| Raw material and energy shortage and environmental pollution aggravation are important problems that people are facing; recycling of waste is an important step to solve these problems. In recent years, comprehensive recycling of waste has become more and more attention by people under the background of process maturity and equalization of profit. Linear cutting waste mortar contains silicon and silicon carbide that are an important industrial raw material. For a long time, waste mortar have been piled up and buried on the ground, which not only waste resources but also pollute the environment. In these years, silicon carbide and cutting fluid in linear cutting waste mortar have been preliminary recovered with the development of recycling technology of linear cutting waste mortar(not yet perfect), but recycling of silicon is still at the exploratory period (rarely reported). However, because silica has features of high purity, low density, high specific surface area and excellent dispersion as well as its superior stability, reinforcing, thickening and thixotropy, it is widely used in rubber, coatings, paints, adhesives and other industries. Therefore, in this paper, recycling of silicon in linear cutting waste mortar have been studied, which thoughts on preparation of white carbon black and recycling silica silicon from waste mortar has been proposed. The following is:Firstly, preparation of white carbon black is used waste mortar and ammonium fluoride as raw materials by recycling of ammonium fluoride (new fluorination). The thermodynamic analysis shows:under 25℃, mole Gibbs function of reaction of ammonium fluoride and elemental silicon (△GmΘ=-338.9 (kJ/mol)) and reaction of ammonia and ammonium fluorosilicate (△GmΘ=-37.542 (kJ/mol)) are both less than zero, therefore, both reaction are feasible from the thermodynamic point of view. Then, the reaction influence laws of various factors were investigated. The results show that mass concentration of ammonium fluoride is 40% to 55%, temperature is 50℃to 60℃and excessive amount of ammonium fluoride is 1.1 to 1.3 for ammonium fluoride reaction; pH Value is no more than 6.9 in the nucleation stage and 7.1 in the growth stage, mass concentration of ammonium fluorosilicate is 12% by dividing two precipitations and filtrations, dropping rate of ammonia water is 2d/s at the initial precipitation stage and 0.5～1d/s at the end precipitation stage, temperature 40℃-50℃, mass concentration of ammonia 5% to 10%, agitator speed 300r/min-400r/min and aging time of 0.5h for ammonia reaction. Secondly, the recovering of ammonium fluoride is more than 95% through a closed loop experiment of ammonium fluoride. Finally, product quality of the obtained silica was tested; test results show that the obtained silica reached the national standard.Secondly, the reaction kinetics of ammonium bifluoride and silicon, which is the key reaction of new fluorination, was studied. The experiment found that the reaction fit second order reaction [-r=k (CNH4HF)2]; and identified the reaction rate constant:k50=3.47×10-4 mol3·L-1·s-1 and k60=5.62×10-4 mol3·L-1·s-1 at 50℃and 60℃, the activation energies Ea=43.16 KJ/mol and the pre-exponential factors A=3288.85.Thirdly, method that is preparation of white carbon black and recycling silica silicon by using high concentrations of alkali solution to dissolve silicon which is in waste mortar was studied. The thermodynamic analysis shows:under 25℃, mole Gibbs function of reaction of ammonium fluoride and elemental silicon (△GmΘ=-490.612 (kJ/mol)) and reaction of ammonia and ammonium fluorosilicate (△GmΘ=-586.596 (kJ/mol)) are both less than zero, therefore, both reaction can be spontaneous. Then, the reaction influence laws of various factors were investigated. The results show that the alkali dissolution reaction conditions:sodium hydroxide concentration of 25% to 35% and the reaction temperature is 55℃or so; acidification reaction conditions:concentration of sodium silicate 9-12%, hydrochloric acid 20% to 25%,13 drops of n-butanol (about 0.6ml) and the reaction temperature is about 80℃.Finally, the home-made silica was modified by using CTAB, n-butanol, N-β-aminoethyl-y-aminopropyl trimethoxysilane, ZH-1108 and methyl triethoxysilane as modifier, and analyzed by using infrared spectroscopy, thermo gravimetry, specific surface area and oil absorption value, the results show that the self-made modified silica was realized, which n-butanol was the most worst modifier and N-β-aminoethyl-γ-aminopropyl trimethoxysilane was much better. |
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