ANODIZING ACID PURIFICATION using RESIN SORPTION TECHNOLOGY at PIONEER METAL FINISHING

Transcription

1 ANODIZING ACID PURIFICATION using RESIN SORPTION TECHNOLOGY at PIONEER METAL FINISHING Paul Pajunen, P. Eng., Group Leader Metal Finishing, Eco-Tec Inc., Pickering, Ontario Jim Harrison, Environmental Engineer, Pioneer Metal Finishing, Green Bay, Wisconsin AESF Compliance Week '97, Orlando, Florida, January, 1997 Introduction The Pioneer Metal Finishing Company (PMF) is the nations largest job shop anodizer having three facilities throughout the country in Minnesota, Wisconsin, and, most recently, a new facility in Michigan. The Green Bay, Wisconsin facility started out in 1974 and currently employs over 275 people. Customers include the papermaking, automotive, aerospace/aviation, defense, computer, electronics, and housewares industries. Anodizing is the process by which an oxide film is formed on an aluminum part to give a hard, corrosion resistant and abrasion resistant coating with excellent wear properties. PMF predominately undertakes Type II conventional anodizing and Type III hardcoat anodizing. Currently, nine anodizing tanks are utilized of which six are dedicated to hardcoat anodizing. On a daily basis, close to 100,000 individual pieces of machined items are processed through these anodizing baths. They range from pizza pans to carpenter levels to automatic braking systems. A major contributor that leads to quality problems in an anodizing bath is the buildup of dissolved aluminum contamination. Implementing bath purification alleviates this buildup and promotes the generation of a consistent, predictable oxide coating. The anodizing acid purification system chosen by PMF utilizes a well established method known as resin sorption made possible via the technique of reciprocating flow ion exchange. This paper will review the design considerations of this method as well as outline the resulting benefits of improved quality control, reduced chemical purchases, and reduced sulfate discharge. Typical Workpiece at PMF

2 Anodizing Purification Benefits A large quantity of free sulfuric acid remains when the anodizing solution is discarded once the aluminum concentration reaches unacceptable levels. Being able to recycle this free sulfuric acid provides a couple of key benefits to PMF: i) Bath Control Benefit Operating an anodizing bath in a dump/decant manner presents a number of potential problems. These are due to the fact that there exists a delicate balance within an anodizing bath - namely, the relationships between the electrical resistance (caused by the formed oxide coating and the anodizing solution conductivity), the voltage being applied, and the desired constant current condition. The electrical resistance increases relative to the thickness of the oxide coating and to the increasing aluminum concentration in the anodizing solution. To compensate for this increased resistance, the rectifier voltage must be increased in order for the current to remain constant. Adding in other variables such as bath temperature, degree of solution agitation, and sulfuric acid concentration can result in upsets and lead to a decline in product quality. Maintaining a consistent, low aluminum concentration removes or minimizes a variable that can affect this balance between resistance, voltage, and current. With so many different workpieces going through the anodizing baths, in conjunction with the tight tolerances that must be maintained among these various products, ensuring a predictable anodizing operation is crucial. The following problems can be alleviated by ensuring bath consistency: Variable process times - a fresh and spent bath scenario means adjustments in the length of time a work piece remains in the bath for processing and contributes to productivity variation. Periodic line shutdown - this is required in order to dump the bath and make up with fresh solution; productivity can then become an issue. Material rework/quality problems - poor bath conditions can lead to rework and surface finish problems such as burning or pitting; a more uniform oxide coating leads to better results when, for example, downstream coloring processes are employed. Labor costs for bath maintenance - operator safety is also an issue here. ii) Reduced Waste Benefit The level of acidic waste discharge prior to bath purification exceeded the level of alkaline waste discharge from this facility. The waste treatment practice at that time was to blend these two wastes together for discharge and then undertake further neutralization with virgin caustic soda to produce a predominately sodium sulfate waste. Recycling the sulfuric acid in order to minimize sulfate discharge is attractive for the following reasons: Excess sulfates going to the local POTW is not ecologically desirable due to the damage that can arise to concrete sewer lines. Sulfates can attack the concrete causing swelling and loss of strength and hastening metal corrosion. Sodium sulfate aggressively attacks concrete pipe at acidic, neutral, and even alkaline ph s. Chemical costs are able to be reduced which includes the sulfuric acid for anodizing and the caustic soda for neutralization. Being able to realize the above benefits resulted in PMF undertaking an evaluation of anodizing acid purification.

3 Acid Purification Historically, a process called acid sorption was introduced in It employs ion exchange resins which sorb strong acids from solution but exclude metallic salts of those acids. The process is reversible in that the acid can be readily desorbed from the resin with water. It is thus possible, by alternately passing contaminated acid and water through the bed, to achieve a chromatographic separation of the free acid from the metal salt. Water Sulfuric acid Aluminum sulfate Sorption Resin Bead The process, despite its appeal, was not successfully commercialized for some time. This was probably due to the limitations of conventional ion exchange technology which tends to cause excessive dilution of the purified acid upon water elution and the large column size which results from the small bed volume throughputs inherent in the process. In 1975, a novel ion exchange technique called reciprocating flow ion exchange was applied to the acid sorption process. This method of ion exchange is characterized by fine particle size resins, countercurrent elution, short column heights, fixed, over-packed resin beds, short cycles, and a number of other features. The process is ideal for treating small volumes of concentrated solutions with a minimum of dilution or fluid intermixing in the resin column. Reciprocating flow ion exchange has seen wide application in the metal finishing industry for the recovery of a wide variety of metals. By employing this method, the performance of the acid sorption process was significantly improved. The first successful commercial application of the acid sorption process using reciprocating flow ion exchange technology was reported in Since that time, over 400 of these systems have been installed around the world in a variety of applications employing sulfuric, hydrochloric, and nitric acids. Unit Operation The acid sorption process is assembled into a skid mounted unit called an Acid Purification Unit (TM). The heart of this unit is obviously the resin bed, which is typically 24 inches (61 cm) in height. Scaleup is accomplished by increasing the diameter of the bed.

4 resin column Aluminum sulfate solution Water resin column Sulfuric acid/ aluminum sulfate Sulfuric acid Acid sorption step Water wash step The basic operation of the unit is illustrated above. There are two main steps in the cycle: sorption and wash. During the sorption step, anodizing acid is pumped upward through the resin column. Free sulfuric acid is sorbed by the resin while aluminum sulfate salt solution passes through. During the desorption step, water passes down through the resin column to extract the free sulfuric acid from the resin. The equipment alternates automatically from sorption to desorption every few minutes. The net result is a virtually continuous process for acid purification and metal salt removal. Purification Alternatives While resin sorption is the most widely used method of anodizing acid purification, other membrane based unit operations such as diffusion dialysis and electrodialysis may be technically applicable. Of the two, diffusion dialysis is the more commercially established, particularly in Japan. General statements pertaining to the evaluation of resin sorption versus diffusion dialysis technologies are as follows: In a single pass, resin sorption systems provide a lower metal removal but a higher acid recovery compared with diffusion dialysis systems. The net result is that resin sorption systems remove dissolved aluminum from an anodizing process with less free acid loss. Running costs for both systems are low consisting only of water usage and energy for transporting the acid and water through the equipment. Membrane replacement costs were reported to be an expensive consumable contributing to a large percentage of the overall system cost. Resin sorption reports replacement by regular plant personnel once every 6-12 years comprising 10-15% of the system cost. The performance criteria for an acid purification system must be based on the ability of the system to remove metal down to desired concentration levels, to effect proper metal removal capacity, and, most importantly, to minimize acid losses. System Operation/Performance Two units are currently in operation at this PMF facility. The first was commissioned in February, 1985 and currently has completed close to 550,000 cycles. A plant expansion resulted in a installation of a second unit which was commissioned in December, 1992 and currently has completed over 120,000 cycles. Both units have the capability to effect an aluminum removal rate of 3 lb/hour (1.36 kg/hour) while holding

5 the aluminum concentration at 7 g/l in the anodizing solution. The mass balance around a unit is shown below. Stream Sulfuric Acid (g/l) Aluminum (g/l) Flow (L/hr) Feed Product Byproduct Water Acid Purification Unit Operating Conditions The operating conditions reflect the following performance standards: 96% of all free sulfuric acid fed to the acid purification unit is recovered for reuse. For every pound of aluminum removed by the unit, 2.5 pounds of sulfuric acid was lost to waste. This translates into a 70% reduction in sulfuric acid purchases and caustic soda neutralization costs when compared to holding the sulfuric acid and aluminum concentrations at the above feed levels under a dump/decant scenario. In terms of system interface, a feed pump transfers anodizing solution from one of the anodizing baths to an acid reservoir located on the skid of the unit. This reservoir is then pressurized with air to force the bath solution through the resin column. Once a predetermined volume of bath solution has been processed, a second water reservoir is pressurized forcing water in the opposite direction to displace the free sulfuric acid back to the anodizing bath. The unit cycles continuously and automatically in this manner. Normal daily analysis of the individual bath solution will dictate when and if the unit is to be shut off.

6 ANODIZING TANK ANODIZING TANK ANODIZING TANK ANODIZING TANK PRODUCT FEED PUMP FEED AIR ELECTRICITY WATER F F ACID PURIFICATION UNIT BYPRODUCT Process Schematic One unique feature is an automatic valve switchover sequence. Since a number of baths exist here and the unit only operates on one bath at a time, the unit can be programmed to cycle through the various baths to effect aluminum removal. Complete flexibility is afforded here - the unit can operate longer or shorter on each of the baths as programmed by the operator depending on the workload through the baths at any given time. Since startup, the units have operated with little more than routine maintenance involving cartridge filter replacement and normal service of valves. A resin bed volume was replaced once on the older unit, not to the degradative effects of the sulfuric acid as one might expect, but because the resin was lost due to an accidental breakage of the screen/gasket assembly holding the resin in place. No resin changeout has occurred with the newer unit.

7 Acid Purification Unit Contamination has consistently been kept at the desired 7 g/l metal concentration level. Problems such as pitting and burning have been avoided as a result of the consistency that arises with bath performance. No bath solution has been dumped since the units have been installed. Summary Savings in sulfuric acid purchases and neutralization costs have been realized. Of greater importance has been the consistent, predictable anodizing operation at all times even in the face of different requirements and tolerances from the wide variety of work pieces being processed. Also, the reduction of sulfate discharge from the facility has proved beneficial to the local POTW in terms of minimizing damage to concrete sewer lines. It is interesting to note that the new plant in Michigan has subsequently gone ahead with such a unit as a result of the positive results from this installation. Also, the sister plant in Minneapolis has had such a system for the last 11 years.

8 References 1. Pernick, J. Problems in Hardcoat Anodizing, Plating and Surface Finishing Magazine, p. 32, (June, 1988). 2. Albertson, O., ph Control - Neutralizing Alkaline Industrial Wastewater - Beneficial or Detrimental?, Industrial Wastewater Journal, p (March/April, 1996) 3. Dejak, M., Acid Purification and Recovery using Resin Sorption Technology - An Update, presented at the AESF/EPA Pollution Prevention Conference, Orlando, Florida (1994) 4. Brace, A.W., Anodic Coating Defects - Their Causes and Cure, Technicopy Ltd., Gloucestershire, England (1992) 5. C. Brown, C. Fletcher, "The Recoflo Ion Exchange Process", Ion Exchange for Industry, M. Streat, Ed., Chichester, UK: Ellis Horwood Limited (1988) 6. C. Fletcher, V. Pace, "New Performance Standards for Demineralization set by Recoflo Technology", presented at the American Power Conference, Chicago, Illinois (1995) About the Authors Jim Harrison has been with the Pioneer Metal Finishing Company, Green Bay, Wisconsin Division, for 3 years. He is the Quality Assurance Supervisor at this facility where he coordinates all environmental compliance activities. Mr. Harrison holds a degree in Biology and Chemistry from St. Norbert College in DePere, Wisconsin. Paul Pajunen has, for the last 12 years, been with Eco-Tec of Pickering, Ontario, Canada, in various roles as process engineer, customer service engineer, and sales/marketing representative. He currently leads Eco-Tec's marketing efforts as it pertains to the electroplating and aluminum metal finishing sectors. Mr. Pajunen holds a degree in chemical engineering from the University of Waterloo, is a registered professional engineer in the Province of Ontario, is a member of the American Electroplaters and Surface Finishers Society, Inc. (AESF), is a member of the Aluminum Anodizers Council (AAC), and is involved with the Metal Finishers Suppliers Association (MFSA), Canadian division, government relations committee.