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ECF Bleaching

ECF means Elemental Chlorine Free bleaching

Elemental chlorine is considered to be C and H Use of chlorine is still primarily because of low cost.

Disadvantages on quality and environmental front.

One of the foremost is the extensive pulp

degradation particularly in the H stage The other is high environmental loads particularly

chloroorganics responsible for the effluent toxicityrepresented by AOX and effluent color and highcolor reversion tendency of bleached papers

one can achieve moderate brightness of 78-82 %depending upon raw material & pulping process

For many end applications particularly printinggrades high brightness pulps are required

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Modern bleaching processes are Elemental ChlorineFree (ECF) and Total Chlorine Free (TCF) bleachingprocesses.

ECF process employ elemental chlorine free (C & Hstages) bleach chemicals like chlorine dioxide (D)and non chlorine bleaching chemicals like oxygen,hydrogen peroxide, enzymes, per acids etc.

In TCF processes ozone and non chlorine chemicals

as used in ECF are employed.

ECF and TCF pulping processes differ mainly on twofronts- pulp strength and environmental loads.

ECF bleached pulps are stronger than TCF pulps.

TCF processes give very low AOX values and theeffluent is easily treatable and recyclable

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Oxygen delignification/ pre-bleaching

It involves the use of molecular oxygen in alkalinemedia at elevated temperatures and under pressure.

Biggest advantage of oxygen delignification (O) isthat lignin content can be reduced by 40-50 %.

Bleach chemical demand in subsequent bleachingstages drops by up to 50 %, reducing formation of

chloroorganics (AOX) effluent color, BOD and COD Oxygen stage effluent can be used for washing of

unbleached pulp effuent reaches chemical recovery,alkali and additional steam becomes available fromorganics present in waste waters

Thus no pollution comes from Oxygen stage

Selectivity good and pulp easily bleached to +90%

Bleaching cost low - lower chemical consumption

oxygen stage an integral part of pulp bleaching

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Chemistry

Oxygen is an unusual molecule

It contains two unpaired electrons and is

considered as free di radical

It is less reactive than other free radicals

but can attack appropriate substrates at

regions of high electron density

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Stepwise reduction of oxygen

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Steps in mechanism of oxygen bleaching

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It involves the formation of per hydroxyl anion, hydrogen

peroxide and free radicals like hydroxyl radical OH., hydro

trioxy radical HOOO.and singlet oxygen O2.

Complex oxidation processes that occur in oxygen bleaching

include radical chain reactions involving organic species fromboth lignin and carbohydrates

The following figure shows the likely initiation, propagation and

termination steps

Substrate is activated under alkaline conditions by ionizing the

free phenolics groups present in residual lignin

Resultant anion is electron rich and vulnerable to attack

oxygen

An added complication and of great significance is the

involvement of trace quantities of transition metals acting as

catalysts for peroxide decomposition.

This decomposition forms hydroxyl radical with poor selectivity

is responsible for attack on cellulose giving pulp degradation

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Initial reactions leading to oxygen delignification

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Reactions of intermediate hydroperoxides

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Reactions of hydroperoxide intermediates to

form muconic acids and quinnones

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Oxidation and cleavage of cellulose chains

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Carbohydrate reactions not leading to chain cleavage

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Effect of addition of magnesium ion on selectivity

Magnesium Sulphate hepta hydrate Epsom salt is normally applied at 0.05-0.1%

Mg 2+on OD pulp. It is believed to function by precipitating magnesium hydroxide which adsorbs

metal ions or making them unavailable for catalysis of peroxide decompositionIt is apparent from the curve that selectivity do not remain constant as thekappa number of the pulp is decreased.

However the selectivity remains constant until about 50% of the lignin is

removed

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Effect of black liquor carryover and dissolved solids recirculation on

oxygen stage selectivity at 25% consistency

Carry over of the black liquor from the brown stock washers in to oxygen stage

adversely affects selectivity

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Treatment of unbleached pulp with nitrogen peroxide

(Prenox process) in the presence of oxygen makes the

oxygen stage more selective and efficient even when the

oxygen conditions are mild Lignin removal in oxygen stage can be as high as 80%

considerably enhancing pollution abatement benefits

Two types of commercial processes have been

developed and are in commercial operation These are Medium Consistency (MC) and High

Consistency (HC) processes

HC processes are the earlier processes, MC processes

are recent addition Now a days all newer units installed are exclusively MC

processes.

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Operating data of oxygen delignification

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Flow sheet of MC oxygen delignification

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Flow sheet of high consistency oxygen delignification

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MoDo-CIL reactor

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Bottom section of HC reactor used for Sapoxal Process

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The change over to MC units is due to lower

capital cost, greater ease of handling andmixing and pumping technology, improved

selectivity in presence of appreciable

amounts of black liquor solids.

However the chemical consumption is higher

in MC than HC oxygen delignification

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Effect of temperature on oxygen delignification rate

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Effect of time, alkali charge and

consistency on kappa number

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Effect of time, alkali charge and

consistency on viscosity

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The general trend is to optimize the

various parameters so as to achieve 40-

50% reduction in pulp kappa number.

Adjustment of alkali charge to target

desired kappa reduction is practiced for

routine adjustments.