Question 1

In a closed-circuit, air-swept ball mill system, which combination is normally used to return coarse material and collect fine product?

Accepted Answer

Introduction / Context:Closed-circuit grinding controls product fineness and improves mill efficiency. In an air-swept ball mill, the gas stream transports ground particles to a separator and then to a collector, while the classifier returns coarse particles to the mill inlet.Given Data / Assumptions:An air-swept mill entrains fines with gas.A classifier performs size separation.A cyclone commonly collects fine product from the gas stream.Concept / Approach:The classifier (static or dynamic) splits fines and coarse; the cyclone serves as a dust collector to separate solids from conveying air. Many practical circuits place a cyclone between the mill outlet and classifier plenum to stabilize flow and recover fines, with dust recirculation as designed.Step-by-Step Solution:Identify unit for size control: classifier.Identify unit for gas–solid separation: cyclone.Select the combined use of both.Verification / Alternative check:Typical cement grinding circuits show air-swept mills connected to cyclones and high-efficiency separators, confirming the configuration.Why Other Options Are Wrong:Classifier alone: offers split but needs a collector.Cyclone alone: no cut-size control; only collection.ESP: overkill for this duty and uncommon for mill recycle.Common Pitfalls:Assuming a single device can both classify and collect with high precision.Final Answer:Both (a) & (b)

Question 2

Which filter type is best suited for gelatinous solids that tend to plug the septum, by first depositing a protective pre-deposited layer on the medium?

Accepted Answer

Introduction / Context:Gel-like or highly compressible cakes can blind a filter medium rapidly. A precoat technique deposits a porous layer (e.g., diatomite or perlite) on the septum before filtration, preventing blinding and stabilizing flow resistance.Given Data / Assumptions:Feed contains gelatinous solids.Objective is to avoid septum plugging.Concept / Approach:Precoat filters operate by first laying down an expendable filter aid layer. During filtration, fine or gelatinous solids lodge in this porous cake instead of directly in the septum, preserving permeability and allowing periodic removal/regeneration of the precoat.Step-by-Step Solution:Recognize plugging tendency of gelatinous solids.Use a sacrificial precoat to intercept fines.Select “Precoat filter.”Verification / Alternative check:Industry practice: kieselguhr or perlite precoats are standard in beverages, pharmaceuticals, and chemical services with slimy solids.Why Other Options Are Wrong:Sparkler/plate-frame/vacuum leaf: can blind quickly without precoat.Cartridge: also prone to rapid fouling by gels.Common Pitfalls:Attempting to filter without filter aids; throughput collapses as resistance skyrockets.Final Answer:Precoat filter

Question 3

Sorting (hydraulic) classifiers that use differential settling separate particles primarily based on differences in which property?

Accepted Answer

Introduction / Context:Hydraulic classifiers segregate particles in a fluid under gravity by exploiting differences in settling behavior. The governing property is the terminal settling velocity, which depends on size, density, and shape.Given Data / Assumptions:Particles settle in a fluid column.Steady, laminar-to-turbulent settling as applicable.Concept / Approach:Terminal velocity v_t encapsulates the combined effects of particle size and density in a given fluid. Classifiers set operating conditions so that particles with higher v_t report differently than those with lower v_t, achieving separation.Step-by-Step Solution:Note that “size” and “density” influence v_t.Recognize the device acts on the resulting v_t differences directly.Choose “Terminal velocities.”Verification / Alternative check:Design correlations (e.g., Stokes or Newton regimes) predict v_t; classifiers are tuned to these predictions.Why Other Options Are Wrong:Size or density alone is incomplete; v_t captures both.Surface charge is relevant to electrostatic separation, not hydraulic classifiers.Common Pitfalls:Treating size-only classification; in fluids, density differences can override size alone.Final Answer:Terminal velocities

Question 4

What is the primary size-reduction mechanism in toothed roll crushers when handling friable or soft rock and coal?

Accepted Answer

Introduction / Context:Toothed roll crushers incorporate intermeshing teeth that grip and pull material into the crushing zone. They are widely used for coal preparation and soft ores, achieving controlled top size with minimal fines.Given Data / Assumptions:Material is friable/soft.Device has toothed rolls rotating toward each other.Concept / Approach:As particles are nipped, the teeth apply compressive forces while the relative motion introduces shear. The combined compression–shear (tearing) mode fractures the material along planes of weakness.Step-by-Step Solution:Identify action of teeth: grip, compress, and shear.Differentiate from smooth rolls (mainly compression) and impact mills (impact/attrition).Select “Tearing (shear) and compression.”Verification / Alternative check:Operation manuals specify shear-compression as the main breakage mode for toothed rolls.Why Other Options Are Wrong:Impact/attrition: characteristic of hammer/pin/ball mills.Cutting only: teeth do not slice like knives; they compress and tear.Common Pitfalls:Assuming impact dominates because of rotation; intermeshing geometry favors compression–shear.Final Answer:Tearing (shear) and compression

Question 5

Which oxide is generally not associated with hematite (Fe2O3) iron ore as a natural constituent or common gangue impurity?

Accepted Answer

Introduction / Context:Hematite iron ore primarily contains iron(III) oxide with various gangue minerals. Understanding typical impurities helps in beneficiation route selection and slag design in ironmaking.Given Data / Assumptions:Common gangue: silica, alumina, phosphorus/sulfur-bearing species.Minor oxides may be present depending on deposit.Concept / Approach:Titanium oxide and manganese oxide can occur as trace impurities; calcium oxide appears from carbonate gangue (e.g., calcite/dolomite) or flux addition, though not intrinsic to the hematite lattice. Cobalt oxide is not a typical component or common impurity associated with iron ore bodies on an appreciable level.Step-by-Step Solution:List likely impurities from geology: TiO2, MnO, SiO2, Al2O3, carbonates.Note that CoO occurrence is rare/negligible in iron ores.Select cobalt oxide as “not present.”Verification / Alternative check:Typical assay tables of hematite ores seldom record cobalt oxides; processing does not account for cobalt removal in iron ore circuits.Why Other Options Are Wrong:TiO2/MnO: observed in some ores.CaO: may occur via carbonates or be considered in gangue/flux context.SiO2: classic gangue mineral.Common Pitfalls:Assuming any listed oxide cannot occur; geology varies, but CoO is distinctly uncommon.Final Answer:Cobalt oxide

Question 6

In jaw crushers, how does choke crushing (operating with a full chamber) compare to free crushing in terms of capacity and power consumption?

Accepted Answer

Introduction / Context:Crusher performance depends on feed conditions. “Choke” operation keeps the crushing chamber full, altering inter-particle breakage and throughput relative to lightly loaded (free) conditions.Given Data / Assumptions:Same machine and feed characteristics, but different chamber loading.Choke implies continuous, full-feed operation.Concept / Approach:Choke crushing enhances inter-particle breakage and increases the product density and throughput (capacity). However, the higher load also raises power draw due to increased work on the material and frictional effects.Step-by-Step Solution:Compare capacity under free vs choke: choke is higher.Compare power: choke draws more due to greater material load.Therefore, (a) and (c) are both true.Verification / Alternative check:Plant data and vendor curves show improved tph in choke operation along with higher absorbed kW.Why Other Options Are Wrong:Less power: contrary to increased load.Single statements omit the coupling of both capacity and power changes.Common Pitfalls:Running starved-feed due to fear of power spikes; this often reduces efficiency.Final Answer:Both (a) and (c)

Question 7

In gas cleanup for coke-oven gas, which device is most suitable for removing trace liquid tar fog (submicron to micron droplets)?

Accepted Answer

Introduction / Context:Coke-oven gas contains mist-like tar droplets after primary cooling. To meet downstream specifications, an efficient fine-aerosol collector is required.Given Data / Assumptions:Droplets are small (fog) and sticky.Pressure drop and fouling are practical concerns.Concept / Approach:Electrostatic precipitators impart electrical charge to droplets and collect them on grounded plates, achieving high efficiency on fine mists with moderate pressure drop. Cyclones are ineffective for submicron aerosols; strainers are for coarse debris.Step-by-Step Solution:Identify particle size regime (mist/fog).Select a device effective for submicron droplets: ESP.Confirm suitability for sticky tar aerosols.Verification / Alternative check:Coke plant gas-cleaning trains routinely include ESPs for tar-fog removal.Why Other Options Are Wrong:Cyclone: low efficiency for very fine droplets.Strainer: not an aerosol device.Bag filter: tar loading would blind quickly.Common Pitfalls:Using only mechanical separators; they cannot meet fine mist removal targets.Final Answer:Electrostatic precipitator

Question 8

During constant-pressure filtration of a slurry, how does the filtration rate vary with time as the cake builds on the medium?

Accepted Answer

Introduction / Context:Filtration performance depends on the driving force and the resistance of both the filter medium and the deposited cake. Under constant pressure, the cake thickness increases over time, changing the overall resistance.Given Data / Assumptions:Pressure drop is fixed (constant-pressure mode).Cake is incompressible to moderately compressible.Concept / Approach:Darcy’s law for filtration: rate ∝ ΔP / (μ (R_m + R_cake)). As filtration proceeds, R_cake increases with cake mass/height; therefore, at constant ΔP, the volumetric filtration rate decreases with time.Step-by-Step Solution:Write: dV/dt = ΔP / (μ (R_m + α V/A)).As V increases, R_cake rises linearly for incompressible cakes.Hence, dV/dt decreases with time.Verification / Alternative check:Filtration plots (V^2 vs t for constant-pressure) are linear for incompressible cakes, reflecting decreasing instantaneous rate.Why Other Options Are Wrong:Constant rate: occurs at constant-flux or constant-rate operation, not constant pressure.Increasing rate: contradicts rising resistance.Common Pitfalls:Confusing constant-pressure and constant-rate control strategies.Final Answer:Rate of filtration decreases with time

Question 9

The angle of nip in crushing rolls primarily depends on particle friction and geometry. Which factor below does it not significantly depend upon?

Accepted Answer

Introduction / Context:The angle of nip is the maximum angle between the roll faces at which a particle can be drawn into the rolls. It is a function of friction and the relative size of the particle to the roll radius.Given Data / Assumptions:Classical nip analysis assumes quasi-static conditions.Friction and size geometry control nipping.Concept / Approach:The condition for nipping can be written in terms of coefficient of friction and the ratio of particle size to roll diameter. While speed affects capacity and power draw, the static nip angle relation is independent of rotational speed in first approximation.Step-by-Step Solution:Relate nip angle to friction coefficient and size ratio.Recognize roll diameter and feed size appear geometrically.Exclude speed as a primary variable for nip angle.Verification / Alternative check:Design charts show nip angle versus size ratio and friction, with no speed term in the basic nipping criterion.Why Other Options Are Wrong:Diameter/feed size/friction: explicitly present in nip equations.Product size correlates with nipping and breakage once drawn in.Common Pitfalls:Assuming higher speed increases nip angle; it can increase throughput but not the geometric limit.Final Answer:Speed of the rolls

Question 10

Compared with a plate-and-frame filter press, a shell-and-leaf (pressure leaf) filter typically offers which combination of advantages in operation?

Accepted Answer

Introduction / Context:Filter selection balances labor, pressure capability, washing efficiency, and ease of cake discharge. Pressure leaf (shell-and-leaf) designs differ significantly from plate-and-frame presses.Given Data / Assumptions:Vertical or horizontal pressure leaf units are considered.Batch operation with precoat or body feed possible.Concept / Approach:Pressure leaf filters provide enclosed operation at relatively high pressures, allow uniform cake washing by spray or reslurry, and often require less manual labor than opening and cleaning a plate-and-frame press. Cake discharge can be automated by vibration or sluicing.Step-by-Step Solution:Assess labor: leaf filters minimize manual plate handling.Assess pressure: designed for high-pressure service.Assess washing: improved access to cake surfaces and spray washing.Verification / Alternative check:Manufacturer literature highlights reduced OPEX and improved CIP/cleanability for pressure leaf units.Why Other Options Are Wrong:Single benefits only: understates the combined advantages.Common Pitfalls:Comparing only cloth cost without considering labor downtime for press opening/closing.Final Answer:All (a), (b) & (c)

Question 11

Bond work index correlation: the work index measured in wet grinding is often adjusted to estimate dry-grinding energy. What factor is used to convert wet-grinding work index to an equivalent dry-grinding value?

Accepted Answer

Introduction / Context:Bond’s work index (Wi) is widely used to estimate energy requirements. Because transport and cushioning differ in wet vs dry circuits, empirical adjustments are made to compare conditions.Given Data / Assumptions:Wi was measured under wet conditions.We need an equivalent dry-grinding Wi.Concept / Approach:Industry practice applies a multiplicative factor to the wet Wi to estimate dry circuit energy, commonly about 1.34, reflecting additional energy to overcome increased coating, dispersion issues, and classification inefficiencies in dry grinding.Step-by-Step Solution:Start with wet Wi.Multiply by 1.34 to obtain dry Wi.Use the adjusted value for dry energy sizing.Verification / Alternative check:Cement and mineral processing design texts cite the 1.34 factor as a rule-of-thumb adjustment.Why Other Options Are Wrong:1.0: ignores known differences.0.5/0.75: opposite trend.4.34: unrealistically high.Common Pitfalls:Treating Wi as universal without noting test conditions (wet/dry, closing screen).Final Answer:1.34

Question 12

Ribbon blenders (U-trough with helical ribbons) are most suitable for homogeneous mixing of which type of materials?

Accepted Answer

Introduction / Context:Ribbon blenders are common in foods, chemicals, and pharmaceuticals. The counterflow motion of inner and outer ribbons promotes convective mixing in free-flowing bulk solids.Given Data / Assumptions:Blender has a U-trough and a double-helix ribbon agitator.Target materials vary in flow properties.Concept / Approach:Ribbon blenders excel with free-flowing dry powders and granules (e.g., flour, polymer pellets, additives). They provide reasonable mixing times and gentle handling with low power compared to high-shear mixers.Step-by-Step Solution:Match mechanism (bulk conveyance) to material flowability.Select “Dry powders.”Exclude viscous/pasty feeds that require high-torque kneaders or planetary mixers.Verification / Alternative check:OEM datasheets specify recommended bulk density and angle of repose ranges for effective ribbon mixing.Why Other Options Are Wrong:High-viscosity/pastes/gums: need sigma-blade or double planetary mixers.Lumps with liquids: not ideal without pre-breaking and may smear.Common Pitfalls:Overfilling the trough, which reduces mixing efficiency.Final Answer:Dry powders

Question 13

For a rotating trommel, how does the critical speed N vary with drum diameter D? Choose the correct proportionality.

Accepted Answer

Introduction / Context:Critical speed is the rotational speed at which material just begins to centrifuge. For rotating cylinders (mills, trommels), it depends on gravity and characteristic radius.Given Data / Assumptions:Trommel approximated as a rotating cylinder.Critical condition derived from centripetal balance.Concept / Approach:At critical speed, ω^2 R ≈ g. Hence ω ∝ 1/√R and N (rpm) ∝ 1/√D because R ∝ D. Therefore, as diameter increases, critical speed decreases with the square root relationship.Step-by-Step Solution:Set ω^2 R = g.Solve ω ∝ 1/√R.Convert to rpm and replace R with D: N ∝ D^-0.5.Verification / Alternative check:Mill design rule Nc ≈ 42.3 / √D (D in metres) shows the same dependence.Why Other Options Are Wrong:Linear or √D dependence contradicts physics.1/D is too strong; correct dependence is 1/√D.Common Pitfalls:Confusing trommel screening speed optimization with true “critical” speed.Final Answer:N ∝ D^-0.5

Question 14

What is the standard process term for removing fine suspended solids from liquids under gravity in a clarifier or settling tank?

Accepted Answer

Introduction / Context:Liquor–solid separation can be achieved by gravity settling, flocculation, filtration, or centrifugation. The terminology matters in water treatment and mineral processing.Given Data / Assumptions:Fine solids are removed from liquids using gravity in tanks.No filter media is involved.Concept / Approach:Sedimentation is gravity settling of discrete or flocculated particles. Decantation is the subsequent removal of clarified supernatant. Flocculation is a conditioning step to form larger aggregates. Classification is size-separation of solids by settling velocity in hydraulic devices.Step-by-Step Solution:Identify the core operation: settling under gravity.Choose the process term: sedimentation.Differentiate from ancillary steps (flocculation/decantation).Verification / Alternative check:Water treatment unit operations name primary settlers as sedimentation basins, confirming nomenclature.Why Other Options Are Wrong:Decantation: liquid withdrawal, not the settling mechanism.Flocculation: pre-treatment aiding sedimentation.Classification: solid–solid hydraulic sorting.Common Pitfalls:Interchanging “sedimentation” and “decantation” in reports; they are distinct steps.Final Answer:Sedimentation

Question 15

According to Rittinger’s law for crushing and grinding, the energy required is directly proportional to which quantity?

Accepted Answer

Introduction / Context:Comminution energy models help estimate power requirements across size ranges. Rittinger’s law applies best to fine grinding where surface area increases significantly.Given Data / Assumptions:Focus on the proportionality statement.No numeric data required.Concept / Approach:Rittinger’s postulate: E ∝ ΔA (increase in surface area). It contrasts with Kick’s law (E ∝ ln size ratio) and Bond’s law (E ∝ 1/√d relationships). Therefore, for fine size targets, Rittinger provides the most accurate estimate.Step-by-Step Solution:Identify the law linking energy to surface area.Select “new surface created.”Verification / Alternative check:Textbook derivations and experimental evidence in fine milling confirm the proportionality to ΔA.Why Other Options Are Wrong:Size ratio or volume change are not Rittinger’s basis.Common Pitfalls:Confusing Bond’s widely used industrial correlations with Rittinger’s fine-grind regime.Final Answer:The new surface created

Question 16

In froth flotation, which reagent acts as an activator that enhances the floatability of specific minerals (e.g., activating sphalerite for subsequent collector action)?

Accepted Answer

Introduction / Context:Flotation selectivity relies on modifiers: activators, depressants, pH regulators, and collectors. Activators render mineral surfaces more responsive to collectors.Given Data / Assumptions:Sphalerite (ZnS) example: activation by copper ions.Collectors respond to altered surface chemistry.Concept / Approach:Copper sulphate provides Cu^2+ ions that exchange with surface Zn^2+, forming a more readily floatable copper-activated surface that adsorbs xanthates. Cresylic acid is a frother; carbonates are pH modifiers/buffers, not activators.Step-by-Step Solution:Identify reagent that specifically increases mineral responsiveness.Match to copper sulphate as a classic activator.Verification / Alternative check:Standard reagent schemes list copper sulphate as activator for ZnS and occasionally pyrite in certain systems.Why Other Options Are Wrong:Cresylic acid: frother.CaCO3/Na2CO3: pH regulation and depression/dispersion roles.Kerosene: hydrocarbon collector/dispersant in some systems, not an activator.Common Pitfalls:Confusing activators with collectors due to similar metallurgical outcomes (higher recovery).Final Answer:Copper sulphate

Question 17

For the same feed size range, which crusher generally gives the least reduction ratio (i.e., produces the least overall size reduction per stage)?

Accepted Answer

Introduction / Context:Reduction ratio (R) is the feed size divided by the product size for a single stage. Crusher selection considers achievable R, product shape, and capacity.Given Data / Assumptions:Typical single-stage ratios: roll (≈ 3–4:1), jaw (≈ 6–8:1), gyratory (≈ 6–8:1), cone (≈ 6–8:1), impact (≈ 10:1, application dependent).Concept / Approach:Because roll crushers rely on compression between smooth or toothed rolls with limited nip angle, the allowable size reduction per pass is modest compared with other crusher types.Step-by-Step Solution:Compare typical R across devices.Identify smallest R for rolls.Select roll crusher.Verification / Alternative check:Vendor catalogs consistently list lower reduction ratios for roll crushers in standard duty.Why Other Options Are Wrong:Jaw/gyratory/cone: higher achievable R per stage.Impact: potentially highest R but material dependent.Common Pitfalls:Assuming toothed rolls can match cone reduction; they prioritize controlled topsize over high R.Final Answer:Roll crusher

Question 18

According to Taggart’s empirical relation for jaw and gyratory crushers (gape 10–60 cm), the capacity in kg/hr is proportional to which expression in terms of L (feed opening length, cm) and S (maximum discharge opening width, cm)?

Accepted Answer

Introduction / Context:Rules of thumb help make preliminary crusher selections. Taggart provided correlations linking crusher capacity to opening dimensions for common primary crushers.Given Data / Assumptions:Applicable for jaw and gyratory crushers with gape 10–60 cm.Units: capacity in kg/hr; L and S in centimeters.Concept / Approach:The empirical proportionality incorporates a constant with dimensional effect so that capacity scales with both feed opening length and discharge width. The commonly cited form uses a factor on the order of 250 for kg/hr when dimensions are in cm, recognizing variability with material and operating conditions.Step-by-Step Solution:Identify the capacity proportional to opening area L*S.Apply Taggart’s empirical constant for the stated unit system.Select “250 L S.”Verification / Alternative check:Preliminary sizing examples in handbooks often back-calculate similar constants for average ores; detailed sizing still requires throughput, bulk density, and power checks.Why Other Options Are Wrong:Bare L S lacks unit scaling.93 L S underestimates capacity for the given units; L/S or S/L are dimensionally inconsistent.Common Pitfalls:Using empirical constants outside their intended unit systems or feed characteristics.Final Answer:250 L S

Question 19

Which comminution energy law is most appropriate for fine grinding where surface area increase dominates the energy requirement?

Accepted Answer

Introduction / Context:Different empirical laws apply across size ranges. Choosing the correct law improves power estimates and equipment selection.Given Data / Assumptions:Target is fine grinding with large increases in surface area.No numeric inputs; conceptual selection only.Concept / Approach:Rittinger’s law states energy is proportional to the new surface created, suiting fine size domains. Kick’s law fits coarse crushing where size ratios matter more than absolute surface. Bond’s law bridges the intermediate range with work indices.Step-by-Step Solution:Match regime: fine grinding → surface area governs.Select Rittinger’s law accordingly.Verification / Alternative check:Handbooks recommend Rittinger for micronization and milling where specific surface dramatically grows.Why Other Options Are Wrong:Kick/Bond: better for coarse and intermediate ranges.“None”: incorrect; Rittinger applies.Common Pitfalls:Using Bond indices blindly at very fine sizes, leading to misestimated energy.Final Answer:Rittinger’s law

Question 20

Comminution principle: for the coarse reduction of hard, strong solids (e.g., run-of-mine ore or clinker), which primary breakage mechanism is preferred for efficient size reduction and control of product shape?

Accepted Answer

Introduction / Context:In particle technology and mineral processing, different breakage mechanisms are favored at different size scales and material strengths. For very hard, strong, and abrasive solids that must be reduced from large lumps to manageable sizes, the dominant and most reliable mechanism is typically compression. This question tests your understanding of how material properties and duty (coarse vs. fine) guide the choice of crusher type and the underlying breakage physics.Given Data / Assumptions:Material: hard/strong solids (e.g., hard ore, clinker, rock aggregates).Duty: coarse reduction (primary or secondary crushing).Goal: robust operation, control of top size, acceptable product shape, manageable wear.Concept / Approach:Compression applies a relatively slow, high force over a contact area; it is effective for large, hard particles and results in predictable fracture along planes of weakness. Primary crushers (jaw, gyratory) and some secondary units (cone, smooth rolls) are compression based. Impact is better for brittle materials at smaller sizes or where cubicity is prioritized; attrition suits intermediate to fine grinding; cutting is for fibrous or soft, ductile materials.Step-by-Step Solution:Identify duty: coarse reduction of hard solids.Map duty to mechanism: compression is favored for primary/secondary crushing of hard rocks.Eliminate alternatives: impact (better for brittle shattering and fines control), attrition (fine grinding), cutting (fibrous/soft).Verification / Alternative check:Flowsheets show jaw or gyratory crushers as standard primaries in mines and cement plants because compression handles large feed with lower catastrophic wear compared to pure impact on hard lumps.Why Other Options Are Wrong:Impact: efficient for brittle materials and shaping at smaller sizes, but high wear on very hard, coarse feed.Attrition: dominant in mills for fine size reduction, not coarse crushing.Cutting: used for fibrous materials (plastics, rubber, biomass), not hard rock.Common Pitfalls:Assuming impact is always “more powerful.” Power and effectiveness depend on material properties and scale; for hard, coarse duty, compression is more controllable and economical.Final Answer:compression

Mechanical Operations Questions

Questions

Practice smarter