but the taconite samples appeared to be dense single particles. This conclusion was verified to some extent by some exploratory runs with -325 mesh Laborador hematite. Reduction times were of the same magnitude as those obtained for taconite. X-ray analysis of taconite concentrate partially reduced a t 450" C. showed a trace of wustite; however, it was much less than that detected in reduced Fe203.
longer tube should be constructed, which would give more reliable data for temperatures as low as 300' C. Detection of wustite in material reduced a t 450' C. suggests that this phase may also be present during reduction of a large lump of ore a t temperatures below 570" C. I t might be possible to verify this if a method were devised to provide a n extremely rapid quench for specimens as soon as they are removed from the reducing atmosphere. O n the commercial scale. one of the drawbacks encountered with fluidized beds is sintering of the iron. which seriously interferes with fluidization. By operating at low temperatures. sintering can be avoided, but reduction is extremely slow Sintering, like reduction, is a rate process which is influenced by temperature. I t may possibly be avoided during reduction in the flow reactor, because a particle could conceivably be reduced and leave the reactor before appreciable sintering can take place. Further, because the solid phase is in streamline flow and very dilute. there is less tendency for particle interaction in the flow reactor than in a fluidized bed.
Discussion I t is particularly interesting that for Fez03 which has been largely reduced to magnetite, a new period occurs where reduction rate is accelerated. The reason for this is not clear; however, it should be pointed out that good data were difficult to obtain a t the lower temperatures where this effect is pronounced. When a low gas velocity was used little product was obtained, even after long periods of operation. Because of this, accurate analysis was very difficult and per cent reduction reported for runs a t the lower temperatures is more uncertain than that for substantially higher temperatures. A reactor with a much
I
1. G. DALLA LANA' and N. R. AMUNDSON University of Minnesota, Minneapolis, Minn.
(Reduction of Powdered Iron O x i d e )
A Tubular Reactor Here, the previous study is extended, but more attention is given to complicating factors which result from large particle sizes
IN
THE PREVIOUS work on reactions of hydrogen with pigment grade Fe?Ol and taconite concentrate, analysis of the reaction kinetics was simplified because the relative geometries of particles and tube could be overlooked. Use of extremely fine particles, about 0.3 micron in diameter, eliminated many experimental difficulties, including the determination of particle velocities, because the dispersed solidsgas mixture could be assumed to possess the flow characteristics of a single
phase. Particle residence times and the kinetic data would not suffer appreciable error from factors such as varying particle diameters, the nature of the velocity profile in the tube, and heat and mass transfer limitations (7). In studying gas-solids reactions, the tubular reactor has certain advantages over other more conventional experimental techniques. With iron oxide powders and a pure hydrogen phase
Present address, Department of Chemical and Petroleum Engineering, University of Alberta, Edmonton, Alberta, Canada.
22
INDUSTRIAL AND ENGINEERING CHEMISTRY
,Conditions may approach those for isolated single particles ,Residence times as low as one second are possible
Acknowledgment Gust Bitsianes gave freely of his time to perform the x-ray diffraction work and aided in many other aspects of the project. literature Cited (1) Bitsianes, G., Joseph, T. L., J . Metals
7 , 639-45 (May 1955).
(2) Chufarov, G. I., Averbukh, B. D., Z. physik. Chem. (Lezpzig) B33, 334-48
(1936). (3) Edstrom, J. O., J . IronSteelIns!. (London) 175, 289-304 (1953). (4) Kolthoff, I. M., Sandell, E. E., "Quantitative Inorganic Analysis" (3rd ed.), Macmillan, New York. 1952. (5) Lloyd, W. A., Ph.D. thesis. Univ. of Minn.. Minneapolis, Minn., 1954. (6) Moreau. J., Bardolle, J., Benard, J., Rez,. met. 48, 486-94 (1951). (7) Specht, 0. G., Zappfe, C. A., Tram. Am. Inst. M e c h . Engrs. 167, 237-80 (1946). f8\ Tennebaum. M.. Sauarcv. C M.. Am. Iron, Steel Inst. Ygarbook, pp. 20840, 1951. (9) U. S. Bureau of Mines, Rept. Invest. 4092, June 1947. \
I
RECEIVED for review February 6, 1960 ACCEPTED October 14, 1960
,Use of pure hydrogen eliminates particle surface mass transfer resistances ,Sensitive and reproducible kinetic data may be obtained with small amounts of a powder b Product sintering can b e eliminated
Changing the experimental conditions from those employed in the first study adds further complexities. Increasing the particle diameters may result in a significant velocity of the particle relative to the gas phase. Flow as a single phase is no longer a reasonable assumption. A powder containing a range of particle diameters results in a distribution of residence times. A gas velocity profile with a n appreciable gradient near the wall combined with transport of the particlesin a radial direction within the tube may produce convective particle paths. Particles may be entrained in the center of the tube, or they may settle near the walls. If they are large and move axially upwards, their inertial mass may suppress radial transport from gas turbulence. Particle-particle and particle-wall collisions may also produce radial transport and such effects can be minimized only by entraining with very small solids to gas ratios. Radial transport of particles has not been studied extensively and, in this work, it has been neglected. This article describes studies similar in scope to those described in the previous
POWDERED IRON OXIDE report, but more attention is given to some complicating factors occurring \vith A large larger particle diameters. tubular reactor and a powder of known size distribution were employed. Ethane adsorption surface-area measurements were obtained for feed and selected product powders. Excess hydrogen and small solids to gas ratios (.5 to 25 grams per standard cubic foot) were employed to eliminate mass transfer resistances and to minimize temperature lags in the flowing mixture. Calculated heat rransfer rates for the particles from 2 to 22 microns in diameter indicated that the time required for heating cold particles to the reactor temperature Tvould be negligible for any residence time that \vould probably be encountered in the scope of the study. Endothermic effects from the chemical reaction were considered negligible: based on heat transfer calculations.
was placed within a steel shell, 2 feet in diameter, filled Lvith vermiculite and flushed with nitrogen gas. Powder feed rates were controlled by v a q i n g the speed of a small horizontal screw reeder. Tests indicated that no measurable attrition of the poivder occurred within the feeder. Chemicals. T h e hydrogen gas used
in all experiments was purchased commercially in cylinders with a capacity of 200 standard cubic feet. T h e ferric oxide poivder :c.P. grade, Fisher Scientific Co.) could not be used as received because of the extremely small particles and their tendency to form verv large cumbersome aggregates. Preliminary treatment involved screening through a
To vent
Apparatus. Basically, the reduction of Fen02 powder was performed by continuous entrainment of the particles lvith hydrogen gas flowing upward in a vertical tubular reactor externally heated. The feed streams consisted of a portion of the total hydrogen flow carrying the cold oxide particles a t room temperature, and a second preheated hydrogen stream comprised the remainder of the total flo~v. The t\vo streams merged in the throat of a \'enturi nozzle located a t the entrance to the heated reaction zone. After passing through the reaction zone the gassolids mixture was partially cooled in a Lvater-jackcted section of narrobv-diameter pipe. The particles Tvere removed from the gas in a small efficient cyclone, the gas being vented. Temperatures were measured with four Alumel-Chrome1 thermocouples encased in l;'d-inch Type 304 stainlesssteel wells. Four wells were axially placed within the reactor, two from the inlet end and two from the outlet end of the reactor tube. Each pair of wells measured a temperature near the tube end and another temperature 1,,'3 of the distance into the tube. The wells were sealed, except the lower thermocouple which extended bare from the well tip to increase its sensitivity to transient temperatures. Any one of the thermocouples could be read continuously or all thermocouple readinqs could be recorded within a recorder cycle of 8 seconds. An isothermal reactor temperature profile was produced by maintaining a constant electrical current flow to each of the ten identical 2-foot long heaters. The entire reactor and heater assembly
Thermocouple Pressure gauge
Expansion
Reduction valve jackel Cyclone
relief valve
co1ie:tc.r
i Shell vent
R e o c t o r gas storage
Experimental
a 8
L
Reactor flow meters
Prehaat
flow
Solids pickup
i'
Manometer
flow
L.-
Variable ; p e e d l screw f e e d e r
&\
Vibrator
The reactor was made from a 26-foot length of 2-inch Schedule 40 pipe, Type 304 stainless steel. Of this length 20 feet was covered with heaters and 16.1 feet comprised the reaction zone. Hydrogen entering through an externally heated side arm, was preheated to a temperature higher than the reaction temperature. However, when the two streams were combined in the Venturi nozzle, the mixture was at reaction temperature
With this process, investigated first with finely powdered iron oxide in a small reactor and then with coarser particles in a 76-foot reactor, the sintering usually encountered in fluidized beds c a n perhaps be avoided. Using a prepared ore, about 9570 reduction was obtained in a remarkably short time, and it i s conceivable that a particle c a n be reduced and leave the reactor before appreciable sintering c a n occur. Also, the tendency for particle interaction i s decreased because the solid phase i s dilute and suspended in gas undergoing streamline flow
VOL. 53, NO. 1
JANUARY 1961
23
35-mesh sieve followed by firing the dry powder a t 1100' C. until suitable mild sintering had occurred. This technique obtained by modifying a wet firing method ( Z ) , formed a cake which easily crumbled into a free-flowing powder with a wide size distribution. By screening and air elutriation. a feed powder with a size distribution ranging from 2 to 22 microns was obtained (Figure 1) and prepared in batches which were combined and homogenized in size distribution with a V-blender. The iron content of the powders averaged
68.8670. Operation. Procuring experimental data with the reactor was largely a question of controlling the equipment heaters so that the desired temperature steady-state existed prior to the entry of the gas-solids mixture. 'Transient temperatures and nonisothermal reaction rates were eliminated by suitable control of the hydrogen preheat. When a suitable temperature steady state was obtained Lvith hydrogen alone flowing, the powder screw feeder was started. After a brief trial operation, a series of product samples were separated a t the cyclone. The control of the test was checked by collecting and analyzing the product from each of several successive test periods from 3 to 5 minutes long. If satisfactory operating conditions existed and if the chemical analyses indicated no differences between products of successive periods, the test was regarded as being reliable. Auxiliary Techniques. Particle size distributions for feed and products were obtained with centrifugal sedimentation equipment ( 7 7). Surface area measurements were obtained by measuring the adsorption of ethane using a standard adsorption apparatus. Linear adsorption curves were obtained for feed and products by using the BET adsorption isotherm. The reoxidation of reduced products before being subjected to chemical analysis was minimized by filling powder collection bottles with nitrogen. Longterm storage protecrion was obtained by acetone-washing followed by airdrying. The amount of chemical reduction was determined by chemical analysis for total iron content before and after reduction. 4 standardized ceric sulfate solution was employed as the oxidizing agent. Residence Times. Reynolds numbers, based upon the gas phase flowing through a 2-inch tube a t the reaction conditions, never exceeded 600. These conditions should produce flow in the viscous regime. If it is assumed that the presence of small quantities of oxide particles does not significantly alter the parabolic velocity profile of the gas, a powder residence time may be estimated. The calculations involve the assumptions that radial transport or par-
24
ticles can be neglected and that settling of particles near the wall occurs without re-entrainment. If the particle velocity upward is - u,,, where U, is velocity equal to of the gas a t the radial position, 7 , and U,,,~, terminal velocity of a spherical particle of diameter, d,, settling in stagnant h>drogen a t reaction conditions, its residence time mav be assumed equal.
where L is length of the vertical tubular reaction zone. The powder residence time, Z, mav be approximated by averaging arithmetically the values of ti for all particles at all radial positions if the dispersion of the powder in the gas is uniform a t the tube inlet. The calculations were performed ( 4 ) by integrating across the tube radius for each d,, and then arithmetically averaging mass-weighted values of t , corresponding to small mass-fraction intervals. An equation \vas thus obtained:
Further simFllification for the case when U , the bulk gas velocity, exceeds all t i n a L .yields
in tvhich k represents the number of mass fraction intervals used in the averaging. Equation 3 was used for residence time calculations with the feed particle-
INDUSTRIAL AND ENGINEERING CHEMISTRY
size distribution. The use of this equation for particle diameters exceeding 30 microns may lead to large error because values of umi become large and Equation 1 may become indeterminate a t large residence times. The calculated residence times are believed reliable even though the presence of a thermocouple well in the reaction zone of the tube would create a more complex velocity profile. Comparison of calculated residence times for a parabolic velocity profile with similar calculations for a plug-flow velocity profile revealed minor differences with the feed po\vder employed. Results
Chemical reduction data were obtained as a function of time a t constant temperature and pressure. A series of such runs was performed at each of four temperatures: 4505, 500'. 550'. and 600' C.. and a t a constant pressure of approximately 1250 mm. of mercury (Figure 2). Reduction above 600' C. could not be performed in the tubular reactor Lvithout jeopardizing the equipment. Residence times were varied from 2 to 13 seconds. Comparison of the size distributions for the reduced products Lvith that of the 2- to 22-micron feed po\vder indicated that particles exceeding 16 microns did not appear in the products. Cpon examining the reactor geometry, it \vas round that some of the feed poivder had settled in the reactor assembl\- prior to entering the reaction zone. Accordingly: the kinetic data Lvere based upon a feed poivder size distribution conform-
99.9 r 99.8 -
99.5 99 98 -
95 -
5
9
-
90-
Figure 1. Particle size distributions for feed and products reduced at 550" C. Ferric oxide powder was mildly sintered into a cake which was easily crumbled Feed ( 2 to16microns) - into a free-flowing From this Feed (2to 22 microns) - powder. powder a feed was - separated b y screening and air elutria- tion
80-
-b
70-
E
60-
0
3
50;
0
.-
40 30
3
20-
h
-
L
a" > ._ *
IO
-
s
5-
5 0
2-
-
l -
05 -
c/
0.2 O l d /
2
'
4
I
6
I
I I
I
I
8 IO 12 14 16 Particie Diameter - microns
I
I8
4
I
I
20
22
1
POWDERED I R O N OXIDE ing to the 2- to 22- micron feed but with all particles exceeding 16 microns elirninated. This distribution [Figure 1). agrees closely Ivith those of the products. Sintering of the particles was believed absent because the product size distributions did not appear to change. T h e curves in Figure 2 are quite consistent and reproduce the t)pe of phenomena observed with very fine po\vders in a small reactor (6). S o evidence of an induction period was noted. contrary to statements of other ivorkers in the field. .It loxver temperatures, the reduction rate becomes very slo~vand could be misinterpreted as being an induction time effect. Other Lvorkers (5) studying initial reaction temperatures lvith batch reduction tests concluded that 1'5" C . is the approximate loxcer temperature for discerning occurrence of chemical reaction. IVith the tubular f l o i v reactor and the powder used. reduction at temperature below 400' C. \vas negligible because of insufficient residence time capacit! . T h e rate of reduction for the 2to 16-micron ferric oxide poivder was less than that obtained by Lloyd and Amundson (7) \vith the very fine ferric oxide averaging 0.3 microns because of the lo\z-er specific surface area. Their results also suggested a discontinuity in the reduction data at small residence times and temperatures of 550' C. and lo~ver. This discontinuit>-could also be interpreted as a n initial dec;rease in reaction rate which becomes more evident at lo\ver temperatures. Figure 2 may be interpreted in this latter fashion for the
450' and 500' C. reduction curves but by no means conclusively. The reduction rates then increased ver)rapidly until a limiting degree of reduction was obtained. T h e existence of a minimum in this limitin3 degree of reduction with respect to temperature was confirmed 16. 8 ) . Evidence exists ( 3 , 70) that H,O vapor is strongly adsorbed on an Fe80J surface a t 400' to 500' C. T h e initial reaction step involves the change, Fes04, and the suggested inFe2C)3 itial decrease may result from water vapor Lvhich covers the Fen04 surface and thus impedes further chemical reaction. Surface areas \vere also measured for the feed and products obtainecl at 550' C. T h e specific surface area for the feed is slightly low because it \vas determined for a 2- to 22-micron po\vder. whereas the products contain 2- to 16micron sizes as discussed previously. T h e very rapid increase in surface area shown in Figure 3 parallels thy accelerating reaction rate u p to a limiting value. T h e constancy of the particle diameter distributionj for all product po\vders suggests that the over-all particle diameter is not altered appreciably by chemical reduction, hence the porosity must increase markedly. Consequently, the chemical reaction zones move into the particle interior as the residence time increases. T h e initial reaction velocities. as obtained by measuring the limiting slope of the reduction curves when t + 0. are applicable to the reaction of the Fez03 phase with hydrogen. If the initial reaction is chemically ratecontrolled, the specific velocity constants. r, for the reaction should be proportional to the reaction velocities measured. By correlating this data as a function of temperature using a n Arrhenius-type equation.
-
7
I
3
,
1
IO Veon h.1055 - Weighted Residence lime, 2
4
6
8
- - 12)
(1)
(3)
path. F e ? 0 3 -+ FeaOk FeO Fe. when the temoerature exceeds 570' C., (4)
(5)
and the path, Fe?On Fe30r Fe. below 570' C., the wuslite phase being ' 5 '. thermodynamically unstable below 0 All of the reaction steps are endothermic and AH' values at 600' and 550' are
Reaction 1
2 3
AH'T, Kral. / G . Mole 0.96 10
4
22 0.6
5
20.5
Temp..
c.
600 600 600 550 550
For endothermic chemical reactions, the heat of reaction must be equal to or less than 1:he activation energy required for the reaction at the given temperature. Comparing the value of E* at t = 0 with the AHoT data for Reactions 1 and 4, shows that E* is considerably larger. O n the other hand, a t t = t , the E* value is lower than AH', for Reactions 3 and 5 but approximately equal to that for Reaction 2. If the initial reaction is chemically rate-controlled, the mechanism changes to a lolver activational energy rate-
(4)
, 12 i - seconds
Figure 2. Reduction curves for coarser ferric oxide powder in the large reactor are consistent and similar to those for very fine powder in a small reactor. Contrary to reports b y other workers, no induction period i s discernible Pressure, 1260 mm. of mercury
= (constant) e - E * , R T
the activation energy. E*, may be estimated. Slopes were also measured on the steep linear portion of the reduction curves. although these regions on different curves represent differing chernical compositions. T h e applicability of such a procedure was based upon the supposition that identical reduction mechanisms may occur a t this point. (The slopes have been replotted as fractional change in weight, w ,w o ~with time.) T h e activation energy a t t = 0 measured 19.5 kcal. 'gram-mole and a t t = t. measured 9.8 to 12.3 kcal. granimole (Figure 4). T h e reacrion sequence follo\vs the
70
y
0
i4 2
L
6
3
C
1
2
1
4
t -seconds
Figure 3. The increase in surface area parallels the increase in reaction rate up to the limiting value
Figure 4. Determination of activation energies. Slopes were measured on the steep linear portions of the reduction curves VOL. 53, NO. 1
JANUARY 1961
25
100
300
,
I
I
I
I
I
,
,
I
I
I
400
500
600
700
800
900
Temperofure
,
I
1
IO00
1100
1200
I
I
1
O C
Figure 5. Up to 570" C., the per cent of water needed for equilibrium between ferric oxide and reduced iron increases (6). Above 570" C., however, the wustite phase appears and complicates the reduction process
controlling process such as might occur from a combination of mechanisms involving transition from a chemical regime to a diffusional regime. This analysis is conjectural without further evidence. I t is known however that the reduction is topochemical in nature ( ? ) . The increased porosity and movement of the reduction zones into the particle interior could create suitable conditions for growth of a gas-phase diffusional regime. A few reduction tests were performed at 550' C. and a lower pressure of 1010 mm. of mercury. Comparison with the data a t 1250 mm. of mercury indicated no difference in the reduction curves during the interval, t = 0 to t = 4 seconds. At values of t = 4. the reduction rate increased slightly a t the higher pressure. The data are fragmentary but support the view that gas-phase diffusion may become the dominant mechanism. Proposed Mechanism
The experimental results strongly indicate that the initial controlling process in the hydrogen reduction of an entrained Fez03 powder is the chemical reaction on the particles' surface. This chemical regime gradually changes with increasing degree of reduction and a diffusional regime develops, eventually becoming dominant. Within this framework the other phenomena Ivhich were observed may be explained. The zone of reaction from its initiation to where the reduction curve rises linearly may be considered to fall within the chemical regime. Surface phenomena account for the shape of the curves-e.g., reaction suppression by H 2 0 adsorption on FesOl a t lower reaction temperatures and reaction acceleration from increases in surface
26
INDUSTRIAL AND ENGINEERING CHEMISTRY
available for further reaction. \Vhen the particle exterior is reduced and its core remains to be reduced, the reaction rate becomes controlled by transport of hydrogen or water vapor along particle pores. I n this region, the reaction rate decreases, eventually approaching a constant limit. I f hydrogen and water vapor pass through the pores by molecular diffusion: their relative tendencies to diffuse may be compared. Diffusion coefficients may be estimated empirically but, in general? experimental values are more reliable. Diffusion coefficients measured for water vapor and hydrogen passing through air a t 25" C. and 1 atm. of pressure have been reported ( 9 ) equal to 0.256 and 0.410 sq. cm. per second. During the development of the diffusional regime, the diffusion would not occur as equimolar counter-diffusion of hydrogen and water vapor. The water vapor with its loiver diffusivity Lvould tend to accumulate until a steady-state counter-diffusion process is approached. This would be the period ivhen the reaction velocity would decrease because of reduced hydrogen transfer. Calculated energies of activation could be ambiguous because chemical and diffusional processes would both be contributing, the emphasis gradually shifting towards the diffusional regime. I\'ith water vapor accumulating within the particle pores, a partial pressure corresponding to the thermodynamic equilibrium value for the reduction of one phase to another (such as FesOl to Fe) may be reached. ,4t this point, the rate diminishes markedly and the degree of reduction approaches a constant value. Thermodynamics indicates that this limiting partial pressure increases with increasing temperature for an endo-
thermic reaction, permitting increased reduction to occur. U p to 570" C., this is very definitely the case (Figures 2 and 5). T h e per cent of H20 for FejOl Fe equilibrium increases until 570' is reached and above this temperature. thc wustite phase appears and the reduction i s complicated by the presence of this additional solid phase. Bitsianes (7) noted that the rate of wustite reduction did not increase as rapidly with temperature as the reduction rates of Fez03 and Fe:Od. Because of this: the increase in extent of the wustite phase a t the Fe304 'Fe boundary above 570' was explained. The equilibrium partial pressure of HzO for Fe304 'wustite increases sharply a t 570" C. from the trend indicated for FesOd 'Fe (Figure 5). Combining these observations, the wustite phase would tend to accumulate because it is not reduced as quickly as it is formed from Fe30d. Its further reduction to Fe a t 600" cannot proceed until the H2O concentration drops from approximately 35 to 237,. This introduces a lag in the rate of reduction and a minimum in the limiting degree of reduction could occur. Subsequent temperature increases again increase the limiting degree of reduction because of the positive temperature coefficients for both chemical and diffusional regimes. Acknowledgment
Gratitude is extended to V. E. Denny for his assistance in equipment construction. Literature Cited (1) Bitsianes. G., Ph.D. thesis, Cniv. of
Minnesota, Minneapolis, Minn., 1951.
( 2 ) Bitsianes, G., Univ. of Minnesota,
Minneapolis, Minn.. private communication.
( 3 ) Chufarov, G. I., Averbukh, B. D., Tatievskoya, E. P., Antonov, V. K., Zhur. Fiz. Khim. 28, 490 (1954).
(4) Dalla Lana, I. G.: Ph. D. thesis, Cniv. of Minnesota, Minneapolis, Minn., 1958. (5) Komarov, V. A . . Drozdova, V. M., Shif. G. .4., Ser. Khim. Nauk. No. 10, 79 (1951). (6) Lloyd, \V. A , , Ph.D. thesis, Cniv. of Minnesota, Minneapolis. Minn., 1954. (7) Lloyd. W. A , , Amundson, N. R., IND.ENG.CHEM.5 2 , 19 (1961). (8) Matoba, S., Otake, Y . , Nagasawa: Y., Bull. Research Inst. Mineral Dressing and .Met. (Sendai) 9,91 (1953). (9) Perry. J. H.? (ed.) "Chemical Eng-ineer's Handbook," p. 539, McGraw-Hill, New York, 1950. (10) Tatievskoya, E. P.. Chufarov, G. I., J . Phys. Chern. (U.S.S.R.) 10, 747 (1937).
(11) Whitby, K. T., Heating, Piping, .4ir Conditioning, 27, 1, 139 (1955). RECEIVED for review July 5, 1960 ACCEPTED October 28. 1960 Work supported by research grants from the University of Minnesota Graduate School.
