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(11) | EP 0 977 918 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
A WOOD DELIGNIFICATION CHEMICAL PROCESS USING AMMONIUM MAGNESIUM BISULPHITE AS THE ACTIVE REAGENT EIN HOLZDESIGNIFIZIERENDES CHEMISCHES VERFAHREN WOBEI AMMONIUM MAGNISEIUM BISULFIT ALS AKTIVES REAGENZVERWENDET WIRD PROCEDE CHIMIQUE DE DELIGNIFICATION DU BOIS UTILISANT COMME REACTIF ACTIF LE BISULFITE D'AMMONIUM ET DE MAGNESIUM |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] Several sulphite wood delignification process with industrial application have been known for a number of years. The distinction between them is based on the active chemical present in the reagent solution (cooking acid).
[0002] The compounds used as active chemicals are all monocationic bisulphites, mainly of calcium, magnesium, sodium or ammonium. The pulping process with calcium bisulphite is an exception, when dolomite is used to prepare the acid.
[0003] In the last 30 years the bisulphite cellulosic pulps have been losing competivness to KRAFT pulps, which are obtained by a chemical process in an alkaline medium.
[0004] The major disadvantage of bisulphite pulps is the weaker mechano-physical characteristics of their cellulosic fibres when compared with those obtained by the KRAFT process.
[0005] The present innovation refers to a chemical process for wood delignification and consists of the use of a reagent solution where ammonium and magnesium bisulphites are together the active chemicals.
[0006] The combination of both ammonium and magnesium cations as bisulphite is the novelty of this application
[0007] This new pulping process is particularly useful to those industrial plants where cellulose pulp is obtained by the magnesium acid bisulphite process.
[0008] The specific cooking acid of magnesium acid bisulphite pulping process is, physically, a gas-liquid system. The liquid phase is a magnesium bisulphite and sulphur dioxide aqueous solution in equilibrium with the SO2 gas phase. The system is kept at temperature and pressure values of about 50°C and 2 Kg cm-2, respectively.
This particular cooking acid will be referred further on this document as the INICIAL COOKING ACID.
[0009] The basis of the present innovation is the inclusion of the ammonium ion in the initial cooking acid, as described by the following equation:
[0010] The cooking acid obtained by this reaction will be referred to in this document as the FINAL COOKING ACID.
FINAL COOKING ACID composition: Mg(HSO3)2 + NH4(HSO3) + [SO2 =f(pH)]
[0012] The addition of ammonium hydroxide must drive the initial pH value of inicial cooking acid (1.5) to a final value between 2 and 4, depending on the needs of the process. This operation could be performed in a tank with the initial cooking acid under pressure (accumulator) or in the reactor (digester).
[0013] In the FINAL COOKING ACID the percentage of total SO2, free plus combined forms, must be kept within 4% to 7% , depending on the requirements of the process.
[0014] The pulping process operated at a higher pH value, such as that attained after ammonium hydroxide addition, decreases the wood degradation process, enables the production of cellulosic fibres with higher mechanico-physical index values, and decreases the specific consumption of wood.
[0015] On the other hand, the presence of ammonium ions accelerate the reaction process decreasing both wood chip impregnation time and percentage of uncooked material. This latter effect improves the wood yield of the process.
[0016] The experimental results obtained with hard wood, namely eucalyptus, show the superiority of the present innovation over the magnesium acid bisulphite pulping process, which is expressed by:
- a - Production of cellulosic fibres with a higher mechanico-physical index values.
- b - Decreased percentage of uncooked material.
- c - Increased wood yield.
- d - Less environmental impact.
a) start from an initial cooking acid at pH 1.5, temperature of 50°C and pressure
of 2 kg cm-2, with the following composition:
Mg (HSO3)2 (aq) + SO2 (g) ;
b) add ammonium hydroxide in order to obtain a final cooking acid with the following
composition:
Mg(HSO3)2 (aq) + NH4HSO3 (aq) + SO2 (g)
with a pH between 2 to 4 and a concentration of total SO2 from 4% to 7%.
a) man zuerst, bei einer Temperatur vom 50°C eine Kochsäure mit der folgenden Zusammensetzung
Mg(HSO3)2 (aq) + SO2 (g)
mit einem pH 1.5, bei einem Druck von 2 Kg cm -2, benutzt;
b)dann Ammoniumhydroxis hizufügt um, als Endprodukt, eine Kochsäure mit der folgenden
Zusammensetzung
Mg (HSO3)2 (aq) + NH4HSO3 (aq) + SO2 (g)
mit einem pH zwischen 2 und 4 und eine SO2 Gesammtkonzentration von 4% bis 7%, zu
bekommen.
a) commencer à partir d'un acide de cuisson initial ayant un pH de 1,5, une température
de 50°C et une pression de 2 kg cm-2, avec la composition suivante :
Mg(HSO3)2 (aq) + SO2 (g) ;
b) ajouter l'hydroxyde d'ammonium afin d'obtenir un acide de cuisson final avec la
composition suivante :
Mg(HSO3)2 (aq) + NH4HSO3 (aq) + SO2 (g)
ayant un pH de 2 à 4 et une concentration de SO2 total d'entre 4% et 7%.