Mechanical Operations Flashcards
Concepts and heuristics necessary for unit operations and processes involving solids (or with fluids) requiring mechanical operations.
Short distance transportation of grain, gravel, sand, ash, asphalt etc. is done by using a ____ conveyor.
screw
Equivalent diameter of a particle is the diameter of the sphere having the same
ratio of surface to volume as the actual volume
If dp is the equivalent diameter of a non-spherical particle, Vp its volume and sp its surface area, then its sphericity is Φs is defined by
Φs = 6 Vp/dp sp
Power required to drive a ball mill with a particular ball load is proportional to (where, D = diameter of ball mill )
D2.5
Pick out the wrong statement.
Size enlargement (opposite of size reduction) is not a mechanical operation
In continuous filtration (at a constant pressure drop), filtrate flow rate varies inversely as the
square root of the velocity
For sizing of fine materials, the most suitable equipment is a
vibrating screen
Which of the following is not categorised as a “mechanical operation” ?
Humidification
Which of the following is a pressure filter?
Plate and flame filter
Tabular bowl centrifuges as compared to disk bowl centrifuges
operate at higher speed
Pebble mills are tumbling mills widely used for grinding in the manufacture of paints & pigments and cosmetic industries, where iron contamination in the product is highly objectionable. Pebbles used in pebble mill are made of
flint or porcelain
Filter aids like asbestos, kieselguhr, diatomaceous earth etc. are used to increase the porosity of the final filter cake & reducing the cake resistance during filtration. Filter aid is
a. added to the feed slurry.
b. precoated on the filter medium prior to filtration.
c. separated from the cake by dissolving solids or by burning it off.
all ‘a’, ‘b’&’c’
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
Pick out the wrong statement pertaining to the roll crushers.
Both the rolls run necessarily at the same speed
Which of the following is not an ultrafine grinder (colloid mill)?
Toothed roll crusher
Two particles are called to be equal settling, if they are having the same.
terminal velocities in the same fluid & in the same field of force
Which of the following is the most suitable for handling fibrous and dense slurries?
Cone type agitator
____ 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
The distribution given by microscopic analysis of powder is
number
Number of particles in a crushed solid sample is given by (where, m = mass of particles in a sample, Vp = volume of one particle, ρ= density of particles)
m/ρ . Vp
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
In washing type plate and frame filter press, the ratio of washing rate to the final filtrate rate is
1/4
Ball mills and tube mills with flint or porcelain balls are used for size reduction of
non-metallic ores
Size reduction of ____ is accomplished in steam heated rollers and roll crushers.
hard rubber
The capacity of a belt conveyor depends upon two factors. If one is the cross-section of the load, the other is the ____ of the belt.
speed
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’?
a. Panning
b. Spiralling’
c. Tabling
None of these
The capacity of a pneumatic conveying system depends upon the
a. bulk density of materials.
b. pressure of the conveying air.
c. diameter of the conveying line.
all (a), (b) and (c)
Pick out the wrong statement.
The equipment used for the removal of traces of solids from a liquid is called a classifier
Gravity settling process is not involved in the working of a
hydrocyclone
Froth floatation is the most suitable for treating
sulphide ores
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/to
Which of the following is the most suitable for cleaning of fine coal dust (<0.5 mm)?
Froth floatation
Pick out the correct statement:
The operating cost of shaking screen is more than that of a vibrating screen
Range of compressibility co-efficient of the commercial compressible cake obtained in filtration operation is
0.2 to 0.8
Highly viscous liquids & pastes are agitated by
multiple blade paddles
The value of ‘angle of nip’ is generally about
32°
Size reduction of the ____ can be suitably done by ball mills, crushing rolls and rod mills.
metalliferrous ores
Sulphuric acid mist is arrested by using a ____ scrubber.
venturi
The main size reduction operation in ultrafine grinders is
attrition
In case of a hammer crusher,
a. crushing takes place by impact breaking.
b. maximum acceptable feed size is 30 cms.
c. reduction ratio can be varied by adjusting the distance from cage to hammers.
all (a), (b) and (c).
Jigging is a technique by which different particles can be
separated by particle size.
Crushing of mineral particles is accomplished in a ‘cage mill’, when one or more alloy steel bars are revolved in opposite directions. It is a type of ____ mill.
impact
Balls for ball mills are never made of
lead
200 mesh screen means 200 openings per
inch
____ is the most commonly used ‘filter aid’.
Diatomaceous earth
Energy consumed for crushing one ton of material ranges from ____ kWh.
0.5 to 1.5
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.
Trommels separate a mixture of particles depending on their
size
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.
Dust laden air can be purified using a
cyclone separator
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
Capacity (in tons/hr) of jaw/gyratory crusher is equal to (where, L = length of the receiving opening, cm S = greater width of the discharge opening, cm)
0.087 L.S
Vacuum is applied in ____ zone, in case of a general type continuous rotary drum vacuum filter.
a. filtering
b. washing
c. drying
all (a), (b) & (c)
General crushing equation is given by https://www.indiabix.com/_files/images/chemical-engineering/mechanical-operations/122-368-1.png . Bond’s crushing law isobtained by solving this equation for n = ____ and feed of infinite size.
1.5
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 mechanical conveyors does not come under the division ‘carriers’?
Screw conveyor
Screen capacity is not a function of
atmospheric humidity
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
Which is the most suitable conveyor for transportation of sticky material?
Screw conveyor
The most suitable equipment for removing the fine dust particle (< 1 micron dia.) from air below its dew point will be a/an
electrostatic precipitator
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
Moore filter is a ____ filter.
leaf
Sphericity of a cubical particle, when its equivalent diameter is taken as the height of the cube, is
1
Ball mill is used for
fine grinding
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
Increasing the capacity of a screen ____ the screen effectiveness.
decreases.
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.
Flow of filtrate through the cake in a plate and frame filter press is best described by the ____ equation.
Kozney-Karman
The controlling resistance in a rotary drum vacuum filter is the ____ resistance.
cake
Which of the following is used for primary crushing of very hard lumpy materials?
Gyratory crusher
Coal is finally pulverised to 200 mesh size for burning in boilers by a
ball mill
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
The crushed material received for separation is called feed or
heading
For spheres, volume shape factor is given by
π/6(=V/D3)
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
The operating speed of a ball mill should be ____ the critical speed.
less than
Use of baffles in agitators help in minimising the ____ tendency.
a. swirling
b. vortexing
both (a) & (b)
____ are used for the separation of coarse particles from a slurry of fine particles.
Classifiers
Fibrous material is broken by a
squirrel-cage disintegrator
____ is the most suitable for compounding rubber and plastic solids.
Banbery mixer
Out of the following size reduction equipments, the maximum feed size can be accepted by the
jaw crusher
Bucket elevators are not suitable for the vertical lifting of ____ materials.
sticky (e.g. clay paste)
The most common filter aid is
diatomaceous earth
For removal of very small amounts of precipitate from large volume of water, the most suitable filter is the ____ filter.
sand
For separation of sugar solution from settled out mud, we use a ____ filter.
centrifugal
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.
0.3
What is the selectivity index, if the grade of tailings & concentrate is the same?
1
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
Colloid mills achieve size reduction mainly by
attrition
Mesh indicates the number of holes per
linear inch
For achieving maximum capacity of the ball mill, the ball charge should be equal to about ____ percent of the ball mill volume.
50
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).
sqrt(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
Electrical energy consumed by a jaw crusher is not a function of the
none of these
Size reduction mechanism used in Jaw crushers is
compression
Trommels employ ____ for screening of materials.
punched plate
A widely used size reduction equipment for ____ is Bradford breaker.
coal
Ultra centrifuges are used for the separation of ____ solid particles.
colloidal
Specific surface area is the surface area of a unit ____ of materials.
either (a) or (b)
A. ____ mill is a revolving mill divided into two or more sections by perforated partitions in which preliminary grinding takes place at one end and the finishing grinding at the discharge end.
compartment
The constants (Kb, Kr and Kk) used in the laws of crushing (i.e., Bond’s law, Rittinger’s law and Kick’s law) depend upon the
a. feed material.
b. type of crushing machine.
both (a) & (b).
Size reduction does not occur due to compression in case of
rod mills
According to Taggart’s rule for selecting between a gyratory crusher and a jaw crusher; the later should be used, if the hourly tonnage to be crushed divided by the square of the gape expressed in cm is less than
0.0184
Which of the following represents the plot of filtrate volume versus time for constant pressure filtration?
Parabola
Driving force in case of filtration by a centrifuge is the
centrifugal pressure exerted by the liquid.
Rod mills employed for grinding
a. employ a steel shell having L/D ratio of 1.5 to 3.0.
b. is useful for handling sticky materials.
c. employ steel rods of 2-12 cms diameter extending over full length of the mill.
all ‘a’, ‘b’ & ‘c’.
Production rate ____ with increased fineness, with a given energy input to the size reduction machine.
decreases
Gold ore concentration is mostly done using
tabling
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
Sphericity is the ratio of the surface area of a spherical particle having the same volume as the particle to the surface area of the particle. Which of the following has the maximum value of sphericity?
Sphere
The unit of specific cake resistance is
gm/cm2
Size reduction of asbestos and mica is done by
hammer mills
A pebble mill
a. is a ball mill.
b. employs flints or ceramic pebbles as the grinding medium
c. is a tube mill lined with ceramic or other non-metallic liner.
both (b) and (c).
The study on washability of coal is done by using the ____ technique.
heavy media separation
Horsepower required for a roll crusher is directly proportional to its
both (a) & (b).
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
____ conveyor is the most suitable for long distance transportation of cold, non-abrasive granular/irregular shape/fine materials.
Belt
The specific cake resistance for compressible sludges is a function of the pressure drop
over cake
Moisture can be removed from lubricating oil using
tubular centrifuge
Tabling process used for separating two materials of different densities by passing the dilute pulp over a table/deck, which is inclined from the horizontal surface at an angle of about
2 to 5
Pick out the wrong statement.
Crushing efficiency is the ratio of the energy absorbed by the solid to the surfaces energy created by crushing.
Addition of filter aid to the slurry before filtration is done to …… of the cake.
increase the porosity
Which of the following terminology is not used for size reduction of materials to fine sizes or powders?
Compression
A tube mill as compared to a ball mill
a. employs smaller balls.
b. gives finer size reduction but consumes more power.
c. has larger length/diameter ratio (>2 as compared to 1 for ball mill).
all (a), (b) and (c).
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.
Critical Speed (Nc) of a ball mill is given by (where R1 and R2 are radii of ball mill and the ball respectively).
Nc = (1/2π)*sqrt(g/(R1-R2))
Gizzlies are used for separating ____ solids.
fine
Which of the following clay mixing devices is vacuum operated for deairation of clay?
Pug mil
Pick out the material having minimum Rittinger’s number.
Quartz
____ mills are termed as impactors.
Hammer
In case of a revolving mill, wet grinding compared to dry grinding
none of these.
To get a fine talc powder from its granules, the equipment used is
ball mill
For a cyclone of diameter 0.2 m with a tangential velocity of 15 m/s at the wall, the separation factor is
230
Width and speed of a conveyor belt depends upon the ____ of the material.
both (a) & (b)
Higher is the mesh number, smaller will be the aperture size of the screen. It means that the aperture size of a 200 mesh screen will be smaller than that of 20 mesh screen. This is valid for
all (a), (b) and (c).
Pick out the wrong statement.
Capacity of crusher in choke feeding is increased.
Which of the following size reduction equipments employs mainly attrition for ultrafine grinding?
all (a), (b) and (c)
Tank filter (e.g., Nutsch filter) is
used for small scale filtration work.
In bag filters, filter fabrics are never made of
metallic wire woven mesh
Use of ‘grinding aids’ is done in ____ grinding.
dry
Height of liquid in agitation tank is normally maintained equal to the tank diameter. However, if the tank is too tall and a large liquid hold up is desired, then two or more impellers mounted on the same shaft may be used. The clearance between the tank bottom and the bottom most impeller should be about (where, D = impeller diameter)
D
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
A ____ mixer resembles a ball mill without balls.
tumbling
____ conveyor is the most suitable for short distance transportation of non-abrasive loose materials like garbage, grain, food wastes etc.
Flight
As the product becomes finer, the energy required for grinding
icreases
Gyratory crushers compared to the reciprocating jaw crushers
have greater capacity per unit of discharge area.
The important dimensional group involved in the power requirement calculation in mixing operation is the ____ number.
both (a) & (b)
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 ∝ Re-1.0
Which of the following grinding mills has the horizontally arranged rods as the grinding elements thereby delivering more uniform granular products with minimum fines?
Rod mill
Rittinger’s crushing law states that
work required in crushing is proportional to the new surface created.
Mechanical conveyors which push the material along an endless trough or tube are called scrappers. Which of the following conveying equipments comes under the category of ‘scrappers’?
Both (b) and (c)
Sphericity of pulverised coal is
<1
