Technical Field This invention relates to clay-based compositions suitable for the bleaching of oil. In particular, the present invention is directed to a method of selecting appropriate raw materials for treatment, as well as the treatment of those raw materials to produce clay compositions suitable for the adsorptive purification, such as bleaching, of oils. Background of the Invention

Fats and fatty oils, commonly called triglycerides, are constituted of triesters of glycerol, and include minor amounts of fatty acids. At ambient temperatures, about 20 degrees Celsius to about 25 degrees Celsius, fats are solids, whereas fatty oils are liquids.

Triglycerides are widely distributed in nature. Some triglycerides are edible while others are not. Many are derived directly from vegetable, animal, and marine sources. Others are obtained, as by¬ products, in the production of fiber from vegetable matter, and in the production of protein from vegetable, animal or marine matter.

Edible vegetable oils include canola, coconut, corn germ, cottonseed, olive, palm, peanut, rapeseed, safflower, sesame seed, soybean, and sunflower oils. Examples of nonedible vegetable oils are jojoba oil, linseed oil and castor oil.

Illustrative sources of edible animal-derived oil include lard and tallow. Examples of nonedible animal-derived oil are low grade tallow and neat's-foot oils.

Some of these oils may have a color that is objectionable to a consumer. Thus, the oil needs to be bleached to improve its color quality. To this end, a great many oils are commonly treated with bleaching clays to reduce oil color values by adsorptive purification. Bleaching clays generally improve oil color quality by selectively adsorbing color impurities that are present. Color impurities typically present in oils include, for example, carotenoids, xanthophylls, xanthophyll esters, chlorophyll, tocopherols, as well as oxidized fatty acids and fatty acid polymers.

It is also desirable to remove color impurities from a nonedible oil to obtain a acceptable color. Natural clays, e.g., Fuller's earth and the bentonites, have commonly been used as bleaching clays to remove both the naturally-occurring and the otherwise-present, e.g., the thermally-induced, color impurities from edible and nonedible oils. It has been suggested that clays containing a zeolite can be used for such a purpose as well.

Acid-activated clays have also been used for this purpose. Such clays generally remove a relatively wider spectrum of color impurities. However, there has been no way of predicting which clays have properties that make them preferable for acid activation methods.

A conventional process for producing acid-activated bleaching clays utilizes calcium bentonite clays and sulfuric acid. The calcium bentonites used in the acid activation process typically are neutral to mildly basic. The acidic salts formed during activation and residual acid can be washed off and separated by filtration from the product clay, if desired. However it is not necessary to do so.

Other suitable naturally-occurring clays are the palygorskite clays. Mineralogically, the palygorskite (attapulgite) clays are distinguishable from the bentonites (smectites, including montmorillonites) .

What is needed is a way to identify clay minerals that are readily susceptible to treatment with acid to produce a useful bleaching clay product suitable for adsorptive purification of an oil. Furthermore, what is needed is a method of making manufacturing a bleaching clay product that has enhanced surface acidity that provides product efficacy for purification, e.g., bleaching, of an oil. Summary of the Invention A method for making a bleaching clay product from a palygorskite - smectite type clay mineral by controlled acidification with sulfuric acid is provided. The present method provides optimal acid activation of the clay mineral without substantial deterioration of the clay mineral structure.

The present method comprises the step of combining a palygorskite - smectite clay with sulfuric acid while maintaining a substantially constant ratio of surface acidity to added acidity for the clay mineral . A preferred starting material is a palygorskite - smectite clay having a basic pH value in aqueous suspension. Sulfuric acid is added to the clay mineral in an amount such that the produced bleaching clay product has no more than about 1.5 milliequivalents of acidity per gram.

Brief Description of the Drawings In the drawings,

FIGURE 1 is a graphical representation of the data of Examples 1 and 2, showing the relationship between added acidity and surface acidity of a slurry

of palygorskite - smectite clay mineral from sources A (open circles) and B (filled squares) ;

FIGURE 2 is a graphical representation of the data from Example 1 showing the relationships of added acidity to surface acidity (open squares) , total acidity (filled circles) , pH of the suspension (filled triangles) and the chlorophyll remaining in bleached oil after treatment (open circles) , respectively; and

FIGURE 3 is a graphical representation of the data from Example 2 showing the relationships of added acidity to surface acidity (open squares) , total acidity (filled circles) , pH of the suspension (filled triangles) and the chlorophyll remaining in bleached oil after treatment (open circles) , respectively. Description of the Embodiments

While this invention is susceptible to embodiments in many different forms, preferred embodiments of the invention are described below. It should be understood, however, that the present disclosure is to be considered as a exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.

Throughout this application, the term "oil," and the grammatical variations thereof, includes vegetable-derived, animal-derived as well as marine source-derived fats and fatty oils that are liquids at the particular temperature that is necessary for desired processing of a particular type of oil. In use, the oil and the present oil bleaching composition are combined in a suitable vessel to produce a slurry. The resulting slurry is maintained at an elevated temperature and at a pressure no greater than about atmospheric pressure, preferably less than atmospheric, for a time period sufficient to reduce the

amount of color impurities of the oil without causing degradation of the oil, i.e., the oil is bleached substantially without thermal decomposition of the oil. A bleached oil is then recovered from the slurry. The bleaching is performed at a temperature elevated above room temperature, i.e., at about 30 degrees Celsius and higher, but below the temperature that induces thermal decomposition of the oil . A preferred bleaching temperature is in a range of about 50 degrees to about 130 degrees Celsius, more preferably about 60 degrees Celsius to about 125 degrees Celsius.

The pressure at which the bleaching is performed can be atmospheric or less than atmospheric (subatmospheric) , as desired. A suitable reduced pressure is in a range of about 1 to about 26 inches of mercury (3.4 kPa to about 88 kPa) . A preferred reduced pressure is in a range of about 1 to about 5 inches of mercury (3.4 kPa to about 16.9 kPa) . A most preferred reduced pressure is about 4 inches of mercury (13.5 kPa) .

The contact time period sufficient to reduce the amount of color impurities in the oil utilizing the present bleaching compositions usually is in a range of about 5 to about 90 minutes. Oils that can be bleached using the composition of the present invention include both edible and inedible oils. Illustrative oils are those previously mentioned hereinabove.

The term "clay" refers to a naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired. Although clay usually contains phyllosilicates, it also may contain other materials that impart plasticity and harden when dried or fired. Associated phases in clay

may include materials that do not impart plasticity and organic matter.

The term "clay mineral" refers to phyllosilicate minerals and to minerals which impart plasticity and which harden upon drying or firing. See generally, Guggenheim, S. & Martin, R.T., "Definition of Clay and Clay Mineral: Joint Report of the AIPEA Nomenclature and CMS Nomenclature Committees," Clays and Clav Minerals 43 : 255-256 (1995) . Materials suitable for the activating treatment of the present invention are clay minerals, preferably having a free moisture content of no more than about 50 weight percent, and preferably having a basic pH value in aqueous suspension. These clay minerals are characterized by a mineral structure formed by the arrangement of octahedral units and tetrahedral units or by stacked layers formed by an octahedral sheet and one or more tetrahedral sheets of the atoms that constitute the clay mineral structure. Illustrative are the two groups of naturally occurring clay minerals. First is the hormite group, defined here as including palygorskite and sepiolite, which have channels formed by octahedral units and tetrahedral units of the clay mineral structure. Second is the smectite group including montmorillonites and saponite, which are constituted by stacked layers formed by an octahedral sheet and more than one tetrahedral sheet, and mixtures of the foregoing. Palygorskite (attapulgite) , a mineral found in some clay deposits, is a hydrous silicate mineral represented by the ideal formula:

( 0H ₂ ) ₄ (OH) ₂ Mg ₅ S i ₈ O ₂₀ * 4H ₂ 0

See, e.g., Grim, R.E., Clay Mineralogy, 2nd ed. , McGraw-Hill, Inc., New York, N.Y. (1968) , p. 115. Other members of the hormite group of minerals, such as sepiolite, can be associated with palygorskite. Smectite is a generic term that refers to a variety of related minerals also found in some clay deposits. The smectite minerals typically occur only as extremely small particles. Smectite is composed of units made of two silica tetrahedral sheets with a central alumina octahedral sheet. Each of the tetrahedra has a tip that points to the center of the smectite unit. The tetrahedral and octahedral sheets are combined so that the tips of the tetrahedra of each silica sheet and one of the hydroxyl layers of the octahedral sheet form a common layer. See Id.. pp. 77- 78. Smectite minerals exhibit intracrystalline swelling.

In particular, the smectite family of clay minerals includes the various mineral species montmorillonite, beidellite, nontronite, hectorite and saponite, all of which can be present in the clay mineral in varying amounts.

Other minerals, neither of the smectite genus nor of the hormite variety, that can be present in minor amounts in the clay earth materials suitable for the present purpose include opal, apatite, calcite, the feldspars, amorphous silica (diatoms) , kaolinite, mica, quartz, pyrite and gypsum, among others.

The analysis of the relationship between added acidity and acidity found in solution has been found to be useful to minimize the likelihood of over - acidification and attendant deterioration of the clay mineral structure. Thus, the present invention relates to the process of optimizing the acidification treatment for producing bleaching clay products.

Optimized acidification of a palygorskite - smectite clay mineral is depicted graphically in FIGURE 1 where the ordinate is surface acidity (SA) for a clay mineral, expressed as milliequivalents of acidity per gram and the abscissa is the amount of added acidity (A) , also expressed as milliequivalents of acidity per gram.

Acidification of the palygorskite - smectite clay mineral is effected by adding sulfuric acid to the clay mineral in an amount not to exceed about 1.5 milliequivalents of acidity per gram of produced bleaching clay product . Preferably the amount of sulfuric acid added is about 0.5 to about 1.5 milliequivalents of acidity per gram of bleaching clay product (measured on a dry basis) , more preferably about 0.75 to about 1.25 milliequivalents of acidity per gram of bleaching clay product. Sulfuric acid may be added as an aqueous solution or as concentrated sulfuric acid. The term "sulfuric acid" refers herein to either an aqueous solution of sulfuric acid or concentrated sulfuric acid.

The addition of sulfuric acid to the palygorskite - smectite clay mineral can be achieved in any convenient manner. For example, sulfuric acid may be added by spraying sulfuric acid onto an agitated particulate clay mineral bed, by adding aqueous sulfuric acid to a particulate clay mineral as it is passed through a pug mill, and the like.

The resulting bleaching clay product is not washed with water, but instead is dried and comminuted to a desired particle size.

The addition of sulfuric acid is effected so as to maintain a substantially constant ratio of surface acidity to added acidity for the produced bleaching clay product. Surface acidity is calculated as the

difference between total acidity (acidity of a clay suspension) and solution acidity (acidity of a water extract of the clay suspension) .

The addition of sulfuric acid so as to maintain a substantially constant ratio of surface acidity to added acidity is illustrated in FIGURE 1, where the slope of the depicted curve is substantially constant until the added acidity exceeds about 1 milliequivalent per gram of bleaching clay product. Further addition of sulfuric acid does not result in an increase of surface acidity, the desired acidity for bleaching, but instead increases the likelihood of structural deterioration of the clay mineral structure. For bleaching an oil, the amount of the present bleaching clay that is combined with the oil to be bleached usually is in the range of about 1 to about 5 weight percent, based on the weight of the oil. Lesser or greater amounts can be used, depending upon the oil to be treated and the severity of the desired bleaching treatment.

The present invention is illustrated by the following examples.

EXAMPLE 1: Treatment of Clay- Raw palygorskite - smectite clay mineral mixture was obtained from Source A. Each raw clay sample was ground to less than 3 mm in largest dimension by a laboratory hammer mill . Acidity was added in the form of sulfuric acid (H ₂ S0 ₄ ) in the amount of about 0.25 to about 10 weight percent based on the dry weight of the clay mineral. Added acidity, total acidity and solution acidity were noted, and were expressed as milliequivalents of acidity per gram of bleaching clay product . The pH of a 1% slurry of the clay mineral in water was measured. Acidity found on the surface (surface acidity) was calculated as total acidity in

suspension less solution acidity. Slight differences in the values for surface acidity found in Tables 1 and 2 are due to rounding.

After the acid treatment was applied to the samples, they were dried to about 10 weight percent free moisture and ground to 90-92% to -200 mesh and 69-71 % to -325 mesh (U.S. Sieve Series) . These finely ground samples were used for the various analyses and experiments reported below. A sample of 0.200 ± 0.005 g air dried bleaching clay product was suspended to 20 ml distilled water to form a 1 % suspension and equilibrated for 20 minutes before the measurement of suspension pH. A 20 ml sample of the water extract from a 1% clay suspension was used for the measurement of solution pH. The suspension and the solution aliquots were placed in a temperature controlled water bath maintained at 25 ± 0.1 degrees Celsius and titrated with calibrated 0.01N NaOH and 0. IN NaOH solutions using a Dosimat titrator. The base addition rate was in the range of 0.1-0.6 ml/min and 0.01-0.2 ml/min for the 0.01N and 0. IN NaOH solutions, respectively. The rate was selected depending on the sample acidity, and was kept constant throughout each titration. The 0.IN NaOH solution was used for H ₂ S0 ₄ added at 7.5 and 10 weight percent. When using the slowest rate to titrate high-acidity clay, the titration took about 10 hours, compared to the usual duration of 15-25 minutes. At least two replicate titrations were conducted for each clay. The experimental error was generally less than 5% for titration volumes larger than 0.3 ml base.

A combined (glass + calomel) pH electrode and a pH meter were used for pH measurements. Titration curves were recorded using a computerized system. The system was calibrated using standard buffers at pH 4.0

and pH 7.0. The milliequivalents NaOH added were converted to milliequivalents acidity per gram of dry clay.

The specific surface area was measured by the ethylene glycol monoethyl ether (EGME) method. See generally, Bradley, et al . , Clays and Clay Minerals 10: 117-122 (1961) ; Brindley, G.W. and Hoffmann, R.W. , Clavs and Clay Minerals 9: 546-556 (1960); Carter, et al. , Soil Sci. 100: 356-360 (1965) ; Ratner-Zohar, et al . , Soil Science Society of America Journal 47: 1056-1057 (1983) .

Each sample was tested for bleaching activity with canola oil. Canola oil (100 g) containing chlorophyll (26,000 ppb) was combined with 1.5 weight percent of clay and bleached at 108 degrees Celsius for 30 minutes under partial vacuum of 26 inches of mercury (88 kPa; 0.87 atmosphere) . The remaining chlorophyll values in the bleached oils were measured as absorption of chlorophyll pigments determined with a Beckman spectrophotometer.

The results are shown in Table 1 and FIGURES 1 and 2. In FIGURE 1, the open circles indicate the acid found (surface acidity) compared to the acid added, expressed as milliequivalents of sulfur per gram of clay. The acid found was noted to rise as acidity added increased.

The amount of chlorophyll remaining reached a minimum at about 0.7 to about 0.8 milliequivalents of acidity per gram of clay product of added acidity (Table 1) . This range corresponded to about 3.5 to about 4.5 weight percent of added sulfuric acid, based on weight of clay.

FIGURE 2 compares the relationships of added acidity to surface acidity (open squares) , total acidity (filled circles) , initial pH of the suspension (filled

triangles) and the chlorophyll remaining in bleached oil (open circles) . The amount of chlorophyll remaining was optimally reduced in the range of about 0.5 to about 1.5 milliequivalents per gram acidity added. More preferably acidity is added in the range from about 0.75 to 1.25 milliequivalents per gram.

The results of Table 1 also show a decrease in specific surface area due to an attack by excess acid on the crystal structure of the clay minerals.

Table 1

H,SO, Added Added Moisture Specific Surface Initial pH, Initial pH, Total Solution Surface Remaining wt%dry Acidity Content, wt % Area (m'/g EGME) Suspension* Solution' Acidity Acidity Acidity Chlorophyll' ppb basis (meq/g) (meq/g) (meq/g) (meq/g)

0 0013 1244 512 767 877 0 0 0 4075

0 0014 942 495 723 722 0 0 0 4596

025 0043 1065 496 66 665 0048 0011 0037 4400

025 008 103 496 636 636 0058 eon 0047 3546

05 01 1249 476 619 648 0064 0014 0050 3245

OS 011 1139 494 607 6398 0078 0011 0067 3344

075 016 83 461 559 678 0112 0011 0102 2768

075 0157 872 472 556 67 0119 0016 0103 2660

1 0175 1782 491 522 602 0143 0024 0120 2800

1 021 1122 477 48 609 0177 0024 0153 2420

35 0711 1221 465 376 392 0563 0124 0439 1782

35 0706 1042 462 368 378 0594 0142 0452

45 0829 1121 429 352 379 0739 0204 0535 1663

45 0862 1149 451 355 395 0569 0167 0402 1845

55 0914 1209 424 338 347 0904 0358 0546 2022

55 1018 1154 463 338 341 0954 0379 0574 1839

75 136 1042 391 297 308 1295 0724 0571 2323

75 1409 1104 360 302 306 1404 0729 0675 2840

10 1781 115 322 285 292 1678 1067 0612 3312

10 1847 1102 288 290 289 1714 1117 0597 3759

' 1 weight percent slurry

¹ Starting conditions 26000 ppb chlorophyll, 15 weigh! percent clay

EXAMPLE 2: Clay Source B

Raw palygorskite - smectite clay material from Source B was prepared as described in Example 1. Added acidity, acidity found on the surface and the total acidity were noted and expressed as milliequivalents of sulfur per gram of clay. The pH of a 1% slurry of clay in water was measured. Each sample was tested for bleaching activity with canola oil as described in Example 1. The results are presented in FIGURES 1 and 3 , as well as in Table 2.

In FIGURE 1, the filled squares indicate the acid found (surface acidity) compared to the acid added, expressed as milliequivalents of sulfur per gram of clay. The acid found was noted to increase as acidity added increased. FIGURE 3 compares the relationships of added acidity to surface acidity (open squares) , total acidity (filled circles) , initial pH of the suspension (filled triangles) and the chlorophyll remaining in bleached oil (open circles) . As found in Example 1, the amount of chlorophyll remaining is reduced in the range of about 0.5 to about 1.5 milliequivalents per gram acidity added. The amount of chlorophyll remaining reached a minimum at about 0.6 milliequivalents of added acidity per gram of clay product of added acidity (see Table 2) . This range corresponded to from about 0.75 to about 7.5 weight percent of added sulfuric acid, expressed as percent of weight of clay.

Table 2

H,Sθ, Added Specific Initial Total Solution Surface Remaining

Added Acidity Surface PH. Acidity Acidity Acidity Chlorophyll' wt % dry (meq/g) Area (m'/g Suspension' (meq/g) (meq/g) (meq/g) ppb basis EGME)

0 0.025 556 7.22 0 0 0.000 4611

0.5 0.092 556 5.60 0.020 0.001 0.019 3455

0.75 0.150 650 4.95 0.092 0.000 0 092 2938

1 0.192 638 4.49 0.140 0.000 0.140 3462

2 0.357 620 4.01 0.293 0.053 0.240 2340

3 0.562 500 3.58 0.486 0.139 0.347 2281

4 0.733 581 3.47 0.709 0.250 0.458 2667

5 1.006 496 3.24 0.936 0.464 0.471 3590

7.5 1.276 365 3.09 1.266 0.700 0.566 3563

10 1.646 286 3.04 1.696 1.132 0.565 4534

15 2.380 176 2.89 2.488 1.881 0.607 6977

¹ 1 weight percent slurry Starting conditions 26000 ppb chlorophyll, 1 5 weight percent clay

Table 2 also indicates that excessive addition of sulfuric acid has an adverse effect on chlorophyll removal.

This invention has been described in terms of specific embodiments set forth in detail. It should be understood, however, that these embodiments are presented by way of illustration only, and that the invention is not necessarily limited thereto. Modifications and variations within the spirit and scope of the claims that follow will be readily apparent from this disclosure, as those skilled in the art will appreciate.