Question 1

Hammer crusher product top size control
In a hammer crusher, which combination of operating variables primarily governs the final product size distribution?

Accepted Answer

Introduction / Context: Hammer crushers (or hammer mills) are impactors widely used for intermediate to fine size reduction in minerals, cement rawmix, and biomass. Operators can shape the product size distribution (PSD) by adjusting several accessible variables. Recognising these variables and their coupled effects is central to stable operation and specification compliance. Given Data / Assumptions: A conventional hammer crusher with breaker plates (liners) and a grate or classifying section. Material is moderately hard and friable; machine is not overfed. Steady-state operation is assumed. Concept / Approach: Product top size and fines generation in a hammer crusher are controlled by impact energy per collision and residence time. Impact energy scales with rotor tip speed (rotor speed and diameter). The clearance between hammers and breaker plates alters the severity of secondary breakage and the effective classification. Feed rate influences residence time and bed depth, which in turn affects the number of impacts a particle receives before exiting. Consequently, all three variables act together to define the PSD; changing any one often requires compensating changes in others to maintain the same product target. Step-by-Step Solution: Recognise product size depends on impact severity and residence time.Impact severity ↔ rotor speed; secondary breakage ↔ hammer/liner clearance.Residence time ↔ feed rate (and grate condition).Therefore, the correct choice is that all three variables together govern the product size. Verification / Alternative check: OEM manuals specify rotor speed ranges and breaker plate settings for target top sizes, with de-rating limits on feed rate to avoid oversize leakage or excessive fines. Why Other Options Are Wrong: Single-variable answers ignore the coupled nature of impactors. Moisture content affects behavior but is not the primary knob for top size once within a normal operating window. Common Pitfalls: Increasing rotor speed alone to fix oversize, which often raises fines excessively; always balance speed with clearance and feed rate. Final Answer: All of the above together (feed rate, rotor speed, and clearance)

Question 2

Standard sieve conversion (200 mesh)
On the standard sieve scale used in process engineering, a 200-mesh sieve corresponds to approximately how many micrometres (µm)?

Accepted Answer

Introduction / Context: Mesh numbers are ubiquitous in lab work (sieving), quality control, and equipment specification. Converting between mesh and metric opening (micrometres) is essential for comparing to particle size analyzer outputs or setting cut points for classifiers and filters. Given Data / Assumptions: Standard 200 mesh means roughly 200 openings per linear inch in the sieve cloth. Use of standard tables that account for wire diameter and series (Tyler/ASTM), giving 200 mesh ≈ 74 µm nominal opening. Concept / Approach: While a crude inverse of count per inch (25.4 mm/in divided by 200) gives 127 µm, standard sieves use specific wire diameters that reduce the open space. Thus, the accepted nominal aperture for 200 mesh in standard practice is 74 µm. This value is widely used for defining fine product specifications (e.g., “80% passing 200 mesh”). Step-by-Step Solution: Recall standard value: 200 mesh ≈ 74 µm.Compare options and select 74. Verification / Alternative check: Handbooks and ASTM E11 tables list 200 mesh openings at approximately 0.074 mm (that is 74 µm), confirming the selection. Why Other Options Are Wrong: 24 µm and 47 µm: too fine for 200 mesh; nearer to 500–325 mesh apertures. 154 µm or 200 µm: too coarse; align with meshes closer to 100 and 70. Common Pitfalls: Ignoring the role of wire diameter and relying only on 25.4/mm count; always consult standardized tables for nominal openings. Final Answer: 74

Question 3

Most widely used grinder for grains, spices, pigments, sawdust, cork
For comminuting cereals, grains, spices, pigments, saw dust and similar materials at industrial scale, which size-reduction equipment is most extensively used?

Accepted Answer

Introduction / Context: Food and specialty chemical operations frequently need versatile grinding of a wide variety of organic materials—from cereals and grains to spices, pigments, wood-derived products, and cork. The right equipment balances throughput, maintenance, heat generation, and product uniformity. Given Data / Assumptions: Materials are generally brittle to moderately tough and not exceptionally abrasive. Target sizes span coarse to moderately fine (hundreds to a few tens of micrometres). Temperature rise should be controlled, and screens should be easily changed for different products. Concept / Approach: Hammer mills are high-speed impactors that use swinging hammers and replaceable screens to control top size. They are rugged, accept varied feeds, and changeover quickly, making them the standard workhorse for the listed materials. Buhrstone and attrition mills have niche roles, particularly for gentle flour milling, while ball mills are better for mineral or paint base grinding where residence time and contamination control differ. Crushing rolls are for relatively coarse, uniform reduction of friable solids and do not handle fibrous or elastic organics as well as hammer mills. Step-by-Step Solution: Match material list to typical hammer-mill service (impact-friendly, screen-controlled).Recognise flexibility and ease of maintenance/changeover.Select “Hammer mill.” Verification / Alternative check: Industry catalogs for feed, spice, and pigment processing consistently position hammer mills as the most common choice due to cost-effectiveness and robustness. Why Other Options Are Wrong: Buhrstone/attrition: lower throughput, more specific applications. Ball mill: slower, wet/closed-circuit oriented; not primary for listed organics. Crushing rolls: limited to coarse friable solids; poor with fibrous materials. Jet mill: ultrafines with higher cost; unnecessary for general grain/spice milling. Common Pitfalls: Overheating aromatic spices—use cryogenic assists or airflow through hammer mills to control temperature. Final Answer: Hammer mill

Question 4

Definition of screen efficiency in particle sizing
In industrial screening, “screen efficiency” is best described as which of the following?

Accepted Answer

Introduction / Context: Screen efficiency is a key performance metric for sizing operations in mining, aggregates, and chemicals. While different texts present slightly different forms (sometimes defining “effectiveness” using both streams), the most common, practical definition of efficiency focuses on the recovery of the desired size class to its designated product stream. Given Data / Assumptions: A single-deck screen separates undersize (through) and oversize (retained). “Undersize” is the material smaller than the screen aperture. Concept / Approach: Efficiency may be defined for either stream. The most often used is the undersize recovery: the mass of true undersize in the undersize product divided by the total mass of true undersize in the feed, expressed as a percentage. This captures the screen’s ability to pass the material that should pass. A complementary metric is oversize recovery to oversize (rejection), and a combined measure sometimes called “effectiveness” considers both simultaneously. However, the question asks specifically for “screen efficiency,” for which the undersize recovery definition is the standard in many QA calculations. Step-by-Step Solution: Identify the desired stream: undersize to undersize product.Define efficiency = (mass of true undersize in undersize product) / (mass of true undersize in feed) * 100%.Select the option that states this recovery definition. Verification / Alternative check: Plant data sheets and vendor performance guarantees commonly specify minimum undersize recovery (efficiency) at given feed conditions and apertures. Why Other Options Are Wrong: Rejection concerns the oversize stream; useful but not the usual “efficiency” in QA sheets. Recovery/rejection or averages are not standard definitions and can mislead. “None” is incorrect because a clear, accepted definition exists. Common Pitfalls: Confusing “efficiency” with “effectiveness” (which blends both recoveries); always state which stream you are referencing. Final Answer: Recovery: percentage of undersize material correctly reporting to the undersize stream

Question 5

Support structure of plate-and-frame filter presses
In typical chemical plants, the main supporting legs/frame of a plate-and-frame filter press are generally constructed from which material?

Accepted Answer

Introduction / Context: Plate-and-frame filter presses remain workhorses in solid–liquid separation for chemicals, food, and minerals. Their structural frame must withstand large hydraulic closing forces while being economical and durable, prompting common material choices for frames and supporting legs. Given Data / Assumptions: The question refers to the main press frame/legs that bear mechanical load, not necessarily the wetted plates or cloths. Operating environments are typical industrial settings with basic corrosion control. Concept / Approach: Cast iron has historically been favored for the frames and supports of filter presses due to its high compressive strength, rigidity, good castability, and cost-effectiveness. While stainless steel is used for wetted components and special hygiene or corrosion services, the primary support structure in many standard installations remains cast iron or carbon steel. Wood is obsolete for structural frames in modern plants; high-speed steel is a tooling material, not a structural one; FRP lacks the stiffness for large closing loads in classic designs (though FRP can be used for corrosion-resistant components elsewhere). Step-by-Step Solution: Identify need: strong, rigid, economical support for high closing pressures.Match cast iron to these mechanical requirements and common practice.Select “Cast iron.” Verification / Alternative check: Legacy OEM catalogs and many installed units list “cast iron frame” or “cast steel frame” as standard offerings, with stainless optional for special-duty frames. Why Other Options Are Wrong: Stainless steel: used but less common for main frames due to cost unless corrosion dictates. High-speed steel: inappropriate for structural frames. Wood: not used for modern industrial frames. FRP: insufficient stiffness for large presses. Common Pitfalls: Assuming the material of filter cloth/plates is the same as the frame; wetted and structural materials are chosen independently. Final Answer: Cast iron

Question 6

Commercial-scale sedimentation equipment
Sedimentation of slurries on a commercial scale is most commonly carried out in which device?

Accepted Answer

Introduction / Context: Solid–liquid separations may be performed by gravitational settling (sedimentation) or by filtration. Large-volume clarification and concentration of slurries usually rely on sedimentation before polishing steps. Correctly identifying the principal equipment is vital for flowsheet development and cost estimates. Given Data / Assumptions: “Commercial scale” implies large throughputs and continuous operation. Feed is a slurry of solids in liquid, needing thickening/clarification. Concept / Approach: Thickeners (circular or rectangular) provide a large cross-sectional area for gravity settling, with rake mechanisms to move settled solids to a discharge cone and overflow launders to collect clarified liquid. They are the standard for concentrating slurries and recovering process water. Classifiers provide size-based separation with settling but are not primarily used for bulk thickening of mixed fines; cyclones impart centrifugal force and are not sedimentation devices per se; rotary drum vacuum filters are filtration devices, not settling tanks; decanters rely on centrifugal settling in a rotating bowl rather than gravity alone. Step-by-Step Solution: Identify the need: gravity settling at large scale.Match to equipment that offers large area and continuous operation → thickeners.Select “Thickeners.” Verification / Alternative check: Mineral processing and wastewater plants standardize on high-rate/high-density thickeners for this service; filters are placed downstream if needed. Why Other Options Are Wrong: Classifiers: size separation rather than bulk thickening. Vacuum filters: filtration, not sedimentation. Cyclones: centrifugal separation, not gravitational settling tanks. Common Pitfalls: Confusing clarification by sedimentation with dewatering by filtration; the former targets overflow clarity and underflow concentration via gravity. Final Answer: Thickeners

Question 7

Identify the non-screening device
Which of the following is NOT considered industrial screening equipment for size classification?

Accepted Answer

Introduction / Context: Screening separates particles primarily by geometric size using apertured surfaces. Many devices fall under this umbrella (static grizzlies, vibrating screens, trommels), but other separation devices operate on different principles and thus are not “screens.” Distinguishing them avoids specification errors. Given Data / Assumptions: Industrial minerals or aggregates application context. Terminology uses standard equipment names. Concept / Approach: A Sharpies centrifuge is a type of solid–liquid centrifuge used for fine particle separation from liquids via centrifugal force. It does not employ a perforated screening surface to classify dry or wet solids by size. In contrast, vibrating screens and banana screens are dynamic screens with apertures; grizzlies are stationary bar screens for coarse scalping; trommels are rotating cylindrical screens. Therefore, the centrifuge is the outlier and not an industrial screening device. Step-by-Step Solution: List which items employ apertured surfaces: vibrating screens, grizzlies, trommels, banana screens.Recognise that a Sharpies centrifuge performs centrifugal separation, not screening.Select “Sharpies centrifuge.” Verification / Alternative check: Process flow diagrams separate screening/sizing blocks from solid–liquid centrifugation blocks; vendors classify them into different product families. Why Other Options Are Wrong: The other listed devices are all aperture-based classifiers and thus are screening equipment. Common Pitfalls: Assuming any rotating cylindrical device is a trommel; only those with apertures for size-based separation qualify as screens. Final Answer: Sharpies centrifuge

Question 8

Reagents in froth flotation (pine oil, cresylic acid)
In froth flotation of minerals, pine oil and cresylic acid are commonly used as which type of reagent?

Accepted Answer

Introduction / Context: Froth flotation separates minerals via surface chemistry. Reagents fall into functional classes: collectors (render target mineral hydrophobic), frothers (stabilize froth), depressants (prevent undesirable minerals from floating), and modifiers/conditioners (adjust pH or dispersion). Knowing which reagent does what is foundational to circuit design and troubleshooting. Given Data / Assumptions: Standard sulfide and oxide flotation practice is implied. Pine oil and cresylic acid are classical frothing agents. Concept / Approach: Frothers, such as pine oil and cresylic acid (a phenolic mixture), reduce bubble coalescence and produce a stable froth with small, persistent bubbles, enhancing the transport of hydrophobic particles to the froth phase. Collectors (e.g., xanthates) change the mineral surface; depressants (e.g., sodium cyanide, starch) inhibit unwanted flotation; conditioners (e.g., lime, sodium silicate) adjust slurry chemistry. The listed reagents do not act primarily as collectors or depressants but as froth stabilizers. Step-by-Step Solution: Identify reagent function: pine oil/cresylic acid → froth stability and bubble size control.Map to reagent class → frothers.Select “Frother.” Verification / Alternative check: Reagent handbooks classify these substances under frothers; plant dosing systems often label the same storage as “frother tank.” Why Other Options Are Wrong: Collectors change surface hydrophobicity; not the role here. Depressants inhibit flotation of gangue; not applicable. Conditioners modify pH/dispersion; not the primary function of these oils. Common Pitfalls: Confusing commercial blend names that include frothers with collectors; always check MSDS and function. Final Answer: Frother

Question 9

Filter media for corrosive liquids
Which of the following is generally NOT preferred as a filter medium when handling corrosive liquids in chemical processing?

Accepted Answer

Introduction / Context: Filter media must resist chemical attack, retain mechanical integrity, and deliver required clarity. For corrosive services—strong acids/alkalis or oxidizers—material compatibility becomes the dominant selection criterion, superseding cost in many cases. Given Data / Assumptions: Corrosive liquids include strong mineral acids, strong bases, or oxidizing environments at process temperatures. Media are evaluated for chemical resistance and durability. Concept / Approach: Cotton is a natural cellulose fiber that degrades in strong acids and alkalis and can be oxidized, making it a poor choice for many corrosive services. In contrast, nylon (polyamide), glass cloth, and metallic meshes (Monel, stainless) exhibit superior resistance profiles in many corrosive conditions (selection depends on specific chemistry). PTFE felts are highly inert and a common premium option where compatibility is critical. Therefore, among the choices, cotton fabric is generally not preferred for corrosive duties. Step-by-Step Solution: Define “corrosive” and consider cellulose (cotton) stability.Compare to synthetics (nylon, PTFE), glass, and specialty metals.Conclude cotton is not preferred; select it as the answer. Verification / Alternative check: Chemical compatibility charts show rapid strength loss for cotton in strong acids/bases, while PTFE and certain metals remain serviceable. Why Other Options Are Wrong: Nylon/glass/Monel/SS/PTFE have known corrosion resistance niches and are routinely used based on chemistry and temperature. Common Pitfalls: Ignoring temperature: some polymers soften; always cross-check actual process conditions and fluid composition. Final Answer: Cotton fabric

Question 10

Apron conveyors — suitable duty
For which duty are apron conveyors most appropriately used in materials handling?

Accepted Answer

Introduction / Context: Conveyor selection is governed by load, distance, material characteristics, and environmental constraints. Apron conveyors (pan conveyors) consist of overlapping metal pans attached to chains, designed to move heavy, hot, or abrasive lumps where belts would suffer damage or excessive spillage. Given Data / Assumptions: Material is bulky, possibly hot or abrasive (e.g., clinker, sinter, ore lumps). Conveying distance is modest and speed is low to moderate. Concept / Approach: Apron conveyors excel at short-distance transfer of heavy loads, accepting large lumps without spillage and tolerating heat and impact. They are not economical for long runs due to chain wear and power consumption. Small loads or fine powders over long distances are better handled by belt, screw, or pneumatic systems. For long heavy runs, troughed belts or overland belts are typically preferred where feasible. Step-by-Step Solution: Match equipment construction (metal pans on chains) to rugged, heavy-duty service.Recognise economic limitation → best for short runs.Select “Heavy loads over short runs.” Verification / Alternative check: Cement and steel plants routinely use apron conveyors under crushers, coolers, and stockyard reclaimers for short transfers of hot, heavy materials. Why Other Options Are Wrong: Small loads/long runs: belts or pneumatics are more economical. Heavy loads/long runs: belts or rail-mounted systems are favored where possible. Powders at high speed: not an apron duty; spillage and wear are issues. Common Pitfalls: Overspecifying aprons where belts suffice; chains add significant maintenance costs over distance. Final Answer: Heavy loads over short runs

Question 11

Specific surface of spherical particles (per unit mass)
For a collection of identical spherical particles, what is the specific surface area (surface area per unit mass) in terms of particle diameter D and particle density rho?

Accepted Answer

Introduction / Context: Specific surface area (SSA) strongly influences reaction rates, dissolution, heat transfer, and filtration performance. For ideal spheres, SSA can be expressed analytically from geometry, offering a quick check on particle size analyses and mass–transfer estimates. Given Data / Assumptions: Particles are perfect spheres of uniform diameter D. Particle density is rho (mass per unit volume). SSA is defined as total surface area divided by total particle mass. Concept / Approach: For one sphere: surface area A = pi * D^2. Volume V = (pi/6) * D^3. Mass m = rho * V = rho * (pi/6) * D^3. Specific surface S = A / m = (pi * D^2) / (rho * (pi/6) * D^3) = 6 / (rho * D). This compact formula is widely used for monodisperse spheres and provides a consistency check; if D halves, SSA doubles for the same material. Step-by-Step Solution: Write A = pi * D^2, V = (pi/6) * D^3.Compute mass m = rho * V = rho * (pi/6) * D^3.Take S = A / m = (pi * D^2) / (rho * (pi/6) * D^3) = 6 / (rho * D).Select the option matching S = 6 / (D * rho). Verification / Alternative check: Dimensional check: D has length, rho has mass/volume, so S units are area/mass; 6/(D*rho) has (1/length) / (mass/volume) = area/mass → consistent. Why Other Options Are Wrong: 4/(D*rho) or 3/(D*rho): incorrect geometric factors. 6*D*rho: wrong dependence; SSA decreases with larger D. pi*D^2/rho: missing normalization by volume, not per mass. Common Pitfalls: Confusing specific surface per unit volume vs. per unit mass; always check definitions and units. Final Answer: S = 6 / (D * rho)

Question 12

Pick the correct statement: terminology and laws in size reduction
Which of the following statements is correct in standard particle technology terminology?

Accepted Answer

Introduction / Context: Process engineers must use terminology precisely—misstating a law or confusing size reduction vs. enlargement can derail calculations and vendor communications. This question probes three areas: material flow classification (cohesive vs. non-cohesive), the proper form of grinding laws, and the definition of the Bond Work Index (W_i). Given Data / Assumptions: “Plastic chips” are irregular, relatively large, and typically do not exhibit interparticle cohesion like powders. Kick’s law and Bond’s Work Index are as defined in standard texts. Concept / Approach: Non-cohesive solids are free-flowing and show little tendency to stick; chips and pellets normally qualify unless tackified. Comminution is the collective term for size reduction, not enlargement. Kick’s law relates energy per unit mass to the logarithm of size ratio, but the complete form is P/m = K * ln(D_feed/D_product), not an incomplete expression. The Bond Work Index W_i is defined with respect to 80% passing 100 micrometres (not 200 mesh ≈ 74 µm) from an infinite feed size in the standard test context; thus, stating 200 mesh is inaccurate. Step-by-Step Solution: Assess each option independently.(a) Plastic chips → non-cohesive → correct.(b) Kick’s law shown is incomplete and dimensionally ambiguous.(c) Comminution ≠ enlargement; it is reduction.(d) Bond Work Index convention uses 80% passing 100 µm; 200 mesh (74 µm) is not the reference. Verification / Alternative check: Handbooks define W_i with P80 = 100 µm in the Bond ball mill work index test. Kick’s law explicitly uses ln(L1/L2) or ln(D1/D2), a ratio, not a single size term. Why Other Options Are Wrong: (b) Missing the ratio of feed to product size. (c) Reverses the meaning of comminution. (d) Uses the wrong reference size for W_i. Common Pitfalls: Memorizing fragments of laws without the full functional form; always include both feed and product sizes in Kick’s correlation. Final Answer: Plastic chips are considered non-cohesive solids (generally free-flowing).

Question 13

Rotary drum vacuum filter — effect of drum speed on filtration rate
As the rotational speed of a continuous rotary drum vacuum filter increases (within normal operating limits), how does the filtration rate generally respond?

Accepted Answer

Introduction / Context: Rotary drum vacuum filters carry the filter medium through zones of cake formation, washing, drying, and discharge. Drum speed determines the residence time in each zone and the frequency with which new cake is formed, thus influencing the overall filtration rate (throughput per unit area). Given Data / Assumptions: Operation within practical limits (not so fast that cake cannot form properly, not so slow that capacity is wasted). Feed solids concentration and slurry properties are constant. Concept / Approach: Increasing drum speed increases the number of cycles per hour and, up to a point, raises filtrate volume per unit time. However, extremely high speeds may reduce cake thickness (insufficient formation time) and reduce washing or drying efficiency. Thus, the first-order trend is that filtration rate increases with speed in the useful operating window, though not necessarily linearly across the entire range. Step-by-Step Solution: Consider cake formation per revolution and number of revolutions per hour.As speed increases moderately, more cycles lead to more filtrate → higher rate.Recognize nonlinearity and practical maxima but select the general trend: increases. Verification / Alternative check: Vendor performance curves show rising filtrate rate with speed until a plateau or decline due to thin cake and poor dewatering. Why Other Options Are Wrong: Linear across the full range is rarely accurate due to formation/drying limits. Decrease or no effect contradicts observed behavior in the normal operating window. Common Pitfalls: Running too fast and sacrificing cake quality; throughput is not the only objective—washing and dryness matter. Final Answer: Increases

Question 14

Relative coarseness of products from different mills
Arrange the following equipment in decreasing order of average particle size of the product: Jaw crusher, Ball mill, Fluid energy (jet) mill.

Accepted Answer

Introduction / Context: Equipment selection often begins with a target size range. Different comminution devices occupy characteristic ranges: primary crushers produce coarse fragments, tumbling mills produce finer powders, and jet mills produce ultrafines. Knowing the qualitative ordering avoids mismatches in flowsheets. Given Data / Assumptions: Normal operation for each device: jaw crusher (primary reduction), ball mill (fine grinding), jet mill (ultrafine). Comparable, typical materials. Concept / Approach: Jaw crushers reduce large lumps to coarse fragments, typically 100–20 mm. Ball mills grind to fine powders, commonly 300–20 µm or finer in closed circuit. Fluid energy mills can achieve median sizes in the 1–10 µm range and below. Therefore, sorted by decreasing product size (coarsest to finest): jaw crusher > ball mill > fluid energy mill. Step-by-Step Solution: Assign typical product ranges to each device.Order devices from coarsest to finest product.Select “Jaw crusher, Ball mill, Fluid energy mill.” Verification / Alternative check: Plant layouts universally place crushers upstream of mills, and jet mills only after fine grinding when ultrafine cuts are needed. Why Other Options Are Wrong: Any ordering with jet mill coarser than ball/jaw is inconsistent with its ultrafine duty. Common Pitfalls: Forgetting that classifiers attached to ball mills can tighten size distributions but do not shift the mill into the ultrafine regime typical of jet mills. Final Answer: Jaw crusher, Ball mill, Fluid energy mill

Question 15

Dominant breakage mechanism in a hammer crusher
The predominant mechanism responsible for size reduction in a hammer crusher is:

Accepted Answer

Introduction / Context: Correctly identifying the dominant breakage mechanism of a machine helps predict product shape, fines generation, and wear patterns. Hammer crushers (mills) use pivoting or fixed hammers on a rotor to deliver repeated blows to feed particles inside a chamber lined with breaker plates and screens. Given Data / Assumptions: Conventional hammer crusher geometry with hammers and breaker plates. Hardness within typical operating range (no extreme ductility). Concept / Approach: The high tip speed of the hammers delivers impact energy to the particles, causing fracture by dynamic stress well above the material’s strength. Secondary mechanisms (some attrition as fragments rub and are sheared along breaker plates; limited compression between particles) do occur, but the dominant energy input is impact. This is why rotor speed and hammer design heavily influence PSD and fines generation in these machines. Step-by-Step Solution: Recall machine class: hammer mills are “impactors.”Map operating variable (tip speed) to impact severity.Choose “Impact” as the predominant mechanism. Verification / Alternative check: Manufacturer literature classifies hammer crushers under impact crushers (with cage mills and impactors), distinct from compression (jaw/roll) and attrition (disc/colloid) devices. Why Other Options Are Wrong: Attrition is present but secondary. Compression is minimal in free-flight impact chambers. Common Pitfalls: Confusing hammermills with attrition mills due to presence of breaker plates; the kinetic energy transfer indicates impact dominance. Final Answer: Impact (high-velocity blows)

Question 16

Cumulative undersize mass fraction from mixed particle data
Given particles i = 1, 2, 3, … with diameters d_i and masses m_i, what is the expression for the cumulative mass fraction of particles smaller than or equal to a size d_j?

Accepted Answer

Introduction / Context: Particle size distributions (PSDs) are commonly represented in cumulative undersize (passing) form. When raw data consist of individual particle masses and sizes, the cumulative mass fraction at a cut size is the mass-based proportion of particles not exceeding that size. This definition underpins PSD plots, mass balancing, and screen efficiency calculations. Given Data / Assumptions: A set of particles indexed by i with measured diameter d_i and mass m_i. Mass-based PSD (not number-based) is required. Concept / Approach: The cumulative undersize mass fraction at size d_j, denoted F(d_j), is the total mass of all particles with diameters less than or equal to d_j divided by the total sample mass. This directly corresponds to “% passing” if multiplied by 100. Number-based PSDs would instead count particles rather than summing masses, which is a different metric and often less useful for process calculations. Step-by-Step Solution: Compute numerator: sum masses m_i for all particles with d_i ≤ d_j.Compute denominator: total mass = Σ m_i over all particles.Form fraction: F(d_j) = (Σ m_i for d_i ≤ d_j) / (Σ m_i for all i). Verification / Alternative check: Integrating a continuous mass-based size density function up to d_j yields the same cumulative passing function; this discrete form is its sample analogue. Why Other Options Are Wrong: Summing diameters or number counts does not produce a mass-based fraction. Using ≥ d_j yields a cumulative oversize fraction, not undersize. Common Pitfalls: Confusing number-based and mass-based PSDs; always specify the basis when comparing distributions or setting specs. Final Answer: F(d_j) = (Σ m_i for d_i ≤ d_j) / (Σ m_i for all i)

Question 17

Coal washability by float-and-sink testing
A float–and–sink (washability) test determines the suitability of cleaning coal by a process based primarily on what principle?

Accepted Answer

Introduction / Context: Coal preparation routes include gravity separation, froth flotation, magnetic separation, and others. Washability studies guide the choice of route and operating cut points by showing how coal and associated mineral matter partition by density under ideal conditions. Given Data / Assumptions: Float–and–sink tests use heavy liquids of known specific gravities. Coal particles with different ash contents exhibit different densities. Concept / Approach: In the test, a sample is divided into density fractions by immersion; the mass and ash of each density class are measured. Plots of cumulative float and sink versus density (washability curves) reveal potential yield and ash of clean coal for a chosen density cut. This directly informs gravity-based separation equipment such as jigs and dense-medium cyclones. The basis is density difference, not surface wettability (flotation), shape, magnetic, or electrostatic properties. Step-by-Step Solution: Identify mechanism: separation by specific gravity in heavy liquids.Link to equipment: dense-medium, jigs, spirals.Select “Gravity (density) separation.” Verification / Alternative check: Preparation plant design uses washability data to select cut densities (e.g., 1.4–1.8) for cyclones and to predict product yield and ash. Why Other Options Are Wrong: Wettability is used in flotation, a different test. Shape is secondary and not the basis of float–sink partitioning. Electrostatic and magnetic methods are niche and not addressed by washability curves. Common Pitfalls: Confusing “float” in heavy liquid with “float” in froth flotation; the former refers to buoyancy, not bubble attachment. Final Answer: Gravity (density) separation

Question 18

Mills used for flour and cereal milling — identify the exception
Which of the following mills is NOT typically used for grinding wheat into flour or for milling cereals and other vegetable products?

Accepted Answer

Introduction / Context: Food milling uses specific equipment engineered for grain breakage, starch damage control, and flour quality. Traditional and modern choices include buhrstone (stone) mills, roller mills, hammer/attrition mills, and associated sifters. Some mineral industry mills are ill-suited due to contamination, slow throughput, or unsuitable breakage mechanisms. Given Data / Assumptions: Cereal milling targets controlled size reduction with limited metal contamination. Continuous sifting and multiple passes may be used for flour grading. Concept / Approach: Roller mills dominate modern flour milling; buhrstone mills are traditional stone mills; attrition and hammer mills are used for cereals and other vegetable products in various industries. Pebble mills are tumbling mills using flint/ceramic media for fine grinding in minerals or specialty chemicals; they are slow and not typical for flour milling, making them the exception in the list. Step-by-Step Solution: Identify common food milling equipment: roller, stone, hammer/attrition.Recognise pebble mill as a mineral-oriented, slow tumbling device.Select “Pebble mill” as the exception. Verification / Alternative check: Commercial flour mills use roller stands and plansifters; small-scale mills use stones or impact/attrition grinders, not pebble mills. Why Other Options Are Wrong: Buhrstone/roller/attrition/hammer mills are all used in cereal or vegetable product milling contexts. Common Pitfalls: Assuming any mill can produce food-grade flour; hygiene, temperature, and particle damage are critical considerations. Final Answer: Pebble mill

Question 19

Identify the metalliferous mineral among the list
Which of the following is NOT a non-metalliferous (i.e., is a metalliferous) mineral?

Accepted Answer

Introduction / Context: Minerals are commonly grouped as metalliferous (ore minerals yielding metals) and non-metalliferous (industrial, non-metallic minerals). Correct classification matters in beneficiation method selection and market segmentation (e.g., metal recovery vs. filler/flux applications). Given Data / Assumptions: Non-metalliferous examples include common industrial minerals like calcite, fluorspar, quartz, and barite. Metalliferous ore minerals include those like cassiterite (SnO2), the primary ore of tin. Concept / Approach: Calcite (CaCO3), quartz (SiO2), fluorspar (CaF2), and barite (BaSO4) are industrial minerals used as fillers, fluxes, or chemicals; they do not directly yield base or precious metals. Cassiterite is a tin oxide mineral mined specifically for metal extraction. Therefore, it is metalliferous and not part of the non-metalliferous category. Step-by-Step Solution: List mineral types and typical uses.Identify cassiterite as the ore of tin (metalliferous).Select “Cassiterite.” Verification / Alternative check: Mineralogy references list cassiterite as the principal tin ore; the others are sold as industrial minerals. Why Other Options Are Wrong: Calcite/fluorspar/quartz/barite are non-metallic, used industrially, not to extract metals directly. Common Pitfalls: Confusing “fluorspar” with fluorine as a metal—fluorine is a halogen element; fluorspar is a non-metallic mineral. Final Answer: Cassiterite

Question 20

In Indian Standard (IS) test sieves used for particle-size analysis, the “mesh number” is defined as the aperture size expressed to the nearest decamicron (0.01 mm).
Approximately how many micrometres is the aperture width for an IS sieve of mesh n…

Accepted Answer

Introduction / Context: IS (Indian Standard) sieves are widely used in chemical engineering and materials processing for particle size classification. This question checks your understanding of the definition of IS “mesh number” and how to convert it into a practical aperture size in micrometres for calculations or lab work. Given Data / Assumptions: IS mesh number is equal to the aperture size expressed to the nearest decamicron. 1 decamicron = 0.01 mm = 10 micrometres. Required: aperture for mesh number 50 in micrometres. Concept / Approach: The mapping is direct: mesh number = aperture (in decamicrons). Convert decamicrons to millimetres and then to micrometres. Remember 1 mm = 1000 micrometres. This avoids confusion with “mesh per inch,” which is a different convention used in ASTM/BS standards. Step-by-Step Solution: Step 1: Interpret “mesh number 50” as 50 decamicrons.Step 2: Convert decamicrons to millimetres: 50 decamicrons * 0.01 mm/decamicron = 0.50 mm.Step 3: Convert millimetres to micrometres: 0.50 mm * 1000 μm/mm = 500 μm. Verification / Alternative check: If the sieve were specified by “openings per inch,” the number would not match this value. Because the question explicitly uses the IS definition (aperture expressed in decamicrons), the direct conversion to 0.50 mm (500 μm) is correct. Why Other Options Are Wrong: 5 μm and 50 μm are far too fine for a 50-decami﻿cron IS aperture (which equates to 0.50 mm).5000 μm = 5 mm is much coarser than typical lab sieves near this designation.250 μm corresponds to mesh number 25 under the IS decamicron convention, not 50. Common Pitfalls: Confusing IS “mesh number” with “mesh per inch”; omitting the conversion 1 mm = 1000 μm; or misreading decamicron as micrometre can all lead to errors by a factor of 10 or 20. Final Answer: 500

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