Volatilisation of tellurium from gold telluride

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...

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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.

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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

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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