DE2110657A1 - Process for the recovery of silver - Google Patents

Description

i Process for the recovery of silver

The present invention "relates to a method of recovery of silver from silver-containing manufactured organic products such as printed circuit boards, photographic Films and photosensitive paper.

Although it is well known that used products such as photographic Films and papers and various types of printed circuit boards, containing significant amounts of silver, such products are often due to a lack of appropriate methods and procedures for economic recovery of silver discarded.

In recent years, the two most widely used methods (1) have been a sodium thiosulfate method in which the Film is treated in a sodium thiosulfate solution, and (2) a combustion process in which the material is formed of ashes being burned in an open vessel. The sodium thiosulfate process is relatively expensive and is used almost exclusively in the treatment of X-ray films. The combustion process possesses various disadvantages that the education

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significant air pollution and a significant one. Silver loss in the exhaust gases released into the atmosphere, lock in.

This makes it necessary that such a combustion " procedure is carried out away from populated centers. A refinement of the combustion process is in the Ü.C. patent specification 2 94-4-886, with the water on the burning film is sprayed to reduce the loss of silver in the exhaust gases. However, this procedure is only partially satisfactory.

The above problems can be overcome by using a method of recovering silver from such silver-containing organic materials in which the organic materials are initially hot, non-oxidizing. Atmosphere to reduce the organic materials to a carbon residue containing silver and a pyrolytic exhaust gas mixture. The silver-containing carbon residue is then subjected to an oxidizing atmosphere to incinerate the silver-containing carbon residue to form silver-containing ash and to generate hot, non-oxidizing gases. The process is characterized in that the hot, non-oxidizing gases formed during the combustion of the silver-containing carbon residue are used as hot, non-oxidizing gases for the pyrolysis of the organic materials. The process is much more characterized in that the oxidizing atmosphere is preheated before the silver-containing carbon residue is burned. The process is further characterized in that the oxidizing atmosphere contains oxygen and water vapor, the amount of water vapor in relation to the oxygen being controlled in such a way that the temperature of the bed of the burning silver-containing carbon residue is kept ^{between 350 and 800 ° C.}

On the basis of the device shown schematically in the accompanying drawing, a preferred embodiment of the

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lying invention in the following for example explained in more detail * «

A closed retort 10 and a device attached to it are explained in the accompanying drawing in order to To illustrate the description of the present invention in more detail.

The beguiled or the reaction vessel 10 is essentially upright and comprises a lined chamber 11 with a grate 12 located near the lower end of the retort "and carries the solid materials. The side walls 1 $ of the reaction vessel or the retort are essentially vertical arranged to direct the solid materials downwards towards the grate 12 and towards the lower end. A withdrawal valve 14- for the solid particles is arranged at the lower end of the retort or the reaction vessel, around the solid particles can be removed without introducing air. Such a valve is often referred to as an airlock rotary valve.

A similar valve 15 is located at the top of the retort, to introduce the silver-containing, artificially produced organic products into the chamber without access to air. Only for purposes of a more detailed explanation, the method according to the invention in the recovery of silver from photographic film described.

A conveyor belt 17 is indicated in the drawing for feeding the film to the retort in a continuous or discontinuous manner support financially.

The chamber 11 has two reaction zones above the grate, - An oxidation zone 20 and a reduction zone 21 - the reduction zone 21 being adjacent to and directly above the oxidation zone 20 is located.

A gas inlet line 22 extends in the area of the oxidation zone or below the grate into the reaction device or the retort. In the accompanying drawing it is shown that

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the line 22 extends below the grate into the retort.

An exhaust line 23 leads above the reduction zone from the beguiled to the outside in order to discharge the product from gases that are formed by the pyrolytic decomposition of the film.

The method and apparatus are designed to operate in countercurrent with the photographic film on top or is introduced into the fieduction zone and down flows, while the decomposition takes place within the reduction and oxidation zones and that in the form of a silver-containing one Ash is removed from the bottom or the lower part of the reaction device. In the reduction zone, the film is formed to form a silver-containing carbon residue and a j> yiO.ly ti see Pyrolyzed exhaust gas mixture. The pyrolytic exhaust gas mixture flows out of the line 23 and the carbon residue containing silver is moved into the oxidation zone due to gravity and guided by the side walls. In the oxidation zone the carbon residue containing silver in an oxidizing atmosphere burned to turn the residue further into a silver-containing one To decompose ash material, and to form the hot non-oxidizing gases that flow upward into the reduction zone pyrolyze the film.

The burning is done by the supply of a heated air and Causes water vapor mixing through line 22 into the retort, a sufficient temperature and a sufficient mixing ratio is maintained to avoid the silver-containing carbon residue to burn. The resulting silver-containing ash trickles through the grate and is removed via the extraction device 14 taken from the retort. A receptacle 26 is positioned below the retort to receive the ash. The ashes will then worked up in order to separate the silver using conventional flux processes or flotation processes.

In the oxidation zone the air and the water vapor react with it the carbon so that the carbon is reduced to an ash and oxygen from the air is consumed, so that

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a hot non-oxidizing gas mixture of hydrogen, throat forms inonoxide, carbon dioxide, nitrogen and water vapor. Of the Hydrogen reacts with the existing silver salts and reduces the salt to metallic silver. When the silver salt content is essential, it may be desirable to use an alkali metal carbonate, such as sodium carbonate, to reduce the formation of volatile mineral acids.

The hot, non-oxidizing gas mixture then flows through the Film upwards in the reduction zone, causing the film to degrade by removing the high molecular weight organic compounds, such as cellulose nitrate, cellulose acetate or other film carrier materials to volatile components with low molecular weight, such as acetone, acetic acid, hydrogen, methane, carbon dioxide and carbon monoxide. The silver one Carbon residue usually drops because of gravity into the oxidation zone to the extent that the ash material falls through the grate underneath. Every 100 kg of processed film produces around 25 kg of silver-containing carbon residues8 educated.

The exhaust gas mixture is used to heat the air and water vapor before this mixture reaches the retort. Preheating is achieved by adding oxygen or air to the exhaust gas mixture through line 28 to burn off the combustible gases present so that a very hot gaseous product of nitrogen, carbon dioxide and water vapor at a temperature of about 1370 ° C (2500 ⁰ F) is formed. The hot gaseous product is passed through a heat exchanger or chamber 30 which transfers the heat to the conduit 22 and the air and water vapor flowing therethrough. The hot gaseous product is then vented to the atmosphere via line 31 with no accompanying soot or smoke. The amount of air that is combined with the exhaust gases is controlled by valve 32. Excess air can be added to raise the temperature of the gaseous product. steer*

The amount of air flowing through line 22 is through

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the valve 33 is controlled. Water or steam is drawn through the pipe

34 introduced into line 22. The valve 35 controls the amount of water mixed with the air to provide means for regulating the oxidizing and pyrolytic temperatures of the film material during constant flow conditions. The bed temperature of Oxydationsζone should be maintained between 500 and SOO ⁰ C "For optimum efficiency, the bed temperature should be about 7'00 be ⁰ O.

In order to achieve the desired bed temperature and to control the pyrolysis rate and to prevent the loss of silver, the inlet temperature of the air and steam mixture should be kept between 350 and 800.degree. JFür an optimum procedure, the inlet temp he a door should be about 500 'C ⁰ at a volume ratio of 0.47 of steam to air. If a photographic picture is processed with little or no silver salts, direction-oxidizing gases are formed in the oxidation zone in the following approximate volume composition: H ₂ O - 3%; CO ₂ - 7%; CO - 28%; H ₃ - 2QP / o and Ng - 42%.

Thermocouples 35, 56 and 37 extend into the chamber 10 to measure the temperature of the processed material as it moves through the retort. The thermocouple

35 is placed adjacent the grate to measure the bed temperature of the carbon residue. The thermocouple 36 is arranged on the intermediate layer between the oxidation ζ one and the Ked.uktionszone. The thermocouple 37 extends approximately into the upper part of the reduction zone. A good indication of the rate of pyrolysis taking place in the reduction zone can be obtained from the measurements of the thermocouples 36 and 37. It is preferred that the temperature of 700 ⁰ C at the lower end of the zone to 100 ⁰ C 21 changes in the vicinity of the tip of the Keduktionszone, so that a giatte pyrol.v-ical reaction without loss of silver in the i ^ gajsn

Although there are several ways to get the process started, one of the preferred methods is

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ren in initially inserting a carbonaceous material into the
To introduce oxidation zone and then to introduce the film onto it in the reduction zone. A "combustible fuel" such as natural gas is then introduced into chamber 50 via line 40 and ignited with air from line 28 so that the air and water vapor mixture is heated sufficiently to burn the carbonaceous material. The hot gases from the carbonaceous material pyrolyze the film and reduce the film to a carbon residue containing silver and form the desired exhaust gases. Then the valve AO is closed., Whereby
the preheating keeps going by the addition of air via line 28 by itself.

During operation, the grate 20 can be periodic or continuous vibrated or opened and closed to aid the flow of solids through the zones.

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Claims (10)

Claims 1. . Process for the recovery of silver from silver-containing, man-made organic materials such as photographic film in which the organic materials are coated with a hot non-oxidizing atmosphere to pyrolyze the organic materials to form a silver-containing carbon residue and an exhaust gas mixture, the silver-containing carbon residue in an oxidizing atmosphere to form a silbex-containing ash and. hot, non-oxidizing gases burn the silver out of the Separating ash in order to obtain it, characterized in that the by burning the silver-containing carbon residue formed hot, non-oxidizing gases than hot non-oxidizing atmosphere for pyrolysis of organic materials used, 2. The method according to claim 1, characterized in that the oxidizing atmosphere before burning the silver-containing Preheated carbon residue. 3. The method according to claim 1 or 2, characterized in that that the oxidizing atmosphere contains oxygen and water vapor. M-. Process according to Claim 2, characterized in that the exhaust gas mixture is used to preheat the oxidizing atmosphere. 5. The method according to claim 2, characterized in that that one burns the "exhaust gas mixture in order to obtain a hot gaseous product with which one the oxidizing atmosphere on preheated to a temperature between 35O ° C and 80 ° C. 6 »method according to claim 3» characterized in that that the amount of water vapor in the oxidizing atmosphere 109839/1165 controls in relation to oxygen in order to keep the bed temperature of the burning si l'b containing carbon atom between 50O ⁰ C and 800 ⁰ C au. 10 9839/1165 Blank page