Non-ferrous smelting is mainly divided into the following major blocks: aluminum smelting, copper smelting, lead smelting, zinc smelting, nickel smelting, gold and silver, and here we introduce these refractory materials for non-ferrous smelting one by one.
In the production of non-ferrous metals in the world, the annual output of aluminum ranks first, far exceeding other non-ferrous metals. The annual consumption of refractory materials in the aluminum industry is much more than the total amount of refractory materials consumed by copper, lead and zinc smelting. The production method of metallic aluminum is a fixed two-step preparation method: the first step is to prepare alumina from bauxite ore by wet method; the second step is to use industrial alumina as raw material to prepare metal by molten salt electrolysis aluminum. The high-temperature kilns used in the production process include rotary kilns, molten salt electrolytic cells, aluminum melting furnaces, etc.
Aluminum industrial furnaces consume a lot of refractory materials. The reason is that when preparing AI2O3, the erosion of the refractory material of the rotary kiln by the alkaline substances in the material is particularly serious. In the process of smelting aluminum, even at relatively low temperature, metal aluminum still has a strong penetrating ability. Once it penetrates into the brick, it will react with SiO2 in the brick and restore Si, destroying the structure of the refractory material. The furnace lining produces a metamorphic layer, which is loose, peeled off and damaged. Its reaction: 3SiO2 + 4Al —2Al2O3 + 3SiO
Therefore, refractory materials containing SiO2 are not suitable as furnace materials for aluminum smelting equipment. Therefore, refractory materials for general aluminum industrial furnaces, except high alumina bricks, are commonly used carbonaceous products.
1. Refractories for alumina rotary kiln
At present, due to raw materials, most of the production processes of alumina in my country adopt sintering method and combined method. The drying and calcination of bauxite and the roasting of aluminum hydroxide mostly use rotary kilns. In recent years, the imported fluidized roasting device has been widely used, but the rotary kiln still accounts for a large proportion in some old factories.
Rotary kiln is a sintering kiln for alumina clinker. When preparing alumina, firstly put bauxite, soda ash and lime in proportion into the rotary kiln, and then leave the kiln after being calcined at 1200~1300°C, and then make aluminum hydroxide and mother liquor through proper treatment; aluminum hydroxide Put it into a rotary kiln and calcine at a high temperature of 1200°C to make it. The calcination process in the rotary kiln is as follows: the high-temperature flame and the heated material make reverse movement in the furnace. Soda-lime-aluminum rock slurry (water content 40%) or aluminum hydroxide (water content 12% ~ 18%) is added from the kiln tail, after low-temperature drying and dehydration, heating, and high-temperature calcination, the material is discharged from the kiln head and the high-temperature gas is discharged from the kiln head flow to the end of the kiln. Therefore, the kiln is divided into a pre-tropical zone and a high-temperature stewing zone. In order to prevent the slurry from adhering to the kiln lining and enhance the heat transfer process during the heating and calcination process, a chain is set between the refractory masonry, and the material and the lining brick are continuously hit during the kiln body’s rotating motion, and the kiln lining have a certain impact on the service life.
The rotary kiln body for alumina production is a cylinder body welded by steel plates and lined with refractory materials. The working environment of refractory materials is harsh and the conditions are harsh. It should have the following characteristics: strong alkali corrosion resistance; can work at a high temperature of 1200~1300 ℃ for a long time without damage; can withstand the impact of dynamic load; can resist the erosion of furnace materials; can withstand the erosion of high temperature airflow. The refractory materials used in the rotary kiln are mainly high-alumina bricks, and the low-temperature drying kiln is lined with clay bricks.
At present, unshaped refractory materials have been widely used in the aluminum industry. Due to the high temperature wear and thermal shock stress of the material at the kiln mouth of the rotary kiln, the kiln mouth is easily deformed and damaged; in the transition zone of the alumina clinker kiln , the ambient temperature is 400 ~ 1000 ℃:, subject to alkali corrosion and mechanical damage (vibration, distortion) serious, the lining often falls off. The rotary kiln also has steel fiber reinforced castables, which are mainly used in the preheating zone, kiln mouth, kiln tail and cooler.
Aluminum hydroxide roasting is the last process in the alumina production process. It is mainly to dry the attached water and crystal water in the aluminum hydroxide filter cake, and convert a part of β-alumina into α-alumina. At present, the aluminum hydroxide roasting of major domestic alumina manufacturers has adopted the imported fluidized roasting device in whole or in part. Fluidized roasting equipment is divided into three furnace types: fluidized flash roaster, circulating fluidized bed roaster, and suspension roaster. Or refractory castables), the amount of which accounts for 50% to 70% of the refractory materials used. The monolithic refractory materials for the fluidized flash roaster are all imported from Germany, and the refractory materials for the circulating fluidized bed roaster are all domestically produced.
Alumina gas suspension roasting furnace is a special equipment for roasting aluminum hydroxide. Its technology and automation level are very high. The alumina material has high hardness and good fluidity, and has very strict requirements on the quality of alumina products. The mixing of any impurities in the lining material will directly affect the function of the product. Therefore, the refractory material must meet the following conditions: high temperature resistance, Wear resistance, high strength, good thermal stability, good integrity and strong sealing.
The electrolytic cell is the core equipment for the production of electrolytic aluminum. The electrolytic cell is usually a rectangular steel shell lined with carbon bricks. There is a carbon anode suspended in the electrolytic cell, and the carbon bottom of the cell is the cathode. Aluminum electrolysis uses cryolite, aluminum fluoride, lithium fluoride and other melts as electrolytes, melts Al2O3 at about 970 ° C, ionizes under the action of electric field force, and the metal aluminum melt recovered by electrolysis is deposited on the cathode and anode at the bottom of the tank The released oxygen 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, and the molten aluminum is released from the cell for a certain period of time, and a certain amount of alumina and cryolite are added to the cell; the electrolysis temperature is 900~1000°C.
The working layer at the bottom of the electrolytic cell is generally built with carbon blocks. However, due to the reaction of carbon and sodium to form new compounds, the brick lining structure is loose, the strength is reduced, and cracks appear in the carbon block. Then the electrolyte and aluminum liquid enter along the cracks, and the aluminum and carbon react to loosely combine with carbon at high temperature, so that the cracks expand, and eventually lead to deformation of the electrolytic cell shell and severe corrosion of the inner lining and shorten the service life. Therefore, the cathode material at the bottom of the electrolytic cell is changing from the original amorphous carbon brick to semi-graphitized carbon brick or graphitized carbon brick.
The main causes of damage to the side wall lining of aluminum electrolytic cells are: oxidation of materials caused by inhalation of air from between the steel shell and brick lining; corrosion of cryolite, NaF and molten aluminum at high temperatures; erosion caused by melt flow; temperature fluctuations and thermal expansion cause thermal stress.
The side walls of aluminum electrolytic cells have always been made of amorphous carbon blocks, graphite carbon blocks, etc. The most fatal disadvantage of this type of material is poor oxidation resistance and low strength. In order to make the sidewall not oxidized and have a larger resistance, the sidewall is developing towards the use of SiC materials in part or in full. Silicon nitride bonded silicon carbide bricks are best used. Silicon carbide bricks combined with silicon nitride have excellent high-temperature mechanical properties, good thermal conductivity, and easy to form condensation slag on the inside; high resistivity, reducing current loss on the side wall; the material is not easy to be oxidized; it is not compatible with aluminum liquid and ice crystals Melts such as stone react; high mechanical strength, can also greatly reduce the thickness of the lining brick, increase the volume of the electrolytic cell, and stabilize the operation. For example, when using carbon bricks, the thickness of the side wall is about 200~400mm, but the thickness of the side wall is only 75 mm after using silicon carbide bricks combined with silicon nitride.
2. The barrier layer under the bottom of the tank
In the production of electrolytic aluminum, the vapor and liquid of Na and NaF can pass through the cathode material at the bottom of the tank and enter the lower heat insulation layer. After the heat insulation layer enters NaF, etc., the thermal conductivity increases, the thermal efficiency of the electrolytic cell decreases, and the working condition deteriorates until the cell is damaged. The “barrier layer” under the cathode material is a layer of material that can prevent electrolyte penetration between the cathode refractory material and the insulation material, and has good insulation performance. A new type of “barrier” material, dry impervious, is now in good use.
3. Refractory materials for melting furnace and holding furnace
The smelting furnaces commonly used for the melting and alloying of primary aluminum ingots and scrap aluminum mostly use gas or fuel oil fixed or tilting reverberatory furnaces, and resistance reverberatory furnaces and induction crucible furnaces are also used. Although the temperature of aluminum liquid and aluminum alloying in the smelting furnace is only 700~800°C, the magnesium, silicon and copper in aluminum and its aluminum alloys are very active, and it is easy to react with some components in the refractory material, resulting in refractory material damage. The corrosion damage mechanism of aluminum smelting furnace is mainly:
Liquid aluminum is easy to enter the refractory material;
The alloying elements in aluminum and its alloys have a strong recovery ability to some oxides, and the resulting redox reaction is a strong exothermic reaction; some alloying elements such as magnesium have a high vapor pressure, and its vapor is easier than molten aluminum Enter the refractory material, and then be oxidized after entering the refractory material, which will eventually lead to qualitative change, loose structure and damage of the refractory material;
During the smelting process of a large aluminum melting furnace, due to the continuous addition of aluminum ingots and alloys, the impact and wear of aluminum ingots and alloy blocks on the furnace mouth, furnace bottom and furnace wall are very serious;
The addition of aluminum ingots and alloy blocks, the outflow of molten aluminum, and fluctuations in furnace temperature, etc., cause thermal shock damage to the refractory lining.
Refractory materials for reverberatory furnaces for aluminum smelting are required to resist the entry of molten aluminum and magnesium vapor, and have excellent anti-wear and thermal shock resistance. The lining of the aluminum smelting reverberatory furnace contacting the molten aluminum is generally made of high-alumina bricks with an AI2O3 content of 80% to 85%; when smelting high-purity metal aluminum, mullite bricks or corundum bricks are used. Silicon carbide bricks combined with silicon nitride are used in the parts that are prone to corrosion and wear, such as the slope of the hearth and the waste aluminum material. The parts such as the aluminum flow tank and the aluminum outlet are severely scoured by the molten aluminum. Generally, self-bonded or silicon nitride-bonded silicon carbide bricks are used, and malatite bricks are also used as linings. For the obstruction of the aluminum outlet, it is better to use vacuum-cast refractory fiber. For furnace linings that do not touch molten aluminum, clay bricks, clay refractory castables or refractory plastics are generally used. The lining of the flow aluminum tank is generally made of silicon carbide bricks, and prefabricated bricks of fused silica foam bricks can also be used.
Now, with the enlargement of aluminum melting furnaces and the requirement of intensified tempering, the high-strength anti-aluminum infiltration castable has excellent anti-aluminum and magnesium vapor entry functions, excellent anti-wear and thermal shock resistance, etc. use.
