Metal removal apparatus and method

A first metal is removed from solutions such as polluted water by contacting the solution with a metallic exchange medium. The metallic exchange medium comprises a plurality of buoyant elements, each comprising a substrate coated with a second metal sufficiently reactive to undergo an electrochemical exchange reaction with the first metal. The substrate can be expanded polystyrene beads and the metal can be iron such as sponge iron of small particle size.

CROSS-REFERENCE 
This application is related to my copending application entitled "Silver 
Removal Apparatus and Method," filed on the same day as this application, 
Ser. No. 266,066, which is incorporated herein by this reference. 
BACKGROUND 
This invention is directed to removing metals from solutions, and 
particularly, polluted or contaminated water. 
Dissolved metals are commonly removed from solutions to recover the metal 
as a valuable product and/or to remove the metal as a pollutant. For 
example, silver is removed from photographic solutions to recover the 
silver as a valuable product as well as to permit the solution to be 
discharged without causing pollution. 
Numerous devices have been developed for removing silver from used 
photographic fixer solutions. Such devices are described in U.S. Pat. Nos. 
3,369,801; 3,692,291; 4,035,181; and 4,156,604; all of which are 
incorporated herein by this reference. 
These devices utilize an electron exchange reaction, i.e., an 
electrochemical exchange reaction which occurs between a more noble metal 
and a less noble metal in order to recover the silver in the used 
solution. The less noble metal in these devices is iron in the form of 
inexpensive steel wool or a window screen. When the silver rich solution 
is passed over the iron, an electron exchange exchange occurs wherein the 
iron metal Fe.sup.O is oxidized to ferric or ferrous ion and the silver 
ion (Ag.sup.+) is reduced to silver metal (Ag.sup.O). The silver 
precipitates from the solution to form a sludge, from which the silver is 
recovered. The spent solution from the device is discharged to a sewer or 
returned to a fixing tank. 
The use of steel wool or window screen as the silver source has significant 
disadvantages. Among these problems is inefficient use of the iron, a 
substantial portion of the iron not being used to replace silver. Further, 
frequent clogging and channeling have been experienced. Channeling is a 
particular problem as the iron in the steel wool or window screen is 
depleted, resulting in voids forming in the exchange medium. These voids 
provide a path of lower resistance to flow and loss of efficiency in 
silver recovery and material use. 
A further disadvantage is that these units can require the use of steel 
wool or window screen of different sizes or weights dependent upon the 
silver content or pH of the solution being treated. This can require the 
availability of two or more types of silver recovery devices, or the use 
of a plurality of devices. 
Another disadvantage of existing recovery devices is that it is difficult 
to purify the recovered silver. The recovered silver is mixed in with 
large particles of window screen or steel wool. In order to refine the 
silver, it isfirst necessary to comminute the silver sludge containing 
particulate iron into a fine powder, such as by ball milling. This is a 
time-consuming and costly step. 
A further disadvantage of the existing devices is that leakage between the 
container lid and body can occur. A monocoque container cannot be used 
because the steel wool or window screen needs to be placed in the 
container. Such leakage can cause significant property damage. 
In view of these problems, it is apparent that there is a need for a metal 
recovery apparatus that makes efficient use of the exchange medium, avoids 
clogging and channeling in use, does not leak, maintains good contact with 
the metal containing solution, can be used for a wide variety of solutions 
having different metal contents and different pH, and recovers metal in a 
form that does not require comminution for refining. 
SUMMARY 
The present invention is directed to an apparatus having these features. 
The apparatus is useful for recovering metals from a solution, such as 
silver from a photographic fixer solution. The apparatus comprises a 
container formed of a material substantially non-reactive with the 
solution. The container has an inlet for introducing an influent solution 
containing a first metal into the container and an outlet for removing 
spent solution from the container, the spent solution having first metal 
removed therefrom. The container contains a metallic exchange medium that 
comprises a plurality of buoyant elements. The elements are sufficiently 
buoyant to float in the solution. Each element comprises a substrate 
coated with a second metal, the substrate being substantially non-reactive 
with the solution. The second metal is sufficiently reactive to undergo an 
electrochemical exchange reaction with the first metal. 
A preferred substrate is a polymeric substrate such as expanded polystyrene 
beads, which are hollow and contain air. The metal can be iron. Preferably 
the iron is sponge iron, secured to the substrate with an adhesive. 
Preferably the sponge iron is from about +150 mesh to about -10 mesh, and 
more preferably from about +80 mesh to about -20 mesh. Preferably the 
beads are from about 1/8 to about 1/2 inch, and more preferably from about 
1/4 to about 3/8 inch in diameter. Preferably the iron used is 
substantially free of zinc, aluminum, and copper to insure that these 
pollutants do not go into solution. 
Preferably, the container is not filled with the exchange medium, so that a 
void can be left at the bottom of the container for precipitated first 
metal to collect. This can be achieved by using an amount of metallic 
exchange medium so that the medium comprises from about 70% to about 90% 
of the volume of the container. 
The precipitated metal can easily be recovered by withdrawing the contents 
of the container and pyrolyzing the polymeric substrate in a single 
heating step. This leaves substantially only first and second metal 
particles that usually can be refined without comminution. 
This apparatus and method have general applicability, also being useful for 
recovering not only silver, but also metals that are more noble than iron. 
Further, the exchange medium can be formed using metallic coatings other 
than iron coatings, so that metals less noble than iron can be recovered.

DESCRIPTION 
The present invention is directed to an apparatus and a method for removing 
and recovering metals from solutions. Although the present invention will 
be principally described with regard to removing and recovering silver 
from used photographic fixing solutions, the apparatus and method can also 
be used for removing other metals from solutions, and for removing silver 
from solutions other than photographic fixing solutions. 
With reference to FIGS. 1 and 2, an apparatus 10 for removing silver from 
photographic fixing solutions comprises a pail shaped container 12 that 
includes a monocoque body portion 14 forming the side and bottom walls of 
the container 12, and a top lid 16. The top edge of the body portion has 
an integral strengthening rib 18 over which the lid 16 snap fits. There is 
a gasket 20 between the rib 18 and the lid 16 to prevent leakage of 
photographic fixing solution from the container 12. 
There are two holes or ports through the lid 16, an inlet hole 22 in the 
center of the lid and an outlet hole 24 toward the outer edge of the lid. 
The inlet hole and the outlet hole are provided with a gasketed plastic 
fitting 26 and 27, respectively, with a gasket 29 on the underside of the 
lid to prevent leakage of photographic fixing solution around the outside 
of the fittings 26 and 27. Both gasketed fittings 26 and 27 are connected 
outside of the container 12 by means of plastic tubing 28 to a rotatable 
T-connector 30. The T-connector 30 for the inlet port 22 is connected to a 
length of tubing 32 through which influent liquid passes. Likewise, the 
T-connector 30 for the outlet port 24 is provided with tubing 33 for 
passage of the effluent from the container 12. The T-connectors are 
rotatable so that influent and effluent liquid can pass to and from the 
container in any desired direction. 
The two T-connectors 30 are joined by an inverted V-shaped assembly 36 
comprising two pieces of tubing 38, each piece of tubing being connected 
at one end to one of the T connectors 30 and at the other end to a 
90.degree. elbow fitting 40, which is at a higher elevation than the 
elevation of the inlet 22 and the outlet 24. An anti-siphon hole 41 is 
provided at the apex of the elbow fitting 40. The assembly 36 serves as an 
emergency gravity bypass for rapid discharge of influent solution during 
flow surges or in case there is a plug in the container 12. 
The inlet plastic fitting 26 has connected thereto on the inside of the 
container 12 a vertically oriented tube 42 which extends almost to the 
bottom of the container 12. In the container is a metallic exchange medium 
44. Both the bottom of the inlet tube 42 and the outlet fitting 27 are 
provided with a fine mesh screen or strainer 46 made of sufficiently small 
mesh to prevent the metallic exchange medium 44 from plugging the inlet 
tube 42 or passing out of the container through the outlet 24. 
The container can be any size convenient to handle, and generally from 
about 1 to 10 gallons, and more preferably from about 3 to about 7 
gallons. Alternatively, rather than using a container 12 that is suitable 
for only one use, a reusable column that can be repeatedly be refilled 
with metallic exchange medium 44 can be used. For example, a 5-1/2 feet 
tall column 13 inches in diameter is satisfactory. Such a tall and narrow 
column is advantageous in that influent liquid contacts a larger amount of 
exchange medium 44 than if the container were short and wide. 
Rather than using a container 12 comprising a body portion 14 and a lid 16, 
the container can be provided as a single integral monocoque structure. 
This has the advantage that leaks cannot occur between the body portion 14 
and the lid 16 as can occur with prior art units. By using the metallic 
exchange medium 44 rather than steel wool or window screen, it is possible 
to fill a sealed container through the inlet 22 and/or outlet 24. 
The choice of the materials for the apparatus 10 is important, in that all 
of the components of the apparatus, with the exception of the metallic 
exchange medium 44, need to be substantially non-reactive with the 
influent liquid. For photographic fixing solutions, preferably all 
materials used are polymeric. For example, the container 12, including the 
body portion 14 and the lid 16, is formed of a plastic material such as 
polyethylene, polypropylene, polystyrene, nylon, or polyvinylchloride. 
This can be done by forming the container 12 from such a material, or 
forming the container from a reactive material which is a lined with a 
non-reactive plastic material. Likewise, the fittings 26 and 27, tubing 
32, 34, 38, and 42, as well as the T-connectors 30 and elbow 40 are formed 
of a substantially non-reactive material such as polyvinylchloride. The 
gasket 20 between the lid 16 and body portion 14 of the container and the 
gasket of the gasketed plastic fittings 26 can be formed of neoprene. The 
screens 46 can be formed of nylon or fiberglass, among other materials. 
From FIGS. 1 and 2, it can be seen that liquid is introduced into the 
container 12 at the bottom of the container in a void space 48. The liquid 
then passes up through the metallic exchange medium 44 toward the top of 
the container, to pass out from the container through the outlet 24. An 
advantage of this configuration is that the influent liquid is distributed 
evenly across the cross-sectional area of the container 12 at the bottom, 
thereby insuring even flow upwardly through the metallic exchange medium 
44 with minimum channeling occurring. 
The exchange medium 44 comprises a plurality of bouyant elements 50, one of 
which is shown in FIG. 3. The bouyant elements 50 can be of any shape, 
including oval and spherical. Each element comprises a substrate 52 coated 
with metal 54. The substrate 52 is chosen so that the elements 50 are 
buoyant in the container when the container is filled with the solution 
from which metal is being removed. As used herein, the term "buoyant" 
means that an element 50 floats in a solution that the element is being 
used for treating. 
The substrate is substantially non-reactive with the liquid being treated. 
For a photographic fixing solution, a non-reactive polymeric material such 
as polyethylene, nylon, polypropylene, polystyrene, or polyvinylchloride 
can be used. Preferably the beads are hollow, and filled with air 56 to 
insure buoyancy. The preferred substrate is expanded polystyrene beads. 
Such beads are available from Vertex of Vernon, Calif., under Catalog 
Designation EPS. 
The substrate preferably has a diameter of from about 1/8 inch to about 1/2 
inch, and, more preferably, of from about 1/4 inch to about 3/8 inch. If 
the buoyant elements are too small, plugging of the metallic exchange 
medium can occur. If the buoyant elements are significantly larger than 
about 3/8 inch to about 1/2 inch, then the rate of recovery of metal is 
decreased due to less available surface area of the metallic exchange 
medium. 
The metal 54 can be held on the beads 50 by means of an adhesive 55 such as 
a synthetic organic elastomer type adhesive. A suitable adhesive is 
available from Industrial Polychem, Inc. of Gardena, Calif., under Catalog 
No. 5031. The adhesive chosen needs to be one that when cured does not 
attack the substrate, and is also stable and substantially non-reactive 
when exposed to the liquid being treated. Since photographic fixer 
solutions can be acid or alkaline, the adhesive should be stable in both 
mild acid and mild alkaline solutions. 
For recovery of silver, the metal 54 can be a metal that is less noble than 
silver such as aluminum, zinc, copper or iron. A disadvantage of zinc, 
aluminum and copper is that each is a water pollutant in its own right. 
Therefore, for recovery of silver, preferably iron is used. Preferably, 
the iron used is substantially free of zinc, aluminum, and copper to avoid 
introducing pollutants into the solution being treated. Moreover, 
preferably the iron is substantially free of carbon, i.e., steel is not 
used. This is because the presence of carbon can interfere with the 
recovery of silver. The recovery of silver depends upon the acidic 
photographic solution etching the metallic exchange medium, resulting in 
sloughing off of precipitated silver. If the iron contains carbon, less 
etching can occur, and the silver does not slough off, thereby preventing 
iron underlying deposited silver from entering into the electrochemical 
exchange reaction. 
Preferably, sponge iron is used. Preferably, the sponge iron is from about 
+150 mesh (does not pass through a 150 mesh sieve) to about -10 mesh 
(passes through a 10 mesh sieve). This is because with particles 
substantially smaller than about 150 mesh, reduction in recovery of silver 
is noted. It is believed that this results from generation to hydrogen due 
to the acidic photographic fixer solution quickly reacting with the iron. 
Such quick reaction occurs because of the high surface area of the small 
iron particles. The generation of hydrogen can lower the pH of the 
photographic fixer solution, which results in increased usage of iron and 
undesirable break-up of the ammonium thiosulfate present in the 
photographic fixing solution. 
When the iron particles are larger than about 10 mesh, there is inadequate 
surface area for quick and efficient recovery of silver. 
In view of these considerations, more preferably the iron is from about +80 
mesh to about -20 mesh. 
The buoyant elements 50 can be formed by putting 1 gallon volume of 
expanded polystyrene particles in a cement mixer with 8 fluid ounces of 
adhesive solution. The adhesive solution is formed by diluting 0.4 parts 
by volume adhesive 5031 from Industrial Polychem, Inc. from Gardena, 
Calif., with 0.06 parts by volume methyl ethyl ketone and 0.54 parts by 
volume hexane. The adhesive coating 53 and expanded polystyrene particles 
52 are mixed for about 1-1/2 to 2 minutes. Then, the sponge iron 54 is 
added, in an amount of 2 pounds per 1 gallon volume expanded polystyrene 
beads. The cement mixer continues to run until the expanded polystyrene 
particles are substantially completely coated with iron, and then the 
adhesive is permitted to cure. 
Preferably the container 12 is not completely filled with the metallic 
exchange medium 44 so that there is a void space 48 at the bottom for even 
distribution of incoming liquid, and to provide a space for recovered 
silver to collect. An adequate void space can be provided by using an 
amount of metallic exchange medium having a volume from about 70% to about 
90% volume of the container 12. 
To use the apparatus 10, influent liquid is introduced through tubing 32, 
into the inlet tube 42 for discharge adjacent the bottom of the container 
12. The liquid percolates upwardly through the metallic exchange medium 
44, where a first metal in the influent liquid undergoes an 
electrochemical exchange reaction with a more noble second metal 
constituting the reaction exchange medium 44. Spent liquid of a lower 
content of the first metal than the influent liquid is discharged through 
the outlet 24. 
The apparatus 10 is suitable for photographic fixer solutions. As used 
herein, the term "photographic fixer solution" refers to both fixer 
solutions and bleach-fixer solutions. A fixer solution conventionally is a 
solution comprising water, sodium or ammonium thiosulfate, sodium or 
potassium sulfite, sodium or potassium hydroxide, and acetic or sulfuric 
acid. It can also contain aluminum sulfate. A bleach-fixer solution 
contains the same constituents, but also includes ferric ammonium ethylene 
diamine tetracetic acid. 
The optimum pH for recovering silver from photographic fixing solution is 
from about 4.5 to about 5.5. The pH of an available photographic fixing 
solution can be as low as 4 or, in the case of a bleach-fixer solution, as 
high as 8. It is possible to modify the pH of the influent liquid to a pH 
of from about 4.5 to about 5.5 for effective recovery of silver. 
More than one container can be used. If desired, containers 10 can be 
connected in series and/or in parallel. 
In use, an apparatus 10 comprising a five-gallon container 12 can recover 
about 120 ounces of silver from a photographic fixer solution at an 
influent rate of from about 300 to about 500 milliliters per minute, with 
about 95% to about 99% recovery of the incoming silver. This is a 
significant improvement over comparable prior art devices using steel wool 
or window screen which can treat only a maximum of about 300 milliliters 
per minute. 
As the electrochemical exchange reaction occurs, the iron coating 54 is 
depleted and silver precipitates on the exchange medium 44. As the 
reaction further precedes, the silver sloughs off into the void 48 at the 
bottom of the container, forming a silver sludge. 
The silver can be recovered by removing the contents of the container from 
the container, then drying the contents and pyrolyzing the substrate. This 
can be effected by drying at an elevated temperature sufficiently high to 
pyrolyze the plastic substrate 52 and the adhesive coating 55. Materials 
such as polystyrene can be pyrolyzed in about 30 minutes at a temperature 
of about 500.degree. F. After drying and pyrolysis, which can be effected 
in a single heating step, all that remains is a fine powder principally 
constituting particulate silver and small iron particles that originally 
constituted the metallic coating 54 on the substrate 52. This powder can 
be refined to recover the silver according to conventional techniques, 
without the need for any further comminution. 
The apparatus 10 provides substantial advantages compared to prior art 
devices for recovering silver from photographic fixing solutions. 
Principally, more photographic fixing solution can be treated with the 
apparatus 10 than with comparable prior art devices. It is believed that 
this results from reduction in channeling, avoidance of plugging, and 
better contact between the fixing solution and the iron than is possible 
with prior art devices. A further advantage is that a sealed container can 
be used, thereby minimizing chances that the container will leak. Further, 
the recovered product can be refined without the need for an expensive and 
difficult comminution step. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions thereof, other versions are 
possible. For example, rather than supplying an integral container 
containing the reaction medium 44, the reaction medium 44 can be supplied, 
separately for use in a reusable container, such as the tall and narrow 
column previously described. Therefore, the spirit and scope of the 
appended claims should not necessarily be limited to the description of 
the preferred versions contained herein.