An Alternative Alkaline Fusion Process for the Production of Heavy Rare Earth, Thorium, Uranium and Phosphate from Malaysian Xenotime

Transcription

1 An Alternative Alkaline Fusion Process for the Production of Heavy Rare Earth, Thorium, Uranium and Phosphate from Malaysian Xenotime Meor Yusoff M. S., Kaironie M.T., Nursaidatul K., Ahmad Khairulikram Z. and Nur Aqilah S., Malaysian Nuclear Agency, Bangi, Kajang, Selangor, Malaysia corresponding author: Abstract - Xenotime is a rare earth phosphate mineral which contains significant amounts of yttrium. Currently, the method of xenotime cracking involves the use of concentrated sulphuric acid, which is hazardous towards the environment. This paper presents a study where an environmental friendly, alternative process to extract rare earths is developed. This method uses diluted hydrochloric acid (HCl) during the dissolution stage to replace hazardous sulphuric acid currently in use. This change was made possible by incorporating an alkaline fusion stage prior to the acid dissolution stage, which helps improve the leachability of yttrium and other rare earths elements. Selective and fractional hydroxide precipitation stages were also included in the process to separate the yttrium from the natural radioactive uranium and thorium elements that are also present in the xenotime mineral. An added advantage to this process is the production of trisodium phosphate and uranium concentrate as by-products from the process. Key words: heavy rare earths, thorium, uranium, xenotime, alkaline fusion 1. INTRODUCTION The rare earth elements, also known as lanthanides, comprises of elements from Lanthanum to Luthetium in the periodic table along with yttrium. These elements exhibit special electronic, magnetic, catalytic and optical properties, thus having a profound effect on the final performance of technologically advanced products [1]. The rare earths can be generally grouped into the light and heavy rare earths, with the most widely used convention stating that the rare earth elements from Lanthanum (atomic number =57) to Samarium (atomic number = 62) are considered light rare earth elements (LREE), while rare elements from Europium (atomic number = 63) to Luthetium (atomic number =71), including Yttrium (atomic number = 39) are categorized as heavy rare earth elements (HREE) [2]. Due to its simplicity, this grouping convention is also the most commonly used both by rare earths production and applications in industrial complexes. Yttrium has found wide applications as a phosphor material that is used in energy efficient lighting, colourtelevision, and computer monitor and laser materials.on top of the phosphor application, yttrium is also used as stabilizers for ceramics and as superconducting materials [2,3,4]. Due to the scarcity of available HREE mineral deposit, Jack Lifton [4] had predicted that the demand for this rare earth groups would exceed its available supply in the coming five years. This is particularly true for the rare earth elements yttrium, dysprosium, terbium and europium. The higher demand for this rare earths are attributed to its applications in green technology such as; energy efficient light bulbs, wind turbine generator, and the growing importance of hybrid and electric vehicles. Rare earths mineral can come from carbonatite and placer deposits [5]. The most common rare earths minerals found in placer deposit are monazite (Ce,La,ThPO 4 ) and xenotime (YPO 4 ).In Malaysia, these minerals are produced as a by-product from the alluvial tin mining industry, and reports stated that Malaysia is one of the most important rare earths producing countries, with an estimated reserves of 30,000 tons [6,7]. A major difference between monazite and xenotime is the former is LREE mineral, whilst the latter is a HREE mineral. In order to obtain yttrium from xenotime, breaking up or cracking of the mineral is required. Sulphuric acid digestion is by far the more common and popular method for monazite mineral due to the resistance of the mineral towards caustic soda [1,3]. There are ISBN:

2 several drawbacks in using sulphuric acid digestion method. Firstly, the operation can cause environmental pollution from the release of atmospheric SO x gases, as well as sulphuric acid leaching solution to the water system. Another notable drawback is the large amount of radioactive waste produced from this process, as the volume is related to amount of undissolved solid in the postleaching stage. Drawing from prior experiences in dealing with the Asian Rare Earth and Lynas Advanced Materials plants in Malaysia, the production of radioactive waste in the process could spell controversy apart from being very costly in storing and disposal of the materials [8]. In this study, an alternative process was developed using the alkaline fusion method, which overcomes the acid leaching resistance of the xenotime mineral. Then, yttrium will be separated from uranium and thorium via selective precipitation during the leaching solution. Bearing in mind the potential of uranium and thorium as present and future strategic nuclear fuel, respectively, where they have a wide room for growth as a clean energy source (7), the invention also intends to produce uranium and thorium as important by-products of the process. 2. EXPERIMENT In this study, xenotime samples obtained from the Malaysian Mining Corporation plant in Kampar, Perak were used for the alkaline fusion process. The mineral was cracked using the alkaline fusion process; where it was first mixed with sodium hydroxide pellets and fused in a furnace. Several parameters were considered, such as the fusion temperature, time and also particle size of the mineral. The optimum fusion temperature and time were determined by varying the temperature used for each of the fusion processes. The effect of particle size was determined by grinding the mineral to 125 microns using the Fritch pulverisette and corresponding appropriate sieves. A fused complex is formed as a result of the fusion process and this complex will undergo washing to remove the loose phosphate component. The yttrium component will then be placed in a leaching reactor to be leached by dilute HCl at 80 o C. Yttrium hydroxide will be precipitated by adding ammonia to the yttrium chloride solution. A selective precipitation procedure was also done on the solution to remove uranium and thorium, which are present in the leaching solution. Yttrium oxide (yttria) was obtained by calcining the yttrium hydroxide or oxalate at 900 C. All the characterization analysis carried out in this study was done at Malaysian Nuclear Agency using Baird Ex energy-dispersive XRF spectrometer. Reference standards used for the quantitative analysis were xenotime and monazite standards obtained from South East Asia Tin Research and Development Centre based in Ipoh, Malaysia. 3. RESULTS AND DISCUSSION The two main rare earths mineral found in Malaysia are monazite and xenotime. Monazite is more common and widely available mineral compared to the latter, where it exists in most of the local tin district. Analysis of these two minerals showed the difference in their respective chemical compositions (see Table I), where monazite is comprised of mainly LREE, while xenotime is mainly made up of HREE. The Yttrium content in xenotime is at 35.17%, and is significantly higher than that in monazite (1.77%). The natural radioactive elements of thorium and uranium also show differences in their amount in the two minerals, with monazite containing higher thorium content as compared to uranium, while xenotime has more uranium than thorium. The amount of phosphate present in these two minerals is almost similar. TABLE I: ELEMENTAL COMPOSITION OF LOCAL XENOTIME AND MONAZITE Element Xenotime Kampar Monazite Bidor Light rare earths Heavy rare earths Yttrium Thorium Uranium Phosphate Alkaline fusion is a well-known mineral processing technique that has been used in other minerals, notably in the production of zirconium chemicals from zircon [9]. Sodium hydroxide was chosen as the fusion agent due to its relatively lower fusion temperature requirements, and it is also cheaper than most of the other chemicals. In this process, sodium yttrium phosphate complex will be formed if sufficient temperature and fusion agents are used. ISBN:

3 The effectiveness of this fusion process was measured by the value of degree of reaction (α), calculated from the following (1) [10]; Concentration of yttrium in yttria α = Concentration of yttrium in xenotime (1) One of the factors influencing the efficiency of this process is the fusion temperature. In this study, the fusion temperature varies from 200 C to 450 C (see Fig. 1). The degree of reaction increases linearly through 200 C to 300 C. As the fusion temperature increases further to 350 C, only a slight increase is registered, while further increase does not show any noticeable changes. The result shows that the fusion temperature of 350 o C being the optimum temperature for the fusion process, and at this temperature, an almost complete reaction takes place. Comparison on the effect of particle size of the starting material was also carried out. The original xenotime Kampar sample seems to have mainly a particle size of microns, and this was then compared to the result obtained by ground samples of less than 125 microns particle size. The results from this study show that there are not much of differences in degree of reaction for the original and ground samples. Degree of Reaction (alpha) 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 alpha (original) alpha (grind) 0, Fusion Temperature (celsius) Fig. 1. Graph of Effect of fusion temperature and particle size on degree of reaction. The ratio of xenotime to sodium hydroxide also influences the result of this process, where the degree of reaction tends to increase drastically with the initial ratio of 1:1, until the ratio of 1:2 was used. Further increase of sodium hydroxide content does not show much effect on the degree of reaction, but analysis of the final products tends to contain higher content of excess sodium. After the fusion process, the sodium yttrium phosphate complex formed is washed to separate the yttrium from the phosphate. This was done by washing the complex with warm water, where the soluble phosphate will dissolve in the solution, while yttrium, which is insoluble, will be left behind as solids. Trisodium phosphate (50.8% phosphate) is produced by crystal growth of the phosphate solution. Analysis of the phosphate product shows that it has a relatively low content of rare earths and uranium, with no thorium present. This implies that a good separation process had been achieved between these elements and phosphate during the washing stage. TABLE II: ELEMENTAL COMPOSITION OF TRISODIUM PHOSPHATE Element Content (%) Phosphate Rare Earths 0.33 Silicon 0.93 Iron 0.46 Uranium 0.03 Moisture 45.0 The next stage of the process is the hydrochloric acid leaching of the yttrium complex; carried out in a leaching reactor at 80 o C. Using a 4MHCl solution, a leaching efficiency of 98.5% was obtained for yttrium, 98.4% for dysprosium, 98.6% for erbium, 98.7% for ytterbium, 98.1% for uranium and 98.6% for thorium. These results show a relatively high recovery of the major elements obtained from the alkaline fusion process. If yttria is to be produced directly from this solution, a high concentration of impurities, including the radioactive elements, will present in the final product, and this is unacceptable. ISBN:

4 The radioactive elements of uranium and thorium needs to be reduced to a level that complies with the regulation imposed by many importing countries. Malaysia, through its Atomic Energy Licensing Act, states that the total uranium and thorium content in this material should be less than 500 ppm, or 0.05% [7]. The process to reduce the uranium and thorium was done by using both selective and fractional precipitation methods. In the selective precipitation study, thecontent of yttrium as well as uranium and thorium precipitates at ph from 3.5 to 10.0, as shown in Fig. 2. The yttrium content in the yttria product varies at the different ph, with its concentration registered at 32.5% for ph3.5, and rises as the ph was increased until the optimum level of 45.2% at ph6.0. Thereafter, any increase in ph will result in the reduction in yttrium content. Both uranium and thorium exhibit different trend from that of yttrium, where their content of 4.5 and 6.2%, respectively, were registered at ph3.5, and then drops to 3.0% at ph6.0. Increases in ph after this will result in the increase of the content of both uranium and thorium. From this result, we decided to choose the precipitation ph of 6.0 for further fractional precipitating study. content(%) thorium 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10 ph yttrium uranium Fig. 2. Graph of yttrium with thorium and uranium present at different ph. The fractional precipitation study involved the repeated use of the selective precipitation procedure for four stages, as shown in Fig. 3. As the fractional precipitation stage increases, the content of yttrium increases from 45.2% in Stage 1, to 73.2% in stage 4. The uranium and thorium content however, shows an opposite trend, with their total concentration decreasing from 6.25% to 0.03%. As the total concentration of uranium and thorium of 0.03% is much lower than the regulatory requirement of 0.05% imposed by importing countries, both the selective and fractional precipitation methods are proven useful to produce a product with high concentration of yttria and low radioactive elements. TABLE III: YTTRIUM WITH THORIUM AND URANIUM PRESENT AT DIFFERENT FRACTIONAL PRECIPITATION STAGES No. of Stages Yttrium (%) Thorium (%) Uranium (%) The final stage of the process is to precipitate the radioactive component of uranium and thorium. This was done by using the hydroxide precipitation at ph 8.0, where the uranium will be converted into a uranium hydroxide concentrate. Analysis of the uranium concentrate shows that it has a concentration of 24.7% uranium and 19.8% thorium. Further separation of the uranium and thorium by the solvent extraction technique is required for both the uranium and thorium to be qualified as nuclear fuel grade material. The overall process is shown in Fig. 4, where the picture shows how xenotime was converted to the intermediate yttrium hydroxide after the selective and fractional precipitation methods. Then, calcination of the yttrium hydroxide convert this compound into its yttria form. Also, shown in Figure 4; the two byproducts produced from the processes which are trisodium phosphate and uranium concentrate. ISBN:

5 XENOTIME PHOSPHATE Fig. 4. Flowchart and picture of the xenotime alkaline fusion process. 4. CONCLUSION YTTRIUM HYDROXIDE BY-PRODUCTS URANIUM YTTRIUM OXIDE The results from this study show that the alkaline fusion method could be a good alternative for the cracking of xenotime. Among its advantages are the use of a more dilute HCl as the dissolution medium, production of trisodium phosphate as a by-product of the process, and the viable use of uranium and thorium for nuclear fuel materials. This will eventually result in a more environmental-friendly process for recovering of valuable HREE. for the electronic industry, Materials Science Forum, vol , pp , T.M. Mahadevan, Rare Earths Application and Technology, Materials Science Forum, vol.30, pp.13-32, Kingsnorth D., Rare earths: Facing New Challenges in the New Decade", Presented at SME Annual meeting, Phoenix, Arizona, USA,28 Feb 3 March J. Lifton, Recent dynamics in the rare earth sector, in Proceedings on International Symposium on Rare Earths, Academy of Sciences Malaysia, Kuala Lumpur,2011, pp Y. Kanazawa et al., Rare earth minerals and their crystal structure, Materials Science Forum, vol.70-72, pp.43-80, T. Haridas, The Th-U-REE-P Linkages, Presented at World Thorium Resources Meeting, IAEA, Thiruvananthapuram, India, st October Meor Yusoff M.S., The Asian Rare Earth Experienced and Related Research Activities at Nuclear Malaysia, Academy of Sciences Malaysia, Kuala Lumpur, pp , May Clough D.J., ZrO 2 powders for advanced and engineered ceramics, Ceram. Eng. Sci. Proc., 1985, p Wadsworth M.E., Hydrometallurgical Processes, In Rate processes of extractive metallurgy 1979, Sohn H.Y. and Wadsworth M.E. (eds.): Plenum Press, New York, p REFERENCES 1. Academy of Sciences Malaysia & National Professors Council, Rare Earth Industries: Moving Malaysia s Green Economy Forward, ISBN ,,Academy of Sciences Malaysia, Kuala Lumpur, August R.A Taylor, Australian rare earth resources ISBN: