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Trans. Nonferrous Met. Soc. China 272017 197−203 Vanadium recovery from stone coal through roasting and flotation Chun LIU1, Yi-min ZHANG1,2,3, Shen-xu BAO1,3 1. College of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China; 2. College of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China; 3. Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan 430070, China Received 11 November 2015; accepted 4 May 2016 Abstract A new process for vanadium recovery from stone coal by roasting−flotation was investigated based on the mineralogy. The process comprised four key steps decarburization, preferential grinding, desliming and flotation. In the decarburization stage, roasting at 550 C effectively avoided the negative effect of the carbonaceous materials in raw ore and generation of free CaO from calcite decomposition during roasting. Through preferential grinding, the high acid-consuming minerals were enriched in the middle fractions, while mica was enriched in the fine and coarse fractions. Through flotation, the final concentrate can be obtained with V2O5 grade of 1.07 and recovery of 83.30. Moreover, the vanadium leaching rate of the final concentrate increased 13.53 compared to that of the feed. The results reveal that the decarburization by roasting at 550 C is feasible and has little negative impact on mica flotation, and vanadium recovery from stone coal is conducive to reducing handling quantity, acid consumption and production cost. Key words vanadium-bearing stone coal; roasting decarburization; mineralogy; preferential grinding; flotation 1 Introduction In China, stone coal is a specific vanadium-bearing resource. The gross reserve of vanadium in terms of V2O5 in stone coal is 118 million tons, which accounts for over 87 of the domestic reserve of vanadium [1,2]. Hence, various vanadium extraction techniques from stone coal are investigated. Generally, these techniques involve long processes like roasting, acid leaching, ion purification, precipitation and calcination [3−6]. However, due to the low vanadium grade, complex chemical composition and various occurrences of vanadium of stone coal, vanadium extraction from stone coal is commonly confronted with the problems of enormous ore handling quantity, high acid consumption and high production cost [7]. For the sake of relieving these problems, pre-concentration of vanadium in stone coal before leaching is an effective . ZHAO et al [8] took gravity separation to pre-concentrate vanadium from stone coal. Although the V2O5 loss was low in gravity separation stage, the yield of final tailings was not high and the separation for the acid-consuming minerals, especially calcite, was not satisfactory. In order to obtain satisfactory separation results, WANG et al [9] chose flotation for beneficiation due to its high handling capacity and high selectivity, and obtained satisfactory flotation results as the stone coal was weathered. Under that condition, little carbon existed in the raw ore and the chemical composition and mineral texture were comparatively simple, which was easy to obtain relatively higher flotation efficiency and better separation results. However, most stone coal in China exists as primary ore, where the carbon content usually ranges from 8 to 12, and the chemical composition and mineral texture are rather complex. The carbonaceous materials are disseminated among various minerals such as oxides, carbonates, silicates and sulfides, which closely coexist as fine-grained particles. Due to the coating of carbonaceous materials on the surfaces of mineral particles, the floatability differences among different minerals decrease significantly and flotation separation effect is not satisfactory. Even for the severely Foundation item Project 2015BAB03B05 supported by the National Key Technology R Project 51404177 supported by the National Natural Science Foundation of China Corresponding author Yi-min ZHANG; Tel 86-27-87882128; E-mail zym126135 DOI 10.1016/S1003-63261760022-0 万方数据 Chun LIU, et al/Trans. Nonferrous Met. Soc. China 272017 197−203 198 metamorphic stone coal that carbon mainly exists as graphite, the negative effect of carbon on flotation of stone coal still resulted in a long process, huge reagent consumption and production cost, so technique of decarburization by roasting before floatation was developed [10−13]. Usually the roasting temperature was set over 650 C so as to remove the carbon quickly without considering the negative effect on flotation. Our previous research [14] found that, with the increase of roasting temperature, reactions like the oxidation of pyrite, combustion of carbon and calcite decomposition occurred successively in vanadium-bearing stone coal. At 600−700 C, carbon and pyrite disappeared, and octahedral structure in mica was not damaged, while the remaining free CaO after sulfur-fixation of calcite made the pH value and concentration of Ca2 increase in flotation pulp. Under that condition, it is not conducive to flotation because the fine particles coagulated in the non-selective state, and much fatty acid was consumed, as well as the quartz was activated. Given that the negative effect of carbonaceous materials and free CaO on flotation should be balanced, reasonable roasting temperature is important for optimizing the flotation process. The focus of this study is to determine a simple and reasonable flotation process for typical primary vanadium-bearing stone coal at relatively low roasting temperature 550 C. The flotation results are interpreted within the perspective of the effect of the mineralogy on the flotation process and vise versa. 2 Experimental 2.1 Materials and reagents The vanadium-bearing stone coal used in this study was obtained from Hubei Province, China. The raw ore was firstly crushed to below 2 mm in size with two-stage jaw crusher and one-stage roll crusher. The crushed ore was blended and then split into 200 g samples for mineralogy and flotation tests. The analytical grade reagents used in this study and their abbreviations are listed in Table 1. Table 1 Reagents used in flotation Reagent Function Sulfuric acid pH regulator Fatty acid YS Reverse flotation collector Sodium fluosilicate Depressant Acidic water glass Depressant Dodecyl amine DDA Direct flotation collector 2.2 Procedure The crushed raw ore was firstly decarbonized in a SXZ-10-B muffle furnace at 550 C for 90 min and then wet-ground in a XMB-70 laboratory rod mill at 50 solids, until a particle size distribution of 84.62 passing 74 μm was achieved. The rod mill product was subjected to desliming by free settling and then the flotation tests was conducted in a 0.5 L flotation cell at an agitation speed of 1992 r/min, in which pH regulator, depressant and collector were added. Two-stage flotation experiments were carried out containing reverse and direct flotation. The detailed conditions and process flowsheet are given in Fig. 1. The flotation products contained slime, V concentrate, tailings 1 and 2. The slime and V concentrate are merged into final concentrate, and the rest are rejected as final tailing. The vanadium of preferential grinding product and final concentrate were leached by 5 CaF2 and 15 volume fraction H2SO4 at 95 C for 6 h. Fig. 1 Flotation conditions and process flowsheet of decarbonized sample 2.3 Test s The vanadium grade was measured by Test s of Vanadium in Coal Standard GB/T 19226−2003. X-ray diffraction XRD analysis was conducted by D/Max-IIIA X-ray diffractometer with Cu Kα radiation, voltage 40 kV, current 30 mA and at the scanning rate of 15 /min from 5 to 70 . The phases were identified by comparison of the peak positions and d values with data published by the international centre for diffraction data ICDD. The chemical analysis was pered with the Xios advanced X-ray fluorescence XRF analyzer. Detailed mineralogy of the raw ore and decarbonized samples was investigated using Leica DMLP polarization microscope and quantitative uation of minerals by scanning electron microscopy QEMSCAN. Valence distribution was measured on ZDJ-4A automatic potentiometric titrimeter by ammonium ferrous sulfate titration [15]. Vanadium chemical phase analysis 万方数据 Chun LIU, et al/Trans. Nonferrous Met. Soc. China 272017 197−203 199 was carried out according to the sequential extraction procedures [16]. Sizing analysis was conducted on rod mill product using laboratory wet screening and classification by free setting. Coarse particles 38 μm were classified by wet screening and the fine particles were classified by free setting. 3 Results and discussion 3.1 Mineralogy of raw ore and decarbonized sample 3.1.1 Chemical composition The chemical composition of stone coal is shown in Table 2. It can be seen that the main components in the raw ore are SiO2 and Al2O3, but the content of V2O5 is only 0.71. As the content of CaO in the raw ore is over 5, this ore belongs to high-calcium vanadium-bearing stone coal. The high content of CaO not only severely decreases the water leaching rate of vanadium in the traditional technique of NaCl roasting−water leaching, but also increases the acid consumption in the process of acid leaching. Therefore, the CaO should be removed in the reverse flotation. Table 2 Chemical composition of raw ore and decarbonized sample mass fraction, Sample V2O5 SiO2 Al2O3 Fe2O3 K2O Raw ore 0.71 57.09 11.03 2.60 2.42 Decarbonized sample 0.79 63.45 12.24 3.97 2.78 Sample Na2O CaO MgO SO3 C Raw ore 0.11 5.45 1.48 3.62 8.12 Decarbonized sample 0.14 5.92 1.67 2.69 0.12 3.1.2 Mineral composition The XRD pattern of the raw ore Fig. 2 indicates that the main mineral phases are quartz, mica and calcite. In addition, feldspar, pyrite and kaolin exist as the accessory minerals. The mineral composition and content Table 3 are obtained based on the XRD analysis, microscopy and chemical composition analysis. The results indicate that reducing minerals like coal and pyrite, disappeared in the decarbonized sample, while pyrite was changed into hematite and some decomposed calcite was converted into anhydrite. For decarbonized sample, calcite and hematite belong to high acid consuming minerals, which make it difficult to regulate pH value for mica flotation under acidic condition. Therefore, it is necessary to remove them before the mica flotation, especially calcite because of its much easier dissolution in acid solution compared to that of hematite. Fig. 2 XRD patterns of raw ore and decarbonized sample 3.1.3 Occurrence of vanadium The result of vanadium valence distribution measured by potentiometric titration measurement shows that VIII is dominant which accounts for 79, and VIV accounts for 21, while VV is undetected. Vanadium chemical phase analysis Table 4 reveals that vanadium mainly exists in silicate, but little in calcite and iron oxide. Previous studies showed that VIII and VIV usually substitutes for AlIII in the crystal lattice of mica type minerals in the of isomorphism [3]. Hence, the major gangue minerals are quartz, calcite, feldspar and pyrite, and the main target mineral is mica in the beneficiation process. 3.1.4 Lithological feature of decarbonized sample The mineralogy of the raw ore is complex with several mineral phases including a variety of silicates, oxides, carbonates as well as sulfides. Besides that, the Table 3 Mineral composition of raw ore and decarbonized sample mass fraction, Mineral Quartz Coal Calcite Mica Feldspar Pyrite Hematite Kaolin Silicate Sulfate Raw ore 37 13 11 15 10 7 0 5 0 0 Decarbonized sample 38 0 9 15 11 0 11 0 8 3 Table 4 Chemical phase analysis of vanadium in raw ore mass fraction, Mineral Vanadium adsorbed state and free vanadium oxide Organism Calcite Iron oxide Silicate Others Total vanadium Vanadium occupation ratio 1.2 2.2 0.4 0.2 88.2 7.8 100.0 万方数据 Chun LIU, et al/Trans. Nonferrous Met. Soc. China 272017 197−203 200 raw ore texture is also complicated. The micro disseminated carbonaceous materials cover on the surface of various minerals especially the fine clay minerals. Mica usually presents in the of flake and strip, and it is classified into muscovite and illite according to the particle size and associated minerals. The muscovite presents in the of flake or strip, and it is often associated with quartz and pyrite, and some is even locked in quartz, which makes it difficult to break and recover. The average size of muscovite is 30−45 μm. Small amount of very fine stripy illite particles are generally less than 5 μm and closely associated with kaolin, carbon, fine-grained quartz and feldspar. Most calcite is irregular granular and closely associated with coal, clays and quartz, and some even is locked in quartz vein. Pyrite is associated with quartz and muscovite in the of regular and semi-regular particle shape. There are three main types of quartz based on the particle size and associated minerals. The vein quartz in the size of 70 μm is often associated with calcite, whereas the lenticular quartz displaying aggregate structure in the size of 20−40 μm is associated with muscovite. The final type whose size is nearly 10 μm is closely associated with clays and carbon. As the composition and texture of the raw ore are so complex, it is difficult to obtain satisfactory flotation result without high enough grinding fineness. In order to remove the negative effect of carbonaceous materials on flotation and make full use of thermal energy for subsequent leaching, roasting decarburization is necessary for the raw ore before flotation. The changes of chemical and mineral constituents between the decarbonized sample and raw ore are shown in Tables 2 and 3. The results indicate that most carbon was removed, while pyrite was changed into hematite and some decomposed calcite was changed into anhydrite, whereas the free CaO was not found and the octahedral structure of mica was not damaged in the decarbonized sample. QEMSCAN analysis Table 5 was pered on the decarbonized sample to determine the average grain diameters of minerals. It reveals that minerals are finely disseminated in the decarbonized sample, and it is difficult to separate valuable mineral mica from the associated gangue minerals. What we should note here is that muscovite usually presents in the of thin strip or flake, so the average mica size here refers to the length of the vein. 3.2 Preferential grinding Comminution of complex ores is typically necessary to liberate the sought after minerals to allow fo
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