FOCUS: Recovery/Recycling
Acid Recycling at a Captive Finishing Shop
Pitney Bowes uses diffusion dialysis to recycle its acids and save $80,000 annually...By DONALD OFFINGER
Pitney Bowes, Stamford, Connecticut and DANIEL E. BAILEY Pure Cycle Environmental Technologies, Inc., Palmer, Massachusetts
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 1995. 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, metal-bearing solution.
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.
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Phase III 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. |
- 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.
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