Mineral resources are important basic raw materials for human survival and development. About 95% of China's energy and 80% of industrial raw materials are directly or indirectly derived from mineral resources. With the advancement of technology and the development of industrial production, people are The demand for mineral resources will further increase. This undoubtedly means that the number of tailings will be growing tailings not only pollute the environment, endanger human life, but also take up a lot of land, consumption of capital, the impact of the economic efficiency of enterprises. However, based on historical and technical reasons, such as outdated equipment and poor sorting technology, tailings still contain a lot of useful materials, even rich mines, and the idleness of these resources has caused huge waste. Therefore, the recovery of valuable elements is of great significance for the comprehensive utilization of tailings [1 , 2] . This paper mainly introduces the effect of hydrochloric acid pickling on the enrichment of rare earth elements in Baotou Steel's rare earth tailings, which lays a foundation for the next step of extracting single rare earth elements.
First, the experiment
(1) Main raw materials
The main raw material used in this experiment is Baotou Steel's rare earth tailings. The chemical composition analysis is shown in Table 1, and the mineral composition is shown in Figure 1.
Table 1 Elemental content of rare earth tailings in Baotou Steel
M 2 O
Al 2 O 3
K 2 O
Na 2 O
Figure 1 Phase analysis of raw material minerals
(two) experimental principle
Mainly based on the following chemical reactions, to achieve the purpose of removing impurities and enriching rare earths.
The dissolved rare earth reacts with hydrogen fluoride in the solution to form a rare earth fluoride and precipitates in the undecomposed rare earth mineral. Since the REF 3 solubility product (K sp = 8 Ã— 10 - 16 ) is smaller than the CaF 2 solubility product (Ksp = 217 Ã— 10 - 11 ), the chemical reactions represented by the formulas (1), (2), and (3) Continuously, it can remove a lot of iron and calcium, and the loss rate of rare earth is very small  .
(3) Experimental methods
On the basis of crushing and sieving, the effects of pickling concentration, pickling time, solid-liquid ratio and stirring time on the enrichment effect were investigated. The optimal parameters were determined by a large number of comparative experiments.
Second, the results and discussion
(1) The effect of hydrochloric acid concentration
The tailings were pickled with different concentrations of hydrochloric acid, and the effect of the obtained hydrochloric acid concentration on the content of each element in the pickling slag is shown in Fig. 2.
Figure 2 Relationship between element content and hydrochloric acid concentration in pickling slag
It can be seen from Fig. 2 that as the concentration of hydrochloric acid increases, the weight loss rate of iron decreases first and then increases, which is caused by the following reaction:
When the concentration of hydrochloric acid is 60%, CaSiO 3 is completely reacted, and the content of iron in the pickling slag reaches a maximum. When the concentration of hydrochloric acid increases, the reaction with FeS and Fe 2 O 3 becomes more and more complete. The iron content in the washing slag is gradually reduced, so the leaching rate of iron is gradually increased. The changes of calcium and silicon are basically similar. When the concentration of hydrochloric acid is less than 60%, silicon is mainly precipitated in the form of silicic acid. Calcium is dissolved by hydrochloric acid and removed in the form of CaCl 2 . Therefore, when the concentration of hydrochloric acid is less than 60%, silicon is used. And the content of calcium in the pickling slag gradually decreases. When the concentration of hydrochloric acid is more than 60%, with the increase of the concentration of hydrochloric acid, the iron can be more and more fully reacted with hydrochloric acid, and the content of calcium and silicon is obviously increased, mainly CaF 2 and SiO. The form of 2 exists, and therefore, the larger the concentration of hydrochloric acid, the more obvious the effect of removing impurities.
(2) Influence of solid-liquid ratio
In the unstirred state, different solid-liquid ratios were controlled, and the comparative test was as shown in Fig. 3. It can be seen from Fig. 3 that the weight loss rate increases first and then decreases with the increase of solid-liquid ratio. When the solid-liquid ratio is 1:3, the weight loss rate is the largest, reaching 48.6%. From Figure 3, the following can be obtained. Some conclusions: (1) Before the solid-liquid ratio is 1:3, the weight loss rate increases sharply because the amount of hydrochloric acid is relatively small and the mineral reaction is incomplete; (2) after the solid-liquid ratio is 1:3, The weight loss rate is getting smaller and smaller because the acid reacts with iron in the mineral to form a Fe(OH) 3 colloidal solution, and a part of silicon exists in the form of colloidal silicic acid. As the amount of acid liquid increases, there is no stirring. In the case of liquid-solid separation, some of the acid cannot be sufficiently contacted with the mineral, so that the mineral cannot be sufficiently reacted.
Figure 3 Relationship between solid-liquid ratio and mineral weight loss rate
(3) The influence of mixing time
Controlling different mixing times, the comparative test is shown in Figure 4.
Figure 4 Relationship between stirring time and mineral weight loss rate
It can be seen from Fig. 4 that as the stirring time increases, the weight loss rate gradually increases. When the stirring time is 6h, the curve tends to balance, because during the pickling process, as the stirring time increases, the mineral and hydrochloric acid increase. It can fully react. When the stirring time is 6h, the reaction is basically complete, so the weight loss rate does not change much with the increase of the stirring time. The change in the content of each element in the pickling slag is shown in Fig. 5 with the stirring time.
Figure 5 Relationship between stirring time and element content in pickling slag
It can be seen from Fig. 5 that the calcium content is first decreased and then increased, because at the beginning of the reaction, the calcium compound increases with the stirring time, and the reaction becomes more and more sufficient. When the stirring time is 3 hours, the calcium is increased. reaction is substantially complete, with the lower iron content, calcium content gradually increased with increasing cerium content increased mixing time, when the stirring time is longer than 5h, little change.
Third, the conclusion
(1) In the case of grinding of the tailings of Baotou Steel, it is better to wash with concentrated hydrochloric acid. The solid-liquid ratio is 1:3, which can make the leaching rate of iron reach 91.97%.
(2) The optimum stirring time for pickling is 6h, the iron content in the pickling slag is reduced to 212%, and the strontium content is increased to 515%.
(3) This method is simple in process and suitable for the low-grade rare earth minerals of the coated steel tailings, which lays a foundation for the further extraction of rare earth elements, especially strontium.
 Hu Tianxi. Cong Ming, Chen Mingjie, et al. Some progress in the comprehensive utilization of tailings in China [J]. Foreign metal ore dressing , 2006, 43 (2): 152181.
 Xiang Wu. A new approach to comprehensive utilization of tailings [J]. Nonferrous Metals Minerals and Exploration, 1998, 7 (2).
 Wu Wenyuan. Rare Earth Metallurgy [M]. Beijing: Chemical Industry Press, 2005. 482491.
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