[en] The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a testfluid, which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong divertor recycling), causing in the divertor case impurity accumulation especially near the divertor throat. In addition self-sputtering at the target plates is calculated, showing the importance of frictional impurity acceleration in addition to the charge state dependent electrostatic energy gain. (orig.)