ENVIRONMENTAL MAGNETIC CHARACTERIZATION OF FLY-ASH FROM COAL BURNING POWER PLANTS

The magnetic properties of concentration, magnetic grain size (i.e., domain state) and mineralogy were measured for coal fly-ash samples obtained from power plants located throughout the United States. Fly-ash is the finely divided mineral residue resulting from the combustion of powdered coal in electric generating plants. Different types of coal are used to generate electricity ranging from bituminous, subbituminous, to lignites. Each type of coal contains varying amounts of sulfur, iron, and Ca that should in turn affect the magnetic properties of the fly-ash. By analyzing the magnetic properties from known coal types (bituminous, subbituminous, and lignites), it was possible to determine which magnetic properties result from the combustion process and which properties result from the type of coal being combusted.

Magnetic grain size parameters for the three different types of fly-ash have very similar values, all suggesting that the remanence-bearing magnetic grains range from stable single domain to pseudo single domain in size. Magnetic mineralogy parameters also reveal similar values for all three different types of fly-ash and indicate the presence of low coercivity minerals such as magnetite. These results indicate that it is the coal and grinding combustion process that produces fly-ash with fine magnetic grain size and low coercivity mineralogy regardless of the type of coal being combusted. On the other hand, magnetic concentration of the fly-ash is affected by the varying amounts of iron sulfide and Ca found in the different types of coal. Fly-ash from mid-continent bituminous coals contained the highest magnetic concentration owing to the higher iron sulfide and lower Ca content in these Paleozoic marginal marine coals. Fly-ash from Powder River Basin subbituminous coal and southern lignites have lower magnetic concentrations. These freshwater Cenozoic subbituminous coals and lignites contain less iron sulfide minerals and thus produce lower concentrations of remanence-bearing iron oxides upon combustion. In addition, these freshwater coals and lignites have a higher Ca content which is diamagnetic and thus has a dilutional effect on the magnetic concentration.