Solubility of Calcium Sulfate Scale in Cooling Waters

摘要

Because of the increasing costs of makeup water and sewage discharge, cooling tower operators have been challenged to reduce water consumption and blowdown. These reductions lead to increasing cycles of concentration at high levels. For systems with high calcium hardness and high sulfate in makeup, or for systems using sulfuric acid (H_2SO_4) for pH control, the formation of calcium sulfate (CaSO_4) scale can be a potential problem. Unlike calcium carbonate (CaCO_3) scale, which can be easily dissolved with acids, CaSO_4 scale is difficult to remove. To prevent the formation of CaSO_4 scale, the common practice within the industry is to keep calcium and sulfate in recirculating water below the solubility limits. Several simple guidelines have been proposed, but they vary over a wide range. For example, the suggested upper limits of concentration product for the calcium and sulfate (in ppm as CaCO_3) change from 500,000' to as high as 3,000,000. On the other hand, it is suggested that the calcium limit would stay almost unchanged once it has reached a minimum of 900 ppm (as CaCO_3)-even at a sulfate concentration in excess of 25,000 ppm.1 In addition, there are some recommendations based only on calcium hardness. The suggested maximum calcium hardness does not exceed 1,200 ppm (as CaCO_3) at pH 7 to 8 and does not exceed 700 ppm if the chloride exceeds 4,000 to 5,000 ppm.