This invention relates to a novel process for selectively removing
slimed (colloidal) particles in kaolin clay, from larger particles by chemical means.
The processing effects the selective flocculation and sedimentation of the coarser
particles from a dispersed aqueous pulp into a coherent gelled lower layer which
is a concentrate of deslimed particles and an upper dispersed layer which is a dilute
concentrate of the slimed particles. When applied to processing kaolin clay, the
gelled lower layer is readily dispersed to produce a useful deslimed kaolin clay
product having a desirable narrower particle size distribution than the starting
clay as a result of desliming.
In particular, the invention relates to the use of the combination
of controlled amounts of a high molecular weight, high charge density anionic polymeric
flocculating agent, polyvalent cations, an anionic clay dispersant, and pH control,
to effect the removal of slimes as a dilute fluid supernatant layer from the lower
gelled layer containing the deslimed particles.
BACKGROUND OF THE INVENTION
Kaolin clay is a widely used industrial mineral. The clay occurs as
an ore from which grit must be removed for virtually all end uses of the clay. The
resulting degritted crude kaolin is composed largely of kaolin particles that usually
have a wide range of sizes ranging from slimes (finer than 0.3 microns) up to about
10 microns. Frequently discrete mineral impurities such as titania, various ferruginous
minerals, mica and nonkaolinitic clays such as bentonite and attapulgite, are present.
Such nonkaolin minerals can be removed at least partially by means such as froth
flotation, selective flocculation, magnetic separation, bleaching and combination
of such steps. The purified kaolin particles are polydisperse, i.e., the size of
the particles represent a wide range of sizes. For example, a kaolin that is 100%
by weight finer than 2 microns, may contain kaolin particles ranging in size from
submicron through 2 microns.
WO-A-9617688 and WO-A-9811993 disclose methods of purifying kaoling
Most uses of purified kaolin require stringent control of the size
of the kaolin particles. For example, a #2 coating clay must be about 80% by weight
finer than 2 microns. A #1 coating clay must be about 90% by weight finer than 2
microns. A high glossing (#0) grade is nearly 100% by weight finer than 2 micron.
Particle size distribution is also controlled for many commercial uses. Thus, two
clays may be #1 coating clays as defined by the minus 2 micron parameter. However,
a #1 grade with a narrow particle size distribution will generally provide greater
opacity and gloss than a #1 grade with a broad particle size distribution. Narrowing
of particle size distribution can be achieved by fractionation to reduce oversize
and/or desliming to reduce undersize.
Conventional centrifuges, such as Bird solid bowl machines, are widely
used to fractionate kaolins. Typically, such centrifuges operate at "g" forces of
about 800-1200. Centrifuges that apply greater "g" forces than the Bird machines
are needed to produce deslimed kaolins. An examples of a high speed centrifuge is
a horizontal three-phase centrifuge, such as one commercially available from Alfa
Laval Co.(Greenwood, Indiana). Such centrifuges can be operated in the range of
about 1,000 to 10,000 "g" forces, preferably in the range of about 1500-3000. A
"g" force of about 5000 is typical. These high speed centrifuges can effect a sharp
separation of kaolin particles finer than about 0.3 microns from larger kaolin particles.
However, the capital and operating costs are high. Also, these centrifuges are subject
to excess wear, especially wear of the nozzles, necessitating frequent repair or
Most uses of kaolin mineral particles require that the kaolin particles
be dispersed or dispersible in water. This is true whether the kaolin particles
are intended for use as an intermediate in one or more beneficiation schemes or
whether the kaolin particles are intended for a commercial end-use, such as paper
coating. Nondispersible kaolin products or by-products have limited commercial value.
It has long been the practice of the industry to fractionate kaolin
into kaolin particles of one size range from kaolin particles having a different
size prior to and/or subsequent to certain beneficiation treatments carried out
with dispersed aqueous pulps. For example, the flotation beneficiation process known
as ULTRAFLOTATION requires the use of a fraction of kaolin that is finer than typical
degritted kaolin crudes of the type frequently referred to as "coarse" or "soft"
crudes. See U.S. Patent No. 2,990,958. Conventional (Bird) centrifuges can operate
effectively to produce typical #1 and #2 fractions as a step prior to carrying out
this beneficiation process. On the other hand, some processes practiced in the kaolin
industry require or are improved by removing slimes before or after carrying out
the unit operation. An example is the use of mechanical means to remove slimes before
or after delamination. Reference is made to copending application, USSN 08/384,973,
filed February 7, 1995, (and refiled as USSN 08/677,758), entitled "DELAMINATED
KAOLIN PIGMENT, THEIR PREPARATION AND USE", Behl, et al. When true desliming, i.e.,
removal of essentially all slimed particles is required, the use of centrifuges
operated at very high "g" values was necessary in the past.
In contrast to mechanical systems employed in minerals processing
industries to effect separations based on differences in mineral species or size,
chemical means are also used in processing industrial minerals. Froth flotation,
referred to above, is an example of a chemical system. In selective flocculation,
charged inorganic or organic molecules are used to selectively flocculate minerals
from each other based on difference in mineral species. This is fundamentally different
from separating minerals of one species from minerals of the same species, e.g.,
the separation of fine kaolin particles from coarser kaolin particles.
A recent example of the use of selective flocculation to separate
minerals based on differences in species is U.S. Patent No. 5,535,890, Behl, et
al. This patent relates to a flocculation process especially useful in separating
colored mineral impurities, especially titania, from kaolin particles. The processing
includes the use of a dispersant to provide a fluid pulp, conventional kaolin froth
flotation reagents such as oleic acid, a source of calcium ion, and a high molecular
weight anionic polymeric flocculating agent. The impurities settle as a lower flocculated
layer from an alkaline pulp and the purified kaolin is recovered as a dispersed
fluid suspension which is separated from the sedimented impurities by decantation.
Organic polymers have also been proposed to achieve selective flocculation
of kaolins into dispersed and flocculated fractions differing in viscosity. An early
example is U.S. Patent No. 2,569,680, Leek, which utilizes the difference in surface
chemistry between some kaolin particles and other kaolin particles giving rise to
high viscosity. A more recent example is U.S. Patent No. 4,334,985 Turner, et al.
Turner, et al., seek to remove small amounts of coarse aggregates of kaolin from
kaolin particles to improve the viscosity of the remaining kaolin. The processing
uses an anionic organic polymer to bring about separations comparable to those achievable
with conventional (e.g., Bird) centrifuge. Thus, coarse aggregate are removed as
a sediment from a kaolin material and the remaining kaolin, the desired product
is recovered as a dispersed slip. Turner, et al., do not fractionate a kaolin into
a slimed and nonslimed fractions such as is accomplished using an Alfa Laval separator
as the dispersed phase material. Rheology data in the patent indicate that the flocculated
aggregates are not readily dispersible.
All particle sizes referred to herein are determined by a conventional
sedimentation technique using a SEDIGRAPH® 5100 analyzer analysis. The sizes,
in microns, are reported as "e.s.d." (equivalent spherical diameter).
IN THE DRAWING
The accompanying figure contains particle size distribution curves
for a feed kaolin and the fine and coarse fractions obtained by practice of this
invention. The distribution is expressed, in conventional manner, as a weight percentage.
The present invention comprises a process for selectively removing
slimed kaolin particles (i.e. particles finer than 0.3 micron) from larger kaolin
(for example, particles in the range of 0.3 to 5 microns) as defined in claim 4
and in a manner such that the resulting deslimed particles are easily dispersible
in water using conventional dispersion chemicals and equipment. The processing includes
the use of controlled amounts of at least one anionic dispersant, polyvalent cations,
a high molecular weight, anionic polymeric flocculating agent and a pH of at least
7 and below 10.
The processing results in the separate recovery of two different size
fractions of kaolin, one of which (the slimes) are recovered as a dilute suspended
dispersed phase and the other, normally the product of principal value, is recovered
as a coherent gelatinous bottom layer. Upon decanting the supernatant upper layer
from the coherent gelatinous bottom layer, a deslimed kaolin product is obtained
without using a high speed centrifuge to effect the separation. A unique and valuable
aspect of the processing is that the gelatinous layer, although composed of a tenacious
network of flocs, is readily dispersed, whereby the deslimed kaolin is useful for
commercial applications of high grade kaolins, especially kaolins having a narrow
particle size distribution as a result of the removal of slimes. The slimed fraction
can be blended with hydrous grades of kaolin or it can be used as calciner feed
to make metakaolin or fully calcined kaolin pigments.
In practice of the invention, a high recovery of deslimed but dispersible
kaolin can be obtained at an extremely low operating cost. This result is obviously
not achievable by the Turner et al.
Another aspect of the invention relates to novel deslimed kaolin pigment
products obtained by practice of the invention as defined in claim 1.
Kaolin pigments of the invention are especially useful for providing
the combination of opacification and gloss when coated on paper. The pigments have
an average particle size in the range of 0.5 to 5 micron, usually 0.5 o 1.5 micron
and a narrow particle size distribution such that the ratio of the weight of the
fraction of particles finer than 2 microns divided by the weight of the fraction
of particles finer than 0.3 microns is greater than 4.0, preferably greater than
4.5. The pigments contain residual anionic polymer from the fractionation process
in amount in the range of 50 to 500 ppm and, when calcium is the polyvalent cation,
contain from 0.025% to 0.1% by weight calcium, expressed as CaO, based on the dry
weight of the pigment.
DESCRIPTION OF PREFERRED EMBODIMENT
Clay dispersants useful in practice of the invention are sodium polyacrylate,
sodium condensed phosphate, sodium silicate, alum silicate hydrosols, soda ash and
combinations thereof, as well as combinations thereof with sodium hydroxide.
In practicing the invention using flotation beneficiated kaolins which
already have a suitably high calcium ion concentration as a result of addition of
a calcite carrier, increments of dispersant are added using conventional "laddering"
techniques known in the industry to determine an optimum dispersant concentration.
When applying the process to nonfloated clays, it is preferable to add dispersant
and thereafter add a source of calcium at a fixed level, for example, 2#
CaCl2 per dry ton clay. The dispersant level is optimized and then the
calcium level is optimized.
In one preferred embodiment of the invention, using feed clay that
is about 80% by weight finer than 2 microns, at least 60% by weight of the slimed
kaolin charged to the process is recovered as a deslimed product in the lower gelatinous
layer. Preferably, recovery of deslimed product is at least 80%, most preferably
90% or higher.
In another embodiment, the feed is about 90% or more finer than 2
microns and the recovery of deslimed product is lower than when using coarser feed
clay; thus recovery of slimed material is higher. For example, 60% by weight of
feed may be converted to deslimed product and 40% by weight slimed co-product useful
as calciner feed.
To produce strong flocs that are readily dispersible reagent control
is required. As mentioned, polyvalent (e.g., Ca**) concentration in the aqueous
phase should be at least 5 ppm. At least 0.025% by weight of polymer, based on the
dry weight of the clay should be added. The pH should be less than 10 and greater
than 7. The polymer should contain both amide and acid groups, the acid groups predominating.
To maximize the recovery of deslimed kaolin without significantly
changing the coarse end of the particle size distribution curve, polymer should
be added in 0.005% increments until the selectivity of the particle separation is
observed by the yield of particles in the gelatinous phase reaching a maximum. Additionally,
when the surface charges and number of counterions are optimized the molecular weight
of the polymer can be varied to shift the degree of slime particle mass removed
to meet manufacturing requirements.
In practice of the invention, kaolin clay composed of a range of particle
sizes is formed into a well dispersed, fluid aqueous pulp by adding at least one
anionic dispersant, such as a sodium silicate, condensed phosphate salt such as
sodium hexametaphosphate, soda ash, a water soluble acrylate salt polymer having
a molecular weight in the range of 1000 to 5,000, or alum silicate hydrosol. Sodium
hydroxide may be added for Ph control. The dispersant is used in amount sufficient
to form a fluid pulp.
Preferably, the zeta potential of the dispersed pulp is such that
it is essentially constant (does not vary by more than about 5 millivolts) when
increments of dispersant are added. It may be necessary to add additional dispersant
when using as feed clay, a stream that is optimally dispersed for a particular unit
operation, such as magnetic separation. Typically, zeta potential is in the range
of -25 to -50 millivolts, usually -30 to -45 millivolts, and most usually -35 to
Generally the Brookfield viscosity (as measured using a No. 2 spindle
at 20 rpm) is less than 600 centipoises, typically is in the range of 20 to 50 cp
for a 40% solid pulp. The quantity of dispersant needed to form a fluid pulp will
vary with the type of kaolin, the solids and the species of dispersant. An excess
of dispersant could flocculate the pulp; this is avoided.
To the well dispersed aqueous pulp there is added a source of multivalent
cations, preferably divalent cations, most preferably calcium ions, unless the pulp
already contains a sufficiently high concentration of polyvalent cations; the concentration
of calcium ions in the solution phase (before the addition of polymer) must be at
least 5 ppm and should be up to about 30 ppm as defined in claim 1, and is preferably
6 to 25 ppm. Most of the calcium reports with the flocculated kaolin. In the case
of calcium chloride, the preferred source of polyvalent cations, the quantity of
such salt is generally in the range of 1 to 4 pounds per ton of dry clay, preferably
2 to 2.5 pounds per ton of dry clay. Zero (0) pounds per ton is suggested for use
with clays previously beneficiated with a calcite carrier.
A high molecular weight, high charge density anionic acrylic acid
polymer or acrylic acid/acrylamide polymer is added to the pulp with mixing. The
resulting pulp is typically at a clay solids content of about 20% to 25% (weight).
The pulp, after dilution with polymer solution, is allowed to settle until it separates
into a lower flocculated gelatinous layer and an upper dilute suspended layer. Typically,
the gelatinous layer contains a coarser fraction of kaolin particles having an average
size above 0.5 microns and is at about 35% solids. The dispersed suspended layer
of finer kaolin particles typically has an average size below 0.5 micron, preferably
below 0.3 microns, for example 80% by weight finer than 0.5 micron and is typically
at 3-10% solids. The flocculated layer and the dispersed layers are separately recovered.
Removal of the supernatant dispersed layer from the gelatinous layer is by decantation.
Filtration is not a suitable means for effecting the separation.
The flocculating polymer used in the process is highly anionic and
is a homopolymer or co-polymer of carboxylic acid, carboxylic anhydride and carboxylic
acid salt monomer with a suitable non-ionic monomer. Examples of a non-ionic monomers
are carboxylic acid amide and carboxyl alkyl esters. A co-polymer of acrylic acid
(or salt thereof) and acrylamide is preferred for kaolin processing. Since the polymer
is highly anionic, it consists predominately of the acid acrylic group. Recommended
is a polymer in the ratio of acid acrylic group to amide group is about 80/20.
Typically, the molecular weight of the highly anionic polymer is at
least 1 million, preferably at least 5 million, and most preferably in the range
of 10 million or higher. The quantity of the high molecular weight polymer is typically
in the range of 0.1 to 1.0 #/ton of kaolin (based on the dry weight of the kaolin).
The term "molecular weight" as used herein, refers to the weight average molecular
weight as measured using the mark HOUWINK™ equation which uses intrinsic viscosity
versus molecular weight relationships in a capillary instrument.
Polymers used in the accompanying examples were obtained from Sharpe
Specialty Chemical Co. and included Sharpfloc™ 9990, 9993, 9950, 9954 and
8581. The method of production of these polymers is proprietary. In theory they
can be prepared by either co-polymerization of acrylamide and acrylic acid (anionic
monomer) or by partial hydrolysis of polyacrylamide.
The anionic polymeric flocculating agent is different from a low molecular
weight anionic polymer which may be used as a dispersant or as a component of the
dispersant. Dispersant grades are suitably sodium or ammonium polyacrylate salts
having a molecular weight below 20,000, preferably in the range of 1000 to 8000,
and most preferably in the range of 3000 to 5000.
The quantity of dispersant, which, as mentioned can be any conventional
anionic clay dispersant, may be used to control the amount of minus 0.3 micron particles
that are removed from the coarser kaolin particles. Preferably, maximum selective
removal of slimes is desired. The high molecular weight polymer, on the other hand,
may be used to control the recovery of the coarser fraction. Thus, a desired degree
of desliming can be achieved by adjusting the relative proportions of dispersant
and flocculating agent and recovery of deslimed kaolin can be maximized by controlling
the proportions of these materials.
In one preferred embodiment of this invention, the chemically deslimed
clay (flocculated phase) is dispersed and subsequently used as feed to a delaminator
to produce a glossing, coating clay, as described in copending USSN 08/677,758.
The starting clay is suitably approximately 70% by weight (e.g., 65-75%) finer than
2 microns and about 25-30% by weight finer than 0.3 microns. Chemical desliming,
in accordance with this invention results in a flocculated (coarse fraction) that
has about the same weight percentage of particles finer than 2 microns as the starting
feed (i.e., 70% finer than 2 microns for a crude that is 70% finer than 2 microns)
but contains only from 0 to less than 20% by weight of particles finer than 0.3
microns. Thus, desliming can be selective in that it does not change the coarse
end of the particle size distribution curve more than 5%. This means that there
are essentially no losses of kaolin particles in the desirable particle size range.
The slimes remain dispersed and may be discarded or used as a blend component in
standard processing. The flocculated kaolin is then dispersed using conventional
clay dispersants and low shear equipment such as blungers.
In a second preferred embodiment, an ultrafine clay such as a Tertiary
clay, also referred to as "hard" kaolin, is used as feed to the chemical desliming
treatment of the invention. Optionally, the product is calcined or converted to
a cationically bulked hydrous kaolin coating pigment. The kaolin crude is typically
about 90% by weight (e.g., 85-95% by weight) finer than 2 microns and contains about
30% by weight or more of particles finer than 0.3 microns. Average particle size
is generally in the range of 0.4 to 0.5µ. Significant amounts of the particles smaller
than 0.3 microns are separated as a suspended layer during the chemical desliming
treatment. The flocculated deslimed material is typically 90% finer than 2 microns
and usually is free from particles finer than 0.3 microns. In some cases the chemically
deslimed kaolin can contain up to about 20% by weight of particles finer than 0.3
The chemical desliming treatment of the invention can be applied to
various other processing streams in a kaolin plant. For example, the treatment can
be applied to a dispersed beneficiated kaolin product obtained from a flotation
plant or it can be applied to dispersed kaolin purified by selective flocculation.
Frequently, sodium silicate or sodium silicate/alum hydrosols are
used as a primary dispersant in producing product streams subsequently processed
by the method of this invention. The pH of such kaolin streams are frequently in
the range of 5 to 7. After addition of sodium metasilicate or other dispersant such
as sodium hexametaphosphate, or sodium or ammonium polyacrylate the pH of the slurry
should be at least 7 but below 10. After addition of primary dispersant and secondary
dispersant, the dispersed kaolin pulp is a thin fluid having the appearance of a
milkshake. When maintained quiescent, essentially no stratification or appearance
of flocs takes place. The slurry is dispersed in the sense that particles are not
aggregated. The degree of dispersion may not be the same as that of a slurry dispersed
to minimum viscosity. We prefer to disperse the pulp using a quantity of dispersant
such that when zeta potential is measured in conventional manner and a zeta potential
curve is drawn, the curve is substantially flat.
The salt containing a polyvalent metal cation is added to the pulp
simultaneously with or immediately after the addition of the secondary dispersant.
When treating a kaolin material which provides a sufficiently high concentration
polyvalent cations in pulp, it may not be necessary to add any other source of polyvalent
cations. Suitable salts containing polyvalent metal cations are soluble in water
at the pH of the pulp to which the salt is added. Especially preferred are salts
containing divalent metal cations, particularly calcium and magnesium. Other polyvalent
metal cations that may be used include aluminum, ferric, tin, titanium, manganese
and rare earth. The preferred anion of the salt is chloride, although nitrate, acetate
or formate salts may be used. The salt is added dry or as an aqueous solution; salt
is added in the amount generally in the range of about 0 to 4 pounds/ton, most preferably
about 2 pounds per ton of dry clay. Sufficient Ca++ concentration results
in an aqueous phase (after settling of the gelled layer) containing at least about
5 ppm Ca++. When excess salt is used, undesired nonselective flocculation
of the pulp may occur and this may interfere with the ability of the polymer to
flocculate the kaolin selectively, based on particle size. When no salt is added,
the flocs formed are very small and this would adversely affect the separation process.
The dispersed pulp is typically at 10 to 50 percent solids prior to
addition of high molecular weight polymer. Minimal dilution occurs when these reagents
are added, whereby the solids of the pulp may remain essentially unchanged. The
pH of the slurry typically ranges from 7 to 8.5 after addition of all reagents.
The high molecular weight polymers are subject to degradation when
aged. In a 0.3% concentration they are stable for several days. The shelf like of
0.025% is about one day, sometimes one-half day, due to temperature. Additionally,
to achieve optimum "uncoiling" of the polymer concentrations in the range of 0.025%
provide an optimum configuration for attachment to the clay surface. At very low
concentrations, the volume of water added becomes too large, thereby causing handling
problems and undesirable hydrolysis of the polymer. In making up the polymer solution,
water with a low content of calcium and magnesium is recommended. Agitation should
be sufficiently moderate to avoid degradation of the polymer while it is being solubilized
in water. At higher concentrations, the flocculated material may aggregate due to
Virtually immediately after the solution of the high molecular weight
polymer is added to the well dispersed pulp preconditioned with metal salt, the
formation of a lower gelatinous layer can be observed. It is preferable not to agitate
the contents of the vessel in order for layering to take place. However, agitation,
even severe, will not impair floc formation. Within a few minutes of standing under
quiescent or semiquiescent conditions, the floc settles as a well-defined viscous
gelatinous bottom layer, which tends generally has the same color as the suspended
layer. However, the suspended layer is semitransparent; the lower layer is homogeneous
and opaque. In case of East Georgia kaolin, the iron content of the clay remains
essentially unchanged. However, in the case of kaolins containing liberated iron
and titanium minerals, the iron and titanium may concentrate in the gelatinous layer.
Thus, the use of kaolin pretreated by means such as froth flotation, or selective
flocculation to remove colored impurities is desirable when producing kaolin pigments
requiring high brightness. Unless the clay has been degritted before treatment,
grit will report in the flocced layer when processing kaolin crude. Thus, the clays
undergoing treatment should be degritted. Most of the water in the pulp appears
in the supernatant slime-rich upper layer.
After polymer addition, a fluid dispersion of the slimed matter can
be decanted in a cone bottom classifer, a cylindrical tank, column, etc., with the
underflow containing the gelatinous mass containing the desired coarser particles.
Mechanical devices such as a drag box or a low shear centrifugal device, may also
be used to separate the flocs from the dispersed product. Use of such devices are
included in the term "decantation" as employed herein.
Processing downstream of decantation can provide numerous opportunities
to optimize the overall process yield and decrease the amount of residual impurities
which remain in the dispersed phase.
The flocs are agitated with the addition of standard kaolin dispersants,
typically using from 1 to 6 #/ton of dispersant (dry basis). This tends to break
the floc structure and the slurry becomes workable. In plant practice, this step
may be achieved by using standard centrifugal pumps.
The process feed clay was previously beneficiated in a commercial
flotation plant. The feed clay to the flotation plant was obtained by blunging a
Georgia kaolin crude with a hydrosol dispersant, degritting and fractionation by
a known manner in a Bird Centrifuge to obtain a #2 fraction (70-80% <2um) as
the overflow. The #2 fraction was beneficiated by ULTRAFLOTATION using a calcite
carrier substantially as described in U.S. 2,990,958 (Greene, et al.)
The beneficiated #2 fraction from the flotation process had a pH value
of 8.6 and a solids content of 21.8% by weight. The product slurry was then subjected
to an ozone treatment to decolorize residual flotation reagents. This ozonated slurry
with a pH value of 8.1, represents the control sample for the following tests carried
out at this pH. A portion of the ozonated slurry was treated with caustic to increase
the pH value to 10.3 to act as the control sample for the higher pH tests.
The following dispersants were added at the disclosed levels:
- 1. Sodium polyacrylate, supplied by Rhone Poulenc Corp. with a designation of
C-211, was added at a level of 0.25#/ton clay on a dry/dry basis.
- 2. Sodium hexametaphosphate (SHMP) was added at 0.75#/ton on a dry/dry basis.
- 3. Sodium Metasilicate (MS) was added at 0.25#/ton clay on a dry/dry basis.
To a portion of the high pH control slurry, the following was added:
- 1. Sodium hexametaphosphate at 0.25#/ton
- 2. Sodium polyacrylate at 0.1#/ton, both added on a dry/dry basis.
The following measurements were performed on each sample before the
addition of the flocculating reagent:
- 1. Slurry pH
- 2. Zeta potential using a PEN Kem Lazer Zee Meter Model 501
- 3. Concentration of calcium ion in the aqueous layer using the conventional
While each of the samples, including the controls, was being stirred,
there was added 0.3#/ton of clay (dry/dry basis) of the flocculating polymer, Sharpfloc
9950. This polymer has a molecular weight of 10 million and a ratio (mole) of polyacrylic
acid/polyacrylamide of 80/20.
The concentration of the polymer solution was 0.025%. The stirring
of samples was stopped and allowed to settle at a at the rate of 30 seconds/inch-depth
of slurry. The nonflocculated supernatant, or slimes, was decanted from the flocculated
mass. Soluble calcium concentration of the slimes was 2.6 ppm; soluble calcium concentration
of the flocculated mass was 18.3 ppm.
The relative strength, or "redispersion characteristics" of
the flocculated layer was measured by a stress sweep on a Bohlin Rheometer, model
CS 50, in oscillatory mode. The critical stresses are reported at angular frequencies
of 0.1Hz. and 1.0Hz. These measurements were made to identify any potential differences
in the flocculated mass structure and energy requirements to redisperse. The oscillatory
rheological measurements provide an important insight into the structure of the
flocculated masses. The critical stress values relate to the strength of the floc
structure which, in turn, relates to the ease of redispersion. It was found that
the flocs obtained at the lower pH value will require about 10% of the energy of
that required by the high pH floes for redispersion.
The flocculated product was treated with a dispersant mixture designated
"SAP" in order to achieve a fluid, dispersed slurry. SAP is composed of 30% sodium
hexametaphosphate, 45% soda ash and 25% C211 sodium acrylate, on dry weight basis.
The redispersed slurries were screened through a 325-mesh sieve and spray dried
using a laboratory spray dryer.
The following measurements were obtained on the spray dried products:
- a. Particle size distribution (ESD) by Micromeritics Sedigraph 5100
- b. pH of a 10% solids slurry
- c. Brookfield viscosity of a 68% solids slurry (#1 spindle @ 20 RPM)
The data are presented in Table I.
Separation of Slimed Kaolin Using Organic Polymer
Control Low pH
Initial pH of the slurry
Zeta potential, mv
Soluble Calcium Conc. (ppm)
Treated Slurry ESD Slimes
% @ 2um
% @ 0.18um
ESD Flocs % @ 2um
% @ 0.18um
% floc recovered
B'field vise. (#2/20)
The data show that in all low pH tests the viscosity of the final
products were significantly lower than products prepared using a similar process
but with a pH above 10 at the onset. The data also show that the selectivity of
the process to separate fine from coarse particles comparing similar chemical reagents
at the two pH regions shows poor results when the pH is above 10.
Example 2 (Not the invention)
Tests similar to those performed above were completed with a non floated
#1 kaolin slurry having nominally 88 -92% <2um PSD. This feed slurry had a solids
content of 22.1% and a pH value of 8.3. The dispersant used to prepare the slurry
The feed slurry was the control to which no reagents or dispersants
were added. The following reagent and dispersant were added to portions of the feed
slurry and stirring continued for 3 minutes:
- 1. Calcium Chloride at 2#/ton of clay and sodium polyacrylate, C-211, at 0.25#/ton
- 2. Calcium Chloride at 2#/ton of clay and sodium hexametaphosphate at 0.50#/ton
At the end of 3 minutes each sample, including the control, was treated
with 0.3 dry #/ton of clay (in the form of a 0.025% solution) of the flocculating
polymer, Sharpfloc 9950. The stirring was stopped and the slurries settled at a
rate of 60 seconds/inch-depth. The nonflocculated supernatant was decanted from
the flocced mass. The flocculated mass was then treated with SAP to redisperse the
flocs, screened through a 325 mesh sieve and spray dried in a laboratory spray dryer.
The measurements made on these products were identical to those in previous examples.
The data obtained is shown in Table II.
Separation of Slimed Kaolin Using Organic Polymer
Initial Slurry pH
Soluble Calcium Level (ppm)
% @ 2um
% @ 0.30um
B'field visc. (#2/20)
The results shown in Table II again demonstrate a process with high
selectivity and efficient control characteristics for the segregating of kaolin
particles in an aqueous slurry. The data from the test with no additives shows a
reduction in particle size finer than 2 micron from the nominal 90% finer than 2
microns (feed) to 78% finer than 2 microns for the underflow product versus the
results with additives of 86%. This represents a 4% versus 12% difference which
is economically unfavorable in an industrial application. Additionally, the yield
to the underflow product was less than 60% in the test without additives versus
greater than 85% in the process of the invention.
This example illustrates the effect of dispersant in controlling the
removal of colloidal fines from a kaolin crude and its effect on the rheology of
the product. This example illustrates the application of the invention to a coarse
feed fraction of kaolin which has a classical booklet morphology.
A coarse white based crude slurry that was beneficiated by the ULTRAFLOTATION
process was treated with increasing amounts of C-211 sodium polyacrylate. It may
be noted that the ULTRAFLOTATION product slurry has a significant concentration
of calcium ions as a result of the use of a calcite carrier used in the flotation
process. In accordance with the invention, 0.3 lb/ton of Sharpfloc™ 9950 polymer
at a concentration of 0.025% was added to this slurry (at 20% solids) under mild
agitation. Flocs began to appear immediately. As soon as agitation was stopped the
flocs began to settle very rapidly. The flocs were settled at a rate of 7.62 cm/minute
(3 inches/minute). The floc phase (gelatinous phase) constituted about 30% of the
volume of the slurry.
Each dispersed slurry was decanted to separate it from the flocced
layer. The flocced phase was separated and re-dispersed with 5 lb/ton/ of sodium
metasilicate using a drill press laboratory blunger. This slurry was then flocced,
using alum and sulfuric acid in conventional manner and filtered in a Buchner funnel.
The filter cake was washed with equal volume of water and a small amount of the
cake was dried in a microwave oven. The dried sample was pulverized and the PSD
was analyzed using the Sedigraph 5100 analyzer. The filter cake was re-dispersed
using a mixture of soda ash, C211 and sodium hexametaphosphate (SAP) and spray dried.
Rheological testings were subsequently made on the pigment.
Table 3 gives the PSD of the dispersed phase and the feed. It can
be seen that the greater the amount of C-211 dispersant in the system, the lower
the recovery of the coarser phase (l.a), more of the fines were removed. At a dosage
of 1.0 lb/ton of C-211 no flocculation was observed. It may be noted that by using
0.1 lb/ton of the C-211 in the system the colloid content (% smaller than 0.3 microns)
of the product changed from 21 % to 18%. Also noted was that there was no significant
difference in the brightness or the iron and Ti02 contents of the feed and the product.
As expected the Hercules (high shear) viscosity became poorer as increasing
amount of fines were removed, while the Brookfield viscosity was less than 500 cps
at 70% solids even when significant amount of fines were removed as in the example
where 0.5 lb/ton of C-211 was used. Relatively low Brookfield viscosity (less than
500 cps at 70% solids using Number 2 Spindle @ 20rpm) is an indication that the
pigment slurry is useful for coating applications.
Effect of C-211 on Removal of Fines and Pigment Properties
This example is a comparison of the efficiency of a high speed Alfa
Laval disc-nozzle centrifuge operating at nominally 5000"g" as compared to the process
of the invention in the production of a deslimed kaolin pigment.
A coarse white clay fractionated to nominally a #1 clay prior to desliming
using an Alfa Laval was sampled and was used to compare the commercial mechanical
process and the invention. The product from the commercial Alfa Laval was sampled
simultaneously with the feed.
The Alfa Laval feed sample was treated with 0.2 #/ton C211 and 2 #/ton
calcium chloride. The procedure used in Example 3 was followed.
Table 4 illustrates that the fraction made by the commercial Alfa
Laval can be simulated by the chemical process of this invention. The two deslimed
products were subsequently processed (flocculated with 8 #/ton alum and sulfuric
acid to 2.5 pH, 10 #/ton sodium dithionite bleach, filtered, washed and redispersed
with 5.5 #/ton soda ash/SHMP). Testing showed that there was no significant differences
in the particle size and quality of the resulting products.
COMPARISON BETWEEN CHEMICAL AND MECHANICAL DESLIMING
0.3 lb/ton of SF 9950 (Chemical)
Alfa Laval (Mechanical)
Wt.% finer than
PSD - 2u
Properties of Pigment Products
Alfa Laval (Mechanical)
Surface Area (m2/g)
Black Glass Scatter, 577 nm @ 60% solids
Rheology @ 70% solids. Brookfield No. 2 spindle at 20 rpm);
Hercules Viscosity A bob, rpm/dyne
Note that the similarity in surface areas between the products reflect
the similarity in particle size distribution.
The accompanying figure contains particle size distribution curves
for a representative feed clay as well as deslimed product and the slimed fraction.
In conventional manner the distributions are all expressed on a weight basis.
To more fully appreciate the potential extent of slime removal when
evaluated on the basis of the number of particles involved, calculations were performed
to estimate the number of particles involved.
The feed particle size (78.2%-2 micron and 24.7%-0.3 microns) has
3.2 x 1012 particles per gram of kaolin as calculated using a derivative
of Stokes Law.
- N=liquid viscosity in poise
- g=gravity acceleration constant
- h=sedimentation height
- t=time in seconds
- D=diameter in cm of the particle
Using the above equation, the mass fraction in a interval can be calculated
based on the number of equivalent spheres. During a particle size test 250 intervals
are measured and the cumulative mass fraction determined.
This calculation does not consider the fraction finer than 0.3 microns
due to the difficulty of accurately measuring particle size by sedimentation procedures
in the colloidal size range. Note the feed referenced in the graph has almost 25%
of the particles in this very fine particle size.
Using this technique, it was calculated that the deslimed product
(74.5%-2 and 18.4%-0.3 microns) contains 2.8 x 1012 particles per gram
and that the slime product (98.4%-2 and 61.2%-0.3 microns) contains 4.0 x 1012
particles per gram.