Note: Descriptions are shown in the official language in which they were submitted.
2 0 3 2 6 27 60818-17
PROCESS FOR PRODUCING SODIUM CARBONATE
AND AMMONIUM SULPHATE FROM SODIUM SULPHATE
This inventlon relates to manufacturing of sodium car-
bonate (soda ash). More particularly, this invention relates to
manufacturing of sodium carbonate and ammonium sulphate.
Sodium carbonate (Na2CO3) is a white crystalline
hygroscopic powder known in the chemical trade as ash, soda ash,
soda and calcined soda. Most of the world's soda ash is produced
by the ammonia-soda process, also known as the Solvay process. In
this process soda ash is produced from common salt (sodium
chloride), ammonia, carbon dioxide and limestone by a sequence of
reactions involving recovery and reuse of practically all the
ammonia and part of the carbon dioxide. In what is conventionally
considered as the first step, limestone is calcined in a kiln to
produce lime and carbon dioxide. Carbon dioxide is dissolved in a
purified brine containing additionally the ammonia, with resultant
precipitation of sodium bicarbonate. The latter is filtered off,
dried and thermally decomposed, at about 200C, to sodium carbon-
ate. The comparatively expensive ammonia is recovered from ammon-
ium chloride contained in the filtrate by adding to the heatedfiltrate a slurry of lime. Calcium chloride produced in this step
is a major byproduct.
The Solvay process suffers from several disadvantages.
Firstly, the process consumes large amounts of fuel (to burn
limestone, calcine sodium carbonate and produce steam for recovery
of ammonia from ammonium chloride). Secondly, the chlorine from
common salt is not recovered but discarded in the form of calcium
chloride effluent, which pollutes the environment. Largely
2032627 60818-17
because of high energy costs and strict pollution controls, most
of the synthetic production of soda ash by the Solvay process in
North America has been or is being abandoned in favour of the
product obtained from natural deposits.
The most abundant natural form of sodium carbonate is
trona (Na2CO3.NaHCO3.2H2O). Two processes simpler than the Solvay
process are used to refine trona ore. In the first, so called
monohydrate process, the trona ore is calcined to impure soda ash
which is then purified. In the second, a sodium sesquicarbonate
process, the soda ash is produced by calcination of sodium
sesquicarbonate which had previously been purified. Both pro-
cesses are characterized by relatively favourable production costs
and reduced environmental hazards.
Soda ash is also produced in the United States from the
natural brine at Searles Lake, California. More complicated
processes are required in this case than for processing of trona,
because of the complex nature of the brines. In one of the pro-
cesses, brine is evaporated to give sodium carbonate in the form
of burkeite (Na2CO3.2Na2SO4), which is then separated in a complex
sequence of fractional crystallizations into sodium carbonate and
sodium sulphate. In another process brine is first treated with
carbon dioxide in carbonation towers, the precipitated sodium
bicarbonate is filtered off, washed and converted by heating to
soda ash.
Several processes described in the prior art, but not
commercialized, propose to make use of sodium sulphate for sodium
carbonate. Canadian Patent No. 1,099,892 discloses a process for
2 0 3 2 6 27 60818-17
dry conversion of alkali metal sulphate to alkali metal carbonate.
In the process alkali metal sulphate is reduced with carbon to
alkali metal sulphide which is subsequently converted with carbon
dioxide and steam to alkali metal carbonate. Similar processes
comprising the step of reducing alkali metal sulphate with carbon
are disclosed in U.S. Patents No. 1,979,151 and 3,127,237. A
process for producing an alkali metal carbonate by reacting solid
alkali metal sulphate with a gas containing hydrogen and carbon
monoxide is described in U.S. Patent No. 3,401,010.
U.S. Patent No. 3,134,639 discloses a wet process for
converting alkali metal sulphates into alkali metal carbonates
using lime, hydrogen sulphide and carbon dioxide. No ammonia is
used in this process.
U.S. Patent No. 3,493,329 discloses a wet process for
producing soda ash from sodium chloride which process eliminates
the use of calcareous material. The process comprises a liquid
state cycle and a solid state cycle. A solution of sodium sul-
phate is used in the liquid state cycle to precipitate sodium
bicarbonate under conditions similar to those of ammonia-soda
20 process. Ammonium sulphate produced in this step is then precipi-
tated in the following dissolving-crystallizing step by adding
solid sodium sulphate to the solution from which the precipitated
sodium bicarbonate has been separated. The ammonium sulphate
precipitated by addition of the solid sodium sulphate, either pure
or in the form of a double salt of ammonium sulphate and sodium
sulphate, is reacted in its solid form in the solid state cycle by
heating it in the presence of sodium sulphate to produce ammonia
2032627 60818-17
and sodium bisulphate which is subsequently reacted in the form of
fused salt with sodium chloride to produce regenerated sodium
sulphate and hydrochloric acid. It should be noted that in this
process sodium chlorlde is the actual process feed material,
whereas sodium sulphate is in fact used only for recovery of
ammonia and is itself finally recovered and reused.
Canadian Patent No. 543,107 discloses a process for the
recovery of ammonium sulphate from the filtrate obtained by fil-
tering off sodium bicarbonate precipitated in the ammonia-soda
process using sodium sulphate as a starting material. Ammonium
sulphate is precipitated from the filtrate by dissolving in it
ammonia under superatmospheric pressure and filtering off the
precipitated salts under the ammonia pressure. However, the
product so obtained contains less than 75% of ammonium sulphate
and over 25% of sodium sulphate. Such a product has a very little
practical use because it is a mixture of salts. It cannot be
used, for example, as a fertilizer because of the high percentage
of sodium sulphate. The patent does not provide any indication
whether and how this mixture of salts can be separated to produce
essentially pure and therefore marketable ammonium sulphate.
A process for the separation of sodium sulphate and
ammonium sulphate from aqueous solutions of a mixture of them to
obtain substantially pure sulphates is disclosed in Canadian
Patent No. 821,457. The process uses an evaporator-crystallizer
system which produces a crystal mixture in which crystals of each
salt have different granulometric and density characteristics.
This mixture is separated, e.g. by dry or wet screening, into
2 0 32627 60818-17
fractions consisting mainly of sodium sulphate and ammonium sul-
phate crystals, respectively, which fractions are subsequently
washed to dissolve away the sulphate present in the least quantity
and leave the other sulphate substantially pure. The process, even
though finally producing substantially pure sodium and ammonium
sulphate from a mixture thereof, is lengthy and requires a rela-
tively complicated equipment to separate the mixture of crystals
obtained in the evaporation-crystallization step.
Demonstrated worldwide demand for sodium-based chemicals,
particularly sodium carbonate (soda ash) has been on the rise in
recent years. This strong demand, which is forecast to continue,
keeps soda ash in a tight supply position thereby holding the price
at a high level. Because of environmental concerns and the vast
reserves of natural sodium carbonate, the production of soda ash in
the United States is primarily from natural sources. In Canada,
the known natural reserves of sodium carbonate are not as vast as
those in the United States. However, Canada is endowed with vast
deposits of sodium sulphate, located mostly in Southern
Saskatchewan, that could potentially become a source of soda ash.
Moreover, statistics indicate that the demand for sodium sulphate
is dwindling and its prices declining. This declining trend in the
demand for and prices of sodium sulphate together with strong
demand for and relatively high prices of soda ash created a need
for an economical process for producing sodium carbonate (soda ash)
from naturally-occurring sodium sulphate as feedstock.
Thus, the present invention provides a continuous pro-
cess for preparing sodium carbonate, which process comprises
2 0 3 2 6 27 60818-17
reacting sodium sulphate in aqueous solution with ammonia and
carbon dioxide to precipitate sodium bicarbonate, separating the
sodium bicarbonate and calcining it to convert it to sodium car-
bonate, sujecting the mother liquor from the precipitation of the
sodium bicarbonate to evaporation to precipitate unreacted sodium
sulphate, cooling the mother liquor to precipitate ammonium sul-
phate, further cooling the mother liquor to precipitate a double
salt of sodium sulphate and ammonium sulphate and adding the
double salt to the mother liquor from the precipitation of sodium
bicarbonate, prior to the evaporation.
The present invention has the considerable advantage
that, in addition to producing sodium carbonate, it also produces
ammonium sulphate in a purity such that it can immediately be used
as a fertilizer. In contrast to the Solvay process, it is not
necessary to seek to recycle all the expensive ammonia used in the
process; a substantial amount of that ammonia appears in a pro-
duct, ammonium sulphate, of high commercial value. Also, the
process does not produce any unwanted byproducts, such as calcium
chloride produced in the Solvay process. The products of the
process are the required sodium carbonate, ammonium sulphate that
can be used as fertilizer, a double salt of sodium and ammonium
sulphate that is recycled and therefore does not accumulate, and
sodium sulphate that can be recycled. As the sodium sulphate
recovered in the process is of enhanced purity when compared with
the raw salt normally used as starting material, it may be a
valuable commercial product. Another advantage over the Solvay
2 0 3 2 6 27 60818-17
process is the avoidance of absorption towers, which have a
tendency to become clogged with precipitated bicarbonate.
To carry out the precipitation of sodium bicarbonate
according to the invention, any continuously fed reactor capable
of operating under superatmospheric pressure and assuring an
efficient gas-in-liquid dispersion can be used. In a preferred
embodiment a closed, cylindrical reactor equipped with two gas
dispersion impellers mounted on a coaxial shaft is used. The
precepitation is normally conducted under superatmospheric
pressure of from 0 kPa to about 700 kPa. A pressure of about
70 kPa is preferred.
Since the reaction generates heat, the reactor is
equipped with a cooling system to maintain a temperature in an
optimum range preferably from about 20C to about 60C. A tem-
perature of about 40C is particularly preferred.
A sodium sulphate solution having a concentration of
from about 400 g/L to about 500 g/L is continuously fed into the
reactor. A low temperature of the brine fed into the reactor
helps in maintaining the content of the reactor in the optimum
temperature range. However, since the solubility of sodium sul-
phate in water decreases with decreasing temperature, this tem-
perature should not be excessively low, to keep the solubility of
sodium sulphate in the indicated range of concentrations. A
temperature for the brine solution in the range of from 20C to
60C is suitable. In a preferred embodiment a saturated solution
of sodium sulphate (concentration of about 488 g/L) having tem-
perature of about 40C is used.
2032627 608l8-l7
Carbon dioxide and ammonia are fed into the reactor in
the liquid and/or gaseous form. According to a preferred embodi-
ment, a major part of the two gases is injected into the brine in
the liquid form which has the advantage of producing a cooling
effect compensating for the exothermic effect of the processes
taing place in the reactor.
It is also preferred that a gaseous mixture of ammonia
and carbon dioxide recovered from the brine after the precipita-
tion of sodium bicarbonate is recycled into the reactor. It is
therefore preferred to heat the mother liquor remaining after the
precipitation of the sodium bicarbonate, to recover dissolved or
unreacted carbon dioxide and ammonia before subjecting the liquor
to evaporation.
The pH of the content of the reactor is maintained in a
range of from about 7.0 to about 9Ø The value of pH of about
8.0 is preferred. The value of pH can be maintained by control-
ling the supply of liquid carbon dioxide and/or ammonia.
In a preferrred embodiment the mother liquor from the
precipitation of the double salt is added to the mother liquor
from the precipitation of sodium sulphate. The mother liquor from
the precipitation of the double salt can be subjected to concen-
tration, for example by evaporation, before it is added to the
mother liquor from the precipitation of sodium sulphate.
The mother liquor from the precipitation of sodium
sulphate may be alternatively added to the liquor from the preci-
pitation of sodium bicarbonate, before this liquor is evaporated.
However, it is preferred to reduce the amount of the recycled
2032627 60818-17
salts by cooling the mother liquor from the precipitation of
sodium sulphate to precipitate ammonium sulphate, further cooling
the mother liquor to precipitate a double salt of sodium sulphate
and ammonium sulphate and adding this double salt to the mother
liquor from the precipitation of sodium bicarbonate, prior to the
evaporation of this liquor.
It is preferred that the mother liquor in the first
evaporation stage shall be at a temperature not lower than about
80C, preferably not lower than about 95C, so that the precipi-
tated sodium sulphate is not contaminated with any significantamount of sodium sulphate. It is preferred that the precipitation
of ammonium sulphate shall take place at a temperature above about
40C, preferably above about 60C, so that is is not contaminated
with any significant amount of the double salt. It is possible to
include a concentration step between the precipitation of the
sodium sulphate and the ammonium sulphate. It is also possible to
include a concentration step between the precipitation of the
ammonium sulphate and the double salt.
The various evaporation, precipitation and concentration
stages can be carried out under reduced pressure, in which case
temperatures different from those mentioned above will be appro-
priate. Determing the appropriate temperatures for a particular
chosen reduced pressure will be within the competence of a person
skilled in the art.
The invention will be further described by way of a
preferred embodiment and with references to the accompanying
drawings in which:
2 0 3 2 62 7 60818-17
Figure 1 represents schematically a process for purifi-
cation of raw sodium sulphate and preparing sodium sulphate brine,
Figure 2 represents schematically a process for produc-
ing soda ash according to one preferred embodiment of the inven-
tion,
Figure 3 represents schematically a process for recover-
ing ammonium sulphate and sodium sulphate according to one pre-
ferred embodiment of the invention,
Figure 4 represents schematically an experimental bench-
scale reactor for conducting the process according to the inven-
tion, and
Figure 5 represents a flow diagram of the experimental
system using the reactor of Figure 4.
The process according to the invention requires as a
feed a solution of sodium sulphate. According to a preferred
embodiment of the invention, this solution is prepared from raw
mined sodium sulphate, in a process shown schematically in
Figure 1. Raw sodium sulphate (Na2SO4.10H2O) is continuously fed
into a classifier 1 and mixed with hot brine (an overflow from
cyclone 5). The heat from the brine is sufficient to dissolve the
crystals of sodium sulphate. Any stones and dirt settle to the
bottom of the classifier 1 and are discarded as waste. The salt
brine from the classifier 1 is put through a 30 mesh screen 6 to
remove any fine particles suspended in the brine.
Once screened, the brine is transferred by a pump 7 to
two submerged combustion evaporators No. 1 and 2 to evaporate some
water, thus causing some sodium sulphate to crystallize out. An
2032627 60818-17
air and natural gas mixture which is burned under the brine in a
burner tank 2 causes violent bubbling action. This causes some
brine to splash over a weir and run down into a settling tank 3.
Here the crystals of sodium sulphate settle towards the bottom of
the tank and are drawn off by a pump 10 and put through cyclones 5
to spin off more of the brine. This brine is the cyclone overflow
which is used to dissolve the raw salt in the classifier 1. The
solid sodium sulphate coming out of the cyclones 5 contains about
20% of water and is pumped by a pump 11 into a rotary dryer 55
shown in Figure 3 to finish drying the salt and produce sodium
sulphate (salt cake).
The brine that comes off the top of the settling tank 3
is normally recycled to the burner tank 2 for further evaporation
when only sodium sulphate is to be produced. In the process
according to the invention, some of this brine is transferred by a
pump 8 into a thickener 4, where a flocculating agent, for example
Percol 156, is added to remove fine clay particles still suspended
in the brine. The flocculating agent causes the clay particles to
stick together and settle out faster at the bottom of the thick-
ener 4, to be removed as a mud. The resulting brine is a cleansaturated solution of sodium sulphate (concentration of about 30%)
in water having temperature of about 40C. This brine is fed into
the reactor 21 shown in Figure 2 to be mixed with carbon dioxide
and ammonia.
A cylindrical reactor 21 is equipped with gas dispersion
impellers 22 and 23 and cooling coils 24. For better agitation
cooling coils in the form of single vertical tubes are placed
2032627 60818-17
radially along the reactor wall. These tubes also act as baffles
when the content of the reactor is agitated.
Liquid carbon dioxide and liquid ammonia are injected
into the brine through injection nozzles 24 and 25, respectively.
A part of carbon dioxide and ammonia used in the process is sup-
plied to the reactor in the gaseous form through a tube 26, as a
mixture recovered from the gas recovery boiler 31, as explained
below.
As carbon dioxide and ammonia dissolve in the brine, the
following reactions take place:
NH3 + H2O = NH40H
4 2 4 CO3
Na2SO4 + 2NH4HCO3 = 2NaHCO3 + (NH4)2SO4
These reactions produce considerable heat which is to some extent
compensated by the cooling effect produced by injecting liquid
carbon dioxide and ammonia into the brine. Any excess heat is
removed by the cooling coils 24 supplied with cooling water having
temperature of about 7C to maintain the temperature of the reac-
tor at an optimum level of about 40C. The pressure maintained in
the reactor 21 is about 70 kPa. The pH of the content of the
reactor is maintained at about 8 by controlling the supply of
carbon dioxide.
Because of its limited solubility in water, sodium
bicarbonate (NaHCO3) produced in the above shown series of reac-
tions precipitates out. It is continuously removed from the
reactor 21 in the form of a slurry by a pump 35 and transferred to
a brine cooler 27, where the slurry is cooled to about 20C to
20~2627 60818-17
further reduce the solubility of sodium bicarbonate and to cry-
stallize out as much of the product as possible. The sodium
bicarbonate is then screened out of the brine on a 21 micron
screen 28 and subsequently washed on a screen 29 with water to
remove any ammonium and sodium sulphate present in the entrained
brine. The washed sodium bicarbonate is then fed into a rotary
dryer 30 to be dried and calcined to sodium carbonate (soda ash).
The brine remaining after screening off the solid sodium
bicarbonate contains a mixture of unreacted sodium sulphate,
ammonium sulphate, ammonium bicarbonate and minor amounts of
sodium bicarbonate. This brine is transferred by a pump 36 into a
gas recovery boiler 31 where it is heated to a temperature of 95
to 100C. Under these conditions the ammonium bicarbonate breaks
down and sodium bicarbonate dissolved in the brine reacts with
ammonium sulphate to produce sodium sulphate, carbon dioxide and
ammonia. Carbon dioxide and ammonia dissolved in the brine boil
off, leaving in the solution a mixture composed mostly of sodium
and ammonium sulphate. The carbon dioxide and ammonia so regener-
ated are cooled in a gas cooler 32 and returned to the reactor 21
by a blower 33 after being further cooled in a gas cooler 34.
This regeneration step minimizes the amount of carbon dioxide and
ammonia used in the process.
The brine remaining after removing ammonia and carbon
dioxide has a temperature of 95 to 100C and contains approxi-
mately 15% of sodium sulphate and 15% of ammonium sulphate. This
brine is added by a pump 37 to double salt crystals separated on
the screen 61 shown in Figure 3, as explained below. The 1:1
2032627 60818-17
ratio of sodium sulphate to ammonium sulphate in the double salt
is the same as the ratio of these two salts in the brine, so that
the mixing of the double salt with the brine do not affect the
proportion of the two salts in the brine.
The mixture of the double salt from the screen 61 and
the brine from the gas recovery boiler 31 is then fed into the
submerged combustion evaporator No. 3 where the brine is concen-
trated up to the point where both salts reach their saturation
point. Since ammonium sulphate has a much higher situation point
than sodium sulphate, most of the latter crystallizes out before
the brine is saturated with ammonium sulphate. When the brine
approaches saturation also with respect to ammonium sulphate, the
brine is drawn off from the settling tank 52 by a pump 65 and
transferred onto a screen 53, where sodium sulphate crystals are
screened off. These crystals are then washed on a screen 54 to
remove any ammonium sulphate present in the entrained brine.
Although sodium sulphate so recovered could be redissolved in
water and returned to the reactor 21, this product is so pure that
it is more economical to dry it to produce sodium sulphate. This
is normally done by combining the solids from the screen 54 with
the underflow of the cyclone 5 shown in Figure 1 and feeding the
mixture into a dryer 55 for final drying.
The brine remaining after screening out sodium sulphate
has a temperature of about 95C and is saturated with respect to
both sodium and ammonium sulphate. This brine via a pump 67 is
combined with brine from a submerged combustion evaporator No. 4
used to crystallize out ammonium sulphate. The brine from this
14
2032627 60818-17
evaporator is saturated in both salts and contains some crystals
of ammonium sulphate. By cooling the mixture of both brines in a
crystallizer 56 down from 95C to 60C, the solubility of ammonium
sulphate is decreased while the solubility of sodium sulphate
increases. This forces more ammonium sulphate to crystallize out
while keeping sodium sulphate in the solution. The brine is then
drawn off from the crystallizer 56 by a pump 68 and transferred
onto a screen 57, where ammonium sulphate crystals are screened
off. The crystals are then washed on a screen 58 to remove any
sodium sulphate from the entrained brine and finally dried in a
dryer 59.
After removing the ammonium sulphate crystals, the brine
is transferred by a pump 69 to a double salt crystallizer 60 where
it is further cooled to about 15C. At this temperature sodium
sulphate and ammonium sulphate crystallize out in the form of a
double salt (Na2SO4.(NH4)2SO4.2H20). The brine with suspended
crystals of the double salt is transferred from the crystallizer
60 by a pump 70 onto a screen 61 where the solids are screened out
and added to the brine from the gas recovery boiler 31 shown in
Figure 2.
By removing the double salt from the brine the ratio of
sodium sulphate to ammonium sulphate becomes 1:3 in the remaining
brine. This brine is fed into submerged combustion evaporator No.
4 where it is concentrated until sodium sulphate reaches its
saturation point, by which time some of the ammonium sulphate
crystallizes out. At this point the brine passing from a burner
tank 62 into a settling tank 3 is transferred from the settling
2 0 3 2 627 60818-17
tank 63 by a pump 66 into the crystallizer 56 where it is combined
with the brine left after screening out sodium sulphate on the
screen 53.
EXPERIMENTAL
The operating parameters of the process of the precipi-
tation of sodium bicarbonate were studied and optimized in a
bench-scale reactor unit.
Desiqn and Construction of the Laboratory Bench Unit
The bench unit comprises three main sections: reaction
system, feed system, and product recovery systems.
The reaction system consists of three main sections:
reactor, agitator, and cooling jacket.
A schematic of the reactor is shown in Figure 4. The
reactor 1 is fabricated of a 0.076 m (3 in) diameter x 0.457 m (18
in) long mild steel pipe and has a volume of 2 litres. The reac-
tor head 2 carries an agitator 3, a 0.0063 m (1/4 in) NPT feed
inlet port 4, 0.0063 m NPT reaction gaseous vapour outlet port 5,
and 0.0032 m (1/8 in) and 0.003 m NPT ports for thermocouple 6 and
back-pressure gauge 7, respectively. The bottom of the reactor is
a 0.051 m (1 in) long, 45-degree cone 8 to which is attached a
0.0095 m (3/8 in) diameter product drain connector 9. The reactor
is designed for pressures up to 1034 kPa (150 psi) and tempera-
tures up to 30C.
The feed mixer is a variable speed (640-930 rpm) motor-
driven agitator 3 welded onto the reactor head 2 and centrally
located in the reactor. Attached to the part of the agitator
inside the reactor is a 0.069 m (2-3/4 in) wide by 0.28 m (11 in)
16
2 0 3 2 627 60818-17
long 16 x 16 mesh adjustable stainless-steel screen (not shown in
the drawing). The screen enables good liquid/gas mixing by allow-
ing the gas to pass freely through the screen pores to contact the
feed.
The cooling jacket 10 is constructed of 0.102 m (4 in)
diameter by 0.406 m (16 in) mild steel pipe, which is welded to
the reactor in such a way that it is concentric with the reactor.
The coolant (glycol/water mixture) circulates through the system
by the inlet and outlet ports (11 and 12, respectively).
The feed system consists of the feedtank and transfer
line, metering pump and gas delivery manifold. The freshly pre-
pared feed is siphoned into the feedtank 20 shown in Figure 5 from
where it is fed to the reactor. To prevent crystallization of the
feedstock, the tank is maintained at a constant temperature of
about 40C. The feed is delivered to the reactor by means of a
metering pump 21. The feed transfer line is 0.0063 m (1/4 in)
diameter vinyl tubing heat traced at a constant temperature of
about 40C and connected to the reactor by the metering pump.
Heat tracing the feed lines is necessary to prevent plugging
problems caused by crystallization of sodium sulphate in the feed
solution. The gaseous feed line is also heat traced to provide a
constant feed rate.
Gas distribution within the reactor is accomplished by
means of a manifolds 13 and 14. This system has eighteen gas
inlet nozzles: nine for carbon dioxide and nine for ammonia. The
manifolds are located near the bottom of the reactor so that the
gas can be fed counter currently with the sodium sulphate
2032627 60818-17
solution, which is fed into the reactor from the top. The gas
inlet nozzles 15 and 16 are arranged in a circle inside the reac-
tor such that each gas is fed by an alternate nozzle. This design
enables good dispersion of the gas mixture within the reactor.
Feedstock Preparation
The solution of sodium sulphate used in all the experi-
ments was prepared as follows:
Approximately 12.4 kg of fresh natural Glauber salt
crystals (Na2SO4.10H2O) and 12 L of hot tap water were added into
a metal pail and stirred continuously until the aqueous liquid was
saturated. The temperature of the stirred mixture was maintained
at about 40C by means of a drum heater wrapped around the pail.
After about two hours, the stirring was stopped and the
mixture left to settle for approximately four hours at 40C to
remove silt, salt crystals and other fine clays in the mixture
The supernatant (clear sodium sulphate solutions) was withdrawn,
without disturbing the sediments, and transferred to the feed
storage tank 20. This decanting of the solution was accomplished
by siphoning. A sample of the feed solution was analyzed to
determine the sodium sulphate content. It was found to contain
2.13 g mol/L or 24.6 wt % of sodium sulphate. The density of the
solution at 40C was determined to be 1229.7 kg/m3.
Experimental set-uP and Procedure
A flow diagram describing the experimental system is
illustrated in Figure 5. A typical procedure involves feeding the
sodium sulphate solution to the reactor 22 by means of a metering
pump 21 calibrated to maintain the desired flow rate of the
18
2 0 3 2 62 7 60818-17
solution. The feed solution is fed from the top of the reactor
The carbon dioxide and ammonia gases at desired flow rates and
pressures are metered with capillary flowmeters 23 and 24,
respectively, into the reactor 22 through the gas manifold inlets.
The gas manifold is located near the bottom of the reactor en-
abling the gases to flow countercurrently with the downflowing
sodium sulphate liquid solution. The desired speed of the agita-
tor (640-930 rpm) is set so that the reactor contents are ade-
quately mixed.
The heat generated by the exothermic reactions of the
liquid feed/gas mixture is removed by the circulating glycol/water
mixture maintained at a constant temperature of about 20C. The
liquid and gaseous reaction vapours are routed through the liquid
trap 25 and back-pressure regulator (BPF) 26 into the vent. The
sodium bicarbonate/ammonium sulphate product mixture is manually
withdrawn from the coned-bottom of the reactor through the drain
connector at intervals determined by a stopwatch (=51-53 mL/min).
Ideally the product recovery is accomplished at a uniform rate by
means of a metering pump 27.
Process OPeratinq Conditions
The experimental runs were conducted based on the fol-
lowing reactor unit operating conditions:
2 0 32627 60818-17
Feed flow rate, mL/min 55
Ammonia flow rate, L/min 0.96-1.0
Carbon dioxide flow rate, L/min 1.1-1.2
Pressure, kPa 550-620
Temperature, C 23
Agitator speed, rpm 640-930
Product withdrawal, mL/min 51-53
The carbon dioxide flow rate is an estimated flow rate
based on the ammonia rate because the CO2 flow exceeded the maxi-
mum flow capacity of the flowmeter.Results
The aqueous sodium sulphate feedstock was evaluated
primarily for its effectiveness to convert to sodium bicarbonate
when reacted with carbon dioxide and ammonia.
Crystallization of sodium sulphate in the feed solution
occurred at relatively low temperatures (<40 C) necessitating heat
tracing the feed lines (both liquid and gaseous lines) to prevent
plugging problems.
Ammonia flow rate is an important parameter because it
directly affected the quantity and quality of the product. For
instance, excess amount of ammonia inflow resulted in the product
being a mixture of sodium bicarbonate and sesquicarbonate instead
of predominantly the less soluble sodium bicarbonate.
The process reactions were exothermic, requiring con-
tinuous circulation of the glycol/water cooling mixture. An
efficient cooling system had to be installed to provide the
required cooling because of an excessive amount of ammonia gas and
2 0 3 2 6 27 60818-17
C2 had to be fed to the reactor under insufficient cooling condi-
tions. This resulted in the formation of excessive quantities of
intermediate product, ammonia bicarbonate/carbonate.
A vacuum filtration system and a medium speed (3000 rpm)
centrifuge were used as a means of recovering the bicarbonates and
carbonates from the product stream. The vacuum filtration system
produced a purer product (0.86 wt % sulphate after only one wash-
ing of 35 mL washing solution/70 g wet precipitates) than the
centrifugation system after two washings (5.0 wt % sulphate). The
washing solution was a saturated ammonium bicarbonate aqueous
solution.
After the carbonate/bicarbonate products were removed
(at 23C), the supernatant solution contained mainly 281 g/L
ammonium sulphate, and 65.8 g/L of sodium compounds (carbonates,
sulphates, and bicarbonates). This solution was treated by two
different processes to recover the ammonium sulphate. The first
process used one litre of methanol per litre of solution, which
recovered 157 g of precipitates per litre of fluid. The precipi-
tates were found to contain 18 wt % of sodium. In the second
process, ammonia gas was added to the solution. This process
recovered 115 g of precipitates per litre of fluid. The precipi-
tates were found to contain 28 wt % of sodium.
The experimental data are presented in Tables 1 and 2.
The percent recoverable carbonate/bicarbonate products were esti-
mated from the product composition data as follows:
21
2032627 608l8-l7
A = 33.5 wt % NaHCO,/100 x wt % recoverable (Table 1)
= 28.14 wt %
B = 65.2 wt. % Na2C03 . NaHC03 . 2H20/100 x wt %
recoverable (Table 2) = 43.0 wt %
C = 1.3 wt % Na2S04
Thus, the calculated % recoverable of products -
A=B=C = 72.5 wt %.
The actual percentage of recovered products (Table 1)
was 73.6 wt %.
The amount of sodium remaining in solution was 25.8 g/L.
When this solution is heated, the sodium bicarbonate/carbonate
compounds react with the ammonium sulphate to produce Na2S04, C02
and NH3.
Table 1
Percentage Recoverable at 0C, Theoretical vs. Experimental
% Recovery (Calculated Based Experimental Results O
on Solubility Data) (% Recovery Basis Na @ O C)
Wt % (Basis Na) Compound at OC 1 st 2 hr of 2nd 2 hr of
Trial Run Trial Run
74.0 Sodium Carbonate
70.0 Sodium Carbonate,
decahydrate
73.6 73.5
66.0 Sodium Sesquica,l,on~le
(Na2C03-NaHC03.2H20)
84.0 Sodium Bica,l,on~le
2 0 3 2 ~ 27 60818-17
Table 2
Percentage Recovery of Sodium at 23C and OC
SulphateSodium Content in Recovery
pH ContentSu~,e",ala"l g/L (basis Na)
@ 22C(wt %) @ 23C @ 0C @ 23C @ 0C
Supernatant from 7.9 - 65.9 25.8 32.6 73.6
product stream
recovered in 1st two
hours of Trial run
Supernatant from 8.6 - 65.7 25.9 32.8 73.8
product stream
recovered in 2nd two
hours of Trial Run
