Effect Of Transition Metal Oxides And Chemical Bleaching Process On The Whiteness Of Kaolins

Kaolin was widely used in various industry departments such as ceramics, paper,paint,plastic,rubber,stuffing and etc.owing to its excellent technologic performance.However,the iron impurity will decrease the whiteness of kaolin,and reduce its application value correspondingly.Therefore,removing iron from kaolin is necessary and indispensable in the application of kaolin.In this reseach,the effects of eight transition metal oxides on the whiteness of kaolin and the bleaching process by mono-factor and orthogonal experiment are investigated,and the amount of bleaching agent,pH value of slurry,solid-liquid ratio and the reaction time are concerned in the bleaching process.The kaolin was supplied by Longyan Kaolin LTD.and the results are as follows:1.There are eight trasition metal oxides can reduce the whitness of kaolin to some extent respectively,especially after sintered.TiO₂ and ZnO have a relatively less influence,while the other five such as Cr₂O₃,MnO₂,Co₂O₃,and etc have a significant effect on the whiteness of kaolin.And Fe₂O₃ is the biggest one which can reduce the whiteness of the kaolin mostly to the same extent before as after cacination.The results show that,the whiteness of kaolin decreases due to Fe₂O₃ despersing in kaolin particles,or adsorbing on the surface.2.The mono-factor experiments show that the appropriate condition is sodium dithionite dosage 2%～3%,pH 2～3,solid-liquid ratio 1:3～1:6 and reaction time no more than 1.5h.From the orthogonal experiments,the optimal condition is sodium dithionite 3%,pH 2,solid-liquid ratio 1:3 and reaction time 45min.The whiteness of kaolin increases from 69.93%to 81.31%by the optimal bleaching process.During the bleaching process,sodium dithionite dosage is the major factor of influencing the whiteness of kaolin,and pH is the second.The interation among sodium dithionite,pH and solid-liquid ratio can not be ignored.3.According to the research,the optimum conditions are used in scale experiment in the productive practice of kaolin with iron impurity from Zhangzhou.The result shows that the whitness of kaolin increases by 21.7 percent in maximum,the content of Fe₂O₃ can be reduced to less than 0.5wt%,and Al₂O₃ is basically unchanged.And in the end,the project 'Process Design for20,000 tons per year's kaolin with Chemical Bleaching'was done.