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Clarification Principles

Clarification:

The green liquor clarifier, white liquor clarifier and the mud washer are liquid / solids separation devices that work by the same principal of sedimentation. Sedimentation is the removal of suspended solids from a liquid by gravity settling. To achieve a separation of solids from the liquid, the downward velocity of the solids to be removed must be greater than the upward velocity of the liquid. The velocity of the liquid is governed by the clarifier design and the influent flow rate.

The clarifiers have four sections to provide efficient solids / liquid separation: The inlet section to provide a smooth transition from the high inlet piping velocities to the low settling zone velocity; the settling zone to provide a low upward liquid velocity; a transition to the high velocity of the outlet overflow; and the sludge zone to collect, compact and remove the settled solids. Mechanical failures to any of the internals, such as holes in the centerwell wall or plugged overflow weirs, will adversely affect the solids removal efficiencies.

The downward velocity or settling rate of the solids is controlled by their size, density and shape, as well as the viscosity of the liquid. Stoke’s Law gives the terminal velocity of settling spherical particles:

V = [2 r2 (pp – p) g] / 9 u

V = terminal velocity
p = density of the liquid
r = radius of the particle
g = gravitational constant
pp = density of the particle
u = viscosity of the liquid

Stoke’s Law is a balance between the gravitational forces acting on the solid particle and the viscous drag (friction) forces between the particle and the liquid. Analyzing the terms in Stoke’s Law allows us the ability to see how changes in the liquid and solids’ physical properties impact the settling characteristics. Changes that increase (V) will improve settling and changes that decrease (V) will hinder settling.

The rate of solids / liquid separation will then be the difference between the upward velocity of the liquid and the settling velocity of the solid particles. This explains why solids removal efficiency goes down when high flow rates occur through the clarifiers.

In the green liquor clarifier we can improve the settling rate by using polymers to increase the size of the particles (r). As the r term in Stoke’s Law is squared, increases in the particle size will have a large positive impact on settling. Stoke’s Law also tells us that the difference in densities between the fluid and particle will impact settling. If we assume that the particle density (pp) is fixed, increases in the chemical concentration in the green liquor will increase the density of the green liquor (p) and decrease the density difference (pp – p), thereby slowing the settling rate. Poor settling can be caused by low reduction efficiency (conversion of Na2SO4 to Na2S in the recovery furnace), causing the green liquor density to increase.

Green Liquor Clarification:

The purpose of the green liquor clarifier is to remove solids from the green liquor. The solids are primarily unburned carbon particles from the recovery furnace. These solid particles are typically referred to as dregs. Along with carbon, the dregs contain other impurities such as silica, iron, chromium, aluminum and magnesium. These impurities are not part of the causticizing process and are also called Non Process Elements (NPEs). If the NPEs are not removed from the system in the green liquor clarifier, they can cause many operational problems downstream. Problems that can be caused by inefficient removal of NPEs in the green liquor clarifier include:

Dirt in the bleached pulp from carbon and brightness reversion due to Fe
Poor mud settling in the white liquor clarifier
Poor weak wash clarity and settling in the mud washer
Reduced mud dewatering at the mud filter
Increased ball and ring formation in the kiln
Increased fuel requirements at the kiln due to dead load
Reduced causticizing efficiency due to the inhibitory effects of heavy metals
Increased oxidation of sodium sulfide to sodium thiosulfate (catalyzed by metals)
Blinding of mud filter face wire

The primary causes of high dregs carryover in the green liquor clarifier are:

High liquor flow rates
Poor reduction efficiency in the recovery furnace
High green liquor chemical concentration
Muddy weak wash

White Liquor Clarification:

The purpose of the white liquor clarifier is to settle out the mud and provide low-solids-content white liquor to the digester(s). High levels of solids in the clarified white liquor can cause can cause calcium scaling problems at the digester and increased pitch deposition in the pulp mill, bleach plant and paper mill. Poor settling in the white liquor clarifier can also increase sodium levels going to the kiln.

The primary causes of poor mud settling in the white liquor clarifier:

High liquor flow rate
Dregs carryover from the green liquor clarifier
Excessive lime addition to the slaker (target 1-5% over stoichiometric)
Slaker outlet temperature too high or low (target 212 – 2150 F for maximum size crystal growth in causticizers)
Holes in the feedwell or other clarifier internals

Mud Washer:

The purpose of the mud washer is to remove the sodium compounds from the mud. Inefficient sodium removal will increase the formation of rings and balls in the kiln. Muddy weak wash from the mud washer will also increase the solids loading going to the green liquor clarifier.

The primary causes of poor settling and muddy weak wash at the mud washer are:

High levels of dregs carry over from the green liquor clarifier
Pulling white liquor from the white liquor clarifier when pumping mud. (It is important to pump mud from the white liquor clarifier based on the clarifier rake torque reading to prevent pulling white liquor. Weak wash chemical concentration should be less than 1.5 lbs / ft3.)
Cycling of hot and cold wash water causing temperature inversions and thermal currents
Holes in internal mud feed lines and washer internals

   
   
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