EP0684324A1 - Process for manufacturing metal hydroxides - Google Patents

Abstract

Prepn. of metal hydroxides and/or metal oxy-hydroxides from the corresp. metal ions and OH ions comprises forming the metal ions in a membrane electrochemical process by anodic dissolution of the corresp. metal in the anode compartment, forming the OH ions by redn. of water in a cathode compartment bounded by an anion exchange membrane and transferring the OH ions through this membrane into the anode compartment by the driving force of an electric field. The novelty is that the metal is dissolved at pH above 7 in the presence of a ligand (I).

Description

The present invention relates to a method for producing metal hydroxides and / or metal oxide hydroxides from corresponding metal ions and hydroxide ions, the metal ions being formed in a membrane electrochemical process by anodic dissolution of corresponding metals in the anode space and the hydroxide ions by cathodic reduction of water in the cathode space delimited by an anion exchange membrane and the hydroxide ions are transferred to the anode compartment through the anion exchange membrane under the driving force of an electric field.

Metal hydroxides and metal oxide hydroxides are valuable intermediates for the production of inorganic or organic salts of these metals, for the corresponding oxides or the pure metals themselves. Cobalt hydroxide by calcining a cobalt oxide of defined composition e.g. for use in electronics for the production of varistors or in accumulators or by reducing a cobalt metal powder defined particle size distribution. Nickel hydroxides serve as pigments or are used with various doping and particle structures for use in batteries. Zinc hydroxides can serve as precursors for pigments and the copper compounds can be converted into catalytically active materials.

When manufacturing hydroxides for various applications, the primary goal is to produce the most compact and flowable material possible for further processing. Cobalt metal powder, made from cobalt hydroxide or Cobalt oxide hydroxide, due to its particle size distribution and particle structure after sintering, together with tungsten carbide, results in, for example, special hard metal tools.

For the newly developed foam anodes, which are used in particular in nickel hydride storage cells, a nickel hydroxide is required, the physical properties of which are optimized both with regard to the intended use and the processing technology used. The use in high-performance accumulators with nickel foam electrodes based on the paste technology requires a material with high flowability, compact particle shape, narrow particle size distribution and constant quality. Furthermore, the product should look good with the commonly used additives such as Allow cobalt metal powder and cobalt oxide to mix.

A corresponding material and basic features of the manufacturing process are described in the patent JP Hei 4-80513. Nickel hydroxide particles with a diameter between 1 and 100 microns are crystallized by continuously feeding a nickel salt solution and an alkali hydroxide in solid or liquid form into a reaction vessel at a constant pH and at a constant temperature with vigorous stirring. A pH of 11 and a temperature of 48 ° C are given as favorable test conditions.

It is also known that a sufficiently compact nickel hydroxide can be produced by precipitation in the presence of ammonia or an ammonium salt. According to Trans. Faraday Soc. 51 (1955) 961, a nickel amine complex solution is prepared from nickel nitrate and aqueous ammonia solution, from which a nickel hydroxide is obtained by boiling at ordinary or reduced pressure or by treatment with steam, and which is significantly less specific than the nickel hydroxides which are precipitated in the absence of ammonia Surface area (13 to 20 m² / g). The production of compact nickel hydroxides in the presence of ammonia or an ammonium salt is also evident from patent applications JP-A 53-6119 and JP-A 61-18107. In the first-mentioned patent application, the precipitation of nickel hydroxide by adding an alkali metal hydroxide solution to a corresponding solution with a pH of at least 3.0 described. Electrochemical investigations on the material produced in this way revealed particularly high specific charge capacities in comparison to commercially available nickel hydroxides.

However, such products do not yet meet the requirements for particle shape, particle size distribution and flowability mentioned above.

Essential features of the process for producing a compact nickel hydroxide and its use in alkaline batteries are described in EP-A 353 837. A nickel (II) tetrammine salt solution is prepared by dissolving nickel nitrate or nickel sulfate in dilute ammonia solution and decomposed by the controlled addition of sodium hydroxide solution in accordance with the following reaction:



Ni (NH₃) ₄SO₄ + 2 NaOH ⇒ Ni (OH) ₂ + Na₂SO₄ + 4 NH₃



The reaction takes place at temperatures between 40 and 50 ° C and in the pH range between 11 and 13. The pore volume decreases with decreasing pH. It is expressly stated that a pore-free product can only be crystallized at sufficiently slow reaction rates. Furthermore, the nickel hydroxide produced by this process has a high crystallinity, a low specific surface area, a small pore volume and therefore a high physical density. The disadvantages of this product due to the high density are also described. The low specific surface area results in a lower proton conductivity and a higher current density, which promotes the formation of the undesired γ-NiOOH, which leads to the swelling of the electrode. Although the nickel hydroxide crystallized at low pH values has a high density, it tends to form γ-NiOOH. By choosing an average pH value, a compromise can be found between the required high density and the porosity required to a certain extent. This process produces a nickel hydroxide containing 3 to 10% zinc or 1 to 3% magnesium in solid solution. These doping counteract the formation of γ-NiOOH.

JP Hei 4-68249 discloses a continuous process for crystallizing a nickel hydroxide with a spherical particle shape. A nickel salt solution (0.5 to 3.5 mol / l), dilute alkali metal hydroxide solution (1.25 to 10 mol / l) and an ammonia and / or ammonium salt solution are continuously heated with intensive stirring into a heated overflow pipe cylindrical container pumped, the ammonia can also be introduced in gaseous form. The ammonia concentration is given as 10 to 28% by weight and the ammonium salt concentration as 3 to 7.5 mol / l. To complex the nickel, between 0.1 and 1.5 moles of ammonia are added per mole of nickel salt solution. After about 10 to 30 hours, the system reaches a steady state, after which a product of constant quality can be continuously discharged. The residence time in the container is between 0.5 and 5 hours.

An essential feature of this process is that the reaction is carried out at a defined pH, which is kept constant at ± 0.1 pH levels in the range between 9 and 12 by pH-controlled supply of alkali metal hydroxide solution, and at a constant temperature in the range between 20 and 80 ° C, the temperature deviations should not be more than ± 2 K. Under these conditions, the compact spherical particles with a particle size between 2 and 50 µm are obtained. The particle size can be adjusted in particular by varying the NH₃ inflow, the residence time and the stirring speed. With decreasing back speed or increasing NH₃ inflow, the particle size increases. As the dwell time in the container increases, the product becomes coarser and the particle size distribution narrows. The crystals are then filtered, washed with water and dried. The product produced by this process has the properties mentioned at the outset and does not need to be ground.

EP A 462 889 discloses a process for the production of nickel hydroxide. The temperature range of the crystallization is above 80 ° C. Nitrate or sulfate solutions doped with cobalt, cadmium and / or zinc are used. The cobalt content is between 1 and 8% by weight and the cadmium and / or zinc contents are between 3 and 10% by weight. Complexation takes place with the help of an ammonium salt, the molar ratio NH₃ / Ni is between 0.3 and 0.6. This method maintains a pH of 9.2 ± 0.1. Furthermore, a three-bladed stirrer, the diameter of which is half the size of the container diameter and the speed of which is between 300 and 1000 rpm, is used.

As in the processes already described, the product is filtered, washed and dried.

The disadvantages of these processes are on the one hand the inevitable large amounts of neutral salts, which are at least twice the stoichiometric amount of nickel hydroxide and are released into the waste water. On the other hand, in addition to small amounts of complex dissolved nickel, this wastewater also contains large amounts of ammonia, which have to be disposed of.

The chemical process of precipitation crystallization for the production of spherical nickel hydroxide inevitably produces 2 moles of sodium chloride per mole of nickel hydroxide. With regard to stricter environmental guidelines and limit values for wastewater on the one hand and economic aspects due to the high consumption of lye and resulting landfill costs for the resulting salt on the other hand, closed production cycles must be developed.

In such a procedure, nickel is anodically dissolved in a metal salt solution by means of electrolysis and precipitated as nickel hydroxide by the hydroxide ions formed cathodically. After sedimentation and various subsequent washing stages to clean the precipitated product from salts that are still present or are included in the precipitation, the pure product is obtained.

Processes for the production of metal hydroxides have already been described in the following patents. In JP-A 63/247 385 the electrolytic production of metal hydroxides is carried out using a perfluorinated anion exchange membrane from Toyo Soda and the use of inert electrodes. The metal salt of the metal hydroxide to be produced is used as the electrolyte on the anode side. An alkaline solution is used in the cathode circuit. In EP-A 0 559 590 this is described in a comparable arrangement Metal salt continuously added by anodic dissolution of the electrode. The requirements for the process, in particular the membranes to be used, the electrolyte solutions and the test conditions are insufficiently specified.

The object of this invention is to provide a process for producing metal hydroxides and / or metal oxide hydroxides which does not have the disadvantages of the prior art described.

This object is achieved by a process for producing metal hydroxides and / or metal oxide hydroxides from corresponding metal ions and hydroxide ions, the metal ions being formed in a membrane electrochemical process by anodic dissolution of corresponding metals in the anode space and the hydroxide ions by cathodic reduction of water in the cathode space delimited by an anion exchange membrane and the hydroxide ions are transferred under the driving force of an electric field through the anion exchange membrane into the anode compartment, the dissolution of the metals being carried out in the presence of a complexing agent at a pH> 7.

Ammonia and / or organic mono- and / or diamines with a chain length of 1 to 6 carbon atoms are preferably used as complexing agents in the sense of this invention. Metals are in particular one or more from the group Co, Ni, Cu, Fe In, Mn, Sn, Zn, Zr, Ti, Al, Cd and Ni. Co and / or Ni are particularly preferred. The process according to the invention is also described below for the case of the production of nickel hydroxide, without restricting the invention thereby.

The configuration which results in principle for a membrane electrolysis cell and is suitable for carrying out the method according to the invention is shown below. The cathode and anode compartments of the electrolytic cell are separated by an anion exchange membrane, so that two separate circuits result. The circuit on the side of the cathode is called catholyte, that on the anode side is called anolyte. Alkaline solutions such as sodium hydroxide solution or potassium hydroxide solution can preferably be used as the catholyte. For the economy of the process, it is advantageous if the solution itself has a high conductivity and the cation of the alkali used is also used on the anode side. The cathode itself can be made from tempered steel, platinized titanium, nickel or a nickel alloy.

The composition of the anolyte results from the starting materials for the production of nickel hydroxide, i.e. Ammonia, sodium chloride and small amounts of nickel sulfate. The sodium chloride primarily serves to increase the conductivity of the solution and the small addition of sulfate improves the anodic dissolution of the nickel electrode. Particularly good results are achieved if chloride and / or sulfate ions are present in the anolyte. The anode itself consists of pure nickel, preferably an electrochemically produced anode.

When producing other metal hydroxides and / or metal oxide hydroxides, the anode consists of the corresponding metals. Basically, a sacrificial anode is used.

Under active transport conditions due to the external potential applied, nickel dissolves as a Ni²⁺ ion, releasing electrons. The presence of ammonia prevents spontaneous precipitation of Ni (OH) ₂ under alkaline conditions and leads to a divalent nickel-amine complex via various intermediates.

The reaction at the cathode supplies hydrogen, which escapes in gaseous form, and hydroxide ions, which are transported according to their charge via the anion exchange membrane into the anode circuit. The nickel hydroxide then forms and precipitates in the anolyte when the solubility limit is exceeded. The precipitation follows one dynamic equilibrium, where a ligand exchange (ammonia for hydroxide) takes place.

The formation of the spherical product is essentially determined by the crystallization conditions, i.e. determines the concentration of the individual components and the temperature control in the anode circuit. The precipitated product is then continuously separated from the anolyte circuit. The separation can be carried out in a sedimentation container that is easy to implement in terms of process technology, owing to the large difference in density of the product formed and the solvent. To separate a product with a uniform grain size, the separation takes place via a filtration stage (microfiltration). The main advantage of this process variant is that additional, individual process steps for the recovery of the different educts are omitted, since they are kept in the anolyte cycle.

To implement the electrochemical membrane process described, it must be ensured that the anion exchange membrane to be used meets the following requirements:
It must be alkali-stable, in particular chemically stable in the adjacent solutions (against NH₃ to saturation concentration), oxidation-stable (Ni²⁺ / Ni³⁺; Cl⁻, ClO³⁻), temperature-stable up to 80 ° C, it must have a high permselectivity, a low membrane resistance have high mechanical strength and dimensional stability and sufficient long-term stability.

Technically relevant ion exchange membranes usually have a micro-heterogeneous and / or an interpolymer morphology. This is intended to ensure that the mechanical and electrochemical properties can be set in a decoupled manner. Accordingly, a membrane is constructed from a matrix polymer, a fabric or a binder, and from a polyelectrolyte or an ionomer. A distinction is made according to the degree of heterogeneity of the ion exchange membrane between homogeneous membranes, interpolymer membranes, micro-heterogeneous graft or block copolymer membranes and heterogeneous membranes.

The polymer network can be constructed differently in order to have sufficiently good electrical and mechanical properties for most applications. Polyvinyl chloride and polyacrylate are usually used as the charge-neutral matrix polymer. Polyethylene, polypropylene or polysulfone can also be used as further matrix polymers, only these having long-term chemical stability under alkaline conditions.

Thus, in the process according to the invention, preference is given to using an anion exchange membrane based on polyethylene, polypropylene, polyether ketone, polysulfone, polyphenyl oxide and / or sulfide.

The ion-conducting polyelectrolytes of an anion exchange membrane consist of a network with a positive excess charge and mobile, negatively charged counterions. The framework can be composed of weakly basic amino and imino groups, as well as strong basic immonium and quaternary ammonium groups:



-NH₃⁺ -RNH₂⁺ -R₃N⁺ = R₂N⁺



The anion exchange membrane used in the process according to the invention particularly preferably has exchange groups composed of alkylated polyvinylimidazole, polyvinylpyridine and / or alkylated 1,4-diazabicyclo [2.2.2] octane.

Particularly suitable membranes are described in DE-A 42 11 266.

The type and concentration of fusions mainly determines the permselectivity and the electrical resistance of the membrane, but can also affect the mechanical properties, in particular the swelling of the membrane due to the concentration of fusions. The strongly basic quaternary ammonium group is dissociated at all pH values, while the primary ammonium group is only dissociated black. For this reason, quaternary ammonium groups are mostly incorporated into commercial anion exchange membranes, except that a membrane with certain properties is to be produced.

Systems based on chloromethylated polystyrene, styrene / divinylbenzene copolymers and styrene / butadiene copolymers with subsequent quaternization with trimethylamine are used most frequently.

• Zerstörung der Polymermatrix (unzureichende Stabilität des Matrix- oder Interpolymers in alkalischer Lösung)
• morphologische Veränderung des Systems Festionengerüst/Polymermatrix
• chemischer Abbau der Festionen unter alkalischen oder oxidativen Bedingungen

Zur Auswahl einer Anionenaustauschermembran für den Einsatz bei der Herstellung von sphärischem Nickelhydroxid mittels Membranelektrolyse aus ammoniakalischer Lösung müssen sowohl die elektrochemischen, die mechanischen und die chemischen Eigenschaften in gleicher Weise optimiert sein. Dies bedeutet, daß Vorgaben hinsichtlich Membran- bzw. Materialauswahl und den vom Hersteller dargestellten physikochemischen Eigenschaften erstellt und evaluiert werden müssen. Diese Vorgaben lassen sich für die erfindungsgemäß eingesetzten Membranen wie folgt zusammenfassen:
Bezüglich der elektrochemischen Eigenschaften sollte der elektrische Widerstand <10Ω-cm², die Permselektivität >92 %, die Quellung <25 % und die Ionenaustauscherkapazität >1,2 mmol g⁻¹ betragen. The long-term chemical stability of the anion exchange membranes can only be influenced by the following factors:

• Destruction of the polymer matrix (insufficient stability of the matrix or interpolymer in alkaline solution)
• morphological change in the system of the framework / polymer matrix
• chemical degradation of the fixations under alkaline or oxidative conditions

To select an anion exchange membrane for use in the production of spherical nickel hydroxide by means of membrane electrolysis from an ammoniacal solution, both the electrochemical, the mechanical and the chemical properties must be optimized in the same way. This means that specifications regarding membrane or material selection and the physicochemical properties presented by the manufacturer must be created and evaluated. These specifications can be summarized as follows for the membranes used according to the invention:
Regarding the electrochemical properties the electrical resistance <10Ω-cm², the permselectivity > 92%, the swelling <25% and the ion exchange capacity Be> 1.2 mmol g⁻¹.

Regarding the mechanical properties, the fabric should consist of temperature, alkali and oxidation stable polymers (polypropylene, polyethylene, polyether ketone) and have a chemically stable quaternary ammonium salt (vinylimidazole, 4,4'-diaza-bicyclo [2.2.2] octane) as a fixed charge.

Suitable membranes are described in DE-A 44 21 1266. The process according to the invention is particularly preferably carried out continuously, the metal hydroxide and / or metal oxide hydroxide formed being separated from the anolyte and the complexing agent being returned to the anode compartment.

The invention is explained by way of example below, without any restriction being seen here.

Example 1 Preparation of cobalt hydroxides Basic structure of the electrolytic cell

The electrolytic cell is composed of two nickel cathodes, two spacers made of polyethylene, two membranes and the cobalt sacrificial anode and four frames of different thicknesses. The cell is constructed so that the nickel cathodes represent the outer sides of the cell with an area of 120 x 200 mm² effective electrode area. The electrical contact is made on protruding electrode surfaces. There is a PE frame of 5 mm thickness on the cathodes, on which in turn the membrane rests. With a further frame of 10 mm thickness, the distance to the cobalt anode is maintained, which protrudes beyond the frame and is provided with the electrical leads. The cobalt anode consists of pure cobalt with a thickness of 20 mm. The entire structure is pressed together in a liquid-tight manner using a holder. A PE grid is inserted between the cathodes and the membrane, which prevents contact between the cathode and the membrane. The frames that separate the anode and membrane are provided with holes through which the anolyte is fed in and out. The cathodes are also provided with feed lines so that a uniform flow of the catholyte is ensured in the entire cathode space.

The catholyte and anolyte each contain 100 g / l NaCl, the catholyte also 40 g / l NaOH.

The catholyte is repumped at a rate of 100 l / h, which corresponds to a residence time of the electrolyte of 9 seconds in the cathode compartment. The anolyte is pumped during the electrolysis at a rate of 650 l / h, which corresponds to an average residence time of 2.7 seconds in the anode compartment. The temperature of the anolyte is 50 ° C. The ammonia concentration in the anolyte is set to 2 mol / l and losses due to evaporation are compensated for by adding ammonia to the anolyte circuit.

The stationary solid concentration of cobalt hydroxides formed is 80 g / l with an average residence time of 4 h.

The electrolysis conditions are chosen so that a current of 12 A corresponding to 500 A / m² flows, 21 g of cobalt hydroxide in the form of Co (OH) ₂ being formed every hour, which are removed from the circuit in 0.26 l suspension and by filtration be separated. After washing with water, a clean cobalt hydroxide is obtained. The hydrogen formed gases from the catholyte pre-container. pH anolyte: 10.5 - 11.5 Membrane: Neosepta® AMH, manufacturer Tokuyama Soda

Composition of the final product

Cobalt hydroxide, mixture of Co (OH) ₂ with CoOOH in the ratio 80/20 after analysis Bulk density: 1.6 g / cm³ Cobalt content: 63.5% Colour: Dark brown

Example 2 Preparation of nickel hydroxide

In an electrolysis cell, which is constructed in a comparable manner as a stack of electrodes and membranes with the electrode spaces in between, nickel is dissolved electrochemically in the presence of ammonia and the amine complex formed is decomposed to nickel hydroxide.

Electrolyte composition: Anolyte: 16.5 mmol / l NiSO₄ 220 ml of NH₃ (25%) / l 2 mol / l NaCl Catholyte: 1 mol / l NaOH Anode: Pure nickel Cathode: platinum-plated titanium Temperature: Electrolysis 40 ° C decomposition of the complex 70 ° C Current density: 1000 A / m² Distance electrodes / membrane: 2 mm Overflow speed: > 10 cm / s pH anolyte: 10.5 - 11.5 Membrane: Neosepta® AMH, manufacturer Tokuyama Soda

The amine complex formed in the electrolysis is decomposed by increasing the temperature of the electrolyte in a reactor to give nickel hydroxide.
a) Production of a compact, spherical nickel hydroxide
The amine complex is decomposed in a stirred reactor, the decomposition product agglomerating into compact, spherical particles. The agglomerated material is continuously separated from the circulation of the anolyte via an overflow as a suspension.
Nickel hydroxide from overflow: Bulk density: 1.35 g / cm³ average particle size: 10 µm
b) When substrates such as fibers made of nickel or a spherical ion exchange resin with an average particle size of 200 μm are placed in the decomposition reactor, a uniform layer of nickel hydroxide is deposited on the substrate.

Claims (9)

Process for the production of metal hydroxides and / or metal oxide hydroxides from corresponding metal ions and hydroxide ions, the metal ions being formed in a membrane electrochemical process by anodic dissolution of corresponding metals in the anode space and the hydroxide ions by cathodic reduction of water in the cathode space delimited by an anion exchange membrane, and the hydroxide ions under the driving force of an electric field through the anion exchange membrane in the anode compartment, characterized in that the dissolution of the metals is carried out in the presence of a complexing agent at a pH> 7. A method according to claim 1, characterized in that ammonia and / or organic mono- and / or diamines with a chain length of 1 to 6 carbon atoms are used as complexing agents. Method according to one of claims 1 or 2, characterized in that one or more of the group Co, Ni, Cu, Fe, In, Mn, Sn, Zn, Zr, Ti, Al, Cd and U are used as metal. A method according to claim 3, characterized in that Co and / or Ni is used as the metal. Method according to one or more of claims 1 to 4, characterized in that an aqueous alkali metal hydroxide solution is used as the catholyte. Method according to one or more of claims 1 to 5, characterized in that chloride and / or sulfate ions are present in the anolyte. Process according to one or more of claims 1 to 6, characterized in that the anion exchange membrane used is one based on polyethylene, polypropylene, polyether ketone, polysulfone, polyphenyl oxide and / or sulfide. Process according to one or more of claims 1 to 7, characterized in that the anion exchange membrane has exchange groups made of alkylated polyvinylimidazole, polyvinylpyridine and / or alkylated 1,4-diazabicyclo [2.2.2] octane. Method according to one or more of claims 1 to 8, characterized in that the metal hydroxide and / or metal oxide hydroxide formed is separated from the anolyte and the complexing agent is returned to the anode compartment.