Received 12 February 2016; accepted 8 March 2016; published 11 March 2016
Rare earth elements (REEs) have become vital and indispensable components of many advanced products, devices and technologies. The production of rare earth minerals in China increased by an average of 40% annually between 1978 and 1989, and had provided more than 90% of the world’s supply since 2001  -  . Ion-adsorp- tion rare earth deposition was firstly discovered in Ganzhou, Southern China, in 1970. Unlike other rare earth minerals which are in solid state mineral phase and tend to be associated with the radioactive elements uranium and thorium, ion-adsorption rare earth minerals occur at a simple trivalent cationic state, which is simply adsorbed onto clays and could be readily extracted by a simple leaching technique with an aqueous electrolyte solution via anion-exchange process  . The extraction of ion-adsorption rare earth is carried out by surface/ mountain-top mining followed by tank or heap leaching with sodium chloride ammonium sulfate solution.
Ion-adsorption rare earth deposition has a relatively low REEs concentration ranging from 10 to a few hundred parts per million by weight  . Using heap leaching techniques, it is estimated that for the production of 1 tons (t) rare earth oxide, 2000 tons tailings are disposed into adjacent valleys and streams. Due to this, the rare earth miners in Ganzhou region have generated 191 million tons of tailings  . These hazardous tailings are a major environmental concern causing increased water and soil loss, deterioration of water quality and potentially other geological disasters. Therefore, the re-utilization of the tailings not only reduces their impact of environmental pollution but also can be used to develop highly valued end-products for the associated industries  . To realize it, the tailings should be characterized and pre-treated before their re-utilization.
In the present work, the chemical and mineral compositions of the ion-adsorbed rare earth tailings from the Southern China were analyzed in order to determine the feasibility of the tailings as raw materials for the ceramic industry. The treatments of the tailings, such as sieving, iron removal treatment, were also considered.
2. Materials and Methods
The tailings were collected from the ion-adsorption rare earths area in Ganzhou, Southern China. Representative samples were prepared by following Chinese standard method of mixing (China Standard Specification, GB2007.1-87).
The chemical element analysis of samples was performed with an XRF spectrometer (PANalytical Axios- mAX, Netherland).
The bulk mineral composition was performed with Mineral Liberation Analyzer (MLA 650, Czech Republic), which was equipped with environment SEM (FEI, Quanta 650), EDS (Bruker Quantax 200) and mineral parameters automatic analysis software of (MLA2.9). For preparation of grain mounts in epoxy blocks, 10 - 100 g of dried mineral sands is required. Aliquots of 10 - 20 g of a sample will be mixed with pure graphite powder to prevent particle cohesion and embedded in epoxy blocks. The preferred diameter of the round blocks is 30 mm, diameters of 25 mm. The other operational parameters are reported in Reference  .
In the pre-treatment processes, the tailings was sieved sequentially by the copper standard sieves with the sieve sizes of 0.8 mm, 0.4 mm, 0.25 mm, 0.12 mm, 0.075 mm, 0.043 mm. The −0.043 mm fraction of the tailings was used for the process of iron removal. Firstly, the sample was fed to the high gradient magnetic separator (SSS-I-145)   to remove magnetite and ilmenite, followed by a bleaching process of the remained samples (fine kaolinite) using hydrogen peroxide  or sodium dithionite  as the leaching agent.
3. Results and Discussion
3.1. Characterization of the Tailings
3.1.1. Chemical Composition of Tailings Characterized by XRF
The chemical composition of the tailings sample was characterized by XRF. The data is presented in Table 1. The rich chemical composition in oxides in mine tailing are SiO2 (78.32 wt%), Al2O3 (17.50 wt%) and Fe2O3 (2.11 wt%). However, Fe2O3 is not what we want because it gives a brown color to ceramics. Therefore, iron removal is required to the tailings until the Fe2O3 content is under the maximum limit (1.8 wt%) for ceramic applications according to the Chinese standard GB/T 14563-1993.
3.1.2. Mineral Composition of Tailings Characterized by MLA
The tailings are the mixtures of many minerals. Generally, the minerals can be distinguished out by Mineral Liberation Analyzer (MLA). Table 2 shows the mineral composition of the tailings. Quartz and kaolinite are the major minerals of the tailings, which take up 91.98 wt% of the tailings. Quartz and kaolinite are commonly utilized in ceramic application. Therefore, they can be of industrial interest after a proper treatment processing. The other silicates, such as feldspar, biotite, muscovite and amphibole, are also generally used as sintering aid in ce-
Table 1. Chemical composition of the tailings measured by XRF.
Table 2. Mineral composition of the tailings.
ramic applications. The iron- and titanium-contained minerals include titanomagnetite, limonite, pyrite, ilmenite, rutile and leucoxene, which make up 1.72 wt% of the tailings. The total content of rare earth minerals is only 0.04 wt%, which include monazite, xenotime, parisite and colloidal rare earth.
The SEM images of the minerals are shown in Figures 1-5.
The chemical compositions and their concentrations of feldspar measured by EDS are K2O 7.50%, Na2O 4.62%, CaO 0.51%, Al2O3 19.59%, SiO2 67.58% and FeO 0.20% (in weight percent). Fe element is incorporated in the crystal lattice of feldspar. Most of feldspar has been weathered into kaolinite.
Quartz is the major mineral in the tailings according to the data in Table 2. Figure 2 shows the SEM images of the quartz particles. As can be seen, the quartz is associated with magnetite, muscovite, or parisite.
Magnetite is the main iron-bearing minerals in the tailings. EDS results show the chemical elements of the magnetite mineral include Fe 66.28%, Ti 3.18%, Mn 0.51%, Zn 0.49%, K 0.01%, Al 0.95%, Si 0.67% and O 27.92% (in weight percent).
The SEM image of magnetite particles associated with the TiO2 is shown in Figure 3(A). The titanomagnetite mineral forms a grid mark pattern in the cross-section of magnetite grain. Figure 3 shows that it is hard to separate titanomagnetite from magnetite by sieving separation method. Figure 3(B) shows the image of the magnetite associated kaolinite. In fact, Figure 1(D) and Figure 3(B) show that kaolinite and magnetite have the similar associated mineral structure. This associated structure results in the difficulty in the separation of magnetite from kaolinite.
Limonite is another iron-bearing mineral found in the tailings. EDS results show the chemical elements of the magnetite mineral include Fe 53.19%, Ti 0.52%, Mn 0.62%, Zn 0.08%, Ca 0.24%, Al 3.57%, Si 3.05%, Cr 0.11%, S 0.44% and O 38.12% (in weight percent).
Figure 4 shows the SEM image of limonite particles. Limonite particles found to be are granular and dust- like. The particle size is in the range of 0.1 μm - 0.08 mm. 80 wt% of limonite has particle sizes of less than 0.01 mm. And most of the dust-like limonite particles are mixed with kaolinite.
The chemical composition of ilmenite in the sample is 52.38% TiO2, 41.23% FeO and 6.40% MnO (in weight
Figure 1. SEM images of kaolinite: (A) single grains; (B) embedded monazite; (C) embedded biotite; (D) embedded magnetite.
Figure 2. SEM images of the associated minerals of (A) quartz, magnetite and muscovite; (B) quartz, muscovite and parasite.
percent). Ilmenite in the tailings is granular and very fine. The particles size distribution is in the range of 0.1 μm - 0.15 mm. Ilmenite grains are mostly associated with magnetite, kaolinite or xenotimeetc, as shown in Figure 5.
Figure 3. SEM images of the associated minerals of (A) magnetite and TiO2; (B) magnetite and kaolinite.
Figure 4. SEM image of limonite.
Figure 5. SEM images of ilmenite.
3.2. Pre-Treatment of the Tailings
The data in Table 2 show that most of the mineral in the tailing can be utilized in ceramic industry except of Fe2O3. Therefore, the pre-treatment of the tailings before re-utilization is mostly iron removal.
3.2.1. Fe Element Distribution
Kaolinite and quartz together make up 91.98% of the ion-adsorption rare earth tailings. It implies that the tailings can be re-utilized in ceramic application if the Fe content is less than 1.8% according to the requirement of TC-3 grade in the national standard GB/T 14563-1993. However, the content of Fe2O3 in the tailings is 2.11%, thus, further treatment to remove the iron is required.
Table 3 shows the distribution of Fe element in the tailings. It shows that titanomagnetite, kaolinite and limonite are the main Fe-bearing minerals in the tailings. Only titanomagnetite and limonite can be removed by magnetic separator methods. If titanomagnetite and limonite are removed, the Fe content in the tailings decreased to 1.23%, which meets the requirement of the standard of TC-3 grade for ceramic industry applications.
To ascertain the various particle sizes, the particle size distribution of the tailings is characterized by standard sieves method. Table 4 shows the particle size distribution and the grade of SiO2, Al2O3, Fe2O3 and TiO2 in the distribution. The +0.4 mm and −0.043 mm fraction are the main of the particle size distribution. The grade of SiO2 decreases with decreasing particle size. The grade of SiO2 in the +0.4 mm fraction is larger than 91.46%, implying that coarse grains are quartz and they can be separated by 0.4 mm sieve. Correspondingly, the fine grains (<0.043 mm) are kaolinite because the grade of Al2O3 increases with decreasing particle size. The change
Table 3. Distribution of Fe element in the tailings.
Table 4. Particle size distribution of the tailings and the quality of SiO2, Al2O3, Fe2O3 and TiO2.
of the Fe2O3 grade is in accord with that of Al2O3. Considering the discussion in section 3.1, it can be concluded that the iron-bearing minerals is mostly mixed with kaolinite. The methods of iron removal from kaolinite reported in reference  - can be applied in the tailings.
3.2.2. Iron Removal by Magnetic Separator
The −0.4 mm fraction of the tailings is treated by a periodic high-gradient magnetic separator (SSS-I-145). The Fe content of the sorted tailings is measure by XRF. The effect of the magnetic induction on the Fe content is listed in Table 5.
As can be seen, the Fe grade of the sorted tailings decreases with the increase of the magnetic induction. However, the ratio of iron removal is not proportional to the magnetic induction. The ratio of iron removal increases quickly with the increase of the magnetic induction before the magnetic induction is 0.6 Tesla. And then, the ratio of iron removal increases slightly (4.68%) when the magnetic induction increases further from 0.6 T to 1 T. When the magnetic induction is 0.6 T, the Fe grade of the sorted tailings is 1.06 wt% and the ratio of iron removal is 50.41%, indicating most of titanomagnetite and limonite is removed. Therefore, the magnetic induction of 0.6 Tesla is more economical.
To remove the iron in kaolinite, redox method   is attempted in the experiment. The −10 μm, −7 μm and −5 μm fractions of the tailings are obtained by settling method according to Stokes equation. The iron removal of the fine tailings is listed in Table 6.
The results in Table 6 show that the iron removal of the tailings by H2O2 treatment is insufficient in all the three fractions of the tailings, implying that most of Fe element is in the oxidized state. By using the Na2S2O4 treatment, the Fe content of the −7 μm fraction changed from 1.68% to 1.27%, which is equivalent to 25.4% removal, indicating that Na2S2O4 method is the good way to the iron removal of the fine tailings.
The ion-adsorption rare earth mining tailings in Southern China can be re-utilized because kaolinite and quartz share 91.98 wt% of the tailing. The other minerals include feldspar, muscovite, biotite, titanomagnetite, limonite and ilmenite, which are the important raw materials in ceramic industry. The particle size distribution of the tailings is bimodal. 91.46 wt% of the +0.4 mm fraction is quartz and the −0.043 mm fraction is mostly kaolinite. The two minerals can be separated by sieves. The total Fe content of the tailings is 2.11 wt%, which distributes
Table 5. Effect of the magnetic induction on the Fe content.
Table 6. Iron removal of the fine tailings by redox method.
mostly in titanomagnetite, kaolinite and limonite. The iron-bearing minerals can be removed by the periodic high-gradient magnetic separator with the magnetic induction of 0.6 T. The Fe content of the sorted tailings is less than 1.06 wt%, which meets the Chinese national standard of TC-3 grade for ceramic industry application. The Fe content in kaolinite can decrease further after Na2S2O4 treatment.
The authors gratefully acknowledge the financial support provided by the National High-tech Research and Development Program of China (Grant No. 2012AA061903).