How to remove stubborn mortar

[Tomw84]

I've been taking some tiles off my kitchen walls and it's been a bit of a mission because they're fixed in place with a thick helping of mortar, which also needs to come off. Most of it is now gone, but in a few places there are bits which have stubbornly clung on and there are no surfaces on them I can get at with my chisel.

Is there anything I can use to scrape/grind these bits off so that the wall is flat?

 

[Old Salt]

Bolster chisel