Research On Calcination And Reduction Of Pre-prepared Pellets For The Novel Silicothermic Process

Magnesium and its alloys are widely used in automobile industry,electronic components,metallurgical industry,chemical engineering and other fields because of special chemical,physical and mechanical properties.The energy structure and market requirement in our country promotes the wide application of Pidgeon process domestically.Silicothermic process has become the main technology for magnesium production from dolomite.A novel incorporate silicothermic process was put forward in this paper.Dolomite,ferrosilicon,fluorite and binder are mixed to produce pre-prepared pellets in the new process and the pre-prepared pellets are used directly for highefficiency and low-energy magnesium production through calcination-reduction integration process.Calcination and reduction thermodynamics,calcination kinetics,vacuum reduction experiments and isothermal reduction kinetics of pre-prepared pellets were studied in this paper based on the new silicothermic process.(1)Thermodynamic calculation indicates that the combining stable phase of silicon oxide and calcium oxide is dicalcium silicate.Si-Fe can hardly reduce calcium oxide to generate calcium vapor in experimental conditions while calcium disilicide is produced readily.The reducibility of calcium is stronger than that of Si-Fe.When the generated CaSi2 is further heated,the gibbs free energy of calcium disilicide reducing magnesium oxide was lower than that of Si reducing magnesium oxide solely.Slagging reaction can decrease the gibbs free energy of reduction reaction when adding calcium oxide.(2)The DTA and TG curves of pre-prepared pellets and pure dolomite decomposition moved to high temperature region with the increase of heating rate.The decomposition temperature of pre-prepared pellets were lower than that of pure dolomite at all heating rates due to the higher thermal conductivity of reductant ferrosilicon alloy.(3)Calcination kinetics of pre-prepared pellets and pure dolomite were studied in two stages.The values of the activity energy for the two stages decomposition of dolomite are 270kJ·mol-1 and 280kJ·mol-1 respectively in pre-prepared pellets.And the values of the activity energy of pure dolomite were more than 350 kJ·mol-1 for the two decomposition stages.(4)The carrier gas flow rate had no effect on decomposition temperature,but the heat absorption on the two endothermal peaks decreases with the increase of carrier gas flow rate for pre-prepared pellets.The porosity of calcined pre-prepared pellets is higher than initial pellets.(5)Improving reduction temperature,prolonging reduction time,increasing silicon stoichiometric amount and reducing calcination time are effective ways to improve the reduction efficiency of pre-prepared pellets.When reduction temperature is 1573K,reduction time is 3h and silicon stoichiometric amount is 1.15,the reduction efficiency reaches 92.26%.Meanwhile,crystallinity of calcium silicate is higher with higher reduction efficiency.(6)The mechanism function D3 is the best one to explain the reduction experimental results of pre-prepared pellets.The rate constant and apparent activation energy increases with the increase of silicon stoichiometric amount.The apparent activation energy reaches 388.38kJ/mol when the silicon stoichiometric amount is 1.15.(7)The reduction efficiency of pre-prepared pellets increases with the increase of the carrier gas flow rate and the decrease of heating rate under the condition of Ar atmosphere protection.