Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Patent
1995-12-18
1997-04-29
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
205758, 205759, 205770, 205494, C02F 1461
Patent
active
056245459
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of a PCT SE94/00526 filed Jun. 1, 1994.
The present invention relates to a method for the electrolytic treatment of white liquor in a cell comprising separate anode and cathode compartments, sulphides present in the white liquor being oxidised in the anode compartment, and alkali being produced in the cathode compartment. The invention further concerns a method for manufacturing cellulose pulp, as well as an electrolytic cell suited for the treatment of white liquor.
When manufacturing cellulose pulp by alkaline cooking, usually through the so-called sulphate process, white liquor is used as cooking liquor and is obtained by treating the black liquor which is residual product from the cooking. The chemical composition of the white liquor is of major importance for the cooking results.
The demand of the pulp mills for alkali was previously met by sodium hydroxide obtained in the electrolytic production of chlorine gas for bleaching purposes. However, chloroine-gas bleaching has more and more fallen into disuse and is nowadays not used at all in most pulp mills, so that the chlorine gas formed in chlorine-alkali electrolysis has come to be regarded as an undesirable by-product.
In "Some Ideas on Polysulfide Pulping" by E. Venemark, Svensk Papperstidning (Swedish Paper Journal), No. 5, 15 Mar. 1964, it is stated that the yield of wood from alkaline pulp cooking is improved if the white liquor contains polysulphides. Although mention is made of electrolytic oxidation of sulphides, it is evident that tests in that direction have failed.
Further, it is known that the polysulphide content of white liquor can be increased by air oxidation, which is described in, for example, G. C. Smith et al, Paper Trade Journ., 1 May 1975, "All It Takes Is Moxy: Mead Oxidation System Generates Polysulfide Liquor".
Also, Yu. A. Malkov et al, Chemical Abstracts 101(8): 56694, Khimiya in Tekhnol. Dreves, Tsellyulozy, L. 1983, pp 45-50, from Ref. Zh., Khim. 1984, Abstract No. 6T3006 describe electrolytic treatments of white liquor but do not touch upon the production of alkali.
Moreover, the prior art encompasses the absorption of hydrogen sulphide from flue gases and the conversion thereof to sulphide, which is electrochemically oxidised in order to recover sulphur. This is described in, for example, A. A. Anani et al, J. Electrochem. Soc., Vol. 137, No. 9, September 1990, pp 2703-2709, "Electrochemical Production of Hydrogen and Sulfur by Low-Temperature Decomposition of Hydrogen Sulfide in an Aqueous Alkaline Solution"; Yen-Shiang Shih et al, Ind. Eng. Chem. Process Des. Dev., 1968, 25, pp 834-836, "Continuous Solvent Extraction of Sulfur from the Electrochemical Oxidation of a Basic Sulfide Solution in the CSTER System"; B. Dandapani et al, Intersociety Energy Conversion Engineering Conference 1986, pp 262-265, "Electrolysis of Hydrogen Sulfide in Aqueous Alkaline Solutions"; as well as U.S. Pat. No. 3,409,520. A common problem is the passivation of the anodes due to the precipitation of sulphur.
U.S. Pat. No. 4,717,450 describes how chloride is removed from spent liquor by electrolysis.
The present invention aims at providing a solution to the problem of how to produce alkali metal hydroxide from raw material normally accessible in pulp mills without the formation of any undesirable by-products. Also, the invention enables the improvement of processes commonly used for alkaline cooking of cellulose pulp.
Thus, the invention relates to a method for treating sulphide-containing white liquor, as set forth in the appended claims. According to the invention, the white liquor is treated in a continuously operating electrolytic cell having separate anode and cathode compartments, which are separated by a partially permeable barrier. White liquor is introduced into the anode compartment of the cell, and an aqueous solution of alkali metal hydroxide is introduced into the cathode compartment of the cell. In the anode compartment, sulphide is oxidised, while at the same time alkali metal cations are transported
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Venemark, "Some Ideas on Polysulfide Pulping" Mar. 1964, pp. 157-166.
Malkov et al., "Electro Chemical Oxidation of Sulphidic Sulphur in an Alkaline Medium." 1984 (no month) Abstract only.
"Some Ideas on Polysulfide Pulping", E. Venemark, Svensk Papperstidning (Swedish Paper Journal), No. 5, 15 Mar. 1964.
"Electro Chemical Oxidation of Sulphidic Sulphur in an Alkaline Medium", Yu. A. Malkov et al, Chemical Abstracts 101(8):56694, Khimiya in Tekhnol. Dreves, Tsellyulozy, L., 1983, pp. 45-50, from Ref. Zh., Khim., 1984, Abstract No. 6T3006 no month provided.
"Electrochemical Production of Hydrogen and Sulfur by Low-Temperature Decomposition of Hydrogen Sulfide in an Aqueous Alkaline Solution", A.A. Anani et al, J. Electrochem. Soc., vol. 137, No. 9, Sep. 1990, pp. 2703-2709.
"Continuous Solvent Extraction of Sulfur from the Electrochemical Oxidation of a Basic Sulfide Solution in the CSTER System", Yen-Shiang Shih et al, Ind. Eng. Chem. Process Des. Dev., 1968, 25, pp. 834-836 no month provided.
"Electrolysis of Hydrogen Sulfide in Aqueous Alkaline Solutions", B. Dandapani et al, Intersociety Energy Conversion Engineering Conference, 1986, pp. 262-265 no month provided.
"All It Takes Is Moxy: Mead Oxidation System Generates Polysulfide Liquor", G.C. Smith et al, Paper Trade Journ., 1 May 1975, pp. 38-41.
H.ang.kansson Bo
Landfors Johan
Norell Maria
Eka Nobel Inc.
Phasge Arun S,.
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