Aqueous rare earth phosphate collidal dispersion and preparation method

Abstract

The invention concerns an aqueous colloidal dispersion of isotropic particles of a phosphate of at least one rare earth and is characterized in that it comprises either a complexing agent with a pK (cologarithm of the dissociation constant of the complex formed by the complexing agent and said rare earth) of more than 2.5; or an anion of a monobasic acid, soluble in water and with a pKa in the range 2.5 to 5; or said complexing agent and/or said anion of a monobasic acid. In the case of cerium and lanthanum, the mean particle size is at most 20 nm. The dispersions can have a degree of colloid agglomeration of less than 40%.This dispersion can be prepared by forming an aqueous mixture comprising at least one rare earth salt and the complexing agent or monobasic acid anion; by adding a base then phosphate ions to the mixture formed; and heating the mixture obtained. A precipitate is obtained that is re-dispersed in water.

1 . An aqueous colloidal dispersion of isotropic particles of a phosphate of at least one rare earth, characterized in that it comprises either a complexing agent with a pK (cologarithm of the dissociation constant of the complex formed by the complexing agent and said rare earth) of more than 2.5; or an anion of a monobasic acid, soluble in water and with a pKa in the range 2.5 to 5; or said complexing agent or said anion as a mixture, and in which the degree of colloidal agglomeration is less than 40%, more particularly less than 10%.  
     
     
         2 . An aqueous colloidal dispersion of isotropic particles of a phosphate of at least one rare earth, characterized in that it comprises either a complexing agent with a pK (cologarithm of the dissociation constant of the complex formed by the complexing agent and said rare earth) of more than 2.5; or an anion of a monobasic acid, soluble in water and with a pKa in the range 2.5 to 5; or said complexing agent or said anion as a mixture, with the exception of dispersions of cerium phosphate, lanthanum phosphate or cerium and lanthanum phosphate.  
     
     
         3 . A dispersion according to  claim 1  or  claim 2 , characterized in that the mean particle size is at most 20 mn.  
     
     
         4 . An aqueous colloidal dispersion of isotropic particles of a phosphate of cerium and/or lanthanum, characterized in that it comprises either a complexing agent with a pK (cologarithm of the dissociation constant of the complex formed by the complexing agent and said rare earth) of more than 2.5; or an anion of a monobasic acid, soluble in water and with a pKa in the range 2.5 to 5; or said complexing agent or said anion as a mixture, and in that the mean particle size is at most 20 nm.  
     
     
         5 . A dispersion according to any one of  claims 2  to  4 , characterized in that it has a degree of colloid agglomeration of less than 40%, more particularly less than 10%.  
     
     
         6 . A dispersion according to any one of claims  1 ,  2 ,  3  and  5 , characterized in that the rare earth phosphate is a phosphate of lanthanum, cerium and terbium.  
     
     
         7 . A dispersion according to any one of claims  1 ,  2 ,  3  and  5 , characterized in that the rare earth phosphate is a phosphate of lanthanum and europium.  
     
     
         8 . A dispersion according to any one of the preceding claims, characterized in that the complexing agent is selected from acid-alcohols or polyacid-alcohols, aliphatic amino acids, polyacrylic acids or salts thereof.  
     
     
         9 . A dispersion according to any one of  claims 1  to  7 , characterized in that said monobasic acid is acetic acid.  
     
     
         10 . A process for preparing a colloidal dispersion according to any one of the preceding claims, characterized in that it comprises the following steps: 
 bringing a colloidal dispersion of at least one rare earth compound comprising at least one complexing agent or an anion of said monobasic acid into contact with phosphate ions;    heating the medium obtained to obtain a precipitate;    separating the precipitate obtained and re-dispersing it in water, to obtain a colloidal dispersion.    
     
     
         11 . A process for preparing a colloidal dispersion according to any one of  claims 1  to  9 , characterized in that it comprises the following steps: 
 bringing a colloidal dispersion of at least one rare earth compound comprising at least one complexing agent or an anion of said monobasic acid into contact with phosphate ions at least a portion of which is in the form of an alkaline tripolyphosphate;  
 heating the medium obtained to produce a colloidal dispersion.  
 
     
     
         12 . A process according to  claim 10  or  claim 11 , characterized in that said colloidal dispersion of at least one rare earth compound is prepared by forming an aqueous mixture comprising at least one rare earth salt and either said complexing agent or a monobasic acid, soluble in water and with a pKa in the range 2.5 to 5, or a mixture of complexing agent and monobasic acid; and by adding a base to the mixture formed.  
     
     
         13 . A process according to any one of  claims 10  to  12 , characterized in that the medium obtained after bringing said colloidal dispersion of at least one rare earth compound into contact with phosphate ions or phosphate ions and tripolyphosphate is heated to a temperature of at least 60° C.  
     
     
         14 . A process according to any one of  claims 10  to  13 , characterized in that the phosphate ions are provided in the form of a solution of ammonium phosphate, more particularly monoammonium or diammonium phosphate.  
     
     
         15 . A process according to any one of  claims 10  to  14 , characterized in that the pH of the medium obtained after bringing said colloidal dispersion of at least one rare earth compound into contact with phosphate ions is adjusted to a value of at least 10.