The aim of the study was to test commercial and experimental NF membranes for their separation efficiency and acid resistance in a long-term filtration experiment. Several NF membranes (NF 270, Desal-5 DK, Desal KH, BPT-NF-1 and BPT-NF-2) were tested for their separation efficiency and stability when a solution containing 25 g/L CuSO4 and 8 wt.% H2SO4 was continuously filtered at 40degreesC for 2 months. Filtration experiments were carried out with a new five-cell flat-sheet laboratory apparatus. Commercial NF membranes showed good selectivity, retaining most of the copper sulphate and letting most of the sulphuric acid pass into the permeate. However, only the membranes designed to be acid resistant (Desal KH and BPT-NF-2) maintained their separation efficiency during the 2 months of separation. The Desal KH membrane gave better copper retention values (92-95%) than the BPT-NF-2 (60-88%), but the overall selectivity was best with the BPT-NF-2 membrane due to its good sulphuric acid permeation. (C) 2004 Elsevier B.V. All rights reserved.