The filter cake is fed into calciners where they are roasted at temperatures of up to 1100°C to drive off free moisture and chemically-connected water, producing alumina solids. There are different calcination technologies in use, including gas suspension calciners, fluidised bed calciners and rotary kilns.

The following equation describes the calcination reaction:

2Al(OH)3 → Al2O3 + 3H2O

Alumina, a white powder, is the product of this step and the final product of the Bayer Process, ready for shipment to aluminium smelters or the chemical industry.