Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process

Bauxite residue,a byproduct of alumina manufacture,is a serious environmental pollutant due to its high leaching contents of metals and caustic compounds.Four typical anions of CO3^2-,HCO3^-,Al(OH)4^- and OH-(represented caustic compounds) and metal ions(As,B,Mo and V) were selected to assess their leaching behavior under dealkalization process with different conditions including liquid/solid ratio(L/S ratio),temperature and leaching time.The results revealed that washing process could remove the soluble composition in bauxite residue effectively.The leaching concentrations of typical anions in bauxite residue decreased as follows:c(CO3^2-)> c(HCO3^-)> c[Al(OH)4-]> c(OH-).L/S ratio had a more significant effect on leaching behavior of OH-,whilst the leaching concentration of Al(OH)4-varied larger underleaching temperature and time treatment.Under the optimal leaching,the total alkaline,soluble Na concentrations,exchangeable Ca concentrations were 79.52,68.93,and 136.0 mmol/L,respectively,whilst the soluble and exchangeable content of As,B,Mo and V in bauxite residue changed slightly.However,it should be noted that water leaching has released metal ions such as As,B,Mo and V in bauxite residue to the surrounding environment.The semiquantitative analysis of XRD revealed that water leaching increased the content of gismondine from 2.4% to 6.4%.The SEM images demonstrated the dissolution of caustic compounds on bauxite residue surface.The correlation analysis indicated that CO3^2- and HCO3^- could effectively reflect the alkalinity of bauxite residue,and may be regarded as critical de alkalization indicators to evaluate alkalinity removal in bauxite residue.