01. Frontier Overview
Nonferrous smelting is mainly divided into the following major blocks: aluminum smelting, copper smelting, lead smelting, zinc smelting, nickel smelting, gold and silver. Here we introduce these refractory materials for nonferrous smelting one by one.
In the world's nonferrous metal production, the annual output of aluminum ranks first, far exceeding other nonferrous metals. The aluminum industry consumes much more refractory materials each year 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 produce alumina from bauxite by wet method; the second step is to use industrial alumina as raw material and use molten salt electrolysis to produce metallic aluminum. The high-temperature kilns used in the production process include rotary kilns, molten salt electrolytic cells, aluminum melting furnaces, etc.
The consumption of refractory materials in aluminum industrial furnaces is very large. The reason is that when preparing AI2O3, the alkaline substances in the material are particularly serious in corroding the refractory materials of the rotary kiln. In the process of smelting aluminum, even at a relatively low temperature, metallic aluminum still has a strong penetration ability. Once it penetrates into the brick, it will react with SiO2 in the brick, reducing Si, destroying the organizational structure of the refractory material, causing the furnace lining to produce a metamorphic layer, loose, peeling and damage. The reaction: 3SiO2 + 4Al —2Al2O3 + 3SiO
So, in addition to high-alumina bricks, carbonaceous products are commonly used for refractory materials used in general aluminum industrial furnaces.
02. Refractory materials for alumina rotary kilns
At present, due to the reasons for raw materials, the production process of alumina in my country mostly adopts sintering and combined methods. The drying and calcination of bauxite and the roasting of aluminum hydroxide are mostly carried out using rotary kilns. In recent years, the introduced fluidized roasting device has been widely used, but in some old factories, rotary kilns still account for a large proportion.
The rotary kiln is a sintering kiln for alumina clinker. When making alumina, first load bauxite, soda ash and lime into a rotary kiln in proportion, calcine at 1200~1300℃ and then take out of the kiln, and then process it properly to make aluminum hydroxide and mother liquor; load aluminum hydroxide into the rotary kiln and calcine at 1200℃ to make it. The calcination process in the rotary kiln is: the high-temperature flame and the heated material move in the opposite direction in the furnace. Soda lime bauxite raw material slurry (water content 40%) or aluminum hydroxide (water content 12%~18%) is added from the kiln tail, and after low-temperature drying and dehydration, heating, and high-temperature calcination, the material is discharged from the kiln head and the high-temperature gas flows from the kiln head to the kiln tail. Therefore, the kiln is divided into a preheating zone and a high-temperature stewing zone. In order to prevent the slurry from bonding the kiln lining during the heating and calcining process and to strengthen the heat transfer process, a chain is also set between the refractory masonry. During the rotation of the kiln body, the material and lining bricks are continuously hit, which has a certain impact on the service life of the kiln lining.
The rotary kiln body for alumina production is a cylinder welded from steel plates, lined with refractory materials. Refractory materials work in harsh environments and demanding conditions. It should have the following characteristics: strong resistance to alkali corrosion; able to work for a long time at high temperatures of 1200~1300℃ without damage.
Nowadays, amorphous refractory materials have been widely used in the aluminum industry. The kiln mouth of the rotary kiln is easily deformed and damaged due to the high temperature wear and thermal shock stress of the material; in the transition zone of the alumina clinker kiln, the ambient temperature is 400~1000℃:, it is severely corroded by alkali and mechanically damaged (vibration, distortion), and the lining often falls off. The rotary kiln also uses steel fiber reinforced castables, which are mainly used in the preheating zone, kiln mouth, kiln tail, and cooler.
Alumina hydroxide roasting is the last process in the alumina production process, which is mainly to dry the attached water and crystal water in the aluminum hydroxide filter cake and convert part of the β-type alumina into α-type alumina. At present, the aluminum hydroxide roasting of major domestic alumina manufacturers has all or partly adopted the imported fluidized roasting equipment. Fluidized roasting equipment is divided into three types: fluidized flash roasting furnace, circulating fluidized bed roasting furnace, and suspension roasting furnace. Although the refractory materials used are different, a large number of amorphous refractory materials (refractory plastic or refractory castable) are used, and their usage accounts for 50%~70% of the refractory materials used. The amorphous refractory materials of the fluidized flash roasting furnace are all introduced from Germany, and the refractory materials of the circulating fluidized bed roasting furnace are all produced domestically.
Alumina gas suspension roasting furnace is a special equipment used to roast aluminum hydroxide. Its process and automation level are very high. The roasting process is completed under the conditions of high temperature furnace body operating temperature of about 1200℃ and high wind speed. At the same time, due to the high hardness and good fluidity of the processed alumina material, there are very strict requirements on the quality of alumina products. Any impurities mixed into the lining material directly affect the performance 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 with a carbon brick lining. A carbon anode is suspended in the electrolytic cell, and its carbon bottom is the cathode. Aluminum electrolysis uses cryolite, aluminum fluoride, lithium fluoride and other melts as electrolytes, melts AI2O3 at about 970℃, ionizes under the action of electric field force, and the metal aluminum melt restored by electrolysis is deposited on the cathode at the bottom of the cell. The oxygen released by 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 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℃.
The main reasons for the damage of the side wall lining of the aluminum electrolytic cell are: the oxidation of the material caused by the air inhaled from the steel shell and the brick lining; the corrosion of cryolite, NaF and aluminum liquid at high temperature; the erosion caused by the flow of the melt; the thermal stress caused by temperature fluctuations and thermal expansion.
The side walls of the aluminum electrolytic cell have always used 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 prevent the side wall from oxidation and have a larger resistance, the side wall is moving towards the direction of partially or completely using SiC materials. Silicon carbide bricks combined with silicon nitride are the best. Silicon carbide bricks bonded with silicon nitride have excellent high-temperature mechanical properties and good thermal conductivity. They are easy to form condensed slag on the inside. They have high resistivity and reduce the current loss of the side wall. The material is not easy to be oxidized. It does not react with melts such as aluminum liquid and cryolite. It has high mechanical strength and can greatly reduce the thickness of lining bricks, increase the volume of the electrolytic cell, and stabilize the operation. For example, when carbon bricks were used, the thickness of the side wall was about 200~400mm. After using silicon carbide bricks bonded with silicon nitride, the thickness of the side wall is only 75mm.
03. Barrier layer under the bottom of the tank
In the production of electrolytic aluminum, the vapor and liquid of Na and NaF can enter the insulation layer below through the cathode material at the bottom of the tank. After the insulation layer enters NaF, the thermal conductivity increases, the thermal efficiency of the electrolytic cell decreases, and the operating conditions deteriorate until the tank 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 it also has excellent insulation function. A new type of "barrier" material, dry anti-seepage material, is now well used.
04. Refractory materials for melting furnaces and holding furnaces
The smelting furnaces commonly used for melting and alloying primary aluminum ingots and scrap aluminum mostly use gas-fired or oil-fired fixed or tilting reverberatory furnaces, and also use resistance reverberatory furnaces and induction crucible furnaces. Although the temperature of aluminum liquid and aluminum alloy in the melting furnace is only 700-800℃, magnesium, silicon and copper in aluminum and its aluminum alloys are very active and easily react with some components in the refractory, causing damage to the refractory. The corrosion damage mechanism of aluminum melting furnace is mainly:
Aluminum liquid easily enters refractory materials;
The alloying elements in aluminum and its alloys have strong recovery ability for some oxides, and the redox reaction produced is a strong exothermic reaction; some alloying elements such as magnesium have a high vapor pressure, and their vapor is easier to enter refractory materials than aluminum liquid, and after entering the refractory materials, they are oxidized, which eventually leads to qualitative changes, loose structure and damage of refractory materials;
In the smelting 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, furnace bottom and furnace wall are very serious;
The addition of aluminum ingots and alloy blocks, the outflow of aluminum liquid, the fluctuation of furnace temperature, etc., cause thermal shock damage to the refractory lining.
The refractory materials used in aluminum smelting reverberatory furnaces are required to resist the entry of aluminum liquid and magnesium vapor, and have excellent wear resistance and thermal shock resistance. The lining of the aluminum smelting reverberatory furnace that contacts the aluminum liquid is generally built with high-alumina bricks with an AI2O3 content of 80% to 85%; when smelting high-purity aluminum metal, mullite bricks or corundum bricks are used. Silicon carbide bricks bonded with silicon nitride are used in the parts that are prone to corrosion and wear, such as the slope of the furnace bed and the loading of scrap aluminum materials. For the parts such as the aluminum flow trough and the aluminum outlet, where the aluminum liquid is severely eroded, self-bonded or silicon nitride-bonded silicon carbide bricks are generally used, and quartz bricks are also used as linings. For the aluminum outlet obstruction, vacuum-cast refractory fibers are used for better results. The lining of the furnace that does not contact the aluminum liquid is generally made of clay bricks, clay refractory castables or refractory plastics. The lining of the aluminum flow trough is generally made of silicon carbide bricks, and electric fused foam silicon brick precast bricks can also be used.
Now, with the large-scale aluminum melting furnace and the requirement of strengthening training, high-strength aluminum-resistant infiltration castables have been well used because of their excellent resistance to the entry of aluminum liquid and magnesium vapor, excellent wear resistance and thermal shock resistance.
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