An overall view of the cell is shown in figure 8. Direct current was furnished by a Hewlett-Packard model 6466A constant -amperage rectifier (0-600 amp, 0-18 volts). For heating the cell overnight and during weekends, a 75-kva ac welding transformer was used with a Superior Electric powerstat in the primary circuit to control the output. Changing from dc to ac or vice versa was done with a 600-amp Meyers transfer switch. Temperature readings from different parts of the cell were noted with a four-channel Honeywell model C153 recorder. Voltage was checked with a Heathkit IM-102 multimeter, and the number of ampere-hours was determined with a Rapid Electric digital meter.

Operation of the cell was extremely simple. Chlorine, evolved during electrolysis, was drawn through a scrubbing tower (fig. 8). Before electrolysis, a circulating pump was turned on. The dc circuit was then activated, and enough PbC12 added to the cell at 30-min intervals to maintain the bath level. When the run was complete, the rectifier was switched off and the metal produced was drawn by suction into a graphite mold (fig. 8). The transfer switch was then moved from the dc to the ac setting, and the ac

circuit was turned on to maintain the cell at operating temperature until the next run.

As shown in table 5, 379 amp and 11.6 volts were needed to maintain a heat balance during electrolysis. When the cell was not in service, the ac input averaged 539 amp and 5.4 volts; less energy was required because no metal was being produced. These results indicate that about two-thirds of the energy supplied to the cell during electrolysis was being used to maintain the operating temperature. The average current efficiency with the

bipolar arrangement was 86 pct; the average energy requirement was
0.52 kwhr/lb Pb. This cell operated with no significant difficulties for
1 month, during which time about 600 lb of metal was made, using reagent-
grade PbClą.

EFFECT OF IMPURITY BUILDUP

Tests were made next to determine the effect of impurity buildup in the bath. Although a very pure grade of PbCl2 can be made by the method shown in figure 1, it will inevitably contain impurities which will in time affect the grade of metal produced, interfere with the operating conditions, and/or cause the bath to be discarded. To determine how quickly difficulties would occur, 1,200 pounds of PbC12 (equal to 17 bath changes) was prepared by ferric chloride leaching of galena concentrate as shown in figure 1. The cell was then operated for 9 weeks using this PbCl2 as feed. Results are shown in table 6 along with the ASTM specifications for corroding-grade lead. The specifications include upper limits for As, Bi, and Sn (15, 500, and 95 ppm, respectively) but these are not shown in table 6 because all three elements were below the limit of detection (1 ppm).

Some of the results were surprising. According to the standard electromotive series, Ag, As, Bi, Cu, Fe, and Sb all have lower decomposition potentials than Pb and should end up in the deposit. Na, Sn, and Zn have higher decomposition potentials than Pb and should not be found in the metal as long as there is any appreciable amount of PbCl2 left in the bath. Actually, while all of the Ag and Sb deposited with the lead, Cu and Fe did not. Also, some Zn was found in the product. Arsenic, bismuth, and tin could not be accounted for because they were not present in the PbCl2 added to the cell. None of these elements dissolved during the ferric chloride leach, although a measurable amount of each was contained in the galena Nickel dissolved in the leach while cobalt did not. The solubility of any of these elements, however, will depend upon the form in which they are present in the concentrate. Silver and antimony could not be detected in either the PbCl feed or the bath but did show up in the metal. The anomalies could be due to analytical error, to complex formation changing the relative position of the elements in the electromotive series, or to the form in which the elements were present; for example, iron in the feed could have been present as an insoluble oxide rather than as a soluble

concentrate.

chloride.

The sodium content of the bath showed a sudden jump during the first week. This was due to the addition of a batch of PbCl that had not been washed properly. The impurity level also rose in the seventh, eighth, and ninth weeks when another batch of contaminated PbCl was used. Very little effect was noted on the composition of the metal, however. down after about 4 months of operation, and the bath was removed. All The cell was finally shut components were in good condition, although there was some evidence of attack on both the electrodes and the lining. Sludge formation had not occurred, and it appeared that the cell could have been operated for at least a year without

major difficulties.

A

One disadvantage of the LiCl -KC1-PbCl system is the high cost of LiCl, necessitating its recovery if the bath has to be discarded for any reason. small amount of NaCl is usually present in PbCl made by the method shown in figure 1 because the PbCl is crystallized from a brine solution. therefore, build up in the electrolyte, eventually raising the melting point above 450° C. At this point, some of the bath will have to be bled off and the PbCl and Lic1 recovered. Fortunately, the NaCl content of PbCl made by

NaCl will,

ferric chloride-brine leaching rarely exceeds 0.1 pct. As NaCl does not interfere with electrolysis until its concentration reaches approximately 20 wt-pct, an extended period is required for the bath to reach the point where it must be replaced.

CONCLUSIONS

Electrolysis of PbCl2 prepared by ferric chloride leaching of galena concentrate appears to be a feasible method for producing lead metal with a minimum amount of air pollution. Unlike smelting, it can be used on a small scale, requiring less capital investment. Best results were obtained with a bipolar cell having horizontal electrodes. This configuration would definitely lower the cost of electrical equipment and busbar. However, since the cost of electrolysis is so low in any case, a monopolar arrangement might be preferred because of its ease of construction and reliability. evaluation showed the overall cost of producing lead to be 5.7 cents per pound with an interest rate of return on the investment after taxes of 22 pct. The purity of PbCl made by ferric chloride leaching is such that corroding-grade metal can be made for at least several months without any need for discarding the electrolyte or removing sludge from the cell. Materials of construction are readily available at a low cost, and lead emissions, using the LiCl -KC1PbCl2 bath, are negligible.