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Thallium Removal from Mine Waste Waters
Primary Issue Addressed: Trace Metal Removal
Secondary Issues Addressed: Acid Drainage/Water Treatment
Project Site: Testing to be performed at MSE Technology Applications, Inc., facilities using wastewater from ASARCO East Helena lead smelter.
Collaborating Entities: Montana Tech, ASARCO East Helena lead smelter
Cost Share: In-kind services provided by ASARCO East Helena lead smelter.
Project Description
Thallium is more toxic to humans than mercury, cadmium, lead, copper, or zinc. Its chemical behavior resembles the heavy metal lead (Pb) and the alkali metals (potassium, rubidium, cesium); and it occurs almost exclusively in natural waters as monovalent thallium (Tl+1). The solubility of thallous (Tl+1) compounds (e.g., thallous hydroxide) is relatively high so that Tl+1 is readily transported through aqueous routes into the environment. The major sources of thallium are the base metal sulfides and precious metal-bearing sulfides. Therefore, it has been shown to be a frequent contaminant in waters emanating from heavy metal deposits (e.g., sulfide-bearing deposits).
The Mine Waste Technology Program (MWTP) commissioned Montana Tech of the University of Montana (Montana Tech) to review the literature to determine if thallium is an important constituent in mine wastewaters and whether there were appropriate, documented technologies to remove it from mine waste and related waters. A comprehensive review of the literature indicated that:
- thallium is a constituent of concern in waters emanating from sulfide-bearing deposits; and
- only two industrial removal technologies existed for recovering
thallium from process solutions:
- oxidative precipitation of thallic (valence for thallium is +3) hydroxide; and
- reductive cementation of thallium using elemental zinc as the precipitant.
The U.S. Environmental Protection Agency’s Best Demonstrated Available Technology (BDAT) is chemical oxidation of thallium followed by chemical precipitation with hydroxide compounds, settling, and filtration.
Figure 11. EH/pH diagram for thallium, iron, and sulfur.
The results of the project demonstrated the validity of the reductive precipitation concept and evaluated the effect of pH, residence time, and sulfide dosage for a batch application in which metallic iron and sodium sulfide were used in the same vessel for effective removal of thallium. Examination of the solid product from reductive precipitation showed that it was indeed thallium sulfide intermixed with the iron particulate reactant. Application of this technology to an ASARCO wastewater with a thallium concentration of 300 µg/L produced effluent thallium concentrations below the detection limit for thallium by graphite furnace atomic absorption (1 µg/L).
Additional bench tests will focus on generating more information needed for developing a continuous-flow pilot-scale system. The technology will also be evaluated for removing other toxic elements [arsenic (As) and selenium (Se)] and heavy metals [copper (Cu), cadmium (Cd), Pb, nickel (Ni), and zinc (Zn)], as these elements are present in the thallium-bearing water used for the demonstration.
Status
A test plan for performing the planned test program is nearly complete. The ASARCO water targeted for testing has been characterized. Upon completion and approval of the test plan, testing will be initiated and completed.
Original plans for the project included a second phase in which continuous-flow pilot-scale testing would be conducted. Due to program budget limitations and changing priorities, this second phase has been eliminated.