At present, the domestic production of lightweight mullite insulation bricks has certain technical accumulation and experience, but there is still a big gap in the production process and product indicators compared with similar foreign products. One of the obvious differences is that EPS raw material pellets are commonly used as pore-forming agents in China. EPS raw materials are expensive, and some high-molecular organic toxic substances are generated during combustion, which easily cause environmental pollution. The products produced have high thermal conductivity, which fails to achieve the goal of energy saving and environmental protection. Using sawdust as the pore-forming agent, the raw materials come from a wide range of sources, and the cost is low, and there is no environmental pollution during firing. Research shows that pure sawdust products the thermal conductivity is lower. Therefore, sawdust is widely used as a pore-forming agent in foreign countries, and EPS raw materials are not used. However, there are no relevant production reports for this product in China. The reason is that the strength of the product will be greatly reduced when the sawdust is used alone as a pore-forming agent, which is an insurmountable problem; and the firing process shrinks greatly, and the product is easy to crack. The purpose of this experimental study is to use pure sawdust powder as a pore-forming agent to develop an energy-saving and environmentally-friendly lightweight thermal insulation brick with low production cost and low thermal conductivity.
2.1 Selection of raw materials
According to the composition requirements of ordinary lightweight mullite insulation bricks of the same level, try to choose raw materials with less impurity components, mainly because the content of Fe2O3 must be low. The main crystalline phase of the product is the mullite phase, so the raw material is required to have a certain content of Al2O3. Soft clay is used as the plastic binder during molding, and recycled light mullite brick powder and kyanite are used as raw materials for Al2O3. At the same time, hard clay clinker with low impurity content is added to adjust the product's performance. Impurity content. In view of the difficulty of sintering of pure sawdust low-density insulation bricks and the low sintering strength of products, a certain amount of additive A is added. This additive is an inorganic binder, which can effectively promote the sintering of products without increasing the firing temperature. Product strength, and the additive will not introduce impurities that are harmful to product ingredients.
The chemical composition of raw materials of pure sawdust lightweight mullite insulation bricks is listed in Table 1.
Table 1. Chemical composition of raw materials/%
2.2 Test formula
Table 2 lists the test formulations of pure sawdust lightweight mullite insulation bricks.
Table 2. Test ratio/%
In the above formula, sawdust powder is used alone as the pore-forming agent, and the currently commonly used EPS foam ball particles are not added, which greatly reduces the cost and does not cause environmental pollution during calcination. Because the sawdust is added in a large amount, it will produce larger volume shrinkage during calcination. The added kyanite raw material can use the expansion produced by its mulliteization to offset part of the green body shrinkage and prevent the product from sintering cracks.
2.3 Production process
The grouting molding method commonly used in the production of lightweight mullite insulation bricks is adopted. After natural curing until demoulding, it is moved into a forced dryer to dry, and then fired at a high temperature. The fired body is mechanically polished to become a finished product. The process is shown in Figure 1.
Figure 1. Production flow chart of lightweight mullite insulation bricks
The order of adding materials during the mixing process is as follows: First add the refractory raw materials, after the mixing is uniform, add the sawdust powder and then mix uniformly, and then add the water. The amount of water added is generally controlled at about 50%, so that the slurry maintains good fluidity and is easy to shape, and at the same time, it will not cause too much water to affect the curing time. In the process of stirring and mixing, additives such as water reducing agent and green strengthening agent are also added. After the mud is mixed into a mud, add additive A, and mix well again. Inject the prepared mud into the model to form, and leave it to cure at room temperature to disperse and evaporate the excess water in the mud. Generally, it can be demolded after standing for about 24 hours. At this time, the moisture content of the mud is about 35%. When the moisture drops to 25%~30%, the drying car on the green body enters the dryer for forced drying, and the drying temperature can reach up to 110℃; after drying for 24 to 48 hours, when the moisture content of the green body is less than 2%, it will be fired in the kiln. The firing is carried out in a 20m3 gas shuttle kiln. The firing temperature is 1,300°C, and the heat preservation is 4h. After the fire ceases, the temperature is lowered to 200°C and exit the kiln. After the firing, the blank is mechanically polished and processed, and the finished product is qualified after inspection. The appearance of the product is white, and most of the pores are micro pores about 1mm. Compared with the appearance of the product using EPS pellets as the pore former, the pores are smaller and more uniform, and the texture is more delicate.
3 Results and discussion
The physical properties of pure sawdust lightweight mullite thermal insulation bricks were tested according to the testing methods of shaped thermal insulation products, and compared with products mainly containing EPS particles. The results are listed in Table 3.
Table 3. Comparison of related physical properties of products
From the index point of view, compared with foreign products, the strength and thermal conductivity of this test product meet its requirements. Compared with domestic lightweight insulation bricks with EPS particles as pore-forming agent, the room temperature compressive strength and flexural strength are somewhat higher. Reduced, but does not affect the use effect, because for light-weight thermal insulation bricks, as long as they have a certain strength and can meet the transportation, storage and construction conditions, it is obvious that the strength of this product can meet the requirements. From the point of view of thermal conductivity, it is reduced by about 11%, which is very important, which means that it has better insulation effect, can reduce the thickness of the insulation layer of the kiln, and save materials;Or in the case of the same thickness, it is better to reduce heat loss and save fuel, so as to achieve the purpose of energy saving and environmental protection.
Using sawdust alone as a pore-forming agent, using a grouting method and adding a certain amount of additives can produce light-weight mullite insulation bricks with excellent performance. The process is simple, suitable for batch production, and does not use EPS raw materials, which saves costs and is environmentally friendly. The density of the pure sawdust mullite lightweight insulation brick produced is about 0.57g·cm-3, and the thermal conductivity at 400°C is 0.16W·(m·K)-1. The thermal conductivity index of the product is better than that of common products, and the energy saving effect is obvious. improve,It has good economic and social benefits.