NO310426B1 - Carbothermal process for the manufacture of metal - Google Patents

Description

The present invention relates to a new method for carbothermic production of metal such as aluminium, silicon, magnesium and the like. The process takes place inside a reaction chamber where metal oxides or compounds thereof are exposed to intense jet streams of hot combustion gases. The hot gases deliver heat to the materials and set them in an intense rotating motion that creates an artificial centrifugal force and generates frictional forces between the particles that generate additional heat to the process.

The present invention will discuss the method used in the production of aluminum and to some extent silicon.

Aluminum is per today produced in large-scale facilities worldwide by it

electrolytic Hall-Heroult process. The electrolytic bath contains aluminum oxide, cryolite and other additives. The electrolysis temperature is approx. 1000°C. Carbon is used as electrode material.

The main advantage of this method is that it is well developed and operates under stable conditions. It is also considered the cheapest way to produce aluminum from aluminum oxide (AI2O3) which is upgraded from bauxite by the Bayer process.

The Hall-Heroult process is a very energy-intensive process. Theoretical energy consumption is

about. 11 kWh per kg aluminum produced. Today's production technology operates on

the level 13-14 kWh per kg of aluminum produced. Intensive research is taking place worldwide to improve current technology with the aim of reducing energy consumption. In addition to the costs associated with energy consumption, the costs of aluminum are also affected by the costs of the materials used and gas cleaning facilities to meet environmental requirements.

Production of aluminum by carbothermic reduction of alumina has been and is still taking place in order to develop an alternative process by most of the large aluminum producers. But per today, no process has been developed that can meet the technical and environmental requirements.

Aluminum has been produced by direct reduction with carbon. All of these

the carbothermic production processes are static processes. The temperatures in the processes are above 2000°C. Such high temperatures cause construction and material problems. Since reoxidation of metal such as aluminum is a serious

problem at these high temperatures, an accurate control of back-reaction is necessary. On the other hand, the reduction of energy consumption and a significant reduction of capital costs is very interesting and has been the focus of research and development work on carbothermic processes for the production of aluminium.

As is known, the carbothermic process can be divided into two groups. First, pure alumina is produced from impure raw materials and then it is reduced to pure aluminum.

Second, the impure raw materials are completely reduced to form an alloy, usually consisting mainly of aluminum and silicon, and minor amounts of iron.

Aluminum can be produced from this alloy by separation and

refining processes.

In the present invention, however, both materials can be used for production

of aluminum or an aluminum alloy.

The present invention thus relates to a carbothermic process for the production of

metals such as aluminium, silicon, magnesium and the like from metal compounds, characterized by the metal compound(s) such as the oxide(s) or hydroxides being heated by combustion gases and internal friction between the particles in the feed generated by mechanical forces from the jet gases inside a process chamber, together with either free water or water-forming chemicals in the aluminum compound(s) and carbon and/or oil and/or natural gas in such a way that the material in the reactor chamber behaves as a hot mechanically established fluidized bed whereby the water can be split into hydroxyl and hydrogen radicals and causes the hydrogen radicals to react with the oxygen in the hot and now unstable aluminum compound(s) and thus replace aluminum atoms, and the hydroxyl radical reacts back to water and excess oxygen reacts with carbon to CO.

The present invention describes methods for producing aluminium, silicon and similar metals from metal by the carbon reduction process. In addition to using alumina as raw material, it is also possible in this process to use aluminum hydroxide Al(OH)3. The present method is a highly efficient cracking process of aluminum compounds to aluminium, water and CO at low pressure and temperature. The energy requirement is less than all other known methods and does not use cryolite and has limited emission of toxic gases.

The overall reaction for the carbothermic reduction of alumina can be written as:

This equation describes the net reaction, while the physical and chemical processes are far more complex. The reduction can take place in three stages:

In addition to the formation of aluminum carbide and aluminum oxycarbide, the following reactions also take place during the process:

The principle of the present invention, which is also called the Jet Cracking Process (JCP), is to treat the compounds in a pneumatically hot fluidized bed. The fluidized bed gives rise to the following effects to achieve the cracking: 1. In addition to the calorimetric energy from the propellant gases, heat will also be supplied from material friction and hydrodynamic forces. 2. Water, which is present on the raw materials, will evaporate at a reduced rate during the process and thus form microbubbles with extreme pressure and temperature. 3. Rapid cooling of the released heat from the microbubbles to the process ambient temperature. 4. Formation of temperature spots between the moving grains caused by internal frictional forces.

The fluidized bed takes place in the reactor chamber by jet streams of combustion gases that are injected tangentially into the reactor chamber.

The energy for the process is supplied from combustion gases - both thermally and kinetically. This is achieved by pressurized combustion, where the combustion gases are injected into the reactor in such a way that a rotating movement of the material in the reactor is generated. The energy transferred to the material will be a combination of the heat in the gases and the movement of the particles in the reactor. The rotation of the particles in the reactor will generate shear forces between the particles, which generate maximum temperatures and a centrifugal force, which will separate different phases in the reactor.

The aluminum compound is delivered to the process chamber together with the correct molar amount of carbon which is correctly balanced with the energy supply. Carbon can also be supplied from the combustion gas.

When the raw material is aluminum hydroxide instead of alumina, the water will evaporate after the so-called "flash evaporation". During the evaporation, ultrasonic vibration will be generated in the steam caused by the particles in motion and the pressure in the front of the gas streams.

Due to the extreme separation of the fluids in the solids that takes place before they evaporate, the separation in addition to the vibration generates microbubbles in the fluidized bed with extreme temperatures and pressures of several thousand degrees Kelvin and atmospheres whereby water is broken into hydrogen and oxygen radicals.

In previous experiments in a 25 kW reactor, it was found that water can be cracked at a process temperature of 250° and very high movement between the particles in the order of 40-200 m/s.

When the reactants enter the reactor under these conditions, they will be heated to these process temperatures as described above.

During the process, hydrogen radicals will react with oxygen in the alumina and form water. Hydroxyl radicals (OFf<1->) that can be formed will react back to water and oxygen. So-called hydrogen-yielding materials other than water such as natural gas or oil can be added to the process. Alternatively, carbon can be supplied and thus oxygen will react with carbon to form CO.

When alumina (AI2O3) is used, the reactions are written as follows:

Cracking of water:

Backlash:

The oxygen will be converted to CO or CO2 when carbon is added.

One mole of AI2O3 (molecular weight of 102 g) gives 2 moles of Al with a molecular weight of 54.

6 mol H2O with molecular weight 18.

1.5 mol C with molecular weight 12.

Thus, for each kg of aluminum produced:

Moles Al: 1000/27 = 37.04

M0IAI2O3: = 37.04/2= 18.52

M0IH2O: = 18.52<*>6=111

MolC: = 18.52<*>1.5 = 27.79

This gives in kg:

AI2O3: = 18.52<*>102/1000= 1.89/kg Al H20: = 111<*>18.02/1000 = 2 kg

C: = 27.79<*>12/1000 = 0.33 kg

When using aluminum hydroxide Al(OH)3, the main reaction is:

Thus we have: Each mole of Al(OH)3 (molecular weight 77.99) gives one mole of Al with a molecular weight of 27. Thus, 4 mol of Al(OH)3 gives 4 mol of Al and requires 3 mol of C with a molecular weight of 12.

Thus, for each kg of aluminium, we have:

Mol Al: 1000/27 = 37.04 mol

Mol Al(OH)3: = 37.04/1 = 37.04 mol

Moles C: = 37.04/16* 12 = 27.79 moles

This gives in kg:

Al(OH)3): = 37.04<*>77.99/1000 = 2.98 kg/kg Al

C: = 26.79<*>12/1000 = 0.33 kg

When using a mineral such as anatrosite containing silicon and aluminum oxide, the following can be noted:

The reduction of SiO2 to SiC takes place at a significantly lower temperature than the reaction from AI2O3 to AI4O4C. In a combined reaction, it is therefore expected that the first step will be a complete reaction of silicon oxide to silicon carbide:

This selective reaction of silica has been confirmed by the reduction of clay and laboratory experiments by the reduction of fused mullite.

Once all the SiCh has reacted, the next step will be the reaction between alumina and carbon to form oxycarbide:

As previously mentioned for alumina, the oxycarbide will be more or less complete in the molten phase. We can formulate the subsequent reaction step with stoichiometric aluminum oxycarbide, while assuming a charged composition that enables the use of all oxide and all fresh carbon in two subsequent steps. The metal production reaction can then be written:

The term (m) after oxycarbide indicates that this will be present in the melt phase at the temperature necessary for the reduction to take place at an equilibrium pressure of 1 atm. The molar ratio of Al:Si = 4:3. At a higher alumina content, there will be some free alumina left according to the reactions. With more alumina, some AI4C3 will have been formed in the last reaction step.

When the process temperature is reached, the reactants are automatically added to the reactor. The process is balanced by the addition of the material and the addition of energy as follows: When the desired process temperature is reached, aluminum is discharged via a discharge arrangement which can be a rotary valve, pump or other practical equipment. When the process is balanced, reduction and input are more or less continuous.

The gaseous reaction products from the process, steam and CO, are removed from the reactor via a pipe arrangement for heat recovery and gas cleaning system, which preheats the combustion air.

For example, supersaturated steam can be used to preheat the material to reduce energy consumption.

The energy consumption is the sum of the energy required to heat the oxide to the process temperature, to heat and evaporate the water at the same temperature and the energy required for the chemical reactions. We mentioned that the energy used to crack the water is recovered by the exothermic reaction that takes place in the back reaction to water.

The produced aluminum from the reactor can come out either as aluminum powder to a neutral atomosphere or as molten aluminium.

When producing liquid aluminium, we will have a mixing zone in the reactor with unreacted alumina and liquid aluminium. With continuous feeding of the material becomes

in this automatically balanced in the reactor.

The novelty of the present invention is a kinetic and dynamic one

reaction process where all parameters can be controlled such as temperature,

retention time, pressure and the velocity and temperature of the combustion gases.

The reactor is characterized by the fact that it has no moving parts and that it can be isolated

to withstand the operating temperatures which are in the range of 1000-2500°C depending on the metals to be produced.

The energy is delivered to the reactor in the form of pressurized combustion gases, which are burned under pressure in a separate combustion chamber. The gases leave the combustion chamber and enter the reactor through one or more tangentially oriented slits in the reactor.

The process can have many different arrangements and layout of the reactor - either

vertical or horizontal and can take any form where the principle can be applied. In the following description, a preferred embodiment is shown.

Fig. 1 shows a simplified flowchart of the process with the following main elements:

a) is a container with an internal mixer for receiving the reactants, for example alumina, carbon and water. A transport screw is placed at the bottom of the container b)

driven by a variable motor c) which supplies the material to the reactor chamber d). A spiral-shaped extension e) of the combustion chamber is placed around the reactor chamber

f). A valve arrangement g) for metal discharge and a pipe h) for the discharge of steam and CO are placed on the reactor chamber. The gases are fed to a heat exchanger i) where pressurized

combustion air from compressor j) is preheated before it enters combustion chamber f). From heat exchangers i) the gases are led to a condenser k) where the steam is condensed. The cooled CO is passed on to a CO burner 1) for further heat recovery. The produced metal is finally collected in a tank m) where it is sucked off.

Fig. 2 shows an embodiment of the reactor chamber d), the combustion chamber extension

e) which surrounds the reactor chamber d). The dashed lines of the reactor show heat resistant coating. Inside the reactor, a step n) is arranged above the conical part o). In the center

of the reaction chamber, a gas outlet pipe p) is placed. The insulated metal collection tank m) is located at the bottom of the conical part o)., in the embodiment shown, it is intended that the metal should flow from the reaction chamber directly to

the collecting tank m), but depending on operating conditions and the metal being produced, a valve arrangement can be placed between the lower part of the conical part o) and the tank m).

As can be seen from the drawn fig. 2, the reactor has a shape like a vertical cyclone.

When the jet gases enter the reaction chamber and set the material in rotation corresponding to a cyclone, a layer of the material will build up due to step n) which establishes a fluidized bed in the reaction zone.

Fig. 3 shows a cross-section of the reactor d) with the extension e) and the combustion chamber

f). The fuel gas and air are delivered to the combustion chamber via pipe arrangement q). The combustion gases enter the reaction chamber d) via slits r) which have been designed

to give the correct velocity of the gases into the reactor.

Fig. 4 shows a lay-out of a 1000 kW unit designed to produce 50 kg of aluminum per hour with the following elements: The material is brought to the container a) and fed to the reactor d) with a transport screw b) driven by a variable drive c). The compressor supplies the combustion air at a suitable pressure through the line s) to the heat exchanger i). Preheated air is then led to the pipe arrangement g) where it is mixed with gas supplied from a gas compressor t). The combustion gases are injected into the reactor via extension e). The waste gases from the reactor are discharged through the outlet pipe h) to the heat exchanger from which they are led to a condenser k). The non-condensable gas CO is passed on to a CO burner.

The produced metal is collected in the tank m) where it is sucked off.

The entire unit is controlled from the control unit u).

It must be understood that this arrangement is only one of many layouts and options for an aluminum producing unit based on the principles of the invention.

Fig. 5 and 6 show a simplified illustration of the process.

Claims (6)

1. Carbothermic process for the production of metals such as aluminium, silicon, magnesium and the like from metal compounds, characterized by the fact that the metal compound(s) such as the oxide(s) or hydroxides are heated by combustion gases and internal friction between the particles in the feed generated by mechanical forces from the jet gases inside a process chamber, together with either free water or water-forming chemicals in the aluminum compound(s) and carbon and/or oil and/or natural gas in such a way that the material in the reactor chamber (d) behaves as a hot mechanically established fluidized bed whereby the water can be split into hydroxyl and hydrogen radicals and cause the hydrogen radicals to react with the oxygen in the hot and now unstable aluminum compound(s) and thus replace aluminum atoms, and the hydroxyl radical reacts back to water and excess oxygen reacts with carbon to CO. 2. Process according to claim 1, characterized in that the metal oxide(s) can be mixed with other radical-generating liquids. 3. Process according to claim 1, characterized in that different metal oxides can be reduced to metal and thus form a metal alloy. 4. Process according to claim 1, characterized in that the reactor chamber (d) can be conical as illustrated in fig. 2 with a combustion chamber (f) and an extension (e) of the same and where the combustion gases can enter the reaction chamber (o) as can be seen from fig. 3 through one or more tangentially oriented slits in the reactor. 5. Process according to requirement log4, characterized in that the combustion gases can be burned under pressure. 6. Process according to claim 1, characterized in that the metal can be collected in a tank (m) located at the bottom of the conical part of the reactor (d).