Hemihydrate type phosphoric acid process with crystal modifier

Phosphate rock and sulfuric acid are reacted to produce phosphoric acid and calcium sulfate by means of the hemihydrate wet process. In this improved process, the calcium sulfate hemihydrate is crystallized from solution in the presence of an organic sulfonic acid or a derivative thereof. The organic sulfonic acid or its derivative improves the growth of the calcium sulfate hemihydrate crystals and thus improves the filtration rate of the slurry produced in this improved process.

SUMMARY 
The present invention is directed to an improved process for the 
manufacture of phosphoric acid by the wet process. The hemihydrate or as 
it is sometimes called the semihydrate process is employed to produce wet 
process phosphoric acid from phosphate rock and sulfuric acid, the 
improvement being the presence of organic sulfonate materials during the 
crystallization of the calcium sulfate hemihydrate from solution. 
Phosphate rock and sulfuric acid are reacted within the slurry comprising 
sulfuric acid, phosphoric acid, monocalcium phosphate and calcium sulfate 
hemihydrate, therebeing present an organic sulfonate in said slurry which 
acts to increase the crystal growth of the calcium sulfate hemihydrate 
therepresent. 
BACKGROUND 
The present invention is directed to an improved process for the production 
of phosphoric acid by the wet process. The invention is directed to the 
production of phosphoric acid by the calcium sulfate hemihydrate or simply 
the hemihydrate process. The present invention is directed to a process in 
which organic sulfonate reagents are used to increase the crystal size of 
the calcium sulfate hemihydrate crystals formed during the production of 
phosphoric acid. 
The hemihydrate wet process for the production of phosphoric acid is one of 
the processes used in the industry today. Other processes used are the 
gypsum, or the dihydrate process and the anhydrite process. All three 
prcesses are named from the by-product calcium sulfate produced during the 
production of phosphoric acid. The gypsum or dihydrate process is run at a 
temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration 
of about 30% in the liquid portion of the slurry. Increasing the 
temperature to about 90.degree. to 120.degree. C. and the P.sub.2 O.sub.5 
concentration from about 30 to 45% in the liquid phase will yield calcium 
sulfate hemihydrate. If, however, one chooses to run the phosphoric acid 
wet process at 130.degree. C. and a P.sub.2 O.sub.5 concentration greater 
than 30% than one obtains calcium sulfate anhydrite as the by-product. 
Advantages and disadvantages of each process are described in "Phosphoric 
Acid" Part One, edited by A. V. Slack, Marcel Dekker, Inc., New York, 
N.Y., 1968. 
Phosphate rock and sulfuric acid are reacted with a slurry comprising 
phosphoric acid, sulfuric acid, monocalcium phosphate, and calcium sulfate 
hemihydrate. The temperatures and P.sub.2 O.sub.5 concentrations are such 
that the main product from the reaction of the phosphate rock and the 
sulfuric acid will be calcium sulfate hemihydrate and phosphoric acid. 
Little, if any, calcium sulfate dihydrate will be observed in said 
reaction. The slurry so produced is then sent to a recovery section where 
the solids are separated from the liquid. This is usually done by means of 
a filter. The rate of filtration or the filterability of the slurry will 
be dependent among other things upon the size of the calcium sulfate 
hemihydrate crystals produced during the reaction. 
The literature makes reference to the use of organic sulfonic acids and 
derivatives thereof as crystal growth modifiers to be used in wet process 
phosphoric acid processes, as agglomerating agents for use in wet process 
phosphoric acid slurries and as a reagent to moderate the conversion of 
calcium sulfate hemihydrate to calcium sulfate dihydrate or the reverse, 
namely the conversion of calcium sulfate dihydrate to calcium sulfate 
hemihydrate. 
Slack (reference cited page 279) describes the use of a surface active 
agent in the dihydrate process, said surface active agent is used to 
promote the growth of small crystals. It appears as if the surface active 
agent in the dihydrate wet process acts as a nucleation poison to gypsum 
and reduces the number of gypsum nuclei formed, thereby favoring growth of 
large crystals and making the operation more difficult to upset by changes 
in conditions. The formation of larger crystals makes it easier to filter 
the slurry so formed. D. W. Leyshon et al, U.S. Pat. No. 3,192,014 
describes a process for the preparation of phosphoric acid by means of the 
dihydrate wet process in which an additive selected from the group 
consisting of alkylbenzenesulfonic acids having an alkyl group from 9 to 
12 carbon atoms, isopropylnapthalene sulfonic acid and the alkali metal 
salts of said acids are used to improve the filterability of the slurry 
produced. The additive is used from about 0.1 to about 3.2 pounds per ton 
of phosphate rock. 
Slack (reference cited, page 383) describes the use of surface active 
agents in the hemihydrate wet process. Tests have shown that the formation 
of small unstable hemihydrate crystals as observed in previous processes, 
does not occur when a surface active agent or surfactant is added to the 
system. The surface active agent used was an alkylbenzenesulfonic acid. 
A. F. Sirianni et al, U.S. Pat. No. 3,796,790 describes a process for the 
recovery of concentrated phosphoric acid from a suspension of gelatinous 
or finely divided precipitate such as the calcium sulfate in the process 
for producing phosphoric acid. The process involves treating the 
suspension with a particular surface active agent and a particular 
bridging liquid, said bridging liquid is a liquid hydrocarbon such as 
naphtha, kerosene, fuel oil, low viscosity processed oils, gas oils and 
petroleum aliphatic solvents. Agglomerates are formed when the suspension 
is vigorously mixed with the surface active agent and the liquid 
hydrocarbon. Said agglomerates are then removed by decanting, filtering, 
or centrifuging. 
Several patents describe the recrystallization of calcium sulfate from one 
hydrate form to another. The patent issued to H. Akazawa et al, U.S. Pat. 
No. 3,645,677 is representative. Calcium sulfate hemihydrate is hydrated 
to calcium sulfate dihydrate in the presence of at least one surface 
active agent, selected from the group consisting of alkylaryl sulfonic 
acids, alkylnapthalene sulfonic acid, sulfuric acid esters of higher 
alcohols and salts thereof, and a mixed acid comprised of sulfuric acid 
and phosphoric acid from the ratio of about 0.4 or greater. Large, easily 
filterable purified calcium sulfate dihydrate crystals are prepared. 
Other types of additives have been employed which aid in the growth of 
calcium sulfate crystals. Hey et al, U.S. Pat. No. 3,653,727, uses a 
mixture of an amide and a fatty acid as an anti-foam agent. The agent also 
improves the rate of calcium sulfate filtration. 
DETAILED DESCRIPTION 
This invention is directed to an improved process for the production of 
phosphoric acid by the calcium sulfate hemihydrate process. 
Phosphate rock, either calcined or uncalcined, and phosphoric acid are 
added to a first slurry of calcium sulfate hemihydrate, monocalcium 
phosphate, phosphoric acid and sulfuric acid. Preferably, the phosphate 
rock is slurried in the phosphoric acid prior to the addition to the first 
slurry. Phosphate rock, about 95% of +100 mesh, containing at least 32% 
P.sub.2 O.sub.5 is the preferred source of phosphate for the process. 
However, phosphate rock of 95% of -200 mesh can be used. Rock containing 
less than 32% P.sub.2 O.sub.5 is acceptable, and can be employed in this 
process. High alumina phosphate pebble may also be used. The phosphate 
rock is slurried in phosphoric acid that contains from about 13% to about 
47% P.sub.2 O.sub.5. Phosphoric acid, recycled from the separation 
section, containing from about 13% to about 47% P.sub.2 O.sub.5 is usually 
used in the process. However, phosphoric acid from other sources, such as 
other phosphoric acid plants or merchant grade acid may be used. When the 
phosphoric acid is recycled from the separation section it will usually 
contain from about 0.5 to about 3.5% free sulfuric acid. 
The temperature of the phosphate rock-phosphoric acid mixture is maintained 
at about 50.degree. C. to about 100.degree. C., preferably from about 
90.degree. C. to about 100.degree. C. The resulting mixture is from about 
30% to about 40% solids by weight, about 33% being preferred. A defoamer 
is added if and when required. 
The defoamer may be selected from the group consisting of tall oil fatty 
acids, oleic acid, sulfated tall oil fatty acids, sulfated oleic acid, 
silicones and mixtures of a monocarboxylic acid (12-22 carbon atoms) and 
monoalkanoylamide derivatives of the monocarboxylic acid. The preferred 
defoamer is produced and sold by AZ Products Co. of Eaton Park, Fla. and 
referred to as AZ 10A. The amount of the defoamer used is from about 0.05% 
to about 1.5% by weight based on the weight of the slurry transferred to 
the separation section. 
The phosphate rock-phosphoric acid mixture is added to a first slurry of, 
calcium sulfate hemihydrate, phosphoric acid, monocalcium phosphate and 
sulfuric acid in a first reaction vessel. The phosphate rock and 
phosphoric acid may be added separately to the first slurry in the first 
reaction vessel. The phosphate rock-phosphoric acid mixture on being added 
to the first slurry in the first reaction vessel is immediately dispersed 
within the first slurry. A first portion of the second slurry from the 
second reaction vessel is added to the first slurry in the first reaction 
vessel. The second slurry which contains an excess of sulfuric acid is 
also immediately dispersed within the first slurry. A first portion of the 
first slurry is transferred to a second reaction vessel. 
The first reaction vessel is fitted with a draft tube and an agitator. (The 
agitator consists of a shaft fitted with a propeller to the draft tube 
that on activation of the agitator, a second portion of the first slurry 
is drawn from the bottom of the draft tube up through the draft tube and 
out the top of the draft tube. On exiting the draft tube, said slurry 
passes in a downward direction in the space between the draft tube and the 
wall of the first reaction vessel. The direction of circulation through 
the draft may be reversed and is not critical. Circulation is thus 
established within the first reaction vessel. The rate at which said 
slurry is circulated is at least equal to about 50% of the volume of the 
slurry in the first reaction vessel per minute, preferably from about 50% 
to about 150% and most preferably about 100%. This circulation thoroughly 
disperses the phosphate rock-phosphoric acid mixture within the first 
slurry. The first slurry contains sulfuric acid which reacts 
exothermically with the phosphate rock being added. Dilution of the 
sulfuric acid also results in the evolution of heat. These exothermic 
reactions supply the heat required to maintain the temperature of the 
slurry in the first reaction vessel between about 66.degree. C. to about 
113.degree. C. The soluble sulfate content of the first slurry is 
maintained at about +0.7% to about -4%. As measured, soluble sulfate 
values can be either positive or negative. Soluble sulfate values include 
not only the sulfuric acid present in the liquid component of the slurry 
but also the soluble calcium sulfate therepresent. Negative soluble 
sulfate values indicate that an excess of calcium ions are present in the 
solution as is usually observed in the phosphate rock-phosphoric acid 
mixture. Positive soluble sulfate values indicate that excess sulfate ions 
are present. A value of 0.0% soluble indicates that the sulfate ions and 
the calcium ions are equivalent stoichiometrically within the limits of 
the analysis. The residence time of the solids in the first reaction 
vessel is from about 2.0 hours to about 5.0 hours, preferably from about 
2.5 hours to about 4.5 hours. 
A first portion of the first slurry is transferred through a first conduit 
into a second reaction vessel. The second reaction vessel which can be 
subjected to a vacuum, is fitted with a draft tube, an agitator and a 
sulfuric acid inlet. The agitator consists of a shaft fitted with a 
propeller at the bottom thereof. The shaft and agitator are so located 
with respect to the draft tube that on actuation of the agitator a second 
portion of the second slurry is caused to flow from the bottom of the 
draft tube up through the draft tube and out the top of the draft tube. On 
exiting the draft tube, said second portion of the second slurry flows in 
a downward direction in a space between the draft tube and the inside 
walls of the second reaction vessel. The direction of the circulation can 
be reversed and is not critical. The rate at which the slurry is 
circulated is at least equal to about 50% of the volume of the slurry in 
the vessel per minute, preferably from about 50% to about 150% of the 
volume and most preferably about 100% of the volume. Sulfuric acid, 
preferably about 98%, is added through the sulfuric acid inlet into the 
second slurry either as is or mixed with phosphoric acid. The first 
portion of the first slurry is also added into the second slurry. A 
crystal modifier, usually an organic sulfonic acid or a derivative 
thereof, can be added to the slurry in the second reaction vessel. The 
crystal modifier can also be added to the first reaction vessel. The 
crystal modifiers are selected from the group consisting of alkyl, aryl, 
alkylaryl, and alicyclic derivates of sulfonic and sulfuric acids in which 
the organic radical contains from about 12 to about 30 carbon atoms. The 
free acid, salts thereof and mixtures of the free acid and salts may be 
used. The salts can be of alkali metals, ammonium, or of organic amines 
which contain from 1 to about 12 carbon atoms. Polymeric sulfonates and 
sulfates can also be employed. Examples of crystal modifiers which can be 
employed in the present process are alkyl sulfonic acids containing from 
about 12 to about 30 carbon atoms, benzenesulfonic acid, 
alkylbenzenesulfonic acid in which the alkyl group contains from about 8 
to 20 carbon atoms, alkylcyclohexane sulfonic acid in which the alkyl 
group contains from about 8 to 20 carbon atoms, polymeric sulfonates and 
sulfates such as polystyrene sulfonate, and polyvinylsulfonate, said 
polymeric materials having a molecular weight of from about 500 to about 
1,000,000. The crystal modifier is added for the purpose of increasing the 
growth of the hemihydrate crystals formed in the system. The flow of the 
second slurry within the second reaction vessel thoroughly disperses the 
first portion of the first slurry, the sulfuric acid and the crystal 
modifier within the second slurry. (The location of the sulfuric acid 
inlet in the second reaction vessel is not critical. It may be at the top, 
the middle, the bottom or at intermediate points of the second reaction 
vessel. The sulfuric acid conduit attached to the sulfuric acid inlet may 
enter the second reaction vessel from the top, the bottom or points 
intermediate therein, the exact point of entrance into the vessel is not 
critical.) Phosphoric acid, if needed, can be added to the second slurry 
within the second reaction vessel. The surface of the second slurry in the 
second reaction vessel is exposed to a pressure of between about 2 to 
about 29 inches of mercury absolute, preferably from about 3 to about 20 
inches mercury absolute. Water and volatile components added to or 
produced in both the first and second slurries can be removed from the 
second slurry causing a reduction in the temperature of the second slurry 
from which the volatiles are removed. The cooled second slurry is 
thoroughly mixed so that temperature differentials are minimized within 
the total volume of the second slurry. With this evaporative cooling, the 
temperature of the second slurry is maintained between about 66.degree. C. 
to about 113.degree. C. preferably from 80.degree. C. to about 105.degree. 
C. [The process can be run while maintaining both the first and second 
reaction vessels at atmospheric pressure.] Sulfuric acid which is added to 
the second slurry in the second reaction vessel through the sulfuric acid 
inlet can be from about 89% to 99% H.sub.2 SO.sub.4 or more, preferably 
about 98% H.sub.2 SO.sub.4. 
It has been determined that the total sulfate value added to the system is 
the sum of the sulfate values in sulfuric acid added plus the sulfate 
values added in the rock and this total is only about 90% to 100% of the 
stoichiometric amount of sulfate needed to convert the calcium added in 
the rock fed to the first reaction vessel into calcium sulfate 
hemihydrate. See Table 1 for the compilation of sulfuric acid usuage. 
Listed are the tons per day (TPD) of phosphate rock fed, % CaO in the 
rock, % SO.sub.4.sup.-- in the rock, CaO fed (TPD), stoichiometric sulfate 
for the calcium in the rock (TPD), sulfate in sulfuric acid fed to the 
unit (TPD), sulfate equivalent in the rock (TPD), the total used (TPD), 
and total sulfate used as a fraction of the stoichiometric amount of 
sulfate required for the calcium in the rock. The soluble sulfate content 
as measured in the second slurry should be from about +0.7% to about 4.5%, 
preferably from about 2.5% to about 3.5%; provided that when the soluble 
sulfate content of the first slurry is about +0.7% then the soluble 
sulfate content of the second slurry must be +1.0% or more. The specific 
gravity of the slurry in the second reaction vessel is about 1.80 
.sup..+-. .2 grams per cc. The specific gravity of the liquid portion of 
the slurry is about 1.56 .sup..+-. 0.20 grams per cc. The liquid gravity 
corresponds to a phosphoric acid which contains about 42% .sup..+-. 12% 
P.sub.2 O.sub.5. Residence time of the phosphate values in the second 
reaction vessel is from about 0.6 hour to about 2.0 hours, 
TABLE 1 
__________________________________________________________________________ 
Total 
SO.sub.4 
used 
Stoichio- 
Sulfate as a 
metric 
Present frac- 
Sulfate 
in 100% 
Sulfate tion 
CaO SO.sub.4 
(SO.sub.4) 
H.sub.2 SO.sub.4 
Equival- 
Total 
Stoi- 
in in CaO for CaO 
Fed ent in 
Sulfate 
chio- 
Rock Fed, 
Rock, 
Rock 
Fed in Rock 
to Unit, 
Rock, 
Used, 
met 
TPD % % TPD TPD TPD TPD TPD Amount 
__________________________________________________________________________ 
1209.5 
44.90 
0.65 
543.07 
930.97 
844.4 
7.86 852.26 
0.915 
1383.1 
45.97 
0.65 
635.81 
1089.4 
1052.2 
8.99 1061.2 
0.974 
1381.6 
46.76 
0.65 
646.04 
1107.5 
1024.7 
8.98 1033.7 
0.933 
1172.2 
46.81 
0.65 
548.71 
940.64 
844.3 
7.62 851.9 
0.906 
1110.9 
46.89 
0.65 
520.90 
892.97 
804.6 
7.22 811.82 
0.909 
__________________________________________________________________________ 
preferably from about 0.7 hour to about 1.6 hours. 
The excellent mixing obtained with this system is achieved using 
approximately 1/10 of the horsepower required for a comparable wet process 
phosphoric acid plant such as a Dorr-Oliver or a Prayon Plant. 
A first portion of the second slurry flows from the second reaction vessel 
back to the first reaction vessel through a second conduit and is 
thoroughly dispersed within the first slurry. It is the flow of the second 
slurry to the first slurry which aids in maintaining the temperature of 
the first slurry and adds sulfate values (sulfuric acid) to the first 
slurry. Additional sulfate values are added to the first slurry in the 
first reaction vessel with the recycled phosphoric acid. Circulation 
between vessels and within vessel minimizes localized concentration of 
reactants of hot slurry and of cooled slurry thus resulting in a more 
easily controlled process than previously observed. A third portion of the 
second slurry is removed from the second reaction vessel and is 
transferred through a conduit to a reservoir. The third portion of the 
second slurry, on a weight basis, is approximately equal to the phosphate 
rock, the phosphoric acid, and the sulfuric acid added in the first and 
second reaction vessels respectively minus the volatiles (on a weight 
basis) removed from the second reaction vessel which (is) can be subject 
to a vacuum. The third portion of the second slurry is constantly stirred 
in the third vessel to prevent separation of the solids from the liquid 
and is maintained at about 66.degree. C. to about 113.degree. C., 
preferably from about 70.degree. C. to about 100.degree. C. The residence 
time in the third vessel is relatively short, being from about 0.5 hour to 
about 1.5 hours, preferably from about 0.60 hour to about 1.25 hours. The 
soluble sulfate concentration of the slurry in the third vessel may change 
somewhat due to continued reaction of the soluble sulfate values with any 
calcium values therepresent. Sulfuric acid may be added to the third 
vessel to adjust the sulfate values. 
From the third vessel, the slurry is transferred to the separation section 
in which the slurry is separated into solid and liquid components using 
apparatus well known in the art. 
Slurry samples are removed from the system at several locations. A sample 
port is placed in the first conduit at a location between the first and 
second vessels, the distance between the first and the second reaction 
vessel is not critical. Slurry removed from this sample port represents 
the first slurry. A sample port is located in the conduit between the 
second reaction vessel and the reservoir to which the third portion of the 
second slurry is pumped. The location of this sample port in terms of 
distance between the second reaction vessel and the reservoir is not 
critical. Slurry samples obtained from these two ports can be analyzed for 
soluble sulfate concentrations, specific gravities, and crystal size. The 
flow rates of the reactants and of the slurries are adjusted in accordance 
with the analytical values obtained in order to maintain the desired 
sulfate levels within the reaction system. It is to be understood that the 
system described can be run on a continuous basis, the reactants are 
continuously added and the third portion of the second slurry is 
continuously removed from the system prior to separation into phosphoric 
acid and calcium sulfate hemihydrate.

EXAMPLE 1 
The first and second reaction vessels and the accompanying connective means 
such as conduits, pumps, etc., are filled with a slurry consisting of, 
calcium sulfate hemihydrate, monocalcium phosphate, phosphoric acid and 
sulfuric acid. The weight percent of the solids in the slurry is about 
31%, the specific gravity of the slurry in the second reaction vessel is 
about 1.80 .sup..+-. 0.07 g/cc and the specific gravity of the liquid 
portion of the slurry is about 1.53 .sup..+-. 0.06 g/cc. The P.sub.2 
O.sub.5 concentration of the liquid portion of the slurry is about 42% by 
weight. The temperature of the slurry in the first reaction vessel is 
between about 88.degree.-102.degree. C. preferably between 92.degree. and 
105.degree. C., whereas the temperature in the second reaction vessel is 
between 88.degree. and 105.degree. C., preferably between 92.degree. C. 
and 105.degree. C. Soluble sulfate concentration in the first reaction 
vessel is from about +0.7 to about -4% and the soluble sulfate 
concentration in the second reaction vessel is from about 0.7% to about 
+4.5%. 
A mixture of phosphate rock (typical analysis shown in Table 2) of a size 
distribution shown in Table 3, and phosphoric acid is prepared by adding 
phosphate rock to phosphoric acid in the ratio of about 1647 pounds of 
phosphate rock (about 31.2 P.sub.2 O.sub.5 and 45.6 CaO) to about 3700 
pounds of phosphoric acid (about 32% P.sub.2 O.sub.5). The temperature of 
the mixture is about 90.degree. C. A defoaming agent is added as needed to 
reduce the foam caused by partial dissolution of the phosphate rock in 
phosphoric acid. The amount of defoamer varies from about 0.05% to about 
1.5% based on the amount of phosphate rock added. 
This phosphate rock-phosphoric acid mixture is added to the first slurry in 
the first reaction vessel at the rate of about 380 gpm (about 5350 pounds 
per minute). The incoming mixture is thoroughly mixed with the first 
slurry. Intra vessel mixing is accomplished by means of the draft tube and 
the agitator. The first slurry is pumped from the first reaction vessel to 
the second reaction vessel at the rate of about 1640 gallons per minute. 
The first slurry is thoroughly mixed with the second slurry and 98% 
sulfuric acid which is added to the second reaction vessel at about 87 
gpm. An organic sulfonic acid derivative, a crystal modifier, can be added 
to the second reaction vessel in amounts from about 1 ppm to about 1000 
ppm by weight based on the weight of the slurry transferred to the 
separation section where the solids are separated from the liquid portion 
of the slurry; 10 ppm being preferred. This material is added to promote 
the growth of the calcium sulfate hemihydrate crystals. The first slurry, 
the sulfuric acid and the crystal modifier are thoroughly dispersed into 
the second slurry in the second reaction vessel. The second slurry flows 
at the rate of about 1280 gallons per minute from the second reaction 
vessel into the first reaction vessel where it is thoroughly mixed with 
the first slurry. 
About 45 gpm of water and volatile materials (HF, SiF.sub.4, H.sub.2 S, 
CO.sub.2, etc.) is vaporized from the second slurry in the second reaction 
vessel. The second reaction vessel is maintained under a pressure of about 
15 inches of mercury absolute. Approximately 400 gpm of slurry is 
withdrawn from the second reaction vessel and flows to the separator feed 
tank. Thus about 445 gpm of material (vaporized material and the slurry to 
the separator feed tank) is removed from the system. The removed slurry is 
then passed to the separation section where the solid and liquid portions 
of the slurry are separated. 
At these rates, the plant will produce about 350 tons per day of P.sub.2 
O.sub.5 of 35-44% P.sub.2 O.sub.5 phosphoric acid. The recovery data is 
summarized below. 
______________________________________ 
TOTAL LOSS IN FILTER CAKE 
% of P.sub.2 O.sub.5 fed in rock 
______________________________________ 
Citrate insoluble (CI) 
0.76 
Citrate soluble (CS) 
4.64 
Water soluble (WS) 2.34 
Total loss 7.74 
Total Recovery 92.26 
______________________________________ 
A typical analysis of the phosphoric acid produced by this process is shown 
in Table 4. The total resident time, from entering the first reaction 
vessel to exiting separation feed tank is calculated at 7.9 hours. The 
volume of the first reaction vessel is about 120,000 gallons, the volume 
of the second reaction vessel is about 40,000 gallons to normal liquid 
level. 
EXAMPLES 2 to 7 
The following system as described hereinafter was set up in the pilot plant 
to duplicate plant operation in order to investigate the effect of 
defoamers and crystal modifiers on the filterability, and hence the 
crystal size, of the calcium sulfate hemihydrate produced. 
Into a first reaction vessel containing reaction slurry was added phosphate 
rock, recycled phosphoric acid and recycle reaction slurry from the second 
reaction vessel. Defoamer, when used, was added in the first reaction 
vessel. The reaction slurry so formed in the first reaction vessel was 
circulated to the second reaction vessel. Sulfuric acid and crystal 
modifiers were added to the second reaction vessel. Phosphoric acid could 
be added to the second reaction vessel if needed to control the viscosity 
of the reaction slurry. The second reaction vessel was maintained under 
vacuum so as to remove gaseous impurities and water from the slurry. The 
evaporative of water was utilized to cool the reaction slurry. 
The conditions employed in determining the utility of the crystal modifier 
and the defoamer were standardized and are shown below in Table 5. Results 
of the tests are shown in Table 6. 
Phosphate rock is present in the first and in the second slurries in the 
first and second reaction vessels respectively. The amount present is 
quite small and will vary considerably. The value for the "Citrate 
Insoluble" loss of the filter cake is a rough measure of undissolved and 
unreacted phosphate rock. 
Table 2 
______________________________________ 
Typical Phosphate Rock Analysis 
Compound % By Weight 
______________________________________ 
P.sub.2 O.sub.5 31.2 
CaO 45.6 
Fe.sub.2 O.sub.3 1.4 
Al.sub.2 O.sub.3 1.2 
MgO 0.4 
SiO.sub.2 8.7 
F 3.7 
SO.sub.3 0.9 
CO.sub.2 3.6 
Organic 1.8 
H.sub.2 O 1.1 
Na.sub.2 O, K.sub.2 O 
0.4 
______________________________________ 
Table 3 
______________________________________ 
Typical Phosphate Rock Screen Analysis 
Mesh Cummulative Percent 
______________________________________ 
+14 0.4 
+24 2.6 
+28 9.3 
+35 26.6 
+48 64.1 
+65 86.4 
+100 97.7 
-100 2.3 
______________________________________ 
Table 4 
______________________________________ 
Typical Phosphoric Acid Analysis 
P.sub.2 O.sub.5 
37.95% 
SO.sub.4.sup.= 
1.72 
CaO 1.04 
F.sup.- 1.27 
MgO 0.46 
Fe.sub.2 O.sub.3 
0.97 
Al.sub.2 O.sub.3 
0.91 
______________________________________ 
Table 5 
______________________________________ 
GENERAL REACTION CONDITIONS 
(Pilot Plant) 
______________________________________ 
Slurry Density 1.72 g/cc 
Sulfate Concentration 
First Reaction Vessel -2% 
Second Reaction Vessel 
+2% 
Phosphate Rock Feed Rate 
174 g/min. 
Slurry Recycle From Second 
To First Reaction Vessel 
2200 g/min. 
Recycle Phosphoric Acid Feed 
Rate to First Reaction Vessel 
390 g/min. 
Sulfuric acid (93%) Feed Rate 
To Second Reaction Vessel 
150 g/min. 
Defoamer Feed Rate to 
First Reaction Vessel 0.7 - 1.1 g/min. 
Crystal Modifier Feed Rate 
To Second Reaction Vessel 
(1% Soln of CM in Water) 
0.1 - 0.4 g/min. 
Temperature of Slurry 
In Both Reaction Vessels 
195-205.degree. F 
______________________________________ 
Table 6 
__________________________________________________________________________ 
Calcium Sulfate 
Hemihydrate 
Defoamer Crystal Modifier 
Filter Rate 
Crystal Size 
Example 
Type Amount 
Type Amount 
TON P.sub.2 O.sub.5 /ft.sup.2 -Day 
(microns) 
__________________________________________________________________________ 
2 None None None None 0.30 to 0.35 
10-15 
3 None None Actrasol W-40 
10ppm 
0.70 to 0.90 
30-45 
4 None None Actrasol W-40 
100ppm 
0.45 to 0.55 
20-30 
5 AZ10A 
0.46% 
None None 0.40 to 0.50 
20-30 
6 AZ10A 
0.46% 
Actrasol W-40 
10ppm 
0.80 to 1.10 
60-90 
7 AZ10A 
0.46% 
Conoco 10ppm 
0.60 to 0.70 
25-35 
__________________________________________________________________________ 
Actrasol W-40 - primarily sodium dodecylsulfonate, Arthur C. Trask Corp., 
Summit. Ill. 60501 
AZ10A AZ Products Co., P.O. Box 67, Eaton Park, Florida 33840 
Conoco c-560 - primarily sodium dodecylbenzene sulfonate, Continental Oil 
Co., Ponca City, Oklahoma