Non ferrous metallurgy - refractory materials for aluminum smelting

Nonferrous smelting is mainly divided into the following sections: aluminum smelting, copper smelting, lead smelting, zinc smelting, nickel smelting, gold and silver smelting. The editor will introduce these refractory materials used in non-ferrous smelting one by one, combined with our company's product introduction.



In the world's non-ferrous metal production, aluminum ranks first in annual production, far exceeding other non-ferrous metals. The aluminum industry consumes much more refractory materials annually than the total amount consumed in copper, lead, and zinc smelting. The production method of metallic aluminum is a fixed two-step production method: the first step is to produce aluminum oxide from bauxite by wet method; The second step is to use industrial alumina as raw material and use molten salt electrolysis method to produce metallic aluminum. The high-temperature kilns used in the production process include rotary kilns, molten salt electrolysis tanks, aluminum melting furnaces, etc.



The consumption of refractory materials in aluminum industrial furnaces is significant. The reason is that during the production of Al2O3, alkaline substances in the material have a particularly severe erosion on the refractory materials of the rotary kiln. In the process of melting aluminum, even at lower temperatures, metallic aluminum still has strong permeability. Once it infiltrates into the brick, it will react with SiO2 in the brick, reducing Si and damaging the microstructure of refractory materials, resulting in a metamorphic layer, looseness, peeling and damage to the furnace lining. Its reaction: 3SiO2+4A1-2A12O3+3SiO2



Therefore, refractory materials containing SiO2 are not suitable as furnace building materials for metal aluminum smelting equipment. So, refractory materials commonly used in aluminum industry furnaces, in addition to high alumina bricks, are often made of carbon products.



Refractory materials for alumina rotary kilns



At present, due to raw material reasons, the production process of alumina in China mostly adopts sintering and combined methods. The drying and tender roasting of aluminum clay, as well as the roasting of aluminum hydroxide, are mostly carried out using rotary kilns. In recent years, the introduced fluidized bed roasting equipment has been widely used, but it still accounts for a large proportion in some old rotary kilns.



Rotary kiln is a sintering kiln for alumina clinker. When producing alumina, the alumina clay is first mixed with soda ash and lime in proportion and loaded into a rotary kiln. After being fired at 1200-1300 ℃, it is discharged from the kiln and then processed appropriately to produce aluminum hydroxide and mother liquor; Aluminum hydroxide can be loaded into a rotary kiln and fired at a high temperature of 1200 ℃ to make it. The calcination process in a rotary kiln involves the reverse movement of high-temperature flames and heated materials in the furnace. Alkali lime alumina clay raw material slurry (with a water content of 40%) or aluminum hydroxide (with a water content of 12%~18%) is added from the kiln tail, and after low-temperature drying, dehydration, heating, and high-temperature calcination, it is discharged from the kiln head and high-temperature gas flows from the kiln head to the kiln tail. Therefore, the kiln is divided into pre heating zone and high-temperature calcination zone. To prevent the slurry from sticking to the kiln lining and enhancing heat transfer during the heating and calcination process, chains are also installed between the refractory masonry. During the rotary motion of the kiln body, the materials and lining bricks are continuously struck, which has a certain impact on the service life of the kiln lining.



The rotary kiln body used for alumina production is a cylinder made of welded steel plates, lined with refractory materials. The working environment of refractory materials is harsh and the conditions are harsh. It should have the following characteristics: strong resistance to alkali erosion; Can work under high temperatures of 1200-1300 ℃ for a long time without damage; Capable of withstanding impacts from dynamic loads; Can resist the erosion of furnace materials; Capable of withstanding erosion from high-temperature airflow. The refractory materials used in the rotary kiln are mainly high alumina bricks and magnesium chromium bricks, while the low-temperature drying kiln uses clay bricks as the lining. The refractory materials used in the rotary kiln for alumina production are shown in Table 1.



Picture Table 1 Refractory Materials for Rotary Kilns Used in Alumina Production



Nowadays, amorphous refractory materials have been widely used in the aluminum industry. Due to the high temperature wear and thermal shock stress of materials, the kiln mouth of rotary kilns is prone to deformation and damage; In the transition zone of the alumina clinker kiln, the ambient temperature is between 400 and 1000 ℃, and it is severely affected by alkali corrosion and mechanical damage (vibration, distortion), with the lining often falling off. Rotary kilns also use steel fiber reinforced castables, mainly used in preheating zones, kiln entrances, kiln tails, and cooling machines.



Aluminum hydroxide roasting is the final process in the production of aluminum oxide, which mainly involves drying the attached water and crystalline water in the aluminum hydroxide filter cake, and converting a portion of β - type aluminum oxide into A-type aluminum oxide. Currently, the main domestic alumina manufacturers have fully or partially adopted the introduced fluidized bed roasting equipment for the calcination of aluminum hydroxide. Fluidized roasting equipment includes three types of furnaces: fluidized bed flash roasting furnace, circulating fluidized bed roasting furnace, and suspension roasting furnace. Although different refractory materials are used, many of them use amorphous refractory materials (refractory plastic or Refractory castable), which account for 50% to 70% of the fire resistant materials used. All the amorphous refractory materials of fluidized bed flash calcination furnaces were imported from Germany, while all the refractory materials of circulating fluidized bed calcination furnaces were produced domestically.



The alumina gas suspension roasting furnace is a specialized equipment used for roasting aluminum hydroxide, with high technology and automation level. The roasting process is completed at a high operating temperature of about 1200 ℃ in the high-temperature furnace body, under high-speed conditions. At the same time, due to the high hardness and good fluidity of the processed alumina material, there are strict requirements for the quality of alumina products. Any impurities in the lining material directly affect the function of the product. Therefore, it is required that the refractory material must meet the following conditions: high temperature resistance, wear resistance, high strength, good thermal stability function, good overall integrity, and strong sealing.



The electrolytic cell is the core equipment for producing electrolytic aluminum, which is usually a rectangular steel shell lined with carbon bricks. There is a carbon anode suspended in the electrolytic cell, and the bottom of the carbon cell is the cathode. Aluminum electrolysis uses molten solutions such as cryolite, aluminum fluoride, and lithium fluoride as electrolytes. Al2O3 is melted at around 970 ℃ and ionized under the action of electric field force. The molten metal aluminum recovered from electrolysis is deposited on the bottom cathode of the tank, and the oxygen released from the anode reacts with the carbon anode to generate CO2 or CO. The heat released by the electrochemical reaction keeps the electrolytic cell and aluminum in a molten state. The aluminum liquid is released from the cell at certain intervals and a certain amount of aluminum oxide and cryolite are added to the cell; The electrolysis temperature is between 900~1000 ℃.



The working layer at the bottom of the electrolytic cell is generally constructed with carbon blocks. But due to the reaction between carbon and sodium forming a new compound, the brick lining structure becomes loose, the strength decreases, and the carbon block shows cracks. Subsequently, electrolytes and aluminum liquid enter along the cracks, and at high temperatures, aluminum reacts with carbon to form a loose bond with carbon, causing cracks to expand and ultimately leading to deformation of the electrolytic cell shell and severe corrosion of the inner lining, shortening its service life. Therefore, the cathode material at the bottom of the electrolytic cell is being changed from amorphous carbon bricks to semi graphitized carbon bricks or graphitized carbon bricks.



The main reasons for the damage of the inner lining of the side wall of the aluminum electrolysis cell are: air inhalation between the steel shell and brick lining causes oxidation of the data; Corrosion of cryolite, NaF, and aluminum liquid at high temperatures; Scouring caused by melt flow; Temperature fluctuations and thermal expansion cause thermal stress.



The side walls of aluminum electrolysis cells have always used amorphous carbon blocks, graphite carbon blocks, etc. The most fatal drawback of such materials is poor antioxidant function and low strength. In order to prevent oxidation and increase resistance of the side walls, they are developing towards partial or full selection of SiC materials. Silicon carbide bricks combined with silicon nitride are the best for use. Silicon carbide bricks combined with silicon nitride have excellent high-temperature mechanical properties, good thermal conductivity, and are prone to forming condensation slag on the inner side; High electrical resistivity reduces the loss of current on the sidewalls; The data is not easily oxidized; Not reacting with molten aluminum, cryolite, etc; High mechanical strength, can greatly reduce the thickness of lining bricks, increase the volume of electrolytic cells, and ensure stable operation. For example, when using carbon bricks, the thickness of the side wall was about 200-400mm, while the thickness of the back wall of silicon carbide bricks combined with silicon nitride was only 75mm.



Barrier layer below the bottom of the groove



In the production of electrolytic aluminum, the vapor and liquid of Na and NaF can enter the insulation layer below through the cathode data at the bottom of the tank. After the insulation layer enters NaF, the thermal conductivity is added, the thermal efficiency of the electrolytic cell decreases, and the operating condition deteriorates until the cell is damaged. The "barrier layer" below the cathode material is a layer of material that can block electrolyte penetration between the cathode refractory material and the insulation material, and has outstanding insulation function. A new type of "barrier layer" material, dry anti-seepage material, has now been well applied.



Refractory materials for refining furnaces and insulation furnaces



The commonly used smelting furnaces for the melting and alloying of primary aluminum ingots and waste aluminum are fixed or tilting reflex furnaces using gas or oil, as well as resistance reflex furnaces and induction volute furnaces. Although the temperature of aluminum liquid and aluminum alloying in the smelting furnace is only 700-800 ℃, the magnesium, silicon, and aluminum in aluminum and its aluminum alloys are very active and easily react with some components in the refractory materials, causing damage to the refractory materials. The corrosion damage mechanism of aluminum smelting furnace is mainly: aluminum liquid is easy to enter refractory materials;



The alloying elements in aluminum and its alloys have strong restorative abilities towards some oxides, and the resulting redox reactions are strong exothermic reactions; Some alloy elements such as magnesium have a high vapor pressure, and their vapor is more likely to enter refractory materials than aluminum liquid. After entering refractory materials, they are then oxidized, ultimately leading to refractory material transformation, structural looseness, and damage;



In the melting process of large aluminum melting furnaces, due to the continuous addition of aluminum ingots and alloys, the impact and wear of aluminum ingots and alloy blocks on the furnace mouth, bottom, and wall are very severe;



The participation of aluminum ingots and alloy blocks, the outflow of aluminum liquid, and fluctuations in furnace temperature can cause thermal shock damage to the refractory material lining.



The refractory materials used for aluminum smelting reflex furnaces require resistance to the entry of aluminum liquid and magnesium vapor, as well as excellent wear resistance and thermal shock resistance. The lining of the aluminum smelting reflection furnace that touches the aluminum liquid is generally constructed with high alumina bricks with an Al2O3 content of 80% to 85%; When melting high-purity aluminum metal, choose mullite bricks or corundum bricks. Use silicon carbide bricks bonded with silicon nitride in areas prone to corrosion and wear such as furnace bed slopes and waste aluminum materials. In areas such as aluminum flow channels and aluminum outlets, aluminum liquid is severely eroded. Generally, self bonded or silicon nitride bonded silicon carbide bricks are used, and staggered quartz bricks are also used as lining. The use of vacuum cast refractory fibers for aluminum outlet blockage has a better effect. For furnace linings that do not touch aluminum liquid, clay bricks, clay Refractory castables, or refractory plastics are generally selected. The lining of aluminum launder is generally made of silicon carbide bricks, or prefabricated bricks made of fused foam silicon bricks.



Nowadays, with the increasing scale and strengthening requirements of aluminum melting furnaces, high-strength and anti aluminum penetration castables have been well applied due to their excellent resistance to aluminum liquid and magnesium vapor entry, as well as their excellent wear resistance and thermal shock resistance.