Abstract:
A process for converting the more easily synthesized and stored AMP hexahydrate into monohydrate of high purity. The resultant monohydrate (dittmarite) can then be either dried to stabilize it, or used directly in cigarette production such as paper making as filler or a filler component together with calcium carbonate.

Description:
This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Application No. 60/308,865 entitled PROCESS OF PRODUCING MAGNESIUM AMMONIUM PHOSPHATE IN MONOHYDRATE FORM (DITTMARITE) and filed on Aug. 1, 2001, the entire content of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods of producing magnesium ammonium phosphate in monohydrate form (dittmarite), and in particular to methods of converting a hexahydrate form of magnesium ammonium phosphate (struvite) into a monohydrate form of magnesium ammonium phosphate monohydrate (dittmarite). 
     BACKGROUND OF THE INVENTION 
     Ammonium magnesium phosphate (AMP) monohydrate has been discovered as useful in producing reduced harm smoking articles. Advantageous uses of AMP in smoking articles is described in commonly assigned U.S. patent application Ser. No. 09/399,159, which is incorporated herein by reference in its entirety. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for converting the more easily synthesized and stored AMP hexahydrate into monohydrate of high purity. The resultant monohydrate (dittmarite) can then be either dried to stabilize it, or used directly in cigarette production such as paper making as filler or a filler component together with calcium carbonate. 
     More particularly, a preferred embodiment of the present invention provides a process wherein AMP hexahydrate is heated under pressure to convert it to the monohydrate form. In a preferred process, an aqueous slurry of AMP hexahydrate is heated in a pressure vessel (e.g., autoclave) to 100° C. to 110° C. with agitation under pressures from 1 psig to 15 psig. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a layout of processing equipment for effecting the conversion process and subsequent use of the monohydrate form of magnesium ammonium phosphate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Table 1 sets forth parameters used in preparing high purity AMP monohydrate (dittmarite) by converting AMP hexahydrate (struvite) under elevated temperature and pressure. The test runs were carried out using an aqueous slurry having a solids content of 35 wt. % struvite in autoclave equipment available from Lee Industries in Philipsburg, Pa. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Temperature 
                 Time 
                 Pressure 
                 Volume 
               
               
                 Run 
                 ° C. 
                 Minutes 
                 Psig 
                 Gallons 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 100 
                 60 
                 8 
                 25 
               
               
                 2 
                 105 
                 35 to 200 
                 12 
                 50 
               
               
                 3 
                 105 
                 40 
                 15 
                 45 
               
               
                   
               
             
          
         
       
     
     Runs 2 and 3 exhibited higher purity (i.e., higher conversion rates) results than Run 1, indicating the reaction should preferably be run at 105° C. and 12 to 15 psig. The product of the reaction can be used as a slurry or the slurry can be dried to provide the dittmarite in particle form. In a paper making operation, the dittmarite can be supplied directly to a tank in which the dittmarite is mixed with one or more components of the paper making feedstock, preferably at a temperature and/or pressure sufficient to maintain stability of the reaction product. In a cigarette paper making process, the overall process steps can include: 
     
       
                 
         
             
             
         
      
     
     The pressure vessel has a jacket for heating and cooling, is rated for pressure operation, is of sanitary construction for easy clean up, and has at least one agitator, e.g. two agitators. One agitator scrapes the walls and creates a uniform temperature mixture, the second uses high shear to break up agglomerates and control particle size. This vessel is a preferred embodiment of the invention due to its high degree of temperature uniformity, and ability to control particle size, a critical parameter of paper making. Details of operating such a vessel can include: 
     
       
                 
         
             
             
         
      
     
     The conversion process is preferably run without intentional venting of pressure, or unintentional leaks, as undesirable side reactions may occur and the purity of the resultant product is reduced. Preferably, the slurry is contained in a sealed vessel during heating of the slurry. Preferably, the tank is in a filled condition during the conversion process. The conversion process is effective to product at least 95%, preferably at least 95% or 98% or higher conversion of struvite to dittmarite. It is recognized that the initial slurry may contain additional ingredients and/or impurities that may be present provided such ingredients/impurities do not adversely affect the conversion process. 
     In a preferred process, a slurry of up to 50%, preferably 20 to 40 weight % solids of hexahydrate form of magnesium ammonium phosphate is heated under pressure of up to 25 psig, preferably 10 to 15 psig, for time sufficient to convert at least 90%, preferably at least 95%, of the hexahydrate form of the magnesium ammonium phosphate to the monohydrate form thereof. The slurry can be heated to 90 to 135° C., preferably 100 to 110° C., for at least 5 minutes, preferably 5 to 25 minutes, to effect the conversion to monohydrate magnesium ammonium phosphate. The slurry can optionally be cooled but preferably not below a temperature at which the monohydrate magnesium ammonium phosphate will convert back to hexahydrate magnesium ammonium phosphate. Thus, the slurry of monohydrate magnesium ammonium phosphate is preferably maintained above 55° C., preferably above 60° C. until the slurry is further processed such as by incorporation into paper or by drying the slurry to obtain particles of magnesium ammonium phosphate.  FIG. 1  shows a layout of processing equipment suitable for effecting the conversion process and subsequent use of the dittmarite. 
     The monohydrate form of magnesium ammonium phosphate is desirable in paper making manufacture such as cigarette paper. The slurry of the monohydrate form of magnesium ammonium phosphate can be mixed with feedstock of a paper making machine or the slurry can be dried to particle form (e.g., powder) and such powder can be incorporated in the paper making feedstock. In order to prevent the magnesium ammonium phosphate in the monohydrate form from transforming back to the hexahydrate form, it is desirable to maintain the slurry above 55° C. until it is incorporated directly in feedstock (preferably heated above 60° C.) of the paper making machine or until the slurry is dried into particle form such as by flash drying which removes the water from the slurry under elevated temperature conditions. Once dry, the monohydrate form of the magnesium ammonium phosphate remains stable. 
     The struvite slurry hexahydrate magnesium ammonium phosphate can be obtained by any known processes of reacting magnesium hydroxide, ammonia, phosphoric acid and water. The present invention provides an advantageous and effective method of converting such slurry of hexahydrate form to the monohydrate form of magnesium ammonium phosphate usable to form filler for paper making wherein a particle size is preferably in the range of approximately 2 μm to 8 μm, more preferably in the range of 2 μm to 4 μm. If the slurry is dried to particle form, a preferred spin-flash drier can be obtained from APV Anhydro of Tonawanda, N.Y. 
     While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made, and equivalents employed, without departing from the scope of the appended claims.