Silicon bricks are prone to various cracks during production, such as spalling, vertical cracks, horizontal cracks and reticulation cracks. The scrap rate of silica bricks is high and their yield is not as good as other refractory bricks. Please see below for more information.
When a silica brick is subjected to thermal shock, thermal stresses are generated internally and the product will crack if the thermal stresses are too great. Cracks in silica bricks include surface cracks and layer cracks. Surface cracks are divided into horizontal cracks, vertical cracks and web cracks. Generally spalling and vertical cracks are caused by mechanical pressure, while horizontal and reticulated cracks are caused by firing. The main factors affecting cracking are firing and mechanical pressure, but the role of other factors related to the production process should not be ignored.
Cracking during the production of products (including silica bricks) is one of the main reasons for the increase of scrap rate. In this paper, we analyze the causes of cracks in finished silica bricks as an example and show that product quality is related to the whole production process. Only a thorough inspection of all aspects of the production process can reveal the key factors to improve product quality from the traces. This experience can also be learned in the production of other products.
First, the production mechanism
Refractories are subjected to rapid changes in ambient temperature during the firing process, called thermal shock.
Under the effect of thermal shock, thermal stress is generated inside the refractory material, and when its value reaches the strength limit value of the product, the product will crack or fracture. Refractory materials are uneven and brittle materials. Compared with metal products, it is easy to crack during firing due to its large thermal expansion, low thermal conductivity, low elasticity and low tensile strength, and it keeps expanding under thermal shock and eventually fractures. Silica bricks are particularly prone to cracking and even fracture during firing due to the change in crystal, which expands greatly in volume. However, the presence of microcracks in the microstructure improves the elasticity of the structure, weakening the thermal stress and suspending the expansion of cracks.
Cracks that are visible to the naked eye or by conventional methods and affect the use of the product
Cracks in finished silica bricks can be divided into surface cracks and internal cracks (also called layer cracks). Among them, the surface cracks are longitudinal and horizontal and vary in length. In order to facilitate the analysis of the causes of the cracks, it is necessary to make a reasonable classification of them. Practice has shown that the shape of the cracks is closely related to the direction of pressure during the forming process, which in turn is related to the shape of the brick, so they are usually classified according to the direction of pressure associated with the brick.
First, two directions related to the classification are specified, namely the horizontal and vertical (or vertically) directions of the brick. For example, for a standard common brick, the pressure direction is usually the thickness direction, and the other two directions are vertical.
(1) Vertical cracks, i.e., perpendicular to the direction of pressure on the product, usually along the height of the product. Semi-dry machine forming production products, along the pressing direction will produce "layer density", resulting in uneven thermal expansion of the billet, thermal stress, cracks parallel to the dense layer, so also known as the machine pressure cracking.
(2) transverse cracking, that is, parallel to the direction of product compression, generally along the product thickness direction, usually due to uneven heating of various parts of the product during firing, mainly in the outer stacks of bricks, especially the top product surface.
(3) mesh cracks, i.e. closed curve composed of multiple cracks, usually due to high and fluctuating temperatures, high internal thermal stress of the product, high failure stress of the product, microscopic unevenness of the billet itself, etc. . In addition, uneven mixing, raw material changes, etc. can also cause reticulation cracks.
(1) The use of large amounts of rapidly transforming porous silica or vein quartz with excessive component expansion.
(2) Excessive fine particles less than 0.5 mm or excessive large particles in the slurry, one reason being excessive critical particles.
(3) Insufficient addition of silica bricks, poor quality of lime milk, improper addition of mineralizing agent or too little water content in the slurry.
(4) unreasonable model design or improper model operation method.
(5) Uncertain drying system, incorrect direction of brick into the kiln, and too fast heating during firing, resulting in excessive volume expansion.
Silica brick is an acidic refractory material, mainly composed of phosphor quartz, square quartz and glass phase. It has strong resistance to acidic slag, but will be corroded by alkaline slag, not by oxides such as Al2O3, K2O and Na2O. Its load softening temperature is high, between 1640~1680℃, but the disadvantage is low thermal shock stability, but the refractoriness is close to the load softening temperature, no deformation at high temperature for long term use, and it helps to ensure the strength of the masonry structure when used.
Due to the high temperature and fluctuation, the internal thermal stress of the product is high, the product failure stress is high, and the billet itself is microscopically uneven. In addition, uneven mixing or changes in raw materials can also cause reticulation cracks. Strict control of temperature and thermal stress and uniform material mixing can reduce the occurrence of network cracks.
