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Solid separation
processes
Ali Ahmadpour
Chemical Eng. Dept.
Ferdowsi University of Mashhad
2
Contents
Introduction
Physical properties of solids
Separation of particulates & powders
Air classification
3
References
Particle size measurements, T. Allen, 1997
Vol. 1: Powder sampling and particle size measurements
Vol. 2: Surface area and pore size determination
Powder surface area and porosity, Lowell & Shields,
1983
Powder technology: fundamentals of particles, powder
beds, and particle generation, M. Hiroaki, H. Ko, Y.
Hideto, 2007
Air pollution control equipment, H. Brauner & Y.B.G.
Varma, 1981
4
Introduction
Separations involving solids, together with their properties which influence the separation will be discussed.
The removal of solids from gases will be illustrated, to show some of the difficulties in selecting solids separation methods.
Solids come in many forms, shapes and sizes, so some discussions of the main properties of solids which will influence different types of separation processes will be discussed.
5
Mechanical solid separation
techniques
Solids from liquids
Sedimentation:
Principles: gravity, centrifugal, electrostatic, magnetic centrifugation
Examples: gravity settlers, centrifugal clarifiers, hydrocyclones; use
of chemical flocculants or air flotation
Filtration:
Principles: gravity, vacuum, pressure and centrifugal
Examples: sand and cake filters, rotary vacuum filters, cartridge and
plate and frame filters, microfilters, use of filter aids
Solids from gases
6
Physical properties of solids
Solids come in a wide variety of shapes and sizes.
Solids contain moisture ranging from <10% to > 90%.
Some operations where separations from solids is
involved, are:
Cleaning of products,
Sorting and size grading,
Fractionation or recovery of the main components
within the solid bulks.
7
Cont.
Some separation operations is concerned with the fractionation of solids
(in their particulate or powder form), and their recovery from other
materials.
Separation of powders are based on factors such as:
size and shape,
density differences,
flow properties,
color and electrostatic charge,
An important pretreatment for many operations is size reduction, but in
some cases very fine powders provide processing problems, and
agglomeration may be used to improve flow characteristics and
wettability.
8
Physical properties of solids
9
Classification of powders
Particle size and particle size distribution
Particle shape
Particle density
Forces of adhesion
Bulk properties
Bulk density and porosity
Flowability
10
Particle size and PSD
Operations that result in the production of a powder, e.g.
milling or spray drying, will give rise to a product with a
distribution of particle sizes and this distribution is of extreme
importance and will affect the bulk properties.
Particle size can be measured by measuring any physical
property which correlates with the geometric dimensions of
the sample.
geometric characteristics, such as linear dimensions, areas, volumes,
mass (microscopy or image scanning techniques);
settling rates (wet and dry sieving methods);
interference techniques such as electrical field interference and light or laser scattering or diffraction (electrical impedance methods such as the Coulter counter, laser diffraction patterns).
11
Sampling
Since particles can vary in both shape and size, different
methods of particle size analysis do not always give consistent
results.
different physical principles being exploited,
size and shape are interrelated.
Sampling is important to ensure that a representative sample is
taken, usually by the method of quartering.
The results are present in the form of a distribution curves:
Frequency distribution (histogram)
Cumulative distribution
12
Frequency (F) and Cumulative (C)
distributions
13
Cont.
From the distribution curves, mean diameter, median diameter
and standard deviation can be calculated.
Mean diameter:
Median diameter: the diameter which cuts the cumulative
distribution in half.
Standard deviation:
Sauter mean particle diameter (d3/2):
i
ii
n
dnd
n
ddn2
ii
2ii
3ii
2/3dn
dnd
14
Cont.
Sauter diameter (d3/2):
Equivalent diameters: For particles with shapes other than
sphere, the diameter is calculated from the comparison of their
surfaces or volumes to sphere.
2/3d
6
Volume
area Surface
241.16
d6
dV
3/1
v
3v
382.1
6ddS
2/1
s2s
sv3v
2s
v3v
2s
d
6
d
d6S
6/d
d
V
S
15
Cont.
The particle size and distribution has a pronounced effect on
interparticle adhesion, which will affect some of the bulk
properties, such as bulk density, porosity, flowability and
wettability.
16
17
• Feret's Diameter. This is depicted as dimension 'A', it is the overall length from
'tip-to-tail' of the particle.
• Martin's Diameter. This is depicted as dimension 'B', it is the length of a
theoretical horizontal line, which passes through the centre of gravity of the
particle, to touch the outer boundary walls of the particle.
• Projected Area Diameter. This is depicted as dimension 'C' and is the diameter
of a theoretical circle, which would contain the same projected area as the
irregular particle.
• Equivalent Diameter. This is the diameter of a sphere, which would contain the
same volume as the irregular particle.
• Aerodynamic Diameter. This is the diameter of a spherical particle that exhibits
the same settling velocity as the irregular particle.
18
Sampling technique(Coning and quartering process)
19
Sampling
devices
20
Sampling points
21
Sampling devices
22
Unit dose sampler
A popular sampler in the pharmaceutical
industry for taking a small volume
cohesive powder sample.
SLOT SAMPLER
For target, multilayer and
average sampling
23
Powder sampler
24
Sampling from falling streams
25
Sample splitter
26
Particle shape
Sphere has the lowest and a chain of atoms has the highest
surface/volume ratio.
The relation between particle’s surface area and shape can be
shown by assuming two particles with same weights one in
sphere and the other in cubic forms.
spherecubespherecubespherecube VVVVMM
3
rS
6
lSr
3
4l
sphere
spherecubecube3
sphere3cube
cube
sphere
sphere
cube
l
r2
S
S
27
Porosity
Porosity is the summation of surfaces of those pores that their
depths are more than their diameters.
Surface area of non-porous sphere particles:
Particles with r = 0.01 m and = 3 g/cm3 have 100 m2/g surface area.
Particles with r = 0.1 m and = 3 g/cm3 have 10 m2/g surface area.
Particles with r = 1 m and = 3 g/cm3 have 1 m2/g surface area.
But, porous particles with r = 1 m and = 3 g/cm3 have >1000 m2/g surface
area. This shows the importance of porosity.
1i
i2in
2n2
221
21t Nr4NrNrNr4S
1i
i3in
3n2
321
31 Nr
3
4NrNrNr
3
4MV r
3S
Nr
Nr3
M
SS
1i
i3i
1i
i2i
t
28
Particle density
The density of an individual particle is important as it will
determine whether the component will float or sink in water or
any other solvent; the particle may or may not contain air.
Air has a density of 1.27 kg/m3.
Therefore, this equation is not applicable where there is a
substantial volume fraction of air in the particle.
29
Cont.
An estimate of the volume fraction of air (Va) can be made
from:
Differences in particle densities are exploited for several
separation techniques, e.g. flotation, sedimentation and air
classification.
30
Forces of adhesion
There are interactions between particles, known as
forces of adhesion and also between particles and the
walls of containing vessels.
These forces of attraction will influence how the
material packs and how it will flow.
Interparticle adhesion increases with time, as the
material consolidates.
Flowability may be time-dependent and decrease
with time.
31
Fractal geometry
To characterize rough or textured surfaces,
Mandelbrot suggested a new geometry in 1975.
According to him, there are new dimensions
between the common dimensions of 1, 2, and 3
known as fractal dimensions (D).
Brian Kaye (1991) has elaborated the importance
of fractal geometry in particle characterization.
32
Cont.
If we put a irregular shape in a polygon with
length of “”, its perimeter (P) will be increased
by reduction of length.
Polygon with n sides:
Mandelbrot showed that:
Therefore, plotting logP vs. log gives a straight
line with 1-D slope.
nP
D1kP
33
Cont.
34
Cont.
35
36
37
Fractal in
nature
38
Bulk properties
In most operations, the behavior of the bulk particles
is very important.
The bulk properties of fine powders are dependent
upon:
Geometry,
Size,
Surface characteristics,
Chemical composition,
Moisture content, and
Processing history.
39
Cont.
The behavior of powders influenced by forces of attraction (or
repulsion) between particles is called cohesiveness.
For cohesive powders, the ratio of the interparticle forces (F)
to the particles’ own weight is large.
F 1/d2 small particles adhere to each other more strongly
than large particles.
For majority of particles, when the particle size exceeds 100
m, they are non-cohesive (free flowing).
Increase in moisture content makes powders more cohesive.
40
Bulk density and porosity
The bulk density (b) is an important property,
especially for storage and transportation, rather than
separation processes.
b = (mass / total volume occupied by the material).
Total volume includes air trapped between the
particles.
The volume fraction trapped between the particles is
known as the porosity ().
s
b1
41
Cont.
True (Skeletal) density: measured with helium (mass / volume of the solid).
Apparent density: measured by liquid
displacement (mass / voids volume + solid volume).
Bulk densities:
Loose density: (mass / total volume occupied by the material).
Compact (tap) density: (mass / total volume occupied by the
material after mechanical compression).
42
Cont.
The ratio of tapped bulk density to the loose bulk
density is referred to as the Hausner ratio.
Hayes (1987) quotes the following ranges:
43
44
Flowability
The flowability of powders is very important in their handling.
Flowability increases with increasing particle size and decreasing
moisture content.
Factors used to assess flowability are:
Compressibility
Cohesiveness
Slide angle: Placing the powder sample on a flat smooth horizontal
surface and then slow inclination until the powder begins to move
The angle at which movement occurs is the slide angle.
Angle of repose: This is useful in the design of powder handling
systems. Its value depends upon the method of determination (forming
a heap, bed rupture, or rotating drum method). It is affected by
frictional forces and interparticle attractive forces.
45
Cont.
According to Carr:
Angles up to 35° free flowability;
35 - 45° some cohesiveness;
45 - 55° cohesiveness or loss of free flowability;
>55° very high cohesiveness, very limited or zero
flow.
46
Slide angle
47
Angle of repose
48
Angle of repose
49
Cont.
A more fundamental method for flow behavior of powders is
based on the work of Jenike.
A flow cell is used, where the powder is first consolidated to
a particular bulk density and porosity. It is then subjected to a
compressive force (N) and the shear force (S) required to
cause the powder to yield and shear is determined. These
readings are converted to a normal stress () (N/A) and a
shear stress () (S/A).
50
Solid characterization
(a) Jenike flow cell;
(b) normal stress against shear stress, for a non-cohesive powder, = angle of
friction;
(c) yield locus for a cohesive powder for powders compacted to different initial
porosities; porosity 1 > 3;
51
Cont.
Unconfined yield stress (fc)
Major consolidation stress (l)
The ratio of l/fc which is called the Jenike flow
function, is an indicator of the flowability of
powders. Its values correspond to the following
characteristics:
52
Definition of stress
53
Types of stress
Shear Stress
Bending Stress
54
Cont.
The flowability is extremely useful for designing hoppers,
bins, pneumatic conveying systems and dispensers.
The hydrodynamics of powder flow are different to that for
liquids. The pressure does not increase linearly with height,
rather it is almost independent.
They can resist appreciable shear stress and can, when
compacted, form mechanically stable structures that may halt
flow. Also, any pressure or compaction can increase the
mechanical strength and hence the flowability.
55
The behavior of bulk solids in silos
v : vertical stress
h : horizontal stress
: stress ratio
56
Cont.
Pressures in fluids and stresses in bulk solids
57
Cont.
Qualitative courses of wall normal stresses (w) and assumed
trajectories of the major principal stress (1)
58
Cont.
Wall normal stress in funnel flow silos
a. steep border line b. flat border line
59
Cont.
60
Cont.
61
Separation of particulates and
powders
The separation or recovery of solids from within a
solid matrix or from a particulate system is
concerned.
The main emphasis will be in fine particulate form,
so the production of material in a form suitable for
separations is often crucial for the process. In this
respect, size reduction and milling equipment is
important.
62
Size reduction
Size reduction is a very important preliminary
operation for several separation processes, extraction
operations, or expression processes.
Crushing: reduction of coarse material down to a
size of about 3 mm.
Grinding: production of finer powdered material.
The degree of size reduction can be characterized by
the size reduction ratio (SRR).
63
Cont. The main forces involved in size reduction are:
compressive forces,
impact forces,
shear or attrition forces.
The fracture resistance increases with decreasing particle size.
In selection of appropriate equipment for size reduction, two things need to be considered: particle size range required,
hardness of the material.
Hardness can be measured in Mohs, whose scale ranges between 0 and 8.5. very soft ( < 1.5 Moh),
soft (1.5 to 2.5 Moh),
medium hard (2.5 to 4.5 Moh),
hard (4.5 to 8.5 Moh).
64
Cont.
Different mills for processing grains include:
1) Hammer mills: general-purpose mills; impact forces; used for spices,
sugar and dried milk powder.
2) Roller mills: one or several sets of rollers; compressive forces; SRR is
<5; used for milling of wheat; size range 10-1000 m.
3) Disc attrition mills: two discs, one is stationary and the other moving;
peripheral velocity of 4-8 m/s; used for grindings; size range down to
100 m.
4) Ball mills: tumbling mills used for very fine grinding processes; a
horizontal slow-speed rotating cylinder contains steel balls (d=25-150
mm) of hard stones; impact and shear mechanism.
65
Hammer mill
66
Roller mill
67
Disc attrition mill
68
Pin mill
69
Ball mill
70
Cost of milling
The particle size affects the cost of milling and the energy
requirement. Energy is based on the following equation:
where dE is the energy required to produce a small change in
diameter dD and Km is a characteristic of the material. The
three main equations result from different values of n are:
71
Wet milling
Wet milling is achieved by wetting the material and the
feedstock is ground in a suspension in the liquid, which is often
water.
Energy requirements are usually slightly higher than for dry
milling but a finer powder is obtained and dust problems are
eliminated.
Often wet milling is useful as part of an extraction process,
whereby soluble components are transferred from the solid to
the liquid phase.
72
Sieving
Sieving is the easiest and most popular method for size analysis
and separation of the components within powders.
A sieve is an open container with uniform square openings in
the base.
The effectiveness of a sieving process depends upon:
amount of material placed on the sieve,
type of movement,
time of the process.
73
Cont.
The sieving time can be affected by the
following factors:
the material characteristics, e.g. fineness, particle
shape, size distribution, density;
intensity of sieving;
nominal aperture size of the test sieve;
characteristics of sieving medium;
humidity of the air.
74
Air classification
Air classification is a means of using a gaseous
entraining medium, which is usually air, to separate a
particulate feed material (for particles <50 m) into a
coarse and fine stream, on a dry basis.
Separation is based mainly upon particle size,
although other particle properties, such as shape,
density, electric, magnetic and surface properties may
play a part.
75
Simple classifiers
(a)aspiration F = fan;
(b)fractionation L = large; S = small particles;
(c)zig-zag classifier.
76
Commercial air classifiers
In commercial air classifiers, the gravitational force is used
supplemented by a centrifugal force. This is essential for
separating small particles and speeds up the separation
process.
Air classifiers are categorized by factors, such as:
the forces acting upon the particles; e.g. the presence or absence of a
rotor, the drag force of the air and the presence of collision forces;
the relative velocity and direction of the air and particles, controlled
by their respective feed systems;
directional devices such as vanes, cones or zig-zag plates;
location of the fan and fines collection device (internal or external)
77
Cont.
Other important features are:
capacity of the classifier,
energy utilization.
In processing coal dust and cement classifiers,
flow rates of over 100 tonnes/h can be handled.
Classifiers handling foods can process more
than 5 tonnes/h.
78
Commercial air classifiers
79
Cyclone separation
80
Cyclone
81
Cont.
82
83
Hydrocyclones
84
Process characterization
In most cases, air classification work is empirical because of the
difficulties in quantifying the forces acting upon a particle.
One method of characterizing the separation is by means of the
cut size. Ideally, all particles below the cut size end up in the fines
and all particles above the cut size end up in the coarse stream.
The cut size is defined as that size where the weight of
particles below the cut size in the coarse fraction is the same
as the weight of coarse particles above that size in the fines
stream.
85
Cont.
Factors which influence the cut size are:
dimensions of the classifying chamber,
peripheral forces
the spiral gradient.
The cut point can be adjusted by varying:
the rotor speed,
air velocity,
vane setting,
feeding rate.
86
Cont.
By equating these forces when they are in equilibrium,
an equation for the cut size (d) can be derived. This is
based on Stokes’ equation:
= viscosity of air
a = radial speed of air
r = clearance of classifier wheel
= particle density
p = rotational speed
87
Cut size determination
(a) ideal separation;
(b)real separation, weight frequency distribution;
88
Grade efficiency
The cut size alone does not provide information on how sharp
the separation is.
An alternative method of evaluation is grade efficiency, which
also indicate the sharpness of the separation.
The particle frequency distribution is determined by weight for
the coarse stream (qc(x)) and feed material (qf(x)) .
The yield is determined for the coarse stream Yc.
The grade efficiency T(x) indicates for any particle size x, the
mass fraction of feed material appearing in the coarse fraction.
89
Grade efficiency vs particle size
(a) ideal separation; (b) and (c) decreasing sharpness.
90
Cont.
The sharpness of the separation is measured by the
ratio k = [x25t/x75t], i.e. the ratio of the sizes giving
grade efficiencies of 0.25 and 0.75 respectively.
Ideally k = 1.0.
The best industrial air classifiers achieve k = 0.7, but
typically commercial air classifiers show k values
from 0.3 to 0.6