Key Factors For The Casting Process Of Fused Cast AZS

Casting is an important step of the manufacturing process that has great influence on the quality of the fused cast AZS block. Key factors that affect the casting process of fused cast AZS include the casting temperature, casting speed, the casting time and back pour.
The casting temperature is the temperature of the melt when it is poured into the mold. The casting temperature of fused cast AZS can be as high as 1820-1840℃. The viscosity of the melt is up to the chemical composition and temperature of the melt. The chemical composition is up to the formulation. The casting temperature is crucial since the higher the temperature, the lower the viscosity, the better the flow-ability. However, if the temperature is too high, the temperature difference between the casting and the mold is small, which will increase the shrinkage stress and cause cracks.
The casting speed determines the casting time. Every casting products has its best casting time. Improper casting time will cause many defects. If the casting speed is too fast, the stream will be coarse and the flowing speed will be too fast, which will produce big impact force on the mold. The force may destroy or melt the mold and cause protrusions. Additionally, gas will be brought into the mold and produce a thin shell filled with gas and bubbles in the casting. Besides gas, raw materials in the furnace mouth may be brought into the melt and cause defects. Conversely, if the casting speed is too slow, it will produce defects such as loose corners, knots, sand and pouring shortage.
Back pour refers to the process of repouring some melt into the shrinkage cavities after the casting is completed. Generally, small bricks are solidified quickly and do not require back pour. Medium bricks can be repoured. Large bricks require back pour and it takes a long time. Back pour is one of the effective ways to reduce the shrinkage cavities and improve the bulk density.
Zhengzhou Sunrise is a refractory material supplier from China, offering high quality refractory materials for glass furnaces, including fused cast AZS block, fused cast alumina block, fused cast high zirconia block, mullite brick, zircon brick, alumina bubble brick, magnesia brick, sillimanite brick, corundum brick, low porosity fireclay brick, high alumina brick, all kinds of glass furnace ramming masses, etc..

Corrosion And Erosion Wear Of Fused Cast AZS In Glass Furnaces

Nowadays, refractory materials are important in the glass furnace. Refractory materials are considered mechanically and chemically high performance at temperatures above 1400℃. The requirements for refractory materials are good physical and mechanical resistance to thermal shock and to attack by molten glass.
The refractory materials for the glass industry has made significant progress. Refractories for glass furnaces are normally based on A12O3, ZrO2, and SiO2, with or without Cr2O3. Refractories for the glass furnace are expected to have higher corrosion resistance to molten glass, no defects in glass environmental compatibility, reliable properties and improved methodology to identify origins of defects.
The major objective in the glass furnace is to optimize the entire process by reducing the maximum temperature of melting, improving refractory performance and/or improving fuel efficiency. The required physical and chemical properties of refractories are: thermal shock resistance, corrosion resistance, controlled expansion behavior, low thermal conductivity, and adequate creep resistance.
The fused cast manufacturing process was introduced by Fulcher in 1925 based on research conducted at Corning Glass Co. A characteristic of this manufacturing is that the materials have a low porosity. It is of great importance to the refractory, since this minimizes the glass refractory interface. Because the ZrO2 has a high melting point and has good corrosion resistance to the molten glass, fused cast AZS is widely used in the production of glass.
Fused cast AZS block generally consists of 33-41% zirconia crystals, interlocked with corundum, and minor interstitial glass. They perform well in contact with many glasses melted at 1500℃ or lower. But it can react with new developmental glasses, particularly rare earth alumino-silicates which has much higher melting temperatures, resulting in erosion of crystalline particles which cause stones in the glass.
The chemical reactions at the surfaces of refractories may take place between molten slag, fluxing agents, and gaseous substances. The corrosion of the refractories may be due to batch carryover, volatile fluxes, and melt attack, mainly at the metal line. Erosion often follows the initial corrosion, washing away refractory grains after the original bond has dissolved.
Fused cast AZS in contact with molten glass form a viscous silicate layer adjacent to the refractory thus restricting contact, and therefore, increasing corrosion resistance. When fused cast AZS are used in superstructures, they are exposed to high temperature combustion products. Superstructure and crown refractories are subjected to corrosive reactions with the vapor species of the batch components and batch carryover. In the melting of soda-lime glasses, the vapor species are primarily soda and sodium sulfates, while for borosilicate glasses, they are Na-tetraborate and B2O 3 predominate.
Zhengzhou Sunrise is a refractory material supplier from China, offering high quality refractory materials for glass furnaces, including fused cast AZS block, fused cast alumina block, fused cast high zirconia block, mullite brick, zircon brick, alumina bubble brick, magnesia brick, sillimanite brick, corundum brick, low porosity fireclay brick, high alumina brick, all kinds of glass furnace ramming masses, etc..

Suggestions On The Proper Use Of Fused Cast AZS

Based on the campaign life of the glass furnace, the indicators of the furnace operation and the physical and chemical properties and performance of fused cast AZS block, some reasonable suggestions are made on the proper use of the fused cast AZS.
Fused cast AZS is characterized with stable and dense structure and good corrosion resistance to glass. During 900-1200℃, it will have a abnormal volume expansion due to its crystal phase transition.
For structures constructed with fused cast AZS and silica brick (such as the front wall of the feeder pool), each material should have its own structure, because they have different volume changes at high temperature. For the superstructure that is less than 8m, there is no need to leave expansion joints. For the superstructure that is longer than 8m, it is recommended to divide it into several parts by expansion joints(4-6mm).
There are masonry joints between the sidewall block and the bottom block. When the refractory materials are heated, 4-5mm volume expansion per meter will be expanded into the masonry joints.
Fused cast AZS block has a higher thermal conductivity. When used in the sidewall, it parts near the liquid level require bigger cooling air flow.
Zhengzhou Sunrise is a refractory material supplier from China, offering high quality refractory materials for glass furnaces, including fused cast AZS block, fused cast alumina block, fused cast high zirconia block, mullite brick, zircon brick, alumina bubble brick, magnesia brick, sillimanite brick, corundum brick, low porosity fireclay brick, high alumina brick, all kinds of glass furnace ramming masses, etc..

Four Ways for Fused Cast AZS Casting

Casting is a manufacturing process by which the melt is poured into a mold to be solidified. Although the process is short, it is a complex process and every step is related to the quality of the final products.

There are 4 ways of fused cast AZS casting, including normal casting(PT), Tilt casting(QX), Reinforced casting(ZWS) and Void-free casting(WS).

PT-Normal casting

It adopts general casting risers and the riser will be removed in the hot state. The shrinkage cavity in brick is located under the casting scar, due to volume shrinkage from liquid-solid transform and rapid solidification of a casting. The feasible limits of cavity can be evaluated and indicated by the volume-weight testing of blocks in-situ according to the technical specifications.
PT fused cast AZS block is always cheap and used in the areas with no serious corrosion such as top crown, superstructure and feeder channel. The surface with scars should not be used as the working surface.

QX-Tilt casting

In this method, the mold is inclined at an angle. The shrinkage cavity in brick disperses at one end of the brick and a completely dense zone forms at another end. The feasible limits of cavity can be evaluated and indicated by the volume-weight testing of blocks in-situ according to the technical specifications. Meanwhile, blocks which height less than 900mm are not suitable to be manufactured by this process.
QX fused cast AZS block is generally used in the sidewall of glass furnaces.

ZWS-Reinforced casting

Similar to WS, merely a few shrinkage voids remain in upper part of the body. A little part of the cavity will be remained at the end of the block after the header cutting. Block evaluation in-situ begins from the determination for the feasible size and deepness of cavity remainder at first and then to the volume-weight testing of blocks according to the technical specifications.
ZWS fused cast AZS block is always used in the sidewall of glass furnaces.

WS-Void-free casting

Additional liquid from the header fills sufficiently into a casting and the header is then cut off, so no shrinkage cavity exists in brick body. Block evaluation in-situ includes the acceptance or rejection for somewhat trace of cavity remainder which perhaps appeared and the volume-weight testing of blocks according to technical specifications.
WS fused cast AZS block has a wide application in the glass furnaces such as sidewall, tank bottom, feeder channel, doghouse, throat, dam wall, electrode wall and corner wall.
Sunrise is a refractory material supplier from Zhengzhou, China, supplying high quality fused cast AZS blocks manufacturing in different casting methods (PT, QX, ZWS and WS). Sunrise will be present at Vitrum 2015 in Milan, Italy from October 6, 2015 to October 9, 2015. Visit us at Booth U13, Hall 22.

Chemical Composition And The Quality Of Fused Cast AZS

Fused cast AZS block is the essential refractory material used in the demanding areas of glass furnaces. The quality of fused cast AZS can affect the quality and production of glass and the service life of the glass furnaces. The chemical composition of fused cast AZS is one of the key factors that determine the crystal structure and the properties.
Fused cast AZS block has many chemical and physical properties over other refractory materials, such as high temperature resistance, dense crystal structure, low porosity, high mechanical strength, good chemical stability, good corrosion resistance, good thermal stability(1200℃) and high exudation temperature of glass phase(>1400℃). Due to the features mentioned above, it is widely used in the glass industry.
It is important to select high quality raw materials and improve the purity of the raw materials. Any change of the chemical composition can change the crystal structure. Too much SiO2 can cause the formation of mullite and the decrease of glassy phase and corundum, which will destroy the crystal structure and make the structure loose. ZrO2 has the features of high temperature resistance, high mechanical strength, good corrosion resistance to glass and good chemical stability, but the drawback is its bad thermal stability at 1000-1200℃.
The corrosion resistance to glass of fused cast AZS is related to not only the content of ZrO2 but also its micro-structure. The denser its micro-structure, the better its corrosion resistance to glass. Too much ZrO2 will make its micro-structure loose and the baddeleyite will drop into the glass melt, which will cause glass defects such as stones, stripes and bubbles and reduce the service life of the glass furnaces. Therefore, its corrosion resistance to glass is not proportional to the content of ZrO2. Besides, with the increase of ZrO2, the melting temperature and viscosity of the glass melt also increases, which increases the difficulty of manufacturing and casting.
The content of Na2O and the impurities such as Fe2O3, TiO2, CaO, MgO will also affect the quality of fused cast AZS. Therefore, the stability of chemical composition is important for the quality of fused cast AZS and the service life of the glass furnaces. Sunrise offers high quality fused cast AZS block 33#, 36# and 41#.

How To Select Fused Cast AZS Block For Glass Furnaces

Fused cast AZS is made of pure alumina powder and zircon sand(65% ZirO2, 34% SiO2). Its main crystal phase consists of corundum, baddeleyite and glass phase. The use of fused cast AZS expands the life of glass furnaces.
The life of glass furnaces can be expanded from 11-15 months to 3-4years, even longer. The melting temperature of the glass can be increased from 1450 ℃ to 1550 ℃, even up to 1600 ℃. The glass defects are reduced and the quality of glass is improved. The production capacity of the glass furnace can be increased by 20-30%.
The damage of fused cast AZS is mainly caused by chemical attack and thermal stress. The quality and property of fused cast AZS plays a remarkable influence on the life of glass furnaces and quality of glass. The use of poor quality bricks will not only result in frequent shutdown and repair, limiting melting temperature and reducing the furnace production, but also shorten the life of furnaces, cause glass defects (stripes, stones, etc.) and reduce the quality of the glass.
When selecting the size of fused cast AZS, the furnace design principles and the producing conditions of fused cast AZS must be taken into account. When the dimension is too big, it tends to crack and will increase the difficulty of packaging, transport and masonry. For the shaped brick, the feasibility of each production procedure and the specialty of the casting method should be taken into account.
The corner brick, dam block, bubble brick and doghouse always employ the fused cat AZS block 41#. The gaps between bricks should be less than 0.5mm. The bottom of the brick should not have voids. The brick should not contains too many impurities. If the color of the brick is blue, there may be too much C in the brick. The cutting will directly determine the degree of convergence during masonry. The smaller the gaps are, the longer the life of the glass furnace is.
Sunrise offers different types of fused cast AZS block, including AZS 33#, AZS 36# and AZS 41#. Our fused cast AZS block is made of high quality raw materials and advanced technology. It has been widely used in the glass furnaces and improves the life of glass furnaces effectively.

How To Prolong The Service Life of Fused Cast AZS Sidewall Block

The fused cast AZS block is widely used in the sidewall in the glass furnaces. The service life of the fused cast AZS sidewall block determines the campaign life of the glass furnaces. So, it is important to prolong the service life of fused cast AZS sidewall block.
How to decrease the corrosion rate of sidewall blocks and improve the corrosion resistance is the key to prolong the service life of sidewall blocks. It is well-known that controlling the operation temperature is one of the effective approaches to prolong the service life of refractory blocks. In different areas and different periods, different cooling methods should be adopted to achieve the desired the results.
In the top parts of the sidewall where are not insulated. In horseshoe flame glass furnaces, as the flame is heating in the surface of glass melt, the temperature of the upper parts of the glass melt is higher than that of the lower parts. Silicate is formed and the refining of glass melt produces a lot of bubbles. Heat convection is very severe here. As a result, the corrosion rate of is bigger there. To solve this problem, it is necessary to select and design the cooling fan reasonably. The cooling should be started slowly at first and then increases gradually to reduce or avoid the thermal stress caused by temperature gradient.
The service life of fused cast AZS block lasts about 3-5 years. During the preheating of the glass furnaces, cracks may be formed. After 2-year operation, there will be holes in the body of the brick. Glass melt may exude though the holes and cracks. To solve these problems, a single-point, focused and forced air cooling method can be adopted, which can crease the viscosity and reduce the temperature of the bricks.
Some special parts of the melting zone is corroded at the fastest rate and not easy to repair. Due to strong mechanical erosion, the glass furnace can just be used for 2.5 years before cold repair. Air and spray mixed cooling method can be adopted here. It is easy, stable and effective. It avoids the problems caused by the water cooling method.
Shower cooling method is mainly used in the middle and upper parts in the later period of the glass furnace campaign. 3 years after the cold repair, the problem of glass melt exudation may occur in these areas. As there are layers of insulating materials here, forced air cooling and water cooling method can not be used here. Compared to other methods, shower cooling method is more suitable for this parts.
Sunrise offers various high quality refractory materials for glass furnaces, including fused cast AZS, fused cast alumina block, zirconia block, mullite block, silica block, magnesia block,etc..

Fused Cast AZS Exudation And Glass Defects

Fused cast AZS block is the most widely used refractory material in the demanding areas. It is thought as the leading cause of knot and cord defects. Fused cast AZS exudation in glass contact is the main cause of impaired performance of AZS under particular working conditions.

The main crystal phase of Fused cast AZS consists of corundum, baddeleyite and glass phase. The glass phase can reduce the expansion or shrinkage of ZrO2 within the above temperature range and prevent cracking. However, the glass phase is easy to be corroded and cause corrosion of the fused cast AZS and glass defects.
Fused cast AZS exudation exhibits a shiny "wet looking" appearance at the surface of blocks during heat-up process and beginning of a furnace campaign. This situation tends to stabilize all along the campaign life, with a generally moderate dripping of a viscous glassy phase into the smelted glass.

Fused cast AZS exudation at high temperatures, can lead to the formation of defects in the glass, especially during furnace start-up. It is only a short-term cause of glass defects. Fused cast AZS exudation from a glass contact surface is not very visible than that in the superstructure.

In the glass furnaces, a portion of the glass phase of fused cast AZS block exudes onto the superstructure surface during furnace heat-up, until the furnace has achieved stable temperature profile. The exuded glass, often containing small zirconia crystals, tends to run down into the glass bath. This is a major concern to glass producers since this glassy phase often causes defects, such as viscous knots and stones.

To solve this problem, low exudation fused cast AZS has been developed. With modification of chemical compositions and glass phase, it offers low exudation level and maintains all other positive characteristics of fused cast AZS.

Sunrise offers various refractory materials for glass furnaces including fused cast AZS, silica brick, fireclay brick, high alumina brick, silimanite brick, mullite brick, etc..

Fused Cast AZS Block and Glass Defects

The interaction of glass melts and refractories in glass furnaces has a large impact on the resulting glass quality. The mostly applied type of refractory in the glass industry, in direct contact with the glass melt, is fused cast AZS. Due to the interaction of the glass melt with fused cast AZS, glass defects like stones, knots and bubbles can be generated.
Fused cast AZS block exudation is thought as the leading cause of knot and cord defects. Fused cast AZS exudation is only a short-term source of glass defects. Fused cast AZS corrosion is a more potent and long-term source of defects. Glass melt contact refractory corrosion can lessen with time due to boundary layer formation and the effect of external cooling at the metal line.
Fused cast AZS block has no open porosity but there are some closed voids and pores which arise from the manufacturing process. These pores are filled with gases. The gases in these pores have an air like composition, with nitrogen and oxygen as main components and some carbon dioxide. When fused cast AZS is in contact with glass, it reacts with the glass melt and slowly dissolves in this melt. The closed pores will be opened and then release their gas content, forming bubbles.
Impurities, which can oxidize, are for example elemental carbon originating from graphite electrodes of the fused casting operation, sulfur or zirconium carbide originating from raw materials of the AZS. The dissociation/oxidation of nitrides/oxynitrides, produced by reactions with the graphite electrodes during melting of the AZS, could give nitrogen gas.
When fused cast AZS block is brought into contact with a glass melt, an electrical potential is generated between the refractory and the glass melt because generally no thermodynamic equilibrium exists between these materials. The electromotive force (emf) generated between glass melt and refractory interior can cause the formation of oxygen bubbles.
The generation of a knot is due to one or more forces, which drive the fused cast AZS glass phase towards the refractory surface and into the bulk of the glass melt. Those forces can be caused by gravity, thermal expansion of zirconia and gas formation.
Glass defects are typically unavoidable, but their occurrence must be minimized during the production of high-quality glasses. Some alternatives, such as fused cast AZS with modified chemistry and glassy phase and alpha-beta fused cast alumina block are used to replace fused cast AZS to reduce the glass defects and improve the glass quality.

Wear Of Fused Cast AZS Block In The Superstructure And Crown

In the past, wear of superstructure refractories, primarily silica crowns, is probably the most serious concern. Fused cast AZS block has been suggested as an alternative to the traditional silica crown as it has good corrosion against and can slow the wear of superstructure and improve the life of the furnaces.
Superstructure and crown refractories are subjected to corrosive reactions with the vapor species of the batch components and batch carryover. In the melting of soda-lime glasses, the vapor species are primarily soda and sodium sulfates. For borosilicate glasses, Na-tetraborate and B2O3 predominate. Direct attack occurs by condensation of the hot face of the refractory. The chemical changes in this reaction zone result in mechanical stresses and flaking, peeling or spalling of the hot face. Vapor penetration into joints with subsequent condensation and corrosion lowers the mechanical integrity of the superstructure or crown.
Corrosion phenomenon in superstructure have environmental and intrinsic causes. Heavy metals from fossil fuels, volatile component of the glass can reach the refractory surface as vapors, but the most severe attack can be caused by silica, reaching the surface as dust. Silica reacts easily with the corundum crystals of the Fused Cast AZS and, with the contribution of refractory's soda, produce sodium-alumina-silicates, pealing off the external layer of refractory, exposing deeper layers to corrosion and so forth.
At temperatures above 1475 ℃, the bond strength between the embedded ZrO2 crystals and the alumina is reduced, attributed to the variation in the thermal expansion characteristics of A12O3 and ZrO2.
Fused Cast AZS Block that are not exposed to peculiar environmental attack. In some case, due to recurrent thermal cycling and subsequent multiple exudation, the refractory is losing very significant amounts of glassy phase, extruded with a gas-driven mechanism, replacing it with increasing amounts of closed porosity. When this happens, in relatively deep zone under the hot face there is formation of mullite by reaction of silica of the glassy phase and corundum crystals; while minor amounts of alkaline glassy phase are extruded, the increasingly acidic refractory structure undergo progressive mullitization. Volumetric changes and depletion of the liquid phase necessary to absorb the thermal dilatation of crystalline components are concurrent causes of possible cracking and deep spalling of surface, as a consequence of intrinsic corrosion subsequent to exudation.
While the wear of fused cast AZS block in the superstructure has been a potent and long-term source of glass defects, it is proved a longer service life as superstructure refractories in glass melting furnaces and other similar applications, it still encounters wear in the superstructure.