US2457551A - Condensing zinc vapor - Google Patents

Description

Dec. 28, 1948. s. RoBsoN 2,457,551

CONDENSING zINc VAPOR4 i Filed Jan. 14, 1947 A l 2 sheets-sheet- 1 JNVENTOR.

Gbmmwla, MTM w1 @mm 4 @aimais 2 Sheets-Sheet 2 INVENTOR. M MJ? S. ROBSON CONDENS ING ZINC VAPOR Filed Jan. 14. 1947 Patented Dec. 28, 1948 UNITED STATES PATENT OFFICE 2,451,551 A v coNDnNsnzG zinc varon Stanley Robson, Bristol, England, assig'r'ii'` to `The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey Application January 14, 1947,- serial N0. 722,083 In vGreat Britain March L2, 1946 2 Claims. (or. 'z5- ssj This invention relates to an improved method "The diilculties involved in avoiding blue powof condensing liquid zinc from a mixture of aine vapor With permanent gases. It yis adapted to deal with, for instance, the gaseous products evolved when oxidized zinciferous materials are reduced by carbonaceous vreducing agents in externally heated retorts, yelectro-tlflermic furnaces or shaft furnaces. It is an object of this invention to provide an improved method of condensing zinc vapor in which most of the zinc contained in the incomingr gases can be condensed t liquid metal. In particular, besides ensuring thathno large amount of zinc vapor escapes condensation, the invention prevents the formation ofmsany large quantity of zinc dust and dross, consisting of metallic zinc with more or less .zine oxide, commonly known as blue powder. y In retort processes of zinc smeltingfthe` overall reaction occurring is reduction of vvzinc oxide by carbon to give equal Volumes of Aainc vapor and carbon monoxide, according to the equation: v l

ZnO-l-'C-Zn-:FCO

The concentration of zinc vapor in the gases evolved may therefore be about 50%.; As a' typical instance of retort smelting may bementioned the process in which a briquetted charge of yoxidized zinc ores and carbonaceous material is heated in vertical retorts. In this vertical retort process it is customary to admit a certa-in volume of air or other gas or vapor, e.- g. steam; at the bottom of the retort, so thatthe gases finally evolved contain only 30% to 40%v zinc vapor, the balance being chiey carbon monoxide but including some nitrogen, hydrogen, anda; small amount of carbon dioxide. From suchgasesfin the types of condenser usually employed,- itis possible to condense the greater part ofthe zinc as liquid metal, but quite' a considerable-fraction, of the order of -15 is obtained asblue powder. When zinc oxide compounds are reducedby smelting in electric arc furnaces-,the main reaction taking place is the same as in retort smelting, namely, reduction of zinc oxide by carbon, according to the aforementioned equation,V but condensation of the zinc vapor to liquid zinc-.ijs more dilcult. The difference may be due to the fact that more dust and fume, and somewhat` more carbon dioxide. are present in thel gases fromkv an arc furnace than from' a, vertical retort. -In arc furnace smelting with the conventional Y condensers (refractory chambers tted with baiiles-l, upwards of 36% of the zinc isy generally obtained in the form of blue powder.v f fder formation and the manner in Which this invention'overcomes these diii'iculties may be most clearly explained if some account is iirst given of (the theory of blue powder formation. One characteristic of blue .powder is that it consists of small particles. Factors which cause the Zinc `vapor to be condensed as small droplets or particles rather than as coherent liquid metal may be classified as physical causes of blue powder foriii-ation. The blue powder generally contains, besides metallic z'inc, some Zinc oxide. Factors which causeoxi'dation of zinc in the condenser maybe clas'sed as chemical causes of blue powder formation.

Physical causes of blue powder formation operate independently of the chemical composition of the permanent gases with which the zinc ,p`hysicalv blue'powder to be formed. A possible explanation of the factors effecting physical blue powder formation may be given as follows. Irithe conventional types of condenser, all the latent heat of. condensation is removed through the walls, theinner surfaces of which are at a lower temperature than the dew-point of the gas. Adjacent to the` Wall surface is a lm of gas across which `there is a steep temperature gradienuso thatJ the surface, remote from the wall, of this boundary layer, and the bulk of the gas, is hotter thanthe Wall. Condensation of zinc can then take vplace by zinc vapor diffusing through the boundary layer to the Wall, Where it condenses and down to the pool of molten metal `intherbottoin of the condenser. If the bulk of Vthe gas is above the dew-point this is the only way in which the zinc can condense. If the wall is made colder and the bulk of the gas has a temperature at or below the dew point, condensation can` then take place in the body of the condenser to yielddroplets of metallic zinc. Any particles of dust or fume in the gas can act as nuclei for lthe formation'of such droplets. If

these droplets are cooled below their melting point before coalescing, zinc dust or blue powder is formed. 1

Apart from physical blue powder formation in the condenser itself, there is also the question of the zinc vapor which escapes from the con-- denser as such and is usually caught elsewhere as blue powder. To minimize this loss, the gases must be made as cold as possible during their passage through the condenser.

The above described physical mode of fori-f Mrotary type, but the preferred form of the mation of blue powder `is unaiected by thev chemical composition of the permanent gases with which the zinc vapor is mixed. VIf these gases contain any considerable'content of car--v bon dioxide (CO2) or other oxidizing gas, the formation of blue powder is initiatediand'promoted by oxidation direct from the vapor phase,

second-stage condensing unit comprises a stationary chamber in which va bath or pool of molten zinc is maintained and which contains one or moremechanical devices for agitating the molten '.zinc inthe p'ool'tov produce the spray or shower of'molten zinc. vThe gas inlet of the second f'stagecondensingunit and the gas outlet stack and this chemical cause of blue powder formal-w,`

tion becomes serious when the zinc concentration in the gases is low. The present invention is concerned with the condensation of zinc metal from zinc-rich gases of a reducing character, such as those given olf by retort processes or'electric larc furnaces, in which the content of` carbon dioxide (or other oxidizing gas) is low in comparison with the content of carbon monoxide; and therefore for the purposes` of this invention the chemical cause of blue powder formation is of veryv minor importance.` y

It will be clear from theforegoing discussion that the later stages of the conventional condensation processes are the critical ones for the formation of physical blue powder, and the main aim' of the invention is to overcome the defects of the conventional processes inY this particular respect.

In accordance with the present invention, `the zinc vapor-bearing gases are first led from the producing unit directly into an initial condensing. unit, in which condensation is effectedj on are brought into intimate contact with a spray ,I

or shower of molten zinc maintained by mechanical means. The process thus comprises essentiallyv two stages or zonesfirstly a stage of conventional surface condensation and secondly a stage of condensation on or in a shower of molten zinc. For reasons already stated, the'temperatures of the condensing surfaces in the first stage should be relatively high, and, indeed, the higher the better, provided they are not too high `for condensation to take place; vand the temperature of the shower of molten zinc inthe second stage should be relatively low, and preferably as low as possible having regard to the need to keep the metal fluid and to enable it to be tapped. Thus, the method of condensing zinc vapor in accordance with the invention comprises condensing the vapors in a first zone comprising a baffle-type condenser wherein the condensing surfaces are maintained at a temperature sufiiciently high t0 preventr substantial formation of physical blue powder, and effecting condensation of previously ,uncondensed zinc vapor in a second zone comprising a splash-type condenser wherein a substantially continuous shower of molten zinc is hurled by centrifugal action in an upward direction with such violence asv to provide by itself and its splashing against the confining upper portion of the condenser turbulent sheetlike showersof molten lzinc through I which the residual zine vapor-containing gases v- "pass, '1

are-arranged to ensure that the zinc vapor bearing-gases are brought into intimate contact with the' molten zinc shower .in their passage through the condensing chamber of the second stage unit.,y

f The sprayforshower'of molten zinc through which the gases" are caused to pass may be produced by` a 'number of devices. One methodvis to cause arotary paddle-wheel to dip into and agitate apool of molten zinc so as to producea spray or shower.- rAll portions of the paddlewheel and its shaft coming within the condenser arek constructed of, or encased in, a material, such as graphite or silicon carbide, that is not attacked by lzinc liquid or vaporr at vthe temperature at which the condenser works. The paddle-wheel is'enclosed; in a chamber' having at one end an opening for they admission of the zinc vapor and 'gases andiatltheother end an outlet for the gases out of which'the zinc 4*has been condensed.' The chamber is made of'aste'el casing, the lid of which is removable, and islined throughout with 'bricks or a cement that-fisr not attacked by liquid zinc.

- Th'e invention willv be better yunderstood from the following description taken in conjunction with the accompanying drawings in which Fig. 1 is a longitudinal sectional elevation of an apparatus for practicing the invention, IFigJZI`v is a longitudinalsectional elevation of the' apparatus with the second stage condensing unit `divided intoj two chambers,

Fig; 3v transverse sectional elevation of the *Figsfle 5 and 6 are sectional elevations of modified formsofthe paddle wheel. f

illustrated in Fig. 1', the first stage condensing unitmay'beof thesame general construcy -bide. Af'cylindrical'orice Il connects the' higher end of this chamber withthe source of zincvva'por.

lnaties, l2 are hungfrom tneirooftiles. The roof 4tiles andbaiiies can be removedA by hand,thus access can-be' obtainedtoany part so that any deposits or'accre'tions, can be dislodged'and removed b'y'means of suitable'rakes or scraping tools.

' The downwardly sloping' iirststage condensing lunit 'leadfsf directly intothe second stage condensing Aunitgwhich in a preferred forom of construction comprises a stationary chamber lll 'containing a rotating paddle-wheel i4 dipping into la pool o'f -moltenfzina The inlet 'l5 and outlet I6 of ythis chamber are soloca'ted that the gases v pats through a sprayor' showerof molten zinc that is produced by the'vane's vof the wheel as the latter is rotated at 'afapproprate speed. 'The is removed, is situated at such a level a that the paddle-wheel at all timesA dips into the molten zinc bath sufliciently toV produce: the necessary shower of molten zinc; The refractory lining on the bottom of the chamber is made to slope upwards from the licor underneath the paddle- Wheel to form a ramp leading to the tap-hole. A rake or scraper can be inserted through the taphole to remove any blue powder or dross that has accumulated on the bottom of the chamber. At the lowest level of the bottom of the chamber a drain holel I8 is provided, which is1 normally kept closed, but enables all the metal to be drained off if necessary. Theexhaust stack I9 leaves the chamber at theV end opposite to that at which the gases entered, I-nsulating bricks may be placed round the chamber forcontrolling theheat loss, and the chamber has a removable lid 20 per* mitting access thereto for cleaning, etc.

In the rst stage of condensation, in the refractory chamber fitted with baffles, no large amount of Zinc oxide is formed. Most of the zinc condensed during this stage runs down as a stream of molten metal into thev second stage condensing unit. VSome globulesv ci liquid or molten zinc will, however, form during the first condensation stage and there is always sufficient carbon dioxide present toY react with these droplets and coat them with a nlm of zinc oxide, by the reaction l This superficial oxide iilm tends to prevent the droplets coalescing. When these droplets enter the second stage condensing unit, the vigorous scrubbing action to which the gases are subjected by the motion of the paddle-wheel and the resultant spray of molten zinc serves to disrupt the oxide iilm and allow the droplets to coalesce to form molten zinc.

The gases entering the second stage condensing unit still contain some zincy Vapor. They are very rapidly brought into contact with a large superficial area of molten zinc in the form of spray. Consequently the temperature of the gas 50 cooling by and in contact with molten zinc, most of the zinc vapor probably condenses directly to augment pre-existing drops of molten zinc, and any blue powder formed is converted to coherent molten zinc by the violent mechanical action to which it is subjected.

It should be pointed out that while the use of the present invention prevents the formation of ward' to the.mechanical'condenser- (second stage condensing unit) where any liquid. zinc. droplets with which it is associated are largely stripped from it. They zinc` oxide, however, is still left as such, and: must form some dross or accretions. Accordingly, provision should be made for periodicalcleaning. For this reason, the lid of the mechanical condenser may be made removable, so that rakes or scraping tools can be inserted to remove any deposits.

An advantage of this invention when used in combination with continuously operated. vertical retorts is that the mechanical condenser can re,- place the, condenser sump now usually iitted to vertical retorts. The downwardly sloping. member of the condenser now generally fitted can then be left almost unchanged. Thus the condenser of the present invention. can be tted onto exist7 ing. vertical retorts within the floor space now allocated. to the conventional type condenser with.- out modication of the foundations or buildings.

As already mentioned, this invention is in.- tend'ed only for use when the zinc content of the gases is. high and the carbon dioxide ccntentlow. With but/little carbon dioxide present, there is no pressing need to take steps to avoid oxidation during the rst stage of condensation; the loss as actual zinc oxide is small and the mechanical condenser through which the gases subsequently pass ensures that any droplets of zinc which may form round the zinc oxide particles are converted to. coherent liquid zinc and are not collected as blue powder.

In order to eiectas complete condensation as *Y possible, it is desirable to cool the gases to a temperature below that at which liquid zinc can be conveniently .tapped for casting into ingots. According to a feature of the invention, vthis is achieved by conducting the. second stage of con,- densation in two phases in the rst of which a higher temperature is maintained than in the second and through which the zinc vapor bearing eases and the condensed molten zinc now coun,- tercurrently. This feature of the inventionmay be conveniently carried out byy constructing the second; stage condensing unitwith two chambers crcompartments 2l and 22 (Fig. 2), each con,- tainins a rotary paddle-Wheel YIl or other meollanlsrn for Producing a Shower or .Spray of molten zine. the' gases passing from one to the other. The tan hole 23'is situated in the compartment 2l into which the gas enters rst, and the heat insulation round this Compartment iS So oontrolled that the metal. leaves the tap hole at a suitable temperature. It is impracticable to have to tap metal at ,a temperature only just above its melting point, and it is generally Considered convenient to have molten zinc leave the condenser at least above 590 C. In the other condensing compartment 22, where the stackk IB for the exhaust gases is situated, the heat insulation is so adjusted that the temperature is only slightly above the melting point of zine, and as zinc .is condensed in this compartment, it forms a pool there, and then flows over a Weir 24 into the other condensing compartment. Each .com- Dartment 2| and 22 has a removable lid 25 and a drain hole 26.

With the mechanical condenser the temperature of the. gases can be reduced very nearly to that of the liquid metal- The vapor pressure of zinc is such that if the gas entering the condensing chamber, contains only about 5% zine, and-the eases leaving the. .chamber are saturated with .zinc at somewhat over 500o C'., an; ap-

y35 through which theshaft 28 extends.

preciable fraction of the zinc is lost. By using two condensing compartments, with molten zinc and zinc vapor 'bearing gases nowing in. countercurrentthe zincvapor can be condensed till the gas leaving is saturated with zinc vapor at a temperature not greatly in excess of 420 C., while the moltenmetal can still be tapped oif at above The :paddle-wheel or mechanized rotor rei'erred to can have a variety of forms in'all of which .it is'designedto dip beneath the surface a of the pool of molten zinc and by mechanical agitation produce a shower or spray of molten i zinc in the condensing chamber of the second stage condensing unit. It may consist of a .drum with projecting paddles, which may be shrouded by end ilanges. Alternatively, it may have a saw-tooth pronle. Furthermore, the depth of the projections, and the number of them disposed around the circumference, may be varied. In one case, which may be of importance, a notched, fluted, or grooved roller is used; in this case there will be 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 vapor diffusing to vthe glands and solidifying there. AArrangements should be made to force a slow stream of gas `from the outside through the glands to ensure that no zinc vapor can reach them. A suitable gas for this purpose is one consisting vchiefly of carbon monoxide, such as the exhaust. condenser gasafter it has been scrubbed and cooled.

The mechanized rotor I3 in' Figs. 1, 2 and 3 of the drawings has the aforementioned saw-tooth profile 21. The rotor may conveniently be made of graphite, and is separated from direct contact with the rotatably mounted metal shaftv 28 by a sleeve 29 of linsulating cement (Fig. 3). The shaft 28 has an axial bore 33 lthrough which flows a cooling medium, such as water. Each -end of the rotor has an annular endflange 3| whose outside diameter is the same as the outside diameter of the rsaw-tooth profile 21. The rotor has a laterally extending sleeve 32 at each end thereof surrounding the cement sleeve 29 where the later extends through the side walls 33 of the condenser. The outer ends of the concentric sleeves 29 and 32 are enclosed in a gas seal comprising a stationary cap 34 held tightly against the condenser wall by a gland bushing A suitable gas, such as the exhaust condenser gas after it `has been scrubbedjand cooled, is forced into the cap 34 through a pipe 36' to ensure that noA zinc vapor diffuses into and freezes between the openings inthe condenser walls through which the rotor sleevesr 32 extend.

The rotor shown in' Fig. 4 is a metal drum 31 with projecting paddles 38 and circular ends 39. The drum is carried on the cement sleeve 29 by the ends 39 which are of suiiicient diameter to shroud the paddles 38. l

The rotor 40 shown in Fig. 5 is a cylinderof graphite or the like with its cylindrical surface notched with numerous teeth lll. The rotor 42 lshown in Fig. V6 has' a plurality of circumferenwithin the chamber. The rotors 40 and d2 have shrouding end anges similar to the end flanges of rotors I4 and 31. y v l The condensing chambers'orvcompartments I3, 2i yand 22 are provided at each end with depending bailles 44 for forcing the zinc vapor bearingy gases to pass through the cloud of `metal spray set up by the rotor. The rotor Iltin the chamber I3 rotates counterclockwise, aslviewed in Fig. l and as indicated by` the arrow, so that the small amounts of molten zinc picked-up by the saw-toothy profile are thrown upwardly againstl the roof of the chamber and the baille 44 adacentthe gasinlet I5. The rotor Ill` vin the chamber 2I similarly rotates counterclockwise as viewed in Fig. 2 and as indicated by the densing the last traces of zinc vapor in the gases as they leave the chamber 22.

The process of this inventionmay also include the ,step of condensing residual zinc from the gases exhausted through the stack in the form of blue powder and allowingthis vblue powder to fall into the mechanical condenserr for re,- covery as liquid metal, and the apparatus according to thisr invention may be constructed toenable this step to beperformed, as describedin the specification of the copending British patent application No. 7621/46,v Stack` condensation of blue powder.

I claim:

l. In the methodof condensing zinc vapor in which zinc vapor bearing gases are condensed in a, first zone comprising a baille-type condenser, and the residual zinc vaporbearing gases and molten zinc are passed into a second zone withtaining the condensing surfaces in said zone at a temperature suiciently high to prevent substantial formation of physical blue powder,- and eiecting in said second zone condensation of previously uncondensed zincv vapor insaid gases by hurling by centrifugal action a substantially continuous and upwardly-directed shower of said molten zinc of such vviolence as to provide byitself and by 'its A,splashing against the confining upper portion of the chamber turbulent sheetlike showers of molten zinc through which said zinc vapor-containing gases pass, thecondensation in said second condensing zone being conducted in two phases having separate poolsof molten zinc which communicate with one another, the molten zincl in the pool of the first `phase being maintainedl at a temperature of'at least about 500 C. to vpermit tapping of the molten metal therefrom and the temperature of the molten zinc in the pool of the second phase being vthrough the two phases, tapping molten zinc from the poolthereof Vin the first phase, and withdrawing uncondensed vgases from the second phase. y

p 2. In the condensation of zinc vapor in which yzinc vapor bearing'gases are passed through a shower* of moltenv zincthrown upwardly kfrom a body thereofwithin a-condensingchamber, the

improvement which comprises conducting the condensation in two phases having separate bodies of molten zinc which communicate with one another, the temperature of the molten zinc in the body thereof in the irst phase being maintained sufficiently high to permit tapping of the molten metal therefrom and the temperature of the molten zinc in the body thereof in the second phase being maintained approximate the melting point thereof, hurling by centrifugal action a substantially continuous and upwardlydirected shower of molten zinc from each body thereof of such violence as to provide by itself and by its splashing against the confining upper portion of the chamber turbulent sheet-like showers of molten zinc through which said gases pass, causing the zinc vapor bearing gases and the condensed molten zinc to flow counter-currently through the two phases, tapping molten zinc from the body thereof in the first phase, and withdrawing uncondensed gases from the second phase.

`STANLEY ROBSON.

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

UNITED STATES PATENTS

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