Chemical Fixation of Trace Elements in Coal Fly Ash Using Ferrous Sulfate Treatment

Sidhartha Bhattacharyya and Rona J. Donahoe
Dept. of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487-0338

Coal fired electric power plants produce 50% of the electricity consumed in the US and generate large volumes of fly ash and other coal combustion by-products (CCBs). The majority of the CCB materials are disposed of in surface impoundments and landfills located throughout the US. Fly ash contains trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V which can have a negative impact on the environment due to leaching by acid rain and groundwater with time. The potential release of these toxic trace elements into the environment is a big concern for the US power industry due to the high cost involved in lining the old and existing ash disposal sites. As a result, simple and effective treatment techniques are needed to stabilize the coal combustion by-products produced by power plants in the ash disposal sites and also to increase the use of coal fly ash for beneficial purposes.

This paper reports the results of batch experiments designed to chemically treat coal fly ash with ferrous sulfate solution by promoting the formation of insoluble iron oxyhydroxide phases that immobilize the toxic trace elements. Four fly ash samples, three acidic (HA, HB and MA) and one alkaline (PD), were treated with a ferrous sulfate (FS) solution (322 ppm Fe) and a ferrous sulfate + calcium carbonate (FS+CC) solution (322 ppm Fe and 28 ppm CaCO₃) at solid:liquid ratios of 1:3 and 1:30. The effectiveness of this treatment technique was evaluated by the batch sequential leaching of treated and untreated coal fly ash samples using a synthetic acid rain (SAR) solution (USEPA Method 1312B) and also by a 7-step sequential chemical extraction procedure (SCEP) to understand the mechanism of treatment.

The unbuffered FS solution at the 1:30 ratio was highly successful in reducing the mobility of the oxyanionic trace elements As (24-91%), Cr (82-97%), Mo (79-100%), Se (41-87%) and V (55-100%). However, the unbuffered FS treatment failed to reduce the mobility of B, Ni and Sr for the acidic fly ash samples. The buffered FS + CC solution greatly increased the mobility of the oxyanionic trace elements in the acidic fly ash samples, except for Cr in HA and HB fly ash. The buffered FS+CC solution reduced the mobility of all the trace elements except Ni in the alkaline PD fly ash. The ferrous sulfate treatment can be applied directly to the fresh fly ash produced in the electric power plants as well as to the fly ash already placed in the ash disposal facilities. A preliminary estimate indicates that ferrous sulfate treatment of fly ash at a power plant would be cost effective.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas