Acidic gas absorbent and process for production thereof

A molded acidic gas absorbent produced by press molding powders, etc. of calcium hydroxide and one or more powders, etc. of alkali metal hydroxides and alkaline earth metal hydroxides except for calcium hydroxide, impregnating the molded article with water, followed by heat treatment while maintaining a predetermined amount of water therein is effective for absorbing an acidic gas such as CO.sub.2 in air and a narcotic gas.

BACKGROUND OF THE INVENTION 
This invention relates to an absorbent for acidic gases, such as carbon 
dioxide, HCl gas, HF gas, etc., used to clean air, narcotic gas, etc., and 
a process for the production thereof. 
Soda lime is widely used as an absorbent for carbon dioxide contained in 
air and narcotic gas. In general, it is produced by immersing calcium 
oxide in a concentrated sodium hydroxide aqueous solution, subjecting the 
solution to heat treatment and pulverizing the resultant aggregate into 
particles, by mixing calcium hydroxide with other hydroxide and water, 
heating the mixture and pulverizing the resultant aggregate into 
particles, or by some other method. 
However, the soda lime produced as above has an irregular physical form and 
often has an acute angle partially. For this reason, the soda lime has a 
problem in that when it is packed in a container, etc., and shipped, its 
individual particles grind one another by friction to crush the acute 
angles thereof, which results in formation of a fine powder or dust-like 
soda lime. That is, this fine powder or dust-like soda lime is inherently 
corrosive. For example, when there is used a device equipped with a column 
charged with this soda lime as an absorbent to remove carbon dioxide from 
a narcotic gas, the fine powder or dust of the soda lime is fed together 
with the narcotic gas and a narcotized patient might be seriously affected 
by breathing it in. Further, this soda lime may cause a problem in that 
its dust flies away into a room and pollutes air in the room when the 
column is recharged. 
There have been proposed a variety of methods to solve these problems. For 
example, these problems might be solved by coating particle of the soda 
lime with, e.g. a gelatin-like substance such as dextran, by adding 
carboxymethyl cellulose to the soda lime (Japanese Patent Publication No. 
42-20464), by forming the soda lime into semisperical particles, or by 
some other method. 
Soda lime particles produced according to these methods are not necessarily 
satisfactory as an absorbent in view of performance and cost. For example, 
the coating method not only increases the production cost, but also causes 
a problem in that the capabilitiy of absorbing carbon dioxide is reduced 
depending upon a gelatin-like substance used to coat the particles. The 
method using carboxymethyl cellulose is costwise unsatisfactory. Further, 
although the method of forming semispherical soda lime makes it possible, 
to some extent, to reduce physical shape-induced formation of a fine 
powder or duct in transportion, such a semispherical soda lime has a 
problem in that the formation of a fine powder or dust cannot be 
satisfactorily prevented due to its low hardness. 
SUMMARY OF THE INVENTION 
This invention has been made by taking the above situations into 
consideration, and the present object is to provide a process for the 
production of an absorbent which is almost free from the formation of a 
fine powder and dust and which has a high capability of absorbing acidic 
gas such as carbon dioxide. 
The present invention provides a process for producing a molded acidic gas 
absorbent, which comprises 
press molding at least one selected from the group consisting of powders, 
particles and granules of calcium hydroxide and at least one member 
selected from the group consisting of powders, particles and granules of 
alkali metal hydroxides and alkaline earth metal hydroxides except for 
calcium hydroxide, 
impregnating the resulting molded article with water, and 
heat treating the resulting article so as to maintain a predetermined 
amount of water therein. 
The present invention also provides a molded acidic gas absorbent 
comprising calcium hydroxide, and at least one member selected from the 
group consisting of alkali metal hydroxides and alkaline earth metal 
hydroxides except for calcium hydroxide, and water, and having an average 
hardness of 900 to 2600 g and a melt solidified surface in an area of 50% 
or more.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present inventors have diligently studied a process for the production 
of an acidic gas absorbent containing soda lime as a main component, in 
which the formation of a fine powder or dust due to friction and vibration 
during the transportion is reduced without decreasing the capability of 
absorbing carbon dioxide in particular. As a result, it was found that an 
acidic gas absorbent having a high capability of absorbing acidic gas and 
being almost free from the formation of a fine powder and dust due to 
friction and vibration during the transportion can be produced by a 
process which comprises press molding powders, particles and/or granules 
containing calcium hydroxide and hydroxide of an alkali metal, etc., (the 
resultant product is abbreviated as "a molded product" hereinbelow), then 
impregnating the molded product with water and subjecting the molded 
product to heat treating. 
The calcium hydroxide, hydroxide of an alkali metal, etc., and water used 
in this invention are not particularly limited if they do not contain 
impurities which cause a problem when the acidic gas absorbent of this 
invention is used as an absorbent of carbon dioxide contained in air, 
narcotic gas, etc. Thus, these components are not required to have a 
specially high purity. 
Examples of the hydroxide of an alkali metal used in this invention are 
hydroxides of alkali metals such as lithium, sodium, potassium, etc. 
Examples of the hydroxide of an alkaline earth metal other than calcium 
are barium hydroxide, magnesium hydroxide, etc. These hydroxides of alkali 
metals, etc., may be used alone or in combination. 
The acidic gas absorbent of this invention may be produced as follows. 
First, a mixture of a powder, particles and/or granules of calcium 
hydroxide with a powder, particles and/or granules of hydroxide of an 
alkali metal, etc., or powders, particles and/or granules of a mixture of 
calcium hydroxide with hydroxide of an alkali metal, etc., is press molded 
into a suitable form, e.g. a tablet form (circular plate, disk, etc.). The 
molded product is then impregnated with water according to an ordinary 
method such as spraying using a sugar coater, coating pan, or the like. 
Thereafter, the resultant product is heat treated, whereby an acidic gas 
absorbent molded product of this invention can be obtained. 
The process of this invention has characteristics in this heat treating 
wherein the molded product is heated so as to maintain a predetermined 
amount of water therein. It is considered that this treatment brings a 
state in which at least 50% or more, usually 70% or more, preferably 80% 
or more, of the resultant acidic gas absorbent molded product surface is 
melt-solidified to increase the hardness of the absorbent, whereby the 
formation ratio of a fine powder and dust during the transportion can be 
consequently reduced. 
When the powders, particles and/or granules containing calcium hydroxide 
and hydroxide of an alkali metal, etc., are press molded, the mixing ratio 
of the calcium hydroxide and hydroxide of an alkali metal, etc., in the 
powders, particles and/or granules is preferably adjusted in such a range 
that the resultant absorbent of this invention can effectively work. Based 
on 100 parts by weight of the calcium hydroxide, in general, the amount of 
the hydroxide of an alkali metal, etc., is about 1 to 7 parts by weight, 
preferably 3 to 5 parts by weight. 
The method of press molding is not particularly limited, so long as there 
is produced a molded product having almost no acute angles which are 
liable to form a fine powder due to friction, etc. For example, a method 
using a commercially available tablet-making machine is preferable in view 
of machine availability and operationability. 
The size of the molded product is not particularly limited if the molded 
product can be filled, in a suitable density, in a column, etc., into 
which a usual acidic gas absorbent is filled. In general, for example, a 
tablet of the absorbent has a diameter of 1 to 10 mm, preferably about 2 
to 7 mm, and a thickness of 1 to 10 mm, preferably about 1.5 to 3 mm. 
The pressure for the press molding is very important. That is, with 
increasing the pressure, the acidic gas absorption capability of the 
resultant absorbent decreases, although the formation of a fine powder and 
dust is reduced. With decreasing the pressure, the formation of a fine 
powder and dust increases due to low hardness, although the acidic gas 
absorption capability of the resultant absorbent increases. When the 
acidic gas absorbent of this invention is produced, the pressure for the 
molding is usually 200 to 1,300 Kg/cm.sup.2, preferably 250 to 800 
Kg/cm.sup.2, and more preferably 250 to 400 Kg/cm.sup.2. The molded 
product in a circular plate form has an average hardness of 200 to 2,000 
g, preferably 300 to 1,200 g, and more preferably about 500 g measured by 
using a jelly strength measuring apparatus of Japanese Society of Agar 
Fisheries type. 
In general, the molded product is allowed to contain water by fully 
spraying the water by means of, e.g. a coating pan, and the amount of 
water contained therein is usually 5 to 21 W/W%, preferably 14 to 19 W/W%. 
In the heat treatment of the molded product containing water, the heating 
temperature is usually 50.degree. to 120.degree. C., preferably 75.degree. 
to 85.degree. C., and the heating time is usually 1 to 24 hours, 
preferably 4 to 15 hours. When the heat treating is carried out, it is 
necessary to take care so as to make the water contained in the molded 
product hardly evaporate or dissipate. This is because it is an essential 
requirement of the present acidic gas absorbent to maintain the specified 
amount of water so as to exhibit the function as the gas absorbent. 
Therefore, it is preferable to carry out the heat treating in a state that 
water vapor is nearly saturated. The most simplest method is carried out, 
for example, by packing the above molded product in a sealable container 
or bag, bringing it into a sealed or nearly closed state, and carrying out 
the heat treating in such a state. There is no special limitation to be 
imposed on methods to be taken actually, if the heat treating can be 
carried out under such conditions. In addition, the amount of water 
contained in the acidic gas absorbent of this invention is usually 5 to 21 
W/W%, preferably 14 to 19 W/W%. 
The acidic gas absorbent of this invention, produced as specified above, 
has characteristics in that not only the acidic gas absorption capability 
thereof is high but also the amount of a fine powder and dust formed 
during the transportation is very small. The acidic gas absorbent of this 
invention, e.g. in a circular plate-like tablet form, has an average 
hardness, usually, in the range of from 900 to 2,600 g. Such an absorbent 
that has a hardness of 900 to 1,400 g in particular is more preferable, 
since the amount of a formed fine powder and dust is small and the acidic 
absorption capability thereof is high. 
In the acidic gas absorbent of this invention, as can be assumed from its 
purpose in use, the amounts of individual components contained therein are 
not required to be constant, and their weights are not required to be 
constant, either. Nor is it necessary to prepare its appearance, size, 
etc, uniformly. 
An indicator may be incorporated into the acidic gas absorbent of this 
invention to detect a residual capability of absorbing acidic gas. For 
example, if an acid-alkali indicator such as ethyl violet, titanium 
yellow, Congo Red, or the like is incorporated into the acidic gas 
absorbent of this invention, the absorbent undergoes a color change when 
the alkalinity of the absorbent decreases by absorbing acidic gas, or in 
other words, when the acidic gas absorption capability thereof lowers, 
whereby the time for absorbent replacement can be determined by observing 
the color change with the eyes. The above acid-alkali indicator can be 
incorporated into the acidic gas absorbent of this invention by a method 
of mixing the indicator at a molding time, by a method of dissolving the 
indicator in water to be contained in the molded product, or by some other 
method. Any method of these can be used. 
The form of the acidic gas absorbent of this invention is not critical, if 
it is not a form partically having such an acute angle that is liable to 
form a fine powder due to friction and attrition. Specific examples of the 
form are spherical, semispherical, and tablet-type forms. The tablet-type 
form is, in general, circular plate or disk-like as shown in FIG. 1 and 5. 
And, forms shown in FIGS. 2 to 4 may be also employed. When the absorbent 
is formed in one of these forms, the absorbent can have a large surface 
area, and therefore, it can naturally have improved capability of 
absorbing acidic gas. 
This invention will be explained more in detail by reference to Examples 
and Comparative Examples, in which all percents are by weight unless 
otherwise specified. 
EXPERIMENTS 1-6 
(1) Preparation of tablet-type absorbent of acidic gas 
Calcium hydroxide (6 kg) in a powder form and 280 g of sodium hydroxide in 
a powder form were fully mixed, and then divided into six equal portions. 
These portions were molded into tablets having a diameter of 5 mm and a 
height of 2.5 mm by using a commercially available tablet-making machine 
under a given pressure. The resultant tablets were sprayed and impregnated 
with water so as to have a water content of 17% by using a commercially 
available coating pan. Then, the tablets were sealed in a polyethylene 
bag, and the bag was allowed to stand in a constant temperature oven at 
80.degree. C. for 15 hours to give tablet-type absorbents for acidic gas 
having a melt solidified surface in an area of 80% or more. 
In addition, the tablet-type absorbents were measured for a water content 
(W/W%) to show that the water content was nearly the same as the content 
of water impregnated by using the coating pan. 
(2) Test on carbon dioxide absorption capability 
The carbon dioxide absorption capability of the tablet-type absorbents 
obtained in the above (1) was tested in the following manner. 
An oxygen gas containing 4.6% of carbon dioxide was flowed, at a rate of 
6,000 ml/min., through a canister (volume 950 ml) charged with 810 g of 
the tablet-type acidic gas absorbent by using a respirator (Cape Bristol 
ventilator, supplied by Nippon Medico K.K.), and the time was measured 
until a trace amount of carbon dioxide leaked in a gas flowing out of the 
absorbent canister. In addition, the carbon dioxide in the gas was 
measured by gas chromatography. 
The results are shown in Table 1. 
(3) Measurement of hardness of tablet-type acidic gas absorbent 
The hardness of the tablet-type acidic gas absorbents obtained in the above 
(1) was measured in the following manner. 
Five tablets were selected from each of the tablet-type acidic gas 
absorbents prepared under predetermined conditions, and the hardness of 
each of the tablets was measured according to an ordinary method by using 
a jelly strength measuring apparatus of Japanese Society of Agar Fisheries 
type (mfd. by Kiya Seisakusho, Ltd.). 
The measured values were statistically treated, and shown in Table 1. 
(4) Measurement of formation ratio of powder 
The formation percentage of a powder formed from the tablet-type acidic gas 
absorbents obtained in the above (1) due to vibration and friction was 
measured in the following manner. 
The tablet-type acidic gas absorbent (20 g) which was accurately weighed 
out was put in a glass bottle (volume 250 ml), and the bottle was set in a 
reciprocating shaker (supplied by Takahashi Seisakusho) and shaken for 7 
hours at a shake width of 90 mm and a shake rate of 60 times/min. The 
resultant substance was sieved to measure a formation percentage (W/W%) of 
a powder of passing 12 mesh. 
Table 1 shows the results. 
COMATIVE EXAMPLE 1 
A commercially available, massive absorbent of carbon dioxide (composed 
mainly for soda lime) was tested on carbon dioxide absorption capability 
in the same way as in Experiments 1 to 6. It was also measured for a 
hardness and powder formation percentage in the same way as Experiments 1 
to 6. 
Table 1 shows the results. 
COMATIVE EXAMPLE 2 
A commercially available, semipherical absorbent of carbon dioxide 
(composed mainly of soda lime) was tested for carbon dioxide absorption 
capability in the same way as in Experiments 1 to 6. It was also measured 
for a hardness and powder formation rate in the same way as Experiments 1 
to 6. 
Table 1 shows the results. 
TABLE 1 
______________________________________ 
Experiment 
Tablet Tablet Powder 
Nos. and making Leakage hardness 
formation 
Comparative 
pressure point (n = 5) 
percentage 
Examples (Kg/cm.sup.2) 
(hr) (g) (%) 
______________________________________ 
Experiment 1 
150 6 600 1.2 
Experiment 2 
300 6 1150 0.3 .dwnarw. 
Experiment 3 
700 5 1150 0.3 .dwnarw. 
Experiment 4 
1200 4 1150 0.3 .dwnarw. 
Experiment 5 
1600 3 2100 0.3 .dwnarw. 
Experiment 6 
2800 1 3000 0.3 .dwnarw. 
Comparative 
-- 4 1000 1.5 
Example 1 
Comparative 
-- 5.5 300 0.8 
Example 2 
______________________________________ 
Table 1 clearly shows that the tablet-type acidic gas absorbents produced 
according to the process of this invention at a tablet-making pressure of 
300 to 1,200 Kg/cm.sup.2 exhibit a clearly lower power formation 
percentage than commercially available, carbon dioxide absorbents, and 
have a carbon dioxide absorption capability equivalent to or higher than 
those of the commercially available, carbon dioxide absorbents. 
EXAMPLES 1-3 
(1) Preparation of tablet-type acidic gas absorbent 
Calcium hydroxide (3 kg) in a powder form and 140 g of sodium hydroxide in 
a powder form were fully mixed, and the mixture was molded into tablets 
having a diameter of 5 mm and a height of 2.5 mm by using a commercially 
available tablet-making machine at a tablet-making pressure of 300 
kg/cm.sup.2 (the resultant tablets had an average hardness of 500 g). The 
molded tablets were separated into three equal groups, and each group of 
the tablets was sprayed and impregnated with water so as to contain a 
predetermined amount of water by using a commercially available coating 
pan. Then, the tablets were sealed into a polyethylene bag, and the bag 
was allowed to stand in a constant temperature oven at 80.degree. C. for 
15 hours to give tablet-type acidic gas absorbents having a melt 
solidified surface in an area of 80% or more. 
The tablet-type acidic gas absorbents were measured for a water content 
(W/W%) to show that the water content was nearly the same as that of water 
impregnated by using the coating pan. 
(2) Performance test 
The tablet-type acidic gas absorbents obtained in the above (1) were tested 
on carbon dioxide absorption capability in the same way as in Experiments 
1 to 6, and measured for an average hardness and a powder formation ratio 
in the same way as in Experiments 1 to 6. 
Table 2 shows the results. For comparison, Table 2 also shows the results 
of comparative Examples 1 and 2. 
TABLE 2 
______________________________________ 
Tablet Powder 
Water Leakage hardness 
formation 
content point (n = 5) 
percentage 
Example No. 
W/W % (hr) (g) (%) 
______________________________________ 
Example 1 10 5 1150 0.3 .dwnarw. 
Example 2 15 5 1150 0.3 .dwnarw. 
Example 3 18 5 1150 0.3 .dwnarw. 
Comparative 
-- 4 1000 1.5 
Example 1 
Comparative 
-- 5.5 300 0.8 
Example 2 
______________________________________ 
Table 2 clearly shows that the tablet-type acidic gas absorbents of this 
invention obtained in Examples 1 to 3 exhibit a clearly lower powder 
formation percentage than those of the commercially available carbon 
dioxide absorbents, and have carbon dioxide absorption capability 
equivalent to or higher than those of the commercially available carbon 
dioxide absorbents. 
EXAMPLES 4-5 
(1) Preparation of tablet-type acidic gas absorbent 
Calcium hydroxide (3 kg) in a powder form and 140 g of sodium hydroxide in 
a powder form were fully mixed and the mixture was molded into tablets 
having a diameter of 5 mm and a height of 2.5 mm by using a commercially 
available tablet-making machine at a tablet-making pressure of 300 
kg/cm.sup.2 (the resultant tablets had an average hardness of 500 g). The 
molded tablets were sprayed and impregnated with water so as to have a 
water content of 17% by using a commercially available coating pan. Then, 
the tablets were separated into two equal portions in number and sealed 
into a polyethylene bag, and the bag was allowed to stand in a constant 
temperature oven at 80.degree. C. for a given period of time to give 
tablet-type acidic gas absorbents having a melt solidifed surface in an 
area of 80% or more. 
The tablet-type acidic gas absorbents were measured for a water content 
(W/W%) to show that the water content was nearly the same as that of water 
impregnated by using the coating pan. 
(2) Performance test 
The tablet-type acidic gas absorbents obtained in the above (1) were tested 
on carbon dioxide absorption capability in the same way as in Experiments 
1 to 6, and measured for an average hardness and a powder formation 
percentage in the same way as in Experiments 1 to 6. 
Table 3 shows the results. 
COMATIVE EXAMPLE 3 
Calcium hydroxide (3 kg) in a powder form and 140 g of sodium hydroxide in 
a powder form were fully mixed, and the mixture was then molded into 
tablets having a diameter of 5 mm and a height of 2.5 mm by using a 
commercially available tablet-making machine at a tablet-making pressure 
of 300 kg/cm.sup.2 (the resultant tablets had an average hardness of 500 
g). The molded tablets were sprayed and impregnated with water so as to 
have a water content of 17% by using a commercially available coating pan 
to give tablet-type absorbents. 
These tablet-type absorbents were tested on carbon dioxide absorption 
capability in the same way as in Experiments 1 to 6, and measured for an 
average hardness and a powder formation ratio in the same way as in 
Experiments 1 to 6. 
Table 3 shows the results. For comparison, Table 3 also shows the results 
of Comparative Examples 1 and 2. 
TABLE 3 
______________________________________ 
Heat Tablet Powder 
treating Leakage hardness 
formation 
time point (n = 5) 
percentage 
Example No. 
(hr) (hr) (g) (%) 
______________________________________ 
Example 4 4 6 1000 0.3 .dwnarw. 
Example 5 15 6 1150 0.3 .dwnarw. 
Comparative 
0 5 560 1.2 
Example 3 
Comparative 
-- 4 1000 1.5 
Example 1 
Comparative 
-- 5.5 300 0.8 
Example 2 
______________________________________ 
Table 3 clearly shows that the tablet-type acidic gas absorbents of this 
invention obtained in Examples 4 and 5 exhibit a clearly low powder 
formation percentage than those of the commercially available carbon 
dioxide absorbents, and have carbon dioxide absorption capability 
equivalent to or higher than those of the commercially available carbon 
dioxide absorbents. 
Further, Table 3 also shows that the absorbent obtained in Comparative 
Example 3, which was treated so as to contain water but was not heat 
treated, had a powder formation percentage nearly equivalent to those of 
the commercially available carbon dioxide absorbents, although it had a 
higher carbon dioxide absorption capability than the commercially 
available carbon dioxide absorbents. 
As described above, the acidic gas absorbent of this invention has 
characteristics in that it has not only a higher capability of absorbing 
acidic gas but also a lower fine powder formation ratio than conventional 
acidic gas absorbents. Therefore, the acidic gas absorbent of this 
invention is industrially very useful since it makes it possible to 
overcome the problems of conventional absorbents caused by a fine powder 
of soda lime, e.g. air pollution in a room caused at a time of recharging 
the absorbent, risk that a narcotized patient might breathe in a soda lime 
fine powder when the absorbent is used to remove carbon dioxide from a 
narcotic gas, and the like.