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. 5. Influence of coke breeze in a coal charge on tensile strength of coke

The purpose of this study was to determine the influence of different proportions and different particle sizes of coke breeze in a coke-oven charge on the tensile strength of the coke. The diametrical-compression test was used to determine the tensile strength of the coke produced in a 10-t test oven and the results obtained were considered in relation to the composition of the oven charge, the coke micum indices and to parameters describing the coke texture. It was established that breeze additions caused measurable but nonsystematic changes in the coke tensile strength and that decreasing the breeze particle size generally increased the coke tensile strength. These changes could not however be directly related to changes observed in the density, porosity, pore-wall thickness or mean pore size of the cokes. The previously established relations between micum indices and the tensile strength of foundry cokes were also found to be inapplicable. The conclusion was drawn that the behaviour described is associated with some, at present unestablished, factor of the blend composition, one possibility being the relative proportions and compatibility of the ‘binder’ and inert material acting through their influence on those aspects of the coke microstructure which control the coke breakage.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

炼焦工艺条件对焦炭反应性和反应后强度的影响

通过40 kg焦炉炼焦实验,研究了加热速率、焦饼终温、焖炉时间、入炉煤堆密度及入炉煤细度等对焦炭的CRI(焦炭反应性)、CSR(反应后强度)的影响。结果表明:为保证焦炭成熟和获得较低的CRI值,较高的CSR值,焦饼终温应控制在1000～1050℃范围内。炼焦时焖炉时间应控制在3 h以上。提高入炉煤堆密度,可显著改善焦炭的热性质。入炉煤细度控制在90%左右时,CRI、CSR值较佳。提高加热速率,特别是粘结阶段的升温速率,有利于改善焦炭的热性质。     

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.     

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.     

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.     

焦炭气孔率对焦炭热性能的影响

通过研究40kg试验焦炉单种煤焦炭、配合煤焦炭及工业焦炉焦炭的气孔结构及焦炭热性能,得出气孔率对焦炭热性能的影响。焦炭气孔率对焦炭热性能有较大影响,随着气孔率的增加,CRI增加,CSR降低;工业焦炉焦炭气孔率与焦炭热强度之间关系密切,气孔率每增加1％,CRI增加0．48％,CSR降低1．46％。用气孔率预测焦炭热性能,对指导焦炭生产、控制焦炭热性能具有指导意义。     

Study of the relation between the phosphorus content of coal and coke

Established methods for the determination of phosphorus in coal and coke were compared and found to give results in satisfactory agreement. The method for the determination of phosphorus described in BS 1016, ‘Methods for the analysis and testing of coal and coke’, Part 9, 1977 was used to study the relation between the phosphorus content of coals and their corresponding cokes. The cokes were prepared on laboratory, test oven and industrial scales, by the carbonization of various bituminous coals within the range of volatile matter yield of 16–40 wt%. The determined values of the phosphorus contents of these cokes and their parent coals indicated that the phosphorus present in the coal is completely retained in cokes carbonized to temperatures between 900 and 1050 °C. On the basis of these experimental results it is suggested that the phosphorus content of coke can generally be calculated from a knowledge of the phosphorus content of the coal and the coke yield with an accuracy which is sufficient for normal requirements.     

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.     

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.     

单种煤细度对焦炭热强度的影响

通过对单种煤细度进行预处理,利用70kg焦炉及配套干熄炉进行试验,在保证焦炭热强度的前提下,确定了预粉碎的煤种及最佳细度范围,研究表明,随着细度的增加,不同细度的瘦煤和气煤对焦炭热强度的影响不同,控制出料细度在60%左右时,配合煤堆密度增大,有利于提高焦炭产量及质量。     

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.