【China Aluminum Industry Network】1 Overview

Although China’s aluminum profile production has ranked first in the world for five consecutive years, there is still a large gap between the production technology of the aluminum profile industry and those of the United States, Japan, Germany, and Italy, and the task of energy saving and emission reduction is significant. Mainly in the following areas:

(1) Energy Consumption and Pollutant Emissions of Aluminum Profile Industry

The average industrial water consumption per ton of aluminum profiles consumed is approximately 16-18 tons; the waste water produced by the surface treatment contains a variety of metal impurity ions; high energy consumption; a large amount of waste residue, only an oxidation coloring production line with an annual output of 2,400 tons of aluminum profiles, Each year it produces about 150,000 tons of sludge and 2000 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 Profile Industry

1 Smelting and recycling: At present, the electric furnace in the aluminum smelting furnace accounts for 5%, the oil furnace accounts for 91%, and the gas furnace accounts for 4%, resulting in a fuel consumption of re-melting to produce 1 ton of extruded round ingots which is 55.17% higher than that of industrial advanced countries. The average smelting energy consumption of ingots (rolled slabs and extruded pellets) is much higher than that of industrially 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 clean production technology of aluminum melting furnace

2.1 Aluminum melting furnace working process

The smelting process of the aluminum melting furnace can be roughly divided into four stages, that is, charging the material into a softened bed, softening the bed until the material is charged, flattening the material to be fully melted (oxidation scum generated at this stage), and heating the aluminum liquid. 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 aluminum melting furnace, in the melting period of aluminum, the temperature of the furnace is generally controlled at 1200°C. The temperature of the flue gas at this time is the temperature of the furnace, and the heat removed by the flue gas accounts for about 50-70 of the heat load of the furnace. %, taking into account 10% of other heat losses, effective heat utilization is only 30 - 40%, if you do not make full use of this part of waste heat, it will inevitably cause great waste, so that the thermal efficiency of the furnace is 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 air passing through the ceramic honeycomb body is preheated to above 1000° C. and injected into the furnace at an appropriate speed. Combine 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 low oxygen concentration (lower 5%-6.5%) fluid and burned under high temperature air conditions. It can achieve low air coefficient combustion and reduce the oxidation loss of aluminum.

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 outgassing speed of the combustor outlet gas mixture is about 60m/s, the flame length is about 2.5-3m, and the flame diameter is about 0.5-0.7m. 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 stoves in use, the addition of a pair of regenerative burners worth 300,000 yuan is more difficult for companies to accept.

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. high-speed burner products, we have developed high-performance burners with a high flow rate of 180m/s, a load regulation ratio of 1:20, and an excess air ratio of 0.65 to 10.8. Adjust between.

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 Aluminum Furnace Automatic Control Technology

The control system is an important guarantee for improving combustion, reducing energy consumption, ensuring process requirements, and increasing product output and output. The ultimate goal is to achieve automatic detection and process control of combustion equipment flow, temperature, pressure, and atmosphere parameters.

2.4.1 furnace pressure automatic adjustment control

A reasonable flame furnace should achieve micro-pressure operation.

3 extruder bar furnace energy-saving technology

The energy consumption of extruder rod furnaces accounts for 12 - 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 rod 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 furnace is assumed to be 60%, and natural gas is also consumed 22 m3 more. /t aluminum rod, energy consumption of the product is 26m3/t. 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 Energy Saving Technology

3.1.1 Renovation furnace

According to theoretical calculations, the volumetric thermal strength of the furnace is much lower than that of industrial furnaces. The furnace is too large and the temperature in the furnace is low, which is unfavorable to combustion. It is necessary to increase the combustion power to reach a certain volumetric thermal intensity, which inevitably increases the unit product. Gas consumption. We adopted the method of reducing the volume of the furnace and increasing the vault bricks to transform the furnace.

3.1.2 Replace Burner

The existing DC burners were all replaced by a patented product, the Swirl Burner (second generation), made by a good combustible company. Increase flame stiffness, increase flame temperature, and enhance heat transfer.

(1) Working principle of swirl burner

The burner is equipped with various types of swirl generators (referred to as cyclones). 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) Good combustible swirl burner structure

Swirl nozzle, compared with the traditional long rod heat shear furnace nozzle, has a large flame stiffness, large heat transfer, high combustion efficiency.

3.1.3 Cancelling the circulation fan

Extruder rod furnace in the case of a circulating fan, the furnace negative pressure is too high, the amount of cold air intake, the nozzle severe off the fire, greatly reducing 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 - 66,000 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 sends the oxygen content data to the PLC. The fixed air-fuel ratio data adjusts the opening of the gas and air-conditioning valves so that the oxygen content is always maintained at an appropriate ratio to ensure accurate organization of combustion.

3.1.5 strengthen insulation

After the furnace is reformed, new insulation materials are used in the exterior of fire nozzle bricks and roof bricks to enhance insulation, which is to reduce the temperature of the furnace wall below 80° C. and 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 Reconstruction of the combustion chamber

The bar side of the multi-rod furnace was converted into a combustion chamber, the original burner system was eliminated, and the high-speed burner was used as a heat source device to directly inject the high-speed gas stream of the high-speed burner into the combustion chamber to reduce the heat exchange process with the air.

In order to avoid direct contact of the flame with the aluminum rod, a melt rod accident occurs, in the burner flame

A refractory partition is placed at the bottom so that the flame spreads rapidly to all corners of the furnace.

3.2.2 Using High Speed ​​Burners

Similar to aluminum melting furnace.

3.2.3 Cancelling Hot Air Circulation System

After the combustion chamber is reformed, all the circulating fans, combustion engines, and the original combustion chamber will be eliminated. To reduce the workload of the transformation, 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 of all, it is specified that the pressure range of the combustion chamber is 10-20 Pa, and a baffle plate is installed on the chimney. When the furnace pressure is high, the baffle opening degree 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 Adding Plate Air Preheaters

In order to ensure that the temperature of the flue gas is maintained at about 150°C and full use is made of flue gas residual heat, an air preheater is installed at the bottom of the chimney to preheat the combustion air, increase the combustion temperature, reduce the excess air coefficient, and increase the efficiency of the rod furnace. 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 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 Adding Automatic Control System

(1) Basic composition of a heating furnace temperature control system

The basic composition of the heating furnace temperature control system is shown in Figure 3-10. It consists of 5 parts: PLC master control system, phase-shift trigger module, rectifier SCR, heating furnace, and sensors.

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 cancel the circulation fan

Short rod heating furnaces are 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) Cancel the circulation fan

After replacing with a high-speed burner, because the high-speed burner has a high jetting speed, it has a strong stirring effect on the atmosphere in the furnace, and the temperature of the furnace can be ensured without a fan. Therefore, the circulating fan can be completely eliminated to save power consumption.

(2) Renovation of the combustion chamber at the bottom of the hearth

Reduce the volume of the bottom furnace to maintain the flue gas flow rate. 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.

Through the energy saving effect of the short rod furnace of an aluminum processing enterprise, the energy saving rate of gas is 10 to 35%.

4 Oxidized Sludge Deep Processing Technology

A lot of colloidal waste liquid will be produced during the aluminum surface treatment process. After sludge treatment, it is commonly called sludge, and after further dehydration, it is aluminum-containing waste residue. This kind of waste residue is very large, only one

With an annual output of 2,400t of aluminum oxide coloring lines, it produces about 150,000 tons of sludge annually and 2000 tons of waste residue. 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 Production of Active Alumina

The process flow 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 Conclusion

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 was studied. Some practical experiences were summarized and certain results were 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. .

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