Heating furnaces and reverberatory furnaces are high-temperature equipment, and the lining masonry cannot be separated from refractory materials. Reverberatory furnaces are traditional equipment for heating materials in the furnace or completing smelting processes such as oxidation and reduction.
Large continuous heating furnaces are high-temperature furnaces used to heat steel billets. There are many varieties, but the furnace structure is composed of several parts such as the furnace top, furnace wall, furnace bottom, flue, and heat exchanger. According to the heating system of the continuous heating furnace, the heating furnace can be divided into 800~900℃ preheating section (1100~1400℃) and soaking section (1100~1400℃). There are also differences in the refractory materials used for different segmented working temperatures.
Refractory materials for heating furnaces
1.Furnace roof
The furnace roof of medium and small heating furnaces is mostly an arched structure. Large modern heating furnaces adopt a suspended flat roof structure. The furnace roof of the heating furnace is a weak link, and its life is a sign of the life of the furnace.
The temperature of the furnace is about 1400℃, so it is mostly made of clay refractory materials, which can be made of clay bricks or castables or plastics to make an integral furnace roof. Only high-aluminum refractory materials such as high-aluminum bricks or phosphate high-aluminum castables are used in high-temperature key parts. The life of the furnace top is generally 10 years. In order to reduce the loss, a layer of insulation material is laid on the furnace roof. The anti-arch part of the furnace top is mainly affected by thermal stress and gas erosion, and sometimes it is hit by the arched blank. Generally, hanging brick roof, refractory castables or plastic products are used.
2.Furnace wall
The furnace wall is divided into side wall and end wall. The working lining of the side wall is mainly clay bricks, and the non-working layer is mainly refractory bricks. The side wall working lining of modern large-scale heating furnaces can use high-alumina bricks, the non-working layer is clay bricks, and the lightweight brick insulation layer is added. In order to save investment, refractory materials can be selected according to the temperature.
In recent years, refractory castables and plastic linings have been widely promoted and applied all over the world, with long service life and good energy-saving effect.
3.Furnace bottom
The total effective area of heated steel at the bottom of the furnace. The bottom of the heating furnace includes a soaking bed, a slide rail support structure of the preheating section and the heating section. It must bear the load, friction and vibration of the billet. The high-temperature section is also corroded by iron oxide.
The soaking bed is usually built with magnesia bricks or high-alumina bricks, or a molten cast material with more than 70% Al2O3 or a zirconium-aluminum material doped with Zr02.
The working lining of the furnace bottom of the heating section is the same as that of the preheating section. Generally, the same or similar materials as the side walls are selected, generally clay bricks. Sometimes high-alumina bricks can also be used for heating. The furnace bottom is generally a solid bottom. The guide rail is made of silicon carbide or water-cooled pipe, or cast iron beam.
4. Flue
The flue lining is generally clay brick or refractory castable block, with clay brick arched out, or covered with a cover made of refractory castable.
Refractory materials for reverberatory furnace
The reverberatory furnace is composed of refractory bricks and metal skeletons. The high-temperature flue gas of pulverized coal or heavy oil used in the smelting operation is heated or completes smelting operations such as oxidation-reduction.
Existing types of reverberatory furnaces: 1. Small furnace capacity of 10~50 tons, furnace width of 23 meters, length of 3~5m, length-to-width ratio of 1.5~3, and molten pool depth of 0.4~0.6m. When burning coal or lump coal, a combustion chamber (or fire chamber) is set on the furnace head. A fire wall is built between the combustion chamber and the molten pool, and the fire wall is 200~300mm higher than the molten pool liquid level. 2. Large-scale refined reverberatory furnace with a capacity of 100~400t. Length 10~15m, width 3~5m. Large reverberatory furnaces do not have combustion chambers, and directly use nozzles to burn pulverized coal, heavy oil or natural gas.
Basic structure of reverberatory furnaces The reverberatory furnace is a horizontal rectangular furnace body, consisting of a furnace base, furnace bottom, furnace wall, furnace top and metal brackets. The following is the basic structure of the reverberatory furnace.
Working principle of reverberatory furnace The reverberatory furnace uses a one-stage method to handle the smelting of miscellaneous copper, which is generally carried out in a fixed reverberatory furnace. Therefore, the reverberatory furnace is actually both smelting and refining. And it is the same as the principle of pyrometallurgical copper ore smelting. However, due to the high impurity content of crude copper, it also has its own characteristics in operation. The reverberatory furnace handles miscellaneous copper, and the entire refining process includes melting, oxidation, reduction, slag removal, pouring and other operations. The core of this process is oxidation and reduction. In the entire melting process, the key to oxidation and reduction is to be fast and completely remove impurities in the copper liquid. Efforts should be made to strengthen the oxidation process so that the Cu2O content in the copper reaches saturation. The main impurities are: iron. Nickel. Zinc. Lead. Arsenic (Sb), etc.
The damage mechanism of refractories used in reverberatory furnaces is an important indicator of furnace age. The inner wall of a copper furnace is subjected to stress during use. The main factors of damage are:
1. Chemical factors:
1) Erosion caused by melt penetration; these substances mainly come from slag and also contain impurities in scrap copper;
2) Erosion caused by the diffusion of SO2 gas in copper;
3) Oxidation-reduction caused by changes in oxygen pressure or low oxygen partial pressure;
4) Special circumstances, such as using raw concentrate or re-lining with furnace lining.
2. Thermal factors:
1) Temperature values determined by exothermic reactions during furnace heating and smelting;
2) Severe intermittent thermal shock caused by abnormal charging or furnace operation;
3) Infiltration of copper in the molten pool.
3. Mechanical factors:
1) Wear caused by the movement of materials in the furnace (such as metal, scrap copper, slag, charge, dust-filled exhaust gas, etc.), smelting, smelting, wear caused by reduction;
2) Impact stress caused by loading;
3) Stress caused by improper construction of the furnace lining. Again, since the reverberatory furnace is an indoor flame furnace, the heat transfer in the furnace not only depends on the reflection of the flame, but more importantly, it is through the radiation heat transfer of the furnace roof, furnace wall and hot gas. The temperature in the furnace is 1800℃ and the temperature of the molten pool is 1350℃. Therefore, the above-mentioned damage mechanism of the reverberatory furnace refractory requires the reverberatory furnace refractory to have corrosion resistance, thermal shock resistance and high compressive strength.
The roof of the reverberatory furnace with refractory materials in various parts: the roof masonry can currently be divided into two forms: arch roof and hanging roof. The hanging ceiling is further divided into the hanging roof of the simple hanging furnace, the pressure beam type thrust hanging furnace roof, and the vertical rod type thrust hanging furnace roof. Furnace roof thickness: generally between 230~380mm; the thickness of the rib brick is 380mm-460mm. The arch masonry uses silica refractory bricks. The suspended ceiling uses magnesia-alumina refractory bricks, but foreign countries have directly used magnesia-chrome bricks combined with phosphates. Arch: Advantages of using silicon refractory bricks: simple structure, less steel consumption, convenient construction and short cycle.
Disadvantages:
1. The structural stability is not good. When the arch is shut down, it is necessary to pay attention to adjusting the looseness of the tie rod frequently, which is labor-intensive;
2. The life of the furnace roof is short, generally about 3 months;
3. The working conditions are not good when the furnace roof is hot repaired. Use: Suitable for reverberatory furnaces, the road width is less than 5m. Suspended ceiling: Magnesium-aluminum bricks and high-aluminum refractory bricks are used for masonry. Advantages: The advantage of the suspended ceiling is that it can reduce the height of the furnace arch, which is conducive to strengthening the heat transfer of furnace gas to the charge, reducing the pressure of the furnace roof on the top furnace wall, simplifying the furnace roof structure, and will not collapse in a large area, which is conducive to extending the furnace life. Disadvantages: 1. High cost and large amount of steel;
Complex structure, unchanged installation and adjustment operations, high labor intensity, and inconvenient cleaning of the furnace roof. Use: Suitable for smelting in large-scale copper-melting reverberatory furnaces. Furnace wall: Most of the inner walls of melting reverberatory furnaces use magnesium bricks. Some important parts, such as the burner and slag removal port of the copper melting furnace, are built with chrome-magnesia bricks to extend their service life. The insulation layer is usually built with clay refractory bricks. Method of building the copper melt reverberatory furnace wall: The thickness of the upper part of the molten pool is generally between 460 and 690 mm. In order to extend the service life of the furnace wall, the lower part of the molten pool gradually adopts the method of staggered thickening, and the furnace wall thickness can reach 900~1290 mm, and the thickness of the end wall and lower wall can reach 10001400 mm. For the furnace working in cycles, due to the large temperature fluctuation of the furnace, in order to improve the stability of the furnace wall, the furnace wall is often built into an arc to prevent the furnace wall from collapsing in the furnace. Schematic diagram of the furnace wall structure: the copper melting copper reverberatory furnace bottom is directly built on the basis of heat-resistant concrete. The base requires a temperature of more than 850℃. The bottom of the furnace is the bottom of the entire sintering furnace. The sintering furnace bottom is generally composed of two layers, the upper and lower layers, and the total thickness is generally between 1100 and 1400 mm. (115m) clay refractory brick layer (about 50mm) asbestos board and quartz sand (about 50mm) magnesia-alumina brick layer (345~460mm thick) sintering layer (200~500mm thick). Note: The material of the sintering layer can be selected from quartz and magnesia iron according to the composition of the slag. Slag removal port: There is a slag removal port on the other side or the other end of the furnace wall, 400×400mm in size, and a molten pool is formed around the furnace wall below the slag removal port. The material is the same as the furnace wall material. Copper discharge port: The side wall or end wall of the molten pool is equipped with a copper discharge port. The external copper discharge port is divided into inner and outer layers. The inner layer is about 150×150mm in size and 360~400mm in depth. The outer wall is a round hole, about 25~30mm in size and 200mm in depth. The outer layer of the copper discharge port is detachable for easy maintenance. The entire copper discharge port can be built with magnesia-chrome bricks.
Requirements for the construction of reverberatory furnaces:
- Construction of furnace bottom and molten pool 1) The thickness of the refractory bricks for the furnace bottom anti-arch should be less than 1~1.5mm. 2) The seams of the furnace wall bricks below the slag line should be within 1~1.5mm, and the seams of the furnace wall bricks above the slag line should generally not exceed 1.5~2.5mm. 3) All holes in the molten pool (such as slurry ports, metal discharge ports, etc.) must be wet-laid according to one type of masonry. 4) The lateral expansion joints of the molten pool anti-arch are usually left on both sides of the anti-arch according to the expansion amount. The shape of the expansion joint should be determined according to the actual situation, and it is better to be wide and narrow at the bottom. The longitudinal expansion joints of the reverse arch can be concentrated at both ends, and can be dispersed for furnaces with longer lengths (one for every three refractory bricks in the upper anti-arch, that is, one expansion joint is set every 115mm*3=345mm, and one for the second layer of anti-arch can be divided into four pieces.
5) Except for the flat seams, no other through seams shall be produced in the molten pool masonry.
6) Before laying magnesium bricks on the furnace bottom, the filler layer (rough furnace bottom) should be dried.
7) Magnesium brick anti-arch is generally dry-laid. After the masonry is completed, magnesium powder is used to sweep and fill the brick joints, or magnesium powder is mixed with tung oil to sweep and fill the brick joints. - Furnace roof masonry 1) The furnace roof is generally wet-laid, and the thickness of the brick joints does not exceed 2mm. When using dry masonry (mostly used for suspended ceilings), the brick masonry around all reserved holes in the furnace roof (such as feeding holes, instrument holes, and copper plate holes, etc.) is wet-laid.
2) The masonry of the metallurgical furnace roof is usually divided into two types: ring masonry and staggered masonry. The construction of the ring structure is relatively simple and easy to maintain, so the smaller span and hanging furnace roof are all ring-laid. However, the strength and airtightness of the ring-laid arch structure are poor, so staggered masonry is usually used when the furnace span is large.
1). The small reflective furnace uses expansion joints to build longitudinal joints on the furnace roof. The longitudinal cracks on the furnace roof are generally concentrated at both ends of the furnace roof and at the junction of the horizontal arch and the inclined arch of the furnace roof. There are two types of longitudinal expansion joints in the large-diameter reflective furnace roof: centralized and distributed. The centralized retention method is to divide the furnace roof into several sections according to the furnace temperature distribution, so that the expansion of each inner furnace roof masonry is concentrated in an expansion joint. The length of the high-temperature zone arch section is usually 2~3 mm, the length of the low-temperature zone arch section is usually 4~5 mm, and the width of the expansion joint between the two arch sections is generally 20~40 mm. In order to prevent the furnace gas from escaping from the expansion joint when the furnace is opened, a layer of refractory bricks is covered on the furnace roof through expansion joint. The advantage of centralized direct buried expansion joints is that the construction is convenient, but the expansion is not good, which affects the airtightness of the furnace roof and is also one of the first parts of the furnace roof to be damaged. Distributed reservation means that 2~4 mm expansion joints (clamping cardboard) are reserved every 1~2 bricks. Distributed expansion joints have better expansion and improve the airtightness of the furnace roof, but the construction is difficult.
2) The horizontal expansion joint of the furnace roof is reserved. The reverberatory furnace brick arch generally does not leave a horizontal expansion joint, and the horizontal expansion is adjusted by the elastic tension rod, so the thread length of the tension rod should leave a certain space to meet the needs of adjusting the tension rod. The large reverberatory furnace is suspended from the top, with a “radial” expansion joint reserved on the top of the furnace, and cardboard is sandwiched between two refractory bricks. - The distance between the pull rod on the furnace and the brickwork on the top of the furnace should be less than 0.4mm. 2) The height of the civil foundation of the electric furnace should avoid positive errors. The side of the foundation should be flat to facilitate the installation of columns.
In order to increase the service life of the furnace, refractory materials should be reasonably selected according to the degree of local damage in the furnace.