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 low-density 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 bindercontentvariedbetween5 and 100 wt %. The barrels of the twins crew extruder were kept at a temperature of 220°C to melt the binders while forcing the mixture through a 10 mm die. Theextrudatescontaining10%or more binder were strong and homogeneous, while a 5% binder addition proved to be too low to producehomogeneousextrudates.Theextrudatescontaining10%LDPEand 10%PP showed compressive strengths of 17.5and 7.9 MPa before breaking, respectively. The rest of the extrudates(>10%plasticaddition)did not break but merelyflattenedas the plasticloadincreased.The compressivestrengthof all the extrudatesshowedno significantdifferenceafterbeingexposedto water.Furthermore,the extrudatesabsorbedless than5% waterafterbeingsubmergedfor 24 h. Thermogravimetricanalysisof the extrudateswas conductedundera nitrogenatmosphereup to 900°C. Threeiso-conversionalmethods,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 co-extrusion of recycled plastic with coal fines will produce solid carbonaceous fuels with high hydrophobicity, heating value, and high mechanical strength compared to coal fines.