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Recycling oil and steel from grinding swarf
Resources, Conservation and Recycling 49 (2006) 191–201
Recycling oil and steel from grinding swarf James I. Chang ∗ , J.J. Lin, J.S. Huang, Y.M. Chang Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, ROC Received 2 December 2005; received in revised form 23 March 2006; accepted 24 March 2006 Available online 22 May 2006
Abstract The aqueous washing process using nonionic surfactants and detergent builders was investigated for recovering cutting oil and alloy steel from swarf generated during high-speed grinding operations. Bivariate regression and the response surface methodology were applied to find the optimal concentrations of surfactant and detergent builders. Experimental results showed that 95% of oil removal could be achieved. Nonylphenol decaethoxylate (NPE-10), Tergitol 15-S-7 and 15-S-9 with low oil/water interfacial tension were most efficient in oil removal. Washed samples containing less than 3% of oil and 0.03% of phosphorous were acceptable for recycling in a smelter. A hypothetical 40-t per month processing unit installed at the manufacturer’s site would break even, if 24.7 and 58.6% of the recoverable oils were recycled as cutting oils for the base and the alternative cases, respectively. Higher profit would be obtained if more recoverable oil could be recycled as cutting oil. © 2006 Elsevier B.V. All rights reserved. Keywords: Surfactants; Aqueous washing; Bivariate regression; Economic feasibility
1. Introduction Grinding swarf is a mixture of small metal particles, metal removal fluid, lubricants, and residuals from grinding media such as stone wheels produced during the manufacture of high-speed metal cutting tools and machine operations in automobile and aerospace industries. The exact amount of the world generation of dry metallic swarf is unknown due ∗
Corresponding author. Tel.: +886 7 601 1000x2315; fax: +886 7 601 1061. E-mail address: [email protected] (J.I. Chang).
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to lack of statistical data. It is estimated to be between 2,300,000 and 5,800,000 t based on the amount of the world utilization of metal removal fluids (339,200,000 gal). To reduce the consumption of metal removal liquids and quantities of solid wastes, many manufacturers have installed oil filtration and recycling systems in last 10 years (OPP, 1998; ORC, 2002; Unidrive, 1997). Dried swarf contains 20–50% of oil, 45–80% of metals and 1–5% of other materials. The contaminants found in tool grinding swarf cause technical problems in a remelt process. Nonmetallics create undesired slag in the metal remelt process. Oil, if presents above 3%, may burn in the melt. If the oil content is too high, the oil can burn explosively. Phosphorus, which is present in the cutting oil, typically remains on the metal alloy particles as well, in levels greater than 0.03%. This level is unacceptable for recycling of the metal in a smelter or other recovery alternative (CWC, 2000). Swarf is currently landfilled either as a solid or a hazardous waste depending on its composition and regulatory definitions in different nations. Historically, extreme pressure additives such as chlorine and sulfur have been added at various levels to oils to provide increased tool life and improve surface finish. The chlorine or sulfur content in the cutting oil may be higher than 1.5%. If high-chlorine cutting oils were used in the grinding operation, swarf would definitely have high-chlorine content. In the United States, swarf or coolant with high-chlorine content is classified as hazardous waste regulated by RCRA. In Europe and Japan, where environmental regulations are as stringent as those in the United States, swarf with high-chlorine content is also classified as a hazardous waste. As landfills have closed at an alarming rate and disposal cost have increased sharply in last 10 years, the disposal of swarf has become a serious problem for many manufacturers. In addition, landfill causes a serious loss of high-quality alloy steel. Aqueous washing process using commercial surfactants has been successfully implemented in several soil washing projects (Deshpande et al., 1999; Edwards et al., 1994; Gannon et al., 1989; Gotlieb et al., 1993; Pennell et al., 1992; Verma and Kumar, 1998; Yeom et al., 1995). If an aqueous process could be implemented successfully for the separation of metal and oil from raw swarf, metallic fines would be reused by the metal industry and the oil would be recycled as cutting oil or fuel. Swarf would then become an asset rather than a waste for manufacturers. Bench scale studies of aqueous surfactant washing conducted by Fu et al. (1998) and CWC (2000) showed that a strong dependence of the aqueous washing efficiency on the choice of a suitable surfactant. Amway SA8 and A&W CDE/A6 out of seven commercial surfactants tested removed sufficient oil from swarf (86 and 98%, respectively) and yielded recyclable samples after a reasonable number of washings. Fu et al. (1998) also did a preliminary cost estimation of a hypothetical swarf cleaning facility with a capacity of processing 3,000,000 lb of raw swarf. The processing cost was $ 0.156/lb, which was higher than the income of $ 0.147/lb. The commercial processing of swarf would be a losing proposition, as neither the processor nor the swarf generator would be able to recover their costs. It would be economically feasible if swarf was regulated as a hazardous waste (Fu et al., 1998). The purpose of this work is to study if the economics of the aqueous washing process can be improved. To reduce the operating cost, less expensive surfactants other than industrial cleaners are desired. Detergent builders including sodium tripolyphosphate (STPP), sodium metasilicate, sodium citrate and sodium carbonate are added to improve the oil removal efficiency. Bivariate regression analysis is used to find optimal concentrations of
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the surfactant and the builder. The economic feasibility of a swarf cleaning facility with a capacity of 40 t per month is also conducted and discussed.
2. Materials and method 2.1. Swarf Swarf generated at a bearing manufacturer in northern Taiwan was used in this study. The metal used for bearing manufacture was a high-carbon chrome bearing steel (SUJ2) with 1.30% of chrome. The No. 2T cutting oil used in the grinding operation was supplied by Yushiron chemical industry, Tokyo, Japan. It contained 80–99% of mineral oil, 0–9% of anticorrosion agent, 0–9% anti-atomization agent and 0–9% chlorine additive, and has a HLB value of 8–20. 2.2. Surfactants and builders Commercial nonionic and anion liquid surfactants were obtained from Sino-Japan Chemical Corporation. Tergitol 15-S-7 and 15-S-9, mixtures of secondary alcohol ethoxylates with the alcohol group located at various positions along a chain of 11–15 carbon atoms and with an average ethylene oxide (EO) number of 7.3 and 9, respectively, were obtained from Dow Chemical Company. Detergent builders including sodium metasilicate, sodium tripolyphosphate, sodium carbonate, and sodium citrate were purchased from Sigma–Aldrich Company. 2.3. Experimental procedures Oil removal efficiencies of surfactants were tested in a 250-ml beaker. Weighted samples of dry swarf were ground to small particles and added in the beaker with a known amount of water. Surfactants were added in the beaker to achieve the desired surfactant strength. The solution was agitated at a speed of 200 rpm for 15 min, the solution was let stand for several minutes to allow the solid precipitate. The liquid fraction was then decanted and the precipitated solid was washed with water for several times. The solid was put into an oven and dried at 105 ◦ C for 8 h. The residual oils in washed swarf samples were measured using the Soxhlet extraction method. The surfactants with higher removal efficiencies were then selected and subject to further investigation such as the temperature effect, optimal concentration, multiple and recycle washings. The effects of detergent builders were also studied. Bivariate regression analysis was also conducted to obtain the optimal combinations of the surfactant and the detergent builder. 2.4. Analytical methods The composition of swarf was measured by a Philips PW1610 wave-dispersive X-ray fluorescence spectrometer (WD-XRF, Pananalytical, Almelo, the Netherlands). The liquid
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viscosity was measured by a CAV-4 (Cannon Instrument Company, State College, PA, USA) automatic viscosmeter using the ASTM D-445 method. The liquid density was measured by a DA-5200 automatic densitometer (Kyoto Electronics, Tokyo, Japan). The liquid color was measured by a PS-2121 colorimeter (Roseville, CA, USA) using the ASTM D-1500 method. 2.5. Experimental design and data analysis A full factorial central composite experimental design and a response surface methodology (Box et al., 1978) were conducted. The concentrations of the surfactant and the builder were selected as influencing parameters (independent variables), and the oil removal efficiency was selected as the corresponding parameter (dependent variable). The effects of independent variables on the dependent variables were analyzed as a quadratic function: Y1 = a0 + a1 X1 + a2 X2 + a11 X12 + a22 X22 + a12 X1 X2
(1)
where Y1 is the predicted response (dependent variable); X1 and X2 the independent variables; a0 the offset term; a1, and a2 the linear coefficients; a11 and a22 the squared terms; and a12 is the interaction coefficient. The best value of Y in Eq. (1) was nonlinearly evaluated using the Newtonian method of the “Solver” function in Microsoft Excel v. 5.0. The Windows software Statistica was employed for bivariate analysis to obtain best values of coefficients in Eq. (1). The graphical representation of Eq. (1) called the response surface could be used to determine the mutual interactions between test variables and their subsequent effect on the response (Box et al., 1978). The response surface contour plots were constructed using Igor Pro v. 3. The statistical diagnosis of the above parameters was based on the approach reported by Wen et al. (1994).
3. Results and discussion 3.1. Composition of swarf Swarf contains 47.12% of alloy steel, 51.40% of grinding oil, and 1.48% of residues of grinding media and impurities as shown in Table 1. In general, the physical properties of the unused cutting oil, the oil extracted from the dried grinding swarf before the filtration and recycling was installed and the oil extracted from the dried swarf after the filtration and recycling system was installed are similar (Table 2). The color of the oil extracted from the dried swarf after the filtration and recycling system is darker and the chlorine content is 25% lower. Table 1 Composition of swarf (wt.%) Metals (alloy steel) Fe
Cr
C
Mn
Si
P
S
44.15
1.30
1.03
0.32
0.28
0.02
0.02
Cutting oil
Residue
51.40
1.46
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Table 2 Physical properties of cutting oils Item
Unused cutting oil
Oil recovered from swarf before the filtration and recycling system was installed
Oil recovered from swarf after the filtration and recycling system was installed
Color Viscosity (centistoke) Specific gravity Flash point (◦ C) Pour point (◦ C) Sulfur (%) Chlorine (%)
Pale yellow 11.71 0.852 156 −12 0.211 1.174
Pale yellow 11.12 0.850 158 −9 0.230 1.154
Deep brown 11.43 0.85 160 −12 0.246 0.870
3.2. Preliminary testing Among a dozen surfactants tested, only nonylphenyl polyethoxylate (NPE-10), Tergitol 15-S-7 and 15-S-9 with low oil/water interfacial tension were able to remove 40% or more oil from swarf after the first washing. After a reasonable number of washing, all three surfactants removed more than 95% of the oil in swarf and yielded recyclable samples that met Timken-Latrobe’s criteria stated in CWC’s report (CWC, 2000) as shown in Table 3. The pH value had little effect to the cloud point and the oil removal efficiency as found by CWC (2000). To avoid the solublization of metals in the acidic solution, the pH value of the washing solution was adjusted to 8 or above, when a weakly acidic surfactant such as NPE-10 was used. The oil removed from swarf floated on the surface of the surfactant solution and only 0.02 g of oil dissolved in 250 ml of surfactant solution. Nonionic polyoxyethylene alcohols with low oil/water interfacial tension and a high solid/water adhesion tension perform well in removing oil from soil and polyester fibers. The effects of the O/W interfacial tension are the major factors in determining the relative effectiveness (Schick, 1987). The oil removal efficiency for nonionic polyethoxylates surfactant was somewhat dependent upon the temperature of the surfactant solution. When the temperature was below the cloud point of the nonionic surfactant, the higher the temperature was, the more the oil was removed. Nonionic surfactants become water-soluble by the hydration of ether oxygens of the polyoxyethylene group. An increase in temperature
Table 3 Removal efficiencies of selected surfactants Surfactant
Cloud point (◦ C)
Optimal temperature (◦ C)
Surfactant concentration (wt.%)
No of washing
Removal efficiency (wt.%)
References
NPE-10 Tergitol 15-S-7 Tergitol 15-S-9 A&W CDE/A6 SA8
60–66 35–40 58–63 – –
50–55 35–40 55–60 – –
11 10 10 2–2.6 2
3 4 4 1; 11 3
95 95 95 81; 98 86
This work This work This work CWC (2000) Fu et al. (1998)
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caused the cleavage of the hydrogen bond between the ether oxygen of the ethylene oxide group and the hydrated hydrogen to the ether oxygen. When the temperature was above the cloud point, the solution became turbid and separated into two phases. No improvement in oil removal was obtained; therefore, the wash temperature for subsequent studies was set at a few degrees below the cloud point. The cloud point had to be determined in each experiment, since the cloud point was affected by the concentrations of surfactants and detergent builders. 3.3. Selection of detergent builders Four detergent builders including sodium metasilicate, sodium tripolyphosphate (STPP), sodium carbonate, and sodium citrate of different concentrations from 0.5 to 3% were added to 11% of NPE-10 solution to improve its cleaning properties. As shown in Fig. 1, the addition of 1% of sodium metasilicate was able remove up to 93.9% of oil from swarf, which was 21.9% of improvement over NPE-10 alone. When 0.5% of STPP was added, 90.4% of oil could be removed from swarf. The addition of more than 1% of sodium metasilicate removed less oil. A similar trend of was also found, when other builders with concentration was over 0.5% were added. STPP, a popular ingredient in laundry products before 1970 and a main source of nutrient enrichment of surface water, was excluded for further study because of its phosphate content and the deleterious effect in eutrophication processes, despite its excellent performance. Sodium metasilicate was selected because of the higher removal efficiency and the economic reason.
Fig. 1. The effects of surfactant (NPE-10) and builders on cutting oil removal efficiency.
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Table 4 Full Factorial central composite experimental design Run
1 2 3 4 5 6 7 8 9 10 11 12 a b
Codea
Concentration (wt.%)
X1
X2
Surfactantb
Builderb
−1 −1 1 1 0 0 −1 1 0 0 0 0
−1 1 −1 1 0 0 0 0 −1 1 0 0
3 3 17 17 10 10 3 17 10 10 10 10
0 3 0 3 1.5 1.5 1.5 1.5 0 3 1.5 1.5
Removal efficiency (%)
67 68 76 88 95 93 76 94 71 87 96 92
X1 and X2 are surfactant and builder, respectively. Surfactant: NPE-10; builder: Na2 SiO3 ·5H2 O.
3.4. Optimal combination of nonionic surfactants and sodium metasilicate A total of 12 experiments were performed to a full factorial design for two independent variables, each at three levels with four replicates of the center value (Box et al., 1978). The matrix of the central composite design experiment with corresponding results is listed in Table 4. Multivariate regression analysis was used to obtain the best regression models as shown in Table 5. A regression model is considered to be statistically significant if the calculated regression test statistic, the F value, is larger than the value of F (μ, ν, α) in the F distribution at a probability of α (Box et al., 1978), where μ is the number of coefficients (parameters) less 1, ν is the degree of freedom, and α is a probability level. The degree of freedom is defined to be the number of data less the number of coefficients. In the regression equation, there are six coefficients (a0 , a1 , a2 , a11 , a12 and a22 ) need to be evaluated by regression analysis and 12 sets of data; therefore, μ is equal to 5 and ν is equal to 6. All calculated F values are from 26.67 to 156.82, which are larger than the F (5, 6, 0.05) value of 4.28 at 95% confidence level (α = 0.05) with six parameters (μ = 5) and 6 degrees of freedom (ν = 6). All r2 values are between 0.957 and 0.992, which also indicate the regression models are in fair agreements with the experimental data.
Table 5 Regression equations Equation YNPE-10 = 39.083 + 6.317X1 + 6.389X2 − 0.265X12 − 5.611X22 + 0.367X1 X2 (F = 26.666; r 2 = 0.957) Y15-S-7 = 29.516 + 3.940X1 + 28.899X2 − 0.139X12 − 6.277X22 + 0.249X1 X2 (F = 29.963; r 2 = 0.961) Y15-S-9 = 31.315 + 2.118X1 + 46.126X2 − 0.077X12 − 9.764X22 − 0.006X1 X2 (F = 156.82; r 2 = 0.992)
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Fig. 2. Contour lines of (A) cutting oil removal efficiency (%) vs. NPE-10 (wt.%) and Na2 SiO3 ·5H2 O; (B) cutting oil removal efficiency (%) vs. Tergitol 15-S-7 (wt.%) and Na2 SiO3 ·5H2 O (wt.%); and (C) cutting oil removal efficiency (%) vs. Tergitol 15-S-9 (wt.%) and Na2 SiO3 ·5H2 O (wt.%).
Contour plots were constructed using the equations listed in Table 5. As shown in Fig. 2, each response surface has a maximum value of 100% removal. To confirm the predicted models of oil removal, additional experiments were carried out. In practice, 95% seemed to be the maximum level for NPE-10 and Tergitols. More washing did not remove more oil from swarf. A recyclable sample with 2.57% of residual oil content (95% removal) meets the 3% remelt requirement. Phosphorous contents in all washed samples were 0.02% or less, which also meets the 0.03% requirement of remelt. The washing solution could be recycled and reused for five times before the removal efficiency went below 95%. 3.5. Economic feasibility A financial analysis for a 40 t per month aqueous washing unit for swarf recycling was prepared. A processing unit of this size could process the amount of swarf generated by a typical bearing and tool manufacturer in Taiwan and in the Southeastern United States as stated by Fu et al. (1998). This unit would be designed based on a flow chart depicted in Fu et al. (1998) and the equipment would be fabricated and assembled by local machine shops. To reduce the cost, prefabricated vessels and equipment would be used. The equipment cost of this processing unit was US$ 150,000 based on the average cost quoted by three local machine shops. The equipment cost (70%) would be borrowed from a bank with an
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interest rate of 6% per year and a payback period of 5 years. This hypothetical processing unit would be installed in swarf generators and operated by plant personnel. Two hypothetical cases, a base case and an alternative case, were studied. The base case assumed that raw swarf contained, by weight, 50% of cutting oil, 48% of recoverable alloy steel, and 2% of impurities. The alternative case assumed that raw swarf contained 20% of cutting oil, 50% of recoverable alloy steel, and 30% of impurities as studied by Fu et al. (1998). 95% recovery for the metal and 90% recovery for the oil were assumed. The recoverable metal would be sold as alloy steel at a price of $ 493/t (80% of the alloy price at $ 616/t) or as scrap iron at $ 160/t, and the recoverable cutting oil would be recycled either as the cutting oil at $ 1640/t or as fuel at $ 180/t. Only NPE-10 and sodium metasilicate would be used due to economic consideration. The unit processing costs as shown in Table 6 were $ 0.379/kg and $ 0.308/kg, respectively. The cost of the chemical reagents (the surfactant and the builder) was the highest among the costs. The profits could be expressed in terms of the fraction of the metal recycled as alloy (X3 ) and the fraction of recoverable oil recycled as the cutting oil (X4 ): Y2 = 0.152X3 + 0.657X4 − 0.162
(2)
Y3 = 0.153X3 + 0.263X4 − 0.154
(3)
Table 6 Monthly cost, income and profit for a hypothetical swarf recovery unit (in US$) Item
Base casea
Alternative caseb
Cost 1. Surfactant + builder 2. Utilities + labor 3. Operating cost 4. Operating cost (equipment loan)
9569 3490 13059 2030
6698 3490 10188 2030
90
90
15179 0.379
12308 0.308
5. Unearned interest
6. Subtotal 7. Unit cost ($/kg) Income 8. Scrap iron 9. Cutting oil 10. Fuel oil 11. Avoided disposal cost
12. Subtotal 13. Unit income ($/kg) 14. Profit per month 15. Unit profit ($/kg) 16. Return of investment (month) a b
2918 8691 2286 2534
16429 0.411 1,250 0.031 36.0
3040 8301 385 1834
13560 0.339 1252 0.031 36.0
Swarf contains 50% cutting oil, 48% alloy and 2% other materials. Swarf contains 20% cutting oil, 50% alloy and 30% other materials.
Sum of items 1 and 2 70% loan ($ 150000 × 70%) with 6% discount interest rate 30% cash investment ($ 150000 × 30%); 2% interest rate Sum of items 3–5
29.4 and 70.3%, respectively 70.6 and 29.7%, respectively $ 70 per tonne; 36.2 t for base case and 26.2 t for alternatives case Sum of items 8–11
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Y2 , Y3 were the profits for the base and alternative cases, respectively; X3 the fraction of the recoverable metal sold as alloy, and X4 was the fraction of recoverable oil sold as cutting oil. The generators of swarf would recycle the recoverable oil back to manufacturing processes if the recoverable oil passes the field test, but they would have little use of the recoverable metal powders. The worst possible scenario would be that the recoverable metal is sold as the scrap iron. Eqs. (2) and (3) would then be reduced to the following: Y2 = 0.657X4 − 0.162
(4)
Y3 = 0.263X4 − 0.154
(5)
The investment would be economically feasible if more than 24.7 and 58.6% of the recoverable oils were recycled as cutting oils for the base and the alternative cases, respectively. The more the recoverable oil is recycled as cutting oil, the more the profit is. As shown in Table 6, the investment would be recovered in 36 months if 29.4 and 70.3% of the recoverable oils were recycled as cutting oils for both cases, respectively.
4. Conclusion The feasibility of an aqueous washing process for swarf recovery was studied. Bivariate regression analysis and surface response methodology were used to find optimal concentrations of nonionic surfactants including NPE-10 and Tergitols, and sodium metasilicate. The results obtained from bench scale experiments showed that 95% of oil in dry swarf could be removed. The residual phosphorous contents in washed samples were 0.02% by weight or less. The washed samples were acceptable for recycling in a smelter. The recovered oil had similar physical properties as the cutting oil currently used inline. A preliminary financial analysis for a swarf recovery unit with a capacity of 40 t per month was conducted. The economic feasibility is dependent upon the recyclability of recoverable oil as cutting oil. The capital investment would be recovered in 36 months if 29.4 and 70.3% of the recoverable oils were recycled as cutting oils for both cases, respectively. Further field test is required to find out the maximum allowable percentage of recycling. Alkylphenol polyethoxylates (APEs) are biodegradable, but they breakdown into less biodegradable products. These products are more toxic to aquatic organisms than the unbroken APEs. They also remain in the aquatic environment for some time. Their persistence, bioaccumulation and oestrogenic effects are clear (Warhurst, 1995). In the late 1980s, European countries began to ban the use of APEs, replacing them with linear alcohol ethoxylates that are readily biodegradable under both aerobic and anaerobic conditions, but the use of APE surfactants in the United States remains widespread. Switzerland has already banned the use of APE surfactants (Maki et al., 1994). Polyoxyethylene secondary alcohols such as Tergitols exhibit excellent oil removal and good biodegradation properties, but they are much more expensive than NPE-10. Detergency range polyoxyethylene primary alcohols are less expensive than secondary alcohols and have better biodegradation properties. Preliminary study showed that they also yielded satisfactory results. Future work will focus on searching for ideal surfactants among afford-
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able primary and secondary alcohol ethoxylates surfactants with low oil/water interfacial tension and on finding applications for the recoverable metal particles.
Acknowledgements The authors wish to express their gratitude to Prof. L.H. Chiu, department of mechanical engineering, National Kaohsiung First University of Science and Technology for suggesting the research project, to Dr. W.K. Yu of Tung Pei Industrial Company for providing swarf and technical information, and to National Science Council, ROC for financial support.
References Box GE, Hunter WG, Hunter JS. Statistics for experimenters: an introduction to design, data analysis, and model building. 1st ed. New York: Wiley–Interscience; 1978. CWC. Recovery of a recyclable metal alloy from high-speed steel grinding swarf. Seattle, WA: CWC, Pacific Northwest Economic Region (PNWER); 2000. Deshpande S, Shiau BJ, Wade WJ, Harwell JH. Surfactant selection for enhancing in-situ soil washing. Water Res 1999;33:351–60. Edwards DA, Liu Z, Luthy RG. Surfactant solubilization of organic compounds in soil/aqueous systems. J Environ Eng 1994;120:5–22. Fu H, Matthews MA, Warner LS. Recycling steel from grinding swarf. Waste Manage 1998;18:321–9. Gannon OK, Bibring P, Raney K. Soil clean up by in-situ surfactant flushing. Part III. Laboratory results. Sep Sci Technol 1989;24:1073. Gotlieb I, Bozzelli JW, Gotlieb E. Soil and water decontamination by extraction with surfactants. Sep Sci Technol 1993;28:793–804. Maki H, Masuda N, Fujiwara Y, Ike M, Fujita M. Degradation of alkylphenol ethoxylates by Pseudomonas sp. Strain TR101. Appl Environ Microbiol 1994;60:2265–71. OPP. The Timeken Company. 1997. Governor’s pollution Prevention Award, The Office of Pollution Prevention. Columbus, Ohio: Environmental Protection Agency, The State of Ohio; 1998. ORC. Management of the metal removal fluid. Washington DC, USA: Organization Resources Counselors; 2002. Pennell KD, Abriola LM, Ang CC. Surfactant-enhanced solubilization of residual dodcane in soil columns. Part 1. Experimental investigation. Environ Sci Technol 1992;27:2332. Schick MJ. Detergency. Nonionic surfactants — physical chemistry. New York: Marcel Dekker; 1987. Unidrive. Cleaner production-machining coolant recycling. Clayton, Australia: Unidrive Pty. Ltd.; 1997. Verma S, Kumar VV. Relationship between oil–water interfacial tension and oily soil removal in mixed surfactant systems. J Colloid Interface Sci 1998;207(1):1–10. Warhurst AM. An environmental assessment of alkylephenol ethoxylate and alkylphenol. London, UK: Friends of Earth; 1995. Wen TC, Cheng SS, Lay JJ. A kinetic model of a recirculated upflow anaerobic sludge blanket treating phenolic wastewater. Water Environ Res 1994;66:794–9. Yeom IT, Ghosh MM, Co CD, Robinson KG. Micellar solubilization of polycyclic aromatic hydrocarbons in coal tar contaminated soils. Environ Sci Technol 1995;29:3015–21.
Recycling oil and steel from grinding swarf James I. Chang ∗ , J.J. Lin, J.S. Huang, Y.M. Chang Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, ROC Received 2 December 2005; received in revised form 23 March 2006; accepted 24 March 2006 Available online 22 May 2006
Abstract The aqueous washing process using nonionic surfactants and detergent builders was investigated for recovering cutting oil and alloy steel from swarf generated during high-speed grinding operations. Bivariate regression and the response surface methodology were applied to find the optimal concentrations of surfactant and detergent builders. Experimental results showed that 95% of oil removal could be achieved. Nonylphenol decaethoxylate (NPE-10), Tergitol 15-S-7 and 15-S-9 with low oil/water interfacial tension were most efficient in oil removal. Washed samples containing less than 3% of oil and 0.03% of phosphorous were acceptable for recycling in a smelter. A hypothetical 40-t per month processing unit installed at the manufacturer’s site would break even, if 24.7 and 58.6% of the recoverable oils were recycled as cutting oils for the base and the alternative cases, respectively. Higher profit would be obtained if more recoverable oil could be recycled as cutting oil. © 2006 Elsevier B.V. All rights reserved. Keywords: Surfactants; Aqueous washing; Bivariate regression; Economic feasibility
1. Introduction Grinding swarf is a mixture of small metal particles, metal removal fluid, lubricants, and residuals from grinding media such as stone wheels produced during the manufacture of high-speed metal cutting tools and machine operations in automobile and aerospace industries. The exact amount of the world generation of dry metallic swarf is unknown due ∗
Corresponding author. Tel.: +886 7 601 1000x2315; fax: +886 7 601 1061. E-mail address: [email protected] (J.I. Chang).
0921-3449/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2006.03.014
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to lack of statistical data. It is estimated to be between 2,300,000 and 5,800,000 t based on the amount of the world utilization of metal removal fluids (339,200,000 gal). To reduce the consumption of metal removal liquids and quantities of solid wastes, many manufacturers have installed oil filtration and recycling systems in last 10 years (OPP, 1998; ORC, 2002; Unidrive, 1997). Dried swarf contains 20–50% of oil, 45–80% of metals and 1–5% of other materials. The contaminants found in tool grinding swarf cause technical problems in a remelt process. Nonmetallics create undesired slag in the metal remelt process. Oil, if presents above 3%, may burn in the melt. If the oil content is too high, the oil can burn explosively. Phosphorus, which is present in the cutting oil, typically remains on the metal alloy particles as well, in levels greater than 0.03%. This level is unacceptable for recycling of the metal in a smelter or other recovery alternative (CWC, 2000). Swarf is currently landfilled either as a solid or a hazardous waste depending on its composition and regulatory definitions in different nations. Historically, extreme pressure additives such as chlorine and sulfur have been added at various levels to oils to provide increased tool life and improve surface finish. The chlorine or sulfur content in the cutting oil may be higher than 1.5%. If high-chlorine cutting oils were used in the grinding operation, swarf would definitely have high-chlorine content. In the United States, swarf or coolant with high-chlorine content is classified as hazardous waste regulated by RCRA. In Europe and Japan, where environmental regulations are as stringent as those in the United States, swarf with high-chlorine content is also classified as a hazardous waste. As landfills have closed at an alarming rate and disposal cost have increased sharply in last 10 years, the disposal of swarf has become a serious problem for many manufacturers. In addition, landfill causes a serious loss of high-quality alloy steel. Aqueous washing process using commercial surfactants has been successfully implemented in several soil washing projects (Deshpande et al., 1999; Edwards et al., 1994; Gannon et al., 1989; Gotlieb et al., 1993; Pennell et al., 1992; Verma and Kumar, 1998; Yeom et al., 1995). If an aqueous process could be implemented successfully for the separation of metal and oil from raw swarf, metallic fines would be reused by the metal industry and the oil would be recycled as cutting oil or fuel. Swarf would then become an asset rather than a waste for manufacturers. Bench scale studies of aqueous surfactant washing conducted by Fu et al. (1998) and CWC (2000) showed that a strong dependence of the aqueous washing efficiency on the choice of a suitable surfactant. Amway SA8 and A&W CDE/A6 out of seven commercial surfactants tested removed sufficient oil from swarf (86 and 98%, respectively) and yielded recyclable samples after a reasonable number of washings. Fu et al. (1998) also did a preliminary cost estimation of a hypothetical swarf cleaning facility with a capacity of processing 3,000,000 lb of raw swarf. The processing cost was $ 0.156/lb, which was higher than the income of $ 0.147/lb. The commercial processing of swarf would be a losing proposition, as neither the processor nor the swarf generator would be able to recover their costs. It would be economically feasible if swarf was regulated as a hazardous waste (Fu et al., 1998). The purpose of this work is to study if the economics of the aqueous washing process can be improved. To reduce the operating cost, less expensive surfactants other than industrial cleaners are desired. Detergent builders including sodium tripolyphosphate (STPP), sodium metasilicate, sodium citrate and sodium carbonate are added to improve the oil removal efficiency. Bivariate regression analysis is used to find optimal concentrations of
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the surfactant and the builder. The economic feasibility of a swarf cleaning facility with a capacity of 40 t per month is also conducted and discussed.
2. Materials and method 2.1. Swarf Swarf generated at a bearing manufacturer in northern Taiwan was used in this study. The metal used for bearing manufacture was a high-carbon chrome bearing steel (SUJ2) with 1.30% of chrome. The No. 2T cutting oil used in the grinding operation was supplied by Yushiron chemical industry, Tokyo, Japan. It contained 80–99% of mineral oil, 0–9% of anticorrosion agent, 0–9% anti-atomization agent and 0–9% chlorine additive, and has a HLB value of 8–20. 2.2. Surfactants and builders Commercial nonionic and anion liquid surfactants were obtained from Sino-Japan Chemical Corporation. Tergitol 15-S-7 and 15-S-9, mixtures of secondary alcohol ethoxylates with the alcohol group located at various positions along a chain of 11–15 carbon atoms and with an average ethylene oxide (EO) number of 7.3 and 9, respectively, were obtained from Dow Chemical Company. Detergent builders including sodium metasilicate, sodium tripolyphosphate, sodium carbonate, and sodium citrate were purchased from Sigma–Aldrich Company. 2.3. Experimental procedures Oil removal efficiencies of surfactants were tested in a 250-ml beaker. Weighted samples of dry swarf were ground to small particles and added in the beaker with a known amount of water. Surfactants were added in the beaker to achieve the desired surfactant strength. The solution was agitated at a speed of 200 rpm for 15 min, the solution was let stand for several minutes to allow the solid precipitate. The liquid fraction was then decanted and the precipitated solid was washed with water for several times. The solid was put into an oven and dried at 105 ◦ C for 8 h. The residual oils in washed swarf samples were measured using the Soxhlet extraction method. The surfactants with higher removal efficiencies were then selected and subject to further investigation such as the temperature effect, optimal concentration, multiple and recycle washings. The effects of detergent builders were also studied. Bivariate regression analysis was also conducted to obtain the optimal combinations of the surfactant and the detergent builder. 2.4. Analytical methods The composition of swarf was measured by a Philips PW1610 wave-dispersive X-ray fluorescence spectrometer (WD-XRF, Pananalytical, Almelo, the Netherlands). The liquid
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viscosity was measured by a CAV-4 (Cannon Instrument Company, State College, PA, USA) automatic viscosmeter using the ASTM D-445 method. The liquid density was measured by a DA-5200 automatic densitometer (Kyoto Electronics, Tokyo, Japan). The liquid color was measured by a PS-2121 colorimeter (Roseville, CA, USA) using the ASTM D-1500 method. 2.5. Experimental design and data analysis A full factorial central composite experimental design and a response surface methodology (Box et al., 1978) were conducted. The concentrations of the surfactant and the builder were selected as influencing parameters (independent variables), and the oil removal efficiency was selected as the corresponding parameter (dependent variable). The effects of independent variables on the dependent variables were analyzed as a quadratic function: Y1 = a0 + a1 X1 + a2 X2 + a11 X12 + a22 X22 + a12 X1 X2
(1)
where Y1 is the predicted response (dependent variable); X1 and X2 the independent variables; a0 the offset term; a1, and a2 the linear coefficients; a11 and a22 the squared terms; and a12 is the interaction coefficient. The best value of Y in Eq. (1) was nonlinearly evaluated using the Newtonian method of the “Solver” function in Microsoft Excel v. 5.0. The Windows software Statistica was employed for bivariate analysis to obtain best values of coefficients in Eq. (1). The graphical representation of Eq. (1) called the response surface could be used to determine the mutual interactions between test variables and their subsequent effect on the response (Box et al., 1978). The response surface contour plots were constructed using Igor Pro v. 3. The statistical diagnosis of the above parameters was based on the approach reported by Wen et al. (1994).
3. Results and discussion 3.1. Composition of swarf Swarf contains 47.12% of alloy steel, 51.40% of grinding oil, and 1.48% of residues of grinding media and impurities as shown in Table 1. In general, the physical properties of the unused cutting oil, the oil extracted from the dried grinding swarf before the filtration and recycling was installed and the oil extracted from the dried swarf after the filtration and recycling system was installed are similar (Table 2). The color of the oil extracted from the dried swarf after the filtration and recycling system is darker and the chlorine content is 25% lower. Table 1 Composition of swarf (wt.%) Metals (alloy steel) Fe
Cr
C
Mn
Si
P
S
44.15
1.30
1.03
0.32
0.28
0.02
0.02
Cutting oil
Residue
51.40
1.46
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Table 2 Physical properties of cutting oils Item
Unused cutting oil
Oil recovered from swarf before the filtration and recycling system was installed
Oil recovered from swarf after the filtration and recycling system was installed
Color Viscosity (centistoke) Specific gravity Flash point (◦ C) Pour point (◦ C) Sulfur (%) Chlorine (%)
Pale yellow 11.71 0.852 156 −12 0.211 1.174
Pale yellow 11.12 0.850 158 −9 0.230 1.154
Deep brown 11.43 0.85 160 −12 0.246 0.870
3.2. Preliminary testing Among a dozen surfactants tested, only nonylphenyl polyethoxylate (NPE-10), Tergitol 15-S-7 and 15-S-9 with low oil/water interfacial tension were able to remove 40% or more oil from swarf after the first washing. After a reasonable number of washing, all three surfactants removed more than 95% of the oil in swarf and yielded recyclable samples that met Timken-Latrobe’s criteria stated in CWC’s report (CWC, 2000) as shown in Table 3. The pH value had little effect to the cloud point and the oil removal efficiency as found by CWC (2000). To avoid the solublization of metals in the acidic solution, the pH value of the washing solution was adjusted to 8 or above, when a weakly acidic surfactant such as NPE-10 was used. The oil removed from swarf floated on the surface of the surfactant solution and only 0.02 g of oil dissolved in 250 ml of surfactant solution. Nonionic polyoxyethylene alcohols with low oil/water interfacial tension and a high solid/water adhesion tension perform well in removing oil from soil and polyester fibers. The effects of the O/W interfacial tension are the major factors in determining the relative effectiveness (Schick, 1987). The oil removal efficiency for nonionic polyethoxylates surfactant was somewhat dependent upon the temperature of the surfactant solution. When the temperature was below the cloud point of the nonionic surfactant, the higher the temperature was, the more the oil was removed. Nonionic surfactants become water-soluble by the hydration of ether oxygens of the polyoxyethylene group. An increase in temperature
Table 3 Removal efficiencies of selected surfactants Surfactant
Cloud point (◦ C)
Optimal temperature (◦ C)
Surfactant concentration (wt.%)
No of washing
Removal efficiency (wt.%)
References
NPE-10 Tergitol 15-S-7 Tergitol 15-S-9 A&W CDE/A6 SA8
60–66 35–40 58–63 – –
50–55 35–40 55–60 – –
11 10 10 2–2.6 2
3 4 4 1; 11 3
95 95 95 81; 98 86
This work This work This work CWC (2000) Fu et al. (1998)
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caused the cleavage of the hydrogen bond between the ether oxygen of the ethylene oxide group and the hydrated hydrogen to the ether oxygen. When the temperature was above the cloud point, the solution became turbid and separated into two phases. No improvement in oil removal was obtained; therefore, the wash temperature for subsequent studies was set at a few degrees below the cloud point. The cloud point had to be determined in each experiment, since the cloud point was affected by the concentrations of surfactants and detergent builders. 3.3. Selection of detergent builders Four detergent builders including sodium metasilicate, sodium tripolyphosphate (STPP), sodium carbonate, and sodium citrate of different concentrations from 0.5 to 3% were added to 11% of NPE-10 solution to improve its cleaning properties. As shown in Fig. 1, the addition of 1% of sodium metasilicate was able remove up to 93.9% of oil from swarf, which was 21.9% of improvement over NPE-10 alone. When 0.5% of STPP was added, 90.4% of oil could be removed from swarf. The addition of more than 1% of sodium metasilicate removed less oil. A similar trend of was also found, when other builders with concentration was over 0.5% were added. STPP, a popular ingredient in laundry products before 1970 and a main source of nutrient enrichment of surface water, was excluded for further study because of its phosphate content and the deleterious effect in eutrophication processes, despite its excellent performance. Sodium metasilicate was selected because of the higher removal efficiency and the economic reason.
Fig. 1. The effects of surfactant (NPE-10) and builders on cutting oil removal efficiency.
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Table 4 Full Factorial central composite experimental design Run
1 2 3 4 5 6 7 8 9 10 11 12 a b
Codea
Concentration (wt.%)
X1
X2
Surfactantb
Builderb
−1 −1 1 1 0 0 −1 1 0 0 0 0
−1 1 −1 1 0 0 0 0 −1 1 0 0
3 3 17 17 10 10 3 17 10 10 10 10
0 3 0 3 1.5 1.5 1.5 1.5 0 3 1.5 1.5
Removal efficiency (%)
67 68 76 88 95 93 76 94 71 87 96 92
X1 and X2 are surfactant and builder, respectively. Surfactant: NPE-10; builder: Na2 SiO3 ·5H2 O.
3.4. Optimal combination of nonionic surfactants and sodium metasilicate A total of 12 experiments were performed to a full factorial design for two independent variables, each at three levels with four replicates of the center value (Box et al., 1978). The matrix of the central composite design experiment with corresponding results is listed in Table 4. Multivariate regression analysis was used to obtain the best regression models as shown in Table 5. A regression model is considered to be statistically significant if the calculated regression test statistic, the F value, is larger than the value of F (μ, ν, α) in the F distribution at a probability of α (Box et al., 1978), where μ is the number of coefficients (parameters) less 1, ν is the degree of freedom, and α is a probability level. The degree of freedom is defined to be the number of data less the number of coefficients. In the regression equation, there are six coefficients (a0 , a1 , a2 , a11 , a12 and a22 ) need to be evaluated by regression analysis and 12 sets of data; therefore, μ is equal to 5 and ν is equal to 6. All calculated F values are from 26.67 to 156.82, which are larger than the F (5, 6, 0.05) value of 4.28 at 95% confidence level (α = 0.05) with six parameters (μ = 5) and 6 degrees of freedom (ν = 6). All r2 values are between 0.957 and 0.992, which also indicate the regression models are in fair agreements with the experimental data.
Table 5 Regression equations Equation YNPE-10 = 39.083 + 6.317X1 + 6.389X2 − 0.265X12 − 5.611X22 + 0.367X1 X2 (F = 26.666; r 2 = 0.957) Y15-S-7 = 29.516 + 3.940X1 + 28.899X2 − 0.139X12 − 6.277X22 + 0.249X1 X2 (F = 29.963; r 2 = 0.961) Y15-S-9 = 31.315 + 2.118X1 + 46.126X2 − 0.077X12 − 9.764X22 − 0.006X1 X2 (F = 156.82; r 2 = 0.992)
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Fig. 2. Contour lines of (A) cutting oil removal efficiency (%) vs. NPE-10 (wt.%) and Na2 SiO3 ·5H2 O; (B) cutting oil removal efficiency (%) vs. Tergitol 15-S-7 (wt.%) and Na2 SiO3 ·5H2 O (wt.%); and (C) cutting oil removal efficiency (%) vs. Tergitol 15-S-9 (wt.%) and Na2 SiO3 ·5H2 O (wt.%).
Contour plots were constructed using the equations listed in Table 5. As shown in Fig. 2, each response surface has a maximum value of 100% removal. To confirm the predicted models of oil removal, additional experiments were carried out. In practice, 95% seemed to be the maximum level for NPE-10 and Tergitols. More washing did not remove more oil from swarf. A recyclable sample with 2.57% of residual oil content (95% removal) meets the 3% remelt requirement. Phosphorous contents in all washed samples were 0.02% or less, which also meets the 0.03% requirement of remelt. The washing solution could be recycled and reused for five times before the removal efficiency went below 95%. 3.5. Economic feasibility A financial analysis for a 40 t per month aqueous washing unit for swarf recycling was prepared. A processing unit of this size could process the amount of swarf generated by a typical bearing and tool manufacturer in Taiwan and in the Southeastern United States as stated by Fu et al. (1998). This unit would be designed based on a flow chart depicted in Fu et al. (1998) and the equipment would be fabricated and assembled by local machine shops. To reduce the cost, prefabricated vessels and equipment would be used. The equipment cost of this processing unit was US$ 150,000 based on the average cost quoted by three local machine shops. The equipment cost (70%) would be borrowed from a bank with an
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interest rate of 6% per year and a payback period of 5 years. This hypothetical processing unit would be installed in swarf generators and operated by plant personnel. Two hypothetical cases, a base case and an alternative case, were studied. The base case assumed that raw swarf contained, by weight, 50% of cutting oil, 48% of recoverable alloy steel, and 2% of impurities. The alternative case assumed that raw swarf contained 20% of cutting oil, 50% of recoverable alloy steel, and 30% of impurities as studied by Fu et al. (1998). 95% recovery for the metal and 90% recovery for the oil were assumed. The recoverable metal would be sold as alloy steel at a price of $ 493/t (80% of the alloy price at $ 616/t) or as scrap iron at $ 160/t, and the recoverable cutting oil would be recycled either as the cutting oil at $ 1640/t or as fuel at $ 180/t. Only NPE-10 and sodium metasilicate would be used due to economic consideration. The unit processing costs as shown in Table 6 were $ 0.379/kg and $ 0.308/kg, respectively. The cost of the chemical reagents (the surfactant and the builder) was the highest among the costs. The profits could be expressed in terms of the fraction of the metal recycled as alloy (X3 ) and the fraction of recoverable oil recycled as the cutting oil (X4 ): Y2 = 0.152X3 + 0.657X4 − 0.162
(2)
Y3 = 0.153X3 + 0.263X4 − 0.154
(3)
Table 6 Monthly cost, income and profit for a hypothetical swarf recovery unit (in US$) Item
Base casea
Alternative caseb
Cost 1. Surfactant + builder 2. Utilities + labor 3. Operating cost 4. Operating cost (equipment loan)
9569 3490 13059 2030
6698 3490 10188 2030
90
90
15179 0.379
12308 0.308
5. Unearned interest
6. Subtotal 7. Unit cost ($/kg) Income 8. Scrap iron 9. Cutting oil 10. Fuel oil 11. Avoided disposal cost
12. Subtotal 13. Unit income ($/kg) 14. Profit per month 15. Unit profit ($/kg) 16. Return of investment (month) a b
2918 8691 2286 2534
16429 0.411 1,250 0.031 36.0
3040 8301 385 1834
13560 0.339 1252 0.031 36.0
Swarf contains 50% cutting oil, 48% alloy and 2% other materials. Swarf contains 20% cutting oil, 50% alloy and 30% other materials.
Sum of items 1 and 2 70% loan ($ 150000 × 70%) with 6% discount interest rate 30% cash investment ($ 150000 × 30%); 2% interest rate Sum of items 3–5
29.4 and 70.3%, respectively 70.6 and 29.7%, respectively $ 70 per tonne; 36.2 t for base case and 26.2 t for alternatives case Sum of items 8–11
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Y2 , Y3 were the profits for the base and alternative cases, respectively; X3 the fraction of the recoverable metal sold as alloy, and X4 was the fraction of recoverable oil sold as cutting oil. The generators of swarf would recycle the recoverable oil back to manufacturing processes if the recoverable oil passes the field test, but they would have little use of the recoverable metal powders. The worst possible scenario would be that the recoverable metal is sold as the scrap iron. Eqs. (2) and (3) would then be reduced to the following: Y2 = 0.657X4 − 0.162
(4)
Y3 = 0.263X4 − 0.154
(5)
The investment would be economically feasible if more than 24.7 and 58.6% of the recoverable oils were recycled as cutting oils for the base and the alternative cases, respectively. The more the recoverable oil is recycled as cutting oil, the more the profit is. As shown in Table 6, the investment would be recovered in 36 months if 29.4 and 70.3% of the recoverable oils were recycled as cutting oils for both cases, respectively.
4. Conclusion The feasibility of an aqueous washing process for swarf recovery was studied. Bivariate regression analysis and surface response methodology were used to find optimal concentrations of nonionic surfactants including NPE-10 and Tergitols, and sodium metasilicate. The results obtained from bench scale experiments showed that 95% of oil in dry swarf could be removed. The residual phosphorous contents in washed samples were 0.02% by weight or less. The washed samples were acceptable for recycling in a smelter. The recovered oil had similar physical properties as the cutting oil currently used inline. A preliminary financial analysis for a swarf recovery unit with a capacity of 40 t per month was conducted. The economic feasibility is dependent upon the recyclability of recoverable oil as cutting oil. The capital investment would be recovered in 36 months if 29.4 and 70.3% of the recoverable oils were recycled as cutting oils for both cases, respectively. Further field test is required to find out the maximum allowable percentage of recycling. Alkylphenol polyethoxylates (APEs) are biodegradable, but they breakdown into less biodegradable products. These products are more toxic to aquatic organisms than the unbroken APEs. They also remain in the aquatic environment for some time. Their persistence, bioaccumulation and oestrogenic effects are clear (Warhurst, 1995). In the late 1980s, European countries began to ban the use of APEs, replacing them with linear alcohol ethoxylates that are readily biodegradable under both aerobic and anaerobic conditions, but the use of APE surfactants in the United States remains widespread. Switzerland has already banned the use of APE surfactants (Maki et al., 1994). Polyoxyethylene secondary alcohols such as Tergitols exhibit excellent oil removal and good biodegradation properties, but they are much more expensive than NPE-10. Detergency range polyoxyethylene primary alcohols are less expensive than secondary alcohols and have better biodegradation properties. Preliminary study showed that they also yielded satisfactory results. Future work will focus on searching for ideal surfactants among afford-
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able primary and secondary alcohol ethoxylates surfactants with low oil/water interfacial tension and on finding applications for the recoverable metal particles.
Acknowledgements The authors wish to express their gratitude to Prof. L.H. Chiu, department of mechanical engineering, National Kaohsiung First University of Science and Technology for suggesting the research project, to Dr. W.K. Yu of Tung Pei Industrial Company for providing swarf and technical information, and to National Science Council, ROC for financial support.
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