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IJESRT
INTERNATIONAL JOURNAL OF ENGINEERING SCIENCES & RESEARCH
TECHNOLOGY
IMPROVEMENTS AND INTENSIFICATION OF INDUSTRIAL CO-
CRYSTALLIZATION PROCESS FOR CADMIUM REMOVAL FROM WET
PHOSPHORIC ACID
Kamal Samrane,1,2 Rachid Boulif,1 Driss Dhiba,1 Ahmed Bouhaouss2
1 Department of Chemical and Biochemical Sciences, Mohammed VI University Polytechnic,
Morocco
2 Department of Chemistry, Mohammed V University, Faculty of Sciences, Morocco
DOI: 10.5281/zenodo.1476369
ABSTRACT
Cadmium is a natural element present in the environment, including in the soil, in the air and in seawater. The
main source of cadmium in the soil is the bedrock, and the most significant man-made cadmium input comes from
industrial waste. Cadmium is also naturally present in 95% of global phosphate reserves, which consist of
sedimentary rock laid down in seabeds over thousands of years. The purpose of this work consists, first, to
establish a baseline knowledge of the status of the cadmium removal technologies from the wet phosphoric acid
and, second, to present recent results on the improvement and the intensification of the co-crystallization process
for cadmium removal from phosphoric acid, which is the most studied and patented technology in this field. Thus,
we have developed new co-crystallization modes (DDS & DDC) that have significantly increased the cadmium
removal and the P2O5 recovery from wet phosphoric acid, saved the heat energy cost and reduced the process
design. The new co-crystallization modes have upgraded the cadmium removal respectively by 34% and 37%,
within a high positive impact on the P2O5 content in the wet phosphoric acid which has been increased respectively
by 0.56 and 1.46 P2O5 points.
KEYWORDS: Cadmium; Calcium sulfate; Co-crystallization; Phosphoric acid.
1. INTRODUCTION
Cadmium is a natural element – a metal like iron or copper – that is naturally present in the environment, including
in the soil, in the air and in seawater. The main source of cadmium in the soil is the bedrock, and the most
significant man-made cadmium input comes from industrial waste – including from the steel and iron industry –
improper waste disposal, vehicle emissions, atmospheric deposition, sewage, manure, and the use of untreated
effluents for irrigation and fertilization [1]. Cadmium is also naturally present in 95% of global phosphate reserves,
which consist of sedimentary rock that was laid down in seabeds over thousands of years [1]. There is no direct
correlation between cadmium levels in the soil and cadmium levels in crops and in human diets. Cadmium uptake
by plants is affected primarily by a wide range of factors, including soil acidity, its content of organic matter, clay
and chloride, the presence of metal ions such as iron and zinc, an overuse of nitrogen, and crop species and
varieties – for instance, differences in cultivars alone can affect cadmium uptake by the plant by up to 10 times.
Cadmium levels in food are affected by a wide range of factors such as food processing and diet – for instance,
wheat products have less than half the cadmium content of wheat grains and flour, and wheat has ten times more
cadmium than eggs [1] [2] [3]. The WHO, FAO [4] and the European Food Safety Authority [5] have found no
evidence of adverse health effects attributable to cadmium in any groups in the global populations, including high
risk groups, at current exposure levels. The only significant case of cadmium toxicity worldwide occurred in the
1950s in Japan with subsistence farmers who grew rice on soils that were contaminated with industrial wastes [1]
and there have been no documented cases, since then, of cadmium affecting public health as there is no scientific
correlation between cadmium in fertilizers and health issues [3]. Nonetheless, major countries/states have
established cadmium limits in their fertilizers on the basis of comprehensive risk assessments. These
countries/states’s limits include the following: USA (Washington) 889mg of Cd/kg of P 2O5, Canada 889mg of
Cd/kg of P2O5, USA (Oregon) 338mg of Cd/kg of P2O5, USA (California) 180mg of Cd/kg of P2O5, Japan 148mg
of Cd/kg of P2O5, Australia 131mg of Cd/kg of P2O5, New Zealand 122mg of Cd/kg of P2O5, Belgium 90mg of
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Cd/kg of P2O5, Austria 75mg of Cd/kg of P2O5, Denmark 48 of Cd/kg of P2O5 and Sweden 44mg of Cd/kg of
P2O5 [1]. As for the European Union, there are currently no cadmium limits on fertilizers. Cadmium’s final content
in fertilizers depends both on the type and origin of the phosphate raw material as well as on the phosphoric
fertilizers manufacturing processes. The cadmium removal from WPA is a priority for researchers and the
industry. In spite of the fact that health risks associated with cadmium are not a cause for concern, various
technologies continue to be researched for the removal of cadmium in phosphate rock and in phosphoric acid,
including co-crystallization, ionic flotation, precipitation, solvent extraction, ion exchange and membrane
separation. Nonetheless, there is still no proven and economically viable decadmiation technology at the industrial
scale. Additionally, the management of the removed cadmium remains a big challenge since the corresponding
environmental issues for its safe disposal pose a significant constraint. The purpose of this work consists first to
establish a baseline knowledge of the status of the cadmium removal technologies from the wet phosphoric acid,
and second to present recent studies and researches on the improvement of the co-crystallization. The later has
been the most considered and studied technology for the cadmium removal from phosphoric acid. Thus, the
present work aims to deepen the study of the thermodynamics and the kinetics of the co-crystallization, and to
find novel parameters that have increased the cadmium removal efficiency of the original.
Cd in phosphate rock
Phosphate rock (PR) is naturally occurring mineral assemblages containing high concentration of phosphate
minerals, which after processing is used to produce phosphoric acid and fertilizers. It is a natural mineral deposit
of phosphorus and calcium, which belongs to the species of the apatite family. PRs resources occur principally as
sedimentary marine phosphorites. The largest sedimentary deposits are found in northern Africa, China, the
Middle East, and the United States [7]. More than 40 countries all over the world produce PRs which are mainly
used for the manufacturing of phosphate fertilizers. The major ones are China, Morocco and United States [7]. In
term of cadmium concentration, sedimentary PRs are generally more enriched than the igneous deposits, typically
in the range of 3–150 mg Cd per kg of rock [8]. The amounts of Cd vary widely not only among various PRs
sources but also even in the same deposit. The composition of phosphate rocks largely depends on its type and
origin. Table 1 shows examples of Cd concentration in PRs.
Table 1. Examples of Cd concentration (mg/Kg P2O5) in phosphate rocks [6]
Phosphate rocks
Davister, 1996
Oosterhuis et al, 2000
Igneous rock
Russia (Kola)
<13
0.25
South Africa (Phalaborwa)
<13
0.38
Sedimentary rock
USA (Florida)
Jordan
Morocco (Khouribga)
Syria
Algeria
Egypt
Israel
Tunisia (Gafsa)
Togo
USA (North Carolina)
Senegal (Taiba)
23
>30
46
52
60
74
100
137
162
166
203
24
18
55
22
61
173
147
120
221
Technologies of Cd removal
In the last decade several methods for the Cd removal from phosphoric acid and phosphate ore have been
developed. Here below, we give a baseline knowledge of the status of Cd removal technologies, with a special
focus on the co-crystallization, the most studied way for Cd removal during phosphoric acid manufacturing. The
aim is to deepen the understanding and the knowledge about the physico-chemistry of the co-crystallization
process, and present recent experiences and improvements of the original co-crystallization process for Cd
removal from wet phosphoric acid.
Calcination
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The removal of cadmium from phosphate rock by calcination is governed by the known physical properties of
cadmium metal and cadmium oxide. Cadmium can be removed from phosphate rock by calcination most
effectively at temperatures between approximately 850° and 1150°C under either an inert atmosphere or reductive
conditions. The higher end of the temperature range is needed for the most effective cadmium removal.
Calcination in the presence of chloride salts allows a temperature as low as 700°C to be used but this results in
excessive corrosion [9]. The study of calcocadmated apatites treated at different temperatures (700-900 °C) under
H2S shows the extraction of Cd from apatite structure, essentially in the form of CdS. In the case of the calcination
of apatites, the OH- or F- ions are replaced by S2- ions coming from the decomposition of H2S gas [10]. Significant
research has been performed in the area of calcination, and several patents have been granted, for example the
CERPHOS process for Cd removal by calcination on two steps, developed in Morocco, decreased the cadmium
content, when heated in the range of 750 to 850°C under controlled pressure and atmosphere to reduce and
volatilize the cadmium [11]. It appears that the only facility that has been built in the world to remove cadmium
was installed (75 mtph) on the Island of Nauru for Nauru Phosphate Corporation and decreased the cadmium
content of Nauru PR from about 600 to less than 120 mg/kg P, the exact cost of calcination is not known [12].
Cadmium sulphide precipitation
The earliest methods for the removal of cadmium from wet process phosphoric acid were based on the
precipitation of CdS with H2S or a soluble sulphide salt [13,15]. In some of the patent applications an overpressure
was used to obtain low residual cadmium concentrations. Chemische Fabrik Kalk GmbH [13] reported a reduction
of the cadmium content of 75 w% H3PO4 from 65 ppm to 13 ppm using an overpressure of 5 bar at 20°C. The
precipitate was separated by filtration. More recent patent applications report a partial neutralization of the acid
before the addition of the sulphide source [16,18]. Boliden AB [17] reported a reduction of the cadmium content
of 62 w% H3PO4 from 20 mg/l to 2.5 mg/l after neutralization with NaOH to Na/P = 1:10 mol/mol (60°C). Hoechst
AG [17] reported the removal of cadmium from Odda acid (30-35% HNO3, 20-28% H3PO4, 7-10% CaO) after
neutralization with ammonia until pH = 1. The cadmium content was reduced from 3-6 to 0.2 ppm. The major
constraint of this way is that H2S is a weak acid with low solubility and higher vapor pressure under phosphoric
acid media. So, working under higher pressure is required to reduce H2S emission from phosphoric acid. Another
way which is suitable to fertilizer process production and that was recently patented by OCP, consist to increase
the pH of the phosphoric acid in order to enhance H2S solubility and then the uptake of Cd as sulfide form [19].
Cadmium precipitation, adsorption and flotation with organic sulfur containing compounds
Solid diphenyldithiocarbamate salts, Ph2NCSSM, or their aqueous solutions were used to remove cadmium ions
from 65% H3PO4. The Cd containing precipitate was separated by filtration [20]. For Cd adsorption, A diorganyl-
O,O-ester of dithiophosphoric acid adsorbed on active carbon or silica (20-60% w/w (RO)2PSSH on adsorbent)
was used to remove cadmium ions from a 40% H3PO4 solution. The Cd complexing ligand and the adsorbent can
also be added separately to the H3PO4 solution. Apart from diorganyl-O,O-esters of dithiophosphoric acid, O-
esters of dithiophosphonic acid ((RO)RPSSH) and diorganyldlthiophosphinic acids (R2PSSH) were used for the
removal of cadmium from Odda-acid. For application at high temperatures (50-80°C) the addition of a large
amount of a reductive compound is necessary [21]. About 1 w% Fe powder was used to remove Cd from 72%
H3PO4 at 70°C. For flotation, After reduction of the phosphoric acid (Fe2+/Fe3+> 6) a diorganyl-O,O-ester of
dithiophosphoric acid was added to the phosphoric acid. The Cd-containing precipitate was removed by flotation
[22] [23].
Cadmium removal by liquid-liquid extraction
A 10% w/w solution of a dialkyl-O, O-ester of dithiophosphoric acid (RO)2PSSH, in kerosene was used to remove
cadmium ions from 40-70% w/w H3PO4 [24,25]. Re-extraction of the loaded organic phase was performed with
30% w/w HC1 or 48% w/w HBr solutions. Apart from diorganyl-O,O-esters of dithiophosphoric acid,
((RO)2PSSH, O-esters of dithiophosphonic acid, R(RO)PSSH, and diorganyldlthiophosphinic acids , R2PSSH,
were used for the extraction of cadmium ions from Odda acid (30-35% HNO3, 20-28% H3PO4, 7-10% CaO). All
compounds mentioned above are not stable in the phosphoric acid solution at higher temperatures (> 80°C) and
decompose more or less.
Cadmium removal by ion exchange
For the removal of cadmium by means of ionic exchange, both cation and anion exchangers may be used, while
cation exchangers are not selective to other cations (Fe, Mg, Ca) [26]. Ion exchange requires pre-removal of
insoluble substances, which otherwise could clog ion exchanger and reduce bed life time; this is frequently omitted
aspect of the application of this process. Ion exchanger regeneration generates large volumes of the solution of
low metal concentration. The removal of cadmium using anion exchanger requires first its complexing to
compounds of structure [CdXn]2-n where X=Br, Cl, I and n varies from 1 to 4, which exhibit high affinity to
anionic resins and thus, they are strongly and quite selectively retained. Achieving the same degree of Cd removal
requires much lower amount of Br- or I- than Cl-, but it is required to maintain reductive environment. Anion
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exchange method for removal of cadmium in form of chloride complex has been tested at the technical scale by
Hydro Agri Rotterdam BV in the HDH Extra process [27]. The studies showed no blocking or damaging ion
exchange bed, despite using only pre-clarification of acid. HDH Extra process was recommended by Lin and
Schorr [28] for consideration as BAT (Best Available Techniques), however only phosphates rock of content up
to 50 mg Cd/kg P can be used as raw materials.
Cadmium removal by electrodeposition
Electrodeposition is an elegant method for the removal of Cd, since the element is obtained in a concentrated form
without the use of additional chemicals. De Jong and Schmal [29] have performed an investigation on the
electrolytical removal of cadmium from concentrated phosphoric acid solutions. The experiments were performed
with a rotating disk electrode (2000 rpm) and with parallel flat - plate electrodes (cathode and anode). Graphite or
lead were used as the cathode and platinum as the anode. The reference electrode is a saturated calomel electrode
(SCE). Cd was reduced at -750 mV SCE with an initial current efficiency of 100%. The current efficiency for
cadmium dropped to a value of a few percent after deposition of a layer of cadmium on the cathode. This is caused
by the lower hydrogen overvoltage of the cadmium coated electrode compared with a clean graphite or lead
electrode. The presence of copper in the phosphoric acid solution caused a tremendous increase of the hydrogen
reduction at -500 mV SCE and at the graphite electrode. The lead electrode gave a much smaller hydrogen
reduction at potentials more positive than -800 mV SCE.
Cadmium removal by adsorption (ELICAD process)
This process would eliminate cadmium from a continuous flow of phosphoric acid by a selective adsorption of
cadmium on an activated material. Once this material is saturated, it can be regenerated five times by a physico-
chemical treatment. The process allows also the removal of other trace metals like arsenic, mercury, nickel,
copper, zinc, vanadium, chromium and lead and most probably uranium [6]. According to estimates in 2009, the
investment costs should be below 1.2 million Euros for an installation treating 1200 tons of phosphoric acid per
day and the fertilizer price increase from the use of this technology would be around 12 to 32 Euros/ton P2O5 for
a 90 % effectiveness [6]. However, these figures need to be refined when data from a pilot plant will be available.
This was expected by mid 2010, however no project for constructing a pilot plant found the necessary funding
and it is uncertain when this will happen [6].
Cadmium removal by co-crystallization
The co-crystallization process is the most studied and patented technology for the cadmium removal from wet
phosphoric acid. The process consists on the co-crystallization of cadmium with anhydrite (CaSO4) when a
concentrated phosphoric acid (50%-54% P2O5), including its sludge content, is heated and further treated with
calcium phosphate and sulfuric acid. Cadmium shows the highest affinity to anhydrite (CaSO4) – hundred-times
greater than to Dihydrate gypsum [30]. One the major process is the CERPHOS process. It has been patented in
1996 and involved the removal of cadmium in the feed acid by co-crystallization into a precipitate of anhydrite
and separation of the latter by settling and/or filtration [31]. A product acid with a cadmium concentration of less
than 23 mg kg-1P was consistently produced from a feed acid with a cadmium concentration of 170 mg kg-1P
(Davister, 1996) [27]. However, Van Kauwenbergh [12] has pointed out that in evaluating the costs some caution
should be used in applying the value of the cadmium-containing rejects as a way of reducing the costs per ton of
phosphorus for the co-crystallization process. He emphasizes the very variable and decreasing value of cadmium
metal between 1995 and 1999, and the cost of disposing of reject material as a hazardous waste. The disadvantage
of this method is a production of significant amounts of phosphogypsum with considerable P2O5 losses. This will
impact significantly both investment and operating cost at the industrial scale. So, this has yet to be taken to the
semi-industrial pilot plant scale to estimate the effective cost of co-crystallization process.
2. MATERIALS AND METHODS
Experimental
Thermodynamics of the cadmium removal by co-crystallization
Uptake of Cd2+ in calcium sulfate proceeds by isomorphous substitution of Ca2+ ions [32/33]. The degree of uptake
is given by the partition coefficient D that takes into account the competition of Cd2+ and Ca2+ for the same lattice
site. D is defined as:
[Cd2+][Ca2+](solution)
⁄
[Cd2+] [Ca2+]
⁄ (Crystal) (i)
A constant D-value implies that the resulting Cd2+ concentration in the crystals can be influenced by either the
Cd2+ or the Ca2+ concentration in the solution. Thermodynamically, an expression has been derived for D:
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D = γ(Ca2+)s
γ(Cd2+)s ∗γ(Cd2+)l
γ(Ca2+)l∗Ksp(CaSO4)
Ksp(CdSO4)∗ e(-ϵel RT
⁄ )*𝑒(-∆μ(CdSO4 → CaSO4)RT
⁄ ) (ii)
D comprises the activity coefficients γ of Cd2+ and Ca2+ in the solution (i) and in the solid phase (s), the
thermodynamical solubility products of calcium and cadmium sulfate, the energy of elastic lattice deformation ϵel
and the free energy required to make a CdSO4 lattice isomorphous with the corresponding CaSO4 lattice. The
activity coefficients represent in fact the deviation from ideality, which can also be expressed as an excess free
energy (ΔG=-RTlnγ). Changes in energy affecting the activity coefficients can thus also be written as an exponent.
For pure calcium sulfate γ(Ca2+)s equals 1, while γ(Cd2+)s is related to the bonding energy of Cd2+ in the calcium
sulfate lattice. If all parameters in (ii) were known, D could be calculated. Mostly, however, the values of these
parameters are not available, which hampers the use of equation (ii) for prediction of the D-value. It is hard to
make an a priori prediction of the D-value on basis of (ii), but under certain conditions the effect of some
parameters can become visible. For instance, at constant temperature and pressure the exponential terms as well
as the solubility product remain constant. If in addition the uptake does not exceed a value of about 1%, the activity
coefficients in the solid are also unaltered. In that case the D-value only depends on the activity coefficients in the
solution and thus solely on the solution composition. Although an a priori prediction of D is as said not possible,
the observed D-values may be understood by comparing the physical and chemical properties of Cd2+ and Ca2+.
Therefore in table. 2, some characteristics of these two ions are listed. The radii of Cd2+ and Ca2+ are almost equal,
enabling replacement of Ca2+ by Cd2+ without introducing much lattice strain. It is therefore expected that the ϵel
term in equation (ii) is small.
Table 2. Comparative properties of Ca2+ and Cd2+
Properties
Ca2+ Cd2+ Unit
Ref.
Radius (8-coordination)
112
107
pm
[35]
Hydration enthalpy
-1602
-1833
KJ/mol
[36]
Hydration enthalpy sulfate salt
Pitzer constant sulfate salt
β(0)
β(1)
β(2)
Solubility of sulfate salt
18
0.20
2.65
-55.7
2.10-2
52
0.2053
2.62
-48.07
6
KJ/mol
Mol/Kg
[37]
[38]
Kinetics of the cadmium removal by co-crystallization
The incorporation of an impurity takes place at the surface. Uptake is therefore mostly influenced by surface
processes as growth [33]. The first is that the growth rate is so high that any Cd2+ or Ca2+ near the crystal surface
has an equal chance to become incorporated with equal diffusion coefficients for these two ions, The resulting D
value is equal to 1. For Cd2+ in gypsum where D is smaller than 1 this means an increase with respect to the
equilibrium situation, due to entrapment at the surface. The second situation is equilibrium without growth, but
with an exchange of ions between the solid state and the solution. Since this will only occur at a very large time
scale, equilibrium partition coefficients can only be obtained by extrapolation to growth rate zero. In practice, D
will lie between the thermodynamical value and the value determined by the diffusion rates. The kinetical aspects
have long been recognized. In many studies, efforts are made to explain the trapping of impurities as a function
of growth rate. For a proper explanation the reactions occurring during growth of the crystals and uptake should
be considered for each case separately. By comparing the respective reaction rates of, in this case, Cd2+ and Ca2+
some prediction of the kinetical behaviour of uptake may be done. The reactions that can be considered are bulk
diffusion, adsorption desorption, surface diffusion, transport along a step, bonding at the kink site, or direct
attachment from the solution. Since during these reactions dehydration occurs, the reaction rates are somehow
related to well-known processes as ligand exchange, dehydration and diffusion.
Materials & experiments
A typical co-crystallization process for cadmium removal from phosphoric acid consists on cadmium
incorporation during hemihydrate/anhydrite co-crystallization. The degree of incorporation depends on the
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supersaturation, which is affected by the sulfuric acid excess [33/34], the free sulfuric acid content and the
phosphoric acid concentration as a driving forces for co-crystallization. For that, the concentrated phosphoric acid
(50%-54% P2O5), is heated at 75°C under specific conditions in terms of free sulfate and solid content, and further
treated with phosphate rock to adjust the final free sulfate content. The free sulfuric acid content points out the
amount of sulfate ions in excess of the calcium ions, and is given as weight percentage of free H2SO4 in solution.
This percentage can thus be either positive for excess sulfate ions or negative for excess calcium ions. To improve
the original co-crystallization process for cadmium removal from industrial phosphoric acid, several semi batch
lab scale experiments were done. All semi-batch experiments were conducted typically to the process as shown
in Fig.2 below, using small reactors of one-liter capacity, filter and raw materials:
Phosphoric acid industrial grade (52 wt % to 54 wt % P2O5).
Sulfuric acid H2SO4 (96 wt %).
Phosphate rock (31 wt % P2O5).
Phosphogypsum slurry (35 wt % solids).
The experiments were based on a phosphate rock sample containing already a very low 10 ppm cadmium content,
which is well below the average for sedimentary rock that typically ranges from 23 to 203 mg Cd/kg P2O5 (see
Table 1). Basically, for each experiment, the operating conditions such as temperature, free sulfate and solid
contents during co-crystallization steps, and final free sulfate content at the free sulfate removal step, were
carefully studied and chosen to reach a high kinetics of Dihydrate/Hemihydrate/Anhydrite co-crystallization.
Finally, solids were separated by press filtration to recover the phosphoric acid with reduced cadmium
concentration. The chemical analysis of the P2O5 content was carried out by UV/VIS Spectrometry, and the
cadmium content by Inductively Coupled Plasma Spectrometry (ICP), the free sulfate was determined by titration
with standard barium chloride solution. The calcium sulfate crystallization form was characterized by X-Ray
Diffraction (XRD) and Scanning Electron Microscopy (SEM).
Methodology for the original co-crystallization process (OCC)
There are many patents concerning the cadmium removal (decadmiation) from phosphoric acid by co-
crystallization. For this experimental and improvement work we have referred to the process block diagram given
in Fig.1 below.
Fig.1. Original co-crystallization process for cadmium removal from phosphoric acid
The process concerns the cadmium removal from concentrated phosphoric acid (54 wt % P2O5) and involves
several steps:
The co-crystallization step: phosphogypsum slurry and concentrated sulfuric acid (98 wt%) were added
to the phosphoric acid to reach respectively 6 wt% of free sulfate and 6% wt of solid. The working
temperature is 70°C.
The free sulfate removal step (desulfation): the free sulfate is removed by precipitation using fine
phosphate rock within particle size distribution less than 160 µm. The working temperature is about
90°C. CaO (Phosphate) + SO42- + xH2O CaSO4, xH2O at 90°C
With x equaling 0.5 or 2 depending on working conditions (temperature and P2O5 content).
The desursaturation step: after the free sulfate removal step, the treated phosphoric is given an additional
residence time for desurtaturation in order to complete kinetics of reactions and having therefore very
lower free sulfate content.
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The solid removal: at the end of the process, the treated phosphoric acid is clarified to separate solids
from the phosphoric acid with reduced cadmium concentration. The sludge is filtered by press filter to
increase the P2O5 recovery and dispose the calcium sulfate solid.
Methodology for the improved of the OCC process
Besides the original process philosophy (Fig.1), we have investigated two new improvements of the OCC process
to explore how both P2O5 recovery and cadmium removal could be enhanced. For the first improvement, we have
particularly developed DDS mode (Separated Decadmiation Desulfation) in which the decadmiation and the
desulfation steps are separately conducted, each one has had its specific process parameters (T°C, P2O5, residence
time, sulfate and solid contents).
The second improvement developed consists on the DDC mode (Combined Decadmiation Desulfation) in which
the decadmiation and the desulfation steps are conjointly conducted during the de desulfation step. The major
improvement related to the DDS and the DDC modes in comparison to the OCC process, is that the
phosphogypsum solid which is calcium sulfate Dihydrate, is not brought as phosphogypsum slurry. The point is
typically to avoid the dilution of the CPA out by the addition of phosphogypsum slurry containing about 75 wt%
of WPA at 29 wt% P2O5. This will consequently increase the kinetics of the co-crystallization, save the heat energy
cost and reduce process design by eliminating the phosphogypsum stream (tank, agitator, pumps and piping).
3. Results and discussions
OCC process Test
The driving force for the cadmium removal by co-crystallization is the recrystallization of calcium sulfate
Dihydrate or Hemihydrate to Anhydrite. This is why in the OCC process (Fig.1 above), phosphogypsum slurry
(calcium sulfate Dihydrate) and concentrated sulfuric acid (98 wt%) were added to the ACP 54% (CPA Feed). In
that case, the CPA Feed’s free sulfate and solid contents were increased to reach 6 wt% each. The working
temperature is about 70°C. After the co-crystallization, the free sulfate is removed by precipitation at 90°C using
fine phosphate rock within particle size distribution less than 160 µm (desulfation step).
Table 3 gives the chemical composition of the CPA Feed used for the experiments. Referring to the X-Ray
Diffraction analysis in Fig.3, the CPA Feed’s sludge is mainly composed of Anhydrite (CaSO4) with small
quantities of Hemihydrate (CaSO4.1/2H2O) and Sodium fluosilicate (Na2SiF6). Fig. 2 and 4 show X-Ray
Diffraction and Scanning Electron Micrographs of phosphogypsum (CaSO4.2H2O). In the final stage of the
process, the CPA is given an additional residence time for desurtaturation in order to complete kinetics of reactions
and having therefore very lower free sulfate content. Solid is then removed by clarification from the CPA out.
The sludge is filtered by press filter to increase the P2O5 recovery.
Table 4 gives the process parameters and results according to the OCC process tests. The results show a low Cd
removal of about 31.6% (based on mgCd/kgP2O5 ratio) within a significant negative impact on the P2O5 content
in the CPA Out which has been decreased by 1.34 P2O5 points due to the dilution by the added phosphogypsum
slurry. As mentioned in Fig. 5 and 6, the CPA Out’s sludge still containing a high quantity of phosphogypsum
(calcium sulfate Dihydrate) which involves an inefficient recrystallization Dihydrate-Anhydrite during the OCC
process, and therefore a low cadmium uptake by Co-Crystallization. The presence of Anhydrite is mainly due to
the CPA Feed.
Fig.2. Scanning Electron Micrographs of phosphogypsum
Table 3. Chemical analysis of CPA Feed
Element
Wt % in WPA
P2O5
CaO
SO3
SiO2
MgO
Fe2O3
Al2O3
Na2O3
F-
K2O
C org
Cd
52,34
0.37
1.94
0.43
0.21
0.29
0.66
0.10
0.79
0.05
0.01
0.0012
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Fig.3. X Ray Diffraction of CPA Feed’s sludge
Fig.4. X Ray Diffraction of phosphogypsum.
Fig.5. X Ray Diffraction of CPA Out’s sludge (OCC process).
Fig.6. Scanning Electron Micrographs of CPA Out’s sludge (OCC process)
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Table 4. Results of CPA decadmiation through the OCC process
Specific consumption
Step
Wt% P2O5
T°C
Wt% SO42-
Wt% Solid
Cd (ppm)
Cd (mg/KgP2O5)
Sulfuric Acid
93.78g/Kg P2O5
CPA Feed
52.34
55
4.00
0.4
12
22.93
Co-crystallization
50.00
70
6.00
6.0
11
22.00
Phosphogypsum
492g/Kg P2O5
Desulfation
50.43
90
1.00
13
9
17.85
CPA Out
51.00
60
0.67
0.5
8
15.69
Phosphate Rock
157g/Kg P2O5
Cd removal
31.6%
DDS co-crystallization mode Test
In order to improve the cadmium removal and P2O5 recovery of the OCC process, we have developed the DDS
mode. The point is that the decadmiation and the desulfation steps still separately conducted as in the OCC
process, with the same process parameters (T°C, P2O5, residence time, sulfate and solid contents). The key
difference compared to the OCC process is the elimination of the phosphogypsum stream, which brings calcium
sulfate Dihydrate solid, and consider only the CFA Feed’s solid content (1.6 wt%) for cadmium removal by co-
crystallization. This improvement aims to avoid the dilution of the CPA caused by the addition of the
phosphogypsum slurry containing about 75 wt% of phosphoric acid with 29 wt% P2O5, and also to maintain a
high P2O5 content during the co-crystallization so as to improve the deshydration of the calcium sulfate Dihydrate
and then the kinetics of co-crystallization. Table 5 gives the process parameters and the obtained results according
to the DDS mode.
Fig.7. DDS mode for cadmium removal from phosphoric acid
Table 5. Results of CPA decadmiation through the DDS mode
Specific consumption
Step
Wt% P2O5
T°C
Wt% SO42-
Wt% Solid
Cd (ppm)
Cd (mg/KgP2O5)
Sulfuric Acid
103g/Kg P2O5
CPA Feed
52.34
55
4.10
1.6
12
22.93
Co-crystallization
52.00
74
6.10
2.6
10
19.23
Phosphate Rock
187g/Kg P2O5
Desulfation
52.50
90
1.00
8.0
8
15.24
CPA Out
52.90
60
0.70
0.5
8
13.23
Cd removal
42.3%
As prospected, the results point out the good effect of the DDS mode for improving the performance of cadmium
removal and P2O5 recovery relatively to the OCC process. In fact, the DDS mode allows a cadmium removal of
about 42.3% (based on mgCd/kgP2O5 ratio), which constitutes an increase of cadmium removal of about 34%
compared to the OCC process, within a positive impact on the P2O5 content in the CPA Out which has been
increased by 0.56 P2O5 point. Thus means that the elimination of the phosphogypsum slurry stream and
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considering only the CPA Feed’s solid content (1.6 wt%) has limited the dilution of the CPA Feed, and induced
a high P2O5 content during the Co-Crystallization step. Accordingly, there were good conditions for calcium
sulfate dehydration, and consequently a sufficient kinetics for Hemihydrate-Anhydrite recrystallization. As a
result, the removal by co-crystallization has been significantly enhanced.
DDC co-crystallization mode Test
Depending on the phosphate ore origin, especially the impurities content, sometimes the calcium sulfate
precipitated during phosphoric acid concentration would mainly be Anhydrite instead of Hemihydrate. As regards
our case, we have found that the CPA Feed’s solid (1.6 wt%) is mainly Anhydrite (see Fig.3 above) with a small
quantity of Hemihydrate. For that reason, we have thought that using CPA Feed free solid (0.4 wt%) could be a
good choice for increasing the CPA Feed’s P2O5 content. As a result, the dehydration of calcium sulfate
Hemihydrate during the desulfation step will be enhanced and then the cadmium removal by co-crystallization.
To confirm that, we have investigated the DDC co-crystallization mode (Combined Decadmiation Desulfation)
shown in Fig.8, in which the decadmiation and the desulfation steps are conjointly conducted during the
desulfation step (Reactor 2 in Fig.8). In fact, the DDC mode will benefit fundamentally from the Hemihydrate-
Anhydrite co-crystallization during desulfation step at the condition of the high P2O5 content of the CPA, which
ensures good kinetics for the cadmium removal by co-crystallization and consequently improve the efficiency of
the cadmium uptake from the CPA.
Table 6 gives the process parameters and results according to the DDC mode. As mentioned in the table, the
results highlight and confirm the expectations, the cadmium removal was upgraded to about 43.2% (based on
mgCd/kgP2O5 ratio) which represents an increase of cadmium removal of about 34% compared to the OCC
process, within a significant positive impact on the P2O5 content in the CPA Out which has been increased by
1.46 P2O5 points.
Fig.8. DDC mode for cadmium removal from phosphoric acid
Table 6. Results of CPA decadmiation through the DDC mode
Specific consumption
Step
Wt% P2O5
T°C
Wt% SO42-
Wt% Solid
Cd (ppm)
Cd (mg/KgP2O5)
Sulfuric Acid
95g/Kg P2O5
CPA Feed
52.34
55
4.00
0.4
12
22.93
Co-crystallization
52.20
70
6.00
1.0
9
17.24
Phosphate Rock
202g/Kg P2O5
Desulfation
53.00
90
1.00
6.0
7
13.21
CPA out
53.80
60
0.50
0.5
7
13.01
Cd removal
43.2%
4. CONCLUSION
In this experimental study, we have developed new solutions for the intensification of the industrial and original
co-crystallization process, the most studied and patented technology for the cadmium removal from phosphoric
acid. Basically, we have found out new Co-Crystallization modes (DDS & DDC) that have significantly increased
the cadmium removal and the P2O5 recovery from wet phosphoric acid, saved the heat energy cost and reduced
the process design by eliminating the phosphogypsum stream (tank, agitator, pumps and piping). In fact, the
driving force for the cadmium removal by co-crystallization is the recrystallization of calcium sulfate Dihydrate
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or Hemihydrate to Anhydrite. Accordingly, suitable conditions are required for a high calcium sulfate
deshydration, and consequently a faster kinetics for recrystallization and cadmium Co-Crystallization.
Referring to the OCC process (Original Co-Crystallization), the DDS mode (Separated Decadmiation Desulfation)
has allowed an increase of cadmium removal of about 34% within a positive impact on the P2O5 content in the
CPA Out which has been increased by 0.56 P2O5 point. The key improvement was the elimination of the
phosphogypsum slurry stream and considering only the CPA Feed’s solid content (1.6 wt%), and thus has limited
the dilution of the CPA Feed, and induced a high P2O5 content during the Co-Crystallization step. Accordingly,
there were good conditions for calcium sulfate dehydration, and consequently a sufficient kinetics for
Hemihydrate-Anhydrite recrystallization.
Concerning the DDC mode (Combined Decadmiation Desulfation), additionally to the phosphogypsum slurry
elimination, we have increased the CPA Feed’s P2O5 content by using CPA Feed free solid (0.4 wt%). Therefore,
the Hemihydrate-Anhydrite co-crystallization during desulfation step was carried out at the condition of the high
P2O5 content of the CPA, and thus has occurred faster kinetics for the cadmium removal by co-crystallization
and consequently improved the efficiency of the cadmium uptake from the CPA. As a result, compared to the
OCC process, the cadmium removal was upgraded by 37% within a considerable high positive impact on the P2O5
content in the CPA Out which has been increased by 1.46 P2O5 points. Table 7 below gives a sum up of the co-
crystallization processes investigated in this present work.
Table 7. Sum up of Cd removal from CPA: Original co-crystallization process Vs Improved modes (DDS&DDC)
Process
OCC
DDS
DDC
P2O5 in CPA Feed
52.34 wt%
52.34 wt%
52.34 wt%
Cd in CPA Feed (ppm)
(mg Cd/KgP2O5)
12 ppm
22.93
12 ppm
22.93
12 ppm
22.93
Cd in CPA out (ppm)
(mg Cd/KgP2O5)
8 ppm
15.69
7 ppm
13.23
7 ppm
13.01
Cd removal
31.6 %
42.3%
43,3%
P2O5 CPA out
51.00 wt%
52.90 wt%
53.80 wt%
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[163]
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CITE AN ARTICLE
Samrane, K., Boulif, R., Dhiba, D., & Bouhaouss, A. (2018). IMPROVEMENTS AND
INTENSIFICATION OF INDUSTRIAL CO-CRYSTALLIZATION PROCESS FOR CADMIUM
REMOVAL FROM WET PHOSPHORIC ACID. INTERNATIONAL JOURNAL OF ENGINEERING
SCIENCES & RESEARCH TECHNOLOGY, 7(10), 152-163.
