Table of Contents
During electrolysis, metal was produced from solid lead chloride in contact with the cathode rather than from lead in solution. Relatively high current densities—up to 300 amp/ft²-were possible when operating in this way.
Primary lead is now usually produced from sulfide concentrate by a smelting process that involves sintering, blast furnace reduction with carbon, and refining. Unfortunately, smelting generates gaseous sulfur oxides and particulate lead, which must be controlled to avoid air pollution. Meeting environmental regulations requires production curtailment at times for some smelters, and for others even threatens eventual closure.
To avoid such problems, the Federal Bureau of Mines has investigated a hydrometallurgical procedure (fig. 1) for producing lead metal from sulfide ores. In this method, galena concentrate is leached with hot ferric chloride-brine solution to obtain lead chloride and elemental sulfur:
At 100° C, over 99 pct of the PbS in the concentrate is converted to PbCl2 in less than 15 min. The leach solution, consisting of brine (NaCl) containing the theoretical amount of FeCl3, dissolves the PbCl2 as it is formed, leaving a residue of elemental sulfur and gangue which is easily washed free of entrained liquor. High-purity PbCl2 (>99.9 pct) crystallizes from the leach solution on cooling and may be electrolyzed in a fused-salt cell to obtain high-purity lead (>99.999 pct Pb) and gaseous chlorine, which is used to regenerate the leach solution. Because fused-salt electrolytes can be
operated effectively at high current densities (>1,000 amp/ft²), a high production rate can be obtained with relatively small equipment. A continuous operation is possible, and energy requirements are low (<1.0 kwhr/lb metal).
Aqueous electrolysis, the subject of the present study, may be more suitable for the small operator, however, because it requires less expensive equipment and can be conducted at ambient temperature with fewer safety hazards. The equipment necessary for the aqueous electrolysis of solid PbCl2 (fig. 2) is extremely simple, merely a tank constructed of some nonconducting material with a horizontal lead cathode at the bottom and a vertical graphite anode at the top. The tank can be filled with hot pregnant solution (fig. 1) which has been purified by passage through a column of lead shot. Lead chloride, crystallizing out on cooling, will then form a layer on the bottom of the tank, covering the cathode. During electrolysis, chlorine produced at the anode reacts with ferrous ions to generate ferric ions in the leach solution, thus eliminating the need for chlorine-handling equipment. At the same time, the PbCl2 is converted to a metallic powder which is not attached to the cathode and can be raked out of the cell, washed, and melted to obtain high-purity lead. While the method is essentially a batch process, very little labor is required. There is also no need for the crystallizer, centrifuge, and absorption tower shown in figure 1.
Investigation of Variables
Variables selected for investigation in the electrolysis of PbCl2 were electrolyte composition, temperature, current density, and electrode spacing. Electrolytes tested included spent leach solution (FeCl2-NaCl), HCl, NaCl, and NH4Cl—these being considered the most practical media. Spent leach solution was prepared by cooling pregnant solution (fig. 1) to room temperature to crystallize out as much PbCl2 as possible. The resulting liquor contained approximately 95 g/l FeCl2, 230 g/l NaCl, and 25 g/l PbCl2. Solutions other than spent leach solution were investigated because they hold the potential of improved electrolytic results. When spent leach solution is electrolyzed, some of the ferric ion formed at the anode is reduced to ferrous ion at the cathode or else reacts with the deposit, lowering the current efficiency and raising the energy requirement. This disadvantage must be weighed against the additional processing required when other electrolytes are used.
Experimental Procedure
Small-scale tests were made with the apparatus shown in figure 3. The electrolytic cell consisted of a 1,000-ml beaker containing a 1-inch-diam vertical graphite anode and a cathode assembly made up of a ¼-inch-thick graphite plate cemented into the bottom of a truncated 800-ml beaker. A 3/8-inch graphite rod protected by a glass sleeve connected the cathode to the dc source. A Hewlett-Packard model 6427B constant-amperage rectifier supplied the dc current, which was measured with a Leeds and Northrup model 8690 potentiometer in conjunction with a 10-amp, 50-mv shunt. Voltage was checked with a Heathkit model 1M-102 multimeter.
In all tests, 35 grams of PbCl2 were placed in the cathode compartment, forming a layer about 1/8 inch thick. After 800 ml of solution was carefully added, electrolysis was started, using a controlled bath composition, temperature, current density, and electrode spacing. Each test was run for 5 amp-hr, equivalent to 75-pet conversion of the PbCl2 to metal. After electrolysis, the deposit was washed, dried in a vacuum oven, and melted at 450° C under an inert salt cover. This eliminated the excess PbCl2, which was mixed in with the lead granules. The button was then weighed to calculate the current efficiency and energy requirement.
Electrolyte Composition
The first series of tests was made to determine the optimum electrolyte composition. Solutions with varying concentrations of HCl, NaCl, and NH4Cl were electrolyzed at 15 amp/ft² using an electrode spacing (anode-to-PbCl2 distance) of 1 inch and a temperature of 25° C. Results are shown in table 1 and figure 4. No investigation was made with spent leach solution because this has a fixed composition as noted previously. However, under the above conditions, spent leach solution gave 67.2-pct current efficiency and an energy requirement of 0.37 kwhr/lb Pb. Of all the electrolytes tried, HCl required the least energy. As expected, the voltage, and therefore the energy requirement, appeared to be a function of the conductivity of the electrolyte; in general, the energy requirement decreased as the solute concentrate increased.
The solubility of PbCl2 in all of the electrolytes was low, usually far below that in water (1.0 wt-pct at 20° C). With increasing amounts of HCl, the solubility reached a minimum of 0.1 wt-pct at a concentration of 5 wt-pct HCl and then gradually increased to a maximum of 1.1 wt-pct at 25 wt-pct HCl. With NaCl, the minimum was also 0.1 wt-pct PbCl2 at a concentration of 5 wt-pct NaCl, increasing to a maximum of 1.9 wt-pct at 25 wt-pct NaCl. NH4Cl gave similar results, the solubility of PbCl2 decreasing to 0.07 wt-pct at 5 wt-pct NH4Cl and then increasing to a maximum of 0.6 wt-pct at 25 wt-pct NH4Cl. During electrolysis, the concentration of PbCl2 in the electrolyte remained constant as long as there was an excess of solid PbCl2 in the bottom of the cell. It was noted that if the solid PbCl2
was completely converted to metal, the deposit thereafter became black and spongy, indicating that deposition from solution was taking place. At the higher salt concentrations, where more PbCl2 was present in solution, the deposit adhered to the cathode, indicating that reduction of both solid PbCl2 and lead in solution was occurring simultaneously.
Temperature
To determine the effect of temperature, tests were run as before using the electrolyte concentration (20 wt-pct) that gave the lowest energy requirement in the preceding work. The temperature of the solutions was varied from 25° to 75° C, using a constant-temperature water bath to maintain the temperature within ±1° C. No tests were made with spent leach solution because pregnant solution (fig. 1) must be cooled to room temperature to crystallize out as much PbCl2 as possible; otherwise, when it is recycled its capacity to dissolve PbCl2 is reduced and a greater volume must be used. Electrolysis with spent leach solution is, therefore, preferably conducted only at ambient temperature. Results of the tests are shown in table 2. Increasing the temperature had no significant effect on the energy requirement in the case of HCl and had only a limited effect in the case of NaCl and NH4Cl.
Current Density
Tests were made at current densities ranging from 15 to 750 amp/ft², using the conditions found best in the preceding work. Except for spent leach solution, 20.0-wt-pct concentrations of solute were employed. Temperatures were held at 25° C for HCl and FeCl3-NaCl and at 75° C for NaCl and NH4Cl, using a 1-inch electrode spacing. Results are given in table 3, and the effect of the current density on the energy requirements is shown in figure 5. Even with no external heating, HCl gave the best overall results. As
expected, FeCl2-NaCl had the highest energy requirement; however, no chlorine was given off when spent leach solution was electrolyzed, even at the highest current density used. Actually, the results with FeCl2-NaCl were not so different as to preclude its use, considering the other advantages obtained. As the current density was increased in the tests, hydrogen evolution occurred at the cathode, resulting in a rapid reduction in the current efficiency above 300 amp/ft². Interaction of ferric ion and the deposit caused a further reduction in current efficiency in the case of spent leach solution.
Electrode Spacing
Conditions used to investigate the effect of electrode spacing were exactly the same as in the previous tests except that the current density was fixed at 15 amp/ft² and the electrode spacing was varied from ½ to 4 inches. Results are shown in table 4. As would be expected, the cell voltage and energy requirement increased with the spacing; however, the change was surprisingly small. Whereas spacing is a very important factor in fused-salt electrolysis, in this case, the effect was almost negligible.
Metal Purity
Metal purity was determined by operating the test cell (fig. 3) with spent leach solution (FeCl2-NaCl) at 25° C, using a current density of 15 amp/ft² and an electrode spacing of 1 inch. The PbCl2 was obtained by crystallization from pregnant solution (fig. 1) after purification with lead shot. The deposit was washed, dried, melted under an inert salt cover, and analyzed. Results are shown in table 5 together with ASTM specifications for corroding-grade lead. Even though a large amount of iron was present in solution, very little entered the deposit because of the large difference in electrode potential (Eo) for deposition of iron and lead (-0.44 and -0.12 volt, respectively, at 25° C). Elements that fall below lead in the electromotive series include Ag, As, Bi, Cu, and Sb. Arsenic and bismuth did not dissolve in the leach (fig. 1), although they were present in the galena concentrate. Silver, copper, and antimony did dissolve but could be reduced to <1 ppm by passing the pregnant solution, before crystallization, through a bed of lead shot. It is apparent that the product met all specifications for corroding-grade metal (>99.94 pct Pb).
Conclusions
In conjunction with ferric chloride leaching of galena concentrate, aqueous electrolysis appears to offer the small operator the potential for a simple, inexpensive method for producing lead metal on a limited scale at a reasonable capital cost. While fused-salt electrolysis is more suitable for a large operation, aqueous electrolysis requires less sophisticated equipment and less technical skill. More labor is involved, however, and the energy requirement is higher.
