Magnetite Spheroidization Using an Alternating Current Arc Heater Maurice G. Fey, Charles B. Wolf, and Francis J. Harvey' Arc Heater Project, Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania 75 I 12
A series of experiments has been conducted on the spheroidization of magnetite grit in a three-phase electric arc heater operating on air. A standard self-stabilizing ac heater with a 1000 kW rating was used in the tests, but the particle injection system, heat transfer zone, and cooling/collection chambers were specially designed to suit the process. The results show that over 90% heat affection of the grit is readily achieved with a yield of acceptable sized product of approximately 20% and a specific energy requirement of 2.18 kW-h/kg. The spheroidized product is nearly dust-free with effectively no change in oxygen content.
Introduction In a myriad of industrial materials processes, the addition of heat at very high temperatures is necessary to provide the endothermic heat of reaction, or to melt the product, or a combination of both. At present, the source of heat in most cases is the combustion of gaseous or liquid hydrocarbon fuels. As the supply of these fuels becomes increasingly short and their prices rise accordingly, industry is beginning to look for alternate energy sources. I t has been suggested (Fey and Harvey, 1976) that electrical energy, generated from the abundant supplies of coal and uranium, be substituted for oil and gas firing. In many systems, electric arc heating would be more efficient than firing with hydrocarbon fuels because of the higher temperature available. The amount of energy which is normally discharged as waste heat could be considerably reduced when electric arc heating is utilized. One such application in which electric arc processing can be substituted for oxy-fuel combustion technique is the area of fusion and/or spheroidization of bulk solids. Flame and plasma spraying is, of course, commercially established with individual units of about 50 kW which can fuse a wide range of powdered materials a t flow rates up to about 10 kg/h (Metco Corp.). Scaleup to capacities approaching the requirements for industrial bulk processing has been reported (Wilks, 1974) for thermal dissolution of zircon, with demonstrated capacity of about 100 kg/h. The latter development employed to good advantage another important characteristic of thermal plasmas-the rapid quenching of species leaving the plasma environment in order to preserve the high-temperature equilibrium and prevent back reactions. In the series of tests described in this paper, the process involves the melting and spheroidization of 44-125-p magnetite grit which was injected into an arc-heated air stream. In the relatively short time (less than 50 ms) that the particles remain a t the melting temperature it was not expected that the product would be significantly oxidized. The arc heater used was a standard self-stabilizing ac unit which will be described later, but the particle injection geometry, heat transfer zone, and cooling/collection chambers were designed to suit this particular process. The design of the heating section was based on a study of the heat and momentum transfer which takes place in the arc heater exit (Harvey et al., 1975). The study assumes an axially flowing cylindrical geometry and determines the net rate of heat transfer to the particles injected into the arc-heated air stream and their resultant temperature as a function of flight distance within the heating zone. Input data include the gas enthalpy and flow rate, as well as the properties of the arc heated gas and the particles. Apparatus Description The test apparatus shown in Figure 1is a vertical arrange108
Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 1, 1977
ment with the three-phase arc heater a t the top. Air is introduced into the arc heater and heated to about 4200 K. Two diametrically opposed jets of magnetite grit are injected into the arc-heated air stream through the feed plate connected to the exit flange of the arc heater. The particles are heated and become molten spheres in the heating chamber, then fall through a large diameter cooling chamber where they are solidified before dropping into the product receiver a t the base of the unit. The arc heater used for the tests described here is a threephase ac self-stabilizing device with 1000 kW rating and is shown in Figure 2. The three electrodes shown are copper and are cooled by high-velocity water. The internal cavity of the arc heater contains two electrodes which are tubular with internal dimensions of 85 mm diameter and 406 mrn total length. These electrodes are separated by a gap of approximately 1.0 mm. The third electrode is toroidal and located a t the upper end of the arc chamber. The outer surface of this third electrode is separated from the upper tubular electrode by a 1.0 mm radial gap. As previously described (Fey and Hirayama, 1970), these small gaps are an important feature in arc heaters of this type, as they enable the unit to be selfstarting when energized and to be self-stabilizing after arcing is established. Air enters the arc chamber through the gaps a t a velocity of approximately 200 m/sec and is heated as it flows through the arcing region. To minimize vorticity in the arc heater effluent, the supply air entering the arc chamber is tangentially admitted through the two small gaps in opposing directions. The magnetic field coils shown are energized by a dc power supply, creating a cusp-shaped field with a flux of about 0.10 T, and thus it parallels the arcing surface of the electrodes. This magnetic field interacts with the arc current, and causes the arc to rotate within the arc chamber a t speeds up to about 1000 revolutions per second. The rapid arc rotation prevents excessive wear of the electrodes, and also promotes uniform, efficient heating of the air flowing through the arc heater. The feed plate, also shown in Figure 2, is essentially a water-cooled copper flange attached to the exit of the arc heater. The magnetite grit is pneumatically conveyed into the central opening through two radially directed passages which are diametrically opposed. There were five different feed plates used during the test series. Four had circular central openings which varied frorn 44 to 92 mm in diameter. The fifth plate had a rectangular central opening 14 mm wide by 76 mm long, with the material feed ports entering on the 14 mm side of the rectangle. Tests were run with each feed plate to determine the effect of gas velocity on gadparticle mixing, and thus optimize the particle heat affection and spheroidization. Figure 2 further shows the cylindrical steel heating cham-
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Figure 1. Magnetite spheroidization test apparatus.
ber, which has an inside diameter of 203 mm. It is jacketed and water cooled. Two heating chambers, one 305 mm long and one 610 mm long, were tested to best determine the particle residence time required to attain complete melting and spheroidization. At the start of the test series the longer heating chamber contained a 19 mm thick magnesium oxide liner, intended to provide a hot internal surface, thereby reducing the radiation heat losses from the particles to the wall. As the series of tests progressed, the liner eroded away and was replaced by a thin coating (about 6 mm thick) of solidified magnetite which served the same purpose. In the initial tests, a quench ring was located a t the outlet of the heating chamber. It was observed that the quench air admitted through this ring tended to propagate the formation of stalactite-like deposits at the exit of the heating section, and this feature was abandoned early in the test program. The cooling chamber is a vertical water-jacketed tank with internal dimensions of 1.22 m diameter by 2.75 m high. The cooling chamber was designed and built with provisions for tangential air admission at four axial locations spaced 610 mm apart which were used for dilution and cooling of the hot gases. These tangential inlets were connected to a compressed air line manifold with individual flow regulation, consisting of critical flow orifices with pressure gauges and control valves. The lower end of the tank contained a 51 mm wide opening into a circumferential manifold for the collection of exhaust gases. A restrictive baffle plate was installed within the manifold to ensure that the exhaust gases were removed uniformly around the periphery of the tank. The exhaust manifold also had two tangential openings which could be used for the admission of additional cooling air to dilute and cool the exhaust gases. Upon leaving the cooling chamber manifold, the exhaust gases were ducted to a cyclone separator as shown in Figure 3, which was installed to remove solid particles prior to discharge to the atmosphere. In operation, there was a very small amount of solids collected in the cyclone; however, the exhaust
Figure 2. Section view of arc heater, feed plate, and heating sec-
tion. gas was discolored with ultrafine particles which require removal by a cloth type dust separator. A removable product receiver is located at the bottom of the cooling chamber. Actual product collection was in a separate removable liner of the product receiver. A provision was made for admission of a cooling fluid from a central location near the top of the receiver. It was possible to admit either air or water through this slot, and thus collect the product in either wet or dry form. A commercially available dense phase pneumatic conveyor shown in Figure 3 was used to inject the magnetite grit into the arc heated air stream. In essence, this system consists of a closed tank containing a bed of the raw material. Air was injected into the bottom of the bed, which served to pressurize the tank and partially fluidize the grit. The powdered solid then flowed through identically sized orifices and was intercepted by jet streams of air which conveyed it through hoses to the feed plate. Transport velocities ranged from about 38 to about 64 m/s. During the initial tests 13-mm i.d. hoses were used for feeding, and later, to reduce the flow rate of carrier gas needed, the hoses were changed to 10-mm i.d. Total carrier gas flow rates were reduced from about 44 m3/h to about 27 m3/h. The magnetite feed rate was determined by measuring weight changes over specific time periods, and feed rate regulation was accomplished by varying the injector tank pressure. The arc heater air was supplied by a portable air compressor. Total air flow, ranging up to 190 m”h, was measured using a sub-sonic orifice. Separate supply lines were connected to the upstream and downstream electrode gaps, and dome loaded pressure regulators were used for flow control. Individual flow regulation was necessary to optimize power input and arc heater efficiency and to minimize the net vorticity of arc heated air in the particle heating section. Compressed air was also used to supply the quench ring and the tangential cooling chamber inlets. Industrial type blowers were used to supply sweep air into the product receiver and dilution/cooling air into the exhaust system. Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
109
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Figure 4. Arc heater electrical schematic. A 56 kW, 10 l./s pump supplied the high pressure cooling water for the arc heater electrodes, field coils, and feed plate. The heating chamber, cooling chamber, and product receiver were cooled by low-pressure water. Flow rate and temperature rise to the individual arc heater components and feed plate were monitored so that the arc heater efficiency and the enthalpy of the arc heated air stream could be determined at the point of material admission. Power for the magnetic field coils was supplied by three commercial dc welding units connected in parallel with a field coil current capacity of about 2000 A. Individual coils were connected in a series-parallel arrangement providing a mag110
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
netic field strength ranging from 0.1 to 0.2 T over the electrode surfaces. Three-phase 60-Hz electrical power was supplied by an available 2450-kW motor-generator test set, which contained two generators operating in tandem. The generators were connected to provide an output voltage which was adjustable up to 4800 V. Arc current limitation is provided by air core inductors having 60-Hz impedances of 1.45,1.85, and 3.5 ohms as shown in Figure 4. By varying inductance and resultant arc currents, it was possible to nearly balance the power to the individual phases thus compensating for any arc voltage imbalance within the arc heater. In this way, it was possible to
Table I. Magnetite Spheroidization Test Data Test number Measurement Arc power Arc heater air flow rate Quench ring air flow rate Cooling chamber air flow rate Carrier gas flow rate Feed plate inside diameter Arc gas velocity thru feed plate Feed hose inside diameter Feed carrier gas velocity Length of heating section Magnetite feed rate Arc heater efficiency Air enthalpy Fraction heat affected Yield Deposit at heating section exit Energy requirement
Units
1
2
3
kW m”1h ms/h
568 196.1 136.2
552 167.5
m:’h
0
mvh
42.8 91.9 112
656 185.2 0 28.3 32.9 68.6 223 12.7
mm
mls mm mls
12.7
61.3
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610
217.7 68.6 5959 0.90+ 15 Large
kglh %
Jk %
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2.60
47.1 610
211.5 68.9 7331 0.90+ 16
Small 3.09
4
0
34.0 41.6 54.9 288 12.7 59.5 305 279.0 66.4 6570 0.40 15
Medium 1.98
696 182.6 0 31.4 26.5 68.6 247 9.5 38.0 610 318.4 71.1
8131 0.9ot 15 to 20 Small 2.18
provide nearly uniform outlet temperature, subject to the small ripple factor resulting from the three-phase, 60-Hz, ac operation. Test Results and Discussion Operating conditions and results are shown in Table I for several typical tests, all of which were run a t essentially atmospheric pressure. Spheroidization was examined for several arc power levels, arc heater gas enthalpies and velocities, magnetite grit and its transport gas flow rates, and heating section length (particle residence time). In Test 1most of the particles were heat affected, hut a large “stalactite” of frozen magnetite was formed at the exit of the heating section. The large formation was eliminated in Test 2 by substituting a modest vortex flow into the cooling chamber for the severe air quench, in addition to raising the arc heated gas enthalpy and stream velocity. The latter tended to improve the mixing hetween the particles and arc heated jet in the manner suggested by Boulos and Gavin (1974). The effect of heating section residence time was investigated in Test 3 by shortening the section from 610 to 305 mm. The effect on spheroidization was drastic, as only about 40% of the product was heat affected. The longer heating section was reinstalled for Test 4, and the enthalpy of the arc heated air stream was increased modestly (over that of Test 2). This permitted a marked increase in magnetite flow rate while still maintaining an acceptable level of heat affection. The specific energy requirement of 2.18 kW hlkg is commercially acceptable, hut can most probably be improved. Electron micrographs of the magnetite grit and the acceptable spheroidized product are shown at 2OOX magnification in Figure 5. Although a substantial amount of fines were produced during the tests as might he expected under such severe thermal conditions, it may be noted that the product is nearly dust free. This is primarily due to the suhstantial gas.velocitieswithin the heating and product collection chambers. The high gas velocities available make the dust laden exit stream quite amenable to cleaning by inertia type separators. It was recognized that a significant cost savings would he achieved if air could he wed as the process gas, compared with heating devices requiring protective atmospheres. At the same time, oxidation of the molten magnetite was predicted by equilibrium considerations as shown in Figure 6 where the relationship between the oxygen partial pressure and the composition of liquid iron oxide is given at 1873 K (Darken
a. MAGNETITE GRil
b SPHEROIDIZEDMm3NETITE PRWUCT
Figure 5. Electron micrographs of magnetite grit and spheroidized product: a, magnetite grit; b, spheroidized magnetite product.
and Gurry, 1946). The composition corresponding to magnetite is an equilibrium with a gas containing 5.9% oxygen; thus an increase in the oxygen content of the melt would he expected by using arc-heated air. However, because of the short residence time in the molten state, oxidation did not occur to any detrimental extent. This was evidenced by measurements of the magnetic moment before and after arc heater processing. Although an acceptable level of heat affection and spheroidization (over 90%) was attained, the existence of a substantial amount of agglomerated and sintered material Ind Eng. Chem.. Process Des Dev., Val. 16.No 1, 1977 111
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diminished the yield of acceptable sized product to about 20%. The high fraction of agglomerates resulted primarily from the fact that particles were injected radially into the arc-heated gas stream with the resultant high probability of wall impingement. Sintering of the spheroidized product in the receiver located a t the bottom of the collection tank resulted primarily from a longer than expected persistence length of the arc-heated jet. Both problem areas are being addressed in an equipment redesign presently underway. The collection chamber is being lengthened to about 9.6 m in modules of 2.4 m each. In this way it will be possible to optimize the collection chamber or to extend the chamber length still further a t modest expense, should that be necessary to avoid sintering of the product. Figure 7 illustrates a three-phase arc heater being specifically designed for solids processing. In essence, it consists of three self-stabilizing ac arc heaters whose exit gas jets impinge upon each other a t the centerline of a cylindrical plenum. The particulate to be processed is axially injected through the
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Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
impingemen$ region of the arc-heated jets, flows through a high velocity heating region, and then into the cooling/collection chamber. Electrically the upstream electrodes of the three arc heaters will be wye connected to the three phases of a line frequency power source. The three downstream electrodes and the plenum to which they attach will be commonly connected to the grounded neutral of the circuit.
Conclusions Magnetite spheroidization has been demonstrated a t commercially acceptable levels of production using an ac arc heater as the process heat source. The material produced was nearly dust free, as fines were carried out of the collection chamber by a high-velocity gas jet. The magnetic properties of the product were unaffected by fusion in arc heated air; thus significant savings are possible over process heating routes which require protective atmospheres. Commercialization will include apparatus for axial injection of solids and a sufficiently long collection chamber to reduce agglomeration and product sintering to acceptable levels. Literature Cited Boulos, M. i., Gauvin, W. H., Can. J. Chem. Eng., 52, 355 (1974). Darken, L. S.,Gurry, W. R . , J. Am. Chem. SOC.,68, 798 (1946). Fey, M. G., Harvey, F. J., Met. Eng. Quant., 16 (2),27 (1976). Fey, M. G., Hirayama. C., Division of Fuel Chemistry, 156th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1970. Harvey, F. J., et al., Proceedings of International Round Table on Study and Applications of Transport Phenomena in Thermal Plasmas, Paper No. I). 6, Odiello, France, Sept 1975. Wilks, P. H., Pure Appl. Chem., 39, 415, (1974).
Received for reuiew F e b r u a r y 23, 1976 Accepted August 16,1976
