CIAT1(UNIT I-III)

MECHANICAL OPERATIONS QN AND ANS BY CHIEF. GORONGA SIR

1. Explain the working principle of a vibrating screen with a neat

sketch.
Ans:

Working Principle

A vibrating screen works on the principle of gyratory motion.

 The sieve assembly, placed on rubber mountings, is set into

oscillatory motion by an eccentric flywheel or counterweights.

 The rotation of the unbalanced top weight generates horizontal

vibrations, while the lower weight induces vertical vibration and
inclination.

 This combined motion makes the feed material move upward,

downward, and sideways in a continuous circular path.

 Balls fitted below the sieves bounce and strike the mesh underside,
preventing clogging.
  The circular/near-circular movement of particles causes
undersized material to pass through the sieves under the influence
of gravity and directional shifts, finally collecting in the receiving
pan.

Construction

 Shape: Box-like, either round or square.

 Screens: Series of sieves stacked vertically with decreasing mesh

openings from top to bottom (coarsest at top, finest at bottom).

 Support: The screen assembly is mounted on a table supported by

springs, allowing free oscillation.

 Motor: Located beneath the table, with a double shaft extension

carrying unbalanced weights to generate gyratory motion.

 Auxiliary Devices:

o Ball trays – prevent mesh clogging.

o Ultrasonic attachments – improve fine separation.

Working

1. Feed material is introduced onto the top sieve and covered to avoid
loss during vibration.

2. Sieves are arranged with coarsest mesh at the top and

progressively finer meshes below.

3. During vibration, feed rolls over the sieve surface:

o Coarse particles are retained on upper sieves.

o Medium-sized particles pass to intermediate sieves.

o Finer particles pass to the lowest sieve/pan.

4. The separation process is efficient due to the combined vertical,

horizontal, and circular motions of the screen.

Advantages

 Simple design and easy handling.

 Low maintenance and power consumption.

  Accurate separation with high processing rate per screen area.

 Prevention of mesh clogging by bouncing balls/ultrasonic devices.

Disadvantages

 Particle collisions may cause attrition, leading to unwanted size

reduction and false results.

 Moist feed may clog sieve apertures, reducing efficiency.

2. Discuss the different techniques applied for determining particle size

distribution.

A. Particle Size Determination Methods (Scientific/Industrial)

These are used to measure particle size distribution, surface area,

or porosity.

1. Sieves

 Principle: Physical separation through mesh of defined size.

 Use: Coarse particles (e.g., mining).

 Pros: Cheap, simple.

 Cons: Not suitable for fine powders, emulsions, or sprays;

agglomerates (clay) hard to measure.

2. Sedimentation

 Principle: Stokes’ law – particles settle at rates proportional to size

and density.

 Use: Historically in clay/pottery industries.

 Pros: Simple.

 Cons: Time-consuming, not good for emulsions or dense/mixed-

density particles.

3. Electro Zone Sensing (Coulter Principle)

 Principle: Change in electrical resistance as particles pass through

an aperture.
  Use: Biological cells, suspensions.

 Pros: Accurate for uniform systems (e.g., blood cells).

 Cons: Needs electrolyte; costly calibration; powders/emulsions hard

to measure.

4. Laser Diffraction

 Principle: Scattering pattern of a laser beam by particles.

 Use: Powders, sprays, emulsions, suspensions.

 Pros: Fast, accurate, no calibration, wide size range, repeatable.

 Cons: Expensive instruments.

5. BET (Brunauer–Emmett–Teller) Gas Adsorption

 Principle: Adsorption of nitrogen gas on particle surfaces at 77 K;

calculates surface area.

 Use: Fine powders, nanomaterials, catalysts.

 Pros: Very accurate for submicron particles; gives surface area &
porosity.

 Cons: Requires cryogenic/vacuum setup; indirect size estimate.

6. Hg Intrusion Porosimetry

 Principle: Pressure forces mercury into pores; intrusion pressure ↔

pore size.

 Use: Porous solids (ceramics, catalysts, filters).

 Pros: Wide pore size range (3 nm – 100 µm).

 Cons: Toxic mercury; destructive test; assumes ideal pore geometry.

B. Separation Techniques (Everyday / Basic Industry)

These are not precise size measurement methods, but practical

mixture separation methods. Some rely on size or density
differences, others on solubility, boiling point, or magnetism.

1. Handpicking → Picking impurities manually.

2. Threshing → Separating grains from stalks.

3. Winnowing → Using air current to separate lighter/heavier

particles.

4. Sieving → Using a porous sieve to remove larger impurities.

5. Sedimentation → Heavy particles settle at bottom.

6. Decantation → Pouring off supernatant after sedimentation.

7. Filtration → Using filter paper to separate solid from liquid.

8. Evaporation → Removing solvent, leaving solid residue (e.g.,

salt from seawater).

9. Condensation → Cooling vapors to liquids.

10. Sublimation → Direct solid-to-gas change (e.g., ammonium

chloride).

11. Distillation → Separating miscible liquids with different boiling

points.

12. Fractional Distillation → Distillation with fractionating column

(boiling point difference < 25 °C).

13. Funnel Separation → Separating immiscible liquids (e.g., oil +

water).

14. Magnetic Separation → Using magnets to separate magnetic

components (e.g., iron).

3. Factors Affecting the Performance of a Vibrating Screen

1. Particle Characteristics

 Size distribution: If feed contains a high percentage of near-

size particles (close to the aperture size), separation
efficiency decreases because such particles have difficulty
passing through.

 Shape of particles: Spherical particles pass more easily,

while elongated or flat particles may get stuck in apertures.

 Moisture content: Wet or sticky feed leads to clogging

(blinding) of sieve apertures, reducing efficiency.

 Density: Heavier particles pass through more easily under

vibration compared to very light particles.
2. Screen Properties

 Aperture size and shape: Determines the cut size; square or

slotted openings influence efficiency.

 Screen surface area: Larger screen area improves capacity

and efficiency.

 Deck arrangement: Multiple decks allow separation into

different size fractions.

 Screen inclination: Steeper angles increase throughput but

reduce accuracy of separation; lower angles improve
efficiency but reduce capacity.

 Mesh blinding and wear: Worn-out or blocked meshes lower

separation accuracy.

3. Operational Parameters

 Vibration amplitude: Higher amplitude → better stratification

of particles but may cause oversize carry-over.

 Frequency (rpm): High frequency favors fine particle

separation; low frequency is better for coarse materials.

 Gyratory motion: Combination of vertical and horizontal

vibrations enhances stratification and passage of undersize
particles.

 Feed rate: Excessive feed load overwhelms the screen,

reducing efficiency; optimal feed rate ensures proper
separation.

 Feed distribution: Even spreading of feed across the entire

width of the screen improves performance.

4. Environmental and Material Handling Conditions

 Temperature: High temperature may affect material

stickiness and screen tension.

 Dust and airflow: Air currents or static charges may cause

fine particles to float instead of passing through.
 Auxiliary devices: Ball trays or ultrasonic attachments help
prevent clogging and maintain efficiency.

Q4. Compare actual and ideal screen performance with a

graph.
5. What is the power required to crush 150 tonne per hour of lime
stone if 85 % of the feed passes in 2 inch screen and 85 % of the
product in a 1/8 inch screen?
6. Derive an expression to find critical speed of a ball mill.
7. Explain the empirical relationship between Ritinger’s law and Kick’s
law.
8. Derive an expression to co-relate the diameter of rolls with size of
the feed for a smooth roll crusher.

Crushing Rolls / Roll Crushers Smooth Roll Crusher:

Principle: Size reduction is achieved by compression (i.e., it employs

compressive force for size reduction).
9. Illustrate how will you separate the particles based on the electrical
properties.
10. Discuss the setting characteristics of fine solids from liquid
using batch sedimentation test.
11. Explain the characteristics of membranes and their
applications.

Characteristics of Membranes

1. Selective Permeability

o Membranes allow certain molecules or ions to pass while

restricting others.

o Separation is based on size, charge, solubility, or other

physical/chemical properties.

2. Driving Force Requirement

o Transport across membranes occurs due to a driving force

such as pressure difference, concentration gradient,
temperature difference, or electrical potential.

3. Structure and Morphology

o Can be dense (non-porous, separation by diffusion) or porous

(separation by size exclusion/filtration).

o Common forms: flat sheets, hollow fibers, spiral-wound

modules.

4. Material of Construction

o Made from polymers (cellulose acetate, polysulfone,

polyamide), ceramics, metals, or composites depending on
application.

5. Thickness and Mechanical Strength

o Must be thin for efficient transport but strong enough to

withstand pressure differences.

6. Chemical and Thermal Stability

 o Should resist fouling, chemical attack, and temperature
variations in industrial use.

7. Hydrophilic/Hydrophobic Nature

o Surface properties influence water flux, fouling tendency, and

compatibility with solvents.

Applications of Membranes

1. Water and Wastewater Treatment

o Microfiltration (MF), Ultrafiltration (UF), Nanofiltration

(NF), Reverse Osmosis (RO): Used for desalination,
softening, removal of bacteria, viruses, organic matter, and
salts.

2. Gas Separation

o O₂/N₂ separation from air.

o CO₂ removal from natural gas/biogas.

o Hydrogen recovery in refineries.

3. Food and Beverage Industry

o Milk protein concentration.

o Juice clarification.

o Alcohol removal from beverages.

4. Biomedical Applications

o Hemodialysis (artificial kidney).

o Controlled drug delivery systems.

o Sterile filtration for pharmaceuticals.

5. Energy Applications

o Proton Exchange Membranes (PEM) in fuel cells.

o Membranes for battery separators.

6. Industrial and Chemical Processes

o Organic solvent nanofiltration for separating valuable

chemicals.
 o Pervaporation for separating azeotropic mixtures (e.g., water–
ethanol separation).

o Electrodialysis for salt recovery and brackish water

desalination.

12. Elaborate separation by magnetic and impingement method.

1. Magnetic Separation

Principle

 Based on the difference in magnetic properties of particles.

 Magnetic particles are attracted by a magnetic field, while non-

magnetic particles are not.

 Separation occurs when a mixture is passed through a magnetic

field.

Types

1. Low-intensity magnetic separation (LIMS):

o Magnetic field strength < 2000 gauss.

o Used for strongly magnetic minerals (e.g., magnetite).

2. High-intensity magnetic separation (HIMS):

o Field strength > 10,000 gauss.

o Used for weakly magnetic materials (e.g., hematite, ilmenite).

Equipment

 Magnetic drum separator

 Magnetic pulley

 Belt-type magnetic separator

 High-gradient magnetic separator

Applications

 Beneficiation of iron ores.

 Removal of tramp iron from coal, glass, food, and chemical

industries.

 Recovery of magnetic catalysts.

Advantages
 Simple, energy-efficient.

 Can handle dry or wet feed.

Limitations

 Only works when magnetic susceptibility difference is significant.

 Not effective for non-magnetic materials.

2. Impingement Separation

Principle

 Based on inertia of particles or droplets in a gas stream.

 Gas stream containing liquid droplets or solid particles is forced to

change direction by baffles, plates, or meshes.

 Gas follows the streamline, but heavier particles/droplets impinge

(strike) on the surface and get separated.

Mechanism

 Droplets/particles impact on surfaces due to inertia.

 They coalesce (liquid) or accumulate (solids).

 Separated material drains away or is collected, while clean gas

passes through.

Equipment

 Impingement separators (zig-zag baffles).

 Mesh pad demisters.

 Cyclone separators (similar principle of inertia).

Applications

 Removal of liquid droplets from gas streams (gas scrubbers, steam

separators).

 Used in chemical industries to separate entrained liquid from vapor.

 In air pollution control (removing particulates).

Advantages

 Effective for removing fine droplets and mists.

 Simple construction, no moving parts.

Limitations

 Pressure drops across baffles.

 Efficiency reduces for very small particles (< 1 μm).

📑 Quick Comparison

Impingement
Aspect Magnetic Separation
Separation

Basis of Magnetic properties Inertia of

Separation (magnetism vs non) droplets/particles in gas

Solids (magnetic Gas-liquid or gas-solid

Medium
minerals, metals) mixtures

Main Magnetic drum, pulley, Baffles, demisters,

Equipment belt, HGMS cyclone separators

Mineral processing, Mist eliminators, air/gas

Application
tramp iron removal cleaning