Sulphite pulping

Sulphur dioxide is used as a preservative for beer, wine and meats in the produetion of sulphites and hydrosulphites in solvent extraetion of lubrieating oils as a general bleaehing agent for oils and foods in sulphite pulp manufaeture in the eellulose and paper industries and for disinfeetion and fumigation. 

Cellulose produds There is little scope for the use of austenitic steels in the alkali processes for the digestion of wood pulp, but both molybdenum-free and molybdenum-bearing steels are extensively used for sulphite pulp digestion, choice depending on concentration, temperature and working experience. 

Table 3 Current national discharge limits and proposal levels for production of bleached Kraft and bleached sulphite pulp [5]... 

Bleached sulphite pulp Exist 40 kg/t Exist 35 mg/L No values Exist 2.5 kg/t... 

Bleached sulphite pulp No limit 90% removal or 50 mg/L No limit 0.1 mg/L... 

Bleached sulphite pulp No achievable guidance levels proposed 10-50 mg/L 10-50 mg/L <1.5 kg/t... 

Environmental limits or guidelines for the pulp and paper industry vary significantly between European countries, despite efforts to create a more uniform system [5]. As an example the discharge limits for bleached Kraft and bleached sulphite pulp are given in Table 3. In some countries, such as Austria or France, there is different legislation depending on the type of paper mill. Normally, the most restrictive corresponds to the bleached Kraft pulp. In other countries, such as Ireland, there are no requirements or limits for some parameters. 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

Calcium bisulphite is largely used in the manufacture of sulphite pulp, from which paper and viscose silk (see p. 264) may be produced. Wood chips or shavings are boiled with the bisulphite under pressure and so yield cellulose, which at the same time is bleached. The bisulphites of magnesium and the alkali metals are also sometimes used. 

According to Bryde and RAnby [20] the degree of polymerization of sulphite pulp varies from 845 to about 3200. 

In the sulphite pulping process wood cut into pieces is subjected to kiering in a calcium bisulphite solution under a pressure of 3-8 kg/cm2, at 120-145°C for 7-30 hr. The higher the temperature, the shorter the time of boiling. Substances accompanying cellulose dissolve in the liquor. Lignin is converted into a soluble lignin sulphonic acid. Cellulose is isolated from the solution, known as sulphite cellulose lye, and washed with water. 

The average composition of crude cellulose sulphite pulp is as follows ... 

Our experimental work started from the following original cellulose samples (a) acetate-grade, bleached cotton linters, DP 1800 (b) hot, refined, spruce, sulphite-dissolving pulp, machine dried, ca. 93% a-cellulose, DP — 750 (c) never-dried, normal, rayon-grade, beech sulphite pulp, ca. 90% a-cellulose, DP = 825 (d) commercial cellulose powders obtained by hydrolytic degradation and/or mechanical disintegration of cotton linters or spruce sulphite pulp. 

Beech sulphite pulp (bleached, chem. grade) 1.8 2.6... 

Gellerstedt, G., "The Reactions of Lignin During Sulphite Pulping", Svensk Paperstidn., 1976, 79, 537. 

Gellerstedt, G., and Gierer, J. (1971). The reactions of lignin during acidic sulphite pulping. Sven. Papperstidn. 74, 117-127. 

Sjostrom, E., Haglund, P., and Janson, J. (1962). Changes in cooking liquor composition during sulphite pulping. Sven. Papperstidn. 65, 855-869. 

Wood, J. R., and Goring, D. A. I. (1973). The distribution of lignin in fibres produced by kraft and acid sulphite pulping of spruce wood. Pulp Pap. Mag. Can. 74, T309-T313. 

Kratzl K, Billek G (1957) Synthesis and testing of lignin precursors Tappi 40 269-285 Kyrklund B, Palenius I (1964) The total lignin content of sulphite pulps and its relation to the kappa number Pap Puu 46 513-520... 

Back E (1960) Tracheidal and parenchymatous cells in Picea abies (Karst ) pulpwood and their behavior in sulphite pulping Sven Papperstidn 63.695- 698... 

Janson J, Sjostrom E (1966) Sulphur content in relation to the degree of sulphonation in birch and spruce sulphite pulps Sven Papperstidn 69 107-110 Katz S, Beatson RP, Scallan AM (1984) The determination of strong and weak acidic groups in sulfite pulps Sven Papperstidn 87 R48-R53... 

Loras V, Loschbrandt F (1961) Determination of lignin in sulphite pulps Nor Skogind 7... 

Sjostrom E, Enstrom B (1966) A method for the separate determination of sulphur and carboxyl groups in sulphite pulps Sven Papperstidn 69 55-59... 

Sulphite Pulping. This is a generic term used to describe various sulphite chemical woodpulping processes carried out at different pH s and pulp yields (37). (The term yield refers to the quantity of fiber recovered from the original wood.)... 

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