Use of Batchwise Extraction Procedure for Coal Ash Disposal Evaluation
Authors: Villaume JF , Middlesworth BC , Unites DF 
Published: March 2009

To provide a steam electric station with information regarding the chemical nature of leachate from a proposed dry fly and bottom ash disposal site, accelerated laboratory testing of ash leachability and contaminant adsorption in the site soils was performed. These tests involved sequential batchwise extractions of various combinations of new alkaline and acidic fly ash, aged fly ash, bottom ash, and site soil to simulate leachate from the disposal site. The testing procedure, originally developed by Houle and Long at the Dugway Proving Ground, Dugway, Utah, has been shown to closely simulate leached columns, with the advantage of requiring less time and effort than column tests. These tests allowed the simulation of years of field leaching in a period of several weeks.

Three different combinations or scenarios of fly and bottom ash and site soil were chosen for accelerated testing. These scenarios represent variations of potential site conditions. Scenario 1 consists of a combination of new alkaline and acidic fly ash obtained from the precipitator outlet hoppers challenging three layers of site soil. This scenario represents the interaction of a worst-case ash (that is, highest trace metal content) in direct contact with site soil. Scenario 2 consists of the same fly ash combination used to challenge a layer of aged fly ash and then bottom ash. Scenario 3 involves a new composite acidic fly ash layer challenging a layer of aged fly ash and then bottom ash. Scenarios 2 and 3 both yielded a leachate which could be expected from the disposal site given a condition of saturation. Scenario 3 was considered to be an absolute worst-case in that acidic leachate from the ash would cause the maximum solubility of heavy metals.

The test procedure involved eight extractions of each ash or soil layer. The ratios of ash to water and soil to water were graded by size to accelerate testing, that is, the volume of each extraction after the second was doubled. Each extraction mixture was slowly agitated with a paddle for a predetermined time period. Conductivity and pH were monitored at intervals throughout the period. At the end of the extraction period, the mixture was filtered in a vacuum. An aliquot of the filtrate was then filtered through a 0.5 Millipore filter and preserved for metals analysis. The filter cake was transferred back to the beaker and mixed with the required volume of water for the next extraction.

The test results agreed closely with the types of results expected from saturated column tests, and considering the fine-grained, low permeability of the fly and bottom ash and site soil, took significantly less time. As such, the accelerated testing method provides a good approach to modeling the actual environmental effects of materials disposal. The site soil showed a potential for adsorbing arsenic and selenium from the ash layers, and then desorbing or releasing these metals into solution. The site soil also demonstrated the capacity for complete removal of aluminum, boron, total chromium, iron, and zinc from solution.