Carbon blocks used for blast furnace hearth lining
Carbon blocks used for blast furnace hearth lining
One of the largest users of refractory materials in a blast furnace (BF) is the BF hearth. This region of the BF exhibits more varied designs, conflicting practices and vastly different performance histories of the hearth lining. The traditional materials used in hearth construction have been carbonaceous in nature. Various grades of amorphous and hot pressed carbon conventionally baked and hot pressed semi-graphite, semi-graphitized carbon and fully graphitized materials, are the basic refractories used in any modern BF hearth refractory lining design. However, the nomenclature of these materials must first be clarified, because they represent an entire family of materials with varying compositions, processing and resulting properties. The words, carbon and graphite, are often used interchangeably, but the two are not synonymous. The following briefly describes the major differences and characteristics of the carbonaceous materials used as refractories in the blast furnace.
- Carbon refractories – Carbon, formed carbon, manufactured carbon, amorphous carbon and baked carbon are the terms which refer to those refractories that result from the process of mixing carbonaceous filler materials such as calcined anthracite coal, petroleum coke or carbon black with binder materials such as coal tar or petroleum pitch. These mixtures are formed by moulding or extrusion, and the formed pieces conventionally baked in furnaces at temperatures between 800 deg C to 1400 deg C to carbonize the binder. The resulting product contains carbon particles with a carbon binder. Typically, conventionally baked carbon is manufactured in relatively large blocks. As the binders carbonize and the liquids volatilize, they escape through the block, resulting in porosity. This porosity results in a permeable material that can absorb elements from the BF environment such as alkalis. These contaminants use the same passages for entering the carbon and chemically attack the structure that the volatilizing binders used for escaping the block. Conventionally baked carbon can be densified and thus, permeability improved and pore sizes reduced. This can be accomplished by the introduction of additional binders, impregnated into the baked carbon under a vacuum, and the resultant product re-baked to carbonize the impregnation. Multiple impregnations are also possible to double or triple densify the end product. Each densification, however, adds additional cost and results in a higher priced refractory product. Some manufacturers also add special raw materials to the carbonaceous mix prior to baking to improve the end product’s properties. Silicon carbide or silicon metal can be added to improve permeability, reduce pore sizes and improve abrasion resistance. Artificial or natural graphite can also be added to improve thermal conductivity. Some manufacturers also impregnate the baked carbon with silicon carbide to improve thermal conductivity. However, each of these steps also results into a higher priced refractory product.
- Hot pressed carbon refractories – In hot pressed method of production carbon refractories, a special pressing/carbonizing operation is utilized. In this process, carbon particles and binders are mixed and are then introduced into a special mould. A hydraulic ram then pressurizes the mixture while, simultaneously, an electric current passes through the mould, carbonizing the binders. Unlike conventionally baked carbons that take a period of weeks to bake out the binders, this process carbonizes the binders in minutes. More importantly, as the liquids volatilize, the hydraulic ram squeezes the mixture together, closing off the pores formed by escaping gases. This forms an impermeable carbon compared to conventionally baked carbon, usually at least 100 times less permeable. This impermeability makes it difficult for BF contaminants, such as alkalis, to enter the hot pressed brick and makes hot pressed carbon an ideal hearth wall or bosh lining refractory material. To make this product even more alkali resistant, special silica and quartz additions are also made. The combination of hot pressing and raw material composition results in a superior alkali resistant carbon. Hot pressing also results in a higher thermal conductivity than conventional carbon, which makes this refractory product a desirable bosh or hearth wall lining. This is because the higher thermal conductivity promotes the formation of a protective skull of frozen materials on its hot face because of its ability to maintain a hot face temperature that is below the solidification temperature of iron and slag. The protective skull protects the wall from chemical attack and erosion from moving liquids. Because of the special manufacturing process required for hot pressing, the refractory blocks are restricted to sizes not exceeding a dimension which is around 500x250x120 mm.
- Graphite material – The term, graphite, also called synthetic, artificial or electro-graphite, refers to a carbon product that has been further heat treated at a temperature between 2400 deg C and 3000 deg C. This process of graphitization changes the crystallographic structure of carbon and also changes the physical and chemical properties. Graphite is also found in nature in flake form, and, if used in a refractory product, usually forms part of a mixture of ceramic materials for the binder. This ceramic bonded, natural graphite containing refractory is considered a ceramic product. However, artificial or synthetic graphite refractories begin as a baked carbon material, similar in manufacture to the carbon refractory material. However, after carbonizing of the binder is completed, this baked carbon is then loaded into another furnace to be graphitized at a high temperature. Graphitization changes the structure not only of the carbon particles but also the binder. The resulting product contains graphitized particles as well as a graphitized binder. Different grades of graphite blocks differ with regard to raw materials, grain sizes, purity, density, etc. For denser versions, the porosity of the material can be filled with additional binder materials such as tar or pitch by impregnation under a vacuum. Then, the impregnated material is re-graphitized, forming a less porous product. Multiple re-impregnations/graphitizations can be performed to provide additional densification. Purification can be utilized to reduce the ash levels of graphite for high purity requirements. In addition, manufacturing methods and techniques can also be used to minimize ash or iron contamination of graphites. Since iron is a catalyst for oxidation of graphite in a BF, graphites intended for use as a refractory should contain relatively low iron. Graphite products are manufactured in large blocks or rounds are usually cut and machined into blocks for use as a refractory. Tight tolerances can be maintained with machined graphite components due to its easy machinability.
- Semi-graphite material – The term, semi graphite, is used to describe a product that is composed of artificial graphite particles mixed with carbonaceous binders such as pitch or tar and baked at carbonization temperatures of 800 deg C to 1400 deg C. The resulting product is composed of carbon bonded graphite particles in which the graphite particles had previously been manufactured at temperatures close to 3000 deg C but with binders that have only been baked in the 800 deg C to 1400 deg C range. The resulting refractory product, a true carbon bonded graphite, exhibits higher thermal conductivity than the carbons but, because of the carbon binder, not as high as 100 % graphite. Thermal conductivities will vary with baking temperature and can be increased by re-baking at higher temperatures. These refractory products are also conventionally baked, which results in a relatively porous material. However, these conventionally baked semi-graphites can also be densified and re-baked to carbonize the impregnated binder. Thus porosity and, consequently, permeability can be reduced. Some conventionally baked semi-graphite products are also impregnated with or combined with silicon metal and silicon carbide for greater abrasion resistance and lower permeability. These refractory products, however, are intended for use in the bosh and stack.
- Hot-pressed semi-graphite material – Hot-pressing method is also used to make a true semi-graphite refractory product. This product is considerably less permeable and has a higher thermal conductivity than conventionally baked semi-graphites. Different products are available for a variety of applications. One grade is composed of crushed graphite particles, which were previously processed at graphitization temperatures, with a carbonaceous binder and the addition of silica and quartz materials for alkali resistance. Another grade is a silicon carbide containing hot pressed semi-graphite refractory. It is composed of the same graphite component as the first product and the same carbonaceous binder. However, silicon carbide is substituted for the silica and quartz. The resultant refractory product is more abrasion resistant and even less permeable than the first product. It has proven especially resistant to thermal shock and cyclic operation. Another grade is hot pressed semi-graphite developed for use as a bosh and stack refractory. Like the first two grades, it is composed of graphite particles with a carbonaceous binder with a smaller addition of silicon carbide. This results in a product that is a true, low ash, semi-graphite but with improved properties that hot pressing provides, compared to conventionally baked semi-graphites. Because of the special manufacturing process required for hot pressing, the resultant refractory products are restricted to sizes which are not exceeding the dimension around 500x250x120 mm.
- Semi-graphitized material – The term, semi-graphitized material, refers to a baked carbon that has been further heat treated at a temperature between 1600 deg C and 2400 deg C. This process only begins to change the crystallographic structure of the carbon and alters its physical and chemical properties. However, because this additional heat treating occurs at temperatures below graphitization temperatures, the product is considered to be semi-graphitized. It contains carbon particles and a carbon binder which are both semi-graphitized. This is different than a semi-graphite refractory product which is composed of true graphite particles with a carbon binder. It has a higher thermal conductivity and resistance to chemical attack (alkali or oxidation) than carbon or semi-graphite. This is because the binder is usually attacked first and the semi-graphitized binder is more resistant to attack than the carbon binder of semi-graphite. These semi-graphitized refractory products are also manufactured in large blocks or rounds and are normally cut and machined into blocks for use as a refractory. However, because of their semi-graphitized bonding, they are more difficult to machine than true graphite.
Widely used refractory products of carbon
The refractory products of carbon that are widely used in BF are as given below.
- Super micro-pore carbon block – It is made of calcined anthracite under high temperature and other additives by extruding, baking and processing. Typical properties are high thermal conductivity, high micro-pore porosity and good permeability which may reduce erosion to the body of BF, and avoid the iron liquid erosion and permeability. Super micro-pore carbon block can meet the requirements of the large BFs.
- Micro-pore carbon block – It is made from calcined anthracite under high temperature with other additives by extruding, baking and processing. Typical properties are better micro porosity, permeability, and anti-erosion from iron liquid. Micro-pore Carbon Block is used in lay hearth of BF.
- Partial graphite carbon block – It is made of calcined anthracite under high temperature and graphite. Medium pitch acts as binder. Main processes are extruding, baking and processing.
Typical properties are with good thermal conductivity and anti-alkaline. Partial graphite carbon block is used in bottom of BF.
- Ultra-thermal conductivity graphite block – It is made of low ash pet-coke and pitch acts as binder by extruding, baking, impregnation, graphitization and processing. The product is with higher thermal conductivity which is helpful in reducing the temperature, cooling and slowing the erosion to the furnace bottom. Ultra-thermal conductivity graphite block is used in the hearth and bottom of BF.
- High thermal conductivity carbon block – It is made of high thermal conductivity materials and partial graphite materials. Pitch acts as binder. Main processes are extruding, baking, and processing. The product is with higher thermal conductivity. High thermal conductivity carbon block is used for the bottom of BF.
Typical properties of carbon refractories
Typical properties of carbon refractories are given in Tab 1
|Tab 1 Typical properties of carbon and graphite blocks|
|Item||Unit||Super micro-pore block||Micro-pore block||Semi graphite block||Graphite block||High thermal conduc-tivity block|
|Iron liquid melting index||Max||%||30||32||32|
|Average pore diameter||Max||Micro meter||0.1||0.25||1.5|
|Pore capacity under 1micro metre||Max||%||80||70||20|
|300 deg C||–||W/m.k||10|
|600 deg C||–||W/m.k||20||14||30|
|800 deg C||–||W/m.k||12||7|
Fig 1 shows carbon blocks on test assembly bench.
Fig 1 Carbon blocks on test assembly bench