FI87091C - FOERFARANDE FOER BEHANDLING AV ALKALILOESNING - Google Patents

Description

87091

Method for treating an alkaline solution

The invention relates to the production of cellulose by a sulphite-based cooking process. In particular, the invention relates to the treatment of alkaline solutions formed in the regeneration of cooking chemicals to remove harmful sulfur compounds therefrom.

In the regeneration of cooking chemicals in the sodium sulfite process, the chemical melt 10 from the incineration of the waste liquor is dissolved in water to a green liquor containing sodium carbonate, sodium hydroxide, sodium sulfides, and sodium thiosulfate and sodium sulfate. In the regeneration process, the sulfur bound to the sodium sulfide and bisulfide in the green liquor is separated off as hydrogen sulfide, leaving an alkaline solution containing sodium carbonate.

The separated hydrogen sulfide is burned, and the sulfur dioxide thus obtained is used to sulfite the alkali solution to prepare a cooking solution.

However, after separation of the hydrogen sulfide, the alkali solution still contains sodium thiosulfate and sodium sulfides as impurities, which interfere with the cooking by causing condensation of lignin. Therefore, the alkali solution as such cannot be used to prepare the cooking solution.

It is known to crystallize sodium carbonate or sodium bicarbonate from an alkaline solution and to use the pure crystal slurry thus obtained for the preparation of the cooking solution. This is done, for example, in the Sivola, Rauma and Tampella processes implemented in practice. The Rauma process is described, for example, in patent publications FI 61214 and 30 FI 59275. Crystallization purification requires, for example, carbonation, evaporation or cooling, as well as separation and further processing of the crystalline mass. In addition, a mother liquor solution containing sodium thiosulfate and residual sulfides remains. As a whole, the processes are quite cumbersome 35 to implement.

It is also known to treat an alkali solution with oxygen to oxidize sodium sulfides and thiosulfate. However, such a 2 87091 process requires a high pressure and temperature (about 15 bar and about 150 ° C), and is therefore cumbersome and expensive to carry out.

A method according to claim 1 has now been invented. Some preferred embodiments of the invention are set out in the dependent claims.

According to the invention, harmful sulfur compounds are oxidized with hydrogen peroxide or ozone to sodium sulfate, which is a process-inert and harmless compound. The amount of oxidant used is such that the concentrations of harmful sulfur compounds are brought to the level required by the process or pulp quality. If desired, the compounds can be completely oxidized.

The oxidation reaction of the process can be carried out in an open vessel and at normal or slightly elevated temperature. The process is thus considerably easier to carry out than crystallization purification or oxidation with oxygen.

An additional advantage is that the oxidation according to the invention also removes colored impurities from the alkali solution, and thus the need for bleaching the pulp is reduced.

The following examples further illustrate the invention:

Example 1

Soda solution from the removal step of sodium 25 sulphide and sodium bisulphide in the Rauma recovery process (FI 61214, FI 59275), so-called intermediate soda, was oxidized with various stoichiometric amounts of H 2 O 2 in a beaker. The oxidation was performed in a non-pressurized, open vessel at a slightly elevated temperature (41 ° C), stirring was performed with a magnetic stirrer. The test results are shown in Table 1.

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Table 1. Oxidation of soda ash with H 2 O 2 Soda ash H 2 O 2 Na 2 ° Na 2 S 2 ° 3 Amount of temperature 5 volume rise (ml) (50%) (ml) (g / l) (g / l) (° C) 100 0 132.0 3.32 0 100 1.0 137.0 2.21 7 10 100 2.0 134.0 0.79 17 100 2.5 133.0 0.32 18 100 3.0 133.0 0 22 15 The test results show that the reaction is highly exothermic; the temperature rise is 22 ° C at the highest dose of H 2 O 2.

When a sufficient amount of hydrogen peroxide is used, the entire amount of thiosulfate is oxidized. Oxidation does not affect the active amount of Na 2 CO 3 in solution 20; The Na2CO3 concentration (expressed as Na2O) does not change by more than the titration accuracy.

Oxidation can also be performed at normal room temperature.

Example 2 The intermediate soda described in Example 1 (diluted 2: 1 with water) was oxidized at 33 ° C with the experimental arrangements of Example 1. The amount of intermediate soda used was 500 ml per test point. Example 2 has been made to show that all sulfur compounds can be selectively oxidized with a sufficiently high dose of hydrogen peroxide to sulfate if necessary. Table 2 shows the titration and analysis results of Example 2.

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Table 2. Oxidation of Soda ash Sulfur Compounds to Sulfate 5 Intermediate H 2 O 2 Titr. Na 2 CO 3 Na 2 S Na 2 SO 3 Na 2 S 2 O 3 SO 4 soda. (50%) Na, 0 ί ......

increase (mi) (° c) (mi) (g / i) (g / i) (g / D (g / i) (g / i) g / i 5 500 0 0 109.3 186.9 2 , 1 0.6 11.5 17 500 7 3 106.2 181.2 0 1.0 9.6 19 500 14 6 106.2 181.2 0 0.9 5.4 23 500 20 9 101.5 173 .5 0 0.1 2.1 28 500 26 12 99.2 169.6 0 0 0 30 10 500 27 15 100.0 171.0 0 0 0 30

Table 2 of Example 2 confirms the result of Example 1; thiosulfate is selectively oxidized from the intermediate soda by H 2 O 2. In addition, the example shows that all the sulfur compounds analyzed are oxidized with hydrogen peroxide to sulfate when a sufficient amount of peroxide is added. Oxidation proceeds logically in the direction of a higher degree of oxidation; sulfide, sulfite, thiosulfate, sulfate, Na 2 CO 3 concentration unchanged. The titration method yields both carbonate, sulfite, and thiosulfate; when all the sulfur compounds have been oxidized, the titration result no longer changes (additions of 12 ml and 15 ml).

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Claims (7)

A process for treating an alkali solution containing sodium carbonate, which is enhanced by the regeneration of the cooking chemicals in a sulfite cellulose process, and which alkali solution impurities containing sodium thiosulfate or sodium sulfide, characterized in that the aqueous solution is treated with oxide or oxide with hydrogen oxide or treated with hydrogen oxide. sodium sulphate or sodium sulphide in sodium sulphate. Process according to claim 1, characterized in that the alkali solution is treated with hydrogen peroxide. Process according to claim 1 or 2, characterized in that the sulfide sulfur is removed from the alkali solution as hydrogen sulphide prior to the oxidation. Process according to any of claims 1-3, characterized in that the oxidized alkali solution is conducted directly without purification to the solubilization. Process according to any of claims 1-4 for treating an alkali solution used in the preparation of the boiling solution in a multiphase sodium sulfite cellulose process, characterized in that the oxidized alkali solution is used as a boiling solution for the boiling alkali phase. Process according to any of claims 1-5, characterized in that the oxidation is carried out at room pressure and temperature or at a slightly elevated temperature. Process according to any of claims 1-6, characterized in that the heat generated by the oxidation is recovered. l! 35