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Dissolution of Phosphate Rock by Mixtures of Sulfuric and Phosphoric Acid.

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Dev.Chem. Eng. Mineral Process., 6(5),pp.273-293, 1998.
Dissolution of Phosphate Rock by Mixtures of
Sulfuric and Phosphoric Acid
M. Jamialahmadi and S.H. Emam
University of Petroleum Industry, National Iranian Oil Co.
Ahwaz, IRAN
and H. Miiller-Steinhagen*
Dept. of Chemical & Process Engineering, University of Surrey,
Guildford, GU2 5XH, UK
The dihydrate process for the production of phosphoric acid is increasingly important
as the basis for phosphate fertilizer plants. The process involves dissolution of
phosphate rock in an aqueous mixture of phosphoric and sulfuric acid and subsequent
crystallization of calcium sulfate dihydrate from the product phosphoric acid solution.
In this study the mechanisms of dissolution of phosphate rock are studied using reagent
grade and planr acih. The eflects of various operating parameters such as
temperature, particle size and excess sulfuric acid concentration on the rate of
dissolution of rock are investigated. The results show that the dissolution of the rock
occurs in two stages of different rates. Initially, the dissolution process is fasr and is
controlled by chemical reaction between rock and acid. In the second slage, when the
particles are coated by calcium sulfate dihydrate the dissolution process is slow and
controlled by mass transfer of acid through the coating layer.
Introduction
Inorganic fertilizers are chemical compounds added to the soil to supply elements
required for plant nutrition. About 60 elements are found in the structure of the
various plants; air and soil are the sources of the plant nutrients. By the mechanism of
photosynthesis, plant takes up carbon, its principal element from air in the form of
carbon dioxide, whereas it derives from the soil water and various minerals. The
three elements of phosphorus, nitrogen and potassium which stimulate the processes of
metabolism in the plant cells, growth and its fruit are needed by the plant in relatively
large quantities.
Intensive farming of the land results in depletion of the soil from these three
elements which are soluble in water and in acids contained in the soil. This leads to
small crops and low quality of plant products. Therefore, the reduction of the content
of these elements in the soil must be continually compensated by the addition of
fertilizers. The primary function of the fertilizer industry all over the world is to
supply these nutrients to the farmers. Production of phosphoric acid by a wet process
occupies a central position in the fertilizer industries. Phosphoric acid can easily be
*Authorfor correspondence (email: hms@surrq.ac. uk).
2 73
M.Jamialahmadi, S.H. Emam and H.Miiller-Steinhugen
combined with ammonia to form ammonhrn phosphate fertilizer.
There is no lack of phosphate rock resources, and the ore will not run short in the
near future. However the fertilizer industry will have to adapt its process units to
accept phosphate rock of lower quality as the high grade phosphate rock becomes
depleted and its price increases steadily. It is prudent for a country like Iran, with
large deposits of low grade phosphate rock, to undertake studies that may assist in the
exploitation of its own natural resources.
The Prayon dihydrate process is one of the most commercially viable processes for
the production of phosphoric acid from phosphate rock. During the production, huge
amounts of calcium sulfate dihydrate are precipitated as a byproduct. The operating
conditions for the dissolution of the phosphate rock with sulfuric acid are chosen such
that large and uniform calcium sulfate dihydrate crystals with high filtration rate are
formed. The Razi Petrochemical Complex in Iran uses this process for the production
of phosphoric acid. At present, the phosphate rocks are imported from different
countries including Togo and Senegal. It is also planned to utilize the local phosphate
rocks in the near future. Due to the fact that the quality and composition of different
phosphate rocks vary significantly, the operational conditions of the plant need to be
adjusted according to the type of the rock.
The aim of the present investigation is to investigate the influence of phosphate
rock particle sue, excess sulfate concentration and temperature on the extent of
extraction of P20, from various phosphate rocks and on the quality of crystallization of
calcium sulfate dihydrate from the product phosphoric acid under Prayon process
conaicions.
Figure 1. Schematic of test apparatus.
2 74
Dissolution of phosphate rock by mixtures of sulfuric and phosphoric acid
Experimental Equipment and Procedure
The apparatus which is used to study the dissolution of phosphate rock in mixtures of
phosphoric and sulfuric acids is shown in Figure 1. The reactor, which is the main
part of the apparatus, is made from stainless steel and has a volume of about 10 liters.
It is baffled and equipped with a six flat blade turbine mixer. The mixer is connected
to a DC motor which allows the speed of the mixer to be varied between 50 to 500
rpm. A controlled band heater covers the reactor allowing constant slurry temperature
between 65 to 80°C. The reactor can be operated continuously or batchwise. Sulfuric
acid and phosphoric acid are stored in two different tanks which are also equipped with
temperature controlled band heaters. A small hopper with a vibrating feeder is used
for ground phosphate rock feeding. The hopper is loaded continuously with preweighted rock to ensure constant flow of phosphate rock to the reactor.
Unless stated otherwise, Iranian phosphate rock was used in the described
investigation. However other standard commercial rocks like those from Togo and
Senegal are also included in the study for comparison. The chemical analyses of the
Iranian and other rocks are given in Table 1.
wt%
Iran
Togo
Senepal
pz05 F
39.5
36.7
36.4
2.9
3.7
3.8
CO,
SO,
SiO,
CaO
A1203
Fez03
MgO
NazO
0.9
1.8
1.6
0.14
1.1
5
0.12
0.3
4.5
1.3
0.9
1
_____
0.3
0.063
1.1
1
0.15
-----
50.3
50
50.5
0.1
0.2
Table 1. Chemical analysis of various phosphate rocks.
The rocks were ground and classified into different sizes using standard laboratory
sieves. Particle size covered the range from 30 to 400pm. Acid concentration ranged
from 29-32 wt% P20, and 1 5 5 . 0 wt% excess sulfuric acid, obtained by dilution of
concentrated acids. Both reagent and plant grade acids were used.
The stoichiometric amount of consumed sulfuric acid is calculated in accordance
with the raw material composition using the following overall reaction equation:
[3Ca,(PO&CaF,
+ lOH,SO, + 2OH,O -+
6H3P0,
i
10CaS0,.2H20 i2HF
(1)
10 moles of sulfuric acid (980 g) are required per 3 moles of P,05 (426 g), hence 2.3
parts by weight of sulfuric acid. The amount of acid required for decomposition of the
impurities present in the rock is small and can be ignored. In phosphoric acid plants,
sulfuric acid is used 1.5-4wt% in excess of the stoichiometric value. In each
experiment samples are removed from the reactor at predetermined time intervals,
cooled and diluted with a known volume of de-ionized water. The solution is filtered
and the cake washed with de-ionized water for several times. The concentration of
P20, in the filtrate is determined using standard analytical procedures. The samples
were also analyzed by the analytical laboratory of the Razi Petrochemical Complex for
cross-checking.
To obtain information on the quality of the calcium sulfate dihydrate crystals, the
experiments were performed long enough to achieve steady-state, at which time a
sample of the slurry was taken. The slurry was filtered, washed with de-ionized water
and then the cake was washed several times with acetone. The samples were dried
275
M. Jamialahmudi, S.H.Emam and H.Miiller-Steinhagen
overnight in an oven at 65°C. After cooling in a desiccator, the funnel was weighed
and a small sample was taken for scanning electronic microscopy and petrographic
examination. To identify the crystalline phases present in the cake, the sample was
immersed in a liquid having a specific gravity of 2.5 (a mixture of carbon
tetrachloride-tetrabromethane). Thls liquid differentiates between calcium sulfate
dihydrate and hemihydrate because the specific gravity of dihydrate is lower, and of
hemihydrate is higher, than 2.5.
Data Reduction
The variation in the phosphate rock particle size as a result of continued grinding is
generally presented by a size frequency curve. A typical particle size frequency curve
of the rock used in the Razi Petrochemical Complex and in t h ~ sstudy is shown in
Figure 2, where the maximum depends on the structure and the source of the rock.
0.3 t
Particle size (+m)
Figure 2. Size distribution of phosphate rock.
In particulate systems, the distribution of particles is generally presented by their
moments [ 11:
where pj represents the distribution moments. With the interpretation of third
moment, and the assumption that perfect mixing conditions exist in the dissolution
tank, it is evident that:
2 76
Dissolution of phosphate rock by mixtures of sulfuric and phosphoric acid
c
R
P3
= -.
(3)
P3(,
Cflil
where CBOand ~ 3 are
0
the initial rock Pz05 concentration and its corresponding third
moment respectively. Therefore, measuring the P205concentration of the rock as a
function of time, the h r d moment distribution of the particles can be obtained from
Equation (3). To determine the particle size distribution as a function of time the
calculated third moments (p3) can be related to the initial population density, n,,(L,,,O),
and particle size distribution as follow [2]:
LLc = L,, - L
500
40
Y
400
U
2
30
A
Q
.-C
Q
N
0
300 5
0
$
9)
a”
c
0
U
20
200
c
C
U
Q
C
I
al
K
al
CL
T
2
0
.c
100 2
10
v
0
a
0
0
0.1
0.2
0.3
0.4
0.5
0
0.6
l i m e (hr)
Figure 3. Variation of P,O, concentration of the rock particles and their size with time.
277
M. Jamialahmadi, S.H. Emam and H.Miiller-Steinhugen
The initial population, n,-&,O),
can be determined from the screen analysis by:
where o iis the weight fraction of rock particles found in the screen increment AZi;
is a shape factor and assumed to be equal to one. Using Equations (3) and (4) and the
initial grain size distribution of the phosphate rock, the value of AL and consequently
the particle size distribution can be determined as a function of time during the
dissolution process. Typical measurements of the variation of P20, concentration of
the rock and its corresponding calculated AL are shown in Figure 3 for one set of
operating conditions. For comparison, some of the measured AL values are also
included in this figure. Good agreement can be seen between the predicted and
measured values. However, since the dissolution process is very fast and accompanied
by deposition of calcium sulfate, measuring particle size distribution during the
dissolution process is difficult and time consuming.
Process Description
The flowsheet of the production of phosphoric acid by a typical Prayon dihydrate
process is shown in Figure 4.
Sea water
Demineralized water
93% H S 0 4
<
Barometric
condenser
a
*
0
3
2
7'
P
B
X
Phorphata
rock
Fume scrubbs
v
I
-1
SI urry t a n k
Figure 4 Simpl@ed dihydrate process flow sheet
Phosphate rock is ground to 60% minus 150 mesh (76 pm) and fed to a chute
where a recycle stream of weak phosphoric acid washes it into the dissolution tank.
278
Dissolution of phosphate rock by mixtures of sulfirric and phosphoric acid
Strong sulfuric acid is metered with automatic control which keeps the acid and rock
feed ratio at the desired setting. A single reactor can be designed by proper baffling
and residence time capacitance to permit 96-98% conversion in 6-9 hours. In another
design such as the Razi Petrochemical complex, 6 to 9 continuous mixing tanks can be
used with better efficiency of extraction by minimizing back mixing. The heat of
reaction is removed by the flow of cooling air across the mixing tanks. The calcium
sulfate dhydrate-phosphoric acid slurry passes to a traveling pan vacuum filter where
29-32% acid is removed and the cake washed with water. The resulting filtrate is
returned to the reactor. The calcium sulfate dihydrate is finally disposed into the
Persian Gulf.
Process Chemistry
The basic chemical reactions for the production of the phosphoric acid by the dihydrate
process can be subdivided into two parallel and simultaneous reactions.
1. Dissolution of Phosphate Rock
First the non-soluble tri-calcium phosphate of the rock is attacked by phosphoric acid
to form the water soluble mono-calcium phosphate:
[3Ca3(POJ,].CaF2+ 14H$o4
*
10Ca(H2P04)2+ 2HF@
(7)
2. Precipitation-C~stallization
In the second stage, the monocalcium phosphate is reacted with excess sulfuric acid to
form calcium sulfate dihydrate and more phosphoric acid:
10Ca(HzP04), + 10H2S04+ 20H20
-+
20H3P04+ 1OCaSO4.2H2O
(8)
Crystallization of calcium sulfate from the phosphoric acid solution is the most
important operation in the phosphoric acid process.
Calcium sulfate can exist in three crystalline states, anhydrate (AH), hemihydrate
(HH) and dihydrate (DH). The state in which calcium sulfate precipitates depends on
the operating conditions of the process as can be seen from the equilibrium phase
diagram in Figure 5. It is the differences in operating temperature and phosphoric acid
concentration which differentiates the dihydrate process from other processes. Taking
rock quality into consideration, the phase diagram shows that in the dihydrate
phosphoric acid plant, 29-32% P,O, acid is produced at temperatures below 80°C.
Results and Discussions
Dissolution of phosphate rock in phosphoric-sulfuric acid mixtures is a typical
heterogeneous non-catalytic process in a multiphase gas-liquid-solid system. The
dissolution rate is fast and strongly dependent on the operational parameters such as
particle size, acid concentration, temperature and intensity of agitation and has been
the subject of many investigations. Previous works on this subject are summarized in
the books by Slack [4] and Becker [ 5 ] .
279
M.Jamialahmadi, S.H. Emam and H. Miiller-Steinhagen
120
100
0
-
2
80
3
c
F
Q)
E
60
al
i-
40
20
0
0
20
10
30
40
50
1
3
P 2 0 , concentration %
Figure 5. Phase diagram of CaSO, hydrates in phosphoric acid (31.
Typical results for the extent of dissolution of phosphate rocks from different
sources versus time are shown in Figure 6. It is interesting that more than 90% of the
phosphate rock is decomposed in less than 0.2 hr. The results illustrate that the
reaction between phosphoric-sulfuric acid mixture and phosphate rock is characterized
by two regimes with different rates. The first regime corresponds to the diffusion of
H+ ions which are formed from the dissociation of acids towards the particles,
followed by a fast chemical reaction with the phosphate rock forming more phosphoric
acid:
Ca,(PO&CaF',
+ 8H+ -+
2H3P04 + 2HF
+ 4Ca2+
(9)
The second stage corresponds to the asymptotic part of the curve where the
dissolution rate is very slow. Since the rate of diffusion of Hf ions is much higher
than that of SO:- ions, it can be expected that solution in the vicinity of the particles
becomes gradually supersaturated with respect to calcium sulfate dihydrate. The
surface of the phosphate rock particles can provide ideal conditions for nucleation of
calcium sulfate. The resulting nuclei will grow into large crystals and gradually cover
the entire particle surface, protecting them from further attack by the acid.
280
Dissolution of phosphate rock by mixtures of sulfuric and phosphoric acid
Time (hr)
Figure 6. Dissolution efficiency of various rocks as afisnction of time.
Figure 7. Coaling of a rock particle observed during a dissolution expenmew
(I 10 x magnificarion).
281
M, Jamialahmadi, S.H.Emam and H.Miiller-Steinhagen
The photograph in Figure 7 was obtained from the microscopic studies of the
dissolution process. It clearly illustrates the calcium sulfate coating around the
phosphate rock core. Coating of the phosphate rock depends on operating
temperature, free sulfuric acid concentration and rock particle size distribution. Based
on Figure 7, the following mechanism can be proposed for the dissolution of phosphate
rock.
(i) Mechanism of Dissolution of Phosphate Rock
Referring to Figure 8 (which is a schematic of Figure 7) the dissolution process is
characterized by the following steps:
1. Approach of acid to the exterior surface of the phosphate rock particles.
2. Penetration of acid through the coating layer to the surface of the unreacted core of
phosphate rock.
3. Reaction of acid with solid at the reaction surface.
4. Diffusion of the product through the coating back to the exterior surface of the
solid particle.
5 . Dispersion of the product into the main body of the fluid by diffusion through the
film surrounding the particle.
Figure 8. Represenration of various concentrations around a panicle.
282
Dissolution ofphosphate rock by mixtures of sulfuric and phosphoric acid
Dissolution processes are generally considered as irreversible. Therefore, steps 4
and 5 can be neglected in the analysis. Steps 1, 2 and 3 are in series and for the
simultaneous action of these three resistances the rate of dissolution of a typical grain
of the phosphate rock in acid can be obtained.
The rate of mass transfer to the external surface of the particle is given by:
For the diffusion step through the coating film covering the particle it can be. written:
N
= Df ~L ’ ( C A ~ - C A , )
bi
where t is the thickness of the coating layer, which is a function of time, and increases
as the dissolution process progresses.
The dissolution reaction at the reaction surface can be considered as zero-order in
solute and first-order in acid concentration [ 6 ] . Therefore, the rate of decrease in the
volume of the unreacted core of a particular grain can be written as:
At steady state, the three individual rates are equal. They can be combined at any
particular stage of conversion to give:
D, P k ,
Equation (13) shows that at any instant the total resistance against the dissolution
process is equal to:
The relative importance of the three individual resistances varies as the dissolution
progresses or the particle size decreases. For instance, at the early stage of dissolution
when the particles are free from coating, only two resistances namely the external
liquid film and surface reaction need to be considered. Therefore, Equation (14)
reduces to:
1 1
R (I =-+P kr
(15)
Knowing values of AL as a function of concentration and time, its derivative and the
total resistance, R,, can be determined from Equation (13).
283
M. Jamialahmadi, S.H.Emam and H. Miiller-Steinhagen
0.8
n
E
>
0.6
s
v
e
6
U
S
0
.= 0.4
-El
c
v)
L
Q
t
0
t
0.2
0.0
4
100
200
Mixer speed, rprn
300
4 I0
Figure 9. Variation of total resistance against dissolutionprocess with mixer speed.
(ii) Effectof Mixer Speed on Rt
For fast reactions, like the dissolution of phosphate rock in a mixture of phosphoric
and sulfuric acid, mixing of the reactants plays an important role [7]. Since sulfuric
acid ionizes instantaneously, very efficient agitation is required to avoid local
supersaturation with respect to calcium sulfate. Consequently, coating of the unreacted
rock particles occurs in most situations. In bench scale experiments, however, perfect
mixing conditions can be readily attained. In this work the effect of mixer speed on
the rate of dissolution is studied, to determine the importance of the external mass
transfer resistance on the dissolution rate. The results of these measurements are
summarized in Figure 9. As the speed of the mixer is increased, the boundary layer
thickness surrounding the particles and hence the external resistance, is decreased.
When the speed of the mixer reaches about 200 rpm or higher, the external resistance
reached its minimum value of b. Therefore, to minimize this resistance, all
experiments were performed at a speed of 300 rpm. Scaling down the speed of mixers
in plant dissolution tanks reveals that their rpm are also in this range.
(iii) Effect of Rock Quality on R,
Figure 10 shows typical measurements of the total resistance R, against the dissoiution
process, as a function of time for various rocks. Two distinctive regimes can be
observed. At the early stage of dissolution the total resistance remains constant and
almost independent of the rock quality. In this regime, particles were not yet coated
and the total resistance follows Equation (15). At the later stage of dissolution, when
the concentration of calcium sulfate in the vicinity of the particles is high, it
precipitates gradually on the particle surface. As a result, the total resistance against
the dissolution process increases sharply. The extent of the variation in total resistance
284
Dissolution of phosphate rock by mixtures of sulfuric andphosphoric acid
with time is strongly affected by the quality of the rock and the adjusted operating
conditions. With increasing temperature and excess sulfuric acid concentration, the
maximum value increases and the operation time needed to reach the maximum value
decreases. In experiments with Iranian rocks it is observed that the total resistance
increases steeper than for other rocks. This indicates a higher tendency for coating.
In experiments with Senegal and Togo rocks with a mean particle size below about 150
pm it was observed that the total resistance dropped sharply towards its initial value of
R, after reaching its maximum. The behavior of these rocks may also be explained by
the coating phenomenon. When the calcium sulfate covers the entire particle surfaces,
the resistance against the dissolution process reaches its maximum value. If the
calcium sulfate crystals detach from the reacting surfaces or the particles disintegrate
as a result of dissolution reactions, the coating resistance would be diminished and,
consequently, the total resistance should drop toward its initial value of &.
Microscopical studies of the dissolution process of the various rocks confirm that the
Senegal and Togo phosphate rocks with diameter equal to or below 150 pm
disintegrate in a mixture of phosphoric and sulfuric acids solutions whde this behavior
was not observed with Iranian rock. The high quality of the Senegal and Togo rocks
seems to be due to the fact that these rocks are sedimentary with high porosity and are
soft, while the Iranian phosphate rock is igneous and metamorphic with low porosity
and it is hard.
1
n
E
0.8
1
I
v
f
Y
0.6
0.4
I
0
c
0
+
0.2
0
Figure 10. Effect of rock origin on total resistance agaimt dissolution process.
285
M. Jamialahmadi, S.H. Emam and H. Miiller-Steinhagen
(iv) Eflect of Operating Temperature
In the absence of an external resistance Equation (14) reduces to:
where 6* =D,lk, is that thickness of the coating at which the resistances to reaction and
mass transfer through the coating are equal. Therefore, when 6, is less than 6* the
dissolution process should be controlled by surface reaction and it should increase
strongly with increasing operating temperature. However, when 6, is larger than 6*
the dissolution process is controlled by mass transfer through the coating and its
dependency on the operating temperature should be weak. This is due to the fact that
processes controlled by chemical reactions are much more temperature sensitive than
mass transfer processes.
The effect of temperature on the extraction of P205from phosphate rock is shown
in Figure 11. As expected the rate of extraction increases as the operating temperature
is increased. The effect of temperature is more pronounced at the early stage of the
process; as the dissolution time exceeds about 0.2 hr, the effect of temperature is less
pronounced. Therefore, Figure 11 indicates that at the early stage of dissolution, when
acids are in close contact with the solid phase in the absence of coating, the dissolution
process is controlled by chemical reaction. Subsequently, the dissolution process shifts
from reaction control to diffusion control through the coating layer.
The optimum operating temperature also depends on form and shape of the calcium
sulfate crystals in the dissolution and crystallization tanks. It is the difference in the
operating temperature and phosphoric acid concentration which mainly differentiates
between dihydrate, hemihydrate and anhydrate processes. Curve (a) in Figure 5
represents the dihydrate-hemihydrate transition equilibrium. Different dihydrate
processes are generally operated somewhere along this curve to achieve:
1. Recovery yields in the range between 95 % - 98 9%.
2. Phosphoric acid concentration of about 29%- 32% P205
3. Large rhombic calcium sulfate dihydrate crystals with good filtration rate.
However, the analysis of calcium sulfate crystals formed under various operating
temperatures shows that the solubility diagram is not the only factor responsible for the
different calcium sulfate dihydrate formations. A typical relationship between
temperature and the percentage of calcium sulfate dihydrate formation is shown in
Figure 12. It is interesting to realize that while the solubility diagram (Figure 5 )
indicates that at 75°C all the formed crystals should be dihydrate, in reality, only 90%
of the total crystals are dihydrate and the rest henuhydrate (Figure 12).
286
Dissolution ofphosphate rock by mixtures of sulfuric and phosphoric acid
100
-0
Q,
c
80
U
z
c
X
a
60
0
"
e
"
c
0
40
c
c
Q,
U
I
a)
a 20
0
0
0.06
0.12
0.3
0.24
0. 18
Time (hr)
Figure I I . Effecr of remperarure on the rare of dissolurion of rock partictes.
00
100
U
a0
80
-!i6
al
c
e
60
60
",
-E
E
al
c
40
40
5
c
c
0)
bJ
L
20
20
:
al
c
I-
0
50
60
70
80
Temperature, "C
90
0
100
Figure 12. Effect of temperature on the formation of calcium sulfate dihydrate and
hemihydrare.
287
M.Jamialahmadi,S.H. Emam and H. Miiller-Steinhagen
(v) Effect of Excess Sulfuric Acid Concentration
According to the stoichiomeuy of Equation (l), the rate of dissolution of phosphate
rock is directly proportional to the sulfuric acid Concentration. Therefore, an increase
in acid concentration should increases the rate of extraction of P205from the phosphate
rock. On the other hand, the amount of excess sulfuric acid in the system is the
paramount factor which controls the extent of coating of phosphate rock particles and
the quality of the calcium sulfate dihydrate crystallization.
Figure 13 shows the weight percent of P20, extracted as a function of excess
sulfuric acid concentration at an operating temperature of 75°C. It is characterized by
a gradual increase to a maximum recovery at about 2.5% of excess acid and a
subsequent decrease of P205 recovery as the amount of excess acid increases beyond
2.5%. The filterability of dihydrate crystals and the recovery of phosphoric acid by
washing of the filtrate depends strongly on shape and size of the formed crystals.
Dihydrate crystals may be rhombic, cluster, needle or plate-shaped. The latter two
crystals types are the most difficult to filter [8]. The appearance of calcium sulfate
dihydrate crystals formed for different excess acid concentrations varied widely, as can
be seen from the photographs in Figure 14. At low amounts of excess acid (0.5 to 1.5
wt%) small thin plate-shaped crystals are observed (see Figure 14a). At slightly
higher excess acid concentrations (2 to 3 wt%) large individual rhombic calcium
sulfate dihydrate crystals with a length not larger than twice their width are formed
(see Figure 14b). At excess concentrations higher than 3 wt%, needle-shaped calcium
dihydrate crystals with very low filtration rate are formed (see Figure 14c). These
effects seem to be caused by changes in nucleation and growth rate since excess sulfate
concentration has a strong effect on the solubility of calcium sulfate and, therefore, on
the degree of supersaturation at any constant phosphoric acid concentration.
Despite Figure 13 showing that the maximum recovery (97% P205)and large
calcium sulfate crystals with best filtration rate can be obtained when the excess
sulfuric concentration is controlled to between 2-3%, excess acid concentrations up to
4% are common practice in plant operation. However, the resulting phosphoric acid
with high excess sulfate content will create corrosion and scaling problems downstream, especially in the phosphoric acid concentration heat exchangers.
(vi) Effect of Particle Size
The particle size influences the dissolution process in various ways. The smaller the
sue, the greater is the interfacial area between phosphate rock and solution and,
therefore, the higher is the rate of transfer of materials. Referring to Equations (10) to
(12) it can be concluded that the time needed to achieve the same extent of extraction
of P20, from phosphate rock of a given particle size is:
(i) For external film diffusion control: t cc L’.5-2
(ii) For coating diffusion control:
tccL2
(iii) For chemical reaction control:
tKL
In plant operation, the resistance due to the diffusion of acid through the fdm
surrounding the particles is minimized by providing good mixing of the solution.
288
Dissolution of phosphate rock by mixtures of sulfuric and phosphoric acid
b
0
0,
t
-
95
0
L
X
0
0
"
a" 90
+
0
Poor digestion
Thin plat.-like
crystal
Gypsum unfilterablo
c
c
0
U
Spontanoous
Nucloation
Noodlo-ltk.
85
0-
Rhombic crystal
80
0
1
2
3
4
Wt% o f excess t12S04
Figure 13. Effect of excess sulfuric acid concentration on the eficiency of phosphare
rock dissolution.
Figure 14a. Calcium sulfate dihydrate crystals observed at ercess sulfuric acid
concentrations between 0.5 and 1.5 wt%.
289
M. Jamialahmadi, S.H. Emam and H.Miiller-Steinhagen
Figure 14b. Calcium sulfare dihydrate crystals observed at excess sulfuric acid
concentrations between 1.5 and 3.0 wt%
' .
Figure 14c. Calcium sulfare dihydrate crystals observed at excess sulfuric acid
concentrations above 3.5 wt 5%.
290
Dissolution of phosphate rock by mixtures of sulfuric and phosphoric acid
100
(T
U
Q)
=
75OC
90
c
U
0
L
c
X
W
80
0
"
n.c
0
+
I=
70
W
V
L
W
a
qn
V"
.
0
0 lronian phosphate r o c k
Excess IH2S041= 1.5 wt%
[P2051 =28.5-31 wt%
I
100
.
.
.Togo phosphate rock
OSenegal ,phosphate rock
.
&
2 00
particle size, dp (urn)
300
4 00
Figure 15. Effect of rock particle size on the eficiency of dissolution.
The results indicate that Iranian rock particles smaller than 100 pm, break down
immediately. Therefore, the access of acid to the core of the particles is not impeded
by the coating phenomenon. Particles larger than 100 pm, have a strong tendency to
coating. On the contrary, Senegal and Togo phosphate rocks up to a particle size of
about 150 pm may be used without coating problem. However, in practical
dissolution processes a mixture of various particle sizes sirmlar to the frequency curve
of Figure 2 is used. The particle size distribution has a specific range which generally
depends on rock origin and the process operating conditions. The effect of particle
size on the percentage of phosphate rock reacted is given in Figure 15. Since particles
with uniform size require approximately the same time for dissolution, a rock with
minimum size distribution is more desirable. Figure 16 shows the changes in
phosphate rock particle size distribution during a typical dissolution experiment. The
reduction in size spread of the particles with time is quite clear. Fine particles have a
high dissolution rate and disappear very fast, whereas large particles have a strong
tendency to coating and consequently their dissolution rate decreases sharply with time.
291
M.Jamialahmadi, S.H. Emam and H. Miiller-Steinhagen
0.5
. _._....
...._.
(Excess [H2SO4l=2 wt%
1
:\.
0.4
+
c
8
0.3
.”
c
I
:
0.2
;
0.1
0
0
100
200
Particle size,
300
43
400
500
(prn)
Figure 16 Change in phosphate rock particle size distribution with time during
dissolution experiment
Conclusions
The mechanisms of dissolution of phosphate rock in a mixture of phosphoric and
sulfuric acid and the subsequent crystallization of calcium sulfate dihydrate from
product acid have been investigated. The results suggest that the observed reduction in
the digestion rate is due to a shift of the dissolution process from reaction controlled to
mass transfer controlled, as a result of a coating mechanism. The dissolution rate
increases as excess sulfuric acid concentration and/or temperature are increased, but it
decreases as the rock particle size is increased. The optimum operating range is
achieved when the free sulfuric acid concentration is controlled to between 2-3.5wt%
and the operating temperature is maintained between 7578°C.
Acknowledgment
The authors are indebted to Razi-Petrochemical Complex for supporting this work.
292
Dissolution of phosphate rock by mixtures of sulfiric and phosphoric acid
Nomenclature
Orders of calcium sulfate dihydrate crystals growth and nucleation rates
.
Concentration (wt %)
Mass diffusivity through coating (m’ s“)
Reaction rate constant (m hr-I)
Particle diameter at time zero and time t, respectively (m)
Concentration of calcium dihydrate in the solution (kg m”)
Population density (number m-I)
Initial population density (number m-l)
Rate of mass transfer (m3 wt% hr-l)
Total resistance at time zero and time t, respectively (hr m-’)
Time (hr)
Subscripts and Superscripts
A
acid
€3
rock
b
bulk
C
coating surface
S
reaction surface
*
saturation
I
solid phase
Greek Symbols
Mass transfer coefficient (m hr-’)
I3
4”
Volumetric shape factor
Calcium sulfate dihydrate crystal density (kg m”)
PS
AL
Reduction in particle size (m)
Weight fraction of crystals in specified size range
Am,
AZ
Screen increment (m)
4
Coating thickness (m)
P3
Third moments (number m3)
T
Residence time (hr)
References
1. Coulson, J.M. and Richardson. J.F. 1976. Chemical Engineering, Volume 2. Pergarnon Press, UK.
2. Najar. M.. Szucs. F.. Blickle. T. and Mihalyko, C. 1992. Mass transfer in sulfuric acid digestion of
phosphate rock. Hungarian 1. Ind. Chem., 20. 71-79.
3. Meyers. R.A. 1986. Handbook of Chemical Production Processes. McGraw Hill, New York.
4. Slack. A . V . 1968. Phosphoric Acid (Fen. Sci. and Tech. Series, 1). Marcel Dekker Inc., New York.
5. Brcker. P. 1989. Phosphates and Phosphoric Acid, Raw Marerials,Technology and Economics of the
Wer Process (Fen. Sci.and Tech. Ser., 3). Marcel Dekker Inc., New York.
6. Fopler. H.S. 1992. Nemenrs of Chemical Reacrion Engineering. Prentice-Hall, New York.
I. Levenspiel. 0. 1976. Chenucal Reacrion Engineering. John Willey & Sons, New York.
8. Sluis. S.. Leenhousrs. W.P. and Wesselingh. J.A. 1989. Filtration and washing of calcium
sulfare/phosphoric acid slurrtes. Proceeding of the Filtration Society, 105-113.
Received: 18 November 1997; Accepted after revision: 1 April 1998.
293
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