US1994355A - Volatilizing zinc - Google Patents

Description

Patented Mar. 1.2, 1935 I .v

UNITED' sTATEs PATENT oi-FICE voLA'rnJzlNG zlNc Willis McGerald Peirce and Robert Kerr Waring, Palmerton, Pa., asslgnors to The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey "application May 12, 1933, serial No. 610,648

Y9 claims. (ci. 'i5-2s) This invention relates to the volatilizing of drical retorts, generally resembling the retorts zinc metal and has for its object the provision of of the Belgian spelter furnace, or in somewhat an improved method of volatilizing zinc metal. larger bottle-shaped retorts. In a. small retort, The invention particularly aims to provide imsuch as the retorts or bottles commonly used provements in volatilizing relatively large quanin the prior art, the percentage of wall surface 5 titles of zinc metal in a substantially continuous in contact with liquid zinc is limited by the necesoperation. While the invention is applicable sity of maintaining a vapor space at the top of wherever zinc metal is to be volatilized, as for the retort. If the size of the retort is increased example in the manufacture of French process in order to provide a unit of large capacity, sev- 10 zinc oxide, it can be applied with particular aderal dimculties are encountered: 10 vantage in redistilling impure zinc metal in the 1. Retorts or bottles of the form used in the purification thereof by controlled distillation or prior art will, if made of large size and lled by rectification treatment. to the customary degree, contain deep baths of We have found that the rate at which zinc liquid Zine- The hydrostatic head 0f such deep can be vaporized in a refractory container debaths of the heavy liquid zinc rapidly becomes pends on the percentage of the area of the heattoo great for a refractory Container Safely to SuS- ed wall which has liquid zinc in contact with tain. its inner surface. This contact maintains the 2. A monolithic container becomes structurinside wall temperature at approximately the ally impossible; and if a retort similar in form` boiling point of zinc and thereby increases the to those of the priorart is built up of separate 2C, temperature gradient between the heating chambricks, the Joints between the brokS Will be, aober temperature andthe inside wall tempera- Cording t0 experience, unable to withstand the ture. The increased temperature gradient inpressure of liquid Zine Without leakingcreases the rate of heat transfer from the heat- 3. Carrying out VaDoriZation in a deep bath ing medium to the zinc to be vaporized. of aheavy liquid, Such as Zine, results in increased 25 In accordance with the present invention, the vboiling point at the bottom and also results in liquid zinc is dispersed in a, plurality of baths violent ebullition, which is objectionable because so that a high percentage of the inner wall of it contaminates the vapor with a spray of imthe refractory container in which the zinc is Dure metal.

volatilized is in Contact with liquid zinc, and so The method and apparatus 0f the present in- 30 that no large individual area of inner wall survention obviate these difficulties and thus permit face is free from contact with liquid zinc. Thus, of the volatilization of yZine in units of larg in one of its practical aspects, the invention is capacity and high outputy aS folloWsI characterized by heating a series of vertically 1- The distribution of the liquid (molten) Zinc spaced pools or baths of molten zinc metal to in a plurality of baths divides the hydrostatie 35 a suiiiciently high temperature to volatilize the head 0f the molten Zine s0 that it is not sufmetal, and maintaining these pools of molten ciently great at any point to endanger the remetal by charging molten metal into the top of the fraetory Containerseries and permitting overflow of molten metal 2. The apparatus uSed in the Practice of the 4 0 from pool to pool. The vertically spaced pools present invention may be vbuilt up of individual 40 of molten metal may be conveniently maintained units in such a way that no joint is exposed to the in an upright refractory chamber internally pressure of liquid zinc, thus obviating leakage packed to permit countercurrent flow of deof liquid zinc through the joints. scending molten metal and ascending vapor. /3. The distribution of the liquid zinc in a plu- The chamber may advantageously be packed with rality of baths renders each bath relatively shal- 45 superposed and vertically spaced transverse pots low, and thus makesl negligible the increase in or trays having appropriate openings permitting the boiling point of zinc at the bottom of the the passage of descending molten metal and asbath brought about by the hydrostatic head 0f cending vapor from tray to tray. l the liquid zinc. The relative shallowness of the The improved method of the present invention individual baths diminishes the violence of ebul- 50 permits of the advantageous use of relatively lition, and furthermore, the degree of ebullition large zinc volatilizing retorts or zinc'boilers compresent here is not disadvantageous in the methpared with the usual practice of the prior art. od and apparatus of the present invention, in

Zinc metal has heretofore been commonly view of the fact that the molten metal flowing volatilized in horizontal or slightly inclined cylindown from bath to bath in counter-current with 55 points and higher densities than zinc, such as lead and iron, will tend to concentrate and remain in the individual troughs. We have discovered two means of limiting this concentration by continuously removing these impurities from the troughs.

1. By providing a suillcient violence of ebullition, these impurities are kept in suspension and carried from one trough to the next and finally out of the last trough, together with a limited quantity of zinc.

2. By so designing the troughs and overflows that the ilow of liquid metal from one unit to the next carries with it such impurities, which because of their greater densitytend to settle out in the bottom of the trough.

In practicing the present invention as a part of a puriiication system to remove high boiling impurities, it is known that these impurities, for example lead, are carried into the vapor stream by mechanical entrainment of'the mist due to boiling. The amounts so carried increase the impurity content of the vapor to an amount greater than would otherwise be present in the vapor by direct vaporization. By suitable arrangement of the retort units, to'bame and scrub the vapor, we may accomplish the removal of a substantial proportion of such entrained mist. By keeping the inner walls of the retort covered with liquid metal, we may also the superheating of the vapor and consequent vaporization oi' such mist. If such vaporization does occur to some extent, thereby increasing the content of impurity vapor in the vapor stream, a means of reabsorbing this excess'impurity vapor by contact with the liquid metal is provided.

We may also decrease the amount ofentrained mist by so adjusting the proportions of the trough as to avoid violent ebullition. This may, if desirable, be carried to a point where the unit becomes a shallow tray, though a loss` 1n vaporizing capacity results.

The invention may be carried out in various forms and types of apparatus, and in the accompanying drawings we have illustrated certain apparatus with which we have secured excellent results in the practice of the invention. In the drawings,

Fig. 1 is a sectional elevation of an apparatus for volatilizing zinc metal 'in accordance with the principles of the invention,

Fig. 2 is a plan of one of the transverse 4trays of the apparatus of Fig. 1,

Fig. 3 is a sectional elevation yoi.' an apparatus with a modied form of transverse tray in conjunction with a lead-eliminating rectifying column,

Fig. 4 is a plan of the transverse tray of the "retort or volatilizing chamber of the apparatus of Fig. 3,

Figs. 5, 6, 7 and 8 illustrate still further modied forms of transverse trays adapted as inte!- nal packing for the 'volatilizing chamber or retort of the invention, and

Fig. 9 `is a vertical section on the line v9-9 of F18. 5.

Referring to Fig. 1 of the drawings, there is shown an upright furnace structure l1li having a heating chamber or laboratory 11 provided with burner openings 12 and an outlet 13 for the exhausted heating gases. Pyrometers 14 extend through the walls of the furnace structure into the heating chamber 11 for indicating and recording the temperature of the heating gases therein. It will, of course, be understood that any other appropriate means of heating maybe used in lieu of the'burners and hot gases of combustion indicated in the apparatus oi Fig. 1.

A vertical volatilizing chamber or retort' is built within the heating chamber 1l. 'I'his retort is built up of superposed rectangular trays or pots 15 of silicon carbide or equivalent re'- fractory material of good heat conductivity. The top and bottom edges of the trays are bevelled as at 16 so that they may be stacked one above the other to form the vertical retort. The joints between the trays are made zinc-vaportight by an appropriate cement, such as a cement of silicon carbide with a small amount o f clay. 'I'he bottom of each tray has a transverse opening 17 approximate one end thereof. The upper surface of the bottom of each tray is provided with-transverse ribs 18 parallel to the opening 17; the Iribs being of progressively increasing height from the end nearest the slot to the end furthest from the slot. The trays 15 are stacked in staggered relationship with respect to the openings 17 so that the bottoms thereof form-a column of superposed and vertically spaced bafiles through which metallic vapor ascends and molten metal descends in tortuous or zig-zag paths. The ribs 18 serve to retain shallow pools of molten metal on the bottoms of the trays. The cascade effect of molten metal pouring over the ribs and falling from tray to tray facilitates intimate contact between ascending vapor and descending molten metal.

The bottom of the retort is formed by a tray 19 having a lateral opening communicating with a discharge pipe 20, the oriilce or tap hole of which is closed by a plug 21. An outlet 22 is mounted on the top of the retort for withdrawing the zinc vapor evolved therein. The outlet 22 is operatively connected with any appropriate apparatus or device (not shown) for utilizing or treating the zinc vapor evolved from the retort. At its top, the retort is provided with a charging device 24 for the introduction of molten zinc metal under conditions preventing the escape therethrough of any vapor evolved in the retort.

In operating the apparatus of Fig. 1 in accordance with the present invention, molten' zinc metal is introduced into the top of the vertical retort (l5) through the charging device 24. As molten metal is added to the charging device, either continuously or at frequent intervals, the trap or charging well thereof overflows into the top transverse tray 15 of the vertical retort or boiler. Molten metal ows downwardly through the openings 17 from tray to tray, and pools of molten metal are retained on the bottoms of the trays by the ribs 18. 'I'he vertical retort is heated to a suillciently high temperature by the hot gases surrounding it in the heating chamber 11 to actively boil the shallow pools of molten metal held on the trays. The heated wall of the retort are wetted with4 a plurality of baths oi' molten zinc metal over a relatively large proportion of the wall area, and there are a plurality'of baths of molten metal of relatively large surface area exposed to radiant heat, and in consequence of these two factors (particularly the first) very efficient volatilization of the molten metal takes place. Molten zinc metal is charged into the top of the retort in suicient amount to maintain pools cf molten metal on all of the trays of the retort, and this condition is established and maintained when a small amount of molten metal continuously overflows into the bottom tray 19 of the retort. In general, the rate of feed of molten metal to the retort approximates its boiling capacity. Where this rate of feed of molten metal is exceeded, the excess molten metal withdrawn from the bottom of the retort (if not too high in impurities) may be recharged into the top.

In Fig. 3 of the drawings the vertical retort or boiler is mounted within la heating chamber as in the apparatus of Fig. 1, but the retort is built up of trays cr pots 25 having a V-shaped peripheral trough or groove 26 for holding molten metal. This groove is formed between the outer wall of the pot and an inner inclined wall 27 of the bottom 28 of the pot. It will be understood that all these parts -of the tray orl pot constitute an integrally molded article. The bottom 28 has an opening 29 approximate one end thereof, and the upper edge of the opening has a rib 30 for retaining a shallow layer of molten metal on the tcp surface of the bottom member 28.

A lead-eliminating reflux or rectifying column is mounted above the top of the vertical retort or boiler formed by the trays (25) to receive vapor therefrom. 'Ihe lead-eliminating column comprises a vertical series of spaced transverse trays or baffles 31 generally resembling in conguration, structure and mounting the trays shown in Figs. 1 and 2 of the drawing. An outlet 32 at the top of the column formed by the trays 31 conveys zinc vapor (purified with respect to lead) to a condenser or to other appropriate apparatus for the recovery, utilization or subsequent treatment of the vapor. v

Molten zinc metal to be volatilized in the retort formed by the trays (25) is introduced into one of the lower trays of the rectifying column through a pipe 33 communicating with a charging well 34. A continuous stream of molten zinc metal flows into the charging well 34 through an opening 35 in the bottom of a superposed charging well 36. The amount of molten metal flowing through the opening 35 is regulated by adjustment of the position of a cooperating valve stem 37. An appropriate level of molten metal is continuously maintained in the upper charging well 36.

Molten metal descends from tray to tray of the retort through the openings 29. Any impurities that may tend to collect in the troughs 26 are kept in suspension by violent ebullition prevailing therein and are in consequence swept out of the troughs and carried down through the retort by the descending stream of molten zinc. The impurities present less volatile than zinc, along with a limited quantity of zinc, escape from the retort through the discharge tube and the taphole thereof.

The rectifying column formed by the trays (3l) is thoroughly insulated throughout its length, except at the extreme top thereof. This uninsulated top of the rectifying column serves as a dephlegmator or condens'er to supply molten metal for refluxing back through the column; The

retort or boiler formed by the trays operates continuously to volatile large quantities of impure zinc metal, and the resulting vapor is sub- Jected to a rectification treatment in the rectifying column for removing lead therefrom as described in the copending application of Messrs. Holstein and Ginder, Serial No. 540,566 filed May 28, 1931. If desired the lead-free zinc vapor flowing from the top of the rectifying column may be conducted to a cadmium-eliminating rectifyi ing column for removing cadmium therefrom as described in the copending application of Messrs. Ginder, Peirce and Waring, Serial No. 620,634, flied July 2, 1932.

In Figs. 5, `6 and 9there is shown a still further modiiied form of tray or pot for the vertical retort or boiler of the invention. The tray 38 is rectangular in section and has an inner peripheral trough or groove 39 for holding molten metal to lie volatilized. At one end of the tray the groove 39 has spaced curtains 42 and spaced dams 40; the molten metal flows under the curtains 42 and over the dams 40, and escapes from the tray thro-ugh an opening 41 in the bottom of the groove and between the dams 40. In flowing under the curtains 42, the molten metal sweeps out of the grooves any impurities denser than molten zinc (such as lead or iron) that may tend to collect therein. With this form of tray the volatilized zinc metal passes freely up through the open center ofthe retort while the descending molten metal flows through the openings 41 from tray to tray and i'n zig-gag fashion through the grooves 39 thereof.

In Figs. 'l and 8 there is shown a type of tray resembling that shown in Figs. 5, 6 and 9 except that the tray is circular instead of rectangular and is not provided with curtains. Thus, the circular tray 38 has an inner circular peripheral groove 39 with spaced dams 40 and opening 4l' in the bottom of the groove and between the dams.

In the actual operation of a five foot high retort built up of ten trays of the type shown in Figs. 3 and 4, 12" by 24 external dimensions and a wall thickness of 11A, we have volatilized zinc metal at the rate of about 20,000 pounds per day of 24 hours. Such a retort has an outside heated surface of square feet, so that the boiling rate per square foot of outside heated surface is about 666 pounds per day. This boiling rate per square f oot of outside heated surface is from 3 to 6 times as great as in 'retorts of the type heretofore commonly used in volatilizing zinc metal.4

The invention is of advantage wherever large quantities of zinc metal are to be volatilized, and is also adapted for the production of zinc vapor low vin lead by controlled redistillation of zinc metal with relatively high lead content. For this application of the invention the internal packing of the retort or boiler should give intimate contact between the ascending vapor and descending molten metal, such as provided in the apparatus of Figs. 1 to 4 of the drawings so as to remove superheat and metallic mist from-the zinc vapors. The retort is charged with impure zinc metal containing lead and/or other contaminants with boiling points exceeding that of zinc at a rate somewhat in excess of the boiling capacity of the retort. The retort may be so operated that the zinc vapor evolved from the retort contains an amount of lead that approaches'that corresponding to the partial pressure of lead in the alloy charged into bri the retort at the boiling point of the alloy. The relative purity of the zinc vapor is due to the following factors:

1. The scrubbing action of the shower of molten zinc metal descending through the ascending vapor tends to remove superheat and thus a portion of the lead vapor evaporated by this superheat is condensed or redissolved, and at the same time the scrubbing action removes metallic mist from the ascending vapor.

2. Superheat is substantially decreased lnot only by the scrubbing action. but also in concomprises passing -molten zinc metal downwardlyV sequence of the large bath surface exposed to heat radiation from the walls of the retort and by the great extent to which the walls of the retort are covered with or scrubbed by molten metal.

Furthermore, the metallic vapor leaving the retort or boiler at the top has on account of its intimate contact withthe descending molten metal ample opportunity to approach closely to a state of equilibrium with the descending molten metal. In view of this circumstance the vapor leaving the retort may have a lead content approaching closely to the lead content of vapor in equilibrium with the molten, metal charged into the retort. Thepurest zinc-- vapor evolved in the retort is the vapor that is evolved by the molten lead-zinc alloy just entering the retort. The vapor evolved from a boiling zinc-lead alloy contains a lower percentage of lead and a greater percentage of zinc than the molten zinc-lead alloy. In consequence, the molten zinc-lead alloy becomes gradually enriched in lead as it descends through the retort. Moreover, the vapor evolved from the molten metal also becomes richer in lead as the lead content of the molten metal is enriched during its descent through the retort. The metallic vapor evolvedv at the bottom of the retort is thus richer in lead than that evolved at the top. However, the excess lead content in the vapor evolved from molten metal near the bottom of tlie retort tends to be removed from this vapor by the scrubbing action of the descending molten metal so that the lead content of the vapor as it leaves the retort is nearly in equilibrium with the entering molten metal.

When operated in the foregoing manner, the vertical retort of the invention is capable of producing large amounts of relatively pure zinc vapor from impure zinc by distillation without refluxing `or rectification, scrubbing by the descending molten metal charged into the retort being relied upon to secure relatively pure zinc vapor. In many cases this zinc vapor will be suiilciently pure for direct utilization thereof and may be conveyed to any appliance for utilizing it, such as a canister for producing zinc dust, a combustion chamber for the production of zinc oxide etc. Where a further removal of lead from the vapor evolved from the retort is necessary, the apparatus of Fig. 3 of the drawings with its superposed lead rectifylngcolumn is employed. l

Relatively greater ebullition takes place in the troughs 26 of the pots of Fig. 3 than does' on the more shallow pools held on the trays 15 of Fig. 1. The trough type of pot 25 has a greater boiling capacity thanl does the tray 15. The opencenter pots of Figs. 5 to 8 of the drawings may often be used with advantage where it is unnecessary or undesirable to operate the retort or boiler with particular attention to evolving vapor' of a lead content in-equilibrium with the'. molten metal charged into the retort. g

`W e claimz- 1. The method of volatilizing zinc metal which comprises heating a series of vertically spaced pools of molten zinc metal to a sumciently high temperature to volatilize the metal, maintaining said pools of molten' metal by charging molten zinc metal intothe top of the series and permitting overilow of molten metalfrom pool to pool. and withdrawing the resulting vapor.

2. 'I'he method of volatilizing zinc'metal which through an upright chamber having a series of y vertically spaced receptacles adapted to hold relatively 'shallow pools of molten metal, heating said receptacles to a sufliciently high temperature to volatilize the zinc. metal, permitting molten metalI to overilow., from pool to pool in its vdownward passage through said chamber and the y resulting vapor to flow upwardly through the chamber, and'withdrawing the volatilized metal from near the top of the chamber.

3. 'I'he method of volatilizing zinc metal which ,comprises heatingI a series ot vertically spaced p'ools of molten zinc metal to a sumciently high temperature to volatilize the metal, charging moltenzinc metal into the top of said series and permitting overflow of molten metal from pool to pool, and withdrawing the resulting vapor from near the top of said series.

4. 'I'he method of volatilizing zinc metal which comprises passing molten zinc metal downwardly through an upright chamber internally packed to provide common tortuous paths for descending molten metal and ascending vapor, heating said chamber to a sufiiciently high temperature to volatilize the zinc metal, and withdrawing the volatilized metal from near the top of the chamber.

5. The method of volatilizing zinc metal which comprises maintaining a multiplicity of supermolten metal into the top of said series at a rate approximating the volatilizing capacity of the entire series of pools and permitting overflow of molten metal from pool to pool, 'and withdrawing the resulting vapor from near the top of said series. 1

'7. 'Ihe method of volatilizing zinc metal which comprises feeding molten zinc metal into the top of a vertically disposed and externally heated retort at a rate approximating the volatilizing capacity of the retort, impeding the passage of the molten metal downwardly through the retort and increasing the area of heated retort wall surface in contact with molten metal by causing the molten metal to iiow through a plurality of baths in contact with the heated retort walls, and withdrawing the volatilized metal from near the topv of the retort.

8. The method of volatilizing zinc metal which comprises feeding molten zinc metal into the top of a vertically disposed and externally heated aanwas retort whose walls are wetted with a plurality of baths of molten metal, and withdrawing the volatilized metal from near the top o! the retort.

9. The method of volatilizing zinc metal contaminated with lead which comprises feeding the molten zinc.metal into the top of a vertically disposed and externally heated retort whose heated walls are wetted with a plurality of baths l0 of molten metal and in which there are a plurality of baths of molten meutal of relatively large surface area exposed to radiantl heat, the Arate o! heating of the molten'zinc metal into the retort approximating the volatilizing capacity of the retort, withdrawing the volatilized metal froml near the top of the retort, and withdrawing a lead-high zinc residue from near'the bottom of the retort. v

- WILLIS MOGERALD PEIRCE.

ROBERT KERR WARING.

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