Kaolin is an important industrial mineral which often needs upgrading for special applications. The upgrading process, commonly known as beneficiation, is aimed at removing impurities like feldspar, quartz, iron oxide, etc. In this study, a kaolin sample was collected from a deposit located at Mutaka, South-Western Uganda, – the biggest kaolin deposit in the country. With the aim of upgrading this sample, it was processed by a laboratory hydro cyclone to produce a kaolinite concentrate and this has been characterized to ascertain its use as an industrial raw material. Characterization of the beneficiated sample was carried out by XRD, SEM, particle-size distribution, density and volume measurements, chemical analysis, whiteness index test, thermal analysis (DTA/TG) and surface area measurements. Results show that the laboratory beneficiation exercise improves the kaolin sample to a very high grade with a chemical composition close to that of ideal kaolinite. The major impurity after beneficiation, iron oxide, was further reduced by acid leaching. Results show that the properties of the final product are close to that of ideal kaolin.
Basically, two different processes are used in beneficiating of kaolin – a dry process and a wet process. The dry process is the simpler and less expensive of the two but yields a lower quality product. In the dry process, kaolin is crushed, dried, pulverized and air floated. The air floating process removes most of the coarse particles from the fine ones, but in order to really obtain a high quality product, the wet process must be used. Consequently, the latter process was used in the present work.
A combination of wet screening and hydro cycloning was used. The 100 kg sample with size fractions less than 2.36 mm was attrition scrubbed to make it ready for the hydro cyclone test rig. The fraction was fed into a receiving cone of a laboratory 2-inch stub hydro cyclone test rig and water was added to make a pulp of 16% solids. The pulp was left to run for ten minutes to allow for scrubbing action on the material before collection of the cyclone products. The pressure was set to 0.21 MPa (30 Psi) and the spigot was 6.3 mm. The vortex finder was set to 11.1 mm. The under size from the screening operation was fed into a hydro classifier (rougher-stage). From its overflow, a product rich in kaolin, was collected and fed to another cleaner hydro classifier. There it was further processed in order to obtain a cleaner product. The underflow fraction normally consists of sand and other heavy mineral impurities but also contains an appreciable amount of kaolin in vibrating screen. In order to recover this kaolin it was fed into a scavenger which completes the kaolin extraction from the waste. The extracted kaolin from the scavenger was fed into the rougher hydro classifier stage together with the underflow fraction from the cleaner stage.
According to chemical analysis results, the beneficiated kaolin contained iron oxide as the main impurity. In order to explore the possibility of reducing the iron oxide content, about 200 g were leached with oxalic acid of concentration of 40 kgm -3 . The mixture was maintained at 80°C for 5h while stirring. It was then filtered and dried at 110°C.