Product development and processing of sugarcane wax from dissolved air flotation (DAF) mud

Abstract

The South African sugar industry is one of the world’s leading cost competitive producers of high-quality sugar. However, this industry is currently experiencing a gradual decline due to numerous challenges such as unfavourable climate conditions, economic decline, cheap sugar imports, the Health Promotion Levy (“sugar tax”), and lack of required capital for innovation (SASA, 2022). This crisis threatens tens of thousands of jobs and hundreds of thousands of livelihoods. As a result, this study seeks to contribute to the mitigation of the crisis faced by this industry. Recently, the Illovo’s Sezela furfural production plant downstream to the sugar mill was commissioned. This plant produces a waste with a sugarcane wax content of about 30% (R.D Gent, 2012), which is higher than the wax (8.3%) extracted from press mud reported by (Paturau, 1982). The aims of this study were to assess methods of separating the wax from DAF mud, as well as a method to refine the crude wax. Furthermore, the resulting waxes were to be characterised to allow for comparison with conventional sugarcane wax as well as other plant-based waxes such as carnauba wax. Finally, a preliminary process layout was proposed, and a mass balance of the overall process presented. This serves as a basis for future equipment sizing and costing exercises. Two methods of producing crude wax from DAF mud were compared in this study, namely a solvent extraction method and the, heating and melting method. The solvent extraction method involved dissolving DAF mud in five different solvents namely, turpentine oil, toluene, butanol, 2-butanone or ethanol. The heating and melting method involved heating the DAF mud to a melting point of crude wax and collecting the resulting wax by decanting. The crude wax yield obtainable via solvent extraction was in a range of 80 - 87.3 %, while the crude wax yield obtained via heating and melting was approximately 56%. Despite the relatively low attainable yield of crude wax via this latter method it was preferred over solvent extraction because eco-friendliness, and being more cost effective, simpler, and faster. The crude wax obtained via this method was a solid at room temperature, whilst the crude wax produced via the solvent extraction method was not in a solid at room temperature. Therefore, crude wax in a solid form is suitable for refined wax production as it can be directly treated with charcoal and a green solvent to obtain refined wax. Crude wax from the heating and melting method was used in the refining step. Both the crude and refined wax were characterized for their physico-chemical properties. Results show that crude wax produced by heating and melting method has an acid value of 155 ± 2.2 (mg KOH/g wax), saponification value of 227 ± 10 (mg KOH/g wax), % FFA of 78 ± 1, ester value of 72 ± 10 (mg KOH/g wax), Iodine number of 53 ± 5 (g I2/100 g), unsaponifiable matter (%) of 21.5 ± 2, melting point (°C) of 76 ± 2, density (g/cm3 ) of 0.850 – 0.882 (at temperatures between 25 and 80 °C) and refractive index (26°C) of 1.4923. Crude wax was further treated with activated charcoal and ethanol to obtain refined wax, at a crude wax: ethanol: activated charcoal ratio of 1:04:02. After crude wax was refined, it was then characterised for its physical and chemical properties. Refined wax characterization results showed that it has an acid value of 23 ± 3 (mg KOH/g wax), saponification value of 59 ± 7 (mg KOH/g wax), % FFA of 12 ± 1, ester value of 35 ± 7 (mg KOH/g wax), Iodine number of 44 ± 8 (g I2/100 g), melting point (°C) of 75 ± 2, density (g/cm3 ) of 0.787 – 0.814 (at temperatures between 25 and 80 °C) and refractive index (26°C) of 1.4867. The GC-MS analysis revealed that, similar to crude wax, refined wax predominantly consists of five main classes of compounds namely, fatty acids, alkanes, alcohols, aldehydes, and esters. Fatty acids contributed about 50% to the total composition for both crude and refined wax samples. Furthermore, both crude and refined wax samples were found to contain policosanol, which can be used to support the evidence of the applicability of sugarcane wax derived from Illovo’s Sezela DAF mud in various applications such as pharmaceuticals industry. In preparation for the scale up of the process a process layout was proposed and a process flow sheet for the whole process and subsequent mass balances derived. Mass balances will be useful in equipment sizing and overall project costing which is part of the future work and beyond the scope of this study. It was concluded that the crude sugarcane wax obtained from DAF mud is quite different from that obtained from filter mud. However, the ester number is in the range given in the literature for filter mud-derived sugarcane wax. The iodine number of the DAF mud-derived raw wax is much higher than that of the conventional filter mud-derived wax. The unsaponifiable matter was lower and the melting point is in the range reported for filter mud-based crude wax. The DAF mudderived refined wax of this study is compared to different wax fractions derived from conventional filter mud-based waxes. The saponification value lies within the ranges of “hard wax” and “refined wax”. The iodine number and the melting point of the refined wax from DAF mud lie within the ranges of “soft wax” and “hard wax”, respectively. Furthermore, the DAF mud-derived refined wax’ properties was found to resemble those of candelilla wax rather than carnauba wax, with the acid and saponification values as well as iodine number being in the same ranges. The melting point is 2-4°C higher than that specified for candelilla wax. Future studies should evaluate the economic feasibility of the process by costing the overall project, investing in the equipment to produce a completely eco-friendly refined sugarcane wax.