THE EFFECT OF QUALITY OF ALUMINA ON THE ENVIRONMENTAL EMISSIONS AT HILLSIDE ALUMINIUM

Hillside Aluminium Smelter consumes approximately 1000 kt of alumina per annum during the production of aluminium. Alumina is supplied from two different sources.

Evolution of fluoride from the reduction cells is approximately 40 kg F/t Al. The air pollution permit limits the fluoride emissions to 1.0 kg F/t Al. The pot gas containing the fluorides is scrubbed in a Gas Treatment Centre (GTC) before being emitted to atmosphere. Alumina is used as scrubbing medium in the GTCs. At present, the fluoride emission from GTC 1 and GTC 2 is higher than from GTC 3 and GTC 4.

The reason for the differences could be due to several factors i.e. plant design, concentration of fluoride in pot gas, plant operation, process control and alumina properties.

The objective of this investigation is to explain the difference between the emissions at the GTCs, focusing on the influence of the two different types of alumina on the process. Alumina from one supplier (Type A) is used on Potline 1, connected to GTC 1 and GTC 2. Alumina from a different supplier (Type B) is used on Potline 2, connected to GTC 3 and GTC 4.

Typical analyses of the two types of alumina are given as well as the effect of alumina properties on fluoride emission. Experience, at Hillside, on the influence of the two types of alumina in the GTCs is discussed in terms of grain size distribution, B.E.T. surface area and loss on ignition.

Some calculations are shown, related to the fluoride content of the alumina in the reactors and the alumina leaving the GTCs. The actual fluoride content is compared to the calculated fluoride content and the comparison is used to show that alumina in GTC 1 and GTC 2 reactors is closer to the adsorption capacity of the alumina. This is one of the main contributing factors to the difference between GTC emissions.

It is recommended that the fresh alumina flow rate at GTC 1 be increased by reducing the charged alumina flow rate.