A process for debismuthizing lead with magnesium
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I.
INTRODUCTION
B E T T E R T O N [2] claimed that after the removal of silver, arsenic, and antimony, lead could be treated with calcium and magnesium to reduce the concentration of bismuth to less than 0.005 wt pct. Betterton and Lebedeff~3l discussed the treatment of lead with magnesium alone for the removal of bismuth. They reported that the addition of 1.0 wt pct Mg to a lead alloy containing 0.6 wt pct Bi (see point [] in Figure 1) produced an alloy containing 0.991 wt pct Mg and 0.15 wt pct Bi (point 9 in Figure 1) after controlled crystallization, whereas for lead containing 0.18 wt pet Bi with the addition of 0.4 wt pct Mg (point A in Figure 1), the concentration of bismuth increased slightly to 0.188 wt pct after controlled crystallization. Davey [41 reviewed the pyrometallurgical processes for the removal of bismuth from lead. Based on the data presented by Betterton and Lebedeff, [3] he proposed that the eutectic trough between the lead and magnesium bismuthide phase fields was isothermal and terminated at a ternary point (0.027 wt pct Bi and 2.5 wt pet Mg). Esdaile e t al.lS] investigated this system and showed that the composition at the ternary point was 0.0247 wt pct Bi and 2.334 wt pct Mg at 252 ~ which is in fair agreement with that proposed by Davey. Data from Esdaile e t al. also showed that an alloy on the eutectic trough, containing 1.0 wt pct Mg, has 0.124 wt pct Bi (point 9 in Figure 1), which is in fair agreement with the value given by Betterton and Lebedeff (point 9 in Figure 1), while an alloy containing 0.18 wt pct Bi and 0.4 wt pct Mg was in the primary lead phase field and crystallization of lead from this alloy increased the concentration of bismuth. An investigation of the Mg-Pb system by Siviour and Ng 161 confirmed the existence of an intermetallic compound/3' (35.1 at. pct Pb) which was originally reported by Eldridge e t al. 171 Eldridge e t al. proposed that /3' transforms to/3 and lead on cooling to 291 ~ by a series K. NG, Experimental Scientist, and N.G. SIVIOUR, Principal Experimental Scientist, are with CSIRO, Division of Mineral and Process Engineering, Clayton, Victoria 3169, Australia. Manuscript submitted March 26, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
of catatectic reactions. However, Siviour and Ng reported several additional phase transformations in the system. Of these, the transformations at 257 ~ and 268 ~ are of particular interest and will be discussed in Section IV-B-2. The investigation of the Pb-Mg-Bi system by Esdaile e t al. m was extended by Siviour and Ng lu to include alloys containing up to 5 wt pet Mg and 0.3 wt pct Bi. The peritectic reaction between magnesium bismuthide (Mg3Bi2) and magnesium plumbide (Mg2Pb) showed a transition from an even to an odd reaction, as defined by Ricci, t8J at a composition of approximately 4.2 wt pct Mg and 0.2 wt pet Bi. This change in the nature of the reaction was due to the incorporation of bismuth from the magnesium bismuthide into the magnesium-lead intermetallic compound and provided the basis for t
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