STUDIES OF DEHYDRATION AND FLUORINATION

It was pursued to examine basically the properties and the dehydration of hydrated uranium tetrafluorides, and the fluorination of the dehydrated uranium tetrafluorides.

5.1. Properties of hydrated uranium tetrafluorides

The hydrated uranium tetrafluorides produced by reaction with UC1⁴ and HF in the PNC process are mainly U F4| H2O crystals, but three crystal structures could be obtained under different conditions. These are the orthorhombic type (UF4-2.5 H2O: R-type), the cubic type (UF4-1.5 H2O: C-type), the monoclinic type ( U F4J H2O: M-type) and the mixture of these types.

The properties of hydrated uranium tetrafluorides are shown in Table IV, and Fig. 3 shows microscopic and scanning electron microscopic photographs of U F⁴| H²O which has been used for the dehydration in the fluidized-bed reactor.

60 TAKENAKA and KAWATE

Hydrofluorinator

UCI4 or U(SO4) 5 0 % HF

Heat exchanger

Overflow

to Scrubber

_UF<-.3/4HzO

FIG.2. The hydrofluorinator at the Ningyo-toge Plant.

IAEA-AG/33-1 61

TABLE IV. PROPERTIES OF HYDRATED URANIUM TETRAFLUORIDES

Type

62 TAKENAKA and KAWATE

I ' > • 1 1

50 ,um

F1G.3. Microscopic and scanning electron-microscopic photographs of UFA' I ИгО.

lAEA-AG/33-1 63

Specific surface area (mJ/g) Apparent density (g/ml)

5.2. Dehydration of hydrated uranium tetrafluorides

As the UF4 • I H2O indicates excellent fluidizing properties, it is easy to control the fluidizing velocity in the reactor. The grain sizes of the U F4 -| H2O are shown in Table V.

The dehydration products and the temperature of phase transition were studied by differential thermal analysis (DTA) and thermogravimetric analysis (TGA) in a stream of nitrogen as shown in Fig. 4, and X-ray diffraction as shown in Fig. 5. The forms and crystal system of UF4-| H2O before and after dehydration were the same, but the specific surface area was increased after dehydration.

The crystal grain sizes estimated with the peak half-width of X-ray diffraction patterns (Warren's method) are shown in Table IV.

The oxygen from the decomposition of hydrated uranium tetrafluorides promotes the formation of UO2, and the peaks in the X-ray diffraction patterns demonstrate that uranium dioxide, UO2, is formed in a stream of nitrogen above 350°C. In the dehydration test in the fluidized-bed reactor only small quantities of UO2F2 and UO2 are formed, for example, 0.83 wt% of UO2F2 and 0.03 wt%

of UO2 under the following conditions:

Fluidizing gas dried nitrogen gas Linear velocity of gas 11.4 cm/s Heating rate 3°C/min Maximum temperature 35O°C Holding time at maximum temperature 2 h

The moisture content after dehydration was decreased to several hundred ppm from 4.45 wt%

before dehydration.

64 TAKENAKA and KAWATE

R-type(UF4.2.5H2O)

Cubic

Orino. I мы T6A

DTA

400 eoo»c

C-type (UF4 . 1 5 H2O) M j M ( s.

Cubic Monocli.

200 400

M-type(UF4.3/4H2O) M | M ( s )

Monocti.

eoo

400 600 800 °C

FIG.4. DTA and TCA curves ofhydrated uranium tetrafluorides.

IAEA-AG/33-1 65

s í

3- о

tfg

S ^w

1

x"о

3-5

66 TAKENAKA and KAWATE

TABLE VI. CHANGES IN IMPURITY CONTENTS BY FLUORINATION

5.3. Fluorination of dehydrated uranium tetrafluorides

The physical properties, such as the particle size distribution, remained the same before and after dehydration and therefore the fluidizing properties on UF⁴ fluorination were as excellent as in dehydration. In the ordinary temperature range of 400-450°C in the reactor the UF4 produced by the PNC process had almost the same fluorination reactivity as that produced by the conventional dry process. Continuous fluorination was performed and the maximum value of fluorine utilization efficiency obtained in one stage was about 88%. The behaviour of impurities in the process was also investigated. The changes of impurity content during the conversion are shown in Table VI.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the contributions of their PNC and ACI colleagues and the Mitsubishi Metal Corporation staff members to this work. The authors also thank Dr. T. Kamiyama, Director of PNC, for permission to publish this paper.

REFERENCE

[1 ] TAKADA, S., AMANUMA, T., FUKUDA, G., "Uranium processing pilot mill at the Ningyo-toge Mine", The Recovery of Uranium (Proc. Symp. Sâo Paulo, 1970), IAEA, Vienna (1971) 97-110.

IAEA-AG/33-1 67

DISCUSSION

S. AJURIA-GARZA: What approximately is the cost of processing, say, one tonne of ore?

S. TAKENAKA: That is very difficult to say, because the cost will depend on the kind of mineral treated.

H.E. JAMES: It would seem to me that production capacities at uranium mills throughout the world tend to be rather too small to support a commercial facility based on the PNC process, at the mine site.

F.R. HARTLEY: To follow up that remark, I should like to ask you to comment on what the potential market is for UF4, assuming that a producer at a mill was to install such a plant. There does not seem to me at this moment to be a particularly attractive market which would induce a milling operator to extend his operation to the UF4 stage.

S. TAKENAKA: Of course, the UF4 has to be converted to UF6. Yellowcake is the current market product produced at mine sites. We intend to produce UF4 at the site and then convert it to UF6 somewhere else, at whatever place is convenient.

F.R. HARTLEY: What would the market be in Japan for UF4 ? That is, if the mill were to put in a plant for UF4, would this be a marketable product as far as Japan is concerned?

S. TAKENAKA: PNC has a big project for an enrichment plant in the future. As you know, we have a centrifuge plant in operation. In the future we hope that the UF4 and UF6 that we produce will be used as experimental materials for our enrichment plant. As a next stage we are thinking of producing these materials on a commercial basis.

IAEA-AG/33-16

PROCESS DEVELOPMENT STUDY ON URANIUM-BEARING CARBONATITE

M. SHABBIR, NAEEM-UL-ZAMAN