Silica-Mullite-Andalusite Brick

Introduction of GMH Bricks
The cement industry is witnessing larger rotary kilns and rising output with faster rotation and bigger diameters. Therefore, refractory materials shall have better ability to deal with mechanical stress. We have used magnesite bricks and spinel bricks for burning zones and transitional zones of domestic rotary kilns; anti-stripping, high-tensile, and alkali-resistant bricks are used in preheating zones. These materials can basically meet requirement of kiln operation, but have also shown some shortcomings; due to high heat conductivity coefficient, spinel bricks will raise temperature in the walls of the kiln, imposing high pressure on the riding wheel due to expansion with high temperature on supporting tiles, which will affect normal running of the equipment; the high density of anti-stripping bricks have increased load on the kiln, affecting the power unit; the high-tensile and alkali-resistant bricks which used for preheater and tertiary air duct cannot protect the equipment and save energy due to bad heat insulation. Therefore, our company has made intensive efforts to prolong the life of refractory materials with simultaneous application of them in the whole kiln. As to refractory materials for 2500T/D, 5000T/D, and rotary kilns above the two series, we have conducted detailed technical analysis with reference to leading technologies and theories at home and abroad. Based on the research in refractory materials of kilns, we have developed an alternative product with better technical advantages: silica-mullite-andalusite bricks. We have applied the patent and name it GMH Bricks. The new material can fully replace imported spinel bricks and anti-stripping bricks at the kilneye and the front and back transitional zones. The company has accumulated abundant experiences in the reasonable use of refractory materials in the whole cement rotary kiln. After years of wide application by customers in recent years, refractory materials can be simultaneously used throughout the whole kiln.
 
Product features
    As a patent product of our company, GMH bricks are made of super-grade bauxite, silicon carbide, fused white corundum, and andalusite, and formed under high pressure after being fired in the high temperature tunnel kiln with natural gas. The bricks are mainly used to replace spinel bricks, anti-stripping bricks, and other refractory materials on the front and back kilneyes, transitional zones, and preheating zones of 2500T/D, 5000T/D, and rotary kilns above the two series. The product does not fall off and is well resistant to pressure and abrasion with high mechanical strength, long lifecycle, and good energy conservation. With outstanding quality and reputation, we have established the "GMH" brand.
 
Technical performance
 
 
Application of GMH Bricks in 5000T/D large cement kilns
    The large size and high daily output of cement rotary kilns have set a high bar for resistance to thermal shock, mechanical stress, and chemical erosion. Based on the 20 years of our production experiences in refractory bricks for rotary kilns, and domestic and overseas leading technologies and theories, our company had developed silica-mullite-andalusite bricks which we name it GMH bricks (Patent No.: 200610097276). The product has high mechanical strength and refractoriness under load, and low heat conductivity with good resistance to fire, erosion, and thermal shock. The product is widely applied in the front and back kilneyes, transitional zones, and preheating zones of 2500T/D, 5000T/D, and rotary kilns above the two series. With over one year operation, the surface temperature of the kiln shell has dropped 60-80ºC with efficient energy conservation and protection of the barrel. The product has effectively improved the operation environment of cement product machine, raised efficiency of large cement rotary kilns, and realizes simultaneous use of refractory materials throughout the kiln.
    Main clients: Conch Group, Yatai Group, Huaxin Group, China National Building Material Group, China Resources, Tianrui Group, Liaoyuan Jingang, Tapai Group, Hongshi Holding Group, Jinfeng Group, Qingshi Group, SDIC Hainan, Sichuan Xingchuancheng Cement, and Taiwan Cement.
    1. Except the burning zone, and castable of kilneyes and kiln ends, GMH bricks can be applied in all the parts of the large dry cement kiln shell with high mechanical strength and refractoriness under load,  low heat conductivity with good resistance to fire, erosion, and thermal shock. This is decided by mechanical features of rotary kilns and features of heating zones: the mechanical strength delivered on the lining refractory bricks due to oval deformation of the kiln shell require high resistance to pressure and bending; As the central part of the rotary kiln, the pre-heating zones needs refractory materials with low heat conductivity coefficient and high resistance to alkali deposit to fight against heat and chemical erosion; the high temperature in the safety zone requires refractory materials with high refractoriness and high refractoriness under load; due to low material temperature in the back transitional zone, the side kiln coating has low strength and weak integration with walls of the rotary kiln, often causing large areas of falling coating. Refractory materials are often exposed in high temperature gas with big thermal load and wide fluctuation of temperatures. In addition, the materials are also washed by strong clinker in bare state from time to time. Therefore, production of refractory materials shall consider better resistance to fire, high temperature, and thermal shock. Given strong eroding of clinker and more frequent temperature fluctuation, refractory materials of the front transitional zone shall have high refractoriness and refractoriness under load with particular stability in thermal shock.
    2. In the past, imported magnesite chrome bricks or spinel bricks are used in the front and back transitional zones, and anti-stripping bricks are used in the pre-heating zone of large cement kilns. These materials have some shortcomings in application: 
(1) Short service cycle: Klin coating often falls off in the front and back transitional zones, especially areas around the tyre due to frequent temperature changes. Spinel bricks are weak against chemical erosion of clinker due to its property limitation (bad resistance to high temperature, pressure, bending, and thermal shock). In addition, the bricks also suffer from serious physical abrasion, making it hard to serve a long cycle due to loose surface grains and broken surface. Frequent maintenance outrage will further peel off:brick surface, resulting in low efficiency in a vicious circle to affect the annual output target;
    (2) Affect equipment operation: Klin coating often falls off in the transitional zones, especially those around the tyre due to frequent temperature changes. It is also difficult for  magnesia-alumina spinel bricks to stick firmly with the klin coating because of high heat conductivity coefficient (≥2.7W/m·K), and bad stability in thermal shock. As a result, the expansion caused by the high temperature of the kiln shell (around 350ºC, and even 400ºC) will exert high pressure on the riding wheel in the middle of the kiln, deforming supporting tiles in high temperature and affecting normal running of equipment;
    (3) Unsatisfactory energy conservation: magnesite chrome bricks or spinel bricks have high heat conductivity coefficient (≥2.7W/m·K), causing high temperature on the kiln shell (above 350ºC) and raising unit cost of clinker. 
    (4) Not cost saving: magnesite chrome bricks or spinel bricks (2.9-3.0g/cm3) are 10% heavier than GMH bricks (1.35t/m) Such heavy material will not protect the kiln shell, but also increase cost expenditure.
    (5) Difficult to store and reduce manage fees: magnesite chrome bricks or spinel bricks are alkaline materials, which will cause pulverization and hydration of bricks in South China. This will affect usage, and even make products complete useless and hard to store; in addition, it takes a long time to produce and is hard to prepare magnesite chrome bricks and spinel bricks, which will take a large amount of fund.
    3. Traditionally, anti-stripping bricks are used as refractory materials for large cement kilns. Anti-stripping bricks have a long service life, and can be used for 2 years in the pre-heating zone of the kiln with high stability against thermal shock. However, as the main component of anti-stripping bricks, zircon sand is seeing a surging price in recent years. However, the price of the bricks remains stable. Brick producers have to use recycled zircon sand and even waste materials to pass standard tests with low cost, but such bricks cannot meet requirements in thermal shock stability and anti-stripping performance. Sometimes, anti-stripping bricks produced with waste zircon sand will react with ash content, melting slag, steam, and other alkaline materials, causing serious erosion and alteration of the crystal structure, which will greatly reduce the service life. Currently, the service life of anti-stripping bricks is just over one year, far below the design requirement. In extreme situations, acid gas may be generated to erode the kiln shell. On the contrary, GMH bricks are neutral refractory materials with no erosion, and can meet and even exceed designed service cycle.
    Our company has traced the situation of refractory materials ever since the introduction of the new 5000T/D large cement rotary kilns in the domestic market. Based on our abundant production experience of refractory materials for cement producers and their feedbacks, we conclude that previous solutions for bricks inside kilns have the following shortcomings:
    (1) There are many types of bricks inside kilns, increasing management difficulty and cost;
    (2) The bricks inside kilns have different service lifes, and those of kilneyes and front transitional zones can only last for 10 months or even less than that. The maintenance intervals of kilneye bricks, magnesite chrome bricks, spinel bricks, and anti-stripping bricks are 4 to 6 months in general, which mean refractory bricks of the whole kiln will be maintained 2 to 3 times a year. This will raise maintenance times, work load, clinker cost, and weaken the competitiveness.
    Based on years of exploration and customers' application and feedbacks, our company has realized simultaneous use of refractory materials throughout the kiln. This will reduce production and management cost and maximizes enterprise profits.
In order to reduce production cost, raise profits, and enhance scientific management, the company proposed the concept of "cycle of rotary kilns", i.e. the safe service life of kiln lining will be regarded as the operation cycle of rotary kilns (1 year). Centering on this objective and based on previous experiences, we have made detailed and comprehensive analysis, judgement, and prediction on material selection and lining workmanship, to ensure kiln operation can meet our objective. Successful optimization of refractory materials of rotary kilns:
 
 
 
The configuration has the following advantages: 
    (1) Long service life: GMH bricks have good stability in thermal shock and strong resistance to bending and pressure in high temperature, which can effectively stand up to thermal stress, mechanical stress (especially in position of second-gear tyre), and chemical erosion. Outstanding abrasion resistance and good mechanical strength have substantially extend the service cycle of kilns to reduce maintenance, and improve operation efficiency and output;
    (2) Self-protection of equipment: GMH bricks have a lower heat conductivity coefficient (1.7W/m·K). Therefore, the lower temperature of the kiln shell (about 320ºC) exerts less impact on the riding wheel in the middle of the kiln. The low temperature of supporting tiles and less deformation of the kiln shell are conducive to normal running of equipment;
    (3) Obvious energy conservation: GMH bricks have a lower heat conductivity coefficient (1.7W/m·K). The temperature of the kiln shell can maintain around 320ºC,, which can reduce unit cost of clinker. According to authorities, compared with spinel bricks, 2077.2Kg standard coal can be saved each day after application of GMH brick on the front and back transitional zones of the 5000T/D kiln. GMH bricks can use inferior coals, which is particular important in shortage of coals and rising prices.
    (4) Easy storage and low-cost management: GMH bricks have stable property and do not absorb water, facilitating storage and supply. In addition, it takes less time to produce GMH bricks, which can save storage and fund. Only two types of bricks are used in the whole kiln, another way to reduce management cost.
 
Joint part between the burning zone and transitional zone (front transitional zone)
    Magnesia bricks and magnesia-alumina spinel bricks are widely used in domestic rotary kilns. Some problems have been found in actual use: kiln coating often fall off due to frequent temperature changes of the transitional zone, especially parts around the tyre; the high heat conductivity coefficient and bad stability in thermal shock will cause high surface temperature of the kiln and heat expansion, which will exert huge pressure on the riding wheel in the middle of the kiln and affect normal operation equipment; the bricks even have shorter service life at areas around the tyre than the burning zone. In order to solve this problem, our company has worked with related research institutes to develop GMH bricks. Thanks to a low heat conductivity coefficient, the temperature of the kiln shell with GMH bricks can drop about 40ºC than other bricks; with pure materials and a scientific formula, GMH bricks have high temperature strength with good abrasion resistance and stability in thermal shock, which are particularly suitable to parts around the tyre and the transitional zone; the components and structure of GMH bricks can work well with kiln materials, which can prevent additional kiln coating on brick surface caused by penetration of elements between bricks and kiln materials, and avoid peeling bricks with the falling coating. GMH bricks have high refractoriness under load and good abrasion resistance as high alumina refractories. The well-distributed temperature gradient inside the bricks can effectively extend the service life. In addition, the lower temperature of the kiln shell can save coals, protect power equipment, and reduce unit clinker cost.