Gel with high dispersibility and method for preparing same

Abstract

A novel alumina gel is described having an elevated dispersibility index, and in particular a dispersibility index greater than 70 %, a crystallite size between 1 and 35 nm, and a sulphur content between 0.001 % and 2 % by weight, and a sodium content between 0.001 % and 2 % by weight, the weight percentages being expressed in relation to the total mass of alumina gel. The present invention also discloses the method for preparing said gel comprising at least one step of precipitating at least one aluminium salt, at least one step of heating the suspension obtained and a final heat treatment step for forming the alumina gel.

1 . Alumina gel having a dispersibility index greater than 70%, a crystallite size between 1 and 35 nm, and a sulphur content between 0.001% and 2% by weight, and a sodium content between 0.001% and 2% by weight, the weight percentages being expressed in relation to the total mass of alumina gel. 
     
     
         2 . Alumina gel according to  claim 1  having a dispersibility index between 80 and 100%. 
     
     
         3 . Method of preparing the alumina gel according to  claim 1 , said method comprising at least the following steps:
 a) at least a first step of precipitating alumina, in an aqueous reaction medium, of at least one basic precursor selected from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and of at least one acidic precursor selected from aluminium sulphate, aluminium chloride, aluminium nitrate, sulphuric acid, hydrochloric acid, and nitric acid, in which at least one of the basic or acidic precursors includes aluminium, the relative feed rate of the acidic and basic precursors is chosen so as to obtain a pH of the reaction medium between 8.5 and 10.5 and the feed rate of the acidic and basic precursor(s) containing aluminium is adjusted so as to obtain a progression rate of said first step between 40 and 100%, the progression rate being defined as the proportion of alumina formed in Al 2 O 3  equivalent during said first precipitation step relative to the total quantity of alumina formed at the end of the precipitation step(s), said first step taking place at a temperature between 10 and 50° C., and for a duration of between 2 minutes and 30 minutes,   b) a step of heat treating the suspension obtained at the end of step a), at a temperature between 50 and 200° C. for a duration of between 30 minutes and 5 hours,   c) a step of filtering the suspension obtained at the end of heat treatment step b), followed by at least one step of washing the gel obtained.   
     
     
         4 . Preparation method according to  claim 3 , in which the basic precursor is sodium aluminate. 
     
     
         5 . Preparation method according to  claim 3 , in which the acidic precursor is aluminium sulphate. 
     
     
         6 . Preparation method according to  claim 4 , in which the mass ratio of said basic precursor to said acidic precursor is between 1.6 and 2.05. 
     
     
         7 . Preparation method according to  claim 3 , in which the progression rate of said precipitation step a) is between 45 and 90%. 
     
     
         8 . Preparation method according to  claim 3 , in which said precipitation step a) takes place at a temperature between 20 and 45° C. 
     
     
         9 . Preparation method according to  claim 3 , in which, in the case where the progression rate obtained at the end of the first precipitation step a) is below 100%, said preparation method preferably comprises a second precipitation step a′) after the first precipitation step. 
     
     
         10 . Preparation method according to  claim 9 , in which a step of heating the suspension obtained at the end of precipitation step a) is performed between the two precipitation steps a) and a′), said heating step taking place at a temperature between 20 and 90° C. and for a duration of between 7 and 45 minutes. 
     
     
         11 . Preparation method according to  claim 9 , in which said second step a′) of precipitating the suspension obtained at the end of the heating step is carried out by adding to said suspension at least one basic precursor selected from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and at least one acidic precursor selected from aluminium sulphate, aluminium chloride, aluminium nitrate, sulphuric acid, hydrochloric acid, and nitric acid, in which at least one of the basic or acidic precursors includes aluminium, the relative feed rate of the acidic and basic precursors is chosen so as to obtain a pH of the reaction medium between 8.5 and 10.5 and the feed rate of the acidic and basic precursor(s) containing aluminium is adjusted so as to obtain a progression rate of the second step between 0 and 60%, said progression rate being defined as the proportion of alumina formed in Al 2 O 3  equivalent during said second precipitation step a′) relative to the total quantity of alumina formed at the end of the two precipitation steps and preferably at the end of step a′), said second step a′) taking place at a temperature between 40 and 90° C., and for a duration between 2 minutes and 50 minutes. 
     
     
         12 . Preparation method according to  claim 11 , in which the mass ratio of said basic precursor to said acidic precursor is between 1.6 and 2.05, the basic and acidic precursors being respectively sodium aluminate and aluminium sulphate. 
     
     
         13 . Preparation method according to  claim 11 , in which the second precipitation step a′) takes place at a temperature between 45 and 70° C.