Method for producing aluminum by means of metallothermic recovery of aluminum trichloride with magnesium and a device for its realization   

Abstract: Here, a method has been offered of aluminum reception by means of metallothermal magnesium recovery of aluminum trichloride in the flow of inactive gas at the temperature of 900-1150° C. and the total pressure of 0.01-5 atm., the mass correlation of aluminii chloride and magnesium in the parent mix being 3.69:1.00. The process is realized in a cylinder-shaped reactor with thin-walled attachments (6) made of ceramics and located inside the reactor. The latter is supplied with a cone-like bottom part. A cauldron-evaporator of magnesium into the flow of inactive gas is mounted in front of the reactor, while behind it there is a unit for separating liquid magnesium from the residual mix of magnesium and aluminii chloride, all the components of the device having been lined inside with fireproof materials. The technical result is growth of efficiency at the expense of guaranteed uninterrupted process of recovery and the best of ecological specifications.

TECHNICAL FIELD

The invention deals with non-ferrous metallurgy, in particular, with technologies of aluminum production.

BACKGROUND ART

In non-ferrous metallurgy, the so-called method of Heroult-Hall, which is an electrolysis of cryolitho-alumina melts, is used to produce aluminum. Electrochemical apparatus and cells in particular being characterized by rather low factor indices of the consumed useful volume as the operations are registered only on the surfices of electrode-electrolyte rather than all through the whole volume of a reactor in chemical technology.

work done when the process is carried out according to the methodology of Heroult-Hall. As a result, the cells are characterized by rather a low productivity per 1 cell, no more than 3-4 tons per 24 hours\' period. Therefore, plants producing aluminum are equipped with hundreds and thousands of cells which occupy vast areas and are quite investment consuming while being constructed.

The cells are not hermetic due to their construction features and the whole process is accompanied with emissing natrii, aluminum and hydrogenii fluorides, carcinogenic polyaromatics compounds, great volumes of greenhouse gases, carbon dioxide and perfluorinecarbons in particular, into the atmosphere. All the above mentioned being taken into consideration, it is quite out of date to produce aluminum by means of electrolysis of cryolitho-alumina melts, because it is not in correspondence with either occurrence frequency of aluminum in the Earth crust (the leading place among the metals) or a unique set of its physical, construction and technical properties.

There are classical solutions of the problem of aluminum reception basing on metallothermic methodology by means of its recovery with potassium from aluminii chloride (F. Wohler, 1828) or sodium (S\'K. Deville, 1854) following the reaction:

AlCl3+3M=3MCl+Al   (1),

where M is an alkaline metal.

However, when alkaline metals are used in correspondence with Reaction 1, the consumption of the metals and electric power is quite high. Thus, when it concerns potassium, it takes 4.33 kg of the alkaline metal and 35 kW/h of power to recover 1 kg of aluminum. For sodium it is necessary 2,555 kg metal and 25,5 kw/h of power to recovery 1 kg of aluminum.

As for other common similar methods, N. N. Beketov\' s suggestion is the most substantial one. According to it, aluminum could be received through its recovery by means of magnesium from aluminum fluoride or cryolite which is a component of Greenlandic cryolithionite:

2(3NaF×AlF3)+3Mg=6NaF+3MgF2+2Al   (2).

Fluorides are more refractory compounds, and the process of recovery as well as the devices to implement this method are more complicated. Besides, the only deposit of Greenlandic cryolithe was exhausted as far back as in the 19th century.

The closest prototype to this method is the production of metallic titanium as a result of its recovery by means of metallic magnesium from tetrachloride.

TiCl4+2Mg=2MgCl2+Ti   (3)

This process is very complicated, as titanii chlorides with different valencies participate in it, the participants of the reaction being in different states of aggregation: titanii chlorides as gaseous, magnesium and magnesii chloride as liquids, whereas titanum as a solid. As a consequence, this method calls for depressurizing the devices from time to time to extract titanium which decreases the productivity of the process and makes worse its ecological characteristics.

The technical result of our invention introducing is a higher productivity due to the continuity of the recovery process and better ecological characteristics due to the provision of the device hermeticity.

Technically, the purpose is met by using the reaction of metallothermic recovery of aluminum trichloride by means of magnesium:

2AlCL3(g)+3Mg(g)=3MgCl2(1)+2Al(1)   (4),

is the index of liquid state. Thus, according to reaction (4) the parent substances are introduced into the process as gaseous ones, and the resulting products, which are aluminum and magnesii chloride are realeased as molten liquid ones. The recovery is performed in the flow of inactive gas at the temperature of 900°- 1150° C. and the total pressure of 0.01-5 atm., the ratio of aluminii chloride and metal magnesium masses in the original mixture being 3.69 to 1.00, correspondingly. In this case, the consumption of magnesium will amount only to 1.35 kg per 1 kg of aluminum, while the power demand for electrolytic magnesium will be of the order of 17.9 kW/h per 1 kg of aluminum. The results are better when magnesium received in a metallothermical way is used, that is, magnesium having been preliminarily recovered from dolomite by means of ferrosilicon according to the technology of Pidgeon (V. A. Lebedev, V. I. Sedykh.Metallurgy of Magnesium. Irkutsk, 2010, p. 149). In accordance with Reaction (4), the total consumption of energy in this case will not exceed 13 kWh per 1 kg of aluminum which is comparable with the values of consumption regarding the Heroult-Holl\'s method and can be considered as a very good index at the stage of generating a magnesium-thermal mode of aluminum production.

Aluminum recovery from trichloride by means of magnesium according to Reaction (4) is, however, an independent scientific and engineering problem. Reaction (4) which is the foundation of the invention, is much simpler to realize than Reaction (3) as a prototype of magnesium-thermal recovery of titanium. However ironic it may seem, but higher temperatures and pressure are needed to produce aluminum as a result of its recovery by means of magnesium from trichloride.

Indeed, magnesium as a recovery agent has the boiling temperature of 1103°-1107° C. (the pressure of steam makes up 1 atmosphere), its melting temperature being 651° C. As for other participants of Reaction (4), aluminum melts at 660° C. A particularly wide range of liquid state is characteristic for aluminum with the boiling temperature of 2497° C. It means that aluminum practically does not evaporate at boiling temperatures of magnesium (1107° C.). Magnesii chloride melts at 708°-714° C. and boils at no less than 1412°-1417° C., that is it has a relatively wide temperature range of liquid state. And finally, aluminii trichloride is sublimated at the temperature of 179.7° C. and cannot be in a liquid state at a normal atmospheric pressure. Thus, the parent substances, i.e. aluminum trichloride and magnesium, are in a gaseous state at the temperatures above 1107° C. while metallic aluminum and magnesii chloride are in a liquid state at the same temperatures, which is a convenient situation to organize a continuous highly effective production.

correspond to gaseous and liquid state following the Le Shatelier rule and Law II of thermodynamics in the conditions of a higher pressure, is to have a balance considerably shifted to the right. As for its kinetics, the reaction can run with an explosion, so to control its speed the parent elements, aluminii chloride and magnesium, should be supplied in separate flows of inactive gas with the temperature lower than the one to be supported in the reactor.

The process of recovery can take place at the temperature of 900° C. because in these conditions the saturated vapor tension of magnesium is considerable and makes up, for example, about 0.19 at for 927° C. At the same time it is not reasonable to raise temperature considerably above the boiling point of magnesium (1103°-1107° C.), as this temperature raise will be accompanied with far too high speed values of the process, so 1150° C. can be set as the highest limit.

The total pressure of the gaseous phase in the reactor is assigned within 0.01-5.0 at, optimum partial pressure of aluminii chloride and magnesium being identified experimentally. It is preferable to get oriented at the upper values of the total pressure, but avoid reaching its explosion limits.

As for the composition of the gas mixture supplied to recover, it should correspond to the stoichiometric ratio of masses involved in Reaction (4) and make up 3.69: 1 in terms of aluminii trichloride and magnesium mass flows as supplied to the reactor.

Realizability of the invention claimed gives rise to no doubt, since there is a similar production of titanium from its tetrachloride in the magnesium-thermal way. Besides, the claimed mode of receiving aluminum is going to be much simpler. Magnesium is also a much more electronegative metal than aluminum. Power inputs can be rather small when magnesium is received from dolomite by means of its recovery in combination with the traditional method of magnesii chloride electrolysis. Moreover, the process is autogenous.

The mode, which uses gaseous aluminii chloride and magnesium as parent substances for reception of the resulting products of magnesii chloride and aluminum which are liquid, can be realized in hermetic apparatus. It is automation-friendly and does not require any manual labour input or application of any mechanical devices to serve the process. The highly approved ecological characteristics of the inventions in terms of employing hermetic apparatus seem to be evident. The possibility to design a device with a high productivity per reactor, low construction and production inputs is one of substantial advantages of the mode offered.

The realizability of the invention is proved by the existence of powerful and effective industries of titanium extraction from its tetrachloride using the magnesium thermic methodology and also by the smoothly realized recovery of zirconium and hafnium from their chlorides in the run of their magnesium thermic process.

Regarding the magnesium thermic way of titanium reception, cylindrical vessels of steel were originally used as recovery devices. They could be made of, for example, chrome-nickel steel lined with a molibdenum plate. Later on, the lining got to be made of low-carbon steel. The process is realized at temperatures lower than the metal melting one, so titanium is received in its solid-phase state which is like a sponge. To extract the sponge the reactor is to be cooled. So, the whole process is inevitably characterized as a periodic one which in its turn calls for manual labour inputs, decreases the reactor efficiency, raises energy inputs and aggravates the ecological characteristics of the production.

In the claimed invented device intended for metal thermic recovery of aluminum by means of magnesium the process takes place at higher temperatures reaching 1100°-1150° C. In these conditions the both received products of recovery, aluminum and magnesii chloride, are in a liquid state and flow down into the bottom portion of the reactor. The melting temperature of aluminum being 660° C. while the one of magnesii chloride being 708°-714° C. with the boiling temperatures of 2497° and 1412°-1417° C., correspondingly. For this reason, the top portion of the reactor is made cylindrical to increase the useful volume of the reactor, while its bottom part—conical to collect liquid aluminum and magnesii chloride.

The most of the reaction zone is made hollow where gaseous aluminii chloride and magnesium are introduced to mix them more thoroughly, though the volume adjacent to the conical part is filled with thin-walled hollow ceramic pieces of the Raschig ring type attachments. The use of the attachments accelerates the processes of condensation and coalescence forming aluminum and magnesii chloride drops.

A flow of inactive gas connects the reactor with the cauldron evaporating magnesium and with the apparatus separating liquid magnesium from the residual mix of magnesium and aluminii chloride vapors. Both the cauldron, which is a magnesium evaporator, and the apparatus separating liquid magnesium are an integral part of the device. They are located separately from the reactor, though rather close by.

The reaction of aluminum recovery from its chloride by means of magnesium in its gaseous phase is an exothermal one and is accompanied with the release of heat in great amounts, the process being autogenous. To control closely the temperature and speed of the recovery the reactor and magnesium separation device are equipped with the system of transpiration boiling water cooling.

Fluid phases (liquids and gases) are used in the invention under consideration. They react in turbulent flows at high temperatures thus providing for high efficiency of the process. Besides, financial expenses intended for the creation of production facilities become lower along with the ones for maintenance of the equipment.

FIG. 1 gives the general view of the reactor.

In FIG. 2 one can see the cauldron—evaporator.

FIG. 3 shows the apparatus of separation. All of them are in their vertical sections.

The reactor (FIG. 1) is made as a cylinder of steel 1 which transforms itself into a cone-like bottom part 2 intended for collection of recovery products, i.e. aluminum and magnesii chloride. A false bottom 3 is located between its cylinder and cone parts. The reactor is sealed with a lid 4. All the inner surfaces of the reactor are lined with refractory materials 5 such as magnesite, graphite and other inert stuff.

Attachments (6) made of thin-walled ceramics of Raschig ring type and usually applied in chemical absorbtion technologies are placed on the false bottom 3. The attachments are made of fireproof materials such as magnesite, carbonitride, etc.

Nozzles and injectors, 7 and 8, fixed in the top hollow part of the reactor tangential the horizontal section circumference of the reaction zone and facing towards each other are used to introduce the parent substances of gaseous aluminii chloride and magnesuim metallic (gas) into the reactor. The recovered aluminum 9 and magnesii chloride 10 are collected in the cone part 2 of the reactor and on escaping through a tap hole 11 magnesii chloride 12 and aluminum 13 collect in the pan 14. There is a branch pipe 15 installed in the lid of the reactor to withdraw the mixture of aluminii chloride and magnesium that has not been involved into the reaction from the reactor.

The cauldron—evaporator (FIG. 2) is a hemisphere of steel 16 sealed with a lid 17, the both being lined with the same material as the reactor. Magnesium is introduced into the cauldron in its liquid state. An electric heater 18 is submerged into the metal. It can be formed as a set of silicon carbide rods and is surrounded with a protective magnesite coating. Inactive gas (argon or nitrogen) is fed into the cauldron—evaporator. Magnesium in its vapor state is directed into the reactor together with the inactive gas. The cauldron—evaporator is equipped with the system of transpiration water cooling like other elements of the device given in FIG. 1 and FIG. 3.

A separation device (FIG. 3) intended for the separation of liquid magnesium from its residual mixture with aluminii chloride is a reactor of a smaller size lined with fireproof ceramics, 19. A branch pipe, 20, feeds a gas mixture into the device, fluid magnesium condensate collecting in the bottom portion of the apparatus. The residual aluminii chloride not involved into the reaction is withdrawn from the apparatus through another branch pipe, 21, which returns it back into the reactor. The apparatus of separation like the reactor is equipped with a lock unit 22 and the system of transpiration cooling 23. The condensate of magnesium is returned back into the cauldron-evaporator and then further into the reactor.

The device works continuously feeding the reactor with aluminii chloride vapor and gaseous magnesium received in a single cauldron-evaporator or in a set of such evaporators. Aluminii chloride and gaseous magnesium are transported and fed into the reactor in oncoming turbulent flows of inactive gas which is a deliverer and provides for ideal conditions for contacting reacting particles, removing diffusion barriers and enabling high speeds of the recovery process. To achieve a greater speed of withdrawing final products, i.e. aluminum and magnesii chloride, from the reactor it is necessary to use greater surfaces and a greater number of active centers of liquid phase formation. This is achieved as a result of using the above mentioned thin-walled and uneven attachments of the Ruschig ring type made of magnesite. Aluminum 13 released from the reactor in continuous or periodical mode is protected by the superior coating of molten magnesii chloride 12. The separation of the recovery products is feasible with ease. For instance, with the drop of temperature down to 680°-700° C. the coating of magnesii chloride transforms its phase into a solid one while liquid aluminum can be easily directed to get involved in other technological operations.

The separation apparatus intended for the separation of aluminii chloride from magnesium in their residual gaseous mix works as a result of a pressure drop in the system and a drop of temperature below the magnesium boiling one. Liquid magnesium in the form of condensate is directed to be refined or to the cauldron to be evaporatorated if the condensate is clean enough, and then further to the reactor.

To provide high but controlable recovery speed both the reactor and the separation device are equipped with the system of water transpiration cooling. The working principles of such equipment used in many fields of technologies, metallurgy including, are well-known and fine-tuned. The systems of inactive gas circulation are also quite controlable both in chemical technologies and in rare metal metallurgy.

Technological use of the invention requires gaseous phase of aluminii chloride to be introduced through the injectors 7 into the top portion of a cylinder of steel 1 pertaining to the reactor (FIG. 1). Gaseous or vaporous magnesium mixed with inactive gas is introduced the same way in the counter-current flow through the injectors 8. This mix is prepared in advance in the cauldron-evaporator (FIG. 2). Parent agents, i.e. inactive gas (for example, argon) and liquid magnesium are introduced there separately, whereas a two-component flow of vaporous magnesium and inactive gas is withdrawn from the cauldron and sent to the reactor (FIG. 1).

The formed liquid aluminum and magnesii chloride condense and coalesce on the attachement surfaces 6 (FIG. 1), trickle down into the cone part of the reactor and further through a tap hole (11) they get into the pan (14) as recovery products, i.e. aluminum (13) and magnesii chloride (12).The residual gaseous mix is expelled through a branch pipe 15 (FIG. 1) .This mixture is introduced into the separation unit through a pipe 20 (FIG. 3).then magnesium removed from the separation unit through a lock unit 22 returns back into the cauldron-evaporator, whereas aluminii chloride expelled through a pipe 21, as a gaseous phase returns back into the reactor through an injector 7 (FIG. 1).

Realizability of the invention submitted causes no doubt and is confirmed by the fact that a similar though much more complicated process of magnesium-thermal recovery of titanum from its tetrachloride is available and widely used in the USA, Russian Federation and some countries of the UIS.

The invention submitted provides for a number of advantages regarding new equipment and technologies of aluminum reception. This is an indefinitely high unit efficiency of the device and low financial expenses in building new production facilities. Hermeticity and ecological safety of the production are guaranteed. Hard manual labour is excluded from the production and a complete automatization of the process is feasible. Aluminum recovery in the device takes place with a considerable positive thermochemical and heating effect and runs in an autogenous mode, practically without any energy consumption from the outside.

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