Silver recovery cartridge for used fixer of neutral or higher pH

A method and article for substantially preventing premature obstruction to flow in a canister of a silver recovery device comprising placing a porous pad at the inlet surface of the metallic filler of the canister.

This invention relates to a method and device for the prevention of 
premature obstructions to flow in a silver recovery device. 
During the processing of silver halide photographic materials, silver is 
carried out of the photographic elements and into solution, particularly 
the fix and bleach/fix solutions. Numerous devices have been developed for 
removing the silver from spent solutions. One such device utilizes a 
container within which is positioned a metallic filler, e.g., U.S. Pat. 
No. 3,369,801. The container is connected to the photographic processing 
unit so that the spent silver containing solution flows completely through 
the container holding the metallic filler prior to the solution being 
sewered or returned to a mixing tank. While the solution is being 
circulated, the silver is deposited throughout the metallic filler. This 
type of system utilizes an electronic transfer which occurs between the 
more and less noble metals in order to recover silver in the spent 
solution. The less noble metal (i.e., the metallic filler) in this type of 
recovery unit is usually iron in the form of inexpensive steel wool. When 
the silver rich solution is passed over the iron, an electron exchange 
occurs wherein the iron metal (Fe.sup.o) is oxidized to ferrous 
(Fe.sup.+2) or ferric (Fe.sup.+3) ion and the silver ion (Ag.sup. +) is 
reduced to silver metal (Ag.sup.o). 
With more aggressive acidic solutions (pH 4 to 6) and with the neutral but 
aggressive bleach-fix solutions plating takes place in the interior as 
well as the inlet surfaces of the steel wool and the filamentary structure 
of the steel wool is quickly disintegrated by dissolution, corrosion, and 
the physical effects of embrittlement. While these effects contribute to 
early loss of capacity for silver recovery, they also serve to maintain 
flow pathways for the solution, making unnecessary the insertion of a 
porous pad as herein described. 
It has been found in the use of such silver recovery devices when the 
silver-containing solution is neutral or slightly alkaline (pH of 6 to 8) 
that a dense impermeable layer of silver tends to form at the inlet side 
of the steel wool which prematurely obstructs the flow before all the 
steel wool can be utilized. Electrons which are released by action of the 
fixer solution upon the steel wool in the interior of the cartridge are 
conducted to the region where silver-rich solution first enters the steel 
wool. There the silver ions are reduced and silver is preferentially 
plated on silver already deposited. This highly localized deposition leads 
to pore blockage and stoppage of flow.

The present invention substantially prevents premature obstruction to flow 
by the introduction of a porous pad at the inlet surface of the metallic 
filler within a silver recovery device. 
The present invention may be described as a silver recovery cartridge 
having an inlet port, an outlet port, and a metallic filler therein where 
the improvement comprises a porous pad adapted to be positioned against a 
substantial portion of the surface of the metallic filler nearest the 
incoming silver containing solution. 
It is believed that the mechanism by which the porous pad prevents 
premature obstruction to the flow is that of providing a three-dimensional 
support network for the growth of the silver dendrites, thereby allowing 
for the maintenace of flow pathways for the incoming solution. Dendrites 
tend to form because the points at which silver is newly deposited are the 
most electrolytically energetic, and more silver deposits on these points 
preferentially, forming growing needles of silver. If these needles are 
not directed away from the surface of the metallic filler, they will be 
impacted at the surface by the incoming stream, resulting in early 
blockage of flow. 
The blockage-preventive device is preferably comprised of a porous 
pad-shaped structure. The material may be any substance which (1) is 
chemically inert to the chemicals present in the photographic fixing 
solution, i.e., thiosulfates, buffers, silver salts, (2) does not release 
pollutants to the influent streams, and (3) can be formed into porous pads 
having sufficient mechanical integrity to support, if need be, the weight 
of the steel wool spool. 
The preferred materials for use as porous pads of the present invention are 
wood products, expanded paper, lofted cellulose fibers, glass wool, nylon 
webbing, resin bonded sawdust, polypropylene padding, etc. The materials 
can be electronically conductive as well as nonconductive. Porous carbons 
may also be used. The pad is shaped to fit within the container of the 
silver recovery device in contact with and fully covering the metallic 
filler preferably near the solution inlet side. The pad should have at 
least an in place thickness of 1/4 inch with a thickness of about 1/2 inch 
being preferred. Although thicker pads may be utilized, it has been found 
that if the thickness exceeds 1/2 inch, no proportionate benefit is 
obtained. 
If an "apparent pad density" (Dp) is defined as weight per unit volume, 
with weight in grams and volume in cubic centimeters of the space occupied 
by the pad as positioned in the silver recovery device, then an acceptable 
value of Dp is between 0.0650.times.Dm and 0.0033.times.Dm, wherein Dm is 
the density of the solid material of which the pad is constructed in grams 
per cubic centimeter. It has been found when the pad is too porous (e.g., 
cellulose, with a Dp less than 0.005 g/cc), there will be insufficient 
filamentary support for silver dendritic deposition, while too dense a 
structure, i.e., Dp greater than 0.1 g/cc (e.g., cellulose of 
Dm.perspectiveto.1.52), prevents free dendritic structure growth leading 
to early obstruction of liquid passages and premature flow blockage. The 
preferred "apparent density" range is about 0.0065.times.Dm to 
0.0033.times.Dm. 
The cartridge of the present invention may be placed in contact with either 
the top or bottom of the metallic filler, the positioning being dependent 
on the direction of flow of the silver-rich spent fixer stream. This 
positioning allows the spent solution to pass through the porous pad 
whereby the silver ions are reduced and deposited on the material of the 
pad in a manner which is governed by the orientation of the material.