recovery of locked up u in slag disc by comelting in mtr for correction1

7/29/2019 Recovery of Locked Up U in Slag Disc by Comelting in MTR for Correction1

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Trans. Indian Inst. Met.

Vol. 61, Nos. 2-3, April-June 2008, pp. 103-106

TP 2189

Recovery Of Locked-up Uranium In Slag Disc

By Co-melting In Magnesio-Thermic ReductionY.S Ladola, S. Chowdhury, S. Sharma and S.B. Roy

Uranium Extraction Division,

Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, India

E-mail : [email protected]

(Received 5 December 2007 ; in revised form 7 February 2008)

MTR reaction mechanism is a complex one. A large number of

side reactions as well as parallel reactions also occur during theconversion of UF

4to U. This reaction is exothermic and final

temperature of the molten product mass i.e. U and MgF2

goes

up to around 1600-1700oC. U settles down at bottom due to

large density difference with slag. Good separation is very

important for the better yield.

Interface of U metal and slag is rich in U content because

freshly reduced U metal droplets, which do not get chance to

coalesce with the bulk of the metal due to the formation of

firm crust at the interface of metal and slag end up getting

accumulated at the interface. This interface is removed before

subsequent vacuum induction melting and fuel fabrication. This

cut interface, which contains entrained metal that could not

coalesce with parent metal, some amount of parent metal andquite a good amount of MgF

2, is called the slag disc. It is

desirable to recover U locked-up in these slag discs as the

recovery of U will not only augment current U inventory but

also will reduce the burden of radioactive material storage. To

recover this U, experiments were conducted using co-melting

in MTR operation along with charge by utilizing the heat

generated during exothermic MTR reaction. Experiments have

been also conducted to find the optimum weight of the slag

disc and its location inside the reactor along with charge to

maximize the U recovery. Results obtained are encouraging, as

it has been observed that purity of finished product doesnt

get affected. This method has advantage over other alternative

methods, as it is simple, cost effective, and doesnt demand

additional process step, setup and energy.

Magnesio-thermic Reduction (MTR) of Uranium tetra Fluoride

(UF4) is one of the main industrial methods for producingcommercial pure uranium metal in massive form. Nuclear grade

natural Uranium (U) metal ingots are produced regularly in

UED, BARC following MTR route for fuelling research

reactors in BARC. This is a bomb type reaction and is

represented by

UF4

+ 2Mg = U + 2MgF2-

('Ho298

= 83.5 Kcal/gm mole)

Small excess of magnesium is required to achieve maximum

yield. This thermite type reduction is carried out in a closed

reaction vessel, popularly known as MTR reactor, lined with

magnesium fluoride powder. MTR reactors are made of boiler

quality steel. Use of MgF2, a reaction by-product, as lining

material completely eliminates the chance of foreign elementcontamination. This lining of MgF

2not only prevents direct

contact of the molten metal and slag with the reaction vessel

but also acts as an insulating material immediately after firing

and holds the hot molten mass for longer period, thereby

facilitating adequate metal-slag separation. This is a batch

process and stoichiometric quantity of UF4

and Mg chips are

blended and charged inside the lined reactor. Once the charging

is over, the surface is covered with fine MgF2

powder and

sealed by fixing a lid. This sealed reactor is then heated inside

an electric furnace at a predefined heating schedule for the

reduction to take place. The initiation of reaction is called

Firing.

1. INTRODUCTION

Uranium (U) metal can be produced in a number of ways.

Reduction of Uranium tetra fluoride (UF4) by magnesium

(Mg) or calcium (Ca) has been used for large-scale production

of nuclear grade Uranium. When UF4 is reduced under

specific conditions, a solid regulus of material is formed

under cover of slag. For obtaining massive uranium, the

products of the reaction, the uranium and slag should be

sufficiently fluid and remain so, long enough for the

dispersed particles of freshly produced uranium to come

together, coalesce and merge to the primary interface. The

heat of reaction should be enough to melt uranium and slag

to a condition of sufficient fluidity and compensate heat

losses. We are using Magnesiothermic Reduction (MTR) of

UF4 in UED, BARC for production of nuclear grade uranium

metal ingot.

Magnesio-thermic Reduction reaction is carried out in boiler

quality reaction vessel popularly called MTR reactor. The

reactor is lined with refractory material i.e. magnesium fluoride

(MgF2) to protect vessel from melting due to the heat of

reaction and prevent contamination of the U metal with the

material of the reaction vessel. The blended UF4 and Mg

charge is packed in lined reactor. Top of the charge is capped

with MgF2 powder to protect lid. The lid is then bolted to

the reactor. This sealed reactor is loaded into the furnace for

preheating. Preheating is done following predefine heating

ABSTRACT


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Trans. Indian Inst. Met., Vol. 61, Nos. 2-3, April-June 2008

104 | Ladola et al. : Recovery of locked-up Uranium in slag disc by co-melting in Magnesio-Thermic Reduction

schedule for the initiation and completion of reaction. The

initiation of reaction is generally called firing of the charge.

Firing is observed by temperature rise in temperature recorder.

MTR reaction mechanism is a complex one. A large number

of side reactions as well as parallel reactions also occur

during the conversion of UF4 to U. The reaction is exothermic

and final temperature of the product mass i.e. U and MgF2goes up to around 1600-1700oC. At this temperature, both Uand MgF2 are in molten state. U settles down at bottom due

to large density difference with MgF2. This separation is

very important for effective yield.

The interface of U metal and slag is rich in U content due

to the presence of the freshly reduced metal droplets that

could not coalesce with the bulk of the metal due to the

formation of firm crust at the interface of metal and slag. This

interface is removed by cutting before subsequent refining

and fuel fabrication. This cut interface, which contains metal

droplets that could not coalesce with parent metal, some

amount of parent metal and a substantial volume of MgF2,

is called slag disc. It is desirable to recover U locked-up in

these slag discs as the recovery of U will not only augment

current U inventory but also will reduce the burden of

radioactive material storage.

Different methods can be planned for recovery of this locked

up uranium. These methods are described in Table 1 with

their merits and demerits. However, a brief mention about

them here will be useful.

Dissolution of slag disc in nitric acid is associated with the

problem of huge NOx generation. Slag discs have to be cut

into small pieces to enable their effective dissolution with

controlled addition. Moreover, considerable amount of

harmful fluoride goes in nitric acid stream. Fluoride makes

complex with uranyl ions that adversely affect impurity

Table 1Different methods of U recovery from slag disc

Method

Dissolution of slag disc

in nitric acid.

Direct melting of slag disc

for slagmetal

separation.

Co-melting with MTR

charge.

Advantage

1. Recovery of uranium with desired

purity.

1. Good amount of slag discs can be melted

together in single batch.

2. If slag disc contains some impurity, then

it can be treated as separate batch.

1. No additional process setup, step or

energy is required.

Disadvantage

1. Huge amount of NOx generation.

2. Size reduction of slag disc is required

3. Considerable amount of fluoride goes in nitric acid

that creates problem in subsequent refining step and

creates corrosion problem.4. Additional setup is required.

5. Recovered U has to pass through all step of uranium

metal production from dissolution.

1. Additional step, process setup and energy are

required.

1. If slag disc contains impurity, it can build-up in

product metal ingot and overall recovery will

be reduced.

2. Limitation on weight of slag disc that can be

co- melted per batch.

Fig. 1 : Schematic diagram of MTR reactor with slag disc.


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Ladola et al. : Recovery of locked-up Uranium in slag disc by co-melting in Magnesio-Thermic Reduction | 105

removal in the subsequent Solvent Extraction (SX) for

purification and refining. It corrodes the stainless steel

reaction tanks also1, which necessitate frequent maintenance.

This demands additional operating system for

accommodating fluoride in the process stream and use of

exotic material of construction for better corrosion resistance.

Second method of uranium recovery is direct melting of slag

discs in high temperature system. This method looks more

attractive as more number of slag discs can be melted in

single batch. This method also requires additional cost-

intensive setup.

Third method is Co-melting of slag disc with MTR charge by

utilizing the heat generated during exothermic MTR reaction.This method has the advantage over other alternative

methods as it is simple, cost effective, and doesnt demand

additional process step, setup and energy. It has the

limitation on the maximum weight of slag disc/discs that can

be melted in a single batch.

2. CALCULATION FOR DETERMINING THE

MAXIMUM WEIGHT OF SLAG DISC THAT

CAN BE CO-MELTED WITH MTR

CHARGE

For simplification, basic assumptions are made

a. No heat loss through out the process.

b. Average reaction temperature is 450oC.

c. No heat is utilized for post reduction increase in

temperature beyond melting point of MgF2

(1263oC).

When the MTR reaction is initiated at 25oC, reaction heat is

not sufficient to melt reaction products (U & MgF2)

completely and an additional heat of 6.8kcal/per gm mole

must be supplied to effect their complete melting2.

Preheating is done to supply this additional heat. As

temperature changes, enthalpy of reactants (UF4 and Mg)

changes as shown in Fig. 2. Generally, firing occurs after

630oC of set temperature, measured at outer wall of the

reaction vessel. Temperature is not uniform through out the

charge.

Fig. 2 : Enthalpy of reactant (UF4

and Mg) at different

temperature.

Table 2Results of slag discs co-melted in MTR batches.

Sr Slag disc Separation Ingot Wt. Position of

No Wt. (kg) (kg) disc from

top H, cm

1 4.0 Good 199.5 71

2 5.0 Excellent 198.5 64

3 6.3 Excellent 200.0 64

4 6.8 Good 206.0 64

5 8.3 Good 201.5 64

6 6.3 Excellent 191 56

7 5.9 Good 208 64

8 11.4 Good 212.5 38

9 8.2 Excellent 203.5 38

10 7.0 Good 188.0 46

11 8.3 Good 169.0 46

12 9.1 Excellent 210.0 46

13 4.2 Good 207.0 56

14 3.8 Excellent 202.0 46

15 2.0+3.5 Excellent 200.0 51

16 11.8 Excellent 205.0 51

17 11.4 Excellent 207.0 51

18 7.6 Excellent 203.0 56

19 9.7 Excellent 192.0 64

20 4.1 Good 200.0 43

21 6.7 Good 205.0 53

22 13.7 Excellent 192.0 51

23 7.0 Excellent 198.5 51

24 8.6 Excellent 206.0 48

25 8.9 Very good 197.5 51

26 7.3 Good 191.5 58

27 7.4 Good 207.0 51

28 8.8 Excellent 189.0 43

29 16.2 Very good 205.0 43

30 16.1 Very good 217 64

245.4 kg Average. Wt

~200 kg


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106 | Ladola et al. : Recovery of locked-up Uranium in slag disc by co-melting in Magnesio-Thermic Reduction

2.1 Heat balance

Average temperature of reactants can be assumed as 450oC.

Enthalpy of charge mixture (UF4 + 2Mg) at 450oC is 17.2kcal/

gm-mole of uranium as shown in Fig. 2. Therefore, extra heat

available for co-melting of slag disc is 10.4 kcal/ gm mole of

uranium. Basis of this calculation is for production of 1

kmole of uranium.Melting points of U and MgF2 are 1133

oC and 1263oC

respectively. However, immediately after the reaction, both

will be at the molten state at a temperature beyond their

respective melting points. However, one of the basic

assumptions is that no heat is utilized for post reduction

increase in temperature beyond melting point of MgF2(1263oC) and the extra heat available is completely used for

slag disc melting and subsequent recovery of uranium.

Now, Heat required for melting MgF2 [i.e. converting MgF2 (s)

at RT (298K) to MgF2 (l) at its melting point (1536K)] is

estimated to be 37.08 kcal/gm mole 2 and that of U

[i.e. converting U (s) at RT (298K) to U (l) at 1536K, the MP

of MgF2] is 16.35 kcal/gm mole2.

Considering all available extra heat is fully utilized for co-

melting, amount of MgF2 (slag) and U that can be co-melted

is 0.28 kmole (10.4 / 37.08) and 0.636 kmole (10.4 / 16.35)

respectively.

Assuming that 0.84 kmole of Uranium ingot is produced in

one batch, maximum feasible value for MgF2 (slag) and U

that can be co-melted can be given as 14.34 kg (0.84 X 0.28

X 61) and 127.15 kg (0.84 X 0.636 X 238) respectively.

Considering general composition of slag disc to be 20% of

MgF2 and 80% of U, maximum weight of the slag disc that

can be co-melted as per calculation is estimated to be 49 kg.

But, for all practical purposes, initial assumptions for basic

calculation do not hold true fully and needs actual

experimentation for process standardization

3. EXPERIMENTS

Slag discs were weighed, numbered and co-melted with

regular MTR production batches. One disc per MTR batch

has been co-melted except experiment number 15 where two

small slag discs were co-melted in a single MTR batch. Slag

discs were put vertically in charge and their positions were

measured and recorded from the top as shown in Fig. 1.

Positions of slag discs were varied to understand the effect

of positioning. In this series of experiments, slag discweighing up to 16kg has been co-melted. Each ingot was

weighed and observed for slag separation. Excellent slag

separation was found with minimum thickness or minimum

weight slag disc.

4. RESULTS AND DISCUSSION

30 experiments have been conducted and the results are

tabulated in Table 2. Average weight of an ingot produced

through MTR route is around 192kg with a recovery of 96%.

But, it has been observed that average weight of ingot where

slag discs were co-melted was more and was around 200 kg.

This increase in ingot weight is a definite indication ofrecovery of uranium from the slag disc. Through these

experiments, around 180kg of locked-up U could be

recovered. Around 245 kg of slag disc with 80% uranium

value were co-melted in 30 MTR batches. Results obtained

for determining optimum position/location of the disc also

appear to be satisfactory for all the position of slag disc.

There are good to excellent slag separation observed in all

the experiments. No adverse effects have been observed

during these experiments except experiment no. 11, where

weight of ingot reduced to 169 kg. Some times, lower yield

has been obtained during normal MTR operation too and so,

this can be considered a stray case. Analysis shows that

there is no impurity build-up due to slag disc co-melting withMTR charge and all the ingots are pure and qualified for fuel

fabrication. Though there was chance of contamination due

to long term storage of the discs in an uncontrolled

atmosphere, practically that didnt affect the purity of the

finished product. Maximum weight of single slag disc that

was co-melted with MTR charge is 16.2 kg. Average recovery

of metal from slag disc is calculated to be 92%.

5. CONCLUSIONS

This locked up uranium can be recovered by different

methods as described in Table 1 with their merits and

demerits. Experiments were conducted for co-melting in MTR

operation by utilizing the heat generated during exothermicMTR reaction. Experiments have shown that 16 kg of slag

disc can be co-melted with MTR charge. Findings of these

experiments are encouraging. It has been observed that purity

of the finished product doesnt get affected while recovering

U from slag discs by co-melting with MTR operation.

Satisfactory results have been obtained for all the positions

of slag disc. Co-melting method appears to have huge

potential as an alternative for recovery of U from old stock.

It is also very simple, cost effective, and doesnt demand

additional process step, setup and energy.

REFERENCES

1. Harrington C D and Ruehle A E (ed.) Uranium ProductionTechnology. D. Van Nostrand co. Inc. (1959).

2. Bendict M, Pigford T H and Levi H W, Nuclear Chemical

Engineering, McGraw Hill Co. 2nd Edition (1981).

recovery of locked up u in slag disc by comelting in mtr for correction1

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