Separation of unburned carbon from coal conversion ash: Development and assessment of a dry method

Abstract

Unburned carbon found in coal conversion ash has the potential to be used in a variety of value-added applications. However, it is first necessary to separate the unburned carbon from the rest of the ash phases. This poses challenges due to the relatively low starting carbon-in-ash values (0.4 to 7.8 wt. % reported for South African sources), and the high carbon grades (ideally ~90 wt. % carbon) and recoveries that need to be achieved. In this paper, a dry method for the separation of unburned carbon from coal ash was developed and assessed using fly, bottom, and gasification ash samples obtained from various South African coal conversion utilities. The idea is to subsequently use the unburned carbon product as a precursor for synthetic graphite manufacturing. The separation consisted of a combination of size, electrostatic (CoronaStat), and magnetic separation steps, and the carbon grades (as loss on ignition - LOI) and recoveries were assessed for each step. Pre-characterisation included X-ray diffraction (XRD), LOI, and petrography (carbon-mineral associations) analyses. The carbon grades increased from 4.01 and 7.04 wt. % to 56.55 and 65.74 wt. % respectively for two fly ash samples. The process was less efficient for the bottom and gasification ash samples, where final carbon grades of 53.21 and 45.10 wt. % were achieved from a starting base of 5.47 and 6.90 wt. % respectively. The carbon recoveries for all samples were low (<35 %), possibly due to the mineral inclusions that form part of the carbon matrix. The suggested separation method is promising but still requires further modification. _Article history: Received 4 February 2019; Received in revised form 15 April 2019; Accepted 7 May 2019_ © 2019 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.