Question 1

Agitation similitude — Froude, Reynolds, and power number: During liquid agitation in baffled tanks, which set of statements about dimensionless groups and power correlations is correct?

Accepted Answer

Introduction / Context:Mixing scale-up relies on dimensionless groups: Reynolds number (inertial/viscous), Froude number (inertial/gravity), and the power number (P/(rho*N^3*D^5)) that captures impeller loading. Understanding when each group matters avoids scale-up surprises like unexpected power draw or vortex formation in unbaffled tanks.Given Data / Assumptions:Baffled, Newtonian liquid systems.Standard impeller types (e.g., Rushton, pitched-blade) in tanks with typical baffle configurations.Large Reynolds number regime implies fully turbulent flow.Concept / Approach:For baffled, fully turbulent systems, the power number becomes essentially independent of Reynolds number; it plateaus to a constant that depends on impeller geometry and clearance. Froude number is invoked when a free surface is present because gravity and surface deformation (vortex depth) influence power draw and aeration; hence Fr is used to correlate surface effects alongside Po. The phrasing in (a) notes that, in power vs. Reynolds plots for baffled tanks, Fr is not the varying axis but a separate parameter in surface-effect correlations; (b) is the classic asymptotic behavior; (c) is the recognized use of Fr for vortex effects.Step-by-Step Solution:Recognise turbulent baffled regime → Po ≈ constant, geometry-dependent.Acknowledge surface effects → need Fr to capture vortex behavior.Conclude all statements together describe accepted practice.Verification / Alternative check:Power number charts show Po plateau at Re > 10^4–10^5; correlations with free surfaces employ Fr to predict vortex depth and associated power penalties.Why Other Options Are Wrong:(e) is incorrect because each listed statement reflects standard mixing theory.Common Pitfalls:Using Re-only correlations for unbaffled or free-surface cases; neglecting Fr yields underpredicted power with deep vortices.Final Answer:All (a), (b) and (c).

Question 2

Pick the wrong statement — cakes and explosive materials: Identify the incorrect statement among the following about filtration cakes and crushing practice.

Accepted Answer

Introduction / Context:This question mixes two topics: filtration cake behavior and safe comminution of sensitive materials. It checks whether you can spot a statement that contradicts widely accepted engineering safety practice, while also recalling how cake resistance behaves with pressure for compressible cakes.Given Data / Assumptions:Compressible cakes compact with increasing pressure drop.Explosive/dusty materials require special handling to avoid ignition.Gyratory crushers are used in primary, coarse crushing.Concept / Approach:Safe handling dictates that explosives and impact-sensitive powders are processed wet whenever feasible to control dust and dissipate heat; dry grinding increases dust explosion risk and frictional heating. Thus, the statement that explosive materials are crushed by dry grinding is incorrect. Regarding cakes, for compressible cakes, voidage and specific cake resistance change with pressure; assuming them constant is invalid. Also, cake resistance depends on pressure drop for compressible cakes. However, the single most clearly wrong and safety-critical statement here is the claim about dry grinding of explosive materials.Step-by-Step Solution:Recall safety rule: explosive powders → wet milling preferred.Identify (c) contradicts this rule → pick as wrong.Note that (d) correctly describes gyratory crushers as coarse crushers.Verification / Alternative check:Process safety guidelines recommend inerting, wet milling, and dust control to mitigate explosion hazards during size reduction.Why Other Options Are Wrong:(a) Not strictly true for compressible cakes (they change), but (c) is the clearly wrong and unsafe assertion.(b) For compressible cakes, resistance depends on pressure; again (c) is the definitive incorrect practice.(d) Correct by definition.Common Pitfalls:Focusing only on filtration theory and missing the obvious safety violation in (c).Final Answer:Crushing of explosive materials is carried out by dry grinding.

Question 3

Closed-circuit vs open-circuit grinding: Compared with open-circuit operation, which statement about closed-circuit grinding is generally true?

Accepted Answer

Introduction / Context:Grinding circuits can run open (mill discharge goes to product) or closed (mill discharge passes through a classifier; coarse returns to the mill). Understanding the qualitative effects of classification—on fineness, size uniformity, throughput, and economics—helps in selecting a circuit configuration for target specifications and energy costs.Given Data / Assumptions:Comparable feed and target limiting size.Functional classifier with steady circulating load.Similar mill conditions aside from the classifier loop.Concept / Approach:In closed circuit, fine particles are preferentially removed as soon as they are generated, reducing overgrinding of fines while forcing additional breakage of coarse particles via recycle. For a given top size, this produces a product with more material near the fine end, increasing specific surface area. It also typically improves size uniformity (contradicting option (b)), increases production at a given limit by avoiding wasted energy on already-fine material (contradicting (c)), and is often more economical (supporting (d)). Among single statements, (a) is a robust, unambiguous truth about closed circuits compared with open circuits.Step-by-Step Solution:Recall classifier function: remove fines, recycle coarse.Infer higher surface area due to accumulation of fines meeting spec.Select statement (a) as generally true.Verification / Alternative check:Plant data routinely show higher Blaine/PSSA at the same top size when operating in closed circuit with balanced circulating load.Why Other Options Are Wrong:(b) Wrong: closed circuit tends to improve, not reduce, size uniformity.(c) Wrong: production rate is typically higher at a set cut size.(d) Often true but can be case-dependent; (a) is the clearest general statement.Common Pitfalls:Confusing “higher surface” with “overgrinding”; closed circuits reduce overgrinding of fines by removing them promptly.Final Answer:The specific surface area of the product is higher for a given top size limit.

Question 4

Pick the wrong statement — size-reduction mechanisms and jaw crushers: Identify the incorrect statement about crushing mechanisms and jaw-crusher types.

Accepted Answer

Introduction / Context:Most size-reduction devices can be mapped to a few dominant mechanisms: compression, impact, attrition, and cutting (shear). Knowing which mechanism a machine uses informs material suitability and expected product shape. This question also checks recognition of common jaw-crusher variants and their usage prevalence.Given Data / Assumptions:Standard definitions of mechanisms and jaw types are assumed.“Cutting machines” refers to guillotines, rotary cutters, knife mills, etc.Concept / Approach:Cutting machines remove material by shear along a narrow zone using sharp edges; they do not rely mainly on attrition (surface rubbing). Attrition mills (e.g., colloid mills) reduce size primarily by rubbing and abrasion between surfaces. Blake jaw crushers are indeed more common than Dodge type for primary service, and universal jaws share motion characteristics of both classic types (top pivot and complex motion). Therefore, the lone incorrect statement is that cutting employs attrition.Step-by-Step Solution:Differentiate mechanisms: shear vs attrition.Confirm usage prevalence: Blake more common than Dodge.Select the statement that mislabels shear as attrition.Verification / Alternative check:Equipment catalogs: knife cutters list shear angles, blade speeds; attrition mills specify gap and abrasive surfaces.Why Other Options Are Wrong:(a), (b), (d) are standard facts in unit operations.Common Pitfalls:Assuming any rubbing in a cutter equates to attrition; the dominant energy mode is shearing across a cutting edge.Final Answer:Cutting machines mainly employ attrition for size reduction.

Question 5

Best filter for tiny precipitate from very large water volumes: Which filter type is most suitable for removing a very small amount of precipitate from very large volumes of water?

Accepted Answer

Introduction / Context:Water-treatment operations often must polish huge flow rates containing very low solids concentrations. The correct choice of filtration technology balances hydraulic capacity, solids loading, maintenance, and cost. Rapid/slow sand filters are designed precisely for this service, unlike industrial cake filters which excel at higher solids slurries.Given Data / Assumptions:Very large volumes of water with a very small amount of precipitate (low turbidity).Continuous operation and low operating cost desired.Concept / Approach:Sand filters (rapid or slow) pass water through a deep granular bed, capturing fine particulates throughout the depth (not only on the surface). They accommodate very high throughputs with periodic backwashing and modest pressure drop, making them ideal for polishing large volumes. Plate-and-frame, rotary, or vacuum filters are cake filters suited to higher solids slurries; they are inefficient and expensive per cubic metre treated when solids are trace. Candle filters are niche for clarity-critical, lower flow industrial services.Step-by-Step Solution:Characterise duty: very low solids, very high flow.Match to deep-bed filtration → sand filter.Reject cake filters as overkill for trace solids.Verification / Alternative check:Municipal and industrial water plants universally employ sand (and dual-media) filters for this duty.Why Other Options Are Wrong:Vacuum/rotary/plate-and-frame: better for slurries with significant cake formation.Candle filter: not economical for very large water volumes.Common Pitfalls:Confusing clarity requirements with solids load; ultraclean but low-load streams still favour deep-bed filters over cake filters.Final Answer:Sand filter

Question 6

Not a tumbling (revolving) mill: Which of the following is NOT classified as a tumbling/revolving mill for size reduction?

Accepted Answer

Introduction / Context:Tumbling (revolving) mills reduce size by cascading/cataracting media and particles inside a rotating cylindrical shell. Examples include ball, rod, pebble, and compartment mills. It is useful to distinguish these from impactor-style mills, which achieve reduction through high-speed particle impacts without a rotating, media-filled drum.Given Data / Assumptions:Tumbling mills possess a rotating cylindrical shell.Charge motion (cascading/cataracting) is the dominant mechanism.Concept / Approach:Cage mills are impact mills; they use one or more high-speed cages that fling material into counter-rotating impactors and breaker plates. There is no media-filled rotating cylindrical drum as in ball/rod/pebble/compartment mills. Therefore, the cage mill is not a tumbling mill, whereas the others listed are classic tumbling types.Step-by-Step Solution:Identify the common geometry of tumbling mills (rotating shell).Recognise that cage mills are impactors with spin cages.Select “Cage mill.”Verification / Alternative check:Equipment taxonomies in unit-ops texts list cage and hammer mills under “impact mills,” separate from tumbling mills.Why Other Options Are Wrong:Compartment/pebble/rod/ball mills all have rotating cylindrical shells and are tumbling mills.Common Pitfalls:Assuming any rotating device is a tumbling mill; internal mechanics matter.Final Answer:Cage mill

Question 7

Best conveyor for gummy and sticky materials (e.g., molasses, sugar pastes): Which conveyor is generally preferred for transporting sticky, pasty materials?

Accepted Answer

Introduction / Context:Material handling selection depends strongly on rheology. Sticky and pasty materials such as molasses-bearing mixes or moist sugar pastes can pack, smear, or bridge in open conveyors. Enclosed positive-displacement devices that force material along a trough with a rotating helix are widely used to overcome these issues.Given Data / Assumptions:Material exhibits stickiness and some plasticity.Short-to-moderate conveying distances.Desire for containment and simple cleanup.Concept / Approach:Screw conveyors provide continuous, forced conveying in an enclosed trough, tolerating sticky or pasty consistencies better than drag or slat systems which can experience carryback and chain fouling. Ribbon screws (a variant) improve mixing and reduce buildup for especially gummy products. Drag and slat conveyors are excellent for granular/packaged materials but struggle with smear-prone pastes. Vibratory troughs work with free-flowing powders/granules; pasty materials may rat-hole or adhere excessively.Step-by-Step Solution:Match sticky rheology to positive-displacement enclosed conveying.Identify screw conveyors (including ribbon screw variants) as industry standard.Select “Screw conveyor.”Verification / Alternative check:Food and chemical plants routinely specify screw/ribbon screws for molasses, wet sugar mixes, and similar pastes.Why Other Options Are Wrong:Drag/slat: chains and flights foul with sticky materials.Ribbon conveyor (as a type of belt): belts slip and cake; “ribbon screw” is different.Vibratory: best for free-flowing solids.Common Pitfalls:Confusing “ribbon conveyor” (a belt variant) with “ribbon screw” (a screw type). The latter indeed helps with pastes, but it is still a screw conveyor.Final Answer:Screw conveyor

Question 8

Gravity stamp mill duty: A gravity stamp mill is primarily intended for which stage of crushing?

Accepted Answer

Introduction / Context:Historic ore-processing circuits used gravity stamp mills to break mined rock before concentration. While modern plants rely on jaw and gyratory primaries, the role of stamp mills in early flowsheets remains a common exam reference and helps contextualize how equipment evolved.Given Data / Assumptions:Stamp mills deliver heavy, repeated blows to break large lumps.Downstream stages provide finer reduction and concentration.Concept / Approach:The duty of a stamp mill is coarse breakage of run-of-mine ore into smaller fragments suitable for further grinding (e.g., in pans or ball mills). The mechanism is impact from falling stamps, comparable to primary crushing in today’s terminology. It is not suitable for fine or ultrafine grinding, which require sustained abrasion/attrition in mills.Step-by-Step Solution:Map impact blows of stamps to coarse size reduction.Relate to stage terminology → primary crushing.Select “Primary crushing.”Verification / Alternative check:Classical gold circuit descriptions place stamp batteries ahead of amalgamation or milling, analogous to primary crushers today.Why Other Options Are Wrong:Intermediate/fine/ultrafine: outside the practical range of stamp mills.Tertiary shaping: a modern VSI duty, not stamps.Common Pitfalls:Equating “impact” with “fine”; impact energy per particle depends on mass and control of collisions.Final Answer:Primary crushing

Question 9

Ribbon blenders — intended service: Ribbon blenders are designed primarily for which type of mixing duty?

Accepted Answer

Introduction / Context:Ribbon blenders are ubiquitous in food, pharma, and chemicals for dry blending and paste mixing. Recognising their operating mode and suitable materials helps avoid misapplication where high shear, emulsification, or continuous throughput is required.Given Data / Assumptions:Horizontal U-trough with inner and outer helical ribbons on a central shaft.Batch operation with top loading and bottom discharge.Materials are powders or pastes with moderate to low flowability.Concept / Approach:Counter-rotating ribbon flights create axial counterflow and radial mixing, ideal for homogenising powders, adding small liquids, and handling thin pastes. They do not provide the intense shear of rotor–stator mixers nor the continuous, steady-state operation of in-line static mixers. Therefore, the exclusive intended service is batch mixing of powders/pastes.Step-by-Step Solution:Identify geometry and motion of ribbon blender.Match to material: powders/pastes; batch duty.Select option (b).Verification / Alternative check:OEM datasheets specify batch volumes, blend times, and suitability for powders/pastes; not for continuous liquid blending.Why Other Options Are Wrong:(a), (d) refer to continuous liquid/slurry mixing; not a ribbon’s forte.(c) High-shear emulsification requires rotor–stator or colloid mills.(e) Gas–liquid dispersion uses impellers or ejectors, not ribbons.Common Pitfalls:Expecting ribbons to deagglomerate hard lumps; pre-milling may be required.Final Answer:Batch mixing of non-flowing powders and thin pastes

Question 10

Taylor 200-mesh opening (screen size conversion): What is the nominal opening of a 200-mesh screen in the Taylor series?

Accepted Answer

Introduction / Context:Mesh numbers must often be converted to metric openings for process calculations, equipment selection, and lab reporting. The Taylor (Tyler) series and ASTM E11 provide standard correspondences; 200 mesh is a common fine sieve used in mineral and cement labs.Given Data / Assumptions:“200 mesh” means 200 openings per linear inch.Taylor series nominal opening for 200 mesh ≈ 0.074 mm.Concept / Approach:One inch equals 25.4 mm. A 200-mesh screen therefore has nominal clear openings on the order of 25.4/200 mm, modified by wire diameter selection in the standard series, giving approximately 0.074 mm. Converting 0.074 mm to centimetres gives 0.0074 cm. Beware distractors that shift the decimal or mix units (milli-microns are nanometers). Option (e) also states 0.074 mm explicitly and may be considered equivalent numerically; however, within the provided choices, the canonical Taylor listing is often given as 0.0074 cm, which equals 0.074 mm.Step-by-Step Solution:Compute 25.4/200 ≈ 0.127 mm before considering wire; standard table gives 0.074 mm.Convert 0.074 mm to cm: 0.074 / 10 = 0.0074 cm.Select 0.0074 cm.Verification / Alternative check:Reference tables list 200 mesh ≈ 74 micrometres (0.074 mm), confirming 0.0074 cm equivalence.Why Other Options Are Wrong:0.0074 mm: off by a factor of 10.0.0047 cm: corresponds to 47 µm, not 74 µm.74 milli-microns (74 nm): off by three orders of magnitude.Common Pitfalls:Mistaking mesh count as a direct inverse without considering standard tables and wire diameters; always consult standardized values.Final Answer:0.0074 cm

Question 11

Cake washing behavior — resistance trends: During the washing of a formed filter cake under steady conditions, which statement best reflects what typically happens?

Accepted Answer

Introduction / Context:After a cake forms, a wash step is often used to displace mother liquor. Engineers must anticipate how hydraulic resistance evolves during washing to size pumps and set wash times. While detailed behavior depends on cake compressibility and chemistry, field practice observes that resistances rarely decrease spontaneously during washing.Given Data / Assumptions:Existing cake on medium; washing at roughly constant pressure/temperature.Possibility of fines migration and partial blinding.Cake is not dissolving significantly during wash.Concept / Approach:Hydraulic resistance is the sum of the medium resistance and cake resistance. Washing can drive fines deeper into the medium and alter pore structures, effectively increasing the medium resistance. Compressible cakes often densify under sustained pressure, raising cake resistance. Therefore, statements that “all resistances remain constant” or “resistances decrease” are generally not reliable. The safest generalisation is that resistances tend to increase or, at best, remain similar; among the provided options, the most defensible trend is an increase in medium resistance via blinding/migration during washing.Step-by-Step Solution:Identify components: R_total = R_medium + R_cake.Anticipate fines migration → R_medium increases.Select option (b) as the best general statement.Verification / Alternative check:Pilot washing often shows higher pressure drop for a given wash flux compared to initial filtration, indicating increased effective resistance.Why Other Options Are Wrong:(a) Unrealistic for compressible cakes and fouling media.(c) Decrease is uncommon absent active cleaning or dissolution.(d) Cake resistance usually increases under sustained pressure.Common Pitfalls:Assuming cleaner liquor automatically implies lower resistance; structure and fouling dominate hydraulics.Final Answer:Filter medium resistance increases (e.g., due to blinding or fines migration).

Question 12

Coal washability (float–sink) curves: A washability curve derived from float-and-sink tests primarily assesses cleanability based on which property?

Accepted Answer

Introduction / Context:Coal preparation depends heavily on density (specific gravity) differences between coal and mineral matter. Float-and-sink testing uses heavy liquids to partition a coal sample into density classes and then constructs washability curves that guide circuit selection (jigs, dense-medium cyclones, spirals) and predict product yield vs ash at given cut densities.Given Data / Assumptions:Standard float–sink laboratory procedure.Coal and its associated mineral matter have distinct densities.Concept / Approach:By immersing particles in liquids of known specific gravity, one observes whether they float or sink and thus infers their density class. Washability curves plot cumulative float/sink mass and quality metrics (e.g., ash) versus density, enabling assessment of how effectively a gravity-based plant can clean the coal. Hence, the primary basis is density, not wettability (flotation), size (screening), volatile content (proximate analysis), or magnetic properties.Step-by-Step Solution:Identify test mechanism: separation by specific gravity.Link to coal cleaning devices that use density differences.Choose “Density separation.”Verification / Alternative check:Plant cut densities predicted from washability curves (e.g., 1.5–1.8) correspond directly to dense-medium separations.Why Other Options Are Wrong:Wettability: flotation tests use reagents and air, not heavy liquids.Size: assessed via screening, not float–sink.Volatile/magnetic: unrelated to float–sink partitioning.Common Pitfalls:Confusing float–sink (density) with froth flotation (wettability); they are distinct beneficiation routes.Final Answer:Density separation

Question 13

Role of the fluid energy (jet) mill: A fluid energy mill is chiefly used for which comminution duty?

Accepted Answer

Introduction / Context:Fluid energy (jet) mills accelerate particles in high-velocity gas streams, causing interparticle collisions that fracture brittle materials. They are designed to achieve very fine sizes with narrow distributions, often without moving mechanical parts in the grinding zone—ideal for heat-sensitive or contamination-intolerant products.Given Data / Assumptions:Gas jets generate supersonic velocities and strong turbulence.Classifying nozzles or internal classifiers may be present for cut control.Concept / Approach:Because breakage occurs via high-energy particle–particle impacts, fluid energy mills can reach micron and even submicron sizes (ultrafines) that are impractical in conventional tumbling mills without excessive residence times. They are not intended for coarse crushing or mere cutting; their forte is ultragrinding with tight control and low contamination when constructed of suitable materials.Step-by-Step Solution:Recall mechanism: gas jet acceleration and collisions.Map mechanism to achievable fineness → ultragrinding.Select option (c).Verification / Alternative check:Pharma and specialty chemical datasheets list median sizes of 1–10 µm routinely achieved in jet mills.Why Other Options Are Wrong:Cutting/crushing: different mechanisms and size ranges.“Grinding (general)” is too broad; jet mills specifically target ultrafine ranges.Common Pitfalls:Ignoring high energy and heat generation—cool, dry gas and controlled feed are essential for thermosensitive materials.Final Answer:Ultragrinding (very fine to micron/submicron)

Question 14

Dust removal from gas using wire mesh: Metallic wire mesh as a filtering medium for dust-laden gas is characteristic of which device?

Accepted Answer

Introduction / Context:Particulate removal technologies rely on different media and mechanisms. Wire-mesh screens are used in certain air filter elements (e.g., prefilters, oil-wetted screen filters) where the mesh provides impaction/impingement surfaces and structural rigidity for low-to-moderate dust loads.Given Data / Assumptions:Gas-phase dust removal at relatively low pressure drop.Reusable or cleanable metal elements preferred.Concept / Approach:Wire-mesh air filters capture particles by inertial impaction, interception, and sometimes with tackifier oils. Bag filters instead use fabric felts or woven textiles, not metal mesh, and rely on a dust cake for high efficiency. Venturi scrubbers are wet collectors; hydrocyclones are inertial separators without a porous medium; electrostatic precipitators use electric fields and plates/wires, not mesh as the primary medium. Therefore, a device explicitly using metallic mesh as the medium is an air filter of the wire-mesh type.Step-by-Step Solution:Match medium (metal mesh) to device type.Exclude devices that employ fabric, liquids, or fields instead.Select “Air filter (wire-mesh type).”Verification / Alternative check:HVAC/industrial prefilter catalogs list layered metal mesh filters for coarse dust and oil mist service.Why Other Options Are Wrong:Bag filters: fabric media.Scrubbers/hydrocyclones/ESPs: different collection principles and media.Common Pitfalls:Assuming all “filters” use fabric; metals are common for cleanable prefiltration and high-temperature service.Final Answer:Air filter (wire-mesh or metal-screen type)

Question 15

Stage identification from size range: Feed size 300–1500 mm reduced to product 100–300 mm corresponds to which crushing stage?

Accepted Answer

Introduction / Context:Crushing stages are often identified by the characteristic size ranges handled. Correctly naming the stage helps with equipment selection, liner choice, and downstream sizing of screens and conveyors.Given Data / Assumptions:Feed lump size: 300–1500 mm (run-of-mine scale).Product size: 100–300 mm (coarse fragments for secondary crushers).Concept / Approach:Primary crushing accepts very large run-of-mine rock and reduces it to sizes suitable for secondary crushing. Typical primary crushers (jaw or gyratory) produce products in the 100–300 mm range, exactly matching the stated output. Secondary crushers then reduce further to tens of millimetres; fine/tertiary stages and grinding produce even smaller sizes.Step-by-Step Solution:Map input size to ROM → primary stage.Match output 100–300 mm to typical primary product.Select “Primary.”Verification / Alternative check:Flowsheets in quarry/mining texts show ROM → primary jaw/gyratory → 100–300 mm → secondary cone.Why Other Options Are Wrong:Secondary/fine/tertiary/ultrafine: handle much smaller feeds and deliver finer products.Common Pitfalls:Judging by reduction ratio alone; absolute size ranges are more informative for stage naming.Final Answer:Primary

Question 16

Name of density-based separation: Separation of solid particles primarily according to their densities is called what?

Accepted Answer

Introduction / Context:Different separation unit operations are named for the dominant property exploited: size (sizing), density (sorting or gravity separation), surface chemistry (flotation), magnetism (magnetic separation), etc. Correct terminology streamlines communication in plant design and operation.Given Data / Assumptions:Bulk mixture of solids differing mainly in density.Mechanisms may include jigs, dense-medium, spirals, shaking tables.Concept / Approach:In classical mineral processing nomenclature, “sorting” and “gravity separation” refer to density-based separations. Depending on context, “hand sorting” is visual/manual, but in many exam banks “sorting” is used generically for density-based mechanical separations. Sizing is based on geometric size using screens; clarification is removal of suspended solids from liquids; dispersion is mixing or phase distribution; flotation exploits wettability differences.Step-by-Step Solution:Identify the property: density.Match to term used in gravity-based devices → sorting.Reject alternatives that rely on different properties.Verification / Alternative check:Text references pair “sorting/gravity separation” for jigs, spirals, dense-medium cyclones—the density-based family.Why Other Options Are Wrong:Sizing: screens use size, not density.Clarification: solid–liquid separation, not solid–solid density sorting.Dispersion: mixing term.Flotation: surface chemistry (wettability), not density.Common Pitfalls:Interpreting “sorting” only as manual picking; in many curricula it denotes density-based mechanical separation as well.Final Answer:Sorting

Question 17

Making fines from soft, non-abrasive materials: Which mechanism is best suited to produce fines from soft, non-abrasive solids?

Accepted Answer

Introduction / Context:Choosing the dominant size-reduction mechanism based on material characteristics improves efficiency and product quality. Soft, non-abrasive materials do not require heavy compression or high-impact devices; instead, rubbing/abrasive contact (attrition) more gently produces fines with controlled top size.Given Data / Assumptions:Material is soft and not highly abrasive (low wear risk).Goal is fine powder rather than coarse fracture.Concept / Approach:Attrition mills (e.g., pin, disc, or colloid mills) subject particles to sliding/rubbing between surfaces, leading to progressive surface removal and fine production. Compression devices (rolls, jaws) create larger, blocky fragments; cutting devices are for fibrous or sheet-like materials, not fine powders. Pure impact can overproduce ultra-fines with poor control and may heat-sensitive materials adversely.Step-by-Step Solution:Match soft, non-abrasive property to gentle surface reduction.Select attrition as the appropriate mechanism.Exclude compression/cutting/impact for this objective.Verification / Alternative check:Applications like milling soft organics or pigments use attrition or pin mills to achieve fine, narrow distributions.Why Other Options Are Wrong:Compression: coarse crushing rather than fine powdering.Cutting: suited to fibrous materials and particle length control.Impact-only: may lead to wide distributions and heat issues.Common Pitfalls:Overmilling soft materials with impact mills causing melt/smear; attrition offers better thermal control.Final Answer:Attrition

Question 18

Specific cake resistance for incompressible sludges: How does the specific cake resistance of an incompressible sludge depend on pressure drop ΔP across the cake?

Accepted Answer

Introduction / Context:In filtration theory, the specific cake resistance alpha characterises how difficult it is for fluid to pass through a unit mass (or thickness) of cake. Whether alpha depends on pressure distinguishes incompressible from compressible cakes and greatly affects design equations and scale-up predictions.Given Data / Assumptions:Sludge/cake is incompressible over the operating pressure range.Fluid properties (viscosity) are constant.Concept / Approach:An incompressible cake maintains its structure under increased pressure; pore geometry does not collapse, so alpha remains constant with respect to ΔP. Hence, the pressure drop affects the volumetric rate linearly through Darcy’s law, but alpha itself does not change. In contrast, compressible cakes densify under pressure, making alpha an increasing function of ΔP.Step-by-Step Solution:Recall Darcy’s-type equation: ΔP = (μ * v * (R_m + α * m/A)).For incompressible cakes, α = constant independent of ΔP.Therefore select “Independent of ΔP.”Verification / Alternative check:Plotting α versus ΔP for rigid granular cakes yields a flat trend; for compressible sludges, α rises with ΔP.Why Other Options Are Wrong:Proportionalities (a–c,e) describe compressible or nonphysical behaviors for incompressible cakes.Common Pitfalls:Applying compressible-cake correlations to rigid cakes, leading to overdesign.Final Answer:Independent of ΔP

Question 19

Paddle agitator characteristics: Which statement best describes the typical operating domain of a paddle agitator?

Accepted Answer

Introduction / Context:Paddle agitators are simple, flat-blade impellers used in tanks for blending and maintaining uniformity. Recognising their strengths and limitations helps select the right impeller for viscosity, shear sensitivity, and mixing objectives (blending, suspension, or gas dispersion).Given Data / Assumptions:Flat or slightly pitched paddles mounted on a central shaft.Baffled tank for effective mixing.Service: blending and mild heat transfer in low-viscosity liquids.Concept / Approach:Paddle agitators run at moderate speeds and generate predominantly radial and tangential flow patterns in laminar to transitional regimes. They are best for low-viscosity blending and maintaining homogeneity, not for strong axial pumping, deep-circulation solids suspension, or gas dispersion. High-shear applications require rotor–stator or Rushton-type impellers; axial-flow pitched-blade or hydrofoils are chosen when strong top-to-bottom circulation is needed.Step-by-Step Solution:Match flat-blade paddle geometry to gentle, low-viscosity blending.Eliminate options implying high shear or strong axial pumping.Select option (a).Verification / Alternative check:Mixing guides specify paddles for blending at Re ranges where laminar/transitional correlations apply; axial-flow impellers are specified when bulk circulation is required.Why Other Options Are Wrong:(b) Axial flow is characteristic of pitched/hydrofoil impellers.(c) Paddles do not run at very high tip speeds nor produce intense shear.(e) Gas–liquid dispersion uses Rushton or high-shear designs, not simple paddles.Common Pitfalls:Using paddles in high-viscosity service without close-clearance anchors or helical ribbons; choose geometry appropriate to viscosity.Final Answer:It is suitable for mixing low-viscosity liquids (laminar to transitional flow).

Question 20

In mineral processing, which equipment is classified as a coarse crusher because it handles large feed sizes and performs primary crushing before secondary reduction stages?

Accepted Answer

Introduction / Context:The first stage of comminution in mining and process engineering is primary crushing. A coarse crusher must accept very large lumps, reduce them to manageable sizes, and deliver steady throughput to downstream secondary crushers or mills.Given Data / Assumptions:The question asks for a “coarse crusher.”We compare typical duties of common crushers and mills.No numerical calculation is required; this is a machinery selection concept.Concept / Approach:Primary (coarse) crushers include jaw and gyratory crushers. They are designed for high capacity and large feed openings. Secondary and tertiary crushers or grinding mills (rolls, ball, tube) work on already-reduced feed and are not considered coarse crushers in standard flowsheets.Step-by-Step Solution:List the options and map them to duty: gyratory (primary), roll crushers (secondary/tertiary), tube and ball mills (grinding, not crushing).Confirm gyratory crushers accept the largest feed and deliver coarse product.Select gyratory crusher as the coarse crusher.Verification / Alternative check:Classic plant layouts show a primary gyratory or jaw ahead of secondary cone/rolls and ball or SAG mills; this confirms the classification.Why Other Options Are Wrong:Smooth roll: mainly secondary, produces finer, controlled product.Toothed roll: medium-duty primary for soft coal, but not general-purpose coarse crusher for hard rock.Tube/Ball mill: grinding devices, not crushers.Common Pitfalls:Confusing “primary” for coal (where toothed rolls may be used) with hard-ore practice, where gyratory dominates.Final Answer:Gyratory crusher

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