IMPROVEMENTS IN AND RELATING TO THE PROCESSING OF POWDERS

Abstract: The invention provides comminuting and/or homogenising apparatus comprising a vessel (3) provided with a rotatable shaft, the shaft being provided with one or more arms (17) extending therefrom, the bottom arm (27) on the shaft being provided with a bottom face corresponding to the opposing base (5) of the vessel and/or provided with one or two end faces corresponding to the side (9) of the vessel (3). The invention results in more effective comminuting and/or homogenising and minimises hold-up in the apparatus after processing.

Description: IMPROVEMENTS IN AND RELATING TO THE PROCESSING OF POWDERS This invention concerns improvements in and relating to the processing of powders, and particularly but not exclusively the processing of powders for forming nuclear fuel . During the processing of powders, such as nuclear fuel materials, it is necessary to thoroughly and effectively comminute and/or mix different components together in a short space of time. The comminuting process may be needed to ensure the desired material size for subsequent stages. The homogenising process may need to evenly distribute a relatively small level of one component in a far greater amount of one or more other components . Precautions must be taken to avoid situations where not all parts of the material are mixed and variations could consequently occur in the product material . To achieve such comminuting and/or homogenising an attrition mill is employed. The attrition mill must, however, be carefully designed to avoid a variety of sources of complication or reduced performance. The present invention aims to provide an attrition mill design which offers more reliable operation and reduced material handling and hold up difficulties. According to a first aspect of the invention we provide a comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms extending therefrom, one or more of the arms defining an incline in the direction of rotation, the vessel being provided with comminuting and/or homongenising media in use . The incline may be defined by a face of an arm and/or the incline may be defined by the relative spacing of two or more arms to one another, for instance, a arm may be provided above another arm, the lower arm being ahead of the upper arm, in the direction of rotation. According to a second aspect of the invention we provide a comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms , one or more of the arms , preferably the bottom arm of the arms, are provided with a lead face, in the direction of rotation in use, which is inclined relative to the axis of the shaft, the vessel being provided with grinding media in use. Preferably the inclined face is reclined, for instance such that its upper portion is behind its bottom portion in the direction of travel . The bottom arm may be provided with a bottom face corresponding to the opposing base of the vessel and/or provided with one or both end faces corresponding to the side of the vessel . According to a third aspect of the invention we provide comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms extending therefrom, the bottom arm on the shaft being provided with a bottom face corresponding to the opposing base of the vessel and/or provided with one or two end faces corresponding to the side of the vessel. Preferably one or both sides of the arm may be inclined, for instance relative to the axis of the shaft and/or relative to the perpendicular from the base. Preferably at least the leading side, relative to the direction of rotation in use is inclined. The first and/or second and/or third aspects of the invention may include the following features, possibilities or options . The angle of inclination of the side(s) of the arm is preferably between 10° and 45°. The arm may have a trapezial cross-section. The upper face is preferably shorter, in cross-section, than the bottom face. The upper face may be less than half the width of the bottom face. A planar upper face, preferably parallel to the bottom face, may be provided. Both bottom and end faces may be provided corresponding to the opposing part of the vessel . Preferably the bottom arm extends across the vessel from one side to the other. The bottom and two ends are ideally provided corresponding to the opposing part of the vessel . One or both ends may be curved to match the curvature of the side of the vessel. The bottom surface of the arm is preferably planar, most preferably parallel to the base of the vessel. The end or ends of the arm are preferably concentric with the vessel. The junction between the end of the arm and the bottom of the arm may be curved. The curved junction may correspond to a curved junction between the side and base of the vessel. One or both sides of the arm may be planar. A straight edge junction may be provided between a side and the bottom face and/or end of the arm. The bottom rotor may be solid. The clearance between the bottom face and the base of the vessel may be less than 20mm and is preferably less than 15mm. The clearance between the ends and the side of the vessel may be less than 20mm and preferably less than 15mm. Preferably the bottom face of the arm is flush. The bottom arm may be fixed to the shaft by a fixing received in the bottom face of the arm. Preferably the fixing and/or the shaft are made flush with the bottom face, for instance by machining down. The apparatus may be provided with non-bottoir arms or rotors according to the fourth and/or fifth aspects of the invention and/or with a discharge opening according to the sixth and/or seventh aspects of the invention. According to a fourth aspect of the invention we provide comminuting and/or homogenising apparatus comprising ;.. vessel provided with a rotatable shaft, the shaft being prov ied with two or more arms extending therefrom, wherein at least one of the arms has a greater radial extent from the shaft than at least one of the other arms, at least one of the arms of greater radial extent being provided above at least one of the other arms . According to a fifth aspect of the invention we provide comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with two or more arms extending therefrom, wherein at least one arm is provided above another on the shaft, the upper arm being provided behind the lower arm, relative to the direction of rotation. Preferably the upper and lower arms form a pair. The axial separation of the upper and lower arm of a pair may be between 5 and 60mm and more preferably between 10 and 40mm, and ideally between 20 and 30mm.. The axial separation of the upper arm of one pair and the lower arm of the next pair up the shaft may be between 5 and 60mm and more preferably between 10 and 40mm, and ideally between 20 and 30mm. The shaft may be provided with at least three arms . Preferably one or more of the upper arms having a greater extent than one or more of the lower, non-bottom, arms are provided. The arms may extend perpendicularly relative to the shaft . The radial direction of the arms may be thought of as a diametrical direction. It is preferable that the shaft be provided with a between 5 and 9 arms . The arms, particularly non-bottom arms, may be provided in pairs. Three pairs may be provided. The arc about the axis of rotation defined by members of a pair may be less than the arc between adjacent pairs, the arc between the rear of one pair and the lead arm of the next . Arms of a pair may be define an arc of between 0 U and 20°. An arc of between 30° and - 5 - 120° may be provided between adjacent non-pair arms and/or adjacent pairs, more preferably an arc of between 40° and 110°, and still more preferably between 45° and 90° is provided. The spacing along the axis of rotation may be less for the two members of a pair than between adjoining pairs. Three or more pairs may be provided with alternate pairs being provided at approximately 90° to the adjacent pairs. The lowest pair is preferably substantially aligned with the bottom arm. The non-bottom arms may extend across between 65% and 95% of the vessel diameter, and more preferably between 75% and 90%. One or more of the lower arms preferably extends across at least 82% and more preferably at least 86% of the diameter of the vessel . One or more of the upper arms preferably extends across at most 80%, and more preferably at most 78% of the vessel diameter. The gap size between the end of an arm and the vessel, perpendicular to the axis or rotation and parallel to the vessel base may be greater for at least one of the upper arms than for at least one of the lower arms. The gap size is preferably at least 60mm and more preferably at least 120mm. The gap size between the end of the arm and the vessel, perpendicular to the axis of rotation and parallel to the vessel base are precisely calculated based on the diameter of the milling agent. The arms preferably have a circular cross-section. The shaft may also be provided with a bottom arm according to the first aspect of the invention and/or with a discharge opening according to the sixth aspect of the invention. According to a sixth aspect of the invention we provide comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms extending therefrom, the vessel being provided with a closeable opening for discharging material from the - 6 - apparatus, the opening including one or more apertures which, in the discharge direction, increase in at least one dimension. The opening may include a grid. The grid may incorporate one or more slits or slots which expand in size in the discharge direction. Preferably the width of the slits or slots increases in the discharge direction. The length of the slits or slots may remain substantially constant in the discharge direction. The slits or slots may extend from a side facing the interior of the vessel to a side facing the discharge route for the material. The slit or slot width on the discharge side may be between 25% and 100% greater than the width on the interior side. The increase in dimension may occur at a constant rate in the discharge direction or at a non-constant rate. Other possibilities, features and options for the first and/or second and/or third aspect included the following details . The vessel preferably has a circular cross-section. The vessel may be in the form of a truncated cone. The base and/or the lid may be provided perpendicular to the axis of the cone. The apex angle for the cone may be between 6° and 40° . The lid for the vessel may include a closeable aperture for feeding material to the apparatus . The aperture may be provided towards the periphery of the lid and/or vessel. The drive means, such as a motor, for the shaft may be mounted on the lid of the vessel or provided above the lid of the vessel . Preferably the drive means are provided outside a radiological contaminant, such as a glove box, the mill being provided within such a contaminant . The vessel is preferably provided with a discharge outlet in its base. The discharge outlet is closeable. Preferably the outlet is dimensioned to retain balls within the vessel during the discharge of the processed material . The discharge outlet is preferably provided towards the periphery of the base . According to a seventh aspect we provide a method of processing powders comprising the addition of one or more powders to an attrition mill, operating the attrition mill and discharging the processed powder therefrom, wherein the attrition mill is provided according to any of one or more of the first, second, third, fourth, fifth or sixth aspects of the invention. The apparatus is preferably loaded with a comminuting and/or homogenising media, such as balls in use. The ball level may be between 80% and 95% of the height of the vessel between the base and lid. Preferably the inclined front face of the bottom arm lifts at least a portion of the comminuting/homogenising media as the shaft rotates. Preferably the rear face of the bottom arm allows at least a portion of the comminuting/homogenising media to drop as the shaft advances the arm past that portion. Preferably the lower and upper arms of a pair of arms provided on the shaft serve to lift at least a portion of the comminuting/homogenising media as the shaft rotates. Preferably the upper arm of a pair of arms provided on the shaft allow at least a portion of the comminuting/homogenising media to drop as the shaft advances the arm past that portion. The method is preferably conducted dry. The material fed to the apparatus may comprise or consist of uranium oxide, such as U0 2 , and/or plutonium oxide, such as Pu0 2 , and/or fuel additives and/or processing additives, such as pore forming agents and/or lubricants . Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which : - Figure 1 illustrates a first embodiment of an attrition mill according to the invention; - 8 - Figure 2a is a side view of the bottom arm of the mill of Figure 1; Figure 2b is an end view of the bottom arm of Figure 2a; Figure 2c is a bottom plan view of the bottom arm of Figure 2a and 2b; and Figure 3 illustrates a cross-section through a discharge grid according to a further embodim e nt of the present invention. A wide variety of process industries need to comminute and/or mix and / or homogenise materials as part of the production route. The comminuting process may be needed to ger .rate the optimum size distribution for subsequent processes. Accurate control over the size distribution produced is needed. Equally fast comminuting is required but without reducing the iize too far. Mixing may be needed for a single material to nsure it is homogeneously provided and/or mixing two or more lifferent materials together to ensure that they are evenly m ed. In the production of nuclear fuel both comminuting and mixing processes are employed. In the production of mixed oxide fuel (MOX it is necessary to homogenise together precise quantities c U0 2 and Pu0 2 together with precise, but small quantities o various additives . The homogeneous mixing of these comp<- nents is essential if the fuel pellet production line is to work efficiently and the optimum product is to be o 1 ained. Additionally the materials need to be comminuted dove to the size range which offers the best performance during s" ^ sequent stages and in the final product . For this proce 3 to be achieved effectively it is important that no parts o the mill offer locations for material which would prevent full treatment . Due to the nature of the materials involved it is desirable to achieve the comminuting , αd homogenising as quickly as possible as in that way the inventory of material within the process apparatus, for a given throughput, is minimised. It is important in achieving this that the mill does not jam or otherwise be impaired from smooth operation. To comminute and homogenise the material a high energy attrition mill is employed. Material is introduced in to the attrition mill which contains a load of balls and a series of arms extending from a shaft . Rotating the shaft rotates the arms, which moves the balls which comminutes and homogenises the powder around them. After processing the material is discharged to the next stage, usually a homogenising or spheroidising process. During discharge it is important that all the material in the mill exits to the next stage. Material hold up is undesirable as this could lead to material from one batch being introduced into the next. It is also important to know, for regulatory reasons, where all the material is at all times and any hold up would introduce a variable unknown into this consideration. The attrition mill is illustrated in Figure 1. The mill 1 is formed of a processing vessel 3 in the form of a truncated cone. The vessel 3 thus has a base 5, lid 7 and inclined side wall 9. The lid 7 is provided with a sealable inlet 11 through which the material to be produced is introduced. The inlet 11 is provided towards the side wall 9 of the vessel 3. Mounted above the lid 7 is the drive motor 13 which rotates central shaft 15 in use. The motor 13 is mounted outside a glove box for ease of maintenance and to avoid unnecessary contamination. The motor 13 is connected to the shaft 15 by a number of linkages. The shaft 15 extends down through the lid 7 and into the vessel 3. A series of arms 17 are mounted on the shaft 15 and extend out from it in a perpendicular manner . The arms 17 are of circular cross-section and are generally provided in pairs . The lower of a pair 17a is provided in advance of the upper of a pair 17b relative to the direction of rotation. The arms 17 in a pair are of the same - 10 - length, but the arms 17 have an increasing length as they progress up the vessel 3 towards the lid 7. In this way the gap 19 between the ends 21 of the arms 17 and the side wall 9 increase, but do not increase as much as the gap between the shaft 15 and the side wall 9. The provision of the arms in pairs in this way causes the lower, forward arm 17a to push the grinding media it contacts upwards . This in turn pushes the grinding media above that upwards too. The gap between the lower and upper arm is such that the upper arm also pushes the grinding media up, before it has had the chance to drop from the lift given by the first arm. The two arms act as an inclined surface in effect. Once clear of the upper arm the grinding media drops and impacts on the material. Accentuated ball to ball impact and hence milling action arises as a result. The base 5 of the vessel 3 includes a discharge grid 23 which is opened to release the material following processing, but which is closed during comminuting/homogenising. The junction 25 between the base 5 and side wall 9 is curved to give a smooth transition. To ensure that no material can remain in the corners or adhere to the side wall 9 or base 5 the bottom arm 27 is carefully designed. The arm 27, Figure 2a, has end 29 which flare upwards from corner 31 to edge 33 in a manner corresponding to the side wall 9 in this part. The corner 31 is rounded to match the junction 25 between the side wall 9 and base 5. The bottom face 35 of the arm 27 matches the base 5 of the vessel 3. The tight clearance between all these surfaces ensure that material in proximity to the base 5 of the vessel 3 is also fully treated. In cross-section, Figure 2b, the arm 27 is trapezial . The bottom face 35 is generally flat to correspond to the base 5, but this face 35 has a greater width than the width of the upper face 37 due to the inclined side faces 39, 41. As the arm 27 rotates the leading inclined face 39 serves to lift both balls and material and once clear of the rear face this is - 11 - allowed to fall once more so increasing the effectiveness of the processing. The arm 27 is also mounted on the shaft 15 in a carefully designed manner. In assembly a specially designed fastener is inserted through aperture 43, 45 into the shaft 15. This fixes the two components together. The end of the fastener is then machined down, as necessary, to give a flat flush finish to the bottom face of the arm 27. This avoids the possibility of material becoming held up in this area and also prevents a ball from working its way into this location. The assembly gives improved reliability and problem free running as a result. After processing when the discharge grid 23 is opened it is important that all the material in the mill pass through without the need for external interference. Constant sized slots in the grid can give rise to material hold up because the milled product can easily be compacted to bridge such slots during discharge. To counteract the above mentioned problem the discharge grid 50 illustrated in Figure 3 has been developed. This grid in cross-section, has an upper face 52 matching the base 5 and over which the bottom face 35 of the arm 27 passes. The slots 54 provided in the grid 50 are wider at the bottom face 56 of the grid 50 than at the upper face 52. The flared slots 54 provided by this structure mean that material passing into the slot 54 has a greatly reduced chance of contacting the side walls 58 of the slot 54 and consequently there is a greatly reduced chance of the material sticking or compacting to form a bridge in the slot 54. Freer flow of material during the discharge and on to the next stage arises as a result.

Claims: - 12 - CLAIMS : 1. Comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms extending therefrom, the bottom arm on the shaft being provided with a bottom face corresponding to the opposing base of the vessel and/or provided with one or two end faces corresponding to the side of the vessel . 2. Apparatus according to claim 1 in which the bottom arm extends across the vessel and the bottom and two ends of the bottom arm are provided corresponding to the opposing part of the vessel. 3. Apparatus according to claim 1 or claim 2 in which both ends of the bottom arm are curved to match the curvature of the side of the vessel. 4. Apparatus according to any of claims 1 to 3 in which the bottom surface of the arm is planar and parallel to the base of the vessel. 5. Apparatus according to any preceding claim in which one or both sides of the arm are inclined relative to the axis of the shaft . 6. Apparatus according to claim 5 in which the angle of inclination is between 10° and 45°. 7. Apparatus according to any preceding claim in which the bottom arm has a trapezial cross-section. 8. Apparatus according to any preceding claim in which the bottom arm is fixed to the shaft by a fixing received in the bottom face of the arm, the bottom face and/or fixing being machined down to a flush finish. - 13 - 9. Comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with two or more arms extending therefrom, wherein at least one of the arms has a greater radial extent from the shaft than at least one of the other arms, at least one of the arms of greater radial extent being provided above at least one of the other arms . 10. Apparatus according to claim 9 in which the shaft is provided with at least three arms, one or more of the upper arms having a greater extent than one or more of the lower, non-bottom, arms. 11. Apparatus according to claim 9 or claim 10 in which the non-bottom arms are provide in pairs . 12. Apparatus according to claim 11 in which the spacing along the axis of rotation may be less for members of a pair than between adjoining pairs. 13. Apparatus according to claim 11 or claim 12 in which three pairs of arms are provided, the intermediate pair being provided at approximately 60° to the other pairs. 14. Apparatus according to claim 11, claim 12 or claim 13 in which the lowest pair is substantially aligned with the bottom arm. 15. Apparatus according to any of claims 9 to 14 in which the non-bottom arms extend across between 65% and 95% of the vessel diameter. 16. Apparatus according to any of claims 9 to 15 in which the gap size between the end of an arm and the vessel, perpendicular to the axis of rotation and parallel to the vessel base is greater for at least one of the upper arms than for at least one of the lower arms . - 14 - 17. Comminuting and/or homogenising apparatus comprising a vessel provided with a rotatable shaft, the shaft being provided with one or more arms extending therefrom, the vessel being provided with a closeable opening for discharging material from the apparatus, the opening including one or more apertures which, in the discharge direction, increase in at least one dimension. 18. Apparatus according to claim 17 in which the apertures are defined by a grid, the grid incorporating one or more slits or slots which expand in size in the discharge direction. 19. Apparatus according to claim 17 or claim 18 in which the width of the slits or slots increases in the discharge direction. 20. Apparatus according to any of claims 17 to 19 in which the slits or slots extend from a side facing the interior of the vessel to a side facing the discharge route for the material, the slit or slot width on the discharge side being between 25% and 100% greater than the width on the interior side . 21. A method of processing powders comprising the addition of one or more powders to an attrition mill, operating the attrition mill and discharging the processed powder therefrom, wherein the attrition mill is provided according to any of claims 1 to 8 and/or according to any of claims 9 to 16 and/or according to any of claims 17 to 20. 22. A method according to claim 21 in which the apparatus is loaded with balls in use. 23. A method according to claim 21 or claim 22 in which the material fed to the apparatus comprises U0 2 and/or Pu0 2 and pore forming agents and a lubricant .

Inventors: WATSON LEONARD CROSS, WATSON, LEONARD, CROSS
Applicants: BRITISH NUCLEAR FUELS PLC, WATSON LEONARD CROSS, WATSON, LEONARD, CROSS
Application year: 1999
Main classification: B02C17
IPC codes: B01F7, B02C17