WOOD DELIGNIFICATION CHEMICAL PROCESS USING AMMONIUM MAGNESIUM BISULPHITE AS THE ACTIVE REAGENT

Abstract

The present innovation consists of the use of a reagent solution (FINAL COOKING ACID) where ammonium magnesium bisulphite is the active chemical. This new process is particularly useful to those industrial plants where the cellulose pulp is obtained by the magnesium acid process. The present innovation consists of the introduction of ammonium ions in the cooking acid used in such plants (initial cooking acid). The final cooking acid is obtained by the reaction of the initial cooking acid with ammonium hydroxide. Experimental results show the superiority of the present innovation over the magnesium acid bisulphite pulping process which is expressed by production of cellulosic fibres with higher mechanico-physical index values, decrease in the percentage of uncooked material, increase in wood yield and less environmental impact.

Description

[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] SU-A-1359387 discloses a pulping process for wood from deciduous trees. This process is a semi-chemical process, the liquor pH is alkaline (8-9.5), the liquor composition is MgSO3+(NH4)2SO3+[NH4+=f(pH)] and the active chemical agente is ammonium magnesium sulphite. U.S. Pat. No. 463,499 discloses a special hardwood sulphite delignification process using exclusively the ammonium base. GB-A-734216 discloses a process for the treatment of wood chips with sulphur dioxide prior with digestion with alkali, namely caustic soda. U.S. Pat. No. 4,141,787 discloses a special acid bisulphite process using calcium or magnesium base, followed by the feed of liquid So2. DD-151581 discloses a special wood delignification process using a monocationic bisulphite.

[0006] 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.

[0007] The combination of both ammonium and magnesium cations as bisulphite is the novelty of this application.

[0008] This new pulping process is particularly useful to those industrial plants where cellulose pulp is obtained by the magnesium acid bisulphite process.

[0009] The expression “ammonium magnesium bisulphite” means an aqueous unsaturated solution containing an ionic mixture of ammonium and magnesium bisulphite salts, fully dissociated: HSO3−+Mg2++NH4+, a pH=2-4 kept through an excess of dissolved sulphur dioxide.

[0010] The bisulphite cooking is an aqueous solution of bisulphite containing an excess of SO2 called “True free”.

[0011] The composition of bisulphite cooking acid is determined by the total amount of dioxide sulphur in liquor and by ratio of Combined SO2 to Free SO2. The amount of Free SO2 is the total of the True Free plus one half of SO2 in bisulphite form. It is the amount of SO2 in excess of the amount theoretically required to form monosulphite. By the same token, the Combined SO2 is equal to one-half of SO2 in the bisulphite, or equal to the amount of SO2 held as monosulphite.

[0012] 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 Kgcm−2, respectively.

[0013] This particular cooking acid will be referred further on this document as the INICIAL COOKING ACID.

[0014] The basis of the present innovation is the inclusion of the ammonium ion in the initial cooking acid, as described by the following equation:

INITIAL COOKING ACID:        Mg(HSO3)2 (aq) + SO2(g) +
Ammonium hydroxide addition: NH4OH(aq) =
Reaction product:            NH4(HSO3)(aq)

[0015] The cooking acid obtained by this reaction will be referred to in this document as the FINAL COOKING ACID.

[0016] FINAL COOKING ACID composition:

Mg(HSO3)2+NH4(HSO3)+[SO2=f(pH)]

[0017] The amount of SO2 stoichiometrically in excess is a function of pH.

[0018] 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).

[0019] 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, the percentage of combined SO2 being between 1.8% and 3.0%. Preferably, the total percentage of SO2 total should be between 5% and 6%, the percentage of combined SO2 being between 2.2% and 2.7%.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] a—Production of cellulosic fibres with a higher mechanico-physical index values.

[0024] b—Decreased percentage of uncooked material.

[0025] c—Increased wood yield.

[0026] d—Less environmental impact.

Claims

1. Chemical process for wood delignification characterised by the use of an aqueous unsaturated solution containing an ionic mixture of ammonium and magnesium bisulphite salts (herein called ammonium magnesium bisulphite reagent), fully dissociated: HSO3−+Mg2++NH4+, a pH=2-4 kept through an excess of dissolved sulphur dioxide, where [SO2]=f(pH)], the percentage of total SO2, free plus combined forms, being kept within 4% to 7%, wherein the percentage of combined SO2 is from 1.8% to 3.0%.

2. Chemical process for wood delignification, according to claim 1, characterised by a Final Cooking Acid containing: % SO2 Total 5-6, % SO2 Combined 2.2-2.7 and a pH value between 2 and 4 at which the process must be run by the controlled addition of NH4OH to the initial cooking acid, whose pH is nearly 1.5.

3. Chemical process for wood delignification, according to claim 2, which being applied to eucalyptus and taking a monocationic acid bisulphite process, namely magnesium, as reference, is characterised by a pulp production having cellulosic fibres with higher physical properties, mainly tensile Index end Tear Index respectively 25% and 15%, at least higher

4. Chemical process for wood delignification, according to claim 2, which being applied to eucalyptus and taking a monocationic acid bisulphite process, namely magnesium, as reference, is characterised by a decrease in the percentage of uncooked material nearly 30% and an increase in the wood yield nearly 4-5%.

5. Chemical process for wood delignification, according to claim 2, which taking a monocationic acid bisulphite process, namely magnesium, as reference, is characterised by the significant improvement of the environmental impact resulting from the lower amount of sulphur dioxide released to the air when the cook is discharged.

6. Process for preparing the ammonium magnesium bisulphite reagent referred to in claims 1 to 5, characterised by having as reaction reagents the initial cooking acid and ammonium hydroxide, according to the equation:

7. Process according to claim 6, characterised by a Final Cooking Acid containing: % SO2 Total 5-6, % SO2 Combined 2.2-2.7 and a pH value between 2 and 4 at which the process must be run by the controlled addition of NH4OH to the initial cooking acid, whose pH is nearly 1.5.″