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EG0800322 Leaching and Group Separation of Lanthanides from Phosphogypsum S. A. El-Reefy, A. A. Nayl and H. F. Aly Hot Laboratories Center, Atomic Energy Authority, 13759, Egypt

ABSTRACT

Phosphogypsum (PG) is obtained as a by-product solid waste as a result of decomposition of phosphate with H2SO4 for production of phosphoric acid. Lanthanides content in PG produced from Sebaiya is found to be about 0.022%. Leaching of total lanthanides from this PG has been studied and optimized. Maximum leaching of lanthanides 76 % was obtained using a mixture of 2-3 M nitric acid and 0.8 M calcium nitrate. Purification of lanthanides from the leached solution was carried out by both and exchange processes. The efficiency of lanthanides recovered by solvent extraction using TBP was 93 % whereby the efficiency using ion exchange was 98%. A flow sheet for the two processes is given. Key Words; Leaching/ Separation/ Lanthanides/ Phosphogypsum

INTRODUCTION

Industrially, phosphoric acid is mostly produced from phosphate ores by attacking with sulphuric acid in the so-called, wet process. In this process, the large amounts of lanthanides (80- 85%) are precipitated in the phosphogypsum (PG) produced (CaSO4.2H2O)(1). The reminds of the lanthanides are dissolved in the crude phosphoric acid. PG is considered as a source of lanthanide elements. Lanthanides elements are used in a large scale in industrial and nuclear fields(2). Accordingly, recovery of lanthanides from phosphate ores as a by-product is a step towards the conservation of natural resources. The present work was directed to the recover of some lanthanides elements from the solid phosphogypsum produced as a by-product from the production of phosphoric acid by the wet process in Abu-Zaabal Company, Egypt. Studies on leaching PG were first carried out and optimized. Further, studies on the recovery of lanthanides from the leached solution were carried out .using solvent extraction by TBP and ion exchange chromatography using the cationic resin N,N,N',N'- tetramethylmalonamide (TMMA) (3).

EXPREMENTAL

Chemicals and materials:

All chemicals used are of analytical grade, unless otherwise stated. The phosphogypsum was obtained from Abu-Zaabal Company, Egypt. It was first washed with distilled water to remove excess H3PO4 and left to be air dried.

Leaching process:

The total concentration of the lanthanides in the PG is taken as 0.022 % as previously repoted(4). Leaching were prepared from nitric acid of different concentrations and admixture of nitric acid and calcium nitrates of known concentrations. Leaching experiments were carried out by taken known weight of PG with a certain volume of the leaching solution and mixed thoroughly for a predetermined period. The mixture was left for settling and the concentration of the lanthanide was determined in the leach solution.

Separation processes:

Solvent extraction procedure was carried out by taken equal volumes of the aqueous phase containing lanthanides and organic phase containing TBP. Stripping of the extracted lanthanides was performed by 0.05 M HNO3 aqueous solution.

Column chromatographic separation was performed using a column of 7.0 cm bed height and 0.8 cm internal diameter packed with the cation exchanger TMMA resin. The flow rate of the eluent solution was 0.3 ml/min. The column was loaded with lanthanides leach solution , this is followed by lanthanides elution with distilled water.

Analysis

Lanthanides concentration was determined spectrophotometrically by Arsenazo (III) method(5).

RESULTS AND DISSCUSSION

The lanthanides content in phosphogypsum produced from Sebaieya ore is nearly 0.022 g/kg. A 2- 3+ 2+ typical analysis of phosphogypsum is 29.2% Ca, 41.39% SO4 , 0.10% Fe , 0.20% Mg , 0.8% (4) P2O5, …etc. . In this work, leaching of total lanthanides from phosphgypsum was studied. Further, solvent extraction and column chromatography were studied for separation of lanthanides.

Leaching studies 2+ It has been found that the lanthanides are present in isomorphous substitution with Ca in the gypsum(6). To optimize the conditions for phosphogypsum leaching, the influence of different factors affecting the degree of lanthanides extraction into leach solution was studied.

Selection of acid media and acid concentration

The effect of HNO3, H2SO4, and HCl concentration ranging from 0.5M to 5M on leaching lanthanides, as percent recovery, from phosphogypsum for 8hr and solid to liquid ratio of 1:1 at room temperature was investigated. The recovery percent of lanthanides into the leach liquor are plotted against the different concentrations of each acid as shown in Fig.(1). The figure shows that the % leached of lanthanides increases with the increase of acid concentrations to reach a highest value at 2M HNO3 (46%) and at 4M H2SO4 or HCl (about 30%). Accordingly, further studies were carried out using 2M HNO3. 60

50

HNO3

40

30 HCl

H2SO4

20

Recovery Percent of Nd (%) Recovery 10

0 0123456 Acid conc., M Fig.(1): Effect of acid concentration on the recovery of lanthanides from phosphogypsum Time = 8 hr S : L = 1 : 1 T = 25 oC

Effect of solid-to-liquid rat

The effect of solid(g)-to-liquid(ml) ratio (S:L) on the leach abikity of lanthanides from phosphogypsum by 2M HNO3 for 8hr of mixing at room temperature was studied in the range from 1 : 1 to 1 : 5 ratio. The results obtained given in Fig. (2), show clearly that decrease the S/L ratio up to 1/3 increases the lanthanides leached to reach a maximum value of 56%. Further increase in the S/L values produced lower leaching values. The use of successive fresh portions of HNO3 (2M) at a fixed S : L ratio (1 : 1) gives a high cumulative recovery of 68% after three stages.

Effect of additives

The recovery percent of lanthanides from phosphogypsum to the leach liquor may be depended upon the various fluoride present in the gypsum. If this is so, then it is evident that the 3+ concentration of lanthanides (Ln ) in solution cannot exceed those dictated by the solubility product equilibrium of the lanthanides fluorides:

3+ + 3F LnF3 …………..….……(1)

60

55

50

45 Recovery Percent ofRecovery Nd (%)

40 0.0 0.4 0.8 1.2 (Solid / Liquid) ratio

Fig.(2): Effect of (solid / liquid) ratio on the recovery of lanthanides from phosphogypsum by 2M

HNO3 solution Time = 8 hr T = 25 oC

On the other hand the free fluoride ion concentration itself is a function of other equilibrium, such as;

+ - HF H + F ………..….……...……(2) 2+ - SiF 6 SiF4 + 2F …….....……...…….(3) 2+ Ca + 2F CaF2 ……………...…….(4)

A logical extension to this line of thought is to consider the effect of the addition, to the leach solutions, of appropriate amount of substances known to diminish the free fluoride concentration, 3+ 3+ either by the formation of stable metal complexes (Al , Fe , etc.,) or by precipitation ( e.g., 2+ Ca ) . Fig. (3) shows the effect of addition of calcium or (in the form of their nitrates) to o phosphogypsum that is leached with 2M HNO3 for 8h at a solid-to-liquid ratio of 1:1 at 25 C. It can be seen that as the concentration of additives increases the concentration of lanthanides in the leach liquor increases till 0.8M calcium nitrate and 1.6 M sodium nitrate then a plateau is formed with further increase in the concentration of nitrates.

It can be seen that addition of sodium nitrate had a slight effect while the addition of calcium nitrate is highly beneficial for the recovery of lanthanides. It is reported that the addition of further nitrate ions in the form of ammonium nitrate (2.1M) enhanced the recovery process and improved the efficiency of the extraction process. This is due to the fact that a high calcium concentration in the solution will naturally tend to displace the rare earths from the solid phase, in which they are heterovalentily substituting for calcium ions (7,8,9).

80

70 Ca(NO3)2

60

NaNO3 50

Recovery Percent of Nd (%) 40

30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Concentration of additives, M Fig.(3): Effect of additives concentration on the recovery of lanthanides from

phosphogypsum by 2M HNO3 Time = 8 hr S : L = 1 : 1 T = 25oC

Group Separation of Lanthanides from phosphogypsum

Based on the results obtained, 1 kg of phosphogypsum was taken and leached with 3 liters of o 2M HNO3 solution containing 0.8M Ca(NO3)2. The leaching process was carried out at 10 C for 8 hours with continuous stirring. This represents the feed solution after to bring a clear solution containing 16.7 mg total lanthanides (based on Nd determination). Two processes were performed to separate the lanthanides from the leach solution using solvent extraction based on the TBP system and ion exchange chromatography. The solvent extraction process was carried out by equilibrating one liter from the leach solution with equal volume of 100% TBP (3.66M) by shaking at room temperature for 5 minutes. The organic phase containing lanthanides was scrubbed with equal volume of 3M HNO3 solution to remove impurities. Finally, the lanthanides were recovered in distilled water aqueous phase. The efficiency for recovery was 93%. The second process was carried out by ion exchange chromatography using N,N,N',N'- tetramethylmalonamide as a chelating resin. A feed solution of 900ml of the lanthanides leach solution was loaded with a flow rate of 0.3 ml/min. After loading the different lanthanides, the remaining leach solution and impurities were removed with 10ml of 3M HNO3. The lanthanides were then eluted with distilled water (80 ml). The process of loading, washing and elution is illustrated in Fig.(4).The efficiency of recovery was found to be 98% . In Fig.(5), a flow sheet for the lanthanides recovery from phosphogypsum for the whole process is given.

1.0

0.8 ) g solution 3 (mg)

+3 0.6 Loading solution (phosphogypsum

leached by 2M HNO3 solution) 10ml washin 10ml HNO (3M 0.4 (900 ml) Concentration of LnConcentration of 0.2 Washing

0.0 -150 -100 -50 0 50 100

Volume Added, ml H2O Fig.(4): Loading and chromatogram of the separation of lanthanides from phosphogypsum leached by 3M

HNO3 contain 0.8MCa(NO3)2

Phosphogyps 1.0 Kg um phosphogypsum . (0.022g Ln-

2-3 M HNO3 for 8 hrs (S/L = 1/3) Leaching with

0.8M Ca(NO3) at Room Temperature

Filtration Leach Solution 76% Ln (NO3)3 0.01672gLn(NO3)3

Filtration Extraction (100% TBP)

Organic Phase [Ln(NO3)3. 3TBP]

Stripping by H O0.01655gLn(NO 3)3 (98%) 2 Concentrat ed (93%)Concentrated lanthanide lanthanides solution 0.01555gLn(NO3)3 ssolution

Fig.(5): Flow sheet of the recovery of lanthanides from phosphogypsum

From this flow sheet, it is clear that column chromatographic separation is more efficient than solvent extraction and on the other hand, chromatographic separation is more time consuming

REFERANCES

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