NEW DEVELOPMENTS IN ALUMINA CALCINATION

Schmidt, H.W. and Taylor, J.C.

In the first chapter a general overview and description of modern stationary calciners, such as fluid flash, circulating fluid bed and gas suspension calciner will be given. The theoretical design parameters, gas velocities, slip velocities, retention time, heat- and mass transfer coefficient, cross section area of the reactors, temperature distribution, are compared. The possibility and the theory of the combustion of different fuels such as bunker-C-oil, high, medium and low BTU-Gas is explained.

In another chapter the factor of particle break down and influence from the hydrate quality and the calcination systems will be evaluated. With a given hydrate the influence on the particle break down of the calcination systems, depending from their operating conditions will be shown. It will be explained, how other product quality parameters, such as specific surface area, alpha-content, loss on ignition, could be achieve by different process conditions.

The production of special alumina with a specific surface of less than 3 m2/g is possible in a two-stage calcining system. In a stationary calciner the alumina is calcined up to a specific surface area of approx. 10 m2/g. With a temperature of approx. 1100 degree C the alumina is discharged to a flash calciner stage where it is heated up to temperatures of over 1500 degree C, so that a specific surface area of less than 3 m2/g is achieved. Advantages and operating possibilities of this combined system are explained.

In a final section a comparison of the stationary calcination systems with a modern "retrofitted" rotary kiln system is given including an economic evaluation.