DE2523768A1 - METHOD OF DETERMINING THE CONCENTRATION OF SOLUBLE MATERIALS AND DEVICE FOR ITS IMPLEMENTATION - Google Patents

Description

Patent attorneys Munich 2 2 Steins do rf straße 21 - 22 Telephone 089/29 84

COMALCO LIMITED 95 Collins Street, Melbourne, Victoria / Australia

Method for determining the concentration of dissolved substances and device for its implementation

The invention relates to a method for determining the concentration of dissolved substances in condensed systems using a measuring arrangement and a device for carrying out this process.

Dr .Pr

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Extensive literature covering the current state of the art aluminum production by electrolysis of aluminum oxide in cryolite-based baths using a large number of similar cells connected in series, is available. A major routine is loading alumina to maintain its concentration, usually in a range of 1-8 weight percent.

Traditionally, this loading is done by hand, with the amount and time of addition of the aluminum oxide from Workers are determined to be a job that has considerable experience and considerable prior knowledge of expected performance of each individual cell. Some systems are electromechanical or electronic Computer built in to assist the worker in this task.

If the cell is overloaded with aluminum oxide, the concentration can decrease increase in aluminum oxide until the melt is saturated. The excess aluminum oxide settles on The bottom of the cell decreases and possibly causes a "sick" or "muddy" condition of the reactor, resulting in a loss Effectiveness and leads to overheating of the cell.

If the concentration of aluminum oxide falls too low Value, generally below 2%, changes the type of cell operation and the well-known "anode effect" occurs. Of the "Anode effect" is generally understood to mean that it is through Electrolysis of the fluoride bath is triggered at these low aluminum concentrations. When this occurs, it becomes an isolating one gaseous or polymeric fluoride film on the surface between the anode surface and the molten electrolyte formed ο The high electrical resistance of this film

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causes the cell voltage to increase by a factor of about to; the resulting excessive and irregular energy consumption is unproductive for aluminum and disrupts the overall load of the series system. This is how electricity can be fall that the entire series-connected system with reduced Production speed works.

Because routine anode effects for the overall operation of the concerned Cell and the serially connected cell row are less harmful, the cells are normally after operated on a loading schedule that eliminates the above-mentioned "disease state" and the occurrence of anode effects should regulate to about 1 per day and per cell.

There would be an improvement in the effectiveness of the method because the "anode effects" and "diseased cells" are eliminated would. A further improvement would be achieved if the concentration of alumina was at an optimal level could be controlled. Such improvements would be made by the Development of a method for measuring the aluminum oxide concentration of the bath in situ will be facilitated for control purposes the feed rate of the cell could be used.

Many attempts have been made to improve the efficiency of the cell by eliminating the anode effects. Some of these Experiments use the observation that the electrical resistance of the cell normally lasts for a period of time Addition of alumina falls but increases before an anode effect, with the increase on immediately before the anode effect is highest. Panebianco and Bacchiega developed a tension bridge based on the resulting rise in tension to warn of the "pre-anode effect state", discuss

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tert in "Le Metallurgie" Italiana "No. 5, 1968. The US-PS 3 712 857 takes advantage of electronic computers to analyze the resistance curve and before the upcoming Warn anode effect. Other systems rely on the fact that at higher than average current densities operated anodes reach the anode effect a little earlier than the other anodes.

While all of these techniques allow a certain improvement in the efficiency of the process, they can neither predict the approach to the state of a "diseased cell" nor control or eliminate the strong fluctuations, which can and do occur in the aluminum oxide concentration in normal aluminum production plants.

U.S. Patent 3,400,062 discloses a system for controlling the aluminum oxide concentration by regulating the feed rate to one of two predetermined values. One A larger energy source is used to energize a control or control anode to a current density greater than the cell current density after two pretreatment stages supply, in which a preliminary anode effect is suppressed by reversing the polarity of the external energy source. The presence or absence of an anode effect on the control anode is used to select the most suitable of the two loading values.

Recent laboratory tests have shown that the concentration on aluminum oxide depends not only on the current density applied, but also on the time that the control electrode needs to achieve the anode effect. Consequently, the determination of only the presence or absence of the anode

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effects only a rough control of the aluminum oxide concentration due to the low inherent low level of the instrumental technology Enable resolution. The resolution would be even more further reduced by using the unshielded measuring arrangements with low current density resolution, as suggested

U.S. Patent 3,471,390 discloses an instrument and measuring arrangement for determining the instantaneous concentration of alumina in the bath. A measuring arrangement containing a small control cell is immersed in the bath and the cell voltage is increased in stages until an anode effect is induced on the control anode. The sudden change in cell resistance is detected by a programmer and the resulting electronic signal is used to record the voltage jump at which the anode effect occurred and to reset the instrument. This voltage level or readout number ¹¹ , used in conjunction with calibration curves determined by laboratory analysis, enables a quantitative value to be assigned to the aluminum oxide concentration. In the preferred form, the voltage is automatically graduated every three seconds, with manual grading being provided. The measuring arrangement contains a Stetter anode, the area of which is carefully defined by means of a boron nitride sleeve and a steel cathode concentric in close proximity to the anode.The resolution and reproducibility obtainable with this instrument depends very much on the necessity, both the anode-cathode relative configuration and the Voltage gradation constant, as well as the need to construct the measuring arrangement with the anode and the cathode as close together as practicable.

B 7k 22

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Any change in the anode-cathode configuration as a result of metering equipment wear or manufacturing changes will introduce changes in the known IR component of the cell voltage and, consequently, the readout number. Further errors come in due to any change in the conductivity of the melt or the effective cathode voltage (as a result of the formation of an alloy with the electrolytically deposited aluminum or the corrosion by the melt). Another significant shortcoming is the low value of the sweep rates used. At these low tilt rates, cell currents develop
and voltage large short-term fluctuations as a result of the development of gas bubbles. This leads to a reduction in the resolution of the readout number.

The invention is primarily intended to provide a system for determining concentrations of dissolved substances, which in operation is more effective than those described so far.

The invention relates to a method for determining the concentration of dissolved substances in condensed systems using a measuring arrangement with at least one anode and a cathode, in which an anode and a cathode are connected to this
Voltage rise is applied, which increases rapidly up to at least such a "Mung, in which an anode effect is induced at the anode, at least the value of the current corresponding to the current at which the anode effect occurs and the concentration of the solute from the is detected The invention also relates to a device for carrying out the method with a measuring arrangement having at least one anode and a cathode, devices for supplying energy which are connected to the measuring arrangement in order to provide the anode with a voltage increase which rapidly increases to at least one such voltage increases at

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an anode effect is induced at the anode, and means for determining at least the maximum value of the The currents supplied to the anode and corresponding to the current at which the anode effect occurs, from which the concentration of dissolved substances can be determined.

The invention is thus characterized by the application of a voltage increase to the anode of a measuring arrangement, used in the determination of concentrations of dissolved substances, which increases rapidly to a value which is sufficient to cause an anode effect on the anode. The current flow through the anode is just before the anode effect designed to provide an indication of the concentration of the solute.

In a preferred arrangement, the measuring arrangement includes a Reference electrode to allow close control over the slope imparted to the anode.

In the two-electrode systems used previously, the Counter electrode (the cathode in the case of alumina determinations) both as a current collector and as an unstable one Reference electrode, so that the response of the control anode is influenced by the configuration of the measuring arrangement, the bath resistance and the operating procedures. System according to the preferred embodiment of the invention overcomes this problem by using a reference electrode which is at a steady potential with respect to the bath is held. The use of this third stable reference electrode reduces the influence of the geometry of the arrangement and the IR waste in the electrolyte to a minimum and allows so the very desired close control over the tax

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anode induced voltage fluctuation rate (rate of voltage sweep). Another advantage of the three-electrode system is the more flexible design of the electrochemical control cell, which can be achieved especially the cathode of the reduction cell can be used as the counter electrode of the control cell, which simplifies the design of the measuring arrangement and allows the current for the control cell to be drawn from the main reduction DC circuits deduct.

Thus, in a preferred form, the invention can be applied to a system for determining the concentration of in in aluminum reduction cells molten electrolyte used can be applied to dissolved alumina. Such a system can therefore can be used advantageously to control the aluminum concentration in such cells.

Further advantages, features and embodiments, details and objects of the invention emerge from the following Description to be read in conjunction with the attached figures:

Figure 1 is a schematic block diagram of the invention embodied analytical measuring system in connection with the circuit of an aluminum reduction cell;

Figure 2 is a partial longitudinal cross-section of a practical embodiment of a measurement arrangement for the system;

FIG. 3 shows a typical current-time curve of the control anode;

Figure 4 shows the typical current-voltage curve and

5 shows a typical calibration curve from which the Al ₀ O - concentration

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can be read off if the value for Q is given

An analytical measurement system embodying the invention is shown in blookschematic form in Pigur 1 each in The block mentioned in the following description consists of standard electrical / electronic components well known to those skilled in the art, and reference to a complete circuit diagram is therefore not necessary here.

The illustrated system comprises three main components Die Power supply A, the measuring arrangement B and the output θ «yetera C.

The power supply A comprises a power source P, which can be either separate or that used for the reduction cell, a waveform generator 1, such as a single pulse generator that generates a triangular or SH tooth waveform, a voltage regulator circuit 2 and a switching and reset circuit 3 t of the Veilenforragerator 1 and the voltage regulator circuit 2 with the arrangement B and an amplifier circuit 4 (power boosting circuit) connects «The measuring system B comprises three electrodes! A control anode 5 $ a reference electrode 6 and a counter electrode or cathode 7 t the latter can be the cell cathode

In one embodiment, the voltage regulator circuit 2 is in the form of a potentiostat. This device is a feedback amplifier with two inputs, namely the potential difference between the control anode 5 and the reference electrode 6 as one and the potential difference between the control anode 5 and the output of the waveform generator as the other «Die The feedback nature of this control circuit narrows the output from this circuit (and ultimately the flow of current in the control anode) to such a value that the two entrances are closer within

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- Io -

Boundaries to each other remain. In its simplest form, this potentiostat can be an operational amplifier and thus include connected feedback resistors. In the preferred Form electronic filter circuits are included to avoid unwanted effects of any unwanted or volatile To reduce signals that can get into the system.

The switching and reset circuit 3 is provided to control the flow of current trip in the system and a variety of arrangements like B from the same voltage regulator circuit and waveform generator from operate.

The control anode 5 has a more positive potential with respect to the cathode due to current allocation from the amplifier circuit k. The reference electrode 6 has a constant potential with respect to the bath for practically long periods of time and provides a stable voltage reference point for the successful operation of the potentiostat. The cathode 7 completes the current-carrying circuit of the measuring arrangement.

One form of construction of the measuring arrangement is shown in FIG. In this arrangement, the control anode 5 comprises a rod made of graphite or other carbonaceous material, such as carbon from glass or pyrolysis coal, which is surrounded by a boron nitride screen 8. The screen or the sleeve is used to define the area of the anode 5 in a suitable manner and to isolate the anode 5 from the reference electrode. The reference electrode 6 is tubular graphite (or any other suitable conductor that builds up a steady potential and at the same time the cell attack resists), which surrounds the shield or the tube 8. The shield 8 becomes as thin as practical made so that the reference electrode 6 is close to the anode

B 7422.

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is Tim any disturbing effect dee Stromflueses in the anode 5 to a minimum to reduce "As can be seen, separates other material 9, the anode 5 and the reference electrode 6 and insulates the anode 5 of the conductive coating Io, which is connected to the reference electrode 6 is. In this embodiment the material 9 is mullite while the cladding Io is Inconel · Boron nitride could be used through and through, but the cheaper mullite material has been found to perform equally well. The reference electrode 6 is also enclosed in a boron nitride sheath 11 to provide additional protection for the reference electrode The counter electrode or cathode 7τ! Η of the arrangement shown is the cathode of the reduction cell, but the cathode 7 can be used as another graphite tube or other tube made of suitable material , for example metal, titanium carbide, surrounding the casing 11, be installed.

The assembly B may have any other suitable physical shape provided the area of the anode is carefully defined to enable an accurate knowledge of the operating current density therein. The assembly is immersed in the electrolyte bath at any suitable location and depth, either continuously or repeatedly. In view of the corrosive ^{effects of E 1} => V + rolyte, immersion operation is currently preferred.

Although the potential applied to the control anode can be adjusted to follow any of the various waveform functions, the preferred form is a single triangle or sawtooth wave which provides a linearly increasing control anode voltage with respect to the reference electrode voltage. This waveform function varies with a predetermined one

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uniform rate. Although the value of this rate itself is not important provided it is within for Maximum and minimum values set limits, it is important that this rate is reproducible, as the size of the The readings obtained from the output system C depend on the rate of voltage change as well as the aluminum oxide concentration. The voltage of the control anode 5 used to achieve the outputs (readouts) is determined by a residual potential of the reference electrode 6 varies in a direction which, in the case of aluminum oxide, would make it more anodic. Typically this change would be within the range of 0-20 volts. The speed at which the tension builds up should be greater than 2 V / sec, since it has been found that the measurement accuracy and service life of the control electrode contribute faster scan speeds is increased. The preferred scanning speed used is between about loo and 2oo V / sec, but speeds of around 5o to 4oo V / seo were used successfully. There is an optimal scanning speed for every arrangement design and reduction cell, above which an improvement in the information of the output system does not occur. Hence it suffices while scanning speeds of 400 V / sec and above are possible, in most cases lower speeds.

Although the rising and falling voltage sections of the wave can have the same inclinations or slopes, requires The system does not, and any slope can be used for the falling voltage, provided it interferes not with the electrochemistry of the system. It is highly desirable that the electrode pass through to the residual potential brought back the waveform generator and voltage regulator circuit as this has been found to be improved

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Accuracy leads. This return of the control anode voltage through the cycle to the residual potential can be accompanied by a small increase in current (about in the order of magnitude of 0.5 A) which contributes to the removal of undesired deposits from the surface of the arrangement and the arrangement for prepared for the next cycle

Referring again to Figure 1, the output system C includes a current sensing circuit 12, connected via a suitable current measuring resistor R in the control anode supply line. A voltage sensing circuit is installed between the anode and the reference electrode, and a current-time integration circuit 18 is self-connecting is connected via R «

A typical current-time waveform is shown in Figure 3, while a typical current-voltage difference (V anode -V reference) waveform is shown in FIG. The smaller curves show the return to the residual potential including that small increase in current mentioned above. The curves apply to an anode 3 »5 mm in diameter and a triangular wave shape with a scanning speed of 2oo V / sec.

It can be seen that the current rises to a peak or critical current I in each cycle, and since this current is important for the determination of the Al ₀ O concentration, a storage and comparison circuit 14 with a peak value detector 15 is provided Current sensing circuit 12 connected.

The output system can be any system that can sample when the power signal is present Peak value I has reached, and a command issued that this peak current, the voltage signal at that moment and the current integral up to this time from switching on the

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Current selected and these values to a suitable one Image memory or a data processing system.

In one system, current and voltage signals are transmitted to an electronic computer in an appropriate manner is programmed to do the above scanning, processing and storing.

In another system that is convenient 'in installations can be used that do not have access to electronic computers or microprocessing equipment, the signals are processed further in a manner described below.

In this system, the current and voltage signals are checked at high speed and converted into numerical signals Transferred with the help of a digital-to-analog converter for handling by the following operations. The current signal is also converted into a pulse frequency signal with the help of a voltage frequency converter.

The output signal from the D-A converter corresponding to the value of the current signal sample is used for temporary storage transmit a comparator. This also stores the highest value of the current signal, which is the special one Sampling has occurred.

If a later value of the current signal is less than the value in the comparator, record logic connected to the comparator Elements the fall and give command: (l) The higher current signal to a memory / view module such as 16; (2) the instantaneous value of the current integral, the total number of Representing pulses from the voltage to frequency converter generated from the start of the current flow up to the moment on

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another storage / viewing module, such as B. 19, to be transferred; (3) the value of the stress test at the moment to transmit corresponding signals to a third memory / vision module, such as 2o «

If this peak turns out to be an IntermediMr peak because unwanted signals get into the system, the current comparison circuit will continue to search for the higher values until the absolute maximum of the current signal is reached. In this way, the final values contained in the memory / display modules correspond to the I _{1 of} the voltage at the moment of I and the current integral from the beginning of the current flow up to the moment

from I c

By calibrating the arrangement for known weight concentrations of AIgO-, an Eioh curve of the form shown in Figure 5 can be obtained for each of the final output signals, namely for I, the voltage at that moment and the current integral · Sb can be a direct value for the output obtained from a bath of unknown concentration the aluminum oxide concentration can be attributed · This process can be performed manually or automatically with the help of a computer can be performed with the calibration information · The computer, in turn, can control the pause / feed cycles of the cell to be in a suitable way to adjust the concentration to a programmed optimal concentration value

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Claims (1)

PATENT CLAIMS 1.) Method for determining the concentration of solute Substances in condensed systems using an arrangement with at least one anode and one cathode, after which the anode and cathode are given a voltage rise which rapidly increases the voltage to at least one level brings, in which an anode effect is induced at the anode, at least the value corresponding to the current Current is detected at which the anode effect occurs, and the concentration of the dissolved substance is determined from the current value. 2. The method according to claim 1, characterized in that the charge flowing from the beginning of the current flow to the occurrence of the anode effect in the anode is determined and this Value related to the value of the current corresponding to the anode effect of the solute. 3. Method according to claim 2, characterized in that a reference electrode is introduced in addition to the anode and cathode and the voltage difference between the reference electrode and the anode is determined at the time of the occurrence of the anode effect and this value in connection with the value of the current corresponding to the anode effect is used to determine the concentration of the dissolved substance. 4. The method according to any one of claims 1 to%, characterized in that the voltage rise at a rate of about 5o to about 4oo V / sec is increased. 5. Method according to claim 4, characterized in that the B 7422 509850/0905 Voltage rise is increased at a rate of about 100 to about 2oo V / sec. 6. Apparatus for carrying out the method according to any one dar claims 1 to 5 _t, characterized in that it has a measuring arrangement with at least one anode and a cathode, energy supply devices connected to the arrangement for transmitting a voltage rise to the anode, which rapidly changes to at least one such voltage at which an anode effect is induced at the anode, and has means for detecting at least the maximum value of the current supplied to the anode and corresponding to the current at which the anode effect occurs, from which the concentration of the solute can be determined 7. Device according to claim 6, characterized in that the arrangement further comprises a reference electrode and the energy supply devices means for maintaining the voltage difference between the control and Reference anode within narrow limits of the potential difference between the control anode and the voltage of a waveform generator. 8. Apparatus according to claim 6 or 7, characterized in that that the energy supply means means for resetting the voltage applied to the anode to the output voltage, so that a current in the anode during the Reset cycle can flow, creating unwanted deposits on the surface of the anode that are during the application of the voltage rise to the anode form, are essentially removed. B 7422 509850/0905 9. Device according to ρ ir · em of the preceding claims 6 to f », characterized by the fact that for the abbreviation of the Voltage rise a waveform generator with a Triangular or saw tooth output is used. Io device according to one of claims 6 to 9, characterized characterized in that the arrangement has an insulating medium around, e planar anode, which is its working surface and has a reference electrode. 11. The device according to any one of claims U to 1o, characterized in that s.:I e additionally has devices for determining the charge flowing in the anode from the beginning of the current flow to the current value at which the anode effect occurs . 12. Device according to one of claims 6 to 11, characterized in that the rate of increase of the voltage rise is approximately *> ο to approximately 4oo V / sec. 13. The apparatus according to claim 12, characterized in that the rate of increase is about 1oo to 2oo V / jgee. B 7 509850/0905