Fully bleached softwood dissolving pulps as a function of

energy input in a Frotapulper [35,36].

Composition of Hot Caustic Extract

Hot caustic extract contains a large amount of low molecular-weight hydroxycarbonic

Acids,with glucoisosaccharinate as the main component, derived frompurification of

softwood sulfite pulp [37]. A typical extract composition is shown in Tab. 8.4.

Tab. 8.4 Typical composition of hot caustic extract [37].

Compounds % of solids as sodium Salts

Chloridea 3.3

Formate 17.0

Acetate 3.4

Glucoisosaccharinate 27.0

Other Hydroxy acids 38.0

“Complex” acids 11.3

A. C stage preceding HCE treatment.

The combined saccharinic acids and other hydroxy acids constitute about 65%

Of the hot caustic extract. These compounds are readily biodegradable in a wastewater

Treatment plant. However, the COD load is significant and calculates to

about 180 kg odt–1, assuming an average yield loss of about 15% on bleached pulp

across the stage (150 kg carbohydrates/odt. 1.185 kg COD/kg carbohydrates). As

Hot Caustic Extraction 961

A consequence, several sulfite dissolving pulp mills have recently installed evaporation

Plants and recovery furnaces (soda boiler) to concentrate and burn the filtrates

From hot caustic extraction. To date, no products are prepared from the thick

Liquor, mainly because of the high costs to isolate, purify, and modify the saccharinic

Acids. Reintjes and Cooper have proposed a scheme to utilize these compounds

Where the acids are lactonized and converted to amides; they may then be

Further processed to anionic and nonionic surfactants by reaction with chlorosulfonic

acid or ethylene oxide [37].

8.4.2

MgO as an Alternative Alkali Source

The main disadvantage of using NaOHas an alkali source forHCE is that the evaporated

Caustic extract cannot be recycled to the spent sulfite liquor (SSL) of a Mg-based

Cooking process due to the formation of low-melting Na-Mg eutectic mixtures. Thus,

Efforts were undertaken to investigate the possibility of using Mg(OH)2 as an alkali

Source for hot caustic extraction, as this enables the combined recovery of hot caustic

extract and Mg-based SSL [38]. As known fromweak bases such as sodiumcarbonate,

Sodium sulfite and others, a higher temperature than is used for NaOH is required

For the purification. The time and temperature of MgO-based HCE (EMgO)

Are the two main parameters that determine the degree of purification, rather

than the Mg(OH)2 charge. Charges higher than 15 kg odt–1 have no effect on purification,

Due mainly to the low solubility of Mg(OH)2 in aqueous solution.

130 140 150 160 170

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