Purification of phosphoric acid solutions by reverse osmosis and nanofiltration

Purification of phosphoric acid by reverse osmosis (RO) and nanofiltration (NF) was studied using different commercial membranes of common industrial use. Both techniques represent an interesting alternative in purifying industrial phosphoric acid solutions. RO is a good choice to purify solutions up to a concentration of 2 M. Among the RO membranes, SX01 (Osmonics) yields the best global performance: permeate flux of 13.5 L/m2h, at 1800 psi, 46.3% of acid permeation and 99.3% of global rejection of cationic impurities. The performance of membrane was better by applying a simple pretreatment with a solution of phosphoric acid/ethanol/water. However, NF is a better alternative since it can yield good results at concentrations up to 8 M. The DS5DL (Osmonics) is the NF membrane with the best global performance: permeate flux of 3.02 L/m2h at 1000 psi, 94.2% of acid permeation and 99.2% of cationic impurities rejection.

Fig. 3. Kedem–Katchalsky’s model: a) SX01 membrane and b) DS5DL membrane. The results obtained in this work show that both RO and NF are viable to purify industrial phosphoric acid solutions. However, NF is the best choice because it is possible to treat solutions with high H3PO4 concentrations with a good global performance. A linear relationship was observed with the application of an irreversible thermodynamics model for the different acid concentrations in both membranes. The experimental concentration of the H3PO4 in the permeate increases with the concentration of feeding solution as predicted by the ion-exclusion model.