The minus 20μm (superfines) content in alumina feed is an important property for aluminium smelters as high superfines content causes lower energy efficiencies, and health and environmental issues at the smelters. Although superfines are generated in calcination the root cause lies within precipitation. The bridge between precipitation conditions and superfines content in shipped alumina is hydrate toughness. A routine hydrate toughness test method is not available and needs to be developed.
In calcination, cracks at grain boundaries and within primary crystals of product hydrate are formed at 400°C. These cracks, combined with mechanical forces in calciners result in particle breakdown and are impacting the superfines content in smelter grade alumina. During calcination gibbsite dehydrates but the particle shrinkage is slower than the change in solids density inducing tensile stresses and resulting in the formation of cracks and pores. This tensile stress should be simulated in the toughness test method to be developed.
A hydrate toughness test method based on confined compression is being developed. Confined compression simulates the random distribution of tensile forces well, as shown by both the same size profile in particle breakdown and the same morphology of breakdown products after calcination and compression. The confined compression test method can be applied to hydrate powders from all sections of precipitation. The test method encompasses a force of 4500N giving a compression pressure of 13MPa. The particle size distribution before and after breakage is measured. The extent of breakage, or toughness, is expressed as Breakage Index (BI).
The breakage index clearly indicates that agglomeration is the dominant factor in making a tough product hydrate. The trend of breakage index of agglomeration overflow matches very well with the trend, inclusive a 10-day lag, of the superfines content in shipped alumina.