Metallurgical coke is a carbonaceous solid made from destructive distillation of bituminous coal. It is used as a heat source for blast furnaces. In addition, it is also used as a reducing agent for ore. Several valuable byproducts can be obtained from its production, including coal tar, light oils, ammonia, coal gas and amino acid liquor. The global metallurgical coke market is expected to grow during the forecast period.
During the production of metallurgical coke, the ash of the raw material is removed by baking it in an oven without oxygen. This results in the fusion of the fixed carbon and the residual ash. Carbonaceous materials are classified according to their reactivity, which is mainly determined by the nature of the distribution of mineral matter in the coke matrix. Some of the moderately reactive minerals that are found in coke include illite, montmorillonite and siderite. These minerals act as catalysts when they are highly dispersed in the C matrix. Coke can be classified into graphitizing and non-graphitizing types.
Graphite is an original mineral in raw coal. Graphite crystals are formed from hexagonal rings of C atoms. Coke is a porous, silver-black solid that contains metallic iron oxides, anorthite, diopside, calcium iron oxide and spinel. A number of studies have investigated the relationship between the structure of coke and its properties. Generally, the mineral matter that is present in coke has an amorphous phase, and the pore size is controlled by the plastic temperature range of the carbonization process. However, the pore shape and volume may differ. Depending on the chemistry of the organic matter, the coke can have either macropores or micropores.
Pores are typically observed in particles larger than 125 microns. In this sample, the micro-pores are present in nm-order. At higher temperatures, the pores become multi-pored. Increasing the thermal value of the coke will increase the number of micro-pores and its size. Moreover, the micro-pores are often found in particles that are close to the softening point, where the particle is swollen. Several practical tests have demonstrated that the strength of coke is weakened after the gasification reaction.
The rate of gasification in the outer part of the coke lump is selectively advanced by the introduction of iron-particles. Iron-particles are generally stronger than coke particles. They can reduce the amount of iron ore required for blast furnaces. Besides, their effect can also advance the chemical reactivity of the gasification reaction.
In a study of coke, the spatial gradient of the gasification ratio is related to the pulverization resistance and fracture resistance. This spatial gradient is influenced by the presence of adsorbent gases such as CO and N2. Gasification reactions occur in two adsorption sites. The first site adsorbs CO and the second one adsorbed carbon monoxide. Both adsorption sites compete with each other. Anions such as nitrogen are less kinetically active than CO. Hence, the diffusion of nitrogen into the narrow micro-porosity is limited.
Coke can be divided into three categories based on pore size. Non-graphitizing coke has large, rounded particles and a medium size. Graphitizing coke has medium-sized, irregularly shaped particles.
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