DE2318936A1 - ALUMINOTHERMAL PRODUCTION OF MAGNESIUM AND AN OXYD SLAG THAT CONTAINS EXTRACTABLE ALUMINUM - Google Patents

Description

Julian Miles Avery

Chestnut Hill, Mass.

United States of America · . P 68/25 / B ¹

M-metal thermal production of magnesium and an oxide slag, which contains recoverable aluminum oxide

The invention is based on a process for the production of magnesium in a reaction-condensation system by introduction of magnesium oxide and a metallic reducing agent in the reaction zone of a reduction furnace in the presence a molten oxide slag bath at melting temperature, the resulting magnesium vapor from the reaction zone into the Condensation zone is transferred and condensed there.

There are two technical processes for metallothermal production known from magnesium, which are carried out discontinuously under a very high vacuum. Neither will carried out on a large scale, which is the general production of magnesium by electrolysis of molten Magnesium chloride would be comparable.

In one of the well-known metallothermal processes, referred to as the "Pidgeon process", perrosilicon ( containing about 75% silicon) and dolomite lime are placed in a battery of externally heated horizontal tubular retorts, and the magnesium is formed as the "crown" by condensation in one

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Extension of the retort obtained. Because the retorts are very high Not withstanding temperatures, the reaction is necessarily a solid reaction at a relatively slow rate. This process requires high capital investments and operating costs.

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In the other metallothermal process, which is known as the "Magnetherm process", dolomite lime is also used as the magnesium oxide source and ferrosilicon (about 78 % Si) as the reducing agent: The process is carried out at about 1500 ⁰ C under high vacuum in the presence of a molten calcium aluminum silicate slag in an electric furnace with internal heating. The proportion of slag in relation to magnesium is very high, around 6 to 1, which creates the problem of how to accommodate it when it is not sold or used to make cement for which it is low in value can. The process is not carried out continuously since the reaction has to be stopped in order to break the vacuum and cut off the slag. Under the very high vacuum, air seeps into the system and 'magnesium is lost not only through oxidation and nitrite formation, but also through the need to remove and clean or replace the capacitor and crucible at the end of each batch.'

The various thermal reduction processes including those described here are described with reference to the relevant patents and publications in "Principles of Magnesium Technology", Ε.Έ ¹ . Emley, Pergamon Press (London 1966), pages 46-64. There is also a comparison between the electrolytic

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and the metallothermal process on pages 65-67.

The invention is based on the object of creating a metallothermic process for the production of magnesium, which he— successful with the electrolytic process in large or can compete to a small extent and that also has advantages over the known processes for the metallothermically ^ production of Has magnesium. -

This object was achieved by the process according to the invention, which can be referred to as an aluminothermic process for the production of magnesium metal and an oxide slag (calcium aluminate slag) containing recoverable aluminum oxide by reducing magnesium oxide in a mixture of calcium oxide and magnesium oxide using a metallic reducing agent , which contains at least 85 % aluminum, in the presence of a molten calcium aluminate slag bath at a temperature of about 1300 to about 170O ⁰ O. The magnesium escapes from the molten slag as vapor. The process is carried out continuously at atmospheric pressure. The molten slag residue is tapped periodically when the KgO level is below 5 % .

The oxide filling contains MgO and CaO in a weight ratio from about 1.1 to about 2.3. The weight ratio of slag to magnesium can be kept at around 2-3 to 1.

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The slag formed as a by-product of the reaction is hauptsäoblich from the soluble calcium aluminates 12OaQeAIoO- ^ and Ga and has a composition -from about 35 - 65 °, Aluminiumoxjdj ί 33 - 55 ° ^'a calcium oxide and .0 - 10 ° / o- and Siliziurnoxyd contains is recovered from the slag at distance- from the system less than $ 5 magnesia "lenn aluminum, the" ratio of the remaining Gago ^ is to magnesium is generally in the range from about 2.8 ι 1 to I ₁ A; 1. in addition, the remaining ■ GaGQ, can be converted into OaO and recycled, whereby the solid residue is reduced to almost zero.

The process according to the invention enables a fairly complete utilization of the magnesium oxide and the reducing agent used. In the description it is shown 'that' the relation of Magnesium oxide to calcium oxide in the batch and the amount and addition ' composition of the metallic reducing agent in the way with each other are related, that once both the reducing agent and the magnesium oxide are consumed in the reduction reaction and further that the obtained aluminum aluminate slag has a composition which gives a sewn feeling of pure Alumina in a very high yield allows.

As already mentioned, the metallic reducing agent should contain at least 85 ° aluminum. It can consist of metalXisohem aluminum or one or more aluminum alloys that contain at least 85 percent by weight of aluminum »

The general features of the method according to the invention are:

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a) The Reduktxonsreaktxon is carried out in an electric furnace with internal heating at a temperature of about 1300 ° C to about 1700 ° C, preferably at about 1500 ⁰ O.

b) The magnesium evaporating during the reaction is stored in a condenser recovered and collected in a crucible, preferably as molten magnesium.

c) The entire system of furnace, condenser and crucible is made preferably held at about atmospheric pressure. The magnesium escapes from the molten slag under partial pressure of about atmospheric pressure and is condensed by suitable measures, or optionally at a low partial pressure in a system containing inert gas such as hydrogen, helium, argon or mixtures thereof. Either of the two Possibilities allows a continuous or almost continuous mode of operation, since the molten one at atmospheric pressure Slag and the magnesium can be easily removed from the furnace and the crucible.

d) The proportion of oxidic and metallic starting materials is chosen so that a slag is formed with a composition that the aluminum oxide contained in the molten slag crystallizes when the slag cools, "mainly in the form of soluble calcium aluminates, e.g. OaO-Al ₂ O ₅ (64.5 % Al ₂ O ₅ ) or 12OaO >> 7Al ₂ O ₅ (51.5 % Al ₂ O ₅ ) or both.

The formation of such a slag requires that the

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weight ratio in the oxide filling of MgO to OaO about 1.1 to 2.3 amounts. When dolomite is used as a source for the GaO the corresponding weight ratio of dolomite to magnesium oxide is about 1.2 to 3 »8. But it can do magnesium oxide (MgO) and lime (CaO) can be used.

In the context of the present invention, such slags can formed by the use of lower quality magnesium oxide be e.g. B. of silicon-containing magnesite, through use of metallic silicon as an additive to aluminum as a reducing agent, or by using an aluminum alloy that somewhat Contains silicon. However, the production of a silicon- Oxide-containing slag should not be a preferred form of the invention, since without it the extraction of aluminum oxide is higher, it provides but a useful modification for the recovery of cheap Low quality magnesite or scrap aluminum containing silicon or it can be used if the recovery of aluminum oxide is not desired.

Although the extraction of AlpO from the calcium aluminate slag is not necessarily decisive for carrying out the present invention, it is an important part of the process. It has been found that it is not necessary to produce a trickling silica-containing slag. In fact, slags composed of the pure oxidic compounds CaO-Al ₂ O-, and 12030-7AIpO ₅ , as well as mixtures thereof, with almost 100% effectiveness with NagGOj solution. (-without free NaOH) at 70 ⁰ O in about 1 hour

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leached v / ground., and that practically without residue except for the GaCO ₅ ,, which has been formed as sodium aluminate by the conversion of NagCO to the solution of AlgO ^.

Saturation with carbon dioxide to separate the Al (OH) from which Solution regenerates the Na-CO ^, and the solution can with so much ' processed Na ^ CO ^, as is necessary because of the loss, be returned »,

A comparison of this extraction process with the usual processes (Bayer and Pedersen) leads to the conclusion that the present process is simpler and cheaper to carry out and requires less capital investment. Another advantage of the slag proposed according to the invention, containing small amounts of silicon oxide or free of silicon oxide, is that it enables the formation of high-quality aluminum oxide suitable for the electrolytic reduction of AlO ^ to aluminum without separate removal of the silicon, which is the case with processes with silicon oxide Bauxite is sometimes necessary. ' So the Al _o 0 _x content per unit is more valuable than that. Aloo-z content of in the Bayer process used high-quality bauxite, and far more valuable than the Al ₀ O ^ content of the used for the Pedersen process low-value bauxite · Barüberhinaus it seems quite possible that the Auslaugevorgang be carried out in an existing Aluminiumoxydanlage can, even if part of the system becomes superfluous.

An important feature of the present process is that that used cheap waste aluminum as a reducing agent

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- either waste at aluminum manufacturing plants or purchased waste from aluminum processing plants. Immediately the question arises whether the use of this waste could adversely affect the technology of the process or the quality of the magnesium detail produced because of the alloying agents present in many types of alloyed aluminum used in its manufacture. An examination of the compositions of the conventional alloys is that the alloy metals used primarily magnesium, silicon and copper, manganese, chromium and zinc in amounts generally less than 5% but up to 12 are available fo in at least one alloy. -. ■ "- .." ■ '■■ - "

Finally, there is the problem of the, presence of aluminum since aluminum itself as Verunreinigung- in the magnesium product a · - has vapor pressure of about 10 mm Hg at 15OG ⁰ O. Bas means that - * the magnesium produced by the present process inevitably contains aluminum. The amount depends on the operating temperature. For example, at 1400 ° 0 it would contain about 0.5 $ aluminum, at 150O ⁰ O about 1.2 fo and at 160O ⁰ O about 2 ^c / o _m. However, this is not a serious problem and in fact it can be advantageous , because:

a) The main use for magnesium today is in aluminum alloys for further processing., and

b) a predominant part of that used for magnesium products Magnesium contains a significant amount of aluminum, generally 3 to 9 #

Thus, the presence of a small amount of aluminum is in

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not harmful to the magnesium produced by the present process, especially if the magnesium process is associated with the production of aluminum, which will probably be _{the pail because of the advantage of recovering Al 2} CU from the slag and the possibility of using waste aluminum as a reducing agent.

From this point of view, the process according to the invention in its preferred form could be characterized by its overall results as a process for the conversion of waste aluminum into pure magnesium, combined with the conversion of magnesium oxide into high-quality aluminum oxide suitable _{for the electrolytic reduction of AlpO 5 to aluminum,} It is particularly suitable for a company that produces and processes aluminum because of the availability of waste aluminum, the ease with which the aluminum oxide can be extracted from the slag and the need for a high quality aluminum suitable for the electrolytic reduction of ALGCU to aluminum. But it can also be done independently, particularly on the basis of an adequate supply of scrap aluminum at low cost.

A so-called modified aluminothermic process, called the MO process (cf. EE ^1. EmIey, Pergamon Press, London 1966, p. 50; British patent specification 922,30Oj and Light Metals (February 1964), p. 44) is also mentioned and briefly described been. This method and its modifications have several disadvantages. The one created in such a process

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Since it contains 15% MgO, slag (cf. British patent specification 922,300) can practically not be used for the recovery of Al ₂ O ₃ , but this is the case with the slag according to the process according to the invention.

Trans. Can. Inst. Min. Met., 1962, 65, pp. 221-224. The purpose of the experiments was to determine the vapor pressure of the magnesium product obtained by reducing magnesium oxide, dolomite and olivine with aluminum. In this work it is concluded that the reaction. product of the reduction of magnesium oxide, magnesium aluminate, and that the reaction product of reduced dolomite either 12OaO.7Al ₂ O ₃ (below 1,200 ⁰ O) or 3OaCAl ₂ O ₃ (above 1200 ° 0). In contrast to these results, I have found that it is possible to obtain a calcium aluminate slag which predominantly contains 12 GaO ₂₃ CaO «Al ₂ O, or mixtures thereof, at a temperature of about 100-1700 ° C. Aluminum oxide can easily be obtained from both oxide products or their mixtures, while that in the above. Process specified product 3OaO-Al ₂ O _{3 is} insoluble and an extraction of aluminum oxide is not possible therefrom.

The present invention offers numerous advantages over the aforementioned metallothermic process. First In fact, all of the aluminum reducing agent can be consumed in the primary reaction, eliminating the need for Recycle or disposal of significant amounts of spent metal, such as iron alloys, is avoided, though

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Ferrosilicon or ferroaluminum can be used. Second, can the magnesium oxide filling, preferably a mixture of magnesium oxide and calcined dolomite, have an advantageously high proportion of MgO relative to OaO. Overall, these benefits mean that the performance of the furnace is much higher than the previous one Procedure is. Third, the aluminum reductant is far more reactive, and fourth, the molten slag at a be flowable at a relatively low temperature. Allow these advantages operating at a relatively low temperature in a molten slag environment with the formation of magnesium vapor approximately at atmospheric pressure (reaction equilibrium), whereby a continuous or semi-continuous process is carried out can be. A fifth and very important advantage is the recoverability of aluminum oxide from the resulting slag, what in turn, the net slag to magnesium ratio is substantially reduced, to between about 3 to 1 and 2 to 1 or even lower.

On a small scale, the already mentioned "Pedersen process" is used for the production of aluminum oxide for the electrolytic reduction of Al ₀ O- to aluminum from low-grade bauxite (ferrous or silicon-containing or both) by adding bauxite together with lime and a carbon-containing reducing agent melted in a covert arc furnace to produce a low grade iron product and calcium aluminate lacquer that can be made into Al ^ O *. Unfortunately, the AlpO ^ SiC ^ obtained contains what is used for the electrolytic reduction of AlgO ^ to aluminum

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is disadvantageous and makes a separate removal of SiO ₂ necessary. See "HE Blake, Jr., Bur. Mines Kept. Inv. 6959 (1967).

The attached figures serve to illustrate an advantageous embodiment to explain the method according to the invention.

Figure 1 is a block diagram of the portion of the present "process, in the magnesium oxide and calcined dolomite in a magnesium furnace. implemented essentially pure aluminum to obtain magnesium through evaporation and condensation and a calcium aluminate slag as a by-product.

Figure 2 is a high level schematic of the portion of the present process in which the by-product, calcium aluminate slag, is leached to obtain purified alumina.

In the following, some of those used in this description are intended Expressions are defined in more detail:

The reducing agent in the magnesium furnace is "essentially pure aluminum". This should not only include pure aluminum but "also aluminum alloys that contain at least 85 percent by weight aluminum (including alloys with the following standard numbers: 13, 380, 43, 195, 214 and 220), as well as waste aluminum, which generally contains aluminum of at least 95 ^c / o (standard numbers: 1100, 3003, 2017, 2024, 5052, 6063 and 7075) In general, silicon can be ^{present in the reducing agent up to 10 c} / o or even slightly more (e.g. 12 ° / o in alloy 13) "if a decomposing slag is desired. The other components, such as manganese, calcium, chromium or magnesium, should preferably contain less than 5 percent by weight of the

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Reducing agent. Magnesium can be present in the reducing agent, but for the purposes of the invention it is for the composition of the reducing agent is irrelevant since it is distilled off.

The term "calcium aluminate slag" refers to the portion of the reactants which form the oxidic by-products of the reaction. These oxides are principally calcium oxide and aluminum oxide, but can also include silicon oxide and other oxides. Calcium oxide, aluminum oxide and silicon oxide are important, as stated here, and the composition contains various oxides: 35-55 ° / ° CaO, 35-65 io Al ₂ O ₅ and 0-10 ti SiO ₂ . Up to 5 % or even more of other oxides can be ignored. However, if the slag is tapped, it must not contain more than 5% MgO and, if aluminum oxide is to be obtained in a satisfactory yield - not more than 2% MgO, preferably as little as possible. in connection with this slag means that at least the greater part of the slag is in the liquid state. This occurs generally in the range of the composition of the slag for the present invention at I300-1700 ⁰ O.

Other terms used herein such as "inert gas" and "essentially static" are in my patent application P 20 02 544.9-24- and in German patent specification 2 II5 324 explained.

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Las byproduct calcium alaminate slag

The essential element in the present proceedings is the ivalziumaluminat-Üchlscke ,. which together with the ^ eschmolzerion reducing agent that forms liquid hedium in the l'iognesium furnace, The composition: the varnish is important for two reasons. First, it is essentially the reaction medium in which. Magnesium oxide reduced to magnesium and at the same time the reduc- ' .tionsmittel aluminum is oxidized to aluminum oxide. (By doing Silicon present in the reducing agent may or may not be oxidized). Second, the slag can, if its .composition properly regulated, a valuable lifting product its what high quality aluminum oxide for the electrolytic Reduction to aluminum or for other purposes, such as high quality refractories, supplies.

In the AlpO ^ -GaO-SiOp quadratic system are CaO.AIpO ₂ , and 12CaO. 7AIpO ₂ . Almost completely soluble in a lye of sodium carbonate, but 3CaO «AlpO-, is ^{soluble to less than 50 C} , O , CaO. 2Al ₀ O ^ is almost insoluble and 2Ca0 «Al _o 0 ₂ oSi0 _o is insoluble. Around

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to increase the extraction of aluminum oxide as much as possible, it is desirable to form a spent oxide, its alumina content as much as possible in the form of crystalline Calcium aluminate is present.,

Controlled cooling of the molten slag increases to a large extent the extraction of aluminum oxide. The appropriate cooling rate is on the composition of the slag

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addicted; in general, a slag with a substantial silicon oxide content must be cooled more slowly than a slag with little or no silicon oxide content. In addition, it is desirable that the cooling rate "is so slow that the two-phase transformations of dicalcium silicate (2CaOSiOp), alpha to beta at 14-20 ⁰ C and beta to gamma at 675 ° C, can take place, since the inversion to the gamma form of one Increased volume is accompanied by 10 <;i> , whereby the cooled residue can be shattered into a fine dust, a phenomenon commonly referred to as "dusting". Cooling too quickly prevents crystallization and a glassy solid is formed , from which the aluminum oxide geins "is.

The silicon dioxide content of the slag must be at least 5 ^c ß> in order to cause dust. Dusting considerably reduces the amount of grinding required before the alumina can be extracted. However, it is as shown in the examples, a very high recovery of aluminum oxide - up to 97 · 5 i'ö -möglich when the Siliziumdioxydgehalt than 4 ° / o is less, which is low for an appreciable dust to. Thus, the advantage of having enough silica for dusting must be weighed against the advantage of a high total recovery of alumina. If the slag contains enough SiO ₂ to cause dusting, then the ÄlgG ^ product will likely contain more SiO ₂ than is allowable for a high quality Al ₂ O, and separate removal of SiO ₂ may be required. The silicon oxide cannot only come from silicon, it can come from it

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can also be the silicon oxide from the original oxide filling (e.g. magnesite) *

The alumina content of the slag should be high to that Amount of AIpO ^ to be treated per unit of recovered Reduce slag as much as possible. The calcium oxide content should be low; he needs them to form more soluble ones Calcium aluminate not to exceed required amount.

The presence of magnesium oxide in the slag, however, considerably impairs the recovery of aluminum oxide, since it has been found that 1 mole of MgO can bind 4 moles of AlpO ,, in the insoluble complex 6OaO-4-Al ₂ O ^ .MgO-SiO ₂ . Therefore, the slag is tapped periodically when its MgO content has decreased to less than 5 #. Most of the magnesium oxide is withdrawn from the reaction zone so that less than 2 % remains before the slag is removed «,

In view of all these factors, the composition of the spent tertiary oxides, apart from small amounts of other oxides (if present) as follows: 35 - 65% alumina, from 55 to 55 ° / ° of calcium oxide, and 0 - 10% silicon dioxide.

Alumina recovery from slag by-product

In-depth studies of the Pedersen Process have been conducted with the aim of determining whether it will be successful in the recovery of Al ₂ O * from low grade aluminous ores such as low grade bauxite and clay at a reasonable cost

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can be applied. The melting process for the production of the slag, to which the present process has no comparable parallel, is not to be discussed here, only the leaching process. ^H

It is not necessary to produce a disintegration slag for a high leaching capacity. The oxidic compounds GaO-AIgO ^ and 12CaO.7A ^ O; * are, if they are synthetically produced in crystal form and properly ground, completely soluble or almost soluble in NagCO ^ solutions without free NaOH at temperatures of about 70 ⁰ G and one Leach time of one or two hours. All other calcium aluminate compounds are either insoluble or sparingly soluble in such a solution.

Within the range of slag compositions listed above the two specified calcium aluminates are the only ones that are formed. To get them in crystalline form, the slag must be cooled very slowly, but this is easy to do.

If the slag contains a considerable amount of SiOo, the utility of the AlpO ^ for the electrolytic reduction to aluminum is limited. In some cases, this problem must be overcome by a separate removal of SiOo, which increases the cost of the A ^ O ;. A preferred embodiment of the method of the present invention is to avoid this problem and to produce a slag that is substantially free of SiO ₂ .

If MgQ is present in the slag, it has even a small amount

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Quantities have a very adverse effect on the. Obtaining AIpCU. When the slag cools, a complex oxidic compound is formed, namely 6CaO-4Al ₂ O ^ -MgO-SiO ₂ J, which is practically insoluble in the leaching process. Thus, 1 mole of MgO in the slag not only binds 4 moles of Al ₀ OZ, which results in a loss in extraction, but it also binds 6 moles of CaO. However, the effectiveness of the Al in the system is so great that a final slag with little or no MgO content can be produced.

The essentially pure aluminum reducing agent

Essentially pure aluminum enables a large reduction from magnesia to magnesium and the escape of magnesium vapor from the molten slag at a total pressure of ■ about 1 atm. As mentioned above, the aluminum can be made from pure metal, high aluminum alloys, or from Waste aluminum including alloys. The compositions of the main aluminum alloys and the waste generated during their processing - aluminxumm materials are given in Table 1 below.

It could be assumed that the presence of metallic impurities in the scrap aluminum used as the reducing agent for the present process could pose serious or even insurmountable problems in its use for this purpose. In fact, it is, it is not, and most of these "impurities" provide benefits in terms of either manufacturing cost or the total value of the magnesium _{BQU / QU6 produced}

table I.
Addition of the aluminum alloys

Al Mn Cu Or Zn Si Mg

1100 99+ - 3003 Remainder 1.2 2017 Remainder 0.5 4.0 2024 Remainder 0.6 4.5 5052 rest ■ - * 6063 rest 7075 rest 1.6 13th rest 380 rest 3.5 43 rest 195 rest 4.5 214 rest 220 rest _

1.5

0.25 - 2.5

0.4 0.7 0.3 5.6 - 2.5 - - 1? -

-9.

—— —— 5 -

0.8

3.8

10

The cheap scrap materials can readily be used in the present process, and this factor is of great importance to its economy. The major impurities in the aluminum alloys and scrap materials are magnesium, silicon, copper, manganese, chromium and zinc, generally less than 5 percent by weight. Alloy 13 has 12 # silicon and alloy 220 has 10 $
Magnesium, but these impurities are not a problem and can be fully utilized in the present process.

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Copper is available in various alloys in proportions up to $ 4-5 available; it is inert in the process and forms part of it of the molten metal residue at the bottom of the furnace. The same thing applies to chromium, which is present in small amounts in some alloys.

Manganese is present in amounts ranging from $ 6 to $ 1.2 in some alloys, and since its vapor pressure is about 10 mm Hg at 1500 ⁰ O, some manganese vapor is carried into the product with the magnesium vapor Using a mixture of all types of scrap, most of which do not contain manganese, this contamination is unlikely to cause serious problems that contain manganese in addition to magnesium.

The pollution “that could cause trouble is Zinc, which has a boiling point of 907 ° C, so that in the scrap Any zinc present is carried away into the magnesium produced * However, it appears that only one alloy contains zinc (about 5.6 #).

The oxide filler containing magnesium oxide;

The oxide filling contains magnesium oxide and preferably calcined dolomite. However, the magnesia is absorbed by the im essential pure aluminum reducing agent * Das

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Weight ratio of MgQ to OaO in the filling is about 1.1 to 2.5. Dolomite supplies both components in a 1: 1 mol- relationship; Magnesium oxide can be used to make MgO, and Lime to provide GaO.

The calcium oxide is carried along to the slag hub product, where, when solidified, it combines with AIoQ * to produce the desired Form calcium aluminates. Some or even all of the calcium oxide required can be added as lime, in general however, this is not preferred as dolomite lime is usually "available" to provide more than just what is needed Calcium oxide but also the magnesium oxide to deliver.

Inexpensive magnesium oxide can be obtained in suitable places in various ways; (a) Production of MgO from sea water, Brines or mother liquors according to generally known methods; (b) Use of inferior magnesite for the manufacture refractory materials not suitable and therefore too low Prices is available. These ores are generally available as waste from refractory magnesia processes or as low-value ore deposits that are not currently being mined. These low grade ores usually contain CaO and in a relatively low ratio to MgO,

The presence of a small amount of OaO in these ores is of course not harmful as OaO is required in the process, but the presence of SiOp requires care careful attention. Is the in Considered type SiOg is present, so it forms when the

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Slag solidifies, the relatively insoluble compound 2OaO-SiO ₂ . With proportions of about 5 to 10 percent by weight SiOg, a slag is formed which disintegrates or "atomizes" when it is cooled very slowly in order to allow phase changes in the crystalline SiIicate.

The construction of the furnace

A significant problem with the present process is the provision of a refractory furnace lining that can ^{withstand the molten slag to 150O 0} O or higher by forming a strong, everlasting lining between the graphite lining that protects the furnace shell and the molten slag that otherwise ^ which would attack graphite with devastating consequences. The solid, self-sustaining lining is created by the fact that a layer of the slag solidifies on the furnace walls.

In the Magnetherm process, magnesium oxide is used, which is very suitable for this purpose, since it is hardly attacked by the calcium-aluminum-silicate slag with a high lime content used in this process, while it is to be expected that the one used in the present process Calcium aluminate slag attacks MgO readily. However, an examination of the slag phase diagram for calcium aluminate slags shows that when alumina is used as the refractory S ^ off, any attack by the molten slag causes an increase in the Al ₂ O ^ content of the slag layer on the surface of the lining, with the result did that after all

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contains an inner Schlaekenmantel, of more than 65% Al ₂ O _5, is formed and solidified · This is due to the favorable fact that the lowest melting point of the calcium aluminate slag, at about 5 * 1- # aigo ₅₁ 1400 ° C is which is close to the mean of the proposed slag composition range; and as the AloO ^ content of the slag rises, the melting point rises steadily to 155O ⁰ G at 60 # Al ₂ O ₅ and to 165O ⁰ O at 70 ° β> Al ₂ O ₅ .

Modifications based on economic considerations

The economic conditions of the process according to the invention are such that the slag can be discarded or sold cheaply if, under special circumstances, waste aluminum and other raw materials are cheap enough and the Al ₂ Ox is below its normal value. In this case, it may be desirable to use an alloy with a silicon content up to 10 # to make a decomposing slag.

It is also possible that under certain circumstances low-value Magnesite and the like are available at such low cost that it affects the overall economics of the process will. Such magnesites are available in numerous places, particularly as scrap or scrap from the manufacture of refractory MgO. Occurrence of magnesite or magnesite-containing dolomite, which is a source of fire-resistant magnesium oxide are unsuitable, are also available for use in the present process *

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These magnesites usually contain both SiOg and GaO as undesirable impurities. For use in the present Process, the GaO is not only acceptable but also to a certain extent Dimensions valuable. The SiO ^ content can be within acceptable Limits are used to produce a decay slag, from which AIoO ^ can be obtained. As in the above Case it is a question of profitability whether the low The cost of the MgO in addition to the cost of grinding a non-decomposing slag, the somewhat lower production of Al ^ O ^ and compensate for the somewhat higher leaching costs in addition to the disadvantage of an undesirably high SiOo content of the Al ^ O ^ produced.

Other conditions

That. present in the slag during the "termination period" remaining aluminum is distilled over into the magnesium.

During the "termination operation", neutral gas is introduced into the System directed to displace the magnesium vapor and maintain atmospheric pressure. But in a suitably constructed System does this automatically and does not provide any Serious operational problem. If slag and magnesium are tapped from the furnace or crucible, the pressure of the neutral gas can be increased to more than atmospheric pressure to drive them out of the system without using a pump. If the use of scrap aluminum involves an accumulation of a non-volatile metal, e.g. B. copper, so can you pile this up to the level of the slag tapping hole leave, whereupon it is let out together with the slag

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and at the bottom of the cinder pot forms a regulus. The use of inert gas

Usually the diffusion of magnesium vapor alone is sufficient to bring about the mass transfer from the reactor to the condenser. However, if desired, a stream of inert gas can be introduced into the furnace and passed through the condenser to to increase the flow of magnesium to the condenser, where a recycle system is desired to recover the inert gas "

In a further embodiment of the present invention, the magnesium product is evaporated and escapes from the reaction medium through an essentially static atmosphere of inert gas and moves through it primarily by diffusion inert gas passes through from the reaction zone to the condensation zone and is then collected. This modification enables to work at higher absolute pressure and results in a magnesium product, which is relatively pure compared to commercially available magnesium, which contains less impurities, especially silicon Concentration, contains. See German patent specification 2 115 324.

Other embodiments using inert gas are also possible, e.g. B. to increase the partial pressure of the Systems, e.g. B. Carrying out the process in the presence of an inert gas at a partial pressure of 0.1 atm, "as in mine Patent application P 20 02 544.9-24 is described, or as a Another method of maintaining positive pressure in the system during slag tapping. In this lall it would

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excess gas is added to the system immediately before the Slag should be tapped and removed for example by tapping or by means of a vacuum pump before filling the reactants are taken up again,

Examples

The following operations are carried out in an electric reduction furnace connected to a condensation chamber. leads. The procedure is that the furnace is charged with the feedstock to form a slag of the desired composition, and that heat is supplied until ^{the correct viscosity is reached at a temperature above about 1300 0} G, whereupon the oxide feedstock and the reducing agent continuously fed in small batches at a time. When a large amount of slag has accumulated, the reaction continues until all or almost all of the magnesium oxide in the slag has been reduced to magnesium. The bulk of the molten slag and, if any, spent alloy is then tapped, leaving enough slag and alloy to repeat the operation. The addition of oxide and reducing agent and the tapping of the slag and the alloy are carried out in such a way that - apart from the fluctuations in the magnesium oxide level - the composition of the slag is kept essentially constant. The process is carried out essentially continuously.

The attached Table II summarizes the stoichiometric data of Examples in which raw materials and slag together

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settlement are chosen to illustrate: limited and normal slag compositions; the use of pure aluminum (> 99 ° / °) and. of three alloys, the compositions of which are extreme in terms of alloy additions; and the use of low-grade magnesia made by calcining magnesite with a SiC ^ content of about 8%. It is believed that all of the OaO required for the slag comes from calcined dolomite, ie dolomite lime, and that any additional MgO is obtained as fairly pure MgO from seawater, mother liquors and other sources. These examples are intended to illustrate the flexibility of the process with regard to the raw materials on the one hand and to the narrow range of slag compositions, from which almost complete extraction of Al ₀ O ^ is possible, on the other hand.

Example A shows a slag which solidifies as a crystalline 12CaO · 7AIpO ^, which is completely soluble in calcined soda solutions. It represents the lower desired limit of Al ₀ OZ for a silicon oxide-free slag, because if the OaO is more than 50 i> , the relatively insoluble compound 2CaO-AlpO ₇ - is formed, whereby the recovery of Al _o 0_ is reduced.

Example C shows a slag that is produced as crystalline CaO-Al ₀ OZ

solidifies, which is also completely soluble in calcined soda lye. It represents the upper desired limit of Al ₀ O-, for a silica-free slag, because if less than 35 "ß> GaO is present, the relatively insoluble * compound 2GaO · 2AIpO_ is formed, whereby the recovery of Al ₀ O ^ is reduced

will.

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Table II ν

Stoichiometric data for the reactions below use of typical alloys, dolomite lime and M s gn e s i umoxy d

example A. B. C. D. 1.80
0.92 1.55
1.11 Ό, 97 E. ^ # "220
90
10 G 1.65
0.85 • 1.48. ■
0.96 - Alloy. #
Al fo
Mg %
Si % ■
Zn JlS
Cu fo
other 1100
100. # 1100
100 # 1100
100 # 1100
100 0.75 o, 75 0.75 # 15
88
12th # 7075
90
2.5
i ', 6
0.5 gr 0.77 0.82 slag 57.5 Al ₂ O ₅ 50 57.5 65 55 2.47
1.00 2.19
1.00 2.58
1.00 • 55 42.5 60 2.27
1.00 2.51
1.045 OaO
SiO ₂ 50 42.5 55 57.9
7.1 56.5
8.5 -v-1550 40 Melting point
⁰ C 1400 w 1550 ^ 1600 -V155O -V155O -v 1550 raw materials Units based on the produced magnesium Dolomite-
lime
magnesium
oxide
alloy 2.45
0.64 1.58
1.10 Products 0.75 0.75 slag
magnesium Remarks 2.84
1.00 2.21
1.00 •

The magnesium produced contains up to .20 % Al. This is not taken into account in the data given above, since the reactions are not influenced by the distillation of aluminum from the reaction zone to the condenser. .

30 9 8 44/0 446.

Slag A is almost exactly 12OaO-7Al ₂ O ₅ (50.5 ρ slag 0 is almost exactly CaO-Al ₂ O-, (64.6 % slag ß is an average composition between A and slag D conducts SiO- of 1, 14 units of magnesium oxide from,

which contains 16>& ^ iO ₂ . Slag E requires a 100 / year utilization of the Si content of the alloy.

Slag G - 1.00, units of magnesium and 0.045 units of zinc. Slags D and S disintegrate.

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2318938

Example B shows a medium slag between A and 0, the solid slag being a mixture of crystalline 12CaO »7Al ₂ O, and _{-CaO-Al 0} O ^, both of which are completely soluble. The preferred slag of the present invention is this type of mixed composition, but it is of course possible to operate somewhat outside of the preferred limits set forth without departing from the subject matter of the invention.

ν
t * -

Examples D and E show slags that contain enough SxOq to cause disintegration, thereby avoiding the need to grind a non-disintegrating slag. But in the case of such slags, the Al-pO ^ content cannot be fully recovered, and there is also a sludge problem. However, in example D, the SiOp is obtained by using inferior silicon-containing magnesium oxide, which could bring a considerable cost advantage at certain points. The magnesite used to make calcined magnesia could contain almost 9% SxOq. In example E, the SiO _{2 is} obtained by using the silicon contained in a silicon-aluminum alloy as an additional reducing agent, which can also bring about a significant cost advantage.

As a further example, as a modification of Example D, a calcined magnesium oxide containing about 27 % SiOp could be used to produce a slag of the following composition: Al ₂ O ₅ 49 ° fa CaO 41 #; SiO ₂ 10 ^. This requires additional dolomite lime to provide CaO to bind the additional SiO ₂ , and about 1.46 units of silicon-containing magnesium oxide after equalization for the MgO-

3 09844/0446

Content of the additional dolomite lime. The slag ratio is thus about 2.9 P ^ o magnesium unit, but it could also low grade magnesite containing about 15 ⁰ P contains siop be -Uses to deliver the silicon-containing magnesium oxide.

If Al _n O- is removed from the slag and the remaining GaGO- is calcined and recycled, essentially no residue remains. The process is then a closed cycle in which MgO and Al are supplied and Mg and AlpO _{x are} recovered.

Table III summarizes the data for Examples 1, - stated 7 · In the reducing agents are substances other than aluminum and _t is not specified Silxziummetall, and it is based on that the magnesium oxide and the Dolomxtkalk in Oxydfüllung pure MgO or MgO "Ga. because the impure oxides are not specified. The composition of the slag, which results from the complete reaction of the reducing agent with the MgO in the oxide filling, is given. To obtain the appropriate calcium aluminate slag for Example 3, an additional amount of lime was added. ■

It can be seen that a very high recovery of aluminum oxide is achieved. It can also be seen how the removal of the recoverable alumina from the slag reduces the total slag ratio as represented by the resulting waste slag to a very low level _} . ·. in some cases almost 1 part by weight of waste slag per 1 part of produced magnesium is reached or even exceeded.

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In Example 4- the insoluble is 2GaO.SiOg in the slag about 18 percent by weight. As the resultant waste slag ratio is increased, it is evident that the essentially pure aluminum in the present process as opposed to Reducing agents with relatively high amounts of silicon metal is preferred.

The painting

The accompanying drawings are a schematic representation of an aluminothermic process divided into two parts, which embodies the present method. One can see that the products of the main part are magnesium, calcium maluminate and a small amount of residual metal (Figure 1). The calcium maluminate can be treated (Figure 2) to make Älumiiiiumöxyd for the to produce electrolytic reduction to aluminum and GaOO ^ or OaO, the latter can be recycled to some of the lime of the first part (Figure 1). deliver. Thus, the The usefulness, somewhat simplified, consists in producing magnesium and high-quality aluminum oxide from aluminum and magnesium oxide. deliver, whereby the products can be pure, but the reactants do not need to be pure.

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Example 1 2 3-4 5 67

Reducing agent <8

Al 93.2 84.4 85.0 90 86.2 94.6 89.3

Si 6.8 15.6 15.0 10 13.8 5.4 11.7

Oxide filling

Magnesium, 502, 597 1.68 0.90, 858, 832 1.08

oxydtL

Dolomite 2.79 2.58 0 1.88 1.95 1.50 1.42

lime <D

added 0

OaCM-- '
added 0

0 1.31 0 0 0 0 0 0 0 0 0 0

Slag*

Al ₂ O ₃ 43.1 40.0 42.9 50.0 46.5 57.6 55.2 OaO 53.7 ^ 52.6 48.5 43.8 45.0 38.7 _Λ 36.9 SiO ₂ ■ 3.2 ^ 8.4 8.6 6.2 8.5 3.7 © 7.4

produced 3.06 2.87 2.69 2.52 2.52 2.30 2.24 slag

recoverable <\ 62.7 7.4 ^ 89.8> 90 91.0 97.5 44.2 Al ₂ O ₅ in - / o ^ 2J

produced 2.22 2.04 1.67> 1.39 1.45 0.99 1.68 Waste slag

1 x unit weight per unit of the produced magnesium.

2. The slag in % is calculated as follows: SiOp + AlpO ^ + CaO-

(OAO equivalent to SiO _2, if present) = ° 100 ° / o _t

3 · 15 * 0 g slag leached with 11.0 g Na _? C0 ^ in 250 ml H _p 0 at 25 ⁰ C ± 50 over a period of 16 hours.

4. The waste slag produced is calculated as follows:

produced slag -. (weight Al ₂ 0 ^) x (^ wirmable AIpO ^) ₀

5 · Examples 2 and 3 show the difficulty that arises

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if a slag is not adequately crystallized, to improve the extraction of aluminum oxide. Slag # 2 was not cooled slowly enough to produce a to allow sufficient crystallization - while £ ^ 3 cooled slowly under adequate control.

6. The silicon dioxide content of these slags is too low to cause any noticeable formation of dust, even when the slag is slowly cooled. ■ - ■■ ". · ._.

309844/0446

Claims (12)

P at ent an s ρ r u c h e 1. Alumlnothermal process for the production of magnesium in a reaction condensation system. Introducing magnesium oxide and a metallic reducing agent into the reaction zone of a reduction furnace in the presence of a molten oxide slag bath at melting temperature, removing the resulting magnesium vapor from the reaction zone in the condensation zone and separating the magnesium, characterized in that in the reaction zone a mixture of magnesium oxide and calcium oxide and a metallic reducing agent with a content of at least 85 $ aluminum is introduced, that the molten calcium aluminate slag bath formed from the oxides mentioned is maintained in the reaction zone, and that when the magnesium oxide content in the slag bath has fallen ^{to less than about 5 c} / ° , the calcium aluminate with a content of 35 - 65 % Al ₂ O ₅ , 35 - 55 ^ CaO and O - 10 % SiO _{2 is} withdrawn, while the magnesium vapor escapes from the reaction zone into the condensation zone and the magnesium is condensed and deposited there . 2. The method according to claim 1, characterized in that the oxide mixture of MgO and CaO in a weight ratio of between 1.1: 1 and 2.3: 1 is introduced » 3. The method according to claim 1, characterized in that the calcium aluminate with a content of 51-65 % Al ₂ O, and 309844/0446 55 _ 49 Cj GaO is withdrawn. 4-. Method smell claim 1, characterized in that .da £ the Oxide mixture contains magnesium oxide and calcined dolomite. 5. The method according to claim 1, characterized in that the Magnesium oxide "at least partially from a silicon-containing Magnesite originates. . ■ ■. 6. The method according to claim 1, characterized in that the oxide mixture contains free lime. 7. Process according to Claims 1 to 6, characterized in that that the process in the presence of an inert gas at a Partial pressure of 0.1 atm., Is carried out. 8. The method according to claims 1 to 3, characterized in that that the escape of magnesium vapor is affected by a stream of inert gas extending from the reaction zone moved into the condensation zone. 9. The method according to claim 5 »characterized in that the Transfer of the magnesium vapor from the reaction zone to the condensation zone predominantly by diffusion through a substantially static atmosphere of inert gas he follows. 10. The method according to claim 1, characterized in that one the withdrawn calcium aluminate solidifies to fine Particles crushed. Or atomized, the crushed Calcium aluminate leaches to 'extract' Al (OH) and that 30 98 44 / 044G - calcined to purified aluminum oxide. 11. The method still claim'10, characterized in that calcium oxide is separated from the crushed calcium aluminum and recycled to the reaction zone to form part of the oxide mixture. 12. The method according to claim 10, characterized in that purified aluminum oxide is reduced to aluminum metal and recycled to the reaction zone to form part of the metallic reducing agent. 309844 / 0Λ46 sr Lee rs e ι te