Pulp Mill processes

Type of pulp and mill

Pulp type

• Virgin pulp

– Chemical pulp

• Sulphate process, kraft*

• Sulphite process*

• Soda

– Semi chemical pulp, NSSC

– Mechanical pulp

• TMP, CTMP

• SGW

• Recycled fibre

Mill type

• Integrated mill

– Produce pulp and paper on

the same site

– No drying machine for the

pulp

• Non-integrated mill

– Sell pulp normally in bales,

”market pulp”

– Have a drying machine

*Dissolving, viscose is becoming

more common again.

Chemical pulp

• Wood consist of cellulose, hemicellulose and lignin

• The chemical pulping process is made to remove lignin

• If production of viscose/dissolving, the hemicellulose is also

removed

CelluloseHemi

cellulose

Lignin

Kraft Pulping Process (Sulphate)

Chemical recovery in kraft pulp

Make-up chemicals-

addition in recovery

Boiler: NaOH

Black Liquor

After evaporation

Greater than 75%

Solids,

Modern mill 85%

LHT

Liquor Heat Treatment, to

Bring down the viscosity

Chemical Pulping - Sulphate process – Kraft

process• Wood chips are treated with ”white liquor” and heated in a cooking process.

Black liquor is developed and then evaporated.

• Active cooking molecules;

– Sodium sulfide and Sodium hydroxide, Na2S and NaOH

– (Earlier use of Sodium sulphate, is the reason for the name Sulphate process)

• The high consistency black liquor is used as fuel in the recovery boiler

– Oxygen deficient environment, Na2S is formed

– 4 C (s) + Na2SO4 (l,s) -> Na2S (l,s) + 4 CO (g)

• A smelt (contains Sodium Sulphite, Sodium Carbonate, Sodium Sulfate) is created in the boiler, and

will become green liquor.– Smelt removed from boiler through smelt spots, into dissolving tank mixed with water, create green liquor, (danger

smelt-water explosion)

• Green liquor sent to causticizing plant, reacts with lime CaO, to convert Na2CO3

to form NaOH (Na2CO3+CaO+H2O->NaOH+ CaCO3)

• The CaCO3 (lime mud) is washed and sent to lime kiln, heated, to regenerate

CaO (CaCO3 (s) + heat -> CaO (s) + CO2 (g))

• Recovery of boiler chemicals (97%), and white liquor is again created

• Process is alkaline

Trend with higher

Pressure and temperature, today

110 bars and 510 C

Steam generation

3,8 kg steam per kg black liquor

(BLds)

0,6 kWh/kg BLds

Modern mill, back pressure

turbine combined with a

condensing turbine to maximize

power generation)

Cl and K removal

• Disposal of of substantial portion of the precipitator ash

• NCG collection (Non-condensate gas) – for reducing odours

in mill

Chemical Pulping - Sulphite process• Earlier the most common pulping process (Invented 1867)

– Smells less, more easy to bleach, cheaper investment, higher yield

– But, due to high levels of effluent, not suitable for pine wood, the fiber is

weaker and has a higher manufacturing costs, the technology was reduced

• Active cooking cooking chemical: sulfite SO32- or bisulfite HSO3

-

• Use of different ”bases”

– Ca2+ - used to be the most common, not possible to recover cooking chemicals

– Na+ - cooking chemicals can be recovered, used in Aditiya Birla Domsjö

– Mg2+ - cooking chemicals can be recovered, today most common, Magnefite bi

sulphite

– NH4+ - cooking chemicals can not be recovered

– K

• The liquor is called red liquor or sometimes brown liquor

• Process is most often acidic (the acidic condition can hydrolyze the cellulose,

giving a weaker fibre)

• During the recovery of the liquor, thick liquor is oxidatively burned, resulting in

magnesium oxide dust and SO2 gas. The MgO dust is taken up in water and

brought into contact with the SO2-containing waste gas from the liquor

combustion to form magnesium bisulphite liquor again – much more simple

process than kraft

Semi Chemical Pulping - NSSC

• NSSC = Neutral sulfite semichemical pulp• Two stage process

– Mild chemical treatment, Sodium Sulpite

– Mechanical treatment

Soda pulping process

• Sodium hydroxide as cooking chemical

• Invented 1851, first mill 1866

• The soda process gives pulp with lower tear strength than

other chemical pulping processes (sulfite process and kraft

process), but has still limited use for easy pulped materials

like straw and some hardwoods

Tall oil

• Tall oil is an important byproduct separated from the black

liquor with skimming before it goes to the evaporators or

after the first evaporator stage.

Na/S balance in Kraft mill

• Normally excess Sulfor and low levels of Na

• Control sulfidity . Purge electrostatic ash from the recovery

boiler