Selective flocculation of coal and shale
The work presented within this thesis describes an investigation into the selective flocculation behaviour of coal/shale slurries using commercially available polyacrylamide polymers. It is considered that selective flocculation may eventually offer a method of recovering the valuable combustibles which are presently discarded. To date, although numerous, mainly laboratory scale, investigations using organic polymers have been conducted on a variety of mineral combinations, little work has been undertaken on coal/shale slurries. Selective flocculation depends essentially on differences in the adsorption of a flocculant on different minerals, which in turn depends on the comparative mineral surface chemistry. The effective dispersion of one component is also essential i.e. it must form a relatively stable suspension, either inherently or after modification with a suitable reagent. The effect of these two factors on both single specie flocculation and selective flocculation is discussed in some detail. Testwork was conducted throughout using pure coal and shale samples in order to minimise any natural variation in surface properties which may occur. In order to define possible conditions under which a separation may be feasible, the flocculating ability of various types of polyacrylamides on separate coal and shale slurries was initially established. However, the predictions were not entirely successful with the results indicating that, in a 50/50 w/w mixed slurry, under approximately neutral conditions, coal was the preferentially flocculated component. This was apparent for polymers of various molecular weights and ionic characters. Variations in the surface chemistry of the coal and shale may be influenced by several factors such as pH, dispersant concentration and promoter effects. Solids concentration is another important physical aspect. Each of these were studied and discussed. Zeta potentials of the individual coal and shale samples at various pH levels were also determined and related to the selectivity and efficiency of separation under similar conditions. It was apparent that although coal was preferentially flocculated, the major problem was that of entrainment. This inevitably arises during the formation of the coal floes, trapping some unwanted, and essentially dispersed, shale. In addition polyacrylamide may possess a weak affinity for the shale surface. These two mechanisms lead to products with a minimum ash content of between 31 - 36%. It was also found that the floes formed were so weak that they ruptured even on gentle agitation - hence trapped material could not be released. The effect of pre-conditioning the coal surface prior to flocculant addition was also assessed and achieved some degree of success, but again products of below 30% ash were not possible. It was therefore concluded that while coal may be selectively flocculated from a coal/shale slurry, using commercial polyacrylamide polymers, without an additional cleaning stage(s) the system is not economically viable.