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Sedimentation
tendency for particles in suspension to settle out of the…
Sedimentation
- tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier by gravity.
- Sediment: a particulate matter transported by fluid flow and deposited as a layer of solid particles on the bottom.
Sediments in Geology
- sediments transported by aeolian and glaciers.
- gravitational collapse also creates sediments such as talus and mountain slide deposits.
- seas, oceans and lakes accumulate sediment over time. The material can be;
terrigenous: originating on land
marine: originating in ocean
- deposited sediments are the source of sedimentary rocks.
- lake bed sediments that have not solidified into rock can be used to determine past climatic conditions.
Settling
- gravity separation of heavy from light materials.
- settling out of dense solids or heavy liquid droplets from a liquid carrier.
- settling out of heavy solid grains from a mixture of solid grains of different densities.
How?
- accomplished by decreasing the velocity of the water which the particles will no longer remain in suspension.
- when the velocity no longer supports the transport of the particles, gravity will remove them from the flow.
- this is due to their motion through the fluid in response to the forces acting on them
Forces: gravity, centrifugal acceleration, electromagnetism
Process
- removal of solid particles from the surface of the liquid.
- successful due to the difference in the density of the solid and the liquid.
- used to determine the particles size in many chemistry lab experiments.
Flocculation
- removal of a sediment from a fluid.
- many irregular-shaped molecules join together to form a larger, and therefore heavier, near-spherical particle.
- increase particle size, thus aid sedimentation.
- polymers being joined, or very tiny particles, perhaps only a few microns in diameter, being linked.
- Eg: grease traps in commercial kitchens, centrifuge for processing blood, extracting salt from seawater, soured milk.
- refers to the process by which destabilized particles actually conglomerate into larger aggregates so that they can be separated from the wastewater.
- depends on: frequency of collision of the destabilized colloids to form larger fine particles.
- frequency of collision depends on the intensity of the agitation and the shear rate
however, too high a value of the agitation intensity and shear rate may break up the floc just formed.
Coagulation
- a mass of minute particles of a solid dispersed throughout a liquid, followed by the precipitation or separation of the solid from the liquid.
- usually involves chemical reaction
- Eg: casein in milk is coagulated by the addition of acetic acid or citric acid, albumin in egg white is coagulated by heating, clotting of blood.
- coagulant: used to promote the growth of large, rapid-settling floc which can then flocculate.
- coagulant used in wastewater treatment: aluminium salts, ferric & ferrous salts, lime, cationic polymers, and anionic & non-ionic polymers.
- indicates the process through which colloidal particles and very fine solid suspensions are destabilized so that they can begin to agglomerate if the conditions are appropriate.
- depends on: rapid reaction through which the coagulant destabilizes the colloids.
- rapid dispersion of the coagulant throughout the wastewater that is accomplished through very intense agitation and mixing.
Decanting
- separation of liquid phase
- 3 layers usually formed;
top: light liquid phase
between: dispersion phase
bottom: solid or heavy phase
- density & viscosity of the 2 phases determines;
- settling velocity of the heavy phase
- settling time
- size of the settling tank
- residence time
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Stoke's Law
- shows the force needed to move a small sphere through a continuous, quiescent fluid at a certain velocity.
- based primarily on radius of the sphere and viscosity of fluid
- Eg: important to understand swimming of microbes and sperm, understand sedimentation, suspension of liquid in air as clouds.
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Ostwald viscometer
- instrument used to measure viscosity of fluid
- sphere of known size and density is allowed to descend through the liquid until it reached terminal velocity, which can be measured by the time it takes to pass 2 marks on the tube.
- knowing terminal velocity, size & density of sphere, and density of liquid, Stoke's Law can be used to calculate the viscosity of the fluid.
Formula
Fd = frictional force acting on the connection between fluid and particle (N)
M = fluid's viscosity (kgm^-1s^-1)
R = radius of spherical object (m)
V = particle's velocityAssumptions;
- particles are spherical
- all particles present are the same size
- flow is laminar, both horizontally and vertically
Terminal/Settling Velocity
- if particles are falling in the viscous fluid by their own weight due to gravity, the terminal/settling velocity is reached when frictional force combined with buoyant force exactly balance the gravitational force
Vs = particles' settling velocity (m/s)
g = gravitational acceleration (m/s^2)
Pp = mass density of the fluid (kg/m^-3)
Pf = mass density of the fluid (kg/m^-3)
R = radius of the spherical object (m)
Gravitational Acceleration
- In physics, gravitational acceleration is the acceleration on an object caused by gravity.
- Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass.
-This equality is true regardless of the masses or compositions of the bodies.
- At different points on Earth, objects fall with an acceleration between 9.78 and 9.82 m/s2 depending on latitude, with a conventional standard value of exactly 9.80665 m/s2 (approx. 32.174 ft/s2).
- Objects with low densities do not accelerate as rapidly due to buoyancy and air resistance.
Particles Settling Rate
- desliming is performed to remove a slime fraction from an ore sample.
- the settling rates for particles with the equivalent diameter at which desliming is desired can be obtained.
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