Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-12-17
2001-03-13
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C205S488000
Reexamination Certificate
active
06200448
ABSTRACT:
The present invention relates to manufacturing nickel hypophosphite under conditions enabling it to be used directly as the sole active component in the chemical (or “electroless”) nickel-plating process.
BACKGROUND OF THE INVENTION
Traditionally, the chemical nickel-plating process makes use of nickel salts and a hypophosphite, e.g. hexahydrated nickel sulfate and monohydrated sodium hypophosphite. It has already been recommended to replace those two components with a single component, namely nickel hypophosphite. It turns that using this single component leads to an increase in the lifetime of solutions for chemical nickel plating and consequently to decreasing the amount of pollution by nickel; in addition, this new formulation gives rise to the resulting nickel deposits being of better quality, in particular by reducing internal tensions.
Nickel hypophosphite can be produced by causing hypophosphorous acid to react with nickel hydroxide or with nickel carbonate or with some other suitable nickel salt. That purely chemical manufacturing technique suffers from being very expensive.
Proposals have already been made in document EP 693 577 to produce nickel hypophosphite by a method that implements the following steps, namely: putting a nickel anode in contact with a solution of hypophosphite anions; applying electrical current from said anode to a cathode that is itself in electrical contact with said solution so that the nickel of the anode is dissolved into the hypophosphite solution, thereby forming a solution of nickel hypophosphite; and recovering and concentrating said solution of nickel hypophosphite.
The same method can be implemented in an electrodialysis cell having three compartments separated by ion exchange membranes, one for cations and the other for anions. In the receptacle acting as an electrolyzer, the central compartment contains a hyposulfite solution of an alkali metal; it is separated from the compartment containing the nickel anode by an anionic membrane, i.e. a membrane that enables anions to diffuse, but that prevents cations from diffusing; in addition, it is separated from the cathode compartment by a cationic membrane, i.e. a membrane that enables cations to diffuse while preventing anions from diffusing. The solutions contained respectively in the anode compartment (referred to as “anolyte”) and in the cathode compartment (referred to as “catholyte”) are selected to be conductive so that electricity can pass through these solutions when a voltage is applied. Specifically, the anolytic solution contains hypophosphorous acid, and the catholytic solution contains caustic soda, both at predetermined concentration and pH. Nickel hypophosphite forms in the compartment containing the anolytic solution.
Document U.S. Pat. No. 5,716,512 proposes a method and an installation for manufacturing nickel hypophosphite from a solution of nickel sulfate and a solution of sodium hypophosphite by using the electro-membrane technique. In example 8, as illustrated in FIG. 9 of that document, both electrodes are insoluble, a second cell delivering the anolyte to the first under conditions comparable to those that are obtained in example 1 from a nickel anode that is soluble. In that method, a pH regulator is provided to activate or deactivate the electrodialysis cell to adjust and control the pH, at least of the first electrolyte. Doubtless the purpose of monitoring the pH of the solutions contained in the various compartments of the first cell is to avoid unwanted and harmful precipitation of nickel hydroxide at the interfaces of the various membranes.
OBJECT AND SUMMARY OF THE INVENTION
The object of the present Applicant is to provide a method of manufacturing nickel hypophosphite that implements an electro-membrane technique that is different from that described in document EP 693 577, the electrodialysis not requiring the use of a soluble nickel anode, and different from that described in document U.S. Pat. No. 5,716,512, the electrodialysis not requiring the pH of the solutions to be regulated. This object is achieved by the method of the invention which consists:
a) in introducing respectively the hexahydrated nickel sulfate solution and the monohydrated sodium hypophosphite solution into each of two dilution circuits of four-compartment electrodialysis cells formed by alternating stacks of cationic and anionic homopolar membranes in an electrodialysis apparatus having an anode and a cathode that are insoluble;
b) in applying an electrical current from the anode to the cathode without regulating the pH of the solutions contained in the dilution and concentration circuits but regulating the electricity supply, either in voltage or in current; and
c) in recovering a hexahydrated nickel hypophosphite solution from one of the concentration circuits.
Thus, in the method of the invention, there are no electrochemical reactions at the electrodes, and in particular no dissolution of the anode. By using ion transport through the membranes, the initial salts, i.e. sodium hypophosphite and nickel sulfate, are both subjected to decomposition leading to ion pairs recombining in two ways: nickel hypophosphite and sodium sulfate. Preferably, the anode is made of platinum-plated titanium and the cathode is made of stainless steel.
With reference to document U.S. Pat. No. 5,716,512, the installation described in example 8 and shown in FIG. 9 dozes not make use of four-compartment electrodialysis cells made by stacking cationic and anionic homopolar membranes in alternation; even though FIG. 9 does show four compartments C1 to C4, those four compartments include the two electrode compartments, and they are separated by three membranes only, two cationic membranes and one anionic membrane.
The electrical power supply is said to be regulated in that the electrical parameters of the electrodialysis are controlled so that at no stage during the method of the invention is nickel hydroxide observed to precipitate at the membrane interfaces.
More particularly, the electrical voltage applied between the cells must necessarily be less than 18 volts since otherwise nickel hydroxide will be caused to precipitate and it is possible that the membranes will be damaged.
Since the manufacturing method is a discontinuous method, in which it is desired to increase the concentration of the nickel hypophosphite solution, the electrical power supply is switched off once the indicated voltage is equal to 18 volts.
Starting from a nickel hypophosphite solution at 1.1 grams per liter (g/l), and after operating for a period of the order of 405 min to 415 min, the resulting nickel hypophosphite solution had a concentration of the order of 202 g/l to 212 g/l.
The current applied to the electrodialysis while implementing the method is at a density of up to 150 amps per square meter (A/m
2
) for a voltage density of up to 5 volts per cell.
Preferably, the concentration of hexahydrated nickel sulfate in the first dilution circuit lies in the range 0 to 2 M, the concentration of the monohydrated sodium hypophosphite solution in the second dilution circuit lying in the range 0 to 4 M, the concentration of the sodium sulfate solution obtained in the first concentration lying in the range 0 to 1 M, and the concentration of the hexahydrated nickel hypophosphite solution obtained in the second circuit lying in the range 0 to 1 M. Both electrodes are disposed in electrode compartments which are separated from the electrodialysis compartments by respective cationic membranes, the anode compartment being in register with an anionic membrane and the cathode compartment with a cationic membrane.
Both electrode compartments are filled with a solution of sodium sulfate, which constitutes the electrically conductive electrolyte for electrodialysis in the various cells. During electrodialysis, both electrode compartments constitute ion-transfer receptacles, for transferring sodium ions in the cathode compartment and sulfate ions in the anode compartment. Identical ions are thus transferred into these two electrode compartme
Phasge Arun S,.
Richard Etablissements A.
Wolf Greenfield & Sacks P.C.
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