Q: Correct the statement for comminution laws: According to Bond’s law, the specific energy required to reduce a material from size D_f to D_p varies approximately with which expression?

Introduction / Context: Comminution energy laws provide empirical relationships between energy input and particle size change. Three classical laws—Kick, Bond, and Rittinger—apply over different size ranges and mechanisms. Accurately recalling their mathematical forms is vital for ball mill sizing and power estimation. Given Data / Assumptions: D_f: characteristic feed size. D_p: characteristic product size. Homogeneous material; bond work index concept applies. Concept / Approach: Bond’s law states that the energy required is proportional to the new crack length generated and correlates with the reciprocal of the square root of size. The commonly used form is: E = k_B * (1/√D_p − 1/√D_f), where k_B includes the Bond work index for the material and machine constants. By contrast, Kick’s law gives E ∝ ln(D_f/D_p) (size ratio), and Rittinger’s law gives E ∝ new surface area, often represented as E ∝ 1/D_p − 1/D_f for uniform particles. Step-by-Step Solution: Identify target law: Bond.Recall dependence: inverse square-root of size.Select expression with (1/√D_p − 1/√D_f). Verification / Alternative check: Ball mill design references and Bond’s original correlation consistently use the square-root reciprocal form. Why Other Options Are Wrong: (S/V) difference: aligns with Rittinger’s surface-area concept, not Bond’s.ln(D_f/D_p): Kick’s law, appropriate for coarse crushing.1/D_p − 1/D_f: a common Rittinger-type representation. Common Pitfalls: Mixing up the three laws; remember: Kick → log, Bond → inverse square root, Rittinger → inverse size (surface). Final Answer: E ∝ (1/√D_p) − (1/√D_f)

Q: In mineral processing of gold ores, which concentration method is most widely used for fine liberated particles?

Introduction / Context: Gold beneficiation involves liberating valuable minerals and concentrating them before extraction (e.g., cyanidation). For finely disseminated gold associated with sulphides (e.g., pyrite, arsenopyrite), flotation is a dominant pre-concentration method. Given Data / Assumptions: Fine liberated particles are present. Ore may contain sulphide carriers of gold. Objective: upgrade feed prior to downstream recovery. Concept / Approach: Froth flotation selectively attaches hydrophobic particles to bubbles, separating them from hydrophilic gangue. It is highly effective at sizes below a few hundred microm, exactly where gravity-based devices lose efficiency due to low settling velocities. Jigging and tabling work best for coarser, high-density contrasts; elutriation is a specialized hydraulic classification step, not a primary gold concentrator for fines. Step-by-Step Solution: Identify particle size regime: fine.Match separation principle: surface chemistry (flotation) excels for fines.Select flotation as the widely used method in modern gold concentrators. Verification / Alternative check: Many refractory and sulphide-associated gold circuits employ bulk or selective flotation before oxidation and leach. Why Other Options Are Wrong: Jigging/Tabling: gravity concentration loses efficiency on fine sizes.Elutriation: classification, not primary gold concentration. Common Pitfalls: Assuming gravity always dominates because gold is dense; at fine sizes, hydrodynamics favour flotation. Final Answer: Froth flotation

Q: A pulverised coal sample passing a 200-mesh screen has a nominal particle size of 0.074 mm (74 microm). What is the approximate nominal particle size of material that just passes a 50-mesh screen?

Introduction / Context: Screen mesh numbers denote openings per linear inch. Approximate relationships allow quick estimation of nominal aperture sizes to translate between mesh counts and particle sizes in classification tasks. Given Data / Assumptions: 200 mesh ≈ 74 microm (0.074 mm). We seek the size for 50 mesh. Use standard sieve series approximations. Concept / Approach: As mesh decreases (fewer openings per linear inch), aperture increases. A common reference: 50 mesh ≈ 297 microm, i.e., about 0.297 mm. Rounding to two significant digits yields 0.30 mm. Step-by-Step Solution: Recognise 200 mesh → 0.074 mm (given).Recall 50 mesh aperture ≈ 0.297 mm.Select the closest option: 0.30 mm. Verification / Alternative check: ASTM E11/ISO sieve tables list 50 mesh near 300 microm. Why Other Options Are Wrong: 0.007 and 0.014 mm: too fine for 50 mesh; those correspond to very high mesh counts.50 mm: off by orders of magnitude. Common Pitfalls: Confusing linear inch basis with square inch count; mesh is per linear inch. Final Answer: 0.30 mm

Q: Among common size-reduction equipment, which accepts the largest maximum feed size for primary breakage?

Introduction / Context: Choosing the first-stage comminution device depends on feed size. Primary crushers must accept run-of-mine or large quarried rocks, whereas mills and fine pulverisers require much finer feeds. Given Data / Assumptions: Standard industrial equipment: jaw crusher, ball/tube mills, jet pulveriser. We compare maximum acceptable feed size. Concept / Approach: Jaw crushers are primary crushers designed to accept very large feeds (hundreds of millimetres to over a metre) and reduce them to manageable secondary sizes. Ball and tube mills require much smaller feeds (typically <25 mm). Jet pulverisers handle sub-millimetre feeds for ultrafine grinding. Step-by-Step Solution: Identify feed size domain of each device.Primary crusher domain → jaw crusher.Select jaw crusher as correct. Verification / Alternative check: Manufacturer datasheets show jaw crushers with large gape dimensions for coarse feeds. Why Other Options Are Wrong: Ball/Tube mills: require pre-crushed material.Jet pulveriser: only for very fine feeds; unsuitable for large lumps. Common Pitfalls: Confusing maximum feed size with final product fineness capability. Final Answer: Jaw crusher

Q: Sphericity (phi) is defined as the surface area of a volume-equivalent sphere divided by the actual particle surface area. Which shape has the maximum possible sphericity?

Introduction / Context: Sphericity is used to characterise particle shape effects on drag, packing, and surface-area-dependent phenomena. It normalises shapes against a perfect sphere with the same volume. Given Data / Assumptions: Sphericity phi = (surface area of volume-equivalent sphere) / (actual surface area). 0 < phi ≤ 1 by definition. Concept / Approach: The sphere minimises surface area for a given volume. Therefore, the numerator and denominator become equal only for a sphere, giving phi = 1. All other shapes have larger surface areas for the same volume, producing phi < 1. Step-by-Step Solution: Recall isoperimetric principle: sphere minimises area for fixed volume.Thus the ratio reaches its maximum (1) for a sphere.Select sphere as the answer. Verification / Alternative check: Empirical shape factors confirm cubes, cylinders, and engineered packings have phi significantly less than 1. Why Other Options Are Wrong: Cube/Cylinder: greater surface area per unit volume.Raschig rings: deliberately non-spherical, very low sphericity. Common Pitfalls: Confusing roundness (edge smoothness) with sphericity (global shape metric). Final Answer: Sphere

Q: For a jaw crusher, which of the following is NOT a determining factor for its electrical energy consumption during operation?

Introduction / Context: Crusher power draw is influenced by duty conditions and setting. Accurately assessing what factors influence energy consumption is important for energy audits and optimisation. Given Data / Assumptions: Jaw crusher operating at steady conditions. Variables: feed size, product size (closed side setting), capacity (throughput). Concept / Approach: Power required correlates with the amount of breakage performed per unit time. Finer product (smaller CSS) increases breakage intensity, raising energy. Larger feed requires more energy to achieve the same product. Higher capacity means more mass processed per unit time, raising total power (though specific energy may vary). Therefore, each listed factor influences energy consumption. Step-by-Step Solution: Evaluate feed size: larger feed → more fracture work.Evaluate product size: finer setting → more energy per mass.Evaluate capacity: more tonnes/h → more power draw.Conclude all three affect energy; hence “none of these”. Verification / Alternative check: Empirical crusher power models include size reduction ratio and throughput explicitly. Why Other Options Are Wrong: Selecting any single variable as irrelevant contradicts practical crusher power behaviour. Common Pitfalls: Confusing specific energy (kWh/t) with absolute power (kW); both still depend on size targets and throughput. Final Answer: None of these (all affect energy consumption)

Q: For estimating the work required to grind coal to minus 200 mesh in a ball mill, which comminution law is considered most accurate over this size range?

Introduction / Context: Different energy–size relationships are applied depending on size range and mechanism. For ball milling from coarse to moderately fine sizes, Bond’s law is widely adopted for engineering estimates. Given Data / Assumptions: Target size: minus 200 mesh (≈ 74 microm). Equipment: ball mill. Material: coal (typical Bond work index available). Concept / Approach: Kick’s law suits coarse crushing where size ratios matter; Rittinger’s law suits very fine grinding dominated by new surface creation. Bond’s law bridges these regimes and is empirically correlated with mill performance through the Bond work index, making it the practical choice for ball mill power estimates around 100–75 microm. Step-by-Step Solution: Identify size range: intermediate-to-fine (ball milling domain).Select law with best empirical support: Bond.Answer: Bond’s law. Verification / Alternative check: Design handbooks use Bond equation with W_i for ball mill sizing to 200 mesh. Why Other Options Are Wrong: Rittinger: too fine-focused; may overpredict in this range.Kick: coarse-size focused; underpredicts for fine grinding.No law: Bond is well established. Common Pitfalls: Applying Rittinger universally to all fine grinding; empirical validation favours Bond here. Final Answer: Bond’s law

Question 1

In comminution (size reduction), how does production rate change as product fineness is increased when the energy input to the machine is held constant?

Accepted Answer

Introduction / Context: Size reduction operations (crushing, grinding, milling) consume substantial power. Throughput, specific energy, and target fineness are tightly coupled. Understanding these trade-offs is essential for selecting mills, setting operating points, and estimating operating costs. Given Data / Assumptions: Energy input to the machine is fixed (power constraint). Target product becomes finer (smaller characteristic size). Same material and feed characteristics. Concept / Approach: Work input generally increases nonlinearly as the required product size decreases. Rittinger’s law ties energy to new surface area (dominant at fine grinding), while Bond’s law is often used for ball mills over a broad range. For a fixed power draw, achieving finer product consumes more energy per unit mass; therefore, less mass can be processed per unit time, reducing production rate. Step-by-Step Solution: Hold power constant → fixed energy per time.Finer product → higher energy per mass required.Thus, mass processed per time (production rate) must fall. Verification / Alternative check: Plant data routinely show throughput drops when grind is tightened (e.g., moving from 150 microm to 75 microm cut). Why Other Options Are Wrong: Increases/Unchanged: contradict energy-per-mass rise at finer sizes.Depends only on machine: material grindability and energy–size laws dominate, not just machine choice. Common Pitfalls: Confusing power (kW) with energy per unit mass required to hit a finer P80; they are not interchangeable. Final Answer: Decreases

Question 2

State the unit of specific cake resistance (alpha) used in filtration calculations when expressed in centimetre–gram units.

Accepted Answer

Introduction / Context: Specific cake resistance alpha links filtration pressure drop to flow through a porous cake. Using proper units ensures consistent scale-up and design. Alpha is widely reported either in SI (m/kg) or in cgs equivalents. Given Data / Assumptions: We work in cgs-style units for clarity. Darcy-type filtration equations relate deltaP to mu, alpha, cake mass per area, and superficial velocity. Concept / Approach: In constant pressure filtration, the cake contribution to resistance can be written as R_cake = alpha * (M/A), where M/A is mass of solids deposited per unit area. Since R has dimensions of length (in cgs, cm) and M/A has dimensions of g/cm^2, the dimensions of alpha must be (length) / (mass per area) = cm / (g/cm^2) = cm^3/g·cm = cm/g. Step-by-Step Solution: Start with R_cake = alpha * (M/A).[R_cake] = length; [M/A] = g/cm^2.Thus [alpha] = length / (g/cm^2) = cm * (cm^2/g) = cm^3/g per cm of area basis → simplifies to cm/g. Verification / Alternative check: In SI, alpha is often reported as m/kg. Converting m/kg to cm/g yields the same dimensional form. Why Other Options Are Wrong: g/cm^2 and g/g: mass ratios, not resistance factors.cm/g^2: incorrect mass exponent. Common Pitfalls: Confusing alpha with medium resistance R_m; the latter has dimension of length, while alpha couples length with mass loading. Final Answer: cm/g

Question 3

Ultra-centrifugation is primarily employed to separate which class of suspended particles from a liquid?

Accepted Answer

Introduction / Context: Centrifuges exploit centrifugal acceleration to enhance settling. Ultra-centrifuges achieve extremely high rotational speeds and fields (>10^5 g), enabling separation of particles that would be essentially non-settling under gravity, such as macromolecules and colloids. Given Data / Assumptions: Device: ultra-centrifuge (very high g). Objective: separate extremely small particles. Fluid properties are typical for aqueous/organic media. Concept / Approach: Stokes-law settling velocity scales with particle size squared. For colloidal particles in the 1 nm to <1 microm range, gravitational settling is negligible; Brownian motion dominates. Ultra-centrifugation amplifies the effective body force, producing measurable sedimentation even for macromolecules and viruses. Step-by-Step Solution: Identify particle size regimes: coarse, fine, colloidal.Match capability: only ultra-centrifuges generate sufficient g to sediment colloids in practical time.Exclude dissolved species; they require separation by diffusion/membranes, not centrifugation. Verification / Alternative check: Analytical ultracentrifugation is standard in biochemistry for proteins, nucleic acids, and viruses. Why Other Options Are Wrong: Coarse/fine particles: lower-speed centrifuges or thickeners suffice.Dissolved species: no particulate phase to sediment. Common Pitfalls: Assuming any centrifuge can separate colloids; only very high g and carefully chosen run times make this feasible. Final Answer: Colloidal particles (nanometre to sub-micrometre)

Question 4

Correct the statement for comminution laws: According to Bond’s law, the specific energy required to reduce a material from size D_f to D_p varies approximately with which expression?

Accepted Answer

Introduction / Context: Comminution energy laws provide empirical relationships between energy input and particle size change. Three classical laws—Kick, Bond, and Rittinger—apply over different size ranges and mechanisms. Accurately recalling their mathematical forms is vital for ball mill sizing and power estimation. Given Data / Assumptions: D_f: characteristic feed size. D_p: characteristic product size. Homogeneous material; bond work index concept applies. Concept / Approach: Bond’s law states that the energy required is proportional to the new crack length generated and correlates with the reciprocal of the square root of size. The commonly used form is: E = k_B * (1/√D_p − 1/√D_f), where k_B includes the Bond work index for the material and machine constants. By contrast, Kick’s law gives E ∝ ln(D_f/D_p) (size ratio), and Rittinger’s law gives E ∝ new surface area, often represented as E ∝ 1/D_p − 1/D_f for uniform particles. Step-by-Step Solution: Identify target law: Bond.Recall dependence: inverse square-root of size.Select expression with (1/√D_p − 1/√D_f). Verification / Alternative check: Ball mill design references and Bond’s original correlation consistently use the square-root reciprocal form. Why Other Options Are Wrong: (S/V) difference: aligns with Rittinger’s surface-area concept, not Bond’s.ln(D_f/D_p): Kick’s law, appropriate for coarse crushing.1/D_p − 1/D_f: a common Rittinger-type representation. Common Pitfalls: Mixing up the three laws; remember: Kick → log, Bond → inverse square root, Rittinger → inverse size (surface). Final Answer: E ∝ (1/√D_p) − (1/√D_f)

Question 5

In mineral processing of gold ores, which concentration method is most widely used for fine liberated particles?

Accepted Answer

Introduction / Context: Gold beneficiation involves liberating valuable minerals and concentrating them before extraction (e.g., cyanidation). For finely disseminated gold associated with sulphides (e.g., pyrite, arsenopyrite), flotation is a dominant pre-concentration method. Given Data / Assumptions: Fine liberated particles are present. Ore may contain sulphide carriers of gold. Objective: upgrade feed prior to downstream recovery. Concept / Approach: Froth flotation selectively attaches hydrophobic particles to bubbles, separating them from hydrophilic gangue. It is highly effective at sizes below a few hundred microm, exactly where gravity-based devices lose efficiency due to low settling velocities. Jigging and tabling work best for coarser, high-density contrasts; elutriation is a specialized hydraulic classification step, not a primary gold concentrator for fines. Step-by-Step Solution: Identify particle size regime: fine.Match separation principle: surface chemistry (flotation) excels for fines.Select flotation as the widely used method in modern gold concentrators. Verification / Alternative check: Many refractory and sulphide-associated gold circuits employ bulk or selective flotation before oxidation and leach. Why Other Options Are Wrong: Jigging/Tabling: gravity concentration loses efficiency on fine sizes.Elutriation: classification, not primary gold concentration. Common Pitfalls: Assuming gravity always dominates because gold is dense; at fine sizes, hydrodynamics favour flotation. Final Answer: Froth flotation

Question 6

A pulverised coal sample passing a 200-mesh screen has a nominal particle size of 0.074 mm (74 microm). What is the approximate nominal particle size of material that just passes a 50-mesh screen?

Accepted Answer

Introduction / Context: Screen mesh numbers denote openings per linear inch. Approximate relationships allow quick estimation of nominal aperture sizes to translate between mesh counts and particle sizes in classification tasks. Given Data / Assumptions: 200 mesh ≈ 74 microm (0.074 mm). We seek the size for 50 mesh. Use standard sieve series approximations. Concept / Approach: As mesh decreases (fewer openings per linear inch), aperture increases. A common reference: 50 mesh ≈ 297 microm, i.e., about 0.297 mm. Rounding to two significant digits yields 0.30 mm. Step-by-Step Solution: Recognise 200 mesh → 0.074 mm (given).Recall 50 mesh aperture ≈ 0.297 mm.Select the closest option: 0.30 mm. Verification / Alternative check: ASTM E11/ISO sieve tables list 50 mesh near 300 microm. Why Other Options Are Wrong: 0.007 and 0.014 mm: too fine for 50 mesh; those correspond to very high mesh counts.50 mm: off by orders of magnitude. Common Pitfalls: Confusing linear inch basis with square inch count; mesh is per linear inch. Final Answer: 0.30 mm

Question 7

Among common size-reduction equipment, which accepts the largest maximum feed size for primary breakage?

Accepted Answer

Introduction / Context: Choosing the first-stage comminution device depends on feed size. Primary crushers must accept run-of-mine or large quarried rocks, whereas mills and fine pulverisers require much finer feeds. Given Data / Assumptions: Standard industrial equipment: jaw crusher, ball/tube mills, jet pulveriser. We compare maximum acceptable feed size. Concept / Approach: Jaw crushers are primary crushers designed to accept very large feeds (hundreds of millimetres to over a metre) and reduce them to manageable secondary sizes. Ball and tube mills require much smaller feeds (typically <25 mm). Jet pulverisers handle sub-millimetre feeds for ultrafine grinding. Step-by-Step Solution: Identify feed size domain of each device.Primary crusher domain → jaw crusher.Select jaw crusher as correct. Verification / Alternative check: Manufacturer datasheets show jaw crushers with large gape dimensions for coarse feeds. Why Other Options Are Wrong: Ball/Tube mills: require pre-crushed material.Jet pulveriser: only for very fine feeds; unsuitable for large lumps. Common Pitfalls: Confusing maximum feed size with final product fineness capability. Final Answer: Jaw crusher

Question 8

Sphericity (phi) is defined as the surface area of a volume-equivalent sphere divided by the actual particle surface area. Which shape has the maximum possible sphericity?

Accepted Answer

Introduction / Context: Sphericity is used to characterise particle shape effects on drag, packing, and surface-area-dependent phenomena. It normalises shapes against a perfect sphere with the same volume. Given Data / Assumptions: Sphericity phi = (surface area of volume-equivalent sphere) / (actual surface area). 0 < phi ≤ 1 by definition. Concept / Approach: The sphere minimises surface area for a given volume. Therefore, the numerator and denominator become equal only for a sphere, giving phi = 1. All other shapes have larger surface areas for the same volume, producing phi < 1. Step-by-Step Solution: Recall isoperimetric principle: sphere minimises area for fixed volume.Thus the ratio reaches its maximum (1) for a sphere.Select sphere as the answer. Verification / Alternative check: Empirical shape factors confirm cubes, cylinders, and engineered packings have phi significantly less than 1. Why Other Options Are Wrong: Cube/Cylinder: greater surface area per unit volume.Raschig rings: deliberately non-spherical, very low sphericity. Common Pitfalls: Confusing roundness (edge smoothness) with sphericity (global shape metric). Final Answer: Sphere

Question 9

For a jaw crusher, which of the following is NOT a determining factor for its electrical energy consumption during operation?

Accepted Answer

Introduction / Context: Crusher power draw is influenced by duty conditions and setting. Accurately assessing what factors influence energy consumption is important for energy audits and optimisation. Given Data / Assumptions: Jaw crusher operating at steady conditions. Variables: feed size, product size (closed side setting), capacity (throughput). Concept / Approach: Power required correlates with the amount of breakage performed per unit time. Finer product (smaller CSS) increases breakage intensity, raising energy. Larger feed requires more energy to achieve the same product. Higher capacity means more mass processed per unit time, raising total power (though specific energy may vary). Therefore, each listed factor influences energy consumption. Step-by-Step Solution: Evaluate feed size: larger feed → more fracture work.Evaluate product size: finer setting → more energy per mass.Evaluate capacity: more tonnes/h → more power draw.Conclude all three affect energy; hence “none of these”. Verification / Alternative check: Empirical crusher power models include size reduction ratio and throughput explicitly. Why Other Options Are Wrong: Selecting any single variable as irrelevant contradicts practical crusher power behaviour. Common Pitfalls: Confusing specific energy (kWh/t) with absolute power (kW); both still depend on size targets and throughput. Final Answer: None of these (all affect energy consumption)

Question 10

For estimating the work required to grind coal to minus 200 mesh in a ball mill, which comminution law is considered most accurate over this size range?

Accepted Answer

Introduction / Context: Different energy–size relationships are applied depending on size range and mechanism. For ball milling from coarse to moderately fine sizes, Bond’s law is widely adopted for engineering estimates. Given Data / Assumptions: Target size: minus 200 mesh (≈ 74 microm). Equipment: ball mill. Material: coal (typical Bond work index available). Concept / Approach: Kick’s law suits coarse crushing where size ratios matter; Rittinger’s law suits very fine grinding dominated by new surface creation. Bond’s law bridges these regimes and is empirically correlated with mill performance through the Bond work index, making it the practical choice for ball mill power estimates around 100–75 microm. Step-by-Step Solution: Identify size range: intermediate-to-fine (ball milling domain).Select law with best empirical support: Bond.Answer: Bond’s law. Verification / Alternative check: Design handbooks use Bond equation with W_i for ball mill sizing to 200 mesh. Why Other Options Are Wrong: Rittinger: too fine-focused; may overpredict in this range.Kick: coarse-size focused; underpredicts for fine grinding.No law: Bond is well established. Common Pitfalls: Applying Rittinger universally to all fine grinding; empirical validation favours Bond here. Final Answer: Bond’s law

Question 11

For constant-pressure filtration of a slurry forming a cake, the qualitative plot of filtrate volume V versus time t most closely resembles which curve?

Accepted Answer

Introduction / Context: Filtration under constant pressure features increasing cake thickness over time, which raises resistance and slows the filtrate rate. Recognising the V–t relationship aids in data reduction and scale-up. Given Data / Assumptions: Constant transmembrane pressure. Cake-forming slurry with negligible medium compressibility. Standard cake filtration model applies. Concept / Approach: The filtration equation for constant pressure can be expressed as: t = a * V + b * V^2, where the linear term represents medium resistance and the quadratic term represents cake resistance. Rearranged, V vs t shows a concave-down increase with diminishing slope (rate dV/dt decreases). While mathematically V ≈ k * sqrt(t) at long times (cake-dominated), in qualitative MCQs this is often described as a “parabolic” trend in the V–t plane (since t scales with V^2). Step-by-Step Solution: Write t = k1 * V + k2 * V^2.As t grows, the V^2 term dominates → t ∝ V^2.Hence V grows sublinearly with t (concave-down), commonly labelled parabolic. Verification / Alternative check: Plotting typical filtration test data yields a straight line for t/V vs V and a curved V vs t with diminishing slope. Why Other Options Are Wrong: Straight line: corresponds to constant-rate filtration, not constant pressure.Hyperbola/Exponential: do not reflect the t ∝ V^2 dependence. Common Pitfalls: Confusing the diagnostic plot t/V vs V (linear) with the raw V vs t curve (curved). Final Answer: Parabola (V increases with t but with diminishing slope)

Question 12

For removing trace quantities of precipitate from a large volume of water, which filter is most suitable as a polishing step?

Accepted Answer

Introduction / Context: In water treatment, polishing steps remove small residual solids at high flow rates and low cost. The selection must balance filtration area, backwashing capability, and operating simplicity. Given Data / Assumptions: Very small amount of precipitate remains. Large water throughput is required. Economical, continuous operation is preferred. Concept / Approach: Rapid sand filters provide deep-bed filtration with periodic backwashing to restore headloss. They are ideal for clarifying large flows with low solids loading. Pressure or vacuum leaf/drum filters are suited for higher solids slurries and cake recovery; plate and frame presses are batch, labour-intensive, and designed for higher solids capture, not trace polishing of municipal-scale flows. Step-by-Step Solution: Identify service: polishing of large volumes at low solids.Select deep-bed granular filter: rapid sand filter.Reject batch/cake filters as ill-suited to high-volume low-solid duties. Verification / Alternative check: Conventional water treatment trains routinely use rapid sand filters after clarification. Why Other Options Are Wrong: Plate and frame / rotary vacuum / shell and leaf: used for slurries with significant solids and cake recovery, not polishing huge clean-water flows. Common Pitfalls: Choosing high-solids equipment for low-solids polishing tasks increases cost and complexity unnecessarily. Final Answer: Rapid sand (gravity) filter

Question 13

In sieve analysis terminology, “mesh” indicates the number of openings per which reference length?

Accepted Answer

Introduction / Context: Understanding mesh terminology is essential for interpreting sieve sizes and correlating with particle diameters. Mesh counts define screening cut points and relate to standard aperture tables. Given Data / Assumptions: Standard wire cloth/sieve nomenclature. “Mesh” is a count of openings. Concept / Approach: By definition, mesh is the number of openings per linear inch of screen. For example, 50 mesh has 50 openings per linear inch. Aperture size also depends on wire diameter, so two screens with the same mesh may have slightly different clear openings if wire gauges differ. Step-by-Step Solution: Recall definition: mesh = openings per linear inch.Select “Linear inch”. Verification / Alternative check: ASTM and ISO sieve standards define mesh on a linear basis, with companion tables giving nominal apertures. Why Other Options Are Wrong: Square inch/foot and linear foot: not the conventional reference. Common Pitfalls: Assuming “per square inch,” which would misstate the count by a factor roughly equal to mesh. Final Answer: Linear inch

Question 14

Colloid mills reduce particle size predominantly by which mechanical action within the narrow rotor–stator gap?

Accepted Answer

Introduction / Context: Colloid mills are widely used to produce emulsions, suspensions, and fine dispersions in food, pharmaceutical, and chemical processing. Knowing the dominant mechanism helps in selecting suitable equipment for rheology targets. Given Data / Assumptions: High-speed rotor–stator device with narrow gap. Fluid feed containing droplets or soft particles. Concept / Approach: In a colloid mill, fluid accelerates through a small gap, generating very high shear rates. The intense shear and turbulence cause droplet breakup and solid deagglomeration primarily via attrition mechanisms. Impact and compression are minimal compared to shear; cutting applies to knife mills rather than rotor–stator dispersers. Step-by-Step Solution: Identify equipment geometry: rotor–stator with narrow gap.Relate to mechanism: high shear → attrition-based size reduction.Select “Attrition (intense shear)”. Verification / Alternative check: Process literature correlates droplet size with shear rate and residence time in colloid mills. Why Other Options Are Wrong: Impact/Compression: dominant in crushers/mills with media.Cutting: characteristic of knife/cutter mills. Common Pitfalls: Assuming “impact” because of high speed; in liquids, shear dominates. Final Answer: Attrition (intense shear)

Question 15

A “pebble mill” is best described by which combination regarding its grinding media and lining?

Accepted Answer

Introduction / Context: Pebble mills are used where iron contamination must be avoided or where a gentle grinding action is preferred. They are common in ceramic, pigment, and certain chemical industries. Given Data / Assumptions: Grinding media can be natural flint pebbles or engineered ceramic cylinders/spheres. Linings often non-metallic to prevent contamination. Concept / Approach: Unlike steel ball mills, pebble mills use non-metallic media and typically non-metallic linings. A tube-mill geometry (length ≫ diameter) is common. This combination minimises metallic pickup and yields a clean product surface. Step-by-Step Solution: Identify media: flints/ceramics → correct.Identify lining: ceramic/non-metallic → correct.Choose combined option to capture both attributes. Verification / Alternative check: Materials handling references describe pebble mills as ceramic-lined, pebble-charged tube mills. Why Other Options Are Wrong: Steel ball mill: that is not a pebble mill by definition. Common Pitfalls: Assuming “pebble” refers to product, not the grinding media. Final Answer: Both (b) and (c) together

Question 16

Which equipment is commonly used to remove dispersed water (moisture) from lubricating oil in refinery or lube-conditioning service?

Accepted Answer

Introduction / Context: Water contamination in lubricating oils reduces film strength, promotes corrosion, and accelerates oxidation. Efficient separation of water droplets from viscous oils is essential in maintenance and refining operations. Given Data / Assumptions: Oil contains dispersed water (emulsified or fine droplets). We need a continuous, compact device with high g-force. Concept / Approach: Tubular-bowl centrifuges produce very high centrifugal accelerations, rapidly separating water from oil based on density difference. Gravity clarifiers are too slow for fine droplets; sparkler and vacuum leaf filters remove solids, not immiscible liquid droplets. Coalescers may also be used, but among listed options, the tubular centrifuge is the standard choice. Step-by-Step Solution: Identify separation type: liquid–liquid (immiscible) with fine droplets.Match to equipment: high-g centrifuge enhances droplet migration.Select tubular centrifuge. Verification / Alternative check: Lube oil purifiers often employ disc-stack/tubular centrifuges for water removal. Why Other Options Are Wrong: Clarifier: too slow for fine dispersion.Sparkler/Vacuum leaf: solid–liquid filtration devices, not suited to separating water droplets from oil. Common Pitfalls: Assuming any filter removes water; filters generally target solids unless paired with coalescing media. Final Answer: Tubular centrifuge

Question 17

Identify the most commonly used filter aid added to form permeable cakes and prevent blinding in filtration.

Accepted Answer

Introduction / Context: Filter aids are precoat or body-feed materials that create a porous, permeable skeleton on the medium, capturing fine solids while maintaining transmissivity. Choosing the right aid avoids excessive pressure rise and short cycles. Given Data / Assumptions: General-purpose clarification in process industries. Fine particulates liable to blind the medium. Concept / Approach: Diatomaceous earth (kieselguhr) consists of frustules of diatoms with intricate, high-void structures, providing excellent permeability at moderate strength—hence its widespread use. Perlite and cellulose are also used, and calcium silicate appears in specialty applications. Sodium carbonate is not a filter aid; silica gel is a desiccant, not typically used as a structural filter aid in pressure filters. Step-by-Step Solution: Match requirement (porous, permeable, inert) with candidate materials.Select diatomaceous earth as the standard choice. Verification / Alternative check: Precoat filtration in beverage, pharma, and chemical sectors commonly uses kieselguhr. Why Other Options Are Wrong: Calcium silicate: used but less common than DE.Sodium carbonate: reagent, not a filter aid.Silica gel: moisture adsorbent, not a structural cake aid. Common Pitfalls: Confusing desiccants or reagents with structural filter aids. Final Answer: Diatomaceous earth (kieselguhr)

Question 18

For maintaining the fibrous or flaky nature during size reduction, asbestos and mica are preferably ground in which mill?

Accepted Answer

Introduction / Context: Some materials, such as asbestos and mica, have valuable properties due to their fibrous or flaky morphology. Size reduction must preserve these shapes to retain performance in downstream uses (e.g., reinforcement, insulation, pearlescent pigments). Given Data / Assumptions: Materials: asbestos (fibrous), mica (platy). Objective: reduce size while preserving morphology (minimal excessive shattering). Concept / Approach: Rod mills provide preferential grinding that tends to break at grain boundaries and minimize overgrinding and slabbing compared to ball mills or impact devices. The linear contact of rods promotes selective grinding, which is better for fibrous/flaky materials. Hammer mills and crushers impose high impact/compression, which can pulverise and destroy morphology. Step-by-Step Solution: Recognise the need to avoid excessive impact.Select a gentler, preferential grinding device: rod mill.Reject high-impact crushers for this duty. Verification / Alternative check: Mineral processing texts recommend rod mills for mica/asbestos to maintain aspect ratio. Why Other Options Are Wrong: Hammer/gyratory/rolls: impact and compression cause plate fracture and fiber shortening. Common Pitfalls: Assuming the highest energy device produces the best product; morphology preservation requires controlled breakage. Final Answer: Rod mill

Question 19

To achieve maximum practical capacity in a ball mill, the total ball charge (by volume) should be approximately what fraction of the mill volume?

Accepted Answer

Introduction / Context: Ball mill performance depends on charge level, lifter design, speed, and media size. The volumetric filling of grinding media directly affects cataracting/cascading motion and thus throughput and grind. Given Data / Assumptions: Conventional overflow or grate-discharge ball mill. Typical operating speeds (65–80% of critical). We seek the general rule-of-thumb for media volume fraction. Concept / Approach: A common rule is to operate with total ball charge near one-half of the internal volume. This promotes an effective cascading and cataracting action without excessive cushioning or power draw instability. Lower fills underutilise the mill; higher fills can reduce active lift and void space for slurry/particles, impairing breakage and increasing power without proportional throughput gains. Step-by-Step Solution: Consider media packing and movement dynamics.Recall industry guideline: ~50% volumetric filling for media.Select the closest option: about 50%. Verification / Alternative check: Plant practice and design texts cite 40–50% as typical; the maximum practical capacity is often near the upper end of this range. Why Other Options Are Wrong: 10%/25%: too low for efficient breakage and throughput.75%: excessive crowding; poor cataract/cascade regime and limited interstitial space. Common Pitfalls: Confusing % media charge with % critical speed or total charge including slurry; the question refers to ball volume fraction. Final Answer: About 50% of the mill volume

Question 20

Rotary drum vacuum filter: what is a typical operating drum speed at room conditions for filtration duty (order of magnitude, in rpm)?

Accepted Answer

Introduction / Context: Rotary drum vacuum filters (RDVFs) are continuous filters widely used for slurries in chemicals, minerals, and wastewater treatment. Drum speed sets the residence time for cake formation, washing, and drying on each sector. Understanding the order of magnitude of drum rotational speed is essential to size filtrate piping, vacuum capacity, and cake washing times. Given Data / Assumptions: Conventional rotary drum vacuum filter with segmented sectors, vacuum box, and discharge by scraper or belt. Room temperature service; non-explosive, non-foaming slurry. Question asks for typical speed magnitude, not the exact optimized value for a specific slurry. Concept / Approach: RDVFs operate slowly so that each sector experiences sufficient submergence, cake buildup, optional wash, and air-drying before discharge. Typical circumferential speeds are modest; hence rotational speeds are usually well below 10 rpm, commonly in the 0.2–6 rpm band depending on cake thickness and washing requirements. Very high speeds would reduce cake residence time and cause discharge and vacuum problems. Step-by-Step Solution: Recognize the need for long residence time for forming, washing, and drying → slow drum rotation.Industry practice: RDVFs often run around 0.5–3 rpm for many slurries; values near 1 rpm are very common.Therefore, pick the order-of-magnitude value closest to typical operation: 1 rpm. Verification / Alternative check: Vendors’ sizing curves and operating manuals routinely show sub-10-rpm operation, with 1 rpm frequently used in design examples for medium-thickness cakes. Why Other Options Are Wrong: 50 rpm and 100 rpm: far too fast; would severely curtail cake residence time and disrupt vacuum sealing.500 rpm: unrealistic and mechanically unsuitable for a vacuum-filter drum. Common Pitfalls: Confusing RDVF with rotary vacuum precoat filters (still slow) or high-speed centrifuges; assuming higher speed always increases throughput—filtration is mass-transfer limited and needs residence time. Final Answer: 1

Mechanical Operations Questions