Recovery of Chemicals from Kraft Pulping Process Chemical Engineering Notes | EduRev

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Chemical Engineering : Recovery of Chemicals from Kraft Pulping Process Chemical Engineering Notes | EduRev

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Recovery of Chemicals from Kraft Pulping Process

Chemical recovery system is an integral part of pulp and paper units and the efficiency of chemical recovery plays an important role in economics. Efficiency of chemical recovery units in terms of thermal efficiency, energy recovery and conversion of thermal energy to electrical energy has tremendous impact on overall economy of pulp production . Although earlier agro-based mills were not having recovery furnace due to high silica content of black liquor and smaller capacity, however now chemical recovery in agro-based mills have been made mandatory. Important terms in recovery of chemicals are given in Table M-TTT 3.1.

Table M-III 3.1: Important terms in Recovery of Chemicals 


Active alkali

NaOH +Na2S

Effective alkali

NaOH+1/2 Na2S

Total titratable alkal

NaOH +Na2S+Na2Co3


Na2S/Titratable alkali


NaOH/ NaOH +Na2S

Causticity Efficiency

[NaOH/ NaOH +Na2S )] x 100


Na2S/( Na2S04 + Na2S )

Over all recovery

Recovery = black liquor stock/ fresh liquor consumed

Kraft Recovery

Recovery of chemicals from black liquor is very important for overall economy of paper industry. Kraft recovery process flow diagram is shown in Figure M-III 3.1. Various sections in kraft recovery plant are

  • Evaporation section and further concentration: Multiple effect evaporator, direct contact, cascade evaporator, Cyclone evaporator, venture scrubber
  • Recovery furnace: Furnace, ESP [Figure M-III 3.2]
  • Dissolving Tank and Green liquor generation
  • Causticizing and clarification
  • Lime sludge reburning 

Recovery of Chemicals from Kraft Pulping Process Chemical Engineering Notes | EduRev  

Figure M-III 3.1: Kraft Recovery Process 

Recovery of Chemicals from Kraft Pulping Process Chemical Engineering Notes | EduRev

Figure M-III 3.2: Kraft Recovery Furnace

Black Liquor Oxidation: Black liquor oxidation has become integral part of recovery section for reducing the emission of volatile sulphur compounds. Oxidation of black liquor by air reduces the emission of odorous sulphur compounds. Mecrcaptans and other sulphur compounds are oxidized into less volatile chemicals. Both weak black liquor and concentrated black liquor oxidation has been reported. Typical Composition of Black Liquor from Wheat Straw Pulping is given in Table M-III 3.2.

Various steps involved in kraft recovery section are 

  • Concentration of black liquor with inlet 12-18percent and outlet 40-50 percent long tube evaporator. Normally five to six effect evaporator is used. As the black liquor become highly viscous, further concentration is done by direct contact evaporator and falling film evaporator
  • Further concentration in direct contact evaporator to 60%. Using cyclone evaporator, cascade evaporator and venturi scrubber. In some of the mill Falling film evaporator is being used to achieve higher concentration of black liquor which is not possible with conventional evaporator. Other
  • The concentrated black liquor is sprayed in the furnace where black liquor is incinerated to yield smelt and flue gases. A typical recovery furnace consists of air preheater, economizer, furnace and boiler, cyclone, ventury scrubber/ electrostatic precipitator.
  • Addition of makeup chemicals Na2SO4 (salt cake) in thick concentrated black liquor which is reduced to Na2Sin the furnace in presence of carbon  
  • Incineration of Black Liquor in recovery furnace
  • Reducing zone in bottom to burn organic
  • Dying zone
  • Oxidation zone 

Reactions in recovery of chemicals from black liquor   

The main reactions are (1-3 predominantly) 


Na+ + H+ + S2-→NaHS

Na + OH-→NaOH

  Na2O+SO2+1/2 O2→Na2SO4



and other reactions: 



  • Dissolving smelt in dilute wash liquor or water to produce green liquor. Containing sodium carbonate and sodium sulphide.
  • Causticising of green liquor with lime to produce white liquor, lime sludge. A typical causticising section consist of lime slaking with green liquor, dreg clarifier, causticising tank, clarifiers, rotary filter, lime kiln.

Na2CO3+CaO +H2O   2NaOH+ CaCO3

  • Burning of lime sludge to produce lime. 

CaCO3 →  CaO + CO2

Table M-III 3.2: Typical Composition of Black Liquor from Wheat Straw Pulping 



Wheat straw Black liquor

Total solids


Swelling volume ratio ml/g


Gross calorific value


Silica as SiO2% , w/w


Acid insoluble as SiO2 %, w/w


Chloride as Cl, %, w/w


Potassium as K, %w/w


Sodium as Na%, w/w


Calcium as Ca %, w/w


Inorganic as NaOH%, w/w


Organics%, w/w



Typical Characteristics of Black liquor

1 ton of black liquor - 2.2 ton of steam

Black liquor solid=1.30 ton /ton of pulp.

Weak black liquor per ton of pulp -10 -12 m

Calorific value of black liquor: 3080 cal/gm

Black liquor-Organic 70%, Inorganic 30%

Weak black liquor: Na 28 gm/lit

RAA10.85 gm/lit  

Recovery of Chemicals in Agro-Based Mill 

With the shortage of forest based raw material and to preserve the forest, the paper industry in Asia and other parts of the world is forced to use non woody fibrous raw materials. India is one of the leading countries using substantial proportion of non wood materials which constitute about 50% of total raw material furnish . Non-wood raw materials are characterized by high silica content which varies from 1.5 to 20%. Most of the silica present in raw material passes into black liquor. Presence of silica in relatively high concentration creates numerous problems in chemical recovery plants.  Evaporator, recovery furnace, and lime kiln operation are adversely affected by presence of silica in black liquor .The process of distillation of bamboo and straw black liquors by carbonation has been developed by central Pulp and paper Research institute, Saharanpur,

Due to high silica content of the agricultural residues recovery of chemical from agrobased mills have caused major concern in Indian paper mills. Table M-III 3.3 gives the details of processes for recovery of chemicals from agricultural residues black liquor. Black liquor from cooking agricultural residues has high silica content and cannot be concentrated to high solid contents to enable its burning in conventional recovery 

systems. Although, technologies are available and being practiced in many mills in other part of world; however, due to poor infrastructure still recovery of chemicals is not being practiced by majority of mills in Indian posing serious environmental pollution problems. Pollution load in agro-based small mills having no chemical recovery is almost three times to that of pollution load in large integrated mills

Some of the currently accepted technologies for recovery of chemicals for agro-based black liquor are: Rotary roaster, Roaster cum smelter, Broby smelter, Dry pyrolysis Process, Wet air oxidation process, Copeland reactor. Some of the emerging technologies are Wet cracking process, Chemsec process, Thermo chemical conversion reactor system, Super critical wet oxidation process, Direct causticization process, ABC process, Direct alkali recovery system (DARS) or Ferrite process. In the fluidized bed soda recovery system where the weak black liquor containing high silica content is concentrated to about 45percent concentration in multiple effect evaporator, venturi scrubber and cyclone system and is sprayed in the fluidized bed combustion system.

Table M-III 3.3: Processes for Recovery of Chemicals from Agricultural Residues Black liquor 




Ferrite process (DARS process)

30% conc. Black liquor burnt with ferric oxide at temp 850 0c.

Na2CO3 +Fe2O3                   2NaFeO3 +CO2

2NaFeO3 +H2O                     2NaOH + Fe2O3

Wet air oxidation process

Oxidation of organic matter at high temp 320-320 0c pressure 175-250 kg/cm2.

Ultra Filtration and

reverse osmosis

Sodium compound is removed as permeate. High molecules

weight lignin fraction is rejected in interface.

COPE LAND process

34-35% Solid liquor is burnt in fluidized bed reactor. 650-



BKMI Process

Evaporation 35-37% in evaporator. Last two effect falling

film. Upto 50% in direct contact evaporator, then concentrated

black liquor is burnt.

Smelter                   cyclone


18-20% evaporation in multiple effect evaporator upto 50%

inn cyclone evaporators. Concentrated black liquor is fired in


Destomax              recovery


Similar to Smelter cyclone process

Burning of Black liquor

after desalination

Removal of silica by carbonation followed by concentration

and burning

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