Separation of titanoferrous impurities from kaolin by high shear pretreatment and froth flotation

Abstract

Separation of colouring titanoferrous impurities from cryo-filtered kaolin was tried in the laboratory so as to achieve brightness improvement and maximum clay recovery. The process consists of high shear agitation with the dispersant (high shear dispersion—HSD) and with the collector (high shear conditioning—HSC) as pre-flotation steps followed by reverse flotation to float out the impurity. While HSD and HSC were carried out at 65% solids, the flotation was done with 20% solids. The flotation feed was prepared from a raw clay collected from the western part of India. The raw clay was processed by blunging, de-sanding followed by three-stage hydrocycloning for sizing and finally a two-pass cryo-filtration. The cryo-filtered clay forms the feed for flotation and consisted of 90% particles below 2 μm and about 70% below 1 μm. This material assayed 1.06% of TiO 2 and 0.29% Fe 2O 3 and measured a brightness of 83.4 % and a yellowness of 7.1% ISO. The flotation test results showed that the major players for the impurity removal are HSD, HSC, dosage of dispersant and collector and temperature of flotation. Even though as high as 35% of TiO 2 could be removed from the clay by flotation, the brightness increased only by 0.5%. However, when flotation product was bleached with sodium hydrosulphite (hydros), the brightness enhancement by the combined processing is 1.8 units. If only reductive bleaching of the feed clay is done without resorting to flotation, the brightness increase is merely 0.4%. Comparing these three results, it can be inferred that the low enhancement in brightness by flotation alone may be due to the fact that the liberated iron/titania particles during HSD and HSC may be getting ‘re-coated’ on the clay particles during flotation and/or drying. Reductive bleaching after flotation may be partially removed this ‘re-coated’ iron resulting in additional improvement in brightness. Further studies are planned to identify the type of ‘coating’ or ‘re-coating’, if any, so that methods can be adopted to prevent the same.