DESTRUCTION OF BAYER PROCESS OXALATES AND CAUSTIC REGENERATION VIA HYDROTHERMAL TREATMENT

During the production of alumina from bauxite via the Bayer process, the decomposition of high-molecular-weight organic compounds found in the feed produces sodium oxalate, which accumulates within the recirculating Bayer liquor and negatively impacts hydrate nucleation and growth within the precipitation circuit, various properties of the Bayer liquor, and the operability of the precipitation and evaporator circuits. Typically, sodium oxalate is removed from the circuit via evaporation and precipitation, forming a hazardous salt cake requiring disposal, or by co-precipitation with the hydrate and washing, requiring disposal of the oxalate-bearing wash water. Hydrothermal treatment is a process, employed in biomass treatment and materials synthesis, in which high-temperature and high-pressure water is used as a reaction medium. A novel hydrothermal process has been developed to convert this sodium oxalate into sodium hydroxide, which can be recycled to the Bayer process and non-hazardous end products, while recovering alumina losses to the oxalate cake. This process relies on high-pressure (95-110 Bar) hydrothermal decomposition to fully convert sodium oxalate to sodium carbonate and carbon monoxide at temperatures of 300-315 °C, with a reaction residence time of 10-15 minutes. With the goal of maximising energy efficiency, reducing maintenance requirements, and reducing safety risks, the hydrothermal treatment system is configured as a tubular reactor with a closed-loop heat recovery circuit and non-flashing pressure reduction coils. Employing in-line filtration of the sodium oxalate slurry and controlling the rate of feed slurry dilution, circuit liquor concentrations are maintained below the solubility limit of sodium carbonate and sodium oxalate. Following hydrothermal decomposition, the produced carbon monoxide is catalytically converted to CO2 exhaust, while the sodium carbonate product is causticized with calcium hydroxide to sodium hydroxide, which is recycled to the Bayer process, and non-hazardous calcium carbonate, which is mixed with the plant’s bauxite residue output. This technology has been tested and validated at laboratory and pilot scales with real Bayer liquor. With hydrothermal treatment, the hazardous sodium oxalate waste produced by a Bayer plant can be converted to recycled hydroxide without requiring the injection of oxygen or steam, enabling the reduction of a Bayer plant’s environmental footprint and operating costs.