FR2556374A1 - Device for maintaining the silver concentration at a determined value in fixing baths in cinematographic or photographic laboratories - Google Patents

Abstract

Device with silver electrodes, one 6 being immersed in a receptacle 1 containing a reference bath, the other 7 being immersed in the fixing bath situated in the electrolysis tank 2. A liquid contact is established by a U-shaped tube 4 filled with the reference bath and stoppered at the ends by cotton plugs 5. A Mariotte bottle 3 supplies the reference bath to the container 1 at a constant level. The electrodes are connected to an amplifier which, by means of a switch, cuts or establishes the electrical supply for the recovery, according to whether the voltage between electrodes is negative or positive.

Description

Device for maintaining the silver concentration of fixing baths in cinematographic or photographic laboratories at a determined value.

 In modern laboratories treating sensitive surfaces based on silver halides, the recovery of the dissolved silver by the fixing baths is carried out by electrolytic means.

 The recovery devices are either fixed cathodes or mobile cathodes and their efficiency depends at the same time on the quantity of silver to deposit, on the arrangement of the electrodes, on the current densities, on the agitation of the liquid subjected to the 'electrolysis...

 For installations operating in a closed circuit, it is desirable, in order to obtain a good recovery yield, that the concentration of silver in the fixing baths is as low as possible and that the intensity of the electrolysis current can deposit within a given time. the amount of silver dissolved in the treatment tanks during the same time.

Theoretically 1 amp # re-hour corresponds to a cathodic silver spite of
108 X 3.600: 96.500 = 4 g 03
In recovery processes, the sulfurization phenomena which occur when the concentration of Agfi ions in the fixing bath subjected to electrolysis drops below a critical value, which depends on the material used.
Sulfurization phenomena not only alter the quality of the silver deposit but risk irreversibly polluting the fixing bath subjected to continuous recovery.

 A well-conditioned installation must make it possible to maintain a concentration of silver in the fixing bath at O g 2 per liter without any sulfurization effects appearing.

 The present invention consists in using a device with silver electrodes connected to an amplifier which by means of a circuit breaker stops electrolysis when the potential difference between electrodes reaches the value 0 and switches on when this- this becomes positive.

Figure I shows the block diagram of this device
A container (i) in contact with the electrolysis tank (2) is kept at constant level by a Mariotte bottle (3 filled with a reference fixing bath. The constant level slightly overhangs that of the tank bath and a liquid contact is established between the container and the tank by a glass or plastic tube (4) filled with reference bath and closed at both ends by cotton swabs (5)
One silver electrode (6) is immersed in the reference bath, the other (7) in the bath of the electrolysis cell.

 The potential difference between the two electrodes is a function of the difference in concentration of Agf ions between the reference solution and the bath subjected to electrolysis.

 The reference solution has a composition identical to that of the circulating fixing bath but with a silver concentration which must be greater than the critical value for which the sulfurization phenomena appear.

In FIG. 11 we have represented the potential differences between electrodes as a function of the silver concentrations of a fixing bath of the following composition
ammonium thiosulfate S203 (NH4) 2: 95 g / liter
.sodium sulfite 503 Na2: 15 g / liter. acetic acid CH3 Co2H: qspf Ph = 5.50
Curve (8) was established with a reference bath of O silver, curve (9) with a bath of O g 2 / liter, curve (10) with a bath of O g 4 / liter.

 FIG. III represents a diagram of an electrolytic recovery installation operating with a recycled fixing bath having the device of the invention.

The fixing bath circulates continuously between the treatment tanks and the recovery tank (lt by the action of a centrifugal pump ss0 the silver electrode device <13) is connected to the amplifier t45 fitted with 'a millivoltmeter Q5) indicating the voltage between electrodes. The amplifier, which includes a timer system to avoid pianotage phenomena, controls a circuit breaker t14 which, depending on whether the voltages between electrodes are negative or positive, cuts or establishes the electrical supply of the rectifier transformer o # and of the motor 09 # which activates the rotation of the anodes (in the example chosen)
An installation of this type makes it possible to maintain, whatever the rate of treatment, the silver concentration of the fixing baths at the minimum compatible with the recovery devices used.

 As a result, the use of this invention makes it possible to substantially improve the yield of the silver recovered and dispenses with making amperage adjustments as a function of the analytical results of the silver contained in the fixing baths.

Claims (3)

1) Device with silver electrodes determining a potential difference between a fixing bath subjected to an electrolytic recovery and a reference fixing bath, this device consisting of a container (1) placed in contact with the electrolytic cell (2) kept at constant level by a bottle of Mariotte (3) containing the reference bath, this constant level slightly overhanging that of the bath of the electrolysis tank and a liquid contact being established between the container and the tank by a glass or plastic tube (4) filled with of the reference bath and closed at both ends with cotton swabs (5), one of the silver electrodes (6) being immersed in the reference bath the other (7) in the bath subjected to electrolysis . 2) electrolytic silver recovery installation operating with a recycled fixing bath using the device according to claim 1, characterized in that the bath circulates continuously between the treatment tanks and the recovery tank (il) by the action of a centrifugal pump (12), the silver electrode device (13) being connected to an amplifier (14) provided with a millivoltmeter (15) indicating the voltage between electrodes and a system timer avoiding the effects of tapping, the amplifier controlling a circuit breaker (16) which, depending on whether the voltages between electrodes are negative or positive, cuts or establishes the electrical supply to the rectifier transformer (17) and to the motor (18) which actuates the movement of the anodes or cathodes.  3) electrolytic recovery installation according to claim 2 characterized in that the silver concentration of the reference bath must be greater than the critical value for which in the installation considered appear sulfurization phenomena, the silver concentration of the bath subjected to electrolysis in an installation equipped with the device object of claim 1 being conditioned by that of the reference bath and not in any case being between less than this.