Antiscalant Control of Calcium Fluoride Deposition in RO Plants
By Suresh Patel, Ph.D., Anna Chomiak, and Marta Farriols
Fluorides occur both naturally (e.g., rock weathering and volcanic emissions) as well as because of human activities (e.g., phosphate rock mining, steel industry, and drinking water fluoridation). In natural waters, the concentration of fluoride is low, ranging up to 5 parts per million (ppm). However, certain wastewaters from steel or mining plants have increased fluoride content of up to 80 ppm. In order to comply with environmental regulations, as well as to economize the costs, steel and mining process waters are often recycled in reverse osmosis (RO) plants operating at high recovery (70% to 80%). In these conditions, brine waters that have highly concentrated fluoride and calcium contents that exceed the supersaturation of calcium fluoride, and form precipitate because of low calcium fluoride solubility in water (16 ppm at 20oC). This article reports on results of laboratory studies carried out to investigate additives that can control calcium fluoride precipitation in waters with high calcium fluoride saturation indices of up to 300. Furthermore, in addition to calcium fluoride precipitation, other scales like calcium carbonate and calcium sulfate can also be found in RO plant. The additives evaluated for calcium fluoride were also tested for calcium carbonate and calcium sulfate scale control.
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