Silica brick
Silica bricks are the most commonly used and important brick type in various types of glass kilns. Silica bricks are mainly used in the roof roof, breast wall, end wall, small furnace and regenerator roof of the upper space of the glass melting furnace. The raw material for producing silica bricks is mainly crystalline quartzite, with a small amount of iron oxide and calcium oxide added as mineralizers to promote tridymite transformation as completely as possible and keep it fired at a stable temperature range. The SiO2 content of the silica bricks is 93%. is the lower limit, and high-end products are 96% to 98%.
Silica bricks have high high-temperature structural strength, such as high load softening point (1640~1700℃) and low creep rate [1550℃-50h-0.2MPa-(6~7)×10-5], and can absorb a small amount of Except for very slight corrosion after the alkaline component is added, the structural strength of the kiln roof will not be reduced.
The main disadvantage of silica bricks is their low thermal shock resistance.
The firing of silica bricks is a key link in the entire silica brick production process. During the firing, we must strive to complete the phase change process to the maximum extent to form an ideal microstructure and have better performance.
According to the phase transformation characteristics of quartz, the following stages should be controlled during firing:
(1) The 500~700°C stage is the range where β-quartz transforms into α-quartz, and the volume expansion is 0.82%. Calcium hydroxide decomposes and undergoes solid-phase reaction with the surface of quartz particles to generate CS.
(2) In the range of 800~1000°C, a small amount of liquid phase can be generated under the action of mineralizer. According to the measurement results of the electronic dilatometer, there is slight shrinkage at this stage, and the firing at this stage is relatively safe.
(3) The temperature range above 1250℃ is the formation stage of metastable cristobalite. For quartz, there will be a volume expansion of about 15%. The temperature rise rate should be slowed down as much as possible. However, the amount of liquid phase formation will also gradually increase, which will slightly balance the huge volume effect.
(4) 1350~1450℃ is the stage of massive crystallization of tridymite. The transformation from α-quartz to tridymite will have a volume expansion of 16.0%. However, quartz cannot be directly transformed into tridymite, but metastable cristobalite is transformed into For tridymite, the volume effect of this phase change is not large. However, the heating rate must also be controlled at this stage because: ① The phase transformation of quartz into metastable cristobalite still occurs at this stage. ② Give sufficient time to ensure that the tridymite crystallization is as complete as possible. Therefore, the firing and holding time above 1350°C is very important for perfecting the formation of microstructure and obtaining the best high-temperature volume stability.
Depending on the production method and usage conditions, the performance requirements of silica bricks vary. YB/T 147-2007 Silica Bricks for Glass Kilns Silica bricks for glass kilns are divided into three grades: BG-96A, BG-96B, and BG-95 according to physical and chemical performance indicators.
The standard also stipulates the appearance as follows:
① When a crack crosses an edge, it is only allowed to cross one edge, and cracks across edges are not included in the calculation.
②Cracks on silica bricks parallel to the working surface: the length shall not be greater than 1/5 of the length of the edge shared by the surface where the crack is located and the working surface.
③Small cracks: Bricks with a unit weight of no more than 20kg will be assessed according to the working surface crack index; bricks with a unit weight of more than 20kg will be assessed according to non-working surface cracks.
④The technical conditions of bricks with a unit weight greater than 40kg shall be agreed upon by both parties.
China's building materials industry standard (JC/T 616-2003 High-quality silica bricks for glass kilns) divides high-quality silica bricks for glass kilns into three grades according to unit weight: XBG-96 for unit weight <15kg; 15kg< unit weight≤ 25kg is ZBG-96; 25kg<unit weight≤40kg is DBG-96.
The standard also stipulates the appearance as follows:
① Cracks are only allowed to span one edge, and cracks across edges are not included in the calculation.
② Cross-section spalling: Bricks with a unit weight less than or equal to 15kg will be assessed according to the non-working surface crack index; bricks with a unit weight greater than 15kg will be assessed according to the working-face crack index, but cross-section spalling is not allowed to extend to the brick surface.
③For bricks with a unit weight greater than 40kg, the technical conditions shall be negotiated between the supply and demand parties.
clay bricks
Clay bricks are one of the main types of aluminum silicate products. They are refractory products with an Al2O3 content of 30% to 48%, made of clay clinker as aggregate and refractory clay as binder. The refractory fluctuation range of clay products is 1580~1750°C. The load softening temperature varies greatly due to different chemical mineral composition and density, and the thermal shock resistance is good. Generally used in kiln parts where the working temperature is below 1300℃.
The clinker proportion of kaolin products fluctuates between 50% and 90%. To manufacture products with high density, high strength and good high temperature performance, a multi-clinker formula should be used, that is, the clinker proportion reaches 80% to 90%, which requires Correspondingly adjusting the reasonable particle size composition, high-pressure forming and appropriate high-temperature firing, the products can obtain good physical properties. Since the process parameters vary greatly, the product properties also vary within a wide range.
Large clay refractory bricks for glass kilns are clay refractory bricks with a unit weight of not less than 50kg for building glass kilns.
This standard also makes the following provisions on the cross-section cracking of bricks.
(1) Brick cross-section cracks
A. Bricks with a length ≤ 400mm: when the spacing width is 0.26~0.50mm, the length is no more than 40mm; when the spacing width is 0.51~1mm, the length is no more than 25mm.
B. Bricks with a length >400mm: when the spacing width is 0.26~0.50mm, the length is no more than 80mm; when the spacing width is 0.51~1mm, the length is no more than 50mm.
(2) For cracks with a crack width less than 0.25mm, the length is not limited. It can be seen that producing clay bricks is relatively simple, but producing high-quality clay bricks is not easy.
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