Gypsum scale formation in continuous neutralization reactors

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