Fischertechnik has made several different types of conveyor elements, which are also used as tank tracks, over the years. All of them clip onto chain links in one fashion or another and are driven by sprockets. The half-width ones you can see in the pictures are not the obvious choice for this project, and they necessitate two sets of chains and sprockets, but I've been collecting my fischertechnik bits second-hand on eBay and those are all I had available at the time. That's why the conveyor appears to be two narrow conveyors running side by side - it is.
It would seem more logical to pull a conveyor belt rather than push it. However, space considerations here made it much easier to fit the drive mechanism at the start of the belt rather than at the end and it doesn't seem to have made any difference. If necessary I could have used a long drive shaft to transmit the drive to the other end.
It is necessary to know how far the belt has travelled in order to push draughts off at the right point: doing it on the basis of elapsed time would have been too tacky. I would have preferred some sort of optical mechanism to measure distance travelled, but I started out with the intention of using fischertechnik as far as possible and the available mechanism uses a toothed wheel to operate a microswitch five times for every rotation of a shaft. It's crude and noisy, but it works provided the switch is de-bounced in software. You can see (and hear) the mechanism working in the video and you can see the delays inserted for de-bouncing in the program listing.
Modularising this section was easy. The sorter is built on two black bases, and the conveyor belt and the boxes occupy one of them. There is no mechanical connection between the two parts, just the two cables connecting the belt drive motor and the microswitch to the controller.