Table of Contents
The closure of zinc plants has reduced smelting capacity in the United States from 1,300,000 tons in 1969 to approximately 700,000 tons in 1974. At the same time, U.S. consumption of zinc has increased, as shown in figure 1. As a result, the United States must import over 50 pct of its zinc supply from foreign sources. New zinc smelting capacity is expected to use electrolytic processing technology. Although electrolytic plants produce purer metal and cause less air pollution than retort plants, the product is either High Grade or Special High Grade zinc. Prime Western zinc, used for galvanizing,
originally came from retort operations but now must be made by debasing this high-purity metal with lead and/or aluminum additions. This adds to costs, which are already high because of low zinc recovery (89-97 pct), solution purification, handling of cathodes, and air pollution control. It would be desirable to reduce both the cost and energy requirement by use of a more efficient leaching process combined with fused-salt electrolysis.
Zinc is especially well suited for fused-salt electrolysis because its low melting point (419° C) makes it possible to produce zinc in the molten state and remove it from the cell without interrupting production. Handling and melting of cathodes are eliminated, and the size of equipment is reduced because of the high current density (>1,000 amp/ft²) possible with fused salts. Purification of cell feed is also not as critical as in aqueous electrolysis; in fact, various grades of zinc may be produced with fused salt simply by changing the composition of the feed. Zinc chloride appears to be the best starting material for this application because of its low melting point (319° C) and ease of preparation.
While only a limited amount of work has been reported on the electrolysis of fused ZnCl2 , numerous publications cover its physical chemistry as well as that of the ZnCl2-KCl, ZnCl2-NaCl, and ZnCl2-KCl-LiCl systems. Mellor discusses early attempts to electrowin zinc from ZnCl2. Some metal was produced, but there was no effort to optimize variables. As a result, problems were encountered and current efficiencies were low. Hames and Plambeck investigated the electrochemistry of zinc in an AlCl3-NaCl-KCl eutectic at temperatures below the melting point of the metal. Fray appears to have made the most thorough investigation, using a ZnCl2-NaCl bath to study the effect of ZnCl2 concentration, electrode configuration, and other parameters. Optimum results were obtained with ZnCl2 concentrations of 60 to 75 wt-pct, using a current density of 5 amp/in² at 500° C.
Zinc chloride for fused-salt electrolysis could be obtained in a number of ways, such as anhydrous or aqueous chlorination of sphalerite flotation concentrate. The Federal Bureau of Mines is presently developing the method shown in figure 2, which uses a pressure leach with chlorine plus oxygen to dissolve zinc while leaving lead, iron, elemental sulfur, and gangue as an insoluble residue. Copper, cadmium, and silver are extracted and can be recovered by conventional means. The purified zinc chloride solution is evaporated to obtain anhydrous ZnCl2 for addition to the electrolytic cell. The chlorine-oxygen leaching method is being studied separately; the present paper is concerned only with the results of fused-salt electrolysis in a small- scale laboratory cell using either reagent-grade ZnCl2, technical-grade ZnCl2, or ZnCl2 prepared by chlorine-oxygen leaching as described above.
The laboratory study was undertaken to determine optimum conditions for recovering zinc from ZnCl2 by fused-salt electrolysis. Preliminary tests showed that the materials used in the work would have to be carefully prepared to obtain consistent results. All salts (ZnCl3, NaCl, KCl, LiCl) were, therefore, vacuum-dried at 150° C for at least 1 week, then melted and further purified by either (1) addition of HCl gas followed by an argon purge, (2) addition of NH4Cl with stirring, or (3) preelectrolysis.
In the initial tests, vertical, graphite plates were used as the anode and cathode. Current efficiencies averaged less than 65 pct, however, due to recombination of the electrolytic products (zinc metal and chlorine gas). The arrangement shown in figure 3 then was developed which gave current efficiencies in excess of 95 pct under optimum conditions. The metal in the bottom of the cell served only as a conductor and a means of protecting the deposit. Metal beads, which formed on top of the graphite cathode, rolled down the sides and coalesced with the metal in the bottom of the cell, and chlorine, evolved at the anode, rose through the bath and was removed by an exhaust hood. All tests were made in an open beaker with no noticeable effects from the presence of air.
The test apparatus consisted of a 1,000-ml, tall-form Pyrex beaker inside a 316 stainless steel container. Heat was furnished by a crucible furnace equipped with an automatic temperature controller, the temperature being monitored by a Chromel-Alumel thermocouple connected to an electronic recorder. A constant-current rectifier supplied the dc amperage, which was measured with a potentiometer in conjunction with a 10-amp, 50-mv shunt.
In a typical test, a beaker was placed inside the furnace, which had previously been brought to the desired temperature. The electrode assembly then was
lowered into place, and 1,300 grams of molten salt was added. The assembly (fig. 3) consisted of a graphite disk, 1-½ inches in diameter by 2 inches thick, separated by glass spacers from a similar disk, 1-½ inches in diameter by ½ inch thick. The upper disk served as the anode, being connected by a ¾-inch-diameter graphite rod to the dc power supply. A ¼-inch graphite rod protected by a glass sleeve joined the metal pool at the bottom of the cell to the negative side of the rectifier. After the temperature had stabilized, 350 grams of high-purity zinc metal was added to the cell and electrolysis started. This procedure was repeated in all tests with appropriate variations in bath composition, temperature, current density, and electrode spacing. Bath composition was predetermined by weight but also checked by X-ray analysis. After each run, the contents of the beaker were poured into a mold. The weight of the deposit then was determined by subtracting the weight of zinc metal added to the cell at the start of the run from the weight of the metal button.
Investigation of Variables
Variables selected for investigation were those that affect the energy requirement such as electrolyte composition, temperature, current density, and electrode spacing. Other factors considered important were current efficiency and the amount of fuming (volatilization of ZnCl2).
Electrolyte Composition and Temperature
Molten ZnCl2 by itself is not a suitable electrolyte because it is extremely hygroscopic and has a low conductivity (0.09 ohm-¹cm-¹ at 500° C) and a high vapor pressure (9.4 mm Hg at 500° C). The conductivity can be improved by raising the temperature, but this also raises the vapor pressure. An alternative is to combine the ZnCl2 with an alkali metal salt or salts. This will increase the conductivity as much as 10 times, at the same time reducing the moisture absorption and vapor pressure. The degree of improvement is determined by the cation size of the alkali metal salt added, the greatest increase in conductivity being obtained with LiCl. Larger cations, however, have a greater effect in lowering both vapor pressure (fig. 4) and water absorption. ZnCl2 forms a very strong complex with CsCl and progressively weaker complexes with RbCl, KCl, NaCl, and LiCl. Hamer found the standard electromotive force required to decompose pure ZnCl2 to be 1.629 volts at 450° C. This is raised slightly by the addition of alkali chlorides, but the effect is minor compared with the large increase in conductivity obtained.
To produce zinc in the molten state, electrolysis must be conducted at some temperature between its melting point (419° C) and boiling point (907° C). Elevated temperatures are not desirable because they result in increased volatility of ZnCl2, greater solubility of zinc metal in ZnCl2, and an enhanced back reaction between zinc and chlorine. It is necessary, however, to provide an adequate differential between the melting point of the deposit and the operating temperature to allow for variations in heat input. A minimum operating temperature of 500° C, therefore, appears advisable, with the maximum depending upon other factors.
On the basis of these data, a series of tests was made to determine the optimum electrolyte composition and temperature. The salts used were confined to chlorides which gave mixtures with ZnCl2 melting below 500° C and which had good electrolytic conductivity, a reasonable cost, negligible vapor pressure, and a higher decomposition potential than ZnCl3. This limited possible combinations to NaCl-ZnCl2, KCl-ZnCl2 , LiCl -ZnCl2, and KCl-LiCl-ZnCl2. Equimolar and 2:1 molar compositions were tried with all other variables except temperature fixed. Tests were made at a current density of approximately 5 amp/in³, an electrode spacing of 1 inch, and a time of 3 hours. Temperatures ranged from 500° C, as noted above, to 650° C; excessive fuming occurred with all baths at temperatures above 650° C. Results are shown in table 1. The data, although erratic, did indicate that good results could be obtained with all systems except ZnCl2.
Zinc chloride by itself gave very poor results. Fuming, as indicated by loss in weight of the bath, was excessive. Some of the metal produced also redissolved in the bath, causing a very rapid drop in current efficiency as the temperature was raised. Tests showed the apparent solubility of zinc in ZnCl2 to range from 1.7 mole-pct at 500° C to 4.6 mole pct at 600° C. At 650° C, all systems except 2KCl-ZnCl2 fumed badly. An assessment of ZnCl2 volatilization from the various baths was obtained by holding the molten salts at 500° C for prolonged periods of time. Weight losses from a 4-inch-diameter surface were as follows: ZnCl2, 22 g/hr; 2LiCl-ZnCl2, 6.6 g/hr; 2NaCl-ZnCl2, 0.78 g/hr; and 2KCl-ZnCl2, 0.11 g/hr. Based on these data, a temperature of 500° C was selected for use in all of the subsequent work.
The LiCl-ZnCl2 system had the best conductivity of any combination tried. However, LiCl-ZnCl2 acted much like ZnCl2 ; that is, the mixture was hygroscopic, fumed badly at all temperatures tried, and gave low current efficiences due to dissolution of metal in the bath. Mixtures containing KCl and ZnCl2 fumed the least and could be stored in air for short periods without picking up moisture. Compound formation (2KCl · ZnCl2 ), however, resulted in a higher decomposition voltage and, therefore, a higher energy requirement. Results obtained with NaCl-ZnCl2 were intermediate between those obtained with LiCl-ZnCl2 and KCl -ZnCl2. The best overall results were obtained with
KCl-LiCl-ZnCl2 which also had an advantage in that the ZnCl2 content could be varied over a wide range without any possibility of freezing of the bath.
Effect of ZnCl2 Concentration
More extensive tests were made next to determine the effect of ZnCl2 concentration on the voltage, current efficiency, and energy requirement, using the different bath compositions. All experiments were conducted at 500° C with a current density of approximately 5 amp/in² and an electrode spacing of 1 inch.
The phase diagram for the NaCl-ZnCl2 system is shown in figure 5. Figure 6 shows the effect of ZnCl2 concentration on the voltage, current efficiency, and energy requirement. Data from the experiments indicate that the NaCl-ZnCl2 combination is suitable for recovery of zinc from ZnCl2 as long as the concentration of ZnCl2 is kept within narrow limits. The bath freezes when the ZnCl2 concentration drops to 31 mole-pct and fumes when the concentration exceeds 45 mole-pct.
The phase diagram for the KCl-ZnCl2 system is shown in figure 7. Figure 8 shows the effect of ZnCl2 concentration on the voltage, current
efficiency, and energy requirement. Freezing of the bath occurred when the ZnCl2 content decreased to 28 mole-pct, and fuming took place above 55 mole-pct. Because of complex formation the voltage, and therefore the energy requirement, was higher than with other systems, varying from 1.61 kwhr/lb zinc at 50 mole pct ZnCl2 to 2.00 kwhr/lb zinc at 77 mole-pct ZnCl2. For this reason, the KCl-ZnCl2 system cannot be considered a suitable electrolyte for recovering zinc from ZnCl2.
No phase diagram was available for the LiCl-ZnCl2 system. Figure 9 shows the effect of ZnCl2 concentration on the voltage, current efficiency, and energy requirement. The zinc chloride in the bath behaved as if it were not complexed by lithium chloride as evidenced by fuming, water absorption, and low current efficiency. Predictably, the voltage for the system was lower than that of any of the others. Because of the poor current efficiency, however, the energy requirement was no better. The bath froze when the ZnCl2 concentration reached 21 mole-pct and fumed badly above 33 mole-pct. The bath appeared to pick up moisture from the air, and better current efficiencies probably would have been obtained in a closed system. Not as many tests were made in this case because it was difficult to obtain reproducible results and it was apparent that the system was not suitable for the electrowinning of zinc.
KCl -LiCl -ZnCl2 System
No phase diagram was available for the KCl-LiCl-ZnCl2 system. Figure 10 shows the effect of ZnCl2 concentration on the voltage, current efficiency, and energy requirement with KCl and LiCl present in eutectic proportions
(41 mole-pct and 59 mole-pct, respectively). In general, the system had the advantages of both KCl-ZnCl2 (reduced fuming and water absorption) and LiCl-ZnCl2 (good conductivity) without their disadvantages. Also, as noted before, a wide range of ZnCl2 concentrations could be used without any possibility of freezing the bath. The maximum current efficiency (96 pct) and minimum energy requirement (1.26 kwhr/lb zinc) occurred at a concentration of approximately 13 mole-pct ZnCl2. Current efficiencies averaged over 95 pct between 1 and 25 mole-pct ZnCl2; optimum energy requirements, however, were restricted to 2 to 15 mole-pct. Although freezing of the bath did not occur at any ZnCl2 concentration, fuming did take place above 33 mole-pct ZnCl2; also, there was poor coalescence of the deposit below 2 mole-pct, and some alkali metal deposited with the zinc. The KCl-LiCl eutectic proved to be the preferred medium for dissolving ZnCl2 because of its low melting point (348° C), high decomposition potential, good conductivity, and compatibility with zinc metal.
Current Density and Electrode Spacing
Current density and electrode spacing are two of the most important variables, determining both the size of equipment and the energy requirement. Tests to determine their effects were conducted at 500° C with a KCl-LiCl-ZnCl2 bath containing 13 mole-pct ZnCl2 with KCl and LiCl present in eutectic proportions. Experimental results shown in table 2 indicate that the current efficiency is not significantly affected by electrode spacing or current density. Extremely close spacings, such as 1/8 inch, were required to produce any significant effect. Electrode spacing and current density did affect the energy requirement, however, as shown in figure 11. The graph indicates that a 0.5-inch spacing with the lowest possible current density gives optimum results.
Industrial production of zinc is now largely divided between Prime Western (PW) and Special High Grade (SHG), PW zinc is used principally in galvanizing, while SHG is required for diecasting. The object of the present work was to produce SHG, which is easily changed to PW by the addition of impurities. To determine what grade of metal was possible, tests were made with (1) reagent-grade ZnCl2, (2) technical-grade ZnCl2, and (3) ZnCl2 made by aqueous chlorine-oxygen leaching of sulfide concentrate (fig. 2).
To prepare the latter, purified ZnCl2 solution (500 g Zn/l) was evaporated under vacuum until the temperature of the boiling liquid had reached 160° C. The pregnant liquor then was cooled to room temperature to obtain crystals of ZnCl2·H2O. The hydrate could not be dried in air because it melts on heating and retains its water of crystallization. It was, therefore, necessary to vacuum-dry the salt before electrolysis; otherwise, poor current efficiency and sludge formation resulted. It was determined that at least 2 hours at 150° C and a vacuum of 25 inches Hg was needed to obtain acceptable electrolytic results.
Conditions in the tests were those found to be optimum in the preceding work. However, in this case, just enough metal was added to the cell before electrolysis to obtain electrical contact. The metal was analyzed before and after electrolysis, and the impurity content was corrected for the dilution factor. In all cases, electrolysis was continued to near ZnCl2 depletion as indicated by a sudden change in voltage. Results are shown in table 3 together with ASTM specifications for PW and SHG zinc. An analysis of the Bunker Hill metal added to the cell before electrolysis is included also. The results indicate that Special High Grade zinc can be made by fused-salt electrolysis provided high-purity ZnCl2 is used as cell feed.
Electrolysis of ZnCl2 in combination with other halides appeared to be a promising method for the production of zinc metal. Optimum results were obtained with a bath containing 51.3 mole-pct LiCl, 35.7 mole-pct KCl, and 13.0 mole-pct ZnCl2 at 500° C, using a ½-inch electrode spacing and a current density of 5 amp/in² or less. Under these conditions, a current efficiency of 94 pct was achieved with an energy requirement for electrolysis of 1.2 kwhr/lb zinc. Other salt combinations showing promise were NaCl-ZnCl2 and KCl-ZnCl2. These combinations gave results comparable to KCl -LiCl-ZnCl2, but possible ZnCl2 concentrations were confined to a narrower range (30-50 mole-pct) because of either freezing of the bath or fuming (ZnCl2 volatilization). The high percentages of ZnCl2 required with these systems also gave problems with water absorption. Fused-salt electrolysis had several advantages over aqueous electrolysis. Less energy was required, even though higher current densities were used. The handling and melting of cathodes was eliminated, and the size of equipment was reduced for the same rate of production. However, further work is needed to develop a cell design for a scaled-up electrowinning operation, and the effect of moisture and other impurities in cell feed (ZnCl2) must be determined.