FR2716683A1 - Uranium extraction process. - Google Patents

Abstract

A process for the extraction of uranium from a uranium containing material by a leaching process which is catalysed by a catalytic agent. In a process where dissolution of a uranium mineral present in the material occurs due to action of ferric ions on the uranium bearing material, ferric ions, and a desired ratio of ferric to ferrous ions, is generated by catalysed oxidation of ferrous ions formed during acid attack on the mineral.

Description

 The present invention relates to the extraction of uranium, in particular from its ore, by means of a preparation method by acid or alkaline leaching.

 Two leaching processes are well known for extracting uranium from its ore. The first process is the alkaline or carbonate leaching process and the second process is the acid extraction process.

The alkaline leaching process has been described by AR Burkin in "Extractive Metallurgy of Uranium" and follows the following route:
UO2 + Ȯ2UO3 (1)
U03 + 3Na2CO3 + H2Oi Na4UO2 (CO3) 3 + 2NaOH (2)
2Na4UO2 (CQ3 + 6NaOH-> Na2U2O7 + 6Na2CO3 + 3H2O (3)
This process is very selective and is therefore preferably adopted in cases where the uranium ore requires a high consumption of acid. The uranium is collected, by this process, by precipitation of sodium diuranate which is filtered and dried, and exported for the rest of the process.

 However, despite the advantages brought by the alkaline leaching process, from the point of view of a reduction in selectivity and corrosion, it has significant disadvantages, as regards costs, compared to the acid leaching process , since it is necessary that the ore is finely ground. In addition, carbonate reagents are expensive.

For this reason, the most widely used preparation method worldwide is the acid route, which is as follows:
Fe + H2SO4O-> Fe2 ++ SO42- + H2 (4)
MnO2 + 2H2SO4 + 2Fe2 + e2Fe3t + Mn2 + 2H2O + 2SO42- (5)
UO2 + 2Fe3 + UO22 + + Fe2 + (6) UO3 + 2H + UQ + H2O + H2O (7)
In this way, in the case of ores based on oxides such as uraninite and pechblende, Uranium oxide is attacked by ferric ions which are generated by the oxidation of ferrous ions generated by the dissolution iron present in uranium ore by acid, usually sulfuric acid, although other mineral acids, such as nitric acid or hydrochloric acid can be used for this purpose. Another source of iron is the grinding materials used to grind the uranium ore to give it a size suitable for leaching. This attack converts U (IV) ions present in the ore into ions
U (VI) in the form of soluble uranyl cations (UO2 +), which are capable of being collected by extraction with a solvent or by any other suitable operation.

 Currently, the oxidation of ferrous ions to ferric ions is carried out by adding, in the leaching circuit, pyrolusite (MnO2), sodium chlorate or other metallic oxidizing components. The introduction of these oxidants presents real difficulties. First, pyrolusite can be expensive or must be finely ground to guarantee the efficiency of the process. As a result, uranium processing plants may have to spend large sums on pyrolusite or must have crushing facilities that mobilize a lot of capital in order to perform the crushing. In some uranium mines, it is necessary to grind up to the 80% level, passing 0.074 mm. This crushing is, of course, very expensive and uranium producers would be very advantageous if it could be avoided. Second, the pyrolusite introduced from manganese or a non-recoverable element in the leaching process. In addition to the fact that it cannot be recovered, manganese undergoes undesirable side reactions which reduce the efficiency of the process and which can affect the efficiency of a solvent extraction process to recover uranium. In the case of sodium chlorate, the expense caused by the existence of undesirable side reactions is reason enough to avoid the use of this product. In particular, chlorate ions tend to degrade into chloride ions which can attack the electrodes used for the electrolytic extraction of copper or other metals downstream of the leaching circuit.

 Despite the unfavorable nature of the oxidants currently used in the industry, they continue to be used because other oxidants do not promote the necessary conversion of tetravalent uranium to hexavalent uranium, or require very expensive equipment to allow their use.

 The present invention therefore aims to provide a method of extracting uranium allowing the use of economic oxidants, unknown until now.

 To this end, the present invention proposes, according to a first aspect, a method of extracting uranium from a material containing uranium, by means of a leaching process comprising the dissolution of said material, this dissolution being catalyzed by a catalytic agent.

 The leaching process may include, for example, acid leaching, using an acid such as sulfuric acid or Caro acid, or alkaline leaching, using an agent alkaline, for example an alkaline carbonate.

 According to a second aspect, the present invention provides a method of extracting uranium from a material containing uranium, by means of an acid leaching process, comprising dissolving the uranium ore present in uranium-containing material by ferric ions, these ferric ions being generated by the oxidation of ferrous ions, at least part of which is formed by reaction of the acid with iron or an iron-containing ore present in uranium-containing material, this oxidation being catalyzed by a catalytic agent in order to allow, thus, obtaining an effective rate of ferric ions compared to ferrous ions, in order to allow a substantial dissolution of the ore d 'uranium.

 Catalytic adsorbent agents such as those based on carbon, for example activated carbon, and ion exchange resins, are particularly suitable catalytic agents which are easy to obtain for use in the process. Other catalytic agents can also be used. It should be observed that, although the contaminating metal ions are difficult and / or expensive to extract from solutions, the extraction of solid catalytic agents, such as adsorbent activated carbon, is as simple as any physical separation; for example by sieving or filtration.

 In a preferred embodiment, gaseous oxidants, which are more economical and less detrimental to the efficiency of the process than chorate and pyrolusite, but which are not currently used in the uranium industry, such as air, oxygen, ozone or a gas containing elemental oxygen, having oxidizing properties, can be used at ambient pressure conditions, in the presence of an adsorbent catalytic agent such as activated carbon. These gases are advantageous, since conventional gas supply equipment can be used to send them to a leaching container.

 Such equipment is significantly more economical than grinding equipment, and the operating costs can be favorably compared to the costs associated with oxidants commonly used. An important point is that there is little or no addition of contaminating metal ions, such as manganese, such as chlorate or manganese chloride, in the leaching process, and the consumption of acid, in the case of acid leaching processes, can be reduced by avoiding undesirable secondary dissolution reactions, and absorption caused by the addition of conventional metal oxidants.

 In addition, when gaseous oxidants are used, conventional gas supply equipment can be used, which requires less maintenance than specialized grinding or chemical storage facilities, which reduces investment.

 Other oxidants which can be used are solid oxidants chosen, for example, from manganese dioxide, permanganates, peroxides, chlorates, chlorides, hypochlorides, chromates, dichromates and persulfates. Salts of an alkali metal, such as sodium or potassium, may be especially preferred.

 It should be noted that the present invention is more effective within a certain range of redox potential (POR). The use of solid oxidants may not allow the required POR range to be reached quickly; thus, gaseous oxidants, which usually allow this range to be reached quickly, should be preferred. Another advantage which can be added lies in better control of the leaching process, since the delay in obtaining the desired POR can be avoided if gaseous oxidants are used. The two types of oxidants can be used together, advantageously, if desired.

 If a solid oxidant is used, this can be introduced directly during the leaching phase, via the leaching agent or the feedstock, or also in the form of an aqueous solution.

 Other sources of ferrous ions can be used than mineral iron found in uranium ore. For example, iron or ferrous components may be added to the leaching solution, but this may not be economical, and it is therefore preferred that a sufficient amount of ions to carry out the process be present in the ore treat.

The invention will be better understood from the following detailed description of a preferred embodiment thereof, with reference to the drawing, in which
Figure 1 is a diagram of an acid leaching process for the extraction of uranium.

 With reference to FIG. 1, a uranium ore comprising, for example, uraninite (whose theoretical formula is UO2) is transported from the well to a primary gyratory crusher, from which the ore to be treated is collected after a sieving to a smaller size, and is stored in a fine ore silo. The refusal of the ore is recycled to the grinding stage.

 Subsequent grinding includes grinding the fine ore in a bar mill, with the addition of water. Separation with a cyclone is then carried out in order to separate an upper layer which is thickened in a thickener, and a lower layer which is sent to the leaching step. The bottom layer of the cyclone is introduced into a bilge mill to obtain an additional reduction in size.

 The leaching is carried out in tanks in which the ore is agitated with sulfuric acid and an oxidant, in the presence of an adsorbent catalytic agent which catalyzes the oxidation of ferrous ions formed by the attack by the acid of the mineral iron present in the ore, ferrous components present in the leaching solution or iron introduced by the grinding materials, into ferric ions which oxidize uranium (IV) oxide into hexavalent uranyl (UOç +) cations which are soluble in acid solution.

 In a conventional leaching installation, the oxidant is manganese dioxide in pure or mineral form (pyrolusite). Otherwise, the oxidant generally used is sodium chlorate. Air or oxygen were not used as oxidants, because their dissolution rates in an acid solution are considered too slow, in acid solution, to allow a significant rate of oxidation of ferrous ions in ferric ions and, therefore, uranium tetravalent to hexavalent uranium.

 This situation can be changed, thanks to the process according to the invention, by the addition of a catalytic adsorbing agent, advantageously activated carbon, although any resin or adsorbent having sufficient reactivity can also be used. The addition of the adsorbent catalytic agent in a suitable amount catalyzes the oxidation of ferrous ions to ferric ions, allowing sufficient availability of the acid solution of oxygen obtained from oxidants, as described above and, in particular, oxidizing gases such as air, oxygen, ozone, gases containing elemental oxygen, or mixtures of these gases, in order to obtain a redox potential of at least + 300 mV during the acid leaching process, and that economically viable uranium extraction rates are obtained. The amount of activated carbon added that is most desirable appears to be 10 to 200 g C / t of ore, preferably 14 to 70 g / t, but other additions may be possible, depending on the type of ore and the conditions. of operation of the installations.

 However, pH and temperature conditions can affect the effectiveness of activated carbon and other agents in the catalysis of the leaching process. Regarding pH, a range of 0.7 to 2 is acceptable. Below a pH of 0.7, carbon can be degraded. Above pH 2, base metals such as copper, lead, zinc and silica dissolve, with possible harmful effects.

 With regard to temperature, the temperature may be maintained, with acceptable extraction, at 30 "C or even at a lower temperature. Heating at higher temperatures, for example at 60" C or higher; in leaching vessels, by adding live steam or by using heating devices, can be used to obtain even better kinetics.

 Table 1 in the appendix makes it possible to clearly distinguish that the addition of 25 g / l of carbon in an acid solution containing a total of 2 g / l of iron makes it possible to obtain an oxidizing environment, measured in terms of E, and conversion d ferrous / ferric ions over time, when oxygen is introduced at a rate of 6 1 / min, which is better when oxygen is introduced in an unsatisfactory manner at the same rate into the leaching solution, in the presence of a small amount or almost no catalyst.

TABLE 1
Comparison of the conversion of ferrous ions to ferric ions in the event of addition
1.25 g / l and 25 g / l of activated carbon and 6 I / min of oxygen as
that oxidizing 1.25 g / l of activated carbon 25 g / l of activated carbon
Time (h! IFe (II) 1% conver-, [Fe (ll)]% con vers
sion sion
(mg / L) (Fe (II) in (mg / L) (Fe (ll) in
Fe (III)) Fe (III)
0 2000 0 563 2000 0 563
i 2000 0 578 1714 14.3 628
2 2000 0 598 1350 32.5 663
3 2000 0 599 1036 48.2 668
4 2000 0 608 821 58.95 679
20.5 1930 3.5 649 50 97.5 789
It should be noted in this regard that the conversion of uranium (IV) to uranium (VI) cannot be achieved economically if the required conversion rate and the required amount of conversion of ferrous ions to ferric ions are not obtained. This depends on the dissolution rate and the availability of the oxidant.

 Another advantage which can be obtained by using the present process is that, by using pyrolusite, manganese dioxide or sodium chlorate, there can be a period, during leaching, during which is not useful to introduce these reagents. This period corresponds to the time when ferrous ions begin to form a solution following the acid attack of iron, pyrite or other minerals containing iron in uranium ore. During this period, the effect of these metal oxidants on the oxidation of uranium (IV) is negligible and can possibly be counterproductive because they can react with components such as hydrogen sulfide and the hydrogen emitted during acid leaching. It is clear that these reactions would lead to excessive consumption of reagent. This should not represent a significant problem if gaseous oxidants, such as those preferred in the present invention, are used. Consequently, these gaseous oxidants can, in the presence of a catalyst, be introduced at the start of the leaching process, which makes it possible to achieve a low rate of oxidation of uranium (IV) to uranium (VI) at during a period during which metal oxidants conventionally used are not effective. At this level, the catalytic oxidation reaction described above can be obtained in parallel with the direct oxidation of uranium (IV), but independently of the latter.

 At the end of the leaching step, the sterile ore and the catalytic adsorbing agent, for example activated carbon, can be separated from the spent liquor.

Advantageously, this separation is obtained by a multi-stage settling process against the current, using different thickeners. The lower components, sterile ore and activated carbon, are separated, the sterile ore being generally neutralized and evacuated or recycled. The activated carbon is sent back to the leaching stage, but it is possible that regeneration or treatment may be necessary to remove the adsorbed species before recycling.

The bottom layer from the thickeners is clarified by sand filtration to ensure that suspended solids cannot get inside the uranium recovery line.

 According to another embodiment, the activated carbon can be kept in the leaching tanks by sieves, with occasional regeneration if necessary. If necessary, additional rectification steps can be implemented in order to recover species adsorbed on the activated carbon.

Naturally, the activated carbon can be replaced by other catalytic agents, such as ion exchange resins, if desired.

 The clarification of spent lye is particularly recommended in the case of uranium collection by extraction using a solvent or using an ion exchange resin.

 In the case of an ion exchange resin, strongly basic anionic resins are used to adsorb uranium complexes which exclude metal cations.

 Solvent extraction is also used to treat clarified acid liquors. Usually, the acid bath is passed through a series of mixing / settling units in which the spent detergent is brought into contact with an organic solvent, such as, for example, those mentioned in A.R.

Burkin "Extractive Metallurgy of Uranium", such as an amine consisting of 5% Alamine 336 and 2% isodecanol. The process described therein included four steps and makes it possible to recover a detergent enriched in uranium containing 34 g / l of U308. This uranium is collected from lye enriched by precipitation with ammonia, in order to form ammonium diuranate.

 The precipitation reaction is as follows 2UO2SO4 + 6NH4OHO (NH4) 2U207ffi + 2 (NH4) 2SO, + 3H.0 (8).

 This precipitate is then thickened, washed and drained by calcination in order to obtain a uranium product containing at least 90% by weight of U3Os.

 According to another embodiment of the present invention, an ore rich in uranium or a concentrate containing precious metals such as gold or silver can also be processed. In this case, an acid leaching step will be preferred as the first step in the process in which the uranium is extracted. In the second step, a leaching process, for example with cyanide, is used to allow the extraction of precious metals from the pulp obtained by acid leaching of the ore or concentrate. In cases where such an ore or concentrate contains cobalt, this preparation method is preferred, in order to avoid the formation of cobaltcyanide ions which interfere with the collection processes by ion exchange.

 The invention will be better understood with the aid of the following description of an exemplary embodiment of the latter.

Example
Copper / uranium flotation residues containing 0.8 0.9 kg / t U308 were treated, in the form of 55% uraninite (uranium oxide), 25-30% uraninite disseminated in minerals based on haematite and sulphide, and 15 to 20% of coffinite and brannerite, the latter minerals being considered as generally refractory to leaching. In this respect, the complex brannerite [(U, Ca, Fe, Th, Y) (Ti, Fe) 2OJ is considered non-leachable and the silicate coffinite [U (SiO4) íX (OH) ,,] does not dissolves only slowly, approximately 33% of uranium being estimated to be leachable in this mineral. The presence of these minerals reduces the proportion of uranium capable of being extracted from the tailings to 65-70% of total uranium.

 A sufficient quantity of residue was obtained in mud, by stirring, in water, in order to produce a mud containing 50% by weight of flotation residue. Sufficient sulfuric acid solution was then added to the pulp to obtain a pH of 1.5. Leaching was carried out at 30 "C and 60" C, the temperature being kept constant for the duration of the leaching process.

Comparative leaching tests were carried out for the following cases
(a) A sufficient quantity of residues, approximately 2 kg / t, of sodium chlorate of industrial type (NaC1OS has been added to the pulp in order to obtain a redox potential (POR), at the start of leaching, of 1300 mV.

 (b) Carbon, in the form of activated carbon, and an oxygenated oxidant (to replace the chlorate) were added to the pulp at rates of 0, 14, 35 and 70 g / t and 1.5 I / min respectively .

 Oxygen was introduced into the leaching container through a single nozzle located just below the stirring propeller. Ideally, oxygen should be introduced into the dispersion by spraying or by a similar process. The addition rate can be controlled to obtain a desired POR.

 The leaching test was carried out for 24 hours, samples being taken for analysis at the end of this period. The results are shown in Table 2 below.

Copper / uranium flotation leaching
Uranium collection (% by weight of recoverable uranium)
Leaching for 24 h Acid consumption (kg / t)
(100% H2S0 ,,)
Conditions Temp ("C) Temp (" C) 30 "C 60" C 30 "C 60" C NaCIO3 (2kg / t) 81.2 75.5 7.71 8.04
O2 with C (g / t) 02/14 T = 35 C 78.7 T = 35 C NA

63.3 6.22 13.54 02/35 T = 35 C 71.6 T = 35 C
68.8 7.28 11.94 02/70 80.3 89.4 9.20 14.08
The amount of uranium collected with chlorate is comparable to that achieved with an addition of carbon of 14 g C / t of ore and an oxygen supply of 1.5 I / min. However, this result is obtained without the drawbacks encountered when using chlorate or pyrolusite, as described above. You can also get better control of the PRO.

 The present invention is not limited, in its application, to a uranium extraction process as described above, and modifications can be made by specialists. For example, the invention can equally well be applied to an extraction of uranium by alkaline leaching. These modifications are within the scope of the present invention.

Claims (19)

Claims  1. A method of extracting uranium from a material containing uranium, by means of a leaching operation, comprising dissolving the material, this dissolution being catalyzed by a catalytic agent.  2. Method according to claim 1, in which an oxidizing agent is introduced during the acid or alkaline leaching operation.  3. Method according to claim 2, in which the oxidizing agent is a gas containing at least one of the elements of the group comprising oxygen, ozone, air, a gas containing elemental oxygen, and a mixture of at least two of these gases.  4. Method according to claim 1, 2 or 3, wherein the catalytic agent is chosen from adsorbents containing carbon and resins.  5. The method of claim 1, 2 or 3, wherein the uranium-containing material contains precious metals, further comprising a leaching step for the extraction of precious metals.  6. The method of claim 5, wherein the extraction of precious metals takes place after the extraction of uranium.  7. A method of extracting uranium from a material containing uranium, by means of an acid leaching operation, comprising dissolving mineral uranium present in the material containing uranium, by ferric ions, the ferric ions being generated by the oxidation of ferrous ions, at least part of which is formed by the reaction of the acid with iron or the iron-containing mineral present in the material containing uranium, this oxidation being catalyzed by a catalytic agent, in order to obtain an effective ratio between ferric ions and ferrous ions allowing a quasi-dissolution of the mineral uranium.  8. The method of claim 7, wherein the oxidation is obtained by means of an oxidizing agent introduced during the leaching process.  9. Method according to claim 8, in which the oxidizing agent is a gas containing at least one of the elements of the group comprising oxygen, Ozone, air, a gas containing elemental oxygen, and a mixture of at least two of these gases.  10. The method of claim 7, 8 or 9, wherein the catalytic agent is chosen from adsorbents containing carbon and resins.  11. The method of claim 7, 8 or 9, wherein the uranium-containing material contains precious metals, further comprising a leaching step for the extraction of precious metals.  12. The method of claim 11, wherein the extraction of precious metals takes place after the extraction of uranium.  13. Method according to any one of claims 7, 8 or 9, wherein the oxidizing agent oxidizes part of tetravalent uranium ions into hexavalent ions.  14. A method of extracting uranium from a material containing uranium, by means of an alkaline leaching process, comprising dissolving the material by means of an alkaline agent, this dissolution being catalyzed by a catalytic agent.  15. The method of claim 14, wherein an oxidizing agent is introduced during the alkaline leaching operation.  16. The method of claim 15, wherein the oxidizing agent is a gas containing at least one of the elements of the group comprising oxygen, ozone, air, a gas containing elemental oxygen, and a mixture of at least two of these gases.  17. The method of claim 14, 15 or 16, wherein the catalytic agent is chosen from adsorbents containing carbon and resins.  18. The method of claim 14, 15 or 16, wherein the uranium-containing mineral contains precious metals, further comprising a leaching step for the extraction of precious metals.  19. Method according to any one of claims 1 to 19, in which a solid oxidant is introduced during the leaching operation, the solid oxidant being chosen from the group comprising manganese dioxide, permanganates, peroxides , chlorates, chlorides, hypochlorides, chromates, dichromates and persulfates.