Introduction of main refractory brick and material for Coke Oven

Introduction of main refractory materials for coke oven

1. Silica brick

   Silica bricks are acidic refractory materials. It has  good acid resistance erosion ability. Feature with  good thermal conductivity , high load softening temperature.  generally above 1620 ° C,  The thermal conductivity of silica bricks increases with the rising  of working temperature and has no residual contraction.  During the perorid  of  furnace drying , the volume of silica bricks increases with the temperature rising. Therefore, Silica bricks are the ideal refractory products on the coke oven.  The important parts of modern large and medium -sized coke oven (such as the combustion chamber, oblique road, and regenerator ) are made of silica bricks.

   During  the perorid  of  furnace drying, the maximum expansion of silica bricks occurred between 100 ~ 300 ° C, and the expansion volume before 300 ° C was about 70%to 75%of the total expansion. Becuase  the SIO2 has four crystal conversion points of 117 ° C, 163 ° C, 180 ~ 270 ° C and 573 ° C  during  the perorid  of  furnace drying, of which between 180 ~ 270 ° C, the volume expansion caused by square quartz is the largest.

   The key to determining the stability of silica bricks is the true density . The real density is one of the important siganificant  of determining its quartz conversion. The smaller the real density of the silica brick, the more complete the lime transformation, and the smaller the residual expansion produced during the furnace drying process.

  For the silica brick , the true density of the squamous quartz crystals is the smallest, the line expansion rate is small, the thermal stability is good for quartz and quartz, the residual resistance is strong, the heating is good, and the heavy softening temperature is high. It is the most stable form in the quartz. Essence Among the better silicon bricks, the content of lumatos is the highest, accounting for 50%to 80%; Fang quartz is only 10%to 30%; while the content of quartz and glass is fluctuated at 5%to 15%.

   When the working temperature is less than 600 ~ 700 ° C, the volume of the silica brick is large, the resistance of anti -cold and hot heat is poor, and the heat stability is not good. If the coke oven has worked at this temperature for a long time, the masonry will easily break and damage.

2. Fireclay brick

Fireclay  bricks refer to clay products with Al2O3 content of 30%~42% aluminum silicate materials.Fireclay  bricks  are batched according to certain particle size requirements. The mineral composition of the fireclay  is mainly kaolinite (Al2O3·2SiO2·2H2O) and 6%~7% impurities (oxides of potassium, sodium, calcium, titanium and iron). The firing process of clay bricks is mainly the process of continuous dehydration and decomposition of kaolinite to form mullite (3Al2O3·2SiO2) crystals. SiO2 and Al2O3 in fireclay bricks form eutectic low-melting silicates with impurities during the firing process, surrounding the mullite crystals.

Fireclay bricks are  weak acid refractory products, which have good resistant to  the erosion of acid slag and acid atmosphere , and have slightly poor resistance to alkaline substances. Fire clay bricks have good thermal properties and  good thermal shock resistance.

   The refractoriness of  Fireclay bricks is comparable to that of silica bricks, up to 1690~1730℃, but the softening temperature under load is more than 200℃ lower than that of silica bricks. Because in addition to the mullite crystals with high refractoriness, the  Fireclay bricks also contain nearly half of the low melting point amorphous glass phase.

   In the temperature range of 0~1000℃, the volume of  Fireclay bricks expands uniformly with the rising of temperature, the linear expansion curve is similar to a straight line, and the linear expansion rate is 0.6%~0.7%, which is only about half of that of silica bricks. When the temperature reaches 1200 ℃ and then continues to heat up, its volume will start to shrink from the maximum expansion. Residual shrinkage of clay bricks leads to loosening of masonry mortar joints, which is a major disadvantage of clay bricks. When the temperature exceeds 1200 °C, the low melting point in the clay brick gradually melts, and the particles are closely attached to each other due to the action of surface tension, resulting in volume shrinkage.

   Due to the low softening temperature of clay bricks and shrinkage at high temperatures, the thermal conductivity is 15%~20% lower than that of silica bricks, and the mechanical strength is also worse than that of silica bricks. Therefore, clay bricks can only be used in the secondary parts of coke ovens. Such as regenerator  wall,  flue lining bricks and regenerator checker bricks, furnace door lining bricks, furnace roof and rising pipe lining bricks, etc.

3. High alumina  brick

High alumina bricks are aluminum silicate or alumina refractory products with Al2O3 content more than 48%,

    The refractoriness and softening temperature of high alumina bricks are higher than those of fireclay bricks, and the slag corrosion resistance (especially for acid slag) is better, and these properties increase with the increase of Al2O3 content, but the thermal stability is not as good as fireclay bricks. High alumina bricks have high density, low porosity, high mechanical strength and wear resistance. The burner head of the coke oven combustion chamber and the burner head of the bottom brick of the carbonization chamber are built with high-alumina bricks, and the effect is better; but it is not suitable for the wall of the carbonization chamber, because the high alumina bricks are prone to curling and warping at high temperatures.

4. Refractory  mortar 

Refractory mortar is an unshaped refractory material composed of powder materials and binders for ready to use mortar . It is mainly used as a binder and coating material for masonry of refractory bricks. Most of the refractory mortar  is mixed with water (or aqueous solution) to make mud. It should have the corresponding brick properties for masonry, and the refractory clay for coke oven should meet the following requirements:

   (1) After construction and use, it should have the necessary adhesion to ensure that it is integrated with the masonry or surrounding layers, so that it should have the effect of resisting external force, gas resistance, and slag corrosion resistance.

   (2) It must have good fluidity and plasticity to facilitate construction.

   (3) It has the same chemical composition as the masonry or the surrounding layer material, so as to avoid the first destruction of the refractory mud and avoid adverse chemical reactions between different materials.

   (4) It has the same thermal expansion as the masonry or surrounding material, so as to avoid separation from each other and rupture of the mud layer.

   (5) The volume should be stable and have less shrinkage to ensure the integrity and tightness of the masonry.

   (6) Sintering can occur at the service temperature to increase the mechanical strength of the masonry.

   (7) It has a certain degree of refractoriness and softening point under load.

   The corresponding refractory mortar  should be selected according to the brick type and operating temperature, that is, clay fire mortar is used when building fireclay bricks, and silica mortar is used when building silica bricks. For all the masonry parts in contact with the metal embedded parts, concentrate powder must be added to the fire mud. When laying coke oven top bricks, the hydraulic cement that can increase the strength - Portland cement and quartz sand should be added to the clay fire mud.

   Silica mortar is a powder made from silica, waste silica bricks and refractory clay (raw clay). Silica is the main component of silicoa mortar  . The higher the SiO2 content in the silica, the higher the refractoriness of the mortar . Adding waste silica bricks can improve the high-temperature bonding properties of mortar  and silica bricks, because the silica brick powder has a similar thermal expansion curve to that of silica bricks. The ability to adhere to silica bricks is good. Generally, the content of silica brick powder is 20%~30%. Adding raw clay to silicon fire clay can increase plasticity, reduce air permeability and water loss rate, but the addition amount should not be too large, otherwise the refractoriness of silicon fire clay will decrease and the shrinkage rate will increase, generally no more than 15%~20 % is appropriate.

   The requirements for particle size are: no more than 3% of the particles above 1mm, no less than 50% of those less than 0.074mm. The particle composition of silicon fire mud affects the performance of fire mud. If the particles are too large, the mud loses water quickly, the bricklaying operation is difficult, and precipitation and segregation are likely to occur in the ash tank; worse. Generally, after the good mortar is hit on the bricks, the bricks can be kneaded and beaten at will for about 15-20s, and this time is related to the particle composition. Therefore, the performance of silicon fire clay can be expressed by this time.

   Clay mortar is made from clinker or crushed clay bricks plus refractory clay (raw clay) from the calcination process. Clinker is the main component of clay mortar , accounting for about 75% to 80%. Raw clay is a binder. Adding raw clay can increase plasticity, reduce air permeability and water loss rate, but increase shrinkage. Too much raw clay will easily cause cracks, so the ingredients account for about 20% to 25%.

   The use temperature of clay mortar  is generally lower than 1000 ℃. Clay mortar  for coke oven is generally of fine and medium particle size, and the percentage of particles passing through 0.5mm and 1mm sieves should be greater than 97%. Clay mortar  is not only used for laying clay bricks, but also for repairing coke ovens.