Abstract: Two kaolins were given five successive processing steps designed to remove a number of impurities: (1) removal of nonclay and soluble fractions; (2) removal of organic material; (3) removal of iron oxides; (4) removal of allophane; and (5) removal of three-layer lattice minerals. After each of these steps, consistency curves were obtained for each of the clays at both 6.5 and 9.0 pH on slurries containing up to 50 weight % solids.
Photomicrographs and electron micrographs showed that Ka-7 from Bath, South Carolina, contained generally smaller particles than Ka-2 from Macon, Georgia. Lowering the pH from 9.0 to 6.5 in Ka-2 produced no change in plastic viscosity at the lower concentrations and a decrease of about 30 per cent at higher concentrations after processing step 1. A comparison of the same evaluations of step-1-treated Ka-7 showed that the decrease in pH reduced the plastic viscosity by 50 per cent at 5 per cent clay concentration and increased the plastic viscosity by 100 per cent at 35 per cent clay concentration. After complete processing, lowering the pH from 9.0 to 6.5 increased the plastic viscosity at all concentrations for both kaolins by 5–20 per cent. At a clay concentration of 30 per cent, complete processing decreased the plastic viscosity of Ka-2 from 0.4 to 0.02 poise and of Ka-7 from 0.4 to 0.06 poise. Ka-7 showed progressive viscosity changes due to the processing, while Ka-2 showed only a marked change in viscosity for processing step 5. The rheological changes of Ka-2 and Ka-7 with processing are supported by the X-ray diffraction data at the various processing stages. X-ray diffraction data confirm that processing step 5 effectively removed a three-layer lattice expandable mineral from Ka-2, while processing step 3 effectively removed a different three-layer expandable mineral from Ka-7.