The AquaCritox® AlOH technology involves the hydrothermal destruction of sodium oxalate contained in the waste steams from the Bayer process used in alumina refining. SCFI engaged in laboratory and on-site pilot studies to investigate the most efficient way of handling the toxic sodium oxalate-containing waste stream. The technology leverages the solubility limits of various components contained within the generally complex waste stream and maximizes the amount of useful material recycled back to the main Bayer process circuit. The process reduces the size of the toxic waste stream while offering the possibility to convert the toxic sodium oxalate back to sodium hydroxide for reuse in the alumina extraction process.
The Application
The formation and management of oxalates in the Bayer process are important aspects of alumina production from bauxite, and the control of oxalates directly contributes to the operating cost of the Bayer process. Furthermore, the long-term sustainability of the alumina refining process is dependent on how well this toxic waste stream can be managed to prevent any impact on the environment.
On-site Piloting
Pilot-scale testing was conducted at an alumina refinery in Ireland using a containerised demonstration unit (80 kg/h nominal capacity, 12 min residence time). This unit was designed to pre-heat the feed to 250-300 °C and use controlled oxygen addition to heat the solution up to 300 °C with the exothermic reactions between the oxygen and organics in the feed. Results from the pilot testing indicated that sodium oxalate is formed from the oxidation reactions of long chain organic molecules. At temperatures below 300°C, a net production of oxalate was observed while at temperatures in excess of 300°C, oxalate destruction was reported. The destruction of long-chain organics in the waste stream was limited to about 40% at the specific test conditions, and salt precipitation was evident in the tubular heat exchangers.
Laboratory Testing
The results from the pilot led to subsequent laboratory tests and the development of the AquaCritox AlOH technology. From the pilot tests, it was clear that the destruction of organics results in the production of more oxalate, increasing the load of toxic waste to be treated. SCFI developed a process that removes the bulk of the organics to be recycled back to the main Bayer process circuit. This leaves the sodium oxalate-containing stream to be treated. Sodium oxalate decomposes at high temperatures (>290°C) and our laboratory trials showed that 99.7% of the oxalate can be destroyed at reactor operating temperatures of 320°C and residence times of 12 minutes. Sodium Oxalate decomposes to form sodium carbonate and carbon monoxide. Sodium carbonate is then converted to sodium hydroxide by dosing lime milk, forming a non-hazardous by product, calcium carbonate. Carbon monoxide is converted to carbon dioxide using a catalytic converter.
The Success
The patented AquaCritox AlOH technology was developed to decompose hazardous sodium oxalate salt produced by the Bayer process into non-hazardous side products and to regenerate caustic and alumina to feed back into the Bayer digestion step.
Overall, this hydrothermal process offers a novel approach to treating and reclaiming the oxalates that are unavoidably formed in the Bayer process, enabling the reduction of the plant’s environmental footprint and operating costs.
