Note: Descriptions are shown in the official language in which they were submitted.
I 337 1 ~0
P~OC~SS FOR 3ENEFICIATING PA~TICULATE SOBI~S
Fleld Of The Invention
The present invention relates to an improved
process for beneficiating coal fines and for predicting
05 the efficiency of separation of density separation
processes.
Background Of The Invention
The burning of fossil fuels, including coal, is
necessary to meet the energy requirements of our
soclety. However, the combustion of coal, and in par-
ticular, many lower grades of coal, produces sulfur
oxides which are emitted to the atmosphere. The
release of these compounds produces many detrimental
environmental effects. Respiration of these pollutants
can cause human health problems ranging from mild
respiratory irritation to more serious chronic dis-
eases. Sulfur oxides can also react with other com-
positions in the atmosphere to form acid preciDitation
which has the effect of acidifying bodies of water and
destroying the wildlife which live in such habitats.
Acid precipitation also can destroy manmade structures
such as buildings and statues.
Industry has sought to burn coal with low sulfur
content to avoid the problems associated with sulfur
oxides emissions. However, such fuel is not always
readily available and the costs to recover and trans-
port such high quality coal is in many cases prohibi-
tive. Therefore, to meet the objective of environmen-
tally acceptable coal combustion, effective methods areneeded to remove sulrur compounds from the coal before,
during, and arter combustion.
Recent revisions in the Federal Clean Air Act
require a ninety percent reduction in pounds of sulfur
dioxide per million Btu for high sulfur coal before
release to the ~tmosphere of combustion byproducts for
new sources or air pollution. Some states have applled
stringent reauirements for reduc~ion of sulfur dioxide
to existing facilities. Federal and state legislation,
! (l 337 1 ~0
there~cre, make it necessary to achieve high reductlons
in the amount or sulfur compounds emitted during the
combustion of coal.
A method of reducing the sulfur content of coal
05 before combustion includes~ grinding the coal to a
small particle size to liberate the inorganic sulfur
containing compounds and other ash forming minerals
from coal; and (2) separating the inorganic material
bearing sulfur from the organic portion, coal. A major
limitation in this technique is that when coal is
ground fine enough to liberate substantial quantities
of sulfur minerals and ash-forming minerals, separation
of the coal from the unwanted material and subsequent
recovery of the coal become difficult.
The grind size required to enable a ninety percent
pyrite reduction and eighty-five percent Btu recovery
for most coals is less than 0.5 mm and frequently flner
than 0.1 mm. At these sizes, reported beneficiation
techniques are not consistently effective in separating
coal at acceptable efficiencies.
Jigs, hydrocyclones and tables are inefficient for
separation of minus 0.5 mm coal. Froth flotation is
ineffective when applied to o~idized coals because
their surface character is not sufficiently hydrophobic
to be activated by collecting reagents. For unoxidized
coals, good Btu recovery is attainable by froth flota-
tion, but pyrite rejection is difficult because of the
relative ease with which pyrite floats.
Ergun, U.S. Patent No. 3,4~3,310 discloses a
method of cleaning fine coal material (0.400 mm - 0.037
mm) by subjecting a mi~ture of coal and pyrite to
electro-magnetic radiation which selectively magnetizes
pyrite. Pyrite is then removed by magnetic means.
This process is limited to magnetizable refuse material
such as pyrite. Other materials frequently found in
coal, such as silica, canr.ot be removed by this method.
Dense media cyclones are efficient devices for
separating coal in the quarter inch to O.S mm range
--2--
- 1 337 1 90
from refuse material on the basis of coal and refuse
material having different densities. A mixture of the two
materials is suspended in a dense media to form a sink
product and a float product. A dense media, or a psuedo-
heavy liquid, is necessary because the specific gravities ofcoal and refuse material are greater than one, and
therefore, cannot be separated by water alone. A media with
an effective media specific gravity between that of coal and
of refuse material is required. A common media useful for
coal beneficiation is a suspension of magnetite particles in
water. By introducing coal-refuse material mixture into a
magnetite media, clean coal floats and refuse material
sinks. Separation of these materials is hastened by using
a dense media cyclone which increases the nominal
gravitational acceleration on the mixture.
The use of dense media cyclone separations for
beneficiating coal is well known. For example, Miller, et
al., U.S. Patent No. 3,794,162, is directed toward a heavy
medium beneficiating process for coal particles greater than
150 mesh (about 0.1 mm). Horsfall, U.S. Patent No.
4,140,628, is also directed toward a dense medium separation
process. Horsfall discloses the use of magnetite particles
less than 0.100 mm for beneficiation of coal fines having a
particle size less than 1.000 mm and, in particular, less
than 0.500 mm. This process involves separation of
materials in a suspension with a dense media to form two
fractions and a series of subsequent screenings and washings
of magnetite from the two fractions. Horsfall, however,
does not address the question of efficiency of separation of
the two products.
Previous attempts to extend the performance of dense
media cyclones below 0.5 mm have generally met with limited
success and, in particular, have been unsuccessful in terms
of teaching a general method for efficient separation. One
parameter which is useful in assessing the effectiveness of
separation of coal fines
~ ;
- ~ 33Zl 90
and refuse material by dense media and cther separation
technicues is the Ecar~ P-obable (Ep). The Ep ~alue is
defined as the difference between the particle density
of that f action of the cyclone feed having a 75~
05 chance of reporting to the overflow minus the particle
density of that fraction of the cyclone feed having a
25% chance of reporting to the overflow divided by two.
~ The separation gravity is defined as the specific
gravity of a small increment of the feed which reports
fifty percent to the clean coal overflow and fifty per-
cent to the refuse underflow. The Ep value is a
measure of the sharpness cr efficiency of the separa-
tion, while the separation gravity defines the specific
gravity at which the separation occurred. This separa-
tion gravity is different for different size fractlonsof feed coal even though all size fractions are cle~ned
in the same dense media cyclone. Generally, a smaller
size ft~ction has a higher separation gravity. Also,
the specific gravity of the dense media is generally
less than the separation gravity.
A typical dense media is a suspension of magnetite
particles in water. The magnetite can be natural mag-
netite which has been milled. Magnet~te is also
recoverable from fly ash. For e~ample, Aldrich, U.S.
Patent No. 4,432,868 discloses that magnetite particles
less than 325 mesh in diameter, having 90% magnetics,
and a specific gravity between 4.1 and 4.5, can be ob-
tained from fly ash. Aldrich further discloses that
such magnetite contains a high proportion of round par-
ticles which are desirable for heavy medium separationbecause round particles reduce the viscosity of the
heavy medium and facilitate separation.
Fourie, et al., The Beneficiation of Fine Coal by
Dense-Medium Cyclone, J. S. African Inst. Mining and
Metallurgy, pp. 357-61 (October 1980), discloses dense
media cyclone separ~tion of a 0.5 mm - 0~075 mm coal
~ action in a heavy medium cyclone with milled ~ag-
netite with at least fifty percent less than 0.010 mm
1 337 1 90
`~ using z 150 mm diameter cyclone. E~ val~es from 0.020
to 0.031 were achieved. While accepta~le ~eparation
efflciences were achieved by Fourie, et al., the
reference does not address cleaning the minus 0.075 mm
os coal fraction or provide a general method for determin-
ing operational parameters necessary to achieve accept-
able efficiences.
Extending the capability of density separation
beyond reported limits to effectively separate coal
fines smaller than 0.5 mm and particularly smaller than
0.075 mm is highly advantageous. Substantial reduc-
tions in sulfur content and high Btu recovery can be
achieved with such coal sizes. The ability to clean
such fine coal is also economical because waste coal
fines which were previously unrecoverable can ncw be
used as an additional fuel source. Accordingly, there
is a need for an improved procsss for the beneficiation
of minerals to effectively recover fine coal.
Brief Desc-iption Of The Drawings
Fig. 1 is a graph of coal to magnetite diameter
ratio values at differing specific gravities using a
specific gravity for coal of 1.3 and a specific gravity
for magnetite of 5.1;
Fig. 2 illustrates the relationship between prob-
able error (Ep) and divergence (difference between par-
ticle specific gravity and effective media specific
gravity);
Fig. 3 illustrates the relationship between par-
ticle size and divergence of actual data from the pub-
lished works of Deurbrouck; and
Fig. 4 illustrates the relationship between
cyclone diameter and apparent distance (as defined in
the specification) as determined by the data in the
published wor~s of Deurbrouc~.
` 1 337 1 ~0
Fig. 5 illustrates a flowchart detailing the steps
followed in one embodiment of the invention using a
diameter ratio deter-ination
//
- 5A -
~ 337 1 ~0
~ Summ2ry O The Tnvention
One aspect of the presen~ in~entio~ involves a
method ~or selecting magnetite to form a dense media
for a dense media separation to benefic ate particulate
05 solids. Particulate solids are provided having a pre-
determined minimum particle size and a known specific
gravity. The method involves calculating a diameter
ratio value applicable to the particulate solids, mag-
netite and the dense media. A diameter ratio value
represents a particulate solid to magnetite particle
diameter ratio for particles having equal but op-
positely directed settling velocities in dense media of
a given specific gravity. The method further involves
selecting magnetite having a particle diameter such
that the actual particulate solid to magnetite diameter
ratio is greater than the diameter ratio value. This
method is particularly useful for beneficiating coal
having a particle size less than about 0.15 mm.
The present method is also direc'ed toward using
magnetite having a particle diameter of less than about
0.005 mm and a mean particle diameter of about 0.0025
mm. Such fine sized magnetite is particularly useful
for beneficiating fine coal particles at low media
specific gravities. Magnetite of this size can be pro-
duced by a process which involves providing an aqueousiron (ferrous) chloride solution. A gas phase pyro-
hydrolysis reaction is then conducted on the solution
to form a mixture of magnetite and hematite. By con-
ducting the reaction in an oxygen restricted atmos-
phere, substantially only magnetite is produced. If thepyrohydrolysis reaction is conduc~ed in an atmosphere
with unrestricted oxygen, a substantial portion of the
product is hematite. For such mixtures, the method
further includes chemically reducing sufficient hema-
tite in the mixture to obtain a mixture comprisin~ atlezst about ~5 percent magnetite.
Another aspect of the present ~nvention invclves
determining the efficiency of separation of a de~se
- I 3371 90
- media seoaratlon process for beneficiating particulate
solids. This method uses, as an indication or effi-
ciency, a "divergence value". This term indicates the
difference between the specific gravity of the particle
05 to be separated and the effective media specific
gravity. This method involves determining an apparent
distance a particle must travel within a dense media
cyclone or centrifuge to be c~rrectly beneficiated.
From the apparent distance and the residence time of
particles in the cyclone or centrifuge, an apparent
velocity a particle must achieve to be correctly bene-
ficiated is calculated. Using the apparent velocity and
other known cyclone geometry and operational para-
meters, a divergence value is calculated to indicate
the efficiency of separation of the system.
A further aspect of the invention involves a
method for selecting cyclone geometry and operating
parameters for improved efficiency of separation in a
dense media cyclone separation process. This method
involves determining a proposed separation efficiency
in terms of a prcposed divergence value. A set of
Cyclone geometry and operating parameters are selected.
A divergence value for the selected cyclone geometry
and operating parameters is determined and compared
with the proposed divergence value. If the selected
divergence value is greater than the proposed diver-
gence value, a new set of cyclone geometry and opera-
tional parameters are selected and a new divergence
value determined. This process is iterated until the
selected divergence value is less than the propose~
divergence value. The step of selecting new cyclone
geometry and operating parameters includes selecting
greater cyclone length, smaller inlet diameter and
greater inlet velocity at constant flow rate, decreased
dense media viscosity, larger particle size and lower
_pec-~ic gr~vity.
A still further aspect of the invention involves a
method for beneficiating particulate solids. This
-7-
1 337 1 90
~ metho~ involves providing magnetite having a diameter
such that the par~iculate solids h~ve a buoyancy with
respect ~o the dense media. Cyclone geometry and
operating parameters are then selected and a divergence
05 value for the geometry and parameters is determined.
The particulate solids are then beneficiated in a
cyclone having the cyclone geometry and operating
parameters with dense media formed from the provided
magnetite.
Detailed Description Of The Invention
The present invention is directed toward an im-
proved method for beneficiating particulate solids from
refuse material in a dense media cyclone. By practice
of the invention, particulate solids, and in parti-
cular, coal, can be effectively cleaned down to a par-
ticle size on the order of tens of microns. When
cleaning coal at such fine particle sizes, more than 60
percent of the pyrite and more than 60 percent of the
ash can be removed, while retaining more than 60 per-
cent of the heating value.
In one aspect of the present method, e.Ytremely
fine magnetite is used to form a dense media for
beneficiating coal in a dense media cyclone. Magnetite
is selected having a particle size such that the
buoyant force of the coal with respect to the dense
media is great enough to provide effective separation.
It has been recognized that effective separation of
small coal particles requires the use of magnetite fine
enough so that the coal particle to magnetite diameter
ratio is greater than a diameter ratio value. Magnetite
having about a 2.5 micron mean diameter is generally
effective for cleaning coal fractions down to 0.015 mm.
In another aspect of the invention, a method for
predicting the efficiency of separation in a dense
media c-~clone is provided. This method involves the
use of three equations which have been derived that re-
late divergence values (difference between specific
~3371-qO
grav~ty cf a p2rticle and effective media specific
gravity) to a number of terms including cyclone g~o-
metry factors and operating parameters. Divergence
values have been recognized to be a measure of the ef-
05 ficiency of seoaration of a system. One of the terms ineach of the equations is V, apparent velocity that a
particle must travel to be correctly beneficlated. To
solve the three divergence equations, a value for V
must be obtained.
V, however, cannot be directly measured. To
determine V for a given system, the following procedure
is used. Using the divergence equations, the term V is
calculated from known data, such as that published by
Deur~rouck, at an arbitrarily selected cyclone radius
lS for sets of data corresponding to different size
cyclones. These velocity terms represent actual radial
particle velocities at the selected radius. However,
for the purpose of simplification o. analysis, radial
part-cle ve~oclty is assumed to be constant and is as-
sumed to be represented by the actual velocities at theselected radius. These velocities are termed "apparent
velocities" and can be determined at ar.y radius as long
as the same radius is used consistently throughout any
analysis. These apparent velocities from Deurbrouck
are used along with particle residence time to calcu-
late an "apparent distance" a particle must travel to
be correctly beneficiated. For coal, "correct bene-
ficiation" is to the overflow, and for refuse material,
"correct beneficiation" is to the underflow. The ap-
parent distances thus calculated have been found to belinearly related to cyclone diameter. From this linear
relationship, an "apparent distance" a particle must
travel to be correctly beneficiated can be determined
for any diameter cyclone. From the apparent dlstance,
an apparent velocity can be determined for any given
sys~e~. In conjunction with known parameters of the
given sys.~m, a di~Jergence value, representing ef-
- -- 13~7190
~ ficiency, can be calculated and can be used in a com-
parative analysis of pro~osed or existing systems.
Grinding coal to a small particle si~e is neces-
sary for ef~ective liberation and separation of coal
05 from refuse material with a density separation method.
Density separation operates by suspending an admi~ture
of coal and refuse material in a dense media of a par-
ticular spec~fic gravity which has an effective media
specific gravity between the specific gravities of the
coal and refuse material. Particles in the suspension
having a specific gravity of pure coal or pure re~use
are most likely to report correctly to the overflow or
underflow because such particles have specific grav-
ities which are either much greater or much less than
the effective media specific gravity. Particles in the
suspension having specific gravities about equal to
that of the effective media specific gravity are
e~ually likely to report to the overflow with the coal
or to the underflow with the refuse. The specific
gravity of particles which include coal and refuse
material physically bound together is between that of
coal and refuse material, and are, therefore, less
likely to report to either the overflow or underflow
than coal and refuse material, respectively. In either
case, the mixed particle will either carry some refuse
to the overflow or some coal to the underflow, thereby
reducing the separation efficiency. By grinding coal
to a small particle size, a high percentage of par-
ticles comprising coal and refuse material are broken
apart into seoarate particles of only coal and only
refuse material. Such separate particles are likely to
report correctly to the overflow and underflow, respec-
tively, because the specific gravity of each is suffi-
ciently different from that of the effective media
specific gravity to form either a float or sink par-
ticle w,th respect to the dense medium.
A primary difficulty with grinding coal to a small
particle size, however, is efficient separation of coal
1 3'37 1 90
~ frsm re~use material. As used herein, "refuse mater-
ial" or "refuse" means any non-car~onace~us substance
entrapped in coal deposits or inadvertently added to
the coal during mining, including, but not llmited to,
05 clays, shales, pyrite, and other precursors to ash.
Coal which is sufficiently fine to obtain accept-
able levels of refuse material rejection can be
produced by grinding coarser coal by conventional
means. The grind size required to enable at least
about a ninety percent by weight pyrite reduction and
at least about ninety percent Btu recovery for most
coals is less than abc~t 0.6 mm and frequently finer
than about 0.1 mm. The present invention is par-
ticularly directed toward cleaning of coal ground fine
enough to allow at least about sixty percent by weight
pyrite rejection with at least about si~ty percent Btu
recovery, more preferably at least about eighty percent
by weight pyrite rejection with at least about eighty
per-ent Btu recovery, and most preferably at least
about ninety percent by weight pyrite rejection with at
least about ninety percent Btu recovery. Alternatively,
fine coal can be obtained from other sources. For e~-
ample, coal found in silt ponds of conventional coal
preparation systems is generally less than about 0.5
mm. Most coal preparation plants currently operating
dense media cYclone separation circuits produce a minus
0.5 mm raw coal slimes product which can be used in the
present process. Additionally, coal derived from co~-
minuting e~isting coal preparation plant refuse, i.e.,
aob or culm bank material, can be used. A substantial
portion of any such coal source will consist of fine
coal material having particle sizes less than about
.150 mm.
The present invention involves the separation of
particulate solids ~rom refuse ma~érials by a density
separation method. ~he preferred em~odiment of the in-
vention discussed herein is the separatlon of fine co~l
particles from refuse material in a dense medium with
-~- 1 3373-90
dense media cyclone. It is ccntempl2ted that the in~Jen-
tion is a~plicable to beneficiati~n ~f par~icuiate
solids other than coal. It is also contemplated that
the present method is applicable to beneficiation by
05 other types of separating systems which employ cent-
rifugal force including devices not normally considered
to be gravity separators, such as centrifuges.
While magnetite dense media is discussed herein as
a preferred embodiment, it should be recognized that
the general principles discussed herein are equally ap-
plicable to other types of dense media. For example,
dense media can be formed from suspensions of sand,
barites and ferrosilicon. For e~ample, with ferro-
silicon, dense media can be formed having specific
gravities which cannot be formed with magnetite dense
media.
One aspect of dense media separation is that the
light (or float) particles must ke less dense than the
effective media specific gravity for separation to oc-
cur. That is, the specific gravity of the float par-
ticles must be less than the effective media specific
gravity. The buoyant force on a particle is a runction
of the difference between the specific gravity of the
particle and the specific gravity of the media. Rela-
tively large coal particles displace dense media, i.e.,a suspension of magnetite in water. As coal partlcles
become smaller and approach the size of magnetite, they
increasingly displace primarily water. Since coal is
not buoyant in water, separation will not occur for
such small particles.
The present invention provides a method for deter-
mining an acceptable magnetite diameter for forming
dense media for the beneficiation of particular c~l
size distributions down to a minimum particle size.
3s More particularly, the present method is useful for
determining the size of magnetite required to ~roduce a
dense media in whicn a particular coal fracticn is
buoyant. This method has the following theoretical
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1 3371 90
~ basis. To be buoyant, a coal particle must have a
velocity in the direction of the center of a cycione.
Such a velocity is a result of a buoyant force, acting
toward the center of the cyclone, minus a resistance
05 force. Particle velocity is a function of the particle
diameter, the difference between its specific gravity
and the specific gravity of the fluid it displaces and
the "g" acceleration arising from rotation of the coal
and media in the cyclone. Accordingly, the velocities
of coal, refuse and magnetite particles can be written
as follows:
[ 1 ] Vc o a 1 = K ( SpGrC o a 1 ~ SpGrf d ) D~: o a 1 m
[Z] Vrefuse = K (SpGrrefuse ~ SpGrfd ) Drefusem
[3~ Vmagneti te = K (SPGrmacne~i te ~ SpGrfd ) Dmagneti te~
where
V = terminal velocity
K = term including components for acceler-
ation and viscosity
SpGrf d = specific gravity or the fluid displaced
SPGrcO a 1, r e f u s e o r = specific gravity of the
m a g n e t i t e coal, refuse and magnetite
D = diameter of the particles
m = exponent ranging from 2 under laminar
flow conditions to 1 for turbulent
conditions
It is known that in dense media systems using
centrifugal force, particles which form the dense
media, e.g., magnetite, have a component of velocity in
the direction of centrifugal force. For the present
invention, the assumption is made that, at a minimum,
the velocity of coal in the direction oppcsite the
centrifugal force is equal to the component of veloci.y
of magnetite particles ccmprising the dense media in
the direction of centrifugal force. For refuse
material, a similar assumption is made eYcept that
refuse material moves in the same direction as mag-
netite. Addition~ , for refuse material having a
buoyant fGrce equal to that of coal, the refuse
material enccunters less resistance force than coal be-
cause it moves in the same direction as magnetite, and
1 3371 90
~ consecuently, has a higher velocity. As a li~it ng
case, nowever, refuse mat~_ial velocity must be ~t
least as great as ~agnetite velocity. The ollowing
equalities can be established based upon the above
05 discussion:
[4] V~oa~ 1) Vmagneti te
[ ~i ] Vr e f u 5 e = Vm a g n e t i t e
By substitution of Equations 1, 2, and 3 into the
above e~ualities, the following ratios are derived
D o a 1 SpGrr, a g n e t i t e ~ SPGrw a t e r 1 / m
[6] = (-1)
Dmagnet1 te SpGr~:oal ~ SpGrfd
- _
Drefuse ~SpGrmagneti te ~ SPGrwater l/m
t7i
Dmagneti te SpGrrefuse SpGrfd
These equations, with the exception of the negative
factor in equation 6, are the same as the equal set-
tling relation given by Gaudin, A.M., Principles of
Mineral Dressing, McGraw-Hill Book Co., Inc., New York,
N.Y., p. 186 (1939).
A value for m in the equations 6 and 7 depends
upon the applicable flow regime: turbulent, transi-
tional or laminar. The Reynolds number of a particle
is a criterion which indicates whethe~ the flow regime
is laminar or turbulent. For Reynolds numbers greater
than 500, flow is turbulent; between 500 and 2,
transitional; and less than 2, laminar. Reynolds num-
ber depends directly upon a particle's diameter, its
veloclty, specific gravity of the fluid it displaces
and inversely upon the fluid viscosity.
To calculate the Reynolds number of a particle,
its ter~inal settling velocity must be known. S.okes
iaw, modified wi~h correction factors for the slmul-
taneous movement of many particles, may be used.
Gaudin, A.M., Principles of Mineral Dressing, McGraw-
-14-
I 337 1 90
ook Co., Inc., New YorX, ~.Y., p. 188 (1939).
[8] Vpar~ ~ s2~3) (1 - s) (1 - 2 5s)
18
05 g(SpGrp a r t ~ spGrf 1 u i d ~ D2
U
where
s = volume fraction of solids
u = viscosity of the fluid
Reynolds number is given by the following
equation:
D V SpGrflui d
[9] Re
Equations 6 and 7 provlde limiting particle
diameter ratios for coal and refuse to be correctly
beneficiated in magnetite heavy media. For particle
dizmeter ratios less than that given by equations 6 and
7, beneficiation cannot occur. The ratios provided by
Equations 6 and 7 are termed "diameter ratio values".
For coal or refuse particles to have the same buoyancy
as though they were immersed in a true liquid having
the same specific gravity as the media, the coal or
refuse to magnetite particle diameter ratios must be
greater than diameter ratio values given by equations 6
and 7. Equations 6 and 7 can be used to construct
Diameter Ratio Partition Curves which plot diameter
ratio values for a range of media specific gravities.
For example, with reference to Fig. 1, a Diameter
Ratio Partition Curve is illustrated wherein the
specific gravity of the dense media is on the abscissa
and the ratio of coal to magnetite particle diameter is
cn the ordinate. Diameter ratio values forming the
curve indicate coal-to-magnetite particle diameter
ratios for coal and magnetite particles having equal
al.h~ug:~ oooosi'e'y dir~cted velocities in a dense
medium of a particular spec7fic gravity for a given
flow regime. If coal partlcles have a velocity in the
1 3371 9~
_ dense medium toward the cyclone center less than mag-
netite particle velocLty in ~he opposite direction, ef-
fective separation of coal by the magnetite dense
medium is not possible because the coal will not
05 "float" with respect to the dense medium.
Two curves are shown in Fig. 1. The upper curve 1
represents the theoretical minimum coal to magnetite
particle diameter rat os for separation in dense media
of given specific gravities for turbulent flow. The
lower curve 2 represents the same information for lam-
inar flow in the cyclone. The graph in Fig. 1 defines
three important regions relevant to effective coal
beneficiation: (1) the region above the turbulent curve
I; (2) the region between the laminar and turbulent
curves II; and (3) the region below the laminar curve
III. Points on the graph in region I allow efficient
separation, while points in region III are ineffective
for coal separation. Points occurring in region II
produce separation efficiencies which are difficult to
predict precisely, but in general depend on the flow
regimes of the particles.
Fig. 1 illustrates the relationship that as the
dense media specific gravity decreases, the ratio be-
tween the coal and magnetite particle diameters must
increase asymptotically for effective separation. In
view of this relationship, processes using dense media
with low specific gravities should have high particle
diameter ratios, i.e. small magnetite with respect to
the coal, for the diameter ratio points to be greater
than diameter ratio values.
Equal settling curves similar to those depicted in
Fig. 1 can be generated by selecting appropriate values
for coal specific gravity and coal size and solving for
magnetite particle size diameter according to Equations
6 or 7. The turbulent curve of Fig. 1 was generated
using a specific g av~ty for magnetite of 5.1 and for
coal of 1.3 for various dense media specific gravities.
For e~ample, in a dense media having a specific gravi~y
-~6-
1 337 1 90
of 1.6, a coal/magnetite diameter ratio value is
approximately 14:1. Accordingly, to effectively clean
coal particles having a 0.14 mm diameter, a dense media
comprising magnetite particles less than 0.01 mm in
diameter is required.
The use of a Diameter Ratio Partition Curve in the
manner described above is useful for beneficiation of
coal from refuse material with magnetite dense media.
While the process is particularly useful for separation
of fine coal, it is applicable to any density separation
for cleaning coal. The present process is also useful for
any separation of solid materials generally on a density
separation principle.
A flowchart for one embodiment of the invention
using a diameter ratio is shown in Fig. 5. Feed material
(1), contains particulate solids to be beneficiated and
refuse particles to be removed. The particulate solids
and refuse particles are of known size and specific
gravity. A fluid (2) and suspension material (3) are to
be used to prepare dense media (4).
The dense media is prepared by first selecting a
minimum particle size (5) of particulate solids to be
beneficiated. A diameter ratio is then determined (6)
for the ratio of particle diameter of particulate solids
to be beneficiated to particle diameter of suspension
material particles required for the particulate solids to
be buoyant in the dense media. Based on the determined
diameter ratio, a maximum particle size for suspension
material is selected (7) corresponding to the minimum
particle diameter to be beneficiated. Suspension
material particles having a particle diameter no larger
than the maximum particle diameter (8) are mixed with the
fluid (2) to prepare the dense media (4), which is then
used for dense media separation (9) of feed material (1),
resulting in beneficiated particulate solids (10) and
refuse (11).
A ~
1 337 1 90
Acceptable separation efficiencies in dense media
cyclone systems depend on the economics of a given
process. However, an Ep value of 0.035 or less generally
indicates a separation efficiency acceptable for
economical recovery of coal, while an Ep value of 0.10 or
more is generally unacceptable for effective recovery of
coal.
The present invention is particularly effective for
coal beneficiation systems in which a low specific
gravity of separation is desired. The specific gravity
of separation (or separation gravity) is the specific
gravity of that portion of the feed reporting fifty
percent to the underflow and fifty percent to the
overflow. The separation gravity is related, but not
equal, to the specific gravity of the dense medium. If,
for example, it is desired to operate a beneficiation
process with a low separation gravity to clean a specific
size coal fraction, the present process is useful for
determining the magnetite particle size necessary for
effective separation. Over a small change in dense media
specific gravity, the coal to magnetite particle diameter
ratio for effective separation can vary greatly.
In accordance with the present invention,
1~
- 17 A -
, .
A
1 337 1 90
~ partlcle size. While t~e present process is appiicaDle
to beneficiation or coal of all sizes, the proce~s be-
comes more critical at smaller coal si~es. For such
coal, correspondingly smaller magnetite is required for
05 effective beneficiation. It is contemplated that minus
0.010 mm magnetite can be used for cleaning down to
small coal particle diameters. conventional grinding
of magnetite to such small particle sizes for purposes
of coal beneficiation by dense media cyclone separation
processes is prohibitively e~pensive. Grinding costs
rise e~ponentially as magnetite particle size
decreases.
Magnetite of the present invention can be produced
by the oxidative pyrohydrolysis of iron (ferrous)
chloride according to the following reaction:
[10] 3 FeCl2 + 3 H2O + 1/2 2 _, Fe3O~ + 6HCl
magnetite
Production of fine magneti'e in this manner avoids high
costs of grinding larger size magnetite. An iron
chloride solution is sprayed into a reaction chamber or
roaster at elevated temperatures and oYygen is supplied
for the reaction to produce magnet te. Such magnetite
has a particle diameter less than about 0.010 mm, and
substantially all of such magnetite has a particle
diameter less than about 0.005 mm. "Substantially", as
used above, means at least about ninety percent and
more preferably about ninety-five percent.
A suitable source of iron (ferrous) chloride solu-
tion can be obtained by dissolving scrap ferrous metalwith hydrochloric ac~d. The hydrochloric acid can be
recovered from the pyrohydrolysis reaction and can be
recycled to dissolve additional scrap. Spent steel
making liquors are also a convenient source of iron
(ferrous) chloride. Additionally, iron (ferrous)
chlor de can be -ecovered from the dissolution of il-
menite wlth hydrochlorlc acid. It should be recognized,
however, that any of various solutions containing iron
1 337 1 qO
- (ferrous) chLoride c~n be use~ in this inventlcn.
If the oxygen content in the pyr~hydrolYsis rezc-
tion is not controlled, the product mi;~ture contains
largely hematite particles with some magnetite, accord-
05 ing to the following reaction:
[11] 2 FeCl2 + 2 HzO + 1/2 02 ~ Fe203 + 4HClhematite
However, by subjecting the mixture to reducing condi-
tions, the iron oxide particles can be converted toprimarily magnetite. For e~ample, the product mixture
can be heated in a carbon monoxide ~r a hydrogen atmos-
phere to reduce hematite to magnetite at relatively low
temperatures of between about 300C and about 400OC.
An acceptable magnetite content in such a mixture
depends upon the economics of a system. However, for
example, it is contemplated that such a mixture have at
least about 85% magnetite and more preferably at least
about 95% maanetite.
Initially, magnetite particles formed by this
process may be fused together into aggregates. Such
fused particles are broken apart into separate par-
ticles as they are initially run through a separation
circuit.
Production of magnetite by these gas phase pyro-
hydrolysis reactions produces substantially rounded
magnetite particles because the temperature of forma-
tion of the particles is close to the fusion tempera-
ture of magnetite or hematite. Rounded magnetite par-
ticles are more efficient for dense media separation
because they create a lower effective dense media vis-
cosity for a given particle size and concentration than
does angular magnetite produced by, for example, grind-
ing. A lower viscosity is more efficient because the
cleaned coal and refuse material move more easily
th oush the heavy medium. Ancther benefit of lowered
viscosl y is that the medium is less cos.ly to pump.
Rounded magnetic particles are also more easily washed
.
1 337 1 90
free from coal than are angular particles because coa_
particLes themselves have ~lat an~ular surraces.
Rounded particles are also much less abrasive to the
internal components of the system, such as pumps,
05 cyclone, and magnetic separators.
It is also contemplated that more effective separ-
tion between coal and refuse material can be achieved
by treating the magnetite particles in the heavy medium
suspension with a surfactant to decrease the effectiJe
viscosity of the heavy medium. Surfactants should be
added to the dense media in the dense media sump prior
to introduction into the cyclone. It is believed that
both coal and refuse material particles move more
freely in the suspension in the presence of a surfac-
tant and are thus more likely to report to the overflowand underflow, respectively.
The concept of buoyancy discussed above is nec-
essary to achieve separation between particulate solids
and refuse material. Particulate solids must approach
the buoyancy they would have if they were immersed in a
true liquid of the same speclfic gravity as the dense
media to be correctly beneficiated, and refuse material
must have a negative buoyancy with respect to the media
to correctly report to the underflow. Positive or
negative buoyancy, however, only indicates the direc-
tion of particle velocity. For a given system, dense
media separation methods also have a time limitation in
that a given particle has a limited residence time.
Therefore, the forces acting upon the given particle
must cause it to travel far enough in the medium to be
correctly beneficiated during the residence time.
Poor separation efficiencies are often encountered
due to a lac~ of understanding of factors involved in
dense media separation processes. Another aspect of
the present invention includes a method for predicting
the separation efficiency of a given system or propcsed
system for the purpose of achieving improved separati~n
effic encies. While this method is discussed in terms
-20-
1 337 1 90
~ of a masnetite dense media Frocess, it should be recog-
ni ed that other types of ~edia can be used equally
well. For example, other types of dense media, such as
suspensions of sand, barites, or ferrosilicon, c~n be
OS used. The method is also applicable to true heavy liq-
uids, such as solutions of halogenated hydrocar~ons or
aaueous salt solutions.
The present method uses, as a measure of effic-
iency of separation, the difference between the par-
ticle specific gravity (SGp) and effective mediaspecific gravity (SG.m). SGe~ is defined as the lowest
specific gravity of separation of any size fraction
treated. For practical purposes, SGem is slightly
higher than the specific gravity of the media. This
measure of efficiency, the difference between SGp and
SGem, is termed "divergence".
It has been found that a direct relationship
e~ists between divergence values and Ep values (a
widely recognized measure of efficiency). Data have
been taken from two published works by Deurbrouck, and
divergence values have been plotted against Ep values
in Fig. 2. For the 20" and 24" cyclones, each data
point represents an average of four actual data points.
Deurbrouc~, A.W., "Washing Fine Coal In A Dense-Medium
Cyclone", U.S. Dept. of Interior, U.S. Bureau of Mines,
Report of Investigation 7982, 1974, six pages.
Deurbrouck, A.~., "Performance Characteristics of Coal
Washing Equipment-Dense-Medium Cyclones", U.S. Dept. of
Interior, U.S. Bureau of Mines, Report of Investiga-
tion" 7673, 1972, 34 pages. As seen in Fig. 2, as
divergence values increase, Ep values increase (less
efficient se~aration). Therefore, by minimizing diver-
gence values, efficlency of separation is increased.
Data from the Deurbrouck wor~s, much of which is used
in the following discussion are shown in Tables 1, 2,
and 3.
-21-
1 337 1 90
O r~ ~ ~ ,1~ ~ 1'
O Ul
~,
Q o ~ ~ ~ ~ tn
1-- ID H~
rt 1-- : ~ 1-' r~ ~ '
O O U~ n
C 3 -- O O rt
W 1-- tt 1- 3 ,.
. )D (D C
~ ~ It c tn
:~ ~ O,
n ~ .
t~ C
Z Q
C (D` (D tD
O ID ID O O O O O ~ Pl O
~ r~ C - . . . . . . 3 ~ 3
a~ 1-- 1-- ~ ~ ~ 0 1-- 3 tD
~D O O V O
n cn -~ o o ~ 1-- It I
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W 1~ 1~ N ~Jl Ul >~ ~ W W t 1'- --
~ rt rD _l w ~ I-- ~ w )D rt ~ H
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tl ~) 3 I-h cn rn
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'D ~ 3 ts~
)D t~ 3 ~I C~
rt
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1'- :I rt o o o o o o rt rt ~
o ~ t ~ r
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n ~ o ~ ~ W o ~ ~ 3 o r!
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1337~90
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U.
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OO ~
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~W ~ ~ ~n ~ O ~- ~ ~ W
rt ~
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n~ s 3 t~
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a~ IJ- ~ ~ o n r~ - 3 Q
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~ww 3 a~ I N Q
pJ~O-- ~T
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a ~ o
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w r o ~ J
a~ ~ w ul --
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5~ ~.
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- 1337190
. .
~ ~ Ul --rD
W (~1 0 C ~ 1~~: 1~ t k
5D ~h ~ N) ~ ~ I-t
00 CD ~P o ~D
C~
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ID ~ 111 C tD O
t'~ 3
'D
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0~ ~ ~h ~
a~ ~ O ~ ~:
3 3 3
~r Hl (D O ~ w O~ O
g r~ O ~ ~ 3 3 3
O 1~ 0 (D ~ C~
3 I t It1~ t' 1-1 ~ .
N ~
n 1-- 1_ 0 ID O H
It~ 3 C 3
Il 1~ U~ tl) 3
W
W~
;n C u~
O O t~
_
D ~' ~
,~--- pJ _1 W ~0 CO D r~ -h 2
~ ~t U7 ~ O w 1-- n
IDn tD o O
n~ ~t 3 ~h Z
n Y- r~
~D~ rn
rD~ Y- '3
~tO n W w
IDt rn C~ 3 t~ C~ rn ~ ~
~ ~ rD ~ ~- H
1'~ 0 0
~ 3 ' (D C D tl)
tD D n ~- t ~-
Q rt O O O O rt ~t ID UD t
~ O ~t C~
C~ Y~ o o O O
n ~o Ul ~ o C rD 3 0
n r,l '3 w w w
ID~ D rD ~ _
1-- 1- n ID ~: n ~
S ~ Q
~ ~ ,3 H
Y ~ n ~D ID Y- Q
ttO ~ ~ ~ t~ 3 ~ ~t 3 e~3
~D3 ~3 w ~ ~1 ~ ~ ~
I rn n Y- 3
_~ C~ o ~ ~ rD -- n ~;
tJ~y- o o W ~D n ~1-3
--~ ~ O ~: tD
W ~ ¦ N
t~ ` tD ~
ID
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w t:n w ~ ~ tt
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S (D Y-
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<: n, ~;l'f
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1 337 1 90
- The Deur~rouck data have als~ ~e~ plotted as
dlvergence values versus par_icle size in Eig. 3. I'
is apDarent that for smaller size particles, divergenco
values increase. Given the direct relationship between
05 divergence values and Ep, it would be expected and is
borne out by data that smaller particle sizes have
higher divergence values since it is widely known that
efficiency deteriorates at smaller size frac~ions.
The present method involves the use of three equa-
tions which have been derived relating divergencevalues to a number of factors (referred to as "diver-
gence equations"). These factors include the followinq:
1. V--Velocity for a given size cyclone that a
particle must achieve in the direction of
buoyancy to be correctly beneficiated;
2. D~--Particle Diameter;
3. G--Acceleration;
4. u--Dense Media Viscosity; and
5. S&em--Effective Media Specific Gravity
Each of the three equations is applicable to a
different flow regime, turbulent, transitional, or
laminar, which depends upon particle Reynolds Number.
These three eauations are shown immediately below. The
derivation of the equations is shown before the Ex-
perimental Section.
-25-
- 1 337 1 90 ~
,` W
H v A Z v
0 ~3 0 ::~
O H O Z
O O O
'Q 'Q
c Q ~ C tl
Il 11 11 11 11 11 11 1~ 1
o ~
hi C ~ ~ 3 ~ ~ o
C Ul r~ O O
~- n ~
n c ~c C C C
;n ~ 1--3
n
o ~ o
n ~ O O "~1
~t O O Q
O ~ ~ +
O
+ + O
''D ~ O
0 01 0
Y
n ~R
c
o o
o ~ o
O O cn
Q ~I Q
u~ cn Q
Q Q
Q Q
+ + ~
O O O
~Q 'Q w
I-- W
W
O C~
2(~
1 337 1 ~0
For any given cyclone/coal system, the Particle
Diameter, Viscosity, and Fluid Specific Gravity terms are
constant. The Velocity (V) and Acceleration (G) terms vary
as a particle moves within a cyclone. Since the particles
accelerate, particle velocities change. Moreover, the G
term varies as particles are at different radii. It is
contemplated that the divergence equations can be used to
determine divergence values by accounting for this
variability. However, such mathematical precision is not
required for effective use of the divergence equations.
Instead, the present method involves using the concepts of
apparent distance and apparent velocity to predict
separation efficiency. As discussed below in more detail,
apparent velocity is calculated from apparent distance which
is calculated on the basis of actual data using an
arbitrarily selected radius of interest. Accordingly, when
apparent velocity is used to calculate a divergence value
from equations 12, 13, and 14, the acceleration term, G, for
that divergence value must be calculated at the same radius
of interest.
The present method involves assuming that apparent
velocity is a constant velocity throughout the entire
residence time in the cyclone. Apparent velocity is a
function of residence time and apparent distance travelled
by a particle. Of the terms residence time and apparent
distance, residence time can be directly calculated from the
cyclone geometry and the media flow. The apparent distance,
however, cannot be directly calculated because the flow
dynamics and actual paths that particles travel within a
cyclone cannot be accurately determined. It is noted that
the apparent distance a particle must travel to be separated
is the same for particles of any size and that the apparent
distance is a function of the diameter of the cyclone.
To overcome the limitation of not being able to
directly determine either apparent distance or apparent
~'
1337190
veloc-ty, the present method includes deter~lning an
apoarent distance based u?on known da~a, such 2S that
reported by Deurbrouck. The apparent d s.ance is ~hen
used to calcuLat~ apparent velocity for use in the
oS divergence e~uations. The acceleration term in the
divergence equations must then be determined at a
radius of interest corresponding to the selected radius
of interest used in conjunction with the known data to
provide the basis for determining apparent distances,
as described below. In this manner, divergence values
for separation systems can be determined. These pre-
dicted divergence values can be analyzed to determine
whether the efficiency of a proposed system is accept-
able.
Apparent distances are determined from the ~eur-
brouck data in the following manner. Initially, the as-
sumption is made that the distance travelled by a par-
ticle in a cyclone is a function of cyclone diameter.
Then, from the Deurbrouc~ data, divergence and fluid
specific gravity are known and viscosity is assumed to
be 1 centipoise, for each particle fraction. Accelera-
tion varies with radius, but for present purposes is
determined at an arbitrarily selected radius of inter-
est of 1/3 the cyclone radius. With this information,
the divergence equations are solved for V, and average
values for V are calculated for the 8", 20", and 24"
cyclones. These values are actual velocities of par-
ticles at one-third the radius of the cyclones. These
values are also termed apparent velocities and are as-
sumed to represent a constant velocity a particle musttravel to be correctly beneficiated.
The apparent distance that particles travel in
each of the three cyclones is then calculated by multi-
plying V by the residence time. The apparent distances
calculated in this manner are then plotted against
cyclone diame~er as sh~wn in Fig. 4. The three data
points on this graph rorm an approximately st-aisht
line. This relationship suggests that the starting as-
-28-
~ 337 1 90
sumpt7on was cor-ect, and that a linear relationshi?
e~ists between cyclone di~met~r ~nd the apparent dis-
tance particles, regardless of size, must traveL to ~e
separated. By conducting a linear regression of the
05 data points in Fig. 4, the following e~uation for
determining apparent distance from cyclone diameter was
calcllated .
[15] y = 3.05 x + 42.07
where
y = apparent distance, centimeters
x = cyclone diameter, inches
According to the present method, the efficiency of
separation of a dense media cyclone for each partlcle
size fraction can be predicted in the following manne-.
The residence time of a particle is calculated by
dividing cyclone volume by flowrate. The apparent dis-
tance a particle must travel to be correctly bene-
ficiated is calculated by applying the cyclone diameter
to Equation 15. A value for apparent velocity is then
detenmined by dividing the apparent distance by the
residenc~ time. G is determined at a radius of inter-
est or 1/3. The appropriate equation, depending on
Reynoids number, of Equations 12, 13, and 14 is then
solved using the value for V and other knowh values. A
value for the logarithm of the divergence value ap-
plicable to the cyclone for a given particle size and
its operating conditions is then obtained.
The divergence value thus obtained can be used in
a comparative analysis with divergence values appli-
cable to cyclones having different geometry or operat-
ing conditions.
The effect of proposed changes in a separation
system on efficiency of the system can be detèrmined by
the above method. Any changes in cyclone geometry and
operat_ng raramete-s in a separation system for im-
proved ef-iciency wiil normally be selectea on the
basis of efficiency improvement per cost. Various
-29-
1 337 1 qO
separaticn systems having di~ferent cyclone geomet~v
and cpe~~ting paramet~r, have ~een analyzed ~or pre-
dicted e-ficlency as measured by divergence values by
the present method. The results of these anal~ses are
05 desc-ibed in the E:~perimental Section. By examining the
amount of efficiency improvement from each of the
changes analyzed and relative cost, it has been ~eter-
mined that certain changes for improving efficiency are
more cost effective.
Increasing residence time by using longer cyclones
or decreased cone angles has been found to be a cost
effective manner for improving efficiency of separa-
tion. As seen in Example 2, substantial decreases in
divergence values are achieved by increased residence
time. The cost associated with this change is simply
the capital cost for buying longer cyclones. This cost
is minimal in terms of efficiency improvement per ton
of coal. It should be noted that residence time can
also be inc~eased by using larger diameter cyclones.
However, any benef ts from this method of increasing
residence time are virtually completely offset by lower
efficiency frcm dec~eased accelerati~n.
Another cost effective -hange in a separation sys-
tem is to increase acceleration, without at the same
time dec~easing residence time. While acceleration can
be increased by smaller cyclone diameters, residence
time is reduced by such a change. Ac^eleration, hc~J-
ever, can be increased without decreasing residence
time by some combination of decreasing the inlet dia-
meter and increasing the inlet velocity, while keepingflowrate constant. The additional cost of such changes
includes costs of equipment modification and increased
pumping costs to achieve higher inlet pressures. The
improvements in divergence values from smaller inlet
diameter at constant flowrate are shown in Example 4.
Re~ucing dense media viscosity is another cost ef-
fective method for imDroving separat-on efficLency.
The effect of reduc-ng the media viscosity on dLver-
-30-
1337190
gence values s illustrated in Example 3. As d-scusse~
above, viscosity reduction can ~e ach~eved by a~diticn
of a su-fac~ant, such as Lomar D,* p:oduced by Diamcnd
Shamroc~. In this manner, magnetite particles move
os more freely with respect to each other. The use of
rounded magnetite particles also reduces viscosity, as
discussed above. Viscosity reduction can also ~e
achieved by heat~ng the dense media. For e~ample, by
raising the temperature of the media in a heated cir-
cuit from 680F to 140F reduces the viscosity of water
from about 1 to about 0.47 centipoise. Moreover, the
use of a heated circuit has other benefits, such as
reduced drying time of filtered coal. The cost of
achieving viscosity reductions by these methods is ac-
ceptable in view of the improvements in efficiencies.
These methods of viscosity reduction can be used alone
or in combination.
Particle size also has a strong effect on separa-
tion efficiences. As seen in all of the Examples, much
lower divergence values are achieved for larger par-
ticle sizes. Accordingly, coal should be ground only to
as small a size as is necessary for acceptable libera-
tion. Moreover, grinding methods which generate the
least amount of extreme fines should be used, such as
rod rather than ball mills.
The general principles disc-~ssed above and equa-
tions 12, 13, and 14 relating to the use of cyclones
for dense media separation (separation of solids based
on different specific gravities) arE also applicable to
the use of cyclones as thickeners (separation of solids
from liquid) and classifiers (separation of solids
based on size). These principles and equations are
also useful for other mineral processing systems which
use ce~trifugal force, such as spirals and hydro-
cyclones. These principles and equations are also ap-
piicable t~ mineral processing systems which do not use
cen.riEusal force for processing. Such systems include
the use of vertical currents, e.g., jigs, the use of
* Trade-mark
-31-
- :
1 3371 90
streaming currents, e.g., tables, and the use cf
launde-s. These princi31es c~n be ~sed to predict per-
formance ef~ectiveness or such systems and to select
operational parameters for improving performance.
05 Equations 12, 13, and 14 can be used directly for
such other systems. Howeverl instead of using
Deurbrouc~'s data, similar data for the appropriate
system would be used to determine apparent distance and
apparent velocity. In the case of systems not using
centrifugal force, acceleration would simply be
gravitational acceleration.
After the coal is separated from refuse material
in the dense media cyclone, the overflow portion con-
taining clean coal is separated from the magnetite par-
ticles by magnetic separation. Coal particles having adiameter less than about 0.6 mm are typically separated
from magnetite particles using magnetic separators.
The underflow portion containing refuse is typically
fed to a separate magnetite recovery circuit where the
dense media is separated, for eY~mple, by magnetic
separators and recycled.
The reductions in ash forming material by the
present invention are highli advantageous and economi-
cal for coal combustion processes. For example, foul-
ing and slagging of furnaces caused by ash is decreasedwith a decrease in ash forming materials in the fuel.
Additionally, ash removal costs are reduced when the
total ash burden is reduced. Costs are also associated
with the transportation of ash forming material to a
utility and movement of ash forming material and ash
through the combustion process. Such costs are reduced
by use and combustion of clean coal.
A clean coal from the present process is par-
ticularly advantageous for mixing with various addi-
tives and forming aggLomerations prior to combustion.Such coal is suited 'o agglomeration because of its
fine size. As used herei~, "agglomeration" refers to
methods for forming fine particles of coal into larger
-32-
13371~0
size units, such as pelletizing, comDaction, or agita-
tlon. Advantages of agslomer~tLon include impro~ed
nandling of coal material, particularly during the
transpor~ation of fuel products. Agglomerations are
o5 particularly advantageous for coal-fired utilities
which use pulverized coal (PC) boilers in which coal
material is pulverized before combustion to a particle
si~e less than about 0.075 mm. Energy savings in this
pulverizing process are made by using agglomerations or
clean coal from the process because agglomerated coal
is more easily pulverized than solid coal pieces and a
large percentage of the coal particles in the pellets
already meet the size requirements for the crushing
process.
There are additional advantages to using clean
coal from the present process when additives are incor-
porated with the coal material. Such additives can in-
clude materials for air pollution reduction, such as
alkaline sorbents for sulfur capture, sulfation pro-
moters, catalysts for intermeaiate reactions in air
pollution reduction processes, or anti-slagging agents.
While such additives can form ash u~on combustion, the
overall ash burden is sufficiently reduced by the
present process that ash formed from additives is ac-
ceptable. Additionally, because of the fine size ofthe coal particles, additives are partlcularly effec-
tive due to ease of dispersion of ad~itives and in-
t-mate mixture with the fine coal particles.
Derivation of Divergence Eauations
The force acting on a coal particle in a cyclone
system in the direclion of the interior of the cyclone
is te~med the "~uoyant force" and is provided by E~ua-
tion A.
[A~ F~ = Volp (SGp - SGfd) G
1 3~371~9
whe-oin, F = buoyznt force
Volp = part cle volume
SGp = art-cle specific gravi~I
SGf d = specific gravity of fluid displaced
G = G acceleration
05
The G acceleration is a radial acceleration which
is caused by the circular motion or the
coal/refusejmedia stream inside the cyclone. This ac-
celeration is a function of tangential velocity of the
stream. As can be seen from Equation A, to increase
the buoyant force on a given particle in a given dense
media, the G acceleration must be increased.
Bradley, D., The Hydrocyclone, Pergamon Press
Ltd., London, 1965, discusses cyclones and provides two
equations which, solved simultaneously, give the fol-
lowing e~uation for G acceleration.
~3.7 R. 2
Rc Vj 2 Rc 2 n +
[B] G = V~ang /r = _______________ ___
Rc ~ r
20 wherein, R~ = radius of in7et, ft
R~ = radius of cyclone, ft
Vj = velocity of feed in inlet, ft/sec
r = radius of in~erest, ft
Vt a n ~ = tangential velocity of stream
inside cyclone, ft/sec
For purposes of the present discussion, the radius
of interest, i.e., the radius within the cyclone at
which acceleration is determined has been selected as
1/3. As mentioned previously, selec~ion of this ~alue
for r is not critical to the present invention and
other values work equally well The term n is an ex-
ponent, the value of which depends on cyclone geometry.
A value of 0.8 is typical and will be used in this
derivation.
In opposition to the buoyant force, is a resis-
tance force. The resistance force is a func'ion ofmany varia~',es and deDends! in part, upon the flcw
regime of partlcles inside the cyclone. As is known,
particles can have turbulent, transitional, or laminar
flow regimes. The particular flow regime for a par-
-3~-
1 337 1 90
-
ticle depends upon the properties of fluid in which the
particle is travelling, viscosity and specific gravity, as
well as on the particle's velocity and diameter. The
Reynolds number of a particle is the criterion which
determines flow regime. For Reynolds numbers less than 2,
particles will travel in laminar flow. For Reynolds numbers
between about 2 and about 500, flow will be in a
transitional phase. For Reynolds numbers greater than about
500, turbulent flow occurs. The formula for Reynolds number
is provided in Equation C.
Dp Vp SGf d
[C] Re =
u
wherein, Re = Reynolds number
Dp = particle diameter
Vp = particle velocity
SGf d = specific gravity of fluid displaced
u = viscosity of fluid displaced
The coefficient of resistance of a particle is a
measure of resistance experienced by a particle as it
travels through a fluid. The formula for coefficient of
resistance is provided in Equation D.
25 [D] Q = 4 Dp (SGp - SGf d) G
3 Vp2 ( SGf d )
whereQ = coefficient of resistance
Dp = particle manner
SGp = particle specific gravity
SGf d = specific gravity of fluid displaced
G = G acceleration
Vp = particle velocity
The relationship between coefficient of resistance and
Reynolds number can be described by three equations, one for
each flow regime.
[E] Turbulent (Re >500) log Q = log 0.44
[F] Transitional (500 ~Re >2) log Q = log 18.5 - 3/5 log Re
[G] Laminar (Re ~2) log Q = log 24 - log Re
-
1 3371 90
The partic'e velocity can oe determined in the
following manne~ uations C ~nd 3 3re sol~ed for the
velocit-~ term and then set equal. The following rela-
tionship is derived f,om this procedure.
05
4 Dp ~SG;, - SGfd ) G
[H] log Q = log ______ +
3 SGf d
Dp SGf d
2 log _ - 2 log Re
u
The log ~ term in Equations E, F, and G can be
substituted into E~uation H and the resulting Reynolds
number may then be solved for particle velocity in
tenms which are generally known. These equations for
each of the three flow regimes are provided in Equa-
tions I, J, and K.
4 D? (SG;, - SGFd ) G
[I] (Re >500) 2 log Vp = log ___________________ - log 0.44
3 SGf d
4 Dp (SGp - SGf d ) G
[J] 500>Re>2 1. 4 log Vp = log - ----------
3 SGf d
Dp SGf d
+ 0.6 log _ - log 18.5
u
4 Dp (SGp - SGf d ) G
tK] Re<2 log Vp = log _______ _____ +
3 SGf d
Dp SGf d
log _______ - log 24
u
Equations I, J, and K can be algebraically manipu-
lated to the form of Equations 12, 13, and 14.
-36-
- 1 337 1 90
~ EXP~RIMENTAL
E:~ample 1
Observed divergence values for difrerent size
05 fractions of coal from DeurbroucX's work with an 8 inch
cyclone are compared with divergence values predicted
by the present method using the actual test parameters
of Deurbrouc~'s worX. The actual conditions of
DeurbroucX's test are shown in Table l-A.
Table l-A
Value Value
Parzmete{ (Actual Conditions) (New Conditions)
Cyclone Diameter 8 inches 8 inches
Inlet Diameter 1.5 inches 0.75 inches
Cyclone Length 8 inches 32 inches
Cone Angle 12 degrees 12 degrees
Flowrate 110 gpm 141.6 gpm
Viscosity 1 centipoise 0.4688 centipoise
(assumed)
Effective Media 1.33 1.33
Gravity
Based upon the actual conditions in Table l-A,
predicted divergence values were calculated according
to the present method. These predicted values are com-
pared with observed divergence values for each size
fraction considered by DeurbroucX. This comparison is
shown in Table l-R.
-
1 337 1 90
~able 1-3
Predicted Predicted
Divergence Divergence
Size Observed (Actual (New
05 Fraction Divergence Conditions) Conditions)
0.814 mm 0.03 0.017 0.001
O.420 mm O.08 0.053 0.002
O.250 mm O.11 0.121 0.006
0.177 mm 0.20 0.210 0.010
0.105 mm 0.24 0.483 0.022
O.074 mm -- 0.846 0.039
0.037 mm -- 2.565 0.119
A new set of test conditions, varying four factors
from Deurbrouc~'s 8 inch cyclone data, were selected
for improved separation. These new conditions were
analyzed according to the present invention to deter-
mine predicted divergence values to illustrate the
potential for improved ef.iciency of separation. The
values for the new corditions are shown in Table 1-A
and the predicted divergence values under the new con-
ditions are shown in Table 1-B. The improvement in
predicted divergence values from the modifications in
the four changed conditions is substantial. Acceptable
cleaning efficencies, represented by a divergence value
of 0.119, are obtained for particles even as small as
37 microns (400-mesh).
Example 2
For Examples 2-8, a series of simulated separation
runs were conducted using Equations 12, 13, and 14, to
~ ;ne the effect on separation efficiency, as indi-
cated ~y divergence values, cf variations in different
parameters.
-38-
1 337 1 90
ln ~xample Z. four s mulatlon runs we~e c~nducted
with the residence tL~e ~i ng varied ~y up t~ a ,ac~r
of 6. The results of this comparison and the effects
on divergence values are shown in Table 2-A. It should
os be noted that this simulation illustrates an increase
in residence time by either an increase in the length
of the cyclone or by a decrease in the cone angle. If
the residence time increase had been achieved by in-
creased cyclone diameter, the acceleration value would
have dec-eased at higher cyclone diameters.
Table 2-A
Run 1 2 3 4
lS Cyclone Diameter, inches 8 8 8 8
Inlet Velocity, ft/sec 68.4 68.4 68.4 68.4
Alpha, 3.7 Dj n 1 e t /Dc y c 1 o n e O . 60 0.60 0.50 0.60
Flowrate, G~M 281.8 282.0 282.0 282.0
Acceleration, g's 2735 2735 2735 2735
Residence Time 0.939 1.409 2.817 5.635
Residence Time Factor, 1 1.5 3 6
x std cyc.
Minimum Vel. for 70.8 47.2 23.6 11.8
Se?aration, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.383 0.217 0.082 0.031
O.037 mm (400-mesh) 1.2 0.65g 0.250 0.095
O.0185 mm 3.5 2.0 0.757 0.287
Example 3
The effect of viscosi~y on efficiency of separa-
t on, as measured by divergence values, was e~amined in
simulation runs 1-4. All other factors were held con-
stant with viscosity being varied from 1.0 to 0.3565
-39-
1 337 1 90
centipoise. The media temperatures represented by the
simulated changes in viscosity are approximately 20C, 40C,
60C, and 80C. The results of these test runs and effects
on divergence are shown in Table 3-A.
Table 3-A
Run 1 2 3 4
Cyclone Diameter, inches 8 8 8 8
Inlet Velocity, ft/sec 68.4 68.4 68.4 68.4
Alpha, 3.7 Dinlet/Dcyclone 0.60 0.60 0.60 0.60
Flowrate, GPM 281.8 282.0 282.0 282.0
Acceleration, g's 2735 2735 2735 2735
Residence Time 0.939 0.939 0.939 0.939
Residence Time Factor,
x std cyc.
Minimum Vel. for 70.8 70.8 70.8 70.8
Separation, cm/sec
Viscosity, Centipoise 1.0 0.656 0.468 0.3565
0 8
Specific Gravity of 1.5 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.383 0.298 0.243 0.206
0.037 mm (400-mesh) 1.2 0.902 0.737 0.626
0.0185 mm 3.5 2.7 2.2 1.9
Example 4
The effect of varying the term alpha on particle
separation efficiency, as measured by divergence values, was
examined in simulation test runs 1-4. The alpha value is
equal to 3.7 (Dinlet/DCyclone). Since cyclone diameter was held
constant, only the inlet diameter was varied in each of the
runs. The effect of making the inlet diameter smaller,
given a constant flowrate, is to increase inlet velocity
and, therefore, acceleration. As can be seen from the
results in Table
-40-
1 3371 90
a-A~ divergenc values were significantly decreased by
dec-eases in the value of ~lFha.
Table 4-A
~5 Run 1 2 3 4
Cyclone Diameter, inches 8 8 8 8
Inlet Velocity, ft/sec 20 30.4 54.1 121.7
Alpha, 3.7 Dj n 1 e ~ /Dc y c 1 o n e 74 0.60 0.45 0.30
Flowrate, GPM 125.3 125.3 125.3 125.3
Acceleration, g's 356 541 961 2163
Residence Time 2.11 2.11 2.11 2.11
~esidence Time Factor,
x std cyc.
Minimum Vel. for 31.5 31.5 31.5 31.5
Separation, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.948 0.623 0.351 0.156
0.037 mm (400-mesh) 2.9 1.9 1.1 0.472
0.0185 mm 8.7 5.7 3.2 1.4
E~ample 5
The effect of increased inlet velocity at constant
inlet diameter on particle separation efficiency, as
measured by divergence values, was ~x~mi ned with the
results shown in Tables 5-A, 5-B, 5-C, and 5-D. As can
be seen from the following results, the increased ac-
celeration has a beneficial effect on divergence
values.
-41-
-
1 337 1 90
Table 5-A
Run 1 2 3
05 Cyclone Diameter, inches8 8 8
Inlet Velocity, ft/sec20 45 70
Alpha, 3.7 D1nle~/Dcyclone 0.74 0 74
Flowrate, G~M 125.3 282.0 438.7
10 Acceleration, g's 356 1800 4357
Residence Time 2.11 0.939 0.603
Residence Time Factor,
x std cyc.
15 Minimum Vel. for 31.5 70.9 110.2
Separation, c~/sec
Viscosity, Centipoise 1.0 1.0 1.0
S~ecific Gravity of 1.5 1.5 1.5
Fluid Dispiaced
Divergence at
0.074 mm (200-mesh) 0.948 0.583 0.447
0.037 mm (400-mesh) 2.9 1.8 1.4
0.0185 mm 8.7 5.4 4.1
_a7-
1 337 1 90
Table 5-3
Run 1 ~ 3
05 Cyclone Diameter, inches 8 8 8
Inlet Velocity, ft/sec 30.4 68.4 106.5
- Alpha, 3.7 D~nlet/Deyclone 0.60 0.60 0.60
Flowrate, GPM lZ5.3 282.0 438.7
Acceleration, g's 541 2739 6678
Residence Time 2.11 0.939 0.603
Residence Time Factor,
x std cyc.
Minimum Vel. for 31.5 70.9 110.2
Separation, cm/sec
Viscosity, CentiDoise 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.623 0.383 0.294
0.037 mm (400-mesh) 1.9 1.2 0.891
0.0185 mm 5.7 3.5 2.7
1337190
Table 5-C
Run 1 2 3
05 Cyclone Diameter, inches 8 8 8
Inlet Velocity, ft~sec 54.1121.7 189.3
- Alpha, 3.7 Din~et/Dcyclone 0.45 0-45
Flowrate, G~M 125.3Z82.0 438.7
Acceleration, g's 961 4869 11,783
Residence Time 2.110.939 0.603
Residence Time Factor,
x std cyc.
Minimum Vel. for 31.570.9 110 2
Separation, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5
Fluid Displaced
Divergence at
0 074 mm (200-mesh) 0.351 0.216 0.165
0.037 mm (400-mesh) 1.10.653 0.501
0.0185 mm 3.2 2.0 1.5
1 337 1 90
Table 5-9
Run 1 2 3
o5 Cyclone Diameter, inches 8 8 8
Inlet Veloc t-~, ft/sec 121.7 273.8 425.9
Alpha, 3.7 Di n 1 e t /D~y c 1 ~ n e 30 0.30 0.30
Flowrate, G~M 125.3 282.0 438.7
Acceleration, g's 2163 10,955 26,511
Residence Time 2.11 0.939 0.603
Residence Time Factor,
x std cyc.
Minimum Vel. for 31.5 70.9 110.2
Separation, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.1560.096 0.073
0.037 mm (400-mesh) 0.4720.290 0.223
O.0185 mm 1.4 0.880 0.675
Example 6
The effect of increased flowrate, at constant in-
let velocity, was ~x~mined in Table 6-A. ~s can be
seen from the following results, in contrast to Example
5, divergence values increased as flowrate increased.
4S
1 337 1 90
Table 6-~
Run 1 2
05 Cyclone Diameter, inches 8 8 8
Inlet Velocity, ft/sec 45 45 45
Alpha, 3.7 D1nl~t/D~yclone0.493 0 74 0.923
Flowrate, GP~ 125.3 282.0 438.7
Accelerationl g'S 800 1800 2801
Residence Time 2.11 0.939 0.603
Residence Time Factor,
x std cyc.
Minimum Vel. for 31.5 70.9 110.2
Separation, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0
Specific Gravity of 1.5 1.5 1.5
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.421 0.583 0.696
0.037 mm (400-mesh) 1.277 1.8 2.180
0.0185 mm 3.872 5.4 6.4
Example 7
The effecL of varying cyclone diameter on effi-
ciency, as measured by divergence values, was ~x~mi ~ed
in simulation runs 1-6. The results of these runs are
provided below in Table 7-A. It can be seen that at
smaller cyclone diameters, which have an included cone
angle of 12, there is, for all practical purposes, no
effect on divergence values. For c-~clones having a cone
-46-
1 33~-1 90
angle of 20O, some small improvements in diver~ence
.~aLues is obser~ed at smaller cyclon2 diameters. The
lack of substantial improvements is due to decreased
residence time.
05
-47-
-
1 337 1 90
n ~ ~ ~ ~. ~ ~ ~ Q ~1
n ~ ~ ~ ~ c
': (D 0 ~ -n n :~ C ~ :~ ~ 3
~D ~ Q ~ - (D 5 ~ --
~ ~ ~ I~ ~t ~D
-- Q ~t ~ Q ID rD ~ (D
r ~: c
~ o 1~.
~ o c~ n
ID ~ Q ~ ~- 3 ~ ~ ~ 3
3 3 3
1 3 ~ ~ (D ' ~ rt
O o O ~t ~t O
o o o ~ ID Ul ~ H~
~ w ~ o o n n ~ ~
01 ~ O n ~
3 3 ~I tl) ~ ~ ~< ~D ~
3 3 3 ~~ t - o Q
. ~t X o :n
o o o u~
o o ~ ~~ rt
3 u~
~ ~D ~ Q ~
3 ~:
D ~ n
~ Q
~ U
Ul ~ W CJ~
_~ ~ o ~ ~ ~ ~ ~ ~ a~ o c3 ~ ~
... . . . ~ 3
W ~ ~ ut o ~ I_ ~ ~ ~ ~ _
~D W O o ~J
o
1~ 1_~ Q :t'
u~ O a a~ ~ 3 t~
c~ ~ o 1~ 1~ ~ ~ o t~ O ~D
... . . . . ,~ . .
n o ~ --J ~ a~ ~
O V
j
Ul a~ --
~ I' w
Vl ~ O ~-- ~ Q 1~ ~ W W O
n o w ~ o
W w ,p O
-
_~ w c~
w ~ o ~-- ~ o ~ o ~n ~ o ~ w
C~ W O
W ~D
U~
O
3 U~
W 1--0 1~ ~ O ~O O O ~ ~ (D
Ul tV W ~n o a~ ~ ~ o~ ~
CD - T O V
0
~2
_
~ ~D w a a~
W ~ ~ ~ O W ~D O a~ w
C1~ ~ W ~n O ~.n w a- a ~
~ ~n o
w ~
(
1 337 1 90
E~amDle 8
The effect or media spe~ific gravity on e~ iency
of se?aration, as measured by divergence vaiues, was
Q~mined. Litlle effect W25 observed on diver~ence
05 values by variations in this factor.
Table 8
Run 1 2 3
10 Cyclone Diameter, inches 8 8 8
Inlet Velocity, ft/sec 68.4 68.4 68.4
Alpha, 3.7 Dinl~t/Dcyclone 0.60 0.60 0.60
Flowrate, GPM 281.8 282.0 282.0
Acceleration, g's 2735 2735 2735
Residence Time 0.939 0.939 0.939
Residence Time Factor,
x std cyc.
Minimum Vel. for 70.8 70.8 7C.8
Separation, cm/sec
Viscosity, Centipoise 1.0 1.0 1.0
Specific Gravity of 1.5 1.4 1.3
Fluid Displaced
Divergence at
0.074 mm (200-mesh) 0.383 0.373 0.362
0.037 mm (400-mesh) 1.2 1.1 1.1
0.0185 mm 3.5 3.4 3-3
While various embodiments of the present invention
have been described in detail, it is apparent that
modific~tions and adaptaticns of those embodiments will
occur to those skilled in the art. ~owever, it is to be
exDressly understood that such modifications and adap-
tations are within the scope of the present invention,
as set forth in the foilowing claims.
-49-
