Process for the production of a uranium trioxide yellowcake from a uranian peroxide precipitate

Abstract

The present invention provides a process for the production of a uranium trioxide yellowcake from a uranium peroxide precipitate, the peroxide precipitate being in the form of a low solids content, uranium rich feed slurry, the process including the stages of: a. thickening the feed slurry to produce a thickener underflow with a solids content in the range of 15 to 50% w/w and a thickener overflow; b. dewatering the thickener underflow to produce a solids cake with a solids content of at least 50% w/w and a dewater overflow; and c. calcining the solids cake at a temperature in the range of 450° C. to 480° C. to produce a calcined uranium trioxide yellowcake.

Claims

1. A process for production of uranium trioxide yellowcake from a uranium peroxide precipitate, the uranium peroxide precipitate being in the form of a low solids content, uranium rich feed slurry, the process including the stages of: a. thickening the feed slurry to produce a thickener underflow with a solids content in the range of 15 to 50% w/w and a thickener overflow; b. dewatering the thickener underflow to produce a solids cake with a solids content of at least 50% w/w and a dewater overflow; c. calcining the solids cake at a temperature in the range of 450° C. to 480° C. to produce a calcined uranium trioxide yellowcake; and d. washing the feed slurry to remove water soluble impurities while monitoring the dewater overflow with a conductivity probe to determine when the water soluble impurities have been reduced to an acceptable level.

2. A process according to claim 1, wherein the washing of water soluble impurities from the feed slurry occurs during dewatering so that at least a substantial portion of the water soluble impurities exit with the dewater overflow.

3. A process according to claim 1, wherein the process includes recycling dewater overflow back to the thickener.

4. A process according to claim 1, wherein the dewatering occurs in a filter press, a pressure filter or a centrifuge.

5. A process according to claim 4, wherein the dewatering occurs in an automatic pressure filter operating semi-continuously.

6. A process according to claim 4, wherein the dewatering occurs in a centrifuge.

7. A process according to claim 1, wherein the solids cake is transferred from dewatering to a bi-directional conveyor that is capable of being controlled to discharge either to calcination or back to dewatering.

8. A process according to claim 1, wherein calcination occurs in an indirectly heated kiln.

9. A process according to claim 8, wherein the kiln is a horizontal rotary kiln.

10. A process according to claim 8, wherein the kiln is a sealed kiln.

11. A process according to claim 1, wherein the solids cake has a solids content of greater than 50% w/w.

12. A process according to claim 1, wherein the solids cake has a solids content of greater than 65% w/w.

13. A process according to claim 1, wherein the process does not include a reduction step or reducing agents.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Having briefly described the general concepts involved with the present invention, a preferred embodiment will now be described that is in accordance with the present invention. However, it is to be understood that the following description is not to limit the generality of the above description.

(2) In the drawings:

(3) FIG. 1 is a schematic flow diagram showing a process for the calcining of uranium peroxide yellowcake in accordance with a first preferred embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

(4) FIG. 1 provides a flow diagram of a preferred embodiment of the process of the present invention, being a process for the calcining of a uranium peroxide precipitate to form a uranium trioxide yellowcake. The main product flow path has been bolded in FIG. 1 to distinguish it from ancillary flow paths and little description is provided (or is necessary) for these ancillary flow paths, including paths that show: movement of process water (2) through, amongst other things, the storage tank (TK-001), the cooling tower (CT-001) and the heat exchanger (HX-001); and movement of the combustion and exhaust gases (8) from the kiln (KN-001).

(5) The main product flow path shows a low solids content, uranium rich feed slurry (1) of the uranium peroxide type having been processed through a conventional thickener (not shown). In this embodiment, the underflow from the thickener, being the thickened slurry (1), will have a solids content in the range of 15 to 50% solids by weight. In this respect, it will be noted that the above description of general aspects of the invention, and the subsequent description of this preferred embodiment, generally refer only to ideal ranges of various normal operating parameters of a process such as that of the invention, such as for temperatures, compositions and concentrations. A skilled addressee will understand the wide variation that exists in the normal operation of a processing plant, dependent upon the type of feed material and its characteristics, and thus the wide variation that must be allowed for when describing preferred operating parameters for an invention of this type.

(6) The dewatering means in the present invention is a centrifuge, a pressure filter or a filter press that separates the solid and liquid phases of the feed slurry. In this embodiment, the dewatering means is a centrifuge (CF-001) that receives thickened slurry (1) via stream (3) from a storage tank (TK-001). In the centrifuge (CF-001) the separation is performed according to specific gravity differences and/or particle size. In this embodiment, the centrifuge (CF-001) produces a centrate (4) (being the dewater overflow referred to in the above general description, and which is essentially a water discharge) that is recycled back to the thickener (not shown) via stream (4), and a solids cake (5), being the solid phase formed in the centrifuge (CF-001), which is essentially the dewatered solids.

(7) The centrifuge (CF-001) consists of a fixed base or casing which carries a rotating element and its drive motor, the details of which are not illustrated in the flow diagram of FIG. 1. The rotating element (or rotor) is preferably made up of a cylindrical section (or bowl) where the liquid is clarified and a conical section (or beach) from where the solids are conveyed out of the liquid by, for example, a screw conveyor (a scroll) inserted into the cylindrical section through an open end of the conical section. Centrifuges of this type are sometimes referred to as decanter centrifuges, solid bowl centrifuges or scroll centrifuges.

(8) The feed slurry is preferably introduced to the centrifuge (CF-001) at an infeed end of the rotating element through a stationary feed tube. The centrifugal force caused by the rotation of the cylindrical section of the centrifuge (CF-001) results in the formation of a continuous solid layer over the inside surface thereof. As will be appreciated, because of the centrifugal forces created by the rotation, the heavier particles will move towards the wall of the cylindrical section, leaving the lighter solids and liquid in a liquid phase (the centrate) in the inner section of the rotating layer. The heavier particles form the dewatered solids cake (5) mentioned above are preferably then transferred by the internal screw conveyor, from the cylindrical section via the conical section, which forms a barrier to the transfer of the liquids, to a solids discharge port.

(9) The centrate preferably flows in the same direction as the dewatered solids cake and returns to the liquid feed end of the cylindrical section of the rotating element of the centrifuge (CF-001) via return tubes, to where it discharges over adjustable weir plates. The centrate and the solids cake can then be collected in separate compartments of the casing of the centrifuge (CF-001), from which they fall by gravity into their respective discharge chutes (all of which occurs internally in the centrifuge (CF-001)).

(10) The centrate (4) discharged from the centrifuge (CF-001) is directed to the thickener (not shown) to prevent the loss of any yellowcake precipitate that might remain in the centrate (4) from the liquid phase, or may alternatively be sent back to a leach (also not shown) if it contains large quantities of impurities. The liquid phase may be able to be reused as process water, depending upon its impurity levels

(11) It should also be noted that if the dewatering means were a filter press or a pressure filter instead of a centrifuge, and a wash cycle was thus required, then a conductivity probe may be used to monitor conductivity in the filtrate (the dewater overflow) to determine when water soluble impurities such as chlorides and sulphates have been reduced to an acceptable level. When the conductivity in the filtrate reduces it is implied that the impurities have been removed from the solids cake. The actual conductivity will be determined by the type and number of impurities and by the required convertor specification.

(12) Indeed, it will be appreciated that at this point, the solids cake (6) may have levels of impurities (in terms of water soluble impurities) that will be in an acceptable range for the subsequent requirements for the desired yellowcake.

(13) Before turning to a more specific description of the kiln (identified as KN-001 in FIG. 1), it should be noted that the bi-directional screw conveyor (SF-001) is also ideally able to recycle the dewatered solids cake (6) back through the dewatering stages should its impurity content exceed that desired for the process or for maintenance shutdowns of the kiln (KN-001) or process upsets, or during start-up and shut-down operations when the cake is out of specification.

(14) In relation to the operation of the kiln (KN-001), the solids cake (6) enters the kiln (KN-001) at a minimum solids concentration of about 50% w/w. Heat is applied to the solids cake in the kiln (KN-001), either electrically or using fossil fuels (10) to evaporate the remaining water (both free water and some bound water), and the evaporated water is drawn out of the kiln (KN-001) in the off gas (20) using liquid ring vacuum pumps (FN-001 and FN-002). The kiln (KN-001) is shown as a horizontal rotary kiln that is indirectly heated and that includes a sealing system to prevent interchange between its internal atmosphere and local ambient conditions as they operate under negative pressure.

(15) The off gas (20) from the kiln (KN-001) is passed through a large spray condenser (CD-001) to cool and condense the vapour prior to passage through a venturi scrubber (VS-001). Apart from cooling the off-gas, the spray condenser (CD-001) removes the bulk of the dust load from the off-gas stream and passes it to a seal tank (TK-002). The cooled off-gas from the spray condenser (CD-001) passes through the venturi scrubber (VS-001) and then a cyclonic separator (CS-001) where large water droplets and fine solids are removed from the gas stream and discharged to the seal tank (TK-002), for eventual flow through to a sump via stream (12).

(16) The liquid ring vacuum pumps (FN-001 and FN-002) used to draw the off gas (20) from the kiln (KN-001) maintain a negative pressure in the kiln (KN-001) preventing the escape of any off-gas generated therein.

(17) After passage through the spray condenser (CD-001) and the venturi scrubber (VS-001), the discharge from the vacuum pumps (FN-001 and FN-002) passes through a fine mist eliminator (FT-002) to remove small liquid droplets and any entrained dust (for discharge to the seal tank (TK-002)), subsequently allowing clean air (7) to be removed to a baghouse (not shown).

(18) Returning to a description of the kiln (KN-001), the solids cake (6) produced by the centrifuge (CF-001) in this embodiment is of course calcined in the kiln (KN-001) to produce calcined uranium trioxide yellowcake (9), being the final calcined yellowcake product.

(19) By way of explanation, and without wishing to be bound by the theory, in the kiln (KN-001) the solids cake (6) (of course, containing water and solids, and moving through heat zones within the kiln) is gradually heated to a typical temperature within the range of from about 100° C. to about 145° C. As the solids cake (6) is heated, it passes through a boiling zone as the water approaches its boiling temperature where the solids cake resembles a pot of slowly boiling “porridge”. Once the boiling point of the water is reached, excess water vapour (free water that is not bound) is evaporated and drawn away from the kiln (KN-001) by the off gas system mentioned above, which preferably maintains the kiln (KN-001) under a slight vacuum. That is, to this point, the reaction is simply represented as moving from UO.sub.4.2H.sub.2O plus water to UO.sub.4.2H.sub.2O plus about 1% moisture.

(20) Typically, beyond about 150° C. thermal decomposition of the uranium peroxide begins, with gradual conversion to the probable composition of UO.sub.3.5.H.sub.2O at about 290° C. Further heating to a temperature within the range of from about 450° C. to about 480° C. completes the dehydration and the formation of UO.sub.3 with the removal of the bound H.sub.2O. By about 480° C., the product bulk density has increased from about 1.5 kg/L (dried at 145° C.) to about 1.85 kg/L.

(21) The overall reaction in the kiln can then be designated in the following manner:
2(UO.sub.4.2H.sub.2O)+(Heat).fwdarw.2UO.sub.3+O.sub.2+4H.sub.2O

(22) The final calcined uranium trioxide yellowcake product (9) from the kiln (KN-001) preferably discharges by gravity into a storage hopper (not shown) incorporating any known type of drum filling and weighing system from which the calcined uranium trioxide yellowcake is packaged into drums.

(23) In conclusion, it must be appreciated that there may be other variations and modifications to the configurations described herein which are also within the scope of the present invention.