FR2473557A1 - Carbothermic prodn. of aluminium (alloy) - with separate feed of alumina and reductant giving rapid prod. removal from reaction zone - Google Patents

Abstract

The process comprises supplying carbonaceous material along a path separate from the supply path of alumina (opt. mixed with other oxides) and contacting the carbonaceous material and alumina in a zone to which electrical energy is supplied to attain a high temp. and effect carbothermic reduction. Pref. The carbonaceous material is placed in carbonaceous material containers stacked on each other with a tight fit and positioned in a shaft on the top of which is a supply of alumina which is gravity fed by a guiding shell, electrodes being positioned in and extending through the alumina. The carbonaceous material is supplied by ascending movement to the reaction zone. The process is used in the prodn. of aluminium or its alloys, e.g. aluminium-silicon alloys. The process is energy efficient and the liquid metal is immediately drained from the vicinity of the electrodes so that the build-up of a molten metal pool is prevented and heat generation takes places in the arcs between the electrodes and the top layer of carbon and in the alumina-carbon mixt. where the heat is needed for reaction.

Description

 The invention relates to the production of aluminum by carbothermal reduction of alumina comprising the production of aluminum in alloy with other metals (such as silicon) by reduction of alumina in mixture with other oxides (such as silica ).

 It has been known for at least twenty years that aluminum can be produced by carbothermic reduction of alumina, for example in a submerged electric arc furnace. The scientific principles involved in the chemistry and thermodynamics of reactions are now well known. However, no other commercial process has been established based on these principles.

: It is well known that the reduction of aluminum by the carta-er when it takes place under reduced pressure, passes through the formation of intermediate products, aluminum oxycarbide and aluminum carbide:
2 Al.203 + 3 C = A1404C + 2 CO (g) (1)
Al404C + 6 C = A14C3 + 4 CO (g) (2)
Below 19000C, all reagents and products, except CO, are solids. In order to arrive at an equilibrium gas pressure of 1 kg / cm, however, temperatures of about 20000C are required, the reaction mixture partially melts, and the simple equations (I) and (2) are not more directly applicable. Likewise, the final step, producing the metal can be written
Al4O4C + Al4C3 = 8 Al (1) + 4 CO (g) (3).

The equilibrium pressure of the gas, calculated for this reaction amounts to 1 kg / cm, at around 2100 C. In a reduction oven which is operated at atmospheric pressure, the reaction zone must be maintained at a temperature at least sufficient to give a CO equilibrium pressure equal to 1 kg / cm2. If a certain overpressure is allowed to cause the reaction, this in this case means a temperature of around 21500C. At this temperature, the system consists of solid carbon plus two liquids, that is to say an oxide-carbon in fission and a metallic part in fusion. Equation (3) is not applicable, and the reaction producing the metal can between written schematic
(Molten oxide-carbide) + C (s) = (molten metal) + CO (g) (4).

 it also forms, concurrently with the production of carbon monoxide and condensed products, volatile substances causing aluminum, A120 (g) and Al (g). In the first stages of the reaction formulated by equations (1) and (2), the equilibrium pressures of A120 and Al amount only to a small percentage of the equilibrium pressure of CO.

In the final step, represented by equations (3) or (4), the proportions of A120 and Al in the gas equilibrium are higher but not excessive. however, it has been shown that the reaction between alumina and carbon takes place via.

a mechanism that involves a gas phase, with a high proportion of A120 and Al; it follows that the volatilization losses are greater than what was expected from the equilibria. In addition, the molten metal mass has a lower density than that of the molten oxide-carbide mass, and consequently floats on the latter. The gas released by the reaction (4) must pass through the molten metal, which further increases the losses by volatilization.

 It should be noted that the volatilization of Al and A120 from the hot zone does not necessarily entail a loss of metal. In a submerged arc furnace, the reaction gases rise through layers of cooler charge where the entrained metal vapors can condense, at the same time as they provide preheating of the charge. However, if the fraction of metal vapors in the gas is high, the charge rises to too high a temperature, and volatilization losses occur.

An additional difficulty in the carbothermal production of aluminum is caused by the high solubility of carbon in the metal at the reaction temperature, about 20 atomic% of carbon when the molten metal mass is in equilibrium with solid carbon. . When the molten metal mass cools, this carbon precipitates in the form of aluminum carbide. We realize, from the equation
(12 Al + 3C, molten mixture) = A14C3 (s) + 8 Al (1) (5) that approximately one third of the quantity of metal is precipitated in the form of carbide. it is therefore necessary to operate a subsequent separation step, which constitutes an economic disadvantage.

 Another difficulty encountered in the carbothermic reduction of alumina in a submerged arc furnace concerns the introduction of energy and the transfer of heat. As explained above, the molten metal mass floats on the surface and is located directly under the electrodes. Due to the high electrical conductivity of the metal, the resistance in the oven circuit is low, and there are difficulties in maintaining an adequate introduction of energy into the oven.

In addition, heat creation occurs predominantly on the surface of the metal, which means that the metal is brought to a very high temperature and that there is significant evaporation. This metal returns well in the hot zone where it is again evaporated insofar as it condenses in the charge which is above the molten mass. The net result of this vaporization and condensation operating cycle is that a large fraction of the generated heat is transferred upward in the furnace, instead of being driven downward, to the molten mass of oxide- carbide, where this heat is necessary for the endothermic reaction (4).

 For thermodynamic considerations, it can be concluded that the reduction of a mixture of alumina and silica which will produce an aluminum and silicon alloy will be more favorable than the reduction of alumina alone with regard to the reaction temperature and vapor losses, mainly due to the decrease in the activity of aluminum and silicon in their alloy in the liquid state. The density of silicon is however even lower than that of aluminum, and the difficulties caused by the fact that there is a layer of metal which floats will also prevail in this case.

The difficulties encountered in reducing alumina in a common submerged arc furnace and described above are avoided by the process of the invention characterized by the following arrangements
The carbon (coke) necessary for the reduction is supplied to the reaction zone by coming via a route separate from that by which the oxides which are to be reduced arrive.

This carbon is supplied, continuously or intermittently, by an upward movement, vertically or at an angle opening upwards.
The gas produced by the reactions is transmitted downward through the carbon.

The invention will be better understood with reference to the description below and to the accompanying drawings showing an embodiment of the invention, drawings in which
FIG. 1 is a schematic drawing illustrating the essential characteristics of the process,
FIG. 2 shows a vertical section of a recommended embodiment of the invention,
FIG. 3 is a sectional view along AA of FIG. 2,
The process according to the invention is set out below with reference to FIG. 1. The reducing agent, in the form of metallurgical coke (1), is supplied from below, and performs an upward movement, as indicated by the lower arrow.

The oxides (2) are supplied from above and descend by gravity when the reaction takes place. The upper part of the construction must be essentially gas tight. Energy is supplied by means of the electrodes (3). The gap between the electrodes is filled with an electrically insulating oxide, and heat formation occurs only at the lower tip of the electrodes and in the adjacent coke. The intense emission of heat in this zone melts the oxide and causes it to react immediately with the carbon. The primary product is a gas which has a high vapor content of Al and Al2o, in addition to CO. This gas is forced to flow downward through the charge of carbon or coke and, consequently, cools progressively. The aluminum vapors then condense into liquid aluminum, while the aluminum sub-oxide reacts with carbon to give aluminum and carbon monoxide.

 liquid oxide-carbide and liquid metal are thus formed directly. However, the melting points of these two phases are very different. The oxide-carbide phase solidifies and is retained in the carbonaceous charge a short distance below the reaction zone while the liquid metal continues to descend.

 The liquid metal that first forms at high temperature contains dissolved carbon. Passing through the carbon charge, this metal gradually cools, so that the solid aluminum carbide precipitates and adheres to the surface of the carbon. At the lower end of the carbonaceous charge, the temperature must be kept below 1000C: at this temperature, the solubility of carbon in liquid aluminum is practically zero and the metal collected at the lower end is practically free of carbon. .

 During the gradual cooling of the gas and liquid metal by the carbon load, there can also occur, to a certain extent, different side reactions, such as the reaction between the liquid metal and carbon monoxide gas, shown from right to left by reaction (3), and the reaction between liquid aluminum and solid carbon which gives solid aluminum carbide. it has been shown experimentally, however, that the rates of these reactions are low at the temperatures in question. In addition, the products of these reactions are solid, so that they will be retained in the carbonaceous charge.

 We can say that the process is based on the low melting point of aluminum (66O0C). With temperatures below 1000C at the lower end of the carbonaceous charge, aluminum is the only liquid still in the system. All the other reaction products are solids which are retained by the carbonaceous charge, and these solids are brought back into the reaction zone by the upward movement of carbon. The process is also suitable for the production of aluminum alloys with a low melting point by reduction of the corresponding oxides, such as, for example, aluminum-silicon alloys with more than 40% by weight of Si (melting point of this composition 9500C).

 A notable advantage of the process is that the liquid metal, as it forms in the hottest reaction zone, is immediately removed from the vicinity of the electrodes.

This prevents a mass of liquid metal from forming, and the formation of heat occurs in the arcs which form between the electrodes and the upper layer of carbon and in the oxide-carbon mixture, where this heat is necessary. for the reaction.

 To further elucidate the process, calculations have been made to show the amounts of energy required, and the distribution of this energy in the carbonaceous charge. The calculations presented apply to the reduction of pure alumina. The energy required to heat the alumina from room temperature to the melting point, the heat of fusion of the alumina mass, the additional heating to the reaction temperature and the reaction heat have been calculated as energy expenditure. . Heating carbon has not been calculated as an energy expenditure since it will be heated by the counter-current flow of the reaction gases. The cooling of the gases is considered to lower them to 1100 K (8270C), which is a suitable temperature when the aluminum must remain liquid.

From the presentation that we have just made, it appears that it is not possible to know the importance of the aluminum metal fraction which is formed directly in the liquid form, nor the importance of the fraction which is initially formed as aluminum vapor with subsequent condensation in the carbonaceous charge. This mainly depends on the kinetics of the reactions, and cannot be reduced to a quantitative calculation in the current state of knowledge. The energy calculations are consequently given separately for the two borderline cases s all the metal produced directly in the liquid form, and all the metal produced initially in the form of vapor. Thermodynamic data are taken from JAMAF
Thermochemical Tables (Natl. Bur Stand., Washington 1971), temperatures given in Kelvin (or). The results of the calculations are given on the following page
Alternative I: All metal is presented as liquid Al
at 24000K.

A1203: heating from 298 to 23270K 61.1 kcal / mol
fusion at 23270K 28.3 dO
heating from 2327 to 24000K 2.5 dO
A1203 (1) + 3 C (s) - 2 Al (l) + 3 CO (g) 282.2 dO
Primary energy requirements 374.1 kcal / 2 mol AL
= 8.05 kWh / kg Al
Cooling 2 Al (l) 2400 to 11000K 19.7 kcal
3 CO (g) 2400 to 1100 K 33.2 kcal
52.9 kcal
Heating 3 C (s) 298 to 24000K 32.4 "Excess heat" in the carbonaceous load 20.5 kcal / 2 mol Al
= 0.44 kWh / kg Al Alternative II: All the metal takes the form of Al vapor
at 2600 K.

A1203: heating from 298 to 23270K 61.1 kcal / mol
fusion at 23270K 28.3 dO
heating from 2327 to 26000K 9.5 dO
A1203 (1) + 3 C (s) = 2 Al (g) + 3 CO (g) 419.6 dO
Primary energy requirements 518.5 kcal / 2 mol Al
= 11.16 kWh / kg Al
Condensation 2 Al 26000K 139.9 kcal
Cooling 2 Al (1) 2600 to 1100 K 22.8 dO
3 CO (g) 2600 to 1100 K 38.5 dO
201.2 kcal
Heating 3 C (s) from 298 to 26000K 36 dO "Excess heat" in the carbonaceous charge 165.2 kcal / 2mol Al
= 3.55 kWh / kg Al
It will be seen that the SOilt reagents and products are the same for both alternatives. Consequently, the net energy requirements, equal to the primary energy requirements minus excess heat, must be the same regardless of the mechanism and reaction temperature adopted
Alt. I Alt. II
Primary energy requirements 8.05 11.16 kWh / kg Al
Excess heat 0.44 3.55
Net energy requirements 7.61 7.61 kWh / kg Ai
The excess heat must be removed in order to keep the temperature below about 10000C in the lower part of the carbonaceous charge. This will be done in part by natural heat loss through the chimney walls and in part by forced air cooling or water cooling, in the latter case some of the heat can be recovered for auxiliary heating effects.

 When considering the energy saving in the process, one must also take into account the chemical energy of carbon monoxide. For each kg of Al, 1.36 m3 of CO gas is produced (1 kg / cm2, 250C), having a heat of combustion of 4.35 kwh. If we accept a 70% efficiency in the use of this heat, we must subtract 3 kWh / kg Al from energy expenditure. On the other hand, heat losses occur by conduction, by the walls of the reactor and by the electrodes, but, all things considered, the energy requirements of the process compare favorably with those of the electrolytic processes currently in use, which are in an order of magnitude from 14 to 17 kWh / kg Al.

 The main technical problem which arises in the process of the invention is constituted by the implementation of an upward movement of the carbonaceous charge against the current of the reaction gas and of the liquid metal. FIG. 2 represents an arrangement which makes it possible to solve this problem, while FIG. 3 shows a horizontal section of this arrangement. In FIG. 1, the reference 1 represents a box made of carbonaceous material, charged with coke. The bottom of this box must allow the passage of gas and molten metal, while it must retain the coke. For this purpose, the bottom is constituted as a grid established with bars of carbonaceous material. When the caissons move upwards while the coke charge is consumed, the bottom of the upper caisson arrives at the reaction zone and the material which constitutes it takes part in the reaction as a reducing agent. Reference 2 designates a steel cylinder which acts as a container and guide for the oxide which is loaded from above. The reference 3 designates the carbon walls of the box after the bottom has been consumed during the reaction: we detach them, and we can assemble them to reuse them with a new bottom, or we can crush and use them for the reduction. In 4, we see the electrodes (three phase). In 5, we see a receptacle made of refractory material, capable of containing the liquid metal, supported by the column 7, which can be raised and lowered by means of the hydraulic cylinder 8.The column 7, and consequently the stack of carbon boxes, goes up to a speed corresponding to the rate of carbon consumption by the reaction, until the bottom of the lower carbon box has arrived at about the level of the ceiling of the enlarged lower part of the chamber. reactor. This carbon box is then locked in this position by means of a mechanical device (which is not illustrated), and the column 7 is lowered with the metal container 5. At the same time, the one on the left of the two doors 6 and pushing the carriage 9 carrying the two hydraulic cylinders 8 to the right, until the metal container 10, empty, and the carbon box loaded with coke are placed directly under the carbon boxes which are in the chimney. The boxes 10 and 11 are then reassembled until they support the upper stack of carbon boxes and the feeding continues at a speed suitable for the reaction. The right door 6 is then closed, so that the reaction gas is forced to pass through the left discharge port, illustrated in FIG. 2. The right cover 12 is removed, the metal is removed from the container by means of the vacuum in a known manner, another container loaded with carbon (coke) is placed on this container, these two containers are lowered, the cover is put back down, and the assembly is ready for the new change which will take place. in the opposite direction. The duration of the transfer, from the descent of a metal container to its replacement by the next one is short if we compare it with the duration of the consumption of the contents of a carbon box2 so that the process can be considered practically continuous.

 The upper face of the carriage 9 is protected by layers of bricks, refractory and heat insulating, as are also the walls of the chamber. A labyrinth closure is arranged between the movable and fixed brick structures in the manner known in common tunnel ovens. The carriage 9 moves in a closed gas-tight space to prevent it from escaping carbon monoxide gas through the labyrinth seal.

 The device illustrated in FIGS. 2 and 3 is based on a vertical displacement of the carbon charge and must be considered as an example which cannot impose any limitation on the method of the invention; the displacement of the carbonaceous charge can, as a variant, take place at an angle below or tangentially to the reaction chamber, although these models of apparatus are not illustrated here.

Claims (10)

R E V E N D I C A T I O N S  10) Process for the carbothermic reduction of alumina, alone or in mixture with other oxides, characterized in that it comprises phases of supply of carbon or carbonaceous material, which will be designated here by the term carbon , by making it follow a path which is separate from the path of arrival of the oxide, of bringing this carbon and this oxide into contact in an area where energy is supplied by an electric current, which makes it possible to obtain a high temperature in this zone and to cause the reaction between the carbon and the oxide (s).  20) Process according to claim 1, characterized in that the carbon is supplied to the reaction zone by an upward movement, vertically or at an angle formed with the vertical.  30) Method according to claim 1, characterized in that the carbon is placed in boxes made with a carbonaceous material which can itself act as a reducing agent, so that these boxes will be consumed, in whole or in part by the reaction with the oxide (s).  40) Method according to claim 3, characterized in that the bottom of each of the carbon boxes has the form of a grid made of bars of carbonaceous material.  50) Method according to claim 1, characterized in that the oxide contains from 100 to 60% by weight of alumina and O to 40% by weight of silica.  60) Device for carrying out the method according to claim 1, characterized in that it comprises an elongated lower chamber, and a chimney placed centrally above this chamber, the chamber and the chimney being part of the same structure and being lined with refractory material, while inside the chimney, boxes (1) are placed in carbonaceous materials, loaded with carbon, these boxes being constructed so that they can be superimposed with a tight seal between the edge of the bottom of one and the upper edge of the one immediately below, and that there is a supply of oxide placed at the top of the chimney so that the oxide descends by gravity into a shell guide 2, electrodes 4 extending in the oxide layer.  70) Device according to claim 6, characterized in that the carbon boxes are stacked above one another f this stack of boxes being supported by a container of refractory material 5 capable of receiving the liquid metal, this refractory container being provided with holes or grooves, on its upper edge, in order to give a passage to the gases.  80) Device according to claim 7, characterized in that the container of refractory material 5 is supported by a column 7 of steel which can be raised and lowered by means of a hydraulic or mechanical mechanism (8).  90) Device according to claim 6, characterized in that the lower chamber is provided with two vertical doors (6) which1 by a horizontal sliding movement, can ensure a substantially gas-tight closure, so that the chamber is divided into three separate compartments, each of these compartments having internal dimensions greater than the internal cross section of the chimney which is above the chambers.  100) Device according to one of Claims 8 and 9, characterized in that two steel columns (7) are provided with their operating mechanisms (8), mounted on a common carriage (9) which can be moved horizontally .