The dependence of cohesion strength on consolidation stress and moisture content is the stress- moisture-cohesion (SMC) function and depends on the size distribution, ash content (especially clays) and the surface properties of the coal particle (rank related). Several authors have examined the influence of ash, in particularly non-swelling and swelling clays, on handleability. Figure 1 shows how ash influence the  unconfined strength of 19 coals tested in ACARP project 15070.  There is a general trend for higher ash contents to lead to higher unconfined strengths, that is poorer handleability. The data only showed weak linear correlations of ash (%d) with the unconfined strength on as received sample and with maximum unconfined strength  with r squared's of 0.52 and 0.58 respectively.

Black and others (2005) [ACARP Project C11016] conducted ECT testing on 7 pulverised coals to determine handleability in PCI distribution systems.  For the seven coals tested by Black and others the size distribution of the PF was similar and all coals had no unconfined cohesion strength at the moisture content after milling.  Figure 2 shows the SMC function for two coals of different rank (Aus L16 & Aus M17) all other coals were only tested at two moisture levels.

At higher moisture contents, one could be expected the cohesion strength of the PF at different moistures would follow similar SMC function due to the similar size distribution. This holds true for five of the coals at a consolidation of 70 kPa, as shown in Figure 2, whereas coals L15 and L18 do not follow the trends of the other coals. Mikka and Smitham [1] showed that in coarse coals clays could contribute to handleability problems. Clays in the PF may also be the cause of the higher unconfined strength of coals L15 and L18 at higher moistures levels.

[1] Mikka, R., Smitham, J., (1985), Coal handleability assessment, Third Australian Coal Preparation Conference, Wollongong, Nov., 1985.