The Rate of Reduction of Iron Ores with Carbon Monoxide Effect of Particle Size, Gas Velocity, and Gas Composition W. H. WETHERILL AND %. C. FURNAS, Yale University, New Haven, Conn. Two new methods of calculating fhe rate of metallization from gas analysis data (ire developed. The probable mechanism of the reaction is indicated. New data on the effect of gas velocity, gas composition, and particle size upon the rate of metallization are obtained. The relative rate of reduction and metallization is shown to increase as density decreases. This confirms the work of others. The hypothesis that the metallization zone advances into the ore particle at a constant linear rate is well substantiated. The rate of advance of the metallization zone is shown to be roughly proportional to the square root of the gas velocity. The eflect of gas composition upon the rate of metallization is shown for two ores. The rate of metallization decreases as the
T
HE reduction of iron ore is probably second only to the combuvtion of carbon in importance to industry. Saturally E,uch a reaction has received a great deal of attention from many investigators in the past. Almost all of the work upon iron ore reduction has, however, taken the form of a study of various aspects of the blast furnace and its operation. Because of the great complexity of the processes which a blast furnace carries out, it is difficult to obtain specific data upon the rates of a single reaction from the study of blast furnace data. Moreover, the greater part of such data has been concerned with the fuel requirements under various conditions or with the effect of operating variables upon the quality of the pig iron produced. ‘Thus the rates of the reactions have received little attention. More recently, considerable attention has been given to the low-temperature reduction or “sponge iron” process. In such low-temperature reduction processes, the reactions of slag formation and melting are absent, so the results might be used in calculating the rate of the reduction reactions. As a matter of fact, however, most of the available data on sponge iron processes are not sufficiently complete to give fundamental information on actual rates of reduction. Most of the laboratory investigations on the rate of reduction of iron ores h:tve been carried out under conditions that have little relation to practice, and thus the data obtained are not applicable to the design or operation of commercial equipment. KISETICFOF HETEROGENEOUS REAC‘rIoNs In a single-phase, or homogeneous, reaction the reaction velocity is approximately proportional to the “concentration distance” from equilibrium of the reactants. The actual chemical reaction is the only process involved. In a heterogeneous reaction, such as is encountered in the reduction of iron oxide, the conditions are quite different. Besides the chemical reaction, there is also the transfer of materials across a phase boundary and diffusion of one phase
carbon dioxide concentrat ion increases. Increasing the percentage of carbon dioxide has a very different effect upon different ores. The effect of particle size is shown for three ores. The rates vary with size so that the time for complete metallization is practically independent of particle size over the range studied. All three sizes were below 1 cm. in diameter. The time required to reach the composition ferrous oxide is given for the various ores, particle sizes, and gas velocities. The data are not extensive enough to make a n y prediction as to the mechanism of the first two steps of the reaction. I n general, the same factors which came rapid metallization cause a rapid reduction to ferrous oxide. through another. Thus, in the reduction of iron oxide the over-all reactions consist of five processes in series: diffusion of the reacting gas through the gas film and through the solid, the chemical reaction, and diffusion of the gaseous products out through the solid and the gas film. Under a given set of conditions there will be a definite concentration gradient across each of these “resistances.” There is no simple relation between the over-all rate of reaction and the concentration gradient. As in the case of heat transfer where there are several resistances in series, one resistance may be large relative to the others and will thus be the controlling factor. The effect of changing some one condition upon the over-all rate will depend upon which of the processes is the controlling one. Thus, if gas film diffusion were the controlling factor, increase in gas velocity would have a great effect upon the over-all rate, whereas if the chemical reaction or solid diffusion were the controlling factor, increase in gas velocity would have almost no effect upon the over-all rate. In the same way, if gas film or solid diffusion were controlling, a change in temperature would cause only a relatively small change in the rate because diffusional coefficients change only slightly with temperature. If, however, chemical reaction were controlling, change in temperature would have a very great effect upon the rate of the reaction. Thus in heterogeneous reactions, while the equilibrium conditions naturally set the limits, they are not necessarily the chief factor in the control of the reaction velocity as they are in the case of homogeneous reactions. The mechanism of the reaction within the solid ore particle can be fairly well predicted. Langmuir (9) has shown that in a gas-solid reaction which involves two solid phases, giving a gaseous product, the gas evolution takes place a t the phase boundary. If it can be assumed that, a t the time metallization starts, the ore particles have the uniform composition ferrous oxide, then the reaction will occur a t the phase boundary between iron and ferrous oxide. I n the case of iron and iron oxide,
983
INDUSTRIAL AND ENGINEERING CHEMISTRY
984
solid solutions are formed so that the phase boundary, and thus the reaction zone, will be a narrow band of finite thickness instead of a sharp line. This reaction band will appear first a t the surface and advance radially inward toward the center of the particle. Work on other reactions, particularly the I'
a
CAUSTIC SOLUTION
# a
FIGURE1. APPARATUS FOR STUDY OF RATEOF REDUCTION OF IRONORES
calcination of limestone and the roasting of sulfide ores, also shows this same reaction mechanism. It is thus seen to be rather general for gas-solid reactions.
Vol. 26, No. 9
a t temperatures around 500' to 600' C . and the rate increases rapidly as the temperatureis increased. Above 1000" C . the temperature has less effect. The chemical composition of the ore has a marked effect a t low temperature, the ores high in slag-forming constituents showing low rates of reduction. At the higher temperatures, chemical character of the ore has almost no effect upon the reaction rate. All investigators have shown that the physical character of the ore has a great deal of influence on the reduction rate. Several properties have been used as a means of correlating relative rates of reduction for different ores. I n general the dense, impervious ores are reduced slowly while the light porous ores show rapid rates of reduction. There is no such complete agreement in the matter of the effect of gas composition upon the rate of reduction. It is generally recognized that the rate of reduction decreases as the concentration of carbon dioxide in the reducing gas increases. Concerning the quantitative relation, however, the different authors disagree. The work of Bone, Reeve, and Saunders (8) would indicate that the rate of reduction is inversely proportional to the carbon dioxide concentration, Isibi and Hirano (6) indicate in their work that the rate decreases much more rapidly as the carbon dioxide content increases. Stalhane and Malmberg (18) give an equation relating reduction rate and percentage of carbon dioxide. The equation is rather complex and, while it fits their data very well, requires several constants that would be difficult to evaluate and would have little significance for general application.
REVIEWOB PREVIOUS WORK This paper is confined entirely to a consideration of the reactions between iron oxides and carbon monoxide. The equilibrium conditions in the reaction of carbon monoxide with iron oxides have been investigated by many authors among whom may be mentioned Bauer and Glassner ( I ) , Matsubara (11),Eastman (4),Schenck ( l 7 ) ,and Bone, Reeve, and Saunders ( 2 ) . Ralston (14) gives an excellent summary of the work done up to 1929. Although slight differences occur in the values of the equilibrium constants obtained by the different investigators, the general conditions are well established. The r e duction of iron oxide by carbon monoxide is shown to proceed in three steps: 3Fe20, CO -+ 2Fe804 COS (1) Fe80c CO e 3Fe0 COS (2) FeO CO e Fe COZ (3)
+ +
+
+
+
+
Reaction 1 is almost irreversible and will proceed in the direction indicated in the presence of small traces of carbon monoxide. Reactions 2 and 3 are definitely reversible. The equilibrium value of carbon dioxide concentration is considerably higher for reaction 2 than for 3. There are no sharp breaks between the three steps, for the solid phases show distinct solid solubility in each other. I n this paper the first two steps (1 and 2) are treated together as "reduction to ferrous oxide." The last step (Equation 3) is referred to as "metallization." Among the investigators who have studied the rate of the reduction of iron ores and sinters may be mentioned Bone, Reeve, and Saunders @), Meyer (I!?), Diepschlag and Zillgen (S),Isibi and Hirano (8), Stalhane and Malmberg (18), Kamura (8), Joseph, Barrett, and Wood ( 7 ) , Eastman ( 5 ) , Weinert (I@, and Royster, Joseph, and Kinney (15). From the results of the work done with carbon monoxide, the effect of temperature and physical character of the ore on the rate of the reduction reaction has been qualitatively established. The reaction rate is shown to increase considerably as the temperature rises. The reduction is very slow
FIGURE 2.
TYPICAL RUN IN STUDY OF RATEOF REDUCTION OF IRONORE
Material Port Henry einter through 6 on 10 mesh; depth of bid, 10 cm.; rate of gas flow, 18.0 standard liters per sq. cm. per hour; average temperature, 823O C.
The effect of gas velocity upon the rate of reduction has received very little attention. As far as could be determined, no quantitative results have been described, and only a few writers have even mentioned the fact that the rate did increase with increasing gas velocity. The effect of particle size, again, is a subject upon which there is some uncertainty. The general opinion is that with fine particles, up to 1 cm. in diameter, particle size has no effect upon the time required for complete metallization. With the larger size, the time for complete metallization, particularly with dense ores, does increase as the particle size is increased. Stalhane and Malmberg (18), working with a magnetite ore, show that the time for complete reduction is about proportional to the diameter in the range of sizes from 1 to 4 cm. Kamura (8), working with a hematite ore, shows that the time for complete reduction was doubled when the particle size had been increased about 3.5 fold. Relative to the assumption that the ore particles had the uniform composition ferrous oxide a t the time that iron first appeared, Diepschlag and Zillgen (3) show that for all par-
September, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
ticles under 1 cm. practically no ferric iron is present in samples which contain metallic iron. This indicates that the supposition that the particle is entirely ferrous oxide before the metallization begins is correct for the small particles. Stalhane and Malmberg (18) state that three zones--iron, ferrous oxide, and ferroferric oxide-were present in all ore particles during the reduction, but their particles were all over 1 cm. in diameter. Thus the hypothesis that the particle is entirely ferrous oxide at the start of the metallization period has a sound basis if the largest particles are below 1 cm. Concerning the progress of metallization, Stalhane and Malmberg (18) state that, for both hydrogen and carbon monoxide reduction, the iron zone advances radially at a constant rate from the surface towards the center. With all fine-grained ores the distinction between the zones is very sharp. I n the case of very porous ores the distinction becomes less sharp. Weinert (19) states that the metallization starts from several points within the lump and proceeds outward from these points. Most of the evidence, however, supports the Stalhane and hlalmberg conclusions (18). I n any study, whether of equilibrium or rates, in which carbon monoxide is involved, the reaction, 2CO c COa (4)
= +
must be considered. This reaction is important a t all temperatures between 250" and 750" C. It has a maximum rate around 450' C. Iron or iron oxide6 are excellent catalysts for this reaction. Bone, Reeve, and Saunders ( 2 ) have shown that for temperatures below 600" C. carbon deposition limits the extent of reduction which can be reached with any given ore. The carbon deposition increases as the stage of reduction increases until eventually all of the carbon monoxide is being used up in the decomposition reaction. Meyer (12) also shows that crirbon deposition increases as the stage of reduction advanceu. Bone, Reeve, and Saunders (Z), Meyer ( I d ) , and Diepschlag and Zillgen (3) all show that the carbon deposition increases greatly as the reducibility of the ore d e creases.
SCOPEOF PRESENT INVESTIGATION
985
cent of the total reduction time being required for this last step. A few data were also obtained on the length of time required for the first two steps (Equations 1 and 2). The variables studied were gas velocity, gas composition, particle size, and type of ore. The gas velocity was varied from 1.34 to 200.0 liters per sq. cm. per hour. The reducing .IW
402 .IW
i u
wo
f
ow 4s4
0
3
2
492 QDO
o
IO
x)
Y)
40
M w 70 eo TIME I N MINUTES
w
100110
ILO
FIGURE 4. TYPICALCALCULATION BY SECONDMETHOD (RUN51) Material Moose Mountain briquet through 6 on 10 mesh; gas velocity 8.47'atandard litera per 0q. om. per hour; average temperature, 824' b.
gases were various mixtures of carbon monoxide and dioxide dried over calcium chloride. Three particle sizes were used: 14 to 20 mesh, 6 to 10 mesh, and 3 to 4 mesh (Tyler screen sizes). The average screen openings were 0.09, 0.216, and 0.575 cm., respectively. Four ores were used : a natural Mesabi ore, a briquet made from Moose Mountain ore, a sinter made from a concentrate of this same Moose Mountain ore, and a material known as Port Henry sinter, The analysis of the ores is given in Table I.
APPARATUB AND EXPERIMENTAL PROCEDURE
The general arrangement of the apparatus is shown in Figure 1: The electric furnace was 2 feet (61 cm.) long with a uniform nichrome winding. The reduction tube was made from a section of one-inch (2.5-cm.) standard iron pipe. The inlet and outlet tubes were 0.25-inch (0.635-cm.) brass pipe. The length of 5.0 the one-inch iron section vaned according to the depth of bed which was to be used. Most of the runs were made with an 8inch (20.4cm.) bed although a few runs were made with 4inch 4.0 (10.2-cm.) and 12-inch (30.5cm.) beds. The section of oneinch pipe was about 3 inches (7.6 cm.) longer than the depth of the bed. 5 p 3.0 The brass inlet tubes were necessary because in entering and leaving the furnace the gas passed through the 250" to 750' C. 5 temperature range in which there is extensive carbon deposition, as shown by Equation 4. If iron pipes were used, they quickly 5 2.0 became clogged with carbon. Carbon deposition reaction did not occur to any detectable extent on the brass surface. The furnace was hinged along the back to facilitate the inser$ 1.0 tion of the reduction tube. The reduction tube itself was made with unions on each end. The current to the furnace was regulated by a water-cooled rheostat. Temperatures were measured by a chromel-alumel thermocouple which was inserted to the center of the ore bed inside the reduction table. The thermoFIGURE3. EVALUATION OF CONSTANTS FOR PARABOLIC couples had t o be renewed quite frequently. .All couples were EQUATION OF DATAOF FIGURE 2 made from a single batch of wire. Samples of these wires were calibrated against a Bureau of Standards platinum couple and The reaction involved is that of Equation 3. At 825" C. found to agree within 5' a t 800" C. the equilibrium conditions are reached when carbon dioxide By properly placing the one-inch-diameter section of the reequals 37 per cent in an atmosphere of carbon dioxide and duction tube and by inserting refractory plu s around the inlet and outlet tubes above and below this one-inokdiameter section, monoxide. From a practical standpoint this step is by far the it was possible to keep the tem rature radients to less than most important because it accounts for two-thirds of the total 5" C. in a length of 20 cm. In case ofthe 30.6-cm. beds the oxygen removal and because its rate is slow, about 90 per temperature gradients were about 20' C. The purpose of the present investigation is to fill in some of the gaps left by the previous experimenters, but it has been practically limited to a study of the reduction of ferrous oxide by carbon monoxide a t one temperature only, 825" C.
tg
INDUSTRIAL AND ENGINEERING CHEMISTRY
986
The carbon monoxide was formed by the decomposition of formic acid in the presence of phosphoric acid a t about 200" C. The gas was scrubbed with sodium hydroxide and stored in a 50-cubic-foot (1.4cubic-meter) gas holder. During each run the exit gas from the reduction tube was passed through the sodium hydroxide scrubber to remove the carbon dioxide formed, and the carbon monoxide led back t o the holder.
Vol. 26, No. 9
At the end of the run the sample was allowed to cool in the furnace, but, owing t o the small amount of carbon monoxide in the tube a t any one time, there was no appreciable reduction after the gas stream had been stopped. Because the reduced material is subject to reoxidation, it was cooled practically t o room temperature before it was removed from the t,ube.
In practically all cases the material had jammed in the tube and had to be dug out with a pointed steel rod. Thus, there was considerable disintegration of the lumps of ore in removing them from the tube.
I
hfETHODS OF CALCULATION
TIME IN M I N U T E S
FIGURE 5 . CALCULATED REDUCTION CURVES Material, hloose Mountain briquet; g&s velocity, 13 2 standard liters per sq. cm. per hour
The gas was analyzed in a thermal-conductivity gas analyzer similar t o that described by Palmer and Weaver ( I S ) for carbon monoxide-dioxide mixtures. The cells were calibrated t o read directly in percentage carbon dioxide and were checked by frequent Orsat analysis. Samples of the ore after the reduction were analyzed for metallic iron by the hydrogen evolution method as described by Martin (IO). I n making a run, the general procedure was as follows: The tube wab charged with 2 inches ( 5 cm.) of crushed fire brick. A cop er screen was placed over the brick and the desired depth orore was added. The thermocouple was inserted from above into the center of the ore bed. The crushed brick distributed the gas stream evenly over the cross section of the bed and heated the incoming gas t o the temperature of the bed before it reached the ore. After charging, the tube was placed in the furnace and connected to the gas line. The temperature of the furnace was regulated by adjusting the current. About 2 hours were required to bring the temperature up to the desired value of 825" C. Because of the large heat capacity of the furnace and reduction tube the starting of the gas stream had very little effect upon the temperature. 60
I
I
I
I n working up the data of this investigation, three methods of calculating the rate of metallization were used. Two of these were based on a consideration of the analysis of the entrance and exit gases. The third method was based upon the analysis of the solid material a t the end of the reduction period. FIRSTMETHOD. On the basis of the assumed reaction mechanism-namely, that the particles approximate spheres, that the particle is entirely ferrous oxide when metallization begins, and that the metallization occurs in a narrow band which advances at a constant linear rate from the surface t o the center of the particle-a new method of calculating the rate of advance of the metallization zone has been developed: Let R = radius of the unreduced zone; then dR/dT = K Volume of ore reduced per unit time = ~HRZK (5) Liters of CO, formed Der unit time = 22.4 4 r N K (R1 - K T ) ' S I K8 (6) (7)
where N
= = =
No. of particles in the bed
weight of ore in the charge sp. gr. of original charge % ' total iron in ore = 100 = rate of advance of reduction zone, cm./hour = initial radius of particle, cm. = time, hours In the experimental data, the percentage and not the total amount of carbon monoxide is known: % COz = (volume of COz formed X 10O)/gas volume flowing Therefore, from Equations 6 and 7: I (R1 - K T ) z % COz = 120.4 m -K (8) V Ri where Ti = gas velocity, standard liters/hour TI Let K I = 120.4 3VR Equation 5 then becomes: % COZ = KKzR12 - 2KgKzRlT + K'KZT' (9) W S I K Rl T
Equation 9 is a parabola in t e r m of T
\DISTANCE U P THE BED I N INCHES 0
1
2
3
4
5
6
7
I8
FIGURE. 6. TIME REQUIREDFOR REDUCTIOX TO FERROUS OXIDE Material, hloose Mountain briquet; gas velocity, 13.2 standard liters per s q . ern. per hour
The gas velocity was regulated by adjusting the by-pass value on the compressor or by adjusting the value on the outlet side of the reduction tube. The constancy of gas velocit was noted by watching the slant gage connected around the orizce on the inlet t o the reduction tube. The total volume of gas flowing in a given run was read from the dry gas meter.
.
Thus, if the percentage of carbon dioxide plotted against time is a parabola, it indicates that the metallization band does advance a t a constant rate. I n a large number of cases the data confirm this hypothesis very well. As a n example, the data of a typical run are shown in Figure 2. The curve beyond the ferrous oxide point appears to be a parabola. A siniple method used for evaluating constants of empirical equations - (16) will show that this is correct. = Y - Yo,where Xoand Yoare Let X = X - Xa and the coordinates of some arbitrary point on the curve. The plot of ( Y / x ) vs. will be a straight line if the original curve values for the is a parabola. I n Figure 3 the ( F / x ) vs. curve of Figure 2 are given. Although the points spatter somewhat, it is obvious that a straight line fits them better than any other simple curve.
x
September, 1934
INDUSTRIAL
The slope of the ( y ,2)vs.
PLND E N G I N E E R I N G C H E M I S T R Y
x line ia the value of the constant,
R 3 = R13 [l
c, in the general parabolic equation:
Y =a
+ bX
987
2.49 V
- rJ(1' - Y,)d T ]
(16)
4- c X 2
In Equation 9, however, c is equivalent to K3K'!. Thus:
where L
=
4
LTIR13 120.4I the slope of the (P/x)vs. =
line
The fact that, in computing the and 2 values, an arbitrary point Xo and YOwas used is of great advantage in the application of the method. If the values of Ywere shifted, which is equivalent to adding a constant amount to the percentage of carbon dioxide, the calculated rate of advance of
From the data of a run the values of R1, 1', TP, and I are known. The values of L T ( Y - Y QdT ) are obtained by evaluating the area under the Y - Y CVS. time curve, up to the time, T. Thus R, the radius of t'he unreduced zone, can be calculated for any time. The time to ferrous oxide is calculated from the gas analysis by an oxygen balance. This value is taken as the zero time for determining the area under the ( Y - YO)vs. T curve. An example of a calculation b y this method is shown in Figure 4 and Table 11.
TABLEI. ANALYSISOF ORES LOSS O N
SP. GR. MOISTURE IGNITION Fez03 % %
hkTERIAL
3.25 4.25 4.86 4.7s
hlesabi Mooee Mt. briquet
M o o s e Mt. %inter
Port Henry sinter
1.30 0.0 0.0 0.0
5.3(1 0.0 0.0 0.0
81.0 81.5 68.8 68.2
FeO
Si02
1In304
CaO
1IgO
rilzoa
SOa
P2Oa
0.0
7.43 11.25 0.10 3.99
1.05 3.31
3.11 0.44
0.08 0.19
2.51 1.46
0.03 0.02
0.045 0.032
3:35
1:;s
0:iS
2:44
T'race
0:027
2.58 30.9 23.4
the metallization zone would not be affected. Thus, any constant source of carbon dioxide other than iron ore reduction would not affect the calculations. This method depends entirely on the rate of change of the gas composition. SECOND METHOD. In applying the method of calculation just outlined, some of the experimental data gave excellent results. There were a number of runs, however, in which the values of (Y/.%) vs. spattered so badly that it was difficult to establisli the best straight line. In such cases no reasonable accuracy in the calculated rate of advance could be expected. T o meet this situation, a new method of calculation was developed which would not be as sensitive to small variations in the data. This method is based upon an oxygen balance. Consider the reaction: FeO CO eFe COJ Let V = rate of gas flow, liters/hour 7= COzino the exit gas y
x
+
+
LO0
Yo
yo C02 due to entrance
= ____
gas or side
100
Rate of formation of COZ in moles/hour
reactions -_
V ( Y - Yo) (11) 22.4 Fe GO,, one mole of =-
+
+
From the equation: FeO CO COzis equivalent t o one mole of Fe: Rate of change of iron in grams/hour = 55.8 Ti ( Y
22.4 Volume rate of ore reduction = 2.49 (Y - Yo) IS 4 Total volume of ore = N 5 ?rR13 3w N = No. of particles = 4a R13S
- Yo) (12)
1
Total volume reduced at any time
=
31W
Then at any time,
TFRoMJr
GRAPH
RATEOF A\Dv.kXCE INTEGRATIOS"
CALCULhTION O F
(E'-Yl)dT
1,27'5JT
1- 1 . 2 7 5 J T
0.0710 0.0557 0.1268 0.0995 0.1775 0.1393 0.227 0.1780 0.277 0.217 0.324 0.254 0.291 0.371 Zero time for the integral limits is 60 minutes.
60 70 80 90 100 110 120
['(Y
-
Yo)dT (14)
J"
4/3uR3 = 4/3uRi3 - 4/3nRi3 49 V l T ( Y - YO)dT (15) IW d " where R = radius of unreduced zone R1 = original radius of the particle Expanding this and collecting terms,
BY
GRAPHICAL
~ l - - 1 . 2 7 5 ~ T R 0.977 0.957 0.937 0.918 0.898 0.878 0.857
In this run, gas velocity = 55.2 standard liters per hour, TV = 182.6 grams, I = 0.59, and Rl = 0.108 cm. (2.49 V ) / IW = 1.275. Since T.t7 refers to the weight of the entire bed, the gas flow is for the entire bed rather than for unit crosssectional area : R = 0.108
41 -
1.275JT ( Y
- YO)dT
The values of R vs. T give a very good straight line, from the slope of which the rate of advance of the metallization zone is shown to be equal to 0.0135 cm. per hour. COMPARISON OF THE Two METHODS.Of the two methods of calculation, the graphical integration method is seen to be more direct. It gives better results in runs in which the data vary slightly from the parabolic form. Also, the graphical integration method makes no assumption as to the mechanism of the reaction but tends to show what the mechanism actually is. It has one disadvantage, however, in that the value of YO,the percentage of carbon dioxide coming from sources other than iron ore reduction, must be known in order to obtain accurate results. The sources of carbon dioxide other than reduction were carbon deposition, carbon dioxide formed by air leaking into the system, and carbon dioxide formed in the reaction CO H20 = C02 HZ.Since the gas was dried and there was no appreciable amount of hydrogen in the exit gas, tlii3 last source of carbon dioxide was of no importance. THIRDMETHOD. The parabolic and the graphical integration methods have very definite limitations. They are both based upon the analysis of the exit gas. Since the exit gas shows the summation of all the carbon dioxide forniation in the entire bed, any rate of metallization which is calculated from the exit gas analysis must be an average rate for the entire bed. Actually, as the percentage of carbon dioxide increases in going up the bed, the rate of metallization decreases.
+
For a single particle the volume reduced = 4aR13'2'49 ___-
TABLE 11.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
988 I O
I
I
I
I
Vol. 26, No. 9
oa
2
$
07
0
y
OB
$
4 INCHES U P
g
05
@
5 INCHES U P
6 I N C H E S UP
a r
04
03
: c
02
L o
IO
I 20
I
I
30
40
PERCENT
I
I I 50 60 UETALL1ZATION
70
I ao
so
I
loo 01
FIGURE7. RATIOOF (TIMETO FERROUS OXIDE)/(TOTAL OF PERCENTAGE OF METALLIZATION TIME)AS A FUNCTION 50
I
IO
20
30
IO
50 60 70 ao T I M E IN MINUTES
so
100
110
120
FIGURE 9. THICKNESS OF METALLIC ZONE us. TIME All values are corrected to a common baais of 13.2 standard liters per sq. om. per hour.
\
10
0
1
o
1
2 3 4 5 DISTANCE UP THE BED IN INCHES
FIGURE 8. SOLIDANALYSIS OF RUN 63 Material, Moose Mountain briquet through 6 on 10 meah: weight of ore, 176.6 grams:. length of run. 17.0 minutes; gad veloclty, 62.9 standard liters per sq. om. per hour
The difficulty of changing gas composition could have been avoided by using very short beds. In such a case, however, the change in composition between the entrance'and exit gas would have been so small that the analytical errors would have been greatly magnified. T o eliminate these discrepancies, a third method of calculation was devised, based upon the analysis of the solid material at different levels in the bed. At the end of each run, samples were taken from six different levels. The distance which the metallization zone has advanced at any time is a simple function of the percentage of metallization of the sample, as is shown by the following derivation:
GAS VELOCITY
STANOARD u T i R s PER
saw
PER HWR
FIGURE10. RATEOF METALLIZATION vs. GAS VELOCITY
subtracted from the total time of the experiment. In order to compute the time required to reach the composition ferrous oxide, two assumptions were made: (1) the time to reach ferrous oxide us. the distance up the column is a straight line; (2) the ratio (time to reach ferrous oxide)/(total time) is a function of the percentage of metallization only and is independent of position in the bed, gas velocity, etc. This latter assumption implies that any factor which increases the metallization rate will increase the rate of reduction to ferrous oxide by a proportional amount. Figure 6, which was obtained by taking the time for 0 per cent metal and plotting against distance up the bed as obtained from the extrapolated curves of Figure 5, shows that the first a s s u m p Volume reduced = 4/3rRla - 4/3irRa (18) Volume reduced = 4/3rRla M (19) tion is essentially correct. The curves of Figure 5 were also R1 = radius of the particle, R = radius of unreduced ore used to compute the data of Figure 7 which confirms the % metallization second assumption. In this figure the ratio (time to reach M = *LVU nn ferrous oxide)/(total time) is plotted against percentage of , etallization = % metallic iron x 100 metallization for various positions in the bed. Although yototal iron the data spatter, there is no consistent variation with posi4/3irR1~- 4/3rR3 4/3aRla M tion in the bed and the points may be best represented by a single curve. D=R.To illustrate the use of this method, consider Figure 8 where D = distance the metallization zone has advancd which gives the solid analysis for run 63. The metallization a t the top of the bed is 4 per cent and a t the bottom is 42 per (22) D = R1 (1 - 4 1- M) cent; the length of the run is 17 minutes. From Figure 7, Figure 5 shows the experimental results of a number of nhen percentage metallization = 42 per cent, the ratio runs which were made to determine the relation between (tinie to reach ferrous oxide)/(total time) = 0.29. Therepercentage of metallization and time for various levels in the fore tinie to reach ferrous oxide at the bottom of the bed = 5.0 bed. The original data points are not shown here, as the minutes. The percentage of metallization a t the top of the results had to be corrected to a common gas velocity as will bed = 4 per cent, from which the ratio (time to reach ferrous oxide)/(total time) = 0.80. Therefore time to reach ferrous be explained later. TIMEREQUIRED TO REACHTHE COMPOSITION FERROUSoxide at the top of the bed = 13.6 minutes. Since the time OXIDE, To obtain the rate of advance, the time during which to reach ferrous oxide varies linearly up the bed, the average metallization was taking place must be known. Thus the time to reach ferrous oxide would be 9.3 minutes. From an time required to reach the composition ferrous oxide must be oxygen balance based upon the gas analysis, the average-
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
September, 1934
989
time to reach ferrous oxide is 9.1 minutes. This is a satis- different levels in the bed. Although the points spatter somewhat, it can easily be seen that the lines are for the most factory check. From the total length of the run (17 minutes in the case part substantially straight. In the series of runs from which of run 63) the time to reach ferrous oxide a t each level in the Figure 9 was computed, the same material and particle size bed is subtracted. From the total distance advanced a t were used in all cases. All rates of advance were corrected each level (calculated from the solid analysis by Equation to a common gas velocity by assuming that the rate of ad22) and the time since metallization started, the average vance of the metallization zone is proportional to the square rate of advance of the metallization zone can be determined root of the gas velocity. The correctness of this assumption will be shown later in this article. for each position in the bed. CALCULATION OF THE GAS COMPOSITION TN THE BED. From the curves of percentage metallization VS. position in GAS VELOCITY I N STANDARD L I T E R S the bed, i t is possible by an oxygen balance to calculate the PER SQ.CM. PER HR. values of percentage in carbon dioxide in the gas for all positions in the bed. The bed is divided into equal zones so that a sample is taken from each zone for solid analysis. From -t the average percentage metallization and the weight of ore in each zone, the 1,otal carbon dioxide formed in that zone during the metallization period can be determined. From the gas velocity and the length of the run after ferrous oxide had been reached for each zone, the total amount of gas which passed through each zone during metallization can be 0 I 2 3 1 5 6 7 0 9 IO II I2 13 PCPCLNT CO2 determined. From the carbon dioxide formed and the total gas flowing, FIGURE12. RATEOF ADVANCEOF REDUCTION ZONE 1's. the average increase in percentage of carbon dioxide caused PERCENTAGE CARBON DIOXIDEFOR PORTHENRYSINTER (THROUGH 6 ON 10 MESH) by the gas passing through each zone is determined. The average percentage of carbon dioxide a t any point in the bed Thus the constancy of the rate of advance of the metallizais equal to the percentage of carbon dioxide in the entrance gas plus the summation of the increments of percentage car- tion zone is shown by the results of calculations both from the bon dioxide due to all the zones below the point in question. gas analysis data and from the solid analysis data. The curves of Figure 5 were computed from the straight The resultant curve of percentage carbon dioxide us. position in the bed represents the average conditions for each point lines of Figure 9 by means of Equation 22. This means of deriving the curves of Figure 5 was used because the data during the entire metallization period. Thus, for each run for which solid analyses are available, of Figure 9 had been corrected to a common gas velocity SO both the rate of advance of the metallization zone and the that the percentage metallization us. time curves computed average percentage of carbon dioxide can be determined for from t k m would be on a common basis, whereas the original data points were obtained a t slightly different gas velocities. each level in the bed. EFFECTOF GAS VELOCITY.I n Figure 10 the rates of metallization for the different materials and particle sizes EXPERIMENTAL RESULTS are plotted against gas velocity. For the Meaabi ore, the CONFIRMA4TION O F THE REACTIONMECHANISM. The Moose Mountain briquet, and the Moose Mounta n sinter parabola method of calculating the rate of advance of the the rate of advance of the metallization zone increases apmetallization zone was based upon the assumption that this proximately as the square root of the gas velocity. In the case of the Port Henry sinter the effect is somewhat differ0 RUN 61. GAS VELOCITY 222.5 STD. L / SQ.CM./HF. ent. Over low ranges of gas velocity the rate of advance 5 2 . 9 STD. L / SQ.CM./HFI. R U N 6 1 GAS VELOCITY increases with gas velocity. At about 50 liters per sq. cm. 36.9 STD.L/SQ.CM./HFI.l RUN71 GASVELOCITY per hour the rate of advance approaches an asymptote, and further increase in gas velocity has no further effect upon the rate of advance of the metallization zone. This change d in the effect of gas velocity, in the case of the Port Henry f 08 "I sinter, is probably due to the shift of the controlling process from diffusion through the gas film to diffusion through the y solid as described in the section on the kinetics of hetero3 < geneous reactions. ; The curves of Figure 10 were computed from the exit gas i 02 analysis by the graphical integration method. As mentioned in the discussion of those methods, the rates so computed are average rates for the entire bed. Owing to chang ng gas 0 2 4 8 1 0 1 2 1 1 6 1 8 velocity, the percentage carbon dioxide in the exit gas and PERCENT COz THE also the average percentage carbon dioxide in the bed will FIGURE 11. R 4 T E O F ADVANCEO F REDUCTION Z O V E VS. vary from one run to another. As gas composition has a PERCEVTAGE C A R B O N DIOXIDE IN THE GAS FOR M O O S E definite effect upon the rate of advance of the metallization ~'IOUNTAIN BRIQUET (THROUGH 6 ON 10 MESH) zone, this will affect the results of Figure 10, as will be dislinear rate of advance was constant. The results of the cussed later. calculations of the rate of advance by the graphical integraCOMPARISON OF THE DIFFERENT ORES. The comparative tion method showed that this rate actually mas constant rates of metallization for the different ores are best shown (Figure 4). in Figure 10. Considering the 6 to 10 mesh particle size, the In Figure 9 the results of a group of runs in which the cal- Mesabi ore shows the highest rate, the Moose Mountain culations were based upon the results of solid analysis are briquet is slightly less, the Port Henry sinter is considerably shown. This figure shows distance advanced us. time for lower, and the Moose Mountain sinter is slightly below the
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CAS
IYDUSTRIAL AND ENGINEERING CHEMISTRY
990
Vol. 26, No. 9
Port Henry sinter. The ores thus follow the usual order of decreasing rate of reduction with increasing density. The relation between rate of metallization and density is not linear. With data on only four materials, no general quantitative relation between metallization rate and density can be made. li 0 8
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FIGURE 13. RATEOF ADVANCE OF REDUCTION ZONE us. PERCENTAGE CARBON DIOXIDEFOR MOOSEMomTAIN BRIQUET
EFFECTOF GAS COMPOSITIOS.I n Figures 11 and 12 the effect of gas composition upon the rate of advance for constant screen size and different gas velocities is shown for Moose Mountain briquet and Port Henry sinter. For both materials the rate decreases as the percentage of carbon dioxide increases, but the shape of the curves is different for the two materials. I n neither case is the relation linear. The data are hardly consistent enough to warrant the development of equations for the curves of rate of advance us. percentage carbon dioxide. The Port Henry sinter gives slightly more consistent results than does the Moose Mountain briquet. The data for rate of advance of reduction zone vs. percentage carbon dioxide for different particle sizes but constant gas velocity are plotted in Figures 13 and 14. EFFECT OF GASVELOCITY UPON THE RATEOF ADVANCE OF THE METALLIZATION ZONE AT 0 PERCENTCARBONDIOXIDE. I n Figure 15 the rates of advance of the metallization zone for 0 per cent carbon dioxide are plotted against gas velocity. Data are shown for Moose Mountain briquet and Port Henry sinter. These curves were computed by taking the values a t 0 per cent carbon dioxide from Figures 11 and 12. The shapes of the curves are the same as those of Figure 10. The Moose Mountain briquet shows a rate of adirance which increases approximately as the square root of the gas velocity. For the Port Henry sinter, the rate first increases with increasing gas velocity and then approaches an asymptote. Further increase in gas velocity has no effect upon the rate. COMPARISON OF THE DATAOF FIGURES 10 AND 15. The data of Figure 10 were computed from the analysis of the entrance and exit gas by the graphical integration method. Thus the rates shown are the average rates for the entire bed. Since the exit gas contained carbon dioxide, these rates are for some average carbon dioxide value and might be expected to be lower than they would be if no carbon dioxide were present. The data of Figure 15 were taken from an extrapolation to 0 per cent carbon dioxide of data from Figures 11 and 12. As would be expected, they show higher rates of advance than does Figure 10. EFFECT OF PARTICLE SIZE. The effect of particle size upon the rate of metallization is shown by Figures 10, 13, and 14. In all cases the rate of advance of the metallization zone, for a given gas velocity, is shown to be approximately proportional to the diameter of the particle. This implies that, over the range studied, the time for complete metallization would be independent of particle size. This unusual effect is difficult to explain. The rate is
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FIGURE 14. RATE O F ADVANCE OF REDUCTION ZONE CARBON DIOXIDEFOR P O R T HENRY SINTER
VS. PERCENT.4GE
Gas velocity, 28.6 standard liters per sq em. per hour
I n comparing the different ores, the order of rapidity of reduction to ferrous oxide is the same as that for rate of metallization. This is the result that would be expected, since the character of an ore which caused a rapid rate of metallization would also cause a rapid rate for the first two steps of the reduction process. For the Mesabi ore and the Moose Mountain briquet, the time required for reduction to ferrous oxide for all three particle sizes was the same for a given gas velocity. For the Port Henry sinter this relation did not hold. The time required for reduction to ferrous oxide was about one and onehalf times as great for the 3 to 4 mesh size as it was for the 6 to 10 mesh size. NUMERICAL EXAMPLE.The relative importance of the time required to reach the composition ferrous oxide compared to the time for metallization is shown by the following ex-
991
HEoucnoN ZONE us CARBON
1iiylir:r aut/ l ~ ~ w ctcrnpcratiirw r woiilil be valnable. Further investigxiiims of 21 wider range of partic,Se sizes and gas ver d t,l:e gas velocity into the Iwities :mi riceilcil. Incre range wed i n Gl:%st Eiiriiscc prar:t,iit! ~ w u l dgive useful results. At h i g h gas velocities the hlcsalii ore and the Moose Mniintain ixiqiiet. would pmliaijly approat:ii a maximum rate similar to that shown by the l'ort I h r y sinter a t 50 liters per sq. cin. per lionr. A further aturly of tire eflcct of gas couilmsition is also dcsirnble. The range uf mripusiLion of carbon monoxidodioxide niixturcs sliouid be inrreaseil. Gas mixtures coutaining various percentages of inert gas, such as nitrogen, should be studied. Finally the effect of sinall amounts of hydrogen and water vapor on tlie reaction sliould be given some attention.
I)IoxrDE i\CKNUWLEDG\lRWl
2~111plc: The iroii oxide in a lump of Mesabi ore 0.5 cm. in
diameter is to be reduced to 95 per cent metal in a gas stream The authors wish to express appreciatiun tu R. W. Cox for having a low percentage of carbon dioxide and a gas \~elooity the samples of Moose Mountain briquet and Moose Mountain oi 100 liters per sq. em. per hour at a temperature of 825" C. sinter used in this investigation; to the i3ethlchem Steel ComI-iiiler these conditions the time to reach ferrous oxide is pany for tlie Mesabi ore; and to tile Witherbee, Sherman Company for the samples of Port Henry sinter. LITERATURE CITED (1) I%m~or and Glilasner, 2. p h y ~ i k Cham. . 43, 354 (1903). (2) Bono, Reeve, and Saimilers, J . Iron SleclInst., 115. 127 (1927); 121, 36 (1930). (3) Diepsohiap, Ziilgen. and Pootter, Staid U. Eben, 52, 1154 (1932).
(4) EiLstman, J . Am. Chom Soc., 44, 975 (1922). (5) Eastinan. U. S. Sur. of Mines, Rcyl. of Znvcrtioalzon 2485 (June. 19Y3). I&i and Hirimo. Testu Waoane, 18, 913 (Sept., 1932). Joseph, Barrott, tLnd Wood, Blavt Furnace Steel Plant, 21, 147, 207, 260. ail, 336 (1933). Kamurn. Trana. Am. Insl. Mining M e l . E%'nym.. 71, 549 (1925). 1,angmuir. J . Am. Cheni. Soc., 38, 2263 (1916). Martin, H. G.. Ihid., 29, 1211 (1907). Matnubme. ?'?an% Am. I r ~ s l .Miniiio M e t . Bny7s.. 67, 3 (1922). hfeyer, it. H., .iriu. ~ ~ i ~ ~ ~~ - ~~~ ~ii i~~ h ~ to ~a ~i.a s ~ f e~ ~ ~ do& 10,107 (1928). Painter arid Weaver, U. S. Sur. Stmdsrds, Teoh. Paper 249
,
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shown by Figure 16 to he 8 minutes. For 95 per cent metallization the zone must advance G2 per cent of the radius, or for a radius of 0.25 cm. the total distance to be advanced is 0.155 cm. (Equation 22). The rate of advance of the metallization znne is shown by the 3 to 4 mesh curve of Figure 10 to be 0.145 em. per hour. Time for 95 per cent metallization is equal to 64 minutes, while the time for reduction to ferrous oxide under the same conditions is only 8 minutes.
(14) Raluton, U. 5. Bur. Minos, Bdl. 296 (1929). (15) Roystcr, Joseph. and Kinney. Blast Furnace Steel Plant, 12, Z5, 98, 154. 200, 246, 274 (1924). (16) Running, T. R., "Ernpirid Fonnulhs." p. 21. John Wiley and Sons. Now York, 1917. (17) Sohenck,Slahl u. Biun, 46, G65 (1926); Z.anorg. d l ~ e mCham., . 164, 145 (1927). 166. 113 (1927). (18) Stdhano and Mdrnberp, SliLhZ U . Eiaen, 49, 1835 (1929); 50, 909 (1930); 51, 716 (1931). (19) Weinert. Arch. Ebanhilllenw., 7, 275 (1933).
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