I. Introduction

China is a country with abundant magnesium resources, accounting for the world's largest reserves and the world's largest magnesium producer. Of metallic magnesium production process used is more than 99% Pidgeon (also known as silicon thermal reduction). Pi Jiangfa was a magnesium smelting process invented by Canadian professor Lioyd M. Pid-geon in 1941. The method is ground into fine powder after a rotary kiln or a shaft kiln calcining dolomite iron and silicon, according to a certain mixing ratio of the pressure reducing tank is loaded into briquettes made of heat-resistant steel at around 1200 deg.] C and evacuated The vacuum is reduced to an absolute pressure of 10 to 20 Pa to obtain magnesium vapor, which is condensed to become crystalline magnesium, which is then melted and cast into a magnesium ingot. Its main advantages are low investment and operating costs, low prices of raw materials, fast construction, and good product quality. However, the main problem of the traditional Pijiang method of magnesium smelting is that it consumes a lot of energy and pollutes the environment. Therefore, energy saving, improving technology, reducing emissions, and improving the level of magnesium in China's Pijiang process are the fundamental ways to consolidate and strengthen China's metal magnesium industry's leading position in the world and achieve sustainable development.

Rotary kiln and reduction furnace are the main thermal equipment in the production of magnesium metal by Pijiang method, and also the main energy consumption equipment. This paper systematically analyzes the current status of energy use in these thermal equipments and processes, and discusses the energy saving direction and specific measures of China's metal magnesium industry.

Second, the diagnosis of the use of major thermal equipment

(1) Calculation of heat utilization rate of rotary kiln

The rotary kiln is a device for calcining dolomite. During the calcination process, dolomite undergoes endothermic decomposition reaction in the range of 630-780 °C to form CaCO ₃ and MgCO ₃ , and then MgCO ₃ (750-800 ° C) continues to decompose into MgO and At CO ₂ , the endothermic decomposition of CaCO ₃ occurs at about 910 ° C (in the range of 780 to 930 ° C), and finally CaO and MgO are decomposed. The main reactions of dolomite in the rotary kiln are:

CaCO ₃ ·MgCO _3(s) =CaO _(s) + MgO _(s) +2CO _{2 (g)}

The subscripts s and g represent the solid state and the gaseous state, respectively. The reaction is carried out at about 910 ° C. According to the thermal properties of the respective substances in the above reaction, the enthalpy values ​​of the respective substances in the equation at normal temperature (25 ° C) and 910 ° C can be obtained, as shown in Table 1.

Table 1 The enthalpy of matter at room temperature (25 ° C) and 910 ° C KJ / mol

Before the reaction occurs, the high temperature flue gas in the kiln first heats the dolomite to about 910 ° C. In this process, the heat absorbed by the dolomite is equal to its enthalpy change.

Then, the reaction (1) occurs, and the enthalpy of the reactant is subtracted from the enthalpy of the product at 910 ° C to obtain the heat of reaction of the reaction (1) at this temperature.

The energy consumption of the rotary kiln can be obtained by calculation as shown in Table 2.

Table 2 Energy consumption of rotary kiln

(2) Calculation of heat recovery rate of reduction furnace

The main reactions occurring in the reduction tank in the reduction furnace are:

MgO _(s) +CaO _(s) +1/2Si _(s) =Mg _(g) +1/2(2CaOSiO ₂ ) _(s) (2)

The reaction is carried out at about 1200 ° C. According to the thermal properties of the materials before and after the reaction, the enthalpy of each substance at normal temperature (25 ° C) and at the reaction temperature (1200 ° C) can be determined, as shown in Table 3.

Table 3 The enthalpy of matter at room temperature (25 ° C) and 1200 ° C KJ / mol

The furnace flame and high temperature flue gas firstly heat the reactants in the reduction tank to about 1200 ° C. In this process, the heat absorbed by the material is equal to its enthalpy change. Then, the reaction (2) occurs, and the enthalpy of the reactant is subtracted from the enthalpy of the product at 1,200 ° C to obtain the heat of reaction of the reaction (2) at this temperature.

In actual production, the reaction does not proceed to completion, and some of the reactants still do not participate in the reaction. At present, in the actual operation of magnesium smelting in China, the reduction rate of magnesium is about 71%, and thus the energy consumption of the reduction furnace can be calculated as shown in Table 4.

Table 4 Energy consumption of the reduction furnace

Third, the main thermal equipment energy-saving measures

From the above calculations, it is known that the two main thermal equipments, rotary kiln and reduction furnace, have low thermal efficiency and thus have great energy saving potential. In particular, the heat recovery rate of the reduction furnace is surprisingly low, and the energy consumed by the reduction furnace accounts for the largest proportion of the energy consumed by the whole system, so it is more significant for the energy saving of the reduction furnace.

(1) Energy-saving measures for reducing furnace

Too low thermal efficiency is the key to restricting the development of magnesium plants. The bottleneck is that the thermal efficiency of the reduction furnace is too low. To improve the overall thermal efficiency of the magnesium plant, the thermal efficiency of the reduction furnace must be solved, so how to improve the thermal efficiency of the reduction furnace. It is extremely important and the key to solving the problem.

1. Research on new type of reduction furnace

The traditional reduction furnace can only arrange two layers at most when the reduction tank is arranged. The heating area in the furnace is limited, and the temperature drop of the flue gas is small, so that the exhaust temperature of the furnace is as high as 1100 °C. If the number of rows of reduction tanks is increased in the furnace, the heat receiving area in the furnace can be increased, thereby reducing the exhaust gas temperature and improving the thermal efficiency of the reduction furnace. At the same time, the flow field and temperature field in the furnace can be better organized by improving the furnace shape, so that the temperature in the furnace is uniform, thereby increasing the reaction speed and shortening the reaction cycle.

2. Reasonable design of reduction tank structure

Pijiang method magnesium smelting is a batch reduction process with a long cycle of reduction. If the time required for the reduction can be shortened, the thermal efficiency of the reduction furnace is greatly improved. The reduction tanks that are widely used today must be transferred from the outside to the inside during the reduction process, so the reaction must also be carried out from the outside and inside until the pellets in the center of the tank are completely reacted, and the reaction is completely completed. After the pellets participate in the reaction, the heat transfer of the remaining residue is deteriorated, which is not conducive to heat transfer and reaction. Therefore, as the reaction progresses, as the reaction proceeds, the mass fraction of the pellets that can be reduced as the heat passes through the unit radius is reduced, and the reaction rate is slower and slower. In order to speed up the reaction, it is possible to use double-sided heating, that is, heating both inside and outside.

3. Fully and rationally use the high temperature flue gas of the reduction furnace

The temperature of the flue gas coming out of the reduction furnace is as high as l100 ° C, and its grade is very high. This part of the heat is generally generated by a waste heat boiler, and then vacuumed with steam. In this way, the flue gas is also utilized to some extent. However, the use of flue gas of l100 ° C directly through the recovery of waste heat does not fully utilize the exhaust energy of the flue gas.

The most reasonable method of utilization is to use this part of the high-temperature flue gas heat reasonably according to the quality and make full use of it according to the quantity. For example, the material may be preheated with high-temperature flue gas, the combustion air may be used, and then the waste heat of the flue gas may be recovered by the waste heat recovery device, and finally discharged. This can greatly reduce fuel consumption, both to reduce thermal energy losses and to reduce the loss of available energy.

4, the use of magnesium slag heat

The magnesium slag coming out of the reduction tank has a temperature of up to 1000 ° C and will take away a lot of heat. Because the slagging process of the reduction furnace is not continuous, this part of the heat is not used at this stage, but it is wasted. A waste heat boiler can be used to recover this part of the energy. A layer of water pipe is laid under the inlet and outlet of the reduction tank, and the slag is discharged from the tank and covered on the water pipe. The sensible heat of the slag is absorbed by the water, and the heated water is stored in the waste heat boiler, and the slag is cooled and then transported away. .

(2) Energy-saving measures for rotary kiln

The heat loss of the rotary kiln is mainly the heat taken away by the flue gas, the heat taken away by the calcination and the heat lost from the rotary kiln body. The reduction or effective use of these heat can improve the heat utilization rate of the rotary kiln.

1, using flue gas waste heat to preheat materials

The temperature of the flue gas in the rotary kiln still has about 400 °C, and it can still be used to further reduce the exhaust gas temperature. The most direct use of flue gas in the rotary kiln is to preheat the material (dolomite mine). The simpler and more effective method is to add a vertical preheater, the flue gas flows from bottom to top, and the dolomite from above. Moving down to form a countercurrent, the temperature of the flue gas can be lowered very low, so that the residual heat of the flue gas can be fully utilized.

2. Install the cylinder heat exchanger to recover and reduce the heat dissipation of the rotary kiln

Because the temperature of the kiln is high, it is inevitable to dissipate heat outwards. This part of the heat is only partially used, and most of it is still lost to the environment. The installation of an air preheating device on the rotary kiln to preheat the air is a good way to use the simplified heat. The preheating zone of the rotary kiln is also equipped with an air preheater, and these preheaters are taken from the kiln to the end. The kiln heads are connected in series to preheat the air to a higher temperature.

3. Residual calcined white sensible heat preheating combustion air

Because the whitening temperature from the rotary kiln head is very high (1100 ~ 1200 ° C), the hot calcined white will take away a lot of heat, this part of the sensible heat is not used, but directly lost in the environment. The temperature of the calcined white from the kiln head should obviously be much higher than the temperature of the simplified one. Therefore, it is conceivable to take out the air from the preheating of the external body and further preheat the air by the hot calcination.

Fourth, the conclusion

(1) The rotary kiln and the reduction furnace, which are the main thermal equipments for silicon thermal magnesium smelting, have low thermal efficiencies, especially the reduction furnace, and the thermal efficiency is extremely low, so the potential for energy saving and consumption is extremely great.

(2) The energy saving of the reduction furnace is the key to the energy saving of the magnesium plant. For the characteristics of the silicon thermal reduction method, the energy saving and consumption reduction of the reduction furnace should be studied from the research of the new type of reduction furnace and reasonable reduction tank structure, and the efficient use of high temperature flue gas waste heat. And the reasonable recovery of the sensible heat of magnesium slag.

(3) According to the thermal diagnosis analysis of rotary kiln, the energy saving countermeasures of rotary kiln are proposed, for example: preheating materials by using flue gas waste heat, installing simplified heat exchanger to recover and reduce heat dissipation of rotary kiln body, and preheating by calcining sensible heat Combustion air.

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