Of sodium carbonate from the smelt to sodium hydroxide needed for cooking. As

A part of the cooking chemical cycle, the preparation of white liquor consists of

Several process steps, and is accompanied by a separate chemical loop, the lime

Cycle (Fig. 9.10). The generated white liquor ought to contain a minimum of residual

Sodium carbonate in order to maintain the dead solids load in the cooking

Chemical cycle as low as possible.

Cooking /

Washing

COOKING

CHEMICAL

CYCLE

Evaporation

Recovery

Boiler

Smelt

Dissolving

Green liquor

filtration /

Slaking clarification

Causticising

Whilte liquor

Filtration

LIME

CYCLE

Lime reburning

Lime mud

Washing

Fig. 9.10 Major unit operations of causticizing and lime

Reburning in the context of the kraft chemical recovery cycle.

Process wise, the smelt coming from the smelt spouts of the recovery boiler

Drops into the smelt dissolving tank and becomes dissolved in weak wash, thereby

Forming green liquor. Since the smelt carries impurities which disturb the subsequent

Process steps, those must be removed by clarification or filtration of the

Green liquor. Then follow slaking, causticizing and white liquor filtration. After

Separation from the white liquor, the lime is washed and reburned for re-use in

Causticizing.

Chemistry

The basic chemical reactions in the causticizing plant and lime kiln start with the

Exothermic slaking reaction, where burned lime, CaO, is converted into calcium

hydroxide, Ca(OH)2 (slaked lime):

CaO _ H2O→Ca_OH_2 DH _ _65 kJ kmol_1 _12_

Recovery

Then the causticizing reaction transforms sodium carbonate from the smelt,

Na2CO3, to sodium hydroxide needed for cooking, thereby giving rise to calcium

carbonate, CaCO3:

Na2CO3 _ Ca_OH_2 _ 2NaOH _ CaCO3 DH≈ 0 kJ kmol_1 _13_

Calcium carbonate is separated from the white liquor and reburned at a temperature

above 820 °C following the endothermic calcination reaction:

CaCO3→CaO _ CO2 DH _ _178 kJ kmol_1 _14_

From a chemical perspective, white liquor is fundamentally characterized by

Active or effective alkali concentration, by sulfidity, as well as by causticizing and

Reduction efficiencies (see Section 4.2.2). In the causticizing plant, the total titratable

Alkali (TTA) is also of interest. Causticizing efficiency, CE, and TTA are

defined as follows:

CE _

NaOH

NaOH _ Na2CO3 _ 100_ _15_

TTA _ NaOH _ Na2S _ Na2CO3 _16_

The concentrations of the sodium salts in Eqs. (15) and (16) are, by convention,

expressed in g L–1 and in terms of NaOH or Na2O equivalents.

Lime (CaO), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3), also

referred to as “lime mud”, all have a very low solubility in water. Reactions related

To these components are basically happening in the solid phase.

Дата добавления: 2015-10-21