Application of bag filters in the control of copper dust

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Application of Bag Filters in the Control of Copper Processing Dust Authors: Cai Xiude, Dai Wenhua, Li Liangfeng, Lin Xianghua Abstract: This paper briefly describes the principle and characteristics of bag filters as well as their application in the control of dust generated during copper processing. The treatment efficiency of bag filters in controlling copper dust, as well as the factors affecting their dust removal performance, were analyzed. The areas that need improvement when using bag filters to control copper smelting dust were proposed. Keywords: bag filter, copper processing, dust. The copper processing industry is a supporting sector of the valve industry; the large amount of waste gas generated during copper processing, if released untreated, can cause environmental pollution and seriously affect the physical and mental health of people. Bag filters can effectively treat the waste gas produced in copper processing. 　　1 Pollution sources: There are two types of copper smelting furnaces: one is an intermediate-frequency furnace or a power-frequency furnace, which uses electric heating and represents a relatively clean production method ； Another type is the crucible, commonly known as an earth furnace; it uses coke as fuel for heating. This type of furnace generates a large amount of smoke, and its use has been restricted. The waste gas generated during copper processing is mainly powder (dust), which contains CuO, PbO, ZnO, Cu, Pb, Zn, Sn, etc., with particle sizes mostly below 0.5 μm. The Environmental Monitoring Station of Yuhuan County, Zhejiang Province, once conducted monitoring on the uncontrolled emissions of dust from copper smelting furnaces; the dust levels detected were as high as 32.1 mg/m3, while the **allowed emission standard is only 5 mg/m3, meaning the levels exceeded the standard by more than 5 times. 　　2 Principles and Characteristics of Bag Filters When dust-laden air passes through the filter bags, the smaller gas molecules pass through the gaps in the filter fabric covered with a layer of dust, while larger dust particles are prevented from passing through; due to gravitational settling, they fall into the ash hopper. As the dust layer on the surface of the filter cloth becomes thicker, the resistance increases, the gaps shrink, and it becomes difficult for air to pass through. When the dust accumulation reaches a certain level, various dust removal methods must be used to clear the filter bags. 　　Bag filters have the following characteristics: ① High dust removal efficiency, which can reach over 99% ； ②It has strong adaptability, can handle different types of particulate matter, and its processing capacity can be adjusted to varying levels ； ③It has high operational flexibility; changes in the dust concentration of the inlet gas have little impact on the dust removal efficiency ； ④It has a simple structure, is flexible to use, facilitates the recovery of dry materials, and eliminates the need for sludge treatment. 　　3 General process for treating copper powder (fume) dust The general process for treating copper powder (fume) dust is as follows: Some processes include a cooler installed at the main exhaust pipe to cool the air using water. 　　4 Statistics and Analysis of Monitoring Results 4.1 Monitoring Results The Environmental Monitoring Station of Yuhuan County conducted monitoring on 12 enterprises equipped with bag filters to control copper dust; the monitoring results are shown in Table 1. 　　Table 1 Monitoring Results of Bag Filters
Serial Number Average Dust Concentration at Inlet/(mg·m-3) Average Dust Concentration at Outlet/(mg·m-3) Dust Removal Efficiency/%
1 228.04 1.88 1.9
2 704.04 9.79 0.0
3 988.03 2.69 7.2
4 618.17 5.68 8.7
5 1249.07 8.89 93.3
6 1011.09 1.38 90.0
7 728.04 1.88 1.9
8 800.05 1.99 93.9
9 1026.02 8.89 98.0
10 283.02 1.99 1.9
11 1174.01 19.49 78.8
12 1105.01 105.09 0.5

4.2 Analysis of Monitoring Results
Based on the monitoring results conducted by the Yuhuan County Environmental Monitoring Station on 12 enterprises equipped with bag filters, the dust concentration at the inlet of these treatment facilities ranged from 228.0 to 1249.0 mg/m3, with an average inlet concentration of 826.2 mg/m3 ； The outlet concentration range was 19.4–105.0 mg/m3, with an average outlet concentration of 53.2 mg/m3 ； The dust removal efficiency ranges from 81.9% to 98.0%, with an average dust removal efficiency of 91.1%. Based on China’s (GB 9078—1996) Emission Standards for Air Pollutants from Industrial Furnaces, the secondary emission standard for Phase II in non-ferrous metal melting furnaces is 100 mg/m3. Of the 12 dust removal systems, 11 met this standard, resulting in a compliance rate of 91.7%, which indicates that bag filters are highly effective in controlling copper dust. 5 Factors Affecting the Dust Removal Efficiency of Bag Filters (1) Selection of filter bags: Filter bags should be made of materials that are resistant to wear and high temperatures; options include glass fiber cloth/felt, NOMEX, needle-punched felt, P84 needle-punched felt, etc. Considering factors such as wear resistance, tolerance to high temperatures, and cost, the copper processing industry generally chooses NOMEX needle-punched felt. 　　(2) Temperature control: Generally, NOMEX filter bags can withstand temperatures of up to 200 ℃; therefore, the temperature at the inlet of the treatment facility should be kept below 200 ℃. By the time the flue gas passes through the long smoke ducts and reaches the dust collector chamber, its temperature has already dropped below 200 ℃. In special cases, water cooling can be employed by installing coolers at the main smoke ducts. 　　(3) Fan selection: Exhaust fans are more effective than supply fans. 　　(4) Mechanical dust removal is more convenient and cleaner than manual dust removal, but it is costly; therefore, the pulse dust removal method is widely used nowadays. 　　(5) Air leakage and resistance: Theoretically, the dust removal efficiency of bag filters can reach 99%, but this figure is not achieved in practice due to air leakage and resistance; the lower the air leakage rate, the better the dust removal performance ； Resistance has a certain impact on the dust removal efficiency; regularly emptying the filter bags to reduce resistance can improve dust removal performance. 　　(6) The dust collection hood should be as close to the burner as possible, so that dust can more easily enter the hood, increasing dust collection and reducing pollution from uncontrolled emissions. 　　(7) Equipment maintenance: Instrumentation and equipment, as well as the filter bags, are prone to damage; they should be repaired promptly and the filter bags replaced as needed. 　　(8) Air flow velocity: The best dust removal efficiency is achieved at an air flow velocity of 1–3 m/s, with the wind speed generally controlled between 2–4 m/s. 　　(9) Filtration area: The larger the filtration area, the higher the dust removal efficiency, but the cost will increase accordingly. Therefore, the air volume should be calculated in advance, and then the appropriate dust collector model should be selected. 　　(10) Secondary dust removal: A sedimentation chamber is installed in front of the dust collector box to achieve secondary dust removal. The sedimentation chamber can remove some large-particle dust, lower the temperature, and reduce the pressure in the dust collector. 　　6 Economic Benefit Analysis: When the 12 dust treatment systems for copper smelting furnaces in these 12 enterprises operate normally for one month, they can remove 12 tons of dust. The installed capacity of each treatment system is approximately 10 kW, which is 5% of the installed capacity of the intermediate frequency furnace equipment. Since intermediate frequency furnaces are designed with a margin in terms of power capacity, the additional installed capacity of these treatment systems will not result in an increase in the companies’ electricity costs. 　　Total equipment investment: 12×6=720,000 yuan. Value recovered each year: 12×12×0.14=201,600 yuan. Salary for part-time operators each year: 12×12×0.02=28,800 yuan. Reduction in salary due to improved working conditions each year: 12×12×0.02=28,800 yuan. Equipment maintenance costs each year: 12×12×0.02=28,800 yuan. It can be seen that an economic benefit of 172,800 yuan is generated each year, and the initial equipment investment can be fully recovered after 4 years. 　　References 1 Jiang Zhanpeng. Environmental Engineering. Beijing: Higher Education Press, 1992. Reposted at China Thesis Download Center http://www.studa.net