A process for making acid-activated bleaching earth from a crude
attapulgite clay. A crude is selected which is mildly acidic and
contains at least about 90% attapulgite. The process involves treating
such clay with low levels of activating acid which are mixed with
the dried and ground clay, or spray dried from slurries containing
the clay-acid mixture. Advantages include: lower acid costs/unit
mass of clay treated, lower production costs (no washing, filtering,
or waste treatment steps) and environmental soundness (no harmful
environmental waste products are produced).
1. A method for producing bleaching earth which comprises selecting
a naturally-occurring acidic attapulgite clay having a pH in the
range of 5 to 7 and a pore volume in the range of 0.25-0.50 cc/gm,
mixing said clay with an acid solution in amount corresponding to
an acid dosage in the range of 10 to 30%, heating said mixture at
a temperature in the range of 77.degree. to 220.degree. F. to react
said clay with said acid and, without washing the resulting reaction
product, recovering it for use as a bleaching earth.
2. The method of claim 1 wherein said selected clay contains no
more than about 5% CO.sub.2 by weight on a moisture free basis.
3. The method of claim 1 wherein said selected clay contains less
than 1% CO.sub.2 on a moisture free basis.
4. The method of claim 1 wherein said selected clay is dried and
ground before mixing with said acid.
5. The method of claim 1 wherein said mixture of clay and acid
is heated while it is spray dried.
6. The method of claim 1 wherein said mixture of clay and acid
is formed by spraying acid solution onto dried clay.
7. The method of claim 1 wherein said clay is dried and ground,
mixed with acid solution, filtered after heating, and filtrate is
8. The method of claim 1 wherein said acid is sulfuric.
9. The bleaching earth product obtained by the method of claim
This application is related to USSN 352578 filed concurrently
1. Field of the Invention
The invention relates to a process for making acid-activated bleaching
earth from attapulgite clay and to novel acid-activated bleaching
earth products obtained thereby.
Acid-activated clays of high activity are used to adsorb colored
pigments (carotenoids, chlorophyll) and colorless pigments (phospholipids)
from edible and inedible oils. This process is called "bleaching"
and serves both cosmetic and chemical purposes. Thus, bleaching
reduces color, whereby very clear, almost water white oils are produced
that meet with consumer expectations. Bleaching also stabilizes
the oil by removing colored and colorless pigments which tend to
"destabilize" the oil, resulting in oils that rancidify
more easily if they are not removed. The current, and expected long
term trend, favors the use of highest possible bleaching efficiency
clays with this process because: (i) the smaller amounts of the
high activity clays needed to produce desired refined oil properties
mean that lower inventories can be maintained by the oil refiner;
(ii) refined oil losses depend somewhat on the amount of clay used
because less of high activity clay needs to be used and therefore
oil losses are lower; and (iii) less spent clay is produced when
using high activity clay, and hence land-fill disposal costs are
The conventional process for producing acid-activated bleaching
clays utilizes calcium bentonite clays and requires relatively high
acid dosages to achieve maximum bleaching efficiencies. The calcium
bentonites used in the process are hydrated sodium calcium aluminosilicates
which typically are mildly basic. The manufacture of highest quality
commercial bleaching earths typically require 70-90 grams of 96%
H.sub.2 SO.sub.4 /100 grams of dry clay or 67.2-87.4% acid dosage
where: % Acid ##EQU1## Extensive leaching of the clay structure
in the form of solubilized salts takes place and these are removed
in the process. Because of these high acid dosages, and the extensive
leaching that takes place during the leaching process, the yield
of bleaching clay is low (typically in the range of 75-85 wt %).
The acidic salts formed during activation and residual acid must
be washed off and separated by filtration from the product clay.
If high levels of unused acid and acidic salts (iron and aluminum
sulfates) are left in the clay, the quality of the bleached oil
is impaired. High residual acid levels generate undesirable free
fatty acids from the fatty acid triglycerides in the oil. Finally,
the leachate (acidic salts and residual acid) is a waste stream
that contains materials harmful to aquatic life and therefore must
be neutralized or otherwise disposed of in an environmentally acceptable
manner. This constitutes an additional expense of producing bleaching
clays from pure calcium bentonites.
Clay sources used in the past to provide acid-activated bleaching
clay of high activities have been primarily restricted to calcium
bentonites, i.e., clays in which the principal exchangeable cation
is a calcium ion, and these are sometimes also referred to as sub-bentonites.
Another type of naturally-occurring clay simply requires heat to
impart bleaching activity. These are the clays rich in the minerals
attapulgite or sepiolite, now frequently classified as palygorskite
clays. Mineralogically, the palygorskite clays are readily distinguishable
from the bentonites (smectites or montmorillonites) and rarely are
palygorskites and bentonites used interchangeably.
It has been the general belief that palygorskite clays do not respond
to the conventional acid-activation treatment. The same is true
of certain bentonites, namely sodium (swelling) bentonites, such
as Wyoming bentonites.
The following publications pertain to the art of preparing bleaching
earths from naturally-occurring clays.
A.D. Rich, "Bleaching Clay", Industrial Rocks & Minerals,
3rd. Ed., AIME, N.Y. pp 92-101 (1960).
R. Fahn, "Bleaching Earths-Preparation, Properties, Practical
Applications", Chapter 1 Internal Symposium, Brussels, April
L.L. Richardson, "Use of Bleaching Clays in Processing Edible
Oils", JAOCS, 55 777 (1978).
G.M. Clarke, "Special Clays", Ind. Minerals, Sept., 25
D.R. Taylor, D.B. Jenkins, "Acid-Activated Clays", Soc
Mining Eng Of AIME, Transactions, 282 1901 (1988).
R.L. Grim, "Applied Clay Mineralogy", pp 320-326 (1962).
A.C.D. Newman, "Chemistry of Clays and Clay Minerals,"
pp 107-114 (1987).
The following patents relate to the production of acid-activated
U.S. Pat. No. 1397113 (1921); Prutzman
U.S. Pat. No. 1579326 (1924); Kauffman
U.S. Pat. No. 1642871 (1927); Chapel
U.S. Pat. No. 2470872 (1949); Secor
U.S. Pat. No. 2472489 (1949); Pierce
U.S. Pat. No. 2484828 (1949); Hickey
U.S. Pat. No. 2553239 (1946); Christianson
U.S. Pat. No. 2563977 (1949); Van Horn, Kahn
U.S. Pat. No. 2574895 (1951); Stecker
U.S. Pat. No. 2671058 (1952); Mickelson
U.S. Pat. No. 2872419 (1959); Farnand
U.S. Pat. No. 2892800 (1959); Taipale
U.S. Pat. No. 2981697 (1961); Mickelson, et. al.
U.S. Pat. No. 3617215 (1971); Massaire, et. al.
EPA No. 0276954 (1988); Alexander
Generally, in the patents listed above, calcium bentonites are
the source clays and high acid dosages, typically above 40-50 gms
of 96% H.sub.2 SO.sub.4 /100 gms of dry clay, are required for maximum
improvement in bleaching efficiency. The acid treated clay is invariably
washed to remove soluble salts and entrained acid. See, for example,
U.S. Pat. No. 1397113 U.S. Pat. No. 1642871 and the recently
published EPA (0276954).
It is known to add citric acid to oils that are bleached with mixed
attapulgite/calcium bentonite bleaching earths in order to enhance
chlorophyll adsorption. Citric acid is not used to activate the
clay. U.S. Pat. No. 3029783 (Sawyer, et al), directed to an improved
animal litter composition, describes an acid treatment, preferably
using an attapulgite clay, which employs relatively low acid dosages
without washing. The processing requires a calcination treatment
at 700.degree.-1000.degree. F. prior to the acid treatment and
a second calcination at 750.degree.-1100.degree. F. after acid
treatment. The patent is not directed to the manufacture of a bleaching
earth and we have found that the procedure does not lead to the
preparation of a high efficiency bleaching earth.
3. The Invention
Surprisingly, it has been found that mildly acidic uncalcined palygorskite
such as attapulgite clay, hereinafter referred to as "high
susceptibility attapulgite clays" (HSAC) requires significantly
lower acid dosages (e.g., 10-30 grams of 96% H.sub.2 SO.sub.4 /100
grams clay) to achieve their maximum bleaching levels. Because so
little acid needs to be used with these clays, residual acid levels
left on these clays are quite low and subsequent washing, filtration
steps or post-calcination steps are unnecessary. In fact, it has
been found that the requisite acid can simply be sprayed on dry
powdered clay, or a clay-acid slurry can be mixed and then spray
dried, to produce a high activity bleaching clay.
Palygorskite clays include attapulgite clays also known as Attapulgus
clay, or Georgia-Florida fuller's earth. These clays are usually
frequently composed principally of the mineral attapulgite, a crystalline
hydrated magnesium aluminum silicate, but may also contain significant
amounts of other minerals such as montmorillonite, calcium carbonate,
quartz (silicate) and feldspar, and in some cases sepiolite. Those
attapulgite clay used in the practice of this invention contain
at least about 90% of the mineral attapulgite and are limited to
those clays which are low in their content of carbonate minerals.
This excludes most commercial deposits of primary and sedimentary
sources of attapulgite clay which are usually associated with limestone.
The results of experiments conducted with high purity, low carbonate
attapulgite showed that it took 10-30 wt % acid dosages to achieve
maximum activity with these material. Higher acid dosages (i.e.
70-90 wt %) are required to achieve maximum adsorptive capacities
for bentonite clays.
DESCRIPTION OF PREFERRED EMBODIMENTS
By high susceptibility attapulgite clay, we mean those naturally
occurring attapulgite crudes which: (1) contain at least 90% (wt)
attapulgite content; (2) possess a slurry pH less than 7; and (3)
have pore volume greater than about 0.20 cc/gm.
Generally, suitable high susceptibility attapulgite clay contains
no more than 5% by weight CO.sub.2 and preferably less than 1%
by weight CO.sub.2 on a moisture free basis.
In defining clays the terms "volatile matter" (V.M.)
and "Loss on Ignition" (L.O.I.) must frequently be used.
Volatile material is classified according to three levels of thermal
treatment: loosely-held water of hydration known as free-moisture
(F.M.) which is measured by heating to constant weight at 220.degree.
F., structural water that is held chemically in the molecular structure
of the clay and is measured by heating from 220.degree. F. to constant
weight at 1200.degree. F., and other volatile matter such as inorganic
carbonates, principally calcium carbonate, which release CO.sub.2
at 1800.degree. F.
Various methods related to water content or thermal treatment may
be used to express percentages of components in the clay. A moisture-free
weight, or dry weight, is the weight of the clay after heating to
constant weight at 220.degree. F. A volatile-free basis weight is
the weight of the clay after heating to constant weight at 1200.degree.
F., and a loss-on-ignition or L.O.I. basis weight is determined
by heating to constant weight at 1800.degree. F. Also, volatiles
content may be expressed on an as received basis.
The difference between loss on ignition weight and volatile-free
weight is a fairly accurate representation of carbonate content,
since carbonates generally account for virtually all of the non-aqueous
volatile matter in the clay. An alternative method for measuring
the carbonate content (as % CO.sub.2) is to finely pulverize the
clay and heat in the presence of a strong mineral acid such as hydrochloric
acid to release carbon dioxide. The gas can then be collected and
its volume measured, from which the weight percent of CO.sub.2 can
be calculated. This former method was employed to determine the
carbonate content of clays used in the examples described hereinafter.
X-ray diffraction is used to determine the attapulgite content
of the attapulgite/crude clays. To do this, a comparison is made
between the peak areas of the 110 peak of attapulgite and the 001
peak of the montmorillonite in the region 3.degree.-20.degree. 2.theta.
when XRD machine conditions are as follows:
______________________________________ beam slit - .2.degree. baseline
- .15V window - 1V Cu K - .alpha./Ni filter beam current - 31 mA
voltage - 40KV rate - 1/2.degree./min. chart speed - 1 cm/min. range
- 1K or time constant - 1 sec. 10K CPS ______________________________________
In order to calibrate the intensity of these two peaks, a series
of mixtures of pure crude attapulgite (Emcor.RTM. 66 low-carbonate
clay) and pure crude bentonite (Filtrol Gr 2) were prepared by mixing
known quantities of the finely ground powders (100% T-325 mesh)
whose moisture contents were also known. With this information (i.e.--weight
and moisture content of the clays used) and the resultant XRD patterns
for each of the mixtures, it is possible to calibrate the relative
response of these two peaks to the x-ray beam. The assumption is
made that the response of these peaks in a physical mixture is the
same as it will be in the naturally occurring mixture of these two
clays. Using this method, it was determined that the intensity correspondence
between the 110 peak of attapulgite and the 001 peak of montmorillonite
In one embodiment of the invention, 10-30% by weight of concentrated
acid (based on the volatile free weight of clay being activated)
is added to enough water and clay so that the volatile free solids
content of the resultant clay/acid/water slurry is in the range
20-25 wt %. Thereafter, the slurry can be heated for 1-5 hours with
gentle stirring, filtered (washing is not necessary, but can be
practiced), dried, and ground to yield a finished product suitable
for use as described below. Although higher acid dosages may be
employed, no significant advantages are imparted by this effort,
and since acid is a costly reagent, lower acid dosages are preferred.
As can be seen from TABLE 1 treatment with acid tends to reduce
CaO, MgO, and P.sub.2 O.sub.5 content at all dosage levels (accompanied
by an apparent increase in SiO.sub.2 content). Only at the higher
acid dosages is Al.sub.2 O.sub.3 significantly removed from the
clay. This suggests that exchange of Ca.sup.++ and Mg.sup.++ for
protons is the primary role of the acid at lower dosages, but that
actual leaching of the clay structure (i.e.--removal of structural
ions) does begin to occur at the higher acid dosages.
In general, the process of the invention comprises selecting an
acidic naturally-occurring attapulgite clay, crushing, preferably
subjecting the clay to a preliminary drying step; typically to 15
to 30 wt % VM, grinding the dry clay, mixing the clay with a diluted
acid, drying and thereafter pulverizing unless the acid treated
clay is already in desired particulate form. Preliminary drying
is carried out under conditions such as to render the clay amenable
to the grinding method used, e.g., heating at 200.degree.-300.degree.
F. to reduce to 15 to 25 wt % VM. Optionally, the source clay is
extruded before drying and grinding. In some instances, grinding
can take place before drying.
The following discussion illustrate advantages and essential differences
between the processes made possible by the use of high susceptibility
Conventional processing entails crushing, predrying (necessary
so the clay particles will break apart or "slake" properly
when added to water to give a uniform, colloidal dispersion suitable
for activation), a mixing step (where clay, acid and water are combined),
a treater or "leach" step (where the clay/acid/water slurry
is heated to near boiling with gentle agitation for about 5 to 51/2
hours), and a washing/filtration step (where entrained unused acid
and acidic salts are extracted from the filter cake). Some of the
liquid waste stream from this step can be recycled back to the mixer,
but the greater majority must be disposed of by other means. Since
aluminum containing salts formed from the reaction between the acid
and bentonite clay during the leaching process are highly toxic
to aquatic life, the waste stream from this process cannot be discharged
to surface waters or to the subsurface water table. In some cases,
an expensive deep well injection facility must be maintained to
dispose of these wastes, and in other cases, the acidic wastes are
precipitated with lime or caustic, and the neutral solids produced
are placed in land-fill. In any case, some method of waste disposal
is required for an environmentally sound operation. The extracted
filter cake from this operation is then dried ground, and bagged
or placed in bulk storage.
In contrast to the process just described, processes of the invention
utilizing high susceptibility source clays are simpler, more economical
and do not generate waste products which are difficult to dispose
of and add to production costs.
In accordance with the spray coating embodiment of the invention,
the coarse feed from the crusher (nominal 1/4" diameter particles)
is feed directly to a drying and grinding operation where it is
dried to a level somewhat dryer than desired for the finished product,
and ground to the desired average particle size. Typically, the
production has 10-15 wt % VM and 80-85 wt % is finer than 200 mesh.
An acid/water mixture is then sprayed directly onto the dried and
ground powder using such proportions of acid and water that optimized
bleaching performance of the finished product is attained. Preferably,
the acid/water spray is added directly to the powder while it is
still warm and before it takes up moisture from the surrounding
atmosphere. Intimate mixing of the raw clay powder and the acid/water
spray can be achieved by any number of methods, including, for example,
spraying the acid/water into the clay as it is mixed in a glass-lined
Pfaudler mixing vessel, by spraying acid solution into the tumbled
clay powder in a rotating rotary pan spray-coating machine, by spraying
onto the clay as it travels along a moving belt conveying system,
or by spraying onto the clay as it is moved by a screw conveyor.
The dried, ground powder, now impregnated with the acid/water mixture,
is bagged or placed in bulk storage as finished product.
The following outlines the essential steps of the spray drying
process to produce acid-activated bleaching clay from high susceptibility
source clays. In this case, raw clay is first crushed, then dried,
typically to 20-30 wt % VM at 200.degree.-300.degree. F., and pulverized
to produce a finely ground powder (e.g., 99%-200 mesh) suitable
for spray drying using either high pressure nozzles or a spinning
disk as commonly used in these apparatus. The finely ground clay
is mixed with sufficient water and requisite sulfuric acid to form
a slurry which can be spray dried. The high susceptibility source
clay used in this process will be activated instantaneously during
the spray drying operation; therefore, heating of the slurry before
the spraying operation need not be practiced although such heating
will not be deleterious to the final product. The spraying conditions
are set to produce spray-dried particles whose average diameter
falls in the range 15-30 microns. These particles of acid-activated
high susceptibility source clay can be bagged or placed in bulk
storage as finished product.
The essential steps of the modified conventional process to produce
acid-activated bleaching clay from high susceptibility source clays
is as follows. This process uses the same equipment and process
train as that previously described for a conventional acid-activation
process, but with the following essential differences: (i) much
lower acid dosages are employed (i.e., 3-10 grams of 98% H.sub.2
SO.sub.4 /100 grams of dry clay); (ii) shorter reaction times can
be employed (1-3 hours); (iii) 100% of the "sour" water
which is recovered from the filtration step can be recycled back
to the mixing step where only enough additional water plus fresh
mineral acid is added to achieve the desired levels of bleaching
activity; and (iv) since all of the sour water is recycled, there
is no waste stream of acidic salts and unused acid requiring disposal.
When edible (and inedible) animal or vegetable oils are treated
with bleaching clays, the objective is to reduce the levels of certain
trace constituents (including carotenoids such as B-carotene, pheophytins
and chlorophyll, and peroxides, among others). Color pigments, such
as B-carotene (reddish-orange pigment) and chlorophyll (green pigment)
must be removed if the oils is to be of suitably light color to
meet with consumer acceptance; peroxides (highly reactive molecules)
must be removed in order to obtain an oil exhibiting good photolytic
and chemical stability (i.e.,--one which will not rancidify easily).
Additionally, it is desired that levels of free fatty acids produced
when contacting vegetable oils with acid-activated bleaching clays
should not be excessively high since they constitute a refining
Those familiar with the art of bleaching are aware of these and
other quality control tests to monitor oil quality during bleaching.
Red and yellow color is commonly monitored using an automatic tintometer
according to the procedures listed in the American Oil Chemists'
Society Official and Tentative Methods (AOCS Official Method Cc
13b-45); chlorophyll (AOCS Official Method Cc 13d-44); peroxides
(AOCS Official Method Cd 8-53; rev. Ja 8-87); and free fatty acids
(AOCS Official Method Ca 51-40). In all cases, the lower the values
obtained, the better the quality of the resultant oil. Typically,
for instance, when bleaching a caustic refined soybean oil, refiners
find that chlorophyll reduction is the most important quality parameter,
and over time, it has been found that adequate bleaching has occurred
if this constituent can be reduced to the 50-90 ppb range. When
this level is obtained, other oil constituents are usually well
below the levels of which they would cause problems with regard
to achieving satisfactory finished oil quality.
The following examples are presented in order to more fully explain
and illustrate the invention. The examples are not be construed
as limiting the invention.
Three different source clays (A--high purity attapulgite clay;
B--high purity bentonite clay; and C--high susceptibility attapulgite
clay) were subjected to acid-activation with sulfuric acid at varying
acid dosages for 51/2 hours, and then washed, filtered, dried and
ground to finished products. Following are chemical and physical
properties of clays A, B and C along with a summary of activation
These materials were then used at constant dosage either (0.5 wt
% or 0.7 wt % clay, as is, based on amount of oil) to treat a typical
caustic refined soybean oil. The oil quality parameters previously
discussed were then measured as a function of activating acid dosage
as shown in TABLES 2 and 3 and compared to those obtained when
using two commercial bleaching clays, high activity Filtrol Gr 105
and very high activity Filtrol Gr 160.
Data in TABLES 2 and 3 show that all of the clay samples tested
benefited from the acid-activation process. For example, compare
the results obtained at 0% acid dosage (raw clay) versus those obtained
at higher acid dosages. Nevertheless, the high purity attapulgite
which contained 3% carbonate required at least 20% acid dosage to
produce a product capable of achieving an oil quality where the
most important quality parameter, chlorophyll, falls in the desired
range (50 to 90 ppb). The high purity bentonite required even higher
dosages (in the range 45-90%) to achieve comparable bleaching activity.
In contrast, the high susceptibility attapulgite clay (HSAC) which
contains practically no carbonate achieved a chlorophyll bleaching
efficiency in the desired range with as little as 10% acid dosage.
A sample of high purity attapulgite containing 3% calcium carbonate
(Engelhard 200 UP/LVM, source clay A) and a sample of high purity
bentonite (Filtrol GR 2 source clay B) were dried at 110.degree.
C. to the range 10-11 wt % LOD (loss on drying, 300.degree. C.),
ground (90%<200 mesh) and then broken into aliquots which were
spray-coated with dilute solutions (10%-25%) of various acids to
yield samples having been subjected to acid dosages between 1-8
wt %. The spray-coating was accomplished by spraying a mist of the
dilute acid solution into a rotating vessel containing the dried,
powdered samples of the above mentioned clays. After allowing the
samples to equilibrate at room temperature for three days in closed
containers, they were redried (at 110.degree. C.) to the range 10-11
wt % LOI, and any agglomerates broken up so that the final sample
was at least 90%<200 mesh.
As shown in TABLE 4 dosages as low as 3 wt % sulfuric acid applied
to source clay A (high purity attapulgite containing 3% calcium
carbonate) were sufficient to produce a material which removed at
least 1/2 the red color and somewhat more than 2/3's of the chlorophyll
in that oil. Even better results would be expected when using a
high susceptibility attapulgite clay (such as source clay C) because,
as already shown, this clay reaches optimum activity levels at lower
acid dosages than does an attapulgite containing significant carbonate
levels. Even at that, quite acceptable Lovibond red, chlorophyll,
and peroxide reductions were achieved with 3-8 wt % acid dosages
as can be seen by comparing these data to those obtained with a
commercial, high activity bleaching clay such as Filtrol Gr 105.
In addition, % FFA rise was actually much superior to the commercial
bleaching clays because whereas they are net generators of free
fatty acids (compare to FFC level in uncontacted oil), the spray-coated
attapulgite is actually a net adsorber of free fatty acids.
In contrast, source clay B (high purity bentonite) showed essentially
no improvement at these low acid dosages, regardless of the level
of acid (up to 8 wt %) and type of acid.
In these cases, all of the acid added to the clay remained associated
with that sample, although conversion to calcium and magnesium salts
via reaction with the high purity attapulgite source clay is highly
likely. At any rate, adsorptive activities of these materials as
well as their tendency to generate free fatty acids were quite acceptable
even though none of the samples received any subsequent washing
(a salient feature of being able to use this process with a high
susceptibilty attapulgite clay). The superiority of sulfuric acid
over the other acids utilized is clearly evident.
Although the spray-coated attapulgite used here was not as efficient
as Gr 160 bleaching clay for removing red and green color (chlorophyll)
on an equal weight basis, it would still be considered adequate,
particularly in view of its superior free fatty acid reduction characteristics,
and because it can be manufactured much more simply and cheaply
than present commercial bleaching clays such as Gr 160.
The processes just described have numerous economic and procedural
advantages over the process normally employed: (1) lower acid costs
per unit mass of clay treated; (2) lower production costs (no washing,
filtering, or waste treatments steps); and (3) environmental soundness
(i.e.--no harmful environmental waste products are produced). Considering
the fact that attapulgite clays are not normally used as the source
of acid activated clay and almost a century of effort has been directed
to manufacturing and improving acid-activated bleaching earths,
the results were unexpected. It is believed that the vast number
of prior art investigators overlooked the effect of carbonate (limestone)
impurities on the activation process, thereby failing to note that
certain palygorskite clays (those naturally acidic, hence low in
free carbonate) would be amendable to a simple, cost effective treatment.