Precipitation of calcium oxalate is a common occurrence in mammalian urinary tract deposits and in various industrial processes such as paper making, brewery fermentation, sugar evaporation, and tannin concentration. Between pH 3.5 to 4.5 the driving force for calcium oxalate precipitation increases almost by three fold. It is a complicated process to predict both the nature of a deposit and at which stage of a multi-effect evaporator a particular mineral will deposit, as this depends on temperature, pH, total solids, and kinetics of mineralization. It is quite a challenge to inhibit calcium oxalate precipitation in the pH range of 4-6. A13+ions provide excellent threshold inhibition in this pH range and can be used to augment traditional inhibitors such as polyphosphates and polycarboxylates.
The formation and dissolution of calcium oxalate hydrates as a common constituent of pathological deposits in the urinary tract of mammals have been -3) These sparingly soluble the subject of numerous studies and publications . oxalates are also of considerable interest to analytical chemists engaged in quantitative separation of various ions. Precipitation of calcium oxalate occurs commonly in the processing equipment of many industries, such as paper- making and food and beverage processing. In paper-making calcium oxalate is often seen in treatment towers, washer drums, face wires, washer vats, stock lines and pumps, extraction screens, and in filtrate tanks lines and pumps. Calcium oxalate formation in these areas reduces production by decreasing bleaching/delignification efficiency and causing equipment down time to remove the scale. Beer-stones, hard deposits of calcium oxalate combined with proteinaceous and resinous material, are often encountered in ferments. Beer- stones not only adversely affect flow, but also become ideal growth sites for microorganisms which give beer an offensive taste and odor.
Although there are many other areas where calcium oxalate poses deposition problems, the focus of this paper is on the fundamental aspects of calcium oxalate deposit control in sugar evaporators. However, this knowledge is widely applicable in other industries, as demonstrated in the case history describing the benefits of developing a proper treatment for the inhibition of calcium oxalate deposits in a tannin production process.
Deposits in beet sugar evaporators generally consist of multiple components(4) made up of materials brought in with the liquor (juice) or formed in place. These deposits vary with location in multi-effect evaporators due to changes in temperature, total solids, and PH. Deposits found in beet sugar evaporators can contain a variety of minerals such as, calcium carbonate, calcium sulfate, calcium phosphate, calcium oxalate, and various silica and silicate compounds. In the earlier effects, calcium carbonate and calcium sulfate are the most common minerals causing deposition due to high temperature. Silica/silicate and calcium oxalate deposits are generally found in the latter stages of the evaporators. Volubility of silica decreases with increasing sugar concentration. Silica is half as soluble in a 60 brix sugar solution compared to water at the same temperature. Thus the lower temperature and high sugar loading in the last effect makes it an ideal site for silica, calcium silicate, and magnesium silicates deposits.
In the beet sugar industry, calcium oxalate deposition in evaporators is a common occurrence. In spite of a significant reduction in the oxalic acid concentration during initial juice purification, thin juice of 12% dissolved solids contains approximately, 120-460 PPM oxalic acid. During evaporation, additional o
