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An experimental study was carried out to determine the rate and extent of scale formation of gypsum (calcium sulphate dihydrate) on stainless steel surfaces in a continuous hydrometallurgical sulphuric acid-calcium carbonate partial neutralization reactor. The purpose of the study was to determine the effects of pH, temperature, residence time, presence of metal sulphates (Fe, Al, and Ni), addition of surfactants and addition of gypsum seed on scale growth. The rate of scale formation was found to be reduced by lower temperatures, longer residence times and the presence of nickel. pH had no significant effect. The effect of these variables on the rate of scale formation was related to the degree of gypsum supersaturation. Scale reduction through the addition of sulphonated anionic surfactants was found to be effective only under certain conditions. However, gypsum seeding was found to be an effective method of scale reduction. Scale was reduced by 50% or more at a seed concentration of 10 g/L.

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