In the uranium ore alkali heap leaching, carbonate is used as the leaching agent. Therefore, the components of the wastewater are different from uranium ore leaching wastewater except uranium and radium. In addition, sodium carbonate or ammonium carbonate has no solubility in sodium cyanide. It is highly toxic, so its wastewater treatment is relatively simple. Under normal circumstances, it is only necessary to remove radioactive components such as uranium and radium, and adjust the pH of the wastewater to 6-9.

This wastewater SO ₄ ^{2 -} concentration is low, combined with relatively high pH wastewater, typically between 9 and 10, it can not be used in lime and barium oxide and radium addition, instead of using a soft and radium adsorbed manganese ore Flocculation and sedimentation removes uranium.

1. Pyrolusite adsorption Ra

Pyrolusite is a natural material, mostly crystalline or cryptocrystalline, also in powder form. The true density is 4.7-4.9g/cm ³ , and the source is widely available. It is an effective adsorbent for removing radium from alkaline wastewater, but its mechanism of adsorption to radium is different. Most of them are considered to be cation exchange adsorption. That is to say, the action of pyrolusite and wastewater, the surface must be hydrated to form hydronium. Manganese oxide with a hydroxide (OH) group. This group has a dissociation tendency under alkaline conditions, and the dissociated hydrogen ions become exchangeable ions exchanged with Ra ^{2 +} . However, it is also believed that the pyrolusite is porous and has a large specific surface area, and its adsorption to radium is physical adsorption.

The factors affecting the adsorption of pyrolusite are mainly the size of the pyrolusite itself, the pH value of the wastewater, and the contact time.

Under the same conditions, the smaller the particle size of pyrolusite, the more radium it adsorbs; or the same flow rate, the same radium concentration in wastewater, the breakthrough volume of wastewater with the same pH value decreases with the size of pyrolusite. And increase.

In alkaline wastewater, the effect of pH on the adsorption of radium by pyrolusite is shown in Figure 1. It can be seen from Fig. 1 that the higher the pH value of the wastewater, the larger the breakthrough volume. When the pH value of the wastewater rises from 8.8 to 9.25 to 9.9 pairs, the penetration volume increases by 4 times.

Fig.1 Effect of pH on adsorption of Ra of pyrolusite

Pyrolusite particle size: 0.5 ~ 6.7mm, contact time: 25min

Ra concentration in solution: 2.9Bq∕L

The pyrolusite adsorbed with radium becomes a highly radioactive solid material-waste residue, which must be handled with great care. It should be placed in the mine or sent to a stirred immersion factory for oxidant, then diluted by a large amount of pulp. To the tailings pond. In order to transport safety and the health of the operators, the saturated capacity of the pyrolusite adsorption radium should not be too high, generally controlled at 407Bq∕g pyrolusite.

Second, flocculation precipitation in addition to uranium

Alkaline waste water at pH 9 to 11, the uranium ions to form a complex with carbonate, if the pH of adding an acidic electrolyte such as an acid, ferric chloride, aluminum sulfate, polyaluminum chloride and the like, waste water will Immediately lower, when the pH is lowered to 7-8, the uranium is hydrolyzed to uranyl hydroxide. However, due to the low concentration of uranium in the wastewater (0.5 to 3 mg ∕L), the formed uranyl hydroxide is in the form of a gel and is suspended in the wastewater and cannot be precipitated. The final precipitation of uranium is formed by the hydrolysis of iron or aluminum ions to form iron hydroxide or aluminum hydroxide. Taking the electrolyte FeCl ₃ as an example, the following process may exist.

Ferric chloride is added to the alkaline wastewater. At pH 7-8, the formed Fe(OH) ₃ adsorbs water and has the following structure:

{[Fe(OH) ₃ ] _m ·nOH ^- ·(n-x)H ⁺ }xH ⁺

When the wastewater is hydrolyzed by acid or a salt formed by a strong acid weak base to neutralize H ⁺ to a pH of less than 7, the hydrocolloid of Fe ^{3 + has} the following structure:

{[Fe(OH) ₃ ] _m ·nH ⁺ ·(n-x)Cl ^- }xCl ^-

The whole micelle is composed of a rubber core, an adsorption layer and a diffusion layer. The rubber core is a collection of a large number of Fe(OH) ₃ molecules [Fe(OH) ₃ ] _m , and there are two positive and negative ions outside the rubber core. The particles consisting of the core and the adsorbent layer are called colloidal particles. It is obvious that the colloidal particles are charged, and there are a number of ions opposite to the colloidal particles. The charged ions around the rubber particles are far away from the rubber core, so the gravitational force of the rubber core is small, and it can move freely in the aqueous solution. Therefore, this layer of ions is called a diffusion layer. The structure of colloidal particles and diffusion layers is called an electric double layer structure.

The potential between the colloidal adsorption layer and the homogeneous solution is conventionally referred to as the zeta potential. The stability of the colloid in the wastewater is determined by the zeta potential. The higher the potential, the more stable the colloid. If the electrolyte is added, the ion opposite to the surface charge of the rubber core is increased, and the zeta potential is lowered until the zeta potential is zero, and the colloid is electrically neutral, and the electrical property of the colloid is said to be "isoelectric point" at this time.

Now come back to discuss the hydrolysis of uranium ions in wastewater. The hydrolysis of uranyl ions has been studied a lot. It is generally believed that when pH>3 starts to hydrolyze, when pH>4, UO ₂ (OH) ₂ appears, which can be as follows: :

In fact, the hydrolysis product of UO ₂ ^{2 +} is very complicated. People use HO ₂ ^{2 +} hydrolysis process to produce H ⁺ , titrate H ⁺ with alkali, and roughly judge the state change of hydrolyzed product from the curve of potential titration. As the pH increases, UO ₂ ^{2 +} hydrolysis gradually produces UO ₂ (OH) ⁺ , UO ₂ (OH) ₂ , U ₃ O ₈ (OH) ₃ ^- and the like. In the study of UO ₂ ^{2 +} hydrolysis in nitric acid solution, it was proved that when [OH ^- ]/[UO ₂ ^{2 +} ]=1, multinuclear ions [UO ₂ (UO ₃ ) _n ·(OH)] ⁺ were formed in the solution. When [OH ^- ] / [UO ₂ ^{2 +} ] is from 1 to 1.5, a polymer forming a metastable state of {[UO ₂ (UO ₃ ) _n ·(OH)] ⁺ } _m is difficult to settle. When ferric chloride, aluminum sulfate or polyaluminum chloride is added, the uranium flocculates and precipitates because the colloid is at an isoelectric point.

Chen Mingyang et al. studied the treatment of alkaline low-concentration uranium mine wastewater, and considered that the type of electrolyte, the amount of water, the temperature of the water, the stirring time, and the mud content in the wastewater were the main influencing factors for de-uranium. In terms of the type of electrolyte, the order of uranium removal efficiency is

Polyaluminium chloride>ferric chloride>aluminum sulfate>ferrous sulfate

The effect of water temperature is that at low temperatures, the viscosity increases and is not conducive to electrolyte hydrolysis, so when the water temperature is lower than 10 ° C, the settling time is very long. The amount of mud in the water contributes to the rapid sedimentation of the sediment. Stirring time and manner are key factors. It must be stirred quickly for 1 min, up to 3 min, then slowly stirred for 10 min. The table below shows the results of the test for removing uranium from wastewater.

Table Flocculation precipitation method for de-uranium expansion from wastewater

Note: The amount of ferric chloride added is 100g ∕m ³ wastewater.

It can be seen from the above table that the obtained sediment uranium has a higher grade and can recover uranium; the flocculation sedimentation method has high uranium removal efficiency, but the deraching efficiency is poor. Therefore, it is recommended to use soft manganese ore to radium for alkaline uranium heap leaching wastewater. Then, the uranium is removed by flocculation precipitation. The process is shown in Figure 2.

Figure 2 Process for treatment of uranium ore alkali heap leaching wastewater

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