Recovery of rare earths from sludges containing rare-earth elements

Recovery of rare earths from sludges containing rare-earth elements

Journal of Alloys and Compounds 425 (2006) 145–147 Recovery of rare earths from sludges containing rare-earth elements Tetsuji Saito ∗ , Hironori Sat...

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Journal of Alloys and Compounds 425 (2006) 145–147

Recovery of rare earths from sludges containing rare-earth elements Tetsuji Saito ∗ , Hironori Sato, Tetsuichi Motegi Department of Mechanical Science and Engineering, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan Received 26 September 2005; accepted 9 January 2006 Available online 10 February 2006

Abstract To develop a process for the recovery of rare-earth elements from sludges, a chemical recycling process was applied to neodymium-containing boron trioxide. It was found that the neodymium was successfully extracted as either sodium neodymium sulfate hydrate or neodymium hydroxide from neodymium-containing boron trioxide by this process. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth; Sludge; Recycling

1. Introduction Rare-earth elements are widely used in various applications and have become an essential part of modem life. One of the most common applications of rare-earth elements is in metals and alloys for such products as permanent magnets and rechargeable batteries [1]. Although the production of rare-earth compounds has significantly increased, large amounts of scrap from these compounds are being stockpiled due to the lack of a cost-effective recycling process. A number of attempts have been made to recycle rare-earth compounds, especially those used as rare-earth permanent magnet materials [2–7]. Another major application of rare-earth elements is for materials such as glass formulation and polishing powders [1]. Oxides containing rare-earth elements are generally used in these applications. To date, there has been no cost-effective method of recycling rareearth-containing oxides such as those in optical lenses and in polishing sludges. In this study we investigated the possibility of extracting rare-earth elements from oxides containing rareearth elements. Neodymium-containing boron trioxide prepared by the glass slag method was used as the oxide for this study. The experimental details of the glass slag method have been described elsewhere [8]. The rare-earth separation process was then applied to the neodymium-containing boron trioxide to separate the ∗

Corresponding author. E-mail address: [email protected] (T. Saito).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.011

neodymium from the oxide. Sulfuric acid has been reported to be effective for dissolving rare-earth alloys [2]. Thus, the neodymium-containing boron trioxide was first dissolved in sulfuric acid. The addition of sodium sulfate to the solution resulted in the formation of a precipitate. 2. Experimental The process of separating the rare earth from the neodymium-containing boron trioxide is shown in Fig. 1. First, 10 g of neodymium-containing boron trioxide was dissolved in 30 ml of an 80% sulfuric acid solution and heated to 423 K for 3 h. Next, 50 ml of a 20% sodium sulfate solution was added to the first solution, which was then heated to 333 K for 0.5 h. The precipitate formed by the addition of the sodium sulfate was collected by a filter and named specimen I. A second precipitate was obtained by the addition of 50 ml of a 20% sodium sulfate solution and 20 g of sodium hydroxide and subsequent heating to 373 K for 0.5 h. The precipitate formed by the addition of the sodium sulfate and sodium hydroxide was collected by a filter and named specimen II. The external appearances of the specimens were examined under an optical microscope. The phases in the specimens were identified by X-ray diffraction (XRD) using Cu K␣ radiation. The compositions of the specimens were determined by chemical analyses using the inductively coupled plasma (ICP) method.

3. Results and discussions Fig. 2 shows the external appearances of the boron trioxide and neodymium-containing boron trioxide used in this experiment. Unlike the boron trioxide, the color of the neodymiumcontaining boron trioxide is purple. The chemical analyses revealed that the neodymium content of the neodymiumcontaining boron trioxide was 25.5 wt%. It is known that the

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T. Saito et al. / Journal of Alloys and Compounds 425 (2006) 145–147

Fig. 1. Flow of the rare-earth separation process.

presence of rare-earth ions can change the color of a compound through a change in the electronic structure. Thus, the change in the color of the boron trioxide is believed to be due to the presence of the neodymium ion (Nd3+ ) in the neodymium-containing boron trioxide.

Fig. 2. External appearances of (a) the boron trioxide and (b) the neodymiumcontaining boron trioxide used in this experiment.

Fig. 3. XRD pattern of (a) boron trioxide and (b) the neodymium-containing boron trioxide.

Fig. 3 shows the XRD pattern of the neodymium-containing boron trioxide. For comparison, the XRD pattern of the boron trioxide is also shown. The XRD pattern of the boron trioxide shows diffraction peaks of the B2 O3 phase. However, halo-like peaks are also evident in the pattern, suggesting that the boron trioxide contains some amorphous phase, most probably an amorphous B2 O3 phase, together with the crystalline B2 O3 phase. On the other hand, the neodymium-containing boron trioxide shows no indication of the B2 O3 phase but exhibits two broad halo peaks. This suggests that the boron trioxide is amorphous. The broad halo peak in the XRD pattern of the neodymiumcontaining boron trioxide is slightly shifted to a higher angle than that in the XRD pattern of the boron trioxide. This is due to the change in the chemical composition of the boron trioxide. The rare-earth separation process was applied to the neodymium-containing boron trioxide to separate neodymium from the neodymium-containing boron trioxide.

Fig. 4. XRD pattern of specimen I obtained by the rare-earth separation process from neodymium-containing boron trioxide.

T. Saito et al. / Journal of Alloys and Compounds 425 (2006) 145–147

Fig. 5. XRD pattern of specimen II obtained by the rare-earth separation process from neodymium-containing boron trioxide.

Fig. 4 shows the XRD pattern of the first precipitate (specimen I) prepared by the rare-earth separation process. Unlike the case of the neodymium-containing boron trioxide, clear diffraction peaks are noted in the XRD pattern of specimen I. The diffraction peaks are well indexed to the sodium neodymium sulfate hydrate (NaNd(SO4 )2 ·H2 O) phase in the XRD pattern. This indicates that the neodymium in the neodymiumcontaining boron trioxide was successfully extracted as sodium neodymium sulfate hydrate by the rare-earth separation process. The chemical analysis revealed that the neodymium content of the specimen I was 37.9 wt%. This value is comparable to the neodymium content of sodium neodymium sulfate hydrate (38.2 wt%), confirming that the precipitate obtained was sodium neodymium sulfate hydrate. Since the amount of precipitate obtained was 5.0 g, the amount of neodymium in the precipitate was 5.0 × 0.379 = 1.90 g. The recovery rate of neodymium from the 10 g of neodymium-containing boron trioxide (neodymium content: 25.5 wt%) was 1.90/2.55 × 100 = 74.5%. Although three-quarters of the neodymium in the neodymium-containing boron trioxide was extracted by this process, the resultant precipitate consisted of a so-called sodium neodymium sulfate double salt, which is a relatively complex compound. It would be desirable for the neodymium to be extracted as a simple compound such as neodymium oxide or neodymium hydroxide. Thus, a further study was carried out to seek the possibility of obtaining other neodymium compounds. As described earlier, a second precipitate, specimen II, was obtained by the addition of sodium sulfate and sodium hydroxide to the solution (see Fig. 1). Fig. 5 shows the XRD pattern of specimen II. The XRD pattern is well indexed to the

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neodymium hydroxide (Nd(OH)3 ) and sodium sulfate (Na2 SO4 ) phases. This indicates that the neodymium was successfully extracted from the neodymium-containing boron trioxide as neodymium hydroxide. Since neodymium hydroxide is not a complex compound, it is more desirable as the final product than sodium neodymium sulfate hydrate. The chemical analysis revealed that the neodymium content of the specimen II was 10.5 wt%. Since the amount of precipitate obtained was 13.9 g, the amount of neodymium in the precipitate was 13.9 × 0.105 = 1.46 g. The recovery rate of neodymium from the 10 g of neodymium-containing boron trioxide (neodymium content: 25.5 wt%) was 1.46/2.55 × 100 = 57.3%. Although more than half of the neodymium in the neodymium-containing boron trioxide was extracted by this processing technique, the rest of the neodymium still remained in the solution. It is necessary to improve the recovery rate by optimizing the processing conditions. Furthermore, it was found that the neodymium hydroxide was attracted by commercially available high-performance Nd-Fe-B magnets. Magnetic separation from the mixture of neodymium hydroxide and sodium sulfate can therefore be applied to the separation of the specimen II precipitate prepared by the rare-earth separation process. 4. Conclusions Our study has demonstrated that this rare-earth separation process is useful for the separation of neodymium from neodymium-containing boron trioxide as either sodium neodymium sulfate hydrate or neodymium hydroxide. This process could be applied to other rare-earth containing sludges by optimizing the processing conditions. Acknowledgement This work was partly supported by Grant-in-Aid for Scientific Research (C) No. 15510076 from the Japan Society for the Promotion of Science. References [1] J.M. Tourre, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Applications, Dresden, 1998, p. 31. [2] J.W. Lyman, G.R. Palmer, High Temp. Mater. Proc. 11 (1993) 175. [3] N. Sato, Y. Wei, M. Nanjo, M. Tokuda, Shigen-to-Sozai 113 (1997) 1082. [4] T. Uda, Mater. Trans. 43 (2002) 55. [5] Y. Xu, L.S. Chumbley, F.C. Laabs, J. Mater. Res. 15 (2000) 2296. [6] T. Saito, H. Sato, S. Ozawa, J. Yu, T. Motegi, J. Alloy Compd. 235 (2003) 189. [7] T.H. Okabe, O. Takeda, K. Fukuda, Y. Umetsu, Mater. Trans. 44 (2003) 798. [8] S. Ozawa, H. Sato, T. Saito, T. Motegi, J. Yu, J. Appl. Phys. 91 (2002) 8831.