AGH University of Science and Technology Faculty of Non-Ferrous Metals Laboratory of Physical Chemistry and Electrochemistry THERMODYNAMICS OF OXYGEN IN DILUTE LIQUID SILVER-TELLURIUM ALLOYS Justyna Nyk, Bogusław Onderka, Krzysztof Fitzner
MOTIVATION ANODIC SLIME a by-product of the electrolytic refining of copper. ANODIC SLIME a source of silver, gold, platinum group metals, selenium and tellurium. Composition of anodic slime silver Gold, Pt-group selenium, tellurium
DIAGRAM OF SILVER PROCESSING Electrolytic refining of copper Cu Slime containing Ag Treatment of anodic slime Kaldo furnace charge Kaldo furnace
DIAGRAM OF SILVER PROCESSING Kaldo furnace Oxidation Refining Removal of Te by sodium carbonate Electrolytic refining of silver Ag
Refining of silver 1 OXIDATION 2 REMOVAL OF SELENIUM 3 REMOVAL OF TELLURIUM
Tellurium elimination from Ag The tellurium elimination by oxidation: [ Te ] + 2 [ O ] = ( TeO ) + = (1) Ag Ag 2 slag L slag / Ag Te γ [ Te ] ( n) Ag slag = K 1 γ [ n] TeO 2 Ag a 2 [ O ] Ag (2)
Tellurium elimination by sodium carbonate Product of reaction depends on oxygen pressure: - In reducing conditions product is sodium telluride (Na 2 Te): [ ] ( ) Te + Na CO = Na Te + CO + 0.5O Ag 2 3 2 slag 2 2 (3) - In oxidizing conditions product is sodium tellurate (Na 2 TeO 3 ): [ ] ( ) Te + Na CO + O = Na TeO + CO (4) Ag 2 3 2 2 3 slag 2 M.Perez-Tello and M.Prito-Sanchez, A Kinetic Model for the Oxidation of Selenium and Tellurium in an Industrial Kaldo Furnace,JOM, 56(12), 52-54 (2004)
Removal of tellurium by sodium carbonate Distribution coefficient of tellurium between slag and metallic phase for reaction (3) and (4): - Reducing conditions: log L = - 1 2log a + slag / Ag Te,(3) [ O] Ag const (5) - Oxidizing conditions: log L = log a + slag / Ag Te,(4) [ O] Ag const (6) For constant P CO2 and temperature an activity of Na 2 CO 3 was assumed to be 1 (n Na >> n Te ).
Oxygen activity determination The coulometric titration measurements of galvanic cell with YSZ solid oxide electrolyte: Ag-(Te)-O ZrO 2 +Y 2 O 3 O 2 (in air) P O 2 P O 2 = 0.213 bar (I) Method aim: Determination of effect of tellurium concentration on the activity coefficient of oxygen in dilute liquid Ag alloys - (γ o Ag ) in defined temperature range.
Scheme of assembly <3> <2> <1> Fig 1. The scheme of electrochemical cell assembly for the coulometric titration measurements.
Examplesof changeof electric current with time for experiments with liquid silver tellurium alloys 2% at. Te 4% at. Te T=1285 K T=1285 K Q=1.4570 C Q=4.5953 C
Theoretical basis The coulometric titration method bases on measure of EMF change dependent on partial pressure of oxygen, as follows: E 1 R T 0.21 = ln 4 F p O 2 (7) Activity coefficient of oxygen γ 0 in liquid metallic phase: O 1/2 2 γ = (8) 0 (p ) C 1
Theoretical basis The change of concentration of oxygen in sample during titration experiment: 100 M Q jon C1 C2 = (9) 2 W F Activity coefficient of oxygen γ 0 in liquid solution: γ 0 2E F 0.21 ln R T C 1 1 = exp + (10) The standard Gibbs free energy of oxygen dissolution in liquid metallic melt: G = RT ln γ (11) 0 O 0
Experimental results I. Measurements of activity coefficient of oxygen in liquid silver reference and test of this method.
Experimental results for pure liquid Ag Experimental temperature range: 1285-1485 K (step: 50 K) Table 2. The example of obtained results at 1285 K T= 1285, K G E 0 O, 1, V E 2, V Q jon, C C 1, at. % O ln γ O O J/mol 0.103 0.291 0.73025 0.15561-0.75899-8108 0.113 0.306 0.61579 0.12989-0.73256-7826 0.113 0.307 0.57968 0.12989-0.75567-8073 0.101 0.295 0.54681 0.16133-0.74138-7921 0.100 0.245 0.66284 0.16427-0.73209-7821 0.116 0.310 0.45696 0.12304-0.76581-8182 0.108 0.302 0.49750 0.14217-0.75134-8027
Temperature dependence of standard free energy of oxygen dissolution in pure liquid Ag Fig. 2.
II. Measurements of activity coefficient of oxygen for dilute liquid Ag-Te-O alloys
Phase diagram of Ag-Te Experimental Te concentration range was limited to dilute liquid solution of Te in Ag. 1250 Temperature e, 0 C 1125 1000 875 L 1 +FCC L 1 LIQUID L 1 + L 2 L 1 + Ag 2 Te L 2 FCC + Ag 2 Te L 2 + Ag 2 Te 750 0 10 20 30 40 50 Ag mole %Te mole %Te Fig. 3.
Experimental results for dilute liquid Ag-Te-O alloys Table 3. Experimental results for 2 at.% Te liquid Ag-Te-O alloy. T = 1385 K No. E 1, V E 2, V Q jon, C C 1, at. % O G ln γ 0 O, O o J/mol 1 0.145 0.323 1.327 0.0880-0.749-8623 2 0.161 0.338 0.997 0.0673-0.750-8641 3 0.161 0.34 1.273 0.0673-0.752-8656 4 0.152 0.333 1.098 0.0783-0.749-8621 5 0.142 0.325 0.975 0.0926-0.730-8405 6 0.167 0.347 0.957 0.0609-0.749-8622 7 0.187 0.368 1.311 0.0436-0.740-8523
Experimental results for dilute liquid Ag-Te-O alloys - example for two tellurium concentrations in Ag-Te-O alloys. 2 at. % Te 3 at. % Te Fig. 4. The standard Gibbs free energy of oxygen dissolution in Ag-Te liquid solution as a function of temperature.
Interpretation The change in oxygen activity coefficient in dilute solution caused by the addition of the other metallic component defined as a function of Te concentration: 0 (Te) O O O O Te ln γ = ln γ ln γ = ε X where: (Te) lnγ ε O O = X Ag Te X 1 is a first order interaction parameter. The mole fraction of tellurium addition is denoted by X Te.
ln γ o vs. X Te and temperature dependence Table 4. ln γ o vs. X Te and temperature dependence (0 < X Te < 0.06) 1285K 1335 K 1385 K 1435 K 1485 K X Te lnγ o lnγ o lnγ o lnγ o lnγ o 0.01-0.1466-0.1254-0.1182-0.1504-0.1220 0.02-0.1159-0.0828-0.0796-0.0647-0.0665 0.03-0.2339-0.1490-0.1123-0.1113-0.0506 0.04-0.3227-0.3247-0.2825-0.2599-0.2486 0.05-0.2537-0.2207-0.1853-0.2052-0.1823 0.06-0.2592-0.2764-0.3528-0.2497-0.1944
lnγ o vs. X Te dependence (0 < X Te < 0.06) determined for different T values 1385 K 1335 K Fig.5. The dependence of ln γ o vs. X Te (0 < X Te 0.06) determined for different temperatures.
Temperature dependence of a first-order interaction parameter ε O Te Table 5. T, K 1/T, K -1 Te ε O 1485 6,734-3.823 1435 6,968-4.590 1385 7,220-5.216 1335 7,490-5.272 1285 7,78-5.708 Fig. 6. Interaction parameterε O Te as a function of (1/T).
Conclusions 1. The standard free energy of oxygen dissolution in liquid Ag-Te alloys showed negative values. Its absolute value increases with increase of Te concentration and decreases with the growth of temperature. 2. At given temperature tellurium concentration enlargement causes the depression of activity coefficient of oxygen in liquid Ag-Te solutions.
Influence of lead on thermodynamics of oxygen in Ag-Te liquid solutions Some hot results Ag-Te-Pb-O alloy Ag-Te-O alloy 4 at. % Te 10 at. % Pb 4 at. % Te
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Wnioski 1. Energia swobodna Gibbsa rozpuszczania tlenu w ciekłym roztworze Ag-Te ma wartości ujemne i jej wartość bezwzględna rośnie z zawartością telluru i maleje ze wzrostem temperatury. 2. W danej temperaturze wzrost stężenia telluru powoduje obniżenie współczynnika aktywności tlenu w ciekłym roztworze. Oznacza to zmniejszenie rozpuszczalności tlenu w ciekłym roztworze Ag z dodatkiem telluru.
lnγ o vs. X Te dependence (0 < X Te < 0.06) determined for different T values 1335 K 1385 K Fig.5. The dependence of ln γ o vs. X Te (0 < X Te 0.06) determined for different temperatures. 90% confidence intervals were calculated.
Scheme of the cell air Fig 2. Scheme of solid oxide galvanic cell.
Distribution Coefficient The distribution coefficient describes the partition accompanying elements between slag and metal and is therefore an indicator for the efficiency of the metal extraction [8]. (1) In the slag the different metals M (e.g. Cu, Ni, Zn, Pb, Sn etc.) exist in the form of oxides, as demonstrated by the reaction in equation 2. Equation 3 shows the corresponding equilibrium constant to this reaction. The distribution coefficient is directly proportional to p O2 n/2 if the behavior of the metal M in the copper and of the metal oxide MO in the slag obeys Henry`s law.
The distribution coefficient of a solute between two The distribution coefficient of a solute between two phases is calculated as the ratio of the concentration of the solute in one phase to the concentration of the solute in the other phase under equilibrium conditions.
Usuwanie telluru za pomocą sody W atmosferze redukcyjnej tellur można usunąć jako tellurek sodu Na 2 Te. Metodę tą opisuje reakcja: [ ] ( ) Te + Na CO = Na Te + CO + 0.5O Ag 2 3 2 żużel 2 2 Współczynnik podziału telluru między fazę żużlową i metaliczną: K g żużel / Ag 2 [ Te] Ag żużel log LTe = log + log ana2 ( CO3 ) g Na Te[ n] Ag 2 ( n) æ1 ö - ç log po - log çè 2 ø p CO 2 2 (2) Aktywność Na 2 CO 3 można przyjąć za równą 1, co dla ustalonego ciśnienia parcjalnego CO 2 i temperatury prowadzi do zależności: log żużel / Ag LTe,(2) = - 1 2log a[ O] + const Ag
Usuwanie telluru za pomocą sody W atmosferze utleniającej tellur można usunąć jako telluryn sodu Na 2 TeO Metodę tą opisuje reakcja: [ ] ( ) Te + Na CO + O = Na TeO + CO Ag 2 3 2 2 3 żużel 2 Współczynnik podziału telluru między fazę żużlową i metaliczną: K g żużel / Ag 3 [ Te] Ag żużel log LTe = log + log ana2 ( CO3 ) g Na TeO [ n] Ag 2 3 ( n) æ1 ö - ç log po + log çè 2 ø p CO 2 2 (3) Aktywność Na 2 CO 3 można przyjąć za równą 1, co dla ustalonego ciśnienia parcjalnego CO 2 i temperatury prowadzi do zależności: log L = log a + żużel / Ag Te,(3) [ O] Ag const
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