Process of preparing alumina for use in catalyst carrier

Disclosed is a process of preparing an alumina carrier which comprises kneading an amorphous alumina hydrate, whose alumina concentration is in the range of 28-35 wt. % and which has been obtained by neutralizing an aluminum sulfate solution with a sodium aluminate solution at two stages, while endowing said hydrate with sufficient shearing effect and alternate compression and expansion effect; extruding a resulting dough; drying and calcining the extrudates. According to this process, it becomes possible to control the growth of pseudoboehmite grains contained in the alumina hydrate by controlling the time required for kneading, and accordingly it becomes possible to control the pore distribution of the finally obtained alumina carrier.

BACKGROUND OF THE INVENTION 
The present invention relates to a process of preparing alumina useful as a 
catalyst carrier, and in more detail relates to a process of preparing 
alumina designed so that in the course of kneading an amorphous alumina 
hydrate cake produced by the neutralization reaction of aluminum sulfate 
with sodium aluminate, the degree of growth of pseudoboehmite grains 
contained in said cake may be controlled. The gamma alumina or eta alumina 
obtained by drying and calcining a pseudoboehmite gel-containing amorphous 
alumina hydrate has hitherto been widely utilized as a catalyst carrier 
because said alumina has a large specific surface area and is also 
superior in thermal stability. As is generally known, the performance of a 
catalyst supported on alumina owes much to the physical and chemical 
properties of the alumina used in the carrier, in particular its physical 
properties, namely the specific surface area and porous characteristic, 
because these are important factors which control the performance of a 
catalyst supported on alumina. However, as the specific surface area and 
pore characteristic required for alumina carrier vary depending on the 
particular catalysts for which said alumina is used, the specific surface 
area and pore characteristic called for in the alumina carrier are not 
uniform. 
The greatest importance of the alumina carrier used for a hydrocarbon 
reforming catalyst, for instance, consists in the fact that said alumina 
carrier has a large specific surface area, rather than the nature of its 
pore characteristic. In contrast, in the case of the alumina carrier used 
for a desulfurizing catalyst, it is important that said carrier has a pore 
characteristic suitable for the molecule taking part in the 
desulfurization reaction, more specifically, that the carrier has an 
average pore diameter a pore volume suitable for the desulfurization 
reaction. Referring to the specific surface area in this instance, 
although the catalytic activity also increases as the specific surface 
area increases, the pores of less than 50 .ANG., which are very remarkably 
attributable to increase in specific surface area, are easily plugged by 
coke deposit. In order that the catalyst may hold its stability even if 
the specific surface area somewhat deteriorates, it is rather preferable 
that micro-pores of less than 50 .ANG. should not exist. On the other 
hand, an alumina carrier containing little impurities is desirable, 
because impurities present in a catalyst carrier exert an enormous 
influence upon the chemical properties depending on its usage. 
In these circumstances, when preparing an alumina carrier from the 
amorphous alumina hydrate, there has usually been employed the process 
which comprises stirring an aqueous slurry of said amorphous alumina 
hydrate at elevated temperatures and under weak alkaline conditions to 
thereby control the degree of growth of pseudoboehmite grains contained in 
said slurry, thereby regulating, as requested, the pore characteristic and 
specific surface area of the finally obtained alumina carrier. However, 
the aforesaid prior art process is not always advantageous in that it 
takes a relatively long period of time to make the pseudoboehmite grains 
grow in the aqueous slurry of the amorphous alumina hydrate. 
BRIEF SUMMARY OF THE INVENTION 
The inventors have obtained a new finding concerning the process of 
preparing an alumina carrier. That is, the inventors have found that since 
a slurry of an amorphous alumina hydrate obtained by two-stage 
neutralization of aluminum sulfate with sodium aluminate under the 
specific conditions referred to afterwards is markedly superior in 
filtrability, there can be obtained a filter cake having about 30 wt. % of 
Al.sub.2 O.sub.3 concentration, and accordingly by kneading this filter 
cake it is made possible to grow pseudoboehmite grains in a short time. 
The process of preparing an alumina catalyst carrier according to the 
present invention comprises the steps of: (a) simultaneously pouring an 
aluminum sulfate solution and a sodium aluminate solution into deionized 
water in a vessel for reacting aluminum sulfate with sodium aluminate at a 
pH 6.0-8.5 and temperature of 50.degree.-65.degree. C. thereby to prepare 
a first aqueous slurry containing an amorphous alumina hydrate; (b) 
adding, to this aqueous slurry, an aqueous sodium aluminate solution in an 
amount sufficient to neutralize the first aqueous slurry, the sum of the 
sodium aluminate used in the steps (a) and (b) corresponds to 0.95-1.05 
equivalent of the aluminum sulfate used in the step (a) thereby to prepare 
a second aqueous slurry having an Al.sub.2 O.sub.3 concentration of 7 wt. 
% or more; (c) filtering off an amorphous alumina hydrate contained in 
said second aqueous slurry, and washing a resulting filter cake firstly 
with a dilute aqueous ammonia and then with a dilute nitric acid solution 
and washing the same again with a dilute aqueous ammonia so as to regulate 
the pH of the filter cake to range of 7.5-10.5; (d) dehydrating this 
filter cake on a filter press for increasing its Al.sub.2 O.sub.3 
concentration to be in the range of 28-35 wt. %, and thereafter supplying 
the same to a self-cleaning type mixer for kneading it for the residence 
time of 10 seconds or more; and (e) extruding a dough obtained from the 
step (d), thereafter drying and calcining the resulting extrudates.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be explained in more detail in the order of 
steps hereinafter. As the aluminum sulfate solution and the sodium 
aluminate solution used in the present invention, there may be employed 
those obtained by dissolving commercially available aluminum sulfate and 
sodium aluminate in water respectively. Taking the cost into 
consideration, however, it is preferable to prepare an aluminum sulfate 
solution by digesting gypsite in a sulfuric acid and a sodium aluminate 
solution by digesting gypsite in a sodium hydroxide solution. In the 
preparation of the sodium aluminate solution, it is desirable to obtain 
the sodium aluminate solution having the Na/Al atomic ratio in the range 
of 1.15-1.25 by using an about 50% sodium hydroxide solution. 
The first step of the present invention comprises reacting aluminum sulfate 
with sodium aluminate under constant conditions including the pH of 
6.0-8.5 and the temperature of 50.degree.-65.degree. C. to thereby obtain 
the first aqueous slurry containing the amorphous alumina hydrate. The 
neutralizing reaction condition of this step can be achieved for instance 
by pouring simultaneously and at a constant flow rate the aluminum sulfate 
solution and the sodium aluminate solution heated to 50.degree.-65.degree. 
C. into deionized water heated to 50.degree.-65.degree. C. in a vessel. 
The reaction in the first step results in neutralization of the aluminum 
sulfate with sodium aluminate in an amount of 2/3-2.5/3 equivalent of 
aluminum sulfate used. 
The second step of the present invention comprises completing the 
neutralizing reaction by adding the sodium aluminate solution to the above 
mentioned first aqueous slurry and obtaining the second aqueous slurry 
whose Al.sub.2 O.sub.3 concentration is 7 wt. % or more. It is preferable 
that the neutralizing reaction in this step should also be carried out at 
a temperature of 50.degree.-65.degree. C., and in the addition of the 
sodium aluminate solution it is desirable that said solution should be 
added gradually in order to prevent the pH from rising rapidly. The amount 
of sodium aluminate used in the second step must be, in total including 
the amount of sodium aluminate used in the first step, the amount 
corresponding to 0.95-1.05 equivalent of the amount of sodium sulfate used 
in the first step. The reason is that in case this amount of sodium 
aluminate is less than 0.95 equivalent, it becomes difficult to wash and 
remove the sulfate anion, while in case the amount of sodium aluminate is 
over 1.05 equivalent, the stability of slurry is deteriorated exceedingly. 
In this connection, it is to be noted that in the case of using 0.95 
equivalent of sodium aluminate, the final pH of the obtained second 
aqueous slurry at 60.degree. C. is 9.2, while in the case of using 1.05 
equivalent of sodium aluminate, said final pH is 10.2. However, when the 
pH of slurry is over 9.5, the stability of slurry deteriorates. In view of 
this, it is recommended to complete the second step within 30 minutes and 
transfer the resulting second aqueous slurry immediately to the third 
step. 
The amorphous alumina hydrate-containing second aqueous slurry is subjected 
to filtration in the third step, and the obtained filter cake is 
conventionally washed first with an aqueous dilute ammonia solution and 
then with a dilute nitric acid solution. The washing with ammonia in this 
instance mainly aims at removal of the sulfate anion and sodium, while the 
washing with nitric acid mainly aims at removal of sodium. Generally, the 
former washing is effected by sprinkling the filter cake with about 1% 
ammonia solution and the latter is effected by repulping the filter cake 
in an about 1% nitric acid solution. Accordingly, the filter cake is 
filtrated again after the washing with nitric acid. The thus obtained 
filter cake is washed again with a dilute ammonia solution, but this 
washing with ammonia aims at further removing the sodium still remaining 
in the filter cake and regulating the pH of the cake to be in the range of 
7.5-10.5. 
In the fourth step of the present invention, the filter cake of the 
amorphous alumina hydrate, from which the impurities have been removed and 
whose pH has been regulated to be in the range of 7.5-10.5 in the 
preceding third step, is dehydrated by means of a filter press to such an 
extent that the Al.sub.2 O.sub.3 concentration is in the range of about 
28-35 wt. %, and thereafter is kneaded for 10 seconds or more by means of 
a self-cleaning type mixer. 
The "self-cleaning type mixer" referred to herein implies a mixer which is 
capable of endowing the cake of the amorphous alumina hydrate with 
sufficient shearing effect, alternate compression and expansion effect. 
The mixer of this sort is characterized in that it is equipped with two 
parallel shafts of corotating agitators and the blades of one agitator 
assembly maintain close clearances with the second assembly as well as 
with the walls of the barrel. This provides a self-cleaning or self-wiping 
action for the agitator blades. For this purpose, there can be used the 
continuous mixers described in Brennan, Jr, U.S. Pat. Nos. 3,419,250 and 
3,618,902, and in addition thereto there can be used the ZSK twin-screw 
machines (Werner & Pfleiderer Corp.) and the multipurpose mixer (Baker 
Perkins Inc.) outlined in Chemical Engineer's Handbook by Perry and 
Chilton, Fifth Edition, Section 19-21. 
The pH condition in the fourth step is essential for contriving to grow the 
pseudoboehmite grains by kneading. Generally speaking, it is preferable 
that kneading is carried out in the vicinity of the isoelectric point of 
alumina, but it is possible to grow the pseudoboehmite grains provided 
that the pH is in the range of 7.5-10.5. 
Referring to the temperature condition, furthermore, it can be said that 
the higher the temperature at the time of kneading is, the more the 
growing velocity of pseudoboehmite grains can be increased. According to 
the present invention, as the amorphous alumina hydrate is kneaded by 
using the self-cleaning type mixer, it is not only possible to contrive to 
grow the pseudoboehmite grains contained in said hydrate, but also 
possible to control the degree of growing grains, namely the pore 
characteristic of the final product, alumina carrier, by controlling the 
kneading time. It may generally be said that the increased kneading time 
increases the pore volume and average pore diameter, and somewhat reduces 
the specific surface area. And, the increased kneading time, furthermore, 
localizes the pore distribution in a narrow scope. 
The dough taken out of the self-cleaning type mixer is extruded, in the 
fifth step of the present invention, by means of a conventional extruder, 
into extrudates having a desired size, thereafter dried and calcined in a 
known manner, whereby a final product, alumina carrier, can be obtained. 
The drying conditions employed in this instance generally comprise 
350.degree. C. and 1 hour and further 600.degree. C. and 2 hours. 
As explained above, the present invention can obtain the amorphous alumina 
hydrate, which is exceedingly superior in filterability, by means of the 
two-stage neutralizing method, whereby the impurities contained in this 
hydrate can be removed very easily by washing, and furthermore said 
hydrate can be economically dehydrated to a high level by using the filter 
press. In addition thereto, the present invention can readily prepare the 
alumina carrier having the pore characteristic satisfying the object of 
use, because the growth of pseudoboehmite grains can be attained by 
kneading the amorphous alumina hydrate dehydrated to a high level by means 
of the self-cleaning type mixer under the predetermined weak alkaline 
condition and further the degree of growth of said grains can be 
controlled by selecting the kneading condition. 
Examples will be given hereinafter, but the present invention should not be 
limited thereto. 
EXAMPLE 1 
3575 g of an aluminum sulfate solution (Al.sub.2 O.sub.3 concentration 8 
wt. %) and 2644g of a sodium aluminate solution (Al.sub.2 O.sub.3 
concentration 27% and Na/Al atomic ratio 1.20) were simultaneously poured 
in 5546 ml of deionized water with stirring from each different pouring 
port and at a definite flow rate. The aluminum sulfate solution was 
completely poured in 60 minutes, and the sodium aluminate solution was 
completely poured in 80 minutes. During this reaction, the temperature was 
controlled so as to be held at 60.degree. C. 
The neutralizing reaction was carried out at the pH of 6.5-7.0 until the 
pouring of the aluminum sulfate was completed, thereafter the pH rose 
gradually, and an amorphous alumina hydrate slurry (pH 9.5 and Al.sub.2 
O.sub.3 concentration 8.5 wt. %) was obtained at the time when pouring of 
the sodium aluminate solution completed. 
This slurry was supplied immediately to a vacuum filter for obtaining a 
filter cake, thereafter said cake was washed by sprinkling 10 l of a 0.1% 
aqueous ammonia thereon, and then this cake was repulped in 10 l of a 0.7% 
nitric acid solution. 
Thereafter, the resulting slurry was treated again by the vacuum filter to 
obtain a filter cake, and this cake was washed by sprinkling 10 l of a 
0.1% aqueous ammonia thereon. The Al.sub.2 O.sub.3 content of the thus 
obtained cake was 21 wt. %, and its pH was 9.0. The contents of sulfate 
anion and sodium cation in this cake were 0.03% respectively based on 
Al.sub.2 O.sub.3. 
Next, the above mentioned cake was subjected to a filter press for 
dehydrating to obtain a dehydrated cake having Al.sub.2 O.sub.3 content of 
31 wt. %. This cake was provided to a self-cleaning type mixer and was 
kneaded under the conditions including jacket temperature of 100.degree. 
C. and residence time of 35 seconds, to obtain a dough having Al.sub.2 
O.sub.3 content of 33 wt. %. 
This dough was treated by an extruder to thereby obtain a 1.8 mm.phi. of 
extrudates. This extrudates were dried at 120.degree. C. for 12 hours, 
thereafter was calcined at 350.degree. C. for 1 hour and further at 
600.degree. C. for 2 hours, thereby obtaining Alumina Carrier (A). 
COMATIVE EXAMPLE 
A dehydrated cake obtained according to the same procedure as Example 1 was 
treated directly by the extruder without kneading to thereby obtain 
Alumina Carrier (X). 
EXAMPLE 2 
By repeating the exactly same procedure as Example 1 except that as the 
residence time in the mixer, 15 seconds and 210 seconds were employed, 
there were obtained Alumina Carriers (B) and (C). 
The pore characteristics of the respective alumina carriers obtained by the 
above mentioned Examples 1-2 and Comparative Example are as shown in the 
following table. 
______________________________________ 
Sample A B C X 
______________________________________ 
Specific surface 
242 249 247 252 
area* 
(m.sup.2 /g) 
Pore volume** 
0.701 0.540 0.812 
0.227 
(cc/g) 
Average pore 106 93 112 70 
diameter** 
(.ANG.) 
______________________________________ 
*BET method 
**High Pressure Mercury Porosimeter 
In addition, the pore distribution curve of alumina carriers A-C and X 
obtained by a mercury porosimeter under pressure is shown in the 
accompanying drawing.