CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102019000011532 filed on 11/07/2019, the entire disclosure of which is incorporated herein by reference .

TECHNICAL FIELD

The present invention concerns a process for producing metal aluminum, and optionally other metals, from aluminum compounds containing oxygen and possibly other metals.

BACKGROUND ART

Metal aluminum is generally produced on an industrial scale from mineral, in particular from bauxite (so-called primary aluminum) ; or from recasting and recycling of aluminum scrap (so-called secondary aluminum) .

The production of primary aluminum from mineral typically comprises two stages: first alumina (AI2O3) is obtained from bauxite treated with sodium hydroxide (Bayer process); subsequently pure aluminum is extracted from the alumina through an electrolytic process (Hall-Heroult process) .

Despite the significant technological progress that has been made in the sector, the known processes are not fully satisfactory, in particular due to the high energy consumption involved in electrolysis of the alumina.

Furthermore, the known processes are not completely suited to treating other aluminum compounds as raw material instead of aluminum oxide.

DISCLOSURE OF INVENTION An object of the present invention is to provide a process for the production of metal aluminum from aluminum oxides or, more generally, from aluminum compounds containing oxygen (and possibly other metals), which is without the drawbacks of the known art .

In particular, one object of the invention is to provide a metal aluminum production process which is fully effective, particularly efficient in energy terms, and versatile (allowing a variety of aluminum compounds to be treated) .

The present invention therefore concerns a metal aluminum production process as defined in essential terms in the attached claim 1 and, in its additional characteristics, in the dependent claims.

The process of the invention is fully effective in the production of pure metal aluminum and in addition to aluminum oxide can use as raw material other aluminum compounds containing oxygen, also containing other metals.

The process of the invention also allows the process to be started directly from bauxite, skipping the traditional Bayer process with caustic soda. This option is important as it avoids formation of the notorious "red mud" which, as known, is highly contaminating and difficult to dispose of. Furthermore, the process of the invention can treat different types of loads containing aluminum, and can therefore dispose of the highly contaminating residues resulting from the traditional work processes.

The process of the invention is particularly efficient in terms of energy, in particular when compared with the traditional electrolytic processes.

The process of the invention is based essentially on a reduction reaction of the aluminum carried out by heat (namely by means of heat treatment and not electrolytic treatment) .

The reagents of the reduction reaction are a raw material containing aluminum (in particular in the form of aluminum oxides, and optionally other metal oxides); and at least one reducing agent, preferably a reducing agent containing carbon.

In general, the raw material containing aluminum treated in the process according to the invention can comprise one or more aluminum compounds containing oxygen, also in the presence of other metals, alone or in a mixture with one another in any weight ratio.

For example, the raw material treated can comprise one or more of: aluminum oxide AI2O3, Al ₂ Mg0 ₄ spinel, mixed compound AlMgFeO spinel, bayerite A1(0H) ₃ , and mixtures thereof.

The raw material treated can comprise, in particular, magnesium oxide and/or other metal oxides in any weight ratio with the compounds containing aluminum.

For example, a common raw material can comprise a mixture containing 56% of A12Mg04 spinel, 12% of AlMgFeO spinel and 8% Bayerite (melting temperature 300°C) .

The reduction reaction is carried out in particular in a reaction chamber that houses the reagents (raw material containing aluminum and reducing agent) and where a reducing atmosphere is maintained and a predetermined reaction temperature is reached and maintained.

The reduction reaction in a reducing atmosphere is carried out in particular using as reagent a reducing agent containing carbon, in the form for example of coal, methane or other compound or substance containing carbon; clearly, one or more reducing agents can be used.

Advantageously, the reagents (raw material containing aluminum and reagent containing carbon, for example coal or methane) are placed in the reaction chamber in solid particle form, with particles having dimensions preferably smaller than a few millimeters or even smaller than one millimeter.

Preferably, the reagents are brought into contact with one another, preferably dispersed in one another so that the particles of the reagents are in intimate contact.

Preferably, the reaction chamber is kept at a minimum reaction temperature of approximately 2040°C.

The reaction temperature can be higher, in particular up to approximately 2500°C.

If the raw material containing aluminum which is treated in the reaction chamber contains, in addition to the aluminum oxide, other aluminum compounds and/or mixed compounds of aluminum and other metals, and/or compounds of other metals, the reaction temperature will be consequently modulated, also based on the weight ratios of the various metals present in the raw material, to obtain reduction of the metals present.

The process of the invention can be continuous or discontinuous. In any case, the reaction temperature must be maintained for a contact time between the reagents sufficient to reach predetermined energy thresholds on the entire mass of raw material involved in the reduction reaction.

In particular, the reduction reaction is carried out with:

Variation in Gibbs Free Energy Delta G (2040°C) = - 2.5 kJ

(spontaneous reaction at this negative value),

Variation in Enthalpy Delta H (2040°C) = +1307.4 kJ (positive value for endothermic chemical reaction) .

If the process of the invention is applied on relatively small masses of reagents (raw material containing aluminum and reagent containing carbon) , the conditions indicated above are obtained in a relatively simple manner also with the pressure being equal; very short contact times, in the order of seconds or even fractions of a second, are therefore sufficient.

If, on the other hand, larger reagent masses are used, as typically occurs at industrial level, it can be more difficult to maintain the above conditions stationary at all points of the mass in which the reagents are in contact, and therefore longer reaction times are required, in the order of a few minutes. The reaction times can be shortened by operating with reagents in the form of fine particles (with mean dimensions smaller than one millimeter) in intimate contact.

For reasons of safety (to avoid possible risks of explosion) it is also possible to operate with longer contact times and larger reagent particle dimensions (a few millimeters) .

In certain embodiments, in particular if the raw material to be treated comprises Al ₂ Mg0 ₄ spinel (having a melting temperature of 2135°C) and/or AlMgFeO spinel, which have reduction reaction temperatures different from AI ₂ O ₃ , the reaction temperature is selected according to the specific weight ratios.

The reaction temperature for a specific raw material can also be identified, for example, by measuring the flow rate of gaseous carbon monoxide (CO) produced: in fact, the gaseous carbon monoxide is the common by-product of the reduction reaction of all the metal oxides present, and measurement of the flow rate of gaseous CO instantly identifies the degree of progress of the reaction. When a flow of molten metal aluminum is observed, this confirms that a reduction temperature suitable for the mixture of the oxidized compounds has been reached .

The method of the invention, as already highlighted, is valid for any weight ratio among the various oxide molecules and the reciprocal percentages do not affect the reduction reaction.

For any mixture the most appropriate reaction temperature can be selected, but in any case the reaction temperature will be higher than the minimum of 2040°C but will not exceed 2500°C.

In this way, if MgO is present (around 5% weight ratio is common) it will be well below the magnesium oxide melting temperature (2852°C), thus separating and purifying it from the metal aluminum melt.

In accordance with the invention, the reduction reaction is carried out in a plasma chamber in which the predetermined temperature required for the reduction reaction is reached and maintained by means of a plasma torch.

As is known, plasma is the fourth state of matter, after solid, liquid and gaseous: in plasma, there are very weak chemical-physical bonds between the particles of matter.

In practice, the particles of matter between two electrodes at high temperature reach the state of plasma, thus the bonds of metals with oxygen, even those that are strong in common ambient conditions as in A1203 and similar, are weakened, allowing a reducing agent like carbon to substitute the metals in the bond with the oxygen.

The plasma torches proposed so far for the production of aluminum have proved unsatisfactory in terms of overall process efficiency, and therefore cannot be effectively used at industrial level.

The present invention, on the other hand, achieves high efficiency in terms of the progress of the metal reduction reaction of all the compounds with the presence of oxygen, simultaneously blocking the triggering of possible competing reactions which would cause the metal aluminum to revert back to oxide.

The plasma chamber has therefore been designed to fulfill the purpose of producing metal aluminum from oxides, with the possible presence of other elements. Only in this way is it possible to apply the process on an industrial scale.

A reducing atmosphere is maintained in the reaction chamber by appropriately dosing the reducing agent containing carbon (coal, methane or other reducing element) in an amount stoichiometrically in excess with respect to the metal oxides to be reduced (present in the raw material to be treated) . This facilitates shifting of the reduction reaction. Removal of the reaction products from the equilibrium acts as a further accelerating element.

In particular, it is advantageous to extract CO from the exhaust gases (synthesis gas), cooling the area where sublimation of the reduced metal and recombination thereof with the oxygen can occur.

The gas used in the plasma is an inert gas (or mixture of gases), such as argon or nitrogen or other inert gas.

In addition to being inert in order not to interfere in any way with the reactions that take place in the reaction chamber, the inert gas is dosed so as to always maintain the percentage ratios between the other elements present outside possible explosive limits. In the reaction chamber, reduced metal aluminum is produced; the latter is collected from the reaction chamber in the liquid state, together with any other metals present and also reduced .

If necessary, the various metals are then separated and/or purified.

In the reaction chamber a gaseous phase develops containing fumes, exhaust gas, any non-reacted compounds, inert gases, etc., which is collected from the top of the reaction chamber.

Advantageously, the gaseous phase flowing out of the reaction chamber undergoes, as said, a rapid cooling phase, for example to a temperature of approximately 500°C or lower, always maintaining a reducing atmosphere, to avoid the aluminum and/or any other metals present in the gaseous phase sublimating or reacting with oxygen.

Since the gaseous phase flowing out of the reaction chamber is at high temperature, it can also be advantageously exploited to recover heat in a heat recovery phase, for example in a heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will appear clear from the following description of a non-limiting embodiment example thereof, with reference to the figures of the attached drawings, in which:

- figure 1 is an external perspective view of an apparatus for carrying out the metal aluminum production process in accordance with the invention;

- figure 2 is a longitudinal section view of the apparatus of figure 1. BEST MODE FOR CARRYING OUT THE INVENTION

With reference to figures 1 and 2, the metal aluminum production process of the invention is performed in an apparatus 1 comprising a casing 2 having an inner reaction chamber 3 and incorporating a plasma torch 4.

The casing 2 extends along and around a longitudinal axis A between an upper end 5 and a lower end 6 axially opposite, closed by a top wall 7 and by a bottom wall 8 respectively.

The casing 2 has a lateral wall 9, for example cylindrical around the axis A which extends between the top wall 7 and the bottom wall 8.

The walls 7, 8, 9 of the casing 2 are made of suitable materials, preferably with a multilayer structure including one or more refractory and insulating layers.

The casing 2 comprises a collection chamber 10, located at the lower end 6 below the reaction chamber 3 and separated from the reaction chamber 3 by a partition wall 11 provided with ducts 12 that establish communication between the reaction chamber 3 and the collection chamber 10.

The partition wall 11 is provided, on an upper face facing the reaction chamber 3, with a conveying structure 13, for example with cylindrical collar shape, positioned around the axis A and radially spaced from the lateral wall 9 of the casing 2. The conveying structure 13 has an upper peripheral edge 14 and an outer perimeter collecting channel 15, positioned around the edge 14 and below the edge 14.

The conveying structure 13 delimits an inner central zone 16 and a radially outer annular zone 17, adjacent to the lateral wall 9. The casing 2 has at least one feed inlet 20 for raw material to be treated (aluminum compounds containing oxygen and optionally other metals), communicating with the reaction chamber 3.

Preferably, as shown in the figures, the casing 2 has at least two inlets 20 located at different heights along the axis A and obtained through the lateral wall 9 of the casing 2. The inlets 20 are associated with respective ducts 21 which project into the reaction chamber 3.

The casing 2 has a gas inlet 22 for supplying to the reaction chamber 3 a flow of inert gas (typically argon and/or nitrogen) .

The gas inlet 22 is connected to a gas supply duct 23 which projects axially into the reaction chamber 3 parallel to the axis A, for example along the axis A, as far as a predetermined height above the upper peripheral edge 14 of the conveying structure 13.

The gas supply duct 23 is associated with a gas cooling device 24.

The device 24 comprises in particular an annular duct 25 positioned around the gas supply duct 23 in which a cooling fluid circulates.

The casing 2 has a molten metal outlet 27, communicating with the collection chamber 10 to withdraw molten metal from the collection chamber 10; and a vent outlet 28, communicating with the reaction chamber 3 and, more precisely, with the collecting channel 15.

Also the outlets 27, 28 are, for example, formed through the lateral wall 9 of the casing 2. The vent outlet 28 is connected to a discharge duct 29 from which a gas discharge duct 30 branches.

Optionally, the collection chamber 10 also has a bottom drain 31, positioned through the bottom wall 8.

The casing 2 also has a gas outlet 32, located at the end 5 through the lateral wall 9 or the top wall 7 and communicating with the reaction chamber 3 to extract the reaction by-product (CO) from the reaction chamber 3.

The plasma torch 4 is positioned inside the reaction chamber 3 and comprises a plurality of electrodes 33, 34, 35 (cathodes and anodes) which extend into the reaction chamber 3 through respective openings in the casing 2.

The electrodes 33, 34, 35 are formed of respective bars which project into the reaction chamber 3 and terminate with respective free ends in the reaction chamber 3.

In the preferred embodiment illustrated, the plasma torch 4 comprises at least one axial cathode 33 and at least one axial anode 34 which extend into the reaction chamber 3 along respective axes substantially parallel to each other and to the axis A.

In particular, the cathode 33 and the anode 34 extend axially into the reaction chamber 3 from the top wall 7 and are positioned facing each other on opposite sides of the axis A. The cathode 33 and the anode 34 have respective free ends 37, 38 above the inner central zone 16 delimited by the conveying structure 13 and axially and radially spaced from the peripheral edge 14.

The plasma torch 4 comprises a plurality of radial anodes 35 which extend radially into the reaction chamber 3 through the lateral wall 9 and are angularly spaced around the axis A.

For example, the plasma torch 4 comprises at least a pair of diametrically opposite radial cathodes 35 aligned with each other and perpendicular to the axis A and therefore to the axial cathode 33 and anode 34.

The anodes 35 have respective free ends 39 located in the vicinity of the lateral wall 9 and below the free ends 37, 38 of the axial cathode 33 and anode 34 and above the edge 14 of the conveying structure 13.

The reagents (aluminum compounds containing oxygen and possibly other metals) are appropriately dosed and fed to the reaction chamber 3 in the reciprocal ratios favorable to the progress of the reaction and opposed to the occurrence of competing reactions.

Advantageously, the reagents are mixed before being placed in the reaction chamber 3, for example by means of a pneumatic loading and transport system with explosion-safe transport fluid, so that the reagents enter the reaction chamber 3 already balanced.

The reduction reaction is triggered when the ideal reaction conditions are rapidly reached due to the plasma torch 4.

The aluminum reduced to metal is prevented from reverting back to oxide by cooling at a speed higher than the reoxidation kinetics, by means of the gas cooling device 24, which introduces gas at a temperature such that the induced heat exchange lowers the temperature of the upper part of the reaction chamber 3, where the metal aluminum could revert back to oxide, and removes it from the reaction equilibrium by causing it to drop separately into the radially external annular zone 17 adjacent to the lateral wall 9, outside the influence of the electrodes 33, 34, 35.

Said separation is furthermore favored by the rapid extraction of the reaction by-product, namely carbon oxide (CO) , which is extracted from the top through the gas outlet 32 and sent, for example, to an after-burner for conversion to C02.

The metal aluminum collects in the lower part of the reaction chamber 3, passes into the chamber 10 through the ducts 12 and is then collected through the outlet 27 (and if necessary, the drain 31 ) .

On the bottom of the reaction chamber 3 the slag also collects and, since it is lighter than the aluminum, tends to float above the aluminum melt. The slag therefore collects firstly together with the aluminum but then, since it is lighter, it separates and migrates to the central zone 16 which it fills; here it overflows from the edge 14 and is conveyed by the collecting channel 15 to the outlet 28.

The slag can trap gases, which are vented through the gas discharge duct 30.

The apparatus 1 can be provided with appropriate heat and energy recovery systems, so as to further reduce the production cost of the end metal.

In a variation, the sublimated metal aluminum (or a part thereof) is extracted from the top of the reaction chamber 3, through the gas outlet 32, and flows through an outer pipe together with the by-product CO and possibly other off-gases. In this case there is an energy advantage (because it is possible to avoid or reduce cooling of the upper part of the reaction chamber 3 and consequent additional energy to maintain the high temperatures required by the reaction) , leaving a fraction of aluminum reduced to metal, sublimated, in the gaseous state until it leaves the reaction chamber 3. The subsequent cooling, carried out on the gaseous product outlet line, is simple and can be performed with an ordinary heat exchanger, which recovers heat and also produces steam if required. The by-product CO and the other off-gases remain in the gaseous state and are thus separated from the metal aluminum which, having returned to a liquid state, is recombined with the melt obtained from the lower part of the reaction chamber 3.

Lastly it is understood that further modifications and variations that do not depart from the scope of the attached claims can be made to the process described and illustrated here .