Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/20—Automatic control or regulation of cells

Definitions

• the invention relates to aluminum production cells by electrolysis of alumina dissolved in an electrolyte based on molten cryolite, in particular according to the Hall-Héroult process. It relates in particular to a device and a method for detecting the effects of anode.
• Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a bath based on molten cryolite, called electrolyte bath, in particular according to the well-known Hall-Héroult process.
• the electrolyte bath is contained in cells, called “electrolysis cells”, comprising a steel box, which is coated internally with refractory and / or insulating materials, and a cathode assembly located at the bottom of the cell. Anodes are partially immersed in the electrolyte bath.
• electrolysis cell normally designates the assembly comprising an electrolysis cell and one or more anodes.
• the electrolysis current which circulates in the electrolyte bath and the liquid aluminum sheet via the anodes and cathode elements, operates the aluminum reduction reactions and also makes it possible to maintain the bath. electrolyte at a temperature of around 950 ° C by the Joule effect.
• the electrolysis cell is regularly supplied with alumina so as to compensate for the consumption of alumina produced by the electrolysis reactions.
• One of the essential factors ensuring the regularity of operation of an aluminum production tank by electrolysis of alumina dissolved in a molten electrolysis bath based on cryolite is the maintenance of an appropriate content of alumina dissolved in this electrolyte and therefore the adaptation of the quantities of alumina introduced into the bath to the consumption of alumina from the tank.
• an alumina defect can in particular cause the appearance of the "anode effect", that is to say the polarization of an anode, with sudden rise in the voltage across the terminals of the cell. and release in large quantities of fluorinated and fluorocarbon (CF X ) products, whose high absorption capacity of infrared rays promotes the greenhouse effect.
• CF X fluorinated and fluorocarbon
• the alumina content regulation methods consist in modulating the supply of alumina as a function of the value of R and of its evolution over time. This basic principle has been the subject of numerous patents until very recently (see for example the French application FR 2,749,858 corresponding to the American patent US 6,033,550).
• the subject of the invention is a method for early detection of the effects of anode in an aluminum production cell by molten salt electrolysis, in which a first electrical voltage signal Ul and at least a second electrical voltage signal are measured. U2 at two separate locations in said cell, and in which the value of at least one risk indicator for the appearance of an anode effect A (or “anticipated anode effect indicator A”) is determined at from an analysis of said signals Ul, U2, ..., capable of signaling in advance, or even early, a high risk of occurrence of an anode effect.
• An anticipated anode effect indicator A is typically determined from a comparison of the signals Ul, U2, ... More specifically, the indicator A (or the indicators Al, A2, ...) is (are ) typically determined from a function
• comparison (U1, U2, U3, 7), called comparison, which is preferably able to quantify the spreading of the signals, and more specifically the differences E between the signals Ul, U2, U3, ...
• an indicator A can be given by an algebraic difference between the two electrical voltages when two voltage signals are measured, or by an algebraic difference between extreme values (for example between the most separate signals) or enters at least two signals when measuring more than two voltage signals.
• an indicator A can be determined statistically, for example by a standard deviation between all the signals. It can also be determined by more sophisticated analog or digital processing.
• the indicator (s) A are preferably determined from the time course of the comparison function F (U1, U2, 7), typically from the time course of at least one difference E between Ui signals (for example, an algebraic deviation, a standard deviation, ).
• an advance anode effect indicator A can be given by a time evolution indicator B of the comparison function.
• the Applicant has observed that, unexpectedly, a large part of the anode effects started long (up to several tens of minutes) before the effective arrival of the anode effect and that this primer corresponded to a beginning of polarization which result in a modification of the distribution of the electric voltage in the cell, in particular in the vicinity of the anode likely to be polarized. It also found that voltage measurements in at least two separate places of an electrolysis cell made it possible to reliably and early detect the initiation of an anode effect.
• the subject of the invention is also a method for regulating a molten salt electrolysis cell for the production of aluminum, comprising the method of early detection of anode effect according to the invention.
• the invention also relates to a device for early detection of the effects of anode in an aluminum production cell by molten salt electrolysis, capable of implementing the detection method according to the invention, comprising at least a first means for measuring a first electrical voltage signal Ul on said cell, at least second means for measuring at least a second electrical voltage signal U2 on said cell, and at least means for determining an anode effect indicator A from an analysis of said electrical voltage signals Ul, U2, ...., typically from a comparison of these and, possibly, from a quantification of the temporal evolutions of the differences between them .
• the subject of the invention is also an electrolysis cell and a system for regulating a molten salt electrolysis cell for the production of aluminum comprising a device for early detection of anode effect according to the invention.
• FIG. 1 represents, in cross section, a typical electrolysis cell using prebaked anodes made of carbonaceous material.
• Figure 2 illustrates a method of measuring the voltage across an electrolytic cell according to the invention.
• FIG. 3 schematically illustrates a device for early detection of anode effect according to the invention
• FIG. 4 schematically illustrates part of a device for early detection of anode effect according to the invention.
• Figures 5 and 6 show voltage and current signals measured according to the invention on an electrolysis cell.
• the invention advantageously applies to an electrolysis cell (1) for the production of aluminum by electrolytic reduction of the alumina dissolved in an electrolyte bath (15) based on cryolite, in particular by the process of Hall-Héroult electrolysis.
• an electrolysis cell (1) for the production of aluminum by the Hall-Héroult electrolysis process typically comprises a tank (20), at least one anode (13), at least a cathode (5) and means for supplying alumina (18).
• the tank (20) has internal side walls (3) and is capable of containing a bath of liquid electrolyte (15).
• the cell (1) is able to circulate in said bath a so-called electrolysis current having an intensity I.
• the aluminum produced by said reduction normally forms a sheet called "liquid metal sheet" (16) on the cathode (s) (5).
• the anodes (13) are typically supported by the fixing means (11, 12) to an anode frame (10), which can be mobile.
• the tank (20) normally comprises a steel casing (2), interior cladding elements (3) and cathode elements (5, 6), which include connecting bars (or cathode bar) (6) to which the electrical conductors (7, 8) for conveying the electrolysis current.
• electrolysis whose total intensity is Io
• the electrolysis current flows through the electrolyte bath (15) via the anode (s) (13) and the cathode (s) (5). It passes from one electrolysis cell to the next via the connection conductors (7 to 12), and more precisely via the cathode connection conductors (6, 7, 8) of a tank, called upstream, and anodic connecting conductors (9, 10, 11, 12) of the next tank, called downstream.
• the supply of alumina to the cell is intended to compensate for the substantially continuous consumption of the cell which essentially comes from the reduction of alumina to aluminum metal.
• the supply of alumina, which is carried out by adding alumina to the liquid bath (15), is generally regulated independently.
• the feeding means (18) typically include metering sticks (19) capable of piercing the alumina crust (14) and of introducing a dose of alumina into the opening (19a) formed in the alumina crust by drilling.
• the aluminum metal (16) which is produced during electrolysis normally accumulates at the bottom of the tank and a fairly clear interface is established between the liquid metal (16) and the bath based on molten cryolite ( 15).
• the position of this bath-metal interface varies over time: it rises as the liquid metal accumulates at the bottom of the tank and it drops when liquid metal is extracted from the tank.
• the method for early detection of an anode effect in a cell (1) for the production of aluminum by electrolysis in molten salt is characterized in that it comprises:
• the detection method according to the invention includes the measurement of N electrical voltage signals Ui, N being advantageously greater than 2.
• N being advantageously greater than 2.
• the use of several signals makes it possible to increase the reliability of the early detection and to locate more precisely the zone of the tank which is likely to lead to an effect of 'anode.
• a preventive treatment of the anode effect could include, for example, a local modification of the supply of alumina (typically in the area detected by the measurements).
• said electrical voltage signals Ui are normally measured as a function of time. They are typically measured analogically and then converted into digital signals for processing.
• the comparison function F (U1, U2 ,. march) can be given by an equivalent function F '(TU1, TU2, 7) which uses as arguments preprocessed signals TU1, TU2, ...., that is ie the signals TU1, TU2, .... originating from a pre-processing of the signals Ul, U2, ...
• the pre-processing comprises sampling, at a determined frequency Fe, of the real signals Ul , U2, ..., and possibly one (or more) additional processing operation (s) of at least one of the signals.
• An anti-aliasing low-pass filter is advantageously included in the pretreatment. Signals can be processed analog and / or digital. It is also possible that only certain
• the frequency filtration operation can be of different types. It has been found advantageous to use a low-pass type filter.
• the cutoff frequency of this filter is advantageously between 0.001 and 1 Hz.
• the low and high cut-off frequencies of the band pass type frequency filter are advantageously respectively between 0.001 and 1 Hz and between 1 and 10 Hz (typically 0.5 and 5 Hz).
• the preprocessing comprises two frequency filters, one of the low pass type (with a cutoff frequency typically equal to approximately 0.5 Hz) which gives a first preprocessed signal TUi, l 'other of the bandpass type (with a low cut-off frequency typically equal to approximately 0.5 Hz and a high cut-off frequency typically equal to approximately 5 Hz) which gives a second pre-processed signal TUi'.
• the method comprises two comparison functions F, one relating to the signals TUi, the other relating to the signals TUi '.
• the preprocessing comprises three frequency filters: a first of the low-pass type (with a cutoff frequency typically equal to approximately 0.003 Hz) which gives a first pre-processed signal TUi, a second of bandpass type (with a low cutoff frequency typically equal to approximately 0.003 Hz and a high cutoff frequency typically equal to approximately 0.5 Hz) which gives a second pre-processed signal TUi ', and a third of the bandpass type (with a low cut-off frequency typically equal to approximately 0.5 Hz and a high cut-off frequency typically equal to approximately 5 Hz) which gives a third pretreated signal TUi ".
• the method includes three comparison functions F , the first relating to the signals TUi, the second relating to the signals TUi 'and the third relating to the signals TUi ".
• said at least one comparison function F (U1, U2, ...) (or possibly F '(TU1, TU2, ...)) is given by a difference E between said signals (Ul, U2, U3, .%) or between the preprocessed signals (TUI, TU2, ).
• the comparison function F (U1, U2, ...) can be given by a difference E between at least two voltage signals Ul, U2 ,.esc. or between at least two pre-processed voltage signals TUI, TU2, ....
• the difference E can be given by an algebraic difference between the signals Ui or pre-processed signals TUi, for example by the greatest difference between all the signals Ui or pre-processed signals TUi (typically the difference between the most separated signals, at a given time, or over a given period of time).
• the deviation E can also be given by a standard deviation between the signals Ui or preprocessed signals TUi.
• At least one anticipated anode effect indicator A can be equal to a comparison function F (U1, U2, %) or F '(TU1, TU2, ).
• the value of at least one risk indicator for the appearance of an anode effect A can also be determined on the basis of temporal changes in the one or more comparison functions F or F ′. These changes can be given by a time change indicator B of a comparison function F (U1, U2, ...) or F '(TU1, TU2, ).
• the comparison function F (U1, U2, 7) is given by a difference E between at least two voltage signals Ul, U2, .... or between at least two preprocessed voltage signals TUI, TU2, ....
• the evolution indicator B can be proportional to the difference between the value E (t) of a deviation E at time t and its value E (t - to ) at time t - to, where to is an adjustable parameter.
• Indicator A can signal a strong risk of an anode effect appearing when its value is greater than a given threshold value S.
• the process signals this strong risk when the value of a deviation E (and more generally E (t)) is greater than a threshold value Se given or when the evolution of the value of the comparison function F or F ' is greater than a given threshold value St.
• the detection method further comprises a test operation capable of revealing the susceptibility of an electrolysis cell to the triggering of an anode effect.
• This test operation typically involves a temporary reduction in the rate of supply of the cell with alumina
• the regulation method according to the invention advantageously comprises an operation for the preventive treatment of the anode effects capable of eliminating the anode effects which are detected in advance, which can be activated when an anode effect has been detected from anticipated way.
• This operation is normally triggered as a function of the value of the function F (or F '), typically when a difference between at least two signals Ui or between at least two pre-processed signals TUi exceeds a given threshold Se, or when the time evolution of this deviation exceeds a given threshold
• the preventive treatment typically includes a modification of the position of the anode (s) relative to the cathode (s), an over-supply of alumina compared to the normal supply rate, or a combination of these operations.
• the regulation process advantageously takes into account the operating operations which are likely to give disturbed values for the function F (or F ′), and therefore for the indicator (s) A, such as the changes of anode.
• the cell (1) advantageously comprises at least one adjustment means such as a mobile anode frame (10) to which the anode (s) (13) is fixed or a means control of the alumina supply means (18, 19).
• adjustment means such as a mobile anode frame (10) to which the anode (s) (13) is fixed or a means control of the alumina supply means (18, 19).
• the regulation method further comprises: -
• the regulation method further comprises:
• Intensity I is typically the total intensity lo circulating in the cells. It is also possible to use the intensity I of other currents flowing in a series of electrolysis cells, such as the current flowing in an anode, in a connecting conductor or in a cathode bar.
• This variant of the invention notably makes it possible to reduce the so-called “signal / noise” ratio.
• the device for early detection of an anode effect in an aluminum production cell by electrolysis in molten salt is characterized in that it comprises:
• - at least a first means (321 to 344) for measuring a first electrical voltage signal Ul between a first cathode measurement point (301 to 304) on a cathode link conductor (6, 7, 8) and a first anode measurement (311 to 314) on an anode link conductor (9, 10, 11, 12);
• - at least a second means (321 to 344) for measuring a second electrical voltage signal U2 between a second cathode measuring point (301 to 304) on a cathode connecting conductor (6, 7, 8) and a second point of anode measurement (311 to 314) on an anode link conductor (9, 10, 11, 12), at least one of these second measurement points being distinct from said first measurement points;
• the device may also include means for determining the value of at least one risk indicator for the appearance of an anode effect A on the basis of the temporal evolutions of the said comparison function (s) F or F '.
• the means for measuring the electrical voltage signals Ul, U2, ... advantageously include electrical conductors (32, 321, 322, 323, 324, ..., 33, 331, 332, 334, ...) - typically in the form of wires or cables - one end of which is connected to a measuring point (30, 301, 302, 303, 304, ..., 31, 311, 312, 313, 314, ...) on the cell and another end is connected to a voltage measuring means (34, 341, 342, 343, ...), such as a voltmeter.
• the measurement points (30, 301, ..., 31, 311, ...) of the electrical voltage can be achieved by any known means, such as by screws, notching, etc.
• Certain voltage measurement means (30, 31, 32, 33, 34, 7) can be permanently fixed to the cell. They are advantageously installed on the fixed parts of the cell, such as the fixed conductors (7, 8, 9, 10), which, in particular, makes it possible to avoid measurement interruptions and the reinstallation of the measurement means during anode changes.
• Said electrical voltage signals Ul, U2, U3, ... are advantageously measured between a collector (8) and a rise (9), preferably in the lower part (9a) of said rise (as illustrated in the Figure 2), which in particular makes it possible to simplify the wiring (32, 321, 322, ..., 33, 331, ...) and facilitate access to the measurement points (30, 301, ..., 31, 311, ).
• the means (351-354, 40) for evaluating at least one comparison function F (or F ') of said voltage signals Ui advantageously comprise at least one pre-processing means (401-404) for pre-processing at least one Ui signals or equivalent signals Si.
• the means for pre-processing typically comprises at least one frequency filter, and advantageously a low-pass or band-pass filter.
• the means for pre-processing can also be a means for sampling, at a determined frequency Fe, the signals Ul, U2, ....
• the device can also include one or more elements typically chosen from analog / digital converters (ADC), amplifiers (G), frequency filters (low-pass, band-pass or other), sub-samplers, means for calculating an average on a signal (of RMS type or other), means for calculating an average Um at least one signal Ui or several signals Ui, and known mathematical operators (such as the means for subtracting a reference value Uo, and more precisely for calculating a difference between each signal Ul, U2 ,. .. or preprocessed signal TUI, TU2, ... and a reference value Uo, Uo being typically an average Um).
• the cut-off frequency of the low-pass filter is typically between 0.001 and 1 Hz.
• the low and high cut-off frequencies of the band-pass filter are typically respectively between 0.001 and 1 Hz and between 1 and 10 Hz.
• the device can also include means for determining an average value Um of the signals Ul, U2, ... or of the pre-processed signals TUI, TU2, ...
• the device can include means (40, 411) for determining a deviation E (and more generally E (t)) (such as an algebraic deviation, a standard deviation, ...) between at least two voltage signals Ul , U2, .... or between at least two pre-processed voltage signals TUI, TU2, ....
• the device can also include a means for determining a temporal evolution of at least one function for comparing the signals F (U1, U2, ...) or F '(TU1, TU2, 7), such as the evolution time difference E (and more precisely E (t)) between at least two voltage signals Ul, U2, .... or between at least two pre-processed voltage signals TUI, TU2, ....
• the means for evaluating a function F (or F ') (40, 401, ..., 404, 411) and for determining an anode effect indicator A (50) can advantageously be grouped into one, typically at using a common electronic and / or computer circuit.
• the regulation system of an electrolysis cell according to the invention further comprises:
• the regulation system further comprises:
• a means for measuring at least one intensity signal I of an electrolysis current typically the total intensity lo circulating in the cells
• the electrical voltage and current measurements were carried out on an electrolytic cell in which a current with a total current of approximately 500 kA was flowing. The measurements spanned several weeks. Six voltage signals Ui were measured at 6 different places in the tank, between anodic measuring points and separate cathodic measuring points. The current flowing in 6 separate anodes was also measured as a function of time.
• FIG. 5 corresponds to the current signals Ii (graph A) and of voltage Ui (graph B), as a function of time t, digitized and pretreated using a low-pass filter whose cutoff frequency was 0 , 5 Hz.
• Figure 6 corresponds to the same digitized signals, but pretreated using a bandpass filter whose cutoff frequencies were 0.5 Hz and 5 Hz.
• graph C gives the difference between each filtered voltage signal Ui and the average Um of the 6 filtered voltage signals.
• the letters CA identify when an anode was changed.
• Figure 5 shows that the spread of the low-pass filtered signals gradually increased before the polarization events.
• the spread increased significantly (from 9 mV to more than 30 mN) from 90 minutes before the strong polarization observed after the temporary cessation of the supply of alumina (noted SA in FIG. 5 ).
• SA temporary cessation of the supply of alumina
• the spread increased significantly (from 7.5 mN to 12 mN) from 30 minutes before the anode effect noted EA in Figure 5.
• the comparison function could then be given by the greatest difference between two signals Ui - Um.
• FIG. 6 makes it possible to make another diagnosis on the behavior of the signals filtered in bandpass. There was also an increase in spread (which went from 0.2 mN to more than 0.4 mN in this case) in situations of risk of anode effect.
• the combination of this information can be used to develop synthetic anode risk indicators that allow early detection the anode effects with great reliability and to implement treatments capable of avoiding them.

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• 2001
  • 2001-12-07 FR FR0115871A patent/FR2833274B1/fr not_active Expired - Fee Related
• 2002
  • 2002-12-03 AR ARP020104671A patent/AR037624A1/es active IP Right Grant
  • 2002-12-04 CA CA002468737A patent/CA2468737A1/fr not_active Abandoned
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• 2004
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