Removal of Ca²⁺ and Mg²⁺ from Dilute Hydrochloric Acid by Nanofiltration Membrane

In order to solve the present problems of social resource recovery and environmental protection pressure, the diluted hydrochloric acid containing calcium and magnesium ions is treated by the nanofiltration technique to realize the reuse of the diluted hydrochloric acid. The effects of process conditions such as operation time, salt content, salt species, metal ion containing ratio, operating pressure, flow rate and liquid temperature on the removal rates of Ca²⁺ and Mg²⁺ and membrane flux were discussed. At the same time, the performance of single-component and two-component inorganic salt pure water solutions and acidic solutions were compared, and the effects on the removal of Ca²⁺ and Mg²⁺ were studied as well. The results showed that the removal rate of Ca²⁺ was 80%—90%, and that of Mg²⁺ was 90%—100% in the 3% (mass fraction) dilute hydrochloric acid solutions with salt mass fraction of 3×10⁻⁴, 2×10⁻³ and 3×10⁻³ (bi-component, m_\rmCa^2+\text∶ m_\rmmg^2+=1∶1), respectively. The suitable operating conditions of the nanofiltration membrane are as follows, the pressure of 0.4 MPa, temperature of 30 ℃, the maximum flow of 396 L/h. Compared with aqueous solution, acidic conditions are more effective to the removal of Ca²⁺ and Mg²⁺, and the removal rate increases by about 25%. Therefore, the nanofiltration membrane can be applied to the purification and reuse of acid solution.