Tiejun Chen

Comparison of the mechanisms of microwave roasting and conventional roasting and of their effects on vanadium extraction from stone coal

Experiments comparing microwave blank roasting and conventional blank roasting for typical vanadium-bearing stone coal from Hubei Province in central China, in which vanadium is present in muscovite, were conducted to investigate the effects of roasting temperature, roasting time, H2SO4 concentration, and leaching time on vanadium extraction. The results show that the vanadium leaching efficiency is 84% when the sample is roasted at 800°C for 30 min by microwave irradiation and the H2SO4 concentration, liquid/solid ratio, leaching temperature, and leaching time are set as 20vol%, 1.5:1 mL·g−1, 95°C, and 8 h, respectively. However, the vanadium leaching efficiency achieved for the sample subjected to conventional roasting at 900°C for 60 min is just 71% under the same leaching conditions. Scanning electron microscopy (SEM) analysis shows that the microwave roasted samples contain more cracks and that the particles are more porous compared to the conventionally roasted samples. According to the results of X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses, neither of these roasting methods could completely destroy the mica lattice structure under the experimental conditions; however, both methods deformed the muscovite structure and facilitated the leaching process. Comparing with conventional roasting, microwave roasting causes a greater deformation of the mineral structure at a lower temperature for a shorter roasting time.

Microwave roasting with size grading based on the influence of carbon on vanadium extraction from stone coal via microwave roasting-acid leaching

Carbon in stone coal has a double-edged function in the microwave roasting-acid leaching of stone coal; it provides stone coal with good heating characteristics, but it hinders the oxidization of vanadium and causes the sintering phenomenon. Adjustment of the particle size and microwave input power is performed to use the advantages of carbon and avoid its disadvantages. Fine particles are not suitable for direct microwave roasting due to their low heating rate, low final temperature and serious sintering phenomenon. However, for coarse particles, a high microwave input power causes more carbon to be retained in the stone coal in the heating stage to obtain a high heating rate and final temperature. These carbon particles are removed in the temperature holding stage because of the unique cracks and pores of coarse particles under the high microwave input power condition. Comprehensively considering the heating characteristics and vanadium leaching efficiency of stone coal, a 1–3 mm particle size and 1200–1800 W microwave input power are suitable for the microwave roasting of stone coal. The vanadium leaching efficiency of the roasted sample in this condition is 14% higher than the roasted sample without size grading.

Source separation of V and Fe by two-stage selective leaching during V extraction from stone coal

A method of two stage selective leaching for separation of V and Fe during V extraction from stone coal was developed. In the primary procedure stone coal was roasted in an inert atmosphere at 900 °C for 1 h, and then the roasted product was leached for Fe removal in 7% (vol%) H2SO4 at 30 °C for 30 min when the liquid-to-solid ratio (L/S) was 3 : 1 mL g−1. The Fe removal residue was then leached under the conditions of 20% (vol%) H2SO4, L/S = 2 : 1 mL g−1, at 95 °C for 4 h. In the first stage, 98.1% Fe could be removed from the roasted product with only 2.0% V loss. In the second stage, a 97.8% V leaching efficiency was obtained, which was 10.4% higher than the leaching efficiency of the roasted product using one-stage leaching. Based on the different solubilities in acid of muscovite and FeS, the decomposition product of pyrite at high temperature in an inert atmosphere, this method admirably prevented Fe from entering the V leaching liquor, realizing source separation of V and Fe during V extraction from stone coal.

Comparison of microwave and conventional blank roasting and of their effects on vanadium oxidation in stone coal

ABSTRACT The oxidation behaviour of vanadium in stone coal and its mechanisms in conventional blank roasting (CBR) and microwave blank roasting (MBR) are investigated. In both roasting methods, vanadium oxidation rates are increased and the controlling step of vanadium oxidation reaction is changed from chemical control to internal diffusion control with increasing temperature. Compared to CBR, MBR was more efficient in vanadium oxidation under same roasting conditions. The change of controlling step occurred at the temperature range of 500–600 °C and 600–700 °C for MBR and CBR, respectively. This oxidation behaviour of vanadium was mainly dependent on the reducing materials in stone coal and their reaction characteristics. Due to efficient and the lower temperatures of mineral transformations (including reaction of reducing materials and deformation of muscovite structure), MBR had higher vanadium oxidation efficiency and lower temperature range of changing the controlling step than CBR.