We present the generalization of the minimal model fur surface flows of gra
nular mixtures proposed by Boutreux and de Gennes [J. Phys. I France 6, 129
5 (1996)]. The minimal model was valid for grains differing only in their s
urface properties. The present model also takes into account differences in
thr: sire of the grains. We apply the model to study segregation in two-di
mensional silos of mixtures of grains differing in size and/or surface prop
erties. When the difference in size is small, the model predicts that a con
tinuous segregation appears in the static phase during the filling of a sil
o. When the difference in size is wide, we take into account the segregatio
n of the grains in the rolling phase, and the model predicts complete segre
gation and stratification in agreement with experimental observations.