Inorganic pigments and process for preparing same
BACKGROUND OF THE INVENTION 
     As is known, the most commonly used yellowish-green pigment is the green chromic oxide Cr 2  O 3 . Such pigment has the drawback of being toxic. 
     It is known too that it is possible to obtain from the titanium dioxide pigment of rutile structure (that it is white) pigments based on titanium compounds showing different colours by reacting said TiO 2  pigment with compounds of different elements such as, for example, Ni, Cr and Co. 
     For instance, a bluish-green pigment has been obtained by calcination of TiO 2  with a cobalt salt and, optionally an aluminium salt (in the latter case the Al.sub. 2 O 3  /Co molar ratio in the starting mixture shall be lower than 1). 
     OBJECTS OF THE INVENTION 
     Thus it is an object of the present invention to provide new yellowish-green pigments free from chrome. 
     Another object is to provide the above-said pigments in the form of titanium compounds. 
     A further object is to provide a process useful to obtain the above-said pigments. 
     GENERAL DESCRIPTION OF THE INVENTION 
     These and still other objects are achieved by coloured pigments based on titanium compounds, which pigments consist of the crystalline phases rutile TiO 2 , perovskite CaCO 3  and trigonal CoTiO 3  ; contain from 2 to 12% by weight of cobalt and from 4 to 20% by weight of calcium referred to the total TiO 2  (i.e. the TiO 2  present both in the form of TiO 2  and in the form of CaTiO 3  and CoTiO 3 ) and have a dominant wave length λ D  comprised between 530 and 560 mμ. 
     The pigments according to this invention may also contain aluminium; in such amount, that the molar ratio between Al.sub. 2 O 3  and Co is generally comprised between 0.01 and 0.20. Therefore, calculated in respect of the total TiO 2  of the pigments, the Al amount generally ranges from 0.018 to 2.197% by weight, depending on the Co amount contained in the pigment. 
     In fact it has been found, according to this invention, that while the pigments based on titanium compounds containing Co or Co and Al 1 (with Al.sub. 2 O 3  /Co molar ratio lower than 1) exhibit a bluish-green colour, the addition of Ca to the above-said systems permits one to obtain yellowish-green products, i.e., having a λ D  of 530-560 mμ. 
     The preponderant crystal phase of the pigments according to the present invention is that of rutile TiO 2 . Calcium titanate CaTiO 3  having the monoclinic crystal structure of perovskite and cobalt titanate CoTiO 3  having a trigonal crystal structure are present as well. The perovskite amount increases as the calcium amount in the pigment increases. Similarly, the CoTiO 3  phase increases as the cobalt amount increases. The pigments containing Al do not show any crystalline phase of an aluminium compound. This depends at least in part on the fact that the quantity of Al present is low. It is also supposed that at least a part of the Al has been entered into the crystalline lattice of the rutile TiO 2 , modifying said lattice, according to a known phenomenom, described, e.g., in the United States Patent 3.022.186. 
     The pigments of this invention exhibit a roundish shape and have a diameter generally comprised between 0.5 and 3.0μ. 
     The colour of the pigments has been defined by their dominant wave length λ D  (see A. G. Hardy: Handbook of Colorimetry; Massachusetts Institute of Technology; Cambridge, Massachusetts, 1936, in particular page 11). 
     The λ D  of the obtained products covers the entire range of the yellowish-green shades, namely from 530 to 560 mμ. 
     Thus it is possible to obtain also products having the same λ D  as the commercial green chromic oxides, i.e., a λ D  of about 554 mμ. 
     The cobalt amount ranges from 2 to 12% by weight in respect of TiO 2  (we always refer to the total TiO 2 , as defined previously). It is possible to use lesser amounts of cobalt, but in such case the resulting shades are too light. It is also possible to use higher amounts thereof, but it is preferable not to exceed 12% in order to limit the cost of the product. Cobalt amounts ranging from 5 to 12% are preferably employed. Under these conditions the products obtained exhibit a good colour strength comparable to that of the chromic oxide green pigments. 
     The calcium amount ranges from 4 to 20% by weight in respect to TiO 2 , the amount preferably used ranging from 8 to 20%. 
     In general, the aluminum amount is such as to obtain an Al.sub. 2 O 3  /Co molar ratio comprised between 0.01 and 0.20. 
     As it will appear later on, the presence of aluminum permits, among other things, to prepare the pigments at on the average lower temperatures, wherefore it is easier to control the pigments&#39; sizes. 
     The cobalt amount being the same, the increase of the aluminum amount, in the range from 0.01 to 0.20, causes the λ D  to decrease, shifting it towards bluish-green. 
     Aluminum amounts higher than a ratio of 0.20 are generally undesired, because they can lower the λ D  to values below 530 mμ. The preferred ratio is comprised between 0.10 and 0.20. 
     Preferred pigments are those containing from 5 to 12% by weight of Co and from 8 to 20% by weight of Ca based on TiO 2  ; such preferred pigments may also contain Al the Al.sub. 2 O 3  /Co molar ratio ranging from 0.01 to 0.20 and, more preferably, from 0.10 to 0.20. 
     The process according to the present invention consists in preparing a mixture of TiO 2 , CoO and CaO and, in case, Al.sub. 2 O 3  or compounds of the same metals capable of yielding the above-mentioned oxides in the successive calcining operation, and in calcining the mixture so obtained, at temperatures generally ranging from 950° to 1150° C, in the presence of air. 
     The amounts of Co, Ca and Al compounds to be used are selected as a function of the desired pigment composition. Al; the aluminium Al.sub. Generally, the amounts employed are such that the element content, calculated in respect of the starting TiO 2 , is comprised between 2 and 12% for Co, between 4 and 20% for Ca and between 0.018 and 2.197% for A1;the aluminum amount is such that the A1 2  O 3  /Co molar ratio ranges from 0.01 to 0.20. 
     TiO 2  or compounds capable of providing it during calcination mean in particular: 
     TiO 2  having a rutile or anatase structure, whose particles have, for example, a mean diameter comprised between 0.2 and 0.4μ; 
     The hydrated forms of TiO 2  ; particularly suited to this purpose is the gel of the meta-titanic acid TiO 2 .H 2  O purified from iron, that is obtained as an intermediate product in the processes for preparing the TiO 2  pigments via titanyl sulphate (see, e.g., Kirk Othmer, Encyclopedia of Chemical Technology, II Edition, Vol. 20, pages 401-404, Interscience Publisher, New York 1969). 
     Besides CoO, CaO and Al 2  O 3 , it is possible to use the cobalt, calcium and aluminium compounds that decompose to CoO, CaO and Al 2  O 3  at temperatures not higher than the abovesaid calcination temperatures: for example--cobalt sulphate, nitrate, acetate and sulphide; calcium nitrate and carbonate and aluminum sulphate. 
     The preferred source of TiO 2  is the gel of TiO 2 .H 2  O. 
     Admixing of the starting compounds is generally effected in water, for example, at room temperature; admixing is carried on for a time period sufficient to ensure a good homogenization, such, for example, a 10 to 60 minutes mixing period. 
     The mixture can then be dried, for example, at 110°-130° C. The dried product is then homogenized by grinding and subjected to calcination. 
     When use is made of a furnace capable of ensuring a good mixing of the material to be calcined, for example, a rotary furnace, drying can be omitted. The furnace can be directly fed with the paste obtained at the conclusion of the admixing step. 
     Calcination is generally conducted at 950°-1150° C, in the presence of air, employing a residence time preferably comprised between 1 hour and 3 hours. Generally it is preferable to avoid working at temperatures higher than 1150° C, as in such case the pigment particles could tend to become too large. 
     Calcination temperatures are selected as a function of the pigments to be obtained, taking the following factors into account: 
     In the presence of aluminum, calcination can be carried out at lower temperatures; 
     Either in the absence or in the presence of aluminum, calcination can be carried out at lower temperatures when the product contains relatively high amounts of both calcium and cobalt. 
     Calcination temperatures are preferably selected -- as a function of the desired product -- in the ranges reported on Table A. 
     Thus, the preferred products, i.e., those containing from 5 to 12% of cobalt and from 8 to 20% of calcium, and optionally aluminum, are generally obtained at temperatures comprised between about 1000° C and about 1150° C. 
     
                                           TABLE A                                 
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Product composition                                                       
Cobalt  Calcium                                                           
(% b.w. in res-                                                           
        (% b.w. in res-               Calcination                         
pect to TiO.sub.2)                                                        
        pect to TiO.sub.2)                                                
                   Aluminium          temperature                         
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2-5     4-20     absent or present in an Al.sub.2 O.sub.3 /Co             
                                     about 1100 to about                  
                 lower than 0.10     1150° C                       
&gt;5-12   ≧4 and lower                                               
                 absent or present in an Al.sub.2 O.sub.3 /Co             
                                     about 1100 to about                  
        than 8   lower than 0.10     1150° C                       
&gt;5-12   ≧8  up to 20                                               
                 absent or present in an Al.sub.2 O.sub.3 /Co             
                                     about 1000 to about                  
                 lower than 0.10     1150° C                       
2-6     4-20     present in an Al.sub.2 O.sub.3 /Co ratio of              
                                     about 1000 to about                  
                 to 0.20             1150° C                       
&gt;6 up to                                                                  
        ≧4 and lower                                               
                 present in an Al.sub.2 O.sub.3 /Co ratio of              
                                     about 1000 to about                  
12      than 12  to 0.20             1150° C                       
&gt;6 up to                                                                  
        ≧12 up to 20                                               
                 present in an Al.sub.2 O.sub.3 /Co ratio of              
                                     about 950 to about                   
12               to 0.20             1150° C.                      
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     The calcined and cooled product is successively ground until, e.g., its residue on a 325 mesh screen does not exceed 0.1% by weight. 
     The pigments object of the present invention can be used in the fields in which the coloured inorganic pigments are commonly used, i.e., in particular, as coloured pigments for paints and plastic materials. 
     The following examples are given to better illustrate the inventive idea of the present invention. 
     SPECIFIC DESCRIPTION OF THE INVENTION 
    
    
     EXAMPLE 1 
     As a TiO 2  -generating compound use was made of a gel of metatitanic acid at 32.8% of TiO 2 , purified from iron, obtained as an intermediate product in the production of TiO 2  via titanyl sulphate; its impurities total content was lower than 200 ppm calculated on TiO 2  . 11.85 g of Co(NO 3 ) 3 .6H 2  O dissolved in 20 cc of water and 6 g of CaCO 3  were added to 61 g of gel, corresponding to 20 g of anhydrous TiO 2 . The paste was homogenized under stirring at room temperature and dried at about 110° C. The dry product was pulverized in a mortar; it was placed into a crucible and calcined in a muffle furnace, in the presence of air, at 1000° C. The product was kept at such temperature for 1 hour, whereupon it was cooled and pulverized in a mortar. 
     The product contained 11.4% by weight of cobalt and 12.0% of calcium (in respect to TiO 2 ). 
     Its colorimetric characteristics were determined on a dry paint film prepared starting from a vehicle made up of: 
     
         ______________________________________                                    
Aroplaz 1279 (commercial alkyd resin:                                     
  68% by weight; soybean                                                  
  oil: 32%           76.3  %                                              
FL 30 (boiled linseed oil)                                                
                     19.0  %                                              
Drying mixture       4.7  %                                               
 The drying mixture had the following composition:                        
Calcium naphthenate  1.77 %                                               
Zirconium naphthenate                                                     
                     5.31 %                                               
Cobalt naphthenate   6.9  %                                               
White spirit         86.02 %                                              
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     On a dry paint film of 50μ  thickness, containing 50% by weight of pigment (calculated on the dry paint), tristimulus values X, Y, Z were measured in respect of a standard calibrated to magnesium oxide by means of a differential tristimulus colorimeter Ducolor, mod. 220, manufactured by Neotec Instruments Corp. 
     The obtained values were processed according to the C.I.E. (Commission Internationale de l&#39;Eclairage) coordinates system and the dominant wave length was determined graphically (for further details see the above-cited treatise by A. G. Hardy). The λ D  resulted to be of 555 mμ. A commercial pigment of green chromic oxide Cr 2  O 3 , examined according to the same method, exhibited a λ D  of 554 mμ. 
     The X-ray diffractogram of the product according to this invention revealed the presence of rutile TiO 2 , that represented the predominant phase, of CaTiO 3  having the perovskite structure and of trigonal CoTiO 3 . 
     The mean size of the ground pigment particles was of about 1.0 μ. 
     By way of comparison, the test was repeated without adding --calcium carbonate. The product obtained, which contained 11.4% of cobalt, exhibited a λ  D  of 495 mμ. 
     Thus it is clear that the presence of calcium in the pigment shifted the λ D  from bluish-green to yellowish-green. 
     EXAMPLE 2 
     11.85 g of Co(NO 3 ) 2 .6H 2  0 dissolved in 20 cc of H 2  0, 6 g of CaCO 3  and variable amounts of a solution of Al 2  (SO 4 ) 3 . 18H 2  O at 259 g/l were added, to 61 g of meta-titanic acid gel, as described in Example 1. More precisely, 10 cc of the aluminum sulphate solution were added in test 2a, and 20 cc in test 2b. It was operated according to the modalities of Example 1, calcining having been conducted at 1000° C. The characteristics of the resulting products are reported on Table B. 
     
                       TABLE B                                                     
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     %         %         %                                                
     b.w. of Co                                                           
               b.w. of Ca                                                 
                         b.w. of Al                                       
                                 Al.sub.2 O.sub.3 /Co                     
                                         λ.sub.D                   
Test in respect                                                           
               in respect                                                 
                         in respect                                       
                                 molar   in                               
No.  to TiO.sub.2                                                         
               to TiO.sub.2                                               
                         to TiO.sub.2                                     
                                 ratio   m μ                           
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2a   11.4      12        1.05    0.1      552                             
2b   11.4      12        2.10    0.2      545                             
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     The x-ray diffractometric analysis revealed the presence of rutile TiO 2 , of perovskite CaTiO 3  and of trigonal CoTiO 3 . 
     From Table B it can be inferred that the value of λ D  lowers as the Al amount increases. 
     EXAMPLES 3 TO 13 
     The modalities of Example 1 were followed, but varying the calcium and aluminum amounts (while the cobalt amount remained unchanged) and the calcining temperatures. 
     By way of comparison, tests 3 and 4 were carried out without calcium. 
     
                       TABLE C                                                     
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     %       %       %                                                    
     of Co   of Ca   of Al                                                
     in res- in res- in res-                                              
                           Molar                                          
Test pect to pect to pect to                                              
                           ratio   Calcining                              
                                           λD                      
No.  TiO.sub.2                                                            
             TiO.sub.2                                                    
                     TiO.sub.2                                            
                           Al.sub.2 O.sub.3 Co                            
                                   temp.   in mμ                       
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3    11,4    --      --     --     1000° C                         
                                           495                            
4    11,4    --      --    --      1100° C                         
                                           498                            
5    11,4     4      --    --      1000° C                         
                                           536                            
6    11,4     4      --    --      1100° C                         
                                           554                            
7    11,4     4      2,10  0,2     1000° C                         
                                           536                            
8    11,4     8      --    --      1000° C                         
                                           555                            
9    11,4    20      --    --      1000° C                         
                                           534                            
10   11,4    20      --    --      1100° C                         
                                           554                            
11   11,4    20      2,10  0,2     1000° C                         
                                           551                            
12   11,4    20      2,10  0,2     1100° C                         
                                           551                            
13   11,4    20      2,10  0,2      950° C                         
                                           551                            
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     By comparing tests 3-4 with test 5-6 one may notice that the addition of calcium to the TiO 2  -Co system shifts the wave length from bluish-green to yellowish-green. By comparing test 5 with test 6 and test 9 with test 10 one may notice that the rise of the calcining temperature causes the λ D  to increase. By comparing tests 9-10 with tests 11-13 one may notice that in the presence of aluminum it is possible to calcine at lower temperatures. 
     The product of test 8 exhibited a particle&#39;s mean diameter of 0.8 μ. 
     EXAMPLES 14 to 30 
     It was operated according to the modalities of Example 1, but varying the amounts of Co, Ca and Al and the calcining temperatures. 
     These temperatures and the characteristics of the resulting products are reported on Table D. 
     
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Test 21          1.0 μ                                                 
Test 22          1.3 μ                                                 
Test 23          1.0 μ                                                 
Test 30          0.5 μ                                                 
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                       TABLE D                                                     
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     %       %       %                                                    
     of Co   of Ca   of Al                                                
     in res- in res- in res-                                              
                           Al.sub.2 O.sub.3 /Co                           
                                   Calcining                              
                                           λD                      
Test pect to pect to pect to                                              
                           molar   temper- in                             
No.  TiO.sub.2                                                            
             TiO.sub.2                                                    
                     TiO.sub.2                                            
                           ratio   ature   m μ                         
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14   2        4      --    --      1100° C                         
                                           548                            
15   2       20      --    --      1100° C                         
                                           558                            
16   2        4      0,36  0,2     1000° C                         
                                           552                            
17   2       20      0,36  0,2     1000° C                         
                                           559                            
18   4        4      --    --      1100° C                         
                                           552                            
19   4       20      --    --      1100° C                         
                                           555                            
20   6        4      --    --      1100° C                         
                                           553                            
21   6       12      --    --      1000° C                         
                                           549                            
22   6       12      --    --      1100° C                         
                                           557                            
23   6       20      --    --      1000° C                         
                                           558                            
24   6        4      1,08  0,2     1000° C                         
                                           540                            
25   6       12      0,54  0,1     1000° C                         
                                           556                            
26   6       12      0,54  0,1     1100° C                         
                                           556                            
27   6       12      0,54  0,1      950° C                         
                                           556                            
28   6       12      1,08  0,2     1000° C                         
                                           550                            
29   6       12      1,08  0,2     1100° C                         
                                           553                            
30   6       12      1,08  0,2      950° C                         
                                           550                            
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