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The leaching of uranium from pitchblende ores by aqueous oxidation techniques Peters, Ernest

Abstract

Uranium exists in carbonate solution in its hexavalent state as the complex ion U0₂(C0₃)₃---- . In most ores uranium occurs as pitchblende (U₃0₈). It has been found possible to dissolve uranium in carbonate-bicarbonate solutions from these ores by leaching them in the presence of oxygen. The overall reaction is as follows: U₃O₈ + ½0₂+ 3C0₃ˉ˭ + 6HCO₃- → 3U0₂(C0₃)₃---- + 3H₂0 The objective of the present research was to examine the kinetics of this reaction with a view to determining the mechanism of the reaction and to establish the variables upon which the rate depends. Two series of experiments were conducted on two types of materials: (a) Pitchblende specimens of measured surface area were suspended in a solution containing sodium carbonate and sodium bicarbonate. Above the solution a desired pressure of oxygen was maintained. (b) A pulp of low grade pitchblende ore in carbonate, solutions was agitated in an autoclave, in the presence of oxygen maintained at the desired pressures. The rate of solution of uranium was measured by sampling and analyzing the solutions at regular time intervals. The effect of oxygen pressure, temperature, and reagent concentration on the rate was studied in each series. The kinetic results were examined by the methods of the absolute reaction rate theory. The following conclusions were drawn from the leaching tests: (1) The absolute reaction rates are about the same for all the materials studied. (2) The reaction rate varies as the square root of the absolute oxygen pressure. (3) The activation energy is about 12,000 - 3000 calories per gram mole. (4) Minimum amounts of carbonate and bicarbonate are necessary to dissolve the oxidized uranium. Further increases beyond this minimum amount result in only minor increases in the rate of the reaction. A mechanism for the leaching rate is proposed in which the oxidation is the rate controlling step.

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