Phase Transformation Characteristics And Mechanisms Of Main Gangue Minerals In The Process Of CO₂ + O₂ In-situ Leaching Of Uranium In Mengqiguer Deposit

Sandstone type uranium deposits have become the main type of Natural uranium production.Mengqigur deposit is the main deposit of Xinjiang thousand ton uranium ore base,which is mined by CO₂+O₂ neutral in-situ leaching.Due to the dissolution,oxidation and precipitation of gangue minerals such as carbonate and Pyrite with high content in the mining process,which have complex effects on the leaching system,it is of great significance to study the changes and mechanisms of these key gangue minerals under the leaching action to guide the optimization and regulation of in-situ leaching process The core samples from undeveloped and developed mining areas of the Mengqigur deposit are used to study the changes in micro morphology,mineral association and occurrence,geochemical composition of the ores and key gangue minerals such as calcite,pyrite,potash feldspar,kaolin under the CO₂+O₂ leaching by using modern analysis techniques and laboratory leaching tests.The main conclusions are as follows:Comparative study on calcite phase of ores from undeveloped and developed mining areas shows that calcite is the gangue mineral with the most significant leaching effect in the CO₂+O₂ in-situ leaching system.The dissolution of calcite in ore samples affected by leaching is obvious,and the growth ring of Calcite is damaged.In the CO₂+O₂ in-situ leaching system,the sudden change of circumstance from the strata to the pumping well would lead to remarkable growth of calcite in the form of multinuclear crystalline layers.This rapid growth process is mixed with other mineral grains carried by the fluid.Compared with calcite from the untouched ore,the content of Ca O in calcite in the developed mining area decreases,while as Fe O,Mn O₂,and Mg O are enriched.Pb and Th slightly enriched,and U depleted.Trace elements are high unevenly distributed in calcite.These characteristics indicate that part of calcite existing in the developed mining area is formed under the hydrodynamic and hydrochemical conditions strongly disturbed by in-situ leaching.The value of pH is a key factor in the dissolution precipitation process of calcite.Calcite shows dissolution in the pH range of 6.2～6.8,and the lower the pH,the greater the calcite dissolution and the faster the dissolution rate.When the pH value reaches 6.80～7.0,it quickly reaches supersaturation after slight dissolution under low salinity conditions.In CO₂+O₂ in-situ leaching system,the pH increases along the streamline due to the consumption of H⁺,which is the most important reason for the dissolution of calcite in upstream,causing the accumulation of liquid Ca²⁺,and leading to the precipitation of calcite downstream.Pyrite is also a gangue mineral that is obviously leached by CO₂+O₂.Although the change of microscopic morphology of Pyrite is not significant,the continuous increase of SO₄^2-in the in-situ leaching fluid,the accumulation of Fe in the ores of the old mining area at local locations,and the Fe rich characteristics of Calcite of in-situ leaching precipitation origin all reflect that Pyrite is oxidized and dissolved by CO₂+O₂ leaching,and then oxidized to form iron hydroxide oxidation and rapid precipitation that is difficult to dissolve in weak acid neutral environment.The leaching test results show that the oxidation of Pyrite increases with the increase of pH value of the system.The effect of CO₂+O₂ fluid on potassium feldspar and kaolin is relatively not significant.In the CO₂+O₂ system,potassium feldspar undergoes a certain degree of clay formation,which can adsorb sodium ions in the system;Kaolinite is one of the sources of Ca²⁺in liquid phase of in-situ leaching system.Both showed a certain reduction effect on the liquid phase HCO₃^-and SO₄^2-in the in-situ leaching system.