The potash mine at Boulby, Cleveland, UK is one of the largest in Europe and currently supplies some 55% of the UK’s potash consumption. The mine produces 2 Mt of process waste each year. 1.8 Mt is a soluble salt residue, with 200 000 t of insoluble clay residue (filter cake) containing trace heavy metals (cadmium and mercury), approximately 50 kg/year. This paper describes research into the use of waste as backfill to provide an alternative, costeffective disposal option with lower environmental impacts than traditional methods of disposal such as discharge into rivers and coastal waters or by storage in large surface disposal facilities.