Bauxite residue valorization - Soil conditioners production through composting with palm oil mill residual biomass

Bauxite residue (BR) is a by-product of Bayer process, which is applied for alumina production. Due to its inherent alkalinity and sodicity, the use of BR is globally limited to 23% of the 150 million tons (Mt) produced annually. Maximizing alternative and large-scale uses of BR is a game changer to promote the sustainability of the aluminum production chain. As a strategy for BR valorization, a soil conditioner composed of BR and palm oil residual biomass was proposed. Here we evaluate the BR (25%, 50% and 75%) batch composting with raw palm oil mill waste (POMW) and palm oil compost (POC). The pH, EC, total N and organic carbon, C:N ratio, water holding capacity (WHC), cation exchange capacity (CEC), granulometry and elemental composition were determined after 90 days of composting. Changes in temperature, pH and EC curves were observed during composting of soil conditioners for 90 days. Composting reduced the alkalinity and sodicity of BR, increasing CEC, moisture, organic carbon and total nitrogen. The formulation containing 25% of BR and 75% of POC showed WHC ≥ 60% and CEC ≥ 200 mmol_c·kg^-1, meeting the Brazilian legislation for production and commercialization of soil conditioners. This strategy could potentially consume 7.6% of all BR produced annually in the largest Brazilian alumina refinery. Concentrations of potentially toxic elements were far below the allowable levels in all formulations. Major and minor plant nutrients were present and the composting aggregated small particles in BR. Composting of BR is a new alternative for the valorization of mining tailings, allowing the development of an environmentally friendly and zero-waste product, which can be applied on a large scale in agriculture to improve soil fertility.