Development and characteristics of magnesia non-fired refractories

Refractories are divided into fired refractories and non-fired refractories. The raw material composition, preparation principle, the use of binders and admixtures of unfired bricks are basically the same as those of amorphous refractory materials, and they are non-fired or lightly fired products. Therefore, non-fired bricks also belong to the range of amorphous refractory materials.

1, the development of non-burning refractory materials

Although amorphous refractories were reported as early as 1925, the greater development was in the 1950s, and the 1970s was a period of vigorous development. Since the 1970s, the total output of refractory materials in Japan has been declining year by year, but the annual output of amorphous refractory materials has still maintained the momentum of nearly 900,000 tons, by 1985, the proportion of amorphous refractory production in Japan has reached 43.5%, becoming one of the rapid development of amorphous refractory materials.

Non-burning refractory bricks actually have a long history, and at the beginning of the 20th century, there were already non-burning magnesium bricks (also known as chemically combined bricks) in foreign countries. High alumina bricks without burning phosphate bond are also widely used.

In the early 1950s, non-burning viscose brick came out in China. In the late 1950s, there were phosphoric acid bonded, water glass bonded, sulfate bonded, chloride bonded and cement bonded unfired bricks. In the late 1960s and early 1970s, with the development of refractory concrete and amorphous refractory materials, production gradually expanded.

In the mid-1980s, the research and production of unfired bricks have made great progress, in addition to the original unfired bricks classified by binder, there have been various types of non-fired refractory bricks modified by additives, such as non-fired magnesium-chromium bricks, non-fired aluminum-magnesium bricks, non-fired aluminum-magnesium-carbon bricks, Al2O3-SiC-C bricks, MgO-SiC-C bricks. Non-fired refractory brick has become a new variety of refractory materials in recent years, and its research, production and application should be paid attention to.

The classification of non-fired refractory bricks can be carried out by three methods: binder, raw material and additive. Classified by binder are phosphate, water glass, sulfate, chloride, cement, carbon binder (such as resin, asphalt, etc.).

According to the raw materials are divided into aluminum silicate, silicon, magnesium, etc., in fact, almost all refractory raw materials can be made of unfired bricks. In recent years, due to the discovery that additives can greatly improve the performance of unfired bricks, there have been unfired bricks named after additives. By definition, non-fired carbon-containing products such as magnesium-carbon, aluminum-carbon bricks and tar bonds belong to the category of non-fired refractory bricks.

2, do not burn the characteristics of refractory materials

Non-fired refractory brick is a refractory material that can be used directly without firing. It has the advantages of energy saving, good thermal shock stability, simple process, etc., and can replace fired refractory products in a wide range of fields. Non-fired refractory bricks have many characteristics different from fired refractory products in the process. This is mainly reflected in the following aspects:

(1) The raw materials are required to be calcined well

Since there is no firing process for non-fired refractory bricks, they are directly put into use after drying, so it is necessary to use well-calcined raw materials to ensure that they do not cause large volume changes due to sintering of raw materials when used at high temperatures. For example, for high aluminum unfired bricks, when using grade I high bauxite clinker, the water absorption of the clinker must be less than 5%, and when using special grade alum clinker, its water absorption should be less than 3%.

(2) There must be a reasonable proportion of particles and apply a high molding pressure

Since there is no densification in the firing process, the densification of unfired bricks must be completed in the forming process. This requires that the non-fired refractory brick must have a very reasonable particle ratio and particle shape. Generally, it should be required to have a flake or angular particle shape.

Often according to 1. Literature review 10 la: fine powder = 7:3 or 75:25 ratio. The fine powder with fineness less than 0.088mm is required to exceed 90%. At the same time, the high pressure is generally required to use more than 630t friction press, and the number of strikes should be more than 6 times.

(3) Select the appropriate binder

The binder must give the unfired brick good cold strength and prevent deliquing, while wishing not to reduce the high temperature performance of the unfired brick. Due to the fact that a single binder often fails to meet the requirements, at present, a lot of progress has been made in the research of composite binders.

(4) Selection of additives

The bigger disadvantage of unfired bricks is that the refiring shrinkage is larger, so the load softening starting temperature is lower. In addition, the surface formation in use will cause spalling after sintering. These shortcomings can be solved by choosing reasonable additives. Suitable additives are of great significance for unfired bricks under different conditions of use.

(5) Control of drying system

In addition to a few unfired bricks (such as cement), generally unfired bricks must be dried. The different baking temperatures required by various binders must be strictly controlled, otherwise it will cause a series of problems such as delixing and performance changes of unfired bricks.