Article pubs.acs.org/EF
Effect of Ultrasonic Pretreatment on Oxidized Coal Flotation Mengdi Xu,† Yaowen Xing,*,†,‡,§ Xiahui Gui,*,‡ Yijun Cao,‡ Dongyue Wang,† and Longwu Wang† †
School of Chemical Engineering and Technology and ‡Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ABSTRACT: In this work, the effect of ultrasonic pretreatment time on oxidized coal flotation was studied. Analytical techniques, such as particle size distribution (PSD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurement, and specific surface area/pore analysis, were used to examine the changes in the physicochemical characteristics of the coal samples before and after ultrasonic pretreatment. The results showed that the flotation kinetics and yield of the clean coal initially increased with ultrasonic pretreatment time. The maximum clean coal yield (79.6%) and maximum value of the flotation kinetics constant were obtained within 10 min of ultrasonic pretreatment. The hydrophilic oxidized layer on the coal surface was effectively removed by ultrasonic cavitation, as identified by PSD, SEM, XPS, and contact angle, which increased the flotability and recovery of oxidized coal. However, prolonged ultrasonic pretreatment proved detrimental to oxidized coal flotation. The fresh exposed surface was reoxidized by the hydroxyl free radicals produced during the cavitation process. It is expected that the results of this work will provide guidance in ultrasonic flotation for oxidized coal.
1. INTRODUCTION Froth flotation is an effective separation method for fine coal cleaning, which depends upon the differences in the surface properties of coal and gangue minerals.1−5 Fresh coal particles with a hydrophobic surface are easy to float, even at low flotation reagent dosages, but their flotation becomes difficult when these particles are oxidized.6−8 Coal particles can be oxidized to different degrees depending upon the coalification process, temperature, and oxidation time.9−11 Usually, the surface physicochemical characteristics, composition of surface functional groups, and surface roughness are significantly changed during the oxidation process. X-ray photoelectron spectroscopy (XPS) analyses have shown that the oxidation process increases the formation of oxygen-containing functional groups, such as carboxyl, phenol, and carbonyl functionalities, on the coal surface. In addition, surface roughness and cracks also increase after oxidation of coal.12 A firm hydration film is formed at the hydrophilic sites on the oxidized coal surfaces by hydrogen bonding, leading to poor hydrophobicity and low flotation recovery.13,14 Therefore, it is necessary to develop an effective flotation separation process to enhance the flotation efficiency of oxidized coal. Several reports have been published that describe efforts to improve the flotation recovery of oxidized coal. On the one hand, new reagents in the form of new collectors, regulators, and surfactants have been widely used to enhance the flotation capability of oxidized coal. Cationic collectors and collectors with oxygen-containing functional groups can change the ζ potential of oxidized coal, which enhances the flotation performance of oxidized coal.15 Gui et al.16 have found that the flotation yield can be greatly improved using α-furanacrylic acid, instead of the traditional hydrocarbon collector dodecane. Xing et al.17 have shown that the flotation recovery and selectivity of oxidized coal were improved using a new type of oxygenated polar compound collector (CC). The improved performance was attributed to the hydrogen bonding between © 2017 American Chemical Society
the hydrophilic sites on the coal surface and the polar oxygencontaining functional groups in the CC. The adsorption of surfactants can also increase the surface hydrophobicity of coal particles via hydrogen bonding and also decrease the surface tension of oil and cause it to disperse into small droplets. In this manner, the energy required to spread the oil onto the coal surface is decreased.18 On the other hand, a number of pretreatment methods, which include grinding, microwave irradiation, thermal treatment, and ultrasonic treatment, have been used to enhance the flotation of oxidized coal. Microwave and thermal treatments remove the moisture content, pore water, hydration water, as well as certain types of hydroxyl functional groups and, thus, improve the flotation capability of oxidized coal.19,20 Ultrasonic treatment has been successfully employed to clean the surface of the coal particles and increase the distribution and activity of flotation agents in the pulp, which promotes the flotation process.21−23 Feng and Aldrich24 have found that ultrasonic pretreatment has a significant effect on the flotation recovery rate and kinetics of oxidized coal, especially for smaller size fractions. XPS results indicated that ultrasonic treatment successfully exfoliated the thin clay layer as well as the oxidized layer.25 Buttermore and Slomka26 observed that ultrasonic pretreatment can enhance the floatability of oxidized coal to a level close to or exceeding that of fresh coal by removing the oxidized surface layer by cavitation. However, the ultrasonic pretreatment method is a complex process that requires further investigation. In this study, the effect of the ultrasonic treatment time during the prewetting stage on oxidized coal flotation was investigated. Particle size distribution (PSD), scanning electron microscopy (SEM), XPS, contact angle measurement, and specific surface area/pore analysis were used to examine the changes in physicochemical Received: July 20, 2017 Revised: November 13, 2017 Published: November 13, 2017 14367
DOI: 10.1021/acs.energyfuels.7b02115 Energy Fuels 2017, 31, 14367−14373
Article
Energy & Fuels
3. RESULTS AND DISCUSSION 3.1. Effect of the Ultrasonic Pretreatment Time on Oxidized Coal Flotation. The flotation results of oxidized coal subjected to ultrasonic pretreatment for different time periods are shown in Figure 1. It is evident that the flotation of
characteristics of the coal samples before and after ultrasonic pretreatment to obtain a complete mechanistic picture of the role of ultrasonic treatment in flotation. The outcome of this paper is expected to provide guidance for effective ultrasonic flotation engineering for oxidized coal.
2. EXPERIMENTAL SECTION 2.1. Materials. The high-metamorphosed anthracite samples were collected from Xuehu Coal Preparation Plant, Yongcheng, Henan, China. Raw coal samples were screened to obtain fractions with sizes of
