CRUSHING AND GRINDING

The initial step in conventional uranium ore milling involves crushing and usually grinding operations to produce a sized ore suitable for acid or alkaline leaching (heap leaching requires crushing only). The feed preparation requirements are nearly always specific to the site.

Grinding in a mill can be wet or dry. The advantage of wet grinding process is that the dust and radiation hazards associated are essentially eliminated. The primary objective of crushing and grinding in the vast majority of uranium milling operations is to liberate the gangue minerals that are intimately associated with uranium deposits to a degree required for effective leaching i.e. expose the surfaces of uranium minerals to the leaching chemicals.

30 Presented at the Third Consultancy Meeting for the preparation of an IAEA publication on the Major Environmental Considerations Associated with Uranium Mining and Milling, 2327 March 2015, Vienna.

FIG. 1. Simplified flow chart of conventional uranium milling.

The degree of grinding required to achieve this objective for different ores may vary considerably depending upon the type and mineralogy of the ore body being processed (sandstone, limestone, gneiss, etc.), leaching type and lixiviant used (acid or alkaline). It is rarely necessary to fracture the grains of a sandstone ore because the uranium occurs primarily in the cementing material between the grains. Other ores, in particular those containing refractory minerals, may require the breakage of mineral grains themselves [1].

Crushing and grinding are significant energy consumers. Grind size will affect the usage of power and chemicals, and the characteristics of tailings. In general, the energy intensity of uranium production is inversely related to the grade of the ore. Lower grade ores are presumed to lead to increased mining and milling energy requirements per unit of product basis [2].

Mining (Open pit

& Underground)

Crushing &

Grinding

Leaching Separate Solids

Extraction Precipitation

Separate Solids

Drying/Calcining

Uranium Ore Concentrate, e.g.

U3O8

Tailings disposal

In situ leach mining

3. LEACHING

Uranium milling is based primarily on hydrometallurgical operations such as leaching, purification (solvent extraction and ion exchange) and precipitation. Leaching is an important step in the processing of a uranium ore where uranium is chemically leached from the ore crushed and ground to the required size or from slurry in the case of underground mining, in order to produce uranium ore concentrate with a uranium content of at least 65%. This material is normally termed yellow cake, despite the fact that not all modern forms are yellow.

Uranium ores are treated by either acid or alkaline reagents with sulphuric acid or sodium carbonate–sodium bicarbonate systems used almost exclusively for commercial uranium recovery. In general, alkaline leaching is milder and more selective than acid leaching and is used for the treatment of high carbonate ores, which would consume excessive amounts of acid.

A general guide has been that if the ore contains more than 2% of carbonates then alkaline leaching is more economical, depending on the grade, but other factors must also be considered [3]. Such factors include the efficiency of uranium extraction, water usage (particularly in remote locations), energy consumption, product quality requirements and environmental considerations.

Although acid leaching is used in the majority of uranium mills, alkaline leaching can have some inherent advantages; these are: (i) the carbonate–bicarbonate solution is more specific for uranium minerals, leaving most of the associated gangue unattacked, which makes possible direct precipitation of uranium from the leach solution without need for further purification;

and (ii) the carbonate solution can easily be regenerated. These characteristics also lead to a number of disadvantages that include the following [1, 4]:

a) Fine grinding is required to expose the uranium minerals;

b) Some gangue minerals such as calcium sulphate and pyrite, if present, can react with the alkaline reagent resulting in a high consumption;

c) The more refractory uranium minerals are not dissolved under alkaline conditions;

d) Elevated temperatures and/or pressures are sometimes required;

e) The kinetics (speed of reaction) is normally slower than acidic leach.

The selection of leaching reagent for dissolving uranium minerals is dependent in part on the physical characteristics of the ore such as: type of mineralization, ease of liberation and the nature of other constituent minerals present [5]. After selection of the reagent the next step is the choice of the leaching system. The following techniques are available.

a) Agitation leaching at atmospheric pressure (acid and alkaline);

b) Pressure leaching (acid and alkaline);

c) Strong acid pugging and curing (acid);

d) Heap leaching (acid or alkaline);

e) In situ leaching (mainly acidic except in the USA where alkaline is the norm).

Terms such as in situ leaching, in place leaching, heap leaching and percolation leaching have been used to denote various modifications of the static bed leaching practices where the ore bed remains static and the solutions are circulated through the stationary bed of ore. In the context of this discussion, the following definitions will be used [1]:

a) Heap leaching: The ore is mined and then piled in a collection system. Leach solutions are distributed over the upper surface of the heap and passed downwards through the

bed of the ore. The bed may be run-of-mine ore (ROM) or coarse crushed material, although in modern practice agglomeration of the crushed ore is the norm;

b) Percolation (vat or tank) leaching: The mined ore is fine crushed (normally to about 25 mm size or less) and then bedded into tanks or vats. The leach solutions are passed through the static ore in either an upwards or a downwards direction;

c) In situ leaching: The ore is not moved from its geological setting. The leaching solutions are forced, usually in a horizontal direction, through the ore body and recovered by a series of wells;

d) In place leaching: The ore is broken by blasting but left in underground stopes. The leach solutions may pass through the static ore bed in a downward direction or the stope may be at least partially filled with the leaching solution. The leach solutions are recovered by a collection system below the bed of broken ore.

In situ leaching is at present limited in application to confined sandstone formations (high permeability) containing comparatively small deposits of low grade ore at a relatively shallow depth. However, it can offer significant economic advantages as the wellfield replaces the mining and crushing operations and leaching equipment associated with conventional processing. Its disadvantages are the relatively low recovery and sometimes stringent requirements to restore the mined area to acceptable conditions [6].

Heap leaching may suffer from poor recovery, without offering the cost savings associated with in situ leaching. The ore must be mined, transported and sometimes crushed before being leached. The preparation of an impervious base is usually required. Improvements in extraction efficiency require innovations such as pelletizing and the provision of leach vats. Where low grade ore is involved, the more modest capital and operating requirements of a heap leaching operation may give it an economic advantage over conventional leaching routes; in particular where bacterial action and in situ sulphide minerals can be used to provide a degree of autolixiviation (i.e. bacterial oxidation of sulphide produces ferric sulphate and sulphuric acid lixiviant capable of leaching uranium). Heap leaching may be attractive for processing below ore grade uranium material (bogum) stockpiled during mining operations. However, bogum is often weathered and too fine for a heap leaching operation without pelletizing or agglomeration, or treatment to remove fines [6, 7].

There is an increasing trend towards optimization of uranium recovery process. Optimization is occurring in several ways, one of which is balancing energy usage and chemical consumption against recovery through increasing application of the heap leach process. Lagoa Real in Brazil is one of the world’s newer uranium processing facility and treats all ore by heap leaching.

Technology transfer from the gold industry, where heap leaching accounts for a substantial percentage of production, has contributed in shifting production focus in uranium industry from conventional milling to heap and in situ leaching in several countries [8].

For mines where ore is physically removed for treatment, the choice between leaching systems (e.g. agitation leaching at atmospheric pressure, pressure leaching, strong acid pugging and curing), is mainly determined by the mineralogy of the uranium and the gangue. Agitation leaching at atmospheric pressure is most commonly used, with pressure leaching employed for refractory uranium minerals and alkaline conditions where the rate of reaction is too slow under conventional conditions.

Pressure acid leaching has been adopted for a few uranium operations treating more refractory ores, especially in the presence of sulphides such as pyrite which can oxidized to form the sulphuric acid and ferric iron required for the process. Pressure alkaline leaching was introduced

to treat more refractory high acid consuming ores and was successfully used at a number of operations.

Bio-leaching has also been applied commercially for pyritic heap leaching operations and for in place leaching of low grade underground mine stopes broken by blasting and to old mine stopes. Since the last uranium boom there have been major advances in the application of both heap bio-leaching of run-of-mine ore (ROM) or crushed ore and agitated tank bio-leaching of concentrates, such as the use of aeration pipes, addition of nutrients, development of more efficient agitators and the development of new ultrafine grinding equipment. This is likely to lead to a greater use of bio-leaching in future uranium projects involving sulphidic ores. Heap or in place systems will be more suitable for lower grade ores, while tank bio-leaching will be more applicable to uranium bearing sulphidic concentrates [7].

Strong acid pugging is generally used for treating refractory ores. Reaction times and acid consumption in this type of process are generally less than in conventional acid leaching and higher extractions from quite refractory ores can be obtained using an ore that is much coarser than is required with conventional processes. The ability to use a coarser ore not only reduces the grinding costs but also makes subsequent separation of the pregnant leach solution from the leach residue easier. This technique is also advantageous for ores where normal agitation leaching results in the rapid breakdown and dispersion of clays and the dissolution of large quantities of silica [1].

In a few cases, uranium ores may undergo roasting to increase the solubility of valuable constituents and to improve the physical characteristics of the ore. As example, ores are roasted to enhance vanadium extraction, to improve the ore settling and filtration characteristics by alteration of clay minerals in the ore and to remove organic carbon which can cause problems in the leaching circuits [1].

Leaching is the main chemical reagent consumer in the milling process. In addition to dissolving uranium, leaching mobilizes a range of potential contaminants, apart from radionuclides. Reagent quantity can be minimized by optimization of leach conditions to minimize dissolution of gangue minerals and leaching at higher slurry density so the amount of acid needed to maintain free acidity is reduced [9].

In uranium leach processes employing sulphuric acid as the lixiviant, only a relatively small quantity of acid is gainfully employed in extracting uranium from the host ore. The remainder of the acid is consumed by the gangue constituent elements. Many new projects that are being considered today have gangue acid consumptions in excess of 95%. In contrast, alkaline leach processes do not experience the same high reagent consumption rates as acid processes. The solubility of many of the impurity elements in uranium ores (such as Fe, Mg, and Al) in the alkaline leach is quite low. However, the alkaline operations do encounter some challenges with gangue element solubility [10].

3.1. Acid leaching

In nature uranium occurs in the tetravalent form, the oxide of which is UO2, and the hexavalent form, the oxide of which is UO3. In its hexavalent form uranium goes directly into solution.

Tetravalent uranium has a low solubility in both dilute acid and carbonate (alkali) solutions. To achieve economic recovery of uranium in the tetravalent state, oxidation to the hexavalent state is essential. It is therefore important to maintain proper oxidizing conditions during leaching of these minerals to achieve high uranium extraction [6, 11]. Various oxidants have historically

been used and are currently employed to oxidize tetravalent uranium to hexavalent uranium in acid and alkaline leaching of uranium ores, these include:

a) Manganese dioxide (MnO2) as milled pyrolusite;

b) Sodium chlorate (NaClO3);

c) Hydrogen peroxide (addition as H2O2 or Caro’s acid (H2SO5));

d) Ferric iron;

e) Oxygen in pressure leach circuits;

f) Sulphur dioxide/air (oxygen) mixture.

Currently sulphuric acid is almost exclusively used for acid leaching, typically combining high leach performance and relatively low cost. Considering its chemical properties, nitric acid is the most capable leaching agent for uranium. Unlike sulphuric acid it has a high oxidation potential and does not generate an insoluble residue (e.g. gypsum). Nevertheless, its high cost and ability to dissolve many associated gangue minerals greatly reduces the utility of nitric acid in uranium leaching practice. Hydrochloric acid (HCl) solution is intermediate between nitric and sulphuric acid, when cost and the intensity of reaction with uranium ore are considered.

Hydrochloric acid does not produce insoluble compounds which plug the porosity. It is, however, much more corrosive to metal and equipment [6, 12].

Ferric iron (Fe³⁺) acts as the principal oxidant of tetravalent uranium in acid leaching. However, other oxidants namely, pyrolusite (manganese dioxide), sodium chlorate and Caro’s acid (a specific mixture of sulphuric acid and hydrogen peroxide) are typically used to assist in oxidation. Pyrolusite consumes more acid and the product from the oxidation reaction, Mn²⁺, has a potential environmental impact. On the other hand, Caro’s acid offers significant environmental advantages in that the residual reaction product is water [9, 11].

Uranium ores containing sulphidic minerals can be leached in the presence of oxygen only at elevated temperature and pressure. This technique is widely known as pressure leaching. These sulphides are converted to sulphuric acid and ferrous sulphate at elevated oxygen pressures.

The ferrous sulphate is subsequently oxidized to ferric sulphate, which is an effective oxidant for the dissolution of tetravalent uranium.

3.2. Alkaline leaching

As indicated earlier, alkaline leaching of uranium ores is usually a viable alternative to acid leaching if: (a) the ore contains significant amounts of acid consumers such as carbonates; (b) uranium mineralization is in the hexavalent form or if it can be readily oxidized. Unlike acid medium in which the rapid oxidation of tetravalent uranium is achieved by the presence of ferric ions in solution, ferric ions cannot be maintained in alkaline carbonate solutions. The lack of such a catalyst that is largely responsible for the very different conditions required in carbonate leaching as compared with acid leaching. Carbonate leaching calls for more severe conditions of pressure and temperature, usually from 70 to 80°C, often a longer leaching time, and a finer grind [1].