On the use of dead-end ultrafiltration for the study of the influence of sulfate and/or silicate on particle size distribution in concentrated and partially prehydrolyzed Al(III) flocculants

Because of their great tendency towards hydrolysis, concentrated and partia lly prehydrolyzed AI(III) flocculants cannot be diluted as required by many physico-chemical characterization methods. In this work, the complete dead -end ultrafiltration has proven to be a useful characterization method for these products. The results presented have shown that the behavior of the m embrane/solution interphase could be described by general ultrafiltration ( UF) theories. In the first part of this work, the most discriminating UF me thod (serial UF) is chosen for the characterization of concentrated floccul ants. Next, prehydrolyzed flocculants with an increasing silicate and/or su lfate content were synthesized and the influence of silicate and/or sulfate on particle size distribution was resolved by serial UF The silicate conte nt is correlated to the size increase of the colloidal particles in such fl occulants. This is explained by the reduced effectiveness of fragmentation of aluminosilicate particles formed during flocculant synthesis as the sili cate content increases. Sulfate also increases the mean size of the particl es, but the mean size is not correlated to the sulfate content. The role of sulfate is apparently to prevent the dissolution of the Al hydroxide parti cles formed in the initial step of flocculant synthesis. UF in presence of silicate and sulfate shows that, in such cases, sulfate could also limit th e aluminosilicate fragmentation leading to a greater increase of the partic le size than that observed with only silicate addition. (C) 1999 Elsevier S cience B.V. All rights reserved.