Alumina-Based Bodies With Large Pores, Produced by Agglomeration

These examples are not given by way of limitation of the invention in that other general methods of agglomerating particulate substances can be used with similar results, such as agglomeration by compression and agglomeration by extrusion.

A solution of sodium aluminate, having 100 g/l of alumina expressed as Al.sub.2 O.sub.3, is precipitated continuously at a pH value maintained at 8.7 and at a temperature of 35.degree. C., by addition of a N nitric acid solution.

The alumina gel suspension, which issues from the reaction vessel, then passes, for purposes of homogenization, into a buffer tank which is maintained at a temperature of 35.degree. C., for which it is passed over a continuous filter. The resulting cake which comprises aluminum hydroxide, which is virtually amorphous in respect of X-rays, is washed with permuted water until the ions in the filtrate disappear, and then drained. The drained cake is then resuspended by stirring in the permuted water so that it can be sprayed as by means of a nozzle at the base of an apparatus, for dehydration by a rising stream of hot gases produced by the burning of propane. The inlet temperature of the gases is 550.degree. C., their outlet temperature is 300.degree. C., and the contact time is about one second. The resulting powder, which is separated from the gases in a cyclone separator, has a water loss measured by calcination at high temperature of 24%, and an apparent density of 0.30g/ccm. It is formed of substantially spherical particles having a diameter of less than 100 microns, with 70% of such particles having a diameter of less than 44 microns. This powder is moistened by means of an aqueous N/2 nitric acid solution and agglomerated in the form of balls in a rotary granulator. The balls are then kept in a closed vat for a period of 8 hours at a temperature of 30.degree. C., dried in a gas flow at 200.degree. C. and then calcined for two hours at 700.degree. C. The balls are then screened so as to retain only those whose diameters are from 2.4 to 4 mm. A fraction of the graded balls is then calcined for 24 hours at 1000.degree. C.

An alumina gel cake is prepared in a similar manner to the mode of operation described in Example 1, except that the pH-value in this case is fixed at around 8.0 and temperature is fixed at about 30.degree. C. The suspension, issuing from the precipitation reaction vessel, passes into the buffer tank at 30.degree. C., before being passed over the filter. The cake, which is also substantially amorphous, is washed, re-suspended, treated in the hot gas flow and converted into balls, in the manner described in Example 1. The balls produced in this manner are dried, graded and calcined in the same manner.

Balls are prepared in a rotary granulator as set out in Example 1, but after agglomeration and before drying, the balls are subjected to a hydro-thermal treatment for 5 hours at 100.degree. C. in a saturating vapor. The balls are then treated as in Example 1. The vapor treatment of the crude balls makes it possible to increase the strength of the balls, by reducing their pore volume.

A mixture is made which contains by weight 50% of the same powder as that produced in Example 1, and 50% of a 5 A molecular sieve of mono-crystals of from 2 to 5 microns. This mixture is agglomerated in a rotary granulator after moistening with a N/2 nitric acid solution. The balls produced are subjected to a hydro-thermal treatment for 5 hours at 100.degree. C. in a closed vessel. Calcination at 700.degree. C. for a period of 2 hours is then effected.

A powder having the formula 2CuO.1 Cr.sub.2 O.sub.3 is prepared by calcination at 400.degree. C. of an intimate mixture of cupric nitrate and chromic anhydride. This powder is mixed with the alumina powder produced as set out in Example 1, in a proportion of 18% by weight of oxides with respect to the alumina. Agglomeration is effected, followed by maturing and calcination under the same conditions as in Example 1. By virtue of its macro-porosity, this catalyst has a high degree of activity as regards the oxidation of carbon monoxide and the hydrocarbons present in very substantial gas flows. It also has a degree of stability which is higher than that of catalysts containing the same oxides deposited by impregnation.

This example is given by way of comparison and relates to a gel which is dried at moderate temperature. An alumina cake is prepared as set out in Example 1. This cake is re-suspended and then dried in an atomizer in which the inlet temperature of the gases is 180.degree. C. and the outlet temperature is 80.degree. C. The powder which is collected, which is virtually amorphous, has a water loss, as measured by calcination at high temperature, of 40% by weight, and an apparent density of 0.85 g/ccm, which is much higher than that of the powder of Example 1. This powder is converted into balls in a rotary granulator and the balls are then treated in the same manner as in Example 1.

This example is also given by way of comparison and concerns a gel which is calcined in an oven. The same alumina cake as that of Example 1 is dried in a drying oven, coarsely crushed and calcined in an oven at 500.degree. C. so that the water loss, by calcination at high temperature of this cake, is 10% by weight. It is then crushed into the form of powder having particles which are smaller than 100 microns. This powder has a density of 0.65g/ccm and is agglomerated in the form of balls in a rotary granulator as in Example 1. The balls produced are treated in an identical manner.

This table clearly shows that, in spite of their large pore volume, all the alumina agglomerates have a good mechanical strength, particularly as regards attrition, and that even the agglomerates which are half composed of molecular sieves (Example 4) still have levels of strength which are sufficient for them to be employed in industrial uses. This table also shows, by comparison between the results of Examples 6 and 7, and the results of Examples 1, 2 and 3, that the large pore volumes are obtained solely by using gels which are dehydrated rapidly in a stream of hot gases .