The Key Lake mine is a former uranium mine in Saskatchewan, Canada, and the site of the largest uranium mill in the world. It was initially developed to open-pit mine two nearby uranium ore deposits: the Gaertner deposit and the Deilmann deposit. Mining of this ore ceased in the late 1990s; the Key Lake mill now processes uranium ore from the McArthur River mine and from existing stockpiles on site. High-grade ore from McArthur river is blended with lower grade local rock before being passed through the mill. The mill has an annual production capacity of 18 million pounds of U₃O₈. In addition, ammonium sulfate fertilizer is produced as a byproduct from used reagents.