A federal judge in Atlanta granted class status Thursday to Southern Co. shareholders suing for more than $1 billion in stock losses related to a failed “clean coal” plant in Mississippi.

Designed to be a “coal gasification” plant, the project was originally slated to cost about $2.4 billion and go online in 2014. Instead, it ballooned into a $7.5 billion money pit, and Mississippi regulators ordered that it be converted to a natural gas facility in 2017.