EP1344847A1 - Regulating of aluminium electrolysis cells - Google Patents

Abstract

Aluminum reduction cell refractory material comprises a bottom and side lining in the form of blocks, slabs and bricks. The bottom and side linings are made of carboceramic bonded by a tar-pitch-binder. An Independent claim is also included for a process for the manufacture of carboceramic blocks, pastes or bricks comprising: mixing at 150-200 degreesC fine calcined oxides with electrode pitch and powdery additives like silicon, nitrides, graphite or coke; molding or forming into preforms like saggars, brickets or extrusion billets; baking the preforms at temperatures of 1000-1200 degreesC for a time sufficient to produce at least 50% carbon oxides; crushing and screening the carbon oxide material into different particle fractions; mixing the carbon oxide particles at 50-200 degreesC with pitches and additives like silicon, nitrides, graphite or coke to form a green mix which can be used as rumming paste; and vibroforming the green mix to green blocks or baking the green mix to baked blocks and machining it into carboceramic blocks.

Description

The invention relates to a method for automatically regulating the bath of an electrolytic cell for the production of aluminum from aluminum oxide dissolved in a cryolite melt.

The technical production of primary or smelter aluminum usually takes place by melt flow electrolysis of aluminum oxide in an electrolyte essentially consisting of a cryolite melt (Na ₃ AlF ₆ ). The electrolyte or the bath is located in a steel liner lined with carbon, the carbon serving as the cathode. A variety of vertically movable carbon anodes are attached above the tub. The electrolysis decomposes the aluminum oxide into aluminum and oxygen, so that the aluminum oxide has to be continuously introduced into the electrolyte as a raw material.

A DC voltage of typically 2-5 V is applied between the anode and cathode. The specific electrical conductivity of cryolite at 1000 ° C is 2.8 ± 0.02 Ω ^-1 . cm ^-1 . When Al ₂ O _{3 is added} , the conductivity drops, which is why the Al ₂ O ₃ content in the melt is expediently set as low as possible, for example less than 4% by weight. Approximately 40% of the electrical energy is converted into heat as a result of the electrical resistance in the bathroom and enables the working temperature of the electrolyte to be maintained. The other part of the electrical energy causes the actual electrolysis, i.e. the deposition of the aluminum on the cathode and the oxygen on the anode. Since the liquid aluminum at the working temperature with a density of 2.3 g / cm ^{3 is} specifically heavier than the cryolite with a density of approx. 2.1 g / cm ³ , the aluminum collects at the bottom of the tub in a metal bath. The oxygen reacts with the carbon to form carbon monoxide and carbon dioxide, so that the anode burns off and is used up after a few weeks. At the top of the bath, a solid crust of cryolite and alumina forms, which must be pierced to add more alumina.

Due to the solubility of the aluminum oxide in molten cryolite, the melt flow electrolysis of the aluminum oxide (alumina) can be carried out at temperatures of approximately 940 ° C. to 980 ° C. with the addition of further melting agents. As a further melting agent, the cryolite melt contains - in addition to the aluminum oxide - substances which lower the melting point, such as, for example, AlF ₃ , LiF, CaF ₂ and / or MgP ₂ . The addition of AlF ₃ can lower the liquidus line of the ternary system cryolite / Al ₂ O ₃ / AlF ₃ with a quadratic relationship, with an addition of, for example, 10% by weight AlF _3, a decrease in the eutectic melting temperature by about 25 ° C causes. Because of the dependence of the eutectic melting temperature on the AlF ₃ concentration, it is obvious to work with high AlF ₃ concentrations.

• durch die tiefere Elektrolyttemperatur sind die Elektrolytkomponenten chemisch weniger agrressiv, so dass die Lebensdauer der Elektrolysezelle verlängert wird. Zudem wird der Anodenverbrauch erniedrigt.
• durch die tiefere Elektrolyttemperatur verringert sich die Löslichkeit von Aluminium im Elektrolyten, was eine höhere Stromausbeute ergibt;

The use of an electrolyte with a high AlF3 concentration and consequently lower eutectic melting temperature also shows further advantages:

• Due to the lower electrolyte temperature, the electrolyte components are less chemically aggressive, so that the service life of the electrolytic cell is extended. The anode consumption is also reduced.
• the lower electrolyte temperature reduces the solubility of aluminum in the electrolyte, which results in a higher current efficiency;

However, the addition of AlF ₃ reduces the specific electrical conductivity of the electrolyte, ie the electrical bath resistance is increased. The specific electrical conductivity of the electrolyte and the electrode spacing determine the cell voltage. Since the electrolysis cells are driven at constant current, the addition of AlF ₃ thus increases the electrolyte temperature, the electrolyte temperature increasing compared to the eutectic melting temperature, reducing the efficiency of the electrolysis cell and thus increasing the energy required to produce 1 kg of aluminum.

In order to optimize the efficiency of the electrolysis cells, a high excess of AlF _{3 is} preferred. This excess is expressed as a molar or weight ratio of NaF to AlF ₃ including the cryolite or as a percentage of the excess free AlF ₃ . The second variant is selected below. Nowadays, an excess proportion of AlF ₃ of more than 10% by weight is usually used.

For electrolytes with an AlF ₃ content of 10% by weight and above, fluctuations in the AlF ₃ concentration generally occur with a wavelength of typically several days, for example 10 to 30 days. The AlF ₃ content slowly fluctuates within wide limits, for example in the range from 6 to 20% by weight. These fluctuations in the AlF ₃ content are also associated with temperature fluctuations, for example in the range from 930 to 990 ° C., in accordance with the dependency described above.

Accordingly, a high AlF ₃ content in the electrolyte generally causes an unstable electrolyte temperature, which reduces the current efficiency, ie the efficiency, and the service life of the electrolytic cell.

Furthermore, there are fluctuations in electrolyte temperature, for example as a result of a change in the cathode-anode spacing because of the unavoidable anode consumption or because of fluctuations in the Al ₂ O ₃ concentration. Such fluctuations typically last from a few minutes to a few hours.

Evaporation and reaction with Na ₂ O contained in the alumina always result in a loss of AlF ₃ during aluminum electrolysis. AlF ₃ was previously added on the basis of tables which listed the units to be added as a function of the bath temperature and the AlF ₃ content to be set. In order to take into account the effective condition of the electrolysis cell and thus to improve the efficiency of the cells, it has been shown that optimal regulation and stabilization of the electrolysis cells necessitates the periodic measurement and consideration of the cell parameters.

EP-B1 0 455 590 describes a method for regulating an AlF ₃ content in the bath of an electrolysis cell for aluminum production which is at least 10% by weight, in which, based on an analysis of the state of the aluminum electrolysis cell, the optimal time shift between the addition of AlF ₃ and its effect in the electrolyte are determined and the addition of AlF _{3 is} calculated taking into account a specific AlF ₃ content in consideration of the time shift and is added to the electrolyte.

According to the teaching of EP-B1 0 195 142, the measurement of the AlF ₃ content in a stationary state of the electrolytic cell can be replaced by a temperature measurement. A substantially linear relationship between AlF ₃ concentration and electrolyte temperature applies in a range of ± 10 ° C around the temperature setpoint.

According to the prior art, either the AlF ₃ concentration or the electrolyte temperature is measured and regulated as a process variable in order to regulate the excess AlF ₃ in the bath of an electrolysis cell for aluminum production. Practice has now shown that regulating the AlF ₃ concentration alone does not guarantee optimal efficiency of an electrolysis cell.

The present invention is based on the object of a known state of the art the technology with regard to efficiency improved method for automatic Specify regulation of an aluminum electrolysis cell.

According to the invention, this object is achieved in that the efficiency of the aluminum electrolysis is optimized by analyzing the individual state of the electrolytic cell from a series of measured values comprising several parameters at certain intervals and, based on the current state and taking into account the earlier states, that of the further electrolysis process optimum electrolyte temperature and the optimal AlF3 excess concentration can be regulated by calculating and appropriately setting the electrolysis target voltage and AlF ₃ feed rate required for the further electrolysis process.

The process according to the invention thus solves the problem of controlling the electrolyte temperature and the AlF ₃ excess concentration in an aluminum electrolysis cell, according to which the AlF ₃ concentration in the electrolyte has a direct influence on the electrolyte temperature, and both process parameters affect the current yield and thus the energy consumption per kilogram of aluminum produced bein influence.

With regard to the method according to the invention, the term “optimal” relates to the best possible electrolyte temperature or AlF ₃ excess concentration for a given electrolytic cell, which economically yield the highest possible efficiency.

The individual condition of the electrolytic cell at a specific point in time is determined by analyzing a series of measured values comprising several parameters. These include, for example, the electrolyte or bath temperature, the bath composition, the set electrolysis voltage, the schedule for changing the anode, etc. Based on these measured values for the current state of the electrolytic cell and taking into account the previous states, the method according to the invention is used to achieve the desired course the electrolyte temperature and the AlF ₃ excess concentration required electrolysis target voltage and the AlF ₃ supply rate are calculated and regulated accordingly.

The method according to the invention permits the automatic regulation of the electrolyte temperature and the AlF3 excess concentration in the bath of an electrolysis cell for the production of aluminum from aluminum oxide dissolved in a cryolite melt, while at the same time minimizing the fluctuations in the AlF ₃ content and the electrolyte temperature,
the AlF ₃ concentration being regulated without the inclusion of neutralizing additives, such as, for example, soda or sodium fluoride.

a) Berechnung einer Standardzugabe F₀(t) in Funktion des Elektrolysenzellenalters t und des Nennstromes I_N, wobei F₀(t) den mittleren AlF₃-Bedarf pro Zeiteinheit für die im stationären Zustand bei einer vorgegebenen Elektrolyt-Solltemperatur T_soll arbeitende Elektrolysezelle bezeichnet;

b) Sofern ein C_n-Wert gemessen und der gemessene C_n-Wert innerhalb vorbestimmter Grenzwerte liegt, Berechnung einer ersten Korrektur F_n aufgrund der Abweichungen von T_n und C_n von entsprechenden, vorgegebenen Sollwerten T_soll und C_soll gemäss der Massgabe: Fn = kT (Tn-Tsoll) - kC (Cn-Csoll),    wobei k_T und k_C für die verwendete Elektrolysezelle charakteristische, vorbestimmte Parameter bezeichnen; und sofern kein innerhalb vorgegebener Grenzwerte liegender C_n-Wert verfügbar ist, Berechnung einer ersten Korrektur F_n aufgrund der Abweichung von T_n vom vorgegebenen Sollwert T_soll gemäss der Massgabe: Fn = kL (Tn-Tsoll),    wobei k_L ein für die verwendete Elektrolysezelle charakteristischer, vorbestimmter Parameter bezeichnet;

c) Ermittlung der während dem Intervall n und dem Intervall n-1 gemessenen, mittleren Elektrolysespannung U_M;

d) Ermittlung der mittleren, während dem Intervall n und dem Intervall n-1 täglich zugegebenen AlF₃-Menge;

e) Für den Fall, dass

(i) die gemäss c) ermittelte, mittlere Elektrolysespannung U_M um nicht mehr als einen vorgegebenen Wert ΔU von einer Standardspannung Us abweicht, und

(ii) die gemäss d) ermittelte, mittlere tägliche Zugabe von AlF₃ geringer als ein vorgegebener Wert C_m ist, Berechnung einer zweiten Korrektur F_Trend gemäss der nachfolgenden Massgabe: FTrend = kTrend · Tn -Tn-1 Δt    wobei Δt die Zeitspanne zwischen der Messung n und der Messung n-1 in Tagen und k_Trend einen für die verwendete Elektrolysezelle charakteristischen, vorbestimmten Parameter bezeichnen; und
falls die Bedingungen (i) und (ii) nicht erfüllt sind, F_Trend = 0 gilt;

f) Berechnung einer neuen, für das während dem darauf folgenden Intervall n+1 gültige AlF₃-Zufuhrrate A_n+1 gemäss der nachfolgenden Massgabe: An+1 = F0(t) + Fn + FTrend,

g) Falls A_n+1 kleiner oder gleich einem vorgegebenem, oberen Grenzwert Ao und grösser oder gleich einem vorgegebenem, unteren Grenzwert A_U ist, und der während dem Intervall n gemessene Wert C_n kleiner als ein vorgegebener Maximalwert C_max ist, Zufuhr von AlF₃ gemäss dem Wert A_n+1 während dem nachfolgenden Intervall n+1, und falls A_n+1 grösser als A_o, Zufuhr von AlF₃ gemäss dem oberen Grenzwert A_o, und falls A_n+1 kleiner als A_U, oder C_n grösser oder gleich dem Wert C_max ist, Zufuhr von AlF₃ gemäss dem unteren Grenzwert A_U;

h) Periodische Wiederholung der Verfahrensschritte a) bis g) bis die Elektrolyse beendet wird.

The supply rate of AlF ₃ is preferably calculated by measuring the electrolyte temperature T _n and preferably also the percentage C _n of free AlF ₃ in the electrolyte at predetermined intervals n and an AlF ₃ supply rate A _{n +} in the case of a normally working cell ₁ for the interval n + 1 is calculated according to the following process steps and the corresponding amount of AlF _{3 is added to} the electrolyte in portions or continuously during the interval n + 1 :

a) calculating a standard addition F ₀ (t) in function of the electrolytic cells age t and the rated current I _N, where F ₀ (t) the average AlF ₃ -bedarf per unit time for the stationary state at a predetermined electrolyte nominal temperature T _nom operating electrolytic cell designated;

b) If a C _n value and the measured C _n value is within predetermined limits, calculating a first correction F _n because of the deviations of t _n and C _n of corresponding predetermined desired values T _soll, and C _should accordance with the proviso: F n = k T (T n -T should ) - k C (C n -C should ) where k _T and k _C designate predetermined parameters which are characteristic of the electrolysis cell used; and if no lying within predetermined limit values C _n value is available, calculation of a first correction F _n due to the deviation of T _n from the preset desired value T _set in accordance with the proviso: F n = k L (T n -T should ) where k _L denotes a predetermined parameter which is characteristic of the electrolysis cell used;

c) determining the mean electrolysis voltage U _M measured during the interval n and the interval n-1;

d) determining the mean amount of AlF ₃ added daily during the interval n and the interval n-1;

e) In the event that

(i) the average electrolysis voltage U _M determined according to c) does not deviate from a standard voltage Us by more than a predetermined value ΔU, and

(ii) the average daily addition of AlF ₃ determined according to d) is less than a predetermined value C _m , calculation of a second correction F _trend according to the following requirement: F trend = k trend · T n - T n-1 Δ t where Δt is the time period between the measurement n and the measurement n-1 in days and k _trend denotes a predetermined parameter which is characteristic of the electrolysis cell used; and
if conditions (i) and (ii) are not met, F _trend = 0 applies;

f) Calculation of a new AlF ₃ supply rate A _{n + 1} valid for the subsequent interval _{n + 1 in} accordance with the following requirement: A n + 1 = F 0 (t) + F n + F trend .

g) If A _{n + 1 is} less than or equal to a predetermined upper limit value Ao and greater than or equal to a predetermined lower limit value A _U , and the value C _n measured during the interval _{n is} less than a predetermined maximum value C _max , supply of AlF ₃ according to the value A _{n + 1} during the subsequent interval n + 1, and if A _{n + 1} greater than A _o , supply of AlF ₃ according to the upper limit value A _o , and if A _{n + 1} less than A _U , or C _{n is} greater than or equal to the value C _max , supply of AlF _{3 in} accordance with the lower limit value A _U ;

h) Periodic repetition of process steps a) to g) until the electrolysis is ended.

The above procedure for calculating and regulating the AlF ₃ supply applies only to the normal working cell. Accordingly, it must be checked in every period n whether the cell is operating normally, ie whether the electrolysis cell is in a normal state within the specified parameter limit values during the period n. If an abnormal condition is found, the operator changes the condition of the electrolytic cell until a normal condition of the electrolytic cell is reached. Only after a normal state has been reached is the calculation and addition of AlF _{3 continued in} accordance with the process steps a) to h) described above.

The parameters k _T , k _C , k _L and k _Trend are characteristic parameters for a given electrolysis cell, which depend among other things on the size of the cell, the amount of electrolyte, the current strength and the desired correction speed of any AlF ₃ and electrolyte temperature fluctuations. The corresponding parameter values can be determined by statistical analyzes of the dependencies of the temperature values T _n and the AlF ₃ excess concentration on the amount of AlF ₃ supplied and on the respective changes in the electrolysis voltage.

i) während dem Zeitintervall Δt die Elektrolyttemperatur T innerhalb eines Wertebereiches zwischen 920 °C < T < 990 °C liegt;

ii) der elektrische Badwiderstand stabil ist;

iii) die normale, d.h. kontinuierliche oder in vorbestimmten Abständen zu erfolgende Tonerdezufuhr in die Kryolithschmelze nicht unterbrochen ist;

iv) das Zellenalter wenigstens 30 Tage, bevorzugt zwischen 30 und 150 Tagen und insbesondere mehr als 100 Tage beträgt.

The normal working state of an electrolysis cell used for the method according to the invention is given if the following conditions i) to iv) are all met during a predetermined time interval Δt:

i) during the time interval Δt the electrolyte temperature T is within a range of values between 920 ° C <T <990 ° C;

ii) the electrical bath resistance is stable;

iii) the normal, ie continuous or predetermined, alumina feed into the cryolite melt is not interrupted;

iv) the cell age is at least 30 days, preferably between 30 and 150 days and in particular more than 100 days.

The time interval Δt in the sense of (i) expediently denotes the time period between the last and the penultimate temperature measurement, with Δt preferably about one Day lasts.

In the context of the invention, the electrical bath resistance in the sense of (ii) typically applies as stable when the electrical bath resistance noise has a value of 0.5 µΩ during 30 minutes a day. This is the electrical bath resistance noise defined as the difference between that measured during a time interval of one minute maximum electrical bath resistance of the measured during the same time interval minimal electrical bath resistance.

For cells that do not work normally, the state of the electrolysis cell must be changed by an operator until all conditions (i) to (iv) are met. The method according to the invention can then be used again. The time-delayed effect after adding AlF3 can be taken into account in accordance with the method described in EP-B1 0 455 590. If the electrolyte is in a non-normal state in which the electrolyte is oversaturated with AlF ₃ , a neutralizing agent such as soda or NaF can be added. Furthermore, the electrolysis voltage can also be adjusted to set a normal state.

The parameters for the daily standard addition F ₀ (t) to compensate for the daily AlF ₃ loss on the electrolyte the approximate knowledge of the mean AlF require ₃ -Bedarfes per unit time at a stationary operating electrolytic cell at the setpoint T _set lying electrolyte temperature and may be prepared by preliminary tests or determined by the known daily additions of AlF ₃ during the previous electrolysis process. The standard addition depends on the cell age.

If the electrolysis cell is operated with constant current, the standard allowance F ₀ (t) can be calculated according to the following requirement: F 0 (t) = k a - k b exp (-t / τ)

T denotes the cell age in days and τ a predetermined time constant, the value of which is between 150 and 350 days. k _a and k _b denote constants, the value of which depend on the nominal current, the geometric dimensions of the electrolysis cell and the cathode, and on the sodium fluoride and sodium carbonate (Na ₂ CO ₃ ) content of the aluminum oxide. As an example, the constant k _a for an electrolysis cell with a nominal current of 130 KA is between 13 and 17 kg AlF ₃ / day, the corresponding value for the constant k _b typically being between 20 and 25 kg AlF ₃ / day.

The nominal current denotes the highest, permanently possible current strength of the electrolytic cell.

The electrolyte nominal temperature T _nom, or the setpoint of the AlF ₃ content _to C is predetermined by preliminary tests based on empirical values or by determining an optimum value due. By specifying the desired temperature and the desired value of the AlF ₃ content C _should clearly predetermined. Conversely, when the target value C target _is specified, the target temperature T target _is also clearly specified. Accordingly, the specification for the AlF ₃ C -Sollwert directed _to after regarded as optimal electrolyte nominal temperature T _nom. This can be obtained, for example, with approximately 12% by weight of AlF ₃ . The setpoint temperature can be given anew during the process due to changes in the electrolysis parameters. Usually, however, a constant target temperature T target _is preferred over the entire life of an electrolytic cell.

The setpoint temperature T _set is preferably at least 950 ° C and at most 975 ° C. T target _is very preferably in the range 960 ± 3 ° C.

The target value C _to the percentage of free AlF ₃ in the electrolyte is preferably at least 10 wt .-% and at most 13 wt .-%. More preferably is C _to at 12 ± 0.2 wt .-%.

The current electrolyte temperature T _n during the period n can be determined in different ways and at different locations of the electrolytic cell. In principle, the electrolyte temperature can be determined by direct temperature measurement in the electrolyte. Because of the short-term fluctuations, however, a temperature measurement in the trough, in particular in the side walls of the electrolysis cell, or in the liquid aluminum on the trough bottom is preferred. The current is usually supplied to the cathode by means of steel carriers inserted into the carbon steel-lined steel trough, so that the temperature measurement can be carried out particularly preferably by means of temperature measuring devices inserted into recesses in the steel carrier.

The current T _n value can be measured continuously or at predetermined time intervals. The current C _n value is expediently measured at predetermined time intervals. The measurement in predetermined time intervals is preferred for the determination of both values, ie for the T _n and C _n values, an interval of 24 hours being generally sufficient for stable process control.

In a preferred embodiment of the method according to the invention, the C _n and T _n values are measured simultaneously.

The constant k _T required to calculate F _n is in the range from 0.4 to 1.3 kg AlF ₃ / ° C and in particular 0.73 ± 0.02 kg AlF ₃ / ° C. The constant k _{C also} required for the calculation of F _n is in the range of 2.5 to 7.0 kg AlF ₃ / ° C and in particular 4.4 ± 0.1 kg AlF ₃ / ° C. The constant k _L required to calculate F _n in the absence of a certain C _n value within certain predetermined limits is in the range from 0.9 to 2.5 kg AlF ₃ / ° C and in particular 1.46 ± 0.04 kg AlF ₃ / ° C ,

The deviation ΔU specified for condition (i) for calculating F _trend is preferably between 0 and 0.2 V and in particular in the range 100 ± 5 mV. The standard voltage U _s is predetermined and can correspond to the value of the electrolysis target voltage.

(iii) seit der vorletzten Temperaturmessung T_n-1 ist kein Soda zugegeben worden.

In a further preferred embodiment of the method according to the invention, the following additional and additional conditions compared to conditions (i) and (ii) are added for the calculation of a second correction F _trend according to method step (e):

(iii) No soda has been added since the penultimate temperature measurement T _n-1 .

The predetermined maximum value C _max for the AlF ₃ content in the electrolyte is preferably in the range from 14 to 17% by weight and is particularly preferably 16% by weight.

The AlF ₃ required for the method according to the invention is supplied, for example, in portions at certain time intervals. The AlF ₃ supply can be adjusted by adjusting the amount of AlF ₃ supplied at intervals and / or by changing the time interval. AlF ₃ can also be supplied continuously by means of a metering device, preferably by means of a program-controlled metering device.

The measurement of the T _n and C _n values at intervals of several hours is usually sufficient to carry out a stable electrolysis. In general, a daily measurement of the T _n and C _n values and a corresponding adjustment of the AlF ₃ supply rate is sufficient.

As an example of an electrolysis cell with a nominal current of 130 kA, the upper limit value A _o for the AlF ₃ feed rate A _{n + 1 is} in the range from 20 to 60 kg AlF ₃ / day, preferably between 30 and 35 kg AlF ₃ / day and in particular in the range 33 ± 1 kg AlF ₃ / day. Another example of an electrolysis cell with a nominal current of 130 kA is the lower limit value A _U for the AlF ₃ feed rate A _{n + 1} in the range from 0 to 20 kg AlF ₃ / day, preferably 0 to 7 kg AlF ₃ / day and in particular in the range 3 ± 0.5 kg AlF ₃ / day.

• falls T_k kleiner als ein vorgegebener unterer Temperaturgrenzwert T_U ist, wird während der nachfolgenden Zeitspanne Δt_U,1 die Elektrolysesollspannung U um einen vorgegebenen Spannungserhöhungswert ΔU_E erhöht;
• falls T_k grösser oder gleich dem vorgegebenen unteren Temperaturgrenzwert T_U und kleiner als ein vorgegebener mittlerer Temperaturgrenzwert T_M ist, wird während der nachfolgenden Zeitspanne Δt_U,2 die Elektrolysesollspannung U um einen vorgegebenen Spannungserhöhungswert ΔU_E erhöht;
• falls T_k grösser als ein vorgegebener oberer Temperaturgrenzwert To ist, wird während der nachfolgenden Zeitspanne Δt_U,1 die Elektrolysesollspannung U um einen vorgegebenen Spannungsabsenkungswert ΔU_A verringert.

The calculation and setting of the target electrolysis voltage is preferably carried out by measuring the electrolyte temperature T _k at predetermined intervals k and, in the case of a normally operating cell, the target electrolysis voltage U for the subsequent interval k + 1 being set in accordance with the following requirement:

• if T _{k is} less than a predefined lower temperature limit value T _U , the desired electrolysis voltage U is increased by a predefined voltage increase value ΔU _E during the subsequent time period Δt _{U, 1} ;
• if T _{k is} greater than or equal to the predetermined lower temperature limit value T _U and less than a predetermined average temperature limit value T _M , the target electrolysis voltage U is increased by a predetermined voltage increase value ΔU _E during the subsequent time period Δt _{U, 2} ;
• if T _{k is} greater than a predetermined upper temperature limit To, the target electrolysis voltage U is reduced by a predetermined voltage reduction value ΔU _A during the subsequent time period Δt _{U, 1} .

The temperature limit values T _u , T _M and To as well as the voltage increase and voltage reduction values ΔU _E and ΔU _A are specified based on the properties of the electrolysis cell, such as the size of the cell, the amount of electrolyte, the nominal current and the desired correction speed of any AlF ₃ and electrolyte temperature fluctuations ,

The lower temperature limit value T _U is preferably in the temperature range from 945 to 954 ° C. and in particular 950 ± 0.5 ° C. The mean temperature limit T _M is preferably in the temperature range from 955 to 964 ° C. and in particular 955 ± 0.5 ° C. The upper temperature limit To is preferably in the temperature range from 965 to 985 ° C and in particular at 975 ± 0.5 ° C.

The voltage increase value ΔU _E is expediently in the range from 0 to 170 mV and preferably at 120 ± 2 mV. The voltage reduction value ΔU _A is expediently in the range from 0 to 80 mV and preferably 60 ± 2 mV.

The time intervals Δt _{U, 1} and Δt _{U, 2} are specified based on the properties of the electrolytic cell. The time period Δt _{U, 1} is preferably between 35 and 50 hours and is very preferably 40 hours. The time period Δt _{U, 2} is preferably between 15 and 25 hours and is very preferably 20 hours.

The time intervals n and k, ie the time interval for the determination of the AlF ₃ supply rate A _{n + 1} and the time interval k for the determination of the future electrolysis target voltage U, can be the same length or different. In addition, the time intervals k of length Δt _k (k is a natural number; Δt _k describes the duration in hours) can be of different lengths. For example, the interval length Δt _{k can} correspond to the value Δt _{U, 1} and / or Δt _{U, 2} . The time intervals n of the length Δt _n (n is a natural number; Δt _n describes the duration in days) are expediently, but not necessarily, of the same length.

Claims (13)

Process for the automatic regulation of the bath of an electrolysis cell for producing aluminum from aluminum oxide dissolved in a cryolite melt,
characterized in that
The efficiency of aluminum electrolysis is optimized by analyzing the individual state of the electrolytic cell from a series of measured values comprising several parameters at certain intervals and, based on the current state and taking into account the previous states, the optimal electrolyte temperature and the optimal AlF ₃ - for the further electrolysis process. Excess concentration can be regulated by calculation and corresponding setting of the electrolysis target voltage and AlF ₃ feed rate required for the further electrolysis process. A method according to claim 1, characterized in that the electrolyte temperature T _n and preferably additionally the percentage C _n of free AlF ₃ in the electrolyte are measured at predetermined intervals n , and an AlF ₃ feed rate A _{n + 1} for the Interval n + 1 is calculated according to the following process steps and added to the electrolyte: a) calculating a standard addition F ₀ (t) t in function of the electrolytic cells age and the nominal current I, where F ₀ (t denotes ₃ -bedarf per unit time for the stationary state at a predetermined electrolyte nominal temperature T _nom operating electrolysis cell) the average AlF ; b) If a C _n value and the measured C _n value is within predetermined limits, calculating a first correction F _n because of the deviations of t _n and C _n of corresponding predetermined desired values T _soll, and C _should accordance with the proviso: F n = k T (T n -T should ) - k C (C n -C should ) where k _T and k _C designate predetermined parameters which are characteristic of the electrolysis cell used; and if no lying within predetermined limit values C _n value is available, calculation of a first correction F _n due to the deviation of T _n from the preset desired value T _set in accordance with the proviso: F n = k L (T n -T should ) where k _L denotes a predetermined parameter which is characteristic of the electrolysis cell used; c) determining the mean electrolysis voltage U _M measured during the interval n and the interval n-1; d) determining the mean amount of AlF ₃ added daily during the interval n and the interval n-1; e) In the event that (i) the average electrolysis voltage U _M determined according to c) does not deviate from a standard voltage Us by more than a predetermined value ΔU, and (ii) the average daily addition of AlF ₃ determined according to d) is less than a predetermined value C _m , calculation of a second correction F _trend according to the following requirement: F trend = k trend · T n - T n-1 Δ t . where Δt is the time period between the measurement n and the measurement n-1 in days and k _trend denotes a predetermined parameter which is characteristic of the electrolysis cell used; and if conditions (i) and (ii) are not met, F _trend = 0 applies; f) Calculation of a new AlF ₃ supply rate A _{n + 1} valid for the subsequent interval _{n + 1} according to the following requirement: A n + 1 = F 0 (t) + F n + F trend . g) If A _{n + 1 is} less than or equal to a predetermined, upper limit value A _o and greater than or equal to a predetermined, lower limit value A _U , and the value C _n measured during the interval _{n is} less than a predetermined maximum value C _max , supply of AlF ₃ according to the value A _{n + 1} during the subsequent interval n + 1, and if A _{n + 1} greater than A _o , supply of AlF ₃ according to the upper limit value A _o , and if A _{n + 1} less than A _U , or C _{n is} greater than or equal to the value C _max , supply of AlF _{3 in} accordance with the lower limit value A _U ; h) Periodic repetition of process steps a) to g) until the electrolysis is ended. A method according to claim 2, characterized in that a normally working electrolysis cell is present when the following conditions i) to iv) are all met during a predetermined time interval Δt, which preferably corresponds to the time span between the last and the penultimate temperature measurement: i) the electrolyte temperature T is within a range of values between 920 ° C <T <990 ° C; ii) the electrical bath resistance is stable; iii) the normal, ie continuous or predetermined, alumina feed into the cryolite melt is not interrupted; iv) the cell age is at least 30 days, preferably between 30 and 150 days and in particular more than 100 days. A method according to claim 2, characterized in that in addition to the conditions (i) and (ii) of step (e) for the calculation of a second correction F _trend , which is different from zero, the condition (iii) No soda has been added since the penultimate temperature measurement T _n-1 , which must be fulfilled. , A method according to claim 2, characterized in that the target temperature T _is at least 950 ° C and at most 975 ° C, and preferably 960 ± 3 ° C. A method according to claim 2, characterized in that the target value C _to the percentage of free AlF ₃ in the electrolyte at least 10 wt .-% and at most 13 wt .-%, and preferably 12 ± 0.2 wt .-% by weight. , A method according to claim 2, characterized in that the predetermined maximum value C _max for the AlF ₃ content in the electrolyte is in the range 14 to 17 wt .-% and in particular 16 wt .-%. , Method according to one of claims 2 to 6, characterized in that the measurements of the C _n and T _n values are carried out simultaneously. , A method according to claim 1, characterized in that the electrolyte temperature T _{k is} measured at predetermined intervals k, and the target electrolysis voltage U for the subsequent interval k + 1 is set in accordance with the following requirement in the case of a normally operating cell: if T _{k is} less than a predefined lower temperature limit value T _U , the desired electrolysis voltage U is increased by a predefined voltage increase value ΔU _E during the subsequent time period Δt _{U, 1} ; if T _{k is} greater than or equal to the predetermined lower temperature limit value T _U and less than a predetermined average temperature limit value T _M , the target electrolysis voltage U is increased by a predetermined voltage increase value ΔU _E during the subsequent time period Δt _{U, 2} ; if T _{k is} greater than a predetermined upper temperature limit To, the target electrolysis voltage U is reduced by a predetermined voltage reduction value ΔU _A during the subsequent time period Δt _{U, 1} . , A method according to claim 9, characterized in that the lower temperature limit value T _U in the temperature range from 945 to 954 ° C and preferably at 950 ± 0.5 ° C, the mean temperature limit value T _M in the temperature range from 955 to 964 ° C and preferably at 955 ± 0.5 ° C, and the upper temperature limit To is in the temperature range from 965 to 985 ° C and preferably 975 ± 0.5 ° C. , Method according to Claim 9, characterized in that the voltage increase value ΔU _{E is} in the range from 0 to 170 mV and in particular at 120 ± 2 mV, and the voltage reduction value ΔU _{A is} in the range from 0 to 80 mV and in particular at 60 ± 2 mV. , A method according to claim 9, characterized in that the time period Δt _{U, 1 is} chosen between 35 and 50 hours, and is preferably 40 hours. , A method according to claim 9, characterized in that the time period Δt _{U, 2 is selected} between 15 and 25 hours, and is preferably 20 hours.