Key points of production technology of magnesia brick

International standard ISO1109-1975 stipulates that MgO≥80% of alkaline products are magnesia bricks. In the ASTM C 455-1984 classification of magnesia bricks, according to the MgO content, magnesia bricks are divided into three levels: 90, 95 and 98, with the corresponding MgO content minimum values of 86%, 91% and 96%, respectively. 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, reserve abundance, magnesite brick quality and cheap, in the domestic and foreign markets enjoy a high reputation.

(1) Raw materials

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 physical and chemical properties of natural sintered magnesite in China depend on the purity of magnesite and the production process of magnesite, which uses magnesite stone ore with MgO content greater than 46%, mixes with solid fuel block, and calcined magnesite in shaft kiln, the MgO content of which 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%.

(2) 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 a little 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.

(3) Mix 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%.

(4) Forming

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.

(5) 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.

(6) Yard bricks

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 the brick stack, the more defective products there are. The influence of the density of the brick is greater than that of the cooling speed.

(7) Burn

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.