CA1107078A - Thermal reduction process of aluminum - Google Patents

Abstract

ABSTRACT

The thermal reduction process for producing metals such as aluminum in a reactor utilizes a dispersed discharge to provide the heat of reaction within the reaction zone in the presence of aluminum vapor to maintain the temperature in excess of 2000°C. The aluminum oxide powder and a reductant in a gaseous medium are introduced with a tangen-tial component into the reactor to create a vortex motion.
A minimum turbulence level within the reactor in the reaction zone is maintained so as to keep the solid particles in suspension and prevent the dispersed discharge from forming electrical arcs. Aluminum oxide is reduced to aluminum vapor which is removed with the effluent stream of gases from the reaction zone. Thereafter, the effluent is rapidly passed through a condenser where the temperature is dropped to liquefy the aluminum vapor which is then discharged in a continuous stream. The effluent stream is monitored for unreacted carbon or aluminum oxide and this information is fed back to the reactor for controlling the input of the starting materials.

Description

7~3 FIELD OF THE INVENTIO~
My invention relates to a process for the production of metals and, more particularly, to a thermal reduction.
process employing a dispersed electrical discharge for heating a reaction zone in the production of metals such as aluminum from their oxides.
DESCRIPTION OF THE PRIOR ART
The traditional method of producing aluminum (Hall Process) is by the electrolysis of aluminum oxide dissolved ~n mo1ten cryolite. This normally takes place in a large number oE electrolytic cells connected in series. Consumable annodes of carbon connected as the positive pole of the cell extend close to the molten metal surface. Heat is developed by the electrical resistance of the bath as low voltage-high direct current electricity is passed therethrough. Each cell requires separate servicing for feeding the cell and for renewal of the annode. In addition, the molten metal must be removed from each cell separately. The cells are open, therefore, creating air pollution problems.
Newly developed methods of producing aluminum include reducing aluminum oxide by carbon to aluminum tri-chloride in a reactor. Thereafter, aluminum is produced from the aluminum ~richloride in a completely enclosed electr~lytic cell.
Processes have also been developed which utilize charging finely divided ore into a reaction zone. However, such systems use either electric resistance heating or an electric arc to generate the necessary temperatures.
Representative of these processes are United States Patents Nos. 3,365,185; 3,563,726 and 3,765,870, with the latter patent being directed to the reduction of metal oxides other

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than aluminum.
SUMMARY OF THE INVENTION
My process for reducing certain metal oxides such as aluminum oxide i9 completely closed, thereby eliminating the pollution problems associ~ted with open cells. In contrast to an arc, my dispersed discharge system uses high voltage and correspondingly much smaller currents than an arc of the same power input. Standard three-phase a.c.
current can be used, thereby saving the cost of conversion ~ 10 to d.c~ power. The dimensions of the electrodes are much smaller than those of an arc system and the electrode con-sumption is minimal. The feed materials are easily intro-duced into the high tempera~ure zone and the :Length of the gap between the electrodes is not cri.tical in my process.
I provide a long residence time for the reactants, thereby assuring that the oxide is retained until consumed by the reaction and converted into aluminum vapor. As a result, I have combined the functions of a centrifugal separator with a reduc-tion process thereby assuring a high efficiency of operation. The purity of the metal is very high because no impurities are introduced with the reducing agent and the liquid metal is obtained by condensation.
I further provide a feedback control system by monit.oring the radiation emitted by the effluent stream.
This feedback control ties in with the flow rate of the reactants or an oxygen input into the reactor thereby assuring the purity of the product. By maintaining high temperatures in the area of the discharge through the dis-persed discharge mode of heating, I am able to elilrlinate aluminum carbide and other undesirable by-products rom the effluent stream.

7~7~

.
My ~herrnal reduction process for reducing aluminum in a reactor consists of establishing a dispersed discharge within the high temperature reaction zone in the presence of aluminum vapor. Aluminum oxide powder and a reductant such as natural gas or solid carbon in a gaseous medium is intro~
duced into the reaction zone with a tangential component so as to create a vortex motion. This vortex motion is main-tained above a minimum turbulence level so as to keep the solid particles in suspension and maintain the dispersed dîscharge. Aluminum oxide powder is reduced to aluminum vapor which is removed from the reaction zone with the eEfluent stream. Therea~ter, the aluminum vapor is condensed into a continuous liquid stream.
BRIEF DESCRIPTION QF THE DRAWINGS
Fig. 1 is a vertical section of a reduction furnace for carrying out my process;
Fig. 2 is a horizontal section through the furnace of Fig. l; and Fig. 3 is the oxygen potential diagram for aluminum oxide.
THERMAL REDUCTION PROCESS FOR THE PRODUCTION OF AL~INUM
. ~
The thermal reduction process described herein ut~ .es the dispersed electrical discharge for heating ~he reduction furnace. In contrast to an electric arc, high power input into a distributed discharge is maintained at conveniently high voltages (about 3000 to 5000 volts) and moderate current levels. The example described below utilizes natural gas as the reducing agent, obviously any other hydrocarbon or solid carbon could be used for this purpose The process is primarily applicable to the reduction of aluminum oxide to aluminum, but other metals such as magnesium ~ ~8 c~ be produced from their oxides by my process.
HE REACTOR
The reactor, generally designated 10, is schematically shown in Figs~ 1 and 2. A reaction chamber 14 wherein the reduction process takes place is defined by a cylindrical refractory wall 12, a dome-shaped refractory roof 13 and a - dome-shaped refractory floor 15. The width of the reaction - ~one is substantially greater than its axial height as a matter of design prefer~nce.
A plurality oE electrodes 16 (six illustrated in Fig. 2) extend through the roof 13 into the reaction zone 14 in spaced relationship. The electrodes 16, generally of cnrbon or graphite, are surrounded by appropriate insulation jackets 18. Also entering the reaction zone 14 through the wall 12 of the reactor 10 are input jet orifices 20. The ; jet orifices 20 are spaced about the wall 12 so each jet enters the reaction zone 14 su~stantially intermediate two adjacen~ electrodes 16. The jets 20 are positioned so that the entrants into the reaction zone through the jets have a moderate tangential velocity component.
An insulated exit duct 24 extends through the floor 15 along the vertical axis thereof. Exit duct 24 commurlicates the reaction zone 14 with a condenser 22. The condenser 22, generally of the surface condenser type, includes a plurality of heat transfer tubes 28 through which pass a cooling medium. The cooling medium enters through inlet 26 and exits through outlet 36. The lower portion of the condenser 22 constitutes a liquid metal-gas separator 30 having a liquid discharge 32 at the bottom thereof and a gas discharge 34 extending horizontally outward therefrom.
Waste heat can he removed from condenser 22 by a heat trans-fer medium, like hydrogen or helium, and circulated in a closed loop.
THER~LAL REDUCTION OF ALUMINUM OXIDE
The conditions necessary for the thermal reduction oE ~1203 are shown in the oxygen potential diagram, Fig. 3.
The lines representing the oxidation of Al to A1203 and of C
to CO cross at 2000DC. Above this temperature, oxygen moves from the oxide to carbon, forming carbon monoxide. For high reaction rate and for decomposition of unwanted by-products like Al20, A10, A14C3 and A120C, the temperature in the reactor must exceed 2000C and preferably above 2200C.
Above this temperature, oxygen mo~es from the oxide to carbon, forming ~arbon monoxide. For high reaction rate and for decomposition of unwanted byproducts like A120, A10, A14C3 and A120C, the temperature in the reactor must exceed 2000C and preerably above 2200C. In the following examples a gas temperature of 2400C is assumed in the high temperàture zone of the reactor. At this temperature, aluminum is in vapor phase as shown by the followlng Table l.

VAPOR PRESSURE OF ALUMINUM
% Condensed from Initial Temperature ~C Pressure mm Hg 136 mm Hg Pressure l900 75 44 The aluminum vapor leaves the reactor with the exit gases, and must be recovered by condensation.

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The overall reaction is represented by the equation:
1 A1203 + 3 CH~ = 2 ~ 3 C0 ~ 6 H2 The reaction requires 20 cubic feet of natural gas per po~md of aluminum produced and yields 60 cubic feet of carbon monoxide and hydrogen mixture as a by-product.
~leat o reaction is at 298K 375.0 kcal/g mole of Al Heat of content of the reaction products at 2700~ 338.2 kcal/g mole of Al Total heat require~ent 713.2 kcal/g mole of Al I~l large units the heat losses and compressor work are approximately compensated by preheating the feed material to about 300C.
The total electrical power requirement is therefore 7.0 lcwh/l lb Al Waste heat available from the aluminum condenser above 600C is 294 kcal/g mole. With 35% conve~sion efficiency this amounts to 1.0 kwh/l lb Al Ne~ electric power requirement without utilization of the by-product gas is 6.0 kwh/l lb Al Power generated from by-product gas ~ith 35% efficiency amounts to 1.85 kwh/l lb Al ~et power requirement 4.15 kwh/l lb Al It may become more economical to utilize the by-product gas for the manufacture of liquid fuel or other chemical products. In this case the energy requirement of the process would be 6 ~h per pound of aluminum.
DISPERSED ELECTRIC DISCHARGE
Powerful electrical discharge currents are prevented from concentrating into narrow arc filaments by turbulent mixing if the following dispersion criterion is fulfilled:

~`~

(d ~e) ( ~ ~ 0 24 ke x E ~1 :
here Ne is the number of electrons per cm3 T is the gas temperature, ~K
u' is the intensi,y of turbulence, cm/sec is the characteristic time of turbulence, sec u is the charge of the elec~ron, 1.59.1019 coulomb ~ is the density of gas ~ g/cm3 ~ is the scale of turbulence, cm ke is the mobility of electrons ~ T-m Cp is the heat capacity of gas, cal/g C
E is the voltage gradient, volt/cm The ionization potential of Al is low, only S.984 volts. The ion-electron concentration of the product gas, consisting of 136 mm Hg Al, 200 mm Hg C0 and 400 mm Hg }I2, due to thermal ionization of aluminum vapor is given below in Table 2 as a function of the gas temperature.

ION-ELECTRON CONCENTRATION
In Product Gas Containing 136 mm Hg Al 200 mm Hg C0 400 mm Hg H2.
/ dNe ~ 1 Temperature K Ne l/cm ~ dT J cm C _ 2300 0.34 x 1013 0.29 x 10 2400 0.63 x 1013 0.52 x 10 2500 1.5 x 1013 0.8 x 10 2600 1.97 x 1013 1.31 x 1011 ~700 3.28 x 113 2 0 x .loll 2~00 5.3 x 1013 2.7 ~ 10 2900 8.0 x 1013 4.2 x lOll Turbulence with a characteristic time of about lO 3 seconds or less is maintained by the gaseous jets entering the reactor. At this characteristic time of turbu-lence the critical voltage gradient for dispersion o-E the ~ischarge is calculated to be 31.6 volts/cm at 2500K gas temperature. As shown below in the numerical example of a 30,000 kw reduction furnace the design value of the voltage gradien~ is only a fraction of this critical value. The des.ign of the reduction furnace is governed by the desired residence time and not by the permissible voltage gradient.
~ te probability for concentration of the discharge current into an arc filament is further reduced by the heat ~bsorbed by the reaction which stabilizes the temperature, and by continuous stretching of fluid lines by the vortex - motion of the gases in the furnace.
; In contrast to an arc the heat input of a dispersed discharge is spread out over the entire interior high tempera-ture zone of the reactor. Consequently, no fraction of the ~ee~ material can pass through the furnace unreacted and .: contaminate the product stream.
THE REDUCTION PROCESS
A 30,00~ kw reduction unit is used as an example for tlt~ description of the process. Aluminum oxide powder is carried by a stream of natural gas or recycled product gas and injected with a moderate tangential velocity compo-nent into the furnace 10. The gaseous jets entering into the furnace maintain strong turbulence in the medium. The turbulent motion keeps the solid particles in suspension and prevents the discharge currents from concentrating into arc _~_ ~ilaments.
The dust laden gases circulate around the vertical ~xis of the furnace and move slowly inward. They are pre~
heated in the outer zone of the furnace by heat radiated from the hot inner zone of the furnace. Natural ga~" or other hydrocarbons, are decomposed in this preheat zone into hydrogen and fine carbon particles. The A1203 and carbon particles are retained in the furnace by the centrii`ugal force of the vortex motion until they are consumed by the reaction. Thus, while the residence time of the gases in the hot zone of the furnace is in the order of a second, the particles are retained for a much longer time, depending upon their size. The gaseous reaction products leave the furnace 10 through the central exit 24 and pass into condenser 22 where approximately 97,' of the aluminum vapor is condensed while the temperature of the gas stream is reduced from 2000C to 1600C. The condensed liquid aluminum is separated fr~m the gas stream in liquid-gas separator 30 and discharged in a continuous stream of liquid metal. Waste heat is removed from the condenser 22 by a heat transfer medium, like hydrogen or helium, circulating in a closed loop. A
substantial fraction of this waste heat may be reconverted into elec~rical power by a gas turbine. -The gas stream may pass through a second condenser (not shown) where the rest of the metal containing impurities is condensed. The gas stream leaving the condensers passes through a cooler and a cyclone separator, where the solid products of the back reaction, which are in the form of fine dust particles, are removed. These solids, consisting mainly of A1203 A14C3 and A120C are recycled with the feed material into the reduction furnace. In the condenser 22 -a.ld in the separator 30, the high surface tension of molten aluminum prevents the fine oxide and oarbide parti.cles from entering into the molten metal.
Electric power, for example, in the form of three-phase 6Q cycle a.c. is fed into the reduction furnace 10 through the carbon electrodes 16. The electric current may leave the electrodes 16 in the form of arc filaments which pass through the cooler outer zones and disperse rom there in the form of a dispersed discharge into the hot reaction 10 v~lume Thermal radiation originating from the hot reaction volume is intercepted by the clouds of Al2O3 and carb~n particles carried by the gas in the outer regions of the furllace. Thereby, the walls of the furnace are protected Erom strong heat radiation and heat losses from the furnace are kept a~ a moderate level. P~adiated heat absorbed by the particles is utilized for preheating the feed s-tream.
Experience with liquid sprays shows that coagulation of droplets remains insignificant as long as the volume of the gas in which the droplets are dispersed is more than 5, noo times the total volume of the liquid droplets. At the temperatures where the oxide particles begin to soten, the ratio Gf gas volume to the volume of solid particles is much l~r~,er ~han the above limit. Consequently, significant coagulation of the oxide particles in the furnace is not to be expected.
Aluminum production rate of a 30,000 kw reduction furnace would be, with 7 kwh energy input into the furnace per pound of Al, approximately 2 t/h or 16,000 t/year.
Power from the waste heat recovery turbine 4,300 kw 7~

-Power from exhaust gas powered tur-bine 8,000 kw Outside power supply 17,700 kw 30,000 k~J
Assuming a residence time of one second for the gases in the high temperature reaction zone, the dimensions of the furnace are calculated to be:
Active volume 18 m3 Total volume 36 m Diameter - 4.5 m = 15' Height 2.25 m = 7.5' Diameter of electrode circle 3.6 m Electrode diameter (6 electrodes) 6"
Power input density 2 watts/cm3 Average voltage gradient 15 volts/cm Phase voltage 2700 volts Electrode current (6 electrodes) 1850 amps.
: Line voltage 4700 volts Line current 3700 amps Furnace exit duct diameter 25 cm = lQ"
Circumferential velocity at exit 100 m/sec Circumferential velocity at the wall 6 m/sec Centrifugal acceleration at the exit 10,000 g Particles larger than 10 diameter are retained by the centriugal force.
Radiation loss through e~it area 200 Heat loss through the walls 300 kw Condenser for ~1 vapor:
Inlet Outlet Temperature 2,000C 1,600C
Pressure of Al vapor 136 mm Hg 4 mm Hg Flow velocity 600 m/sec 400 m/sec ; :

~iameter of con~enser tubes 2.5 cm Number of tubes 80 Length o~ tubes 75 cm Pressure drop 0.15 atm Residence time in condenser tube 1.5.10 3 sec Required compressor work 60 ~
Below 2000~C, the reaction reverses and Al is oxidi~ed by CO. The xate of this back reaction is not kno~n. However, most of the metal is condensed in a frac-tion of the time required for the product gases to passthrough the condenser tube, that- is, in a fraction of a mill.isecond. The time element for reducing the temperature o the Al vapor is very important and should be as short as possiblc. The time required to reduce the temperature from 2000C to 1600C can be further shortened by using tubes of smaller diameter and length and allowing higher pressure drop in the condenser. The condenser may have other geometri-cal configuration to reduce the residence time in the critical temperature interval. For example, it may consist of a bundle of tubes or ribs over w'nich the hot gas stream is passing.
l~e conditions for the existence of the dispersed electric discharge in the furnace may be established by heating the furnace and the gas stream by natural gas-oxygen flllles or by electric arcs and evaporating some aluminum in the furnace. Once the discharge has been started the required ionization level is maintained by the long residence time of the recirculating hot gases.
The temperature in the reduction furnace at t:he exit therefrom is well above the decomposition temperature of aluminum carbide (2200C~. Downstream o:E the furnace, ..~uminum ~nd ccarbon are not in contact any more. Aluminum carbide formation is therefore not expected in the reduction process.
Particles of Al203 and carbon are retained in the furnace longer than the gases. The amount of carbon present could therefore at times deviate lrom the amount required to complete the reaction. Fine particles of excess carbon can be carried out from the furnace ~ith the product gases. The characteristic radiation of carbon emitted from the exit gas may be us~d for the automatic control of the natural gas flow rate and the power input rate into the furnace. For e~a~ple, as soon as radiation of carbon particles is detected, the CH4 low rate into the reactor is slightly reduced or some oxygen can be introduced into the reactor to burn the excess carbon to C0. This feedback control assures the purity of the product. The electric power input is con-trolled by the voltage impressed on the electrodes.
The thermal reduction process described above is a continuous process well suited for large capacity units and automatic controls. Liquid metal is collected at one point and discharged in a continuous stream. Power consumption of the process will be significantly lower than tha~ of the best Hall Process plants, and comparable with the ultimately expecte~ power consumption rate of the new chlorine process.
Stc~ndard three-phase a.c. current can be used, saving the cost of conversion to d.c. power. Interruptible power supp'y is acceptable as no molten materials, except aluminu~, are handled in bulk. Purity of the metal will be exceptionally high because no impurities are introduced with l.he reducing agent and the liquid metal is obtained by condensation.
El~ctrode consumption will be minimal. The system is ti. ~lly closed and has no harmful emissions. The production capacity of ~wo or three reduction furnaces will equal that of the largest Hall Process pot lines. The area of reduc.tion plants and the required capital investment will be substantial-ly lower. The use of interruptible power, smaller plant, and absence of air pollution will allow greater-freedom in the choice of plant location. This in turn can reduce the cost o~ transportation of alumina and ingot.

Claims (19)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A thermal reduction process for reducing metal oxides in a reactor comprising:
A. introducing the metal oxide in powder form and a reductant in a gaseous medium into a reaction zone of the reactor with a tangential component to create a vortex motion;
B, maintaining a dispersed electrical discharge in the reaction zone through a minimum turbulence level of the gaseous medium;
C. maintaining a temperature in the reaction zone above a reduction-reaction tempera-ture of said oxide;
D. reducing said powder to metal vapor;
E. retaining said powder in the said reaction zone through centrifugal force until reduced;
F. removing an effluent stream of gases including the metal vapor from said reaction zone; and G. converting the metal vapor to the liquid state.

2. The process of Claim 1, said metal oxide being aluminum oxide.

3. The process of Claim 2, said temperature being maintained in excess of 2000°C.

4. The process of Claim 2 including the step of establishing the dispersed discharge within the reactor zone in the presence of aluminum vapor.

5. The process of Claim 2 wherein the con-verting step comprises condensing the aluminum vapor into the liquid state external of the reaction zone.

6. The process of Claim 2 including the further step of monitoring the effluent stream for excessive reductant feed.

7. The process of Claim 6 including controlling the flow rate of the reductant into like reaction zone in response to a signal that reductant is in the effluent stream.

8. The process of Claim 6 including intro-ducing oxygen into the reaction zone in response to a signal that reductant is in the effluent stream.

9. The process of Claim 2 wherein the reduc-tant is selected from the group consisting of natural gas, hydrocarbon gas other than natural gas and solid carbon.

10. The process of Claim 5 wherein the condensing step includes passing the effluent stream through a condenser wherein the temperature of the stream is rapidly reduced to at least 1600°C.

11. The process of Claim 10 wherein the effluent stream enters the condenser at a flow velocity of at least about 600 m/sec.

12. The process of Claim 10 including separating the liquid aluminum from the effluent stream of gas in a liquid-gas separator and discharging a stream of liquid aluminum.

13. The process of Claim 3 wherein said temperature in the reaction zone in the area of the effluent stream removal is maintained at about 2400°C.

14. The process of Claim 4 including establish-ing said dispersed discharge between spaced electrodes by filling said space with aluminum vapor and ionizing said aluminum vapor.

15. The process of Claim 2, said reductant comprising the gaseous medium.

16. The process of Claim 2, said introducing step comprising introducing said oxide powder, reductant and gaseous medium through a plurality of spaced and aligned jets.

17. The process of Claim 6 wherein a free carbon content of the effluent stream is monitored.

18. The process of Claim 2 wherein the minimum turbulence level is defined by a characteristic time in the reaction zone on the order of 103 seconds or less.

19. A process for thermally reducing alumi-num oxide powder to aluminum in a reactor comprising:
A. establishing a dispersed electric dis-charge within a reactor zone in the presence of aluminum vapor to maintain a center portion of said zone of at least 2000°C;
B. introducing aluminum oxide powder and natural gas as introductants into the reaction zone with a tangential compo-nent to create a vortex motion;
C. maintaining the dispersed discharge in the reaction zone through a minimum turbulence level of the introductants;
D. reducing said powder to aluminum vapor and retaining said powder in said reac-tion zone until reduced;
E. removing an effluent stream of gases including aluminum vapor from said reaction zone; and F. reducing the temperature of the effluent stream in a condenser rapidly to 1600°C
to condense the aluminum vapor to molten aluminum.