tailings

The tailings facility is where waste from mining is stored.

"The tailings release and breach was promptly brought under control within hours of detection," the firm said in a statement on 3 September.

The spill happened when a tailings dam, used to store toxic waste and heavy metals, collapsed into the Kafue River, a key drinking water source, following heavy rain.

At much higher concentrations, which may be relevant for testing environmental contamination such as mine tailings, the accuracy is within 4 percent.

Rare earth metals are extracted by digging enormous pits and dissolving rocks in caustic acids, sometimes leaving behind toxic and radioactive tailings.