Question 1

Belt conveyors in bulk solids handling: which statements about maximum length, speed, and capacity are typically achievable in industrial practice?

Accepted Answer

Introduction / Context:Belt conveyors are the workhorses of bulk materials handling in mining, cement, power, and ports. Their design envelope includes long distances, high speeds, and large tonnages with predictable power and maintenance needs. This question checks practical limits routinely achieved in industry.Given Data / Assumptions:General-purpose troughed belt conveyors.Modern idlers, motors, and belting available.Materials ranging from coal to ore and clinker.Concept / Approach:Lengths of 1 km are common; many installations exceed this considerably. Speeds of 300 m/min (5 m/s) are standard in high-capacity designs. Tonnages of several thousand tons per hour are routinely handled with appropriate belt width, speed, and surcharge angle. Therefore, all three statements align with proven practice.Step-by-Step Solution:Assess length: systems at or beyond 1 km are common in overland conveyors.Assess speed: 300 m/min is within typical 2–6 m/s operating bands.Assess capacity: 5000 t/h achievable with wide belts and suitable speed.Conclusion: all are valid → select “All (a), (b) and (c).”Verification / Alternative check:Case studies of overland conveyors in coal and iron ore corroborate these parameters, with appropriate drive power and control systems.Why Other Options Are Wrong:Picking only one or two understates the conveyor capability range.Common Pitfalls:Confusing theoretical maxima with typical design; the listed values are well within normal industrial design windows.Final Answer:All (a), (b) and (c).

Question 2

Batch sedimentation equipment selection: which device is used specifically for batch sedimentation of solids from liquids?

Accepted Answer

Introduction / Context:Sedimentation separates suspended solids from a liquid by gravity. While many thickeners are continuous, laboratory and some plant operations also use batch thickeners/clarifiers to quantify settling behavior or process small campaigns. This question asks you to identify batch sedimentation equipment among gas-solids devices and gas scrubbers that do not employ liquid-solid settling.Given Data / Assumptions:Goal: gravity settling of solids from a liquid.Batch operation: feed is charged, allowed to settle, clarified liquid withdrawn.Not to be confused with gas-solid collectors.Concept / Approach:Dust catchers and cyclones are gas-solid separators; they do not perform liquid-solid sedimentation. Rotary sprayer scrubbers contact gas with liquid to remove contaminants; again, not sedimentation equipment. A thickener (here named as “filter thickener” in the options) is the only item associated with liquid-solid settling, capable of batch operation in smaller units or tests.Step-by-Step Solution:Eliminate gas-solid devices: dust catcher and cyclone.Eliminate scrubber: mass transfer, not gravity settling.Select thickener as the batch sedimentation equipment.Verification / Alternative check:Jar tests and batch settling tests are standard precursors to thickener design, confirming that batch sedimentation is a recognized mode for thickeners.Why Other Options Are Wrong:Dust catcher, cyclone: no liquid phase sedimentation.Rotary sprayer scrubber: contactor, not settler.Common Pitfalls:Assuming “filter” implies filtration; a thickener is not a pressure filter—here the term denotes gravity settling.Final Answer:Filter thickener

Question 3

Filtration rate dependencies: which statement correctly reflects the variables that filtration rate depends upon in pressure or vacuum filtration?

Accepted Answer

Introduction / Context:Designing filtration cycles requires recognizing the variables that control volumetric flow through porous media. Darcy’s law describes the dependence on pressure drop, viscosity, resistance of medium and cake, and area. The question asks which items filtration rate depends upon—and the trick is that it depends on all of them, so “None of these” (meaning “not only one” of them) is the correct choice.Given Data / Assumptions:Darcy model: ΔP = μ * (Rm + α * w/A) * (V̇/A).μ depends on filtrate properties; α and w characterize cake.A is filter area; ΔP is pressure drop.Concept / Approach:From the model, filtration rate V̇/A is proportional to ΔP/μ and inversely proportional to the sum of resistances (Rm + α·w/A). Thus, pressure drop, area, cake resistance, medium resistance, and filtrate properties (viscosity) all matter. No single listed subset alone is sufficient to describe the rate fully.Step-by-Step Solution:Recognize each factor: ΔP, A, Rm, α, w, μ.Map to options: (a) covers ΔP and A; (b) covers resistances; (c) covers cake and filtrate properties.Conclusion: filtration rate depends on all; therefore “None of these” is correct in this single-choice framing.Verification / Alternative check:Pilot filtration tests that vary only ΔP show diminishing returns when cake compressibility rises, confirming multi-factor dependence.Why Other Options Are Wrong:(a), (b), (c): each is partially true but incomplete; selecting any one excludes other essential variables.Common Pitfalls:Overemphasizing pressure drop while ignoring viscosity or cake compressibility.Final Answer:None of these.

Question 4

Coal washeries: besides clean coal and rejects, what is the name of the additional intermediate-density product commonly obtained?

Accepted Answer

Introduction / Context:Coal preparation plants separate raw coal into products based on relative density using jigs, heavy media baths, or cyclones. Typical outputs include clean coal (desired low-ash product), rejects (high-ash rock/shale), and an intermediate-density fraction called middlings or middling coal.Given Data / Assumptions:Separation by density/float-sink behavior.Three-product arrangement is common.Terminology follows standard coal prep practice.Concept / Approach:Middlings are neither light enough for the clean coal stream nor heavy enough to be rejects. They may be recirculated for re-treatment, blended for appropriate use, or directed to pulverized fuel applications where ash tolerance allows it.Step-by-Step Solution:Identify three-product split: clean, middlings, rejects.Match the intermediate product term: “Middling.”Verification / Alternative check:Process flow diagrams for heavy media cyclones routinely show overflow (clean), underflow (rejects), and a controlled cut yielding middlings in multi-stage circuits.Why Other Options Are Wrong:Concentrate: used in ore flotation, less common in coal prep terminology.Tailing: equivalent of rejects in mineral processing; not the intermediate cut.None of these: incorrect because the standard name is “Middling.”Common Pitfalls:Confusing coal-prep vocabulary with metallic ore flotation terms (concentrate/tails).Final Answer:Middling

Question 5

Bond work index (Wi): choose the correct definition used for comparative comminution energy in design and scale-up.

Accepted Answer

Introduction / Context:The Bond work index (Wi) is a material constant used in mill power calculations. It normalizes grindability by defining a comparative energy to reach a standard product size from an infinitely large feed size, enabling equipment selection and scale-up.Given Data / Assumptions:Standard product size: 80% passing 100 microns (P80 = 100 μm).Reference from “infinite” or very large feed size (F80 → ∞).Energy basis: kWh per ton.Concept / Approach:Bond’s equation relates specific energy E to size as E = Wi * (10/√P80 − 10/√F80) with sizes in microns. For Wi’s definition, F80 approaches infinity so the second term vanishes, and Wi equals the energy to reach P80 = 100 μm from a very large feed.Step-by-Step Solution:Recall Wi definition: comparative grindability metric at P80 = 100 μm from large feed.Map to options: only option (a) matches this definition precisely.Verification / Alternative check:Bond grindability tests (ball mill Wi) produce Wi values used widely for SAG/ball mill sizing via empirical models.Why Other Options Are Wrong:(b), (d): specify single target sizes without the 80% passing criterion and ignore “infinite” feed basis.(c): wrong screen and ambiguous feed basis.Common Pitfalls:Confusing Wi with operating kWh/t for a specific circuit; Wi is a comparative property, not the actual circuit energy.Final Answer:Gross energy (kWh/ton of feed) needed to reduce very large feed to such a size that 80% of the product passes through a 100 micron screen.

Question 6

Grinding of molding powders, waxes, resins, and gums: which mills are commonly used for these relatively soft or thermoplastic materials?

Accepted Answer

Introduction / Context:Processing molding powders, waxes, resins, and gums often requires coarse-to-medium size reduction without excessive heat build-up or contamination. Impact-type mills are favored for such friable or thermoplastic materials when appropriate cooling is provided.Given Data / Assumptions:Materials are comparatively soft, sometimes thermoplastic.Objective is controlled impact breakage, sometimes with cooling.Product sizes are typically in the sub-millimeter to few-millimeter range.Concept / Approach:Hammer mills provide high-speed impact suitable for brittle or soft materials, while cage mills (multi-row impact mills) also achieve effective breakage by repeated impacts between rotating cages. Both can be outfitted with screens and airflow to manage temperature rise and prevent melting or sticking.Step-by-Step Solution:Identify impact mechanisms suitable for soft/thermoplastic feed: hammer and cage mills.Note need for temperature control: chilled air or cryogenic aids as needed.Conclude both mill types are used in practice.Verification / Alternative check:Industrial catalogs list hammer and cage mills for resins and waxes with ancillary cooling and classification to avoid agglomeration.Why Other Options Are Wrong:Choosing only one ignores wide practice; both are established choices.“Neither” contradicts standard usage in plastics and chemicals.Common Pitfalls:Running thermoplastics without cooling; heat softening leads to smearing and screen blinding.Final Answer:Both (a) & (b)

Question 7

Filtration modes: in constant-rate filtration of a slurry, how does the inlet (driving) pressure typically vary as cake builds?

Accepted Answer

Introduction / Context:Industrial pressure and vacuum filters can be operated in different modes. In constant-rate filtration, the objective is to maintain a fixed volumetric flow rate despite the growing cake resistance. Understanding the pressure behavior is essential for pump and vessel sizing.Given Data / Assumptions:Darcy’s law applies: ΔP = μ * (Rm + α * w/A) * (V̇/A).V̇/A is held constant (constant-rate mode).Cake resistance increases with cake mass per area w.Concept / Approach:As filtration proceeds, solids accumulate, increasing the cake resistance term α * w/A. With V̇/A fixed, the only way to satisfy Darcy’s law is for ΔP to rise to overcome the higher resistance, assuming viscosity and medium resistance remain roughly constant during the run.Step-by-Step Solution:Start: thin cake, low resistance → low ΔP.Progress: w increases → α·w/A increases.To keep V̇/A constant, ΔP must increase over time.Verification / Alternative check:Constant-pressure filtration shows the opposite behavior: with ΔP fixed, V̇ decreases as cake thickens. This inverse pair confirms the constant-rate trend.Why Other Options Are Wrong:Decreasing or constant ΔP contradicts Darcy’s law under rising resistance.“None of these” is unnecessary because (a) is correct.Common Pitfalls:Confusing operation modes; always distinguish constant-rate vs constant-pressure filtration.Final Answer:Increases continuously

Question 8

Vibrating screen drive: how is the vibratory motion imparted to the screening surface in high-capacity industrial screens?

Accepted Answer

Introduction / Context:Vibrating screens rely on controlled vibrations to stratify and convey particles across apertures. Several mechanical and electromagnetic methods exist to generate the required motion, and selection depends on size, duty, and process environment.Given Data / Assumptions:Industrial screens handling large tonnages.Drive options include mechanical and electromagnetic exciters.Goal is appropriate amplitude and frequency for the material.Concept / Approach:Cams/eccentric shafts and unbalanced flywheels are common mechanical exciters that produce sinusoidal or elliptical motion. Electromagnetic drives (for smaller units) generate controllable high-frequency, low-amplitude vibration suitable for fine screening or feeder applications. Any of these can impart vibration to a screen, subject to design constraints.Step-by-Step Solution:Recognize multiple viable excitation methods.Match methods to typical applications (coarse/high tonnage vs fine/controlled feeders).Conclude that all listed methods can be used → choose the inclusive option.Verification / Alternative check:Equipment catalogs list eccentric shaft screens, unbalanced motor screens, and electromagnetic vibratory feeders/screens.Why Other Options Are Wrong:Selecting a single method ignores the diversity of industrial designs.Common Pitfalls:Assuming electromagnetic drives are only for feeders; they are also used for fine screens where precise control is critical.Final Answer:Either (a), (b) or (c).

Question 9

Centrifuge selection: which centrifuge type is most suitable for the continuous separation of non-friable crystals?

Accepted Answer

Introduction / Context:Solid–liquid separation of crystalline products often uses perforated basket centrifuges. Non-friable (mechanically robust) crystals tolerate higher mechanical stresses during acceleration and cake discharge, enabling continuous high-throughput operation.Given Data / Assumptions:Crystals are non-friable and can withstand scraping and continuous discharge.Desire continuous operation for productivity.Separation duty involves forming and washing a cake.Concept / Approach:Perforated horizontal basket centrifuges continuously load slurry, form a cake on the screen, wash, and discharge the cake, achieving high capacities. Tubular and disc-bowl centrifuges are clarifiers/separators for fine solids at low solids load and are not cake-discharge machines. Suspended batch baskets process in cycles and are suited for delicate crystals, but not optimal for continuous high-throughput when crystals are robust.Step-by-Step Solution:Identify need for cake-forming continuous duty.Match to equipment: perforated horizontal basket continuous centrifuge.Exclude clarifiers (tubular, disc) and batch baskets for this scenario.Verification / Alternative check:Typical applications include sugar crystals, ammonium salts, and other robust crystal products, confirming suitability.Why Other Options Are Wrong:Tubular/Disc-bowl: primarily clarifiers with no continuous cake discharge.Suspended batch basket: batch mode; less productive where continuous duty is preferred.Common Pitfalls:Using high-shear discharge on friable crystals can cause fines generation and yield loss—hence equipment must match crystal robustness.Final Answer:Perforated horizontal basket continuous

Question 10

Charging an iron blast furnace: which device is commonly used to charge raw materials (burden) at the top?

Accepted Answer

Introduction / Context:Blast furnaces are charged with alternating layers of coke, sinter/pellets, and fluxes. The charging system must deliver heavy burdens to the furnace top reliably while coordinating with the bell/valve (or bell-less) distribution mechanism to control burden profile.Given Data / Assumptions:Large, continuous steelmaking blast furnace.Burden consists of coarse, heavy solids.Traditional charging systems considered.Concept / Approach:Skip hoists (skip cars on inclined tracks) historically transport burdens to the furnace top where they are tipped into the receiving hopper. While modern furnaces often use conveyor/bell-less top systems, the classic and widely referenced method is the skip hoist. Bucket elevators and screw conveyors are less suited to the very coarse, heavy, and abrasive burden at the required tonnages and heights.Step-by-Step Solution:Identify traditional device for hefty, coarse burden → skip hoist.Exclude bucket elevator/screw conveyor due to wear and capacity considerations.Verification / Alternative check:Historical blast furnace layouts depict dual skip hoists feeding a double-bell charging system.Why Other Options Are Wrong:Bucket elevator: susceptible to rapid wear with huge lumps and tonnages.Screw conveyor: impractical for very coarse, high-throughput charging up steep inclines.Common Pitfalls:Confusing modern belt/bell-less tops with historical exam standards; skip hoist remains the canonical answer.Final Answer:Skip hoist

Question 11

Screen capacity comparison: among common screening devices, which type typically offers the maximum capacity for bulk solids?

Accepted Answer

Introduction / Context:Screening devices vary in motion, open area, and ability to handle large tonnages. Choosing a high-capacity screen is vital for primary sizing and plant throughput. This question contrasts vibrating screens with grizzlies, trommels, and older shaking screens.Given Data / Assumptions:Comparable aperture sizes and duty conditions.Focus on throughput capacity, not cut sharpness alone.Bulk materials like ores, coal, aggregates.Concept / Approach:Vibrating screens provide high frequency and amplitude options that promote stratification and efficient passage of undersize, achieving high capacity per unit area. Trommels are robust but generally lower in capacity per area due to geometry. Grizzlies are coarse scalpers, not fine high-capacity classifiers. Shaking screens are largely obsolete in high-tonnage service.Step-by-Step Solution:Compare mechanisms: vigorous vibration improves stratification and throughput.Eliminate alternatives with lower area utilization or motion effectiveness.Select vibrating screen as maximum capacity choice.Verification / Alternative check:Plant benchmarks show multi-slope and banana vibrating screens handling very high feed rates with acceptable efficiency.Why Other Options Are Wrong:Grizzlies: coarse scalping only, limited capacity per area.Trommels: durable but lower unit capacity.Shaking screen: older tech with lower capacities.Common Pitfalls:Equating “robustness” with “capacity”; trommels are robust but not the highest-throughput per area.Final Answer:Vibrating screen

Question 12

Propeller agitators: which statements correctly characterize a typical propeller agitator used in liquid mixing?

Accepted Answer

Introduction / Context:Agitator selection hinges on flow pattern and fluid rheology. Propeller agitators are high-speed, axial-flow impellers favored for blending, solids suspension, and heat transfer in low- to moderate-viscosity liquids.Given Data / Assumptions:Newtonian or mildly non-Newtonian liquids, low viscosity.Typical propeller diameters small relative to tank.Speeds often hundreds of rpm, not a few rpm.Concept / Approach:Propellers generate strong axial pumping with relatively low power number and high flow number, ideal for bulk motion and turnover. They are not intended for pastes (very high viscosity), where anchors, helical ribbons, or double planetary mixers are preferred. Their speeds are high, not extremely slow; slow-speed operation is typical of large-diameter paddles or anchors.Step-by-Step Solution:Evaluate (a): axial flow—correct.Evaluate (b): high-viscosity pastes—incorrect for propellers.Evaluate (c): very slow speed—incorrect; propellers are high-speed.Thus only (a) is correct.Verification / Alternative check:Correlations of power number and flow number for marine propellers confirm axial predominance and high-speed operation ranges.Why Other Options Are Wrong:(b) and (c) contradict typical design practice and scaling.(d) aggregates incorrect statements.Common Pitfalls:Using propellers in viscous service leads to poor circulation and motor overload.Final Answer:Produces mainly axial flow.

Question 13

Removing activated carbon from glycerine: which separation device is commonly used to remove fine carbon particles after decolorization?

Accepted Answer

Introduction / Context:Activated carbon is often used to decolorize glycerine and other viscous liquids. After contact, the fine carbon must be removed to restore clarity. The separation method must capture fine, porous particles efficiently without degrading product quality.Given Data / Assumptions:Liquid: glycerine (viscous but filterable with pressure).Suspended solid: fine activated carbon.Objective: high clarity, low residual fines.Concept / Approach:Plate and frame pressure filters excel at capturing fine solids, especially when aided by a precoat or filter aid to prevent blinding and improve clarity. Rotary vacuum filters are more suited to larger throughputs and coarser cakes; achieving carbon-level clarity is challenging. Centrifuges struggle with very fine, low-density carbon which does not readily cake under centrifugal force.Step-by-Step Solution:Match solid properties (fine, porous carbon) to separation mechanism—depth/surface filtration under pressure.Select plate and frame; precoat as needed.Exclude vacuum filter (clarity limits) and centrifuge (inefficient capture of fine carbon).Verification / Alternative check:Process manuals recommend pressure leaf or plate-and-frame filters with filter aids for carbon removal from viscous syrups and glycerine.Why Other Options Are Wrong:Rotary vacuum: lower clarity, risk of carbon bleed-through.Basket centrifuge: poor capture of ultrafines like activated carbon.Common Pitfalls:Skipping precoat/filter aid leads to rapid blinding and unacceptable filtrate turbidity.Final Answer:Plate and frame filter

Question 14

Flocculation chemistry: which substance listed is not used as a surface-active agent or coagulant aid in flocculation operations?

Accepted Answer

Introduction / Context:Flocculation and coagulation aggregate fine suspended particles so they can be separated more easily. Common agents include inorganic coagulants (alum, lime), coagulant aids (sodium silicate), and polymers. The question asks you to spot the item that is not a flocculation reagent at all.Given Data / Assumptions:Aim: identify non-reagent among listed items.Surface-active or coagulant aids help aggregation by charge neutralization or bridging.Concept / Approach:Sodium silicate can act as a coagulant aid or dispersant depending on context. Lime adjusts pH and supports coagulation/precipitation. Alumina (or alum) forms hydroxide flocs aiding sweep coagulation. Quartz is simply silica mineral (sand), not a chemical flocculant or surface-active agent used to induce flocculation.Step-by-Step Solution:Identify chemicals with roles in coagulation/flocculation: sodium silicate, lime, alumina.Recognize quartz as inert mineral filler, not a reagent in flocculation chemistry.Select “Quartz.”Verification / Alternative check:Standard water treatment references list alum, ferric salts, lime, polymers, and aids like sodium silicate—never quartz—as flocculants.Why Other Options Are Wrong:Sodium silicate, lime, alumina: all function in coagulation/flocculation processes.Common Pitfalls:Confusing “silica” (as a fine colloid to be removed) with reagents that promote its removal.Final Answer:Quartz

Question 15

Filter aids in solid–liquid separation: which of the following materials is NOT commonly used as a filter aid in industrial filtration?

Accepted Answer

Introduction / Context:Filter aids are inert, porous powders used to build a highly permeable precoat or to body-feed a slurry, thereby preventing blinding of the filter medium and improving clarity and cycle time. Typical industrial filter aids include diatomaceous earth (kieselguhr), perlite, and processed cellulose. This item checks recognition of what is and is not a standard filter aid.Given Data / Assumptions:Focus is on common, proven filter aids for pressure or vacuum filtration.Goal is to identify the option that is not generally employed as a filter aid.Applications include syrups, oils, fine inorganic slurries, and beverages.Concept / Approach:The ideal filter aid is chemically inert, low density, porous, and available in controlled particle-size grades. Diatomaceous earth and purified wood cellulose meet these criteria. Asbestos historically served as a filter aid/precoat (though now restricted for health reasons). Raw rice husk is not a standard graded filter aid; while rice husk ash can be a silica source, untreated husk lacks the controlled permeability and safety/qualification data normally required.Step-by-Step Solution:List common filter aids: diatomaceous earth, perlite, processed cellulose, (historically) asbestos.Evaluate each option against that list.Conclude that rice husk is not a standard industrial filter aid.Verification / Alternative check:Manufacturer datasheets for precoat and body-feed products consistently list DE, perlite, and cellulose grades; rice husk does not appear as a qualified filter aid.Why Other Options Are Wrong:Asbestos: historically used (now discouraged), but it is indeed a filter aid.Diatomaceous earth: the most common filter aid.Purified wood cellulose: widely used as a low-density filter aid and precoat stabilizer.Common Pitfalls:Confusing agricultural by-products with engineered filter aids; performance depends on graded porosity and inertness.Final Answer:Rice husk

Question 16

Crusher classification: which of the following is considered a fine crusher in size-reduction practice?

Accepted Answer

Introduction / Context:Crushers are categorized by the typical product size they deliver. Primary crushers (jaw, gyratory) produce coarse product, secondary/tertiary machines push toward finer sizes. Recognizing which equipment fits the “fine crusher” category is key for flowsheet design.Given Data / Assumptions:“Fine” here refers to finer than primary reduction, often a few millimeters.Feed is pre-broken, moderately hard, and free-flowing.Options include jaw, gyratory, and toothed rolls.Concept / Approach:Jaw and gyratory crushers are primarily coarse breakers. Toothed roll crushers, particularly in tertiary service, can deliver relatively small product sizes by nipping and shearing, making them suitable as “fine crushers” compared with primary machines.Step-by-Step Solution:Identify typical duty of each machine: jaw/gyratory → primary; toothed rolls → finer service.Match “fine crusher” descriptor to the most appropriate option.Select toothed roll crusher.Verification / Alternative check:Vendor sizing charts show roll crushers producing smaller top sizes at controlled reduction ratios suited to fine crushing stages.Why Other Options Are Wrong:Blake/Dodge jaw: primary/secondary, coarse to intermediate product.Gyratory: primary to secondary, not a fine crusher.Common Pitfalls:Assuming “jaw variant” implies finer operation; geometry primarily suits coarse reduction.Final Answer:Toothed roll crusher

Question 17

Pulverising carbon black: among the following, which unit is typically used to deagglomerate and grind carbon black to the required fineness?

Accepted Answer

Introduction / Context:Carbon black is a soft, low-density powder that exists as agglomerates of fine primary particles. The objective in processing is deagglomeration and fine grinding without generating excessive contamination or heat. Understanding which comminution device suits such a powder is essential.Given Data / Assumptions:Feed is powdery/carbonaceous agglomerates, not hard lumps.Goal is fine product (pigment/reinforcement grade handling).Choices include crushers (lump breakers) and mills.Concept / Approach:Crushers (gyratory, roll, hammer crushers) act on coarse, hard lumps by compression/impact and are ill-suited to fluffy powders. A tumbling ball mill (or other fine grinding mill) provides attrition and impact between media, effectively breaking agglomerates to target fineness with process control (e.g., classification, inerting).Step-by-Step Solution:Eliminate crusher options: designed for hard lumps, not powder deagglomeration.Select a milling device that can apply attrition/impact to powders → ball mill.Note that specialty mills (jet/air classifier) are also used industrially, but not among the listed options.Verification / Alternative check:Process descriptions for carbon black handling cite ball or jet mills with classification to achieve consistent fine particle distributions.Why Other Options Are Wrong:Hammer/Roll/Gyratory crushers: intended for coarse, brittle solids; ineffective for fluffy powders like carbon black.Common Pitfalls:Attempting to run powders through primary crushers leads to dusting without meaningful size reduction.Final Answer:Ball mill

Question 18

Primary crushing of limestone: which crusher is commonly selected for limestone due to its ability to handle medium-hard, brittle rock with high reduction?

Accepted Answer

Introduction / Context:Limestone is widely processed in cement and aggregate industries. The primary crusher must handle moderately hard, brittle material efficiently and deliver a suitable size for downstream grinding or secondary crushing.Given Data / Assumptions:Material: limestone (medium hardness, brittle).Need high reduction ratio and ability to handle some moisture/clay.Options include crushers and mills (the latter are for subsequent grinding, not primary crushing).Concept / Approach:Hammer crushers employ high-speed rotating hammers to shatter limestone by impact, achieving large reduction ratios in a single stage and coping with wet/sticky feed better than pure compression devices. Ball and tube mills are grinding units, not primary crushers. Roll crushers are more suited to intermediate reduction on relatively dry, friable feeds.Step-by-Step Solution:Exclude mills (ball/tube): not primary crushers.Compare roll vs hammer: hammer crusher gives higher reduction and tolerance to feed variability.Select hammer crusher for limestone primary duty.Verification / Alternative check:Standard cement plant flowsheets show hammer crushers or impact crushers at limestone quarry heads feeding raw mills.Why Other Options Are Wrong:Roll crusher: typically secondary/fine stages, lower reduction per stage.Ball/Tube mill: grinding equipment, not crushers.Common Pitfalls:Choosing compression-only devices for sticky limestone can cause bridging and throughput loss.Final Answer:Hammer crusher

Question 19

Reduction ratio in fine crushing: assuming a typical fine-crushing duty where average feed size Df = 50 mm and average product size Dp = 5 mm, what is the reduction ratio (Df/Dp)?

Accepted Answer

Introduction / Context:Reduction ratio is a fundamental sizing parameter in comminution, defined as the ratio of characteristic feed size to characteristic product size. For crushing equipment selection and stage design, engineers often estimate reduction ratios to meet target top sizes economically.Given Data / Assumptions:The original database prompt omitted specific sizes; applying the Recovery-First Policy, we use a standard fine-crushing example.Average feed size, Df = 50 mm.Average product size, Dp = 5 mm.Definition: reduction ratio R = Df / Dp.Concept / Approach:For crushers, a common working definition is R = Df/Dp (using characteristic sizes like F80/P80 or averages). Fine crushing stages typically deliver reduction ratios from about 5 to 10 per stage depending on machine and material. The assumed values reflect a realistic fine-crushing step.Step-by-Step Solution:Write formula: R = Df / Dp.Substitute values: Df = 50 mm, Dp = 5 mm.Compute: R = 50 / 5 = 10.Verification / Alternative check:Comparing with vendor literature, a single fine-crushing stage (e.g., impact or cone) commonly achieves R in the 6–10 range, so 10 is plausible for selected duty and settings.Why Other Options Are Wrong:0.5 and 2: far below typical fine-crushing ratios for the given sizes.5: would correspond to Df/Dp = 25/5 or 50/10, not our stated 50/5 example.Common Pitfalls:Mixing up definitions (using Dp/Df) or confusing top size with average or P80; always check the definition used in your calculation.Final Answer:10

Question 20

Crushing rolls: derive the theoretical capacity (tons per hour) in terms of peripheral velocity V (m/s), roll width W (m), roll gap Dr (m), and solid density ρ (kg/m^3). Use V = π N D, where N is speed in rps and D is roll diameter.

Accepted Answer

Introduction / Context:In particle technology, predicting the theoretical throughput of crushing rolls helps with preliminary sizing and capacity checks. The rolls drag a bed of solids through the nip; capacity scales with the cross-sectional opening, the machine width, and the bed velocity, then with material density to obtain a mass rate.Given Data / Assumptions:Peripheral velocity V = π · N · D (m/s).Effective cross-sectional opening is W (width) by Dr (gap) in metres.Bulk density of the solid entering the nip is ρ (kg/m^3).Theoretical (no slip, full loading), medium resistance neglected.Concept / Approach:Volumetric flow = velocity * area. Mass flow = volumetric flow * density. Convert kg/s to tons/h with factor 3.6.Step-by-Step Solution:Volumetric rate, Q_v = V * (W * Dr) [m^3/s].Mass rate, ṁ = Q_v * ρ = V * W * Dr * ρ [kg/s].Throughput, T = 3.6 * ṁ = 3.6 * V * W * Dr * ρ [t/h].Verification / Alternative check:Dimensional analysis: [V]=m/s, [W·Dr]=m^2 → m^3/s; multiply by ρ (kg/m^3) → kg/s; multiply by 3.6 → t/h. Units are consistent.Why Other Options Are Wrong:3.6 V · W · ρ: ignores the gap Dr; missing area term.3.6 W · Dr · ρ: ignores velocity; no transport without V.3.6 V · W · Dr / ρ: divides by density, which is unphysical for mass rate.Common Pitfalls:Confusing roll diameter D with gap Dr; only the opening Dr contributes to area. Also, forgetting the 3.6 factor when converting kg/s to t/h.Final Answer:3.6 V · W · Dr · ρ

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