This thesis is concerned with the effects of oil on the cohesion of wet coal. Two approaches are described - the analysis of the behaviour of an ideal system of equal spheres and an experimental investigation of the spreading of oil on wet coal. Chapter 1 describes some experiments and calculations carried out to determine radial distribution functions at small separations for dense random packings of equal spheres. The measurement of a packing of table tennis balls cast in agar yielded the near neighbour distribution function to a greater accuracy than has been achieved hitherto. Chapter 2 is concerned with the properties of liquid bridges between particles particularly for the limiting cases of two spheres and a sphere and a plane. For these cases the bridge volumes, neck radii, tensile strengths, pressure deficiencies, rupture distances, and the work required for rupture of the bridges have been measured experimentally assuming a zero contact angle. In addition the conditions for a zero force bridge between unequal spheres have been calculated. The results from Chapters 1 and 2 have been combined in Chapter 6 in an attempt to explain the tensile strength behaviour of packings of wet spheres. The assumption that the cohesion is due to forces exerted by liquid bridges between particles is examined. It is proposed that the tensile strength remains at a constant value over a critical extension of the packing. It is argued that this extension is a little more than one particle diameter. The tensile strength is derived from the work required to rupture the bed which is spread over this extension. The results are in agreement with experimental results. The addition of fuel oil or gas oil is known to improve the flow from bunkers of many wet high rank fine coals. Not all coals respond to oiling and a coal which is responsive to oiling when fresh becomes unresponsive when weathered. Chapters 3, 4 and 5 are an account of experiments to determine whether oil spreads spontaneously on wet coal by measuring the advancing contact angle of oil over wet coal. An original modification of a suction potential technique, which obviates the need to pack identical beds of wet coal, is described. A mechanism whereby oiling improves flow is proposed and discussed in the light of these measurements. The contact angle for weathered surfaces is always less than for fresh surfaces. A critical angle of 36° is the demarcation between responsive and unresponsive coals.
|Date of Award||1968|