Distributional Fate of Elements during the Synthesis of Zeolites from South African Coal Fly Ash

Abstract

The synthesis of zeolites from South African coal fly ash has been deemed a viable solution to the growing economical strain caused by the disposal of ash in the country. Two synthesis routes have been studied thus far namely the 2-step method and the fusion assisted process. Fly ash contains several elements originating from coal which is incorporated in the ash during combustion. It is vital to determine the final destination of these elements in order to unveil optimization opportunities for scale-up purposes. The aim of this study was to perform a material balance study on both synthesis routes to determine the distributional fate of these elements during the synthesis of zeolites. Zeolites were first synthesized by means of the two synthesis routes. The composition of all raw materials and products were determined after which an overall and elemental balance were performed. Results indicated that in the 2-step method almost all elements were concentrated in the solid zeolite product while during the fusion assisted route the elements mostly report to the solid waste. Toxic elements such as Pb, Hg, Al, As and Nb were found in both the supernatant waste and washing water resulting from each synthesis route. It has also been seen that large quantities of Si and Al are wasted in the supernatant waste. It is highly recommended that the opportunity to recycle this liquid waste be investigated for scale-up purposes. Results also indicate that efficiency whereby Si and Al are extracted from fused ash is exceptionally poor and should be optimized.

Integrated Synthesis of Zeolites Using Coal Fly Ash: Element Distribution in the Products, Washing Waters and Effluent

“…Increased levels of As (75%), Cd (40%), In (35%) and Se (30%) were found in zeolite 4A, while Ag (85%) and V (55%) were transferred in greater volume to the final effluent. The distribution of elements during zeolite synthesis using CFA is reported by Du Plessis et al 46 The authors used South African fly ash for zeolite synthesis and, as occurred in our study (for the elements quantified in both works), reported that Mn, Mg, Ca, Ti, S, Ba, Ce, Co, Cu, Sr, Y and Zn present in the ash were fully incorporated into the zeolite formed (Na-P1). In this same study, toxic elements such as Pb, Hg, Al, As and Nb were found in the supernatant (synthesis effluent) and washing water, indicating the need for reuse or the appropriate treatment before disposal.…”

“…In this same study, toxic elements such as Pb, Hg, Al, As and Nb were found in the supernatant (synthesis effluent) and washing water, indicating the need for reuse or the appropriate treatment before disposal. Du Plessis et al 46 found that large amounts of the elements comprising the basic structure (framework) of zeolites (Al, Na and Si) are wasted in the supernatant, indicating that the synthesis process can be improved to use these remaining components. In the present study, this realization prompted the reuse of the supernatant from the first stage (F), generating zeolite 4A and therefore improving the performance of the zeolitization process.…”

“…Du Plessis et al 46 investigated the distribution of elements during zeolite synthesis and reported that, according to the mass balance, 42% of the initial quantity of Na was found in the washing water. Belviso et al 45 used the same rinse solution (at ambient temperature) employed in the present study and recorded concentrations of As, Cr, Cu, Mn, Pb, Se, V and Zn ranging from 0.0010 to 0.0203 mg L -1 , similar to those obtained in the present study (Table 4).…”

“…[45][46][47][48] To the best of our knowledge, only Belviso et al 45 conducted a complete characterization of CFA and synthetizes zeolites, including determining trace elements, whereas other authors [46][47][48] assessed only major elements. Recent studies conducted by Behin et al 49 related to transfer of trace elements from zeolites NaP type.…”

“…46 Assessing the presence of extra framework elements and how they behave during zeolite application may provide answers to several questions regarding the quality of the product generated. These studies can also provide insights in free-contaminants synthesis routes even when using alternative raw materials.…”