The strength of industrial cokes. 7. Further studies of the influence of additives in a coke-oven charge on the tensile strength of coke

The objective of this investigation was to determine if the previously established dependence of the tensile strength of coke on the breeze content and particle size of coke breeze in the coke-oven charge was applicable to different types of breeze additives when used in a size range similar to that of commercial practice. Using a small-scale oven to obtain the desired close control of the charge preparation and carbonization conditions, cokes were prepared from a Yorkshire strongly-caking coal blended with either coke-oven breeze, petroleum-coke breeze, or silica sand. The tensile strength of the cokes was determined by the diametral-compression test and some details of their porous structure were obtained from density measurements and mercury pressure porosimetry. The results confirm that the tensile strength of coke varies systematically with the coke-oven breeze content of the oven charge, and in the present case, for a breeze of the particle size range used in commercial practice the tensile strength is increased at low additions and then progressively reduced at higher levels of addition. Different sources of coke-oven breeze behave in a similar manner and appear to act as an inert filler material. On the other hand petroleum-coke breeze additions progressively increase the coke tensile strength, the additive being bonded into the walls of the coke matrix. The changes in tensile strength are accompanied by systematic variations in apparent density and in porosity.     

The strength of industrial cokes. 6. Further studies of the influence of coke breeze in a coal charge on tensile strength of coke

The objective of this investigation was to ascertain if there was any pattern in the dependence of the tensile strength of coke on the proportion and particle size of coke-breeze in an oven charge and to establish if it was possible to interpret the changes in tensile strength in terms of coke structural features. Using a small-scale oven in order to obtain the optimum in close control of the charge preparation and carbonization conditions, cokes were prepared from each of two coking coals blended with coke breeze. The tensile strength of these cokes was determined by the diametrical-compression test and some details of their porous nature were determined from density measurements, mercury porosimetry and optical microscopy. The results clearly demonstrate that the tensile strength of coke is, in general, systematically reduced with increasing breeze content of the oven charge, the more coarsely ground breeze leading to a greater reduction of the tensile strength at any level of breeze addition. But very finely ground breeze at relatively low levels of addition can lead to an improvement in the tensile strength. These changes correlate with variations in the apparent density and the total porosity and possibly also with the average pore size.     

The strength of industrial cokes: Part 4. The influence of carbonizing conditions on the tensile strength

The objective of this study was to ascertain if the observed differences in strength behaviour of blast-furnace and foundry cokes could be attributed to the different carbonizing conditions used in their production. Two coal blends, one being representative for blast-furnace coke production and the other for foundry coke production, were carbonized in a small-scale test oven using a wide range of heating conditions which included those used in the industrial production of the two types of coke. Coke tensile strengths were determined by the diametrical-compression test and a small-scale drum test was used to derive strength indices comparable to standard micum indices. The tensile strengths and material constants obtained by Weibull statistical analysis, when related to those drum-test indices which assess the resistance of coke to attrition and to corresponding data for equivalent commercial cokes, demonstrated that the cokes fell into two distinct sets according to the coal blend used. It was concluded that changes in coke strength caused by different carbonizing conditions could not account for the different strength behaviour of blast-furnace and foundry cokes. The alternative hypothesis that the nature of the coal blend is the predominant factor is supported by the correlations established for each of the coal blends.     

Examination of some effects of dry cooling on coke quality

A series of coals were carbonized, on the 250 kg scale under standardized conditions, to provide both dry-cooled and wet-quenched cokes which were subsequently subjected to reactivity and strength testing. The data from the tests of reactivity to CO₂ at ≈1000 °C support the view that dry coke cooling leads to lower reactivity, but examination of the porous structure in the pore size range > 5.45 μm and of the optical anisotropy of the coke carbon revealed no changes to account for this effect. Although the micum test indices were sometimes improved by dry coke cooling, the differences were not statistically significant. On the other hand, there was clear evidence of increased coke tensile strength. The effect of dry coke cooling on coke properties appears to be sufficient to exert some influence on the blast furnace coke rate and thereby on the economy of the dry-cooling process.     

Preliminary studies of the relation between the carbon texture and the strength of metallurgical coke

Preliminary attempts to relate the carbon texture to the tensile strength of metallurgical cokes are described. Two series of cokes made by carbonizing blended coal charges in pilot scale ovens were examined. The diametral compression test was used to determine the tensile strength of the cokes and the composition of the coke carbon was measured by applying a point-counting technique to the examination of atomic-oxygen etched surfaces. The strengths and textural compositions could be related by a single equation derived by multi-linear regression analysis.     

A semi-industrial scale study of petroleum coke as an additive in cokemaking

The addition of petroleum coke to a typical industrial coal blend used in the production of metallurgical coke was studied. Cokes were produced at semi-industrial scale at the INCAR coking plant, using petroleum coke of different particle size distribution as an additive. Special attention was paid to changes caused in the textural properties (porosity, pore size distribution, fissures at the interface between metallurgical coke and petroleum coke) which have been found to be responsible for variations in the metallurgical coke quality parameters (e.g., mechanical strength and reactivity towards CO₂). Variation in porosity was found to depend on particle size and the proportion of the additive. The decrease in the microporosity (i.e., pore radius<3.7 nm) of the metallurgical cokes observed when petroleum coke is added to the coal blend, is postulated to be one of the main factors responsible for the decrease in the reactivity of these cokes. The variation of the mechanical strength indices can be explained by the changes in porosity and the quality of the interfaces between petroleum coke and metallurgical coke.     

The characterization of interfaces between textural components in metallurgical cokes

Six coals, representing the rank range normally encountered in commercial coking, were carbonized in a small oven to give dense cokes, of tensile strength comparable with that of good-quality blast-furnace coke. Interfaces between the different textural components in the cokes were studied by polarized-light microscopy. It proved possible to classify interfaces according to their perceived quality, to quantify their occurrence by point-counting and to calculate interface quality indices for the coke as a whole or for interfaces involving individual textural components. Interfaces between vitrinite-derived reactive coke components were superior to those involving inerts, but the inerts content of a coke did not have a marked influence on the coke interface quality index. The highest coke interface quality index was observed for the coke from the coal with the highest dilatation. No clear evidence of an influence of interface quality on coke tensile strength is apparent from the present data.     

Oil-cum-gas-fired experimental coke oven developed at CFRI and the correlation studies of its coke with commercial oven coke

Assessment of the coking behaviour of coals and blends by conducting coking tests in experimental coke ovens still continues to be the most reliable method and is extensively used all over the world. The oil-cum-gas fired experimental coke oven developed at CFRI has a coal charging capacity of 1100 kg and simulates industrial carbonising conditions. The oven is capable of intermittent operation and can be brought up to working temperature within 36 hours.Correlation studies of coke quality were carried out by conducting a series of coking tests on the same blend, carbonised under similar conditions in the CFRI experimental coke oven and the commercial coke ovens of Bokaro Steel Plant. The study has revealed that the physical strength of the CFRI oven coke compares favourably with the Bokaro oven coke. M₄₀ and M₁₀ indices of the commercial oven coke can be predicted fairly accurately from the results of CFRI oven coke.T-tests performed on the shatter results showed that at 5% probability level there was no significant variation between the shatter indices of both cokes. The quality of the gas produced from the CFRI test oven was very similar to that of the gas produced from the Bokaro ovens.     

Fissure formation in coke. 2: Effect of heating rate, shrinkage and coke strength

We investigate the effects of the heating rate, coke shrinkage and coke breakage strength upon the fissure pattern developed in a coke oven charge during carbonisation. This is done principally using a mechanistic model of the formation of fissures, which considers them to be an array of equally spaced fissures, whose depth follows a “period doubling” pattern based upon the time history of the fissures. The model results are compared with pilot scale coke oven experiments. The results show that the effect of heating rate on the fissure pattern is different to the effect of coke shrinkage, while the effect of coke breakage strength on the pattern is less pronounced. The results can be seen in both the shape and size of resulting coke lumps after stabilisation. The approach gives the opportunity to consider means of controlling the carbonisation process in order to tune the size of the coke lumps produced.     

Pitch and petroleum coke additions to coke oven charges

Several pitch materials and a petroleum coke were added to coke oven charges in an attempt to make good metallurgical coke from Canadian coal of poor coking quality. Coal and petroleum pitches were added to a low fluid western Canadian coal of medium volatile bituminous rank, and the blends coked in a technical-scale moveable wall test oven having a 230-kg charge capacity. Pitches improved coke tumble test indices, the principal coke quality parameter related to blast furnace performance. Varying levels of petroleum coke were added to an eastern Canadian coal of high volatile bituminous rank, and the blends, some partially briquetted, were carbonized in a test oven. Tumble indices of coke from the partially briquetted charges approached an acceptable level. These investigations confirm that petroleum products as well as coal derivative can play a useful part in the production of a metallurgical strength coke from poor or non-coking coals.     

Effect of low-volatile additives on the structure and strength of cokes

Blends of medium-volatile or high-volatile coals have been carbonized in a 7 kg oven with low-volatile coals (6–16% VM, dmmf). A comparison is made of the strength and structural properties of these cokes with those of the cokes made under corresponding conditions from the medium or high-volatile coals alone. With increasing levels of addition of the low-volatile coals the tensile strength of the blend cokes generally attains a maximum and then decreases. These strength changes are related to changes in porosity, pore-wall thickness and pore dimensions. Coals which display some degree of plasticity and which are weakly caking improve coke quality by altering the pore-structure due to the combined effects of decreasing the pore diameter and slightly increasing the wall thickness. Those additives which are non-caking act primarily as wall thickeners.     

Carbonization and liquid-crystal (mesophase) development. 21. Replacement of low-volatile caking coal by petroleum pitch in coal blends for metallurgical coke

Cokes were prepared in a 7 kg oven from blends of high-volatile and low-volatile caking coals, using ratios of 1:1 and 3:7. To the 1:1 blend was added 7.5% of either Ashland A240 or A170 petroleum pitch or SFBP petroleum pitch 1. Micum m₃₀ and m₁₀ indices were determined on cokes from the 7 kg oven, using the $\text{1}\text{5}$ Micum drum. Optical textures were assessed using polarized light microscopy of polished surfaces of cokes. The effect of additive is to increase the strength of cokes. The pitch can be an effective replacement of low-volatile caking coal. The analysis by optical microscopy shows that with the stronger cokes from the 7 kg oven there has occurred an interaction between the coal and pitch at the interface of coal particles to produce a solution or fluid phase which carbonizes to a coke with an optical texture of fine-grained mozaics. This material could be responsible for the enhancement of coke strength, being associated with pore wall material rather than with a change in porosity. The results agree with previous work using cokes prepared in the laboratory on a small scale.     

Effects of additions of petroleum coke in coals upon strengths of resultant cokes

Coals of NCB rank 301, 401 and 502 were co-carbonized with pitch-coke breeze pre-carbonized to temperatures between 900–1200 K, in the ratio 9:1. The objective was to provide fundamental information concerning the effect of inert components upon strength of metallurgical coke; these inert components occur naturally in coals and may also be added to coking blends as coke breeze. Polished surfaces of resultant cokes were examined by optical microscopy and fracture surfaces were examined by SEM to investigate the coal-coke/pitch-coke interface for bonding between components and fissure propagation across the interface. Strengths of cokes were measured using a micro-strength apparatus. For three coals, pitch-coke breeze (900 K and highest volatile content) bonded best to the surrounding coal-coke. The interface became increasingly fissured with increasing pre-carbonization temperature of pitch-coke.     

焦粉替代瘦煤的配煤炼焦试验研究

通过煤岩显微分光光度计系统对焦粉配煤炼焦进行了系统的研究,研究了焦粉的配入量对配合煤的镜质组最大反射率及区间变化和显微组分变化的影响,结合小焦炉产品各项质量指标的变化,得到太原煤气化公司焦化厂焦粉的最佳配入比例为1.0%～1.7%,最佳粒度范围为<3mm占98%～100%、<1mm占78%～80%、<0.2mm占40%～50%。煤岩显微分光光度计系统的应用,不但促进了焦粉回配炼焦技术更加系统和精细,也保证了在焦粉配入后焦炭质量的稳定。     

A scanning electron microscope study of fractured and etched metallurgical coke surfaces

The object of this work was to attempt to link more closely coke strength and structure by establishing whether features visible on fracture surfaces could be identified with coke carbon textural constituents revealed either by polarized light microscopy of polished surfaces or by scanning electron microscopy of atomic oxygen-etched surfaces. The cokes used were produced in a laboratory furnace from coals covering the whole range or rank normally encountered in metallurgical coke production in the UK. Fracture surfaces were created by tensile fracture during diametral compression. In all three surfaces examined, the appearance of components derived from reactive coal constituents varied with the rank of the coal carbonized. A clear similarity was evident between features visible in the etched and fracture surfaces. The marked variation of fracture features imply that the textural composition of the coke carbon may make some contribution, as yet unquantified, to the variation in strength among cokes.     

Transformations of pyrite during formation of metallurgical coke

The desulfurization of pyrite during the coking process leads to the formation of phases of varying size, shape and composition. The phases are represented mostly by Fe and S-bearing associations, which can be divided into two categories: those represented only by of Fe-S phases (three varieties), and aggregates containing both sulfides and almost pure iron. There are also Fe-O and Fe-S-O phases, which were probably formed after the coke was pushed from the coke oven. It is suggested that the formation of Fe and S-bearing associations can cause the appearance of cracks and cavities in the coke matrix, which, together with the pressure of the released SO₂ gas, will detract from the strength of the coke. Large grains of pyrite can create weaker spots than do smaller ones, and the incomplete decomposition of pyrite will cause migration of the remaining part of the sulfur to the blast furnace, affecting the reactions there. This may be more common in cokes of relatively low porosity and small pore size and in those made from coals with large pyrite grains.     

Composition of pore-wall material in metal-lurgical coke: Considerations of strength,gasification and thermal stress

The optical texture of metallurgical cokes consists of anisotropic carbon made up of mozaics, 0.5–10 μm in size of flow-type anisotropy, 10–60 μm in size, as well as inert and isotropic material. Cokes from different coal sources possess optical textures which are different, being composed of different extents of the above components. The study examines the optical texture of polished surfaces of cokes and relates changes in surface topography caused by gasification by carbon dioxide at 1173 K, by heat treatment to 2073 k and by etching with atomic oxygen at 293 k to the optical texture. The results support a model to explain the strength of coke and its resistance to breakage caused by gasification, mechanical and thermal stresses, in terms of the size, orientation and bonding of the varied components which constitute the composite structure of coke material.     

Simulation of the strength of metallurgical coke

An algorithm is given for the optimization of the strength characteristics of metallurgical coke with the variation of charge mixture composition. Various versions of the computer simulation of the strength of coke are analyzed with the use of a particular example.     

Molasses and air blown coal tar pitch binders for the production of metallurgical quality formed coke from anthracite fines or coke breeze

The effect of the type and the amount of hardeners, such as ammonium nitrate, ammonium carbonate and nitric acid on the molasses bonded briquettes prepared from anthracite fines or coke breeze were investigated. Amongst the hardener studied the best results were obtained with 2.5% ammonium nitrate hardener. The briquettes produced with this hardener were highly water resistant but not waterproof and their tensile strengths were not adequate to be used as a substitute for the metallurgical coke. Therefore, the briquettes were prepared with molasses containing 2.5% ammonium nitrate hardener and air blown coal tar pitch blended binder. When the blended binder was used for the production of anthracite fines or coke breeze briquettes, after curing at 200 °C for 2 h, they became waterproof and their tensile strengths were found to be sufficient to be used as a substitute for coke oven coke. The briquettes after curing could be directly charged into the blast furnace without carbonizing them at high carbonization temperatures. Since molasses and coal tar pitch, are relatively cheap and readily available materials, the process investigated could be economical way of producing high quality formed coke.     

Upgrading lignites via thermal reduction with coke-oven gas

Three Turkish lignites of varying rank were processed by using coke-oven gas under various processing conditions. Proximate and ultimate analyses and microscopic investigations with a polarized-reflected light microscope were carried out for all of these samples. Gaseous products were also determined after each process. Blends of processed lignites with coking coals were subjected to dilatation tests and cokes were produced in a laboratory scale coke oven using the same blends. The tensile strengths of the cokes produced were determined. The chemical and physical data showed that there are useful changes in lignite structures upon treatment with coke-oven gas under certain processing conditions. The dilatation and tensile-strength results showed that it would be possible to blend processed lignites with coking coals in significant proportions to produce metallurgical grade cokes.