Methods for the preparation of alumina beads formed by dewatering a highly dispersible gel

A process for the preparation of an alumina in the form of beads with a sulphur content in the range 0.001% to 1% by weight and a sodium content in the range 0.001% to 1% by weight with respect to the total mass of said beads, said process comprising at least the following steps:

• a) at least one first step for the precipitation of alumina, in an aqueous reaction medium, using 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 comprises aluminium, the relative flow rate of the acidic and basic precursors being selected in a manner such as to obtain a pH of the reaction medium in the range 8.5 to 10.5 and the flow rate of the acidic and basic precursor or precursors containing aluminium being regulated in a manner such as to obtain a percentage completion of said first step in the range 40% to 100%, the percentage completion being defined as being the proportion of alumina formed in equivalents of Al2O3 during said first precipitation step with respect to the total quantity of alumina formed at the end of the precipitation step or steps, said first precipitation step being operated at a temperature in the range 10° C. to 50° C., and for a period in the range 2 minutes to 30 minutes;
• b) a step for heat treatment of the suspension obtained at the end of step a) at a temperature in the range 50° C. to 200° C. for a period in the range 30 minutes to 5 hours in order to obtain an alumina gel;
• c) a step for filtration of the suspension obtained at the end of the heat treatment step b) followed by at least one step for washing the gel obtained;
• d) a step for drying the alumina gel obtained at the end of step c) in order to obtain a powder;
• e) a step for shaping the powder obtained at the end of step d) by drop coagulation in order to obtain the green material;
• f) a step for heat treatment of the green material obtained at the end of step e) at a temperature in the range 500° C. to 1000° C., in the presence or absence of a stream of air containing up to 60% by volume of water.