Chemical Beneficiation Process of Obtaining Rare Earth Elements - Research Paper Example

Chemical Beneficiation Process of Obtaining Rare Earth Elements Rare earth elements are becoming increasingly important in the material industry for various applications for example in United States where they are used as automobile and petroleum refining catalysts, in phosphors in color television and flat panel displays, permanent magnets, and rechargeable batteries for hybrid and electric vehicles among other applications ( Humphries 6). Rare earth elements are grouped into two major categories; the first category belongs to a group called lanthanides which contains 15 of the rare earth elements. The second group contains only two of the rare earth elements which are yttrium and scandium. Rare earth elements are not rare as suggested by the name but are abundant in the earth’s crust even more than some metals (see table 1). However, they are dispersed and their concentration on the earth’s crust is very low and therefore they cannot be easily exploited. In addition, each rare earth deposit is unique and contains different ore bodies which contains several rare earth elements in varying proportions. Rare earth elements do not occur naturally in elemental state as most metallic elements but are found in a large number of minerals types including halides, oxides, carbonate, phosphate and silicate (Eriksson and Olsson 5). In this project we want to address beneficiation as one of the aspects of chemical process of obtaining rare earths form the naturally occurring ores. Beneficiation is the second step of mining after the ore is removed from the ground. Beneficiation process is divided into two steps i.e. physical beneficiation and chemical beneficiation. Chemical beneficiation is applied after the physical beneficiation. Depending on the composition of the ore, different chemical techniques can be applied to separate the rare earth elements from other minerals contained in the ore. There are various types of ore bodies which include; monazite, euxenite, gadolinite, xenotime, bastnasite, and Elliot lake uranium (Gupta and Krishnamurthy149). Different ore bodies have different compositions and hence different chemical beneficiation processes. Table 1: Abundance of Elements in the Earth’s Crust (EPA 13) Element Crustal Abundance (parts per million) Nickel (28Ni) 90 Zinc (30Zn) 79 Copper (29Cu) 68 Cerium (58Ce)a 60.0 Lanthanum (57La) 30.0 Cobalt (27Co) 30 Neodymium (60Nd) 27.0 Yttrium (39Y) 24.0 Scandium (21Sc) 16.0 Lead (82Pb) 10 Praseodymium (59Pr) 6.7 Thorium (90Th) 6 Samarium (62Sm) 5.3 Gadolinium (64Gd) 4.0 Dysprosium (66Dy) 3.8 Tin (50Tn) 2.2 Erbium (68Er) 2.1 Ytterbium (70Yb) 2.0 Europium (63Eu) 1.3 Holmium (67Ho) 0.8 Terbium (65Tb) 0.7 Lutetium (71Lu) 0.4 Thulium (69Tm) 0.3 Silver (47Ag) 0.08 Gold (79Au) 0.0031 Promethium (61Pm) 10-18 MONAZITE (Ce, La, Y, Th . There are two different chemical beneficiation ways of obtaining rare earth elements from monazite ore. This difference comes from the fact that either sulphuric acid or hydroxide can be used to first attack the ore (Gupta and Krishnamurthy 149). Acid treatment. In this treatment (see fig.1), concentrated sulphuric acid is used to heat the monazite sand up to a temperature of between 120 and 150°C ( Bongaerts 67). Both thorium and rare earth can be dissolved depending on the acid/ore ratio, the concentration of the acid, amount of water added and the temperature to which the ore is heated. The insoluble residue is separated from the solution. Thorium and rare earth will then be recovered from the solution by the process of double sulphate precipitation. First the solution is partially neutralised with ammonium hydroxide to obtain a precipitate of thorium oxide which can be separated from the solution. Next, the solution is again added to sodium sulphate to obtain the rare earth as precipitate and a solution of sulphate of heavy Ln. Figure 1: chemical process of acid monazite beneficiation In this case a hot caustic soda (78%NaOH) is used to attack the finely grounded monazite after the physical beneficiation. This takes place at around 140 (Bongaerts 68), this is a more recent process which allows the recovery of phosphate as a by-product in the form of trisodium phosphate. The mixture of rare earth and thorium hydroxide is then processed for thorium and rare earth recovery by several treatments. The residue containing thorium and lanthanide hydroxides is treated by hydrochloric acid to obtain a solution of lanthanide chlorides and an insoluble and thorium hydroxide which comes out as an insoluble solid. Figure 2: chemical process of alkaline of monazite. BASTNASITE When extracting rare earth from bastnasite, we can either start with crude oil or the bastnasite itself. Starting with bastnasite, we obtain bastnasite at the end of the physical beneficiation which is at a concentration of 60% rare earth oxide (Gupta and Krishnamurthy 151). This rare earth oxide is leached with hydrochloric acid so that it is upgraded to about 70% by removing calcium and strontium carbonates. Since the concentrate is mixed with carbon, this 70% rare earth oxide is further upgraded to about 85 – 90% by dissolving in calcium carbonate to remove carbon dioxide (Gupta and Krishnamurthy 151). Starting with the ore, direct roasting of the ore can be done so that the fluoride in the ore can be converted into soluble alkaline fluoride. This can then followed by water leaching and then the separation of the unconverted barite is done by floatation. XENOTIME Xenotime can sometimes follow the same route as monazite but the more preferred route after the physical beneficiation of crushing and grinding involves the process of hot froth floatation and then digesting the ore by 65% NaOH at a temperature of approximately 140 degrees (Bangearts 70). Water is then added to extract the phosphates from the mixture. This leaves behind a slurry of impure hydrous oxides of La, Ln, Th. This impure mixture can then be added to HCl and boiled up to a pH of 3.5 (EPA 3 ) so that crude thorium oxide can be obtained as a precipitate and removed from the solution. This solution will now be containing a solution of (La, Ln) Chlorides. In the final stages, the solution is added to aqueous solution of barium sulphate which will carry down radioactive RaSO4 as a precipitate. Xenotime can as well undergo acid treatment as monazite Figure 3: chemical process of extracting xenotime CHEMICAL PROCESSING OF GADOLINITE In gadolinite, rare earth elements are obtained through leaching the ore with either nitric acid or hydrochloric acid. We discard the residue and then precipitate the solution with oxalic acid. The precipitate will contain the rare earth oxalates and a solution of Be, Fe. Other different chemicals such as concentrated sulphuric acid or a mixture of sodium hydroxide with sodium peroxide can also be used to attack gadolinite (Gupta and Krishnamurthy 153). CHEMICAL PROCESS OF EUXENITE In this case we first start with chlorination to obtain the chlorides of the rare earth. This is then followed by dissolution so that rare earth, titanium niobium and tantalum can be separated (Gupta and Krishnamurthy 153). Solvent extraction is then applied to remove tantalum – niobium chlorides and rare earth metal from the mixture of the chlorination residue. CONCLUSION Rare earth elements has a wide range of application, with the use of chemical analysis, a higher percentage of recovery can be obtained. Finding the best chemical method is key to future exploitation of the rare earth elements for the increasing market demand. At the same time the environment impact of most of the chemical used should be analysed since some of the chemicals used such as heavy metals if not well controlled and properly managed may contaminate the environment. Some chemicals also may be harmful to human and therefore should be handle with care and properly managed. References Humphries, M. “Rare Earth Elements: The Global Supply Chain”. Environmental issues. 2011. Print. Gupta, C.K. and Krishnamurthy, N. “Extractive Metallurgy of Rare Earths” CRC Press.2004. Print. Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated elements. EPA/600/R-12/572. 2012. Print. Eriksson T and Olsson D. “The product chains of rare earth elements used in permanent magnets and NiMH batteries for electric vehicles.” Chalmers University of technology. Report No. 2011:8 ISSN No: 1404-8167.2011. Print. Bongaerts John C. “Production Process and Recycling Of Rare Earth Elements.” 7 (2), 2013. Print. Read More