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Separation by Filtration

  • Last Updated : 14 Jul, 2021

Making tea is the most typical example. A filter or sieve is used to separate tea leaves from water when making tea. Only water will flow through the sieve holes. The filtrate is the liquid collected following filtering; in this example, water is the filtrate. Paper, cloth, cotton-wool, asbestos, slag- or glass-wool, unglazed earthenware, sand, or any other porous material can be used as a filter. Filtration is employed in the treatment of water and sewage.

What is Filtration?

Filtration is a separation technique that uses a porous medium that retains the solid substance but allows the fluid to penetrate through it. It is the process of removing insoluble substances from the solution. 

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This process is carried out using a filter medium, for instance, filter paper. The pores of the used filter medium are lesser in comparison to the size of the particles to be extracted. This process can generally be used for the separation of any heterogeneous mixture. 



Principle of Filtration: The filtration process relies on the principle of difference in sizes between the particles. It chooses a solvent in such a way that one substance dissolves in the solvent and the other does not. Therefore, the latter is collected as a residue over the chosen filter media. 

Process of Filtration 

  • An insoluble solid is dispersed into the liquid to form a solution.
  • A filter paper cone is fixed into a funnel using the glass rod.
  • The pores of the filter paper are smaller in comparison to the particles of the insoluble solid. Therefore, the solid is extracted out as the residue in the filter paper.
  • The filter paper is then placed upon a mesh to provide support.
  • Gravity acts over the liquid medium.
  • The slurry is obtained over the filter medium. The fluid passes through the filter medium and collects as the solvent at the bottom of the beaker owing to the created pressure difference.

Factors affecting Filtration

  • Properties of the solid used:
    • Particle shape and size
    • Particle charge
    • Particle size distribution
    • Property of the particle to adhere together
    • Density
    • Rigidity on the application of pressure
  • Properties of the liquid used:
    • The density of the used liquid
    • Viscosity- An increase in the viscosity of the filtrate will increase the resistance of flow in the fluid leading to a decrease in the rate of filtration.
  • Temperature of suspension
  • Properties of solids in the slurry: The filter cake formation rate during the initial stages of filtration.
  • The surface area of the filter medium: The rate of filtration is inversely proportional to the specific surface of the filter bed. However, it offers a direct proportionality to the filter medium. Therefore, using a large filter would increase the rate of filtration. It can also be increased by connecting a number of small units in parallel.
  • The pressure of the filter medium: The rate of evaporation is directly proportional to the pressure of both the filter cake and the medium. A pressure difference can be obtained by maintaining ahead of the slurry on top of the filter medium.

Types of Filtration 

The filtration can be broadly classified into two distinct categories: 

  • Surface or screen filtration
    • It is a screening action by which the holes of the medium disrupt the movement of solids.
    • This type of filtration involves the technique of straining. This is a technique where particles bigger than the pore size of the filter medium will be retained on the filter medium. This is done by the usage of plates with pores or the woven sieves.
    • The efficacy of this approach is defined with respect to the mean or maximum pore size.
    • Usually, the mechanical strength of the filter medium is less.
    • Low capacity is generally observed.
    • The size of the retained particles is considered to be greater than the medium pore size.
    • Equipment is generally expensive.
    • e.g. Cellulose membrane filter.
  • Depth filtration 
    • This is a penetration process where the slurry gets deep into a point where the diameter of solid particles is greater than that of the tortuous void or channel.
    • This type of filtration involves the technique of entanglement. This is a technique where the particles get entangled as a mass of fibers in case the filter medium is a cloth or porous felt.
    • The size of the particles is considered to be smaller than the pore size.
    • Usually, the mechanical strength of the filter medium is high.
    • High capacity is generally observed.
    • Equipment is generally cheap.
    • e.g. Ceramic filters.

Applications of Separation by Filtration

Some applications of separation by filtration method are:

  1. Preparing coffee and tea: The hot coffee is obtained as the filtrate while the coffee beans remain as residue on the top of the filter.
  2. A mixture of chalk powder and water can be separated by this technique.
  3. Vacuum cleaners are fitted with filters in order to absorb the dust particles.
  4. Filtration of sand particles from water or chalk powder.
  5. Effluents and wastewater treatment: Filtration is a useful technique to simulate the process of water treatment, however, the process is costly and cumbersome.
  6. Used by furnaces to simulate the prevention of furnace elements from fouling with particulates.
  7. Air filters are used to weed out the airborne particulate matter in building ventilation systems and combustion engines.
  8. Potable wastewater treatment is carried out using biofilm filtration in slow sand filters.
  9. Production of liquid dosage
  10. Blood filtering in kidneys

Sample Problems

Problem 1: How can the problem of a decrease in the rate of evaporation due to increasing viscosity be overcome? 

Solution : 

The problem of a decrease in the rate of evaporation due to increasing viscosity be overcome by following methods:



1. Heating the liquid to increase its temperature.

2. Dilution can be simulated in order to increase the rate of evaporation. 

Problem 2: What do you mean by filter cake?

Solution:

A filter cake is formed by the particles or substances that are retained on a filter during the process of filtration. The filter cake usually comprises filter aids, such as activated carbon in order to increment the flow rate or achieve a smaller micron filtration. The filter cake grows thicker than particulate matter as the process of filtration proceeds. 

Problem 3: Define slurry?

Solution:

The slurry is basically a liquid-solid fluid mixture with a value of specific gravity greater than 1. The slurry is made to pass through each of the filtration chambers, under pressure delivered by a slurry pump.

Problem 4: Briefly explain the types of filter media.

Solution:



The two major types of filter media are: 

  • Thin barriers, exemplified by a filter cloth or filter paper;
  • Thick barriers, for instance, sand beds, or porous metal.

Problem 5: How is filtration used in preparing tea? 

Solution :

A sieve or a mesh filter is used as filter paper in order to separate tea leaves from the water. As a result, the tea leaves remain as residue on the sieve whereas the water with the flavor of tea leaves comes out as filtrate. 

Problem 6: We have a mixture of sand, salt, and water. How will you separate it?

Solution :

Salt gets dissolved in water and forms a solution. It can be separated from the solution using evaporation. Now, the mixture of sand and water remains. The sand settles at the bottom and remains undissolved as a residue. It can be separated by passing the mixture through a filter paper which results in the collection of sand as a residue and water collects as a filtrate. 

Problem 7: What are the basic requirements for the process of filtration? 

Solution :

The basic requirements for filtration are as follows:

  • Filter medium;
  • Fluid with suspended solids
  • Pressure difference to cause fluid to flow;
  • Filter that holds the filter medium, contains the fluid.



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