Technical Market Support » Metallurgical Coal
This project was carried out to address the problems of conventional tumble drum coke strength tests, which, despite their convenience, rank cokes differently and raise the question of which test best reflects the strength of coke under the breakage conditions of interest. These were addressed by analysing the mechanisms of breakage for different tests. Fracture mechanics theory, novel mechanical testing techniques and petrology of fracture products identified the key strength controlling properties of cokes made from a number of Australian coals. Major findings were:
At least 25% more .25mm fines were generated from cokes made from low rank coals (RoMax<1.0) than from higher rank coals (RoMax>1.3). However the proportion of the -10μm fraction (ultrafines) in the .25mm fines fraction of the cokes made from higher rank was up to 5.1%, and was substantially greater (showing between a 3 to 5-fold increase) than for cokes made from lower rank coals.
There are at least three mechanisms of coke breakdown, which occur at the following indicative size ranges. Above about 40 mm the characteristic fissuring of coke provides planes of weakness. Between 2 and 30 mm, size distribution after breakage follows the Rosin-Rammler form, indicating that breakdown at this scale is not directly related to either coke strength or abrasion resistance. Below 2mm, breakage is dominated by abrasion, although IMDC (inert maceral derived coke) and RMDC (reactive maceral derived coke) behave differently. Weakly bound IMDC are released as discrete particles similar in size to the IMDC in the original coke. RMDC abrades by elimination of very fine dusts, the size of which is related to the crystallite size within the domains in the coke. The boundaries between the IMDC and RMDC appeared to be the weakest points in the coke, although this was based on observations of crack lengths and has not yet been quantified.
The drop height of cokes in tumble drums increased with the rank of the source coal, possibly resulting in lower tumble drum indices for cokes made from higher rank coals because of the test procedure rather than the inherent coke strength. This could be either because the smaller fines generated from cokes made from higher rank coals are retained by the drum lifters, slowing the free movement of coke lumps from the lifters or possibly due to differences in the shapes of coke lumps influencing whether the lumps leave the lifters by rolling or sliding.
The strength of RMDC is derived in part from its ability to plastically deform under load, via the generation of micro-cracks along the fine grain boundaries. Strain energy is absorbed locally, producing ultra-fine dusts from a small volume of material. This toughening mechanism is most pronounced for high rank coals where the coarser RMDC grains, observed in the resultant cokes, provides the greatest degree of micro-cracking.
The fracture toughness of cokes, measured using compact tension measurements were broadly in agreement with most drum indices. Toughness generally increased with rank related coke variables, with the exception of one coke (coke D). Hence at a broad level, drum indices reflect a fundamental measure of coke strength.
Sub-micron indentation was developed to study the properties of the individual coke textures. The hardness and elastic modulus of each coke micro-texture was broadly similar, irrespective of the coke. On most measures the coarse mosaic textures were strongest, followed by inerts, then fine mosaics.
Obtaining objective and comparable measures of texture strength for all textures was challenging because of the strong differences in the mechanisms of fracture. Medium and coarse RMDC showed significant plastic deformation. Fine RMDC and IMDC textures showed almost classic brittle fracture behaviour. Sub-micron indentation, with further development for fracture toughness measurement and boundary strength measurement is a valuable means of differentiating the contribution of the components of coke to its strength.
The image analysis technique developed in this project enabled measurement of the microstructure of the cokes, including coke pore wall thickness and pore dimensions. The reproducibility of the technique was generally very good. The results also compared well with experimentally determined porosity. The degree to which drum indices correlated with image analysis data was variable, with some evidence of links with ?abrasion? indices but no link with ?breakage? indices, suggesting that fissures in the coke significantly affect breakage behaviour. A clearer picture of the contribution of coke microstructure to drum index results may be possible if the large variation in microtextures in the cokes used in this study is avoided, for example by using cokes manufactured from two-component blends or single coals but varied coking conditions.
The results suggest that the drum indices that relate best to the fundamental strength properties and blast furnace behaviour are the ?abrasion? drum indices. However, ?abrasion? indices have the limitation that the fines influence the coke behaviour in the tumble drum in a manner different to that in the blast furnace because of the effect of differences in drop height and abrasion mechanisms of coke types. This discrepancy can be minimised by continuous fines removal (e.g. the SBRC test), and avoiding long duration tests which accentuate the difference in total work done on coals of different rank.
The deficiencies of the conventional tumble drum strength tests were highlighted by this work, including:
- The lack of consistency between the results from different types of tumble drum tests,
- The effect of different amounts of fines generation on test results, including the increase in the drop height inside the drum for cokes made from high rank coals, which introduces a bias in tumble drum data because there is no such behaviour in the handling of coke,
- The absence of a direct and consistent link between tumble drum indices and measurements of coke mechanical strength, as shown in compact tension studies, and fundamental coke micro-property measurements as determined from ultra-micro indentation and image analysis, and
- The indirect nature of the relationship between tumble drum parameters and the mechanisms of coke degradation which were identified in this study.
These deficiencies represent areas for future investigation.