Arsenic Solidification Mechanism of Fayalite Phase in Copper Flash Smelting Slag
Copper smelting slag contains a considerable amount of fayalite,which has a high potential for arsenic solidification dur-ing flash smelting process.In order to clarify the conditions of arsenic solidification in fayalite in flash smelting furnace,the formation process of fayalite was studied by high temperature sintering using Fe powder,Fe2O3,SiO2 and CaHAsO4 as raw materials,and the ef-fects of atmosphere including air and argon as well as temperature(1000～1300 ℃)on the arsenic solidification behaviors of fayalite were discussed.X-ray fluorescence spectroscopy(XRF),scanning electron microscopy(SEM-EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and inductively coupled plasma-mass spectrometry(ICP-MS)were used to characterize the com-position,phase,morphology and chemical states of the copper slag and the sintered samples.Toxicity characteristic leaching proce-dure(TCLP)and first-principles calculation were used to analyze the stability of arsenic solidified in fayalite.The flash smelting slag contained 41.07％～42.59％Fe,27.54％～30.96％SiO2,2.07％～2.35％CaO,0.92％～1.28％MgO and 3.13％～3.77％Al2O3.According to the composition of the copper slag,the liquidus temperature was 1100 to 1200 ℃.XRD and SEM results showed that the main phas-es of slag were fayalite(Fe2SiO4),magnetite(Fe3O4)and silicates,and the arsenic was solidified in fayalite,but not in magnetite.In the sintering experiments,when sintered in the air,the sintered product was granular at 1000 to 1100 ℃,and the phase compositions were SiO2,Fe2O3 and Fe3O4.When the sintered temperature at 1200 to 1300 ℃,the sintered product was molten,and the phase com-positions contained Fe3_SixO4,Fe2O3 and SiO2.When sintered in the argon,the products appeared to be porous at 1000 to 1100 ℃,while the sintered products presented a completely molten state at 1200 to 1300 ℃.XRD patterns showed that the fayalite phase was formed at 1000 to 1300 ℃.The results of TCLP showed that the arsenic leaching concentration of the solidified slag sintered at 1000 and 1100 ℃ in air reached 188.7 and 243.3 mg·L-1,respectively,which was far higher than the arsenic leaching standard(5 mg·L-1)stipulated in the national standard for the identification of dangerous substances(GB 5085.3-2007).When the sintering temperature was 1200 and 1300 ℃,the arsenic leaching concentration of sintered products were 2.916 and 3.394 mg·L-1,and the arsenic leaching rate were only 1.42％and 1.74％,respectively,which indicated the low leachability of arsenic solidified sintered slag.When sintered in the argon,the arsenic leaching concentration of the sintered products at 1100,1200 and 1300 ℃ were 3.563,0.958 and 0.199 mg·L-1,and the arsenic leaching rate were 1.66％,0.87％and 0.19％.Compared with the slag sample sintered in the air,the slag sample sin-tered in argon showed higher immobility of As.XPS results showed that the silicon atoms in the slag products sintered at 1100 ℃ in air were in the form of SiO2,without bonding with other atoms.With the increase of sintering temperature,Si-O-Si bond could be trans-formed into Si-O-Fe and Si-O-As bond.When sintered in argon,the stable Si-O-As bond was not formed at 1000 ℃.With the in-crease of sintering temperature form 1100 to 1200 ℃,the proportion of Si-O-As increased from 11.56％to 15.76％.The relative per-centages contents of As(Ⅲ)and As(V)in the sintered product were 29.3％and 70.7％at 1100 ℃,and the values at 1200 ℃ were 20.7％and 79.3％,respectively.The increase of As(V)contents was favorable to the arsenic solidification in SiO4 structure of fayalite.Due to the high content of As(Ⅲ)in the fayalite phase and the lack of stable Si-O-As bond,the slag sample sintered at 1000 ℃ in the argon showed a high arsenic leaching concentration.The first-principles calculations showed that the diffusion rates of As,Ca and O atoms in(Ca0.5Fe0.5)H(As0.5Si0.5)O4 were higher than those in CaHAsO4 at 1600 K.And the part of AsO4 tetrahedron and SiO4 tetrahe-dron could be bonded by Si-O-As bond in the simulated sintering structure of(Ca0.5Fe0.5)H(As05Si0.5)O4.The results revealed that the formation of Si-O-As bond made it difficult for As atoms to escape from the crystal structure,and the molten fayalite at high tempera-ture promoted the diffusion rate of atoms,which was favorable for the replacement of As atoms for Si atoms in the SiO4 tetrahedron structure to form stable Si-O-As bond.The two-dimensional charge density distribution of CaHAsO4 and FeAsO4 showed that almost no charge interaction occurred between Ca atoms and their neighboring O atoms,while there were significant interactions between Fe atoms and O atoms.The results indicated that the strong interaction between Fe atoms in fayalite and O atoms in AsO4 structure could improve the stability of As-solidified copper slag.
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