Abstract:
Disclosed is a method for producing hydrotalcite in high yield including reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions. The method further includes a first step of heating magnesium carbonate or magnesium hydroxide to a temperature between about 500°-900° C. to form activated magnesia or magnesium oxide. The method is suited to producing synthetic hydrotalcite from industrial Bayer liquor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 595,374, filed Mar. 30, 1984, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for producing synthetic hydrotalcite. 
     Hydrotalcite is a naturally occurring mineral having the formula 6 MgO.Al 2  O 3 .CO 2 .12 H 2  O or Mg 6  Al 2  (OH) 16  CO 3 .4 H 2  O. Known deposits of natural hydrotalcite are very limited and total only about 2,000 or 3,000 tons in the whole world. Natural hydrotalcite has been found in Snarum, Norway and in the Ural Mountains. Typical occurrences are in the form of serpentines, in talc schists, and as an alteration product of spinel where, in some cases, hydrotalcite has formed as pseudomorphs after spinel. 
     The upper stability temperature of hydrotalcite is lower than the lower limit for spinel. Spinel and hydrotalcite theoretically never would appear together in stable condition. If equilibrium has been established, the spinel would be completely changed to hydrotalcite. However, naturally occurring hydrotalcite is intermeshed with spinel and other materials. 
     Natural hydrotalcite is not present as pure product and always contains other minerals such as penninite and muscovite and potentially undesirable minerals such as heavy metals. Conventional practice recognizes that it is practically impossible to remove such impurities from a natural hydrotalcite. 
     Previous attempts to synthesize hydrotalcite have included adding dry ice or ammonium carbonate (a) to a mixture of magnesium oxide and alpha-alumina or (b) to a thermal decomposition product from a mixture of magnesium nitrate and aluminum nitrate and thereafter maintaining the system at temperatures below 325° C. at elevated pressures of 2,000-20,000 psi. Such a process is not practical for industrial scale production of synthetic hydrotalcite by reason of the high pressures. Furthermore, the high pressure process forms substances other than hydrotalcite, such as brucite, boehmite, diaspore, and hydromagnesite. 
     Ross and Kodama have reported a synthetic mineral prepared by titrating a mixed solution of MgCl 2  and AlCl 3  with NaOH in a CO 2  free system and then dialyzing the suspension for 30 days at 60° C. to form a hydrated Mg-Al carbonate hydroxide. The mineral product has been associated with the formula Mg 6  Al 2  CO 3  (OH) 16 .4 H 2  O while having the properties of manasseite and hydrotalcite. X-ray diffraction powder patterns have indicated that the mineral more closely resembles manasseite than hydrotalcite, while the differential thermal analysis curve of the precipitate has been characterized as similar to that given for hydrotalcite. 
     Kerchle, U.S. Pat. No. 4,458,026, discloses a preparation of Mg/Al/carbonate hydrotalcite which involves the addition of mixed magnesium/aluminum nitrates, sulphates or chlorides as an aqueous solution to a solution of a stoichiometric amount of sodium hydroxide and carbonate at about 25°-35° C. with stirring over a several-hour period producing a slurry. The slurry is then heated for about 18 hours at about 50°-200° C. (preferably 60°-75° C.) to allow a limited amount of crystallization to take place. After filtering the solids, and washing and drying, the dry solids are recovered. 
     Kumura et al. U.S. 3,650,704, reports a synthetic hydrotalcite preparation by adding an aqueous solution of aluminum sulfate and sodium carbonate to a suspension of magnesium hydroxide. The suspension then can be washed with water until the presence of sulfate radical becomes no longer observable. The suspension is heated at 85° C. for three hours and dried. The magnesium component starting material is reported as any member of the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, and water-soluble magnesium salts, e.g., such as mineral acid salts including magnesium chloride, magnesium nitrate, magnesium sulfate, magnesium dicarbonate, and bittern. 
     It is an object of the present invention to produce synthetic hydrotalcite in high purity. 
     It is another object of this invention to produce hydrotalcite in high yield at atmospheric pressure. 
     SUMMARY OF THE INVENTION 
     The present invention includes a method for producing hydrotalcite including reacting an activated magnesia with an aqueous solution of aluminate, carbonate, and hydroxyl ions. The method can be carried out at atmospheric pressure to form hydrotalcite in high purity and high yield. Activated magnesia is formed by heating a magnesium compound such as magnesium carbonate or magnesium hydroxide to a temperature between about 500°-900° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical depiction of a powder X-ray diffraction pattern obtained from synthetic hydrotalcite produced by the method of the present invention. 
     FIG. 2 is a graphical depiction of the differential thermal analysis of synthetic hydrotalcite obtained by the method of the present invention. 
     FIG. 3 is a photographic representation of synthetic hydrotalcite obtained by the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention produces synthetic hydrotalcite by reacting activated magnesia with an aqueous solution of aluminate, carbonate, and hydroxyl ions. The magnesia must be activated to produce hydrotalcite in high purity. Otherwise, i.e., in the event that an unactivated magnesia is used, the resulting product will include substantial amounts of mineral forms other than hydrotalcite. 
     The activated magnesia can be formed by activating magnesium compounds such as magnesium carbonate or magnesium hydroxide at temperatures of between about 500°-900° C. Below 500° C, the magnesium salt will not activate sufficiently and will contain inhibiting amounts of the starting material. Above 900° C, the resulting magnesium oxide takes on a form which is insufficiently active. The insufficiently active magnesia could be characterized as dead burnt. Such a form of magnesia will not form hydrotalcite predominantly over other mineral forms. The insufficiently active form of magnesia which is nonspecific to forming hydrotalcite will be avoided by heating the magnesium salt starting materials to elevated activating temperatures, but which must not exceed about 900° C., to form the activated magnesia or magnesium oxide (MgO). 
     The activated magnesium oxide is added to a solution containing ions of aluminate, carbonate, and hydroxl. Preferably the activated magnesium oxide is added to an aqueous solution having a pH above about 13. For example, a suitable solution may contain alkali hydroxide, alkali carbonate, and aluminum oxide. Industrial Bayer process liquor used for the production of alumina from bauxite is a suitable solution containing sodium hydroxide, sodium carbonate, and aluminate ions. A Bayer process liquor containing excess alumina also is suitable. 
     By way of example, 5-25 grams per liter of activated MgO can be added to 120-250 g/1 NaOH (expressed as Na2C03) &#39; 20-100 g/1 Na 2  CO 3 , and 50-150 g/l Al 2  O 3  in an aqueous solution. The mixture should be agitated at a temperature of about 80°-100° C. for 20-120 minutes. 
     It has been found that magnesium compounds other than the activated magnesia of the present invention produce less than desirable results. For example, MgSO 4 , MgCl 2 , or MgNO 3  added to Bayer liquor yields Mg(OH) 2  and Al(OH) 3 . Similarly, Mg(OH) 2  added to Bayer liquor remains mostly unreacted. 
     The process of the present invention produces hydrotalcite in high yield. By high yield is meant a conversion yield greater than about 75% and preferably greater than about 90%. 
     The mineral produced by the method of the present invention can be analyzed by powder X-ray diffraction. The product formed by Example 2 of this specification was analyzed in powder form in a Siemens X-ray diffractometer having Model No. D-500 supplied by Siemens AG (W. Germany). The resulting X-ray diffraction pattern is depicted in FIG. 1. The diffraction pattern indicates that the product is hydrotalcite at high purity. The dÅ spacing obtained by X-ray diffraction is shown in Table I for the mineral obtained from the method of Example 2 and is compared to (1) the ASTM standard for hydrotalcite and (2) natural hydrotalcite as reported by Roy et al. American Journal of Science, Vol. 251, at page 353. By these indications, the process of the present invention produces hydrotalcite in high purity. 
     High purity in the context of the present invention is established by the absence of diffraction lines attributable to compounds other than hydrotalcite. The absence of diffraction lines indicates that such other compounds are not present in any significant amount. By way of contrasting example, the material produced in Example 1 described hereinbelow using a non-activated magnesium oxide contains lines or peaks indicating the presence of compounds other than hydrotalcite. These lines are observed in the data in Table I for the dÅ spacing of the product from Example 1. 
     
                                           TABLE I__________________________________________________________________________X-RAY DIFFRACTION    Natural HydrotalciteASTM (22-700)    (Snarum, Norway)               Example 1 Example 2dÅ    I/I Max.    dÅ         I/I Max.               dÅ                    I/I Max.                         dÅ                             I/I Max.__________________________________________________________________________7.84    100  7.63 100   12.4676                    4.3  8.8729                             3.73.90    60   3.82 50    12.3128                    4.8  7.7348                             99.22.60    40   2.56 10    12.1094                    4.2  7.6746                             100.02.33    25   2.283         5     11.8579                    5.5  6.0944                             5.01.990    30   1.941         10    11.5907                    4.2  6.0194                             4.71.950    6    1.524         5     11.3070                    4.7  5.9257                             5.91.541    35   1.495         5     11.1268                    4.2  4.0786                             8.61.498    25              10.9421                    4.2  3.9498                             30.01.419    8               10.5889                    4.1  3.8387                             60.91.302    6               4.7678                    45.7 3.8192                             64.51.265    10              4.6131                    6.9  2.6644                             4.01.172    2               4.5742                    6.0  2.5765                             80.10.994    4               4.5429                    3.9  2.5204                             25.20.976    6               4.5093                    5.3  2.5102                             21.7               4.4645                    4.9  2.4960                             14.9               4.4154                    3.3  2.4840                             13.0               4.3161                    3.3  2.4643                             10.8               4.2944                    3.0  2.4526                             11.4               4.2552                    3.2  2.4364                             10.0               4.2163                    5.9  2.0677                             3.7               4.1814                    5.4  2.0530                             5.7               4.1349                    7.4  2.0477                             3.3               4.1009                    6.9  2.0467                             3.9               4.0676                    9.7  2.0401                             4.9               3.9759                    13.9 2.0318                             7.4               2.7284                    5.4  2.0221                             6.7               2.6458                    4.1  2.0191                             6.6               2.5774                    30.4 2.0041                             12.4               2.4920                    7.3  1.9976                             10.3               2.4800                    6.6  1.5239                             38.8               2.4660                    8.0  1.5115                             18.4               2.4372                    19.9 1.4963                             34.1               2.3703                    100.0                         1.3209                             2.0               2.3191                    15.5 1.3180                             2.8               2.2869                    17.1 1.3161                             4.1               1.9616                    5.2  1.3114                             4.1               1.9465                    9.7  1.3099                             3.3               1.9372                    8.3  1.2771                             4.1               1.9302                    8.2  1.2722                             5.2               1.9244                    7.3  1.2692                             4.3               1.8194                    5.0  1.2689                             5.6               1.7953                    27.1 1.2662                             6.8               1.5740                    29.2 1.2632                             4.1               1.5614                    3.0               1.5557                    4.2               1.5347                    4.7               1.5225                    18.2               1.5102                    7.9               1.4918                    87.7               1.3745                    4.9               1.3719                    5.2               1.3692                    3.0               1.3176                    2.2               1.3121                    7.8               1.3089                    8.4__________________________________________________________________________ 
    
     The product of Example 2 was analyzed by differential thermal analysis (DTA). FIG. 2 presents a graphical illustration of the DTA for the product of Example 2 which represents hydrotalcite in a high purity. 
     The synthetic hydrotalcite produced by the present invention is a highly porous mineral. A photograph by scanning electron micrograph was taken of the product of the process carried out in Example 2 and is presented as FIG. 3. The photograph illustrates the mineral product at a 5,000X magnification. The mineral can be seen to have a high surface area and high porosity. 
     Synthetic hydrotalcite produced by the process of the present invention has utility in one aspect in purification applications such as a filter aid. The synthetic hydrotalcite is adaptable in other aspects as a fire retardant material which releases water and CO 2  on heating. Other applications include a filler material for paper or as a drying, bleaching, or absorbent material after activation by heating to over about 500° C. Synthetic hydrotalcite produced by the process of the present invention also is useful in purification and catalytic applications by virtue of an anion exchange capability wherein carbonate anion can be replaced with other anions without destroying the structure of the compound . 
     EXAMPLE 1 
     Magnesium carbonate in an amount of 25 grams was heated to about 1,100° C. for about 45 minutes and allowed to cool. The resulting magnesium oxide was added to a Bayer liquor prepared by digesting Suriname bauxite in a ratio of about 0.65 (defined as Al 2  O 3  /caustic expressed as Na 2  CO 3 , as used in industrial practice) at blow off and then filtered. One liter of Bayer liquor was heated to about 95° C. Ten grams of the magnesium compound treated at 1,100° C. were added. The mixture was agitated for one-half hour and then filtered. The residue was washed and dried at 105° C. overnight. 
     The resulting product weighed about 16.7 grams which indicates a yield of less than 67%. The product of this Example 1 was analyzed by powder X-ray diffraction and was found to contain predominant amounts of Mg(OH)2 and MgO. 
     EXAMPLE 2 
     Activated magnesia was produced by heating 25 grams magnesium carbonate to about 600° C. for 45 minutes. The heating period of 45 minutes was selected to facilitate complete activation. For varying amounts and temperatures, the heating period should be adjusted to achieve an active product. Typical heating periods will range from about 30 to about 120 minutes. 
     Ten grams of the activated MgO were added to one liter of the same Bayer liquor used in Example 1. The mixture was heated to about 95° C. and agitated for about one-half hour. The mixture was filtered, and the residue was washed and dried at 105° C. overnight. The resulting precipitate had a white appearance, weighed about 22.5 grams, and had a refractive index of 1.50. The precipitate was a fine, free-flowing crystalline powder insoluble in water and organic solvents. 
     The precipitate was analyzed by powder X-ray diffraction and found to be hydrotalcite in high purity. 
     The 22.5 grams compares to a theoretical yield of 24.95 grams and indicates a high yield conversion of over 90%.