| Black aluminum dross is a hazardous solid waste generated in aluminum processing and regeneration,and China’s aluminum industry produces about 1 million tons of black aluminum dross every year.The generated black aluminum dross can only be disposed of by landfill,which wastes resources,causes environmental pollution,and endangers human health.black aluminum dross contains many aluminum and other valuable elements,but it is challenging to utilize resources because of its complex phase and composition.The harmless disposal and resourceful utilization of black aluminum dross is the key to guarantee the green ecology of China’s aluminum industry.There is more research and mature technology on the recovery technology and equipment of aluminum metal in black aluminum slag at home and abroad.However,there are few kinds of research on the resource utilization technology of alumina,aluminum nitride,and other valuable elements in the black aluminum dross produced after aluminum extraction.There are problems such as more secondary severe pollution in the disposal process and low value of products.Therefore,there is an urgent need to develop a resource utilization technology with a short process,high recovery rate,and high value for black aluminum dross.Black aluminum dross contains Al,Si,and various valuable metal elements and contains harmful substances such as nitrogen and chlorine.This paper proposes and realizes the research goal of preparing aluminum-based alloys by molten salt electrolysis using black aluminum dross as raw materials to recover valuable elements.The critical scientific problems such as the assignment characteristics of valuable/hazardous elements in black aluminum dross,the deep removal of nitrogen and chlorine elements from black aluminum dross,the dissolution performance of black aluminum dross in the aluminum electrolyte,and the preparation of aluminum alloy by multi-component co-precipitation electrolysis have been studied in depth.Innovatively developed a short process,high-value resource utilization technical route,and critical technologies for preparing aluminum-based alloys by catalytic hydrolysis denitrification-coordinated salt removal-calcination phase inversionmolten salt electrolysis.This study analyzed the content,state of residence,and distribution characteristics of valuable/harmful components in black aluminum dross.The composition of black aluminum dross is complex,the embedded shape of various minerals is complex,and the separation and recovery are complicated.Especially the deep removal of nitrogen and chlorine elements is the crucial link that restricts the electrolysis of black aluminum dross molten salt.The thermodynamics of aluminum nitride removal from black aluminum dross was investigated,and a new process of catalytic hydrolysis for nitrogen removal and synergistic salt removal was proposed,and the optimal process conditions were determined as follows:2 wt%NaOH,temperature>90℃,liquid-solid ratio 6 mL/g.particle size less than 100μm,and time 3 h.Under the optimal conditions,the recoveries of nitrogen and salts were 90.25%and 99.84%,respectively,realizing the efficient separation of nitrogen and chlorine in black aluminum dross.The unreacted core model was used to study the kinetic behavior of aluminum nitride hydrolysis,and it was found that the hydrolysis process of aluminum nitride conformed to the mixed control mode of diffusion and chemical reaction.The products of aluminum nitride hydrolysis are mainly bayerite and pseudo-boehmite.After calcination at 700℃ for two h,the bayerite and pseudo-boehmite generated by the hydrolysis of aluminum nitride can be directionally converted into active γ-Al2O3,which is easily soluble in aluminum electrolyte.The electrolysis process requires good dissolution characteristics of black aluminum dross,i.e.,high solubility and fast dissolution rate.Based on the principle of the Tyndall effect combined with high-speed photography,the dissolution behavior of black aluminum dross and its main components in the electrolyte system was investigated in this study by high-temperature transparent observation,and it was proved that aluminum nitride was the main factor affecting the dissolution performance of black aluminum dross.The transformation law of aluminum nitride in the electrolyte is expounded.After adding aluminum nitride into the electrolyte,a small part is converted into α-Al2O3.The converted α-Al2O3 is partially dissolved in the aluminum electrolyte,and the rest is precipitated at the bottom together with the aluminum nitride.The isothermal saturation method determined the solubility of aluminum nitride in the aluminum electrolyte.The test results show that aluminum nitride is almost insoluble in the aluminum electrolyte,and the solubility is only 0.0022%.The dissolution performance of the black aluminum dross in the electrolyte before and after denitrification was compared,and the dissolution rate of the black aluminum dross in the electrolyte was greatly improved after denitrification and desalination.In the dissolution process in the electrolyte,temperature plays a positive role in the dissolution rate of black aluminum dross,and the mole ratio of aluminum electrolyte has little effect on its dissolution rate.The dissolution performance of black aluminum dross in the aluminum electrolyte was optimized by changing the amount of aluminum oxide blended in black aluminum dross to meet the electrolysis demand.When the proportion of black aluminum dross was 25%,the dissolution rate of the material in the electrolyte was improved by about 60%compared with the proportion of black aluminum dross of 100%.Under laboratory conditions,using Na3AlF6-AlF3-Al2O3 as the electrolysis system,the aluminum-based alloy was prepared by direct electrolysis with the pretreated black aluminum dross as the raw material.The influence of the proportion of black aluminum dross in the material on the alloy composition,cell voltage,and current efficiency was investigated.Each valuable element’s migration and transformation law in the black aluminum dross during the electrolysis process was analyzed.The results showed that 95%of the valuable elements in the black aluminum dross were effectively recovered under the electrolysis conditions of 980℃,1 A/cm2 current density,2.6-mole ratio,and Na3AlF6-AlF3-Al2O3 electrolyte system.There are mainly three phases of Al,Si,and Al-Si-Fe in the aluminum alloy.None of the elements in the aluminum alloy exceeds the saturation concentration,the overall distribution is uniform,and no obvious local segregation phenomenon is found.Almost all elements with positive electronegativity than aluminum,such as Fe,Si,Cu,and Zn in black aluminum dross,enter the aluminum alloy and will not accumulate in the electrolyte.The Ca and Mg elements in the black aluminum dross are converted into CaF2 and MgF2 and remain in the electrolyte.Based on laboratory research,the industrialized process of producing aluminum-based alloys by molten salt electrolysis was determined after the black aluminum dross was disposed of harmlessly,and the industrial test research was carried out on a 200 kA electrolytic cell.Under industrial test conditions,the removal rate of aluminum nitride in black aluminum dross after catalytic hydrolysis is 89.05%,and the removal rate of chloride salt is 96.04%.The black aluminum dross after denitrification and desalination,is calcined in a rotary kiln(about 800°C),so that all-aluminum hydroxide produced by the hydrolysis of aluminum nitride is converted into active γ-Al2O3.The industrial electrolysis test ran smoothly on the whole,and the produced aluminum alloy liquid was used to produce ADC 12 alloy.Consistent with the laboratory research results,the elements with lower precipitation potential in the black aluminum dross are preferentially precipitated under industrial test conditions and all enter the aluminum alloy,while Mg and Ca with higher precipitation potential are converted into fluorides and enter the electrolyte.Various valuable elements in black aluminum dross are recovered efficiently in the whole process.The ammonia gas generated by the hydrolysis of aluminum nitride is used to prepare ammonia water for desulfurization and denitrification.The chloride salt enters the filtrate and is re-recovered as refining agent salts by evaporation and crystallization.The remaining high alumina materials disposed of are all returned to the electrolyzer as raw materials to prepare aluminum alloy so that all valuable elements such as aluminum,silicon,copper,nickel,and iron in them are recovered.In summary,this paper developed a process route for high-value resource utilization of various valuable elements of black aluminum dross,which is a short process,low-carbon and clean,without secondary pollution,realizes a high recovery rate and high-value resource utilization of valuable components of black aluminum dross,solves the environmental severe pollution problem of this kind of hazardous solid waste,and provides the certain theoretical basis and technical support for large-scale industrialization and promotion... |
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