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Volatilisation of tellurium from gold telluride
632
SHORT COMMUNICATIONS
Volatilisation
of tellurium
from gold telluride
Gold telluride occurs fairly widely as the mineral calaverite from which metallic gold is extracted. During the extraction process the ore is roasted in air. This may volatilise some tellurium as such and oxidise some to TeOa. It is thus of interest to measure the vapour pressure of tellurium over compositions close to AuTez. Nature of the volatile species In separate experiments AuTez containing some Au197 as radioactive tracer was heated in vacuum to 4o0°, 470’ and 62o’C. In no case could radioactivity be detected in the condensate. The volatile species was therefore assumed to be Tezl. Vapour
pressure of tellurium over solid gold tell&de In the present work vapour pressure-composition data has been obtained at 4oo”, 415”, 430’ and 447°C by the transportation method for the composition range 66.6-66.8 at.% tellurium. About IO g of sample were used and placed on a sintered disc of porosity 2 through which the carrier gas (nitrogen) passed. Otherwise the usual precautions were observedz. Typical isotherms for 400’ and 447°C are shown
-1.0
-I* 5
Log P
-2.0
-2.5
t
60
66.65
Fig. 1. Vapour pressure-composition
J. Less-Common Metals,
66’10 ATOM% TELLURIUM isotherms near Au
13 (1967) 632-634
I
66.75
66.80
: Te = I : 2 for 400’ and 447’C.
SHORT
COMMUNICATIONS
633
in Fig. I. Table I shows the apparent limits of composition for single-phase gold telluride. These values may not be accurate to better than +o.oz at.% composition. Thus the existence of single phase gold telluride over a range of composition T.AALE LIMITS
I OF
COMPOSITION
‘to” 4’5 4.70 447
OF
“XuTeo”
upper limit
LOZkY limit -
66.69 Oh.71 66.71 66.69
66.64 66.67 66.69 66.67
(at.“/” Te)
I .2
I ,I
I.0
0.9
0.8
0.7 P mm 0.6
0.5
0’4
0.3
0.2
0.1
58
60
62
64
66
68
70
72
ATOM O/olELLUfWM Fig. I. Vapour tellurium.
prcssurc-composition
isotherms
for
the
liquid
system
J. Less-Common
containing
Metals,
60-72 at.l:
13 (1967) 632-634
SHORT
634
COMMUNICATIONS
is proven only at 400°C. Previous information was confined to a statement that less than 0.3 wt.% excess tellurium could dissolve in gold telluridea. Vapour pressures in the two-phase region gold telluride+gold conform to the relationship log p,,,,=
10.27 -8700/T
This equation is closely similar to that from Knudsen effusionb. Using the GibbsHelmholtz equation and entropy values quoted by MILLS~, the present data yields a value of -4.2 kcal/mole for the heat of formation of gold telluride. This compares with the value of -3.7 kcal/mole obtained from the effusion data4. In the two-phase region gold telluride + tellurium, vapour pressure values are in good agreement with those of solid tellurium, thus showing that solid solubility of gold telluride in tellurium is very small. Vapour presswe
ovey liquid Au-Te
alloys near AuTen
Again using the transportation technique, vapour pressures have been measured in the range 59-73 at.% tellurium from 477” to 570°C. Isotherms are shown in Fig. 2. For the composition Au: zTe the vapour pressure of tellurium over the liquid is given by log pInIn = 7.773 -6579/T Activity
coefficients increase from 0.25-0.3 at 60 at.% tellurium to 0.4-0.5 at 70 at.%. Within the composition range under study no evidence was found for liquid immiscibility over the liquidus curve. A redetermination by DTA shows that this liquidus is even more nearly flat than that shown previously5. An excess of 9 at.% tellurium depresses the freezing point of the compound by less than 5°C. Flat peaks on liquidus curves have been found for some III-V semiconducting compounds and attributed to high entropies of fusion resulting from relatively small departures from ideality in the liquid state697. Qualitatively a similar explanation can be offered in the present case where activity coefficients are considerable and where, owing to the lower melting points, the radius of curvature of the liquidus will be large. Materials Unit, National Physical Laboratory,
Inorganic
Teddington,
I 2 3 4 5 6 7
Middlesex
(Gt. Britain)
B. SIEGEL, @uzvt. Rev., q (1965) 77. C.B. ALCOCK AND G.W. HOOPER, Met.Soc.Conf., 7 (1959) 325. L. J. CABRI, Econ.Geol., 60 (1965) 1569. K. C. MILLS, to be published. G. PELLINI AND E. QUERCIGH, Atti Accad. Nazi. Lincei, 19 (1910) 445 C. WAGNER, Acta Met., 6 (1958) 309. W.F. SCHOTTKYANDM.B.BEVER, Acta Met.,6 (1958) 320.
Received
July rgth,
J. Less-Common
1967
Metals, 13 (1967)632-634
C. R. VEALE
M.F.
BARRETT
SHORT COMMUNICATIONS
Volatilisation
of tellurium
from gold telluride
Gold telluride occurs fairly widely as the mineral calaverite from which metallic gold is extracted. During the extraction process the ore is roasted in air. This may volatilise some tellurium as such and oxidise some to TeOa. It is thus of interest to measure the vapour pressure of tellurium over compositions close to AuTez. Nature of the volatile species In separate experiments AuTez containing some Au197 as radioactive tracer was heated in vacuum to 4o0°, 470’ and 62o’C. In no case could radioactivity be detected in the condensate. The volatile species was therefore assumed to be Tezl. Vapour
pressure of tellurium over solid gold tell&de In the present work vapour pressure-composition data has been obtained at 4oo”, 415”, 430’ and 447°C by the transportation method for the composition range 66.6-66.8 at.% tellurium. About IO g of sample were used and placed on a sintered disc of porosity 2 through which the carrier gas (nitrogen) passed. Otherwise the usual precautions were observedz. Typical isotherms for 400’ and 447°C are shown
-1.0
-I* 5
Log P
-2.0
-2.5
t
60
66.65
Fig. 1. Vapour pressure-composition
J. Less-Common Metals,
66’10 ATOM% TELLURIUM isotherms near Au
13 (1967) 632-634
I
66.75
66.80
: Te = I : 2 for 400’ and 447’C.
SHORT
COMMUNICATIONS
633
in Fig. I. Table I shows the apparent limits of composition for single-phase gold telluride. These values may not be accurate to better than +o.oz at.% composition. Thus the existence of single phase gold telluride over a range of composition T.AALE LIMITS
I OF
COMPOSITION
‘to” 4’5 4.70 447
OF
“XuTeo”
upper limit
LOZkY limit -
66.69 Oh.71 66.71 66.69
66.64 66.67 66.69 66.67
(at.“/” Te)
I .2
I ,I
I.0
0.9
0.8
0.7 P mm 0.6
0.5
0’4
0.3
0.2
0.1
58
60
62
64
66
68
70
72
ATOM O/olELLUfWM Fig. I. Vapour tellurium.
prcssurc-composition
isotherms
for
the
liquid
system
J. Less-Common
containing
Metals,
60-72 at.l:
13 (1967) 632-634
SHORT
634
COMMUNICATIONS
is proven only at 400°C. Previous information was confined to a statement that less than 0.3 wt.% excess tellurium could dissolve in gold telluridea. Vapour pressures in the two-phase region gold telluride+gold conform to the relationship log p,,,,=
10.27 -8700/T
This equation is closely similar to that from Knudsen effusionb. Using the GibbsHelmholtz equation and entropy values quoted by MILLS~, the present data yields a value of -4.2 kcal/mole for the heat of formation of gold telluride. This compares with the value of -3.7 kcal/mole obtained from the effusion data4. In the two-phase region gold telluride + tellurium, vapour pressure values are in good agreement with those of solid tellurium, thus showing that solid solubility of gold telluride in tellurium is very small. Vapour presswe
ovey liquid Au-Te
alloys near AuTen
Again using the transportation technique, vapour pressures have been measured in the range 59-73 at.% tellurium from 477” to 570°C. Isotherms are shown in Fig. 2. For the composition Au: zTe the vapour pressure of tellurium over the liquid is given by log pInIn = 7.773 -6579/T Activity
coefficients increase from 0.25-0.3 at 60 at.% tellurium to 0.4-0.5 at 70 at.%. Within the composition range under study no evidence was found for liquid immiscibility over the liquidus curve. A redetermination by DTA shows that this liquidus is even more nearly flat than that shown previously5. An excess of 9 at.% tellurium depresses the freezing point of the compound by less than 5°C. Flat peaks on liquidus curves have been found for some III-V semiconducting compounds and attributed to high entropies of fusion resulting from relatively small departures from ideality in the liquid state697. Qualitatively a similar explanation can be offered in the present case where activity coefficients are considerable and where, owing to the lower melting points, the radius of curvature of the liquidus will be large. Materials Unit, National Physical Laboratory,
Inorganic
Teddington,
I 2 3 4 5 6 7
Middlesex
(Gt. Britain)
B. SIEGEL, @uzvt. Rev., q (1965) 77. C.B. ALCOCK AND G.W. HOOPER, Met.Soc.Conf., 7 (1959) 325. L. J. CABRI, Econ.Geol., 60 (1965) 1569. K. C. MILLS, to be published. G. PELLINI AND E. QUERCIGH, Atti Accad. Nazi. Lincei, 19 (1910) 445 C. WAGNER, Acta Met., 6 (1958) 309. W.F. SCHOTTKYANDM.B.BEVER, Acta Met.,6 (1958) 320.
Received
July rgth,
J. Less-Common
1967
Metals, 13 (1967)632-634
C. R. VEALE
M.F.
BARRETT