Process for recovering silver from photographic chemical effluent

A process for absorbing silver from photographic hypo solutions involves passing the hypo solution through a bed consisting of a multitude of a sponge product confined within a vessel. The sponge product is derived from an open-celled cellulosic sponge into which there has been incorporated 30% to 80% by weight of a polymer produced by the thermal interaction of polyethyleneimine (PEI) with a polycarboxylic acid. The polymer further contains an activating multivalent cation and between 90% and 300% water. Silver is eluted from the sponge product employing aqueous solutions of a complexing agent such as an ammonium compound or a cyanide compound. Following a water wash, the bed of sponge product is ready for its next cycle of silver absorption.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns the recovery of dissolved silver from solutions 
having been employed for the processing of silver-containing 
photosensitive materials. 
2. Description of the Prior Art 
Photosensitive materials such as films and papers employed for 
black-and-white photography generally have an emulsion layer which 
contains a finely divided silver halide such as the chloride or bromide. 
In the course of the development and fixing of the image within the 
emulsion layer, molecules of silver halide which are not converted to 
metallic silver by the developer are dissolved out of the emulsion layer 
by a "hypo" solution which generally employs a thiosulfate. 
Following a certain amount of use, the hypo solution becomes spent, and 
must be discarded. Although it is desirable to recover the dissolved 
silver for its economic value, it is more important to recover the silver 
because it is highly toxic, and should not be indiscriminately dumped into 
a convenient sewer line. The United States Environmental Protection Agency 
has indicated that industrial effluents should contain less than 5 parts 
per million (ppm) silver, and drinking water should contain less than 0.05 
ppm silver. Techniques generally employed for removing silver from spent 
hypo include electrodeposition, and treatment with a sacrificial metal 
such as steel wool. Hypo solutions emergent from an electrolytic cell 
which removes silver by electrodeposition upon electrodes will have silver 
concentrations between about 50 and 200 ppm. Hypo solutions emergent from 
steel wool systems will have unpredictably high levels of silver, 
depending upon the state of exhaustion of the steel wool, and will contain 
undesirably high levels of dissolved iron. 
Attempts have been made to utilize ion exchange resins to remove the silver 
from spent hypo. Because the dissolved silver is in an anionic form such 
as silver thiosulfate Ag(S.sub.2 O.sub.3).sub.2.sup.-3, an anion exchange 
resin must be employed. However, ordinary anion exchange resins are not 
sufficiently selective to discriminate between the silver thiosulfate 
anion and unreacted thiosulfate and sulfite anions present in the hypo 
solution. 
U.S. patent application Ser. No. 07/808,884, filed Dec. 18, 1991, now U.S. 
Pat. No. 5,180 concerns a specialized chelation-type ion exchange resin 
having the ability to selectively remove silver thiosulfate from spent 
hypo solutions. Although a worthwhile accomplishment, such removal of 
silver from hypo solutions would be even more practical if the silver 
could be eluted from the resin, permitting repeated recycling of the 
resin. Such elution and recycling is not disclosed in said Patent 
Application. 
The usually employed procedure for eluting absorbed species from an anion 
exchange resin and re-using the resin involves treatment of the resin with 
strong solutions of anions that can replace the absorbed anion. In the 
case of the aforesaid chelation-type resins, especially when disposed 
within an open-celled sponge, treatment with strong solutions of the usual 
anions has bee-,i found to be poorly effective in eluting absorbed silver. 
In the several aforesaid techniques for removing silver from 
photoprocessing effluents, the efficiency of silver removal is generally 
inversely dependent upon the rate of flow of the effluent. The efficiency 
of silver removal is also dependent upon the degree of saturation of 
whatever device or material is employed. 
It is accordingly an object of the present invention to provide a process 
for removing silver from photoprocessing hypo solution. 
It is a further object of this invention to provide a process as in the 
foregoing object employing a chelation-type ion exchange resin which 
removes said silver by selective absorption. 
It is another object of the present invention to provide a process of the 
aforesaid nature wherein the silver absorbed on said resin can be eluted, 
and the resin can be re-used in said process. 
It is a still further object of this invention to provide a process of the 
aforesaid nature wherein the efficiency of silver removal has reduced 
dependency upon the flow rate of said hypo solution and the degree of 
saturation of said resin with silver. 
It is yet another object of the present invention to provide a process of 
the aforesaid nature which involves low capital investment and low 
operating costs. 
These and other beneficial objects and advantages will be apparent from the 
following description. 
SUMMARY OF THE INVENTION 
The above and other beneficial objects and advantages are accomplished in 
accordance with the present invention by a process for removing silver 
from a photoprocessing hypo solution comprising: 
a) passing said solution through a bed of open celled sponge product 
containing a gel form of a polymer produced by the thermal interaction of 
polyethyleneimine (PEI) with a polycarboxylic acid, said polymer 
containing chemically bonded thereto a substantially saturation amount of 
an activating multivalent metal cation and containing between 90% and 300% 
of water based upon the dry weight of the polymer, the rate of passage of 
said solution through said bed being between 0.05 and 2.0 bed 
volumes/minute (bvm), 
b) eluting silver from said bed by passing through said bed an aqueous 
solution of a complexing agent selected from the group consisting of 
ammonium compounds and cyanide compounds, and 
c) washing said bed with water, thereby removing said complexing agent and 
preparing said bed for re-use, beginning at step a. 
Preferred multivalent cations include Mg.sup.++, Ca.sup.++, Al.sup.+++, 
Cu.sup.++, and Fe.sup.+++. In a particularly preferred embodiment, the 
sponge product comprises between 30% and 80% of polymer based upon the 
"overall dry weight of said sponge product. The sponge product is 
preferably of cuboid configuration, having an average volumetric size less 
than a cubic inch. 
Suitable complexing ammonium compounds include ammonium hydroxide, ammonium 
nitrate, ammonium acetate and other ammonium salts which ionize in water. 
Suitable cyanide compounds include sodium cyanide, potassium cyanide, 
ammonium cyanide and other compounds which provide the cyanide anion 
(CN.sup.-) in water. Mixtures of complexing agents may be employed in the 
elution step.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The polymer employed in the process of the present invention is preferably 
one produced by the insolubilization of polyethyleneimine (PEI) with a 
multi-functional carboxylic acid. Polyethyleneimine, a water-soluble 
branched chain polymer having recurring secondary amino nitrogen atoms, is 
commercially available in molecular weights ranging from 1200 to 60,000. 
Preferred polycarboxylic acids are those which additionally contain amino 
nitrogens, typical embodiments including iminodiacetic acid, 
ethylenediamine tetraacetic acid and nitrilotriacetic acid. The resultant 
insolubilized or cross-linked PEI preferably contains pendant carboxyl 
groups by virtue of the multi-functional carboxylic acid. 
The polymer is produced within the sponge by initially dissolving the 
otherwise insoluble multi-functional carboxylic acid in an aqueous 
solution of PEI to produce a homogeneous precursor solution. An 
open-celled cellulosic sponge is impregnated with the precursor solution. 
The sponge is then subjected to thermal curing at temperatures in the 
range of 130 degrees C to 170 degrees C, preferably in an oxygen-free 
environment, to achieve an amide-type condensation cross-linking reaction 
which produces a water-insoluble gel polymer that swells in water. The 
extent of cross-linking of the gel polymer is controlled primarily by the 
ratio of PEI/polycarboxylic acid and the time and temperature of the 
curing treatment. The gel polymer product absorbs water in amounts 
generally indicative of the extent of cross-linking. For example, highly 
cross linked Polymers absorb less than 90% of water based upon their dry 
weight. Very slightly cross-linked polymers can absorb as much as 500% of 
water. It has been found that, for the purposes of the present invention, 
polymers having a water absorption capacity less than 90% lack adequate 
ion exchange capacity, and polymers having a water absorption capacity 
over 300% lack adequate cohesive strength. 
The amount of water absorbable by the gel polymer is measured by blotting 
excess water off a mass of fully waterswollen polymer prepared in the 
absence of the sponge, weighing, heating to dryness, and re-weighing. The 
percent water absorption is expressed as the weight of water divided by 
the dry weight of the polymer times one hundred. 
When the polycarboxylic acid is nitrilotriacetic acid (NTA), the preferred 
weight ratio of NTA/PEI to achieve crosslinking in the desired range is 
between 0.9 and 1.4. 
Incorporation of the activating metal cation into the polymer may be 
accomplished either by inclusion of the cation within the precursor 
solution, or by an aftertreatment of the polymer. The metal cations become 
chemically bound to the polymer by formation of ionic bonds with carboxyl 
groups and formation of coordination bonds with amine groups. Regardless 
of the exact manner of chemical bonding, the polymer interacts with a 
stoichiometric amount of the metal ion. By this it is meant that each 
polymer type can reproducibly saturate with a specific quantity of metal 
ions. Any metal ions present beyond the stoichiometric amount are not 
chemically bound, and can be removed by physical methods such as 
extraction with water. It is to be understood however, that the manner of 
chemical bonding may be governed by general considerations of chemical 
equilibrium. Accordingly, excessive extraction of a metal-containing 
polymer with water may in some instances cause a slight loss of metal 
ions, the magnitude of the loss being dependent upon an equilibrium 
constant. 
Polymers prepared for use in accordance with the present invention, 
containing stoichiometric quantities of metal cations, generally contain a 
weight of metal ion in the range of 2% to 20%, based upon the dry weight 
of the polymer. The exact weight of metal content is dependent upon the 
particular nature of the polymer and the valence and atomic weight of the 
metal ion. 
For the proper treatment of photoprocessing effluent solution, a bed of the 
sponge product is preferably confined within a tube, column or drum 
structure wherein the ratio of height or long axis to diameter is between 
about 1.5 and 10. It has been found that greater efficiency of silver 
removal is achieved when the sponge product is in a compacted state within 
said confining structure. The extent of said compaction is such that, in 
the compacted state, the sponge product occupies between 40% and 80% of 
the volume that would be occupied by the uncompacted sponge product, 
corresponding to compactions of 60% and 20%, respectively. The flow of 
solution to the treated is preferably in the direction of the height or 
long axis of the confining structure, said flow traveling from an inlet 
port, thence through the sponge product, and thence through the exit port. 
The rate of flow through the bed should be in the range of 0.05 to 2.0 
b.v.m. 
The following examples are presented for illustrative purposes without 
intending to be limitative of the scope of the invention. All parts and 
percentages are by weight. 
EXAMPLE 1 
A vertical column of 4" inside diameter was filled to an uncompacted bed 
height of 40" with polymer-containing sponges having a 9 mm cubic 
configuration. The sponges are comprised of 72% by weight of a gel polymer 
produced by the thermal cross-linking of PEI with NTA, the 
water-absorption capacity of the polymer being 252%. Calcium was 
incorporated into the polymer by treating the sponges with lime water in 
the column, followed by washing with water. 
A photographic hypo solution containing 977 ppm Ag(S.sub.2 
O.sub.3).sub.2.sup.-3, 847 ppm of (S.sub.2 O.sub.3).sup.-2 and 513 ppm 
SO.sub.3.sup.-2 was passed downwardly through the bed at a flow rate of 
0.08 bed volume/minute. The treated solution emergent from the column 
initially contained 2.1 ppm silver, as measured by atomic absorption. In 
the course of passage of four bed volumes of the photographic solution 
through the column, the silver content of the emergent treated solution 
rises to 5.0 ppm. Passage of the photographic solution through the column 
was continued until the bed of sponges was substantially saturated, as 
evidenced by a silver content in the emergent solution of 952 ppm. The bed 
was then washed with 5 bed volumes of water, and a sample of the saturated 
sponge was removed from the top of the bed. The sample was found to 
contain 5.4% silver (dry weight basis). 
In separate experiments, different aqueous eluting solutions were employed 
in attempts to remove the absorbed silver from the bed and enable the 
sponge to be re-used for silver absorption. Following each elution trial, 
the bed was re-saturated with silver from the photographic solution. In 
each trial, the effectiveness of the eluting solution was determined by 
measuring the percent of silver removed from the sponge. The eluting 
solutions tried, and results obtained are shown in Table 1 below. 
TABLE 1 
______________________________________ 
Eluting Solution % Ag Removed 
______________________________________ 
1.6% Sodium Thiosulfate 
12 
5.0% Sodium Nitrate 25 
Ammonium Hydroxide (saturated) 
78 
Ammonium Acetate (saturated) 
79 
Ammonium Nitrate (saturated) 
82 
1.0% Sodium Cyanide 93 
______________________________________ 
As the data indicate, desorption of silver is not produced as a consequence 
of a simple anion exchange reaction. If such were the case, then the 
sodium nitrate solution should have been effective. Instead, desorption of 
silver appears to require the presence of a complexing moiety such as 
cyanide or ammonium ions. Despite the different efficiencies of the 
several eluting solutions, in each case the sponge, following passage of 
several bed volumes of wash water, was found to be capable of reabsorbing 
its initial saturation level of silver, and such recycling ability is 
achievable for at least ten cycles. 
EXAMPLE 2 
The polymer-containing sponge product of Example 1 containing aluminum 
instead of calcium was employed to form a bed of initially 40" height in a 
vertical column of 4" inside diameter. In different trials, the bed was 
subjected to various degrees of compaction prior to receiving the hypo 
solution of Example 1. The beds were subjected to vacuum deaeration to 
remove air trapped within and between sponges. The flow rate of the hypo 
solution through the bed was maintained at 0.15 bed volume/minute for each 
trial. The initial four bed volumes of treated solution emergent from the 
column were collected as a single sample for silver analysis. The results 
obtained are reported in Table 2 below. 
TABLE 2 
______________________________________ 
Concentration of Ag in the 
% of bed compaction 
treated solution (ppm) 
______________________________________ 
0 7.8 
12 6.3 
19 4.7 
28 3.5 
43 2.8 
______________________________________ 
As the data of Table 2 indicate, greater compaction of the bed produces 
greater efficiency of silver removal. The compacted beds of sponge of this 
example were found capable of discharging at least 90% of the absorbed 
silver into a 1.0% sodium cyanide eluting solution. Following a water 
wash, the sponges were capable of repeated cycles. 
The elution solutions may be employed at ambient room temperature or at 
elevated temperatures up to about 95 degrees C. The elution solutions 
containing silver can be passed through an electrolytic cell which removes 
silver, and enables the elution solution to be utilized again in 
subsequent elution cycles. Because the elution step requires procedures 
and chemicals generally unfamiliar to photoprocessing installations, it is 
preferable that the sponge product be packaged in returnable drums 
amenable to shipment by ordinary modes of transportation. In this manner 
of use, when the sponge in the drum is saturated with silver, it would be 
returned to a processing facility where elution of silver and washing 
would be performed, and the sponge-containing drum would be returned to 
the user for the next cycle of use. Such drums preferably have inlet and 
exit ports, thereby enabling the sponge to be utilized, eluted and washed 
without removal from the drum. 
While particular examples of the present invention have been shown and 
described, it is apparent that changes and modifications may be made 
therein without departing from the invention in its broadest aspects. The 
aim of the appended claims, therefore, is to cover all such changes and 
modifications as fall within the true spirit and scope of the invention.