Study On Binding Relationship Between Mineral Phases And Heavy Metals,Harmlessness Process And Mechanism Of Copper Smelting Slags

Smelting slag rich in fayalite structure,represented by copper smelting slags（CSS）,is widely existed in metallurgical industry and pyrometallic secondary metal recovery industries.However,due to the lack of efficient and economic treatment methods,most of the CSS is stacked in the open air.After rain erosion,the heavy metals in the CSS are continuously enriched in the water environment and soil environment,which causes serious pollution to the environment and threat to human health.Therefore,the efficient harmlessness and reduction of CSS is an important issue to alleviate the current disposal of CSS.In recent years,with the increasingly severe environmental protection requirements,the demand for efficient and harmless technology of CSS has become more urgent.However,due to the lack of sufficient understanding for the binding state of heavy metals in CSS,it is difficult to design an efficient,stable and economic harmless treatment process,so it is of great significance to carry out research on the binding state and mechanism of heavy metals in CSS.To solve the problems of tough activiation for harmlessness in CSS:（1）heavy metals in CSS was taken as the research object in this work to reveal the binding state of heavy metals in copper smelting slag by various means;（2）based on this,strategy of alkali disaggregation coupling with dilute acid oxidation leaching was developed;and（3）the lattice substitution mechanism of heavy metals in fayalite was explored.The main research results are as follows:（1）In order to reveal the distribution of heavy metals in copper smelting slag and the binding relationship between the heavy metals and the main substances,the main crystalline phases in copper smelting slag are identified as fayalite（47.4%）,hedenbergite（38.7%）,magnetite（11.0%）and iron（2.9%）by XRD Rietveld method.The results of EPMA and SEM-EDS show that the distribution of heavy metals in copper smelting slag is related to the structure of mineral phase.Specifically,the distribution of arsenic,lead and copper is closely related to the distribution of fayalite and hedenbergite,and lead and copper are enriched in iron phase.It was confirmed that fayalite is the main phase of heavy metals in copper smelting slag by several techniques.The improved BCR sequential extraction method was used to analyze the binding relationship between mineral phases and heavy metals in copper smelting slag.The proportion of Cu,Pb and As bound in fayalite was 76.9%,85.0%and 89.7%,respectively.It is indicated that the fayalite structure has the strongest binding relationship to heavy metals,while the heavy metals in other phases and amorphous structures can be breaken free with the change of environment.（2）In order to expose the heavy metals bound by fayalite phase in copper smelting slag,the idea of alkali disaggregation of fayalite in copper smelting slag by Na OH was put forward,and the effect of alkali disaggregation on improving the binding relationship between the main substances in copper smelting slag and heavy metals was explored.The results show that Na OH disaggregation results in the transformation of fayalite phase to magnetite structure in copper smelting slag,which represents that the binding relationship of heavy metals in copper smelting slag decreases from strong to weak,indicating that the disaggregation process improves the binding relationship in copper smelting slag and exposes the heavy metals bound in fayalite.Alkali disaggregation was evaluated in dilute acid leaching system,the results show that when the concentration of nitric acid is 1.2 M,the reaction temperature is 90℃and the concentration of H₂O₂ is 0.4 M,the total leaching rates of As,Cu and Pb are 99.7%,99.9%and 99.1%,respectively.In the dilute nitric acid leaching system,compared with the total leaching rate of copper smelting slag without disaggregation,the total leaching rate of As,Cu and Pb in the copper smelting slag treated by alkali disaggregation is significantly increased by 73.3%,72.2%and 22.4%,respectively.It is mainly attributed to that alkali disaggregation improves the binding state of heavy metals in copper smelting slags,promotes the migration and transformation process of heavy metals among minerals,and fully exposes the surface of minerals with weak binding relationship such as magnetite.Dilute acid oxidation leaching system can reduce the leaching effect of heavy metals in slag and improve the total leaching rate of heavy metals in the system.（3）In order to further understand the binding mechanism of heavy metals in fayalite,fayalite containing Cu,Pb and As named Cu-Fe₂Si O₄,Pb-Fe₂Si O₄ and As-Fe₂Si O₄ was synthesized,and the occupation rules and lattice substitution mechanism of heavy metals in the formation of fayalite minerals were investigated.Compared with fayalite,the XRD of Cu-Fe₂Si O₄,Pb-Fe₂Si O₄ and As-Fe₂Si O₄ synthesized successfully is shifted and morphologies of those have changed.Further analysis showes that Cu and Pb entered the lattice structure of fayalite and occupied the M2 site in the structure of fayalite,while As in As-Fe₂Si O₄ may occupy the O or Si site as well as the M2 site in the structure of fayalite.The above heavy metal occupation rules are further confirmed in the alkaline disaggregation experiment.After the reaction of Cu-Fe₂Si O₄ with Na OH,the structures of ferric oxide,sodium silicate and copper spinel are formed.Pb-Fe₂Si O₄ reacts with Na OH to form iron oxide,sodium silicate and lead oxide structures.However,As-Fe₂Si O₄ is only partially disaggregated to form sodium silicate,and no arsenic compounds are found.It meant that As enteres the lattice of fayalite in a different way than Cu and Pb.DFT calculation confirms the above rule of Cu,Pb and As occupied in fayalite.Cu（II）and Pb（II）enter the lattice of fayalite mainly by replacing M2 site of Fe（II）,while As（III）simultaneously replacs M2 site of Fe（II）and partial Si atoms for charge compensation.The disaggregation results of copper smelting slag samples prove the lattice rule and lattice substitution mechanism of heavy metals such As Cu,Pb and As occupying fayalite.In summary,in this study,the distribution of heavy metals in CSS and their binding relationship with the main phases are revealed,and a new harmless process of alkali disaggregation coupled with dilute acid oxidation is developed,and understanding of the relationship between heavy metals and fayalite is deepened.The principle of inter-lattice substitution provids a reference for the phase identification of iron-containing fayalite smelting slags,and concerns a new strategy and the theoretical basis for the harmlessness of iron-containing fayalite smelting slags.