| Metallurgical-Grade silicon（MG-Si）is a basic raw material that is widely used in photovoltaic,organosilicon,and silicon alloy industries.In the global environment of developing new energy and reducing carbon emissions,demand of MG-Si grows year by year and develops broad prospect.As a solid waste produced in MG-Si secondary refining process,yield of molten silicon refined slag（MSRS）is increasing year by year.Due to high viscosity and low fluidity of the slag,approximate 15%of silicon is trapped in MSRS.At present MSRS is mainly treated in form of solid waste dump or low-value casting paving materials.In MSRS,silicon particles with small and uniform size are tightly embedded in the slag.This increased the difficulty of fully liberating and recycling of silicon,resulting in a huge loss of silicon resources.While the recovery and reuse of silicon from MSRS has considerable economic and environmental value.Therefore,the problem of silicon recovery from MSRS needs to be solved urgently.In this thesis,based on the physical and chemical properties of MSRS,smelting and flotation combined technology to recover silicon from MSRS was proposed.The study focusing on formation and regulation of MSRS,full liberation,and modification and regulation of silicon and slag interface were carried out.The specific research results are as follows:（1）Formation and regulation mechanism of MSRS were clarified.The formation mechanism of MSRS and crystal structure of calc-aluminosilicate were analyzed in the process of MG-Si secondary refining.Ca2Al2Si O7slag was formed by adjusting the molar ratio of Ca O:Al2O3:Si O2to 2:1:1.This slag system has low viscosity,which can reduce the entraining of silicon in the process of MG-Si secondary refining and be benefited to silicon recovery.（2）The phase and existing state of MSRS raw materials were characterized.The MSRS was mainly composed of silicon（Si）,gehlenite（Ca2Al2Si O7）and silicon carbide（Si C）by phase analysis.Microscopic analysis shown that silicon particles in MSRS were tightly wrapped by the slag,but the interface was clear.Hence silicon particles can be liberated from slag by grinding.However,due to uneven distribution of silicon particles and existence of plenty particles with only a few microns in size,it is difficult to fully liberate silicon from MSRS.While the difference of hydrophilicity and hydrophobicity between silicon and slag indicates that the silicon particles can be recovered by flotation.（3）The content of silicon in MSRS was determined.According to different phases of MSRS with different solubility to various acids,a two-step method was proposed to determine the grade of silicon of MSRS.Hydrochloric acid was primarily used to dissolve calc-aluminosilicate,and then nitric acid and hydrofluoric acid were used to dissolve silicon.The optimal process conditions were explored.Hydrochloric acid was used to dissolve silicate in MSRS with particle size of-74μm.The amount of hydrochloric acid was 2m L/g MSRS,and the reaction time was 2 h.Silicate was dissolved,and the mixed particles containing Si and Si C were left.Subsequently,the mixture of nitric acid and hydrofluoric acid was used to dissolve Si.The volume ratio of HNO3:HF was 1:1 and the reaction time was 1h.The remaining particles were Si C.The content of silicon in MSRS was obtained.（4）The technology of silicon and slag interface sufficient liberation has been completed.Through MLA analysis,the liberation degree of silicon in MSRS reached79.47%,which was 7.94%higher than that of 5 min.The results showed that most of silicon particles can be liberated from MSRS by extending the grinding time and increasing the grinding concentration,and the grinding particle size reached 22.94μm.The main cleavage plane of silicon particles is（111）plane,and that of gehlenite is（211）plane.During wet grinding,gehlenite and calcium silicate hydrolyzed,resulting in the p H value of pulp reached 8.68.（5）The optimum technological parameters for flotation separation and recovery of silicon from MSRS were obtained.By addition of surfactants,the hydrophilic and hydrophobic difference of silicon and slag interface was increased.Then silicon particles in MSRS were efficiently recovered by flotation technology.Molecular dynamics calculation showed that the binding energy of Si-Si bond on the silicon surface with the surfactant was the strongest.The flotation results showed that the 2#oil selectively adsorbed and enveloped silicon,and has the dual function of foaming and collecting.When concentration of 2#oil was 200 g·t-1,the flotation recovery reached 77.42±4.40%,which was more than 50%higher than that without 2#oil.Through the study of flotation dynamic process,a flotation first-order matrix distribution dynamic model was established.According to the model,the flotation can be divided into two stages.In the first stage,the cumulative recovery rapidly increased to 62.24%,the recovered silicon grade was more than 76%.This stage is the key to improve the flotation effect.（6）Improvement recovered silicon grade by interface control was realized.The surface of calc-aluminosilicate was regulated by adding sodium silicate（SS）.It was found that the main hydrolysate of sodium silicate were H2Si O3and HSi O3-,with strong hydrophilic properties.They can adsorb on the surface of calc-aluminosilicate,forming a hydrophilic layer.Thus,the hydrophilic of calc-aluminosilicate surface was enhanced,and they were inhibited to floating.When concentration of SS was 300 g·t-1,the grade of recovered silicon increased from 66%to 82.04%.The silicon ingot was obtained by smelting the recovered silicon product,which could be further purified and utilized.Based on the research of technological route and mechanism of silicon recovery from MSRS,this thesis realized the efficient recovery of silicon from MSRS through separation and recovery technology.This provided theoretical support for recovery and high-value utilization of silicon. |
