Abstract

One of the potential sources of alumina and mesoporous silica is the coal-fired thermal plants waste known as the coal fly ash (CFA). The studies of the alumina extraction from CFA are often focused on the preliminary desilication, but the efficiency of the alkali desilication is low due to formation of the desilication product—Na6[Al6Si6O24]·Na2X (DSP). This research is focused on the possibility of CFA desilication without formation of DSP using a leaching process with higher liquid to solid ratios (L/S) and alkali concentrations. The experimental data were analyzed using an artificial neural network (ANN) machine learning method and a shrinking core model (SCM). The investigation of the CFA morphology, chemical and phase composition before and after leaching were carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The present work shows that it is possible to avoid formation of DSP if using the L/S ratio >20 and concentration of Na2O—400 g/L during CFA leaching. The kinetics analysis by SCM showed that the process is limited by the surface chemical reaction at T <100 °C, and by diffusion through the product layer at T >100 °C, respectively. The SEM images of the solid residue after NaOH leaching under conditions that prevent the DSP formation show mullite particles with an acicular structure.

Highlights

  • Coal is the main source of energy worldwide

  • The main purpose of this study is to show the possibility of increasing the SiO2 extraction degree from coal fly ash (CFA) by NaOH leaching while reducing the loss of NaOH with solid residue by keeping Al2 O3 in the liquor

  • The possibility of CFA leaching by highly concentrated alkaline solutions (HCAS)

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Summary

Introduction

Coal is the main source of energy worldwide. The ever-increasing energy demand means that coal will remain a key component in energy generation for the nearest future [1], with proven reserves of over 1000 billion tons [2]. As a result of coal combustion on thermal power plants (TTP), the solid residue is formed, which is usually referred to as coal fly ash (CFA). The CFA content of most coals is around 10–20%, while it reaches ~40% in the brown coal from the Ekibastuz basin of Kazakhstan. The volume of generated CFA will increase in upcoming years due to the increasing need for energy from coal plants. The degree of CFA utilization is increasing, but it still does not exceed

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