RU2486292C1 - Method to create wet coating of carbon hearth in aluminium cell - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes high-temperature electrochemical deposition of coating components from melted electrolyte and synthesis of carbides and borides of refractory metals on the surface of coal and graphite blocks of the hearth. Supply of coating components into the cell, their electric deposition and synthesis of coating compounds are carried out in stages with initial supply of refractory metals into the electrolyte melt, their electric deposition and synthesis of carbides of refractory metals on coal and graphite blocks and subsequent supply of boron into the electrolyte melt, its electric deposition and synthesis of coating compounds in the system of refractory metal-boron-carbon and refractory metal-boron on the hearth surface. Duration of the first stage is determined by reduction of refractory metal concentration in electrolyte from 5.0 to 0.1 wt %. Duration of the second stage is determined by reduction of boron concentration in aluminium from 0.5 to 0.01 wt %.

EFFECT: guaranteed presence of a protective coating on a hearth soaked with aluminium after start-up of a cell, reduction of specific energy consumption for production of aluminium, prevention of damage of carbon lining integrity.

5 cl, 4 dwg, 1 tbl

Description

The present invention relates to the electrolytic production of aluminum and can be used in the technological process of electrolysis of cryolite-alumina melts.

Improving the technical and economic indicators of the electrolysis process is the main task of aluminum production. One of the directions for solving this problem is the creation during operation of the electrolyzer on the cathode carbon surface of the shaft wetted with aluminum coatings. The main functions of wettable coatings (SP):

- providing reliable and efficient electrical contact between the aluminum layer and the carbon material of the cathode, which minimizes the specific consumption of technological electricity, stabilizes the shape of the working space, current distribution in the cathode device and the electrolysis process as a whole;

- protection of carbon-graphite cathode blocks from the damaging effects of the aggressive environment of molten electrolyte and aluminum, which ensures an increase in the life of the cell.

A patent is known for an electrolyzer and a method for producing metal, which describes a method for creating a thick inactive layer of a mixture of boride, carbide, nitride particles of heavy metals with aluminum on the surface of an inclined drained cathode [1].

The mobility of the pulp between the carbon surface and the aluminum layer with a yield strength of 10-100 N / m ² and bulk density of 2.5 g / cm ^{3 is} ensured by the content of particles in the layer 25-70 vol% and their size in the range of 0.1-1 μm or 5 500 microns. Depending on the particle size, an aluminum-wetted pulp layer is formed from 1 to 10 mm. The preferred composition of the pulp is considered a mixture of Al / TiB ₂ .

The patent provides almost all known methods of placement and pulp formation on the surface of the hearth:

- in the form of powder TiB ₂ or briquettes of the composite Al / TiB ₂ before starting the bath,

- in the form of TiB ₂ powder or Al / TiB ₂ composite briquettes after start-up,

- synthesis of TiB ₂ in the electrolysis process by adding TiO ₂ and B ₂ O ₃ to the electrolyte,

- simple boronation of aluminum by additives B ₂ O ₃ to the electrolyte.

An option is also provided for creating Al / TiB ₂ pulp during destruction of the TiB ₂ / C composite layer, if such was previously created and degrades on the surface of the hearth. It is reported that the application of the method can be extended to conventional industrial electrolyzers with a large layer of aluminum on the surface of the cathode.

The disadvantage of this method is the instability of the viscous flow layer of pulp on the surface of the carbon hearth. Any disturbing effects on this layer (anode effects, technological maintenance operations, replacing anodes, etc.) in an electrolytic cell with a drained cathode will lead to its mixing with the electrolyte, short circuits of the interpolar distance (MPR), disruption of the process and loss of aluminum. Ultimately, the unstable operation of the electrolyzer will lead to a decrease in the technical and economic indicators of electrolysis.

There is a method of applying a wettable coating of the bottom of an aluminum electrolyzer during its start-up by joint electrodeposition from cryolite-alumina melts of refractory metals and boron on a carbon cathode [2]. Moreover, when the concentration of alumina in the electrolyte is 2-8 wt.%, The parameters of electrodeposition of its components (for example, titanium and boron) are selected in such a way that metal aluminum does not co-precipitate:

- deposition is carried out at a current density of 0.1-0.9 A / cm ² , with a cryolite ratio of 2.3-2.9 and a temperature of 950-970 ° C;

- as a source of boron, oxygen and / or fluorine-containing derivatives of boron are used, namely boron oxide, tetraborates of alkali and alkaline earth metals, tetrafluoroborates of alkali and alkaline earth metals in an amount of 0.5-2 wt.% based on boron oxide;

- as a source of refractory metals - additives of oxygen- and / or fluorine-containing derivatives of these metals, namely oxides, oxygen- or fluorine-containing compounds with cations of an alkali or alkaline earth metal in an amount of 0.5-4 wt.% calculated on the metal oxide.

The mechanism of formation of the boride layer is not specified, but in the description of the method it is declared that special conditions are created under which the density and connectivity of the coating layer with the surface of the hearth is ensured, regardless of the difference in the thermal expansion coefficients of the coating layer and the hearth material. At the same time, other conditions and parameters, other than those described in the description and claims, are not given. At the same time, the listed parameters of the coprecipitation of boron and refractory metals, including an alumina concentration of 2-8 wt.%, Create ideal conditions for the simultaneous electrodeposition of aluminum, which reacts with the surface of the carbon hearth to form aluminum carbide. The layer of this compound on carbon blocks prevents the formation of a boride coating on the cathode or, at least, its good adhesion to the surface of the hearth.

The main obstacle in ensuring a uniform distribution of current over the surface of the hearth and, therefore, uniform, frontal and layer-by-layer growth of the synthesized boride phase is the extremely heterogeneous surface of carbon-graphite blocks with numerous micro- and macro-irregularities, cracks and pores. Therefore, the conditions and parameters of the joint electrodeposition of boron and refractory metals in the known method of applying a wettable coating to the bottom of an aluminum electrolyzer will allow one to obtain only powdered boride compounds or, in the best case, coatings with poor adhesion to a carbon bottom with a boride layer of dendritic morphology. The quality of such a coating cannot ensure stable operation of the electrolyzer with a drained cathode.

A known method of creating a joint venture on the cathode surface of an aluminum electrolyzer and preserving this coating during electrolysis, comprising the following main steps [3]:

- To initiate wetting, it is recommended to initially introduce titanium-containing compounds into the cryolite-alumina melt and to deposit titanium on the cathode aluminum surface. Saturation of aluminum with titanium is supposed to chemically synthesize a TiC film on the surface of carbon blocks, which favors the subsequent formation and growth of a TiB ₂ film.

- To form a joint venture, oxides and salts of boron, titanium, zirconium, hafnium, chromium, vanadium, niobium, tantalum, molybdenum, tungsten and their mixtures are supplied, followed by their dissolution and electrochemical reduction of ions on the cathode aluminum surface to create a supersaturated solution of metals and boron in the cathode layer of aluminum;

- In the process of contacting a supersaturated solution of the near-cathode layer of aluminum with a carbon hearth, an aluminum-boride boride layer forms on the carbon surface.

The rationale for the coating process is the very low solubility of refractory metal borides in aluminum, in contrast to the high solubility of their constituent elements in aluminum. For this reason, according to the logical assumption of the author, the prevailing formation of carbides and borides of refractory metals from atoms of constituent elements will occur on a coal surface, but not in the volume of a thin cathode layer of aluminum. It is stipulated that this process has not been studied and it is proposed to characterize it as “supersaturated platinization” of the carbon surface.

By technical nature, the presence of similar features, this solution is selected as the closest analogue.

The main disadvantage of the known technical solution is that when implementing the method, the consequences of the possible interaction of the coating components in the volume of aluminum are not taken into account. At 750-1000 ° C, the presence of refractory metals in aluminum or the combined presence of these metals and boron in aluminum in any amount is accompanied by a thermodynamically favorable process of their interaction with aluminum and with each other to form dense boride crystals (e.g., TiB ₂ ) and intermetallic compounds (e.g., AlTi ₃ ). This process has been well studied and is used to clean aluminum from impurities of refractory metals (for subsequent use of aluminum for electrical purposes) and to modify the internal structure of deformable aluminum grades (for its subsequent rolling into thin sheets or foil). Crystals of borides and intermetallic compounds of refractory metals, having a relatively high density relative to aluminum, will settle in the cathode layers and / or be removed with the poured aluminum. The interaction between them and the carbon surface is absent.

Thus, the proposed technological regimes for supplying titanium and boron to the electrolyzer will lead to the formation in the cathode layer of a viscous-fluid mixture of aluminum with particles of refractory compounds of borides and intermetallides, but not a wettable boride coating on the carbon cathode.

The objective of the invention is to increase the technical and economic indicators of the process of electrolytic production of aluminum.

Technical results - the guaranteed presence of an aluminum-wettable protective coating on the carbon lining of the cathode device after the start-up period of the bath, a decrease in the specific energy consumption for aluminum production, and prevention of the integrity of the carbon lining.

Technical results are achieved by the fact that in the method for creating a wettable coating of the carbon bottom of an aluminum electrolyzer, including high-temperature electrochemical deposition of coating components from a molten electrolyte and the synthesis of carbides and borides of refractory metals on the surface of carbon-graphite blocks, the coating components are supplied to the electrolyzer, their electrodeposition and synthesis of coating compounds are carried out in stages with the initial supply of refractory metals to the cryolite melt, their electrodeposition synthesis and carbides of refractory metals on the carbon blocks and then supplying the boron in the molten cryolite, its synthesis and compounds electrodeposition coating system refractory metal-boron-carbon and the refractory metal-boron on the hearth surface.

Refractory metals and boron are transferred to the electrolyte in the form of pure components or their salts and oxides, and titanium, zirconium, tungsten, vanadium, or mixtures thereof can be used as input metals.

The technical essence of the proposed solution is as follows.

The creation during the start-up period of the electrolyzer on the cathode carbon surface of the electrolyzer shaft of aluminum wettable coatings (SP) is a task whose solution provides an increase and stability of high technical and economic indicators of the process, and an increase in the service life of the electrolyzer. The method of creating such a coating should be technologically advanced, reliable, reproducible and low cost.

In the proposed technical solution, on the cathode carbon surface in the starting period of the electrolyzer, an aluminum-wettable coating of refractory metal borides (Ti, V, Zr, Mo, W) is created in two stages.

At the first stage, the introduction and dissolution of metal-containing components into the cryolite melt is carried out by electrodeposition of refractory metals (Me) at the cathode

in two $$Me{F}_6^{2-}+e\to Me{F}_6^{3-}\left(A\right)$$

$$Me{F}_6^{3-}+3e\to Me+6F^{\text{-}}\left(B\right)$$

or in one stage $$Me{F}_6^{2-}+\mathrm{four}e\to Me+6F^{\text{-}}\left(C\right)$$

with their subsequent interaction with carbon until the formation of carbides of refractory metals in the near-surface volume of the cathode

. $$Me+C=MeC\left(D\right)$$

.

Moreover, the AD processes occur on the entire surface of the cathode, in pores, irregularities, micro- and macrocracks, gradually leveling the surface layer of the cathode and, thereby, preparing it for the next stage - the stage of interaction with boron.

To prevent the formation of aluminum carbide Al ₄ C ₃ in the surface layer of the carbon cathode as a competing process, which complicates the carbidization process D and excludes the cathode surface leveling only by its product, the presence and, especially, additives of alumina are excluded in the cryolite melt. In addition, in the presence of alumina in the electrolyte, the solubility of the metal-containing components is reduced.

The duration of the first stage is determined by a decrease in the concentration of refractory metal in the electrolyte from 5.0 to 0.1 wt.%. At concentrations of refractory metal in the electrolyte of more than 5 wt.%, Difficulties may arise in ensuring the normal process of the start-up period in the form of “hot” or “cold” »The course and precipitation at the bottom of the cell. The decrease in the concentration of refractory metal in the electrolyte to 0.1 wt.%, And below prepares the transition to the second stage, since its activity does not affect the purity of aluminum as a commercial product.

At the second stage, the introduction and dissolution of boron-containing components in the cryolite melt is carried out by electrodeposition of boron at the cathode together

. $$B{F}_{\mathrm{four}}^{-}+3e\to B+\mathrm{four}{F}^{\text{-}}\left(E\right)$$

.

Boron will be reduced directly on the surface of the carbon cathode. The absence of aluminum in the melt and on the surface of the hearth eliminates the competing processes of the discharge of aluminum ions and the formation of aluminum carbide, which prevents the interaction of boron with titanium compounds uniform in the area of the hearth. Such a mechanism for the delivery of boron to the bottom provides a uniform and irreversible flow to the reaction carbide-metal surface. Thus, the solid-phase interaction of boron with refractory metals and their carbides will occur in the near-surface volume of the cathode. In fact, the boronation of the surface cathode layer containing refractory metals is carried out. The formation of a wettable coating passes through the stages of formation of compounds in the Me-B-C system, then of the MeB intermediate compounds and, finally, stoichiometric MeB ₂ . Accordingly, the surface layer of the carbon cathode during the start-up period will acquire a layered partially mixed structure and composition. The outer layer (to aluminum) will be completely selected to MeB ₂ , the next will consist of compounds close in composition to MeB. The deeper layers will have the composition Me _x B _y C _z . Such a structure of a near-surface wettable layer has sufficient strength, good bonding, and almost complete correspondence of physicochemical properties with a carbon base.

The duration of the second stage is determined by a decrease in the concentration of boron in the electrolyte from 0.2 to 0.005 wt.%. At concentrations of refractory metal in the electrolyte of more than 0.2 wt.%, Difficulties may arise in ensuring the normal technological process of the start-up period in the form of a “hot” stroke and the possibility of establishing a so-called non-extinguishing “flash” - a difficult to remove anode effect. A decrease in the concentration of boron in the electrolyte to 0.005 wt.% And below allows us to switch to the production of aluminum as a commercial product that meets the requirements of standards and consumers.

A comparative analysis of the proposed solution with the closest analogue revealed the following. The proposed solution and the closest analogue are characterized by similar features:

- materials containing boron and refractory metals are fed into the electrolyzer;

- metal- and boron-containing components are introduced into the electrolyte with their subsequent dissolution and electrochemical reduction of ions on the cathode surface;

- it is recommended to initially introduce titanium-containing compounds into the electrolyte and to deposit titanium on the cathode surface;

- a wettable protective coating of refractory metal borides is created directly during electrolysis.

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

- during the creation of the joint venture at the cathode, metal and boron-containing components are fed into the electrolyzer in an amount that ensures a decrease in the metal content in the electrolyte from 5.0 to 0.1 wt.% and boron in the electrolyte from 0.2 to 0.005 wt.%, and in a known technical solution, the content of boron and refractory metals in aluminum is brought and maintained to saturation, which can lead to contamination of commercial aluminum and a breakdown in electrolysis technology;

- preparation for the creation of a joint venture is carried out by the initial introduction of metal-containing components into the electrolyte in the absence of aluminum alumina in the bath and the aluminum is coprecipitated on the carbon cathode surface, and in the known technical solution, refractory metals are introduced in the presence of alumina in the electrolyte, aluminum in the bath and aluminum is simultaneously deposited and refractory metals on the aluminum cathode surface, which leads to the formation of aluminum carbide on the carbon cathode and, therefore, uneven growth TiC layer on the bottom area and the deterioration of its adhesion to the carbon cathode;

- the creation of a joint venture at the second stage is accompanied by the introduction of only boron-containing components into the electrolyte without the presence of aluminum and the reaction of boron with a surface carbide-titanium layer on the carbon cathode, while in the known technical solution, metal and boron-containing components are introduced with coprecipitation of titanium and boron on the aluminum cathode, which is inevitably accompanied by the interaction of boron and titanium with each other and with aluminum to form dense boride crystals (e.g., TiB ₂ and AlB ₁₂ ) and intermetallic compounds (e.g., AlTi ₃ ), precipitation by the deposition of these crystals into the near-cathode layer of aluminum without interaction with the bottom carbon and without the formation of a boride wettable coating.

The presence in the proposed technical solution of signs that are distinct from the signs characterizing the closest analogue allows us to conclude that the proposed patentability condition is “novelty”.

In the process of searching and comparative analysis, no technical solutions were identified that would be characterized by a combination of features similar to the proposed technical solution and would give similar results when used, which allows us to conclude that the proposed solution meets the patentability condition of the invention “inventive step”.

Testing and testing the proposed solution to create a wettable coating of the carbon hearth was carried out in laboratory conditions at plants with the starting conditions of an aluminum electrolyzer.

As the anode and crucible for the electrolysis cell, blanks from industrial hearth blocks were used. The dimensions of the crucible made it possible to melt about 1000 g of the electrolyte of the claimed composition and to establish the MPR between the electrodes of 25-30 mm. Samples of hearth blocks with sizes образцы = 30 and h = 50 mm were also used as carbon cathodes. Using threaded contact, the samples were connected to the cathode current supply and after melting the dry electrolyte mixture with the addition of electroactive components (Ti and B) they were lowered into the melt. The installation was connected to electrolysis. At the end of the stages of creating a joint venture to check the wettability of the cathode, alumina was introduced into the electrolyte for the electrodeposition of aluminum on the cathode. After the experiment, the cathode located above the anode was removed from the cell, cleaned of the electrolyte, photographed, and sent to the XRD. Test conditions and XRD results of the surface layer of cathode samples after experiments on electrodeposition and the synthesis of wettable coatings are presented in the table.

Electrodeposition test parameters and XRF results

Test parameters Test number one 2 3 four Stages Stage 1 2 stage Stage 1 2 stage Stage 1 2 stage Stage 1 2 stage Test temperature ° C 990 990 970 970 Current density, A / cm ² 0.7 0.82 0.82 1.3 Duration, hour 40 42 48 48 - 1 stage deposition of titanium twenty twenty 24 24 - 2 stage deposition of boron twenty 22 24 24 The composition of the electrolyte: Cryolite ratio 2,5 2,5 2,5 2,5 Al ₂ O ₃ initial concentration,% 0 0 0 0 TiO ₂ initial concentrate.% 9,4 4.4 6.5 8.0 Additives Na ₂ B ₄ O ₇ · 10H ₂ O,% 3.0 1,2 1.8 1,5 The frequency of additives, hour 3 3 3 3 The XRD results of the surface of the samples (wt.%)

Phase Mineral one 2 3 four TiB ₂ 1.44 2.44 1.47 Ti ₆ C _3.75 1,54 1.93 Ti ₈ C ₅ 3.46 TiC _0.981 Khamrabaevite 2,30 SaB ₆ 1.22 4.26 0.66 TiO ₂ Rutile syn 0.93 0.58 TiO ₂ Anatase 1.47 TiO 0.60 Ti 1.65 Na ₃ AlF ₆ Cryolite syn 2.91 35,4 27.7 34.6 Na ₅ Al ₃ Fl ₄ Chiolite syn 43.6 5.65 17.8 0 NaF Villiaumite syn 2.86 7.60 CaF ₂ Fluorite 0.54 0.48 C Graphite-2 \ ITH \ RG 8.7 38.1 34.6 23.7 Al Aluminum, syn 8.46 3.10 10.1 Al ₂ O ₃ Comndum syn 21.1 1.47 1.23 5.03 Al ₄ C ₃ 3.82 5.27 17,2 CaTiO ₃ Perovskite, syn 0.7 Unknown phase (crystal) 7.5 6.44 The x-ray was taken on an automated X-ray diffractometric equipment company Shimadzu XRD-6000 (radiation CuK _α , graphite monochromator). X-ray powder diffraction was carried out using an information retrieval system for x-ray phase identification of materials (IPS RFI), combining qualitative and semi-quantitative analysis. For semi-quantitative SAR, the fundamental parameter method was used (wave X-ray fluorescence spectrometer from Shimadzu XRF-1800). Note: the number of compounds of the wettable coating (TiB ₂ , TiC) is insignificant due to the need to remove a 1-3 mm layer from the cathode surface for analysis.

In experiment 1, an initial concentration of titanium in the electrolyte of more than 5 wt.% (9.4 wt.% In terms of TiO ₂ ) was used, and in the second stage, the concentration of boron in the electrolyte is more than 0.2 wt.%. During the tests, the normal electrolysis process was periodically interrupted by a sharp increase in voltage - anode effects. This is due to the formation of large amounts of lower titanium oxides in the first stage and the formation of sparingly soluble compounds of the type 2Al ₂ O ₃ · B ₂ O ₃ in the second stage, which passivate the surface of the electrodes. These compounds were detected by XRD precipitation after the experiment. As a result, the cathode was not wetted (see photo 1).

In experiments 2-4, the concentrations of electroactive components were used in accordance with the declared limits. After testing, each of the samples revealed a wetting effect - an aluminum layer with satisfactory adhesion to the cathode surface (see photo 2-4). The reason for the wetting was detected by XRD - the presence in the surface layer of cathodes of borides and titanium carbides.

The test results indicate the fundamental possibility of commercial implementation of the proposed method for producing and maintaining a wettable coating on the carbon blocks of the cathode device of the electrolyzer for aluminum production.

The proposed method for creating a wettable coating of the carbon bottom of an aluminum electrolyzer is preferably as follows:

1. Filling the cell of the electrolyzer after firing with molten cryolite taken from the mother cells before the next technological treatment to minimize the alumina content in the electrolyte.

2. The introduction into the electrolyte of additives of refractory metal in the form of its oxides at the upper declared concentration limit with the expectation of the required duration of the first stage of creating a wettable coating.

3. Electrochemical deposition of refractory metal on a carbon cathode with standard electrolysis parameters in the start-up period, followed by synthesis of refractory metal carbide on the surface, micro and macro-irregularities, cracks and pores of bog blocks.

4. Upon reaching the lower declared limit of the concentration of the refractory metal, the introduction of the boron-containing component into the electrolyte according to the upper declared concentration limit with the expectation of the required duration of the second stage of creating a wettable coating.

5. Electrochemical deposition of boron at the cathode with standard parameters of electrolysis in the start-up period, followed by the synthesis of refractory metal boride on the surface of the hearth.

6. Upon reaching the lower declared limit of boron concentration in aluminum, the process flow of the electrolyzer is brought into operation with standard parameters.

The proposed method is an effective and reliable technical solution for creating an aluminum-wettable coating of the carbon bottom of an aluminum electrolyzer and, ultimately, increasing the technical and economic indicators of electrolysis. The method of creating a joint venture can be applied to the existing technology for the electrolysis of cryolite-alumina melts and to create a technology of new generation electrolyzers with a drained cathode.

Information sources

1. Watson K.D. et al. Electrolysis Cell and Method for Metal Production. // US petent 5658447, 1997.

2. Abakumov A.M. and others. A method of applying a wettable coating of the bottom of an aluminum electrolyzer. // Patent of Russia RU 2299278, 2007.

3. Townsend D.W. Supersaturation plating of aluminum wettable cathode coatings during aluminum smelting in drained cathode cells. // US patent 5028301, 1991.

Claims (5)

1. A method of creating a wettable coating of the carbon bottom of an aluminum electrolyzer, including high-temperature electrochemical deposition of coating components from a molten electrolyte and the synthesis of carbides and borides of refractory metals on the surface of carbon-graphite blocks of the hearth, characterized in that the coating components are supplied to the electrolyzer, their electrodeposition and synthesis of coating compounds in stages with the initial supply of refractory metals to the electrolyte melt, their electrodeposition and synthesis of car ide refractory metals on carbon and graphite blocks, and then supplying the boron in the molten electrolyte, its synthesis and electrodeposition coating compounds in the refractory metal-boron-carbon and the refractory metal-boron on the hearth surface. 2. The method according to claim 1, characterized in that titanium, zirconium, tungsten, molybdenum, vanadium or a mixture thereof are used as refractory metals. 3. The method according to claim 1, characterized in that the refractory metals and boron are fed into the electrolyte in the form of pure elements, or their salts and oxides. 4. The method according to claim 1, characterized in that the duration of the first stage is determined by reducing the concentration of the refractory metal in the electrolyte from 5.0 to 0.1 wt.%. 5. The method according to claim 1, characterized in that the duration of the second stage is determined by reducing the concentration of boron in the electrolyte from 0.2 to 0.005 wt.%.

Images