Note: Descriptions are shown in the official language in which they were submitted.
~ll,'j4~
This invention relates to a method for the production
of monocalcium phosphate and phosphoric acid by the acidulation
of phosphate rock with phosphoric acid in the presence of
silicon dioxide and potassium ion wherein fluorides are converted
to potassium fluosilicate and the calcium is converted to
monocalcium phosphate from which potassium dihydrogen phosphate
may be formed.
Phosphoric acid plants are currently operated
utilizing a basic and well known process for the acidulation of
phosphate rock which comprises reaction of the rock with sulfuric
acid to form phosphoric acid with subsequent reaction of the
phosphoric acid, with for example ammonia to produce monoammonium
phosphate tMAP) and diammonium phosphate (DAP). The phosphoric
acid formed in this process is called wet process phosphoric
acid. In this reaction, a by-product is gypsum having the
chemical formula CaSO4 2H2O. Essentially, all phosphate rock
contains some fluoride, normally in the 3.0 to 4.0~ range, and
the acidulation reaction usually generates gaseous fluorides.
Because of the fluoride content, an important problem in the
operation of these wet process phosphoric acid plants has been
in the expensive methods for handling the large amounts of
fluorine compounds which are liberated in the gaseous and
aqueous effluents from such plants. It is only in recent years
that studies have been made on the effects of fluorides contained
in the final product and indications seem clear that they may
have a deleterious effect on the long range producing ability
of the soil when present in fertilizers.
lllS4~;3
It i5 accordingly an object of this invention to
produce relatively pure phosphoric acid and relatively pure
monocalcium phosphate which are essentially free of fluorides,
iron, aluminum, magnesium and other impurities, in such manner
as to eliminate or greatly reduce K2O losses and concentrate
insoluble fluoride compounds in recoverable form so that they
can be processed for fluorine and K2O recovery and reuse, and
minimize contamination of the environment and final products
by the presence of fluorine compounds.
Thus, by one aspect of the present invention there
is provided a process for the acidulation of phosphate rock and
the production of phosphoric acid and monocalcium phosphate
which may subsequently be converted to potassium dihydrogen
phosphate, a valuable fertilizer, as well as the recovery and
isolation of the fluoride compounds initially as K2SiF6 and
ultimately as calcium fluoride. This process comprises, in
its broadest embodiment, acidulating phosphate rock with an
excess of phosphoric acid in the presence of added silicon
dioxide and potassium ion to produce a first slurry of insoluble
potassium fluosilicate in a solution of monocalcium phosphate in
phosphoric acid; subjecting this mixture to separation to produce
a clarified solution of monocalcium phosphate in phosphoric acid
and a second slurry comprisin~ monocalcium phosphate in phosphor-
ic acid which contains insoluble potassium fluosilicate; subject-
ing said second slurry to hydrolysis at an elevated temperature
to regenerate a KH2PO4/H3PO4 solution and produce calcium
fluoride and silicon dioxide; recovering the calcium fluoride
and silicon dioxide and recycling the KH2PO4/H3PO4 solution to
.~ - 3 -
,
4~3
the acidulation reaction. Preferably, a major portion of the
monocalcium phosphate/phosphoric acid solution is reacted with
sulEuric acid to precipitate calcium sulfate hydrate which is
removed from the system, and phosphoric acid, a portion of
which may be removed as product, with the balance being
recycled to the acidulation reactor as determined by material
balance considerations.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the drawings accompanying
this application which are diagrammatic flow sheets wherein:
Figure 1 shows the main embodiment of the process
of this invention; and
Figure 2 shows alternative embodiments for further
processing of the monocalcium phosphate/phosphoric acid
product.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As indicated above, this invention is concerned with
a multi-step procedure for the preparation of essentially
fluoride-free products, preferably alkali metal phosphates
and phosphoric acid, by the acidulation of phosphate rock,
which procedure is conducted in the substantial absence of
fluorine pollution and wherein the fluorides may be recovered
in usable form, and wherein phosphoric acid may be regenerated
for reuse in the system and/orseparated as product.
- 4
~: , . ' . ~ ... ~,. . ' : -
~. - . . :
; ., ; ~ : ~ .:
:. . : ~
As is known, most of the commercially important
phosphate ores mined in this country, and particularly those
mined in Florida, contain 3-4% fluorine after beneficiation.
The fluorine is a constituent of fluoapatite which is commonly
expressed as Ca9(PO4)6 CaF2 and may also be present as calcium
fluosilicate (CaSiF6). Silica is a component of phosphate rock
and is usually abundant in most grades of rock that are commonly
used in the production of wet process phosphoric acid. In
usual processes the fluorine compounds in the phosphate rock
react with sulfuric acid during the attack on the rock so that
the fluorine appears in vapor form as hydrofluoric acid (HF),
silicon tetrafluoride (SiF4), or other gas, and in the phosphoric
acid solution as fluosilicic acid (H2SiF6) and/or fluosilicate
salts or other forms. Acids from a rock low in reactive silica
may also contain free hydrogen fluoride. The present invention
provides a significant solution to problems of fluoride pollu-
tion by providing a procedure for minimizing fluoride evolution
while recovering substantially all of the fluorides in usable
form thereby preventing the fluorides form contaminating the
environment and desired products. The present invention also
provides a series of substantially purer and useful products as
well as novel procedures for obtaining these products without
pollution.
In one aspect, the process of this invention is
concerned with the preparation of alkali metal phosphates and/or
phosphric acid and in a main embodiment of the invention, the
alkali metal phosphate is an alkali metal dihdyrogen phosphate.
A preferred product is KH2PO4 and/or its admixture with
phosphoric acid, which contain high plant food
- 5 -
S~3
nutrients, and is hi~hly valued as a ertilizer. Na~l2PO~, an
alternative product, is widely used in tile detergent industry
and other areas. However, potassium products are preferred
and the reaction is described with respect to potassium reactants
and products. The process of the present invention is carried
out in a continuous manner in the optimum embodiment.
In the initial step of the process of this invention,
- phosphate rock from any origin, but usually of the type described
above containing at least some fluorides, is acidulated with a
solution of phosphoric acid containing potassium ion recycle
values from room temperature up to about 95C., and preferably
about 70 to 90C., for a sufficient time to achieve substantially
complete acidulation, usually about 1/2 to 4 hours depending
on the reaction temperature and using a sufficient amount of
the phosphoric acid solution to completely solubilize the calcium
phosphate formed. Sufficient potassium ion is present in the
mixture to cause precipitation of the fluorides as a precipitate,
primarily as K2SiF6 together with SiO2 and impurities. In the
preferred embodiment, the potassium ion values are provided
by KH2PO4 salts contained in recycle phosphoric acid solution.
In conducting this initial step, the phosphoric acid
solution is utilized in sufficiènt excess to effect subs-tantially
complete acidulation and solubilization oE the calcium in
phosphate rock. The P2O5 content of the phosphoric acid should
range from about 20-55~ and preferably about 25~~0~ by weight.
In general, there should be used an excess of phosphoric acid
and preferably about 35 to 90 moles of phosphoric acid for
each 6 moles of phosphate in phosphate rock, or a molar ratio of
P2O5 in the acid to P2O5 in the rock, of about 6:1 to 15:1,
respectively. Also, about 1.0 to 10 moles of K2O, preferably
4~3
more than about one mole, to provide a slight excess of K2O,
should be present for each three moles of phosphate rock
of the formula ca9(po4)6 CaF2 The K2O or potassium ion is
preferably added as KH2PO4.
As pointed out, the phosphoric acid is present in
sufficient amounts to solubilize the calcium phosphate contained
in the phosphate rock. Further, the K2O values such as the
KH2PO4 salt are contained in the phosphoric acid in a sufficient
amount to precipitate the fluorides present as dense crystalline
solids which may be recovered readily. Thus, during the
acidulation step, while the calcium phosphates are solubilized,
there is precipitated a mixture of solids from which the fluorides
may be recovered. This precipitate contains the fluorides
essentially as K2SiF6.
It is to be appreciated that the phosphoric acid as
the treating acid is to be distinguished from the stronger mineral
acids such as sulfuric acid, nitric acid, hydrochloric acid,
and the like. As is shown in many standard reference books,
phosphoric acid has a weaker ionization constant than stronger
mineral acids. By use of the term phosphoric acid, it is meant
that it is an acid that is ionized at less than 90~ at a strength
of concentration of 0.1 Normal, and has an ioni~ation constant
of no more than 7.5 x 10 3.
11 154~3
In conducting the initial step of the reaction,
the phosphate rock and phosphoric acid are reacted in the
plesence of reactive silica. There is also present a recycle
solution comprising a solution of potassium dihydrogen
phosphate and phosphoric acid. In general, there is
sufficient potassium ion and reactive silica present in this
initial reactor to convert fluorides contained in phosphate
rock to potassium fluosilicate.
The silica added during the reaction of this in-
vention may be amorphous silicon dioxide in any suitable formso long as it is not delete~ious to the reaction under con-
sideration. The silica is preferably obtained from materials
combinable with the phosphate rock, such as slag, or commerci-
ally available products such as those sold under the trade
mark "Dicalite,"~ sold by Grafco Corporation.
The product resulting from the initial reaction
comprises a relatively low concentration of suspended solids
(e.g., in the range of 3 to 15 wt.%), in the solution of
monocalcium phosphate and phosphoric acid. This mixture is
preferably passed to a thickener for separation of the solids
and solution to produce a clarified monocalcium phosphate
solution. This clarified monocalcium phosphate may then be
treated as described herein to produce phosphoric acid and/or
potassium dihydrogen phosphate.
An important feature of the invention is the
utilization of the calcium ion from phosphate rock to remove
j"':~'
......
::~ ', . :
: ~ '' . :
11154~3
fluorides as 3CaF2 and/or 3CaF2/SiO2 and thereby eliminate
the need of using an external source of calcium such as
limestone. While the potassium ion is a critical component
of this system, it is not consumed, but simply recycled to
perform the required fluoride removal function. As a con-
sequence, the cost of K2O in fluoride removal is no longer
a significant factor since only makeup K2O will be needed as
governed by losses.
It is also within the scope of the invention to
utilize an external source of phosphoric acid and/or an ex-
ternal source of K2SiF6 in the initial acidulation reaction.
However, in the preferred embodiment, recycle of these mater-
ials is especially preferred for purposes of economics.
The underflow, when a thickener is used, is a slurry
of monocalciurn phosphate/phosphoric acid solution which con-
tains the fluorides, usually as potassium fluosilicate, and
any slirnes. A feature of this invention is that this mix-
ture is hydrolyzed, preferably by heatin~ at 100-115C. or
up to the reflux point, to form potassium dihydrogen phos-
phate in phosphoric acid and convert the fluorides to cal-
cium fluoride and silicon dioxide. As shown, this hydrolysis
reaction proceeds as illustrated by the following equation:
2 4)2 + lOH3PO4 + K2SiF6 + R2O3/P2O5 + 2 H O >
2/3caF2 + R2O3/P2O5 + 2KH2PO4 + 14H PO
wherein R is a metal such as Fe or Al.
As may be seen from this equation, the fluorides,
lli54~3
in the form of K2SiF6, are converted to SiO2/3CaF2as a solid
in admixture with A12O3, Fe2O3, etc. This solid mixture is
separated from the solution of 2KH2PO4 + 14H3PO4 and valuable
fluorides may be recovered from the solids as described here-
in.
The resulting solution is suitable for recycle to
the system to provide at least a portion of the potassium ion
necessary to produce additional potassium fluosilicate and
also provide a source of phosphoric acid. As a result,
some of the SiO2 and K2O are not consumed in the reaction
but rather are recycled in the continuous process. It is, of
course, to be understood that additional amounts of potassium
ion and SiO2 from external sources may be added to the acidula-
tion reactor as may be required by the system. An external
source of phosphoric acid may also be used.
In one embodiment, a portion of the resulting clari-
fied monocalcium phosphate and phosphoric acid solution is
reacted wlth potassium sulfate, potassium bisulfate or mix-
tures thereof to produce KH2PO4/H3PO4 solutions from which
KH2PO4 may be recovered as a fertilizer grade material. Phos-
phoric acid may also be produced in this embodiment and may
be recovered or recycled as makeup phosphoric acid.
The remaining monocalcium phosphate/phosphoric acid
solution is reacted with sulfuric acid to produce calcium
sulfate hydrate which may be recovered and the phosphoric
acid regenerated as a result of this reaction may be recovered
as product and/or recycled to the main reactor to effect
acidulation of the phosphate rock feed.
-- 10 --
: ..... : ;.. : - . .
lllX4~3
The essential steps described above for the reaction
provide a number of advantages in the process. Thus the
process serves to regenerate valuable hydrogen ions as
illustrated by the following equation:
K2SiF6+ 3Ca(H2PO4)2~ 10H3PO4 ~ 3caF2 + SiO2+ 2KH2PO4+ 14H3PO4
Thus the phosphoric acid concentration increases from 10 to
14 moles or an increase of 40%. More importantly, this 14
moles of free H3PO4 can now accommodate additional unreacted
phosphate rock. In effect, approximately 3CaO/30CaO or 10%
of the original rock feed can be acidulated in this manner;
3 4)2 14H3Po4 = 3ca(H2P4)2 + 10H3P
The process of the invention also removes unreacted
phosphate rock from the acidulation reaction and subjects
this rock to much more vigorous acidulation conditions to provide:
~ 15
; a) increased phosphate acid concentration as illustrated
above,and b) increased temperatures from 80-90C. The process
accomplishes these functions using a relatively modest defluo-
rination/hydrolysis loop which is only 10% of the main loop
or system. Further it permits recovery of the considerably
more dense Fluorspar component, and will also separate
unhydrolyzed K2SiF6 with the ÇaF2. In this instance, subsequent
- treatment with NH~OH can be utilized to produce a chemical
grade Fluorspar. The process also eliminates the R2O3 component
after removal of the dense CaF2/K2SiF6 components - preferably by
the addition of clean gypsum to assist in the separation
(centrifuge) step and to simulate the 0-20-0 NSP grade. The
110 115C. temperatures involved in hydrolysis will help
::`
li:lS4~3
flocculate the R2O3 component and simplify separation.
Reference is now made to Figure 1 accompanying the
application wherein there is shown a schematic diagram of
the main embodiment of the process of the present invention.
In the drawing, phosphate rock from line 1 and phosphoric
acid from line 2 are reacted in acidulation reactor 3. The
reaction is conducted at a temperature in the range of about
40-95C. and the materials are reacted utilizing an excess
of the phosphoric acid. The phosphoric acid contains potas-
sium, usually added as KH2PO4, in sufficient amounts to
react with fluoride contained in the phosphate rock and pro-
duce potassium fluosilicate. In addition, reactive silica
is added by line 4 to provide sufficient reaction with
potassium to form the potassium fluosilicate. In this
reactor 3, monocalcium phosphate is formed as a solution
in phosphoric acid with an insoluble precipitatecomprising slimes
and a portion of the potassium fluosilicate. Sufficient
phosphoric acid is present to dissolve the monocalcium phos-
phate.
The reaction mixture is then passed by line 5
directly to defluorination reactor or thickener 6 for re-
.
moval of the fluorides.
In defluorination thickener 6, a product or under-
flow is removed which is a slurry of potassium fluosilicate, sio2
slimes, and other solids in a solution of monocalcium phos-
phate in phosphoric acid. In accordance with a main embodi-
ment of the invention, the potassium fluosilicate in the
slurry is withdrawn by line 7 to hydrolyzer 8. The hydrolysis
- 12 -
! .: ', ' ,
,' ;, , ~;, , ' ' :
~llS4~;~
reaction in hydrolyzer 8 is conducted by heating at a
temperature in the range of 100-115C. or at the reflux
point of the system preferably by introduction of steam
at 9, to convert the potassium fluosilicate to silicon dioxide,
calcium fluoride, and potassium dihydrogen phosphate and/or phos-
phoric acid-using monocalcium phosphate. The resulting mixture
is passed by line 10 to separator 11 where calcium fluoride
and some silicon dioxide are recovered at line 12. In a
preferred embodiment, the mixture from separator 11 is
passed to separator lS by line 13 after addition of a suit-
able amount of gypsum by line 13. Thereafter, there is
recovered from separator 15 an 0-20-0 fertilizer by line
16 which contains most of the R2O3 components or slimes.
The gypsum is added primarily as substrate to provide a
filterable solid 0-20-0 (N-P-K) product, and to facilitate
the separation of slimes from the solution in separator 15.
KH2PO4/H3PO4 solution, which may contain some SiO2, is then
recycled by line 17. While the bulk of the R2O3 is removed
here, it can also be expected that portions will be removed
with other products.
In the meantime, the overflow or solution from de-
fluorinator or thickener 6 is recovered in line 18 as a
solution of monocalcium phosphate in phosphoric acid. This
product may be processed by any of several alternative
embodiments to recover valuable products, including mono-
calcium phosphate, phosphoric acid including recycle H3PO4,
and gypsum, all of which are substantially free of fluoride
contamination.
- 13 -
4~33
As a result of this process, there is recovered
from the defluorinator 6 by line 18 the product from the
reaction of this invention. This reaction product comprises
a solution of monocalcium phosphate in phosphoric acid,
which is a valuable reaction product of high quality sub-
stantially free of fluoride contamination. This product
solution may be treated by various alternative processing
techniques to recover monocalcium phosphate and/or phosphoric
acid, which products may also be converted to other valuable
products including KH2PO4and recycle phosphoric acid. Pre-
ferred further processing techniques are shown in Figure 2.
In the embodiment of Figure 2, the monocalcium phos-
phate/phosphoric acid solution product from line 18 is passed
to intermediate storage 19 where the stream may be divided
into two portions for further processing. The division of
the ~CP/H3PO4 stream at this point may be in a desired ratio,
~ e.g., about 40 to 60 wt. ~ of the stream may be removed, and
; ~ processed to recover XH2PO4/H3PO4. In this aspect, a portion
of the stream is withdrawn by line 20 and passed to reactor
21. In reactor 21, the stream is reacted with a potassium
` sulfate reactant such as potassium sulfate, potassium hydro-
gen sulfate or a mixture thereof, added by line 22. The
potassium sulfate reactant may be added as a solid or aque-
ous solution and is added in sufficient stoichiometric
amounts to react with all the monocalcium phosphate present.
As necessary, for solution purposes, water may be added by
line 23. This reaction is conducted at a temperature of
about 50 to 100C. with agitation.
- 14 -
11~54?;33
In reactor 21, the monocalcium phosphate and
potassium sulfate react to produce potassium dihydrogen
phosphate as product together with gypsum and phosphoric
acid as illustrated by the following equation when the re-
actant is potassium sulfate:
4H20
2 4 2 (YH3PO4)
where Y is the amount of phosphoric acid in the system.
The resulting reaction slurry is then transferred
by line 24 to separator or filter 25 and a solution of
KH2PO4 in phosphoric acid is removed by line 26 and the
gypsum is removed by line 27. The solid filter cake is
washed by water from line 23 and the wash water may be re-
cycled by line 29 to reactor 21.
The product recovered at line 26 contains potassium
dihydrogen phosphate and has a fertilizer value of 0-24-6.
The KH2PO4 may be recovered from this solution by evapora-
tion and precipitation with a water miscible solvent such
as methanol or extraction with a water immiscible solvent
such as butanol.
In the meantime, the other portion of the clarified
monocalcium phosphate/phasphoric acid solution from inter-
mediate storage l9 is passed by line 30 to crystalli~er 31
and reacted with at least a stoichiometric amount of-sulfuric
acid from line 32. The sulfuric acid reacts with the MCP/H3PO4
solution to produce phosphoric acid and calcium sulfate
hydrate and this slurry is passed by line 33 to thickener 34
wherein concentration of the slurry is achieved and the under-
flow slurry is then passed by line 35 to filter 36. The solid
- 15 -
ll.i 54~3
calcium sulfate hydrate in substantially pure form is re-
covered by line 37.
After removal of the calcium sulfate hydrate, the
phosphoric acid solution/filtrate is transferred by line 39
to evaporator 40 where water is removed from the system at
41 as required. The remaining phosphoric acid may then be
recovered as product by line 42 or may be combined with line
38 overflow from thickener 34 via dotted line 43 to meet the
recycle phosphorlc acid needs of line 2 in the phosphate rock
acidulation carried out in reactor 3.
In a further embodiment of the present invention
(not shown), the monocalcium phosphate/phosphoric acid solu-
tion may be processed to recover solid monocalcium phosphate
from the phosphoric acid and each product may then be re-
covered or further processed. In one aspect, the monocal-
cium phosphate/phosphoric acid clarified solution from de-
1uorinator 6 is passed to a crystallizer. Up to this point,
the monocalcium phosphate/phosphoric acid solution has been
maintained at a temperature in the range of 80-95C. to main-
tain the solution. However, in the crystallizer, the solutionis cooled via evaporation to about 25-55C., preferably about
40C., to cause crystallization of solid monocalcium phos-
phate from the phosphoric acid solution. Therefore, it is
preferred that the mixture be cooled by a temperature differ-
ence of about 35-55C. The resulting slurry is then passed
from the crystallizer to a separator where a separation is
effected between solid monocalcium phosphate and the mother
liquor MCP/H3PO4. The solid monocalcium phosphate from the
separator is then passed,for example to reactor 21,wherein
- 16 -
~.1154~3
reaction is carried out with a potassium sulfate reactant
such as potassium sulfate, potassium hydrogen sulfate, or
a mixture thereof as described above for the MCP/H3P04 solu-
tion. In this reactor 21, the monocalcium phosphate and
K2S04and/or KHS04 reactant produce potassium dihydrogen
phosphate and/or phosphoric acid as a product together with
gypsum~ The resulting mixture is then filtered and the gyp-
sum removed by line 27. The product recovered at line 26
is an aqueous solution of potassium dihydro~en phosphate
and/or phosphoric acid. This solution may be further pro-
cessed into desired products.
In this reaction, the monocalcium phosphate reacts
with the potassium sulfate or potassium hydrogen sulfate as
-~ illustrated by the following equations:
2 4)2 8K2S4~ 16~12P04 + 8CaS04 2H2o
b) 8ca(H2P4)2 + 8KHS04 ~ 8KH2P04 + 8H3 4 4 2
.-
In reaction (a) with K2S04, the KH2P04 product is
a liquid 0-15-10 fertilizer which may be further concentrated,
and in reaction (b) with KHS04, the KH2P04/H3P04 product is
~ .
`~ 20 a liquid 0-24-8 fertilizer.
~ In the meantime, the MCP/phosphoric acid from the
;~ separator is passed to the calcium sulfate hydrate crystal-
~, lizer and reacted with sulfuric acid to produce phosphoric
acid product and/or recycle mother liquor and calcium sul-
~i 25 fate hydrate as described above for the process of Figure 2.
-` This reaction for recycle is illustrated by the following
equation:
- 17 -
4~3
~ 2 4)2 9~l3Po4~l9~l2so4+38l~2) > 128H3PO4+l9caso4 21120
The "128 H3PO4" portion represents the phosphoric acid
available for recycle.
It will therefore be understood that this approach
also leads to valuable fertilizer products and recycle
phosphoric acid.
The following example is presented to illustrate the
invention but it is not considered to be limited thereto.
:~n this example and throughout the specification, parts a
by weight unless otherwise indicated.
EXAMPL~ I
In this example, l,278 grams (= 9 moles) P2O5 in
phosphate rock are reacted with 10,224 grams (= 72 moles)
P2O5 as 35~ recycle phosphoric acid for a P2O5 (acid)
/P2O5 (rock) weight ratio of 8/l. This reaction mixture
provides enough excess phosphoric acid to dissolve essentially
all of the calcium i~n~the phosphate rock as monocalcium
phosphate whereln the P2O5/CaO weight ratio should approach
6.75/l. ~The acidulation reactio~ ls conducted at 80-90C. and
contains a minimum of l mole o~ K2O and sufficient external
reactive silica (SiO2) to remove substantially all of the
fluoride as insoluble potassium Eluosilicate. Sand, some
R2O3 slimes and unxeacted phosphate roc]c also remain insoluble.
Small amounts (up to 3-4 ppm) of a flocculatin~ agent such as
Nalcalite 670 are helpful in the settling the solids from this
sys teM.
- 18 -
. .
, ,., . . . . ~ : -
ll~S4F~3
This thin reaction slurry, still at 90C., is
then separated via a decanter/thickener (separatory funnel
may be used in the laboratory) wherein approximately 10% of
the MCP/H3PO4 solution remains with the underflow insolubles.
The now thickened slurry, is directed into the hydrolysis
sector wherein the temperature is raised to 110-115C., e.g.,
by use of low pressure steam. Under these conditions, the
hydrolysis reaction is essentially completed in 1 to 2 hours.
The slurry now contains dense crystalline Fluorspar (CaF2)
; 10 which is readily separated from the unreactive but somewhat
flocculated R2O3/P2O5 components such as by a hydraclone or
by suitable gravity separation means. Sufficient clean
gypsum is then added to the remaining finely dispersed
R2O3/P2O5 to achieve a 0-20-0 grade fertilizer which simu-
lates NSP. This requires approximately 3.64 grams of CaSO4
per gram of P2O5 slimes to be recovered. The R2O3/P2O5
component has alread~ been flocculated/coalesced to a con-
~; siderable degree during the 110-115~C. hydrolysis step.
:
1 However, the utilization of clean gypsum provides additional
.
substrate so that separation of this material presents no
undue difficulties. The product is readily separated via
suitable means, e.g., a centrifuge or a precoat filter.
After separation of the solids, the remaining solu-
tion of 2KH2PO4 + 14 H3PO4, which also contains a small amount
of silicon dioxide, is recycled to the acidulation reactor
as regenerated phosphoric acid containing potassium ion.
-- 19 --
EXAMPLE II
The clarified monocalcium phosphate/phosphoric acid
overflow from the K2SiF6 thickener is thus passed to a crystal-
lizer wherein the temperature is lowered to 40C. to crystal-
lize monocalcium phosphate. The solid monocalcium phosphate
and the remaining MCP/H3PO4 solutions are then separated via
a filter, centrifuge or other separator. The solid monocalcium
phosphate is removed and reacted with a stoichiometric
amount of potassium hydrogen sulfate in an aqueous medium at
~ a temperature of 90C. In this reaction, the monocalcium
; 10 l~hGsphate is converted to KH2PO4 + H3PO4 and gypsum- The
gypsum is removed and the IC~l2PO4 + H3PO4 liquor separated and
recovered as a 0-24-8 fertilize solution.
The phosphoric acid solution which still contains mono-
calcium phosphate from the separator is reacted with sulfuric
acid in stoichiometric amounts at85C. to produce calcium sulfate
hydrate which crystallizes from solution. This solid is then filtered
and removed from the system. The resulting phosphoric acid
; is then recycled to the acidulation reactor.
EXAMPLE III
In an alternative reaction, the solid monocalcium phQsphate
is reacted with potassium sulfate to yield primarily KH2PO4 with
little or no H3PO4 coproduct. Conversely, if a portion of the
(uncrystallized) MCP/H3PO4 liquor is reacted with potassium
sulfate the resulting KH2PO4/H3PO4 solution will have a plant
food value of 0-24-6. A portion of any of the K2O products may b~
recycled back to the acidulation vessel to provide makeup for the
K2O lost in the hydrolysis sector.
- 20 -
