Clay and High Alumina Refractory Castables

Composition Mainly includes binder, refractory aggregate powder and admixtures.

Binder The selected binder clay has different chemical composition, physical and refractory properties due to different origins. The technical requirements for clay binder used as refractory castables are: Al2O3 accounts for 29%~35%, Fe2O3 is less than 1.8%, R2O is less than 1.1%, loss on ignition is 10%~14%, refractoriness is greater than 1670℃, and has a wide sintering temperature range. In the mineral composition, it should contain a small amount of substances such as montmorillonite and mica to give it good bonding properties and plasticity. Clay powder with a fineness of less than 0.088mm should account for more than 60%, and its dosage is 7%~13%.

 

Refractory aggregates and powders Qualified calcined silicon-aluminum clinker or corundum mullite is made by crushing or grinding. The most commonly used refractory aggregates are clay clinker and high-alumina bauxite clinker, with a maximum particle size of generally 10mm and a dosage of about 70%. Refractory powder is an important component of clay refractory castables. Special grade or first grade high alumina clinker should be selected for grinding, and sometimes alumina powder or corundum powder is also added. The fineness of refractory powder is less than 0.088mm, accounting for more than 85%, and its dosage is 17%~25%.

There are many types of admixtures. According to the characteristics of clay binders and construction requirements, dispersants and hardening accelerators are mainly selected (see admixtures). In clay colloid dispersions, alkali ions can form alkali ion-clay when alkali metal ions are added. Because the alkali ions are highly positively charged and have a large potential difference, the repulsion between clay particles is also large, exceeding the van der Waals attraction, that is, the clay has more cations and the particle spacing is increased, which promotes the dissolution of clay colloids, releases free water, and enhances the fluidity of clay refractory castables, which is beneficial for mixing and molding. As a hardening accelerator for clay refractory castables, high alumina cement or pure calcium aluminate cement is generally selected, and its dosage is 0.2%~3.0%. When clay refractory castables are mixed and stirred, two admixtures are added at the same time, and it is required to ensure good fluidity during molding, so that the refractory castables 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 high refractory performance, because it undergoes hydration and hydrolysis after contacting water, and the time required for precipitation of Ca(OH)2 is sufficient to meet the molding time requirements of clay 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 clay sea injection materials. In addition, in order to improve certain properties of clay refractory castables, additives such as ultrafine powder and kyanite are sometimes added.

Performance Compared with cement 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.

Application 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. Ordinary clay-bonded refractory castables can be used as furnace linings for thermal equipment such as steel rolling heating furnaces, soaking furnaces and sintering machine ignition furnaces. High-strength clay-bonded refractory castables are used for key parts such as burner bricks, water-cooling pipe wrapping, soaking furnace mouth and protruding belts, desulfurization spray guns of mixing iron cars and certain parts of rotary kilns.

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