A systematic study was performed in which both hvAb and mvb coals were weathered for up to 500 days at 25-80 °C under closely controlled conditions in the laboratory. Periodic samples were analyzed by a wide range of elemental, spectroscopic, physical, and empirical methods. These systematic and closely controlled experiments confirmed many conclusions reached by others using less rigorous experimental designs. Gieseler plastometry and Audibert-Arnu dilatometry were found to be the most responsive to the early stage of weathering, thus demonstrating that moderate weathering will destroy coal thermoplasticity. The higher rank coal weathered significantly more slowly. The weathering rate is highly dependent on temperature. This study also allowed several new conclusions regarding coal weathering. Rates of change with time of log Gieseler fluidity, heating value, and carbon and oxygen contents show similar dependencies on temperature. The principal chemical changes from organic matrix weathering were the loss of aliphatic groups and the production of carbonyl groups. These reactions occurred rapidly at 80 °C, moderately fast at 50 °C and slowly, if at all, at 25 °C. Froth flotation tests, performed on the hvAb coal, showed that losses in flotation recovery from weathering are strongly affected by pyrite oxidation at 25 °C and by organic matrix oxidation at 80 °C. The different temperature dependencies of pyrite and organic matrix oxidation underscore the importance of using realistic temperatures to study natural weathering.

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