Study On Enhancing The Whiteness Of Kaolin By Bleaching And Calcining

Kaolin is an important non-metal raw material, which has been widely used in various industry departments such as ceramics, paper, paint, plastic, rubber, stuffing and etc.The quality of kaolin is measured in function of iron content, since this element give an undesirable reddish color to this mineral. Therefore, removing iron from kaolin is necessary and indispensable in the application of kaolin. The using of bleaching and calcination for iron removal from different kaolins has been investigated to find out the type of kaolin which is suitable for enhancing the whiteness by bleaching and calcining, and the optimum technological conditions. On this basis, the middle test study of Zhangzhou kaolin and the experiment of middle test enlargement of Longhai kaolin have been completed. The main results were as follows.1. The effect of bleaching and calcining is related to the mineralogical characteristics of kaolin. On the optimum technological conditions of mineral slurry solid-liquid ratio 1:5, pH 2, sodium dithionite dosage 3%, reaction time 45min and oxalic acid dosage 1%, the whiteness of Zhangzhou kaolin was improved significantly from 60.8% to 73.68%, increased approximately 13 percentage points. In the process of calcination, it is evident that ammonium chloride is most effective in improving the whiteness of calcined kaolin. When the bleached kaolin mixed with 5% NH₄Cl and calcined on 900℃for 1h, the whiteness reached 85.31%. In order to reduce the content of Fe₂O₃ and increase the whiteness of kaolin by bleaching and calcination, the raw material has to select by its mineralogical characteristics.2. The middle test shows that the whiteness of kaolin was improved to 82-84%, 1280℃sintering whiteness was improved to 90-92%, the content of Fe₂O₃ has been decreased high from 0.94 to 0.49, but the concentration of Al₂O₃ changed appreciably. When the bleached kaolin mixed with 3% NH₄Cl and calcined on 950℃for 1h, the whiteness reached 90.1%, compare with the whiteness 68.8% of raw material calcined on on 950℃for 1h, increased approximately 21 percentage points.3. The experiment of middle test enlargement shows that on the condition of concentration 20%, pH 2-4, the sodium dithionite dosage 3%, reaction time 40min, oxalic acid 0.7-0.9%, washing cycle 14-18h, the whiteness of kaolin was improved from 60.51% to 82%, the content of Fe₂O₃ decreased from 1.03% to 0.51%, 1280℃sintering whiteness reached 91%. Then mixed with 3% NH₄Cl and calcined on 950℃for 1h, the whiteness reached 92% and fineness was 2500 mesh, has enhanced its market value and the usage in paint industry.