Thermodynamics of removing selenium and tellurium from liquid copper by sodium carbonate slags
- PDF / 220,769 Bytes
- 2 Pages / 614.28 x 794.28 pts Page_size
- 72 Downloads / 176 Views
L~.{e = (pct Me)/[pct Me]
[1]
between the sodium carbonate slag and metallic copper as a function of the prevailing oxygen partial pressure in the system are shown in Figure 1. As one can see, very high distribution coefficients for both selenium and tellurium are found in reducing conditions where L~'; re is of the order of 1000 at Po, = 10 ~5bar. In oxidizing conditions, however, much smaiter distribution coefficient values are obtained, Ur; reaching 50 and L~'; ~- 2 at the oxygen saturation of the copper bath. As Figure 1 shows, the distribution coefficient plots exhibit a refining minimum at Po,_ ~ 10-6 bar and Po: = 10 7 bar for selenium and tellurium, respectively. A thermodynamic resolution of the distribution equilibria indicates that the minority elements Se and Te can be removed from the metal bath to the sodium carbonate flux either in a cationic or in an anionic form, depending on the prevailing oxygen partial pressure of the system. The corresponding reactions can be written as [Me] + n/2 O2(g) = (MeO,)
[2]
[Me] + m/2(Na20) = (Na~Me) + m / 4 0 , ( g ) ,
[3]
where [ ] and ( ) represent metal and slag phases, respectively. Equations [2] and [3] lead to the following forms of the distribution coefficient: lg L~,~ = lg(pct Me)/[pct Me] = lg K~ + n / 2 lg Po: [4t and lg L~'~ = lg(pct Me)/[pct Me] 2 _ m /4 lg Po_, = lg K3. . 9. .ar~d,o
[5]
where K; and K~ are constants if the concentration of Me in +3
log
SIC
LSe.Te +2
\\
+1
It -1
: 1200"C
i CO 2 { g , 1 b a r ) [ c, 9
\
Se
o,O,v,I
9
Te
L
-2 -15
'D
_
,\
r
Oxygen saturation ,
L
J
j
J
i
I
t
-10
J
-5 log
po 2 / b a r )
Fig 1--Equilibnum d~smbuuon coefficient of selenium and tellurium between liquid copper and sodmm carbonate at 1200 ~ independent runs are represented by different symbols and the sohd hnes show the calculated equilibria according to Eqs [11] and [12] XOLUME 16B. MARCH 1985--171
both the phases is small, i.e., Henrian behavior can be assumed for Me and MeXr in metal and slag, respectively. K; and K; can be written as K[ = K, " (U[~176
" (NcJN~2C03),
[6]
where f~MeX,~and fi~ are the activity coefficients of MeX~ and Me in the slag and metal phases, respectively, and N, is the atomic or molar weight of the species i. The first term in the right-hand side of Eq. [5] was approximated to be constant in the present experimental conditions, because the activity of Na20 in pure sodium carbonate is determined by Pco2 only and in all runs Pco, = 1 bar. This assumption is justified also by the fact that t[ae maximum impurity concentration in the slag was always less than 2 pct. The slope of the lg L~e vs lg Po2 plots for selenium and tellurium in reducing conditions corresponds to m = 2 indicating a formation of Na2Se and Na2Te species in the slag. In oxidizing conditions, the slope for tellurium and approximately also for selenium corresponds to n = 2, giving an evidence of the formation of tetravalent selenate and tellurate species in the sodium carbonate flux. Thus, the following activity coefficients of the m
Data Loading...
