Technical Market Support » Metallurgical Coal
A two-stage study was undertaken to establish what effect, if any, perchloroethylene has on the coking properties of coal. In Stage 1, the fluidity of 5 samples of a high fluidity coal (Illawara coal measure, mid-high rank) was studied over a 64-week period. Samples A, C, D and E were stored in open trays at ambient conditions. Sample B was stored immersed in water. Samples A and B were untreated. Sample C was immersed in perchloroethylene for 2 h and then air dried immediately prior to fluidity testing. Samples D and E were both soaked in perchloroethylene and then air dried at the start of the trial. Then Sample E was further soaked in water for 2 h and air dried before storage.
Sample B showed significantly less loss of fluidity versus time compared to Sample A. Samples C, D and E all had a lower average fluidity than Sample A, however, this difference was only significant on 4 out of 27 occasions. The large variability in the fluidity measurements meant there was no statistically significant difference between the three different perchloroethylene exposure protocols. Combined with Stage 2 results, a clear trend emerged that perchloroethylene did cause a reduction of fluidity, but that the effect only became significant for coals of low fluidity, as described by the equation: Loss in log10(Max. Fluidity) = 0.52 - 0.12 × [Initial log10(Max. Fluidity)]
Seven different coking coals were studied in Stage 2, covering three different Australian coal measures and a wide range of coking rank. In each case, a 40 kg clean coal composite was formed using laboratory water-based methods, a Mintek Jig for -50 +16 mm and -16 +2 mm material and the Reflux Classier for the -2.0 +0.25 mm and -0.25 +0.038 mm material. The Mintek jig produced yield-ash curves that matched the float-sink data down to ash levels of between 6 and 10 wt%, depending on the coal and the size range. The Reflux Classifier matched the float-sink yield-ash curve nearly perfectly and can replace float-sink methods as a reliable means for producing clean coal composites without exposing the coal or personnel to hazardous organic chemicals. The 40 kg composite was then subdivided into four sub-samples. Samples A and B were coked immediately, whereas Samples C and D were stored in open trays at ambient conditions for a month before coking. Samples B and D were soaked in perchloroethylene for 2 h and then air dried prior to coke testing, whereas Samples A and C were left untreated.
For 4 of the 7 coals, exposure to perchloroethylene caused a significant increase in NSC Coal Reactivity Index (CRI) and lowering of NSC Coal Strength after Reaction (CSR). Coal rank (based on vitrinite reflectance) did not correlate with the perchloroethylene effect. In general coals with lower fluidity and poorer coke strength were those most severely affected by perchloroethylene. The most seriously affected coals were all from Rangal coal measures and had lower vitrinite and higher inertinite contents. Whether this is linked to the mechanism by which perchloroethylene causes an effect is unknown.
This work confirms that perchloroethlyene does detrimentally affect the coking properties of many coals, particularly those of lower fluidity and poorer coke strength. Hence coking tests on clean coal composites formed using heavy organic liquids may often under predict coking properties. This could lead to some resources being undervalued.