TDM 94- , 1994

CPPA Bleaching Committee

Technical Session, May 7, 1996

1. Seven Steps to Chlorine Freedom - Murray Walters, Harmac Pacific (see Appendix 3)

1100 tpd bleached kraft, 3 lines of everything except recaust
Motivation

European market demands
BC government legislation
Simplify grade structure (11 grades, depending on Cl₂)
Industrial hygiene (release hazard)

Constraints

ClO₂ plant capacity
operating costs
pulp quality

Seven process modifications to provide reductions in chlorine dioxide demand

H₂O₂ reinforced extraction stages

3 kg/UBDt
1:1 replacement
low ROI, but a strategic decision

D₀ stage pH control

optimum pH near 2.5 before ClO₂
dilute H₂SO₄
lab/mill trials to determine the benefit
projected 10% reduction in kappa factor

C Bleach prewasher

> 30 kg/UBDt COD from Chemiwasher (reduced to 5)
give up Papricycle washer
projected 5.5 kg/BADt reduction in D_o ClO₂ usage

C Bleach medium consistency D₀ stage

new upflow to existing downflow tower
40 minutes in upflow alone, 145^oF (typically operators run at 60 min, 165^oF)
projected 13% reduction in ClO₂ demand
sequence now DE_opDED

Full time operation of Kamyr line Oxygen Delignification (basically added MgSO₄)

30% kappa number reduction
significant strength loss and blending restrictions solved by magnesium sulphate system and C bleach prewasher
projected 10% reduction in kappa factor in B bleach (at same CEK = 4)

Mini Oxygen Delignification for Chemiwasher line (650 tpd)

similar to Kamyr line system
30% kappa number reduction
C bleach prewasher as post-oxygen washer
commissioning scheduled for November 1996

A and B Bleach medium consistency D₀ stages

projected 13% reduction in ClO₂ demand
implementation dependant on HPI strategic plan

Economic Impact

planned steps 1-5: $2.94 million capital ($6.09)/BADt op cost 61% ROI
actual steps 1-5: $2.86 million capital ($6.86)/BADt op cost 72% ROI

Operating Experience

Positive:

ClO₂ demand reduced to 37 tpd
upflow/downflow easy to operate
less vulnerable to BSW upsets
H₂O₂ = increased operating flexibility
fewer grade changes

Negative

pH probes require frequent recalibration
dropleg corrosion

Conclusions

ClO₂ demand reduced to target level
chlorine eliminated from bleaching process
AOX reduced to about 0.4 kg/BADt
fibre line modernized
mill operability increased
operating costs reduced
attractive economics

2. Interaction of Hydrogen Peroxide and Chlorine Dioxide Stages in ECF Bleaching - Dave Senior, ICI

(paper was presented at 1996 International Pulp Bleaching Conference, Washington - see Appendix 3)

DEopDPP sequence consistently reached higher brightness than DEopPDP, despite similar chemical charges
Objectives

confirm the DP vs PD effect on other pulps
understand the mechanism
provide basis for improved bleach sequences

Literature Search - not much there

D(EO)D(EP)D = 91.1 ISO vs. D(EOP)DED = 90.2 ISO
location of Eop stage
position of (PO) in bleach sequence impacts brightness
TAPPI Jan/96 - ClO₂ before H₂O₂ improves P stage

Experimental

full bleaching of southern softwood

DP brightness consistently higher than PD
final brightness was the same, but P1 vs D1 different

limited bleaching of NBSK (DEPD vs DEDP)
limited bleaching of hardwood (DEPD vs DEAPD vs DEPAD)
extraction of lignin, compare ³¹P-NMRs

pulps prepared from each stage of bleaching in DE
theoretical says can predict that peroxide is more efficient after ClO₂
DED pulp consistently lower phenolics and condensed lignin than DEP

Conclusions

brightness consistently higher when D stage preceded P stage

Questions:

does this ³¹P-NMR show the tradeoff between ClO₂ and peroxide? Maybe

3. Survey Results: Doug Reeve - University of Toronto (see Appendix 3)

1. Substitution of Chlorine Dioxide for Chlorine in Canadian Bleached Chemical Pulp Mills;

2. ECF Practice in Canadian Bleached Kraft Pulp Mills

Substitution of Chlorine Dioxide

ECF continues to increase, TCF close to zero
softwood 1991 vs. 1996: slight change to lower kappa numbers into bleach plant
hardwood 1991 vs. 1996: more change to lower kappa numbers into bleach plant
ECF has almost caught up to 50% substitution in 1996 (ECF = those who campaign or are totally ECF)
nobody using less than 50% substitution
dominant non-ECF sequence DcEoDED (E₁ may have H₂O₂ )
dominant ECF sequence DEopDED (E₂ may have H₂O₂ )
ECF brightness typically 89-90, so no problem getting high brightness on ECF

ECF Practice in Canada

Softwood only, 20 conventional pulping, 9 with O₂ delignification
conventional

most still measure saltcake, not COD, about 5-10 kg
28-32 kappa to bleach plant
0.20 kappa factor, PEKappa 4.8

O₂ delignified pulp requires more chemical per change in kappa number (as expected, since starting at a lower kappa number)
How can some mills operate at the bottom of the graphs? These are the benchmarks
Where is the best place to put your oxidizing power? It depends - not everybody’s plants have the same strengths and weaknesses

Can we identify the critical parameters that make a mill do well? Not yet. The intention is to present another questionnaire for back end of bleach plant

4. Bleach Washing Evaluation Methods - Rae Cunnington, Dennis Matson

(compendium of six presentations)

Oxygen delignification washing

pre-oxygen washing well understood; require sufficient pre-oxygen stage washing to keep below a COD of 25 gpl in liquor from oxygen stage reactor - no viscosity loss at this level
post-oxygen washing requirements - 2 drum filters generally not enough, need 2 wash presses, or 2 pressure washers, or 2 stages of diffusion washing, or equivalent washing in multi-stage washers to provide low carryover to bleach plant

Bleach Plant Washing

requirements not well understood, many mills now looking seriously at the savings that are possible from optimizing bleach plant washing
Important Wash Parameters

Norden Number (for definitions see MR291, or the new Pulp Bleaching Principles and Practice text)
Modified Norden (Ek)
Displacement Ratio (DR)
see Appendix 3 for comparison of wash parameters for different types of washers

What are we trying to do?

Do - wash out spent hydrochloric acid and some chlorinated dissolved lignin
Eo - wash out dissolved organic material

Paprican

Displacement ratios on a D₀ washer for Na, Ca, Mg, Cl, TDS, lignin show lots of variability in efficiency as measured by displacement ratio

Ca and Mg have strong tendency to adsorb into pulp fibres with increasing pH, so do not use divalent metals as your method to measure washing efficiency will show high removal, even with no washing
lignin is insoluble under acid conditions, so do not use removal of organics to measure efficiency
sodium can adsorb into pulp fibres, so can be used, but may give high efficiency values
chloride is best - it has very low interaction with pulp fibres, and is what you actually want to remove in the washer

Displacement ratio on Eo washer shows very little difference for Na, TDS, and lignin, all around 0.85 displacement ratio

organics are soluble, so can be used to measure washing efficiency
the concentration of soluble Na is much higher, so can be used
divalent metals not significantly soluble, so do not use divalent metals

Northwood experience in measuring washing efficiency

2 bleach plants: same sequence - DE_OPDE_PD upflow
tried chloride on Do as a measure of efficiency, but it did not work because of recycle to dilution, so used dissolved solids (Marco - dissolved solids may not all be detrimental to following stages, so may not be best test)
decided to use displacement ratio using dissolved solids - tedious since must evaporate filtrate down to dry solids, so hired Co-op student for 4 months to do nothing but displacement ratios
Displacement ratio as % for both A and B bleach plants of (average of 3 tests)

Do A/B 64/28
E1 A/B 37/64
D1 A/B 66/32
E2 A/B 86/93
D2 A/B 89/96
important variables

vat level (poor efficiency if low)
air entrainment in vat 1.2% (1.0 - 2.0%) noted poor efficiency above 1.5%

costs related to chemical wastage due to carryover
no shower flow measurement, so no dilution factor calculated

Conclusions

will continue to optimize the lower efficiency vats
will provide increased operator training
will reactivate vat level control
will get ultrasonic flowmeter to measure shower flows

Questions - LaValley showers, Northwood doesn’t use defoamers in bleach plant

Weyerhauser, Kamloops experience

evaluated Do and Eop stages to measure impact of changes
visual examination very important

run washer as low as possible for maximum consistency
set showers for maximum rate
minimize air to air doctors to keep sheet from breaking
watch for filtrate re-wetting
look at wire condition

Do stage

sample sheet, measure consistency, measure sheet filtrate pH, titrate with NaOH to 10.5 pH, ensure you compare at same bleaching conditions and throughput
evaluated before and after shower bar change
lower carryover with new shower bars, so can run with lower shower flow and improve consistency
try it!!!
graph shows came way down in caustic usage for reduced shower flow

Eop stage

sample sheet, measure consistency, determine liquid KNo, ensure at same conditions
liquid Knumber is not as good a test as the Do test, but worth trying
graph shows large scatter

Remedies

replace whistle nozzles with spoon type that disturb the sheet less
keep washer wires in good repair
review wire mesh size
consider anti-rewet deck on washer (high cost, but may be able to justify this)

Weyerhauser - Grande Prairie

Do washer study - replacing washer drum, so wanted to know how much increase in efficiency would result with new drum (this data for existing drum)
attempted to quantify Do washer performance
neither TDS nor Ca provided an accurate basis for determining displacement ratio
chloride concentration was found to be a good indicator to determine the displacement ratio eg. DR = 62, 45, 68, 71% on four trials, which is lower than would be expected for the new drum

Mill X

two similar bleach plants, but very different kappa number of filtrate in Eop

Line 1 Line 2

Kappa No pulp 5 5
Kappa No filtrate 1.5 5
Total Kappa 6.5 10

this explains the high D1 chemical consumption in line 2
old drum washer with lots of re-wetting on line 2 Eop. Mill is working with vendor to make improvements

Bill Miller (Ingersoll Rand at TAPPI round table)

Effect of Dc Washing on Chemical Consumption and Eo Brightness
Increased D1 and Eo shower flow to Dc stage significantly, resulted in 40% reduction in caustic, increased Eo stage brightness
Three mills tried a number of tests to measure efficiency, including conductivity, Na, Cl, TDS. Resulted in very variable results

Conclusions

potential for saving a lot of money
worth trying some efficiency measurements

What can be done? What are people doing?

install high efficiency end valves
add satellite seal tanks to increase air removal (want about 3 minutes for vacuum drums or follow vendor recommendations)
install new shower bars, replace whistle type with spoon type showers
install anti-rewetting drums
increase number of shower bars
add vacuum assists
wire mesh changes
avoid shower water falling into vat
replace water doctors with air doctors
add press rolls (new control methods may help operation)
automatic vat level control
get washer discharge consistency up
add CO₂ on last brownstock washer
change filtrate recycle scheme (are you adding dirtier water)
baffle filtrate tanks
ensure drop-leg configuration and diameter is proper
do not over add chemicals
with diffusion washers, have stroking times compatible with pulp production rates
install baffles over vat inlet pipes for good even distribution of pulp over the drum (are they still there?)
NOTE - different wood species have different washing performance

Are many mills using vacuum assist? Gold River does, and finds excellent washing efficiencies, high consistency (16% consistency E₂ stage after chemical addition), 0.8-0.9 displacement ratios, mill really likes them;
On upflow towers is there a problem with air entrainment? Elk Falls has problems drawing in air at W15, so need to add defoamer. Add level control? Northwood took it off B bleach plant because of plugging. Northwood found no difference between swirl mixer and conventional dilution to the washer in terms of air entrainment in the vat.

5. Update on Bleach Filtrate Recycle - Rogelio Vega Canovas, Champion International (see Appendix 3)

635 tpd pine bleach plant at Canton, NC

chloride removal process (CRP)

started Sep/95
precipitator ash diluted, evaporated, saltcake filter, purge brine solution
built to remove 120 tpd ash, currently at about 100 tpd
satisfactory chloride, potassium removal
on-going optimization
chlorides in WL dropped from 3.2 gpl to 1.7 gpl. When recycle process starts up this will increase again, but hopefully not above the 3.2 gpl level (for comparison, mills on coast run about 30 gpl chloride)

metal removal process (MRP)

fibre strainer, sand filters, softeners (ion exchange)
normally achieve design throughput

metals removal efficiency >90%

high media (sand) filter on-line time
some significant mechanical difficulties
trace metals decreasing, especially potassium
Filtrate Recycle

D1 stage open
Eop stage 20-40% closed
no impact on bleaching
filtrate recycle is a challenge

Next steps

achieve consistent MRP operation for prolonged periods of time
complete Eop stage closure by May, while learning to balance filtrates
complete D1 stage closure by July
Full bleach plant closure hopefully by the Committee visit in the fall