Treatment of fluoride containing effluent generated during uranium metal production

A. Soman, Y. S. Ladola, S. Sharma, S. Chowdhury, V. H. Rupawate

Uranium Extraction Division, BARC, Mumbai Abstract:

Magnesio-thermic Reduction (MTR) of Uranium tetra Fluoride (UF4) is one of the main industrial methods for producing commercial pure uranium metal in massive form. Production of UF4 from uranium dioxide (UO2) is prior step to MTR process. Production of UF4 is carried out in an electrically heated rotary inclined tubular converter at 450oC by passing anhydrous Hydrogen Fluoride (HF) gas counter-currently. Hydro fluorination of UO2 is exothermic and reversible reaction.

UO2 + 4HF UF4 + 2H2O (ΔH0298 = – 41.3 kcal/gm mole)

At least 98.5% conversion of uranium dioxide is required for better recovery of uranium in MTR process. Excess HF is used to achieve required conversion of UO2 as the reaction is reversible. Excess HF with off gases is condensed by water cooling, for HF recovery. Off gases from condenser is scrubbed with KOH (potassium hydroxide) solution for complete removal of HF before release to atmosphere. KOH solution is circulated in scrubber using magnetically coupled pump. HF is neutralized by KOH and converts to potassium fluoride. When this solution becomes acidic, it requires KOH regeneration process. The solution is treated with Ca(OH)2 (hydrated lime) to neutralize dissolved HF and to regenerate KOH.

2HF + Ca(OH)2 CaF2 + 2H2O 2KF + Ca(OH)2 CaF2 + 2KOH

As indicated in the reaction calcium fluoride is precipitated, CaF2 is filtered and disposed as radioactive solid waste. However, filtration of CaF2 is a challenging task as the solid waste obtained even after filtration or centrifugation contains high level of moisture which makes it difficult to dispose. This paper discusses the different methods and parametric effects on reduction of moisture in resultant CaF2. Key words: Fluoride effluent, Uranium production, Radioactive solid waste 1. Introduction:

Magnesio-thermic Reduction (MTR) of Uranium tetra Fluoride (UF4) is one of

the main industrial methods for producing commercial pure uranium metal in massive

form. Production of UF4 from uranium dioxide (UO2) is prior step to MTR process.

Production of UF4 is carried out in an electrically heated rotary inclined tubular converter

at 450oC by passing anhydrous Hydrogen Fluoride (HF) gas counter-currently. Hydro

fluorination of UO2 is exothermic and reversible reaction.

UO2 + 4HF UF4 + 2H2O (ΔH0298 = – 41.3 kcal/gm mole)

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At least 98.5% conversion of uranium dioxide is required for better recovery of uranium

in MTR process. Excess HF is used to achieve required conversion of UO2 as reaction is

reversible. Excess HF with off gases is condensed by water cooling, for HF recovery. Off

gases from condenser is scrubbed with KOH (potassium hydroxide) solution for complete

removal of HF before release to atmosphere. KOH solution which reacts with HF to form

KF is most suitable scrubber solution compared to other alkaline base as summarized in

Table 1. Solubility of potassium fluoride produced in HF neutralization is nearly ten

times higher than other salt of alkaline base except ammonia. Ammonia is not suitable for

scrubber solution due to higher partial pressure of ammonia in water solution which leads

to release of ammonia gas in atmosphere. KOH solution is circulated in scrubber using

magnetically coupled pump. In course of time scrubbed solution becomes acidic, it

requires KOH regeneration process. For regeneration, the solution is treated with

Ca(OH)2 (hydrated lime) to neutralize dissolved HF and to regenerate KOH.

2HF + Ca(OH)2 CaF2 + 2H2O

2KF + Ca(OH)2 CaF2 + 2KOH

As indicated in the reaction precipitated calcium fluoride is filtered and disposed as

radioactive solid waste. CaF2 produced after treatment can find industrial applications

like (i) reuse as raw material for the HF production (ii) filler material in concrete,

enamels for ceramic substrates and (iii) flux in the manufacturing of steel and cast iron.

This requires extra decontamination process and special processing before taking in

commercial domain. However, filtration of CaF2 is a challenging task as the solid waste

obtained even after filtration or centrifugation contains high level of moisture which

makes it difficult to handle and transport. High affinity of CaF2 to the water may be due

to formation of gelatinous precipitation of CaF2. Harrison has discussed in his patent

about this characteristic of CaF2 and claimed that carbon dioxide bubbling during

neutralization process can completely eliminate gelatinous precipitation of CaF2[1]. It

was therefore decided to study initially the particle size and particle size distribution of

CaF2 as they are important parameters in formation of gel.

Besides nuclear industry, fluoride waste water is a typical effluent in electronic

industry [2, 3] and petrochemical industry [1] and fluoride fixation through neutralization

is essential in eliminating its hazardous effects. Over the years different methods of

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treatment and different neutralizing chemicals have been reported in literature as

summarized in Table 2. Lime treatment process has been adopted for large scale

commercial operation due to its cost effectiveness. Present study was conducted to find

the effect of operating parameters on moisture content of product cake.

2. Experimental:

A Polypropylene (PP) beaker of 2 liter capacity, fitted with baffles was used as the slurry reactor. One Stainless steel stirrer with speed controller, specially designed for slurries, was used for agitation. Since the solution used for experiment was highly corrosive, the shaft and the blades were coated with Teflon. Lime slurry was prepared in this reactor using pre-weighed lime and water. Lime was taken as per stoichiometric requirement. Fluoride solution containing KF and HF was collected from the plant. Combined fluoride strength of solution was 200 g/l. Measured volume of fluoride solution was taken in a separating funnel with plug at the top to prevent escape of fluoride vapour from the solution. Suitable tubing was used to connect the bottom of the funnel to the reactor. This was done to prevent any spillage and for controlled addition of the solution to the lime slurry. The pH was monitored using pH paper. The vacuum filtration was carried out in Buchner funnel using vacuum pump. A vacuum gauge was used to measure the vacuum created by the pump during filtration. Vacuum was kept between 650-680 mm of Hg. Filtration was done using Whatman filter paper 41 (pore size 20-25 microns). Particle size is measured by laser diffraction particle size analyzer CILAS 1180. Moisture content of filtered CaF2 cake was measured by moisture analyzer Sartorious MA 100. 3. Results and discussion :

Moisture content of filtered CaF2 cake generated during different experiments is given Table 3. It has been observed that moisture content of cake does not vary with concentration of lime slurry. It does not even change when reaction has stopped at pH 6. Physical appearance shows all cakes are like paste. It suggests that filtered cake of different experiments have similar water content. The particle size distribution of filtered CaF2 cake of different experiments is similar as shown in Figure 1. Cake particle size distribution is not changing with lime slurry concentration and with final pH.

Reaction of hydrated lime with HF and KF may have either of these two possible reaction mechanisms viz i) homogenous reaction, where dissolved lime reacts with fluoride solutions and gets precipitated out as CaF2 due to laters low solubility or ii) heterogeneous reaction, where fluoride ion diffuses to outer as well as inner lime surfaces and follows unreacted shrinking core model. Particle size distributions of hydrated lime and CaF2 cake as shown in Figure 1, reveals that lime particles below 5μm get dissolved

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completely or partially. Hence, homogeneous reaction mechanism is also working for dissolved lime and fluoride. Precipitation of CaF2 can occur on new nucleate of CaF2 or on unreacted lime particles which are available in large quantity. It seems that CaF2 also precipitates on unreacted lime particles due to very high concentration of slurry and Fluoride. Once CaF2 layer is formed on lime particles, dissolution of lime will be hindered. In such condition due to higher concentration of fluoride and availability of large number of unreacted lime particles, heterogeneous reaction mechanism starts functioning as unreacted shrinking core model. When both reaction mechanisms work simultaneously, particle size distribution of CaF2 will definitely shift to the higher side compared to original particle size distribution of lime. This can be clearly observed in Figure 1. 4. Conclusion :

From obtained experimental results it may be concluded that moisture content of filtered

CaF2 cake and cake particle size distribution are not changing with lime slurry

concentration as well as with final reaction pH. Particle size distributions of lime and

CaF2 cakes suggest that both heterogeneous and homogenous reaction mechanisms work

simultaneously. However, more experiments with low lime slurry concentration and with

designed lime particles are required to conclude the reaction mechanism. Further studies

on surface morphology are also required to understand the moisture retention property of

precipitated CaF2.

Acknowledgements

The authors would like to thank HBNI, DAE for allowing to do this work.

5. Reference :

1. J. P. Harrison, Process for calcium fluoride production from industrial waste water, US Patent No. 4, 414, 185, 8th Nov. 1983

2. Min Yang, Takayuki Hashimoto, Nobuyuki Hoshi and Haruki Myoga, Fluoride removal in a fixed bed packed with granular calcite, Water Research, 33 (16) (1991), 3395-3402

3. C. Jane Huang and J. C. Liu, Precipitate flotation of fluoride-containing wastewater from a semiconductor manufacturer, Water Research 33 (16) (1999), 3403-3412

4. Daniel Simonsson, Reduction of Fluoride by Reaction with Limestone Particles in a Fixed Bed, Ind. Eng. Chem. Process Des. Dev., 18 (2) (1979) 288-292.

5. R. Aldaco, A. Garea, and A. Irabien, Fluidized bed reactor for fluoride removal, Chem. Eng. Journal., 107 (2005) 113-117

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6. R. Aldaco, A. Garea, and A. Irabien, Fluoride Recovery in a Fluidized Bed: Crystallization of Calcium Fluoride on Silica Sand, Ind. Eng. Chem. Res., 45 (2006) 796-802

7. A. Garea, R. Aldaco, and A. Irabien, Calcium fluoride recovery from fluoride wastewater in a fluidized bed reactor. Water Research, 41 (2007), 810-818

8. en.wikiepidia.org 9. www.Hfacid.com Table: 1. Possible alkaline scrubbing agent and solubility of product salts

Reactant Salt produced by HF Solubility of salt in water (W/W)

(from reference 8 & 9)

KOH KF 40%

NaOH NaF 4%

Na2CO3 NaF 4%

NaHCO3 NaF 4%

CaO CaF2 0.004%

Ca(OH)2 CaF2 0.004%

CaCO3 CaF2 0.004%

NH3 NH4F 45.3%

Table: 2. Summary of literature survey on fluoride treatment.

Neutralizing chemical

Concentration of fluoride

effluent (fluoride

compound)

Final fluoride

concentration

Process equipment

Remarks Ref.

Ca(OH)2& CO2

4.42 gm/l (KF)

0.18 g/l Mechanical agitated tank

with CO2 bubbling

1) No formation of gel like CaF2

2) Excellent filtration characteristics

1

CaCO3(Granules)

0.025-3 gm/l (NaSiF6,

NH4F, HF & NaF)

0.005-0.02 g/l

Fixed bed Reactor

1) Molar ratio of H+ ion to F- ion is important for complete conversion

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NaOH & Ca(OH)2

1-3 gm/l (HF)

0.0015-0.02 g/l

Flotation tank

(surfactant used for flotation)

1) Molar ration of Ca+ to F- is critical for complete removal of fluoride

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5

CaCO3(lime stone)

4-100 gm/l (NH4F, HF &

NaF)

1-10 g/l Fixed bed Reactor

1) Rate of reaction increase with temperature

2) Rate of reaction does not change with fluoride concentration

3) Unreacted shrinking core model proposed for reaction

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Ca(OH)2 0.3-2gm/l (synthetic fluoride effluent)

---- Fluidized bed reactor (silica Sand as seed for

CaF2 precipitation)

1) Liquid- liquid reaction

2) No formation of sludge

3) Precipitation of CaF2 on silica sand

5 & 6

CaCO3(Granules)

0.3-2gm/l (synthetic fluoride effluent

---- Fluidized bed reactor

1) Solid-liquid reaction 2) CaF2 can be directly

reused for HF production

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Table: 3. Moisture content of filtered cake at different reaction conditions. Exp no

Concentration of lime slurry

(% w/w)

Final pH

Thickness of cake after filtration (cm)

Moisture content of filtered cake

(%) 1 10 8 1.3 51.19 2 20 8 1.4 48.19 3 30 8 1.4 48.58 4 30 8 1.4 48.19 5 30 6 1.3 48.83

6

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12Particle size in micron

% N

o. o

f par

ticle

sHydrated Lime

10% slurry

20% slurry

30% slurry

30% slurry and reactionstopped at 6 pH

Figure: 1. Particle size distributions of hydrated lime and CaF2 at different reaction

conditions

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