Purification of aqueous zinc sulphate solution

DESCRIPTIVE MEMORY filed in support of a request for

PATENT OF INVENTION in the name of:

Mitsui Mining &. Smelting Co., Ltd.

for:

Method and apparatus for purifying zinc sulfate solutions

Priority 'of two patent applications in Japan filed on June 25, 1974, under No. 72458/1974, August 1, 1974, under No. 88274/1974 and of a utility model application in Japan filed on July 4 1974, under N ° 79189/1974.

The present invention relates to a process for the automatic purification of an aqueous zinc sulphate solution used as an electrolyte for the electrolytic production of zinc and to an apparatus used according to this process.

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An aqueous zinc sulfate solution used as an electrolyte in the electrolytic production of zinc is generally obtained by leaching zinc calcine with sulfuric acid. During this leaching process, an oxidizing agent is added to the zinc sulphate solution to cause coprecipitation of the impurities in the calcine with iron compounds, in order to remove these impurities. However, despite the addition of the oxidizing agent for removing impurities, the zinc sulfate solution still contains impurities such as copper, cadmium, nickel, cobalt, antimony and arsenic which disruptively affect the electrolysis carried out subsequently for the production of electrolytic zinc.

Hitherto, a method has been commonly used in which zinc dust is added in several stages to the zinc sulphate solution containing undesirable impurities, so as to eliminate them by substitution and reduction using the difference between the tendency ionization of zinc and that of its impurities. Although this substitution and reduction treatment with zinc dust has been effective in removing most of these impurities, extreme difficulties have been encountered in removing cobalt among these impurities, because its concentration cannot be reduced to less than a few milligrams per liter thanks to the substitution and reduction treatment with zinc dust. It has therefore been common practice to add a-nitroso-ßnaphthol [C. (H) (NO) OH] to the zinc sulphate solution so as to remove the cobalt by causing it to precipitate in the form of nitroso- ß-naphthol cobaltique [Ο ^ θΗθ (NOjOj ^ Co. There is a great demand for an automation of the process of purification of the a · * ♦ * • · · ·

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aqueous zinc sulfate solution. However, the desired automation of the purification process for the zinc sulfate solution was difficult to achieve for the reasons described above, despite this high demand. First, several steps must be incorporated for the removal of unwanted impurities. Secondly, it was impossible to automatically and continuously measure the pH value of the aqueous zinc sulphate solution despite the fact that an accurate measurement of this value for the solution is required for precise control of the quantity. of cc-nitroso-ß-naphthol to be added to the solution. Third, a fully satisfactory device capable of supplying zinc dust in a constant quantity and of automatically measuring this quantity has not yet been designed.

The basics of the technique used for the automatic supply of zinc dust will now be described in detail. Zinc calcine is leached into a solid / liquid separation section during the electrolytic production of zinc. In this leaching section, unwanted impurities such as nickel, cadmium and cobalt are also leached together with the zinc. These impurities must be removed by an elimination treatment in the purification section and the purified aqueous zinc sulphate solution practically free of these impurities must be supplied to the electrolysis section, because the current yield and the purity electrolytic zinc obtained by electrolysis in the electrolysis section is reduced if these impurities are still present in the electrolyte. In the scrubbing section, therefore, the concentration of the impurities in the aqueous zinc sulfate solution supplied from the leaching section is reduced to a value below a predetermined allowable limit.

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According to the present invention, the purification of the leached solution is carried out by means of a continuous process in four stages, as follows:

first and second steps:

removal of copper, nickel, cadmium, etc. by the usual method of using zinc dust;

third step:

cobalt removal using d-nitroso-ß-naphthol; fourth step:

elimination of the excess of reactive agent, added during the third stage using activated carbon.

In order that the current efficiency in the last section cL electrolysis can be kept practically constant, the supply control for zinc dust must be carried out with precision. However, due to the fact that the weight of zinc dust transported per unit length of the transport means such as a feeding device is not constant even when the transport speed is constant, it is extremely difficult to measure with precision the weight of zinc dust transported by the feeder.

Various methods have been used so far in the hit to precisely control the feed. For example, prior art methods have incorporated the use of an electromagnetic feeder, a constant feed pump for supplying a constant weight of zinc dust in the form of slurry, and a device feed table with variable rotation capacity or variable feed speed. However, none of these systems has been satisfactory for obtaining direct measurement and control. For example, a prior art method for supplying zinc dust using an electromagnetic feeder similar to that also used in accordance with the present invention has been to charge a predetermined amount of zinc dust into a hopper intended to supply this zinc dust to the feed device, discharging zinc dust onto the feeder from the lower terminal opening of the hopper, and measuring the weight of the zinc dust remaining in the hopper after a predetermined period of time, thereby determining the weight zinc dust actually supplied to the feeder. Therefore, this prior art method called for indirect measurement. With such an indirect measurement, it was extremely difficult to accurately measure the weight of zinc dust actually supplied and the need to have operators for the measurement constituted a significant obstacle - to the desired automation of the whole system for the production of zinc é-

<img file="BE830643A_D0002.tif" />

One of the reasons causing difficulties for the desired automatic feeding of zinc dust is that not only the weight of this zinc dust supplied in the first and second purification stages is relatively low, but also that the zinc dust has a relatively high bulk density of about 3.2, is hygroscopic and tends to coagulate easily when subjected to excessive friction.

The basics of the technique used for continuous measurement of the pH value of the purification solution will now be described in detail. In the third stage of the electrolyte purification section, in which the cobalt is removed&gt; ".mut dd W'WH WliiMflWBBMffBIWMli '6 ·· · * ··" · · ·· · "··"

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by adding α-nitroso-ß-naphthol (the so-called NN reagent) to the solution, a test paper has so far been commonly used to measure the pH value of the solution in order to control this value . With such a method, however, the pH value could not be accurately measured and the impossibility of continuous measurement resulted in insufficient pH control. In order to overcome this drawback, there has been a strong demand for continuous measurement of the pH value using a process pH meter. However, the process using this process pH meter has also been defective and encrustations in that oxides adhere to the glass surfaces of the electrodes of the pH meter a few hours after the start of the measurement, which leads to an impossibility of precise measurement. In order to carry out the desired continuous measurement with such a pH meter, the oxides must be efficiently removed, without adversely affecting the function of the electrodes of the pH meter. However, the oxides or incrustations adhering to the surface of the glass electrodes appear so ·&gt; the shape of a gom-

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meuse formed by polymerization of α-nitroso-ß-naphthol and generally adheres to an extent such that 1

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very difficult. Although means of brushing have been. made and have proved to be very useful for effective removal of such adherent encrustations, automatic brushing means suitable for use with electrodes of the kind previously described have not been designed so far. This is mainly due to the difficulty of assembling an automatic brushing device in the pleat measuring device, for the following reasons, among others. First, the electrodes of the pH measuring device are small and covered with a glass film, so that they do not have

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··· sufficient mechanical resistance to withstand the force of cleaning. Second, careful handling is required since the pH meter is a very precise and sensitive measuring instrument. Third, the electrodes should be periodically removed from the pH meter for maintenance. For the foregoing reasons and others, it has been difficult to incorporate automatic electrode brushing means into the process pH meter.

The basics of the technique used to estimate the purity of the aqueous zinc sulfate solution used for the electrolytic production of zinc will now be described in more detail. Drawbacks including undesirable reductions in current efficiency and product quality are caused and a long period of time is required to overcome these drawbacks when the purified aqueous zinc sulfate solution supplied to the electrolysis section of the electroplating plant for zinc is not sufficiently purified and contains ions of metallic impurities which will deposit together with the zinc and reduce the overvoltage of hydrogen at the cathode. Due to the fact that the current efficiency of the electrolysis section is thus greatly disturbingly affected by the presence of such metal ions of impurities, it is necessary to provide suitable means for estimating the purity of the solution of aqueous zinc sulfate supplied to the electrolysis section of the electrolytic zinc installation. So far, individual impurities in the aqueous zinc sulfate solution have been analyzed to estimate the purity of this purified solution.

However, this method of estimating the purity is troublesome · * »4« »♦ *» t · · · · ·· • · · · ··· · · · * · · * · · · ····. · * · · · · ·· · · ·· ·· »· ·· ·· · ·· and takes a long time and an accurate estimate cannot be obtained simply on the basis of the individual analyzed values, this estimate is not not always being precise and reliable because of the fact that the disturbing effect of the impurity is variable according to the mutual action of these impurities. Other means of estimating the purity have been proposed up to now, means which consist in appropriately adjusting the acidity of the purified aqueous sulphate solution, in carrying out a deposition on an anode and a cathode in a container. enclosing the electrolyte, supplying the electrodes and directly measuring the hydrogen overvoltage at the cathode. However, these means have not been widely used due to the fact that the device used for the measurement is complex and expensive in structure and is not perfectly reliable.

A main object of the invention is to offer a new and improved process for the purification of an aqueous zinc sulphate solution used as an electrolyte during the electrolytic production of zinc, which solves various technical problems outlined above. and which can be used economically to purify the aqueous solution of sulphate I sine in the desired manner with a minimum of operators, so that this solution can be supplied to the electrolysis section of the electrolytic zinc production installation without practically being accompanied by undesirable impurities.

According to one aspect of the invention, there is provided a method for purifying an aqueous zinc sulfate solution used as an electrolyte in the electrolytic production of zinc, which consists in precisely controlling the weight of dust of zinc supplied by the first stage feeder

<img file="BE830643A_D0006.tif" />

purification and the following stages, in order to remove the impurities from these purification stages, to measure continuously by means of a pH meter, the pH values ​​of sample solutions obtained from the final purification stage, to be controlled precisely using the aforementioned measured pH values, the quantity of α-nitroso-ß-naphthol to be added so as to remove the cobalt ion contained in the abovementioned aqueous zinc sulphate solution of l 'final purification stage, sampling the purified aqueous zinc sulphate solution from the final purification stage separately, in order to subject it to electrolysis and to measure the content of hydrogen gas produced by the electrolysis, so that one can estimate the degree of purity of the purified aqueous zinc sulphate solution.

According to another aspect of the invention, an apparatus is provided for automatically supplying zinc dust to the first purification stage and to the following purification stages for the purification of an aqueous zinc sulphate solution used as an electrolyte in the electrolytic production of zinc, which comprises a table, an endless belt type feeder passing around a head pulley and a rear pulley spaced apart on the table , a base arranged below this table, a support member mounted on this base and having to be connected by a pivot to the table in order to support it by allowing an oscillating movement, means mounted on the table in order to detect the weight communicated to this so as to measure the weight of zinc dust transported by the feeding device of the endless belt type, a hopper arranged above the endless belt type feeder for storing zinc dust therein, and a

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···· ·· * ·· t * ·· »* ·« • · · · Φ · 4 ♦ · 9 «· 9 · · ·» ··· · · • · «· · · · · · · · · * ·· ·· · *. 9 · · positive of electromagnetic supply arranged below

1¹ hopper lower end opening to supply the endless belt type feeder with zinc dust discharged from the hopper lower end opening. The automatic zinc dust supply apparatus according to the invention is very effective in solving the problems encountered hitherto in automating the supply of zinc dust, due to the fact that the weight zinc dust supplied can be measured directly and accurately without requiring any operator.

In the apparatus of the kind previously described, the means intended to measure the weight of zinc dust may comprise a rigid rod fixed to the table in a position situated in front of the support member, and a load cell provided on the abovementioned base opposite the rod and comprising at least one extensometer in pressure engagement with this rod.

In the apparatus of the kind depicted, the means for measuring the weight of zinc dust may include a spring interposed between the table and the base in order to receive the load communicated by this table, a rigid rod of magnetic material mounted on the table. and a differential transformer arranged in the vicinity of the rod, so that the variation in the inductance of the differential transformer caused by the vertical displacement of the rod is detected to measure the weight of zinc dust transported.

In the apparatus of the kind depicted, a chute having a storage capacity of zinc dust less than that of the hopper can be arranged between the latter and the electromagnetic supply apparatus. Providing this gou-

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ft ft

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• ft * · ft ft * · *; · “·” Monkfish is advantageous because the cross-sectional area of ​​the lower end opening of the hopper can be increased thereby avoiding unwanted snagging and bridging of zinc dust.

According to yet another aspect of the present invention, there is provided an apparatus for automatically cleaning the surface of the electrodes of a pH meter used to measure the pH value of a solution purified with a-nitroso-ß- naphthol in order to remove cobalt therefrom in the final purification stage among several during the electrolytic production of zinc, which comprises a measuring tank containing a sample solution supplied for a purified section to be subjected to the measurement of the pH value, a cleaning tank arranged in the vicinity of the measuring tank and in which a cleaning brush is mounted on a rotary shaft in order to accompany it in rotation, means for vertically movably supporting the electrodes of the pH measuring device in order to bring them to the measuring position in the measuring tank, in which the pH value of the purified solution is measured by the measuring device, and at the cleaning position in the cleaning tank where the electrodes are cleaned by the cleaning brush, and means for oscillating support horizontally the electrode support means for moving the electrodes of the pH measuring device between the position for measuring the pH value in the measuring tank and the cleaning position in the cleaning tank. The automatic electrode cleaning apparatus according to the invention is advantageous because the pH value of the purified solution can be measured continuously and automatically and the actual pH control can be reliably obtained so that the Weights of α-nitroso-ß naphthol supplied can also be ordered precisely. In addition, the amount of

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"· **. * ·. ·· *** 'cobalt present in the purified solution can be reduced to less than a few milligrams per liter and it is therefore possible to obtain a neutral solution free of undesirable impurities which affect so disruptive current efficiency in the last section d. 'electrolysis.

According to yet another aspect of the present invention, there is provided a method for estimating the purity of an aqueous zinc sulfate solution purified in the final purification stage among several during the electrolytic production of zinc, by determining that the purity of the electrolyte used in the hydrometallurgical production of zinc is in a predetermined ratio with the quantity of hydrogen gas produced by the electrolysis of the electrolyte, this process consisting in sampling a part of the purified aqueous zinc sulphate solution coming from the final purification stage in order to subject this sampled part of the solution to electrolysis so as to produce a mixture of hydrogen and oxygen gaseous, to send this gaseous mixture into a closed combustion chamber and to ignite the gaseous hydrogen in the gaseous mixture, and measuring the difference between the internal pressures of the combustion chamber before and after the combustion of the hydrogen gas in this chamber, which makes it possible to measure the content of hydrogen gas in the gas mixture.

According to another aspect of the invention, an apparatus is provided for estimating the purity of an aqueous purified zinc sulphate solution which has been purified in the final purification stage among several during the electrolytic production of zinc, which comprises an electrolytic cell for the electrolysis of part of the purified aqueous zinc sulphate solution sampled from the final purification stage, thereby producing

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• 4 · a mixture of hydrogen and gaseous oxygen, a cover member ·· »» ·· · »• - en * · · i * • * en · * ·· ^Λ "• t 9 w * *" * * ·· • · · ·· · * "· * en *" * ^r »'Ture covering the electrolytic cell so as to define a closed space between it and the electrolytic cell, a combustion chamber connected to the aforementioned closed space by a duct having a valve allowing and blocking the flow of a fluid so that the hydrogen gas in the mixture produced by the electrolysis in the electrolytic cell can be ignited while the combustion chamber is completely closed, means for sucking into this combustion chamber the gaseous mixture produced by electrolysis in the electrolytic cell and filling the closed space, a firing heating element arranged in the combustion chamber in order to ignite the hydrogen in the gas mixture and a pressure gauge connected to the combustion chamber in order to measure the internal pressure therein before and after the combustion of the hydrogen gas, to consequently measure the content of hydrogen gas in the gas mixture to determine the purity of the purified aqueous zinc sulfate solution based on the content of hydrogen gas thus measured.

In the device which has just been depicted, the means intended to draw the mixture of gaseous hydrogen and gaseous oxygen into the combustion chamber comprise a U-shaped connection duct connected by one of its ends to a overflow and its other end to the lower wall of the combustion chamber in order to supply a liquid to the latter from the overflow tank to a level of liquid determined by the level of overflow of this tank, a drainage pipe connected to a lower part of the connection pipe and comprising a valve for draining the liquid from the combustion chamber, a level gauge arranged in

<img file="BE830643A_D0012.tif" />

<img file="BE830643A_D0013.tif" />

the connection duct in order to detect the complete evacuation of the liquid from the combustion chamber when the liquid is evacuated from this combustion chamber, a valve arranged in the connection duct in a position adjacent to the other end of this conduit connected to the combustion chamber in order to allow and block the flow of the liquid through the connection conduit, and a vent pipe connected to the upper wall of the combustion chamber having a valve for sending the burnt gases into the atmosphere from the combustion chamber.

The depicted appliance may also include a first magnet member disposed inside the combustion chamber in order to agitate the gas mixture introduced into this chamber, a second magnet member disposed outside the combustion chamber. combustion opposite to the first and an engine carrying the second magnet member at one end of its rotary shaft.

The apparatus of the kind previously described may also include a differential pressure transmitter intended to transmit to a recorder a signal representing the differential pressure in the combustion chamber, as detected by the pressure gauge.

Other details and particularities of the invention will emerge from the description below, given by way of nonlimiting example and with reference to the appended drawings, in which:

FIG. 1 is a flow diagram illustrating a sequence of operations in an embodiment of the method according to the invention, intended for the automatic purification of a solution of zinc sulphate used as an electrolyte during the electrolytic production of zinc.

Figure 2 is a schematic elevational view illustrating the basic structure of an apparatus according to the invention, intended ·&gt; • -.

<img file="BE830643A_D0014.tif" />

. · · · · · · T ·· · · - ·· ·· zinc dust.

FIG. 3 is a section view on a large scale of a hopper part illustrated in FIG. 2.

FIG. 4 is a schematic view illustrating the basic principle of measuring the total weight of the pulverulent material transported * by a feed device of the endless belt type illustrated in FIG. 1.

Figure 5 is a circuit diagram of extensometers used according to the invention as a form of means for measuring the weight of the powdery material.

Figures 6 to 9 are right side elevation, left side elevation, front elevation and plan views, respectively, of a preferred embodiment of the automatic powdery material supply apparatus according to the present invention .

Figure 10 is a block diagram of a control system preferably used with the automatic powdery material supply apparatus according to the invention.

FIG. 11 is a graph illustrating the weight data of the pulverulent material measured by the weight measurement means and recorded on an instantaneous weight recorder illustrated in FIG. 10.

Figure 12 is a perspective view of an apparatus for automatically cleaning the electrodes of a pH measuring apparatus used according to the invention.

FIG. 13 is a perspective view of means intended to cause a rotational movement of a support block illustrated in FIG. 12.

FIG. 14 is a perspective view on a large scale of a cleaning brush and associated parts, illustrated in FIG. 12.

FIG. 15 is a flow diagram of the final stage of a purification section during the electrolytic production of zinc, illustrating the position of the apparatus according to the invention intended for automatically cleaning the electrodes of the apparatus pH measurement.

Figure 16 is a graph giving the measured data for the pH value of the purified aqueous zinc sulfate solution when the electrodes of the pH meter are cleaned using the automatic cleaning apparatus according to 'invention.

Figure 17 is a graph for comparison showing the measured data for the pH value when the measurement is made without cleaning the electrodes of the pH meter.

FIG. 18 is a schematic view of an apparatus according to the invention, intended for the estimation of the purity of the purified aqueous zinc sulphate solution used for the electrolytic production of zinc.

FIG. 19 is a large-scale front elevation view of a part of the apparatus illustrated in FIG. 18.

The figure _£20 is a plan view along line III-III of FIG. 19.

The process for the purification of an aqueous zinc sulfate solution with zinc dust will now be described. To automatically supply the zinc dust used during the electrolytic production of zinc, a hopper 1 containing a pulverulent material such as zinc dust 2 is fixed in position by means of fixing means (not shown

<img file="BE830643A_D0015.tif" />

• 4 in figure 2). The hopper 1 is provided at its upper end with a zinc dust loading opening 3 and at its lower end with a discharge opening 4. The cross-sectional area of ​​the hopper 1 is gradually reduced from the top towards its lower part, as illustrated in figure 2. In order to avoid so-called hooking or bridging phenomena caused by the coagulation of zinc dust 2 in the lower end part of the hopper 1, a vibrator 5 is mounted on an appropriate part of the hopper 1 , as shown.

Zinc dust 2 has a very high apparent specific gravity, it is hygroscopic and it tends to coagulate easily when it is subjected to excessive friction, as indicated above. Consequently, bridge formation tends to occur in particular in the lower end part of the hopper 1, due to the displacement under the effect of the gravity of the zinc dust 2. The lower end or discharge opening 4 of the hopper 1 must have a predetermined cross-sectional area which corresponds to the desired weight of zinc dust 2 continuously discharged from this discharge opening 4. According to the Invention, a chute 6 is provided as a lower end extension of the hopper i in order to avoid such an undesirable formation of zinc dust bridge 2 in the hopper 1. This chute 6 has a capacity lower than that of the hopper 1 and serves as secondary storage means. Consequently, the zinc dust 2 is not discharged directly from the hopper 1 and the predetermined weight of zinc dust 2 is kept in the chute 6 before being discharged. The upper end opening of the trough 6 has a cross-sectional area

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• · · · ·· ·· - ·· · · · ”, transverse greater than that of the discharge opening 4 of the hopper 1, in order to avoid obstacles to a free movement downwards of the dust of zinc 2 as a result of the progressively smaller cross-sectional area of ​​the hopper 1. As a result, the unwanted entrapment or bridging of zinc dust 2 in the lower end part of the hopper 1 can be avoided. In addition, the zinc dust 2 in the trough 6 is also free from undesirable bridging, due to the fact that any significant charge causing coagulation is communicated to it indirectly.

Unwanted bridge formation can still be more surely avoided by placing a chain 7 on the inner wall surface of the trunking 6, as illustrated in FIG. 3. This chain 7 is welded or riveted to the inner wall surface of the trunking 6 in order to ensure a uniform downward movement of the zinc dust 2. The cross-sectional area of ​​the lower end opening 8 of the chute 6 can be made less than that of the lower end opening 4 of the hopper 1 by virtue of a connection of the chute 6 with the lower end hopper 1, as shown in Figures 2 and 3. An electromagnetic supply device 9 is arranged below the lower end opening 8 of the chute 6, as illustrated in FIGS. 2 and 3, so that? the quantity of zinc dust 2 discharged from the chute 6 can be controlled by appropriately determining the amplitude of vibration of the electromagnetic supply device 9.

Referring to Figure 2, a table 10 extending horizontally is arranged below the electromagnetic feed device 9 and a head pulley 11 and a pulley

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shank 12 are mounted, respectively, on a front end part and a middle part of the upper surface of the table

10. A feeder of the endless belt type 13 passes around the head pulley 11 and the tail pulley 12 in order to transport the zinc dust 2 discharged towards a purification section (not shown), passing through a conduit 14 fixed to a body part (not shown in Figure 2). A motor 15 is mounted on a rear end part of the upper surface of the table 10, and a belt 16 passes around the tail pulley 12 and a pulley 17 on the motor shaft 18, in order to drive the belt type feeder

13. A pair of brackets 19 is fixed to an appropriate part of the lower surface of the table 10 and these brackets pivot with the aid of a pivot pin 22 on a support member 20 disposed on a base 21, so that the table 10 can tilt or oscillate around the pivot pin 22.

A rigid rod 23 extends downward from a suitable part of the lower surface of the table 10, so that the weight of the zinc dust 2 transported on the belt type feeder 13 can be measured by detecting the charge communicated in table 10. A load cell 24 consisting of four G ^ extensometers, as illustrated in FIG. 5, is fixed to the upper surface of the base 21 and it is engaged with the rod 23 so as to detect the load which is communicated to the latter .

A balancing weight 25 is fixed to a rear part of the lower surface of the table 10 in order to keep the latter in a horizontal position. A retaining member 26 is fixed to any suitable part of the lower surface of the table 10 in order to maintain an adjustment balance weight 27 therein to allow fine adjustment of the balance. The value of • · »·

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balance balance weight 27 is related to the weight detected by the load cell 24.

The load cell 24 detects the charge communicated to the rod 23 by detecting variations in the resistance of the extensometers at G ^. To compensate for errors of the detected load due to the temperature coefficient of the resistance, the four extensometers G ^ to G ^ are arranged so as to constitute a Wheatstone bridge, as illustrated in FIG. 5. The load on the rod 23 is communicated to one of the extensometers or to the pair of extensometers arranged diagonally, for example G ^ and G₃ or G₂ and G ^.

Another suitable means for measuring the weight of zinc dust may comprise a helical spring interposed between the lower end of the rod 23 and the base 21 so as to receive the load from the table 10 and a differential transformer arranged in the vicinity of the rod 23, so that the charge communicated to the rod 23 can be detected by determining the variations in the inductance of the differential transformer caused by the vertical displacement of the rod 23. Yet another means can be constituted by a lever connected by one of its end parts to the rod 23 so as to detect the vertical displacement of the latter.

A method for measuring the total weight Q of the zinc dust transported by the belt-type feeder 13 for a predetermined period of time by measuring the charge communicated to the rod 23 will now be described with reference to the figure. 4. An instantaneous weight q (t) [kg / sec. ] transported by the belt type feeder 13 is given by the following equation:

q (t) = W (t) · V (t) .............. (1) in which W (t) [kg / m] is a unit of weight of the dust from i

Cf zinc on the belt type feeder 13 and V (t), expressed in meters per second, is a transport speed of the belt type feeder 13.

The instantaneous pressure F (t), expressed in kg, communicated to the load cell 24 is given by the following equation:

F (t) = W (t) · T¹ .................. (2) l₂ in which expressed in meters is half the total length of the belt type feeding device 13 and L₂expressed in meters is a horizontal distance between the pivot point and the center of the load cell 24.

Consequently, the total transported weight Q expressed in kg is given by the following equation:

q = i^t2 q (^fc) * clt ................ (3) ti = ç^fc2 W (t) · V (t) · dt ^ 1

Since V (t) is a constant, Q is expressed as follows:

q = ^ 2. _vjt_{2 F (t}.) . _{dt = K} f-2 _{F (t}) . _dt........ (4)

Iq tl

It will be seen from equation (4) that the total weight transported Q is determined by measuring the force F (t)_z by integrating this force F (t) for the period at t₂ and multiplying the integrated value by K which is a constant.

Another preferred embodiment of the apparatus for automatically supplying zinc dust according to the invention will be described with reference to Figures 6 to 9.

The body 30 of the automatic feeding apparatus is mounted on a reinforced beam structure 31 and a chassis structure 32 made of steel profiles is firmly fixed to the beam structure 31. Another chassis structure 33 made of steel profiles steel is rigidly mounted on the chassis structure 32. A hopper 1 with an opening for loading zinc dust

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is firmly fixed to the chassis member 33 and a chute 6 constituting an extension of the lower end of the hopper 1 is connected to the body 30 of the device by a flexible seal 34. A ladder 35 extends upwards to from there. beam structure 31 to a suitable position above the hopper 1, so that an operator can observe the state of the zinc dust 2 in the hopper 1.

The body 30 of the automatic feeding device is constructed from several profiled steel elements and a table 10 is supported in this body 30 so as to be able to pivot on a support member 20. The table 10 is constituted by a pair of members 36 of profiled steel extending parallel to each other in the longitudinal direction of the body 30 and several members 37 of profiled steel extending in the lateral direction so as to connect the members together 36. A shaft 38 is supported at opposite ends by a pair of bearings mounted on a rear part of the members 36 and another shaft is carried at opposite ends by a pair of bearings 41 mounted on a front part of the members 36. A head pulley 11 and a tail pulley 12 are mounted on the respective shafts 40 and 38 and the belt type feeder 13 passes around the pulleys 11 and 12. A mad pulley 42 is disposed between the members 36 so as to be engaged by the endless belt 13, as illustrated in FIG. 8. It will be appreciated from Figure 8 that a pair of spaced vertical members 43 is attached to one of the members 44 constituting the body 30 of the apparatus and a horizontal member 45 connects the lower ends of the vertical members 43 so as to constitute support means 46 intended to support a tension pulley 47 which communicates a tension to the endless belt 13. An adjustment screw • · extends through the horizontal member 45 in order to adjust. . "Ii" -A * -i * '· ft # "4 ·; · · ♦ *" 4 # ·

- - * · · :· ♦ · ♦&lt;

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- · · · · #. ·· ·· * ♦ # »• * # ·· • · · • · the vertical position of the tension pulley which can be moved vertically along the inner wall surface of the vertical members 43.

In the structure illustrated in the figure

8, a differential transformer 49 is arranged in the body 30 so as to serve as means for measuring the weight of the zinc dust transported “by the supply device 13. The differential transformer 49 is associated with a rigid rod of material such as iron (not shown), fixed to a connecting member 50. The latter is connected to the table 10 by a pair of support members 51 and it is also connected by a helical tension spring 52 to another pair of support members 53 fixed to the body 30.

A suitable balancing weight 27 is supported by a retaining member 26 on the table 10 and a covering member 54 is hinged to the upper surface of the body 30 so that the balancing weight can be removed.

The tension pulley and the table support member 20 are arranged so as to align with the same vertical line so that the resistance imparted by the tension pulley 47 to the moving belt 13 cannot affect perceptibly the balance of the table 10 by opposing the oscillating movement of the latter up to the desired horizontal position.

The rear end part of the table 10 is formed in steps downward relative to the front end part as seen in FIG. 8, in order to provide space for the mounting of an engine 15. The latter is mounted on the rear end portion in steps of the table 10. A pulley 17 is mounted on

1¹ motor shaft 18 and a belt 16 passes around this pulley 17 and the rear pulley 55 mounted on the shaft 38. A

<img file="BE830643A_D0020.tif" />

»* ·« 444 4 · φ »φ · ♦ ··« 4 4 4 b 4 · «» 4 4 «444 * · '« • 4 4 9 4 4 · φ «4 · _ν * · · · · “4 · φφ φ tension roller 56 is provided for communicating tension to the belt 16. The shaft of this roller or roller 56 is slidably housed in a slot 57 formed in a support plate 58 welded to the table 10, so that the tension imparted to the belt 16 can be adjusted appropriately.

An electromagnetic supply device 9 intended to supply the zinc dust coming from the chute 6 on the belt 13 is mounted by means of a vibration isolation spring 59 on a pair of brackets 60 fixed to the body 30 of the device, as shown in Figure 8.

FIG. 10 represents a control system preferably used for the apparatus illustrated in FIG. 2.

The charge communicated to the rod 23 is transformed into a voltage signal appearing as an output of the bridge circuit of FIG. 5, or of the differential transformer. This signal is applied via an AMP amplifier to a control device 61 which is supplied via a power transformer 62. The value of the load communicated to the rod 23 is indicated on an instantaneous weight indicator 64 connected to 1 ƒ control unit 61 and, at the same time, recorded on the instantaneous weight recorder 63 connected to the control unit 61 A weight integrator indicator 65 is also connected to the controller 61 to provide a digital display of the weight integration. The electromagnetic feeder 9 is connected to a control panel 66 through which the weight of the zinc dust supplied through the hopper 1 on the belt type feeder 13 can be adjusted . A control signal is applied to the control panel 66 from the control unit 61 and via an indicator which indicates the ** ·

<img file="BE830643A_D0021.tif" />

»* - 44 ·· ·· o en -« t. · Fr 4 I »fr. * • fr fr fr ·· ♦» «• · · · fr · · ♦ ·· ♦ ·« · · · · 4 4 ♦ ··· ·· 4 4 «4 4 * *« , predetermined weight value and the weight of zinc dust supplied through the hopper while automatically controlling the weight of the zinc dust transported to make it equal to the predetermined weight value.

FIG. 11 is a graph giving the weight values ​​obtained during 24 hours and indicated by the instantaneous weight indicator 64 of FIG. 10. In FIG. 11, the predetermined weight value is changed from 75 kg per hour to 63 kg. per hour after 18 hours.

Referring now to FIG. 12, a sample solution is supplied from a purification section of an electrolytic zinc production installation in a measurement tank 68, via a pipe. flexible 69, and a cleaning tank 70 is disposed juxtaposed with the measuring tank 68. Part of the sample solution supplied to the measuring tank 68 is evacuated by overflow from this tank 68 or by means of suitable evacuation means (not shown) arranged at the base of the measuring tank 68. A cleaning liquid or washing water is supplied via flexible pipes 72 to the cleaning tank 70 and discharged by overflow from this tank 70 by means of suitable drainage means (not shown) arranged at the base of the cleaning tank 70.

Another flexible hose 4 supplying compressed air extends to the cleaning tank 70. A cleaning brush 73 is arranged in the cleaning tank 70 and is mounted on a rotary shaft 74. A motor 75 is mounted on a chassis member 76 and drives the rotary shaft 74.

A support member 77 is arranged in the vicinity of the measuring tank 68 and the cleaning tank 70 in order to support a rotary support block 78. As illustrated in FIG. 13, a connecting rod 79 connects the base of the block support 78 to a rod of. · * · ♦ · “♦ * ♦ &lt;·“ ♦ ”- ···“

- ' ’ - » ♦ - * 4 4 4 4 - ·

Z ~ S J-. 4 4 "4 4 4 4" 444 ·

4 4 4 * 4 4 4 4 4 · piston 80 of a fluid cylinder, such as a pneumatic cylinder disposed horizontally in a cylinder box 82. Thus, the support block 78 pivots around the axis of the support 77 in response to the movement of. horizontal back and forth of the piston rod 80 of the pneumatic cylinder 81. Another fluid cylinder such as a pneumatic cylinder 83 is fixed vertically on one side of the support block 78. A piston rod 84 extends vertically downward from the pneumatic cylinder 85 so as to support at its lower end an electrode holder 86 which holds the electrodes of a pH meter. A control panel 87 is arranged on the other side of the support block 77 and it contains means intended to control the reciprocating movement of the piston rods 80 and 84 of the respective pneumatic cylinder 81 and 83, as well as means for controlling the oscillating movement of the support block 78. A pneumatic unit 88 is connected to the control panel 87 so as to be driven in response to the application of a control signal from the control panel 87 and to the supply of compressed air into the air hose 71 and the pneumatic cylinders 81 and 83.

The pneumatic cylinder 83 is actuated so as to immerse the electrodes of the pH measuring device in the sample solution of the measuring tank 68 when measuring the pH value of this sample solution in the tank 68. After a predetermined period of time during which the electrodes are used for measuring the pH value of the sample solution in the measuring tank 68, the pneumatic cylinder 83 is again actuated to spread the electrodes upwards from the position immersed in the measuring tank 68. The support block 78 is then made to pivot by the pneumatic cylinder 81 to bring the electrodes into the position located above the cleaning tank 70. The pneumatic cylinder 85 is again actuated to immerse the electrodes in the cleaning water

O ····

<img file="BE830643A_D0022.tif" />

of the cleaning tank 70 and, in the submerged position, the cleaning brush 73 is in cleaning contact with the surface of the electrodes of the pleat measuring device. The compressed air supplied via the air hose 71 into the wash water of the cleaning tank 70 produces air bubbles in the wash water so as to clean the surface of the electrodes, while simultaneously the surface of the electrodes is subjected to brushing by the rotary cleaning brush 73. Cleaning with air bubbles continues for approximately 15 minutes and brushing with the cleaning brush 73 for approximately 2 minutes. Upon completion of the cleaning operation, the pneumatic cylinder 85 is again actuated so as to spread the electrodes of the pH meter upwards from the position of immersion in the cleaning tank 70 and then the pneumatic cylinder 81 is actuated to bring the electrodes to the position located above the measuring tank 68, the pneumatic cylinder 83 then being actuated so as to lower the electrodes in the position for measuring the pH value in the measuring tank, 68.

The pH measuring apparatus equipped with the automatic electrode cleaning apparatus is arranged in the third stage of the purification section for the electrolytic production of zinc, as illustrated in FIG. 15. With reference to In this FIG. 15, this third stage of the cobalt purification section comprises first to fourth purification tanks, a filter press 89 and a filtered product tank 90. ​​The reference 90a designates a reagent tank NN. The reference 90b designates a spent electrolyte tank. The reference 90c designates an active carbon tank. The reference 90d designates a pipe connected to the second stage oy of the purification section. In the third stage of the purification section, the purified solution from which impurities such as copper, cadmium and nickel have been removed in the first and second stages of the purification section by adding dust zinc and another reagent, is used as the starting iiqui.de. The α-nitroso-ß-naphthol, called NN reagent, is added to this starting liquid to remove the 8 to 18 mg of cobalt contained therein per liter. In short, in the purification process described, the spent electrolyte is added to the starting liquid in the tank of the first stage of the purification section so that the pH value (of around 6) of the is adjusted to the predetermined value. The reagent NN is also added to the starting liquid of the tank of the first stage of the purification section and reacts with the cobalt ion so as to form a precipitate which is chemically stable. The pH measuring device equipped with the automatic electrode cleaning device is arranged at the outlet of the tank of the first stage of the purification section and the sample solution is continuously supplied by a sampling pump. (not shown) at the measuring tank, so that the pH value of the sample solution leaving the tank for the first stage of the purification section can be measured continuously.

The results of the measurement by the pH measuring device equipped with the automatic electrode cleaning device are given in FIG. 16 for comparison with the results indicated in FIG. 17 and which are obtained without cleaning the surface of the electrodes. It will be seen from FIG. 16 that the waveform a representing the pH value of the sample solution subjected to a cobalt purification by a-nitroso · ♦ · ···· • ··· · ♦ »♦ · • i ·· • · · ·

<img file="BE830643A_D0023.tif" />

• "•" · "er" r • · F • ♦ ♦ ß-naphthol is practically uniform and free from any distortion even after 7 days. The waveform a seems almost rectangular due to the alternating repetition of the measurement and the cleaning. Thus, the continuous measurement for one week is carried out reliably thanks to the use of the automatic electrode cleaning apparatus according to the invention. By way of comparison with the aforementioned case, that illustrated in FIG. 17 without cleaning sees the pH value given by the curve a gradually increase over time up to about 115% of the initial pH value, despite a pH control. sufficient.

It will be appreciated from the foregoing description that the pH meter equipped with the automatic electrode cleaning apparatus is advantageous because continuous measurement of the pH value can be performed reliably during more than a week without requiring any maintenance, while continuous measurement with the prior art pH meter has been limited to a short period of time on the order of a few hours. In addition, the present invention provides additional advantages in that 1-a dirt on the electrodes of the pH meter can be easily removed and the buffer test (test with standard solution) during electrode replacement of the pH meter as well as maintenance of this meter can be easily performed in a short period of time.

Referring now to Figure 18, part of the purified aqueous zinc sulfate solution is sampled from the flow path of this solution to industrial electrolytic cells (not shown) and its acidity is adjusted so as to remain constant by adding appropriate quantities of sulfuric acid and water or solutions of · * ♦ ·· ♦ ““ 4 • · ““ · t

<img file="BE830643A_D0024.tif" />

aqueous zinc sulfate containing free sulfuric acid. Such a sample (purified zinc sulphate solution) of electrolyte 91 is shown as contained in a small electrolytic cell 92 used for measuring the purity of the purified aqueous zinc sulphate solution 91.

An anode 93 in the form of a plate (10 mm thick) of lead containing silver and a cathode 94 in the form of a plate (4 mm thick) in aluminum are immersed in the electrolyte 91 contained in the small electrolytic cell 92 in order to electrolyze the sample of electrolyte 91 (purified zinc sulphate solution). This small electrolytic cell 92 is placed in a container 95 and is covered by a cover member 96 which defines a closed space 97 between it and the electrolytic cell 92. Water 98 is placed in the container 95 so that that the temperature of the electrolyte 91 in the electrolytic cell 92 can be kept practically constant and that the gases produced by the electrolysis can be isolated from atmospheric air.

A duct 99 extends from the upper wall of the covering member 96 so as to connect the closed space 97 above the electrolytic cell 92 to a combustion chamber 100, so that a mixture of hydrogen and gaseous oxygen produced by the electrolysis of the electrolyte 91 in the electrolytic cell 92 can be sent by a line 99 to a combustion chamber 100. A three-way solenoid law valve is arranged in the line 99. This valve 101 is carried by a support plate 102 mounted on the body of the combustion chamber 100, as illustrated in FIGS. 19 and 20. This three-way solenoid valve 101 has three operating positions. In the first, it allows me31 to flow. es

- * "·· ·" "4 ·· ** * - ♦ - ♦ 9 · · *" • 4t "" 44 ""

4 4 4 4 4 "" "*" • · "" é ·· · j "r" 4 "4 4 ·. 44 4 »gas mixture produced in the electrolytic cell 92 towards the combustion chamber 100 and, in the second position it cuts off the flow of the gas mixture towards the combustion chamber 100, while in the third position it acts so as to evacuate the gas mixture to the atmosphere.

A U-shaped connecting pipe 103 is connected at one end to an overflow tank 104 having an overflow orifice 105 and, at its other end, to the bottom wall of the combustion chamber 100. A drainage pipe 106 with a solenoid valve 107 is connected to the U-shaped connecting pipe 103. A liquid supply pipe 108 with a solenoid valve 109 terminates above the overflow tank 104 so as to supply a liquid to the latter, then via the connection pipe 103 to the combustion chamber 100, up to a liquid level which will be determined by the overflow level L in the overflow tank 104. Thus, the gas mixture produced by electrolysis in the electrolytic cell 92 and occupying the closed space 97 below the covering member 96 is sucked into the combustion chamber 100 when the liquid filling the combustion chamber 100 is discharged. and the level of liquid in the latter then drops to the level of the bottom.

A solenoid valve 110 is disposed in the U-shaped connecting pipe 103, near its end connected to the combustion chamber 100 and it cooperates with the solenoid valve 107 on the drainage pipe 106, so as to control the introduction and evacuation of the liquid in and from the combustion chamber 100. The solenoid valve 107 is arranged at a level lower than that of the solenoid valve 32

<img file="BE830643A_D0025.tif" />

t * · ""

· ♦ “· · 4 ♦ · r Μ V · of 110. A level detector 111 is arranged in the U-shaped connecting pipe 103, at a level slightly higher than that of the solenoid valve 107 but below the position of the solenoid valve 110, so as to detect whether or not the liquid in the combustion chamber 100 has been completely evacuated. The solenoid valve 109 disposed in the liquid supply line 108 is controlled by a timer so as to ensure the supply of liquid to the overflow tank 104 from the line 108. Any excess liquid supplied to the overflow tank 104 after the overflow level L has been reached therein is discharged through the overflow orifice 105. Thus, the constant liquid level is maintained in the overflow tank 104. A gas vent pipe 112 with a solenoid valve 113 extends from the top wall of the combustion chamber 100 to send the gases therein to the atmosphere. This solenoid valve 113 is also mounted on the support plate 102, as seen in Figures 19 and 20. The solenoid valve 113 is brought to the open position when desired to exhaust the gases from the combustion chamber 100 to the atmosphere. At this time, the solenoid valve 110 is also in the open position, while the solenoid valve 107 is in the closed position.

The solenoid valve 113 is closed while holding the solenoid valve 110 in the open position after the timer controlled solenoid valve 109 has been brought to the closed position to stop the supply of liquid to the overflow tank. 104 from the liquid supply pipe 108. The solenoid valve 107 is then opened to discharge the liquid from the fuel chamber 100, through the solenoid valve 110 and the part as33

O '' • R ♦ ”·· 99 ··“

<img file="BE830643A_D0026.tif" />

associated with the U-shaped connecting pipe 103. Then, the gas mixture in the closed space 97 is sucked into the combustion chamber 100 because of the evacuation from the latter of the liquid. The three-way solenoid valve 101 is brought to the position in which it cuts off the flow of gas through the conduit

99. The solenoid valve 110 is brought to the closed position to completely close the combustion chamber 100.

Ignition means or a heating element 114 is mounted on the internal surface of the upper wall of the combustion chamber 100 in order to ignite the gaseous mixture enclosed in this chamber. An agitator 115 in the form of a magnetic bar is placed in the combustion chamber 100 in the vicinity of its bottom wall, in order to agitate the gas mixture subjected to combustion, which shortens the period of time required for the combustion of the gas mixture and ensures complete combustion of the hydrogen gas. An engine 116 is arranged below the bottom wall of the combustion chamber 100. A magnet is mounted on the upper end of the motor shaft, opposite the magnetic rod stirrer 115 in the combustion chamber

100, to cause the rotation of this agitator 115. A pressure detector 117 is carried by a support member 118 and it is connected to the combustion chamber 100 by a conduit 119 passing through the upper wall of the combustion chamber 100 to there penetrate so as to detect the internal pressure in this chamber. The differential pressure in the combustion chamber 100 before and after the combustion of the hydrogen gas is measured by the pressure detector 117 in order to know the content of hydrogen gas in the mixture and the purity of the purified aqueous zinc sulfate solution. is then calculated on the basis of the detected hydrogen gas content. A pressure transmitter

Co

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differential 120 is incorporated in the pressure sensor

117, as illustrated in FIG. 19, in order to transmit the detected differential pressure to a recorder (not shown).

During operation, the electrolytic cell 92 is subjected to predetermined electrolytic conditions. For example, the current density is fixed at 600 A / m, the distance between the two anodes 93 and the cathode 94 at 34 mm, the composition of the electrolyte 91 is 70 g per liter of zinc and 90 g per liter of sulfuric acid, and the temperature of the electrolyte 91 is fixed at 35 ° C. A current is then applied to the electrodes in order to cause electrolysis, thereby generating hydrogen gas at cathode 94 and oxygen gas at anodes 93. The air or gases previously present in space 97 are purged through the three-way solenoid valve 101 until this space 91 is completely occupied by the gas mixture produced during the electrolysis. This is obtained when the electrolysis is carried out continuously for a few minutes.

After the above step, the solenoid valves 110 and

113 are open. While holding the solenoid valve 107 in the closed position, the solenoid valve 109 is opened to supply liquid to the overflow tank 104 through the liquid supply line 108. The liquid supplied to the liquid tank overflow 104 flows through the U-shaped connecting pipe 103 into the combustion chamber 100 in order to reach the overflow level L in this tank 104. Thus, unnecessary gases previously present in the combustion chamber 100 are evacuated via the vent pipe 112 and the valve 113. The purging of unnecessary gases from the combustion chamber 100 can also be obtained by reversing the sou.ε ** · xr ** o

<img file="BE830643A_D0027.tif" />

three-way solenoid 101 to the position in which it establishes a communication between the combustion chamber 100 and the space 97 and by supplying the gaseous mixture produced by electrolysis in the combustion chamber 100. In a in such a case, therefore, electrolysis can be initiated after supplying the liquid to the combustion chamber 100. In this case, the unnecessary gases and the air previously present in the combustion chamber 100 are purged via the vent pipe 112 and / or the gas mixture supplied via the valve 101.

Then, the liquid supply to the overflow tank 104 via the liquid supply conduit

108 is interrupted and the valve 107 is brought to the open position while closing the valve 113 in order to evacuate the liquid from the combustion chamber 100, via the conduit 103 and the drainage pipe 106. Due to the evacuation of the liquid from the combustion chamber 101 under the effect of gravity, the mixture of hydrogen and oxygen gas · produced by the continuing electrolysis is sucked into the combustion chamber 100 to fill the space previously occupied by the liquid.

The level detector 111 detects the complete evacuation of the liquid from the combustion chamber 100. The valve

110 is then closed. The internal pressure of the combustion chamber 100 is now negative because of a total evacuation of the liquid from the combustion chamber 100. The quantities of gaseous mixture continuously introduced into the combustion chamber 100 are preferably as large as possible in order to improve the accuracy of the measurement. Therefore, the three-way solenoid valve 101 is kept in * · ♦ ·· ·· ·· · ·· ·· · · ¥ * the position in which it allows communication between space 97 and the combustion chamber 100 and the gaseous mixture produced gradually by the continuation of the electrolysis is introduced into the combustion chamber 100, so that the internal pressure of the latter gradually approaches zero (relative pressure) to finally establish a positive pressure in this combustion chamber 100. However, an extremely high positive internal pressure is undesirable because it results in a loss of the joint effect with water 98 for the gas mixture produced by the electrolysis. It is therefore desirable to close the valve 110 ^ which brings the combustion chamber 100 to a fully closed state after the level detector 111 has detected a complete discharge of the liquid from this chamber 100. Thus, the detector of pressure 117 detects a zero differential pressure in the combustion chamber 100. The gas mixture thus introduced into the combustion chamber 100 is then ignited by the ignition heating element 114 in order to measure the hydrogen gas content of the gas mixture to estimate the pure t of the solution of purified zinc sulphate. .

The ratio between hydrogen gas and oxygen gas produced by electrolysis is generally close to 1/99. According to the present invention, the hydrogen gas is ignited in the combustion chamber 100 of a predetermined volume and while it is in a completely closed state. Therefore, combustion of the hydrogen gas results in a decrease in the amount of the gas mixture and a reduction in the internal pressure of the combustion chamber. It will for example be assumed that the quantity of the gaseous mixture is reduced by 1%, that the temperature of this mixture is constant and that the initial pressure of the mixture is approximately equal to atmospheric pressure. Then, the internal pressure of the combustion chamber 100 is reduced to about 100 mm of water column. Thus, a very large variation in the internal pressure of the combustion chamber 100 is caused by the combustion of hydrogen gas in very small quantities. This pressure variation is detected by the pressure detector 117.

According to the process according to the invention, a check with the use of an agent such as a standardized gas is unnecessary. Thus, the method of the invention is more advantageous than the other known methods because the measurement is carried out more easily than with a process gas chromatograph commonly used for automatic measurements. In addition, the combustion of the hydrogen gas does not cause any appreciable rise in the temperature of the gas, because the heating element 114 used to ignite the hydrogen gas has a very low calorific value and that it produces no appreciable amount of heat, since it is excited for a very short period of time and the heat produced by the combustion of hydrogen gas is also very low, because the amount of hydrogen gas is very small. The temperature of the gases in the combustion chamber 100 is slightly increased due to the combustion of the hydrogen gas by the heating element 114. The temperature is reduced practically until it is equal to the ambient temperature in a very short time after the heating element 114 has been switched off, due to the stirring effect of the agitator 115 which acts so as to evacuate the heat in the combustion chamber 100 via the walls of the latter. In addition, the pressure of

<img file="BE830643A_D0028.tif" />

• · ······ t • · ·· ····· · effective gas in the combustion chamber 100 can be measured with precision because * after the combustion of the hydrogen gas, it is measured during a predetermined period of time after switching off the heating element 114.

According to the present invention, the hydrogen gas in the gas mixture introduced into the combustion chamber 100 is burnt entirely to provide drops of water which are independent of a variation in the internal pressure of the combustion chamber 100. Therefore, the internal pressure of the combustion chamber 100 before and after the combustion of the hydrogen gas can be accurately and quickly detected and the amount of hydrogen gas produced by the electrolysis can be easily detected based on the pressure reduction corresponding to the hydrogen gas content. This hydrogen gas content detected in the manner described above can then be applied to a conversion table or in a formula transforming or giving the relationship between the purity of the electrolyte and the hydrogen gas content, in%, as calculated previously. Thus, the purity of the electrolyte can be easily determined.

After measuring the content of hydrogen gas in the gas mixture, the zinc deposited on the cathode is removed by reversing the direction of current flow in the electrolyte. The completion of the operation is easily detected by a sudden increase in tension. Thus, the zinc deposited on the cathode can be removed easily and automatically.

The table below gives the ratio between the current efficiency and a content of hydrogen gas in a gas mixture and the differential pressure in the combustion chamber before and after the combustion of the corresponding hydrogen gas.

Cf • · ♦ · .- · 4 · “• ♦ ·. ···· * · · • · · · · · · · · · · · · · · · · · · · · ·. ·· ·· ·· ·· · ·· with various hydrogen gas contents. BOARD

Pressure variation.

<td>mm E ^ O</td><td> 218</td><td> 450</td><td> 598</td><td> 800</td>

<td>Hydrogen gas content,%</td><td> 1,4</td><td> 2,9</td><td> 3,85</td><td> 5,2</td>

<td>Current efficiency,%</td><td> 92,5</td><td> 91,5</td><td> 90,5</td><td> 89,0</td>

The purity P above is calculated by the following formula in which the hydrogen gas content is designated by V, expressed in%:

<img file="BE830643A_D0029.tif" />

It should be understood that the present invention is in no way limited to the above embodiments and that many modifications can be made without departing from the scope of this patent.