Silver Recovery – Profitability and Compliance

David Fenton
CPAC Equipment Division
Leicester, New York

Abstract

Electrolytic silver recovery continues to be the most popular and cost effective form of silver recovery for high volume photofinishers. It is not only an essential part of the environmental compliance process but is also required for many chemical regeneration and reduction programs.

This paper will review several film and color paper processes, and the silver recovery equipment required for each. Film processes include: closed-loop fix recirculation and batch desilvering. Color paper processes include: regenerated bleach-fix, regenerated bleach-fix followed by a silver wash solution, and non-regenerated (NR) bleach-fix.

Silver recovery is automated through the use of electronic controls, which are designed specifically for the processing configuration and the chemistry. The initial section of this paper discusses the different chemical processes and required controls. The latter portion addresses the costs of silver recovery and the impact of equipment design.

Bleach-Fix

Bleach-Fix is generally desilvered in a "batch" process. Overflow chemistry is stored in the collection tank. When the tank reaches a predetermined volume, the entire contents are transferred into the desilvering tank. The chemistry is then desilvered for a programmed time and amperage. At the end of the desilvering cycle, chemistry is discharged to a post treatment tank. Silver recovery equipment should control the entire batch process, without requiring operator intervention to start or stop the process.

After electrolytic desilvering, bleach-fix can be sent to a "tailing" system where the last traces of silver are removed, or the chemistry can be combined with special regeneration chemicals and reused as bleach-fix replenisher. The advantages of regeneration include: reduced chemical costs, less bleach-fix to be disposed of, and lower silver levels in the processing tanks when compared to NR bleach-fix. Figure 1 shows a typical batch desilvering system. Components and connections required only for regeneration are shown as dotted lines.

Figure 1. Bleach-Fix Desilvering

Electronic Controls for Desilvering Bleach-Fix:

1. Automatic batch control (with manual override), to automatically transfer chemistry between the collection tank, desilvering tank, and post treatment tank.
2. Hold discharge option to stop the batch process after desilvering is complete. This allows the operator to sample the desilvered chemistry and determine if it is ready for discharge or needs further desilvering.
3. Programmable time and amperage settings (with ability to lock in settings and prevent unauthorized changes).
4. Provisions to add a caustic doser metering pump to increase the pH of the chemistry.
5. Automatic cell current reduction during the desilvering cycle.

Silver Wash

For high speed paper processing, silver wash technology can be used in conjunction with regenerated bleach-fix to achieve low silver concentrations in the final wash. Preventing silver from reaching the final wash is a cost effective alternative to desilvering wash water.

More than 10% of silver in the paper process is lost through the final wash. To recover it, a silver wash tank is inserted between the final bleach-fix tank and the wash tanks. Special silver wash chemistry acts as a washing aid but also concentrates the silver, allowing it to be removed by electrolysis. An effective silver wash system will recover 80% of the silver that escapes from the bleach-fix tanks. This means 98% of the total silver is recovered and only 2% is lost through the wash. Figure 2 shows a typical silver wash configuration.

Figure 2. Closed-loop Silver Wash Desilvering

To operate efficiently, closed-loop silver recovery equipment must monitor the silver concentration in the loop or calculate the amount of silver being added to it. Silver-specific ion electrode sensors are used to continuously measure silver concentration. Unfortunately, these sensors do not work well in solutions containing high levels of iron, such as bleach-fix and silver wash that follow a bleach-fix tank.

For a silver wash loop following bleach-fix, the most accurate method to control desilvering is through developer usage. The desilvering unit calculates the volume of developer replenisher added to the loop, and then calculates the amount of silver added. Plating current is automatically adjusted to remove the silver released into the silver wash.

Electronic Controls for Desilvering Silver Wash Chemistry:

1. Automatic desilvering control based on developer replenishment (with manual override controls).
2. Programmable processing parameters and plating parameters.
3. Ability to monitor multiple processors.

Fix

Fix can be desilvered in a batch system (identical to bleach-fix) or a closed-loop system. Fix does not require a pH adjustment prior to desilvering.

A closed-loop system continuously removes silver from the fix tank as film is processed. This reduces the amount of silver carried into the final wash and allows fix replenishment rates to be reduced by over fifty percent. Two methods of desilvering control are available for closed-loop fix systems: 1) continuous silver sensing using silver- specific ion electrode sensors; and 2) monitoring the activity of one or more film processors.

Continuous silver sensing is the most accurate method to monitor and maintain the silver level in the loop. It does require added capital equipment investment and periodic calibration of the sensor. Controlling the desilvering current based on the silver level provides optimal results.

Fix desilvering can also be controlled by the drive signals from one or more film processors. Using the film size and processor speed, desilvering current is calculated for each processor. This is a very simple and inexpensive method, and if monitored properly, can be very effective. Figure 3 shows a typical closed-loop fix desilvering system.

Figure 3. Closed-Loop Fix Desilvering

Electronic Controls for Desilvering Fix:

1. Ability to accept information from a continuous silver sensing device. Plating current should automatically adjust as the silver level changes. The rate of adjustment must be programmable.
   

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Ability to interface with processor drive signals. This control varies the desilvering current based on the processors that are running.

2. Programmable plating current. Programmable start and stop values based on silver levels or number of active processors.
3. Programmable maximum desilvering amperage.
4. Manual override controls.

Electrical controls for all electrolytic desilvering equipment should have the following features:

1. Automatic restarting following an electrical power outage. The electronic controller should retain all programmed parameters and resume the desilvering cycle where it was interrupted.
2. Electronics that are located away from corrosive chemical fumes.
3. Digitally programmable operating parameters that are entered through a membrane keypad. Keypads are well suited for harsh environments, easy to clean, and not susceptible to accidental activation or breakage.
4. Ability to lock in programmed settings to prevent unauthorized changes.
5. Safety provisions that allow the electrical system to be "Locked Out and Tagged Out".
6. Modular electrical design – for easy troubleshooting.
7. Microcomputer-based electronics, providing the flexibility to update the software in the future.

Electrolytic Cell Design

The electrolytic cell assembly consists of anodes, a cathode, a vessel to house them, and a means of circulating chemistry through the cell. This assembly is the core of every electrolytic silver recovery machine. Its design determines the rate at which silver can be plated and also the amount of labor required to harvest the silver from the cathode.

Two common designs:

Rotating cathode – The cathode rotates and the anodes remain stationary.

Advantages:

1. Effective plating with all solutions due to the very high agitation on the surface of the cathode. This results in high recovery rates.

Disadvantages:

1. Requires high maintenance. The design includes a motor, several bearings and brushes. Cathodes, loaded with silver, may weigh over 60 pounds and rotate at speeds over 200 RPM.
2. Cathodes are round, very rigid, and can be difficult to desilver.

Stationary Cathode –The cathode and anodes do not rotate. Pumps are used to create agitation on the surface of the cathode.

Advantages:

1. Requires very little maintenance. The design is simple and incorporates few moving parts. Very clean and quiet.
2. Some stationary cathode designs are very easy to desilver. Cathodes do not have to be rigid or round. Thin, "flexible" cathode designs are especially easy to desilver. They incorporate a thin steel surface that can be flexed to break the bond between the silver and the steel. One such design shown in figure 4.

Figure 4. Flexible cathode

Disadvantages:

1. Agitation on the surface of the cathode may be poor -this varies greatly from design to design. Low agitation results in low recovery rate (Troy Ounces per hour). This can make the initial capital investment prohibitive for some types of solutions.

The Cost of Silver Recovery

The cost of electrolytic silver recovery can be separated into three components: 1) Original equipment cost 2) Labor to operate the equipment 3) Ongoing maintenance and repairs.

Original equipment cost – can be thought of as the capital equipment needed to achieve a given recovery rate. To calculate this, divide the manufacturer’s price by the stated recovery rate. This is the most common method of measuring silver recovery value, but does not reflect total cost since it does not incorporate the ongoing labor and maintenance.

Labor cost to operate equipment – depends on the degree of automation designed into the equipment and the labor required to desilver the cathode. When selecting electrolytic silver recovery equipment, look for state-of-the art automation, but always have manual override controls to ensure that the system allows for complete operator control.

Labor required to desilver the cathode is the most overlooked cost of silver recovery. This varies greatly from model to model and is incurred weekly for the entire operating life of the unit.

To illustrate how desilvering times vary, an informal survey of large photofinishers revealed desilvering times ranging from 10 to 60 minutes. These times relate to bleach-fix desilvering units plating at approximately 6.0 Troy Ounces per hour. Average cathode yield was approximately 30 pounds. The following table calculates minimum and maximum desilvering labor costs over a five year period (costs are calculated using a labor rate of $25 / hr, desilvering once each week).

Desilvering Cost Table

Maintenance and repair cost – this varies greatly depending on the equipment design. It is generally proportional to the number of moving parts in the cell assembly. The informal survey of photofinishers found that monthly maintenance times ranged from 10 minutes to two hours per unit. The maximum and minimum costs are summarized below (costs are calculated using a labor rate of $25 / hr, cost of replacement parts is not included).

Maintenance Cost Table

The cost of maintenance is impacted by the quality of the technical support provided by the manufacturer. Telephone support should be promptly available from technicians capable of instructing maintenance personnel in troubleshooting to the component level. The technicians must also have a strong knowledge of photographic chemicals. The electrolytic process is very dependent on chemical parameters and it is impossible to optimize silver recovery without understanding the chemistry.

Conclusion

Electrolytic silver recovery equipment can improve a lab’s profitability, maintain environmental compliance, and create opportunities to reduce chemical costs through regeneration and replenishment reduction programs. Silver recovery equipment is designed specifically for the solutions to be desilvered and the processing configuration. Selecting chemical processes is an important decision that will significantly impact the operating costs of any lab. Chemical suppliers and silver recovery manufacturers can assist in analyzing the options.

After the processes are defined, silver recovery equipment should be selected based on recovery performance, reliability, initial cost, labor required for normal operation, and costs of maintenance and repairs. Electrolytic silver recovery equipment is a long-term investment. A high quality unit will provide service for ten to fifteen years...or more. This makes operating and maintenance costs very significant factors in the purchasing decision. If the equipment is to be used for chemical regeneration or closed-loop recirculation, it will play an important role in the production process, so reliability is especially important.

There are several electrolytic equipment manufacturers offering many equipment designs. Some features, such as recovery rate, are easily compared. Other features, such as maintenance costs and time required to desilver the cathode, are not as obvious. Carefully review the equipment designs and seek advice from labs that have similar equipment. For certain applications, there are tradeoffs between price, performance and reliability. For other applications, a single unit will have advantages in all areas. It is well worth the effort to research the equipment options available and make an informed decision.