CA1186903A - Hydrometallurgical process for the recovery of lead - Google Patents

Abstract

Abstract A process for selectively leaching lead and silver chlorides from a sulfide ore residue in a rapid time which comprises brine leaching the residue under pres-sure at a temperature above the normal boiling point of the solution, preferably above 100°C.
Modifications are leaching at the agglomeration temperature of sulfur when present in the residue to agglomerate the sulfur for ease of recovery, and flashing from leach temperature to ambient as a lead chloride crystallization recovery step to produce a large crop of lead chloride crystals per pass.

Description

Description PROCESS FOR THE RECOVERY OF LEAD
AND SILVER CHLORIDES
.
Technical Field The invention lies in the field of recovery of the chlorides of lead a~d silver by selectively solubiliz-ing the chlorides from other solid materials and the final recovery of the metals from the s~lubili~ed ehlorides.

Back~round Art Prior Art Stat~emPnt U. S. Patent 4,113,471 disc.lo~es a process for brine leaching oxide ores to non~selectively soluhi-lize non-ferr~us metal values as chlorides, at elPvated temperatures and pressures with the addition of oxygen.
Leaching is required for a time of 1~4 to 12 hours.
There is no sel~ctive leaching of lead or silver ~hlorides from their sulfide ores.
U. S. Patents 4,135,993 and 4,173,623 teach brine leaching lead chloride at temperatures of 80-120C to selectively solubilize ~helead chlorideoutof a sulfidic residue containing the s~lfides of copper~ iron and ~inc as well as elemental sulfur. The leachin~ is not done under pressure. Leach times ~f 1~4 to 2 hours are requir~d in both proc~s~e~.
Internati~nal publication WO 80/00852, class 2Z~

~i

-2-13/00, published under the patent cooperation treaty on May 1, 1980 (01.05.80), discloses the recovery of lead from crystallized lead chloride by reduction with hydrogen or a hydrogen containing compound accompanied by condensing lead chloride volatilized during the reduction process.

Disclosure of the Invention . . _ ..
Lead and silver chlorides are selectively separ-ated from other solid materials by leaching the materials with a brine lea~h at elevated temperatures and pressures to selectively solubilize the chlorides, followed by a liquid~solids separation. A starting material on which the process is particularly effect-! ive is the sulfidic residue obtained by selectively leaching a sulfidic lead ore with a cupric or ahalogenating leach to produce solid leacl chloride.
Temperatures ranging from the hoiling point of the solution to 170C are used ~o selectively solubilize substantially all of the lead and silver chlorides in a rapid time which can be not in excess of about three minutes. If elemental sulfur is present in the starting material, i~ is agglomerated at elevated temperatures for ea~e in separation from the solid residue. Lead chloride is crystallized from solution by flashing from the high temperature of ~he leach to lower tempe~atures to produce a large crop of lead chloride crystals per pass and elemental lead of high purity recovered from the crystallized lead chloride by hydrogen reduction, or otherwise~ Silverchloride is redovered from the mother li~uor and processed for recovery of silver.

Best Mode for Carrying Out the Invention ~ .
Although the process is not limited in its appli-i cation to any particular starting materials containing lead and silver chlorides, it has been found effective for selectively leaching lead and silver chlorides at a rapid rate from residues o~tained ~y cupric chloride leaching o~ pyritic or sulfidic lead ores in which lead and silver chlorides are selectively leached at saturation into solid products. Among other materials, these residues contain the sulfides of copper, iron and zinc.
The process is equally effective for selectively leaching the residues produced from halogen leach of pyritic ores in the processes of U.S. Patents 4,135,993 and 4,173,623.
The operation of the invention will be illustrated by its application for the recovery of lead from the residue resulting from the cupric chloride leach of sulfidic ores.
Pressure Brine Leach The solids from the cupric chloride leach reaction, comprising lead chloride, silver chloride, elemental sulfur, unreacted metal sulfides of other metals, and gangue, are treated ~or the selecti~e separation of lead and silver chlorides. The lead and silver chlorides are selectively solubilized from the residue in the illustrative embodi-ment by leaching with an aqueous brine solution having a sodium chloride concentration of from about 200 gpl to saturation. Suitable substitutes for sodium chloride are the other alkali metal chlorides~ lithium and potassium chlorides, as well as the alkaline earth metal chlorides, calcium and magnesium chlorides.
When other solutes than sodiu~ chloride are used the upper limit o~ the amount used will change, the ;3 I

the minimum amount of solute preferably being above about 200 gpl. A leach pH of about 0-7 is preferred. Use of too high a pH will precipitate lead compounds.
Hydrochloric acid (hydrogen chloride,) may also be used as one of the chlorides. The solute must be a chloride which provides maximum chloride ion concentration to the saturation point under the reaction conditions.
The solubilization of silver chloride may be enhanced by the use of an oxidant in the leach, such as, sodium ~ chlorate or oxygen.
The brine leach is conducted at a temperature in excess of the solution boiling temperature, which, of course, requires a pressurized system. The tempera-ture is maintained between about 100C to about 170C, the system pressure being selected so as to accommo-date the solution temperature while preventing solution boiling. Pressures from about 30 to about 150 psig are suitable to accomplish 'chis purpose. If elmental sulfur is presentin the residue the agglomeration temperature o-E sulfur'is used. This is about 130C to 140C. It was found that.-~hen the sulfur is agglomer-ated during the leach and separated from the liquid chlorides in this form with the other solids it can be readily separated from the other solids by physical methods, such as, wet screening.
Th~ brine leach, conduGted under the described temperatures and pressures, accomplishes a rela~ively high solubility of lead and ~ilver chlorides in a relatively short period of time, while leaving the elemental sulfur and unreacted metal sulfides in the residue ~hase. Retention times of from about 30 seconds to abou-t 5 minutes are generally ade~uate to dissolve lead chloride to solution concentrations of at least about 130 grams per liter of lead. A
preferred leach time is not in excess of about three ~ 3 minutes. Increased lead concentrations as a result of high temperature and pressure brine leach significantly facilitate further separation processing.
The brine leach may be conducted at lower pressures, 05 including atmospheric pressure, and lower temperatures, as in the prior art. However, pressures and temperatures lower than those recited for the preferred range of the brine leach will require more of the brine solution per amount of lead and si.lver chlorides and a longer retention time in order to solubilize the chlorides.
In a typical application~ washed tails or residue as a filter cake from the above-referred-to cupric leach of sulfidic ores was brine leached in an exter-nally heated concentric double pipe pressure leach.
~rine containing 280 g/l of NaCl and PbC12 cake was heated to 135C under 50 psig pressure with a one minute retention time to dissolve PbC12 up to a con-i centration of 145 g/l Pb. This procedure reduces the size of the crystallizer used in subsequent PbC12 crystalli~ation and circui-t flows to a fraction of that of an ambient pressure system with a corresponding reduction in heating and cooling needs.
To explore the efectiveness of the brine pres-sure leach at high temperatures in rapidly solubilizing large amounts of lead chloride, the solubility system PbC12-NaCl-H2O was extended to 144C at two brine con-centrations of 240 and 320 g/l of NaC1 used for leaching a lead sulfide ore residue fxom a cupric leach, the composition of -the residue being typified by ~he brine leach residues of ~xamples 1 and 2. The results of the solubility tests reported in Table 1 below show a decided nonlinear increase in the solubility of lead chloride with increase in temperature above the boiling point ~ 33 ~6-of the solution, and particularly above 125 C.

~iluted Diluted Pb Temper- Sample to Sample Total Pb Solubil-NaCl ature Volume Volume ~b in Sample ity g/l C ml ml g~l g g/l _ _ _ _ 240 42 10.0 200 0.746 0.149 14.9 10.0 200 1.450.290 29.0 g4 1/10.0 200 2.g40.588 58.8 100 -87.5 795 9.417.4885.5 124 ~/176.01360 6.20~.43110.9 14~ ~79.01945 7.1313.87175.5 320 47.510.0 200 0.972 0.194 19.4 - 69.010.0 200 2.970.~9459.4 96 10.0 200 6.021.2041~0.4 100 ~ .5 1320 8.9611.8139.1 124 - 8.3 2060 6.6713.7165.5 135 2/9.~1625 11.017.9194.3 143 1/8.22200 ~.8219.4236.6 1/ Sample withdrawn from pressuried autoclave (150 psig N2) using sample bomb.
2/ Repeat run to check data, new solutions.
Since a pipeline brine leach is contemplated in the most feasible commercial application of the pro-cess, a minimum leach time is required in the interest of reducing equipment cost and processing time. Rate of brine leaching tests at high temperatures were made on a residue obtained by the above-referred-to cupric leach of a lead sulfide ore. The brine leach con-tained 250 g/l of NaCl and a pH of about 1.5 was used.
The leach temperature was 140C. The results recorded in Table 2 below indicate that substantially all of the PbC12 i5 leached in a time not in e~cess of about three minutes.

;1 ' .

., ~ 3 T~BLE 2 Rate of Brine Leachlng Tes-t Feed: 50 g cu+2 leached residue of a high grade galena containing sa .68 percent Pb and obtained by leaching 200 g of a high grade ~alena in 1.14 liter of 90 g/1 Cu and 200 g/l NaCl at pH =
1 (HCl) for one hour at 60 C.
Brine Leach Solution: 1 liter 250 g/l NaCl, pH 1.5.
Procedure: Feed added to solution with continuous stirring. Thief samples removed at designated time and immediately vacuum filtered without rinsing.
_ . . . ~.. __. ._. __ . - -Leach Time Pb (min.)_ Vol. Wt % Pb Extraction (%) 1 PF120 ml 26.1 g/l 89 Residue 1.9 g 20.6%

3 PF 110 ml 28.6 g/l 93 Residue 1.3 g 18.7~
PF 625 ml 28.5 g/l 92 Residue 9.1 g 17.2~
__ ~ ~ .. ._ _ .. _ ._ _.. , . .. _ .. ..
Sulfur Agglo _ration The agglomeration of sulfur is accomplished during leaching by operating the brine leach within the ~ulfur agglomeration ternperature range thereby permitting the sulfur to be readily separated from ¦ the remainder of the residue follo~ing liquid-solids separation. ~gglomeration tests were run on a brine leach residue from cupric leach of lead sulfide ore as referred to above containing elemental sulfur to see if the sulfur could be coalesced to a size lar~e 1 Y!~
enough for a wet screen separation. The autoc~lve ili ~J
leach was made a~ 130C. The results recorded in ;i9~

Table 3 below shows that th2 plus 200 mesh fraction contains about 90 percent of the free sulfur with a grade of 82 percent, thus showing that the procedure is feasible for sulfur separation.

Elemental Sulfur Distribution in + 200~mesh Size F ctions of S-Agglornerated Autoclave Leach Residue Feed to We-t Screening 10 5.0 g, S - agylomerated product from Test 1151-107-1:
Conditions: Test 1151-99-1 leach residue pH 11.8 with KOH
2 Hours Di~tribution Size Weight S Weiyht S
Fraction g ~ % %
20 Plus 200 (beads) 1.67 81.833.9 89.5 Minus 200 (fines) 3.26 4.92 66.1 10.5 Total/overall 4.93 (31.0) 100.0 100.0 Liquids-Solids Separation Following the brine leach, the preynant lead and silver chloride solution is separated from the re-maining residue for subsequen-t recovery therefrom of lead and silver values. As high temperatures and pressures are utilized during the leach, the liquid-solid separation must be conducted under pressure in order to prevent flash crystallization of the lead chloride from the solution. One suitable technique to accomplish the separation while avoiding flash crystallization is to employ small diameter pres-surized liquld cyclones in parallel, the hydro-clones operating to permit pressure reduction _9_ to atmospheric as the cyclone operation effe~ts a liquid-solids separation. A pressure drop of about 40 psi across the cyclone system occurs. Hydroclone techniques such as those discussed in ~he Hydroclone, 05 D. Bradley, Pergamon Press, Lrd. 1965 may be utilized in this context. Another solids separation device, such as, an insulated or jacketed pressure leaf filter can be used to accomplish the same objective.
In operation, the pipeline dissolver discharyes through a bank of 10 mm alumina cyclones to remove solids at about a 4 5 micron cut point with a let down from a 50 psig pipeline leach to atmospheric, the pressure being utilized to remove the solids. Floc-culant may be injected at the cyclone inlet to improve clarity of the cyclone vortex flow. The apex flow, containing unreacted s^~lfides and agglomerated sulfur, flashes to atmosheric pressure and mixes with concen-trate and mother liquor from the subsequent PbC12 crystallization to quench the hot slurry and solidify beads of ayglomerated sulfur. The slurry is gravity-fed to A wet screen or similar separation device to make a separation of agglomerated sulfur beads from other solids, principally, unreacted sulfide tails.
The fines are dewatered and final:ly filtered by con-ventional filtration. Filtrate is recycled to theleach feed tank and tails cake is discharged to a solids disposal area. Prior to reaching the leaching tank the leach can be purified by a bleed stream in which copper and lead values are recovered by iron cementation and soda ash used at pH 9 to precipitate Fe, Mg, and Zn to permit recycle of barren brine. The residue from sulfur separation is disposed of or further pxocessed for recovery of metal values if warranted.

Lead Chloride Crys-tallization Lead chloride is crystallized from the liquid phase resulting from the liquid-solids separation on ~he brine leach solution for subsequent recovery of 05 elemental lead by hydrogen reduction, or otherwise.
Two-stage crystallization may be used with the first-stage at atmospheric pressure and the second stage at about 50 mm Hg absolute to cool the feed to about 40C. A pregnant brine containing up to 145 g/l Pb flashes typically from 135C to ambient temperature in the first stage to produce a large crop of crystals per pass. Surface condensers may be used for the second stage, with contaminated lead chloride condensate being recycled to process. Mother liquor overflow and crystal withdrawal elution leg are specific design requirements to elute minus 5-micron impurities not removed in the cyclones. Alternatively, polish filtration techniques could be used to separate minus 5-micron solids.
Crystallizer under-flow is removed through an elution leg at 40-50 percent solids and advanced to a washing centrifuge. A three percent moisture PbCl2 cake is conveyed to a surge hopper above the PbC12 reduction furnace.
Lead Recovery from Lead Chloride The lead chloride is reduced to high purity lead by hydrogen directly without further refining. The remaining solubilized silver chloride is treated for recovery of silver by cementa-tion or other means.
Other co~ventional methods may be used to recover elemental lead from the ]ead chloride. Hydrogen supplying compounds, such as, methane and propane may ~e used as a source for hydrogen.
Since the reduction of PbCl2 is endothermic, heat must be supplied to the reaction~represented by ~11--the formula PbC12 -~ H2- -Pb + 2 HCl when an e~cess of hydrogen over stoichiornetxlc is used.
As lead chloride is extremely corrosive, -the reactor cannot be made of conventional reactor materials but 05 must be made of material which is substantially im-pervious to the corrosive action o~ lead chloride, such as, castable or refractory brick. The materials of which the reactor walls must be m~de have such a low heat conductivity that it is practically impossible to heat the reactor contents with heat applied to the outside of the walls. Accordingly, it was necessary to devise a practical procedure for internally heating the reactor contents to a temperature up to 900C at least. Two alternative procedures were found to be feasible.
In accordance with one procedure a furnace ~r reactor made of refractory brick was used. Heat for the endothermic reaction occurring in the reaction chamber was supplied by fire tubes submerged in molten lead in con-tact with lead chloride and the other reactants in the reaction chamber. Means are pro-vided for introducing reactants into the reaction chamber and for continuously or intermittently tapping pure lead from the furnace. Means are also provided for condensing vaporized lead chloride and returning the vaporized lead chloride to the reaction chamber.
Lead chlori.de does not react with molten lead and having a lesser specific gravity floats on top of the molten lead.
The second proc~dure comprises introducing into the reaction chamber a partially uncombusted gas mixture supplying hydrogen, and completing the com-bustion with oxygen in an endothermic reaction which supplies heat for the endothermic lead chloride re-duction reaction. Heat balance calculations showed tha-t sufficient heat can be brought into the lead chloride reduction reactor to supply -the endothermic heat of reaction and other heat requirements, includlng that caused by heat loss, by using a reducing com-S bustion gas or gas mixture, the term "gas" as usedherein and in the claims including both. An~ hydro-carbon or mixture of hydrocarbons which supply hydro-gen can be used. A mix-ture produced by a partial combustion of hydrocarbons, such as, methane or pro-pane, provides both the hydrogen and the heat neededfor the endothermic reduction of lead chloride. Con-trary to what migh-t be expected, introduction into the reaction area of large volumes of water vapor formed in the partial combustion reaction and diluent gases does not adversely affect the reduction reaction. The above described procedure applies also to the recovery of copper from cuprous chloride by reduction of hydro~
gen.
Illustrative gases and gas mixtuxes found suitable 2 2' 2 CO CO2-H2O, and H2-CO-N2 The gases used may or may not be supplemented by hot reducing combustion gas. Oxygen gas or alr may be used to supply oxygen.
The PbC12 cake was metered to a brick-lined PbC12 reduction furnace as described above operating between 600-900C, preerably at about 800C. A reducing ga~
feed of 98 percent H2 from an on-site H2 plant was used. An excess of 240 perceh-t of theoretical H2 was fed based on lab tests in batch tube furnace runs.
This produces an exit gas consisting of 60 pexcent HCl and 40 percent H2, by volume. Some volatilized PbC12 leaves the reactor zone with the off~as but is refluxed back to the furnace by either a molten lead splash condenser or an air-cooled surface condenser.
Any additional heat requirements for the endothermic reduction reaction and to bring reactants up to temperature may be supplied by indirect firing of submerged fire tubes in the molten lead in the reactor as described above. High puri-ty lead is tapped con~
05 ~inuously or intermittently from the furnace into a casting machine.
Off-gas is scrubbed in a packed tower or similar scrubbing device using liquor from cupric leach, and a large excess of dilutlon air to lower H2 content to a saEe level and also simultaneously consume scrubbed HCl which may be used to reoxidize the cuprous ion to cupric. A water scrubber may also be used to recover the HCl. Exit gas, free of HC1 and particulate matter, is exhausted to atmosphere.
lS Up to over 99 percent of lead was obtained from the starting material. Lead having a purity of ~g9.9 percent was consistently obtained by the process. The recoveries of lead and silver shown in Tables 4 and 5, as produced by Examples 1 and 2, are representative of xecoveries obtained by the process. The lead purity obtained in Example 2 is also typical.

Example 1 Two different 100 gram samples of a lead con-centrate having a composition of 18 percent lead, 26.2 percent zinc, 0.54 percent copper, 5.1 troy ounces of silver ore ton of concentrate, 0.029 percen-t antimony and 14.4 percent iron were treated with 250 milliliters of a cupric chloride leach solution com-prising about 50 grams oE copper per liter as cupric chloride and 200 grams of sodium chloride per liter~
The pH of the leach solution was maintained at about 1 through the addition of hydrochloric acid. After 3 hours, a total of 4~08 and 4.80 grams of hydrochloric acid were added to Sample 1 and Sample 2, respectively.
The cupric chloride leach of Sample 1 was conducted at a temperature of 60C and the cupric chloride leach of Sample 2 was conducted at a temperature of 80C.
The r~idu~ o:E the cupric chlc)r;~.~lo l~(~ch o F ~ h ~f the samples was separately brine leached in a brine solution containing about 250 grams of sodium chloride per liter at a temperature of 80-85C and about one a-tmosphere for one-half hour. Each brine leach slurry was filtered while hot and the residue was washed first with hot brine solution and then with water.
The analyses of the brine leach residue and the results of this extraction are set forth in Table 4. The negative extracted copper percentages are due to a portion of the cupric chlori.de of the leach solution being precipitated to copper sulfide.

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Example 2 A 125 gram sample of a lead concentrate haviny a composi-tion of 25.1 percent lead, 9.57 percent zinc, 0.36 percent copper and 16~3 perc~nt iron was treated with 500 milliliters of a cupric chloride leach solu-tion comprising about 50 grams of copper per liter as cupric chloride, 200 grams of sodium chloride per liter and sufficient hydrochloric acid to maintain a pH of about 1. The cupric chloride leach was conducted at a temperature of 60C for two hours. The residue of the cupric chloride leach was subjected to a 900 milliliter brine leach at a temperature of 80-90C and about atmospheric pressure of one half hour. The brine solution contained about 250 grams of sodium chloride per liter. The analysis of the brine leach residue, which weighed 83 grams, and the results of the ex-traction are set forth in Table 5.
The extraction resulted in 19.0 grams of lead chloride being produced. This lead chloride was reduced to lead in an atmosphere of 175 cubic centi-Tneters per minute of hydrogen, 75 cubic centimeters per minute of carbon monoxide, 75 cubic centimeters per minute oE carbon dioxide at a temperature of 800C
for 35 minutes. The lead metal was assayed by emission spectroscopy. The lead metal was 99.98 per-cent pure. It contained impurities of 0.01 percent silicon, 0.005 pel~cent iron, 0.001 percent copper and 0.001 percent bismuth with no other elements being detected.

~17-Table 5 B _ne Leach Residue Assay ! %
Ag Pb Cu Zn Fe (oz/ton) Sb 0.21 0.80513.9 22.8 2.0 0.33 Extractlon,~
.
Pb CuZn _ Fe Ag Sb 99.4 -~8~5 3.6 7.1 81.6 32

Claims (32)

Claims

1. A process for solubilizing a chloride selected from the group consisting of lead and silver chlorides which comprises subjecting the chloride to a brine leach at a temperature above the normal boiling point under a pressure sufficient to prevent boiling for a sufficient time to solubilize sub-stantially all of the chloride.

2. The process of Claim 1 in which the temperature is between about 100-170°C.

3. The process of Claim 1 in which the time of leach is not in excess of about three minutes and a con-centration of at least about 130 gpl of lead chloride is obtained.

4. The process of Claim 1 in which the brine leach comprises an aqueous solution containing about 200-300 gpl to saturation of a soluble chloride which provides a maximum concentration of chloride ion below saturation.

5. The process of Claim 4 in which the chloride is a member selected from the group consisting of chlorides of alkali and alkaline earth metals and hydrogen chloride.

6. The process of Claim 1 in which the chloride solu-bilized is lead chloride.

7. The process of Claim 1 in which the chloride solu-bilized is silver chloride.

8. A process for selectively solubilizing a chloride selected from the group consisting of lead and silver chlorides contained in a mixture of other solids including metal sulfides which comprises subjecting the mixture to a brine leach under pressure at a temperature in excess of the normal boiling point.

9. The process of Claim 7 in which the temperature is in excess of about 100°C.

10. The process of Claim 9 in which the leaching is performed in a time not in excess of about three minutes and a concentration of at least about 130 gpl of lead chloride is obtained.

11. The process of Claim 9 in which said metal sul-fides include the sulfides of copper, iron and zinc.

12. The process of Claim 11 in which the time of leach is less than about three minutes and a concen-tration of at least about 130 gpl of lead chlor-ide is obtained.

13. The process of Claim 11 in which the brine leach comprises an aqueous solution containing at least about 200 gpl of soluble chloride which provides a maximum concentration of chloride ion below saturation.

14. The process of Claim 13 in which said chloride is a member selected from the group consisting of chlorides of alkali and alkaline earth metal chlorides and hydrogen chloride.

15. The process of Claim 9 in which the chloride solubilized is lead chloride.

16. The process of Claim 9 in which the chloride solubilized is silver chloride.

17. The process of Claim 14 in which said chloride is sodium chloride.

18. The process of Claim 9 in which the solubilized silver and lead chlorides are separated from solids.

19. The process of Claim 18 in which said separation is accomplished by liquid cyclone separation.

20. The process of Claim 18 in which lead chloride is crystallized from the solution and silver chloride is recovered from the mother liquor.

21. The process of Claim 18 in which lead chloride is recovered from the brine solution by crystallization.

22. The process of Claim 21 in which said crystallization includes flashing from the brine leach temperature to a lower temperature.

23. The process of Claim 9 in which said mixture includes elemental sulfur and the temperature of said brine leach is at the agglomeration temperature of sulfur.

24. The process of Claim 21 in which lead is recovered from the crystallized lead chloride.

25. The process of Claim 24 in which the lead is recovered by hydrogen reduction of the crystallized lead chloride.

26. The process of Claim 25 in which the hydrogen and the heat requirement for the endothermic reduction of lead chloride are supplied by a partially combusted hydrocarbon gas.

27. The process of Claim 25 including supplying the heat required for the endothermic reduction by heating molten lead introduced into the reaction area.

28. A method for heating an internal reaction area which comprises introducing into said area a partially com-busted hydrocarbon gas in an exothermic reaction.

29. The method of Claim 28 in which the endothermic reaction is a reduction reaction.

30. The method of Claim 29 in which hydrogen is the re-ducing agent and is supplied by the partially com-busted hydrocarbon gas.

31. The method of Claim 30 in which a compound selected from the group consisting of lead chloride and cuprous chloride is reduced.

32. The method of Claim 31 in which the compound reduced is lead chloride.