| The potential use of PEO/PPO (polyethylene/polypropylene oxide) triblock co-polymers as promoters to improve flotation of coal was investigated in this thesis. The results show that effectiveness of the polymer depended on the rank of coal. Such reagents affect flotation by modifying the surface of coal and by increasing the emulsification of oily collectors used commonly to float coal. For high rank coals, which usually require relatively small amounts of the collector, the surface modifier function of the polymers was dominant over their emulsifier function for the oily collector. The polymer increased ash rejection in flotation primarily because the agglomerates which were observed in the flotation cell were smaller and considered to be more selective. For medium and low rank coals, the polymer acted both as an emulsifier and a surface modifier. Although such coals require larger quantities of oily collector, the clean coal recovery increased substantially while the ash content was lowered when polymers were used. For low rank coals, the aggregates were smaller and more hydrophobic when polymers were present.;To elucidate the changes in the surface properties of the dispersed phases and in the interactions between various sub-processes involved in coal flotation, surface tension, emulsification and contact angle studies were conducted. It was suggested that adsorption of the block co-polymers at coal/water interface occurred by two different mechanisms: (i) adsorption of PPO groups by hydrophobic attraction on the most hydrophobic sites, and (ii) adsorption of PEO groups by hydrogen bonding on the hydrophilic sites. The dominant mechanism in a given system, however, was determined by the polymer concentration and the PPO/PEO ratio of the block co-polymer and by coal rank. The coverage of the hydrophilic sites on the surface by the surfactant molecules was proposed to be the mechanism by which hydrophobicity increased. The best separation was observed for polymers containing about 40% EO and a molecular weight greater than 2900. This conclusion was valid for both the low and high rank coals. However, the amount of surfactant required was a function of coal type since the fraction of hydrophilic sites varied substantially for coal ranks. For the high rank coals, which contain a relatively small number hydrophilic sites, the polymer increased contact angle when added in small quantities. At higher concentrations, the reagent also adsorbed on hydrophobic sites rendering coal more hydrophilic and less floatable. For low rank coals, an increase in the polymer concentration continued to improve coal cleaning by flotation probably because the larger fraction of coal surface consists of hydrophilic sites. |
