Calcining Method for Alumina

This invention relates to processes for the production of alumina. This invention further relates to processes for the production of low-density, high-porosity, high-surface area alumina. This invention further relates to an improvement in such processes whereby alumina having improved color characteristics and improved extrusion properties is obtained.

Numerous processes such as the water hydrolysis of aluminum alkoxide, the alum process and sodium aluminate processes are known for the production of alumina. Such alumina has a variety of uses such as for catalysts, catalyst supports and the like. In many of these applications the usefulness of the alumina is directly related to the pore volume, surface area and density. Generally, lighter alumina having low-bulk density, high-surface area and high-porosity is more desirable. Most processes produce alumina having loose bulk densities greater than about 35 lb/ft.sup.3, a pore volume of less than about 1 cubic centimeter per gram (cc/g) and surface areas lower than about 275 square meters per gram (m.sup.2 /g). Recently it has been discovered that alumina slurries produced by the water hydrolysis of aluminum alkoxide, may be contacted with an effective amount of an organic solvent to form a solvent-aqueous alumina mixture and azeotropically dried to produce an alumina having a loose bulk density from about 7.5 to about 25 lb/ft.sup.3, a surface area from about 275 to about 400 m.sup.2 /g and a pore volume of from about 1 to about 2.75 cc/g. Such alumina is very desirable in catalytic applications and the like. The alumina so produced frequently has a substantial amount of residual carbon contamination as a result of the various production process steps and the like. Substantial amounts of the residual carbon material can be removed by steam contacting as shown in U.S. Pat. No. 3,773,691 entitled "Removal of Residual Carbon Contaminants from Alumina", issued Nov. 20, 1973 by Leach. Although substantial amounts of the residual carbon can be removed by the steam contacting operation trace amounts of the residual carbon apparently remain on the alumina. Upon calcining the remaining residual carbon causes discoloration of the alumina product, results in the formation of odorous components associated with the alumina product and results in an alumina product having inferior extrudability. Accordingly, a method was sought whereby the discoloration, formation of odorous compounds and loss of extrudable properties could be prevented.

In a prior art search conducted on the concept of the present invention, the following references were discovered: U.S. Pat. No. 2,889,268 to Dinwiddie; U.S. Pat. No. 2,749,216 to Dinwiddie; U.S. Pat. No. 2,970,891 to Hinlicky; U.S. Pat. No. 3,394,990 to Weingaertner; U.S. Pat. No. 3,480,389 to Graulier; U.S. Pat. No. 2,903,418 to Kirschenbaum; U.S. Pat. No. 3,042,628 to Cramer; U.S. Pat. No. 3,220,797 to Lester; U.S. Pat. No. 3,416,888 to Notari; U.S. Pat. No. 3,297,414 to Mazdiyasni. The references cited are illustrative of the state of the art.

It is an object of the present invention to provide an improved process for the production of high-porosity, high-surface area, low-bulk density alumina. It is a further objective of the present invention to provide an improved process for the production of high-surface area, high-porosity, low-bulk density alumina having improved color characteristics and improved extrusion properties.

The objectives of the present invention are accomplished in a process for producing low-density, high-porosity, high-surface area alumina from an aqueous alumina slurry produced by the water hydrolysis of aluminun alkoxides by contacting said aqueous alumina slurry with an effective amount of an organic solvent having a lower surface tension than water to form a solvent-aqueous alumina mixture; azeotropically drying said mixture and recovering said low-density, high-porosity, high-surface area alumina by an improvement comprising calcining said alumina in an inert atmosphere and recovering said low-density, high-porosity, high-surface area alumina having improved color properties.

Alumina is generally useful for a variety of catalytic and refractory applications. Low-bulk density, high-porosity, high-surface area alumina is especially useful in catalytic applications such as chemical process catalysts and catalyst supports, catalyst linings for mufflers and the like. Such alumina has been produced by a process comprising contacting aqueous alumina slurries produced by the water hydrolysis of aluminum alkoxides, with an effective amount of an organic solvent having a lower surface tension than water such as ethanol, propanol, isopropanol, butanol, isobutanol and tertiary butanol and the like to form a solvent-aqueous alumina mixture, azeotropically drying said mixture and recovering low-density, high-porosity, high-surface area alumina. Such processes frequently result in an alumina product having desirable properties but containing high levels of residual carbon contamination. Such a process is more fully shown in U.S. Ser. No. 191,085 filed Oct. 20, 1971, entitled "High-Porosity, High-Surface Area, Low-Bulk Density Alumina" by W. C. Ziegenhain now abandoned and U.S. Ser. No. 246,028 of the same title filed Apr. 20, 1972 by W. C. Ziegenhain. One method for reducing the residual carbon levels comprises contacting the alumina so produced with an effective amount of superheated steam as more fully described in U.S. Pat. No. 3,773,691 issued Nov. 20, 1973 entitled "Removal of Residual Carbon Contaminants from Alumina" by Leach.

Residual carbon as used herein includes carbonaceous compounds generally since such residual compounds are typically reported as residual carbon.

Alumina as used herein refers to an apparently dry material which may contain in addition to Al.sub.2 O.sub.3, associated water of hydration, free water and the like. The degree of drying is hereinafter designated by the weight percent Al.sub.2 O.sub.3.

The aqueous alumina slurry produced by the water hydrolysis of aluminum alkoxides is produced by a process comprising;

a. contacting aluminum alkoxides, such as those produced by the Ziegler process with an effective amount of water to produce an organic reaction product and an aqueous alumina slurry.

b. separating the aqueous alumina slurry from the organic reaction product which comprises primarily alcohols and any organic diluent which may have been present in the aluminum alkoxide charge to the reaction zone.

The separation of the organic reaction product and the aqueous alumina slurry can be achieved in a variety of ways such as by gravimetric separations, solvent extractions, steam stripping and the like. One such process comprises:

a. gravimetrically separating the organic reaction product and the aqueous alumina slurry;

b. thereafter contacting the aqueous alumina slurry with a suitable organic solvent such as butanol and thereafter gravimetrically separating the organic solvent from the aqueous alumina thereby reducing the quantity of organic reaction product remaining with the aqueous alumina; and,

c. thereafter steam stripping the aqueous alumina slurry to further reduce the quantity of alcohol remaining with the aqueous alumina slurry.

It should be noted that the steam contacting of the aqueous alumina slurry described above is not the same as the subsequent contacting of partially dried alumina with superheated steam; i.e., after forming the organic solvent-aqueous alumina mixture and azeotropically drying, the alumina is contacted with superheated steam prior to calcining.

It has now been found that when the starting aqueous alumina slurry is formed by the process described above it is desirable that air be present in the calcining operation, but that surprisingly and unexpectedly when no steam stripping is used in the preparation of the aqueous alumina slurry it is necessary that an inert atmosphere be used in the calcining operation. When air is used in calcining alumina produced from such non-steam stripped aqueous alumina slurries the alumina product is discolored, odorous and extrudes poorly. Early attempts to solve this problem included the use of additional amounts of air and oxygen-enriched air. Such attempts increased the discoloration, odor and difficulty in extrusion.

No completely satisfactory explanation for the desirable color, lack of odor and good extrudability obtained when an inert atmosphere is used is now known to applicants. While applicants do not wish to be bound by any particular theory it is hypothesized that the heavier alcohols are substantially removed from the aqueous alumina slurry during steam stripping and that the lighter organic solvents admixed with an aqueous alumina subsequently which remain with the alumina as charged to the calciner in trace amounts are readily oxidized or vaporized and thus removed by the calcining in air. By contrast when no steam stripping is used with the aqueous alumina slurry greater amounts of the heavier alcohols remain with the alumina as charged to the calciner. Such alcohols are not readily vaporized or oxidized to volatile components but rather form colored odorous compounds. Much higher temperatures are required to vaporize such materials and as shown hereinafter such temperatures are detrimental to the product alumina. It is further postulated that such alcohols while not subject to vaporization and oxidation are subject to thermal cracking on the alumina to volatile, colorless, odorless compounds thus explaining the surprising improvement achieved when an inert atmosphere is used. It is again noted that applicants do not wish to be bound by any particular theory and to date no satisfactory explanation for this highly desirable result has been documented experimentally. The discussion hereinafter is directed to aqueous alumina slurries produced as described above wherein no steam stripping was used.

Certain advantages are obtained by omitting the steam stripping step. For instance the alumina is "aged" at an accelerated rate when subjected to the higher temperatures involved in the steam stripping operation. Such "aging" is evidenced primarily by accelerated growth of the alumina crystallite size. "Aged" alumina slurries are also less suitable for subsequent contacting with solvent and azeotropic drying to produce high-pore volume, high-surface area, low-bulk density alumina. In particular slightly higher bulk densities are obtained, and slightly lower pore volumes and surface areas are obtained.

As noted hereinbefore, it has now been found that the discoloration, formation of odorous compounds and the like can be avoided by carrying out the drying operations in an inert atmosphere. The inert atmosphere may consist of any suitable gaseous compound which does not contain free oxygen. Some examples of such gaseous materials are nitrogen, argon, helium, hydrogen, carbon monoxide, superheated steam and mixtures of superheated steam and hydrocarbon compounds containing up to about 30 carbon atoms.

The drying prior to calcining can be carried out by several methods. The first and most obvious method is to dry the solvent-aqueous alumina mixture directly to the Al.sub.2 O.sub.3 concentration desired. In this method it will be found that a suitable inert atmosphere may be generated from the solvent-aqueous alumina mixture itself; i.e., upon heating, water and solvent vapors are evolved and unless removed serve as an inert atmosphere which prevents the formation of the discoloration and the like at higher calcining temperatures (i.e., in excess of 500.degree.F).

A second method involves drying the solvent-aqueous alumina mixture to form alumina containing from about 65 to about 75 weight percent Al.sub.2 O.sub.3 and thereafter calcining said alumina to form an alumina product having the desired Al.sub.2 O.sub.3 concentration. By this method the vapors generated during the first drying stage are suitable for solvent recovery and recycle. The alumina subjected to calcining still retains sufficient water and organic solvent to generate an inert atmosphere and thus prevent discoloration and the like during the calcining operation.

Both the above methods will be found more useful when ethanol, propanol, isopropanol and the like are used as an organic solvent. When butanol, isobutanol, tertiary butanol and the like are used the residual carbon levels are higher, although desirable alumina is obtained. A preferred method for use with the heavier solvents is set out below.

In a third method the solvent-aqueous alumina slurry is dried to form partially dried alumina containing from about 65 to about 75 weight percent Al.sub.2 O.sub.3 ; said partially dried alumina is then contacted with superheated steam to remove and recover hydrocarbons and further dried to about 81 weight percent alumina and thereafter calcined to from about 85 to about 95 weight percent alumina. It is necessary that an inert atmosphere be supplied by an inert gas purge and the like during the final calcining operation. Desirably the inert gas is selected from the group consisting of nitrogen, argon, helium, hydrogen and carbon dioxide. Of these nitrogen is preferred.

In a preferred embodiment the calcining is achieved by

a. heating the solvent-aqueous alumina mixture to about 300.degree.F for at least about one hour to obtain a partially dried product alumina containing from about 70 to about 76 weight percent Al.sub.2 O.sub.3 ;

b. heating to about 300.degree.F and simultaneously contacting the partially dried product with superheated steam for at least about 1 minute to obtain a dried alumina containing from about 79 to about 82 weight percent Al.sub.2 O.sub.3 ;

c. calcining the dried alumina to about 800.degree.F for at least about 4 minutes in the presence of an inert atmosphere to obtain a product alumina containing from about 86 to about 90 weight percent Al.sub.2 O.sub.3 ; and thereafter

d. cooling to at least about 300.degree.F in the presence of an inert atmosphere to obtain low-density, high-porosity, high surface area alumina containing from about 86 to about 90 weight percent Al.sub.2 O.sub.3 and having desirable color characteristics and desirable extrusion properties.

It has been found that when alumina is heated above 500.degree.F in the presence of free-oxygen containing gases with the residual carbon present, discoloration begins to occur. Accordingly, during calcining operations which involve temperatures higher than 500.degree.F, it is necessary that the inert atmosphere be supplied and maintained until the alumina is cooled to below about 300.degree.F and preferably below about 200.degree.F. By this technique it has been discovered that discoloration, formation of odorous compounds and the like is readily avoided.

The alumina may be extruded by any suitable method. Some suitable extrusion techniques are set forth more fully in U.S. Ser. No. 168,420, now abandoned, entitled "Low-Density, High-Porosity Alumina Extrudates and a Method for Producing Said Extrudates," filed Aug. 2, 1971, by Leach et al; U.S. Ser. No. 191,091, now abandoned, entitled "Low-Density, High-Porosity Alumina Extrudates and a Method for Producing Said Extrudates," filed Oct. 20, 1971, by Leach et al; and U.S. Ser. No. 268,246, a continuation-in-part based on the two cases noted hereinbefore of the same title filed July 3, 1972, by Leach et al. Especially desirable extrudates are obtained when the alumina starting material is dried or calcined to an Al.sub.2 O.sub.3 content from about 77 to about 97 weight percent Al.sub.2 O.sub.3. Such properties are lost to a significant degree when discoloration of the alumina and the formation of the odorous bodies occurs. Accordingly, a further advantage is realized in such processes by the improvement of the present invention in that desirable extrusion properties are retained.

An alumina product having reduced discoloration can be obtained by calcining to higher temperatures, i.e. in excess of 1200.degree.F, in the presence of oxygen, but as a result the alumina product is typically 100 weight percent Al.sub.2 O.sub.3 and as a result is less desirable for certain uses. In particular, the alumina marketed for use in the production of catalysts and the like is usually required to be in an uncalcined state, i.e. contain from about 65 to about 97 weight percent Al.sub.2 O.sub.3. The calcining step is then performed by the catalyst manufacturer by a method of his choosing in light of the desired catalyst properties and the like. The extrusion properties are degraded by severe calcining, i.e. above about 1000.degree.F for a substantial length of time, thus it is desirable that the discoloration and the like be avoided, as by the improvement of the present invention in a manner such that severe calcining is not required. Accordingly typical calcining temperatures are from 550.degree. to about 1200.degree.F, preferably from about 700.degree. to about 1000.degree.F.

Many variations and modifications are possible within the scope of the present invention and indeed such modifications and variations may appear obvious or desirable to those skilled in the art based upon the foregoing description of preferred embodiments and the appended examples and the claims.

500 grams of alumina having a pore volume of 1.99 cc/g, a surface area of 284.7 m.sup.2 /g and containing 94.6 weight percent Al.sub.2 O.sub.3 was prepared by the water hydrolysis of aluminum alkoxides produced by the Ziegler process, separation of the aqueous alumina slurry from the organic reaction products without steam stripping by admixing the aqueous alumina slurry with n-butanol to produce an n-butanol-aqueous alumina slurry, azeotropically drying the n-butanol-aqueous alumina slurry in a dryer having an overhead temperature of 313.degree.F and a product discharge temperature of 275.degree.F to produce alumina containing about 75 weight percent Al.sub.2 O.sub.3, contacting said alumina with superheated steam and thereafter calcining at 800.degree.F for about 5 minutes in a nitrogen atmosphere. The alumina after calcining was white and had no odor.

The alumina was then mixed with 500 ml of a solution of water containing 2 volume percent acetic acid. An additional 280 ml of water was added during the mixing step. The alumina, acid and water were intimately mixed to a uniform consistency in a blender manufactured by Baker Perkins, Inc., Saginaw, Michigan, and extruded into 0.125 inch diameter extrudates using a model 2 auger type extruder manufactured by the Bonnot Company, Kent, Ohio. The alumina extruded well, did not have a greasy or slick feeling and typically had a pore volume of about 1.1 to 1.3 cc/g and a surface area of 240-275 m.sup.2 /g after extrusion.

300 grams of alumina having a pore volume of 2.30 cc/g, a surface area of 303 m.sup.2 /g and an Al.sub.2 O.sub.3 content of 90-94 weight percent Al.sub.2 O.sub.3 was prepared in a manner similar to the process in Example 1 except that the alumina was calcined at 800.degree.F for about 4 minutes in an air atmosphere. The alumina after calcining was tan to brown in color and had a slight odor.

The alumina was extruded by a procedure similar to that in Example 1 except that 300 ml of 2 volume percent acetic acid and 200 ml additional water were used. The alumina was slick and did not extrude well. The extrudates shrank upon drying and exhibited a lower pore volume of 0.9 to 1.05 cc/g at least partially as a result of the shrinkage. The extrudate surface area was 280-290 m.sup.2 /g at least partially as a result of the residual carbon still present. In general the extrusion of the air-dried alumina was very difficult due to the slickness and shrinkage.

All extrudates in Examples 1 and 2 were dried after extrusion at 150.degree.F for 2 hours followed by calcining at 900.degree.F for 3 hours in order that the pore volumes and surface areas may be readily compared, i.e. at 100 weight percent Al.sub.2 O.sub.3.

The foregoing examples illustrate that improved color characteristics in the product alumina are realized by the process of the present invention and that improved extrudability is also achieved when alumina produced by the hydrolysis of aluminum alkoxides is used as the starting material.