Abstract
M.Ing. (Engineering Management)
Run-off-Mine (ROM) coal processing will become more difficult in the future as the quality
of raw coal mined continues to decline in the Witbank coalfields. The extensive reserves of
coal that remain in the Waterberg and Soutpansberg coalfields are of relatively low quality
compared to that from the Witbank area. Coal processing plants will have to deal with lower
yields and employ robust processes that can cope with high mounts of waste rejection. The
effective use of water in coal processing plants will also become more important as water
availability is expected to decrease significantly in the future.
Dry sorting techniques such as x-ray transmission (XRT) have been successfully implemented
for de-stoning/waste rejection applications both locally and abroad in mining applications.
Mintek investigated the possibility of using XRT to produce both low-grade Eskomspecification
product and high–grade, niche quality product for the metallurgical sector.
Preliminary laboratory XRT test work results on coal from Vlakfontein mine (4 Upper and 4
Lower seams) indicated that different grades of coal could be distinguished by XRT sensors as
the basis of an upgrading process. However, in order to evaluate sorter separation efficiency at
industrial throughput rates, a production scale XRT sorting plant was commissioned at
Vlakfontein mine with a design capacity of 125 tons/hr of sorter feed.
The 4 Upper seam was not able to achieve high quality coal with calorific values (CVs) in
excess of 25 MJ/kg. This was evident from the feed washability analysis, as the seam is
inherently of a low quality with only 30% of the feed mass having CVs in excess of 25 MJ/kg.
However, this material could be upgraded to a saleable Eskom grade product by XRT.
The 4 Lower seam was more encouraging, the feed washability analysis indicates that 67.5%
of the feed mass has CVs in excess of 25 MJ/kg. The 4 Lower high grade products, are suitable
for Richards bay terminal 3 (RB3) export grade coal. The CV values obtained after XRT sorting
are on-specification for export and the metallurgical markets.
XRT Sorting was able to separate coal from waste and in addition upgrade a low grade coal to
a saleable grade of thermal coal. The process has lower operating and capital expenditure
compared to conventional coal processing methods, however at lower efficiencies.
The Ecart-probable (Ep) of the sorters ranges between 0.1-0.20 which is high in comparison to
the 0.02-0.03 typically expected for dense medium circuits. It is thus necessary to balance
separation efficiency which leads to lower product yields against the lower capital and
operating cost of the dry XRT sorting facilities.
The study has demonstrated that the addition of sensor-based particle ore sorting to the mineral
processing value chain can have significant influence on the overall economic and
environmental aspects of the minerals processing plant.