US1998440A - Manufacture of zinc oxide - Google Patents

Description

April 1935. E. H. BUNCE ET AL MANUFACTURE OF zmc OXIDE Filed March 24, 1930 air/er W ATTORNEY Patented Apr. 23, 1935 NIANUFACTURE OF ZINC OXIDE Earl H. Bunce, Clarence J. Lcntz, and GeorgeT. Mahler, lalmerton, Pa., assignors to The New Jersey Zinc Company, New York, N. Y., a corporaticn of Newfiersey Application. March 24,

10 Claims.

This invention relates to the manufacture of zinc oxide and has for'its object .certain improvements in the method ofand apparatus for manufacturing zinc oxide. The invention relates to the manufacture of zinc oxide more especially from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent.

In'copending application, Serial No. 438,460, filed March 24, 1930, there is disclosed a process of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated retort, in which a flow of evolved gases and liberated zinc vapor is induced through the retort by controlled stack draft, the resulting mixture of gases and vapor being withdrawn from the retort by the aid of said stack draft, and then burning the zinc vapor in the presence of air. The process contemplates more specifically the burning of the zinc vapor in free air. While this process of manufacturing zinc oxide is highly emcient, it is not specifically adapted for the manufacture of a zinc oxide pigment having a minimum particle size; such as is desired, for example, in the compounding of-certain rubber products.

It has heretofore been proposed to manufacture zinc oxide of extreme fine size by oxidizing an issuing stream of metallic zinc vapor by a blast of relativelycool air, which is adapted substantially instantaneously to cool the initially formed particles of zinc oxide. The stream of zinc vapor is obtained by melting and vaporizing zinc'metal confined within a tightly sealed distillation chamber. The jet of blast air moving across the jet of zinc vapor issuing from-the chamber may produce a considerable back pressure in the chamber. Since the retort is tightly sealed, the vol'atilized zinc vapor cannot escape except through an outlet orifice or a nozzle provided for the same. In fact, the issuance of the stream of zinc vapor from the chamber depends upon the diiTerentia-l in pressures of the chamber and of the outside atmosphere. While zinc oxide of the desired 7 particle size may be obtained in the practice of this process, the process is an involved one because it is a twostage process. That is to say, the zinc metal is obtained in a separate and distinct reduction operation; after which, in the second stage, it is melted and volatilized to produce the stream of zinc vapor that is then burned in a blast of compressed air, While this process is capable of producing a very fine grade'of zinc oxide, it is neces- 1930, Serial No. 438,461

sarily more costly and less efficient than a onestage process.

As a result of our investigations, we have determined that the above-mentioned stack draft control step may be utilized advantageously in conjunction with certain other manipulative steps adapted effectively to control the formation" of zinc oxide particle s; more especially in such manner as to regulate their size when employing a charge consisting of ag'glomerates of mixed zinciferous material and carbonaceous reducing agent. Since the process consists essentially of a one-step operation, it represents a marked improvement over the two-stage process'discussed above.

The present invention contemplates a method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined withinan externally heated retort in which a flow of evolved gases and liberated zincvapor is induced throughv the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, preferably augmented by anapplied suction draft, and directing a, blast of oxidizing gas into the said mixture of gases and vapor whereby thezinc vapor is burned to zinc oxide.

In the presentpreferred practice of the invention the process is conducted in a vertically dis-- posed and externally heated retort. The agglomerates of-mixed zinciferous material and carbonaceous reducing agent arefconfined within the retort, the retort having a height adapted to assist at least inpart in setting up the desired stack or chimney draft. An outlet nozzle or conduit is located at or near the upper end of the retort for the escape of the mixture of gases and zinc vapor.

It is provided with means for directing a blast of oxidizing gas, such as compressed air, against and/or into the issuing stream of gases and vapor.

While a substantially continuous circumferen tial blast of air, for example, may be employed, in one practice of the invention we prefer to direct a plurality of streams of air into the escaping zinc vapor i the streams of air being directed at an angle not adapted to build up undesirable backpressures in the retort. A sufiiciently strong draft should constantly be induced upwardly through the retort that is 'adapted'to prevent zinc vapor from being forced through the lower end of the retort. In fact, it is preferred that the stack draft through the retort be sufiiciently strong to cause the influx of regulated amounts of air into the bottom of the retort. Any suitable gas or mixture of gases in suitable amount may be admitted into the retort, preferably at or near its lower end. Thus, air, nitrogen, water-gas, carbon dioxide, even steam and the like may be introduced into the system.

The zinc vapor escaping from the retort is promptly burned to form zinc oxide. Such an amount of air is employed that the initially formed zinc oxide particles are substantially instantly cooled to a point at which they will no longer grow. The retort gases and excess air are present, moreover, in suificiently large amount promptly to suspend widely the newly formed zinc oxide particles, thus permitting them to cool out of contact with one another. In this manner, particle growth of the zinc oxide is substantially hindered, and it becomes possible to produce a pigment of extremely fine size.

These and other features of the present invention will undoubtedly be better understood if reference is made to the accompanying drawing, taken in conjunction with the following description, in which:

Fig. 1 is an elevation in section of an apparatus adapted for the practice of the invention;

Fig. 2 is an enlarged plan view, on the line 2-2 of Fig. 1, of the blasting ring element shown in Fig. 1;

Fig. 3 is an elevational section in part of a modified form of apparatus adapted to effect the formation of zinc oxide; and

Fig. 4 is another modified form of apparatus adapted to effect the formation of zinc oxide.

Referring more particularly to Fig. l, the apparatus shown comprises a furnace structure HJ resting upon concrete foundations II. The furnace structure comprises a bottom l2, side walls I3 and an arched roof !4 constructed of suitable heat-resistant material, such. as refractory brick, preferably of poor heat-conducting capacity. An outer metallic casing l5 completely surrounds the furnace structure. Thespace provided between the arched roof and the metallic casing is filled with a suitable heat-insulating material [6, such as diatomaceous earth.

A heating chamber I! is defined by the inside linings of the furnace structure. A port, or ports, l8 extend completely through the side walls of the furnace structure. A number of ports are preferably employed, being spaced at intervals adapted to provide optimum heating conditions within the chamber. These ports are adapted for the introduction into the heating chamber of air and suitable fuel. For example, air and gas, oil or pulverized fuel may be fed through the ports. A number of regularly spaced openings l9 extend through the furnace walls, through which pyrometric readings may be taken. An outlet 20 is provided through the furnace struc ture walls at or near the upper end of the heating chamber for the withdrawal of spent heating gases. This opening preferably connects with a stack (not shown).

A vertically disposed reduction retort 2| extends completely through and centrally of the heating chamber. It is advantageously built-up with suitable heat-resistant material, such as refractory brick, having good heat-conducting qualities. The retort is supported by the furnace structure at only one point, preferably at its bottom. In this manner the retort may expand and contract independently of the surrounding furnace structure.

A sleeve member 22 is fastened to the outside of the bottom of the furnace structure. Its passageway is in substantial alignment with that of the retort. A revolving platform 23 is located directly below the retort and sleeve member, which is adapted to rotate about a vertical axis. This platform is adapted to withdraw spent residues 24 from the bottom of the retort.

The upper end of the retort extends a convenient distance above the furnace structure, and preferably constitutes a substantial elongation or extension 25 of the same. This prolongation may advantageously consist of a unit in which lead, and the like present in the zinc vapor liberated in the reduction process may be intercepted or retained by hot agglomerates 26 confined Within the prolongation, and which are about to be subjected to the reduction process. In copending application, Serial No. 244,519, filed January 4, 1928, two of us, Bunce and Mahler, have disclosed such a process for the elimination of contaminating lead. According to the disclosure more particularly contemplated in said copending application, the vertically disposed retort has an extension or prolongation of substantial length, which is referred to as an eliminator. The hot agglomerates confined within the eliminator are adapted selectively to remove lead from the rising zinc vapor, after which the agglomerates themselves are progressively moved down to the reduction zone.

The eliminator structure 21 consists of an outer metallic casing 23 of variable size surrounding the eliminator, and providing a space in which is placed a layer of appropriate heat-insulating material 29, such as dust coal. A door, or doors, 30 is advantageously located at or near the lower end of the metallic casing for the withdrawal of heat-insulating material, if that should become desirable in order to regulate the dissipation of heat. The upper end of the eliminator structure is preferably left open so that heat-insulating material may be introduced into and Withdrawn from the same.

A charging device 3! fits into the upper end of the eliminator and extends a convenient distance below the opening of the off-take pipe, so that charge materials may not clog the same. A removable cap 32 fits over the charging device. This cap is removed whenever charge materials are introduced into the charging device. If the apparatus is to be operated in a s bstantially continuous manner, the cap may be permanently removed and a suitable connection made with a source adapted to supply a constant feed of agglomerated charge materials to the charging dev1ce.

An upwardly inclined off-take pipe 33 extends through the eliminator structure at or near its upper end. This off-take pipe or conduit is adapted for the passage therethrough of the gases and vapor rising through the eliminator. The offtake pipe terminates in a nozzle portion 34. A

blasting ring 35 is employed in association with the discharge nozzle. It preferably fits snugly around or above the upper end of the nozzle. The blasting ring is provided with a plurality of spaced holes 35 (see Fig. 2) that are adapted to direct a plurality of separate and distinct streams of oxidizing gas at an appropriate angle against a stream of zinc vapor issuing from the nozzle. An' angle is adapted that tends least to build-up undesirable back pressures while at the same time adequately quenching the issuing stream of zinc vapor. Instead of employing a plurality of streams of air, a circumferential blast of air may beobtained by the provision of. a circumferential slot 36 in the blasting element, (see Fig. 1)

The blasting ring has a connection 31 that connects; with a source of oxidizing gas. under pressure, such, as compressed air. The directionof the streams of air escaping through the holes, or the annular blast of air escaping through the slot, maybe better kept in their course when the metal wall of the ring member isrelativelythick. In our present preferred practice the ring. constitutes a cast-iron member having awallof adequate thickness to provide holes of sufficient depth for this purpose, and with enough metal to retain heat sufiicient in amount to inhibit the condensation of zinc vapor on the ring. We have obtained excellent results with the small holes drilledat an angle of 60 degrees to the plane of the ring. Greater suction effects'are secured with the holes at such an angle than may be obtained when the holes are, for example, drilled at an angle of degrees to the plane of the ring. On the other hand, a more complete and rapid quenching of the issuing zinc vapor with air appears to be possible with the holes at the latterangle,

- which aids materially in securing a pigment of minimum particle size. It is thus seen that the number of holes and their angle of inclination may be suitably adjusted to assist in getting the. results desired.

A hood 38 is located at a suitable distance directly above the blasting ring. It is adapted to receive the 'newly formed zinc oxide particles together with their accompanying gases, and to conduct them throughv a connecting pipe 39 to collecting chambers (not shown) where the particles of zinc oxide are permanently separated from the gases. A suction fan (not shown) is preferably used in association with the pipe for transporting the mixture of gases and zinc oxide particles to the collecting chambers.

Modified forms of apparatus adapted to blast an issuing stream of zinc vapor with compressed oxidizing gas, such as air, are shown in Figs. 3 and 4. These apparatus are designed to inject a jet of air within a jet of zinc vapor, so that the air blows in a direction parallel with the direction of the moving jet of vapor. The two jets preferably move in the main direction of the stack draft within the reduction retort; that is,

. substantially upwards, so that no objectionable back pressure may result from the air blast. Thus,v an air supply pipe extends centrally of the vapor discharge nozzle, which constitutes a constricted member 4| providing a relatively small. orifice 42.

In the case of Fig. 3, the discharge end of the air supply pipe is equipped with a perforated cap 43, located just below the orifice end of the vapor discharge nozzle. In the caseof Fig. 4, the

s 1 discharge end of the air supply pipe is open, and

it is likewise located just below the constricted orifice; an expanding nozzle section M, however, providing an enlarged orifice for the ultimate discharge of the zinc vapor. The latter form of zinc vapor nozzle results in a Venturi tube action, which tends to support the suction produced by the air blast. As the zinc vapor passes through the constriction, it tends promptly to expand. The inertia of this expansion creates a suction in the conduit below. It will of course be apparent that the discharge ends of the air supply pipe may be, located at or above, as well as below, the level at which the zinc vapor finally leaves the nozzles.

The process of the present invention may be practiced as; follows in the apparatus described above:

The: removable cap 32 is-lifted away, and fresh agglomerates of mixed. zinciferous material. and carbonaceous reducing agent 26 are introduced into the charge feeding device 3|, until finally the: retort and the eliminator and the charge 'feeding device are filled. Precautions should be taken, always to have the upper level of the agglomerates extend well within the charging device, preferably at or above the opening into the off-take. conduit 33 The cap 32 is again returned.

Air and. suitable fuel, such as oil, through ports l8 into thechamber H. The high,- ly heatedcombustion, gases circulate around: the retort and finally escape through the exit 20 to the stack or chimney (not shown). readings of the temperature conditions within the chamber are taken from time tov time through the openings Hi. When necessary,

proper changes are made in the amount and quality of the air and. fuel employed in order to regulate temperature conditions to effect optimiun reduction.

As heat is driven through the retort Walls, the charge materials are gradually raised to their reduction temperature- As a result of the reduction step, retort gases are evolved and zinc vapor is liberated. The mixture of gases and vapor is.

permitted to rise upwardly by controlled stack draft, preferably augmented by applied suction draft; such as that obtained when the zinc vapor is appropriately blasted with oxidizing gas.

When reduction of the compoundswithin the lower section of agglomerates has proceeded to completion, the revolving platform 23 is set in.

motion and spent residues 24. are discharged from thelower end of the retort. Agglomerates that do not break down or disintegrate are preferablyemployed, so that the spent residues may be removed as agglomerates. Thecap 32 is again removed, and fresh charge materialsv 26 are introduced into the charging device 3| to make up for the spent residues just'removed. In order to operate the process and apparatus continuously, fresh charge materials may be introduced intothe system as fast asspent residues are removed.

The mixture of retort gases and zinc vapor rises upwardly through the retort and ultimately finds its way through the eliminator 25, where the zinc vapor is brought into intimate contact with hot agglomerates about to be subjected to the reduction operation. As pointed out above,-

if the zinc vapor is contaminated with lead, these hot agglomeratestend selectively to retain the lead while permitting the lead-free zinc vapor to continue its passage. If this lead elimination step is to be practiced, precautions must be taken adequately to: regulate the temperature of the agglomerates confined within the eliminator. The temperature may be controlled by regulating the dissipation of heat from the hot agglomerates. If the agglomerates are too hot, coal dust 29 may be removed from the doors 30. On the other hand, if the agglomerates are not hot enough, additional heat-insulating material is provided within the metallic casing 28 to hinder loss of heat.

- The retort gases and liberated zinc vapor ultimately find their way tothe off-take conduit 33, and upwardly through the nozzle 34.. This upward direction is preferably in line with the. verticaldirection that. the gases and vapor take;

Pyrometricv are fed within the retort and eliminator, in order to provide for the necessary stack draft. Unless adequate provision is made for the upward movement of the gases and vapor, pressures will be built-up within the retort suificiently large to drive zinc vapor downwardly through the bottom of the retort. If such an action should take place, the zinc vapor would, of course, be permanently lost. It is a characteristic feature of the present invention that a natural upward draft be employed to induce the evolved retort gases and liberated zinc vapor to rise upwardly through the system, It is also a preferred practice of the invention that this stack or chimney effect be sufiiciently strong to suck regulated amounts of air into the bottom of the retort. Since the spent residues 24 are in the form of agglomerates, air may be permitted to seep through the voids or spaces provided between contacting agglomerates. It will of course be understood that the seepage of indiscriminate amounts of air is to be avoided. From what has been said above, it will be clear that other desirable gases may be suitably admitted into the system.

Air under pressures, such as compressed air, is fed through the connection 31 into-the blasting ring 35. The pressure of the air is sufficiently strong to cause a plurality of streams of air to escape through the holes 36 of the ring (Fig. 2). In the case of the annular slot shown in Fig. 1, an annular or circumferential blast of air will issue through the slot 36. The zinc vapor rises upwardly through the nozzle 34 in the form of a stream, and the air escaping from the holes impinges against the vapor, whereupon the zinc vapor promptly burns to zinc oxide. The veloc ity with which the air strikes the issuing zinc vapor must be such as not to build up undesirable back pressures. As was pointed out above, the evolved retort gases and liberated zinc vapor are caused to rise upwardly through the retort, the eliminator, the off take conduit, and the nozzle by'a natural upward or chimney-like draft. If the blast of air is of suflicient strength and amount, it may strike the issuing gases and zinc vapor in a manner that will make it impossible for the gases and zinc vapor to continue their upward rise. For example, if the streams of air were to strike the issuing gas and vapor in a horizontal plane, and in great amount, sufficient back pressure might be built-up to cause the retort gases and zinc vapor to escape from the bottom of the retort. In fact, it is preferred that the angle of inclination and the amount of the escaping air be such as'to produce a suction effect down in the retort. It is for that reason that the angle of inclination of the impinging blast of air, the amount and velocity of air must be accurately controlled. If these variable factors are appropriately regulated and coordinated, the total stack draft effect may be rather accurately controlled.

As the zinc vapor contacts with the blast of air, it is promptly burned to zinc oxide. The initially formed particles of zinc oxide are also substantially instantaneously cooled. As a result of the prompt formation of the zinc oxide particles, their instantaneous chilling, and the immediate dilution of the gasesand zinc oxide with excess air, particle growth of the zinc oxide is substantially hindered. This results in the production of a pigment of extremely fine size. It will of course be apparent that the ultimate size of the zinc oxide particles, at least on the average, may be somewhat controlled by regulating appropriately the manner in which the issuing zinc vapor is blasted with oxidizing gas.

The zinc oxide particles, together with their accompanying gases, rise into the hood 38 and are promptly transported through the communicating pipe 3& to the collecting chambers, where the zinc oxide particles are permanently sepa rated from the gases. The transportation of the zinc oxide particles and gases are for the most part helped by a suction draft, provided by a suction fan in association with the pipe 39.

From the foregoing description, it will be clear in what manner the apparatus shown in Figs. 3 and 4 may be employed to effect the combustion of zinc vapor issuing from the nozzles. In the case of the apparatus shown in Fig. 3, the zinc vapor finds its way through the vapor discharge nozzle 4!. Air under pressure passes through pipe 43 and escapes through the perforations in the cap 43. It will be seen that the escape of the air is in direct line with the escaping zinc vapor. It will further be seen that the air does not set up imdesirable back pressures, as it impinges against the issuing zinc vapor. In fact the escaping air sets up a suction effect within the retort.

Much the same procedure takes place when the apparatus shown in Fig. i is employed. The zinc vapor passes through the constricted nozzle 4 i, and escapes through the relatively small orifice 12 only to expand through the nozzle section 44. The sudden expansion of the vapor causes a strong forward action, which tends to support the suction produced by the air escaping through pipe 48, again setting up a suction effect within the retort.

We claim:

1. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated and vertically disposed retort which comprises inducing a flow of evolved gases and vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a plurality of streams of air into said mixture of gases and vapor at an angle adapted to inhibit the building up of undesirable back pressures in the retort whereby the zinc vapor is burned to zinc oxide, said streams of air being controlled to effect the admission of regulated amounts of gas into the lower end of the retort.

2. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, Withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a blast of air into said mixture of gases and vapor at an angle adapted to inhibit the building up of undesirable back pressures in the retort whereby the zinc vapor is burned to zinc oxide, said blast of air also being sufiicient in amount and velocity to exert a suction effect within the retort.

3. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a blast of air into said mixture of gases and vapor at an angle adapted to inhibit the building up of undesirable back pressures in the retort whereby the zinc vapor is burned to zinc oxide, said blast of air being adapted to aid in the maintenance of the stack draft in the retort.

i. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing ablast of air into said mixture of gases and vapor at an angle adapted to inhibit the building up of undesirable back pressures in the retort whereby the zinc vapor is burned to zinc oxide, said blast of air being adapted to exert a suction effect within the retort. 1

5. In a method of manufacturing zinc oxide, the steps comprising subjecting an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined in an externally heated vertical retort to a reduction operation, inducing a flow of evolved retort gases and liberated zinc vapor upwardly through the retort by chimney draft with the admission of regulated amounts of air into the retort, and injecting air under pressure into the mixture of zinc vapor and retort gases coming from the retort whereby said chimney draft is supported and the zinc vapor is burned to zinc oxide.

6. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated and vertically disposed retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a blast of compressed oxidizing gas into said mixture of gases and vapor whereby the zinc vapor is burned to zinc oxide, said blast of compressed oxidizing gas being directed to cause an aspirating action and thereby effect the admission of regulated amounts of gas into the lower end of the retort.

'7. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated and vertically disposed retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a plurality of streams of oxidizing gas into said mixture of gases and vapor whereby the zinc vapor is burned to zinc oxide, said streams of oxidizing gas being directed and controlled to cause an aspirating action which is adapted to eifect the admission of regulated amounts of gas into the lower end of theretort.

8. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within an externally heated and vertically disposed retort which comprises inducing a flow of evolved gases and zinc vapor through the charge in the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and directing a blast of air into said mixture of gases and vapor at an angle adapted to inhibit the building up of undesirable back pressures in the retort whereby the zinc vapor is burned to zinc oxide, said blast of air being so directed and controlled as to cause an aspirating action which is adapted to effect the admission of regulated amounts of gasinto the lower end of the retort.

9. In a method of manufacturing zinc oxide pigment from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined in a vertically disposed retort, the steps which comprise introducing during the progress of the reduction operation a regulated amount of a gas into the retort at its residue discharge end, said gas so introduced being adapted to inhibit the diffusion downwardly of zinc vapor and to assist in establishing a stack draft upwardly through the retort, regulating the stack draft so that an optimum differential gas pressure is maintained between two or more points in the path of the gas-vapor flow, withdrawing the resulting mixture of gas and vapor from the retort by the aid of said stack draft, and directing a blast of compressed oxidizing gas into said mixture of zinc vapor and gases coming from the retort whereby said stack draft is supported and the zinc vapor is burned to form zinc oxide pigment.

10. The method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined within a vertically disposed retort which comprises inducing the flow of gases and zinc vapor through the charge in the retort by controlled stack draft, admitting into the retort at the residue discharge end a gaseous agent in regulated amount, said gaseous agent so introduced being adapted to inhibit the diffusion downwardly of zinc vapor and to assist in establishing a stack draft upwardly through the retort, withdrawing the resulting mixture of gases and vapors from the retort by the aid of said stack draft, and directing a blast of compressed oxidizing gas into said mixture of gases and vapors whereby the zinc vapor is burned to zinc oxide, said blast of air being directed to cause an aspirating action upon the mixture of gases and vapors withdrawn from the retort.

EARL H. BUNCE. CLARENCE J. LENTZ. GEORGE T. MAHLER.

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