Thermodynamic Study on Direct Hydrogen Reduction Process of Molybdenite
Molybdenum is an important non-ferrous metal, widely used in many fields of modern technology. The industrial production route of metal molybdenum is as follows: firstly, the molybdenum concentrate is calcined at 600-650 ° C to form molybdenum oxide, then the molybdenum oxide is purified, and finally the metal molybdenum powder is obtained by reduction with H 2 . However, there are three major problems in the process: 1. The oxidized roasting process of molybdenum concentrate releases a large amount of sulfur dioxide, which pollutes the environment seriously; 2. The molybdenum loss is more serious during the oxidative roasting and further purification of molybdenum ore; 3, MoO 3 It has been significantly sublimated at 600 ° C, and its vapor pressure reaches 0.101 MPa at 1151 ° C. Therefore, MoO 3 must be reduced to stable MoO 2 with hydrogen at a lower temperature (450-650 ° C), and then MoO 2 is Reduction to metal molybdenum at 900 to 950 °C. The process is long and the operation is complicated. Therefore, some researchers are trying to find a new metal molybdenum extraction process. These studies have made a useful attempt to open up a new metal molybdenum production process route, but the research focuses on the reaction kinetic mechanism analysis, the lack of systematic thermodynamic analysis, and the difficulty in selecting the test temperature of the new process route and other working conditions. The authors have conducted thermodynamic analysis of the molybdenum ore and the vacuum non-oxidation roasting route. The use of hydrogen as a clean energy source can solve the problem of product gas emissions. Therefore, this paper will discuss the feasibility of several new technologies for the reduction of molybdenum ore by hydrogen reduction without SO 2 gas emissions through thermodynamic analysis. 1. Direct hydrogen reduction of molybdenum concentrate Without the sulfur-fixing agent, molybdenum ore is directly hydrogen-reduced, and its reaction equation is: Calculate the relationship between temperature and system gas composition at equilibrium, and get the equation: Where: T is the reaction temperature (K); V (% H 2 S) and V (% H 2 ) are the volume percentages of H, S and H 2 in the equilibrium gas composition, respectively. It is known from the formula (1) that the reaction is intended to be carried out within a feasible temperature range, and the PH 2 S/PH 2 ratio must be controlled to a small range so that the reaction Gibbs free energy is less than zero. For example, the reaction is intended to be carried out at temperatures of 1100 K and 1300 K, respectively, and the ratio must be controlled to be lower than 1.06 × 10 -3 and 6.15 × 10 -3 , respectively . It can be seen that the reaction of directly reducing the molybdenum concentrate with H 2 is difficult to carry out without using a sulfur-fixing agent. When the reaction is balanced, the H 2 content in the gas component is high. In actual production, the reaction is far from equilibrium, which results in a higher H 2 content in the gas composition. Therefore, in order to reduce the molybdenum concentrate with H 2 at a feasible temperature, it is necessary to add a sulfur-fixing agent. Second, calcium oxide as a sulfur-fixing agent hydrogen reduction iridium ore Calcium oxide is used as a sulfur-fixing agent, and the hydrogen reduction molybdenite reaction equation is: Where: ηH 2 is the hydrogen utilization rate; V (% H 2 O) and V (% H 2 ) represent the volume percentages of H 2 O and H 2 in the equilibrium gas composition, respectively. Calculate the relationship between temperature and HZ utilization at equilibrium. Get the equation: According to the formula (3) and the formula (5), Fig. 1 can be obtained. It can be seen that when the temperature is less than 1200 K, the H 2 utilization rate increases rapidly with the increase of temperature, and then the growth rate tends to be moderate, indicating that CaO is added as a sulfur-fixing agent ratio during the H 2 reduction of molybdenum ore. The reaction is easy without the addition of CaO, and the use of CaO as a sulfur-fixing agent can fix the sulfur in the form of stable CaS, and the gaseous product is non-polluting H 2 O, which avoids the pollution of the toxic gas H 2 S. Fig.1 Relationship between H 2 utilization rate and temperature of calcium oxide as sulfur-fixing agent Third, sodium carbonate as a sulfur-fixing agent (1) Change law of gas composition in hydrogen reduction process Sodium carbonate is used as a sulfur-fixing agent, and the reaction equation for molybdenum hydrogen reduction is: Under constant pressure, the relationship between the temperature of formula (6) and the volume fraction of equilibrium gas is shown in Fig. 2; the relationship between pressure and volume fraction of equilibrium gas at constant temperature is shown in Fig. 3. Figure 2 Temperature and equilibrium gas composition Figure 3 Relationship between pressure and equilibrium gas composition It can be seen from the Gibbs free energy that the reaction is an endothermic reaction. Under the same pressure, the equilibrium constant increases with increasing temperature, which causes the content of reducing agent H 2 in the equilibrium gas component to gradually decrease. , while the H 2 O and CO gas contents increase. It can be seen from Fig. 2 that when the pressure is 0.1 MPa, the temperature is 1073K and l173 K, the corresponding volume percentage of H 2 is 78.8% and 61.0%, H20 is 14.1% and 26.8%, and CO is 7.1%. And 13.10%. It can be seen from the reaction formula that when the temperature is constant, as the pressure is lowered, the volume fraction of H 2 O and CO in the atmosphere increases, and the volume fraction of H 2 decreases. It can be seen from Fig. 3 that when the temperature is 1173K and the pressure is 0.01 MPa and 0.001 MPa, the volume percentage corresponding to H 2 is 38.5% and 18.6%, H 2 O is 41.0% and 54.3%, and CO is 20.5%. And 27.1%. (2) Influence of temperature and pressure on gas utilization rate Under normal pressure, the reaction starting temperature of formula (6) is 1380K, the product is metal molybdenum and sodium sulfide, sodium sulfide is soluble in water and metal molybdenum is insoluble in water. In addition, the main impurity SO 2 in molybdenite is also easily reacted with sodium carbonate to form sodium silicate, which is also soluble in water. Therefore, pure metal molybdenum powder can be obtained by a water washing method. The equilibrium constants are 2.0×10 -5 and 1.6×10 -3 at 1100K and 1200 K, respectively, and the equilibrium constant is small, indicating that the H 2 content in the gas product is high and the H 2 utilization rate is low. Because the reaction is affected by temperature and pressure at the same time, when the pressure is constant, the equilibrium constant increases and the H 2 utilization rate increases when the temperature rises; when the temperature does not change, the pressure balance decreases, the reaction equilibrium shifts to the right, and the H 2 utilization rate Increase. Therefore, the relationship between temperature and H 2 utilization rate under different pressures is now calculated. Get the equation: According to equations (6) and (7), Fig. 4 can be obtained. Since the melting point of Na 2 CO 3 is 1131 K, when the temperature exceeds the melting point of Na 2 CO 3 , the reaction will have a liquid phase formation, which will stratify and be unfavorable to the reaction, so the reaction is generally carried out at a temperature below the melting point of Na 2 CO 3 . . However, if the temperature is chosen too low, the reaction rate will be slow and the H 2 utilization rate will be low. Under normal pressure, the equilibrium utilization of H 2 is only 15.2% at a temperature of 1073K. And when the pressure drops to. At 0.001 MPa, the equilibrium utilization of H 2 at 1073 K can reach 51.3%. Figure 4 Temperature and H 2 utilization rate under different pressures As can be seen from Fig. 4, at the same temperature, when the pressure is lowered from 0.05 MPa to 0.01 MPa and from 0.005 MPa to 0.001 MPa, the H 2 equilibrium utilization rate is rapidly increased. Fourth, the conclusion (1) The direct hydrogen reduction reaction of molybdenum ore is difficult to carry out without using a sulfur-fixing agent; (2) Using CaO as a sulfur-fixing agent, the hydrogen reduction reaction of molybdenum ore is easier to carry out than when no CaO is added, and the H 2 utilization rate can reach 50%, 60% and 67.7%, respectively, at 1100K, 1200 K and 1300 K; (3) Using Na 2 CO 3 as a sulfur-fixing agent, the molybdenite hydrogen reduction reaction product can be washed with water to obtain pure molybdenum powder. The hydrogen utilization rate is affected by temperature and pressure. As the temperature increases, the hydrogen utilization rate rises rapidly; as the pressure decreases, the hydrogen utilization rate increases.
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