leaching of some lanthanides from phosphogypsum ...4) 19 - 2015...arab journal of nuclear science...

Arab Journal of Nuclear Science and Applications, 48(2), (37-50) 2015

73

Leaching of Some Lanthanides from Phosphogypsum Fertilizers by

Mineral Acids

Z.H. Ismail, E.M. Abu Elgoud, F. Abdel Hai*, Ibraheem.O. Ali*, M.S. Gasser and H.F. Aly

Hot Laboratories Center, Atomic Energy Authority, Egypt *Chemistry Department, Faculty of Science, Al-Azhar University, Egypt

Post Code 13759, Egypt

Received: 1/1/2015 Accepted: 3/3/2015

ABSTRACT

The phosphate rock contains trace amounts of uranium and lanthanides.

When treating this rock with H2SO4 produces phosphoric acid together with 5 times

of its quantity of phosphogypsum (PG) precipitate as a by-product. The lanthanides

in the rock are mostly present in the PG. After a slight modification of the

precipitated PG, some portion of the PG is used as a low-grade phosphate fertilizer

(PGF) containing ~ 2% P2O5. Leaching of different lanthanides from PGF by the

mineral acids solutions namely; HCl, H2SO4 and HNO3 is investigated in relation the

acid concentration, the mixing time, and acid volume to mass of PGF ratio (L/S) as

well as temperature. It is found that the highest leaching efficiency was obtained

using 3.0 M HNO3, L/S = 3.0 and mixing time of 3.0 hr, at room temperature. The

total lanthanides content in the PGF, was found to be about 480 mg/kg. After three

cycle of leaching, the leached out lanthanides from PGF was more than 66 %.

Key Words: Lanthanides, Phosphogypsum fertilizer, Characteristic, Leaching

1-INTRODUCTION

Phosphogypsum fertilizer (PGF) is a waste by-product from processing of phosphate rock by

the "wet process" method for phosphoric acid production [1]. In the wet process, the phosphate content of the rock ([Ca3(PO4)2]3CaF2) is converted by concentrated sulfuric acid to phosphoric acid and a

calcium sulfate residue in either dihydrate (CaSO4. 2H2O) or hemihydrate form (CaSO4.1/2H2O)

according to the following, (Eq.1):

[Ca3(PO4)2]3CaF2 + 10H2SO4 + 10nH2O → 6H3PO4 + 10(CaSO4.nH2O) + 2HF … (Eq.1)

Where n equal ½ or 2

This process widely used industrially. However, the drawback of this process is the large

amounts of PG by-product, where about 5 tons of PG is generated for every ton of phosphoric acid [2].

The phosphogypsum precipitate is mainly CaSO4.2H2O but it also contains impurities such as free

phosphoric acid, phosphates, fluorides and organic matters that adhere to the surface of the gypsum

precipitate [3]. It contains also trace amounts of many other elements, including thorium, uranium and

most of the lanthanides (rare earth elements), which are originally present in the phosphate rock.

Previous studies have focused on reducing impurity levels in PG to make it suitable for use for other

purposes especially as soil stabilizer, building materials, or as set controller in the manufacture of

Portland cement [4-8].

With the increasing demand of lanthanides and their compounds in different technological

applications in electronic devices, lasers equipment, superconductors, batteries and super magnets,

separation and recycling of lanthanides from different secondary resources have recently drawn

extensive attention [9]. Within this merits, different approaches were carried out to separate

lanthanides from phosphate rocks or phosphogypsum as a secondary source for the lanthanide

elements.


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The leaching of the lanthanides from PG has been studied in a number of works by different

methods e.g. acid leaching [10-13]. As early as 1985, Habashi [6] investigated the recovery of

lanthanides from phosphoric acid by-product (PG) using H2SO4. The obtained results showed that,

about half of the lanthanides is recovered by leaching the PG at ambient temperature with 0.5-1.0 M

H2SO4 at a (liquid/solid) ratio of (1/10). Preston in 1995 [14] used HNO3 and H2SO4 as leaching

solutions for the recovery of lanthanides from a phosphoric acid sludge by- product. The obtained

results indicated that, leaching of the sludge of PG with nitric acid was more effective than sulfuric

acid at the same conditions. Lokshin et.al [15], treated the PG by soda ash to form sodium sulfate and

calcium carbonate precipitate containing the lanthanides. The leaching of lanthanides from the

carbonate precipitate was more efficient using nitric acid rather than sulfuric acid.

Ali and Mohammed [16] studied a process for the treatment of phosphate rock from western

Deseret, Egypt (Abu-Tartur) using HNO3 as a leaching agent for the recovery of lanthanides without

interfering with the normal rout proposed for fertilizer production. The results showed a complete

dissolution of the lanthanides in nitric acid. When this solution was treated with sulfuric acid to

produce phosphoric acid, the lanthanides mostly transferred to the co-produced PG. Kandil et al [17]

studied the dynamic leaching of lanthanides from Western Deseret phosphate ore (Abu Tarttur) by

hydrochloric acid, nitric acid and sulfuric acid solutions. They found that HCl and HNO3 are more

efficient for leaching. El-Didamony et al [18] found that by treatment of Phosphogypsum waste with

tributylphosphate in Kerosene, more than 60% of the lanthanides and radioactive elements are

removed. They did not mention the effect of the hydrophobic nature of the organic extractants on the

treated PG.

Recently in 2014, Kouraim et al. [19] studied leaching of lanthanides from PG waste using

Nonyl Phenol Ethoxylate (NPE) associated with HNO3 and HCl. Further, they reported that in the

presence of NPE, the lanthanides leached out better. This study clearly highlights that the association

of NPE with HCl or HNO3 is a potential leaching reagents for the leaching of lanthanides from

phosphogypsum waste.

Since minor and trace elements can vary depending on the origin of the phosphate rock as well

as the process of wet phosphoric acid production, the obtained PG composition is rather heterogeneous

in terms of minor as well as trace elements. In this respect, several processes determine trace elements

concentration and distribution of non- radioactive elements in PG as crystallization of pure PG phase,

formation of solid solutions and adsorption to organic matters and mineral components [20-22].

Therefore, to obtain reliable results, leaching investigations require specific standard product, which is

quite difficult to obtain for PG in terms of the changes of PG characteristic within the differences in

the production technology of the wet process as well as the aging effects on the open-air area of PG

disposal sites. Therefore, it is rather constructive to investigate any treatment investigation on a

standard product of PG. In this concern, ~15% of PG by-product is treated and recycled and

commercialized as agriculture fertilizer with low content of P2O5 (PGF), which is chosen as the target

material in the present study.

Accordingly, the present work is directed to study the leaching of lanthanides present in the low

content of P2O5 fertilizer (PGF), produced and commercialized by Abu-Zaabal Company for

Fertilizers and Chemicals, Egypt, with different mineral acids. The specification of this product is

more homogenous and the leaching conditions obtained can be more reliable.

2. EXPERIMENTAL

2.1. General:

All chemicals used were of analytical grade, unless otherwise stated. Samples of PGF (of density

equals 2.29 g/cm3) were obtained from Abu-Zaabal Company for Fertilizers and Chemicals. The

Shimadzu, UV-visible spectrophotometer model UV-160, Japan was used for measuring the

concentrations of total lanthanides.


73

Extraction experiments were carried out using a water-thermostated shaker (type G.F.L 1083,

Germany). The two phases were separated completely using a centrifuge of (type Z-230 obtained from

Hermle, Germany), with a maximum speed of 5500 ± 5 rpm at the ambient temperature. All pH values

of the different solutions were measured using a bench pH meter of Hanna instruments type having pH

range 0-14 with resolution of 0.01-pH and accuracy ±0.01.

2.2. Instrumental Analysis:

Chemical analysis, X-ray diffraction analysis (XRD), and IR spectra were the main

experimental methods carried out to investigate the PGF samples. Trace concentrations of the different

metals in the PGF samples are determined using the inductively coupled plasma optical emission

spectrometer (ICP-OES) of the model, Shimadzu Sequential Type, Kyoto "Japan". For major

elemental analysis, X–ray fluorescence spectrometry (XRF) Phillips, Holland model PW – 2400

spectrometer was used.

2.3. Leaching Process:

Different acidic solutions namely, HNO3, H2SO4 and HCl of different concentrations were used

as leaching agents. Unless otherwise stated, leaching experiments were carried out by taking 1.0 g of

PGF with a certain known volume of the leaching solution (according to the applied phase ratio) in a

polyethylene vial, and mixing thoroughly for a predetermined period. The mixture is separated by

filtration and the concentration of the produced lanthanides (Cf) was determined in the leaching

solution colorimetricaly by Arsenazo-III method [23]. Factors affecting the desired leaching materials

such as contact time; acid concentration, liquid/solid ratio (L/S) and temperature were studied.

The leaching percent (% L) of lanthanides was calculated from the following equation (Eq.2);

% L = [ Cf / Co] x 100 …………… (Eq.2).

Where, Co is the concentration of the total lanthanides (mg/l) actually present in 1.0 g of PGF.

To determine Co, 1.0 g of PGF was completely dissolved in aqua regia and the total lanthanide

concentration was determined.

3. RESULTS AND DISCUSSION

3.1. Characterization of PGF:

PGF was characterized in terms of its major element concentrations and minor lanthanides,

crystal structure, thermal analysis and infra-red spectrometry.

3.1.1. Chemical Analysis of PGF Sample:

Chemical analysis of PGF using X- ray fluorescence (XRF) is shown in Table (1). This table

indicates that, PGF is composed mainly of CaO (35.9 %), SO3 (44.08 %) with minor contents of SiO2

(9.95 %), Fe2O3 (1.64 %), and P2O5 (2.38 %) as well as traces of Na2O (0.24%), TiO2 (0.15%) and F

(0.36%). The total rare earth elements in the PGF sample as analyzed by ICP-OES equals ~ 481±5

mg/Kg. The concentration of the individual lanthanides present in ppm are Ce (234.1 ppm), La (117.0

ppm), Er (79.1 ppm), Pr (27.1 ppm), Y (21.6 ppm) and Sm (2.0 ppm), Table (2).


04

Table (1): Chemical analysis of PGF by X- ray fluorescence (XRF).

Analyte Compound

Formula

Concentration

(%) Analyte

Compound

Formula

Concentration

(%)

F F 0.36 S SO3 44.08

Na Na2O 0.24 Ca CaO 35.90

Mg MgO 0.24 Sr SrO 0.16

Al Al2O3 0.26 Ti TiO2 0.15

Si SiO2 9.95 Fe Fe2O3 1.64

P P2O5 2.38 Ni NiO 0.12

Drying of PGF sample at 200 oC for two hr indicated a weight loss of about 15%. Part of the weight

loss in the PGF sample may be due to humidity and the other is due to transformation of dihydrate

calcium sulfate (CaSO4.2H2O) to hemihydrate calcium

Table (2): Chemical analysis of lanthanides in PGF by ICP-OES.

sulfate (CaSO4.1/2 H2O) and un-hydrated calcium sulfate (CaSO4) as follows (Eqs. 3 & 4) [24]:

CaSO4 . 2H2O ∆,~ 151 ℃→ CaSO4 . ½ H2O + 3/2 H2O…….. (Eq.3)

CaSO4 . ½H2O ∆,~ 180 ℃→ CaSO4 (- anhydride) + ½ H2O ……… (Eq.4)

The presence of dihydrated calcium sulfate (CaSO4.2H2O) was also observed by Hanna et al

(1999) [24] and others [25] for PG using DTA and TGA analysis. Chemical analysis of the dried

sample is shown in Table (3) which indicates that there is no drastic change in the chemical

composition of PGF after drying. However, loss of water after drying is reflected by slight increase in

the concentration of the main composition of PGF.

Table (3): Chemical analysis of PGF by XRF after drying.

Analyte Compound

Formula Concentration % Analyte

Compound

Formula Concentration %

F F 0.33 S SO3 46.22

Na Na2O 0.30 Ca CaO 39.38

Mg MgO 0.26 Sr SrO 0.27

Al Al2O3 0.26 Ti TiO2 0.19

Si SiO2 10.06 Fe Fe2O3 1.54

P P2O5 2.52 Ni NiO 0.25

Element Concentration

(mg/kg) Element

Concentration

(mg/kg)

La 117.0 Sm 2.0

Ce 234.1 Er 79.1

Pr 27.1 Y 21.6

Nd < 0.1 Yb < 0.1

Total REEs ~ 481.0 mg/kg


04

3.1.2. X-ray Diffraction of PGF:

X–ray diffraction (XRD) analysis of pure CaSO4.2H2O and PGF was carried out to determine

the PGF crystalline structure, Fig. (1a & b) respectively. The spectrum showed that there is a great

similarity between the XRD patterns of pure CaSO4.2H2O (a) and PGF (b) which indicates that the

dihydrated calcium sulfate (CaSO4.2H2O) is

the main phase of the PGF. In addition, silica, quartz and kaolinite are present in small amounts

which is in accordance with that observed by Hanna et al [24 & 26].

Fig. (1): X-ray diffraction Spectrum of (a) chemical CaSO4.2H2O (b) PGF.

3.1.3. IR spectrum of PGF:

The IR spectrum of the studied PGF is given in Fig.(2). It indicates the presence of two

absorption bands at the 3550.6 cm-1 and 3404.5 cm-1; and two absorption bands at 1686.1 cm-1 and

1620.1 cm-1 which are characteristic to water vibrations (H-O-H) (stretching and bending vibration)

[27] confirming the presence of PGF in the form of CaSO4.2H2O. Bands observed at 2361.7 cm-1,

2115.6 cm-1, 1146.7 cm-1, 1115.0 cm-1, 668.8 cm-1 and 601.4 cm-1 are associated with the vibration of

O-S-O in the SO4- group. The weak bands at 798.4 cm-1 and 779.8 cm-1 as doublet could be related to

the absorption of H2PO4- and HPO4

2- groups, which characterize PG from other gypsum. The bands

observed at the regions 668.8 cm-1 and 601.4 cm-1 are related to the presence of metal oxides content

in PGF. The absorption shoulder at 3243.7 cm-1 is attributed to the Si-F bond [27]. The positions of IR

absorption bands of phosphogypsum are available in the literature and mentioned by several authors

such as Hanna et al. [24] and Hassen et al. [26]. This spectrum is almost in agreement with the

reported bands.

0 10 20 30 40 50 60 70

0.0

0.5

1.0

1.5

2.0

0 10 20 30 40 50 60 70

0.0

0.5

1.0

1.5

2.0

Inte

nsity (

kcp

s)

Inte

nsity (

kcp

s)

2-Theta Scale

a

b


04

Fig. (2): IR spectrum of PGF produced from Abu-Zaabal Company for Fertilizers and Chemicals.

3.2. Leaching Studies:

Nitric, sulfuric and hydrochloric acids were used as leaching solutions for lanthanides from

PGF generated as a by-product from the production of phosphoric acid from Abu-Zaabal Company

for Fertilizers and Chemicals using the wet process. It has been proposed that the lanthanides can be

present in isomorphous substitution with Ca+2 in the PG [28 & 29]. To optimize the conditions for

leaching lanthanides from PGF, the influence of different factors affecting the degree of lanthanides

extraction into leach solution was studied as follows:

3.2.1. Type of acid and concentration:

The effect of acid concentration (HNO3, H2SO4 or HCl) on total lanthanides leaching from PGF

was studied at different acid concentration ranging from 0.1 to 4.0 M. Other parameters were fixed at a

leaching time of 3.0 hr, temperature of 25 oC and liquid/solid (L/S) ratio of 2/1.

The obtained results are presented graphically as a relation between total leached lanthanides (%L)

and acid concentration, Fig. (3). For HNO3, it is clear that as acid concentration increased from 0.10 to

3.0 M, the % L increased from 12.6 to 43.3 %. Further increase in nitric acid concentration has a

negligible effect on %L. Using HCl, as leaching solution, the %L increased from 1.0 to 11.9 % with

increase in acid concentration from 0.1 to 2.0 M. Further increase in acid concentration up to 4.0 M,

% L has a slight decrease. In case of leaching with sulfuric acid, the maximum % L was found to be

12.5 % at 4.0 M H2SO4.


07

Fig. (3): Effect of acid concentration on leaching of lanthanides from PGF, at contact time 3.0 hr, L/S

ratio of 2/1 and temperature 25 oC.

Therefore, 3.0 M HNO3, 2.0 M HCl or 4.0 M H2SO4 is used for other experiments of PGF

leaching process to follow the behavior of other parameters.

This high leaching efficiency using nitric acid than sulfuric acid was also observed by Preston et al.

[14], who extracted rare earth oxide from sludge obtained in the manufacture of phosphoric acid from

a South African apatite ore. They found that, leaching of a sludge containing 2.8 % Ln2O3 at ambient

temperature and a liquid/solid ratio of unity with 4.0 M nitric acid for 48 hr gave 40 % leaching

efficiency, but only 7% with 2.0 M sulfuric acid. Further, the work carried out by Nayl et al. [30] on

leaching of lanthanides from PG is most favorable when using nitric acid rather than sulfuric acid.

3.2.2. Contact time:

The effect of contact time on lanthanides leaching from PGF by different acids was studied at

different time intervals ranging from 0.25 to 6.0 hr at a reaction temperature of 25 oC and L/S of 2/1.

The obtained results are given in Fig. (4). From this figure it is clear that, for HNO3, as the time

increase from 0.25 to 3.0 hr, the %L of total lanthanides increased from about 14.1 to 43.7 %, then %

L is almost constant up to 4.0 hr after that the % L slightly decreases as time increases. In case of

H2SO4, the % L gradualy increases as time increases reaches ~ 25.9 % at 6.0 hr. While, for HCl, as

the time increase the % L increase and reaches 18.9 % at 1.0 hr then it decrease with further increase

in the time. This decrease can be explained on the assumption that certain precipitated fluorides such

as Ca2 F is dissolved and released into solution

0 1 2 3 4 5

0

5

10

15

20

25

30

35

40

45

% L

[acid], M

HNO3

HCl

H2SO

4


00

Fig. (4): Effect of contact time on leaching of lanthanides from PGF with 3.0M HNO3, 2.0 M HCl and

4.0 M H2SO4 at L/S ratio of 2/1 and temperature 25 oC.

which subsequently interact with the released lanthanide to form insoluble lanthanide fluoride

according to the following reactions:

2HCl + CaF2 CaCl2 + 2HF

3HF + Ln3+ LnF3 + 3H+

Accordingly, as contact time increased the leached lanthanides are precipitated again as lanthanide

fluoride [31].

3.2.3. Leaching temperature:

The effect of temperature on leaching lanthanides from PGF was investigated at temperature

ranging from 25 -75 oC, Fig. (5). The % L of lanthanides obtained with 3.0 M HNO3 shows slight

increase as temperature increase from 25 -55 oC. With further increase in temperature, % L suffers

from sharp decrease. Therefore, room temperature is used for leaching lanthanides from PGF by 3.0 M

HNO3. In case of leaching with 2.0 M HCl, the % L increase with temperature up to 65 oC then it

remains constant with further increase in temperature. The change in temperature has no effect on %

L, when H2SO4 is used.

The decrease observed at high temperature can again be explained in terms of the release of

fluoride ions at high temperature and the formation of lanthanide fluoride precipitate.

0 1 2 3 4 5 6 7 8 9

0

5

10

15

20

25

30

35

40

45

% L

time , hr

HNO3

HCl

H2SO

4


04

Fig. (5): Effect of temperature on leaching of lanthanides from PGF with: a) 3.0 M HNO3 at contact

time 3.0 hr and L/S ratio of 2/1; b) 2.0 M HCl at contact time 1.0 hr and L/S ratio of 2/1;

c) 4.0 M H2SO4 at contact time 6.0 hr and L/S ratio of 2/1.

3.2.4. Liquid/solid (L/S) ratio:

Optimum L/S ratio of the lanthanides leaching from PGF was determined at ratios 1:1, 2:1, 3:1,

4:1 and 5:1; Table (4). For all acids, the 1:1 L/S ratio suffer from paste formation followed by

solidification and it is highly difficult to mix and separate solid from liquid. By employing HNO3 with

concentration of 3.0 M at leaching temperature of 25 oC and leaching time of 3.0 hr, the optimum L/S

ratio is 3:1. For 2.0 M HCl at 65 oC and leaching time of 1.0 hr, the % L decrease as the L/S increases.

Therefore, 2:1 is more suitable. The same behavior is shown using 4.0 M H2SO4 at leaching

temperature of 25 oC and leaching time of 6.0 hr and the suitable L/S ratio found to be 2:1.

From the aforementioned investigations on the leaching of lanthanides from PGF with different

mineral acids it was found that, the maximum total lanthanides obtained is ~ 47.4 % with 3.0 M HNO3

at L/S 3:1, contact time 3.0 hr and 25 oC. While, the optimum conditions of HCl produce ~ 35 % of

lanthanides at 2.0 M HCl with contact time 1.0 hr and 65 oC at at L/S 2:1. In case of H2SO4, the

maximum leached total lanthanides is ~ 26.1 % with 4.0 M H2SO4, contact time 6.0 hr and 25 oC at

L/S 2:1. Therefore, it is clear that, HNO3 is more effective for lanthanide leaching from PGF.

0 10 20 30 40 50 60 70 80 90

0

5

10

15

20

25

30

35

40

45

50

%L

Temp.,0C

HNO3

HCl

H2SO

4


04

Table (4): Effect of L/S ratio on % L from PGF with: a) 3.0 M HNO3 at contact time 3.0 hr and 25oC;

b) 2.0 M HCl at contact time 1.0 hr and 65 oC; c) 4.0 M H2SO4 at contact time 6.0 hr and

25 oC.

S/L %L

HNO3 HCl H2SO4

1:1 - - -

1:2 43.5 35.0 26.1

1:3 47.4 30.2 24.3

1:4 45.6 22.6 23.1

1:5 41.4 14.3 19.6

3.2.5. Salt addition:

The effect of salt addition on the % L was studied for HNO3 system. The addition of Ca(NO3)2,

NaNO3 and Mg(NO3)2, salts to 3.0 M nitric acid solution were investigated in the concentration range

from 0.1 – 2.0 M, Fig. (6). As shown from the figure, addition of NaNO3 and Mg(NO3)2 has no effect

on % L. On the other hand, increase of Ca(NO3)2 concentration from 0.0 – 1.0 M in 3.0 M HNO3

increased the % L from 47.4 to 59.5 % then it remains almost constant with further increase in

Ca(NO3)2 concentration up to 2.0 M.

Fig. (6): Effect of salt concentration on leaching of lanthanides from PGF with 3.0 M HNO3 at contact

time 3.0 hr, L/S ratio of 3/1 and 25 oC.

0.0 0.5 1.0 1.5 2.0 2.5

30

40

50

60

% L

Conc. ,M

Ca(NO3)

2

NaNO3

Mg(NO3)

2


03

Table (5): Chemical analysis for leaching solution obtained with HNO3 in the absence and in presence

of Ca(NO3)2 by XRF.

Analyte Compound

Formula

Concentration %

Absence of Ca(NO3)2 presence of 1.0 M Ca(NO3)2

Ca CaO 54.5 68.1

Fe Fe2O3 5.1 7.3

S SO3 39.5 23.1

Sr SrO 0.9 1.6

The XRF analysis for the obtained leached solution with HNO3 in the absence and in presence

of 1.0 M Ca(NO3)2 are represented in Table (5). The addition of 1.0 M Ca(NO3)2 to 3.0 M HNO3

increased the % L of lanthanides from 47.4 to 59.4% and decreased the sulfur anions from 39.5 to

23.1% as shown in Table (5). Furthermore, the concentration of strontium in the solution increased

from 0.9% to 1.6%. These variations can be explained in terms of the displacement of the high

concentration of calcium for the lanthanides and strontium species from the solid phase, in which these

species are hetero-valently substituted for calcium ions [32-34]. Subsequently, both lanthanides and

strontium concentrations released in the leach solution with the decrease of the sulfate concentration.

On the other hand the increase in iron concentration in the presence of Ca(NO)3 from 5.1 % to 7.3 % is

related on the possible partial solubility of iron oxide from the nitrate anions into the leach solution.

Developed procedure:

The obtained results indicated that the most suitable studied acid solution for lanthanides

leaching from PGF is HNO3. The optimum conditions are; nitric acid concentration of 3.0 M, L/S ratio

3:1, and contact time 3.0 hr at room temperature that give leaching efficiency of about 47 %. To

increase the leaching efficiency, the PGF sample is exposed to more than one leaching cycle under the

same conditions. Accordingly, 150 ml of 3.0 M HNO3 was added to 50 g of PGF stirring for 3.0 hr at

room temperature then filtered. The residue is directed to the next cycle while the filtrate is analyzed.

The obtained lanthanides leaching efficiency was 47.1 % from the first cycle, 14.4 % from the second

cycle and 4.5 % from the third cycle.

Therefore, the total lanthanides obtained from the above sequential three stages are ~66 % from

the total lanthanides present in the original sample. The obtained acidulate solution was analyzed after

three cycles as total and individual lanthanide, Table (6). The relation between the concentration of

individual lanthanide in the original sample and in leached solution is given in Fig. (7). From this

figure, it is clear that the leaching efficiency is different for individual lanthanide. In this respect La is

the highly leached element from PGF with efficiency more than 73.4%, this is followed by 68.0 % for

Ce, 62.3 % for Er, 55.1 % for Y and finally 39.5 % for Pr.

While Ca(NO3)2 enhancing the leaching of the lanthanides from PGF, the separated lanthanides

solution contains increased amount of Ca(NO3)2 which complicated the purity of lanthanides

produced.


03

Table (6): Chemical analysis of lanthanides in acidulated solution collected after 3 cycles of leaching

of 50 gm PGF with 150 ml of 3.0 M HNO3 at contact time 3.0 hr, L/S ratio 3:1 and 25 oC.

Fig. (7): The relation between the concentrations of lanthanides in the original sample and in leached

solution.

5. CONCLUSION

The total lanthanides content in PGF is about 481 mg/kg. The main component of the lanthanide

elements are Ce, La, Er, Pr, and Y. The leaching behavior of the total lanthanides from PGF has been

investigated using nitric acid, hydrochloric acid and sulfuric acid. Recovery is highest when the PGF

is leached with HNO3. It is found that the highest leaching efficiency was obtained using 3M HNO3,

L/S = 3, and mixing time of 3 hours, at room temperature. After three cycle of leaching, about 66% of

the leached lanthanides was obtained from PGF. The leaching efficiency of individual lanthanide

element was different.

La Ce Pr Sm Er Y

0

50

100

150

200

250

Ln

Co

nc., p

pm

Ln

Ln Conc. after

Ln Conc. before

Element Conc., (mg/l) Element Conc., (mg/l)

La 85.9 Sm --

Ce 159.2 Er 49.3

Pr 10.7 Y 11.9

Nd -- Yb --

Total REEs 317 mg/l ≈ 65.9%


03

REFERENCES

(1) P. Becker, “Phosphates and Phosphoric Acid: raw materials, technology, and economics of the

wet process”, Fertt. Sci. Technol. Ser, Second edition, 6 (1989): 752.

(2) R. Subpart, National Emission Standards for Hazardous Air Pollutants, United States

Environmental Protection Agency (USEPA), (2002).

(3) P.M. Rutherford, M.J. Dudas and J.M. Arocena,"Heterogeneous distribution of radionuclides,

barium and strontium in phosphogypsum by-product", Sci. Total Environ., 180 (3) (1996) 201-

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