OXALATE PROCESS OPTIMISATION IN THE BAYER REFINERY

In Bayer liquor, oxalate is formed mainly by degradation of higher organic compounds. As a relatively insoluble impurity, sodium oxalate often crystallises in the cooler parts of the Bayer circuit. The resulting crystals are usually acicular, difficult to handle and likely to increase hydrate nucleation rate, resulting in hydrate fining and increased liquor flow resistance, scale formation, process instability and ultimately production and hydrate quality loss.
To manage oxalate, some refineries practise oxalate co-precipitation, in which seeded oxalate crystallisation occurs in parallel with hydrate crystallisation in the same slurry. Other refineries choose to remove oxalate via side-stream processes. Either way capital and operating costs are significant, design decisions are not straight forward, and process stability not assured.
Oxalate management is a multi-factorial, multi-scale problem. At the highest level, plant oxalate balance is defined by the relative rates of oxalate input (organic degradation, direct input from bauxite) and output (soluble loss, causticisation, solid occlusion, etc). At the refinery facility level, oxalate recycle, between solid and liquor phase, may be significant. At the particle level, oxalate crystallisation rate, crystal morphology and hydrate nucleation rate are defined by liquor composition, temperature and hydrodynamics.
In recent years, Rio Tinto’s Yarwun refinery has been a valuable test bed for optimisation of oxalate management. Advances in the understanding of oxalate uptake by de-silication product, and of the role of impurities and additives on sodium oxalate crystallisation, underpin improvement in oxalate process optimisation. This paper describes aspects of oxalate process optimisation that allow Rio Tinto to adapt existing and new refineries to more efficiently exploit oxalate-producing bauxite.