Pollution prevention in a zinc die casting company: a 10-year case study

Journal of Cleaner Production 10 (2002) 93–99www.cleanerproduction.net

Pollution prevention in a zinc die casting company: a 10-year casestudy

Eugene Parka,*, Richard Enanderb, Stanley M. Barnetta

a Center for Pollution Prevention, Chemical Engineering Department, University of Rhode Island, Kingston, RI 02881, USAb Office of Technical and Customer Assistance, Rhode Island Department of Environmental Management, Providence, RI 02908, USA

Received 9 August 2000; accepted 12 September 2000

Abstract

In this project, pollution prevention methodologies were applied to a mass finishing process used for the cleaning and polishingof miniature zinc die-casts. While the original project objective had been to rely on the use of acids and filtration methods (cartridgeand membrane) to minimize waste discharges, development testing and improvement data obtained throughout the year-long projectindicated that acid use and cartridge filtration were no longer required; pH levels were found to be the key to simplifying theentire operation.

Pollution prevention was achieved through (1) the elimination of mineral spirits to preclean parts; (2) the elimination of hydro-chloric acid used to settle and remove sludge; (3) the elimination of a 100,000 l/month sewer discharge of metal-bearing wastewater;(4) the recovery of zinc metal for off-site recycling; and (5) the recycling of an aqueous-based soap. Favorable economic paybackand reduced liabilities were also realized while product rejection rates decreased. The company has been using the process for 10years. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Die-casting; Membrane; Recycling

1. Introduction

Vibratory and tubbing operations are mass-finishingtechniques that are found in a variety of industries. Typi-cal industry types include jewelry and machine toolmanufacturing. The size of the metal parts to be finishedrange from small jewelry pieces to large, heavy metalparts that are over 10 cm long.

In order to clean and/or polish metal parts, aqueous-based chemicals and ceramic or plastic media are typi-cally used in either a vibrating basin or a rotating barrel.Occasionally, metal-based media such as steel shot isused. In most cases, the waste effluent from these massfinishing operations consists of soapy water, pulverizedmedia, and both soluble and insoluble metals of the typefound in the metal parts being processed. Traditionalwaste treatment techniques have included chemical

* Corresponding author. Tel.:+1-401-874-4303; fax:+1-401-222-3810.

E-mail address: [email protected] (E. Park).

0959-6526/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0959-6526 (01)00024-5

flocculation and settling to remove metal contaminantsprior to wastewater discharge. In most instances, vibra-tory waste effluent is co-mingled with other types ofmetal-finishing and plating operations from the plant andwith other metal-bearing waste streams prior to pretreat-ment and disposal.

Since its founding in the early 1960s, Miniature Cast-ing Corp. has been involved in the manufacture of pre-cision miniature zinc die-castings. As a fully integratedfacility, the company is responsible for the design andconstruction of their own die-casting equipment as wellas the production of castings. In the early days of oper-ation, all effluents of the die-casting process were dis-charged into the municipal wastewater treatment facilityincluding non-contact cooling water and soapy vibratorywaste. Compliance with the local publicly-owned treat-ment works (POTW) was sometimes difficult due to highzinc levels in the wastewater. When the company relo-cated to the city of Cranston, RI in 1988, a consciouseffort was made to implement an effective environmen-tal management/pollution prevention program. MiniatureCasting sought to determine the most efficient and cost-

94 E. Park et al. / Journal of Cleaner Production 10 (2002) 93–99

effective means of reducing and managing its vibratorywaste effluent.

In 1990, the University of Rhode Island (URI) andRhode Island Department of Environmental Manage-ment (DEM) helped Miniature Casting apply for and wina US$24,000 US Environmental Protection Agency(EPA) Small Business Pollution Prevention Grant tocarry out a year-long study on vibratory solution recyc-ling. Pollution prevention for this type of operation gen-erally consists of source segregation, the elimination oftreatment chemicals, the reduction of sludge generation,water/soap conservation, and metal recovery [1]. Thisreport profiles a typical mass finishing operation and thetypes of technologies and methods that can be used bycompanies to reduce operating/environmental costs andimprove overall production efficiency.

2. Procedure

2.1. Original operation

As part of the manufacturing process, precision zincdie-castings were cleaned with vibratory finishing equip-ment. Ten tons of zinc alloy were purchased on amonthly basis. Before the research study began, approxi-mately 10% of the die-castings had to be precleaned withmineral spirits prior to vibratory finishing. All of theeffluent from the vibratory machines was mixed anddiluted with non-contact cooling water and dischargeddirectly to the sewer (Fig. 1). One objective of the EPA-funded study was to incorporate membrane technology,such as ultrafiltration, into a working pollution preven-tion program for the mass finishing operation. The inves-tigators anticipated that the mineral spirits-based cleanercould be eliminated and replaced with an aqueous sol-ution also; in this way, membranes could be used toclean and recycle all of the spent cleaning solution. Anadditional objective was to recover all die-cast metalconcentrate, sent to the sewer in the original operation(Fig. 1), for off-site recycling.

In 1990, 9,000 kg of zinc alloy bars were melted downevery month in the die-casting operation of the manufac-turing process. Approximately 10% of the die-casts exhi-bited noticeably more “smudge” or oxide build-up thanthe remaining 90% of the parts. While all parts werecleaned in the vibratory machines, 10% of the pieces hadto undergo an additional precleaning step with mineralspirits prior to the vibratory operation. The mineral spir-its cleaner was changed every month (60 l) by an outsidesolvent supplier. On a monthly basis, 20,000 l of waterwere used with approximately 150 l of soap concentratein the vibratory operation. The effluent was filteredthrough 1 µm cartridge filters and mixed with almost100,000 l per month of non-contact cooling water priorto discharge.

2.2. Research work performed

Early stages of the research (phase 1) involved (1) theelimination of mineral spirits, (2) the installation of 10µm cartridge filters for process solution recycle, and (3)the implementation of a rinsewater recycle process usingpH-adjustment for metal precipitation and membranefiltration for water purification. Initially, project teammembers focused on removing as much of the metal aspossible using acid and filtering the supernatant from theprecipitated solution to produce the cleanest water poss-ible for rinsewater reuse (see Fig. 2). Hydrochloric acidand citric acid were used in the tests. Zinc is least solubleat a pH of around 9 [2]. The actual mass finishing orvibratory solution was recycled in a separate loop using10 µm cartridge filters. The soap used to clean the partswas an alkaline cleaner (Okite M3) which resulted in asolution pH of 10. Because of POTW discharge limi-tations [3], the pH of the solution could not exceed 10.A small commercial membrane filtration system withapproximately 1.4 m2 of membrane area capacity wasused for all tests. Both ultrafiltration (UF) and nanofiltr-ation (NF) membranes were used. UF separates largemolecular species and particles down to 0.002 µm. NF, aloose type of reverse osmosis, has been used for smallermolecular species separation and partial removal of ionicspecies; there has recently been an increase in the num-ber of applications using NF [4].

The latter part of the project (phase 2) entailed simpli-fication of the entire process through the elimination of(1) cartridge filters, (2) acid use for pH-adjustment, and(3) the rinsing step entirely (see Fig. 3). In addition,because rinsing was no longer required, membrane fil-tration use was also reduced considerably. Zinc levelsand pH measurements were taken of the recycled sol-ution at various points in the process. Metal analyseswere performed using atomic absorption spectroscopy.An economic analysis was performed to determine costsavings resulting from the new process.

3. Results

3.1. Phase 1

Immediately, the entire mass finishing operation wasset up in a recycling test mode with no sewer discharge.Mineral spirits as a pre-cleaner was eliminated byincreasing the pH of the mass finishing solution to 10–11. The slightly higher alkalinity of the solution in com-bination with the mechanical cleaning from the vibratorymedia was effective in removing the heavy oxidedeposits on the zinc die-casts. Since the water solutionat this point was now being recycled, POTW compliancefor pH or any other parameter associated with this oper-ation could be ignored. In addition, the non-contact coo-

95E. Park et al. / Journal of Cleaner Production 10 (2002) 93–99

Fig. 1. Original plant operation (1990).

ling water from the casting furnaces was set up to recycleback to the furnaces after cool down in a holding tankand was no longer being added to the vibratory oper-ation. This water was no longer being discharged to thesewer since a diluent was no longer needed. Thoughrelatively small, added economic benefits were realized.

Because the initial goals of the project included totalsolution recycle (both vibratory and rinse), tests weredesigned with two separate process loops (see Fig. 2). Tenmicron cartridge filters were used to clean the vibratorysolution prior to reuse, and a small membrane filtrationsystem was installed to clean and recycle the rinsewater.Metal removal had been regarded as the most critical taskto ensure the production of clean water. As a result, acidwas used to adjust the spent process water pH down to 9

and to precipitate as much of the metal (zinc) as possibleprior to any filtration. Zinc content was measured fromgrab samples at the top of the settling tank — dramaticzinc level reductions were typically found (see Table 1).Membrane testing revealed that ultrafiltration was able toreduce the Zn even more (2.3 mg/l) while nanofiltrationbrought the Zn levels down to 1.0 mg/l. For severalmonths, this test process using ultrafiltration was an inte-gral part of the manufacturing at Miniature Casting. Themass finishing operation during this stage of the researchwas functioning well without any major problems; sewerdischarge had been eliminated and materials were beingrecycled. However, one major concern remained — theuse of HCl, a hazardous chemical which posed significantsafety issues in the workplace.


Fig. 2. Flow scheme for phase 1 (1991, 2-month period).

In an effort to eliminate the use of HCl, citric acidwas tested. Since citric acid is known to act as a metal-chelator [5], however, Zn concentrations were observedto actually increase significantly in solution. As seen inTable 1, the concentration of metal in the UF permeatewas still very high as almost all of the chelated zincpassed through the membrane. Only with NF was anyreduction in Zn observed because the NF membrane wasable to reject the smaller chelated molecule. Over time,a slimy substance began to form along the piping andequipment which were directly in contact with the pro-cess stream. Even though the zinc levels in solution werevery high, product quality was for the most part satisfac-tory. The presence of the slimy material, however,

rendered the overall operation much more labor-inten-sive due to extra clean-outs; ultimately, the use of citricacid ceased after several weeks.

3.2. Phase 2

An acceptable in-process recycling operation had beenestablished using HCl and filtration. The high Zn resultsseen with citric acid use, however, provided someadditional insights into improving the operation. Despitethe slime build-up using citric acid and the high levelsof Zn, the parts appeared to be very clean. Building onthis observation, project members instituted a drasticchange in the test program to investigate whether or not


Fig. 3. Flow scheme for phase 2 — present operation (2001).

Table 1pH-adjustment and Zn levels

Total Zn (mg/l)

Acid Before acid After acid After UF After NF

HCl 124 11 2.3 1.0Citric 145 �1000 �1000 1.0


Table 2Economic analysis of vibratory/cleaning operation

Before project (US$) After project (US$)

Capital equipment, including 0 5500membrane systemtankspumpsfilters/housings

Annual operating costs (approximate)Soap 1526 100Mineral spirits 1015 0Cartridge filters 1189 400Membrane replacement 0 300Energy Negligible NegligibleLabor 4742 1700Water 161 0Sewer fees 300 0Analytical tests 224 0Rework of rejected parts 1500 0

Total annual operating costs 10,657 2500

metal removal was really critical. The pH levels, asopposed to metal concentrations, seemed to be moreimportant in this type of mass finishing operation.

In the mass finishing recycle loop, the operation wasmodified by eliminating the acid addition and cartridgefiltration steps. The hypothesis used here was that thepresence of metals and solids may not adversely affectthe cleaning of the zinc die-casts since vibratory media,in any case, would be used to mechanically clean theparts along with the pH 11 solution. In fact, immediateresults indicated that not only were the zinc parts satis-factorily cleaned, but also rinsing was no longer needed.Rejection rates also decreased through this drastic sim-plification of the process. Ultrafiltration use was alsoreduced by over 90% because rinse water was no longerrequired. The simplified process is shown in Fig. 3. Inthis schematic, UF is used only to “ recharge” the systemevery 3–4 months. After approximately 3–4 months ofrecycling, the process solution is no longer effective incleaning the parts. The entire volume (200 l) is trans-ferred to the UF process tank to be processed. A secondbatch of clean soap and water which had been processedfrom the previous cycle is then returned back to the oper-ation loop for another cycle. Fresh tap water is addedperiodically to make up for evaporative losses. MiniatureCasting has been using this process since 1991.

Other improvements in the operation included thereduction in soap use (because acid was not being usedso chemical changes were minimized). Also, unexpec-tedly, a decrease in the product rejection rates wasobserved. It was estimated that approximatelyUS$1500/year was saved by eliminating rejected part re-processing. Since the pH of the solution was no longer

being reduced for metal removal, the integrity of thealkaline soap could be preserved. The quality of the fin-ished product has also been better maintained with alower percentage of the pieces requiring re-processing.This phenomenon could be the subject of anotherresearch study, but the hypothesis developed is that thepresence of suspended solids in the recycled solution canactually contribute to the mechanical cleaning action ofthe metal parts.

Even though metal precipitation through pH adjust-ment was no longer part of the process, settled solidsfrom the abrasion of metal parts and vibratory mediawere periodically removed from the settling tank.Because Miniature Casting generates a significantamount of scrap metal from the die-cast operation, therelatively small quantity of non-hazardous mass fin-ishing sludge is mixed with the scrap metal and returnedto the supplier. Approximately 40 kg of almost drysludge is sent back with 15,000 kg of scrap metal every3–4 months. Solids are removed at the same time thewater from the ultrafiltration process is used to“ recharge” the entire operation.

Since 1991, production at Miniature Casting hasincreased significantly. Approximately 9000 kg of zincalloy was purchased every month in 1991 compared to30,000 kg per month in 2000. Except for minor modifi-cations such as the addition of a specialty heated massfinishing unit, the operation has not changed since 1991and the company continues to recycle process solution.This industrial process has been monitored over a 10-year period during which fine tuning of the operationwas executed to accommodate the growing, changingproduction needs.


3.3. Economics

In addition to the environmental benefits achieved,many cost savings were quickly realized. Table 2 showsa direct cost comparison between the present recyclingoperation and the old, wastewater discharge operation of1990. Not included in the table are the costs incurred bythe test program itself, i.e., university chemical engineersand laboratory costs, which were covered mostly by theEPA grant. Based on capital expense and annualoperating cost calculations, a significant payback (lessthan 1 year) was observed.

4. Conclusion

A simple change in pH of the cleaning solutionresulted in multiple effects — acid use was eliminated,soap was conserved, rinsing was no longer needed, andfiltration requirements were minimized. This projectdemonstrates a cost-effective and environmentally soundsolution to mass finishing waste management. The pro-ject has demonstrated success in pollution preventionand the recycling of vibratory solution in a zinc die-cast-ing company. While the vibratory/tubbing operation isubiquitous, many parameters exist which render eachindividual operation unique. The type of metal to be cle-aned, the type of soap used, the desired finish on the

metal, the type of vibratory media used, and even theincoming tap water supply must all be considered.

An economic analysis of this project displays a favor-able payback. While direct technology transfer hasalready been demonstrated in many other companies, itis anticipated that the data collected and experienceacquired from this project will facilitate new programsto the extent that developmental work and costs are min-imized.

Acknowledgements

This work was carried out in conjunction with Minia-ture Casting Corporation of Cranston, RI, and funded bythe Rhode Island Department of Environmental Manage-ment and the United States Environmental ProtectionAgency.

References

[1] Park E. Pollution prevention in mass finishing operations. MetalFinishing 1994;Nov:14–23.

[2] Bailar JC, Emels HJ, Nyholm R, Trotman-Dickenson, editors.Comprehensive inorganic chemistry. New York: PergamonPress, 1973.

[3] POTW discharge regulations. City of Cranston, RI, 1990.[4] Lien L. Using membrane technology to reduce wastewater. Pol-

lution Engng 1998;30(5):44–7.[5] Catsch A. The chelation of heavy metals. New York: Pergamon

Press, 1979.

Pollution prevention in a zinc die casting company: a 10-year case study

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