Research on the technology of drying high moisture molybdenum concentrate powder with waste heat of roasting kiln

Molybdenum concentrate oxidation roasting is an important part of molybdenum metallurgy process. Aiming at the current situation of high energy consumption in the process of oxidation roasting and drying of molybdenum concentrate powder in China, based on the analysis of traditional mechanism and process of molybdenum concentrate roasting and heat transfer in rotary kiln, the heat is adjusted and a new rotary kiln drying system is designed to make 16-20% high-moisture molybdenum concentrate powder once dried. Drying each ton of molybdenum concentrate powder can save about 308125kJ of heat, equals toabout 14.38M gas or 10.53kg standard coal.


Introduction
Molybdenum is a refractory metal with a melting point as high as 2620°C.Its metal and alloy materials exhibit excellent high-temperature mechanical properties, physical properties, corrosion resistance and good thermal shock resistance, which are widely used in high-temperature structural and functional materials, play a crucial role in various fields, including industry, agriculture, biomedicine, aerospace, and nuclear industry, as a nonrenewable strategic resource [1][2][3][4][5][6][7] .There are various molybdenum minerals, yet only four of them possess industrial significance: molybdenite (MoS2), calcium molybdate (CaMoO4), ferromolybdenite (Fe2O3-3MoO3-7H20) and lead molybdate (PbMoO4).Among them, molybdenite is the most abundant and has the highest industrial value, accounting for almost 90% of molybdenum mining volume.
The processing technique of molybdenum pyroxene has three widely-used industrial applications: (1) Oxidation roasting, which yields industrial molybdenum oxide used in molybdenum iron smelting.
The roasting reaction of molybdenum concentrate is a strong exothermic process, and the reaction gas has high temperature and strong corrosiveness, so the requirement of water content of molybdenum concentrate in rotary kiln is no more than 3%, while the water content of molybdenum concentrate is generally 16-20% in beneficiation and filtration of molybdenum concentrate in molybdenum mine companies.At present, the production process of roasting molybdenum concentrate in China is as follows: firstly, obtain wet molybdenum powder with moisture content of 16%-20% in molybdenum mines, and then dry the wet molybdenum powder for the first time to commercial molybdenum concentrate with moisture content of 6%-8% by using gas heating, and then dry commercial molybdenum concentrate for the second time through the residual heat drier of rotary kiln in the roasting plant to obtain molybdenum concentrate with moisture content of less than 3%, and then send it to the roasting kiln to get the molybdenum oxide final product after roasting.

Fig. 1 Schematic diagram of rotary furnace roasting system
In this paper, through analysis and research of the molybdenum concentrate roasting mechanism, the traditional rotary kiln heat transfer and process, a new type of drying system with utilization of roasting kiln waste heat is designed.Through heat deployment, the current two drying processes of molybdenum concentrate are integrated together, drying high moisture molybdenum concentrate at a time with the use of waste heat from the roasting kiln , which greatly reduces energy consumption and the investment in equipment.

Mechanism of Molybdenum concentrate roasting
Molybdenum pyroxene concentrate undergoes a series of chemical reactions during the oxidation roasting process.These chemical reactions can be categorized into the following four groups: (1) Oxidation of molybdenite to molybdenum trioxide; (2) Interaction between molybdenum trioxide and molybdenite; (3) Oxidation of sulfides of associated elements (iron, copper, zinc, etc.) to produce oxides and sulfates; (4) Interaction of molybdenum trioxide with impurity oxides, sulfides, and sulfates to produce phthalates.The total reaction for oxidative roasting of molybdenite is: This reaction is strongly exothermic: This is a complex gas-solid multiphase chemical reaction, which usually starts to occur at about 400°C.The temperature increases, and the oxidation reaction is rapidly enhanced in air and a large amount of sulfur dioxide gas is produced.In the Mo-S-O system, the reaction is very complex and different reactions may occur under different conditions.
From the thermodynamic data, it follows that, 2MoS2(s)+7O2(g)=2Mo03(s)+4S02(g)   0 /P P -1 The reaction of equation (1.3) is strongly exothermic.At room temperature:T=298K, ΔG01=2008.93kJ/mol,which means that the reaction can be carried out at room temperature, and once the roasting reaction is carried out, it does not require an external supply of heat.

Analysis of traditional rotary kiln heat transfer process and test of roasting thermal
Heat transfer process in rotary kiln mainly includes three parts: kiln, kiln wall, and kiln outside.Rotary kiln heat transfer process mainly includes: gas, materials and kiln wall convection, radiation heat transfer process and due to the chemical reaction in the kiln caused by the heat absorption and exothermic process.The heat transfer process inside the kiln wall is mainly the conduction of heat from the inner surface of the kiln wall to the outer surface.The heat transfer process outside the rotary kiln is the dissipation of heat to the environment by convection and radiation [7] .
In order to understand the energy situation in the traditional molybdenum concentrate roasting systemand to find the problem, a thermal test was did in the 1 # rotary kiln of a high-temperature new materials limited company in Rizhao, using the heat balance analysis method to establish heat balance in the rotary kiln, as shown in Table 1.Calculated by the heat balance, in the rotary kiln of a high-temperature new materials Co., Ltd.In Rizhao , the vast majority of the expenditure of heat from the flue gas out of the flue gas, flue gas out of the heat accounted for 47.4% of the total heat consumption.From the roasting system (Figure 1) , it can be seen that in order to control the gas temperature of this part of the flue gas into the dust collector tail no more than 160 ℃, the system is set up with a special radiator.Therefore, on the basis of ensuring the kiln temperature can meet the level of roasting process, it is important to take effective measures to recover the heat from the roasting flue gas to save energy and reduce the consumption of rotary kiln, and there is a lot of potential to be explored.

Analysis of traditional rotary kiln roasting process
From the analysis of the heat balance test of 1# rotary kiln rotary kiln of a high temperature new materials limited company in Rizhao, although its energy consumption per unit of product and molybdenum roasting product quality level ranked the forefront of the domestic market, the actual roasting process of molybdenum concentrate and molybdenum concentrate roasting mechanism and thermodynamic trends are still contradictory.On the one hand, the roasting rotary kiln produces a large amount of high temperature exhaust gas, which needs to be dissipated through the radiator, and on the other hand, it is impossible to dry the molybdenum concentrate with high water content.The root cause of this contradiction is: ① Insufficient heat exchange capacity of rotary kiln roasting kiln headbox cannot provide the heat required for drying high moisture molybdenum fine powder; ② High moisture molybdenum concentrate is very easy to slate, high moisture molybdenum concentrate using the existing conveyor is easy to clog, dryer airflow design can not meet the drying requirements.Therefore, it is necessary to make full use of the metallurgical gas parameters of molybdenum concentrate roasting, systematically analyze the heat balance and material balance of the roasting process from a new perspective, strengthen the joint research on roasting process and equipment, improve the design of rotary kiln, so as to make it compatible with the temperature and heat regimes of the molybdenum concentrate roasting reaction, so as to reduce the energy consumption and pollutant emissions.

Design of the molybdenum concentrate drying system
In order to solve the above problems, this paper on the roasting kiln waste heat drying molybdenum concentrate heat transfer, feeding mechanism, dryer internal and so on was redesigned, before and after the design of the molybdenum concentrate drying system flow is shown in Figure 2.

Design of the kiln headbox for molybdenum concentrate drying system
The tail gas from the roasting of molybdenum concentrate has a temperature of about 500°C.It is discharged through the exhaust port set in the kiln headbox and is desulfurized after dust collection.The roasting tail gas carries a large amount of heat, if it can be fully utilized, can meet the molybdenum concentrate drying heat needs.Accordingly, the design is to install a high-efficiency heat exchanger in the kiln headbox to improve the utilization rate of waste heat.
Kiln head box is a hollow rectangular box structure, in the kiln head box opened a heat exchange chamber, so that the room temperature gas can be in the heat exchange chamber and the kiln head box in the high temperature exhaust heat exchange, and through the multi-layer wind baffle plate and wind guide plate with, so that the heat exchange chamber in the vertical direction of uniform separation into a plurality of horizontal direction around the inner cavity of the box heat exchange air ducts, and through the wind guide plate in the adjacent two heat exchange air ducts between the role of the guide, so that cold air can be in a plurality of heat transfer ducts in the same direction around the box inner cavity flow, and then in turn into the upper part of the heat exchange duct, so that cold air in the chamber has sufficient flow space and heat transfer time.Can be in a number of heat exchange air ducts in the same clockwise direction around the box body cavity flow, and then flow into the upper heat exchange air ducts, so that the cold air in the heat exchange cavity has sufficient flow space and heat exchange time, to ensure that the cold air and the heat exchange cavity sidewalls are fully in contact with the cold air and the high temperature exhaust gas can be fully heat exchange, improve the heat exchange efficiency, so that the roasting kiln high temperature exhaust in the residual heat of the energy to be effectively utilized.

Design of feeding mechanism of molybdenum concentrate waste heat dryer
Conventional waste heat dryer use screw conveyor to transport wet molybdenum powder to the dryer cylinder, and the wet molybdenum powder is more prone to the phenomenon of slate, shed material, etc. under the higher humidity, , so the wet molybdenum powder with humidity of 16% to 20% obtained directly from the molybdenum mines is easy to be consolidated and clog the screw conveyer when conveyed by the screw conveyor .To solve this problem, the technical solution adopted here is (as shown in Fig. 3 and Fig. 4): the hopper used for storing wet molybdenum powder is glued with anti-adhesive material inside, the bottom of the hopper is equipped with a feeding pipe connected to the side wall of the cylinder, a screw conveyor is installed in the feeding pipe, a bulk material shaft is installed between the bottom opening of the hopper and the screw conveyor, the bulk material shaft and the screw conveyor are both set horizontally, and the axle of the bulk material shaft is aligned with that of the screw conveyor in the vertical direction.The axis of the bulk material shaft is aligned with the axis of the screw conveyor in a vertical direction, and the bulk material shaft is provided with a plurality of bumps, through which the bumps rotate with the bulk material shaft so as to disperse the wet molybdenum powder falling from the bottom opening of the hopper, and the plurality of bumps are divided into a plurality of rows in a circumferential direction along the bulk material shaft, and the number of bumps in each row is the same, and the same row of bumps is set in uniformly spaced rows along the length of the bulk material shaft.

Design of molybdenum concentrate waste heat dryer
In the traditional waste heat dryer, the hot gas enters directly from the bottom of the cylinder body, so that the flow effect of the hot gas in the inner cavity of the cylinder body is poor, and it is difficult to form a spiral air field with high circumferential flow rate, so that the drying effect of the hot gas on the wet molybdenum powder is poor, which restricts the drying capacity of the waste heat dryer.The technical solution adopted in this paper is (as shown in Fig. 5 and Fig. 6): the cylinder body is equipped with an air inlet tube, the air inlet tube is located below the feed pipe, the axis of the air inlet tube coincides with the axis of the cylinder body, the inner wall of the air inlet tube is fitted with the outer wall of the cylinder body to form an annular hot air distribution room, the air inlet tube is connected to an air inlet tube, the axis of the air inlet tube is set horizontally and is tangent to the inner wall of the air inlet tube, the air inlet tube is connected with the residual heat of the kiln head heat exchanger box The air inlet pipe is connected to the waste heat gas outlet of the kiln head heat exchanger box, so that the waste heat gas can flow into the hot air distribution room along the tangential direction and flow around the outer wall of the cylinder body within the hot air distribution room, and the bottom end of the cylinder body is set as an opening, and the edge of the bottom end of the cylinder body is matched with the base plate of the air inlet cylinder to form an annular air inlet slit, so that the waste heat gas within the hot air distribution room can flow into the bottom of the cylinder body along the tangential direction through the annular air inlet slit, and compared to the prior art in which the hot gas flows into the cylinder body from the bottom of the cylinder body, the waste heat gas in the kiln head heat exchanger box can flow into the cylinder body directly.Compared with the existing technology in which the hot gas flows directly from the bottom of the cylinder, the flow effect of the residual heat gas in the cylinder body cavity is improved, and a spiral gas field with significantly increased circumferential gas velocity is formed, so that the residual heat gas can dry the wet molybdenum powder more efficiently during the flow process, and the drying capacity of the residual heat dryer is improved.At the same time in the air inlet cylinder on the bottom plate is provided with a drive shaft, the axis of the drive shaft and the axis of the cylinder body overlap, the drive shaft is equipped with a stirrer, the stirrer is located in the inner cavity of the cylinder body and is located in the cylinder body near the bottom of the end of the air inlet cylinder is installed on the outside of the motor used to drive the drive shaft to rotate, so that the drive shaft can be driven by the stirrer to rotate around the axis of the cylinder body, in order to avoid the accumulation of the wet molybdenum powder at the bottom of the cylinder body.

Application effects and economic benefits
New rotary kiln roasting drying system applied in a high-temperature new materials Co., Ltd in Rizhao successfully, compared with the traditional rotary kiln roasting drying system (see Figure 7), the loss of heat of the flue gas out is reduced by about 3%, which exactly meet the heat requirement of drying the original wet molybdenum powder with moisture content of 16% to 20% in the mining company to the moisture content of 6% to 8% of the commercial molybdenum powder by using gas heating.
With this technology, drying each ton of molybdenum powder can save heat about 308125kJ, which equals to about 14.38M 3 gas or 10.53kg standard coal.According to the annual output of 50,000 tons of Rizhao company, 719,000M 3 gas or 526.5 tons of standard coal can be saved annually, which directly saves the annual cost of about 251.65 million yuan for the company .

Conclusion
(1) Utilizing only the reaction heat of molybdenum concentrate for molybdenum concentrate drying, eliminating the external heat source under production conditions; (2) In order to realize reasonable deployment and flexible control of the temperature in the rotary kiln, the heat exchange grill of roasting kiln head box is designed to heat exchange the room temperature gas to 120℃-160℃ and directly send it to the waste heat dryer; (3) Conveying equipment is designed to quantitatively feed high moisture molybdenum fine powder into the waste heat dryer and fully mix the hot gas with the material in the waste heat dryer to achieve the best drying requirements; (4) The internal structure of the dryer is designed so that the dried materials are automatically discharged under the drive of the hot gas flow, and the exhaust gas is treated by the dust collector and discharged according to the standard.

Fig. 2
Fig. 2 Flow chart of molybdenum powder drying system

Fig. 3
Fig. 3 Structure diagram of molybdenum powder dryer

Fig. 5
Fig. 5 Structure diagram of drying mechanism of molybdenum fine powder

Fig. 7
Fig.7 Comparison of hot income and expenditure items before and after renovation

Table 1
Heat balance of payments in rotary kiln