The preparation method of lightweight refractory materials is usually similar to the preparation method of traditional lightweight refractory materials, mainly composed of aggregate and matrix, according to the different material systems or uses can choose machine compression molding, pouring molding and other molding methods, and then used after firing at medium temperature or high temperature or directly without firing. However, the traditional refractory material uses dense aggregate, while the lightweight refractory material uses lightweight aggregate, and the lightweight aggregate has a decisive impact on the structure and properties of the lightweight refractory. Of course, there are also some lightweight refractory materials whose preparation methods are almost exactly the same as those of their lightweight aggregates, and their integrity and uniformity are better. Considering that lightweight aggregate is the most important part of the preparation of lightweight refractory materials, only some of the preparation methods of lightweight aggregate are described below. In general, the preparation method of lightweight aggregate is mainly borrowed from lightweight thermal insulation refractory materials, which mainly include burnout addition method, foam method, in situ decomposition synthesis method, partial sintering method, gel injection molding method, sol-gel method and nanoparticle sintering method.
Mullite ceramic module
(1) Burn loss method
The burn loss method is also called the pore-making agent burn loss method, which is usually used in the preparation of lightweight refractory materials and thermal insulation materials, and is to add a certain amount of combustible exhaust, such as rice husks, wood chips, starch, organic polymer microspheres, pulverized coal, petroleum coke, coke and polystyrene and other organic substances. This type of material is also known as aporing material and a pore-forming material. At high temperature treatment, these pore-making materials will be burned off, turning into gas volatilization and forming pores. This method is the most common production method for thermally insulated refractory materials. Mohanta et al. prepared alumina porous ceramics with a porosity higher than 20% and 66% and a thermal conductivity of 1.2-2.4W/(m·K) at a heat treatment temperature of 1700°C using bran as a pore-making agent and sugarcane as a binder. Sandoval et al. took kaolin, talc and activated alumina as the main raw materials, used starch as the pore making agent and binder, and prepared a lightweight iolite porous ceramic at a heat treatment temperature of 1330 °C for 4h; Isobe et al. used carbon fiber as the pore making agent to keep warm at 1600 °C heat treatment temperature for 2h, and the pore porosity of the lightweight alumina ceramic prepared was 38%, and its bending strength could reach 171MPa.
The advantage of this method is that the pore size, porosity and stomatal morphology can be controlled by the size, shape and quantity of the pore material. The disadvantage is that uneven mixing of raw materials can lead to uneven pore distribution. When combustibles burn endlessly, black cores are prone to appear. When some organic synthetic pore making agents such as polystyrene pellets are added, some toxic gases are generated when sintered, which will also cause pollution to the environment.
(2) Foam method
The foam method, also known as the foaming method, is a foaming agent, foam stabilizer and water mixed in a certain proportion to prepare a stable, small bubble size and uniform foam liquid. After molding, it is first placed in a constant temperature and humidity curing box, and then placed in a constant temperature drying box for drying, and finally the sample is prepared by heat treatment. Chen Huan et al. prepared a lightweight corundum aggregate with a core-shell structure (dense external and porous internally) at a heat treatment temperature of 1800 °C by foaming method of 3.08 g/cm3 and 10.9%, respectively, and the thermal conductivity of this aggregate at 1000 °C was 0.75 W/(m·K).
This method can regulate the shape, pores and density of the sample relative to the foam impregnation process. The result is a porous material that meets expectations. This method is particularly suitable for the production of closed-cell ceramic products. Compared with the first foam method, this method is easier to produce insulated products with small bulk density, but the foaming method also has disadvantages, that is, the production process is more complex, the production control is more difficult, and the production efficiency is relatively low.
(3) In situ decomposition synthesis method
In situ decomposition synthesis method is a research method for preparing porous materials by using the gas produced by the decomposition of the sample at high temperature, and the gas or crystalline water is removed during the sintering process to form porosity. Li Shujing et al. produced stomata by in situ decomposition of Al(OH)3, and prepared porous mullite materials with small aperture and uniform distribution of pores; Fang Yineng et al. used alumina fine powder as raw material, organic polymer as pore making agent and binder, and added a small amount of Al(OH)3 fine powder, and sintered at 1400 °C to make porous bauxite aggregate; Deng et al. prepared porous alumina ceramics with Al(OH)3 and alumina as the main raw materials, and found that with the increase of aluminum hydroxide content, the strength of the specimen was also improved; Yan et al. used magnesite and Al(OH)3 as the main raw materials, TiO2 as additives, and used in situ decomposition synthesis to obtain lightweight porous spinel ceramics, and the apparent porosity of the specimen reached 53%, and its average pore size, flexural strength and normal temperature compressive strength were 5.95 μm, 8.5 MPa and 21 MPa, respectively.
Although in situ synthesis technology has many advantages, such as lower synthesis cost, small pore size, and more uniform distribution of stomatal pores. But there are also many problems, specifically manifested in: the content of reactants and the composition of the reaction and the reaction speed have a greater impact, and it is more difficult to control, which is more plastic to the sample, and the porosity of the material will not be very high.
(4) Gel injection molding method
The gel injection molding method is to add certain chemical substances to the slurry raw material and obtain a porous material through a certain chemical reaction. The principle of this method is that the organic monomers in the slurry raw material can be fully crosslinked and polymerized into a three-dimensional network structure through the action of initiators and catalysts, which can solidify the slurry raw material. Then after drying, heat treatment to obtain the product. Fukushima et al. dispersed the mullite particles in a gel, then added an ice structure protein additive, and then sintered at 1500 °C after freezing, drying, and then sintering at 1500 °C. The thermal conductivity of the obtained thermal insulation material is 0.23-0.38W/(m·K) at room temperature and compressive strength is 1-22MPa.
Compared with other methods, the gel injection film molding method has many advantages: low cost and requirements for raw materials, and a wide range of applications; Molding is easy to control, able to prepare complex parts of different specifications; The strength of the cross-linked blank is higher than that of the traditional method.
(5) Sol-gel method
The sol-gel method (Sol-Gel method, referred to as the SG method) prepares a gel with a three-dimensional mesh structure by uniformly mixing inorganic substances or metal alkoxides as a precursor, and the raw material undergoes a chemical reaction of hydrolysis and condensation, and then undergoes the polymerization of the rubber particles. After drying and heat treatment, the gel can be prepared to produce a nanostructured material.
The pore size of the material prepared by the sol-gel method is narrow, and the pore size can be adjusted by solution composition and heat treatment process, but the raw material of this method is limited, the production efficiency is relatively low, and the synthesis temperature is high and the agglomeration is serious, which is the more active method of current research.
(6) Nanoparticle sintering method
Nanoparticles generally have a high specific surface area and sintering activity, by introducing nanoparticles can increase the reaction process, and then the stomata are too late to eliminate and exist in the form of closed pores in the specimen, thereby obtaining a lightweight aggregate. Perko et al. coated sub-micron zirconia with 11 nm of zirconia, and insulation at a heat treatment temperature of 1400 °C for 2h to obtain zirconia porous ceramics with a relative density of 60% and a strength of 430 MPa; Fu et al. took alumina micronized powder as the main raw material, used starch as the pore making agent and binder, and then introduced a small amount of alumina sol, kept warm for 3h at a heat treatment temperature of 1800 °C, and prepared a lightweight corundum aggregate with a volume density of 3.05g/cm3 and an average pore size of 0.43 μm. The addition of nanoparticles reduces the pore size from microns to nanoscale, and the nanoliberation of pore size increases the flexural strength and normal temperature compressive strength of the castable by 50% and 125%, respectively. However, due to the high cost of nanoparticles as raw materials for the preparation of specimens, it is difficult to apply them on a large scale.