APPLICATION OF SPLIT GROWTH IN A GREENFIELD ALUMINA REFINERY

Precipitation rate of alumina tri-hydrate (commonly referred to as “Hydrate”) in the growth circuit of an alumina refinery is increased by cooling the hydrate slurry and increasing the seed surface area. Maximizing the yield must be balanced against the requirement to maintain the quality specifications of particle size, strength, and impurity content. This paper outlines the in-house improvement that was done within the existing Al Taweelah alumina refinery oxalate co-precipitation circuit to maintain a similar level of supersaturation (SSAT) and nucleation in lead growth tanks under lower temperature conditions. This has helped in improving particle strength and increasing precipitation yield, while maintaining particle size in control. In the spilt growth application, the operating philosophy of the first two lead growth tanks was modified. These growth tanks were operated in parallel by splitting pregnant liquor (PGL) and coarse seed charge (CSC) equally in both tanks to increase the holding time. The effect of higher holding time on first growth tanks is a reduction in SSAT, which was offset by reducing the lead tank temperature with front-end interstage coolers (ISC). This was initially implemented as a plant trial to study the impact on SSAT, nucleation and oxalate behaviour along with operational flexibility and controls. With Process simulation, plant trial data analysis, and operational experience, split growth was integrated into the existing circuit, helping to improve yield. In process upset conditions, it also acts as a strong lever to control nucleation at moderate growth tank temperatures.