  A dilute suspension of impurities in a low density gas is described by the
Boltzmann and Boltzman-Lorentz kinetic theory. Scaling forms for the species
distribution functions allow an exact determination of the hydrodynamic fields,
without restriction to small thermal gradients or Navier-Stokes hydrodynamics.
The thermal diffusion factor characterizing sedimentation is identified in
terms of collision integrals as functions of the mechanical properties of the
particles and the temperature gradient. An evaluation of the collision
integrals using Sonine polynomial approximations is discussed. Conditions for
segregation both along and opposite the temperature gradient are found, in
contrast to the Navier-Stokes description for which no segregation occurs.
