Properties and Structure of Metallurgical Coke
Properties and Structure of Metallurgical Coke
Metallurgical coke is a porous, fissured, silver-black solid and is an important part of the ironmaking process since it provides the carbon (C) and heat required to chemically reduce iron burden in the blast furnace (BF) to produce hot metal (HM). It is a porous C material with high strength produced by carbonization of coals of specific rank or of coal blends at temperatures around 1100 deg C in coke ovens. It is composed of both the organic and inorganic matter. C is the major component of the organic part. Small amounts of sulphur (S), nitrogen (N2), hydrogen (H2) and oxygen (O2) also occur in the organic part. The inorganic matter in coke is called coke ash (mineral matter) and is typically around 12 % on dry basis. Both the organic and inorganic components influence coke reactivity. Thus, coke characterization is an important aspect to understand the quality of coke formed.
The basic understanding of coke quality is an important task as it determines the high temperature and gasification behaviours of coke in the blast furnace (BF). As the coke moves towards the lower zones of BF, it degrades and generates fines, which affects the bed permeability and the process efficiency. Hence, superior coke quality is critical for a stable and efficient BF operation.
Coke quality is influenced by many factors such as the rank, the maceral composition (leading to isotropic or anisotropic coke structures), the ash composition and the fluidity of the starting coals, the carbonization conditions including peak temperature, heating rate, particle size, pressure and bulk density as well as heat treatment conditions.
The important properties of coke, including mechanical strength and reactivity, are governed by the arrangement of the constituent C atoms. The principal features of the atomic arrangement are the alignment and size of C crystallites. The size of textural components is regarded as indicative of their chemical reactivity.
However, coke properties can be significantly modified by the method by which the coal is heated to form it, and the heating method consequently can affect coke reactivity. Hence, the influence of the carbonization conditions, including maximum exposure temperature, heating rate and time, on coke reactivity are to be carefully considered.
Microscopically coke consists of a solid matrix, organic and inorganic inclusions in the matrix, pores and rnicro-fissures. The processes of the development of the porous structure and the micro-texture of coke take place essentially within the plastic range. The structure formed in the coke by the gas bubbles occupies almost half its volume and influences two properties of coke, the mechanical strength and the bulk density. The solid material forming the pore walls consists of optically-anisotropic entities which are usually observed using polarized light microscopy (PLM). The coke micro-texture influences the coke properties which are essential for its use in the BF.
The vitrinite is the main origin for the formation of the anisotropic C phases during carbonization process. Coke C anisotropy has been found to decrease with increasing inertinite contents. The inertinites are responsible for the formation of the isotropic components in the coke.
The micro-texture of coke is the organization of the material at the intermediate scale from the nano-metric (nm) to the micro-metric (micron) scale. Micro-texture is the result of the mutual orientation in space of poly-aromatic ‘basic structural units’ (BSU) with size about 1 nm (nano metre), formed by poly-aromatic layers (4 to 10 rings) stacked by 2 or 3. Coke is composed of differently sized ‘molecular orientation domain’ (MOD) as coals are generally chemically heterogeneous. Coke micro-texture can be characterized by the distribution of MOD sizes varying from 5 nm to a few micrometers (microns).
Coke micro-texture is characterized using optical microscopy (OM) as well as by transmission electron microscopy (TEM). However, due to the low resolution of (theoretically 0.3 micron) of OM, it is unable to bring out the smaller molecular structures responsible for the various optical textures. Hence, TEM in the 002 dark field modes (resolution around 1 nm) is frequently used to characterize the MODs below the micrometric scale.
The micro-texture of metallurgical coke is characterized by a unique anisotropy of the organic substance as observed by optical microscopy. Microscopic examination is carried out by viewing polished coke sections in plane polarized light. The highest degree of alignment is seen in natural graphite crystals, which consist of extended layers of fused hexagonal rings of C atoms. Carbonaceous solids can be classified as graphitizing or non-graphitizing depending on their behaviour when heat-treated. Graphitizing C is formed generally from substances (including coking coals) which are inherently rich in H2 and poor in O2. In the early stages of carbonization as they pass through the plastic stage, the crystallites retain mobility and cross-linking is relatively weak. This leads to a compact, highly ordered structure in which neighbouring crystallites align themselves as near as possible and parallel to each other.
The structure of coke is directly related to the rank of the parent coal. High rank coals produce coke with large mosaic micro-texture while fine mosaic micro-texture is thee characteristic of the coke made from low rank coals. There is also some evidence that mineral matter modifies coke texture formation. Generally, isotropic micro-texture is believed to react more readily with carbon dioxide (CO2) than the anisotropic micro-texture. Fine mosaic, isotropic micro-texture and inertinite-derived components are known to be the most reactive C phases of coke.
High-rank coal (within the reflectance range of 1.55 % to1.8 %) produces coke with large mosaic micro-texture with larger MOD while fine mosaic micro-texture is characteristic of the coke which is made from low-rank coals (within the reflectance range of 0.8 % to 1.46 %). It is reported that the smaller MOD and the mosaics in the coke can be obtained by lowering plasticity and increasing the O2 content in the parent coal. A detailed classification of coke micro-texture is shown in Tab 1.
Coke reactivity is one of the most important factors which control the bed permeability in the BF. The lower the coke reactivity the higher is the permeability of the burden in the BF. The coke reactivity is influenced by its three major properties of coke namely (i) micro-texture which determine the number of active sites, (ii) micro-structure which controls diffusion and chemical rates, and (iii) constituent minerals which can have a catalytic effect.
The reactivity of coke is well known to be greatly affected by the pore characteristics of the coke such that coke reactivity increases with increasing porosity volume, mean pore size and number of pores as well as decreasing pore wall size. The pore characteristics of coke are particularly important for the chemically controlled reaction regime. The porosity of coke is basically created by the mis-orientation of the MODs (each MOD forming a pore wall). Also, the pore structure of coke is modified by growth and/or coalescence of pores, which is often related to fluidity and swelling characteristics of parent coals.
Coke reactivity is influenced by various factors including the nature of the C matter, inorganic matter and pore structure, which in turn depend on coal properties and process conditions. Coal rank, coal composition, coal rheology and coking conditions are generally considered to be the main factors that influence coke texture, which in turn affects coke reactivity (Fig 1).
Fig 1 Main factors influencing coke texture and reactivity
Coke reactivity is mainly controlled by certain size range of pores. In the case of pores with larger diameters, the specific surface is too small while in the case of smaller pores the diffusion coefficients decisive for the reaction are too small to influence the reactivity behaviour of the coke. It has been found during investigations that only pores larger than 3 nm are accessible for coke reactivity during CO2 gasification in the range of 850 deg C to 1100 deg C. It has also been reported that most of the reaction of coke occurs on the surfaces of pores around 5.5 nm in diameter. Thus, the pore volume distribution in the range between 2 nm and 1000 nm is of great importance for the coke reactivity.
Pore structure development
The pore structure of coke is largely determined within the plastic temperature range of the carbonization process. During carbonization, initially pores appear in large particles at a temperature near the softening point, while the medium size particles become porous at higher temperatures. No pore formation is normally detected at any temperature within particles < 125 microns in size. An increase in temperature induces an increase both in the number and the size of pores, and more particles are observed to have pores with the larger particles becoming multi-pored. With increasing temperature, particles become more rounded and swell into the inter-particulate voids.
The pore structure of metallurgical coke can vary greatly over a wide range. Within a given coke and between one coke and another coke, the individual pores can vary both in size and in shape. A classification of pores on the basis of their average width was proposed in 1967 and subsequently adopted by IUPAC in 1972. As per this classification, there are three types of pores namely (i) micro-pores (less than 2 nm), (ii) meso-pores (less than 2 nm to less than 50 nm) and macro-pores (greater than 50 nm). The micro-pore size range has been further subdivided into the very narrow ultra-micro-pores (less than 0.5 nm) and super-micro-pores (greater than 1 nm to less than 2 nm). However, coke structure is dominated by large pores (greater than 2 nm) namely meso-pores and the remainder appears relatively dense and smaller than 0.5 nm to1 nm. Higher micro-pore surface area of coke is frequently related to higher anisotropic C content.
A number of techniques which include electron microscopy, mercury intrusion porosimetry and physical adsorption of gases, have been developed for analyzing pore structure and surface area. However, the most frequently used technique for porosity characterization of coke is physical adsorption of gases, such as N2 and CO2. Micro-pores are considered as being about the size of the adsorbate molecules and accommodate 1, 2, or 3 molecules. Thus, CO2 (0 deg C) is used to measure micro-pores whereas nitrogen (- 196 deg C) measures meso-pores but only some portion of micro-pores owing to the limited diffusion of N2 molecules into the narrow micro-porosity as well as the lower kinetic energy than that of CO2.
The porosity of coke can be developed during carbonization. When coals are carbonized, volatile matter (VM) is released from the coal skeleton framework to create additional porosity. It has been found that the characteristics of the porosity of coke are largely dependent on the initial coal porosity. The porosity of coke is affected by the rank of the parent coal, maximum fluidity and the reactive macerals. Low rank coals with good fluidity produces coke with high porosity (55 % to 60 % by volume) while medium rank coals makes less porous cokes less porous (less than 54 % by volume).
However, it is difficult to present a general description of the evolution of the micro-pores of coke during carbonization since the micro-pores structure can be modified by growth and coalescence simultaneously, which is related to the carbonization temperature. A study has been carried out to know the effect of carbonization temperature on the micro-pore surface area of two sub-bituminous coking coals having a high VM content. It has been seen that the specific surface area increases with carbonization temperature and reaches a maximum at around 600 deg C, and coke produced at temperatures above 800 deg C possess much lower surface area, due to sealing off of the micro-pore system.
Coke ash is the mineral matter present in the coke. Metallurgical coke typically contains around 8 % to 12 % of mineral phases. The proportion of individual minerals present in coke can vary from coke to coke depending on the mineralogy of the original coals as well as the process conditions employed for carbonization. During carbonization, some minerals decompose and some undergo a number of complex reactions, all of which contribute to the formation of new crystalline minerals as well as amorphous phases.
X-ray diffraction analysis is normally used to identify and quantify the mineral phases present in the metallurgical coke. For X-ray diffraction analysis, a sample of the mineral matter from the coke is prepared by ‘low temperature radio-frequency plasma ashing’ (LTA) as this technique results in minimal alteration of the mineral matter
Most of the mineral matter present in the coke has been found to be present as an amorphous phase (greater than 50 % in LTA samples). Partially decomposed clays and other material which become structure-less on heating due to the removal of constitutional water or other volatiles are responsible for the formation of amorphous alumino-silicate material.
With the exceptions of some artifacts or hydrated minerals such as bassanite, coquimbite and jarosite, the major mineral matter in crystalline phases identified in the metallurgical coke are quartz, mullite, fluorapatite, and pyrrhotite. Metallic iron (Fe), brookite, anatase, rutile, cristobalite, iron oxides (Fe3O4, Fe2O3 and FeO), akermanite and oldhamite are common minerals present in coke. Alumino-silicates, troilite, calcium iron oxide, diopside, fayalite, gehlenite, anorthite, alumina and spinel have been also reported as being present in the metallurgical coke. The formation of pyrrhotite and troilite is mainly due to decomposition of pyrite during carbonization. Siderite decomposition can form metallic Fe, magnetite and wustite. Quartz, fluorapatite, titanium oxides (rutile, anatase and brookite) have been reported to be relatively unaffected by the coking conditions. The alumino-silicates have been reported to form from the decomposition of illite, kaolinite, montmorillonite and chlorite.
The ash of coal is known to affect the micro-texture of resultant coke from the coals. Mineral matter retards growth of mesophase from the isotropic fluid phase of coal so decreasing the size of the anisotropic component of the texture. This effect is higher when the mineral matter is finely dispersed (20 nm diameter) throughout the matrix of the coke. Particularly, the size of anisotropic C has been found to be smaller around pyrite and some clay minerals than that in similar areas that lack minerals.
Minerals in coke can be classified by their possible catalytic effect on coke gasification. This classification is given below.
- Reactive such as metallic Fe, pyrrhotite, magnetite, wustite, hematite, and oldhamite
- Moderately reactive such as illite and montmorillonite.
- Non-reactive such as apatite, quartz, mullite, alkali feldspar, ferrosilicon, fayalite, and cristobalite.
During carbonization, a number of pre-existing minerals in coal can disappear and new mineral phases form, some of which can catalyze gasification and some of which do not. Some of the recent studies have identified the importance of Fe bearing minerals in coal which can transform differently to various Fe minerals in amount and type according to carbonization and heat treatment conditions, and consequently can have the strongest effect on coke reactivity, particularly at low temperatures (around 900 deg C). Pyrite has been found to progressively convert to phases with less S such as pyrrhotite and troilite which can lose further S to form metallic Fe. Metallic Fe is also known to form from the decomposition of siderite. Magnetite and wustite has been found to form from siderite decomposition. Hematite has been reported to form from the oxidation of metallic Fe by traces of O2 during coke quenching. Coke reactivity has been found to increase with increasing magnetite content. Pyrrhotite (Fe1-xS) has also been reported to increase coke reactivity. Metallic Fe is very efficient catalyst for coke gasification. However, other Fe compounds such as iron phosphate and iron silicates are not believed to catalyze coke gasification.
The rate of coke gasification is controlled not only by the amount of catalytic minerals, but also by the particle size and nature of distribution in the coke matrix. In this respect, it is considered that some of the metallic elements chemically bound within the molecular structure of the coke, rather than present as discrete particles of a particular element or compound can have catalytic effect when present at edges of the basal planes of the C structure, thus, suggesting that the presence of trace impurities is also very important. It is also noticed that even alkaline earth metal act as catalysts for gasification when highly dispersed in C matrix. It has been observed that the presence of a relatively high abundance of fine pore inclusions when they are in contact with c matrix appears to have great catalytic effect while discrete agglomerates does not. With the progress of gasification, the catalytic effect of minerals on coke reactivity has been reported to decline. For example, the surface contact between catalytic mineral particle and coke matrix is diminished due to preferential gasification of the C around the mineral matter. Thus, it is concluded that any catalytic effect of mineral matter on gasification decreases as gasification proceeds.