05-10-2012, 11:13 AM
IMPACT OF CRUSHER-TYPE AND PRE-SCREENING ON FINES GENERATION
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ABSTRACT
Fine coal particles create dust problems in material handling and are more costly to clean in the
preparation plant, so minimizing the generation of fines in the crushing process can provide tangible
benefits to the mining company. The hardness of the coal probably has the greatest impact upon
product fineness, but the type of crusher employed and the crusher settings also have significant
impacts upon the ultimate product fineness.
A study was performed to evaluate the difference in crushers on fines generation. In addition, tests
were performed to compare two types of crushers, and to evaluate the impact of pre-screening
ahead of the crusher on fines generation. Test data is presented and discussed.
Data will also be presented to show the impact of coal washing processes on fines generation.
Process degradation, in some cases, may be a more significant source of fines than crushing.
Definition of Fine Coal
ASTM defines fine coal as “that portion of a coal sample being subjected to a washability study
that is smaller than the predetermined particle size, generally between 2.36 mm and 9.5 mm …”.
Fines could then be described as particles finer than 10 mm. For the purpose of this discussion
the focus is on particles that tend to be found in air-borne dust, particles generally 0.5 mm and
smaller.
When comparing two different samples for fineness, one common method is to compare the
percentage of particles finer than some given sieve opening; however, in this study, mass mean
diameter, and Sauter Mean Diameter (also called Average Grainsize) were used.
The mass mean diameter (MMD) is calculated by summing the products of the sieve fraction
weight% and the midpoint diameter of the sieve fraction and dividing by the sum of the weight%
as shown in Table 1. The midpoints of the plus 1-1/2” and minus No. 100 sieve fractions were
estimated.
Background, Samples, and Testing
A large surface mine in southwest Wyoming experienced dust accumulations at the numerous transfer
points throughout the material handling system. The mined coal has a Hardgrove Grindability
Index of 60. Figure 1 shows a flow diagram of the material handling system. Baghouses were
installed at the transfers with the dust discharged down-flow from the transfer point. Coal with a
24” top-size was fed from a truck dump to an inclined vibrating screen. The plus 1-1/2” oversize
coal reported to a Jeffrey Flextooth crusher. Mine personnel had two primary questions:
1. Are there crushers that could produce less fines and dust than the existing crusher?
2. Does pre-screening the feed reduce the percentage of fines generated by crushing?
A review of literature (Lutz 1960, Horsfall, 1980) suggested that two-stage double-roll crushers
would produce a lower percentage of fines at the same top-size than other crushers. Pre-screening
feed to remove most of the undersize material was also identified as a way to reduce the percentage
of fines in the final product. A study on liberation and crusher type (Kestner 1962) provided useful
information on a variety of crushers and settings when crushing identical coal feeds. Figure 2
shows summary data and a chart comparing the results for the different crushers. The black bars in
Figure 2 show crushers/settings that generated product top-sizes as large, or larger, than that of the
single roll crusher. In every case, those crushers produced more fines, as measured by SMD, than
the single roll crusher. In fact, the SMD of the single roll crusher product was more than double
the SMDs of the other products.
Conclusion
Size degradation occurs in material handling systems and at preparation plants due to a variety of
factors. Paying close attention to process design can help reduce the production of fines.