol / mol it is possible to oxidize the As(lll) present in solution. Additionally, the kinetics proceed slightly slower in the presence of As(lll) due to the oxidation reaction necessary to reduce the As(V). This result is important, since it shows that for the arsenic removal process in solutions containing high concentrations of As(lll), the arsenic oxidation processes do not have to be as demanding in terms of the residual concentration of As(lll), since the scorodite precipitation system in the presence of the magnetite and ferrous sulfate couple is capable of oxidizing fractions of As(lll) present in solution, thus maximizing the abatement yields of arsenic in the form of scorodite.

[0106] Figures 26 and 27 show the effect of increasing the initial concentration of As(lll) on the concentration of Fe and Fe(ll) in the treated EPAS solution - sulfuric acid concentration 5.0 g / L - ferrous ion 5 g / L. The results show that in these tests there is a consumption of ferrous ion, which is slightly more pronounced in the presence of As(lll) with respect to the solution with lower concentration of As(lll), probably due to an activation of the oxidative cycle of the ferrous sulfate magnetite couple. In any case, the process acts with a slower kinetics of arsenic reduction because it is necessary to complete the oxidative process of the arsenious ion prior to its precipitation as scorodite.

[0107] Effect of ferrous ion concentration on arsenic precipitation

[0108] Figures 28 and 29 show the effect of lowering the initial Fe(ll) concentration on the As and As(lll) concentrations - sulfuric acid concentration 25-75 g / L - ferrous ion 0.5-2 g / L. The results show that at a Fe(ll) concentration of 0.5 g / L it is not possible to drive As(V) precipitation, probably because the ferrous ion concentration is low enough to proceed with the catalytic reaction. As a consequence, the difference in As(V) and Fe(lll) concentration is high enough to trigger the precipitation reaction. The above is supported by increasing the Fe(ll) concentration to 1.0 and 2.0, which as can be seen in Figure 29 improves the precipitation kinetics in 48 h, even though it does not reach the levels observed for Fe(ll) concentrations higher than 3.0 g / L observed in the previous sections.The As(lll) non-oxidative test (i.e., As(lll) concentration around 10 g / L) fails to precipitate arsenic at a ferrous ion concentration of 2.0 g / L. These results demonstrate that oxidation of the arsenious ion is necessary for precipitation to occur, since the ferrous ion-magnetite system is not sufficient to oxidize all the arsenious ion present in solution, but only a fraction of it. Furthermore, the critical ferrous ion concentration required to achieve arsenic abatement at concentrations in the treated solution below 100 mg / L is close to 3 g / L of ferrous ion.

[0109] Effect of increasing ferrous ion concentration to 3-4 g / L

[0110] Tests were performed to adjust the dosage of magnetite and ferrous ion to minimize the total iron supplied to the system. The doses supplied in this test plan were Fe(lll)Mag / As(V) 0.5 mol / mol is equivalent to FeT Mag / As(V) 0.75 and molar dose Fe(ll) / As(V) 0.63-0.84 mol / mol, which totals FeT / As(V) molar ratios within the range of 1.38-1.59 mol / mol.

[0111] Figure 30 shows the effect of the initial Fe(ll) concentration in the range 3-4 g / L on the As concentration and As(lll) - sulfuric acid concentration 5.0 - Fe(lll) ratio Mag / As(V) 0.5 mol / mol. The results show that a higher ferrous ion concentration improves the precipitation kinetics, as presented in the previous sections. However, total As concentrations at 48 h of precipitation are within the range of 100–200 mg / L, while As(III) concentrations are around 10–20 mg / L.

[0112] Synergistic effect of the addition of magnetite and ferrous ion

[0113] In order to isolate the effects of the addition of magnetite and ferrous ions on the precipitation of As(V) as scorodite, three experiments were carried out with only the addition of magnetite and one experiment with only the addition of ferrous ions. The results are presented in Figure 31 , which show that under neither condition could the arsenic present in the solution be reduced. In this sense, these results demonstrate the synergistic effect of the addition of magnetite and ferrous ions in the abatement of arsenic in the form of scorodite. The mechanisms by which arsenic is reduced in the presence of both elements can be very complex, but they must necessarily be related to a Fenton-type effect between the ferrous ion and the magnetite present in the mixture, which must generate a flow of electrons capable of oxidizing the ferrous present in solution to trigger the precipitation reaction of As(V) in the form of scorodite.

[0114] Figure 32 shows a diffractogram of arsenical residues ReAs generated in 24 h of precipitation with molar ratio Fe(lll) Mag / As(V) 1 ,0 and ferrous ion concentration 5 g / L and sulfuric acid concentration of 5 g / L, where it is observed that the arsenical residue has a scorodite content greater than 95% w / w. Figure 33 shows a scanning electron microscopy image of arsenical residues ReAs generated in 24 h of precipitation with molar ratio Fe(lll) Mag / As(V) 1 ,0 and ferrous ion concentration 5 g / L and sulfuric acid concentration of 5 g / L, where the typical morphology of scorodite crystals of spherical aggregate particles is observed.

[0115] Figure 34 shows the particle size distribution of arsenical residues ReAs generated in 48 h of precipitation with Fe(lll) molar ratio Mag / As(V) 0.5 and ferrous ion concentration 4 g / L and sulfuric acid concentration of 5 g / L, where it is observed that 80% of the particles have a size less than 30 pm.

[0116] Figure 35 shows a scanning electron microscopy image of magnetite used in scorodite precipitation tests.

[0117] In conclusion, based on the results obtained through the process of the present invention, it is possible to state that:

[0118] • It is possible to precipitate or generate arsenical waste using magnetite as a source of Fe 3+ with EPAS solutions (Sulfuric Acid Plant Effluent), generating abatement solutions that have As concentrations lower than 100 mg / L and arsenical waste with arsenic contents over 25% w / w.

[0119] • For the precipitation of scorodite from EPAS, it is necessary to provide at least 0.3 mol FeT / As(V) from magnetite and ferrous ion concentrations of 5 g / L. Additionally, the process requires the use of seed, which has been established at 15% w / w to accelerate the reaction kinetics.

[0120] • The ferrous ion and magnetite pair generates a synergistic effect in the precipitation of scorodite. Based on the results obtained, it is concluded that both agents participate in the precipitation of As(V), which is mostly reported as scorodite according to XRD analysis. (XRD: X-ray diffractometry). Consequently, there must be an oxide-reduction phenomenon that allows the oxidation of the ferrous ion in solution to be precipitated together with the As(V) as scorodite.

[0121] • The ferrous ion and magnetite pairing allows the oxidation of the remaining As(lll) in the EPAS solution. As(lll) concentrations of up to 1.4 g / L can be oxidized to As(lll) concentrations below 50 mg / L using systems that include the presence of ferrous ion and magnetite. • The kinetics of scorodite precipitation depend on a variety of factors, including: o Sulfuric acid concentration, the lower the concentration, the better the abatement kinetics o Ferrous ion concentration, the limiting concentration of Fe 2+ It is set at 3 g / L to facilitate the kinetics of scorodite precipitation using magnetite or Fe(lll) ratio Mag / As(V), higher dosages of magnetite favor the kinetics of arsenic precipitation, however, they leave unreacted magnetite in the arsenical residue. The molar ratio Fe(lll) Mag / As(V) 0.5 is the one that maximizes the conversion of magnetite to scorodite, provided that sufficient ferrous ion is added. or Percentage of seed solids, a higher percentage of solids favors precipitation kinetics. It has been established that a percentage of solids of 15% w / w is sufficient to accelerate the kinetics of scorodite precipitation.

[0122] • Depending on the magnetite and ferrous sulfate dosage, scorodite precipitation generates and / or consumes sulfuric acid. Systems that promote magnetite conversion are acid consumers, while those that react by converting the ferrous ion present in solution release acid. For the system evaluated with EPAS solutions, scorodite precipitation generates acid at a rate of 5 g / L in 6.3 g / L AsT solutions (0.6 mol of sulfuric acid per mole of As(V)).

[0123] • Arsenical waste generated with EPAS solutions are stable for the TCLP test due to both their As and Cd concentrations, both being less than 5 and 1 mg / L, respectively.

[0124] • The arsenical waste generated under the proposed process is the highest As grade and environmentally stable waste evaluated by EcoMetales to date. The results were obtained with real solutions of effluent from sulfuric acid plants.

Claims

CLAIMS 1. A process for obtaining scorodite without the use of neutralizers from solutions comprising arsenic and sulfuric acid, CHARACTERIZED in that it comprises the steps of: i. subjecting a first arsenical solution comprising arsenic in mixtures of arsenite ion and arsenate ion and sulfuric acid, to an oxidation process in order to oxidize the arsenite ion into arsenate ion, to obtain a second arsenical solution; i. adding to the second arsenical solution a first solution comprising ferrous ion, to generate a third arsenical solution; iii. adding to the third arsenical solution a first portion of magnetite to adjust the molar ratio of ferric ion:arsenate ion between 0.3 and 1 to generate a first arsenical pulp; iv. adding to the first arsenical pulp a portion of a scorodite pulp, recycled from step (viii), as a nucleation base for scorodite particles, to obtain a first scorodite pulp that is within the range of 2 to 15% w / w; v. heating the first scorodite pulp to a temperature between 80 and 90°C; vi. maintaining the first scorodite pulp at a temperature between 80 and 90°C for a period of between 5 and 48 h with constant stirring to obtain a second scorodite pulp; vii.sending the second scorodite pulp to a solid-liquid separation stage to obtain a third scorodite pulp and a treated solution; viii. recirculating a first portion of the third scorodite pulp to stage (iv); and ix. filtering a second portion of the third scorodite pulp to obtain a scorodite cake that is sent for final disposal.

2. The method according to claim 1, CHARACTERIZED in that the first arsenical solution has an arsenic concentration that varies between 1 to 15 g / L.

3. The process according to claim 1, CHARACTERIZED in that the first arsenical solution has a sulfuric acid concentration that varies between 5 and 80 g / L.

4. The process according to claim 1, CHARACTERIZED in that the oxidation process of step (i) consists of the addition of one of hydrogen peroxide or sodium chlorite, or a photooxidation process.

5. The process according to claim 1, CHARACTERIZED in that the first solution of step (i) comprises ferrous sulfate.

6. The process according to claim 1, CHARACTERIZED in that in step (i) the ferrous ion concentration is adjusted between 0.5 and 20 g / L.

7. The process according to claim 1, CHARACTERIZED in that in the portion of magnetite that is added in step (iii) 80% of the particles have a size less than 75 microns.

8. The process according to claim 1, CHARACTERIZED in that the temperature of step (v) is 85°C.

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