Abstract
Calcium sulphate dihydrate is a compound important among those which form troublesome scale in distillation apparatus for the production of potable water from the sea. Among methods employed for the reduction of scale-formation is the use of certain substances which inhibit the crystallization of scale-forming materials. The present study is an investigation of some of the factors involved in the inhibition of crystallization from supersaturated solutions. The crystallization of CaSO4.2H2O, both seeded and unseeded, from solutions containing sodium chloride and sodium tripolyphosphate, has been studied as a function of the supersaturation and of the concentration of polyphosphate additive. The processes occurring have been monitored by following the concentrations of calcium and polyphosphate species in solution with time, using compleximetric and colorimetric analysis. Both in the seeded and unseeded crystallization, the presence of polyphosphate led to the inhibition of crystallization. Induction periods have been shown to be logarithmically related to supersaturation and additive concentration and in the seeded case to be dependent on the mass of seed crystals for small amounts of seed. The interaction between tripolyphosphate in solution and calcium sulphate dihydrate has been studied by measurements of concentration loss from solution and the take-up of polyphosphate shown to involve processes other than simple sorption. In contrast with sorption of polyphosphates by less soluble adsorbents, such as barium or strontium sulphates, the take-up by calcium sulphate dihydrate is suggested to involve in initiation of the deposition of a substance containing calcium and polyphosphate on to the crystals. The solid phases involved have been examined by chemical and infrared analysis. X-ray powder diffraction and electron microscopy.