Treatment HW-Sulfite HW-PHK

Unbleached (E/O) O

Untreated 6.0 2.1 2.8

CCE 4.5 1.8 2.3

Cold Caustic Extraction

Clearly, delignification is most pronounced for the unbleached pulp. However,

Significant parts of the residual lignin structures are even removed after oxygen

Delignification through CCE, but this may be attributed to the dissolution of xylan

linked to residues of oxidizable structures (degraded lignin and/or HexA?).

8.3.6

Molecular Weight Distribution

The aim of CCE is selectively to remove short-chain carbohydrates and other alkaline-

Soluble impurities, and this leads to a narrowing of the molar mass distribution.

The effect of CCE on molecular weight distribution (MWD) has been investigated

Using a standard hardwood sulfite dissolving pulp (HW-S). The data in

Fig. 8.14 show that the main part of the short-chain carbohydrates with molecular

Weights ranging from 2.5 to 12 kDa (maximum at 5 kDa) is removed through

CCE. At the same time, the mid-molecular weight region between 30 and 380 kDa

becomes enriched. CCE treatment at low temperature (23 °C) proves to be rather

Selective. Only a very small proportion of the very high molecular-weight fractions

(>1000 kDa) is degraded through CCE. Numerical evaluation of the MWD confirms

The removal of short-chain material (Tab. 8.2). It should be noted that the

Polydispersity and amount of low molecular-weight fractions (below DP50 and

DP 200) are significantly decreased, while the high molecular-weight fraction

Remains largely unchanged (beyond DP2000).

103 104 105 106 107

0.0

0.2

0.4

0.6

0.8

1.0

HW-S CCE treated HW-S Δ (CCE-Untreated)

dW/dlogM

Molar Mass [g/mol]

Fig. 8.14 Molar mass distribution of a hardwood-sulfite dissolving

pulp (HW-S) before and after CCE treatment [12].

CCE-treatment: 80 g L–1 NaOH, 23 °C, 45 min.

Tab. 8.2 Numerical evaluation of molecular weight distribution

of HW-S pulp before and after CCE treatment [12]. DP: degree of

polymerization, I: weight fraction.

Pulp DPw DPn PDI I< P50

[wt.%]

I< P200

[wt.%]

I> P2000

[wt.%]

HW-S 1950 245 8.0 5.1 17.4 27.9

HWS-CCE 1880 355 5.3 2.0 13.4 26.6

8.4

Hot Caustic Extraction

The purpose of hot caustic extraction (HCE) is to remove the short-chain hemicelluloses

(determined as S18, S10 fractions) for the production of reactive dissolving

Pulps based on acid sulfite cooking. In contrast to cold caustic purification, which

Relies on physical effects such as swelling and solubilization to remove short-chain

Noncellulosic carbohydrates, hot alkali extraction utilizes primarily chemical reactions

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