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top zone

refractory

Top Zone

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 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.

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.


برچسب‌ها: Top Zone , fused cast , AZS , Crystalline Phases And Transformation Of Silica Bricks , Silica brick ,
+ نوشته شده در  جمعه 29 آبان 1394ساعت 23:58  توسط bartar  |  | تعداد بازدید : 92