Vanadium-containing rock, also known as black shale type vanadium ore, coal, is a very important part of our vanadium resources, lithology carbonaceous shale, carbonaceous shale siliceous rocks containing or enriched vanadium, nickel, molybdenum, uranium, copper, selenium, gallium, silver, and other 60 kinds of noble metal elements, throughout the country in Hunan, Hubei, Sichuan, Guizhou, Guangxi, Zhejiang, Anhui, Gan, Shaanxi, Gan, Jin, In the ancient strata before the Middle Devonian in more than 20 provinces. Among the rocks in China, the main deposits of industrial deposits are vanadium. According to relevant data, the reserves of V2O5 in Hunan, Hubei, Jiangxi, Zhejiang, Anhui, Guizhou and Shaanxi provinces have exceeded 1.2 billion tons (excluding new proven reserves of the country in recent years), as vanadium and titanium magnetic iron ore of vanadium in total reserves seven times more than the sum of the rest of the world reserves of vanadium pentoxide. The main producing areas of vanadium-bearing rocks in China are shown in Table 1.
In the early 1960s, China began to carry out vanadium extraction from vanadium-bearing rocks. In the 1970s, sodium-salt roasting and vanadium extraction processes were widely used. The total recovery rate of the whole process was usually less than 50%, and the recovery rate was low. The effective use of resources is poor. In the production process, a large amount of soot and HCl, Cl2, SO2 and other corrosive exhaust gases, which are highly polluted by the surrounding environment, are discharged, which makes the environmental treatment after production more difficult, and is sacrificed as a typical The production process at the expense of the environment has been completely banned in China. In recent years, some research and production departments in China have carried out a lot of useful explorations in improving the total recovery rate of V2O5, simplifying the process flow, reducing the generation of pollutants, and achieving clean vanadium production, in order to find a way to completely replace the traditional vanadium extraction process. Clean production process. This evaluation is mainly for the segmental acid leaching and vanadium extraction process research and test conducted by the Guiyang Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources. It uses the comprehensive index method to conduct a preliminary analysis of the resources and pollutants indicators, and the degree of clean production expected by the process. Evaluation, explore a quantitative evaluation method for whether the process can achieve clean production in the laboratory test stage, and provide reference and reference for the next step of expansion test and industrial test to select the corresponding process.
First, the basis of clean vanadium extraction process
Mastering the change state of vanadium in the vanadium occurrence state and vanadium extraction process is an important basis for formulating a reasonable vanadium-bearing process for vanadium-bearing rock, improving the recovery rate of vanadium resources, and developing a low-cost clean vanadium extraction process.
(1) Vanadium occurrence status
Mineralogical studies show that the composition of vanadium-bearing rocks in different regions of China is complex. The vanadium occurrence and valence state vary among different regions or different mines in the same region, but most of the vanadium is mainly present in illite. Among the clay- like minerals, most of them exist in the clay mineral dioctahedral sandwich layer in the form of V3+, and only Al3+ in the Al-O octahedron is replaced by the isomorphism. V4+ is generally metamorphism of vanadium-bearing rocks. After weathering leaching or oxidative calcination, V4+ in the original rock is usually in the form of oxide (VO2), vanadium porphyrin (VO2+) or vanadate in vanadium porphyrin, and is present in aluminosilicate. Salt clay minerals (such as illite, kaolinite, etc.), and VO2 of Al3+ in the Al-O octahedron replaced by the isomorphism. Because the vanadium-containing aluminosilicate clay mineral structure is very stable, it is difficult to be directly dissolved by water, acid and alkali. To extract this vanadium, the crystal structure of these vanadium-containing minerals must be destroyed first, so that vanadium is present in the crystal structure. Release it and oxidize and convert it.
Vanadium-bearing rocks are formed in a reducing environment. V5+ is unlikely to exist in this environment, and a small amount of V5+ may be present in samples of natural weathering (oxidation) zone (oxidized ore) in some mining areas. From the basis of crystallography, since the V5+ ionic radius and the cation radius of the aluminosilicate clay mineral are quite different, there is no premise for the formation of the isomorphism. It can be considered that these small amounts of V5+ are mainly in the free state V2O5 or the bound state. The vanadate form of (xM2O·rV2O5) is adsorbed in iron oxides and clay minerals, soluble in acids and bases, and can be directly leached by acid or alkali.
(2) Clean vanadium extraction mechanism
The study on the chemical migration of the Earth's elements shows that the H+ dissociated from the hydration and hydrolysis during the chemical weathering process is of great significance for the removal of the base ions and silicon of the aluminosilicate minerals in the original rock. Oxidation of some non-oxidized minerals (such as sulfides) in minerals during the weathering process causes the minerals to decompose, while simultaneously activating the oxidized minerals, thereby promoting their migration and transformation. Due to the existence of a large amount of H+ in this process, H2CO3, H2SO4, etc. can be formed with CO2, SO42-, etc. in water, and the existence of H+, in turn, causes H2CO3 to dissociate into H+ and HCO3-, so that the weathering process is always in a bias. In the acidic medium environment, the primary rock is subjected to deep chemical weathering, and finally the crystal structure of the aluminosilicate mineral is destroyed, and the elements constituting the mineral form various compounds to undergo geochemical migration or residue. It can be seen that the presence of acidic medium is the most important reason for the destruction of the crystal structure of aluminosilicate minerals.
In the acidic medium, when H+ enters the aluminosilicate clay mineral lattice, H+ will replace the position of V3+ to replace the Al3+ ion, and the ionic radius will change. The lattice energy of Al-OH octahedron is better than that of Al-OV lattice. It can be smaller and has a tendency to form a more stable structure. However, due to the presence of H+ ions, the Al-OH bond in the aluminosilicate clay mineral aluminoxy octahedron is easily dissociated in a strong acid medium, causing the minerals to undergo a chemical process similar to natural chemical weathering. . Therefore, the presence of H+ will cause V3+ in the aluminosilicate clay mineral to be released into the solution, which can be oxidized to V4+ ions, which can be extracted by subsequent processes:
(wherein x represents an oxide bonded to V2O32+)
Second, clean vanadium extraction process - segmented direct acid leaching vanadium
Based on the characteristics of geochemical behavior of V3+ in acidic media, the Guiyang Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources has successfully studied the vanadium extraction technology by direct acid leaching (Fig. 1). The total recovery of vanadium is generally over 80%. The traditional roasting process is greatly improved. At the same time, due to no roasting in the whole process, the problem of pollution of corrosive gases such as HCl, Cl2, SO2 and the like caused by roasting in the conventional vanadium extraction process is solved.
Figure 1 Segmental direct acid leaching vanadium process
The segmented direct acid leaching vanadium extraction process has the advantages of simple technical process, low cost, high vanadium recovery rate and low environmental pollution. However, there are still some problems to be solved in the process: First, in order to obtain a higher V2O5 leaching rate, it is necessary. Consumption of a large amount of acid (H2SO4, HF, H2SiF6, etc.); Second, the purification of impurities such as impurities, Fe3+ reduction and pH adjustment process consumes a large amount of chemicals, especially ammonia, which leads to the generation and treatment of ammonia nitrogen wastewater; Other metal elements associated with the ore will inevitably enter the leachate together during the acid leaching process. Under the current technical and economic conditions, these metal elements are difficult to achieve comprehensive recycling and are discharged together with the final wastewater, causing pollution problems. .
III. Evaluation of vanadium extraction process by direct acid pickling
(1) Evaluation method
The concept of Cleaner Production was first proposed by the United Nations Environment Development Agency's Industrial and Environmental Planning Activity Center in 1989. It refers to the continuous application of integrated prevention environmental strategies to production processes and products to reduce the need for humans and the environment. The risk. Clean production includes both clean production processes and clean products, that is, not only the production process is not polluted, but also the products produced are not harmful to the environment during use and final disposal. Clean production is based on pollution prevention, and the focus of industrial pollution control is changed from end-of-pipe treatment to new production mode of production process reduction. It mainly focuses on pollution control analysis of production process units to reduce pollutant emissions and reduce waste toxicity. .
Among the various mathematical evaluation models of the current clean production evaluation index system and method, the comprehensive evaluation index method, the average evaluation index method, and the fuzzy mathematics comprehensive evaluation method are the main ones. The latter two evaluation methods are subjectively determined in the evaluation model. Factor weights, if there is not a large amount of detailed testing and statistical data, it is impossible to give a weight that conforms to objective reality, so that the evaluation results have strong subjective factors; second, a large number of complex mathematical calculations are involved in the evaluation process, reducing The operability of the evaluation. As an analysis of the expected clean production level of the laboratory process, the problem can be indicated by simple quantitative or semi-quantitative evaluation conclusions based on the relevant information of the laboratory process only. Therefore, referring to the “Comprehensive Evaluation Index Method†proposed by Lu Changqing et al, the expected clean production level of vanadium in direct acid leaching is evaluated.
(II) Evaluation factors
1. Evaluation index system is determined
The quantitative evaluation indicators of enterprise clean production are divided into two categories: reverse index and forward index: one is the reverse index of energy and resource consumption and pollutant generation as the evaluation factor. The lower the value (small), the more meet the requirements of cleaner production. (such as energy consumption, water consumption, pollutant discharge and other indicators); the other is a comprehensive indicator of the comprehensive utilization of resources, reuse, etc., the higher the value of the indicator (large), the more consistent with the requirements of cleaner production (such as waste water reuse rate, comprehensive utilization of tailings, total recovery rate and other indicators).
One of the main reasons for the introduction of the direct acid leaching vanadium extraction process is to reduce and eliminate the problem of air pollution in the traditional vanadium extraction process. Whether it can achieve clean production is more reflected in the production process. Based on the characteristics of scientificity, strong pertinence, simple and easy quantification, simple calculation process, combined with the key control points of the direct acid leaching vanadium extraction process, refer to the “Vanadium Industry Pollutant Emission Standards†(Draft for Comment) related pollutants. The index proposes the production evaluation of the vanadium-bearing rock segmental direct acid leaching and vanadium extraction process. It mainly focuses on the reverse index, namely the resource index and the pollutants, and uses the corresponding indicators of the unit product to achieve the expected clean production achievability of the process. Conduct an evaluation. See Table 2 for the evaluation factors and evaluation points of the clean production of the direct acid leaching and vanadium extraction process.
Table 2 Evaluation factors and evaluation points of clean production of segmental direct acid leaching vanadium extraction process
2. Evaluation index
One of the most notable features of the “Comprehensive Evaluation Index Method†is the use of an index method to “standardize†the raw data of the evaluation indicators, so that the evaluation indicators can be converted into quantities that can be compared with each other on the same scale. The evaluation index is divided into a single evaluation index, a category evaluation instruction, and a comprehensive evaluation index.
Single evaluation index: Calculated by using the reference value of the corresponding individual indicator of the analog process as the evaluation standard. Its calculation formula is
In the formula, Qi is a single evaluation index; Di is a single evaluation index value (actual or design value) of the process; Ai is a reference value of a single indicator of the analog process.
Category evaluation index: calculated based on the arithmetic mean of each individual index. Its calculation formula is
Where Cj is the category evaluation index; n is the number of individual items under the category indicator.
Comprehensive evaluation index: In order to avoid the subjective influence in determining the weighting coefficient, a comprehensive index that takes into account the extreme value or the maximum value of the maximum is used to ensure that the evaluation is as objective and comprehensive as possible, and can overcome the shielding of the evaluation results by individual evaluation indicators. The calculation formula is
In the formula, Icp is the comprehensive evaluation index of clean production; Qi, max is the maximum value in the evaluation index; Cj, a is the average value of the category evaluation index; m is the number of category indicators under the evaluation index system.
According to the comprehensive evaluation index, the comprehensive evaluation index is divided into four grades by the classification system. According to the level reached by the comprehensive production comprehensive evaluation index Icp, it can provide a reference relative standard for whether the process can achieve clean production (Table 3).
Table 3 Process cleaning production level evaluation
(III) Evaluation of clean production of vanadium extraction by direct acid leaching
1. Process data and reference process
According to the process flow shown in Figure 2, according to the process test analysis and calculation results, the resource and pollutant indicators in the process test are shown in Table 4 and Table 5, respectively.
Table 4 Segmental direct acid leaching and vanadium extraction process resource indicators
Note: Both are the same for the V2O5 output.
Table 5 Contaminant index t/t of segmental direct acid leaching and vanadium extraction process
For the same mining area, the salt-free decarburization roasting-acid leaching, catalytic calcification roasting-acid leaching, sodium roasting-acid leaching, oxygen pressure-acid leaching process as the reference process, obtained in the process of each process test The corresponding data was used as a reference evaluation index (Table 6).
Table 6 Different process reference evaluation indicators t/t
Note: 1 power consumption unit is kW·h/t; 2A～C raw ore V2O5 grade 1.00%, D original ore V2O5 grade 0.658%.
In the evaluation of the pollutant index of the process, because there is no corresponding indicator for the clean production of vanadium industry, the relevant indicator limits in the “Vanadium Industry Pollutant Discharge Standard†(Draft for Comment, hereinafter referred to as “Standardâ€) As a reference value.
2. Evaluation index
The calculation results according to the calculation formulas of the individual evaluation index, the category evaluation index and the comprehensive evaluation index for clean production are shown in Tables 7 to 9.
Table 7 Individual evaluation index of each process resource
Table 8 Fractional acid leaching process pollutant evaluation index
Note: 1 The process in the table refers to the direct acid leaching and vanadium extraction process; 2 The standard refers to the “Vanadium Industry Pollutant Emission Standard†(draft for comments).
Table 9 Comprehensive evaluation index of each process
3. Expected clean production level evaluation
The segmental direct acid leaching vanadium extraction process resource category index value Cj indicates (Table 7E) that the effective utilization rate of the process resource is improved: it is treated with the conventional process of roasting (no salt roasting, calcification roasting, sodium roasting) V2O5 grade 1.00% of raw ore compared (Table 7A, B, C), the process of processing V2O5 grade 0.696% of raw ore resource consumption is significantly lower than the traditional process, and higher than the treatment grade V2O50.658% of the original ore oxygen pressure acid leaching process (Table 7D); the process's pollutant category index is slightly larger than the Standards (Draft for Comment) pollutant category index (Table 8), suggesting that emission levels still need improvement.
According to the analysis of the comprehensive evaluation index value of the direct acid pickling vanadium extraction process Icp (Table 9, E), the process is expected to basically achieve clean production (Table 3); combined with the single evaluation index of the process (Table 7, E) and Compared with the corresponding index of the oxygen pressure-acid leaching process (Table 10), some process parameters in the process (Table 11) were appropriately adjusted, and the process is expected to fully achieve clean production.
Table 10 Comparison of individual index of different acid leaching processes
Table 11 Parameters of the proposed direct acid leaching vanadium extraction process recommended t/t
Note: 1 oxygen pressure - acid leaching using oxygen, segmental direct acid leaching using NaClO3 as oxidant; 2 units are t/t unless otherwise noted.
The segmental direct acid leaching vanadium extraction process is basically at the same level as the “Standard†(Draft for Comment) pollutant comprehensive evaluation index (Table 9). This feature has two meanings: one is to indicate the sewage discharge of the process and the “standard†(request According to the emission limit, in terms of pollutants, the clean production evaluation system adopted in this study is at the same level of clean production; the second is due to the “standard†(draft) emission limit. Mainly based on the roasting process, the comprehensive evaluation index value of pollutants, Icp falls into the scope of the process that should be eliminated (Table 3), the significance of which is that there is still a certain gap between the “standard†(draft for comment) and the realization of cleaner production. Fully consider the need for cleaner production in the vanadium industry and make certain modifications to meet the emission limit standards for cleaner production.
Fourth, the conclusion
(1) Using the comprehensive index method to evaluate the degree of clean production of the vanadium extraction process, based on the individual indicators of the comparable projects in the vanadium extraction process, the indicators have certain comparability, reflecting certain scientific and realistic. The evaluation composite index can qualitatively and comprehensively describe the actual state and level of clean production expected by the process, and can help the research or production department to purposely select the corresponding process for the next experimental study.
(2) The comprehensive index mainly involves the centralized calculation of individual indicators of each evaluation project. The formula is simple, easy to grasp, easy to calculate, and operability. Through the comparison of individual index indicators, process developers can clearly understand the process itself. Levels and issues to promote the selection and smelting process in the laboratory research phase to increase the expected efforts to achieve cleaner production.
(3) The selection of the clean production evaluation method should be matched with the whole process index system. Although the comprehensive index evaluation method can give the evaluation results intuitively and succinctly, it does not consider the constraint relationship between the selected specific indicators. The correlation between the evaluation indicators is not considered enough, and the contribution of the individual indicators to the clean production level of the whole process is not considered. Therefore, the comprehensive index method can be used as a reference for the expected clean production level of laboratory processes, but it should be carefully applied to the clean production level evaluation of industrial production process.
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