CA1305328C - Recovery of silver from photographic film with high shear and caustic - Google Patents
Recovery of silver from photographic film with high shear and causticInfo
- Publication number
- CA1305328C CA1305328C CA000560445A CA560445A CA1305328C CA 1305328 C CA1305328 C CA 1305328C CA 000560445 A CA000560445 A CA 000560445A CA 560445 A CA560445 A CA 560445A CA 1305328 C CA1305328 C CA 1305328C
- Authority
- CA
- Canada
- Prior art keywords
- silver
- caustic
- photographic film
- film
- recovery
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Abstract
TITLE
Recovery of Silver from Photographic Film With High Shear and Caustic Abstract of the Disclosure A process for the recovery of silver from exposed or unexposed photographic film using a caustic alkali solution at elevated temperatures with high shear.
Recovery of Silver from Photographic Film With High Shear and Caustic Abstract of the Disclosure A process for the recovery of silver from exposed or unexposed photographic film using a caustic alkali solution at elevated temperatures with high shear.
Description
1305;:~28 TITLE
Recovery of Silver from Photographic Film With High Shear and Caustic Background of the Invention Photographic film generally comprises a polymeric base, a coating of adhesion-promoting polymer, typically a polyvinylidine chloride based resin and a light-sensitive emulsion layer, usually a silver halide dispersed in gelatin. A wide variety of techniques ha~ been suggested for the recovery of the components of exposed nr unexposed photographic film, to reclaim the component part~, particularly the silver present in the light-sensitive emulsion layer and the polymer base which is often polyethylene terephthalate.
Such techniques have included the processes described in Hittel et al., U.S. Patent 3,652,466, ~nd ~user et al., U.S. Patents 4,602,046 and 4,612,057. However, previous techniques have resulted in separated products which are not entirely satisfactory. For example, the process shown in Buser et al., U.S. Patent 4,602,046 relates to the separation of one or more coating layers from a plastic b~sed material using caustic alkali ~olution. Hittel et al., U.S. Patent 3,652,466, uses caustic liquid to separate polyester based material from subsequently applied coatings. However, the Hittel et al. process is inefficient in the recovery of silver and clean polyethylene terephthalate from photographic film, and the Buser et al. patent is directed only to the separation of the polymeric layer from the ba~e layer and not to the recovery o~ silver.
Summary ~f the Invention The instant invention provides an improved process for the recovery of silver from photographic ~k 130~ 28 film~ using a combination o~ elevated temper~ture, caustic treating solution, and high ~hear.
Specifically, the instant lnvention provides a proces~ for the recovsry of silver from photographic film having a polymeric ba~e, a surface coating which provides adhe~ion for a ~ilver emul~$on layer, and a ~ilver emulsion layer, which proces~ compri~es:
~a) mixing pieces of the ~ilm wlth a caustic alkali solution heated to a temperature of about from 140F to 200F, (b) sub~ecting the mixture to conditions of high shear for a period sufficient to remove the surface coating from the base and to reduce the ~ilver h~lides in the emulsion layer to metallic 6ilver, (c) ~eparating the aqueous liquid containing the removed silver and surface coating from the film pie~es, ~nd (d) acidifying the agueous liquid to separate the metallic silver.
Detailed De~cription of the Inventlon The present process i~ appli~ble to the recovery of silver from photographic ~ilms of the type having a polymeric base layer, an adhesion promoting layer, and a silver-containing layer. Base layers used in such photographic films are often biaxially oriented polyesters such as polyethylene terephthalate, cellulose acetate, or nitrocellulose.
The photographic film i~ generally coated with an adhe6ion promoting layer such as a vinylidine chloride polymer or copolymer, which in turn, is coated with a light-sensitive emulsion layer containing a silver halide.
13()5~28 In accordance with the present invention, the photographic film, whether in an exposed and developed or unexposed state, is contacted with a caustic alkali solution. The caustic used can, for example, be sodium hydroxide or pota sium hydroxide of which 60dium hydroxide is preferred. The concentration of the solution should be at least about 7 wt. percent of the caustic, and maintained at a treatment temperature of about from 140F to 200F.
The photographic film, in the hot caustic solution, is subjected to high shear. High shear is used in its conventional sense, that i8 ~ mixing sufficient to create turbulent as opposed to laminar flow within the treatment vessel. To promote high shear, the photographic film is preferably in the form of small pieces, so that the turbulence of the film pieces aids in the development of shear within the treatment vessel. Particularly preferred are pieces of film ranging in size from about 2 to about 30 millimeters in the longest dimension.
The high 6hear mixing in th~ caustic solution is carried out for a period ~uffic~ent to reduce the silver halides in the emulsion layer to metallic sil-ver. The mechanism of the chemical conversion is not fully understood, but is belleved to be a function of the caustic in combination with the gelatin used in the light-sensitive emulsion layer. The period for treat-ment will necessarily vary with the concentration of silver in the film, the concentration of caustic, the specific temperature used, and other conditions such as the concentration of the film being treated in the caustic solution. However, in general, substantially complete conversion of the sllver halides to metallic silver will be complete within a period of about from 15 to 45 minutes. The conversion to metalllc silver is generally accompanied by a color change.
After conver~ion of the silver halide in the emul~ion layer to metallic silver, the aqueous liquid containing the metallic silver is removed. This liquid is then acidified to facilitate the preclpitation and removal of the metallic ailver. Any convenient acid can be used for the acidification, of which ~ulfuric acid has been found to be particularly satisfactory, and i~ accordingly pre~erred. The removal o the silver metal i5 further facilitated by ~cidification to a pH of le~s than about four.
It was previou~ly thought that treatment wlth caustic would convert the silver halides a~ found in the photographic film to ~ilver oxide rather than elemental silver. It was as6umed, for example, by those skilled in the art, that the Hittel et al.
process described in U.S. Patent 4,652,466, in which caustic was used with low shear, that the deposition of ~ilver on proce~sing eq~ipment wa~ a re~ult of metal interchange from the solution and the treatment vessels.
The process of the instant invent~on is applicable to both exposed and unexposed photographic film. The production of metallic silver from the proce6s, as oppo~ed to a s~lver hallde or oxide, not only results in the removal of metallic sil~er in an efficient manner, but provides an aqueous residue having a very low concentration of silver, for example, less than about five parts per million, which can be disposed in an environmentally safe manner. Moreover, ~he high shear conditions used permit removal of the metallic sllver in an efficient manner as opposed to the Hittel et al. proce6s in which metallic silver was often ~ound to coat the walls of the treatment ~essel.
The present in~ent~on i5 further illustrated by the following ~peci~ic example in which parts and percentages are by weight unless otherwise indicated.
~30~ 8 Example Silver was removed ~rom photographic film having a biaxially oriented polyester base, polyvinylidine chloride adhesion promoting layer, and photographic gelatin containing silver halide. The photographlc film was in the form of flake having a maximum dimension of a~out 25 mm.
Four thousand parts of the flake were introduced into a high ehear mixer in addition to 4700 parts of water maintained at a temperature of 150F.
The agitator was started and 400 part~ of sodium hydroxide in flake form were added to the mixture to provide a caustic solution of 8.5% sodium hydroxide.
The treatment vessel was heated by ~team to a temperature of 195F, and mixing under high shear was continued for about 30 minutes.
The contents of the treatment vessel were emptied into a second treatment vessel and the original vessel flushed with fresh water until clean.
Forty-five hundred parts of water were-added to the- -second treatment vessel and the mixture agitated for a period of two minutes. Four hundred parts of 8ul furic acid was added to the second treatment ves6el to neutrali~e the treatment mlxture to a pH of about 7.
~5 The silver suspended in water i6 separated by means of a continuous centrifuge in which the polyester flake is retained on a rotating screen in a centrifugal filter while rinsing. The filter permits passage of the elemental silver which is suspended in the rinse water and collection of the wet polyester flake.
Rinse water was collected in a holding tank and sulfuric acid was added to the tank to adjust the pH to about three, resulting in the flocculation of the silver metal. Silver flocculates in the r~nse water upon acidification, ~ince at a pH of about three the 13~S328 gelatin used in the photographic film ~ubstantially loses its disper~ing capacity for the sllver.
The rinse water from the holding tank was pumped to two cone-bottom tank~ and allowed to 6ettle for one hour. The acidic aqueou~ solution was removed and found to still exh~bit a pH of about three. Silver metal ~ludge is recovered from the bottom of the cone-bottom tank. The ~ludge compri~e~ the silver ~etal, gelatin and adhesion promoting polymer. This sludge i~ then ~urther treated using silver processing techniques well known in the art to ~eparate the silver metal from the other ~ludge components.
The clean polyester flake was removed from the centrifuge and dried by ~onventional ~eans.
ZO
~ - 6 -
Recovery of Silver from Photographic Film With High Shear and Caustic Background of the Invention Photographic film generally comprises a polymeric base, a coating of adhesion-promoting polymer, typically a polyvinylidine chloride based resin and a light-sensitive emulsion layer, usually a silver halide dispersed in gelatin. A wide variety of techniques ha~ been suggested for the recovery of the components of exposed nr unexposed photographic film, to reclaim the component part~, particularly the silver present in the light-sensitive emulsion layer and the polymer base which is often polyethylene terephthalate.
Such techniques have included the processes described in Hittel et al., U.S. Patent 3,652,466, ~nd ~user et al., U.S. Patents 4,602,046 and 4,612,057. However, previous techniques have resulted in separated products which are not entirely satisfactory. For example, the process shown in Buser et al., U.S. Patent 4,602,046 relates to the separation of one or more coating layers from a plastic b~sed material using caustic alkali ~olution. Hittel et al., U.S. Patent 3,652,466, uses caustic liquid to separate polyester based material from subsequently applied coatings. However, the Hittel et al. process is inefficient in the recovery of silver and clean polyethylene terephthalate from photographic film, and the Buser et al. patent is directed only to the separation of the polymeric layer from the ba~e layer and not to the recovery o~ silver.
Summary ~f the Invention The instant invention provides an improved process for the recovery of silver from photographic ~k 130~ 28 film~ using a combination o~ elevated temper~ture, caustic treating solution, and high ~hear.
Specifically, the instant lnvention provides a proces~ for the recovsry of silver from photographic film having a polymeric ba~e, a surface coating which provides adhe~ion for a ~ilver emul~$on layer, and a ~ilver emulsion layer, which proces~ compri~es:
~a) mixing pieces of the ~ilm wlth a caustic alkali solution heated to a temperature of about from 140F to 200F, (b) sub~ecting the mixture to conditions of high shear for a period sufficient to remove the surface coating from the base and to reduce the ~ilver h~lides in the emulsion layer to metallic 6ilver, (c) ~eparating the aqueous liquid containing the removed silver and surface coating from the film pie~es, ~nd (d) acidifying the agueous liquid to separate the metallic silver.
Detailed De~cription of the Inventlon The present process i~ appli~ble to the recovery of silver from photographic ~ilms of the type having a polymeric base layer, an adhesion promoting layer, and a silver-containing layer. Base layers used in such photographic films are often biaxially oriented polyesters such as polyethylene terephthalate, cellulose acetate, or nitrocellulose.
The photographic film i~ generally coated with an adhe6ion promoting layer such as a vinylidine chloride polymer or copolymer, which in turn, is coated with a light-sensitive emulsion layer containing a silver halide.
13()5~28 In accordance with the present invention, the photographic film, whether in an exposed and developed or unexposed state, is contacted with a caustic alkali solution. The caustic used can, for example, be sodium hydroxide or pota sium hydroxide of which 60dium hydroxide is preferred. The concentration of the solution should be at least about 7 wt. percent of the caustic, and maintained at a treatment temperature of about from 140F to 200F.
The photographic film, in the hot caustic solution, is subjected to high shear. High shear is used in its conventional sense, that i8 ~ mixing sufficient to create turbulent as opposed to laminar flow within the treatment vessel. To promote high shear, the photographic film is preferably in the form of small pieces, so that the turbulence of the film pieces aids in the development of shear within the treatment vessel. Particularly preferred are pieces of film ranging in size from about 2 to about 30 millimeters in the longest dimension.
The high 6hear mixing in th~ caustic solution is carried out for a period ~uffic~ent to reduce the silver halides in the emulsion layer to metallic sil-ver. The mechanism of the chemical conversion is not fully understood, but is belleved to be a function of the caustic in combination with the gelatin used in the light-sensitive emulsion layer. The period for treat-ment will necessarily vary with the concentration of silver in the film, the concentration of caustic, the specific temperature used, and other conditions such as the concentration of the film being treated in the caustic solution. However, in general, substantially complete conversion of the sllver halides to metallic silver will be complete within a period of about from 15 to 45 minutes. The conversion to metalllc silver is generally accompanied by a color change.
After conver~ion of the silver halide in the emul~ion layer to metallic silver, the aqueous liquid containing the metallic silver is removed. This liquid is then acidified to facilitate the preclpitation and removal of the metallic ailver. Any convenient acid can be used for the acidification, of which ~ulfuric acid has been found to be particularly satisfactory, and i~ accordingly pre~erred. The removal o the silver metal i5 further facilitated by ~cidification to a pH of le~s than about four.
It was previou~ly thought that treatment wlth caustic would convert the silver halides a~ found in the photographic film to ~ilver oxide rather than elemental silver. It was as6umed, for example, by those skilled in the art, that the Hittel et al.
process described in U.S. Patent 4,652,466, in which caustic was used with low shear, that the deposition of ~ilver on proce~sing eq~ipment wa~ a re~ult of metal interchange from the solution and the treatment vessels.
The process of the instant invent~on is applicable to both exposed and unexposed photographic film. The production of metallic silver from the proce6s, as oppo~ed to a s~lver hallde or oxide, not only results in the removal of metallic sil~er in an efficient manner, but provides an aqueous residue having a very low concentration of silver, for example, less than about five parts per million, which can be disposed in an environmentally safe manner. Moreover, ~he high shear conditions used permit removal of the metallic sllver in an efficient manner as opposed to the Hittel et al. proce6s in which metallic silver was often ~ound to coat the walls of the treatment ~essel.
The present in~ent~on i5 further illustrated by the following ~peci~ic example in which parts and percentages are by weight unless otherwise indicated.
~30~ 8 Example Silver was removed ~rom photographic film having a biaxially oriented polyester base, polyvinylidine chloride adhesion promoting layer, and photographic gelatin containing silver halide. The photographlc film was in the form of flake having a maximum dimension of a~out 25 mm.
Four thousand parts of the flake were introduced into a high ehear mixer in addition to 4700 parts of water maintained at a temperature of 150F.
The agitator was started and 400 part~ of sodium hydroxide in flake form were added to the mixture to provide a caustic solution of 8.5% sodium hydroxide.
The treatment vessel was heated by ~team to a temperature of 195F, and mixing under high shear was continued for about 30 minutes.
The contents of the treatment vessel were emptied into a second treatment vessel and the original vessel flushed with fresh water until clean.
Forty-five hundred parts of water were-added to the- -second treatment vessel and the mixture agitated for a period of two minutes. Four hundred parts of 8ul furic acid was added to the second treatment ves6el to neutrali~e the treatment mlxture to a pH of about 7.
~5 The silver suspended in water i6 separated by means of a continuous centrifuge in which the polyester flake is retained on a rotating screen in a centrifugal filter while rinsing. The filter permits passage of the elemental silver which is suspended in the rinse water and collection of the wet polyester flake.
Rinse water was collected in a holding tank and sulfuric acid was added to the tank to adjust the pH to about three, resulting in the flocculation of the silver metal. Silver flocculates in the r~nse water upon acidification, ~ince at a pH of about three the 13~S328 gelatin used in the photographic film ~ubstantially loses its disper~ing capacity for the sllver.
The rinse water from the holding tank was pumped to two cone-bottom tank~ and allowed to 6ettle for one hour. The acidic aqueou~ solution was removed and found to still exh~bit a pH of about three. Silver metal ~ludge is recovered from the bottom of the cone-bottom tank. The ~ludge compri~e~ the silver ~etal, gelatin and adhesion promoting polymer. This sludge i~ then ~urther treated using silver processing techniques well known in the art to ~eparate the silver metal from the other ~ludge components.
The clean polyester flake was removed from the centrifuge and dried by ~onventional ~eans.
ZO
~ - 6 -
Claims (5)
1. A process for the recovery of metallic silver from photographic film having a polymeric base, a surface coating which provides adhesion for a silver emulsion layer, and a silver emulsion layer containing gelatin, which process comprises:
(a) mixing pieces of the film with a caustic alkali solution heated to a temperature of about from 140°F to 200°F, (b) subjecting the mixture to conditions of high shear for a period sufficient to remove the surface coating from the base and to substantially completely reduce the silver halides in the emulsion layer to metallic silver, (c) separating the aqueous liquid containing the removed silver and surface coating from the film pieces, and, (d) acidifying the aqueous liquid to separate the metallic silver.
(a) mixing pieces of the film with a caustic alkali solution heated to a temperature of about from 140°F to 200°F, (b) subjecting the mixture to conditions of high shear for a period sufficient to remove the surface coating from the base and to substantially completely reduce the silver halides in the emulsion layer to metallic silver, (c) separating the aqueous liquid containing the removed silver and surface coating from the film pieces, and, (d) acidifying the aqueous liquid to separate the metallic silver.
2. A process of claim 1 wherein the caustic consists essentially of sodium hydroxide.
3. A process of claim 1 wherein the caustic mixture is maintained at a temperature of about from 180°F to 200°F.
4. A process of claim 1 wherein the base of the photographic film is polyethylene terephthalate.
5. A process of claim 1 wherein the photographic film is comminuted to flake of about from 2 to 30 millimeters in the longest dimension.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000560445A CA1305328C (en) | 1987-02-13 | 1988-03-03 | Recovery of silver from photographic film with high shear and caustic |
| US07/168,541 US4799954A (en) | 1987-02-13 | 1988-03-08 | Recovery of silver from photographic film with high shear and caustic |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US1471187A | 1987-02-13 | 1987-02-13 | |
| CA000560445A CA1305328C (en) | 1987-02-13 | 1988-03-03 | Recovery of silver from photographic film with high shear and caustic |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1305328C true CA1305328C (en) | 1992-07-21 |
Family
ID=25671753
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000560445A Expired - Lifetime CA1305328C (en) | 1987-02-13 | 1988-03-03 | Recovery of silver from photographic film with high shear and caustic |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1305328C (en) |
-
1988
- 1988-03-03 CA CA000560445A patent/CA1305328C/en not_active Expired - Lifetime
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed | ||
| MKEC | Expiry (correction) |
Effective date: 20121205 |