On the utilization of waste fried oil as flotation collector to remove carbon from coal fly ash

Highlights

• A decarburization approach was proposed based on “waste control through waste”.

• Waste fried oil was used as flotation collector to remove carbon from coal fly ash.

• Waste fried oil had abundant hydrophilic oxygen-containing groups.

• Waste fried oil could weaken the interaction between fly ash and deionized water.

• The unburned carbon could be removed using waste fried oil with high efficiency.

Abstract

In this paper, the waste fried oil was used to remove the unburned carbon in coal fly ash during flotation process, and found that the waste fried oil could be a novel collector for the removal of carbon from coal fly ash. The results implied that the wetting rate of the fly ash after treated by waste fried oil was decreased, meanwhile the contact angle was increased. A significant decrease in wetting heat was observed, which indicated a weaker interaction between deionized water and fly ash after treatment with waste fried oil. Flotation tests showed that the content of unburned carbon could be reduced effectively through froth flotation when took waste fried oil as collector. FTIR analysis testified that waste fried oil had abundant oxygen-containing groups that could be adsorbed in a carbonaceous matter to achieve hydrophobization. X-ray photoelectron spectroscopy, scanning electron microscope, and energy dispersive analyses showed that the main compositions of flotation concentrate products were unburned carbon, whereas the tailing products consist of aluminum and silicon, which confirmed the superior separation performance when the waste fried oil was used as a collector in coal fly ash flotation. This investigation provides an approach to remove the unburned carbon in coal fly ash based on the principle of “waste control through waste”, which can solve the environmental problems brought by large amounts of both coal fly ash and waste fried oils.

Introduction

Fly ash typically produced in the power and heat supply industries, which followed hot air and collected in the dust removal equipment (Ahmaruzzaman, 2010, Hower et al., 2017, Bartoňová, 2015). In China, fly ash has a considerable annual output at 5.32–6.86 hundred million tons, which is the second highest amount of solid waste over past five years (Guanyan tianxia, 2018). And the multicomponent properties provide its potential use in comprehensive applications, including the construction industry, environmental protection industry, chemical industry and valuable elements secondary recovery (Yao et al., 2015, Flores et al., 2017). Fly ash takes up a significant amount of land space and pollutes the environment (Aldahri et al., 2016, Tsiridis et al., 2015). The main approach to reaching a resource maximization/reserve reduction of fly ash is to add it to building materials in the construction industry (Xu & Shi, 2018). As is well known, fly ash is mainly composed of silica-alumina glass (αSiO₂-βAl₂O₃), neoformed minerals, mineral relics, and variable amounts of unburned carbon. The proportion of unburned carbon affects the activity of fly ash, particularly when added to the building feedstock (Bartoňová, 2015, Xu and Shi, 2018, Mahedi et al., 2019). Therefore, it is necessary to limit the proportion of unburned carbon to facilitate the utilization of fly ash in the construction industry. For example, the ASTM C618-12a defines the required loss on ignition (LOI) computed from organic unburned carbon as less than 10.00%, 6.00%, and 6.00% when added to concrete (ASTM C618-2012), and the GB/T 1596-2017 stipulates that the LOI used for concrete should be limited to 10.00%, 8.00%, and 5.00% (GB/T 1596-2017). Moreover, the unburned carbon remaining in fly ash means that the coal is not burning adequately and lead to energy loss, which is against the principle of resource maximization utilization. Hence, a highly efficient technique to remove unburned carbon is important for the further application of fly ash. From the published literature, researchers have used separation techniques such as classification (Groppo, 1997), electrostatic separation (Zhang et al., 2018, Tao et al., 2009, Tao et al., 2017), gravity separation (Maroto-Valer et al., 2001, Zhang et al., 2020), oil agglomeration (Xing et al., 2019, Gray et al., 2002), and froth flotation (Yang et al., 2019, Zhou et al., 2017) to realize the removal of unburned carbon. Among these separation techniques, froth flotation showed its advantage in terms of removal efficiency and had been successfully applied in the industry (Li et al., 2015, Şahbaz et al., 2008).

The selection of reagents is of vital importance to fly ash flotation process, particularly for the collectors. The conventional mineral oils such as kerosene (Kim et al., 2018, Huang et al., 2003, Liu et al., 2013) and diesel oil (Zhang and Honaker, 2015, Zhang and Liu, 2019) are the most common collectors in the reported studies, and the flotation effect differs from the composition of the fly ash samples. Zhang and Honaker found that larger diesel oil dosages (4.50 kg/t) were required for good flotation performance with the LOI of 13.48% and carbon recovery of 73.13% (Zhang & Honaker, 2015). The oxygen-containing groups like CO and OCO in unburned carbon might generate during the combustion of coal, which increases the polarity and decreases the hydrophobicity of the surface in unburned carbon (Xing et al., 2019, Wang et al., 2009). To further improve the collection efficiency of unburned carbon, which has similar surface properties with oxidized coal and low rank coal, combing mineral oil with new polar reagents such as 4-dodecylphenol (Drzymala et al., 2005) and oleic acid (Eisele & Kawatra, 2002) might be a feasible approach.

Meanwhile, the waste vegetable oil collected from food swilling or waste fried oil used in household kitchens, restaurants, and food-processing factories is processed as a flotation reagent, and is applied in the field of mineral processing (Valdés and Garcia, 2006a, Halek et al., 2013). Waste vegetable oil is mostly derived from over-used natural vegetable oils such as olive oil (Alonso et al., 2000), colza oil (Alonso et al., 2002), peanut oil, corn oil, soybean oil (Serqueira et al., 2014), and sunflower seed oil (Sahinoglu & Uslu, 2015), which are derived from vegetables. Waste vegetable oil has a composition of fatty acids (linoleic, oleic, and palmitic acids) and fatty acid methyl esters (methyl linoleate, methyl linolenate, and methyl oleate), which are favorable nonpolar and heteropolar collectors for unburned carbon with oxygen-containing groups (Abidin et al., 2013). Waste vegetable oil is commonly processed using esterification and transesterification methods with catalysts and purified into a high-purity biodiesel, which can be a complement to traditional fossil fuels (Hums et al., 2016, Peiró et al., 2008). Waste fried oil is a type of waste vegetable oil derived from the remaining over-used vegetable oils after the frying of food, which cannot be used further during food processing owing to its damage to human health. A filtration procedure is necessary to remove the food residues in waste fried oil (Valdés & Garcia, 2006b). During the frying process, some chemical reactions including oxidation, hydrolysis, and polymerization occur, whereas the main physicochemical properties do not change when compared with the original vegetable oil (Valdés and Garcia, 2006b, Mohammed and Bandari, 2017). Currently, waste fried oil is used as an agglomerant and collector for the cleaning of fine coal waste. During the oil agglomeration process, the waste fried oil acts as an agglomerant, bridging the coal particles and aggregating into large coal particles, and the optimal oil concentrations can range from 5.00 wt% to 10.00 wt% (Alonso et al., 2002). Compared with oil agglomeration, the waste fried oil acts as collector that cooperates with the frother in froth flotation. For example, an used olive oil can be applied as a collector to recover coal from fine coal waste, which can obtain a coal concentrate with an ash content of 28.00% and calorific value of 5953 kcal/kg from the fine coal waste with an ash content of 68.84% and a calorific value of 2213 kcal/kg (Alonso et al., 2000). The success in cleaning fine coal waste using froth flotation with the collector of waste fried oil inspired that it might be an effective way to fly ash decarburization. Unburned carbon has similar surface properties as both oxidized and low-rank coal, therefore the waste fried oil might be alternative collector when using a flotation to process fly ash. More than that, the recycling of waste fried oil in the treatment of coal fly ash is of great significance to the reduction of solid waste and kitchen waste, which can improve economic efficiency and reduce environmental pollution simultaneously, that is conform to the concept of “waste-to-resource supply chain” (Pan et al., 2015, Behera et al., 2012).

The conventional mineral oils that commonly used in the decarburization of coal fly ash is precious and nonrenewable, which accompanied with the high consumption. It this paper, waste fried oil was proposed to act as a novel collector to remove unburned carbon from coal fly ash based on the principle of “waste control through waste”. The wetting rate, contact angle, and wetting heat were analyzed in detail for the fly ash treated by waste fried oil. The removal efficiency for the unburned carbon in fly ash when using waste fried oil as collector was also estimated through froth flotation tests. And the interaction mechanism between waste fried oil and fly ash was investigated using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy analysis, scanning electron microscope and energy dispersive analyses of the X-rays measurements.