Nadir toprak elementleri minerallerinden bazı lantanitlerin kazanılması ve seryumun floresans spektroskopisi ile belirlenmesinde yeni bir yöntem

Abstract

Nadir toprak elementlerinin birbirinden ayrılması ve tayinleri, kimyasal özelliklerinin çok benzer olması dolayısıyla zordur. Ortaya çıkan problemlerin çözümünde çoğu zaman iyon değiştirici yöntemleri kullanılmaktadır ve özellikle preparatif ölçüde çalışmalar için yeterli literatür bulunmamaktadır. Bu çalışmada önce nadir toprak elementlerinden hazırlanan bir karışımdan, sonra monazit mineralinden elde edilen ve elektron mikroskopu yöntemi ile bileşimi kısmen tayin edilen bir karışımdan bazı nadir toprak elementlerinin ayrılması ve preparatif ölçüde elde edilmeleri için iyon değiştirici yöntemi kullanılmıştır. DQLüEX 50üJ-XB reçinesi kullanılarak AHIB ile elüsyon yapılmıştır. İM d. HİB ortamında PH ya bağlı bölünme oranlarının incelenmesinden beklendiği gibi önce Y, daha sonra Nd, Pr, Ce ve La elde edilmiştir. Elüsyon süresinin ve elüent hacminin azaltılması koşulları saptanmıştır. Fraksiyonlardaki nadir toprak elementlerinin konsantrasyonları spektrofotometrik olarak tayin edilmiş, her fraksiyon grubundaki PH değişimi belirtilmiştir. Ayrıca seryumun hegzametafosfat kompleksinin floresans özelliği gösterdiği tespit edilmiştir. Literatürde henüz bahsedilmeyen bu özellikten faydalanılarak seryumun kalitatif ve kantitatif tayini için yeni bir yöntem ortaya konulmuştur.

The rare earth group of elements , also called the Ian— thanides is composed of fifteen elements with atomic numbers between 57 and 71. Yttrium atomic number 39 is found naturally with the rare earths. The lanthanides may be generally classified as the light (atomic number 57 to 60) heavy (atomic number 64 to 71) rare earths including yttrium . These terms are derived from the fact that the light Il materials are generally more soluble than the "heavy" ones in a given solvent system. It is difficult to obtain the rare earth elements from their minerals as well as to separate them from each other because of their similar chemical properties. In addition, the detection of those elements are rather difficult due to again the same reason mentioned above. Before the development of the ion exchange methods in 1941 to 1947, fractional crystallization, fractional precipitation and fractional thermal decomposition had been used in order tP get over these problems. But unfortunately, these classic methods are laborious and time consuming. Nowadays, the most effective tool, for the separation and purification of rare earths is the ion exchange method, utilizing synthetic ion-exchange resins. Separation the rare earths using a suitable eluting agent is an important step. The effectiveness of separation for an experimental arrangement depends on such parameters as temperature, type and size of the cation exchange resin particles, dimensions of the column, conceñtration, pH and flaw rate of eluting agent. In this study strong acid type cation exchange resin Dowex 50Ul-X8, 200—400 mesh and elution solution hydroxy isobutyric acid ( 'AH 1B) were used. Commparison of the relative separation factors with other eluents indicates that this eluent is so far the best. At first an artificially prepared mixture of rare earth elements has been studied. The washed resin, after being kept in water overnight, was transferred to the column, the resin was then equilibrated with 1M HI B. Then an elution solution with 3.5 placed in the first flask. The other elution solution with low pH 3.0 was placed in the second flask which was equipped with a magnetic stirrer. The rate of elution was adjusted with a pump, placed after the flasks. b The column was 40cm long by 1.2cm internal diameter with porcelain strainer on the bOttOm. 0<HIB solutions, (with pH 3.0 and pH 3.5) were used as the eluent, respectively and flow rate of 60m I/ hour was chosen. Under these conditions, elution time and eluent volume are better than those found in the literature. Afterwards by considering these results, we were tried to separate some rare earth elements from a mix ture of rare earth oxides obtained from a French monazite. For the separation of rare earth groups, the following procedure was applied: 1- The ground sample was dissolved with perchloric acid, 2— Thorium and rare earth elements were precipitated with ammonium oxalate, 3- The precipitate was dissolved with HN03 -HCIO 14 mixture, 4— Step 2 UJaS repeated, 5- Step 3 was repeated, 6— The solution was boiled, and pH IjaS adjusted to 2 with ammonia and HNO3 • Thorium U.jaS precipitated as benzoate and after ignition it was weighed as ThO 7- The other rare earths were precipitated from the filtrate as their benzoates in the presence of excess ammonia and after ignition weighed as their oxides. -vii- The results obtained by this method are; Residue % 6. 59 ThO % 9.42 2 Total rare earth oxides % 49.15 Later this method was repeated several times in order to have enough rare earth oxides for ion exchange separations of the rare earth elements. The mixture of rare earth oxides (250mg) obtained from monazite, by the method of benzoic acid mentioned before, was dissolved in a small amount of diluted nitric acid by heating. Cerium Was then reduced with 1-1202 by evaporating the obtained solution, It was DUr opinion may be employed without removal of cerium and we kept all cerium in the sample. The sample was added from the top of the equilibrated resin column which was previously rinsed with a small amount of water. The adsorbed sample in the upper part of the resin column was eluted. The elution was continued till Sm (with other rare earth elements) fractions were collected with eluent into the first and second flasks which contained 0011B at pH 3.61 and 3.06, respectively. A f IOW rate of 35m1/hour was used in all runs. At the end of the elution, it has been observed that Y, (with other rare earth Elements) , Nd, Pr, Ce and La were separated preparatively, It has been seen that there are very important contributions to the literature with this experiment. Firstly we used the rare earth oxides as a preparative amount (250mg). Secondly, we kept all the cerium in the sample. Then we obtained cerium fractions without any problems There i sn i t any overlap with other elution picks of lanthanides. Thirdly we determined the place of yttrium in this elution sequence. When the amount of sample is increased, the experimental conditions should be set up again to get rid of the insufficence seen at the elution. The pH of the fractions were determined in the previous experiments. Since the pH of effluent for every element is known these conditions can be used preparatively the processes of purification of La, Ce, Pr and Nd. Since there are empty places between the -viii- fraction groups, this elution can be adopted to some other 111 i rterals th ose include rare earth elements in different percentages. The eluent used is known as an expensive material. The amount of eluent used in this Wark has been decreased as compared to the that used in the literature. At the same time the volume of the eluent can be decreased when the pH of eluent is increased after obtaining cerium fractions. In this case La fractions will come earlier and this will further lessen the amount of the eluent used. It has been found that the elution period and the volume of the eluent is decreased. In this study the determination of the concentration of elements obtained in fractions after elution was performed by the spectrophotometric method using Arsenazo I reagent. Then the fraction groups of every element were collected and precipitated with oxalic acid. The precipitate of each element l.daS ignited to their oxides and the COIOUr of these oxides were used identification of the elements (La: white, Ce: yellow, Pr: dark brown, Nd: light blUe) . At the second part of this study, we investigated the fluorescence properties of cerium hexametaphosphatè complex. It is well known that the analytical problems encountered in the determination of lanthanide ions, especially as traces in solutions, arise from their very similar chemical properties. Therefore, it is extremely difficult to find specific reactions for the individual ions. Spectrophotometric methods produce satisfactory resullts for only high lanthanide concentrations, since the molar absorptivities of the individual ions are rather low. Better results are obtained using optical emission spectra or activation analyses, but it is still difficult to determine traces of individual lanthanides in complex mixtures. Fluorometry has been partially successful because the fluorescence intensity of solutions of common soluble salts of the lanthanides are fairly 1 OW- Sodium hexametaphogoh2te acts as a specific reagent for enhancing the fluorescence intensity of cerium in aqueous solutions. The fluorescence measurement of cerium hexametaphosphate in aqueous solutions shoued that the maximum fluorescence intensity is obtained by irradiating this lanthanide at 314nm in D. 05N sad i ure hexametaphosphate solution. In such solutions the calibration curve is linear for cerium concentrations below 160 g/ ml. The effects of other rare earth on the fluorescence intensity of cerium hexametaphosphate was investigated. It has been seen that there isn't any interferences from other rare earth elements. As a rest-I It, this new spectrofluorometric method can be used successfully for the qualitative and quantitative trace determination of cerium.