The suitability of South African coal discard for the production of carbon based building composites

Abstract

The South African coal mining industries have been discarding their mining and coal preparation waste residues for many years. Estimates suggest that over 60 mega tonnes of coal waste is generated annually in South Africa. When poorly managed these residues are a significant environmental hazard as they occupy useful land and are prone to generating acid mine and spontaneous combustion. The ultimate goal of this research is to transform the coal-based waste problem into a material input for carbon-building materials in construction applications. The research presented in this thesis relates to the characterisation and assessment of coal or carbon ceramic composites produced from the pyrolysis of South African coal waste and preceramic polymer. The composites were prepared by curing and controlled pyrolysis of coal waste mixed with a preceramic polymer (PCP). The different South African coal wastes investigated are Greenside tailing slurry (GTS), Likeflow coal slurry (LCS), Proteas coal discard (PCD), GG1 (a discard from the washing plant), GT (a fine from the mined semi-soft coking coal), GS (from the Witbank coalfield) and fly ash (FA) sample. For the first part of the study, coal composites were produced from coal wastes using Semplastics formulated preceramic polymer (X-MAT®) as the ceramic-forming binder. Later on, commercial preceramic polymer (SPR-212) was investigated as the binder for coal wastes to produce the coal composites. Proximate, ultimate, total sulphur, hydrogen/carbon ratio, oxygen/carbon ratio, X-ray diffraction, X-ray fluorescence, Fourier Transform Infrared spectroscopy, Raman spectroscopy, thermogravimetric and Brunauer-Emmett-Teller analyses were used to obtain descriptions of the coal wastes. The various analyses conducted on the composites, which are up to 80% coal waste, indicated that the physicochemical properties of the coal waste play a huge role in the performance of the composites. For instance, when a propriety PCP was used, GTS and LCS samples containing carbonyl functional groups, a hydrogen/carbon ratio of ~0.7%, volatile matter between 22.6 and 23.7% and ash content between 15.6 and 30.7%, respectively, produced good-quality composites. The GTS composite recorded flexural strength of 36.5 megapascals (MPa), compressive strength of 130 MPa, and water absorption as low as 3.4%. This suggests that a high hydrogen/carbon ratio and the presence of carbonyl groups in these coal wastes favoured binding or fuseability with the PCP during curing and pyrolysis. On the other hand, PCD of a hydrogen/carbon ratio of ~0.3%, volatile matter of 10.7%, and ash content of 39.8% experienced inertness and lower internal heat transfer with the PCP during pyrolysis. As a result, the PCD composite recorded low mass/shrinkage loss but high water absorption of 10.1%, low compressive strength of 83 MPa and low flexural strength of 27.5 MPa. By extending the range of the coal wastes investigated using discarded fine semi-soft coking coal (i.e., GT coal) mixed with a commercial PCP (SPR-212) as the ceramic-forming binder, coal composites of very poor quality were produced. The GT’s hydrogen/carbon ratio was 0.09%, with a volatile matter of 32.7% and ash content of 11.4%. Hence, GT composites recorded volumetric shrinkage up to 36% during pyrolysis, water absorption of 15%, and compressive strength of 278 MPa. This was attributed to the swelling nature of the coal and the residual total carbon of the composite (up to 63%). As a result, the mechanical strength was compromised and the composites’ susceptibility to heat during thermal analysis increased. Conversely, GG1 of 5.4% fixed carbon, 10% volatile matter, an ash content of 81.3% and a hydrogen/carbon ratio of 1.7% bonded well with the SPR-212 polymer. Consequently, quality GG1 composites recorded 19.7% volumetric shrinkage, 11% water absorption, and a compressive strength of 392 MPa. However, due to the high ash content of GG1 (81.3%) and the moderate weight loss that occurred during pyrolysis, the composites recorded densities up to 2.5 grams per cubic centimetre. Additional composite properties reported in this research include textural properties, morphology, the form of carbon, chemical bonds, thermal stability (continuous operating temperature), efflorescence potential, surface properties utilising the angle of contact with water, volume resistivity and leaching potential. Based on the results collated, the coal composites could be designed into technical building parts such as bricks, panels, roofing tiles, etc. Optimisation of the mixing ratio of coal waste and preceramic polymer and the production process is expected to lower the processing temperature and yield coal composites that meet or exceed many conventional building parts.