Electrochemical oxidation of amine complexing agents in waste streams from electroplating processes

Abstract

A method of treating a waste stream comprising an organic amine compound complexed with heavy metal ions. The process comprises the steps of: (1) adjusting the pH of the waste stream to between about 4 and about 10; (2) b) adding a chloride salt to the waste stream to produce a concentration of chloride ions in the waste stream; and (3) circulating the waste stream through the electrochemical reactor. The electrochemical reactor includes an electrode array including alternating anodes and cathodes. The waste stream is recycled through the electrochemical reactor for a sufficient period of time to hydrolyze amine compounds in the waste stream.

Description

Technical Field

[0001] The present invention generally relates to the treatment of waste generated in electroplating processes. Background Art

[0002] In the metal polishing industry, plating baths for electrolytic and electroless plating of metals and metal alloys typically contain metal ions of the desired metal to be plated along with various complexing agents (i.e., chelating agents) and other bath ingredients. Depending on the metal to be plated, these complexing agents may include, for example, ethylenediaminetetraacetic acid (EDTA), aminopolycarboxylic acids such as nitriloacetic acid (NTA), cyanides, polyamines, and other complexing agents capable of creating strong bonds.

[0003] While the use of such complexing agents helps produce the desired plating deposits on substrates, the disposal of these compounds and their byproducts can be problematic and expensive, and in some cases can inhibit the ability of wastewater treatment plants to meet required discharge limits.

[0004] Examples of industrial wastewater include various waste electroless plating baths and electroplating baths present in the wastewater alone or in combination, including, for example, cyanide copper plating wastewater, pyrophosphate copper plating wastewater, acidic copper plating wastewater, bright nickel plating wastewater, potassium chloride zinc plating wastewater, alkaline non-cyanide zinc plating wastewater, cyanide-initiated gold copper-zinc alloy electroplating wastewater, gun-colored tin-nickel alloy electroplating wastewater, alkaline zinc-nickel alloy electroplating wastewater, acidic zinc-nickel alloy electroplating wastewater, hexavalent chromium electroplating wastewater, trivalent chromium electroplating wastewater, electroless nickel plating wastewater, electroless copper plating wastewater, hexavalent chromium passivation wastewater, trivalent chromium passivation wastewater, degreasing wastewater, pickling wastewater, etc. Industrial wastewater usually contains complexing agents (i.e. sodium cyanide, potassium pyrophosphate, sodium citrate, potassium sodium tartrate, sodium malate, diethylenetriamine, hydroxyl-containing organic amines, etc.), heavy metal ions (i.e. copper, nickel, zinc, cobalt, hexavalent chromium, trivalent chromium, etc.) and other pollutants (i.e. sodium phosphate, sodium hypophosphite, brighteners, auxiliary brighteners, surfactants, etc.).

[0005] Depending on the nature of the industrial wastewater produced and the degree of metal removal required, various wastewater treatment options may be used. However, if the industrial wastewater contains strong complexing agents such as EDTA and / or polyamines, conventional treatment options (including precipitation of metal ions) may not be sufficient.

[0006] Examples of conventional options for treating industrial wastewater include, but are not limited to, (1) precipitation of metal hydroxides; (2) use of iron reagents; and (3) use of dimethyldithiocarbamates.

[0007] Hydroxide precipitation of metals is widely used due to its simplicity of use and relatively cheap cost. However, hydroxide precipitation is generally ineffective in breaking down the complex bonds produced by various complexing agents. That is, sodium hydroxide will react with any non-chelated heavy metals present to produce insoluble metal hydroxides. However, coprecipitation with other metals as cations can produce lower solubility. In addition, some complexing agents are specifically designed to prevent hydroxide precipitation from occurring under normal conditions.

[0008] Iron can be used to decompose many complexing agent bonds in the presence of complexing agents. A sufficient amount of iron reagent is introduced into the wastewater to reduce most metals and destroy the complexing agent bonds. The iron reagent is added to the solution in the first reaction tank at a pH of about 2.0 to 3.0, and then the pH of the solution is raised to between 8.5 and 11 in the second reaction tank, where the metal (including iron) is precipitated as a metal hydroxide. One of the major disadvantages of iron reagent chemistry is that the reagent is added on a volume basis, so the dosage must be predetermined and is independent of the actual concentration of the metal in the feed. Therefore, when there is a wide fluctuation in the feed metal concentration, the iron reagent method may be very difficult to optimize and control.

[0009] Dimethyl dithiocarbamate (DTC) can be used in the presence of most, if not all, chelating agents. DTC reacts with metals to form insoluble metal / DTC compounds, which will precipitate as sludge. The concentration of the chelating material has a significant effect on the dosage rate. Therefore, a significant disadvantage of the DTC process is that it cannot be easily and safely adjusted to handle fluctuations in feed metal concentrations. However, the DTC process produces less sludge than the corresponding iron reagent process.

[0010] Although these treatment options have been successful in treating industrial wastewaters (including electroless and electroplating wastewaters), these treatment options have difficulties in treating industrial wastewaters containing complexing agents such as polyamines in an efficient and cost-effective manner. There is still a need in the art for treatment methods for treating industrial wastewaters that can decompose complexing agents, including amine-based complexing agents. In addition, there is still a need in the art for improved treatment methods for treating alkaline zinc-nickel electroplating waste streams and electroless copper plating waste streams containing amine-complexed metal ions for subsequent treatment. It is also desirable to develop a method for treating electroplating wastewater containing complexing agents such as EDTA and amine-based complexing agents complexed with heavy metal ions, and the method can be used in combination with conventional methods for precipitating heavy metal ions and heavy metal ion mixtures.

[0011] For many years, the automotive industry has relied on the production of fasteners and other manufactured steel parts coated with zinc and nickel alloys by electroplating. Typically, the alloy is primarily zinc with a nickel content between 12% and 15%. These alloys have good corrosion resistance, and the corrosion products are dense and do not significantly detract from the aesthetics of the finished product. Zinc-nickel alloys also exhibit favorable contact corrosion properties when in contact with aluminum. Due to the popularity of long-term corrosion warranties on modern automobiles, the use of zinc-nickel plated parts has become essential.

[0012] There are two main methods commonly used to deposit zinc-nickel alloys, depending in part on the type of component being plated.

[0013] For cast iron parts such as brake cylinders, methods close to neutral pH are used. These methods typically have a pH of about 5 and are referred to as "acidic zinc nickel" solutions. In order to buffer the solution pH at the electrode / electrolyte interface and prevent the co-deposition of metal hydroxides due to the increase in pH at the cathode during electrolysis (due to the reduction of hydrogen ions), ammonium ions are used in the electrolyte. Ammonium ions become a problem in waste treatment because when the pH of the waste stream is increased in order to precipitate metal ions in the form of metal hydroxides from the solution, the ammonium ions lose protons and become ammonia. This can act as a ligand, making the nickel ions soluble, thereby preventing precipitation.

[0014] For other parts, including fasteners and other manufactured steel parts, the alkaline zinc-nickel process is often used. This is because the alkaline process is easier to operate and produces a more uniform alloy composition over a wide current density range compared to the typical acidic process. However, unlike the acidic process, which relies on the use of ammonium ions as a pH buffer, the use of an organic amine is necessary in the alkaline zinc-nickel process to make the nickel ions soluble. Ammonia is not suitable for this purpose because it is volatile at the operating pH and does not produce a complex with the kinetic and thermodynamic properties required for the process.

[0015] A typical alkaline zinc-nickel electroplating bath contains a combination of metal ions and an amine-based complexing agent, as well as other bath ingredients added to obtain the desired plating characteristics. Various alkaline zinc-nickel plating electrolytes are known in the art and are described, for example, in U.S. Patent Publication No. 2008 / 0223726 to Eckles et al., U.S. Patent Publication No. 2006 / 0201820 to Opaskar et al., U.S. Patent No. 7,442,286 to Capper et al., U.S. Patent Publication No. 2010 / 0096274 to Rowan et al., and U.S. Patent Publication No. 2020 / 0263314 to Niikura et al., the subject matter of each of which is incorporated herein by reference in its entirety.

[0016] Typical alkaline zinc-nickel electroplating baths contain nickel ions from about 0.5 g / L to about 50 g / L, zinc ions from about 0.1 g / L to about 100 g / L, and complexing agents from about 5 g / L to about 100 g / L, which are typically amine-based complexing agents. In addition, electroplating baths typically also include sufficient amounts of inorganic alkaline components to provide a bath with a desired pH. For example, the inorganic alkaline component can be present in a concentration of about 50 g / L to about 220 g / L to provide a bath with a pH of at least about 10, or at least about 11, or at least about 14. The inorganic alkaline component is typically an alkali metal derivative, such as sodium hydroxide or potassium hydroxide, but may alternatively include sodium carbonate or potassium carbonate or sodium bicarbonate or potassium bicarbonate. A mixture of inorganic alkaline components may also be used.

[0017] Examples of amine complexing agents that can be used in alkaline zinc-nickel electroplating baths include alkyleneamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine; alkylene oxide adducts of the above alkyleneamines such as ethylene oxide adducts and propylene oxide adducts; amino alcohols such as ethanolamine, diethanolamine, triethanolamine, diisopropanolamine, triisopropanolamine, ethylenediaminetetra-2-propanol, N-(2-aminoethyl)ethanolamine, and 2-hydroxyethylaminopropylamine; alkanolamine compounds such as N-(2-hydroxyethyl)-N,N',N'-triethylethylenediamine, N,N'-di(2-hydroxyethyl)-N,N'-diethylethylenediamine, N,N,N',N'-tetra(2-hydroxyethyl)propylenediamine, and N,N,N',N'-tetra(2-hydroxypropyl)ethylenediamine (trade name The complexing agent may include one or more selected from the group consisting of alkyleneamine compounds, alkylene oxide adducts thereof, and alkanolamine compounds. These amine complexing agents may be used alone or in combination of two or more. In the case of an electroless copper plating bath, EDTA is a common complexing agent.

[0018] Amine complexing agents are often not effectively removed from wastewater using conventional oxidation processes, resulting in difficulty in treating electroplating wastewater. A typical alkaline zinc-nickel alloy electroplating bath may contain about 3 wt% amine complexing agents, which have high stability and cannot be effectively decomposed by typical wastewater treatment methods.

[0019] WO2019 / 085128 granted to Guangzhou Ultra Union Chemicals Ltd. discloses a method for treating alkaline zinc-nickel alloy electroplating wastewater, the method including adjusting the pH of the alkaline zinc-nickel alloy electroplating wastewater and adding various reagents including sodium diethyldithiocarbamate, flocculants and sodium hypochlorite. Similarly, US2021 / 0380455 granted to Guo et al. (the subject matter of the patent is incorporated herein by reference in its entirety) describes a method for treating mixed electroplating wastewater containing no cyanide and no cyanide-containing phosphorus reducing agent, and wherein biodegradation technology is used to destroy aliphatic polyamine complexing agents to reduce chemical oxygen demand (COD). However, and in addition to requiring multiple additional steps in the method for treating wastewater so that it is fully free of pollutants before discharge, biodegradation technology generally requires 8 to 24 hours to reach an acceptable COD level.

[0020] Other treatment methods, such as the method described in CN103641207A, rely on the electrolysis of zinc-containing electroplating wastewater using a composite electrolytic cell.

[0021] Because nickel anodes cannot be used in alkaline zinc-nickel electroplating processes, all of the nickel that is plated is added in the form of a nickel salt, such as nickel sulfate or nickel chloride. However, this salt cannot be simply added to the electroplating bath because it is insoluble in an alkaline environment. Instead, it must be pre-complexed in advance using one or more of the above-mentioned amine complexing agents. Therefore, in use, the electroplating bath will accumulate both in terms of the amine complexing agent and the salt, namely sodium sulfate or sodium chloride or potassium sulfate or potassium chloride (depending on the process recipe). This usually has to be controlled by periodically removing a portion of the electroplating bath or by using a "feed and bleed" system, which generates a large amount of waste in addition to the usual "drag-out" losses. Because alkaline zinc-nickel electroplating systems are widely used, the problem of waste disposal is significant.

[0022] As mentioned above, the waste from these alkaline zinc nickel electroplating processes cannot be effectively treated and must be taken to incineration, which is expensive and wastes energy.

[0023] Some electroplating companies attempt to remove nickel from waste by precipitating nickel using dimethylglyoxime, as described, for example, in U.S. Pat. No. 4,500,324 to Vuong, the subject matter of which is incorporated herein by reference in its entirety. This produces an insoluble nickel complex that can be filtered out. However, the process still leaves the amine complexing agent in solution, so the waste solution cannot be mixed with other waste streams and in many cases cannot be safely discharged into the environment due to the toxicity of the amine compounds to fish and other aquatic species. In some cases, large amounts of sodium hypochlorite solution may be added to react with the amine groups in the complexing agent present in the waste stream. However, a large excess of sodium hypochlorite is required, which leads to further waste disposal problems and the formation of large amounts of sludge.

[0024] Organic amines are also used as complexing agents in other processes in the metal polishing industry. For example, the printed circuit industry uses complexing agents containing organic amines (e.g. and EDTA) electroless copper plating process.

[0025] It would therefore be desirable to provide a method for removing amine complexing agents from industrial process wastewater that overcomes the deficiencies of the prior art. Summary of the invention

[0026] In one embodiment, the present invention describes a method for removing organic amine complexing agents from industrial electroplating waste using a chloride-mediated electrochemical oxidation process. The method uses an anode electrode that is insoluble in the wastewater under an applied potential and has a high oxygen overpotential and a low chlorine overpotential.

[0027] Following electrolytic treatment to remove the organic amine complexing agent, the wastewater may be further treated to remove metals such as zinc and nickel from the wastewater stream. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Depicted is a graph showing comparative rates of amine decomposition for examples performed in accordance with the present invention. DETAILED DESCRIPTION

[0029] As used herein, "a," "an," and "the" refer to both the singular and the plural, unless the context clearly indicates otherwise.

[0030] As used herein, the term "about" refers to a measurable value, such as a parameter, amount, duration, etc., and is intended to include variations of + / -15% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less relative to the specifically stated value, as long as such variations are suitable for implementation herein. In addition, it should also be understood that the value referred to by the modifier "about" itself is specifically disclosed herein.

[0031] As used herein, for ease of description, spatially relative terms such as "below," "beneath," "lower," "above," "upper," etc. are used to describe the relationship of one element or feature structure to another or more elements or features, as shown in the figures. It should also be understood that the terms "front" and "rear" are not intended to be limiting and are intended to be interchangeable where appropriate.

[0032] As used herein, the terms “includes and / or comprises” specify the presence of stated features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0033] As used herein, if not otherwise defined herein for a particular element or compound, the term "substantially free" or "substantially free" means that the given element or compound cannot be detected by ordinary analytical means of bath analysis well known to those skilled in the art of metal plating. Such methods typically include atomic absorption spectroscopy, titration, UV-visible analysis, secondary ion mass spectrometry, and other commonly used analytical methods.

[0034] The terms "plating" and "electroplating" are used interchangeably throughout this specification.

[0035] As used herein, the term "immediately" means without intermediate steps.

[0036] The present invention generally relates to at least substantially removing strong complexing agents, such as EDTA and amine-based complexing agents, from industrial process wastes, including, for example, spent alkaline zinc nickel electroplating baths and spent electroless copper plating baths, using electrochemical oxidation. The methods described herein can also substantially remove any cyanide present in the electroplating baths, either as an additive or as a byproduct.

[0037] The oxidation process involves removing electrons from the reactants being oxidized. This can theoretically be achieved by electrochemical oxidation, where an electrical source is used to remove electrons directly from molecules in contact with the anode (balancing the charge by donating electrons to other molecules in contact with the cathode). This approach is advantageous for waste treatment because it does not add new molecules to the waste stream (compared to chemical oxidation) and can utilize electricity from renewable sources. Therefore, the method described in the present invention has the potential to operate in an environmentally acceptable manner.

[0038] The present inventors have found that the use of electrochemical oxidation is complicated and that many difficulties must be overcome in order to successfully use electrochemical oxidation to treat strong complexing agents in waste streams. These difficulties can be summarized as follows:

[0039] 1) Because the reaction occurs at the electrode surface (i.e., it is a heterogeneous reaction), the oxidation rate is limited by the rate of mass transfer of the molecules to be oxidized to the electrode surface.

[0040] 2) The electrodes must be composed of a material with sufficient affinity for the molecule to be oxidized to allow specific adsorption of the molecule so that an electron transfer reaction can occur (via the mechanism of quantum mechanical tunneling).

[0041] 3) The material constituting the electrode must not be oxidized at the electrode potential used in the electrochemical oxidation process.

[0042] 4) The molecule to be oxidized must be capable of being oxidized at a potential lower than that required to oxidize water (this is because the zinc-nickel waste stream contains primarily water).

[0043] 5) The reaction products from the oxidation process must not "poison" the electrode. Electrode poisoning is caused by the adsorption of species on the electrode surface, which prevents further adsorption of the molecules that need to be oxidized.

[0044] Studies related to the electrochemical oxidation of ammonia in waste streams have determined that during the oxidation process, atomic nitrogen produced as an intermediate species remains adsorbed on the electrodes and poisons further oxidation of the ammonia, thus significantly limiting the efficiency of ammonia oxidation.

[0045] The inventors of the present invention have found that this "poisoning" of the electrode surface is also quite significant if one attempts to oxidize an organic amine in this way.

[0046] It is known that in the case of ammonia in waste streams, the inclusion of chloride ions can prevent this poisoning reaction. However, the oxidation reaction mechanism of polyamines and other such strong complexing agents cannot be explained by the proposed ammonia oxidation reaction mechanism.

[0047] Based on this, this reaction mechanism has not been attempted in systems containing amine complexes and heavy metal ions, where the amine complexing agent must be separated from the heavy metal ions to allow for the separate disposal of the amine complexing agent and the heavy metal ions in an efficient manner. In addition, this method has not been applied to complexing agents based on organic amines, because the reaction mechanism for ammoxidation is different from the reaction mechanism for treating / oxidizing organic amines.

[0048] The inventors of the present invention have discovered that organic amines present in zinc nickel waste streams can be electrochemically oxidized by utilizing chloride ions as an intermediate for oxidizing the amines. That is, the inventors of the present invention have discovered a method for treating organic amines present in industrial process waste streams using electrochemical oxidation to oxidize EDTA and amine-based complexing agents in the waste stream. The method can also oxidize any cyanide compounds present in the waste stream.

[0049] In one embodiment, the present invention is generally directed to a method of treating a waste stream comprising an organic amine compound complexed with a heavy metal, the method comprising the steps of:

[0050] a) adjusting the pH of the waste stream to between about 4 and about 10;

[0051] b) adding a chloride salt to the waste stream to produce a concentration of chloride ions in the waste stream;

[0052] c) circulating the waste stream through an electrochemical reactor, wherein the electrochemical reactor comprises an electrode array comprising alternating anodes and cathodes;

[0053] The waste stream is circulated through the electrochemical reactor for a time sufficient to hydrolyze the organic amine compounds in the waste stream.

[0054] Hydrolysis of the amine occurs after an initial oxidation reaction with hypochlorous acid formed at the anode by the electrolytic oxidation of chloride ions.

[0055] Thereafter, the waste stream containing the reaction product of the hydrolyzed amine compound and the heavy metal ion can be subjected to a step of treating the waste stream to remove the heavy metal ions, such as by precipitating the heavy metal ions from the waste stream. Thereafter, if necessary, the waste stream can be subjected to further treatment steps before discharge.

[0056] Without wishing to be bound by theory, the following mechanistic considerations are believed to be the most likely way in which the electrochemical oxidation described in the present invention operates:

[0057] Anode reaction

[0058] Chloride ions can be anodized by the following reaction:

[0059] (1) 2Cl - → Cl2 + 2e - E o = +1.36V vs NHE

[0060] The chlorine gas formed by this reaction then reacts with water to form hypochlorous acid:

[0061] (2) Cl2 + H2O → HOCl + HCl

[0062] At a potential where chloride ions are oxidized, there is a competing oxidation process that produces oxygen:

[0063] (3) 2H2O → O2 + 4H + +4e - E o = +1.229V vs NHE

[0064] However, since this reaction involves the release of oxygen, there will be a considerable oxygen overpotential to add to the standard potential. This is because the standard potential is an equilibrium potential where there is not full release of gas. In order to release the gas, additional voltage is required to provide the activation energy. Typically, the overpotential for chlorine evolution is lower than the overpotential for oxygen evolution.

[0065] Typical overpotentials for oxygen and chlorine at electrodes commonly used for this purpose are as follows:

[0066]

[0067] All of these electrodes preferentially produce chlorine rather than oxygen in chloride-containing environments. However, graphite anodes tend to have a short life as chlorine-producing anodes and have been largely replaced by mixed metal oxide-coated anodes. Due to their catalytic action, these graphite anodes are very efficient at releasing chlorine, so that chlorine release at slightly below standard potentials can be obtained in some cases.

[0068] Hypochlorous Acid Reaction

[0069] Hypochlorous acid will react rapidly with amines to form chloramines, as exemplified by the following reaction:

[0070] (4) R-NH2+HOCl→R-NHCl+H2OPrimary amine

[0071] (5) R2-NH+HOCl→R2NCl+H2O secondary amine

[0072] (6) R3-N+HOCl→R2NCl+ROH tertiary amine

[0073] These chloramines are unstable and hydrolyze into nitrogen (or nitrate to a lesser extent) and other organic moieties in the presence of chlorine or hypochlorous acid. Once the process is complete, the resulting mixture is no longer able to chelate metal ions. Therefore, the resulting heavy metal ions and the reaction products of the hydrolysis can be handled or disposed of separately.

[0074] Here is an example of the decomposition process:

[0075] (7) 3R-NHCl + 2H2O→ N2 + 3Cl - + 3H + + R-NH2 + 2ROH

[0076] (8) 3NR2Cl + 4H2O → R2NH + N2 + 4ROH + 3H + + 3Cl -

[0077] Electrochemical oxidation proceeds through a stepwise mechanism to remove amines from the waste stream, and nitrogen species in solution are ultimately converted to nitrogen gas. Oxidation of organic amines also produces organic alcohols, which can be further oxidized to aldehydes and carboxylic acids. Chloride ions, which were initially oxidized to form hypochlorous acid, are regenerated during the decomposition of chloramines to act as a catalyst for the decomposition.

[0078] During zinc-nickel plating, some amines are partially oxidized to cyanide ions, and these cyanide ions can be problematic in waste streams. However, cyanide can also be electrochemically oxidized in the presence of chloride ions, which have been oxidized to hypochlorous acid according to the following reaction:

[0079] (9) HOCl + OH - → OCl - + H2O

[0080] (10) CN - + OCl - → CNO - + Cl -

[0081] (11) CNO - +3H2O→NH3+HCO3 - Catalyzed by chlorine / hypochlorous acid

[0082] (12) 6ClO - + 2NH3 →N2 + 6H2O + 6Cl -

[0083] Cathode reaction

[0084] The main cathode reaction is the reduction of water to hydrogen gas and hydroxide ions as follows:

[0085] (13) 2H2O + 2e - → H2 + 2OH - E o = -0.8277 V vs NHE

[0086] There may be some side reactions of metal reduction. However, this is likely to be negligible given the low concentration of metal ions in the waste stream. The hydroxide ions produced at the cathode will neutralize the H generated at the anode. + Based on this, the overall change in pH of the waste stream during electrolysis is expected to be small.

[0087] In the case of processing electroless copper waste, it is generally expected that there will be some deposition of copper on the cathode and this will require occasional stripping of the cathode. Therefore, in the case of processing electroless copper waste streams, it is generally preferred to use stainless steel cathodes as the use of such cathodes will promote stripping.

[0088] As described above, the present invention generally comprises the following steps:

[0089] 1. The pH of the waste stream to be treated is adjusted to between 4 and 10, preferably to a pH of 8 to 9.2. At a pH above this range, the hypochlorous acid formed at the electrode surface is converted to hypochlorite ions, which are not effective in oxidizing amines. At a pH below this range, chlorine gas tends to be formed instead of hypochlorous acid.

[0090] 2. Adding a chloride salt (preferably sodium chloride or potassium chloride) to the waste stream such that the concentration of chloride ions in the waste stream is between 10 mg / l and 50,000 mg / l, preferably between 300 mg / l and 3000 mg / l, most preferably between 600 mg / l and 1200 mg / l.

[0091] 3. Passing the waste stream through an electrochemical reactor comprising an array of electrodes comprising alternating anodes and cathodes. Organic amines in the waste stream are oxidized to species that are unable to complex metals (without wishing to be bound by theory, the inventors of the present invention believe that the nitrogen-containing portion is primarily oxidized to nitrogen, with some secondary oxidation to nitrates). The waste stream is circulated through the array of electrodes within the treatment cell for a sufficient time to oxidize the amine complexing agent such that the amine complexing agent decomposes into nitrogen gas. The oxidation period is typically from about 30 minutes to about 180 minutes, more preferably from about 60 to about 120 minutes.

[0092] In one embodiment, the level of breakdown is at least about 50%, preferably at least about 75%, and most preferably at least about 90%.

[0093] The contact time that provides sufficient decomposition of the amine compound will depend on a variety of factors including, for example, the type of amine compound, the concentration of the amine compound in the waste stream, current density, electrode area, flow rate, and temperature, among others.

[0094] 4. Optionally, once the amine compound is hydrolyzed to nitrogen, heavy metal ions such as nickel and zinc ions in the waste stream may be removed from the waste stream by precipitating the nickel and zinc ions or by other removal means. As described above, any cyanide compounds in the waste stream are also hydrolyzed to nitrogen.

[0095] It should be noted that prior art methods are often used to first attempt to remove metal ions such as zinc and nickel from waste streams prior to removing complexing agents from the waste streams by methods such as hydroxide precipitation and DTC methods, and often prove to be ineffective because the heavy metals are complexed with amine compounds, and amine-complexed metal compounds can be decomposed by conventional means for removal and disposal.

[0096] In contrast, the present invention serves to first hydrolyze / decompose the amine compounds so that they are no longer complexed with metal ions, which makes the removal of the amine precipitation compounds and metal ions more efficient. That is, by the methods described herein, the steps of first oxidizing and hydrolyzing the amine compounds and then precipitating the metal ions from the waste stream produce a waste stream that has fewer waste disposal issues and does not produce sludge that must be further processed (by incineration or landfill).

[0097] Preferably, the anode used in the electrochemical cell may be selected from titanium or niobium coated with platinum or mixed metal oxides or any combination of iridium oxide, ruthenium oxide or tantalum. Other anode materials may also be used, including, for example, lead dioxide, graphite or boron-doped diamond electrodes. The main consideration is to select an electrode material with the lowest possible chlorine overpotential and the highest possible oxygen overpotential. This is to maximize the efficiency of the chlorine generation reaction.

[0098] The material of the cathode is not critical, but preferred materials include metals with low hydrogen overpotential. Examples of suitable cathode materials include, for example, mild steel or stainless steel.

[0099] Due to the relatively low conductivity of typical waste streams, the inter-electrode distance (i.e., the distance between adjacent anodes and cathodes) should be as short as possible without the possibility of short circuiting, so as to maximize efficiency. The preferred electrode distance is as short as possible within the design parameters of the treatment cell, and is generally about 0.5 cm to 20 cm, more preferably about 1 cm to 15 cm, more preferably about 2 cm to 10 cm. This will minimize the effect of the ohmic resistance of the waste stream and maximize the efficiency of the process. If the electrode distance is too small, there is a possibility of short circuiting in the treatment cell when the anode and cathode are in contact. On the other hand, if the electrode distance is too large, the process will not work in an effective manner and may not work at all.

[0100] The operating current density of the anode should preferably be between 0.5 amperes per square decimeter (ASD) and 4 ASD, most preferably between 1 ASD and 2 ASD. The surface area of ​​each anode is designed to obtain the desired amine oxidation rate for a particular application.

[0101] The electrochemical oxidation processes described herein may be performed at a temperature in the range of about 20°C to about 40°C, and more preferably at room temperature.

[0102] The electrochemical oxidation cell may be subjected to agitation. In some embodiments, the waste stream may need to be agitated to achieve good efficiency. Various agitation methods may be used depending on the desired degree of agitation. In one embodiment, agitation is accomplished at least in part by pumping the waste stream through the electrochemical cell.

[0103] The invention will now be illustrated with reference to the following non-limiting examples.

[0104] Example

[0105] Example 1

[0106] Oxidation of diethylenetriamine

[0107] Diethylenetriamine contains both primary and secondary amine groups in the same molecule and is therefore a good test for the methods described herein.

[0108] An electrolyte is prepared, comprising:

[0109] 2g / l sodium sulfate;

[0110] 2g / l sodium chloride; and

[0111] 1g / l diethylenetriamine (DETA).

[0112] Assemble a 250 ml beaker containing a surface area of ​​50 cm in the center of the beaker. 2 Platinum-coated niobium anode (25 cm on each side 2 ), with 2 mild steel cathodes on the sides of the beaker. The electrode spacing was about 2 cm on either side of the anode. The beaker was also equipped with a magnetic stirrer.

[0113] 250 ml of electrolyte was added to the beaker and electrolyzed at ambient temperature with a current of 0.5 A, corresponding to 10 mA / cm 2 The anode current density is 1.3V. The battery voltage is 8.5V.

[0114] Take 25 ml samples at 30 minutes, 1 hour and 2 hours intervals and fill the beaker with fresh electrolyte to maintain the liquid level. Titrate the taken samples with 0.05 M hydrochloric acid using bromocresol green indicator and record the titration value.

[0115] The pKa values ​​of neutralized diethylenetriamine are as follows:

[0116] pKa1: 4.42

[0117] pKa2: 9.21

[0118] pKa3: 10.02

[0119] The color of bromocresol green changes from blue to yellow. The color is blue above pH 5.6 and yellow below pH 4.0, so a color change would be expected when neutralizing 2 of the three nitrogen-containing moieties in diethylenetriamine. Therefore, 2M HCl is equivalent to 1M DETA.

[0120] The titration values ​​are as follows:

[0121] 0 minutes 11.1ml Theoretical value 10.8ml

[0122] 7.0ml 36.9% decomposition in 30 minutes

[0123]

[0124] Theoretical current required to decompose 1g / l DETA at 100% efficiency = 2.14Ahr / l

[0125] Therefore, after electrolysis at 0.5A for 30 minutes in 250ml, 1Ahr / l is passed, so at 100% conversion efficiency, the decomposition percentage will be (1 / 2.14) x 100 = 46.7%, but the actual decomposition value is 36.9%, so the conversion efficiency is (36.9 / 46.7) x 100 = 79.0%

[0126] Likewise, after 1 hour of electrolysis, at 100% conversion, the decomposition percentage would be (2 / 2.14) x 100 = 93.5%, but the actual decomposition value is 53.2%, so the conversion efficiency is (53.2 / 93.5) x 100 = 56.9%

[0127] Example 2

[0128] Oxidation of diethylenetriamine in larger cells

[0129] In a system with alternating anodes and cathodes and a total anode area of ​​300 cm 2 The experiment described in Example 1 was repeated in a larger cell using a 2 liter volume of the test electrolyte.

[0130] A current of 5 amperes was applied to the cell at a voltage of 10.6 V. Air jets were used to stir the solution in the cell instead of magnetic stirring. The anode used was platinum-coated titanium.

[0131] Different analytical methods were used as follows:

[0132] A solution containing an acetate buffer solution (2 g / l sodium acetate and 2 g / l acetic acid) at pH 4.5 was prepared. 3 g / l copper sulfate pentahydrate was added thereto (solution A). Solutions containing 0.2 g / l, 0.4 g / l, 0.6 g / l, 0.8 g / l and 1.0 g / l diethylenetriamine were also prepared (solution B).

[0133] 10 ml of solution A was pipetted into a small beaker, 10 ml of solution B was added and mixed thoroughly. The absorbance of the solution was measured at a wavelength of 618 nm (corresponding to the peak absorbance of a scan performed between 700 nm and 450 nm) using a PerkinElmer UV visible spectrophotometer.

[0134] A calibration curve was prepared from different concentrations of Solution B so that the amount of amine in the test solution could be determined in order to evaluate the effectiveness of the test cell. The correlation coefficient (R 2 ) is 1, showing excellent linearity.

[0135] The results are as follows:

[0136]

[0137]

[0138] The conversion efficiency at 30 minutes was 37.5%. This is not as high as the conversion efficiency of Example 1, but the stirring rate in the larger cell was smaller. This suggests that higher flow rates will produce more efficient results.

[0139] Example 3

[0140] N,N,N',N'-Tetrakis(2-hydroxypropyl)ethylenediamine Oxidation

[0141] is an example of a tertiary amine. The method of Example 2 was repeated with a solution containing:

[0142]

[0143] 2g / l sodium chloride

[0144] 2g / l sodium sulfate.

[0145] In this example, an absorption peak was observed to occur at a much higher wavelength than the 618 nm absorption peak recorded for diethylenetriamine.

[0146] exist In the case of , the absorbance peak of the complex is at 753 nm and the extinction coefficient is lower. The analytical method was modified to take these differences into account. The results are as follows:

[0147]

[0148] In this experiment, The decomposition rate of is higher than that of diethylenetriamine. Without wishing to be bound by theory, it is believed that this may be because There are only 2 amine moieties rather than 3 in the case of diethylenetriamine. This example illustrates that the oxidation process works effectively on tertiary amines as well as primary and secondary amines.

[0149] Example 4

[0150] Anodization of Diethylenetriamine Using Mixed Metal Oxides

[0151] The method of Example 2 was used, except that the platinum-plated titanium anode was replaced with titanium coated with a coating consisting of tantalum oxide and iridium oxide. The results obtained were as follows:

[0152]

[0153]

[0154] This example demonstrates that mixed metal oxide anodes can be used to decompose amines.

[0155] Comparative Example 5

[0156] Oxidation of diethylenetriamine in the absence of chloride ions .

[0157] The process of Example 2 was repeated, except that the electrolyte consisted of 5 g / l sodium sulfate, no chloride was added and 1 g / l diethylenetriamine.

[0158] The results are as follows:

[0159]

[0160] From the above it can be seen that in the absence of chloride ions the rate of decomposition of diethylenetriamine by electrolysis is much lower (ie almost 3 times slower) and therefore the process will take more time to work and there will be a corresponding cost in energy consumption.

[0161] The data of Examples 2, 3 and 4 and Comparative Example 5 are shown in Figure 1 In. Figure 1 It can be seen that when the electrochemical oxidation is carried out in the presence of chloride, the concentration of the amine compound remaining after the electrochemical oxidation is much lower than when chloride is omitted from the process.

[0162] Example 6

[0163] Treatment of Industrial Zinc-Nickel Wastes .

[0164] An industrial zinc-nickel waste sample was electrolyzed using the method described in Example 2. The zinc-nickel waste electrolyte contained a 2% dilution of a standard zinc-nickel electroplating electrolyte containing between about 40 g / L and 50 g / L of an amine complexing agent (i.e., about 0.8 g / L to 1.0 g / L of amine complexing agent in a 2% dilution).

[0165] The chloride ion content was adjusted to 2000 ppm chloride ion by adding sodium chloride before starting the electrolysis.The absorbance method shown in Example 2 was used to estimate the amount of organic amine present in the solution.

[0166] The results are as follows:

[0167]

[0168] This example illustrates that similar results can be obtained using electrochemical oxidation of industrial waste containing amine compounds compared to the results obtained from the oxidation of diethylenetriamine in Examples 1, 2 and 4.

[0169] Commercial zinc-nickel electrolytes use a variety of amine complexing agents, which can be used in different concentrations depending on the composition of the bath and the specific type of complexing agent. For example, in some cases, tetraethylenepentamine can be used in the range of about 1 g / L to about 5 g / L, diethylenetetramine can be used in the range of about 12 g / L to 20 g / L, triethanolamine can be used in the range of about 2 g / L to 10 g / L, and can be used in the range of about 15 g / L to 25 g / L. Other amine complexing agents and concentrations of the above complexing agents can also be used, depending on various factors. It is believed that the method described herein can be used with most amine-complexed metals to hydrolyze the amine compound and allow further processing of the metal ion.

[0170] The resulting waste stream may therefore be subjected to further treatment to precipitate out the zinc and nickel ions, such as by hydroxide precipitation.The resulting waste stream will therefore contain zinc and nickel (or other metal) levels below regulated levels and is therefore suitable for discharge.

Claims

1. A method for treating a waste stream comprising an organic amine compound complexed with a heavy metal ion, the method comprising the steps of: a) adjusting the pH of the waste stream to between about 4 and about 10; b) adding a chloride salt to the waste stream to produce a concentration of chloride ions in the waste stream; c) circulating the waste stream through an electrochemical reactor, wherein the electrochemical reactor comprises an electrode array comprising alternating anodes and cathodes, wherein the waste stream is circulated through the electrochemical reactor for a period of time of 30 minutes to 180 minutes to hydrolyze the amine compound; and then d) removing heavy metal ions from said waste stream.

2. The process according to claim 1, wherein the pH of step a) is between 8 and 9.

2.

3. The method of claim 1, wherein the chloride salt is selected from the group consisting of sodium chloride and potassium chloride.

4. The method of claim 1, wherein the concentration of chloride ions in the waste stream is between about 10 mg / L and about 50,000 mg / L.

5. The method of claim 4, wherein the concentration of chloride ions in the waste stream is between about 600 mg / L and about 1,200 mg / L.

6. The method of claim 1, wherein the heavy metal ions are selected from the group consisting of nickel, zinc, copper, and combinations of one or more of the foregoing.

7. The method of claim 1, wherein the anode is selected from the group consisting of: titanium or niobium coated with platinum or mixed metal oxides; a combination of one or more of iridium oxide, ruthenium oxide, and tantalum oxide; lead oxide; graphite; and boron-doped diamond.

8. The method of claim 1, wherein the cathode is selected from the group consisting of mild steel and stainless steel.

9. The method of claim 1, wherein the electrode array is arranged such that the distance between adjacent anodes and cathodes is between about 0.5 cm and about 20 cm.

10. The method according to claim 1, wherein the amine compound is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine; alkylene oxide adducts of the above alkyleneamines, such as ethylene oxide adducts and propylene oxide adducts; amino alcohols, such as ethanolamine, diethanolamine, triethanolamine, diisopropanolamine, triisopropanolamine, ethylenediaminetetra-2-propanol, N-(2-aminoethyl)ethanolamine and 2-hydroxyethyl Aminopropylamine; alkanolamine compounds such as N-(2-hydroxyethyl)-N,N',N'-triethylethylenediamine, N,N'-di(2-hydroxyethyl)-N,N'-diethylethylenediamine, N,N,N',N'-tetrakis(2-hydroxyethyl)propylenediamine, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine; poly(alkyleneimines) obtained from ethyleneimine, 1,2-propyleneimine; poly(alkyleneamines) obtained from ethylenediamine, triethylenetetramine; Ethylenediaminetetraacetic acid, and combinations of one or more of the foregoing.

11. The method of claim 1 , wherein the step of circulating the waste stream through the electrochemical reactor hydrolyzes any cyanide compounds in the waste stream.

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