CA2531169C

CA2531169C - Spent metalworking fluid treatment system

Info

Publication number: CA2531169C
Application number: CA002531169A
Authority: CA
Inventors: Alok Goel; Aviral Goel; Victor Zhang
Original assignee: OMTEC Inc
Current assignee: OMTEC Inc
Priority date: 2005-12-28
Filing date: 2005-12-28
Publication date: 2009-10-20
Anticipated expiration: 2025-12-28
Also published as: CA2531169A1

Abstract

A system and a process is provided for treating spent machining to separate waste oil and biochemical oxygen demand materials and chemical oxygen demand materials from the coolant to produce treated high quality soft water. The process comprises the steps of de-emulsifying emulsified oil in the coolant and separating the de- emulsified oil from the coolant to create substantially de-oiled coolant; mineralizing biochemical oxygen demand (BOD) materials and chemical oxygen demand (COD) materials in the substantially de-oiled coolant to create mineralized de-oiled coolant; and, separating the mineralized BOD materials and the COD materials from the mineralized de-oiled coolant to create treated water.

Description

Field of the Invention This invention relates to a cost-effective and environmental-friendly process for treating spent metalworking fluids. The treatment provides means of breaking emulsions;
separation of oil and wastewater; mineralization of BOD/COD materials including emulsifiers, dyes, and other functional organic additives; and removal of mineralized organic materials and heavy metals. With the process, waste mineral oil in the coolants is recovered and the treated water is high quality soft water, with low BOD/COD
content, free of colors and heavy metals. The water can be reused in preparing new metalworking fluids or discharged into the sewer system.

Background of the Invention Metalworking fluids including straight oils, soluble oils, synthesized oils, and semi-synthesized oils are widely used in metal processing to lubricate and cool cutting tools and metal pieces. Besides water (ca. 90%), spent metalworking fluids (SMF) are composed of mineral oil, emulsifiers, dyes, biocides, anticorrosion, antifoam, and other functional additives as well as dissolved metals and metal chips produced during metal processing. SMF cannot be discharged into the sewer due to its high oil content (1-10%
v/v), high BOD/COD content (> 10,000 mg/L), colors, etc. For middle- and small-size metalwork shops, SMF are generally stored in drums and hauled away for off-site treatment. However, the off-site treatment is expensive since over 90% of SMF
composition is water. Common on-site treatment options are evaporation and biological treatment. Evaporating water in SMF to reduce the volume of waste for disposal is high energy-consuming and thus expensive. The vast land occupation for biological processes inhibits their application in small- and medium-size metalworking shops. Ultra-filtration (UF) membrane has been recently used for treating oily wastewaters including SMF.
However, the membrane has been found be quickly fouled in treating SMF (See Belkacem et al., Journal of membrane science, 106 (1995): 195-205). In addition, many dissolved organic components in SMF cannot be retained onto the UF membrane and I

thus the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of the permeate is greater than the BOD and COD limits for sewer discharge (See Belkacem et al., Journal of membrane science, 106 (1995): 195-205).

A need exists for a cost-effective process for treating SMF, especially for small- and middle-size metalworking shops. For SMF in stable oil-in-water emulsions, de-emulsification is necessary for satisfactory oil/water separation. Coagulants and flocculants are used for achieving de-emulsification. Oil droplets coalesce into an oil layer, which can be recovered by subsequent skimming. After oil-water separation, in addition to some amount of residual mineral oils, water-soluble additives including dyes, biocides, and other BOD/COD forming materials exist in the wastewater. Under stringent environmental protection laws and regulations, the wastewater cannot be discharged into the sewer. Hydrogen peroxide catalyzed with iron catalyst has been found to effectively mineralize water-soluble organic contaminates in various wastewaters to carbon dioxide, organic acids, and water (See Gogate and Pandit, Advances in Environmental Research, 8 (2004): 501-551). Heavy metals dissolved in SMF can cause serious environmental problems by causing toxicity in aquatic animals.
Ion exchange is used to in this invention to remove mineralized organic acids and heavy metals. High quality soft water is produced from SMF and waste oil is recovered as fuel for incineration.

Summary of the Invention The present invention relates to methods for treating SMF by removing mineral oils, water-soluble additives, and heavy metals. As shown in FIG. 1, the system consists of five functional units: emulsion breaking, oil/water separation, BOD/COD
material mineralization, iron catalyst recovery, and mineralized material removal. With the developed process, mineral oils in SMF is recovered and used as fuels in industrial furnaces and the treated water is reused in preparing new metalworking fluids or discharged directly into the sewer. The advantages of the designed system compared with conventional processes are as follows:

1. The volume for off-site disposal decreases up to 90-95%, which saves a considerable amount money on waste disposal;

2. All water-soluble additives including emulsifiers, dyes, biocides, etc. in SMF are decomposed to carbon dioxide and water or mineralized into ions that can be removed using ion exchange columns;

3. Space requirement is much less than that is required for biological processes;

4. The recovered mineral oils can be sold as fuels for industrial furnaces;

5. The produced water is high quality soft water that can be discharged directly into the sewer without adverse environmental effects or `zero-wastewater discharge' can be achieved by reusing the water in preparing new cutting coolants;

6. The catalyst used in mineralizing BOD/COD materials can be easily recovered and reused;

7. SMF treatment with the developed technology is energy-saving and environmentally friendly with low capital investment.

In an embodiment a process is provided for separating waste mineral oil and treating and recycling wastewater from spent machining coolants comprises the steps of:
de-emulsifying the emulsified oil droplets by adding a coagulant of multivalent salt including aluminum (A13+), iron (Fe3-+-) , and calcium (Ca2+); flocculating the de-emulsified oil droplets by doping flocculants; separating and collecting waste mineral oil from spent machining coolants using a bag filter; mineralizing BOD/COD
materials in the separated wastewater using hydrogen peroxide catalyzed by Fenton's reagents; separating and recovering iron from the treated wastewater using a bag filter;
and, passing the mineralized wastewater through ion exchange columns for production of high-quality soft water.

In an embodiment of the process the Fenton's reagent comprises an iron catalyst that is precipitated by adjusting pH 9.5-10.5 and collected by passing the separated mineralized water through a bag filter.

In an embodiment the iron catalyst may be re-used by dissolving the collected iron precipitate in further after adjusting its pH to about 3.0-4.5. In an embodiment of the process the mineralized BOD/COD materials are removed by passing the mineralized waste water through any combination of ion exchange columns.

In an embodiment a process of treating spent metalworking fluids is comprised of the steps of emulsion breaking, oil/water separation, BOD/COD material mineralization, and mineralized contaminant removal. Multivalent salts are used to coagulate emulsified oil droplets in the coolant followed by flocculating the oil droplets into flocs, which are separated with a bag filter from wastewater. The dissolved BOD;COD materials including emulsifiers, dyes, biocides, and other organic additives in the machining coolant are mineralized using hydrogen peroxide with a catalyst of ferrous salt at pH 3.5-5Ø At the end of the reaction, iron catalyst is precipitated by adjusting pH 9.0-10.0 and recovered by passing the content through a bag filter.
Separated from the precipitated catalyst, the wastewater is then passed through a series of ion exchange columns for the removal of the mineralized organic contaminants, dyes, and heavy metals.

3a The emulsified mineral oil in spent metalworking fluids is recovered and the effluent from the process is high quality soft water with a low conductivity and free of oil, colors, and heavy metals. The water can be discharged into the municipal sewer system due to its low BOD and COD content. More importantly, by recycling the water for preparing new cutting coolants, 'zero-wastewater discharge' can be achieved in metal workshops.

3b Detailed Descriptlon of the Invention The present invention relates to methods for rqnoving mineral oils, water-soluble additives, and heavy metals in SMF. After the treatment, clean water can be recovered and reused in preparing new cutting fluids or discharged into the sew system.

EneulsioK Breaking and OiUivQta Separatton Referring to FIG. 1, the collected SMF in a drum 10 is first pumped into the emulsion-breaking tank 20, where coagulants are added as 21. Aluminum Al sup 3+; ferric Fe sup 3+; or calcium Ca Sup 2+ can be used as the coagulant with a charge of I-5%.
The content is thoroughly mixed for 10 min. The anulsron is broken and a subsequent dense oil layer forms on the top of the tank 20. The oil layer is then skimmed off from the tank 20 and collected in the waste oil tank 40. A flocculant shown as 22 is added to the oil-skimmed westcwaGer at a chacge of 0.1-194. ASer thorough mixing, the content stands for 20 min and the flocculated particles are filtered out through a bag filter 30. The mineral oil content of the filtrate from the bag filter is lowered to 40-100 mg/L. The filtered oil particles are transferred into waste oil tank 40. The 5-day biochemical oxygen demand (BODS) of treated water is around 5,000-10,000 mg/L, which is higher than 300 mg/L, the sew discharge limit of most North American cities. The water needs further treatment to be discharged or reused.

BODi1COD Materral MineralitarHon For further removal of BOD/COD materials, the effluent from the bag filters is transferred to a reactor 50. As shown in FIG. 1, a Fenton's reagent, fearous sulfate 51, Fe S 0 sub 4 as 51 is added into the wastewater and the pH value of the wastewater in the reactor is first adjusted to 3.5-5.0 with concentrated sulfuric acid 52.
Hydrogen peroxide 53, H sub 2 O sub 2 of 5-50 g/L is then added into the wastewater to initiate the reaction.
The amount of H202 is dependent on the properties of the water for treatment such as BOD and COD. The ratio of added FeSO4 to H2th is 1:5-1:20.

q The reaction proceeds for 30 min and the temperature is controlled in the range of 25-45 C. The reaction is terminated by adjusting pH 9-10. Ion catalyst is precipitated and collected by passing the content through a bag filter 60. The recovered catalyst can be reused in another treatment cycle as in 61. The BOD value of the filtrate from 60 is in the range of 1,000-2,000 mg/L.

Mineralized Material Removal The mineralized BOD/COD materials and heavy metals in the wastewater are further removed by passing through two ion exchange columns 70 and 80 in series. The effluent from the bag filter 60 is passed through two BOD/COD removal columns as 71 and 81.
The mineralized contaminants and heavy metals in the wastewater are exchanged onto the columns. Thereby, the BOD value of the produced water can be in the range of 50-200 mg/L. After adjusting the pH value of the produced water to 6-8, the conductivity of the water is 50-100 S/cm. The water thus can be reused in preparing new cutting coolant or discharged into the sewer. The adsorbed contaminants can be removed from the columns by passing a small amount of acid and base solutions through the columns as 72 and 82. The columns are thus recharged. The washed-off waste is collected in a waste tank 90 for disposal. The recharge of the columns is monitored by measuring the conductivity value of the effluent.

In one example of the process of this invention, spent COMMCOOL MAX coolant, a stable milky white emulsion is treated. The characteristics of the coolant are listed on Table 1.

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Table 1. The characteristics of spent COMMCOOL MAX and TRIM E 206 coolants Oil content COD
SMF Color pH
(% v/v) (mg/L) COMMCOOL

8 125,000 Milky white 8.6 MAX

TRIM E206 10 186,000 Milky blue 8.8 The coolant is first added a said amount of multivalent coagulant to break the emulsion and mixed for 10 min. A said amount of flocculant is then dosed and the content stands for 20 min. The oil and flocs formed are filtered through a bag filter. The filtrate has a tint of yellow. The oil content and the COD value of the filtrate are in the range of 40-100 and 20,000-30,000 mg/L, respectively. The water-soluble COD materials are mineralized with a said amount of hydrogen peroxide and ferrous sulfate at pH
3.5-5.0 for 40 min. The iron catalyst is recovered by adjusting pH 9.5-10.0 and passing the content through a bag filter. Mineral oil is not identified in the filtrate and the COD value of the filtrate is in the range of 7,000-9,000 mg/L. The water is then passed through two ion exchange columns. The filtrate is transparent without any color. After being adjusted pH 6-8, the BOD and COD values of the treated effluent are 60-200 and 400-700 mg/L.
The conductivity of the water is in the range of 50-100 S/cm. The water can be reused in metalworking shops for preparing new coolant or directly discharged into the sewer.
The iron catalyst is dissolved with oil-separated SMF after adjusting pH 3.5-5.0 and used as catalyst in a new batch for hydrogen peroxide reaction. The results of the treatment using the recovered catalyst are as good as the fresh one.

In one example of the process of this invention, spent TRIM E206 coolant, a stable milky blue emulsion is treated. The characteristics of the coolant are listed on Table 1.

The coolant is first added a said amount of multivalent coagulant to break the emulsion and mixed for 10 min. A said amount of flocculant is then dosed and the content stands for 20 min. The oil and flocs formed are filtered through a bag filter. The filtrate is blue.
The oil content and the COD value of the filtrate are in the range of 40-100 and 20,000-30,000 mg/L, respectively. The water-soluble COD materials are mineralized with a said amount of hydrogen peroxide and ferrous sulfate at pH 3.5-5.0 for 40 min. The iron catalyst is recovered by adjusting pH 9.5-10.0 and passing the content through a bag filter. Mineral oil is not identified in the filtrate and the COD value of the filtrate is in the range of 7,000-9,000 mg/L. The water is then passed through two ion exchange columns. The filtrate is transparent without any colors. After being adjusted pH 6-8, the BOD and COD values of the treated effluent are 60-200 and 400-700 mg/L. The conductivity of the water is in the range of 50-100 S/cm. The water can be reused in metalworking shops for preparing new coolant or directly discharged into the sewer.

The iron catalyst is dissolved with oil-separated SMF after adjusting pH 3.5-5.0 and used as catalyst in a new batch for hydrogen peroxide reaction. The results of the treatment using the recovered catalyst are as good as the fresh one.

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Claims

WHAT IS CLAIMED IS:

1. A process for treating spent machining coolant comprising the steps of:
de-emulsifying emulsified oil in the coolant and separating the de-emulsified oil from the coolant to create substantially de-oiled coolant;
mineralizing biochemical oxygen demand (BOD) materials and chemical oxygen demand (COD) materials in the substantially de-oiled coolant to create mineralized de-oiled coolant;
separating the mineralized BOD materials and the COD materials from the mineralized de-oiled coolant to create treated water.

2. The process of claim 1 wherein the step of de-emulsifying emulsified oil comprises adding a coagulant to the coolant.

3. The process of claim 2 wherein the coagulant comprises a multivalent salt.

4. The process of claim 3 wherein the multivalent salt is selected from salts of aluminum (Al3+), iron (Fe3+) or calcium (Ca2+).

5. The process of claim 4 wherein the multivalent salt is added to a concentration of 1-5%.

6. The process of claim any of claims 1 to 5 wherein the step of separating the de- emulsified oil comprises skimming an oil layer from the coolant.

7. The process of any of claims 1 to 6 wherein the step of de-emulsifying emulsified oil further comprises adding a flocculant.

8. The process of claim 6 wherein the step of separating the de-emulsified oil further comprises filtering flocculated oil from the coolant.

9. The process of claim 8 wherein the flocculated oil is filtered using a bag filter.

10. The process of claim 1 wherein the step of mineralizing comprises:
adding a Fenton's reagent to the substantially de-oiled coolant, adjusting the pH to about 3.5-5.0, and then adding hydrogen peroxide.

11. The process of claim 10 wherein the step of mineralizing is terminated by adjusting the pH to about 9-10.5 to precipitate the Fenton's reagent.

12. The process of claim 11 wherein the Fenton's reagent is collected after the step is terminated by filtering with a bag filter,

13. The process of claim 12 wherein the collected Fenton's reagent is reused in a subsequent treatment process.

14 The process of claim 10 wherein the Fenton's reagent comprises ferrous sulphate.

15. The process of claim 14 wherein the ratio of ferrous sulphate to hydrogen peroxide is in the range of about 1:5 to 1:20.

16 The process of claim 1 wherein the step of separating the mineralized BOD
materials and the COD materials from the mineralized de-oiled coolant comprises:
passing the mineralized de-oiled coolant through at least two ion exchange columns in series.

17. A method for separating high quality soft water and waste oil from spent machining coolant comprising the steps of:
transferring the spent machining coolant to an emulsion-breaking tank and adding coagulants to the emulsion-breaking tank;
mixing the spent machining coolant and the added coagulants in the emulsion-breaking tank;

allowing time for an oil layer to form above the coolant;
skimming off the oil layer and transferring the skimmed oil layer to a waste oil tank;
adding a flocculant to the emulsion-breaking tank;
mixing the flocculant with coolant in the emulsion-breaking tank;
allowing time for flocculated oil particles to form in the mixture;
filtering the flocculated oil particles from the mixture and transferring the flocculated oil particles to the waste oil tank to create a substantially de-oiled coolant in the emulsion-breaking tank;
transferring the substantially de-oiled coolant to a reactor to create a mineralized substantially de-oiled coolant;
adding a Fenton's reagent to the reactor and adding an acid to the reactor to lower the pH to about 3.5-5.0;
adding hydrogen peroxide to the reactor;
allowing time for a reaction to proceed;
terminating the reaction by adding a base to the reactor to raise the pH to about 9-10.5 and precipitate the Fenton's reagent;
filtering the precipitated Fenton's reagent from the reactor; and, passing the mineralized substantially de-oiled coolant through a series of ion exchange columns to remove mineralized contaminants and heavy metals to produce high quality soft water.

18. The method of claim 17 wherein the Fenton's reagent comprises ferrous sulphate.

19. The method of claim 17 wherein the coagulant comprises a multivalent salt of aluminum, iron or calcium.

20. The method of claim 17 wherein the reaction proceeds at a controlled temperature in the range of 25-45°C.

21. The method of claim 17 further comprising the step of adjusting the pH
level of the high quality soft water to about 6-8 to allow for discharge in a public sewer.

22. The method of claim 17 further comprising the step of adjusting the pH
level of the high quality soft water to about 6-8 to allow for re-use as a cutting coolant.

23 A metalworking fluid comprising the high quality soft water produced by the method of claim 19, combined with at least mineral oil.

24. An oil treatment system for treating spent machining coolant comprising:
an oil-water separation tank for separating waste oil from the spent machining coolant to produce substantially de-oiled coolant;
at least one bag filter for removing emulsified oil or flocculated oil in the oil-water separation tank and transferral to a waste oil tank;
a mineralization reactor for mineralizing contaminants in the substantially de-oiled coolant to produce mineralized substantially de-oiled coolant;
at least one bag filter for collecting a Fenton's reagent used in the mineralization reactor for re-use; and, at least two ion exchange columns arranged in series to separate mineralized contaminants from the mineralized substantially de-oiled coolant to produce high quality soft water.