Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2001-04-10
2003-06-03
Kastler, Scott (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C266S101000, C266S170000
Reexamination Certificate
active
06572677
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the recovery of silver from waste photographic solutions. Although typically most of the silver removed from silver-halide imaging elements occurs in the fixing step, leaching out during the fix step, some quantity of silver is also removed in the preceding steps, such as the developer and bleach steps. There are three principle means that are used for silver recovery: (1) chemical precipitating agents, (2) electrolysis and (3) the use of a galvanic cell or metal—metal exchange. This invention relates to a method employing metal—metal exchange.
Recovery of silver ions from solution can occur when the silver ions come into contact with a metal having a higher electropositive oxidation potential than silver. The reaction proceeds as follows:
Me→Me
n+
+ne
nAg
+
+ne→nAg
where n is the valence of the metal (Me) that has a higher electropositive potential than the Ag
+
ion.
Metals that fall into this category include Al, Fe, Zn. In most commercial recovery schemes, iron is the metal of choice, for health, safety, and environmental reasons and also for economic reasons. Typically, steel wool or chopped up steel wool is used in a canister. The silver-bearing waste solutions are contacted either in a batch mode or a continuous mode.
Several patents disclose methods and apparatus for recovering silver by the metal—metal exchange reaction. For example, U.S. Pat. Nos. 2,194,056; 3,705,716 and 4,065,313 disclose methods and apparatus for recovering silver from waste photographic processing solutions by an exchange reaction with a metal, such as iron or copper, that is more electropositive than silver. European publication EP27769 discloses a method of monitoring the redox potential of the silver-bearing solution while the metal—metal exchange reaction is taking place. This is done to minimize the re-dissolution of silver when the ferrous ions are oxidized to Fe
3+
ions.
U.S. Pat. Nos. 5,188,662, and 5,310,629 disclose substrates coated with a hydrophilic colloid containing physical development nuclei to remove soluble silver from developer solutions during the development step, in order to maintain the performance of the developer solution. Useful hydrophilic colloids include proteins, gelatin, and polysaccharides such as dextrin, to name a few. In U.S. Pat. No. 5,188,662, the development nuclei are noble metals that are less electropositive. In U.S. Pat. No. 5,310,629, the development nuclei also include sulfides of metals. While this technology is also capable of use to recover silver from waste photoprocessing solutions, noble metals are substantially more expensive than the electropositive metals listed above. Sulfides of metals are, in general, undesirable because of the possibility of developing an odor due to generation of hydrogen sulfide. Also, since the object of the aforementioned patents is to remove silver from functioning developing solutions, one would not want to use electropositive metals, which could ionize in the developer solution and change its functionality. Japanese patent application 01-050047 discloses a cleaning film that comprises a substrate coated with a hydrophilic colloid (like gelatin) containing a hydrophobic organic compound that is capable of adsorbing silver on the substrate. This film is also used to remove soluble silver from functioning developer solutions.
In view of the above, the prior art that discloses the removal of silver ion from solution in the presence of a hydrophilic colloid either use a crystallization phenomenon or simple physical adsorption, which methods are less effective and more expensive than the metal—metal exchange reaction.
In a typical photoprocessing operation, silver-bearing waste from the operation is passed through a cartridge containing an electropositive metal. For example, steel wool in a canister is known for this purpose.
PROBLEM TO BE SOLVED BY THE INVENTION
It is highly preferred that the hydrophilic binder used in the present invention be crosslinked to some degree, to prevent disintegration/dissolution of the binder when it comes into contact with an aqueous solution. For example when polyvinyl alcohol is used as the hydrophilic colloid, it can be crosslinked with sodium borate (Borax). In the case of the preferred hydrophilic colloid, gelatin, several types of crosslinkers or hardeners are known in the art. These are described in “The Theory of the Photographic Process” 4
th
Ed., Ed. T. H. James, pg. 77-87, 1977. The class of inorganic crosslinkers includes salts of chromium and some salts of aluminum. These typically crosslink via the free carboxylic acids in gelatin, and the degree of crosslinking is pH sensitive and also reversible. It may not be desirable, however, to use these salts for absorbents because of the impact these materials have on the environment. Organic crosslinkers are, therefore, preferred. The organic crosslinkers act via the &egr;-amino function of lysine and hydroxylysine. There are, on the average, about 0.35-0.4 mmol of lysine per gram of dry gelatin and about 20% of that amount of hydroxylysine per gram of dry gelatin. Classes of organic crosslinkers include, but are not limited to, aldehydes and blocked aldehydes, ketones, carboxylic and carbamic acid derivatives, sulfonate esters and sulfonyl halides, s-triazines, epoxides, aziridines, isocyanates, carbodiimides and isoxazolium salts. Polymeric crosslinkers are generic polymer molecules bearing one or more of the above moieties in their chain. The selection of the crosslinker type depends on the efficacy of the crosslinking, its toxicity in the native state, the residuals in the absorbent, and cost.
SUMMARY OF THE INVENTION
It has been found that the problems encountered using fine metal particles for the recovery of silver from a silver-bearing photographic waste solution can be alleviated by encapsulating or embedding metal particles in a matrix of a hydrophilic colloid that allows for the permeability of water and silver ions. When contacted with the silver-bearing photographic waste solution, the hydrophilic colloid will imbibe water and swell. At the same time, the silver ions can permeate the colloidal matrix and come into contact with the metal particles, preferably iron particles, at which point a metal—metal exchange reaction takes place to reduce the silver ion to silver metal.
By embedding, dispersing, or encapsulating the metal particles within a hydrophilic colloid, the reduced silver is precipitated within a colloidal matrix, whereby the hydrophilic colloid also functions as a filter. The hydrophilic colloid/metal combination can take various physical forms or shapes, such as, for example, a coating or coated film, a coated substrate, granules, a block of variable shape, or a flow-through filter. The recovery element may be attached/affixed to a surface or other object or remain unattached. If used in a filter, the filter may be of any conventional design. See, for example, U.S. Pat. No. 5,310,629, hereby incorporated by reference.
The advantages of the present invention over other methods of recovering silver include the fact that, since it involves metal—metal exchange, it allows the use of a metal with a high specific surface area, but without the problems associated with handling of powders as well as the oxidation of such powders over time. It also obviates the need for a filter to separate the silver from the solution. Finally, it increases the ease of handling the recovered silver, i.e., one can deal with granules or coatings (containing relatively little or no liquid) rather than slurries containing finely divided silver and oxidation products of the metals themselves. Another advantage is that, by encapsulating the fine metal particles within the hydrophilic colloid, it was surprisingly found that the amount of metal needed to maintain the same reaction kinetics is much less than that needed when the metal is in the form of chopped or shredded wool, as currently used in the trade.
DETAILED DESCRIPTION OF THE INVENTIO
Donovan Kevin M.
Lobo Lloyd A.
Eastman Kodak Company
Kastler Scott
Konkol Chris P.
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