Potential ph diagrams of Cr-H 2 O system at elevated temperatures

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1 Potential ph diagrams of -H O system at elevated temperatures YOU Hai-xia ( ) 1,, XU Hong-bin( ) 1, ZHANG Yi( ) 1, ZHENG Shi-li ( ) 1, GAO Yi-ying( ) 1 1. National Engineering Laboratory for Hydrometallurgical Cleaner Production echnology, Beijing , China;. Graduate School of the Chinese Academy of Sciences, Beijing , China Received 6 July 009; accepted 30 December 009 Abstract: he potential ph diagrams of the -H O system at temperatures of 98, 33, 373 and 3 K were established through thermodynamic calculation with coupling of experimental results. he potential ph diagram at 98 K agrees well with the previously reported results. Based on the potential ph diagrams of the -H O and e-h O systems, IR spectra and XRD pattern, the existence forms of iron in the potassium chromate were analyzed, and the methods of iron removal were investigated. he results show that the existence forms of iron in the potassium chromate are potassium ferrate. hese results are instructive and helpful to the hydrometallurgical processing of chromite ore. Key words: -H O system; thermodynamic calculation; potential ph diagram; iron removal; potassium chromate 1 Introduction As a powerful tool in hydrometallurgy, potential ph diagram obtains a wide application. In the process of leaching, purifying, oxidation and reduction, precipitation and dissolution, replacement of metal deposition and electrolytic refining, the potential ph diagram is used for analyzing the physicochemical principle and determining the thermodynamic condition. In addition, the potential ph diagram is also very important in the fields of analytical chemistry, mineral geology, biology, nuclear power technology and corrosion and anticorrosion of metal[18]. Unfortunately, the utilization of published diagrams is somewhat limited that they had been constructed at a single temperature and the thermodynamic data at higher temperatures are lacking in most cases. his article has done some work in order to extend the effect of the diagram. Based on the obtained data at lower temperatures, calculations using empirical formulas must be made to obtain the necessary data that can not be measured at higher temperatures. Experimental.1 hermodynamic calculation he following half cell reaction represents a general electrochemical reaction. When the coefficient (n) of e is zero, Eq.(1) represents a chemical reaction. aamh ne=bbch O (1) where a, m, b and c are the stoichiometric coefficients for species A, H, B, and H O in the electrochemical reaction, respectively; and n is the electron transfer number of the electrode reaction. In dilute solution, when a H O = 1 for the activity of water, the equilibrium potential can be simplified as follows according to the Nernst isothermal equation: E = E.303[ Rm /( n)] ph b.303[( R /( n))]lg( a B / a ) () where E or E is the electrochemical potential or standard electrochemical potential for reaction at a certain temperature ; is the araday s constant and R is the molar gas constant; and a A or a B is the overall activity of species A or B. a A oundation item: Project(007CB613501) supported by the National Basic Research Program of China; Project(006BAC0A05) supported by the National Science and echnology Pillar Program of China; Project( ) supported by the National Natural Science oundation of China Corresponding author: XU Hong-bin; el: ; ax: ; ipecas_hbxu@63.net

2 YOU Hai-xia, et al/rans. Nonferrous Met. Soc. China 0(010) s6s31 he relationship between the potential and ph value is shown explicitly. G = R ln K = ne (3) where G is the standard free energy of reaction at temperature, and K is the reaction equilibrium constant. rom Eq.(), we can obtain the diagram of potential ph curves at a certain temperature as long as whenever G, K and E can be known.. hermodynamic calculation of high-temperature aqueous solution method selection At arbitrary temperature and pressure, the standard free energy of substance formation can be calculated through the relationship of the thermodynamic parameters. G f ( ) = Gf,98 K ( 98) S 98 K d p c pd c p Vdp () p0 where G f () is the standard free energy of formation at a certain temperature ; at 573 K, the volume V is believed to have small impact, and p Vd p can be p0 neglected, p 0 = Pa; S 98 K is the standard entropy of substance in 98 K, c p is the constant-pressure molar heat capacity of substance in the temperature range from 98 K to, and can be expressed as follows: 0 x1 x x3 c p = x (5) s7 able 1 hermodynamic data of main substances in -H O system Specie G / f,98 K (kj mol 1 ) S 98 K / (J mol 1 K 1 ) c / p (J mol 1 K 1 ) H (g) O (g) H O OH H e (s) O 3(s) O 3(s) H O HO O O (OH) OH O where x i (i=0, 1,, 3) is a constant. herefore, as long as the values of S 98 K and c p are obtained, the standard free energy of formation can be calculated from Eq.()..3 Correspondence principle Based on many results of experiments, CRISS[911] summarized a correspondent principle as follows: ( K S i,abs) = a b S 98 ( i,abs) (6) where 98 K ( i,abs) refers to the ionic entropies on the absolute scale at 98 K; a and b refer to the constants at a certain temperature. S 3 Results and discussion 3.1 Potential ph diagrams of -H O system at elevated temperature he data of the thermodynamic calculation in this paper are obtained from Refs.[116] except as otherwise indicated, and the thermodynamic data of the main substances in -H O system are listed in able 1. In able 1, G f, 98 K is the standard Gibbs free energy of formation at 98 K; and S 98 K is the third law entropy at 98 K; and c p is the standard specific heat capacity. he various considered reactions are listed in able. he diagrams for other values of activity and pressure can be easily obtained by ph potential functions listed in able. he ph potential relations are shown in igs.1. he potential ph diagram at 98 K agrees well with the previously reported results in Ref.[1]. he principal effects of temperatures can be summarized as follows. 1) he / equilibrium is seldom affected in the temperature range of 983 K. ) he stability region for the and 3 decreases, and the stability region for the O and O increases. 3) he stability of 3 increases apparently in low ph value region. However, in high ph value region its stability decreases.

3 s8 YOU Hai-xia, et al/rans. Nonferrous Met. Soc. China 0(010) s6s31 able Electrode reaction and standard potential value ( refers to ph value in standard state) Reaction E /V Reaction E ph equation No. 98 K 33 K 373 K 3 K a.303r.303r O (g) H e H O E = E ph lg po b.303r.303r H e H (g) E ph R e (s) E = E lg a R (s) H e (s) H O E = E ph H O H ph = lg K / a 3 R e E lg = E a R.303R H O e H E = E ph lg a R.303R OH H O e H E = E ph lg a OH R e E = E lg a R.303R 3 H e H O E = E ph lg a R.303R H e H O E = E ph lg a H O OH H = K a 3 a OH OH H O H = K a a OH H O 3 H ph = lgk lg( a / a ) H O H ph - = lgk lg( a / a ) a 3 R R HO HO 6H 3e H O E = E 3 a R.303R a HO HO 7H 3e H O E p lg = E H 3 3 a R.303R a HO HO 6H 3e OH 3H O E = E 3 a OH a R R HO 5H 3e H O E = E 3 3 a HO H 3e.303R.303R a HO 3 H O E = E 3 3 a 1.01 HO O 5H 3e O H 3e 3 H O 5.303R.303R a O E = E ph lg 3 3 a R.303R a O E = E ph lg 3 3 a HO HO H ph = lgk lg( a / a ) 0.6 HO HO HO O H HO O O H H O H e H O 3H O 3 HO H 3e O 5H 3e O 8H 3e O ph = lg K lg( a / a ) ph = lg K lg a R E = E ph H ph =lg K /3 lg a 3 / H O 3 3H e.303r.303r E = E ph lg a HO H O E E a O H O 5.303R.303R = R.303R a O E = E ph lg 3 3 a R.303R 3H O E = E ph lg a

4 YOU Hai-xia, et al/rans. Nonferrous Met. Soc. China 0(010) s6s31 s9 ig.1 Potential ph diagrams of -H O system at 98 K ig. Potential ph diagrams of -H O system at 3 K ig. Potential ph diagrams of -H O system at 33 K ig.3 Potential ph diagrams of -H O system at 373 K 3. orms of iron in potassium chromate and methods of iron removal 3..1 orms of iron in potassium chromate he ph potential dagrams[117] are plotted in ig.5 Potential ph diagrams of -H O system and e-h O system at 98 K ( are reactions of e-h O system; (1)(1) are reactions of -H O system)[117] ig.5. It can be seen that, when the ph value is higher than 7.0, the line of eo /e(oh) 3 equilibrium is higher than that of O / 3 equilibrium. he eo ion demonstrates stronger oxidative performance than the O ion, and the two ions can coexist in the alkaline solutions. ig.6 shows the IR spectra of the high pure potassium ferrate[18]. he wide and strong peak at (807±3) cm 1 is the characteristic absorption peak of the anti-symmetric stretch vibration of iron-oxygen bond, and the weak peak at (1 110±5) cm 1 is stretch vibration characteristic peak of the iron-oxygen bond. rom the -IR spectra of the potassium chromate product from the demonstration plant shown in ig.7, the characteristic peak of iron-oxygen bond appears at cm 1. Because of the effect of the strong characteristic peak of potassium chromate which is the main component at 880 cm 1, the anti-symmetric stretch vibration peak of ironoxygen bond of a spot of potassium ferrate is shaded.

5 s30 YOU Hai-xia, et al/rans. Nonferrous Met. Soc. China 0(010) s6s Method of iron removal he above analysis indicate that the potassium ferrate may exist in the potassium chromate. he decomposition of ferrate ion is given by the following reaction. eo H O e(oh) 3 O OH (7) ig.6 IR spectra of high pure K eo Based on the thermal instability (see Eq.(7)) and photolysis of potassium ferrate, a method is brought forward that potassium ferrate can be removed by the sequential process of heat dissolution, laying in light, filtration and re-crystallization. ig.9 shows the removal rate of iron by re-crystallization. In the experiment two samples of potassium chromate with iron contents of 1.0 % and 0.7% are respectively dissolved and heated and laid in light for 60 min. he separating experiment results show that the content of iron drops under the detection line. ig.7 -IR spectra of reagent K O and demonstration K O ig.8 shows the XRD pattern with some peaks of potassium ferrate. It can be concluded that the iron in the potassium chromate product from our demonstration plant exists in the form of potassium ferrate. ig.8 XRD pattern of demonstration K O ig.9 Removal rate of iron by re-crystallization Conclusions 1) he / equilibrium is not much affected in the temperature range of 983 K. he stability region for the and 3 ions decreases, and the stability region for the O and O ions increases. here is a considerable increase in the stability of 3 in low ph value region. However, in high ph value region its stability decreases. ) he ph potential dagrams, IR sepctra, and XRD pattern show that K eo and K O can coexist under alkaline condition. Based on the thermal instability and photolysis of potassium ferrate, a method is brought forward that potassium ferrate can be removed by the sequential process of heat dissolution, laying in light, filtration and re-crystallization. he iron content of potassium chromate, after laying in light for 60 min, drops under the detection line.

6 YOU Hai-xia, et al/rans. Nonferrous Met. Soc. China 0(010) s6s31 s31 References [1] LINKSON P B, PHILLIPS B D, ROWLES C D. Computer methods for the generation of Eh ph diagrams [J]. Min Sci Eng, 1979, 11(): [] ANGUS J C, ANGUS C. Computation of Pourbaix diagrams using virtual species: Implementation on personal computers [J]. J Electrochem Soc, 1985, 13(5): [3] ANGUS J C, LU B, ZAPPIA M J. Potential ph diagrams for complex systems [J]. J Appl Electrochem, 1987, 17(1): 11. [] YOKOKAWA H, SAKAI H, KAWADA, DOKIYA M. Generalized electrochemical potential diagrams for complex aqueous(m-x-h-o-e-) systems [J]. J Electrochem Soc, 1990, 137(): [5] ANDERKO A, SANDERS S J, YOUNG R D. Real-solution stability diagrams: A thermodynamic tool for modeling corrosion in wide temperature and concentration ranges [J]. Corrosion, 1997, 53(1): 353. [6] HOMPSON W, KAYE M H, BALE C B, PELON A D. In Uhlig s corrosion handbook [M]. New York: John Wiley & Sons, 000: [7] KINNIBURGH D G, COOPER D M. Predominance and mineral stability diagrams revisited [J]. Environ Sci echnol, 00, 38(13): [8] ISHIK I. hermodynamic stability relations in redox systems [J]. Environ Sci echnol, 006, 0: [9] CRISS C M, COBBLE J W. he thermodynamic properties of high temperature aqueous solutions. IV. Entropies of the ions up to 00 and the correspondence principle [J]. J Am Chem Soc, 196, 86(): [10] AYLOR D. hermodynamic properties of metal-water systems at elevated temperatures [J]. Journal of the Electrochem Society, 1978, 15(5): [11] YANG X W, HE A P, YUAN B Z, HU H. he thermodynamic calculation in high-temperature aqueous solution [J]. Nonferrous Metals: Extractive Metallurgy, 1979(6): 737. [1] BALL J W, NORDSROM D K. itical evaluation and selection of standard state thermodynamic properties for chromium metal and its aqueous ions, hydrolysis species, oxides, and hydroxides [J]. Journal of Chemical and Engineering Data, 1998, 3(6): [13] LIANG Ying-jiao, CHE Yin-chang. Handbook of inorganic thermodynamics [M]. Shenyang: Northeastern University Press, (in Chinese) [1] ZHONG Z Q, MEI G G. Application of diagrams of chemical potential in hydrometallurgy and purification of waste water [M]. Changsha: Central South University of echnology Press, (in Chinese) [15] ENDOR S, LA ORCE M J, LI Guang-chao. Heavy metals in the environment-temporal changes in soil partitioning and bioaccessibility of arsenic, chromium and lead [J]. Journal of Environmental Quality, 00, 33(6): [16] BARNER H E, SCHEUERMAN R V. Handbook of thermochemical data for compounds and aqueous species [M]. New York: John Wiley & Sons, 1978: 173. [17] WOOD R H. he heat free energy and entropy of the ferrate (VI) ion [J]. Journal of the American Chemical Science, 1958, 80(9): 038. [18] PEI Hui-xia. Study on stability and preparation of potassium ferrate (VI) [D]. aiyuan: aiyuan University of echnology, 007: 01. (in Chinese) (Edited by LONG Huai-zhong)

Ruthenium redox equilibria 3. Pourbaix diagrams for the systems Ru-H 2 O and Ru-Cl - -H 2 O

J. Electrochem. Sci. Eng. 6(1) (216) 145153; doi: 1.5599/jese.229 Original scientific paper Open Access : : ISSN 18479286 www.jeseonline.org Ruthenium redox equilibria 3. Pourbaix diagrams for the systems