Point Defects in PbS and Their Effects in the Decopperization of Lead with Sulfur
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fur at about 330°C that the effect of impurities upon the decopperization process may be considered from the view of the point defects created in PbS by the impurity sulfide and by Cu 2S. In particular, if the impurity sulfide creates a defect of opposite type to that created by the Cu 2S in PbS then the solubility of the Cu2S is enhanced while the creation of the same type of point defect by the impurity sulfide as that created by the CU2S lowers the solubility of the Cu2S in PbS and, hence, decreases the decopperization effectiveness of the "cold" drossing process.
IN industrial practice':" it is common to decopperize lead, in a final step, by introducing a small amount of sulfur into molten lead at about 330°C, stirring the melt, and after a relatively short time (10 to 30 min) removing the" cold" dross which consists of entrapped lead, solid lead sulfide and impurity sulfides which mayor may not be in solid solution in the PbS formed. Of some concern is the effect that the impurities present in the lead may have upon this process. In particular, the effects of the impurities, Ag, As, Bi, Sb and Sn have received some attention since one or more of these elements is frequently found as an impurity in the lead and the effects found by several investigators appear to be in contradiction.v" In the following it is shown that consideration of the point defects in the solid PbS along with dopant effects of the impurities allows one to at least partially rationalize the seemingly contradictory results obtained previously.
In converse to this the dissolution of Ag 2S into PbS occurs as follows:
[4] where the dash superscript indicates an excess negative charge in comparison to the pure, undisturbed defe ct free lattice. Equations [3] and [4] are the normally accepted manner of the respective dissolution of Bi 2S g and Ag 2 S into PbS. It is anticipated that As and Sb act in a similar manner to that of Bi. Contrary to the case of Ag 2S cuprous sulfide may occupy either a cationic lattice site position or an interstitial site dependent upon the temperature and time at temperature." At high temperature and at long times at low temperatures copper ions have been shown to substitute for lead ions in the cationic sublattice. This is given as: Cu2S
THEORETICAL
=
Pbint +
[5]
+ 2Pbint + 1/2S 2
with the equilibrium condition:
Lead sulfide in equilibrium with lead has been shown to be an n-type semiconductor with singly charged cation interstitials, Pblpt, and excess electrons, e, as compensating detects." The formation of these defects is given as: PbS
= 2Cu(Pb)'
e + 1/2 S2
[1]
with the equilibrium condition:
{K5
=
[Cu(Pb)'] . [Pbint]} constant aCu2S, P
s2 .
Bloem 4 has shown that at short times at low temperatures, copper moves very rapidly into PbS via interstitial sites. This dissolution process is then given as: Cu2S = 2CUint + 2e + 1/2S 2
[2]
{K = [Pbint]' [e]}PS2 = constant' 1
The dissolution of BbSg into PbS makes the PbS a more n-type semiconductor and, therefore, is represented by: Bi2S g = 2Bi(Pb)'
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