General Description

The main active chemical agents in the kraft process are hydroxide and hydrosulfide

anions which are present in the kraft cooking liquor, an aqueous solution of

caustic sodium hydroxide and sodium sulfide, denoted as white liquor. The hydrosulfide

ion plays an important role in kraft pulping by accelerating delignification

and rendering nonselective soda cooking into a selective delignifying process.

Delignification can be divided into three phases, namely the initial, bulk, and residual

or final phases. In the initial phase, delignification is caused by the cleavage

of a-aryl and b-aryl ether bonds in the phenolic units of lignin which accounts for

approximately 15–25% of native lignin. In this stage, the predominant part of the

total carbohydrate losses can be observed. In the bulk delignification phase the

main part of the lignin is removed while at the same time only minor carbohydrate

losses occur. The cleavage of b-aryl bonds in nonphenolic units of lignin is

assumed to be the main delignification reaction. In the residual delignification

phase, only approximately 10–15% of the native lignin is removed. However, with

continuous delignification, the dissolution of carbohydrates extensively increases.

In order to maintain high yields and to preserve a sufficiently high quality of the

pulp, delignification is limited to a certain degree of delignification, targeting

kappa numbers of about 25–30 for softwood and 15–20 for hardwood kraft pulps.

After cooking, the pulp and the black liquor (which is white liquor enriched

with degraded wood components with a residual hydroxide ion concentration of

4.2 Kraft Pulping Processes 111

ca. 0.25 mol L–1) are discharged at reduced pressure into a blow tank. After removing

the knots through screening on knotter screens, the black liquor is removed

after countercurrent washing of the pulp and further processed within the recovery

line. The washed pulp is mechanically purified by pressurized screens prior

entering the bleach plant.

The volatile fraction of thewood extractives – the crude turpentine – is removed during

presteaming and condenses from the relief condensates (the average yield of

crude turpentine of pine is 5–10 kg t–1 pulp, with monoterpene compounds as the

main fraction). The tall oil soap, which originates fromthe nonvolatile fraction of the

wood extractives, is removed during evaporation of the black liquor by skimming.

Through the addition of sulfuric acid, the resin and fatty acids are liberated to yield

crude tall oil (CTO) in an amount of approximately 30–50 kg t–1 of pulp. Further purification

of the main fractions of the CTO is achieved by vacuum distillation.

During kraft pulping, malodorous and toxic compounds such as methyl mercaptan

(CH3SH), dimethylsulfide (CH3SCH3), dimethyldisulfide (CH3SSCH3) and

other reduced sulfur compounds, referred to as “total reduced sulfur” (TRS), are

formed during the course of nucleophilic substitution reactions with predominantly

lignin moieties. Great efforts must be made to collect TRS containing gases

(relief gases from cooking, blowing, evaporation of black liquor, etc.) to convert

them to harmless compounds by oxidation, mainly through incineration.

The regeneration of the black liquor to fresh white liquor comprises the following

principal steps of the recovery line:

_ Evaporation of the black liquor

_ Incineration of thick liquor

_ Causticizing of smelt from recovery boiler

_ Calcination of the lime

The weak black liquor, including the filtrates from oxygen delignification, must be

evaporated to a solid content of up to 80% by multiple-effect evaporators; this

includes a concentrator prior to entering the Tomlinson-type recovery furnace.

Make-up sodium sulfate is added to the concentrated black liquor to compensate

for the losses of sodium and sulfur. Combustion of the dissolved organic compounds

generates heat, which is transformed to process steam and electric power.

A modern kraft pulp mill is designed such that it is self-sufficient with respect to

both power and heat. In fact, it even has the potential to generate surplus energy.

The inorganic smelt, which contains mainly sodium carbonate and sodium sulfide,

is dissolved in water to yield the so-called green liquor which, after clarification,

is subjected to the causticizing reaction where sodium carbonate is converted

to sodium hydroxide by treatment with calcium hydroxide (slaked lime) according

to the Eq. (1):

Ca(OH) + Na2CO3_ 2NaOH + CaCO3 (1)

After calcium carbonate is precipitated and separated from the liquor, the resultant

white liquor is ready for re-use for cooking. To close the cycle, the lime is

112 4 Chemical Pulping Processes

4.2 Kraft Pulping Processes 113

reburned in a rotary kiln (lime kiln) to produce calcium oxide, which in turn is

slaked to form calcium hydroxide [Eq. (2)]:

CaCO3 __

heat CaO + CO2

CaO + H2O __ Ca(OH)2

(2)

Make-up calcium carbonate is added to the lime kiln to compensate for the losses

of calcium hydroxide.

4.2.2

Дата добавления: 2015-10-21