[China Aluminum Industry Network] 1 Overview Although China's aluminum profile production has ranked first in the world for five consecutive years, but aluminum production industry production technology compared with the United States, Japan, Germany, Italy and other countries there is still a big gap, energy-saving emission reduction tasks major. Mainly in the following areas:
(1) The energy consumption and pollutant discharge of the aluminum profile industry consumes about 16 to 18 tons of industrial water per 1 ton of aluminum profiles. The waste water generated from the surface treatment contains a variety of metal impurity ions; it has high energy consumption; it produces a large amount of waste residue. Only an oxidation coloring production line with an annual output of 2,400 tons of aluminum profiles produces about 150,000 tons of sludge and 2,000 tons of waste residue. The composition of waste water sludge is relatively complex, and most factories currently use landfill methods to treat these sludges. This not only occupies limited land resources but also wastes resources and pollutes the environment.
(2) Reasons for high energy consumption and high pollution in aluminum industry 1 Smelting and recycling: At present, there are 5% of electric furnaces in aluminum smelting furnaces, 91% of oil furnaces, and 4% of gas furnaces, causing re-melting to produce 1 ton of extruded round ingots. The fuel consumption is 55.17% higher than that of the industrialized countries, and the actual smelting energy consumption of actual ingots (rolled slabs and extruded round bars) is much higher than that of industrial developed countries. In addition, the overall energy consumption of aluminum smelting furnaces is still at a medium to low level.
2 Casting, rolling and extrusion: lack of high-grade products; small machine tables, extrusion equipment, backward, low efficiency; low degree of automation equipment, can not achieve isothermal rapid extrusion, production of precision profiles; mold quality is poor.
3 Surface treatment: Surface treatment is a process of high power consumption, high water consumption, and high pollution in aluminum processing.
Energy-saving and emission reduction has become an urgent problem for aluminum processing companies today. This paper starts with aluminum melting furnaces, holding furnaces, extruder rod furnaces, and deep processing and utilization of oxidized sludge, and proposes some new methods and strategies for aluminum processing. The enterprise achieves the goal of energy conservation, emission reduction and cleaner production.
2 Energy Saving and Emission Reduction and Cleaner Production Technology of Aluminum Melting Furnaces 2.1 Melting Aluminum Furnace Working Process The melting process of the molten aluminum furnace can be roughly divided into 4 stages, ie, charging the material into the softened stay, softening the stay to the charge material leveling, and charging the material leveling. To the full melting (oxidation scum generated at this stage), the molten aluminum heats up. Heating of the aluminum material is accomplished by convective heat transfer of the burner flame, radiant heat transfer from the flame and the furnace wall, and conductive heat transfer between the aluminum materials.
Throughout the process, the ratio between the three is constantly changing. In solid state, the blackness of aluminum is small, and its thermal conductivity is strong. As the smelting process proceeds, the charge enters a critical state of semi-liquid semi-solid, its thermal conductivity decreases, and the thermodynamic properties have undergone fundamental changes. The thermal conductivity of liquid aluminum is only 40% of solid aluminum, and the conduction and heat transfer process from the upper part of the molten pool to the bottom is very slow. The loose scum floating on the metal mirror surface constitutes an insulating barrier to heat transfer. At this point, the oxide film on the surface of the bath opens and loses its protective effect. The tendency of oxidation and inhalation tends to increase. For the fused-aluminum furnace, in the melting period of aluminum, the temperature of the furnace is generally controlled at 1200°C. At this time, the temperature of the flue gas at the furnace is the temperature of the furnace, and the heat removed by the flue gas accounts for about 50 to 70% of the heat load of the furnace. Taking into account 10% of other heat losses, effective heat utilization is only 30-40%. If this part of the waste heat is not used fully, it will inevitably cause great waste, making the thermal efficiency of the furnace very low.
In summary, the effective method of strengthening the heating method and recovering the residual heat of the flue gas to preheat the combustion air is an effective way to improve the thermal efficiency of the furnace and ensure less direct fuel consumption in the smelting process.
2.2 Using High-Temperature Air Combustion Technology High-temperature air combustion changes the traditional combustion method, using the flue gas recirculation method or direct injection combustion in the fuel furnace. The main manifestation is that the combustion-supported air passing through the ceramic honeycomb body is preheated to over 1000°C. It is sprayed into the furnace at an appropriate speed and mixed with the combustion products in the furnace under the action of high-speed airflow suction and stirring. 21% of the oxygen in the air is diluted and burned in a fluid with low oxygen concentration (lower 5% to 6.5%). Combustion under high temperature air conditions can achieve low air-coefficient combustion and reduce aluminum oxidation burn.
The regenerative combustion system mainly includes a pair of burners equipped with a regenerator, a set of reversing devices, a set of control systems, and a piping system.
When the temperature of the furnace gas is 1000-1200°C, the temperature of the combustion air can be preheated to 800-1000°C, which can save about 50% of fuel compared with the fuel-melted aluminum furnace using the inter wall type air heat exchanger.
The actual outlet velocity of the mixture gas at the burner outlet is about 60 m/s, the flame length is about 2.5 to 3 m, and the flame diameter is about 0.5 to 0.7 m. The paired burners in the furnace are operated in a commutation manner, and high-temperature zones are frequently exchanged to ensure that the temperature in the furnace is uniform and no low-temperature zone is formed.
2.3 High-Speed ​​Burner Technology For the old stove in use, it cost more than 300,000 yuan to add a pair of regenerative burners.
Using high-speed burner nozzles, gas is sprayed to the furnace at a speed of 100m/s or more, combustion air is used to support combustion at a speed of 90m/s, and a strong impact is exerted on the aluminum stack to accelerate the melting. The spout placed a long lighter.
Based on the digestion and absorption of U.S.-day high-speed burner products, we have developed a high-performance burner with excellent performance. The flue gas flow rate can reach 180m/s, the load regulation ratio can reach 1:20, and the excess air coefficient can be between 0.65 and 10.8. Inter-regulation.
The technical features of high-speed gas burners are as follows:
a) accurate organization of combustion, combustion efficiency 99.9%;
b) wide operating conditions: heat load ratio 1:20, air ratio 0.5 ~ 10;
c) The use of staged combustion, harmful gas (NOx) emissions in line with national environmental standards;
d) With the function of ejecting smoke back, it can bring the flue gas back from the furnace back into the furnace;
e) Full metal structure, continuous service life of 3 years.
2.4 The automatic control technology of aluminum melting furnace control system is an important guarantee for improving the combustion, reducing energy consumption, ensuring process requirements, increasing product output and output. The ultimate goal is to achieve the automatic flow parameters, temperature, pressure, atmosphere and other parameters of the combustion equipment. Inspection and process control.
2.4.1 The furnace pressure automatically adjusts to control a reasonable flame furnace should achieve micro-pressure operation.
3 Energy-saving technology for extruder rod furnaces The energy consumption of extruder rod furnaces accounts for 12 to 20% of the total energy consumption of aluminum processing companies. Extruder rod furnace can be roughly divided into three types, single rod furnace, multi-rod furnace and short rod furnace. The pictures are shown in Figure 3-1, Figure 2-3, and Figure 3-5, respectively.
According to theoretical calculations, heating 1t aluminum bar to 450°C requires only 13m3 of natural gas (thermal efficiency of the furnace is 100%). Taking into account the thermal efficiency of the furnace and the production process of intermittent heating, the heating efficiency of the heating furnace is assumed to be 60%, and natural gas is also consumed more than 22m3/ t aluminum rods, energy consumption of products converted to 26m3 / t products. However, at present, the product energy consumption of aluminum rod heating furnaces is equivalent to 45m3/t natural gas products, and some even up to 70m3/t products. Energy-saving space is still great.
3.1 Single-rod furnace energy-saving technology 3.1.1 Renovation furnace According to the theoretical calculation, the current furnace volumetric heat intensity is much lower than the industrial furnace standard, the furnace hearth is too large, the furnace temperature is low, unfavorable to the combustion, it is necessary to reach a certain volumetric heat intensity must Increase the burning power, which will inevitably increase the gas consumption of the unit product. We adopted the method of reducing the volume of the furnace and increasing the vault bricks to transform the furnace.
3.1.2 Replacing the Burner Replace all existing DC burners with the patented Swirl Burner (second generation), a proprietary product produced by the Good Combustion Company. Increase flame stiffness, increase flame temperature, and enhance heat transfer.
(1) Swirl burner operating principle The burner is equipped with various types of swirl generator (referred to as cyclone). The mixture of fuel and air rotates as it passes through the cyclone, and it emerges from the spout to form a swirling jet. Using a rotating jet, a high-temperature flue gas recirculation zone can be formed that facilitates the ignition and the air flow is strongly mixed.
(2) The swirl nozzle with good flammable swirl burner structure, compared with the traditional long rod hot shear furnace nozzle, has the characteristics of large flame stiffness, large heat transfer power and high combustion efficiency.
3.1.3 Cancellation of circulating fan Extruder In the case of a circulating air blower, the negative pressure of the hearth is too high, the quantity of cold air sucked in is large, and the nozzle is severely deflaming, which greatly reduces the thermal efficiency of the furnace.
After canceling the circulation fan, the furnace can be operated at a slight positive pressure to ensure the safety of the furnace.
Economic Operation. Can also save electricity 45,000 ~ 66000 yuan / Taiwan. Year.
According to our retrofit experience, the Φ180mm bar furnace can cancel the circulation fan. Φ180mm or more bar furnaces are not suitable for canceling the circulation fan. Instead, plate heat exchangers are used to fully utilize the residual heat of the flue gas to heat the combustion air and improve the overall thermal efficiency of the heating furnace.
3.1.4 Installation of Oxygen Detector In order to more accurately adjust the combustion, accurately control the air-fuel ratio, and get rid of the passive situation of empirical adjustment, an oxygen detector is installed on the pipe behind the Venturi mixer, and the oxygen detector continuously sets the oxygen content. The data is sent to the PLC, and the PLC adjusts the opening of the gas and air-conditioning valves according to the set air-fuel ratio data so that the oxygen content is always maintained at an appropriate ratio to ensure accurate organization of the combustion.
3.1.5 After the transformation of the heat-retaining furnace is intensified, new heat-preservation materials are adopted outside the fire nozzle brick and the top-cover brick to enhance heat preservation, which is to reduce the temperature of the furnace wall below 80° C. and to reduce heat loss.
Since 2010, we have conducted energy-saving reforms on more than 80 single-rod furnaces, with a good energy saving rate of 49% and a poor rate of 12%. We promise to our customers that after our energy-saving transformation, the gas-fired energy-saving rate will reach over 10%.
3.2 Multi-barrel energy-saving technologies 3.2.1 Transformation of the combustion chamber The rod side of the multi-barrel furnace is converted into a combustion chamber, the original burner system is eliminated, and the high-speed burner is used as a heat source device to directly inject the high-speed gas stream of the high-speed burner into the combustion chamber. Reduce the heat exchange process with air.
In order to prevent the flame from directly contacting the aluminum rod and causing the accident of the melting rod, a refractory partition is laid at the bottom of the burner flame so that the flame spreads to various corners of the furnace at high speed.
3.2.2 Use of high-speed burners similar to aluminum melting furnaces.
3.2.3 Abolishing the hot air circulation system After the combustion chamber is reformed, all the circulating fans, combustion engines and the original combustion chamber will be cancelled. In order to reduce the transformation workload, the internal structure will not be changed and only the original circulation air duct will be blocked. . This not only saves the investment in fans and burners, but also saves power consumption due to circulating fans.
3.2.4 Furnace pressure control system Furnace temperature control, exhaust temperature and furnace pressure control are inseparable. First, the pressure range of the combustion chamber is set to 10 to 20 Pa. A baffle plate is installed on the chimney. When the furnace pressure is high, the baffle opening is increased, and vice versa. In the normal range of furnace pressure, the baffle opening degree is adjusted according to the furnace temperature, especially the exhaust temperature. This is the so-called cascade control.
3.2.5 increase plate air preheater In order to ensure that the exhaust temperature is maintained at about 150 °C, make full use of the residual heat of the flue gas, install an air preheater at the bottom of the chimney to preheat the combustion air, increase the combustion temperature and reduce excess Air coefficient, improve bar furnace efficiency, reduce gas consumption.
The efficiency of the multi-rod furnace energy saving transformation that we have done for an aluminum processing company is significant. The original ton product consumes 53m3 of natural gas, only 31.8m3 after transformation, the energy saving rate reaches 40%, and the furnace temperature uniformity is improved, and the heating time is greatly shortened. , increased equipment productivity.
3.3 Short Rod Oven Energy Saving Technology The short rod heating furnace is a convection heating furnace, as shown in Figure 3-9. Convection heating of aluminum rods is performed using high-temperature gases generated by combustion of the burner.
A thermocouple is set in the furnace of the aluminum rod heating furnace to control the temperature of the furnace to meet the requirements of the heating of the aluminum rod. Since the control system is pulsed, that is, after the temperature exceeds the set temperature, the burner stops working. When the temperature is lower than the set temperature, The burner must be ignited for a few tens of seconds before the cold air is blown into the furnace. This affects the thermal efficiency of the furnace and increases energy consumption. In addition, the air-fuel ratio of the combustion engine cannot be controlled automatically. Generally, it is a large air-fuel ratio operation, resulting in increased energy consumption.
3.3.1 Using High-Speed ​​Burners to Replace Burners (See Multi-Bar Burners)
3.3.2 Addition of automatic control system (1) Basic composition of heating furnace temperature control system The basic composition of heating furnace temperature control system is shown in Figure 3-10. It consists of PLC master control system, phase-shift trigger module, rectifier SCR, heating furnace, Sensors and other five parts.
The short rod heating furnace is the furnace temperature as an indicator of temperature control, and the furnace temperature is preset to the corresponding temperature.
3.3.3 Canceling the circulation fan The short rod heating furnace is generally equipped with 1~4 circulating fans, trying to make the temperature of the furnace uniform and increase the flow rate of the flue gas.
(1) After canceling the replacement of the circulation fan into the high-speed burner, because the high-speed burner discharges at a high speed, it has a strong stirring effect on the atmosphere inside the furnace, and the fan can ensure the furnace temperature is uniform without using a fan. Therefore, the circulation fan can be completely eliminated. To save power.
(2) Renovate the combustion chamber at the bottom of the hearth to reduce the volume of the bottom hearth and maintain the flow rate of the flue gas. The furnace is reduced with special refractory bricks, and the upper part is opened in the direction of the flow of the flue gas in ascending order to ensure uniform heating of the aluminum rod.
According to the energy saving effect of the short rod furnace of an aluminum processing enterprise, the gas-fired energy-saving rate reaches 10 to 35% or more.
4 Oxidized Sludge Deep Processing Technology During the surface treatment of aluminum profiles, a large amount of colloidal waste liquid will be produced. After sludge treatment, it is commonly referred to as sludge, and after further dehydration, it is aluminum-containing waste residue. This kind of waste residue is very large. It only produces an oxidation coloring line with an annual output of 2,400 tons of aluminum profiles. It produces about 150,000 tons of sludge and 2000 tons of waste residue each year. Therefore, comprehensive utilization is of great significance.
Aluminum waste water treatment process is simple in principle, convenient in operation and management. The current problem is the disposal of waste slag. Aluminum sludge still contains more hydroxides after dewatering by a filter press. Any secondary disposal will cause secondary pollution. In fact, the precipitate of aluminum waste water contains a large amount of aluminum hydroxide, and if it is developed and utilized, active alumina products are produced and have a wide range of uses.
4.1 Oxidation Sludge The process for producing active alumina is shown in Figure 4-1. By roasting in a fluidizing furnace, the wet aluminum hydroxide is first sent to a dryer, sent to a fluidized roasting furnace and injected into a fuel to be calcined into alumina. After granulation, health preservation, activation, and classification, it is more complete. product.
5 Conclusions Through the energy-saving technological transformation of aluminum melting furnaces and rod furnaces in the aluminum processing industry, the deep processing and utilization of oxidized sludge has been studied. Some practical experiences have been summarized and certain results have been achieved. However, because aluminum processing companies are mostly full of orders, there is seldom enough time for a thorough energy-saving transformation, and there is still much room for energy conservation. It is suggested that aluminum processing enterprises increase their awareness of energy conservation and fully realize that energy conservation and emission reduction are not only tough measures formulated by the state to complete the national convention, but also have important significance for improving the economic efficiency of enterprises, improving the management level of enterprises, and improving the level of technological equipment of enterprises. .