CA1331099C - Removal of thallium from impure lead - Google Patents

Abstract

REMOVAL OF THALLIUM FROM IMPURE LEAD

Abstract Molten thallium-containing lead, pumped from beneath the surface of a bath of the lead, is passed through a layer of molten chloride salt in a contactor. After contacting the molten salt, the molten lead returns to the bath through a valve port at the bottom of the contactor. Thallium extracted from the circulating lead accumulates in the salt layer.
Thallium-enriched molten salt is removed from the contactor by throttling the valve port and pumping lead into the contactor, forcing the layer of molten salt to rise in the contactor until it overflows through a side tube into a ladle. The preferred chloride salt is anhydrous zinc chloride.

Description

This invention relates to a method and apparatus for removing thallium from thallium-containing lead and, more particularly, relates to an improved method and apparatus for contacting molten thallium-containing lead with a molten chloride salt for removing thallium from a molten bath of thallium-containing lead.

Thallium is present in many lead ores as a minor constituent.
The thallium content of lead sulphide concentrates may range from trace amounts to levels as high as 600 ppm. In the recovery of lead from chloride-containing lead sulphide concentrate by blast furnace smelting, most of the thallium volatilizes as thallous chloride during the sintering step prior to smelting. The unvolatilized thallium remaining in the sinter largely reports to the blast furnace bullion and is retained in the lead during subsequent purification incorporating conventional pyrometallurgical and electrorefining steps. The thallium concentration in the resulting refined lead is acceptably low, generally about 10 ppm or less.

However, in the recovery of lead from thallium-containing lead sulphide concentrates by a direct smelting process, for example . :
3.

by the Queneau Schuhmann Lurgi (QSL) process, in which furnace dusts are recycled to the furnace, substantially all of the thallium in the concentrates eventually reports to the furnace bullion. Subsequent conventional purification, including drossing and electrorefining, will not remove the thallium.
This would result in unacceptably high thallium levels carrying through into the refined lead. In order to benefit from the advantages offered by direct smelting, it is necessary to find an efficient method of removing thallium from thallium-containing metallic lead.

Methods of removing thallium from lead are known. Of these known methods, the most practical one for use in large-scale lead production is to contact molten thallium-containing lead with a suitable molten chloride salt. Thallium is taken up by the molten salt. With appropriate operating conditions, the thallium content of the lead can be lowered to a few parts per million. The salts which are known to effect thallium removal are zinc chloride, lead chloride, ammonium chloride and mixtures thereof.
. .
Polish Patent No. 46,715 (Styrski, et al, 1963; CAS citation 59:14939b) describes addition of solid zinc chloride to a stirred molten lead bath at 400-420C. The amount of zinc chloride added is 2-2.5 kg per 1000 kg of lead. Stirring is - ~331099 4.

continued for 30 minutes, the bath is cooled to 380C and the dross, containing 4- 5S thallium, is collected from the surface of the bath.

Japanese Patent Appl'n. No. 52/143,918 (Miyata, 1977; CAS
citation 89:63103u) describes a series of tests using zinc chloride additions. Excess zinc chloride is stirred into a liquid lead bath held at 350-450C. After 30-60 minutes of stirring, the liquid salt slag is drawn off the surface of the lead bath.

Japanese Patent Appl'n. No. 53/71,623 published June 26, 1978 (Sumita, 1978: CAS citation 89:201055h)-describes the use of one or more chlorides, such as lead chloride, zinc chloride or ammonium chloride. The chloride, or mixture of chlorides, is added to the top of the molten lead bath. In a test using lead chloride, the bath temperature is raised to 505C to melt the lead chloride. Upon completion of thallium removal, the . - . . .: . - .
temperature is lowered to 380C and the layer of solid lead chloride is removed from the surface of the molten bath. In a test with zinc chloride, the bath temperature is 350C during , ... .
mixing, then the bath temperature is lowered to 300C to solidify the lead. The molten zinc chloride layer on top of `-the solid lead is removed. ;~

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These prior art methods, while successful in lowering the thallium content of the lead bath, do not succeed in providing a satisfactory method of subsequently separating the salt from the lead. In the prior art methods it is necessary to skim a thin layer of liquid or solid thallium-rich chloride salt from the surface of a liquid or solid lead bath. Typically, the salt layer would have a thickness of the order of one millimeter. Attempting to skim such a thin layer off a lead bath having an area of several square metres would not be practical on a routine basis. It has been suggested to add a bulking agent, such as sawdust or powdered vermiculite or charcoal, to the salt layer to facilitate skimming. Even if a bulking agent is used, however, the skimming method is still unsatisfactory because it would be impossible to achieve complete separation of the salt from the lead. A certain amount of residual salt would inevitably remain on the lead surface. During ensuing processing of the lead, the thallium in the residual salt could re-contaminate the lead. Also, the residual salt could easily become dispersed into the working environment to present a health hazard.

A further failing of the skimming method is that it would be difficult to collect and handle the skimmings without some degree of exposure to workers and dispersal into the surrounding area. This is an important consideration in view of the highly toxic nature of thallium salts.

~331099 ~
6. - ~-The fact that molten lead oxidizes rapidly in air is the cause - -of additional difficulties with the prior art methods. A lead bath open to the air soon acquires a surface layer of dross, up to several centimeters in thickness, comprising powdery lead monoxide or a viscous mixture of lead monoxide and metallic ~-lead. Before addition of a chloride salt for thallium removal, ~ ~
the bath would have to be skimmed essentially free of dross, ~ -otherwise the salt would be absorbed by the dross and not make ~ ;
effective contact with the molten lead surface. A further : -:; . : . : :
problem arises if zinc chloride is to be used for thallium removal. Zinc chloride will react with lead monoxide present on the bath to form lead chloride, which is known to be : :, . ,.:
significantly less effective than zinc chloride for removing i~ ~;

thallium from lead.
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The prior art methods thus require two labour-intensive skimming steps: first, to remove the dross layer immediately ~ `
prior to chloride salt addition; second, to remove the thallium-enriched salt layer.

Although the bath temperatures in the prior art methods are - `-kept as low as possible to minimize the vapour pressures of the - -chloride salts, while still retaining a satisfactory rate of thallium removal, the vapGur pressures would be nevertheless significant. To maintain hygienic conditions with production . ,. ::.

1~3109~

scale lead baths several meters in diameter, extensive ventilation would be required.

The present invention provides an improved method for removing thallium from impure thallium-containing lead by contacting molten impure lead with a molten chloride salt and subsequently separating spent thallium-enriched molten chloride salt from purified thallium-depleted molten lead.

In the present invention, molten lead is pumped from an intake submerged in a bath of molten thallium-containing lead and fed into the upper end of a tubular contactor which has its lower end immersed in the bath. The lead falls through a layer of molten chloride salt in the contactor and returns to the bath through an opening in the lower end of the contactor. Thallium is removed from the circulating lead by contacting the salt.
Thallium enriched molten salt is separated from the purified lead by feeding molten lead into the upper end of the contactor while restricting the outflow of lead from the lower end of the contactor, forcing the salt layer to ascend until it overflows through a side tube located on the upper end of the contactor.
The discharged salt is caught in a ladle.

This method reduces or eliminates the difficulties of the prior art methods. There is no need to skim the dross from the bath ~ ;

~ 331099 8.
:
before thallium removal. Skimming of the bath to remove the spent salt is eliminated. Efficient separation of the spent salt from the purified lead is achieved. No residual spent salt remains on the purified lead. Since the chloride salt is confined to a compact volume throughout the process, ventilation and fume handling requirements are greatly reduced. The exposure of workers to thallium and the escape of thallium into the environment are virtually eliminated. The discharged spent salt is in a compact fluid state which facilitates immediate treatment for recovery of the thallium content in a concentrated form. Since the content of oxidized and metallic lead in the spent salt is substantially lower than that in the prior art methods, recovery of the thallium content is simplified.

It has been found that anhydrous zinc chloride is preferred over lead chloride or lead chloride-sodium chloride eutectic for removing thallium from lead. Advantages of zinc chloride include high thallium removal efficiency, low operating temperature and low vapour pressure. If lead chloride or the eutectic is used to remove thallium from lead, it is necessary to recover and recycle the lead chloride after thallium recovery. Recycling is not required with zinc chloride because a relatively small amount is used. This advantage more than compensates for the cost of continually buying or manufacturing fresh zinc chloride.

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In its broad aspect, the method of the invention for removing thallium from thallium-containing lead comprises forming a molten bath of said lead, immersing the lower end of a vertical contactor having a lower end and an upper end in said bath to form a column of molten lead in the lower end of the contactor, forming a layer of molten chloride salt within said contactor on the molten lead, feeding said molten lead from below the surface of the bath of molten lead into the upper end of the contactor onto the molten chloride salt whereby the molten lead descends through the layer of molten chloride salt for accumulation of thallium in the molten chloride salt and return of lead to the lead bath through the contactor lower end, restricting the return of lead through the contactor lower end to the molten bath during feed of the molten lead when a desired accumulation of thallium in the molten chloride salt is reached for upward displacement and substantial discharge of the thallium-enriched molten chloride salt from the contactor, and replenishing the discharged molten chloride salt while resuming the return of lead through the contactor lower end to the lead bath.

The molten chloride salt preferably is anhydrous zinc chloride. The molten lead is maintained at a temperature in the range of 400 - 425C, preferably about 400C, and is fed into the upper end of the contactor at a controlled, substantially constant rate.

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1 331~9 1 0 . ' " ' .": ~.
The apparatus of the invention for the removal of thallium from a molten bath of thallium-containing lead comprises a contactor having an upper end and a lower end and a side discharge in proximity to the upper end, means for suspending said contactor in said molten bath of lead whereby the lower end of the contactor is immersed in the molten lead, said contactor having a layer of molten chloride salt therein on the molten lead below the level of the side outlet, means for feeding molten lead from the bath below the surface thereof into the upper end of the contactor onto the layer of molten chloride salt for downward passage through the molten chloride salt for accumulation of thallium in the molten chloride salt and return to the molten lead bath, and means for restricting the return to the molten lead to the molten lead bath at the lower end of the contactor whereby the layer of molten chloride salt can be displaced upwardly during feed of the molten lead for discharge of the molten chloride salt containing accumulated thallium - :.
from the contactor through the side outlet.
. .
The means for restricting the return of molten lead to the lead ;~`~
bath comprises a valve port formed at the lower end of the ~-contactor and a valve stem having a valve plug formed at the lower end thereof adapted for vertical reciprocal movement to ; ~~
and from the valve port for closing and opening the valve port. , .~t,.l ;~

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The contactor is cylindrical in shape and includes a circular perforated distributor plate affixed to the contactor transversely thereof below the side discharge. A plurality of steel packing elements may be positioned in the contactor on the layer of molten chloride salt below the circular plate.

The method and apparatus of the invention will now be described with reference to the drawings, in which:

Figure 1 is a schematic cross-sectional side elevation of the apparatus of the invention, shown in cross-sectional side elevation;

Figure 2 is a schematic illustration of another embodiment of the apparatus of the invention used in pilot-scale tests;

Figure 3 is a graph illustrating the effect of circulation rate on thallium removal;

Figure 4 is a graph illustrating the effect of the presence of packing on thallium removal; and ~-Figure 5 is a graph illustrating the effect of a two-stage contact of molten salt with the molten lead.

With reference to Figure 1, a vertically-disposed tubular contactor 10 is positioned relative to a holding kettle 12 with ~331~

the lower end of the contactor located near the bottom of the kettle. The contactor is supported by horizontal support beams, not shown, resting on the edge of the kettle. The body 14 of the contactor is a tube with a ventilation hood 16 above the open upper end and flow restricting means shown as closure 18 at the lower end. Closure 18 contains an opening, ie valve port 20, which may be opened or restricted by raising or lowering valve stem 22 to which valve plug 24 is attached. A
downwardly inclined side tube 26 is attached to the upper portion of the contactor body.

' -:
During thallium removal, the kettle contains a bath of impure molten lead 28, the top surface of which is indicated by line 30. The lower portion of the contactor contains a column of molten lead 32, to a depth indicated by line 34, in communication with lead bath 28 through open port 20. A layer 36 of molten chloride salt floats on the lead column 32 inside the contactor. Line 34 thus represents the interface between the liquid lead and salt phases. A horizontal perforated distributor plate 38, fixed to the body of the contactor, is located below the point of attachment of side tube 26. Plate 38 has a central collar 40, providing an opening for vertical reciprocal movement of valve stem 22. Fresh molten chloride salt is poured into the open top end of the contactor when necessary. Pump 42, submerged in the lead bath, and with 331~9~

13. ~ ;

intake 44 located near the bottom of the bath, provides a flow of lead into the upper end of the contactor via pipe 46 and discharge pipe 48. Lead discharged from pipe 48 falls on the distributor plate, trickles downward through the layer of molten salt, enters the lead column 32 in the lower portion of the contactor and finally returns to the bath through port 20.
The circulation of lead is continued until the desired degree of thallium removal has been reached.

Thallium-enriched salt can be removed from the contactor by pumping lead into the contactor and simultaneously throttling down port 20 by lowering valve stem 22, until the flowrate of lead into the contactor exceeds the outflow sufficiently to cause the lead level to rise in the contactor. This forces the molten salt layer upward until it begins to overflow through side tube 26. The overflowing salt is caught in a ladle or other suitable container of appropriate size, equipped with a ventilation hood. When the required amount of salt has overflowed, the salt flow is stopped either by turning off pump 42 or by re-opening port 20.

When the purified lead is removed from the kettle, the kettle is not completely emptied. Enough lead is left in it to prevent residual salt from escaping from the contactor into the kettle through port 20.

~ ~ 331~99 14.

In practice, it is necessary to circulate the lead through the contactor at a controlled, approximately constant rate for uniformity of operation. Pump 42 may be a positive-displacement pump having the required capacity, or it may be a centrifugal pump with excess capacity. In the latter case, provision is made for controlling the flow rate, such as the constant-head assembly described below in relation to Figure 2. With a suitable piping arrangement, pump 42 may also be used for removing the purified lead from the kettle.

- . . .

The thallium removal rate can be improved by placing packing ~ -elements in the space occupied by the molten salt layer 36.
Steel balls several centimeters in diameter or other steel objects of a similar size are suitable. The packing elements ` -can be placed loose in the lower part of the contactor, or they :
can be held in the zone of the salt layer by suitable means, an `
example of which will be shown in relation to Figure 2. If the elements are loose, they will float on lead column 32. When the spent salt is discharged, the elements are prevented from reaching side tube 26 by distributor plate 38.

It is not necessary for valve plug 24 to completely seal off -opening 20 when the spent salt is to be discharged. All that -~-is required is that the opening be choked down to sufficiently ;~
restrict the outflow of lead from the contactor. In practice, , ~ ' : 13310~
15.

a complete seal would be difficult to achieve. Plug 24, situated inside the contactor, is preferable to an external plug as shown in Figure 2 because it can be pulled up through the contactor to provide easier access for repair or replacement.

In the simplest case, one batch of lead is purified with one lot of fresh salt, with removal of the spent salt at the end of the run. The procedure is used in Examples 1 and 2. Other procedures are possible using the apparatus of the invention.

One batch of lead may be treated with two or more lots of fresh salt, with removal of the spent salt before addition of the next lot of fresh salt.

A series of two or more batches of lead may be treated with one lot of initially fresh salt, leaving the salt in the contactor between batches and removing the spent salt at the end of the series.
, The salt may be used counter-currently, for example, by initially contacting a batch of lead with already-used salt from the preceding batch, removing the spent salt, contacting the batch with fresh salt and leaving the used salt in the contactor in readiness for the next batch of lead. The feasibility of this method is demonstrated by Example 3.

~331~9~ .
16.

A procedure which is more favoured than any of the preceding ones is to maintain continuously an approximately constant base amount of used chloride salt in the contactor throughout the purification of an indefinitely long series of lead batches, with discharge of a fraction of the base amount at the end of the purification of each batch and addition of a portion of fresh chloride salt at the beginning of the purification of the succeeding batch. The base amount of used salt is several times larger, preferably by a factor of about 3 to about 6, than the amount of fresh salt that would be required to purify one batch of lead. The fraction of used salt discharged, called ~spent salt~, contains an amount of thallium equivalent to the amount removed from one batch of lead. The size of the portion of fresh salt added is a predetermined quantity, the amount that would be required to purify one batch of lead. The respective weights and volumes of the added fresh salt portion and the discharged spent salt fraction are not equal because the density and volume of the molten salt changes during purification. In practice, it is convenient to control the amount of spent salt discharged by metering the overflowing salt into a calibrated ladle. The amount to be discharged and the optimum base amount depend on the operating conditions and are determined empirically. The relatively large base amount of salt in the contactor provides a buffer against fluctuations in the process, such as variations in the thallium content of ~331~9~
- -17.

the impure lead and inaccuracies in controlling the amounts ofadded fresh salt and discharged spent salt. The base amount also provides an increased surface area for contact of the molten lead and salt. The contactor operator is freed from the problem of trying to discharge the spent salt without also letting some lead overflow.

The preferred procedure, a combination of the preceding method and counter-current salt use, is to maintain continuously an approximately constant base amount of used chloride salt in the contactor throughout the purification of an indefinitely long series of lead batches, with discharge of a fraction of the base amount and addition of a portion of fresh chloride salt at a predetermined time during the purification of each batch.
Except for the time at which spent salt is discharged and fresh salt is added, the same comments apply as in the preceding procedure. The inclusion of counter-current salt use increases the efficiency of thallium removal, by shortening the time required to lower the thallium content of the lead to a given final level, or by the attainment of a lower final thallium level in a given time, or both. The time at which the spent salt is discharged and fresh salt added is chosen on the basis of obtaining a satisfactory balance between the rate and degree of thallium removal.

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18.

An approximation based on pilot testing using zinc chloride indicated that, with the preferred procedure, the thallium content of a 200 metric ton batch of molten lead at 400C can be lowered from 100 ppm to 10 ppm under the following conditions: contactor diameter = one metre; lead circulation rate = 600 metric tons per hour; one hour of contact with 600 kg of zinc chloride-based salt initially containing 4 wt.%
thallium, followed by discharge of 80 litres of spent salt containing 6 wt.% thallium and addition of 180 kg of fresh zinc chloride, followed by 3 hours of further contact.

' ,- - ' As is known from the prior art previously described, zinc -~ -chloride, lead chloride, ammonium chloride and mixtures thereof can be used in the molten state to extract thallium from molten lead. We have found that lead chloride alone or lead chloride-sodium chloride eutectic mixture removed thallium from lead at 550C and 500C, respectively. The eutectic mixture gave more efficient thallium removal than lead chloride alone.
However, an excessive amount of the eutectic was required to obtain sufficiently low final thallium levels in the lead. On a commercial scale, it would be essential to remove and recover the thallium rom the used eutectic and to recycle the lead chloride. Although this was found to be technically feasible, the economic aspects were not favourable.

~331~3~

19 .

Ammonium chloride was not seriously considered, either by itself or as a component of salt mixtures for thallium removal, because of its high vapour pressure at temperatures above the melting point of lead.

Preliminary tests with chemically pure anhydrous zinc chloride showed it to be more efficient than lead chloride-sodium chloride eutectic. In addition, zinc chloride is effective at a lower temperature (about 400C) than the eutectic. An .
additional advantage of zinc chloride is that removal and recovery of thallium from the used salt is technically and economically feasible on a commercial scale as disclosed in co-pending application Ser. No. 566,166 . A further advantage is that it is not necessary to recycle the zinc chloride because the amount used is relatively small.
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Zinc chloride is the preferred salt. The presence of a certain amount of lead chloride in the zinc chloride is not disadvantageous. The initia`l zinc chloride may contain up to 5 mole % lead chloride. ;~
' ~. ~ . . . ~, In lead production by a direct smelting process, for example by ;~
the QSL process, the lead bullion tapped from the smelting -`
furnace is cooled to about 425C and ~drossed~ to remove certain impurities, not including thallium. Since the -:-' . ~ :

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preferred temperature for thallium removal with zinc chloride is about 400C, it is then convenient to remove thallium.
While still molten, the lead may then be cast into anodes for electrorefining. A less preferred alternative is to electrorefine the drossed lead and remove thallium from the cathode lead.

Using zinc chloride, a series of tests was carried out with pilot-scale apparatus as shown in Figure 2. The body 14 of contactor 10 consisted of a steel tube 60 cm long with an internal diameter of 10 cm. Kettle 12 had an effective capacity of about 110 litres and was heated with natural gas, under temperature control. A sheet metal canister 50, closed at the top by a perforated distributor plate 52 and closed at the bottom end by perforated bottom plate 54, was filled with a packing of steel cap screws. The canister was fixed to the body of the contactor at a height such that a molten zinc chloride layer 36, floating on lead column 32, occupied the free volume of the lower portion of the packed canister. The packing in the upper part of the canister, above the zinc chloride layer, helped distribute the flow of lead evenly across the horizontal cross-section of the canister. A pipe 56 passed through the canister to provide an opening for valve stem 22 to which valve plug 58 was attached. Valve port 20 was opened or restricted by respectively lowering or raising valve ~3310~

21.

stem 22. Lead circulation was provided by a centrifugal pump 42 discharging via pipe 60 to a constant-head headbox 62 located adjacent to the upper end of the contactor. Lead flowed by gravity into the contactor via valve port 64 and discharge pipe 66. The flow rate into the contactor was controlled by valve plug 68 which was raised or lowered by means of valve stem 70. A constant head of molten lead, indicated by dotted line 72, was provided by pumping lead into the headbox at a rate higher than the drainage rate through port 64. The excess flow returned to the kettle through overflow pipe 74. Inclined side tube 26 was located 8 cm above distributor plate 52.

The same batch of lead, weighing 1250 kg, was used in all the tests. The desired initial thallium concentration, about 80 to 100 ppm, was obtained by dissolving an appropriate weight of metallic thallium in the lead bath. The bath temperature was maintained at about 400C throughout the test series. Zinc chloride was prepared by reacting metallic zinc with lead chloride. The zinc chloride thus produced contained about 5 mole % lead chloride, or about 7% lead on a weight basis.
During each test, the lead circulation rate was checked periodically by swinging discharge pipe 66 out of the contactor body and measuring the time required to fill a hand ladle of known capacity. Valve stem 70 was ad]usted accordingly in 1331~9 .~, , , 22. -order to maintain an approximately constant lead circulation rate. At fixed time intervals, a sample of lead was taken from the bath for thallium analysis. Prior to sampling, the bath was stirred to ensure homogeneity. At the end of each test, substantially all of the thallium-containing chloride salt layer was removed from the contactor using the procedure previously described. After solidification, the discharged chloride salt was weighed and sampled for analysis. In addition to thallium, the discharged salt contained, on a weight basis, 30 to 35~ zinc, 20 to 25% lead, 30 to 35%
chlorine and 1 to 2% oxygen. The increase in the lead concentration of the salt is attributed to dissolution of lead oxide contained in the pumped molten lead and to reaction of -the contained lead oxide with zinc chloride.

The following examples illustrate the results obtained in the pilot-scale tests.

Example 1 Effect of Circulation Rate Three tests were carried out at different lead circulation rates, namely, 32, 45 and 61 kg/min. Fresh zinc chloride, 1300 g, was used in each test. The results, shown in Figure 3, indicate that thallium elimination improves as the lead circulation rate is increased. At 61 kg/min, the thallium 1331~39 content of the lead was lowered to 10 ppm after 2 1/2 hours of circulation. The discharged chloride salt in these tests contained about 6 to 6.5 wt.% thallium.

.
Example 2 Effect of Packing ~'' ' A test was done with the packed canister removed from the contactor. Fresh zinc chloride (1300 g) was placed directly on . . ', ~
the molten lead surface in the contactor. The thickness of the zinc chloride layer was about 5.5 cm. A perforated distributor trough, similar to plate 38 in Figure 1, was located about 10 cm above the top surface of the zinc chloride layer. The lead `~
circulation rate was 61 kg/min. The results, shown in Figure 4 -in comparison with the final test of Example 1, indicate that the initial rate of thallium removal is lower when there is no ;~
packing present.

Example 3 Effect of Two-Stage Contact .--:' .' ~;
A test was carried out to simulate counter-current use of zinc ~ -chloride. As in Example 2, the canister was replaced by a distributor trough. A 1400 g portion of used zinc chloride ~- `
salt containing 7 wt.~ thallium was placed in the contactor and ~ ;
the lead was circulated for 50 minutes. The chloride salt was then discharged and 1300 g of fresh zinc chloride was added to ' ~.: .:

-- ~L331~
24.

the contactor. Circulation was continued for a further period of 4 hours. The lead circulation rate was 59 kg/min. The thallium removal rate is shown in Figure 5. The final thallium content in the lead was 5 ppm, the lowest value obtained in the tests. The first portion of discharged chloride salt contained 10.1 wt.~ thallium. The second portion contained 5.2 wt.%
thallium. This example shows that counter-current use of the zinc chloride is feasible for obtaining very low thallium levels and for obtaining more efficient utilization of the zinc chloride.
~ ' It will be understood that modifications can be made in the embodiment of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.

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Claims (8)

1. A method of removing thallium from thallium-containing lead comprising forming a molten bath of said lead, immersing the lower end of a vertical contactor having a lower end and an upper end in said bath to form a column of molten lead in said lower end, forming a layer of molten chloride salt within said contactor on the molten lead, feeding said molten lead from below the surface of the bath of molten lead into the upper end of the contactor onto the molten chloride salt whereby the molten lead descends through the layer of molten chloride salt for accumulation of thallium in the molten chloride and return of lead to the lead bath through the contactor lower end, restricting the return of lead through the contactor lower end to the molten bath during feed of the molten lead when a desired accumulation of thallium in the molten chloride salt is reached for upward displacement and substantial discharge of the thallium-enriched molten chloride salt from the contactor, and replenishing the discharged molten chloride salt while resuming the return of lead through the contactor lower end to the lead bath.

2. A method as claimed in claim 1 in which the molten chloride salt is anhydrous zinc chloride.

3. A method as claimed in claim 2 in which the molten lead is maintained at a temperature in the range of 400 - 425°C.

4. A method as claimed in claim 1, 2 or 3 in which the molten lead is fed into the upper end of the contactor at a controlled, substantially constant rate at a temperature of about 400°C.

5. An apparatus for the removal of thallium from a molten bath of thallium-containing lead comprising a contactor having an upper end and a lower end and a side discharge in proximity to the upper end, means for suspending said contactor in said molten bath of lead whereby the lower end of the contactor is immersed in the molten lead, said contactor having a layer of molten chloride salt therein on the molten lead below the level of the side outlet, means for feeding molten lead from the bath below the surface thereof into the upper end of the contactor onto the layer of molten chloride salt for downward passage through the molten chloride salt for accumulation of thallium in the molten chloride salt and return of the molten lead to the molten lead bath, and means for restricting the return of molten lead to the molten lead bath at the lower end of the contactor whereby the layer of molten chloride salt can be displaced upwardly during feed of the molten lead for discharge of the molten chloride salt containing accumulated thallium from the contactor through the side outlet.

6. An apparatus as claimed in claim 5 in which said means for restricting the return of molten lead to the lead bath comprises a valve port formed at the lower end of the contactor and a valve stem having a valve plug formed at the lower end thereof adapted for vertical reciprocal movement to and from the valve port for closing and opening the valve port.

7. An apparatus as claimed in claim 6 in which the contactor is cylindrical in shape and in which the apparatus additionally comprises a circular perforated distributor plate affixed to the contactor transversely thereof below the side discharge.

8. An apparatus as claimed in claim 7 in which a plurality of steel packing elements are positioned in the contactor on the layer of molten chloride salt below the circular plate.