To estimate the environmental consequences of the storage and utilization of coal fly ash, in the Netherlands a standard leaching test has been developed. In this test, leach conditions are used which aim at close approximation with field conditions. The standard test consists of three main parts: a column leaching test, a serial batch leaching test, and a test for the determination of maximum leachability. As a time scale the liquid/solid ratio (LS) is used, where LS is the ratio between the total quantity (L) of liquid used for leaching and the weight (kg) of the leached material. The relation between this relative time scale and the actual time scale depends on the time required for a given LS ratio to be reached in the actual situation.
In recent years, 50 ashes from a wide variety of sources were analyzed. Five different ashes were submitted to the complete standard leaching test described. All ashes show common leaching characteristics for several groups of elements. The elements present in the form of anionic species (for example, arsenic, antimony, selenium, molybdenum, tungsten, and vanadium) behave similarly. In contrast with literature information, limited solubility of anions at high pH (>11) has been observed. The metals lead, copper, cadmium, and zinc show minimum solubility at high pH, whereas major elements like aluminum and silicon show two minima in the pH range 7 to 9 and at pH values higher than 11. The latter was related to the formation of new mineral phases (for example, ettringite). To verify the pH dependence of the element concentrations leached, experiments with an artificial fly ash solution, containing all major elements normally found in fly ash extracts at pH 4 and LS ratio 5, have been performed. By stepwise increase of the pH, by adding calcium oxide, the relation between pH and element concentrations in the solution has been established. The results are in good agreement with the concentrations found in normal fly ash extracts. The experimental data of all batch experiments are summarized in three-dimensional graphs relating element concentration in solution, pH, and LS ratio. It shows that pH is the main factor in controlling the leachability.
The leaching behavior of trace elements in products containing fly ash has also been determined. After grinding, the materials were subjected to the standard test described. The leaching data retrieved from the products agreed well with the results from the pure ash/water system, indicating a consistent solubility control in solution. To what extent this information on ash/water mixtures can be used for other waste materials remains to be answered.
