GB2038880A - Reduction of Metal Oxide in Dispersed Electrical Discharge - Google Patents

Abstract

The process comprises 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; maintaining a dispersed electrical discharge in the reaction zone, the gaseous medium in the reaction zone having a turbulence such as to keep the powder in suspension and to prevent the dispersed discharge from forming electrical arcs; maintaining a temperature in the reaction zone above a reduction-reaction temperature of said oxide; reducing said powder in the reaction zone to metal vapour; retaining said powder in the said reaction zone through centrifugal force until reduced; removing an effluent stream of gases including the metal vapour from said reaction zone; and converting the metal vapour to the liquid state. <IMAGE>

Description

SPECIFICATION Thermal Reduction Process This invention relates to a thermal reduction process for reducing a metal oxide in a reactor wherein a dispersed electrical discharge is employed for heating a reaction zone of the reactor. The process is applicable to the production of metals such as aluminium from their oxides.

The traditional method of producing aluminium (Hall Process) is by the electrolysis of aluminium oxide dissolved in molten cryolite. This normally takes place in a large number of electrolytic cells connected in series. Consumable anodes of carbon connected as the positive poles of the cells 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 anode. 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 aluminium include reducing aluminium oxide by carbon to aluminium trichloride in a reactor. Thereafter, aluminium is produced from the aluminium trichloride in a completely enclosed electrolytic 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 necesary temperatures. Representative of these processes are those disclosed in United States Patent Specifications Nos. 3,365,185: 3,563,726 and 3,765,870, with the latter patent specification being directed to the reduction of metal oxides other than aluminium.

According, therefore, to the present invention, there is provided a thermal reduction process for reducing a metal oxide in a reactor comprising 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; maintaining a dispersed electrical discharge in the reaction zone, the gaseous medium in the reaction zone having a turbulence such as to keep the powder in suspension and to prevent the dispersed discharge from forming electrical arcs; maintaining a temperature in the reaction zone above a reduction-reaction temperature of said oxide; reducing said powder in the reaction zone to metal vapour; retaining said powder in the said reaction zone through centrifugal force until reduced; removing an effluent stream of gases including the metal vapour from said reaction zone; and converting the metal vapour to the liquid state.

The process of the present invention may therefore be completely closed, thereby eliminating the pollution problems associated with open cells. In contrast to an arc, the said dispersed electrical discharge 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 to d.c.

power. The dimensions of electrodes used in producing the dispersed electrical discharge may be much smaller than those of an arc system and the electrode consumption is minimal. The metal oxide and reductant can be easily introduced into the reaction zone and the length of the gap between the electrodes is not critical.

A long residence time can be provided for the reactants, thereby ensuring that the oxide is retained until consumed by the reaction and converted into metal vapour. As a result, the functions of a centrifugal separator are combined with a reduction process thereby ensuring a high efficiency of operation. The purity of the metal is very high because no impurities are introduced with the reductant and the liquid metal is obtained by condensation.

A feedback control system may be provided by monitoring the radiation emitted by the effluent stream. This feedback control can be related to the flow rate of the reactants or to an oxygen input into the reactor thereby ensuring the purity of the product. In the case of aluminium oxide, by maintaining high temperatures in the area of the discharge through the dispersed discharge, aluminium carbide and other undesirable by-products from the effluent stream can be eliminated.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a vertical section of a reduction furnace for carrying out a thermal reduction process according to the present invention.

Figure 2 is a horizontal section through the furnace of Figure 1, and Figure 3 is the oxygen potential diagram for aluminium oxide.

The thermal reduction process described herein utilizes a dispersed electrical discharge for heating a reduction furnace. In contrast to an electric arc, high power input into a distributed discharge is maintained at conveniently high voltages (e.g. 3000 to 5000 volts) and moderate current levels. The example described below utilizes natural gas as the reducing agent, although obviously any other hydrocarbon or solid carbon could be used for this purpose. The process is primarily applicable to the reduction of aluminium oxide to aluminium, but other metals such as magnesium can be produced from their oxides by the process of the present invention.

A reactor or reduction furnace, generally designated 10, is schematically shown in Figures 1 and 2. A reaction chamber or zone 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 1 5. The width of the reaction zone is substantially greater than its axial height as a matter of design preference.

A plurality of electrodes 1 6 (six being illustrated in Figure 2) extend through the roof 13 into the reaction zone 14 in spaced relationship. The electrodes 16, generally of carbon or graphite, are surrounded by appropriate insulation jackets 1 8. Also entering the reaction zone 1 4 through the wall 12 of the reactor 10 are input jet orifices 20. The jet orifices 20 are spaced about the wall 12 so that each jet enters the reaction zone 14 substantially intermediate two adjacent electrodes 1 6. 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 1 5 along the vertical axis thereof. The exit duct 24 communicates both with the reaction zone 1 4 and 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 an inlet 36 and leaves through an outlet 26. 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 outwardly therefrom.

Waste heat can be removed from the condenser 22 by a heat transfer medium such as hydrogen or helium, circulated in a closed loop.

The conditions necessary for the thermal reduction of Awl203 are shown in the oxygen potential diagram, Figure 3. The lines representing the oxidation of Al to AI2O3 and C to CO cross at 20000 C.

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 Awl20, AlO, AIDS3 and Al20C, the temperature in the reactor must exceed 20000C and is preferably above 22000 C. In the following examples a gas temperature of 24000C is assumed in the high temperature zone of the reactor. At this temperature, aluminium is in vapour phase, as shown by the following Table 1.

Table 1 Vapor Pressure of Aluminium % Condensed from Initial Temperature OC Pressure mm Hg 136 mm Hg Pressure 2100 400 0 2000 190 0 1900 75 44 1800 30 77 1700 12 91 1600 4 97 The aluminium vapour leaves the reactor with the exit gases, and must be recovered by condensation.

The overall reaction is represented by the equation: 1 Awl203+3 CH4=2 Ai+3 C0+6 H2 The reaction requires 20 cubic feet of natural gas per pound of aluminium produced and yields 60 cubic feet of carbon monoxide and hydrogen mixture as a by-product.

Heat of reaction is at 2980K 375.0 kcal/g mole of Al Heat content of the reaction products at 27000K 338.2 kcal/g mole of Al Total heat requirement 713.2 kcal/g mole of Al In large units the heat losses and compressor work are approximately compensated by preheating the feed material to substantially 3000 C.

The total electrical power requirement is therefore 7.0 kwh/l lb Al Waste heat available from the aluminium condenser above 6000C is 294 kcai/g mole. With 35% conversion efficiency, this amounts to 1.0 kwh/l Ib Al Net electric power requirement without utilization of the by-product gas is 6.0 kwh/l Ib Al Power generated from by-product gas with 35% efficiency amounts to 1.85 kwh/l Ib Al Net power requirement 4.15 kwh/l IbAl 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 kwh per pound of aluminium.

Powerful electrical discharge currents are prevented from concentrating into narrow arc tilaments by turbulent mixing if the following dispersion criterion is fulfilled:

where Ne is the number of electrons per cm3 T is the gas temperature, OK u' is the intensity of turbulence, cm/sec I - is the characteristic time of turbulence, sec u' E is the charge of the electron, 1.59 10'9 coulomb p is the density of gas, g/cm3 I is the scale of turbulence, cm cm/sec ke is the mobility of electrons volt/cm Cp is the heat capacity of gas, cal/g OC E is the voltage gradient, volt/cm The ionization potential of Al is low, being only 5.984 volts.The ion-electron concentration of the product gas, consisting of 136 mm Hg Al, 200 mm Hg CO and 400 mm Hg H2, due to thermal ionization of aluminium vapour is given below in Table 2 as a function of the gas temperature.

Table 2 lon Electron Concentration In Product Gas Containing 136 mm Hg Al 200 mm Hg CO 400 mm Hg H2 dNe 1 l/cm3{ Temperature OK Ne dT cm3 oC 2300 0.34x 1 013 0.29x1011 2400 0.63x1013 0.52x1011 2500 1.5x1013 0.8x1011 2600 1.97x1013 1.31x1011 2700 3.28x1013 2.or 1011 2800 5.3x10'3 2.7x1011 2900 8.0x1013 4.2x1011 Turbulence with a characteristic time of substantially 10-3 seconds or less is maintained by the gaseous jets entering the reactor.At this characteristic time of turbulence the critical voltage gradient for dispersion of the discharge is calculated to be 31.6 volts/cm at 25000K gas temperature. As shown below, in the numerical example of a 30,000 kw reduction furnace, the design value of the voltage gradient is only a fraction of this critical value. The design of the reduction furnace is governed by the desired residence time and not by the permissible voltage gradient.

The probability for concentration of the discharge current into an arc filament is further reduced by the heat absorbed 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 temperature zone of the reactor. Consequently, no fraction of the feed material can pass through the furnace unreacted and contaminate the product stream.

A 30,000 kw reduction unit is used as an example for the description of the process. Aluminium oxide powder is carried by a stream of natural gas or recycled product gas and injected with a moderate tangential velocity component into the furnace 1 0. 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 filaments.

The dust laden gases circulate around the vertical axis of the furnace and move slowly inward.

They are preheated in the outer zone of the furnace by heat radiated from the hot inner zone of the furnace. Natural gas, or other hydrocarbons, are decomposed in this preheat zone into hydrogen and fine carbon particles. The Awl2 03 and carbon particles are retained in the furnace by the centrifugal 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 of 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 exit duct 24 and pass into condenser 22 where approximately 97% of the aluminium vapour is condensed while the temperature of the gas stream is reduced from 2000"C to 1 600cC. The condensed liquid aluminium is separated from the gas stream in the liquid-metal-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, such as hydrogen or helium, circulating in a closed loop. A substantial fraction of this waste heat may be reconverted into electrical 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 Al203, Al4C3 and AI20C are recycled with the feed material into the reduction furnace. In the condenser 22 and in the separator 30, the high surface tension of molten aluminium prevents the fine oxide and carbide particles from entering into the molten metal.

Electric power, for example, in the form of three-phase 60 cycle a.c. is fed into the reduction furnace 10 through the carbon electrodes 1 6. The electric current may leave the electrodes 1 6 in the form of arc filaments which pass through the cooler out zones and disperse from there in the form of a dispersed discharge into the hot reaction volume.

Thermal radiation originating from the hot reaction volume is intercepted by the clouds of Awl 303 and carbon particles carried by the gas in the outer regions of the furnace. Thereby, the walls of the furnace are protected from strong heat radiation and heat losses from the furnace are kept at a moderate level. Radiated heat absorbed by the particles is utilized for preheating the feed stream.

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 5,000 times the total volume of the liquid droplets. At the temperatures where the oxide particles begin to soften, the ratio of gas volume to the volume of solid particles is much larger than the above limit. Consequently, significant coagulation of the oxide particles in the furnace is not to be expected.

Aluminium 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 1 6,000 t/year.

Power from the waste heat recovery turbine 4,300 kw Power from exhaust gas powered turbine 8,000 kw Outside power supply 17,700 kw 30,000 kw 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 m3 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 1 5 volts/cm Phase voltage 2700 volts Electric current (6 electrodes) 1850 amps Line voltage 4700 volts Line current 3700 amps Furnace exit duct diameter 25 cm=1 0" 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 1 owl diameter are retained by the centrifugal force.

Radiation loss through exit area 200 kw Heat loss through the walls 300 kw Condenser for Al vapour: Inlet Outlet Temperature 2,0000C 1,6000C Pressure of Al vapour 136 mm Hg 4 mm Hg Flow velocity 600 m/sec 400 m/sec Diameter of condenser tubes 2.5 cm Number of tubes 80 Length of tubes 75 cm Pressure drop 0.15 atm Residence time in condenser tube 1.5.10-3 sec Required compressor work 60 kw Below 2000 C, the reaction reverses and Al is oxidized by CO. The rate of this back reaction is not known. However, most of the metal is condensed in a fraction of the time required for the product gases to pass through the condenser tube, that is, in a fraction of a millisecond. The time element for reducing the temperature of the Al vapour is very important and should be as short as possible.The time required to reduce the temperature from 2000C to 1 600C can be further shortened by using tubes of small diameter and length and allowing a higher pressure drop in the condenser. The condenser may have other geometrical configuration to reduce the residence time in the critical temperature interval. For example, it may consist of a bundle of tubes or ribs over which the hot gas stream is passing.

The 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 flames or by electric arcs and evaporating some aluminium 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 the exit therefrom is well above the decomposition temperature of aluminium carbide (22000C). Downstream of the furnace, aluminium and carbon are not in contact any more. Aluminium carbide formation is therefore not expected in the reduction process.

Particles of Awl203 and carbon are retained in the furnace longer than the gases. The amount of carbon present could therefore at times deviate from the amount required to complete the reaction.

Fine particles of excess carbon can be carried out from the furnace with the product gases. The characteristic radiation of carbon emitted from the exit gas may be used for the automatic control of the natural gas flow rate and the power input rate into the furnace. For example, as soon as radiation of carbon particles is detected, the Cm, flow rate into the reactor may be slightly reduced or some oxygen can be introduced into the reactor to burn the excess carbon to CO. This feedback control ensures the purity of the product. The electric power input is controlled 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 that of the best Hall Process plants, and comparable with the ultimately expected power consumption rate of the new chlorine process. Standard three-phase a.c. current can be used, saving the cost of conversion to d.c.

power. Interruptible power suppiy is acceptable as no molten materials, except aluminium, are handled in bulk. Purity of the metal will be exceptionally high because no impurities are introduced with the reducing agent and the liquid metal is obtained by condensation. Electrode consumption will be minimal. The system is totally closed and has no harmful emissions. The production capacity of two or three reduction furnaces will equal that of the largest Hall Process pot lines. The area of reduction plants and the required capital investment will be substantially 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 of transportation of alumina and ingots.

Claims (21)

Claims

1. A thermal reduction process for reducing a metal oxide in a reactor comprising 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; maintaining a dispersed electrical discharge in the reaction zone, the gaseous medium in the reaction zone having a turbulence such as to keep the powder in suspension and to prevent the dispersed discharge from forming electrical arcs; maintaining a temperature in the reaction zone above a reduction-reaction temperature of said oxide; reducing said powder in the reaction zone to metal vapour; retaining said powder in the said reaction zone through centrifugal force until reduced; removing an effluent stream of gases including the metal vapour from said reaction zone; and converting the metal vapour to the liquid state.

2. A process as claimed in claim 1, in which said metal oxide is aluminium oxide.

3. A process as claimed in claim 2 in which said temperature is maintained in excess of 20000C.

4. A process as claimed in claim 2 or 3 including the step of establishing the dispersed discharge within the reaction zone in the presence of aluminium vapour.

5. A process as claimed in any of claims 2-4 wherein the converting step comprises condensing the aluminium vapour into the liquid state externally of the reaction zone.

6. A process as claimed in any of claims 2-5 including the further step of monitoring the effluent stream for excessive reductant feed.

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

8. A process as claimed in claim 6 or 7 including introducing oxygen into the reaction zone in response to a signal that reductant is in the effluent stream.

9. A process as claimed in any of claims 2-8 wherein the reductant is selected from the group consisting of natural gas, hydrocarbon gas other than natural gas, and solid carbon.

10. A process as claimed in 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.A process as claimed in claim 10 wherein the effluent stream enters the condenser at a flow velocity of at least about 600 m/sec.

12. A process as claimed in claim 10 or 11 including separating the liquid aluminium from the effluent stream of gas in a liquid-gas separator and discharging a stream of liquid aluminium.

13. A process as claimed in claim 3 wherein said temperature in the reaction zone in the area of the effluent stream removal is maintained at substantially 24000 C.

14. A process as claimed in claim 4 including establishing said dispersed discharge between spaced electrodes by fiiling said space with aluminium vapour, and ionizing said aluminium vapour.

15. A process as claimed in claim 2 in which said reductant comprises the gaseous medium.

1 6. A process as claimed in claim 2, in which said introducing step comprises introducing said oxide powder, reductant and gaseous medium through a plurality of spaced and aligned jets.

1 7. A process as claimed in claim 6 wherein a free carbon content of the effluent stream is monitored.

18. A process as claimed in claim 2 wherein the minimum turbulence level is defined by a characteristic time in the reaction zone on the order of 10-3 seconds or less.

1 9. A process for thermally reducing aluminium oxide powder to aluminium in a reactor comprising establishing a dispersed electric discharge within a reaction zone in the presence of aluminium vapour to maintain a temperature in a centre portion of said zone of at least 20000 C; introducing aluminium oxide powder and natural gas as introductants into the reaction zone with a tangential component to create a vortex motion; maintaining the dispersed discharge in the reaction zone through a minimum turbulence level of the introductants: reducing said powder to aluminium vapour and retaining said powder in said reaction zone until reduced; removing an effluent stream of gases including aluminium vapour from said reaction zone; and reducing the temperature of the effluent stream in a condenser rapidly to 1600C to condense the aluminium vapour to molten aluminium.

20. A thermal reduction process for reducing a metal oxide in a reactor substantially as hereinbefore described with reference to the accompanying drawings.

21. A thermal reduction process for reducing metal oxide 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 temperature of said oxide: D. reducing said powder to metal vapour; E. retaining said powder in the said reaction zone through centrifugal force until reduced; F. removing an effluent stream of gases including the metal vapour from said reaction zone: and G. converting the metal vapour to the liquid state.