Flammability, corrosion resistance, and environmental friendliness of coal composites produced from various coal fines

Abstract

The concept of recycling carbon-containing waste into secondary raw materials is highly promising for fostering a resource-efficient and circular economy, given the increasing scarcity of natural resources and growing population. The effectiveness of coal tar modified by air- blowing technique as a binder “pitch” for coal fines in the production of structural composites is highlighted in this study. In addition, the feasibility of commercial dimethylpolysiloxane as a binder for coal carbon composite production was assessed and compared to that produced using coal tar-pitch. The two coal fines (GG1 and GS) used in this study have an ash content of 84.02% and 62.27%, respectively, and can be classified as rock. Their fixed carbon content ranges between 5.44% and 16.27%, compared to coal tar (35.23%). The coal tar has a volatile matter content of 64.50%, and with the air-blowing pretreatment, the tar was converted to pitch with a low volatile matter content of 11.27%. A pitch with the highest fixed carbon content of 87.68% and total carbon content of 96.01% was produced. Various ASTM standard test methods were used in the investigation to characterise and evaluate samples, including mineral phases and functional groups in the raw and coal composites produced. The composites were fabricated using a circular mould with a diameter of 30 mm and a 40 mm square mould. In the study, it was found that composites with a low H/C atomic ratio had low water absorption. Additionally, composites with high volatile matter content had high water absorption. However, the sample with the highest water absorption (19%) was the GG1 50/50 coal tar pitch 400-composite, which falls within the range (0-25%) for building materials. The composites with an intense O-H group had high compressive and flexural strengths ranging from 106.58 to 344.71 MPa and 48.75 to 159.30 MPa, respectively. The flammability of all composites was low. The highest flammability mass ablation rate and linear ablation rate were found to be 0.008 g/sec and 0.00983 mm/sec, respectively. In terms of the corrosion rate, the GS 80%/20% dimethylpolysiloxane coal composite had the highest corrosion rate (0.081 μmpy), which is minimal compared to some commercial ceramic tiles. The composites' environmental friendliness was determined by leaching them at various pH levels. The test was conducted by comparing the concentration of heavy toxic elements in the solution to the leachable concentration threshold for waste management standards (NEM WA Act No. 59 of 2008). All composites were environmentally friendly, meeting the moderate risk leaching concentration threshold. The composites that were produced in this study from South iii African discard coal can be used in large quantities in the environment without any danger or hazards, as demonstrated. Based on this study's overall results, repurposing South African discard coal as carbon/ceramic composites for building materials could play a role in the country's Just Transition initiatives. In addition to waste reduction, this strategy could reduce operational emissions, improve circularity, and address associated environmental risks.