Method of purifying phosphoric acid

1. A method of purifying phosphoric acid comprising:

(a) providing a pre-treated wet phosphoric acid feed containing:

(i) 2-62% phosphoric acid, expressed as a percent weight of P 2 O 5 in said feed;

(ii) a sulfate concentration of at least 15 ppm, per each 1 percent weight of said P 2 O 5 in said feed; and

(iii) metallic impurities including iron, aluminum, calcium, magnesium, and chromium, a total feed concentration of said iron, aluminum, calcium, magnesium, and chromium being at least 10 ppm, per each 1 percent weight of said P 2 O 5 in said feed;

(b) subjecting said pre-treated wet phosphoric acid feed to a first nanofilter in at least a first nanofiltration stage, to obtain a first permeate and a first reject;

(c) treating said first permeate to produce sulfate solids in a partially treated phosphoric acid stream;

(d) removing at least a portion of said solids from said phosphoric acid stream to produce a sulfate-depleted phosphoric acid stream; and

(e) subjecting said sulfate-depleted phosphoric acid stream to at least a second nanofilter in at least one additional nanofiltration stage, to obtain a purified phosphoric acid in a second permeate, and a second reject, said second permeate containing:

(i) a concentration of 2-62% phosphoric acid, expressed as a percent weight of P 2 O 5 in said second permeate;

(ii) a sulfate concentration of less than 10 ppm per each 1 percent weight of said P 2 O 5 in said second permeate; and

(iii) metallic impurities including iron, aluminum, calcium, magnesium, and chromium, a total concentration of said iron, aluminum, calcium, magnesium, and chromium in said second permeate being at most 5 ppm, per each 1 percent weight of said P 2 O 5 in said second permeate.

2. The method of claim 1 , said pre-treated wet phosphoric acid feed further containing a total organic carbon concentration of at least 5 ppm, per each 1 percent weight of P 2 O 5 in said feed.

3. The method of claim 1 , said purified phosphoric acid further containing metallic impurities, said total concentration of said iron, aluminum, calcium, magnesium, and chromium being at most 4 ppm per each 1 percent weight of P 2 O 5 in said second permeate.

4. The method of claim 1 , said purified phosphoric acid further containing a total organic content concentration of at most 3 ppm per each 1 percent weight of P 2 O 5 in said second permeate.

5. The method of claim 1 , said treating including contacting said first permeate with a barium salt to precipitate said sulfate solids.

6. The method of claim 5 , said barium salt including barium carbonate.

7. The method of claim 5 , said second permeate containing at most 0.05 ppm of barium, per each 1 percent weight of P 2 O 5 in said second permeate.

8. The method of claim 1 , an operating temperature of said first and second nanofiltration stages being within a range of 20° C. to 60° C.

9. The method claim 1 , at least one of said first and said at least one additional nanofiltration stages being operated at a pressure within a range of 30 to 80 bar.

10. The method of claim 1 , said pre-treated wet phosphoric acid feed containing at least 20% of said P 2 O 5 , by weight.

11. The method of claim 1 , said purified phosphoric acid containing at least 20% of said P 2 O 5 , by weight.

12. The method of claim 1 , further comprising performing a backwashing operation on at least one of said first nanofilter and said second nanofilter.

13. The method of claim 1 , further comprising returning said first reject to a process step prior to step (b).

14. The method of claim 1 , further comprising returning said second reject to a process step prior to step (d).

15. The method of claim 7 , a composition of said pre-treated wet phosphoric acid feed containing 20-60% of said P 2 O 5 , and at least one of: 1000 to 3000 ppm of said calcium, 1000 to 3000 ppm of said magnesium, 1000 to 4000 ppm of said aluminum, 1000 to 3000 ppm of said sulfate, 100 to 500 ppm of said total organic carbon, 500 to 3000 ppm of said iron, and 50 to 500 ppm of said chromium.

16. The method of claim 7 , said second permeate containing 30-55% of said P 2 O 5 , and at most 15 ppm of said calcium, at most 15 ppm of said magnesium, at most 3 ppm of said aluminum, and at most 120 ppm of said sulfate.

17. The method of claim 7 , said second permeate containing at least one of: at most 4 ppm iron, at most 2 ppm chromium, and at most 1 ppm cadmium.

18. The method of claim 7 , said sulfate concentration of said second permeate being at least 1 ppm per each 1 percent weight of P 2 O 5 in said second permeate.

19. The method of claim 5 , wherein said phosphoric acid stream contains barium ions, and wherein said subjecting of said phosphoric acid stream to at least said second nanofilter is performed so as to selectively reject to said second reject, with respect to said second permeate, said barium ions.

20. A method of purifying phosphoric acid comprising:

(a) providing a pre-treated wet phosphoric acid feed containing:

(i) 20-62% phosphoric acid, expressed as a percent weight of P 2 O 5 in said feed;

(ii) a sulfate concentration of at least 15 ppm, per each 1 percent weight of said P 2 O 5 in said feed; and

(iii) metallic impurities including iron, aluminum, calcium, magnesium, and chromium, a total feed concentration of said iron, aluminum, calcium, magnesium, and chromium being at least 10 ppm, per each 1 percent weight of said P 2 O 5 in said feed;

(b) subjecting said pre-treated wet phosphoric acid feed to a first nanofilter in at least a first nanofiltration stage, to obtain a first permeate and a first reject;

(c) treating said first permeate with a barium salt to produce sulfate solids in a partially treated phosphoric acid stream;

(d) removing at least a portion of said solids from said phosphoric acid stream; and

(e) subjecting said phosphoric acid stream to at least a second nanofilter in at least one additional nanofiltration stage, to obtain a purified phosphoric acid in a second permeate, and a second reject, said second permeate containing:

(i) a concentration of 20-62% phosphoric acid, expressed as a percent weight of P 2 O 5 in said second permeate;

(ii) a sulfate concentration of less than 10 ppm per each 1 percent weight of said P 2 O 5 in said second permeate; and

(iii) metallic impurities including iron, aluminum, calcium, magnesium, and chromium, a total concentration of said iron, aluminum, calcium, magnesium, and chromium in said second permeate being at most 5 ppm, per each 1 percent weight of said P 2 O 5 in said second permeate;

said second permeate containing at most 0.05 ppm of barium, per each 1 percent weight of P 2 O 5 in said second permeate.