Recovery Behavior of Rare Earth from Bayan Obo Complex Iron Ore
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- Ding, Y., Xue, Q., Wang, G. et al. Metall and Materi Trans B (2013) 44: 28. doi:10.1007/s11663-012-9762-z
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A novel process is presented for recovering rare earth from Bayan Obo complex iron ore. The iron ore was reduced and melting separated to produce iron nugget and rare-earth-rich slag. In order to investigate the influence of cooling rate on mineral components, especially the enrichment behavior of RE-containing mineral, the slag was remelted at 1673 K (1400 °C) and the liquid slags were cooled using three types of cooling conditions, water quenching, air cooling, and furnace cooling. Subsequently, the slags were leached by hydrochloric acid to evaluate the relations between leaching efficiency of rare earth and cooling conditions. The results indicated that the slags under different cooling conditions mainly contained fluorite, cefluosil, and cuspidine. The rare-earth mineral is more fully crystallized when the cooling rate of the liquid slag was decreased. The proportion of Ce (III) to Ce (IV) increases with the increase of heating time and decrease of cooling rate. It has been found that the influence of cooling rate on the leaching rate of the rare earth is slight. From water quenching to furnace cooling, the leaching rate of rare earth increases from 97.00 pct to 99.48 pct. After being filtered, filtrate can be used to produce rare-earth chloride. Leached residue, with CaF2 of 64.45 pct and ThO2 of 0.05 pct, can be used to recover CaF2 and extract nuclear source material.
The Bayan Obo ore deposit in the Inner Mongolia region of North China is one of the best known iron-LREE-niobium deposits in the world. It accounts for 35 pct of rare-earth resource of world’s proven reserves and approximately 80 pct of China.[1–3] Currently, it is mined mainly as an iron ore. After grinding to smaller than76 μm of 90 to 95 pct, a feeble magnetic powerful magnetic-flotation process is used for recovering about 70 pct of iron and less than 10 pct of the rare earth. Most of the rare metal minerals remain in the tailings. Because there is only 230 million tons of the complex iron ore left in Main and East Ore Body, and the current designed capacity is 12 million tons per year, the ore will be used up within 20 years.[4–6] Therefore, the comprehensive utilization of Bayan Obo ore has become an urgent task.
Besides physical concentration method, the blast furnace process has been used for processing of Bayan Obo run-of-mine ore.[7,8] The ore was directly loaded into blast furnace for ironmaking, and rare-earth elements remained in the slag with fluorine. The slag, containing 14 pct RE2O3, i.e., rare earth oxide, has been used for making rare-earth alloys. Investigations of the composition, slow-cooling crystallization and mineral processing of the slags have been conducted. Several mineral and metallurgical processes have been employed to concentrate the rare-earth mineral from the slag, such as gravity separation, magnetic separation combined with gravity separation, and hydrometallurgy.[9–13] Nonetheless, the running condition of blast furnace gets worse with the coke ratio increasing, the productivity dropping, and furnace lining eroded seriously; this process is no longer used.
A new process has been proposed by the authors’ laboratory for the rare earth and iron separation based on carbon-bearing pellet reduction and melting technology.[14,15] In this work, a simulation test of rotary hearth furnace was conducted using Bayan Obo complex ore and pulverized coal as raw materials. The Bayan Obo complex iron ore was reduced and melting separated to produce iron nugget and rare-earth-rich slag. Subsequently, the slag was leached by hydrochloric acid to recover rare earth. The main objective of this research is to investigate the behavior of rare earth in the carbothermic reduction and melting separation process, and to develop a reasonable slag treatment model to make full use of the rare-earth-rich slag without slag modification.
Chemical Composition of Bayan Obo Complex Iron Ore (Weight Percentage)
Analysis and Characterization
The compositions of iron ore, rare-earth-rich slag, and leached residue were measured by chemical analysis. The X-ray diffraction (XRD) patterns of the samples were detected with a Rigaku diffractometer (DMAX-RB 12 kW; Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation; the scanning angles were in the range from 10 deg to 90 deg (2θ) at a speed of 10 deg/min. The mineralogy of slag was observed by scanning electron microscope (SEM; JEOLJSM-6480A, JEOL Ltd., Toyo, Japan) with an energy-dispersive spectroscope (EDS; JEOL JSM-6480A; JEOL Ltd.). The volume fraction and average grain size of RE-containing phase were obtained with the help of Quantiment 500 image analyzer (Leica Microsystems, Wetzlar, Germany) according to color differences (average of 10 fields). In this article, XPS is utilized to study the change of rare-earth elements valence in different cooling types. The XPS analyses were carried out with a Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, U.K.) using a monochromatic Al Ka source (15 mA, 14 kV) and a hybrid lens with a spot size of 700 × 300 μm2.
The recovery of rare-earth experiments involved three procedures, namely, slag remelting, slag cooling, and acid leaching. First, the slag was crushed to a particle size less than 1 mm. Slag remelting was then conducted in a high-purity graphite crucible (40 mm diameter and 50 mm height). Approximately 25 g slag powder was placed into the crucible and heated in a laboratory-scale resistance box furnace. When the furnace reached the desired temperature of 1673 K (1400 °C), the liquid slag was held for 30 minutes to homogenize the composition completely.
The three types of slag cooling processes are water quenching, air cooling, and furnace cooling. For water quenching, the liquid slag was taken out of the furnace and quickly poured into the water, and the generated slag granules were collected at the bottom of the water tank. For air cooling, the crucible for holding the slag was promptly removed from the furnace and cooled to ambient temperature. For furnace cooling, the remelted slag was left in the crucible to cool to room temperature in the furnace. The cooling rate was approximately 10 K·s−1.
The Preparation of Rare-Earth-Rich Slag and Its Property
Chemical Composition of Rare-Earth-Rich Slag (Weight Percentage)
The melting temperature of slag is only 1386 K (1113 °C) measured by the automatic slag melting point tester. Therefore, keeping the temperature at 1673 K (1400 °C) for 30 minutes in the furnace is long enough to realize the homogeneous of liquid slag.
XRD Analysis of the Rare-Earth-Rich Slag with Different Cooling Conditions
Microstructure and Phase Characteristics of the Rare-Earth-Rich Slag with Different Cooling Conditions
EDS of Different Phase in Fig. 4 (mol pct)
Dark Gray Phase
Light Gray Phase
As is shown in Figure 4, divergent crystal structures exist under different cooling conditions. Due to the rapid cooling rate, the RE-containing phase is fine dispersing floc or spicule (Figure 4(b)). The grain size of air cooling slag (Figure 4(c)) is smaller than that of furnace cooling slag (Figure 4(d)). Furnace cooling slag has the lowest cooling rate, and the minerals are crystallized fully into big crystals of strip shape about 70 micrometers wide and hundreds of micrometers long. CaF2 in the slag has a fine crystallization in elliptic shape, and the size of which in the furnace cooling slag can reach 30 μm or so.
The Grain Size and Crystallized Area of RE-Containing Phase with Different Cooling Types
Mean Length (μm)
Mean Breadth (μm)
Mean Area Ratio (pct)
The quantity and size of the crystalline granules increase with the cooling rate slowing down, which provides an effective way to decrease power consumption of beneficiation. In light of the discussions presented above, to obtain a high recovery ratio of rare earth from the rare-earth-rich slag and to enrich RE-containing minerals fully to match the physical mineral separation for rare earth recovery, a lower cooling rate during slag cooling is suggested. In the industrial application, to decrease the cooling rate of liquid slag as much as possible, the complete cooling method in slag is most effective to improve the enrichment of RE-containing phase. However, this method may require a reduction of the rotational speed of rotary hearth furnace, as well as a decrease of the production efficiency and an increase of the costs.
XPS Analysis of the Rare-Earth-Rich Slag with Different Cooling Conditions
In the Bayan Obo iron ore, the content of cerium oxide accounts for above 50 pct of total rare earths in Bayan Obo iron ore. Therefore, Ce is the most primary research object in this study.
Calculations of Ce (III)/Ce (IV)
Percentage of Ce(III)
In the experiments, the slag was held in the graphite plate, and Ce (IV) can be reduced by carbon during heating in the furnace and in the water quenching. The cerium (III) may be oxidized to cerium (IV) partially, existing as cerium oxide (CeO2). Therefore, the relative intensity of the peak V, characteristic feature of Ce (III), increases with the increase of heating time and decrease of cooling rate. In contrast, the intensity of peak V′, characteristic features for Ce (IV) oxidation state, decreases with the heating time increasing and the cooling rate decreasing.
Acid-Leaching Behavior of the Rare-Earth-Rich Slag after Different Cooling Conditions
After being crushed to particle sizes smaller than 74 μm, the slag was charged into conical flasks, and then was leached with hydrochloric acid. The effects of the cooling type, the concentration of hydrochloric acid, the ratio of the solid to the liquid, and the leaching time on the leaching efficiency of RE elements from slag were investigated.
Consequently, an enriched cerium compound is prepared. The leachate satisfies the needs for further process to recover rare earth by precipitation, using oxalic acid.
Characteristic of the Leached Residue
Chemical Composition of Leached Residue (Weight Percentage)
Balance of Materials
The recovery rate of Fe is about 97.52 pct, nearly all the rare-earth enriched in slag during the melting separation process through theoretical analysis, and the rare earth was separated successfully from iron. In the acid-leaching process, for original slag, 98.70 pct rare earth was dissolved into the acid solution, and 88.12 pct F, 90.15 pct Th are in the leached residue.
Mechanism of Leaching Process
The Characteristic of Rare-Earth-Rich Slag with Different Cooling Types
Leaching Rate (pct)
Mean Area Ratio (pct)
Effect of Crystallization and Valence State on the Rare Earth Leaching Rate
In the cooling process of rare-earth-rich slag, the ratio of Ce (III) to Ce (IV) increases with the cooling rate slowing down. Meanwhile, the crystallinity has the similar trend. The noncrystalline substance is in an activated metastable state, and the crystalline mineral is at the lowest energy state. Therefore, more external energy is needed to break the structure of crystalline mineral than the noncrystalline mineral in the leaching process.
For water quenching, slag was rapidly cooled by water, and the formation and growth of the crystalline core was inhibited. Therefore, lots of noncrystalline substances were formed with crystal defect in this slag. It could be conclude that water quenching played a role in activating the slag in the cooling process. This activated slag was more favorable for extracting rare earth by hydrometallurgy process. While the rare-earth-bearing mineral and fluorite of good crystallinity were formed in the original and air cooling slags. Likewise, furnace-cooling slag was cooled in the furnace, and it can achieve better crystallinity. In the acid-leaching process, more energy is needed to destroy the crystalline structure.
The percentage of Ce (III) in the original slag was higher than the water quenching slag, which will counteract partially the disadvantage of higher energy consumption to destroy the crystalline structure in the acid leaching process. For these reasons, under the same leaching conditions, the rare-earth leaching effect on water quenching slag was similar to that of original slag as shown in Table VII.
As is shown in Table VII, there was no significant difference between the crystallinity of original slag and air cooling slag. The greatest difference between them was found where the proportion of Ce (III) is higher in the air cooling slag. In addition, the mean area ratio is 25.68 pct in the furnace cooling slag, which requires more effort to destroy the crystalline structure than water quenching slag. However, since the percent of Ce (III) is 84.91 pct, the leaching rate can reach more than 99.48 pct.
Through the analysis mentioned above, the leaching rate of rare-earth elements from slag relates to their valence state in normal cooling conditions. Under certain conditions, the liquid slag was cooled rapidly, and the leaching rate of rare earth from slag relates to the crystallization of rare-earth elements. Practically, before being discharged from the rotary hearth furnace, the liquid slag needs to be cooled down and solidified, so the water quenching method is infeasible.
In order to obtain a high recovery rate of rare earth from the slag through hydrometallurgy, the ratio of Ce (III) to Ce (IV) should be controlled to a certain range, and a reducing atmosphere during slag cooling is suggested. In the industrial application, a layer of carbon material (such as coal and graphite) is laid on the hearth to keep the reducing atmosphere in the rotary hearth furnace.
Summary and Conclusions
A novel process for recovering rare earth from Bayan Obo iron ore is developed. The rare earth and iron separated well at 1673 K (1400 °C) for 12 minutes. The RE2O3, with a content of 14.19 pct, is enriched into cefluosil (7[(Ca, Ce, La, Nd) 2 SiO4] (F, O)10).
With the decrease of cooling speed and the prolongation of heating time, the crystallinity of cefluosil is significantly increased and the ratio of Ce (III) to Ce (IV) has similar trends too. From water quenching to furnace cooling, the mean area ratio of RE-containing phase in slag increases from 9.17 pct to 25.68 pct and the Ce (III)/Ce (IV) ratio changes from 1.98 to 5.63.
The optimal acid-leaching parameters of rare earth elements from the original slag are suggested as follows: the hydrochloric acid concentration, the ratio of solid to liquid, and the leaching time are 4 mol/L, 1:8, and 4 hours, respectively. After being filtered, the rare earth solution can be used to extract rare earth, and the leached residue, with CaF2 of 64.45 pct and ThO2 of 0.05 pct, can be used as the raw material for recovering CaF2 and ThO2.
The leaching rate of rare earth from slag relates mainly to the valence state of rare earth elements in normal cooling conditions. For water-quenching slag, the leaching efficiency of rare earth closely relates to the crystallization of rare-earth minerals.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51090381) and Baotou Steel for supplying Bayan Obo iron ore for the current work.