Burnt Dolomite Sand Refractory Products
It usually contains more than 40% calcium oxide (CaO), more than 35% magnesium oxide (MgO), and also contains a small amount of silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3) and ferric oxide ( Fe 2 O 3 ) and other impurities
The CaO/MgO ratio of natural dolomite varies greatly, if the CaO/MgO ratio in the brick is less than 1.39, it is called magnesia dolomite brick. According to the production process, dolomite bricks can be divided into: tar (asphalt) combined with unfired bricks, light oil-impregnated burnt bricks and oil-impregnated fired bricks
Dolomite bricks contain free CaO, which are easy to hydrate and crack in the air, and are not suitable for long-term storage. The raw material Dolomite is a double salt of calcium carbonate and magnesium carbonate, the molecular formula is CaCO 3 MgCO 3 , hexagonal crystal system, the specific gravity is 2
80 299, the Mohs hardness is 35 45, the theoretical composition is CaO 30.41%, MgO 21.87%, CO 2 is 47.72% and the CaO / MgO ratio is 1.39
China dolomite is rich in resources and high quality, its composition is shown in Table 1
In recent years, in addition to using natural dolomite sand as a raw material for making bricks, synthetic magnesia dolomite sand made from seawater and milk of lime has also been used to produce high-quality magnesia dolomite bricks.
Table 1 Origin and composition of dolomite in China (%) The brick manufacturing process is divided into two steps: clinker preparation and brick formation. Clinker calcination: dolomite ore must be calcined at a high temperature of 1650 1850 ° C to form dense dolomite sand
The degree of sintering of clinker is generally measured by bulk porosity, bulk density, true specific gravity, and hydration resistance.
Calcining equipment and calcining system mainly include:
Use coke as fuel and use a shaft furnace for calcination
Because the calcination temperature is generally only 1650 1700 , and the kiln temperature is not easy to be uniform, so the calcination quality is poor, and there are many products with poor firing.
In addition, the impurity content of the prepared dolomite sand is as high as 6-10% due to the incorporation of coke ash.
Using heavy oil as fuel, using rotary kiln and shaft kiln for calcination. Because the calcination temperature is as high as 1700 1800 , the kiln temperature is uniform, and there is no fuel ash pollution, so the quality is good.
This method is mainly used to prepare dolomite clinker.
Sometimes, in order to improve the hydration resistance of clinker and lower the sintering temperature, a small amount of iron oxide powder (usually <3%) is also added, it is ground into fine powder, granules or compacts, and then calcined in a high-temperature rotary kiln the so-called semi-stable dolomite sand
Activation and calcination in two steps
The crude ore is first lightly burned at 1000 1100 , and then pulverized, digested (adding mature aquatic lime Ca(OH) 2 ) or partially digested, trapped, compacted, etc.
The resulting “activated material” is calcined with ashless fuel; At this time, the sintering temperature can be lowered by 100-200 than without activation treatment, and the prepared dolomite sand has high purity, compact structure, uniform composition, and strong hydration resistance.
The microstructure of the synthesized magnesia dolomite clinker is shown in Fig.
Reflective two-step calcination synthetic magnesia dolomite clinker × 200 Gray-white matrix is MgO, black dots are CaO, dark gray agglomerates are silicate phase
Brick formation Jar combined dolomite brick: dolomite sand is first preheated or baked, generally large particles with particle size of 8-30mm are baked at 300-400, and fine powder less than 1mm is preheat to about 100 , and then mix according to a certain particle composition, add 5 to 7% hot dehydrated asphalt, stir, press or vibrate at 130 to 170
Sometimes need light heat treatment
Oil immersed sintered dolomite brick: Compared with tar bonded dolomite brick, the main difference is that the boundary particle is finer, generally not more than 10mm; the amount of binder added is less, usually about 3%, commonly used paraffin wax, not regular polypropylene, etc. as binder
The bricks are usually fired in a high-temperature tunnel kiln after pressing and forming
After burning, the brick body is impregnated with vacuum pressurized asphalt to obtain oil-impregnated burnt brick. Composition and performance The composition and performance of various types of dolomite bricks and magnesia-dolomite bricks in China are shown in Table 2.
Table 2 Composition and behavior of dolomite bricks produced in China Uses Dolomite bricks are widely used in core converters
China converter lining mainly uses tar blended dolomite bricks and tar blended magnesia dolomite bricks
Some factories use light burning oil immersion bricks and oil immersed magnesia dolomite bricks in the vulnerable parts
In Western Europe and Japan, converters mainly use oil-dipped and heat-treated dolomite bricks combined with coal tar and magnesia dolomite bricks.
In addition, oil-impregnated magnesia dolomite fired bricks are also used as the inner lining of some off-kiln refining furnaces
