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

Abstract

A process for treating a waste stream containing organic amine compounds complexed with heavy metal ions. The process includes the steps of: (1) adjusting the pH of the waste stream to about 4 to 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 an electrochemical reactor. The electrochemical reactor includes an array of electrodes including alternating anodes and cathodes. The waste stream is circulated through the electrochemical reactor for a time sufficient to hydrolyze the amine compounds in the waste stream.

Description

[Technical field]

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

[0002] In the metal finishing industry, plating baths used for electrolytic and electroless plating of metals and metal alloys generally contain metal ions of the desired metal to be plated along with various complexing agents (i.e., chelating agents) and other bath components. Depending on the metal to be plated, these complexing agents can include, for example, aminopolycarboxylic acids such as ethylenediamine tetraacetic acid (EDTA), nitriloacetic acid (NTA), cyanides, polyamines, and other complexing agents capable of forming strong bonds.

[0003] While the use of such complexing agents is helpful in producing the desired plating deposit on the substrate, disposal of these compounds and their by-products can be problematic and expensive, and in some cases may impede the ability of wastewater treatment plants to meet required discharge limits.

[0004] Examples of industrial wastewaters include, for example, various used electroless baths and electroplating baths including cyanide copper plating wastewater, copper pyrophosphate plating wastewater, acid 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 color tin-nickel alloy electroplating wastewater, alkaline zinc-nickel alloy electroplating wastewater, acid zinc-nickel alloy electroplating wastewater, hexavalent chromium plating wastewater, trivalent chromium plating wastewater, electroless nickel plating wastewater, electroless copper plating wastewater, hexavalent chromium passivation wastewater, trivalent chromium passivation wastewater, degreasing wastewater, pickling wastewater, among others, which may be present in the wastewater alone or in combination. Industrial wastewater typically contains complexing agents (i.e., sodium cyanide, potassium pyrophosphate, sodium citrate, sodium potassium 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 contaminants (i.e., sodium phosphate, sodium hypophosphite, brighteners, co-brighteners, surfactants, etc.).

[0005] Depending on the nature of the industrial wastewater generated and the degree of metal removal desired, various wastewater treatment options can be used. However, when the industrial wastewater contains strong complexing agents such as EDTA and / or polyamines, traditional treatment options involving precipitation of metal ions may be inadequate.

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

[0007] Metal hydroxide precipitation is widely used due to its simplicity of use and relatively low cost. However, hydroxide precipitation is generally not effective at breaking down the complexing bonds formed by various complexing agents; that is, sodium hydroxide reacts with any unchelated heavy metals present to produce insoluble metal hydroxides. However, when co-precipitated as a cation with other metals, it can reduce solubility. Furthermore, certain complexing agents are specifically designed to prevent hydroxide precipitation from occurring under normal conditions.

[0008] Iron can be used in the presence of a complexing agent to break down many of the complexing agent bonds. The iron reagent is introduced into the wastewater in an amount sufficient to reduce most of the metals and break the complexing agent bonds. The iron reagent is added to a solution in a first reaction tank at a pH of about 2.0-3.0, and then the solution is raised to a pH of 8.5-11 in a second reaction tank, where the metals, including the iron, precipitate as metal hydroxides. One of the major disadvantages of iron reagent chemistry is that the dosage must be predetermined since the reagent is added on a volumetric basis and is not related to the actual concentration of the metal in the feed. Thus, the iron reagent process can be very difficult to optimize and control when feed metal concentrations vary widely.

[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 that precipitate as sludge. The concentration of the chelated material has a large effect on the dosing rate. Thus, one significant drawback of the DTC process is that it cannot be easily and safely adjusted to accommodate fluctuations in feed metal concentration. 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 plating wastewater and electroplating wastewater, these treatment options have difficulty efficiently and cost-effectively treating industrial wastewaters containing complexing agents such as polyamines. There remains a need in the art for a treatment method for use in treating industrial wastewaters that can decompose complexing agents, including amine-based complexing agents. In addition, there remains a need in the art for an improved method for treating alkaline zinc-nickel electroplating waste streams and electroless copper waste streams that contain metal ions complexed with amines for subsequent disposal. There is also a desire to develop a process for treating electroplating wastewaters that contains complexing agents, such as EDTA and amine-based complexing agents, that are complexed with heavy metal ions, that can be used in combination with conventional methods for precipitating heavy metal ions and mixtures of heavy metal ions.

[0011] For many years, the automotive industry has relied on the production of fasteners and other engineered steel parts coated with zinc and nickel alloys by electroplating. Typically, the alloys are predominantly zinc with a nickel content of 12-15%. These alloys have good corrosion resistance, and the corrosion products are compact and do not significantly detract from the aesthetics of the finished product. Zinc-nickel alloys also exhibit favorable contact corrosion characteristics when in contact with aluminum. The prevalence of extended corrosion warranties in modern automobiles has made the use of zinc-nickel plated components mandatory.

[0012] Two main processes are typically used for the deposition of zinc-nickel alloys, depending in part on the type of component being plated.

[0013] For cast iron components such as brake cylinders, processes with a near neutral pH are used. These processes are typically around pH 5 and are referred to as "acid zinc nickel" solutions. Ammonium ions are used in the electrolyte to buffer the solution pH at the electrode / electrolyte interface and prevent co-precipitation of metal hydroxides (due to reduction of hydrogen ions) due to the pH rise at the cathode during electrolysis. Ammonium ions are problematic in waste treatment because they lose a proton to ammonia as they increase the pH of the waste stream to precipitate metal ions from solution as metal hydroxides. This can act as a ligand to make the nickel ions soluble and prevent precipitation.

[0014] For other components, including fasteners and other engineered steel parts, alkaline zinc-nickel processes are often used. This is because the alkaline process is much easier to operate than typical acid processes and produces a much more uniform alloy composition over a wide range of current densities. However, unlike acid processes, which rely on the use of ammonium ions as a pH buffer, the alkaline zinc-nickel process requires the use of an organic amine 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 complexes with the kinetic and thermodynamic properties required for the process.

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

[0016] A typical alkaline zinc-nickel electroplating bath contains about 0.5 to about 50 g / L of nickel ions, about 0.1 to about 100 g / L of zinc ions, and about 5 to about 100 g / L of a complexing agent, which is typically an amine-based complexing agent. In addition, the electroplating bath also typically contains an inorganic alkaline component in an amount sufficient to provide the bath with a desired pH. For example, the inorganic alkaline component may be present in a concentration of about 50 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 or potassium hydroxide, but may alternatively include sodium or potassium carbonate or bicarbonate. Mixtures of inorganic alkaline components may also be used.

[0017] Examples of amine complexing agents that can be used in the alkaline zinc-nickel electroplating baths include alkyleneamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine; alkylene oxide adducts such as the ethylene oxide and propylene oxide adducts of the above alkyleneamines; aminoalcohols such as ethanolamine, diethanolamine, triethanolamine, diisopropanolamine, triisopropanolamine, ethylenediaminetetra-2-propanol, N-(2-aminoethyl)ethanolamine, and 2-hydroxyethylamine. 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, and N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (available under the trade name Quadrol®); poly(alkyleneimines) derived from ethyleneimine, 1,2-propyleneimine, and the like; and poly(alkyleneamines) derived from ethylenediamine, triethylenetetramine, and the like. The complexing agent may include one or more selected from the group consisting of alkyleneamine compounds, their alkylene oxide adducts, and alkanolamine compounds. These amine complexing agents may be used alone or in combination of two or more. For electroless copper plating baths, EDTA is a common complexing agent.

[0018] The amine complexing agents in the wastewater generally cannot be effectively removed using conventional oxidation processes, resulting in difficult to treat electroplating wastewater. A typical alkaline zinc-nickel alloy electroplating bath may contain about 3% by weight of amine complexing agents, which are highly stable and cannot be effectively decomposed by typical wastewater treatment methods.

[0019] WO 2019 / 085128 (Guangzhou Ultra Union Chemicals Ltd.) discloses a method for treating alkaline zinc-nickel alloy electroplating wastewater, including multiple steps of adjusting the pH of the alkaline zinc-nickel alloy electroplating wastewater and adding various reagents including sodium diethyldithiocarbamate, a flocculant, and sodium hypochlorite. Similarly, U.S. Patent Application Publication No. 2021 / 0380455 (Guo et al.), the entire subject matter of which is incorporated herein by reference, describes a method for treating cyanide-free mixed electroplating wastewater and cyanide-free phosphorus-containing reducing agents, in which a biological degradation technique is used to destroy an aliphatic polyamine complexing agent to reduce chemical oxygen demand (COD). However, biological degradation techniques typically require a time period of 8 to 24 hours to reach an acceptable level of COD, in addition to multiple additional steps required in the process to treat the wastewater so that it is sufficiently free of pollutants before discharge.

[0020] Other treatment methods, such as that described in CN103641207(A), rely on the use of a composite electrolytic cell to electrolyze zinc-containing electroplating wastewater.

[0021] Since no nickel anode can be used in alkaline zinc-nickel electroplating processes, all of the nickel that is deposited as plating is added as a nickel salt (such as nickel sulfate or nickel chloride). However, this salt is not soluble in the alkaline environment and cannot simply be added to the electroplating bath. Instead, it must be precomplexed in advance using one or more of the amine complexing agents mentioned above. Thus, during use, both the amine complexing agent and salt (i.e., sodium sulfate or sodium chloride or potassium sulfate or potassium chloride, depending on the process formulation) build up in the electroplating bath. This must typically be controlled by periodically removing a portion of the plating bath or by using a "feed and bleed" system, which creates a large amount of waste in addition to the normal "drag out" losses. Because alkaline zinc-nickel electroplating systems are widely used, the waste disposal problem is significant.

[0022] As mentioned above, the waste materials from these alkaline zinc-nickel electroplating processes cannot be effectively treated and must be removed for incineration, which is expensive and energy-wasting.

[0023] Some electroplating companies have attempted to remove nickel from waste by precipitating it using dimethylglyoxime, as described, for example, in U.S. Patent No. 4,500,324 (Vuong), the entire subject matter of which is incorporated herein by reference. This results in an insoluble nickel complex that can be filtered off. However, this process does not allow the waste solution to be mixed with other waste streams, as the amine complexes still remain in solution, and in many cases, the amine compounds cannot be safely discharged into the environment, as they are toxic to fish and other aquatic species. In some circumstances, large amounts of sodium hypochlorite solution may be added to react with the amine groups in the complexing agents present in the waste stream. However, a large excess of sodium hypochlorite is required, which leads to further waste disposal problems and the generation of large amounts of sludge.

[0024] Organic amines are also used as complexing agents in other processes in the metal finishing industry, for example, the printed circuit industry utilizes electroless copper plating processes that contain organic amine complexing agents (e.g., Quadrol® and EDTA).

[0025] It would therefore be desirable to provide a method for removing amine complexing agents from industrial process wastewater that overcomes the shortcomings 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 that utilizes an anodic electrode that is insoluble in wastewater at the applied potential and has a high oxygen overpotential and a low chlorine overpotential.

[0027] Following electrolytic treatment to remove the organoamine complexes, the wastewater can be further treated to remove metals such as zinc and nickel from the waste stream. [Brief description of the drawings]

[0028] [Figure 1] 1 is a graph showing a comparison of the rate of amine decomposition for examples carried out in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0030] As used herein, the term "about" refers to a measurable value of a parameter, amount, duration, etc., and is meant to include no more than ±15% variation, preferably no more than ±10%, more preferably no more than ±5%, even more preferably no more than ±1%, and even more preferably no more than ±0.1% variation of and from the specifically recited value, insofar as such variations are appropriate for the practice herein. Furthermore, it is also to be understood that the values ​​to which the modifier "about" refers are themselves specifically disclosed herein.

[0031] As used herein, spatially relative terms such as "beneath," "below," "lower," "above," "upper," and the like are used for ease of explanation to describe the relationship of one element or feature to another element or feature as illustrated in the figures. It will be further understood that the terms "front" and "back" are not intended to be limiting and are intended to be interchangeable where appropriate.

[0032] As used herein, the terms "comprises" and / or "comprising" 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, "substantially free" or "essentially free" with respect to a particular element or compound means that, unless otherwise defined herein with respect to a particular element or compound, the given element or compound is not detectable by conventional analytical means well known to those skilled in the art of metal plating for bath analysis. Such methods typically include atomic absorption spectroscopy, titration, UV-Vis spectroscopy, secondary ion mass spectroscopy, and other commonly available analytical methods.

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

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

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

[0037] The process of oxidation involves the removal of electrons from the reactants being oxidized. This can theoretically be achieved by electrochemical oxidation, which uses a power source to directly remove electrons from molecules in contact with the anode (the charge is balanced by the supply of electrons to other molecules in contact with the cathode). This method 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. Thus, the method described in this invention has the potential to operate in an environmentally acceptable manner.

[0038] The inventors have found that the use of electrochemical oxidation is complex and that there are many difficulties that must be overcome in order to successfully use it to treat strong complexing agents in waste streams. These can be summarized as follows: 1) Because the reaction occurs at the electrode surface (i.e., it is a heterogeneous reaction), the oxidation rate is limited by the mass transport rate of the molecule to be oxidized to the electrode surface. 2) The electrode must be composed of a material that has sufficient affinity for the molecule to be oxidized and allows specific adsorption of the molecule so that the electron transfer reaction can occur (via the mechanism of quantum mechanical tunneling). 3) The material comprising the electrode must not itself be capable of being oxidized at the electrode potentials used in the electrochemical oxidation process. 4) The molecule to be oxidized must be capable of being oxidized at a potential lower than that required to oxidize water (which is the case since water makes up the majority of the zinc-nickel waste stream). 5) The reaction products from the oxidation process must not "poison" the electrode. Electrode poisoning is caused by the adsorption of species onto the surface of the electrode that prevent further adsorption of molecules that need to be oxidized.

[0039] Studies on the electrochemical oxidation of ammonia in waste streams have revealed that atomic nitrogen produced as an intermediate species during the oxidation process remains adsorbed on the electrode and impairs the further oxidation of ammonia, dramatically limiting the efficiency of ammonia oxidation.

[0040] The present inventors have found that when attempting to oxidize organic amines in this way, this "poisoning" of the electrode surface also becomes very pronounced.

[0041] It is known that in the case of ammonia in waste streams, this poisoning reaction can be prevented by including chloride ions. However, the reaction mechanisms for the oxidation of polyamines and other such strong complexing agents cannot be explained by the reaction mechanism proposed for the oxidation of ammonia.

[0042] Based thereon, this reaction scheme has also not been attempted in systems containing amine complexes with heavy metal ions, where the amine complexing agent must be separated from the heavy metal ions to allow for efficient separate disposal of the amine complexing agent and the heavy metal ions.In addition, the method has not been applied to organic amine-based complexing agents, since the reaction mechanism for the oxidation of ammonia is different from that for the treatment / oxidation of organic amines.

[0043] 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 species to oxidize the amines. That is, the inventors of the present invention have discovered a process for treating organic amines present in industrial process waste streams that uses electrochemical oxidation to oxidize EDTA and amine-based complexing agents in the waste stream. The process can also oxidize any cyanide compounds present in the waste stream.

[0044] In one embodiment, the present invention generally relates to a process for treating a waste stream containing an organic amine compound complexed with a heavy metal, the process comprising: a) adjusting the pH of the waste stream to about 4 to 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, the electrochemical reactor comprising an array of electrodes comprising alternating anodes and cathodes; Including, The present invention relates to a process in which a waste stream is circulated through an electrochemical reactor for a period of time sufficient to hydrolyze organic amine compounds in the waste stream.

[0045] Hydrolysis of the amine occurs after an initial oxidation reaction with hypochlorous acid formed at the anode from the electrolytic oxidation of chloride ions.

[0046] The waste stream containing the reaction products of the hydrolyzed amine compounds and the heavy metal ions may then be subjected to a step of treating the waste stream to remove the heavy metal ions, for example by precipitating the heavy metal ions from the waste stream, after which the waste stream may be subjected to further treatment steps, if required, prior to discharge.

[0047] Without wishing to be bound by theory, the following mechanistic considerations are believed to be the most likely mode by which the electrochemical oxidation described in this invention operates.

[0048] Anode reaction Chloride ions can be anodized according to the following reaction: (1) 2Cl - →Cl2+2e - E o =+1.36V vs NHE The chlorine formed by this reaction then reacts with water to form hypochlorous acid. (2) Cl2+H2O→HOCl+HCl At potentials where chloride ions are oxidized, there are competing oxidation processes that produce oxygen. (3) 2H2O→O2+4H + +4e - E o =+1.229V vs NHE

[0049] However, because this reaction involves the liberation of oxygen gas, there is a significant oxygen overpotential that increases the standard potential. This is because the standard potential is the equilibrium potential at which there is no total liberation of gas. To liberate the gas, an additional voltage is required to provide the activation energy. Typically, the overpotential for chlorine evolution is lower than the overpotential for oxygen evolution.

[0050] Typical overpotentials for oxygen and chlorine at electrodes commonly used for this purpose are:

[0051] [Table 1]

[0052] All of these electrodes preferentially produce chlorine rather than oxygen in chloride-containing environments. However, graphite anodes tend to have short life spans as chlorine evolution anodes and have been largely replaced by mixed metal oxide coated anodes. These liberate chlorine very efficiently due to their catalytic effect, so that in some circumstances it is possible to obtain chlorine evolution at potentials slightly less than standard potential.

[0053] Reaction of hypochlorous acid Hypochlorous acid reacts rapidly with amines to form chloramines, as exemplified by the following reaction:

[0054] (4) R-NH2+HOCl → R-NHCl+H2O Primary amines (5) R2-NH+HOCl→R2NCl+H2O Secondary amine (6) R3-N+HOCl→R2NCl+ROH Tertiary amine

[0055] These chloramines are not stable and are hydrolyzed in the presence of chlorine or hypochlorous acid to nitrogen (or to a lesser extent nitrates) and other organic moieties. Once this process is complete, the resulting mixture is no longer capable of chelating metal ions. Thus, the resulting heavy metal ions and hydrolyzed reaction products can be processed or disposed of separately.

[0056] An example of the decomposition process is as follows: (7) 3R-NHCl+2H2O→N2+3Cl - +3H + +R-NH2+2ROH (8) 3NR2Cl+4H2O→R2NH+N2+4ROH+3H + +3Cl -

[0057] 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 undergo further oxidation to aldehydes and carboxylic acids. Chloride ions, which are initially oxidized to form hypochlorous acid, are regenerated during the decomposition of chloramines to act as a catalyst for the decomposition.

[0058] During the zinc nickel plating process, some of the amines are partially oxidized to cyanide ions, which can be problematic in the waste stream. However, the cyanide can also be electrochemically oxidized in the presence of chloride ions, which are oxidized to hypochlorous acid according to the following reaction: (9) HOCl+OH - →OCl - +H2O (10) CN - +OCl - →CNO - +Cl - (11) CNO - +3H2O→NH3+HCO3 - Catalyzed by Chlorine / Hypochlorous Acid (12) 6ClO - +2NH3→N2+6H2O+6Cl -

[0059] Cathodic reaction The primary cathodic reaction is the reduction of water to hydrogen gas and hydroxide ions as follows:

[0060] (13) 2H2O+2e - →H2+2OH - E o =-0.8277V vs NHE

[0061] There may be some side reactions of metal reduction. However, given the low concentration of metal ions in the waste stream, this is likely negligible. Hydroxide ions produced at the cathode are converted to H produced at the anode. +On this basis, little overall change in the pH of the waste stream during electrolysis is expected.

[0062] When treating electroless copper waste, some deposition of copper on the cathode is generally expected, which will necessitate occasional stripping of the cathode, and therefore the use of stainless steel cathodes is generally preferred when treating electroless copper waste streams, as such cathodes facilitate stripping.

[0063] As mentioned above, the present invention generally comprises the following steps: 1. Adjust the pH of the waste stream to be treated to between 4 and 10, preferably between pH 8 and 9.2. At pHs above this range, hypochlorous acid formed at the electrode surface is converted to hypochlorite ions, which are not effective at oxidizing amines. At pHs below this range, chlorine gas tends to form instead of hypochlorous acid. 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 10-50,000 mg / L, preferably 300-3000 mg / L, and most preferably 600-1200 mg / L. 3. Passing the waste stream through an electrochemical reactor containing an array of electrodes with 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 believe that the nitrogenous moieties are oxidized primarily to nitrogen with some secondary oxidation to nitrates). The waste stream is circulated through the array of electrodes in the treatment cell for a time sufficient to oxidize the amine complexing agent such that it is decomposed to nitrogen. The oxidation time is generally on the order of about 30 minutes to about 180 minutes, more preferably about 60 minutes to about 120 minutes.

[0064] In one embodiment, the level of degradation is at least about 50%, preferably at least about 75%, and most preferably at least about 90%.

[0065] The contact time to fully decompose the amine compounds depends on a variety of factors including, for example, the type of amine compound, the concentration of the amine compound in the waste stream, the current density, the electrode area, the flow rate, and the temperature, among others.

[0066] 4. Optionally, once the amine compounds are hydrolyzed to nitrogen, heavy metal ions such as nickel and zinc ions in the waste stream can be removed from the waste stream by precipitating the nickel and zinc ions or by other removal means. As noted above, any cyanide compounds in the waste stream are also hydrolyzed to nitrogen.

[0067] It is noted that prior art processes generally function to first attempt to remove metal ions such as zinc and nickel from the waste stream prior to removing complexing agents from the waste stream by methods such as hydroxide precipitation and DTC processes, but have generally proven ineffective because the heavy metals form complexes with amine compounds and the metal compounds complexed with the amines can be decomposed for removal and disposal by conventional means.

[0068] In contrast, the present invention serves to first hydrolyze / decompose the amine compounds so that they are unable to further complex with metal ions, resulting in much more efficient removal of both the amine precipitate compounds and the metal ions. That is, the process described herein results in a waste stream that presents fewer waste disposal problems and does not generate sludge that must be further processed (either by incineration or landfilling), as a result of the steps of first oxidizing and hydrolyzing the amine compounds and subsequently precipitating the metal ions from the waste stream.

[0069] Preferably, the anode used in the electrochemical cell can be selected from titanium or niobium coated with either platinum or mixed metal oxides or any combination of iridium oxide, ruthenium oxide, or tantalum. Other anode materials can also be used, including, for example, lead dioxide, graphite, or boron-doped diamond electrodes. The main consideration is to select the electrode material to have the lowest possible chlorine overpotential and the highest possible oxygen overpotential. This is to maximize the efficiency of the chlorine evolution reaction.

[0070] The material of the cathode is not critical, but preferred materials include metals that have a low hydrogen overvoltage. Examples of suitable cathode materials include, for example, mild steel or stainless steel.

[0071] The electrode distance (i.e., the distance between adjacent anodes and cathodes) must be as short as practical without the possibility of shorting due to the relatively low electrical conductivity of typical waste streams, in order to maximize efficiency. The preferred electrode distance is as short as possible within the design parameters of the treatment cell, typically about 0.5-20 cm, more preferably about 1-15 cm, more preferably about 2-10 cm. This minimizes the effect of ohmic resistance of the waste stream and maximizes the efficiency of the process. If the electrode distance is too short, a short circuit may occur within the treatment cell if the anode and cathode come into contact. On the other hand, if the electrode distance is too long, the process may not function efficiently or at all.

[0072] The operating current density of the anodes should preferably be between 0.5 and 4 amps per square decimetre (ASD), most preferably between 1 and 2 ASD. The surface area of ​​each anode is designed to obtain the amine oxidation rate required for a particular application.

[0073] The electrochemical oxidation process described herein may be carried out at a temperature within the range of about 20 to about 40° C., and more preferably at room temperature.

[0074] The electrochemical oxidation cell may be provided with agitation. In some embodiments, agitation of the waste stream is required to achieve good efficiency. Depending on the degree of agitation required, various agitation means can be used. In one embodiment, agitation is achieved, at least in part, by pumping the waste stream through the electrochemical cell.

[0075] The invention will now be described with reference to the following non-limiting examples. EXAMPLES

[0076] Example 1 Oxidation of diethylenetriamine Diethylenetriamine is a good test material for the processes described herein because it contains both primary and secondary amine groups in the same molecule.

[0077] An electrolyte was prepared containing: 2g / L sodium sulfate, 2 g / L sodium chloride, and Diethylenetriamine (DETA) at 1g / L.

[0078] Place a beaker with a surface area of ​​50 cm 2 (25cm per side 2 A 250 mL beaker containing a 1000 cc platinized niobium anode and two mild steel cathodes was placed on either side of the beaker. The electrode spacing was approximately 2 cm on either side of the anode. The beaker was also equipped with a magnetic stirrer.

[0079] 250 mL of electrolyte was added to the beaker and charged to 10 mA / cm 2 The electrolysis was carried out at ambient temperature with a current of 0.5 A, corresponding to an anode current density of 1000 mA. The cell voltage was 8.5 V.

[0080] 25 mL samples were taken at 30 min, 1 h, and 2 h time intervals and the beaker was topped up with fresh electrolyte to maintain the water level. The samples were titrated with 0.05 M hydrochloric acid using bromocresol green indicator and the titer was recorded.

[0081] The pKa values ​​for neutralization of diethylenetriamine are as follows: pKa1: 4.42 pKa2: 9.21 pKa3: 10.02

[0082] Bromocresol green changes color from blue to yellow. Since bromocresol green is blue above pH 5.6 and yellow below pH 4.0, one would expect the color change to occur if two of the three nitrogen-containing moieties in diethylenetriamine were neutralized. Thus, 2M HCl is equivalent to 1M DETA.

[0083] The titration values ​​were as follows:

[0084] [Table 2]

[0085] The theoretical current required to decompose 1 g / L of DETA with 100% efficiency = 2.14 Ahr / L.

[0086] Therefore, after electrolysis in 250 mL at 0.5 A for 30 minutes, if 1 Ahr / L is flowed with 100% conversion efficiency, the decomposition rate will be (1 / 2.14) x 100 = 46.7%, but since the actual decomposition value was 36.9%, the conversion efficiency is (36.9 / 46.7) x 100 = 79.0%.

[0087] Similarly, after 1 hour of electrolysis, the decomposition rate at 100% conversion would be (2 / 2.14)×100=93.5%, but the actual decomposition value was 53.2%, so the conversion efficiency is (53.2 / 93.5)×100=56.9%.

[0088] Example 2 Oxidation of diethylenetriamine in larger cells 300cm 2 The experiment described in Example 1 was repeated in a larger cell containing alternating anodes and cathodes with a total anode area of ​​1000 μm and using a 2 liter volume of test electrolyte.

[0089] A current of 5 amps was applied to the cell and the voltage was 10.6 V. The solution in the cell was stirred using air sparging rather than magnetic stirring. The anode used was platinized titanium.

[0090] Different analytical methods were used as follows:

[0091] A solution containing an acetate buffer solution of pH 4.5 (2 g / L sodium acetate and 2 g / L acetic acid) was prepared. To this was added 3 g / L copper sulfate pentahydrate (Solution A). Solutions containing 0.2, 0.4, 0.6, 0.8, and 1.0 g / L diethylenetriamine were also prepared (Solution B).

[0092] 10 mL of solution A was pipetted into a small beaker and 10 mL of solution B was added and mixed well. The absorbance of this solution at a wavelength of 618 nm (corresponding to the peak absorbance of a scan performed from 700 to 450 nm) was measured using a Perkin Elmer UV-visible spectrophotometer.

[0093] To evaluate the effectiveness of the test cell, a calibration graph was made from different concentrations of solution B so that the amount of amine in the test solution could be determined. The correlation coefficient (R 2 ) was 1, showing excellent linearity.

[0094] The results were as follows:

[0095] [Table 3]

[0096] The conversion efficiency at 30 minutes was 37.5%, which is not as high as that of Example 1, but the stirring speed in the larger cell was lower, indicating that a higher flow rate would yield more efficient results.

[0097] Example 3 Oxidation of N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol®) Quadrol® is an example of a tertiary amine. The method of Example 2 was repeated using a solution containing: 1g / L Quadrol® 2g / L sodium chloride Sodium sulfate at 2g / L.

[0098] In this example, it was observed that the absorption peak occurred at a wavelength much higher than the 618 nm absorption peak recorded for diethylenetriamine.

[0099] In the case of Quadrol®, the complex had an absorption peak at 753 nm and a lower extinction coefficient. The analytical method was modified to take these differences into account. The results were as follows:

[0100] [Table 4]

[0101] In this experiment, Quadrol® decomposed at a faster rate than diethylenetriamine. Without being bound by theory, it is believed that this may be because Quadrol® contains only two amine moieties instead of three in the case of diethylenetriamine. This example shows that the oxidation process works efficiently for tertiary amines as well as primary and secondary amines.

[0102] Example 4 Oxidation of diethylenetriamine using mixed metal oxide anodes. The method of Example 2 was used, except that the platinized titanium anode was replaced with titanium coated with a coating of tantalum and iridium oxide, and the results obtained were as follows:

[0103] [Table 5]

[0104] This example demonstrates that mixed metal oxide anodes can be used for the decomposition of amines.

[0105] Comparative Example 5 Oxidation of diethylenetriamine in the absence of chloride ions.

[0106] The method of Example 2 was repeated except that the electrolyte consisted of 5 g / L sodium sulfate and 1 g / L diethylenetriamine with no added chloride.

[0107] The results were as follows:

[0108] [Table 6]

[0109] From the above it can be seen that in the absence of chloride ions the rate of decomposition of diethylenetriamine by electrolysis is much slower (i.e. almost three times slower) and therefore the method takes much longer to function with a corresponding cost in energy consumption.

[0110] The data from Examples 2, 3, and 4 and Comparative Example 5 are shown in Figure 1. As can be seen from Figure 1, the concentration of amine compounds remaining after electrochemical oxidation is much lower when the electrochemical oxidation is carried out in the presence of chloride than when chloride is omitted from the process.

[0111] Example 6 Treatment of industrial zinc-nickel waste.

[0112] A sample of industrial zinc-nickel waste was electrolyzed using the process described in Example 2. The zinc-nickel waste electrolyte contained a 2% dilution of standard zinc-nickel plating electrolyte with about 40-50 g / L of amine complexing agent (i.e., about 0.8-1.0 g / L of amine complexing agent in the 2% dilution).

[0113] The chloride content was adjusted to 2000 ppm chloride ions by adding sodium chloride before starting the electrolysis. The amount of organic amines present in the solution was estimated using the spectrophotometric method shown in Example 2.

[0114] The results were as follows:

[0115] [Table 7]

[0116] This example compares the results obtained from the oxidation of diethylenetriamine in Examples 1, 2, and 4 to show that similar results can be achieved in the electrochemical oxidation of industrial waste containing amine compounds.

[0117] Commercially available zinc-nickel electrolytes use a variety of amine complexing agents, which can be used at different concentrations depending on the bath composition and the specific type of complexing agent. For example, in some cases, tetraethylenepentamine may be used in the range of about 1 to about 5 g / L, diethylenetetramine may be used in the range of about 12 to 20 g / L, triethanolamine may be used in the range of about 2 to 10 g / L, and Quadrol® may be used in the range of about 15 to 25 g / L. Other amine complexing agents and concentrations of the above complexing agents may also be used depending on various factors. It is believed that the process described herein can be used with most amine-complexed metals to hydrolyze the amine compound and allow for further processing of the metal ions.

[0118] The resulting waste stream can then be subjected to further processing to precipitate the zinc and nickel ions, such as by hydroxide precipitation, and is therefore suitable for discharge because it contains levels of zinc and nickel (or other metals) below regulated levels.

Claims

1. 1. A process for treating a waste stream containing organic amine compounds complexed with heavy metal ions, comprising: a) adjusting the pH of the waste stream to about 4 to about 10; b) adding a chloride salt to said waste stream to produce a concentration of chloride ions in said waste stream; c) circulating the waste stream through an electrochemical reactor, the electrochemical reactor comprising an array of electrodes comprising alternating anodes and cathodes, circulating the waste stream through the electrochemical reactor for 30 to 180 minutes to hydrolyze amine compounds; and thereafter d) removing heavy metal ions from said waste stream; The process includes:

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

2.

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

4. 2. The process of claim 1, wherein the concentration of chloride ions in the waste stream is from about 10 to about 50,000 mg / L.

5. 5. The process of claim 4, wherein the concentration of chloride ions in the waste stream is from about 600 to about 1,200 mg / L.

6. 2. The process 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. 2. The process of claim 1, wherein the anode is selected from the group consisting of platinum or titanium or niobium coated with mixed metal oxides; combinations of one or more of iridium oxide, ruthenium oxide, and tantalum; lead oxide; graphite; and boron doped diamond.

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

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

10. The amine compounds include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine; alkylene oxide adducts, such as ethylene oxide adducts and propylene oxide adducts of the above alkyleneamines; aminoalcohols, such as ethanolamine, diethanolamine, triethanolamine, diisopropanolamine, triisopropanolamine, ethylenediaminetetra-2-propanol, N-(2-aminoethyl)ethanolamine, and 2-hydroxyethylaminopropylamine; alkanolamine compounds, such as N-(2-hydroxyethyl)ethanolamine, and 2-hydroxyethylaminopropylamine.

2. The process of claim 1, wherein the poly(alkyleneimine) is selected from the group consisting of 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(alkyleneimine)s derived from ethyleneimine, 1,2-propyleneimine; poly(alkyleneamine)s derived from ethylenediamine, triethylenetetramine; ethylenediaminetetraacetic acid, and combinations of one or more of the foregoing.

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

Images