Transformation Of Alkaline Mineral And Its Calcium-driven Mechanism In Bauxite Residue

Bauxite residue is a strong alkaline solid waste discharged from alumina production,with a comprehensive utilization rate of<3%,and there is no economically feasible and large-scale disposal technique to date.Soil formation of bauxite residue may be a highly promising ecological disposal method,but the problem of strong alkalinity seriously restricts its development.The alkaline substances in bauxite residue consist of free and chemical bound alkalis,which mainly originate from the pre-desilication,high-pressure dissolution and sedimentation separation processes of bauxite ores.The level of free alkalis（Na Al（OH）₄,Na₂CO₃,Na HCO₃ and Na OH）contents directly determine the alkalinity of bauxite residue,while the alkaline solids maintain a strong acid buffering capacity to acid neutralization for bauxite residue,mainly composed of alkaline minerals such as cancrinite and sodalite.Hence,there are problems of unstable effect and alkalinity rebound in the process of alkalinity regulation,and it is still impossible to realize the effective removal of bauxite residue alkalinity fundamentally,so it is urgent to explore the suitable alkalinity regulation technology.The present study investigated the dissolution behavior of alkaline minerals and their transformation mechanism in the process of acid dealkalization of bauxite residue,afterwards,the dissolution kinetics and stabilization mechanisms of the alkaline minerals cancrinite and sodalite,as well as the interaction mechanisms in the presence of dissolved organic matter,were investigated at macroscopic and microscopic scales through batch experiments（dissolution and adsorption）combined with spectroscopic characterizations and density functional theory（DFT）calculations,respectively.Moreover,a long-term alkalinity regulation method for bauxite residue based on a calcium ion activator was established,and finally its environmental risk was also evaluated.The main results are as follows:（1）The dissolution behavior of alkaline minerals and their embedding characteristics are the key factors affecting the alkaline conversion of bauxite residue.The sequence of acid buffering by minerals in bauxite residue is firstly by the dissolution of minerals containing Na and Ca via ion exchange processes,followed by the hydrolysis of Al-O-Si framework in cancrinite and sodalite.The embedding characteristics of alkaline minerals in bauxite residue will affect its dissolution process.The undissolved minerals block the interface reaction between embedded alkaline solids and acids to result in the diffcult reaction of alkaline minerals,which is induced by ball milling,thus preventing them to dissolve and effectively inhibiting the p H rebound.Moreover,the mechanism of p H rebound was also related to the behavior of mineral surface charge.（2）p H significantly affects the elemental release characteristics and their stoichiometric relationships during the dissolution of alkaline minerals.At different p H conditions（8,9 and 10）,the elemental Na in cancrinite and sodalite showed an increased tendency and then decrease with increasing dissolution time.The stoichiometric relationship of Na,Si and Al released from the two minerals is significantly different,and their release rate in cancrinite is more rapid than that of sodalite.When the solution p H decreases,cancrinite and sodalite reach the saturation state easily,thus shifting from dissolution to precipitation,and their dissolution rate will decrease accordingly.Kinetic calculations by the shrinking core model（SCM）show that both cancrinite and sodalite have slow kinetic processes,with the former being an internal diffusion process controlled by Al（OH）₃ product layers,and the latter being an external diffusion control related to the ion concentration in solution.p H controls the kinetic dissolution rate of the two minerals mainly by changing their coupled dissolution-precipitation processes,which affects the release process of alkaline substances from the minerals.（3）Calcium ions can effectively control the stability of alkaline minerals,thus inhibiting their release of alkalinity and enhancing the ability of alkaline minerals to sequester dissolved organic carbon.It migrates to sodalite and cancrinite by replacing Na sited in the mineral lattice and Na adsorbed on the mineral surface,respectively,which enhances the binding degree of polysaccharide functional groups between minerals and organic matter and the adsorption capacity and stability of minerals to organic matter.Furthermore,the amount of calcium ions entering sodalite and cancrinite controls their acid neutralization ability.Finally,the“dual role”of calcium ions in the remediation of bauxite residue,i.e.,the stabilizing regulation of alkaline minerals and the enhancement of carbon sequestration capacity of alkaline minerals,was further identified.（4）By using Fe Cl₃ combined with EDTA and desulfurization gypsum（calcium ion activator）to achieve better alkaline regulation of bauxite residue,and the treated residue p H was 7.20.The calcium ion activator could effectively promote the stability of dissolved organic carbon in bauxite residue.This treatment effectively increased the saturation of calcium ion exchange capacity,which in turn increased the contribution of Ca to total exchangeable cations（CEC）from 68%to 92%,and then reduced the acid neutralization capacity of the bauxite residue.Following the treatment of EDTA and desulfurization gypsum with Fe Cl₃,the calcium added as gypsum can be retained as exchangeable calcium rather than in the form of chelates.The alkaline minerals embedded on the surface of bauxite residue were subject to phase transformation,and the exogenously added Fe would exist in a stable state after neutralizing the free alkali,while calcium was released from the Ca-EDTA chelate and then stabilized with the alkaline minerals.（5）The potential environmental impacts in the alkaline regulation process of bauxite residue were comprehensively evaluated.The Fe Cl₃combined with EDTA and desulfurization gypsum lowered the environmental risk of metals leaching.Atmospheric CO₂ and rainfall leaching facilitated the alkaline stability of bauxite residue disposal areas.The effloresced materials on the surface of bauxite residue are formed through the migration of soluble alkali along with water.Addition of gypsum shifted the composition of effloresced salts from alkalinity to favor neutrality,and also changed the chemical form of Na-containing minerals in the bauxite residue and the mesoscale spatial distribution of Na,thus reducing the generation of alkaline efflorescence for ameliorating alkaline dust pollution from bauxite residue.In summary,Fe Cl₃ combined with EDTA and desulfurization gypsum to treat bauxite residue is a stable,reliable and safe remediation technology with promising applications.Figures 57,Tables 12,References 220...