Treatment method of uranium ore acid method heap leaching wastewater

The composition of wastewater from uranium ore acid heap leaching is shown in Table 1. It can be seen from Table 1 that if lime is neutralized to pH 7-8, most of the contaminated components in the wastewater (except radium) can be removed by precipitation. The most widely used treatment for uranium ore acid leach wastewater has been the neutralization process, especially the lime neutralization process.

Table 1 Composition of wastewater from uranium ore acid heap leaching (mg∕L)

Uranium, iron, aluminum and heavy metals precipitate component due to hydrolysis, i.e.

Me ^{n +} +nOH ^- Me(OH) _n ↓

The sulfate, phosphate and phosphate, silicon and the like to form a precipitate with calcium

SO ₄ ^{2 -} +Ca ^{2 +} CaSO ₄ ↓

The acidity of the wastewater is neutralized by lime

H ⁺ +OH ^- H ₂ O

Due to the high content of iron and aluminum, their precipitation tends to be gelatinous, so the adsorption is also strong, and some components that are not precipitated can be removed. However, due to the precipitation of a large amount of iron-aluminum hydroxide, the particles are fine. The sediment has a large water content, the sediment has plasticity, the fluidity is also strong, and the solid-liquid separation is difficult, which brings great trouble to the disposal of the sediment, and thus has to adopt a method of slag circulation. Radium can also remove a part of the neutralization process, but it has been proved that lime neutralization can not completely remove the radium in the wastewater, and additional measures are needed.

1. Method for removing radium from wastewater

There are many methods for removing radium from wastewater, such as manganese dioxide adsorption method, potassium permanganate activated wood dust adsorption method, barite adsorption method, barium sulfate coprecipitation method, etc., which can be selected for specific objects and conditions. According to the characteristics of acid flooding wastewater, it is recommended to use barium sulfate coprecipitation.

The principle of the barium sulfate coprecipitation method is to add barium chloride, barium ion and sulfate to form barium sulfate in a wastewater containing a large amount of SO ₄ ^{2 -} . In this process, although the concentrations of Ra ^{2 +} and SO ₄ ^{2 -} still do not reach the solubility product of radium sulfate (4.25 × 10 ^-11 ), radium sulfate and barium sulfate occur isomorphously due to the similar properties of radium and thorium. Coprecipitation allows radium to enter the crystal lattice of barium sulfate precipitation to form a precipitate of Ba(Ra)SO ₄ .

The precipitate particles are very fine and difficult to filter and settle, but the addition of lime accelerates the settling rate. The acid method heap leaching wastewater has high sulfate content and is neutralized with lime, which can meet the premise and necessary conditions of the barium sulfate coprecipitation method. It is true that in order to achieve the ideal radium removal effect, there are still many conditions to be carefully controlled. The main factors affecting the effect of radium removal are the content of SO ₄ ^{2 -} and Ra ^{2 +} in wastewater, the amount of Ba ^{2 +} added, the stirring time, etc. It is especially important to maintain the clarity of the wastewater, if the amount of suspended solids in the wastewater High, the radium removal effect is very poor. Table 2 is the test control results.

Table 2 Effect of suspended solids on radium removal

* In the wastewater, Ra 14.0Bq∕L; SO ₄ ^{2 -} 1015mg/L; pH 7.5-8.0.

** Solid content is 100g∕L.

It can be seen from Table 2 that the addition of a small amount of cesium chloride to the clarified wastewater can remove more than 99% of radium, but when the content of suspended solids in the wastewater is high, the efficiency of radium removal is significantly reduced. The test shows that if the suspended matter content in the wastewater reaches 100g ∕L, when the amount of lanthanum chloride added is equivalent to 100 times the amount added in the clarified wastewater, the radium removal efficiency reaches 95%. It should be emphasized that in the treatment of uranium ore acid method heap leaching wastewater, it is necessary to add cerium chloride to remove radium before the neutralization of lime. If lime is first neutralized, then strontium chloride is added to remove radium, the effect is very poor. .

The relationship between the effect of radium removal and the amount of lanthanum chloride added, and the content of SO ₄ ^{2 -} and Ra in the wastewater are shown in Table 3. It can be seen from Table 3 that when the amount of sulfate added is fixed, the removal rate of radium in the wastewater increases with the increase of the amount of cerium chloride added; when the amount of cerium chloride is fixed, with the increase of the concentration of sulfate in the wastewater, the radium in the wastewater The removal rate also increases; however, in addition to the concentration of strontium ions and sulfates, the ratio of the two should be considered. The amount of strontium ions added in the No. 12 test is the most, and the effect of removing radium is worse, the main reason is sulfuric acid. The ratio of root to sputum is not appropriate.

Table 3 Factors affecting radium removal

* The pH of the wastewater is 2.8.

Second, the slag recycling method

Add cesium chloride to the uranium ore acid heap leaching wastewater, stir for 5 to 10 minutes, then add lime, stir for 10 to 20 minutes, adjust pH=7, toxic substances in wastewater such as uranium, radium, heavy metal elements, arsenic The etc. are all removed in the form of a precipitate. However, due to a large amount of hydrolysis and precipitation of iron, aluminum, etc., the sediment content at this time is extremely high, and it is difficult to settle. If it is left for several days, the sediment volume accounts for 25% to 40% of the volume of the treated wastewater. Even with filter press, the moisture content of the filter cake is still above 70%. The sediment particles are fine, and most of the particles are less than 40 μm, so the fluidity is large, and it is easily washed away by rainwater and pollutes the environment. The performance of the dam, which is precipitated by a large amount of hydroxide composed of iron, aluminum and other metal elements, has a great difference in water permeability and agitation of tailings. The tailings fraction of the stirred leaching is 20% to 30% larger than 74 μm, and only a small part of the particle size is less than 40 μm. Therefore, how to convert this sediment containing radioactive elements, heavy metals, arsenic and other pollutants into a solid substance which is easy to dehydrate, can be stored, and has a small solubility in the natural rainwater, is an acid method. An important issue to be considered in the treatment of heap leaching wastewater.

Yang Chaowen and others have studied the sediment treatment in the uranium acid mine pit water, and proposed the process of “rhenium chloride deionization-sludge cycle-step neutralization”. Zhong Pingyi and others introduced the process into the processing process of the uranium ore acid heap heap leaching wastewater of Dachayuan, which was successful.

The principle flow of the law is shown in Figure 1.

Figure 1 Uranium ore acid method heap leaching wastewater treatment process

The principle of the method can be inferred that the slag acts as a nucleation during the neutralization process, which adsorbs and traps the oxygen oxides formed during the reaction and reduces the transient multinuclear reaction and hydroxide at the time of neutralization. The formation probability of the floc, thus causing the precipitation to change in structure, gradually forming a relatively coarse solid. In another case, some of the hydroxide is gradually converted into carbonate during the aging and alkalization. During the alkalization, there is no doubt that a part of the CO ₃ ^{2 -} exists in the alkaline solution, and during the aging, hydrogen Oxides are converted to carbonates by absorbing carbon dioxide from the air. Although the mechanism has yet to be further studied, in practice, the method can greatly reduce the volume of lime neutralization sediment and greatly improve the permeability of the sediment dam.

Yang Chaowen et al. also studied the re-dissolving properties of toxic components in slag in natural water and weakly acidic water. The results obtained prove that the toxic components in the slag are most soluble at the pH of the water under examination. Cadmium does not pollute the environment. The slag was leached with water having a pH of 7.18 and 3.61 over 150 days, and the cadmium concentrations in the leaching solution were analyzed to be 0.01 and 0.04 mg ∕L, respectively.

The operation process of the method is as follows: uranium ore acid method heap leaching wastewater is added to the corresponding cesium chloride solution, stirred for 5 to 10 minutes, then neutralized with lime milk to pH 7-8, discharged into a thickener to clarify and concentrate, clarified, and discharged The supernatant. At this time, the sediment volume is about 23% to 38% of the volume of the treated water (according to the iron-aluminum content in the water), and the wastewater, the mash and the lime milk are continuously added to the sediment to be stirred and neutralized, and after clarification, the supernatant is discharged. At this time, the sediment volume is about 18% to 30% of the amount of treated wastewater, and the cycle is continued until the total volume of the sediment is constant. This stage is called the scum preparation stage. Thereafter, normal wastewater treatment can be carried out in accordance with the procedure shown in FIG. Several key parameters, such as the amount of ruthenium chloride added, the amount of circulation when the slag is alkalized, and the ratio of the amount of lime milk added to the alkalization tank and the neutralization tank should be determined according to the specific wastewater properties. However, the end point of neutralization should be controlled to a pH of 7-8.

The results of using the strontium chloride deionization radium-sludge cycle-step neutralization method to treat the acid wastewater of the Yanzhou uranium mine are shown in Table 4.

Table 4 Expanded test results of acidic uranium-containing wastewater treatment (mg∕L)

* Ra concentration unit is Bq/L.

Practice has proved that the use of the slag cycle compared with a neutralization, the consumption of neutralizer (lime) is 60% to 80%, the sediment volume is reduced by 90%, the sediment sedimentation and dewatering performance is greatly improved, and the filtration strength is increased by 15 times. .

The US company Bethlehem first used this method to treat heavy metal acid wastewater from steel mills.

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