Banglore 2010, IPPTA

8/8/2019 Banglore 2010, IPPTA

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Desilication of Bamboo and Straw

Black Liquors

A.K.Dixit, B.P.Thapliyal,

R.K.Jain and R.M.Mathur

IPPTA Conference

16TH- 17THJULY, 2010

Banglore

Central Pulp & Paper Research Institute, Saharanpur, U.P.


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Structure of Presentation

Why Desilication ? Desilication Technology Developed by CPPRI

-Concept of Submerse Bubble Reactor

-Process Details

Mechanism of Silica Precipitation by lowering of pH Desilication of Bamboo Black Liquor

Desilication of Rice Straw Black Liquor

Commercial Scale Desilication of Rice Straw

Impact of Desilication on Black Liquor Properties Conclusion


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Why Desilication ?Presence of silica in black liquor causes:

Scaling in Evaporator tubes

Hard deposits on wall tubes- CaAl Silicates

Beehives on furnace walls

Possibly responsible for high viscosity of Black

Liquors in agro residue black liquors Slow settling of lime sludge due to calcium- Silicates

formed during causticization

Low dryness of silica mud from causticizers duringmud washing leads to high oil consumption in lime kiln

Difficulty in lime mud re-burning

Thermal analysis studies have shown that silica forms a sheathover lime particles so the CO2 can not escape from CaCO3molecules during calcination in lime kiln.


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Overview of Desilication Technologies

No. of researchers had tried various processes fordesilication of black liquor.

Additions of Cations ( Ca++ , Mg++, Al+++ )

Lowering the pH of black liquor with CO2 or acid

Combined method by lowering pH & addition of

cations

None of these methods could succeed on commercial scale.


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CPPRI Desilication Technology Lowering of pH with CO2 under controlled conditions

Only silica gets precipitated and no lignin precipitation

occurs.

Concept of Submerse Bubble Reactor

Liquor pumped in a tubular system, is treated with gas,which on its downward path sucks the gas in the form

of discreet bubbles.

The shearing action due to flow of the liquor, presses

the gas bubbles in a kneading fashion. This action continuously exposes new gas liquid

interfaces and thus ensures slow reduction of pH

leading to selective precipitation of silica.


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Schematic of CPPRI Desilication Technology

18

1 Flue Gas from

Boiler2,4,6 Circulation Pum

3,5,7 Submerse

Reactor Tank

8 Hot Retention

Tank

9 Feed Pump to

Filter

10 Clarifier

11 Belt Filter12 Filtrate Tank

13 Feed Pump to Mi

Tank

14 Caustic Mix Tank

15 High pH Tank

16 Feed Pump to

Storage

17 Foam Tank

18 Foam TankLiquor Pump

Desilicated BLto Storage

1

9

10

2 3 4 5 6 7

8

11

1213

14

15

16

17

Weak

L From

torage

Caustic

Sod

a

Precipitated

Silica


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Flow Sheet of Mill Scale Desilication Plant in Rice Straw Based Mill


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CHEMISTRY OF DESILICATION

NaOH + SiO2 Na2SiO3 + H2O

60 122

CO2

+ H2

O H2

CO362

2NaOH + H2CO3 Na2CO3 + 2 H20

80 106

Na2SiO3 + H2CO3 Na2CO3 + SiO2 + H2O

122 + 62 106 + 60 + 18


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Mechanism of Silica Precipitation by lowering of pH

Lowering of the pH of black liquor converts silicates into

insoluble polymeric anhydrides of silicic acid.

Lowering the pH of alkali silicate solutions results in;

First in formation of monosilicic acid.

This monomer undergoes polymerization, at a concentration greater

than the solubility of amorphous silica (i.e. 100-200 ppm), forming

higher molecular weight species.

Above pH7 stabilized particles (Sols) grow to a size of about

100 nm.

When salts are present, to neutralize the charge on growingparticles, aggregation of particles occur with the formation of

chains and ultimately, three dimensional gel net works.


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Mechanism of Silica Precipitation by lowering of pH

Central Pulp & Paper Research Institute, Saharanpur, U.P., INDIA

(Mono silicic acid)


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Desilication of Bamboo Black Liquor

Desilication of Bamboo Black liquor was carried out on

Pilot Plant in a bamboo based mill.

70 litres of weak black liquor was desilicated in each

trial. Flue gas was collected from chemical recovery Boiler

stack.

CO2 content in flue gas was 12%.

Desilicated black liquor was filtered under vacuum. Filtered desilicated black liquor was subjected to re-

alkalisation.


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Black Liquor Characteristics Desilication

(%)Before Desilication After Desilication

pH SiO2, g/l PH SiO2, g/l

12.8 4.2 10.5 2.1 48.810.4 1.5 64.3

10.3 1.2 71.4

10.2 0.8 80.9

10.1 0.5 88.9

10.0 0.4 90.5

9.8 0.4 90.5


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Silica precipitation starts at a pH around 10.4.

90% desilication is achieved at a pH of 10.1.

Results show that residual silica content is almost

same after pH level of 10.0.

Carbonation of black liquor beyond pH 10.1 does notfacilitate silica precipitation.

Optimum pH range for precipitation of silica is 10.1

Residual silica content in black liquor is 0.4 g/l in this

pH range. Residual silica is difficult to remove bycarbonation.

6 gpl NaOH is required for regaining the pH and

viscosity of black liquor.


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PHYSIO-CHEMICAL ANALYSIS OF BAMBOO BLACK LIQUOR

BEFORE AND AFTER DESILICATION

PARAMETERSORIGINAL

BLACK LIQUORDESILICATED

BLACK LIQUOR

1. pH at 3

0o

C12.

8 12.3

2. Total Solids, %w/w 19.0 19.2

3. RAA,gpl as NaOH 7.0 4.0

4. Silica, gpl 4.2 0.4

5. Sulfur, %w/w 2.8 2.85

6. Organics, % w/w 63.2 62.3

7. In organics,% w/w 36.8 37.7


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Viscosity of Desilicated and Original

Bamboo Black liquor

8

22

55

128

240

8

20

40

80

190

50

55

60

65

70

%

V

Desilicated Blackiquor

Original Black iquor


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Swelling Volume Ratio of Black Liquor

Carbonated Black Liquor

Realkalised Black Liquor

Original Black Liquor


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Desilication of Rice Straw Liquor

Black Liquor Characteristics Desilication

(%)Before Desilication After Desilication

pH SiO2, g/l pH SiO2, g/l

12.0 13.3 10.5 6.9 48.1

10.4 5.8 56.3

10.3 1.5 88.7

10.2 1.0 92.5

10.1 0.7 94.7

10.0 0.4 97.0

9.9 0.3 97.7

9.7 0.3 97.7


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Desilication of Rice Straw Liquor

Silica precipitation in rice straw black liquor starts atpH around 10.3.

Around 97% desilication is achieved at pH 10.0.

Below pH 10.0 there is only marginal improvement

in desilication efficiency.

Residual silica content is 0.4 gpl at this pH and 0.3

gpl at pH levels 9.9 to 9.7.

Optimum pH range for silica precipitation in ricestraw black liquor is 10.0 9.9. Around 98% silica is

precipitated in this pH range.


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Commercial Scale Desilication of Rice Straw

A full commercial scale desilication plant

was installed in 2006 in Coastal papers

Limited.

Capacity of theplant is 20M3 black liquor / hr

Flue gas is tapped from Power boiler.

CO2 content of flue gas is 9-11%.


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INPUT CONDITIONS FOR B.L. DE-SILICATION.

1. RAA IN WBL 4 5 gpl

2. SILICA AS SiO2 6 13 gpl

3. pH MAX. REMOVAL OF SILICA 9.9

4. pH OF WBL BEFORE SILICA REMOVAL 11.8 12.0


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PHYSIO-CHEMICAL ANALYSIS OF RICE STRAW BLACK LIQUOR

BEFORE AND AFTER DESILICATION

PARAMETERSORIGINAL

BLACK LIQUORDESILICATED

BLACK LIQUOR

1. pH at 30o C 12.0 10.3

2. Total Solids, %w/w 14.1 9.6

3. RAA,gpl as NaOH 5.0 0.8

4. Silica, gpl 13.3 0.68

5. Sulfur, %w/w 0.06 0.1

6. R2O3, % w/w 0.09 0.05

7. Organics, % w/w 67.65 71.9

8. In organics,% w/w 32.2 28.1

9. Calorific value cal/gram 2578 2940


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PHYSIO-CHEMICAL ANALYSIS OF RICE STRAW BLACK LIQUOR

BEFORE AND AFTER DESILICATION Contd..

PARAMETERS ORIGINALBLACK LIQUOR

DESILICATEBLACK LIQUOR

10. Carbon, % w/w 34.4 36.611. Hydrogen, %w/w

2.5 2.9

12. Nitrogen, % w/w 0.7 1.013. Sodium, % w/w 8.5 11.514. Calcium,% w/w 0.04 0.0615. Sulfate, %w/w as SO4 0.15 0.15


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Viscosity Of Desilicated and Original Rice

Straw Black liquor

33

56

112

316

730

8

12

24

75

130

35

40

45

50

52

%

tl

li

Vi it , . . .

Desilicated Black iquor

Original Black iquor


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Photographs of Mill scale desilication plant

in Rice straw based mill


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DESILICATION PLANT REACTION TANKS


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GAS NOZZLES ASSEMBLY


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FLUE GAS NOZZLES


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CLARIFIER UNDERFLOW PUMP


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PRESSURE FILTER


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SILICA CAKE BEING SCRAPPED


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DUMPED SILICA CAKE


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Impact of Desilication on Black Liquor Properties

Colloidal stability of B.L.- High

Lignin co-precipitation - Nil

Foam generation during gas liquid interaction-moderate and

easily manageable.

Settling of silica sludge- Encouraging results were obtained on

carbonated black liquor.

After three hours of settling the sludge volume stands at only 20% of total

liquor volume and supernatent black liquor has less than 20 ppm

suspended solids.

iscosity of desilicated and realkalized black liquor is fairly close

to the original black liquor in case of bamboo black liquor.

iscosity of rice straw B.L. is reduced significantly after

desilication.

Contd


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Impact of Desilication on Black Liquor Properties..

Contd

No precipitation observed even at 50% Concentration. Original

black liquor gets precipitated at 28% Conc.

There is marginal reduction in the calorific value of realkalized

bamboo black liquor, due to increase in the concentration of

inorganics during realkalisation. However there is no change incalorific value on the basis of organic content of the black liquor.

Calorific value, however increased in case of rice straw black

liquor.

There is a remarkable improvement in the swelling volume ratio(S R) of carbonated & realkalized black liquor when compared to

the original black liquor, which is a pointer towards better

combustibility of desilicated black liquor.


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Conclusion Black liquor properties are improved & burning behaviour of black liquor

is better after desilication.

Successful operation of full commercial scale plant since last four years

has created confidence on CPPRI Technology.

Pilot plant and full commercial scale plant trials have shown that both

Bamboo and straw black liquors containing 2.0 to 14 gpl silica can be

desilicated to a residual silica of 0.5gpl. Successful demonstration of bamboo and rice straw black liquor

desilication has further confirmed the suitability of CPPRI desilication

technology in wide range of silica content of black liquor ( 2-15 gpl).

Adoption of desilication process in bamboo and straw based chemical

recovery will help to; reduce operational problems caused by silica,

save energy during evaporation and

protect environment by making lime sludge suitable for reburning and reducing

solid waste.


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Documents

Banglore 2010, IPPTA