A. C. D. Chaklader

Factors affecting the strength of blast furnace coke

Abstract The influence of coking conditions, with respect to height in a commercial coke-oven, on the compressive strength of blast furnace coke at both ambient and high temperatures has been studied. This involved the determination of density, porosity, the characterization of microstructure and the assessment of the influence of all three on the compressive strength of coke. Coke samples extracted from the sole region of the coke-oven exhibited higher compressive strength than those obtained from a higher region of the oven. This may be due to higher coke-oven wall pressure at the sole, resulting in lower porosity in the coke samples. Coke compressive strength was generally shown to be higher at 1400 °C than at room temperature.

Influence of char porosity on strengths of hot-briquettes

Abstract The effect of char porosity on the strength-porosity relation of coal/char hot-briquettes has been investigated. Six different char and coke samples are characterized with respect to pore-size and pore size distributions, using mercury porosimetry and SEM. An empirical correlation between the ultimate compressive strength ( U ) and total porosity ( p ) shows that Ryshkewitchs expression U = U oe − ap is obeyed. The porosity coefficient ‘a’ is sensitive to interfacial bonding and decreasing values are indicative of penetration of the binder phase into the pores of the chars. The ability of a binder phase to penetrate very small size (submicron) pores of certain types of char appears to be a contributing factor in the strength in the hot-briquetted composites.

Material characteristics affecting formcoke

Abstract The influence of aggregate and binder phase characteristics on formcoke products has been studied by investigating the compaction kinetics of the system and determining the mechanical strength of the briquettes produced. The char phase was characterized in terms of density, hardness and porosity and the binder phase with respect to rheological properties. Results indicate that binder phase fluidity affects compaction viscosity during the particle flow stage of compaction and that char porosity influences final briquette bulk density by affecting the amount of total compaction required to obtain a given bulk density. In general, increased total compaction was shown to result in higher product bulk density with the attendent higher gross composite strength. The latter relationship was seen to be approximately linear over the range of bulk porosity found in this study. A higher briquette strength was found for systems with aggregates carbonized at lower temperatures. Similarly the binder phase fluidity was also seen to affect briquette strength, higher fluidity resulting in higher strength. It was concluded that this was due to increased binder penetration in the highly porous char carbonized at lower temperatures. With no significant pore structure in the aggregate, as found with high temperature char, briquette strength was seen to become approximately constant for the three binder coals used.

Hot-compaction behaviour of coals

Abstract Hot-compaction behaviour of semi-anthracite, low-, medium- and high-volatile caking, oxidized medium-volatile, sub-bituminous and lignite coals was studied under a constant load of 3.45 MPa. Generally, total compaction decreased and initial compaction temperatures increased with increasing coal rank. Initial compaction of the caking coals was noted at temperatures below their Ruhr dilatometer softening temperatures. For the high-volatile sample, this softening temperature was low enough, at about 250 °C, to imply possible thermal breakdown of such coals in preheating processes used before coke-making. The visco-elastic behaviour of coals of different rank over a range of temperature was examined. Densification mechanisms are suggested and the rate-controlling steps are discussed. Devolatilization is an important factor in particle-flow and shrinkage/dilatation mechanisms, as inferred from the similarities of the activation energies associated with these mechanisms and the reported values for devolatilization processes. Non-caking coals exhibit visco-elasticity at temperatures where they are expected to be semi-chars, in contrast with the caking coals.

Hot-compaction behaviour of char/ binder-coal systems

Abstract Hot-compaction behaviour of semi-anthracite coal char, sub-bituminous coal char and coke fines with a moderately caking hvAb binder coal has been studied in the temperature range at which binder coal softened. The binder-coal softening temperature was observed to be significantly lower in the compaction equipment than in the Ruhr dilatometer. Isothermal compaction curves can be fitted to an equation of the form ϵ = ΔL L 0 = K(1 − Ae −αt − Be −βt − Ce −γt ) , from which a viscoelastic model consisting of a series coupling of three Kelvin elements has been developed. Of these six mechanical components four (two viscous and two elastic components) are found to be temperature-sensitive. These temperature-sensitive elements are assigned to 1. (1) the softening of the binder phase (one viscous and one elastic component decrease rapidly to minimum), and 2. (2) interparticle flow (one viscous and one elastic component go through a maximum before being reduced to near zero). The temperature-insensitive components are assigned to the overall press set-up and intrinsic material characteristics. While some of the physical properties such as packing density and particlesize distribution are similar in these char systems, large differences in their viscous and elastic coefficients have been observed. This is probably caused by the wetting behaviour of the binder and some inherent physical properties of the chars.

DC arc plasma synthesis of SiC powder

Attempts were made to synthesize sub-micron SiC powders using a dc arc plasma. This Ar-plasma unit has been designed and constructed indigenously. The precursor materials for synthesizing SiC powders are SiCl 4 and CH 4 and the carrier gas for SiCl 4 was H 2 . The effect of variations of the ratio of SiCl 4 to CH 4 has been investigated. In order to avoid O 2 pick-up by the very fine SiC particles, the powder was collected in a non-polar organic fluid (high boiling point). All powders produced so far are amorphous and the surface reacted with the organic collecting medium. TGA measurements in Ar indicated about 15 to 17% organic compound pick-up by the powder. The powder has also about 5% excess carbon. The amorphous powder started to recrystallize to - SiC above 1000°C.