Size reduction processes, particularly fine grinding systems, in mineral processing and cement
production plants constitute a great portion of energy consumption and operating costs. Therefore, the
grinding systems should be designed properly and operated under optimum conditions to achieve
productive and cost effective operations. The use of simulation based on kinetic mathematical models
of grinding has proven useful in this respect. The kinetic models contain two essential parameters,
namely, breakage rate and breakage distribution functions, that are to be determined experimentally,
and preferably in laboratory, or by back-calculation from the mill product size distribution for a given
feed size distribution.
Experimental determination of the breakage parameters has been mostly carried out in laboratory
batch mills using one-size-fraction material. The breakage rate parameter is obtained from the
disappearance rate of this one-size-fraction material, while the breakage distribution parameters are
estimated from the short-time grinding of the same material. Such laboratory methods using one-size
fraction material, however, are not truly representative of industrial continuous mill operations where
the mill contents have a distribution of particle sizes. There is evidence in the literature that the size
distribution of the mill contents affects the breakage parameters.
This thesis study was undertaken with the main purpose of investigating the effect of the size
distribution of the mill hold-up on the brekage parameters of quartz and calcite minerals in lockedcycle
dry grinding experiments. The locked-cycle and one-size-fraction experiments were performed
in the Bond ball mill instrumented with a torque-measuring device. Different closing screen sizes
were used in the locked-cycle work to produce different size distributions of the mill hold-up, and the
operating conditions were changed in the one-size-fraction experiments to obtain different power
draws. Particle breakage parameters were assessed for these changing conditions.
Prior to the experiments related to the main purpose of the study, preliminary experiments were
conducted for two reasons: (i) to find the power draw of the Bond mill in relation to the operating
conditions with the intention of eliminating the use of costly torque-measuring devices by others / and
(ii) to find the most accurate estimation method of breakage distribution functions among the three
existing methods, namely, the &ldquo / zero-order production of fines&rdquo / method, the BII method, and the G-H
method. The G-H method was found to be more appropriate for the current study.
The locked-cycle grinding experiments revealed that the breakage rate function of coarse fractions
increased with increasing proportion of fines in the mill hold-up. Breakage distribution functions were
found to be environment-dependent and non-normalizable by size in one-size-fraction and locked
cycle grinding experiments. It was concluded that the cumulative basis breakage rate function could
sufficiently represent the breakage characteristics of the two studied materials in a wide range of
operating conditions. Therefore, it would be more appropriate to evaluate the breakage characteristics
of materials ground in ball mills by linearized form of the size-discretized batch grinding equation
using single parameter instead of dealing with two parameters which may not be independent of each
other.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/12615427/index.pdf |
| Date | 01 January 2013 |
| Creators | Acar, Cemil |
| Contributors | Hosten, Cetin |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | Ph.D. Thesis |
| Format | text/pdf |
| Rights | Access forbidden for 1 year |
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