Preparation And Properties Of Biomass Carbon Reducing Agent For Industrial Silicon Smelting

Carbonaceous reductant is the essential material for the production of metallurgical grade silicon (MG-Si), and wood charcoal has always been the ideal reductant for MG-Si production. However, the production of wood charcoal is at the cost of consuming the limited forest resources, furthermore, the price of wood charcoal has been raised severely than never before, which has influenced the economic benefit of plants engaged in MG-Si production. Consequently, in this thesis, the agricultural biomass materials (bagasse, walnut shell, pinenut shell and corncob) with the characteristics of abundant storage quantities, short growth cycle and being renewable have been selected as the raw materials to conduct the following researches.Firstly, bio-char reductant was prepared from the biomass materials with the method of pyrolysis, and investigations focused on the effect of pyrolysis process parameters on the yield, fixed carbon content, volatile matter content, ash content and resistivity of bio-chars were conducted. Meanwhile, pine tree wood was selected as the comparison material for the preparation of wood charcoal(pine wood charcoal). Performance comparison between wood charcoal and bio-char was conducted from the aspects of resistivity changing rule, the content of impurities in the ash as well as the hole structures of bio-chars prepared under different pyrolysis temperature was conducted, and comparison of these indexes between bio-chars and wood charcoal was also performed. The experimental results indicate that pyrolysis temperature had prominent effect on the resistivity of bio-chars, the resistivity of bio-chars prepared under 700℃ decreased slowly with increasing pyrolysis temperature, while it decreased drastically during the temperature range of 700℃～1000℃, and continued to decrease slowly and tended to be flat with increasing prolysis temperature.Furthermore, dynamics researches on the pyrolysis of biomass were conducted by employing Thermogravimetric analysis, the results indicate that the weight losing rate of biomass tended to increase firstly and decrease afterwards. During the temperature range of 233～353℃, the apparent activation energy corresponding to the heating rate of 10,20 and 30℃/min was 68.15、68.62 and 80.83 kJ/mol, respectively. While, during the temperature range of 353～533℃, the apperant activation energy corresponding to the heating rate of 10,20 and 30℃/min was 37.25, 38.82 and 12.10 kJ/mol, respectively. The mechanism function of f(α)=(1-α)1.5 well fitted the pyrolysis process of biomass under these two temperature ranges.Finally, characterizations of the bio-char reductant prepared under different pyrolysis temperatures were conducted by employing the techniques of Fourier transform infrared spectrometer (FT-IR) and X-ray diffraction (XRD) analysis so as to investigate the resistivity change mechanism of bio-chars prepared under different temperatures. The experimental results indicate that the carbonyl and aliphatic series functional groups in the bio-chars were destroyed with increasing pyrolysis temperature, the realignment of the molecular structures in the bio-chars took place, the (002) and (100) peaks gradually tended to be sharper and sharper, the interlayer distance (doo2) between the two crystal lattices tended to decrease, cyrstallinity, the degree of aromatization and carbon structure ordering were enhanced, which consequently contributed to the decrease in the electrical resistivity of bio-chars. Characterization on the morphology of bio-chars was also conducted with the technique of Scanning electronic microscope (SEM), and morphology comparison between bio-chars and wood charcoal was also performed. Experimental results indicates that there appeared obvious burning and erosion phenomenon on the morphology of bio-chars with increasing pyrolysis temperature, and to some extent, high pyrolysis temperature tended to exert certain destruction on the hole structure of bio-chars.Based on the industrial analysis and the comparison of each performance, it can be concluded that the order of bio-chars suitable to be used as MG-Si reductant was: pine nut shell char> walnut shell char> corn cob char> sugar cane char. And bio-chars had developed hole-structure and high specific surface area, bio-chars prepared during the pyrolysis temperature range of 500～700℃ had proper volatile content and low ash content, and their average yield, average resistivity, fixed carbon content and heat value of was>24%、>5600μΩ·m、>80% and>30MJ/kg, respectively, certain performance indexes approached or exceeded those of wood charcoal, making them promising to be applied as carbonaceous reductant for MG-Si production.