refractory

When selecting refractories for regenerator chambers serving natural gas fired furnaces producing soda lime glass, both the functions of the refractories and the operating conditions within the checkers should be taken into consideration.

As heat exchangers, checkers should have high thermal capacity and thermal conductivity. Basic refractories and fused cast refractories are the best solution.

However, refractory selection also depends on the operating conditions, while the operating conditions depend on the position. According to the temperature, checkers can be divided into four zones: Top zone (from the first row to 1,350℃), Mid zone (from 1,350℃ to 1,000℃), Condensation zone (from 1,000℃ to 700℃) and Lower zone (from 700℃ to rider arches).

Top Zone

High temperature and batches and dusts result in chemical attack and gradual corrosion of the basic refractory bricks. If refractories are magnesia bricks, the chemical attack is up to the CaO/SiO2 ratio in the waste gases. if the radio is low, forsterite (Mg2SiO4) will be formed, which results in fissures opening within the bricks. Subsequently, silica penetrates these fissures resulting in the familiar cubic breakdown of the upper checkers. If the CaO/SiO2 ratio in the waste gases is high, a liquid phase enters into the refractory causing deformation. The best solution is magnesia bricks with high Mg content, well developed MgO crystals and a direct bonded structure. Additionally, the refractories should have low iron to avoid FeO oxidation to Fe2O3 and vice versa (Fe2O3 reduction to FeO) with volume variations and resultant brick failure.

Fused cast refractories have no surface porosity thus they are resistant to the corrosive effects of waste gases and carryover and can be used in all the checker zones. Compared to sintered refractories they are more resistant to abrasion due to their dense and homogeneous structure thus they are suitable for the top zone where there is a strong carry-over. Fused cast alumina brick is recommended for its very limited glassy phase. No glassy phase means no exudation therefore no excessive bonding with carry-over thus minimizing the risk of blockages.

Mid Zone

This zone is protected by the top checker area and temperature level is lower, thus 96% MgO with low iron and fused cast AZS 33# are recommended.

Condensation Zone

This is another critical area. The waste gases contain alkaline sulphate and SO3 which will condense out in the 1,000-700℃ range. In presence of sodium sulphate, the predominance of Na2O or SO3 in the waste gases causes chemical attack. Periclase base refractories are not chemically attacked by sodium sulphate or sodium oxide but they strongly react with SO3 forming MgSO4 causing densification of the Structure.

The chemical attack, enhanced by the presence of vanadium pentoxide when using fuel oil, breaks up the refractory and the structure densification lowers thermal shock resistance. Viable substitutes for chrome bearing refractories, which have a high resistance to condensates but cannot be used for environmental reasons, are both the spinel (MgO·Al2O3) and refractories made by periclase (MgO) and zirconia (ZrO2) having good resistance against Na2O and SO3.

When firing with natural gas, since the SO3 quantity is low, basic refractories can be used. When fused cast material is used, fused cast AZS is recommended.

Low Zone

Super duty fire clay brick can be used in non severe working conditions. 90-92% magnesia brick is recommended when firing by natural gas. Fused cast AZS 33# is also used in this zone.

Silica bricks with identical chemical composition can have differing mineralogical compositions which can cause quite different behavior during use. Therefore, it is not always sufficient to evaluate silica bricks only by their chemical composition. It is essential to also consider the degree of transformation and the thermal expansion behavior of the bricks.

Silica brick contains cristobalite, tridymite and some residual quartz. The crystal phases each have a high and low temperature forms which can transform reversibly. The crystal structure of the individual SiO2 crystal phase can differ widely. This is of great importance during heating and cooling because of the change in the volume.

Quartz requires the smallest volume and the quartz glass the largest. During firing above approximately 900℃, quartz transforms into the other modifications and melt completely at 1725℃. It shows such a transformation at 573℃, tridymite at 117℃, and cristobalite between 225℃ and 270℃. The thermal expansion of cristobalite is considerably greater than that of the tridymite.

Because well transformed silica bricks contain little or no residual quartz, their behavior under the influence of temperature is largely determined by the ratio of cristobalite to tridymite. During heating up, silica bricks expand rapidly with the total reversible expansion being completed at around 800℃. Therefore they are insensitive to the temperature fluctuations above 800℃, but very susceptible below this temperature because of the sudden volume expansion. For this reason, sufficient time must be allowed for heating furnaces up to about 800℃.

During slow cooling , reversible volume decreases take place which are a result of the spontaneous transformation of the crystal structure from the high to the low temperature modification. The reversible and irreversible volume effects can cause considerable stress within the refractory brick structure.

During the firing process, the lime reacts with the quartzite components to form wollastonite. The matrix also contains very small quantities of calcium ferrite, hematite, magnetite, calcium olivine and hedenbergite, which are formed from impurities. These crystalline phases are the reason for the discoloration and spot formation on the silica bricks.

The degree of transformation of the bricks can be determined easily and accurately by the density of the residual quartz content. The density of a silica brick is lowest when the degree of transformation is farthest advanced.

The appearance of the bricks also indicates to the degree of transformation. The reversible thermal expansion also depends on the mineral composition. Tridymite and cristobalite do not expand linearly during heating but exhibit sudden changes in length both during heating and during cooling.

The key to production of silica bricks is to select the raw materials and the firing process in which the degree of quartz transformation is suitable for the intended application.

The raw material for silica brick is quartz which must meet certain requirements. If refractoriness or thermal expansion are the main requirements, a quartz of high chemical purity must be selected. If volume stability is the main requirement, raw materials should have good transformation properties.

The chemical composition of quartz is important in its evaluation as a raw material, in particular the content of alumina and alkali, as these lower the melting point and considerably reduce the possibilities of application. In addition the firing behavior of the quartz must be taken into account.

After the raw materials have been crushed, ground and screened to the various grain fractions, they are mixed in predetermined proportions according to the required application properties. In most cases, mixers are used for mixing and special bonding agents. Generally around 2% lime water and some sulphite solution as a temporary binder, are added at the same time. The mixture is then processed on friction presses or hydraulic presses. Complicated shapes or those where short runs are required, are still rammed by hand.

After drying, silica bricks are fired at temperature of about 1450-1500℃. Because the transformation of the silica takes place suddenly, cooling must be carried out slowly, or the brick will crack. It is necessary to maintain a carefully planned time temperature cycle during firing. A strong, well bonded high quality silica brick can be obtained only after passing through critical temperature ranges. During firing, silica brick will have about 4% linear growth.

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Refractories are the main materials used in the glass furnaces and have big influence on the service life of glass furnaces, the quality of glass, the energy consumption and the غير مجاز مي باشدt. The development of the manufacturing technology of glass greatly depends on the the development of refractory technology and improvement of the quality of refractories.

Glass is melt at more than 1600℃, so the refractories will suffer from corrosion during the manufacturing process. The corrosion resistance performance and corrosion rate of refractories determines the service life of glass furnaces. The larger the scale of glass furnaces is, the bigger the production capacity is. The quality and accuracy of refractories has direct influence on the scale of glass furnaces. The application of the oxygen-fuel technology and reduced pressure technology depends on the performance of refractories, since those technologies increase the concentration of alkali vapor and water vapor, make the refractories more serious and have more demanding performance on the performance of refractories. Since the corrosion of refractories can also cause glass defects such as stones, bubbles and stripes, the corrosion resistance performance has a big influence on the quality of glass.

According to the types of glass produced, the size and structure of furnaces, the quality of glass required and the production capacity, different refractories are selected. So far, refractories for the glass industry, especially the float glass furnaces, include fused cast refractories, silica bricks and magnesia refractories.

Fused Cast Refractories

Fused cast refractories are refractories made by melting the raw materials and then casting the melt into a mold. Fused cast refractories for glass furnaces mainly include fused cast AZS, fused cast alumina block and fused cast high zirconia block. Fused cast AZS and fused cast alumina block are the main refractories used in float glass furnaces, Electronic glass furnaces, household glass furnaces, and pharmaceutical glass furnaces. Besides glass contact areas, they are also used in demanding areas such as the beast wall and the crown.

Silica brick

Silica brick is a traditional refractory used in glass furnaces, especially in the crown and superstructure. The quality of glass furnaces will affect the normal operation and service life of glass furnaces.

Magnesia refractories

Magnesia refractories are alkali refractories, including fused cast magnesia brick, high pure magnesia brick, direct bonded magnesia chrome brick and magnesia zircon brick. Magnesia refractories are mainly used in the regenerator.

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Silica brick belongs to acid refractories, mainly consisting of tridymite, cristobalite and a small amount of residual quartz and glass phase. It has good resistance to acid slags.

The raw material for silica brick production is silica. The silica has a high content of SiO2 and high refractoriness. The most harmful impurities are Al2O3, K2O, Na2O, etc. They can seriously reduce the refractoriness of the silica brick. Silica brick is made of silica containing more than 96% SiO2 , with addition of mineralizing agent (such as iron scales, lime) and binder (such as molasses, sulfite pulp waste), by kneading, shaping, drying and calcining.

Silica brick has a high softening temperature under load up to 1640 ~ 1670 ℃ and good volume stability in long term use. The mineralogical composition of silica brick mainly consists of tridymite and cristobalite, as well as a small amount of quartz and glass phase.

Tridymite, cristobalite and residual quartz has a big volume change due to crystal transformation at low temperatures. Therefore, the thermal stability of silica brick is poor at low temperatures. During the use, the silica brick should be heated and cooled slowly in order to avoid cracks. It should not be used in the furnaces that have rapid temperature change below 800℃.

The property and process of the silica brick is closely related to the crystal transformation of SiO2. Therefore, the true specific gravity is an important quality indicator of silica brick. Generally it should be under 2.38. For the high quality silica brick, it should be under 2.35. The low true specific gravity means the quantity of tridymite and cristobalite is high and the amount of residual quartz is small. Therefore, the residual linear expansion is small and the strength decreases.

SiO2 has seven crystalline variants and a amorphous variant. These variants can be divided into two categories. the first category include quartz, tridymite and cristobalite. Their crystal structure is very different and the transformation between them is very slow. The second category are subspecies of the variants above, α,β and γ. They have similar structures and can be transformed to each other quickly.

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..

With the development of technology, a variety of zirconia refractories have been developed. Recently, zirconia refractory, as a new type of refractory, has been widely used in the metallurgy, construction and chemical industry.

ZrO2 is widely used in the refractory for the glass industry, metallurgy industry, construction industry and chemical industry, because it has high melting temperature, good thermal shock resistance and high chemical stability. Zirconia refractories are mainly used in demanding areas of glass furnaces such as the superstructure, the melting zone, the sidewall and the throat.

The main types of zirconia refractories include zircon bricks, dense zircon bricks, fused cast AZS block, fused cast zirconia mullite block and bonded again AZS bricks.

In the metallurgical industry, based on the materials, zirconia refractories can be divided into zirconite refractory, zirconia refractory, aluminum zirconium refractory, calcium zirconate refractory, zirconium diboride refractory and modified zirconia refractory.

Zirconite refractory features high temperature performance, good resistance to acid slags, good corrosion resistance, low thermal expansion coefficient and good thermal shock resistance. It can be used in the ling of the ladle.

The raw materials for zirconia refractories include zirconite refractories in various sizes and various zirconia materials. The molding methods include slurry casting method, machine pressing method and isostatic pressing method. Different molding methods have different requirements for the grains of raw materials. Products made in different methods have different performance and applications, but all have the feature of good resistance to high temperature and corrosion.

Aluminum zirconium refractory is developed based on the aluminum carbon refractory. It is mainly used in the ladle, tundish, slide gate brick, long nozzle, stopper and outlet. Compared to aluminum carbon refractory, it has better resistance to oxidation, thermal shock and corrosion, higher strength and longer service life.

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..

Fused cast AZS block is the most widely used refractory material used in the glass furnace. Its quality and performance is important for the service life of the glass furnace and the glass quality. To select the right fused cast AZS, it is important to know how to judge its quality.

In the corner block, dam block, bubble block and throat, fused cast AZS 41# is used. The bottom part of the brick should not have shrinkage cavity and the numbers of the apparent pores larger than 2mm should be less than 20/m3.

Cracks are not allowed in the bubble block and throat block. The top part of the sidewall does not allow the existence of defects. These requirements are very important, or it will affect the life of the bricks.

The color of fused cast AZS block is generally white or light yellow. Its color can tell the content of impurities, such as the content of the carbon. If the brick is blue, it may contain a high content of carbon. This kind of brick cannot be used, or it will reduce the service life of the furnace. If the surface of the brick has blue spots, there may be iron inside it. This kind of brick should be avoided too.

The content of glass phase should be controlled well. It it is too high, the glass phase may extrude at high temperature, which will reduce the service life of the furnace and pollute glass liquid.

Besides those mentioned above, there are still many other factors that should be paid attention to. The problems fused cast AZS encounters in use have great relationship to its quality. So it is important to select high quality bricks.

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..

Alumina (Al2O3) refractories are the part of alumina- silica group of refractories. They differs from fireclay refractories in term of Al2O3 content and normally have Al2O3 content of more than 45 %. The raw material base for these refractories are different than the fire clay bricks.

As the Al2O3 content is increased, the melting point of the refractory increases to a maximum of 2054℃ which is the melting point of pure corundum. Refractoriness increases with the increase in the Al2O3 content. The only stable compound in the system is mullite, which has a defective space lattice and decomposes into corundum and liquid phase at around 1840℃.

According to the Al2O3 content, alumina refractories can be divided into:

Al2O3 less than 50%

Fireclay belongs to this group. Its main crystal phases are mullite and glass. It can contain free SiO2. Normally it is made from 100% fireclay. High quality fireclay is used at about 1600℃,  Usually it contain 38 % to 42 % Al2O3 and are based on fireclay minerals.

Al2O3 50% or 60%

The main crystal phases of these refractories are mullite as major phase and glass as minor phase. They can contain free SiO2. These refractories cannot be made from 100 % clay since Al2O3 content of clay are lower. They are made from minerals containing 60% Al2O3 and contains some fireclay. They can be made with bauxite and clay and can be used for temperatures greater than 1700℃.

Al2O3 70%

The major phase of these refractories is mullite. They contain corundum, mullite and glass. They are made either from bauxitic clay or calcined bauxite and clay. They can be used for temperatures greater than 1750℃.

Al2O3 80% and 85%

These refractories are made from calcined bauxite. The main phases of these refractories are corundum as the major phase along with minor quantity of mullite and glass.

Al2O3 90%

These refractories consist of corundum as the major phase with a minor amount of mullite and glass. Fused cast alumina block belongs to this group. It is used in contact with liquid steel or glass due to its enhanced abrasion resistance and corrosion resistance. When in contact with molten glass, it barely produces any blistering or stones. so it is ideal for the high quality glass furnace and special glass furnace.

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..

With the improvement of the quality of the fused cast AZS, the traditional masonry method of the sidewall brick in the float glass furnace is replaced by the large sidewall block made of fused cast AZS 33# QX.

The use of large sidewall block of fused cast AZS 33# QX eliminates the horizontal mortar joints, expands the service life of the sidewall blocks, reduces the pollution to glass liquid and improves the quality of glass. When installing the large sidewall block, the following guideline should be followed.

Since the large sidewall block is very heavy, it is better to increase the times of loading and unloading. When transporting, be gentle to avoid damages.

When installing the sidewall block, the bottom block should be even. The nozzle of the large sidewall block of fused cast AZS 33# QX should be placed towards the outside of the bottom of the sidewall.

When preheating the furnace, follow the temperature curve strictly and avoid big temperature fluctuation. Especially at 800-1200℃, the fused cast AZS block expands rapidly. Big temperature fluctuation will cause stress inside the block and may cause cracks.

During the operation, reduce the fluctuation of the level of the glass liquid since it is one of the main reason for the corrosion and erosion of the sidewall block, especially the part around the liquid level where is the junction of the solid, liquid and gas.

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, 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..