| Molybdenum(Mo)is one of the most widely used refractory metals.Because of its attractive physical and chemical properties such as high melting point,high elastic modulus,great high temperature strength,superior thermal conductivity,excellent resistance to corrosion and low coefficient of expansion,molybdenum and its compounds are extensively utilized in iron and steel,nuclear energy and other fields.Particularly,the application of molybdenum in the iron and steel industries accounts for about 80%.The addition of Mo to steel can significantly improve its hardenability,toughness,high temperature strength and creep performance.Mo is added to iron and steel in the forms of molybdenum additives,including molybdenum powder/bar,ferromolybdenum,molybdenum trioxide block,calcium molybdate,carbon-reduced molybdenum.The preparation of these five molybdenum additives all requires the oxidation roasting of molybdenum concentrate(in China,molybdenum is mainly endowed in molybdenite ore,molybdenum concentrate can be obtained after beneficiation of molybdenite ore,and the main component of molybdenum concentrate is MoS2).However,in order to ensure that Mo in MoS2 is converted into MoO3 as much as possible,it is necessary to continuously introduce a large amount of air to guarantee a high oxygen partial pressure,resulting in a low SO2 concentration in the generated flue gas.Therefore,it is particularly urgent to develop a green and short process to extract molybdenum from molybdenum concentrate.At the same time,the development of steelmaking processes and steels such as carbon-containing highspeed steels has made it possible to use molybdenum carbide(Mo2C)with a C content of about 5.89%as a steelmaking additive.Molybdenum silicides have recently attracted great interest in various applications such as the advanced aerospace,electrical heating element,and high temperature structural material owing to their good electrical conductivity,ultrahigh melting point and well oxidation resistance.Popular routes to prepare molybdenum silicides are mechanical alloying and self-propagating high temperature synthesis,both of which use pure Mo and Si as raw materials,with high cost.Therefore,it is of great significance to develop a green-short process to prepare molybdenum silicides by silicothermic reduction of molybdenum concentrate.Firstly,this manuscript supplements the problems existing in the existing CaOassistant carbothermic reduction of MoS2.Secondly,based on the concept of "shortprocess Mo extraction and green desulfurization",a novel route for preparing Mo.Mo2C or their mixture by desulfurizer coating-carbothermic or Si/C synergistic reduction of molybdenum concentrate was proposed,and the key mechanism was studied.Secondly,a novel process for the preparation of ultrafine MoSi2 powder via silicothermic reduction of MoS2 with CaO as the desulfurizer was developed.and the process was successfully extended to the preparation of ultrafine tungsten silicides powder.The following research results were obtained:(1)Mo powder with a C content of about 0.18%was prepared by lime-assisted carbothermic reduction of MoS2,and the reaction mechanism of lime participating in the carbothermic reduction of MoS2 was determined for the first time,when the temperature was between 800 and 1200℃,CaO preferentially reacted with MoS2 to form CaMoO4,Mo and CaS.The resulting CaMoO4 then reacts with the carbon black to form Mo or Mo2C.The reaction between CaO and MoS2 is the key factor to reduce the reaction temperature of carbothermic reduction of MoS2.However,when desulfurizer CaO was mixed with raw materials(composed of MoS2 and carbon black),the generated Mo or Mo2C was mixed with the desulfurization product evenly and is hard to separate green.(2)The goal of short-flow-green preparation of Mo and Mo2C was achieved by carbothermic reduction of molybdenum concentrate with the mixture of lime and carbon black as the desulfurizer.The sulfur-containing gas generated between reducing agent and molybdenum concentrate can be completely captured by the wrapped desulfurizer in the form of CaS,and the target product can be obtained by separating the desulfurization product layer.Both thermodynamic calculations and experimental results show that the main gaseous product of carbothermic reduction of MoS2 is CS2.When the raw material(50 g)with a MoS2:C molar ratio of 1:1.52 and reacted at 1600℃ for 4 hours,the residual S content of the prepared sample was about 0.33%,which was higher than the upper limit of S content in ferromolybdenum(0.15%).Therefore,it is necessary to further reduce the residual S content of the product.(3)Based on the desulfurization strategy of desulfurizer coating,a novel route to prepare molybdenum additives by silicon/carbon synergistic reduction of molybdenum concentrate was proposed to further reduce the residual S content of the product.The S and Si contents of the prepared Mo2C or the mixture of Mo2C and Mo in the scale-up experiment(100 g of raw material reacted at 1600℃ for 4 hours)both meet the requirements of ferromolybdenum national standard.When the molar ratio of MoS2:C:Si in the raw material was 6:6:4,the C,S and Si contents of the product were 5.33%,0.04%and 0.35%,respectively.When the molar ratio of MoS2:C:Si was 6:4:3.2,the C,S and Si contents of the product were 2.19%,0.13%and 0.23%,respectively.When the molar ratio of MoS2:Si in the raw material is between 1:0 to 1:2,MoSi2 is preferentially formed,and MoSi2 will react with MoS2 to form Mo5Si3,Mo3Si or Mo with the increase of reaction temperature.At the same time,C will participate in the reaction to generate Mo2C or Mo.When the molar ratio of MoS2:Si is between 1:0 to 1:4,the S-containing product generated by silicothermic reduction of molybdenum concentrate is a mixture of SiS and SiS2.not just SiS.(4)Ultrafine molybdenum silicide powder was successfully prepared at 1000～1200℃ by lime coating-silicothermic reduction of high-purity MoS2.The asprepared MoSi2 powders inherit the sheet-like morphology of MoS2 and consists of a large number of ultrafine grains(about 300 nm).The equilibrium conditions of SiS-SiS2 were clarified for the first time through thermodynamic calculations and experiments.The reaction between MoS2 and Si should be controlled by the interfacial chemical reaction between gaseous SiS and MoS2,and the activation energy is about 179 kJ/mol.The reaction between CaO and SiS is a strong exothermic reaction,which makes the separation of molybdenum silicide and desulfurization products difficult in scale-up experiments.This problem can be solved by placing graphite powder or carbon felt between the desulfurizer and the sample.In addition,MosSi3 and Mo3Si powders were successfully prepared by adjusting the temperature and the ratio of raw materials.Moreover,the S element in the desulfurization product CaS was converted into an economical FeS product,realizing the cycle of S removal and recovery.(5)Ultrafine tungsten silicides powder was obtained at 1100～1200℃ by silicothermic reduction of high-purity WS2.The as-prepared WSi2 and W5Si3 powders inherit the morphology of raw material WS2 and consist of a large number of ultrafine grains.The reaction between WS2 and Si was controlled by the interfacial chemical reaction between SiS and WS2,and the activation energy is about 184 kJ/mol. |
PDF Full Text Request