RU2359071C2 - Operating procedure of electrolyser for aluminium manufacturing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to non-ferrous metallurgy, particularly to operation of electrolyser for aluminium receiving with application of acid electrolytes. Method includes feeding of alumina on electrolyte crust or in low doses under the electrolyte crust, definition of electrolyte compound, including cryolite relation, detected as molecular relation NaF/AlF₃, and content of calcium fluoride, updating of electrolyte in sprecified range, herewith from the moment of electrolyser launch it is implemented gradual change of electrolyte composition with reduction of cryolite relation up to target value, equal to 2.15±0.15 relative units, herewith simultaneously in electrolyte it is increased content of calcium fluoride up to value no more than 8.5 wt %, and overheating of electrolyte, defined as difference between actual temperature of electrolyte and corresponding liquidus temperature, is kept in the range 13-22°C for electrolyser with feeding of alumina on electrolyte crust, and for electrolyser with feeding of alumina in low doses under the electrolyte crust- in the range 7-15°C, herewith electrolyser operation is implemented at anodic current density 0.7÷0.9 A/cm².

EFFECT: increasing of engineering-and-economical performance of aluminium electrolysis with application of acid electrolytes with KO at the level or less 2,3 relative units with anodic current density 0,7÷0,9 A/cm².

4 cl, 1 ex

Description

The invention relates to ferrous metallurgy, in particular to the operation of an electrolytic cell for producing aluminum using acidic electrolytes.

Currently, there is a tendency in aluminum smelters in Russia and abroad to use electrolytes with a low cryolite ratio (KO), which allows reaching high technical and economic indicators of electrolysis.

So, in well-known US patent No. 3852173 "Method for the reduction of aluminum" (03.12.74, C22d 3/12) and No. 3951763 "Method for the reduction of aluminum" (04.20.76, C25C 3/06) acidic electrolytes with a weight the ratio of NaF to AlF ₃ 1.1 ÷ 1.

US Pat. No. 3,996,117, “Method for the Production of Aluminum” (12/07/76, C25C 3/06) also discusses the composition of acidic electrolytes, but with a weight ratio of NaF to AlF _{3 of} 1.04-1.15 with the addition of 5 to 5 to the electrolyte 10 wt.% LiF.

For electrolyzers with inert anodes, US Pat. No. 5,415,742, “Process and Apparatus for Low-Temperature Electrolysis of Oxide”, proposes an electrolyte capable of operating in the temperature range of 685-900 ° C. and containing 36 wt.% NaF

and 64 wt.% AlF ₃ .

A decrease in KO is accompanied by a decrease in the temperature of the electrolyte and a lower concentration of aluminum dissolved in the electrolyte. Both the one and the other factor, as well as the addition of fluorides, such as CaF ₂ , lead to a decrease in the rate of dissolution of alumina in the electrolyte, which complicates the electrolysis technology. The first requirement for such electrolytes is the alumina solubility acceptable for production (at least 4 wt.%) And its dissolution rate (at least 0.04 mol / m ³ s). (“Aluminum of Siberia - 2006”, p. 46. VA Kryukovsky, “Study of the physicochemical properties of potassium cryolite as the main component of an electrolyte for producing aluminum”).

In the article by Puzanov I.I. et al. “Measurements of electrolyte overheating at OJSC“ SAZ ”for the development of a method for controlling the thermal regime of an electrolyzer” (“Aluminum of Siberia - 2005”, p. 52) indicated that “Further work to reduce the cryolite ratio showed that the current level of electrolysis technology (ceteris paribus), with a cryolite ratio of 2.3 or less, does not provide the optimal heat balance. "

The ability to work effectively with CO at or below 2.3 is ensured by the start-up of the electrolyzer and the features of the first year of its operation. Both of these periods of operation of the electrolyzer should proceed under strict control of the technological parameters provided for by the technological regulations (technological instructions).

The well-known "Method of operating aluminum electrolytic cells" according to the patent of the Russian Federation No. 2104334 (C25C 3/06 of 03/21/1996). According to this patent, the control adjustable electrolysis parameters are:

- cryolite ratio (molecular ratio NaF / AlF ₃ ), rel.

- the content in the electrolyte of calcium fluoride,%;

- average cell voltage, V;

- the height of the cathode aluminum layer, see

For the control and subsequent adjustment of the controlled parameters of the electrolysis process, a number of empirical formulas are proposed, including correction coefficients determined experimentally.

This technical solution was chosen for the prototype (the closest analogue), as the closest in technical essence and the presence of similar features.

The prototype method relates to the operation of the electrolyzer after reaching the optimal target technological mode and is aimed at stabilization within the stated limits of the main controlled indicators. The method of operation of the electrolyzer according to the prototype assumes that the target technological mode is established at the end of the post-launch period, and then the conduct of the electrolysis process involves only corrective actions to stabilize this mode.

Such process management does not allow electrolysis to be realized on “acidic” electrolytes with KO at or below 2.3. Reaching low CO indicators should be gradual, as indicated in the article by Vasyutin S.A. and others. "Research and development of an optimal schedule for lifting metal in the first year of operation of the electrolyzer" ("Aluminum of Siberia - 2005", p.22).

Also a significant disadvantage of the prototype is the lack of control of overheating of the electrolyte. It is known that in order to achieve high electrolytic cell productivity and reduce electric power consumption, it is necessary to use not only the traditional analysis of the electrolyte composition for cryolite ratio (KO) and corrective additives, for example CaF ₃ , but also measurements of electrolyte temperature and liquidus temperature in controlling the heat and energy balance of the electrolyzer. Overheating of the electrolyte (the difference between the working temperature of the electrolyte and the liquidus temperature) affects the condition of the crust and the skull, the solubility conditions of alumina.

The objective of the proposed solution is to increase the technical and economic performance indicators of electrolytic cells for aluminum production.

The technical result is the implementation of aluminum electrolysis in acidic electrolytes with KO at the level of or less than 2.3 relative units with the achievement of high technical and economic indicators on any type of electrolytic cell with an anode current density of 0.7 ÷ 0.9 kA.

The technical result is achieved by the fact that in the method of operation of the electrolyzer for the production of aluminum from cryolite-alumina melt, including feeding alumina to the electrolyte crust or in small doses under the electrolyte crust, determining the electrolyte composition, including the cryolite ratio, defined as the molecular ratio NaF / AlF ₃ , and the content of calcium fluoride, the correction of the electrolyte in a given interval, from the moment the electrolyzer is started, a gradual change in the composition of the electrolyte with a decrease in the "cryolite ratio "to the target value of 2.15 ± 0.15 rel. units, while simultaneously increasing the content of calcium fluoride in the electrolyte to a value of not more than 8.5 wt.%, and the electrolyte overheating, defined as the difference between the actual temperature of the electrolyte and the corresponding liquidus temperature, support within 13-22 ° C for an electrolyzer with alumina feeding to the electrolyte crust, and for an electrolyzer with small doses of alumina feeding under the electrolyte crust, within 7-15 ° C, and operation of the electrolyzer is carried out at anode density current 0.7 ÷ 0.9 A / cm ² .

The claims include two independent points due to the presence of electrolysis features with a different method of supplying the electrolyzer with alumina and corrective additives, specifically: feeding through the APG systems (automatic feeding of alumina) in small doses under the electrolyte crust or flow to the electrolyte crust.

The technical essence of the proposed method is as follows.

The interest in low-temperature (“acidic”) baths for the electrolysis of alumina is associated with their unconditional advantages, the main one being the ability to reach high technical and economic indicators of electrolysis, for example, the current efficiency approaches the maximum possible value. So, in RF patent No. 2234558 “Method for conducting electrolysis in aluminum electrolytic cells with calcined anodes” (С25С 3/06, dated February 19, 2003), the current efficiency index is 94.9% at KO = 2.3, the current strength is 85 kA , voltage 4.3 V, anodic current density 0.85 A / cm ² , electrolyte temperature 950 ° C, electrolyte level 20 cm, metal level 9 cm.

But at the same time, acidic electrolytes have a number of significant drawbacks, including a decrease in the solubility of alumina and its dissolution rate, as well as working with them requires more precise control of the heat balance of the electrolyzer. Too high an overheating creates a threat of melting of the accretion, damage to the airborne units, and is also accompanied by increased energy consumption. Low overheating will not provide acceptable alumina dissolution.

According to the work of Mikhalev Yu.G. et al. “The influence of cryolite ratio, electrolyte overheating and potassium fluoride additives on the dissolution rate of alumina” (“Aluminum of Siberia 2005”, Collection of reports of the XI International conference edited by P.V. Polyakov), as well as data from foreign experts, the dissolution rate of alumina directly proportional to the increase in electrolyte overheating only up to 15 ° C. At higher superheats, the increase in the dissolution rate is negligible. This is due to the fact that at high overheating and high electrolyte temperature, the true value of KO increases due to an increase in the volatility of aluminum fluoride. An increase in KO causes an increase in the temperature of the liquidus, and therefore, a decrease in the electrolyte overheating. To achieve maximum current output values, work with high overheating is impractical. Therefore, to achieve high electrolysis rates, it is necessary to strictly maintain the specified electrolyte overheating in a fairly narrow predetermined range. This is especially important when working at low KO.

With the existing electrolysis technology in the cryolite-alumina melt during the start-up of the cell and in the post-start-up period, it is necessary, firstly, to ensure that sufficient heat arrives, and, secondly, to create more refractory nastily and skull in order to ensure a stable workspace form. In this regard, to start on acidic electrolytes with KO <2.7 ÷ 2.5 is impractical. In this case, the yield on the target electrolyte with KO = 2.15 ± 0.15 rel. should be carried out smoothly, preferably with the same step, for example, 0.05 rel. Moreover, practice has shown that the rate of decrease in CO in the initial period (~ to 2.4 rel.units) can be significantly higher than in the subsequent one.

A phased reduction of QoS is carried out before reaching the target value of QoS. The research work carried out by the authors of the proposed technical solution on industrial electrolyzers showed that a current output of more than 90% can be obtained at KO = 2.15 ± 0.15. With a decrease in KO below 2 rel.ed, a general breakdown of technology was observed, which is confirmed by foreign data. So, in the aforementioned US patent No. 5415742 it is stated that “... attempts to use a salt electrolyte at lower temperatures by using weight ratios of the electrolyte below the usual ratio NaF: AlF ₃ = 1.1: 1 were stopped by the formation of a crust of solidified electrolyte over molten aluminum in electrolysis. This crust greatly increases the resistance of the cathode, reduces the connection of the metal and causes the precipitation of sodium, which, in turn, reduces the current efficiency. Under such conditions, the cell cannot be used effectively. ”

When working on acidic electrolytes, tight control of the heat balance of the electrolyte is necessary. In the context of a gradual decrease in KO, the electrolyte liquidus temperature, which depends on the electrolyte composition, decreases. In this connection, in order to achieve stable high electrolysis rates in the context of a planned decrease in KO, it is necessary to control the heat balance of the electrolyte through stabilization of overheating within the specified technologically feasible limits. Under conditions of sustained overheating, the optimum solubility of alumina and its dissolution rate are ensured. Practice has shown that the set limits of overheating should vary depending on the design of the electrolyzer, the period of its operation, raw materials, as well as the method of supplying alumina and corrective additives to the electrolyzer.

On electrolyzers equipped with APG systems, the feed of raw materials is carried out in small doses, so the level of overheating can be reduced to 15 ° C, as indicated above. The lower claimed limit of overheating at 7 ° C is due to a minimum of heat influx, ensuring the normal course of the electrolysis process.

In the case of continuous processing of electrolyzers, the superheat of the electrolyte must be increased to compensate for heat loss for heating a volley dose of raw materials. It is practically determined that the optimal overheating interval is 13-22 ° C.

The boundary values of the above-mentioned overheating intervals are justified by the following: overheating above the upper limit will not allow reaching high current efficiency indicators due to an increase in unproductive heat losses, and overheating below the lower limit can lead to a breakdown of the technology as a whole.

With decreasing temperature of the electrolyte, its electrical conductivity decreases. To compensate for this decrease, it is proposed to increase the CaF ₂ content in the electrolyte. A.I. Belyaev in "Electrometallurgy of Aluminum" (Moscow, 1953) indicates that the electrical conductivity of the molten electrolyte passes through a maximum corresponding to 10 wt.% CaF ₂ . A further increase in the content of calcium fluoride is also impractical due to an increase in the density of the melt and the convergence with the density of the metal. In the same place, A.I. Belyaev notes that CaF ₂ is a surface-inactive compound, that is, it increases the surface tension of the cryolite melt at the coal boundary, reducing the likelihood of this melt being absorbed into the carbon lining of the bath. This is confirmed by practice: it is noted that the electrolytes of starting baths containing CaF ₂ do not become so "acidic" (that is, they have a smaller excess of AlF ₂ ) than the electrolytes of baths that are started without CaF ₂ additives. This is due to a decrease in the absorption of NaF into the coal bottom. In this regard, the authors propose to increase the content of calcium fluoride gradually, starting from the minimum allowable contents, due to the features of the start-up of the electrolyzer, but not more than 8.5%.

It is known that in order to increase the metal production time, it is advisable to conduct the electrolysis process at elevated current densities. It is known that at present a number of foreign companies conduct electrolysis at an anode current density of more than 1 A / cm ² . When working on acidic electrolytes, the authors determined the optimal interval of the anode current density of 0.7-0.9 A / cm ² .

Comparison of the proposed method of operation of the electrolyzer for the production of aluminum from cryolite-alumina melt with the closest analogue (prototype) revealed the presence of a number of similar signs:

- both methods are aimed at optimizing the electrolysis process by stabilizing the controlled process parameters in predetermined optimal limits;

- both methods are implemented in cryolite-alumina melt;

- in both methods use corrective additives;

- in both methods carry out a systematic determination of the composition of the electrolyte, including the cryolite ratio, defined as the molecular ratio of NaF / AlF ₃ , and the content of calcium fluoride.

However, the proposed method is characterized by the following distinct from the closest analogue features:

- starting the electrolyzer is carried out on an electrolyte with a cryolite ratio higher than that of the target electrolyte, that is, higher than 2.15 ± 0.15 relative units;

- the composition of the electrolyte is changed in order to reduce KO;

- reduction of KO is carried out gradually to the target value of 2.15 ± 0.15 rel.ed .;

- the content of CaF _{2 is} gradually increased to a value of not more than 8.5%;

- control the overheating of the electrolyte;

- in electrolyzers with APG systems, overheating is maintained within 7-15 ° C;

- in electrolyzers with continuous processing support overheating in the range of 13-22 ° C;

- operation of the electrolyzer is carried out at an anode current density of 0.7-0.9 A / cm ² .

The proposed technical solution is characterized by features that are similar to those of the closest analogue, and distinctive features, which allows us to conclude that it meets the patentability condition of "novelty."

A comparative analysis of the proposed technical solutions with known solutions in this technical field, identified in the search process in patent and scientific literature, revealed the following:

1. Conducting electrolysis in accordance with the technological regulations, providing for a change in the control parameters of electrolysis during the year from the start up to reaching the target controlled parameters, it is known, for example, from the article by S. A. Vasyutin et al. year of operation of the electrolyzer ”(“ Aluminum of Siberia-2005 ”, Collection of reports of the XI International Conference edited by P.V. Polyakov). The article indicates a reduction in KO from 2.7 to 2.4 smoothly during the year from the start, which, according to the authors of the article, provides better impregnation of the hearth with sodium, the creation of more refractory nastily, which allows you to form and maintain the correct shape of the workspace . The electrolyte temperature also decreases smoothly throughout the year from 970 ° C to 955 ° C. Moreover, the authors consider the achieved KO and electrolyte temperatures to be the minimum possible for S-8BM electrolyzers with Soderberg anodes.

In the proposed solution for the same type of electrolyzers, it is provided, firstly, to reduce the KO to a lower value of 2.15 ± 0.15, and secondly, the electrolysis process is controlled not by the temperature of the electrolyte, but by the temperature of its overheating.

2. The conduct of electrolysis at KO at or below 2.3 is known from the analogs given in the corresponding section of the description, as well as from RF patent No. 2234558 “Method for conducting electrolysis in aluminum electrolyzers with calcined anodes” (patent holder of OJSC NAZ), according to which KO support in the range of 2.2-2.3.

According to an article by Mikhalev Yu.G. “Anode overvoltage in acidic electrolytes of aluminum electrolysis cells” (“Aluminum of Siberia-2006”, Collection of reports of the XI International conference edited by P.V. Polyakov), the electrolysis process in the laboratory was carried out at KO = 2.1-2.2 rel. . But the industrial output for this KO has not been achieved, in contrast to the proposed solution.

Also, the use of various low-temperature electrolytes in combination with various solutions regarding the anode is known from US Pat. No. 5,415,742, “Process and Apparatus for Low-Temperature Electrolysis of Oxides”: “Preferred salt electrolytes used in connection with this invention include at least one alkali metal fluoride, different from LiF (for example, NaF), and at least one additional metal fluoride such as aluminum fluoride, calcium fluoride, magnesium fluoride or another metal fluoride. Low temperature salt electrolytes are used at temperatures preferably below 900 ° C, most preferably in the range from 685 ° C to 850 ° C in the case of aluminum production. In a preferred embodiment, the salt electrolyte contains NaF and AlF ₃ , preferably 30-60 mol% NaF, and more preferably contains a molar ratio of NaF: AlF _{3 of} about 1: 2 to 1: 3. In one of the most preferred embodiments of the invention, the salt electrolyte contains a mixture of NaF and AlF ₃ in a weight ratio of about 0.5-1.2 (KO 1-2.4) NaF: AlF ₃ . In one of the most preferred embodiments of the invention, the salt electrolyte contains about 36 wt.% NaF and about 64 wt.% AlF ₃ . In one of the most preferred embodiments of the invention, this salt electrolyte is used without any other additives in a weight ratio of 36% / 64%, which corresponds to the eutectic composition of the NaF / AlF ₃ melt. The eutectic mixture has a melting point of about 695 ° C. "

From US patent No. 5114545 "The chemical composition of the electrolyte to improve the characteristics of modern industrial electrolyzers for the reduction of alumina" it is known that preference is given to acidic electrolytes with a weight ratio of NaF to AlF ₃ less than 1 (KO less than 2).

3. According to an article by Mikhalev Yu.G. “Anode overvoltage in acidic electrolytes of aluminum electrolysis cells” (“Aluminum of Siberia-2006”, Collection of reports of the XI International conference edited by P.V. Polyakov), experiments were carried out with a constant overheating of 15 ° С. But there is no industrial outlet for overheating at 15 ° C.

4. The use of the indicator of electrolyte overheating as the most objective indicator in controlling the heat balance of an electrolyzer is known, for example, from an article by Torsten Rieck et al. “Increasing current efficiency and reducing energy consumption at the TRIMET plant in Essen by using controls based on a nine-dimensional matrix table” ("Light metals", 2003). And also from the article by A. M. Nadtochy, L. V. Ragozin and others. “Control of the electrolyzer by regulating the heat balance” (Materials of the scientific-practical conference of OJSC “SUAL”, branch of “IrkA3-SUAL, 2004)

A comparative analysis of the proposed solution with solutions in this area shows the presence of a new set of quantitative indicators of the electrolysis process, including CO, overheating and the content of corrective CaF ₂ , which ensures the normal course of the electrolysis process in industrial conditions with the achievement of high technical and economic indicators when using acidic electrolytes with cryolite the ratio of 2-2.3. Given the foregoing, we can conclude that the proposed technical solution meets the condition of patentability "inventive step".

Compliance with the patentability condition “industrial applicability” is proved by the fact that the authors carried out experiments on industrial electrolyzers of VgAZ. According to the results of the experiment, a "Technological Instruction" was developed, which allows to implement the proposed method of operation of the electrolyzer.

An example of the implementation of the method according to the proposed solution in relation to S8BM electrolyzers:

1. The cryolite ratio in electrolyzers with a service life of more than 14 months is 2.18 ± 0.05.

2. On electrolyzers with a service life of less than 14 months, the cryolite ratio is maintained in accordance with the attached schedule:

Term service 0.5-1 1-2 2-3 3-6 6-8 8-10 10-12 12-14 (month) KO 2,55 2.5 2.45 2,4 2,35 2,3 2.25 2.2

3. The content of calcium fluoride in electrolyzers with a service life of more than 12 months is 8.2 ÷ 8.4%.

4. On electrolyzers with a service life of less than 12 months, the content of calcium fluoride is maintained in accordance with the attached schedule:

Service life (months) 0-6 6-9 9-12 The content of CaF ₂ (%) 6.0-6.5 6.5-7.5 7.5-8.0

5. The working temperature of the electrolyte is 950 ± 7 ° C.

6. The temperature of the superheat of the electrolyte for baths with a flow treatment of 13 ÷ 22 ° C.

7. The temperature of the superheat of the electrolyte for baths equipped with APG, 7 ± 15 ° C.

Subject to the above requirements, the following technological indicators were achieved on the experimental bath:

- current efficiency - 90.14%;

- access to the bath-day - 1191.2 t / day;

- current strength - 163880 A.

Thus, the proposed solution has successfully passed pilot tests at the Volgograd aluminum smelter and can be recommended for widespread implementation on an industrial scale.

Claims (4)

1. The method of operation of the electrolyzer for the production of aluminum from cryolite-alumina melt, including the supply of alumina, determining the composition of the electrolyte, including the cryolite ratio, defined as the molecular ratio of NaF / AlF ₃ , and the content of calcium fluoride, the correction of the electrolyte with corrective additives in a given interval, characterized in that the supply of alumina is carried out on the crust of the electrolyte and from the moment of starting the electrolyzer produce a gradual change in the composition of the electrolyte with a decrease in the cryolite ratio to the target value (2.15 ± 0.15) rel.ed., while simultaneously increasing the content of calcium fluoride in the electrolyte to a value of not more than 8.5 wt.%, and the electrolyte overheating, defined as the difference between the actual temperature of the electrolyte and the corresponding liquidus temperature, support within 13-22 ° C. 2. The method according to claim 1, characterized in that the operation of the electrolyzer is carried out at an anode current density of 0.7 ÷ 0.9 A / cm ² . 3. A method of operating an electrolyzer for the production of aluminum from a cryolite-alumina melt, including feeding alumina, determining the composition of the electrolyte, including the cryolite ratio, defined as the molecular ratio NaF / AlF ₃ , and the content of calcium fluoride, adjusting the electrolyte with corrective additives in a given interval, characterized in that the supply of alumina is carried out in small doses under the crust of the electrolyte and from the moment of starting the electrolyzer produce a gradual change in the composition of the electrolyte with a decrease in cryolite ratio to the target value (2.15 ± 0.15) rel.units, while simultaneously increasing the content of calcium fluoride in the electrolyte to a value of not more than 8.5 wt.%, and the electrolyte overheating, defined as the difference between the actual temperature of the electrolyte and the corresponding liquidus temperature, support within 7-15 ° C. 4. The method according to claim 2, characterized in that the operation of the electrolyzer is carried out at an anode current density of 0.7 ÷ 0.9 A / cm ² .