Fundamental study of respirable dust generation in coal

Black lung disease induced by the formation of fine fragments of coal imposes high human and direct costs. A fundamental understanding of the fracture processes leading to fine fragment formation in coal by applying principles of fracture mechanics is presented. A four-step hypothesis is proposed which sequentially describes the mechanics of fine fragment formation during a generic coal material-coal cutting tool interaction. These steps include: (1) development of a crush zone under the tool tip, (2) macrocrack propagation, (3) shear movement along macrocracks, and (4) additional fragmentation from shear. This model contains two sources of fine fragments, crushing under the tool tip and shearing along the macrocrack surfaces. Three critical assumptions are made that require experimentation. First, the coal material must contain flaws of sufficient density and sufficiently small size to liberate fine fragments by crushing. The following conceptual relationships are proposed between flaw size, spacing and fragmentation: first, the active flaw size must be less than the desired particle size and second, the spacing (or density) must be in the order of the particle size. The second major assumption is that a mixed mode fracture theory will describe the initiation and crack growth direction for the microscopic inherent flaws. The third assumption is that some degree of roughness, damage, and interlocking exist along the newly formed macrocrack surfaces such that a small amount of shear movement along that surface leads to additional particle formation at a microscopic scale.