Process for preparing granulated magnesium hydroxide and magnesia of a large specific surface

Granulated magnesium hydroxide is prepared by agglomerating finely divided magnesia with water as a binding agent to produce granules, the water serving to hydrate the magnesia. The magnesium hydroxide may be thermally decomposed to obtain magnesia of a large specific surface, such as microporous magnesia.

The present invention relates to the preparation of granulated magnesium 
hydroxide by hydrating magnesia and, in one aspect thereof, to the thermal 
decomposition of the magnesium hydroxide to obtain magnesia of a large 
specific surface. 
In one known process, magnesia obtained by thermal hydrolysis is introduced 
into water to hydrate the same, the amount of water exceeding that of the 
magnesia by about 5 to 10 times, the magnesia being suspended in the water 
and the suspension being heated to accelerate the hydration reaction. This 
produces an aqueous suspension of magnesium hydroxide and the magnesium 
hydroxide is then mechanically separated from the suspension, the 
resultant filter cake containing about 50% water. This is then granulated 
with the addition of previously dried, pulverized magnesium hydroxide, if 
desired, and the granules are subjected to heat to dry them. This process 
requires large apparatus and a high expenditure of time and energy since 
the large excess amount of water is heated to accelerate the reaction and 
a relatively large amount of water remains in the filter cake, which must 
be evaporated by a corresponding amount of heat. 
It is the primary object of this invention to improve the production of 
magnesium hydroxide granules from magnesia by reducing the apparatus, time 
and energy requirements. 
It is another object of the invention to provide an improved process for 
preparing magnesia of a large specific surface. 
The above and other objects and advantages are accomplished in accordance 
with the present invention by agglomerating finely divided magnesia by the 
addition of sufficient water to act as a binding agent for producing 
granules, and hydrating the magnesia with the added water. The granules 
may be subjected to heat, either during or after agglomeration, to 
inititate the exothermic hydration reaction. 
If desired, the autogenously dried magnesium hydroxide may be thermally 
decomposed to obtain magnesia of a large specific surface, for instance 
microporous magnesia, in granular form. 
With the process of this invention, magnesium hydroxide in granulated form 
is obtained in a fast and simple step with little expenditure of time or 
energy. The resultant granules have a high stability and, therefore, may 
be handled simple without any substantial dusting problem. The granules 
are practically dry without any need for supplying external heat energy 
for drying. Since the hydration reaction is exothermic, any heat supplied 
to inititate the reaction may be held to a low level. All of these 
advantages are obtained by reversing the conventional procedure of first 
hydrating in an excess of water and then granulating the hydrate, the 
magnesia in the process of the invention being first granulated and the 
water, which serves as a binding agent in the agglomeration, continuing 
then to serve as a hydrating agent immediately following the 
agglomeration. Since the hydration proceeds exothermically, it often 
starts in short time after the water has been added to the finely divided 
magnesia, depending on the grain size of the magnesia or its porosity and 
the like. After hydration has been initiated, the reaction accelerates and 
releases heat, causing evaporation of any excess water needed for the 
agglomeration. The resultant product are autogenously dried granules and 
no external heat is required for drying. 
A preferred starting product is magnesia calcined or burned at temperatures 
below 1200.degree. C because this type of magnesia is quickly and readily 
granulated by adding water as a binding agent, hydration of such magnesia 
starts quickly, the heat produced by the exothermic reaction accelerates 
the hydration and evaporates excess water, and magnesium hydroxide 
granules may thus be prepared from such magnesia without any external heat 
energy. 
The granulation will be further enhanced with the use of a magnesia having 
a particle size of less than 500 microns. Best results are obtained by 
agglomerating the finely divided magnesia to produce granules having a 
size at least 10 times that of the finely divided magnesia, granules 
having a size between about 0.1 mm and 50 mm being preferred. 
An advantageous heat balance during agglomeration and hydration being 
obtained with the use of 0.5 to 1 part by weight of water per part by 
weight of the finely divided magnesia. 
The start of the hydration of the magnesia may be initiated by subjecting 
the granules to external heat which may be minimal in view of the 
exothermic nature of the hydration reaction. Such heating may be 
exceedingly simple, for instance by adding warm water or by heating the 
granulating device wherein the magnesia is agglomerated. This has the 
advantage of initiating the hydration reaction even while the magnesia is 
agglomerated. 
If the granulated mass is conducted through a conveying path, for instance 
on a conveyor band, the granules may be subjected to heat in the conveying 
path to start hydration quickly during the transfer of the granules. The 
heat may be supplied in the form of infrared radiation or by hot air, for 
instance. 
It is also possible to introduce a heated rod into the mass of granules to 
initiate hydration at one site and the thus initiated hydration reaction 
will then spread through the entire mass of granules. 
Furthermore, alone or in conjunction with any of the heating procedures 
outlined hereinabove, the start of the hydration reaction may be initiated 
by adding to the finely divided magnesia concentrated at one site a small 
amount of magnesia of a large specific surface. Magnesia of a large 
specific surface, such as microporous magnesia, is hydrated more readily 
and faster than ordinary magnesia. 
The exothermic hydration reaction may be permitted to proceed without 
further subjecting it to heat after the reaction has been initiated, 
either spontaneously or by external heating. 
It is not necessary to use pure magnesia as the starting material. 
Naturally occurring ore with its usual impurities may be used, such 
magnesia produced from natural magnesite without special purification. 
If a residual amount of magnesia in the magnesium hydroxide end product 
makes no difference, an amount of water may be added to the finely divided 
magnesia sufficient for the agglomeration thereof but insufficient to 
hydrate all of the magnesia, which makes is possible more readily to 
control the granulation. In this case, the amount of water is preferably 
so chosen that at least 50%, by weight, of the magnesia is converted to 
magnesium hydroxide. 
Throughout the specification and claims, a large specific surface is a 
surface, measured according to the B.E.T. method, of more than 75 
sq.m./gram. Magnesia of such large specific surface is designated as 
"highly active". 
In the above-described known process, such highly active magnesia is 
produced from the dried magnesium hydroxide by the thermal decomposition 
thereof, with all the attendant requirements for apparatus, energy and 
time. All this is avoided according to the invention because the 
autogenous dried granules resulting from the hydration of the magnesia may 
be thermally decomposed to become magnesia of a large specific surface. If 
desired, the resultant magnesia may be ground. The thermal decomposition 
temperature may be between about 300.degree. C and 500.degree. C. In this 
range, a product of particularly large specific surface, for instance up 
to 200 sq.m./g., may be obtained. 
Since the magnesium hydrate is in relatively stable granular form, 
practically no dusting problem occurs during the thermal decomposition 
thereof. The thermal decomposition causes no comminutation of the granules 
so that the end product is granulated, highly active magnesia which may be 
readily ground in any suitable comminuting apparatus, the grinding being 
particularly simple because of the high porosity of the product. 
Comminution may proceed to particle sizes below 10 microns. 
The resultant magnesia is very useful, for instance, as a catalyst, a 
catalyst carrier or an adsorbent. It is also useful in finely divided form 
as a filler in elastomers or adhesives.

Without being limitative, the following examples illustrate the practice of 
the present invention. 
EXAMPLE 1 
Fifty kilograms of magnesia having a particle size of less than 500 microns 
and produced by the thermal decomposition of natural magnesite ore at 
about 800.degree. C, commercially available as "caustically burnt 
magnesite", was granulated by adding thereto 28 liters of water as a 
binding agent. The water addition rate was so controlled that the 
resultant granules had a diameter of about 5 mm. (Like results were 
obtained by so controlling the water addition that the resultant granules 
had diameters between 0.1 mm and 50 mm.) 
The mass of granules was divided into two halves. 
One half of the granules was placed into an open container and left there. 
The hydration reaction proceeded slowly and was completed after about 360 
minutes. The exothermic reaction caused excess water to evaporate and, 
after completion of the reaction, only dried granules remained in the 
container, an analysis of the dried mass of magnesium hydroxide granules 
showing a humidity of 0.5% and an ignition loss of 30.8% after calcining 
the granules for 2 hours at a temperature of 1050.degree. C. The humidity 
was determined by comminuting a test sample of the granules to sufficient 
fineness for analysis, weighing the test sample, then washing the weighed 
sample with alcohol to remove all water, removing the alcohol, and then 
again weighing the sample, the difference in the weight between the first 
and second weighing giving the percentage of humidity. 
The product had an SiO.sub.2 content of 0.8%, an Fe.sub.2 O.sub.3 content 
of 0.8%, an Al.sub.2 O.sub.3 content of 0.1%, a CaO content of 4.5% and 
SO.sub.4 " content of 0.3%, and a Cl' content of 0.1%. 
The second half of the granules was also placed into a container but the 
hydration reaction was initiated by locally heating the mass of granules 
for a short time, this heating being effected by heating a small portion 
of the container bottom. This reduced the hydration time to 90 minutes. 
The resultant product was identical. 
EXAMPLE 2 
Fifty kilograms of magnesia produced by calcining seawater magnesium 
hydroxide and commercially available as such was granulated by adding 
thereto 35 liters of water as a binding agent, the water addition rate 
being so controlled that the resultant granules had a diameter of 15 mm. 
As indicated in Example 1, like results were obtained by controlling the 
water addition so as to obtain other granule diameters. The mass of 
granules was again divided into two halves. 
One half of the granules was placed into an open container and left there, 
the hydration being completed within 240 minutes and the resultant 
magnesium hydroxide granules having a humidity of 1.2% and an ignition 
loss of 31.9% under conditions analogous to those of Example 1. The 
product analyszed to 1% SiO.sub.2, 1.4% Fe.sub.2 O.sub.3, 0.4% Al.sub.2 
O.sub.3, 0.8% CaO, 1% So.sub.4 ' and 0.2% Cl'. 
The other half of the granules was also placed into a container and a 
heated rod was introduced into the mass to initiate the hydration reaction 
at one site, thus reducing the hydration time to 60 minutes. The product 
was identical to that of the first half. 
EXAMPLE 3 
Fifty kilograms of magnesia produced by the thermal decomposition of a 
magnesium chloride solution at 650.degree. C was granulated by the 
addition of 25 liters of water as a binding agent, the water addition rate 
being so controlled that the resultant granules had a diameter of 2 to 3 
mm. 
One half of the granules was placed in a container and left there, the 
hydration being completed within about 150 minutes and the resultant 
magnesium hydroxide granules having a humidity of 1% and an ignition loss 
of 32.7% under conditions analogous to those of Example 1. The product 
analyzed to 0.005% SiO.sub.2, 0.02% Fe.sub.2 O.sub.3, 0.01% Al.sub.2 
O.sub.3, 0.9% CaO, 1.5% SO.sub.4 " and 0.6% Cl'. 
The other half of the granules was also placed into an open container and 
treated like the other half in Example 2, with identical results. 
EXAMPLE 4 
Fifty kilograms of the magnesia used as starting material in Example 1 was 
granulated by adding thereto 18 liters of water as a binding agent to 
obtain granules similar to those of Example 1 but the water amount was 
sufficient for the hydration of only a portion of the magnesia, as 
determined by the ignition loss of the granules which was only 20.7%. 
EXAMPLE 5 
Example 2 was repeated with the addition of only 20 liters of water to 
produce only partial hydration of the magnesia, as determined by the 
ignition loss of the granules which was only 24.7%. 
EXAMPLE 6 
Example 3 was repeated with the addition of only 15 liters of water to 
produce only partial hydration, as determined by the ignition loss of the 
granules which was only 19.7%. 
EXAMPLE 7 
Both halves of the autogenously dried magnesium hydroxide granules obtained 
according to Example 1 were subjected to a thermal treatment at about 
450.degree. C in an indirectly heated rotary kiln. At an output of about 5 
kg/hour, highly active magnesia was produced in the kiln. 
After each hour, a test sample was removed from the kiln and the ignition 
loss as well as the specific surface (according to the B.E.T. method) of 
the sample product was determined. The average value of all measurements 
was an ignition loss of about 5.8%, rising to about 8.1% for those samples 
having the largest specific surface. The average specific surface was 183 
sq.m./g., with a maximum value of 204 sq.m./g. 
EXAMPLE 8 
The autogenously dried magnesium hydroxide granules of Example 2 were 
subjected to the same thermal decomposition treatment as used in Example 
7. The test samples removed from the kiln showed an average ignition loss 
of 7.2% rising to 7.6% at the largest specific surface. The average 
specific surface of the product was 150 sq.m./g., with a maximum value of 
161 sq.m./g. 
EXAMPLE 9 
The autogenously dried magnesium hydroxide granules of Example 3 were 
subjected to the same thermal decomposition treatment as used in Example 
7. The test samples showed an average ignition loss of 6.5%, falling to 6% 
at the largest specific surface. The average specific surface of the 
samples was 135 sq.m./g., with maximum value of 144 sq.m./g. 
The data and values of analysis for Examples 7 to 9 are shown in following 
Table I. 
TABLE I 
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Example 7 8 9 
Amount of starting material (kg) 
50 50 50 
Addition of water (liter) 
28 35 25 
Hydration time (minutes-approximate) 
without external heating 
360 240 150 
with external heating 
90 60 60 
Humidity (weight %) 0.5 1.2 1.0 
Ignition loss (2 h at 1050.degree. C) (weight%) 
30.8 31.9 32.7 
Chemical analysis of the dried 
Mg(OH).sub.2 granules (calculated free of 
ignition loss) (weight%): 
SiO.sub.2 0.8 1.0 0.005 
Fe.sub.2 O.sub.3 0.8 1.4 0.02 
A1.sub.2 O.sub.3 0.1 0.4 0.01 
CaO 4.5 0.8 0.9 
So.sub.4 " 0.3 1.0 1.5 
C1' 0.1 0.2 0.6 
Analysis of the activated 
magnesia granules: 
Average ignition loss (weight%) 
5.8 7.2 6.5 
Ignition loss at maximal specific 
surface (weight%) 8.1 7.6 6.0 
Specific surface (according to 
B.E.T. method) (sq.m./g.) - average value 
183 150 135 
maximum value 204 161 144 
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EXAMPLE 10 
The autogenously dried granules of Example 4 were subjected to the same 
thermal decomposition treatment used in Example 7. The tests showed an 
average ignition loss of 8.3%, falling to 5.2% at the largest specific 
surface. The average specific surface of the test samples was 104 
sq.m./g., with a maximum value of 134 sq.m./g. 
EXAMPLE 11 
The autogenously dried granules of Example 5 were subjected to the same 
thermal decomposition used in Example 7, with the test showing an average 
ignition loss of 7.4% falling to 6.5% at maximum specific surface. The 
average specific surface was 122 sq.m./g., with the maximum being 138 
sq.m./g. 
EXAMPLE 12 
The autogenously dried granules of Example 6 were subjected to the thermal 
decomposition treatment of Example 7, with the tests showing an average 
ignition loss of 6%, rising to 7% at maximum specific surface. The average 
specific surface was 78 sq.m./g., with maximum value being 108 sq.m./g. 
The data and values of analysis for Examples 10 to 12 are shown in 
following Table II. 
TABLE II 
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Example 10 11 12 
Amount of starting material (kg) 
50 50 50 
Addition of wter (1) 18 20 15 
Analysis of autogeneously dried 
granules: 
Humidity (weight%) 0.8 1.7 1.5 
Ignition loss (2 h at 1050.degree. C) (weight%) 
20.7 24.7 19.7 
Analysis of the magnesia granules: 
Average ignition loss (weight%) 
8.3 7.4 6.0 
Ignition loss at maximum specific 
surface (weight%) 5.2 6.5 7.0 
Specific surface (according to B.E.T. 
method) (sq.m./g.) 
average value 104 122 78 
maximum value 134 138 108 
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