3.1. Materials and Reagents
The used original samples (acid leaching feed) were from the flotation preconcentration process of vanadium-bearing shale (the content of V
2O
5 in vanadium-bearing shale is 0.83%) in Shangluo, Shaanxi, China. For the three-stage open circuit procedure of the pre-concentration process, the used reagents were sulfuric acid (pH regulator), sodium silicate (dispersant), sodium fluosilicate (inhibitor), and dodecylamine (collector). The main chemical components, valence distribution, and phase of the original samples are shown in
Table 4,
Table 5 and
Table 6, respectively.
Table 4 shows that the content of V
2O
5 in flotation concentrate is 1.96%, which indicates that flotation greatly improves the grade of V
2O
5 and has an obvious preconcentration effect. Nowadays, the high cost of vanadium extraction is mostly due to the low grade of vanadium. The flotation preconcentration process is undoubtedly of great significance to improve the vanadium grade and reduce the cost of vanadium extraction. Additionally, the original samples are based on SiO
2, with the content of SiO
2 as high as 86.49%, followed by an Al
2O
3, which grade is 3.78%. According to this, the original samples should contain a large dosage of quartz (SiO
2) and aluminosilicate minerals. Additionally, alkalinity is R = (CaO + MgO)/(SiO
2 + Al
2O
3) = 0.021. Therefore, from the alkalinity calculation results, it is concluded that the original samples belong to acid ore and are suitable for extracting vanadium through acid leaching.
Table 5 shows that most of the vanadium is in the form of low valence V (III), with the distribution rate as high as 74.55%, whereas the distribution rates of V (IV) and V (V) are very low, 22.73% and 2.72%, respectively. However, V (III) mainly occurs in the lattice of aluminosilicate minerals, replacing Al (III) in six-times-coordinated Al–O octahedron in the form of isomorphism and entering the mineral lattice [
24]. V (III) is restrained by the stable layered aluminosilicate crystal structure. Its state is particularly stable, making it extremely difficult to dissolve in acid and aqueous solutions. Only by breaking the Al–O bond in the lattice of aluminosilicate minerals can vanadium be released and then oxidized into high-valence vanadium, which can form vanadium that is easily dissolved in acid and aqueous solutions. Therefore, in the acid-leaching process, it is necessary to break the mineral lattice and convert low valence of vanadium into high valence of vanadium.
Table 6 shows that vanadium is mainly hosted by muscovite (49.36%), garnet (21.10%), and biotite (17.26%). A lower dosage of vanadium occurs in iron oxide (3.81%) and chlorite (3.03%). In addition, trace vanadium is dispersed in wollastonite, ilmenite, and V
2O
5.
Among them, garnet is an island aluminosilicate mineral, which is difficult to dissolve in acidic or aqueous solution. Therefore, it is difficult to extract vanadium under the conditions of non-roasting and atmospheric pressure. In order to extract vanadium from insoluble aluminosilicate and improve the leaching efficiency of vanadium and, subsequently, the process of vanadium extraction, research on enhanced acid leaching must be discussed.
In these experiments, the following reagents were used: sulfuric acid (H2SO4, 98%), manganese dioxide (MnO2, 1%), and calcium fluoride (CaF2, 3%), which were, respectively used as a leaching agent or pH regulator, an oxidant, and a fluorinating aid (leaching aid). The sulfuric acid was purchased from Chengdu Kelong Chemical Reagent Factory, and the leaching aids were purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. All the reagents were of analytical grade.
3.3. Leaching
The thermostat water bath cauldron was preheated before leaching experiments, and 100 g of original samples were uniformly sampled each time and placed into a beaker (500 mL). A certain dosage of leaching aids, H2O, and H2SO4, were added into the beaker, thoroughly mixed, and then placed into the thermostat water bath cauldron. After leaching experiments, the pulp was filtered, dried, and weighed. The volume and concentration of the leaching solution were obtained to calculate the leaching efficiency. Each group of experiments was repeated thrice, and the results were averaged.
3.5. AFM Analysis
AFM is usually used to describe the surface properties of minerals through imaging technology. The height of the surface cross-section of the leaching residues under the conditions of the original samples, enhanced acid leaching, and non-enhanced acid leaching were observed by AFM (Dimension Icon, Bruker AXS, Germany, Beijing Representative Office, China). The scanning range was 5 μm × 5 μm, and images obtained by AFM were analyzed using the NanoScope 1.5 software to reveal the changes in the mineral surface morphology and cross-section height under different action conditions. Before scanning, the three samples were prepared by water dispersion, and each action condition was consistent with the best parameters of leaching experiments.
3.7. SEM Analysis
SEM is often used to observe changes in the mineral surface micromorphology. First, three samples of the original samples, non-enhanced acid leaching residues, and enhanced acid leaching residues, all of them with a particle size of −37 μm, were sprayed with spray-gold treatment for 45 s. Subsequently, the samples were photographed with a Czech TESCAN MIRA LMS electron microscope, Philips, Netherlands. The accelerating voltage of the instrument was 3 kV, and the selected magnification was ×10,000. Finally, the high-energy electron beam of SEM was targeted at the samples to stimulate a variety of physical information. Through reception, amplification, and display imaging of the physical information, the mineral surface morphology could be observed.