Pressure carbonate leaching

Figure 9.2 presents a number of less commonly used stages, but all of them have commercial application in special cases. Pressure carbonate leaching of ores, which contain limestone can be cheaper than acid leaching and ion-exchange. This is followed by TBP extraction of the concentrate for final purification since any alternative would normally be based upon the same general chemical engineering principles and would only involve a different solvent. The older type of dryway process is then shown, with ammonia precipitation as the first step, since this still has applications for the production of special types of uranium dioxide. The final calcium reduction of oxide finds application on a relatively small scale, where the uranium metal product is required in powder form. 

Temperatures near 100°C are often employed in the sodium carbonate leaching of uranium, with pressures up to about 100 psi. Leaching times vary over a wide range but the object is to reduce them to only a few hours. 

Ammonium carbonate leaching has been developed specifically for uranium extraction from the ore at Grand Junction, Colorado. The technique and plant resembles that used for sodium carbonate pressure leach. 

An ammonium carbonate leaching pilot plant has been described which consists of five towers, each 10 ft high by 4 in. internal diameter, connected in series. The slurry of acid and ore is pumped in a co-current manner from the bottom to the top of each tower and then allowed to pass via 0-5 in. piping from the top of one tower to the base of the next. Air is injected at the base and mid-point of each tower. Typical pressures in the first and last towers are 130 psi and 90 psi. The system is fed from an agitated mixing tank, via a positive displacement piston pump located at the base of the first tower. After leaching, the gases are separated off by means of a pressure release valve and the leach liquor then removed from the spent ore in a hydrocyclone. 

Australian Vanadium—Uranium Ore. A calcareous camotite ore at YeeHrrie, AustraHa, is iU-suited for salt roasting and acid leaching. Dissolution of vanadium and uranium by leaching in sodium carbonate solution at elevated temperature and pressure has been tested on a pilot-plant scale... 

If tungsten is recovered from the wolframite group mineral, the wolframite concentrate is boiled or pressure-digested with 50% caustic soda solution. Alternatively, they may be fused or sintered with caustic soda, caustic potash or sodium carbonate and the fused mass then leached with water. The solution is filtered to separate sodium tungstate solution. The fdtrate is subjected to various treatments to remove molybdenum, phosphorus, and arsenic impurities. The filtrate at this point is essentially a solution of sodium tungstate and is treated in the same way as that obtained from the scheehte concentrate discussed above. 

For this reason, additional studies on carbon tetrachloride flux rates into and out of surface water, as well as refined quantitative estimates of aquatic fate processes would be valuable. The chemical is expected to evaporate rapidly from soil due to its high vapor pressure and may migrate into groundwater due to its low soil adsorption coefficient. No data are available on biodegradation in soil. Additional studies to determine degradation rates and the extent to which adsorption has occurred would be useful. These data are also useful in evaluating the impact of carbon tetrachloride leaching from hazardous waste sites. 

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

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