Cross flow cyclonic flotation column for coal and minerals beneficiation

An apparatus and process for the separation of coal from pyritic impurities using a modified froth flotation system. The froth flotation column incorporates a helical track about the inner wall of the column in a region intermediate between the top and base of the column. A standard impeller located about the central axis of the column is used to generate a centrifugal force thereby increasing the separation efficiency of coal from the pyritic particles and hydrophillic tailings.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to mineral beneficiation by the separation of a 
preferred component from a mixture by froth flotation. More particularly, 
this invention relates to an improved method and apparatus for the 
separation of finely ground minerals and contaminants by combining froth 
flotation separation with density separation techniques. 
2. Description of Related Art 
Flotation, in particular froth flotation, is one of the primary solid-solid 
separation processes for fine particles. The process has been widely 
practiced for almost a century in the mining industry for concentrating 
valuable minerals such as phosphate rock, precious metals, lead, zinc, 
copper, molybdenum, and tin containing ores as well as coal. Typically, 
the froth flotation process has been developed to work in water, with air 
as the froth generating gas, however, other liquid and gas combinations 
can be used 
With the froth flotation process one or more specific particulate 
constituents of a slurry or suspension of finely dispersed particles 
become attached to gas bubbles so that they can be separated from the 
other constituents of the slurry or suspension. The froth flotation 
process exploits the wettability differences of the particles to be 
separated. Differences in the wettability among solid minerals particles 
can be natural, or can be induced by the use of chemical additives. The 
buoyancy of the bubble/particle aggregate, formed by the adhesion of the 
gas bubble to a particle in the slurry, is such that it rises to the 
surface of the flotation vessel where it is separated from the remaining 
particulate constituents which remain suspended in the aqueous phase of 
the suspension. 
The particles to be separated by the froth flotation process are in the 
size range of about 500 .mu.m to 2-10 .mu.m; however, 65 mesh (230 .mu.m ) 
to 270 mesh (53 .mu.m) is typical. Raw ore is comminuted in size from 
boulders of up to 100 cm in diameter to a size range of from about 3 cm to 
about 0.5 cm using jaw crushers, cone crushers, gyratory crushers, or 
roll-type equipment. If ore of this size is to be used in a subsequent 
process, the sized ore is sieved and/or washed to remove impurities that 
concentrate in the fine particle size range. When the entire volume of ore 
is to be processed by froth flotation further size reduction using rod 
mills and ball mills is used to bring the particle size of all the ore to 
finer than about 65 mesh (230 .mu.m). The primary objective of this is to 
generate mineral grains that are discrete and distinct from one another. 
The generation of distinct particles is essential for the exploitation of 
individual mineral properties in the separation process. At the same time, 
particles at such fine sizes can be more readily buoyed to the top of the 
flotation cell by gas bubbles that adhere to them. 
The flotation step is accomplished by the preparation of pulp, consisting 
of a solid-liquid slurry that may contain up to 40% solids, to which 
chemical reagents known as collectors are added in a conditioning tank. 
Selected reagents are added to render some minerals hydrophobic so that 
they selectively adhere to air bubbles introduced into the pulp in a 
flotation cell. On the other hand, some reagents are added to enhance 
selectivity through activation and depression phenomena. Frothers are also 
used to generate a mineral-laden froth layer and enhance particle-bubble 
adhesion. The products from the flotation cell are a concentrate and a 
tailing stream. The concentrate proceeds to the next step for further 
cleaning or treatment. A typical froth floatation process can treat, for 
example, a raw feed that assays 0.5% to a few percent copper to give a 
mineral concentrate analyzing 35% copper with a recovery of more than 85% 
of the copper content of the original ore. 
The actual flotation process occurs in flotation cells usually arranged in 
batteries in an industrial plant. The individual cells can be any size 
from a few to 30 m.sup.3 in volume. Also, column cells have become 
popular, particularly in the separation of very fine particles in the 
minerals industry and colloidal precipitates in environmental 
applications. Such cells can vary from 3 to 9 meters in height and have a 
cross section of 0.3 to 1.5 meters in width. 
Traditionally, in the U.S., only about 5 percent of fine coal is cleaned by 
froth flotation because of technical difficulties and unfavorable 
economics. Fine coal processing by froth flotation is associated with 
difficulties in froth handling, product dewatering, low throughput, and 
inefficient separation of impurities such as pyrites. Therefore, 
traditionally, a majority of the coal in the U.S. is cleaned at coarse and 
intermediate sizes (down to 28 mesh) by gravity separation. A significant 
portion of the coal fines (minus 28 mesh or less than 0.595 
mm.--equivalent to 0.0234 inches) are discarded as waste into tailings 
ponds. Therefore, it is believed, there is a need to augment the froth 
flotation process and the flotation column in particular with an efficient 
secondary separation process based on density separation in order to 
improve the utilization of fine coal. 
It has been noted in the art that the froth flotation process does not 
always provide complete separation of desired materials from unwanted 
impurities. A number of modifications have been suggested to the froth 
flotation process to improve the efficiency of the separation. 
A method and apparatus for separating coal or mineral ore fines by froth 
flotation are disclosed by Miller et al., U.S. Pat. No. 4,744,890. Miller 
discloses a countercurrent flotation device and method that use a 
vertically oriented, cylindrical flotation vessel having a tangential 
inlet at its upper end and an annular outlet at its lower end. A pedestal 
positioned within the lower end of the vessel serves to support the froth 
column formed within the flotation cell and to minimize mixing between the 
froth column and the fluid discharge. The configuration of the flotation 
vessel, with its tangential inlet and annular outlet directs the 
particulate suspension around the vessel in a swirling motion. The froth 
column, which carries one component stream, exits through the top, center 
of the column, while the second component exits around the outer perimeter 
at the bottom of the column. This design has the disadvantage that the 
fluid flow and the centrifugal forces are coincident, making it difficult 
to separate the effects of froth flotation from the secondary separation 
method. Further, this system utilizes a porous column wall to introduce 
gas into the flotation process which acts counter to density separation by 
the use of centrifugal force. 
An alternate method and apparatus for separating coal or mineral ore fines 
by a swirl-flow pattern to develop centrifugal forces on the liquid or gas 
stream are disclosed by Duczmal et al., U.S. Pat. No. 5,224,604. Duczmal 
discloses an air-sparged hydrocyclone flotation device and method for the 
separation of particles in either liquid or gas streams. The fluid stream 
is directed in a swirl-flow pattern in a porous-walled cylinder to develop 
centrifugal forces on the stream. Magnetic or electrical fields can be 
applied to the system to enhance separation of the particles. Air sparging 
may also be employed to further amplify the separation of hydrophilic 
particles from hydrophobic particles in a liquid system. The swirl-flow 
pattern exits the downstream end of the separator where a stream splitter 
is employed to split the swirl-flow pattern stream which splays outwardly 
at the outlet in two or more streams which carry desired particles to be 
recovered. The hydrocyclone of Duczmal has the disadvantage that the fluid 
flow and the centrifugal forces are coincident making it difficult in 
separating the effects of froth flotation from the secondary separation 
technique/method. Further, the hydrocyclone of Duczmal provides minimal 
mixing of the materials to be separated. Also, in one embodiment, both 
impurities and desired products enter from the same end and are discharged 
at different radii from the other end. 
BRIEF SUMMARY OF THE INVENTION 
An object of this invention is to provide an improved froth flotation 
apparatus and process having increased separation efficiency. 
Another object of this invention is to provide an improved froth flotation 
device that augments the froth flotation process for the separation of 
fine particle minerals by adding density separation. 
Another object of this invention is to provide an improved froth flotation 
process that removes higher density hydrophilic material by means in 
addition to froth flotation. 
Another object of this invention is to separate coal from clay and pyrite 
(and other heavy metals and minerals) by taking advantage of the large 
difference in their specific gravities, whereas the specific gravity of 
coal is 1.2, while the specific gravity of pyrite is 5.0. 
These and other objectives of the invention, which will become apparent 
from the following description, have been achieved by a novel froth 
flotation column for the separation of at least two different materials, 
comprising a cylindrical column having at least one helical track adjacent 
to the inner surface of the column and located intermediate between the 
top and the bottom of the column. The helical track is a piece of linear 
material that is attached to the inside of the column. The helical track 
is attached to the column in a spiral fashion with the respective ends 
marking imaginary planes intersecting the column and defining a separation 
zone. The pitch (angle of the incline of the spiral) of the helical track 
is inclined from 10.degree. to 60.degree. from the horizontal, and 
preferably from 25.degree. to 45.degree. from the horizontal. The upper 
surface of the helical track can be horizontal or it can be inclined so 
that it slopes toward the inside of the column wall. This inclination is 
at an angle of from about 20.degree. to about 60.degree. from the 
horizontal and preferably at an angle of from about 40.degree. to about 
60.degree. from the horizontal. 
A central mixing shaft is located along the axis of the column. The mixing 
shaft is attached to a motor to provide for variable speed rotation. The 
other end of the shaft terminates within the column. At least one impeller 
is attached to the shaft and at least an impeller is located within the 
separation zone of the column. The rotation of the mixing shaft can be in 
the same direction as the pitch of the helical track or it can be in the 
opposite direction. For example, when viewed from above, if the helical 
track spirals down in a clockwise direction the mixing shaft can be made 
to rotate in either the clockwise or counter clockwise direction. It is 
preferable to have the pitch of the helical path and the rotation of the 
mixing shaft in opposite directions. The impeller and shaft provide for 
agitation of the mixture within the froth flotation column. A gas inlet or 
air sparger is located near the bottom of the column. The air sparger 
should be spaced at a sufficient distance from the bottom to allow 
hydrophilic tailings and high density particles to exit through the 
discharge port at the bottom of the column. The gas inlet permits the 
introduction of a stream of fine gas bubbles into the column for the 
separation of at least one of the component from the feed material. 
A feed material comprising a slurry of the material to be separated, 
chemical additives, and a liquid solvent are mixed in a separate tank or 
sump and is conveyed to the column through appropriate piping and pumping 
equipment. The slurry is then introduced into the column. The slurry of 
feed material can be introduced at the top of the column or at any point 
intermediate between the top and bottom of the column. The feed material 
is introduced through an opening that is provided for the slurry to be 
introduced perpendicular to or tangentially to the column wall. A first 
discharge port is provided near the top of the column for removing at 
least one component to be separated. A second discharge port is located 
near the bottom of the column for removing at least one of the other 
components to be separated. 
A concentrate holding tank is attached to the first discharge port to 
provide for further cleaning and drying of the hydrophobic material 
removed from the top of the column. A tailing holding tank attached to the 
second discharge port for further processing of the higher density 
material and hydrophilic material from the bottom of the column. 
The apparatus and process of this invention can be used to separate a two 
component mix. However, it is preferable to use at least a three-component 
mix comprising a material that will be made hydrophobic through the use of 
additives, a hydrophilic material, and a higher density material, to take 
advantage of this invention.

The invention is not limited in its application to the details and 
construction and arrangement of parts illustrated in the accompanying 
drawings since the invention is capable of other embodiments that are 
being practiced or carried out in various ways. Also, the phraseology and 
terminology employed herein are for the purpose of description and not of 
limitation. 
DETAILED DESCRIPTION OF THE INVENTION 
Description of the Preferred Embodiment(s) Referring to FIG. 1, a flow 
sheet showing the froth flotation system using the cross-flow flotation 
cell (hereinafter referred to as "CFC") 10 of this invention is presented. 
The feed material 12 to be separated is introduced into sump 14 along with 
the appropriate reagents 16 and a suspension liquid 18, which is typically 
water. Typically, the feed material 12 to be separated by this system 
comprises a three-component mixture; hydrophobic particles or particles 
made hydrophobic after treatment with the appropriate chemical reagents, 
hydrophilic particles or particles made hydrophilic after treatment, and a 
high density particle. The resulting feed slurry 20 is fed through sump 
line 22 via pump 24 into feed line 26. The feed slurry 20 is injected into 
the CFC 10 via an opening 28 located at a place intermediate between the 
top 30 and bottom 32 of the CFC 10. The feed slurry 20 enters the 
separation zone 34 of the CFC 10. The feed slurry 20 can be injected 
tangentially or perpendicular to the wall 36 of the CFC 10, as illustrated 
in FIG. 4. 
Referring to FIG. 2, a detailed discussion of the CFC 10 follows. A helical 
track 38 is placed around the inside wall 40 of the CFC 10. The helical 
track 38 can extend from the top 30 to the bottom 32 of the CFC 10 or from 
any two points intermediate between the top 30 and the bottom 32 of the 
CFC 10. The length of the CFC 10 in which the helical track 38 is located 
is referred to as the separation zone 34. The helical track 38 is a narrow 
strip of material attached to the inside wall 40. The helical track 38 is 
inclined or has a pitch (shown by .varies.) at an angle of from about 
10.degree. to about 60.degree.. Preferably the incline is from about 
25.degree. to about 45.degree.. Preferably, the helical track 38 is 
inclined at an angle of 30.degree.. Individual turns of the helical track 
38 are spaced from each other by one to three times the width (the 
distance from the upper to the bottom surface of the track) of the 
material from which the helical track 38 is fabricated. The helical track 
38 is from about 0.5 inches to 3 inches in thickness (the distance the 
track extends from the inside wall 40). In terms of dimensions relative to 
the CFC 10 diameter, preferably the thickness of the helical track 38 is 
from about 0.05 to about 0.20 times the diameter of the CFC 10. For 
example, when the CFC 10 has a diameter of 4 inches, the helical track 38 
has a thickness of 0.375 inches. The upper surface 42 of the helical track 
38 can be horizontal. Preferably the upper surface 42 is inclined at an 
angle .beta. (as shown in FIG. 3) of from 20.degree. to about 60.degree., 
and more preferably from about 40.degree. to about 60.degree., in order to 
form an incline toward the inside wall 40 of the CFC 10. This stops 
material that falls on the track from easily returning to the separation 
zone 34 of the CFC 10. The bottom surface 43 of the helical track 38 can 
be inclined in a like manner to the upper surface 42 to simplify 
fabrication of the CFC 10, however, this is not required. 
The feed slurry 20 entering the separation zone 34 is agitated by at least 
one impeller 44 attached to a central shaft 46. Rotation of the central 
shaft 46 is provided by motor 48. Air or gas bubbles are introduced into 
the CFC 10 by air spargers 50 located adjacent to the bottom 32 of the CFC 
10. Air or gas is fed from source 52 through air line 54. The air sparger 
50 for use with this invention can be any standard air spargers or air 
sparger systems known in the art, such as porous metal, porous glass or 
porous ceramic. A porous column wall should not be used to provide gas 
bubbles for flotation as this inhibits the efficiency of density 
separation through the use of centrifugal forces to drive the high density 
material to the column wall. 
During flotation, the feed slurry 20 in the CFC 10 is mixed with by a 
series of impellers 44 attached to a central shaft 46. The pitch of the 
impellers is from about 25.degree. to about 60.degree., and preferably 
about 45.degree.. The diameter of the impeller 44 should be from 
one-quarter to one-half of the diameter of the CFC 10. The central shaft 
46 is rotated at suitable rpm to generated sufficient centrifugal force on 
the high density particles to force them against the inside wall 40 of the 
CFC 10. A suitable angular velocity is from about 600 rpm to about 3600 
rpm. Preferably, a suitable angular velocity is from about 1000 rpm to 
about 300 rpm. For example, a high density particle having a specific 
gravity of 5.0 and a CFC 10 having an interior diameter of 4 inches, an 
angular velocity of from about 600 to about 2000 rpm produces suitable 
results. This angular velocity helps to create a string of vortices near 
the central shaft 46 during the mixing. The slurry is moved in a circular 
motion by stirring in a counterclockwise direction and is moving slightly 
upward, while the helical track 38 is arranged in a clockwise direction 
and is slightly downward. The rotation of the central shaft 46 and the 
arrangement of the helical track 38 can be reversed so that the central 
shaft 46 is rotating in a clockwise direction and the helical track 38 is 
in a counterclockwise direction. The helical track 38 and the central 
shaft 46 can be oriented in the same direction, however, it is preferable 
to have the rotation of the central shaft 46 opposite to the arrangement 
of the helical track 38. 
The interrelationship between the impeller diameter, the impeller angular 
velocity, the column diameter and the centrifugal forces necessary to 
throw the high density particles against the inner wall 40 may place 
practical upper limits on the column diameter. A practical upper limit for 
the angular velocity is believed to be about 5000 rpm. This in conjunction 
with the limitation on the diameter of the impeller and the generation of 
centrifugal forces sufficient to throw high density particles to the inner 
wall 40 may limit the column diameter to a diameter less than that 
typically available in larger flotation cells. 
The CFC 10 is a vertically oriented column constructed out of any 
appropriate material for the manufacture of process equipment, such as, 
but not limited to, iron, mild steel, stainless steel, or fiberglass. The 
helical track 38 can be made from any suitable material, such as, but not 
limited to, iron, mild steel, stainless steel, rubber, plastic, or 
fiberglass. The cross-section of the helical track 38 can be square, 
rectangular, triangular or trapezoidal (as shown in FIG. 3). The helical 
track 38 is attached to the inside wall 40 by use of an appropriate 
adhesive, welding, soldering, or riveting with the aid of a support 
device. 
During flotation, as shown in FIG. 5, air bubbles are generated from the 
bottom 32 of the CFC 10 by air sparges 50. The hydrophobic particles 
(light particles) form a lightweight froth through the attachment of the 
hydrophobic particles to the rising air bubbles. The froth, due to its 
relatively light weight, is concentrated near the center of the CFC 10 and 
moves upward. Hydrophilic tailings (shaded particles), such as clay, stay 
with the liquid phase and proceeds down the CFC 10 to the bottom 32 along 
with the net movement of the liquid phase. The higher density particles 
(solid dark particles), due to their high specific gravity, swirl along 
the inner wall 40 of the CFC 10 and are caught in the helical track 38. 
The higher density particles are propelled down the helical track 38 by 
the movement of carrier fluid. 
The hydrophobic particles combined with the froth move to the top 30 of the 
CFC 10 where they enter the defoamer 56 where the hydrophobic particles 
are rinsed with separation liquid fluid from sprayers 57 to free them from 
the foam. The sprayer 57 can be directed onto the froth in order to 
improve the separation efficiency by removing hydrophillic material from 
the froth and returning it to the CFC 10. The resulting clean hydrophobic 
particles or concentrates are conveyed to a storage chamber 58 through 
conduit 60. The hydrophilic tailings and the higher density particles 
proceed down the helical track 38 or through the CFC 10 to the bottom 32 
of the CFC 10. The higher density particles and hydrophilic tailings 
proceed through the tailing conduit 62 through valve 64 and are 
transported by pump 66 to the tailings storage 70, by way of conduit 68 
for later disposal. 
EXAMPLES 
FIG. 1 shows the flow sheet of the flotation column circuit. The laboratory 
CFC used in these tests was 4 inches in diameter and 6 feet in height. A 
series of angular helical tracks was attached to the wall of a 
conventional column to produce the CFC of this invention. During these 
experiments a coal slurry was mixed with a series of impellers attached to 
a central shaft. In the operation of the CFC, air bubbles were generated 
with three air spargers located in the bottom chamber by air provided at 
14 psig. A variable speed motor was used to turn the mixing impeller. The 
impeller speed was set at 1400 rpm for all the tests. The pitch of the 
impellers was set at 45 degrees. Experiments were carried out in a 
semi-continuous mode. 
An Upper Freeport coal from Indiana County, Pennsylvania was used in these 
experiments. The sample was stage crushed and screened to collect the 100 
M.times.325 M size fraction for experiments. The feed sample contained 
26.4% ash and 2.9% sulfur (2.4% pyritic sulfur, 0.06% sulfate sulfur and 
0.5% organic sulfur). The effect of frother concentration on the kinetics 
of coal recovery and the removal of pyrite was evaluated. 
In each Test, 300 grams of coal were premixed in a 1500 ml beaker with an 
addition of 500 ml tap water. The coal and water mixture was conditioned 
for 5 minutes with an addition of variable amounts of methyl isobutyl 
carbinol (MIBC) frother. The column was filled with 9 L water, and then 
the preconditioned coal slurry was charged into the column for flotation. 
Clean coal froths were collected at various predetermined time periods 
until depletion of the froth. 
During flotation, air bubbles were generated from the bottom of the column. 
The clean coal forms a lightweight froth through the attachment of coal 
particles to the rising air bubbles. This created a string of vortices 
near the shaft during the mixing. The slurry was moved in a circular 
motion by stirring in a counterclockwise direction and slightly upward, 
while the helical insert was arranged in a clockwise direction and was 
slightly downward. The froth, due to its relatively light weight, was 
concentrated near the center of the shaft and moves upward. The heavy 
pyrite, due to its high specific gravity, swirls along the wall of the 
column and was caught by the angular helix. The pyrite was washed downward 
along the helix by the movement of water. 
RESULTS AND DISCUSSIONS 
A series of column flotation experiments were run to compare the kinetics 
of coal cleaning using three modes of operation: (1) without mixing and 
without helix attachment. (2) with 1400 rpm mixing but without helix 
attachment, and (3) with helix attachment and 1400 rpm mixing. FIG. 6 
shows cumulative Btu recovery as a function of time for each of the above 
three flotation modes. The column with helix attachment and with mixing 
has superior recovery and superior kinetics. The asymptotes of the 
cumulative recovery curves are 90.6, 87.3, and 78.0 for modes 3, 2, and 1 
respectively. FIG. 7 shows the kinetic plot for the three modes of 
operation. Mode 3 exhibits the highest rate as exemplified by the steepest 
slope. 
Several tests were conducted to compare the pyritic sulfur rejection 
capabilities of the CFC column with those of more conventional flotation 
techniques (Denver Cell and an open column). The results are shown in FIG. 
8. FIG. 8 indicates that the CFC achieved higher pyritic sulfur rejections 
than the other flotation systems, at all levels of frother concentration. 
The Denver cells had the poorest pyritic sulfur rejections, most likely 
because of the turbulent flotation conditions present in a Denver cell, 
which results in significant entrainment of unwanted mineral matter. 
Thus, in accordance with the invention, there have been provided an 
improved froth flotation apparatus and process having an increased 
separation efficiency. There has also been provided an improved froth 
flotation device that augments the froth flotation process for the 
separation of fine particle minerals by the addition of density-based 
separation. There has also been provided an improved froth flotation 
process that removes higher density hydrophilic material by means in 
addition to froth flotation techniques. Additionally, there has been 
provided an improved means to separate coal from pyrite (and other heavy 
metals and minerals) by taking advantage of the large difference in their 
specific gravities, whereas the specific gravity of coal is 1.2, while the 
specific gravity of pyrite is 5.0. 
With this description of the invention in detail, those skilled in the art 
will appreciate that modification may be made to the invention without 
departing form the spirit thereof. Therefore, it is not intended that the 
scope of the invention be limited to the specific embodiments that have 
been illustrated and described. Rather, it is intended that the scope to 
the invention is determined by the scope of the appended claims.