Study On SiC And Si₃N₄ Separation From Silicon Melt During Metallurgical Process Of Silicon Scraps Recycling

Multicrystalline silicon is the main raw materials to produce solar cells.In recent years,the purification of silicon has been deeply developed.In directional solidification process,the crucible is commonly coated a Si3N4 layer to prevent SiO2 pollution.Therefore,the obtained ingot will be polluted by N and even precipitate with Si3N4 particles.Meanwhile,the heating system is commonly graphite,C will evaporated during melting and pollute silicon melt and even precipitate with SiC particles.Therefore,the ingot top,bottom and edge will be cut due to the high content of SiC and Si3N4 particles and the cut parts become silicon scraps which account for 30%of the obtained ingot.As the increasing demand of Si,the traditional process generates multi tens thousands every year and it increases continuously.Moreover,the cutting slurry of silicon ingot during wire sawing process cause lots of silicon slurry wastes.It is obvious that the recycling of these scraps and waste will tremendously reduce production cost.Therefore,the separation of SiC/Si3N4 from silicon is a significant topic to be studied.Considering that there hasn't been developed an effective way to recycle the scraps yet.Effective and efficient method to separate particles from silicon has been an urging problem to solve.It has significant meaning to save silicon resources and will be beneficial to the industrial sustainable.Considering the faultiness of the existed silicon scraps recycling method,this thesis focus on the metallurgical method,to apply physical field and chemical kinetics to study the transfer mechanism of SiC/Si3N4 particles.The study can provide theoretical foundation and high efficient separation process.The methods this thesis applied and the research results are conclude as following:(1)Based on the electricity property difference of SiC/Si3N4 from silicon melt,electromagnetic field is applied to study the migration mechanism of particles under the melt convection and Lorenz force.Particles precipitate at melt bottom and form enrichment region at melt top,a particles accumulation layer is formed at melt edge.Particles coalesce with Ostwald and Quasi-Ostwald ripening mechanisms.The precipitation follows Stokes discipline.The accumulated layer at melt edge is attributed to melt flow,Lorenz force and skin effect.The layer thickness can be increased with increasing melting time.The increasing coil current can condense layer and raise particles amount.The particles migration velocity to edge is mainly depended on Lorenz force.A separation efficiency of 85.7%is obtained with 45min melting.The efficiency with shorter melting time is 20%higher than other studies.Separation efficiency can be raised obviously with increasing coil current.A efficiency of 97.6%is obtained with 45A coil current.The separation of SiC/Si3N4 is also proved to be effective with EM field application and a separation efficiency of 88.3%is obtained.The industrial scale experiment is conducted with 399.9kg and a separation efficiency of 70.2%is obtained.(2)By Al addition,the effect of Al/SiC wetting and interface interaction on SiC migration is studied.Meanwhile,the coupling of EM field with A1 addition is also studied.As the increasing A1 addition,SiC enrichment region migrate to melt bottom and a separation efficiency of 81.4%is obtained.The efficiency is reduced with 7.9%A1 addition.In Al/Si alloy region,there exists incompletely dissolved SiC and Al-C binary phase.In Al/SiC region,there exists incompletely dissolved SiC and as-dissolved SiC.The migration of SiC experiences three statuses:Al/SiC wetting-SiC particles groups precipitated-Al/SiC interface action.The dissolved particles will be reduced with increasing temperature,further increase diffusion coefficient of A1 and C and lower the purity of eutectic silicon.But increasing temperature can improve wetting ability of Al to SiC.By coupling EM filed with A1 addition,SiC tend to migrate to melt top of the center and the efficiency is raised to 88.1%with increasing melting time.Particles form 3D structure and the interface between particles will disappear with longer time.Al4C3 is formed with 45min melting time.It indicates that longer melting time will improve Al/SiC interface reaction.(3)Based on the high vacuum pressure,high vertical temperature gradient and melt convection of electron beam melting(EBM).The transfer behavior of C/N and SiC/Si3N4 are studied.During EBM,fast cooling process can improve the nucleation of SiC on Si3N4.Meanwhile,the high temperature gradient can improve the collision and combination of particles and the formation of particle groups.Particles suspended in center region and precipitated to melt bottom with increasing melting time.By modifying solidification process,the precipitation of particles occurs with slow cooling patterns.C-O and N-O complex migrate from melt to surface and consume O continually.Finally,the industrial scale experiment is conducted to separate particles from silicon melt.Particles precipitate to the bottom of melting crucible and they are separated with silicon melt by pouring silicon to solidification crucible.A particle-free ingot is obtained and it is proved that EBM can be an effective way to recycle silicon scraps.