Process for Reducing the Concentration of Arsenic in an Aqueous Solution Comprising a Fluoroacid

Abstract

A process for reducing the concentration of one or more arsenic-containing compounds in an aqueous solution comprising at least one fluoroacid, which process comprises: (i) contacting the aqueous solution with an oxidising agent to produce one or more Asv-containing compounds; and (ii) removal of precipitated arsenic-containing compounds; wherein the process comprises a step (iii) the addition of an aqueous alkali solution or slurry, which may take place after step (i) and before step (ii) or after step (ii).

Claims

1. A process for reducing the concentration of one or more arsenic-containing compounds in an aqueous solution comprising at least one fluoroacid, which process comprises: (i) contacting the aqueous solution with an oxidising agent to produce one or more As.sup.V-containing compounds; and (ii) removal of precipitated arsenic-containing compounds; wherein the process comprises a step (iii) the addition of an aqueous alkali solution or slurry, which may take place after step (i) and before step (ii) or after step (ii).

2. A process according to claim 1, wherein the aqueous solution comprises fluorosilicic acid.

3. A process according to claim 1 or 2, wherein the fluoroacid is present in an amount of from about 1 to about 50% by weight based on the total weight of the aqueous solution.

4. A process according to any of the preceding claims, wherein the one or more arsenic-containing compounds comprise AsF.sub.3 or a hydrolysed form of AsF.sub.3.

5. A process according to any of the preceding claims, wherein the oxidising agent is selected from chlorine (Cl.sub.2), hypochlorite salts (M.sup.+ClO.sup.), hypochlorous acid (HClO), hydrogen peroxide (H.sub.2O.sub.2) and permanganate salts (M.sup.+MnO.sub.4.sup.) and mixtures thereof.

6. A process according to any of the preceding claims, wherein the removal of the precipitated arsenic-containing compounds comprises gravity-settling, filtration, anion-exchange resin or a combination thereof.

7. A process according to any of claims 1 to 6, wherein step (ii) is preceded by the addition of an aqueous alkali solution or slurry.

8. A process according to any of claims 1 to 6, wherein step (ii) is followed by the addition of an aqueous alkali solution or slurry.

9. A process according to any of the preceding claims, wherein the alkali is calcium hydroxide (Ca(OH).sub.2) or calcium oxide (CaO).

10. A process according to claim 9, wherein the calcium hydroxide or calcium oxide is present in an amount of from about 15 to about 20% by weight of the solution or slurry.

11. A process according to any of the preceding claims, further comprising dilution of the solution prior to the addition of the alkali solution or slurry.

12. A process according to claims 8 to 11 comprising a subsequent step (iv) of removal of the resulting precipitate.

13. A process according to any of the preceding claims, wherein the addition of the oxidising agent is carried out at a temperature of from about 0 to about 35 C.

14. A process according to any of the preceding claims, wherein step (i) is carried out in a time period of from about 1 to about 30 minutes.

15. A process according to claim 14, wherein step (i) is carried out in a time period of from 2 to 5 minutes.

16. A process according to any preceding claim, wherein in step (i) the oxidising agent is used in a stoichiometric excess relative to the quantity of oxidisable arsenic-containing compounds.

17. A process according to claim 16, wherein the stoichiometric excess is 4 to 60 times the quantity of oxidisable arsenic-containing compounds.

18. A process according to claim 17, wherein the stoichiometric excess is 20 to 40 times the quantity of oxidisable arsenic-containing compounds.

19. A process according to any of the preceding claims, wherein step (iii) is carried out at a temperature of from about 0 to about 35 C.

20. A process according to claim 18 or 19, wherein step (iii) is carried out at a temperature of from about 15 to about 30 C.

21. A process according to any of the preceding claims comprising subsequently recycling all or part of the treated aqueous solution and repeating a process as defined in any one of claims 1 to 20.

22. A process according to any of the preceding claims, wherein from about 50 to about 100% by weight of the arsenic-containing compounds is removed from the solution.

23. A process according to any of the preceding claims, wherein the treated solution contains about 5 ppm or less of arsenic-containing compounds.

24. A process according to any of the preceding claims, wherein the treated solution contains about 1 ppm or less of arsenic-containing compounds.

25. A process for the production of fluorosilicic acid comprising a process as defined in any of the preceding claims.

26. A process for the purification of fluorosilicic acid comprising a process as defined in any of the preceding claims.

27. A process for the production of HF comprising a process as defined in any of the preceding claims.

28. A process for the production of aluminium trifluoride (AlF.sub.3) comprising a process as defined in any of the preceding claims.

Description

[0054] FIG. 1 shows a process of the invention where step (iii) is carried out after step (ii).

[0055] FIG. 2 shows a process of the invention where step (iii) is carried out before step (ii).

[0056] As an example of the process of the invention, step (i) may be carried out in a static mixer before the solution is moved to a sedimentation tank. The arsenic-containing sludge is then removed and the treated solution is contacted with an ionic exchanger, i.e. an anion exchange resin as part of step (ii). The arsenic-containing residues and sludge produced in steps (i) and (ii) can then be transferred to a further mixing tank where an alkali slurry is added (step (iii)), before the arsenic containing solids are separated from the solution. This is represented by FIG. 1.

[0057] In an alternative, step (i) may be carried out in a static mixer before the solution is moved to a dilution tank to reduce the concentration of fluorosilicic acid to below 10% by weight. The aqueous solution is then transferred to a sedimentation tank and contacted with an alkali slurry (step (iii)). The arsenic-containing solids that are generated are then separated from the solution in step (ii). This alternative set up is represented by FIG. 2.

[0058] Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all the preferences and options for all other aspects, features and parameters of the invention.

[0059] The invention will now be illustrated with reference to the following, non-limiting Examples.

EXAMPLES

[0060] All percentages are given by weight of the total composition unless otherwise specified.

Example 1

Arsenic Oxidation by 1% KMnO.SUB.4 .Solution, Followed by Filtration and Neutralization

[0061] 1. A sample of aqueous H.sub.2SiF.sub.6 was taken from an HF manufacturing plant that uses high arsenic fluorspar. The sample was analysed and found to contain 25.48% H.sub.2SiF.sub.6, 4.94% HF and 140.028 ppm arsenic

[0062] 2. 100 mL samples of H.sub.2SiF.sub.6 were placed in 4 different beakers and stirred at 350 RPM

[0063] 3. 10 mL of a 1% solution of KMnO.sub.4 were added to each beaker and stirred for varying amounts of time as shown in Table 1.

TABLE-US-00001 TABLE 1 Time (min) KMnO.sub.4 1% (mL) H.sub.2SiF.sub.6 (mL) RPM 5 10 100 350 10 10 100 350 15 10 100 350 20 10 100 350

[0064] 4. After the allotted time the solution was filtered with Whatman filter paper and then filtered again with a filter cloth

[0065] 5. From this filtrate an aliquot was taken to measure the concentration of arsenic, the results of which are provided in Table 2.

TABLE-US-00002 TABLE 2 Time KMnO.sub.4 1% H.sub.2SiF.sub.6 As Removal (min) (mL) (mL) RPM (ppm) (%) 5 10 100 350 38.1 73 10 10 100 350 44.0 69 15 10 100 350 25.0 82 20 10 100 350 26.3 81

[0066] 6. 50 mL of each filtrate was taken and neutralized with 15% Ca(OH).sub.2 slurry. The final pH of each solution is provided in Table 3.

TABLE-US-00003 TABLE 3 Time (min) H.sub.2SiF.sub.6 (mL) Ca(OH).sub.2 slurry (g) pH 5 50 10 14 10 50 7.8 10 15 50 7.07 10 20 50 7.65 10

[0067] 7. The neutralized solution was filtered through Whatman filter paper

[0068] 8. The filtrate was analysed with the results being shown in Table 4.

TABLE-US-00004 TABLE 4 Time H.sub.2SiF.sub.6 Ca(OH).sub.2 slurry As Removal (min) (mL) (g) pH (ppm) (%) 5 50 10 14 0.00 100.00% 10 50 7.8 10 0.01 99.99% 15 50 7.07 10 0.00 100.00% 20 50 7.65 10 0.03 99.98%

Example 2

[0069] Arsenic Oxidation with NaClO (12 wt %)

[0070] 1. A 500 mL sample of aqueous H.sub.2SiF.sub.6 was taken from an HF manufacturing plant that uses high arsenic spar. The sample was analysed and found to contain 25% H.sub.2SiF.sub.6, 5% HF and 100 ppm arsenic

[0071] 2. 10 mL and 5 mL portions of 12% aqueous NaClO were added to two 100 mL samples of the aqueous H.sub.2SiF.sub.6

[0072] 3. The mixtures were stirred at 350 rpm for 10 minutes

[0073] 4. The resulting mixtures were filtered with double filtration Whatman paper

[0074] 5. The arsenic concentration of the filtrates were analysed. The results are presented in Table 5.

TABLE-US-00005 TABLE 5 Sample Filtration As (ppm) Removal (%) 10 mL NaClO Double 4.8 93.3 5 mL NaClO Double 4.2 94.1

Example 3

[0075] Arsenic Oxidation with NaClO, R=Weight Ratio of NaClO (g) to Arsenic (g)

[0076] 1. A sample of aqueous H.sub.2SiF.sub.6 was taken from an HF manufacturing plant that uses high arsenic fluorspar. The sample was analysed and found to contain 25% by weight H.sub.2SiF.sub.6, 5% HF and 100 ppm arsenic

[0077] 2. 1000 mL of the H.sub.2SiF.sub.6 was placed in a polypropylene container

[0078] 3. 110 mL of 12% aqueous NaClO was added to the sample

[0079] 4. The mixture was stirred at 450 rpm for 15 minutes

[0080] 5. The sample was allowed to stand for 10 min before being filtered with two Whatman filter papers

[0081] 6. 37.97 g of white solids are recovered once the filtered solids are dried

[0082] 7. The arsenic concentration of the filtrate was analysed with the results set out in Table 6.

TABLE-US-00006 TABLE 6 Sample As (ppm) Removal (%) NaClO R = 172 4.4 95.6

[0083] 8. Two aliquots of the filtrate were taken and subsequently diluted to obtain an aqueous solution at 10% and 1% by weight of H.sub.2SiF.sub.6

[0084] 9. 15% Ca(OH).sub.2 slurry was added to neutralize the H.sub.2SiF.sub.6 and capture any arsenic still present in the filtrate

[0085] 10. The results are presented in Table 7.

TABLE-US-00007 TABLE 7 Concentration Ca(OH).sub.2 slurry As Removal of H.sub.2SiF.sub.6 (g) pH (ppm) (%) 1% 5.06 10.09 0.09 99.91 10% 53.4 10.22 0.139 98.82

[0086] 11. The samples were filtered again with Whatman paper and the Ca(OH).sub.2 was added to neutralise the samples

[0087] 12. The results are set out in Table 8.

TABLE-US-00008 TABLE 8 Concentration Ca(OH).sub.2 slurry As Removal of H.sub.2SiF.sub.6 (g) pH (ppm) (%) 1% 5.06 10.09 0.06 99.92 10% 53.4 10.22 0.03 99.96

Example 4

Arsenic Removal in Process Effluents Hydrofluoric Acid

[0088] 1. Two samples of 500 mL of aqueous H.sub.2SiF.sub.6 were taken from an HF manufacturing plant that uses high arsenic spar. The sample was analysed and found to contain 25% by weight H.sub.2SiF.sub.6, 5% by weight HF and 100 ppm arsenic

[0089] 2. 1.1 g of 12% by weight NaClO (aq) was added to each sample

[0090] 3. The samples were mixed at 700 rpm for 15 minutes

[0091] 4. The solutions were filtered with Whatman filter paper

[0092] 5. The arsenic content of the filtrates were analysed and the results are set out in Table 9.

TABLE-US-00009 TABLE 9 Sample 12% NaClO (g) As (ppm) Removal (%) 1 1.1671 0.47 69 2 1.0503 0.20 87

[0093] 6. Subsequently, these samples were neutralized with 15% Ca(OH).sub.2 slurry until a pH of 10 was reached. The results are set out in Table 10.

TABLE-US-00010 TABLE 10 Sample Ca(OH).sub.2 slurry (g) pH As (ppm) Removal (%) 1 2.9 10.46 0.014 99% 2 3.5 10.74 0.023 98%

Example 5

Arsenic Removal in Fluorosilicic Acid by Oxidation and Ion Exchange

[0094] 1. A 400 mL sample of effluent was taken from an HF manufacturing plant that uses high arsenic spar. The sample was analysed and found to contain 25% H.sub.2SiF.sub.6, 5% HF and 100 ppm Arsenic

[0095] 2. 20 mL of 12% aqueous NaClO was added

[0096] 3. The mixture was stirred at 750 rpm and left to stand for 15 minutes

[0097] 4. The solution was filtered with 250 g of a strong anion exchange resin type II (Purolite PFA 300)

[0098] 5. The solution was in contact with the resin for 15 minutes before the cycle was repeated

[0099] 6. The results are set out in Table 11.

TABLE-US-00011 TABLE 11 Resin cycle As (ppm) Removal (%) 1 R 1.6 98.40 2 R 1.2 98.80 3 R 0.6 99.40 4 R 0.4 99.60

Example 6

[0100] Arsenic Removal in Fluorosilicic Acid by Oxidation with H.sub.2O.sub.2 and Ion Exchange

[0101] 1. 3800 mL of effluent was taken from an HF manufacturing plant that uses high arsenic spar. The sample was analysed and found to contain 25.13% H.sub.2SiF.sub.6, 5.48% HF and 70 ppm arsenic

[0102] 2. The sample was mixed at 350 rpm

[0103] 3. 3 mL of 30% H.sub.2O.sub.2 was added

[0104] 4. Reaction begins and ends after 18 hours of continuous stirring.

[0105] 5. The sample was introduced to a packed column of 185 g of anionic resin (Purolite PFA 300)

[0106] 6. A liquid outlet flow was adjusted to 5 mL/min

[0107] 7. Samples were obtained every 15 minutes

[0108] 8. The results are set out in Table 12.

TABLE-US-00012 TABLE 12 Sample Time (min) As (ppm) Removal (%) 1 15 1.3 98.1% 2 30 4.6 93.4% 3 45 4.2 94.1% 4 60 12.4 82.3% 5 75 11.8 83.2%