Article pubs.acs.org/jced
Thermodynamic Properties of Ternary Solutions in the Water−Nitric Acid−Rare Earth Nitrate (Pr, Nd, Sm) Systems at 298.15 K Alexander E. Moiseev,† Alexander V. Dzuban,*,†,‡ Alisa S. Gordeeva,‡ Anatoly S. Arkhipin,‡ and Nikita A. Kovalenko† †
Department of Chemistry and ‡Department of Materials Science, Lomonosov Moscow State University, 119991 Moscow Russia ABSTRACT: The vapor pressure measurement unit from borosilicate glass was assembled for determination of partial pressures of water in aggressive acidic media using the transpiration method. Three ternary systems H2O−HNO3−RE(NO3)3 (RE = Pr, Nd, Sm) were investigated. The experimental results in ternary systems were compared with the estimation by the Pitzer model and Zdanovsky rule from the data of binary subsystems. In addition to vapor pressure measurements densities of investigated solutions were determined.
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INTRODUCTION Rare earth elements (REEs) have become vital in modern technological applications including electronics, optics, atomics, metallurgy, chemical industry, engineering, etc. The elements and their compounds act as catalysts in ammonia synthesis and oil cracking, components of superstrong magnets, control rods of atomic reactors, special alloys and glasses, lasers and other optically active and nonlinear elements in optoelectronics. High-purity specific REEs are of particular interest. However, their isolation and separation remain challenging due to the similarity of physicochemical properties. Hence, there is a great variety of methods used: fractional crystallization and precipitation, ion exchange, extraction, selective oxidation and reduction. One of the most common, cost-effective, easy-to-set and operate methods is liquid extraction of REE salts (nitrates, chlorides, sulfates, phosphates) from acidic water media with an immiscible organic agent. Despite it being used for the last 50 years, the process conditions are generally adjusted experimentally, a rigorous physicochemical model does not exist, and thermodynamic properties of phases in extraction systems described in the literature are contradictory, roughly estimated, and inconsistent even for aqueous solutions. The thermodynamic model of a multicomponent water−organic salt system is relevant for optimization of REE salts separation conditions. It requires experimental data describing the properties of the systems of various orders. To the best of our knowledge, binary water solutions of acids and REE salts are studied in detail, in contrast with ternary systems, which makes the construction of the thermodynamic model of a real multicomponent system quite a challenging task. REEs are usually extracted from nitric-acid solutions, and therefore the aim of the present work is studying the thermodynamic properties of aqueous solutions of nitric acid © 2016 American Chemical Society
and light- or medium-REE nitrate (praseodymium, neodymium, samarium). These three elements are the most widely observed after processing of ores with nitric acid. The water−nitric acid binary system is a thoroughly investigated one. Its total and partial vapor pressures at various temperatures were measured by Burdick and Freed,1 Wilson and Miles,2 Lloyd and Wyatt,3 Potier,4 Boublik and Kuchynka,5 Flatt and Benguerel,6,7 Davis and De Bruin,8 Yakimov and Mishin,9 Haase et al., 10 Lemire et al.,11 Hanson and Mauersberger,12 Tang et al.,13 and Massucci et al.14 mainly using static and transpiration methods. Critical review of all previously obtained experimental data and thermodynamic modeling of the system was made by Clegg and Brimblecombe.15 The water−rare earth nitrate binary systems are less well investigated. The Solubility Data Series16 contains critically reviewed data on RE nitrates aqueous solubility. Rard et al.17−19 measured osmotic coefficient of the solutions at 25 °C by the isopiestic method. As far as we know, there are no experimental or theoretical investigations of the ternary systems H2O− HNO3−RE(NO3)3 (RE = Pr, Nd, Sm), apart from the solubility.16 In particular, the work of O’Brien and Bautista20 on the measurements of hydrogen and nitrate ions activities and vapor pressure of binary electrolyte solutions consisting of neodymium nitrate and nitric acid should be mentioned and will be discussed below. A convenient way to obtain partial thermodynamic properties of the components in any system is vapor pressure measurements. Considering the fugacity of two compounds in the systems of our interest (water and nitric acid) we used the Received: April 30, 2016 Accepted: August 1, 2016 Published: August 10, 2016 3295
DOI: 10.1021/acs.jced.6b00357 J. Chem. Eng. Data 2016, 61, 3295−3302
Journal of Chemical & Engineering Data
Article
saturator (3−4) placed in the thermostated (298.15 K) zone (A). The saturator consists of two partsthe presaturator (3) and the main saturator (4). It resolves the problem of changes in solutions concentration during an operation. The primary saturation of inert gas occurs in the presaturator, hence a gas stream composition changes insignificantly in the main saturator keeping the solution concentration that is in it almost constant (see Apparatus Validation section). Then the carrier gas passes through a thermostated hot line into cold U-type traps (5). The first one collects the vapors of volatile components. The second one protects the system against condensation of atmospheric water. A temperature control of the hot line is required to prevent condensation of transferred vapors in it. The cooling system of the traps comprises tandem vessels with ethanol (6) and liquid nitrogen (7) as shown in Figure 1. External cooling of the alcohol container by liquid nitrogen leads to the formation of a two-phase system: solid ethanol with a liquid above it. As a result, a gradual top to bottom fall of temperature appears that extends to the area of condensation. It helps also to avoid partial condensation of the carrier gas and blocking of the traps with ice. Measurements duration varies from 5.5 to 8 h and depends on the fugacity of solution components and their concentration. As the experiment concludes, the condensed vapor from the first trap is weighed for determination of the total mass of the condensate (water and nitric acid), flushed out by distilled water into graduated flask (50 or 100 mL), and analyzed for the amount of nitric acid. Experimental Procedure. A studied solution is poured into both part of the saturator (ca. 70 mL at all) in such a way that a carrier gas stream could pass easily over it. The saturator is placed into the temperature-controlled bath (Huber CC-K6) filled with water at 298.15 ± 0.02 K and then connected to a gas line and traps. Right after switching on the gas flow, all traps should be submerged into the cooling system and a timer is turned on. When the experiment is finished, an operator disconnects the traps and keeps them sealed with Parafilm until they are warmed to room temperature (to avoid atmospheric water condensation). After reaching an ambient temperature, traps are weighed and vapor pressures are calculated using the following expression:
transpiration method which makes it possible to determine vapor composition. An experimental unit similar to that described by Storonkin21 was set up. Its major advantage is the lack of direct contact between acid vapors and any structural elements reacting with HNO3.
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EXPERIMENTAL SECTION Reagents. Ternary solutions were prepared from double distilled water, 70% nitric acid, and rare earth (Pr, Nd, Sm) nitrate hexahydrates (Table 1). The amount of water in salts Table 1. Purities and Sources of Chemicals
chemical name
CAS number
nitric acid (aqueous solution, ca. 70%) praseodymium(III) nitrate hexahydrate neodymium(III) nitrate hexahydrate samarium(III) nitrate hexahydrate
7697-37-2
source
initial mass fraction puritya
analysis methodb
0.9999
15878-77-0
ComponentReaktiv Lanhit
0.9999
ICPAES TG
16454-60-7
Lanhit
0.9999
TG
13759-83-6
Lanhit
0.9999
TG
a
No additional purification was carried out for the chemicals. bICPAES, Inductively coupled plasma atomic emission spectroscopy; TG, thermogravimetry.
was determined thermogravimetrically on a NETZSCH TG 209 F1 Iris by heating the samples up to 800 °C in corundum crucibles under air atmosphere. Also the concentration of rare earth nitrate in the solutions under investigation was confirmed by spectrophotometry (see Analytical Methods section). Standard solutions of REE nitrates for the calibration of the spectrophotometer were prepared by dissolution of a certain amount of Pr6O11, Nd2O3, and Sm2O3 (extra-pure grade, 99.99%) in nitric acid upon heating. A 0.05 mass % water solution of Arsenazo-III (analytically pure, 99%) was taken as an indicator. A Sartorius Research R 160P analytical balance was used for weighing the solids; the corresponding error was less than 1 × 10−2 mg. Apparatus. The scheme of an assembled experimental unit is shown in Figure 1. The carrier gas (1) stream controlled by the flowmeter (2) passes over an investigated solution in the
pi =
ni k
n(N2) + ∑ j = 1 nj
patm (1)
where pi is the partial pressure of the component i, patm is the atmospheric pressure during measurement process, ni is the amount of condensed substance of component i, n(N2) is the amount of flowed gas, and k is the number of volatile components in the solution under investigation. Apparatus Validation. Pure water and aqueous solutions of nitric acid were chosen as test objects for checking the accuracy of measurements by the assembled experimental unit. Various flow rates of inert gas (60−100 mL·min−1) were tried also. The identical results were obtained for all of them indicating that thermodynamic equilibrium was reached. The value of 100 mL·min−1 was chosen then to reduce the experiment time. Results of pure water vapor pressure measurements with comparison to the reference data22 are listed in Table 2. An excellent agreement between these data can be observed, with a deviation of less than 0.3%. Possible lags and inaccuracies in the determination of measurements duration are within a few
Figure 1. Schematic diagram of the assembled unit for vapor pressure measurements by the transpiration method. A, temperature-controlled part; B, zone of vapor condensation; 1, container with inert gas (N2); 2, flowmeter; 3, presaturator; 4, saturator; 5, U-type traps; 6, ethanol container; 7, Dewar vessel with liquid nitrogen. 3296
DOI: 10.1021/acs.jced.6b00357 J. Chem. Eng. Data 2016, 61, 3295−3302
Journal of Chemical & Engineering Data
Article
Table 2. Pure Water Vapor Pressure p Measured by Transpiration Method at Temperature Ta T/K
m(H2O)/g
pexp/kPa
pref/kPa22
Δ(p)/%
293.15 293.15 293.15 298.15
0.637 0.664 0.835 1.137
2.319 2.320 2.315 3.178
2.314 2.314 2.314 3.169
0.24 0.26 0.04 0.30
a
Standard uncertainties u are u(T) = 0.01 K, u(m) = 0.001 g, u(p) = 0.01p. m(H2O) is the mass of condensed water vapor from the first trap.
seconds. This cannot affect significantly the uncertainty of the method, because the whole experiment lasts for 5−8 h. Aqueous solutions of nitric acid were studied up to a concentration of 10 mol·dm−3 at 298.15 K. According to Clegg and Brimblecombe15 the amount of acid in the vapor phase for diluted solutions (
