BACKGROUND
[0001] In the mining industry, nin-of-the-mine (ROM) ores are crushed and pulverized to
detach (or liberate) the valuable components from waste rocks. Although ROM coal is
rarely crushed, a significant portion is present as fine coal. The pulverized ores
and fine coal are then separated using appropriate methods. One of the most widely
used methods of separation is froth flotation. In this method, a pulverized ore (or
fine coal) is mixed with water to form a slurry, to which surfactants known as collectors
are added to render selected constituent(s) hydrophobic. For the case of processing
higher-rank coals such as bituminous and anthracite coals, which are naturally hydrophobic
as mined, no collectors may be necessary. When these materials are not sufficiently
hydrophobic, hydrocarbon oils are added to enhance their hydrophobicity. The hydrophobized
(or naturally hydrophobic) particles are then collected by the air bubbles introduced
at the bottom of a flotation cell. It is believed that the bubble-particle adhesion
is driven by hydrophobic attraction. The air bubbles laden with hydrophobic particle
rise to the surface of the aqueous pulp, while hydrophilic particles not collected
by the air bubbles exit the cell. Thus, flotation produces two products, i.e., floated
and unfloated. The more valuable of the two is referred to as concentrate, and the
valueless is referred to as tailings (or refuse).
[0002] The concentrates are dewatered before they can be further processed or shipped to
consumers, while the tailings (or refuse) are discarded with or without extensive
dewatering. The dewatering process consists of several steps. In the first step, a
slurry is thickened to 35 to 75% solids in a large settling tank, while free water
is removed from the top and recycled back to the plant. In the second step, the thickened
pulp is subjected to a mechanical dewatering process, such as filtration or centrifugation,
to further remove the water. However, this process is inefficient, particularly when
the mineral (or coal) particles are fine. In general, the moisture content in the
dewatered product increases with decreasing particle size, which indicates that the
residual moisture is mostly due to the surface water, i.e., the water molecules that
are strongly adhering to the surface. For sulfide mineral concentrates, the filtered
products contain typically 12 to 18% moisture by weight. For coal, the residual moistures
are higher (20 to 30% by weight) due to its low specific gravity. Very often, these
products need to be further dewatered in a third and the most costly step, i.e., thermal
drying, which may be an option for high-priced materials. However, it is not so for
low-priced commodities such as coal. Even for the high-priced materials, elimination
of the third step has significant economic and environmental advantages.
[0003] At present, the costs of cleaning and dewatering fine coal (finer than 0.5 mm) are
approximately 3 times higher than those for cleaning coarser coal. For this reason,
it is often more economical to discard the fines, if the fine coal constitutes only
a small fraction of the product stream. This is typically the case with many coal
producers around the world. In the U.S. alone, it is estimated that approximately
2 billion tons of fine coal has been discarded in abandoned ponds, while approximately
500 to 800 million tons of fine coal have been discarded in active ponds. On a yearly
basis, the U.S. coal producers discard approximately 30 to 50 million tons of fine
coal to ponds. This represents a loss of valuable natural resources and causes significant
losses of profit to coal producers. The U.S. coal producers are blessed in that the
fines fractions constitute only 5 to 20% of their product streams. In countries where
coals are more friable, the fines fractions can be in the 20 to 50% range. In this
case, coal producers can no longer afford to discard the fines. It is unfortunate
that there are no technologies available today, other than the costly thermal drying,
to lower the moisture of coal fines.
[0004] The difficulty in dewatering fine particulate materials may be explained from first
principles. Those skilled in the art consider that a filter cake consists of a series
of capillaries of different radii, from which water is removed during the process
of vacuum or pressure filtration. The water can be removed only when the pressure
drop applied across the filter cake exceeds the pressure of the water present inside
the capillaries. The pressure, Δ
p, in the capillary of radius,
r, can be calculated using the Laplace equation:
in which γ
23 is the surface tension at the water
3 and air
2 interface and θ is the contact angle of the inner walls of the capillary under consideration.
In filtration, the capillary wall is made of the surfaces of the particles in the
cake, and the effective capillary radius decreases with decreasing particle size.
The contact angle is the most widely used measure of particle hydrophobicity (water-hating
property). In the cessile drop method, a drop of water is placed on the surface of
interest and the angle is measured through the aqueous phase. Thus, the term contact
angle used in the present invention refers to the water contact angle, which increases
with increasing surface hydrophobicity. Eq. [1] suggests that the capillary pressure
increases with decreasing capillary radius, which explains the difficulties encountered
during the filtration of fine particles. If a filter cake contains capillaries of
different radii, it would be more difficult to remove the water from the finer capillaries.
At a given pressure drop applied across the filter cake, one can see that the water
trapped in the capillaries that are smaller than certain critical radius (
rc) cannot not be removed. Thus, the moisture of a filter cake should be determined
by the amount of the water trapped in the capillaries smaller than the critical capillary
radius.
[0005] Eq. [1] suggests three ways of achieving low cake moistures during filtration. These
include i) surface tension lowering, ii) capillary radius enlargement, and iii) contact
angle increase. Various chemicals (dewatering aids) are used to control these parameters.
One group of reagents is the surfactants that can lower the surface tension. Most
of the dewatering aids used for this purpose are ionic surfactants with high hydrophile-lipophile
balance (HLB) numbers. Sodium laurylsulfate and sodium dioctylsulfosuccinate, whose
HLB numbers are 40 and 35.3, respectively, are typical examples. Sing (
Filtration and Separation, March, 1977, pp. 159-163) suggested that the former is an ideal dewatering aid for coal because it does not
adsorb on the surface, which in turn allows for the reagents to be fully utilized
for surface tension lowering. The
U.S. Patent No. 5,346,630 teaches a method of pressure spraying a solution of a dewatering aid from a position
within the filter cake forming zone of a filter just prior to the disappearance of
the supernatant process water. This method, which is referred to as torpedo-spray
system, ensures even distribution of the dewatering aid without becoming significantly
diluted by the supernatant process water.
[0006] It should be noted here that high HLB surfactants are also used as wetting agents
for hydrophobic materials such as coal. Recognizing that dewatering is essentially
a de-wetting process, it is difficult to see how one type of reagents can be used
for both. It is well known that high HLB surfactants adsorb on hydrophobic non-wetting
surfaces with inverse orientation, i.e., with hydrocarbon tails in contact with the
surface and the polar heads pointing toward the aqueous phase. Thus, high HLB surfactants
can lower the surface tension, but they can also dampen the hydrophobicity and decrease
the contact angle. For this reason, the high HLB surfactants used as dewatering aids
can actually cause an increase in moisture content. Furthermore, the reagents remaining
in filtrate eventually return to the flotation circuit and cause adverse effects.
[0007] Various polymeric flocculants are used as dewatering aids. The role of these reagents
is to increase the effective size of the particles in the filter cake, so that the
pore radii are enlarged. This will greatly reduce the capillary pressure and, hence,
increase the filtration rate. However, most of the flocculants used as dewatering
aids are hydrophilic. Therefore, their adsorption dampens the hydrophobicity of the
mineral or coal concentrates that are mildly hydrophobic by virtue of collector adsorption
or by nature. Furthermore, the particles form small capillaries within each floc created
by organic flocculants. Therefore, the method of using polymeric flocculants for dewatering
has limitations. It has been reported that flocculants are capable of reducing dewatering
rate but not necessarily the final cake moisture (
Meenan, Proceedings of the Industrial Practice of Fine Coal Processing, Society of
Mining Engineers, pp. 223-229,1988).
[0008] Various electrolytes can also be used to coagulate the particles to be filtered,
and improve dewatering,
Groppo and Parekh (Coal Preparation, 1996, vol. 17, pp. 103-116) showed that fine coal dewatering improves considerably in the presence of divalent
and trivalent cations. They found this to be the case when using cationic, anionic
and nonionic surfactants.
[0009] The
U.S. Patent No. 5,670,056 teachers a method of using non-ionic (or neutral) low HLB surfactants and water-soluble
polymers as hydrophobizing agents that can increase the contact angle above 65° and,
thereby, facilitate dewatering processes. Mono-unsaturated fatty esters, fatty esters
whose HLB numbers are less than 10, and water-soluble polymethylhydrosiloxanes were
used as hydrophobizing agents. The fatty esters were used with or without using butanol
as a carrier solvent for the low-HLB surfactants. This invention disclosure lists
a group of particulate materials that can be dewatered using these reagents. These
include coals, clays, sulfide minerals, phosphates, metal oxide minerals, industrial
minerals and waste materials, most of which are hydrophilic without suitable pretreatment.
The use of the low HLB surfactants disclosed in the
U.S. Patent No. 5,670,056 may be able to increase the contact angles of the minerals that are already hydrophobic,
but not for the hydrophilic particles.
[0010] There are several other U.S. patents, which disclosed methods of using low HLB surfactants
as dewatering aids. The
U.S. patent Nos. 4,447,344 and
4,410,431 disclosed methods of using water insoluble nonionic surfactants with their HLB numbers
in the range of 6 to 12. These reagents were used together with reagents (hydrotropes)
that are capable of keeping the surfactants in solution or at the air-water interface
rather than at the solid-liquid interface, so that they can be fully utilized in lowering
surface tension. Thus, the role of the low HLB surfactants disclosed in this invention
is different from: that of the surfactants disclosed in the
U.S. Patent No. 5,670,056. They do not to adsorb on the surface of the particles and enhance their hydrophobicity.
The low HLB surfactants, disclosed in the
U.S. Patents Nos. 4,447,344 and
4,410,431, are the reaction products of one mole equivalent of a primary alcohol containing
6 to 13 carbons with 2 to 7 mole equivalents of methylene oxide.
[0011] The
U.S. Patent No. 2,864,765 teaches a method of using another nonionic surfactant, a polyoxyethylene ether of
a hexitol anhydride partial long chain fatty acid ester, functioning alone or as a
solution in kerosene. However, the disclosure does not mention that the nonionic surfactant
increases the hydrophobicity of moderately hydrophobic particles. Furthermore, the
compounds disclosed are essentially not adsorbed upon the solid surface of the ore
particles and remain in the filtrate, as noted in the
U.S. Patent No. 4,156,649. In the latter patent and also in the
U.S. Patent No. 4,191,655, methods of using linear or branched alkyl ethoxylated alcohols as dewatering aids
were disclosed. They were used in solutions of hydrocarbon solvents but in the presence
of water-soluble emulsifiers such as sodium dioctylsulfosuccinate. As has already
been discussed, the use of high HLB surfactants can dampen the hydrophobicity due
to inverse orientation and increase the capillary pressure.
[0012] The
U.S. Patent No. 5,048,199 disclosed a method of using a mixture of a non-ionic surfactant, a sulfosuccinate,
and a defoaming agent. The
U.S. Patent No. 4,039,466 disclosed a method of using a combination of nonionic, surfactant having a polyoxyalkylene
group and an anionic surfactant. The
U.S. Patent No. 5,215,669 teaches a method of using water-soluble mixed hydroxyether, which is supposed to
work well on both hydrophobic (coal) and hydrophilic (sewage sludge) materials. The
U.S. Patent No. 5,167,831 teaches methods of using non-ionic surfactants with HLB numbers of 10 to 14. This
process is useful for dewatering Bayer process alumina trihydrate, which is hydrophilic.
The
U.S. Patent No. 5,011,612 disclosed methods of using Cg to C
20 fatty acids, fatty acid precursors such as esters or amides, or a fatty acid blend.
Again, these reagents are designed to dewater hydrophilic alumina trihydrate.
[0013] The
U.S. Patent No. 4,206,063 teaches methods of using a polyethylene glycol ether of a linear glycol with its
HLB number in the range of 10 to 15 and a linear primary alcohol ethoxylate containing
12 to 13 carbon atoms in the alkyl moiety. These reagents were used to dewater mineral
concentrates in conjunction with hydrophobic alcohols containing 6 to 24 carbon atoms.
The composition of this invention was preferably used in conjunction with polymeric
flocculants. Similarly, the
U.S. Patent No. 4,207,186 disclosed methods of using a hydrophobic alcohol and a non-ionic surfactant whose
HLB number is in the range of 10 to 15.
[0014] It is well known that oils can enhance the hydrophobicity of coal, which is the reason
that various mineral oils are used as collectors for coal flotation. The
U.S. Patent No. 4,210,531 teaches a method of dewatering mineral concentrates using a polymeric flocculant,
followed by a combination of an anionic surfactant and a water-insoluble organic liquid.
The use of flocculant and ionic surfactants may be beneficial in dewatering, but they
could dampen the hydrophobicity of the particles and, hence, adversely affect the
process. The
U.S. Patent No. 5,256,169 teaches a method to treat a slurry of fine coal with an emulsifiable oil in combination
with an elastomeric polymer and an anionic and nonionic surfactant, dewatering the
slurry and drying the filter cake, where the oil reduces the dissemination of fugitive
dusts. The
U.S. Patent No. 5,405,554 teaches a method of dewatering municipal sludges, which are not hydrophobic, using
water-in-oil emulsions stabilized by cationic polymers. The
U.S. Patent No. 5,379,902 disclosed a method of using heavy oils in conjunction with two different types of
surfactants, floating the coal-emulsion mixture, dewatering the flotation product
and drying them for reconstitution. The
U.S. Patent No. 4,969,928 also teaches a method of using heavy oils for dewatering and reconstitution.
[0015] The
U.S. Patent No. 4,770,766 disclosed methods of increasing the hydrophobicity of oxidized and low-rank coals
using additives during oil agglomeration. The main objective this process is to improve
the kinetics of agglomeration and ultimately the separation of hydrophilic mineral
matter from coal. The additives disclosed in this invention include a variety of heavy
oils and vegetable oils, alcohols containing 6 or more carbon atom, long-chain fatty
acids, etc. When these additives were used, the product moisture was lower than would
otherwise be the case. However, the process requires up 300 lb/ton of additives and
uses 45 to 55% by volume of an agglomerant, which is selected from butane, hexane,
pentane and heptane.
[0016] The
U.S. Patent Nos. 5,458,786 disclosed a method of dewatering fine coal by displacing water from the surface with
a large amount of liquid butane. The spent butane is recovered and recycled. The
U.S. Patent No. 5,587,786 teaches methods of using liquid butane and other hydrophobic liquids for dewatering
other hydrophobic particles.
OBJECTS OF THE INVENTION
[0017] From the foregoing, it should be apparent to the reader that one obvious object of
the present invention is the provision of novel methods of decreasing the moisture
of fine particulate materials during mechanical methods of dewatering processes such
as vacuum and pressure filtration and centrifugation.
[0018] Another important objective of the invention is the provision of improving the rate
at which water is removed so that given dewatering equipment can process higher tonnages
of particulate materials.
[0019] An additional objective of the present invention is the provision of novel fine particle
dewatering methods that can reduce the moisture to a level that no thermal drying
is necessary.
[0020] Still another object of the instant invention is the provision of a novel dewatering
method that creates no adverse effects on up- and downstream processes when the water
removed from the dewatering processes disclosed in the present invention is recycled.
SUMMARY OF THE INVENTION
[0021] The invention relates to a process according to claim 1. In particular, the instant
invention discloses methods of rendering the particulate materials suspended in water
hydrophobic and/or enhancing the hydrophobicity of the materials, so that the process
of removing the water by mechanical processes such as filtration and centrifugation
are improved. The improvements will result in lower product moisture and/or higher
throughput. The essence of the invention is to render the particles reasonably hydrophobic
in the first place by suitable means and, then, add non-ionic low HLB surfactants
to significantly enhance the hydrophobicity of the particulate materials, so that
the pressures required to expel the moisture from smaller capillaries are reduced
substantially. This will greatly increase the rate of dewatering and reduce the cake
moisture.
[0022] The hydrophobicity enhancing reagents disclosed in the present invention have HLB
numbers below 15, and are insoluble in water. Therefore, appropriate solvents such
as light hydrocarbon oils and short-chain alcohols may be used in conjunction with
the low-HLB surfactants. The light hydrocarbon oils, which should also be considered
as HLB surfactants, may also act as hydrophobicity enhancing agents. Furthermore,
the packages of the reagents used in the instant invention are capable of lowering
surface tension. Also, the particles coagulate owing to the increased hydrophobicity
and, thereby, increase the capillary radius. Thus, the reagent compositions disclosed
in the present invention is capable of increasing contact angle, lowering surface
tension, and enlarging capillary radius, all of which should contribute to decreasing
capillary pressure and improving dewatering. The instant invention also discloses
reagent dosage by adding cations, and achieving substantial moisture reduction by
spraying reagents to filter cake and applying mechanical vibration during drying cycle
time.
DETAILED DESCRIPTION OF THE INVENTION
[0023] In any mechanical dewatering process, the bulk of the water present in a feed stream
can be removed easily. It is mainly the residual water adhering to the surface of
the particles that is difficult to remove. Clear evidence for this is given by the
fact that residual cake moistures are proportional to the specific surface area of
the particulate materials, which in turn increases with decreasing particle size.
Thus, the difficulty in removing the surface water may be considered to arise from
the fact that water molecules are held strongly to the surface by hydrogen bonds.
This would be particularly the case with hydrophilic particles, which by definition
have an innate affinity to water. One may choose to break the H-bonds by subjecting
the slurry to a heat or a very high mechanical force field created by centrifugation
or pressure. However, these methods entail high energy costs and maintenance problems.
A better option would be to destabilize the water molecules on the surface of the
particles, so that they can be more readily detached (or liberated) from the surface
and be subsequently removed by weaker mechanical forces. The present invention discloses
methods of destabilizing the surface water by rendering the particles substantially
more hydrophobic than usually required for the flotation of minerals and coal using
appropriate surfactants and combinations thereof.
[0024] Thermodynamically, dewatering can be represented as a process in which a solid/liquid
interface, whose interfacial tension is γ
12, is displace by an air/water interface, whose interfacial tension is γ
13. The free energy change, ΔG, associated with the dewatering process can be obtained
using the following relation:
If ΔG becomes negative, the dewatering process becomes spontaneous. The condition
under which ΔG<0 can be found by considering the following relationship, which is
known as Young's equation:
where γ
12 and γ
13 have the same meaning as in Eq. [2] and γ
23 represents the interfacial tension at the air/water interface. Substituting Eq. [3]
into Eq. [2], one obtains the following relationship:
Eq. [4] suggests that ΔG becomes negative, i.e., dewatering becomes spontaneous, when
θ exceeds 90°. The same conclusion can be drawn from Eq. [1], which suggests that
the capillary pressure becomes negative at obtuse contact angles.
[0025] The process of flotation is also based on hydrophobizing mineral particles. Appropriate
collectors are used to render the surface hydrophobic so that air bubbles can displace
the water that has become labile due to the hydrophobization from the surface and
establish a three-phase contact. For the process of bubble-particle adhesion (or formation
of three-phase contact) to be spontaneous, the following relationship must hold:
Substituting Eq. [3] into Eq. [5], one obtains the following relationship:
which suggests that the condition for bubble-particle adhesion (or flotation) is θ>0.
Comparing Eqs. [4] and [6], one can see clearly that the hydrophobicity requirement
for flotation is much less than for dewatering. For this reason; flotation practitioners
have been content with the degree of hydrophobicity obtained by using collectors.
Practically all of the collectors used in industry today are designed to render minerals
hydrophobic with contact angles well below 90°. At the same time, one can now see
why dewatering mineral concentrates has been so difficult. The floated products are
simply not hydrophobic enough for efficient dewatering.
[0026] If water contact angle is less than 90°, the process is no longer spontaneous. One
must supply energy to the system to displace the surface water. Eq. [4] shows that
the free energy requirement is reduced by lowering surface tension and by increasing
contact angle. Of these two variables, control of contact angle is a more powerful
means of reducing cake moisture, particularly if it can be increased above 45°. Consider
a case where one can increase contact angle from 45 to 85° using appropriate means.
This will reduce the energy requirement by 8.1 times. Likewise, the critical capillary
radius (
rc) will be also reduced by 8.1 times, according to the Laplace equation (Eq. [1]).
Let us now consider a case, where surface tension is lowered from 72 to 40 mN/m. This
will reduce the energy requirement and
rc by 1.8 times only. Note also that lowering the surface tension to 40 mN/m requires
a large amount of surfactant, which can cause harmful effects such as hydrophobicity
dampening (due to inverse orientation) and frothing problems.
[0027] In flotation, various collectors are used to render selected mineral constituents
hydrophobic. The collectors adsorb on the surface with normal mode of orientation,
i.e., with their polar heads in contact with the surface and their hydrocarbon tails
pointing toward the aqueous phase. Thus, the collector molecules effectively coat
the surface with hydrocarbon tails (or hydrophobes) that are hydrophobic. However,
the hydrocarbon tails do not usually form a close-packed monolayer at the dosages
normally employed in flotation practice. Even at high dosages, the hydrocarbon tails
of collector molecules do not form close-packed monolayers. The reason is that the
interaction between the polar heads and the surface are site specific and the number
of reactive sites available on mineral surfaces are less than those required to form
close-packed monolayers. For example, the number of negative charge sites available
on mica surface is approximately one half of what is needed for dodecylammonium ions
to form a close-packed monolayer. Thus, collector molecules usually form monolayers
of sparsely populated hydrocarbon tails, the spaces between them being filled with
water molecules. In such cases, contact angles are usually well below 90°. Such moderate
hydrophobicity may be sufficient for flotation but not for spontaneous dewatering.
[0029] In the instant invention, various non-ionic surfactants are used to increase the
contact angle close to or above 90°, so that the efficiency of dewatering fine particulate
materials is greatly improved. This is achieved by using various neutral (or nonionic)
low HLB surfactants that may be useful for producing more complete monolayers. Part
of the surfactants may adsorb in between the sparsely populated hydrocarbon tails
and thereby increase the hydrocarbon chain density on the surface, which is conducive
to hydrophobicity enhancement. Some of the surfactants may adsorb on top of the first
monolayer of hydrophobes, which should also increase the hydrophobicity. Since the
more hydrophobic moiety of a low HLB surfactant is attracted to the hydrophobes on
the surface
via hydrophobic interaction, the more polar part of the molecule may be exposed to the
aqueous phase. However, such an orientation should not dampen the hydrophobicity significantly,
because the polarity of the head groups of the low HLB surfactants disclosed in the
present invention is much lower than that of high HLB surfactants.
[0030] On the less hydrophobic part of the surface, the low HLB surfactants disclosed in
the instant invention may adsorb with their polar parts in contact with the surface,
possibly
via acid-base interactions. Such an adsorption mechanism will have the hydrocarbon tails
point toward the aqueous phase, and thereby convert the less hydrophobic sites to
more hydrophobic ones by covering the sites with hydrophobes.
[0031] The nonionic surfactants disclosed in the instant invention have HLB numbers below
15. These include fatty acids, fatty esthers, phosphate esters, hydrophobic polymers,
ethers, glycol derivatives, sarcosine derivatives, silicon-based surfactants and polymers,
sorbitan derivatives, sucrose and glucose esters and derivatives, lanolin-based derivatives,
glycerol esters, ethoylated fatty esters, ethoxylated amines and amides, ethoxylated
linear alcohols, ethoxylated tryglycerides, ethoylated vegetable oils, ethoxylated
fatty acids, etc. Most of these reagents are insoluble in water; therefore, they are
normally used in appropriate solvents, which are light hydrocarbon oils and short-chain
alcohols whose carbon atom numbers are less than eight. The light hydrocarbon oils
include diesel oil, kerosene, gasoline, petroleum distillate, turpentine, naphtanic
oils, vegetable oils, etc.
[0032] The light hydrocarbon oils may also act as hydrophobicity enhancing reagents. In
addition, both the light hydrocarbon oils and short chain alcohols may act as added
surfactants that can lower the surface tension of water. This is possible because
the surface tensions of the solvents used in the instant invention are in the range
of 20 to 30 mN/m. Thus, the use of a low HLB surfactant in conjunction with a proper
solvent addresses two of the three parameters that are important for improving dewatering,
namely, contact angle increase and surface tension lowering. It seems that the dewatering
in the instant invention also cause particles to coagulate by virtue of increased
hydrophobicity. This phenomenon, known as hydrophobic coagulation, should increase
the capillary radius and help dewatering. Evidence for the hydrophobic coagulation
is given by the fact that cake thickness increases by approximately 10% in the presence
of the dewatering aids. Therefore, combinations of the reagents used in the present
invention is capable of controlling all three parameters suggested by the Laplace
equation (Eq. [1]), i.e., contact angle, surface tension and capillary radius, to
achieve maximum moisture reduction. High HLB surfactants and polymeric flocculants
usually address one, and adversely affect the others, as has been discussed.
[0033] Although hydrophobic coagulation causes the capillary radius to increase, which is
beneficial for dewatering, still another method of achieving the same is disclosed.
In the instant invention, metal ions are added to coagulate particles, which has been
found to drastically reduce the amount of the surfactants required to achieve a desired
moisture reduction. Various metal ions can be used for this purpose. In general, the
higher the valence of the cations, the smaller the amount of the amount of the reagents
needed to obtain beneficial effects. The reagents can be added before, during or after
the addition of the dewatering aids disclosed in the present invention.
[0034] Of the three parameters affecting dewatering, contact angle is probably the most
important. In the instant invention, contact angle is increased by using low HLB surfactants
in conjunction with light hydrocarbon oils and short-chain alcohols. The driving force
for the adsorption mechanism is the hydrophobic attraction. Since the hydrophobic
attraction exists only between two hydrophobic entities, it is necessary that the
particles to be dewatered be rendered hydrophobic prior to or during the addition
of the low HLB surfactants. For hydrophilic particles such as untreated silica and
clay, they are hydrophobized by adsorbing appropriate surfactants on the surface.
After the initial hydrophobization step, a low HLB surfactant can be added to further
enhance the hydrophobicity for improved dewatering. The surfactants that can be used
for the initial hydrophobization step are usually high HLB surfactants whose polar
head groups can interact with the surface via coulombic attraction, chemical bonding,
electron-transfer, or acid-base interactions, while their non-polar tails are directed
toward the aqueous phase. If a mineral concentrate from flotation processes is aged
or oxidized during storage and transportation, it is necessary that the surface is
re-hydrophobized using appropriate amount of collectors (or other high HLB surfactants)
before adding the low HLB surfactants.
[0035] The instant invention also discloses a method of decreasing the final cake moisture
by applying appropriate vibration to the filter cake. It is possible that the vibration
improves the transportation of the water that has become labile by increasing the
hydrophobicity of the particulate materials to be dewatered. This technique is particularly
useful for lowering the moisture from thicker cakes.
[0036] The instant invention discloses still another method of decreasing cake moisture.
This technique involves spraying light hydrocarbon oils and short-chain alcohols on
a filter cake, which is particularly useful for achieving low cake moisture with thick
cakes. It is believed that these reagents decreases the surface tension of the residual
water left in the filter cake. This technique is efficient in lowering the surface
tension of the water that is most difficult to remove. Spraying low HLB surfactant
on to a filter cake is also effective in achieving low cake moistures using very little
incremental reagent consumption.
[0037] An added benefit of using the dewatering aids disclosed in the present invention
is that the kinetics of mechanical dewatering is substantially improved, which will
greatly increase the throughput of dewatering devices. Furthermore, the dewatering
aids of the present invention have the characteristics of anti-forming agents, which
is very important for processing the particulate materials produced from flotation
processes. Also, most of the reagents added as dewatering aids and blends thereof
adsorb on the surface of the particulate materials, so that the water removed from
the dewatering process can be recycled without creating problems at the upstream processes.
TEST PROCEDURE
[0038] Many different samples were used for dewatering tests. These include a silica flower
(<0.037 mm (-400 mesh)), a Brazilian kaolin clay (90% finer than 2 µm), various coal
samples from different sources and sulfide mineral concentrates. The first two were
hydrophilic. Therefore, they were treated by a cationic surfactant to render the surface
moderately hydrophobic. The hydrophobicity was further enhanced using a low HLB surfactant
dissolved in a suitable light hydrocarbon oil before subjecting the sample to a dewatering
test. When the sulfide mineral concentrates were received from Europe, they were superficially
oxidized and became hydrophilic. As a means of regenerating fresh hydrophobic surface,
they were re-floated using a thiol collector and methylisobutyl carbinol (MIBC) as
a frother. The results obtained without the re-flotation step was relatively poor,
indicating that the low HLB surfactants does not adsorb on the samples that have become
hydrophilic due to oxidation during transportation
[0039] Some of the coal samples were used as received. Most of the tests were conducted,
however, after re-flotation using standard flotation reagents such as kerosene and
MIBC. When a sample became hydrophilic due to aging and superficial oxidation during
transportation, it was wet-ground in a ball mill for a short period of time to remove
the oxidation products and regenerate fresh, moderately hydrophobic surface. This
procedure helped the low HLB surfactants work better, indicating that they do not
adsorb on hydrophilic surfaces. In order to eliminate the problems concerning oxidation,
many tests were conducted using coarse coal products from the dense-medium circuit.
These samples were crushed, pulverized, wet-ground in a ball mill, and floated using
kerosene and MIBC. The flotation product was placed in a container and agitated continually.
A known volume of the slurry was transferred to an Elenmeyer flask. A known amount
of a dewatering aid was added to the flask before shaking it for 2 minutes. The conditioned
slurry was then poured into a filter to initiate a filtration test. After a preset
drying cycle time, the product was removed from the filter, dried in an oven for overnight,
and then weighed to determine the cake moisture. During each test, cake formation
time, which is the time it took for bulk of the water is drained, was recorded along
with the cake thickness. For vacuum filtration, a 6.35cm (2.5-inch) diameter Buchner
funnel with medium porosity glass frit was used. To conduct tests at large cake thicknesses,
the height of the Buchner filter was extended. For pressure filtration, a 6.35cm (2.5-inch)
diameter air pressure filter with cloth fabric medium was used to conduct tests under
different pressures. It was made of Plexiglas so that the events taking place during
filtration could be seen.
EXAMPLES
Example 1
[0040] In this example, sorbitan monooleate (Span 80), whose HLB number is 4.3, was used
as a dewatering aid. Since the surfactant is insoluble in water, it was dissolved
in a suitable solvent before use. In this example, dewatering tests were conducted
with the surfactant dissolved in five different solvents, which included diesel oil,
kerosene, fuel oil, gasoline, and butanol. Each test was conducted using one part
by volume of the active ingredient dissolved in two parts of a solvent.
[0041] A 6.35 cm (2.5-inch) diameter Buchner funnel with medium porosity glass frit was
used at 6.35mm (2.5-inch) Hg vacuum pressure with 2 minute drying cycle time and 1.143cm
(0.45-inch) cake thickness. The tests were conducted on a Pittsburgh coal sample.
It was a dense-medium clean coal product, which was crushed, ground, and screened
to obtain a 0.5 mm x 0 fraction. The fine coal sample prepared as such was floated
using a laboratory flotation machine using 0.5 kg/t (1 lb/ton) of kerosene as collector
and 82.7 g/t (75 g/ton) of MIBC as frother. The flotation product was used as a feed
to filtration tests. The feeds to filtration experiments were prepared each day to
ensure that coal surface was fresh and moderately hydrophobic. Sorbitan monooleate
and other low HLB surfactants disclosed in the present invention do not work well
when samples are hydrophilic. Also, their performance deteriorates significantly when
samples are oxidized to become partially hydrophilic.
[0042] Table 1 shows the results of the fiitration experiments. Diesel oil and kerosene
gave the best results. In general, mineral oils gave considerably better results than
butanol, which was used as a solvent for mono-unsaturated fatty esters whose HLB numbers
are less than 10 in the
U.S. Patent No. 5,670,056. At 1.5 (3) to 2.5 kg/t (5 lb/ton) sorbitan monooleate, the moisture reductions where
nearly 50%. Such results are far superior to what can be achieved using conventional
dewatering aids that are designed to control surface tension.
Table 1
Effects of Using Sorbitan Monooleate with Various Solvents for the Vacuum Filtration
of a Pittsburgh Coal (0.5 mm x 0) Sample
Reagent Dosage kg/t (lbs/ton) |
Cake Moisture (% wt) |
Diesel |
Kerosene |
Fuel Oil |
Gasoline |
Butanol |
0 (0) |
25.7 |
25.7 |
25.7 |
25.7 |
25.7 |
0.5 (1) |
15.1 |
15.0 |
16.6 |
16.3 |
17.2 |
1.5 (3) |
13.8 |
13.7 |
14.8 |
14.5 |
15.8 |
2.5 (5) |
12.5 |
13.4 |
14.2 |
14.2 |
15.3 |
Example 2
[0043] Sorbitan monooleate was used as a dewatering aid in the filtration of coal sample
using diesel oil as a solvent. One part of the surfactant by volume was dissolved
in two parts of the solvent before use. The coal sample used in this example was a
0.6 mm x 0 flotation product from Blackwater coal preparation plant, Australia, which
was received in the form of slurry. It was found, however, that the sample was considerably
oxidized during transportation. As a means of regenerating fresh surface, the coal
sample was wet-ground in a ball mill for 1.5 minutes, and re-floated using 0.5 kg/t
(1 lb/ton) kerosene and 82.7 g/t (75g/ton) MIBC. The process of regenerating fresh
surface and re-floating the pulverized coal, rendered the coal surface moderately
hydrophobic, which appeared to be a prerequisite for the dewatering aids disclosed
in the present invention to work more effectively.
[0044] The pressure filtration tests were conducted at different reagent additions, cake
thicknesses, and air pressures. In each test, 2 minutes of conditioning time and 2
minutes of drying cycle time were employed. The results are given in Table 2. The
reagent dosages given in this table refer to the active ingredient only. In general,
the moisture reduction improves with increasing reagent dosage, decreasing cake thickness,
and increasing air pressure. At 200 kPa of air pressure, the cake moisture was reduced
by nearly 50% at 2.16 cm (0.85 inches) of cake thickness and 2.5 kg/t (5lb/ton) sorbitan
monooleate.
Table 2
Effects of Using Sorbitan Monooleate for the Filtration of a Blackwater Coal (0.6
mm x 0) Sample at Different Air Pressures
Applied Pressure (kPa) |
Reagent Addition kg/t (lb/ton) |
Cake Moisture (%wt) |
Cake Thickness (inch) - cm |
(0.25) 0.635 |
(0.50) 1.27 |
(0.85) 2.16 |
100 |
0 (0) |
27.5 |
29.5 |
30.1 |
0.5 (1) |
17.3 |
21.6 |
22.5 |
1.5 (3) |
12.8 |
15.8 |
18.4 |
2.5 (5) |
9.4 |
14.6 |
16.7 |
200 |
0 (0) |
24.5 |
26.2 |
27.8 |
0.5 (1) |
13.2 |
14.6 |
19.4 |
1.5 (3) |
8.4 |
11.9 |
16.4 |
2.5 (5) |
7.9 |
10.5 |
14.2 |
Example 3
[0045] Sorbitan monooleate was also tested as a dewatering aid for zinc (sphalerite) concentrate.
The sample (0.105 mm x 0) was a flotation product, which was oxidized, however, during
transportation. As a means of regenerating fresh hydrophobic surface, the sample was
wet-ground in a ball mill for 1.5 minutes and re-floated using 55.1 g/t (50 g/ton)
sodium isopropyl of xanthate (NaIPX) and 55.1 g/t (50 g/ton) MIBC. The flotation product
was subjected to pressure filtration tests using a 6.35 cm (2.5-inch) diameter filter
at 100 kPa of air pressure and 2 minutes of drying cycle time. The cake thickness
was varied by changing the volume of the slurry, used in the filtration tests. The
results are given in Table 3. The %moisture reductions were 64.1, 54.8, and 52.8%
at 0.508 (0.2), 0.762 (0.3) and 1.524 cm (0.6 inches) of cake thicknesses, respectively,
at 1.5 kg/t (3 lb/ton) sorbitan monooleate. Moisture reduction did not further increase
significantly at 2.5 kg/t (5 lb/ton).
Table 3
Effects of Using Sorbitan Monooleate for the Filtration of a Zinc Concentrate (0.105
mm x 0) at 100 kPa of Air Pressure
Reagent Dosage kg/t (lbs/ton) |
Cake Moisture (% wt.) |
Cake Thickness (inch) - cm |
(0.2) 0.508 |
(0.3) 0.762 |
(0.6) 1.524 |
0 (0) |
14.2 |
15.5 |
18.0 |
0.5 (1) |
6.5 |
8.4 |
9.1 |
1.5 (3) |
5.1 |
7.0 |
8.5 |
2.5 (5) |
4.7 |
6.6 |
8.1 |
Example 4
[0046] Ethyl oleate is another low HLB number surfactant, which was tested as a dewatering
aid in the present invention. This reagent was also used as a dewatering aid in the
U.S. Patent No. 5,670,056, in which butanol was used as a carrier solvent. In the present example, ethyl oleate
was tested for the vacuum filtration of a 0.5 mm x 0 Pittsburgh coal using mineral
oils as solvents. The method of preparing the coal sample and the procedures employed
for the filtration experiments were the same as described in Example 1. The results
obtained with four different mineral oils are given in Table 4 and are compared with
those obtained using butanol as a solvent. As shown, mineral oils produced considerably
better results than butanol.
Table 4
Effects of Using Ethyl Oleate Mixed with Different Solvents on the Vaccum Filtration
of a Pittsburgh Coal (0.5 mm x 0)
Reagent Dosage kg/t (lb/ton) |
Cake Moisture (% wt) |
Diesel |
Kerosene |
Fuel Oil No. 4 |
Gasoline |
Butanol |
0 (0) |
26.4 |
26.4 |
26.4 |
26.4 |
26.4 |
0.5 (1) |
16.6 |
16.6 |
17.2 |
17.0 |
18.7 |
1.5 (3) |
14.2 |
14.5 |
15.4 |
14.7 |
16.5 |
2.5 (5) |
13.3 |
13.4 |
14.1 |
13.8 |
15.7 |
Example 5
[0047] Ethyl oleate was used as a dewatering aid for the vacuum filtration of a bituminous
coal sample from Elkview Mine, British Columbia, Canada. The sample was a 0.21 mm
x 0 flotation product, which was received as a slurry. It was oxidized during transportation;
therefore, the sample was wet-ground in a ball mill for 15 minutes and re-floated
using 0.5 kg/t (1 lb/ton) kerosene and 82.7 g/t (75 g/ton) MIBC before filtration.
A 6.35 cm (2.5-inch) diameter Buchner funnel was used at a vacuum pressure of 635
mm (25 inches) Hg and 2 min drying cycle time. The tests were conducted using different
amounts of ethyl oleate dissolved in 33.3% solutions in diesel oil at 0.635 (0.25)
and 1.27cm (0.5 inches) of cake thicknesses. At 2.5 kg/t (5 lb/ton) ethyl oleate,
the moisture reductions were 71.3 and 57.4% at 0.635 (0.25) and 1.27 cm (0.5. inches)
of cake thicknesses, respectively.
Table 5
Effects of Using Ethyl Oleate for the Filtration of a 0.21 mm x 0 Elkview Coal Sample
at 200 kPa of Air Pressure
Reagent Dosage kg/t (lbs/ton) |
Moisture Content (%wt) |
Cake Thickness (inch)-cm |
(0.25) 0.635 |
(0.50) 1.27 |
0 (0) |
24.0 |
26.3 |
0.5 (1) |
10.3 |
15.2 |
1.5 (3) |
7.8 |
12.6 |
2.5 (5) |
6.9 |
11.2 |
Example 6
[0048] Ethyl oleate was tested as dewatering aid for a lead concentrate (0.074 mm x 0) received
from a flotation plant in Europe. One part by volume of the surfactant was dissolved
in 2 parts of diesel oil before use. The sample, which was received as thickened slurry,
was oxidized during transportation. To generate fresh, hydrophobic surface, the sample
was wet-ground for 1.5 minutes and re-floated using 55.1 g/t (50 g/ton) NaIPX and
55.1 g/t (50 g/ton) MIBC before filtration. A 6.35 cm (2.5-inch) diameter Buchner
funnel was used for filtration at a vacuum pressure of 63.5 cm (25-inch) Hg and at
a drying cycle time of 2 minutes. The tests were conducted at various reagent additions
and cake thicknesses. At 1.5 kg/t (3 lb/ton) ethyl oleate, the cake moisture was reduced
to 6% at 1.524 cm (0.6 inches) of cake thickness. At such low moisture level, it would
not be necessary to dry the concentrate further using a thermal drier.
Table 6
Effects of Using Ethyl Oleate for the Filtration of a Lead Concentrate (0.074 mm x
0) Sample at Varying Reagent Dosage and Cake Thickness
Reagent Dosage kg/t (lbs/ton) |
Moisture Content (% wt.) |
Cake Thickness (inch) - cm |
(0.2) 0.508 |
(0.3) 0.762 |
(0.6) 1.524 |
0 (0) |
9.9 |
11.5 |
13.1 |
0.5 (1) |
5.3 |
5.5 |
7.8 |
1.5 (3) |
4.3 |
5.2 |
6.0 |
2.5 (5) |
4.0 |
5.1 |
5.8 |
Example 7
[0049] Polymethylhydrosiloxanes (PMHS) were disclosed as dewatering aids in the
U.S. Patent No. 5,670,056. However, this disclosure does not teach that better results can be obtained when
the reagents are used after dissolving them in appropriate solvents. Table 7 shows
the results of the vacuum filtration tests conducted using a PMHS whose molecular
weight is 2,900 with and without using various solvents. The filtration tests were
conducted on a Pittsburgh coal in the same manner as described in Example 1. The results
show that use of suitable solvents significantly reduced the cake moisture. This would
be particularly important when using hydrophobic polymers of high molecular weights.
Table 7
Effects of Using a PMHO with Molecular Weight of 2900 Dissolved in Different Solvents
for the Vacuum Filtration of a Pittsburgh Coal (0.5 mm x 0)
Reagent Dosage kg/t (lb/ton) |
Cake Moisture (% wt) |
Diesel |
Kerosene |
Fuel Oil |
Gasoline |
Butanol |
None |
0 (0) |
26.1 |
26.1 |
26.1 |
26.1 |
26.1 |
26.1 |
0.5 (1) |
16.8 |
17.3 |
17.6 |
17.8 |
18.3 |
20.4 |
1.5 (3) |
15.0 |
15.2 |
15.4 |
15.6 |
17.0 |
19.0 |
2.5 (5) |
14.8 |
14.7 |
15.2 |
15.2 |
16.4 |
18.3 |
Example 8
[0050] Sorbitan monooleate with 20 polyoxyethlene (POE) groups (Tween 80) is a nonionic
surfactant with its HLB number at 15, which is higher than those of other non-ionic
surfactants disclosed in the present invention. Nevertheless, the reagent was not
completely soluble in diesel. Therefore, one part by volume of the surfactant was
mixed with two parts of diesel oil and one part of butanol before use. The nonionic
surfactant dissolved in the mixed solvent was used as a dewatering aid for a bituminous
coal (0.84 mm x 0) from Massey Coal Company, West Virginia, using a 6.35 cm (2.5-inch)
diameter pressure filter. The coal sample was a spiral product, which was wet-ground
in a ball mill and floated using 0.5 kg/t (1 lb/ton) kerosene and 110.2 g/t (100 g/ton)
MIBC. The filtration experiments were conducted at 200 kPa air pressure by varying
reagent addition and cake thickness at 2 min drying cycle time. The best results were
obtained at 0.5 (1) and 1 kg/t (2 lb/ton). At 1 kg/t (2 lb/ton) Tween 80 and 2.032
cm (0.8 inches) cake thickness, the moisture reduction was 54.9%. At smaller cake
thicknesses, higher levels of moisture reductions were achieved. Interestingly, the
moisture reduction deteriorates at higher reagent dosages, which may be due to the
inverse orientation of the surfactant molecules with their polar heads (BO groups)
pointing toward the aqueous phase. Such orientation should make the surface less hydrophobic,
which is detrimental to dewatering. The inverse orientation is possible with a nonionic
surfactant with a relatively high HLB number, particularly with EO groups.
[0051] Vacuum filtration tests were also conducted using Tween 80 dissolved in diesel alone
and in butanol alone. The results were not as good as those obtained using the mixed
solvents as shown in Table 8.
Table 8
Effects of Using Sorbitan Monooleate with 20 EQ Groups Dissolved in a Mixed Solvent
for the Filtration of a Massey Coal (0.85 mm x 0) Sample at 200 kPa of Air Pressure
Reagent Addition kg/t (lb./ton) |
Moisture Content (% wt.) |
Cake Thickness (inch) - cm |
(0.2) 0.508 |
(0.4) 1.016 |
(0.8) 2.032 |
0 (0) |
22.2 |
23.5 |
25.3 |
0.25 (0.5) |
10.3 |
12.2 |
13.8 |
0.5 (1) |
8.2 |
9.7 |
11.6 |
1.0 (2) |
7.8 |
9.6 |
11.4 |
1.5 (3) |
9.8 |
10.5 |
13.7 |
Example 9
[0052] Phosphate esters constitute an important group of low HLB surfactants. They can also
be used as dewatering aids for coal and other mineral concentrates that are moderately
hydrophobic. Table 9 shows the results obtained using tridecyldihydrogen phosphate
(TDDP) (a phosphoric acid mono-tridecyl ester) as a dewatering aid in the vacuum filtration
of a Pittsburgh coal (0.5 mm x 0) sample. Various mineral oils and butanol were used
as solvents for the low HLB surfactant. Mineral oils, particularly diesel oil and
kerosene, gave better results than butanol. With diesel oil, the moisture reduction
was 50%. The sample preparation and the experimental procedures employed were the
same as described in Example 1. All of the filtration tests were conducted at a 1.143
cm (0.45-inch) cake thickness.
Table 9
Effects of Using Tridecyldihydrogen Phosphate (TDDP) for the Vacuum Filtration of
a Pittsburgh Coal (0.5 mm x 0) Sample Using Various Solvents
Reagent Dosage kg/t (lbs/ton) |
Cake Moisture (% wt) |
Diesel |
Kerosene |
Fuel Oil No 4 |
Gasoline |
Butanol |
0 (0) |
26.2 |
26.2 |
26.2 |
26.2 |
26.2 |
0.5 (1) |
17.4 |
16.8 |
17.8 |
18.0 |
19.5 |
1.5 (3) |
13.9 |
14.2 |
14.8 |
15.4 |
17.5 |
2.5 (5) |
13.1 |
13.4 |
13.9 |
14.3 |
14.3 |
Example 10
[0053] The effectiveness of TDDP may be attributed to the likelihood that it enhances the
hydrophobicity of coal. To confirm this possibility, contact angle measurements were
conducted on a polished Pittsburgh coal sample, and the results are given in Table
10. Also shown for comparison are the results of the vacuum filtration tests conducted
on the Pittsburgh coal sample and the surface tensions of the filtrate. The filtration
tests were conducted using a 6.35 cm (2.5-inch) diameter Buchner funnel at 63.5 cm
(25-inch) vacuum pressure, 2 minute drying cycle time, and 1.143 cm (0.45-inch) cake
thickness. The sample was a dense-medium product, which was crushed and ground to
obtain a 0.5 mm x 0 fraction. The fine coal sample was floated using 0.5 kg/t (1 lb/ton)
kerosene and 110.2 g/t (100 g/ton) MIBC.
[0054] As shown, the reagent addition caused an increase in contact angle and a decrease
in surface tension, both of which are conducive to improved dewatering. It is interesting
that contact angle increased from 12° to 90° at 1.5 kg/t (3 lb/ton). Thermodynamically,
water should recede spontaneously from a solid surface when its contact angle exceeds
90°. The fact that water is still left in the cake at such high contact angle may
be a reflection of the slow kinetics of transporting the water 'liberated' from the
surface through filter cake. The primary role of the low HLB surfactants is to help
liberate the water molecules adhering on the surface of coal by further increasing
its hydrophobicity. Both the nonionic surfactant and the solvent may have contributed
to the surface tension lowering.
[0055] It may be noteworthy that at 1 lb/ton kerosene the moisture was reduced from 28.4
to 25.3%, which is far less than the cases of using mixtures of TDDP and diesel oil.
Even when the dosage of kerosene (or any other mineral or vegetable oil) was increased,
the moisture reduction did not exceed more than 5%. When the kerosene dosage was increased
to very large amounts, moisture content actually increased. This may be attributed
to the likelihood that water traps within the flocs of coal created in the presence
of large amounts of oil. Only when judicious amounts of low HLB surfactants, such
as TDDP and others disclosed in the present invention, are used in conjunction with
appropriate solvents, significant moisture reductions can be achieved.
Table 10
Effects of TDDP on the Surface Chemistry Parameters for the Filtration of a Pittsburgh
Coal Sample
Reagent Type |
Reagent Dosages
kg/t (lb/ton) |
Contact Angle
(Degree) |
Filtrate Surface Tension
(mN/m) |
Moisture Content
(%wt) |
None |
0 (0) |
12 |
71 |
28.4 |
Kerosene |
0.5 (1) |
40 |
70 |
25.3 |
TDDP |
0.5 (1) |
74 |
67 |
16.2 |
1.0 (2) |
84 |
65 |
14.0 |
1.5 (3) |
90 |
61 |
12.8 |
2.5 (5) |
92 |
57 |
11.9 |
Example 11
[0056] A bituminous coal from Elkview Mine, British Columbia, Canada, was used for a series
of pressure filtration tests using TDDP as dewatering aid. One part by volume of the
reagent was dissolved in two parts of diesel oil before use. The tests were conducted
using a 6.35 cm (2.5-inch) diameter pressure filter at 200 kPa air pressure and 2
min drying cycle time. The coal sample was a flotation product (0.21 mm x 0) received
as a slurry. The sample was re-floated using 0.5 kg/t (1 lb/ton) kerosene and 82.7
g/t (75 g/ton) MIBC as a means of regenerating fresh, hydrophobic surfaces. The filtration
tests were conducted at different cake thicknesses using different amounts of reagents.
At 2.5 kg/t (5 lb/ton) TDDP and 0.635 cm (0.25 inches) cake thickness, the moisture
was reduced from 25.8 to 5.8%, which represents a 77.5% reduction. The moisture was
reduced to less than 10% even at 1.27 cm (0.5 inches) cake thickness.
Table 11
Effects of Using TDDP on the Filtration of an Elkview Coal at 200 kPa Air Pressure
Applied Pressure
(kPa) |
Reagent Dosage
kg/t (lbs/ton) |
Moisture Content (%wt) |
Cake Thickness (inch) - cm |
(0.25) 0.635 |
(0.50) 1.27 |
200 |
0 (0) |
25.8 |
27.1 |
0.5 (1) |
9.3 |
12.0 |
1.5 (3) |
7.4 |
10.4 |
2.5 (5) |
5.8 |
9.8 |
Example 12
[0057] Various ionic surfactants are used as dewatering aids for fine coal dewatering. Brooks
and Bethel (1984) used cationic surfactants (amines) to obtain significant improvements
in fine coal dewatering. It would, therefore, be of interest to compare the performance
of the low HLB surfactants used in the present invention with those obtained using
amines. Table 12 compares the results of the vacuum filtration tests conducted on
a bituminous coal from the Middle Fork coal preparation plant, Virginia, using two
different cationic surfactants (diaminecyclohexane and dodecylammonium chloride) of
high HLB numbers and two different low HLB No. nonionic surfactants (sorbitan monooleate
and TDDP). The coal sample was a dense-medium product, which was crushed and ground
to obtain a 0.6 mm x 0 fraction. All tests were conducted using a 6.35 cm (2.5-inch)
diameter Buchner funnel at 6.35 mm (25-inches) Hg vacuum pressure, 2 min drying time,
and 1.143 cm (0.45-inches) cake thickness. The results given in Table 12 show that
the low HLB surfactants used in the manner disclosed in the present invention are
substantially more efficient than the high HLB surfactants.
Table 12
Results of the Vacuum Filtration Tests Conducted on a Middle Fork Coal Sample Using
High and Low HLB Surfactants
Reagent Dosage
kg/t (lbs/ton) |
Cake Moisture (% wt) |
Diamine |
Dodecyleamine |
Span 80 |
TDDP |
0 (0) |
22.6 |
22.6 |
22.6 |
22.6 |
0.25 (0.5) |
20.6 |
19.1 |
16.5 |
16.9 |
0.5 (1) |
20.5 |
18.6 |
15.0 |
15.3 |
1.0 (2) |
19.7 |
17.9 |
12.6 |
12.2 |
1.5 (3) |
19.8 |
17.4 |
11.4 |
11.1 |
2.5 (5) |
20.9 |
17.1 |
10.9 |
10.2 |
Example 13
[0058] As discussed in Example 10, it is one thing to liberate the water molecules from
the surface of the particles to be dewatered using low-HLB surfactants, but it is
another to transport the liberated water droplets through a filter cake. The latter
problem becomes more serious with thicker cakes. One way to minimize the second problem
is to apply vibration during filtration. Therefore, a bituminous coal (0.6 mm x 0)
from Massey Coal Company was subjected to a series of vacuum filtration experiments,
in which a 6.35 cm (2.5-inch) Buchner funnel was vibrated during the 5 min drying
cycle time. The feed to the filtration tests was prepared in the same manner as described
in Example 8. The vibration was created by placing an ultrasonic probe at the bottom
part of the funnel. Varying amounts of sorbitan monooleate were used as dewatering
aid at 0.635 (0.25) and 1.27 cm (0.5 inches) of cake thicknesses. One part by volume
of the surfactant was dissolved in two parts of diesel oil before use. The results,
given in Table 13, show that very low levels of cake moisture can be achieved by combining
the methods of using low HLB surfactants and mechanical vibration.
Table 13
Effects of Ultrasonic Vibration on the Vacuum Filtration of a Bituminous Coal (0.6
mm x 0) Using Sorbitan Monooleate
Reagent Addition
kg/t (lb/ton) |
Cake Moisture (% wt.) |
0.635 cm (0.25 Inch) Cake |
1.27 cm (0.5 Inch) Cake |
w/o Vibration |
w/ Vibration |
w/o Vibration |
W/ Vibration |
0 (0) |
25.5 |
19.2 |
26.4 |
21.7 |
0.5 (1) |
15.2 |
10.3 |
17.7 |
12.1 |
1.0 (2) |
12.3 |
8.5 |
16.5 |
10.3 |
1.5 (3) |
12.2 |
6.4 |
15.6 |
9.2 |
2.5 (5) |
11.5 |
5.5 |
15.2 |
8.5 |
Example 14
[0059] As suggested by the Laplace equation, surface tension lowering is useful in decreasing
capillary pressure and, hence, improving dewatering kinetics. Conventional wisdom
is, therefore, to add surfactants to a feed slurry before it enters a filter. However,
the bulk of the water present in the feed stream is easily removed at the beginning
of a filtration process. It may be stated, therefore, that much of the surfactants
added to the feed stream are wasted and do not contribute to reducing the final cake
moisture. A more effective method of using a surfactant may be to add it when it is
needed most, i.e., during drying cycle time. Some of the water trapped in finer capillaries
is removed during drying cycle time. Therefore, a series of experiments were conducted
in this example, in which different surface tension lowering reagents were sprayed
over filter cake during drying cycle time.
[0060] Table 14 shows the results obtained by spraying approximately 1 kg/t (2 lb/ton) of
butanol, ethanol, and diesel oil at the beginning of 2 min drying cycle time. The
surface tensions of n-butanol and ethanol are 20.6 and 22.77 mN/m, respectively, at
20°C. The surface tension of diesel oil should also be low, as most other hydrocarbon
liquids are. Therefore, spraying these reagents should lower the surface tension of
the water left in filter cake and help reduce the moisture. The filtration experiments
were conducted on a 0.6 mm x 0 bituminous coal sample from Middle Fork, Virginia,
at 1.143 cm (0.45 inch) cake thickness. Two sets of tests were conducted using sorbitan
monooleate and TDDP as dewatering aids. When using the former, butanol was sprayed
on the cake, while ethanol and diesel oil were sprayed when using the latter. As shown,
the spray technique further reduced the cake moisture substantially.
Table 14
Effects of Spraying Different Regents Over Filter Cake When Using Sorbitan Monooleate
and TDDP as Dewatering Aids
Reagent Dosage
kg/t (lbs/ton) |
Moisture Content (% wt.) |
Sorbitan Monooleate |
Tridecyldihydrogenphosphate (TDDP) |
No Spray |
Butanol Spray |
No Spray |
Diesel Spray |
Ethanol Spray |
Butanol Spray |
0 (0) |
23.1 |
18.1 |
22.3 |
20.7 |
20.1 |
17.4 |
0.5 (1) |
13.8 |
8.3 |
12.4 |
11.9 |
11.4 |
7.3 |
1.0 (2) |
12.2 |
7.1 |
11.8 |
10.0 |
9.5 |
6.2 |
1.5 (3) |
10.1 |
6.1 |
10.3 |
8.5 |
8.1 |
5.2 |
2.5 (5) |
9.7 |
5.6 |
10.0 |
7.7 |
6.9 |
4.8 |
Example 15
[0061] In this example, the methods of applying vibration and spraying surface tension lowering
reagents, as disclosed in Examples 13 and 14, respectively, were combined to be able
to obtain low cake moistures at large cake thicknesses. The tests were conducted using
a 6.35 cm (2.5-inch) diameter Buchner funnel with its height extended to 15.24 cm
(6 inches), so that 300 ml of coal slurry at 18% solids could be filtered in each
test. This allowed the cake thickness to be increased to 3.048 cm (1.2 inches). The
coal sample used in these experiments were a dense-medium product from Massey. Coal
Company, which was crushed and wet-ground in a ball mill to obtain a 0.6 mm x 0 fraction.
The fine coal was floated using 0.5 kg/t (1 lb/ton kerosene) and 110.2 g/t (100 g/ton)
MIBC to obtain a feed to the filtration experiments. The tests were conducted at varying
amounts of TDDP and 5 minutes of drying cycle time. It can be seen that the combined
use of i) low HLB surfactant in diesel oil, ii) butanol spray, and iii) mechanical
vibration achieved very low moistures at an industrial cake thickness of 3.048 cm
(1.2 inches).
Table 15
Effects of Using Reagent Spray, Vibration, and a Combination There of at 3.048 cm
(1.2-inch) Cakes Thickness Using TDDP
Reagent Dosage
kg/t (lbs/ton) |
Moisture Content (%wt) |
None |
Spray |
Vibration |
Spray and Vibration |
0 (0) |
25.6 |
22.4 |
22.2 |
20.0 |
0.5 (1) |
18.2 |
44.3 |
14.5 |
12.3 |
1.0 (2) |
15.8 |
12.0 |
12.7 |
10.1 |
1.5 (3) |
14.9 |
11.0 |
10.8 |
8.8 |
2.5 (5) |
14.7 |
10.8 |
10.6 |
8.1 |
Example 16
[0062] Surprising results were obtained when the low HLB surfactants disclosed in the present
invention were used in conjunction with electrolytes. It appears that the use of electrolytes
can substantially decrease the amount of the surfactant needed to achieve a given
level of moisture reduction. Table 16 shows the results of a series of vacuum filtration
tests conducted using TDDP in the presence of aluminum chloride, chromium chloride,
and copper nitrate. Before filtration, each coal sample (0.2 mm x 0 flotation product)
was conditioned with a known amount of electrolyte for 5 minutes. A known amount of
TDDP dissolved in diesel oil (in 1:2 volume ratio) was then added and conditioned
for another 2 minutes. The conditioned coal slurry was poured into a 63.5 cm (2.5-inch)
diameter Buchner funnel for filtration experiments at 63.5 cm (25-inch) vacuum pressure,
2 min drying cycle time, and 1.016 cm (0.4 inch) cake thickness. The coal sample was
received from Massey Coal Company, West Virginia.
[0063] The results show that in the presence of the electrolytes the amount of TDDP required
was substantially reduced. For example, 1.5 kg/t (3 lb/ton) the reagent was required
to achieve 16.0% cake moisture. In the presence of 11.02 g/t (10 g/ton) aluminium
chloride and chromium chloride, however, only 0.25 kg/t (0.5 lb/ton) TDDP was required
to obtain similar (16.3 and 16.0%) cake moistures. In the presence of 55.1 g/t (50
g/ton) copper nitrate, cake moisture of 16.2% was obtained at 0.5 kg/t (1 lb/ton)
TDDP. Thus, electrolytes of trivalent cations seem to be more efficient than those
of divalent cations. It is possible that the cations introduced with the electrolyte
coagulate coal particles, which in turn results in a decrease in the population of
micropores in filter cake.
Table 16
Effects of Using Electrolytes for the Filtration of a Bituminous Coal (0.2 mm x 0)
Reagent Dosage
kg/t (lb./ton) |
Moisture Content (% wt.) |
None |
Al3+
11.0g/t (10 g/ton) |
Cr3+
11.0 g/t (10 g/ton) |
Cu2+
55.1 g/t (50 g/ton) |
0 (0) |
28.1 |
23.2 |
23.0 |
23.4 |
0.125 (0.25) |
22.5 |
18.2 |
17.6 |
18.4 |
0.25 (0.5) |
20.6 |
16.3 |
16.0 |
17.2 |
0.5 (1) |
19.3 |
15.4 |
15.2 |
16.2 |
1.0 (2) |
17.2 |
14.2 |
14.7 |
15.4 |
1.5 (3) |
16.0 |
13.6 |
14.2 |
15.3 |
2.5 (5) |
14.6 |
13.5 |
13.8 |
14.8 |
pH |
7.5 |
5.5-7.5 |
5.5-7.5 |
4.5-6.5 |
Example 17
[0064] The objective of this example is to demonstrate that combination of several different
methods disclosed in this invention can be used to achieve high levels of moisture
reduction at a cake thickness of approximately 2.54 cm (1 inch). A series of vacuum
filtration experiments were conducted using different combinations of i) a low HLB
surfactant (sorbitan monooleate) mixed with an appropriate carrier solvent, ii) an
electrolyte 11.02 g/t (10 g/ton) aluminum chloride), iii) spray of a surface tension
lowering reagent (1.0-1.5 kg/t (2-3 lb/ton) butanol), and/or iv) mechanical vibration.
The tests were conducted on a flotation product (0.6 mm x 0) using a specially designed
Buchner funnel that can handle large volumes of coal slurry, as described in Example
15. The coal sample was a dense-medium product from the Middle Fork coal preparation
plant. It was crushed, ground, and floated using 0.5 kg/t (1 lb/ton) kerosene and
110.2 g/t (100 g/ton) MBIC. results, given in Table 17, show that almost any level
of cake moisture can be achieved at an industrial cake thickness by combining the
various methods disclosed in the present invention. For example, 14.2% cake moisture
can be achieved using only 0.125 kg/t (0.25 lb/ton) sorbitan monooleate, 11.02 g/t
(10 g/ton) aluminum chloride, 1.0 to 1.5 kg/t (2 to 3 lb/ton) butanol, and mechanical
vibration.
Table 17
Effects of Using Electrolyte, Regent Spray, and Vibration on the Filtration of a Middle
Fork Coal (0.6 mm x 0) at 2.54 cm (1-inch) Cake Thickness Using Sorbitan Monooleate
as a Dewatering Aid
Reagent Addition2Al3+, kg/t (lb./ton) |
Moisture Content (% wt.) |
None1 |
2Al3+ |
2Al3+ and Spray3 |
2Al3+, Spray3 and Vibration4 |
0 (0) |
25.2 |
22.8 |
21.0 |
18.7 |
0.125 (0.25) |
20.1 |
18.0 |
16.7 |
14.2 |
0.25 (0.5) |
18.7 |
15.2 |
13.6 |
11.7 |
0.5 (1) |
16.2 |
14.3 |
12.5 |
10.2 |
1.0 (2) |
15.3 |
13.6 |
11.7 |
9.5 |
1.5 (3) |
14.7 |
13.2 |
10.6 |
8.2 |
2.5 5 |
13.8 |
13.0 |
10.3 |
7.4 |
1sorbitan monooleate in diesel oil (1:2); 2aluminium chloride (11.0 g/t (10 g/ton)); 3butanol (1.0-1.5 kg/t (2-3 lb/ton)); 4mechaical vibration |
Example 18
[0065] It has been shown in Example 7 that the use of PMCH dissolved in a suitable solvent
such as diesel oil gives superior results as compared to the case of using it directly.
It will be shown that the use of PMCH in vegetable oils further improves its performance.
To demonstrate this, a series of filtration tests were conducted on a bituminous coal
from Massey coal company, West Virginia, using a 6.35 cm (2.5-inch) pressure filter
at 100 kPa of air pressure. The coal sample was a flotation product (0.5 mm x 0) obtained
directly from an operating plant. It contained considerable amount of clay and other
ash-forming minerals that have not been completely removed. Also, the sample was oxidized
to some extent. The tests were conducted at a 1.27 cm (0.5-inch) cake thickness and
a 2 min drying cycle time using: i) soybean oil dissolved in diesel oil in 1:2 volume
ratio, ii) PMCH dissolved in diesel oil in the same manner, and iii) PMCH dissolved
in soybean oil and diesel oil. The molecular weight of the PMCH used in this example
was 2,900. The results given in Table 18 show that the combined use (Case iii) exhibited
a synergistic effect in that the results are superior to the Case i or ii.
[0066] As has already been discussed in the present invention, the role of PMCH is a hydrophobizing
agent that can reduce the capillary pressure and facilitate the process of dewatering.
It is possible that the triacylglycerols present in the vegetable oil may act as additional
hydrophobizing agents.
Table 18 Effects of Using PMCH in a Mixed Diesel Oil-Soybean Oil Solvent for the Filtration
of a Bituminous Coal (0.5 mm x 0) at 1.27 cm (0.5-inch) Cake Thickness
Reagent Addition
kg/t (lb./ton) |
Moisture Content (% wt.) |
Reagent Type |
Soy Bean Oil |
Polymethyl Hydrosiloxane |
Combination |
0 (0) |
27.5 |
27.5 |
27.5 |
0.5 (1) |
22.6 |
21.5 |
20.8 |
1.0 (2) |
21.0 |
20.4 |
18.5 |
1.5 (3) |
20.3 |
19.6 |
16.7 |
2.5 (5) |
20.7 |
19.8 |
14.2 |
Example 19
[0067] Many of the examples given hitherto give evidence that the low HLB surfactants work
well only when the particles to be dewatered are reasonably hydrophobic. The use of
the surfactants in the manner described in the instant invention further enhances
the hydrophobicity close to the level that is conducive for spontaneous removal of
surface water. The hydrophobicity of the particles produced from flotation is usually
not high enough for the spontaneous removal of water. In order to demonstrate these
points clearly, a series of dewatering tests were conducted with a 0.038 mm x 0 silica
sample. A 6.35 cm (2.5-inch) diameter Buchner funnel was used for two sets of vacuum
filtration tests at 63.5 cm (25 inches) Hg, 2 min drying cycle time, and 1.143 cm
(0.45 inches) cake thickness. The results are given in Table 19.
[0068] The first series of tests were conducted using various amounts of sorbitan monooleate
(Span 80) dissolved in diesel oil. These reagents were used as a 1:2 mixture by volume.
In the absence any dewatering aid, the cake moisture was 26.1% and the cake formation
time was 158 seconds. At 1 kg/t (2 lb/ton) Span 80, the moisture was reduced to 20.9%
and the cake formation time increased to 179 seconds. The moisture reduction is not
as good as those obtained in other examples with hydrophobic particles. Probably,
the relatively small moisture reduction is due to the surface tension lowering. The
next series of tests were conducted on the silica sample floated using 220.5 g/t (200
g/ton) of dodecylammonium hydrochloride as collector at pH.9.5. The hydrophobization
by the collector coating reduced the cake moisture from 26.1 to 18.9% and the cake
formation time from 158 seconds to 27 seconds. When the low HLB surfactant was added
to the flotation product, the moisture was further reduced. At 1 kg/t (2 lb/ton) Span
80, the cake moisture was reduced to 8.4% and the cake formation time to 18 seconds.
The improved dewatering brought about by the low HLB surfactants is most likely due
to the hydrophobicity enhancement.
Table 19
Effects of Hydrophobizing a Silica Sample (0.038 mm x 0) before Using Sorbitan Monooleate
as a Dewatering Aid
Reagent Dosage
kg/t (lb/ton) |
w/o Flotation |
w/ Flotation |
Moisture Content (%wt) |
Cake Form. Time (sec) |
Moisture Content (%wt) |
Cake Form. Time (sec) |
0 (0) |
26.1 |
158 |
18.9 |
27 |
0.25 (0.5) |
22.6 |
152 |
11.2 |
21 |
0.5 (1) |
20.9 |
167 |
9.4 |
20 |
1.0 (2) |
20.7 |
175 |
8.4 |
18 |
1.5 (3) |
20.9 |
179 |
8.6 |
18 |
Example 20
[0069] In the kaolin clay industry, fine clay is dewatered using vacuum drum filters. The
cake moistures are in the range of 55 to 60%. Typically, part of the filter cake is
spray dried in a natural gas flame, so that it can be added to the wet cake to obtain
a 70 to 75% solids slurry. The spray drying is costly, but it is the only way to produce
highly loaded slurries for shipping. In this example, a series of filtration tests
were conducted on a Brazilian clay (80% finer than 2 µm) using the method disclosed
in Example 19. The sample was floated using 771.6 g/t (700 g/ton) dodecylammonium
hydrochloride and 132.3 g/t (120 g/ton) MIBC. The pH was adjusted to 9.3 using lime.
The flotation product was subjected to vacuum filtration tests using a 6.35 cm (2.5-inch)
diameter Buchner funnel at 63.5 cm (25 inches) Hg, 0.914 cm (0.36 inches) cake thickness
and 3 min drying cycle time. As shown in Table 20, the cake moisture was 50.4% and
the cake formation time was 39.4 minutes, when no dewatering aid was used. At 3.5
kg/t (7 lb/ton) sorbitan monooleate (Span 80), the moisture content was reduced to
28.6%, and the cake formation time was reduced to 18.4 minutes. These results suggest
that the dewatering methods disclosed in the instant invention may be able to eliminate
the use of spray dryers in the clay industry. With further optimization of the process,
the reagent consumption can be reduced to significantly lower than used in the present
example.
Table 20
Effect of Hydrophobizing a Brazilian Kaolin Clay Sample before Using Sorbitan Monooleate
as a Dewatering Aid
Reagent Dosage
kg/t (lb/ton) |
Moisture Content
(%wt) |
Cake Form. Time (Min.) |
0 (0) |
50.4 |
39.5 |
0.5 (1) |
46.2 |
34.2 |
1.0 (2) |
39.1 |
26.3 |
1.5 (3) |
33.9 |
20.2 |
2.5 (5) |
30.2 |
19.2 |
3.5 (7) |
28.6 |
18.4 |
REFERENCES CITED
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Other Publications
[0071]
Brooks and Bethel, "Zeta Potential, Contact Angle and the Use of Amines in the Chemical
Dewatering of Fine-Floated Coal," Powder Technology, vol. 40, pp. 207-214, 1984.
Groppo, J.G. and Parekh, B.K., "Surface Chemical Control of Ultra-Fine Coal to Improve
Dewatering," Coal Preparation, vol. 17, pp. 103-116, 1996.
Meenan, G.F., "Fine Coal Dewatering Equipment," Proceedings of the Industrial Practice
of Fine Coal Processing, R.R. Klimpel and P.T. Luckie, eds., Society of Mining Engineers,
Inc., pp. 223-229, 1988.
Singh, B.P., "The Influence of Surface Phenomena on the Dewatering of Fine Clean Coal,"
Filtration and Separation, pp. 159-163, March, 1977.
Smith, R.W., "Coadsorption of Dodecylamine Ion and Molecule on Quartz," Transactions
of Americal Institute of Mining Engineers, vol, 226, pp. 427-433, 1963.
Yoon and Ravishankar, "Long-Range Hydrophobic Forces between Mica Surfaces in Dodecylammonium
Chloride Solutions in the presence of Dodecanol," J. Colloid and Interface Science,
vol. 179, pp. 391-402, 1996.
1. A process for dewatering a slurry of fine particulate material being smaller than
2 mm in diameter, which process comprises the steps of:
i) rendering the fine particulate material moderately hydrophobic so that its water
contact angle is increased to a value considerably below 90° by using appropriate
surfactants and collectors in an initial hydrophobization step,
ii) adding a nonionic surfactant having a hydrophile-lipophile balance (HLB) number
of less than 15 and dissolved in a appropriate solvent or mixture of solvents,
iii) agitating the slurry to allow for the surfactant molecules to adsorb on the surface
of the moderately hydrophobic material so that its hydrophobicity is enhanced and
the contact angle is increased close to or above 90°, and then
iv) subjecting the conditioned slurry containing the particulate material, whose water
contact angle has been increased, to a suitable mechanical method of dewatering.
2. The process of claim 1 wherein the fine particulate material to be dewatered is a
material whose surface has become less hydrophobic due to aging or superficial oxidation.
3. The process of claim 1 wherein the fine particulate material includes minerals, coal,
plastics, metals, metal powders, fly ash, and biological materials.
4. The process of any of the preceding claims wherein the said suitable mechanical method
of dewatering includes vacuum filtration, pressure filtration, centrifugal filtration,
and centrifugation.
5. The process of any of the preceding claim wherein the nonionic surfactant is selected
from fatty acids, fatty esters, phosphate esters, hydrophobic polymers, ethers, glycol
derivatives, sarcosine derivatives, silicon-based surfactants and polymers, sorbitan
derivatives, sucrose and glucose esters and derivatives, lanolin-based derivatives,
glycerol esters, ethoxylated fatty esters, ethoxylated amines and amides, ethoxylated
linear alcohols, ethoxylated triglycerides, ethoxylated vegetable oils, and ethoxylated
fatty acids.
6. The process of claim 5 wherein the nonionic surfactant is blended with a vegetable,
fish or animal oil containing triacylglycerols to obtain synergistic improvement in
dewatering the fine particulate material.
7. The process of any of the preceding claims wherein the said appropriate solvents include
light hydrocarbon oils and short-chain alcohols.
8. The process of claim 1 wherein the said appropriate surfactants are high HLB surfactants
whose polar heads can interact with the surface of the particulate materials.
9. The process of claim 1 wherein the said collectors are thiols for sulfide minerals
and metals.
10. The process of claim 1 wherein the said collectors are hydrocarbon oils and the particulate
material is coal or an other naturally hydrophobic substance.
11. The process of any of the preceding claims wherein an electrolyte or mixture of electrolytes
selected from salts of monovalent, divalent and trivalent cations and anions is added
after the initial hydrophobization step (i) and before step (ii).
12. The process of claim 11 wherein the said electrolytes are the salts of aluminum ions.
13. The process of claim 11 or 12 wherein the reagents used in step (i) and step (ii)
and said electrolyte or mixture of electrolytes can be added in a single step.
14. The process of any of claims 1 to 10 wherein the suitable mechanical method of dewatering
is a filtration process in which the filter cake is subjected to an appropriate vibratory
means, so that a higher degree of moisture reduction is achieved at a given cake thickness.
15. The process for claim 14 wherein the appropriate vibratory means include ultrasonic,
mechanical and acoustic means.
16. The process of any of claims 1 to 10 wherein the suitable mechanical method of dewatering
is a filtration process in which a suitable surface tension lowering reagent is added
to the filter cake in the form of fine mist or spray, so that a higher degree of moisture
reduction is achieved at a given cake thickness.
17. The process for claim 16 wherein the suitable surface tension lowering agent is selected
from short-chain alcohols, light hydrocarbon oils, and surfactants.
18. A process of any of claims 11 to 13 wherein the suitable mechanical method of dewatering
is a filtration process in which a suitable surface tension lowering reagent is added
to the filter cake in the form of fine mist or spray and, at the same time, the filter
cake is subjected to an appropriate vibratory means, so that a substantial moisture
reduction is achieved at high cake thicknesses using minimum amounts of reagents.
1. Verfahren zur Entwässerung einer Aufschlämmung von feinem teilchenförmigen Material,
das im Durchmesser kleiner als 2 mm ist, wobei das Verfahren die Schritte umfasst:
i) moderates Hydrophobieren des feinen teilchenförmigen Materials durch Verwendung
geeigneter oberflächenaktiver Substanzen und Sammler, so dass der Wasserkontaktwinkel
des Materials auf einen Wert von erheblich unterhalb 90° erhöht wird, in einem anfänglichen
Hydrophobierungsschritt,
ii) Hinzufügen einer nichtionischen oberflächenaktiven Substanz mit einem Hydrophilie-Lipophilie-Gleichgewicht
(HLB-Wert) von weniger als 15, die in einem geeigneten Lösungsmittel oder einer Mischung
von Lösungsmitteln gelöst ist,
iii) Rühren der Aufschlämmung, um die oberflächenaktiven Moleküle an die Oberfläche
des moderat hydrophoben Materials adsorbieren zu lassen, so dass seine Hydrophobie
erhöht wird und sein Berührungswinkel auf nahezu oder oberhalb von 90° vergrößert
wird, und dann
iv) Unterziehen der konditionierten Aufschlämmung, die das teilchenförmige Material,
dessen Wasserberührungswinkel vergrößert worden ist, enthält, einem geeigneten mechanischen
Verfahren zur Entwässerung.
2. Verfahren nach Anspruch 1, wobei das feine teilchenförmige Material, das zu entwässern
ist, ein Material ist, dessen Oberfläche aufgrund von Alterung oder Oberflächlichenoxidation
weniger hydrophob geworden ist.
3. Verfahren nach Anspruch 1, wobei das feine teilchenförmige Material Mineralien, Kohle,
Kunststoffe, Metalle, Metallpulver, Flugasche und biologische Stoffe umfasst.
4. Verfahren nach einem der vorstehenden Ansprüche, wobei dieses geeignete mechanische
Verfahren zur Entwässerung Vakuumfiltration, Druckfiltration, Zentrifugalfiltration
und Zentrifugation umfasst.
5. Verfahren nach einem der vorstehenden Ansprüche, wobei die nichtionische oberflächenaktive
Substanz ausgewählt ist aus Fettsäuren, Fettsäureestern, Phosphatestern, hydrohoben
Polymeren, Ethern, Glykolderivaten, Sarcosinderivaten, oberflächenaktiven Substanzen
und Polymeren auf Siliciumbasis, Sorbitanderivaten, Sucrose und Glukoseestern und
-derivaten, Derivaten auf Lanolinbasis, Glycerinestern, ethoxylierten Fettsäureestern,
ethoxylierten Aminen und Amiden, ethoxylierten linearen Alkoholen, ethoxylierten Triclyceriden,
ethoxylierten Pflanzenölen und ethoxylierten Fettsäuren.
6. Verfahren nach Anspruch 5, wobei die nichtionische oberflächenaktive Substanz mit
einem Pflanzen-, Fisch- oder Tieröl vermischt wird, das Triacylglycerine enthält,
um synergistische Verbesserung bei der Entwässerung des feinen teilchenförmigen Materials
zu erzielen.
7. Verfahren nach einem der vorstehenden Ansprüche, wobei dieses geeignete Lösungsmittel
leichte Kohlenwasserstofföle und kurzkettige Alkohole umfasst.
8. Verfahren nach Anspruch 1, wobei diese geeigneten oberflächenaktiven Substanzen oberflächenaktive
Substanzen mit hohem HLB-Wert sind, deren polare Köpfe mit der Oberfläche der teilchenförmigen
Materialen wechselwirken können.
9. Verfahren nach Anspruch 1, wobei diese Sammler Thiole für Sulfidminerale und Metalle
sind.
10. Verfahren nach Anspruch 1, wobei diese Sammler Kohlenwasserstofföle sind und das teilchenförmige
Material Kohle oder eine andere natürlich hydrophobe Substanz ist.
11. Verfahren nach einem der vorstehenden Ansprüche, wobei ein Elektrolyt oder eine Mischung
von Elektrolyten, ausgewählt aus Salzen von monovalenten, divalenten und trivalenten
Kationen und Anionen, nach dem anfänglichen Hydrophobierungsschritt (i) und vor Schritt
(ii) zugegeben wird.
12. Verfahren nach Anspruch 11, wobei diese Elektrolyte die Salze von Aluminiumionen sind.
13. Verfahren nach Anspruch 11 oder 12, wobei die Reagenzien, die in Schritt (i) und Schritt
(ii) verwendet werden, und dieser Elektrolyt oder diese Mischung von Elektrolyten
in einem einzigen Schritt zugegeben werden können.
14. Verfahren nach einem der Ansprüche 1 bis 10, wobei das geeignete mechanische Verfahren
zur Entwässerung ein Filtrationsprozess ist, in welchem der Filterkuchen geeigneten
vibrierenden Mitteln ausgesetzt wird, so dass ein höherer Grad an Feuchtigkeitsreduzierung
bei einer gegebenen Kuchendicke erreicht wird.
15. Verfahren nach Anspruch 14, wobei die geeigneten vibrierenden Mittel Ultraschall-,
mechanische und akustische Mittel umfassen.
16. Verfahren nach einem der Ansprüche 1 bis 10, wobei das geeignete mechanische Verfahren
zur Entwässerung ein Filtrationsprozess ist, in welchem ein geeignetes Reagenz zur
Verringerung der Oberflächenspannung zu dem Filterkuchen in Form von feinem Nebel
oder Spray gegeben wird, so dass ein höherer Grad an Feuchtigkeitsreduktion bei einer
gegebenen Kuchendicke erreicht wird.
17. Verfahren nach Anspruch 16, wobei das geeignete Mittel zur Verringerung der Oberflächenspannung
aus kurzkettigen Alkoholen, leichten Kohlenwasserstoffölen und oberflächenaktiven
Substanzen ausgewählt ist.
18. Verfahren nach einem der Ansprüche 11 bis 13, wobei das geeignete mechanische Verfahren
zur Entwässerung ein Filtrationsprozess ist, in welchem ein geeignetes Reagenz zur
Verringerung der Oberflächenspannung zu dem Filterkuchen in Form von feinem Nebel
oder Spray gegeben wird und gleichzeitig der Filterkuchen geeigneten vibrierenden
Mitteln ausgesetzt wird, so dass eine wesentliche Feuchtigkeitsreduktion bei hohen
Kuchendicken unter Verwendung minimaler Mengen von Reagenzien erreicht wird.
1. Procédé de déshydratation d'une pâte de matériau particulaire fin inférieure à 2 mm
de diamètre, ledit traitement comprenant les étapes consistant à :
i) rendre le matériau particulaire fin modérément hydrophobe, de sorte que son angle
de contact de l'eau soit augmenté à une valeur considérablement inférieure à 90° en
utilisant des tensioactifs et collecteurs appropriés dans une étape d'hydrophobisation
initiale,
ii) ajouter un tensioactif non-ionique ayant un numéro d'équilibre hydrophile-lipophile
(HLB) inférieur à 15 et dissous dans un solvant approprié ou un mélange de solvants.
iii) agiter la pâte pour permettre aux molécules de tensioactifs d'adsorber sur la
surface du matériau modérément hydrophobe, de sorte que son hydrophobie soit accrue
et l'angle de contact soit augmenté proche de 90° ou plus, et ensuite
iv) soumettre la pâte conditionnée contenant le matériau particulaire, dont l'angle
de contact de l'eau a été accru, à un procédé mécanique adapté de déshydratation.
2. Procédé selon la revendication 1, dans lequel le matériau particulaire fin à déshydrater
est un matériau dont la surface est devenue moins hydrophobe du fait du vieillissement
ou de l'oxydation superficielle.
3. Procédé selon la revendication 1, dans lequel le matériau particulaire fin comprend
des minéraux, du charbon, du plastique, des métaux, des poudres métalliques, des cendres
volantes, et des matériaux biologiques.
4. Procédé selon l'une quelconque des revendications précédentes, dans lequel ledit procédé
mécanique adapté de déshydratation comprend la filtration sous vide, la filtration
sous pression, la filtration centrifuge, et la centrifugation.
5. Procédé selon l'une quelconque des revendications précédentes, dans lequel le tensioactif
non-ionique est choisi parmi les acides gras, les esters gras, les esters de phosphate,
les polymères hydrophobes, les éthers, les dérivés de glycol, les dérivés de sarcosine,
les tensioactifs à base de silicium et des polymères, des dérivés de sorbitane, des
esters et des dérivés de saccharose et de glucose, des dérivés à base de lanoline,
des esters de glycérol, des esters gras éthoxylés, des amines et amides éthoxylés,
des alcools linéaires éthoxylés, des triglycérides éthoxylés, des huiles végétales
éthoxylées, et des acides gras éthoxylés.
6. Procédé selon la revendication 5, dans lequel le tensioactif non-ionique est mélangé
à une huile végétale, de poisson ou animale contenant des triacylglycérols pour obtenir
une amélioration synergique de la déshydratation du matériau particulaire fin.
7. Procédé selon l'une quelconque des revendications précédentes, dans lequel lesdits
solvants appropriés comprennent des huiles hydrocarbures légères et des alcools à
chaîne courte.
8. Procédé selon la revendication 1, dans lequel lesdits tensioactifs appropriés sont
des tensioactifs HLB élevés dont les têtes polaires peuvent interagir avec la surface
des matériaux particulaires.
9. Procédé selon la revendication 1 dans lequel lesdits collecteurs sont des thiols pour
des minéraux de sulfure et des métaux.
10. Procédé selon la revendication 1, dans lequel lesdits collecteurs sont des huiles
d'hydrocarbure et le matériau particulaire est le charbon ou une autre substance hydrophobe
naturelle.
11. Procédé selon l'une quelconque des revendications précédentes, dans lequel un électrolyte
ou un mélange d'électrolytes choisis parmi des sels de cations et anions monovalents,
divalents et trivalents est ajouté après l'étape d'hydrophobisation initiale (i) et
avant l'étape (ii).
12. Procédé selon la revendication 11 dans lequel lesdits électrolytes sont les sels d'ions
aluminium.
13. Procédé selon la revendication 11 ou 12, dans lequel les réactifs utilisés dans l'étape
(i) et l'étape (ii) et ledit électrolyte ou le mélange d'électrolytes peuvent être
ajoutés en une seule étape.
14. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le procédé mécanique
adapté de déshydratation est un procédé de filtration dans lequel le gâteau de filtration
est soumis à un système vibratoire approprié, de façon à atteindre un degré supérieur
de réduction d'humidité à une épaisseur donnée de gâteau.
15. Procédé selon la revendication 14 dans lequel le système vibratoire approprié comprend
des systèmes à ultrasons, mécaniques et acoustiques.
16. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le procédé mécanique
adapté de déshydratation est un procédé de filtration, dans lequel un réactif de réduction
de tension de surface adapté est ajouté au gâteau de filtration sous la forme d'un
brouillard fin ou d'une vaporisation, de sorte qu'un degré supérieur de réduction
d'humidité soit atteint à une épaisseur de gâteau donnée.
17. Procédé selon la revendication 16, dans lequel l'agent de réduction de tension de
surface adapté est choisi parmi des alcools à chaîne courte, des huiles hydrocarbures
légères, et des tensioactifs.
18. Procédé selon l'une quelconque des revendications 11 à 13, dans lequel le procédé
mécanique adapté de déshydratation est un procédé de filtration, dans lequel un réactif
de réduction de tension de surface adapté est ajouté au gâteau de filtration sous
la forme de brouillard fin ou de vaporisation et, en même temps, le gâteau de filtration
est soumis à un système de vibration approprié, de sorte qu'une réduction sensible
d'humidité soit atteinte à des épaisseurs de gâteau élevées en utilisant des quantités
minimes de réactifs.