Iron from Coal Minerals George A. Jensen' and George 1.Austin Washington State University, Pullman Washington 99 163
The carbon in coal fflinerals can be gasified with steam in a fluid bed reactor to produce carbon monoxide and hydrogen for fuel or synthesis gas. Elemental iron is produced within the coal mineral particles during the gasification process. Hydrogen made available by the steam-carbon reaction probably reduces the iron. The process is intermediate between diffusion controlled and rate controlled. Free iron can be produced from the mineral residues of coal mineral gasification by proper addition of a flux such as calcium oxide. With care, nearly quantitative recovery of the iron can be obtained.
Introduction The solvent refining of coal produces a liquid product which contains 90% of the carbon contained in the original coal, gases, and a solid residue. The residue has not been oxidized at any time during processing and, therefore, the components exist in a reduced state.This coal mineral residue is not an ash but a potential source of valuable products. If oxidized, the sulfur can become a pollutant. It is desirable to devise methods for utilizing these coal minerals in such a fashion that the sulfur is not released to the atmosphere. The composition of typical coal minerals is shown in Table I. Recovery of the iron and sulfur would greatly reduce the weight of coal mineral residue formed and would yield useful products. The residue would be suitable for light weight aggregate, glass, mineral wool, or similar products. The present investigation involved the reduction of the iron contained in Kentucky No. 9 coal minerals. Solvent refined coal minerals from coals other than Kentucky No. 9 were not available, but should be similar. Previous Work Coal minerals have not been available for study prior to this work. A literature has therefore not been developed concerning these materials. The literature was investigated to ascertain possible steps for recovering iron and sulfur from coal minerals (Kirk-Othmer, 1966; Mellor, 1934). Kloepper et al. (1965) show that iron and sulfur exist in a 1:l mole ratio in coal minerals and they are presumed to be iron sulfide (FeS). Since the components of coal minerals, including the iron sulfide, are contained in a finely divided matrix, the methods (Auld et al., 1969; Brennan, 1967;Kirk-Othmer, 1966; Squires, 1967) which are usually used for removing inorganic iron sulfides, usually pyrites (Fe,S,), are not applicable. Water reacts with iron sulfides to produce iron or iron oxides, sulfur, and gaseous products (Kirk-Othmer, 1966; Mellor, 1934). Investigators have studied the reaction of iron pyrites with water vapor in mixtures of coal, amorphous carbon, and the oxides of magnesium and calcium (Al'tschuler et al., 1968; Grunert, 1930; Kunda et al., 1968; Siddiqui, 1957). Mixtures of pyrite and amorphous carbon, when treated with steam a t 500-600 "C, produce limited quantities of sulfur as hydrogen sulfide ( H2S).Pyrites treated a t elevated temperatures with a 1:2 mole ratio of oxygen ( 0 2 ) to steam yielded 90% of elemental sulfur and magnetite (FesOd). Iron sulfide, when reacted with water vapor in the presence of calcium or magnesium oxide a t 1180 "C, yielded iron oxides and elemental sulfur, hydrogen sulfide, sulfur dioxide ( S o p ) ,and hydrogen (H2).Sulfur removal exceeded 99%. Iron recovery exceeded Address correspondence to this author at the Chemical Technology Department, Battelle-Northwest Laboratories, Richland, Wash. 99352. 44
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
80% when the iron oxides were reduced with a mixture of hydrogen and steam. Iron sulfides or pyrites are oxidized to sulfur dioxide by air (Kirk-Othmer, 1966; Mellor, 1934). Ordinarily magnetite is formed, but if the temperature is high enough, ferric oxide (FeaOy) forms instead (Mellor, 1934). Vohra (1972) attempted differential oxidation as a means of sulfur removal in the temperature range of 180-370 "C. He found coal minerals ignited spontaneously if the temperature exceeded 370 "C, and a carbon-sulfur separation by selective oxidation was not possible. Dry sulfur dioxide reacts with iron sulfide, forming sulfur. A t 900 "C, sulfur removal has reached 100%.Sulfur or sulfur dioxide and carbon monoxide are produced when carbon dioxide is used as a reducing agent Mellor, 1934). Iron sulfide also reacts with hydrogen at 1100 "C to produce hydrogen sulfide and free iron (Mellor, 1934). Iron oxides are reduced in the blast furnace by reaction with carbon, carbon monoxide, and hydrogen (Kirk-Othmer, 1966). Auld and Masdim (1969) suggest that direct reduction of iron from the ore using CO and H2 may become attractive in the future. Feinman (1958), 1964) used fluid beds for reduction of iron oxides. In most of the experiments, hydrogen was the primary reducing gas. The direct reduction of iron sulfide by carbon, carbon monoxide, and hydrogen is possible by the following reactions (Kirk-Othmer, 1966; Mellor, 1934).
+ Hz 2FeS + C FeS
FeS
-
Fe
+ HpS
(1)
2Fe
+ CS2
(2)
+ CO -Fe + CO + S
(3)
Carbon, hydrogen, and carbon monoxide will be present in a fluidized bed if gasification of the carbon in the coal minerals by reaction with water is attempted. Reaction 2 may occur in two steps as follows FeS FeS
+C
+ CS
-
-
Fe
+ CS
Fe -k CS2
(4) (5)
Carbon monosulfide (CS) and carbon disulfide (CSp)are not stable and probably react immediately with hydrogen or carbon monoxide to form carbonyl sulfide (COS) and hydrogen sulfide by the following reactions (Kirk-Othmer, 1966; Mellor, 1934)
+ H:! CS + HzS CS + H:!-+ C + H2S csp + co cos + cs CS2
+
+
(6)
(7) (8)
Table I. Analysis of Kentucky Coal Minerals" Kentucky sample 57215, Component
71
%
Ignition loss Si02
55.23 18.42 10.07 10.55 0.20 1.66 1.09 1.13 0.78 0.21 0.62 0.066 0.068 0.074
A1203
Fez03 Ti02
MgO CaO Kz0 NalO B203 CUO v20s
ZrOz BaO MnO PbO SrO NiO Cr2O;j coo M00:j Ga2Os SnO Ag2O GeO:!
0.035
0.022 0.027 0.027 0.024 0.015
0.014 0.012 0.004 0.0004
...
The samples as received were dried overnight at 100 OC prior to analysis. The ignition loss was determined by ashing in a muffle furnace at 900 O C , to constant weight. Silica, iron oxide, alumina, and titania were determined in duplicate by wet chemical methods. A separate sample was ashed for quantitative spectrochemical analysis. Analysis was carried out for the Pittsburg and Midway Coal Mining Co. by the Materials Chemistry Section of the College of Engineering Research Division at Washington State University. a
Experimental Apparatus a n d Procedure A flow diagram of the experimental apparatus is shown in Figure 1. Vohra (1972)and Jensen (1970-1973) found that coal minerals in fixed beds tended to fuse. A batch fluid bed reactor was selected for this work because solids reacting in fluid bed reactors have less tendency to agglomerate. The reactor consisted of three sections: a stainless steel top disengaging section, a 2-in. i.d. 1-ft long mullite reactor tube contained in a gas fired furnace, and a heated steel bottom section to evaporate water with a removable stainless steel tube having a porous stainless steel bed support and gas distributor. Asbestos compression seals were used as packing to hold the reactor parts together and form a gas tight seal. Distilled water was fed to the lower reaction section through a calibrated rotameter and evaporated to steam. The product gases were piped through a heated cyclone, condenser, cold traps, and a soap bubble flowmeter, and noncondensable gases were then exhausted. Gas samples were obtained directly by drawing a stream through a Carle sampling valve and injecting this stream directly into the gas chromatograph. Additional intermediate sampling was done using gas-tight syringes. The coal minerals, as received, exist as large, hard, black lumps ranging in size to in. in diameter. They were ground to all pass 20 mesh and sieved for experiments. The fractions used were 30/35 mesh (0.50-0.59 mm), 50/60 mesh (0.21-0.30 mm), and 200-270 mesh (0.05-0.07 mm). One hundred grams of coal minerals were charged to the reactor for each run. Nitrogen gas was used as a fluidizing gas to keep the bed fluid and to avoid slugging the coal minerals until reaction temperature was reached. Nitrogen flow was
v2
11 I1
II
"I; METER
J1 STAINLESS STEEL GAS DlSTRlWTOR SOLENMO SWlTCmYG VALVE TUBEFURNACE
TO GAS ICE BATH
ttRCUh1W
H
ROTAMETERS
L__-i
Figure 1. Flow diagram of experimental apparatus. then stopped and steam was fed to the reactor. On completion of each experiment, steam flow was stopped and nitrogen was fed to the reactor to prevent oxidation of any iron formed during the gasification of the carbon. The minimum steam rate for fluidizing the coal mineral bed was determined by cold tests in a lucite model reactor. Hydrogen and air were used as fluidizing gases. The height of 100 g of fluidized coal minerals was 7 in. Gas rate to achieve fluidization was 20 l./min. Entrainment losses did not exceed 5% and were nominally 2% or less. Calculations showed that steam rates of >25 l./min at the reaction temperatures could be obtained by volatilizing 4 ml of water/min. A total radiation pyrometer measured reactor temperature. It was impossible to measure the bed temperature, for a fine black dust constantly obscured the bed. Reactor temperature was, therefore, measured at the reactor surface. Manual control and later automated burner control held the reactor surface to f 2 5 "C of the desired temperature. Reaction temperatures were 1040,1150, and 1320 "C. In several experiments, the furnace temperature was increased at this point to melt the bed and agglomerate as much iron as possible within the reactor. Other experiments were also performed where carbon monoxide, carbon monoxide-. hydrogen mixture, or carbon dioxide was fed to the reactor after gasification of the carbon by water was completed. This was done to determine if these gases would reoxidize any iron which formed during gasification of the carbon. Analytical Procedure Analysis of the gases was by gas chromatography. Analysis of the solid feed and products was done under the direction of Dr. Charles Wright a t The Pittsburg and Midway Coal Mining Co. Laboratories in Merriam, Kansas. There is a minimal amount of data published on the analysis of the gas mixtures, and most methods require that one or more components must be removed prior to analysis by gas chromatography. Usually a separate analysis has been used for the determination of hydrogen (Jones, 1967; Purcell, 1965). Similar analyses have used a dual column system with thermal conductivity detectors using back flushing (Jones, 1967). These analyses use a modification involving two chromatographic columns operated in series or individually. Carle switching and sampling valves were used for sampling and switching the columns into and out of the eluting gas streams. No back flushing was necessary. Porapak Q-S was used for separation of carbon dioxide and other larger or polar compounds from the gases. Molecular sieves are useful for analysis of air, and a molecular sieve 13X 5 8, column was found satisfactory for separation of hydrogen, nitrogen, oxygen, methane, and carbon monoxide. Calibration using samples of known compositions of the effluent gases was repeated every few months to assure con-
+
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
45
Table 11. Reactor Solid Products
Temp, "C
Size of feed coal minerals, mm
Solids Recovery Reactor,
1038 1038 1038
0.50-0.59 0.21-0.30 0.05-0.07
71.3 48.4 59.2
g
Ov'hd, g 0.3
g
Magnetic solids, %
Melted" solids in reactor, %
71.6 50.2 62.5
10 50 50
10 10 40
Iron,
Total,
g
Similar to charge Similar to charge Gray, Bk, sinter and 3.3 powder nil Bk. granular solids 1149 0.50-0.59 55.0 nil NA 55.0 100 1.7 Bk. sinter and slag & 35.2 40 1.7 38.6 90 1149 0.50-0.59 powder 46.6 Black powder and slag 1149 0.21-0.30 3.9 NA 50.5 50 30 38 50.2 Black slag and powder 1149 6.6 NA 56.8 50 0.05-0.07 5.6 41.4 100 70 35.8 Couldn't separate 1149 Lenses 0.05-0.07 iron 36.5 0.7 37.2 100 53 Black slag and iron 1316 0.50-0.59 Lenses Couldn't separate 36.7 0.6 0 100 0.50-0.59 1316 37.3 iron 52 Bk. slag and powder 42.6 100 39.2 3.4 1316 0.21-0.30 Balls small balls Black slag and powder 45.0 0.05-0.07 100 28 31.5 11.7 1.8 1316 nil 38 51.3 100 Black slag and powder Mixed 1316 0.7 52.0 32 6.3 0 Black slag and powder Mixed 1316 48.9 42.4 0 25.6 1.2 85 Black slag and powder Mixed 28.7 1.9 80 1316 80 ? Bk. slag on tube Slag Mixed 7.5 0.4 80 1316 couldn't detach iron lenses Black slag and powder 10.0 59 Mixed 39.0 Balls 49.0 80 1316 small balls 0 Approximate quantity. bNA = reactor not heated to melting point of solids after completion of experiment. 1.8
NA NA NA
Comments
sistent results. Losses of COS and H2S in the cold traps was checked during early experiments and found to be negligible. Analysis of a sample could ordinarily be completed in 12-15 min if the Porapak column temperature was programmed to rise 15 "C/min from 45 to 210 "C after hydrogen had been eluted. The molecular sieve column was heated from ambient to 100 "C as rapidly as the Beckman Thermotrak oven would heat. This gave consistent chromatograms using this method of heating. Experimental Results Carbon monoxide and hydrogen were the major products of the steam gasification of the carbon in the coal minerals at 1150 and 1320 "C. Carbon dioxide concentrations were less than 2% during the early stages of each experiment and the COdCO ratio increased as the experiment progressed. At 1040 "C, H2, CO, and CO2 were the major products. As the experiment progressed, the hydrogen concentration remained constant a t approximately 50%of the effluent gas while the COJCO ratio increased. The rate of gas evolution was initially high and decreased steadily throughout each experiment. The hydrogen sulfide content of the product gas increased slowly during each experiment. If the experiment was long, the H2S concentration peaked, then rapidly dropped. Small quantities of methane, nitrogen, carbonyl sulfide, and water were also detected in the gasified products. Concentration of these gases ordinarily totaled less than 4%of the product gas mixture. Slaglike masses containing iron nodules and magnetic particles were found in experiments where the reaction temperature was 1320 "C and in those experiments at 1150 "C where the reactor was heated to 1320 "C upon completion of the gasification of the carbon. Yields were low, rarely over 15% of the iron charged (Table 11).All of the particulate residue and much of the slag was magnetic, indicating the presence of free iron or magnetite. The magnetism of the slag was sig46
Ind. Eng. Chem., Process Des. Dev., Vol. 1 6 , No. 1 , 1977
nificantly reduced if the water feed was continued beyond maximum H2S production and/or if COz concentration in the reactor increased beyond 10%.Conversion of magnetic iron or iron oxide to nonmagnetic iron oxides (FeO or Fe2O3) thus occurred. To determine whether iron or magnetite was the cause of the magnetism, samples of the slag were mounted for metallographic analysis and examination. Microscopic examination showed shiny metallic materials in the matrix. The particles were hard, gave a cone-shaped dent when punched with a microhardness tester, and showed a metallic crystalline structure when etched with nitric acid. These tests confirm the existence of free iron in the solid reactor product. To identify the form of the iron in the particle residue, the solid reaction products were heated in a vacuum furnace to between 1300 and 1450 "C after addition of calcium oxide (CaO) as a flux. The calcium oxide required for a slag having a eutectic of 1170 "C was determined from the A1203-Si02CaO phase diagram (Reser, 1954). The CaO required for each sample was calculated based on the A1203-Si02 ratio present in the original coal minerals. A maximum of 10 g of residue could be charged to the vacuum furnace. Graphite crucibles were used for most experiments but a ceramic crucible was used in one experiment to determine whether the graphite influenced iron deposition. The results of these experiments are shown in Table 111. When slaglike residues were charged to the furnace and temperatures were sufficiently high, nearly quantitative iron removal was obtained. In experiment 2, Table 111, where the ceramic crucible was used, the maximum temperature of the sample was significantly lower than in other experiments and the slag did not get sufficiently fluid for complete iron removal. The crucible was destroyed and the residue was recovered and recharged in a graphite crucible into the furnace. All of the iron in the original sample appears to have been recovered since a yield of 107%was obtained (experiments 2 and 6, Table 111).It was impossible to remove the
Table 111. Vacuum Furnace Experiments Ex p e r i ments
Temp, "C
Feed"
Estimated iron content,
Iron recovery,
Iron recovery,
g
g
%
Comments ~
1 2
3 4 5
6 7
8 9
10
A A B
1371 1149 1260
2.06 2.06 2.05
2.28 1.92 -
0
Reheat of3 Reheqtof3 t more B Remelt 2
1482 1454
-
2.00
-
0 0
1399
2.06
0.27
13
C D E F
1349 1343 1422 1439
2.10 2.04 2.05 1.29
2.17
103 0 116 0
-
2.39 -
Clear glass. Black glass. No melting-particles appear t o explode, substantial losses. No melting-substantial losses. No melting-substantial losses.
111
94
Clear dark glass. Total recovery 2 t 6 = 107%. Hard rock-like slag. Results same as for 3 and 4. Hard rock-like slag. No melting-substantial losses.
( I A = slag-like residue-30-35 mesh fluid bed reactor feed, 1149 "C, no excess water. B = particle residue-30-35 mesh fluid bed reactor feed, 1149 "C, no excess water. C = slag-like residue-200-270 mesh fluid bed reactor feed, 1149 "C, no excess water. D = particle residue-200-270 mesh fluid bed reactor feed, 1149 "C, no excess water. E = slag-like residue-200-270 mesh fluid bed reactor feed, 1149 "C, excess water. F = residue B t 10 g silica sand + 1.8 g CaO.
iron from the graphite crucibles without some graphite remaining on the contact surface. Since the iron-carbon eutectic contains 2-5% carbon, and carbon content of the products is thus explained (experiments 1 , 2 and 6 , 7 , and 9, Table 111). Particle residues did not melt upon heating (experiments 3 4 8 and 10,Table 111),but burst instead, causing weight loss in the furnace during outgassing. Iron Reduction in Coal Minerals. Iron Sulfides are the principal sulfur-containing compound in coal minerals. The Gibbs free energy for iron sulfide reaction with reducing agents was calculated using available thermodynamic data (Janaf, 1965; Kelley, 1960; Perry, 1963; Rossini et al., 1952). These data indicate that only reactions 1 to 5 would occur in the reactor. Reactions 4 and 5 can be summed to give the overall reaction 2FeS
+C
-
2Fe
+ CS2
R
[l - (1 - X)1'3]
reacted-coreshrinking model.
Wen (1968) assumes that D " e ~= r,/R time for conversion as
A plot of In t vs. In [I - (1- X ) 1 / 3 can ] be used to indicate the controlling mechanism. A slope of 1 indicates chemical reaction controls; a slope > 1 shows that ash diffusion is affecting the rate process, and other data or models may be required to properly evaluate the data. For particles of different but unchanging size, the time needed for the same conversion will be
t t
a
a
R
R2
(for ash film controlling)
(13)
(for chemical reaction controlling)
(14)
Wen (1968) uses the general and homogeneous models to point out that if chemical reaction rate is slow and controlling (diffusion is rapid),the concentration of component A a t the reacting surface will nearly equal the bulk stream concentration of component A, thus C'A = CAO
_ cs - 1- X
cso
and for ash film resistance controlling
+ 2(1 - X ) ]
(11)
de^ and obtains the (12)
(10)
ksCA0
t = - aR2Cso [I - 3(1 - X)*l3 6 D e ~ C ~ ~
Figure 2. Schematic diagram of concentration profile for the un-
(9)
The direct reaction of water with iron sulfide appears unlikely since there is a large positive free energy for this reaction. Any CS2 or CS which is produced would immediately react with the hydrogen or CO by reactions 6,7, and 8, and only COS and H2S would be present in the product gas. Since only small quantities of COS were observed (2% maximum, usually less removal of sulfur by reactions 1and/or 2 seems most than l%), probable. Iron reduction could thus occur simultaneously with the steam-carbon reaction once hydrogen or carbon monoxide is available from carbon gasification. The kinetic analysis of iron reduction was based on data for H2S production using the unreacted shrinking core model (Yagi and Kunii, 1955; Levenspiel, 1972; Wen, 1968; Ishida et al., 1971). This model visualizes a solid spherical particle reacting with a gas. An unreacted shrinking core is surrounded by a growing layer of inert porous residue solid, Figure 2. The rate of reaction is controlled either by diffusion of the gas through the ash layer, chemical reaction a t the surface of the unreacted shrinking core, or by a combination of the two. All of these authors arrive a t the simple solid conversion time, expressed as eq 10 and 11, for the shrinking unreacted core model. For chemical reaction controlling t=-
SOLID
'REACTA NT
(for 0
<
