Separation and Purification Technology 55 (2007) 212–216
Purification of wet process phosphoric acid by solvent extraction
with TBP and MIBK mixtures
Hannachi Ahmed ∗ , Habaili Diamonta, Chtara Chaker, Ratel Abdelhamid
Department of Chemical-Process Engineering, National Engineering School of Gabes, Omar Ibnelkhattab St., 6029 Zrig, Gabes, Tunisia
Received 24 November 2006; received in revised form 15 December 2006; accepted 16 December 2006
Abstract
In the present work, the purification of the wet process phosphoric acid (WPA) with mixtures of methyl isobutyl ketone (MIBK) and tri-butyl
phosphate (TBP) was investigated. Based on a three step purification process, the optimal solvent mixture composition that yielded the highest
purified acid with the greatest possible P2 O5 recovery was obtained for an MIBK percentage of 55%. Acid recovery increased for higher solvent
MIBK contents whereas the impurities’ contents were lower for near equal TBP and MIBK percentages in the binary solvent mixture. The phase
diagram of the ternary system H3 PO4 –water–optimal solvent mixture was determined. The partition ratios for H3 PO4 as well as three trace metallic
impurities were also obtained. Relative purification was better for Mg. The lowest selectivity was that of Fe which, unlike for the other trace
elements does not improve when the solvent rate increases. The best purifications are obtained for the higher solvent mixture rates. While TBP has
the highest selectivity, compared with MIBK, the solvent mixture is more selective for extracting H3 PO4 .
© 2007 Elsevier B.V. All rights reserved.
Keywords: Wet process phosphoric acid; Purification; Solvent extraction; TBP and MIBK mixtures; Equilibrium distribution isotherm
1. Introduction
Several techniques are used in the manufacture of industrial
phosphoric acid. The so called thermal process produces a pure
phosphoric acid at the expense of very large energy consumption. The wet process phosphoric acid (WPA) is produced by
a chemical reaction of the phosphate rock with a mineral acid
[1–3]. This method is known to be the most economical way of
getting phosphoric acid [1]. Three mineral acids can be used:
nitric, hydrochloric and sulfuric acid. In Tunisia, sulfuric acid
is used to transform the local phosphated minerals according
to a dihydrate process. Due to the presence of impurities in the
raw material the produced phosphoric acid inevitably contains
many chemical species. Some of these elements are detrimental to the quality of the acid for its end uses in the fertilizers
or food industries [4]. The WPA has to be purified. Several
physicochemical treatment techniques based on precipitation,
adsorption, ion exchange and solvent extraction are available.
None of these techniques is solely capable of carrying such task.
Some, such as membrane techniques are yet at an early stage of
∗
Corresponding author. Tel.: +216 75 392 100; fax: +216 75 392 190.
E-mail address: [email protected] (A. Hannachi).
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2006.12.014
development [5]. The best practical results with respect to the
lowering impurities’ contents of the industrial phosphoric acid
were obtained when combining chemical and solvent extraction
methods [6,7].
Many organic solvents can be used for the purification of
the WPA [3,4,6,8–13]. Unfortunately, none of these solvents
is highly selective to phosphoric acid and the purification process is generally curried out in several steps. According to their
nature, these solvents act differently: some are capable of precipitating part of the impurities; others can extract cationic or
anionic impurities. A number of solvents such as certain alcohols, ketones, ether oxides and phosphoric esters are able of
extracting the phosphoric acid. Because of their effectiveness,
the last ones are widely used in WPA purification by solvent
extraction processes [3,4].
Purification processes involving methyl isobutyl ketone
(MIBK) are widely reported in the literature [7,10,13,14]. The
use of tri-butyl phosphate (TBP) was also reported [8,10,15,16].
Both solvents are attractive because of their immiscibility with
aqueous solutions, good selectivity to phosphoric acid and easy
recovery. Since they are also reasonably non-toxic and noninflammable, their use is almost non-hazardous if appropriate
control measures are taken [17]. While phosphoric acid extraction with MIBK could be carried out at room temperature, the
A. Hannachi et al. / Separation and Purification Technology 55 (2007) 212–216
213
Table 1
Wet process phosphoric acid composition
P2 O5 (wt.%)
Al (ppm)
Ca (ppm)
Fe (ppm)
Mg (ppm)
V (ppm)
SO4 2− (ppm)
Zn (ppm)
Cd (ppm)
Cr (ppm)
54
4076
405
3108
9581
74
9582
209
20
319
extraction with TBP should be conducted at higher temperatures because of its relatively high viscosity. However, TBP has
a better selectivity in phosphoric acid extraction than MIBK.
Therefore, our goal in this work was to investigate the purification process of WPA with MIBK and TBP mixtures in order
to eventually profit from the advantages of each solvent while
avoiding their disadvantages. The equilibrium distribution of
phosphoric acid and some trace metallic impurities between
organic and aqueous phases has been considered.
of Cr2 O7 2− was back titrated with a standard solution of Fe2+ .
The relative error was below 2.5%. The raffinate fluoride content
was analyzed within a ±2.5% accuracy range by titration with
thorium nitrate the fluoride formed while decomposing H2 SiF6
entrained by steam distillation of the raffinate sample and sulfuric acid mixture with an excess of SiO2 . Concentrations for
organic and aqueous phases were obtained as weight ratios.
2.4. Liquid–liquid equilibrium at 30 ◦ C of the system
H3 PO4 –water–solvent mixture
2. Experimental procedures
2.1. Raw material
The raw material of this work was a treated industrial WPA.
The treatment was intended to reduce the organic matter as well
as some mineral impurities in the acid. After the treatment the
WPA was analyzed and showed the results given in Table 1.
2.2. Choice of the solvent mixture
The best solvent mixture should be able to yield a highly
purified acid with the maximum P2 O5 possible recuperation.
Solvent mixtures performances in a WPA purification process
were compared. Based on our previous experience with purification of the WPA using TBP and MIBK separately, a three step
acid purification process was adopted. The process was carried
out at 25 ◦ C. The first step consisted of an extraction with a
solvent to WPA volume ratio of 3.5. The acid was mixed with
the solvent mixtures in conical flask and intensively shaken for
20 min and allowed to settle. Then the extract was collected and
washed with distilled water at a 5% weight ratio as in the first
step. The third and the final step consisted of a stripping of the
phosphoric acid with distilled water at a 20% weight ratio for
which the intensive mixing was continued for 15 min. The aqueous phase, being the purified phosphoric acid, is recovered and
analyzed for P2 O5 and impurities.
2.3. Analysis techniques
The P2 O5 content was determined by volumetric titration
with NaOH using bromocresel green and phenolphthalein indicators with an accuracy of ±0.2%. The water composition was
determined by the Karl–Fisher method with a 4% experimental
error [18]. Metallic impurities’ contents were determined using
a Jobain Yvon ICP 2000 spectrometer previously calibrated with
adequate standards within the concentration ranges of the samples with an accuracy of ±0.2%. The raffinate organic matter
content was determined by using a redox titrimetric method
using a hot mixture of K2 Cr2 O7 in sulfuric medium. The excess
In order to develop the phase diagram of the ternary system H3 PO4 –water–solvent mixture, a series of initial mixtures
weighing 250 g were prepared. They were vigorously shaken
for 20 min and allowed to settle in a thermo stated bath at 30 ◦ C.
After separation, the conjugated phases were weighed and the
P2 O5 and water contents of both the extract and the raffinate
were determined.
2.5. Solvent rate optimization in batch extraction
operations
As in the previous section, several initial mixtures with different solvent rates were prepared and undergone a similar
treatment as described earlier. After separation, the conjugated
phases were weighed and analyzed for P2 O5 and impurities.
3. Results and discussion
3.1. Solvent mixture composition optimization
The P2 O5 yield of the adopted WPA purification process earlier described is given in Fig. 1. As the solvent MIBK content
increased, the P2 O5 yield improved. The composition of some
trace metallic impurities in the purified acid as a function of the
solvent mixture MIBK content is revealed in Fig. 2. The concentrations are given for an equivalent P2 O5 purified phosphoric
acid content of 60%. For all trace impurities, the concentration in
the acid passes through a minimum as the MIBK percentage of
the solvent increased. This was also true for fluoride and organic
matter contents as the weight ratios P/C and P/F were highest
for the same solvent MIBK concentration range as can be seen
in Fig. 3. The best purification performances were obtained for
an optimal MIBK weight fraction of 0.55 in the solvent mixture.
3.2. Phase diagram of the system water–H3 PO4 –MIBK and
TBP at 30 ◦ C
Since solvent mixtures allowed obtaining purer phosphoric
acids, it was interesting to determine the distribution of H3 PO4
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A. Hannachi et al. / Separation and Purification Technology 55 (2007) 212–216
Fig. 1. P2 O5 yield of the WPA purification process vs. MIBK solvent mixture
content.
and impurities between solvent and aqueous phases for extraction operations of the WPA with the optimal composition solvent
mixture. It was then necessary to establish the phase diagram
of the pseudo-ternary system water–H3 PO4 –MIBK (55%) and
TBP (45%) solvent mixture. All extractions were performed at
30 ◦ C. As described previously, several WPA and solvent initial
mixtures were prepared. In each case after separating, weighing
and analyzing the conjugated phases, a material balance was
curried for each key component. The deviations did not exceed
1.8% in the worst case.
The equilibrium phase diagram of the system water–
H3 PO4 –MIBK and TBP solvent mixture is shown in Fig. 4 on
which, for each mixture, the conjugated phases were connected
with tie-lines. The plait point shown in the same figure was esti-
Fig. 2. Purified WPA impurities’ contents vs. MIBK content of the solvent
mixture.
Fig. 3. Effect of solvent MIBK content on P/C and P/F weight ratios of the
purified WPA.
mated according to the Hand method [19]. As for most organic
solvents used for WPA purification, the slope of the tie-lines
changes as the mixture approaches the plait point. Fig. 5 gives
the H3 PO4 distribution between the organic and the aqueous
phases at 30 ◦ C as compared to that of pure TBP at 45 ◦ C [16]
and pure MIBK at 25 ◦ C [14]. Although the WPA quality was not
the same and the extraction temperatures were not identical, it is
clear that the acid distribution curve for the solvent mixture lies
between those of pure solvents. That is, for high WPA concentrations and with regarding H3 PO4 extraction capability, MIBK
performs better than the solvent mixture which in turn is superior
Fig. 4. Equilibrium phase diagram of the pseudo-ternary system H3 PO4 –
water–solvent mixture (55% MIBK, 45% TBP).
A. Hannachi et al. / Separation and Purification Technology 55 (2007) 212–216
215
Fig. 6. Trace metal distribution isotherm for the extraction of the WPA with a
solvent mixture (55%MIBK, 45%TBP) at 30 ◦ C.
Fig. 5. H3 PO4 distribution isotherm for solvent extraction of the WPA.
to TBP. This trend is inversed for lower WPA concentrations.
Similarly to most organic solvents used for WPA purification,
the phosphoric acid partition ratio (KH3 PO4 = yH3 PO4 /xH3 PO4 ),
defined as the ratio of H3 PO4 content in the extract and that in
the raffinate, increases when the acid concentration increases.
3.3. Batch extraction operations
The optimum solvent rate was sought within the range defined
by values obtained through graphical constructions based on the
achieved ternary diagram for a single batch extraction operation
[20]. As for the determination of the equilibrium isotherm, several initial mixtures with different solvent rates were prepared
and undergone a similar treatment likewise in Section 2.3. The
conjugated phases were separated, weighed and analyzed for
P2 O5 and impurities. In Fig. 6, the distribution of some trace
components (Al, Fe, and Mg) is given. The partition ratio for
all three trace metals decreases as the metal concentration in the
aqueous phase increases.
The relative purification or selectivity towards H3 PO4 with
respect to any impurity is an important factor for the choice of
the best solvent to conduct the extraction operation. It is defined
as the ratio of partition coefficients of H3 PO4 and that of the
H3 PO4 = K
impurity (Simp
H3 PO4 /Kimp ). The higher is the selectivity, the purer is the phosphoric acid stripped from the extract
phase. Fig. 7 shows the relative purification for the selected
Fig. 7. Relative purification of H3 PO4 with respect to some impurities for the
extraction of the WPA with a solvent mixture (55%MIBK, 45%TBP) at 30 ◦ C.
trace metallic impurities as a function of the phosphoric acid
concentration in the raffinate. For all impurities the selectivity
increased as the acid concentration decreased. The WPA purification capacity of the solvent mixture is compared to that with
pure solvents in Table 2 which shows the average selectivity of
Table 2
Selectivity of acid extraction with respect to some impurities in the WPA (for an extract H3 PO4 weight fraction: 10–12%)
Solvent
TBP
55%MIBK, 45%TBP
MIBK
Extraction temperature (◦ C)
45
30
25
Average selectivity, SXH3 PO4 (KH3 PO4 /KX )
X = Mg
X = Al
X = Fe
X = SO4
280
97
48
184
42
31
73
14
10
25
5
4
216
A. Hannachi et al. / Separation and Purification Technology 55 (2007) 212–216
H3 PO4 extraction. Despite the fact that the WPA and the extraction temperature were different for each solvent, the selectivity
with respect to all impurities for the solvent mixture lies between
those of pure solvents. The TBP is more selective to H3 PO4
extraction then the solvent mixture and MIBK. In every single
case, the relative purification was better for magnesium than for
the other two considered metallic impurities which in turn were
superior to sulfates. For the solvent mixture the relative purification for iron and sulfates do not improve when the solvent rate
increases. The best purification can be obtained for the highest
solvent rate, i.e. low acid contents in both phases.
Unfortunately, because the purification process is actually
carried out in three steps, the overall performance of any solvent would depend not only on the extraction process but also
on the washing and stripping operations. The presented equilibrium data for the distribution of H3 PO4 and other impurities
between the organic and the aqueous phases is valuable information that can be used to predict the overall performance, to
locate the best operating conditions and to design the needed
extractors for conducting the purification process. These objectives can be accomplished more easily through the simulation
of the purification process while adopting batch or continuous
column liquid–liquid extraction approaches [7,14,16,21].
4. Conclusion
In this work purification of the wet process phosphoric acid
(WPA) with mixtures of methyl isobutyl ketone (MIBK) and tributyl phosphate (TBP) was considered. Solvent mixtures that
yielded the highest purified acid with the greatest possible P2 O5
recovery for a typical three step purification process were found.
The purification process consisted of an extraction, a washing
and stripping operations. The P2 O5 recovery increased for higher
solvent MIBK content whereas the impurities’ contents were
lower for near equal TBP and MIBK weight fractions in the solvent mixture. The best purification performances were obtained
for a solvent mixture having an optimal MIBK content of 55%.
Then, for the optimal solvent mixture, the phase diagram of
the ternary system H3 PO4 –water–solvent mixture was determined. The partition ratios for H3 PO4 as well as Al, Fe and Mg
taken as trace metallic impurities were also obtained. For all key
components, the partition coefficient decreases as the element
concentration in the aqueous phase increases. The relative purification between H3 PO4 and each trace metallic impurity was also
derived. For all impurities, the selectivity increases as the acid
concentration decreases. Relative purification was better for Mg
than for the other two metals. The lowest selectivity was that of
Fe which, unlike for the other trace elements does not improve
very much when the solvent rate increases. The best purification
was obtained for the highest solvent mixture rates. The solvent
mixture is more selective than the MIBK but less selective than
TBP with respect to acid extraction.
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