Abstract

This paper reports a laboratory study of the oxidative destruction by sodium hypochlorite (NaOCl) of ammonium ions adsorbed on relatively reduced south Texas uranium ore. Included are an assessment of reaction stoichiometry, determination of some major reaction pathways and side reactions, and identification of several pathways and side reactions, and identification of several intermediates. Adsorbed ammonium ions were completely removed by 0.5 % NaOCl, with the concentration of NH3 in the effluent falling to a very low value after 10 to 15 PV NaOCl oxidant. A small fraction (5 to 10%) of NaOCl was utilized in reacting with NH3. After the NH3 was nearly depleted, mono-, di-, and trichloramines, the expected intermediates in NaOCl oxidation of NH3, were observed. Chloramine decomposition studies showed that all three decomposed completely within 12 days. Since the ore was relatively highly reducing, the major pan of the NaOCl was, not unexpectedly, consumed in side reactions. Substantial quantities of sulfate, reflecting oxidation of sulfide minerals such as pyrite, were formed, large amounts of uranium were leached out, and substantial amounts of calcium and magnesium ions were also produced during the presaturation with NH4HCO3 preceding the oxidation stage.

Introduction

A leachate that has sometimes been used for in-situ leaching of uranium ores is a solution of ammonium bicarbonate (NH4HCO3) containing an oxidant-usually hydrogen peroxide (H2O2) or oxygen (O2). The ammonium ion (NH4+) introduced into the ore body upon injection of this leachate is exchanged for cations such as calcium (Ca+2 ) and sodium (Na+), which are associated with mineral species in the formation possessing available cation exchange sites. As the indigenous groundwaters reinvade the leached zone, the adsorption process is reversed with NH4+ ions being displaced from process is reversed with NH4+ ions being displaced from the cation exchange sites and returned to the ground-waters. In general, this latter process maintains the ammonia (NH3 (or NH4+ ion) concentration well above the baseline (pre-mining) value in groundwater for extended periods of time in waters produced from wells in or near the mined zone following cessation of leachate injection. Prior to abandonment of an in-situ leach-mining site by the operator, satisfactory restoration of groundwater quality must be demonstrated. Requirements for this demonstration vary with the geographical area. A summary of applicable regulations has been provided by Kasper et al. A review of the state of restoration demonstrations to Sept. 1979 has been given by Tiepel.

Most of the in-situ leach operations in south Texas have been conducted in aquifers containing indigenous waters with TDS contents in the 700- to 3,000-ppm range. Ca+ 2, magnesium (Mg+2), and bicarbonate ion (HCO3 ) concentrations are high in these slightly alkaline waters. These equilibrium water compositions indicate that an appreciable fraction of the interlayer ion exchange sites of the smectite clays in the formation are occupied by Ca+2 or Mg+2 ions.

SPEJ

P. 387

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