Removal of heavy metals from fly ash and the impact on its quality

Abstract

Most coal¯y ashes currently produced in The Netherlands contain leachable trace elements in amounts higher than allowed by Dutch law for its free application as a granular building material. To improve their technical and environmental quality, these ashes were subjected to forced leaching using water, citrate, oxalate, EDTA and carbonate solutions. With water alone, the free lime and substantial amounts of Mo (30%), Se (20±40%) and SO 4 2À (40±50%) are removed. Using the extraction agents, these percentages increase to 60±90% and in addition 15±20% Cr, V and Sb are removed. Most of the extraction agents used form stable complexes or compounds with Ca 2 , thus prewashing with water followed by treatment with extraction agents resulted in higher removal and/or lower reagent consumption. The environmental quality of the washed¯y ash was evaluated using the Dutch standard leaching test (NEN7343). Leachability of oxyanions decreased by a factor of 2±4 after washing with water and by between 2 and 18 after washing with extraction agents. However, combined washing did not further decrease leaching but in some cases caused increased leaching. Selection of the washing procedure thus depends on the subsequent application of the ash, that is whether low concentrations or low mobility of trace elements are required. Removing extractants and metals remaining in the moisture after ®ltration will further decrease leaching. For a feasible process reduction of reagent consumption and simple methods for process water recycling are needed.

Analysis of arsenic and some other elements in coal fly ash by X-ray photoelectron spectroscopy

“…The main purpose of leaching test in this study was the confirmation of surface enrichment of As in CFA. Therefore, powerful leachant conditions, such as adding EDTA and HNO 3 , were used [9,10]. Fig.…”

Effect of water-extraction on characteristics of melting and solidification of fly ash from municipal solid waste incinerator

“…Extraction is one of the common processes used as a pretreatment to remove soluble elements and heavy metals. Various extraction technologies have been used by different researchers [12][13][14][15][16], such as extracting with acid and/or alkaline, water followed by moderate phosphoric acid addition, and ferrous sulphate solution followed by ferrous sulphate oxidation. These extraction processes were reported to be able to remove most of the chlorides in the form of alkali chlorides such as NaCl, KCl, and CaCl, leachable salts, and amphoteric heavy metals from MSWI fly ash.…”

Speciation of some heavy metals in coal fly ash

“…Chemically, fly ash is composed of ferro -alumino -silicate minerals containing considerable quantities of Ca, K and Na, along with other trace elements such as Cu, Zn, Mn, Mo, Ni and Se (Jastrow et al, 1981). Co, Cr, Cu and Ni remain associated with the iron oxide fraction and the elements with relatively low mobility (Cr, Cu, Pb and V) show affinity to silicates and glass fraction (Nugteren et al, 2002). These elements can become bioavailable to the plants and animals in their process of geocycling under varying natural environmental conditions such as Eh, pH, water action.…”