目前，粒状耐火纤维浇注料已在在包头钢铁公司－第二炼钢厂２００吨钢包永久层上应用。2004年元月15日在两个钢包上置换原来永久层65mm厚,钢包外壁温度有以前的240C°-280C°减到180C°-220C°.由于在冬天施工没采取保温措施,用到200多包次后拆除,目前为达到钢包扩容目的,永久衬将采用粒状耐火纤维浇注料75mm厚,减薄工作层去掉保温层.

     2003年5月15日,鞍钢弓长岭球团一厂链蓖机炉顶浇注300mm厚粒状耐火纤维浇注料,烘炉仅用了4天,炉内温度1250C°时炉顶外壁温度仅有80C°左右.

Granular Ceramic Fiber Based Refractory Castable

Keming Zhang¹, Yongyue Deng²

(1-Anshan Hui-ming science and technology company, Anshan, Liaoning province, China, 2-Wuhan university of science and technology, Wuhan 430081, Hubei province, China)

Abstracts: A kind of new ceramic fiber based refractory castable was introduced in this paper. The ceramic fibre cottons were first shaped into globular fibre granula, then the granular ceramic fiber based refractory castable was fabricated when was substituted the fiber grains for the aggregates in refractory castable. The properties, high-strength, high-temperature born, of heavy-quality refractory castable and the ones, low density of volume and low heat conductance, of light-quality heat insulation refractory castable were collected in this granular ceramic fiber refractory castable whole.

Keywords: granular ceramic fiber based refractory castable; heat insulation; high-strength; low heat conductance

1 INTRODUCTION

By far, the technical process of the ceramic fiber based refractory castable at home and abroad is basically: the ceramic fibre cottons were added into various non-shaped refractory castables after fell to pieces. Different kinds of matrixes and combinative agents were chose according to different refractoriness, and then fragmental ceramic fibers were added into the refractory castables. Dispersal of ceramic fibers in the castables were not uniform after were churned up, watered (the quantity of being watered is about 40-90%) and cast, the forms of existence of the ceramic fibers were mostly cloud slice shapes and agglomerative which are non-uniform. Although the sort of ceramic fibre based castables were possessed the properties that are light and heat insulated, strength was less and linear change was bigger. So the merit, heat insulation, of ceramic fibre based castables wasn’t brought into play. That is, the porous structures were destroyed with refractory slurries in the mixed process of ceramic fiber with non-shaped refractory castables. It is known that the form and structure of a sort of material in shaped products influence directly the whole strength and heat insulation of the product.

But, granular ceramic fiber based refractory castable was possessed of high refractoriness, high strength and low linear shrinkage at high temperature, the properties of heavy-quality refractory castables, and low heat conductance and good performance of thermal shock resistance, the ones of light-quality heat-insulation refractory castables. The coefficient of heat-conduction (CHC) of this ceramic fiber based refractory castable is about 0.48w/m.k at the temperature of 500℃. Although heat-preservation or heat-insulation of furnace walls was improved upon condition that high-strength light-quality refractory castables were emerged, the applied fields were restricted within lower temperature furnace because of their disadvantage: low strength and refractoriness. Despite of cool crushing strengths (CCS) of alumina hollow balls based refractory castables with mullite based light-quality refractory castables are reached 12-18MPa, the incremental strength is only 7-13MPa. However coefficient of heat-conduction of two refractory castables is more than 0.8w/m.k at 500℃ which is two times as much as one of granular ceramic fiber based refractory castable.

2 EXPERIMENTAL

All the samples were molded into shape of 160mm×40mm×40mm.The coefficient of heat-conduction was calculated by two methods Heat-linear Method and Flat Method. The thermal shock resistance of samples was measured by the following method: The cold modulus of rupture (CMOR) of samples after oven drying at 110℃ for 24h were meant original strength. And then samples were plunged into cooling flowing water for about 5 minutes after firing at 1100℃ with 15 minutes of soaking time in electric oven, the experimental data were collected on alternate 10 times.

The representative physical and chemical data of granular ceramic fiber refractory castable was listed in table 1. The Statistic data were average values that were calculated from fifty samples.

Table 1 Different data of granular ceramic fibre refractory castable

3 RESULTS AND DISCUSSION

3.1 THERMAL SHOCK RESISTANCE

The data of thermal shock resistance were showed in figure 1. The causes of good thermal shock resistance of granular ceramic fiber refractory castable are: there are about 10% crystal-silks (length is 15-52μm) because of high-porous structures in fiber grains in matrix of castable when fiber grains were added in, which could make bridge-connected structures formed and induce crackles to excursion. Besides, fibrous crystal-silks restrict stress released.

Stress could be moved to surface along circumference of grain when the fiber grains were applied as aggregates, and expanded energy of crackles was enervated. On the other hand, stresses of entering particulate inside were released as followed in figure 2. So the thermal shock resistance was improved.

Fig.1 Relation of water cooling times and relative residual strength

Above all, fibre grains were formed by uniform dispersed fiber (ration of length and diameter is 10-20), the microcosmic structure of grains is interleaved by fiber poles and high-porous. In this instance, the counteractive effect and function of releasing stress should be brought into play when castable was stood stress.

3.2 PROPERTY OF STRENGTH

  The strength and effect of heat-insulation of this castable was molded were decided by the fibre form in ceramic fiber based refractory castable. And the columned granular ceramic fibers have prominent influence on properties of whole product which are existed as aggregates in the matrix of refractory castable. The included influences are:

  ① Stresses engendered by columned are less while by needle-like angle and non-regular aggregates are greater.

  ② Special surface of columned is least on the condition that all aggregates are possessed same volume, and decrease of effective areas of skeleton are least.

  ③ The columned have no destroyed effect on continuity of products. But the skeleton are destroyed greatly because the sheet and non-regular aggregates are distributed across and impenetrate the matrix of castable with non-shaped refractory fiber cottons.

   Because of above three causes, 70-90% is made use of skeletal strength by columned fiber aggregates while about 30% is used by other sheet or needle-like ceramic fiber aggregates.

3.3 COEFFICIENT OF HEAT-CONDUCTION

Except of hydrogen, most gases including air are low heat-conductive substance in a still state. The heat-conductive coefficient of ceramic fiber is closed to one of gas because the ceramic fiber is made up of complecting solid fiber poles, they are filled with air (ratio of air is reached about 90%). Continuous netty structure is destroyed by a great deal of air that make the ceramic fiber is took on excellent heat-insulation. It is known that almost heat-insulation materials are non-organic nonmetal refractory ceramics that are multiphase, asymmetric, multi-crystal and porous. The volume proportion of gas in product is 70-90% on most conditions and the materials are possessed with continuous pore distribution on the condition. There are continuous pore phase and discontinuous (conjoint each other) pore storehouses in microcosmic structure of ceramic fiber too.

Figure 3, the sketch map being magnified, was showed the mechanism of heat-conductive simulation of fiber grain, and fig.4 was microcosmic structure of fiber grain. It was known that the ratio of length and diameter of fiber poles were about 10-20 as showed in fig.4, the distributive density of air storehouses were high and porosity every volume was high too.

Fig.3 Sketch map of mechanism of heat-conductive simulation in broke surface of fiber grain (×3000)

Fig.4 Microcosmic structure of granular ceramic fiber (×4000)              　

  The fiber pole is non-directive and heat-conduction of solid phase proceeded along the direction of fiber pole. So heat-conduction was not plumbed the hot-surface. Sometimes transferred route was zigzag and circular, which affect the heat-conductive effect of solid-phase in fiber.

In addition, there are 80% contacts were in the form of dot-line between solids, so the transferred process of solid heat-conduction was also an increscent one of thermal-resistance. More gas-phase and higher porosity, better the effect of heat-conduction resistance. Convective strength was related with difference in temperature and was in independent of average temperature. Because difference in temperature between two ends of every pore was little, the heat-conductive effect of being brought by convection was least. Hot air-current entered into inside of fiber was almost in an actionless state because comminuted and baffled by pore. Pressure in dispersing porous storehouse is invariable, so the air pressure and solid fiber pole are formed into compact body to resist hot-gas in intruded.

  There is difference in heat-conductive coefficient between granular ceramic fiber based castable and single refractory fiber. The former is a linear function related with temperature: and the latter is logarithm curve function: , as showed in fig.5.

The heat-conductive coefficient of two materials were almost similar when average temperature of oven-wall was in an over 700℃ state from fig.5. It explains that comprehensive properties of granular ceramic fiber based castable should be superior to ones of single ceramic fiber when were applied in work-layer of oven. So the best choice was that the granular ceramic fiber based refractory castables were applied in work-layer and ceramic fibers were applied in heat-insulation layer in an over 1250℃ of oven structure state.

4 CONCLUSIONS

The product was adapted to apply on thermal equipments used under the circumstance of high-temperature and bearing stress for heat-insulation, such as: permanent layer of ladle, heat-insulation layer of hot metal mixer, wall of isolated heat room of regenerative steel rolling furnace and wall of firebox, furnace wall of tubular furnace, insulation layer of high-temperature rotary kiln, lining of hot blast stove and ladle cover. The granular ceramic fibers as aggregates can be applied in magnesia based refractory castable and middle or heavy-quality spray to achieve the effect of high-toughness, wear-resistance and low heat-conduction. The idiographic effects were showed as following:

1.         Ladle ektexine could drop in temperature by 96℃ when the castables were molded 80mm in thickness in the permanent lining of ladle, and however less than original temperature of ektexine when were molded 60mm. So about 5-8% ladle volumes could be added.

2.         About 30-50% thermal losing could be achieved when the castables were applied by holding original thickness in enswathing of steel rolling furnace water pipes, and the used life could be prolonged by one times.

3.         About 50% thermal losing could be achieved and weight of roof could be decreased by 30% when the castables were molded full by holding original thickness in the roof of steel rolling furnace. At the same time, the less model structural sections for girder could be used. This design of the material provides light-quality high-strength refractory with roof of big span steel rolling furnace.

4.         The oven time is shortened: the castables have been molded whole by 300 in thickness on the roof of steel rolling furnace of grate-kiln at the first iron ore pellets factory Gong Chang Lin Anshan, and there were not heat-insulative layer on the roof. The highest temperature in furnace and ektexine of the roof were separately 1250℃ and ＜200℃. The oven time was shortened by 50% more than former after they have been applied for one year. The energy saving effect was marked.

5.         The granular ceramic fiber based refractory castables could be applied for heat-insulative layer in the following stoves: hot metal mixers, rotary kilns and shaft furnaces. Effect of heat-insulation and dynamic crush-resistance was better.

6.         If were applied in petrochemical tubular furnace such as split-decomposition stoves and reformer furnaces, the effect of heat-insulation and strength should be better. In addition, the oven time should be shortened and applying life be prolonged for over 8 years.

7.         If were applied in low-temperature melting furnaces such as Al-melting furnaces, the insulative effect should be better and energy consume be decreased, effect of erode resistance and insulation should be improved if were applied in high-speed and high-temperature pipelines.

8.         The castables were suitable to apply in wall of beehive type regenerative steel rolling furnace or firebox that are frequently changed in temperature.

References

1 Keming Zhang. Much-crystal mullite refractory fibers and the applied techniques. Metallurgy Industry Press: 1972.

2 Shentai Song. Special ceramics with refractories. Metallurgy Industry Press: 2004.