US2464262A - Production of zinc - Google Patents

Description

March 15, 1949. s. RoBsQN ET AL 2,464,262 l 4PRODUCTIQN OF ZINC Filed May 12, 1944 2 sheets-sheet 1 mill 'HIII lll lll

March 15, 1949. s. RoBsoN ET AL 2,464,262

PRODUCTION OF Z INC Filed May 12, 1944 2 Sheets-Sheet 2 @mwst Patented Mar. 15, 1949 PRODUCTION OF ZINC Stanley Robson, Clifton, Bristol, and Leslie Jack Derham, Severn Beach, England, assignors to The National Smelting Company Limited, London, England, a British company Application May 12, 1944, Serial No. 535,290

` In Great Britain March 13, 1943 Section 1, Public Law 690. August 8, 1946.

' Patent expires March 13, 1963 1 Claim. (Cl. 75-86) In the production of Zinc it: is, often necessary to condense. zincvapour from a gaseous mixture and difculty is generally encountered in obtaining all the zinc as molten metal andavoiding the formation of zinc dust. Particularly when the concentration of Zinc in the gas islow, as is the cas-e when zinc is reduced from a sintered ore in a blast `furnace, the diculty of collecting the zinc as molten metal becomes a major problem and it has been an important factor in the failure of the ina-ny attempts sofar made to produce zinc by blast furnace operation. A similar diniculty has ariseninthe electrotherrnic. sinelting of. Zinc. The principal Ycause o the dimculty is oxidation vby carbon dioxide by the reaction This reaction, unless the concentration ocarbon dioxide is srnall, can occur above the dew-point. Even if only a fraction of the total zinc is thus oxidised, it is found that the diiculty of condensing the remainder to molten metal is increased, A- possible'explanation is that When the dew-point is reached any particles of zinc oxide act as Anuclei tor the condensation of metallic Zinc. nThese droplets of metallic zinc can then be further oxidised on the surface. This superficial oxide film then probably tends toprevent the droplets coalescing, even if they are brought into/contrast with molten zinc.

a' An important object of this inventionis to provide an improved process for condensing zinc vapour lfrom a gaseous mixture.

Another obj ect of this invention is to maxe use ofv the properties Yof. lead or lead-rich alloys in the condensation of* zinc.

AIn our inventionA zinc vapour is condensed by bringingV the gases into intimate contact with moltenv lead'or molten alloy rich in lead that is ata temperature which is not greater than 550 C. We place the condensing apparatus as close as possible to the outlet by which the` gases leave the retort or furnace, tofensure that the zincbearing gases, While still hot, are brought into contact with the molten lead as soon as possible. Rapid chillingy of the gas is thereby effected, and any considerable amount of oxidation oi zinc by carbon dioxide or other gases present is avoided.

The lead must be at a relatively lov temperature for tWo reasons. Firstly, it is necessary to shock-chill the zinc, and secondly the gases will necessarily retain an amount of zinc proportional to the vapour pressure of Zinc over the solution of zinc in lead that is formed by the condensation, and the lower the temperature the lower this prop ortionvwill be. If the initial gas,- eous mixture containsY only a small amount of Zinc vapour, the fraction of this that cannot be condensed Will be high unless the liquid leadv isY at a loW temperature. For these reasons the lead is preferably at a temperature below '500G C. and is generally at a distinctly lower temperature.

Zinc is sparingly soluble in lead, the solubility increasing with the temperature, so the use of molten lead for condensingrthe zinc presents the great advantage that the zinc dissolved in the lead can be recovered when thev lead is` cooled. The zinc may be obtained either directly as a liquid or as a s oldioating on'4 the surface of the molten lead.` After the separation of` the con'- densedz'inc the lead maybe usedagain for the condensation of further zinc vapour, that is to say, it may be continuously circulated asfa liquid; this cany easily' be vdone in `view oi the lojv melting point of vlead'. v Y

The very sparing solubility of Zinc in lead, Which is advantageous for the reason just given, makesthe lead have only a small povver to remove Zinc by solution frorn` a gaseous mixture at temperatures above the dew-point of the gaseous mixture. In our inventionthe low tempera-l ture at which the lead is maintained serves to reduce the temperatureof the zinc toA below the dew-paint and so the zinc is easily condensed dissolved. Y i W It is generally found advantageous to main'- tain a considerable degree 'of turbulence'in the lead. Av possible explanation of its eiTect is that, if any zinc vapour is condensed as dust and becomes superiicially oxdised, vigorous agitation of the lead helps to rupture or prevent the formation of a coherent oxidelm, so that the metallic particlesvcan dissolve in the liquid lead.

As a rule, ordnary commercial lead is used, but the condensation canbe effected equally Well by means of a lead-rich alloy provided that the other element or elements of the alloy dissolve-ln lead in preference to zinc so that they will not contaminate the zinc to any great extent after it has been separated from solution in the lead,

Some plants in which the present invention may be carried out are 'shovvn` diagrammatically in the annexed drawings, in Which- Figure l is a longitudinal vertical section through one plant; A

Figure 2 is a similar section through another plant; and

Figures '3 and e show two other forms off condenser.

'In the plant shown in Figure l zincV is produced from oxides or ores by reduction in a blast furnace comprising a shaft I with tuyres 3. As the charge moves down this shaft, a gaseous mixture of zinc vapour, nitrogen, carbon dioxide and carbon monoxide is evolved and passes to a heated coke column I0, in which Some reduction of carbon dioxide to carbon monoxide takes place, all as described in detail in the application of Stanley Robson, Stephen E. Woods and Leslie J. Derham, Serial No. 535,288, of even date herewith and now abandoned. The gases leave the coke column through a duct I5 and at this point the composition of the gaseous mixture may, for example, be 5% Zn, 63% N, 5% CO2 and 27% CO. The condenser shown in Figure 1 is constituted by a packed tower up which the gases are passed in counter-current with molten lead. The gaseous mixture rst enters a chamber I8 which is packed with coke 22 and is surmounted by the tower proper 20, which is packed with somewhat ner coke 2I. Liquid lead is introduced above this complete coke column by a pipe 21 that terminates in a spraying nozzle 28, so that a ow of liquid lead is maintained down the coke column. A baille 24 prevents the lead from flowing laterally into the duct I5. The incoming gases ow upwards through the column in countercurrent with the molten lead, and most of the zinc vapour is condensed out and dissolves in the lead. A substantial part of the heat that must be removed from the gas in order to cool and condense `the zinc vapour is taken up by the lead, which may rise in temperature by from 100 to 200 C. during its passage through the condenser. The gases leave the top of the tower by a pipe 26, by which they are conducted to a dust chamber (not shown) to recover the remainder of the Zinc as metallic dust. The lead containing, say, 1.5% zinc in solution and at a temperature of about 500 C. forms a pool 23 at the bottom of the chamber I9 and leaves by an outlet pipe 29 to a cooling bath 30. The heat loss from this is so controlled, for example by providing different thicknesses of insulation, that the upper surface 32 of the lead in the bath is at a temperature greater than 418 C., say about 500 C., but the lead at the bottom of the bath is at a lower temperature, say about 340 C. Zinc crystals come out of solution from the lead at the bottom and float to the top where they melt so that 'n a layer` of liquid zinc is formed at the top and is withdrawn from time to time through an outlet 33. From the bottom of the bath the lead, containing about 1% of zinc in solution, is carried by a pipe to a reservoir 36, whence it is conveyed by a hoist 31 or other appropriate means to the pipe 21 and thence back to the top of the condensing tower. A convenient rate of circulation of the lead is from 100 to 200 times by weight the amount of zinc removed from the gas.

It will be understood that other packing materials may be used, e. g. refractory materials such as carborundum.

In the form of condenser shown in Figure 2, the gases bubble through the lead beneath a baffle that projects downwards into a bath of the liquid lead. As before, gases from a blast furnace I arrive by a duct I5 and they enter a closed chamber 40 containing a baile wall 4I. A pool of molten lead 42 is maintained in the chamber in an amount sufcient to keep the bottom of the wall 4I immersed. Suction is applied to the outlet pipe 43 by a pump (not shown), and in consequence the gases bubble through the 4 pool of molten lead and in passing under the wall 4I they produce a spray of liquid metal which is particularly effective in scrubbing the gas. Some of the lead is Withdrawn continuously and circulated through a cooler such as that shown in Figure 1. The type of condenser shown in Figure 2 is simple, but not as eicient as that shown in Figure 1, probably because bubbles of gas tend to surge rapidly through the lead beneath the wall 4 I.

The form of condenser shown in Figure 3 is particularly eicient in extracting a large proportion of the zinc vapour from the gases as molten zinc. In principle, the gases are caused to pass through a spray or curtain of molten lead. This is produced by causing a rotary paddle-wheel to dip into a pool of molten lead 5I in a box 52 having an outer steel casing 53 and a brick lining 54. The gases enter at and leave at 56, and in their passage they are forced to pass through a spray of lead that is forced upwards by the vanes 51 of the wheel as the latter is rotated anti-clockwise at a speed of, say, 100 to 120 revolutions per minute. These vanes are enclosed within a perforated plate 58 through which the molten lead can easily pass. Of course, the wheel 50 must extend from one side of the box 52 to the other so as to ensure that the spray extends completely across the box. The lead is fed in at 65 and discharged at 06.

If it should be found that a single spray is insuicient two paddle-wheels 60 and 5I may be used in series as shown in Fig. 4, the gases passing from one to the other through an opening 64 in a partition 59. Some of the lead thrown up by the second paddle-wheel 0I will pass into the compartment in which the ilrst wheel 60 works, so the incoming lead may simply be introduced into the second compartment through an inlet 62 and removed from the rst compartment by an outlet 63.

Naturally the paddle-wheels shown may be replaced by any equivalent devices which will produce a stray and promote turbulence.

The invention may usefully be applied to any mixture of zinc vapour and other gases from which diiiculty is experienced in obtaining an adequate recovery of molten zinc. It is, however, particularly valuable in the condensation of zinc from gases obtained by smelting zinc ores in a blast furnace as described in the said application of Stanley Robson, Stephen E. Woods and Leslie J. Derham. It is also valuable in the condensation of zinc from the gases from an electrothermic arc furnace in which oxidised zinciferous materials are reduced by carbon. When treating such gases, relatively rich in zinc vapour as com'- pared with those from a blast furnace, it is not so important that the lead should be maintained at below 500 C., but it is still preferable that its temperature should be kept as low as the other operating conditions permit.

When zinc is smelted in externally heated vertical retorts or in vertical electric resistance furnaces, it is found desirable to introduce some gas or air into the bottom of the retort or furnace. The amount of such gas that can -be introduced is limited because large quantities would dilute the zinc vapour so that an unduly large amount oi zinc dust would be formed in the customary types of condenser. If the gases from such vvertical retorts or furnaces are treated by the condens-ation methods of the present invention, the amount of gas or air introduced at the bottom of the retort or furnace can be increased to the extent required to give the best operating results from the point of view of the furnace or retort itself, and the zinc evolved can still be eciently condensed to liquid metal.

What we claim is:

The method of recovering Zinc from Zinc ore which comprises smelting the ore in a blast furnace with resultant formation of a gaseous mixture containing zinc vapor, carbon monoxide and a substantial amount of carbon dioxide, bringing the gaseous mixture while still hot into intimate shock-chilling contact with circulating molten lead in a condensing zone maintained at a temperature not greater than 550 C. to cause the gaseous mixture to cool to a temperature below that at which the carbon dioxide can react with the zinc to form objectionable zinc oxide and condensing the zinc vapor as a solution of zinc in the lead, accumulating a body of the solution, cooling the lower portion of said body of solution to a temperature below 418 C. but not below the melting point of lead to precipitate zinc therefrom, while maintaining the upper portion of said body of solution above 418 C., permitting the REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,513,280 Schmidt Oct. 28, 1924 2,130,886 Kemmer Sept. 30, 1938 2,183,535 Betterton Dec. 19, 1939 2,208,586 Kemmer July 23, 1940 2,238,819 Neve Apr. 15, 1941 2,238,909 McConica Apr. 22, 1941 2,381,405 Griswold, Jr Aug. 7, 1945

Images