In-Situ Leaching of Crownpoint, NM, Uranium Ore: Part 7--Laboratory Study of Chemical Agents for Molybdenum Restoration
Authors: Strom E. Thomas , Vogt Thomas C. 
Published: December 2007

One possible drawback to the use of in-situ leaching to recover uranium is the potential release of previously insoluble chemical species into the formation water. Before a pilot test of in-situ uranium leaching at Crownpoint, NM, was begun, extensive laboratory studies were undertaken to develop chemical methods for treating one possible contaminant, molybdenum (Mo). New Mexico regulations restrict the amount of Mo permissible in formation waters after leaching to less than 1 ppm. permissible in formation waters after leaching to less than 1 ppm. Two techniques to restore Mo after leaching were studied with core and pack tests. These studies suggest that if Mo restoration problems occur in the field, the use of precipitating agents such as Ca 2 + or reducing agents such as Fe 2 + may be helpful in ameliorating such problems.

Introduction

Typically, uranium has been produced by conventional underground mining and surface milling methods, but insitu leaching has recently emerged as an attractive alternative for uranium recovery. Before an in-situ leach operation to produce uranium in the Westwater Canyon member of the Morrison formation near Crownpoint, NM, was begun, comprehensive laboratory studies of ore mineralogy and leachate formulations were undertaken. The resultant pilot test was the deepest (2,000 ft [610 m]) successful in-situ uranium leach test carried out in the U.S. and the first operation of its type conducted in New Mexico.

While in-situ leaching has significant advantages over conventional uranium recovery methods, one possible drawback is the release of previously insoluble chemical species into the formation water. Of course, conventional mining also has the potential to mobilize hazardous trace metals in the environment, as indicated by the elevated concentrations of Mo in waters downstream from the large Mo deposit at Climax, CO. Before the start of the Crownpoint pilot test, laboratory testing was undertaken to develop chemical methods for treating one possible contaminant, Mo. Ore analyses from the Westwater sands at the Crownpoint pilot site revealed Mo occurrences in the intended leach zone.

In-situ production of uranium entails oxidizing uranium from the insoluble +4 oxidation state to the soluble, readily complexed +6 state, i.e.,

However, this process also transforms insoluble Mo 4+ minerals such as molybdenite or jordesite, both MoS2, into the soluble +6 form, molybdate, MoO4 2-, i.e.,

Current New Mexico environmental regulations restrict the amount of Mo permissible in formation waters after leaching to less than 1 ppm. Restoration of formation water to acceptable levels for each of the dissolved solids is the final phase of leaching operations.

Three recent reports deal with restoration operations in in-situ uranium leaching in a most comprehensive manner. The main difficulty is that the original chemicalreducing environment underground has been changed to an oxidizing one during the leaching process. Undesirable species can continue to be slowly released from underground rock surfaces long after the primary leaching has ceased. Obviously, the contaminants need to be rendered insoluble. To reduce Mo levels in groundwater after leaching operations, there are at least two methods of restoration:the MoO4 2- may be precipitated by addition of a cation orthe oxidizing environment can be changed to a reducing one, converting the Mo back to the less soluble +4 oxidation state, perhaps to Mo3 O8.

The latter technique has been characterized by Henry et al. as recreating the original mineralization process.

Both techniques were studied in the experiments described in this report. Calcium ion (Ca2+ was used to precipitate MoO4 2 -. Ferrous ion (Fe2+ a reducing precipitate MoO4 2 -. Ferrous ion (Fe2+ a reducing agent, was studied to see whether it could render Mo insoluble.

JPT

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