Geochemical Metallogenic Mechanism Of Arsenate Double Salt And Its Application In Arsenic-alkali Slag Disposal

Arsenic containing solid waste from nonferrous smelting in China has the characteristics of large output,high content of valuable metals and strong toxicity.Although the traditional process of removing arsenic by alkaline leaching can realize the efficient separation of arsenic and valuable metal components,it can not solve the problem of selective separation of arsenic containing components and alkali components,so it can not be applied in large-scale industrialization.In view of this problem,this paper studid the crystal chemistry and precipitation thermodynamics of arsenate double salt in the complex system based on the metallogenic principle of stuvite type minerals.On this basis,the paper further formed the regulation technology of arsenate double salt mineralization（NH₄-Mg-As O₄ and K-Mg-As O₄）,and solved the problem of selective separation of arsenic containing components and alkali components in the high alkali/salt system.In this way,the resource utilization of alkali components and the safe disposal of arsenic components are realized.Taking this technology as the core,the new process of“The resource treatment of arsenic-alkali slag by double salt mineralization”and“The resource utilization of copper dust by cyclic alkali leaching”had been formed,which achieved successful industrial application.This study will provide new solutions for the harmless,reduction and resource treatment of arsenic containing solid waste.Thus,it can alleviate the problem of arsenic pollution and the safety of non-ferrous metal resources,and promote the industrial upgrading of non-ferrous metal industry.The main conclusions are as follows:1.Basic research on the precipitation thermodynamics of arsenate double salt mineralsArsenate double salt minerals（Mg NH₄As O₄·6H₂O,Mg KAs O₄·6H₂O and Ca KAs O₄·8H₂O）belong to orthogonal crystals.They are formed by hydrogen bonding of tetrahedral As O₄^3-,octahedral Mg²⁺·6H₂O and tetrahedral NH₄⁺,and the stability of their crystal structure is related to the radius of positive monovalent cations and the hydration radius of divalent cations.The solubility product constants（Ksp）of Mg NH₄As O₄ and Mg KAs O₄ were calculated to be 10^-10.55 and 10^-9.81,respectively,based on PHREEQC software and the homologous linear law.The thermodynamic equilibrium diagrams of NH₄⁺-Mg²⁺-As O₄^3--OH^--H₂O system and NH₄⁺-Mg²⁺-As O₄^3--CO₃^2--OH^--H₂O system drawn by thermodynamic calculation show that with the change of reaction p H,the possible precipitation phases in the two systems are one or more of Mg HAs O₄,Mg NH₄As O₄,Mg CO₃,Mg₃（As O₄）₂ and Mg（OH）₂.Under the optimal precipitation conditions of Mg NH4As O4（p H is about 8.5）,the arsenic removal rate reaches more than99%,while the alkali loss is less than 5%.In addition,the initial CO₃^2-concentration,the molar ratio of magnesium to arsenic and the molar ratio of ammonium to arsenic in the system will have a significant impact on the arsenic removal rate and the phase of the precipitation,and affect the grade of arsenic slag by affecting the proportion of Mg NH₄As O₄ and Mg CO₃ in the precipitation.2.Research on the regulation mechanism of arsenate double salt precipitation kineticsUnder the best conditions,the arsenic removal experiment shows that the arsenic removal efficiency and arsenic residue grade of magnesium ammonium system in different alkaline media are Na OH>Na HCO₃>Na₂CO₃,and the law of magnesium potassium system is Na OH>Na₂CO₃>Na HCO₃.The index of magnesium ammonium system in the same alkaline medium is better than that of magnesium potassium system.The crystallization process of arsenate double salt in Na OH solution conforms to the first-order kinetic fitting curve.The reaction time,temperature,reagent dosage,impurity ions(cations:Ca²⁺,Al³⁺,Cu²⁺,Pb²⁺,Zn²⁺,Fe³⁺;anions:PO₄^3-,F^-,Si O₃^2-,S^2-),stirring strength and reagent system（Mg Cl₂+NH₄Cl,Mg CO₃+（NH₄）₂CO₃,and Mg O+NH₄HCO₃）in the mineralization process of arsenate double salt will have a significant impact on the arsenic removal efficiency and the physical and chemical properties such as particle size,morphology,composition and arsenic content of arsenate double salt.3.Research on deep resource utilization of arsenic slag based on the characteristics of arsenate double saltThe direct roasting experiment of arsenate double salt shows that with the increase of roasting temperature,the crystal transformation path of magnesium ammonium arsenate double salt is Mg NH₄As O₄·6H₂O→amorphous→Mg₂As₂O₇→Mg₃（As O₄）₂,while the roasting slag of magnesium potassium arsenate double salt is mainly amorphous.In addition,the arsenic leaching toxicity of roasting slag under different phases is significantly different.When the roasting slag phases are Mg NH₄As O₄·6H₂O and Mg₃（As O₄）₂,the arsenic leaching toxicity is 1901mg/L and 0.7 mg/L respectively.The arsenic slag formed in Na₂CO₃solution contains Mg CO₃,and its thermal decomposition product Mg O has the effect of fixing arsenic.Therefore,the leaching toxicity of arsenic is less than that of arsenic slag formed in Na OH solution.The thermodynamic analysis and reduction roasting experiment show that under the optimal reduction roasting conditions,arsenate double salt can be reduced to elemental arsenic with purity of 99.85%and Mg O roasting slag which can be recycled as arsenic removal agent.4.Study on the comprehensive treatment of arsenic containing solid waste based on the mineralization of arsenate double saltThe new process of“the resource treatment of arsenic-alkali slag by double salt mineralization”was applied to the the harmless treatment of arsenic-alkali slag left over from history in Xikuangshan area,and a 20000t/a arsenic alkali slag treatment line was built.The new process produced Na HCO₃ products with purity of more than 98%and high arsenic slag,which can be rigidly landfilled,with arsenic content of more than 29%.The new process realized the recycling of industrial water,and the treatment cost is only 2228.4 yuan/ton.The new process of“the resource utilization of copper dust by cyclic alkali leaching"was applied to the disposal of copper smelting dust produced in western mining industry.The leaching rate of arsenic was more than 80%,while the loss rate of valuable metals was less than 0.5%in the new process.Moreover,the separation efficiency of arsenic and alkali was maintained at more than 90%,and the grade of arsenic residue was more than 25%.Most importantly,the alkali solution after arsenic removal can be recycled,which provides a new solution for the resource utilization of arsenic containing dust from copper smelting.