DE2951959C2 - Process and device for the production of aluminum or aluminum alloys by carbothermal reduction of aluminum oxide or mixtures of aluminum oxide and other oxides - Google Patents

Description

Process and device for the production of aluminum or aluminum alloy by carbothermal Reduction of aluminum oxide or mixtures of aluminum oxide and other oxides.

The invention relates to the production of aluminum by means of carbothermal reduction of aluminum • ♦ o oxide In the electric arc furnace including the manufacture of alloys of aluminum with other metals (e.g. silicon) by reducing aluminum oxide In a mixture with other oxides (e.g. Silicon dioxide)

For at least 20 years it has been known that aluminum is produced by means of a thermal thermal reduction of

Alumina is producible. z. B. In a light bulb oven. The scientific principles

4S regarding chemistry and thermodynamics of the reactions concerned have now also been developed.

Nevertheless, it has not yet been possible to develop a technical and economic solution based on these principles

indicate practicable procedure. j

When the reduction of alumina using carbon is carried out under reduced pressure, |

Alumlnlumoxldcarbld and Alumlniumcarblc arise as intermediate products:

2 ΑΙ: Ο, -ι-3 C = Al ₄ O, C + 2 Co (g) (I) I

AI4O4C4 + 6C = A1 ₄ C, + 4CoIg) (II) I

Below I90CC, all reactants except for the CO are in the solid state in order to achieve equilibrium for to obtain a gas pressure of atm. A temperature of around 2000 "C is required. The reaction mixture is then partially melted and the simple equations (I) and (II) are no longer directly applicable. The finite metal-generating step can also be described as follows:

Al.OX + Al.C, = 8 Al (IMCO (g) (111)

The calculated Glelchgewlchtsgasdruck for this reaction reaches 1 atm at about 2100 ⁰ C. in an operating under atmospheric pressure reduction furnace, the reaction zone must be maintained at a temperature at least sufficient to maintain the Glelchgewlchlsdruck of CO equal to 1 atm. In the case of a slight overpressure to favor the reaction, a temperature of about 2! SO "C is required. At (, s, this temperature the system consists of solid carbon and two liquids, namely an oxide-carbide melt and a Molten metal. Equation (111) is not applicable for this and the metal-generating reaction can be described sonically as follows:

(Oxld-Carbld-Melt) + C (s) = (MetallschmelzeJ + CO (g) (IV)

_{Volatile AI 2} O (g) and AI (g) are formed in competition with the production of carbon monoxide and condensed products. In the first reaction stage, which is formally described by equations (I) and (II), the equilibrium pressures of Al ₂ O and Al reach only a few percent of the equilibrium pressure of CO. In the final stage represented by equations (III) or (IV), the proportions of Al ₂ O and Al in the equilibrium gas are higher but not excessive. It has been shown that the reaction between aluminum oxide and carbon proceeds via a mechanism which includes a gas phase with a high proportion of Al ₂ O and Al. Consequently, the volatilization losses are higher than one should expect from the equilibria. Furthermore, the metal melt has a lower density than the oxide ίο carbide melt and therefore floats on top. The Co-gas developed in reaction (IV) crosses the metal melt, which causes further losses through volatilization.

However, the volatilization of Al and Al ₂ O does not necessarily have to lead to metal losses. In a submerged arc furnace, the reaction gases flow upward through layers of colder feed, where the metal-containing vapors can condense. This also causes the feed to be preheated. However, if there is a high proportion of metal vapors in the gas, the charge becomes too hot, so that losses occur as a result of volatilization.

Another difficulty in the carbothermal production of aluminum is due to the considerable Solubility of carbon in metal at reaction temperature. This is about 20 atomic percent C, when the molten metal is in equilibrium with solid carbon. When the. , hmelze beats this carbon turns out to be aluminum carbide. low. From the equation

(12 Λ1 + 3 C, molten mixture) = Al ₄ C, (s) + 8AI (1) (V)

It can be seen that about a third of the metal turns out to be carbide.: L. This requires a subsequent split 2s ruling and the repatriation of the aluminum carbide. This is detrimental to the economy of the Process.

Another difficulty with the carbothermal reduction of alumina in an electric arc furnace concerns the energy supply and the heat transfer. As mentioned above, rile molten metal floats on top and is located directly under the electrodes. As a result of the high electrical conductivity of the metal, the resistance in the furnace circuit is reduced and this has resulted in difficulties Maintain adequate energy input in the furnace. Furthermore, the development of heat takes place predominantly on the surface of the metal, resulting in a very high metal temperature and significant volatilization leads. Evaporated metal condenses in the charge located above the melt and runs descends into the hot zone and is evaporated again. This cyclical process of evaporation and condensation As a result, a large part of the heat generated is transferred to the upper part of the furnace, instead of being passed down to the oxide-carbide melt, where the heat for the endothermic reaction (IV) is required.

From theoretical considerations of dynamics one could conclude that the reduction of a mixture of Aluminum oxide and silicon oxide for the production of an aluminum-ninlum-Slllclum-Leglerung would be more advantageous than the reduction of alumina alone, in terms of reaction temperature and losses by volatilization, mainly due to the mutual degradation of the activities of aluminum and silicon in their liquid alloy. However, the density of silicon is lower than that of aluminum and the difficulties due to the floating metal layer are predominant.

From dc DE-OS 23 03 242 is eli. Process for the carbothermal production of Silclum and Ferroslllclum 4S is known, in which the silicon dioxide and the carbon are fed separately, preferably by means of a hollow electrode, and the furnace gases are diverted upwards. This method is, however, not for the production of aluminum carbothermlsche useful would be well occur by the accumulation of a bath of superheated liquid aluminum to hor.i evaporation losses, and the extracted aluminum has a high content of dissolved carbon would have v>

The application is based on the task of specifying a method with which aluminum or aluminum
Umalloys can be produced by carbothermal reduction of aluminum oxide or mixtures of aluminum oxide and other oxides while avoiding the difficulties listed above, and in the case of the aluminum oxide or the Oxiclßemlsch introduced from above in the electric arc furnace, spatially separated from the required carbon-containing reducing agent, through the counter-current the reaction gases are removed, is supplied.

The invention also relates to a device for carrying out this method.

The problem posed is achieved by a method with the characterizing features of the claim 1 In claim 2 a further development of the method according to AA is given. Claim 3 relates to a Device for carrying out the proposed method. In claims 4 to 6 are advantageous Formations of this device indicated.

The invention is explained in more detail with reference to the drawings.

Flg. I shows schematically the main features of the method according to the invention.

Flg. Int Vertlkalschnltt shows a preferred embodiment.

FI g. 3 Is a section along the line AA of FI g. 2.

The method according to the invention is explained with reference to FIG. 1 explained. The reducing agent in the form of metallurgical coke 1 is fed from below and, as indicated by the lower arrow, upwards mages. The oxide 2 is fed in from above and moves through as the reaction proceeds

Gravity down. The upper part of the furnace system is essentially gas-tight. The energy required will be fed through the electrodes 3. The space between the electrodes is electrically insulating Oxide filled. The heat generation takes place only at the lower tips of the electrodes and In the neighboring coke instead. The intense heat generation in this zone causes the oxide to melt and ! 5 an immediate reaction with the carbon. The primary product is a gas with a high content of Al · and

AljO steaming in addition to CO. This gas is forced through the carbon or coke feed to flow downwards, gradually cooling it. Aluminum vapor is then turned into liquid aluminum

nium condenses, while aluminum suboxide reacts with the carbon, with liquid aluminum and Carbon monoxide arise.

ίο In addition, liquid oxide carbide and liquid metal can form directly in the reaction zone. the However, the melting points of these two phases are very different. The Oxld-Carbld solidifies and remains in the coke charge in a short distance below the reactor zone. while the liquid |

Metal weller hcrahfllcl.it

The liquid metal, as it is first formed at high temperature, contains dissolved carbon. At the

The metal is gradually cooled down by the coke charge, with solid aluminum carbide precipitating and depositing on the surface of the coke. At the lower end of the coke feed the temperature below K) O (VC to be maintained at this temperature of carbon is the solubility in liquid aluminum practically zero and the effluent from the end unleren metal is substantially free of carbon I \

in During the progressive cooling of gas and liquid metal as they pass through the coke bed |.

Various reverse reactions can occur to a certain extent, e.g. B. Reaction I.

between liquid metal and gaseous carbon monoxide according to equation III from the right and Us and |

the reaction between liquid aluminum and solid carbon, resulting in solid aluminum carbide. |

Experiments have shown, however, that the extent of these two reactions in the following: '

Temperatures are low. In addition, the products of these reactions are solids that In the Koksbeschlk- |

lag behind |

The process is based on the lowest melting point of the surrounding area (660 ° C). At temperatures below e

Halfway through 1,000 ³ C at the bottom of the coke charge, aluminum is the only remaining liquid in the |

System All other reaction products are solids trapped in the coke charge and |

These solids are returned to the reaction zone with the upward movement of the coke feed. ϊ; ι

The method is also applicable for the production of low-melting alloys of aluminum t,

by reaction of the corresponding oxides. Examples are aluminum-silver alloys with up to 40 |

Weight ». S: (The melting point for this composition is 95O ° C). s

A particular advantage of the process is that the liquid metal to the extent of its formation In |

the hottest reaction zone immediately flows away from the vicinity of the electrodes. This prevents |

a pool or continuous layer of molten metal accumulates and the f

Heat is generated in the arcs between the electrodes and the top layer of the carbon I

Substance that generates the heat required for the reaction in the oxide-carbon mixture. |

To further explain the invention, calculations are given on the energy requirement and the «

Energy distribution in the coke feed. The calculations were made for the reduction to pure aluminum |

employed The energy required to heat the aluminum oxide fj was taken as the energy expenditure

from room temperature to the melting point, the heat to melt the aluminum oxide, the further | Heating to the reaction temperature and the heat of reaction. The warming of the carbon was called The energy requirement is not counted as it is caused by the countercurrent flow of the reaction gas. The cooling of the Gas was assumed to be HOOK (827 ° C) because this is a reasonable temperature if that Aluminum should remain liquid.

From the above it follows that it is not possible to express how big a one is directly as a liquid Metal formed fraction of aluminum metal is and what fraction is initially as aluminum vapor with Subsequent condensation was formed in the coke feed. This is primarily dependent on the kinetics

so the reactions; .nd With the current state of knowledge, a quantitative calculation is not accessible Ute energy calculations were therefore made for two limiting cases:

a) all metal is produced directly as a liquid and

b) all metal is initially produced as steam.

The thermodynamic data were taken from JANAF Thermochemical Tables (Natl. Bur. Stand., Washington 1971) Temperatures are given in Kelvin (K), which shows the results of the calculations following tables.

Alternative I: All metal formed as liquid Al at 2400 K.

AhO ₃ : heating 298-2327 K 61.1 kcal / mol

Melting at 2327 K 28.3

Heating 2327-2400 K 2.5

AI ₂ Oj (I) + 3 C (s) = 2 Al (I) + 3 CO (g) 282.2

Primary energy demand

Cooling 2 AKl) 2400-1100 K 3 CO (g) 2400-1100 K

Heating 3 C (s) 298-2400 K "excess heat" in the coke charge

Alternative II: All metal formed as Al vapor at 2600 K.

Al ₂ O ₃ : heating 298-2327 K melting at 2327 K

Heating 2327-2600 K Al ₂ O ₃ (D + 3 C (s) = 2 Al (g) + 3 CO (g) Primary energy requirement

Condensation 2 Al, 2600 K, cooling 2 AI (I) 2600-1100 K 3 CO (g) 2600-1100 K

Heating 3 C (s) 298-2600 K "excess heat" in the coke charge

374.1 kcal / 2 mol Al > 600 K kcal 8.0f > kWh / kg Al kcal / mol 19.7 kcal 33.2 kcal 52.9 kcal 32.4 kcal / 2 mol Al 20.5 kcal / 2 mol Al 11.16 kWh / kg Al 0.44 kWh / kB Al 139.9 fbeii 22.8 61.1 38.5 28.3 201.2 9.5 36.0 419.6 518.5

IO

20th

165.2 kcal / 2 mol Al
3.55 kWh / kg Al

Note that the reactants and products were the same for both alternatives. Consequently, the net energy requirement is equal to the primary energy requirement minus the excess heat, regardless the assumed reaction mechanism and the reaction temperature, namely:

AlLI

Alt. II

Primary energy requirement 8.05 excess heat 0.44

Net energy demand

7.61

11.16 kWh / kg Al

7.61 kWh / kg Al

The excess heat was removed to a temperature below about 1000 ° C in the lower part of the Maintain coke loading. This can happen in part due to normal heat loss from the Furnace wall, partly also by appropriate air or water cooling. In the latter case, the discharged Heat can be supplied for auxiliary purposes via exchangers.

When considering the energy economy of the process, the chemical energy of the carbon monoxide is also important to be taken into account. For every kg of Al, 1.36 m 'CO gas (1 atm, 25 ° C) are generated with one Combustion heat of 4.35 kWh. Assuming that 70% of this heat is harnessed, so are Deduct 3.0 kWh / kg AI from the energy expenditure. On the other hand, there are heat losses as a result Dissipation through the furnace walls and the electrodes. However, when all circumstances are included, the energy requirement is of the method of the invention is comparatively cheap over the currently used electrolytic Processes in which the effort is in the range from 14 to 17 kWh / kg Al Hegt.

The most important technical problem in performing the method is the upward movement of the Carbon feed In countercurrent to the reaction gas and the liquid metal. Figures 2 and 3 show a device with which this object is achieved.

In this device, containers 4 are used, which are made of carbonaceous material and with Be filled with coke. The bottom of these containers 4 allows the passage of gas and molten material

45

50

55

60

65

Metal while he carries the coke. The ground is therefore z. B. formed as a grate auj rods made of carbon material. As these containers are pushed up and the coke charge is consumed, the bottom of the top container reaches the reaction zone and its carbon material participates in the reaction as a reducing agent. A steel cylinder 5 serves as a container and for supplying the oxide, which is fed in from above. The wall of the carbon-containing container is denoted by 6 after its bottom has been consumed in the reaction. The wall is removed and, provided with a new bottom, can be reused as a container. It can also be crushed and then used as a reducing agent. Number 7 denotes * the electrodes Pphaslg).
The container 8 made of refractory material suitable for holding liquid metal is carried by a stand 10 which can be raised and lowered by means of the hydraulic cylinder. The stand 10 and with it a stack of carbon-containing containers 4 are moved upward at the rate caused by the carbon consumption of the reaction until the bottom of the lower container is approximately level with the cover of the lower extended part of the reaction chamber. The carbon container is then held in this position (the corresponding mechanical device is not shown) and the stand 10 with the metal container 8 is lowered. At the same time the left of the two gates 9 is opened, and the carriage 12 carrying the two hydraulic cylinders 11 is shifted to the right until the empty metal container 13 and the carbon container 14 with the coke charge are directly below the carbon container in the furnace shaft. The containers 13 and 14 are moved upwards so that they support the stack of containers above. Gate 9 on the right is now closed so that the reaction

2n tlon gas can only flow out through the left exit. Then the lid 15 of the right compartment is removed, and the metal container is emptied in known catfish by means of vacuum. Then a new one Carbon container put on. After lowering these two containers, the lid 15 is put back on, and the device is ready for the next shift in the opposite direction. The transfer tent from .1 Lowering a metal container until it is replaced by the next Is short compared to the consumption tent of the

j | 25 contents of a carbon container, so that the process is practically continuous.

The upper side of the car 12 is protected by fire-resistant and heat insulating stones. Between the Moving and standing masonry, a labyrinth sluice is provided, as is the case with conventional tunnel ovens known 1st. The carriage 12 moves within a gas-tight enclosure to prevent escape Prevent carbon monoxide through the labyrinth sluice.

With the system according to Flg. 2 and 3 the feeding takes place in vertical movements. However, this is not the case Condition. The feed and discharge paths can also be at an angle to the reaction chamber or tangentially be arranged.

For this purpose 3 sheets of drawings

Claims (6)

Patent claims: 1. Process for the production of aluminum or aluminum coatings by carbothermal reduction of aluminum oxide or mixtures of aluminum oxide and other oxides, in particular from mixtures See with 100 to 60% by weight of aluminum oxide and 0 to 40% by weight of silicon dioxide, Im electrical Arc furnace in which the oxide or the oxide mixture is introduced from above and spatially separated of which the required carbonaceous reducing agent, through which the reaction gases in countercurrent are discharged, is supplied, characterized in that the carbon-containing reducing agent continuously or intermittently from below vertically or at an angle to the vertical upwards is guided and by the rising carbonaceous reducing agent in countercurrent the liquid Reduction product and the reaction gases are discharged downwards, with the lower part of the coal charge the temperature below 1000 ° C, but above the melting point of aluminum or the Aluminum alloy is held. 2. The method according to claim 1, characterized in that the carbon in containers made of carbonaceous Material is supplied, which itself act as a reducing agent and when reacting with oxide whole or partially consumed. 3. Apparatus for performing the method according to claim 1 to 2, containing an electric arc furnace marked with feed devices for the charging and discharge device for the reaction products Hurch an elongated lower chamber with a central furnace shaft, both with refractory Ma; srlal are lined, one on a carriage (12) displaceable in the elongated chamber and up and down stand (10), the a stack with containers (4) for the carbonaceous Material and a container (8) for collecting the liquid aluminum or aluminum alloy carries, as well as a steel cylinder (5) arranged in the upper part of the furnace shaft for supply and Pick-up of the oxide and the electrodes (7). 4. Apparatus according to claim 3, characterized by container (4) made of carbonaceous material, the bottom of which is in the form of a canister made of rods made of carbonaceous material. 5. Apparatus according to claim 3 or 4, characterized in that the elongated lower chamber is divided into three chambers by two horizontally displaceable and gas-tight closing gates (9), wherein Each chamber is larger in its internal dimensions than the internal cross-section of the furnace shaft and the two outer chambers have removable covers (15) on their upper side. 6. Device according to one of claims 3 to 5, characterized in that the horizontally displaceable Wagon (12) between y | Stänc-r (11) carries, which are mechanically or hydraulically adjustable in height.