  We present the generalization of the minimal model for surface flows of
granular mixtures, proposed by Boutreux and de Gennes [J. Phys. I France 6,
1295 (1996)]. The minimal model was valid for grains differing only in their
surface properties. The present model also takes into account differences in
the size of the grains. We apply the model to study segregation in
two-dimensional silos of mixtures of grains differing in size and/or surface
properties. When the difference in size is small, the model predicts that a
continuous segregation appears in the static phase during the filling of a
silo. When the difference in size is wide, we take into account the segregation
of the grains in the rolling phase, and the model predicts complete segregation
and stratification in agreement with experimental observations.
