3.2. Effect of calcination temperature on whiteness
Organic matter is one of the factors depressing the whiteness of
bauxite tailings. Calcinations aimto eliminate the organics. Effect of calcination
temperature on whiteness of sample from the post-reaction
calcination was also investigated. Fig. 2 shows the relation between
the whiteness of samples with 4.5% phosphorous content and calcination
temperature. It can be seen that the whiteness depended on calcination
temperatures. The whiteness reached the maximum 73% at a
calcination temperature of 550 °C, and decreased at calcination temperatures
higher than 600 °C. This is because the organic matters of sample have been burned at 550 °C, leading to a considerable increase in whiteness.
In further calcination processes at temperatures higher than
600 °C, interactions between a variety of metals and phosphates, as
well as yellowish-white iron polyphosphate formation may have occurred
[10], resulting in decreasing whiteness.