Clay Bonded Refractory Castables

Clay-bonded refractory castables are castable amorphous refractory materials made of soft clay as a binder and refractory aggregates such as alumina-silica or corundum-mullite, powder and admixtures.

Clay-bonded refractory castables are classified into clay, high-alumina, corundum-mullite and high-alumina lightweight according to the type of refractory aggregate. According to their performance characteristics, they are divided into ordinary and high-strength clay-bonded refractory castables.

Clay-bonded refractory castables are mainly composed of binders, refractory aggregates, powder and admixtures. The chemical composition, physical and refractory properties of the binding clay used in clay-bonded refractory castables vary depending on their origin. The technical requirements of clay binder used as refractory castables are: Al2O3 29%~35%, Fe2O3<1.8%, R20<1.1%, ignition loss 10%~14%, refractoriness greater than 1670℃, and a wide sintering temperature range. In the mineral composition, it should contain a small amount of montmorillonite and mica to give it good bonding performance and plasticity. Clay powder with a fineness of less than 0.088mm should account for more than 60%, and its dosage is 7%~13%.

 

The manufacturing process of clay-bonded refractory castables is to crush or grind the calcined qualified silicon-alumina clinker or corundum mullite into refractory aggregate and powder. The most commonly used refractory aggregates are clay clinker and high-alumina bauxite clinker, with a maximum particle size of 10mm and a dosage of about 70%. Refractory powder is an important component of clay refractory castables. Special or first-grade high-alumina bauxite clinker should be selected for grinding, and sometimes alumina powder or corundum powder is also added. The refractory powder with a fineness of less than 0.088mm should account for more than 85%, and its dosage is 17%~25%.

There are many varieties of clay-bonded refractory castable admixtures. According to the characteristics of clay binders and construction requirements, dispersants and hardening accelerators are mainly used. In the clay colloid dispersion system, alkali ion-clay can be formed when alkali metal ions are added. Because the positive charge of alkali ions is strong and the potential difference is large, the repulsion between clay particles is also large, which exceeds the van der Waals attraction, that is, the clay has more cations and the particle spacing increases, which promotes the dissolution of clay colloids, releases free water, and enhances the fluidity of clay-bonded refractory castables, which is beneficial for mixing and molding. As a hardening accelerator for clay-bonded refractory castables, high-alumina cement or pure calcium aluminate cement is generally used, and its dosage is 0.2%-3.0% (external addition). When the clay-bonded refractory castable is mixed and stirred, the two admixtures are added at the same time, and it is required to ensure good fluidity during molding, so that the bonded refractory castable can obtain sufficient strength within a limited time after molding. Therefore, a delayed hardening accelerator is often used, which does not affect the effect of the dispersant during the mixing and molding process, and gradually causes the refractory castable to condense and harden after molding. Calcium aluminate cement has the above characteristics and has higher refractory performance, because the cement undergoes hydration and hydrolysis after contacting water, and Ca(OH) is precipitated, and the time required is sufficient to meet the molding time requirements of clay-bonded refractory castables. At the same time, the active alumina formed reacts with the silica in the material, and secondary mullite can occur when a certain temperature is reached, which also helps to improve the volume stability and refractory performance of the clay castable. In addition, in order to improve the stability of clay-bonded refractory castables, additives such as ultrafine powder and kyanite are sometimes added.

Compared with cement-bonded refractory castables, clay-bonded refractory castables have the characteristics of high medium-temperature strength, high hot strength, good volume stability at high temperature and strong spalling resistance.

According to the operating temperature and other conditions of thermal equipment such as furnaces, different types of clay-bonded refractory castables can be reasonably selected, and they can also be used together with metal or non-metallic anchors. On thermal equipment such as steel rolling heating furnaces, soaking furnaces and sintering machine ignition furnaces, ordinary clay-bonded refractory castables can be used as furnace linings. Its key parts such as burner bricks, water-cooling pipe wrapping, soaking furnace mouth and protruding belts, iron mixing car desulfurization spray guns and certain parts of rotary kilns, etc., use high-strength clay-bonded refractory castables.

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