| Summary: | A thesis submitted to the Faculty of Engineering, University of the
Witwatersrand, Johannesburg, in fulfilrMnt of the requirements for the
degreel of Doctor of Philosophy. === This thesis has investigated ways of exerting control over coal breaking during
blasting. Its ultimate objective has been to optimise the use of explosive energy in
blasting so as to ensure reduced production of coal fines in the comminution
process.
The investigations reported were conducted on laboratory models at a scale of a
controllable energy to enable the development of a theoretical background for the
thesis, However, the bulk of the work was undertaken insitu in surface and
underground coal operations. The research was conducted in three phases. The
first phase dealt with the approach to modelling the primary crushing mechanism
which generates the coal fines during the blasting operation. A laboratory
apparatus to simulate blast-generated dynamic loading was devised. Dynamic coal
crushing test conducted on a set of over 150 samples of coal showed an
exponential relationship; between the comminution energy and the subsequent coal
damage. A comminution index (Com ) characterising the ability of coal to generate
fines was derived.
The second phase of the research concerned the field investigations. This had the
following double aim: validating the hypothesis of the influence of the dynamic
loading of the shock waves and that of the inherent discontinuities imbedded in
coal on fines generation; and quantifying the problem of fines in the blasting
techniques currently used in the local mining industry. Three sites were used,
namely Kleinkopje of AMCOAL, Greenside of Goldfields and ATC, Tavistock of
JCI.
The investigations undertaken at Kleinkopje (surface mining operation) involved the
determination of discontinuities by logging inherent fractures appearing on 75
diamond drilled cores of coal and the monitoring of 76 blasts conducted on the
sites sampled for discontinuities. These investigations indicated a good correlation
between the fines observed in blasted piles and the fracture frequency of the pre"
existing fractures per metre of coal.
The work undertaken in underground collieries (Greenside and Tavistock)
demonstrated the enormity of the fines' problem in the local mining operations.
Approximately a third of the coal broken underground was observed, in 30 blasts
sampled and analysed, to fall into the category of fragments of under 6 mm termed
'coal fines', Which generate low income.
The third phase of the research dealt with the optimisation of coal breaking during
blasting. A series of 100 trial blasts was undertaken with the aim of searching for a
blasting technique that enables the generation of fewer coal fines. A number of
options were investigated. Trial blasts carried out were evaluated and their results
are discussed in this thesis.
The results of the fieldwork undertaken confirmed that fines generation is site
speolflc, However, the decoupling technique used with high VOD explosive
products may be successfully employed to reduced coal fines generation in strong
coals (few inherent fractures). === Andrew Chakane 2018
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