Technology and equipment for roasting molybdenum concentrate by rotary kiln

The technology of rotary-kiln roasting molybdenum concentrate has existed for many years in China, which is divided into external combustion and internal combustion. This article deals only with the latter. Early internal combustion rotary kiln roasting molybdenum concentrate technology has been applied in the former Soviet Union (1), a domestic factory uses this technology to produce ammonium molybdate, due to the simple operation technology and supporting equipment, the use effect is not ideal. So the technology is not widely used. In recent years due to the development of enterprises and the requirements of production environment. People have researched and developed the application technology and supporting equipment. And a good application effect has been obtained. The technology of rotary-kiln roasting molybdenum concentrate becomes mature day by day. The technical characteristics of internal combustion rotary kiln roasting molybdenum concentrate are good roasting quality, and the output of a single set of equipment can reach a new level of 6 ~ 10 tons/day. The service life of the equipment is long, which can reach more than a few years. Save energy and facilitate environmental governance. Supporting equipment with a high degree of mechanization can be automatic control, the side dryer a machine with multiple functions of molybdenum concentrate drying, crushing and screening, air conveying materials and other roles. Different combustion devices can be used to heat the rotary kiln with various fuels. Exhaust gas treatment system can recover materials. Purifying harmful gases to produce environmental by-products. Internal combustion rotary kiln roasting molybdenum concentrate engineering technology and supporting equipment, belongs to the domestic advanced technology, advanced molybdenum product processing equipment.
1. Operation technology and parameters of rotary kiln roasting molybdenum concentrate
1.1 Thermal engineering system
Molybdenum concentrate roasting in rotary kiln should fully satisfy the objective law of MoS2 desulfurization process. The heat of chemical reaction of MOS2 in the kiln and the thermal effect of combustion of fuel in the kiln. They work together to form a complete heating system. According to the requirements of the thermal system of roasting molybdenum concentrate, it needs to be divided into three sections, that is, the preheating section, the heating reaction section, and the insulation section (the residual sulfur section). There is a temperature gradient between the temperature of the material molybdenum concentrate in the kiln and the lining of the kiln and the combustion product. Due to the viscosity of the material, the kiln adheres to the material, so the kiln only considers the temperature of the material and the combustion product (including the material heat release) temperature, the temperature difference between the two is 100℃ ~ 150℃, practice has proved that the kiln can be controlled to form three temperature sections, the material temperature preheating section is about 250℃ ~ 400℃. The heating section is about 650℃ ~ 750℃, and the insulation section is about 600℃. Because the heating of internal combustion rotary kiln is characterized by a little heat supply at the discharge end. Good control of the temperature of the insulation section can ensure the relative high temperature of the heating section, so that the preheating temperature is not too high to ensure the smooth operation of the system. When the produced molybdenum oxide is used for molybdate, the temperature in the above temperature segment should be reduced by about 50℃. When the reaction section is 6 ~ 9 meters away from the discharge end, the change of temperature interval when the temperature shifts back can adjust the working state of the kiln tail fan, reduce heat supply and feed. In serious cases, the kiln should be stopped and restarted.
1.2 Gas and gas flow in rotary kiln
Molybdenum concentrate is roasted in rotary kiln because of low temperature roasting and chemical reaction heat release, it should belong to the heat processing equipment based on convection heat exchange. Therefore, it is an important operation process to analyze and control the gas composition and gas flow state in the kiln. Molybdenum concentrate in rotary kiln is oxidation reaction, only rely on fuel combustion of excess air amount is not enough. Excessive air excess coefficient will damage the effect of fuel combustion, so it is necessary to set up additional oxygenation (air) equipment, so that it can achieve the purpose of oxidation roasting. The oxygen content of roasting tail gas should be controlled at 5% ~ 8%.
The existence of moisture in the combustion gas has an adverse effect on the dust removal system of the kiln tail, which will block the equipment in the dust removal system due to humidity. The presence of water combined with SO2 will cause corrosion to the pipeline of the dust removal equipment. The flow of gas in the rotary kiln directly affects the effect of roasting and removing S. Too small flow speed will make the output of rotary kiln is too low, too large flow speed will make the material in the kiln with the airflow entailing serious. The appropriate gas flow rate should be controlled at about 1.5 ~ 1.8M/S. The micro negative pressure formed along the lower edge of the discharging end operating furnace door should be about 500Pa.
Rotary kiln and supporting equipment
2.1 Rotary kiln
Rotary kiln as metallurgical equipment has been existed for a long time, which is widely used in nonferrous metallurgical industry in our country. Initially applied to molybdenum concentrate roasting is mostly external heating, the development of today's internal heating rotary kiln roasting molybdenum concentrate technology has made great progress. The practice of several years has proved that the use of internal combustion rotary kiln for roasting molybdenum concentrate is technically feasible and economically reasonable.
The following factors should be considered for the structural parameters of rotary kiln.
According to the product use, yield, material (molybdenum concentrate) characteristics and production conditions and other factors to determine the diameter of the rotary kiln φ length L, tilt Angle X and revolution n.
A. Rotary kiln diameter φ
Where, G -- production capacity tons/hour
φ-filling coefficient 0.04 ~ 0.06[primary roasting 0.05 secondary roasting 0.06]
ω material - air in the kiln high temperature state of axial movement speed of 2 ~ 6 m/s
Roasting 2 ~ 3 m/s at a time
Second roasting 5 ~ 6 m/s
γ material - material pile gravity ~ 1.6 tons /m3
The above φ, ω selection should fully consider the respective roasting characteristics of the materials, materials in the rotary kiln will be heated in the cold, hot state of the chemical composition viscosity changes. These factors affect the running speed of the material in the kiln. The shape of the kiln is also very important.
B. Length of rotary kiln L
L = G
The residence time of the material in the kiln is an important parameter. Practice has proved that the residence time of the material in the kiln can not be less than 6 hours. When molybdenum oxide calcine is used for smelting ferro molybdenum, it is more than 7 hours.
C. The setting Angle of rotary kiln X and revolution number N
Under normal circumstances, the setting Angle of rotary kiln X≤ 1.0 RPM =0 ~ 1.5 RPM should be adjustable. According to the different output of different materials, the purpose of normal production can be achieved by adjusting the running revolution of rotary kiln, that is, adjusting the running speed ratio of materials.
2.2 Fuel combustion and discharge device
The fuel combustion and discharge device of internal combustion rotary kiln has great influence on the production process and product quality of molybdenum concentrate roasted by rotary kiln. Due to the different fuels used, the combustion device is also different. The fuel used, combustion device and fuel consumption are generally shown in the table below.
Our country coal resources abundant with coal burning machine as the heat source equipment, can be applicable to various kinds of coal combustion, the enterprise application fuel cost is low.
According to the situation of energy supply, the combustion device should be selected reasonably as the internal combustion rotary kiln to roast molybdenum concentrate. However, due to the initial heating of the material will produce different roasting effects due to the origin of grain size, grade. Therefore, the discharging device should be adjustable, which can be continuous and discontinuous. A technical introduction in China uses the method of secondary hearth to solve the problem of continuous and brief discharge, but it needs manual turning out. This is a defect for the engineering technology of roasting molybdenum concentrate in internal combustion rotary kiln with high degree of mechanization and automation. The authors of this paper jointly developed a continuous and discontinuous discharging machine.
2.3 Schede drying system
After system and structure adjustment, the Side dryer has been successfully applied to roast molybdenum concentrate in rotary kiln and become a special equipment. The system has shown its superior performance (2). Molybdenum concentrate needs to be crushed, screened, dried and sent directly to the feeding bin of rotary kiln before being roasted by rotary kiln. The system is flexible and can be discontinuous and continuous. The system has good sealing performance and good production environment. On the specific function of the Side dryer. Effect, one of the authors of this paper, Wang Yi, has made a specific introduction in the 3rd issue of "China Molybdenum Industry" in 2003, and this paper will not be described here.
2.4 Dust Removal System
The dust removal system is to recycle as much material as possible from the tail gas of rotary kiln. The roasting tail gas enters the cyclone dust collection system from the kiln tail ash box to obtain primary dust removal. Through the fan and then into the cloth bag dust collector or electric dust collection. According to the specific operating conditions of the using unit, either choice. The adjustment of fan air volume makes the tail gas flow rate in the system gentle, which is beneficial to stabilize the gas flow in the rotary kiln.
3 Comprehensive evaluation
Molybdenum concentrate roasting by internal heating rotary kiln is a domestic advanced technology, and its supporting equipment through continuous research and development and rotary kiln body combined with each other to give full play to the superior performance of rotary kiln roasting molybdenum concentrate, its characteristics. The output of single equipment is large, the service life of the equipment is long, the energy is saved, the mechanization and automation degree is high, the labor intensity of the operator is reduced, and the production environment is good, which is an engineering technology worth popularizing and applying. Master the technology to operate each set of equipment in the system requires skilled operation skills.
At present, the problems of increasing the sulfur dioxide concentration in the tail gas of rotary kiln and increasing the oxygen content in the roasted air are being deeply studied. These research results will be applied in the production in the near future, so that the molybdenum roasting technology can reach a higher level.

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