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Feb. 10, 1942.
'
2,272,402
G. DL_J B015
MANUFACTURE OF PHOSPHORIC ACID
Filed May 22; 1959 -
Flames NO. l
Heuzé No. 5
Gas/on DUBa‘Q INVENTOR
BY
ATTORNEY
atented Feb. 10, 1942
2,272,402
_
MANUFACTURE or rnosrnomc Acm
Gaston Du Bois, Clayton, ’ Mo., assignor to
Monsanto Chemical Company, a corporation of
Delaware
Application May 22, 1939, Serial No. 275,130
5 Claims. (jCl. 23—165)
must be such as to provide ?owability and at the
same time provide suf?cient viscosity to impart
corrosion resistance to the metal. The property
This invention relates to the manufacture of
phosphoric acid and also to apparatus therefor.
One of the objects of the present invention is
the provision of a simpli?ed and economical
process for the manufacture of strong phosphoric
acids. Another object is the provision of a proc
ess for the manufacture of various polyphos:
phoric acids. A further object is the provision
of apparatus suitable for carrying out such proc
of ?owability is necessary to permit a sufficient
?ow of heat through the metal walls and viscous
?owing ?lm of acid in order that the cooler may
function e?iciently. It is evident that if the vis
cosity of the acid deposited upon the tubes is too
great, an excessively thick ?lm is deposited which
esses.
10 would greatly impede the flow of heat. On the
other hand if the viscosity of the acid is too low
In known processes for the manufacture of
_then the rate of heat ?ow. would be so great that
phosphoric acid involving the combustion of ele
mental phosphorus and the subsequent hydra
the metal surface would be heated to a tempera—
' tion of the combustion products, it has been cus
ture at which corrosion takes place with rapid
tomary to remove a large part of the heat gen
erated by such combustion by the evaporation of
, water.
15 destruction of the equipment.
The nature of the acid ?lm, that is its viscos
ity, is dependent upon the acid concentration and
the temperature. It has been found that the
In accordance with these earlier proc
esses it has been the practice either to spray the
requisite quantity of water into the hot combus
acid should have a concentration of at least 95%
tion gases to cool these gases, or else to pass the 20 HsPO-i and preferably higher. For most satis
factory operation yielding an acid of high purity,
combustion gases directly into a large‘ body of
that is free of corrosion products, I prefer to op
water or weak acid. Such prior processes pro
erate with a minimum acid strength of 104.9%
duced only the relatively weak grades of phos
H3P04 (76% P205). Acid containing less than
phoric acid, by which I mean-acids having a
15 p. p.’ m. of nickel may be condensed upon
strength of less than 85% or 95% H3PO4. Be
cooled Monel tubes provided that the acid con
cause of the relatively large quantities of water
tains at least 76% P205.
’
evaporated in cooling the combustion gases down
The upper limit of the acid is 116% equiva
to collecting temperatures, it was di?icult, if not
lent H3PO4 content which limit is ?xed by the
impossible, to produce acids having strengths
ranging from 95% to as high as 116% H3PO4 30 viscosity of the acid. This acid is also known
as tetraphcsphoric acid, H6P40l3.
’ The acid collected by my process, in addition to
Other processes have been proposed in which
‘containing .metaphosphoric acid, has an appre- phosphorus combustion gases containing an ex
ciable content of pyrophosphoric acid and also
cess of water vapor are cooled to lower tempera
tures to produce the meta, pyro and orthophos 35 orthophosphoric acid. The stoichiometric com
position of the acid of the limits herein speci?ed
phoric acids. Processes involving the condensa
is as follows:
tion of these higher acids upon cooled metallic
surfaces cannot be carried out because of the
high temperatures necessary to produce such .
Calculated
Concentration of——
acids in the presence of .excess quantities of 40
equivalent
H3P 0 4
water vapor.. Should the temperatures of the
gases be lowered then the acids stronger than
equivalent.
orthophosphoric acid cannot be obtained due to
Percent
Percent
Percent
Percent
the ‘excess of water vapor.
' 104. 9
2. 0
50. 0
‘43. 0
47. 0
55. 0
116. 0
_ 7
~
In conducting experiments on the combustion 45
of elemental phosphorus and the hydration and
cooling of the products of combustion down to '
collecting temperatures, I have found that phos
phoric acids of certain strengths have the prop
erty' of adhering to and covering with a continu
ous, viscous, ?owing ?lm, cooled metallic sur
faces without corroding such surfaces._.v
. The nature of the acid ?lm deposited upon the
3. 0
The viscosity of acid having an equivalent
H3PO4 content of 104.9% is 200 .Saybolt seconds,
while that of an acid of 116.0% H3PO4 is 7000
60 Saybolt seconds, both determinations having,
been made at 163° F.
By controllof the moisture content of the air
present during combustion of the phosphorus, or
added'prior to condensation, I may condense di
cooled metallic surfaces as obtained by my proc
ess is important because the viscosity of the acid 55 rectly in ?owable form, upon the cooling sur
2
2,272,402
faces, various polyphosphoric acids. Such poly
covery that when phosphorus is burned in air
phosphoric acids as are directly condensable in
containing a quantity of water vapor such that
the acid contains at least three mols of water for
each two mols phosphoric oxide (P205) an acid
mist is produced which is easily condensed and
liquid ?owable form are the triterohexaphos
phoric acid H'IP3O11; the penterohexaphosphoric
acid, H5P3O1o, and the tetraphosphoric acid
HsP4O13.
'
?ltered out of the gas stream and consequently
The triterohexaphosphoric acid has a viscosity
in the neighborhood of 250 Saybolt seconds, and
a greater efficiency of collection is obtained.
While such processes are capable of operation
the penterohexaphosphoric acid a viscosity of
using the ordinary materials of construction, that
1500 Saybolt seconds, all measurements having 10 is carbon and acidproof brick, it has been found
been made at a temperature of 163° F.
>
that the acid dissolves small amounts of such
The workability of my herein described process
construction, materials, that is, alumina, silica,
depends upon the discovery that phosphoric
acids within the concentration and temperature
and iron oxides, which substances contaminate
contact therewith and continually drops or flows
oif therefrom. Because of the strength of the
.phosphoric acid constituting the ?lm covering
on nickel-chromium steel and high nickel, alloys
is favored by the low viscosity of the acid, which
the acid produced. In cases where carbon is used
limits described above exhibit practically no cor 15 as a material of construction it must be kept at
rosion upon certain metallic surfaces. At the
a temperature below the oxidizing temperature
same time the viscous ?lm of acid formed when
or it must be maintained saturated with acid.
acids within the above concentration limit are
In construction of, plants for the process here
deposited upon cooled metallic surfaces has a
contemplated both of these requirements are dif~
sui?cient heat conductivity so as to permit of 20 ?cult to meet, with the result that the purity of
the application of this discovery‘ to the construc
the acid produced by prior known processes is
tion of a practical cooler for cooling phosphoric
somewhat lower than-that‘ produced by my pres
acid containing gases down to the usual collect
ent invention.
‘
ing temperatures. In the operation of my proc
As a result of experiments which I have insti
ess phosphoric acid is continually deposited upon 25 tuted on the process herein disclosed, I have dis
the cooled metallic surfaces from the gases in
covered that corrosion by ‘orthophosphoric acid
prevents the formation of a ?lm of the acid ad
the .metallic cooling surfaces, the layer of acid 30 hering to the metal, with the result that acid at
immediately adjacent the metal is immobile
high temperature is in contact with the metal
while the layers of acid farther removed from
surface. If the viscosity of the phosphoric acid
the metal surfaces are ?owable due to their
is increased by either increasing the strength of
higher temperature; the, temperature and hence
such acid, or by lowering the temperature there
the flowability increasing with the distance the 35 of to below 180° C. and preferably below 140° C.
layer is removed from the metal surface. This
characteristic of an acid ?lm varying in ?ow
ability as well as temperature outwardly from the
metal surface is an important feature of my
the corrosion may be substantially prevented be
cause a ?lm of appreciable thickness will be
formed on the metal surface. Consequently the
acid at the interface of the metal and the acid
process.
I
40 ?lm would be of such low temperature due to
By way of further explanation it may be stated
the temperature drop across the ?lm that corro
that the acid deposited upon the cooled metal
sion is substantially eliminated.
surfaces is pure and uncontaminated with solids
In the application of this discovery to the here
as in prior art processes.
The presence of a pro
tective viscous layer of phosphoric acid upon the
metal cooling surfaces is dependent solely upon
in described process I provide a surface of
. chrome-nickel alloy or other suitable metal with
the high strength of the acid thus deposited.
Metals which may offer satisfactory resistance
to corrosion at the temperatures which prevail at
the metal-liquid interface when strong phos 50
phoric acid is maintained as a viscous ?lm on
a ?lm of viscous acid' of such viscosity and
strength as to be within the herein described
limits. I maintain the viscosity and temperature
of the ?lm of acid by the application of a cool
ing ?uid to the chrome-nickel alloy in a manner
“to be hereinafter more fully described.
the surface of the metal, according to my inven
Referring to Figure 1 of the drawing, I have
tion, comprise nickel-chromium steels with or
shown by numeral in a brick-lined combustion
without silicon, titanium, molybdenum or man
chamber provided with an inlet pipe II by. which
ganese, alloys containing a high percentage of 55 molten yellow phosphorus is supplied to said
nickel and certain casting alloys which maybe
chamber. By means of pipe I2, I introduce a
combinations of iron and silicon or complex mix
quantity of previously humidi?ed air from a
tures of iron with nickel, chromium, copper mo
source not shown, the quantity of air thus sup
lybdenum, manganese, tungsten and silicon. Al
plied being so proportioned with respect to the
though pure nickel itself is unsuited for the pres
phosphorus supplied by pipe II, that a substan
ent purpose, its alloys, especially those which 60 tial excess of oxygen over that required to burn
contain iron, copper or ‘chromium, are suitable.
such phosphorus is present in chamber 10. The
Monel metal is particularly valuable for this pur
amount of water ‘carried by said humidi?ed air is
pose.
such that an acid having a strength of from
In accordance with the' present invention, I
104.9% to 116% H3PO4 equivalent is produced.
. provide cooling or heat exchange units positioned 65
Combustion chamber It may contain ba?le
within the combustion chamber or between the
walls i 3, the function of which is to thoroughly
combustion chamber and the absorbing system,
mix the phosphorus, air and moisture undergoing
such units being fabricated for the most part of
reaction in said chamber. The products of the
relatively thin alloy sheets of compositions as
reaction taking place in said chamber In pass
70
hereinafter more particularly speci?ed.
outwardly through duct H, which, in accordance
My process is particularly applicable to those
with my invention, is made of nickel-chromium
phosphorus combustion processes wherein pure
alloy steel sheet or drawn tube or of a high nickel
elemental phosphorus is burned with prehumidi
alloy in similar form and provided with cooling
fled air. These processes are based upon the dis 16 water sprayed upon the exterior thereof, as by
2,272,402
pipe 15 supplying sprays It. The gases pass from
duct or tube It into chamber ll which is pro
by the prior art or as particularly pointed out in
the appended claims.
This application is a continuation in part of
my earlier ?led application, Serial No. 160,398,
vided with‘a ?lter medium I9 consisting of ?nely
divided coke supported upon a grid [9. The gases
pass upwardly through the ?lter bed l8 and out
?led August 23, 1937.- '
of the chamber I‘! by means of stack 20.
. -What I claim is:
'
1. A process for producing pure phosphoric
In accordance with my invention chamber I1 is
constructed of nickel chromium steel sheet in a‘
acid of strength within the range of 104.9% to
age tanks.
tion between 104.9% and 116% equivalent H3PO4,
said ?lm being substantially immobile adjacent
116% equivalent H3PO4 content, comprising
similar manner to duct l4, and is provided with
spray cooling means 2i, said means consisting in 10 burning pure yellow phosphorus in air, adjust
ing the water vapor content of the hot combus
this instance of a perforated pipe supplying cool
tion gases so that the oxidation products are
ing water to the exterior of the steel sheets or
hydrated to the extent of containing phosphoric
plates comprising ?lter chamber H.
acid having a concentration of between 104.9 %
The cooling water from sprays l6 and 2| is
collected by means of pipe 22 and then pumped 15 and 116% equivalent H3PO4 content, contacting
said hydrated oxidation products with a resistant
back to pipes I5 and 2| by pumps not shown in
metallic surface, said surface being maintained
the drawing.
constantly at a temperature below 180° 0., ‘con
The strong acid collected in conduit I4 and
~densing upon said cooled metallic surface a ?ow
?lter chamber I1 is removed from the latter by
means of outlet 23 and conveyed to suitable stor 20 able ?lm of phosphoric acid having a concentra
v
A further embodiment of my invention is illus
said metallic surface and increasing in ?owability
with distance outwardly from said metallic sur
25 face, and recovering acid from said ?owable ?lm.
tion as ‘above described.
2. A process for producing‘ pure phosphoric
In the operation of the apparatus shown in
acid of strength within the range of 104.9% and
Figure 2, the combustion gases pass out of com
116% equivalent H3PO4 content, comprising
bustion chamber I0 by means of duct I4 and
burning pure’ phosphorus in air, adjusting the
enter bricklined chamber25. This chamber is
provided with a multitudeof pipes or pipe coils 30 water~vap0rcontent of the hot combustion gases
so that the oxidation products are hydrated to
26 connected to headers 21, which in 'turn are
the extent of containing phosphoric acid having
connected to a main header 28. The pipes 21 are
a concentration within said limits, contacting
maintained full of water‘, thermal circulation
said hydrated oxidation products with the sur
being maintained with the water in drum 28 in
trated by Figure 2, wherein parts numbered sim
. ilarly to thosein Figure 1 have the same func
the usual manner. The temperature of the 35 face of a water cooled Monel tube, said cooling
‘water being maintained at a temperature below
water in coils 26 is maintained below 180° C. and
180° C., condensing upon said cooled Monel tube
preferably below 140° C. Said coils are con
a ?owable ?lm of phosphoric acid within said
structed of the metals or alloys hereinafter
limits, said ?lm being substantially immobile ad
enumerated and during operation are covered
with a ?owable ?lm of viscous acid in accordance 40 jacent said Monel surface and increasing in ?ow
vability with distance outwardly from said Monel
with my herein described invention. Phosphoric
surface, and maintaining said acid ?lm by con
acid of high strength and of high viscosity is con
densation from the hydrated combustion prod
densed upon coils 26, said coils being below the
nets.
dew point of the vapors and the ?owable ?lm of
acid serves to protect the material of said coils 45 3. A process for producing pilre phosphoric
acid of strength within the range of 104.9% to
against corrosion.
‘
116% equivalent H3PO4 content, comprising
The gases and vaporsrwhich are thus cooled by
burning pure yellow phosphorus in air, adjust
contact with said coils 26 pass into duct'29 and
ing the water vapor content of the'resulting hot
thence into chamber I'I provided with ?lter ma
terial l8 supported upon grid l9. Acid condensed 50 combustion gases so as to contain an equivalent
phosphoric acid of ' 104.9% to 116% H3PO4, con
in chamber 25 passes into chamber l1 and to
tacting said hot hydrated combustion gases with
gether with acid collected in said chamber, ?ows
the exterior surface of an internally Water-cooled
out through port 23 into suitable storage tanks.
Monel tube, said cooling water being maintained
‘As materials suitable for the construction of
duct I4, chamber l1 and tubes 26, shown in the 65 below 180° C., condensing upon said cooled Monel
tube a ?owing ?lm of acid of strength within said
above apparatus in the described process, I
recommend the following:
'
limits, said- ?lm of acid being substantially im
mobile adiacent said Monel surface-and increas
ing in ?owability with distance outwardly from
chromium, 8-35% nickel and small amounts of
said
Monel surface, and recovering" acid con
molybdenum or titanium, the balance being sub 60
densed upon said Monel surface.
stantially all ‘iron. Such ma?erial is known as
4. A process for producing pure phosphoric
stabilized stainless steel.
acid of strength within the range of 104.9% to
Various alloys in which nickel and copper such
116% equivalent H3PO4 content, comprising
as Monel metal are the predominating metals._
burning pure yellow‘ phosphorus in air, adjusting
65
These alloys may also contain appreciable'quan
the water content of the hot combustion products
tities of iron, manganese, silicon and aluminum.
so that the contained phosphorus pentoxide is'
Casting alloys which contain nickel, molybde
hydrated to a degree corresponding to an acid of,
num, chromium and iron as their main constit
from 104.9% to 116% equivalent H3PO4, contact
uents with smaller quantities of copper, manga
ing said hydrated combustion products with one
Nickel-chromium
steel containing
10-30%
nese, tungsten and silicon and casting alloys con
sisting mainly of iron and silicon.
Having now particularly described my inven
70 side of a resistant cooled metallic sheet, said
metallic sheet being maintained below 180° C. by
contact with cooling water maintained at a tem
perature below 180° C. upon the other side there
desire that it not be limited except as required 76 of, condensing upon said metallic sheet a ?lm of
tion and the manner in which it can be used, I
4
-
2,272,402
phosphoric acid of strength within said limits,
said ?lm being substantially immobile where it
to said cooling water, and recovering acid or said
strength from said ?lm.
‘contacts said metallic sheet and said ?lm in;
5. The process de?ned in claim 4 in which the
creasing in'?owability and temperature with distemperature of the cooling water in contact with
tance outwardly from said metallic sheet, and 5 said‘metallic sheet is maintained at a tempera
conducting heat of condensation of said'acid
through said ?lm and through said metallic sheet
ture below 140° C.
‘
GASTON DU BOIS.
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