Production technology of magnesia brick

According to the different production technology, magnesia brick is divided into firing magnesia brick, non-firing magnesia brick and recombination magnesia brick. Fired magnesia brick, usually referred to as fired magnesia brick, magnesia brick, is the largest production, the most widely used basic brick. Our country magnesite quality, abundant reserves, magnesite brick quality and cheap, in the domestic and foreign markets enjoy a high reputation.

A raw material

There are two kinds of magnesia for producing magnesia brick: natural magnesia and seawater magnesia. The vast majority of magnesia bricks in China are made by the former. The content of MgO in magnesia ranges from 89% to 98%. The physicochemical properties of natural sintered magnesite in China depend on the purity of magnesite and the production process of magnesite. The MgO content of magnesia stone ore with MgO content greater than 46.0% is mixed with solid fuel block and calcined in shaft kiln, and the MgO content is 89%~92%. Magnesite with MgO content greater than 46.5% is lightly burned in the reverberatory furnace, then finely ground, mixed with water, pressed ball, and mixed with solid fuel block into the magnesite calcined in the shaft kiln. The content of MgO is about 95%, which is also called mid-range magnesite. Magnesite with MgO content of more than 47%, calcined in ultra-high temperature oil kiln by light burning, fine grinding and dry pressing, is called high-purity magnesite with MgO content of 96.5%~98%. All grades of magnesia fired bricks are produced with one or both magnesia. The quality criteria of sintered magnesite are particle volume density, MgO content, impurity content and CaO/SiO2 ratio. The particle volume density of high-purity magnesite in China is 3.25~3.30g/cm, which is lower than that of high-purity magnesite abroad. The method to improve the particle volume density can be solved by increasing the fineness of light burning MgO and increasing the sintering temperature, but it will increase the production cost. Because the melting point of dicalcium silicate with high CaO/SiO2 is up to 2130℃ and its high temperature performance is good, it is also beneficial to resist erosion when it is used in steel lining, lime kiln and refractory kiln. However, high CaO/SiO2 magnesia used in the production of glass kiln products or MGO-Al2O3 series, MGO-SiO2 series products, will produce more low melts in the use, but is unfavorable. In the case where the former requires high CaO/SiO2, CaO is a harmful impurity.

B particle size composition

The grain size composition should accord with the principle of maximum stacking density and be favorable for sintering. The critical particle size is generally 3~5mm. When the brick size is large, the critical particle size can be larger. The choice of grain size ratio should also take into account the balance of each grain size. When selecting two kinds of magnesite ingredients, the high-quality magnesite with high purity can be added in the form of fine powder to improve the slag erosion resistance and permeability of the product matrix. A typical particle size ratio is: 2.5~0mm, in which 70%~80% and 0.088mm respectively (less than 0.088mm and 95% respectively) are added into the mixture (greater than 2mm and less than 1mm), and 20%~30% are added.

C mixed practice

Mixing can be carried out in SJH-28 wet mill, planetary force mixer, EIRICH mixer or strong countercurrent mixer. Binder can be used sulfurous acid pulp waste liquid, brine, magnesium sulfate liquid, etc., adding about 3%. When the latter two are mixed, the feeding sequence is as follows: first, add the coarse particles, then add the binder liquid, mix for 1-2min, add the fine powder, and then mix for 10-20min. When the content of CaO in brick is high, the trapping process can be adopted as appropriate to prevent the brick from cracking.

D shape

It can be molded by friction press and pressed by hydraulic press on both sides, so that the density of the blank can be even up and down. First light then heavy, multiple pressure is conducive to exhaust, prevent the brick layer due to elastic aftereffect crack. The pressure is generally 100~200MPa, and the density of the billet is generally about 2.95g/cm. In the design of brick-making mould, it is necessary to take into account the laying ruler of brick, and the laying rate of ordinary magnesia brick: the pressing surface of yard brick is 1.5%~2.5%, and the non-pressing surface of yard brick is 0.5%~1.5%. When high purity magnesia brick is fired at high temperature, it will shrink and the brick ruler will decrease accordingly.

In order to improve the yield of finished products, the requirements of tolerances and appearance of brick should be more strict than those of national standards and contracts.

E dry

The purpose of drying is to remove the physical water from the binder, so as to improve the strength of the brick and reduce the bricklaying and burning waste. Tunnel dryer is generally used for drying, drying medium is usually hot gas extracted from the tunnel kiln cooling zone. The inlet temperature of the drying medium is 100~110℃, and the outlet temperature is 40~60℃. The drying time is generally no less than 16h, and the moisture content of the brick is controlled below 0.8% after drying.

F code brick

The density of brick has an important influence on the air distribution in tunnel kiln and the quality of fired products. The higher the density of brick stack, the higher the efficiency of tunnel kiln. However, with the increase of brick stack density, the gas resistance increases significantly. The more air it takes to make a 1kg product. The higher the density of brick stack, the more defective products, the greater the influence of the density of brick brick on the waste products of cooling fracture than the influence of the cooling speed.

Each size of brick must have its own pattern of loading. As a rule of thumb, about 50% of the kiln's broken surface code must be emptied so that the flue gas or cooling air can flow through the brick smoothly. 50% free sections should be distributed as evenly as possible on the brick. The side gap (between the brick and kiln wall) and the top gap (between the brick and kiln roof) should not exceed 80mm and 50mm respectively. If the kiln car brick is too dense in the direction of advance, the gas will pass through the side gap and the top gap at high speed. The consequence is that the temperature difference on the brick section increases and the flue gas temperature is too high, which means the energy loss. Accordingly, the more complex the shape is, the brick with large dimensions of external form, should leave enough space more. It should also be noted that before laying the bricks, the trolley should be kept dry and in good condition. Every time the trolley passes through the kiln, it should be ground and smoothed to avoid the uneven of the trolley will cause indentation to the bricks. If roughness appears, use appropriate dry sand to level.

G firing

The firing of magnesia bricks can be carried out in tunnel kiln or inverted kiln. The latter has low productivity and the thermal system is not easy to control. Tunnel kiln is the most effective and widely used thermal equipment for firing magnesia bricks. The structure, thermal operation and fluid mechanics of tunnel kiln are the important conditions to ensure the properties of products and achieve low energy consumption and high efficiency.