Recovery of Metallic Iron from Flotation Tailings by Pneumatolytic Transport I. Reduction by Carbon Monoxide in Fluidized Bed Chung S. Kim,’ Chin S. Rhee,* Kun Li,3 and Robert R. Rothfus D e p a r t m e n t of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, Pa. 15213
Pneumatolytic transport is a promising process for conserving natural resources by recovering valuable metallic iron as a product from iron ore flotation tailings presently disposed of as a solid waste. As the first step this finely divided material was reduced by carbon monoxide in a fluidized bed. The particles were not observed to react topochemically. The reduction data a t pressures up to 4.4 atm and temperatures from 593-704°C were interpreted by means of a simple two-step model which postulates that the overall rate of reaction is controlled by both chemical reaction of the homogeneous type and mass transfer between the gas and solid phases with a decreasing effective transfer area due to sticking and defluidization. During reduction, carbon deposition occurred at an approximately constant rate after an initial delay. The total carbon and combined carbon were found to increase approximately linearly with the overall reduction a t a given temperature. It is believed that the combined carbon, mainly cementite, is formed by the reaction between metallic iron and carbon monoxide and that the reaction does not play a direct role in reducing oxides. Pneumatolytic transport generally involves reaction between a metallic compound and a gaseous reactant with the formation of a vaporous intermediate product in one zone, and transport of the product vapor to another zone where it decomposes under different conditions into a gas and a final metallic product. In some cases, the same reaction is involved in both the formation and decomposition steps; in others, different reactions are involved. Pneumatolytic transport has been successfully applied to the extraction and purification of a variety of metals. Tailings from iron ore flotation processes, containing up to 40% iron by weight, are presently considered to be solid wastes. Pneumatolytic transport appears to afford an attractive means of extracting metallic iron from such materials. I t is suggested that iron of high purity can be produced from flotation tailings through the formation and decomposition of iron pentacarbonyl. The process is to be carried out in three steps: reduction of iron oxide to iron by carbon monoxide, formation of iron pentacarbonyl from the reduced iron, and decomposition of pentacarbonyl to yield pure iron as the final product along with carbon monoxide which can be recycled. Literature indicates that carbonyl decomposition is a fast step. The present study therefore deals primarily with the rates of reduction (in a fluidized bed) and carbonylation (in a fixed bed).
Literature Reuieu: Reduction of iron oxide in a fluidized bed has been a subject of interest for many years, but only a few studies are in the literature, and kinetic analyses are largely qualitative in nature. Meissner and Schora (1961) observed
that the rate of oxygen loss from the bed was directly proportional to oxygen concentration in the bed when iron ore was reduced by a c 0 - H z - N ~ mixture. The same relationship was also postulated by Okura and Matsushita (1964) for reduction of iron ore powder in Hz. Their experimental data indicated that a volume-based rate mechanism rather than a surface-based topochemical model is valid and that the particle size has no effect on reduction rate. Meissner-Schora confirmed the lack of area dependency by demonstrating that all particle size fractions had undergone the same degree of reduction. These results are in agreement with those of Ess and Wild (1966), and Feinman (1964) for fluidized bed reduction of iron ore with hydrogen. Udy and Lorig (1943) and Bogdandy and Riecke (1958) also found in fixed bed reduction with hydrogen that the time required for a given degree of reduction varied little with particle size. No study has been reported on the reduction of finely divided impure iron oxide in a fluidized bed.
Experimental M e t h o d s The starting materials used in this investigation were two different grades of flotation tailings, designated as Samples’ I and 11, supplied by Jones & Laughlin Steel Corp. Their mineralogical composition and sieve analysis are listed in Tables I and 11. These data were taken on the dried, crushed, and well-mixed samples used in actual experiments. A schematic diagram of the complete apparatus is shown in Figure 1. The reactor was made of a 71-cm long, 6.27-cm i.d. Type 304 stainless steel tube with a 6.4-mm thick Aloxite porous distributor (with average opening of 2.5 p ) at the botton. The top was screwed to a circular cover plate which also served as a flange. A cylindrical stainless steel porous filter, 15 cm long and 5 cm in diameter, was placed under and attached to the cover plate for removing particles entrained in the off gas. The reactor
Table I. Composition of Raw Materials Fe (sol.) Fe304 Fen03 SiOz AIzO3, CaO, etc. Carbona Carbonb a
Present address, Graham Research Laboratory, Jones & Lauehlin Steel Coro.. Pittsburgh. Pa. 15230. * h e s e n t address, Electronic Associates, Inc., West Long Branch, N.J. 07764. To whom correspondence should be addressed.
Sample I i
39.22 54.20 0 41.75 3.90 0.15 0.07
26.54 33.76 3.01 56.99 5.98 0.26 0.04
Before Preheating. After Preheating.
Table II. Size Analysis of Raw Materials Wt Yo
Size
1
Sample I
(mesh)
+ 200 + 230
-200 -230 f 270 -270 f 325 -325
Sample i
Sample i i
1.79 12.72 9.34 21.71 54.44
0.57 0.67 2.51 23.98 72.27
Volume 7, Number 8, August 1973
725
Results
)
7. PREHEATER F I L L E D D :REMOVAL O I XD IE COLUMN
2. MOISTURE REMOVAL 3.
ROTAMETER 4. OXYGEN REMOVAL COLUMN 51.. INSULATOR 6. RESISTANCE HEATER
Figure 1.
W I T H ALUMINA BALLS
8. REACTOR 9 . POROUS STAINLESS STEEL FILTER
10. MANOMETER 11. CHROMATOGRAPH 12 CARBONYL DECOMPOSER 13. BUNSEN BURNER
Schematic diagram of apparatus
tube fitted into an outer tube of Type 316 stainless steel, 102 cm long and 7.3 cm i.d., and was fastened to the outer tube by means of flanges with seals provided by a compressed asbestos gasket. Filled with 1-cm alumina spheres, the bottom 31-cm-long section of the outer tube was used as a preheater for inlet gas. Heating of the entire unit was accomplished by eight 15 cm long, 12.7 cm i.d. semicylindrical resistance heaters placed around the outer tube. Four of them supplied heat to the preheater section and four to the reactor section, each section being individually controlled. The heaters and about 20 cm of the reactor section were insulated with castable cement. A run was started by charging the reactor with 300 grams of ore sample, which occupied a bed height of about 7.6 cm, placing the reactor in the outer tube, and tightening the flanges. Heat was turned on with nitrogen flowing through the reactor. When the temperature reached the desired level, the flow of nitrogen was replaced by carbon monoxide whose impurities such as carbon dioxide, oxygen, and moisture were removed by pretreatment. The inlet gas temperature was measured by a Chromel-Alumel thermocouple located 1.3 cm below the reactor distributor and the temperature within the reactor by two thermocouples, 3.8 cm and 7.6 cm above the distributor. Two static pressure probes, one located 1.3 cm below the distributor and the other above the bed, connected to a manometer, measured the pressure drop across the distributor and the bed. The progress of reduction was followed by injecting a t 5-min intervals a stream of exit gas into a chromatograph column packed with silica gel as the adsorbent. Subsequent peak heights of CO and COz were recorded. For a given composition of COz, the reproducibility of peak height was *290. Fluctuations in reactor temperature and pressure, and flow rate during a run were all within 1 or 2%. At the completion of a run, usually lasting for 2 hr, the reduced material was removed from the reactor for weighing and carbon analysis. Carbon was determined by a Leco carbon analyzer, total carbon on a direct sample and free carbon on the insoluble residue left by a sample treated with dilute nitric acid. The amount of combined carbon was obtained by difference. All determinations were made in duplicate and in most cases the deviation from the average value was less than 3%. 726
Environmental Science & Technology
To achieve reasonable rates of reduction, superficial gas velocities from 1.2-9.6 cm/sec were used. These were an order of magnitude higher than the estimated incipient fluidization velocities based on equations known to be valid for large particles. The behavior of pressure drop across the bed during the reduction process generally falls into any one of the three typical patterns as shown in Figure 2 , where the ratio of pressure drop to the weight of bed per unit cross-sectional area ( A P / A P u ) is plotted against time. Type 1 shows a relatively constant pressure drop, although significantly lower than APK,,throughout a run, type 2 a continuously increasing, and type 3 a fluctuating pressure drop. The behavior of type 2 appears to be a result of continuous sticking of solid particles to each other and to the reactor walls, while that of type 3 may be attributable to periodic formation and collapsing of gas channels in the bed. Approximately 6090 of the runs follow the type 2 behavior while the remaining 40% are evenly divided between types 1 and 3. Few runs with Sample I exhibited type 3 behavior, indicating that higher iron content tends to form stable gas channels. Sample I1 with more gangue material showed a greater tendency toward type 3 behavior. In any case, the pressure drop curves indicate that the bed was poorly fluidized. Fractional reduction, defined as the fraction of the oxygen in the initial sample removed up to a given time, was calculated from the exit gas composition, under the assumption of negligible accumulation of the gas components in the bed, by the material balance
where Y d is the mole fraction of COz attributable to total carbon deposition including carbon deposited on reactor walls. A comparison of the fractional reduction from Equation 1 with the mineralogical analysis of the reduced sample from one run showed good agreement. The fractional reduction based on initial and final weights of the solid is generally 4-1570 higher than that calculated from. exit gas analysis. This corresponds to a loss of 1-4 grams of solid material out of 300 grams originally charged due to particles sticking to the reactor walls and distributor. The calculated fractional reduction for three runs is shown in Figure 3. Analysis of all the runs made in this investigation under different conditions leads to the observation that the total fractional reduction in 2 hr generally increases with increasing temperature and, with increasing pressure, increases sharply a t first and then gradually as the gas velocity increases. The data are difficult to reproduce, owing perhaps to the inherent differences in fluidization. Ten runs made under identical operating conditions showed distinctly different fractional reduction in 2 hr: six from 52-5890, three from 74-7570, and one 84%. The extent of reduction could not, however, be correlated quantitatively with the quality of fluidization as reflected in the pressure drop behavior. Qualitatively, high fractional reduction is generally associated with the pressure drop behavior of type 3. The reduced material showed sticking phenomena frequently associated with iron ore reduction in a fluidized bed (Agarwal and Davis, 1966; Bogdandy and Engell, 1971; Okura and Matsushita, 1964). It is believed that sticking is caused mainly by freshly reduced metallic iron since the sample treated by nitrogen under the same operating conditions for the same period of time exhibited no tendency toward sticking. A reduced sample (Sample 11) showed a decrease in total surface area from 1.9 m2/g ini-
1.6
l4
1 t
9 0 -superficial velocity 1.22 cm/sec 80 e 2 . 6 4 cm/sec ~ 4 . 3 cm/sec 6
1. Run No. 76 2 . Run No. 82 3 . Run N O . 9 2
70 3 1.1.
’A.
.A
1 *.....e..
O0 . 24 I
O
L 0
’ 20
I
I
I
‘
40
60
80
100
‘
1
120
TIME, MINUTES
Figure 2. Typical pressure
TIME, MINUTES
drop during reduction
Figure 3.
Reduction progress for sample I I at 704’C,3.0 atm
tially to 0.97 m2/g after 46.5% reduction. No attempt could be made to relate sticking of the bed to fractional reduction because there was no meaningful way of defining the degree of sticking quantitatively. It was observed, however, that the agglomerates formed from reduction a t 704°C were markedly denser than those a t 593°C. Appreciable amounts of carbon were deposited under the existing experimental conditions. There are three possible reaction paths for carbon deposition during reduction by cabon monoxide:
3Fe,0
+ ( 2 x + 3)CO 3Fe + 2CO 2C0
--
xFe,C
-+ c
Fe,C
+ (x + 3)CO,
+ CO,
co,
(2) (3) (4 )
Although the fractional reduction, as defined, depends only on the total amount of carbon deposited, the final state of the reduced material is considerably affected by the separate amounts of free and combined carbon. The combined carbon determined by chemical analysis is considered to be cementite. As can be seen in reactions 2 and 3, cementite can be formed from wustite or metallic iron. In the former case, each atom of carbon deposited as cementite results in the formation of (3 + x ) / x or approximately 4 moles of C02 while in the latter case only 1 mole of C02 is formed. The number of moles of C02 corresponding to the number of atoms of carbon deposited according to reaction 2 exceeded the total amount of C02 in the exit gas in most of the runs. Experimental results also indicate that a t a given level of reduction, the combined carbon deposited is independent of pressure and gas flow rate. It is therefore believed that cementite is formed by reaction 3. The experimental data indicate that the amount of combined carbon formed in 2 hr increases approximately linearly with overall fractional reduction and that carbide formation is favored by low temperature as shown in Figure 4. This is in good agreement with the findings of Stelling (1958). Generally, the amount of free carbon formed by reaction 4 is quite small compared to combined carbon.
Proposed Model f o r Reduction Development of Model. As seen in Table I1 and observed under the microscope, the flotation tailings consisted of particles of sizes from submicron to + 74 k , of compositions from almost pure silica to pure magnetite, and of shapes from spherical to needle-like. Obviously, it would not be possible to develop a model starting with the kinetics of individual particles, even disregarding the
01 0
I
1
I
I
1
I
I
I
10
20
30
40
50
60
70
80
9
REDUCTION, %
Figure 4.
Total combined carbon deposited vs. reduction, sam-
ple I1
complexity of fluidization. What follows is the development of an “overall” model for the reduction of finely divided, impure iron oxide in a fluidized bed. The reduction data of individual runs exhibited approximately the linear relationship of -In (1 - f) with time proposed by previous investigators (Boetticher et al., 1967; Meissner and Schora, 1961; Okura and Matsushita, 1964) suggesting that the rate of oxygen removal is proportional to the oxygen content of the solid. The noted dependence of the fractional reduction in 2 hr on temperature and flow rate indicates that both the chemical reaction and mass transfer resistances are important. A threedimensional stereo picture of a partially reduced sample taken under a scanning electron microscope revealed no characteristic features of topochemical reaction; thus the resistance to intraparticle diffusion of gas may be assumed negligible. Based on these observations, a model with reaction and mass transfer as controlling steps is proposed. The rate of oxygen removal by the reduction reaction is assumed to be given by the rate equation
where w is gram-atoms oxygen in the solid per gram total iron. This equation is analogous to McKewan’s (1961) except that the rate is expressed here on a mass, instead of area, basis. According to McKewan, the apparent equilibrium constant, K e , is close to that for the wustite/iron reaction, hence the thermodynamic value is used. Since the rate equation is assumed applicable to the entire mass of solid, k must be regarded as an “overall” rate constant. For mass transfer between the bulk gas and solid phases Volume 7, Number 8, August 1973
727
, *'.
22-
2.0
Ifl
-
*'.,
/'
.'r"
Run 101
18-
,*' 6
-
the specific area is not constant during reduction due to the sticking of particles and defluidization. As more oxygen is removed, the tendency for sticking increases as is evident in the pressure drop behavior of type 2. Assuming a linear decrease in the specific area with fractional reduction, the rate of transfer in terms of COn can be written
Run 97
d.'i
dncocs,
.-.-.#'.*'
Run
R u n 72
. . . . I
14-
' a . ,
R u n 41
/* R u n 55
1.0 I 120
I 140
I 160
I 200
I
180
I
I
I
I
I
I
220
240
260
280
300
320
1
f-1,
Figure 5. Analysis of reduction rate data according to Equation 9
1800k
1600k
Sample
I
Sample
1500
nearest to 25%)
1649 C
Pressure Drop
I
...' 1
1300 -
1 dnc
1
Fe,
%)(mk,a,(l
- &;it) (7 )
- f;C)(%
+
where nc is gram-moles of total carbon deposited. Equation 7 can be integrated if an expression for d n c / d t is known. The experimental data on carbon deposition show that the approximately linear relationship between combined carbon and fractional reduction beyond 25-3070 reduction (Figure 4) is also exhibited by the total carbon, since the amount of free carbon is usually a small fraction of the total. Below 25% there are few data points, but as an approximation, the total carbon deposited is assumed to be linear in fractional reduction, with the condition nc = 0 a t t = 0. Thus, the rate of total carbon deposition may be written as dnc df
e593C
1400
kPu! dt 1 -k
U
0593C 0649C A 704 C a704 C Note t Indicates Type 3
1700
(f,
(6) dt = h,a&l - f)FeTC(ys - y * ) where nco,2 , is gram-moles of C 0 2 . By means of the stoichiometric relationship among CO2, oxygen removed, carbon deposited, and a steady state mass balance for C 0 2 over the reactor, Equations 5 and 6 are combined to give _ _ _1_ _du: --1 +
dt=G
200
-
100
-
where 0 has different values below and above a certain fractional reduction f~ which is taken to be 0.25 for all runs except those with Sample I at 704°C. Upon substitution of Equation 8 and the definition u: = uo(1 - f ) , the integrated form of Equation 7 becomes 1
I
I
I
r&Fe,woR T u,AP (f -
-In(%)+
I
fJ
=
r&t - tJ (9)
where
ro
(
=
P KunRT
i?)(&),
K = 1+*-649'CiType 12
20
40
60 80 TIME, MINUTES
100
=(1
120
140
Figure 7. Comparison of calculated and experimental exit gas-sample I I , 3.0 atm, 2.64 cm/sec Points indicate data and lines calculated values
728
05
f
5
fi
(11)
2 Pressure Dropl
r..-.704'CiType 3 Pressure Drop)
0
(10)
Environmental Science & Technology
CO2
in
+ &)(l +
&), P2
f > f,
(12)
Evaluation of Parameters. From Figure 5 where -In [(l - f ) / ( l - f l ) ] / ( f - fl) is plotted against ( t - t l ) / ( ffi), it can be concluded that the experimental data are in excellent agreement with the proposed model. The constant, ro, determined from the slope for each run, is not only a function of temperature and pressure but increases directly with flow rate. Most of the published reports (Kettenring et al., 1950; Resnick and White, 1949; Ricetti and Thodos, 1961; Richardson and Szekely, 1961) of heat and mass transfer in fixed or fluidized beds indicate that the transfer coefficient is proportional to the superficial velocity raised to some power. Recently, a bubbling bed model (Kunii and Levenspiel, 1968) and a channeling model (Kunii and
Suzuki, 1967) have been proposed for fine-particle fluidized beds, and in both cases direct, proportional relationships between the mass transfer coefficient and gas velocity have been theoretically derived. Since these models represent the present system, it is reasonable to assume that k,
=
mu
(13)
Insertion of Equation 13 into Equation 10 leads immediately to
P- 1 ro - 2
+
KwORT 1 ___mao u
(14)
To determine the reaction rate constant, k , and the proportionality constant, m , the quantity P/ro was plotted against l/u, as shown in Figure 6. The data for the runs with type 1 and type 2 pressure drop behavior follow an approximately linear relationship. Best straight lines are drawn a t the points with a common intercept for Samples I and I1 a t the same temperature. The value of k at each temperature is given by the common intercept and KwoRTlrnao by the slope. The constant, m , is calculated with a0 based on the estimated average particle size of 50 j i for Sample I and 30 j i for Sample 11. Table I11 lists the values of k , m, (31 and p2.
Discussion The reaction rate constants, k , obtained in the present work are comparable to those reported by Meissner and Schora (1961) who postulated a similar reaction model. Their overall rate constants vary from 0.00725-0.0255 min-1 for temperatures between 740" and 890°C. These are believed to be chemical reaction constants since the high superficial gas velocity (90 cm/sec) used in their experiments should result in little mass transfer resistance. The activation energy determined from an Arrhenius plot is 10,200 cal/g-mol and the frequency factor 0.821 min-1 atm-1. This value of activation energy is within the range reported previously by other investigators (Hansen et al., 1960; Osman et al., 1966; Smith and McKewan, 1962; Themelis and Gauvin, 1963) whose results vary from 1750 to 18,820 cal/g-mol depending on the experimental system used. Since the proportionality factor, m , for the mass transfer coefficient was determined only from runs with types 1 and 2 pressure drop behavior, it is not expected to be applicable to type 3 runs which show a rather inconsistent trend as seen in Figure 6. The mass transfer coefficients obtained for types 1 and 2 pressure drop behavior are comparable to those obtained from some other correlations. As a superficial velocity of 3 cm/sec, for example, the present correlation yields at k , of 6.0 x 10-4 cm/sec for Sample I and 1.5 X 10-3 cm/sec for Sample 11. At the same conditions, values of 4.5 X 10-4 and 2.3 X cmjsec Samples I and 11, respectively, are predicted by the Resnick and White (1949) correlation, and 2.l(lO-3) and 1.6(10-3) cm/sec according to the correlation of Kettenring et al. (1950). The difference in the constant, m, between Samples I and I1 stems from the different fluidization characteristics of the two systems, even with the same type of pressure drop behavior. According to the channeling model (Kunii and Suzuki, 1967), the constant, m contains a length-ofchannel factor which is likely to depend on the iron content of the bed as well as the particle size. Sample I1 exhibited more type 3 behavior, but also higher mass transfer coefficients for types 1 and 2 behavior than did Sample I. From the standpoint of mass transfer, type 3 pressure drop, although highly irregular, is preferred.
Table 111. Average Values of Reaction Rate Constant, m, PI, and P Z m X io4 k X 103,
Temp, "C
min-'atm-'
593 649 704
Sample I 1.9 2.0 2.05
2.35 2.97 4.29
/3, (gram-atom C, Temp, "C 593 649 704
Sample I 0.533 0.300 0.03(fi 0.3)
Sample I I
5.0 5.1
p2 (gram-atom C, f - l )
t-l)
Sample I I 0.363 0.110 0.20
Sample I 0.876 0.905 0.890
Sample I I 0.443 0.505 0.458
With the rate constants and mass transfer coefficients obtained from the fit of experimental data, it is now possible to predict the COz concentration in the off gas or the reduction rate. The unsteady state C02 balance equation can be solved numerically along with Equations 5 and 6 subject to the initial condition yo = 0 a t t = 0. If the accumulation of COS is assumed negligible except in the very early period, the psuedosteady state solution for y h is KFeww-dl
= u,AC(l
- f)
+ l / K ? ) + KFe,w,r,(l
- f)O
+ l/Kp) (15)
where ro is given by Equation 10 and K by Equation 11 or 12 depending on whether f is greater or smaller than f l . The fractional reduction, f , as a function of t can be determined from Equation 9. The calculated values of yh using the correlated model parameters are in good agreement with the experimental data containing deviations smaller than 10% for all the runs with type 2 behavior and most of the type 1 runs. Comparison of typical runs in each category of pressure drop behavior is shown in Figure 7. Some of the type 1 runs need smaller mass transfer coefficients for better fit of the data, indicating that mass transfer may be controlling because of poor fluidization. Even in these cases, the mass transfer with the (1 - f ) term represents the data better than a constant k,ao. For most of the type 3 runs, the predicted COS mole fraction is lower than the actual value. This suggests that mass transfer may offer negligible resistance, and hence in some runs, the overall rate is governed solely by the chemical reaction rate. In most of the runs the initial rate is higher than predicted, probably owing to uncertainties in starting time, initial unsteady state operating conditions, initial carbon deposition rate, and transitional fluidization behavior. What causes variation in pressure drop or fluidization behavior, even a t otherwise identical operating conditions, has not been ascertained. The parameters determined for the present model can be applied only to poorly fluidized channeling beds which are typical of fine iron ore.
Acknowledgment Jones and Laughlin Steel Corp. supplied the samples and their analyses. U.S. Bureau of Mines contributed to the determination of composition of reacted material. Nomenclature A = cross-sectional area of the bed, cm2 a0 = initial surface area of iron oxide per gram total iron, cm2/gram-Fe C = gas concentration, for ideal gas C = P / R T f = fractional reduction Fer = total iron in the bed, grams K = constant, Equations 11and 12 Volume 7, Number 8, August 1973
729
K , = equilibrium ratio of COz/CO k = reaction rate constant, min-1 atm-’ k , = mass transfer coefficient, cm/sec m = proportionality constant, k , = m u nc = gram-moles of carbon deposited ncOn = gram-moles of COS P = total pressure, atm A P = pressure drop across the bed Apw = weight of the bed per unit cross-sectional area R = gasconstant ro = constant associated with reduction rate (gram-oxygen removed)/(gram-oxygen) (min) T = temperature t = time, min u = superficial velocity U P = superficial velocity corrected for carbon deposition u: = oxygen concentration in iron oxide, gram-atom oxygen/gram-Fe tu0 = initial oxygen concentration in iron oxide, gramatom oxygen/gram-Fe Y d = mole fraction of Con generated because of carbon deposition ys, yb = mole fraction of CO2 a t the particle surface and in the bulk phase, respectively Greek Letters
PI,
p2 = constants for carbon deposition rate, gram-atom carbon deposited per unit fractional reduction
Phys. Chem., 53,240-55 (1967); CA, 66,107136~(1967). In Ger. Bogdandy, L. V., Engell, H. J., “The Reduction of Iron Ores.” pp 256-63, Springer-Verlag, New York, N.Y., 1971. Bogdandy, L. V., Riecke, H. G., Arch. Eisenhuttentc., 29, 603-9 (1958); CA, 53,2025b (1959). In Ger. Ess, S. Y. M., Wild, R.. J . Iron Steel Inst.. London, 194, 211-21 ( 1960). Feinman, J . , Ind. Eng. Chem., Process Des. Decelop., 3, (3), 241-7 (1964). Hansen, J. P . Bitsianes, G., Joseph, T. L.. Blast Furn. Coke Oven, Raw Mater., Proc., 19, 185-99 (1960). Kettenring, K. N.,Manderfield, E. L., Smith, J. M., Chem. Eng. Propr. 46. 139-45 (1950). Kunii:D. Levenspiel, O., Ind. Eng. Chem., Process Des. Decelop., 7 (4),481-92 (1968). Kunii. D.. Suzuki. M.. Int. J. Heat Mass Transfer. 10. 845-52 (1967). McKewan, W. M.. Trans. AIME, 221, 140-5 (1961). Meissner, H . P., Schora, F. C., ibid., 221,1221-5 (1961). Okura. A , , Matsushita, Y., Tetsu To Hagane, 50, 159-65 (1964); CA, 63,6646f (1965). In Jap. Osman, M. A,, Manning, F. S., Philbrook, W. O., Amer. Inst. Chem. Eng. J., 12,685-92 (1966). Resnick, W. E., White R. R., Chem. Eng. Progr. 45, 377-90 (1949). Ricetti, R. E., Thodos, G., Amer. Inst. Chem. Eng. J . , 7, 442-4 (1961). Richardson, J. F.. Szekely, J., Trans. Inst. Chem. Eng., 39, 21222 (1961). Smith, N. D., McKewan, W. M., Blast Furn. Coke Oven Ratc Mater. Proc., 21,3-13 (1962). Stelling, D.. J . Metals, 10, 290-5 (1958). Themelis, N. J., Gauvin, W . H.. Trans. AIME, 227, 290-300 (1963). Udy. M., Lorig, C., ibid., 154, 162-81 (1943).
Literature Cited Agarwal, J. C., Davis, W. L., Jr., Chem Eng. Progr Symp. S e r , 62 (67), 101-10 (1966). Boetticher, H., Bogdandy, L. V., Foerster, E., Schierloh, U., Z
Receiced f o r revietc October 26, 1972. Accepted April 11. 1973. Work supported bx a grant from the Cnited States Department of the Interior.
Recovery of Metallic Iron from Flotation Tailings by Pneumatolytic Transport II. Formation of Iron Pentacarbonyl from Partially Reduced Iron Oxide in Fixed Bed Chin S. Rhee,’ Chung S. Kim,* Kun Li,3 and Robert R. Rothfus Department of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, Pa. 15213
A conceived process for recovery of valuable metallic iron from waste iron ore flotation tailings by pneumatolytic transport involves reduction followed by carbonyl formation. In this work, the formation of iron pentacarbonyl from partially reduced flotation tailings in a fixed bed was studied experimentally at temperatures from 93-177”C, pressures from ’7.8-21 atm, and gas flows from 0.14-0.52 cm/sec. The surface reaction was found to control the overall reaction rate which varies approximately as the second power of carbon monoxide pressure. A deactivation term was introduced to account for the increasing difficulty of interaction of carbon monoxide molecules and iron atoms as reaction proceeded. A model based on surface reaction and deactivation adequately describes the kinetics of carbonyl formation. The existence of an optimum temperature a t 120°C is believed to be associated with the mobility of adsorbed carbon monoxide molecules. Reduction of iron oxide by carbon monoxide a t low temperature and low pressure produces more reactive iron than reduction a t higher temperatures and pressures. Present address, Electronic Associates, Inc., West Long Branch, N . J . 07764. Present address, Graham Research Laboratory, Jones & Laughlin Steel Corp.. Pittsburgh, Pa. 15230. 3 To whom correspondence should be addressed. 730
Environmental Science & Technology
Iron pentacarbonyl is known to be unstable a t high temperatures, decomposing readily to iron and carbon monoxide. The kinetics of decomposition have been studied recently (Carlton and Oxley, 1965), but kinetic information on formation is limited. The only kinetic equation reported in the literature is that of Dufour-Berte and Pasero (1967) for carbonyl formation from hydrogen-reduced iron, apparently developed by curve fitting of conversion data using a topochemical model. No work has been found in the literature on the kinetics of formation of carbonyl from carbon monoxide-reduced iron, even though from a practical standpoint the use of carbon monoxide for reducing iron oxide might be preferable to the use of hydrogen. The rate of formation of iron pentacarbonyl from hydrogen-reduced iron is approximately proportional to the second power of carbon monoxide pressure according to Stoffel (1914) and Dufour-Berte and Pasero (1967). For a given pressure there exists an optimum temperature (Mond and Wallis, 1922; Okamura et al., 1949) or a range of optimum temperatures (Lewis et al., 1958). Too high a temperature is not favorable for carbonyl formation mainly because the equilibrium carbonyl concentration decreases rapidly with increase in temperature. Lewis et al. (1958) indicated that there is a maximum conversion attainable a t a given temperature and pressure regardless of the duration of reac-
