Kinetics of calcium sulfate formation in aqueous media: effect of organophosphorus compounds

The kinetics of spontaneous precipitation of calcium sulfate dihydrate at pH 7.0, 30 degrees C were investigated in the presence of organophosphorus compounds. The measurement of the kinetics parameters was done at conditions of constant supersaturation probing the conductivity of the supersaturated solutions. Aqueous solutions of 1-hydroxy-ethylidene-1,1-diphosphonic acid (EHDP), nitrilotrimethylene phosphonic acid (NTMP) and N,N,N',N'-ethylene diaminotetramethylene phosphonic acid (ENTMP) were included in unstable supersaturated solutions of calcium sulfate. NTMP and ENTMP at concentration levels between 0.1 and 1.0 mu M increased drastically the induction times preceding precipitation while the rates of precipitation were reduced by 90% at concentrations below 1 mu M. EHDP was effective in reducing the precipitation rates but at considerably higher concentrations (70% reduction at 0.1 mu M). Examination of the morphology of the precipitated crystals suggested that the kinetic results obtained could be attributed to the adsorption of the organophosphorus compounds onto different gypsum crystal faces. EHDP did not inhibit the [0 2 0] face resulting in elongated prismatic crystals, while NTMP and ENTMP favored the development of [0 2 0] resulting in shorter and broader crystallites. (C) 1998 Elsevier Science B.V. All rights reserved.