Mechanical Operations — Part 1 MCQs with Explanations

The ratio of the area of openings in one screen (Taylor series) to that of the openings in the next smaller screen is given by the square root of 2, denoted as √2. This ratio is based on the Taylor series for mesh sizes in screens commonly used in mechanical operations. Therefore, in this context, the correct option is Option C: √2 (Square Root of 2) .

The correct answer is B: Thickeners . Let's break down why: Sedimentation is the process where solid particles settle out of a liquid due to gravity. Thickeners are specifically designed to perform sedimentation on a large, industrial scale. They are large tanks where solids are allowed to settle to the bottom, forming a concentrated sludge, while the clarified liquid is removed from the top. Here's why the other options aren't the best fit: A: Classifiers : Classifiers separate particles based on size and density, often using a combination of sedimentation and hindered settling, but they're more about separation into different fractions than just bulk removal of solids from a liquid like in sedimentation alone. C: Rotary drum filters : Rotary drum filters use a rotating drum covered with a filter medium to separate solids from liquids. This is a filtration process, not sedimentation. D: Cyclones : Cyclones use centrifugal force to separate solids from gases or liquids. While they can separate solids from liquids, they are not primarily based on gravity-driven sedimentation. They are more efficient for separating fine particles. Therefore, thickeners are the primary equipment used for sedimentation on a commercial scale.

Gizzlies are heavy-duty screens used in mechanical operations for separating large-sized solids from the feed material. They consist of parallel bars or rods spaced at specific intervals to allow only particles below a certain size to pass through. Typically, gizzlies are used at the initial stages of processing to remove large lumps or coarse materials before finer screening or crushing. Therefore, gizzlies are specifically designed to separate coarse solids .

The correct answer is Option A: Specific surface of product is more . Here's why: Closed circuit grinding involves a classifier (like a sieve or cyclone) that separates the ground material. Only the material *larger* than the desired size is sent back for further grinding. Open circuit grinding , on the other hand, grinds everything in one go without separation. Think of it like this: In closed circuit: Only the coarser particles are re-ground. This continuous re-grinding of coarse particles ensures a finer product . Finer product means more surface area per unit mass (specific surface). So, the specific surface is higher. In open circuit: Everything is ground together, leading to a wider range of particle sizes . Here's why the other options are incorrect: Option B: Product has lesser size uniformity: Closed circuit gives *better* size uniformity because oversized particles are re-ground. Option C: Production rate at a given limiting size is lower: Closed circuit often has a *higher* production rate at a specific size because it efficiently processes only oversized particles. Option D: Operation is economical: While the initial investment might be higher, closed-circuit grinding can be more economical in the long run due to *increased efficiency and reduced over-grinding* but it's not the primary benefit directly related to the grinding process itself. The increased specific surface area is the most direct result.

In a grinding operation, the goal is to reduce the size of particles so that they all pass through a specified size limit. The performance of the grinding process is judged by whether the largest particles in the sample are reduced to or below the target size. Therefore, the limiting particle size in a grinding operation is determined by the size of the largest particle in the sample, since this dictates the extent of grinding needed to achieve the desired fineness. If the largest particle is still above the required size, further grinding is necessary; hence, the largest particle size is the limiting factor.

The Richardson-Zaki equation relates the hindered settling velocity (V h ) to the terminal settling velocity (V t ) through the expression: V h = V t × (1 - φ) n where φ is the volume fraction of solids (here, φ = 0.30) and n is the Richardson-Zaki index (given as 4.5). First calculate (1 - φ): 1 - 0.30 = 0.70 Then raise it to the power of n: (0.70) 4.5 ≈ 0.2008 Rearranging the equation for terminal velocity gives: V t = V h / (1 - φ) n Substituting the values: V t = 4.44 μm/s / 0.2008 ≈ 22.1 μm/s Therefore, the terminal velocity of a sand grain is 22.1 μm/s.

A ball mill is a type of grinder used in mechanical operations to grind and blend materials for use in mineral dressing, paints, pyrotechnics, ceramics, and selective laser sintering. The design of a ball mill is typically cylindrical with a horizontal axis, partially filled with the material to be ground plus the grinding medium (like steel balls). The length to diameter (L/D) ratio of a ball mill is an important design factor as it affects the grinding efficiency and operational stability. For ball mills, the L/D ratio is usually less than 1 , meaning the diameter of the mill is greater than its length. This configuration helps in achieving a better cascading action of the grinding media and efficient grinding performance. Therefore, the correct answer is: < 1 (Option C).

Given : A sphere falling through a fluid in the constant drag-coefficient (Newton’s law) regime , i.e., high Reynolds number where C_D ≈ constant. Forces acting at terminal velocity v_t : Gravitational (effective weight) – Buoyancy = Drag Write each term : Effective weight = \( \left(\rho_p - \rho_f\right) g \times \text{(volume)} = \left(\rho_p - \rho_f\right) g \left(\frac{\pi}{6} d^3\right) \) Drag (Newton’s regime) = \( \frac{1}{2} C_D \rho_f A v_t^2 \), where frontal area \( A = \frac{\pi}{4} d^2 \) Force balance at terminal velocity : \( \left(\rho_p - \rho_f\right) g \left(\frac{\pi}{6} d^3\right) \;=\; \frac{1}{2} C_D \rho_f \left(\frac{\pi}{4} d^2\right) v_t^2 \) Simplify (cancel \( \pi \) and powers of \( d \)): \( \left(\rho_p - \rho_f\right) g \left(\frac{1}{6} d^3\right) \;=\; \frac{1}{8} C_D \rho_f d^2 \, v_t^2 \) Multiply both sides by \( \frac{8}{C_D \rho_f d^2} \): \( v_t^2 \;=\; \frac{8}{C_D \rho_f d^2} \cdot \left(\rho_p - \rho_f\right) g \left(\frac{1}{6} d^3\right) \;=\; \frac{4}{3} \cdot \frac{\left(\rho_p - \rho_f\right) g}{C_D \rho_f} \, d \) Take square root : \( v_t \;=\; \sqrt{ \frac{4}{3} \cdot \frac{\left(\rho_p - \rho_f\right) g}{C_D \rho_f} } \;\sqrt{d} \) Conclusion : Since the bracketed factor is constant in the constant-\( C_D \) regime, \( v_t \propto \sqrt{d} \). Contrast with Stokes (creeping flow) regime : There, drag \( \propto \mu d \, v \) and one gets \( v_t \propto d^2 \). But this question specifies constant \( C_D \) (Newton’s regime), so \( v_t \propto \sqrt{d} \) .

Rittinger's law is the correct answer to “Which of the following gives the work required for size reduction of coal to -200 mesh in a ball mill most accurately”. This question is standard in competitive examination syllabi for Mechanical Operations.

Clarifier is the correct answer to “Traces of solids are removed from, liquid in a”. This question is standard in competitive examination syllabi for Mechanical Operations.

Cage is the correct answer to “__________ mills are termed as disintegrators.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Humidification is the correct answer to “Which of the following is not categorised as a "mechanical operation"”. This question is standard in competitive examination syllabi for Mechanical Operations.

1 is the correct answer to “Sphericity of a cubical particle, when its equivalent diameter is taken as the height of the cube, is”. This question is standard in competitive examination syllabi for Mechanical Operations.

Surface area per unit mass is the correct answer to “Size measurement of ultrafine particles can be best expressed in terms of”. This question is standard in competitive examination syllabi for Mechanical Operations.

All of the above is the correct answer to “Rittinger number which designates the new surface prduced per unit of mechanical energy absorbed by the material being crushed, depends on the”. This question is standard in competitive examination syllabi for Mechanical Operations.

Laminar is the correct answer to “In case of a plate and frame filter press, filtrate flow through the cake follows __________ flow.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Atmospheric humidity is the correct answer to “Screen capacity is not a function of”. This question is standard in competitive examination syllabi for Mechanical Operations.

Rotor shaft carrying hammers can be vertical or horizontal is the correct answer to “In case of a hammer crusher, the”. This question is standard in competitive examination syllabi for Mechanical Operations.

30 is the correct answer to “Percentage of drum submerged in the slurry in case of rotary drum filter is”. This question is standard in competitive examination syllabi for Mechanical Operations.

Collector is the correct answer to “In froth floatation, chemical agent added to cause air adherence is called”. This question is standard in competitive examination syllabi for Mechanical Operations.

Rate of filtration decreases with time is the correct answer to “In constant pressure filtration,”. This question is standard in competitive examination syllabi for Mechanical Operations.

Metalliferrous ores is the correct answer to “Size reduction of the __________ can be suitably done by ball mills, crushing rolls and rod mills.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Peclet is the correct answer to “In a mixer, the quantity, (v. L/D) is termed as __________ number(where, v = longitudinal velocity of material, L = length of the mixer, D = diffusivity in axial mixing).”. This question is standard in competitive examination syllabi for Mechanical Operations.

Banbery is the correct answer to “__________ mixer is used for devulcanisation of rubber scrap & making water dispersion & rubber solution.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Flocculation is the correct answer to “The process opposite to 'dispersion' is termed as the”. This question is standard in competitive examination syllabi for Mechanical Operations.

Rod mills is the correct answer to “Size reduction does not occur due to compression in case of”. This question is standard in competitive examination syllabi for Mechanical Operations.

Mulling is the correct answer to “Mixing mechanism employed in a pan mixer is by”. This question is standard in competitive examination syllabi for Mechanical Operations.

At the end of filtration is the correct answer to “During washing of filter at the end of constant pressure filtration, the rate of washing equals the rate of filtration”. This question is standard in competitive examination syllabi for Mechanical Operations.

Ultra grinding is the correct answer to “A fluid energy mill is used for”. This question is standard in competitive examination syllabi for Mechanical Operations.

Strainer is the correct answer to “A __________ employs a set of screen across a flow channel for the separation of dirt/rust from a flowing liquid stream.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Electro-static precipitator is the correct answer to “The most efficient equipment for the removal of sub-micronic dust particles from blast furnace gas is the”. This question is standard in competitive examination syllabi for Mechanical Operations.

Over cake is the correct answer to “The specific cake resistance for compressible sludges is a function of the pressure drop”. This question is standard in competitive examination syllabi for Mechanical Operations.

Minimum size is the correct answer to “__________ balls capable of grinding the feed in a ball mill gives the maximum efficiency.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Increases continuously is the correct answer to “The inlet pressure in a constant rate filtration”. This question is standard in competitive examination syllabi for Mechanical Operations.

Impact is the correct answer to “Crushing of mineral particles is accomplished in a 'cage mill', when one or more alloy steel bars are revolved in opposite directions. It is a type of __________ mill.”. This question is standard in competitive examination syllabi for Mechanical Operations.

Cake is the correct answer to “The controlling resistance in a rotary drum vacuum filter is the __________ resistance.”. This question is standard in competitive examination syllabi for Mechanical Operations.

1.5 is the correct answer to “Shape factor for a cylinder whose length equals its diameter is”. This question is standard in competitive examination syllabi for Mechanical Operations.

Both B & C is the correct answer to “Vertical transportation of materials can be done by a/an”. This question is standard in competitive examination syllabi for Mechanical Operations.

Sphere is the correct answer to “Sphericity is the ratio of the surface area of a spherical particle having the same volume as the particle to the surface area of the particle. Which of the following has the maximum value of sphericity”. This question is standard in competitive examination syllabi for Mechanical Operations.

Ultrafine grinder is the correct answer to “Fluid energy mill comes in the category of”. This question is standard in competitive examination syllabi for Mechanical Operations.