Study on the effect of sodium removal from citric acid pretreated red mud on the physical properties of red mud.

Red mud is a highly alkaline solid waste discharged from the alumina industry, and its high sodium content is the key factor limiting its wide utilization. Therefore, effective control of the "frosting" phenomenon during the application of red mud has received significant attention. In this study, the changes of particle size, phase, morphology, and pore size of red mud after sodium removal with different amounts of citric acid pretreatment were investigated. The single-factor experiment shows that the Na+ leaching rate is 86.33% under a citric acid dosage of 15%, liquid-to-solid ratio of 7 mL/g, leaching temperature of 80 °C, stirring speed of 300 rpm, and leaching time of 10 min. The leachate is characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. The results reveal that Na+ mainly exists in a combined state in the form of cancrinite. With the increase of citric acid dosage, red mud agglomerates, calcite, and cancrinite are dissolved, and new phases such as calcium oxalate and magnesium aluminum hydroxide are formed. The specific surface area, pore volume, and pore diameter show irregular changes with the increase in the citric acid dosage. Citric acid pretreatment can effectively reduce the sodium content in red mud, the treatment cost of leaching solution is low, and the leaching residue is neutral, which is helpful to promote the practical application of red mud.

Study on application and environmental effect of phosphogypsum-fly ash-red mud composite cemented paste backfill.

Phosphogypsum (PG) cementitious paste backfill (CPB) was prepared by using PG and fly ash (FA) as the main raw materials, red mud (RM) as the alkaline activator, Portland cement (OPC) as the binder, and silica fume (SF) as the additive, and its properties were investigated to achieve the objective of "treating harm with waste." The results showed that the addition of OPC facilitated the flowability of the slurry, while the addition of RM and SF had the opposite effect. The slurry presented ideal flowability when the water/binder ratio was 0.2 and the superplasticizer (SP) content was 0.7%. The mechanical properties and water resistance were improved significantly with increasing OPC, RM, and SF doping. The strength of the CPB material exceeded 22 MPa after curing at room temperature for 28 days, which met the mine filling requirements. Changes in the ion concentrations of the solution were first monitored during immersion. The dissolution rules of Ca2+ and SO42- at different immersion ages confirmed that RM promoted the continuous hydration of CPB, which was the key to improve water resistance. Microstructural analysis showed that the main hydration products were AFt and C-S-H, which played an important role in the strength development of the material. The leaching results demonstrated that the metal ion content satisfied the requirements of the III categories of Chinese environmental standards (GB/T 14848-2017), indicating that the technology is a reliable and environmentally friendly technology for PG, FA, and RM recovery that can simultaneously support safe mining.