Coal Preparation » General
The effect of various constituents of coal on the handleability of both raw and clean coal is generally unknown. There is some information on the influence of moisture content, size distribution and clay content on coal handleability. However the overall influence on handleability of the recovery of additional fines or the specific effects of the use of various chemical additives or processing agents used in the preparation of clean coal is generally unknown.
The purpose of this project was to identify the extent of the problem and make recommendations concerning possible solutions. In some cases attention to design detail for various items of handling plant may be sufficient. In other cases management action in modifying the extent to which various chemical additives or processing agents are used, may reduce handling problems in existing plant.
Specifically, the research has involved flow properties testing to characterise coal containing various proportions of additives at different moisture levels.
The project has dealt, in some detail, with the influence of water on the properties of coal. At the more fundamental level, the mechanisms for water retention by coal particles as influenced by particle size and size distribution has been investigated. From a more practical point of view, a significant finding of the research has been the way in which moisture content of coal influences the unconfined yield strength and bulk density.
It has been known for some time that the strength of coal increases marginally with increase in moisture content over the lower moisture range, but then increases rapidly reaching a maximum strength several times in magnitude as the moisture further increases only a few percent. As the moisture content increases beyond that corresponding to the maximum strength and approaches the saturated moisture content, the strength of the coal decreases. As a general rule, the moisture content at which coal reaches the maximum unconfined yield strength usually occurs within the range 65% to 85% of the saturated moisture content.
The research has shown that for a given major consolidation pressure, the bulk density of coal has a minimum value at the moisture content for which the unconfined yield strength is a maximum. This is an important finding that has not previously been known.
The assessment of the handling characteristics of wet coal fines with oil additives has been undertaken. The test work involved trials using four oil types, a light oil (Singer sowing machine oil), a heavy oil (automotive engine oil), unleaded petroleum and diesel oil. The addition of oil to moist coal increases the bulk density and decreases wall friction angles. The tensile strength is increased slightly with added oil. The addition of light oil and heavy oil has opposite effects on the unconfined yield strength of moist coal; the light oil reduces the bulk strength while the heavy oil increases bulk strength.
The results for the diesel oil as a coal additive are the most encouraging. The diesel oil increases the bulk density and reduces the bulk strength, both desirable characteristics. The application of diesel oil as an additive reduces wall friction, the reduction being significant in the case of stainless steel liners. Again, a desirable result. The improvement in handling characteristics are quite pronounced for diesel contents up to 2%, but only marginal for diesel contents in the range 2% to 4%. Beyond 4% moisture content the bulk density increases substantially with increase in diesel content.
The project has involved the development of additional specialised test equipment to complement the standard flow property equipment comprising the Jenike type direct shear test apparatus and compressibility tester. The additional equipment includes an apparatus for measuring the tensile strength of consolidated coal samples, an associated powder packing apparatus, a surface friction coefficient test rig for liquid bubbles and a large Jenike type, direct shear tester which incorporates a 250mm diameter shear cell. It is noted that the standard Jenike shear tester utilises 95mm and 65mm diameter shear cells. The 250mm cell has the advantage of allowing more representative coal size ranges to be tested.
The large shear tester is a sophisticated machine that incorporates a unique, inverted loading arrangement in which the normal load is applied from underneath rather than from the top as in the standard shear tester. This has the advantage of permitting much lower values of normal stress to be applied during shear, a factor relevant to chute design and analysis.
An important advantage of the large shear tester is the hydraulic loading arrangement which allows much higher consolidation pressures to be obtained than is possible with the standard shear tester. This feature, combined with the larger cell diameter to permit more representative particle size ranges to be tested, makes this device particularly suitable for the design of large expanded-flow bins and gravity reclaim stockpiles. For stockpiles, it is possible for consolidation pressures as high as 1 MPa to be experienced at the base of the piles.
Some teething problems have been encountered during the development of this tester these been mainly associated with the stiffness of the machine. However, these problems have been substantially overcome.
The concept of a 'handleability or flowability indices' to rate coals in terms of their degree of difficulty during handling has been developed and is outlined in Appendix B. This can assist coal operators to determine strategies which may be applied for handling different coal types in specific installations. The indices highlight the influences on the coal handling properties of such parameters as particle size, fines content, moisture, temperature and storage time. Apart from providing information on the degree of handling difficulty that may be experienced, the indices can be an indicator as to what conditioning needs to be performed on particular coals to improve its handleability.
For coal, it is considered that the most appropriate test for 'handleability' should be based on the unconfined compression test. This test is simpler to apply than the Jenike direct shear test as used for the flow property tests. Although not part of the original project brief, a prototype tester has been constructed and is yielding promising results.