ChE 441

Refining

Introduction

Mechanical treatment of pulp fibers to develop their optimum papermaking properties

Optimum paper properties depend on the product being madeThere is always a tradeoff between various properties

Introduction

Refining increases the strength of fiber to fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each otherHowever, individual fibers are weakened and made shorter due to cutting actionRefining is generally a tradeoff between improving fiber to fiber bonding and decreasing the strength of individual fibers

Paper of Increasingly Refined Pulps

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Paper of Increasingly Refined Pulps

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Fiber Brushing

Refining at high consistency with a relatively large distance between the refiner platesThis tends to roughen the fiber surface, with minimal fiber cutting for improved fiber-fiber bonding

Fiber Cutting

Operating refiners at low consistencies with a minimal distance between the refining surface Increases fiber-bar contact resulting in fiber cuttingDesired with certain woods containing long fibers to increase the quality of formation

Drainage

Is the ease of removing water from pulp fibers – by gravity or mechanical means

CSF is a measure of drainage and a useful means for determining the level of refining

Fibrillation

Is the production of rough surface on fibers by mechanical actionRefiners break the outer layer of the fiber (primary cell wall) causing the fibrils from the secondary wall to protrude from the fiber surfaces

Fibrillation

Source: www4.ncsu.edu

Canadian Standard Freeness, CSF

Refining is monitored by the drainage rate of water through the pulpHigh drainage rate means high freenessThe CSF test measures the drainage of 1 liter of 0.3% consistency pulp slurry through a calibrated screen

CSF Test in Progress

Mechanism of Refining

What Happens During Refining?

Primary EffectsRemoval of primary wallPenetration of water into the cell wallBreaking of some intra-fiber bonds –replacement by water-fiber hydrogen bondsIncreased fiber flexibilityExternal fibrillation and foliationFiber shortening

What Happens During Refining?

Secondary EffectsFractures in the cell wallFiber stretching or compressionPartial solubilization of surface hemicelluloses into gelsStraightening of fiber (at low consistency)Curling of fiber (at high consistency)

Net Specific Energy

pulp oft unit weighappliedenergy net energy specificnet =

Refining Intensity

area refining effectiveenergy specificnet intensity refining =

Intensity Factor

area refining effectiveappliedenergy net factor intensity =

Variables Affecting Refining

Raw MaterialsFiber morphologyPulping methodDegree of pulpingBleaching treatmentPrior processingFiber coarsenessEarlywood/latewood ratioChemical composition (lignin/cellulose/hemicellulose)

Variables Affecting Refining

Equipment CharacteristicsBar size and shapeArea of bars and groovesDepth of groovesPresence or absence of damsMaterials of constructionWear patternsBar anglesSpeed of rotation

Variables Affecting Refining

Process VariablesTemperaturepHConsistencyAdditivesPretreatmentsProduction rateApplied energy

Refiners

Conical refinersLow-angle types – Jordan refinersHigh-angle types – Claflin refiners

Disc refiners

Jordans

Claflins

Single-disk Refiners

Disc Refiner with Two Opposing Rotating Discs

Refiner with Double-sided Rotating Disc between Two Stationary Discs

Mono-Flo and Duo-Flo

Disc-Refiner Plates

Effects of Refining on Paper Properties

Most strength properties of paper rely on fiber to fiber bonding and increases with refiningTear strength depends highly on the strength of the individual fibers – therefore decrease with refining

Typical Strength Development During Refining

Effects of Refining on Paper Properties

The paper stock becomes slower (more difficult to drain)

The resultant paper sheets become denser, with reduced porosity

Effects of Refining on Paper Properties

After a certain point the liming factor of strength is not fiber to fiber bonding – but the strength of the individual fibersRefining beyond this point begins to decrease other strength properties besides tear

Control of Refining Process

Generally there are two specific objectivesOptimum strength developmentControl of sheet formation and wire drainage

For ease of control and minimum energy consumption these objectives are best met in two-stage system

Refiner Control System