Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1~855~3
The invention relates to a process for concentrat-
ing dilute phosphoric acid, which is heated by an indirect
heat exchange to a temperature below its boiling point and
is subsequently sprayed and directly contacted with hot gases,
whereafter water vapor and fluorine compounds are withdrawn
with the exhaust gas and the fluorine compounds are removed
from the exhaust gas.
- The treatment of raw phosphates with sulfuric acid
in a wet process often results in a dilute phosphoric acid,
10which contains about 26 to 32% P2O5 and must be concentrated
to a P2O5 content above about 45~ for the production of
fertilizer. The phosphoric acid is concentrated by an
evaporation of water in direct contact with hot gases or by an
indirect supply of heat in a vacuum and this may be accom-
panied by crystallization (A.V. Slack, "Phosphoric Acid", Vol.
II, 1968, Marcel Dekker, Inc., New York, pages 581-634). When
he phosphoric acid is concentrated by an indirect supply of
heat in a vacuum, expensive equipment is required to produce
the vacuum, the boiling may be delayed because the operation
must be performed in the boiling range, and primary energy or
steam is required. When the phosphoric acid is concentrated by
a direct contact with hot gases, primary energy at considerable
cost is required in the known processes for producing the hot
gases. Steam is also used in part for indirectly heating the
acid before it is contacted with the hot gases. The use of
submerged burners results in the production of acid fumes at
a considcrable ratc, and these fumes can be scparatcd only
with difficulty and are withdrawn with the exhaust gascs.
It is an object of the invention to avoid the use
of primàry energy or steam in a concentrating process which
involves a direct contact with hot gases and also to minimize
the operating costs as well as the acid fume content of the
exhaust gas.
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This object is accomplished according to the inven-
tion in that the dilute phosphoric acid is heated by an
indirect heat exchange with heat to be extracted from the
absorption system of a sulfuric acid contact process plant,
the heated dilute phosphoric acid is sprayed into and
phosphoric acid is circulated through two successive concen-
trating stages and in said stages is directly contacted with
a heated gas mixture consisting of the tail gas of the
sulfuric acid contact process plant and of admixed air, the
gas mixture is heated by an indirect heat exchange with
surplus gas heat of the gases which have left the first
contacting stage of the sulfuric acid contact process plant
and have partly been converted to S03 and have not yet entered
the high-temperature interstage absorption system, the heated
gas mixture is conducted through the first and second
concentrating stages in succession and the exhaust gas is
subjected to a plurality of scrubbing stages for removing the
fluorine compounds by absorption.
To heat the gas mixture by an indirect heat exchange
with the surplus gas heat of the gases which have left the
first contacting stage and have been partly converted to S03,
the end gas may be heated and may be mixed with air before
entering the concentrating unit, or the air may be properly
heated and may subsequently be mixed with the end gas. The
gas mixture is suitably heated in the second heat exchange
stage in the flow path of the S03-containing gases. In the
operation of the interstage absorption system of the sulfuric
acid contact process plant, the gas phase and the sulfuric
; acid flow concurrently and the gas exit temperature of the
S02-containing gases is approximately as high as the
temperature of the draining sulfuric acid and the temperature
of thP draining sulfuric acid and the temperature of the
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draining sulfuric acid is at least 95C. The first and second
concentrating stages succeed one another in the direction of
flow of the hot gases. The phosphoric acid is generally
concentrated to a P2O5 content of 45 to 60~.
According to a preferred feature, the dilute phos-
phoric acid is circulated, the fresh dilute phosphoric acid
is fed to the second concentrating stage, the circulating
acid is heated by the heat which is to be withdrawn from the
sulfuric acid circulating through the interstage absorption
system of the sulfuric acid contact process plant, the
circulating phosphoric acid is sprayed into the second
concentrating stage in a countercurrent to the rising gas
mixture, part of the circulating acid is fed from the sump
of the second concentrating stage into the sump of the first
concentrating stage, the remaining part of the circulating
acid is fed from the sump of the second concentrating stage
to the acid circulating through the secondconcentrating stage,
the acid circulating through the first concentrating stage is
heated behind the sump thereof by means of the heat which is
to be removed from the sulfuric acid circulating in the final
absorption system and is then sprayed into the first concen-
trating stage concurrently to the descending gases, and
phosphoric acid of higher concentration is withdrawn from the
ac`id which circulates through the first concentrating stage.
This operation results in a favorable transfer and supply of the
heat which is required for the concentrating treatment whereas
a formation of deposits in the concentrating unit is avoided
because heat at a higher temperature is supplied through the
gas phase to the first concentrating stage and only heat at
a relatively low temperature is supplied by the heated acid
.~
to the second stage. Generally, the heating of the acid
circulating through the phosphoric acid-concentrating stages
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in the interstage and final absorption systems may be
interchanged.
According to a preferred feature, the acid is
concentrated in the first concentrating stage in a vertical
Venturi tube. In that case, a favorable and fast transfer
of heat and mass is effected in a small unit because the gas
and liquid phases are intimately mixed. Because all walls are
constantly flushed with acid, the solids which are formed as
the concentration of the acid is increased cannot deposit but
remain in suspension. Besides, the pressure loss in the gas
is low.
According to a preferred feature, the acid circulat-
ing through the first concentrating stage is heated by heat to
be removed from the sulfuric acid which circulates in a high-
temperature final absorption system. In the final absorber of
the sulfuric acid contact process plant, the gas phase and the
sulfuric acid flow concurrently at least in a first absorption
stage. The second stage may be operated with uncooled sulfuric
acid, which flows in a countercurrent to the gas so that the
exit ~emperature of the end gas corresponds to the temperature
of the draining sulfuric acid, which is at least 95C. This -~
operation results in a removal of heat at a higher rate by the
gases leaving the two absorption systems of the sulfuric acid
contact process plant so that heat at a correspondingly lower
rate must be removed from the acids which circulate in said -
absorption systems. As a result, the area of the heat
exchangers for cooling the sulfuric acid can be reduced and
~` the surplus heat from the absorption systems of the contact
~ .
process plant is utilized in a desirable manner to concentrate
~;~ 30 the phosphoric acid.
:~
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1085~83
According to a preferred feature, the tail gas
from the final absorption system of th~ sulfuric acid contact
process plant is heated with surplus gas heat of the gases
which have left the first contacting stage of the sulfuric
acid contact process plant, said heating is effected in a
second heat exchange stage in the gas flow path, and the air
is admixed to the heated tail gas. This operation avoids a
temperature drop of the S03-containing gases at the heat-
exchange surfaces of the heat exchanger below the dew point.
According to a preferred feature, gases which have
a high SO2 content and become available at a low temperature
are processed in the sulfuric acid contact process plant and
part of the gas mixture is heated in the final heat exchanger
succeeding the last contacting tray. That operation will
afford advantages particularly when the SO2-containing gases
which are processed in the sulfuric acid contact process plant
come from metallurgical plants and contain more than 8.5% SO2
and surplus heat is available for thermally self-sufficient
operation of the contact process plant. The final heat
exchanger has preferably two stages, the first stage in the
flow path of the hot gases serves to preheat gases for the
contact process plant and the second stage serves to heat the
gas mixture. Either the air or the tail gas may be heated.
According to a preferred feature, the hot gases are
fed to the first concentrating stage at a temperature of 70 to
250C, preferably 80 to 220C. This results in a good heat
supply to the first stage without a formation of crusts.
According to a preferred feature, the hot gases are
fed to the first concentrating stage at a temperature of 100
to 280C, preferably 120 to 250C. When gases having a high
So2 content are processed in the contact process plant, this
feature results in a very good heat supply to the first stage
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without a formation of crusts.
According to a preferred feature, the phosphoric
acid is sprayed into the first concentrating stage at a
temperature of 60 to 100C, preferably 75 to 90C. As a
result, additional heat at a relatively low temeprature is
fed to the first stage as heat of the acid, and a good evapo-
ration of water in the first stage is accomplished whereas
there is no danger of a formation of crusts.
According to a preferred feature, the fluorine
constituents are absorbed in an absorption unit in two
successive stages, fluosilicic acid is injected in the first
stage of the absorption unit into a vertical Venturi tube
absorber to flow concurrently with the gases, fluosilicic acid
; is injected in the second stage into an empty tower to flow in
a countercurrent to the gases, and a considerable part of the
inside surfaces of the absorption unit is contacted by the
injected fluosilicic acid. To contact the inside surfaces
r
with fluosilicic acid, the latter is injected with turbulence
so that separated SiO2, which is formed by the hydrolysis of
SiF4, cannot deposit on the wall. Besides, an optimum phase
boundary surface between the gas and liquid is formed and
results in a good absorption.
According to a preferred feature, the fluorine
constituents are absorbed in two series-connected absorption
units, water is continuously fed to the acid which circulates
` in the second stage of the second absorption unit, acid from
the sump of the second stage of the second absorption unit
flows over into the sump of the second stage thereof, acid
from the sump of the first stage of the second absorption unit
flows over into the sump of the second stage of the first
absorption unit, acid from the sump of the second stage of
the first absorption unit flows over into the sump of the first
. . .
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stage thereof, and fluosilicic acid is withdrawn as product
from the sump of the first stage of the first absorption unit.
The overflow rates are controlled so that a continuous
equilibrium of flow is achieved and the steady-state concen-
trations of the fluosilicic acid circulating in the several
absorption stages decrease from the first stage to the last.
This results in constant optimum conditions for the absorption
of fluorine. Besides, the adjustment of a continuous equi-
librium of flow minimizes the residence times of the acid in
the several sumps so that an increased aging accompanied by a
deposition of the separated SiO2 is avoided and a large propor-
tion of the SiO2 is discharged with the product from the
absorption units.
According to a preferred feature, the continuous
feeding of water to the second stage of the second absorption
unit is controlled so that the product acid withdrawn from the
first stage of the first absorption unit has the desired fluo-
silicic acid concentration. This enables a simple adjustment
of the desired fluosilicic acid concentration in the product
which is discharged.
According to the preferred feature, the bottoms of
the sumps slope toward the inlets of the suction conduits
leading to the acid-circulating pumps so that the silica gel
and/or SiO2 which is separated during the formation of
fluosilicic acid is kept in suspension and flows with the acid
through all stages of the absorption units and is continuously
discharged from the absorption system together with the
fluosilicic acid product. In that case there is a turbulent
flow of acid in the sumps and regions of stagnant liquid are
avoided so that the deposition of SiO2 suspended in the liquid
phase is further opposed.
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1085~83
The invention will be described more fully and by
way of examples with reference to the drawings.
The drawings are flow schemes of processes of
concentrating dilute phosphoric acid in conjunction with a
sulfuric acid contact process plant, which is fed in
FIG. 1 with SO2-containing gases produced by the
burning of sulfur, in
FIG. 2 with SO2-containing gases from a roasting
- plant and in
FIG. 3 with gases having a high SO2 content and
formed in a pyrometallurgical plant.
Raw phosphate is fed in conduit 1 and sulfuric acid
is fed in conduit 2 to a wet-process plant 3. Dilute
phosphoric acid is fed from a filter unit 4 in conduit 5 to
an agitating vessel 6 and in a conduit 7 to the second
concentrating stage %, which consists of an empty tower.
From the sump 9 of the second concentrating stage 8, phosphoric
acid is fed by means of a pump 10 through conduit 11 into the
heat exchanger 12 and is heated there and sprayed from conduit
13 into the second concentrating stage 8. Part of the
phosphoric acid which has been collected in the sump 9 flows
in conduit 14 into the sump 15 of the first concentrating
stage 16, which consists of a vertical Venturi tube. Phospho-
ric acid from the sump 15 is fed via conduit 17, pump 18, and
conduit 19 into the heat exchanger 20 and is heated there and
then sprayed from conduit 21 into the first concentrating
`~ stage 16. Concentrated phosphoric acid is fed in conduit 22
to the agitating vessel 23.
The SO2-containing gases are fed in conduits 24 and
24a into the contact process tower 25, in which the gases are
catalytically reacted in known manner. From the first
contacting stage, the partly converted gases, which contain
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iO85583
S03, are fed in conduit 26 to the first heat exc~ange stage 27
and in conduit 28 into the second heat exchange stage 29. The
cooled gases which contain S03 are fed in conduit 30 to the
interstage absorber 31, in which a large part of the S03 is
removed by a treatment with sulfuric acid. The gases which
contain the residual S02 are fed in conduit 32 to the first
heat exchange stage 27 and are heated there to the operating
temperature of the next contacting stage and are then fed in
conduit 33 to the second contacting stage of the contact
process tower 25, in which their conversion is completed in
known manner. The converted gases which contain S03 are fed
in conduit 34 to a heat exchanger 35 and are cooled therein
and are then fed in conduit 36 to the final absorber 37, in
which the S03 is removed from the gases by a treatment with
sulfuric acid. The end gases from the contact process plant
are fed in conduit 38 to the second heat exchange stage 29, in
which they are heated by a heat exchange with the S03-contain-
ing gases, and are then fed in conduit 39 to a blower 40, which
is supplied with air through filter 41 and conduit 42.
In accordance with Fig. 3, filtered air is first
heated in the second heat exchange stage 35a in a heat exchange
with the S03-containing gases which are fed to the final
absorber 37. The heated gas mixture is fed in conduit 43 to
the first concentrating stage 16 in which the gas mixture and
the concurrently sprayed phosphoric acid are cooled by an
adiabatic evaporation of water and most of the concentrated
phosphoric acid is collected in the sump 15 whereas the qas
which has been enriched with water vapor is ~ed to the second
concentrating stage 8, in which it rises in a countercurrent
to the sprayed phosphoric acid. This results in a further
adiabatic evaporation of water to cool the phosphoric acid
which is then collected in the sump 9.
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The exhaust gas from the concentrating unit contains
water vapor and fluorine compounds and is fed in conduit 44
to the first absorption unit 45 and in said unit is treated
in the first stage 45a, consisting of a vertical Venturi tube,
with concurrently flowing fluosilicic acid and in the second
stage 45b with countercurrently flowing fluosilicic acid.
These treatments are correspondingly repeated in the second
absorption unit 46. After the removal of most of the fluorine
compounds, the gas is fed into the atmosphere through conduit 47.
Water is fed in conduit 48 to the acid which circulates in the
second stage 46b of the second absorption unit 46. In conduit
49, the fluosilicic acid which has been formed is withdrawn
from the acid circulated in the first stage 46a of the second
absorption unit 46 and is then added to the acid which circu-
lates in the second stage 45b of the first absorption unit 45.
The entire fluosilicic acid product is withdrawn in conduit 50
from the acid circulating in the first absorption stage 45a of
the first absorption unit 45.
The phosphoric acid is heated in the heat exchangers
12 and 20 by the sulfuric acid which is fed to the heat exchan-
gers 12 and 20 in conduits 51 and 52, respectively, and when
cooled by the heat exchange is returned to the absorbers 31 and
37 in conduits 53 and 54, respectively.
The SO2-containing gases are produced in a sulfur-
burning furnace 55a, a fluidized-bed roasting furnace 55b or
a flame cyclone reactor 55c. The hot gases which contain SO2
are cooled in a waste heat boiler 56. The heat which has been
removed is used to produce steam. In accordance with Fig. 1,
the cooled gases which contain SO2 are directly fed to the
contact process tower 25 in conduit 24. In accordance with
Figs. 2 and 3, the cooled gas which contains SO2 is purified
and dried in known manner and is then heated to the operating
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temperature of the first contacting tray by the heat which is
released in the contact process tower as a result of the cata-
lytic oxidation of SO2 to SO3. This heat is used to produce
steam in accordance with Fig. 1. In all cases, the air
required to burn sulfur and to oxidize SO2 is dried before.
Examples
In all cases, the sulfuric acid contact process
plant is designed for a production capacity of 1500 (metric)
tons of H2SO4, calculated as 100% H2SO4, per day.
; 10 The conditions which are obtained at the positions
indicated by the reference numbers of Figs. 1 to 3 will now
be indicated.
;' .
Position Unit Fig.l Fig.2 Fig.3
1 Raw phosphate feed
rate t/h 70.4 70-4 70-4
P2O5 content % by weight 34 34 34
1~ . . . _
2 Sulfuric acid
(calculated as
100~ H2SO4 t/h 62.5 62.5 62.5
7 Fresh dilute
~ phosphoric acid t/h 77.6 83.3 80.4
Z P2O5 content % by weight 29 27 28
22 Concentrated
phosphoric acid
rate t/h 41.7 41.7 41.7
P2O5 content % by weight 54 54 54
. ~
H2SiF discharge
rate 6 t/h 33.4 33-4 33-4
l~2Si~6 contcnt % by weight 20 20 20
48 Water feed rate t/h 29 29 29
.:
24 Gas flow rate standard 145484 175500 102000
m /h
S2 content % by vol. 10 8 14
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Position ~nit Fig.l Fig.2 Fig.3
38 Gas flow rate standard 124375 154416 80453
m3/h
Gas temperature C 140 140 140
39 Gas temperature C 254 235 305
_
42 Air flow rate standard
(calculated as 3
dry air) m /h 50866 15590 89548
Air temperature C 20 20 165
. 43 Gas flow rate standard 170000 170000 170000 (calculated as 2
dry gas) m /h
Gas temperature C 184 215 231
. .
I 44 Gas temperature C 75 75 75
Water vapor3g/standard
content m of dry gas 188 224 209
44 Fluorine 3g/standard 5.2 5.2 5.2
content m of dry gas
47 Gas flow ratestandard m3 170000 170000 170000
(calculated as 3
dry gas) m /h
Gas temperature C 63 63 63
Water vapor3g/standard 240 240 240
contentm of dry gas
21 P2O5 content % by weight 54 54 54
TemperatureC. 85 85 85
- ,:
19 Phosphoric acid O
temperature C 75 75 75
:
13 P2O5 content % by weight 41 39 40
:~ Temperature C 80 80 80
'.
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Position Unit Fig.l Fig.2 Fig.3
- 11 Phosphoric acid C 71 71 71
temperature
51 H2SO4 content % by weight 98.5 98.5 98.5
Temperature C 140 140 140
53 Temperature C 111 111 111
52 H2O4 content % by weight 98.5 98.5 98.5
Temperature C 140 140 140
54 Temperature C 121 121 121
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The advantages afforded by the invention reside
mainly in that dilute phosphoric acid can be concentrated
without a need for primary energy or steam and in such a
manner that surplus heat from a sulfuric acid contact process
plant can be economically utilized with a low expenditure.
The operating costs of the combined system are low because
primary energy or expensive steam is not required for concen-
trating the dilute phosphoric acid and costs involved in theextraction of surplus heat from the absorption systems of the
contact process plant are saved. Besides, the end gas from
the contact process plant is purified once more without an
additional expenditure. The fluorine constituents of the raw
phosphoric acid which is to be concentrated are removed to a
very high degree in the course-of the concentrating treatment
and are recovered as a usable fluosilicic acid in a succeeding
unit for absorbing the fluorine compounds. Whereas the high-
temperature heat which becomes available in the production of
the SO2-containing gases and in the catalytic reaction in the
contact process plant is usually employed to produce steam for
use in the process of concentrating the phosphoric acid which
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becomes available, said heat is now freely available for a
production of energy in as much as the heat is not required
for heating initially cold gases which contain S02 to the
operating temperature of the first contacting tray.
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