(1) Energy consumption and pollutant discharge of aluminum profile industry
An average of 1-18 tons of aluminum profiles consumes about 16-18 tons of industrial water; waste water from surface treatment contains a variety of metal impurity ions; high energy consumption; generates a large amount of waste residues, only one oxidation coloring production line with an annual output of 2400 tons of aluminum profiles. Every year, about 150,000 tons of sludge and 2,000 tons of waste residue are produced, which is a huge amount. The composition of wastewater sludge is relatively complex, and most plants currently use landfills to treat these sludges. This not only takes up limited land resources, but also wastes resources and pollutes the environment.
(2) Reasons for high energy consumption and high pollution in the aluminum profile industry
①Smelting and recycling: At present, electric furnaces account for 5%, oil furnaces account for 91%, and gas furnaces account for 4% of aluminum smelting furnaces. The fuel consumption for remelting 1 ton of extruded round ingots is 55.17% higher than that of industrially developed countries. The average melting energy consumption of ingots (rolled flat ingots and extruded round ingots) is much higher than that of industrially developed countries. In addition, the overall energy consumption of aluminum smelting furnaces is still at a low level.
② Casting, rolling and extrusion: lack of high-end products; many small machines, relatively backward extrusion equipment, low efficiency; low degree of automation of equipment, unable to achieve isothermal rapid extrusion, production of precision profiles; mold quality is poor.
③Surface treatment: Surface treatment is a link of high power consumption, high water consumption and high pollution in aluminum processing.
Energy saving and emission reduction has become an urgent problem to be solved by today’s aluminum processing enterprises. This article proposes some new methods and strategies from the aspects of aluminum melting furnace, holding furnace, extruder rod furnace, oxidation treatment sludge deep processing and utilization, so that aluminum processing The enterprise achieves the goals of energy saving, emission reduction and cleaner production.
2 Energy-saving emission reduction and cleaner production technology of aluminum melting furnace
2.1 Working process of aluminum melting furnace
The melting process of the aluminum melting furnace can be roughly divided into four stages, that is, the charge is loaded into the softening bed, the softening bed is until the charge leveling, the charge leveling is completely melted (oxidized scum is generated at this stage), and the aluminum liquid is heated. The heating of the aluminum material is accomplished by the convection heat transfer of the burner flame, the radiant heat transfer of the flame and the furnace wall, and the conductive heat transfer between the aluminum materials.
Throughout the process, the ratio between the three is constantly changing. In the solid state, the blackness of aluminum is small, and the thermal conductivity is strong. As the smelting process progresses, the charge enters the critical state of semi-liquid and semi-solid, its thermal conductivity decreases, and the thermodynamic properties undergo a fundamental change. The thermal conductivity of liquid aluminum is only 40% of that of solid aluminum, and the heat transfer from the top to the bottom of the molten pool is very slow. The loose scum floating on the metal mirror surface constitutes an insulating barrier for heat transfer. At this time, the oxide film on the surface of the molten pool is opened, and the protective effect is lost, and the tendency of oxidation and inhalation is enhanced. For the flame melting aluminum furnace, during the melting period of aluminum, the furnace temperature is generally controlled at 1200 ℃, the temperature of the flue gas at this time is the furnace temperature, and the heat taken away by the flue gas accounts for about 50-70 of the furnace heat load %, considering the other heat loss of 10%, the effective heat utilization is only 30-40%. If this part of waste heat is not fully utilized, it will inevitably cause a lot of waste and make the furnace thermal efficiency very low.
In summary, selecting an effective enhanced heating method and recovering flue gas waste heat to preheat combustion air is an effective way to improve the furnace thermal efficiency and ensure less direct fuel consumption during the smelting process.
2.2 Using high temperature air combustion technology
High-temperature air combustion has changed the traditional combustion method, using flue gas recirculation method or direct injection combustion method in the fuel furnace. The main performance 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 , Mixed with the combustion products in the furnace under the action of high-speed airflow entrainment and stirring, 21% of the oxygen in the air is diluted, burned in a fluid with a low oxygen concentration (lower 5%-6.5%), and burned under high temperature air conditions It can achieve low air coefficient combustion and reduce oxidative burning loss of aluminum.
The regenerative combustion system mainly includes a pair of burners equipped with regenerators, a set of reversing devices, a set of control systems and piping systems.
When the furnace gas temperature is 1000-1200°C, the combustion air temperature can be preheated to 800-1000°C, which can save about 50% of fuel compared with the fuel-melting aluminum furnace using a partition wall air heat exchanger.
The actual gas injection speed at the burner outlet 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 reversing operation, and the high temperature area is frequently exchanged to ensure that the temperature in the furnace is uniform, and no low temperature area is formed.
2.3 High-speed burner technology
For the old furnace in use, it is difficult for enterprises to accept a new pair of regenerative burners for 300,000 yuan.
A nozzle with a high-speed burner is used. The gas is sprayed into the furnace at a high speed of more than 100m/s. The combustion-supporting air participates in the combustion-supporting at a speed of 90m/s, which has a strong impact on the aluminum stack and accelerates the melting. In order to prevent fire, the gas The spout is equipped with a long bright ignition gun.
On the basis of digesting and absorbing American Tianshi high-speed burner products, we have developed a high-speed burner with excellent performance, the flue gas velocity can reach 180m/s, the load regulation ratio can reach 1:20, and the excess air coefficient can be 0.65-10.8 Adjust between.
The technical characteristics of high-speed gas burners are as follows:
a) Accurately organize the combustion, the combustion efficiency is 99.9%;
b) Wide operating conditions: heat load regulation ratio 1:20, air coefficient 0.5—10;
c) It adopts staged combustion, and the emission of harmful gas (NOx) complies with the national environmental protection standards;
d) It has the function of flue gas ejection and reflux, which can lead the waste flue gas back from the furnace and put it back into the furnace;
e) Full metal structure, continuous service life of 3 years.
