Refractory materials refer to a class of inorganic non-metallic materials with a fire resistance of not less than 1580 ° C. Magnesia-carbon bricks are widely used in various fields of China's industrial production due to their stable physical and chemical properties under high temperature conditions, such as steel and metals , Cement, petrochemical, military, etc., it is an important basic supporting material to ensure the normal operation of the industry and technological innovation and development.

        Castable construction

        There are many types of classification of refractories. According to the form of supply, they can be divided into shaped refractories and unshaped refractories. Refractory castable is a typical unshaped refractory material. It is a mixture composed of a reasonable gradation of aggregate particles, fine powder, a binder and an admixture. Refractory castables are usually dry and have no uniform shape. Water or other liquids can be used to make slurry, paste and loose shapes for pouring construction. After casting for a period of time, it will obtain a certain strength after hydration and solidification, and it can be used directly without baking after baking. Refractory castables have the advantages of simple production technology, convenient construction, and long service life. They are often used as lining materials for thermal furnaces and equipment and are widely used. In recent years, new varieties of refractory castables have continuously appeared. By changing the combination ratio and production process of materials, the dosage and type of binding agents, the choice of admixtures, and the update of preparation technology, the comprehensive use effect has been continuously improved. The application field is also expanding. The author takes domestic and foreign reports on refractory castables as an example to systematically introduce the composition, development and application of amorphous refractory castables.

        1. Composition of amorphous refractory castable

        1.1 refractory aggregate

        There are many types of aggregates for refractory castables, including clay aggregates, high alumina aggregates, alkaline aggregates, siliceous aggregates, mullite aggregates, corundum aggregates, thermal insulation aggregates, etc. Widely used are high alumina aggregate, mullite aggregate and corundum aggregate.

        1.1.1 High alumina aggregate

        High-aluminum aggregate is made of natural raw high-alumina bauxite. It is made by homogenization forming, and is baked and calcined. It is an important refractory material suitable for industrial kiln lining. It has the characteristics of rich raw material sources, uniform and stable ingredients, good strength, high fire resistance and excellent abrasion resistance, and is well received by the market.

        Refractory products based on high alumina aggregates are widely used in the cement industry. Zhang Wei and others used low alumina aggregate and fine powder as raw materials, and used silica fine powder and calcium aluminate cement as binders to prepare low cement castables for cement kilns. The results show that as the heat treatment temperature increases, the bulk density of the refractory castable product decreases first and then increases, while the room temperature flexural strength and compressive strength first increase, then decrease and then increase, indicating that the product has Good thermal shock resistance. Shan Yuxiang and others used high-alumina bauxite clinker as the main raw material and calcium aluminate as the binder when analyzing the use and damage mechanism of refractory materials inside the rotary kiln, while adding alumina fine powder, silica fine powder, and brown corundum Kyanite, zircon, silicon carbide, spinel, steel fiber, polypropylene fiber and composite admixtures, etc., through orthogonal experiments, reasonable data were measured. The results show that the compound admixture and reasonably graded high-aluminum aggregate particles can significantly improve the performance of the castable, and this castable is more feasible for use in the cement rotary kiln.

        Bauxite aggregate

        1.1.2 mullite aggregate

        Mullite is a series of minerals composed of aluminosilicates. Natural mullite crystals are fine needle-like and radiant clusters. It has excellent structural properties, high strength mechanical properties, high temperature corrosion resistance and creep resistance, high fire resistance and other characteristics, so it is widely used in ceramics, glass, cement and refractory materials.

        Zhang Wei et al. Dried the castable samples prepared with mullite aggregate at 110 ° C and then heat-treated them at a gradient temperature of 10001500 ° C. It was found that the flexural strength decreased first and then increased with the temperature gradually increasing. Large, the compressive strength increases first and then decreases. Qin Zhenmin, through continuous improvement of the formula design, found that the high temperature mechanical properties of sintered materials using the mullite-corundum system are superior to pure corundum materials or pure mullite. When studying the relationship between self-flow performance and particle composition, it was found that when the content of fine powder (<0.074 amount) was 22% and 39%, the flow value was better. When the expansion agent was added in a composite form, the addition amount was 5%. At 8%, the linear shrinkage of the castable after burning is the best.

        1.1.3 Corundum aggregate

        Corundum refractory castable has good high temperature abrasion resistance, and only has excellent corrosion resistance to acid and alkali slag and metal glass solution, so it is widely used in high temperature industrial fields such as building materials and metallurgy. He Zhiyong, Wei Qingfeng, etc. studied the effect of the amount of SiO2 fine powder on the performance of ρ-Al2O3 fine powder combined with corundum refractory castables. The research shows that: 2% SiO2 fine powder is mixed to form a network flocculent structure after hydration, and it reacts with ρ-Al2O3 fine powder to form mullite. And drying intensity. Li Zhigang et al. Studied the effect of nanometer calcium carbonate on the properties of corundum refractory castables: The experimental results show that at a processing temperature higher than 900 ° C, the nanometer calcium carbonate has a smaller particle size, higher dispersion uniformity, and is easy to form in situ bonding The calcium aluminate mineral, therefore, the corundum refractory castable containing nano calcium carbonate has a higher flexural strength than the castable containing calcium aluminate cement. He Zhonggong and other studies found that: when the amount of water reducing agent is added-timing, the flowability and flexural strength of the castable decrease with the increase of the amount of nano-calcium carbonate, and the apparent porosity gradually increases; when the castable flows When the value is constant, the apparent porosity and cold and hot strength of the castable treated at 1000 ° C and 1600 ° C significantly increase with the increase of the amount of nano-calcium carbonate added. Wang Zhoufu and others pointed out that the introduction of a small amount of nano-silica can significantly improve its physical properties at room temperature, but the nano-silica enhances the sintering degree of the castable, which reduces its thermal shock resistance.

        1.2 Binder

        A binder is a substance that is added to an indefinite refractory material to give it workability and green strength or dry strength. Refractory castables generally require the binder to have good dispersibility, lubricity and high hardening strength, so the reasonable choice of the binder determines the compactness of the formed body and even the excellent performance of the refractory castable.

        1.2.1 Calcium Aluminate Cement

        There are many binders for refractory castables. Commonly used binders are water glass, calcium aluminate cement, phosphoric acid and phosphate, starch, dextrin, and the like. Refractory castables using calcium aluminate cement as a binder are currently widely used. The binder (such as calcium aluminate cement) hydrates with water under certain temperature and humidity conditions, and the resulting hydration product generates gelation. Cooperative use, therefore has the advantages of high early strength, low production costs, good high temperature performance and so on. Jia Quanli et al. Used corundum as aggregate and pure calcium aluminate cement as binder to study the effect of pure calcium aluminate cement on the properties of corundum-spinel castables. The results show that with the continuous increase of the amount of pure calcium aluminate cement, the flexural strength of the sample increases first, and then decreases rapidly after 1000 ° C. Research by Li Shengyou and others found that when the cement content was about 25%, the castable had the highest apparent porosity and the highest flexural strength; when the cement amount was gradually increased, the apparent porosity and flexural strength of the castable gradually decreased, and the stability gradually increased. restore.

        1.2.2 Phosphates

        Another commonly used binder is a phosphate binder. The phosphate binding mechanism is mainly a chemical reaction and cementation with the material. When the phosphate binding agent adheres to the surface of the material particles, it forms a layer of colloid film with low strength and high adhesion. As the temperature increases, the concentration of the binding agent It increases with the decrease of water in the material, and chemical reaction with the oxide in this process, which changes the adhesion effect into a chemical effect and improves the strength of the material.

        Aluminum dihydrogen phosphate is a typical phosphate binder. It has plasticity before coagulation and hardening, convenient construction, low cost, and can be hardened at room temperature. It is suitable for making refractory materials with indefinite shapes. Jia Jiangyi and others studied the effect of aluminum dihydrogen phosphate on the performance of Al2O3-SiO2 based castables. The results showed that as the molar ratio of P2O5 / Al2O3 decreased, the solubility of the binder in water decreased, the cementation performance decreased, and the Reduced strength. When the amount of aluminum dihydrogen phosphate is less than 12%, the material is difficult to form, and the bonding agent cannot play the role of cementing; when the amount is more than 15%, the porosity of the sample will increase, and the strength will decrease. Therefore, the amount of aluminum dihydrogen phosphate has a great impact on the performance of the product.

        1.3 Additives

        The admixture is a material that strengthens the action of the binder and improves the properties of the matrix phase. It is a material other than the basic components composed of refractory aggregate, refractory powder and the binder, so it is called an admixture. There are many types of admixtures, which are divided into coagulant, dispersant, water reducing agent, inhibitor, early strength agent, retarder, anti-explosion agent, quick-drying agent, sintering agent and expansion agent. The author takes water reducing agent and dispersant as examples for discussion.

        1.3.1 Water reducing agent

        The use of water reducing agent greatly improves the flowability of the castable, which can be formed without mechanical vibration under the condition of low water consumption, effectively improving the comprehensive performance of the castable. Common superplasticizers are polyphosphates and organic salts. Zhang Haibo et al. Proposed that refractory mud without water reducing agent needs to add 8% water to produce thixotropic flow, while refractory mud containing polyphosphate water reducing agent needs to add 6.0% 6.5% water; while organic The effect of the water-reducing agent is more significant than that of polyphosphate, as long as 5% 5.5% of water can meet the needs of artesian flow. Xue Haitao and others used polycarboxylate as a water reducing agent to explore its effect on the hydration process of calcium aluminate cement. It was found that the addition of polycarboxylate will lead to an increase in flow value and lower water demand, and can improve the flowability of concrete. And hardening. Wei Juncong et al. Explored the effects of various water reducers on the viscosity of corundum self-flowing castable slurry, and obtained the optimal amount of various water reducers (acid sodium tripolyphosphate 0.08%, sodium tripolyphosphate 0.05%, Sodium hexametaphosphate (0.04%, complex sodium phosphate 0.05%, calcium lignosulfonate 0.10%), and it was found that 0.04% sodium hexametaphosphate was added as a water reducing agent in an aqueous solution of pH = 10, and the mud had a better dispersion effect .

        1.3.2 Dispersant

        Dispersants are a type of compounds that can be added to dry materials and mixed with water, which can keep the flow value of the castables substantially unchanged, and can significantly reduce the amount of water used for mixing. Dispersants can be broadly divided into inorganic dispersants and organic dispersants. When an inorganic electrolyte is used as a dispersant, if the amount is too small, the desired effect cannot be achieved; if the amount is too large, it will have a coacervation effect and even bring in impurities, so the amount of dispersant added affects the refractory castable. An important factor in performance.

        Li Zi et al. Used pure aluminate cement-free SiO2 micropowder corundum castables as the research object and explored the effects on the performance of the castables by adding different dispersants for comparison. It was found through experiments that all four dispersants were able to reduce the viscosity of the slurry, the dispersion effect was good, and the physical properties of the samples were better. Li Ning et al. Studied the effect of different dispersants on the rheological properties of matrix slurry of corundum castables in three different systems, and found that in the corundum fine powder-cement-silica micropowder system, the amount of dispersant added was 0.1% 0.3% , FS20 has the best dispersion effect, and the least amount of water is added (the amount of water is 12% of the matrix). Similarly, when Cheng Peng and others explored the effect of different dispersants on the performance of Al2O3-SiC-C series ultra-low cement self-flowing castables, it was found that in order to achieve the self-flowing properties of the castables, the surface viscosity of the slurry should be less than 375mPaS, and 0.06% was added After sodium polyacrylate, the slurry viscosity reduction effect is the best.

        Magnesium castable

        2. Application status and development trend of unshaped refractory castables As the most commonly used type of unshaped refractory, unshaped refractory castables have been widely used in many industries such as petrochemicals, building materials, kiln structures and thermal equipment. Such as high temperature sintering belts and coal injection nozzles in cement kilns, heat-resistant linings; riser reactors, high-temperature linings for other chemical conversions in petroleum refining; water-cooled tubes in heating furnaces, and electric furnace ladle nozzles. Through the introduction of binding agents, additives, etc., supplemented by baking and sintering heat treatment, the refractory castable has the development characteristics of enriching product varieties, purifying materials, and rationalizing particle size grading.

        Unshaped refractory castables will develop in the following areas in the future:

        1) The types of binders and admixtures are increasing;

        2) Diversified products and optimized performance of refractory castables;

        3) The application field of refractory castables is constantly expanding;

        4) The use of micro-powder and sub-micro-powder in the refractory castable will gradually replace cement and enhance physical properties.

        In recent years, the refractory castable technology has been continuously updated, and has achieved good application value and economic benefits in many fields. In the future, if relevant technologies such as nano-powder can be combined and industrialized, more refractory castables will have more new varieties with the lowest energy consumption and the best quality, which will drive the high-temperature industrial field to develop faster and better.