Mechanical and Thermal Properties of Extrudates Produced from Discarded Coal Fines and Recycled Plastics as Binders

Abstract

This study focuses on the extrusion of discarded coal fines from the Highveld coalfield together with recycled lowdensity polyethylene (LDPE) and polypropylene (PP) which are used as binders to produce agglomerates with better handling properties than the coal fines for industrial use. The binder content varied between 5 and 100 wt %. The barrels of the twin screw extruder were kept at a temperature of 220 °C to melt the binders while forcing the mixture through a 10 mm die. The extrudates containing 10% or more binder were strong and homogeneous, while a 5% binder addition proved to be too low to produce homogeneous extrudates. The extrudates containing 10% LDPE and 10% PP showed compressive strengths of 17.5 and 7.9 MPa before breaking, respectively. The rest of the extrudates (>10% plastic addition) did not break but merely flattened as the plastic load increased. The compressive strength of all the extrudates showed no significant difference after being exposed to water. Furthermore, the extrudates absorbed less than 5% water after being submerged for 24 h. Thermogravimetric analysis of the extrudates was conducted under a nitrogen atmosphere up to 900 °C. Three iso-conversional methods, Kissinger−Akahira−Sunose, Starink, and Flynn−Wall−Ozawa, were used to determine the activation energy of the extrudates and raw materials. The lower activation energy and conversion temperatures found for the extrudates indicate a synergy between plastic and coal fines when the extrudates are pyrolyzed. Results from this study suggest that the coextrusion of recycled plastic with coal fines will produce solid carbonaceous fuels with high hydrophobicity, heating value, and high mechanical strength compared to coal fines.

Slow Pyrolysis Products Derived from Extrudates Produced from Discard Coal Fines and Recycled Plastics as Binders

“…The extrudates were pressed out of a 10 mm die, and the long strands were cut to a length of 2 cm after cooling in a water bath at room temperature. Further details on the extrusion method can be found in Marais et al 15 2.2. Pyrolysis Experiments.…”

“…Furthermore, the calorific value of both the chars and condensable products was measured using ISO 1928:2009b. Characterization of the raw materials was done previously by Marais et al 15 (proximate analysis, ultimate analysis, and calorific value) can be found in Table 1.…”

“…This was done to better illustrate how the composition of coal fines and plastics affects the pyrolysis characteristics. Since the plastics produce significantly more volatile matter than the coal fines during pyrolysis, 15 expressing the characteristics as gram per gram extrudate clearly indicates the weight of each property that will be produced at the end of a pyrolysis process. Therefore, the proximate and ultimate analysis results (air-dried basis) were multiplied with their respective pyrolysis product yield (char and condensable product yields) to convert the characteristics to gram per gram extrudate before the statistical analysis was used to model these results.…”

“…Furthermore, some plastics [including polyethylene (PE) and polypropylene (PP)] cannot be recycled without decreasing the polymer quality . Plastics, especially low-density polyethylene (LDPE) and PP have a high heating value, are hydrophobic, and contain negligible sulfur and mineral contents, making them potential binders . Currently, pyrolysis is the main method of producing chemicals and fuels from plastic wastes .…”

“…14 Plastics, especially low-density polyethylene (LDPE) and PP have a high heating value, are hydrophobic, and contain negligible sulfur and mineral contents, making them potential binders. 15 Currently, pyrolysis is the main method of producing chemicals and fuels from plastic wastes. 16 The highenergy content of plastic wastes enables them to be chemically repurposed to produce environmentally friendly and valuable products.…”