Main compound chemistry of uranium

Uranium has a high chemical activity and is a very active element. It is almost, but also with metal to form metal and non-metallic effect of all intermetallic compounds. It also reacts with many acids, bases, and salts. Therefore, the content of uranium compound chemistry is very rich, and there are many monographs. The only basic knowledge of the chemistry of some uranium compounds related to the uranium heap leaching process is presented here.

First, the valence of uranium

It is generally believed that uranium has four valence states: trivalent, tetravalent, pentavalent, and hexavalent. Trivalent uranium is purple-red, very unstable and highly oxidizable. Pentavalent uranium has only academic significance because it disproportionates in aqueous solution, so only tetravalent and hexavalent uranium ions and compounds are common. Tetravalent uranium is blue-green and relatively stable, but it is also easily oxidized and can only be stably present in reducing media. Hexavalent uranium is the most stable and yellow-green. Unless there is a reducing agent, hexavalent uranium can be stably present in solution, solid phase or gas phase. However, in natural uranium minerals, uranium exists in tetravalent form, such as asphalt uranium ore. In the leaching and conversion process, most of the processes have to take advantage of the characteristics of hexavalent uranium, so tetravalent and hexavalent uranium have important value for heap leaching production and research.

Second, uranium oxide

There are many uranium oxides, and UO ₂ , U ₂ O ₈ and UO ₃ are closely related to the heap leaching process. UO ₂ It is a dark brown powder, insoluble in hydrochloric acid, dilute sulfuric acid, dilute nitric acid, but soluble in concentrated nitric acid or aqua regia. In nature, UO ₂ is present in the uranium ore in the form of crystals. In the presence of a strong oxidant, UO ₂ can react chemically with dilute sulfuric acid or sodium carbonate (ammonium). Uranium ore heap leaching often utilizes these two chemical reactions:

U ₃ O _{8 has} long been mistaken for uranium salt U(UO ₄ ) ₂ or uranyl uranyl salt (UO ₂ ) ₂ UO ₄ , which is considered to be tetravalent and hexavalent uranium. mixture. The study found that U ₃ O ₈ is a mixture of pentavalent and hexavalent uranium, and its composition should be U ₂ O ₅ ·UO ₃ . When the insulating air heats U ₃ O ₈ in concentrated sulfuric acid, the disproportionation of pentavalent uranium into tetravalent and hexavalent uranium dilute sulfuric acid is weak even when heated under conditions of U ₃ O ₈ , but when oxidant is present , the reaction speed is very fast. Sodium carbonate or ammonium carbonate can only dissolve hexavalent uranium in U ₃ O ₈ .

At 250 to 300 ° C, oxygen can oxidize U ₃ O ₈ to become UO ₃ . However, at low temperatures, the reaction of oxygen to oxidize UO ₃ is very weak.

UO _{3 is} present in certain natural oxidation states and often occurs as hydrates. Its morphology is amorphous and four crystal varieties. UO ₃ is a very active uranium oxide, but its characteristic reaction is still easy to be reduced. Hydrogen, carbon, alkali metals and alkaline earth metals can reduce UO ₃ to UO ₂ . UO3 reacts with water to form a series of hydrates. . Such as H ₂ UO ₄ (UO ₃ ·H ₂ O), H ₄ UO ₅ (UO ₃ ·H ₂ O), Wait.

UO ₃ is amphoteric, reacts with acid to form a yellow-green uranyl salt; reacts with a base to form a water-insoluble urate; it also reacts with carbonate to form a uranyl carbonate ion. These reactions are often used in uranium heap leaching, namely:

Third, uranyl ions and their complexes

Uranyl ions and their complexes are of great significance in uranium ore heap leaching. When sulfuric acid, soda ash, or ammonium carbonate is used as a leaching agent in the leaching stage, the uranium in the ore is transferred into the leaching solution in the form of uranyl complex ions. In the purification concentration stage, it is also adsorbed by the resin in the form of uranyl complex ions or extracted by an organic solvent.

UO ₂ ^{2 +} uranyl ion has a strong affinity with water and shows a distinct acidic reaction. The uranyl ion is easily hydrolyzed in aqueous solution, so its morphology in aqueous solution varies with pH. It is stable only when the pH is less than 2.5. When pH > 2.5, UO ₂ ^{2 +} begins to hydrolyze to form a series of polymers such as UO ₂ (OH) ₂ . The higher the pH, the more the product. Factors affecting the hydrolysis of UO ₂ ^{2 +} , in addition to the pH, there is temperature and its own concentration.

Another characteristic of UO ₂ ^{2 +} is that it forms complexes with many anions. Its order of ability to form complex ions with different kinds of anions of different valence states is:

Monovalent anion F ^- >NO ₃ ^- >Cl ^- >ClO ₄ ^- ;

Divalent anion CO ₃ ^{2 -} > C ₂ O ₄ ^{2 -} >SO ₄ ^{2 -} ;

In general, the ability of UO ₂ ^{2 +} to complex with a divalent anion is greater than its ability to complex with a monovalent anion.

The stability of various complex ions of UO ₂ ^{2 +} varies widely, and is usually characterized by a stability constant or an unstable constant. Since the generation of complex ions is carried out step by step, there is also a stepwise (stepwise) stability constant, and the product-cumulative stability constant of the stepwise stability constant of the complex ions is the total stability constant of the complex ions. The stability constant of a complex ion is the equilibrium constant of the reaction of generating a complex ion, ie

UO ₂ ^{2 +} +2SO ₄ ^{2 -} UO ₂ (SO ₄ ) ₂ ^2- (6)

The instability constant is the reciprocal of the stability constant.

Table 1 shows the stability constants of commonly used uranyl complex ions.

Table 1 Stability constants of commonly used uranyl complex ions

Uranyl carbonate ion There are two important complex ions, namely uranyl dicarbonate and uranyl tricarbonate. Alkali metal or ammonium carbonates react with uranyl hydroxides, diuranes or other hexavalent uranium compounds to form the corresponding uranyl tricarbonate complex

Uranyl ammonium tricarbonate is the most significant compound in the uranyl carbonate complex. It is a yellow crystal, easy to filter, has a stable chemical structure, and is easily soluble in water. Uranyl ammonium tricarbonate is a common compound in the leaching and purification process and is an important intermediate product in uranium hydrometallurgy.

It reacts with acid:

It reacts with a base to form a diuranic acid, and the reaction is a reverse reaction of the formula (8).

It is thermally decomposed:

There are two other characteristics of uranyl tricarbonate that deserve attention: First, its presence in aqueous solution and its conversion form are closely related to pH. When the pH is between 6.5 and 11.5, it can exist stably; when the pH is between 4.5 and 6.5, it exists as uranyl dicarbonate; when the pH is less than 4.5, carbon dioxide is evolved and converted into uranyl hydroxide. The second characteristic is that when ammonium carbonate is present in the aqueous solution, the amount of uranyl tricarbonate dissolved is significantly reduced. Table 2 shows the dissolution data of it in ammonium carbonate solution.

It can be seen from Table 2 that when we use ammonium carbonate as a leaching agent to heap leaching uranium ore, we should not take the conventional idea to think that the higher the concentration of the leaching agent, the better. According to the author's experience of heap leaching tests on sandstone and carbonate rock uranium mines, when the ammonium carbonate concentration reaches 10%, there are three major problems, namely (1) slow leaching rate, and (2) when adsorbed by anion exchange resin. , CO ₃ ^{2 -} has strong adsorption competitiveness, affecting the adsorption of uranium; (3) is prone to calcium carbonate scaling.

Table 2 (NH ₄ ) ₄ [UO ₂ (CO ₃ ) ₃ ] Dissolution in ammonium carbonate solution

These uranium sulfate complex ions can be adsorbed by anion exchange resin and are also well extracted by amine reagents.

Know from Table 1, three kinds of sets of objects uranyl sulfate complex _{UO 2 SO 4, UO 2 (} SO 4) 2 2 - and ^{_{UO 2 (SO 4) 3 4}} - for the stability constant of 50, 350 and 2500, respectively. Therefore, as long as the amount of SO ₄ ^{2 -} in the leachate is measured, the corresponding percentage of the three complexes in the leachate can be determined. Table 3 shows the relationship between various uranyl chloride ion contents and SO ₄ ^{2 -} concentration.

It can be seen from Table 3 that when the concentration of SO ₄ ^{2 -} is very low, such as 0.01 lmol/L, not only the uranyl complex ion exists in the solution, but also mainly exists in the form of UO ₂ ^{2 +} cation, which is apparent for adsorption and extraction. It is unfavorable. However, when the concentration of SO ₄ ^{2 -} is too high, although the formation of uranyl chloride complex ions is favorable, the adsorption competition of SO ₄ ^{2 -} can not be ignored.

Table 3 Relationship between various uranyl complex ion content (%) and SO ₄ ^{2 -} concentration

Fourth, heavy uranium

An important property of diuranic acid salts is that they are insoluble in water and in an alkali solution free of CO ₃ ^{2 -} . It is a yellow amorphous precipitate, commonly known as yellow cake, which is a product of the heap leaching process. Heap Leaching was passed through ion exchange adsorption, and then rinsed by filling and resultant leaching rich ammonia, sodium hydroxide or magnesium oxide will effect obtained yellowcake. The leaching rich liquid is acidic, and the reaction is as follows:

If the leaching rich liquid is uranyl tricarbonate, the reaction is:

The factors affecting the precipitation of diuranic acid are pH, temperature, type of precipitant, precipitation time and stirring strength.

If it is precipitated in an acidic medium, ammonium hydroxide reacts faster than sodium hydroxide, and the produced diuranate particles are coarse, which is convenient for filtration. The reaction temperature is preferably controlled at 50 to 65 °C. The pH of the reaction end point is preferably 6.5 to 7.5. The precipitation time should be kept above 2h. The agitation intensity should not be too large to avoid breaking up the formed ammonium diurate particles.

If precipitation is carried out in an alkaline medium, sodium hydroxide is often used as a precipitant. At this time, the reaction temperature is required to be 70 to 90 ° C, and the pH of the reaction end point is preferably greater than 10.5. It should be noted that when a high concentration of vanadium is present in the solution (for example, 2 to 3 g/L of V ₂ O ₅ ), the reaction is inhibited and the precipitation of uranium is seriously affected, and vanadium must be removed in advance.

5. Uranium peroxide

UO ₄ is a yellow crystal whose one characteristic is that it can be precipitated from a uranyl-containing acidic solution. Utilizing this property, uranium can be separated from the acidic solution containing vanadium, molybdenum and a large amount of potassium, sodium, calcium, iron , sulfate and chloride to achieve the purpose of uranium purification. Specifically, hydrogen peroxide is added to a solution having a pH of about 2, and the following reaction occurs:

The reaction product has a high uranium content (72% to 76%), a low impurity content, a high particle density (1.25 times larger than ammonium uranium), and is easy to filter and wash. Due to the relatively high price of hydrogen peroxide, the industrial application of this method of precipitating uranium has been limited.

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