CLAIM FOR PRIORITY

[0001 ] This PCT International Application claims the benefit of priority of U.S. Provisional Application No. 62/746,368, filed October 16, 2018, the subject matter of which is incorporated herein by reference in its entirety.

FIELD

[0002] The present disclosure concerns partition plates for rotary tube reactor vessels, rotary tube reactors comprising one or more partition plates, and methods of processing materials using rotary tube reactors.

BACKGROUND

[0003] Rotary tube reactors, such as rotary kilns, are used in the processing of materials such as granular materials. In particular, rotary tube reactors are used in the thermal processing of materials, such as in the calcination or hot chlorination purification of silica powder.

[0004] During processing, a material flows sequentially through a series of rotary tube reactor vessel chambers partitioned by partition plates. The partition plates typically include one or more apertures for enabling transport of the material being processed between chambers or for enabling flow of gases/reagents inside the reactor vessel.

[0005] Dependent on the size, shape and arrangement of the apertures, partition plates can be used to control the rate of flow of material through the reactor, the depth of material in a reactor vessel chamber at a given time, and the length of time material spends in the reactor (i.e. the retention time). These parameters can determine the effectiveness of thermal treatment processes. Partition plates can also assist in restricting heat transfer between neighbouring chambers, enabling chambers to be held at different temperatures, or have different atmospheres throughout the reactor vessel.

[0006] Partition plates are known which provide moderate material bed depth control, but which also typically have large open aperture cross-sections and therefore provide poor control of heat transfer and gas flow. Partition plates are also known which have reduced open aperture cross-sections, but which also typically hinder material flow through the reactor vessel or are unable to achieve material bed depth control. Some partition plates which enable material bed depth control and improve mixing have complex three-dimensional structures which are difficult and costly to manufacture.

SUMMARY OF INVENTION

[0007] According to a first aspect, there is provided a partition plate for partitioning chambers of a rotary tube reactor vessel, the partition plate having one or more off-centre material feed apertures extending therethrough, the one or more off-centre material feed apertures being spaced apart from a peripheral edge of the partition plate to thereby define a material bed depth in one or more of said chambers when in use in the rotary tube reactor vessel.

[0008] It has been found that the presence of one or more off-centre material feed apertures spaced apart from the peripheral edge of the partition plate enables the material bed depth in the one or more chambers of the rotary tube reactor vessel to be controlled effectively (and therefore also the length of time taken for material to flow through the reactor vessel (i.e. the retention time)), without use of excessive open aperture cross-sections which negatively affect heat and gas transport and without complex three-dimensional plate structures which are difficult to manufacture. In particular, it has been found that a greater material bed depth in the one or more chambers is achievable, without a significant reduction in material flow rate or retention time, by use of the one or more off-centre material feed apertures spaced apart from the peripheral edge. A deeper material bed for a given size of chamber allows more material to be processed in a given time period. A deeper material bed also typically permits improved mixing of the material in the one or more chambers and enables better transfer of heat to and from the material in the reactor vessel. The material bed depth can be controlled through adjustment of the size, shape, and arrangement of said one or more off-centre material feed apertures.

[0009] The rotary tube reactor vessel typically forms part of a rotary tube reactor. The rotary tube reactor is typically configured for processing, for example refining, a material.

[0010] The material processed in the rotary tube reactor vessel may be solid. In particular, the material processed in the rotary tube reactor vessel may be a (i.e. solid) granular material such as a powder. The rotary tube reactor vessel may be a rotary tube reactor vessel for use in processing a granular material. Accordingly, the one or more off-centre material feed apertures may be one or more off-centre granular material feed apertures and the material bed depth may be a granular material bed depth.

[0011 ] Additionally or alternatively, the material processed in the rotary tube reactor vessel may be liquid, i.e. a material provided in its liquid state. The rotary tube reactor vessel may be a rotary tube reactor vessel for use in processing a liquid material. Accordingly, the one or more off-centre material feed apertures may be one or more off-centre liquid material feed apertures and the material bed depth may be a liquid material bed depth.

[0012] The material processed in the rotary tube reactor vessel may comprise a mixture of solids and liquid, for example a mixture of a granular material and liquid. For example, the material processed in the rotary tube reactor vessel may comprise a suspension of solids in a liquid, for example a suspension of a granular material in a liquid, such as a paste or a slurry. Accordingly, the rotary tube reactor vessel may be a rotary tube reactor vessel for use in processing a mixture of solids and liquid, for example a mixture of granular and liquid materials (such as a suspension), the one or more off-centre material feed apertures may be one or more off-centre solid and liquid material feed apertures, and the material bed depth may be a solid and liquid material bed depth.

[0013] It may be that the material processed in the rotary tube reactor vessel changes form or physical state as it moves through the rotary tube reactor vessel. For example, it may be that the rotary tube reactor vessel receives a (i.e. solid) granular material as an input and that said granular material transitions partially or fully to a liquid state (e.g. melts) as it moves through the rotary tube reactor vessel. Accordingly, the rotary tube reactor vessel may be a rotary tube reactor vessel for use in processing a molten or semi-molten material, the one or more off-centre material feed apertures may be one or more off-centre molten or semi-molten material feed apertures, and the material bed depth may be a molten or semi-molten material bed depth. Additionally or alternatively, the material processed in the rotary tube reactor vessel may undergo one or more solid phase transformations (for example, between different crystal phases) as it moves through the rotary tube reactor vessel.

[0014] It may be that the rotary tube reactor vessel received a wet granular material as an input and liquid (e.g. water) is evaporated from the granular material as it moves through the rotary tube reactor vessel.

[0015] It will be appreciated that the material processed in the rotary tube reactor vessel is not necessarily chemically or physically homogeneous. The material may comprise a mixture of two or more chemically and/or physically distinct components. For example, the material may comprise a mixture of two or more different solids, each of the said solids having a different chemical composition and/or structural form.

[0016] It will be understood that the term“off-centre” indicates that the one or more off-centre material feed apertures are spaced apart from the centre of the partition plate. That is to say, the one or more off-centre material feed apertures do not overlap with or extend across the centre of the partition plate.

[0017] The partition plate may be substantially planar, i.e. flat. The partition plate may have first and second substantially planar (i.e. flat) faces. The first and second substantially planar faces are typically substantially parallel with one another. A (i.e. perpendicular) distance between the first and second substantially planar faces may define a thickness of the partition plate. The thickness may be substantially uniform across the partition plate. Nevertheless, the dimensions of the partition plate parallel to the first and second substantially planar faces are typically sufficiently greater than the thickness of the partition plate that the partition plate can be considered to be substantially planar and a plane can be defined in which the partition plate lies. A planar partition plate is particularly simple to manufacture compared to partition plates having complex three-dimensional shapes or protrusions. A simple design, and consequently a simple manufacturing process, is particularly important for the production of partition plates from refractory materials, for example refractory glasses such as quartz glass, which must be held at high temperatures (typically over 1000°C) during shaping. Complex, three-dimensional designs are very difficult to form with dimensional accuracy under such conditions.

[0018] The partition plate may have a generally rounded shape (i.e. in the plane). Each of the first and second substantially planar faces may have a generally rounded (i.e. curved) perimeter.

[0019] For example, the partition plate may be circular in shape (i.e. in the plane). Each of the first and second substantially planar faces may have a circular perimeter. That is to say, the partition plate may be a disc. Accordingly, the peripheral edge may be a circumferential edge of the partition plate which defines a diameter and a radius of the partition plate.

[0020] Alternatively, the partition plate may be elliptical in shape. Each of the first and second substantially planar faces may have an elliptical perimeter. The size of the partition plate may be quantified by a minor dimension measured along the minor axis of the elliptical shape and a major dimension measured along the major axis of the elliptical shape.

[0021 ] Alternatively, the partition plate may be polygonal in shape, i.e. the partition plate (e.g. each of the first and second substantially planar faces) may have a polygonal perimeter. For example, the partition plate may have a regular polygonal shape. The size of the partition plate may be quantified by one or more characteristic dimensions of the polygonal shape. For example, the polygonal shape may be a cyclic polygon and the size of the partition plate may be characterised by the circumradius of its circumscribed circle.

[0022] It will be understood that the centre of the partition plate is the region of the partition plate which intersects with a central, longitudinal axis of the rotary tube reactor vessel when the partition plate is installed for use in said vessel, i.e. the centre of the partition plate is the centre of rotation of the partition plate when installed in the rotary tube reactor vessel. The centre of the partition plate may therefore be located at the centre of mass of the partition plate, for example in embodiments in which the mass of the partition plate is distributed symmetrically about the centre of rotation. In embodiments in which the partition plate is circular in shape, the centre of the partition plate may be located at the centre of the circular shape. More generally, however, the centre of the partition plate may be located at the centroid (i.e. the geometric centre) of the planar shape of the partition plate.

[0023] It will be understood that the peripheral edge of the partition plate is the outer boundary of the region of the partition plate (i.e. the outer boundary of the regions of the first and second substantially planar faces of the partition plate) which is exposed to the interior of the rotary tube reactor vessel when installed and, therefore, which is exposed directly to material being processed in the rotary tube reactor vessel in use. That is to say, the peripheral edge of the partition plate can be defined by the lines of contact between the faces of the partition plate and the interior walls of the rotary tube reactor vessel. [0024] It may be that a radially outermost extent of the partition plate is not exposed to the interior of the rotary tube reactor vessel in use. For example, it may be that the partition plate extends radially in a mounting region for a small distance beyond the peripheral edge to the outermost extent of the partition plate. However, this mounting region, where present, is typically received in a corresponding recess in the interior walls of the rotary tube reactor vessel when the partition plate is installed in use, such that the mounting region is not exposed to the interior of the rotary tube reactor vessel and therefore does not come into direct contact with material being processed in the rotary tube reactor vessel in use. The peripheral edge may therefore be defined as the boundary of the region of the partition plate (i.e. of the first and second substantially planar faces of the partition plate) which is not in contact with the interior walls of the rotary tube reactor vessel in use. In alternative embodiments, however, the peripheral edge may define the radially outermost extent of the partition plate, i.e. it may be that the partition plate does not comprise a mounting region beyond the peripheral edge.

[0025] It will be appreciated that the one or more off-centre material feed apertures being spaced apart from the peripheral edge of the partition plate indicates that the one or more off-centre material feed apertures do not intersect or overlap with the peripheral edge. That is to say, a region of solid partition plate material typically extends between each of the one or more off-centre material feed apertures and the peripheral edge.

[0026] It may be that the partition plate comprises a central region which extends around and includes the centre of the partition plate. It may be that the partition plate comprises a peripheral region which extends around the partition plate at its periphery and includes the peripheral edge. It may be that the partition plate comprises an intermediate region located between the central region and the peripheral region. It may be that the one or more off-centre material feed apertures are located in the intermediate region of the partition plate. It may be that the central region and/or the peripheral region do not contain any apertures, i.e. that the central and/or the peripheral region are continuous, aperture-free regions of solid material. Alternatively, it may be that the central region and/or the peripheral region contain one or more apertures, but said central region and/or the peripheral region do not typically contain any material feed apertures.

[0027] It may be that a shortest distance between the centre of the partition plate and a boundary between the central region and the intermediate region is at least about 10 %, for example at least about 15 %, or at least about 20 %, or at least about 25 %, or at least about 30 %, or at least about 35 %, or at least about 40 %, of a shortest distance between the centre of the partition plate and the peripheral edge. Additionally or alternatively, it may be that a shortest distance between the peripheral edge of the partition plate and a boundary between the peripheral region and the intermediate region (thereby defining a thickness of the peripheral region) is at least about 5 %, for example at least about 10 %, or at least about 15 %, or at least about 20 %, or at least about 25 %, or at least about 30 %, of the shortest distance between the centre of the partition plate and the peripheral edge. Additionally or alternatively, it may be that a thickness of the intermediate region, defined as a shortest distance between the boundary between the central region and the intermediate region and the boundary between the peripheral region and the intermediate region, is at least about 10 %, for example at least about 15 %, or at least about 20 %, or at least about 25 %, or at least about 30 %, of the shortest distance between the centre of the partition plate and the peripheral edge.

[0028] It may be that the partition plate comprises one single off-centre material feed aperture. Alternatively, it may be that the partition plate comprises more than one off-centre material feed aperture. It may be that the partition plate comprises at least two, at least three, at least four, at least five, at least six, at least seven, or at least eight off-centre material feed apertures.

[0029] In embodiments comprising more than one off-centre material feed aperture, it may be that the said off-centre material feed apertures are spaced apart from one another around the partition plate. For example, the off-centre material feed apertures may be spaced equally apart from one another around the partition plate, that is to say, the shortest distance between each adjacent (i.e. closest) pair of off-centre material feed apertures may be the same.

[0030] It may be that the off-centre material feed apertures are spaced equally apart from the peripheral edge. That is to say, the shortest distance between each material feed aperture and the peripheral edge may be the same.

[0031 ] Throughout this specification and the appended claims, unless stated otherwise, references to distances from a first aperture to a second aperture should be interpreted as straight-line distances measured from the solid boundary (i.e. perimeter) of the first aperture, at the point closest to the second aperture, to the solid boundary (i.e. perimeter) of the second aperture, at the point closest to the first aperture. Similarly, unless stated otherwise, references to distances from an aperture to another partition plate feature, such as the peripheral edge or the centre of the partition plate, should be interpreted as straight-line distances measured from the solid boundary (i.e. perimeter) of the aperture, at the point closest to the said other feature, to the point on the other feature closest to the aperture.

[0032] It may be that the off-centre material feed apertures are arranged symmetrically about the centre of the partition plate. For example, it may be that the symmetrical arrangement of the off-centre material feed apertures is defined by one, two, three, four or more mirror planes extending substantially perpendicular to the plane of the partition plate. The said mirror planes may intersect at the centre of the partition plate.

[0033] It may be that the arrangement of the off-centre material feed apertures is rotationally symmetric about the centre of the partition plate. It may be that the symmetrical arrangement of the off-centre material feed apertures has a rotational axis of symmetry which extends substantially perpendicular to the plane of the partition plate at its centre. The arrangement of the off-centre material feed apertures may have 2-fold, 3-fold, 4-fold, 5-fold, 6-fold, 7-fold, 8-fold or more-fold rotational symmetry about the said rotational axis of symmetry.

[0034] It may be that the off-centre material feed apertures are arranged in a ring, i.e. it may be that the off-centre material feed apertures are arranged to lie on a (i.e. imaginary) circle. The ring or circle may be centred on the centre of the partition plate.

[0035] It may be that the off-centre material feed apertures are arranged at the vertices of a (i.e. imaginary) regular polygon. The regular polygon may be centred on the centre of the partition plate. That is to say, the regular polygon may have a centroid or circumcentre coincident with the centre of the partition plate. The regular polygon may be an n-sided regular polygon, wherein n is greater than 2. Accordingly, the regular polygon may have three, four, five, six, seven, eight or more sides. For example, the regular polygon may be a triangle, a square, a pentagon, a hexagon, a heptagon, or an octagon.

[0036] It may be that each of the off-centre material feed apertures has (i.e. substantially) the same shape. For example, it may be that each of the off-centre material feed apertures is substantially circular in shape. Alternatively, it may be that one or more (e.g. each) of the off-centre material feed apertures is substantially polygonal in shape.

[0037] It will be appreciated that the “shape” of an off-centre material feed aperture refers to the shape of the off-centre material feed aperture when viewed along a direction substantially perpendicular to the plane of the partition plate. That is to say that the shape of the off-centre material feed aperture refers to the shape of the boundary of the off-centre material feed aperture in the plane of the partition plate, i.e. the cross-sectional shape of the off-centre material feed aperture in the plane of the partition plate. It will be appreciated that, given the non-zero thickness of the partition plate, each off-centre material feed aperture typically also has a three-dimensional prismatic (e.g. cylindrical) shape with a longitudinal axis extending substantially perpendicular to the plane of the partition plate, but throughout this specification and the appended claims, the term“shape” applied generally to the one or more off-centre material feed apertures does not refer to this three-dimensional prismatic shape.

[0038] In addition to, or as an alternative to, having the same shape, it may be that each of the off-centre material feed apertures has (i.e. substantially) the same size. For example, it may be that each of the off-centre material feed apertures has (i.e. substantially) the same open aperture cross-section (e.g. open aperture cross- sectional area).

[0039] It may be that each of the off-centre material feed apertures has (i.e. substantially) the same characteristic dimension or dimensions. For example, in embodiments in which the off-centre material feed apertures are substantially circular in shape, it may be that each of the off-centre material feed apertures has (i.e. substantially) the same aperture diameter. In embodiments in which the partition plate is substantially circular in shape, it may be that the aperture diameter of each off-centre material feed aperture is at least about 5 %, for example at least about 10 %, or at least about 15 %, or at least about 20 %, of the radius of the partition plate. In embodiments in which the off-centre material feed aperture are non-circular (for example, where the off-centre material feed apertures are substantially polygonal in shape), it may be that each of the off-centre material feed aperture has (i.e. substantially) the same maximum dimension and/or the same minimum dimension (for example, the same width and/or breadth).

[0040] It may be that a shortest distance, for example a radial distance, between the centre of the partition plate and each of the off-centre material feed apertures is at least about 30 %, for example at least about 35 %, or at least about 40 %, or at least about 45 %, of the radius of the partition plate. It may be that the shortest distance, for example the radial distance, between the centre of the partition plate and each of the off-centre material feed apertures is no greater than about 70 %, for example, no greater than about 65 %, or no greater than about 60 %, or no greater than about 55 %, of the radius of the partition plate. It may be that the shortest distance, for example the radial distance, between the centre of the partition plate and each of the off-centre material feed apertures is from about 30 % to about 70 %, for example from about 35 % to about 65%, or from about 40 % to about 60 %, of the radius of the partition plate.

[0041 ] It may be that a shortest distance, for example a radial distance, between the centre of the partition plate and each of the off-centre material feed apertures is at least the aperture diameter, for example at least about 1.5 times the aperture diameter, or at least about 2 times the aperture diameter.

[0042] It may be that each of the off-centre material feed apertures is spaced apart from the peripheral edge by a distance which is at least about 10 %, for example at least about 15 %, or at least about 20 %, or at least about 25 %, or at least about 30 %, of the radius of the partition plate. It may be that each of the off- centre material feed apertures is spaced apart from the peripheral edge by a distance which is at least about 50 %, for example at least about 75 %, or at least about 100 %, of the aperture diameter. The distance between the off-centre material feed apertures from the peripheral edge typically defines the material bed depth in the rotary tube reactor vessel in use.

[0043] It may be that the distance between adjacent (i.e. closest) off-centre material feed apertures is at least 50 %, for example at least 75 %, or at least 100 %, of the aperture diameter.

[0044] The partition plate typically comprises no material feed apertures other than the one or more off-centre material feed apertures. For example, the partition plate does not typically comprise a central material feed aperture (i.e. a material feed aperture located at the centre of the partition plate and intended for transferring material to be processed, such as granular material, between chambers of the rotary tube reactor vessel). However, it may be that partition plate comprises one or more other apertures in addition to the one or more material feed apertures. [0045] For example, it may be that the partition plate comprises one or more drain apertures. The one or more drain apertures may be located adjacent or at the peripheral edge of the partition plate. The one or more drain apertures may be smaller in size than the one or more material feed apertures.

[0046] Additionally or alternatively, it may be that the partition plate comprises a central aperture located at the centre of the partition plate. The central aperture may be smaller in size than the one or more material feed apertures. The central aperture is typically configured (e.g. sized) to receive one or more gas supply pipes and/or one or more gas exhaust pipes when installed in the rotary tube reactor vessel.

[0047] The partition plate may have a total open aperture cross-section of no greater than about 30 %, for example, no greater than about 25 %, or no greater than about 20 %, or no greater than about 15 %, or no greater than about 10 %, or no greater than about 5 %. The total open aperture cross-section is expressed as the percentage of the total cross-sectional area of the partition plate (i.e. in the plane of the partition plate) which is open aperture (and therefore does not contain solid material). For example, an entirely solid partition plate having no apertures would have a total open aperture cross-section of 0%. It will be appreciated that the total open aperture cross-section includes contributions to the open aperture cross- section from all open apertures in the partition plate, including the one or more material feed apertures and any drain apertures or central aperture where present. The thermal barrier effect, and the effect on control of gas currents in the reactor vessel, of the partition plate is typically greater for smaller total open aperture cross- sections

[0048] The partition plate is typically substantially solid (excluding any apertures through the partition plate). It may be that the partition plate is made at least predominantly, for example entirely, of a refractory material, for example a refractory glass such as quartz glass or silica glass, i.e. fused quartz or fused silica. The refractory material typically has a melting temperature, or softening temperature in the case of a refractory glass, of greater than about 800°C, for example greater than about 1000°C, or greater than about 1200°C, or greater than about 1400°C, or greater than about 1600°C. The refractory glass may have a strain temperature of greater than about 800°C, for example greater than about 1000°C. The refractory glass may have an annealing temperature of greater than about 800°C, for example greater than about 1000°C, or greater than about 1100°C. For example, quartz glass can have a softening temperature of about 1683°C, a strain temperature of about 1070°C and an annealing temperature of about 1140°C. Use of refractory glasses such as fused quartz or fused silica is typically required for rotary tube reactor vessels used in refining silica powder.

[0049] For the avoidance of doubt, the term “silica” as used throughout this specification and the appended claims is to be interpreted as referring to silicon dioxide, i.e. S1O2. Silica can form different crystalline polymorphs. For example, quartz (i.e. a-quartz) is a common, naturally-occurring crystalline form of silica. Other crystalline forms of silica include b-quartz, coesite, stishovite and tridymite, which are stable across different ranges of temperature and pressure. Silica may also take up an amorphous form, typically referred to as silica, synthetic silica, precipitated silica, and also“quartz” glass.

[0050] Silica is also available in many different granular forms characterised by the particle size. Throughout this specification and the appended claims, unless stated otherwise, references to “silica powder” or “quartz powder” are to be interpreted as covering granular silica or quartz, irrespective of particle size, including silica or quartz sand, grain, powder or flour.

[0051 ] According to a second aspect, there is provided a rotary tube reactor comprising a reactor vessel having first and second chambers partitioned from one another by a first partition plate according to the first aspect. The first and second chambers and the first partition plate may be arranged so that in use material moves from the first chamber into the second chamber, by way of one or more material feed apertures in the first partition plate, as the reactor vessel rotates.

[0052] The reactor vessel may further comprise a third chamber partitioned from the first chamber or the second chamber by a second partition plate according to the first aspect. The first, second and third chambers and the first and second partition plates may be arranged so that in use material moves sequentially from the first chamber into the second chamber, by way of one or more material feed apertures in the first partition plate, and from the second chamber into the third chamber, by way of one or more material feed apertures in the second partition plate, as the reactor vessel rotates.

[0053] The rotary tube reactor may receive a granular material as an input. The rotary tube reactor may be a rotary tube reactor for processing said granular material. Processing the granular material may comprise refining or purifying the granular material.

[0054] Processing the granular material may comprise exposing the granular material to a process gas. The process gas may be a halogen-containing process gas, such as a fluorine-containing gas, a chlorine-containing gas or a bromine- containing gas. The process gas may comprise F2, Br2, CI2, HF, HBr, and/or HCI. Processing the granular material may comprise exposing the granular material to an atmosphere which may be oxidising, neutral or reducing. The atmosphere may be made oxidising by introducing an oxidising gas such as O2. The atmosphere may be made reducing by introducing a reducing gas such as FI2, CO2, and/or H2S. Additionally or alternatively, the process gas may comprise N2 or a noble gas such as Ar.

[0055] Additionally or alternatively, processing the granular material may comprise heating the granular material, for example to a temperature exceeding about 600°C, for example exceeding about 800°C, or exceeding about 1000°C, or exceeding about 1200°C, or exceeding about 1400°C. [0056] Additionally or alternatively, processing the material may comprise cooling the material in the first chamber and/or the second chamber by contact with the rotary tube reactor and/or exposure to an air or gas stream. When used as a rotary cooler, the reactor vessel may include partition plates with one or more off-centre material feed apertures located on an axis (or axes) parallel to the longitudinal axis of the reactor vessel,, a central material feed aperture located on the longitudinal axis of the reactor vessel, or both. Cooling water may be used to improve heat transfer from the rotary cooler.

[0057] The first chamber may be an inlet chamber for receiving the granular material as input. The second chamber may be a process chamber for processing the granular material, for example by exposing the granular material to process gas in said process chamber and/or by heating the granular material in said process chamber. The third chamber may be an outlet chamber for receiving the processed granular material from the process chamber.

[0058] The reactor vessel may be made at least predominantly, for example entirely, of a refractory material, for example a refractory glass such as quartz glass or silica glass, i.e. fused quartz or fused silica. For example, the interior walls of the reactor vessel may be made at least predominantly, for example entirely, of the refractory material. The refractory material typically has a melting temperature, or softening temperature in the case of a refractory glass, of greater than about 800°C, for example greater than about 1000°C, or greater than about 1200°C, or greater than about 1400°C. The reactor vessel may be made at least predominantly, for example entirely, from one or more of the following: silica glass, silicon carbide (i.e. SiC), silicon-filled silicon carbide (i.e. silicon-filled SiC), graphite, silicon nitride (e.g. ShN ₄ ), alumina (i.e. AI2O3), an aluminosilicate such as mullite, a carbon fibre reinforced material such as carbon fibre reinforced carbon or a carbon fibre reinforced ceramic. [0059] The reactor vessel may be substantially prismatic in shape. For example, the reactor vessel may be substantially cylindrical in shape. The reactor vessel is typically rotatable about a longitudinal axis.

[0060] A third aspect provides a method of processing a material in a rotary tube reactor according to the second aspect, the method comprising: supplying material to the first chamber; and transferring material from the first chamber to the second chamber through the off-centre material feed apertures of the first partition plate during rotation of the reactor vessel.

[0061 ] In embodiments in which the reactor vessel comprises a third chamber, the method may further comprise: transferring material from the second chamber to the third chamber through the off-centre material feed apertures of the second partition plate during rotation of the reactor vessel.

[0062] The method may comprise processing the material in one or more of the first, second and third chambers.

[0063] Processing the material may comprise heating the material, for example, to a temperature exceeding 600°C, for example exceeding 800°C, or exceeding 1000°C, or exceeding 1200°C, or exceeding 1400°C.

[0064] Additionally or alternatively, processing the material may comprise cooling the material in the first chamber, the second chamber, and/or the third chamber by contact with the rotary tube reactor and/or exposure to an air or gas stream. When used as a rotary cooler, the reactor vessel may include partition plates with one or more off-centre material feed apertures located on an axis (or axes) parallel to the longitudinal axis of the reactor vessel, a central material feed aperture located on the longitudinal axis of the reactor vessel, or both. Cooling water may be used to improve heat transfer from the rotary cooler. [0065] Additionally or alternatively, processing the material may comprise exposing the material to a process gas. The process gas may be a halogen- containing process gas, such as a fluorine-containing gas, a chlorine-containing gas or a bromine-containing gas. The process gas may comprise F2, Br2, CI2, HF, HBr, SF6, and/or HCI. Processing the granular material may comprise exposing the granular material to an atmosphere which may be oxidising, neutral or reducing. The atmosphere may be made oxidising by introducing an oxidising gas such as O2. The atmosphere may be made reducing by introducing a reducing gas such as FI2, CO, and/or H2S. Additionally or alternatively, the process gas may comprise N2 or a noble gas such as Ar.

[0066] It may be that the first chamber is an inlet chamber for receiving material as input to the reactor vessel. It may be that the second chamber is a processing chamber for processing the material. It may be that the third chamber is an outlet chamber for outputting processed material. Accordingly, the method may comprise processing the material in the second chamber, for example by heating the material and/or exposing the material to process gas.

[0067] The method may be a method of purifying quartz or silica powder. Supplying material to the first chamber may comprise suppling quartz or silica powder to the first chamber. Transferring material from the first chamber to the second chamber may comprise transferring quartz or silica powder from the first chamber to the second chamber through the off-centre material feed apertures of the first partition plate during rotation of the reactor vessel. Processing the material in the second chamber may comprise heating (for example, to a temperature exceeding about 600°C, for example exceeding about 800°C, or exceeding about 1000°C, or exceeding about 1200°C, or exceeding about 1400°C) the quartz or silica powder and/or exposing the quartz or silica powder to process gas (for example, a halogen-containing process gas such as gaseous chlorine or gaseous HCI). Transferring material from the second chamber to the third chamber may comprise transferring processed quartz or silica powder from the second chamber to the third chamber through the off-centre material feed apertures of the second partition plate during rotation of the reactor vessel.

[0068] The method may be a method of calcining silica (for example silica powder, e.g. quartz powder) to reduce hydroxyl content. Silica powder (e.g. quartz powder) typically contains bound water in the form of hydroxyl groups prior to calcining. Hydroxyl groups can reduce silica glass network stability, resulting in reduced viscosity and working temperatures, or reduced optical transmission, particularly in the infra-red. The method of calcining silica typically includes the step of heating the quartz or silica powder (for example, to a temperature exceeding about 600°C, for example exceeding about 800°C, or exceeding about 1000°C, or exceeding about 1200°C, or exceeding about 1400°C). However, the method of calcining silica typically does not include the step of exposing the quartz or silica powder to process gas such as a halogen-containing process gas.

[0069] The method may be a method of purifying silica (for example silica powder, e.g. quartz powder) by hot chlorination. Hot chlorination typically reduces the hydroxyl content of the silica. Hot chlorination also typically reduces the content of lattice-bound metal impurities. Metal impurities tend to negatively affect structural, optical and electrical properties of silica glass. The method of purifying silica by hot chlorination typically includes both steps of heating the quartz or silica powder (for example, to a temperature exceeding about 600°C, for example exceeding about 800°C, or exceeding about 1000°C, or exceeding about 1200°C, or exceeding about 1400°C) and exposing the quartz or silica powder to process gas such as a halogen- containing process gas, preferably to a chlorine-containing process gas such as CI2 or HCI.

[0070] The material, for example the quartz or silica powder, may change form or physical state as it moves through the reactor vessel. The material may at least partially melt as it moves through the reactor vessel. Accordingly, the method may comprise transferring molten or semi-molten material (e.g. molten or semi-molten quartz or silica) from the first chamber to the second chamber and/or from the second chamber to the third chamber. Additionally or alternatively, the material, for example the quartz or silica powder, may change phase as it moves through the reactor vessel. For example, solid silicon dioxide can exist in many different phases including quartz, cristobalite or tridymite. Sub-phases of quartz, such as alpha quartz or beta quartz, are also possible.

[0071 ] The steps of the method may be carried out sequentially. For example, the method may comprise: first, supplying material to the first chamber; and second, transferring material from the first chamber to the second chamber through the off- centre material feed apertures of the first partition plate during rotation of the reactor vessel; and optionally, third, transferring material from the second chamber to the third chamber through the off-centre material feed apertures of the second partition plate during rotation of the reactor vessel. For example, the method may be carried out as a batch process.

[0072] Alternatively, the steps of the method may be carried out concurrently (i.e. as partially or entirely overlapping steps, for example steps carried at the same time). For example, the method may comprise: supplying material to the first chamber; and concurrently transferring material from the first chamber to the second chamber through the off-centre material feed apertures of the first partition plate during rotation of the reactor vessel; and, optionally, concurrently transferring material from the second chamber to the third chamber through the off-centre material feed apertures of the second partition plate during rotation of the reactor vessel. For example, the method may be carried out as part of a continuous process.

[0073] The skilled person will appreciate that, except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and/or combined with any other feature described herein. FIGURES

[0074] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0075] Figure 1 is a schematic representation of a rotary tube reactor in use refining silica powder;

[0076] Figure 2 is an example partition plate for use in a rotary tube reactor shown in views (a) perpendicular to the plane of the partition plate and (b) parallel to the plane of the partition plate;

[0077] Figure 3 is an alternative view of the partition plate of Figure 2 indicating certain geometrical features of the partition plate;

[0078] Figure 4 is an alternative example partition plate for use in a rotary tube reactor;

[0079] Figure 5 is a further alternative example partition plate for use in a rotary tube reactor;

[0080] Figure 6 is a further alternative example partition plate for use in a rotary tube reactor;

[0081 ] Figure 7 is a further alternative example partition plate for use in a rotary tube reactor;

[0082] Figure 8 is an example end plate for use in a rotary tube reactor;

[0083] Figure 9 is schematic cross-section through a rotary tube reactor vessel chamber indicating a“cake slice” and silica powder bed;

[0084] Figure 10 is a plot of silica powder bed profile for a rotary tube reactor as calculated using the Saeman approximation for partition plates having one central material feed aperture (dashed lines) or eight off-centre material feed apertures (solid lines).

[0085] Figure 11 is a plot of the dio of silica powder exiting a rotary tube reactor as a function of time

[0086] Figure 12 is a plot of the temperature of material exiting a rotary cooler as a function of angular speed. [0087] Figure 13 is a plot of the temperature of material exiting a rotary cooler with partition plate as a function of material feed rate and rotary cooler angular speed.

[0088] Figure 14 is a plot of the percentage area of a rotary reactor in contact with quartz as a function of effective dam height.

DETAILED DESCRIPTION

[0089] Rotary tube reactor apparatus

[0090] With reference to Figure 1 , a rotary tube reactor (i.e. a rotary kiln) 1 includes a reactor vessel 2 extending between an inlet end 3 and an outlet end 4. The interior space of the reactor vessel 2 is divided into three chambers - inlet chamber 5, processing chamber 6 and outlet chamber 7 - by first and second partition plates 8 and 9. The partition plates 8 and 9 are provided with apertures such that the inlet chamber 5, processing chamber 6 and outlet chamber 7 are in fluid communication with one another. The reactor vessel 2 is supported by a housing (not shown) such that a longitudinal axis of the reactor vessel is inclined at an angle of about 3° with respect to the horizontal.

[0091 ] A feeding hopper 10 is provided at the inlet end 3 and includes a chute 11 which extends through an inlet aperture 12 of the reactor vessel 2. An outlet chute 13 is provided at the outlet end 4 around an outlet aperture 14 of the reactor vessel 2. A gas supply pipe 15 extends into the processing chamber 6. A gas exhaust pipe 16 extends into the outlet chamber 7.

[0092] A heater 17 is located around the outside of the reactor vessel 2 within the housing. The heater 17 contains electrical heating elements and may function by either direct or indirect gas heating methods, although indirect methods are preferred. The electrical heating elements may be formed from a metal (for example a refractory metal such as tungsten), graphite, silicon carbide or intermetallic molybdenum disilicide. Silicon carbide heating elements which have a maximum operating temperature of about 1550°C, or molybdenum disilicide heating elements which have a maximum operating temperature of about 1800°C, are most preferred. Graphite or tungsten heating elements require use of neutral or reducing atmospheres. Heating elements made of metals such as platinum or Ni-AI-Cr alloys require use of oxidising atmospheres.

[0093] The reactor vessel 2 is generally cylindrical and the partition plates 8 and 9 are disc-shaped. Both the walls of the reactor vessel 2 and the partition plates 8 and 9 are formed from fused silica glass having a softening temperature of around 1600°C.

[0094] Processing method

[0095] The rotary tube reactor 1 is used to process granular materials. For example, the rotary tube reactor 1 may be used, at outlined in the following method, to purify silica powder using high-temperature reactions.

[0096] For example, in a hot chlorination method, silica powder 18 (such as powdered, naturally-occurring quartz) is fed as an input material into the inlet chamber 5 of the reactor vessel 2 from the feeding hopper 10 through the chute 11. The reactor vessel 2 is rotated around its longitudinal axis and the heater 17 heats the reactor vessel 2.

[0097] The inlet chamber 5 is heated to around 1000°C. As the reactor vessel 2 rotates, heated silica powder is tumbled within the inlet chamber 5. Due to the inclination of the reactor vessel 2 with respect to the horizontal, silica powder is progressively transferred from the inlet chamber 5 to the processing chamber 6 through apertures in partition plate 8 as the reactor vessel 2 rotates.

[0098] A chlorine-containing process gas, such as hydrogen chloride, is pumped into the processing chamber 6 through supply pipe 15 and the processing chamber 6 is also heated to around 1200°C. Under these conditions, lattice-bound impurities (such as alkali metal, alkali earth metal or transition metal impurities) in the silica powder diffuse across silica grains to the grain surfaces where they react with the process gas and are thereby removed from the silica powder. For example, sodium impurities present in the silica powder are removed by exposure to hydrogen chloride through the following reactions:

[0099] Si-O-Na + HCI Si-O-H + NaCI

Al-Na + HCI = Al-H + NaCI

[0100] As the reactor vessel 2 continues to rotate, processed silica powder is progressively transferred from the processing chamber 6 to the outlet chamber 7 through apertures in partition plate 9. The outlet chamber is heated to a lower temperature than the processing chamber, for example to around 800°C. Under these conditions, process gas adsorbed on the surface of the silica powder desorbs and is pumped out of the chamber through the exhaust pipe 16. The processed silica powder then leaves the reactor vessel though the outlet chute 13.

[0101 ] The various steps of the method are typically carried out simultaneously, i.e. as a continuous process, for example as seen in Figure 1 where silica powder continuously passes through the reactor vessel. However, the method may also be performed as a batch process.

[0102] Additionally or alternatively, the rotary tube reactor 1 may be used as a rotary cooler. A granular material, such as, for example, silica powder 18 is fed as an input material into the inlet chamber of the reactor vessel 2 from the feeding hopper 10 through the chute 11. In this process, processing chamber 6 is cooled instead of heated, causing the granular material to cool as it passes through the reactor vessel 2.

[0103] Reactor vessel 2 is rotated around its longitudinal axis, causing the granular material to be progressively transferred from inlet chamber 5, to processing chamber 6, and to outlet chamber 7. The granular material may be cooled, for example, by contact with the rotary tube reactor and/or exposure to an air or gas stream passing through the rotary cooler. The apertures restrict natural air flow, which is good to help avoid contamination. Any one or more of air, oxygen, and nitrogen may be added to help remove any residual reagent gasses.

[0104] In some exemplary embodiments, partition plates 8 and 9 include one or more off-centre material feed apertures, as described below. Partition plates 8 and 9 may additionally or alternatively include a central material feed aperture located on the longitudinal axis of the reactor vessel 2.

[0105] Cooling water may be used to improve heat transfer from the rotary cooler.

[0106] Partition plates

[0107] The effectiveness and the efficiency of the method described above depend in part upon the depth of material (i.e. the material bed depth) retained within each chamber, the length of time material is retained within the reactor vessel (i.e. the retention time), the amount of mixing for reactions and heat transfer, and on the rate of flow of material through the reactor vessel. The material bed depth, the retention time and the flow rate are determined at least in part by the structure of the partition plates 8 and 9 which partition the chambers from one another.

[0108] The partition plates 8 and 9 also act as thermal barriers between adjacent chambers and the structure of the partition plates is therefore important for achieving accurate temperature control in each chamber. Similarly, the partition plates regulate flow of gases between chambers. Generally, it is desirable for partition plates to have low total open aperture cross sections since this provides better thermal control and reduces gas currents in the chambers.

[0109] An example structure for partition plate 8, which separates chambers 5 and 6, is shown in detail in Figures 2 (a) and (b). The partition plate 8 has a generally disc-shaped body 19 formed from solid fused silica glass. The thickness t of the partition plate 8 is substantially smaller than the external diameter d such that the partition plate can be considered to be substantially planar.

[0110] A circumferential mounting portion 20 of the body 19, bounded by dashed line 21 , is shaped to fit into a corresponding slot in the walls of the reactor vessel to hold the partition plate in place in use. Accordingly, the dashed line 21 represents an effective peripheral edge of the partition plate when the plate is installed in the reactor vessel, i.e. dashed line 21 bounds the region of the partition plate which is exposed to material and gas flow inside the reactor vessel. The partition plate 8 may also be attached to the reactor vessel by welding using a quartz glass welding tool or, alternatively, interior walls of the reactor vessel may (at high temperatures) be necked down around the mounting portion of the partition plate to hold it in place.

[0111 ] Eight open material feed apertures 22A to 22H extend through the entire thickness of the partition plate body 19. The material feed apertures 22A-22H are circular in shape and are arranged in a ring around the centre of the partition plate, i.e. such that the ring is concentric with the peripheral edge 21. Each of the material feed apertures 22A-22H are spaced apart from one another, from the centre of the partition plate, and from the peripheral edge 21. An open drain aperture 23 also extends through the entire thickness of the partition plate body 19. The drain aperture 23 is located outside the ring of material feed apertures 22A-22H, adjacent to the peripheral edge 21. The drain aperture 23 is used to empty the reactor vessel of residual material when a processing run is completed.

[0112] The geometrical arrangement of the material feed apertures and the drain aperture is shown in more detail in Figure 3. In particular, the partition plate 8 may be divided conceptually into three regions: a central region 24, an intermediate region 25 and a peripheral region 26. The central region 24 is a circular region which is centred on and includes the centre 27 of the partition plate 8 and is bounded by boundary 28. The peripheral region 26 is an annular region which extends around partition plate 8 adjacent the peripheral edge 21 and is bounded by said peripheral edge 21 and boundary 29. The intermediate region 25 fills the space between the central and peripheral regions and is consequently bounded by boundaries 28 and 29. As shown in Figure 3, the material feed apertures 22A-22FI are located in the intermediate region 24 and the drain aperture 23 is located in the peripheral region 26.

[0113] The location of boundary 28 can be defined by radius r ₃ . The location of boundary 29 can be defined by radius r ₂ . The location of the peripheral edge 21 can be defined by radius r _x . The size of each material feed aperture can be defined by radius r _m .

[0114] As can be seen in Figure 3, the shortest distance between each material feed aperture and the centre is approximately 3 r _m . Similarly, the shortest distance between each material feed aperture and the peripheral edge is approximately 2r _m . The shortest distance between each adjacent pair of material feed apertures is approximately 1.5 r _m . The thickness of the intermediate region (i.e. r ₂ - r ₃ ) is approximately 0.37 The thickness of the peripheral region (i.e. r ₁ - r ₂ ) is approximately 0.2?v The total open aperture cross-section of the partition plate is approximately 17 %. The material feed apertures 22A-22H are arranged symmetrically about the centre of the partition plate with an angular separation Q of approximately 45°.

[0115] An example structure for partition plate 9, which separates chambers 6 and 8, is shown in detail in Figure 4. The partition plate 9 has substantially the same structure as partition plate 8 (including a generally disc-shaped body 30 formed from solid fused silica glass, a circumferential mounting portion 31 bounded by peripheral edge 32, eight open material feed apertures 33A to 33H and drain aperture 34). Flowever, partition plate 9 also includes a central aperture 35 for permitting the process gas supply tube 15 to access the processing chamber 6.

[0116] As will be explained in more detail below, use of a partition plate having the structure of partition plate 8 or partition plate 9 enables a relatively constant material bed depth to be developed in chambers upstream of the plates. In particular, the distance between each material feed aperture and the peripheral edge (i.e. the thickness of the peripheral region, r ₁ - r ₂ ) determines the material bed depth. The inventors have found that, due to the symmetrical arrangement of the material feed apertures around the plate, as the material feed rate is increased such that more material is added to a chamber, the said material is exposed to a progressively larger open aperture cross-section such that the material bed depth in effect remains substantially constant. Control of the material bed depth in turn permits control of the material retention time. A deeper material bed, and consequently a higher retention time, results in better mixing of the material and also more effective heat transfer to the material grains. However, it will be appreciated that alternative partition plate or material feed aperture geometries and arrangements are possible while still obtaining an improvement in material bed depth and retention time control, so long as the material feed apertures are spaced apart from the peripheral edge.

[0117] For example, with regard to partition plate 8 in Figure 3, the shortest distance between each material feed aperture and the centre is typically at least about 10 %, for example at least about 20 %, or more typically at least about 30% of r _{l t} up to about 70% of r _x . The radius of each material feed aperture, r _m is typically at least about 10 % of r _x . Similarly, the shortest distance between each material feed aperture and the peripheral edge is typically at least about 10%, for example at least about 50%, of r _{l t} or at least about 100 %, for example at least 150 %, of r _m . The shortest distance between each adjacent pair of material feed apertures is typically at least about 100%, for example at least about 150 %, of r _m . The thickness of the intermediate region (i.e. r ₂ - r ₃ ) is typically at least about 10 %, or more typically at least about 20 %, of r _x . The thickness of the peripheral region (i.e. r ₁ - r ₂ ) is typically at least about 5 %, or more typically at least 10 %, or at least about 20 %, or at least about 30% of r _x . The thickness of the peripheral region determines the material bed depth and consequently the amount of material mixing in the reactor vessel. The material bed depth required, and consequently the thickness of the peripheral region required, depends on the nature of the particular material being processed, including the flow properties of the material and the amount of friction between the material and the reactor vessel walls. The total open aperture cross- section of the partition plate is typically no greater than about 30 %, and preferably is less than about 20 %, or even more preferably less than about 10 % .

[0118] The number and shape of the material feed apertures may also be varied, so long as the material feed apertures are located in the intermediate region, spaced apart from both the centre of the partition plate and peripheral edge. As the number of material feed apertures may be varied, so may the angular separation Q between material feed apertures. However, the material feed apertures are typically arranged symmetrically or in a periodically repeating pattern around the centre of the partition plate.

[0119] Examples of alternative partition plate geometries are illustrated in Figures 5 to 7. As shown in Figure 5, disc-shaped partition plate 36 includes four circular material feed apertures 37A to 37D located in the intermediate region spaced apart from both the centre of the partition plate and the peripheral edge. As shown in Figure 6, disc-shaped partition plate 38 includes two semi-annular (i.e. bow-shaped) material feed apertures 39 located in the intermediate region spaced apart from both the centre of the partition plate and the peripheral edge. As shown in Figure 7, polygonal partition plate 40 includes eight polygonal materials feed apertures 41 located in the intermediate region spaced apart from both the centre of the partition plate and the peripheral edge. Such a polygonal partition plate would be suitable for use in an alternative rotary tube reactor vessel having a polygonal rather than circular cross-sectional shape. In each case, the respective material feed apertures are spaced apart from both the peripheral edge and the centre of the partition plate. This enables material bed depth control without an excessive increase in open aperture cross-section.

[0120] Variations [0121 ] In one variation of the apparatus, an end plate may also be provided at the outlet end 4 of the reactor vessel 2 shown in Figure 1. The outlet aperture 14 may be provided in the end plate. However, Figure 8 shows an example end plate 42 which includes four circumferentially spaced outlet openings 43A-43D located at the periphery of the end plate 42. The end plate 42 also includes a central aperture 44.

[0122] In another variation, the inlet end 3 and/or the outlet end 4 are provided with dome-shaped end pieces having central inlet and outlet apertures smaller than the cross-sectional area of main body of the reactor vessel 2. Use of such dome- shaped end pieces enables inlet and outlet chambers to hold a greater volume of granular material at any given instant without loss of granular material from the reactor vessel. This design is particularly useful when the material bed depth and the material retention time is high.

[0123] In another variation, the rotary reactor may be a calcination kiln for calcining silica powder, rather than the hot chlorination kiln described above. Calcining typically involves heating the silica powder to an elevated temperature (for example, exceeding about 1200°C) such that hydroxyl groups are liberated from the silica grains. Calcining does not require supply of a halogen-containing process gas. Accordingly, a calcination kiln includes a reactor vessel extending between an inlet end and an outlet end, the interior space of the reactor vessel being divided into an inlet chamber, processing chamber and outlet chamber by first and second partition plates similar to the hot chlorination kiln. However, the calcination kiln does not require the presence of a gas supply pipe. Both first and second partition plates may therefore take the same form as partition plate 8, shown in Figure 2, which lacks a central gas-supply aperture.

[0124] It will be understood that the invention is not limited to the embodiments described above and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub- combinations of one or more features described herein.

[0125] Bed depth and retention time control

[0126] Mathematical models have been developed to study the transport of a material, such as granular material, through a rotary tube reactor vessel. The residence time of a granular material in a rotating cylindrical reactor vessel, where no partition plates are present, can be determined using the following empirical expression:

[0127] _T = U

f·ά·w

[0128] Where: t is the residence time in seconds; l is the length of the reactor vessel in metres; Q is the angle of repose in degrees; f is the angle of inclination of the reactor vessel in degrees; d is the diameter of the reactor vessel in metres; and w is the rotary speed of the reactor vessel in rpm. In such a system, the retention time is independent of the density of the granular material.

[0129] The calculation of the retention time for a reactor vessel including partition plates, which function as dams, is more complex. In such a scenario, it is first necessary to determine a layer thickness profile, for which the Saeman approximation can be used:

[0131 ] Where: — is the change in layer thickness with distance along the length of the reactor vessel, in metres; Q _v is the volume flow of material through the reactor vessel, in m ³ s ¹ ; w is the angular speed of rotation of the reactor vessel, in s ¹ ; ^a re _P ose is the angle of repose, in radians; R _tube is the radius of the reactor vessel, in metres; 9 _inclination is the inclination of the reactor vessel, in radians; and /i(x) is the layer thickness in the tube at a distance x, in metres.

[0132] The Saeman approximation is used to determine the slope of the granular material bed in the longitudinal (i.e. axial) direction. Starting from a dam which locally fixes the layer thickness, the layer thickness profile can be calculated by applying the Saeman approximation in an upstream direction. The bed of granular material upstream of the dam is typically slightly higher than the height of the dam and can be measured empirically. The retention time can be calculated from the volume of the granular material bed upstream of the dam as follows.

[0133] Assuming a more or less flat granular material bed, the material bed volume can be calculated by considering the schematic representation shown in Figure 9 which is a cross section through a reactor vessel perpendicular to the longitudinal axis. The shaded region represents the granular material bed and the lateral boundaries of the granular material bed define a sector of the circular cross- section (or a “cake slice” of the cross-section) having a central angle f . By subtracting the area of the triangular portion of the sector above the granular material bed from the sector, the following expression for the cross-sectional area of the granular material bed depth may be determined in terms of f :

[0135] The central angle f may be written in terms of the granular material bed depth, h, as:

[0137] Accordingly, the cross-sectional area of the granular material bed is:

[0138] ^A = R ² · ( arccos (l - ¾ - (l - ¾ · sin (l - ¾) [0139] Where: A is the cross-sectional area of the granular material bed, in m ² ; h is the depth of the granular material bed, in m; and R is the radius of reactor vessel, in m.

[0140] Integration along the axis of the reactor vessel gives the following expression for the volume of the granular material bed.

[0142] If the volume flow rate through the reactor vessel is constant, the average retention time is given by the dividing this expression for the volume by the volume flow rate. However, in practice it is found that the volume flow rate varies along the length of the reactor vessel since the bulk density of the granular material tends to vary as it is undergoes processing (for example, due to the granular material drying). For example, the loose bulk density of a wet quartz powder (having a moisture content of 3.5 %) may be about 800 kgrrr ³ , whereas the loose bulk density of the same quartz powder when dry may be about 1230 kgrrr ³ . Similarly, the tapped bulk density of wet quartz powder may be about 1100 kgrrr ³ , whereas the tapped bulk density of the quartz powder when dry may be about 1380 kgrrr ³ .

[0143] Figure 10 shows the depth of quartz powder in a reactor vessel as a function of distance from the inlet to the reactor vessel as determined using the preceding equations. In the calculations, it is assumed that the bulk density of the quartz power is 800 kgrrr ³ up to 450 mm from the inlet to the reactor vessel, beyond which the quartz is assumed to be dry and the bulk density is assumed to be 1230 kgrrr ³ . The calculations take into account the presence of a first dam and a second dam indicated at points D and D ₂ respectively. The results shown were calculated assuming each dam consists of a partition plate having (a) one central material feed aperture (dashed lines) or (b) eight off-centre material feed apertures (solid lines) of the type shown in Figure 2. [0144] Taking into account the change in granular material density along the axial direction of the reactor vessel, the theoretical retention time was calculated according to the following expression:

[0146] Where: t is the retention time in s, p _{Q buik} (l) is the bulk density of the granular material at a position l, in kgrrr ³ ; Q _m is the mass flow of the granular material feed in kgs ¹ ; R is the tube radius in m; h(l) is the granular material bed depth at a position l, in m; and dl is an incremental distance along the reactor vessel axis in m.

[0147] The retention time was also measured experimentally using partition plates having the same (a) central material feed aperture or (b) eight off-centre material feed apertures as used for the theoretical calculation. The retention time was determined by adding material having a different grain size to a feed of quartz powder. In particular, 500 grams of a finer natural quartz powder was added to a feed of a coarser natural quartz powder coming from a dewatering filter (while simultaneously about 500 grams of the coarser powder was removed from the feed, to keep the total feed rate constant). The finer powder had a dio of about 75 pm, while the coarser powder had a dio of about 120 pm. The product stream coming out of the reactor vessel was sampled and the grain size was measured using laser diffraction (and in particular using a laser diffraction instrument from CILAS S.A.). The grain size of samples, as evaluated by a measurement of particle dio values, as a function of time passed since adding the fine particles to the input, and as shown in Figure 11 , was used to determine the residence time in the reactor vessel.

[0148] Figure 11 shows experimental results obtained for flow through a reactor vessel having (a) no dams (filled circles in Figure 11 ), (b) a partition plate having one central material feed aperture (filled squares in Figure 11 ), and (c) a partition plate having eight off-centre material feed apertures (open circles in Figure 11 ). The details of the experimental set up is contained in Table 1 below.

[0149] Table 1.

[0150] As can be seen from Figure 11 , adding dams increases the granular material retention time but also increases the spread in retention time, which implies that more mixing of the granular material takes place. The effective dam height for

(b), the partition plate having one central material feed aperture, was higher than for

(c), the partition plate having eight off-centre material feed apertures. Accordingly, the retention time for (b) is slightly higher than for (c), although the retention time for (c) is still substantially higher than for (a). Structure (c) also has a significantly lower open aperture cross-section than (b) and enables a higher throughout of material as more powder can be loaded into the reactor vessel without it backing up and falling out of the inlet. Accordingly, the partition plate with eight off-centre material feed apertures is able to achieve a higher throughput, with reduced open aperture cross- section, and only a moderately reduced retention time compared to the partition plate with a single central material feed aperture. In addition, by spacing the material feed apertures further apart from the peripheral edge, the retention time of partition plate (b) could also be increased beyond that of (c) without a corresponding increase in open aperture cross-section.

[0151 ] Table 2 shows that the retention times predicted by theory correlate with those ascertained by experiment.

[0152] Table 2.

[0153] Figure 12 shows experimental results obtained for a flow through a reactor vessel being used as a rotary cooler having (a) no partition plates and (b) a single partition plate having a central material feed aperture.

[0154] Figure 12 compares the temperature of the material coming out of the cooler versus angular speed of the cooler. The feed rate was between 160-170 kg/h and the temperature of the quartz fed into the cooler was around 560C. Cooling water varied from 65 to 70 l/min at a temperature of 6 C.

[0155] The rotary cooler’s reactor vessel had an inner diameter of 300 mm. The circular material feed aperture, located along the longitudinal axis of the reactor vessel, was 200 mm in diameter.

[0156] Figure 13 shows temperature of the material coming out of the rotary cooler (with partition plate) as a function of material feed rate and rotary cooler angular speed. The temperature of the quartz entering the rotary cooler was 764 C As shown in Figure 13, the feed rate was changed from 100 to 150 kg/h. Post- cooling temperatures increased with increasing feed rate and decreased with increasing angular speed, RPMs.

[0157] Higher tube speed (angular speed, which may be measured in RPMs) may increase mixing and heat transfer when combined with partitions and material feed apertures consistent with this invention. Increased heat transfer may be important for heating and cooling of reagents and reactants. It is generally taught that lower speed gives longer residence time and longer residence time is desirable. The general art in this area does not teach increasing speed to increase heat transfer. Speeds of over 6 rpms and over 12 rpms may be desirable for improved heat transfer. Speeds up to about one-half of the critical speed (the speed at which centrifugal forces hold the grain against the wall of the reactor) may also be desirable. The critical speed is when the reactor becomes a centrifuge. The critical speed, CS, in rpms is defined as 76.63 divided by the square root of the inside diameter of the reactor vessel, measured in feet. For an exemplary reactor vessel with a 250-mm (0.82 ft) inner diameter, the CS is about 85 rpms.

[0158] The temperature of the cooling water varied from 7.8 to 11.2 degrees centigrade. The flow rate of the cooling water varied from 50 to 57 I/m in

(per cooling tube). The highest flow rate occurred when the cooling water had the highest temperature

[0159] As illustrated in Figure 14, varying the number and location of partitions may impact the contact area of the granular material within the rotary cooler. The rotary cooler tested to prepare Figure 14 was 300 mm in inner diameter, and had partitions with central material feed apertures located on a longitudinal axis of the reactor.