RU2287026C1 - Multi-cell electrolyzer with bipolar electrodes for production of aluminum - Google Patents

Abstract

FIELD: electrolyzers for production of aluminum.

SUBSTANCE: proposed electrolyzer is used for production of aluminum by electrolysis of cryolite-alumina melt; electrolyzer is provided with bath lined with refractory material, mono-polar electrodes mounted approximately vertically and secured by means of part electrically insulated from electrolyzer; bipolar electrode is made from cathode and anode plates interconnected by means of conducting strips, thus forming vertical passage filled with electrolyte; length and total section of strips are sufficient to exclude electrolysis in said vertical passage between cathode and anode plates of bipolar electrode; each second bipolar electrode is secured by adjusting screws for performing vertical motion; surfaces of anode and cathode plates are deflected from vertical so that movable bipolar electrode is narrowed to bath bottom and immovable bipolar electrode is widened to bath bottom; located under electrolyzer bath is U-shaped magnetic circuit connected with electrolyzer bus duct or with additional DC source; bath has recess in center which is connected with pipe line for discharge of aluminum melt ; fitted in lower part of this pipe line is electromagnetic pump. Anode plates are made from conducting materials resistant to electrolyte.

EFFECT: avoidance of edge break of bipolar electrodes; reduced power requirements.

3 cl, 4 dwg, 1 ex

Description

The invention relates to aluminum electrometallurgy in general and to devices in which electrolysis cells are connected in series with each other in the space of the electrolysis bath, and each electrode of such a cell, except for those connected to the supply buses, is bipolar, that is, at the same time plays the role of the cathode and anode in two adjacent interpolar gaps (MPR).

A known design of the electrolyzer, in which the bipolar electrodes are horizontally located flat graphite vessels, filled with liquid metal obtained during electrolysis, serving as the cathode side of the electrode, and the graphite surface at the bottom of the vessel - the anode [1]. Of these electrodes, separated from each other by the pole distance, the feet are formed, filled with electrolyte, from N electrolysis cells. This leads to a reduction in the operating current by N times in comparison with a conventional electrolyzer with a single interpole space of the same capacity and to a decrease in electric losses in the external circuit.

The disadvantage of the analog design is the increase in the polar space during electrolysis due to the oxidation and destruction of the anode surface of the electrodes, that is, the “ceilings” in the polar space, and the increase in electrical losses per unit weight of the obtained aluminum, as well as the need for more frequent replacement of electrodes than identical designs of Eru-Hall electrolyzers.

The prototype of the invention is a multi-cell electrolyzer with bipolar electrodes for producing aluminum [2], equipped with almost vertically mounted carbon electrodes, suspended or supported by parts electrically insulated from the electrolyzer and attached with plates inert to the electrolyte.

This design, in comparison with the analogue [1], has the advantage of making all bipolar electrodes available for dismantling separately, and the presence of a certain inclination of the surface allows the interpole distance in the electrolysis space to be kept constant by vertical displacement of the electrodes relative to each other. Its advantage is also the ability to organize directional circulation of the electrolyte and to separate aluminum into prefabricated cells, increase productivity, reduce specific energy consumption and increase the environmental friendliness of the apparatus.

The disadvantage of the prototype is a high density of Joule heat and temperature inhomogeneity, which impede the creation of more powerful electrolyzers, as well as the phenomenon of inhomogeneous electrode actuation, which is significant in this design, compared with the Herou-Hall electrolyzers. Thus, the bipolar electrolyzer of the prototype [2], reflected in the variations in [3, 4], is a device, in general, with a lower unit capacity compared to existing Eru-Hall electrolyzers in modern factories. Testing the prototype circuit as a material for bipolar electrodes of modern materials, such as cermets, confirmed the phenomenon of intense destruction of the latter at the edges of the electrode plates, explained by the phenomena of electric transfer in a sufficiently strong electric field in the edge zone [6-8].

The objective of the invention is to reduce the electric field in the edge zone of the electrodes so that the potential drop along the edge of the plates is less than the contact potential difference, i.e. so that "parasitic electrolysis" at the edge of the plates is impossible while maintaining the well-known advantages of electrolyzers with bipolar electrodes - a significant reduction in energy consumption per kilogram of aluminum produced, creating a mode of uniform heat intake in the volume where electrolysis takes place.

The problem is solved using the signs specified in the 1st claim that are common with the prototype, such as a multi-cell electrolyzer for producing aluminum by electrolysis of cryolite-alumina melt, includes a bath lined with refractory materials, monopolar and bipolar electrodes mounted almost vertically, fixed at the help of parts electrically insulated from the electrolyzer, and distinctive essential features, such as a bipolar electrode made of cathode and anode plates connected each other with conductive jumpers with the formation of a vertical channel filled with electrolyte, and the length and total cross-section of the jumpers is sufficient to prevent electrolysis in the aforementioned vertical channel between the cathode and anode plates of the bipolar electrode, and every second bipolar electrode is mounted on the adjusting screws with the possibility of vertical movement, with the surfaces of the cathode and anode plates are deviated from the vertical so that the movable bipolar electrode is narrowed to the bottom at the bathtub, and a fixed bipolar electrode with an extension to the bottom of the bathtub, while under the electrolyzer’s bathtub there is a magnetic circuit in the form of an inverted letter “P” connected to the electrolyzer busbar or with an additional DC source, and the bathtub is made with a center recess connected to a pipeline for outputting the molten aluminum, and an electromagnetic pump is installed in the lower part of the pipeline. The design feature of the anode and cathode plates is reflected in paragraph 2 of the claims, namely, the anode plates are made of electrically conductive materials that are resistant to the electrolyte of an aluminum electrolyzer, for example cermet, the cathode plates and jumpers between the anode and cathode plates are made of structurally conductive material, for example bimetallic steel, and are located outside the electrolyte, while structural steel baths are used as part of the magnetic circuit.

The design of the lid of the electrolyzer was reflected in paragraph 3 of the claims, namely, the bath is equipped with a lid in which a system of heat pipes is installed, the lower ends of which are located in the superelectrode space outside the electrolyte, and the upper ends enter the pipe with a circulating coolant.

The above distinguishing features individually and all together are aimed at solving the problem and are significant. The use of the proposed combination of essential features in the prior art is not found, therefore, the proposed technical solution meets the patentability criterion of "novelty."

A single set of new essential features with common, known, provides a solution to the problem, is not obvious to specialists in this field of technology and indicates the compliance of the claimed technical solution with the patentability criterion of "inventive step".

The invention is illustrated by a description of a specific, but not limiting example of implementation.

To achieve the above task, the design uses the internal circulation of the electrolyte itself using MHD interaction with the magnetic field of the electrolyzer itself, while accelerating the evacuation of droplets of molten aluminum into the transportation pipeline to the mixer, and the external heat is taken at low entropy, i.e. at a high consumer cost of heat, for example using heat pipes with MHD control. The bipolar electrode is replaced by a bipolar element consisting of a cermet anode and a graphite cathode connected to each other by jumpers to form vertical channels or a space freely filled with electrolyte. The condition for choosing the length and cross section of the jumper connecting the anode and cathode is a condition under which the potential drop along the operating current is less than the electrolysis polarization voltage, that is, in other words, electrolysis should not occur in this gap, and the electrolyte will move in the vertical direction , which is of great importance for uniform heat removal and for a uniform distribution of alumina introduced into the electrolyte.

The electric current in the device has a horizontal direction and passes sequentially from one anode-cathode electrolysis zone to another, thereby reducing the specific energy loss per joule heat in the busbar, there is no more molten aluminum cathode in the interpolar gap, as in the Herou-Hall electrolyzers, therefore , there are no phenomena of MHD instability of the MPR. In the proposed device, the total electrical resistance can be significantly reduced by reducing the conductor layer with conductivity an order of magnitude lower than all the others in the working circuit. A large horizontal working current creates a circulation of the magnetic field in the electrolyte, which, thanks to the massive casing of magnetically soft material of ordinary quality steel, forms a uniform horizontal magnetic field, and the entire electrolyzer can be considered as a large conductive electromagnetic pump with the Ampere force directed downward and with an excess of this force inside droplets of aluminum almost doubled.

To separate the electrolyte and aluminum, an electromagnetic pump is provided in the aluminum intake pipe, supplying aluminum to a predetermined height to the mixer of the foundry.

The invention is presented in the following figures

Figure 1 - electrolyzer for producing aluminum with bipolar electrodes (longitudinal section).

Figure 2. - a cross section through the area of electrolysis and the action of MHD forces down (arrows indicate the direction of movement of the electrolyte).

Figure 3 is a cross-section through the area of the return movement of the electrolyte and the input of alumina (electrolysis in this area does not occur, the arrows indicate the direction of movement of the electrolyte).

4 is a diagram of the action of forces on droplets of aluminum and gas bubbles in the region where electrolysis occurs.

The following notation is introduced in the figures:

1 - busbar, 2 - monopolar electrodes, 3 - electrolyte, 4 - cathode plate made of bipolar electrode graphite, 5 - jumper wire, 6 - anode plate made of cermet bipolar electrode, 7 - pipe with circulating heat carrier, 8 - heat pipe, 9 - cover baths, 10 - channels where electrolysis takes place, 11 - aluminum melt, 12 - bath, 13 - magnetic circuit, 14 - electromagnetic pump, 15 - skull, 16 - gas bubble, 17 - drop of aluminum melt, F _apx - Archimedes force acting up on the generated anode gases in the space between the anode and cathode plates of the bipo yarnogo electrode, F _el - electromagnetic force (MHD force) acting down on a drop of molten aluminum in the same space between the anode and cathode plates of the bipolar electrode.

The basis of the proposed electrolyzer are bipolar electrodes (Figure 1), consisting of parallelly located cathode plates 4 of graphite and anode plates 6 of cermet, connected by conductive bridges-jumper 5, through which current I flows. Each similar cell is separated from the adjacent channel 10, in which there are no jumpers 5 and in which there is electrolysis. In it, droplets of molten aluminum 17 and bubbles 16 of the anode gas are formed. The electromagnetic forces F _{el of the} interaction of the current and the external magnetic field are directed downward, and the Archimedes force F _apx acting on the gas bubbles is directed upward. In general, the electrolyte 3 moves downward in the channel 10. (Electrolyte movement is indicated by arrows.)

Figures 2 and 3 show a cross-section of the electrolyzer and bipolar electrodes, consisting of parallelly located cathode plates 4 of graphite and anode plates 6 of cermet, connected by conducting bridge bridges 5, through which current I. The horizontal operating current flowing in the cell, closes on the busbar 1, which together with the working current forms a magnetizing coil for the magnetic circuit 13. In the embodiment of the proposed device, in order to strengthen the magnetic field, the busbar 1 can make several revolutions around the magnet of the wire 13, also laid between the ceramic bottom of the electrolysis bath 12 and the magnetic circuit 13. In the upper part of the electrolysis bath 12, it is proposed to install a loading device for raw materials (alumina) according to the patent [5], not shown in the illustrations. Utilization of the Joule heat of the bath is carried out using heat pipes 8, which are connected to a pipe with a circulating heat carrier 7. For heat pipes 8, operating in the temperature range 800-960 ° C, it is advisable to use alkaline liquid metals (Na, K, Li) as heat carriers That will also allow to apply the Institute of Physics of the Academy of Sciences of Latvia developed at the time. SSR heat pipes with MHD-adjustable performance.

Depicted in figures 1, 2, 3, the device operates as follows.

, где r - электрическое сопротивление проводящей k перемычки 5, k - число перемычек, I₀ - рабочий ток электролизера, Е_пол - напряжение поляризации, то есть ЭДС гальванического элемента кермет-электролит-графит, направленная против рабочего тока I₀. Это условие обеспечивает отсутствие явлений электролиза электролита в том промежутке, где находятся проводящие перемычки 5, а также на порядок уменьшает краевые токи вокруг биполярного электрода. Таким образом, в целом, происходит управляемое конвективное движение, направленное в канале 10 вниз под действием сил Ампера F_e1, образованными рабочим током и внешним магнитным полем от внешнего магнитопровода 13. Причем эта сила, действующая на капли алюминиевого расплава 17, примерно в два раза больше, чем сила, действующая на электролит - j₀·B. Здесь j₀ - плотность рабочего тока, а В - индукция магнитного поля. Пузырьки 16 анодных газов, образующиеся в канале 10, в межполюсном пространстве (МПР) будут испытывать с одной стороны силу Архимеда, образованную гидростатической и электромагнитной силами,

где а - радиус пузырька, g - ускорение силы тяжести, δ_el - плотность электролита, а с другой стороны, силу Стокса, увлекающей эти пузырьки вниз, F_Стокс=6πaVη, где η - вязкость электролита. Таким образом, в МПР образуется два весьма выгодных движения газовых пузырьков, определяемые их критическим размером

при котором пузырек как бы зависает относительно системы координат, связанной с корпусом электролизной ванны. В начале образования пузырька 16 анодных газов, когда его радиус меньше критического радиуса а_кр, пузырек 16 увлекается движением электролита 3 вниз, а при достаточном росте, когда его размер будет больше критического радиуса, всплывает вверх, захватывая на своем пути более мелкие пузырьки. Отбор джоулева тепла в электролите 3 выгоднее всего произвести сверху в его слое над пакетом электродов. При этом тепловая конвекция между МПР и пространством в биполярном электроде будет усиливать вышеописанную конвекцию за счет электромагнитных сил [9, 10]. С точки зрения термодинамики и условий утилизации джоулева тепла в предлагаемой конструкции верхний слой электролита, то есть слой над пакетом биполярных электродов, является самым выгодным. Поэтому в крышке 9 ванны 12 предлагается разместить систему тепловых трубок 8 с теплоносителем из щелочных металлов (Na, К, Li). Преимуществом такого типа отбора тепла является малый вес и надежность, а также возможность управления интенсивностью теплоотвода МГД-методами. Такой способ отбора тепла, кроме его низкой энтропийности, то есть технологической ценности, имеет преимущество в виде сокращения теплоотвода через боковые стенки ванны 12. В проекте целесообразно поднять температуру магнитопровода 13 примерно до 600°С, то есть ниже точки Кюри стали обыкновенного качества. Целесообразно разместить слой изоляции снаружи магнитопровода 13, на фигурах не показанный.In the electrolysis bath 12 of a rectangular type, electrolyte 3 is poured and a layer of liquid aluminum is located on the bottom 11. The hydrostatic pressure of the liquid media is compensated by an electromagnetic pump 14, the pressure drop in which is maintained so that the upper level of electrolyte 3, that is, the electrolyte - anode gas boundary, forms a free cavity under the lid of the electrolysis bath 12. Into the electrolysis bath 12 are loaded, using the protrusions in the lower part of the bath, not shown, bipolar electrodes consisting of pa Allelic plates disposed cathode 4 and anode plates 6, joined by bridges-conductive webs 5 on which a current flows I. The electrical resistance of conductive parallel crosspieces 5 is chosen so that the voltage drop across them satisfies

where r is the electrical resistance of the conductive k jumpers 5, k is the number of jumpers, I ₀ is the working current of the electrolyzer, E _floor is the polarization voltage, that is, the emf of the cermet-electrolyte-graphite cell directed against the working current I ₀ . This condition ensures the absence of electrolyte electrolysis phenomena in the interval where the conductive jumpers 5 are located, and also reduces the edge currents around the bipolar electrode by an order of magnitude. Thus, in general, a controlled convective movement occurs, directed downward in the channel 10 under the action of Ampere forces F _e1 formed by the working current and external magnetic field from the external magnetic circuit 13. Moreover, this force acting on drops of aluminum melt 17 is approximately twice more than the force acting on the electrolyte - j ₀ · B. Here j ₀ is the density of the working current, and B is the induction of the magnetic field. The bubbles 16 of the anode gases generated in the channel 10 in the interpolar space (MPR) will experience, on the one hand, the Archimedes force formed by hydrostatic and electromagnetic forces,

where a is the radius of the bubble, g is the acceleration of gravity, δ _el is the density of the electrolyte, and on the other hand, the Stokes force, which carries these bubbles down, F _Stokes = 6πaVη, where η is the viscosity of the electrolyte. Thus, two very favorable motions of gas bubbles are formed in the MPR, determined by their critical size

in which the bubble, as it were, hangs relative to the coordinate system associated with the body of the electrolysis bath. At the beginning of the formation of the bubble 16 of the anode gases, when its radius is less than the critical radius a _cr , the bubble 16 is carried away by the electrolyte 3 moving downward, and with sufficient growth when its size is larger than the critical radius, it floats up, capturing smaller bubbles on its way. The selection of Joule heat in electrolyte 3 is most advantageous to produce from above in its layer above the electrode stack. In this case, thermal convection between the MPR and the space in the bipolar electrode will enhance the above-described convection due to electromagnetic forces [9, 10]. From the point of view of thermodynamics and the conditions for utilizing Joule heat in the proposed design, the upper electrolyte layer, that is, the layer above the bipolar electrode stack, is the most advantageous. Therefore, in the lid 9 of the bath 12 it is proposed to place a system of heat pipes 8 with a heat carrier of alkali metals (Na, K, Li). The advantage of this type of heat removal is its low weight and reliability, as well as the ability to control the heat sink intensity using MHD methods. This method of heat selection, in addition to its low entropy, that is, technological value, has the advantage of reducing heat dissipation through the side walls of the bath 12. In the project, it is advisable to raise the temperature of the magnetic circuit 13 to about 600 ° C, that is, below the Curie point of steel of ordinary quality. It is advisable to place the insulation layer outside the magnetic circuit 13, not shown in the figures.

In the design, the input of raw materials (alumina, etc.) is supposed to be carried out through the top cover 9, for example, according to the patent [5]. Placing the latter conveyor next to the coolant can be used to preheat the latter in order to save energy.

As an example of the described construction, we cite extracts from the program for constructive calculation of the electrolyzer for producing aluminum with bipolar electrodes, equivalent in performance to the Eru-Hall electrolyzer (I ₀ = 150 kA), compiled with reference to the cermet plates tested at NIFTI Kr.

one The composition of the cermet 47% NiFe ₂ O ₄ , 36% Fe ₃ O ₄ , 3% Cu, 4% Ni 2 Dimensions of cermet plates 250 × 240 mm ² 3 The number of plates on the anode 25 four Aluminum productivity per hour. M _al HrΣ = 46.3 kg / h 5 The number of bipolar elements Ni = 5 6 Capillary constant of molten aluminum in an electrolyte a CAR = 1.3 cm 7 Interpolar distance MPR = 1.5 cm 8 Thermal power of the device Pap _i = 23.8 kW 9 Operating current, "total current", i.e. product of the working current by the number of cells connected in series. I ₀ = 36 kA,
Ni I ₀ = 180 kA 10 Total polarization EMF E _floor = 8.25V eleven The length of the active part of the bath 1,533 m 12 Bath Width and Height 1.50 m × 1.50 m 13 The output of aluminum in kg per Joule 2.44 · 10 ^-8 c ² m ^-2 fourteen The output of aluminum in kg per Joule in the best existing Eru-Hall machines 1.83 · 10 ^-8 s ² m ^-2 fifteen Relative output as a percentage of the output in existing devices 133% 16 The average magnetic field in the cell B = 0.016 Tesl 17 The average speed of the electrolyte in the MPR 1.42 m / s eighteen Reynolds number (turbulent flow in the MPR, contributing to the separation of gas bubbles and equalization of temperatures and alumina concentration) Re = 1.5 · 10 ⁷ 19 Temperature imbalance in the bath 1.2 K

The above specific example indicates the industrial applicability of the proposed technical solution.

From the description and practical application of the present invention, other particular forms of its implementation will be apparent to those skilled in the art. This description and examples are considered as material illustrating the invention, the essence of which and the scope of patent claims are defined in the following claims by a combination of essential features and their equivalents.

Information sources

1. US Pat. 3554893 Janv. 12, 1971 / US C1. 204-244 "Electrolytic furnaces having multiple cells formed of horizontal dipolar carbon electrodes" Giuseppe de Varda, Milan, Italy. Montecatiny Edison S.p.A.

2. A.S. 199781, C 25 C 3/08 of 07/13/1967, BI No. 15 "Multi-cell electrolyzer with bipolar electrodes for aluminum production", J. De Ward, J. Olah de Garab.

3. A.S. 225105, С 25 С 3/08 "Multi-cell electrolyzer with bipolar electrodes for aluminum production" J. Olah de Garab, J. De Ward.

4. A.S. 314361, С 25 С 3/08 dated 03/06/1972 "Multi-cell electrolyzer for aluminum production", by Giuseppe De Varda.

5. A.S. 629248, M.C. ² C 25 C 3/08, Zuev N.M., Melnikova G.V., Sharunova G.M., Ladaria O.V. "Electrolyzer with bipolar electrodes for aluminum."

6. V.A. Blinov, L.A. Isaev, A.M. Kurmangaleeva, P.V. Polyakov, M.M. Sharafutdinov, S.A. Shcherbinin. "Mathematical modeling of thermal and electric fields in an aluminum electrolyzer with bipolar electrodes." Non-ferrous metals, 1993, No. 1, p.31-34.

7. Patent RU 2106431 "Charge for the manufacture of inert anodes", Ivanov VV, Ivanov Vl.Vl., Polyakov PV, Blinov VA, Kirko VI, Savinov VI

8. Patent RU 2108204 "Method for the manufacture of fireproof anodes", Ivanov V.V., Ivanov V.Vl., Polyakov P.V., Blinov V.A., Kirko V.I., Savinov V.I.

8. Mintsis M.Ya., Polyakov P.V., Sirazutdinov G.A. "Electrometallurgy of aluminum", Novosibirsk, Nauka, 2001, 368 S.

9. Blinov V.A., Isaeva L.A., Kurmangalaeva A.M., Polyakov P.V., Sharafutdinov M.M., Shcherbinin S.A. Mathematical modeling of thermal and electric fields in an aluminum electrolyzer with bipolar electrodes - Non-ferrous metals, 1993, No. 1, pp. 31-34.

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Claims (3)

1. A multi-cell electrolyzer for producing aluminum by electrolysis of a cryolite-alumina melt, including a bath lined with refractory materials, monopolar and almost vertically mounted bipolar electrodes fixed by parts electrically insulated from the electrolyzer, characterized in that the bipolar electrode is made of cathode and anode plates, interconnected by conductive jumpers with the formation of a vertical channel filled with electrolyte, and the length and total cross section of the jumpers sufficient to prevent electrolysis in the aforementioned vertical channel between the cathode and anode plates of the bipolar electrode and every second bipolar electrode is mounted on the adjusting screws with the possibility of vertical movement, while the surfaces of the cathode and anode plates are deviated from the vertical so that the movable bipolar electrode is made narrowing down to the bottom of the bath and a fixed bipolar electrode with an extension downward of the bath, under the bath of the electrolyzer there is a magnetic circuit in the form of an inverted letter "P", connected to the busbar of the electrolyzer or with an additional source of direct current, and the bathtub is made with a recess in the center connected to the pipeline for outputting molten aluminum, and an electromagnetic pump is installed in the lower part of the pipeline. 2. The device according to claim 1, characterized in that the anode plates are made of electrically conductive materials that are resistant to the electrolyte of an aluminum electrolyzer, for example cermet, the cathode plates and jumpers between the anode and cathode plates are made of structural conductive material, for example bimetallic steel, and are located outside the electrolyte, and structural steel bath is used as part of the magnetic circuit. 3. The device according to claim 1, characterized in that the bath is equipped with a lid, in which a heat pipe system is installed, the lower ends of which are located in the superelectrode space outside the electrolyte, and the upper ends enter the pipe with a circulating coolant.

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