FI80734B - Method for the fractionating of spent liquors form alkaline pulp cooking - Google Patents

Description

1 80734

The present invention relates to a process for fractionating waste liquors formed during the alkaline delignification of wood-5 or other cellulosic plant material to recover the aliphatic acids contained therein.

Alkaline delignification of wood, such as sulphate cooking, aims to remove lignin that binds cellulose fibers to another, but at the same time a considerable proportion of the carbon hydrates are broken down and dissolved (E. Sjöström, Wood Chemistry, Fundamentals and Applications, Academic Press, New York, 1981). In addition to lignin dissolution products and extractants, the organic matter in the waste liquor consists of decomposition products of carbon-15 hydrates, mainly various aliphatic acids. Most of the spent sodium binds to these acids while the majority of the sulfur is in the inorganic material.

The aliphatic acid fraction of the waste liquor contains both volatile acids (e.g. formic acid and acetic acid) and hydroxy acids with either one or two carboxyl groups (e.g. mono- and dicarboxylic acids) (R. Alen, K. Niemelä and E. Sjöström, J. Chromatogr. 301 (1984) 273). Hydroxymonocarboxylic acids form the most significant acid fraction in the waste liquor, which is a mixture of more than 20 different components. The total amount of acids formed in hardwood soup is almost the same as in softwood soup, but the relative proportions of acids differ. In addition, the composition of acid-30 varies somewhat depending on the cooking conditions. The chemical utilization of aliphatic acids requires that they can be isolated from the other constituents of black liquor in pure form. Recovery has been studied especially recently, but isolation of the pure acid fraction has proved difficult (see R. Alen, V-P. Moilanen and 2,80734 E. Sjöström, Tappi £ 9 (2) (1986) 76).

This invention relates to a process for fractionating waste liquors for the purification of aliphatic acids using a chromatographic process in which separation 5 is performed on a column packed with a cation exchange resin.

The main features of the invention appear from the appended claims.

In this chromatographic separation, the ionized fraction of the feed solution, due to ion exclusion 10, passes through the cation exchange resin faster than the neutral compounds, which elute more slowly (J.S. Fritz, D.T. Gjerde and C. Pohlandt, Ion Chromatography,

Dr. Alfred Hlithing Verlag GmbH, Heidelberg, 1982). The method has been known since the early 1950s 15 (RM Wheaton and WC Bauman, Ind. Eng. Chem. £ 5 (1953) 228) and the industrial applications based on it include, for example, the separation of sugars from molasses (IP Chernikova and MS Rutskaya, Gidroliz, Lesokhim Prom.

(8) (1983) 20). The applicability of the method has been investigated for the separation of sugars and lignosulfonates contained in waste liquors from the manufacture of sulphite cellulose (H. Hassi, P. Tikka and E. Sjöström, The Ekman-Days 1981, June 9-12, 1981, SPCI Report No. 38, Vol. 5, 1981, p. 65), but so far it has not been possible to use a corresponding chromatography for the fractionation of waste broths formed in temporal cellulose soups.

During the development of the process according to the invention, it turned out that the chromatographic purification of the acid fraction contained in the alkaline waste broth is possible if the waste broth is first acidified with, for example, mineral acid. During acidification, most of the lignin precipitates, which substantially reduces the total amount of organic impurities in the fraction subjected to chromatographic separation. In addition, the extractants dissolved in the broth can be removed from the waste broth as raw crude before acidification.

The preparation of the impure acid fraction required for this chromatographic separation can be carried out according to a previously developed method (FI 850208) and the recovery of acids can thus be linked to the pulping process. It is preferable to carry out the precipitation of lignin in two steps so that, before the addition of the mineral acid (sulfuric acid or hydrochloric acid), the concentrated waste solution is carbonated by evaporation. The corresponding salt (sodium sulphate or sodium chloride) formed in the neutralization of the mineral acid can be recovered and can compensate for the sodium and sulfur losses of the plant (sodium sulphate) 10 or it (sodium chloride) can be electrolytically decomposed into chemicals (chloride and sodium) in both bleaching and cooking. The hydrogen sulphide released during the acidification of sulphate waste liquor is absorbable in the so-called green liquor or may alternatively be oxidized to sulfur dioxide or trioxide. In the economic recovery of acids, the total amount of effluent subjected to chromatographic separation is thus determined primarily on the basis of inorganic chemical balances.

Chromatographic purification of the acidified effluent is performed on a separation column packed with a strong cation exchange resin in Na form. During the separation process, the resin used is in a state of equilibrium depending on the sodium and hydrogen ion content of the waste liquor. Column 25 is eluted with water and the resin does not need to be regenerated between successive separation cycles. Fractionation can be performed in either one or two steps. In the case of a two-stage separation, in the first stage most of the inorganic salt formed during the acidification (sodium sulphate or sodium chloride) is removed and in the second stage the pure hydroxy acid fraction is isolated. Much of the sodium sulfate can also be crystallized from the acidified waste liquor prior to chromatographic separation. In addition, it is possible to divide the hydroxy acid fraction 35 of the second fraction into sub-fractions of somewhat different composition.

The following examples illustrate the method of the invention in more detail.

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Example 1 Pine (Pinus sylvestris) plant wood chips were boiled to 36 (active alkali 22% (as NaOH) from wood at a liquid / wood ratio of 4.5 l / kg). Most of the extracts were removed from the resulting waste-5 broth as a crude soup, after which the waste broth was evaporated to a dry matter content of 22.4% (Tables I and II) and 63 wt% sulfuric acid was added until the pH was 1.2. The lignin precipitate was separated by centrifugation (5000 rpm, G-number 2100), and the clarifier was filtered using diatomaceous earth as an excipient. Finally, the filtrate was concentrated by evaporation to a dry matter content of 35%. The resulting mother liquor, which had stabilized at pH 0.8, was used as a feed solution in the chromatographic separation. The column (bed height 350 cm and inner diameter -15 and 60 cm, 65 ° C) was packed with a strongly acidic polystyrene-backed cation exchange resin (average particle size 0.28 mm and crosslinking rate 4% DVB).

In the first separation (load (sodium sulphate + aliphatic acids + lignin) 37 kg dry matter-3 3 20 ta / m resin, flow rate 250 m / h and feed volume 3 102 dm) the acidified and evaporated waste liquor was removed as a separate fraction. the resulting sodium sulphate and part of the non-precipitated lignin. The resulting acid concentrate (Table III, fraction a) was divided according to the order of elution (partial fractionation of the acids also took place at this stage) into two equal parts (Table III, fractions c and c), which were purified by further separation as before. The total yield of hydroxy acids was over 90% of the initial amount.

Figure 1 shows graphically the typical chromatographic fractionation of the waste liquor, the separation being monitored by determining the corresponding fractions by gas chromatography (R. Al £ n, K. Niemelä and E. Sjöström, J.

35 Chromatogr. 301 (1984) 273 and R. Al§n, P. Jännäri and E. Sjöström, Finn. Chem. Letters 1985 190) aliphatic acids, 5 80734, by UV spectrophotometry (R. Al £ n, P. Patja and E. Sjöström, Tappi 62 (11) (1979) 108 and R. Al§n, E. Sjöström and P. Vaskikari , Cellulose Chem. Techn.19 (1985) 537) lignin and atomic absorption spectrophotometrically (SCAN-N 5 29:84) sodium content and pH.

Table I

Composition of alkaline waste liquor (dry matter 22.4%)

Ingredient_ g / 1_ 10 Lignin 108.4

Sodium 39.4

Volatile acid ^ 18.4

Hydroxy acids 42.9 15 a)

Formic and acetic acid Table II

Composition of the aliphatic acid fraction of alkaline waste liquor (dry matter 22.4%) 20

Acid_%_

Formic acid 15.4

Acetic acid 14.6

Glycolic acid 8.7 25 Lactic acid 11.0 2-hydroxybutanoic acid 2.9 2-deoxytetronic acid 0.2 3-deoxytetronic acid 1.1 2.5-dihydroxypentanoic acid 6.2 30 3-deoxypentonic acid 5.1

Xyloisosaccharic acid 1.5 3.6-dideoxyhexonic acid 0.6

Anhydroglucososaccharic acid 1,3 α-glucososaccharic acid 6.0 35 3-glucososaccharic acid 14.5

Dicarboxylic acids 6.6

Other 4.3 6 80734

Table III

Fractionation of acidified pine waste liquor

Feed Fraction _broth_a_b__c 5 Volume, 1 102.0 404.3 193.9 210.4 pH 0.8 1.7 1.2 2.0

Dry matter, g / la 485.6 68.5 112.7 27.9

Sodium, eq / l 3.44 0.53 0.91 0.17

Lignin, g / l 9.0 1.1 1.2 1.0 10 Hydroxy acids, g / l 87.2 25.2 26.0 24.5

Composition of hydroxy acids

Glycolic acid 10.3 10.3 8.5 14.8

Lactic acid 14.2 14.2 12.3 16.7 15 2-hydroxybutanoic acid 3.8 3.8 2.3 4.5 2-deoxytetronic acid <0.1 <0.1 0.1 0.2 3-deoxytetronic acid 1 .4 1.4 1.1 1.6 2.5-Dihydroxypentanoic acid 8.2 8.2 7.8 7.8 3-Deoxypentonic acid 9.2 9.2 8.2 7.5 20 Xyloisosaccharic acid 2.3 2.3 2 .0 2.4 3.6-dideoxyhexonic acid 1.0 1.0 0.9 0.9

Anhydroglucososaccharic acid 3.3 3.3 4.0 1.4 α-glucoisosaccharic acid 10.6 10.6 12.3 6.8 25 β-glucososaccharic acid 24.2 24.2 23.9 18.0

Dicarboxylic acids 5.5 5.5 11.1 5.8

Other 6.0 6.0 5.5 11.6 30 ___________

a105 ° C

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Example 2

The acid concentrate obtained in the case of Example 1 (Table III, fraction a) was purified by a similar but smaller-scale separation. Since 5 ko. the fraction had a sodium sulfate content of more than 55%, crystallization occurred during cold storage, and the salt content decreased to less than 25% (before storage, the fraction had evaporated to 31%; the pH was 1.5). The crystallization of the salt also caused small changes in the proportions of the other components. The pH of the mother liquor was checked to 0.8 using sulfuric acid. This solution was applied to a column (bed height 500 cm and inner diameter 22.5 cm, 65 ° C) packed with the resin used in Example 1. Load 3 was varied in the range of 14.4 to 59.1 kg dry matter / m hart-15. At lower loads the flow rate was 20 1 / h and at the highest 25 1 / h.

Figure 2 shows graphically the difference made with a load of 3 59.1 kg dry matter / m resin. As in the separation of Example 1, the acid concentrate was divided into two portions (at approximately the peak of the acid peak) to determine how much acid enrichment has occurred. The purities of the recovered acid fractions (proportion of hydroxy acids in the total amount of hydroxy acids, lignin and sodium sulfate) were high: 25 in fraction d) 80% and in fraction e) 95%. Glucoisosaccharic acid was slightly enriched in the first fraction, while e.g. lactic and glycolic acid to the latter fraction (Table IV).

Analytical assays were performed as mentioned in Example 30 1. In addition, the concentration of the solution was evaluated on the basis of its refractive index, using the correlation between the refractive index and the concentration observed for sucrose solutions.

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Table IV

Fractionation of the acid fraction after pre-fractionation and salt crystallization

Feed- Fraction 5 _broth_d_e

Volume, 1 34.9 36.5 78.4 pH 0.8 1.32.1

Dry matter, g / la 290.7 126.9 69.8

Sodium, eq / l 1.12 0.21 0.02 10 Lignin, g / l 10.6 3.4 1.8

Hydroxy acids, g / 1,239.8 76.0 67.7

Composition of hydroxy acids,%

Glycolic acid 12.9 8.2 14.3 15 Lactic acid 15.9 10.7 17.4 2-hydroxybutanoic acid 3.3 2.0 3.7 2-deoxytetronic acid 0.1 <0.1 0.2 3-deoxytetronic acid 1 5,5 1,6 2,5-Dihydroxypentanoic acid 8.7 7.1 9.6 20 3-Deoxypentonic acid 7.5 7.3 8.1

Xylisosaccharic acid 2.3 1.8 2.6 3.6-dideoxyhexonic acid 1.1 1.2 1.1

Anhydroglucososaccharic acid 2.6 4.0 2.1 α-glucososaccharic acid 9.4 13.7 7.3 β-glucososaccharic acid 20.0 25.5 18.3

Dicarboxylic acids 9.6 11.3 8.7 30 Other 5.1 6.2 5.0

a105 ° C

Claims (9)

9 80734 A process for purifying and fractionating an impure aliphatic carboxylic acid-containing acid fraction soluble in corresponding waste liquors and separated by corresponding methods in time-consuming processes of wood or other cellulosic plant material by known methods, characterized in that the purification is carried out with strongly acidic at pH 0.5-3 using water as eluent. Process according to Claim 1, characterized in that the pH is from 0.5 to 2, preferably from 0.8 to 1.5. Process according to Claims 1 and 2, characterized in that the impure aliphatic carboxylic acid fraction contains, in addition to the acids in question, varying amounts of other cleavage products of carbohydrate origin, lignin, extractants and inorganic material. Process according to claims 1-3, characterized in that said waste liquor is derived from sulphate, soda-anthraquinone, soda or neutral sulphite soup. Process according to Claims 1 to 4, characterized in that the known processes for the preparation of the impure aliphatic acid fraction are based on the acidification, ultrafiltration or electrodialysis of the waste liquor. Process according to Claims 1 to 5, characterized in that the pH of the acid fraction is adjusted by adding mineral acid. Process according to Claim 6, characterized in that the mineral acid is sulfuric acid-35 po. 10 80734 Process according to Claim 1, characterized in that the sodium sulphate is crystallized before the chromatographic separation. Process according to Claim 1, characterized in that part of the lignin is precipitated before the chromatographic separation. 11 80734