Cold-pressed Preparation Of Carbonaceous Reducing Agent Pellets And Forming Mechanism For Industrial Silicon Production

Charcoal is the best reducing agent of industrial silica because of its low ash content and high resistivity. The increasing scarcity of forest resources and the growth of environmental awareness caused charcoal prices rising and even ending production, and it had an effect on production of industrial silicon. Therefore, it is necessary to find new carbonaceous reducing agent instead of charcoal completely.The carbonaceous reducing agent used in the production of industrial silica requires following performances. First, its chemical composition requires high fixed carbon content, low water content, right volatility and low ash content (ash content<5%, iron content< 0.2%). Second, it should have considerable mechanical strength. Third, right particle-size composition is nessary. Last, it should have high resistivity and chemical reactivity. Two kinds of raw materials were used in this paper. A kind of material was bituminous coal which was provided by surrounding Yunnan province, and the other kind of material was the powders that was produced a large amount of powders during processes of transit, crush and cleaning (short for powders). The performances of pelletizing were reflected by compressive strength of fresh pelletizing and dry pelletizing, the shatter strength, the heat intensity and porosity except chemical position.Ash content and Fe content of bituminous coal was 16.09 and 1.44% respectively, so it should be preprocessed. The chemical composition of powders could meet the requirements of carbonaceous reducing agents, and it was no need to pre-process powders. First, pre-treatment of bituminous coal and the choice of raw material was studied. The processes of flotation and acid leaching were chosen to take off ash and impurities of the bituminous coal. The results show that:when three roughings and two cleanings processes were adopted, the best dosage was 120g/t if terpenic oil was used as forther, the best dosage was 600g/t if diesel oil was used as collector, the best dosage was 2300g/t if terpenic oil was used as depressant. Under the optimum process parameters, the ash content was 4.68% and Fe content was 0.32%. Thus it could be seen that it was not satisfied with the requirements of chemical composition. Acid leaching was a necessary process because of high iron content. Effects of these factors such as acid kinds, acid concentration, temperature, time and liquid-solid ratio on ash content and leaching ratio of iron were studied in the experiment. The optimum conditions were as follows:hydrochloric acid concentration was 4mol/L, the leaching temperature was 60?, leaching time was 60 min, liquid-solid ratio was 5:1. Under these conditions, the ash content was 3.89% and Fe content was 0.17%. Its chemical composition could satisfy the requirements. The pelletizing was prepared by the above materials respectively, through comparing the performances of two types pelletizing, powers was better than bituminous coal when they used.Second, the optimum technological parameters were obtained in the forming process when R binder was used. The forming pressure was 25MPa, the concentration of caustic soda solution was 4.8mol/L, and its content was 8.0%, the content of silica fume was8.0%, the best proportion of anthracite:Petroleum coke:charcoal was9.3:8.8:1, the maximum particle size range was 3.3mm?4.75mm which ration was 60%; R binder content was 3.5%. Under these conditions, compressive strength of fresh pelletizing was 698.ON, compressive strength of dry pelletizing was 6985.ON, the shatter strength was 93.68%, the heat intensity was 55.25%, porosity was 14.69%. Production practice have proved that all of the performances of pelletizing should be improved.Third, effects of binders such as asphalt, NaHA, sodium lignin sulfonate, sodium silicate solution and different kinds of starch on performances of pelletizing were studied. The results showed that best performances except porosity were obtained when starch D was used as binder. Its optimal dosage was 1.96%, under these conditions, compressive strength of fresh pelletizing was 873.2.ON, compressive strength of dry pelletizing was 8091.4N, the shatter strength was 99.21%, the heat intensity was 94.44%, porosity was 5.86%. These performances except porosity could satisfy the production requirements, therefore, it was necessary to study the effects of pore-forming agent of biomass-free such as hollow float beads and calcium oxide and pore-forming agent of biomass such as rice husk, corn straw, wheat straw, rice straw and wood chips on pelletizing performances further. The results showed that pelletizing performances could meet the production requirements when pore-forming agents were used except hollow float beads. Rice straw was used as pore-forming agent and the content was kept in 2.5%, the maximum particle size was 4.75mm, module was 0.5, under these conditions, compressive strength of fresh pelletizing was 4700.2N, compressive strength of dry pelletizing was 9536.1N, the shatter strength was 99.07%, the heat intensity was 96.21%, porosity was 30.78%.Mechanism of effects of factors on pelletizing was studied in moulding process, and the factors contained water content, wetting agent and binder, pore-forming agent, forming pressure and composition of particle sizes. The results showed that caustic soda was able to react chemically with starch D, adhesion force between powders and binder was improved that was influenced by the new chemical bond. Pelletizing moulding was the results of mechanical adhesion and physical-chemical adhesion between binder and particles, and mechanical adhesion played more important role in the process. Porosity of pelletizing was improved effectively because rice straw density was small and easy to deformation. The composition of particle size accorded with GGS equation, the best performances of pelletizing was obtained.Last, the pelletizing with best performances was placed into Semi-closed rotary submerged arc furnace which power was 25.5MVA. Through analysis of industrial production data, it could be seen that pelletizing performances could satisfy production requirements, the best substitution amount was 80%.