Table 1: Studies of industrial-grade phosphoric acid purification.

Process Efficiency/energy requirements/power consumption (kWh/kg)Reference

Selective precipitation63% of Pb and As was precipitated from the phosphate solution. Purification was carried out at 49°C[4]

Selective precipitationMg was removed by mixing the crude acid with to form MgSiF6·6H2O. Also, 25% Fe and 10% Al were removed. Filtrate had 0.37–0.51% of MgO in 72% P2O5. The temperatures range was 20 to 30°C[5]

Selective precipitationAmmonia was added to crude acid. Iron precipitates as Fe3(K,NH4)H8(PO4)6·H2O. The concentrations of Al, Fe, and Mg were decreased by ten times their initial values. The process was continued for 8 h at 70°C[6]

Solvent extraction The purified acid contained between 50 and 60% phosphate and approximately 1%  and 0.2% F. The concentration of the cation impurities was approximately 0.2%. Crude phosphoric acid (P2O5 52%) was fed at rate of 19.1 mL/min at temperature of 65°C. The process produced 37.5% phosphoric acid, and the impurities were extracted by solvent extraction at 110°C[7]

Solvent extractionThe process removed approximately 95% of Cd after 260 h of operation. The method used three hydraulic pumps, an extraction module (1.4 m2), and a back-extraction module at pressure of 4 psi[8]

Flotation and selective precipitation95% of the Cd and metals impurities ( ) were removed by neutralizing the solution to pH 1.4–2.0 and cooling it to 5–40°C. The metals were precipitated as metal xanthates (MXan). The xanthate having the longest and most branched organic part produced the best results[9]

Nanofiltration by membranesThe filtering process operated at a temperature of approximately −1°C to about 30°C and at a pressure between 600 and 1000 psig. The feed solution was filtered through a nanofiltration membrane to remove approximately 90% of the metal ion impurities[10]

Solvent extractionResin impregnated with extractants was put into contact with phosphoric acid for 24 h using a reciprocal shaker (150 rpm) at 25°C. Solvent extraction removed approximately 90% of the Cd from concentrated phosphoric acid solutions. The presence of cations strongly decreased the Cd removal efficiency.[11]

Reverse osmosis and nanofiltration Nanofiltration was more efficient than reverse osmosis. The permeate flow was 13.5 L/m2h at 1800 psi. The acid permeation was 46.3%, and 99.3% of the cationic impurities were globally rejected. The concentration of the impurities was reduced by approximately 100 times the initial value. However, the acid concentration was reduced to half its original value.[12]

Selective precipitationThis method produced an acid solution with an As concentration of less than 1 ppm. The purification method produced P2O5 concentration of at least 72.4%. The temperatures range was 130 to 150°C. Examples of the method produced concentrations of impurities in industrial 84-85% phosphoric acid were as follows: Fe (0.2–1.6%), Cr (0.1–0.8%), Ni (0.1–0.6%), Mo (<0.5%), and Na (0.1%)[13]

Nanofiltration by membranesAn 8 M acid solution was filtrated at 1000 psi for different periods of time. For periods longer than 120 h, the rejection of impurities decayed considerably. The acid permeation at 25°C was approximately 90% and the metal impurities’ retention was approximately 97%[14]

Solvent extraction Phosphoric acid was purified using solvent mixtures at 30°C. The most efficient P2O5 recovery was obtained using 55% methyl isobutyl ketone and 45% tributyl phosphate. A phase diagram was created for the ternary system of H3PO4-water-optimal phosphate solvent. The concentration of impurities decreased 1000 times from their initial values. Mg2+ purification was higher than that for the other impurities[15]

Complex formation and zeolites as adsorbentsBentonite and potassium amyl xanthate were used to remove organic impurities and Fe from crude phosphoric acid. The efficiency of potassium amyl xanthate in reducing the iron content was 79.19% and was accompanied by a small decrease in the P2O5 concentration. Bentonite clay was used to remove the organic impurities. Na2SiO3 and Na2CO3 reduced the F content at 70°C at a defluorination efficiency of 90.56%. The Fe content was reduced to 0.52% of its initial value at a P2O5 loss of 3%. Cd and Cu were removed at a P2O5 loss of 1.70%. The Fe minimizing efficiency was 10.95–52.38% at the temperatures of 25 to 70°C[16]

Selective precipitationPrecipitation occurred at 60°C. The process produced 85% acid with 200 ppb of Sb. The impurities were removed by adding H2S. However, the content was 220 ppb[17]

Solvent extractionThe phosphoric acid was purified using n-butanol, n-hexanol, and n-octanol. Octanol was the most efficient alcohol. The final concentration of H3PO4 was 9.2 M. 98% of F was removed and the final concentration of Fe was 10 ppm. The Cu, Cd, Mn, and Zn concentrations were too low to be determined. The temperature (5°C and 60°C) had a small effect[18]