Preliminary study on clean production of small and medium magnesium plants

I. Introduction

In the face of the ever-expanding global economic development and the double challenges of resource depletion and environmental damage, the Rio Conference of Brazil has established a strategy for coordinating economic and environmental sustainability and its programme of action, Agenda 21, and will be cleaned. Production is promoted as a key measure to implement a sustainable development strategy. The countries in the world have responded positively and widely adopted, and successively introduced various environmental regulations, policies and legal systems, and launched a wave of clean production activities on a global scale. Since 1990, the United Nations Environment Program has held six international seminars on cleaner production, and in October 1998 issued the International Declaration on Cleaner Production, which has achieved great environmental and economic benefits.

In 1992, China put clean production into the "Top Ten Countermeasures for Environment and Development"; in 1993, the State Environmental Protection Administration and the State Economic and Trade Commission jointly organized the second national industrial pollution prevention work conference to further clarify the need for industrial pollution prevention and control. changes to the production process, along with the support of the World Bank and the United Nations Environment program, the State environmental Protection Administration of China organized the first systematic implementation of cleaner production of B-4 demonstration project: "promoting cleaner production in the prince Yu"; in 1999 in May issued a "notice on the implementation of cleaner production demonstration pilot program" to select the petrochemical, metallurgy, chemical industry, light industry, shipbuilding industry, 5 cleaner production demonstration pilot for the industry; and in January 2003 the formal implementation of the "Chinese Law of the People's Republic of China on Cleaner Production Promotion.

As the world's largest producer of magnesium metal, China's metal- magnesium industry in one of the metallurgical industries is very important in promoting clean production. However, most of the small and medium-sized magnesium plants are still stalled due to financial and technical problems, and even many aspects are not in line with clean production. The requirements, and the corresponding research carried out very little. In view of this situation, this study takes a small metal magnesium plant as an example to carry out clean production analysis, and provides a scientific basis for the future engineering design of the small and medium magnesium plant and the environmental management decision after the project is put into operation.

Second, research examples

The plant uses the Pijiang method to produce magnesium metal. In pulverized coal as fuel in the kiln calcined dolomite was added to produce calcined dolomite. The calcination temperature is 1200 ° C to 1250 ° C, and the chemical reaction is: CaC0 ₃ ·MgC0 ₃ =Ca0·Mg0+ 2CO ₂ . Calcined dolomite, silicon iron, fluorite proportionally mixed and milled to <100 mesh to form a crude evacuation magnesium reduction stage, the main chemical reaction: 2 (MgO · Ca0) + Si (Fe) = 2Mg + 2CaO · SiO 2 + (Fe ). Fine or magnesium alloys can be produced by refining or adding adjuvants. The production process is shown in the following figure:

G-atmosphere; D-waste residue; T-smoke, dust;

W-waste water; N-noise; S-sulfur dioxide.

Diagram of metal magnesium and magnesium alloy production process flow chart

Third, clean production analysis

Cleaner production refers to continuous improvement of design, use of clean energy and raw materials, adoption of advanced process technology and equipment, improvement of management, and enhancement of comprehensive utilization, reducing pollution from sources, improving resource utilization efficiency, and reducing or avoiding production, service and The generation and discharge of pollutants during the use of the product to mitigate or eliminate harm to human health and the environment.

(1) Analysis of the selection of major production equipment

1. Calcination kiln

At present, vertical kiln and rotary kiln are mainly used at home and abroad. Now compare and analyze its various technical and economic indicators, see Table 1.

Table 1 Comparison of technical and economic indicators between vertical kiln and rotary kiln

Rotary kiln is a clean production equipment, which meets the requirements of China's environmental protection conditions, rational use of energy, mechanization and automation. The use of rotary kiln has become a trend. The plant still uses vertical kiln due to limited investment in smaller scale. It is recommended to improve on the technology of using the vertical kiln to calcine dolomite. In addition to improving operations and selecting raw materials, it is also necessary to improve the structure of the furnace, improve the degree of mechanization, change the energy structure, use the semi-gas external combustion chamber, and the bottom discharge, and then support the corresponding environmental protection measures.

2, reduction furnace

At present, domestic reduction furnaces are mainly divided into coal-fired reduction furnaces and gas reduction furnaces according to different heating methods. The technical and economic indicators of these two kinds of equipment are compared and analyzed, as shown in Table 2.

Table 2 Comparison of technical and economic indicators of coal-fired reduction furnace and gas reduction furnace

The gas reduction furnace is a clean production equipment. The factory adopts a coal-fired reduction furnace and the facilities are backward. It is recommended that the plant be replaced with a reduction furnace unit.

3, refining furnace

There are three heating methods. Electric heating: no combustion exhaust gas, but need to increase the electric load; use gas as energy: combustion exhaust gas can directly discharge to the standard, and must build a gas generating furnace device to increase investment; the plant uses raw coal as fuel: small investment but burning smoke The gas must be further dedusted and purified to meet the standard discharge. The device is relatively backward and does not meet the requirements for cleaner production. It is recommended to change the heating method of the refining furnace according to the local resource advantage or energy advantage.

(II) Comparative analysis of production processes

The production methods of magnesium in the world are electrolysis and silicon thermal methods (Pijiang method, internal heat method and semi-continuous silicon heat method). Electrolysis method: The output can be continuously produced with good economic benefits. However, the power consumption is large, the investment is large, the quality of magnesium is slightly poor, and a lot of toxic pollutants such as Cl ₂ and HCI are produced in the production process, and the environmental pollution is heavy; semi-continuous silicon thermal method and internal heat method: good economic benefit and low operating cost However, the construction of the factory is large; the Pijiang method: simple process, low investment, good quality of magnesium, mainly produces smoke (powder) dust and sulfur dioxide in the production process, which is relatively less harmful to the environment, and environmental pollution is relatively easy to handle, which is relatively mature. Production process. At present, 70% of the magnesium plants in China use the Pijiang method. In view of the economic and technical level of the plant, the production of magnesium metal by the Pijiang method is more reasonable.

(III) Analysis of raw and auxiliary materials and energy consumption

Since the clean production industry standard for metal magnesium smelting has not yet been released, this study has collected the main technical and economic indicators of representative manufacturers at home and abroad, as shown in Table 3, as a reference indicator.

Table 3 Main technical and economic indicators for the production of magnesium by silicon thermal method at home and abroad

It can be seen from Table 3 that China's Pijiang method smelting magnesium raw materials consumes high energy and has high energy consumption. In addition to the equipment level gap, the quality requirements of raw materials such as dolomite, ferrosilicon and fluorite are not strict, and the ratio of ingredients is The accuracy of the ingredients is not controlled accurately, the process conditions are not up to standard, the quality of the products is unstable, and the magnesium recovery rate is far lower than the foreign advanced indicators. This is where we should pay attention. At the beginning of the new century, it is very correct and necessary for China to develop the world's leading magnesium smelting technology with its own intellectual property as a major scientific and technological project, especially to develop an advanced internal heat reduction furnace with its own intellectual property rights. The magnesium smelting technology will play a major role in improving China's magnesium metallurgy technology. Because it not only saves a lot of energy, improves the environment, but also greatly reduces the production cost of magnesium smelting. Preliminary calculation, using advanced internal heat reactor magnesium reduction technology, can reduce production costs by 1000-1500 yuan / t-Mg, saving bituminous coal 10t / t-Mg. According to China's annual production of 200,000 tons of metal magnesium, the economic benefit of reducing the annual cost of China is 200-300 million yuan, saving 2 million tons of coal resources (bituminous coal).

The power consumption of the plant is very large. The comprehensive utilization of the residual heat of the reduction furnace should be strengthened. The steam generated by the waste heat boiler directly drives the compressor to vacuum and change the power consumption of the vacuum.

Due to the adoption of coal-saving measures, the coal consumption of the production of tons of magnesium products is reduced by 0.79×10 ⁴ KJ/t magnesium compared with the domestic average, but it is still higher than the advanced level in Japan.

(4) Analysis of pollution control measures

The process sewage node is shown in the figure above.

1. Exhaust gas

The plant uses the sedimentation chamber humidification dust collector to remove the high-altitude after desulfurization and dust removal treatment for the calcined flue gas; the desulfurization and dust removal by the high-efficiency spray desulfurization dust remover for the reduction furnace flue gas; the refining furnace flue gas reduction and purification furnace purification system Uniform treatment; the material is dusty during crushing, fine grinding and transportation. In addition to strengthening the equipment sealing, the plant also sets a dust collecting device for each material falling point to prevent dust from spilling out in the workshop and causing environmental pollution; the factory is in the same industry in China. Exhaust gas treatment is still a good level, and the exhaust gas treatment effect is remarkable. The desulfurization efficiency of the kiln is ≥75%, and the dust removal efficiency is ≥96%.

Flue dust concentration after treatment ≤150mg / Nm ^3, S0 ₂ emissions concentration ≤214mg / Nm ^3, emissions compliance "industrial furnace air pollutant emission standards". Considering the high temperature of the reducing furnace flue gas, it is recommended that the plant set up a waste heat boiler to exchange the steam generated by the heat of the flue gas to make the vacuum pump vacuum pumping power, thereby saving energy.

2. Waste water

The plant carries out corresponding technical treatment according to different water quality. For the equipment cooling water, due to the temperature rise change before and after use, it will be directly recycled without any treatment; for industrial wastewater, it will be neutralized and precipitated into the sedimentation tank after neutralizing its acidity and alkalinity. The supernatant is recycled or periodically discharged, and the precipitated sludge is sent to the slag plant for storage. After the above treatment, the new water consumption of the plant is l42m ³ /d, and the circulating water consumption is 4880m ³ /d. The secondary utilization of water is 39m ³ /d. The reuse rate of water is as high as 97.2%. Sewage discharge complies with the secondary standard of Integrated Wastewater Discharge Standard. Considering that magnesium production wastewater contains a large amount of salts, especially calcium chloride and active chlorine, fluorine, heavy metal ions. It is recommended that the magnesium plant construct desalination facilities, including equipment for processing the obtained brine into a commercial solution of calcium chloride.

3. Comprehensive utilization of solid waste

The reduction furnace slag of the plant contains CaO, MgO. SiO ₂ and Fe ₂ O ₃ are used as cement raw materials to replace the utilization of some clinker, and can also be used as comprehensive materials for building materials; the refining slag contains NaCl, MgCl2 and KC1, etc., as a comprehensive utilization of raw materials for chemical enterprises; Kiln, reduction furnace and refining furnace burning coal to produce coal ash slag containing SiO ₂ and Al ₂ O _{3 ,} etc., as comprehensive utilization of building materials; dust recovery system for recycling of dust removal system in raw material workshop; circulating spray water for desulfurization and dust removal system The sludge discharged from the sedimentation tank is mainly coal ash slag, and the slag is stored. It has met the requirements of reducing, harmless and recycling waste in clean production.

4. Suggestions and measures for clean production in small and medium-sized magnesium plants

Enterprises implementing clean production must proceed from the principle of production-environmental protection-corporate. From the analysis of the current situation of small and medium-sized magnesium plants in China, the recommendations are as follows:

(1) Cleanliness of raw and auxiliary materials

From the source of production, the pollution source reduction control, that is, the selection of non-toxic, low-toxic, less polluting raw and auxiliary materials to prevent the harm of raw materials and products to humans and the environment, in the raw material dolomite, ferrosilicon, sulfur powder, sulfur powder is Toxic substances, replaced with 2 ^# flux.

(2) Cleanliness of process equipment

It adopts new processes and equipments with high conversion utilization rate, low emission coefficient, energy saving and consumption, and is conducive to automation of production operation control, achieving the purpose of energy saving, consumption reduction and pollution reduction. Through the analysis of the clean production of the plant equipment in the previous article, it is recommended that the small and medium-sized magnesium plant invest in the introduction of advanced and clean process equipment in the long run.

(3) Pollution prevention and treatment

In addition to the “three wastes” prevention and treatment methods analyzed above, the ball mill should be isolated and controlled to prevent noise; the harmful fumes such as chloride, hydrogen chloride and sulfur dioxide generated in the refining process should be improved to reduce local smoke and add purification facilities. Smoke pollution hazards.

(4) Control and management of the production process

Production process control, mainly to control important production indicators, such as increasing the activity of calcination, selecting a reasonable ratio of ingredients, increasing the vacuum degree during reduction to increase the yield of magnesium reduction; reducing the ratio of magnesium to magnesium, reducing the consumption of ferrosilicon, and reducing Reducing tank consumption to reduce material consumption; reducing energy consumption by rationally controlling air consumption coefficient, rationally selecting gas burner, improving furnace structure and waste heat recovery; reducing pollutant emissions in flue gas; increasing water reuse rate and pollution Comprehensive emission compliance rate, solid waste reduction, resource utilization and harmlessness, improve the integrity and development rate of environmental protection facilities. Production process management is mainly to strengthen the awareness of cleaner production; strengthen the post and process according to standardized operations; use administrative, economic and other means to prevent or reduce the "running and leaking" of the production process.

V. Conclusion

Metal magnesium works with high energy consumption, high material consumption and waste. Through the implementation of cleaner production, using clean raw and auxiliary materials, advanced process equipment and production operation process and effective pollution prevention measures, improve the environmental and economic benefits of small and medium-sized magnesium plants, realize resource-based production, and embark on the road of sustainable development. .

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