Hammer mill – principle, construction, working, uses, merits, demerits MCQs With Answer

Hammer mill – principle, construction, working, uses, merits, demerits MCQs With Answer

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Hammer mill – principle, construction, working, uses, merits, demerits MCQs With Answer is an essential topic for B.Pharm students studying pharmaceutical unit operations. This introduction explains the hammer mill’s impact-based size reduction principle, main constructional parts (rotor, hammers, screens, casing), working parameters (rotor speed, feed rate, screen aperture) and pharmaceutical uses such as disintegration, granule size adjustment and excipient processing. It also covers merits like high throughput and simple design, and demerits including heat generation, fouling and broad particle size distribution. Keywords: hammer mill, principle, construction, working, uses, merits, demerits, pharmaceutical milling, particle size, B.Pharm. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary principle by which a hammer mill reduces particle size?

  • Compression between two plates
  • Impact and attrition by rapidly moving hammers
  • Centrifugal separation without contact
  • Chemical dissolution of particles

Correct Answer: Impact and attrition by rapidly moving hammers

Q2. Which of the following is NOT a main component of a typical hammer mill?

  • Rotor with hammers
  • Screen or grate
  • Centrifugal pump
  • Feed inlet and discharge

Correct Answer: Centrifugal pump

Q3. Which hammer type offers the best flexibility and wear distribution in many hammer mills?

  • Fixed steel blocks welded to the rotor
  • Swinging or pivot-mounted hammers
  • Polymer-coated stationary paddles
  • Magnet-mounted hammers

Correct Answer: Swinging or pivot-mounted hammers

Q4. How does the screen aperture size in a hammer mill affect the product?

  • Smaller apertures yield coarser particles
  • Smaller apertures produce finer particles
  • Screen size does not affect particle size
  • Larger apertures increase the temperature but not size

Correct Answer: Smaller apertures produce finer particles

Q5. Increasing rotor speed in a hammer mill most directly affects which parameter?

  • Color of the product
  • Impact energy and generally reduced particle size
  • Decrease in electrical consumption
  • Elimination of fines

Correct Answer: Impact energy and generally reduced particle size

Q6. Which grinding mechanism predominates in a hammer mill?

  • Electrostatic separation
  • Impact and some attrition
  • Solvent-based dissolution
  • High-pressure compaction

Correct Answer: Impact and some attrition

Q7. What material is commonly used for hammers to resist wear and impact?

  • Soft aluminum
  • Hardened or high-chromium steel
  • Pure copper
  • Unreinforced polypropylene

Correct Answer: Hardened or high-chromium steel

Q8. Which of the following is a main advantage of hammer mills in pharmaceutical processing?

  • Produces ultra-narrow particle size distributions without classifiers
  • High throughput with simple, robust construction
  • No heat generation during milling
  • Always suitable for all APIs without testing

Correct Answer: High throughput with simple, robust construction

Q9. A key disadvantage of hammer mills when processing heat-sensitive powders is:

  • Excessive cooling of the product
  • Heat generation during milling causing possible degradation
  • Complete sterilization of material
  • Elimination of electrostatic charge

Correct Answer: Heat generation during milling causing possible degradation

Q10. In pharmaceutical manufacturing, hammer mills are commonly used for:

  • Final sterile filling
  • Particle size reduction of excipients and some APIs
  • Tablet coating application
  • High-vacuum lyophilization

Correct Answer: Particle size reduction of excipients and some APIs

Q11. How does high feed moisture generally affect hammer mill performance?

  • Improves flow and reduces clogging
  • Leads to agglomeration and screen blocking
  • Reduces hammer wear markedly
  • Turns the product into powder instantly

Correct Answer: Leads to agglomeration and screen blocking

Q12. Which parts are most susceptible to wear and require regular replacement in a hammer mill?

  • Control panel and wiring
  • Hammers, screen and rotor liners
  • Feed hopper paint
  • External frame bolts only

Correct Answer: Hammers, screen and rotor liners

Q13. The particle size distribution produced by a hammer mill is typically described as:

  • Narrow and mono-disperse
  • Broad and multi-modal unless classification is applied
  • Perfectly Gaussian for all materials
  • Constant irrespective of operating conditions

Correct Answer: Broad and multi-modal unless classification is applied

Q14. Which operating change would reduce the proportion of very fine particles (fines)?

  • Reduce screen aperture size
  • Increase rotor speed significantly
  • Use a coarser screen or lower rotor speed
  • Increase feed temperature

Correct Answer: Use a coarser screen or lower rotor speed

Q15. Specific energy consumption in a hammer mill is influenced most by:

  • Color of feed material
  • Rotor tip speed, feed rate and hardness of material
  • Ambient humidity only
  • Brand of electrical motor only

Correct Answer: Rotor tip speed, feed rate and hardness of material

Q16. Which strategy helps control heat generation during milling of thermolabile APIs?

  • Use smaller hammers at higher speeds
  • Implement cooling, reduce feed rate or use intermittent operation
  • Remove the screen entirely
  • Increase moisture content to maximum

Correct Answer: Implement cooling, reduce feed rate or use intermittent operation

Q17. For GMP compliance, what is crucial when using a hammer mill for multiple products?

  • Never clean between batches
  • Robust cleaning validation and changeover procedures to avoid cross-contamination
  • Rely solely on visual inspection for cleanliness
  • Use random cleaning agents without documentation

Correct Answer: Robust cleaning validation and changeover procedures to avoid cross-contamination

Q18. Which dust control measure is commonly applied to hammer mills in pharmaceutical plants?

  • Open-air operation without enclosures
  • Use of local exhaust ventilation and dust collectors/filters
  • Relying on settling of dust over several days
  • No containment due to low dust generation

Correct Answer: Use of local exhaust ventilation and dust collectors/filters

Q19. What is the likely consequence of worn hammers in a hammer mill?

  • Finer particle size and reduced throughput
  • Reduced impact efficiency and broader particle size distribution
  • Complete sterilization of the product
  • Decrease in operating temperature

Correct Answer: Reduced impact efficiency and broader particle size distribution

Q20. Which of the following materials is generally problematic to mill with a hammer mill?

  • Brittle crystalline excipients
  • Soft, elastic, very ductile polymers prone to smearing
  • Dry friable powders
  • Low-density flake materials

Correct Answer: Soft, elastic, very ductile polymers prone to smearing

Q21. Which design feature helps reduce re-circulation of oversize particles in a hammer mill?

  • A smaller feed inlet only
  • Properly designed screen and discharge arrangement or air classification
  • Removing all screens altogether
  • Sealing the discharge permanently

Correct Answer: Properly designed screen and discharge arrangement or air classification

Q22. What effect does increasing feed rate beyond optimum have?

  • Improves fineness and reduces power draw
  • May overload mill, reduce residence time and produce coarser product
  • Eliminates noise completely
  • Automatically reduces dust emission

Correct Answer: May overload mill, reduce residence time and produce coarser product

Q23. Which maintenance practice extends hammer and screen life?

  • Running the mill dry with abrasive contaminants
  • Regular inspection, balancing rotor and timely replacement of worn parts
  • Increasing rotor speed without limits
  • Using incorrect hammer alignment to speed wear

Correct Answer: Regular inspection, balancing rotor and timely replacement of worn parts

Q24. For pharmaceutical hygiene, what material is preferred for screens and product-contact parts?

  • Uncoated carbon steel with rust
  • Food-grade or pharmaceutical-grade stainless steel (e.g., SS316)
  • Untreated wood
  • Painted mild steel inside contact surfaces

Correct Answer: Food-grade or pharmaceutical-grade stainless steel (e.g., SS316)

Q25. Hammer mills are generally classified as which type of mill?

  • Centrifugal impact mills
  • Hydraulic compression mills
  • Cryogenic ball mills
  • Ultrasonic disintegrators

Correct Answer: Centrifugal impact mills

Q26. During scale-up of a hammer mill process, which parameter is most often kept constant to maintain similar grinding performance?

  • Plant floor color
  • Rotor tip speed (tip speed similarity)
  • Screen thickness only
  • Ambient noise level

Correct Answer: Rotor tip speed (tip speed similarity)

Q27. Compared to a ball mill, a hammer mill typically provides:

  • Finer, narrower distributions for all materials
  • Faster, impact-driven size reduction with broader PSD and higher throughput
  • Wet grinding capability only
  • No maintenance requirements

Correct Answer: Faster, impact-driven size reduction with broader PSD and higher throughput

Q28. What is the effect of increasing hammer mass (heavier hammers) while keeping rpm constant?

  • Decrease in impact energy
  • Increase in impact momentum and possible coarser-to-finer change depending on dynamics
  • Does not affect the milling action
  • Automatically reduces noise to zero

Correct Answer: Increase in impact momentum and possible coarser-to-finer change depending on dynamics

Q29. A typical operating rotor speed range for many industrial hammer mills is:

  • 20–100 rpm
  • 200–900 rpm
  • 2,000–9,000 rpm
  • 50,000–100,000 rpm

Correct Answer: 2,000–9,000 rpm

Q30. Which measure is effective to control noise from a hammer mill?

  • Increase rotor speed without shielding
  • Use acoustic enclosures, vibration isolation and maintenance of clearances
  • Remove all guards
  • Paint the mill with noise-absorbing paint only

Correct Answer: Use acoustic enclosures, vibration isolation and maintenance of clearances

Q31. Why is dust explosion risk a concern with hammer mills and how is it mitigated?

  • Dust cannot explode; no mitigation needed
  • Fine combustible dust can ignite; mitigate with grounding, inerting, and dust control systems
  • Only a concern for liquids
  • Mitigate by increasing humidity to 100% at all times

Correct Answer: Fine combustible dust can ignite; mitigate with grounding, inerting, and dust control systems

Q32. What role does feed particle size play before entering a hammer mill?

  • No role — feed size is irrelevant
  • Smaller feed reduces energy consumption and improves product uniformity
  • Large feed always yields finer product
  • Feed size only affects color changes

Correct Answer: Smaller feed reduces energy consumption and improves product uniformity

Q33. Which property of materials increases the difficulty of milling in a hammer mill?

  • Brittleness
  • High hardness and toughness (ductility)
  • Low melting point below ambient
  • Porosity alone

Correct Answer: High hardness and toughness (ductility)

Q34. Adding an air classifier to a hammer mill system helps to:

  • Increase the mill’s electrical consumption only
  • Produce narrower particle size distribution by removing fines or oversize particles
  • Completely eliminate the need for screens
  • Cool the product to cryogenic levels

Correct Answer: Produce narrower particle size distribution by removing fines or oversize particles

Q35. Where is the screen placed in most hammer mill designs?

  • Inside the casing around the rotor for size selection at discharge
  • Above the feed hopper outside the mill
  • Only at the motor shaft
  • At the building exit door

Correct Answer: Inside the casing around the rotor for size selection at discharge

Q36. The main difference between impact milling (hammer mill) and attrition milling is:

  • Impact mills use chemical reagents
  • Impact mills primarily impart kinetic impact forces; attrition relies on rubbing and shear between surfaces
  • Attrition produces no particle size reduction
  • Impact mills operate only at cryogenic temperatures

Correct Answer: Impact mills primarily impart kinetic impact forces; attrition relies on rubbing and shear between surfaces

Q37. Which hammer geometry feature influences breakage and airflow?

  • Color of hammer paint
  • Shape and thickness of the hammer blade
  • Number of rivets only
  • Type of lubrication used on the motor

Correct Answer: Shape and thickness of the hammer blade

Q38. Hammer mills are most suitable for which type of materials?

  • Ductile, highly elastic rubbers only
  • Brittle, friable materials like some excipients and brittle APIs
  • Liquids with zero solids
  • Pure gases

Correct Answer: Brittle, friable materials like some excipients and brittle APIs

Q39. A hammer mill can be used to break up agglomerates in a feed. Which parameter is most useful to adjust for deagglomeration?

  • Increase humidity dramatically
  • Optimize rotor speed and screen size to provide sufficient impact without overheating
  • Remove all hammers
  • Seal the discharge to trap agglomerates

Correct Answer: Optimize rotor speed and screen size to provide sufficient impact without overheating

Q40. A common failure mode in hammer mills related to rotating parts is:

  • Bearing failure due to imbalance and insufficient lubrication
  • Failure of static signage
  • Paint chipping only
  • Cooling system flooding only

Correct Answer: Bearing failure due to imbalance and insufficient lubrication

Q41. Which analytical measure best evaluates hammer mill performance for particle size?

  • pH of milled powder
  • Particle size distribution (PSD) and percent passing specific sieve sizes
  • Colorimetric absorbance
  • Electrical conductivity only

Correct Answer: Particle size distribution (PSD) and percent passing specific sieve sizes

Q42. Hammer mills are generally used for dry milling. Which statement is true about wet milling in hammer mills?

  • Wet milling is standard and preferred in hammer mills
  • Hammer mills are not suitable for wet milling due to clogging and corrosion risks
  • Wet milling eliminates the need for screens altogether
  • Wet milling turns the hammer mill into a centrifuge

Correct Answer: Hammer mills are not suitable for wet milling due to clogging and corrosion risks

Q43. In wet granulation processes, a hammer mill is most often used to:

  • Dry the wet granules to constant weight
  • Size-reduce dried granules to break agglomerates prior to further processing
  • Spray binder solution onto granules
  • Coat tablets in a drum coater

Correct Answer: Size-reduce dried granules to break agglomerates prior to further processing

Q44. For allergen control and cleaning validation, which practice is recommended for hammer mills?

  • Cross-process different allergens without cleaning
  • Documented cleaning procedures, sampling and analytical verification between allergenic products
  • Rely on visual cleaning only
  • Use only water with no validation

Correct Answer: Documented cleaning procedures, sampling and analytical verification between allergenic products

Q45. Which routine maintenance task helps ensure consistent particle size and safe operation?

  • Never inspect the mill
  • Periodic balancing of rotor, hammer replacement and screen inspection/replacement
  • Running the mill at maximum speed permanently
  • Painting internal parts monthly

Correct Answer: Periodic balancing of rotor, hammer replacement and screen inspection/replacement

Q46. Choosing the correct hammer shape can influence:

  • Only the color of the discharge
  • Breakage pattern, airflow and wear characteristics
  • Electrical tariff classification
  • Whether the mill can grind liquids

Correct Answer: Breakage pattern, airflow and wear characteristics

Q47. To reduce airborne contamination and operator exposure, best practice is to:

  • Operate the mill in an open, uncontrolled room
  • Use enclosed milling, HEPA filtration and proper PPE
  • Turn off ventilation during operation
  • Rely on natural room ventilation only

Correct Answer: Use enclosed milling, HEPA filtration and proper PPE

Q48. What happens if oversized lumps enter the hammer mill feed continuously?

  • The mill self-cleans and improves fineness automatically
  • Possible blockages, increased wear and inconsistent product size or damage to the mill
  • Instant cooling of the product
  • Reduction of noise to zero

Correct Answer: Possible blockages, increased wear and inconsistent product size or damage to the mill

Q49. How does the shape of screen holes (round vs square) affect milling?

  • Screen hole shape has no effect
  • Hole shape influences flow, likelihood of blinding and exit particle shape/PSD
  • Only color changes occur
  • Square holes always clog less than round holes for all materials

Correct Answer: Hole shape influences flow, likelihood of blinding and exit particle shape/PSD

Q50. Which pharmaceutical materials are commonly processed in hammer mills?

  • Only sterile injectable solutions
  • Excipients like lactose, starch and some brittle APIs for size reduction
  • Live cell cultures for fermentation
  • Finished coated tablets for packaging

Correct Answer: Excipients like lactose, starch and some brittle APIs for size reduction

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