Kaolin tailings ponds play an important role in reclamation of land and water. Discharges impounded from kaolin plants include oversize fractions consisting of coarse kaolin, mica, quartz as well as magnetically extracted contaminents, acid filtrates, and plant washdown streams. Tailing ponds perform an essential function in sedimentation of particulates, blending and reoxygenation of liquid waste, and neutralization and clarification of discharge water. During the past fifteen years, most kaolin firms have segregated their coarse filler waste to allow for possible reclamation for production of delaminated coating clay. This excess coarse filler fraction constitutes a reserve for future coating clay production.
In most cases, the pH of the filtrate is neutralized to pH 6-8 by the addition of lime prior to discharge in the tailings pond. In this condition, ferrous hydroxide is precipitated along with waste kaolin and other suspended solids. The resulting effluent must meet strict environmental standards as to pH, turbidity (0 NTU) , dissolved solids, and chemical oxygen demand.
From the onset of large scale waterwashed kaolin production about 50 years ago until 1965, crude soft or Cretaceous kaolin, which typically averages 55% minus two microns, has been fractionated into fine coating fractions and coarse filler fractions. The coating fractions have ranged from 70-90% minus two microns and the filler fractions have ranged from a low of 10% minus two microns to 35% minus two microns.
The delaminated coating clay product had superior coating efficiency, especially for light weight publication grades of paper made from southern softwoods. The high covering efficiency of delaminated clay and its excellent printing and optical properties rapidly established this grade of clay as a premium coating pigment. In 1983 delaminated coating clay accounted for over 13% of the total water-washed production clay and represented one of the most rapidly growing grades of coating clay.
One of the most important methods of converting coarse filler fractions into delaminated clay is to use fine media such as 20-30 mesh sand or glass beads in an attrition mill. In one process, it is possible to convert a filler fraction of about 40% minus two microns into a coating fraction about 70% minus two microns in one pass with 100% conversion.