J Therm Anal Calorim DOI 10.1007/s10973-017-6822-2 Model studies on the separation of Ca2+ and Nd3+ ions using ethylenediaminetetraacetic acid 1 1 1 Z. Jagoda • A. Pelczarska • I. Szczygieł Received: 13 July 2017 / Accepted: 1 November 2017 Ó The Author(s) 2017. This article is an open access publication Abstract Studies on the separation of calcium and neo- Introduction dymium ions by using ethylenediaminetetraacetic acid (H4EDTA) as a complexing agent were performed. This Rare earth elements (RE) constitute a group of great research was undertaken due to the possibility of H4EDTA importance for technology. There has been a considerable applying to isolate rare earth elements from the solution increase in applications of RE due to their desirable after acidic leaching of phosphogypsum, and because of the chemical, catalytic, electrical, magnetic and optical prop- similarity of coordination properties of calcium and lan- erties. These applications widely range from polishing thanides ions. The experiment was carried out in model agents [1, 2] through lasers [3, 4] and magnets [5, 6]or systems containing Ca2? and Nd3? ions in hydrochloric or batteries [7] to modern technologies such as those of solar sulphuric acid. The content of calcium and neodymium panels [8, 9], high–temperature superconductors [10] and metals, phase composition and thermal behaviour of the plasma display panels [11, 12]. obtained products were determined by ICP-OES, FTIR, The RE, in contrast to the designation, in fact, are not XRD and TG/DTA techniques. During the separation uncommon—for instance, the cerium content in the Earth’s process, the precipitates of a light pink colour were crust is similar to that of copper and nickel [13]. However, obtained. The obtained results show that the neodymium the problem appears with their large dispersion which ethylenediaminetetraacetate has been successfully formed makes the mining and separation be difficult and ineffi- and that the isolation of neodymium ions was more effi- cient. These elements occur in nature mostly in the form of cient in chloride medium. The precipitate included 72.2 phosphates and silicates in so-called rare earth minerals and 3.9% of the starting amount of neodymium and cal- (e.g. monazite, rhabdophane, xenotime). Currently, most cium used in the experiment, respectively. However, in exploited deposits of the elements are those in China. sulphates medium, these amounts were equal to 73.8 and However, the China export used to be limited to 40% in 53.5%, respectively. Moreover, the obtained powder was 2012. This may cause serious problems for technologies polluted with sulphates. The addition of the EDTA in an outside of China, hence alternative RE sources, are being excess (15%) contributed only to an increase in calcium searched for [14–18]. One of the possible sources can be content in the complex. phosphogypsum which is a by-product (End-of-Life material) in the process of phosphoric acid production from Keywords Rare earth elements Á EDTA Á Separation phosphate rocks, phosphorites and apatites, by the wet method. Phosphogypsum contains RE metals in an amount dependent on the origin of rocks used in the process, usually, there is * 0.5–1% [19]. The largest content is & I. Szczygieł observed for lanthanum, cerium and neodymium [19–21]. [email protected] Attempts to obtain RE from the waste materials of phos- phate fertilisers production were made in the past [22–25]. 1 Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wrocław University of Economics, Those ways chiefly consist in the leaching of phospho- Komandorska 118/120, 53-345 Wrocław, Poland gypsum with an inorganic acid to be followed by 123 Z. Jagoda et al. precipitation of the RE salts from the obtained solution. Test S1: CaSO4Á2H2O water suspension (pHS1 = 5.7) Usually rare earth elements are recovered by precipitation was mixed with H4EDTA in the molar ratio 1:1 with a as RE oxalates or sodium-RE double sulphates, separated small amount of water. Then the mixture was heated and by hydrofluoric acid and then converted into desired refluxed for 2 h. The precipitate was filtered, washed with commercial rare earth salts. Unfortunately, the precipitates distilled water and dried in the air at room temperature. contain not only lanthanides but large amounts of calcium Test C1 was carried out in a similar way, with a solution the primary component of phosphogypsum. Due to a sim- of CaCl2Á6H2O and at pHC1 = 5.2. ilar chemical reactivity and coordination properties of Ca In the S2 and C2 test, neodymium oxide was dissolved and RE, separation of those elements is difficult. in the stoichiometric amount of 1 M H2SO4 (pHS2 = 6.5) Therefore, in the present investigations, we described and 1 M HCl (pHC2 = 2.5), respectively. Then the H4- 3? the first step leading to separation of calcium and neody- EDTA acid was added ensuring the molar ratio Nd :H4- mium ions by using H4EDTA as the strongly chelating EDTA to amount 1:1. The mixtures were refluxed for 2 h. agent and the difference in the stability of Ca(II) and The precipitates were filtered, washed and dried. Nd(III) with ethylenediaminetetraacetic acid complexes. The next stage was an attempt to separate calcium and Citric acid and amino acids similar to EDTA were found to neodymium ions through chelating of sulphates (test S3) separate some REE but could not separate as many as and chlorides (test C3). Neodymium oxide was dissolved in EDTA [26, 27]. Ethylenediaminetetraacetic acid (EDTA) a similar way as in the case of tests S2 and C2. The may be an improvement over the use of other chelating obtained solutions were treated with the appropriate cal- agents. The experiment was performed in model systems cium salt (pHS3 = 6.5; pHC3 = 2.3) and H4EDTA. The 3? 2? which consisted of calcium and neodymium in sulphates or molar ratio Nd :Ca :H4EDTA was 2:1:2 and 1:1:1 in chlorides medium. The task was undertaken in view of the tests S3 and C3, respectively. possibility to apply H4EDTA to the selective separation of RE from the solution after leaching process of phospho- Experimental methods gypsum as well as for the known similar coordination properties of calcium and neodymium ions. Chemical The content of C, H, N, S and Cl was determined by atomic effect of Nd ions in examined model systems should be absorption spectrometry (AAS) using the analyser CHNS typical of whole RE group elements owing to their electron Vario EL III (Elementar). Ca and Nd analysis was per- configuration (n–1) d1ns2. The choice of the salts was made formed by the inductively coupled plasma-optical emission for the possibility of using sulphuric(VI) or hydrochloric spectrometry (ICP-OES) using the apparatus ARL3410 acids as the phosphogypsum leaching agent. The studied ICP with argon plasma excitation. Moreover, the obtained Nd and Ca separation process is considered to be applied in products were characterised by Fourier transform infrared the future for the recycling technology of RE elements (FTIR) spectroscopy, powder X-ray diffraction (XRD), and from the waste phosphogypsum in Poland. thermal analysis (TGA/DTA). The FTIR spectra were measured with a Perkin-Elmer System 2000 FTIR spec- trophotometer in the medium IR range (4000–400 cm-1)at Materials and methods room temperature. The samples were prepared in the form of KBr pellets. The XRD measurements were performed Chemical synthesis with a SIEMENS D5000 diffractometer (copper X-ray tube) in the range 2h of 5–50° with a 0.04° step and at least The following reagents, commercially available, were used 2 s per step. The TGA/DTA analyses were carried out without any additional purification: calcium chloride hex- using a derivatograph 3427 (MOM, Hungary) in a tem- ahydrate (analytically pure), ammonium sulphate (analyti- perature range of 20–1000°C (heating rate 7.5°C/min, cally pure), ethylenediaminetetraacetic acid platinum crucible, air atmosphere). (H4EDTA; C 99.0%), neodymium(III) oxide (99 ?), sul- phuric(VI) acid (96%), hydrochloric acid (35-38%). Dihydrate calcium sulphate(VI) was obtained via reaction Results and discussion of calcium chloride hexahydrate with ammonium sulphate and its purity was confirmed by XRD. To obtain RE from the leaching solution, chelating agents In the first stage of the study, the calcium and neody- can be used. One of the widely applied agents is mium complexes with H4EDTA were obtained by treating ethylenediaminetetraacetic acid (H4EDTA). This agent is the relevant sulphates or chlorides with ethylenedi- capable of complexing the ions of most metals; it is aminetetraacetic acid. applied, in the form of sodium salts, in quantitative chemical analysis owing to a simple stoichiometry of the 123 Model studies on the separation of Ca2? and Nd3? ions using ethylenediaminetetraacetic acid reaction with metals. The capability of H4EDTA agent to carboxyl groups are non-ionised and non-coordinated, this chelate metal ions depends on the pH value of the solution, signal appears at 1750–1700 cm-1. As a result of metals which affects the protonation of H4EDTA acid as well as coordination, the band is shifted towards low frequencies to the equilibrium of complexes formation. In a neutral or the 1650–1590 cm-1 range. In the range from slightly alkaline medium, an HEDTA3- ion will be the 1630 to 1575 cm-1, bands of free and ionised carboxyl prevalent form of the acid. Complexing will proceed groups occur [30]. according to equation: An FTIR spectrum of a precipitate obtained in the C1 test is shown in Fig. 1a. In the figure, a band of stretching Mnþ þ HEDTA3À $ ½MEDTA nÀ4þ Hþ ð1Þ vibration at the wave number 1697 cm-1 is visible. This The formation reaction of metal complexes with H4- means that the carboxyl groups of H4EDTA were non- EDTA at the pH in the range from 3.5 to 5.5 can be coordinated to the metal.