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China's scheelites account for 70.5% of the country's total tungsten ore reserves, most of which are skarn-type. Tungsten often coexists with molybdenum, copper, sulfur, lead, zinc, arsenic and other elements. In addition to tungsten, copper, molybdenum, sulfur is the target elements that can be recovered as by-products, non-target elements can enter the tailings through beneficiation, Liu et al. through the study of the migration and transformation of toxic and harmful elements in the tungsten ore beneficiation system, the results show that lead, zinc, arsenic are the main non-target toxic and harmful elements and highly concentrated in the tailings, after calculation, they account for 96.64%, 79.21%, 85.27% of the mass of the element in the tailings, respectively.
In this paper, the occurrence forms of toxic and harmful elements (lead, zinc and arsenic) in the skarn-type stungsten ore tailings reservoir of a mining company in Jinpingshan, Hubei Province, were compared and sat at different times, namely fresh tailings just discharged, tailings submerged in the reservoir (referred to as immersed tailings), and dry tailings (aged tailings) aged in the reservoir, in order to reveal the distribution of toxic and harmful elements in the tailings at different times, and to find measures to prevent the loss of toxic and harmful elements from the tailings to the environment and the safe comprehensive utilization.
Source and sampling method of tailing sand
The tailings samples were taken from the fresh tailings, immersed tailings and aged tailings in the tailings reservoir of the white tungsten ore processing plant of a mining company in Jinpingshan, Hubei Province, and the sampling method and sample status of the tailings are shown in Table 1.
Table 1
Extraction method of toxic and harmful elements in tailing sand
There is currently no unified definition and classification method for the existence of heavy metals, and the BCR three-step extraction method proposed by the European Community Standards Bureau and the improved BCR continuous extraction method proposed by Rauret et al. are used to extract the occurrence form of heavy metals in tailings in tailings of tailings ponds. At present, this method is recognized as simple and easy, high recovery, good reproducibility, and no serious channeling between morphomorphes, and can divide the chemical forms of heavy metals into exchangeable states, reducible states, oxidizable states and residual states, respectively using HAc and NH2OH· HCl and H2O2 + NH4OAc were extracted, and the extraction methods of each morphology were as follows.
The part of the element that can be extracted from neutral salts or mild acids is called the exchangeable state. This form is the most active part of toxic elements, which is easily transferred to the aqueous phase and is easy to cause water pollution. Test method: accurately weigh 1.000g of tailings into a centrifuge tube (50mL), add 0.1mol/L acetic acid solution (40mL) and shake at room temperature (20°C±2°C) for 16h; Then centrifuge on the centrifuge for 20min (speed 3000r/min), transfer the separated supernatant into a 50mL volumetric flask, add 1mL concentrated HNO3, and store it in a fixed volume; The residue is washed with deionized water (20 mL), shaken for 15 min, centrifuged on a centrifuge for 20 min (speed 3000 r/min), discard the supernatant, and retain the solid residue.
A form in which an element can be released under reducing conditions is called a reducible state. The behavior of this morphological element is greatly affected by redox conditions, and the chemical activity is obviously controlled. Test method: add 50mL of newly prepared 0.5mol/L hydroxylamine hydrochloride (pH 1.5) to the centrifuge tube with the residue of the previous step, and stir for 12h at a speed of 30r/min; Then centrifuge on the centrifuge for 20min (speed 3000r/min), transfer the separated supernatant into a 50mL volumetric flask, add 1mL concentrated HNO3, and store it in a fixed volume; The residue is washed with detached water (20 mL), shaken for 15 min, centrifuged on a centrifuge for 20 min (speed 3000 r/min), discard the supernatant, and retain the solid residue.
Elements in the solid residue retained in the previous step in the form of complexes (chelated) such as organic, sulfide, etc., and substances that can be oxidized by oxidants are called oxidizable states.
Test method: 30% H2O2 50mL was added to the centrifuge tube with the residue of the previous step, and the reaction was carried out for 1h at room temperature; Then heat the reaction in a water bath at 85 °C ± 2 °C for 1h, and stir intermittently within the first 30 minutes until the volume is less than 3mL, and then add 30% H2O210mL for the reaction; When the reaction solution is concentrated to 1mL, add 25mL of 1.0mol/L ammonium acetate solution (adjust the pH value to 2 with concentrated HNO3), centrifuge on the centrifuge for 20min (speed 3000r/min), separate and move the supernatant into a 50mL volumetric flask, add 1mL of concentrated HNO3, and store in a fixed volume; The residue is washed with detached water (20 mL), shaken for 15 min, centrifuged on a centrifuge for 20 min (speed 3000 r/min), discard the supernatant, and retain the solid residue.
The residual state of the elements in the tailings is mainly silicate minerals, which are relatively stable in the environment because the crystal structure and particle size of the minerals have a strong fixation effect on toxic and harmful elements. The content of toxic and harmful elements in the residual state can be obtained by subtracting the sum of the three forms obtained by the above method from the total amount of corresponding elements in the tailings sample. The extracted solution stored at 4 °C obtained in this test was uniformly determined by inductively coupled plasma atomic emission (ICP-AES) spectrometry to determine the content of toxic and harmful elements lead, zinc and arsenic. The instrument used is ICAP6300 (ThermoFisher production plant).
The contents of various forms of toxic and harmful elements in the tailings reservoir of white tungsten ore of a mine Co., Ltd. in Hubei Province were obtained by ICP-AES spectrometry, and the distribution characteristics of different forms of lead, zinc and arsenic were mainly analyzed on this basis. The content and proportion of lead in different forms of tailings are shown in Table 2.
Table 2
It can be seen from Table 2 that whether it is exchangeable, reducible or oxidizable, with the extension of the tailings in the tailings reservoir, the total amount of lead in the tailings is decreasing, indicating that the tailings pond has a certain purification effect on the unstable lead in the discharged tailings. However, the proportion of lead in the residue state is increasing, and the residual state in the aged tailings has accounted for 41.44%, indicating that the lead in the aged tailings has relative stability.
However, it is worth noting that not only fresh tailings, but also immersed tailings, the proportion of exchangeable lead content is high, and there is still 24.61% exchangeable lead in aged tailings, and attention should be paid to prevent the migration of lead to the environment. The content and proportion of zinc in different forms of tailings are shown in Table 3.
Table 3
It can be seen from Table 3 that with the extension of the storage time of tailings, the total amount of zinc in the exchangeable, reducible and oxidizable states in the tailings of the tailings of the tailings reservoir basically shows a decreasing trend, reflecting the same distribution law as lead. Similarly, with the extension of the storage time of the tailings, the proportion of zinc content in the residual state increased, indicating that the zinc stability in the aged tailings was better.
However, it is worth noting that the oxidizable zinc in the tailings at different times accounts for the highest proportion, which may be due to the fact that a large part of the zinc in the tailings is more widely contained in the primary sulfide sphalerite (the secondary mineral in the Edong tungsten ore is sphalerite), although it is relatively stable under general environmental conditions, but it can be released under oxidation conditions and affect the surrounding environment. The content and proportion of arsenic in different forms of tailing sand are shown in Table 4.
Table 4
It can be seen from Table 4 that whether it is exchangeable, reducible or oxidizable, with the extension of the arsand placement time in the tailings reservoir, the total amount of arsenic element in the tailings is decreasing, while the residual arsenic is increasing, which is the same as the distribution law of lead elements, reflecting that the tailings pond has a certain purification effect on the arsenic in the discharged tailings; Whether it is fresh tailing, immersed tailings or aged tailings, the proportion of exchangeable arsenic in the tailings is very low, all below 6%, while the proportion of arsenic content in the residual state is very high, all above 50%, and the aging tailings are as high as 66.23%, the reason may be that the lattice structure of arsenic in the raw ore is difficult to release arsenic, and the arsenic morphology has been relatively stable in the beneficiation process, so arsenic mainly exists in a stable residual state in the tailings.
Tailing sand pollution prevention and comprehensive utilization enlightenment
According to the morphological distribution of toxic and harmful elements in tailings at different time periods shown in this research work, the following enlightenment is obtained for the prevention and control of tailings pollution and its comprehensive utilization: In general, whether it is exchangeable, reducible or oxidizable, with the extension of the place time of tailings in the tailings reservoir, the total amount of toxic and harmful elements in the tailings is decreasing, while the residual state is increasing, reflecting that the tailings pond has a certain purification effect on toxic and harmful elements. Therefore, the tailings in the sedungsten ore tailings reservoir must be aged for a long time in the tailings reservoir and then comprehensively utilized, which will help prevent secondary pollution.
All kinds of toxic and harmful elements are in the exchangeable state of fresh tailings, because the company's beneficiation wastewater is directly discharged into the tailings reservoir without purification. Therefore, a certain degree of pretreatment of mineral processing wastewater can be considered in the circulating tank, which will be conducive to the reduction of pollution load in tailings reservoirs.
The three main toxic and harmful elements (lead, zinc and arsenic) in the tailings of the tailings of the tungsten mine tailings reservoir are generally relatively stable, and the normal comprehensive utilization conditions are safer for the environment, of which the most stable is arsenic, and the poor stability is lead, but there is still 24.61% exchangeable lead in the aged tailings, and attention should be paid to preventing the migration of lead to the environment during the comprehensive utilization process. In addition, zinc oxidizable states account for the highest proportion in different periods of tailings, and attention should be paid to preventing the oxidative precipitation of zinc deposited with primary sulfides in tailings during comprehensive utilization.
Using the BCR three-step extraction method proposed by the European Community Bureau of Standards and the improved BCR continuous extraction method proposed by Rauret et al. in parallel with ICP-AES spectroscopy, the occurrence forms of lead, zinc and arsenic associated with skarn-type scheeliteite of a mining company in Jinpingshan, Hubei Province were compared in the fresh tailings, immersed tailings and aged tailings in the tailings reservoir, and the results showed that the unstable morphological contents of lead, zinc and arsenic in the tailings decreased with the extension of the sand placement time. It shows that the tailings reservoir has a certain purification effect on polluting elements.
These three toxic and harmful elements are high in the exchangeable state of fresh tailings, so a certain degree of pretreatment of mineral processing wastewater in the circulating tank can be considered to reduce the proportion of exchangeable state.
The three toxic and harmful elements in the tailing sand mainly exist in a relatively stable form, and the normal comprehensive utilization conditions are relatively safe for the environment. Among them, the most stable tailings are arsenic, which may be due to the lattice structure of arsenic in the raw ore as arsenic sand that is difficult to release; Lead element with poor stability in tailings can be considered, and it can be considered to reduce its unstable form as much as possible and increase the proportion of residual state by purifying the mineral processing wastewater and prolonging the placement time of tailings in the tailings reservoir, so as to ensure the safety of comprehensive utilization of tailings. In addition, the oxidizable state of zinc in the aged tailings accounted for 40.30%, indicating that most of the zinc in the tailings was deposited in the primary sulfide sphalerite, so the release of zinc under strong oxidation conditions should be prevented during the comprehensive utilization of tailings.