Bipolar electrode

Abstract

The invention relates to the field of non-ferrous metallurgy. A bipolar electrode contains two electrodes made of a nickel-chromium alloy containing not less than 10% chromium, which constitute an anode and a cathode. One electrode is in the form of a metal pipe and the other is in the form of a metal rod arranged coaxially inside the pipe on insulators. Welded to the upper part of each electrode is a plate that forms a contact element. These contact plates are offset from one another in the plane of the plates by an angle of 35°. In the upper part of the wall of the pipe, below the insulators, holes are formed to equalize the surface area of the electrodes. The insulators are configured in the form of two rectangular ceramic plates that are tensioned together, one of which is disposed on the pipe, beneath the contact plate thereof, and the other of which is also disposed on the pipe, on top of said contact plate, with the contact plate of the rod attached thereto. The result is an increase in the service life of the bipolar electrode in electrolytic processes occurring at 950-1000°C.

Description

[0001] Bipolar electrode

[0002] Field of technology

[0003] The invention relates to the field of non-ferrous metallurgy and concerns, in particular, the design of a bipolar electrode used in the design of electrolyzers for the production of primary purified aluminum.

[0004] Prior art

[0005] In recent years, there has been a fairly clear trend towards the use of electrolyzers with bipolar electrode connection. This was facilitated by the widespread introduction of low-wearing anodes and diaphragms with an increased service life into industry. Electrolyzers with unipolar electrode connection are more compact and allow saving production space. Their manufacture requires less consumption of construction materials. During operation, they are distinguished by greater ease of maintenance, since they have an insignificant number of control and regulation points.

[0006] Refusal of carbon anode and transition to non-consumable

[0007] (low-consumption) metal or other conductive bipolar electrode, resistant to cryolite-alumina melt, solves the environmental problems of the process, replacing the emission of greenhouse gases CO and

[0008] CO2 is oxygen released at the anode during the decomposition of alumina.

[0009] A bipolar electrode is known, used in electrolyzers for refining non-ferrous metals in melts, containing electrode capacities, an anode, bipolar and cathode located sequentially from top to bottom, wherein the electrode capacities are made cylindrical and are located concentrically one inside the other with a gap between them, the capacity of the bipolar electrode is made of a cast dielectric, and its base is made of a porous one, on the side surface of the capacity of the bipolar electrode there are holes located symmetrically around the circumference and above the base of the anode capacity (SU 799512,

[0010] C25C3 / 34, published 10.11.2011). Also known is a bipolar electrode containing two active electrodes, the first of which, being the anode, is made of a tubular shape, and the second, being the cathode, is made of a rod and is located coaxially on an insulator inside the first, which is made with holes in the wall to equalize the surface area of ​​the anode to the surface area of ​​the cathode, while the anode and cathode are made the same length (RU 220188, C25C3 / 06, published 31.08.2023).

[0011] This decision was adopted as a prototype.

[0012] The disadvantage of this solution is the short service life of the bipolar electrode. This is due to the fact that the released pure aluminum and the oxygen obtained as a result of aluminum release interact with the surfaces of the anode and cathode. And this interaction is aggressive. Oxygen is a by-product of electrolysis, this product has no independent value and is released into the atmosphere. The reaction rate of electroreduction of dissolved oxygen is limited by the low solubility of oxygen in solutions, especially at high temperatures. But the process of oxygen release at the anode is accompanied by oxidation of the anode material with the formation of surface oxides. Therefore, during prolonged electrolysis, the discharge of anions occurs not on the metal, but on the oxidized surface. Over time, the overvoltage of oxygen release increases somewhat until it reaches a constant value after a long period of time.

[0013] According to established practice, the following requirement is imposed on electrode materials: the overvoltage of hydrogen and oxygen evolution on them should be as low as possible. The choice of electrode materials is dictated by the need to reduce the unproductive consumption of electricity for polarization of electrodes. It is known that the best cathode material is platinized platinum, however, due to the high cost and instability of the spongy layer, platinum cannot be used as an electrode material. Iron group metals are stable in solutions, have low overvoltage and are suitable as cathode materials.

[0014] The overvoltage on iron and cobalt is several tens of millivolts less than on nickel. The nature of the anodic polarization curves of oxygen release from the solution shows that the overvoltage of oxygen release on iron group metals is also small. Consequently, this group of metals is quite suitable as materials not only for cathodes, but also for anodes. Ordinary steel is used to manufacture cathodes. The cathode is sometimes activated by depositing sulfur-containing nickel or platinum group metals on its surface. Carbon steel is used as anodes in the electrolysis of aqueous alkaline solutions, onto which a nickel coating 100 μm thick is electrochemically applied. Such an anode retains sufficient corrosion resistance in solutions in the presence of 1.5 10 3 pores per 1 m 2. Low wear of such an anode even with greater porosity of the galvanic coating is explained by clogging of the pores with corrosion products of the steel base. Nickel plating of the anode transfers it to a passive state and makes it insoluble in the potential range at which oxygen is released.

[0015] But in reality, the situation is somewhat different, which reduces the service life of the bipolar electrode. The cathode and anode, made of steel coated with nickel or platinum group metals, are a composite consisting of two metals / alloys with different physical properties and which, under high temperature conditions, exhibit their distinctive properties. For example, the temperature coefficient of linear expansion (TCLE) of steel is on average 10 to 18 10 at temperatures from 27 to 100 °C. -6 hail -1. Steel in a heated state (depending on the type) at 900-1000°C can have a TCLE of up to 24.6-10 -6 hail -1 When carbon steel is heated, its TCLE increases and can reach 19.8 10 -6 hail -1 (for U8 steel) in the temperature range of 27-650°C.

[0016] The thermal coefficient of linear expansion of nickel is 13.3 - 10 -6 at a temperature of 100 °C, and platinum - 9.1 10 -6 .

[0017] In known designs, nickel with sulfur is used as a coating. Burning in sulfur vapor, nickel forms sulfides of the compositions NiS and Ni3S2. occurring in nature. The most stable is the sulfide Ni3S2, having a melting point of 788 ° C. Monosulfide NiS can be obtained by heating NiO with sulfur. It is clear that such a coating is not suitable for electrolysis processes occurring at temperatures of 950-1000 ° C.

[0018] During high-temperature heating, deformation changes in the shape of the electrodes occur, manifested in a change in shape and bending. It should be taken into account that when the base is deformed, the coating is also deformed, since it has a molecular bond with the base, which is locally heterogeneous and directly depends on the quality of the base surface treatment. Deformation of the coating leads to deformation of the pores and local destruction of the coating, which become bridges for oxygen penetration to the base.

[0019] It should also be taken into account that a bipolar electrode is a design of electrode capacities in which the cathode and anode are located coaxially, i.e. at a small distance from each other to form a narrow elongated area of ​​electrolysis. In this narrowed area, a complex process of separating the cryolite-alumina melt into pure aluminum and oxygen takes place. If we consider that aluminum accumulates on the surface of the cathode, and oxygen is grouped on the inner wall of the anode, then the melt transformation zone is simultaneously the melt decomposition zone. In this regard, the application of established rules for composite cathode and anode designs does not give the desired result and should be revised. Taking into account the influence of deformation loads and high-temperature heating, it is advisable to exclude spraying on the base.

[0020] Disclosure of invention

[0021] The present invention is aimed at achieving a technical result consisting of increasing the service life of a bipolar electrode for electrolysis processes occurring at temperatures of 950-1000°C.

[0022] The said technical result is achieved by the fact that in a bipolar electrode used in electrolyzers for refining non-ferrous metals in the melt and containing two electrode capacities, which are the anode and cathode, one of which is made in the form of a metal pipe, and the other is made in the form of a metal rod and is located coaxially on insulators inside the first, which is made with openings in the wall to equalize the surface area of ​​one capacity to the surface area of ​​the other capacity, both electrode capacities are made of a nickel-chromium alloy with a chromium content of at least 10%, a plate is welded to the upper part of each capacity, which is a contact element for connecting the corresponding pole of a DC voltage source, said plates are rotated relative to each other in the plane of these plates at an angle of 35°, the openings in the pipe are made in its upper part under the insulators,which are made in the form of two rectangular ceramic plates, one of which is placed on the pipe under its plate, and the other on the same pipe on the plate above, to which the rod plate is attached, and both ceramic plates are pulled together at the corners with aluminum fasteners.

[0023] The best embodiment of the invention

[0024] The present invention is explained by a specific example of implementation, which, however, is not the only possible one, but clearly demonstrates the possibility of achieving the required technical result.

[0025] Fig. 1 shows a general view of the bipolar electrode, side view; Fig. 2 - the same as in Fig. 1, top view; Fig. 3 - general view of the cathode; Fig. 4 - view of unit A according to Fig. 3; Fig. 5 - general view of the anode; Fig. 6 - view of unit B according to Fig. 5; Fig. 7 - histogram of the nickel content in some alloys; Fig. 8 - histogram of the chromium content in some alloys.

[0026] According to the present invention, a new design of a bipolar electrode is considered, used in an electrolysis plant, allowing for an environmentally friendly and safe method for obtaining primary aluminum by electrolysis of a suspension of alumina in an aluminum melt, in which a so-called distributed cathode is used, directly aluminum itself in the composition of the electrolyte - aluminum melt.

[0027] The production of aluminum by electrolysis of alumina suspension in aluminum melt is based on passing an electric current in the electrolyte between the cathode and a non-consumable anode. The electrolysis process is carried out at a melt temperature of 700-750°C and direct current between the cathode and non-consumable anode. The design of the gas-electric bath for electrolysis involves the use of bipolar electrodes for the direct electrolysis process of cryolite melt separation of alumina dissolved in cryolite into molten aluminum at the cathode and gaseous oxygen at the anode.

[0028] Structurally, the bipolar electrode (Fig. 1) consists of an anode 1, which is made of metal and tubular in shape, to the end part of which a plate 2 is welded (Figs. 5 and 6), which is a contact element (for connection to the positive pole of the cable / power cable). Inside the anode tube, a rod element is coaxially located, which is a cathode 3, and to the end of which, brought out through the hole in the anode tube, a plate 4 is welded (Figs. 3 and 4), which is also a contact element (for connection to the negative pole of the cable / power cable). On the lower side of plate 2, an insulating ceramic gasket 5 is located on the anode tube, the same gasket 6 is located on the tube on plate 2 from above and between plates 2 and 4 so that the central holes in these gaskets 5 and 6 position the coaxial arrangement of the cathode and anode (the hole in gasket 5 is made equal to the outer diameter of the anode tube, and the hole in gasket 6 is made equal to the diameter of the rod element of the cathode).The gaskets are made rectangular (square) in plan in the form of plates and are fastened together with bolts 7 with nuts 8 and washers 9.

[0029] It has been experimentally established that plates 2 and 4 should be located in the direction of their planes with an angular displacement relative to each other of 35° (Fig. 2). This angle was chosen based on the safety of electrical connections to cables and wires and the exclusion of electromagnetic parasitic coupling between the connected cables / wires. In addition, with such a displaced position of plates 2 and 4, it became possible to fasten the anode tube and its plate 2 between two gaskets 5 and 6 with two bolt fastenings in isolation from the cathode (fastening points at the ends of one diagonal of the gaskets) and to fasten the cathode rod and its plate 4 between the same two gaskets 5 and 6 with two other bolt fastenings in isolation from the anode (fastening points at the ends of the other diagonal of the gaskets). Alternatively, the cathode plate 4 is secured on top of the insulator (with bolts) at a given angle, and the entire insulating unit is tightened with bolted fasteners along the ears of rectangular gaskets 5 and 6.

[0030] The peculiarity of this design is that, depending on the polarity of the connection, the anode turns into the cathode, and the cathode becomes the anode.

[0031] In the upper part under the insulator in the anode tube there are holes 10, which, on the one hand, allow the surface area of ​​one container to be equal to the surface area of ​​the other container, and on the other hand, are passages for the oxygen to exit from the electrode area to the outside. In the bath, the bipolar electrode is immersed in the melt for approximately two-thirds of its length. In this connection, a cavity is formed under the insulator in the tube, into which the released oxygen (removed from the bath) rushes.

[0032] The bipolar electrode is designed to operate in an electrolytic bath, the design and characteristics of which are known to be unknown. However, during design, the bath is filled with molten electrolyte, which is a molten alumina in cryolite with the addition of aluminum fluoride, the operating temperature of the electrolyte is 950-1000 ° C, the electrode must ensure the deposition of aluminum on the cathode and its continuous flow to the bottom of the bath, and oxygen is released on the inner surface of the anode, which rises upward and must freely leave the system. In this regard, certain requirements must be imposed on the materials from which the electrodes are made. To produce an inert (non-consumable) anode operating in an aggressive electrolyte environment at temperatures up to 1000 ° C, it is necessary to use heat-resistant alloys. The choice of material for the manufacture of an inert anode was based on foreign scientific articles.Based on the studies of oxidation processes of Ni-Fe-Cr-containing alloys with different St content at 960°C and the evaluation of the electrode properties of pre-oxidized anodes during short-term aluminum electrolysis at 960°C in a molten electrolyte, it was found that after oxidation, the surface oxide scale of the Ni-Fe alloy consisted of (Ni,Fe)3O4 oxide with some Fe2O3. With the addition of 10 and 15 wt % Cr, the oxide scales formed on the Ni-Fe-Cr alloys contained a three-layer oxide structure with an outer spinel layer of (Ni,Fe)3O4, the middle spinel layer had an oxide structure of the (Ni, Fe, Cr)3O4 type, and the inner layer was Cr2O3. The inner Cr2O3 layer obviously improved the oxidation resistance of the Ni-Fe-Cr alloys. It is important to note that the layered oxide scale formed on the Ni-Fe-15Cr anode not only has acceptable electrical properties, but is also resistant to oxidation during aluminum electrolysis (Metallurgy of Aluminum by Yu. V.Borisoglebsky, G.V. Galevsky, N.M. Kulagin, M.Ya. Mintsis, GA. Sirazutdinov, Novosibirsk, “Science”. 1999).

[0033] Thus, for the material to be resistant to oxidation in the electrolyte, it must contain a sufficient amount of chromium. Histograms of nickel and chromium content were constructed for all materials in the table (Figs. 6 and 7). The orange line shows the nickel and chromium content of the experimental sample.

[0034] Chromium (Cr) increases strength, hardenability and heat resistance, cutting properties and abrasion resistance, but reduces the viscosity and thermal conductivity of steel. The content of a large amount of chromium (in ordinary steel grades reaches 2%, and in special ones - up to 25%) makes steel stainless and ensures resistance to magnetic forces. Nickel (Ni) increases viscosity, strength and elasticity, but slightly reduces the thermal conductivity of steel. A significant nickel content makes steel non-magnetic, corrosion-resistant and heat-resistant. Nickel does not form carbides in steels. In steels, it is an element that promotes the formation and preservation of austenite. In combination with chromium and molybdenum, nickel further increases the ability of steels to thermal hardening, helps to increase the viscosity and fatigue strength of steels. Nickel increases the corrosion resistance of chromium-nickel austenitic steels in non-oxidizing acid solutions.

[0035] From these histograms it is evident that the optimal options for the manufacture of an inert anode are the following materials: ХН78Т, ХН45Ю, ХН70Ю,

[0036] 10Х25Н25ТР. These materials have a large amount of nickel and chromium in their composition, which has a positive effect on the service life of the inert anode, but due to the high cost and rarity of the materials, the production of an inert anode from these alloys requires high costs. On the other hand, to reduce the cost of anode production, it is proposed to use alloys with a smaller amount of nickel, with the same amount of chromium in the composition. Such alloys are marked 20Х23Н18 and 10Х23Н18. The use of these alloys allows to reduce the cost of anode production by several orders of magnitude while maintaining a long service life at high temperatures under conditions of interaction with pure oxygen.

[0037] This utility model is industrially applicable and allows increasing the service life of a bipolar electrode when operating at temperatures up to 1000C under conditions of pure oxygen release at the anode.

[0038] This conclusion was obtained as a result of the experimental use of a bipolar electrode in the electrolysis process, the description of which is given in the prototype.

Claims

Invention formula A bipolar electrode used in electrolyzers for refining non-ferrous metals in a melt and containing two electrode capacities, which are an anode and a cathode, one of which is made in the form of a metal pipe, and the other is made in the form of a metal rod and is located coaxially on insulators inside the first, which is made with openings in the wall to equalize the surface area of ​​one capacity to the surface area of ​​the other capacity, characterized in that both electrode capacities are made of a nickel-chromium alloy with a chromium content of at least 10%, a plate is welded to the upper part of each capacity, which is a contact element for connecting the corresponding pole of a DC voltage source, said plates are rotated relative to each other in the plane of these plates at an angle of 35°, openings in the pipe are made in its upper part under the insulators, which are filled in the form of two rectangular ceramic plates in plan,one of which is placed on the pipe under its plate, and the other on the same pipe on the plate above, to which the rod plate is attached, and both ceramic plates are tightened at the corners with bolted fasteners.

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