Coke oven construction, oven baking and start-up (2)

2. Refractory materials for coke ovens

(1) Silica bricks. Silica bricks are made of quartzite, which is crushed, mixed, molded, dried and heated according to a plan.

Silica bricks contain more than 93% SiO2 and are acidic refractory materials with good resistance to acidic slag corrosion. Silica bricks have good thermal conductivity, a refractoriness of 1690~1710℃, a load softening point of up to 1640℃, and no residual shrinkage. Its disadvantages are poor resistance to rapid cooling and heating, and strong thermal expansion.

SiO2 (silicon dioxide) can exist in different crystal forms at different temperatures. When the crystal form transforms, it will produce volume changes and internal stress. Therefore, the manufacture, performance and use of silica bricks are closely related to the crystal form transformation of SiO2.

SiO2 can exist in three crystal forms, namely quartz, cristobalite and tridymite, and each crystal form has several allotropes. That is:

α quartz, β quartz;

α cristobalite, β cristobalite;

α tridymite, β tridymite, γ tridymite.

The three forms and their allotropes are distinguished from each other by the different densities of the crystal forms. They are stable within a certain temperature range. Beyond this temperature range, a crystal transformation occurs.

For example: α quartz with a density of 2.53 is transformed into a new α tridymite with a density of 2.2 when heated to 870°C, and transforms into quartz glass when the temperature reaches 1710°C.

This transformation can be divided into two categories. One is a lateral slow transformation, which is a transition from one crystal structure to another new crystal structure. This transformation starts from the edge of the crystal and proceeds slowly to the center of the crystal. It takes a long time and can only be completed within a certain temperature range. It generally only proceeds in one direction. However, in the actual firing process, SiO2 does not simply transform from α-quartz-α-tridymite-quartz glass:

Another transformation is up-down transformation, called high-low transformation. This transformation does not involve lattice rearrangement, but only lattice distortion or straightening, so the transformation speed is fast and reversible.

The transformation temperature and volume change of various forms of SiO2 are different, as shown in Figure 2: cristobalite transforms at 180-270℃, with the most dramatic volume change, while quartz transforms at 570℃ with a smaller volume change. Tridymite has two crystal transformation points: 117℃ and 163℃, with the smallest volume change. Therefore, it is hoped that silica bricks used in coke ovens will be transformed into tridymite as much as possible during the manufacturing process. However, the actual production of silica brick products always has three crystal forms at the same time. Since tridymite has the lowest density among the three quartzes, the higher the tridymite content of silica bricks, the lower its true density.

Silicon bricks with low true density have more complete quartz transformation, a smooth expansion process, and small residual expansion, which is conducive to keeping the furnace tight. In addition, tridymite has a high load softening temperature and good thermal conductivity, so coke ovens should try to use silica bricks with low true density, generally required to be below 2.38, and the true density of high-quality silica bricks should be between 2.34 and 2.35.

The thermal expansion of silica bricks is uneven. There are more crystal transformations before 600℃, so the volume changes are large. Moreover, at several transformation points such as 117℃, 163℃, 180~270℃ and 570℃, the volume changes are particularly significant, which is most likely to cause deformation and cracking of the masonry. Therefore, this is of great significance to the selection of materials for various parts of the coke oven, the masonry, oven baking, production maintenance and cooling of the coke oven. Since silica bricks have many advantages, they can be used in coke ovens to increase the temperature of the combustion chamber, shorten the coking time, increase the production capacity of the coke oven, and extend the service life of the furnace body. Therefore, modern coke ovens are mainly built with silica bricks.

Below 600~700℃, silica bricks have poor resistance to sudden changes in temperature. This is due to the sudden expansion or contraction of volume due to the transformation of high and low crystal forms. Therefore, silica bricks are not suitable for areas with drastic temperature changes. However, at temperatures above 700°C, the volume change of silica bricks is relatively stable, so they can better adapt to temperature changes.

At present, there is a trend to use high-density silica bricks to build coke ovens. High-density silica bricks refer to silica bricks with a porosity range of 10% to 13%. They are characterized by high density and low porosity, so their thermal conductivity and strength are better than ordinary silica bricks.

(2) Clay bricks The main raw materials of clay bricks are refractory clay and kaolin. The main component is kaolinite (Al2O3·2SiO2·2H2O), and the rest are impurities such as K2O, Na2O, CaO, MgO and Fe2O3. They account for about 6% to 7%. Clay bricks are made by crushing, mixing, drying and firing calcined hard refractory clay (clinker) and some plastic clay. The purpose of adding clinker is to reduce the shrinkage during firing and use, and to improve the yield and service life. The firing process is a process in which kaolinite loses water continuously and decomposes to form mullite (3Al2O3·2SiO2) crystals.

The impurities in the clay form eutectic low-melting silicates with alumina and silicon oxide during the firing process, and form an amorphous glass phase surrounding the mullite in the fired clay bricks.

Generally, fired clay products contain 30~45% mullite crystals, and around them, in addition to the above-mentioned amorphous glass phase, there are also some cristobalite.

Clay bricks are acidic refractory materials that can resist the erosion of acidic slag very well, but have poor corrosion resistance to alkaline slag. Although they have high refractoriness, their load softening start temperature is low, and the softening deformation temperature interval is large, up to 200℃. In fact, high-temperature creep begins to occur far below the load softening start temperature. This is because in addition to the highly refractory mullite crystals, clay bricks also contain almost 50% of glass phase, the latter of which has a very low softening starting temperature, but the viscosity of the melt is very large, so the above situation occurs.

Compared with silica bricks, the total thermal expansion of clay bricks is smaller than that of silica bricks, and the expansion is basically proportional to the temperature. The volume change of clay brick coke ovens within the temperature change range of the carbonization chamber is larger than that of silica brick coke ovens.

Since the total expansion of clay brick coke ovens heated to 1100°C is small and uniform, and the resistance to sudden temperature changes is strong, the baking period of clay brick coke ovens is short, but when heated to above 1200°C, residual shrinkage will occur. This is due to the continued recrystallization of minerals in clay products, and the gradual melting of low-melting-point compounds in products at high temperatures, which brings solid particles closer to each other under the action of surface tension. The size of the shrinkage is related to the composition of the ingredients and the firing temperature. Therefore, during the long-term use of clay brick coke ovens at high temperatures, gaps may appear in the brick joints, destroying the tightness of the masonry.

Due to the above characteristics, clay bricks are not used in high-temperature parts of large coke ovens. They are mainly used in parts with lower temperatures and greater fluctuations, such as furnace doors, riser lining bricks, small flue lining bricks, heat storage chamber sealing walls and furnace roofs.

Clay bricks have a wide source of raw materials, are easy to make and have low cost. Therefore, some small coke ovens can be built with clay bricks, but the operating temperature must be strictly controlled to avoid damage to the coke oven.

(3) Heat-resistant concrete It is a special concrete that can withstand high temperatures for a long time. It is made of refractory aggregates, appropriate binders (sometimes also mixed with minerals and organic admixtures) and water in a certain proportion to form mud, which is rammed or vibrated into shape, then solidified, hardened, demoulded, cured and dried to produce a high-temperature resistant product with a certain strength.

Usually, alumina, non-refractory bricks, blast furnace slag, etc. are used as aggregates, and alumina cement, silicate cement, phosphoric acid and water glass are used as cementing materials. According to the different aggregates and cementing materials, heat-resistant concrete is divided into many types, with different compositions and properties, and therefore different scopes of use. Compared with refractory bricks, this refractory product has the following advantages:

1) It does not need to be sintered before use, which reduces the complex process of manufacturing refractory bricks. The preparation process is simple and can be made into various required shapes on site.

2) It quickly generates strength at room temperature and maintains strength at operating temperature without decreasing.

Refractory concrete has been used in coke ovens for many years, mainly for furnace doors and riser lining bricks.

(4) Refractory mud Refractory mud is a binder that makes masonry a whole. It should have the same properties as masonry bricks. The following requirements are imposed on refractory mud for coke ovens:

1) It has good adhesion at room temperature.

2) It has small shrinkage to prevent cracking when the brick joints are dry.

3) It has a certain refractoriness and load softening start temperature.

4) It has a certain water retention, which is convenient for construction and ensures quality.

5) The sintering temperature that meets the operating conditions of the refractory mud is used to make it sinter at this temperature to increase the mechanical strength of the masonry. Therefore, the corresponding refractory mud should be used according to the brick type and operating temperature.

For masonry parts that are in contact with metal embedded parts, concentrate powder needs to be added to the fire mud. When used for masonry of coke oven top bricks, hydraulic binders that can increase strength, such as silicate cement and quartz sand, should be added to the fire mud.

Silica fire mud is a powder made of silica, waste silica brick powder and refractory clay (raw clay). Silica is the main raw material of silica fire mud. The higher the SiO2 content in silica, the higher the refractoriness of the fire mud. The addition of waste silica brick powder can improve the high-temperature bonding performance of fire mud and silica bricks. This is because silica brick powder has a thermal expansion curve consistent with silica bricks. Therefore, when the volume changes caused by quartz and crystal transformation, the possibility of fire mud detaching from silica bricks is small, and the ability to adhere to silica bricks is good. Generally, the content of silica brick powder is more suitable at 20~30%. Adding raw clay to silica fireclay can increase plasticity, reduce air permeability and water loss rate, but the amount added should not be too large, otherwise the refractoriness of silica fireclay will be reduced and the shrinkage rate will increase. Generally, it is appropriate not to exceed 15~20%.

The requirements for particle size of silica fireclay: no more than 3% for those above 1mm, and no less than 80% for those below 0.2mm. In actual use, there is also the problem of performance, that is, the bricks with mortar can be kneaded and knocked at will during masonry. After masonry, the bricks and mortar should be firmly combined. Generally, good mortar should be able to move for 15~20s. The performance can be expressed by "time", and the performance is related to the particle composition. Practice shows that the particle size is too fine or too coarse, and the water loss rate is fast, which is not suitable for use. The more suitable particle size composition is as follows:

Clay fireclay: It is made of calcined block clinker or crushed clay bricks with combined clay (raw clay). Clinker is the main component of clay fireclay, accounting for about 75~80%. Raw clay is a binder. Adding raw clay can increase plasticity, reduce air permeability and water loss rate, but increase shrinkage. Too much raw clay is prone to cracks, so the proportion of raw clay should not be too large, about 20~25%.

The use temperature of clay fire clay is generally lower than 1000℃. Clay fire clay for coke ovens is generally fine-grained and medium-grained, and the particle size should be greater than 97% through the sieve holes of 0.5 and 1mm respectively.

In addition to being used for masonry of clay bricks, clay fire clay is also widely used to repair coke ovens.

(5) Other masonry materials

1) Thermal insulation materials Building materials with a thermal conductivity of less than 0.837Kj/m·h·K. Generally, it has the characteristics of large porosity and small pores, low mechanical strength, and low volume density. Common thermal insulation materials and their main properties are shown in Table 7.

Among the various insulation materials listed in the table, lightweight clay bricks are clay bricks made of clay as raw material, in which a certain proportion (30~50%) of sawdust is added and fired. There are many brands, and their volume density is 0.4~1g/cm3, and the refractoriness is 1690℃.

Diatomaceous earth bricks are products made of diatomaceous earth as raw materials. A certain amount of combustibles can also be added to increase the porosity of the product to increase its thermal insulation performance. Diatomaceous earth bricks can only be used in parts below 1000℃, and they will shrink and melt if the temperature is too high. Diatomaceous earth bricks can be divided into several levels according to their physical and chemical indicators. Their volume density is 0.5~0.7g/cm3, the refractoriness is 1280℃, the apparent porosity is 73~18%, and the compressive strength is (49~108)×104Pa.

Diatomaceous earth is divided into raw material and clinker. The former is used for bricklaying and insulation layer plastering, and the latter is used as insulation layer filler.

In short, various insulation materials can be used directly in bulk or mixed with water to form a paste for application. When selecting, their maximum allowable use temperature should be considered. If the specified temperature is exceeded, the insulation material will lose strength or break.

2) Water glass is a block of solid sodium silicate obtained by mixing finely ground quartz sand or quartz powder with sodium carbonate or sodium sulfate in a certain proportion and melting at 1300~1400℃. If it is vaporized, liquid sodium silicate is obtained.

Na2O·nSiO2 is the molecular formula of water glass, where n is the modulus of water glass, indicating the SiO2/Na2O molecular ratio. It is generally 1.5~3.5. Water glass with high modulus is difficult to dissolve in water, and is very inconvenient in practical application; low modulus has no practical value. Water glass used for furnace construction and furnace repair is an aqueous solution of water glass.

Water glass is a mineral gelling agent with bonding ability, which is related to its modulus, concentration and temperature. Because the dissolution of water glass is a complex process consisting of hydration, initial dissolution, hydrolysis (producing free NaOH), peptization (SiO2 precipitated by decomposition of sodium silicate is peptized by NaOH) and ionization (generating ions and SiO2 composite micelles formed by SiO2 hydrate adsorbing Na+ ions), the solution of water glass contains both crystalline components and colloidal components. Water glass with higher modulus has stronger bonding ability because it contains more colloids. Water glass solution with lower modulus is mainly crystalline components with low colloidal content, so the bonding ability is poor. Increasing the temperature will reduce the viscosity of water glass. The higher the modulus, the greater the effect of temperature on viscosity.

In addition, the viscosity of water glass solution also increases with the increase of density, and the density of water glass solution is related to its chemical composition and the total amount of solid matter dissolved in water. Therefore, when the total amount of solid matter in the solution is constant, the ratio of Na2O to SiO2 can be different, so the density of water glass solution is related to its modulus and the content of SiO2 and Na2O.

Water glass is an air-hardening cementitious material. Due to the effect of CO2 in the air and the cementing effect of SiO2 precipitated during drying, it hardens at room temperature and gives the masonry early strength. The addition of water glass can also reduce the sintering temperature of masonry and mud.

According to experience, the addition of water glass has a great effect on the strength of mud.The refractoriness of the material is slightly affected. If too little is added, the cold bonding is poor and it is easy to wear. If too much is added, even if the amount of water glass is increased, the mud will become brittle and easy to fall off. Generally, the mud should contain about 1.5% Na2O.

In addition to the above-mentioned furnace building materials, there are also ordinary cement and red bricks, which are mainly used in coke oven foundations and resistance walls.