Uranium ore acid heap leaching waste

Acid heap leaching uranium produced by waste water, waste, emissions heap leaching is much more complicated than gold ore cyanide pollution of the environment, much more serious. On the one hand, the uranium mine itself is radioactive, on the other hand, the acid has a greater impact on the environment than the alkali, whether it is on the ore.

First, wastewater

The wastewater generated by uranium ore acid heap leaching is basically acidic, and the total dissolved solids far exceeds the total dissolved solids in the gold mine cyanide heap leaching wastewater. According to the source of wastewater generation, it also includes three circulation systems (or two circulation systems).

First, the wastewater generated by an adsorption cycle is leached.

Second, the effluent-(extraction)-precipitation cycle produces wastewater. There are two processes for rinsing. Most of them use sodium chloride and low-concentration sulphuric acid as eluent, and they are directly precipitated without extraction. However, a small amount of sulfuric acid (10%) is used as the eluent, and the leaching rich liquid is extracted, back-extracted, and the uranium is crystallized from the stripping solution into uranyl tricarbonate or precipitated as a heavy uranium salt.

It can be seen from the above discussion that when 0.8-1.0 mol/L sodium chloride and 1% sulfuric acid are used as the eluent, only two circulating system wastewaters are used, when 10% sulfuric acid is used as the eluent, followed by extraction. And the back extraction process, there are three cycles of wastewater, namely the leaching-adsorption cycle system, the leaching-precipitation cycle system or the leaching-extraction-reverse-crystallization cycle system.

As for the effluent washing of the ore heap with lime water before the unloading in the uranium ore acid heap leaching, it is generally not recycled, but is reserved for the next heap to prepare the leaching solution.

The most important pollutants in wastewater are radioactive uranium and radium. Because of their long half-life, their environmental impact is long-term. In addition, certain ores contain heavy metal elements such as cadmium , which will also seriously pollute the environment. Arsenic is also a common pollutant in uranium ore acid leach wastewater.

The following table shows the results of chemical analysis of the main elements in the wastewater produced by the leaching-adsorption cycle system during the 724 mineral off-site mineral acid heap leaching.

Table 724 Components of the heap leaching adsorption tail (mg∕L)

* Ra concentration unit is Bq∕L.

When leaching with salt, sulfuric acid, and ammonia precipitation, the uranium concentration of the wastewater of the second circulation system is about 5-10 mg ,L, and the chlorine and ammonia ion contents are greatly increased. When extraction leaching processes, waste water also contains organic substances, such as coal, oil, and amine.

The effluent leaching of uranium ore is simpler. In addition to uranium, it is mainly molybdenum and arsenic.

Second, waste residue

The slag of uranium ore heap leaching, especially the slag of acid leaching heap, is more serious to the environment. After the acid leaching of the ore, although most of the uranium is removed, and the leaching of radium is only a small part, the radioactivity in the slag is still strong. Due to leaching, the ore is decomposed and the release of helium is more serious than the original ore.

In addition, a large amount of hydroquinone sludge is produced during the wastewater treatment process. This type of sediment has a high water content, is larger than the surface, has poor sedimentation, is extremely easy to flow, and is difficult to store. This type of sediment is not only radioactive, but also contains heavy metal elements. It can be seen that most of the toxic and harmful substances in the wastewater are transferred to the waste residue, causing the slag of the uranium heap to constitute a major source of pollution to the environment.

Third, the exhaust

Mainly helium, followed by acid mist, as the ammonia in the process and the organic phase in the leaching process volatilize, due to the small amount, will not have a big impact on the environment.

Radon is a radioactive gas that is harmful to human health and animals. According to the conclusion of the "Third Years of Radiation Environmental Quality Assessment of China's Nuclear Industry", the harm caused by the practice of uranium mining and uranium smelting in China is far below the maximum acceptable level, that is, within the acceptable range of harm. However, a survey of the annual effective dose equivalents of individuals in the key population group indicates that some units still exceed the annual dose limit of 1 mSv.

According to the survey, the nuclides emitted by the uranium ore processing enterprises in China account for 92.0% of the gaseous nuclides produced by humans, and the liquid accounts for only 8%. Among the doses of gaseous nuclides to humans, cockroaches accounted for 82.6%, showing that sputum is the main pollutant harmful to the environment.

Radon is a decay product of uranium and thorium in uranium ore, there are four isotopes, i.e., ²³² Th decay into the ²²⁰ Rn ²³⁸ U decay into the ²²² Rn and ²¹⁸ Rn, and ²³⁵ U decay into the ²¹⁹ Rn. Among the four isotopes of strontium, ²²² Rn is the most harmful to humans and is the main subject of consideration. The generation and decay of ²²² Rn is shown in Figure 1.

Figure 1 Uranium (4n+2) decay

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