TREATMENT OR RECOVERY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Chinese Patent Application Serial No.

202110542747.X, filed May 18, 2021, which is incorporated herein by reference.

FIELD

[0002] This specification relates to water treatment, in particular treatment to remove a quaternary ammonium hydroxide such as tetramethylammonium hydroxide (TMAH) from water such as electronics manufacturing wastewater.

BACKGROUND

[0003] Tetramethylammonium hydroxide (TMAH) is a quaternary ammonium hydroxide with the molecular formula (CH ) NOH. In electronics industries, TMAH is widely used as a substrate etchant or as a developer to remove a photoresist. For example, TMAH may be use in making integrated circuits, flat panel displays, printed circuit boards, capacitors, sensors, silicon solar cells and many other electronic components. TMAH is often sold as a 25% solution. TMAH may be sold in either an industrial grade or an electronics grade depending on the purity of the solution. TMAH may be diluted for use. For example, in wafer fabrication a 2.38% TMAH solution is widely used as developer to dissolve photoresists, for example g-line, i-line and DUV photoresists.

[0004] TMAH is considered to be a hazardous material (HAZMAT class 8, corrosive materials). TMAH is corrosive to skin, eyes, and the respiratory tract. TMAH exhibits high acute toxicity on oral or dermal exposure. TMAH contaminated wastewater is harmful to humans and the environment, and also causes eutrophication in recipient water bodies. EU directive 2010/75/EU decrees a TMAH concentration limit of 7 ppm in discharged industrial wastewater. Hence, electronics manufacturing plants require a way to remove TMAH from wastewater. SUMMARY

[0005] This specification describes systems and methods for removing a quaternary ammonium hydroxide, such as tetramethylammonium hydroxide (TMAH), from water. Optionally, the quaternary ammonium hydroxide removed from the water can be re used in an electronics (i.e , integrated circuit, flat panel display, printed circuit board, capacitor, sensor, silicon solar cells etc.) or other manufacturing process. The systems and methods may be located onsite at an electronics manufacturing facility to treat wastewater produced at the facility. Alternatively, the systems and methods may be located in a remote facility that receives water transported from a manufacturing facility. In some examples, 80% or more or 90% or more of the TMAH in water may be removed from the water. In some examples, TMAH removed from the water can be re-used as an industrial grade or an electronics grade TMAH solution for further manufacturing.

[0006] In some examples, the system includes a membrane filtration unit, an ion exchange unit and an electrodialysis unit. The membrane filtration unit may include nanofiltration membranes. The electrodialysis unit may include a two-compartment bipolar electrodialysis unit, a three-compartment electrodialysis unit, or both. A source of wastewater containing a quaternary ammonium hydroxide is connected to an inlet of the membrane filtration unit. A permeate outlet of the membrane filtration unit is connected to an inlet of the ion exchange unit. A regeneration outlet of the ion exchange unit is connected to an inlet of the electrodialysis unit. In some examples, the regeneration outlet of the ion exchange unit is connected to the dilute chamber inlet of a three-compartment bipolar electrodialysis unit and the base compartment outlet of the three-compartment bipolar electrodialysis unit is connected to another electrodialysis unit.

[0007] In some examples, the process includes filtering wastewater to preferentially remove contaminants other than the quaternary ammonium hydroxide. For example, the filtration step may selectively remove cations and organic molecules with a molecular weight of more than about 300 Daltons (Da). The filtered water is treated through ion exchange to produce a regeneration stream with an increased concentration of quaternary ammonium cations. The regeneration stream is treated through electrodialysis to form a quaternary ammonium hydroxide solution. Optionally, the quaternary ammonium hydroxide solution is concentrated by electrodialysis. BRIEF DESCRIPTION OF THE FIGURES

[0008] Figure 1 is a schematic drawing of a quaternary ammonium hydroxide recovery system and process.

[0009] Figure 2 is a graph of molecular weight related rejections for various NF membranes.

[0010] Figure 3 is a schematic diagram showing the working mechanism of a three- compartment bipolar electrodialysis system (3C-BPED) for quaternary ammonium hydroxide production from a quaternary ammonium brine.

DETAILED DESCRIPTION

[0011] The systems and methods described in this specification are applicable to the removal of any quaternary ammonium hydroxide from any source of water. As an illustrative example, examples will be described below relating to the removal of tetramethylammonium hydroxide (TMAH) from electronics manufacturing wastewater. In some examples, the removed TMAH may be at a concentration suitable for re-use in electronics or other manufacturing. The systems and methods described below may optionally be provided at the electronics manufacturing facility. A person or ordinary skill in the art will be able to adapt the illustrative example below to the removal of other quaternary ammonium hydroxides from other sources of water.

[0012] Table 1 gives the concentration of TMAH and various contaminants in wastewater (WW) produced in an electronics manufacturing facility that produces electrical or photonic circuits on semiconductor wafers ("wafer fab"), and the target concentrations of purified water for reuse in the wafer fab. Table 2 gives the concentration of TMAH and various contaminants in wastewater (WW) produced in a liquid crystal display (LCD) manufacturing facility, and the target concentrations of purified water for reuse in LCD manufacturing. Table 3 gives the limits of contaminant concentration for TMAH solutions in industrial grade and electronics grade. Table 1 : Water chemistry of waste and purified TMAH solution in a wafer fab

Table 2 Water chemistry of waste/purified TMAH solution in LCD manufacturing Table 3: Standards for industrial and electronics grade TMAH solutions

[0013] Figure 1 shows a system 10 for treating water containing a quaternary ammonium hydroxide such as tetramethylammonium hydroxide (TMAH). Wastewater 12 is collected, for example from an electronics manufacturing facility, for example an integrated circuit, flat panel display, printed circuit board, capacitor, sensor, or silicon solar cell manufacturing plant. The wastewater 12 may contain, for example, 0.2-2.5 wt% TMAH. The wastewater 12 is likely to also contain other contaminants, for example metal ions, multivalent ions, and photoresist compounds. Contaminant ions include, for example, Na, Mg, Ca, Fe and Al ions. Photoresist compounds include, for example, g-line, i-line and DUV photoresists.

[0014] The wastewater 12 is treated in a nanofiltration unit 14. The nanofiltration unit

14 contains nanofiltration membranes, for example in spiral wound membrane modules. The nanofiltration unit 14 separates the wastewater 12 into a permeate 16 that passes through the membranes, and a concentrate 18 that is retained by the membranes.

[0015] The nanofiltration membranes selectively reject multivalent ions, for example

Mg ²⁺ , Ca ²⁺ , Fe ³⁺ , Al ³⁺ and SO4 ² ions. Most, for example 80% or more or 90% or more, of the multivalent ions in the wastewater 12 are retained in the concentrate 18. Most, for example 80% or more or 90% or more, of photoresist compounds are also rejected by the nanofiltration membranes and retained in the concentrate 18. For example, the molecular weight (MW) of i-line photoresist (C25H25O3) is 373 Da and the molecular weight of DUV photoresist is over 500 Da. Figure 2 shows the rejection (in percent) for five types of membranes (DK, DL, RL, HL and KH), which are types of nanofiltration membranes available from Suez Water Technologies & Solutions. As shown in Figure 2, each of these types of nanofiltration membranes has over 90% rejection of organic compounds with a molecular weight (MW) over 300 Da. Since the pH of the wastewater 12 is usually above 12, caustic tolerant nanofiltration membranes are preferred.

[0016] Most, for example 80% or more or 90% or more, of the TMAH that was present in wastewater 12 passes through to the permeate 16. Permeate 16 optionally contains less than 100 ppm of multivalent ions and only traces of photoresist compounds. Retentate 18 contains less than 20% or less than 10% of the TMAH that was present in wastewater 12 as well as concentrated multivalent ions and photoresists compounds. The retentate 18 is mixed with other discharged flows of water to create a combined waste stream 22. The combined waste stream 22 optionally has less than 0.05 wt% TMAH, as well as photoresist compounds, metal and multi-valent ions, chloride and sulfate ions. The combined waste stream 22 is sent to a discharge 20, which may be for example a sewer, receiving water body, holding tank, transportation tank, or further treatment process.

[0017] The permeate 16 is fed into an ion exchange unit 24. The ion exchange (IX) unit 24 has a proton type ion exchange unit resin bed. TMA ⁺ ions and metal ions are absorbed by the resin and accumulate in the resin bed as permeate flows through the ion exchange unit. An IX effluent 32 flows out of the ion exchange unit 24. The IX effluent 32 has a very low concentration of TMA ⁺ and metal ions but contains most of the chloride and sulfate ions that were present in the permeate 16, and most of the remaining photoresist compounds that were present in the permeate 16. The IX effluent 32 is mixed with the combined waste stream 22 and sent to the discharge 20.

[0018] Periodically, for example when the resin bed is nearly saturated, TMA ⁺ ions are washed out of the resin bed by regeneration acid solution 26, for example an HCI or

H2SO4 solution. An eluate 28 with a high concentration of one or more TMA salts (called

TMAX), for example TMACI or (TMA^SCU. The eluate 28 is fed to an electrodialysis unit 30.

In some examples, the eluate 28 may have a TMAX concentration of 10-20 wt%. A second eluate 29 is created during the regeneration of the IX bad after the TMAX has been collected in the eluate 28. The second eluate 29 contains metal ions that were absorbed in the resin. The addition of an ion exchange step greatly reduces the concentrations of many contaminants.

[0019] The electrodialysis unit 30 is used to generate a TMAH solution. In some examples, the purity of the TMAH solution is also further enhanced relative to the purity of the eluate 28. In the example shown, the electrodialysis unit 30 is a three-compartment bipolar electrodialysis (3C-BPED). Figure 3 shows the arrangement of cells and the influent and effluent streams of a 3C-BPED. Figure 3 shows only one complete cell (surrounded with a dashed line), and parts of some other cells, to simplify the figure, though many cells are typically provided in an electrodialysis unit. A series of cation exchange membranes 54, anion exchange membranes 52 and bipolar membranes 56 (each with a cation exchange membrane side 50 and anion exchange membrane side 48) are spaced apart between a cathode 46 and an anode 44. An additional cation exchange membrane 54 is provided next to the cathode 46. An additional anion exchange membrane 52 (not shown) is provided next to the anode 44.

[0020] Referring to Figures 1 and 3, either eluate 28 or deionized water 38 are fed to each compartment of the electrodialysis unit 30 and three product streams are produced: a base product stream 32, a dilute stream 40 and an acid product stream 42. Optionally, one or more of the base product stream 32, dilute stream 40 and acid product stream 42 can be recirculated to the inlet of the compartments that produced the stream. Inside the 3C-BPED, salt anions (X ^_ ) 62 move from the eluate 28 through anion exchange membranes 52 into the acid product stream 42. TMA cations (TMA ⁺ ) 64 move from the eluate 28 through cation exchange membranes 52 into the base product stream 32. Hydroxide anions (OH-) 60 are produced against the cathode 46 and in water within the bipolar membranes 56 and added to the base product stream 32. Hydrogen cations (H ⁺ ) 66 are produced against the anode 54 and water within the bipolar membranes 56 and added to the acid product stream 42.

[0021] The base product stream 32 contains TMAH, optionally at a concentration of

10-20 wt%. If the system 10 is located onsite at an electronics manufacturing facility, the base product stream 32 may be diluted for re-use within the same electronics manufacturing facility. Alternatively, the base product stream 32 may be concentrated further, for example in a second electrodialysis unit 34. The second dialysis unit is optionally a conventional electrodialysis unit or a two-compartment bipolar electrodialysis unit with bipolar membranes and cation exchange membranes. The second electrodialysis unit 34 produces a concentrated TMAH solution 36, for example a 20-25 wt% TMAH solution, or any other concentration that fits a customer requirement. The concentrated TMAH solution 36 is typically more cost effective when shipping and storage are required before the concentrated TMAH solution 36 can be reused. Even if the base product 32 or concentrated TMAH solution 36 do not meet the electronic grade standard they will be suitable for industrial grade reuse or may be upgraded to electronic grade with further processing. Optionally, a portion of the base product stream 32 may be recirculated to base product compartments of the electrodialysis unit 30 to increase the concentration of the base product stream 32.

[0022] The dilute stream 40 still contains TMAX, for example at 2-10 wt%. A portion of the dilute stream 40 is recycled to the eluate 28 inlet of the electrodialysis unit 30. In the example shown, the recycled portion of the dilute stream 40 is concentrated, for example through an optional third electrodialysis unit 58, to produce a second dilute stream 41 and a portion of the second dilute stream 41 is recirculated to the electrodialysis unit 30. The second dilute stream 41 may have a TMAX concentration, for example, of 10-20 wt%. Optionally, the dilute stream 40 is recirculated to the electrodialysis unit 30 without being concentrated, and the third electrodialysis unit 58 may be omitted or only used to concentrate a portion of dilute stream 40 that is sent to discharge 20. Whether the recirculated portion of the dilute stream 40 is concentrated before being fed to the electrodialysis unit can be based on the TMAX concentration in the mixture of the eluate 28 and the recycled portion of the dilute stream 40. For example, if the TMAX concentration in the mixture would be below 10 wt%, then the recycled portion of the dilute stream 40 can be concentrated as required to produce a mixture with a TMAX concentration of 10 wt% or more. A portion of the second dilute stream 41 (or optionally the dilute stream 40 if a third electrodialysis unit 58 is not used) is mixed with the combined waste stream 22 and sent to the discharge 20.

Recirculating the dilute stream 40 increases TMA recovery. In some examples, 90% or more of the TMA in the wastewater 12 is recovered for reuse.

[0023] The acid product stream 42 may be, for example, a HCI or H2SO4 solution with about 3-10 wt% acid. The acid product stream 42 can be reused as ion exchange resin regeneration acid 26. Alternatively, the acid product stream 42 can be sold as a product or reused for another onsite application. Optionally, a portion of the acid product stream 42 may be recirculated to acid product compartments of the electrodialysis unit 30 to increase the concentration of the acid product stream 42. [0024] In an alternative method, after the IX step, the concentrated TMAX can be converted to TMAH though an OH type ion exchange resin. However, this alternative method is will consume more chemicals and will generate a larger amount of wastewater. In another alternative method, nanofiltration is replaced with carbon adsorption or acid precipitation. However, this alternative method also requires more chemical inputs and/or create increased amounts of waste. Optionally, electrodialysis unit 30 may be replaced with any other one or more electrodialysis units capable of producing a base and/or hydroxide salt product. However, a three-compartment electrodialysis unit such as the 3C-BPED also produces an acid product that can be used to regenerate the ion exchange resin bed.