US3607232A

US3607232A - Refining lead

Info

Publication number: US3607232A
Application number: US831698A
Authority: US
Inventors: Yurii E Lebedeff; William C Klein
Original assignee: American Smelting and Refining Co
Current assignee: American Smelting and Refining Co
Priority date: 1969-06-09
Filing date: 1969-06-09
Publication date: 1971-09-21
Anticipated expiration: 1988-09-21

Abstract

Softened lead, prior to desilverizing, is detellurized (i.e. tellurium is removed from the lead) by adding sodium or potassium to a pool of the molten softened lead, agitating the molten pool while maintaining the temperature of the molten lead below 1,200* F. to remove tellurium from the lead and incorporate the removed tellurium into a layer of slag overlying the pool, discontinuing the agitation before an undesired amount of the removed tellurium is reincorporated into the molten lead from the overlying slag and removing the slag from the molten lead pool.

Description

United States Patent [72] Inventors Yurii E. Lebedeii Edison, N..l.; William C. Klein, deceased, late of Menlo Park, NJ. [21] Appl. No, 831,698 [22] Filed June 9, 1969 [45] Patented Sept. 21,1971 [73] Assignee American Smelting and Refining Company New York, N.Y.

[54] REFlNlNG LEAD 9 Claims, No Drawings [52] US. Cl 75/78, 75/79 [51] Int. Cl C22b 13/06 [50] Field of Search 75/78, 79

[56] References Cited UNITED STATES PATENTS 2,133,433 10/1938 Deitz...

Primary Examiner-Delbert E. Gantz Assistant Examiner-Veronica OKeefe Attorneys-Elwood J. Schaffer and Roger J. Drew ABSTRACT: Softened lead, prior to desilverizing, is detellurized (Le. tellurium is removed from the lead) by adding sodium or potassium to a pool of the molten softened lead, agitating the molten pool while maintaining the temperature of the molten lead below 1,200 F. to remove tellurium from the lead and incorporate the removed tellurium into a layer of slag overlying the pool, discontinuing the agitation before an undesired amount of the removed tellurium is reincorporated into the molten lead from the overlying slag and removing the as esfremthsm ltenl e 1 U BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to lead refining and more particularly to refining lead to remove tellurium therefrom.

2. Description of the Prior Art l-leretofore, for want of a commercially feasible process for separately removing tellurium from lead during the kettle refining of lead, tellurium has unavoidably reported in the silver dross obtained by the conventional desilvering of the lead with zinc, together with other impurities that persist in softened lead such as, for example, copper and bismuth in addition to the silver. An important disadvantage of the tellurium in such silver dross is that the tellurium is removed and recovered therefrom with great difficulty and expense and conventionally results in a high circulating load of tellurium in the lead circuit in the lead refinery. Another important disadvantage of the tellurium conventionally present in the silver dross is that it greatly increases the load and causes large decreases in the efficiency of the subsequent conventional retorting and cupellation of the silver dross.

One of the advantages of the invention is that it overcomes the above described difficulties of the prior art. Another advantage is that the invention provides a method for refining lead in which tellurium is removed from the lead before desilverizing the latter and readily produces a lead for desilverizing which contains less than .01 percent and as low as .005 percent and as little as .001 percent or less, by weight of tellurium. In addition, the invention provides a process in which tellurium is quickly removed from lead at temperatures between the melting of the lead and 1,200 F. and especially at temperatures between 850 F. and the melting point of the lead, thereby providing a process for refining lead in which the tellurium can be removed from the lead at the temperatures which conventionally prevail between the softening and the desilverizing step in lead refining. These and other advantages will become apparent from the following description of the invention.

SUMMARY OF THE INVENTION Broadly, the invention comprehends a method of refining lead, particularly lead requiring softening and desilverizing, which comprises adding an alkali metal from the group consisting of sodium and potassium and mixtures thereof to a pool of the molten lead, agitating the molten pool to remove tellurium from the lead and incorporate the removed tellurium into a layer of slag overlying the molten lead, and removing the slag from the molten lead pool. in the case of lead which requires softening or softening and desilverizing, the instant process is practiced after softening and prior to desilverizing. Also, in another aspect of the invention, the agitation is discontinued before an undersired amount of the removed tellurium is reincorporated into the pool from the slag.

The invention is based on the unexpected discovery that, instead of proceeding slowly, tellurium is removed from molten lead which has been softened and from lead which does not require softening, at an extremely rapid rate at the relatively low temperatures between the melting point of the lead and the temperature of the lead from the conventional pyrometallurgical lead softening processes, i.e. at temperatures which ordinarily are below 1,200 F. and which, depending on the softening process employed, may be as low as 850 F. The invention is also based on the further unexpected discovery that removed tellurium which separates from the molten lead as a slag which floats on the lead is, with continued agitation of the lead, reincorporated into the molten lead.

Conventionally lead is pyrometallurgically softened in the industry by either the so-called Parkes or Harris processes; softening being practiced to remove antimony, arsenic and tin from the lead. The Parkes process is conducted in a reverberatory furnace in which air is blown through the molten lead to be softened while maintaining the latter at a temperature above 1,300 F. and usually at a temperature of about 1,500 F. to form a litharge slag into which the removed antimony, arsenic and tin are incorporated. During the blowing a major portion of theimpurities removed from the lead are collected in the litharge slag; the remainder of such removed impurities escaping from the slag as a fume. The temperature of the thus softened lead, after the same is tapped into a kettle and ready for further refining, is ordinarily below 1,200 F. The Harris process, on the other hand, is conducted in a kettle. In this process, molten lead to be softened is charged to the kettle and a mixture of sodium hydroxide and sodium nitrate is added to a pool of molten lead. The added mixture is melted in the molten lead and the molten pool is stirred while maintaining it ata temperature of about 850 F. to remove tin, arsenic and antimony from the lead and incorporate the removed impurities into the mixture of such molten hydroxide and nitrate, after which the latter is removed from the molten pool and the remaining lead is desilverized and further refined, as required, in the kettle. The Harris process and any subsequent kettle refining and the kettle refining after softening in the lead by the Parkes process are conventionally conducted in either ordinary steel or cast iron kettles; and, as is known in the art, kettles of such construction, due to corrosion in the case of steel and cracking in the case of cast iron, do not afford a satisfactory kettle life when temperatures above 1,200 F. are employed in kettle refining. An important advantage of the present process is that a satisfactory kettle life is obtained when our process is conducted in steel or cast iron kettles. Accordingly, as already indicated, the process is preferably conducted at temperatures below 1,200 F. and above the melting point of the lead to be detellurized, preferably at a temperature above 650 F. Temperatures below 1,100 P. and above 750 F. are more preferred; within this range, temperatures below 1,000 F. are still more preferred. Temperatures in the range of 800 850 F. are at present most preferred.

The amount of sodium or potassium employed in practicing the process will depend upon the amount of contained tellurium, which is to be removed from the lead. Preferably, the amount of sodium or potassium or mixtures thereof employed in practicing the process is an amount equivalent to 0.3 to 0.8 pounds of sodium per pound of tellurium to be removed from the lead, and more preferably equivalent to 0.3 to 0.8 pounds of sodium per pound of telluriam contained in the lead. Thus; for example, an amount of equivalent to 0.3 to 0.8 pounds of sodium means 0.3 to 0.8 pounds of sodium when the latter is employed to remove the tellurium and is a quantity of potassium equivalent to 0.3 to 0.8 pounds of sodium when potassium is used to remove the tellurium. Such equivalent amounts of sodium are, still more preferably, in the range of 0.01 0.3 to 0.6 pounds of sodium per pound of tellurium contained in the lead; the most preferred equivalent amount of sodium being 0.45 pounds of sodium per pound of tellurium contained in the lead to be detellurized. It has been found that, in the instant process, sodium and potassium are equivalent to each other in proportion to their atomic weight. Accordingly, for purposesof this application and in the claims, 1 pound of sodium is equivalent to 1.7 pounds of potassium. It has also been found that, in practicing the process, selenium and sulfur are removed from the lead along with the tellurium; and, it will be understood that appropriate amounts of sodium or potassium in addition to those indicated above should be used to compensate for the consumption of the sodium and potassium by selenium and sulfur and other constituents which react with the sodium and potassium and which may be present or persist in the lead which is to be detellurized. For convenience, such constituents in the lead may be considered to be tellurium for purposes of determining the amount of sodium or potassium to be employed in practicing the process; thus, for example, a lead containing, by weight, 0.04 percent Te and .01 percent Se is considered to contain 0.05 percent Te. ln general, the amount of such sodium or potassium consuming constituents other than tellurium that are encountered in softened primary lead or primary lead which does not require softening, are so small that they may be ignored.

In Practicing the invention with lead requiring softening, it is desirable that the lead be softened suffieiently to remove arsenic to a value less then 0.02 percent, by weight, in the lead so as to avoid the formulation of arsine during the performance of the detellurizing step of the invention. It is also desirable to remove at least the bulk of the antimony during the softening because it has been found that, where the arsenic and antimony content of the lead is such as to require softening i.e. above about 0.02 percent lead of arsenic and antimony, by weight, relatively large quantities of antimony as well as arsenic are incorporated into the tellurium during the instant detellurizing procedure.

Most tellurium containing lead, and particularly most primary lead or lead bullion i.e. lead produced directly by smelting lead concentrates as opposed to lead scrap, will generally contain, by weight, more than 0.01 percent and usually at least 0.02 percent tellurium. Generally it will contain, by weight, up to 0.06 percent tellurium and usually it will contain from 0.04 to 0.06 percent tellurium. However, at present in the lead refineries practicing pyrometallurgical refining, most such lead contains, prior to desilverizing, as much as 0.1 and as high as 0.2 percent or more, by weight, of tellurium; such amounts of tellurium building up as a recirculating load in the lead in the lead circuit due to the absence, prior to the present invention, of a commercially feasible process for removing tellurium at an early stage in the lead refining prior to the desilverizing step. Such recirculating of tellurium is quickly eliminated upon initiation of the practice of the present process in a refinery and thereafter the concentration of the tellurium in the lead to be detellurized is that of the primary lead or lead bullion without such recirculating load. Practice of the invention with or without lead containing such recirculating tellurium or with other tellurium containing lead readily produces a lead containing, by weight, less than 0.01 percent and as low as less than 0.005 percent lead and as little as 0.001 percent or less.

in general for any given set of conditions, the agitation period required to reduce the tellurium in the lead to a given amount, will vary inversely with the vigor of the agitation i.e. with more vigorous agitation, a shorter agitation period can be used and vice versa. In practicing the invention, it has been found that the sodium and potassium react so rapidly with the tellurium during the agitation period that regardless of the vigor or lack thereof the agitation, the tellurium content in the lead is reduced to the minimum content for the particular con ditions employed (temperature of the molten lead, amount of sodium or potassium used and character of the agitation employed), in a relatively very short period of time, which in our experience to date with the process has been less than about 5 minutes and in as little as less than 3 minutes and as low as 1 minute or less. lt will be understood that such agitation period is measured as the time elapsed after all of the sodium or potassium has been stirred into the molten pool i.e. such time period is measured beginning with the time when all of the added sodium has been incorporated into the molten lead.

In conducting the detellurizing step, the sodium and potassium may be incorporated into the lead in any appropriate manner employing any appropriate mode of agitation. Thus, the sodium or potassium or mixtures thereof may be incorporated into the lead by adding them to the molten pool as a master alloy of lead which may be added to the pool in a liquid or solid form; the agitation period being measured from the time the master alloy is added when the latter is added to the bath as a molten alloy and from the time the master alloy is melted when a solid master alloy is added to the molten pool. While any suitable agitation may be employed during the agitation period as well as in incorporating the sodium or potassium into the molten lead it is preferred, at present to add the sodium or potassium as such to the lead by feeding them into a vortex formed by mechanically stirring the molten pool with an impeller having the impeller blades submerged below the surface of the molten lead. With such mode of addi tion, the agitation period is measured from the time the sodium or potassium are incorporated into the molten lead i.e. when sodium or potassium no longer appear on the surface of the molten bath. While the sodium or potassium is being added to the lead, the impeller is operated at a sufficient speed to provide a vortex which will suck the sodium or potassium into the lead. Preferably, however, the speed of rotation of the impeller is below that speed at which appreciable quantities of gas from the atmosphere above the molten pool is sucked into the lead.

As indicated earlier, it has been discovered that, after the agitation period is required to reduce the tellurium content of the lead to the minimum afforded by the conditions employed during the agitation, continued agitation causes tellurium in the slag to be reincorporated into the lead. It has also been found that as much as 50 percent or more of the removed tellurium can be reincorporated into the lead by continuing the agitation after the agitation period. It has been found further that employment above the pool of an atmosphere of a reducing gas such as nitrogen or a reducing gas such as natural gas will diminish; and, depending upon the care with which such an atmosphere is maintained, will greatly diminish the rate at which the tellurium is reincorporated into the lead. The benefit provided by this discovery is obtained by discontinuing the agitation before an undesired amount of the tellurium is reincorporated into the lead. in this connection, it is commercially important, at least at present, to reduce the tellurium contained in lead, and particularly primary lead to be refined, to a value below 0.01 percent tellurium, by weight. Accordingly, in practicing this feature, the stirring is discontinued before tellurium is reincorporated into the lead in amounts to raise the tellurium content of the lead to a value above 0.01 percent by weight.

The invention may be practiced in the presence of a layer comprising molten sodium or potassium hydroxide. The layer may be formed by adding to the molten lead, either or both, sodium hydroxide and potassium hydroxide, as such, or mixtures of such hydroxide or by adding either or both of such hydroxides in admixture with other slats or other salts of sodium or potassium to provide an added mixture having a melting point such that the added mixture is molten at the temperature of the molten pool at which the detellurizing step is to be conducted. Thus for example the added mixture may be a known admixture of either or both sodium and potassium hydroxide together with carbonates or chlorides of sodium or potassium or of other metals, in such quantities as to provide a mixture having an appropriate melting point. Such layer may be present during any part or all of the detellurizing step. Thus the molten layer may be established in the pool before, during or after the addition of the sodium or potassium or during or after the agitation period. When such a layer is employed, enough of such added material is fed to the surface of the bath to form, with the tellurium removed lead, a slag containing, by weight, at least 10 percent Te calculated on the basis of a slag which is free of metallic lead. ln practicing this feature, it has been found that slags containing up to 35 percent Te can readily be produced. Such slags are advantageous in that they are more efficiently treated subsequently to recover the tellurium therefrom. Preferably in practicing this feature a slag is produced which contains 20 to 30 percent tellurium on said metallic lead free basis. In the presently preferred procedure, no such added material is employed and the sodium or potassium are added as such or in the form of a master alloy. Such preferred procedure affords higher rates of tellurium removal from the lead and readily provides a tellurium containing slag which contains up to 35 percent tellurium on the basis of a slag free of metallic lead.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is further illustrated in the accompanying examples. It should be understood, however, that the examples are given for purposes of illustration and that the invention in its broader aspects is not limited thereto.

EXAMPLE 1 50.5 pounds of lead containing 0.034 percent tellurium, by weight, was melted in a cast iron crucible and heated to 725 F. The molten pool was mechanically agitated with a conventional motor driven stirrer. The agitation was obtained by submerging the blades of the impeller in the molten lead and rotating the impeller to produce a vortex. 5.1 grams of metallie-sodium were incorporated into the lead by adding the sodium to the vortex and adjusting the speed of the impeller to a speed at which the vortex sucked the sodium into the lead without sucking appreciable amounts of air into the lead. lncorporation of all of the sodium into the lead took place in about 1 minute as was evidenced by the disappearance of the sodium from the vortex. Agitation'of the molten lead was continued in this manner after the sodium was incorporated into the lead and samples of the lead were taken at the time intervals set forth in the following table; the intervals being measured from the time all of the sodium was incorporated into the lead. The thus taken samples were analyzed for tellurium; and the tellurium found in each of the samples is set forth in said table. After addition of the sodium a wet dross (i.e. a wet mushy slag) formed on the surface of the pool. The dross readily separated from the lead when the agitation was stopped after the end of the 60 minute period and the dross was easily skimmed from the surface of the molten lead.

Sample Time In Minutes After Incorporation of Na Te Content of Lead By Weight EXAMPLE 2 50.75 pounds of lead containing 0.044 percent tellurium, by weight, were melted in a cast iron crucible and heated to 850 F. Thereafter 0.5 pounds of sodium hydroxide were added to the surface molten lead and was melted thereon. The molten pool was agitated as described in Example 1 with the blades of the impeller submerged in the molten lead beneath the layer of the molten hydroxide. The impeller was rotated at a sufficient speed to form a vortex. 5.21 grams of sodium were added to the vortex and the speed of the impeller was adjusted to a speed at which the vortex sucked the sodium into the lead without sucking air into the lead. The sodium was incorporated into the lead in about 1 minute as was evidenced by the disappearance of the sodium. Thereafter, as in Example l, the stirring was continued and samples of the lead were taken at the intervals set forth in the following table, The samples were analyzed for tellurium and the results are tabulated in the table below. The molten liquid hydroxide slag layer became more viscous as the agitation proceeded. The liquid slag read ly separated from the lead when the agitation was stopped and was easily skimmed from the surface of the molten lead.

Te Content of Lead by Weight Sample Time In Minutes Al'ter Incorporation of Na Table Continued It will be noted that the tellurium in the lead was reduced to a minimum in less than 5 minutes and that thereafter continued stirring caused removed tellurium to be reincorporated into the lead.

EXAMPLE 3 Sample Time In Minutes After incorporation of Na Tc Content of Lead By Weight It will be noted that again the tellurium in the lead was reduced to a minimum amount within 1 minute and that thereafter removed tellurium was reincorporated into the lead.

EXAMPLE 4 The procedure of Example 1 was repeated using 53.25 pounds of lead containing 0.048 percent Te, by weight. The temperature of the molten pool was maintained at 850 F. However in this example the sodium was introduced in the form of a molten master lead alloy which was poured into the vortex. The master alloy contained, by weight, 97.3 percent lead and 2.7 percent sodium. 207 grams of the master alloy were employed. The intervals at which the samples were taken and the tellurium content of the samples are set forth in the following table; the time at which the samples were taken being measured from the time all of the master alloy was poured into the vortex.

To Content of Lead By Weight Sample Time in Minutes After Adding Master Alloy EXAMPLE 5 The procedure of Example 1 was repeated employing 52 grams of lead containing 0.056 percent Te by weight; but, instead of metallic sodium, 8.9 grams of metallic potassium were used. The intervals at which the samples were taken and their tellurium content are set forth in the following table.

Te Content Of Lead By Weight Sample Time In Minutes After Incorporation of K Table -(ontinued It will be noted that within 1 minute after incorporating the potassium, the tellurium content of the lead was reduced to a minimum and that thereafter continued stirring caused the tellurium to be reineorporated into the lead.

EXAMPLE 6 The procedure of Example 2 was repeated employing 50.75 pounds of lead containing 0.047 percent Te, by weight and 5.18 grams of metallic sodium; 6 instead of sodium hydroxide, 0.5 pounds of potassium hydroxide were used. The intervals at which the lead samples were taken and their tellurium content are set forth in the following table.

'l'c Content of Lead By Weight Sample Time In Minutes After Incorporation of Na It will be noted that the tellurium in the lead was reduced to a minimum within 1 minute and that thereafter removed tellurium was reineorporated into the lead.

Example 7 Sample Time In Minutes Te Content of After Incorporation of Na Bullion By Weight It will be noted that the tellurium content of the bullion was reduced to a minimum within 2 minutes; that continued stirring thereafter caused 21 percent of the removed tellurium to be reineorporated into the bullion within 10 minutes and 42 percent of the removed tellurium to be reineorporated therein within 40 minutes.

EXAMPLE 8 The procedure of example 7 was employed in the softened lead bullion produced in a lead refinery over a period of several weeks except that agitation of the bullion in each kettle was discontinued 2 minutes after the sodium was incorporated into the bullion and, thereafter, the slag was skimmed from the bullion in the kettle. Typical of the procedure during such period, one of the kettles of the 240 tons of the softened bullion contained 0.064 percent tellurium, by weight. The bullion in such kettle also typically contained, by weight, 0.06 percent copper, 0.3 percent bismuth, 0.01 percent antimony, 0.001 percent arsenic, 36 parts of selenium per million parts of bullion, 225 troy ounces of silver er ton of bullion and l troy ounce'of gold per ton of bullion. wo hundred pounds of sodium hyroxide were added to and melted on the molten bullion in such kettle and 151 pounds of metallic sodium were thereafter added for the removal of tellurium from the bullion. After the 2 minute agitation period the bullion in such kettle was found to contain 008 percent tellurium, by weight, and the slag skimmed from the bullion was found to contain approximately 30 percent tellurium, by weight, on a metallic lead free basis. The detellurized bullion also typically contained, by weight, 0.06 percent copper, 0.3 percent bismuth, 0.005 percent antimony, 0.001 arsenic, 1 part of selenium per million parts of bullion, 225 troy ounces of silver per ton of bullion and l troy ounce of gold per ton of bullion. The dctellurized bullion produced during said period of several weeks was conventionally desilverized with zinc and the silver dross was subjected to conventional retorting and cupellation. It was found that the efficiency of these latter two procedures were markedly increased.

In practicing the invention, various modifications may be made in the procedure without departing from the essential principles of the invention, which is intended to be limited only by the scope of the appended claims.

What is claimed is:

l. A method of refining a tellurium containing lead which comprises establishing a pool of molten lead to be refined, adding a metallic alkali metal from the group consisting of metallic sodium and metallic potassium and mixtures thereof and a master alloy containing said metallic alkali metal to the pool, agitating the pool for a time sufficient to remove tellurium from the lead to a value below 0.01 percent by weight tellurium and incorporate the removed tellurium into a layer of slag overlying the molten lead, discontinuing the agitating before an undesired amount of tellurium is reineorporated into the lead from the slag to raise the tellurium content of the lead to a value above 0.0] percent by weight tellurium, and removing the slag from the molten pool.

2. A methqqfaccording to claim 1 in which the lead to be refined is softened and undesilverized lead.

3. A method according to claim 2 in which metallic alkali metal along is added to said pool.

4. A method according to claim 3 in which said alkali metal is metallic sodium.

5. A method according to claim 3 in which said alkali metal is metallic potassium.

6. A method according to claim 2 in which said agitation is conducted in the presence on the surface of the molten lead of a material comprising at least one material selected from the group consisting of the hydroxide of sodium and potassium.

7. A method according to claim 6 in which said material is added to the surface of said pool of lead prior to adding said alkali metal to said molten lead.

8. A method according to claim 7 in which said material consists of sodium hydroxide.

9. A method according to claim 7 in which said material consists of potassium hydroxide.

mg A UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 6()7 232 Dated September 2 l, 1971 Inv n fl Yurii E. Lebedeff and William G. Klein It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 2, line 36, a dash should be inserted after "800" and before "850F'."; line 45, "telluriam" should read --telurium--; line 51, "0.01" should be deleted. Column 3, line 3 "Practicing" should read --practicing--; line 37, --tellurium-- should be inserted after "percent" and before "and; line 38,

"lead" should read --tellurium--; line 47, --of-- should be inserted after "thereof and before "the". Column 4, line 41, "slats should read --salts--. Column 6, line 3, should read --30--; line 3, should be deleted; line 29, "10" should be read in the left hand column entitled "Sample Time In Minutes After Incorporation of Na"; line 29, --.0057-- should be inserted in the right hand column entitled "Te Content of Lead Z, By Weight"; line 30, 0.0057 should be deleted; line 58, '2" should be read in the left hand column entitled "Sample Time In Minutes After Adding Master Alloy"; line 66, "0.056" should read --0.05--. Column 7, line 3, "5" should be read in the left hand column; line 16, "6" should be cancelled and --but,-- substituted therefor; line 51, "10" should be read in the left hand column entitled Sample Time In Minutes After Incorporation of Na". Column 8, line 15, "008" should read --.008--; line 50 "along" should read --alone--.

Signed and sealed this 30th day of May 1972.

(SEAL) Attest:

L EEL JAR D I LFLETCHERJR. ROBERT GOTI'SCHALK J Atbesting Officer Commissioner of Patents

Claims

2. A method according to claim 1 in which the lead to be refined is softened and undesilverized lead.
3. A method according to claim 2 in which metallic alkali metal along is added to said pool.
4. A method according to claim 3 in which said alkali metal is metallic sodium.
5. A method according to claim 3 in which said alkali metal is metallic potassium.
6. A method according to claim 2 in which said agitation is conducted in the presence on the surface of the molten lead of a material comprising at least one material selected from the group consisting of the hydroxide of sodium and potassium.
7. A method according to claim 6 in which said material is added to the surface of said pool of lead prior to adding said alkali metal to said molTen lead.
8. A method according to claim 7 in which said material consists of sodium hydroxide.
9. A method according to claim 7 in which said material consists of potassium hydroxide.