Vanadium is a transition metal element, distributed widely dispersed in nature, it is also known scattered elements. Vanadium is widely used in the iron and steel industry, nonferrous metals, chemicals, alloys, superconducting materials, automobile, etc. are indispensable elements. The addition of a certain amount of vanadium to steel, non-ferrous metals and alloys can change its microstructure, greatly improve the wear resistance and red hardness of steel, reduce the weight of materials and prolong the service life. The vanadium catalyst is produced in the chemical industry at a low price. Stable, anti-poisoning performance; at the same time, the colorful color of vanadium compounds can be used to make pigments, paints, etc.; in superconducting materials, vanadium and silicon, gallium compounds have higher superconducting transition critical temperature characteristics. Therefore, the comprehensive development and utilization of vanadium resources has very important strategic significance and industrial needs.
I. China's vanadium resources and its regional distribution
(1) Reserves and regional distribution of vanadium resources in China
According to the statistics of mineral reserves, as of the end of 2006, there are 18 provinces and autonomous regions in China with vanadium resources, 123 production sites, and reserves of about 34 million tons of resources (based on V 2 O 5 , the same below), and accumulated resources. The reserves are about 36 million tons. Mainly distributed in Hunan, Hubei, Anhui, Shaanxi, Sichuan, Guizhou, Hebei and other provinces, including Sichuan, Shaanxi, Hunan, Anhui and Hubei provinces, the reserves of reserves are 1, 855.9, 454.4, 384.8, 234.2 and 1.433 million t, accounting for 54.4%, 13.3%, 11.3%, 6.87% and 4.20% of the country's retained resources, respectively; the accumulated resource reserves were 1 941.4, 455.1, 385.4, 277.8 and 1.433 million tons, respectively. They accounted for 53.9%, 12.6%, 10.7%, 7.71% and 3.98% of the total accumulated resource reserves in the country. The reserves of the vanadium mines in these five provinces accounted for 90.1% of the national vanadium resources, and the accumulated resource reserves accounted for 88.9% of the national total.
China's large vanadium ore (≥1 million t V 2 O 5 ) is not much, mainly distributed in 9 mining sites in a few areas such as Shaanxi, Hunan, Sichuan and Gansu, with reserves of 16.894 million tons, accounting for 49.6% of the total reserves; Medium-sized vanadium ore (10-1 million t V 2 O 5 ) is widely distributed in 11 provinces including Sichuan, Shaanxi, Hunan and Hubei, with a total of 41 mining sites, with a reserve of 153.56 million tons, accounting for 45.0% of the total reserves; Vanadium mines (≤100,000 t V 2 O 5 ) have the largest number, with 73 mining sites, but the reserves are only 1.843 million tons. The reserves of large and medium-sized vanadium ore account for 94.6% of the national reserves, and the reserves of small vanadium ore only account for 5.4% of the national reserves.
(II) The common, associated characteristics and regional distribution of vanadium resources in China
There are fewer vanadium-rich ore in nature, mostly symbiotic and associated ore. According to statistics, there are only 30 vanadium mines alone, with a total reserve of 6.651 million tons, accounting for 19.5% of the country's total reserves; 93 symbiotic and associated vanadium mines, with reserves of 27.444 million tons, accounting for 80.5% of the total reserves. The total reserves of 1.0% of the national vanadium ore grades are 28.846 million tons, accounting for 94.6% of the total reserves. Among them, the reserves of grades ranging from 0.6% to 1.0% are 8.903 million tons, accounting for 29.2% of the total reserves. According to the data, the average grade of V 2 O 5 in vanadium resources is higher in Hubei, Shaanxi, Hunan and Zhejiang provinces, 0.89%, 0.82%, 0.80% and 0.78%, respectively, and the highest grade is above 1%. The Shangnan County mining area in Shangluo City, Shaanxi Province has a grade of more than 1.5%; these vanadium resources have high industrial utilization value and provide abundant resource reserves for the extraction of metal vanadium.
Vanadium ore mainly vanadium iron ore, coal, uranium, vanadium, vanadate mineral, rock phosphate, green sulfur vanadium ore, rock asphalt, crude oil and bauxite. Of vanadium resources it is mainly composed of vanadium iron ore and coal mines stone composition, with the value of mining vanadium ore stone coal. The vanadium iron ore is mainly vanadium- titanium magnetite. According to the general industrial requirements of minerals, the V 2 O 5 mass fraction of vanadium iron ore can be comprehensively recovered when it is 0.15% to 0.2%. China's iron ore with a V 2 O 5 mass fraction of 0.15% or more has a reserve of 21.56 million tons, accounting for 72.7% of the total reserves, mainly distributed in Panzhihua, Sichuan, Chengde, Hebei, Hanzhong, Shaanxi, and Fuyang, Hubei. Xiangyang, Guangdong Xingning and Shanxi Daixian, among them, Panzhihua is the main distribution area, with proven ore reserves of 10 billion tons and V 2 O 5 reserves of 15.78 million tons. Vanadium-titanium magnetite is currently mainly used for titanium refining, and vanadium metal is mainly extracted from steel slag during smelting. Other forms of vanadium-containing resources are not widely distributed in China, and there are not many reports.
According to statistics, the reserves of V 2 O 5 in China's stone coal are about 11.28 million tons, accounting for 37.0% of the total vanadium reserves. They are mainly distributed in Guizhou, Shaanxi, Hunan, Jiangxi, Henan, Hubei, Anhui and Zhejiang. Among them, the areas with concentrated distribution are mainly Hunan, Hubei, Zhejiang and Guizhou. The resources of the four provinces of stone coal and vanadium ore account for 53.5% of the national reserves of stone coal and vanadium reserves (in terms of V 2 O 5 ).
Second, the conventional process of extracting vanadium from stone coal
At present, vanadium-titanium magnetite and stone coal are mainly used as raw materials for vanadium extraction. Vanadium-titanium magnetite is mainly used for smelting titanium and by-product vanadium. Vanadium-bearing coal is a unique vanadium resource in China. Due to its relatively low grade, its mining and comprehensive utilization is far from enough, but vanadium-bearing coal is an important development direction of vanadium resource utilization in China.
(1) Mineralogy and existence of vanadium in stone coal
Stone coal is a coal resource that exists in ancient strata and is formed by the action of lower organisms such as algae and fungi in a shallow sea environment. Compared with general coal, stone coal has the characteristics of high ash, high sulfur, low carbon and low calorific value. It is both an energy source and a potential polymetallic mineral resource, mainly based on V metal. Some stone coals are associated with metal elements with high industrial value such as Ag, Cu, Mo, Na, Ni, U, Zn; in some horizons, one or several associated elements reach the industrial separate mining grade or boundary grade. It can be mined separately as a mineral resource.
The presence of vanadium in stone coal is various and generally divided into three types, namely vanadium mica , vanadium-bearing goethite, hematite and carbonate, vanadium-containing tourmaline and kaolin . Vanadium is found in vanadium mica in most stone coals, and is associated with Si, Al, and K. The vanadium-bearing goethite, hematite and vanadium (companion) are mostly Fe; carbonate minerals. It contains many elements such as Al, Ba, Ca, Cu, Fe, K, Mg, Na, P, Pb, Si and Zn. Vanadium has various valence states in these minerals. In vanadium mica, vanadium is usually present in V(III) and V(IV), and V(III) is the majority. The trivalent vanadium can replace the trivalent aluminum into the silicate mineral crystal lattice in the form of a homogeneous phase, and the tetravalent vanadium can also exist in the silicon tetrahedral structure in the form of a homogeneous phase. In vanadium-bearing hematite and vanadium kaolin, vanadium mainly exists in the form of adsorption, mainly V(IV) and V(V).
The key to the choice of vanadium ore smelting method is determined by the state of vanadium in such ore. If the vanadium in the stone coal is mainly in an adsorbed state, it may be directly leached with an acid or alkali solution, so that the vanadium is dissolved in the solution in the form of various vanadate ions, or an oxidizing or reducing substance may be added to assist the leaching; The vanadium is mainly present in the silicate mineral crystal lattice in the form of a homogeneous phase. Therefore, such ore is difficult to be leached. To leaching trivalent or tetravalent vanadium, the crystal structure must first be destroyed and allowed to exist in the crystal structure. Vanadium is released. Therefore, it is a prerequisite for vanadium smelting to find out the state of vanadium in the ore (including the presence of various compounds and minerals of vanadium, the valence state and its distribution). Since most of China's stone coal is difficult to be immersed in vanadium ore, many researchers are working on how to release vanadium from silicate crystals in an economical and simple way. At present, the vanadium extraction process mainly includes a fire method-wet method and a wet method.
(2) Fire method-wet method
The fire method-wet method is a technology widely used in the industrial extraction of vanadium from stone coal, mainly including sodium roasting-water immersion process, calcification low-sodium roasting-alkali leaching process, blank roasting-alkali dip process ( Direct calcination) and acid roasting ice immersion process.
The sodium roasting-water immersion process is the most widely used process in the industry. The process technology is mature, the basic principle is to convert the multivalent vanadium into a water-soluble sodium salt by calcination using NaCl or Na 2 CO 3 as an additive, such as Na 2 O·yV 2 O 5 , NaVO 3 , and then sodium. The calcined product is directly immersed in water to obtain a vanadium-containing leaching solution, and then ammonium chloride is added to carry out neutral vanadium precipitation, and the precipitate is calcined to obtain crude V 2 O 5 . The reaction during the roasting process is as follows:
Using sodium roasting-water immersion process, the recovery rate of vanadium is low, only 40% to 60%, and harmful gases such as Cl 2 , HCl and SO 2 are generated during the sodium roasting process, which is harmful to the environment.
The calcification low-sodium roasting-alkali leaching process is to add the additive CaO in the traditional sodium roasting process, so that the vanadium in the stone coal is oxidized and then combined with CaO to form calcium vanadate, and then leached with a Na 2 CO 3 solution, and the calcium is more soluble. Small CaCO 3 , vanadium enters the solution in free state, and the final vanadium leaching rate can reach 67.6%. The reaction mechanism of the calcification low sodium roasting-alkali leaching process is as follows:
After calcification roasting, sulfuric acid leaching can obtain more than 85% vanadium leaching rate. The vanadium recovery rate of the calcification low-sodium roasting-alkali leaching process is still not high, but the addition amount of NaCl is reduced, and the atmosphere is still polluted.
The blank roasting-alkali leaching process (direct roasting) refers to the use of oxygen in the air as the oxidative power to directly destroy the crystal structure of the vanadium mineral, thereby oxidizing vanadium to V(V) and converting into soluble vanadate and metavanadate; The calcined product was leached with a NaOH solution. The blank roasting-alkali leaching process avoids the acid gas pollution caused by sodium roasting and saves the additive, but the leaching time must be ensured that the leaching rate of vanadium can reach 75% or more in more than 3 hours.
The vanadium leaching rate of sodium roasting and blank roasting process is not high, so researchers have discussed the feasibility of adding acid roasting-water leaching process. The process is to add 10% sulfuric acid during calcination, calcination for 3 hours, natural cooling and then leaching with water for 2 h, and finally the vanadium leaching rate is over 95%. For the sulfuric acid roasting process, researchers have proposed a low temperature sulfuric acid roasting-water immersion process. 2%。 The leaching rate of the vanadium leaching rate of 78.2%.
In the combined method of fire method and wet method, sodium roasting-water immersion, calcification low-sodium roasting-alkali leaching and blank roasting-alkali leaching are relatively mature, but vanadium recovery rate is low, and there are serious environmental pollution problems. In particular, the generation of harmful gases such as Cl 2 , HCl, and SO 2 , and the high concentration of ammonia nitrogen wastewater discharged in large quantities are relatively difficult problems in the current vanadium smelting industry. The vanadium leaching rate of the acid roasting-water leaching process is relatively high, which is a process worthy of further study.
(3) Total wet process
The process of extracting vanadium from stone coal by the full wet method is currently not much research, and all of them are developed around acid leaching. The acid leaching method mainly includes direct acid leaching, addition of leaching agent acid leaching and pressure acid leaching.
Direct acid leaching is the replacement of Al 3+ by H + into the silicate mineral crystal lattice, which changes the ionic radius, thereby releasing V 3+ , and V 3+ is further oxidized to V 4+ and then leached with sulfuric acid. After direct acid leaching, the V 2 O 5 recovery is between 70% and 85%. The basic principle of direct acid leaching is as follows:
Direct acid leaching relies solely on H+ action to destroy the crystal structure. Since vanadium has high morphological stability in stone coal, the effect of direct acid leaching is not satisfactory, the leaching time is long, and the leaching efficiency is low. Adding a certain reagent, ie adding a dipping agent, can promote the leaching of vanadium and obtain a higher vanadium leaching rate. When leaching stone coal with hydrochloric acid, a certain amount of ferrous salt is added to dissolve most vanadium into the solution, and the vanadium recovery rate can reach more than 85%.
Another improvement in direct acid leaching is pressurized acid leaching. The pressurized conditions improve the vanadium leaching kinetics, greatly shorten the reaction time, and the vanadium leaching rate can reach more than 90%. However, this method is highly corrosive to equipment and requires high equipment.
In recent years, there have been other reagents for direct leaching of vanadium from stone coal. Among them, sub-molten salt leaching is a new method developed for the problems of environmental pollution, high energy consumption and low vanadium conversion rate during roasting. Alkylene molten sodium compound formulation and the formulation including sodium chloride, the chloride salts with mineral oxides, such as V 2 O 5, Fe 2 O 3, SiO 2 and other reaction Cl 2, Cl 2 has higher activity and can be The mineral crystal structure is destroyed, and V(III) and V(IV) are oxidized to V(V). The vanadium leaching rate of the sub-molten salt method can reach more than 90%. The sub-molten salt leaching method shortens the reaction time with respect to direct acid leaching, and can obtain a higher recovery rate of vanadium. At the same time, the leaching solution does not contain acid, and relatively easy to be post-treated, and is a new process worthy of further improvement and development.
(4) Bioleaching technology
Bioleaching technology is environmentally friendly and simple in process. It has developed rapidly in recent years and has been tried to extract vanadium from stone coal.
The vanadium in the refractory stone coal is present in the form of a silicate. Studies have shown that the dissolution of silicate in the bioleaching process will increase the pH of the reaction system, thus affecting the bioleaching effect; the toxic effect of vanadium on bacteria is also mainly affected by pH rather than the metal element itself. The effect of toxic effects indicates that it is important to control pH during bioleaching. When cultivating a vanadium-resistant strain, V 2 O 5 , VOSO 4 , Na 3 VO 4 and NaVO 3 are used as a domestication in a medium to which organic matter is added, and buffered with a phosphate buffer to control the pH in the range of 8.0 to 8.9. The temperature is maintained between 24 and 37 ° C, and finally a better domestication effect can be obtained. Katarina et al. studied the reduction of pentavalent vanadium in spent catalyst and petroleum fly ash into tetravalent vanadium for waste detoxification and recovery of vanadium using Acidithiobacillus ferrooridans and Acidithiobacillus thiooxidans strains. FeSO 4 ·7H 2 O was added to the medium at 30 ° C. And the elemental S, the tolerance of V 2 O 5 and NaVO 3 to the two strains were 0.003 mol / L and 0.01 mol / L, respectively, and the highest vanadium tolerance to the produced tetravalent vanadium was 4 mol / L. Pradhan et al. studied the use of sulfur-oxidizing bacteria and iron-oxidizing bacteria to extract spent catalyst from the petroleum refining process by two-stage leaching. In the first stage, the pH is controlled between 2 and 3, the mass concentration of the catalyst is 15 g/L, and the leaching rates of V, Mo, and Ni are 32.3%, 58.0%, and 88.3%, respectively. Between 9 and 1.0, the catalyst mass concentration is 50 g/L, and the final metal leaching rates are 94.8% V, 46.3% Mo and 88.3% Ni, respectively. The bioleaching process is not limited to the use of conventional bacteria, and the fungus-Aspergillus niger can also leach the heavy metals V, Ni, Fe, Al, Sb in the spent cracking catalyst. Sucrose was added to the thermophilic medium, and the stirring speed was 120 r/min in a water bath at 30 ° C. The leaching rates of V, Ni, Fe, Al, and Sb were 36%, 9%, 23%, 30%, and 64%, respectively. Although the leaching rate is not high, it is much better than chemical leaching. It can be seen that the bioleaching method is feasible for leaching vanadium from stone coal, but this technology is still in the preliminary exploration stage and needs further research and development.
Third, the outlook
Due to the low calorific value, complex composition and low grade of valuable metals, stone coal has certain difficulties in its development and utilization. Vanadium is present in most of China's stone coal. Vanadium is mainly present in the silicate minerals in the form of isomorphism. It is difficult to leach. Therefore, the mineralogy and related chemical reactions of stone coal are strengthened, and the appropriate vanadium extraction method and rational development are developed. The use of stone coal is very important.
At present, the process of extracting vanadium from stone coal is relatively backward, and is still in the laboratory research and development stage in China. The scaled sodium roasting-water immersion process has serious atmospheric and water pollution, and does not meet the requirements of green technology. In addition, other high-value metals such as Mo in stone coal are not properly utilized. If you do not recycle, it will not only bring a heavy burden to the environment, but also cause waste of resources. Therefore, the development of new environmentally friendly and efficient extraction processes is a key issue that needs to be addressed in the comprehensive utilization of stone coal.
Due to the low grade of valuable metals in stone coal, the use of bioleaching technology with low cost, simple process and environmental friendliness is a good choice. However, vanadium is more toxic to the strains, and the smaller amount is more lethal. Therefore, the key to adopting the bioleaching method is the domesticated strains. If the strains are domesticated successfully, the bioleaching technology will be a considerable development. The green craft of the foreground.
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