DEWATERING

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DEWATERING

• PREPARED BY
• MD.ALAM (492/08)

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INTRODUCTION

• The separation of solids from fluids
  (dewatering) is one of the important subjects in
  the field of mineral dressing
• Water purification
• In mining and smelting operations

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WHAT IS DEWATERING?

• A simple answer would be the process of removing
  water from sludge (suspension of solid matter in
  liquid).
• Dewatering is the process of natural, chemical,
  or mechanical removal of water from sludge,
  thereby reducing it to a damp solid with the
  lowest level of moisture attainable .

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• Mechanical separation of solids from liquids is
  accomplished by one of two general procedures
• Thickening.
• filtration.
• (Ultimate moisture removal is obtained by
  drying.)
• Separation of solids from gases
• settling.
• Filtration.

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THICKENING

• A suspension of solids in a liquid is allowed to
  settle until a clear liquid layer tops a mud
  layer.
• A volume reduction of approximately 30 80 can
  be reached with sludge thickening before a
  further treatment.

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IMPORTANT TERMS

• Flocculation it is a process of contact and
  adhesion whereby the particles of a dispersion
  form large size clusters.
• Floccules or flocs these are rounded aggregates
  of solid particles.
• Pulp any soft or soggy(soft and watery) mass.
• Slime any thick viscous matter.
• Flocculant Particles finer than 0.1 µm (10-7m)
  in water remain continuously in motion due to
  electrostatic charge (often negative) which
  causes them to repel each other

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THICKENING

•  At smaller wastewater treatment plants, where
  the sludge is driven off regularly,
• thickening usually takes place directly in the
  sludge storage tank. The sludge is compressed at
  the tank bottom only by the force of gravity,
  while above the sludge a cloudy water layer is
  formed, which is taken off and led back into the
  inlet.

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STEPS IN PROCESS OF THICKENING

• Flocculation of the minute solid particles so as
  to form aggregates or flocs of many particles.
• Sedimentation of the liquid-laden flocs,leaving
  clear liquid.
• Compaction of the sedimented flocs.
• Elimination of the clear fluid and of the
  thickened mud.

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PROCESSES INVOLVED IN DEWATERING FLOCCULATED PULP

• Free settling of flocs.
• Hindered settling of flocs.
• Exudation of water from the settled flocs,under
  the influence of pulp pressure.
• Exudation of water from the settled and naturally
  exuded flocs by application of outside pressure
  or suction.

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SLIME SETTLING ZONE IN DORR THICKENER.
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DORR THICKNER

• Although less expensive per square foot of
  settling area they are
• Less effective and more difficult to operate.
• More power consumption.

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WORKING

• The revolutionary EIMCO E-CAT
  Clarifier-Thickener combines optimized
  flocculation, high-rate clarification and
  high-density thickening in a single compact unit.
  It streamlines liquid-solid separation flowsheets
  by optimizing chemical settling aids and
  providing a dedicated escape route for displaced
  free liquid. Even more impressive, the E-CAT
  Clarifier Thickener system accomplishes all of
  this with no moving parts. 

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FEATURES AND BENEFITS

• No moving parts - Lower capital cost - Lower
  energy consumption - Less maintenance
• Smaller surface area than conventional
  thickeners - Lower capital cost - Smaller
  footprint
• Self diluting feed - Optimal flocculant
  utilization - No external dilution pumps required
• Good overflow clarity - Effluent re-use
• Dense underflow - Lower energy requirement for
  drying - Less ponding area required - Reduced
  handling volume

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FILTRATION

• Filtration is that separation of finely divided
  solid particles from a fluid which is
  accomplished by driving the pulp to a membrane or
  septum, porous to the fluid but impervious to the
  solid, through which the fluid passes, and by the
  removal of the solid cake from the septum.

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MECHANISM OF FILTRATION

• The simplest type of filter consists of a tube of
  small bore through which the fluid is sucked
  while the solid particles accumulate at the
  entrance. As the device is operated, solids at
  first pass through the tube, but they quickly
  arch or bridge across the opening, allowing only
  clear liquid to pass

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RATE OF FILTRATION

• The filtering area.
• The difference in pressure between two sides of
  the filter.
• The average cross section of the pores within the
  filter cake.
• The number of pores per unit area of the septum.
• The thickness of the filter cake.

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TYPES OF FILTERS

• Pressure type pressure is applied to push the
  liquid through the filter bed.
• (for hydrometallurgical work in which interest
  centers on the filtrate, as in cyanidation.)
• eg. Hardinge filter thickener.
• Suction type suction is applied to pull the
  liquid through the filter bed.
• (used in chemical laboratories, for mineral
  dressing in which interest centers on the cake.)
• eg. Genter thickener.

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PRESSURE AND SUCTION FILTERS

• Adv
• higher fluid capacity per square foot of
  filtering surface,
• better control of cake formation,
• possibility of better washing of cake.
• Disadv
• A great disadvantage is that they are
  intermittent (stopping and starting at regular
  intervals) instead of continuous.

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BENEFITS FROM DEWATERING

• A good reason would be cost savings and
  environmental awareness .
• Lower Transport and Disposal costs .
• Dewatered solids are cheaper and easier to
  incinerate.
  
• Ease of handling with dewatered solids.
• Less offensive smells.
• Reduced volume in landfills .
• Reduced leachate production in landfills .
• Lower amount and cost for bulking agents for
  composting

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CONCLUSION

• Keeping in mind the optimum sludge removal and
  dewatering technique a site and sludge specific
  evaluation process should be performed with the
  objective of developing the safest, most
  economical, and environmentally responsible
  approach that meets or exceeds the objectives and
  expectations of the client.

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THANK YOU
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REFERENCES

• PRINCIPLES OF MINERAL DRESSING
• BYA.M. GAUDIN.
• www.FLSmidthMinerals.com
• www.lenntech.com