Patent Publication Number: US-6036933-A

Title: Calcium carbonate precipitation method

Description:
FIELD OF THE INVENTION 
     This invention relates to methods for producing crystalline materials by precipitation. According to one aspect of the invention it relates to the production of precipitated calcium carbonate. The invention thus relates particularly, but not exclusively, to the production of high purity calcium carbonate from relatively impure calcium source material. 
     BACKGROUND TO THE INVENTION 
     Calcium carbonate is a natural carbonate which is present in large quantities particularly in calcitic and dolomitic limestone. In raw impure form these products are used in various industries including the mining and cement industries. In purified crystalline form calcium carbonate is used in the production of paint, in toothpaste, paper coating and sizing and as an acid neutraliser or as a filler in pharmaceutical products, amongst other applications. 
     Despite the natural abundance of calcium carbonate as such pure calcium carbonate is a relatively expensive product due to the difficulties and expenses associated with conventional purification methods. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to provide a new method for producing precipitated calcium carbonate from a natural source material containing calcium values. 
     EP-A-49666 [Kalk Chemische Fabrik] teaches the preparation of calcium carbonate [vaterite] by reaction of a calcium ion containing aqueous solution with carbonate ions containing aqueous solution. Preferred are the use of a solution containing calcium nitrate and a solution of ammonium carbonate. 
     Chemical Abstracts, AN: 116:258510H of CN-A-1055718 [Zou et al] discloses the preparation of calcium nitrate from calcined [dolomite] ores [i.e. lime] by impregnation of the calcination product with ammonium nitrate solution. The resultant calcium nitrate solution is used for the preparation of calcium carbonate by carbonisation with CO 2  -containing air. 
     WO-A-7900931 [Anderson] discloses a process for the preparation of white liquor for the sulphate pulping process, wherein quicklime is slaked with green liquor and the slaked lime formed is reacted with the sodium carbonate in the green liquor to sodium hydroxide in a causticizing process, the lime sludge formed being separated. According to that invention, the process from the slaking of the lime up to the separation of the white liquor and the lime sludge is carried out at a pressure above the atmospheric pressure. In a preferred embodiment, the process is also carried out at a temperature above the normal boiling temperature of the system. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method for producing precipitated calcium carbonate by reacting an aqueous solution of calcium nitrate [Ca(NO 3 ) 2  ] with an aqueous solution of ammonium carbonate [(NH 4 ) 2  CO 3  ] and allowing calcium carbonate to precipitate from the resultant mixture containing nitrate [NH 4  NO 3  ] in the mother liquor, the process being characterised in that 
     [i] the calcium nitrate [Ca(NO 3 ) 2  ] solution utilised in the processes is prepared by slaking lime [CaO] in water in the presence of ammonium nitrate [NH 4  NO 3  ] to form calcium nitrate [Ca(NO 3 ) 2  ] and ammonium hydroxide [NH 4  OH] in solution, filtering the solution to render it solids free, and heating the filtrate to dissociate the ammonium hydroxide [NH 4  OH] and to drive ammonia gas [NH 3  ] from the solution; 
     [ii] the ammonium carbonate (NH 4 ) 2  CO 3  solution utilised is prepared by absorbing ammonia gas [NH 3  ] and carbon dioxide gas in water, the ammonia gas preferably being derived from the step in [i] above in which the Ca(NO 3 ) 2  solution is heated; 
     [iii] the ammonium nitrate used is derived from the precipitation phase during which calcium carbonate is precipitated from the mother liquor containing ammonium nitrate. 
     The precipitation reaction between the two reagent solutions which react to form the precipitated CaCO 3  is preferably performed by feeding the solutions into intimate contact with one another in a contact zone of a reactor arrangement and immediately displacing the mixture from the contact zone, and allowing the resultant crystalline reaction product to precipitate from its mother liquor and separating the precipitated crystals from the mother liquor. 
     In a preferred form of the invention the reagent solutions are introduced into a reactor arrangement which is in the form of an elongated tubular conduit into which the reagent solutions are fed under a pressure of between 1 and 5 bar gauge pressure. It has been found that the application of feed pressure in this range results in a decrease in crystal particle size. 
     In one application of this aspect of the invention there is thus provided a method for selectively producing precipitated calcium carbonate crystals in which a preferred crystal size distribution predominates, the method comprising the steps of intimately contacting an aqueous solution of Ca(NO 3 ) 2  with an aqueous solution of (NH 4 ) 2  CO 3  whilst controlling the pressure of the reaction to remain within a range at which the formation of the required particle size and distribution of the CaO 3  crystals is favoured, allowing the resultant crystalline reaction product to precipitate from its mother liquor and separating the precipitated crystals from the mother liquor. 
     The method may be used to produce precipitated calcium carbonate which is predominantly of the vaterite crystalline form which has been found to be the default crystalline form when the temperature was kept between 15° C. and 75° C. The reaction is for this purpose most preferably carried out at between 34° C. and 40° C. 
     With residence in slurry form the vaterite crystal form has been found to convert into calcite and is known in the art. 
     According to a further aspect of the present invention there is provided a method for producing precipitated calcium carbonate crystals of a selected crystal size comprising the steps of reacting an aqueous solution of calcium nitrate with an aqueous solution of ammonium carbonate and controlling the crystal size of the precipitated calcium carbonate by providing at least one of the reagent solutions at a concentration favouring the formation of the desired crystal size, allowing the resultant crystalline reaction product to precipitate from its mother liquor and separating the precipitated crystals from the mother liquor. 
     Such concentration control for influencing crystal size may of course be combined with reaction pressure control as described above. 
     In one embodiment of this aspect of the invention it has been found that by reacting 20% [by mass] solution of Ca(NO 3 ) 2  with a stoichiometric equivalent quantity of (NH 4 ) 2  CO 3  it is possible to produce calcium carbonate crystals having a d 50  value of less than 6 microns, and that an increase in the concentration of the Ca(NO 3 ) 2  leads to the formation of finer crystals. 
     The various aspects of the invention will now be illustrated with reference to the accompanying examples: 
     EXAMPLES OF THE INVENTION 
     Example 1 
     Selective Preparation of Different CaCO 3  Crystalline Forms 
     [a] Preparation of Stock Solutions 
     50 Kg NH 4  NO 3  was dissolved in 120 liters water with stirring, to which was added 17.5 kg lime sourced from Lime Acres in South Africa. 100 ml of triethanolamine was added to the dissolved NH 4  NO 3  before the addition of the lime. The mixture was filtered to remove undissolved solids. The clear Ca(NO 3 ) 2  /NH 4  OH filtrate was transferred to a separate vessel and heated to a temperature of 80° C. and held at 80° C. for 30 minutes to drive off ammonia gas. [Care must be taken to ensure the pH of the Ca(NO 3 ) 2  solution remains above 9.5]. 
     A solution of approximately 10% (NH 4 ) 2  CO 3  was prepared in 100 liters of water by absorbing NH 3  [released by heating the filtered Ca(NO 3 ) 2  /NH 4  OH solution as described above] and pure CO 2  gas in the water in an absorbtion column. 
     The stock solutions of Ca(NO 3 ) 2  and (NH 4 ) 2  CO 3  as prepared above were used to demonstrate the invention. 
     It will be appreciated that the ammonium carbonate solution may in the alternative have been prepared by feeding ammonia gas from another source with CO 2  gas into a scrubber arrangement to absorb such gases in water thereby to render a solution of ammonium carbonate. 
     [b] Influence of Reaction Pressure 
     The stock solutions prepared as described above were fed by means of constant displacement pumps at various pressures into an elongated reaction chamber of about 5 m in length. It was found that the pressure in the reaction chamber, which was controlled by means of a valve at the outlet end thereof, had a marked influence on the size of the crystals of CaCO 3  formed by the reaction between the concentrated stock solutions. Reaction temperatures were measured by determining the temperature of the final reaction slurry at the end of the reaction chamber. The results of typical pressure and temperature influences on crystal size are summarised in Table 1, the crystal size values being the d 50  values in microns. 
     
                       TABLE 1                                                     
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Effect of Pressure:                                                       
                GAUGE PRESSURE [BAR]                                      
REACTION TEMPERATURE                                                      
                  0        3        5                                     
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17° C.     ND       2.44     1.86                                  
50° C.     7.27     2.38     2.38                                  
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     It will be seen that changes in reaction pressure affects particle size and distribution specifically, pressure above zero bar reduces particle size and narrows distribution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The tightening or narrowing effect of the present invention on particle size distribution is graphically illustrated in FIG. 1. 
     In FIG. 1 Graph A represents the particle size distribution and composition ranges for a commercially available ground calcium carbonate. 
     Graph B represents the same for a precipitated calcium carbonate produced by the method of the invention at 50° C. and 3 bar pressure. 
    
    
     It will be seen that the product of the method of the invention displays a sharper gaussian distribution of crystal sizes than the commercial product. 
     [c] Influence of Concentration of Feed Solutions 
     It has been found that increased concentration from 20% Ca(NO 3 ) 2  to 30% Ca(NO 3 ) 2  typically results in a decrease in mean particle size, e.g. d 50  6 μm to d 50  2 μm. 
     Example 2 
     The purity of the product produced by the method as described in Example 1[a] and [b] is extremely high and is in many respects comparable with the industry spectrographic standard for CaCO 3  marketed by Johnson Matthey. In Table 2 below the relative values for various impurities are compared against the Johnson Matthey standard [taking the quantities of impurities in such standard as being 1] and against two commercially available products identified as A and B. 
     
                       TABLE 2                                                     
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                                Product of                                
         JM  A           B      Invention                                 
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SiO.sub.2  1     2,8         2,7  0,61                                    
Al.sub.2 O.sub.3                                                          
           1     1,1         2,5  0,67                                    
Fe.sub.2 O.sub.3                                                          
           1     2,2         3,5  0,83                                    
Mn.sub.2 O.sub.3                                                          
           1      1,16        1,37                                        
                                  1,08                                    
MgO        1     19,8        17,7 1,03                                    
P.sub.2 O.sub.5                                                           
           1      1,15        0,47                                        
                                  1,09                                    
SO.sub.3   1      0,97        0,89                                        
                                  2,65                                    
Cl.sup.-   1      1,17        1,15                                        
                                  1,41                                    
K.sub.2 O  1      1,33        1,76                                        
                                  1,93                                    
Nm.sub.2 O 1      1,12        0,61                                        
                                  1,06                                    
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     From the above table it will be seen that the product of the invention contains only 61% of the quantity of SiO 2  present in spectrographic grade CaCO 3  while other products in the market contain almost three times such amounts. For many of the other impurities similar observations may be made. 
     If one bears in mind that the process by which the product of this purity is achieved starts from very impure CaO, this achievement is remarkable.