Reclaiming sodium silicate into diatom

The low cost and non-toxicity of sodium silicate sand have a broad application prospect in the future high-precision green casting industry. However, a longstanding problem for the sand is how to effectively recycle and prepare high-quality reclaimed sand. In this study, we reported a biological reclamation technology that diatom communities absorb Na₂O and SiO₂ from the sodium silicate waste foundry sand (SSWFS) surface. Optimal environmental conditions were established, allowing the diatom, water, and sand after reclamation to be recycled and reused. Optimization studies revealed that at a temperature of 25°C, a light intensity of 5000 lux, and a lighting time of 14 h/day, with 80 mg/L N, 50 mg/L P, 30 mg/L K, 16 mg/L Mg, 2.28 mg/L Fe, 2.28 mg/L Mn and a 10:1 N/P ratio, the diatom communities achieved optimal bloom conditions with 36.3% Na⁺ reduction (36.3% demolding rate) and 46.0% SiO₃^2- reduction in solution during the spring-summer transition. The environmental factors had the most significant influence on the diatom communities, whereas nutrients had a lesser effect. Carbohydrates and a small amount of protein, which are metabolites produced by diatom communities, were proven to be surfactants, promoting the dissolution and deskinning of residual sodium silicate on the SSWFS surface and facilitating “bioassisted dissolution”. The microwave-hardened bioreclaimed sand exhibited promising moisture resistance, with a similar initial strength to that of the raw sand, and the storage strength (98% RH in humidistatat for 24 h) increased by 13.3% to 1.87 MPa. Powered by sunlight, the process enabled the dual recovery of sand and sodium silicate, while also reusing wastewater, offering a cost-effective and environmentally friendly solution to traditional SSWFS recycling methods.