WO2009013759A1

WO2009013759A1 - Single stage purification for uranium refining

Info

Publication number: WO2009013759A1
Application number: PCT/IN2007/000305
Authority: WO
Inventors: A. Bhowmik; P. Shanmugavelu; D. Dhavamani; Ankur Agrawal
Original assignee: India Atomic Energy Department of
Current assignee: India Atomic Energy Department of
Priority date: 2007-07-24
Filing date: 2007-07-24
Publication date: 2009-01-29
Anticipated expiration: 2010-01-24
Also published as: ZA201001122B; CA2707460A1; AU2007356815B2; AU2007356815A1; CA2707460C

Abstract

A process for refining yellowcake to produce nuclear grade uranium using a single step precipitation route for the simultaneous removal of heavy metals, boron and other rare earth metals comprising dissolving the yellowcake in nitric acid under mild agitation and adding hydrogen peroxide at pre-defined pH and temperature to selectively precipitate uranium peroxide hydrate. Also described is a process for producing nuclear grade uranium starting from uranium ore utilizing the above process.

Description

SINGLE STAGE PURIFICATION FOR URANIUM REFINING

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of nuclear grade pure uranium dioxide, natural metallic uranium and uranium hexafluoride from yellow cake containing boron, rare earth and other metallic impurities. More particularly, the present invention relates to a process for the preparation of pure uranium dioxide that meets the nuclear grade specifications of the equivalent boron content being less than 4μg/g on uranium basis as per ASTM C - 753-99.

BACKGROUND AND PRIOR ART

Yellowcakes are uranium concentrates, which represent an intermediate step in the processing of uranium ores. Yellowcakes are usually obtained through the milling and chemical processing of uranium ore forming a coarse powder, which is insoluble in water and contains about 60-80% of uranium oxide depending on type of Yellowcakes such as Magnesium Di-uranate, Ammonium Di-uranate or Uranium peroxide. In the process conventionally used within the art, the ore is first crushed to a fine powder by passing the starting raw uranium ore through crushers and grinders to produce the pulped ore. The pulped ore is thereafter processed with concentrated acid or an alkaline solution to leach out the uranium and the eluate is subjected to precipitation of Uranium concentrates that is then filtered and dried to produce yellowcake. This yellowcake usually contains boron, rare earths and other metallic impurities.

The yellowcake thus produced is thereafter converted to nuclear grade pure uranium dioxide, natural metallic uranium and uranium hexafluoride using various processes conventionally known in the art. In one of the known processes i,e Solvent Extraction, the yellowcake is first dissolved in nitric acid and thereafter feed preparation is done to adjust the nitric acid and uranium concentration. This feed is thereafter passed through a multi-stage counter current slurry extractor wherein uranyl nitrate is extracted using a mixture of 33% tributyl phosphate and kerosene leaving behind impurities in the mother liquor known as raffinate. The organic phase containing pure uranyl nitrate is further subjected to another separation step using de-mineralized water to produce pure uranyl nitrate solution. This pure urnayl nitrate solution is co-precipitated with ammonia to produce ammonium diuranate (ADU), which is thereafter converted to produce nuclear grade uranium dioxide or metallic uranium.

The conventionally known solvent extraction process, which involves the use of carcinogenic materials such as tributyl phosphate, highly inflammable kerosene and hazardous ammonia needs lots of monitoring and safety regulations for industrial scale operations. This process also produces degraded tributyl phosphate due to reaction with nitric acid and radioactivity, which requires complicated disposal method involving further treatment and incineration facilities, generates lots of solid and liquid wastes containing nitrates which are difficult to dispose off. The generated solid waste contains a preponderance of nitrates, which therefore cannot be recycled for the recovery of uranium without the removal of nitrates. The removal of nitrates from the solid wastes generated requires special treatment steps as nitrate contamination of the ground water may lead to methemoglobinemia and stomach cancer. The conventionally used solvent extraction process is also disadvantageous in that it generates multiple streams of wastes. The liquid waste stream typically contains about 100 ppm of uranium along with the soluble nitrates which are disposed off in large solar ponds. The disposal of the liquid wastes in the large solar ponds requires large space and a continuous monitoring of the ground water around the solar pond.

In another "dry refining process", the starting yellowcake is directly palletized and reduced with hydrogen to produce uranium dioxide at a temperature between 550 - 650 ⁰C in a fluidized bed reactor. The uranium dioxide is thereafter converted to uranium tetrafluoride and uranium hexafluoride in the fluidized bed/Flame reactor. The thus produced uranium hexafluorides are thereafter "refined" using a two-stage pressure distillation process. Further, this process of refining uranium fluorides by pressure distillation is a technically difficult and potentially hazardous process.

Refining is a process aimed at reducing the harmful impurities to an acceptable level, particularly to meet the nuclear grade specification that the equivalent boron content (EBC) may not exceed 4μg/g on uranium basis as per ASTM C - 753-99. The process according to the present invention surprisingly brings down the initial EBC of 180 μg/g in the starting yellowcake to about less than 1.0 μg/g of EBC in the final product.

A further approach for the production of pure uranium grades has been to use purer forms of the starting yellowcakes using hydrogen peroxide for the reduction of Mo, V, P₁ Zr, As, Ca, Mg, Na, Si and other sulfates to produce purer yellowcakes in the form of uranium peroxide from eluate solutions of sulfate nature compared with Ammonium diuranate and Magnesium diuranate are used as the starting materials. However, it has been found that the yellowcake produced in the form of uranium peroxide using the aforesaid approach also contains substantial levels of rare earth impurities that remain in the yellow cake thus necessitating further refining steps.

Without wishing to be bound by theory, the inventors believe that hitherto, it has been impossible to remove both heavy metals and rare earth impurities such as boron, gadolinium, cadmium, europium and samarium in a single refining step using Hydrogen peroxide as precipitation route because the process involves stringent pH control by the addition of alkaline solutions, which interferes with the simultaneous removal of rare, earth impurities such as boron, gadolinium, cadmium, europium and samarium in single refining step. US 4 024 215 describes a process for the preparation of yellowcake, with a reduced content of sodium and vanadium from the eluate. However, the disclosed is not suitable for the removal of the rare earth impurities. Further, the yellowcake produced is further refined using the wet solvent extraction process to produce nuclear grade uranium, which suffers from the above identified deficiencies.

US 2 770 521 teaches the treatment of sulfated uranium with a peroxide to produce uranium peroxide di-hydrate and separation of relatively pure uranium peroxide.

However, the starting material for the process disclosed in this US patent is uranium present in the slag after the magneto-reduction of uranium tetrafluroide, which is already free of the rare earth impurities sought to be removed according to the present invention. A further disadvantage of the process disclosed in this US patent is that the disclosed process requires strict pH control for the complete precipitation of uranium peroxide, which is cumbersome.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide a single step precipitation route for refining yellow cake to produce nuclear grade pure uranium.

It is a further object of the invention to provide a process for refining yellowcake that is eco-friendly producing only one waste liquid stream that contains a ppm level of uranium and wherein the level of nitrate impurities is substantially less than that obtained by the solvent wet extraction process.

It is a further object of the present invention to provide a single step precipitation route for refining yellowcake to meet Nuclear Purity Standard as specified by ASTM C 753 - 99 that avoids the use of hazardous and toxic chemicals. It is a further object of the present invention to provide a process for refining yellowcake to produce nuclear grade pure uranium that permits a simultaneous removal of heavy metals in addition to rare earth metals such as boron, gadolinium, dysprosium, samarium, europium and the like.

It is a further object of the present invention to provide a process for refining yellowcake to produce nuclear grade pure uranium that brings down the level of rare earth metals such as boron, gadolinium, dysprosium, samarium, europium and the like to less than 0.1 ppm in a single precipitation step followed by washing.

It is a further object of the present invention to provide a process for refining yellowcake to produce nuclear grade pure uranium that brings down the Equivalent boron concentration (EBC) of the yellowcake to less than 1.0 microgram per gram of uranium.

It is a further object of the present invention to provide a process for refining yellowcake to produce nuclear grade pure uranium that does not require stringent pH control conditions.

SUMMARY OF THE INVENTION

A process for refining yellowcake to produce nuclear grade uranium using a single step precipitation route followed by washing for the simultaneous removal of heavy metals, boron and other rare earth metals comprising dissolving the yellowcake in nitric acid under mild agitation and adding hydrogen peroxide at pre-defined pH and temperature to selectively precipitate uranium peroxide hydrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for refining yellowcake to produce nuclear grade uranium using a single step precipitation route followed by washing for the simultaneous removal of heavy metals, boron and other rare earth metals comprising dissolving the yellowcake in nitric acid under mild agitation and adding hydrogen peroxide at pre-defined pH and temperature to selectively precipitate uranium peroxide hydrate.

According to the present invention, the starting yellowcake is preferably magnesium diuranate (MDU)₁ which may be prepared from the uranium ore by processes that are conventionally known in the art. The starting material MDU typically contains various impurities having an equivalent boron concentration as high as 180 μg/g on uranium basis.

In the process of the present invention, the starting MDU is dissolved in nitric acid to produce crude uranyl nitrate containing all the impurities in dissolved form.

In a preferred embodiment, the said nitric acid used has strength of about 2 N to about 6 N.

In a subsequent step of the process of the present invention, uranium peroxide hydrate is selectively precipitated out by the addition of commercial hydrogen peroxide to above uranyl nitrate solution at a predetermined pH. The pH of the solution at the time of addition of hydrogen peroxide is selected such that all the impurities remain in soluble form in the reaction system.

In a preferred embodiment, the commercial hydrogen peroxide is about 30 to 70% w/w. The pH of the reaction system is preferably less than about 2, at which pH, it was surprisingly found that all the impurities were present in the reaction system in soluble form, which enabled precipitation of surprisingly pure uranium peroxide hydrate. It was surprisingly found that the process outlined above led to the end product i.e. uranium peroxide hydrate that had a substantially reduced equivalent boron concentration less than 1.0 μg/g of EBC, which is well below the specification depicted in ASTM C 753 - 99. It was also found that the process herein described led to a simultaneous reduction of the heavy metal as well as rare earth metal content such that of gadolinium, cadmium, europium, samarium in addition to boron and other heavy metals such as iron, nickel, cobalt, calcium and magnesium in one refining step, which was hitherto not possible using hydrogen peroxide precipitation route.

A qualitative analysis of the refined uranium peroxide hydrate according to the process described herein presented the following results.

The present inventors have also found that completion of precipitation as well as the efficiency of refining depended on the stoichiometric ratios of the reactants, system

. pH and temperature. While the role of pH in the purity of the final product is not clear, the present inventors believe that the reaction according to the invented process leads to an in-situ formation of nitric acid, which increases the tendency for

■ the precipitated end product to get dissolved back favoring a reverse reaction. A careful monitoring of the pH to below 2 presumably strikes balance between the prevention of co-precipitation of impurities along with uranium peroxide hydrate, and a comparatively slow reverse reaction favoring a low dissolved uranium peroxide hydrate content in the filtrate. It is also believed that a low temperature of the reaction system favors lower uranium content in the solution as the solubility of uranium peroxide hydrate in nitric acid is found to be low at room temperature. This ensures that a concentration of uranium content in the filtrate could be maintained below 300 ppm.

In a further optional embodiment, the dissolved uranium content (less than 300 ppm) could be optionally further reduced to less than 50 ppm by further adding hydrogen peroxide over a sufficient time period to allow delayed precipitation of uranium peroxide hydrate with addition of suitable flocculants like INDFLOC or by Ion Exchange with suitable cation or anion based resins. Thus, it was found that the overall uranium content loss from the system could be kept at as low as 20- 50 ppm which is well below the overall loss of uranium content in the conventional solvent extraction process.

The suspended uranium content in the system could be possibly be also removed by micro filtration after heavy metal removal.

In a preferred embodiment, the amount of commercial hydrogen peroxide added was 1.5-6 times the stoichiometric amount of the uranium in the starting MDU. The temperature at which hydrogen peroxide is added preferably varies from about 15 to about 30⁰C to produce relatively pure uranium peroxide hydrate.

The reaction according to the invented process is instantaneous and is preferably_Λ carried out in a stainless steel container under controlled addition of hydrogen peroxide. The entire system is kept under mild turbulence by turbine type agitator at 150 - 200 rpm to complete the precipitation process. The yellow colored uranium peroxide thus produced needs a proper separation from the residual solution, which is carried out in a neutche filter backed up by a vacuum pump or by centrifuge followed by de-mineralized water washing cycles. It was found that preferably two washing cycles with de-mineralized water was sufficient to produce uranium peroxide hydrate having desired purity levels.

The washed solid is fed to a co-current spray drier to remove the additional moisture content at an inlet air temperature of 200 - 300⁰C and an exhaust temperature of 100 - 130⁰C. The dried powder in the size range of 2 - 10 micron was the end product obtained by the herein described process.

An advantage of the process according to the present invention is that there is a single waste stream (liquid) that is more convenient to handle and dispose off vis-avis the solid and the liquid waste streams conventionally produced in the state of the art. This wastewater stream was found to carry a few ppm of uranium, which could be micro filtered to discharge a very clear permeate as effluent containing only soluble rare earths.

Another significant advantage of the process according to the present invention is that the entire process requires only nitric acid and commercial grade hydrogen peroxide as the reaction chemicals thereby eliminating the need for tributyl phosphate, kerosene, sodium carbonate, ammonia and other waste treatment chemicals, which are conventionally used in the solvent extraction process.

The present process therefore provides an efficient refining of yellowcake in a much more simple way, reducing load of waste streams, chemicals inventory and offering much more in-built safety features. The process is also compact, both capital cost and running cost saving and could be integrated to milling operation thus eliminating requirement of separate refining facility.

The invention shall now be described with reference to the following specific examples. It should be noted that the example(s) appended below illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, the word 'comprising' does not exclude the presence of other elements or steps than those listed in a claim. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Other than in the operating examples provided hereunder, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions are to be understood as being modified in all instances by the term "about".

EXAMPLE 1

MDU powder was dissolved in nitric acid (2 - 6 N) at 80 - 90⁰C and the uranium content in initial solution kept at 250 - 300 g/lit and free acidity up to maximum of 3N. The solution was then diluted with DM water to keep uranium concentration in the range of 10 - 50 g/lit. Commercial hydrogen peroxide in the concentration range of 30 - 70% (w/w) was then gradually added to the crude uranyl nitrate solution at temperature range of 15 - 30⁰C. A turbine type agitator kept the whole system in mild turbulence (150 - 200 rpm) so as to maintain uniform reaction as well as unhindered growth of particles. The whole reaction was carried out in SS container. The yellow colored uranium peroxide hydrate precipitated out instantaneously and was subjected to filtration. This part required efficient separation of solid and liquid followed by DM water washing cycles . The washed cake was collected and fed to a co-current spray drier (in S No. 5^* & 6* below) at inlet air temperature of 200 - 300⁰C and exhaust air temperature of 100 - 130⁰C in effect to a dry granular product. The ' filtrate contained 150 - 300 ppm of dissolved uranium content, which was further brought down to less than 50 ppm before discharging to solar pond by proper pH adjustment, and addition of little quantity of hydrogen peroxide and flocculants causing delayed precipitation.

Claims

1. A process for refining yellowcake to produce nuclear grade uranium using a single step precipitation route for the simultaneous removal of heavy metals, boron and other rare earth metals comprising dissolving the yellowcake in nitric acid under mild agitation and adding hydrogen peroxide at pre-defined pH and temperature to selectively precipitate uranium peroxide hydrate.

2. A process for producing nuclear grade uranium starting from uranium ore, said process comprising:

(a) passing the starting raw uranium ore through crushers and grinders to produce a fine pulped ore powder;

(b) processing said pulped ore with concentrated acid, alkaline solutions to leach out the uranium;

(c) precipitation of uranium concentrates from eluate;

(d) filtering and drying the Uranium concentrates to produce yellowcake; and

(e) subjecting said yellowcake to a refining step as claimed in claim 1 to produce nuclear grade uranium.

3. A process as claimed in claim 1 or claim 2, wherein said yellowcake is magnesium diuranate.

4. A process as claimed in claim 3, wherein said magnesium diuranate has an equivalent boron concentration as high as 180 μg/g on uranium basis.

5. A process as claimed in any preceding claim, wherein said nitric acid used has a strength of about 2 N to about 6 N.

6. A process as claimed in any preceding claim, wherein hydrogen peroxide is added to the uranyl nitrate solution at a pH selected such that all the impurities remain in soluble form in the reaction system.

7. A process as claimed in claim 6, wherein said pH is less than about 2.

8. A process as claimed in any preceding claim, wherein said commercial hydrogen peroxide is about 30 to 70% w/w.

9. A process as claimed in any preceding claim, wherein the amount of commercial hydrogen peroxide added was 1.5-6 times the stoichiometric amount of the starting MDU.

10. A process as claimed in any preceding claims, wherein the temperature at which hydrogen peroxide is added preferably varies from about 15 to about 30⁰C.

11. A process according to any preceding claim, which is carried out in a stainless steel container.

12. A process as claimed in any preceding claim, wherein said refining is carried out under mild turbulence by turbine type agitator at 150 - 200 rpm.

13.A process as claimed in any preceding claim, wherein uranium peroxide hydrate produced is separated from the residual solution using a neutche filter backed up by a vacuum pump or by centrifuge followed by one or more de- mineralized water washing cycles.

14. A process as claimed in claim 14, which comprises two water-washing cycles.

15. A process as claimed in claim 13 or claim 14, wherein said washed solid is fed to a co-current spray drier to remove the additional moisture content at an inlet air temperature of 200 - 300⁰C and an exhaust temperature of 100 -

130⁰C.

16. A process as claimed in claims 14-16, wherein the particle size of the end product is in the size range of 2 - 10 micron.

17. A process as claimed in any preceding claim further comprising an optional step of further adding hydrogen peroxide over a sufficient time period to allow delayed precipitation of uranium peroxide hydrate with addition of suitable flocculants or by Ion Exchange with suitable cation or anion based resins.

18. A process as claimed in claim 18, wherein said flocculant is INDFLOC.

19. A process substantially as described herein with reference to the examples.