DE2649716A1 - DEVICE FOR MONITORING SODIUM CONCENTRATION IN A STREAM OF MERCURY-SODIUM-AMALGAM - Google Patents

Description

PATENT LAWYERS

DR. WALTER KRAUS DIPLOMA CHEMIST. DR.-ING. ANN EKÄTE WEISERT DIPL-ING. SPECIALIZATION CHEMISTRY D - 8 MUNICH 19. FLÜGENSTRASSE 17 TELEFON 089/177061 TELEX O5-215145 ZEUS

TELEGRAM CRAUS PATENT

1391 Wt / r

Societa Italiana Resine S.I.R. S.p.A., Milan / Italy

Device for monitoring the sodium concentration in a stream of mercury-sodium amalgam

The invention relates to the determination of the sodium concentration in mercury-sodium amalgams.

The invention relates in particular to a device or a device (these terms are used interchangeably in the following) for continuous monitoring of sodium metal concentration values in a stream of mercury-sodium amalgam. The device according to the invention comprises a reference electrode, a solid electrolyte made up of

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a wall of ß-aluminum oxide between the reference electrode and attached to, consists of, or contains the amalgam, and a stress sensitive device with electrical inlets connected to the amalgam and electrode respectively, the sodium metal concentration in the amalgam as the only function of the electromotive force formed simultaneously by the electrode and the amalgam Forces is monitored or measured.

In the above type of devices, the reference electrode, the solid electrolyte and the amalgam constitute one electrochemical battery cell, whose electromotive force (abbreviated as e.m.f. from the Anglo-Saxon expression "electromotive force" is an indication of the actual sodium concentration in the amalgam. The device is particularly suitable together with mercury-chlorine cells in which an aqueous solution of sodium chloride (Saline) is electrolyzed and a continuous stream of sodium laden procedural amalgam obtained from which aqueous sodium hydroxide is then isolated by decomposition of the amalgam with water. For one Successful operation of the electrolytic cell requires the sodium concentration in the effluent To continuously monitor the amalgam of the process and to regulate the operation of the cell (through suitable controls), if the concentration values determined differ significantly from the previously determined nominal value. The latter value is usually in the range of 0.15 to 0.30 wt .- 6. mainly because lower Concentrations than 0.15 wt. # Not economical

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and concentrations above 0.30% by weight degrade the flow properties of the amalgam. The most preferred nominal value is 0.25% by weight.

The use of β-alumina as the solid electrolyte in the aforementioned type of device was discussed by L. Msueh and DN Bennion in ¹ NT. Electrochem. Soc.% 7, 1128 (1971). These authors suggested sodium metal as the reference electrode. However, the use of sodium metal has been found to be impractical because extremely high purity sodium is required and because the sodium metal is extremely sensitive to moisture and oxygen in the atmosphere.

According to the invention, a device is provided as previously indicated, which is characterized in that the reference electrode is a reference amalgam or comparative amalgam with a known sodium metal and mercury concentration, the reference amalgam being an accumulation or ¹ poOl "in a depression or a Forms hole of an impermeable electrically insulating material, wherein a wall part of the material is in contact with the reference amalgam or comparative amalgam (these terms are used synonymously in the present application) and wherein this previously described wall part is formed from ß-alturinium oxide.

In a preferred embodiment of the invention is the amalgam of the previously described current the sodium-laden procedural amalgam that comes from a mercury-chlorine cell flows out and a nominal sodium concentration

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in the range of 0.15 to 0.30 wt%. The sodium concentration in the reference amalgam is selected so that it is in the range of 0.01 to 0.7 % by weight and differs from the nominal value by an amount corresponding to at least 0.05% by weight sodium concentration . If the nominal value of the process amalgam effluent from the process is 0.25 wt % sodium, the concentration of sodium in the reference amalgam will be at least 0.30 wt % or, alternatively, will be less than 0.20 wt%. In this case the preferred value is about 0.1% by weight and the preferred rule is that the sodium concentration in the reference amalgam is generally selected to be below the nominal sodium concentration value in the process amalgam.

In a preferred embodiment of the invention, in particular In connection with mercury-chlorine cells, the device or device contains a container into which the amalgam stream flows while a hydraulic sluice or hydraulic lock is being formed, which acts between the inlet of the stream into the container and the outlet of the stream from the container, the The aforementioned recess is immersed in the amalgam in the sluice. It is further preferred that the walls of the container, which form the downward part of the hydraulic lock, are extended upwards and for any saline solution carried away by the amalgam flow from the mercury-chlorine cell, a collection chamber form.

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The subject matter of the invention is also a probe or a button (these terms are used in the present application used synonymously), which can be used in the device described above. The probe according to the invention comprises a tubular stem or tube made of an impermeable electrically insulating material a closed lower end and a measuring head or a bracket or a mounting bridge at its upper End, where the accumulation of mercury-sodium amalgam is present in the lower end part of the stem, the sodium metal concentration in the amalgam has a selected value in the range from 0.01 to 0.7% by weight and a part of the Trunk wall - in contact with the amalgam collection, the TaL deir trunk wall through an insert made of ß-aluminum oxide is formed with an exposed outer surface, an electrical connection carried by the measuring head, and an electrical path or conductor in the cavity of the trunk through which the cluster connects to the Connection is conductively connected. When in use, the lower end of the probe is placed in the stream of the amalgam (procedural amalgam) immersed and then the connection is made to the appropriate inlet of the monitoring device or measuring device connected, for example with a millivoltmeter and / or a process computerj a separate deliberately provided electrical conductor connects the second inlet of the measuring device with an area or part of the Amalgam flow proximal to the ß-aluminum oxide insert. In a preferred probe according to the invention, however, this separate electrical conductor is avoided and such a probe comprises a second electrical connection the measuring head and a second electrical path along the

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Stem, the second path being electrically isolated from the first path and insert and separate from the second terminal extends into the vicinity of the insert with the lower end of the second path exposed to the outside of the Trunk is exposed so that it directly contacts the amalgam stream when the lower end of the part of the trunk enters the Electricity is immersed. Both the single way or the single ladder and the double way or double ladder of the probe are paths or conductors, which are preferably made of platinum wire.

The production of ß-aluminum oxide suitable as a solid electrolyte is well known. Typically an aluminum compound, such as common alumina, becomes α-alumina or aluminum hydroxide, with a suitable sodium compound, for example the oxide or hydroxide, or another easily decomposable salt, such as carbonate, citrate or oxalate. Implementation can be in solid State or in the molten state. In the latter case, the temperature is excessive in order to avoid it Sodium oxide evaporation losses preferably below 165O ° C.

beta-alumina can be generally prepared with alkali metal salts in the molten state at about 1000 ⁰ C also by thermal decomposition of sodium aluminate (NaAlO ") at elevated temperatures in the order of 165o ° C or by reaction of a-alumina.

Either of the above processes produces β-alumina with the basic formula NagO 11Al ^ O ,. To form a solid

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ψ -

Electrolytes, particles of β-aluminum oxide are sintered together, even without mechanical compression, at a temperature of approximately 1700 ^° C. or higher within a short time, so that loss of sodium oxide is avoided. For the production of beta-alumina-tablets which are used for the present invention, preferably, the sintering is preferably performed at a temperature below 1700 ⁰ C, preferably in the range from 1000 to 130o ⁰ C under a mechanical pressure of 1000 to 3000 kg / cm carried out. Tablets produced in this way have a density of 3.15 to 3.25 g / cm3 and show very good mechanical strength. They are also resistant to the diffusion of amalgam through them. The thickness of the tablets or, in general, the thickness of the walls of the β-aluminum oxide in the device according to the invention is not particularly critical. A thickness of 1 to 3 mm is sufficient, at least in the preferred device according to the invention. The electromotive force developed by the device is essentially independent of the thickness and density of the wall of β-alumina and practically completely independent if the above-mentioned values are observed.

Preferred embodiments of the present invention are illustrated with reference to the accompanying drawings Embodiments explained in more detail.

Fig. 1 is a graph showing the electromotive Force in millivolts (plotted as the ordinate) from a device according to the invention as Function of the concentration of sodium metal in the current to be monitored from amalgam is plotted.

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> - σ -

Figure 2 is an illustration of a probe according to the invention.

Figure 3 is an illustration of a portion of the probe of Figure 2 with the lower end modified.

Fig. 4 is a device according to the invention shown together with a mercury-chlorine cell.

In FIG. 1, the sodium concentration is shown as a logarithmic function both in mole fraction below the abscissa and in percent by weight (above the abscissa). Obviously, the values can be converted into one another in a simple way. A 0.1 wt .-% concentration corresponds to a molar fraction of 8.8 χ 10 ", whereas a 0.5 wt .-% concentration corresponds to a molar fraction of 0.87 × 10". A 0.01% by weight concentration corresponds to a molar fraction of 4.2 χ 10. The electromotive force curves in Figure 1 are determined using a tablet of β-alumina with a diameter of 20 mm, 2 mm thick and a density of 3.24 g / cm3. The sodium concentration in the amalgam is 1 reference χ 10 "(mole fraction) for the curve A, and 1.2 χ 10" - ⁵ for the curve B, which corresponds to about 0.12 wt -.% And 0.04 wt-96. An Orion model 801-A digital voltmeter is used as the measuring instrument. The temperature of the amalgam to be tested is 75 ° C. This temperature is the typical temperature for procedural amalgam given off by mercury-chlorine cells.

Some values (from a series of 80 readings) related with curve A are listed below:

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Molfraction Na
(χ ΙΟ " ³ ) emf
mV 0.174. -140.2 1.245 -72.1 2.172 -53.8 3,099 -44.1 4.113 -33.5 5.103 -24.0 6,800 -14.4 9,857 -1.4 10,560 +2.7 15,440 +17.1 20.876 +32.5 31.505 +55.5 40.296 +71.5 55.518 +100.2

A series of parallel curves that are identical to curve A are compared with reference amalgam with sodium concentrations of 1.2 10 ", 1.4 χ 10", 6.8 χ ΙΟ "* ⁵ , 22.4 χ 10 * " ³ and 62.6 χ 10 ^-3 (as the mole fraction). Curve B is one of this series of curves.

Area C in FIG. 1 corresponds to the typical concentrations of sodium metal in procedural amalgams leaving a mercury-chlorine cell. 0.15% sodium concentration values correspond to a molar fraction of 1.29 x 10, whereas a 0.30% sodium concentration corresponds to a Molfraction of 2.56 χ 10 "corresponds.

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The probe shown in Figure 2 comprises a tubular stem 10 or a hollow tube approximately one length of 60 cm, an outer diameter of 20 mm and an inner diameter of 10 mm. The stem 10 is associated with a measuring head 12, which is a cylindrical tubular Body 14 having a length of 50 mm and a pair of mounting flanges 16 and 18 comprises. The outside diameter of the body 14 is 40 mm and the internal cavity forms a direct, upwardly extending extension of the Inner cavity 11 of the trunk. The trunk 10 and the body 14 may be integrally molded from polypropylene, although other materials (e.g. ceramic or glass) are used can be. For clarity of illustration, the flanges 16 and 18, which are also formed from polypropylene, are shown somewhat removed from the body 14. With the actual In use, the body 14 clamps between them when the probe is in situ with fastening screws (not shown). Bolts or pins extending downward through opposing openings 20 and 22 in the flanges extend. An O-ring 24A is between the upper side of the body 14 and adjacent to the flange 16 for sealing the cavity 15 with respect to the Outside atmosphere provided.

The lower end portion of the stem provides a well for the accumulation 24 of the control amalgam with a concentration of sodium from 0.01 to 0.7% by weight, preferably 0.1% by weight. The depth of the accumulation or "pool" 24 is approximately 6 to 7 cm. The lower end of the trunk is closed and the closure wall ^, - consists of one Disc 26 made of ß-aluminum oxide with a diameter of

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10 mm, a thickness of 2 mm and a density from 3.15 to 3.25 g / cm-. ⁵ The diameter of a part 11A of the cavity 11 adjacent to the disc 26 is reduced to 4 mm.

The upper flange 16 is centrally equipped with an electrical connection 28 which is formed by an outer bracket 28A and an inner bracket 28B. A platinum conductor 30 extends from the inner bracket 28B in the cavities 15, 11 and immersed in the accumulation 24 with its lower end ^5. The upper bracket 28A is used for connection to the measuring instruments.

In the illustrated embodiment, the head body 14 includes another bracket 32. Another platinum conductor 34 is embedded in the body 14 and the wall of the trunk 10 and extends downward from bracket 32 to the lower end of the trunk. The lower end 34A of the conductor 34 protrudes a few mm from the lower front of the trunk in the vicinity of the disc 26 and can be slightly curved towards the latter.

In the embodiment shown in FIG. 3, in which the same parts are provided with the same reference numerals as In Figure 2, the β-alumina disk 26A is fitted laterally into the lower end of the stem 10 and is the reference amalgam in the accumulation 24 the disk is in contact through or via a lateral or radial passage 11B a diameter of 4 mm. The lower end 34B of the platinum conductor 34 extends laterally from the trunk into the Neighborhood of disk 26A.

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The term "neighborhood" as used herein means that the lower end 34A or 34B of the conductor 34 is sufficiently close to the β-alumina disc that it immersed in the stream of procedural amalgam along with the disc. Typically the lower end of the trunk emerges about 6 to 8 inches into the stream. However, there is no benefit if the lower end of the conductor 34 from the Disk is more than a few cm away ",

In Figure 4, 40 is a typical mercury-chlorine cell with a sloped bottom 40A and an amalgam pickup tube 40B at the end of the cell in the direction where the bottom 40A is inclined. The amalgam level in the Cell labeled 42. The tube 40B empties or opens in the direction of an upright container or a chamber 44 through one (44a) of the side walls of the latter. The horizontal cross section of the chamber is generally rectangular. The chamber is closed at the top by a cover plate 44B. In wall 44C of the Chamber 44, facing wall 44A, is fitted with a tube 46 through which the procedural amalgam PA is continuous is removed from the chamber in the direction of the not-shown amalgam decomposer. The tubes 4OB and form the inlet and outlet for the procedural amalgam in and out of chamber 44. Preferred is the height where the outlet tube branches off from the chamber, slightly lower than the height where the inlet pipe 40B opens into the container. The interior of the chamber 44 is divided into two spaces by a perpendicular impermeable wall 44D that is parallel to the walls 44A, 44C and extend from the cover plate

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44b extends down to a level L which is between the floor 44E of the chamber and the level where the outlet pipe 46 branches off from the chamber. The two aforementioned spaces are in the chamber through a gap 48 with each other in communication formed between the lower end of the inner wall 44D and the bottom 44E of the chamber. To this Type flows the procedural amalgam, although the chamber is one in the hydraulic field as a "hydraulic sluice" The designated lock forms where the flow of liquid (amalgam) assumes a U-shape and one that runs downwards Branch 50 and an upward branch 50A forms. The lock is between the inlet and the outlet the chamber 44 effective, so that the tubes 40B and 46 are always separated by a "plug" of amalgam in the lock are. The walls 44A and 44D, which form the downward branch of the hydraulic lock, extend deliberately upwards through a substantial amount, so that above the amalgam level a collection chamber 52 for any Saline solution is formed, which is carried away by the flow of amalgam from the cell 40. The saline solution 54 separates in this chamber due to the gravity and is removed via the pipe 56.

The chamber cover plate 44B contains fitted therein the probe of Figure 2 with the stem 10 of the probe extending downward extends through the cover plate and immersed in the amalgam flow in the hydraulic lock. Preferred the trunk dips into the downwardly extending branch 50 of the sluice and extends through the collection chamber 52. This makes the trunk constant at one temperature

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which is close to the temperature of the amalgam coming out of the cell 40. The tube 40B that the chamber 44 with the cell connects is preferably short or the chamber is directly connected to the end wall of the cell, so that an uncontrolled accidental cooling of the amalgam flow on its way from the cell into the chamber is avoided.

An electrical cable containing two conductors 60 and 62 connects brackets 28A and 32 to their respective ones Insertion into a measuring instrument 64, for example a digital millivoltmeter, its reading scale directly in the form of the sodium concentration in the amalgam in the hydraulic lock can be calibrated. Further conductors 66 and 68 branch off from conductors 60 and 62 and are connected to a process computer through which the operation of the cell 40 is controlled.

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Claims (10)

Claims 1. Device for continuous monitoring of sodium metal concentration values in a stream of mercury-sodium amalgam comprising a reference electrode, a solid electrolytes consisting of a wall of ß-aluminum oxide, which is attached between the reference electrode and the amalgam, and a voltage-sensitive device with inlets that are electrically connected to the amalgam and the electrode, the sodium metal concentration in the amalgam as a clear function of the electromotive force, which is formed jointly by the electrode and the amalgam is monitored, characterized in that the reference electrode is a reference amalgam made of sodium metal and is mercury of known sodium concentration, the reference electrode being an accumulation in a recess of impermeable electrically insulating material, a wall portion of which is in contact with is the reference amalgam and wherein the wall consists of or contains ß-aluminum oxide. 2. Apparatus according to claim 1, characterized in that the amalgam in the stream with one Sodium laden procedural amalgam flowing out of a mercury-chlorine cell is a nominal sodium concentration having a value in the range of 0.15 to 0.30 wt% that the sodium concentration value in the reference amalgam is in the range from 0.01 to 0.7% by weight and differs from the nominal value by an amount -16- 709818/0839 separates, which corresponds to at least a sodium concentration of 0.05 wt .- ^. 3. Apparatus according to claim 2, characterized in that the nominal value is approximately 0.25 % by weight and that the value for the sodium concentration in the reference amalgam is approximately 0.1% by weight. 4. Device according to one of claims 1, 2 or 3, characterized in that it has a container includes, in which the stream of amalgam flows while it is a hydraulic lock between the inlet in the Container and the outlet from the container, and that the recess is immersed in the amalgam in the lock. 5. Apparatus according to claim 4, characterized in that the walls of the container which the form downward part of the hydraulic lock, are extended upwards and a collection chamber for any saline solution that is carried away by the amalgam flow from the mercury-chlorine cell. 6. Device according to at least one of claims 1 to 5, characterized in that the wall made of ß-aluminum oxide has a density of 3.15 to 3.25 g / cwP and is 1 to 3 mm thick. 7. probe which are used in a device according to at least one of the preceding claims 1 to 6 can, characterized in that -17- 709818/0839 _ 26A971δ it comprises a tubular stem (10) of impermeable electrically insulating material with a closed one lower end and a measuring head (12) at its upper end, an accumulation (24) of mercury-sodium amalgam deposited at the lower end part of the stem, the sodium metal concentration in the amalgam being im Range of 0.01 to 0.7 wt .- # has a selected value and wherein part of the wall of the trunk is in contact with the amalgam collection, and through a pad of β-alumina with an exposed outer surface, an electrical connector carried by the measuring head and an electrical path into the cavity of the trunk which conductively connects the accumulation to the terminal. 8. Probe according to claim 7, characterized in that the insert made of ß-aluminum oxide a density of 3.15 to 3.25 g / cnr and a wall thickness of 1 to 3 now owns. 9. Probe according to one of claims 7 or 8, characterized in that it additionally comprises a second electrical connection on the measuring head and a second electrical path along the stem, the second way is electrically isolated from both the first mentioned path as well as from the insert and extending from the second connection into the vicinity of the insert, the lower end of the second path being freely exposed to the outside of the trunk. -18- 709818/0839 10. A probe according to claim 9, characterized in that each of the two paths consists of one Platinum conductor is made. 709818/0839