chapter  2
18 Pages

Crystal Growth Inhibition of Calcium Sulfate and Calcium Oxalates in Aqueous Systems

The crystallization of solid material from solution is one of the most frequently used unit operation and one of the oldest separation and puri cation operations employed in chemical industries. Crystallization determines chemical purity and physical properties of substances including crystal habit, crystal size distribution, crystal structure, and the degree of crystal imperfection. It serves not only to separate and purify substances, but also to produce crystals with the required shape. Models for the growth of crystals by condensation from the vapor are well established, but crystal growth and dissolution from solution is much more complicated. Despite its almost universal applicability, the mechanism involved in many crystallization processes is still not completely understood. Nevertheless, the elucidation of the mechanisms controlling these reactions is extremely important because of their involvement in both industrial and biological processes [1]. Calcium-containing minerals are the most abundant because of their relatively low solubility with ions such as oxalates, sulfates, phosphates, and carbonates [2]. The precipitation of calcium sulfate onto the walls of water-handling equipment, e.g., boilers and heat exchangers, is a serious problem encountered in many industrial processes [3a,b,c,d]. These deposits are formed from salts that are dissolved in the

process feed water. During the process, the process water becomes supersaturated with respect to these salts, which then precipitate out onto equipment walls. The supersaturation of process water usually arises from an increase in temperature or from an accumulation of dissolved salts in recycling stream. These deposits lead to a loss of heat transfer ef ciency, to partial or even total blockage of water ow, and are the cause of boiler cracking and boiler explosions [4].