phosphoric acid in kerosene

Chemical Engineering and Processing 49 (2010) 159–164

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Removal of iron from Cr-electroplating solution by extraction with di(2-ethylhexyl)phosphoric acid in kerosene Y.A. El-Nadi ∗ , N.E. El-Hefny Hot Laboratories Centre, Atomic Energy Authority, 13759, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 29 May 2009 Received in revised form 12 December 2009 Accepted 5 January 2010 Available online 11 January 2010 Keywords: Extraction Iron DEHPA Chromium Electroplating

a b s t r a c t The extraction of iron(III) from aqueous sulphate solution was studied using di(2-ethylhexyl)phosphoric acid (HA) mixed with kerosene. Distribution ratios were investigated as a function of the concentration of sulphuric acid, extractant, metal, hydrogen ion as well as the phase ratio and loading capacity of the extractant. The extracted iron species was suggested as [Fe(HA2 )3 ] and the extraction constant was found to equal (9.1 ± 0.5) × 102 . The thermodynamic functions calculated from the temperature dependence data referred to the endothermic nature of the extraction process. The method of extraction was successfully applied to remove the iron from the chromium electroplating solution giving purification percent of about 97.5%. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA or simply HA) is widely used in hydrometallurgical processes for the separation and purification of a number of metals [1,2]. Its importance as a solvent extraction reagent is very well recognized as reported in detailed studies of the extraction of the majority of the first row transition elements, notably vanadium [3], manganese [4], iron [5,6], cobalt [7–10], nickel [7–9] and copper [7–11]. The greater resistance to hydrolysis and lower aqueous solubility make DEHPA an excellent reagent for solvent extraction [12–14]. Iron is a very common impurity in raw industrial acid metal solutions and, therefore, it is of importance to know its properties in a given extraction system. The extraction of Fe(III) with DEHPA from different acid solutions was studied in detail [6,15–19]. Thus, its extraction from perchlorate medium by DEHPA was investigated from the kinetic viewpoint by several authors [6,15,16]. Similarly, Sato et al. [20,21] reported similar extraction investigations from sulphate, nitrate and chloride solutions, while MeleSˇ ˇ and ProStenik [22] used phosphoric acid medium to study the extraction system. The extraction of acid and iron values from sulphate waste pickle liquor of steel industry was investigated [23]. It was found that, after extraction of acid, iron values in the raf-

∗ Corresponding author. Fax: +20 2 44620784. E-mail address: yael [email protected] (Y.A. El-Nadi). 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.01.004

finate were extracted with a binary solvent mixture consisting of methylisobutyl ketone (MIBK) and DEHPA which shows a synergistic effect on iron extraction. Principe and Demopoulos [24] studied the removal of iron(III) from strong zinc sulphate – sulphuric acid solutions using octylphenyl acid phosphate (OPAP) and DEHPA. At 50 ◦ C, DEHPA yielded a comparable iron loading capacity as that of OPAP at 20 ◦ C. Singh et al. [25] developed a process employing oxalic acid to separate iron from the organic phase composed of DEHPA and tri-n-butylphosphate (TBP) obtained during the processing of phosphoric acid. The stripping of iron was found to follow first order kinetics with an activation energy of 77 kJ mol−1 . On the other hand, Agrawal et al. [26] studied the extractive removal of Cr(VI) from chloride solutions using Cyanex 923 mixed with kerosene. Under the optimum experimental conditions 98.6–99.9% of Cr(VI) was extracted in 3–5 min at O/A of 2 with initial feed concentration of 1 g/L of Cr(VI) and experiments were performed with the synthetic effluent from an electroplating industry. A simple extractive separation method was developed [27] for the determination of chromium(VI) based on its extraction as ionpair with tribenzylamine (TBA). The extracted chromium could be stripped to the aqueous phase using NaOH as the stripping agent and the validity of the method was checked in real tannery effluent, electroplating waste water and spiked water samples. Silva et al. [28] reported a study on the extraction and recovery of chromium from the wastes generated by a galvanic manufacturer. Commercial HCl at room temperature was employed, and the conditions of the extraction process were optimized according to a sequential experimental design. Sequential designs were used to find satisfactory

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Y.A. El-Nadi, N.E. El-Hefny / Chemical Engineering and Processing 49 (2010) 159–164

experimental conditions for chromium recovery from electroplating sludge giving a recovery of 97.6%. In a previous study [29], the extraction of chromium(VI) by Aliquat-336 and Alamine-336 from acidic sulphate medium was compared to that from alkaline carbonate medium. The results indicated that the extraction of Cr(VI) from acidic medium is more favoured than from alkaline medium and the extraction method was applied for the removal and recovery of Cr(VI) from electroplating waste solution. After passing a time, the iron accumulated in the solution affecting the efficiency of the electroplating process. So, it is necessary to find a method to get rid of the iron without affecting the chromium concentration. Therefore, the present work is directed to investigate in detail the iron extraction isotherm from sulphate medium using DEHPA to reach to the optimum conditions for the removal and recovery of iron from the chromium electroplating solution and purifying it. 2. Experimental Potassium dichromate (K2 Cr2 O7 ) analytical reagent grade (AR) was supplied by Fluka, analytical pure sulphuric acid and DEHPA were purchased from Merck, while ferric sulphate (Fe2 (SO4 )3 ·9H2 O) was a product of BDH. As diluent of the organic phase kerosene (non-aromatic) was supplied by Misr Petrol Ltd., Egypt. Iron samples in Cr-electroplating baths were obtained from Helwan Engineering Industrial Company, Cairo, Egypt. They were composed of 12.5 g/L iron(III) and 95.4 g/L chromium(VI) in 0.1 M sulphuric acid [30]. The stock solutions of iron(III) were prepared by dissolving a certain weight of ferric sulphate in 0.1 M H2 SO4 solution. The concentrations of iron(III) were determined using thiocyanate method [31] while chromium(VI) concentrations were measured by its own colour spectrophotometrically. The results show that Beer’s law was obeyed for both cases under the experimental conditions used. The effect of change in hydrogen or sulphate ions concentrations on the extraction was performed by preparing different solutions containing different H+ at constant SO4 2− and vice versa. The extraction procedure was carried out by vigorously shaking equal volumes of the two phases in stoppered glass tubes using a thermostated water bath shaker adjusted at 25 ± 1 ◦ C. The equilibrium was achieved within 40 min and the metal concentration in the organic phase was determined by the difference in its concentration in the aqueous phase before and after extraction. The distribution ratio (D) was calculated as the ratio of the Cr(VI) concentration in the organic phase to that in the aqueous phase.

Fig. 1. Effect of DEHPA concentration on the extraction of iron(III) (1 g/L) from 0.1 M H2 SO4 medium at phase ratio = 1 and 25 ◦ C.

solution was studied at organic/aqueous phase ratio = 1. The distribution ratio was found to increase linearly with increasing the DEHPA concentration, Fig. 1, as a log–log relation. The resulted slope indicates the participation of three DEHPA molecules in the extracted species. 3.1.2. Effect of H2 SO4 The extraction of Fe(III) (1 g/L) from different sulphuric acid concentrations (0.01–1 M) by 0.05 M DEHPA in kerosene was studied at phase ratio = 1. The results illustrated in Fig. 2 show that the distribution ratio decreases with the aqueous acidity which is in agreement with the behaviour reported before [21,33]. In addition, since Fe(III) in 0.1 M H2 SO4 solution gives high extraction, the experiments were carried out using this concentration particularly it is within the range found in the real electroplating solution. 3.1.3. Effect of hydrogen ion concentration Different iron(III) solutions each containing 1 g/L and consisting of various H+ concentrations in the range of 0.01–0.1 M at constant 0.1 M [SO4 2− ] were prepared and shaken with 0.05 M DEHPA in kerosene at phase ratio = 1. As observed in Fig. 3, the log–log relation between the distribution ratio and the equilibrium hydrogen ion concentration give a negative slope nearly equals 3. This indicates the release of three hydrogen ions through the extraction process.

3. Results and discussion 3.1. Extraction investigation Preliminary experiments were performed to test the extraction of iron(III) from H2 SO4 solution using different extractants. DEHPA extractant was chosen as it gives high extraction percent towards iron(III) from 0.1 M H2 SO4 (pH 0.32) with negligible extraction of chromium(VI) which represents the major constituent of the electroplating solution. This is in agreement with that previously obtained and reported [32] for the extraction of chromium from aqueous solution (pH 2–4) using kerosene containing organophosphorus extractant (TOPO). The various parameters affecting the extraction of iron(III) from sulphate medium by DEHPA extractant were thus studied in detail as follows. 3.1.1. Effect of DEHPA The effect of change in DEHPA concentration in the range of 0.01–0.1 M on the extraction of 1 g/L Fe(III) from 0.1 M H2 SO4

Fig. 2. Effect of sulphuric acid concentration on the extraction of iron(III) (1 g/L) by 0.05 M DEHPA in kerosene at phase ratio = 1 and 25 ◦ C.


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Fig. 3. Effect of hydrogen ion concentration on the extraction of iron(III) (1 g/L) from H2 SO4 medium by 0.05 M DEHPA in kerosene at phase ratio = 1 and 25 ◦ C.

Fig. 5. Relation between the iron(III) transferred to the organic DEHPA phase to that remained in the aqueous sulphate solution at phase ratio = 1 and 25 ◦ C.

3.1.4. Effect of sulphate concentration At fixed 0.1 M hydrogen ion concentration, the extraction of Fe(III) solutions composed of 1 g/L and dissolved in different sulphate concentrations in the range of 0.1–1 M was almost not changed when shaken with 0.05 M DEHPA in kerosene solutions at phase ratio = 1. This means that the sulphate ion is not participated in the formation of extracted iron species within the concentration range investigated.

3.1.7. Extractant loading capacity The loading capacity of DEHPA extractant was determined for the extraction of Fe(III) from sulphuric acid solution by shaking equal volumes of the organic and aqueous phases, separating the two phases and determining the amount of iron transferred to the organic solution. The remained aqueous phase was discarded and a fresh aqueous solution was used for another round of extraction (cross-current extraction). The steps were repeated until the extractant becomes unable to extract more amounts of the metal ions. As appear from Fig. 6, this was reached after five extraction steps using 0.05 M DEHPA and the quantity of iron accumulated in the organic phase was found to equal about 0.94 g/L (0.016 M). This means that the mole ratio of the extractant in the organic phase to the total iron extracted is about 3:1 which is in consistent with the data obtained from the slope analysis method used in Figs. 1 and 3.

3.1.5. Effect of phase ratio In order to study the effect of organic/aqueous phase ratio (O/A) on the extraction of Fe(III) (1 g/L) from 0.1 H2 SO4 medium by 0.05 M DEHPA in kerosene at 25 ◦ C, different volumes of the two phases were used covering the O/A range of 1:5 to 5:1. Fig. 4 shows that the extraction percent of iron increases with the increase in the phase ratio till O/A = 2 then remains constant.

3.2. Extraction equilibrium 3.1.6. Effect of metal ion concentration The effect of iron(III) concentration on its extraction from 0.1 M sulphuric acid medium using 0.05 M DEHPA at 1:1 phase ratio was investigated. The amount of the iron transferred to the organic phase (Corg ) was found to increase with that remained in the aqueous phase (Caq ) as shown in Fig. 5. The maximum value was reached when the initial concentration is about 7 g Fe(III)/L.

In contrary with that mentioned by previous workers [18,22,34] who suggested hydrolyzed iron species (FeOH2+ ) and based on the slope values obtained in Figs. 1 and 3, the iron(III) species in acid medium can be considered as Fe3+ . This is also supported by the diagram shown in Fig. 7 which is produced by MEDUSA program [35]. In this figure at pH = 0.32 (0.1 M H2 SO4 ), it is clear that Fe3+

Fig. 4. Effect of organic/aqueous phase ratio on the extraction of iron(III) (1 g/L) from 0.1 M H2 SO4 medium by 0.05 M DEHPA in kerosene at 25 ◦ C.

Fig. 6. Change of the accumulated iron in the organic phase with the stage number of extraction by 0.05 M DEHPA in kerosene at phase ratio = 1 and 25 ◦ C.

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Fig. 7. Iron(III) species at different pH values.

represents more than 92% of the total iron species while the presence of FeOH2+ does not exceed 3%. On the other hand, Baker and Baes [36] showed that DEHPA is a dimer (HA)2 in n-octane and this is almost the same in a hydrocarbon kerosene diluent. Therefore, the extraction equilibrium can be represented as Fe3+ + 3[(HA)2 ] [Fe(HA2 )3 ] + 3H+

(1)

+ 3

[Fe(HA2 )3 ][H ] [Fe3+ ][(HA)2 ]

Kex =

D[H+ ]

(2)

3

(3)

3

The average extraction constant was thus calculated at various concentrations of hydrogen ion and extractant and found to equal to (9.1 ± 0.5) × 102 . Although, the extraction trend in this work is consistent with that reported by Sato et al. [21], the extraction percent of iron(III) gave higher value at the same sulphuric acid concentration. Furthermore, the extraction of iron at 25 ◦ C in the present work was better than that obtained by Principe and Demopoulos [24] who performed the extraction at 50 ◦ C using much higher concentrations of DEHPA. Moreover, this work covered the lack of detailed studying of the effect of temperature on the extraction of iron(III) that was not reported in Refs. [21,23,24] and calculating the thermodynamic functions as shown in the subsequent section. 3.3. Temperature effect The increase of temperature in the range of 15–45 ◦ C was found to increase the extraction of Fe(III) (1 g/L) when extracted by 0.05 M DEHPA in kerosene from 0.1 M sulphuric acid medium at phase ratio = 1. The Van’t Hoff equation [37] used to calculate the enthalpy change (H) and the entropy change (S) associated with the extraction of this metal is ln Kex =

S H 1 R

(5)

of the value −16.9 ± 0.2 kJ mol−1 gives an idea about the spontaneous nature of the extraction process. 3.4. Stripping investigation

3

[(HA)2 ]

the entropy change is found to equal to 147 ± 4 J mol−1 K−1 indicating the increase in the disorder of the system upon extraction. On the other hand, the free energy change (G) calculated from the following equation G = −RT ln Kex

where the overbars refer to the species in the organic phase. So, the extraction constant is given by Kex =

Fig. 8. Effect of temperature on the extraction constant of iron(III) extracted from 0.1 M H2 SO4 medium by 0.05 M DEHPA in kerosene at phase ratio = 1.

−

R

T

(4)

where R is the universal gas constant (8.314 J mol−1 K−1 ). The plot of ln Kex vs. 1/T for the extraction of the metal gave straight line, Fig. 8. The positive value of the enthalpy change (26.8 ± 1.2 kJ mol−1 ) calculated from the slope of the linear relation refers to the endothermic nature of the extraction process. Furthermore, using the intercept value generated from the above figure,

After loading of the DEHPA extractant with iron(III), different reagents were tested for one stage stripping of the iron from the organic phase. These stripping agents include mineral acids, ammonium chloride, ammonium nitrate, sodium hydroxide, ammonia and water. As observed in Table 1, the acids generally give higher results compared to the other reagents and the detailed study of stripping using sulphuric acid shows that 4 M H2 SO4 is quite efficient reagent to recover the iron from the organic solution.

Table 1 Stripping of iron(III) from loaded organic phase after extraction with 0.05 M DEHPA using different stripping agents at phase ratio = 1 and 25 ◦ C. Stripping agent

Stripping percent (%S)

H2 O

11.1

HCl 0.1 M 1M

3.1 31.3

HNO3 0.1 M 1M

3.8 12.8

H2 SO4 0.1 M 0.5 M 1M 2M 3M 4M 6M

5.3 13.9 37.2 44.6 63.7 88.9 91.2

NH4 Cl, 1 M NH4 NO3 , 1 M NH3 , 1 M NaOH, 1 M

9.4 9.4 Turbidity formed Turbidity formed


163

Fig. 9. McCabe–Thiele diagram for the extraction of iron(III) from 0.1 M H2 SO4 medium by 0.05 M DEHPA in kerosene at phase ratio = 2 and 25 ◦ C.

3.5. Application to electroplating solution A synthetic solution containing 1 g/L iron(III) and 8 g/L chromium(VI) (the same ratio in the real solution) in 0.1 M sulphuric acid was prepared and shaken with 0.05 M DEHPA in kerosene to find out the effect of the existence of chromium on the extraction of iron. It was found that the extraction percent of iron slightly decreased from 80% to 74.5% while the chromium extraction was only 2.6%. Moreover, the stripping of iron from DEHPA phase by 4 M H2 SO4 solution was not accompanied by the presence of chromium ions. To extend the extraction and separation method for application on the real solution resulting from chromium electroplating unit and to predict the number of the theoretical stages required for iron extraction, a McCabe–Thiele diagram was constructed [38] for O/A ratio = 2, Fig. 9. From this figure, it is clear that complete extraction of Fe(III) with 0.05 M DEHPA is reached after five extraction stages which is in agreement with that obtained before (Section 3.1.7). On the other hand, the bench scale experiments on the stripping of iron from the loaded organic solution with 4 M H2 SO4 showed efficient and complete stripping after three stages. Therefore, as shown in Fig. 10, the real electroplating solution containing 12.5 g/L iron(III) and 95.4 g/L chromium(VI) in 0.1 M sulphuric acid [27] was diluted to 1 + 1 with the same concentration of the acid, shaken with 0.05 M DEHPA in kerosene at phase ratio = 2 for 40 min. After separating the phases, the extraction step was repeated five times, then the total organic phase containing iron(III) was stripped with 4 M H2 SO4 solution in three steps process according to the data summarized in Table 2. Finally, iron(III) in the aqueous phase could be precipitated by addition of NaOH and increase the pH > 3 for complete precipita-

Table 2 Extraction and stripping data for the recovery of iron(III) from real Cr-electroplating solution using DEHPA in kerosene at phase ratio = 2 and 25 ◦ C. Step No.

Process Extraction

Stripping

%E

g/L

%S

g/L

1 2 3 4 5

40.6 57.0 69.6 82.1 86.5

2.54 2.12 1.11 0.40 0.08

72.8 88.5 95.0

4.55 1.70 0.20 0.01

Total

Extracted

6.25

Remained

0.01

Fig. 10. A schematic diagram represents the removal of iron from Cr-electroplating solution by extraction with DEHPA in kerosene and stripping with sulphuric acid.

tion. As a result, the iron was almost completely removed from the initial aqueous electroplating solution containing chromium giving purification percent not less than 97.5%. The organic phase containing the small amount of extracted Cr(VI) could be treated with ammonium carbonate solution to strip chromium [39], washed with dil. H2 SO4 and water to remove the residual of NaOH or (NH4 )2 CO3 and reused. 4. Conclusion The extraction of iron(III) from sulphate solution using DEHPA in kerosene was investigated. The extraction was found to be accompanied with participation of three molecules of the extractant and release of three hydrogen ions. The extraction percent of iron(III) in the present work gave higher values compared to that previously obtained whether carried out at the same sulphuric acid concentration [21] or performed at 50 ◦ C using the same extractant [24]. Besides, this work studied in detail the effect of temperature on the extraction of iron(III) that was not reported in Refs. [21,23,24]. In this concern, increasing the temperature enhanced the extraction indicating the endothermic nature of the process (H = 26.8 kJ mol−1 ). The free energy (G) and the entropy change (S) associated to the extraction process were found to equal −16.9 kJ mol−1 and 147 J mol−1 K−1 , respectively. Stripping was efficiently performed by using of 4 M H2 SO4 solution. The method of extraction and stripping was applied to the removal of iron(III) from chromium(VI) electroplating solution. Quantitative extraction of iron(III) was achieved with DEHPA at a phase ratio = 2 in five cross-current stages. The organic phase was stripped with H2 SO4 solution and the iron was precipitated using

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