Mechanical Operations Flashcards
Pick out the wrong statement.
Size enlargement (opposite of size reduction) is not a mechanical operation.
Power required to drive a ball mill with a particular ball load is proportional to (where, D = diameter of ball mill )
D»2.5
Which of the following relationships between co-efficinet of friction (μ) between rock & roll and a (half of the angle of nip) of the particle to be crushed is correct ?
μ ≥ tan α
Which of the following is not an ultrafine grinder (colloid mill)?
Toothed roll crusher
__________ centrifuge is normally used in sugar mills.
Suspended batch basket
Solid particles separation based on the difference in their flow velocities through fluids is termed as the
classification
Diatomaceous earth is a/an
filter aid
For a non-spherical particle, the sphericity
is defined as the ratio of surface area of a sphere having the same volume as the particle to the actual surface area of the particle.
Which of the following is not a part of the Blake jaw crusher?
Hanger
If a force greater than that of gravity is used to separate solids & fluids of different densities, the process is termed as the
centrifugation
Filter aid is used to
increase the porosity of the cake.
Which of the following mineral dressing operations is termed as ‘comminution’?
None of these
The energy required per unit mass to grind limestone particles of very large size to 100 μm is 12.7 kWh/ton. An estimate (using Bond’s law) of the energy to grind the particles from a very large size to 50 μrn is
18 kWh/ton
Which of the following is the most suitable for cleaning of fine coal dust (<0.5 mm) ?
Froth floatation
Range of compressibility co-efficient of the commercial compressible cake obtained in filtration operation is
0.2 to 0.8
Size reduction of the __________ can be suitably done by ball mills, crushing rolls and rod mills.
metalliferrous ores
The main size reduction operation in ultrafine grinders is
attrition
Balls for ball mills are never made of
lead
200 mesh screen means 200 openings per
inch
For transporting pasty material, one will use a/an
screw colaveyor
Where the density difference of the two liquid phase to be separated is very small (as in milk cream separator), the most suitable separator is a
disc bowl centrifuge.
A tube mill compared to a ball mill
has a higher length/diameter ratio.
Basic slag is not ground in
jaw crushers
Grindability of a material does not depend upon its
size
Shape factor for a cylinder whose length equals its diameter is
1.5
In case of a hammer crusher, the
rotor shaft carrying hammers can be vertical or horizontal.
Rittinger’s number designates the new surface created per unit mechanical energy absorbed by the material being crushed. Larger value of Rittinger’s number of a material indicates its
easier grindability
Which of the follwoing mechanical conveyors does not come under the division ‘scrapers’ ?
Bucket elevators
__________ mixer is used for devulcanisation of rubber scrap & making water dispersion & rubber solution.
Banbery
Which of the following is a vacuum filter ?
Rotary disc filter
Pine oil used in froth floatation technique acts as a/an
frother
Length/diameter ratio of a ball mill is
< 1
Feed size of ≥ 25 cms can be accepted by
jaw crusher
Laminar flow region is said to exist during agitation of a liquid in an agitator, when the value of Reynolds number is
<10
Filtration of water in a paper mill is done by a/an __________ filter.
open sand
Sphericity of a cubical particle, when its equivalent diameter is taken as the height of the cube, is
1
In case of a ball mill,
coarse feed requires a larger ball.
Maximum size reduction in a ball mill is done by the, __________ action.
impact
The power number for a stirred tank becomes constant at high Reynolds number. In this limit, the variation of power input with impeller rotational speed (N) is proportional to
N2
Bond crushing law
calls for relatively less energy for the smaller product particles, than does the Rittinger law.
Crushing efficiency is the ratio of the
surface energy created by crushing to the energy absorbed by the solid.
The resistance offered by the filter used in a bag filter is proportional to (where, c = dust concentration, and s = particle size )
c/s
Cement clinker is reduced to fine size by a
tube mill
Which of the following agitators having a large blade area, rotating at slow speed is used for mixing high viscosity liquids ( > 50000 centipoise) ?
Helical screw
__________ is defined as the geometric mean of the relative rejections and the relative recoveries of two minerals.
Selectivity index
__________ is the most suitable for compounding rubber and plastic solids.
Banbery mixer
Bucket elevators are not suitable for the vertical lifting of __________ materials.
sticky (e.g. clay paste)
For removal of very small amounts of precipitate from large volume of water, the most suitable filter is the __________ filter.
sand
The sphericity of a cylinder of 1 mm diameter and length 3 mm is
0.9
Pulverised coal passing through 200 mesh screen has a diameter of 0.074 mm (74 micron). The same passing through 50 mesh screen will have a dia of __________ mm.
0.30
The most suitable equipment used to devulcanise rubber scrap and to make water dispersion & rubber solution is a
boundary mixer.
Traces of solids are removed from, liquid in a
clarifier
__________ balls capable of grinding the feed in a ball mill gives the maximum efficiency.
Minimum size
The speed of a rotary drum vacuum filter may be about __________ rpm.
1
For achieving maximum capacity of the ball mill, the ball charge should be equal to about __________ percent of the ball mill volume.
50
Colloid mills achieve size reduction mainly by
attrition
According to Bond crushing law, the work required to form particle of size ‘D’ from very large feed is (where (S/V)p and (S/V)f are surface to volume ratio of the product and feed respectively).
«_space;(S/V)p
Which of the following gives the work required for size reduction of coal to -200 mesh in a ball mill most accurately ?
Rittinger’s law
Trommels employ __________ for screening of materials.
punched plate
Ultra centrifuges are used for the separation of __________ solid particles.
colloidal
Which of the following represents the plot of filtrate volume versus time for constant pressure filtration ?
Parabola
Mixing of light fine powder such as insecticides is done by
impact wheels
Run of mine (ROM) coal is crushed by a __________ for use in domestic ovens.
hammer crusher
The unit of specific cake resistance is
gm/cm2
Size reduction of asbestos and mica is done by
hammer mills
The study on washability of coal is done by using the __________ technique.
heavy media separation
A suspension of glass beads in ethylene glycol has a hindered settling velocity of 1.7 mm/s, while the terminal settling velocity of a single glass bead in ethylene glycol is 17 mm/s. If the Richardson-Zaki hindered settling index is 4.5, the volume fraction of solids in the suspension is
0.6
The specific cake resistance for compressible sludges is a function of the pressure drop
over cake
Pick out the wrong statement.
Crushing efficiency is the ratio of the energy absorbed by the solid to the surfaces energy created by crushing.
Which of the following terminology is not used for size reduction of materials to fine sizes or powders?
Compression
Kick’s law assumes that the energy required for size reduction is proportional to the logarithm of the ratio between the initial and the final diameters. The unit of Kick’s constant is
kW. sec/kg
Which of the following gives the crushing energy required to create new surface ?
Rittinger’s law
In filtration, the use of ‘filter aid’ helps in
deplugging the filter medium.
Gizzlies are used for separating __________ solids.
fine
Which of the following clay mixing devices is vacuum operated for deairation of clay?
Pug mill
Pick out the material having minimum Rittinger’s number.
Quartz
In case of a revolving mill, wet grinding compared to dry grinding
none of these.
For a cyclone of diameter 0.2 m with a tangential velocity of 15 m/s at the wall, the separation factor is
230
Pick out the wrong statement.
Capacity of crusher in choke feeding is increased.
In bag filters, filter fabrics are never made of
metallic wire woven mesh
Use of ‘grinding aids’ is done in __________ grinding.
dry
Which of the following is the most suitable filter for separation of abrasive solids suspended in a corrosive liquid ?
Vacuum filter
All resistances during washing of cake
remain constant
__________ conveyor is the most suitable for short distance transportation of non-abrasive loose materials like garbage, grain, food wastes etc.
Flight
At low Reynold’s number, the power (P) required for agitating a fluid in a stirred tank becomes independent of inertial forces. In this limit, indicate which of the following relations is satisfied:
Po = ρ/ρN3 D5 : Power number
Re = ρ N D2/μ: Reynold’s number
N is the impeller rotational speed, and D is the impeller diameter.
Po ∝ Re0.0
Rittinger’s crushing law states that
work required in crushing is proportional to the new surface created.
The reduction ratio for fine grinders is
as high as 100
A straight line is obtained on plotting reciprocal of filtration rate vs. the volume of filtrate for __________ flow of filtrate.
incompressible cake and laminar
The work index in Bond’s law for crushing of solids has the following dimensions
kWh/ton
Ore concentration by jigging is based on the difference in the __________ of the particles.
specific gravities
Size measurement of ultrafine particles can be best expressed in terms of
surface area per unit mass
Which is a secondary crusher for a hard & tough stone ?
Cone crusher
Separation of isotopes is generally done using a/an __________ centrifuge.
ultra
Reciprocating screens are normally inclined at an angle of 5° with the horizontal and employ gyratory motion at feed end & reciprocating motion at the discharge end. They are not suitable for the screening of the
heavy tonnages of rocks or gravel.
Molten ammonium nitrate is mixed with ground limestone in fertilizer plant in a
pug mill
For a sphere falling in the constant drag co-efficient regime, its terminal velocity depends on its diameter (d) as
d»2
Dust collection efficiency of electrostatic precipitator can be as high as 99.9%. Maximum temperature and pressure of dust laden gas that can be cleaned in an electrostatic precipitator is respectively.
1000°C and 10 atm.
In a size reduction crushing operation, the feed size is 10 to 50 mm, while the product size is 2 to 10 mm. This is a case of __________ crushing.
fine
For spheres, the surface shape factor is given by (where, A = area, V = volume, and D = diameter)
π(=A/D2)
Pick out the wrong statement pertaining to the turbine agitator.
Pitched blade turbine agitator gives only radial flow with complete absence of the axial flow.
Dry powdery solid materials are transported by a __________ conveyor.
screw
During washing of filter at the end of constant pressure filtration, the rate of washing equals the rate of filtration
at the end of filtration.
Optimum ratio of operating speed to critical speed of a trommel is
0.33-0.45
Grinding efficiency of a ball mill is of the order of __________ percent.
1-5
During agitation of liquids, power consumption during laminar flow is not proportional to the
density of the liquid
Trommels are revolving screens which normally operate in the range of __________ rpm.
15 - 20
The value of work index does not change materially from one equipment to another. If the value of work index determined for close circuit grinding is Wi, then the same for open circuit grinding will be
1.34 Wi
To remove dirt from the flowing fluid, we use a
strains
Which of the following crushers can be considered as a combination of a jaw crusher and a roller crusher ?
Gyratory crusher
Crushing efficiency of a machine ranges between __________ percent.
0.1 to 2
Screen capacity is expressed in terms of
tons/hr-ft2
What is the critical rotation speed in revolutions per second, for a ball mill of 1.2 m diameter charged with 70 mm dia balls ?
0.66
Percentage of drum submerged in the slurry in case of rotary drum filter is
30
Which of the following oxides is not present in hematite iron ore?
Cobalt oxide
In closed circuit grinding as compared to open circuit grinding, the
operation is economical.
Pick out the correct statement.
A hopper is a small bin with a sloping bottom.
Gelatinous solid (which plug the septum) can be filtered by a __________ filter.
precoat
During the washing of cake
all the resistances are constant.
A gravity decanter is meant for the separation of two __________ density.
immiscible liquids of different
The most suitable filter for the removal of very small amount of precipitate from very large volumes of water is the __________ filter.
sand
__________ are mixed using ribbon blenders.
Dry powders
Activators are those chemicals which help buoying up one mineral in preference to the other in the froth floatation process. Which of the following is an activator?
Copper sulphate
In a size reduction crushing operation, feed size is 300 to 1500 mm while the product size is 100 to 300 mm. This is a case of the __________ crushing.
primary
As per Taggart’s formula, the capacity (kg/hr) of Jaw & Gyratory crushers (for gapes of 10 to 60 cms) is equal to (where, L = Length of feed opening, cms S = Maximum width of discharge opening, cms )
93 LS
Which of the following crushing laws is most accurately applicable to the fine grinding of materials ?
Rittinger’s law
The process by which fine solids is removed from liquids is termed as
sedimentation
Metallic wire mesh is used as a filtering medium for the separation of dust from dust laden gas in case of a/an
air filter
Sorting classifiers employing differential settling methods for separation of particles make use of the differences in their
terminal velocities
The specific cake resistance for incompressible sludges is (where ΔP = pressure drop over cake)
independent of ΔP
Angle of nip of the crushing rolls does not depend upon the
speed of the rolls
In constant pressure filtration,
rate of filtration decreases with time
The critical speed of a trommel (N) is related to its dia (D) as
N = 1/«D
Which of the following is not a revolving/tumbling mill used for size reduction?
Cage mill
Which of the following achieves the least reduction ratio for a given feed size?
Roll crusher
For coarse reduction of hard solids, use
compression
Work index is the gross energy (kWh/tonne of feed) necessary to reduce a very large feed to such a size that 80% of product particles will pass through a 0.1 mm screen. The value of work index determined for wet grinding should be multiplied with __________ to get the same for dry grinding.
1.34
Choke crushing (in case of a Jaw crusher) compared to free crushing
consumes more power.
The specific surface of sherical particles is proportional to (where, Dp = diameter of particle).
1/Dp
Wheat is ground into flour in a
roller crusher
Vacuum filter is most suitable for the
none of these.
Separation of materials into products based on the difference of their sizes is called
sizing
Hot, lumpy & abrasive materials are best transported by using a/an __________ conveyor.
apron
The maximum slope of a belt conveyor can be
30°
The unit of filter medium resistance is
cm-1
During size reduction by a jaw crusher, the energy consumed decreases with the
increasing size of feed at constant reduction ratio.
Which of the following can be most effectively used for clarification of lube oil and printing ink?
Sharpies supercentrifuge
Sizing of very fine particles of the order of 5 to 10 microns is done by elutriation, which is a __________ operation.
classification
Size reduction of fibrous materials like wood, asbestos, mica etc. is done by a disintegrator exemplified by the
cage mill
The equivalent diameter of channel of a constant non-circular cross-section of 3 cm by 6 cm will be __________ cms.
2
Which of the following may prove unsuitable for filtering volatile liquids ?
Vacuum filter
Mass flow of granular solid (M) through a circular opening of dia, D follows
M ∝ D»3
Sauter mean diameter is the same as the __________ mean diameter.
volume-surface
Cake resistance is
none of these.
In paint industries, blending of light paste is done by using a
charge can mixer
A sand mixture was screened through a standard 10-mesh screen. The mass fraction of the oversize material in feed, overflow and underflow were found to be 0.38, 0.79 and 0.22 respectively. The screen effectiveness based on the oversize is
0.50
__________ is used for producing a thick suspension from a thin slurry.
Pressure filter thickener
Wet seiving is employed, when the product contains __________ materials.
large quantity of very fine
Energy requirement (per unit mass of material crushed/ground) is highest for
fluid energy mill
In a mixer, the quantity, (v. L/D) is termed as __________ number(where, v = longitudinal velocity of material, L = length of the mixer, D = diffusivity in axial mixing).
Peclet
Cartridge filters are termed as ‘edge’ filters, because of the fact that the
bulk of the solids are removed at the periphery of the disks.
Cumulative analysis for determining surface is more precise than differential analysis, because of the
assumption that all particles in a single fraction are equal in size, is not needed.
Screw conveyors are
suitable for sticky materials.
Weber number is significant and is concerned with the
dispersion of liquid in liquid.
Power consumption during turbulent flow in agitation tank is proportional to the __________ of the liquid.
density
For grinding of cereals, grains, spices, pigments, saw dust, cork etc., the most extensively used size reduction equipment is a
Buhrstone mill
Sedimentation on commercial scale occurs in
rotary drum filters
With increase in drum speed, in a rotary drum filter, the filtration rate
increases
What is the usual value of angle of nip for crushing of ordinary rock in smooth steel crushing rolls ?
32°
For a turbine agitated and baffled tank, operating at low Reynold’s number (based on impeller diameter), the power number (Np) varies with NRe as
Np ∝ 1/NRe
Which of the following is not accomplished by agitation of liquids in agitators?
Blending of immiscible liquids.
Which of the following parts of a jaw crusher is subjected to maximum wear and tear during its operation ?
Jaw plates
A 30% (by volume) suspension of spherical sand particles in a viscous oil has a hindered settling velocity of 4.44 μm/s. If the Richardson Zaki hindered settling index is 4.5, then the terminal velocity of a sand grain is
1 μm/s
A cottrel precipitator makes use of the __________ for dusty air cleaning.
corrona discharge
If radius of a batch basket centrifuge is halved & the r.p.m. is doubled, then the
centrifugal force is doubled.
The ratio of the area of openings in one screen (Taylor series) to that of the openings in the next smaller screen is
none of these
For Indian standard (IS) screens, the mesh number is equal to its aperture size expressed to the nearest deca-micron (0.01 mm). Aperture width of IS screen of mesh number 50 will be approximately __________ microns.
500
__________ mills are termed as disintegrators.
Cage
Wet grinding in a revolving mill __________ as compared to dry grinding.
increases capacity
A compressible cake has the
maximum porosity at the upstream side.
Which of the following is not used as a filter medium in case of corrosive liquids ?
Cotton fabric
In ball mill operation, the feed size (Df in meters) and the ball diameter (Db in metres) are related as ( where, K = grindability constant (varying from 0.9 to 1.4 in increasing order of hard ness))
Db^2 = K.Df
Pine oil and cresylic acid are used as __________ in the froth floatation process.
frother
Grinding characteristic of a material is given by its
HGI
Float and sink test determines the possibility of cleaning of coal by a process based on the
gravity separation
Mixer used for rubber compounding is
banbury internal mixer
__________ mill is not used for grinding wheat into flour and for milling of cereals & other vegetable products.
Pebble
Which of the following screens has the maximum capacity ?
Vibrating screen
Stamp mills are generally used for crushing
gold ores
Which of the following is a batch sedimentation equipment ?
Filter thickener
The most suitable equipment for the transportation of 200 mesh size particles is a
pneumatic conveyor
Which of the following is not used as filter aid?
Rice husk
Raw materials are charged in the iron blast furnace using
skip hoist
The inlet pressure in a constant rate filtration
increases continuously
Particle size range in which dust catcher (gravity settling chamber) works most effectively and efficiently is __________ microns.
< 74
A propeller agitator
produces mainly axial flow.
In a grinding operation, the limiting particle size is the size of the __________ particle in the sample.
neither (a) nor (b)
In coal washeries, three products namely the valuable product (i.e. clean/washed coal), discarded product (i.e. mineral matter) and an additional concentrated product called __________ is produced.
middling
Actual operating speed of a ball mill may vary from 65 to 80% of the critical speed. Which of the following duties would require the ball mill to be operated at maximum percentage of critical speed ?
Dry grindng of large particles in un-baffed mills.
Reciprocal of sphericity is termed as the
shape factor
With increase in the pressure drop across the cake, the specific cake resistance for the compressible sludge
increases
Pick out the wrong statement.
200 mesh screen has 200 openings per linear cm.
Xanthates are used in the froth floatation process as a/an
collector
The ratio of the actual mesh dimension of Taylor series to that of the next smaller screen is
«_space;2
Filtrate flow rate in case of a rotary drum vacuum filter (in which Rm < < Rc) is proportional to __________ and the cycle time (where, μ = filtrate viscosity Rm = filter medium resistance Rc = cake resistance ).
1/«_space;u
Introduction of slurry in a plate and frame filter press is done through a plate in each frame. The plate of this filter has a __________ surface.
ribbed
Use of grinding aids results in the
neither (a) nor (b).
__________ mill is not a revolving mill.
Cage
Which of the following is a fine crusher?
Toothed roll crusher
Removal of activated carbon from glycerine is done by
plate and frame filter
