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Metallurgical Principles and Practice of Engineering (PE) Study Guide: Question #18

It Is the Policy of The Minerals, Metals & Materials Society (TMS) to Encourage the Professional Registration of Metallurgical Engineers.

An Imperial Pb Smelter shown in Figure 1 reduces a feed of PbO and ZnO. The reduction is performed using coke composed of C and inert material. Molten hot metal tapped from the bottom of the furnace is a Pb-Zn alloy. Most of the Zn leaves the furnace in the top gas consisting of N2, CO, CO2, and Zn. This gas is passed through a shock quencher where liquid Pb is sprayed through the top gas so as to capture the Zn vapor by condensation and subsequent dissolution into the liquid Pb. Some of the Zn vapor reacts with the CO2 according to the reaction

Zn(g) + CO2(g) = ZnO(s) + CO(g)


Figure 1. Imperial Smelting Furnace

Table 1 below summarizes the gas amounts entering and exiting the quencher. Liquid Zn is recovered from the liquation unit by phase separation upon cooling of the Pb-Zn alloy from the quencher.

TABLE 1. Gas amounts entering and exiting the quencher.
Item
Top Gas
Gas Out
N2 (g)
48.29 lb moles
57.43 %
48.29 lb moles
CO2 (g)
3.61 lb moles
3.61 %
3.47 lb moles
CO (g)
25.82 lb moles
30.71 %
25.96 lb moles
Zn (g)
6.37 lb moles
7.58 %
0.09 lb moles
ZnO (solid)
 
-
-
Volume @STP
30,188 ft3
 
27,934 ft3
T
1000 °C
 
650 °C

Data:

CO2(g) + Zn(g) = ZnO(s) + CO(g) DG°(cal/gmole) = - 45065+28.289*(T, Kelvin)

TABLE 2. Zinc Vapor Pressure
T, ºC
P°Zn, atm
377
5.5×10-5
650
0.045
923
1.16


TABLE 3. Heats of Formation at 77ºF and Heat Capacities
Gas
DH°Formation, 77 °°F
(BTU/(lbmole)
Cp
(BTU/ (lbmole*°F)
N2 (gas)
-
7.0
CO2 (gas)
-169,290
11.0
CO (gas)
-47,556
7.2
O2 (gas)
-
7.1
Zn (gas)
56,104
5.0
ZnO (solid)
-149,706
12.0


TABLE 4. Data for Liquid Pb-Zn Alloys at 650 °C
Mole Fraction of Zn
Activity of Zn in Liquid Pb-Zn Alloys
Relative to Pure, Liquid Zn
Activity Coefficient
0
0
7.94
0.1
0.627
6.273
0.2
0.942
4.709


Figure 2. Sketch of the Pb-Zn Phase Diagram

What conditions would make the reaction below LESS likely?

CO2(g) + Zn(g) = ZnO(s) + CO(g)

(A)
Higher T & Higher Ptotal
(B)
Lower T & Lower Ptotal
(C)
Higher T & Lower Ptotal
(D)
Lower T & Higher Ptotal


The information on this page is maintained by the Nellie Luther (natale@tms.org).

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