Abstract

Mineralogical studies of ore samples from Mobil uranium properties in Grants mineral belt (NM) revealed a correlation between certain mineralogical characteristics of the ore and its leaching behavior in the laboratory. Mild alkaline carbonate solutions containing dissolved oxygen gas or hydrogen peroxide were used to leach ore samples. Leaching effects of the uranium minerals and reactive gangue material were investigated by comparing the mineralogical characteristics between pre- and postleach samples. Analyses were made by using the X-ray diffraction method, petrographic microscopic, and scanning electron microscopic (SEM) petrographic microscopic, and scanning electron microscopic (SEM) examinations. In one ore trend where uraninite was the principal uranium mineral, studies showed that (1) the majority of uranium minerals was readily accessible to the leachate on a microscopic scale, (2) the predominant, 1-micron size uraninite crystals were dissolved rapidly during predominant, 1-micron size uraninite crystals were dissolved rapidly during leaching, (3) the minor but locally abundant uranium vanadate species was insoluble in mild alkaline carbonate leachate, and (4) pyrite crystals, with dimensions 10 times greater than the uraninite crystallites, showed some surface etchings. These results indicate that the solubility and microscopic occurrence of uraninite are favorable for high and rapid uranium recovery, while the fraction of uranium contained in the insoluble uranium vanadate phase cannot be recovered under mild leaching conditions. The uraninite apparently dissolved at a much faster rate than pyrite, indicating a favorable initial selectivity of leachate for uranium. Mineralogical studies of the pre- and postleach ore samples correlate well with the laboratory leach tests, which showed 80 to 90% uranium recovery as well as low vanadium and sulfate levels in the effluents.

Introduction

In-situ uranium mining is built on the concept that an oxidized solution injected into the ore zone can oxidize uranium from its insoluble tetravalent state to its soluble hexavalent state. The solubility of the oxidized uranium is enhanced further by tonning complexes with the simultaneously injected carbonate or sulfate species in solution. The solubilized uranium, moving along with the fluid, is recovered from the production wells and transported to the surface processing circuits. The production wells and transported to the surface processing circuits. The success of an in-situ leaching operation depends to a large degree on whether the injected fluid can contact the uranium minerals efficiently, whether the uranium can be oxidized and dissolved selectively, and finally whether the dissolved uranium can be transported to the production wells efficiently, The fulfillment of all these conditions is governed by the inherent physical and chemical characteristics of the ore body, factors such as the permeability profile of the ore zone, the fraction of uranium minerals occurring on the fluid path, the types of uranium minerals, and the types and abundance of other coexisting reactive minerals. Understanding these factors, which are obtainable through mineralogical study of ore samples, can provide valuable guidance not only in predicting the performance of an in-situ leaching prospect but in helping the selection of leaching agents and processes for optimal uranium recovery. A simple example of applying the knowledge of mineralogy to the selection of leaching process is the choice of carbonate leachate over acid leachate, when the ore is known to be high in calcite, so that excessive acid consumption is avoided. Furthermore, data on mineralogy, particularly the texture and composition of ore and gangue minerals, make it possible (1) to relate the properties of the ore to its leaching performance during laboratory and field leaching tests, (2) to evaluate and anticipate potential problems during leaching operations, and (3) to establish an potential problems during leaching operations, and (3) to establish an effective in-situ ore grade as distinct from total contained uranium value. The previously mentioned approach of combining mineralogy and process engineering for industrial process evaluation has gained popularity in recent years. This relatively new science of process mineralogy has greatly advanced with the aid of high-resolution, high-magnification instruments such as SEM's and electron microprobes. These instruments are essential for studying uranium ores, which are often extremely fine-grained and finely disseminated in the host rocks. In this paper we describe the application of mineralogical studies to evaluate the laboratory leaching performance of a uranium property in Grants mineral belt. This mineralogical study had been included as part of the prepilot plant development program. The samples we studied included both unleached core samples and the ore samples originally from the same core depths that have been leached in the laboratory. By comparing the characteristic changes from pre- to postleach samples, we were able to establish a correlation between certain mineralogical characteristics of the ore and its leaching behavior in the laboratory.

SPEJ

p. 563

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