An apparatus for facilitating particle separation by density, the apparatus comprising: a separator (12, 302, 452) having an inner surface (100, 306, 456) surrounding a rotation axis (14, 304, 454) of the separator and defining a particle path from an input end (102, 308, 458) of the separator to an axially spaced output end (104, 310, 460) of the separator, wherein the inner surface includes a plurality of axially spaced dividers (120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 320, 322, 324, 326, 328, 330, 470, 472, 474, 476, 478, 480) having respective inner positions, the dividers defining at least in part respective axially spaced retainers (150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 340, 342, 344, 346, 348, 490, 492, 494, 496, 498) for collecting particles during rotation of the separator, each of the retainers including at least one fluid inlet for fluidizing particles in the retainer during operation; each of the inner positions includes a radial position and an axial position, wherein for each of the dividers, the radial position is a radial distance from the innermost surface of the divider to the rotation axis and the axial position is a distance projected onto the rotation axis from the innermost surface of the divider to the innermost surface of a first divider of the dividers, the first divider being nearer the input end; wherein the plurality of dividers include a first pair of adjacent dividers and a second pair of adjacent dividers, the first pair of adjacent dividers nearer the input end than the second pair of adjacent dividers, each of the divider slopes includes a ratio of radial position difference over axial position difference between the inner positions of the adjacent dividers; characterised in that: a first divider slope relative to the rotation axis between inner positions of the first pair of adjacent dividers is greater than a second divider slope relative to the rotation axis between inner positions of the second pair of adjacent dividers; the first divider slope is positive, and the second divider slope is positive; wherein the plurality of dividers includes a third pair of adjacent dividers, the third pair of adjacent dividers being nearer the output end than the second pair of adjacent dividers, wherein a third divider slope relative to the rotation axis between inner positions of the third pair of adjacent dividers is zero or positive and is less than the second divider slope. a separator (12, 302, 452) having an inner surface (100, 306, 456) surrounding a rotation axis (14, 304, 454) of the separator and defining a particle path from an input end (102, 308, 458) of the separator to an axially spaced output end (104, 310, 460) of the separator, wherein the inner surface includes a plurality of axially spaced dividers (120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 320, 322, 324, 326, 328, 330, 470, 472, 474, 476, 478, 480) having respective inner positions, the dividers defining at least in part respective axially spaced retainers (150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 340, 342, 344, 346, 348, 490, 492, 494, 496, 498) for collecting particles during rotation of the separator, each of the retainers including at least one fluid inlet for fluidizing particles in the retainer during operation; each of the inner positions includes a radial position and an axial position, wherein for each of the dividers, the radial position is a radial distance from the innermost surface of the divider to the rotation axis and the axial position is a distance projected onto the rotation axis from the innermost surface of the divider to the innermost surface of a first divider of the dividers, the first divider being nearer the input end; wherein the plurality of dividers include a first pair of adjacent dividers and a second pair of adjacent dividers, the first pair of adjacent dividers nearer the input end than the second pair of adjacent dividers, each of the divider slopes includes a ratio of radial position difference over axial position difference between the inner positions of the adjacent dividers; characterised in that: a first divider slope relative to the rotation axis between inner positions of the first pair of adjacent dividers is greater than a second divider slope relative to the rotation axis between inner positions of the second pair of adjacent dividers; the first divider slope is positive, and the second divider slope is positive; wherein the plurality of dividers includes a third pair of adjacent dividers, the third pair of adjacent dividers being nearer the output end than the second pair of adjacent dividers, wherein a third divider slope relative to the rotation axis between inner positions of the third pair of adjacent dividers is zero or positive and is less than the second divider slope. a first divider slope relative to the rotation axis between inner positions of the first pair of adjacent dividers is greater than a second divider slope relative to the rotation axis between inner positions of the second pair of adjacent dividers; the first divider slope is positive, and the second divider slope is positive; wherein the plurality of dividers includes a third pair of adjacent dividers, the third pair of adjacent dividers being nearer the output end than the second pair of adjacent dividers, wherein a third divider slope relative to the rotation axis between inner positions of the third pair of adjacent dividers is zero or positive and is less than the second divider slope.