Research On The Basic Theory Of Microwave Oxidation Roasting Of Molybdenite

Microwave heat energy has various advantageous performances.Based on the principle of microwave-matter interaction,this paper discussed respectively the thermal decomposition characteristics,the phase transition and the dielectric properties of molybdenite,and the intensity distribution of the electromagnetic field and the temperature field in the molybdenite material during microwave heating and heat source heating was studied by means of finite element numerical simulation.The temperature rising behavior of the molybdenite in the microwave field was investigated.Molybdenum oxide products obtained from oxidation roasting at different temperatures were characterized for their composition and morphology.Main study discussions as follows:(1)In the experimental range of 0.9~1.4g/cm~3,the dielectric constant,dielectric loss coefficient and loss tangent of molybdenite increase with the increase in its densities;in the range of 25~500°C,the dielectric constant of molybdenite increase slowly in an increase of temperature before 300°C,and rises sharply after that.However,the dielectric loss coefficient and loss tangent increase slightly with the increase of temperature before 300°C and show a little of fluctuations,then increase sharply.Microwave penetration has a decrease trend with the increase of densities and temperatures,from 16.28cm to 8.51cm and from 16.3cm to 4.4cm,respectively.The penetration depth shows a slow upward trend with the increase of dielectric constant,but shows an outstanding one with the increase of dielectric loss.(2)From the single-module or multi-field coupling analysis conducted in Ansys Mutiphysics,we can know that a rectangular Cartesian coordinate system can be matched with the vertical relationship among the electric vector,the magnetic vector and the propagation direction of microwave in a material.The heating uniformity of molybdentie material heated by microwave which is transmitted in a rectangular waveguide is better than that of the circular,but the temperature rise rates brought from circular waveguide is bigger than that of the rectangular,and the heating efficiency produced by circular waveguide is better than that of the rectangular.The temperature rise rates of materials increase in the increase of radius of waveguide and the thickness of materials.During heat conducting process,the heat energy is gradually transferred from the external heat source to the interior of material.Conversely,the heat energy is generated inside the material and diffused gradually outwards.The heating uniformity of material through microwave is better than that of heat source,and corresponding heating rate is also higher.(3)Molybdenite has good heating properties by using microwave:it can be heated from the room temperature above to 1000°C within 20min through 0.9kW microwave power for 100g material mass;the temperature rise rates increase in the increase of microwave power and material thickness,but decrease with the increase of material mass.From the characterizations for the molybdenum trioxide obtained at different roasting temperature by SEM,XRD and EDS,it can be known that the molybdenum trioxide with high-purity and good crystallinity has been obtained at the condition of 800°C;it is feasible to prepare molybdenum trioxide by microwave heating molybdenite.(4)The heat analysis curves under different temperature rise rates indicate:molybdenite continues to decompose and lose weight before 500°C,after which the rate of weight loss rise slightly at 15°C/min and 20°C/min.The oxidation reaction rate of molybdenite reaches the maximum in the temperature range of 500~700°C,and the smoothness of TG curve is not good,indicating that the purity of molybdenite sample is not high.The Flynn-wall-ozawa method and the Kissnger method have good linearity to the thermogravimetric analysis data.The activation energy of the reaction shows a first decrease and then an increase with an increase of the conversion rate,and exist a large span for the activation energy trend,which points out the reaction mechanism is not unique,and the oxidative roasting process should compose of multiple and different reaction types,with side reactions and the formation of intermediates.The average activation energies calculated by the two methods are respectively 76.1kJ/mol and 70.4 kJ/mol.The confidence level is high.