China’s efforts to curb the smog polluting its cities and clean up its steel industry are remaking the global iron-ore market, driving billion-dollar investments by mining companies and shifting the fortunes of previously unloved pits.
The world’s leading steel-producing country is mothballing more than 100 old, inefficient furnaces that helped build its economic boom, seeking to reduce overcapacity and improve air quality. They are being replaced by larger plants that require less coal for smelting, but are best fed with...