Heap leaching

Uranium is recovered from crushed ore heaps by using a leaching solution applied in situ. This is an alternative first stage of ore processing compared to the more traditional treatment, in which the ore is fed continuously into a chemical engineering process where leachate is contained within recovery tanks. While most of the radiation protection considerations applicable to heap leaching are similar to those for other uranium mining and processing activities, some need particular attention, as outlined in this section. The principal concerns requiring assessment relate to exposure to ore dust during the construction and subsequent

removal of the heap, exposure to gamma radiation and the inhalation of RDP for workers who carry out maintenance tasks on the heap during the leaching period.

6.5.1. Process description

Heap leaching involves the application of a leaching solution to the top of a heap of crushed ore to extract the metal contained within (see Fig. 11).

Metals that are soluble in the leaching solution (typically an acid) are drawn into solution as the liquid percolates downwards through the heap. The metal bearing solution, or leachate, is collected at the bottom of the heap, where it meets an impervious base, and then pumped away to be reapplied or, eventually, to be further processed to separate out the metal. This process is slow and has to be continued for a long period in order to extract as much of the available metal as possible. For heap leaching of uranium ore, a continuous process of many months may be needed, using dilute sulphuric acid.

6.5.2. Design and operation

Crushed ore may be placed in discrete heaps, with each being leached in sequence, or a continuous process may be adopted by progressively constructing a linear heap with the leaching process following construction (see Fig. 12). The leaching solution is applied through a network of slow release drippers at the top of the heap. An impervious pad at the base of the heap has to be constructed to ensure that leachate does not percolate to the underlying ground. This may include a clay base, for example, covered with a plastic membrane, and may be slightly contoured to allow the leachate to run to the drainage pipework. To protect the membrane and to allow leachate to be drawn off from the base of the heap, a layer of screening material can be used to host the pipework for leachate transport. In

FIG. 11. Heap leaching construction (simplified schematic).

addition, some heap leaching processes may employ positive pressure aeration from the base of the heap to improve the leach rate; the pipework for this can also be embedded in the screening layer.

Additional infrastructure is needed for the operation of a heap leaching process. The supply of fresh leaching solution involves a storage and delivery network. The intermediate solutions (leachate that is partially loaded with uranium and which will be reapplied) and pregnant liquor (fully laden leachate) require impermeable retention ponds. Provision also needs to be made to handle natural rainfall, including containment within the site of unusual rainfall events. In the simplest designs, the pregnant liquor will be pumped to a separate processing plant for treatment and the production of uranium oxide concentrates.

Some preliminary processes, such as solvent extraction of the uranium, may be carried out within the heap leach area.

6.5.3. Principal exposure pathways

In addition to the more commonplace health and safety concerns relating to heap leaching, such as the potential for physical injury during building and removing the heaps, working at heights and on uneven and unstable surfaces, and handling pumped acidic solutions, the leaching of uranium ore raises radiation protection issues, which are discussed in the following.

FIG. 12. Heap leaching process (simplified two stage schematic).

6.5.3.1. Exposure to external radiation

The intensity of gamma radiation depends on the uranium ore grade.

Typical exposure rates of ore stockpiles lie between a few μSv/h for low grade ore (below ~0.1%) and a few tens of μSv/h for higher grade ores (~1%), and they vary with the area, depth and geometry of the heap. There is little that can be done to restrict exposure to external gamma radiation other than to control the amount of time that workers need to spend on, or close to, the heap, although the cabins of the heavy vehicles used in construction and removal operations afford a degree of shielding for the driver. Optimizing protection from this pathway focuses on restricting the time spent on, or close to the heap.

6.5.3.2. Exposure from inhalation of radon and radon decay products

The rate radon gas diffuses from a heap of uranium ore depends on factors such as ore grade, crushed rock size, moisture content of the ore, meteorological conditions and, where applicable, the rate of any aeration applied. Under dry conditions, exhalation rates are around 50 Bq·s⁻¹·m⁻² per % ore grade; under normal operation, with the heap saturated with leaching solution, the exhalation rates will be an order of magnitude lower. When aeration is applied to the heap, exhalation rates are expected to rise.

Due to natural dispersion processes and dilution of the exhaled radon, the exposures of persons remote from the heap will usually be negligible. However, some workers carry out periodic inspection and maintenance tasks for the dripper system at the top of the heap, and drivers of vehicles engaged in constructing or removing the heap will be in close proximity to the ore. Optimization of protection will focus on limiting the time spent on, or close to, the heap and on providing filtered air cabins for vehicle drivers.

6.5.3.3. Exposure from inhalation of radioactive material

Once constructed and in operation, an ore heap is kept wet by applying the leaching solution, and ore dust from the heap is minimal. Hence, inhalation of ore dust will only be significant if there are interruptions to the feeding of the leaching solution or breakdowns in the feed pipes, which can lead to the core drying out. However, the potential for inhalation of ore dust will need to be addressed during construction of the heap or its removal on completion of the leach cycle. Continual wetting of the heap surface at the point of delivery or removal can be part of a strategy for optimization of protection, as can the use of filtered air cabins for ore handling and ore transport vehicles.

6.5.3.4. Exposure from ingestion of radioactive material

Ingestion of radioactive material is not expected to be a significant pathway of exposure, provided good occupational hygiene practices are followed, such as no eating, drinking or smoking while working directly on heap management operations.

6.5.4. Control mechanisms

A heap of uranium ore in the open presents a large source of relatively low concentration radioactive material for which little can be done in the way of shielding from external radiation or of mechanical containment of any emissions.

The control of occupational exposure will depend primarily on restricting the time spent on, or close to, the heap and on ensuring that the heap remains saturated to restrict the emission of radon and ore dust.

During construction of the heap and its removal (e.g. using haul trucks and front end loaders), vehicles need to be fitted with filtered air cabins, and drivers have to avoid opening windows and doors as far as practicable. At these times, site access other than in filtered air vehicles needs to be controlled, and dust masks worn to restrict dust intake. Workers who perform maintenance and inspection tasks are expected to organize their work to restrict the time spent on the heap by careful planning prior to access. Where working conditions create a need for frequent hydration, provisions for drinking from uncontaminated water bottles or water fountains need to be made.

6.5.5. Monitoring and dose assessment

A risk assessment and exposure pathway analysis needs to be performed to establish the types and frequency of monitoring required to demonstrate compliance with health and safety standards and with the principle of optimization of protection. Provided that good occupational hygiene practices are observed, intakes of radioactive material other than by inhalation are expected to be negligible. The three key pathways of exposure that require attention are thus external gamma radiation, inhalation of radon and RDP and inhalation of ore dust. If chemical processing of the leachate takes place near the heap, additional monitoring may be necessary.

Prior to construction of the heap, estimates of the exposure conditions likely to be encountered need to be made based on earlier experience within the operation or from other sources, including a knowledge of the ore grade and construction method. As a precaution, some personal monitoring is advisable for all three pathways of exposure in the initial stages of construction until reliable

data on exposure conditions become available, although this does not need to be done for every worker. Similarly, during the initial operation of the heap with the application of leaching solution, personal monitoring will assist in establishing an appropriate monitoring programme for subsequent routine operation.

For exposure to RDP, it may also be necessary to carry out an intensive area monitoring programme (in a representative area) at the start of leaching operations to establish the radon exhalation characteristics of the heap and to provide information on the variation of radon concentrations with height above the heap surface. Some of the maintenance work on the dripper feed system for the leaching solution will involve crouching close to the heap surface. The radon dispersion and dilution levels may be lower near ground level than at the breathing zone when standing, although this is unlikely to be significant, except in very still air conditions or during atmospheric inversions. Furthermore, if personal monitoring of RDP shows intakes to be small, more comprehensive monitoring might not be needed.

For external gamma radiation in routine operation, personal dosimeters are readily available for the relatively few workers who need to work on, or close to the heap and the cost will be modest. All workers can be issued with personal dosimeters and their external dose records maintained. For routine monitoring of the inhalation pathway, the situation is more complex. While personal monitoring equipment is available, its use can be complicated, cumbersome and moderately costly, and it is unlikely to be necessary for every worker to carry personal monitoring gear. For workers in the cabins of construction vehicles, monitoring within the cabin can provide a good alternative to wearing personal monitoring equipment. Given the proximity to the surface exhalation of radon, the disequilibrium of RDP can vary considerably, depending on factors such as location and wind conditions. Hence, a radon decay product monitoring technique that accounts for disequilibrium is preferable.

The results of the initial monitoring and dose assessment programme will allow the subsequent routine programme to be designed and implemented. For example, decisions can be taken on the fraction of the workforce who needs personal monitoring to be confident that the results could be used for the dose estimation for the whole group. Similarly, the extent and frequency of any area monitoring that is required can be determined.

6.6. PROCESSING FACILITIES