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Upgrading Refractory for electric furnace cover
Feb 05, 2018

Upgrading Refractory  for electric furnace cover

Working conditions of furnace cover refractory

As is shown in the diagram below, the lining of the furnace cover is the weak link of the whole furnace.

                                            

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diagram 1 Application of refractory  for electric furnace cover

1-refractory;  2- Water cooling coil; 

3-Activity oriented connector; 4-spring; 5-connecting bolt

 

Refractory furnace cover is affected by Operatingand structure.Operating factors include operation efficiency, maximum power consumption, oxygenconsumptionvolume ratio, ratio of deoxidized furnace and compoundslag furnace, etc.Main structure factors concern the furnace size, furnace cover thickness and molten pool, the distance between the furnace cover, etc. To sum up, the factors affecting the service life of furnace cover are as follows:

(1) Thermal shock. It refers to theradical temperature change caused by arc heating, the output of steel and furnace cover mobility.

(2) Chemical erosion.Steel slag splash to brick surface, causing chemical erosion, and the reaction among CO, CO2 and SO2 in the furnace, etc;

(3) Arc radiation. Melting will be caused by thermal radiation during arc heating.

(4)  worn out caused by the high-speed airflow in furnaceformed during the working of dust fan.

(5)loose arch structure caused by brick crash under the gravity, etc.

The combination of several factorshas significant influence on the life of furnace cover.chemical erosion will be fuelled by ultra high temperature of arc radiation; while steel slag erosion leads to the thinning of refractory lining thicknessand structure change, accelerating the spalling damage of refractory materialsunder thermal shock.

Therefore, refractory for furnace cover require high resistance of thermal shock  and the high temperature furnace slag splash erosion, and good integrity as well as firm structure, etc.

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The trend of refractory for furnace cover is: brickprecast integral casting. countries have different specific conditions usingdifferent materials.

(1) Silica brick furnace cover. In the 1950s furnace covers were constructed by silica (SiO2 93% - 93%), the proportion of which is low of 2.37-2.39. They were light weight and low price, with good high temperature structural strength and creep resistance.But after high temperature melting, SiO2 generated continuous drop, leading to decrease on slag reduction degree, which affects the smelting operation, and thus high alumina brick with better performance were developed.

(2) High alumina brick furnace cover.High alumina brick masonry furnace cover began in the late '50s. For examples, there were unburnt high alumina brick formed by sintering alumina brick (Al2O3 80%) and phosphates, and burned mullite bricks or  unburnt phosphate combined with mullite brick. what they have in common  is that they have good thermal shock resistance.

(3) soaking furnace cover.In the 1970s,in Japanperformance of furnace cover were greatly improved by using with impregnation techniquesto improve the high aluminum brick.Using MgO style, Cr2O3, CaO and SiO2 oxide as impregnant to dip into the cracks to make up for the substrate susceptible to corrosion defects, improving the structure of the products and increasing its durability.Using impregnant can reduce the invasion oflow melting on the furnace cover brick, and improve the creep resistance.

(4) the alkaline furnace cover.Electric furnace coverswere made of magnesia brick.In order to ensure the stabilityof the structure and reduce the magnesite brick deformationand peeling, use iron magnesia brick or insert into plate during construction.Because the iron induced current (high power electric furnace cover especially) and formed local overheat, and thus switch tonon-magnetic steel brick shell. Besides, linear expansion coefficient of pure magnesium brick is too large, dosage are not much.

Other basic brick, such as magnesium, chromium, chromium magnesite, dolomite, magnesium, aluminum, etc have been used. There were also high temperature sintering brick or air brick, or through dipping dolomite brick, etc.They have excellent fireproof performance and the ferric oxide and slag erosion resistance. Their performance is better than that of high alumina brickunder harsh conditions.

(5) the integral casting furnace cover.Adoption of the integral casting hasmade further improvement in the service life of furnace cover.After furnace coverwater cooled down, water around the electrode was also cooled. Choosing material containing 3% Cr2O3 of high aluminum low cement castable and stainless steel fiber, in the case of slight repair, furnace cover were used in 594 furnaces, whose life expectancy increased about 4 times.

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In order to improve the quality of refractory castable, the following measures can be taken:

1, Aggregate plays a skeleton and supporting role, while the substrate are used to fill in the pore, wrap and connect the aggregate. Therefore,  the primary concerns are high purity and high temperature stability of raw material.

2, In order to obtain good high temperature performance, control CaO contentas far as possible, that is, to reduce the dosage of cement as could as possible. Once the CaO content increased the amount of liquid phase increases with temperature increase.

3, Add proper amount of alpha Al2O3 to improve the strength of medium temperature. At medium temperature stage, reactions ofα-Al2O3with CaO and Ca are as follows: CaO + Al2O3 ---> CaO·Al2O3, inflation was 19.6%.CaO· Al2O3 + Al2O3 --->CaO·2Al2O3, inflation was 12.86%.The expansion made up for castable volume shrinkage caused by dehydration and type transformation, improving the intensity of medium temperature.

4, Adding suitable amount of soft clay as sintering agent to promote the liquid generated and sintering to form ceramic combination.

5, To improve the volume contraction in the sintering process, add additives (kyanite and sillimanite, etc.) that expand  under the high temperature.

6, Add heat resistant stainless steel fiber to improve thermal stability and enhance toughness.

7, Add suitable amount of antiknock to dischargewater vapor smoothly and improve the baking quality.

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From the long-term production practice, although steelmaking technology development may result in limit choice of alkaline materials on electric furnace cover, however,  selected high-quality raw materials made of high aluminum furnace cover material, including "three kinds of stone" of new varieties,  jade, mullite, and other new materialsmade of super fine powder of castable and chromium,  are adapted to the needs of the development of new technology.