How Uranium Concentrate Is Produced
Uranium concentrate is the first industrial product on the path from a uranium deposit to nuclear fuel. It is produced by processing that separates uranium from most of the associated material and collects it in a concentrated solid product. This product is known as yellowcake.
Before processing, uranium is part of the geological environment. It is bound within minerals and distributed through the rock, and the characteristics of the deposit determine how it can be recovered. A sequence of mining, mechanical and chemical operations therefore lies between the deposit and a container of concentrate.
In Part 1, we looked at a map of the entire fuel cycle. Here, we follow its opening stages: how we access the uranium, how we bring it into solution, and why we then recover it as a solid product.
The Deposit Determines the Recovery Method
Knowing that a rock contains uranium is not enough to select a recovery method. The depth and geometry of the deposit, the distribution of uranium, the types of uranium minerals and the properties of the surrounding rock all matter. Groundwater conditions, site accessibility, recovery costs and environmental requirements must also be considered.
In conventional mining, ore is extracted from an open-pit or underground mine and transported to a processing plant. With in situ recovery, uranium is dissolved directly within the deposit, and the solution containing the dissolved uranium is pumped to the surface. The terms in situ leaching (ISL) and in situ recovery (ISR) are both used for this approach.
In the first route, the processing plant receives mined ore; in the second, it receives a solution. In both cases, the plant must recover uranium concentrate from the incoming material.
A simple calculation helps illustrate the quantities involved. One tonne of ore containing 0.1% uranium by mass contains one kilogram of uranium. This is an example based on a selected ore grade, not an average for all deposits. The amount of that uranium actually recovered depends on the recovery efficiency of the process. The mass of the final concentrate also includes the other elements in the uranium compounds and any remaining impurities.
What Milling Means in Uranium Processing
The term uranium milling refers to the overall processing of ore into uranium concentrate. In this context, milling means more than grinding alone. A uranium mill carries out chemical treatment and separation as well as mechanical ore preparation.
In conventional processing of mined ore, crushing and grinding are the initial mechanical stages. Large pieces of rock are reduced to smaller particles, increasing the surface area available for contact with the process solution and making the uranium minerals more accessible. The resulting material is often mixed with water to form a slurry that can be pumped between different parts of the plant.
The ground ore still contains the accompanying minerals. Uranium concentrate is produced only through further chemical processing and separation.
Leaching Transfers Uranium into Solution
During leaching, the prepared ore comes into contact with a solution that allows uranium to pass from the minerals into the liquid phase. In the resulting solution, uranium is present as dissolved chemical species. Fine rock particles remaining in the slurry form another component of this process stream.
Both acid and alkaline processes are used. Acid leaching often uses sulphuric acid, while alkaline leaching uses solutions based on carbonates and bicarbonates. The choice depends on the mineral composition of the ore and the response of the accompanying material. Ore containing a high proportion of acid-consuming minerals may require a different approach from ore with low acid consumption.
The chemical state of the uranium also matters. For some ores, effective leaching requires oxidation, which converts uranium into a chemical form that dissolves more readily in the selected medium. This changes its chemical state; it is not isotopic enrichment.
Selecting a process therefore requires an understanding of the ore as a whole. Other substances may dissolve along with the uranium, affecting reagent consumption, subsequent solution purification and residue management.
Other leaching arrangements are also used. In heap leaching, mined ore is placed on a suitably engineered pad, and process solution percolates through it. The collected solution then requires further treatment. This is another reason why a single plant design cannot describe every route from mined ore to concentrate.
Separating the Liquid and Solid Phases
After leaching, the process stream contains a uranium-bearing solution and the remaining solid material. In a conventional processing sequence, these are separated, for example by settling and thickening, or by filtration. Washing the solid residue helps recover solution that would otherwise remain trapped between the particles.
This step removes undissolved particles, while other dissolved substances remain in the liquid. The solution therefore generally requires further treatment. Solid-liquid separation and solution purification perform different functions.
This sequence is a simplified description of a conventional process route. Individual plants may combine certain stages or arrange them differently, depending on the ore and the technology used.
Purifying the Solution and Concentrating the Uranium
The next task is to separate uranium from other dissolved substances and prepare the liquid stream for uranium recovery. The methods used include ion exchange and solvent extraction. A plant may use either method or a combination of the two.
In ion exchange, dissolved uranium species bind to a suitable ion-exchange resin. Subsequent treatment transfers them from the resin into another solution. The resin thus enables selective uptake and subsequent release of uranium.
In solvent extraction, the aqueous solution comes into contact with an organic phase containing a suitable extractant. Uranium is preferentially transferred into the organic phase and then, in a subsequent step, transferred back into a different aqueous solution.
These processes separate uranium from accompanying substances and concentrate it in a stream suitable for further treatment. No individual technology removes all impurities equally well, so the plant design must match the composition of the process solution.
Precipitation Produces a Solid Once Again
Uranium is recovered from the prepared solution in the form of a sparingly soluble compound. This process is called precipitation, and the resulting solid is called a precipitate. The choice of reagents and process conditions influences its composition and properties.
The precipitate is separated from the liquid and processed into the final product. This includes moisture removal and, depending on the required composition of the concentrate, further heat treatment. Drying primarily removes water; calcination may also change the chemical form of the uranium compound.
During processing, uranium is therefore first transferred from the minerals into solution and then recovered from that solution as a solid product. This transition allows it to be separated from much of the original accompanying material. The concentrate is prepared and packaged for shipment to the next stage of processing.
Yellowcake Is Not Always Yellow
The name yellowcake comes from the yellow colour of some uranium concentrates. The term nevertheless covers a group of industrial products whose composition depends on the recovery method and final treatment.
Concentrate may be yellow, greenish, brown or dark in colour. It is often associated with the uranium oxide U3O8, but the name yellowcake alone guarantees neither this chemical composition nor a particular purity. In its report on radiation protection in uranium mining and processing, the IAEA explicitly notes the differences in the colours and chemical forms of uranium concentrates.
A similar colour does not mean the same substance. A photograph of yellow crystals of a uranyl salt is not necessarily a photograph of uranium concentrate. Selecting and labelling images requires information about the substance shown and its origin.
Concentration and Isotopic Enrichment
Producing concentrate increases the proportion of uranium relative to accompanying substances. Isotopic enrichment increases the proportion of uranium-235 within the uranium itself. These are two different ratios.
Conventional ore processing does not separate uranium isotopes. The uranium in the concentrate therefore remains natural uranium, in which uranium-235 accounts for approximately 0.7%. Purification and concentration do not, by themselves, turn it into enriched uranium.
In the fuel cycle for light-water reactors, conversion takes place between concentration and isotopic enrichment. Conversion changes the chemical form of uranium, for example into uranium hexafluoride, UF6, suitable for subsequent isotopic enrichment.
| Stage | What changes |
| Processing into concentrate | The proportion of uranium relative to accompanying substances. |
| Conversion | The chemical form of uranium. |
| Isotopic enrichment | The proportion of uranium-235 within the uranium. |
Enrichment is not part of every nuclear fuel cycle. Some reactors, such as CANDU reactors, can use natural uranium fuel. This fuel still requires further chemical processing and the manufacture of fuel elements.
In Situ Recovery Moves Leaching into the Deposit
With in situ recovery, the ore remains underground. Process solution is introduced into the uranium-bearing formation through injection wells, and the uranium-bearing solution is brought to the surface through recovery wells. Uranium separation and concentrate production then take place at the surface. This route does not require the conventional crushing and grinding of mined ore.
The method is applicable only under suitable geological and hydrogeological conditions. The uranium-bearing formation must allow the solution to flow through it and come into contact with uranium minerals. Groundwater flow directions, rock permeability and connections between aquifers must be understood. Low-permeability layers can help limit the spread of solutions, but effective control of flow must be verified by measurements.
Baseline groundwater conditions must be established before operations begin. During operation, groundwater composition, the movement of process solutions and well performance are monitored. Injection and pumping must be managed so that the area through which the solutions move can be controlled.
Once recovery ends, the required groundwater treatment or restoration, well closure and verification of the resulting conditions follow. The objectives and procedures depend on the site and the requirements of the relevant authorities. Simply stopping the pumps does not mean that site remediation is complete.
In situ describes where the process takes place. Its environmental impacts still depend on conditions within the deposit, the chemical process, operational controls and the success of remediation.
What Remains After Uranium Is Removed?
In conventional uranium recovery, a distinction must be made between waste rock and ore-processing residues. Waste rock is excavated rock that is not classified as ore for processing because its uranium content is too low or because of other properties. It may still contain uranium and other substances, so its characteristics and the measures needed to manage it are assessed on the basis of its actual composition.
Ore processing leaves a fine-grained material from which most of the uranium has been removed. This is known in English as mill tailings and in Slovenian as hidrometalurška jalovina. ARAO also uses this Slovenian term when describing the residues from uranium ore processing at the Boršt disposal facility.
Mill tailings retain some uranium, radionuclides from the natural decay chains, including thorium-230 and radium-226, as well as accompanying substances and residual process reagents. Removing most of the uranium therefore does not remove all sources of radioactivity. The decay of radium produces radon, which may be released from the material.
Tailings management requires control of dust, radon and water, as well as measures to ensure the stability of disposal facilities. Limiting seepage and the leaching of substances, rehabilitating the surface and monitoring the site over the long term are all important. These tasks must be incorporated into the planning of uranium recovery and processing.
In situ recovery does not generate the same mill-tailings stream as a conventional mill. It does, however, produce other waste streams, including process liquids, used filters and contaminated equipment. Groundwater management also remains a major responsibility. A comparison of recovery methods must therefore cover the entire process and the condition of the site after operations have ended.
Safety Begins Before the Reactor
From the very start of the fuel cycle, conventional mining and industrial risks, chemical hazards and radiation protection requirements all need to be addressed. Their significance depends on the type of workplace, the ore composition and the technology used.
Radiological considerations include external exposure, inhalation of radioactive dust, and exposure to radon and its short-lived decay products. Preventing the accumulation of radon and its decay products in poorly ventilated spaces is particularly important. Uranium is also chemically toxic, and the reagents used may introduce additional hazards.
Protective measures include ventilation, dust control, containment of substances within process equipment, proper management of solutions and exposure monitoring. When handling dry concentrate, preventing the dispersal of dust and the intake of material into the body is important. Personal protective equipment supplements engineering and administrative controls.
The natural origin of uranium does not determine the level of exposure at a particular workplace. This must be established by assessing conditions, taking measurements and monitoring the actual process.
From Deposit to Feed Material for the Next Stage
At the end of the route described here, we obtain uranium concentrate with properties suitable for further processing. Uranium has been transferred from the geological environment into a controlled industrial process stream, and most of the original accompanying material has been separated from the product.
Responsibilities for worker protection, water management and the site remain after the concentrate has been shipped. Residue management, remediation and long-term monitoring are integral parts of the overall recovery process.
In the next part, we will follow the concentrate through further chemical processing. We will examine conversion and then isotopic enrichment in the fuel cycle for light-water reactors. This will distinguish a change in the chemical form of uranium from a change in the proportions of its isotopes.
Terminology
The nuclear fuel cycle uses precisely defined technical terminology. The terms used in this article are explained below.
Uranium milling
The industrial processing of uranium ore into uranium concentrate. The term includes mechanical ore preparation, leaching, separation, solution purification and concentrate production, rather than crushing and grinding alone.
Slurry
A mixture of finely ground solid particles and liquid that can be pumped between processing stages.
Leaching
A chemical process in which uranium passes from minerals, or from the solid phase, into a process solution.
In situ leaching (ISL) or in situ recovery (ISR)
Uranium recovery by dissolution directly within the deposit. Process solution is injected and recovered through wells, while most of the rock remains underground.
Process solution
A liquid process stream that may contain reagents, dissolved uranium and other substances transferred from the ore.
Ion exchange
A process in which dissolved uranium species bind selectively to an ion-exchange resin and are subsequently released into another solution through further treatment.
Solvent extraction
A separation process in which uranium is transferred between aqueous and organic phases because of differences in solubility or chemical affinity.
Precipitation
A process in which dissolved uranium forms a sparingly soluble solid compound. The resulting solid product is called a precipitate.
Calcination
Heat treatment that removes water and other volatile constituents and may also change the chemical form of the uranium compound.
Uranium concentrate, or yellowcake
A solid industrial product with a high uranium content relative to the original ore. Its chemical composition and colour depend on the production process, so it is not necessarily yellow or pure U3O8.
Mill tailings
The fine-grained residue from the chemical processing of uranium ore, after most of the uranium has been removed. The material retains radionuclides from natural decay chains, other ore constituents and residual process substances.
Concentration
Increasing the proportion of uranium relative to accompanying substances. Concentration does not, by itself, change the isotopic composition of uranium.
Conversion
Transforming uranium into a chemical form suitable for the next stage of the fuel cycle, for example uranium hexafluoride, UF6. Conversion does not increase the proportion of uranium-235.
Isotopic enrichment
A process that increases the proportion of uranium-235 within the total amount of uranium.
Note on Illustrations
Cover photograph: Ranger uranium mine, Northern Territory, Australia. Source: iStock.
The diagrams “From Uranium Ore to Uranium Concentrate” and “Concentration Is Not the Same as Enrichment” were created by Elite Studio 3D / By the Protocol specifically for this article.
The first diagram provides a simplified comparison of conventional mining and ore processing with in situ recovery, showing uranium moving from the solid phase into solution and then back into a solid concentrate. The second distinguishes uranium concentration, the change in its chemical form during conversion, and the subsequent change in its isotopic composition during enrichment.
Intermediate stages, individual process configurations and side streams are simplified. The diagrams do not constitute design or operating documentation for any particular mine, processing plant or in situ recovery site.
References
NRC. Uranium Recovery (Extraction) Methods.
IAEA. Uranium Extraction Technology. Technical Reports Series No. 359, 1993. In particular, Chapters 2, 4–8 and 10.
NRC. Conventional Uranium Mills.
Cameco. Uranium 101: Milling.
IAEA. Manual on Laboratory Testing for Uranium Ore Processing. Technical Reports Series No. 313, 1990. Chapter 4 on leaching.
IAEA. Production of Yellow Cake and Uranium Fluorides. STI/PUB/553, 1980. Papers on precipitation, concentrate composition and subsequent processing.
IAEA. Occupational Radiation Protection in the Uranium Mining and Processing Industry. Safety Reports Series No. 100, 2020. For the colours and chemical forms of concentrates, see also Section IV.3.2, printed page 179.
Cameco. Uranium 101: Refining & Conversion.
IAEA. Manual of Acid In Situ Leach Uranium Mining Technology. IAEA-TECDOC-1239, 2001. In particular, Chapters 2, 4 and 11.
NRC. Backgrounder on Uranium Recovery.
NRC. Uranium Mill Tailings.
ARAO. ARAO has assumed long-term management of the closed Boršt disposal facility at the former Žirovski Vrh mine. In Slovenian; article title translated here.
NRC. Comparison of Conventional Mill, Heap Leach, and In Situ Recovery Facilities.
from ore to energy in situ recovery leaching mill tailings nuclear fuel cycle radiation protection uranium uranium concentrate uranium milling uranium mining uranium ore yellowcake
Last modified: October 7, 2026
