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Separation Process of Fine Coals by Ultrasonic Vibration Gas-Solid Fluidized Bed 时间:2017-09-19 16:35 来源:未知作者:admin 点击: 次 Abstract:Ultrasonic vibration gas-solid fluidized bed was proposed and introduced to separate fine coals (0.5–0.125 mm fraction). Several technological methods such as XRF, XRD, XPS, and EPMA were used to study the composition of heavy products to evaluate the separation effect. Results show that the ultrasonic vibration force field strengthens the particle separation process based on density when the vibration frequency is 35 kHz and the fluidization number is 1.8. The ash difference between the light and heavy products and the recovery of combustible material obtain the maximum values of 47.30% and 89.59%, respectively. The sulfur content of the heavy product reaches the maximum value of 6.78%. Chemical state analysis of sulfur shows that organic sulfur (-C-S-), sulfate-sulfur (-SO4), and pyrite-sulfur (-S2) are confirmed in the original coal and heavy product. Organic sulfur (-C-S-) is mainly concentrated in the light product, and pyrite-sulfur (-S2) is significantly enriched in the heavy product. The element composition, phase composition, backscatter imagery, and surface distribution of elements for heavy product show concentration of high-density minerals including pyrite, quartz, and kaolinite. Some harmful elements such as F, Pb, and As are also concentrated in the heavy product.
1. Introduction
Coal is an important primary energy source worldwide, especially in China. In China, coal-fired power is predominant in the production of the Chinese electric power and accounts for more than 70% [1]. Coal contains appreciable quantity of inorganic minerals and harmful elements, such as sulfur, lead, arsenic, and mercury; these minerals and elements can be transformed into inhalable particles, acid rain, and other pollutants and can be discharged into the atmosphere during coal combustion [2], thereby resulting in serious pollution to the atmosphere and large economic losses [3–6]. Thus, desulfurization and deashing of coal prior to combustion are important to prevent fog haze weather.
Although lump coals are separated prior to milling to remove large pieces of waste rock, fine waste rocks containing harmful elements inlay in the coal are not always removed. If coal is sufficiently crushed to fine particles, then the mineral particles can be fully dissociated from the coal. Such condition is favorable to the separation process. Fine coals are currently separated mainly by flotation, which can effectively separate <0.5 mm fraction coal [7–9]. The development of cyclonic-static microbubble flotation column and new reagent systems has enabled good results of low-rank coal separation [9–13]. However, the flotation process consumes large amounts of water, and its development in arid regions is limited by water resource deficiency. Thus, high-efficiency dry separation technology of fine coals should be investigated.
Dry separation technologies, especially the separation technology of the gas-solid fluidized bed, are currently used to separate coal [14–19]. For example, air dense medium fluidized bed is used to effectively separate coal of 50−6 mm size fraction [20–24]. Xu and Zhu [25] examined the influence of vibration parameter on the fluidization characteristics of fine materials. Luo et al. [26] separated coal of 6−1 mm size fraction by use of air dense medium fluidized bed and analyze the particle force condition. Yang et al. [27, 28] used vibrated fluidized bed to separate coal of 6−3 and 3−1 mm size fractions without a dense medium. These abovementioned dry separation methods can effectively separate >0.5 mm fraction coal but present difficulty in separating <0.5 mm fraction coal and exhibit many limitations. Thus, new dry separation methods to deal with fine coal of <0.5 mm fraction should be explored.
This study investigated the separation process of the 0.5−0.125 mm fraction coal with ultrasonic vibration gas-solid fluidized bed. The composition of products under different experimental conditions was studied by advanced analysis and test methods to evaluate the separation results.
2. Materials and Methods
2.1. Sampling and Experimental Device
The 0.5−0.125 mm fraction coal was chosen to study the separation process. The ash content of the coal was 36.21%, and its sulfur content reached 2.82%. The coal was obviously of high sulfur content. The schematic of experimental system is shown in Figure 1. The system included air supply and separation systems. The air supply system included a roots blower, an air reservoir, a rotor flow meter, and an air valve. The separation system included a gas-solid fluidized bed and an ultrasonic vibration device. The fluidized bed was made of organic glass (radius of 75 mm and height of 300 mm), and the ultrasonic vibration device included one ultrasonic transducer and one ultrasonic power supply. The ultrasonic transducer was fixed to the bottom of the air distribution plate. The air came from the blower and enters the fluidized bed through the pipe, air reservoir, rotor flow meter, and air distributor. The vibration force field came from the ultrasonic vibrator, which was controlled by ultrasonic generator. The height of the static bed containing the material was 100 mm. The product was divided into five layers, and the thickness ratio of each layer from upper to lower was 1 : 1 : 1 : 1 : 1.