Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2000-11-20
2001-11-27
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S556000
Reexamination Certificate
active
06322690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing hydrogen peroxide in an electrochemical cell by cathodic reduction of oxygen. The invention further relates to production of chlorine dioxide.
BACKGROUND OF THE INVENTION
Alkaline solutions of hydrogen peroxide are commonly used for bleaching and/or delignification of cellulose pulp. The hydrogen peroxide is normally produced according to the anthraquinone process in large plants and transported to the pulp mills. The anthraquinone process is very effective, but requires high investments and is not suitable for small scale on-site production.
Electrochemical production of alkaline hydrogen peroxide solutions is disclosed in U.S. Pat. No. 5,702,585, “Process for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen”, P. C. Foller et al, Journal of Applied Electrochemistry, 25 (1995), p. 613-627 and in C. Oloman, “Electrochemical Processing for the Pulp and Paper Industry”, The Electrochemical Consultancy 1996 p. 143-152. However, none of the documents disclose a process that is flexible to obtain various ratios NaOH
−
:H
2
O
2
in combination with production of one or more other chemicals that can be used at pulp mills.
Also chlorine dioxide is a commonly used bleaching agent at pulp mills. Due to the chemical instability of chlorine dioxide it is always produced on-site and many different processes are used commercially, such as those described in U.S. Pat. Nos. 5,770,171, 5,091,166, 5,091,167 and EP 612686. Most of the commercial processes involve reaction of sodium chlorate with a mineral acid, normally sulfuric acid, and a reducing agent, such as chloride ions, sulfur dioxide, methanol or hydrogen peroxide, in an acidic reaction medium. Sodium sulphate is normally obtained as a by-product, either as an acid or neutral solid salt cake or in the form of an acidic residual solution. In many modern pulp mills the sulphate by-product is considered as a useless waste material that has to be disposed, although there still is a demand for some of the sulphate for make up of the cooking liquor.
The formation of sulphate by-product can be decreased or completely avoided by replacing part of or all the sodium chlorate with chloric acid, as described in WO 93/25470. It is also possible to electrochemically acidify the solid sodium sulphate obtained, as described in U.S. Pat. No. 5,198,080, or to electrochemically acidify depleted reaction medium, without crystallisation, as described in U.S. Pat. No. 5,487,881. However, none of these methods have so far been commercialised.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electrochemical process for producing alkaline hydrogen peroxide solutions that can be used in a great variety of bleaching processes, such as bleaching and/or delignification of cellulose pulp, particularly in ECF (elemental chlorine free) or TCF (totally chlorine free) sequences, or for brightening of mechanical pulp.
It is another object of the invention to provide an electrochemical process for producing alkaline hydrogen peroxide solutions, in which one or more other chemicals are obtained that can be used at a pulp mill, particularly an acidified chlorate containing solution for use in the production of chlorine dioxide.
It is still another object of the invention to provide an electrochemical process for producing alkaline hydrogen peroxide solutions, in which one or more by-products from chlorine dioxide production can be used as feed materials.
It is still another object to provide a method for production of hydrogen peroxide and chlorine dioxide that awards for flexibility as to the amount of sulphate by-product obtained.
These objects are achieved by the process defined in the appended claims.
GENERAL DESCRIPTION OF THE INVENTION
According to one aspect, the invention concerns a continuous process for production of an alkaline hydrogen peroxide solution and an acidified alkali metal salt solution in an electrochemical cell including an anode compartment provided with an anode and a cathode compartment provided with an oxygen reducing cathode. The process comprises the steps of:
(a) feeding an aqueous alkali metal salt solution containing chlorate to the anode compartment;
(b) reacting at the anode said aqueous alkali metal salt solution to obtain H
+
and to form an acidified aqueous alkali metal salt solution in the anode compartment;
(c) transferring H
+
and alkali metal ions from the anode compartment to the cathode compartment;
(d) feeding oxygen or oxygen containing fluid and water to the cathode compartment;
(e) reacting at the cathode said oxygen and water to form an alkaline aqueous solution containing hydrogen peroxide (H
2
O
2
) and alkali metal hydroxide (MOH) in the cathode compartment;
(f) withdrawing acidified aqueous alkali metal salt solution formed in the anode compartment; and,
(g) withdrawing alkaline aqueous hydrogen peroxide solution formed in the cathode compartment, wherein the molar ratio MOH:H
2
O
2
for the net production of alkali metal hydroxide and hydrogen peroxide in the cathode compartment in step (e) is maintained from about 0.1:1 to about 2:1, suitably from about 0.1:1 to about 1.8:1, preferably from about 0.1:1 to about 1.7:1, most preferably from about 0.5:1 to about 1.7:1. M refers to alkali metals such as sodium, potassium or mixtures thereof, among which sodium is most preferred.
The terms anode compartment and cathode compartment as used herein also include optional recirculation loops for anolyte and catholyte. The anode- and cathode compartments are suitably separated by at least one barrier, such as a membrane or a diaphragm, permeable for H
+
and alkali metal ions, but preferably not for anions such as hydroxide- and perhydroxyl ions. The barrier is preferably at least one ion-exchange membrane, most preferably at least one cation-exchange membrane. Standard polymeric ion-exchange membranes are preferred, but also high ion conducting membranes such as ceramic membranes can be useful. In most cases a two compartment cell is used, but also cells comprising one of more compartments between the anode- and cathode compartments may come into consideration.
Any oxygen reducing cathode can be used, although it is preferably made of carbon, such graphite or carbon black, optionally with other materials incorporated, like inert chemically resistant polymers (e.g. PTFE) or catalytically active materials (e.g. gold, zinc or the like), the latter suitably being employed as a partial coating on the carbon. Due to the low solubility of oxygen in the catholyte a three dimensional electrode is preferred as a cathode. One group of three dimensional electrodes include fixed beds with large porosity, for example reticulate (e.g. Reticulated Vitreous Carbon), particulate or felt (e.g. graphite felt) beds, or beds of non-woven fibres (e.g. carbon fibres), which all are commercially available. These kinds of fixed beds may be designed as a trickle-bed cathode. Another group of three dimensional electrodes are micro-porous, optionally layered, gas diffusion electrodes, which may, for example, be made of porous carbon. Such gas diffusion electrode are commercially available and are similar to those used in, e.g. fuel cells.
For the anode, any standard type of electrodes can be used, such as titanium coated with precious metal oxides, e.g. DSA O
2
™, or gas electrodes like polarised hydrogen anodes.
The aqueous solution fed to the anode compartment in step (a) is an optionally acidic solution containing alkali metal chlorate, optionally also containing alkali metal sulphate and/or other alkali metal salts. The solution suitably contains from about 0.5 moles/I up to saturation, preferably from about 1 to about 12 moles/I of alkali metal salts. The acidity is preferably from about 0 to about 11 N, most preferably from about 0 to about 9 N. In one preferred embodiment the solution originates directly or indirectly from a chlorine dioxide generator. For example, the alkali metal s
Hammer-Olsen Roy
Sokol John
Sundqvist Johan
Sundstrom Goran
Akzo Nobel N.V.
Phasge Arun S,.
Serbin David J.
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