RU2742633C1 - Method for producing aluminum by electrolysising cryolito-aluminum melts - Google Patents

Abstract

FIELD: method for producing aluminum by electrolysis of cryolite-alumina melts.SUBSTANCE: invention relates to the method for producing aluminum by electrolysis of cryolite-alumina melts. The method includes preparing an electrolyte with a ratio of electrolyte and aluminum densities more than 1, adjusting the electrolyte composition, supplying and removing direct current from the cathode and the anode located vertically in an electrolyzer, adjusting the interpolar distance between the electrodes. At the same time electrolysis is carried out with immersion of the solid anode in the upper layer of liquid aluminium and partially in the lower layer of the electrolyte melt. The interpolar distance between the solid anode and the liquid aluminium cathode is regulated by the height of the aluminum layer.EFFECT: invention makes it possible for electrolyzers of average power with current efficiency of ~94% to achieve specific power consumption of about 8 kWh/kg at an operating voltage of 2.32.5 volts and with specific capacity which is 2.8 times higher than the specific capacity provided by existing designs of electrolyzers with horizontal electrodes.1 cl, 2 tbl, 2 dwg

Description

The proposed invention relates to the electrolytic production of aluminum and can be used in the technological process of electrolysis of cryolite-alumina melts in energy-saving designs of aluminum electrolyzers.

Improving the technical and economic indicators of the electrolysis process is the main task of aluminum production, which focuses on increasing productivity and reducing energy and material consumption indicators.

The first industrial aluminum electrolysers of the 1900s required over 40 kWh / kg of aluminum produced and had a current efficiency ranging from 75-78%. The entire further history of the development of the aluminum industry testifies to a continuous decrease in electricity consumption for the implementation of the electrolysis process. During the 20th century, the current efficiency on new series increased to 95 ± 1%, and the specific power consumption decreased to 15 ÷ 13 kWh / kg Al. The long-term goals of aluminum producers determine the achievement of this figure of 11 kWh / kg Al. Their implementation in 2 kWh / kg Al requires a decrease in the voltage on the electrolyzer by 0.6-0.7 V, which corresponds to a voltage loss in the electrolyte layer of about 2.5 cm.It is these values that the fluctuation of the aluminum surface reaches with the existing level of electrolysis technology ... Therefore, a decrease in the interpole distance (MPD) from 4-5 cm to 2.0-2.5 cm is possible only with a radical change in the design of the cathode and the introduction of new materials. And this required the designers to include in the design of electrolyzers the technology of aluminum-wetted cathodes or vertically arranged electrodes with a reduced MPR "anode-cathode".

The first attempts to create high-performance electrolyzers using solid electrodes based on graphite elements in the form of cylinders [1] and plates [2] with a low MPD did not receive commercial distribution due to the lack of a fundamental property of cathodes - aluminum wettability.

Interest in solid cathode cell designs revived after the publication of C.E. Ransley on the use of borides and carbides of refractory metals as cathode elements with aluminum wettability and resistance in an aggressive working environment [3, 4]. Despite the absence of a commercial application of these inventions in industry, subsequently, numerous attempts were made to use the unique properties of products and coatings based on carbides and borides of refractory metals in the designs of traditional aluminum electrolyzers and new generation electrolysers with drained cathodes and vertical electrodes.

The patent [5] presents an original anode-cathode structure in the form of a vertical wedge-shaped cathode made of a refractory aluminum-wetted material with carbon anodes adjacent to its surfaces. It is argued that such an anode-cathode structure improves the conditions for the removal of anode gases from the pole gap between the anode and cathode surfaces, allows the movement of the anodes to control its minimum value up to 1.3 cm. These design advantages make it possible to significantly reduce energy consumption for aluminum production.

The main disadvantage of the known invention is high thermal cracking, that is, low resistance to thermal shock of cathode elements made of refractory materials. There is no possibility of their replacement without shutting down the electrolyzer. This depreciates the successful design of the electrolyzer and the method of conducting the electrolysis process with a low MPR.

The patent for the design of a multicellular electrolyzer for producing aluminum by electrolysis of cryolite-alumina melt [6] solves the problem of reducing the intense destruction at the edges of the electrode plates by reducing the electric field strength in the edge zone of the electrodes. The solution consists in a special arrangement of bipolar electrode elements in the volume of the electrolyzer, made of cathode graphite and anode cermet plates, interconnected by conductive steel plates. Bipolar electrodes are located vertically in the electrolyte with a distance between the cathode and anode plates of 1.5 cm. Every second bipolar electrode is fixed on adjusting screws with the possibility of vertical movement and adjustment of the MPR. Thus, the well-known advantage of electrolysers with bipolar electrodes is retained - a significant reduction in electricity consumption per kilogram of aluminum produced.

The disadvantage of the known solution is the extremely complex design of the electrolyzer, which the authors consider as a large conduction electromagnetic pump. Obviously, for this reason, there are no examples of experimental confirmation of the solution to the problem and the possibility of practical implementation of melt electrolysis in this design solution. In the presented calculations of the process, the original method confirmed only the known decrease in electricity consumption for the production of aluminum by 1.33 times relative to the best values achieved on conventional Erou-Hall electrolysers.

The work [7] presents the so-called low-temperature floating electrolysis of aluminum (LFEA), which is characterized by the use of an electrolyte Na ₃ AlF ₆ -AlF ₃ -BaF ₂ -CaF ₂ with a density of 2.7 ÷ 2.9 g / cm ³ and a liquidus temperature of about 640 ° C. Therefore, the process of electrolysis of cryolite-alumina melt was carried out at a temperature of 750 ÷ 800 ° C in an inert atmosphere, an alumina concentration of 2.4 ÷ 2.7 wt. % with the reduction of aluminum on a vertically located graphite cathode, the surface of which was previously coated with a layer of titanium diboride TiB ₂ . Graphite and a low-consumable anode based on a Cu-Ni-Fe alloy were used as positive electrodes. The interpolar distance of the MPR between the cathode and the anodes was 2 cm. After reduction, aluminum, having a lower density relative to the electrolyte, floats to the surface of the melt in the cathode region, which is bounded by an insulating corundum screen. According to the results of laboratory experiments on installations with a current of 100 amperes and 1000 amperes, an average current efficiency of 90.6% was recorded, and the specific DC energy consumption was 12.3 kWh / kg Al.

By its technical nature, the presence of similar features, this solution was chosen as the closest analogue. The main disadvantage of the known technical solution is the complexity of organizing the collection of aluminum on the near-cathode surface of the electrolyte in a space limited to 2 cm between the anode and the cathode. The small volume of the metal collector requires its continuous removal from the electrolyzer, the mechanical, thermal and chemical nature of the integrity of the insulating corundum shield is possible, the consequences of which are associated with a short circuit between the electrodes. When organizing the process on an industrial scale, these disadvantages will cause a decrease in current efficiency, an increase in energy consumption and, in general, affect the operability of electrolyzers. In addition, it is known that a noticeable obstacle to the effective flow of electrochemical processes on a solid cathode is the chemical and physical inhomogeneity of polycrystalline electrodes, as a result of which the direct current is unevenly distributed over their cross section, concentrated in areas with the lowest resistivity (protrusions, faces and edges of crystallites), flowing around pores and microcracks. A high fluctuating current density appears on the surface of the electrodes, which, during prolonged operation with the development of physical microdefects, is transformed into a real one and exceeds not only the calculated (geometric) current density, but also the limiting (diffusion) one for the discharge of aluminum and oxygen. At such high real current densities under conditions of concentration polarization, the probability of recovery at the cathode of the most electronegative impurity elements and electrolyte components increases, with corresponding negative consequences for cathodic processes and the state of electrodes. These consequences, under conditions of electrode passivation by impurity deposits and enrichment of the melt volume and near-electrode space with sodium fluoride, will be expressed in the interaction of impurity elements with the cathode material and a local increase in the melting temperature in the near-cathode space. As a result, the surface will not only change its composition and physical state, but also passivate by the electrolyte freezing on the electrode, refractory compounds of impurities. Under these conditions, the normal process of electrolysis of cryolite-alumina melts is impossible. In fact, these phenomena on an inhomogeneous cathode surface are one of the main reasons for the lack of progress in the commercial development of the technology of electrolysis of cryolite-alumina melts using inert electrodes.

The objectives of the proposed invention are to improve the technical and economic indicators of the process of electrolytic production of aluminum, to create technical and technological conditions for the use of low-consumption anodes and improve the environmental consequences of production.

The technical results are an increase in the productivity of an aluminum electrolyzer based on the area of the occupied surface, stabilization of the cathodic and electrolysis processes in general, a decrease in the consumption of anodes and electricity,

The solution to this problem and the set technical results are achieved by the fact that in the method of electrolysis of cryolite-alumina melts, including the supply and removal of direct current from polarized electrodes, correction of the electrolyte composition, adjustment of the pole-to-pole distance between the electrodes, electrolysis is carried out with a density ratio of electrolyte and aluminum of more than 1, with immersion solid anode into the upper layer of liquid aluminum and partially into the lower layer of the electrolyte melt, and the pole-to-pole distance between the solid anode and the liquid aluminum cathode is regulated by the height of the aluminum layer.

Comparison of the proposed technical solution with the closest analogue shows the following. To solve the set tasks, both methods are characterized by common features:

• Carry out electrolysis of cryolite-alumina melt when the ratio of the density of electrolyte and aluminum is more than 1;

• The surface of the electrodes is located vertically to minimize the pole-to-pole distance and voltage losses in the MPR;

• Reactive and inert electrodes are used as anode;

• Reduced at the cathode aluminum is concentrated in a layer on the surface of the heavy electrolyte.

The proposed solution is also characterized by features that differ from those of the closest analogue:

• A vertical, chemically and physically homogeneous surface of liquid aluminum is used as a cathode;

• The solid anode is placed vertically in the aluminum layer and partially in the lower layer of the heavy electrolyte;

• The pole-to-pole distance between the solid anode and the liquid metal cathode is regulated by the ratio of the wetting characteristics of the anode surface with electrolyte and aluminum, the height of the aluminum layer and the depth of the anode into the lower electrolyte layer.

The presence in the proposed technical solution of features that are different from those characterizing the closest analogue, allows us to conclude that the proposed solution meets the condition of patentability of the invention "novelty".

The search and comparative analysis did not reveal technical solutions characterized by a set of features similar to the set of features of the proposed technical solution and giving similar results when used, which allows us to conclude that the proposed solution meets the patentability condition “inventive step”.

The technical essence of the proposed solution is as follows.

When a solid is lowered into a two-layer melt, an interaction occurs between them due to the forces of hydrostatic pressure of the melts (P _hydr ), mutual internal adhesion of liquid molecules to each other (W _adg ) and external adhesion to a solid surface (W _co ). In addition, the Archimedean force (P _arkh ) causes the level to rise or the melt is pushed out by the volume of a rigidly fixed body. In the case of penetration of the electrode through the upper layer of aluminum into the heavy electrolyte melt, the buried volume of the anode pushes out an equal volume of electrolyte.

P _arh = ρ _el × g * × V [H] (1)

R _arch - force of Archimedes, force of pushing out of the body, N;

ρ _el - electrolyte density, kg / m ³ ;

g * - coefficient of gravity = 9.8 N / kg.

V is the volume of the electrode buried in the electrolyte, m ³ .

If there are no gaps between the electrode and the aluminum melt, the level of metal and electrolyte will increase. On the contrary, in the presence of gaps, the squeezed out electrolyte rises into the gaps on both sides of the electrode (Fig. 1), which can be formed as a result of oppositely directed forces of adhesion and cohesion of aluminum layers adjacent to the electrode:

ΔW = W _coh - W _adg = σ _Al × (1 - cosθ _Al ) [N / m] (2)

W _adg - specific work of adhesion, N / m;

W _coh - specific work of cohesion, N / m;

σ _Al - surface tension of aluminum, N / m;

θ _Al - contact angle of electrode wetting with aluminum, deg.

In this case, the volume of the ejected melt into the gaps is equal to the volume of the electrode buried in the electrolyte.

B is the anode width, m;

T is the thickness of the anode, m;

h - height (depth) of immersion of the anode in the electrolyte, m;

H _Al - height (thickness) of the aluminum layer, m;

h _E - thickness of the electrolyte interlayer in the gap (or interpolar distance of the MPR), m.

Nevertheless, the formation of a gap and its stable state is possible at a certain ratio of hydrostatic pressures of the cathode aluminum layer and electrolyte in the gap (h _E ), that is, on the characteristics of wetting the cathode surface with electrolyte and the height of the layer of aluminum and electrolyte melts ΔР _hydr = Р _el - P _Al :

ΔР _hydr = g × H _Al × (ρ _el - ρ _Al ) [N / m ² ] (4)

g - acceleration due to gravity = 9.8 m / s ² ;

ρ _Al - density of aluminum, kg / m ³ .

Under the condition of dynamic equilibrium of the acting forces, we can write the following equation:

ΔР _hydr × S = ΔW × h _E + Р _arch [H] (5)

S is the surface area of the anode in the aluminum layer, m ² .

The left side of equation (5) is the force acting on the aluminum or anode surface, equal to S of the anode surface area in the aluminum layer. The direction of the vector of this force, towards aluminum or the cathode, depends on the ratio of the hydrostatic pressures of the electrolyte and aluminum.

The right-hand side of equation (5) is the sum of the forces aimed at pressing aluminum against the anode or displacing the aluminum surface at a distance h _{E. The} direction of this force depends on the ratio of the adhesion and cohesion forces of the aluminum surface adjacent to the cathode surface and the pushing force of a limited electrolyte volume.

Substituting the parameters of equations (1) - (4) into equation (5), we obtain an equation for calculating the thickness of the gap (MPG) between the solid anode and the liquid, vertically located cathode:

[м] (6)

[m] (6)

Thus, the pole-to-pole distance between the vertical surfaces of the liquid cathode and the solid anode depends on the ratio of the densities of aluminum and electrolyte and the wetting characteristics of aluminum with respect to the anode. But the main influence on the MPR is exerted by H _Al height of the aluminum layer. Preliminary calculations using the formula (6) show that h _E can vary from 4 to 20 mm when the H _Al level of aluminum on the surface changes from 75 to 350 mm (Table 1 and Fig. 2). This means that the required pole gap h _E and, accordingly, the power consumption W are controlled by a certain frequency of pouring aluminum from the electrolyzer while maintaining a constant aluminum level H _Al = const.

To ensure a stable electrolysis process, two main problems are solved - maintaining a constant concentration of aluminum oxide in a small MPR of up to 20 mm and removing a gas film from the anode surface. With the vertical arrangement of the electrodes, they are solved by creating conditions for an unhindered bottom-up gas-lifting effect. For this, the alumina is loaded into the bath and placed on the bottom. Its dissolution in the electrolyte occurs spontaneously under the action of a concentration gradient between the electrolyte volume under the aluminum layer and the cathode volume as it is spent on aluminum reduction. That is, a self-regulating process of dissolution of aluminum oxide and nutrition is carried out in the volume of the MPR. This process is supported by the convection of the electrolyte from bottom to top as a result of gas-lifting initiated by the formation of gas bubbles in the anode space.

The method is carried out as follows.

The electrolysis of cryolite-alumina melts by the proposed method can be organized in an electrolysis cell with the main structural elements and electrolytic cells for refining aluminum by a three-layer method. The cathode device is a steel casing, the bottom of which consists of carbon-graphite blocks. The side lining is made of silicon carbide plates. The cathode aluminum bus bar is closed on a cathode polarized layer of liquid aluminum located on the electrolyte surface. The anode unit consists of an aluminum anode busbar on a steel frame with devices for fastening anode aluminum rods, on which reactive graphite or inert anodes are fixed, for example, cermet ones based on a composition of metal and oxide components (NiFe ₂ O ₄ -NiO-Cu-Ag) or metal based on an alloy of Ni with Fe and the introduction of a significant amount of alloying additives (Cu, Al, Ti, Y, Mn, Si). Alumina with a layer of about 2 cm is loaded into the cathode shaft on the hearth, and liquid aluminum is poured in the amount necessary to create a layer of 150 ÷ 200 mm. Then the electrolyte melt prepared in the uterine bath is poured with a density satisfying the ratio of the density of the electrolyte to aluminum greater than 1, for example, Na ₃ AlF ₆ -AlF ₃ -BaF ₂ -CaF ₂ -Al ₂ O ₃ with cryolite molar ratio KO = 2.15, mass concentrations of BaF ₂ , CaF ₂ and Al ₂ O _3, respectively, 20, 15 and 3.0 wt. %. This electrolyte has a density of about 2600 kg / m ³ and a liquidus temperature of 640 ° C. Therefore, the temperature of the electrolysis process must be maintained at about 740 ÷ 750 ° C. After the electrolyte is poured in an amount sufficient to establish the general level of the melts, at which there is a reliable contact of the aluminum liquid layer with the aluminum cathode busbar, the levels of melts and the completeness of separation are monitored. Anodes are immersed in the aluminum melt and with partial immersion in the lower electrolyte layer and fixed on the anode busbar using fastening mechanisms. The electrolyzer is connected to the general series circuit. The process of electrolysis of cryolite-alumina melts begins with periodic feeding of the bath with alumina by loading it onto the bottom of the cathode shaft.

The theory of the proposed method of electrolysis of heavy cryolite-alumina melts was checked by preliminary calculations and confirmed by experiments in laboratory conditions. The method is illustrated by the following figures and tables:

FIG. 1 - Diagram of an electrolysis cell.

FIG. 2 - Parameters for calculating the pole-to-pole distance (h _E ).

FIG. 3 - Dependence of h _E and specific energy consumption W on the height of the aluminum layer.

FIG. 4 - Parameters and calculation of technical and technological characteristics.

Example 1. Theoretical confirmation of the possible implementation of the proposed electrolysis method was carried out by calculating the interpole distance MPR according to the formula (6). It was assumed that in the electrolysis process, an electrolyte melt Na ₃ AlF ₆ -AlF ₃ -BaF ₂ -CaF ₂ -Al ₂ O _{3 is used} in a cell, the diagram of which is shown in FIG. 1. In the calculations, the values of the parameters presented in FIG. 2. The results of calculating the MPR depending on the height of the liquid aluminum layer is shown in Fig. 3. These calculations make it possible to determine the electrical, energy and production indicators of the proposed method of electrolysis of cryolite-alumina melts, including the voltage across the electrolyzer (U):

U = | E ^rev | + U _buth + U _BU + | η _dC | + | η _dA | + | η _RA | + U _E [B] (7),

and its components according to empirical recommendations [8]

[В] (8)

[AT 8)

U _bath = i _a / χ × (h _E - r) + U _BU [В] (9)

and also according to well-known theoretical formulas [9]:

[В] (11)

[AT 11)

[В] (12)

[AT 12)

[В] (13).

[B] (13).

The calculated values of these parameters, as well as the technical characteristics, are presented in Fig. 4. Theoretical calculations confirm the possibility of implementing the proposed method for electrolysis of cryolite-alumina melts, monitoring the interpolar distance of the MPR within 5 ÷ 15 mm and specific energy consumption for aluminum production within 7.5 ÷ 9.5 kWh / kg Al (Fig. 3) ... At the same time, the specific productivity of the method, based on the occupied working area (the area of the cell shaft), in comparison with the method used on the operating cells of the same power, is not less than 2.5 times higher (Fig. 4).

Example 2. Practical confirmation of the possible implementation of the proposed method of electrolysis in laboratory conditions was carried out on a 20 A installation in a graphite crucible with an inner diameter of 60 mm and a height of 150 mm (Fig. 1). The crucible was inserted into a steel casing with a gap, which was filled with alumina for thermal insulation and protection from possible leaks. At the bottom of the graphite crucible, an alumina layer of about 1 cm was poured, and solid aluminum with a mass of 220 ± 0.5 g was placed to create, after melting, a layer of liquid aluminum of about 50 mm to provide a cathode current density of about 0.7 A / cm ² . Together with a ceramic lid with holes for a steel anode current lead and a thermocouple, an anode in the form of a parallelepiped 70 mm high, 30 mm wide, and 10 mm thick was installed in the crucible. The graphite crucible was filled with Na ₃ AlF ₆ -AlF ₃ -BaF ₂ -CaF ₂ -Al ₂ O ₃ electrolyte with cryolite molar ratio KO = 2, mass concentrations of BaF ₂ , CaF ₂ and Al ₂ O _3, respectively, 20, 15, and 2.5 wt. % (density about 2.6 g / cm ³ ). A steel cathode current lead was screwed into the crucible wall. The assembled cell was installed in a vertical shaft resistance furnace, the current leads were connected to the anode and cathode terminals, respectively, and the furnace was heated with automatic maintenance of the holding temperature of 750 ° C. After the electrolyte and aluminum melted, the anode was positioned in the liquid aluminum layer according to the marks on the anode current lead and the metal level was measured so that about 10 mm was immersed in the lower electrolyte layer. The installation was switched on for electrolysis, the current was gradually raised to 20 A. After a short fluctuation, the establishment of a voltage of 2.2 ÷ 2.4 V indicated the course of the normal electrolysis process. After 8 hours of operation, the furnace and the cell were disconnected from the power sources, the system was frozen, and aluminum was removed from the crucible. The weight gain of aluminum was 45 g (total 265 g), which corresponds to the current efficiency of 84%.

Thus, with the successful implementation of the proposed project on medium-power electrolysers with a current output of 94%, it is possible to achieve a specific electricity consumption of about 8 kWh / kg Al at an operating voltage of 2.3 ÷ 2.5 volts and a specific productivity of 2.8 times higher than on existing designs of electrolysers with horizontal electrodes.

INFORMATION

1. Patent US No. 1070454, 1913

2. US Patent No. 2,480,474 CI. 204-67, 1949

3. GB patent No. 802905, B23n, C22d, 1958

4. US patent No. 3028324, CI. 204-67, 1962

5. US patent No. 4405433, C25C 3/08, 1983

6. RF patent No. 2287026, C25C 3/06, 2006

7. Lu Huimin and Yu Lanlan. Technique and mechanism of aluminum floating electrolysis in molten heavy Na ₃ AlF ₆ -AlF ₃ -BaF ₂ -CaF ₂ bath system // Light Metals. 2003. P. 351-356.

8. Grjotheim K., Kvande H. Introduction to Aluminum Electrolysis. 2 ^nd Edition. - Aluminum-Verlag, Dusseldorf, 1993 .-- 212 p.

9. Vetyukov M.M., Tsyplakov A.M., Shkolnikov S.N. Electrometallurgy of aluminum and magnesium. - M .: Metallurgy, 1987. - 320 p.

Claims (1)

A method for producing aluminum by electrolysis of cryolite-alumina melts, including the preparation of an electrolyte with a density ratio of electrolyte and aluminum of more than 1, adjusting the electrolyte composition, supplying and removing direct current from the cathode and anode vertically located in the electrolyzer, adjusting the pole-to-pole distance between the electrodes, characterized in that electrolysis is carried out with by immersing the solid anode in the upper layer of liquid aluminum and partially in the lower layer of the electrolyte melt, and the pole-to-pole distance between the solid anode and the liquid aluminum cathode is controlled by the height of the aluminum layer.

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