Abstract

The basic fundamentals of ferrous oxalate dihydrate (FeC2O4.2H2O) crystallization including supersaturation, nucleation and crystal growth in simulated dihydrate phosphoric acid product with and without cetyl pyridinium chloride (CPC) additive were studied. Oxalic acid and ferrous sulfate heptahydrate crystals were mixed with dilute phosphoric acid (28% P2O5) at 60 °C and the turbidity of the reaction mixture was measured at different time intervals. Induction time of ferrous oxalate dihydrate crystals was calculated at different supersaturation ratios ranging from 2.5 to 6.7. With increasing the supersaturation ratio, the induction time decreased. The nucleation rates are 46.4 × 1028 nuclei cm−3 s−1 and 50.2 × 1028 nuclei cm−3 s−1 at supersaturation ratio 6.7 with and without CPC addition, respectively. The surface energy increases with CPC addition compared to the baseline. In addition, the formed crystals are modified from cubic shape to rod-like shape with increasing CPC dose.

INTRODUCTION

Iron content in most commercial phosphate rocks is varying according to ore location around the world. Its value has a variable percentage ranging from low 0.1%–0.4% as Fe2O3 in Morocco phosphate (Becker 1989) to high 3.0%–4.0% in Egyptian phosphate (Abdel-Aal 1984, 1989; Abdel-Aal et al. 1991, 1999).

Phosphoric acid is mainly produced by the wet process (Monser et al. 1999).The wet process of phosphoric acid production involves reaction of phosphate rock with an acid (mainly sulfuric acid). This process produces a crude phosphoric acid with a variety of impurities that vary according to the origin of phosphate rocks.

Phosphoric acid is an important intermediate for many industries. It is used as raw material for the production of fertilizers, detergents, toothpaste and food products (Slack 1968; El-Asmya et al. 2008; Guirguis et al. 2008). However, the impurities content is highly related to its usage. High iron content is not preferable for all of the previous industries. Many techniques and methods were developed to remove or decrease iron content in wet-process phosphoric acid with low percentages of P2O5 losses (El-Shall et al. 1999; Hana 1999; El-Bayaa et al. 2011; Amin et al. 2014; Soltani et al. 2018). A new process for decreasing iron content from phosphoric acid using oxalic acid has been recently reported (Hilakos 2015). Within this method, precipitation of ferrous oxalate dihydrate takes place. On the other hand, many inorganic and organic additives were studied to improve gypsum crystallization in phosphoric acid production (Manar 2016; Zhua et al. 2016). Cetyl pyridinium chloride (CPC), sodium dodecyle sulfate and cetyltrimethyl ammonium bromide were studied on calcium sulfate dihydrate crystallization to increase phosphoric acid filtration rate from gypsum cake (Mahmoud et al. 2004; El-Shall et al. 2005). So, in the iron purification methods, residue of inorganic and organic additives used in phosphoric acid production should be taken into consideration. This study takes into consideration presence of CPC surfactant in phosphoric acid medium. Several additives were used for improving crystallization of gypsum in the dihydrate (DH) phosphoric acid manufacturing process. CPC is one of the additives most used. For the first time, effect of residual CPC additions in crude phosphoric acid on the crystallization of ferrous oxalate dehydrate was studied in this work. Precipitation of iron as ferrous oxalate dehydrate is a promising and applicable method for purification of crude phosphoric acid with high iron content.

This work aims to study basic fundamentals of primary nucleation and crystallization for ferrous oxalate dihydrate in phosphoric acid medium (28% P2O5) with and without residual from CPC additive.

EXPERIMENTAL

Materials

Pure chemicals of phosphoric acid H3PO4 (Scharlab, Spain), ferrous sulfate heptahydrate FeSO4.7H2O (ACROS, USA), oxalic acid dihydrate C2H2O4.2H2O (ADWIC, Egypt), 98% sulfuric acid H2SO4 (Sigma-Aldrich) and CPC (Fisher Scientific Co.) were used.

Method

A stock solution of dilute phosphoric acid of 28% P2O5 and 2% H2SO4 contents was prepared. A 100 mL of the stock solution was heated to 60 °C using a water bath. Known amounts of ferrous sulfate heptahydrate (FeSO4.7H2O) were added with stirring at 300 rpm, and then the stoichiometric equivalent amounts of oxalic acid dihydrate powder were added. These conditions simulate the filter acid produced by the DH process for phosphoric acid production. Crystal morphology of FeC2O4.2H2O with and without CPC addition has been investigated using scanning electron microscopy (SEM, S-7200, Hitachi, Japan).

Determination of induction time

Samples of the reaction mixtures were withdrawn for turbidity measurements at different time intervals using a turbidity-meter (HACH 2100A, USA). The time (minutes) versus turbidity (NTU) graph was plotted. The induction time (tind) was corresponding to the point of intersection of the two asymptotic lines on the graph (Abdel-Aal et al. 2015).

Calculation of supersaturation ratio

Ferrous oxalate dihydrate was prepared as shown in the following reaction equation:  
formula
Supersaturation ratio (S) was calculated according to the following equation (Tavare 1995):  
formula
where S: ratio of supersaturation; c: FeC2O4.2H2O concentration, %; c*: FeC2O4.2H2O (solute) solubility in dilute phosphoric acid medium under the experimental conditions = 1.08 g/100 mL, which was determined experimentally. Different supersaturation ratios of FeC2O4.2H2O were prepared ranging from 2.5 to 6.7.

RESULTS AND DISCUSSION

Effect of time on turbidity measurements

The induction time (minutes) versus turbidity (NTU) is plotted. The induction time is determined from the point of intersection of the two asymptotic lines. Turbidity values were measured at different supersaturation ratios with and without 50 mg L−1 CPC addition. The results are represented in Figures 1 and 2.

Figure 1

Effect of time on turbidity measurements of FeC2O4.2H2O nucleation at different supersaturation ratios without CPC addition.

Figure 1

Effect of time on turbidity measurements of FeC2O4.2H2O nucleation at different supersaturation ratios without CPC addition.

Figure 2

Effect of time on turbidity measurements of FeC2O4.2H2O nucleation at different supersaturation ratios with 50 mg L−1 CPC addition.

Figure 2

Effect of time on turbidity measurements of FeC2O4.2H2O nucleation at different supersaturation ratios with 50 mg L−1 CPC addition.

The induction time was determined at different values of supersaturation ratios. These results are given in Table 1. It is clear that the induction time increased with CPC addition compared to the baseline (without CPC) at all supersaturation ratios. These results indicate that CPC reduces the nucleation rate of ferrous oxalate dehydrate. The induction time is decreased with increasing the supersaturation ratios.

Table 1

Induction time of FeC2O4.2H2O at different supersaturation ratios with and without 50 mg L−1 CPC addition

Supersaturation ratio Induction time, minutes
 
Without additive With 50 mg L−1 CPC 
2.5 24 42 
3.3 8.6 12.8 
4.2 7.5 9.1 
3.9 4.1 
5.8 1.85 3.6 
6.7 0.15 2.5 
Supersaturation ratio Induction time, minutes
 
Without additive With 50 mg L−1 CPC 
2.5 24 42 
3.3 8.6 12.8 
4.2 7.5 9.1 
3.9 4.1 
5.8 1.85 3.6 
6.7 0.15 2.5 

Surface energy calculation

On the basis of the classic homogeneous nucleation theory, induction time can be related to supersaturation ratios using the correlation described elsewhere (Myerson 1993; Tavare 1995; Musmara & Prisciandaro 1999; Shanthi et al. 2014). This correlation is a fundamental parameter to understand and calculate the surface energy (interfacial tension) between the crystal and the aqueous solution.

Based on the classic homogeneous nucleation theory, a straight line with the slope (B/T3) is obtained by plotting log t(ind) versus 1/(log2S) over a range of supersaturation ratios (2.5–6.7) at constant temperature. The relation between log t(ind) versus 1/(log2S) with and without 50 mg L−1 CPC is shown in Figure 3. The calculated surface energies are 9.98 × 10−7 and 10.3 × 10−7 J m−2 without and with 50 mg L−1 CPC, respectively. Increasing the surface energy led to decreasing the nucleation rate of FeC2O4.2H2O crystals (Myerson 1993).

Figure 3

Relation between log induction time (tind) and 1/log2S with and without 50 mg L−1 CPC addition.

Figure 3

Relation between log induction time (tind) and 1/log2S with and without 50 mg L−1 CPC addition.

Nucleation rate calculation

The nucleation rate is expressed by the number of nuclei formed per unit time per volume. It can be calculated according to the following equation (Myerson 1993):  
formula
where Js is the nucleation rate and F is a frequency constant and is known as the pre-exponential factor and has a theoretical value of 1030, β is a geometric (shape) factor of 16π/3 for the spherical nucleus, ƒ(θ) is a correction factor – when purely homogeneous nucleation takes place ƒ(θ) = 1 and when heterogeneous nucleation occurs ƒ(θ) = 0.01. Vm is the molar volume (78.9 cm3 mol−1 for FeC2O4.2H2O), T is the absolute temperature (K) and R is the gas constant (J mol−1 K−1), γ is the surface energy (J m2), NA is Avogadro's number (mol−1).

The nucleation rates of FeC2O4.2H2O crystals with and without CPC additive at supersaturation ratio (2.5–6.7) were calculated and are given in Figure 4. It is clear that, with increasing the supersaturation ratio, the nucleation rate was increased with and without CPC addition, respectively. Also, addition of CPC led to decreasing nucleation rates at all the studied supersaturation ratios compared to without additive. Addition of CPC reduces the ferrous oxalate dihydrate crystallization by decreasing the nucleation rate. At supersaturation ratio of 6.7, the nucleation rates are 46.4 × 1028 nuclei cm−3 s−1 and 50.2 × 1028 nuclei cm−3 s−1 with and without CPC addition, respectively.

Figure 4

Correlation between the supersaturation ratio and the nucleation rate (Js) with and without 50 mg L−1 CPC addition.

Figure 4

Correlation between the supersaturation ratio and the nucleation rate (Js) with and without 50 mg L−1 CPC addition.

Calculation of free energy change and critical nucleus radius

The free energy change (ΔGcr) calculation for the formation of critical nucleus sizes are obtained from the Arrhenius type equation described elsewhere (He et al. 1994). Lancia et al. (1999) evaluate the radius of critical nucleus (r) from a relation between ΔGcr and γ.

Based on the classical homogenous nucleation theory, Gibbs free energy is estimated as the free energy barrier to nucleation rate that was surface free energy and volume free energy. The interfacial free energy is the difference between the free energy per molecule of the bulk and that of the surface (Shanthi et al. 2014).

The obtained results of ΔGcr for the formation of critical nucleus size are given in Table 2. The Gibbs free energy change needed for the formation of critical nucleus size was decreased with increasing the supersaturation ratio. However, it was increased with the addition of 50 mg L−1 CPC compared to the base line (without addition).

Table 2

Effect of different supersaturation ratios of FeC2O4.2H2O on free energy change for formation of critical nucleus size

Supersaturation ratio Free energy change for formation of critical nucleus size ΔGcr × 10−21, J
 
Without additive With 50 mg L−1 CPC 
2.5 14.4 16.03 
3.3 8.3 9.28 
4.2 5.9 6.61 
5.0 4.7 5.19 
5.8 3.9 4.32 
6.7 3.4 3.74 
Supersaturation ratio Free energy change for formation of critical nucleus size ΔGcr × 10−21, J
 
Without additive With 50 mg L−1 CPC 
2.5 14.4 16.03 
3.3 8.3 9.28 
4.2 5.9 6.61 
5.0 4.7 5.19 
5.8 3.9 4.32 
6.7 3.4 3.74 

The results of calculation of the radius of the critical nucleus (r) at the studied supersaturation ratios with and without 50 mg L−1 of CPC additive are given in Figure 5. It is clear that the radius of the critical nucleus required for the formation of the stable nucleus decreased with increasing supersaturation ratio. On the other hand, it was increased with the addition of 50 mg L−1 of CPC surfactant compared to without additive at all the studied supersaturation ratios.

Figure 5

Effect of supersaturation ratio on the radius of nucleus (r, cm) with and without 50 mg L−1 CPC addition.

Figure 5

Effect of supersaturation ratio on the radius of nucleus (r, cm) with and without 50 mg L−1 CPC addition.

SEM photomicrographs of FeC2O4.2H2O crystals

The precipitated ferrous oxalate dihydrate crystals with and without different doses of CPC at supersaturation ratio 4.2 were investigated using SEM. The obtained photomicrographs are given in Figure 6. The FeC2O4.2H2O crystals' shape is cubic without CPC addition, while at high CPC concentration of 100 mg L−1, the shape of crystals is modified to rod-like. This may be attributed to adsorption of CPC on the anionic active sites leading to inhibition of the crystal growth.

Figure 6

SEM of FeC2O4.2H2O crystals at supersaturation ratio 4.2 with magnification ×4,000 at different concentration of CPC (0, 1, 50 and 100 mg L−1 CPC for (a), (b), (c) and (d) respectively).

Figure 6

SEM of FeC2O4.2H2O crystals at supersaturation ratio 4.2 with magnification ×4,000 at different concentration of CPC (0, 1, 50 and 100 mg L−1 CPC for (a), (b), (c) and (d) respectively).

CONCLUSION

Nucleation and crystallization of ferrous oxalate dihydrate crystal, FeC2O4.2H2O, in phosphoric acid medium with and without CPC additive were studied. Addition of 50 mg L−1 of CPC increases the induction times at all supersaturation ratios due to a decrease in the nucleation rate. The critical nucleus diameter is larger with addition of 50 mg L−1 of CPC. At high CPC concentration of 100 mg L−1, the FeC2O4.2H2O crystals' shape is modified to rod-like shape. The obtained basic data will be helpful for decreasing iron content for the acid produced by the dihydrate wet process as a cost-effective method after extraction and recycling the precipitating reagent (oxalic acid).

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