FI74498B - FOERFARANDE FOER UTVINNING AV ALIFATISKA SYROR UR AVLUTAR FRAON ALKALISKA CELLULOSAKOK. - Google Patents

Description

1 74498

This invention relates to a process for the partial recovery of aliphatic acids from waste liquors formed during the step of raising the temperature of alkaline soups, in particular pine and birch sulphate soups.

Timber cooking processes remove most of the lignin contained in the raw wood, but sa-10 clubs also decompose the wood to reduce the overall yield of the polymeric carbohydrate, cellulose and hemicelluloses (E. Sjöström, Wood Chemistry, New York, Fundamentals and Applications, Academic Press , 1981). The organic matter dissolved in the waste liquor thus consists, after the crude separation from the wood extracts, mainly of both the lignin decomposition products and the organic acids formed as a result of the cleavage reactions in the carbohydrate chains. The total amount of acids formed in leaf-wood soup is almost the same as in softwood-20 wood soup, but the relative proportions of acids differ in some respects.

The possibilities of recovering these acids from post-cooking waste liquor, so-called black liquor, have been studied to some extent (cf. R. Αίδη, Isolation of 25 aliphatic acids from pine and birch kraft black Liquors, dissertation, TKK, 1981). However, due to the complex chemical composition of black liquor, the unambiguous separation and recovery of acids has not progressed to industrial application except for a few special applications (cf. WA Biggs, Jr., JT Wise, WR Cook, WH Baxley, JD Robertson & JE Copenhauer, Tappi 1961) 385-392). The difficulties encountered in the development work have been primarily related to the disturbances caused by the separation methods in the regeneration of inorganic cooking chemicals. It is thus obvious that the recovery of acids can be smoothly integrated into the normal operation of the plant only by special arrangements.

This invention relates to a process for recovering the above-mentioned acids from waste liquor. The inorganic chemicals added in the process in question are returned to the factory's cooking chemical recovery system. The main features of the invention appear from the appended claims.

During the development of the method according to the invention, it turned out that a substantial advantage is obtained if the waste liquor required for the separation of acids is taken from the step of raising the temperature of the cooking to be carried out. A significant advantage from the point of view of acid separation is that at that stage of the cooking a large proportion of the carbohydrates dissolve relatively selectively without a corresponding lignin removal. The deficiency of the broth after the lifting step can advantageously be arranged by initially increasing the liquid / wood ratio of the soup accordingly.

According to the present invention, the preparation of the acid fraction is carried out by first removing the extractants from these effluents as a crude salt, after which lignin is precipitated from them by acidification, whereby the organic acids are also released from their sodium salts. After the partial crystallization of the inorganic salts as a final step, a concentrate containing mainly aliphatic acids is formed. The incorporation of the acid recovery process into the sulphate process is also not of great importance for the energy economy of the plant, as the calorific value of the recovery process, eg black liquor, is only 2.5 and 3.5% for pine and birch soups.

Precipitation of lignin can be performed in both two-30 and single steps. In the first case, the waste broth is carbonized in the optimum dry matter (R. Alön, P. Patja & E. Sjöström, Tappi £ 2 (11) (1979) 108-1 10) first with carbon dioxide (the pH drops to about 8 and can be used to some extent in the factory flue gases), after which mineral acid (e.g. or HCl, 3 74498 pH is then lowered below 3) to complete the lignin precipitation. Since the ρκ values of aliphatic acids are between 3.5 and 5, at the same time the acids are also released for the most part from their sodium salts. In single-step acidification, lignin is precipitated directly with mi-5 mineral acid.

As the acid recovery is designed to be combined primarily with pine and birch sulphate soups (however, the method can be applied to other alkaline soups if necessary, eg similar soda-AQ boilers), the hydrogen sulphide released during acidification must also be taken into account. Although it can be absorbed into the so-called vi-herl liquor, its oxidation to sulfuric acid is also possible.

The sodium released during carbonation is present as a mixture of sodium carbonate and bicarbonate. Instead, when mineral acid is used, depending on the acid used, either sodium sulfate or sodium chloride is formed. They can be returned, like impure lignin precipitates, to incineration during the regeneration of cooking chemicals. In this case, the sulfur entrapped in the sodium sulphate compensates for the typical chemical losses of the sulphate process and its amount, together with the sulfur bound to the hydrogen sulphide, determines the total degree of deficiency. If hydrochloric acid is used for the acidification, the resulting sodium chloride is crystallized, so that it can be electrolytically decomposed into chemicals required for both bleaching and cooking (chlorine and sodium hydroxide). If the lignin precipitation is carried out in two steps, the amount of sodium sulphate formed is reduced to a certain extent and it is possible to increase the total amount of waste liquor required accordingly.

The post-acidification solution contains, in addition to aliphatic acids, mainly inorganic salts. If the so-called volatile acids (formic and acetic acid) are removed from the resulting solution by distillation, more corresponding salts precipitate during the distillation. However, they can be removed almost completely if the non-volatile acids present as the main distillation residue, the so-called hydroxy acids, are dissolved in concentrated acetic acid (the salts are insoluble). Hydroxy acids can be purified, for example, by distillation or electrodialysis, or chromatographic methods can be used to separate them. In the latter case, it is not necessary to remove formic and acetic acid and inorganic salts before separating the hydroxy acid.

The following examples illustrate the method of the invention in more detail. In the experiments leading to the presented balance sheets, no extractants were removed from the broth prior to acidification.

Example 1 A chip made from pine wood (Pinus sylvestris) was subjected to normal sulphate cooking / active alkali 22% (NaOH) of wood: sulphidity 30% 7 at a liquid / wood ratio of 4.5 l / kg. The cooking broth was deficient after the temperature raising step (30 -> 170 ° C, 90 min) by 22.2 wt%, whereby the final cooking step at maximum temperature (170 ° C, 70 min) was performed after replacement of the active cooking chemicals with a liquid / wood ratio of 3 due to the deficiency. 5 1 / kg. The total amount of organic matter groups and sodium in the reduced broth is shown in Figure 1, where the balances have been calculated assuming the amount of chips added to the soup to 1000 kg. With a view to acid recovery, the waste slurry after evaporation (optimization of carbonation) was first carbonated with carbon dioxide, followed by the addition of sulfuric acid (consumed as a 62% by weight solution in 48.1 l). The compositions of the formed impurities and lignin precipitates to be incinerated are shown in Table 1 30 (no extractants are indicated). The maximum amount of hydrogen sulphide produced during acidification was 6.2 kg. As mentioned above, the acidified waste broth was evaporated (while the volatile acids were partially removed) to crystallize 32.7 kg of sodium sulfate (crystallizes for the most part 35, as an aqueous glauber's salt, but the value given does not include the proportion of water of crystallization). The dry matter of the acid concentrate formed after removal of sodium sulphate, which mainly contains hydroxy-74498 acids, contained 9.3% sodium.

Example 2

The experiments according to Example 1 were repeated with the difference that the acidification was carried out in one step with sulfuric acid (Figure 2; consumed as a 62% by weight solution in 67.8 l). The amount of hydrogen sulfide generated during acidification was the same as in the previous case, but the composition of the lignin precipitate (Table 1) differed in some respects from the previous one. After evaporation of the ha-10 broth, 45.2 kg of sodium sulfate crystallized, whereby the organic matter of the hydroxy acid concentrate contained 10.2% sodium.

If hydrochloric acid was used for the acidification (consumed as a 35.4% by weight solution in 92.7 l), sodium chloride did not crystallize from the post-acidification effluent without extraction with concentrated acetic acid. In this case, 39.7 kg of sodium chloride were removed and the dry matter of the hydroxy acid concentrate contained 8.7% of sodium. With a similar treatment with acetic acid, it was possible to remove almost completely the sodium sulphate formed during the sulfuric acid acidification from the hydroxy acid-20 concentrate.

Example 3

For comparison, the experiments according to Examples 1 and 2 were also performed by depletion of 22.2% by weight of sulphate cooking / active alkali on birch chips (Betula verrucosa / Betula pubescens) in 20% (NaOH) of wood; sulphidity 30% of the waste broth formed during the 7 lifting steps (30 → 168 ° C, 90 min) (Figures 3 and 4). The maximum amount of hydrogen sulfide produced during the acidification was then 5.7 kg (the balance sheets were presented as in the previous examples, starting with 1000 kg of wood chips). After crystallization of the sodium sulfate formed in the two- and one-step acidification (62% by weight sulfuric acid consumed 37.1 and 54.0 l, respectively; the composition of the lignin precipitates is shown in Table 1), 35 (crystallized amounts 32.7 and 46.4 kg, respectively) contained the corresponding solids of hydroxy acid concentrates 9.3 and 10.3% sodium.

6 74498

When hydrochloric acid was used for the acidification (consumed as a 35.4% by weight solution in 89.7 l), 38.4 kg of sodium chloride were removed from the acidified and evaporated waste liquor by the addition of acetic acid. The dry matter of the hydroxy acid concentrate then contained 8.6% sodium.

Claims (2)

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