2023

Principle, Manufacture and Application of Light Bone Refractory (Ⅱ)

Source: Network

In addition, lightweight porous aggregates of calcium aluminate and spinel have emerged.

Lightweight refractories made with this lightweight aggregate exhibit lower bulk density and thermal conductivity than ordinary dense refractories. However, a key challenge is the resistance of the material to slag erosion and the mechanical properties.

In order to obtain lightweight refractories with guaranteed resistance to slag corrosion and stress damage, researchers have made several attempts to manufacture lightweight aggregates with a high proportion of closed pores and small pore sizes.

The superplastic foaming method is a solution that allows the manufacture of ceramics with a high volume fraction of fine closed pores. In addition, lightweight microporous alumina, bauxite and magnesia materials can also be obtained by adding nano-additives.

In these materials, the proportion of closed pores is about 40-70% of the total pores. Due to the reduced pore size, the manufactured lightweight material can show better slag corrosion resistance compared to dense materials.

2. lightweight aggregate and its five manufacturing processes

Since most lightweight refractories are produced by replacing dense aggregates with lightweight aggregates. Therefore, the performance of lightweight refractories depends on the structure and properties of lightweight aggregates.

Generally, the process route for manufacturing lightweight aggregates is suitable for the processing of porous ceramics. However, for the preparation of lightweight aggregates, the cost and operational convenience of large-scale industrial production should be considered.

Complex, elaborate and expensive methods such as sol-gel, gel casting, freeze-drying and the use of replica templates have not been used to manufacture lightweight aggregates. In addition, although the connection structure of pores is set as a target for porous ceramics, a high closed porosity ratio is required in the manufacturing process of lightweight aggregates. Based on the above considerations, the following will introduce five processes for manufacturing lightweight aggregates.

partial sintering

Partial sintering is a more common method of manufacturing lightweight aggregates.

As shown in Figure 3, the main principle of this method is to stabilize the gaps between the starting powder particles by adding certain additives. The additive may be a particle of smaller size, higher surface activity than the starting material, or a sintering aid that may form a liquid phase during heat treatment. With the additive, the starting powder particles are necked down, so that voids remain in the material.

In local sintering technology, the addition of additives should be accurately controlled. If the amount of additive added is small, it is difficult to stabilize the porous structure; however, due to its high surface activity and tendency to form a liquid phase, adding an excessive amount of additive may cause sintering densification of the lightweight aggregate.

In addition, grain coarsening (Ostwald ripening process) may occur during liquid phase sintering. With the formation of the liquid phase, smaller grains may partially dissolve into the liquid phase and precipitate on the larger grains, resulting in coarsening of the grains.

The advantage of local sintering is that the density and strength of the material can be increased, and the wear resistance and corrosion resistance of the material can be improved. The disadvantage is that it may cause deformation and cracks of the material, affecting the performance and life of the material. In addition, local sintering requires a lot of energy and time, which increases the manufacturing cost.

in situ decomposition

In-situ decomposition techniques involve the use of decomposable inorganics such as hydroxides, carbonates and hydrosilicates as raw materials.

The decomposition of these materials during heat treatment causes the volume of the particles to shrink, thereby forming voids.

In addition, the product produced upon decomposition is fine particles having high surface activity.

Due to the good sinterability of the decomposition products, the porosity structure is stable. Therefore, the porosity morphology and performance of the lightweight aggregate produced in this way are affected by the type, addition amount and particle size of the raw materials, as well as the technical parameters of the formation and sintering process.

Researchers have used three different magnesium-bearing minerals (basic magnesium carbonate, hydrotalcite and magnesite) as raw materials to make porous alumina spinel materials.

The higher impurity content in magnesite compared with the other two feedstocks promotes the formation of a liquid phase during heat treatment.

This results in a higher degree of adhesion between the particles, thereby increasing the mechanical strength and reducing the pore size. This effect is particularly pronounced in magnesite, which contains higher levels of silica and alumina impurities.

The amount of liquid phase and new phase are two key factors affecting the pore morphology and properties of the prepared lightweight aggregate.

On the one hand, as the amount of liquid phase generated increases, the sintering process is promoted, thereby reducing the pore size and porosity. On the other hand, volume expansion occurs during the formation of the new phase.

When a small amount of new phase is formed, this volume expansion may fill the voids, thereby reducing the pore size and porosity. However, excessive amounts of new phase can hinder sintering densification, resulting in increased pore size and porosity.

The amount of the liquid phase and the new phase largely depends on the content of the decomposable raw material. Therefore, the composition of the raw material used should be strictly controlled. The more important limitation of the lightweight aggregate prepared by in-situ decomposition method is the high porosity, which leads to the unsatisfactory slag resistance and mechanical properties of the material.

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.

Key words:

Principle, Manufacture and Application of Light Bone Refractory Material