EP0084521A2 - Electrolytic cell for metal recovery and its operation - Google Patents

Abstract

An electrolytic metal recovery cell has a housing (1, 2) with inlet (4) and outlet for a solution from which metal is to be recovered, as well as a cylindrical cathode (9) rotatably mounted in the housing. The housing contains an anode (7) and a cutting knife (22) which is fixed so that it is able to remove deposited metal on the entire side surface of the cathode during operation of the cell. In order to control the operation of the cell, electrical means are provided for driving the cutting knife in predetermined time periods without taking into account the electrolysis conditions and for actuating the cutting knife (22) if the monitored cathode potential deviates from a desired value by a predetermined amount. This avoids the use of previously suggested scrapers, and the cell can be operated easily both under stationary electrolysis conditions and with large fluctuations in the metal ion concentration of the solution. The cell and its operation are described when used to recover silver from spent photographic fixer solutions, but they can also be used to recover other metals.

Description

The invention relates to an electrolytic metal recovery cell of the type described in the preamble of claim 1 and its operation.

An article by D. R. Gabe in the Journal of Applied Electrochemistry 4, (1914) pages 91-108 describes a rotatable cylindrical electrode which can be used as an electrolytic cell for the recovery of metals. Other rotating cathode electrolysis cells are described in U.S. Patents 1,535,577 and 3,560,366 and in French Patent 2,449,734.

A method of using a rotating cylindrical cathode electrolytic cell in such a manner that the metal is deposited on the rotating cylindrical cathode in powder form is described in GB-PS 1 505 736. The cell comprises a cylindrical cathode which is rotatably supported in a housing which has an inlet and an outlet for the solution from which metal is to be recovered, and an anode and a device for removing metal deposited on the cathode in powder form.

The cell described in this GB-PS has been used in practice in particular for the recovery of silver from used photographic processing solutions. If the cell is used in the manner described in GB-PS, silver accumulates as a powdery precipitate on the rotating cathode and is removed by a scraper which is in engagement with the cathode during its rotation. However, the method, the powdered metal precipitate of the rotating cathode by means of a scraper to remove, is not particularly effective, and at the _T to the use of the scraper brings some disadvantages, in particular an excessive load on the bearing of the scraper at higher deposition. of metal on the rotating cathode.

wiedergegeben wird, in welcher

• I die bei der elektrolytischen Abscheidung des Metalls angewandte Ist-Stromstärke in Amperes
• C die Metallionenkonzentration in _Mol je cm³,
• V die Umfangsgeschwindigkeit der zylindrischen Elektrode,
• X einen Exponenten, der unter den für das Niederschlagen des Metalls in Pulverform erforderlichen Bedingungen zwischen 0,7 und 1,0, vorzugsweise aber zwischen 0,80 und 0,85 festgestellt wurde, und
• K eine von den Dimensionen der Zelle abhängige Konstante

The Gabe article mentioned above shows that the current density achievable on a rotating cylindrical cathode for a given cell approximates by function

is reproduced in which

• I the actual current in amperes used for the electrolytic deposition of the metal
• C, the metal ion concentration in _M ol per cm ^3,
• V the peripheral speed of the cylindrical electrode,
• X is an exponent which was determined under the conditions required for the precipitation of the metal in powder form between 0.7 and 1.0, but preferably between 0.80 and 0.85, and
• K is a constant dependent on the dimensions of the cell

means.

• (1) Herabsetzen der Zylinderdrehzahl V,
• (2) Verwendung einer kleineren Kathode, wodurch der Wert von K verkleinert wird, oder
• (3) Herabsetzen des Wertes von x erniedrigt werden kann.

From this function it follows that the current flowing through the cell varies with the metal ion concentration and through

• (1) lowering the cylinder speed V,
• (2) using a smaller cathode, thereby reducing the value of K, or
• (3) lowering the value of x can be decreased.

Changing the speed of the cylinder does not offer a practical solution since it would be necessary to constantly monitor the metal ion concentration and to adjust the value of V accordingly. The use of a cathode of smaller dimensions would result in a significant reduction in current at lower concentrations of metal, but it is desirable to maintain maximum current even at low concentrations. It has now been found that decreasing the exponent x is the most effective means of compensating for the current flowing through the cell at high concentrations; this is accomplished by removing the deposited powdered metal which causes a significant change in the effective area of the cathode surface. This removal is carried out by means of the scraper provided in the cell as described in GB-PS 1 505 736, and since the scraper serves to continuously remove the deposited metal powder, substantial fluctuations in the current intensity are avoided.

As indicated above, there are certain disadvantages to using the scraper, but control of the cell would be difficult to achieve without using it. It is therefore an object of the invention to reconcile these two conflicting requirements.

According to a first aspect of the present invention, this is achieved in a cell of the type described at the outset by the features described in the characterizing part of patent claim 1.

According to another aspect of the invention, this is achieved by a method for operating the aforementioned cell, which comprises the measures described in the characterizing part of patent claim 7.

The use of a cutting instead of a scraping action prevents the gradual build-up of thicker metal deposits on the rotating cathode, as could be done by sliding the scraper away over harder or more adherent deposits.

The commissioning of the cutting knife during predetermined periods should normally suffice to control the current, provided that the periods are selected depending on the parameters of the cell. However, in order to account for possible fluctuations in the metal ion concentration or other changes in the conditions of the electrolysis, the cell is continuously monitored by checking the cathode potential and generating an imposed signal when the controlled potential deviates from a desired value. The overdriving signal is then used to start the cutting knife and in the cell _S ollbedingungen for their operation to restore, regardless of the normal, by a timer controlled _B e-powered.

Further features and objectives of the invention in its two aspects can be seen from the subclaims.

Details of the invention are explained in more detail below with reference to the accompanying drawing, which illustrates an embodiment of the invention.

• _Fig. 1 einen senkrechten Axialschnitt durch eine elektrolytische Metallrückgewinnungszelle,
• Fig. 2 einen horizontalen Schnitt durch die Zelle entlang der Ebene II-II in Fig. l, und
• Fig. 3 ein Schaltbild.

In the drawing:

• _F ig. 1 shows a vertical axial section through an electrolytic metal recovery cell,
• Fig. 2 shows a horizontal section through the cell along the plane II-II in Fig. L, and
• Fig. 3 is a circuit diagram.

In the figures 1 and 2 an electrolytic metal recovery cell is shown, which comprises a cylindrical, cup-shaped housing part 1, which is liquid-tightly closed by a cover 2, so that both parts represent an outer housing of the cell, which is preferably made of plastic, e.g. Polyvinyl chloride is made.

The bottom of the housing part 1 is provided with a passage opening 3, into which an inlet connection 4 is inserted, while the cover 2 also has a passage opening 5 with an outlet connection 6 inserted into it. A substantially annular cylindrical Graphitanod _e 7 is mounted coaxially on the inner wall surface of the housing part 1 with this and has an axial gap 8, so that the anode in the cross section of the shape close to that of a horseshoe. A cylindrical cathode 9 is rotatably mounted in the cell housing and comprises a hollow cylinder 10 made of stainless steel, which is closed at its end with plastic caps 11 which fit snugly into the cylinder but have outwardly projecting flanges 12, the diameter of which is slightly larger than that of the cylinder. The cathode is mounted on a drive shaft 14 which is carried in bearings 15 mounted on the cover 2 and which is driven by drive means (not shown) located outside the cell via a pulley 16. The drive shaft 14 is sealed against the interior of the cell housing part 1 by surface seals 17.

The anode 7 and the cathode 10 are connected to a fixed voltage source via electrical lines indicated at 18 and 19 in FIG. 1, as will be described below in connection with FIG. 3.

A rotatable cutting knife carrier shaft 20 extends axially parallel with the drive shaft 14 and the cathode 9 through the cover 2 and lies essentially centrally in the anode gap 8. The shaft 20 carries a cylindrical cutting knife holder 21, on which a cutting knife 22 is attached. The shaft 20 and the holder 21 are preferably made of stainless steel and the cutting knife of stellite steel. The cutting knife has a triangular cross section and extends helically along the cutting knife holder. For further below for illustrative reasons, these screw line of the cutting blade holder does not extend completely around the circumference, but rather only a part of this circumference, preferably according to an angle of about 120 to 18 ₀ ^0th

The shaft 20 is rotated by means of an electric cutting knife drive motor 23 under the control of a timer 24 and is provided with a cam 25 which actuates a microswitch 26 arranged in a line 27 leading from the anode line 18 to the shaft 20, at the beginning of the rotation the shaft 20 of the cams 25 switches the microswitch 26 to the shaft 20 and thus to the cutting knife 20 by applying an anode current pulse.

The probe 28 of a saturated calomel electrode is passed through an opening in the side wall of the housing part 1 and an opening aligned with this opening in the anode 7 and is sealed therein by means of a seal 29. The end of the probe is set at a certain distance from the cathode cylinder 10.

The cell voltage or the electrode potential is controlled in a known manner by a potentiostat using a calomel electrode as the reference electrode.

Fig. 3 shows the circuit diagram of an electrical arrangement for the purpose of such control of the cell. The electrolytic recovery cell E is shown in dashed outline in the middle of the figure and comprises the anode 7, the cathode 9 and the cutting knife 22. The anode and the cathode are connected to a stabilized current source SPS via electrical lines. A millivolt meter MV indicates a signal indicating the potential difference between the potential of the calomel electrode determined by the probe 28 and the potential generated at the cathode 9 / The potential difference is also passed to a separating stage BA and through a feedback loop FBL to the stabilized current source SPS transfer. The separation stage BA, the feedback loop FBL and the stabilized current source SPS together represent a potentiostat P, which serves to stabilize the potential of the cathode 9 with respect to the calomel electrode.

The cutting knife 22 is shown in FIG. 3 as being able to be put into operation by the cutting knife drive motor 23 under control both by the timer 24 and by the limit signal amplifier LSA. The limit signal amplifier LSA is connected to the output of the isolating stage BA and is arranged in such a way that when the probe potential drops below the limit set on the limit signal amplifier LSA, this amplifier starts the cutting blade drive motor 23 without taking into account the status of the timer 24.

The above-described electrolytic metal recovery cell is said to electrolytically recover metal from a solution containing the metal, and is particularly suitable for the recovery of silver metal from used photographic processing solutions. The solution is introduced into the cell through the inlet nozzle 4 and leaves it again through the outlet nozzle 6, whereby it can be circulated through the cell several times in succession until the recovery is complete.

The cell uses a rotating cylindrical cathode 9 for the recovery, the operation of which has been described in the literature. Briefly, in the operation of such a cell, the cathode is rotated and metal is deposited as a powder on it, the electrolysis conditions being chosen so that the metal is deposited as a powder removable from the cathode. The deposition of silver on the rotating cylindrical cathode has the advantages that the cell is built once It can be said that it has a high recovery capacity in relation to its size, and secondly that the powder can be easily removed from the rotating cylinder and then separated by filtering.

In the preparation of the present cell to the _I nbetrieb- acquisition, the cell is filled with a silver salt solution _and the electrolysis conditions are first selected so thatwhen the rotation of the cathode these "hartplattiert" with a thin layer of silver. The cell is then ready for start-up and the solution to be treated is passed through the cell, the cathode is rotated, and the electrolysis conditions are selected so that the silver in powder form is deposited on the rotating cathode. In periodic periods determined by the setting of the timer 24, the cutting knife drive motor 23 is turned on to rotate the shaft 20 and remove deposited silver powder from the cathode by the cutting knife 22, whereupon the powder from the cell through the Flush outlet port 6 and collected in a suitable filter.

Fig. 2 shows the cutting knife 22 and the shaft 21 stationary, it can be seen that the cutting knife 22 extends helically around that part of the cutting knife holder 21 which is located away from the cathode. The radial distance from the center of the cutting knife shaft 20 to the closest point to the edge of the flange 12 of the end cap 11 is equal to the radial distance from the center of the drive shaft 20 to the outermost part of the cutting knife 22. This removes the cutting knife 22 upon its rotation from the cathode cylinder 10, all the silver deposited thereon, which protrudes over an imaginary cylinder jacket, which passes through the edges of the two flanges 12 is defined. In practice, the flanges 12 now protrude a certain distance beyond the cylinder 10 of the cathode 9, which allows the formation of the "hard-plated" metal layer mentioned above and a very thin layer of powder deposited on it. Since the end caps 11 are made of plastic, it is easy to see that they do not have to be dimensioned exactly, since the first rotation of the cutting knife 22 shaves off the edges of the flanges 12 to the required diameter. The cathode rotates at a constant speed from 200 to 2000 rpm. can be, with practical operation about 1000 U / min. to be favoured. Since the cathode rotates at a relatively high speed, it is clear that the slower the cutting knife rotates, the lower the risk of damage to the cathode by the cutting knife. However, it is desirable to remove the deposited silver from the entire surface of the cathode as quickly as possible so that the electrolysis conditions remain as unchanged as possible. A suitable speed for the shaft 20 and the cutting knife is 1/4 to 3 rpm. The timer 24 and the cutting knife drive motor 23 are set such that they allow the shaft 20 to carry out an entire revolution or an integral number of revolutions, so that the cutting knife and shaft assume the same rest position as shown in FIG. 2 with each revolution.

As an important feature of the cutting knife, when it rotates and removes metal from the cathode, it only makes point contact with the metal. This reduces the stress on the knife and ensures that metal can be removed from the cathode by the knife even if the electrolysis conditions are not satisfactory were selected or are subject to fluctuations and the silver was not deposited on the cathode in powder form but as a coherent coating.

Another feature that helps to influence the cutting force is formed by the pitch angle of the helical cutting knife around the cutting knife holder 21, a larger pitch angle reducing the stress on the knife. However, as mentioned above, it is necessary to provide enough space between the cutting knife holder 21 and the cathode so that a layer of deposited metal can form into which the knife can cut. Therefore, the helical knife should not extend completely around the circumference of the blade holder around, .but it has been found that a meter length over a circumferential angle of 120 to 180 ⁰ represents a useful compromise between the two mutually conflicting requirements.

As the silver settles, it is inevitable that some of it will be deposited on the cutting knife and knife holder. In order to prevent the formation of such a silver layer, the cam 25 actuates a microswitch 26 each time the knife drive shaft is rotated anew, which sends an anode pulse through the holder and knife, as a result of which silver deposited from the solution again dissolves. As shown in Fig. 1, the anode pulse is delivered directly from the anode.

It has been found that the operating potential of the rotating cylindrical cathode is the most important factor in ensuring that the silver is electrolytically precipitated as a powder. For this it is necessary to use the elec keep trical potential on the rotating cylindrical cathode at a constant value or within certain limits with respect to the reference electrode. This is brought about by the potentiostat P, which continuously stabilizes the potential of the cathode with respect to the reference electrode.

In order to produce a powdery precipitate of metal, the potential near the cathode must be measured by the probe of the reference electrode. will be given a value above the cathode potential, which depends on the metal and the distance between the probe and the cathode surface. The probe 28 of the reference electrode RE is therefore at one end as close as possible to the surface of the cathode, i.e. located on the edge (s) of the flanges 12 defined cylinder surface and as far as possible from the influence of the anode.

During the electrolytic deposition of the metal, the current used increases as the metal layer on the cathode thickens or when the metal ion concentration in the solution suddenly increases. The increase in the current intensity causes an increased load on the current source, and the electrolytic deposition can then only be continued with an increased current source. However, the potentiostat places limits on such an increase in the power supply. Apparently, an increase in the output power of the potentiostat also increases the costs for the components used and the complexity of the system. It is therefore desirable to keep the output power as low as possible.

As shown above, the current used in electrodeposition can be controlled by removing the deposited metal. Thus, in the present cell, the control of the current is achieved by operating the cutting knife 22, and this can take place when a maximum, previously selected current value is reached. However, since the period in which this value is reached, is subjected, inter alia, of the metal ion concentration dependent _en fluctuations, and since this will affect the thickness and to some extent also the type of deposition, their removal according to the invention by the start-up of the cutting blade during predetermined periods of time that are selected to ensure that the preselected maximum current is never reached.

However, sudden changes in concentration or other conditions can act on the cell to such an extent that the preselected maximum current value is reached between two successive start-ups of the cutter, resulting in a cut off of the power supply and a consequent decrease in cathode potential with respect to the reference electrode . In order to avoid the occurrence of this condition, the value of the cathode potential is continuously monitored by the millivolt meter MV, and any significant deviation from the nominal operating potential generates a signal through the isolating stage BA, which is sent to the limit switching amplifier LSA, which directly operates the knife drive motor 23 sets and causes the cutting knife to make one or a number of complete revolutions until the cathode potential is returned to its target value.

From this it can be seen that in this way a very significant modulation of the current flowing through the electrolytic cell can be achieved, whereby the metal ion concentration in the solution to be treated can be managed over a wide range without it being necessary for the operating range of the direct current source and of the potentiostat. It has been found that the cell of the invention is capable of treating spent photographic fixer solutions with relatively high silver concentrations using moderate power. If there is no cutting knife operating in the manner described above, a much larger power supply combined with significantly higher equipment costs is required to process the same solutions.

One may wonder why a lower output power source is not used with a current limiting device. As explained above, such an operation would, however, lead to a decline in the cathode potential at higher metal ion concentrations.

If the cell continued to operate with such suppressed potential, the metal would no longer be deposited as a powder. According to the present invention, the method potential is immediately raised to its desired value when the cutting knife is actuated.

While the invention has been described particularly in relation to the electrolytic deposition of silver in the silver recovery from spent photographic fixer solutions, it is also applicable to the electrolytic deposition, recovery and electrolytic production of metals other _H applicable.

Claims (10)

1. Electrolytic metal recovery cell with an inlet (4) and outlet (6) for a solution containing the metal to be recovered housing (1,2), in which an anode (7), a rotatably mounted cylindrical cathode (9) and a device (20,21,22) for removing metal deposited on the cathode in powder form, characterized in that the metal removal device (20,21,22) comprises a cutting knife (22) which, during operation of the cell, of the entire cathode surface (10) is able to remove deposited metal thereon, and that electrical means for operating the cutting knife (22) are provided in predetermined time periods and whenever a monitored cathode potential deviates from a desired value by a predetermined amount. 2. Electrolytic metal recovery cell according to claim 1, characterized in that the cutting knife (22) on a drive means (23) rotatable shaft element (20) is attached and in an at least the length of the cathode (9) corresponding area helically around part of The circumference of the shaft element (20) extends, the cutting knife (22) being arranged such that in the rest position the longitudinal region of the shaft element (20) free of the cutting knife faces the cathode (9) at a distance, and that when the shaft element (20) rotates the cutting knife (22) penetrates at a predetermined depth into metal deposited on the cathode (9). 3. Electrolytic metal recovery cell according to claim 2, characterized in that the cutting knife (22) extends helically over an angle of 120 to 180 ⁰ around the circumference of the shaft element (20). 4. Electrolytic metal recovery cell according to claim 2 or 3, characterized in that the shaft element (20) made of stainless steel and the cutting knife (22) consists of stellite steel. 5. Electrolytic metal recovery cell according to one of claims 1 to 4, characterized in that means (25, 26) are provided by which anodic pulses can be given to the cutting knife (22) after the drive means (23) have been started up. 6. Electrolytic metal recovery cell according to one of claims 1 to 5, characterized in that a potentiostat circuit (P) and a reference electrode (RE) are provided for monitoring the cathode potential to maintain its target value. 7. A method for controlling the operation of an electrolytic metal recovery cell in which a rotating cylindrical cathode (9) is brought into contact with a solution containing ions of the metal to be recovered under conditions such that the metal is deposited on the rotating cathode (9) in powder form , characterized in that a target value of _K is athodenpotentials monitored relative to a reference electrode that cutting means (20,21,22) for removing said deposited on the entire side surface of the cathode metal at predetermined time intervals be put into operation without taking into account the cathode potential, and that an imposed signal for starting up the cutting means (20, 21, 22) is generated if the monitored cathode potential deviates from the desired value by a predetermined amount. 8. The method according to claim 7, characterized in that the cutting means (20,21,22) comprise a drivable shaft element (20) and a cutting knife (22) which helically around in an area corresponding at least to the length of the cathode (9) part of the circumference of the shaft element (20) extends that the cathode (9) at a speed of 200 to 2000 U./min. is rotated, and that in the predetermined periods the shaft element (20) describes a complete revolution in 1/4 to 3 minutes. 9. The method according to claim 7 or 8, characterized in that anodic pulses are given to the cutting knife (22) at the start of the rotation of the shaft element (20). 10. The method according to any one of claims 7 to 9, characterized in that the metal is silver, and the solution is a used photographic fixing solution.

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