2023

Principle, Manufacture and Application of Light Bone Refractory Materials (Ⅲ)

Source: Network

Adding pore-forming agent

When pore formers are added to the starting powder, they burn during the heat treatment, leaving voids in the ceramic material. The porosity level of the fabricated material can be easily controlled using pore formers compared to other techniques.

In this case, a higher sintering temperature can be applied to the ceramic green body, thereby increasing the mechanical strength of the produced lightweight material.

Currently used pore-forming agents can be divided into two categories: organic materials and inorganic materials.

Commonly used organic pore formers include starch, rice hulls, seragos, PMMA microspheres, and walnut shells, while commonly used inorganic materials include coal ash and carbon.

There is a clear correlation between the morphology of the pore-forming agent and the pores in the prepared lightweight aggregate. Thus, the fraction, size and morphology of the pores are directly related to the porogen selected.

The key factors of this technology are the type, amount and particle size of the pore forming agent used.

The addition of pore formers is often used in combination with other techniques to produce lightweight aggregates with improved properties.

direct foaming

In the direct foaming technique, the foam is first produced by foaming a mixture of blowing agent and water. Then, the foam and the raw material slurry are mixed and molded, dried and sintered to obtain a porous material. However, the main disadvantage of this method is the large pore size.

Generally, the average pore size of porous materials manufactured using direct foaming is as high as 200-300 μm; these materials have low mechanical strength and poor resistance to slag attack. Therefore, the porous material manufactured using this method is applied to the insulation layer, not as a wear-resistant lining.

superplastic foaming

Superplasticity is defined as the ability of a material to exhibit considerable elongation under load, indicating that the ceramic may undergo plastic deformation under stress.

First of all, suitable raw materials should be selected to produce ceramic bodies with superplasticity at high temperatures.

Generally, ceramic materials exhibit high temperature superplasticity only when the grain size is less than 1 μm. Thus, the starting materials used are mainly nanoscale or submicron powders.

Second, the technology requires the addition of high-temperature foaming agents, which are inorganic materials that are oxidized or decomposed at high temperatures, such as SiC,Si3N4, and hydroxyapatite.

The raw materials and the high temperature foaming agent are first compacted to obtain a green body. The green body is then sintered at a temperature below the oxidation or decomposition temperature of the high temperature blowing agent to produce a dense ceramic block. Subsequently, the ceramic block is reheated at a higher temperature to generate a gaseous substance by the high-temperature foaming agent.

Due to the partial pressure of the gas and the superplasticity of the ceramic matrix, closed pores are formed in the ceramic material.

The superplastic foaming method is mainly used to manufacture lightweight alumina and zirconia because of their good high temperature superplasticity. The performance of the material is mainly affected by the type, particle size and amount of raw materials and high temperature foaming agent. In addition, certain additives have been introduced to enhance the superplasticity of alumina and zirconia.

In-situ decomposition technology is the more economical method of all process routes, however, the lightweight aggregate manufactured by in-situ decomposition technology usually has a high open porosity.

The partial sintering and porogen addition method provides a simple and inexpensive method for preparing lightweight aggregates with porosity below 20%. The direct foaming technique has a higher cost than the above three methods.

The superplastic foaming route has significant advantages in producing lightweight aggregates with high closed porosity. However, since the starting materials used are mainly nano-scale or sub-micron-scale powders, the cost of superplastic foaming is high.

Based on the above systematic overview of the principle application and production process of lightweight wear-resistant lining refractories, future research directions should focus on the development of new process routes for lightweight aggregates such as the introduction of additive manufacturing technology (3D printing) or new core-shell structures;

In-depth use of lightweight refractory performance and degradation behavior of the internal mechanism and industrial furnace refractory lining of the integrated design.

Key words:

Principle, Manufacture and Application of Light Bone Refractory Material