Thesis M Tech_DP_[R8] (1)

7/23/2019 Thesis M Tech_DP_[R8] (1)

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Study on Recovery of Nitric Acid from Aqueous

Acidic Solution in Presence of Nitrate Ions

By

Sandeep Kumar Jaiswal

(ENGG1A201201040)

Bhabha Atomic Research Centre, Mumbai

A thesis submitted to the

Board of Studies in Engineering Sciences

In partial fulfilment of requirements

for the Degree of

MASTER OF TECHNOLOGY

of

HOMI BHABHA NATIONAL INSTITUTE

October, 2014

aa


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Homi Bhabha National InstituteRecommendations of the Thesis Examining Committee

As members of the Thesis Examining Committee, we recommend that the thesis prepared by

Shri SANDEEP KUMAR JAISWAL entitled Study on Recovery of Nitric Acid from

Aqueous Acidic Solution in Presence of Nitrate Ions be accepted as fulfilling the thesis

requirement for the Degree of Master of Technology.

Name Signature

Member 1 Dr. A. K. Nayak

Member 2 Dr. S. Mukhopadhyay

Member 3 Dr. R. C. Bindal

Examiner Dr. G. Sugilal

Guide / Convener Dr. D. Mandal

Chairman Dr. P. K. Tiwari

Final approval and acceptance of this thesis is contingent upon the candidates submission of

the final copies of the thesis to HBNI.

I hereby certify that I have read this thesis prepared under my direction and recommend that

it may be accepted as fulfilling the thesis requirement.

Date:9thOctober 2014 Dr D. Mandal

Place: Mumbai (Guide)


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Declaration

I, hereby declare that the investigation presented in the thesis has been carried out by me. The

work is original and has not been submitted earlier as a whole or in part for a degree / diploma at

this or any other Institution / University.

(SANDEEP KUMAR JAISWAL)


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DEDICATION

To

My loving parents


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Acknowledgements

It gives me immense pleasure and satisfaction to express my sincere and the deepest gratitude to

my guide Dr D. Mandal, Head, Materials Section, Chemical Engineering Division, Bhabha

Atomic Research Centre, Trombay, Mumbai-85 for his valuable guidance and advice throughout

this project work.

I am expressing my deep gratitude to Shri R. V. R. L Visweswara Rao who has inspired and

encouraged me to start my M.Tech project with his invaluable suggestions.

I also express my deep sense of gratitude to Shri C. V. R. Sharma; Chief Engineer; NFC Kota

Project, Shri P. A. Pratap; Project Director, NFC Kota Project, Shri P. B. Ojha; Technical Adviser

NFC Kota project, Shri T. K. Sinha, Scientific Officer-G and Shri Satyadeep, Scientific Officer-

C, NFC Hyderabad for their supports during this project work.

I would like to acknowledge all the staff members of the Analytical Section of Control

Laboratory, NFC Hyderabad for providing all the analytical supports required for this project

work.

Date: 9thOctober 2014

Place: Mumbai (SANDEEP KUMAR JAISWAL)


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Synopsis

Nitric acid is a commonly used dissolving agent in nuclear, chemical and metallurgical

industries and all these industries generate and discharge a huge quantity of effluent containing

free nitric acid and different metal nitrates. Disposal of effluents containing high nitrate

concentration is a serious and global problem. Nitrate contamination in drinking water causes

methemoglobinema, a disease in which oxygen-bearing capacity of blood is reduced, which is

called Blue-Baby-Syndrome. It is possible to recover and reuse nitric acid from effluents

containing free nitric acid. Experiments were carried out to recover nitric acid from aqueous

solution containing nitric acid in absence and presence of different metal ions viz., Na+, Mg+2,

Ca+2 and Al+3 by solvent extraction. In the present work, Tri-butyl phosphate (TBP) was

selected among several extractant because of its better selectivity towards nitric acid (HNO 3),

overall superiority in operation, favourable physical properties and economics.

Experiments were conducted to study the recovery of HNO3from aqueous solution in absence

and presence of different metal nitrates viz.,NaNO3, Ca(NO3)2, Mg(NO3)2 and Al(NO3)3

and of different nitrate concentrations (10-30 g/l) by using TBP (diluted with kerosene) as

solvent. All the experiments were carried out at room temperature. Mixing time for all the

extraction experiments were one minute and for the stripping experiments it was two minutes.

It was found that nitric acid can be extracted from effluent containing free nitric acid by using

dilute TBP (in kerosene) solution of organic to aqueous phase (O/A) ratio of 1:1. The

presence of various metal nitrates viz., NaNO3, Ca(NO3)2, Mg(NO3)2 andAl(NO3)3 of

different concentrations enhance the degree of extraction of nitric acid from aqueous nitric acid

solution. It was also found that the percentage of extraction of nitric acid in presence of

Al(NO3)3was higher than that of other metal nitrates. The extracted nitric acid was stripped


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out by using demineralized water with O/A ratio of 1:1. It was found that 69 to 76% of nitric

acid can be recovered in a single stage extraction process. The percentage extraction of nitric

acid can be increased by using higher concentration of TBP, as the concentration of TBP

increases; the percentage extraction of nitric acid increases. By recovering nitric acid form

effluent containing free nitric acid the total nitrate concentration in the effluent can be reduced,

which in turn can solve the disposal of nitrate bearing effluents. Moreover, the recovered nitric

acid can be reused, which in turn reduce the processing cost.


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Contents

Synopsis .................................................................................................................................................. vi

List of Figures ......................................................................................................................................... xi

List of Tables ........................................................................................................................................ xvi

Nomenclature ...................................................................................................................................... xxiv

Chapter 1 .................................................................................................................................................1

Introduction...............................................................................................................................................1

1.1 Background ................................................................................................................................1

1.2 Objectives ...................................................................................................................................5

1.3

Outline of the Thesis ..................................................................................................................6

Chapter 2 .................................................................................................................................................7

Literature Review ......................................................................................................................................7

2.1

Factors Affecting Extraction Process .......................................................................................11

2.0.1 Effect of Mixing Time ............................................................................................................11

2.0.2 Effect of Nitric Acid Concentration ..................................................................................12

2.0.3

Effect of TBP Concentration on HNO3Extraction ...........................................................13

2.2

Equilibrium Constant for the System TBP-Water-Nitric Acid ................................................18

2.3 Solubility of TBP in Water and HNO3 Solution ......................................................................19

2.4

Studies on Third Phase Behaviour for TBP-Nitric Acid System .............................................22

2.5

Thermodynamics of Extraction of Nitric Acid by Using TBP Solution ..................................25

2.6

Effect of Inextractable Nitrate on Solubility of TBP in Water ................................................29


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Chapter 3 ...............................................................................................................................................32

Experimental Study .................................................................................................................................32

3.1

Brief Process Description .........................................................................................................32

3.2 Methodology ............................................................................................................................32

3.3 Experimental Setup ..................................................................................................................33

3.4

Chemicals & Reagents .............................................................................................................34

3.5

Experiments ..............................................................................................................................35

3.6 Experimental Procedure ...........................................................................................................35

Chapter 4 ...............................................................................................................................................39

Results and Discussions ..........................................................................................................................39

4.1 First Set of Experiments in Absence of Metal Nitrates ............................................................39

4.2

Second Set of Experiments in Presence of 20 g/l Sodium Nitrate ...........................................44

4.3

Third Set of Experiments in Presence of 20 g/l Calcium Nitrate .............................................48

4.4 Forth Set of Experiments in Presence of 20 g/l Magnesium Nitrate ........................................52

4.5 Fifth Set of Experiments in Presence of 20 g/l Aluminium Nitrate .........................................56

4.6

Effect of Nitrate Concentration on the Extraction of Nitric Acid ............................................62

4.7

Nitric Acid Extraction in Presence of 10 g/l NaNO3by TBP Diluted with Kerosene .............64

4.8 Nitric Acid Extraction in Presence of 10 g/l Ca(NO3)2by TBP Diluted with Kerosene .........69

4.9

Nitric Acid Extraction in Presence of 10 g/l Mg(NO3)2by TBP Diluted with Kerosene ........73

4.10

Nitric Acid Extraction in Presence of 10 g/l Al(NO3)3by TBP Diluted with Kerosene..........77

4.11 Nitric Acid Extraction in Presence of 30 g/l NaNO3by TBP Diluted with Kerosene .............80


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4.12 Nitric Acid Extraction in Presence of 30 g/l Ca(NO3)2by TBP Diluted with Kerosene .........84

4.13 Nitric Acid Extraction in Presence of 30 g/l Mg(NO3)2by TBP Diluted with Kerosene ........89

4.14

Nitric Acid Extraction in Presence of 30 g/l Al(NO3)3by TBP Diluted with Kerosene..........93

4.15 Effect of TBP Concentration on the Extraction of Nitric Acid ..............................................100

4.16 Stripping of Nitric Acid from Extract ....................................................................................102

Chapter 5 .............................................................................................................................................104

Conclusions and future work ................................................................................................................104

5.1 Conclusions ............................................................................................................................104

5.2

Future Work ...........................................................................................................................106

References .............................................................................................................................................108


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List of Figures

Fig. 2.1 Effect of extractant type on the extraction efficiency of HNO3 and HAc (O: A =1,

single-contact 5 min, 22C, undiluted extractant, aqueous phase: 279 g/l HAc and 513 g/l

HNO3) [14]. ...............................................................................................................................9

Fig. 2.2 Variation of HNO,with time. TBP = 0.363 M, , 1.13 M, O/A= 1, T = 25 C

[30]. .........................................................................................................................................12

Fig. 2.3 Variation of HNO,with , . TBP= 0.363 M, O/A= 1,T= 25C [30]. ......................13

Fig. 2.4 Variation of ,with , , = 1.125 M. , =3.15 M, , = 6.35

M, O/A= 1 and T= 25 C [30]..................................................................................................14

Fig.2.5 Variation of log {HNO,/ .} with logTBP. , =1.125 M.

, =3.15 M, O/A= 1, T= 25 C [30]. ..............................................................................16

Fig.2.6 Effect of extractant concentration on the extraction efficiency of HNO3 and HAc

(O:A=1, single-stage contact 5 min, 22C, aqueous phase: 279 g/l HAc and 513 g/l

HNO3) [14]. .............................................................................................................................18

Fig.2.7 Variation of organic phase conductivity as the function of concentration of feed nitric

acid and TBP [42]. ...................................................................................................................24

Fig. 3.1 A snapshot of the experimental setup. .....................................................................................34

Fig.4.1 The extraction of nitric acid in the range of (14M) by various concentration of tri-butyl

phosphate/kerosene (30 - 80%) at an organic to aqueous phase ratio (O/A = 1) and at

25C was determined in the absence of nitrate ions ................................................................41

Fig.4.2 Variation of nitric acid concentration in organic phase with TBP concentration (30 -80%)

in the absence of nitrates ion. ..................................................................................................42


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Fig.4.3 The variation of the nitric acid concentration in the extract with the initial concentration

of nitric acid in aqueous phase; which was varied from 14 M. 1.095 M (30 volume %)

TBP diluted with kerosene was used for the extraction with O/A= 1 and extractions were

carried out at 25C. ..................................................................................................................43


phosphate/kerosene (30 - 80%) at an organic to aqueous phase ratio (O/A= 1), and at

25C was determined in presence of 20 g/l NaNO3. ................................................................46

Fig.4.5 Variation of nitric acid concentration in organic phase with TBP concentration in

presence of 20 g/l sodium nitrates. ..........................................................................................47

Fig.4.6 The extraction of nitric acid in the range of (14 M) by various concentration of tri-butyl


25C was determined in presence of 20 g/l Ca(NO3)2. ............................................................50


presence of 20 g/l calcium nitrates. .........................................................................................51

Fig.4.8 The extraction of nitric acid in the range of (14 M) by various concentration of tri-butyl


25C was determined in presence of 20 g/l Mg(NO3)2. ...........................................................55


presence of 20 g/lMagnesiumnitrates. .....................................................................................55

Fig.4.10 The extraction of nitric acid in the range of (14 M) by various concentration of tri-

butyl phosphate/kerosene (30 - 80%) at an organic to aqueous phase ratio (O/A= 1), and

at 25C was determined in presence of 20 g/l Al(NO3)3.........................................................58


presence of 20 g/l aluminium nitrates. ....................................................................................59


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Fig.4.12 Effect of extractant concentration on the extraction efficiency of HNO3(O:A = 1, single

contact 3 min. 22C, aqueous phase; 4 M HNO3). ..................................................................61

Fig.4.13 Effect of TBP concentration on distribution coefficient of nitric acid in presence and

absence of nitrate ions(O: A = 1, single contact 3 min. 22C, aqueous phase: 4 M HNO3) ...62

Fig.4.14 Effect of nitrate ion concentration on the extraction of 2 M Nitric acid with 50% TBP

solution. ...................................................................................................................................64



at 25C was determined in presence of 10 g/l NaNO3. ............................................................67

Fig.4.16 Variation of nitric acid concentration in organic phase with the initial concentration of

TBP in kerosene in presence of 10 g/l sodium nitrate in feed solution. ..................................68

Fig.4.17 Variation of the equilibrium concentration of nitric acid in extract with that in raffinate,

the experiments were carried out at 25oC with O/A ratio of 1:1 and feed solution

contains 10 g/l Ca(NO3)2 and with different concentration of TBP (30 - 80%) in

kerosene. ..................................................................................................................................72

Fig. 4.18 Variation of nitric acid concentration in organic phase with the initial concentration of

TBP in kerosene and in presence of 10 g/lCa(NO3)2in feed solution. ...................................72

Fig.4.19 Variation of the equilibrium concentration of nitric acid in extract with that in raffinate,

the experiments were carried out at 25oC with O/A ratio of 1:1 and feed solution

contains 10 g/l Mg(NO3)2 and with different concentration of TBP (30 - 80%) in

kerosene. ..................................................................................................................................76


presence of 10 g/l Magnesium nitrates. ...................................................................................76


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Fig.4.21 The extraction of nitric acid in the range of (14M) by different concentration of tri-

butyl phosphate/kerosene (30 - 80%) at an organic to aqueous phase ratio (O/A = 1), and

carried out at 25C was determined in presence of 10 g/l Al(NO3)3. ......................................79


presence of 10 g/lAl(NO3)3. ....................................................................................................80

Fig.4.23 The variation of nitric acid concentration in extract with that in raffinate for different

initial concentration of TBP (30 - 80%) in kerosene and in presence of 30 g/l NaNO3,

extraction experiments were carried outbyusingO/A ratio of 1:1 at 25oC nitric acid

concentration in feed solution were in the range of 14 M.....................................................83


presence of 30 g/l NaNO3. .......................................................................................................84



25C was determined in presence of 30 g/l Ca(NO3)2. ............................................................87

Fig.4.26 Variation of nitric acid concentration in organic phase with the initial concentration of

TBP in kerosene in presence of 30 g/l Ca(NO3)2in feed solution. .........................................88



at 25C was determined in presence of 30 g/l Mg(NO3)2in feed solution. .............................92


presence of 30 g/l Mg(NO3)2. ..................................................................................................93

Fig.4.29 Variation of equilibrium concentration of nitric in extract with that in raffinate for

different initial concentration of TBP in kerosene and initial nitric acid concentration in

feed 4 M and in presence of 30 g/l Al(NO3)3. Organic to aqueous phase ratio, O/A= 1

and experiments were carried out at 25

C. ..............................................................................96


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presence of 30 g/l Al(NO3)3. ...................................................................................................97

Fig.4.31 Effect of the presence of NaNO3concentration on of the extraction nitric acid of initial

concentration 2 M by using TBP diluted with kerosene. ........................................................98

Fig.4.32 Effect of the presence of Mg(NO3)2of different concentration in feed solution on of the

extraction nitric acid of initial concentration 2 M in feed solution by using TBP diluted

with kerosene ...........................................................................................................................98

Fig.4.33 Effect of the presence of Al(NO3)3 of different concentrations on the extraction nitric

acid of initial concentration 2M in feed solution by using TBP diluted with kerosene. .........99

Fig.4.34 Effect of the presence of Ca(NO3)2 of different concentrations on the extraction nitric

acid of initial concentration 2M in feed solution by using TBP diluted with kerosene. .........99

Fig.4.35 Effect of the concentration of TBP in kerosene on the extraction of nitric acid of initial

concentration of nitric acid in feed solution 1-4 M, in the absence of metal nitrate ions. ....102

Fig.4.36 Stripping of nitric acid from extract by using DM water with O/A ratioof 1:1 at constant

mixing time of 2 min. The average value of the % recovery of four set of experiments

for the initial nitric acid concentrations in aqueous solution of 1-4 M and extraction with

TBP solution was considered. ...............................................................................................103


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List of Tables

Table 2.1 Flash point of organic solvents, which are reported in literature for nitric acid

extraction. 8

Table 2.2 Effect of sodium nitrate on the solubility of TBP in aqueous phase. 31

Table 4.1 Equilibrium concentration of nitric acid in absence of metal nitraes in aqueos and

organic phase, nitric acid was extracted by using 30% TBP (diluted with kerosene). 39








organic phase, nitric acid was extractedby using 70% TBP (diluted with kerosene). 40



Table 4.7 Equilibrium concentration of nitric acid in presence of 20 g/l NaNO3in aqueos phase,

nitric acid was extracted by using 30% TBP (diluted with kerosene). 44





Table 4.10 Equilibrium concentration of nitric acid in presence of 20 g/l NaNO3in aqueos

phase, nitric acid was extracted by using 60% TBP (diluted with kerosene). 45


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Table 4.13 Equilibrium concentration of nitric acid in presence of 20 g/l Ca(NO3)2in aqueos












Table 4.19 Equilibrium concentration of nitric acid in presence of 20 g/l Mg(NO3)2 in aqueos









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phase, nitric acid was extracted by using 70% TBP (diluted kerosene). 53



Table 4.25 Equilibrium concentration of nitric acid in presence of 20 g/l Al(NO3)3in aqueos












Table 4.31 Ionic radius of cation salting agents used in the experiments 60

Table 4.32 Detail of the Extraction data of 2M initial nitric acid solution with 50% TBP

solution in kerosene with 10-60 g/l concentration of NaNO3. 63

Table 4.33 Equilibrium concentration of nitric acid in presence of 10 g/l NaNO3 in aqueos





phase, nitric acid was extractedby using 50% TBP (diluted with kerosene). 65


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Table 4.45 Equilibrium concentration of nitric acid in presence of 10 g/l Mg(NO3)2in aqueos







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Table 4.65 Equilibrium concentration of nitric acid in presence of 30 g/l Ca(NO3)2 in aqueos















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Table 4.75 Equilibrium concentration of nitric acid in presence of 30 g/l Al(NO3)3 in aqueos











phase, nitric acid was extracted by using 80% TBP (diluted with kerosene).. 95

Table 4.81 Experimental data of log-log plots of HNO,/( .) versus TBPin the

absence of metal nitrate. 101


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Publication

[1] Sandeep Jaiswal, R. V. R. L Visweswara Rao, D. Mandal, Recovery of Nitric Acid from

Aqueous Acidic Solution of Nitrate Ions, CHEMCON14, Chandigarh, 27-30 December 2014.


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Nomenclature

Symbol Notation Unit

Concentration of hydrogen ion [mol/l]

, Initial concentration of nitric acid [mol/l]

HNO, Nitric acid concentration in organic phase [mol/l]

HNO, Nitric acid concentration in aqueous phase [mol/l]

Concentration of nitrate ion [mol/l]

TBP Initial concentration of TBP [mol/l]

TBP Equilibrium concentration of TBP [mol/l]

TBP.HNO Concentration of TBP and HNO3complex [mol/l]

K1Thermodynamic equilibrium constant for

extraction of nitric acid using dilute TBP

[l/mol]

Keq Equilibrium constant [l2/mol2]

Kd Distribution coefficient [-]

Kdim

Equilibrium constant for dimerization of

TBP in organic phase

[l/mol]

T Time [s]

O/A Organic (O) and aqueous (A) phase ratio[-]


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(volume/volume)

YHO Mole fraction of water in organic phase [-]

YTBP Mole fraction of TBP in organic phase [-]

T Mean activity coefficient of TBP [-]

HNO,Mean activity coefficient of nitric acid in

aqueous solution

[-]


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Chapter 1

Introduction

1.1

Background

Uranium dioxide (UO2) is produced by the reduction of uranium trioxide (UO3) and UO3is

produced by the calcination of Ammonium Di-Uranate (ADU). The latter is produced by the

precipitation reaction of Uranyl Nitrate Pure Solution (UNPS) and ammonia or ammonium

hydroxide solution. The raw material either Magnesium Di-Uranate (MDU) or Uranium Ore

Concentrate (UOC) is dissolved in commercial grade nitric acid to obtain Crude Uranyl

Nitrate Slurry (CUNS). The CUNS is refined by solvent extraction process, using Tri-Butyl

Phosphate (TBP) as solvent to produce nuclear grade UNPS. The uranium in UNPS is

precipitated out as ADU by addition of ammonium hydroxide and or ammonia vapour in a

batch reactor at controlled rate.

CUNS is produced by dissolution of MDU or UOC in nitric acid and free nitric acid in

CUNS is maintained at 2.5-3 N. In the process of solvent extraction, uranium present in

CUNS is extracted counter currently by using 30-35 % TBP (diluted in kerosene) in slurry

extractor to obtain UNPS. The impurities along with undissolved solids go out along with

raffinate. The pure and loaded extract is then stripped counter currently with De-Mineralised

(DM) water in box type mixer-settler to obtain UNPS. During the purification by solvent

extraction process, all the chemical and nuclear impurities present in the raw material are

separated as Uranyl Nitrate Raffinate (UNR). The raffinate produced typically contains

uranium above specified environmental limit, due to which its direct disposal is not


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permissible. The raffinate is sent to Active Effluent Treatment Facility (AETF) for further

treatment before disposal. In the AETF the effluent streams from the solvent extraction are

treated to meet the environmental guidelines before disposal through sale.

In AETF the UNR is treated in a second stage solvent extraction process using conditioned

solvent to bring down the concentration from to less than 5 mg/l of uranium for safe

disposal (as per Atomic Energy Regulatory Board, Govt. of India; guidelines, website:

http://www.aerb.gov.in/). This process requires conditioning of the lean solvent obtained

from stripping section and by using this treated solvent for extraction of residual Uranium

(U) from UNR. The Acidic Raffinate Slurry (ARS), produced in this process contains U

concentrations below disposal limit (5 mg/l) is sold off as a by-product. The free nitric acid

present in ARS may be recovered and reused to minimise the consumption of nitric acid as

well as to reduce the loading of nitrate ion concentration in the effluent.

Moreover, nitric acid is a commonly used dissolving agent in nuclear, chemical and

metallurgical industries [1, 2, 3]. It is also a commonly used acid for various organic

processes, fertilizer industries and lithium production from brine [4, 5, 6, 7]. As an end

result, a large quantity of aqueous acidic effluent containing nitrate ions is generated.

Disposal of such acidic effluent is a serious and global problem. Nitrate contamination in

drinking water causes methemoglobinemia [8], a disease in which nitrate ions react with

blood haemoglobin and convert it into methemoglobin [9]. Haemoglobin carries oxygen

from the lungs to other tissues but methemoglobin is incapable of carrying oxygen.

Formation of excess methemoglobin (more than 80% of total haemoglobin) causes death.

Nitrates are also known as carcinogen; as under some abnormal circumstances, it is reduced
http://www.aerb.gov.in/http://www.aerb.gov.in/http://www.aerb.gov.in/


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to nitrite in stomach and form N-nitrosoamine, which is a postulated cause of stomach

cancer [10].

In recent years; various researchers attempted to separate metal ions or recover nitric acid

from acidic effluent containing free nitric acid [11-16]. Sato et al. [11] have developed an

adsorbent with a phosphonic acid di-butyl ester-type group by chloro-methylating styrene

and di-vinyl-benzene copolymer of different di-vinyl-benzene content following

phosphorylation for the preferential adsorption of nitric acid from effluent in presence of

HCl, NaCl, copper sulphate and nickel sulphate. They observed that nitric acid may be

adsorbed up to a concentration of 2 M. Kulkarni [12] had used Emulsion Liquid Membrane

(ELM) produced by using tri-n-octyl-phosphine oxide and sodium carbonate for the

preferential separation of uranium (VI) ions from aqueous nitric acid solution in presence of

Fe+3, Ca+2, and Mg+2ions. Stankovic et al. [13] used calix-4-arene amide derivatives to

separate silver ion from nitric acid solution. Shin et al. [14] developed a process to recover

nitric acid from the waste stream containing acetic acid, hydrofluoric acid, and silicon

generated from wafer industry using solvent extraction with Tri-Butyl Phosphate (TBP).

Zakharchenko et al. [15] developed sorption materials; by using multi-walled carbon

nanotubes; Taunit, a solid-phase extractant and a polymer composites which has high

sorption ability for the recovery of radionuclide from nitric acid solutions.

Lan et al. [16] used diffusion dialysis with homogenous anion exchange membrane for the

recycling of spent aqueous nitric acid solution containing Li+, Na+, K+, Mg+2and Ca+2ions.

Biswas et al. [17] evaluated the use of di-nonyl phenyl phosphoric acid (DNPPA) and its

synergistic mixtures with oxygen donors for extraction and recovery of uranium from

aqueous nitric acid solution.


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Tri-Butyl Phosphate (TBP) is generally used for the purification of crude uranium, thorium

and plutonium from nitric acid medium by solvent extraction [ 18]. It is known that TBP can

extract nitric acid from aqueous solution.

Hesford and Mckay [19] reported that TBP extracts acids in the order

HFHClO4>HNO3>H3PO4>HCl >H2SO4 in a pure system. The extraction of nitric acid

may be enhanced by the addition of H3PO4and nitrate salts in the aqueous phase [ 20] and

the extraction of nitric acid is reduced by increasing the concentration of fluoride ions (e.g.,

HF, MgF

2etc.) in the feed solution [21].

Lee et al. [22] conducted experiments for the recovery of valuable metals and recovery of

nitric acid from the spent nitric acid solutions produced by dissolving Printed Circuit Board

(PCB). They found that 95% extraction of nitric acid was possible by using 50% TBP in

five counter-current stages at the volume ratio of organic (O) to aqueous (A) phase of 3:1,

from a feed solution containing 250 g/l nitric acid. Distilled water was used to strip nitric

acid.

Palatyand Bendova [23] studied the separation of nitric acid and ferric nitrate through an

anion-exchange membrane in a two-compartment mixed cell, and found that below 3%

partial flux of ferric nitrate, the membrane can be considered a very good separator for an

HNO

3and Fe(NO

3)

3mixture. Palaty and Bendova [24] also studied the separation of nitric

acid and sodium nitrate through an anion-exchange membrane in a two-compartment mixed

cell and found that nitric acid permeates well through this membrane, while sodium nitrate

is not efficiently rejected. Bell et al. [25] have studied the feasibility ofN, N,N,N- tetra-

octyl-di-glycol-amide (TODGA) for the extraction of nitric acid, by using activity based

calculations using thermodynamics based models.


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In the solvent extraction of uranium from aqueous nitric acid solution by using TBP (diluted

with kerosene) a fraction of nitric acid is also extracted in the organic phase. The

concentration of nitric acid in raffinate normally varies between 2.5 to 3 N and in general it

contains nitrate ions of Na+, Ca+2, Mg+2, Al+3 etc. of concentration about 50-60 g/l.

Significant works on the extraction of nitric acid from effluent are reported in literature.

Few attempts were made for the recovery of nitric acid from effluent by extracting with

dilute TBP solution. Limited information is available in literature regarding the effect of the

presence of various metal nitrates on the extraction of nitric acid by using TBP (diluted with

kerosene). Some works are reported on the solubility of TBP in water in presence of metal

nitrates but no information is reported about how the distribution coefficient of nitric acid

varies in the presence of various metal nitrates. It seems that in this field much work has not

been carried out and so much information is not available in literature.

In the present study; experiments were carried out for the recovery of nitric acid for reuse

from the effluent containing free nitric acid, so that total nitrate concentration of the effluent

can be reduced. The experimental details and results are discussed in this thesis.

1.2Objectives

The objectives of the present study are as follows.

Investigation on the effect of different metal nitrates on the recovery of nitric acid

from aqueous solution containing free nitric acid.

Investigation on the effect of the concentrations of different metal nitrates on the

recovery of nitric acid from aqueous solution containing free nitric acid.


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To minimise the loading of nitrate ion (NO3) concentration in effluents.

1.3

Outline of the Thesis

The thesis is divided into six Chapters. This chapter (i.e., Chapter 1) covered the

introduction of the thesis topic which includes background and objectives of the project

work. Chapter 2 includes literature review on the project topic. Details of the experimental

works have been presented in Chapter 3. Results and discussions are presented in Chapter

4. Finally, the conclusions and suggestions are discussed in Chapter 5.


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Chapter 2

Literature Review

TBP is a phosphorus-containing compound with molecular formula (C4H9)3PO4, is prepared

by reaction of phosphorus oxy-chloride with butyl alcohol [1]. It is an organic liquid used as

a solvent for cellulose esters, lacquers and natural gums, in the manufacturing of plastics

and vinyl resins, as a metal extractant, as a base stock in the formulation of fire-resistant

hydraulic fluids for aircraft and as an antifoaming agent. During the past few years, the

utilization of TBP as an extractant in the dissolution process in conventional nuclear fuel

reprocessing has increased considerably; TBP is an excellent solvent for the extraction of

different inorganic salts [1]. The metal nitrates consist of one or all of uranyl nitrate,

plutonium nitrate, thorium nitrate, fission product nitrates or salting agents. Although other

solvents may extract these metal nitrates more efficiently, TBP is chosen for its overall

superiority in operation, safety, physical properties, radiation resistance, and economics.

One of the most desirable attributes of TBP is its high flash point (146 C) compared with

other solvents except Tri-octyl phosphate having flash point 216C .Flash points of various

organic solvents are given in Table 2.1.

Tri-octyl phosphate has higher flash point compared to TBP but its extractability is poor for

nitric acid as discussed. Tri-n-Octyl-Phosphine Oxide (TOPO) is a good solvent which

extracts nitric acid better than TBP, but recoverability is poor. Therefore; TBP has been

chosen as a solvent for recovery of nitric acid from aqueous acidic solution.


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Table 2.1 Flash point of organic solvents, which are reported in literature for nitric acid extraction.

Organic solvent Flash point [C]

Decanol 108

Octanol 81.1

Heptanol 73

Hexanol 63-73

2-ethylhexyl alcohol 73

Tri-octyl phosphate 216

TBP 146

TOPO 110

Several neutral solvents were studied for their effectiveness to extract HNO3preferentially.

An undiluted solvent was used as the organic phase in all cases. The results are presented in

Fig.2.1 and it was found that solvents such as 2-ethylhexyl alcohol (EHA), decanol, octanol,

heptanol and hexanol were more selective for CH3COOH (HAc) over HNO3. Tri-octyl

phosphate (TOP) and TBP showed more preferential extraction of HNO3compared to HAc.

The extraction of HNO3was about 46 % against a co-extraction of 32 % HAc in case of

TBP in a single contact. The corresponding values in case of TOP were 35 and 22 % for

HNO3and HAc, respectively. The high polarity of the phosphoryl (P=O) group in TBP and

TOP enables it to act as a strong Lewis base and as a result it can form acidbase complex

when contacted with strong acids. Finally, TBP was chosen for the extraction of HNO3for

its overall superiority in operation, favourable physical properties and economics.

Therefore, all further experiments were carried out using undiluted TBP, except extractant

concentration study where kerosene was used as a diluent [14].


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EHA Decanol Octanol Heptanol Hexanol TOP TBP

0

10

20

30

40

50

60

70

%

Extraction(E,%

)

Solvent

HNO3HAc

Fig. 2.1 Effect of extractant type on the extraction efficiency of HNO3and HAc (O: A =1, single-

contact 5 min, 22C, undiluted extractant, aqueous phase: 279 g/l HAc and 513 g/l HNO3) [14].

The TBP is always diluted in an organic matrix, or diluent, to improve the physical

characteristics of the organic phase [1]. The diluent reduces the viscosity and density of the

organic phase and improves the phase separation characteristics and reduces criticality

concerns by limiting the maximum actinide concentration in the organic phase. The diluent

is chosen on the basis of radiation stability and inertness to the species in the solvent

extraction process. From a purely technical perspective, the alkane hydrocarbon dodecane,

C12H26is the best diluent to use because it is inert and highly radiation resistant. Dodecane

can be purified to be free of aromatics that can react with some of the components in the

solvent extraction environment. However, dodecane is very expensive and for this reason,

purified kerosene or kerosene-like diluents such as; AMSCO-125-90 W, that have properties

nearly equivalent to those of dodecane are used instead.The tridentate ligand TODGA ( N,

N,N, Ntetra-octyl-di-glycol-amide) is the leading example of this class of ligands that can

also be used for the extraction of nitric acid [1].


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In the extraction of uranium from acidic uranyl nitrate solution; not only TBP but also nitric

acid distributes itself between the organic and the aqueous phases. It was observed that the

solubility of TBP decreases with increase in the concentration of nitric acid in presence of

nitrate salt viz., sodium nitrate and calcium nitrate solutions in the aqueous phase [26].

Sodium nitrate and calcium nitrate are salting agent which reduces the solubility of TBP in

the aqueous phase. Sodium nitrate and calcium nitrate do not form solvates with TBP like

other metal nitrates and hence, they are inextractable metal nitrates. Hesford and Mckay

[27] had studied extraction of acidic uranyl nitrate solution with TBP (diluted with

kerosene). They observed that the extraction of extractable nitrates by TBP increases in

presence of inextractable metal nitrates because of lack of competition for complex

formation and salting-out effect of inextractable nitrates.

Alcock et al. [28] have investigated the salting out effect of TBP in presence of nitric acid

and sodium nitrate and found that TBP salted out more in sodium nitrate as compared to

nitric acid because nitric acid is an extractable but sodium nitrate is not extractable by TBP.

The pressure exerted by inextractable metal nitrates to salt out TBP depends on their ionic

radius and ionic strength. The small ionic radius and low ionic strength of sodium nitrate as

compared to calcium nitrate has resulted in lower solubility of TBP in the aqueous phase

and higher distribution ratio value of TBP.

Xianhong and Zhou [29] studied the extraction kinetics of nitric acid by TBP in heptane in

an improved Lewis cell. The activity of component was calculated by Pitzeris equation [29]

for aqueous phase and UNIFAC model for organic phase. From the calculated mass transfer

coefficient and mass transfer resistance, it was concluded that the main mass transfer

resistance in the extraction process came from the diffusion resistance in the boundary layer


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of organic phase. The mean relative deviation between experimental results and calculation

values by using the model was found within 2.5%.

2.1

Factors Affecting Extraction Process

The extraction of nitric acid and hexavalent uranium from nitric acid and nitric acid

containing uranium (VI) media by Tri-Butyl Phosphate (TBP) diluted with kerosene was

studied by Schulz et al. [30] and all factors affecting the extraction process (time of mixing,

uranium concentration, nitric acid concentration, TBP concentration, temperature) were

investigated.

The previous investigators have studied parameters like time of mixing, uranium

concentration, nitric acid concentration, TBP concentration and temperature on the

extraction of nitric acid by TBP diluent system. However, the effects of presence of

inextractable metal nitrates were not studied earlier. In the present experimental work the

effect of the presence of inextractable metal nitrates on the extraction of nitric acid were

investigated.

2.0.1 Effect of Mixing Time

Schulz et al. [30] had used equal volume (25 ml) of 0.363 M TBP (in kerosene) and 1 M

nitric acid and mixed together at 25C for various time intervals to study the effect of time of

mixing to attend the equilibrium. Fig. 2.2 shows the variation of nitric acid concentration in

the organic phase against time. It was found that 0.5 minute was the minimum time to reach

equilibrium.


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2.0.2

Effect of Nitric Acid Concentration

Schulz et al. [30] had studied the extraction of nitric acid from aqueous solution with initial

concentration of nitric acid in the range of 0.56.35 M by using 0.363 M (i.e.,10 volume %)

TBP diluted with kerosene of organic to aqueous phase (in volume) ratio (i.e.,O/A= 1) and

at 25C. Their experimental results are shown in Fig. 2.3, which indicates that the

concentration of nitric acid in the organic phase increases with the increase in the

concentration of nitric acid in the aqueous phase.

0 1 2 3 4

0.00

0.01

0.02

0.03

0.04

0.05

0.06

HNO

3concentrationinorganicphase[mol/l]

Time [min.]

Fig. 2.2 Variation of , with time. = 0.363 M, , = 1.13 M, O/A= 1,

T = 25

C [30].


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Fig. 2.3 Variation of , with , . = 0.363 M, O/A= 1, T= 25C [30].

2.0.3

Effect of TBP Concentration on HNO3Extraction

Schulz et al. [30] had studied the effect of the TBP concentration in kerosene on the

extraction of nitric acid from aqueous solution of initial nitric acid concentration of 1.125,

3.15, 6.35 M with O/A ratio of 1:1 and at 25C. Fig. 2.4 shows the variation of nitric acid

concentration in the organic phase against initial TBP concentration. It was found that the

concentration of nitric acid in the organic phase increases linearly with increase in initial

TBP concentration (in kerosene).

0 2 4 6 8

0.0

0.1

0.2

0.3

0.4

0.5

HNO

3concentrationinorganicphase[mo

l/l]

Initial HNO3concentration in aqueous phase [mol/l]


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Schulz et al.[30] proposed the mechanism for the extraction of nitric acid by TBP as shown

in Reaction (1).

H++ NO3+ TBP TBP.HNO3 (1)

The equilibrium constant for the Reaction (1) can be written as shown in Eq. (2).

Keq= . .. (2)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

HNO

3concentrationinorganicphase[mol/l]

Concentration of TBP in organic phase [ mol/l ]

1.125 M Nitric acid3.15 M Nitric acid6.35 M Nitric acid

Fig. 2.4 Variation of , with , , = 1.125 M. , =3.15 M, , = 6.35 M,

O/A= 1 and T= 25C [30].


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Schulz et al. [30] considered the dimerization of TBP in the organic phase as follows and

the equilibrium constant for dimerization of TBP in organic phase,

Kdim is shown in Eq. (3) and its value determined by them is 2.6.

2TBP (TBP)2

Kdim =() (3)

From the mass balance of TBP, following equations (4-6) may be written.

CTBP= CTBP- 2C(TBP)- CTBP.HNO (4)

CTBP=1+4K (5)

where, = 1 8KdimCHNOorg+ 8KdimTBP (6)

Taking the logarithm of both sides of Eq. (2) and rearranging, we obtain,

log .= logKeq+ logTBP, (7)

where, . =


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The log-log plots of 3/ ( .) withTBPgave two straight lines, with a slope

of 1.3 and 1.18 and regression coefficients of 0.999 and 0.997 for 1.13 M and 3.15 M nitric

acid concentration, as shown in Fig. 2.5, which suggests that one molecule of HNO 3was

extracted with one molecule of TBP [30]. The equilibrium constant for the extraction

reaction of nitric acid by TBP/kerosene was found to be 0.347 0.077 l2/mol2for 1.13 and

3.15 M nitric acid concentration.

The partition of nitric acid between water and TBP/kerosene in absence of metal nitrates at

25C was studied. The extracted nitric acid in the organic phase is in the form of a 1:1

compound HNO3.TBP. The equilibrium constant of the extraction reaction was found to be

Keq= 0.347 l2/mol2.

Fig. 2.5 Variation of log . with log. , =1.125 M. , = 3.15 M, O/A= 1,

T= 25C [30].


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The effect of the TBP concentration on the extraction efficiency of HNO3and CH3COOH

(HAc) with O/A ratio of 1:1 and at 22C has also been studied and the results are shown in

Fig. 2.6. It was observed that the extraction of nitric acid increased from19 to 46% (Kd =

0.230.85) in a single contact when TBP concentration increased from 20 to 100% [14].

During the same period, the co-extraction of HAc also increased from 9.4 to 32.2% ( Kd =

0.100.47). In the extracted species, the protons of an acid can form a hydrogen bond with

an oxygen atom of TBP molecules and thus the co-extraction of HAc together with HNO3 is

quite reasonable. Generally, the extraction of an acid tends to be enhanced by the common-

ion effect of protons that is produced by the dissociation of the acid, and reduced by the

competing extraction of another acid. To study the effect of HAc presence in feed solution,

another experiment was carried out. In that case, a synthetic solution was prepared

containing only nitric acid having the same concentration to that of actual solution. It was

observed that, the extraction efficiency of HNO3 increased by 1214% in case of a single

acid compared to a binary mixture of acids (HNO3 and HAc) by using TBP of same

concentration. This showed that; the extraction of HNO3 was suppressed by the co-

extraction of HAc as both may be chelating with the same functional group.

The log-log plots of 3/ (.) withTBPgave straight lines with a slope of

1.88 and regression coefficients of 0.99, suggesting that two molecules of HNO3 was

extracted by one molecule of TBP. However, the mechanism is much more complicated

than mentioned above, with the involvement of water in the complex formation as suggested

by several investigators [20, 31]. Another parameter that has significant importance for

plant designing is the phase separation time. It was observed that, phase separation time


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increased from 20 to 80 s, when TBP concentration increased from 20 to 100%. No third-

phase formation was observed in the experimental conditions used in the previous studies

[20, 31].

2.2

Equilibrium Constant for the System TBP-Water-Nitric Acid

Collopy and Cavendish [32] determined, the equilibrium constant for the reaction of TBP

with associated nitric acid was calculated as 19.9 0.5. The equilibrium distribution

constant for the partition of associated nitric acid in to TBP was determined as 0.19. The

equilibrium constant obtained were used to calculate the concentration of associated nitric

acid in dilute aqueous nitric acid solutions. These calculations were made from the

equilibrium distribution curve for nitric acid between TBP and water at 25oC over the

concentration range of 0-12 M aqueous nitric acid.

20 40 60 80 100

5

10

15

20

25

30

35

40

45

50

HAc

Perc

entageExtraction,

E[%]

TBP concentration in organic phase [volume %]

HNO3

Fig. 2.6 Effect of extractant concentration on the extraction efficiency of HNO3and HAc (O/A=1,

single-stage contact 5 min, 22C, aqueous phase: 279 g/l HAc and 513 g/l HNO3) [14].


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As we know that a stable, equimolar complex was formed by the reaction of associated

nitric acid with TBP. It was concluded that this same reaction takes place between TBP and

associated nitric acid in two phase system TBP-H2O-HNO3. Collopy and Cavendish [32]

concluded that as the associated nitric acid present in aqueous nitric acid solution these two

phase system then can be represented by an equilibrium reaction in the organic phase as

shown in Reaction (8).

HNO3aq + TBPorg TBP. HNO3org (8)

The equilibrium curve for the distribution of nitric acid between TBP and water at 25 oC

showed that the ratio of total nitric acid in the TBP phase to the total nitric acid in the

aqueous phase decreases with increasing aqueous nitric acid concentration until a constant

value is obtained.

2.3Solubility of TBP in Water and HNO3 Solution

Burger and Forsman [33] have reported the values of the solubility of pure TBP and of TBP

in an inert diluent in water and in solutions of nitric acid of various concentrations. The

solubility of pure TBP in water was about 0.4 g/1 at 25C. When the TBP was diluted with

an inert substance insoluble in water, the solubility of TBP was found to decrease. The

presence of salts in the aqueous phase also decreased the solubility markedly. Similarly, the

solubility of TBP decreased slowly with increasing nitric acid concentration because of

competing effects such as the formation of TBP-HNO3complexes. The solubility of water

in TBP-diluent mixtures varied from 64 g/l in pure TBP to about 0.06 g/1 in the pure

paraffin-type diluent. A comparison of several analytical methods used for determining the

solubility of TBP in aqueous solutions was also done.


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Higgins et al. [34] have studied the effect of different electrolyte and temperature ranging

from 5 to 50C on the solubility of TBP in water. The salting coefficients at 25C for all the

electrolytes tested, excepting nitrates, have been correlated with the Gurney unitary partial

molal electrolyte entropy concept.

Tuck [35] had carried out experiments to measure viscosity of extract solutions of HNO3in

TBP and TBP-water and TBP-anhydrous nitric acid mixtures and discussed the interactions

involved between like and unlike molecules. The viscosity varies with the mole fraction of

the species involved were interpreted in terms of strength of the interaction in the viscous-

flow transition state. Since the species involved have similar molar volumes, assumptions

were made from the viscosity to derive excess free energy of at different mole fractions.

Bullock and Tuck [36] had measured the mutual solubility of the two phase water-TBP

system. Nuclear magnetic resonance studies of solutions of water in TBP showed that the

structure of such solutions was more complex than suspected. The results obtained were

explained in terms of the formation of linear and chain polymers of varying complexity. The

model showed that the solubility of TBP is temperature dependent in both water and

aqueous nitric acid and also dependent of the concentration of nitrate ions in aqueous

solution.

Hardy et al. [37] studied distribution of nitric acid between aqueous phase and 100% TBP

and measured the densities of both the phases at equilibrium. They have determined the

solubility of lithium, sodium, potassium, caesium and ammonium thiocyanate and rubidium

thiocyanate in TBP. They found that the solubility decreased with increasing cation size,

except for ammonium salts. The values of ammonium salts were high due to the hydrogen-

bonding between NH4+

and the phosphoryl oxygen of the solvent.


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Yamamoto [38] has determined the solubility of nitric acid in the organic phase by

measuring the dielectric property of the system. The dielectric properties of the organic

phase i.e. 30% TBP-n-dodecane-HNO3-H2O system and HNO3-H2O-UO2(NO3)2 system

were measured with a concentric capacitance cell, for in-line HNO3 monitoring in the

organic phase. It was found that the variation in the dielectric constant, caused by the

variation in the extracted HNO3, was markedly greater than that caused by the same molar

variations of H2O and UO2(NO3)2.

Swain et al. [39] had measured viscosities and densities of different binary liquid mixtures

of TBP with benzene, toluene and o-xylene at 30, 35 and 40C. The non-idealities reflected

in mixture viscosities have been expressed in terms of excess viscosities. A Redlich-Kister-

type equation was fitted to the binary -X-T data for each system.

Tripathi and Ramanujam [40] had examined the radiation-induced changes in density and

viscosity of 30% TBP-dodecane-nitric acid system. It was observed that the increase in the

density becomes significant with increasing nitric acid concentration in the solvent,

HNO3.TBP concentration, and absorbed radiation dose, which concurrently leads to a much

sharper increase in the viscosity of the solvent. The extent of increase in the viscosity was

found to be significantly enhanced by gamma radiolysis and was a function of absorbed

dose. Gasliquid chromatography (GLC) and infrared (IR) analysis of the treated solvent

has revealed the radiation induced polymerization and nitration of the hydrocarbon diluent

which has resulted in increased viscosity. Tripathi and Ramanujam also found that a

considerable increase in the viscosity of the solvent with the presence of small amount of

radioactive species remaining in the solvent due to incomplete solvent purification.


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Wright and Hartmann [41] have published a review paper by referring to more than 100

publications on physical and chemical properties of TBP-diluent-nitric acid systems. The

data on the kinetics of degradation products of TBP reported by different authors is also

cited. Wright and Hartmann reviewed the works of various investigators who investigated

the effect of physical properties of solution TBP in kerosene; viz., vapour pressure,

solubility and density and the degradation products solution of TBP in kerosene at different

temperatures in TBP-diluent-nitric acid system.

2.4

Studies on Third Phase Behaviour for TBP-Nitric Acid System

Rao and Srinivasan [42] had measured the conductivity of the organic phase after the

extraction of nitric acid by TBP in order to understand the mechanisms for the formation of

third phase. Rao and Srinivasan also conducted experiments with different concentrations of

nitric acid (2-14M) and TBP (20 to 90 volume %) and the experimental results indicated

that the conductivity variation of the organic phase can be correlated to the water content as

well as the species present in the organic phases. Rao and Srinivasan observed a higher

value of conductivity for third phase compared to the other two organic phases which was

observed to be formed at high nitric acid concentration (> 10M). The higher conductivity of

third phase was attributed to the movement of nitric acid in the micro-emulsion globules due

to the extraction of both nitric acid and moisture near third phase formation limit. A

thermodynamic model has also been developed for the calculation of speciation of TBP-

HNO3 complex. Fig. 2.7 shows variation in the conductivity of the organic phase as a

function of concentration of nitric acid aqueous phase and TBP. It is noticed that for a given

TBP concentration the plot of conductivity versus nitric acid concentration in the organic

phase can be classified into three portions and the transition depends upon the composition

of different species of TBP-HNO3 in the organic phase. The conductivity of organic phase


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increases steadily with increase in acidity up to (2-8 M) (portion-1). The initial increase in

conductivity may be attributed to increased concentration of nitric acid in organic phase

leading to increased movement of charges due to percolation phenomenon. The conductivity

of organic phase nearly remains same in the second portion of the plot (8 to10 M) which

may be attributed to the percolation threshold due to formation of the reverse micelles in the

organic phase. Beyond 10 M feed nitric acid concentration (portion-3) conductivity of

organic phase increases steeply.

Rao and Srinivasan [42] also observed that (TBP)2.HNO3 concentration decreases in the

range of 2-8 M nitric acid and beyond 8 M acidity the concentration of this species in

negligible. On the other hand the concentration of TBP.HNO3 complex increases with

acidity and has maximum at about 8 M acidity and then decreases drastically beyond 9 M.

whereas, concentration of TBP.2HNO3 in the organic phase remains small and increases

drastically beyond 9 M acidity. This increased concentration of TBP.2HNO3 indicates that

micro-structure of micro-emulsion globule is changed drastically, hence more nitric acid get

extracted beyond 10 M in aqueous solution.

Comparison of concentration of the (TBP.HNO3)2, TBP.HNO3 and TBP.2HNO3 in organic

for 1.09 M and 2.18 M TBP indicates that the concentrations of all these species are higher

in 2.18 M TBP. In view of this increased concentration of these species in the organic phase

it is expected that the probability of charges movement in the micro-emulsion globules

should be higher when the TBP concentration is high.


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Fig. 2.7 Variation of organic phase conductivity as the function of concentration of feed nitric

acid and TBP [42].

The study of Rao and Srinivasan [42] for the extraction of nitric acid using TBP indicates

that the different species are present in the extract (organic phase). Concentration of these

species changes as a function of feed acidity as well as the concentration of TBP changes in

the organic phase. Soluble water content in the organic phase decrease with increase in acid

to certain value and then increases beyond a certain limit where the formation of the third

phase takes place. Complex of TBP-HNO3 and moisture in the organic phase is employed

for explaining the variation of conductivity of the organic phase, but in this study no third

phase formation is about to take place because the concentration of nitric acid was very

much lower than 10 M and was within 2.5-3 M.


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2.5

Thermodynamics of Extraction of Nitric Acid by Using TBP Solution

The distribution of HNO3 of initial concentration 0-8 M with TBP of concentration 5-100

volume% (in Amsco 125-82, a hydrocarbon) was measured by Davis [43]. He had arbitrarily

chosen the initial concentration of nitric acid within 0-8 M because of uncertainties in HNO3

activity coefficients in TBP diluted in Amsco are accurately described by the equation

logHNO,/{HNO,(3.75493-(HNO,)} = A+ B (HNO,,) where subscripts org and

aqueous refer to organic and aqueous phases.

Codding et al. [44] proposed an antilogarithms to find the constants A for TBP solution of

six different concentrations and described it as a linear function of (YH2O+ YTBP), the sum of

the mole fractions, YH2O and YTBP of water and TBP in the acid-free water-saturated organic

phase. Codding et al. interpreted antilogarithms as the product KIYT, where K1 is the

thermodynamic equilibrium constant for the extraction reaction and TN was the mean

activity coefficient of TBP and TBP.H2O in the acid-free, water-saturated organic phase. As

the concentration of TBP in Amsco 125-82 increases from 0 to 100%, KIYT, in molal units,

varies from 0.2 to 1.5. The value of B in the equation is proportional to (YH2O+ YTBP)1/2

while the product-B(HNO,,) is interpreted as logTN, where, TN is the mean activity

coefficient of the species TBP.HNO3 and TBP.HNO3.H2O. The organic phase water and

acidity values are consistent with formation of the complexes TBP.H2O, TBP.HNO3,

TBP.HNO3.H2O, and TBP.2HNO3.

Codding etal. [44] also found that the complexities of the multicomponent solutions were

so great that no serious efforts had been made to provide quantitative interpretations.

Instead, there was a concerted effort to obtain a quantitative description of solvent extraction


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system, viz., TBP-diluent-HNO3-H2O. With some exceptions these studies were further

simplified by elimination of the diluent, data on HNO3 extraction by TBP have been

interpreted in terms of the formation in the organic phase of a series of addition compounds,

the quantity of each changing as the aqueous nitric acid concentration changes. Thus in

absence of HNO3 the compound TBP.H2O is formed; as the aqueous HNO3 concentration

increases to 15 M, it was postulated that compounds such as TBP.HNO3, TBP.HNO3.H2O,

TBP.2HNO3, TBP.3HNO3 and TBP.4HNO3 were formed successively, although evidence

for complexes containing more than 2 moles of HNO3per mole of TBP is very sketchy. The

study of Codding etal.for the extraction of nitric acid was limited to the regions of aqueous

acidity, primarily 5 M or less, that are normally used in the pilot plant and production

facilities. Codding etal. had performed experiments with 5 to 100 volume % solutions of

TBP in the diluent; Amsco 125-82 which is an odourless mineral spirits.

Technical grade tri-butyl phosphate and Amsco 125-82 were used as starting materials. The

TBP was partially purified (free of acid) by two or three contacts with equal volumes of

2.5% aqueous sodium carbonate; any traces of the latter were removed with three equal

volume distilled water washes. Before being stored, the resulting TBP was degassed and

dehydrated under vacuum to a water content of 1.8 mg/ml. Diluent was used after two or

three water washes.

TBP-Amsco stock solutions were synthesized by weight and then vigorously agitated in 25-

50 ml portions, with equal volumes of the various HNO3 solutions for 5-20 min. These

mixtures were allowed to settle until the two phases appeared to be water clear, the

minimum settling time being 15 min. and the maximum 3 days. After separation of the two

phases density, acidity, and, in some cases, nitrate were determined in the aqueous phase


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and density, water content, acidity, and, in some case, nitrate in the organic phases.

Densities were determined with 10 ml Pycnometer, usually at 25C. Acidity was determined

by electrometric titration with appropriate sodium hydroxide solutions. Water was added to

the organic samples to make their titrations in essentially aqueous media. Water in the

organic solutions was determined by the Karl Fisher method as described by Kelley et al

[45].

Taras [46] determined nitrate by reaction with 2, 6-dimethylphenol and subsequent

colorimetric measurement in a manner similar to the method based on phenol-di-sulfonic

acid. The limit of detection of nitrate in the TBP-Amsco solutions was approximately 5 g.

Taras has listed the analytical data for the extraction of nitric acid by 5, 10, 15, 30, and 65

volume % solutions of TBP in Amsco 125-82 and by diluent-free, i.e., 100 volume %, TBP.

In addition to analytical data there are included values of the mean molar stochiometric

activity coefficients of nitric acid in water calculated from the 250C molal data of

Hartmann and Rosenfeld. Some of the extraction results were not used in the mathematical

analyses mentioned here; either because differences between nitrate and acidity

determinations, in the low-acidity region in the organic phase were large enough to suggest

the presence of other acids, perhaps di-butyl-phosphoric, or because adequate nitric acid

activity coefficient data are not available. Reichardt and Welton [47] corrected the activity

coefficients they calculated from their freezing point measurements to values at 250C only

at concentrations up to 3 M.

The mathematical description based on the extraction of nitric acid by TBP solutions

according to the reaction is shown in Eq. (9).


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HNO3aq + TBPorg TBP. HNO3org (9)

From which is formulated the equilibrium constant (K1) as shown in Eq. (10).

K1 =C.,

C, .C (10)

Alcocket al.

[48] interpreted two different concepts for the data on the extraction of nitric

acid by TBP-diluent solutions. The first interpretation postulates much of the deviation of

the system from ideality to changes in activity coefficients of species in the organic phase as

well as in the aqueous phase. The second interpretation postulates the formation of

complexes of the type TBP.2HNO3, TBP.3HNO3, and TBP.4HNO3, in addition to

complexes such as TBP.H2O, TBP.HNO3, and TBP.HNO3.H2O, without consideration of

changes in activity coefficients.

The relation between H2O concentration and HNO3 concentration in the organic phase at

low values of concentration is shown in Eq. (11).

HNO3aq + TBP. H2Oorg HNO3.TBP.H2Oorg (11)

HNO3aq + TBP. H2Oorg TBP. HNO3 + H2Oaq (12)

However, as the TBP concentration in the diluent increases, Reactions (9) and (11) become

less important while Reaction (12) becomes predominant.


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All the above mentioned investigators [43-47] had focused mainly on why the extraction of

nitric acid with TBP in presence of metal nitrates increases but the exact data and how the

distribution coefficient of nitric acid varies with the presence of these metal nitrates is not

available and also the complex formation with TBP-Water-Nitric acid system is not given.

They have also focused on solubility of TBP in water or in presence of extractable like

uranium nitrate and plutonium nitrates. However, no literature is available on the extraction

of nitric acid by TBP in different concentrations of nitric acid in presence of different

inextractable metal nitrates. Sodium nitrate and calcium nitrate are

Thesis M Tech_DP_[R8] (1)

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