Technical Note Separation of Monazite from Placer Deposit by Magnetic Separation Kwanho Kim and Soobok Jeong * DMR Convergence Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-42-868-3576 Received: 23 January 2019; Accepted: 19 February 2019; Published: 28 February 2019 Abstract: In this study, mineralogical analysis and beneficiation experiments were conducted using a placer deposit of North Korea, on which limited information was available, to confirm the feasibility of development. Rare earth elements (REEs) have vital applications in modern technology and are growing in importance in the fourth industrial revolution. However, the price of REEs is unstable due to the imbalance between supply and demand, and tremendous efforts are being made to produce REEs sustainably. One of them is the evaluation of new rare earth mines and the verification of their feasibility. As a result of a mineralogical analysis, in this placer deposit, monazite and some amount of xenotime were the main REE-bearing minerals. Besides these minerals, ilmenite and zircon were the target minerals to be concentrated. Using a magnetic separation method at various magnetic intensities, paramagnetic minerals, ilmenite (0.8 T magnetic product), and monazite/xenotime (1.0–1.4 T magnetic product) were recovered selectively. Using a magnetic separation result, the beneficiation process was conducted with additional gravity separation for zircon to produce a valuable mineral concentrate. The process resulted in three kinds of mineral concentrates (ilmenite, REE-bearing mineral, and zircon). The content of ilmenite increased from 32.5% to 90.9%, and the total rare earth oxide (TREO) (%) of the REE-bearing mineral concentrates reached 45.0%. The zircon concentrate, a by-product of this process, had a Zr grade of 42.8%. Consequently, it was possible to produce concentrates by combining relatively simple separation processes compared to the conventional process for rare earth placer deposit and confirmed the possibility of mine development. Keywords: rare earth elements; monazite; placer deposit; beneficiation; ilmenite; magnetic separation 1. Introduction Rare earth elements (REEs) are a group of elements belonging to the lanthanide series, which ranges from lanthanum to lutetium; the group also contains scandium and yttrium with similar chemical properties [1,2]. Due to their unique properties, REEs are widely used in applications such as magnets, battery alloys, and metal alloys. The importance of REEs is growing due to their increasing application in modern technology and their consequent role in the fourth industrial revolution. However, despite this increasing demand, the supply of REEs is not stable due to regionally biased production. According to the United States Geological Survey on REE production, more than 80% of the world’s supply since 1998 has come from China, and this ratio increased to 95% in the mid-2000s [3]. In 2009, the export quota and tax restrictions imposed by the Chinese government resulted in an imbalance in the demand and supply, leading to a dramatic increase in REE prices. In response to rising prices, several companies in the United States, Australia, Brazil, and Russia began to produce REEs. Worldwide REE prices have almost returned to normal after five years because of excessive REE supply and the abolition of Chinese restrictions in 2015. Many new companies have Minerals 2019, 9, 149; doi:10.3390/min9030149 www.mdpi.com/journal/minerals Minerals 2019, 9, 149 2 of 11 shut down operations due to economic problems. As a result, China’s influence on the supply of REEs will likely increase, increasing the probability of the return of the aforementioned instability in production. REEs do not exist as natural metals and are contained in various minerals in a substituted state. However, out of over 250 REE-bearing minerals, only three (bastnasite, monazite, and xenotime) are currently produced on a commercial scale. Bastnasite is the main REE-bearing mineral in large mines (Mountain Pass, CA, USA, and Bayan Obo, China), and the other two exist as heavy mineral sand deposits [4]. To develop beneficiation flowsheets of various REE-bearing deposits, extensive research has been devoted by many researchers [5–11]. In particular, the beneficiation process for placer deposits is well established and includes a combination of gravity, magnetic, electrostatic, and, at times, flotation processes [12–14]. Apart from the well-established beneficiation process for placer deposits, the application of the unit process should be modified according to the mineralogy of the sample. In this study, a placer deposit from North Korea was used as a feed sample. However, the mineralogy data and beneficiation characteristics of the North Korean placer deposit sample are not well known. Therefore, in this study, the mineralogy of a placer deposit in North Korea was investigated and a beneficiation process, especially magnetic separation, was applied to separate valuable mineral selectively and examine the feasibility of resource development project. 2. Materials and Methods 2.1. Materials The feed sample used in this study was obtained from a placer deposit of the Sam-Cheon area in North Korea. According to geological data available on North Korea, there are two REE mines, namely Wolbong and Ryeonsan, where the ore body comprises sandy-gravel layers in alluvium [15]. Figure 1 shows the geological map of Rimjingang belt in the middle Korean Peninsula and the location of two REE mines and Sam-Cheon area [16]. As shown in Figure 1, the samples were collected from the bottom of the active river channel and sand bar in the Sam-Cheon area near two REE mines. It was confirmed that in this mining site, the gravity separation methods using a spiral concentrator and shaking table were preliminary conducted to concentrate heavy minerals in the placer deposit. Therefore, in this study, the preliminary concentrated heavy mineral sample was used as the feed sample for the beneficiation process. A total of 200 kg of feed sample was well mixed and divided into four portions, and one of four portions was selected and further divided into four portions repeatedly. The representative sample for sample analyses was obtained by repeating this procedure. Many analytical instruments were used to identify the feed sample before and after the experiment. X-ray diffraction (XRD; X’pert MPD, Philips, Malvern, UK) was used for mineral constituent analysis; inductively coupled plasma optical emission spectroscopy (ICP-OES; Optima-5300DV, Perkin Elmer, Waltham, MA, USA), inductively coupled plasma mass spectroscopy (ICP-MS; Elan DRC-Ⅱ, Perkin Elmer, Waltham, MA, USA), and X-ray fluorescence spectrometer (XRF; MXF-2400, Shimadzu, Kyoto, Japan) were used for the chemical composition analysis; and the mineral liberation analysis (MLA; MLA650F, FEI, Hillsboro, OR, USA) was used to evaluate the degree of liberation and mineral constituents. For ICP-OES and ICP-MS analyses, a sample pretreatment method was basically carried out according to the USGS method for analyzing rare earth elements by ICP-MS. 0.1 g of sample put into carbon crucible with 0.6 g of Sodium peroxide (Na2O2; Sigma–Aldrich) The carbon crucible heated at muffle furnace about 550 °C for 30 minutes. After cooling the carbon crucible, 10 ml of 25% nitric acid (HNO3; 70%, Sigma–Aldrich) was added to dissolve elements for 15 minutes. The remaining 25% nitric acid was evaporated, and then 1% nitric acid and deionized water (DI) were added to make a 100 ml solution. This solution was diluted 10 times or 1000 times according to the concentration of elements to be measured. Minerals 2019, 9, 149 3 of 11 (b) (a) (c) Figure 1. The geological map of the Sam-Cheon area (a) and photographs of sampling location of feed sample (b,c). WB—Wolbong mine; RS—Ryeonsan mine; SC—Sam-Cheon area. Reproduced with permission from Kim et al. [16], Global Geology; published by World Geological Editorial Department, 2012. Minerals 2019, 9, 149 4 of 11 2.2. Methods A laboratory-scale cross-belt magnetic separator (CBMS; Model EE112, Eriez, Erie, PA, USA) that could modulate the applied magnetic field by changing the current supply to the separator was employed to recover iron oxide and REE-bearing minerals. The sample was fed at approximately 200 g/min for 20 min using a vibrating feeder. The moving velocity of the feed carry conveyor was about 7.3 cm/s, and that of the cross-belt conveyor to recover magnetic minerals was about 31.5 cm/s. The applied magnetic intensity was increased from 0.4 T to 1.4 T. After recovering magnetic products at 0.4 T, the remaining non-magnetic sample was fed into a magnetic separator that was adjusted to a 0.2 T higher magnetic intensity for further separation. The magnetic separation tests were carried out sequentially in this way six times until the magnetic intensity reached 1.4 T. The batch test was conducted using 10 kg of feed sample. In this process, besides the target minerals, a gravity separation process for recovering zircon as a by-product was added. To recover high-density zircon from non-magnetic product, a shaking table (No.13 Wilfley table, Humphreys, Jacksonville, FL, USA) was used. The operating conditions were adjusted based on the general conditions. The angle of the shaking table, shaking amplitude, and water flowrate were varied from 0.5° to 4°, from 10 to 20 mm, and from 5 to 15 L/min, respectively. 3. Results and Discussion 3.1. Feed Sample Analysis The XRD pattern profiles of the feed sample in Figure 2 revealed the main constituent minerals, and mineral liberation analysis (MLA) results in Table 1 showed major and minor constituent minerals that had not been detected by XRD analysis owing to their low content. As a result, monazite was the main REE-bearing mineral and ilmenite was the main iron oxide mineral.