CA1077278A - Method for the batchwise reduction of metal ores - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improvement in a method for the batchwise gaseous re-duction of iron ore to sponge iron in a multiple unit reactor system including a cooling reactor and at least one reduction re-actor. Reduction is effected with a gas composed largely of carbon monoxide and hydrogen which may be, for example, generated by re-forming a mixture of steam and natural gas or other gaseous hydro-carbons at an elevated temperature The reducing gas requirement of the system is decreased by adding to the gas fed to one or more of the reduction reactors a minor amount of a hydrocarbon gas or vapor, e.g., natural gas, methane or a combination of methane and recycled spent reducing gas.

Description

07'7;i~78 This invent~on relates to the batchwise gaseous redueti~n of metal oxides at elevated temperatures and more particular}y tG
an improved method of operating a multi-unit reactor system ~or ef~ecting such a reduction process. The in~ention s especi211y use~ul in connection with the d~ect g~seous reduc~ion of ir~
oxide in lu~p or pellet form ~o sponge iron and wilL be illustra~
tively describ~d in connec~ion with this use, alth~u~h as the de~cr~ption proceeds it will become apparent that the invention can be equally well used in processes wherein meta~ ox~de ores ~
10 other than iron oxides are reduced. -~ ¦
Ih one o~ its aspe~ts the present invention is an improvement in a kn~n type of batch process for produci~g sponge iron which employs a reduction system c~pris~ng a .^;
plurality of interchangea~le reactors in which 9eparate bodies of ferrous material are treated simultaneously. A ~rccess of this type is disclosed in Celada U. S. patent 2,900~247; Celada et al. U. S. patent 3,423,201, and Mader et al. U. S. patents 3,136,623; 3,136,624; and 3,136,625. Similar processes are also disclosed in U. S. patents 3~128,174; 3,684,486; 3,827,879;
3,890,142; and 3,9~4,397. The principal operatLons carried out in a reactor system o~ this t~-pe are ~1) reducti~n of the ore . ~ ~o sponge ix~n, (2) cooling o the reduced ore, and (3) discharg-ing of the sponge ir~n from a reactor and recharging it with ~resh iron ore to be reduced. Reduction is effected by a . _ .

~ o 77~7~
reducing gas which i8 commonly a mixture largely composed of carbon monoxide and hydrogen. The gas is typically generated by the catalytic conversion of a mixture of steam and methane, usually in the form of natural gas, into carbon monoxide and hydrogen in a catalytic reformer of known type according to the equation:
CH + H O ~ CO + 3H
The effluent gas from the reformer i~ cooled and passes succes-sively through a cooling reactor and one or more reduction re-actors in series. During the cooling and reduction stages anadditional reactor, sometimes called the charging or "turn around"
reactor and containing previously cooled reduced ore in the form of sponge, is isolated from the system so that the sponge iron can be disaharged from the reactor and the reactor charged with fresh ore. The reactor system is provided with suitable switch-ing valves whereby at the end of each cycle the gas flow can be shifted to cause the cooling stage reactor to beaome the charging reactor, the last reduction ~tage reactor to become the cooling reactor, the charging reactor to become the first reduction stage reactor, and in cases where multi-~tage reduction is used, to cause the first reduction stage reactor to become the second reduction stage reactor, the second reduction stage reactor to become the third reduction stage reactor, etc.
In the reductio~ reactor series of a system of this type the fresh reducing gas initially passes through the bed of iron~bearing materi~l having the highest degree of reduction and then through beds having succes6ively lower degrees of re-duction. Because of this relationship the numbering of the re-actors can give rise to confusion, since they can be numbered either in accordance with gas flow or in accordance with the degree of ore reduction. In an effort to avoid such confusion, the reduction reactors will be numbered herein in accordance with gas flow. Thus the fresh reducing gas flows to the first or primary reduction reactor in which the last stage of ore reduction is oarried out, and in a system having three reduction reactors, the first stage of ore reduction is carried out in the third or tertiary reduction reactor~
In a reduction system of the type here being described the reducing gas fed to each reduction reactor is commonly heated in an indirect heater to a temperature of say 700 to 800C., after which it flows to a comhustion chamber associated with the reduction reactor wherein it is mixed with air or oxygen and a portion of reducing gas is burned to further in-crease its temperature to say 900 to 1100C. before intro-duction into the reduction reactor, The effluent gas from eachreduction reactor is cooled to remove water therefrom and then reheated before being introduced into the next successive re-duction reactor.
In a gaseous reduction system of this general type the cost of the gas generating apparatus usually represents 107727~
a substantial part of the total cost of the system. Hence from the standpoint of minimizing the investment required to produce a given tonnage of sponge iron of a given degree of reduction in a unit period of time, process modifications which decrease the amount of reducing gas required to produce a given tonnzge of sponge iron are especially important since they make possible the use of a smaller, lower capacity catalytic reformer or other type of gas generator.
Most of the currently operating commercial sys~ems comprise four reactors, i.e., one cooling reactor, two reduc-tion reactors and one "turn-around" or discharging and charging reactor. It is evident that the reducing gas economy of such a system can theoretically be increased by increasing the number of reduction reactors. Thus, if the number of reduction reactions is increased from two to three and the same flow of fresh reducing gas is used, it should be theoretically possible to achieve the same degree of reduction in two-thirds the amount of time. However, it has been found in practice that the use of a third series-connected reduction reactor fails to provide a full 50% increase in the productivity of the system and also gives rise to a number of practical problems~ More particularly, it has ~een ~ound that the sponge iron tends to "hang up" in the discharge reactor when a three-stage reduction is used and thus makes discharge of the reduced metal difficult Also the reduction efficiency is relatively low in the third reactor, probably due ~077Z78 to channelling therein. Moreover, it appears that the channel-ling and hang-up phenomena are interrelated. These practical disadvantages more than offset the theoretical advantage of using a third reduction reactor.
It is accordingly a general object of the present invention to provide an improvement in the batch process hereto-fore used to effect gaseous reduction of iron ore to sponge iron in a multi-reactor system of the type referred to above, which improvement decreases the amount of freshly reformed lQ reducing gas required to effect such a reduction. It is another object of the invention to effect such a decrease in the re-formed gas requirement of the system without increasing the operating cost of the system. It i8 still another object of the invention to provide a modification of the prior process which permits the use of three serially connected reduction reactors while avoiding the problems of channelling and "hang up" that have heretofore characterized three reduction reactor systems. Other objects of the invention will be in part obvious and in part pointed out hereafterO
In general, the present invention is based on the ~ finding that a significant decrease in the reformed gas require-; ment of an ore reduction system of the type described can be effected by mixing with the reducing gas fed to at least one re-duction reactor of the reduction reactor series a minor amount

2$ of a fluid hydrocarbon, e.g., a gaseous hydrocarbon such as .,........... , . ~
.. . . . .
' :~ ' ' , . ' ~ " , ' ~07~'Z78 methane or a mixed gas containing a substantial amount of methane or other gaseous hydrocarbon such as ~atural gas or coke oven gas.
As will be pointed out more fully hereafter, by mixing with the re-ducing gas fed to at least one of the reduction reactors, prior to heating the feed reducing gas, a relatively small amount of a fluid hydrocarbon, usually a gaseous hydrocarbon, the productivity of the system in terms of iron of a given degree of metallization per unit quantity of reducing gas per unit time can be substantially increas-ed. It has been still further found that in cases where a third re-duction reactor is used, the addition of methane to the reducing gasfed to the reduction reactors eliminates the channelling and hang-up problems previously noted in such 3-reactor systems~
As conducive to a clearer understanding of the present invention, it may be pointed out that it is known that methane is a relatively ineffective reducing agent for reducing iron ore. It is for this reason that the relatively expensive catalytic con-version processes are used for converting methane into carbon monoxide and hydrogen to provide an efficient gaseous reducing mixture. It i6 evident that on the basis of this prior knowledge the addition of methane to the feed gas to the reduction reactors is aontra-indicated.
It has been found, however, that there are a number of points in the process where methane that may be present in the reducing gas is converted into carbon monoxide and hydro-genl and that if a limited amount of methane is added ' ' ' ' :

~()i77Z71~

to the feed gas to the reduction reactors, the methane willbe converted into more efficient reducing constituents as the gas passes through the series of reduction reactors~
More particularly, it has been found that in the combustion chambers that are associated w.ith the reactors and in which the reducing gas is partially burned by admixture with oxygen or air, methane pre~ent in khe entering reducing gas is pre-ferentially converted to more efficient reducing constituents such as carbon monoxide and hydrogenO Also the metal-bearing material in the first reduction reactor is largely reduced to sponge iron, and at the temperature prevailing in this reactor, this sponge iron acts as a catalyst to promote the conversion of the methane content of the reducing gas to more efficient reducing constituents. Thus, by adding a minor amount of methane or other hydrocarbon gas to the reformed gas fed to the reduction reactor series, the amount of reformed gas that must be supplied by the catalytic reformer is decreased. Also it has been found that the requirement for reformed gas can be still further decreased by adding to the gas fed to the reduc-tion reactors, in addition to the added methane, a minor amountof the effluent gas from the same or a subsequent reactor of the reduction reactor series.
The methane and recyaled spent gas should be added to the reformed gas only in minor amount, since if major amounts of methane and recycled gas are added at this point in the system, .

excessive carbon deposition occurs and/or the desired reduction ', in reformed gas requirement per ton of sponge iron o a given.
degree of metalli~ation is no~ ac'ai~ved, In most cases the ' ,. ,_ . . . ........................................... . .. . . .~` aooregate amount of methane added to the reduction reactors ~ ' 5 desirably falls wlthin the range 270 to 12/~ by volume of the fresh reformed gas, with S% to 10% by ~olume be~ng pre~erre~.
In respect to the recycled spent gas; the quantity to be inco~-porated in the red~ction reactor ~eed gas ~ilL in mos~ csse~
desirably fall within the range 3Y0 to 307~ by volume of t~e reformed gas, with 5~ to 20% by volume being preferrcd.
H~w~ver, as more fully discussed below, in the special case where a single reduction reactor is used, the recycled gas may desirably be increased to 50Z or more of the gas feed to the reactor.
` 15 The obj~cts and a~vantages of the present invention can best be understood and appreciated by reference to the accompanying draw~ngs which illustrate apparatus capable of being used to carry out the process of the invention and where~: ~
FLgure 1 illustrates dia~ram3atic~lly a mu~tiple reactor system comprising an "out-of-line" cooling re~ctor and three reduction reactors connected in series and adapted t~ be used in carrying out a preferred e~bod~ment of the lnventicn, , ~ ' , ' .
.

.

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': ' ' ., ~igure 2 is a dla~ra~mati~ illustration of a m~ltip~e ~eactor system compr~sing an "in-line." cooling ~actor and two reductlon reactors connected in series and adapted to be us~d I ~; in carrying out a~other advantageous embod~ment of th~ ~nvent~on; -Figure 3 is a graph illustrating the ~anner in which the addition of methane to the first reduction reactor feed gas ~ al~d the c~m~ination of ~ethane additi~n and recycled ~as ad~i-I tion to the first red~ction reactor feed gas in accorda~ce ~the present invention can be expected, on the basis o computer simulation and pilot plant tests, to decrease the d~mand for I reformed reducing gas in the systems o~ Flgures 1 and 2;
¦ Figures 4A, 5A, 6A, 7A, 8A, 9A, lOA, llA, 12A a~d 13A
I are simpl~fied diagrammatic representations of additi~naL modi~
fications of the ~nvention illustrating various points in the iF lS reducticn syste~ at which methane can be advantageously intro-duced ~nd spent gas advantageously recycled. In these figures ~a~ represents the reformer, "C" ~he cooling reactor, "P" the `primary reduction reactor, "S" the secondary reduction reactor and "T" the tertiary reduction reactor; and Figures 4~ through 13B correspond, respecti~ely, to , the "A" Figures and a~e graphs illustrating the im~r~ved pro-! ductivity that can bs expected on the basis of co~puter simu-lat~on and pilot plant tests, when using the flow arrangement illustrated in the corresponding "A" Figures.

!
.
g 10'772~ 8 ReLerring to the drawing$ and moro partlcularly to ; ` Figure 1, the syst~m there shown ge~.erally comp~ises the cooling reactor 10, the primary reduction reactor 12, secondary reduction reactor 14J tertiary reducti~n reactor 16 and the charging reac-tor 18. As indicated above, the ore reduction sy-stem s~own is operated in a cyclic or periodic manner~ The ore re~ct~on B~
cooling opPrations, as well as the discharg~ng of cooled sponge iron rom the charging reactor and ~e recharging thereof ~lth fresh ore, are carried out simult.sn~us~y over ~ pred~te~ined poriod of time which may ~ary depend~g upon such ~actors as reducing gas quality and flow rate, reactor si~e, gas recircula-ti~n rates and the like. At the end of each cycle of operatinns~
.
I the reactors are fu~ctionaLly interch~nged in such manner that~, I the charging reactor becomes the tertiary reduction reactor, ,,~Ai~ 15 the tertiary reduct~on reactor becomes the secondary reduct~on~
: reactor, the second~ry reduction reactor beco~e~ the primary ~j~
reduction reactox, the primary reduction reactor becomes the ~;
cooling reactor and the cooling reactor becomes the charging reactor. This functional interchange of the reactors can be effected by an arrangement of valves and pip~ng between thereactors that is kno-~n in the art ~ se and has been omitte~
rom the drawing in order to simpii~y the sh~wing therein.
The flow of reduc~ng gas through the reduction reac- .
tors is generally counter-current. That is to say, the fresh 25 reduc~g gas is fed to the prim~ry reduction reactor which i` --1 o-- .

.~ .

1~)77'~78 contains iron-bearing material that has already been partially reduced in the secondary reduction reactor and tertiary reduc-tion reactor in previous cycles. The tertiary reduction reac-tor, which initially contains fresh ore, is treated with gas that has already passed through the primary and secondary re-duction reactors.
Referring now to the left-hand side of Figure 1, a reducing gas composed largely of carbon monoxide and hydrogen is generated in a reformer 20 of known construction comprising a gas-heated catalytic converter section 22 and stack 24. Methane, natural gas or other hydrocarbon gas or vapor from a suitable ~ource is supplied through a pipe 26 containing a valve 28 and i8 preheated by passing it through a coil 30 near the top of stack 24 and in heat exchange relation with the hot gases flowing through the stack. Hydrocarbon gas, e.g., methane, leaving the coil 30 is mixed with steam in the proper proportions for catalytic con-version into carbon monoxide and hydrogen, typically in a carbon/-steam molar ratio of 1:2 to 1:3. More particularly, steam is sup-plied from a steam drum 32 through a pipe 34 containing a valve 36, and the mixture of steam and methane flows through pipe 38 to a coil 40 in the lower portion of stack 24 wherein it is further preheated. From coil 40 the methane-steam mixture flows through pipe 42 to the converter section 22 of reformer 20, wherein it pacses through externally heated catalyst tubes in known manner to effect the desired conversion to carbon monoxide and hydrogen.

1077'~8 From reformer 20 the hot reducing gas flows through pipe 44 to a tubular waste heat boiler 46 wherein its sensible heat is used to generate steam. More paxticularly, hot water from steam drum 32 flows downwardly through pipe 48 to the bottom of boiler 46 and thence through the tubes thereof, wherein a portion of the water i~ converted to steam by the heat of the hot reducing gas. The resulting mixture of steam and hot water returns to drum 32 through pipe 50.
In order to utilize further the heat in the hot gases passing through stack 24 of reformer 20, hot water is withdrawn from the bottom of drum 32 through pipe 52, then flows through a coil 54 within stack 24, and is returned to drum 32 through pipe 56. The heat recovered in boiler 46 and the coils in stack 24 is more than enouyh to generate the steam required for admixture with methane as feed to the reformer, Hence excess steam i5 available which can be withdrawn from drum 32 through pipe 58 and used for general plant purposes. Make-up feèd water for the steam generating system just described is supplied through pipe 60. The use of the steam drum 32, waste heat boiler 46 and coils 30, 40 and 54 within stack 24 substantially improves the overall thermal economy of the system.
The reducing gas, which has been cooled by passage through boiler 46, flows through pipe 62 to a quench cooler 64 wherein it is cooled and de-watered, and then to the reducing gas header 66. Some of th~ reducing gas from header 66 may be withdrawn through the pipe 68 containing the valve 70 and supplied to the cooling reac~or ~ystem as described hereafter.
The main portion of reducing gas flow~ through pipe 66 which is provided with a valve 72 to a coil heater 74 wherein it i8 heated to a temperature of the order of 700 to 850C. Since the desired reducing gas temperatuxe at the entrance to the primary reduction reactor 12 is of the order of 900 to 1100C., preferably about 1050C., further heating of the gas leaving coil heater 74 i6 re~uired, and this further heating is effected in a combustion chamber 12a which communicates with the top of primary reduction reactor 12. More particularly, the effluent gas from heater 74 flows through a pipe 76 to the combustion chamber 12a wherein it is mixed with an oxygen-containing gas lS supplied through pipe 78 containing valve 80. The oxygen-containing gas may be air or pure oxygen or mixtures thereof.
Within the combustion chamber a portion of the hot reducing gas is burned to provide a mixture having the desired relatively high temperature. The combustion chamber 12a may be of the type disclosed in Celada U.S. Patent number 2,900,247. If desired, the effluent gas from heater 74 may be further heated in a superheater 82 located in pipe 76. The use of a superheater is advantageous in the present process wherein a hydrocarbon ga~ such a6 methane is added to the reducing gas between the reformer and the primary reduction reactor as iO77~

de~crib~d below, since by using a superheater the amount of oxygen-containing gas supplied to the combustion chamber 12a can be advantageously reduced in some case~.
The volume of oxygen-containing gas used, as well as its temperature, depends upon the oxygen content of the gas.
Thus if air is used as the oxygen-containing ga~, it is de-sirably preheated to a temperature of the order of 700C. or higher, whereas if oxygen i~ used, it need not be preheated or may be preheated to a substantially lower temperature. If air is used as the oxygen-containing gas, the volumetric ratio of air to reduaing gas with which it is mixed may be as high as 0~4:1 and is typically in the range O.lO:l to 0.3:1. If, on the other hand, oxygen is used as the oxygen-containing gas, a volumetric ratio within the range 0.02:1 to 0.15:1 will usually give acceptable results.
From the combustion chamber 12a the hot reducing gas enters the top of primary reduction reactor 12 and flows down through the bed of iron-bearing material therein to effect a further reduction of the iron-bearing material to sponge metal.
The effluent gas from reactor 12 leaves the reactor near the bot~om thereof through a pipe 84 and passes through a quench cooler 86 wherein it is cooled and de-watered and then through a pipe 88 containing a valve gO to a coil heater 92, similar to the heater 74. Within the heater 92 the gas is again heated to a temperature of the order of 700 to 850Co and then flows ~077'~7~

through pipe 94 to the combustion chamber 14a of secondary reduction reactor 14 which is slmilar to the combustion chamber 12a. Chamber 14a receives a supply of oxygen-containing gas through a pipe 96 containing valve 98.
Within combustion chamber 14a a portion of the reducing gas is burned to increase the temperature thereof to the order of 900 to 1100C. and the resulting heated gas enters secondary reduction reactor 14 and flows downwardly through the bed of iron-bearing material therein to effect a partial reduction thereof. As in the case of the primary reactor system, the secondary reactor system may be provided with a ~uperheater 100 located in pipe 94.
The effluent gas from secondary reduction reactor 14 flows through a pipe 102, quench cooler 104 and pipe 106 con-taining a valve 108 to a coil heater 110 which is similar to theheaters 74 and 92 and similarly heats the gas passing therethrough.
From heater 110 the' gas flows through pipe 112, which may be provided with superheater 114 to combustion chamber 16a which communicates with the top of tertiary reduction reactor 16. The combustion chamber 16a is similar to and operates similarly to the combustion chambers 12a and 14a. Chamber 16a is supplied with an oxygen-containing gas through pipe 116 containing valve 118. ~ot reducing gas from chamber 16a flows downwardly through the bed of iron-bearing material in tertiary reactor 16 effecting a partial reduction thereof. Effluent gas from the tertiary ~077Z78 reactor flows through a pipe 120 to a quench cooler 122 wherein it i5 cooled and dewatered.
As indicated above, in accordance with the present invention a minor amount of a hydrocarbon gas such as methane is mixed with the reformed reducing gas fed to the first reduc-tion reactor. Such methane can be introduced into the header 66 through a pipe 134 which is connected to the supply pipe 26.
Pipe 134 contains a flow controller 136 which may be adjusted to provide a predetermined regulated flow of methane to the header 66. The amount of added methane is preferably from about 2% to 12%, by volume of the reformed gas fed to reactor 12.
; As will be pointed out below in connection with the discussion of Figure 3, the addition of methane through pipe 134 to the re-formed reducing gas in pipe 66 substantially reduces the re-formed reducing gas requirement of the system.
It has also been found that the requirement for re-formed reducing gas can be still further reduced by recycling a certain amount of effluent gas from one of the reduction re-actors to the feed gas entering a reduction reactor as illus-trated in Figure 1. More particularly, a predetermined regulatedfraction of the effluent gas from cooler 122 is caused to flow through pipe 124 con~aining valve 126 to a pump 128 and thence through pipe 130 containing flow controller 132 to the reducing gas header 66. As indicated above, the quantity of recycled effluent gas is a minor amount by volume of the reformed gas ~77278 fed to reactor 12, usually from 3~ to 30% of the volume of re-formed gas.
Reverting now to the ri~ht-hand portion of Figure 1, the remainder of the effluent gas from tertiary reduction reactor 16 flows to and through a header 138. As indicated in the draw-ing, at least a portion of this effluent gas may be used as a fuel gas to heat the lower section 22 of reformer 20 and the heaters 74, 92 and 110. More particularly, gas from header 138 can be withdrawn through pipe 140 containing valve 142 to supply fuel for heating the lower section 22 of reformer 20; through pipe 144 containing the valve 146 to supply fuel for heating the heater 74; through pipe 148 containing valve 150 to supply fuel for heating the heater 92; and through pipe 152 containing the valve 154 to supply fuel for heating the heater 110~ If the amount of effluent gas from the tertiary reduction reactor i8 more than that required for recycling through pipe 130 and for heating the reformer and reduction reactor heaters, the excess ga~ can be removed through pipe 156 to a suitable point of storage or vented to the atmosphere~
Referring now to the right-hand side of the drawing, there is illustrated a charging reactor which is structurally similar to the reduction reactors 12, 14 and 16 and is similarly provided with a heater 158 having an inlet pipe 160 provided with a valve 162. Effluent gas from heater 158 flows through a pipe 164, which may contain a superheater 166, to a combustion ~ 07 727 8 ~h~1m~er ~8a. O~ygen-containing gas can be supplied to combus-tion cha~iber 18a through a pipe 168 containing a ~alve 170.
H~eYer, during the portion of the cycle he e being described~ ~
.he valves 162 and 170 are closed and the charging reactor ~8 is isolated from the sys~em so that cooled reduced sponge iro~
can be discharg2d fro~ the reactor and a charge of fresh ore introduced therein.
As indicated abo~e, the system of Figure 1 is char~c-teriæed by the ~act that an out-of-lIne cooling reactor is used~
The cooling reactor 10, like reactors 12, 14 and 1~ fs pro-~ded with a heater 172, inlet pipe 1~4 containing a valve ~76, superheater 178 and ~ombustion chamber lOa which, during the part of the cycle here being described, are rendered inoperative by closure of valves 176 and 180, As descri~ed above, the reactor 10 at the ~egin~in~ of a reducti~n cycle contalns hot reduced sponge`iron from a previous reducti~n cycle. This bed af hot sponge iron particles ig c~ooled by clrcul~tion of a cooli.ng gas therethrough. The cooling gas recirculation system comprises a pump 182 which pumps gas through a pipe 184 to the top of cooling reactor 10. The gas flo~s do-~nwardly through the body of reduced metal in th~ reactor and cools it.- The ef1uent gas frGm th~ cooling reactor 10 flows through a pipe 186 to a que~ch cooler 18~ wherein it is cooled and dewatered and is ~he'n returned -through pipe 190 to the suction o~ pump 182. If~it is desired to use a reducing gas as a cooling medium for cooling the reduced ~ ' . - . .
' ;. ~

iO~7~78 ore, gas-may be withdrawn from he~der ~6 thro~gh pipe 68 con-~aining shut-off valve 70 and 1OW con~ro~ler 192 to introduce a predetermined fl~w of reducin~ gas into the coo~ing reactor , -~, .
recircu~ation system. ~n order to prevent an undesired~~uild-up - ~
S of pr~ssure within the cooling system, gas is removed from pi.pe ~84 through a pipe 194 containing a back pressure regulator 196 ~or , ma~nt~ining a des~red pressure in the coo~ing system. The cool-~ng gas removed through pipe 194 may flow either throu~h p~p~ 198 contain~ng valve 200 bac~ to the he~der 66 or th~u~ pipe ~2 cor~taining val~e 204 to the spent gas header 138 or both~
In general, the use of the out-of-line cooling reactor ~ ~ncreases the operating flexibil~y o~ the system since it permits ,. independent control of both the gas flow`ra~e and gas c~mposition . ~n ~he cooling gas loop.
. 15 As indicated above, it is often desirable to use as a cooling gas for coolin~ the`ho~t ore a gas containing constitu~nts capable of depositing a pre~etermined amou~t of carb~n on th~.
~urface of the reduced sponge iron~ Thus it ~ay be desirable to use in the cooling cycle a gas having a -somewhat different composition than that fed~t6 the reduction reactors in order to i achieve an optimum depositi~n of.carbon on the sponge iron. To permit modificatio~ o~ the gas compositi~n within the cooling reactor recirculati~g system, a branch pipe 206 containing a ~alve 208 is connected to the cooling gas recirculating pipe 190.
As indicated in the drawing, ~y of vario~-~s gases, e.g., carbo~

' ' -19- ~
.... . .... .... ... . . . . .... .. ... . .. ...

' ' ' monoxide, methane, hydrogen, nitro~en or carbon d~ox~de may be ~ntroduced into the cooling gas loop through pipe 206, either in place of or in addition to the reformer product gas supplied to -~- p~pe 68. Thus with the system shown the co~position o the cooling gas can ~e readily modified to effect a desired deposition o carbon on the surace of the reduced sponge iron particles.
Also the rate of flow of the cooling gas can be ~aried over a relatively wide range independently o~ the rate of flow of reduc-I ~g gas through the reduction reactors of the system, ¦ 10 ~cferring now to Figure 2 of the drawings, the system there shown is generally similar to the system of Figure 1 b~t differs therefrom in that the cooling reactor is "in-line" and the system comprises t~o reduction reactors, ra~er than three such reactors. In describing the system of Figure 2, the same ; 15 r~ference characters will be used ~or parts~ corresponding to those of Figure 1 with the identifying letter i~" added to indi-cate th~t the part is incorporated in the system o Figure 2.
. In general, the system of Figure 2 comprises the cool-ing reactor lOb, reduction reactors 12b and l4b and the charg~ng reactor 18b. Nat~ral gas enters the system through pipe 26b i and flows to re~ormer 20b wherein it is first ~oxed with steam i and then passed over a catalyst bed to foxm a reformed reducing gas consisting largely of carbon monoxide and hvdrogen. The reformed g25 flows through waste heat boiler 46b, cooler 64b and a pipe 330 containing valve 302.
1 Ç~' . . , F

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: ' ' ' ', ' ~. ' ;

1077Z7~
Dur~g th~ port~on o~ th~ cycle here being de~cribed the valve 302 is closed and ~11 o the reormed gas flows through pipe 304 conta~i~g valve 306 to the top o the cooling rea~or ' lOb and thence d~wardly through the bed of reduced oré t~e~e~n~
- ~ The cooling reactor lOb initially contains hot reduced ore rom a previous reducing operaticn ~aving a degree of metalliza~
of the order of ~57. to 907O~ The reduc~ng gas flowing downwardly throu~h the be~ of metal-beari~5 material in coo~ing reacto~ ~0~
~irably per~orms three f~mc~i~ns, ~amely, cooling of the spo~ge }O iron of the ~ed, -deposi~i~n of car~on on the sponge iron su~faces ~; dus to crack~tg of carbon-containing components of the redu~in~
gas and fu~ther reduction of unreduced ore in the bed.
~ ~n general, carbon deposition within the bed takes place ¦ . for the most part during the early part-of the cycle when the bed !; 15 is at a relatively high temperature. In order to achieve the ! twin object~ves of deposition of a ~esired amount of carbon on ', the sponge iron and rapid cooling ~f the sponge iron, the gas flow is desirably ~aintained at a relat~vély low value ~t the beginn~ng o the cycle and increased dur~ng the latter pa~t of-20 the cycle after the desired carbon dep,osition has been completed and when a max~n cool:~g rate is desirable.~ These ob3ectives j are atta~ned by recycling a portion of the cooiLng gas as . described below.
Effluen~ gas from the bottom of cooling reactor lOb 25 flows through a cooler 188b to a pipe 308 wherein the flow ~s . ., ~ .
~ , - .

, , ' .

~1 ~

. A . .
"~` ' , ' ' ' . . , I
. .. . .

107~7 8 di~ded. A portion of t~e gas flows~to the heater 74b and th~n to the reduction reacto~ 12b wherein it reduces the metal-beaxing ~aterial in the reactor as described in connecti~n with resc-tor l2 of Figure 1. Thë remainder of the effluent gas fro-m~~~~
cooler 188b flows through pipe 310 to the inlet o~ pump 312 and from the discharge of pump 312 through pipe 314 wh~ch is connected ~o pipe 304. Thus a portion of the gas is recycled by the pump 312 and comb~ned wi~h the ~reshl~ re~med gas to fonm the cooLing gas fed to the top of cooling reac~or lOb.
Pump 312~is prov~ded with a by-pass 3L6 containing a pressure controller 318 which maintains a constant but adjustable pressure at the dis~harg~ side of pump 312. ' In essence, cooling of the metal-bearing material in reactor lOb is carr~ed out as disclosed In Celada et al. U. S.
patent No. 3,423,201. During the early portion of the cooling cycle when it is desired to deposit a ~redete~mined deslred amount of carbon on the sponge iron in the bed, li$tle~ if any, gas is recycled by pu~p 312. During the later por~icn of the cycle after carbon deposition has substan~tial b -ceased, the recycle o~ gas by the pump 312 is started or increased to effect a rapid cooling of the metal-bear.ing materia~ within the cool m g reactor. During $his second cooling stage the, volumetric ratio . of recirculated gas to fresh reformed gas 10win~ through pipe 304 is desirably maintained from about 0.5:1 to 4:1.

' . -22~

, .. , ~ . . ... .

~077Z78 As in the case o~ the sys~em of Figure 1, there is admixed with the reformed gas flow~ng to the ~rst reduction reactor mlnor a~ounts o met~ane and, if des;.red~ spent gas recycled from the last reduction reactor of the system.
~ore particularly, a small amount of the natural gas entering the system throu~h pipe 26b flo~s through pipe 134b containir~
the flow cGntroller 136b to pipe 308 wherein ~t is mixed with the refuxmed gas comi~g from cooler 1~8b~ Also a portion of the efflu~nt gas from cooler lO~b of the second reduction reactor 14b is recycted by pump 128b thsough pip~ 130b containing f~ow controller 132b to pipe 308 wherein ~t is mixe~ with the effluent gaa from cooler 1~8b and the methane entering f~om pipe 134b.
The mLxed gas then flows to the heater 74b of t~e first reduc~ion reactor 12~ and is used to effect a reduction of the ore as descri~ed in connection with Figure 1.
As pointed out above, it has been ound that by adding a minor amount of methane to tho reduction reactor feed gas, either with or without addition of recycled spe~t`gas, the amount o fresh reformed gas requ~red to produce a gi~en deg~ee of meta'lization o a given amount of orè can be substantialLy reduced. This effect is illustrated in Figure 3-of the dr2wings wherein percent metallization obtainable with an ore reduction system of the type described is plotted against reformed ga~
composition in terms of thousands of standard cubic feet per ton of iron. In g~neral~ the curves of Figure 3 (~s well as .

,... . - . - , ~077'~78 the curves of Figures 4B to 13B) ar~ based on computer sim~lla-tion with some degree of empirical verification, The curves ~ndicate the approx~mate typical per~ormance that can be ex-~`.;; pected when using the process of the present ~nvention in ~
S syste~s of the type disclosed in Figures 1 and 2. Curves A, ~ B and C relate to the system of Figure 1, and Curves D, E and F¦ relate to the s~stem of Figure 2. Curve A indi~ates the expected . relation between percent meta~lization and reformed gas consump-tion in a system such as that o Figure 1 when no meth~ne ~ddi~lon 10. or spent gas recycle is used. Under these cond~tions approx~a~ely 35JOOO cubic feet of reformed gas are indica~ed ~o b~ need~d to produce a ton of iron having say 89% metallization. Curve B
represents this relationship wh~n ~% by volume methane is added to the reducti~n reactor feed gas. Curve C represents the same r 15 relationship when both an 8% methane addition and a 10% by volume spent gas recycle are used. The curves indlcat~ that by us~ng the combin~tion of methane addition and spent gas recycle, the :~ re~ormed gas requirement can be ~educed from about 35,000 to about 29,000 cubic feet per ton. , --~0 It will, of course, be unders~ood that the numexical ~alues for. gas consumption given in Fig~r 3 will vary 8S a func-i tion o.such ~actors as the compositi~n and ph~sicaL condition of the ore used, gas flow rates, configuraticn of the ore bed and the l~ke. However, the curves indicate that for a given set o~ process conditions, a substantial reduction in ~he demand or ~ ' -'''` ,,'.

~Z ' ' ' ' ' ~: -24-.~ .... . . ... . . . .. .
'',-. '' ' ' ',' : ' ~ , ' ' ~ ' iO77'~7 ~ .

reforme~ gas can be achieved by adding minor amounts of meth~ne or a co~bination o~ methane and recycLed spent gas to the re-ductio~ reactor feed gas as disclosed herein.
.. Still referring to ~igure 3, Curve D indicates~that~ ~
when the system of Figure 2 is used without methane addition or spent gas recycle the refor~ed gas requirement to achieve 8970 metallization in a typical case is about 43,600 cubic feet per ton. Curve E.~ndicates that with 8% by volume of methane added to the reformed gas under the same ~onditions, the ~eformed gas require~ent can be expected to drop to about 3~,000 c~bic ~ee~ per ton, and Cur~e F indicates that by using both methane ~ddition and spent gas recycle, a decrease of the reformed gas requ~rement to about 35,200 cubic feet per ton can be expected.
It w~ll be noted that the economy obtained with the three reduc-tion reactor system o~ Figure 1 is substantially greater thanthat obtained with the two reduction reactor system of Figure 2.
A.s poineed out abo~e, the addition of methznle to the reduction reactor feed gas is especially important in the case of the system of F~gure 1 in order to overcome the channelling and hang-up problems that are encountered in usin~ a third reductio~
reactor without added ~ethane.

Turning now to Figures 4A and 4B, the system illustrated ¦ in Figure 4A c02prises a reformer, an "in-line" coo~ing reactGr and three series-connected reduction rea~tors. In this case 8% by volume methane is added to the reducing gas flo~ing from the , ~0772~78 .
secondary to the textiary r~actor. The methane is added to the effluent reducing gas from the sec~ndary reactor after the reducing gas has been cooled and de-watered and before it is reheated for use in the tertiary reductlcn reactor. The~--ex~
S pected performance of this system is indicated by Curve A o Pigure 4B.
Curves B, C and D are incl~ded or reference purposes and are the same in each of Figures 4B through 13B. Curve ~ ~
illustrates the typical performance of a system having two 1~ reducti~n reactors and no methane additi~n and thus corresponds ; to Curve D o~ Figure 3. Curve C illustrates the typical per-formance of a system havlng three reduction r~actors and no ~thane addition and thus corresponds to Curve A of Figuxe 3. Curve ~
~llustrates the perfor~ance of an "ideal" three reduction reactor s~ste~ with no added ~ethane, i.e., a system in which the use of a third reduction reactox is assumed to produce a full 50% ~n-~rease in reduction capacity.
. . It is evident fr~m a comparison of C~rve A with Curves B, C and D, and particularly with Cur~e C-that additi~n of methane to the feed ga~ to the tertiary reactor produces s~e improvement ~n the ec~nomy of the system, afthough not the ideal economy of Curve D. `.
- Figure 5A illustrates a system with three reducti~n reactors and methane added to the fePd gas to the. primary reducti~n reactor.. In Figure SB Curve A indicates the expected ' .' .. ~

~ ~26-~7 7Z7 ~

re~ult ~sing such a system with 8% by volume of methane added, A comparison of Curves A and D sh~ws that thls syste~ provides an economy approxi~ately the sa~e as that ~llustrated by Curve D, i.e., approx~mately the same as the ~deal performance~o-f=a th~ee ~~
reduction reactor system wlth~ut added methane.
Figure 6A illustrates a two reduction reactor syste~
with methane added to the feed gas to the primary reduction reactor and thus is a two reduction xe~ctor counterpart o th~
system of Figure 5A. The pex~ormance of this system with 8%
by volume o~ methane added is illustrated by Curve A of Figure 6B
wnich corresp~nds with Curve E of Figure 3. It is evident f~m ~igure 6B that the perfor~ance of the system of Flgure 6A is substantially improved by the addition of methane and very ne~rly reaches the performance of the computer simu~ated three reduction : 15 reactor system. Figures 5B and 6B indicate that t~e addition of methane to the primary reduction reactor feed gas is an especially e~fective embodiment of the invention, Figure 7A illustrates a three reduction reactor system with methane being added to the eed gas to the secondary reac-tor. The performance o~ this sys~em with 8~o by v~lume methaneadded is illustrated by Curve A of ~igure 7B. A comparison of Figures 4B, SB and 7B shows that the perfoxm2nce of this system is intermediate between that o the systems w~ere~n the me~h~ne is fed to the primary and ~ertiary reactor feed gases, respec-ti~ely, ~nd that it more nearly appxoaches that of ~ethaneaddition t~ the primary reactor.

.

- .

~077~
Figure 8A illustrates a tw~ reduction r~actor system with ~ethane added to the feed gas to th~ seconda~y reactor.
The performance of ~his syste~ is illustrated by Curve A of Figure 8B, A comparison of Curves A and B of Fî~lre 8B shows S that a signi~icznt improve~ent in performance is attained~
although the improvement is not as great as when the methane is fed to the primary reac~or as illustrated by Curve A of Figure 6B .
Figure 9~ illus~rates a two reduction reactor system ~0 with additio~ o~ methan~ to the feed gas to the secondary .
reactor and rec~cle o effluent gas from the secondary reactor to the secondary reactor feed gas. Curve A o Figure 9B illus-trates the results obtained with 8% by volume of methane bein~
added and a recycle o 16% by volume. A comparison of Curves A
and B of Figure 9B with Curves ~ a~d B of Figure 8B shows tha~
the recycle produces a signiicant additional improvement in ~he perform~nce of the system.
Figur~ lOA illustrates a two reactor system with methane added to the eed gas to both t,he primary and sec~nd~ry reduction reaetors and recycle of efluent gas from the primary reactor back to the primary reactor inlet. ~urve A of Figure lOB
illustrates the performance o this syste~ with 4% by volume meth~ne added to the pr}mary reactor feed ga$, and 870 by volume methane added to the secondary reactor feed gas, and 8% by volume of recycled gas. A further improvPment in perfor~nce in rèspect to the system o Figure 9A is indicated, .
--~8--Figure lL9. illustrates a three reduction reactor system with methane added to the feed gas to each o the ~hr2e reduct;on reactors. The perfor~ce of ehis system - ~hen addinO 4% by volume methane to the ~eed gas to each of ; S th~ three reductio3 reactors is illustrated by Curve A of ~igure llB. It will be noted that Curve A very nearly coincides w~th Curve D which represents the ~deal three reduction reactor , p~rformance without added ~ethane. Thus this system is an ¦ except~ ~lly effective e~bodL~en~ of the i~nt~on.
I 10 Figure 1~ represents a two~reactor counterpart of ¦ the syste~ of Figure lLA ~n that methan~ is added to the feed ~ gas to ~ach of the primary and sec~ndary reactors. The per-! for~ance of this system with 47O by vol~me methane added to the ~eed gas to each of the reduction reactors is illustrated by Curvè A of Figure 12B. A comparison of Curves A and B of Figure 12B shows that the addition o methane produces a 8ubs~antial ~provement in per~ormance in this system.
`, Figure 13A illustrates a system having an out-o-line cooling reactor and only a single reduction reactor with methane added to the feed gas to the reduction reactor and recycle of effluent gas around the reduction reactor. Sincé only a s m gle j reduction reactor is used, a high recycle ratio is required.
'~he performance of this system with 8% added methane and 50%
by vo~ume recycle is illustrated in Curve A. A comparis~n of , 25 Curves A and B of Figure 13B shows that by using a co~binatio~

;, , .
1077Z~8 of me~ ne a~di~ion and ~as recycle! i~ ;s possible to achie~e . ~7ith ~ singLe reduction reactor a perfoxmance super~or to that yielded by a two-reduction reactor system operating withou~
added methane or gas recycle.
- ~hile all nf the systems o~ Figures 4~ through 12A
show in-line cooling reactors, it should be understood that out-of-line coolin~ reactors can be used in such systems with the adva~tages not~d above.
From the foregoing descrlption ~t should be appa~e~t 10 that the present inver~ion provides a method for the gaseous reduction of irorl ore capable of achieving the several objectives set forth at the begi~ning of the present speci~ication. ~y adding minor amounts of methane to the eed gas streams fed to the reduction reactors, the amount of reformed gas required to effect a given metall~zation of a gi~en amount o ore can be substankially reduced. By using a combination of added methane and spent reducing gas recycle, the requirement for reformed gas can be still further reduced. This economy in the require-ment for reLormed reducing gas permits a decrease in the size o. the refor~r required to produce a g~ven amount of sponge iron and hence decreases the initial invest~2nt required for a .~
plant of a given production capacity. This desirable result is achieved with little, if any, increase in the operating cost o~ the system.

~ ' - ""' .' ~ .

. .

~077'~7~

The addition of methane to the reformed gas fed to the reduction reactors is especially important in the case of a three reduction reactor system wherein channelling of the gas in the reactor and hang-up~ of the reduced ore upon discharge have heretofore made utilization of a three reduction reactor system impractical. By using methane addition in accordance with the present invention such a three reduction reactor system becomes commercially practical, and thus it becomes possible to take advantage of the economies inherent in the use of such a three reduction reactor system~
As particularly shown in Figures 3, 9B and lOB, the improved reducing gas economy obtained by adding methane to the feed reducing gas to the reduction reactors can be still further improved by using a combination of methane addition and spent gas recycle. It is difficult to pinpoint the precise manner in which the added methane and ~pent gas cooperate to provide the improved reducing gas economy obtained with this embodiment of the present process because of the fact that the several gas streams interact with each other and with the ore bodies of the several reactors of the system by a considerable number of different reactions, most, if not all, of which have temperature-dependent e~uilibria.
While we do not wish to be bound by any particular theory as to why this combination yields the novel result de-scribed above, it is our present understanding that the im-proved reducing gas economy obtained by using both methane .

10~7~7~
addition and spent gas recycle results from the production of additional effective reducing gae by reaction of the added methane with components of the re~ycled spent gas in accordance with either or both of the following e~uations:
1) CH4 + C02 -~ 2CO + 2H2 2) CH4 + H2O -~ CO ~ 3H2 The recycled spent gas still contains a certain amount of carbon monoxide and hydrogen initially formed in the reformer, as well as carbon dioxide and water vapor formed as reduction reaction products in the reduction reactors of the system. By mixing minor amounts of this spent gas and a hydrocarbon gas such as methane with the cooling react.or effluent gas and heating the resulting mixture, it appears that a substantial amount of useful reducing constituents, i,eO, carbon monoxide and hydrogen, can be regenerated in accordance with the foregoing equations.
It is, of course, to be understood that the foregoing description is intended to be illustrative only and that numerous changes can be made in the disclosed systems without departing from the scope of the invention as defined in the appended claims.
Thus the feed streams to systems of the type disclosed may vary rather widely as to composition and source~ Considering first the feed stream to the catalytic reformers of Figures 1 and 2, the presently preferred feed is methane or natural gas. However, other gaseous hydrocarbons or vaporized hydrocarbons can be used.
Also gas mixtures containing substantial amounts of hydrocarbon, eOgO, purified coke oven gas can be usedO

~07 7 2~ ~

The reducing gas used may also be derived from other sources t~an the catalytic reformers illustratively disclosed.
Thus the reducing gas may be produced by partial oxidation of gaseous liquid or solid hydrocarbons with air, oxygen or natural S oxides. Where economics permit, gases composed largely of hydrogen can be used. In the appended claims the reducing gas is characterized as largely composed of carbon monoxide and hydrogen: this phraseology is intended to cover gas mixtures having a high proportion of hydrogen and little, if any, carbon monoxide.
The fluid hydrocarbon mixed with the reducing gas fed to the reduction reactors is desirably methane. However, as indicated above, it may also be a mixed gas containing a substantial amount of methane, such as for example, purified coke oven gas or natural ga~. It will be understood that where a mixed gas is used the percentage~ of added gas specified relate to the methane content of the added gas and not to the entire value of added gasO
It should further be noted that if, in the system of Figure 1, it becomes necessary to take one reactor out of service for repairs, the remaining reactors can be connected as illustrated in Figure 2 and production can thereby be main-tained while repairs are being made to the reactor that is out of serviceO
Other modifications will be apparent to those skilled in the art.

Claims (27)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for the batchwise gaseous reduction of metal oxides to metals in a multiple unit reactor system of the type in which separate fixed beds of metal-bearing material are simultaneously treated in a plurality of interchangeable reactors including a cooling reactor and at least one reduction reactor, which method comprises preparing a preformed reducing gas composed largely of carbon monoxide and hydrogen outside of the reactor system causing at least a portion of said preformed reducing gas to flow successively through the bed of metal-bearing material in said cooling reactor and then through the bed of metal-bearing material in said reduction reactor or reactors, heating said reducing gas before it is fed to each said reduction reactor or reactors, cooling and de-watering said gas after it leaves each said reduction reactor or reactors, and after said reducing gas has passed through said cooling reactor mixing a small amount of fluid hydrocarbon with the reducing gas fed to at least one reduction reactor of said system prior to the point at which said reducing gas is heated, to increase the productivity of said reduction system.

20. A method according to claim 1, wherein said system comprises a single reduction reactor and a portion of the effluent gas from said reduction reactor, after cooling and de-watering, is re-heated and recycled to said reactor.

3. A method according to claim 2, wherein from 25%
to 75% by volume of the effluent gas is recycled.

4. A method for the batchwise gaseous reduction of metal oxides to metals in a multiple unit system of the type in which separate fixed beds of metal-bearing material are simultaneously treated in a plurality of interchange-able reactors including a cooling reactor and a series of reduction reactors, which method comprises preparing a preformed reducing gas composed largely of carbon monoxide and hydrogen outside of the reactor system causing at least a portion of said preformed reducing gas to flow successively through the bed of metal-bearing material in said cooling reactor and then through the beds of metal-bearing material in the reduction reactors of said series, heating said reducing gas before it is fed to each reactor of said series, cooling and de-watering said gas after it leaves each reactor of said series and after said reducing gas has passed through said cooling reactor mixing a small amount of fluid hydrocarbon with the reducing gas fed to at least one of said reduction prior to the point at which said reducing gas is heated, to increase the produc-tivity of said reduction system.

5. A method according to claim 4, wherein a portion of the effluent gas from at least one of said reduction reactors, after cooling and de-watering, is reheated and recycled to said reactor.

6. A method according to claim 4, wherein a portion of the effluent gas from at least one of said reduction reactors, after cooling and de-watering, is reheated and recycled to a preceding reduction reactor of said series.

7. A method according to claim 4, wherein a small amount of fluid hydrocarbon is mixed with the reducing gas fed to each reduction reactor of said series.

8. A method for the batchwise gaseous reduction of iron oxide to sponge iron in a multiple unit reactor system of the type in which separate fixed beds of iron-bearing material are simultaneously treated in a plurality of interchangeable reactors including a cooling reactor and a series of reduction reactors, which method comprises pre-paring a preformed reducing gas composed largely of carbon monoxide and hydrogen by catalytic reformation of a mixture of methane and steam passing said preformed gas success-ively through the bed of iron-bearing material in said cooling reactor and then through the beds of iron-bearing material in the reduction reactors of said series, heating said reducing gas before it is fed to each reactor of said series, cooling and de-watering said gas after it leaves each reactor of said series and after said reducing gas has passed through said cooling reactor mixing a small amount of gaseous hydrocarbon with the reducing gas fed to at least one of said reduction reactors prior to the point at which said reducing gas is heated, to in-crease the productivity of said reduction system.

9. A method according to claim 8, wherein said gaseous hydrocarbon is mixed with the reducing gas fed to the first reduction reactor of said series.

10. A method according to claim 9, wherein a portion of the effluent gas from the last reduction reactor of said series, after cooling and de-watering, is reheated and recycled to the first reduction reactor of said series.

11. A method according to claim 8, wherein said gaseous hydrocarbon is mixed with the reducing gas fed to the second reduction reactor of said series.

12. A method according to claim 11, wherein the effluent gas from the second reduction reactor of said series, after cooling and de-watering is reheated and re-cycled to said second reduction reactor.

13. A method according to claim 8, wherein said gaseous hydrocarbon is mixed with the reducing gas fed to the last reduction reactor of said series.

14. A method according to claim 8, wherein said gaseous hydrocarbon is mixed with the reducing gas fed to each of the first two reduction reactors of said series.

15. A method according to claim 14, wherein the effluent gas from said first reduction reactor, after cool-ing and de-watering, is reheated and recycled to said first reduction reactor.

16. A method for the batchwise gaseous reduction of iron oxide to sponge iron in a multiple unit reactor system of the type in which separate fixed beds of iron-bearing material are simultaneously treated in a plurality of interchangeable reactors including a cooling reactor and a series of three reduction reactors which method com-prises preparing a preformed reducing gas composed largely of carbon monoxide and hydrogen by catalytic reformation of a mixture of steam and methane, passing said preformed gas successively through the bed of iron-bearing material in said cooling reactor and then through the beds of iron-bearing material in the three reduction reactors of said series, heating said reducing gas before it is fed to each of the three reduction reactors, cooling and de-watering said gas after it leaves each of the three reduction reactors and mixing with the reducing gas fed to at least the first of said three reduction reactors, after it has passed through said cooling reactor and prior to the point at which it is heated, from 2% to 12% by volume of a gaseous hydrocarbon based on the volume of said reducing gas.

17. A method according to claim 16, wherein gaseous hydrocarbon is added to the reducing gas fed to each of the three reduction reactors.

18. A method according to claim 16, wherein a portion of the effluent gas from at least one reduction reactor, after cooling and de-watering, is mixed with the reducing gas fed to one of said reduction reactors, the volume of recycled effluent gas being from 3% to 30% of the volume of said reducing gas.

19. A method according to claim 4, wherein the fluid hydrocarbon is methane.

20. A method according to claim 4, wherein the fluid hydrocarbon is natural gas.

21. A method according to claim 4, and wherein a portion of the preformed gas from said source flows to said cooling reactor and the remainder of the preformed gas from said source flows to the first reduction reactor of said series through a by-pass around said cooling reactor to reduce the pressure drop between said source and said first reduction reactor.

22. A method according to claim 6, and wherein a portion of the preformed gas from said source flows to said cooling reactor and the remainder of the preformed gas from said source flows to the first reduction reactor of said series through a by-pass around said cooling reactor to reduce the pressure drop between said source and said first reduction reactor.

23. A method according to claim 4, wherein the fluid hydrocarbon is purified coke oven gas.

24. A method for the batchwise gaseous reduction of iron oxide to sponge iron in a multiple unit reactor system of the type in which separate fixed beds of iron-bearing material are simultaneously treated in a plurality of interchangeable reactors including a cooling reactor and a series of reduction reactors, which method comprises preparing a preformed reducing gas composed largely of carbon monoxide and hydrogen outside said reactor system, causing at least a portion of said preformed gas to flow successively through the bed of iron-bearing material in said cooling reactor and then through the beds of iron-bearing material in the reduction reactors of said series, heating said reducing gas before it is fed to each reactor of said series, cooling and de-watering said gas after it leaves each reactor of said series and after said reducing gas has passed through said cooling reactor mixing a small amount of fluid hydrocarbon with the reducing gas fed to at least one of said reduction reactors prior to the point at which said reducing gas is heated, to increase the productivity of said reduction system.

25. A method for the batchwise gaseous reduction of iron oxide to sponge iron in a multiple unit reactor system of the type in which separate fixed beds of iron-bearing material are simultaneously treated in a plurality of interchangeable reactors including a cooling reactor and a series of three reduction reactors, which method comprises preparing a preformed reducing gas composed largely of carbon monoxide and hydrogen by catalytic reforma-tion of a mixture of steam and fluid hydrocarbon, causing at least a portion of said preformed gas to flow successively through the fixed bed of iron-bearing material in said cooling reactor and then through the fixed beds of iron-bearing material in the three reduction reactors of said series, heating said reducing gas before it is fed to each of the three reduction reactors, cooling and de-watering said gas after it leaves each of the three-reduction reactors and mixing with the reducing gas fed to at least the first of said three reduction reactors, after it has passed through said cooling reactor and prior to the point at which it is heated, from 2% to 12% by volume of a fluid hydrocarbon based on the volume of said reducing gas.

26. A method according to claim 24, and wherein the heated reducing gas fed to said one reduction reactor is mixed, prior to its entry into said one reactor, with an oxygen-containing gas to cause at least a portion of the hydrocarbon content thereof to be converted to carbon monoxide and hydrogen.

27. A method according to claim 16, and wherein the heated reducing gas fed to said first reduction reactor is mixed, prior to its entry into said first reactor, with an oxygen-containing gas to cause at least a portion of the methane content thereof to be converted to carbon monoxide and hydrogen.