Basic Theories And Key Technologies For Synchronously Utilizing Secondary Aluminum Dross Via Bayer Process

Secondary aluminum dross（SAD）,a hazardous waste with more than 5 million tons of SAD annually worldwide,acts as a secondary resource generally generated from aluminum electrolysis,casting,fabrication,and regeneration.More than 95%of SAD are stockpiled,causing serious environmental risks.Many technologies to recover alumina or comprehensively utilize SAD have been reported.However,owing to the lack of safe methods to recover alumina from active and inert aluminum-bearing substances,together with expensively recoveries of fluorine and chlorine from SAD,great challenge has to be confronted in safe comprehensive utilization of SAD on a large scale.Meanwhile,the changeable chemical composition and phase content in SAD require remarkable adaptability of technology.Therefore,a novel approach for safe efficient utilization has been urgently required.This thesis presents a novel approach to safely and efficiently utilize SAD in combination with the alumina production by Bayer process,achieving a controllable release of hazardous gases,conversion of inert substances at low temperature,economical removal of hazardous elements and simultaneous purification of Bayer liquids.The leaching behavior of the active aluminum-bearing substances from SAD in the sodium aluminate solution was investigated in detail.Meanwhile,clinker sintered at low-temperature based on transformation of the inert aluminum-bearing substances in SAD into sodium aluminate clinker after adding impurities discharged from the Bayer process was illustrated.Precipitation of the coarse sodium fluoride and incorporation of chloride ion into sodalite during desilication in Bayer process were also presented.The main conclusions were listed as follows:（1）A process to synchronously and efficiently extract alumina from the active aluminum-bearing substances,together with removal of fluoride ions and chloride ions from SAD was proposed.A synergistic phenomenon occurred under the condition of high temperature and high dosage of SAD during the leaching active aluminum-bearing substances from SAD in the sodium aluminate solution.The temperature raised exponentially with the increase of SAD dosage.Under suitable conditions of 95°C,300 g/L SAD,150 g/L Na₂O,500 r/min,and 1 h,the leaching efficiencies of aluminum,aluminum nitride,fluoride,and chloride were 99.21%,94.25%,44.17%,and 100%,respectively.Different from the leaching behavior of the pure aluminum powder and pure aluminum nitride powder in the sodium aluminate solution,metastable bubbles,mutually embedded the active aluminum grains and aluminum hydroxide enveloping the active aluminum grains all changed the leaching mechanism of aluminum and aluminum nitride in SAD from chemical reaction model into internal diffusion model,with apparent activation energies of 31.1 k J/mol and 51.1 k J/mol,respectively.The leaching kinetic equation followed Jander model.Therefore,the safe controllable leaching process for the active aluminum-bearing substances was determined by small dosages of SAD with frequent additions.（2）Clinker sintered at low-temperature to efficiently convert the inert aluminum-bearing substances into sodium aluminate clinker was investigated by mixing impurities from the Bayer process.Nucleating agent,mineralizing agent,and catalyst all promoted formation of sodium aluminate clinker from the inert aluminum-bearing substances at 950°C for 1 h.The mechanism of sodium aluminate clinker sintering at low temperature was then elucidated.Magnesium aluminum spinel readily reacted with sodium carbonate compared with corundum.The reaction apparent activation energy of corundum and sodium carbonate was 202.42k J/mol in the range of 860°C to 1310°C,and the reaction was controlled by the solid-solid chemical reaction.Alumina and sodium oxide were then efficiently extracted during the leaching process from clinker at high temperature（≥95°C）for the short time（≤15 min）with extraction efficiency over 94%and 96%,respectively.Consequently,the slag yield was less than 7%.Simultaneous conversion of inert aluminum-bearing substances at low temperature and efficient elimination of Bayer impurities were then achieved.（3）The coarse sodium fluoride from the sodium aluminate solution by seeded precipitation was proposed.After the equilibrium concentration of sodium fluoride in the sodium aluminate solution was determined,the corresponding mathematical model was then presented as a function of caustic soda concentration and temperature.Adopting inhomogeneously distributed at weak agitation（IDWA）and adding the activated seed achieved high precipitation efficiency and generated the coarse sodium fluoride compared to that under perfectly mixed at strong agitation（PMSA）.The agglomeration kinetics and precipitation kinetics of sodium fluoride in the sodium aluminate solution were both investigated.The agglomeration rate was linearly associated with the supersaturation variation of sodium fluoride,and the apparent activation energy of sodium fluoride precipitated from the sodium aluminate solution was 21.29 k J/mol.Agglomeration contributed to the coarsening of sodium fluoride particles.Seed induction,regulation of supersaturation,and weak agitation synergistically improved the fluoride removal by formation of the coarse sodium fluoride with removal efficiency of 91.15%at the initial fluoride anion concentration of 5.2 g/L.（4）Chloride ions removal from the sodium aluminate solution during desilication in the Bayer process was discussed.The formation of Cl-bearing sodalite during desilication mainly contributed to chloride anion removal.The chlorine removal efficiency of 11.18%and a removal capacity of 47.5 mg/g were achieved under the conditions of 8.5 g/L of Cl^-and 20 g/L of kaolin at 100°C for 4 h.The anion incorporated into the sodalite followed the order:SO₄^2->>CO₃^2-≥Cl^->>F^-.Although the cell structure of sodalite was not changed obviously after embedding the chloride anion,decrease in the bond angle of aluminum and silicon to oxygen reduced the cell volume and grain size of the sodalite.Those changes accounted for the efficient removal of chloride anion from the sodium aluminate solution.The dissertation contains 113 figures,15 tables and 248 references...