|
Foreword
Process of Metallic Replacement
Method
Test Procedure
Sample Analysis
Results and Discussion
Mini-Column
Tests
True-Size
Column Tests - Low Flow Rate (110 to 150 ml/min)
Comparing
a Low Flow Rate Metafix MRC with a Competitor's MRC
True-Size
Column Tests - High to Very High Flow Rates (1 to 12 L/min)
Commercial Use of Metafix
MRCs
Summary
Guidelines for Selecting
Metafix MRCs
References
|
Silver
recovery via metallic replacement from used photo processing solutions
is carried out by using metallic replacement cartridges (MRCs).
Several papers review and discuss this method of silver recovery
1, 2 ,3,
4, along with two detailed studies
carried out on black/white fixer by Dannenberg et al. 5
and by Cooley 6.
Metallic replacement occurs when
a solution containing dissolved ions of an active metal such as
silver, contact a more active solid metal such as iron. The more
active metal goes into solution as an ion and is replaced by the
less active metal as a solid. This reaction is shown in the following
equation:
| REACTION
1
Ag(I) + Fe=Ag + Fe(II) |
Iron becomes ionic and
goes into solution while the silver ions, originally in the
solution, become silver metal. Other metals, such as aluminum,
copper, magnesium and zinc, will also work because they are
more active than silver. Iron in various forms is the most widely-used
metal because it is less expensive and has minimal environmental
impact.
In photo processing applications,
the spent silver-rich solutions are metered through the iron-filled
MRC. The silver metal is retained in the cartridge and the ionic
iron is discharged along with the desilvered solution. In this
paper, the terms "column" and "cartridge"
are used interchangeably. When referring to Metafix MRCs manufactured
with MetaWool, the term Metafix MRC is used.
MRCs are known for performing inconsistently and inefficiently.
A problem is the inefficient use of iron. Based on chemical principles,
one gram of iron could theoretically recover 3.86 grams of silver.
In reality, most commercial cartridges have a recovery efficiency
of only 0.1 to 0.5 grams of silver per gram of iron in the cartridge.
When MRCs are used to recover silver
from acidic silver-rich solutions (e.g., fixer), part of the iron
is consumed by the acid; as the acidity of the solution increases,
the efficiency of the iron decreases. With bleach-fix, the ferric-EDTA
consumes iron indirectly as in the following equation:
| REACTION
2
Ag + Fe(III)=Ag(I) + Fe(II)-EDTA |
The silver recovered
in the MRC reacts with ferric-EDTA in the bleach-fix and is dissolved
back into solution. Ferric-EDTA becomes ferrous-EDTA. The silver
is recovered again downstream as in Reaction 1. The net effect
of Reactions 1 and 2 is that metallic iron is consumed by ferric-EDTA,
as shown in the following equation:
| NET EFFECT
OF REACTIONS 1 AND 2
Fe + Fe(III)-EDTA=Fe(II)
+ Fe(II)-EDTA |
A major cause
of the inefficient use of iron in MRCs, when they are used with
silver-rich photo processing solutions (including color and
black/white), is channeling -- a phenomenon where the silver-rich
solution creates distinct paths through which it flows, leaving
much of the iron unused and reducing the amount of silver recovered.
Silver-rich solutions include fixer, bleach-fix, stabilizers
from washless processes and low-flow wash water.
Channeling in MRCs occurs
for two reasons:
-
Iron fibers
(commonly called steel wool) tend to cluster together. Most
manufacturing processes are unable to eliminate gaps between
iron clusters or iron layers.
-
Silver ions and acids in the silver-rich solutions react and
consume iron, thus enlarging the paths through which the silver-rich
solution flows. Silver-rich solutions will preferably flow
along these paths, offering the silver ions less chance to
react with iron, leading to higher silver concentrations in
the cartridge effluent.
Due to channeling,
much of the initial iron in the MRC remains unused when “breakthrough”
occurs. In this study, breakthrough was defined as the point
at which the effluent from the MRC contained more than 5 ppm
of silver. This number was used because it is a typical compliance
limit in municipal sewer ordinances.
The purpose of this
study was to find a medium for MRCs that used the iron more
efficiently, minimized channeling and improved cartridge performance,
when used with silver-rich solutions from photofinishing processes.
PREVIOUS
PAGE | NEXT PAGE
|
