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 organoph osphorus compounds. The measurement of the kinetics parameters was don e at conditions of constant supersaturation probing the conductivity o f the supersaturated solutions. Aqueous solutions of 1-hydroxy-ethylid ene-1,1-diphosphonic acid (EHDP), nitrilotrimethylene phosphonic acid (NTMP) and N,N,N',N'-ethylene diaminotetramethylene phosphonic acid (E NTMP) were included in unstable supersaturated solutions of calcium su lfate. NTMP and ENTMP at concentration levels between 0.1 and 1.0 mu M increased drastically the induction times preceding precipitation whi le the rates of precipitation were reduced by 90% at concentrations be low 1 mu M. EHDP was effective in reducing the precipitation rates but at considerably higher concentrations (70% reduction at 0.1 mu M). Ex amination of the morphology of the precipitated crystals suggested tha t the kinetic results obtained could be attributed to the adsorption o f 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] res ulting in shorter and broader crystallites. (C) 1998 Elsevier Science B.V. All rights reserved.