WO2001034858A1

WO2001034858A1 - Carbothermic process for production of metals

Info

Publication number: WO2001034858A1
Application number: PCT/NO2000/000379
Authority: WO
Inventors: Olav Ellingsen
Original assignee: Metalica As
Priority date: 1999-11-11
Filing date: 2000-11-10
Publication date: 2001-05-17
Also published as: NO310426B1; NO995514D0; AU1422701A; AU777662B2; NO995514L; CA2390943A1; WO2001034858A9

Abstract

A carbothermal process for the production of metals such aluminium, silicon, magnesium and the like from metal compounds, characterised in that the metal compound(s) such as 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 within a process chamber, together with either free water or water-forming chemcials in the aluminium compound(s) and carbon and/or oil and/or natural gas in such manner that the material in the reactor chamber behaves like a hot mechanical established fluidised bed whereby the water can be split into hydroxyl and hydrogen radicals, making the hydrogen radicals react with the oxygen in the hot and now unstable aluminium compound(s) and thus replacing aluminium atoms, and the hydroxyl radical retracting back to water and the surplus of oxygen reacting with carbon to CO.

Description

Carbothermic process for production of metals.

This invention is related to a new process in carbothermal production of metal such as aluminium, silicon, magnesium and the like means The process takes place within a reactor chamber where metal-oxides or compounds of these are subject to intense jet streams of hot combustion gases The hot gases are delivering heat to the materials and putting them into an intense rotating motion which creates and artificial centrifugal force and generates factional forces between the particles which generates additional heat to the process

The present invention will discuss the process utilised in the production of aluminium and to some extent silicon

Aluminium is at present produced in large-scale facilities throughout the world by the electrolytical Hall-Heroult process The electrolyte bath contains aluminium oxide, cryolite and other additives The electrolysis temperature is about 1000°C Carbon is used as electrode material

The main advantage with this the method is that it is well developed and operates under stabile conditions It is further more regarded as the cheapest way to produce aluminium from aluminium oxide (Al₂O₃) which is upgraded from bauxite by the Bayer-process

The Hall-Heroult process is a very energy consuming process Theoretically energy consumption is approx 11 kWh per kg produced aluminium To day's production technology operates at the level of 13-14 kWh per kg aluminium produced Intensive research is going on all over the world to improve to day's technology in order to decrease the energy consumption In addition to the cost of energy, the cost of aluminium is also affected by the costs of the materials utilised and gas cleaning facilities in order to comply environmental requirements

Production of aluminium by carbothermal reduction of alumina has been and is still going on in order to develop an alternative process by most of the major aluminium producer's But until this date ther has not been developed any process which can fulfil the technical and economical requirements

Aluminium has been produced by direct reduction with carbon All these carbothermal production processes are static processes Temperatures in the processes are above 2000°C Such high temperatures causes constructional and material problems Since reoxidation of the metal such as aluminium is a serious problem at these high temperatures, a careful control of back reaction is needed On the other hand, the lowering of the energy consumption and a substantial reduction of capital cost are most interesting, and have been the incentive in the research and development work on carbothermal processes for the production of aluminium

As known carbothermal process can be divided into two groups Firstly, pure alumina is produced from impure raw materials and then it is reduced to pure aluminium Secondly, the impure raw material are completely reduced to form an alloy, usually consisting mainly of aluminium and silicon, and with smaller contents of iron From this allow aluminium can be produced be separation and refining processes

The present invention however, both materials can be used to produce aluminium or an aluminium alloy

The present invention describes methods to produce aluminium, silicon and similar metals from metal by the carbon reduction process In addition to use alumina as raw material, this process also allows the use of aluminium hydroxide Al(OH)₃ The present process is a high-efficiency cracking process of the aluminium compounds into aluminium, water and CO at low pressure and temperature The energy-requirement is less than any other known method and uses no use of cryolite and has limited emission of toxic gases

The total reaction for carbothermal reduction of alumina may be written as

Al₂O₃ (s)+3 C (s) = Al (l) + 3 CO (g) (1)

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

2 Al₂O₃ +3 C = Al₂O₄C + 2 CO (g) (2)

Al₄O₄C + 6 C = A1₄C₃ + 4 CO (g) (3) Al₄O₄C + A1₄C = 8 Al (1) + 4 CO (g) (4)

In addition to the formation of aluminium carbide and aluminium oxycarbide, the following reactions takes also place during the process

A₂O₃ + 2 C = Al₂O (g) + 2 CO (g) (5)

Al₄O₄C + C - 2 Al₂O (g) + 2 CO (g) (6)

Al₂O₂ + 3 C = 2 Al (g) + 3 CO (g) (7) Al₄O₄C + 3 C = 4 Al (g) + 4 CO (g) (8)

The principle of the present invention also called Jet Cracking Process (JCP), is to treat the compounds in a pneumatic hot fluidised bed The fluidised bed gives rise to the following effects in order to achieve the cracking

1 In addition to the calorimetric energy from the driving gases, heat will also be delivered from material friction and hydrodynamic forces

2 Water, which is present on the raw materials, will be evaporated with a reduced speed during the process, thus creation of micro-bubbles with extreme pressure and temperature

3 Quenching of the released heat from the micro-bubbles to the process ambient temperature

4 Creation of temperature spots between the moving grains caused by internal friction forces

The fluidised bed that takes place in a reactor chamber by jet streams of combustion gases, which are injected tangentially into the reactor chamber

The energy for the process is delivered from combustion gases - bot thermal and kinetic This is achieved by pressurised combustion, where the combustion gases are injected into the reactor in such a manner that it generates a rotating motion of the material in the reactor The energy converted to the material will be a combination of the heat in the gases and motion of particles in the reactor The rotation of the particles in the reactor will generate shear-forces between the particles, which generates peak temperatures and a centrifugal force, which will separate different phases in the reactor

Aluminium compound is delivered into the process chamber together with the correct molar quantity of carbon correctly balanced to the energy input Carbon can also be be delivered from the combustion gas

When the raw material is aluminium hydroxide instead of alumina, the water will be be evaporated after the so-called "flashevaporation" During the evaporation it will be generated ultrasonic vibration in the steam caused by the moving particles and the pressure in the front of the gas jets Because of the extreme separation of the fluids in the solids that takes place before they are evaporated, the separation in addition to the vibration, generates micro-bubbles in the fluidised bed with extreme temperatures and pressure of several thousands of degree Kelvin and atmospheres whereby water is cracked into hydrogen and oxygen radicals

Earlier experiments in a 25 kW reactor, it has been found that water can be cracked at a process temperature at 250° and very high motion between the particles in the order of 40 - 200 m/s

When the entering reactants enters the reactor under these conditions, they will be heated to these process temperature as described above

During the process hydrogen radicals will react with the oxygen in alumina and forms water Hydroxyl radicals (OFT) who may be formed will react back into water and oxygen To the process can be added so-called hydrogen delivering materials other than water such as natural gas or oil Alternatively, carbon can be delivered and thus oxygen will react with carbon into CO

When using alumina (Al₂O₃) the reactions are written as following

Al₂O₃ + 6H⁺ → 2A1 + 3H₂O

Cracking of water 6H₂O -» 6H~ +6OH^" Back reaction 6OH^" → 3H₂O + l,5 O₂

The oxygen will react into CO or CO₂ by adding carbon

One mol of Al₂O₃ (molecular weight of 102g) gives 2 mols of Al with molecular weight of 54

6 mols of H O with molecular weight of 18 1,5 mol of C with a molecular weight of 12 Thus we need for each kg aluminum produced

Mols Al 1000/27 = 37,04

Mols Al₂O₃ = 37,04/2 = 18,52

Mols H₂O = 18,52*6 = 11 1 Mols C: 18,52*1,5 = 27,79

This gives in kg:

Al₂O₃: 18,52*102/1000 = 1,89/kg Al

H₂O: 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:

2Al(OH)₃ + 3C → 2AL + 3H₂O + 3 CO

o Thus we have:

Each mol of Al(OH)₃ (molecular weight 77,99) gives one mol of Al with molecular weight of 27. Thus 4 mols of Al(OH)₃ give 4 mols of Al and requires 3 mols of C with molecular weight of 12. 5

Thus we have for each kg of aluminum: Mols Al: 1000/27 37,04 mols

Mols Al(OH)₃: 37,04/1 = 37,04 mols Mols C: 37,04/16*12 = 27,79 mols 0

This gives in kg:

Al(OH)₃: 37,04*77,99/1000 = 2,98 kg/kg AL

C: 27,79*12/1000 = 0,33 kg

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

The reduction of SiO₂ to SiC occurs at a considerably lower temperature than the reaction of Al O₃ to Al₄O₄C. In a combinded reaction it is therefore expected that the first stage would be a o complete reaction of silica til silicon carbide:

SiO2 + 3 C = SiC + 2CO (g).

This selective reaction of silica has been confirmed in reduction of clay and laboratory 5 experiments with reduction of fused mullite. When all SiO has reacted, the nest stage will be the reaction between alumina and carbon to form oxycarbide:

2Al₂O₃ + 3 C = Al₄O₄C + 2 CO (g)

As earlier mentioned for alumina, the oxycarbide will be more or less completely in the molte phase. We can formulate the subsequent raction stage with stoichiometric aliminium oxycarbide, at the same time assuming a charge composition which enables all oxide and all free carbon to be used in two preceding stages. The metal producing reaction may then be written:

Al₄O₄C (m) + 3 SiC = (4 Al + 3 Si)(l) + 4 CO (g).

The term (m) after oxycarbide indicates that this will be present in the molten phase at the temperature necessary for reduction to take place with an equilibrium pressure of 1 atm. The molar ratio of Al:Si = 4:3. With higher alumina content there will be some free alumina remaining according to the reactions. With more alumina, some A1₄C₃ will be formed in the final reaction stage.

When the process temperature is reached the reactants are charged automatically into the reactor. The process is balanced by the input of the material and the energy input as follows:

When the desired process temperature is reached, discharge of aluminium is done via a discharge arrangement that can be a rotating valve, pump or other practical means. When the process is balanced reduction and feed goes more or less continuously.

The gas reaction products from the process, steam and CO is removed from the reactor via a pipe arrangement to heat recovery and gas cleaning system, which preheat the combustion air.

Fore example the over-saturated steam can be utilised to preheat the material to reduce the 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 the same temperature and the energy required for the chemical reactions. We mention that the energy utilised to crack the water is recovered by the exothermic reaction that takes place when reacting back to water. The produced aluminium from the reactor can be discharged either as aluminium powder into a neutral atmosphere or as melted aluminium

By producing liquid aluminium, we will have a mixing-zone in the reactor of not reacted alumina and liquid aluminium By continuously feed of material this is automatically balanced in the reactor

The novelty of the present invention is a kinetic and dynamic reaction process where all parameters can be controlled such as temperature, retention time, pressure and the velocity and the temperature of the combustion gases

The reactor is characterised by that it has no moving parts and that it can be isolated to withstand the operating temperatures which is in the range of 1000 - 2500 °C depending of the metals to be produced

The energy is delivered to the reactor from pressurised combustion gases, which is combusted under pressure in a separate combustion chamber The gases leave the combustion chamber and enter the reactor through one or more tangentially oriented slots in the reactor

The process can have different arrangements and lay-out of the reactor - either vertical or horizontal and can take any shape where the principle can apply In the following description is shown one preferred embodiment

Fig 1 shows a simplified flow diagram of the process with the following main elements

a) is a hopper with an internal mixer for receiving of the reactants, fore example alumina, carbon and water In the bottom of the hopper is arranged a screw conveyor b) driven by a variable motor c) which delivers the material to the reactor chamber d) Around the reactor chamber is located a spiral formed extension e) of the combustion chamber f) On the reactor chamber is arranged a valve arrangement g) for metal discharge and a pipe h) for discharge of steam and CO The gases are passed to a heat-exchanger i) where pressurised combustion air from the compressor j) is pre-heat before entering the combustion chamber f) From the heat exchanger i) the gases are passed to a condenser k) where steam is condensed The cooled CO is passed further 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 one embodiment of the reactor chamber d), the combustion chamber extension e) surrounding the reactor chamber d). The dashed lines of the reactor show the heat resistance coating. Within the reactor a step n) is arranged above the conical part o). In the centre of the reaction chamber is arranged a gas exit pipe p). The isolated metal collecting tank m) is located at the base of the conical part o). In the shown embodiment the metal is supposed to flow from the reaction chamber directly to the collector tank m) but depending upon the operating conditions and metal produced, a valve arrangement can be located between the base of the conical part o and the tank m).

As can be seen of the drawing fig. 2, the reactor has a shape of a vertical cyclone. When the jet gases enters the reaction chamber and put the material in rotation similar to an cyclone, a layer of material will built up due to the step n) which establish a fluidised 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 is delivered to the combustion chamber via the pipe arrangement q). The combustion gases enters the reaction chamber d) via the slots r) which is designed to give the correct velocity of the gases into the reactor.

Fig. 4 shows a layout of a 1000 kW unit designed to produce 50 kg aluminium each hour. With the following elements:

The material is brought to the hopper a) and is passed to the reactor d) by a screw-conveyor b) driven by a variable drive c). The compressor at ratted pressure delivers the combustion air by the line s) to the heat exchanger i). The preheated air is then passed to the pipe arrangement g) where it mixes with gas delivered from a gas compressor t). The combustion gases are injected into the reactor via the extension e). The off gases from the reactor are discharged by the exit pipe h to the heat exchanger from where it is passed to a condenser k). The non-condensable gas CO is passed further 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 has to be understood that this arrangement is only one of many lay-out and alternatives for a aluminium producing unit based upon the principle in this invention.

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

Claims

Patent claims:

1. Carbothermal process for the production of metals such as aluminium, silicon, magnesium and the like from metal compounds, characterised in that the metal compound s such 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 within a process chamber, together with either free water or water-forming chemicals in the aluminium compound/s and carbon and or oil, and/or natural gas in such a manner that material in the reactor chamber behaves like a hot mechanical established fluidised bed whereby the water can be split into hydroxyl and hydrogen radicals making the hydrogen radicals react with the oxygen in the hot and now unstable aluminium compound/s and thus replasing aluminium atoms, and the hydroxyl radical reacting back to water and the surplus of oxygen reacting with carbon to CO.

2. A process according to claim 1, characterised in that the metal oxide/s can be mixed with other radical generating liquids.

3. A process according to claim 1, characterised in that different metal oxides can be reduced to metal thus forming a metal alloy.

4. A process according to claim 1, characterised in that the reactor chamber may 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 seen in Fig. 3 through one or more tangentially oriented slots in the reactor.

5. A process according to claim 1 and 4, characterised in that the combustion gases can be combusted under pressure.

6. A process according to claim 1, characterised in that the metal can be collected in a tank located at the base of the conical part of the reactor.