Abstrict A method for producing an attapulgite clay (Fuller's Earth) that
significantly removes other types of clays and minerals, resulting
in improved performance. A unique dispersant is used that will fully
disperse the individual attapulgite particles in water such that
the particles remain in suspension, and said dispersant will not
fully disperse other clay and mineral species, which then can be
easily separated through various techniques. The attapulgite clay
may be recovered from the dispersion and dried in such a manner
to produce a finely sized powder from ore, without the need for
any grinding or milling operation. The dried attapulgite clay with
adsorbed dispersant is redispersed in water and retain thixotropic
properties.
Claims What is claimed is:
1. A method of processing naturally occurring clay ore to separate
attapulgite clay from other materials comprising the steps of:
crushing the clay ore,
adding sodium polyacrylate to water, the sodium polyacrylate having
a molecular weight between 4000 and 5000
preparing a slurry of the clay ore with the aqueous sodium polyacrylate
to disperse the attapulgite clay in the water,
separating the dispersed attapulgite clay from un-dispersed materials,
and
drying the dispersed attapulgite clay.
2. The method of claim 1 wherein the sodium polyacrylate content
is between 1% and 4% on a dry clay ore basis.
3. The method of claim 1 wherein the separated attapulgite clay
content is greater than 30% by weight.
4. The method of claim 1 wherein the attapulgite clay dispersed
in water is separated from un-dispersed materials by screening.
5. The method of claim 1 wherein the attapulgite clay dispersed
in water is separated from un-dispersed materials by centrifuging.
6. The method of claim 1 wherein the attapulgite clay dispersed
in water is separated from un-dispersed materials by gravity settling.
7. The method of claim 1 wherein the slurry of attapulgite clay
is dried to a moisture content between 6% and 10%.
8. The method of claim 1 wherein the attapulgite clay slurry is
dried in an oven.
9. The method of claim 1 wherein the attapulgite clay slurry is
dried by spray drying.
10. The method of claim 1 wherein the attapulgite clay slurry
is dried by evaporative filtration.
11. The method of claim 1 wherein the dried attapulgite clay contains
sodium polyacrylate sorbed thereon.
12. The method of claim 11 wherein the dried attapulgite clay
is added to water with shear agitation and the attapulgite clay
is redispersed in the water without the addition of a dispersant
or a surfactant.
13. The method of claim 1 wherein the dispersed attapulgite clay
is dried at atmospheric pressure at a temperature between 85.degree.
C. and 250.degree. C.
14. The method of claim 13 wherein the free moisture content is
less than 7%.
15. The method of claim 1 wherein the dried attapulgite clay is
in the form of free flowing particles which are clusters of individual
attapulgite crystals, avoiding the need for a grinding or milling
operation.
16. The method of claim 1 wherein the non-dispersed material is
quartz, dolomite, limestone, feldspar, diatomaceous, earth, mica,
titania, aluminum montmorillonite, sepiolite, bentonite and kaolin.
17. The method of claim 1 wherein the clay ore is gradually added
with shear to the slurry of clay ore in the aqueous sodium polyacrylate.
18. A method of preparing an aqueous dispersion of purified attapulgite
clay comprising the steps of:
crushing naturally occurring clay ore,
adding sodium polyacrylate to water, the sodium polyacrylate having
a molecular weight between 4000 and 5000
preparing a slurry of the clay ore with the aqueous sodium polyacrylate
to disperse the attapulgite clay in the water,
separating the dispersed attapulgite clay from un-dispersed materials,
drying the dispersed attapulgite clay, and
adding the dried attapulgite clay to water with shear agitation
wherein the attapulgite clay is dispersed in the water without the
addition of a dispersant or a surfactant.
19. A purified dried, particulate attapulgite clay comprising attapulgite
clay separated from non-attapulgite clay matter and sodium polyacrylate
adsorbed on the attapulgite clay particles; wherein the sodium polyacrylate
has a molecular weight greater than 4000.
20. The attapulgite clay of claim 19 wherein the non-attapulgite
clay matter includes sepiolite, montmorillonite, bentonite and non-clay
grit.
21. The attapulgite clay of claim 19 wherein the free moisture
content of the attapulgite clay particles is between 6% and 14%.
22. The attapulgite clay of claim 19 which is dispersed in water
with shear agitation to provide an aqueous slurry without the addition
of either a dispersant or a surfactant.
23. The attapulgite clay of claim 19 wherein the separated attapulgite
clay is dried at atmospheric pressure between 85.degree. C. and
250.degree. C. to yield a free moisture content of less than 0.5%,
the dried attapulgite clay retaining thixotropic properties.
24. A purified dried attapulgite clay able to be dispersed in water
and retain thixotropic properties comprising attapulgite clay and
sodium polyacrylate sorbed thereon, the sodium polyacrylate having
a molecular weight greater than 4000.
Description FIELD OF INVENTION
This invention relates to the ability to treat naturally occurring
attapulgite clay deposits and more particularly, to selectively
disperse discrete, individual particles of attapulgite while leaving
other present clay types, such as montmorillonite, sepiolite, bentonite,
and kaolin, in their un-dispersed naturally occurring agglomerated
state.
BACKGROUND OF THE INVENTION
Attapulgite clay (Fuller's Earth) particles naturally occur as
colloidally dimensioned, high aspect ratio needle-like shaped crystals
that are bundled together, and the bundles are agglomerated into
clusters. As the attapulgite deposits were formed, other clay types
were formed simultaneously or were introduced through air and water
movements. Other mineral types also were formed in place or transported
into the deposits during their formation, resulting in a variety
of non-attapulgite materials being present in levels approaching
20 weight percent or more. In order for the attapulgite to be in
a physical form acceptable for commercial use, it needs to be processed
into a powder. The attapulgite clay is historically processed by
a grinding or milling operation, followed by particle size separation
and drying to a moisture content of approximately 10%. The non-attapulgite
matter is also reduced in size and partially removed during the
particle size classification operation. The non-clay minerals can
be abrasive and must either be removed or reduced in size to lessen
their abrasive characteristics in order for the attapulgite powder
to be useful in end use applications.
A grinding/milling operation will partially break up the bundle
clusters with the undesirable result of fracturing of individual
attapulgite particles. The individual attapulgite particles provide
the thixotropic, anti-settling, and/or binding properties to the
end use application. Particles with higher length to width, or aspect
ratios, typically are more efficient than particles with lower aspect
ratios.
In end use applications, a chemical dispersant, predominantly tetrasodium
pyrophosphate (TSPP), will at best, separate a majority of the attapulgite
bundles into individual particles in an aqueous medium under moderate-to-high
shear agitation. However, TSPP will also disperse other clay types
present. Individual clay particles of each clay species present,
will remain suspended in water. U.S. Pat. No. 3569760 has shown
that non-clay minerals will not remain suspended in the water and
will settle to the bottom due to gravitational forces if the clay-water
slurry is low enough in viscosity. The relatively large non-clay
minerals, which by their means of higher hardness, can resist size
reduction and thus can also be removed by screening, centrifuging,
via sedimentation tanks, hydrocyclone, or other physical separation
means. The non-attapulgite clay minerals, such as montmorillonite
and sepiolite, will remain mixed in with the attapulgite clay. The
non-attapulgite clays do not have the same performance characteristics
as attapulgite clay, some which are detrimental to performance in
particular applications. At the present time, the suppliers and
users of attapulgite are dependent on the quality of natural deposits
for the concentration of non-attapulgite clay content. The availability
of high purity attapulgite clay relies on the economically inefficient
selective mining of the deposits.
Gantt et. al. in U.S. Pat. No. 5358120 have shown that other
types of previously commercially available dispersants such as sodium
polyacrylates can be used with clay. However, sodium polyacrylates
with molecular weights less than 4300 while effective on kaolins
and bentonites, are typically not as effective with attapulgite
when compared to TSPP, and thus not generally used.
Attapulgite clay provides thixotropic and binding properties through
the process of re-flocculating after being typically dispersed at
less than 5% solids in an aqueous system, whereby a gel structure
is generated. Various salts, at additive levels, can act as re-flocculating
agents.
Historically, attapulgite clay is dried to a free moisture content
of approximately, 10-16%, with an 8 or 9% minimum. At lower moisture
contents, the attapulgite begins to lose its thixotropic properties
and does not readily re-disperse in water. Brooks, in U.S. Pat.
No. 4966871 has shown that it is possible to vacuum dry attapulgite
down to less than 2% and still retain its thixotropic properties,
the importance being that the attapulgite is dried under vacuum
conditions.
Attapulgite clay is frequently used as an additive product, often
comprising only 1 to 3 weight percent of the final composition.
Attapulgite clay does not disperse completely in a liquid medium
at these low concentrations. The believed cause is the lack of clay
to clay collisions needed to break up the agglomerates and bundles.
A technique used to overcome this behavior is one in which a 20
to 25% slurry is made, and when completely dispersed, it is diluted
with water down to the desired ultimate concentration.
There is a need to be able to efficiently and economically separate
attapulgite clay from other clay types and from non-clay minerals.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an economical and efficient
method to separate attapulgite clay from other clay components and
minerals in clay ore.
It is a further object of the invention to provide a purified attapulgite
clay.
It is yet another object of the invention to provide dry attapulgite
clay particles which are readily dispersed in water without addition
of dispersant or surfactant.
It is a further object of the present invention to provide a dry
particulate attapulgite clay which has less than 0.5% free moisture
content and retains thixotropic properties.
In accordance with the teachings of the present invention, there
is disclosed a method of processing naturally occurring clay ore
to separate attapulgite clay from other materials. This includes
the steps of crushing the clay ore, adding sodium polyacrylate to
water, the sodium polyacrylate having a molecular weight between
4000 and 5000 preparing a slurry of the clay ore with the aqueous
sodium polyacrylate to disperse the attapulgite clay in the water,
separating the dispersed attapulgite clay from un-dispersed materials,
and drying the dispersed attapulgite clay.
Further, in accordance with the teachings of the present invention,
there is disclosed attapulgite clay which has separated from non-attapulgite
type clay matter wherein the non-attapulgite type clay matter includes
sepiolite, montmorillonite, bentonite-type and non-clay grit, the
attapulgite clay being in the form of particulem.
These and other objects of the present invention will become apparent
from a reading of the following specification, taken in conjunction
with the enclosed drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged view of dispersed attapulgite crystals.
FIG. 2 is an enlarged view of a smectite/attapulgite cluster.
FIG. 3 is a diagram of the purification of attapulgite clay.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
We have made the unexpected discovery that the efficiency of a
particular sodium polyacrylate, which preferably has a molecular
weight of approximately 4700 allows for higher concentrations
of attapulgite clay in a water slurry at a lower viscosity than
is attainable when using TSPP or other sodium polyacrylate dispersants
having lower molecular weights. The practical upper limit for the
attapulgite concentration when using TSPP as the dispersant is approximately
25%, above which the viscosity becomes too high for handling, and
less than 25% solids for other sodium polyacrylates with lower molecular
weights. The approximate 4700 MW sodium polyacrylate imparts the
ability to prepare at least 35% concentration attapulgite slurry
and still have acceptable handling characteristics. This property
offers the economic advantage of more efficient manufacture of formulations
that begin with a 25% slurry that is diluted down to a lower concentration
(typically 1-3% used in suspension fertilizers) as fewer slurry
batches need to be prepared to yield the same amount of final product.
When further studied, we have made an additional unexpected discovery
that by using said higher molecular weight sodium polyacrylate,
the attapulgite particles will disperse with significantly improved
efficiency in water under moderate-to-high shear conditions, (see
FIG. 1.) and will disperse other clays present to a less extent
than TSPP will. The result is that more of the non-attapulgite clays
present will remain as relatively large particles, along with the
non-clay matter present. The attapulgite particles will form a colloidal
suspension. The undispersed non-attapulgite clays can be easily
removed by physical means along with the non-clay matter. The result
is an increased purity form of attapulgite that is essentially grit
free. The relatively lower molecular weight sodium polyacrylates
tested will not disperse the attapulgite particles as efficiently,
thus achieving less separation of attapulgite and non-attapulgite
clay particles. See FIG. 2 where the molecular weight of the sodium
polyacrylate used is 2600.
We have further discovered that when attapulgite is dried, the
sodium polyacrylate remains adsorbed on the attapulgite particles.
The dried attapulgite will readily re-disperse into water without
the need for a dispersant or surfactant to be pre-mixed into the
water. In addition, with the removal of non-attapulgite clay, higher
performance is observed in the ability to provide thixotropy, binding,
and suspension behavior.
Another unexpected discovery was the ability of the residual sodium
polyacrylate to allow the attapulgite to retain its thixotropic
properties when dried to less than 0.5% moisture at atmospheric
pressure, without the need for vacuum drying and/or a silicone or
silane type stabilizer.
Attapulgite clay ore is mined and crushed into pieces small enough
so that they can be blunged into a slurry. The sodium polyacrylate
dispersant, having a molecular weight range of between 4000 and
5000 is pre-mixed with water. The sodium polyacrylate concentration
preferably is between 1 and 4 weight % of the attapulgite clay content.
The attapulgite clay is gradually added under moderate-to-high shear
to the water--sodium polyacrylate solution, until fully dispersed.
The attapulgite bundles and clusters become separated into colloidal
particles while some of the other the clay species present, remain
agglomerated in relatively large particles. The undispersed non-attapulgite
clay particles are removed along with the non-clay matter, via physical
means. The de-gritted attapulgite clay is then de-watered or dried
until the desired residual moisture content is achieved. Preferably,
the drying process is at atmospheric pressure at a temperature between
85.degree. C. and 210.degree. C. During the drying process, the
attapulgite particles `self-coalesce` into loosely bound granules.
They do not re-flocculate into tightly bound bundles and agglomerates.
The final form may include a filter cake or a dried powder. The
filter cake and/or the dried powder can be easily re-dispersed into
water without the use of additional dispersant. One effective drying
method which may be employed is spray drying. Spray drying offers
the ability to produce uniformly sized granules whose size can be
controlled by adjusting the process conditions in the spray dryer.
Thus, finely sized attapulgite granules can be made without a grinding
or milling procedure that would fracture the attapulgite needles,
thereby reducing performance.
EXAMPLES 1 THROUGH 9
Samples of various quality attapulgite clay were slurried, using
between 1.0 and 1.8 weight percent, on a dry clay basis, of sodium
polyacrylate with a molecular weight between 4000 and 5000. The
slurries consisted of approximately 25 weight % clay in de-ionized
or distilled water. The sodium polyacrylate was mixed with the water
until a homogeneous solution was achieved, then the clay was gradually
added under moderate-to-high shear, until completely wetted. The
slurries were filtered with a 325 mesh screen and then dried into
a free flowing powder. X-ray and Atomic Absorption analysis methods
were used to compare the compositional quality of the raw material
and the finished material. The following species were identified:
attapulgite, smectite clay (i.e., montmorillonite, sepiolite), quartz,
calcite, dolomite, apatite, illite, mica, kaolinite, as well as
other trace minerals. The results are listed below in Table 1.
Specimen #1 the reference specimen, is a relatively high quality
grade with 85% attapulgite content, with a low 2% smectite content,
without any processing. Specimen #2 the control specimen, which
is specimen #1 that was processed according to the preceding paragraph,
except that TSPP was used as the dispersant in place of sodium polyacrylate.
Specimen #2 did not exhibit any purification other than some grit
removal during the screening. The attapulgite and smectite contents
remained the same. Specimen #3 was also processed from Specimen
#1 according to the preceding paragraph, this time using the above-identified
sodium polyacrylate dispersant. Specimen # 3 tested with an increase
in attapulgite content from 85 to 90%. The smectite content decreased
1%. The remaining 4% is comprised of an additional 1% reduction
of quartz plus reduced amounts of apatite, illite and mica. This
can be attributed to the increased dispersion of the attapulgite
bundles, freeing up more entrapped contaminant particles via physical
separation. Specimens # 4 and # 6 are two other grades of attapulgite
clay, a low and high gelling grade, respectively which have been
processed with TSPP but without the sodium polyacrylate dispersant.
Specimens # 5 and # 7 are the corresponding specimens treated with
the sodium polyacrylate dispersant. In both cases, the attapulgite
content increased from 85 to 90%. Specimen # 8 is a relatively impure
attapulgite, with a high smectite content of 30%. Upon processing
with the sodium polyacrylate dispersant, (Specimen #9) the smectite
content is reduced from 30 to 25%, along with a drop in illite/mica
content from 6% to 1%.
EXAMPLE 10
A specimen of attapulgite clay, with a moisture content of between
8 and 13 weight %, was dispersed in water at 35% solids, using said
sodium polyacrylate dispersant. The viscosity was measured at 300
cPs using a Brookfield RVT viscometer. The slurry was then de-gritted
with a 325 mesh screen and dried at 105.degree. C. at atmospheric
pressure to a powder to approximately the original moisture content.
The de-gritted attapulgite was then re-slurried into water at 35%
solids without the use of any additional dispersant. The viscosity
was then measured at 340 cPs. The slight increase is proportional
to the amount of grit removed and the results can be considered
similar and within experimental reproducibility.
EXAMPLE 11
A specimen of attapulgite clay was dispersed using said sodium
polyacrylate dispersant in water at 35% solids. The slurry was de-gritted
and spray dried to yield a dry, powdered attapulgite. The free moisture
content was determined and the viscosity was measured at an industry
standard quality control test level of 7% solids in water. The attapulgite
powder was then dried further at approximately 105.degree. C. to
produce various free moisture content specimens. These specimens
were subsequently tested for their respective viscosities and ability
to re-flocculate into a gel structure that provides thixotropic
performance.
EXAMPLE 12
A specimen of attapulgite was slurried into water at 35% solids
using various grades of sodium polyacrylate dispersants. The amount
of dispersant used was varied between 1 and 2.5% in order to determine
the most effective level, using the minimum viscosity obtained as
the criteria (an indication of degree of dispersion). The results
were recorded as viscosity vs % dispersant added (Table 3). Note:
non-crosslinked sodium polyacrylates with molecular weights greater
than approximately 4700 are not known to exist. It can be expected
that, if available, they would exhibit similar or improved performance.
In summary, clay ore is purified and separated from non-attapulgite
material by dispersing the ore in an aqueous solution of sodium
polyacrylate having a molecular weight of approximately 4000-5000.
Non-attapulgite clays are not dispersed, or only very slightly dispersed,
in this sodium polyacrylate solution. Also non-clay material ("grit")
is not dispersed. In this manner, the attapulgite clay forms a slurry
and can be separated from non-attapulgite material. The slurry is
dried to provide a powder of attapulgite clay. There is residual
sodium polyacrylate in the dried attapulgite clay such that upon
addition of water, the attapulgite redisperses without the addition
of further dispersant. The redispersed attapulgite clay retains
thixotropic properties. Thus, a purified form of attapulgite clay
is provided which is more economical to use due to the absence of
non-attapulgite material. Also, poorer quality clay ore can be processed
economically to provide purified attapulgite clay.
Obviously, many modifications may be made without departing from
the basic spirit of the present invention. Accordingly, it will
be appreciated by those skilled in the art that within the scope
of the appended claims, the invention may be practiced other than
has been specifically described herein. |