Acid Recycling at a Captive Finishing Shop
Pitney Bowes uses diffusion
dialysis to recycle its acids and
save $80,000 annually...
By DONALD OFFINGER
Stamford, Connecticut and
DANIEL E. BAILEY
Pure Cycle Environmental
Pitney Bowes, Stamford, Connecticut, is a multi-billion
dollar marketer and manufacturer of mailing and office systems, service
and supplies. Its goal of zero discharge initiated a corporate-wide waste
minimization program in 1989. This included eliminating all hazardous
wastes resulting from manufacturing and support operations by the end of
Part of the zero-discharge program includes several diffusion dialysis
acid recycling systems at its metal surface finishing facility. The first
installation was made in September 1991.
Diffusion dialysis is ideally suited for recycling a broad spectrum of
industrial acids and acid mixtures. Successful applications range from
simply hydrochloric or nitric rack stripping solutions to mixtures of
nitric and hydrofluoric acids, ferric chloride etchants, fluoboric acid
and sulfuric acid anodizing solutions.
Prior to implementing the acid recycling systems, a multidisciplinary team
formulated an implementation strategy at the Mailing Systems Division Main
Plant in Stamford. The plan was based on a comprehensive analysis of all
manufacturing processes. Each process was scored and then assigned a rank
to indicate its priority.
Spent solutions of hydrochloric and nitric acids contributed significantly
to the hazardous waste stream. These solutions were generated from
stripping and pickling operations. Prior to acid recovery, spent nitric
acid solutions, rich in nickel metal, were drummed and disposed of off
site as hazardous waste.
Spent hydrochloric acid solutions from rack stripping as well as pickling,
rich in chromium, iron and nickel, were collected in bulk and used in
waste treatment for pH adjustment. Excess bulk hydrochloric acid was
hauled off site as hazardous waste. Diffusion dialysis is a membrane
separation process. An anion exchange membrane acts as a diaphragm. Acid
molecules diffuse into water containing lower concentrations of that acid.
By causing the acid to flow across the membrane in the opposite direction
of the water flow, a high percentage of the acid diffuses into the water.
This results in a metaldepleted acid solution and an aciddepleted,
The left diagram relates a typical, automatically operated acid-recycle
configuration. Once the acid and the water are delivered to the recycling
system, the streams then flow by gravity down into the membrane stack
Once within the membrane stack, the acid diffuses into the water. The
majority of the metal contaminants are left behind. Two streams are
produced from the acid-recycling system: a purified, metal-depleted acid,
and a metal-bearing, acidepleted stream. The recovered acid is recycled
back into the operating bath. The metal-rich, aciddepleted stream is
directed to waste treatment or a recovery/volume reduction system.
Pilot studies were performed separately on nitric and hydrochloric acid
solutions to prove the system's effectiveness in removing metallic
contaminants and producing workable concentrations of acids. The studies
showed excellent results in removing metallic contaminants as well as
generating an acid permeate with sufficient concentration for reuse. The
acid-depleted fraction following dialysis produced a solution rich in
metaland weak in acid concentration.
Phase I. A phased approach was
used to implement acid recovery at the main plant. Phase I outlined
steps to install dedicated acid-recycling units for each of the two rack
strip acids. A 25-gpd diffusion dialysis system was installed on the
250-gal hydrochloric acid tank used for chromium stripping. A 50-gpd
system was installed on the 250-gal nitric acid tank that is used for
nickel stripping. The sizing of these systems was based on the volume of
spent acid previously produced, production rate and the efficiency of the
diffusion dialysis process as determined in the pilot study. The plan
required that, at a minimum. the volume of spent acid previously discarded
must be recycled once through the diffusion dialysis unit over the
same period of time that it took to generate it. The 25-gpd hydrochloric
acid-recycling system was installed in September 1991. The recycling
system was installed directly on the acid process tank. Prior to acid
recycling, the tank was dumped every six weeks. To date, the recycling
unit has operated continuously and the acid has not been dumped. Additions
of virgin acid are made to replenish depleted volume due to dragout and
the minor amounts lost in dialysis. Tables I and II illustrate the
performance of the system. The system recovered about 92 pct of the
hydrochloric acid solution and removed about 80 pct of the dissolved
chromium contaminant. The 50-gpd nitric-acid-recycling system was
installed in October 1991.This recycling system was also installed
directly on the acid process tank. Prior to acid recycling, this tank was
dumped every three to four weeks.
Phase II outlined steps for
procuring and installing a centralized 100-gpd acid-recycling system for
hydrochloric acid dumps from pickling operations in the plating
department. This acid-recycling system came online in October 1992. It was
located in the waste treatment area, directly below the plating
department. Three, 1,000-gal tanks were located adjacent to the
acid-recycling unit. One tank receives acid dumps from any one of eight
hydrochloric acid tanks in the plating room. This tank is plumbed into the
acid-recycling unit and supplies spent acid for dialysis. A second tank
collects the recovered acid from the recycling unit. This tank is plumbed
back into the eight hydrochloric acid process tanks in the plating room.
Once the operator has finished dumping the spent hydrochloric acid from a
particular tank, a pump is activated that supplies recovered hydrochloric
acid back into this tank. Process control personnel sample the recycled
acid to determine its acid strength and then recommend the appropriate
addition of virgin acid solution, if required. A third tank receives
the metalrich waste solution that has been depleted of acid. Nonecycled
sulfuric acid salt solutions are also collected in this tank. This
solution is used for pH adjustment in waste treatment.
outlined steps for procuring and installing a edicated 50-gpd
acid-recycling system for nitric acid dumps from stripping plate-out from
the electroless nickel line. This acid-recycling system came on-line in
April 1993. It is in the waste treatment area below the electroless nickel
plating operations. At Pitney Bowes, proposals for capital projects, such
as the three phased implementation of acid-recycling systems, require a
strict financial review in addition to an environmental-impact review and
a review of the impact on manufacturing. In terms of impact on
manufacturing and operations, the project was required to streamline production
and assure quality. In terms of an environmental review, the project was
required to address the goals of the zero-discharge program as well as to
better ensure worker safety. The financial review included the
determination of the project's merit by calculating its internal rate of
return and net present value. This was accomplished using a computer
model. The following are benefits derived from the implementation of acid
recycling at Pitney Bowes.
Diffusion dialysis for acid recycling has reduced acid purchases by up to
95 pct. The process eliminates or lessens neutralization or hazardous
waste hauling costs and the related liability. Toxic chemical use is
reduced, and the required reporting and handling of hazardous material and
associated labor is reduced. Consistent bath strength yields greater
product uniformity and better quality. The process helped Pitney Bowes
improve quality as well as its economic performance.
- Savings from eliminated disposal costs and reduced inventory purchases are estimated to be $80,000 annually.
- Elimination of production downtime associated with the dumping and recharging of acid baths.
- Elimination of direct operator contact with dangerous chemicals.
- Automatic operation, 24 hrs per day, seven days a week, with minimal operation costs.
- Improved process control, improved quality and waste minimization.
To request an additional copy of this article, write on company letterhead to
"Reprints," c/o PRODUCTS FINlSHING,
6600 Clough Pike, Cincinnati,
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