Article pubs.acs.org/EF
Flotation Behaviors of Coal Particles and Mineral Particles of Different Size Ranges in Coal Reverse Flotation Yonggai Li, Jianzhong Chen,* and Lijuan Shen Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering and Technology, China University of Mining and Technology, No. 1 University Road, Quanshan District, Xuzhou, Jiangsu 221116, People’s Republic of China ABSTRACT: Reverse flotation is conducted as an alternative method for cleaning of fine coal. In the present study, dextrin and Lilaflot D817M (a complex amine) were used as a coal depressant and mineral collector, respectively. Dextrin can be adsorbed by coal particles through hydrophobic bonds, while Lilaflot D817M can be adsorbed by minerals (mainly silica) through electrostatic interactions. The effects of reagent dosages on the flotation performance of coal and mineral particles at different size ranges were examined. The average particle size of all of the concentrates (sink) was much larger than that of the tailings (float). Dextrin has weak depression effects on coal particles of below 38 μm because they become entrained easily to the froth phase. Coal particles −74 + 53 μm respond to dextrin best. More than 80% of minerals −38 μm were recovered when 1 kg/t collector was used, while coarser minerals needed more collectors. For the mineral particles larger than 38 μm, those of −104 + 74 μm have the best flotation performance. When the collector dosage increased from 1 to 6 kg/t, the combustible matter recovery in concentrates decreased from 78.65 to 59.35% because coal particles can also interact with the amine collector and are lost in the froth product. A concentrate with 25.45% ash reduction can be achieved with 4 kg/t dextrin and 6 kg/t collector.
1. INTRODUCTION Coal has natural hydrophobicity, and fine coals are usually cleaned by conventional flotation using oil collectors. Unlike conventional direct flotation, during the process of coal reverse flotation, minerals float to the froth product while coals are depressed and remain in the pulp.1−3 Reverse flotation has been widely used in the separation of metallic ores and non-metallic ores, such as the removal of silicates from diasporic bauxite and quartz from dickite and the separation of magnesite from dolomite and silica from magnetite or hematite.4−11 Coal reverse flotation has been used for the desulfurization of clean coal.12,13 Stonestreet and Franzidis first conducted reverse flotation systematically for fine coal upgrading by dealing with the coal−silica mixture to reduce ash entrainment.1−3 This method has also been used to clean difficult-to-float coals, such as lignite, sub-bituminous coal, and oxidized coal.14−21 Amine is the most popular collector for minerals, while dextrin is often used as a coal depressant.22 It is well-known that different particles with different size ranges have different flotation behaviors during the flotation process.14,21,23−26 The effects of various factors, such as dosages and types of collectors and depressants, pH, and conditioning time, on coal reverse flotation have been investigated, but the flotation behaviors of coal particles and mineral particles during the reverse flotation process were little studied.1−3,14−22 In this investigation, the reverse flotation was introduced as an alternative method for cleaning coal. The effects of reagents on flotation behaviors of coal particles and mineral particles in different size ranges, using dextrin as a coal depressant and Lilafloat D817M, a complex amine, as a mineral collector, were investigated. Dextrin is a derivative of starch produced by partial thermal degradation under acidic conditions. It has higher branched polymeric carbohydrates and composed of dextrose units. The structure of dextrin is shown in Figure 1. The adsorption of © XXXX American Chemical Society
Figure 1. Structure of dextrin.27
dextrin on the mineral surface could be in different ways, such as chemisorption, physisorption, or hydrophobic−hydrophobic interaction.27 Miller et al.12 have studied dextrin adsorption on demineralized coal and oxidized coal. In their work, those investigators found that the existence of mineral matter and increasing oxygen-containing functional groups as a result of oxidization could decrease the dextrin adsorption on the coal surface.12 Therefore, the interaction between dextrin and the coal surface is mainly hydrophobic−hydrophobic interaction. The nonpolar groups of the dextrin molecule interact with the hydrophobic bonding on the coal surface, with the polar groups directed away from the surface. Therefore, the coal surface become hydrophilic and will not float to the froth phase, which makes reverse flotation possible.
2. MATERIALS AND METHODS 2.1. Materials. The coal sample was obtained from the Coalburg mine in West Virginia. The proximate analysis and size distribution of Received: June 21, 2016 Revised: September 27, 2016
A
DOI: 10.1021/acs.energyfuels.6b01516 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels the sample are presented in Tables 1 and 2, respectively. A laser particle analyzer (Brookhaven Instruments, Holtsville, NY) was used
Table 1. Proximate Analysis of the Coal Sample sample
moisture (%)
ash (%)
volatile matter (%)
fixed carbon (%)
total sulfur (%)
Coalburg
2.52
56.21
19.04
22.23
0.55
Table 2. Particle Size Distribution and Ash Content of the Coal Sample particle size (μm)
percentage (%)
ash (%)
>200 150−200 104−150 74−104 53−74 38−53
