Process for the preparation of anhydrous magnesium chloride having a high degree of purity

Anhydrous magnesium chloride containing substantially no magnesium oxide is produced by a process comprising reacting magnesium chloride in the form of its hexahydrate with ammonia in the presence of ammonium chloride in an aqueous media; separating the resultant precipitated magnesium chloride hexammoniate crystals from an aqueous solution containing unreacted ammonia, magnesium chloride and ammonium chloride; washing the crystals with liquid ammonia; decomposing the washed crystals into anhydrous magnesium chloride and gaseous ammonia, and; isolating the anhydrous component from the composition mixture; which process is characterized in that a portion of the separated aqueous solution is evaporated until the concentration of ammonia in the resulting residual liquid becomes 2.5% or less; the residual liquid is mixed with fresh magnesium chloride so as to dissolve it in the liquid; the resultant solution is fed into the reacting step, and; the remaining portion of the separated aqueous solution is directly recycled into the reacting step.

FIELD OF THE INVENTION 
The present invention relates to a process for the preparation of anhydrous 
magnesium chloride. More particularly, the present invention relates to a 
process for the preparation of anhydrous magnesium chloride which has a 
very high degree of purity and contains substantially no water and 
magnesium oxide, and from which pure metallic magnesium can be produced by 
using a molten salt electrolysis method. 
BACKGROUND OF THE INVENTION 
It is known that magnesium chloride hexammoniate (MgCl.sub.2 6NH.sub.3) is 
usable as a material for producing anhydrous magnesium chloride which can 
be converted into metallic magnesium by using a molten salt electrolysis 
method. For example, Japanese Pat. No. 89,519 and U.S. Pat. No. 3,092,450 
disclose a process for producing anhydrous magnesium chloride by reacting 
magnesium chloride with ammonia and by decomposing the resultant magnesium 
chloride hexammoniate. However, the known processes are technically 
disadvantageous in that the resultant anhydrous magnesium chloride 
contains a considerable amount of impurities and causes the degree of 
purity of the metallic magnesium converted therefrom to be poor. Also, the 
known processes on the recovery of the sensible refrigerant energy and 
that of unreacted compounds are economically disadvantageous due to the 
fact that after the reaction for producing magnesium chloride hexammonite 
at a low temperature, unreacted compounds in the reaction mixture are 
neither recovered nor reused, and the sensible refrigerant energy of the 
reaction mixture is discharged without recovery. 
For example, in the process of Japanese Pat. No. 89,519, solid magnesium 
chloride hexahydrate is directly brought into contact with liquid ammonia 
at a low temperature to produce magnesium chloride hexammoniate by 
substituting the hexahydrate ligand group in the magnesium chloride 
hexahydrate by a hexammoniate ligand group. In this ligand group 
substitution reaction, first, the ammonia comes into contact with the 
surface of each solid magnesium chloride hexahydrate particle and, then, 
penetrates into the inside of the particle. The resultant magnesium 
chloride hexammoniate is insoluble in the liquid ammonia. Therefore, 
during the ligand group substitution reaction, the peripheral portion of 
each magnesium chloride hexahydrate particle is replaced by the resultant 
solid magnesium chloride hexammoniate layer which obstructs the 
penetration of the liquid ammonia into the inside of the particle. 
Accordingly, the resultant magnesium chloride hexammoniate particles 
contain, in their center portions, the unreacted magnesium chloride 
hexahydrate as an impurity. When the thus produced magnesium chloride 
hexammoniate is used for producing anhydrous magnesium chloride, the 
magnesium chloride hexahydrate contained in the hexammoniate causes the 
resultant anhydrous magnesium chloride to contain a considerable amount of 
magnesium oxide. Furthermore, in the process of Japanese Pat. No. 89,519, 
after the magnesium chloride hexammoniate is separated from the reaction 
mixture, only unreacted ammonia is recovered from the remaining reaction 
mixture, and the residual liquid which has a low temperature is discharged 
without recovering the sensible refrigerant energy of the residual liquid. 
Also, unreacted magnesium chloride hexahydrate in the reaction mixture is 
neither recovered nor reused. 
In the process of U.S. Pat. No. 3,092,450, an aqueous solution of magnesium 
chloride and, optionally, a soluble ammonium salt such as ammonium 
chloride, is added to an aqueous solution containing ammonia, at a low 
temperature, so as to allow the resulting magnesium chloride hexammoniate 
to precipitate from the reaction mixture, and then, the magnesium chloride 
hexammoniate is recovered. In this process, since the reaction of the 
magnesium chloride hexahydrate with the ammonia is carried out in the 
homogeneous liquid phase, the resultant magnesium chloride hexammoniate 
has a relatively high degree of purity. However, after the recovery of the 
precipitated magnesium chloride hexammoniate, the remaining aqueous 
solution containing unreacted magnesium chloride, ammonium salt and 
ammonia, and having a low temperature, is discharged from the reaction 
system without recovering it. That is, in the above-mentioned process, the 
sensible refrigerant energy of and the unreacted compounds in the 
remaining aqueous solution are not re-utilized for this process. 
Furthermore, in order to recover liquid ammonia from the remaining aqueous 
solution, it is necessary that, first, a mixture of water and ammonia be 
separated from the remaining aqueous solution and, then, the liquid 
ammonia be isolated from the mixture. Accordingly, the isolation of the 
liquid ammonia from the remaining aqueous solution results in the 
consumption of an extremely large amount of thermal energy. 
In connection with the reuse of the remaining aqueous solution containing 
magnesium chloride, ammonium chloride and ammonia, it should be noted 
that, when solid magnesium chloride is dissolved in the remaining aqueous 
solution, the following undesirable reaction may occur. 
EQU MgCl.sub.2 +NH.sub.4 OH.fwdarw.Mg(OH).sub.2 +NH.sub.4 Cl 
That is, this reaction results in undesirable containing of magnesium oxide 
in the resultant anhydrous magnesium chloride. Accordingly, the reuse of 
the remaining aqueous solution should be carried out in such a manner that 
the generation of the magnesium oxide can be avoided. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for the 
preparation of anhydrous magnesium chloride having a high degree of purity 
while reusing unreacted compounds and sensible refrigerant energy 
discharged from the process. 
Another object of the present invention is to provide a process for the 
preparation of anhydrous magnesium chloride which contains substantially 
no magnesium oxide and, therefore, is very useful for producing pure 
metallic magnesium. 
The above-mentioned object can be attained by using the process of the 
present invention which comprises the steps of: 
reacting magnesium chloride with ammonia in the presence of ammonium 
chloride in an aqueous medium at a low temperature while allowing the 
resultant magnesium chloride hexammoniate to precipitate from the reaction 
mixture; 
separating an aqueous solution containing unreacted ammonia, magnesium 
chloride and ammonium chloride from said precipirated magnesium chloride 
hexammoniate; 
washing said magnesium chloride hexammoniate with liquid ammonia; 
decomposing said washed magnesium chloride hexammoniate into anhydrous 
magnesium chloride and gaseous ammonia, and; 
isolating said anhydrous magnesium chloride from said decomposition 
mixture; and which is characterized in that a portion of said separated 
aqueous solution is subjected to an evaporation in which an aqueous 
ammonia solution is removed to such an extent that the residual liquid of 
said evaporation contains 2.5% by weight or less of ammonia; said residual 
liquid is mixed with fresh magnesium chloride; said mixture is fed into 
said reacting step, and; the remaining portion of said separated aqueous 
solution is directly recycled into said reacting step. 
The above-mentioned process of the present invention is effective for 
readily producing anhydrous magnesium chloride having an extremely high 
degree of purity with an economical advantage. That is, the disadvantages 
of the known processes are all eliminated by the process of the present 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the accompanying drawing, in a reacting step 1, ammonia reacts 
with magnesium chloride in the presence of ammonium chloride in an aqueous 
medium at a low temperature, while allowing the resultant magnesium 
chloride hexammoniate to precipitate from the reaction mixture. In the 
start of the reaction, fresh ammonia, magnesium chloride and ammonium 
chloride are fed into the reacting step. 
When the reaction is completed, the reaction mixture is forwarded through a 
path 7 to a separating step 2 in which the reaction mixture is separated 
into the precipitated magnesium chloride hexammoniate and a remaining 
aqueous solution containing unreacted magnesium chloride, ammonia and 
ammonium chloride. 
The separated magnesium chloride hexammoniate is forwarded to a washing 
step 3 through a path 8. In the washing step 3, the separated magnesium 
chloride hexammoniate is washed with liquid ammonia. 
The washed magnesium chloride hexammoniate is forwarded to a decomposing 
step 4 through a path 9 and decomposed into anhydrous magnesium chloride 
and gaseous ammonia. The resultant anhydrous magnesium chloride is 
collected. 
In the process of the present invention, the separated aqueous solution in 
the separating step 2 is recovered through a path 11, and a portion of the 
aqueous solution is subjected to an evaporating step 5 through a path 18. 
In the evaporating step 5, ammonia in the form of an aqueous ammonia 
solution is removed from the separated aqueous solution to such an extent 
that the residual liquid of the evaporating step contains 2.5% by weight 
or less, preferably, 2.0% by weight or less, of ammonia. The resultant 
residual liquid, which consists of an aqueous solution of unreacted 
magnesium chloride, ammonium chloride and the above-mentioned decreased 
amount of ammonia, is withdrawn from the evaporating step 5 through a path 
13 and mixed with fresh magnesium chloride in a mixing step 20. The 
mixture is fed into the reacting step 1 through a path 19. 
The remaining portion of the separated aqueous solution recovered from the 
separating step 2 is directly recycled into the reacting step 1 through a 
path 12. 
In the process of the present invention, it is preferable that the waste 
washing liquid of the washing step 3 be collected and recycled into the 
reacting step 1 through a path 10. This is because the waste washing 
liquid contains magnesium chloride, ammonium chloride and water dissolved 
in liquid ammonia. 
Also, it is preferable that the gaseous ammonia generated in the 
decomposition step 4 be collected through a path 14 and mixed with the 
aqueous ammonia solution removed from the evaporating step 5, and the 
mixture is fed into a step for recovering liquid ammonia through a path 
15. In this recovering step liquid ammonia is collected and waste water is 
discharged into the outside of the process system. 
Furthermore, it is preferable that a portion of the recovered liquid 
ammonia be fed into the washing step 3 through a path 17 and the remaining 
portion of the recovered liquid ammonia is recycled into the reacting step 
1 through a path 16. 
In the case where all of the steps mentioned above are carried out, 
necessary amounts of fresh ammonia and ammonium chloride should be fed 
into the reaction step 1 only in the starting stage of the process, and 
after the process reaches a normal condition, in theory, the fresh ammonia 
and ammonium chloride do not need to be added into the reacting step. 
The magnesium chloride to be fed into the reacting step 1 may be in the 
form of magnesium chloride hexahydrate (MgCl.sub.2.6H.sub.2 O), which may 
be partially dehydrated (MgCl.sub.2.NH.sub.2 O, 0&lt;n&lt;6), and carnallite 
(MgCl.sub.2.NH.sub.4 Cl.6H.sub.2 o). Usually, magnesium chloride 
hexahydrate is used for the reaction step. The concentration of the 
magnesium chloride hexahydrate in the reaction mixture is not limited to a 
special range. However, it is preferable that the concentration of 
magnesium chloride hexahydrate be in a range of from 2 to 20% by weight. A 
concentration smaller than 2% by weight of the magnesium chloride 
hexahydrate sometimes may results not only in a poor efficiency of 
reaction with ammonia, but also, in consumption of a large amount of 
thermal energy for preparing the residual liquid of the evaporating step 
5. A concentration larger than 20% by weight of the magnesium chloride 
hexahydrate in the reaction mixture sometimes may result in production of 
undesired magnesium hydroxide, which will be converted into magnesium 
oxide. 
Also, in the reacting step, it is preferable that the ammonia (NH.sub.3) in 
the reaction mixture have a concentration of from 50 to 90% by weight. 
When the concentration of ammonia is lower than 50% by weight, and 
therefore, concentration of undessociated ammonia is relatively low, 
sometimes undesired magnesium hydroxide is generated in accordance with 
the following formula. 
EQU MgCl.sub.2.6NH.sub.3 +6H.sub.2 0.fwdarw.Mg(OH).sub.2 +2NH.sub.4 
Cl+4NH.sub.4 OH 
This reaction causes the content of magnesium oxide in the resultant 
anhydrous magnesium chloride to be increased. When the concentration of 
the magnesium chloride hexahydrate is larger than 70% by weight, a portion 
of the magnesium chloride hexahydrate may become difficult to dissolve in 
the reaction mixture. In this case, ammonia reacts with solid magnesium 
chloride hexahydrate particles and the resultant magnesium chloride 
hexammoniate deposites on the hexahydrate particles so as to obstruct the 
penetration of ammonia into the inside of the particles. Accordingly, the 
resultant magnesium chloride hexammoniate contains therein a considerable 
amount of unreacted magnesium chloride hexahydrate. 
Furthermore, in the reacting step, it is preferable that the concentration 
of ammonium chloride, which is effective for preventing the formation of 
magnesium hydroxide in the reaction mixture, be in a range of from 1 to 5% 
by weight. A concentration of less than 1% of ammonium chloride is 
sometimes not high enough for preventing the production of magnesium 
hydroxide during the reacting step. Also, if the concentration of ammonium 
chloride exceeds 5% by weight, the precipitated magnesium chloride 
hexammoniate may contain an undesirably large amount of ammonium chloride, 
which may cause the amount of the liquid ammonia to be used for washing 
the precipitate to be undesirably too large. 
The reacting operation is preferably carried out at a temperature of from 
-30.degree. to 0.degree. C. while the reaction mixture is stirred. The 
reaction between the magnesium chloride hexahydrate and ammonia in the 
reaction mixture is carried out along the following course. 
EQU MgCl.sub.2,6H.sub.2 O+6NH.sub.3 .fwdarw.MgCl.sub.2.6NH.sub.3 +6H.sub.2 O 
That is, the hexahydrate ligand group (6H.sub.2 O) coordinated to the 
magnesium ion in magnesium chloride is substituted by a hexammoniate 
ligand group (6NH.sub.3). Generally, the rate of the ligand group 
substitution reaction is variable depending upon the type of metal ion to 
which a ligand group is coordinated. In the case of the magnesium ion, the 
rate of the ligand group substitution reaction is 1.times.10.sup.-5 
seconds, which is a very high rate. Also, the ligand group substitution 
reaction is exothermic. Accordingly, it is clear that the lower the 
reaction temperature, the larger the reaction equilibrium constant. This 
means that the lower the reaction temperature in the range of from 
-30.degree. to 0.degree. C., the larger the yield of the magnesium 
chloride hexammoniate in the reaction step. 
Usually, the reaction step is held for a period of 1/2 hours or longer, 
preferably, from 1/2 to 3 hours. A reaction time shorter than 1/2 hours 
sometimes may cause the precipitated magnesium chloride hexammoniate 
crystals to have such a small size of 50 microns or less that it is 
difficult to separate the crystals from the reaction mixture within a 
short time. A reaction time longer than 3 hours will neither result in any 
technical advantages, nor therefore, results in an economical 
disadvantage. 
The separation of the aqueous solution containing the unreacted ammonia, 
magnesium chloride and ammonium chloride from the precipitated magnesium 
chloride hexammoniate in the reaction mixture is preferably carried out at 
a temperature the same as or lower than the reaction temperature, and 
within the range of from -50.degree. to 0.degree. C. A separation 
temperature lower than -50.degree. C. may cause the consumption of 
refrigerant energy to be excessively large. Also, a separation temperature 
higher than 0.degree. C. may sometimes result in a low yield of the 
precipitated magnesium chloride hexammoniate crystals. 
In the process of the present invention, the evaporating operation is 
carried out until the concentration of ammonia in the residual liquid of 
the evaporation becomes 2.5% by weight or lower, preferably, 2.0% by 
weight or lower. This residual liquid contains concentrated magnesium 
chloride and ammonium chloride which are nonvolatile, and mixed with fresh 
magnesium chloride, usually, its hexahydrate. When this mixing is carreid 
out, the ammonia in the residual liquid may react with the fresh magnesium 
chloride as follows. 
EQU MgCl.sub.2 +NH.sub.4 OH.fwdarw.Mg(OH).sub.2 +NH.sub.4 Cl 
The rate of the above reaction varies depending upon the concentration of 
the ammonia in the residual liquid. For example, when the concentration of 
ammonia is 9.0% by weight of higher, the above-mentioned reaction will 
occur at a very high rate. The formation of magnesium hydroxide will 
result in an undesirably high concentration of magnesium oxide in the 
resultant anhydrous magnesium chloride. However, when the concentration of 
the ammonia in the residual liquid of the evaporation is 2.5% by weight or 
less, it has been discovered that the rate of the above-mentioned reaction 
is extremely low and the production of the magnesium hydroxide during the 
mixing step is substantially prevented. 
The evaporating operation is preferably carried out at a temperature of 
80.degree. to 110.degree. C. under atmospheric pressure. The evaporating 
operation at a temperature lower than 80.degree. C. may cause the 
evaporating time to be excessively long and, therefore, result in an 
economical disadvantage. Also, an evaporating temperature higher than 
110.degree. C. can be obtained only under a pressurized condition, which 
will result in an economical disadvantage and in an undesirable technical 
complexity. 
In the mixing step, it is preferable that the aqueous solution contain 30 
to 70% by weight of the fresh magnesium chloride in terms of its 
hexahydrate. 
The washing operation of the precipitated magnesium chloride hexammoniate 
with the liquid ammonia is usually carried out at a temperature of 
-30.degree. C. or lower, because the liquid ammonia is vaporized under 
atmospheric pressure. The washing operation at a temperature higher than 
-30.degree. C. should be effected under a pressurized condition capable of 
preventing the vaporization of ammonia. Preferably, the liquid ammonia is 
used in an amount of at least 0.5 times, more preferably, from 0.5 to 2.0 
times, the weight of the precipitated magnesium chloride hexammoniate 
crystals to be washed. If the amount of liquid ammonia is less than 0.5 
times the weight of the precipitated crystals to be washed, sometimes, it 
is difficult to completely eliminate water from the precipitated crystals. 
The use of the liquid ammonia in an amount of more than 2.0 times will not 
result in any technical advantages and will result in an economical 
disadvantage. 
The decomposing operation of the washed magnesium chloride hexammoniate is 
usually carried out at a temperature of from 270.degree. to 400.degree. C. 
A temperature lower than 270.degree. C. may result in a very slow 
decomposition of the magnesium chloride hexammoniate, and therefore, is 
economically disadvantageous. A temperature higher than 400.degree. C. 
will cause the resultant ammonia to be thermally decomposed. 
The resultant anhydrous magnesium chloride may be melt-purified at a 
melting-temperature (712.degree. C.) thereof or higher. 
The gaseous ammonia generated in the decomposing step and the aqueous 
ammonia solution obtained in the evaporating step are forwarded into the 
process for recovering liquid ammonia. This process can be effected by 
using any conventional process and apparatus, for example, an ammonia 
recovering column. In this column, the recovery of from 15 to 20 
kg/cm.sup.2, at an overhead temperature of from 38.degree. to 49.degree. 
C. and at a bottom temperature of from 200.degree. to 220.degree. C. 
The process of the present invention has the following advantages. 
1. Since the magnesium chloride in the form of an aqueous solution is 
brought into contact with ammonia in the form of an aqueous solution, the 
reaction takes place in a homogenous single phase, and there occurs no 
such a phenomenon that solid magnesium chloride hexammoniate crystals 
deposit and cover solid magnesium chloride hexahydrate particles. 
Accordingly, the entire amount of the magnesium chloride in the reaction 
mixture can readily react with ammonia and be converted into magnesium 
chloride hexammoniate. Therefore, the resultant anhydrous magnesium 
chloride contains an extremely small amount, for example, 0.05% by weight 
or less, of magnesium oxide, and therefore, has a very high degree of 
purity. 
2. Also, entire amounts of non-volatile compounds, that is, magnesium 
chloride and ammonium chloride, in the separated aqueous solution of the 
separating step can be recycled into the reaction step. 
3. Since the fresh magnesium chloride to be fed into the reacting step is 
firstly mixed with the evaporated residual liquid which contains 2.5% by 
weight or less of ammonia, the production of undesirable magnesium 
hydroxide can be substantially completely prevented. 
4. Since a portion of the separated aqueous solution in the separating step 
is directly recycled into the reacting step, a considerable amount of the 
refrigerant energy of the reaction mixture can be reused. 
5. The thermal energy consumption for recovering the unreacted ammonia is 
relatively small. 
6. A major portion of the water in the separated aqueous solution in the 
separating step is reused without evaporation. Only a minor portion of the 
water is evaporated. Therefore, the energy consumption for evaporating 
water is relatively small. 
7. Since the unreacted magnesium chloride hexahydrate, ammonia and ammonium 
chloride can be reused, the yield of the anhydrous magnesium chloride is 
very high. 
The features and advantages of the present invention will be further 
illustrated by the examples set forth below, which are presented for the 
purpose of illustration only and should not be interpreted as limiting the 
scope of the present invention. In the following examples, the percentages 
are based on weight unless otherwise noted. 
EXAMPLE 1 
A reaction vessel having an inside diameter of 100 mm and an inner volume 
of 500 ml, which had been cooled to a temperature of -20.degree. C. by 
using a coolant consisting of dry ice and methyl alcohol, was charged with 
200 g of liquid ammonia and 78.8 g of 13.2% aqueous solution of ammonium 
chloride. The mixture in the reaction vessel was stirred with a stirrer at 
a rate of 400 rpm. 60 g of a 23.4% aqueous solution of magnesium chloride 
hexahydrate was uniformly fed into the mixture in the vessel by using a 
feeding nozzle over a period of time of 30 minutes, while stirring the 
reaction mixture. 2 hours after the feeding operation was coupled, the 
reacting operation was completed. 
The reaction mixture was discharged into a filter through an outlet located 
at the bottom of the vessel and filtrated in the filter at a temperature 
of -20.degree. C. The resultant magnesium chloride hexammoniate crystals 
were separated from 338.5 g of an aqueous solution containing soon 
unreacted ammonia, magnesium chloride and ammonium chloride. 105.9 g of 
the separated aqueous solution was evaporated at a temperature of 
98.degree. C., under an ambient pressure, for 1 hour. The residual liquid 
had a weight of 34 g and contained 1.2% of ammonia. 71.9 g of evaporated 
ammonia aqueous solution was forwarded to an ammonia recovering column, 30 
g of fresh magnesium chloride hexahydrate was dissolved in the residual 
liquid of the evaporation. The resultant solution was recycled into the 
reaction vessel. 205.9 g of the remaining aqueous solution separated in 
the separating step were also recycled into the reaction vessel. 
The separated magnesium chloride hexammoniate crystals were washed in the 
filter at a temperature of -30.degree. C. with 30 g of liquid ammonia. The 
waste washing liquid was collected and recycled into the reaction vessel. 
The washed magnesium chloride hexammoniate crystals were placed in a 
quartz tube, having an inside diameter of 30 mm, the inside space of which 
had been filled with nitrogen gas. The quartz tube was heated at a 
temperature of 300.degree. C. for 3 hours, to decompose the magnesium 
chloride hexammoniate into anhydrous magnesium chloride and gaseous 
ammonia. The resultant gaseous ammonia was collected and the anhydrous 
magnesium chloride was melt-refined at a temperature of 750.degree. C. for 
0.5 hour. The melt-refined anhydrous magnesium chloride contained an 
extremely small amount of 0.05% of magnesium oxide. The collected ammonia 
was forwarded into the ammonia recovery column. 
EXAMPLE 2 
30 g/hr of magnesium chloride hexahydrate were dissolved in 20.4 g/hr of a 
residual liquid of an evaporating operation, which will be mentioned 
hereinafter. The residual liquid contained 1% of ammonia. The resultant 
solution was mixed with 254.4 g/hr of a separated aqueous solution of a 
separating step which will be mentioned hereinafter, 21.6 g/hr of liquid 
ammonia recovered by an ammonia recovery process which will be mentioned 
hereinafter and 49.2 g/hr of a waste washing liquid of a washing step 
which will be mentioned hereinafter. The resultant reaction mixture was 
fed into a reactor. The reactor comprised a cooling cylinder, four 
reaction pipes having an inside diameter of 16.1 mm and a length of 800 mm 
and arranged within the cooling cylinder in parallel to the longitudinal 
axis of the cooling cylinder, a feed inlet for the reaction mixture 
connected to each of reaction pipe and a discharge outlet for the reaction 
mixture connected to opposite ends of the pipes. The cylinder had an inlet 
for introducing a coolant thereinto and an outlet for discharging the 
coolant therefrom. The coolant inlet was located close to the outlet of 
the reaction mixture and the coolant outlet was located close to the inlet 
of the reaction mixture. Accordingly, the reaction mixture flowed through 
the four reaction pipes in a direction opposite to the flowing direction 
of the coolant through the cooling cylinder. The reaction mixture in the 
reaction pipes was cooled to a temperature of -20.degree. C. by the 
coolant. The reaction mixture discharged from the reactor was fed into a 
filter and filtrated therein at a temperature of -20.degree. C. 29.1 g/hr 
of magnesium chloride hexammoniate crystals and 364.4 g/hr of a filtrate 
were obtained. The obtained magnesium chloride hexammoniate crystals were 
forwarded to a washing vessel and washed therein with 49.2 g/hr of liquid 
ammonia, which had been recovered in the recoverying process to eliminate 
impurities from the magnesium chloride hexammoniate crystals. The entire 
amount of waste washing liquid discharged from the washing vessel was 
recycled into the reactor. 29.1 g/hr of the washed magnesium chloride 
hexammoniate crystals were forwarded into a decomposing furnace and heated 
therein to a temperature of 300.degree. C. 14.1 g/hr of anhydrous 
magnesium chloride and 15.1 g of gaseous ammonia were obtained. The entire 
amount of the anhydrous magnesium chloride was forwarded into a 
melt-refining furnace and refined therein at a temperature of 750.degree. 
C. The refined anhydrous magnesium chloride contained an extremely small 
amount of 0.03% of magnesium oxide. 
A portion of the filtrate which corresponded to 73.4% of 346.4 g/hr of the 
filtrate, was directly recycled into the reactor. The remaining portion of 
the filtrate was subjected to an evaporating operation, at a temperature 
of 105.degree. C., so as to decrease the concentration of ammonia in the 
filtrate to 1%. The evaporated portion of the filtrate was also recycled 
into the reactor. 
55.7 g/hr of ammonia and 16.0 g/hr of water, while were generated in the 
evaporating step, and 15.1 g/hr of gaseous ammonia generated in the 
decomposing furnace, were forwarded all together into a gas compressor. 
The resultant compressed gas was forwarded into a ammonia recovery tower 
and separated therein into 70.8 g/hr of liquid ammonia are 15.6 g/hr of 
water. The water was discharged from the process system. A portion of the 
liquid ammonia was recycled into the reactor and the remaining portion 
thereof into the washing vessel. 
COMATIVE EXAMPLE 1 
The same procedures as those mentioned in Example 1 were carried out, 
except that the concentration of ammonia in the residual liquid of the 
evaporating step was 3%. The resultant refined anhydrous magnesium 
chloride contained a relatively large amount of 2.2% of magnesium oxide. 
COMATIVE EXAMPLE 2 
The same procedures as those mentioned in Example 2 were conducted, except 
that no washing operation for the separated magnesium chloride 
hexammoniate was carried out. The resultant refined anhydrous magnesium 
chloride contained a large amount of 10.8% of magnesium oxide.