  A Coqblin-Schrieffer impurity of spin $S$ coupled to the boundary of an open
SU(N)-invariant $t-J$ chain with $N = 2S+2$ is studied. The model is integrable
as a function of one coupling parameter $v$ for arbitrary spin and band
filling. The system is coupled to a reservoir of electrons and as a function of
the chemical potential the host chain has a critical point corresponding to the
van Hove singularity of the empty band of charges. In the neighborhood of this
critical point we study the low temperature properties of the impurity as a
function of $v$. For constant chemical potential we obtain two critical
coupling values $v_1=-1/2$ and $v_2=0$ separating phases in which the impurity
spin is completely compensated (Kondo effect), partially screened
(undercompensated impurity) or decoupled from the chain (free spin). The
partial compensation occurs as a consequence of a thermally {\it activated
Kondo effect} in the region $v_1 < v < v_2$. The possible realization of such a
{\it finite temperature Kondo effect} in narrow-gap semiconductors is
discussed.
