DE2501056B2 - High strength macroporous clay agglomerates - Google Patents

Description

The invention relates to the macroporous ones specified in more detail in the preceding claim Alumina agglomerates obtained from powders containing bayerite.

It is well known that porous alumina agglomerates find diverse uses, especially in Adsorption and for catalyst purposes. These various uses, especially in catalysis, put in each case certain, well-defined properties with regard to the specific surface and the porosity as well as in almost all cases good mechanical strength, above all crush strength and mostly also one good wear resistance ahead.

For certain applications, such porous alumina agglomerates are desired that have a microporosity, which corresponds to a certain specific surface required for the activity, combined with a considerable macroporosity, which facilitates gas exchange. General leads the value for the total porosity in these cases leads to an excessive deterioration of the mechanical Strength properties of the used alumina agglomerates. This is especially disadvantageous if the agglomerates for the catalytic cleaning or combustion of the toxic exhaust gases from motor vehicles are to be used and are subject to strong vibrations, which cause wear and tear as a catalyst carrier accelerate the agglomerates used.

In addition, the mechanical strength properties should usually be these alumina agglomerates for long periods of time at fairly high temperatures, which can be up to 1000 ° C, maintain sufficient values.

Often these porous alumina agglomerates are obtained by agglomerating powders and there have already been numerous driving Ve ^r recommend this to give the desired macroporosity to such agglomerates or bodies. For example, pore formers can be added to most agglomerated substances, which are removed after shaping into the desired agglomerated bodies and, as a result of the development of gas, leave behind their own volume or an even larger volume than pores. In addition, there are a number of processes that are more closely linked to the special properties of alumina or aluminum oxide (hydrate), which, as is generally known, is present in various amorphous or crystalline, optionally hydrated forms, which can have very different textures or structures .

In the process of FR-PS 10 77 163, which leads to particularly interesting results, are Alumina trihydrate particles are quickly partially dehydrated, the severely disorganized particles on it become after optionally grinding and moistening into different forms and agglomerated by a similar one Setting process as hardened with hydraulic binders. This procedure is applied quite generally to the hydrargillite obtained by the Bayer process or

ίο Gibbsite applied The bodies obtained by agglomeration must be activated by burning so that they obtain the desired specific surfaces.
In the process of US Pat. No. 2,881,051, a still one-part hydrargillite is gently dehydrated, which has been obtained by decomposing a technical waterglass solution at about 30 ^° C. in the presence of alumina gel as a nucleating agent. By rehydrating to bayerite, drying and firing the agglomerates, a greater microporosity is obtained than by direct activation of hydrargillite from the Bayer process.

In the by means of agglomeration of partially dehydrated Hydrargillite bodies can be obtained by controlling the amount of water used to moisten is required and by using a very narrow grain size distribution of the particles a large-pore size Create porosity. The disadvantage of this process, however, is that it is difficult on an industrial scale a narrow grain size distribution can be obtained and this problem can, as is well known, only difficult to solve.

However, it is possible to obtain porous alumina agglomerates with macroporosity if one uses as

r, raw materials that have to be partially dehydrated in hot gas streams, other alumina hydrates are used as hydrargillite. According to an older proposal (DE-OS 24 11 846), a large-pored one is obtained Alumina agglomerates based on precipitated gels, which not only have a considerable macroporosity, but also already have good strength. However, in order to complete the large-scale implementation To obtain reproducible results, this process must be very good in all phases or stages

4j controlled v / earth due to the unstable nature of the precipitated gels, which can develop or change under the influence of numerous factors.

It has now been shown that a different alumina tri

hydrate variety, the bayerite, when dehydrating in

■> () Hot gas flows to severely disorganized clay particles leads that set through rehydration and after activation provide porous agglomerates that form a Have surprisingly high mechanical strength, which is also at fairly high temperatures in the area

) ^r ) of 1000 ° C is retained. It has also been shown that bayerite itself gives these agglomerates only a low porosity, but that the desired modifications of the pore properties are probably achieved as a result of the strength it brings about.

W) the, whereby a macroporosity arises without the mechanical properties of the agglomerates are significantly impaired. These modifications can by the various known methods mentioned above as well as by others less

μ known methods, for example the various hydrothermal treatments can be achieved as well as the fact that together with bayerite other alumina hydrate varieties are used that are suitable

contribute to porosity.

It is also known from the aforementioned US Pat. No. 2,881,051 to dehydrate bayerite in a stream of hot gas and to agglomerate the powder obtained and, after rehydration, to activate it. The practical one However, implementation is not described, nor does this publication provide any information on a resulting macroporosity.

It is surprising that alumina agglomerates, too their production 15 to 80 wt .-% partially dehydrated Bayerite are used, have a practically twice as high proportion of macroporosity such as agglomerates which are partially dehydrated with 90% by weight or only 5% by weight of bayerite in the total mass Alumina hydrate have been produced. So based on Bayerit the most appropriate strongly disorganized clays are obtained, the starting material should continue to be completely dehydrated until the amount of residual water that can be removed by burning at high temperature, makes up a maximum of 10% by weight. Complex mixtures of different alumina hydrate varieties treated, of course, in order to obtain sufficient disorganization of the existing bayerite, the residual water content of these treated mixtures are of the same order of magnitude. Continue to have to the mixtures therefore contain a sufficient amount of dehydrated bayerite because of the strength the agglomerate is due to this component; the other components contribute less to the jo Strength and more to porosity.

Due to the great strength brought about by the bayerite, other porous agglomerates can also be obtained starting from complex starting mixtures to which further components have been added, especially the clays of the various crystalline varieties, optionally treated in hot gas streams, as well Zeolites or molecular sieves. These additives can also be elements or compounds with a catalytic effect of various types, especially oxides and metals that are added to the alumina from the hot gas treatment, after this, after shaping or also to the sodium aluminate solution or the aluminum salts, from their solutions the alumina is precipitated which v, then the hot gas is subjected to exposure.

In practice, the most varied of shaping processes for agglomeration are used Above all, deforming into spheres in a rotary granulator, deforming into rings or cylinders using w Extrusion and compression deformation. You get agglomerates with reproducible properties, what is of great technical importance.

The conditions under which alumina hydrates more or less rich in bayerite are obtained γ are well known. Usual processes consist, for example, in the fact that alkali aluminates or aluminum salts are precipitated at temperatures a little above room temperature and at a relatively high pH of about ^ O to 13; preferably, at these same pH values, gels are allowed to develop or age which have been precipitated at less high pH values. The duration of this further development (maturation), the temperatures at which it takes place, as well as certain other conditions such as the nature and concentration of the reactants change the results in such a way that the products obtained combined very different compositions with regard to the amount of bayerite present and the nature of the others May have alumina hydrate varieties; the latter can be amorphous or crystallized, as norstrandite, hydragillite or boehm ”.

In the following examples, the precipitates were either starting from sodium aluminate or obtained from aluminum sulphate and the summary (general) information relates to the precipitation conditions and the composition of these precipitates.

In the first example, the properties of the spheres are given, starting from the disorganized Clays were produced, soft when treated in the hot gas stream of containing alumina result in different amounts of bayerite. The following examples relate to spheres whose Macropore volume by adding further compounds, which can be removed by firing, has been increased, - also on balls, which starting from a mixture of clays and starting from a mixture of clay and molecular sieve have been obtained. The last example shows that preferably the dehydration of bayerite should be driven sufficiently far so that the best results

example 1

This example mainly explains the relationships that exist between the proportion of bayerite in the clays that are subsequently dewatered in the hot gas stream and the microporosity of the agglomerates that are produced on the basis of the dehydrated clays. In order to clearly show that the reduction in microporosity is due to the special properties of bayerite, the microporosity of the original clays, which were simply dried by spraying and then ^{fired at 500 °} C., and the microporosity of clays after dehydration were also measured before agglomeration, the microporosity of the agglomerates obtained and the total porosity.

The different clays tested are referred to below as A ₁ B, C, D, E, F ₁ G; the manner of their preparation is given below.

Alumina A - 15% bayerite

An alumina gel suspension containing 50 g / l Al2O3 was produced by precipitating a sodium aluminate solution with nitric acid solution at 30 ° C. and at a constant pH of 8.7. This suspension was kept at the precipitation temperature for 6 h and then filtered; the filter cake was washed with demineralized water and resuspended in water and this suspension is atomized in hot gas of 300 ⁰ C.

Alumina B - 39% bayerite

The manufacturing process was analogous; the precipitation was carried out at 43 ° C. and at pH 10; the suspension obtained starting from the washed filter cake containing 150 g / l A ^ Oj and was by sputtering in He ßgase of 450 ⁰ C dried.

Alumina C - 50% bayerite

It was prepared separately in the course of 5 hours and containing 80% bayerite

suspended in water, added to a gel of amorphous clay which had been obtained by precipitating sodium aluminate solution with saleptic acid at 25 ° C. and at pH 7.5; the quantitative ratio was 30% alumina hydrate containing 80% bayerite to 70% alumina gel, in each case calculated as Al2O3. The mixture was allowed to develop or age for 20 hours; it was then filtered and the cake obtained was washed. The cake was suspended again in water and the suspension was dried ^{at 450 ° C. by atomization}

Alumina D - 65% bayerite

The same procedure was used as for Alumina C; just the time to let the The mixture was different and here was 36 hours.

Alumina E - 75% bayerite

It was turned on at 45 ° C and at pH 9.5 Alumina gel, precipitated from water gins and nitric acid, filtered, washed and re-added Suspended in water at room temperature. The suspension contained 150 g / l Al2O3; after a further development (Aging) of 2 days, the suspension was dried by spraying at 450 ° C

Alumina F - 80% bayerite

The same procedure was followed as for Alumina E except that the development and The aging time of the suspension is based here on 3 days.

Alumina G - 90% bayerite

An aluminum sulfate ^{solution was continuously precipitated at 40 °} C. and pH 11 with a sodium aluminate solution. The concentrations of the solutions were such that the suspension obtained contained 50 g / l Al2O3. The suspension was kept at room temperature for 12 h and then filtered and washed Cake was suspended again in water and the suspension was dried at ^{300 ° C. by atomization.}

The various clays A, B, C, D, E, F and G thus obtained were shown in FIG Hot gas flows partially dehydrated so that they still contained 3 to 8% residual water, which could be removed by firing at a high temperature. Under Under these conditions, the bayerite could no longer be seen in the X-ray diffraction diagram.

These dehydrated clays were then re-moistened and added to balls in the rotary granulator Deformed diameter 3 to 4 mm; the balls were matured in the oven at 105 "C for 24 hours.

For activation, some of the spheres were then fired at 650 ° C. for 2 hours and the other part at 1000 ° C. for 24 hours. To clearly show the origin of the microporosity (VmP) in the agglomerates fired at 650 ° C.

This microporosity was measured not only on these (fired) products, but also on the clays partially dehydrated in hot gas streams and on the clays after spray drying and subsequent firing at 500 ^° C. This microporosity was determined with the aid of carbon tetrachloride. Furthermore, the total porosity of the ^{agglomerates fired at 650 °} C. was determined with the aid of a mercury porosimeter, as well as the specific surfaces of the partially dehydrated clays

3ü and the agglomerated and at 650 ⁰ C fired products. Finally, the crush strength was determined grain each grain of the baked at 650 ⁰ C, and fired at 1000 ° C agglomerates.

The results are summarized in Table 1 below

Table 1 After atomization
practice drying
and burning at
500 C After drainage spec.
Surface
m ² / g VmP
cm-Vg After agglomerating and
at 650 C spec.
surface
m ² / g Burn After burning
at 1000 C Clay VmP
cmVg VmP
cm ³ / g 300 0.75 Total-
porosity
cmVg 401 Crushing
strength
kg Crushing
strength
kg 0.60 0.51 310 0.70 0.95 220 4th 2.5 15% Bayerite A 0.55 0.40 310 0.65 0.90 205 5 4th 39% Bayerite B 0.48 0.33 250 0.60 0.80 215 6th 5 50% BayeritC 0.42 0.31 230 0.56 0.78 220 6.5 4.5 65% Bayerite D 0.39 0.28 210 0.50 0.75 210 7th 4.5 75% Bayerite E. 0.35 0.26 185 0.47 0.70 196 15th 7th 80% Bayerite F 0.18 0.13 0.57 18th 7.3 90% Bayerite G.

The comparison of these test results clearly shows that the micropore volumes of the Agglomeration of the different clays obtained spheres change in the same sense as the Micropore volumes of the clays from which it is assumed, in the same sense as in the case of the dehydrated Clays and in the opposite sense as the bayerite content of the clays. The micropore volumes the spheres, however, are larger than those of the clays, probably because of dehydration obtained disorganized clay is rehydrated after bo agglomeration and thereby crystals in Forms of highly porous felts

The increase in strength with the level of bayerite shows the importance of the agglomerates, which starting from clays containing sufficient b5 bayerite.

The results obtained after firing the balls at 1000 ^° C. for 24 hours show that the crush resistance is still sufficiently good.

25 Ol 056

Example 2

This example concerns spheres made from a mixture of clays, one of which is based from a bayerite-rich product and the other from an alumina gel without bayerite.

The gel, which did not contain bayerite and was obtained at a pH of about 8, consisted mainly of pseudoboehmite and amorphous phases. It was washed, spray dried and then dehydrated in a stream of hot gas such that the residual water content (removable only by burning at high temperature) was 8% by weight. 68 kg of this dehydrated gel were _{mixed with 103 kg (calculated as Al 2} O ₃ ) of dehydrated alumina D according to the previous example, which contained 4% by weight of residual water which could be removed at a high temperature. The mixture of the two clays became spheres in the rotary granulator

Deformed diameter 3 to 4 mm; the balls wurdei then ripened and then baked at 650 ⁰ C and 1000 ⁰ C be. The specific surface, the microporosity (VmP), the total porosity and the crush strength were determined for these fired spheres; the results are summarized in the following table:

Table 2

After firing at 650 "C

After burner at 1000 ° C

Special surface itiVg 213

VmP cm ³ / g 0.73

Total porosity cmVg 1.01

Crush strength kg 4.3

126 0.70 0.95 3.3

Claims (1)

Claim: High-strength macroporous alumina agglomerates, especially for adsorption and catalysis purposes, by moistening and agglomerating alumina powders partially in the hot gas stream dehydrated bayerite and subsequent activation by firing have been obtained thereby characterized in that in the production 15 to 80 wt .-% of the total mass partially dehydrated bayerite with a maximum of 10% by weight residual water obtained by burning at high temperature can be eliminated have been used.