Aspects of gypsum precipitation in scaling waters

The focus of this work is the control of the desupersaturation reactor in the Slurry Precipitation and Recycle Reverse Osmosis (SPARRO) process for desalinating water with a high calcium content. Three topics are covered in this paper: quantifying and controlling gypsum morphology, identifying and quantifying the crystallization phenomena in the reactor, and scaling up the reactor on the basis of a mixing time scale analysis. Experiments were carried out in batch mode, and the factors varied were seed morphology and quantity, stirrer speed and sulphate to calcium ratio. The measured responses were the crystal morphology, the time to reach equilibrium and the change in mean crystal size. Two distinct crystal morphologies (needles and platelets) were identified and appropriately quantified using an edge detection method. Both the quantity and the morphology of the seeds had a decisive influence on the seed morphology at equilibrium. The seed morphology at seed concentrations below 4% by volume also affected the time required to reach equilibrium. The proposed mechanism of desupersaturation is primarily a second order growth rate, with no nucleation taking place. There is evidence for the occurrence of attrition at higher Reynolds numbers. It is recommended that scale up take place to ensure that the relative influence of the characteristic mixing scales does not chance. (C) 2004 Elsevier B.V. All rights reserved.