GRADE CLASSIFICATION, PRODUCTION PROCESS AND PERFORMANCE OF SINTERED MAGNESIA

In the refractory industry, light-burned magnesia from magnesite or light magnesia produced by the chemical method of seawater and brine is calcined in a rotary kiln or shaft kiln at a temperature range of 1500~2000℃ to allow the magnesia to pass through The crystal grains grow up and densify, and transform into almost inert magnesia called sintered magnesia or reburned magnesia.
Production Process
According to different sintering characteristics of raw materials and different product technical requirements, one-step calcination or two-step calcination can be used in the production process. Its quality is mainly evaluated by the content of magnesium oxide, bulk density, CaO/SiO2 ratio and crystal size. The magnesia currently sold in the international market is divided into six grades.
In order to produce high-quality magnesia, the purified magnesite concentrate is generally used and sent to a light-burning furnace for light-burning (1000~1100℃) to obtain light-burning (also known as light) magnesia, which is then pressed by a ball press The ball is sintered in a two-step calcination method in a recalcining kiln. The following three methods are commonly used for two-step calcination.
①Multilayer furnace-rotary kiln or shaft kiln method.
This is a method of lightly burning magnesite concentrate [MgO or Mg(OH)2] in a multi-layer furnace, and then reburning in a rotary kiln or shaft kiln. For example, the Belgian NESA company uses the flotation method to prepare the magnesite concentrate (MgCO3), and the dried magnesite concentrate is sent to the multi-layer furnace by the screw conveyor. After preheating, light burning and cooling, light burning is produced. Magnesium oxide.
The multi-layer furnace is composed of an annular furnace and a vertical column. The furnace has an outer diameter of 7850mm, a total of 12 layers, 30 burners, heavy oil as raw material, and a light burning temperature of 900°C. Each layer of the hearth has alternate or peripheral and central leakage holes, and the materials are harrowed from one layer to the next through the leakage holes by the rake teeth installed on the rake arms. The burned light burnt magnesia is supplied to the double-roller ball press with a screw conveyor. The feed temperature is about 700℃, the roller of the ball press is about 750mm, the roller width is about 320mm, the rated speed of the roller is 10.7r/min, and the adjustment range is It is 5~16r/min, the total pressure is 3520kN, when the speed of the ball press is 16r/min, the motor power is 168kW, the cooling water requirement is 9m3/h, the ball is in the shape of apricot kernels, and the volume is about 7~10cm3. The finished product The rate is about 65%. The compressed balls are sent to the shaft kiln for re-fire, the firing temperature is 2000 ℃, the fired balls are discharged from the shaft kiln, and can be transported out after being cooled to about 90 ℃ by the cooler.
The North Korean Magnesium Mine Company uses the large open-pit magnesite near Chongjin Port to prepare the magnesite concentrate by flotation. The slurry is pumped into 9 disc centrifuges for dewatering, and the dewatered concentrate is transported by a conveyor. Enter 8 intermediate troughs and supply 4 multi-layer furnaces for light firing. The prepared light fired magnesia powder is wetted with about 10% of water, and 4 ball presses are used for two stages without the addition of bonding agent. The pressed ball is pre-pressed and then pressed to death. In order to increase the strength, the material ball is preheated and dried on the mobile grate machine with waste gas from the rotary kiln, and finally sent to two 110m long rotary kilns for reburning. The firing temperature is 1900°C.
② Suspension roaster-rotary kiln or shaft kiln method.
In foreign countries, when the Plepol-type suspension roaster-rotary kiln or shaft kiln is used to produce magnesia, a three-stage heat exchanger is generally used to heat the flotation concentrate to a temperature close to the light firing temperature, and then send it to the suspension roaster. burn. After the light-burned magnesia powder is prepared, it enters the ball press at a temperature of 700-900°C, presses it into pellets at a pressure of 15t/cm2, and sends the pellets to the vibrating screen by the elevator. After sieving out the powder, the pellets with a temperature of 500-750°C are put into a rotary kiln or shaft kiln for re-sintering, and the firing temperature is 1800-2000°C. The sintered magnesia discharged from the kiln is cooled to 60~80°C with a mobile grate cooler or a cylindrical cooler.
③ Rotary kiln-rotary kiln method.
This is a method of light-burning in a rotary kiln, pressing the balls through a ball press, and then re-burning in another rotary kiln. The rotary kiln used by the Feinanxiaer Mining and Shipping Company of the Skalistieri consortium in Greece, lightly burns magnesite concentrate at 1000℃, and the lightly burned oxide powder is ball-pressed and then weighed at 2000℃ in the rotary kiln. After firing, the performance of the sintered magnesia is 95% MgO, and the bulk density is 3.4~3.5g/cm3.
Sintered magnesia performance
The performance is mainly evaluated by the MgO content, particle volume density and the ratio of CaO/SiO2. Generally, it is required that the MgO content in magnesia should be high, and the impurity components SiO2, CaO, Al2O3, Fe2O3 should be low. This kind of main phase is mainly periclase, with less cement, and a better direct binding structure of periclase is formed. The ratio of CaO/SiO2 determines the phase composition of the matrix in magnesia. Theoretically, it is expected that the ratio of its mass is less than 0.93 or greater than 1.86. In order to get high melting point minerals.
In addition, when the CaO/SiO2 is low, the silicate becomes the film or shell surrounding the magnesium oxide crystal; when the CaO/SiO2 is high, the film-forming effect of the silicate is not so significant, and the silicate appears in isolation, and the square Magnesia crystals are directly combined with each other, which is beneficial to the high temperature performance of magnesia. The refractory industry generally believes that the CaO:SiO2 ratio is less than 3:1 is not a high-quality product, and the CaO:SiO2=(3:1)~(8:1) is the most popular product on the market, but it cannot Beyond the limit value of this range, because it will bring side effects. The higher the MgO content of sintered magnesia, the lower the content of CaO and SiO2, and the greater the ratio of CaO:SiO2, the better the high-temperature strength, slag resistance and volume stability of sintered magnesia.