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Screening For Quality One mill’s experience
Gary Parobeck Acrowood
Appvion Roaring Spring, PA
From length screening to thickness screening.
It was a long haul but worth the trip.
Where is Roaring Spring PA
Mill Information
End product - carbonless and security papers.
Five Batch Digesters
Hardwood chips – 80% manufactured on site. 20% saw mill and chip mill supply.
The Screening System Before
Getting There
Testing – Testing – Testing
First investigations in 1999
Chips were sent to various suppliers for screening and overs treatment testing
The end product was sent on for independent cook trials
Close review by mill personnel
Testing
Testing
Results
Thickness screening offered the greatest savings and shortest payback
Cracking was shown to provide the mill the best result for their cooking process
A compromised was reached based on available funds and space
New Thickness Screens
Model 1730 Screw Conveyor
Model 1230 Side Fed Screw
Model 55144 DiamondRoll Thickness Screen
Model 35108 DiamondRoll Fines Screen
Note – the new system was designed to send the overs through a sizer and then recirculate this product back over the screening system.
Side View
Installation Photos – During Demo
Installation Photos
Bucket elevator at left feeds the screw conveyor which in turn feeds the infeed screw distributor.
Installation Photos
Installation Photos
#1 and #2 Accepts Chutes below DTS
DFS #2 Accepts bypass - above DFS fines and accepts - below
Installation Photos
Screen Performance
Design Capacity 100 green tons per hour
Performance Guarantee:
Infeed Chip Quality Chip Quality after Screening ≤10% Overthick (+8mm T) ≤2.0% Overthick (+8mm T) >10% and ≤15% Overthick ≤2.5% Overthick >15% Overthick ≤3.0% Overthick
≤1.5% Fines (-3mm RH) ≤0.5% Fines (-3mm RH) >1.5% and ≤2.5% Fines ≤0.6% Fines >2.5% Fines ≤0.8% Fines
Fines removed by DiamondRoll screen ≤0.5% Accepts loss (+2mm thickness)
Mill Classification
To the Digester – Old System
Over 1 ½” RH – 3 %
Over 8mm T: 11%
1 1/8” RH: 7%
7/8” RH:17%
5/8” RH: 27%
3/8” RH: 27%
Pins (3/16 RH): 7% Dust: 1%
Average Overs 6mm 4mm 2mm P&F Accepts
Summer '12 17.63 28.37 39.14 13.74 1.11 81.26
To the Digester New System
Their new stack was 10mm, 8mm, 6mm, 4mm, 2mm and a 3mm RH
After screen with out the sizer product they saw 96.9% Accepts
What did this mean to the mill
Yield improvement
liquor savings of 12,000 gallons per day.
Overall improvement in Digester operation
Cleaner, Safer Working Environment.
The Perfect Storm
The Perfect Storm
The Sizer product is returned to the infeed for re-screening
A fraction of the returned material contained chunks that were just over 8mm thick and as such were rejected as an over.
These pieces were small enough to also pass the Sizer grate again.
The Perfect Storm
As a result overthick built up to a level which over-ran the sizer capacity
The mill could bypass the Sizer and could continue to run
This of course meant no overs removal and everything but the fines went to the Digester
What to do?
Open up the DiamondRoll Thickness Screen to allow the larger pieces to pass
Or – Install a smaller grate in the Sizer thus reducing the size of the product.
Consequences
If the screen is opened up a higher percentage of overthick will make it to the Digester.
If the Sizer is closed down fines and pins will go up, ultimately greater fiber loss
Action Taken – Step One
The mill has resolved this issue (for now) by revising the screen IRO:
First half (infeed) 7.5mm IRO
Second half – 9.5mm IRO
Last two shafts – 13mm IRO.(Gate)
IRO Change
The IRO is changed by shims between the bearing housings.
IRO Change
Discharge end gate
Results
Accepts to the Digester dropped from 94% to 90% due to allowing more overthick.
But it did solve the problem of the overs build up and Sizer plugging
Action Taken Step Two
They worked with the Sizer Manufacturer to make grate changes
They were able to close the DiamondRoll thickness screen back down to a 7.5 MM IRO – All but the last two that Remain at a 13MM IRO
Automated gate at the discharge end of the thickness screen
Overview
They are happy with the system and it has shown great savings.
Currently accepts to the digester are back in 94% range.
Their target is to bring the accepts to the Digester up to 96% or better. They have calculated that for every 1% at the digester they save $000,000 per year.
Questions ?
Slicers & Crackers
Chip Slicers
Chip Slicer Basics
Gravity Feed
Bottom Discharge
Slices Chips to Thickness
Two Co-Rotating Elements: Anvil Rotor and Knife Ring
Typically 95% Reduction Efficiency
Must be protected from contaminants
The anvils push the chips into the knives, while centrifugal force holds them flat against the knife ring
Chip Slicer Function
123 rpm
369 rpm
Chip Slicer Drive
Single 150 HP, 1200 rpm motor
Belt drive to the gear train
Simple gear box design
Shear Pin Coupling
Wear Parts
There are six main wear part components: Anvils
Knives
Counter knives
Knife carriers
Wear plates
Gauge plates
Knife
Anvil
Knife Carrier
Wear Plate
Counter Knife
Gauge Plate
Gear Assembly
Chip Quality
95% Reduction Efficiency
10-15% Fines and Pin chip production
The thickness control is adjustable
Chip distribution into the slicer is adjustable
Summer & winter chip quality is essentially the same
Chip Slicer Efficiency - Evaluation on Southern Pine
Model 1511i Chip Slicer Configured for 10mm Thickness Control
Sample +10mm T +8mm T +6mm T +4mm T +2mm T +3mm RH Pan
Infeed 66.5% 16.6% 12.9% 3.8% 0.2% -- --
Infeed 69.4% 17.3% 10.7% 2.6% <0.1% -- --
Slicer Product 0.4% 31.7% 29.7% 22.9% 12.9% 1.9% 0.5%
Slicer Product 0.9% 36.1% 29.8% 20.7% 10.1% 1.8% 0.5%
Results*: 99.0% >10mm Overthick Reduction Efficiency 1.9% Pin Chip Generation (<2mm T, >3mm RH) 0.5% Fines Generation (<3mm RH)
Chip Quality
* Results based on the average of both Infeed and Slicer Product Samples
Chip Crackers
Reducers And Drivelines
Two reducers drive the rolls
Large universal joint drivelines connect the rolls and reducers
Swing arms support the rolls
Chip Cracker
• Overthick chips are compressed by two diamond tipped rolls which gently crack the chips
• Cracking allows proper liquor penetration in pulping
• Can adjust performance by changing pressure and/or gap between rolls
Cracker Rolls and Segments
• Square central box
• Machined attachment surfaces
• Integrated register lip that locates individual segments and transfers cracking forces from the segment to the central shaft
Cracker Performance
High overthick treatment efficiency –+95% fissuring effectiveness at 8mm chip thickness control.
Low pins and fines production
Typically <2.5% combined pins and fines (pins <7mm, >3mm RH, fines <3mm RH)
Cracker Pulping Response
More effective overthick treatment results in better pulping response
“Hardwood – Cracked overthick chips produced (3.4 to 3.8%) more absolute wood to pulp yield than rechipped overthick chips. Knots were (85% to 89%) less and Shives (22% to 26%) less with Cracked overthick chips compared to Rechipped overthick chips.” Econotech study in 2006
Cracker Pulping Response
More effective overthick treatment results in better pulping response
“Softwood – Cracked overthick chips produced (2.5 to 2.6%) more absolute wood to pulp yield than rechipped overthick chips. Knots were (71% to 86%) less and Shives (6% to 37%) less with Cracked overthick chips compared to Rechipped overthick chips.” Econotech study in 2006