US2473304A - Condensation of zinc from its vapor in gaseous mixtures - Google Patents

Description

June 14, 1949. s. RoBsoN 2,473,304 CONDENSATION OF ZINC FROM ITS VAPOR IN GASEOUS MIXTURES Filed June 18. 1947 4 Sheets-Sheet 2 n m5 -3 km .3 mm x 3. g \v m\\ \\\\\\\\\\.\\R\\\\\\\\\\\\\\\v% .xw w 7 1 a \Q g \\\\\\\\\\\\Q\\\\\\\ Q Q mm x mw v ww w N M June 14,1949; sPRoBS oN Y 2,473,304

CONDENSATION OF ZINC 'FR ITS VAPOR IN GASEOUS MIX S Filed June 18, -l947 4 Shegts-Sheet 3 Patented June 14, 1949 CONDENSATION OF ZINC FROM ITS VAPOR IN GASEOUS MIXTURES Stanley Robson, Redland, Bristol, England, assignor to The National smelting Company Limited, London, England, a British company Application June 18, 1947, Serial No. 755,443

' In Great Britain March 12, 1946 Section 1, PublicLaw 690, August 8, 1946 Patent expires March 12, 1966 4 Claims.

This invention relates to an improved method of condensing liquid zinc from a mixture of zinc vapour with permanent gases. It is adapted to deal with, for instance, the gaseous products evolved when oxidised zinciferous materials are reduced by carbonaceous reducing agents in externally heated retorts, electrothermic furnaces or shaft furnaces. It is an object of this invention to provide a condenser in which most of the zinc contained in the incoming gases can be condensed to liquid metal; in particular, besides ensuring that no large amount of zinc vapour escapes condensation, the practice of this invention prevents the formation of any large quantity of zinc dust and dross, consisting of metallic zinc with more or less zinc oxide, commonly known as blue powder.

In retort processes of zinc smelting the main overall reaction OCCUIlIlg is reduction of zinc oxide by carbon to give equal volumes of zinc vapour and carbon monoxide, according to the equation,

As a typical instance of retort smelting may be mentioned the process in which a briquetted charge of oxidised zinc ores and carbonaceous material is heated in a vertical retort. In this vertical retort process it is customary to admit a certain volume of air or other gas or vapour, e. g.

steam, at the bottom of the retort, so that the gases finally evolved contain only 30% to 40% Zinc vapour by volume, the balance being chiefly carbon monoxide but including some nitrogen, hydrogen, and a small amount of carbon dioxide.

From such gases, in the types of condensers usually employed, it is possible to condense the greater part of the zinc as liquid metal, but quite a considerable fraction, of the order of -15%, is obtained as blue powder.

When zinc oxide compounds are reduced by smelting in electric arc furnaces, the main reaction taking place is the same as in retort smelting, namely, reduction of zinc oxide by carbon,

When zinc ores are smelted in a blast furnace,

of metallic zinc.

the heat required is provided by burning carbon in a furnace that also contains the zinciferous material. Consequently the zinc vapour evolved is mixed with the products of combustion of the fuel. The gaseous products may contain only about 5% of zinc by volume; the carbon dioxide content may also be about 5%, the balance consisting chiefly of carbon monoxide and nitrogen.

Sudden chilling in the conventional types of zinc condenser promotes blue powder formation. In the presence of considerable carbon dioxide, slow cooling promotes the formation of oxidized blue powder by reaction (2). Conflicting. requirements are thus encountered in dealing with a gas mixture whose zinc concentration is relatively small and which contains gases, such as carbon dioxide, that can oxidize zinc.

It is an object of the present invention to reconcile these conflicting requirements more especially when the gas to be treated has a relatively small concentration of zinc vapour and there are present gases, such as carbon dioxide, that can oxidise the zinc. With such gases, slow cooling promotes chemical blue powder formation, by giving opportunity for reaction of zinc vapour with carbon dioxide or other oxidising gases present, while rapid cooling, at least in a conventional surface condenser, tends'to promote physical blue powder formation by causing forma- -tion of droplets of zinc within the gas phase rather than on the condenser walls. This dilemma is encountered in dealing with the gaseous products resulting from treating zinc ores in a blast furnace.

- spray of molten metal; and in the second stage thetemperature of the metallic shower or spray is less thanthe minimum practicable temperature for tapping molten zinc for casting into ingots.

The metal on or in which the zinc from the gas mixture isacondensed int-the ffir'st'stageiof condensation is zinc itself. The metallic shower or spray in the second (and optional third) stage of condensation is lead (in practice having a very small amount of zinc in solution) thefirst stage having a shower or spray of liquid zinc.

without appreciable loss of temperature. Rapid chilling is thereby efiected, heat being abstracted from the gases at a high rate by the rain of mcltedzinc; appreciableioxidation of? zinc by 5 carbon dioxide is substantially avoided,- and blue powder is not formed to any great extent,

The compartment in which the first stage of econdensationis:efiected by a shower of molten :lzinc-is isolated from the second stage compart- For carrying out this process, a preferred form ment except for the gas transfer opening, so as of apparatus comprises a stationary condensing unit, partitioned to provide two or three internal chambers, each of which contains mechanical means for producing a continuouseshower .or spray of the liquid metal. prises an inlet for the gases into. th first of the chambers and a stack outlet from the last'chamher, and the chambers communicate with one another above the levels of the moltenimetal in them.

The spray orshowerof. molten metal through which the gasesarecaused to passmaybe pro- :ducedby a number-cf devices. One methodds "to cause arotary :paddle wheel to dipuinto a 511001 of the molten metal. not the condenser? inuwhich liquid-zinc is'ipresent -all portions of". the paddle-wheel and its -shaft .:are constructed :of, "or encased in, .zaxmaterial, :suchas graphite or: siliconccarbide, that is: not

:attacked'zbye zinc at the temperature -.at which :the condenserzworks. .Therpaddle wheels are enclosed in a box having at one endaan opening tfor: the admission of 'the .zinc vapourr-and gases sandaatath other endtan'outlet'for the gases out f which'zinchasbeen condensed. "The boxais: iimade of a steelucasing, the. l-id1of .which isarer-movable, andis :lined throughout'with bricks orla cement that is not? attacked by. liquid: zinc or lead. One or two similarly :linedpartitions .separate the chambers in-which successive condensation stages are effectedand each partition 'has an opening-to allow the gases-to pass-from one chamber to. the next.

The gas inlet and outlet "openings "of each chamber should be so located with reference to -the paddle-wheels that" the gases :are compelled to traversethe-shower =or spray of moltenrnetal, th final exhaust'stack'being'at the top o'f the last chamber at the end remote from' that'at which the gases enter. The paddle-wheel should have only a small: axialv clearance. at I each end from the sides of the chambenato ensure that' the -'=.shower or spray'of molten metal extendsacross the full width of the chamber. The outletfor .liquid metal from each chamber shouldxbe siturated at a level ensuring adequate immersionof .the paddle-wheel for showering the molten metal at all times. The-liquid meta-l outlet above re- =ferred to is either atop hole or a weir separating -two adjacent condensing chambers both conz-taining the same liquid :metal I as condensing agent, as in athree-stage processwiththe first -stage using a-shower of molten zinc and'ithe second and third stages using showers of: molten .alead,as hereinafter described. Control.of-heat loss from the chambers :may be .obtained'by placing heat insulating bricks round them as xrequired, .or 1:.by .cooling the 'base i of .thexcham- .bers by meansso-f water jackets orpipes.

' The mechanical condensing apparatus should be situated as close as possible to'the'outlet by :which the gases leave the *furnace, to ensure :that thelinc-beafing :gases are lbroughtiinto .-contact with the shower or spray of moltenizinc aas soon as:possible;aterleaving-thetfurnace and i The unit also-ocom- 115111 "toavoid mixing of the zinc and lead baths of ithexfir-staandrsecond stages. The heat insulation of the "first stage condenser compartment ziis preferablyrsol adjusted that all the molten zinc aintaine'd at about the minimum satisfactory vtapping temperature, say from 500 to 550 0.; ,whereasthe temperature of the molten lead in the second condensation stage may be kept as xlowsas 420 C.

The method of condensing Zinc from a mixi-ture of its vapour with permanent =ga-sesr by bringing the gases directly' from the producing -unit-into a stationary chamber in-which-ashower or spray of molten :lead-ismaintained -byi me- Within'compartments tchanical means such as amotarypaddle-wheel .is described in application .Ser. .-N0.---535',290 of ==Robson and Derhanm. filed May L2, .1944, .now .Patent No.2,464 ,262. In this-process the-temperature of the lead is. normally between e500" and/550 Cnand a'very large.-quantity of-lead is employed on account of the low 'lsolubility of. zinc :inlead, which 'decreases as" the temperature is lowered with cyclic operation, vinlwhich the lead ,lisre-circulated. after cooling to. enable dissolved rzinc to be separated, -.the .rate of circulation of leadmust usually be fromr100 to 200 timesthe rate of extraction of zinc by weight. .Inthe-two- .stageprocess ofthe present 'invention, in which the major part -0f-the total zinc extraction is q performed in-the first stagebylmoltenzina the quantityef lead required for-.circulation in the second stage is much less than is requiredin the :single-stage lead-processof=application Ser. No. 535,290, and the lead can be kept at a lower 5 'temperature. Full advantage can in fact be taken of the fact that leadhas a lower melting point than zinc, so that'the;gases'when finally "discharged can have "been'scrubbedat a temperature-below the melting point'of zinc.

--Scrubbing-the gas with "lead "at a low temperature ensures that zinc vapour is almost completely removed'from the gas. 'If, however, the lead leaves'the condenser at a temperature below *the'melting-point of zinc, cooling this lead fur- 'ther brings the con'densed'zinc out of solution as crystals, which have to-be'remelted to produce liquid zinc. It is preferable that, on the-one hand, th gas should be scrubbed bylead at a temperature below the melting point of zinc,

- while, on the otherhand, the lead should leave .'-the'condenser at a temperature above the melt- ;in'g-point of zinc.

To obtain-' the greatest benefit in this respect, the leadcondensation maybe performed in two -stages, making, with the first stage of condensation by molten zinc, three stagesin all. The twostage lead condensation is" performed in two "chambers in' series each provided with mechanizcal shower producing means and counter-current Show of moltenmetal and 'gas, the latter passing "from thefirst to the secon'dof the'two lead conudensation chambers and-the molten lead flowing from the second to the first chamber.

Such a three-stage plant, using molten'zinc ::andlead-inseries issuitablefcr thetreatment of gas from a blast furnace in which sintered zinc ore is reduced. In a typical plant of this kind the first condensing apparatus, containing a device for producing a shower or spray of molten zinc, is situated as close as possible to the outlet by which the gases leave the furnace; rapid chilling is thereby effected, and any considerable condensers, each fitted with apparatus for producing a shower or spray of molten lead, the last of these condensers having an opening in the roof for the stack by which the gases finally leave. Through these two latter condensers molten lead is circulated, in countercurrent with the gas stream; lead is introduced continuously, at a temperature of, say 350 0., into the chamber through which the gases finally pass before leaving by the stack, and flows over a weir into the next compartment, whence it is allowed to flow out continuously, its temperature then possibly being 500 C. The lead, containing some zinc in solution, is cooled under controlled conditions to separate some of its dissolved zinc, and is then re-circulated. A wall between the zinc spraying chamber and the adjacent lead spraying is of sufficient height to prevent flow of liquid metal from one compartment to the other, but there is sufiicient free space above the top of the wall to permit free travel of gases.

The paddle-wheel referred to can have a variety of forms. It may consistof a drum with projecting paddles, which may be shrouded by end flanges. Alternatively it may have'a saw-tooth profile. Furthermore, the depth of the projections, and the number of them disposed round the circumference, may be varied. In one case, which may be of importance, a notched, fluted or grooved roller is used; i this case there will be very numerous indentations or, alternatively regarded, very numerous teeth, all of small size. Whatever type of rotating apparatus is employed, it is advisable to cool the glands through which the shaft is introduced into the condenser by means of water. There is serious risk of zinc vapour diffusing to the glands and solidifying there. Arrangements should be made to force a slow stream of gas from the outside through the glands to ensure that no zinc vapour can reach them. A suitable gas for this purpose is one consisting chiefly of carbon monoxide, such as the condenser gas after it has been scrubbed and cooled.

The spray-producing device need not necessarily revolve round a horizontal axis. For instance, one type of device that may be used comprises a propeller, or series of cones or cups. or a centrifugal impeller, rotated by a vertical central shaft introduced through the roof of the chamber.

Another method, particularly convenient for the stage or stages of condensation for which lead is used, is to allow a stream of the molten metal to fall on to a table which is kept in rapid rotation by a vertical shaft.

The process of this invention may also include the stepof condensing residual zinc from the gases exhausted through the stack in the form of blue powder and allowing this blue powder to fall into the scrubbing zone of the condenser for recovery as liquid metal.

The accompanying drawings illustrate schematically some typical examples of zinc condenser assemblies according to the invention, and since the manner of carrying out our improved process, and various modifications thereof, have already been fully described and the apparatus explained in general terms, a brief description of each condenser assembly illustrated will sufiice for its understanding.

The drawings also include figures illustrating typical examples of paddle-wheels and other showering devices for molten metal for use in condenser assemblies according to the invention.

In the drawings:

Figs. 1 and 2 respectively are schematical central vertical sections of a first and second example of a condenser-assembly;

Figs. 3 and 4 are respectively transverse sections of two alternative examples of paddle-wheel or rotor structure;

Figs. 5 and 6 show two further examples of paddle-wheel or rotor structure in transverse section (outline only) Fig. '7 is a central vertical section of a centrifugal impeller mechanism for showering molten metal, with vertical axis;

Fig. 8 is a transverse section on the line 8-8 of Figs. 7 and 9;

Fig. 9 is a view in elevation of the impeller itself.

In Fig. 1, showing a two-stage condenser, I represents a flue by which the zinc-bearing gases are conducted from the source, which may, for example, be a zinc blast furnace, to the condenser, which is divided into two compartments, 2 and 3. In compartment 2 is a paddle-wheel 4 dipping into a pool of molten zinc 5. In compartment 3 is a similar paddle-wheel ii dipping into a pool of molten lead I. The floor 8 of the condenser is constructed of, or lined with, suitable refractory material that is not attacked by liquid zinc or lead. The lids 9 and H! of the compartments are r removable and are similarly constructed. Be-

tween the two compartments there is a baflie ll depending from the roof; beneath it is a bafiie l2 supported from the floor and of sufficient height to prevent liquid zinc or lead flowing from one compartment to the other. Through the opening between these baffles the gas passes from compartment 2 to compartment 3. At the inlet end of compartment 2 is a depending baffle i3 which compels the incoming gas to pass through an opening I 4 just above the level of the molten zinc. The paddle-wheel 4 is rotated in a direction shown by an arrow such that the lower half is moving towards the opening 14; this ensures that the entering gases are immediately brought into contact with a spray of molten zinc. In compartment 3 the paddle-wheel 6 is moving in the opposite direction (as shown by an arrow), so that its lower half is moving towards the gas outlet I5, which is limited at the top by a depending bafiie it. This arrangement ensures that the gases traverse, firstly a spray of molten zinc in compartment 2, and then a spray of molten lead in compartment 3.

The pool of liquid zinc 5 is connected with an outer well H by an underflow weir l8. From this well I! the zinc is run off continuously or "re moved at frequent intervals, to maintain the desired level of zinc in compartment 2 and thus 7 5 a suitable depth of immersion of thepaddle whee1 7 -4. Should-the tempera-tureeofi thezinc-in compartment 2 rise above thedesired-figure,--which--is .generallyabout 500 C.'-to- 550 C1, this canbe corrected by effectin auxiliary cooling in the well l 'Lor by=cireul-ating coolingwaterin a jacket round the-sump. -Shou1d-the temperature oi -the szinc become too low; this can'be corrected by placing insulation bricks round compartment 2.

- Lead is continuously circulated through-com- ;partment 3, entering byi pipe 28 and-leaving by pipe l9; it takes some zinc into solution-and rises in temperature-during-itspassage. It-is then cooled -under controlled conditions 'to separate a-someotits dissolved-zinc and is returned through the pipe 2 9.

In Fig 2, the first condensing chamber'2l :con- -tains"a. paddle-WheeI- ZZ "which rotates and -=dips intoa pool 230i molten zinc, producing a shower of molten zinc. This efiectsrapid chilling-of the -zinc bearing gasesentering from aflue 24* be- J'neath a baffle 25 to 'be brought into contact with the shower of molten zinc. The zinc condensed can be run-off under a bafile 26 to an outside well 22', whence it can' be: removed byany convenient means; alternatively, the Zinc can be run off directly iroma ta'p hole (not shown) arranged in chamber 2 l. -.The temperature of the molten Zinc is maintained at about550 C. If'it becomes :too cold, insulating bricks may-be placed round chamber 2 I. If the zinc becomes too hot, cooling may convenientlybe efieeted by an immersion water-cooler in well 21. Alternatively cooling may beeiiected by a water-jacket round the sides and bottom of the sump.

From the z inc sprayingcondenser 2 I the gases then passthrough two condensers 23 and 29, each fitted with a paddle-wheel and 3| respectively, 'for pr oducing' a-shower or spray of molten lead. From chamber 29 the gases leave by a flue 32. 'While theopening to flue 32 may be situated directly in the roof of chamber 29, it is found con- "venient to have a'baffle 33 interposed to force the "gas to enterthe'flue near the bottom of chamber 29. Moltenlea'd, aha-temperature of, say, 350 (3., is introduced into chamber 29 by pipe 34 and forms a pool BS-into which-the paddle-wheel 3| 'clips. The-lead then flows over a weir '36 into chamber 28," where it forms a p0ol31, into which "paddle-wheel 3U dips. Thence the lead flows out continuously through pipe 38, its temperature 'then bein 'about 500 C. Thelead, containing some zinc in solution,- is-cooled under controlled conditions to separatesome of its dissolved zinc "and is'then recirculated back by pipe'34. Arrows indicate the directions of rotation of the paddlewheels.

Between the zinc spraying chamber 2| and the adjacent lead sprayingchamber '28 is a wall '39 of sufficient height to prevent flow of liquid metal 'between chambers2l and 22. The space above "this wall 39 up to the roof maybe left open, but it is found preferable to hang a bafile 40 from the roof to prevent spray being thrown from one compartment to another. There is sufficient space hetween bailles -39 and40 to permit free flow of gas "from compartment 2! to compartment 28. A *depending bafile 4i -is also.interposed'between chambers 28 and 29.

Some typical examples of paddle-wheels and other rotary devices for showerin molten metal for use in condensers according to this'invention will now be briefly described. 1 I'n Fig. 3,4! isapaddle-wheel comprisingsa metal drum 42 with-projecting paddles 43, which -mayf'beshrouded 'by end;--flanges-%44; This:--is=suitable only fortuse-in-lead. I

:In- Fig. 4-isshown a rotor ldwitha-saw-tooth profile 41. This may -be-constructed-of graphite and carried by a-waterecooled 'metalshaft -48 which extends through the side walls-ofthev eon- "denser. Separating -the-hollow shaft 48 from --direct.contact with-the graphite is-a sleeve 4913f insulating cementfembeddedinwhich-are several -ribs-'50'-projecting-irom-the shaft 48. Similarly, the cement is-keyed-to the-g-raphite'by providing recesses5|-in-- the graphite whicharefilled with cement.

-'Fig.5- shows the profile of arotor: 56- construct- -edof-=graphite,- so shaped as to provide cups viil which serve to pick up the molten metal into which it 'dips. y,

Fig. 6 shows the profile ofagraphite rotor 52, notched with numerousindentations 5-3, or, alternatively regarded, provided with numerous teeth'M. Thisisshrouded by end flanges 55, also consisting of graphite.

(Fig. -7 shows "a "centrifugal "impeller device "which-consists of-a; generally cylindrical rotor 58 --which-is-carriedbya -hollow metal shaft 59, verrt-ically mountedand extended through the roof 60 of the condensing chamber. The rotor 58 may -be' constructed-of graphite or other; suitable refraetory material and is separated from vdirect :contact withthe shaft .59 by a sleeve BI .01? :insulati-ng cement. Ofone piecewith the rotor is an upwardly extending hollow cylinder .62 .-of graphite which surrounds the shaft -59, with .a layer of insulating scement-in between, up to the roof'fifi'of the condensing chamber. At its lower end the shaft 59 is furnished with splines 63fso *as'to key it to thewinsulating cement. ln the same region there are recesses i l in the rotor "andthese'are filled with cement, so'that'the -shaft,-sleeve and rotor are effectively keyed 'to- -gether. Theshaft 59 is cooled by'water or oth'er cooling medium introduced through a pipe I 65 which terminates just. above the bottom ofthe hollow shaft 59. Leaving the. open end 66 0f this 'pipe, the water flows upwards through-the annulus -61 between the pipe-64 andlthe shaft'59.

,On the outside surface of the rotor areitwo similarand diametrically opposite ledges cutout of -the-cylindrical surface, each, as shown by Figs-7 and 8, extending from apoint 6801' 69 at the top =0f'the rotor toadi-ametricall'y opposite point 18 or H respectively at the bottom of the rotor. Thepath followed'on the cylindrical sur- -face;of the rotor maybe described by considering this surface as unrolled-onto a plane; the ledge from 68 to 10 would then bevertical from -68 to 12, approximately circular, forming a quadrant of a circle, from 12 to '13, and horizontal from T3 to 10. The ledges are recessed into the surface, being cut at a somewhat acute angle.

In horizontal projection the ledges taper .from a maximum depth at'68 (69) through a mean depth at 14 (15) 'toize'r'o depth at '10 (H). The direction. of rotationis shown by an arrow "in .Fig. 8.

The bottom of 'therotor is "situated at Ia level belowthat of the molten zinc to be held 'in"'tlie condensing chamber 'andthe upper end'fof'the body of the rotor .is above the molten zinc level. :.A convenient level for the molten zinc is "shown by the line 16- 11. g

The rotating.sleeve"62, fits,"with only 'a narrow clearance, inside a stationary sleeve I8extending downwards from the. roof fill'of the condenserlarid into a. circular-trough "119 "out into 'tlie top of the rotor. In this trough is maintained liquid zinc, which forms an efiective seal. The upper end of the stationary sleeve 18 is enclosed in a gas seal, comprising a tight fitting box 80 with an opening in the top through which the shaft 59 extends, this being fitted with a bush 82. In the side of the box 80 there is an opening 83 through which a slow stream of gas is pumped; a suitable gas for this purpose is one consisting chiefly of carbon monoxide, such as the condenser gas after zinc has been removed from it. This gas passes down the annulus between the stationary sleeve 18 and the rotating sleeve 62, becoming heated as it does so, and then bubbles through the metal contained in the circular trough 19.

I claim:

1. A process for condensing zinc as liquid from a mixture of zinc vapour with permanent gases, comprising at least two stages of condensation in which the gas-zinc vapour mixture is fed with the minimum possible heat loss from the producing unit to a first condensing stage where the mixture is brought into intimate contact with a shower of molten zinc metal at a temperature high enough to permit tapping of molten zinc for casting into ingots, and the gas mixture is thence fed to a second condensing stage in which the mixture is brought into intimate contact with a shower of molten lead which may contain a residual amount of zinc in solution and whose temperature is below the minimum practical 10' temperature for tapping molten zinc for casting into ingots.

2. A process for condensing zinc as claimed in claim 1, including a third condensing stage in which the zinc vapour-bearing gas mixture is brought into intimate contact with a shower of molten lead which may contain a residual amount of zinc in solution and whose temperature is lower than in the second condensing stage.

3. A process for condensing zinc as claimed in claim 2 in which molten lead for the second condensing stage is provided by the flow of molten lead from the third condensing stage.

4. A process for condensing zinc as claimed in claim 3 in which molten lead is introduced into the third condensing stage at a temperature of about 350 C. and leaves the second condensing stage at a temperature of about 500 C.

STANLEY ROBSON.

REFERENCES CITED 7 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,871,657 Bunce Aug. 16, 1932 2,208,586 Kemmer July 23, 1940 2,238,819 Neve Aug. 15, 1941 2,381,405 Griswold, Jr. Aug. 7, 1945

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