Separation and Purification Technology 28 (2002) 197– 205
www.elsevier.com/locate/seppur
Wet-process phosphoric acid obtained from Kola apatite.
Purification from sulphates, fluorine, and metals
R. Kijkowska *, D. Pawlowska-Kozinska, Z. Kowalski, M. Jodko, Z. Wzorek
Institute of Inorganic Chemistry and Technology, Kraków Uni6ersity of Technology, Warszawska 24, 31 -155 Krakow, Poland
Received 7 August 2001; received in revised form 3 April 2002; accepted 5 April 2002
Abstract
The purification of pre-concentrated up to 80 wt.% H3PO4 phosphoric acid (PA) obtained from Kola apatite in one
of the Polish plants by the wet processing route was carried out by sulphate precipitation, desorption of volatile
components (SiF4, HF) and liquid–liquid extraction method using 4-methyl-2-pentanone (MIBK). The experiment
was carried out on a laboratory scale. The effects of the reagent grade Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O, Ca3(PO4)2,
and technical grade calcium oxide, the molar ratio of Ca2 + to SO24 − (0.8– 1.5), the temperature (343– 363 K), and
the duration of precipitation time (1800–7200 s) on the extent of purification from SO24 − were determined. The most
efficient precipitant was CaHPO4 · 2H2O. The precipitation using CaHPO4 · 2H2O purified phosphoric acid from the
initial SO24 − concentration (1.5–1.8%) to a level of 0.1– 0.2 wt.%. The use of 100% excess of SiO2 over the
stoichiometric ratio (in relation to SiF4), while air bubbling and very intensive stirring of the phosphoric acid at 403
K was carried out, allowed the fluorine concentration to decrease to a level below 0.005 wt.% of F. Purification from
metals was carried out at room temperature using 1:1.22 mass ratio of PA to MIBK. The stripped phosphoric acid,
with a concentration of about 50 wt.% of H3PO4 and 1.5 wt.% of MIBK, contained Fe and Al at a level of 0.01– 0.005
wt.% each, Pb, Th B1 ppm, Cr, Co, Ni B0.1 ppm, As, Cd— not detected. © 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Phosphoric acid; Purification; Sulphate precipitation; Fluorine; Solvent extraction
1. Introduction
Wet-process phosphoric acid contains a number
of undesirable ionic impurities, like fluorine, sulphate, aluminium, iron, and other metals originally present in the phosphate rock. The
impurities interfere in the technological process of
* Corresponding author. Fax: + 48-12-633-3374
E-mail address: [email protected] (R. Kijkowska).
making phosphoric acid and/or liquid fertiliser.
They also precipitate while the acid is concentrated or stored. Without previous treatment, the
acid cannot be applied directly to the production
of some industrial and food grade phosphate
derivatives, for example calcium, sodium or ammonium salts. A wide variety of miscellaneous
applications for the food, beverage, and toothpaste markets, or cleaning markets, require high
purity acid.
1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 4 8 - 5
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R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
The earliest known phosphoric acid purification
methods are based on the precipitation of sparingly
soluble
salts.
De-fluorination
by
(Na,K)2SiF6 or MgSiF6 · 6H2O precipitation are
traditionally used methods [1– 5]. To purify from
sulphate ion precipitation methods at an elevated
temperature using barium or calcium salts are
applied [5,6]. Some companies, for example,
Chemische Fabric Budenheim and R.A. Oetker,
combine precipitation of barium sulphate with
iron. The excess of barium for sulphate precipitation is removed from the acid using cationite [6].
The precipitation of heavy metals as sulphides [7]
constitutes one of the methods.
The precipitation of inorganic salts, frequently
applied in industry, purifies phosphoric acid to a
level at which the impurities, due to their solubility, still remain in the acid. The degree of phosphoric acid purification is higher when extraction
techniques with organic solvents are applied [8–
15]. A few purification processes using nonaqueous solvents have been put into commercial
practice. The available published data concern
purification of wet-process phosphoric acid, which
is derived mostly from African or North American rock phosphates [8– 10,12 – 14].
This paper presents the purification of industrial phosphoric acid obtained from Kola apatite
(PA) in one of the Polish plants. The PA differs
from the acid derived from Moroccan or American rock phosphates. It contains no organic materials or uranium. Instead, it contains a fairly
significant amount of Ti and Th. The acid was
pre-concentrated up to 77– 80% of H3PO4 (56 –
58% P2O5). By weight, it contained about 2% of
SO24 − , 0.1% F, 0.5% Al, 0.3% Fe, 0.15% Ti and
some other metals at a lower level (Table 1).
The purification was carried out on a laboratory scale in three steps:
“ pre-purification by sulphate precipitation,
“ purification from fluoride,
“ purification from metals by solvent extraction.
The wastes obtained from the purification experiments underwent neutralisation treatment.
The obtained products were analysed in order to
assess whether they may be considered as a phosphate-containing mineral fertiliser.
2. Experimental procedure
2.1. Sulphate precipitation
As a precipitant reagent grade (POCH,
Poland): Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O,
Ca3(PO4)2 and technical grade calcium oxide (88%
CaO, 12% CaCO3) were used. The investigated
parameters were:
molar ratio of Ca2 + /SO24 − (0.8–1.5),
time of precipitation (1800–7200 s),
temperature of precipitation (343–363 K).
The experiment was carried out in a glass reactor (0.0005 m3) with a mechanical stirrer. It was
furnished with a heating jacket connected to a
thermostat. Phosphoric acid (0.2 kg), was heated
to the desired temperature, then the precipitant
Table 1
Concentration of metals, (ICP method), in pre-concentrated,
pre-purified, derived from Kola apatite acid before (b) and
after purification using MIBK (c), (units: wt.% or ppm)
Component (a)
Before purification
(b)
After purification
(c)
H3PO4 (P2O5)
SO4
F
Fe
Al
Ti
Mg
Ca
Sr
Ba
Cr
Mn
Co
Ni
Zn
Pb
La
Ce
Nd
Sm
Dy
Er
Yb
Y
As, Cd
Th
77.3 (56.0)%
0.036%
0.041%
0.35%
0.50%
0.15%
620 ppm
236 ppm
1 ppm
3 ppm
2 ppm
350 ppm
3 ppm
2 ppm
12 ppm
20 ppm
14 ppm
24 ppm
31 ppm
10 ppm
10 ppm
10 ppm
10 ppm
140 ppm
Traces
20 ppm
50.0 (36.2)%
0.003%
0.003%
0.01%
0.005%
0.01%
B1 ppm
B10 ppm
B0.05 ppm
B0.05 ppm
B0.1 ppm
B10 ppm
B0.1 ppm
B0.1 ppm
B0.5 ppm
B1 ppm
B0.1 ppm
B0.1 ppm
B0.1 ppm
B0.1 ppm
B0.1 ppm
B0.1 ppm
B0.5 ppm
B1 ppm
Not detected
B1 ppm
R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
Fig. 1. The scheme of a laboratory desorption unit. (1) Vessel,
(2) resistor heater, (3) power regulation device, (4) thermoregulator, (5) temperature sensor of thermoregulator, (6) temperature sensor of digital thermometer, (7) digital thermometer, (8)
stirrer, (9) rotations reducer gear, (10) alternating-current motor, (11) stroboscope, (12) aeration tube, (13) rotameter, (14)
reducing valve, (15) membrane pump.
was slowly added. At intervals of 1800, 3600 and
5400 s, a small sample of slurry was taken out
through a stub pipe located at the bottom of the
reactor and filtered off. After 7200 s, the total
precipitate was filtered off from the phosphoric
acid, dried at 353 K and subjected to X-ray
diffraction (XRD), using X’Pert Philips. The sulphate was analysed in the filter solution: the turbidity was measured at 490 nm for the low SO24 −
concentration (0.01– 0.2%), while for the concentrations higher than 0.2 wt.%, the BaSO4 precipitation method was applied [16].
2.2. Fluoride desorption
The scheme of a laboratory unit equipment is
presented in Fig. 1. While slowly stirred, the acid
(0.42 kg) was placed in a vessel (1) and heated at
the maximum capacity of the heater. Within 8– 12
min, the temperature of the acid increased to
373–403 K. Then, the stirrer was turned off, the
level of the solution was marked and SiO2 was
introduced. While the solution was intensively
stirred (37/s), the bubbling was produced by
pumping the air (1.5 dm3/s) through tube 12.
Every hour, a sample of the acid was taken out,
199
cooled, and the fluorine content was determined.
Before sampling, the stirrer was turned off, an
amount of water was added to refill the volume of
the solution to the level marked, and the solution
was carefully stirred. The same procedures were
repeated before each sampling.
The fluorine content in the phosphoric acid was
determined with the use of an adapted W.C.
Hanson and D.J. Lloyd method [17]. The F content was measured by the potentiometric method
using the multivariable instrument CX-742 (Elmetron, Poland) and a specific fluoride ion electrode (Orion, USA). The standard deviation
(S.D.) of the used analytical method was 0.0025%
F.
2.3. Sol6ent extraction
The pre-purified (0.04% SO24 − ) or crude ( 2%
SO24 − ) phosphoric acid underwent purification by
the liquid-liquid extraction method using 4methyl-2-pentanone, 99% purity (MIBK-SIGMAALDRCH). Several sets of experiments were
carried out in a periodic system. Each set included
four cycles with solutions recycled as illustrated in
Fig. 8. Each cycle was arranged to have several
stages as presented below:
(a) extraction stage and separation of the phases,
(b) washing the MIBK phase (1st washing
stage+2nd washing stage); the solution from
the 2nd washing stage was used for the 1st
washing, while the solution from the 1st
washing was recycled to the extraction stage
in the subsequent cycle,
(c) re-extraction of purified phosphoric acid with
water.
In each stage, the mixture was intensively
stirred for 30 min then left to sit for the next 60
min to get the phases separated. The proportions
of the contacted aqueous and MIBK phases were
determined on the basis of the phase equilibrium
diagram in the H3PO4 –MIBK –H2O system published by Feki et al. [18]. The amounts of the
recycled aqueous solutions were adjusted to keep
the concentration of MIBK close to:
55% MIBK at the extraction stage,
52% MIBK at the 1st washing stage,
50% MIBK at the 2nd washing stage.
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R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
To simplify the experimental procedure, it was
assumed that almost all MIBK inlet passed
through the experimental stages. The approximation was based on the procedure that the washing
solutions from the previous cycle were returned to
the next one, while the amount of MIBK taken
away with the raffinate could be neglected. The
adjustment was usually made between washing
stages. If the amount of the 2nd washing solution
was too large to recycle it to the 1st washing
stage, some of the surplus was removed. If it was
too low to meet the parameters at the 1st washing
stage, some reagent grade phosphoric acid was
added.
To control the amounts of recycled solutions,
the phosphate was analysed in all phases at each
stage of the experiment. For the analysis, the
spectrophotometric method with ammonium
molybdate and 2,4-diaminophenol hydrochloride
(amidol) was used. The extinction was measured
at 660 nm.
increase in P2O5 concentration decreases the
metastable solubility of HH. However, the data
available are limited to acid concentrations below
the level of 55% P2O5, (74.8% H3PO4), [1,19]. To
prove this for more concentrated phosphoric acid,
a supplementary experiment on CaSO4 · 0.5H2O
solubility at 323–363 K, extending the concentration up to 60% P2O5, was carried out. The results
presented in Fig. 4 indicate that an increase in
P2O5 concentration up to 60% should bring the
SO24 − concentration down to a level of 0.2 wt.%.
Actually, the purification effect by sulphate precipitation obtained after 7200 s using
CaHPO4 · 2H2O was even higher than the predicted solubility of CaSO4 · 0.5H2O (Fig. 2D and
Fig. 3). The increased effect might have a kinetic
origin but to explain it some extended research
has to be done.
3. Results and discussion
3.1. Effect of sulphate precipitation
Figs. 2 and 3 present selected results chosen
from 89 phosphoric acid purification experiments.
The
most
effective
precipitant
was
CaHPO4 · 2H2O. At 353 K, using its stoichiometric amount (Ca2 + /SO24 − =1), the SO24 − concentration went down to a level of about 0.1 wt.%
(Fig. 3). An excess of precipitant over the stoichiometric Ca2 + /SO24 − ratio did not give any
significant decrease in SO24 − concentration (Fig.
3). Within the range investigated (343– 363 K),
there was no significant effect of temperature on
phosphoric acid purification from SO24 − ions.
The purification effects can be explained on the
basis of solubility of CaSO4 · 0.5H2O in concentrated phosphoric acid. It is well known that
above 40% P2O5, anhydrous calcium sulphate
(CaSO4) is a thermodynamically stable phase.
However, calcium sulphate hemihydrate (HH),
CaSO4 · 0.5H2O, which shows a high degree of
metastability, is usually formed first [1,19]. The
HH was also forming in our precipitations. An
Fig. 2. Plot of wt.% of SO24 − in phosphoric acid (initial
concentration: 72.5% H3PO4, 1.75% SO24 − ) versus time after
sulphate precipitation at 353 K. Molar Ca2 + /SO24 − ratio = 1.
Precipitants: A, Ca3(PO4)2; B, Ca(H2PO4)2 · H2O; C, CaO
(technical grade); D, CaHPO4 · 2H2O.
R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
201
initial phosphoric acid (a) (75–78% H3PO4) to
MIBK (J) was 1:1.22, which resulted in 55% of
MIBK at the extraction stage (Ex). The Ex is the
point where the a–J line crosses with the line of
55 mass% of MIBK. The concentration of H3PO4
in the mixtures obtained was at a level of 34–
35%. The separated phases were an aqueous
phase (raffinate) (b), with a 68–71% concentration of H3PO4 and about 2.5% of (Fe+Al), and
MIBK phase (extract) (L). The MIBK phase underwent washing stages.
Fig. 3. Influence of precipitation time and molar Ca2 + /SO24 −
ratio on SO24 − concentration in phosphoric acid (initial concentration: 74.2% H3PO4, 1.54% SO4). Precipitant:
CaHPO4 · 2H2O, molar Ca2 + /SO24 − ratio: 0.8 –1.5, temperature 353 K.
3.3.2. First washing stage (Fig. 6)
For the first washing, in the 1st cycle the
reagent grade phosphoric acid was used, while in
the 2nd, 3rd and 4th cycles the solutions (d) from
the 2nd washing stage were used. The amount of
the washing solutions was adjusted to keep the
concentration of MIBK close to 52% (S in Fig. 6).
If the amount of the solution from the 2nd washing stage was not sufficient to meet the proportions determined by S, some amount of reagent
3.2. Effect of defluorination
To transform fluoride into SiF4 the SiO2 was
added in the amount of up to 100% over the
stoichiometric quantity. During SiO2 additions,
phosphoric acid was heated to temperatures of
373, 383, 393 and 403 K, respectively, and bubbled with the air. The defluorination rate was
strongly dependent on both temperature and reaction time. Higher temperature of desorption, resulted in lower F concentration in the acid (Fig.
5). After 2 h of reaction time at 403 K the
concentration of F in the acid was lower than 0.01
wt.%.
3.3. Purification from metals by sol6ent
extraction
3.3.1. Extraction
The extraction carried out at a room temperature is illustrated in Fig. 6, which refers to the
phase diagram of Feki et al. The mass ratio of the
Fig. 4. Solubility of CaSO4 · 0.5H O in reagent grade phosphoric acid at temperatures: A, 343 K; B, 353 K and C, 363 K.
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R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
MIBK phase using between 0.15:1 and 0.2:1 mass
ratio. This located the re-extraction system near
the point (r).
3.3.5. Effects of phosphoric acid purification
The results based on phosphate, Fe and Al
analysis are presented in Fig. 8 for one of the sets
of four-cycle experiments. Depending on the acid
concentration and the amount of washing solution recycled, some scattering of Fe and Al concentrations from one cycle to another was
observed. The numbers in the diagram indicate
the highest and lowest values of the component
obtained at individual stages of the experiment.
To follow the effect of purification while the
concentrations of impurities and the phosphoric
acid were changing from one stage to the next
(Fig. 8), the molar ratios of Al/P and Fe/P in the
aqueous phases were calculated (Fig. 9). It was
Fig. 5. The fluorine content in phosphoric acid after SiF4
desorption with the use SiO2 at temperatures. , 373 K; ,
383 K, , 393 K, 2, 403 K.
grade phosphoric acid was added. After separation, the 1st washing solution (c) (63 – 65%
H3PO4) was recycled to the extraction stage of the
next cycle, and the MIBK phase (Q) underwent
the 2nd washing stage.
3.3.3. Second washing stage (Fig. 7)
The MIBK phase from the previous washing
stage (Q) was contacted with purified phosphoric
acid (product) (g), recycled from the re-extraction
stage, taking such an amount that was sufficient
to approach a concentration of MIBK close to 50
wt.% (i in Fig. 7). After separation, aqueous (f)
and purified MIBK phase (R) were obtained.
3.3.4. Re-extraction of purified phosphoric acid
(Fig. 7)
The amount of water used for the re-extraction
of the acid depended on the composition of the
purified MIBK phase (the position of the point R)
and the designed concentration of the product. To
release purified phosphoric acid with a concentration of 50% H3PO4, water was introduced into the
Fig. 6. Extraction and 1st washing of the extract approximated
to the phase diagram of Feki et al. in the system: H3PO4 –
MIBK– H2O. Concentration is given in wt.%. a —crude phosphoric acid, b — raffinate, c —solution from the 1st washing
stage, recycled to the extraction stage, d —solution from the
2nd washing stage. Ex— the mixture at the extraction stage
(55% MIBK). L —organic phase (MIBK-extract) coexisting
with aqueous phase b (raffinate). S —the mixture at the 1st
washing stage (52% MIBK). Q— MIBK phase coexisting with
aqueous phase c at the 1st washing stage.
R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
203
the majority of the impurities, the coexisting
aqueous-MIBK phases had compositions located
adequately to the H2O–H3PO4 –MIBK system on
binodal curves, close to the compositions determined by Feki connodal lines.
3.4. Potential waste
The purification of phosphoric acid described
above generates potential wastes:
1. the filter cake from sulphate precipitation and
Fig. 7. 2nd Washing stage and re-extraction of purified phosphoric acid. I— the mixture at the 2nd washing stage (50%
MIBK). R — MIBK phase coexisting with aqueous phase (f) at
the 2nd washing stage. Q as in Fig. 6. r— The mixture at the
re-extraction stage. g —Purified phosphoric acid (product).
very characteristic that Al/P increased more than
Fe/P in the raffinate with respect to phosphoric
acid undergoing purification, while the molar ratios of Al/P and Fe/P in the purified phosphoric
acid (product) were on the same low level. To
estimate the effect of the final purification, a full
analysis of the initial (PA) and purified phosphoric acid (product) is presented in Table 1.
Referring to the diagram of the H2O – H3PO4 –
MIBK ternary system described by Feki et al.
[18], the compositions of the separated phases at
the extraction stage were located outside the heterogeneous region (Fig. 6). The MIBK phase (L)
was close to the binodal curve, while the aqueous
phase, raffinate (b), was located in the region
which could be reached by an extrapolation of the
Feki binodal curve. The 1st and 2nd washings of
the MIBK phase and the re-extraction of the acid
led to compositions of the separated phases located inside the heterogeneous region. The deviation from the binodal curve observed at the
extraction stage might be a result of the influence
of the impurities present in the phosphoric acid.
After separation of the raffinate, which took out
Fig. 8. Block flow diagram of phosphoric acid purification
experiment. Mass ratio of the aqueous phase to MIBK at the
extraction stage by weight is 1:1.22. Concentration in wt.%.
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R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
cake or the raffinate were treated with technical
grade calcium oxide using its stoichiometric
amount with respect to monocalcium phosphate
equivalent to the amount of H3PO4 in the potential waste. The heat from the exothermic reaction
raised the temperature of the treated mixture up
to 353 K, and some amount of water and/or
MIBK evaporated. The product obtained was
dried and subjected to chemical analysis and
XRD. No analysis of MIBK content was carried
out. From the equilibrium diagram (Fig. 6), the
amount of MIBK in the raffinate may be about 2
wt.% and should decrease during water evaporation when the phosphoric acid neutralisation is
carried out. The characteristics of the product
obtained, with respect to the requirements of Polish National Standards [20–22], were comparable
or even better than those for single
superphosphate.
Fig. 9. Molar ratio of Al/P and Fe/P in the aqueous phases at
each stage of the phosphoric acid purification using MIBK.
4. Conclusions
2. the raffinate from the extraction stage.
The filter cake obtained at temperatures higher
than 343 K was well crystallised CaSO4 · 0.5H2O.
It was also readily filtered off. Some amount of
Ca(H2PO4)2 · H2O was found in the precipitate
obtained below 333 K. To avoid transformation
of the originally formed calcium sulphate into
another form, the filter cake was not washed off.
That resulted in the presence of phosphoric acid
in the precipitate. The estimated amount of the
filter cake from sulphate precipitation is 35– 50 kg
per 1000 kg of pre-purified phosphoric acid.
The raffinate, an aqueous phase from the extraction stage, was a green oily liquid. During
storage, an easily filtered solid precipitated. The
main component of the solid was an iron compound
identified
by
XRD
as
Fe3(H3O)H14(PO4)8 · 4H2O accompanied by some
amount of CaSO4 · 0.5H2O. The estimated
amount of the raffinate with a concentration of
about 70% of H3PO4 is 20 kg per 100 kg of
purified phosphoric acid (50% H3PO4).
To assess, whether the potential waste might be
considered as a by-product for phosphate containing mineral fertiliser production, the filter
1. CaSO4 · 0.5H2O precipitation at temperatures
353–363 K is an efficient pre-purification
method of pre-concentrated (above 75% of
H3PO4) wet-process phosphoric acid. The application of CaHPO4 · 2H2O as a precipitant
using the stoichiometric molar (Ca2 + /SO24 − =
1) ratio enables purification from the initial
(1.5–1.7%) SO24 − concentration to 0.2–0.1
wt.% in purified phosphoric acid.
2. The desorption of volatile fluorine compounds
(SiF4, HF), at a temperature of 403 K, using
an excess of SiO2 over the stoichiometric
quantity required to transform F into SiF4,
defluorinates phosphoric acid to a level of
0.001–0.002 wt.% of F. The rate of defluorination is higher than 95%.
3. Pre-purified from sulphate and fluorine phosphoric acid was purified from metals by liquid–liquid
extraction
method
using
4-methyl-2-pentanone. The purified product
was phosphoric acid (50% of H3PO4) with
fluorine and sulphate at a level of 0.003 wt.%
each and other impurities as indicated in Table
1, column c. To balance the amounts of the
R. Kijkowska et al. / Separation and Purification Technology 28 (2002) 197–205
washing solutions with the mass of the main
stream passing from the extraction to the reextraction stages, it was necessary to keep the
acid concentration at a level of about 34.5–
35% of H3PO4 at the extraction and 1st washing stages. Some dilution of the system may
result in an excessive amount of the 2nd washing solution. The surplus might be withdrawn
as a phosphoric acid (56– 65% of H3PO4 in
Fig. 8) with Fe and Al at a level below 0.1
wt.%. This intermediate level of purification
might be sufficient, for example, for tripolyphosphate use in detergent production.
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[20] Polish National Standard: Norma PN-85/C-7008.
[21] Polish National Standard: Norma PN-85/C-87015.
[22] Polish National Standard: Norma PN-85/C-87020.