ABSTRACT

U. S. Steel Corporation’s Minnesota Ore Operations is comprised of two taconite processing facilities in northern Minnesota, Minntac and Keewatin Taconite. The process water system at the Minntac facility has been operated in a closed-loop fashion since it began processing taconite in the late 1960s. Water from tailings discharged from the ore concentrating operations, along with minor streams from other processes, collects in a large tailings basin clear pool. This water is reclaimed and pumped back to the processing facilities to meet the nominal 50 million gallon per day process water demand. As a result, Minntac has experienced a gradual buildup of dissolved solids in its process water system over the 35-year life of the facility. The primary dissolved solids constituents are calcium, magnesium, chloride, and sulfate. As expected, the conductivity of the process water has also increased as the dissolved solids concentrations have gone up. Since Minntac operates as a zero liquid discharge facility, there is no opportunity, other than minor seep flows

through the tailings basin dam, to reduce process water dissolved solids concentrations.