DE2900649C2 - Process for the thermal reduction of metal oxides - Google Patents

Description

The invention relates to a method for the thermal reduction of metal oxides and the extraction thereof formed metals, in particular for the reduction of aluminum oxide to aluminum, in a reactor at a Temperature above the reaction temperature of the oxides, d. H. in the case of aluminum oxide above 2000 ° C: that The exhaust gas from the reactor, which contains metal vapors, is condensed and the melt for further processing guided. The proportion of unreacted carbon or metal oxide in the exhaust gas is used as a measured variable for feeding in the starting materials.

Aluminum is produced to a large extent by melt flow electrolysis made of alumina in cryolite. This requires a large number of electrolysis cells, in which carbon anodes are used. The thermal energy required for electrolysis is applied by resistance heating. The melt flow electrolysis is in terms of apparatus and operation Considerable costly and the fluorine-containing exhaust gases are often a considerable cost factor

the need for gas scrubbing.

One is therefore striving. Aluminum by reducing aluminum oxide with carbon to form To obtain aluminum trichloride, which decomposes the latter to aluminum in a closed electrolysis cell can be.

Processes have also become known in which fine ores are placed in a reaction zone for reduction introduced and the necessary temperature generated by electrical resistance heating or an electric arc (US-PS 33 65 185,35 63 726 and 37 65 870).

From US-PS 32 32 746 a method for the reduction of metal oxides is known in which the required Thermal energy is applied by burning common fuels. From DE-OS 26 50 324 is a Process for reducing metal oxides in a plasma torch produced between the cathode and anode Arc in the presence of a reducing agent known. Serves to stabilize the plasma lance a gas flow guided along the electrodes. This stabilization gas is the plasma-forming gas hydrogen or natural gas. Molten ore flows down the chamber wall and in this layer of melt on the The reduction takes place in the furnace wall.

The application is based on the object of a method for reducing metal oxides, in particular Alumina to provide in a completely closed system so that all with air pollution related problems associated with open cells are eliminated.

The problem posed is achieved by a method with the characterizing features of the claim I.

Claims 2 - 5 relate to training courses

J5 driving according to claim 1.

In the method according to the invention, instead of an arc according to the prior art, a Stray discharge applied, which works with a high voltage and accordingly lower amperage required than this with one: arc would be required for the same performance. In the inventive You can work with the usual three-phase alternating current and do not need that Rectification and the associated effort.

In the method according to the invention, the dimensions of the electrodes are significantly smaller than in the case of the electrodes with arc working systems, electrode consumption is minimal. The starting material can simply introduce it into the high temperature reaction zone and the electrode spacing is not a critical factor.

In the process according to the invention, a long residence time of the reactants in the reaction zone is ensured, so that a complete reduction of the oxides to the metal vapor is ensured. Finally, it should also be noted that the function of a centrifugal separator is integrated into the reduction process in the method according to the invention. so that this has a high overall efficiency; the metal obtained is particularly pure because no impurities are introduced with the reducing agent; eventually the metal vapors are condensed.
The feed of the starting material is regulated via the radiation emitted by the exhaust gas as a measured variable. This regulation is closely related to the flow rate of the reactants or the oxygen supply into the reactor, so that a high purity

29 OO 649

the product is guaranteed. By maintaining of high temperatures in the area of the discharge due to the scatter discharge as a heat source no carbides (especially aluminum carbide) or other undesirable by-products associated with the Exhaust gas are discharged. According to the method according to the invention, in particular for the extraction of aluminum, the reduction is carried out in a reactor, with a high temperature reaction zone a stray discharge is maintained in the presence of the metal vapor (aluminum vapor). That Oxide powder (aluminum oxide powder) and a reducing agent such as natural gas, coal or carbon in one gaseous medium are introduced into the reaction zone with tangential movement component to a causing swirling motion. This circular motion is maintained over a minimal amount of turbulence, so that the solid particles are kept in suspension and the stray discharge is maintained. The oxide powder is reduced to metal vapor and this is removed from the reaction scene with the reducing gas discharged and finally condensed

As mentioned, for the process according to the invention for the thermal reduction of metal oxides a electrical stray discharge applied for heating in contrast to an electrical arc takes place the discharge at high power generally with high voltages and moderate amperage (about 300 up to 5000 V). As already mentioned, natural gas can be used as the reducing agent; but they are suitable also other hydrocarbons or coal or carbon. The method according to the invention is described in Primarily used to reduce aluminum oxide to aluminum, but is also suitable for extraction other metals such as magnesium.

The invention is further explained with reference to the drawings; in these shows

F i g. 1 shows a vertical section through the reduction furnace zr- carrying out the method according to the invention.

F i g. 2 shows a horizontal section through the furnace according to FIG. 1.

F i g. 3 is a diagram showing the oxygen potential for aluminum oxide as a function of the temperature.

According to FIG. I and 2, the reduction furnace 10 has a reaction chamber 14 within the cylindrical refractory Oven vessel 12 with a curved bottom 15 and a curved top 13. The width of the reaction zone is much greater than the axial height due to this construction.

A number of electrodes 16 (six are shown in Fig. 2) penetrate the ceiling 13 and reach into it Reaction zone 14. They generally consist of carbon or graphite and are provided with a corresponding Insulation 18 provided. The nozzles 20 feed the starting material into the reaction zone. They are arranged in the furnace vessel 12 that the starting material is substantially in the middle between two adjacent electrodes 16 enters the reaction chamber 14. Your position in the oven will be like this chosen that the starting material! gets a moderate tangential velocity.

The reaction products exit the chamber 14 through the discharge nozzle 24 in the bottom 15 in the vertical axis of the furnace from Mod enter the condenser 22. The condenser ^ is generally a surface cooler which contains a plurality of cooling tubes 28, to which cooling medium is supplied via 26 and is deducted at 36. The separator 30, from which the melt is discharged at 32 and the exhaust gases at 34, is located in the lower area of the condenser. The waste heat from the condenser 22 can be obtained with a heat transfer medium such as hydrogen or helium if it circulates in a closed circuit.
The conditions for the thermal reduction of

to Al ₂ O ₃ are in the diagram of FIG. 3 shown. The straight lines correspond to the oxidation of Al to Al ₂ O ₃ and of C to CO; they cross at 2000 ° C. Above this temperature, carbon absorbs the oxygen from the metal oxide with the formation of CO. For high reaction rate and for the decomposition of unwanted by-products, such as Al ₂ O _3, AlO, Al ₄ C ₃ and Al ₂ OC, the temperature in the reactor over 2000 ⁰ C. should preferably be over 2200 ° C, are. In the following examples, a gas temperature of 2400 ° C. is assumed in the reaction zone. At diev. Temperature reaches the vapor pressure of the aluminum di "values compiled in Table 1.

Tabel'

"C mbar % Condensate at initially 181 mbar 30 2100 532 0 2000 253 0 1900 100 44 1800 40 77 1700 16 91 35 1600 5 97

The aluminum vapor leaves the reactor together with the reaction gas and is then condensed.

The process is based on the following overall equation:

1 Al ₂ O ₃ + 3 CH ₄ = 2 Al + 3 CO + 6 H ₂

0.1243 mVkg natural gas is required for the reaction, 1.698 m ³ CO + H _{2 being formed} as a by-product. Based on 1 g-mol of aluminum, the heat of reaction at 298 K is 1575 kJ and the heat content of the reaction products at 2700 K is 1420 kJ. The total heat demand is thus 2995 kJ. In large systems, the heat losses and the energy requirements of the compressor are roughly compensated for by preheating the raw material to around 300 ° C.

For a total of 0.45 kg aluminum you need of electrical energy 7 kWh; the waste heat from the condenser of over 600 ° C makes you lied to g-mole aluminum 1235 kj. so that with a 35% efficiency 1 kWh results. The net requirement of electrical energy without utilization of the gas produced as a by-product is therefore 6 kWh. the end the by-product can achieve an energy gain of 1.85 kWh with a 35% efficiency, so that the total energy requirement is 4.15 kW η.

It is more economical to use the exhaust gas to manufacture fuels or chemical products. In In this case, the energy requirement of the method according to the invention can be reduced to 6 kWh.

High-energy discharge currents do not concentrate in a narrow arc if

29 OO 649

turbulent mixing takes place according to the following criteria.

( ^άΝ Λ

VJTJ

0.24 Ek _r PC ₁ ,

E ² > 2

N _e T u 'L

Number of electrons per cm ³

Gas temperature, K

Intensity of the turbulence in cm / s

—R characteristic time of turbulence in s

electric charge of the electron 1.59
Gas density in g / cm ³
Degree of turbulence in cm

Mobility of electrons

10 ¹⁹ Cb

V / cm

Cp specific heat of the gas 4.2 J / g · K

^ aI ₁ IiUIi ₆ J ₆ ,

The onization potential of aluminum is low and is only 5.984 V. The ion / electron concentration in the reaction gas containing 181 mbar aluminum, 266 mbar carbon monoxide and 532 mbar hydrogen due to the thermal ionization of the aluminum vapor is shown in Table 2 as a function of the Gas temperature specified.

Table 2 N, 10 ³ / cm ³ / dyV, \ ΙΟ ¹¹ K \ άΤ) cm ³ ■ K 0.34 0.29 2300 0.63 0.52 2400 1.5 0.8 2500 1.97 1.31 2600 3.28 2.0 2700 5.3 2.7 2800 8.0 4.2 2900

The turbulence with a characteristic time of about 10 "'s or less is caused by the in the reactor Maintain incoming gas jet. At this characteristic time of turbulence is calculated the critical voltage gradient for the scattering of the discharge with 31.6 V / cm at a gas temperature of 2500 K. As shown in the following example for a 30,000 kW reduction furnace, the construction is the proportion of the stress gradient to be measured is only part of this critical value. The construction of the reduction furnace is determined by the desired dwell time and not by the permissible stress gradient

The possibility of concentrating the discharge current to form an arc thread is made possible by the Reaction required heat, which stabilizes the temperature and through constant stretching of the flow lines further reduced as a result of the circular movement of the gases in the furnace

In contrast to an electric arc, the thermal energy is spread over the whole with the help of a stray discharge High temperature reaction zone distributed inside the reactor. As a result, no part of the starting material can be used leave the furnace without reacting and contaminate the reaction product.

example

In a 30,000 kW reduction furnace, alumina powder was mixed with natural gas or recycled Rnak-S tion gas introduced with a moderate tangential velocity component. The ones in the oven Incoming gas flows lead to violent turbulence, as a result of which solid particles are held in suspension and the discharge currents are drawn apart so that they do not combine into an arc can.

The powder-laden gases rotate around the vertical axis of the furnace and slowly move along Inside. They are placed in the outer zone of the furnace due to thermal radiation from the hot inner zone of the Oven preheated Natural gas or other hydrocarbons are converted into hydrogen in this preheating zone and fine carbon cakes! decomposed. Al ^ O, and the Carbon particles remain in the furnace in a whirling motion by centrifugal force until they are complete have reacted. Thus, the residence time of the gases in the hot zone is of the order of 1 s while that of the particles is much longer, depending on their size. The reaction gases leave the Furnace 10 through 24 and enter the condenser 22, in which about 97% of the aluminum vapor is condensed, As a result, the temperature of the gas drops from 2000 to 1600 ° C. The aluminum melt separates in the separator 30 from the exhaust gas and is continuously discharged. The waste heat from the condenser 22 is on a heat transfer medium such as hydrogen or helium that circulates in a closed circuit. A A substantial part of this waste heat can be obtained as electrical power in a gas turbine.

The exhaust gas passes into a (not shown) second condenser, in which the rest of the metal is also included the impurities is condensed. The exhaust gas is now passed through a cooler and a cyclone separator guided so that the solids are removed from the opposing reaction as fine dust. This exists mainly as AI2O3, AI4C3 and AI2OC and is returned to the furnace with the starting material. The high surface tension of the molten Aluminum prevents the fine oxides and carbides from entering into the condenser 22 and separator 30 the melt.

The reduction furnace 10 is via the carbon electrodes 16 electrical current, for. B. 3-phase alternating current at 60 Hz, supplied The current leaves the electrodes 16 in the form of an arc, which the cool outside zones penetrates and splits into a scatter discharge in the hot reaction zone

The thermal radiation from the hot reaction zone acts on the clouds of AI2O3 and carbon particles, which are in the gas in the outer furnace area. This means that the furnace walls are protected from high levels of heat radiation protected and the heat losses from the furnace are kept low. The radiant heat is absorbed by the solids, which are preheated with it.

It was found that no larger droplets converge as long as the gas volume in which the droplets are located, is more than 50,000 times the total volume of the droplets At temperatures at which the oxide particles begin to soften, the ratio of gas volume is To solid volume much larger than the above limit value. So there is a noticeable coagulation

29 OO 649

of the oxide particles in the furnace are not to be expected.

In the 30,000 kW reduction furnace at an energy consumption 7 kWh - based on 0.45 kg aluminum - results in aluminum production of about 2 l / h or 16,000 t / y. The 30,000 kW required for the reduction furnace come to 4,300 kW from the waste heat turbine, 8000 kW from the exhaust gas turbine and 17 700 kW from an external power supply.

If one takes a residence time of the gases in the high temperature reaction zone from 1 s onwards, the following furnace dimensions are calculated:

Active volume 18 m ³ Total volume 36 m ³ diameter 4.5 m height 2.25 m diameter of the electrode circuit 3.6 m Electrode diameter (6 electrodes) 152.4 mm Power density 2 W / m ³ Mean stress gradient 15 V / cm Phase voltage 2700W Electrode current (6 electrodes) 1850A Line voltage 4700V Line current 3700A Discharge nozzle 25 cm Peripheral speed upon leaving 100 m / s Peripheral speed on the wall 6 m / s Centrifugal acceleration upon exit 98 067 m / s ² I cnCncfl "> ivfjifTi νγ- * 5Γα £ Ώ uürCii the ZöTitrifügslkr; held back.

Radiation loss at the output 200 kW

Radiation loss through the walls 300 kW

Altiminium vapor condenser:

input Output temperature 2000 ⁰ C 1600 ° C Vapor pressure mbar 181 53 Flow velocity 600 m / s 400 m / s Diameter of the cooling tubes 2.5 cm Number of cooling tubes 80 Length of the cooling tubes 75 cm Pressure drop 0.15 bar Dwell time in the cooling tubes 1.5 x 10 ^-3 s Energy requirement of the compressor 60 kW

Below 2000 ^° C., aluminum is reoxidized again by carbon monoxide. The reaction rate is not known. Most of the metal, however, is condensed within the time required for the reaction gas to flow through the condenser tubes, ie in a fraction of a millisecond. The quenching time of the aluminum fumes is very essential and such !! be as short as possible. The quenching time from 2000 to 1600 "C can be further reduced by using very narrow cooling tubes of greater length at the expense of a higher pressure drop in the condenser. The condenser can also have a different geometric configuration in order to reduce the residence time within the critical temperature range For example, tube bundles or finned tubes can be used around which the hot reaction gases flow.

The conditions for the existence of a stray discharge in the furnace are adjusted by heating

ίο the furnace and the gas flow with the help of natural gas / oxygen flames or electric arcing and evaporation of some aluminum in the furnace. Has once the discharge has ceased, the required ionization is maintained by a long dwell time. the circulating hot gases.

The temperature in the reduction furnace at the outlet connection is considerably higher than the decomposition temperature of aluminum carbide (2200 ^° C.). After the furnace, aluminum and carbon are no longer in contact with one another. An aluminum carbide bond is therefore not to be expected.

The solid particles, namely AI2O1 and carbon, are kept in the furnace longer than the gases. The current carbon content therefore sometimes gives way from the proportion that is required for a complete reaction. Excess carbon is used with the reaction gases discharged from the furnace. The characteristic radiation of the carbon in the reaction gas can be used for the automatic control of the supply of natural gas and the energy supply in pull up the stove. As soon as, for example, radiating carbon particles are detected, the Throttle the methane supply somewhat or introduce some oxygen into the reactor to remove the excess carbon to burn to CO. This control option ensures the high purity of the aluminum

regulated voltage.
The continuous process according to the invention is suitable for large-scale systems with automatic control. The molten metal collects in one place and is continuously discharged. The power consumption is much cheaper than with the best systems that work according to the Hall process.

and can be compared with the power consumption to be ultimately expected from the new chlorine process. at 3-phase alternating current can be used with the method according to the invention, so that no rectification is required is required. A power supply that can be switched off is permissible because, with the exception of aluminum, there is no molten material Material has to be handled in large quantities. The purity of the metal is unusual high, since no impurities are introduced with the reducing agent and the liquid metal passes through Condensation is obtained. The consumption of electrodes is minimal. The system is completely closed and there is no harmful emission. The production output of 2 or 3 furnace units corresponds that of the largest systems based on the Hall process; the space requirements and capital requirements of a system for However, implementation of the method according to the invention are significantly less. The application of an interruptible Power supply, a smaller system and the avoidance of air pollution greater freedom with regard to the choice of location for the systems. This can further reduce costs for the Avoid transporting the clay and aluminum.

For this purpose 2 sheets of drawings

Claims (5)

29 OO 649 claims: 1. Process for the thermal reduction of metal oxides, in particular aluminum oxide, with the aid of a reducing agent in a gaseous medium at a temperature above the reduction temperature and condensation of the metal from the reaction gases, in which the metal oxide as a fine powder in the reaction zone of a reaction furnace for vortex motion introduced and an expanded discharge maintained in the reaction zone is, characterized in that the metal oxide with a tangential Component is introduced into the reaction zone to maintain a circular motion, an expanded electrical discharge is maintained in the reaction zone at a minimal turbulence of the gaseous medium, the oxide powder by centrifugal force in the reaction zone is held until the reduction is complete and then the one leaving the reaction zone gaseous reaction product is quenched to condense the metal vapors. 2. The method according to claim 1, characterized in that in the reduction of aluminum oxide a temperature of at least 2000 ° C is maintained in the reaction zone. 3. The method according to claim 1 or 2, characterized in that the structure of the expanded electrical discharge carried out in the reaction zone in the presence of aluminum vapor will. 4. Procedure according to claims! to 3. characterized by that the metal oxide, the reducing agent and a gaseous medium by im Spaced and aligned streams is introduced into the reaction zone. 5. The method according to claim 1 to 4, characterized in that a minimum in the reaction zone Turbulence is maintained, the characteristic time of which is a maximum of 10-> s is.