Study of the extractability of alumina from fly ash by the lime-sinter process. [78 refs. ; appendix

Abstract

The exploratory work combined with the derived results made it possible to reach the following conclusions. Magnetic separation of fly ash in water will dissolve all the sulfur contained in the original fly ash. Although it will recover less iron in the magnetic fraction than the wet separation, the dry separation is more convenient. It is not necessary to use a very high pressure to make pellets, since the alumina recovery percentage does not increase with the pressure. A pressure of 8000 psi should be good enough for this process. The best lime to alumina ratio for the lime-sinter method for recovering alumina from C-1 fly ash is between 1.7 to 2.0. The most favorable sintering temperature is 1380/sup 0/C and one hour of sintering time appears sufficient. Qualitative analysis of the sinter reveals the complexity and heterogeneity of the system. The system does not reach equilibrium, and a variety of products are found by x-ray study including the calcium aluminates from which the alumina is subsequently recovered by leaching. Pictures based on the electron microprobe analysis show the partial segregation of calcium and aluminum. This fact may suggest that chemical diffusion occurs during sintering. Results of leaching of the sinters with sodium carbonate solution show maximum recovery at 65/sup 0/C and an increase in recovery with more concentrated Na/sub 2/CO/sub 3/ solutions or an excess of this solution. Economic evaluation shows the process to be economically unattractive based on the present degree of development and costs. As bauxite keeps on being consumed and the lime-sinter process is improved, fly ash has the potential of becoming a source of alumina in the future.