Recovery of chromate from spent plating solutions by two-stage nanofiltration processes

A two-stage nanofiltration (TSNF) approach was designed to concentrate and purify the chromium (IV) or chromate for reuse from a spent plating solution. Chromate is a divalent species with pKa2 of 6.5. Therefore, monovalent HCrO4– is dominant for pH < 6.5 but divalent CrO42– is dominant for pH > 6.5. Since NF can remove divalent ions but for monovalent to pass, in the first stage of the TSNF system, NF was used to retain nickel in the concentrate stream, but HCrO4–, the dominant at a low initial pH ≈ 3, can pass through the membrane in the permeate stream, and separation of HCrO4– and nickel was achieved at a high system recovery. The permeate stream from stage 1 was then adjusted to pH > 8 to convert HCrO4– to CrO42–. Thus in the second stage NF can concentrate CrO42– at a high system recovery. The result shows that at a pressure of 80 psi and water recovery 10% more than 90% mass of nickel and chromium were separated and recovered in the TSNF system. Moreover, a reflection factor σ for HCrO4– was determined as 0.2725 in the first stage and 0.9961 for CrO42– in the second stage. With almost the same molecular weight, the reflection factors were completely different. Therefore, the selectivity of these two chromium species (HCrO4– and CrO42–) for the TSNF system was dependent on the charge repulsion instead of the molecular weight.