US2560175A

US2560175A - Fluidizing reduction of zinc compounds with stagewise feeding of solids

Info

Publication number: US2560175A
Application number: US26484A
Authority: US
Inventors: John C Kalbach
Original assignee: American Metal Co Ltd
Current assignee: American Metal Co Ltd
Priority date: 1948-05-12
Filing date: 1948-05-12
Publication date: 1951-07-10
Anticipated expiration: 1968-07-10
Also published as: ES187846A2

Description

J y 10, 1951 J. c. KALBACH 2,560,175

FLUIDIZING REDUCTION OF ZINC COMPOUNDS WITH STAGEWISE FEEDING OF SOLIDS Flled May 12, 1948 ma 7 T I W mm 5 I 6 m a A \EAI. H m

my l. m

Patented lIuly 10, 1951 .U NIT-ED (S TATE-S PATENT OFFICE FLUIDIZINGBEDUCTION OF ZINC COM- POUNDS WITH STAGEWISE FEEDING -Application May 12, 1948, Serial No. 26,484

-9; Claims. "1

This application is H a continuation-in-part of the copending :application SerialzNo; 759,545, filed July 8, 1947, now U. S. 'Patent" ,485,6 wherein I have disclosed aprocess for producing: zinc by reducing comminuted zinc compounds, such asthe oxide and the silicate of zinc, under fluidizi-ng conditions and in a manner such that the gaseous efliuentfrom the reducing zone is rich in zinc vapor. 1 This application is also a continuation-in-part of the copending application-Serial No. 767,548, filed-August 8,1947, which isdirected. to a stagewise-fiuidizing process for the reduction of zinc compounds.

The present invention-is-in the nature-of an improvement-ofthe processes of copending applications Serial- Nos'. 759,545 and '7 67,548.

A primary object of this invention isto conduct the fluidized reduction of zinocompounds with a minimum inventory of solids in the reaction Zone. -A corollaryobject is to use apparatus of decreasedsize-in efiectingthe reduction of zinc compounds by the fiuidizing technique.

Another important object is to ensure good fiuidization throughout the reduction of zinc compounds notwithstanding any inherent tendencies of these compounds to -agglomerate in the course of reduction.

Still another object is to curtail the proportion 7 of carbon dioxide in the gaseous efiluentflowing from the fluidized reducing zone to the condensers employed :for' the: recovery of the zinc contained in the-.gaseousefliuent.

A further object is to-conductthe fiuidizedreduction of zines-compounds.-=-underconditions which-prolong-the life ofthe reducing apparatus.

Additionalobjects and advantages of my invention will becomeevident from the description which follows.

For the purposes of this invention, the term zinc compound, embraces the oxide, hydroxide, silicate. and carbonate of zincsince these zinc compounds: are readily reduced to metalliczinc. It will be'noted that all of the fore- I going zine compounds are solid inorganic oxygen-containing compounds of zinc. Generally, the oxide forinof 'zinc is charged to thereducing zoneJ The mostprevalent type of mineral or ore contains-zinc as a -sulfide which is roasted --to the-oxideformpriorto-reduction for the 1 recovery of metallic zinc. Typical of sulfide :typeores is-spha1erite-, =-the sulfide of zinc. A lower grade ore.or: mineralis manmatita. a double sulfide f azinc. and iron,- which when found substantially free.;of-'i-richer .zincvminreral is .=..considered;ttoo.-. lean in ;zinc for. economic 7 Furthermore, an upwardlyfiared reduction zone 7 Working by conventional processes of reduction.

After roasting, ,,these ores contain not only the ,still have anappreciable contentof impurities.

Some of these impurities like ironpromote agglomera tion of the particles in the fluidized mass as theparticlesbecomedepletedin zinc in the course ofthe reduction. hgglomeration is undesirable because it interferes with good fluidization and decreasesthe reactivity of the solid particles.

.Copendingapplication Serial No. 759,545 sets forth in detail the advantages of conducting the fluidized, reduction of a comminuted zinc compound ina bed of, increasing horizontal cross- ,sec tion,-ythe.lsmallest cross section being at the bottom of the bed. The use of a tall, flared fluidizing zone, or chamber to hold the ,bed of increasing horizontal cross section makes it possible to introduce such small quantities of gas as the, fiuidizing medium;that the gaseous stream leaving the chamber is rich in zinc vapor.

permits themaintenance of a substantially uniform gas velocity therethrough in spite of the ex- ,ponential increase of the volume of gas with the evolution of zincvapor and carbon oxides in the reduction of the zinc compound. Those skilled in the art will appreciate that the recovery of .1 molten zinc from a gaseous stream rich in zinc .vapor is materially facilitated since there is less tendency to form bluepowder.

Copending application Serial No. 767,548 points out that the reduction of zinc compounds under :fiuidizing conditions. is advantageously carried out in a seriesof stages through which the solids and the fluidizing and reaction gases pass concurrently, In accordance with the teachings of theearlier filed application, the preferred em- ,bodiment of the process;ofapplication Serial No.

767,548 involves a first reducing stage in which 1 the fluidized mass is ,;of; increasing horizontal cross-section in the di rectionof the gaseous flow therethrough.

In the processes of my copending applications, the comminuted zinc compound is charged into the reducing zone along with a comminuted reducing agentselected from the-class of carbona- ..ce 0us.solidiuebdik coa pokeali ni ea d ch coal. The ratio of the weights of reducing agent and zinc compound fed to the reducing zone is set to provide suflicient carbon for the reduction in accordance with the reaction:

(A) ZnO+C+Zn+CO undergoing reduction. Furthermore, the reduction of zinc compounds involves the reversible reaction:

(B) ZnO+CO=Zn+COz and it is obviously advantageous to suppress the presence of carbon dioxide in the reaction gases through the producer gas reaction:

For this additional reason, the weight ratio of carbonaceous reducing agent to Zinc compound is generally made high to provide excess carbon and thus minimize the presence of carbon dioxide in the gaseous efiluent leaving the reducing zone. A high weight ratio means that the inventory of solids in the reducing zone is larger than is stoichiometrically required. A large solids inventory, in turn, means large apparatus and large heat losses. I have now found a way to decrease the solids inventory without encountering any of the difiiculties which are avoided by resorting to a higher Weight ratio of carbonaceous reducing agent to zinc compound than is dictated by stoichiometry.

In accordance with this invention the fluidized reduction of a zinc compound is carried out in at least two stages arranged for the concurrent flow of solids and fiuidizing and reaction gases from the first stage to the last. A distinguishing feature of the process of the invention is the introduction of a finely divided nonagglomerating solid, which may be inert like burnt dolomite or sand or reactive like coke, into the fluidized mass beyond the first stage in a reducing stage where the agglomerating tendency of the zinc compound undergoing reduction becomes appreciable or where a solid carbonaceous reducing agent is needed to complete the reduction of the zinc compound. If both the agglomerating tendency has to be corrected and the need for additional solid reducing agent satisfied in a reducing stage subsequent to the first, a finely divided carbonaceous fuel like coke can be used in a dual capacity. Furthermore, a carbonaceous fuel added to the fluidized mass in a reducing stage subsequent to the first serves to decrease the presence of carbon dioxide in the gaseous efiluent flowing to the zinc condensers.

To permit the withdrawal of a gaseous effluent rich in Zinc vapor from the last stage of reduction, the first stage is made the smallest in horizontal cross-section so that a relatively small quantity of fiuidizing medium, say carbon monoxide, injected thereinto can support fiuidization; preferably, the first-stage is of increasing horizontal cross-section in the direction of the gaseous flow therethrough. While the subsequent; stages of reduction are as large or larger in horizontal cross-section than the uppermost horizontal cross-section of the first stage, in general they' necessitate relatively slight upward flaring of thefluidized mass which they hold and often they are substantially uniform in horizontal crosssection. For example, a two-stage reducer may have a first stage wherein the horizontal crosssection increases 16-fold from bottom to top and the second stage has an invariant horizontal cross-section equal to the uppermost section of the first stage. In another illustration, a threestage reducing operation may involve a first stage with a 14-fold increase in horizontal crosssection in the upward direction, a second stage of relatively slight upward flaring such that the overall increase in horizontal cross-section from the bottom of the first stage to the top of the second stage is about 17-fold, and a third stage of the same horizontal cross-section as the up permost section of the second stage. It is advisable to effect reduction and vaporization of at least 60% of the zinc in the compound charged into the first reducing stage and to eflect a decreasing percentage of the overall reduction in each succeeding stage. I have found that optimum results are obtained by proportioning and regulating the several stages of the reduction, two or three stages suificing in most cases, so that the conversion or percentage of reaction effected with the solids entering each reducing stage is approximately constant for all the stages. For instance, if itis desired to recover about 98.5% of the available zinc in a roasted ore of the zinc sulfide type in a threestage reduction, optimum staging is attained when a conversion of about 75% is effected in each stage, i. e., the first stage reduces about 75 of the zinc content of the charged material, the second stage then reduces about 75% of the residual zinc content (25% of that originally available) and the third stage reduces about 75% of the available zinc in the solids passing from the second to the third stage. Thus, of the available zinc in the roasted ore, 75% is reduced in the first stage, 19% in the second and 4.5% in the third, giving an overall recovery of 98.5% for the threestage process.

The advantages of stagewise operation are still realized to a somewhat lesser extent when the conversions for the several stages difier among themselves within reasonable limits. From experience I have determined that it is desirable to limit the deviations in conversion in the several stages to 10% of the constant value which, as hereinbefore stated, gives optimum results. Thus, for optimum operation of a twostage reducer to recover 99% of the zinc content of the charged ore, a conversion of 90% is effected in each of the two stages, while good results are still obtained if, for example, the firststage conversion is (deviation of 5.6%) and the second-stage conversion is 93.3% (deviation of 3.7%). As is frequently the case with a twostage operation, when the agglomerating tendency of the ore is so great as to prevent design for minimum reactor size according to the criterion of equal conversions in the stages and at the same time the use of a low ratio of carbon to ore, the best compromise is in the direction of a departure from equal stage conversion as illustrated by the immediately preceding example of a two-stage process.

As used in this specification and the appended current progress or movement of reaction solids and gases' therethrough. alt. willzberunderstood that any stage Iof sreductionc-may be? carried out in a:-porti'onfof. apreducer, assepar'atetreducerror a' plur'ality lotvessels interconnected: toziunction effectively as. a' isinglerrreducer. A; .pluralitmiofo;

e.: g 1 ;;will iun'ctiona 'ias aesingletzone orcvessel when they/Jere arranged: in..sipar'allel :andrliave 1': Inc-accordance with: thei rpreferredzpracticeg'of first reducing stage with only-iaboutzfioneehalf of fitheiproportion" oiucarbon: ultimately required un the-last reducingi'stage': The'ivolume iof' 'solids in the i'lfiTSt =reducin stage. is thus-:rmade tcomciparatively; sm'all since the :excess: carbon 'l'BQlliI'Bdl to drive":thesreducingtreaction: tolthe -'desired;degree "of.completioniand..the"dilutingtsolids :(which may 'alsoibe carbon) requireda tmir'ninimize fag- 'glomer'ation'r are .not .passediithroughi thefirst are- -:ducing f stagei butr are introduced into. a subse- -quent stage. .-.1As:previouslyistated, azs'olidscar- -bonaceous 11161 in.zcomminuted s-form umay be 'chargedto'the Jsubsequent stage, sayiitheisecond and/orithird stage',to -s'atisfyv both .-.requirements.

For the first stage of reduction; I-rpreferito.

:maintain ':the fiuidized :solids as a deep :bed of -=-upwa'rdly increasing horizontal cross-flseictionloecause I: 'am thenable to: use such small :quantities: -'of 1 gas tas the 'fluidizing medium :that' the zinc :vapor: in the 'gases discharged by the processapproaches closely the-concentration': it would have accordin toiutheory, i: e;; 50% by volume 2:-concentration -=based= on the pure zinc *ox'ide equivalent content of the material -charged to the reducer. 'l-Ioweverg'it is to be -remembered '--"that roasted or *calciried ores and-other forms of zinc compounds -frequently contain reducible compounds of iron-and other metals which --'natura-l1ydecrease that Y concentration- 0f zinc vapor -in' the gases leaving '-the reducing zone which is theoretically" obtainable with the-- reduction of the pure zinc compound. In-any eve'nt', -'"through the use ofa tally flared reducer and the'injectionthereinto of a-relativelyminute quantitycf ga's to support fluidization',-the gaseous eflluent contains not less than 40 %=by volume, and preferably-notless than 45 "by volume;"of "=z-inc vapor; based-on only theinjected fluidizing gas and the-reaction'gases deriving from the pure zinc oxide equivalent content of" the" material "entering*the-reducing zone.

" By referring the concentration "of" zinc vapor the gaseous efilluent *to the pure' zincoxide equivalent content-of the material charged to the -r'educingzone; a common basis is providedfor ZnO Name Formula --Equiva- =-lent zinc-silicate carbonate; andtsilicate: ohzinczchave:wnemquimtclenti content. ofrzincxoxide; :1I'h hydroxiderand carbonate 1 of .rzinci'rwill adoring reduction-erelease water: vapor and carbonngdioxide, 1 respectively, and willvthus naturally-decreasethe-concentration of ..zinc ivaporin: the gases ileaVinglzthe reducer. :Because the water vapor and .carbon dioxide. react with the solid -reducing agentto produce a larger volume-cigas' (hydrogen-and carbon monoxide): which further dilutes -theilzinc vapor; it 2 is generally advisable' to: calcine the purities.

hydroxide and carbonate of zinc to eliminate' -the water and carbon dioxide, respectively; as a-pre- :treatment -step -prion to reduction. l since the silica (SiOz) of zinc silicate is not-yolatilized like the water and'carbon dioxideiof' the-hydroxide and carbonate," there I is no reason ion -first calcining zinc silicate other-than to-drive off physically associated moisture. Furthermore, as alreadymentioned; the zinc'compounds are generally found in nature "associated 'with= im- When these impurities; -for instance, the oxides of iron and lead, are reduced-under the conditions selected for the reduction- 0fthe zinc compound; the gases leaving the reducing vessel will contain carbon monoxide-resulting from the reduction of the-impurities as-well as the carbon-monoxideresultingiromthe reducreduction ofthe -pure zinc-compound.

For the preferred'embodiment of my stagewise -reduction processinvolving a-first stage-of -increasing horizontal cross section, it is advisable to operate in thefirststage with a fluidized mass ofat least 5 foot=depth;"preferably; the depth; is

of the order 'of- '10 -to40- feet. 7

The rate of increase of gas volume; and" hence horizontal cross-section of vessel; with depth of bed depends upon the reactivity of the zinc compound and solid reducing agent usedythe-"gas velocity maintained, the-temperature of, operation and-the-recovery of'zinc metalwhichtisregarded as satisfactory for each-stage (since the 1 fluidized mass of each stage is o-fresiduecomposition forthat-stage) This rate-of increase may-be conveniently expressed" as -the gas doubling height, that is to say, that "height-ofi-bed vvithimwhich the volumeof gas-rpassing there- 5 through isdoubled. In practice; the gas-:dou- -bling height may-vary frorrrabout l"to 20 feet but preferably falls in the range of about 3 to" 12 feet.

It will be appreciated that -the-overa1l--depth "of-bed in the several stagesof a given reduction process and the gas doubling height must in each rzase =be--cor rela-tedso that the-gases discharg'edby the-process contain thezinc vapor in "the high-concentration "sti-pulated hereinabove.

Another ns eful guide -in -properlycorrelating the overall bed depth for a stagewise operation and -the gas-doubling-meight is -to provide -such l a totalbed depth that the:gaseous efilnent -from 'the last st'ageis atleast 8 times; and preferably at: least 12; times, the :volume of the fluidizing .kmediumlcharged at thebottom of the first stage.

3 lfllhexcomminuted zinc :comp'ou nd, notably im ipure'iizinccoxide'iobtained lby 'lroasting a sulfide ctypeiof zinc ore; isigenerally supplied to 'thereducing zoneain'ftheiformi'i'ofsparticles all of i which iupass zrthrough alzfifi mesh-hscreeniand 20% 430140 carbon or solid reducing agent, such as coal, charcoal or coke, is usually supplied to the reducer in the form of particles somewhat coarser than the zinc compound particles because of the lower specific density of carbon and consequent tendency of these carbon particles to become fluidized in the range of l600 to 2300 F., preferably in the range of 1750 to 1950 F.

To facilitate understanding of the invention, reference is now had to the drawing accompanying this specification and forming a part thereof, of which:

Figure 1 is a schematic sectional elevation of one form of reducing apparatus in which a twostage reduction may be carried out;

Figure 2 is a similar view of a three-stage reducer suitable for the process of this invention; and.

Figure 3 is a diagrammatic representation of two-vessels arranged for a two-stage operation conducted in accordancewith the principles of this invention.

Referring to Figure 1, a reducing vessel is is provided with a lower flared section H and an upper straight section I? communicating with lower section H through the openings in a grill 13. The fluidized mass [4 disposed in the contiguous sections II and I2 has an upper pseudoliquid level 15 defining the region where the reaction gases emerge from the fluidized mass and flow through space It wherein the bulk of entrained particles tend to become disengaged from the reaction gases. The filter element l! serves to eliminate residual particles from the gases leaving the, reducer H] by way of outlet l8. In effect, the grill l3 divides thefluidized mass [4 into a lower portion A wherein the first stage of reduction is carried out and an upper portion 13 wherein the second stage of the reduction is performed. A feed hopper IS with a rotary bucket-type valve 20 serves to introduce a cornminuted mixture of zinc compound and carbon to the first reducing section or stage I l by way of screw conveyor 2|. At the lower end of section H an inlet pipe 22 provided with nozzles 23 is used for the introduction of a relatively small quantity of a fluidizing gas such as carbon monoxide, carbon dioxide or nitrogen.

Because of the restricted openings in grill I3, the fluidized solids in flared section II move into the contiguous section l2 with little or no tendency of the solids in section l2 to flow back into section II. Additional reducing and/or inert solids are fed from hopper 24 through standpipe 25 into the second stage 12 to drive the reducing reaction toward completion and/ or to ensure good fluidization in spite of any agglomerating tendency of the reacting solids in this stage. The flow of solids through pipe '25 is regulated by slide valve 26 and is facilitated by injecting an aeration gas, say carbon monoxide or air, into pipe 25 by way of tap 27. The reacted solids are withdrawn from section 12 or the second stage of the reducer through draw-off pipe 28. A slide valve 29 is used to control this withdrawal and tube 30 above valve 29 is used to introduce a small quantity of gas such as carbon monoxide .to ensure the free flow of solids discharging through pipe 28.

The reducer H] has a plurality of fire-tubes 3| set obliquely through the fluidized mass in sections l I and [2 to supply the heat required for the reduction of the zinc compound. A fluid fuel such as natural gas or fuel oil is charged through injectors 32 which cooperate with Venturi-like openings 33 for the aspiration of air to support combustion within the tubes 3|. tion or flue gases leave these tubes through the upper ends 34, discharging into the atmosphere or a suitable stack.

The lower end of reducing vessel II] has a slide valve Ila which may be used to withdraw the solids from the vessel to permit cleaning or repairing of the apparatus. Valve I la may also be used during-the operation of the reducing Vessel for the periodic or continuous withdrawal of such coarse particles which may accidentally enter section I l or be formed therein by agglomeration that they settle out of the fluidized mass and accumulate at the bottom of vessel It]. By withdrawing these coarse particles which drop to the bottom of the vessel, their accumulation to the point at which fluidization within section II would be impaired is thus avoided.

In Figure 2, the reducer 35 comprises a plurality of cylindrical sections 36, 31 and 38 of increasing diameters connected by frusto-conical sections 39 and 40. The lowermost cylindrical section 36 terminates in a tapered section 4| connecting with inlet pipe 42 for the introduction of a small stream of fluidizing gas. Attached to the tapered section 4| is a draw-off pipe 43 having valve 44 for the withdrawal of solids as hereinbefore discussed in connection with valve Ila of Figure 1. The fluidized mass 45 having an upper pseudo-liquid level 45a is divided by perforated plates 46 and 41 into a lowermost portion A to serve as the first stage of the reduction, a middle portion B for the second stage, and an uppermost portion 0 for the third stage of the reduction. The perforations in plates 46 and 41 permit the flow of reaction gases and fluidized solids from the first stage to the-third stage of reducer 35and prevent the solids of the fluidized mass from dropping back from one stage to a lower stage. Accordingly, the reaction gases and fluidized solids move concurrently from the first stage to the last stage of the reducer 35. Hopper 48 is used for charging a mixture of finely divided zinc compound and carbon or like reducing agent into the first stage of the reducer 35. Hopper 48 discharges into portion A of the fluidized mass through standpipe 49 and slide valve 50. A connection 5| is used to introduce a small quantity of gas, say carbon monoxide or air, to keep the solids in line 49 in a free-flowing condition. A further addition of powdered solids like sand or coke is made from hopper 52 by way of rotary bucket-type valve 53 and screw conveyor 54 which discharges into the second reducing stage. The reacted solids are conveyed by entrainment in the reaction gases leaving reducer 35 by way of outlet pipe 55. The reaction gases and suspended particles enter cyclone separator 56 wherein the gases and solids are separated, the gases leaving by way of outlet 51 and the solids flowing out of standpipe 58. The reaction gases emerging from outlet 51 pass to suitable condensers or equivalent means for separating the The combus- I atom-7s zinc vapor fromthe" remainder *ofthegaseous efiillent issuing fr m outlet 51. In this instanoej' the necessary heat for the reductionpfthe zinc compound -is fui'nished" by electrical resistance'-- heaters 59.

Figure 3 presents l an arrangem'ent of reducing-i equipment-comprisinga tapered vessel (in and it cylindrical vessel 6H Vessel Bm-holdsportion- 'A of the fiuidized -mass'- in 'which th e firststage-ofi reduction is efiected and vessel 6| holds portion' reaction mass in vessels 60 and- 6 l i Spe'cifieexamples-of *the'stagewise reduction process of this invention-are given in =the--fo11ow--"-' 5 xing table-in terms ofa reactor of the type shown in Figure 1. Thecoke to oreratio's shown in thetable --for" the -first stage are -based-on the orefed to-thefirst -stage--and-the -coke "fed --to the first stage fithe ratios foi the second stage are based on--the' ore fedto 'thefirst stage'and-the--total- B of the fluidized mass for the second stage-of the coke fed-to both stages:

Table.

Control Example Example Control Example RunX Run '1 Run 2 Run Y Run 3' Roasted Ore (weight analysis) 2' ZnO;' fiper'ceflki 78.3? 78.3. J 78. 31 81.10 81. 0 7n9 do 0.1 c ()1 0.1 0.3 0.3 ZnSO4: do 2. 4 2.4 2.4 3. 6 3. 6 Feado 11.3. 11.3 i. 11.3 4.1 4.1 PM). d 1.9 1.9 1.9 1.5 1.5- otherflomp lu do 6. 0 6.0 6.0 9. 5 9.5 ReactoriDiametersz AtLine 22 fee t 3. 0 3. 2 3. 0 3.1 3. 2- At Gri11"l3 I do 9. 1 9. 7' 9.1 9.0 9.7 Heightgof Fluidized Mass:

Portion A do 7 r 17. 0 6. 3 17. 0 15.7 9. 4 Portion'B d0;;- 7.0 9. 6 7. 0 6. 5 6. 5 'lotal;'1 y do 24.0 15.9 24.0. 22.2 15.9 Volume of Fluidized Maserv Portiou'A -cu;ft 478" 177 478 434 267 PortionB ..cu.-ft 397 547 397 355 355 V Total 011 It- 875 724 875 789 622 Temperatureoi Mass:-

Portion A F 1, 850 1, 850 1, 790 1, 850 1, 850 Y Portion B F 1, 850 1, 850 1, 850 1, 850 1, 850 Roasted Ore Fed to:

Portion A lh /hr-'. 3,140 3,140 3,140 3, 030 3, 030 Coke Fed to:

PortionAle 1b./hr l, 570 785 785 1, 515 758' Portion B 1b./l1r 785 1 785 0 757 Coke to Ore Ratio:

Portion An 1:2 1:4" 1:4 1:2 1:4 Portion B v V 1:2 1:2 1:2 1:2 1 2 Fluidizmg (:las (00) Fed by Line 22 std. cu. ft./hr 3, 980 3, 980 3, 980 4, 200 4,200 Reactedsolids Withdrawn-fmm'Portion B lb./hr 1, 262" 1,262 1, 262' 1, 124' 1, 124" Zinc-Vapor in:

Total Reaction Gases volume pereent 40.0 40. 0 40.0 42.0 42.0 Reaction Gases from-ZnO a1one 1 do 445" 44. 5 44. 5 46. 7 46. 7 Conversion to Zinc:

InPortion A percent 898 85.0 89.8 89. 8 89. 8- In portion B- d0 90.2 93. 4 90. 2' 90. 2' 90.2 Overall.; do 99. 01 99. 0' 99.0 99.0 99.0

reduction. A mixture of zinc compound andsolid 5 Ineach of the tabulated runs:

1. The gases flow up through the reactorat a-n: averagevelocity of 0.45 foot per second.

2. Zinc vapor is-recovered from-the gaseous efiluentleaving the reactor at the rate of 2090 lbs.-

quantity of fiuidizing gasis introduced into -the "'per-hour.-

bottom of tapered vesse1j69 byway .of line 66. f Thereaction gases from the first stage of redue-i tion passthrough line 13intovessel 6| wherein thesecond stage of the reduction is carried out.

3. Heat-is supplied to the endothermic reac tiornmass within the reactor by closely spaced fire-tubes passing therethrough.

Control runX provides. thebasis for evaluating The final reaction gasesincludingzinc vapor are *"the benefits of this invention in example runs 1 withdrawn from vessel 6 I by .wayof outlet pipe 14 and conducted to a condenser for the recovery of molten zinc. The partially reacted solids "flow from vessel through standpipe 61;"controlld 'by and. 2, Whilecontrol run Y bearsthe same rel a-- tion to example-run 3. Control runs X andY are examples of the invention of my copending ape plieationSerialNo; 767;548',the present'inventioh a valve 68; and dischargetbelow the pseudo- 0"'beinganimprovement'thereover.

liquid level." of portionB'of the fluidized mass? within vessel 6|. The reacted solids leave the second stage of the reducing equipment through line 15 and control valve 16; The arrangement" vantageouslyjused where the zinc compound to be reduced is in the form of a material 'which" has a considerable tendency toagglomerate during reduction. .Insucha case, the agglomerates vessel B0 or the first'sta'ge 'of reductio'n'to'vessel 6 l or the second stage "of the reduction and; the residueof'theoperationcan be easily withdrawn throughlinev 15*atthe"bottom"of* vessel'liljj: As 1 I I shownttheapparatusofFigure 31s PIOVidEd Wit 0f -the fluidized"mass"inthe first" stage (6.3 ftJT' Referring to the table, it -willdoeobserved that" the roastedzin'c ore of runs'Xfl'l and '2 base.

fairly-high content of-FezQa and P which'gives the ore "-'appreeiabl'e 'agglomeratin'g propensity of reducing equipment showninFigure 3 isad-" 5"during-reduction. Runs 1 and 2 exemplify two ways ink-which :i-suchr an ore zcans be treatedain 1 accordance with th'eprinciples of" this. invention? Inwrun 1, the conversion to zinc: in the first I reducing stage is limited to.85% because ahigher-- willreadily' pass. with the fluidized mass from'i7dconversion in this stage w ya 1:4 ratio of coke to ore. wouldlead to serious. agglomeration which .in turn would interfere With properfluidf ization. Because of the selected lower 'conver-' ,sion and lower "ratio of coke to" ore, the height of run 1 is 63% less than the corresponding.

height (17.0 ft.) in run X. To effect an overall conversion of 99%, the second stage in run 1 has to operate with a conversion of 93.4% to;

compensate for the relatively low-conversion of 85% in the first stage. This makes the second stage of run 1 somewhat larger than the corresponding stage in run X; the height of the fluidized mass in the second stage (9.6 ft.) of run 1 is thus 37% more than that (7.0 ft.) of run X. In spite of this increase, the total height (15.9 ft.) of fluidized material in both stages of run 1 is 34% less than the total height (24.0 ft.) in run X. Similarly, the volume of fluidized solids'in run 1 is considerably smaller than in run X.

In run 2, the benefit of introducing part of the coke into the first stage and part into the second is taken in a different form. The reactor dimensions'are held the same as those for run X; This permits the coke deficient mixture fed to the first stage of run 2 to have a longer residence time therein than does the mixture fed to essary heat transferareawithin the fluidized:

tubes are disposed horizontally therethrough.

the first stage of run X. To effect 898% conperatures of 1790 F. and 1850 F., respectively.

Because of the lower temperature, the agglomerating tendency is less pronounced and a firststage conversion of 89.8% can be countenanced in run 2 while it was limited to 85% in run 1. In decreasing the temperature of the first stage from 1850 F. to 1790 F., the allowable stress on the metal parts of the reactor may be increased from about 550 lbs. per sq. in. to about 750. This increase of 36% in design stress is of considerable advantage from the points of view of capital investment and maintenance of the reactor.

The roasted ore used in control run Y and example run 3 has appreciably less F6203 and PbO than the ore of runs X, l and 2 and therefore shows less tendency to agglomerate during reduction. In decreasing the proportion of coke fed to the first stage pursuant to this invention, the more agglomerating ore made it necessary in run 1 to limit the conversion in the first stage or in run 2 to lower the temperature in the first stage. With the less agglomerating ore, there is no need to lower either the first-stage conversion (89.8%) or the temperature (1850 F). Comparing run 3 with control run Y, it is seen that the height of the fluidized mass in the first This form of reactor is useful in the present-in- Vention, particularly inasmuch as a relatively large area of heat transfer surface can be buil into the reactor. e V 1 It iscontemplated that under certaincircumstances it may be advisable to maintain diiferent gas velocities and/or different temperatures in the succeeding stages of the reduction. Thus, it may be advisable to'use a comparatively low gas velocity, say 0.5 foot per second, through the first stage in orderto permit the zinc concentration to build up, in the gaseous product stream but to have the gas velocity in the succeeding stage higher, say 1 foot per second, in order to obtain maximum capacity per unit of horizontal cross-section. Where the feed material containing the zinc compound which is to be reduced is in a poorly reactive form, it Will often be beneficial to maintain a higher reducing temperature in the last stage than that in the first stage of the reduction. In the final stages of a reduction, resort tohigher temperatures which generally aggravate agglomeration is made feasible by the introduction of diluting, nonagglomerating solids pursuant to this invention. Thus, solids which are added to the fluidized mass primarily to ensure good fluidization enter the mass at that 7 stage or stages of the reduction where agglomeration requires correction and not at the first or early stages of the reduction where the diluting solid particles serve no useful purpose.

While I have referred to the concurrent flow of solids and gases from one stage to a succeeding one, it is clear that the solids and gases do not progress through the stages at the same velocity because of the slippage or hindered settling V of the solid particles within the fluidized mass of stage of run 3 is 9.4 ft. or 40% less than the corresponding dimension (15.7 ft.) in run Y. The total height of the fluidized mass for run 3 is 15.9 it. while in run Y it is 22.2 ft. the total height is thus decreased 28%. Similar decreases will be noted by comparing the volumes of the fluidized masses in the first stage and in the whole reactors of runs 3 and Y.

The reactor diameters in the vicinity of gas feed line 22 are slightly larger in runs 1 and 3 than in control runs X and Y, respectively, because in these runs the depth of the fluidized mass is less and consequently the fluidizing gas is introduced against less fluid-static pressure. Under these circumstances, a given quantity of gas occupies a somewhat larger volume. upper diameters of the reactors in runs 1 and 3 are also larger than those in runs X and Y, respectively, for the reason that these shorter reactors must accommodate a largernumber of fire-tubes (of shorter length) to provide the neo- The each stage. As a matter of fact, the residence time of the solids in each stage will usually be very much greater than that of the gases therein.

Those skilled in the art will visualize many variations of the invention without departing from its spirit or intent. For instance, the reducible compounds of other metals such as cadmium, usually occurring in small proportions with the zinc compounds, may be simultaneously reduced and their metal vapors recovered along with the zinc vapor. Accordingly, the foregoing disclosure should be interpreted as being illustrativeof the invention and not restrictive; only such limitations should be imposed as are indicated in the appended claims.

What is claimed is: 1. In the stagewise reduction of a solid inorganic oxygen-containing compound of zinc by a solid carbonaceous reducing agent, both said solids being in comminuted form, under fluidizing conditions for'the recovery of zinc, wherein said reduction is conducted in at least two stages with 'concurrentmovement of the gases and solid particles of the fluidized mass from the first stage successively to the subsequent stages while preventing substantially all back-flow of said solid particles from any stage to preceding stages, the

" improvement which comprises introducing into taining the presence of said particulate material 13 duction of said particulate material into said subsequent stage.

2. The process of claim 1 wherein the particulate material is a carbonaceous fuel.

3. The improved process of treating a comminuted solid material containing zinc oxide for the recovery of zinc, which comprises introducing said comminuted material and a comminuted solid carbonaceous reducing agentinto a fluidizing and reducing zone adapted to hold a deep fluidized bedof increasing horizontal cross-section in the upward direction, injecting a fluidizing medium into the base of said bed, passing gases and solids from said reducing zone into a subsequent reducing zone to effect further reduction of zinc oxide while preventing substantially all back-flow of said solids from said subsequent reducing zone to the first said reducing zone, introducing into said subsequent reducing zone a solid nonagglomerating particulate material, maintaining the presence of said particulate material in said subsequent reducing zone by the continued introduction of said particulate material into said subsequent reducing zone, withdrawing from said subsequent reducing zone gases and solids, and recovering zinc from the withdrawn gases.

4. The process of claim 3 wherein at least 60% of the zinc content of the solid material introduced into the reducing zone adapted to hold a deep fluidized bed of increasing horizontal cross-section is vaporized by reduction within said reducing zone and the particulate material introduced into the subsequent reducing zone is coke.

5. In the production of zinc by the fiuidizing process involving the reduction of a solid inorganic oxygen-containing compound of zinc by reaction with a solid carbonaceous reducing agent, both of said reacting solids being in comminuted form, wherein a gaseous reaction stream containing the zinc vapor generated by said reduction is withdrawn from the zone of reduction, the improvement of producing said gaseous reaction stream with a high concentration of zinc vapor, which comprises effecting said reduction while passing said reacting solids and said gaseous reaction stream successively through a series of fluidized beds thereof while preventing substantially all back-flow of said reacting solids from any bed to any preceding bed of said series, charging a fluidizing medium at the bottom of the first of said beds to support fiuidization therein, said first bed being about to 40 feet in height and of increasing horizontal cross-section in the upward direction, introducing into one of said beds subsequent to said first bed a solid nonagglomerating particulate material, maintaining the presence of said particulate material in said subsequent bed by the continued introduction of said particulate material into said subsequent bed, and withdrawing from the last of said beds said gaseous reaction stream after it has attained a volume at least 8 times the volume of said fluidizing medium charged at the bottom of said first bed.

6. The process of claim 5 wherein the solid inorganic oxygen-containing compound of zinc is essentially zinc oxide and both the solid carbonaceous reducing agent and the particulate material are essentially coke.

7. The process of claim 6 wherein the conversion to zinc efiected with the reacting solids entering the first bed is approximately equal to the conversion to zinc eifected with the reacting solids entering each subsequent bed.

8. In the stagewise reduction of a solid inorganic oxygen-containing compound of zinc by reaction with a solid carbonaceous reducing agent, both of said reacting solids being in comminuted form and being introduced into the first reducing stage, under fiuidizing conditions for the recovery of zinc, wherein said reacting solids and the resulting reaction gases concurrently move successively through a series of fluidized reducing stages with substantially no back-flow of said reacting solids from any stage to any preceding stage of said series, the improvement of introducing additional solid carbonaceous reducing agent in comminuted form into a stage subsequent to the first stage of said series.

9. The process of claim 8 wherein the additional solid carbonaceous reducing agent in comminuted form introduced into a stage subsequent to the first stage of said series amounts to about onehalf of the proportion of said reducing agent ultimately required in the last stage of said series.

JOHN C. KALBACH.

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

UNITED STATES PATENTS Number Name Date 694,947 Davies Mar. 11, 1902 2,316,664 Brassert et al. Apr. 13, 1943 2,342,368 Queneau Feb. 22, 1944 2,379,408 Arveson July 3, 1945 2,379,711 Hemminger July 3, 1945 2,393,704 Ogorzaly Jan. 29, 1946 2,397,352 Hemminger Mar. 26, 1946 2,398,443 Munday Apr. 16, 1946 2,425,098 Kassel Aug. 5, 1947 2,431,630 Arveson 1 Nov. 25, 1947 2,475,607 Garbo July 12, 1949 2,478,912 Garbo Aug. 16, 1949 2,485,604 Kalbach Oct. 25, 1949

Claims

8. IN THE STAGEWISE REDUCTION OF A SOLID INORGANIC OXYGEN-CONTAINING COMPOUND OF ZINC BY REACTION WITH A SOLID CARBONACEOUS REDUCING AGENT, BOTH OF SAID REACTING SOLIDS BEING IN COMMINUTED FORM AND BEING INTRODUCED INTO THE FIRST REDUCING STAGE, UNDER FLUIDIZING CONDITIONS FOR THE RECOVERY OF ZINC, WHEREIN SAID REACTING SOLIDS AND THE RESULTING REACTION GASES CONCURRENTLY