A good selectivity, the instant infusion with metals leached mineral chemical reaction rate, and can not afford a chemical reaction with other metal, or chemical reaction rate is slow. Second, the price is low, the source is widely available, and the unit consumption is small. Third, the toxicity is small, or it is easy to take practical precautions. Fourth, can be recycled without changing its physical and chemical properties. 5. It is beneficial to recover metal from the leachate. In practice, comprehensive comparisons should be made. It is not possible to emphasize only one aspect but ignore the overall benefits. Taking gold ore heap leaching as an example, the reaction rate of thiourea, bromine and iodine in chloride and acidic medium with gold is higher than that of cyanide and gold, and cyanide is highly toxic and has a high unit price. The heap leaching basically uses cyanide. The reason is that the sources of compounds such as bromine and iodine are not widely available, and the subsequent processes do not solve the problems of recovery and recycling. The thiourea using a sulfuric acid medium is not only used in a large amount, but also has poor selectivity, and not only leaches gold, but also leaches a large amount of impurities, which brings a lot of troubles to subsequent processes. Although cyanide is highly toxic and has a high unit price, its consumption is low, it does not react with most surrounding rock minerals, the subsequent treatment technology and equipment are mature, and the recovery and recycling methods are simple. Commonly used leaching agents for uranium ore heap leaching are sulfuric acid, ammonium carbonate (or sodium carbonate). Usually, depending on the amount of carbonate in the ore, the ore with a carbonate content of more than 1% is leached with ammonium carbonate (or sodium carbonate), and the ore below this value is leached with sulfuric acid. For heap leaching and agitation leaching of alkaline uranium ore, sodium carbonate was used in the early stage. This is because sodium carbonate has a wide range of sources and is easy to use. Ammonium carbonate is temporarily synthesized on site by using CO 2 and ammonia in western countries, which is inconvenient to use. In terms of environmental protection, the NH 4 + in wastewater is quite demanding, while Na + is required. There is no clear requirement. The in-depth study of the heap leaching and in-situ leaching processes of the nuclear industry showed that ammonium carbonate is superior to sodium carbonate as an alkaline leaching agent for heap leaching and in-situ uranium ore. The reason is: 1 improved the permeability of the ore heap, which is more prominent when the ore contains clay . Because Na + causes the clay to swell and the porosity decreases, Na + deteriorates the permeability of the ore during the leaching process; on the contrary, NH 4 + shrinks the clay and increases the porosity, and NH 4 + makes the permeability obtain during the leaching process. improve. 2 Simplifies the whole leaching and subsequent treatment process, the same reagent is used for leaching and leaching, and the rich liquid is evaporated to make the uranium into a crystalline UO 3 · 2H 2 O, which is easy to dehydrate and ammonia can be reused. The whole leaching and recycling process is greatly simplified, so that the whole process of heap leaching can realize closed circuit circulation. The process is shown in Figure 1. 3 The sanitary conditions are good, and the dehydration performance of UO 3 · 2H 2 O crystal is better than that of diuronate. 4 Under a slightly improved condition, the final small amount of wastewater will be profitable. NH 4 + is a nutrient-rich substance that is directly discharged into water bodies, which reduces oxygen in water and is not conducive to the growth of plankton. Therefore, NH 4 + has strict requirements for NH 4 + in environmental protection. However, after removing the uranium radium from the tail liquid, if the wastewater containing NH 4 + is sprayed, the water is applied to the dry land, which can improve the fertility of the land, promote the growth of the vegetation, and green the environment. Figure 1 Flow chart of uranium heap heap leaching with ammonium carbonate as solvent
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