Coal characteristics that lead to abrasion during grinding
Date
2011-10-18
Authors
Tlotleng, Monnamme Titus
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Abstract
Currently in South Africa there is no acceptable standard method for testing coals for abrasion.
Abrasion is the tendency of coals to wear away machinery. The technique referred to as YGP
(Yancey, Geer and Price), proposed and accepted in 1951, is most commonly used to test for
abrasion. Over the years there have been some modifications to this method by both mining
houses and coal users (such as Eskom) which have resulted in inconsistent and conflicting
results. To this end, this study serves as part of a larger project that will aim at devising a
standard method acceptable for testing coals for abrasion in South Africa. The underlying
principle behind this research was to determine the main characteristics in South African run of
mine (ROM) coals that may cause abrasion. This research work specifically seeks to: (1)
determine if the Abrasion Index (AI) and Hardgrove Grindability Index (HGI), empirical
correlations, developed by Scieszka (1985), can be verified using experimental results; (2)
determine if excluded minerals and included minerals are equally abrasive; (3) establish the type
of abrasive wear that occurred during coal grinding. Five ROM coals from the Witbank Coalfields were analysed. An abrasion index tester pot and
Hardgrove machine were used to determine the grinding properties of the coal samples. XRD
(Rietveld method), XRF, Petrography and SEM-EDS were used to characterise the coals samples
in terms of their inorganic and organic components. SEM-EDS was also used for the particle
morphology analysis, and blade surface topography analysis. TGA, moisture oven and sieve
method were used for proximate analysis, moisture analysis, and particle size distribution
analysis respectively.
The results indicated that the key characteristics that influenced the AI of the coal samples were
moisture, vitrinite, and minerals and mineral associations (excluded and included minerals and
carbominerite) and HGI. Results indicated that coal weathering renders coals less abrasive,
compared to unweathered coals. It was concluded that AI and HGI are experimentally
dependent; excluded and included minerals were equally abrasive; and three-body abrasive wear was established to be the main wear during grinding in an abrasion index tester pot.