EP0084521B1 - Electrolytic cell for metal recovery and its operation - Google Patents

Description

The invention relates to an electrolytic metal recovery cell with a housing provided with an inlet and outlet for a solution containing the metal to be recovered, in which an anode, a cylindrical cathode rotatably mounted in the housing and a cutting knife are accommodated, the cutting knife operating the cell from the whole Removed metal deposited on the cathode surface in powder form, and a method for controlling the operation of such a metal recovery cell, in which a rotating cylindrical cathode 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 in powder form and wherein the deposited metal is removed from the cathode by means of a cutting knife.

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. Patent Nos. 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 of removing the powdered metal precipitate from the rotating cathode by means of a scraper is not particularly effective, and in fact the use of the scraper has some drawbacks, in particular excessive use of the scraper bearing when metal is heavily deposited 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

bedeutet.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 is the metal ion concentration in moles 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. decrease the cylinder speed V,
• 2. Use a smaller cathode, which reduces the value of K, or
• 3. Decrease the value of x

can be lowered.

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 with smaller dimensions would result in a significant decrease 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, causing 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.

It follows from the foregoing that the use of a scraper has certain disadvantages on the one hand, but on the other hand can hardly be avoided, since otherwise the control of the cell would be difficult to achieve.

Priority EP-A-58 537 describes an electrolytic metal recovery cell with a housing provided with inlet and outlet for a solution containing the metal to be recovered, in which an anode, a cylindrical cathode rotatably mounted in the housing and a cutting knife are accommodated, the Cutting knife at Operation of the cell removed metal deposited from the entire cathode surface in powder form. This cell is intended for the recovery of copper, the removal of the copper powder from the cathode surface being carried out continuously. This cell has no monitoring of the cathode potential and no activation of the cutting knife controlled afterwards.

The invention is intended to find a way out of the contradictory requirements set out above and to improve an electrolytic metal recovery cell of the type in question in such a way that the deposition of the metal in powder form is possible under stable operating conditions. H. especially also in cases of possible fluctuations in the metal ion concentration or other changes in the conditions of the electrolysis.

The electrolytic metal recovery cell according to the invention is characterized in that electrical means are provided which put the cutting knife into operation in predetermined time periods and whenever the cathode potential deviates by a predetermined amount from a potential value which characterizes the target conditions for the operation of the cell. The method according to the invention for operating the cell is characterized in that the cathode potential is monitored with respect to a reference electrode, and in that the cutting knife is put into operation at preselected time intervals which preclude reaching a preselected maximum current and also when the monitored cathode potential is increased by a predetermined amount deviates from a potential value characterizing the target conditions for the operation of the cell.

Preferred configurations are described in the dependent claims.

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 overriding signal is then used to start the cutting knife and to restore the target conditions for its operation in the cell, independent of the normal operation regulated by a timer. Details of the invention are explained in more detail below with reference to the accompanying drawing, which illustrates an embodiment of the invention.

• Figur 1 einen senkrechten Axialschnitt durch eine elektrolytische Metallrückgewinnungszelle,
• Figur 2 einen horizontalen Schnitt durch die Zelle entlang der Ebene 11-11 in Fig. 1, und
• Figur 3 ein Schaltbild.

In the drawing:

• FIG. 1 shows a vertical axial section through an electrolytic metal recovery cell,
• Figure 2 is a horizontal section through the cell along the plane 11-11 in Fig. 1, and
• Figure 3 is a circuit diagram.

In 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 such. B. is made of polyvinyl chloride.

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 largely annular, cylindrical graphite anode 7 is fastened coaxially to the inner wall surface of the housing part 1 and has an axial gap 8, so that the anode in cross section comes close to the shape 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 ends 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 9 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 reasons to be explained further below, this screw line does not extend completely around the circumference of the Cutting knife holder around, but rather only around a part of this circumference, preferably corresponding to an angle of about 120 to 180 °.

The shaft 20 is rotated by means of a drive means 23 in the form of an electric cutting knife drive motor 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, whereby at the beginning of the rotation of the shaft 20 of the cams 25, the microswitch 26 switches to the shaft 20 and thus to the cutting knife 22 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 which indicates 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 an isolating stage BA and transmitted through a feedback loop FBL to the stabilized current source SPS. The isolating 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 drive means 23 under the control of both the timer 24 and 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 drive means 23 without taking into account the status of the timer 24.

The above-described electrolytic metal recovery cell is intended 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, on the one hand, that the cell can be constructed to have a high recovery capacity in relation to its size, and, on the other hand, that the powder is easily removed from the rotating cylinder and can then be separated by filtering.

When preparing the present cell for commissioning, the cell is filled with a silver salt solution and the electrolysis conditions are first selected so that when the cathode is rotated, it is hard-plated 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 now selected so that the silver in powder form is deposited on the rotating cathode. At regular intervals determined by the setting of the timer 24, the drive means 23 is turned on to set the shaft 20 in rotation and to remove deposited silver powder from the cathode by the cutting knife 22, whereupon the powder from the cell through the outlet port 6 rinsed out 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 beyond an imaginary cylinder jacket, which is defined by the edges of the two flanges 12. 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 the remaining of 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 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 1 000 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 less 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 drive means 23 are set so that they allow the shaft 20 to perform an entire revolution or an integral number of revolutions, so that the cutting knife and shaft assume the same rest position with each revolution as shown in FIG. 2.

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 have not been selected correctly or are subject to fluctuations and the silver on the cathode has been deposited not as a powder 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 knife holder, but it has been found that a knife length over a circumferential angle of 120 to 180 ° represents a useful compromise between the two 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 on the latter dissolves again. 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 purpose it is necessary to keep the electrical 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 deposit of metal, the potential near the cathode, which is measured by the probe of the reference electrode, must be given a value which is above the cathode potential and which depends on the metal and on 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 cathode surface, i. H. located on the cylindrical surface defined by the edges of the flanges 12 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 be done when a maximum, previously selected current value is reached. However, since the period in which this value is reached may a. is subject to fluctuations depending on the metal ion concentration, and since this affects the thickness and to some extent also the type of deposition, its removal according to the invention is effected by the operation of the cutting knife during predetermined periods of time which are chosen to ensure that the preselected one Maximum current is never reached.

However, sudden changes in concentration or other conditions can affect the cell to such an extent that the preselected maximum current value between two successive startups of the Cutting device is reached, which leads to a blockage of the power supply and a resulting decrease in the cathode potential compared 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 drives the drive means 23 sets and causes the cutting knife to make one or a number of complete rotations 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 cathode potential is immediately raised to its desired value when the cutting knife is actuated.

While the invention has been described in particular with respect to the electrodeposition of silver in silver recovery from spent photographic fixer solutions, it is also applicable to the electrodeposition, recovery and electrolytic manufacture of other metals.

Claims (10)

1. An electrolytic metal recovery cell, comprising a housing (1,2) provided with an inlet (4) and an outlet (6) for a solution containing the metal to be recovered, in which housing there is present an anode (7), a cylinder cathode (9) mounted for rotation in said housing and a cutter blade (22), which cutter blade (22) removes metal deposited in powder form from the whole face of the cathode (10) when the cell is in operation, characterised in that electrical means are provided for operating the cutter blade (22) at predetermined intervals of time and for operating the cutter blade (22) when the cathode potential deviates a predetermined amount from a potential value representing the desired conditions for the operation of the cell.

2. An electrolytic metal recovery cell according to claim 1, characterised in that the cutter blade (22) is mounted on a shaft member (20) which is rotatable by drive means (23), the cutter blade (22) extending for at least the length of said cathode (9) in a helical manner about only part of the periphery of said shaft member (20), the cutter blade (22) being arranged such that in the rest position, the peripheral portion of the shaft member (20) free of the cutter blade (22) is adjacent to but spaced from the cathode (9), and such that, upon turning the shaft member (20), the cutter blade (22) is engageable to a predetermined depth with metal deposited on said cathode (9).

3. An electrolytic metal recovery cell according to claim 2, characterised in that the cutter blade (22) extends for 120 to 180 ° about the periphery of the shaft member (20).

4. An electrolytic metal recovery cell according to either of claims 2 or 3, characterised in that the shaft member (20) consists of stainless steel and the cutter blade (22) of stellite steel.

5. An electrolytic metal recovery cell according to any one of claims 1 to 4, characterised in that means are provided for anodically pulsing the cutter blade (22) upon operation of the drive means (23).

6. An electrolytic metal recovery cell according to any one of claims 1 to 5, characterised in that potentiostat means (P) and a reference electrode (RE) are provided for monitoring the cathode potential and maintaining its desired value.

7. A method of controlling the operation of an electrolytic metal recovery cell, in which a rotating cylinder cathode (9) is contacted 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, which deposited metal is removed from the cathode by means of a cutter blade (22), characterised in that the cathode potential is monitored with respect to a reference electrode, and that the cutter blade (22) is operated at predetermined time intervals chosen to prevent a predetermined maximum- current from being reached and, in addition, that the cutter blade (22) is operated when the monitored cathode potential deviates a predetermined amount from the potential value representing the desired conditions for the operation of the cell.

8. A method according to claim 7, characterised in that a cutter blade (22) is used which extends for at least the length of the cathode (9) in a helical manner about only part of the periphery of a motor-drivable shaft member (20), that the cathode (9) is rotated at a speed in the range from 200 to 2 000 r. p. m., and that the shaft member (20) is rotated at the predetermined time intervals at a speed in the range from one quarter to three r. p. m.

9. A method according to either of claims 7 or 8, characterised in that the cutter blade (22) is anodically pulsed at the commencement of rotation of the shaft member (20).

10. A method according to any one of claims 7 to 9, characterised in that the metal is silver and the solution is spent photographic fixing solution.

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