The Inkai uranium deposit is a roll-front stratiform system. Roll-front deposits are a common example of stratiform deposits that form within permeable sandstones in localised reduced environments. Microcrystalline uraninite and coffinite are deposited during diagenesis by oxygenated and uraniferous groundwater, in a crescent-shaped lens that cuts across bedding and forms at the interface between oxidized and reduced lithologies. Sandstone host rocks are medium to coarse grained and were highly permeable at the time of mineralization.

They form in continental-basin margins, fluvial channels, braided stream deposits and stable coastal plains. Contemporaneous felsic volcanism or eroding felsic plutons are sources of uranium. In tabular mineralization, source rocks for uranium-bearing fluids are commonly in overlying or underlying mud-flat facies sediments.

The Inkai deposit is hosted within the Lower and Middle Inkuduk horizons and Mynkuduk horizon which comprise fine, medium and coarse-grain sands, gravels and clays. The redox boundary can be readily recognized in core by a distinct colour change from grey and greenish-grey on the reduced side to light-grey with yellowish stains on the oxidized side, stemming from the oxidation of pyrite to limonite.

The sands have high horizontal hydraulic conductivities. Hydrogeological parameters of the deposit play a key role in ISR mining. Studies and mining results indicate Inkai has favourable hydrogeological conditions for ISR mining.

Mineralization in the Middle Inkuduk horizon occurs in the central, western and northern parts of the MA Area (MA Area means the 139 km2 area in which JV Inkai currently has the right to mine, as covered by the Mining Allotment, which includes Block 1 and portions of Blocks 2 and 3). The overall strike length is approximately 35 km. Width in plan view ranges from 40 to 1,600 m and averages 350 m. The depth ranges from 262 to 380 m, averaging 314 m.

Mineralization in the Lower Inkuduk horizon occurs in the southern, eastern and northern parts of the MA Area. The overall strike length is approximately 40 km. Width in plan view ranges from 40 to 600 m and averages 250 m. The depth ranges from 317 to 447 m, averaging 382 m.

Mineralization in the Mynkuduk horizon stretches from south to north in the eastern part of the MA Area. The overall strike length is approximately 40 km. Width in plan view ranges from 40 to 350 m and averages 200 m. The depth ranges from 350 to 528 m, averaging 390 m.

The roll fronts mineralization is hosted by three horizons: the Middle Inkuduk horizon; the Lower Inkuduk horizon; and the Mynkuduk horizon.

Regional structures in the Chu-Sarysu Basin have had some control to the development of the sedimentary facies and to the movement of uranium bearing groundwater to form the roll fronts. Structure contour maps, on the surface of the basement Palaeozoic rocks, indicate that perhaps linear depressions in the surface have coincidence with overlying roll fronts; the hydrostratigraphy of the Cretaceous formations being the primary control to mineralization.

The zone of uranium mineralization is located along the geochemical barrier marked by the contact zone of the incompletely oxidized rock and the primary grey-coloured rock. Iron oxides are nearly absent in this zone. Organic carbon content is decreased. Some associated pyrite, and sometimes carbonates, are observed. Four geochemical host rocks types can be identified at the deposit:

• diagenetically reduced grey sands and clays containing coalified plant detritus
• green-grey sands and clays, reduced both diagenetically and epigenetically by “gley” soil (anaerobic organic) processes
• non-reduced initially mottled sediments
• yellow-coloured lithologies that underwent stratal epigenetic oxidation.

The initial colours are typical of channel of flood-plain facies. Diagenetically reduced grey sands and gravel of channel facies are more favourable for uranium deposition compared to greenish-grey or grey-green sands.

Occurrence and development of facies of Upper Cretaceous continental mottled alluvial formation is controlled by syn-sedimentary structures consistent with the tectonic pattern of the depression. Structural-facies control of mineralization is clearly expressed in mineralization of the Mynkuduk horizon. In the upper horizons such control is weakly expressed.

From observations of core, the redox boundary can be readily recognized by a distinct colour change from grey and greenish-grey on the reduced side to light-grey with yellowish stains on the oxidized side, stemming from the oxidation of pyrite to limonite and consumption of organic carbon.

The propagation of the oxidation fronts is affected by hydrostratigraphy (controlling fluid paths and velocities), and rock composition (controlling redox reactions). The implied groundwater movement direction was from the southeast to northwest, leading to the formation of oxidation tongues also oriented to the northwest. It gives rise to characteristic geometries of the redox fronts and associated mineralization described in more detail in the following section.

The main uranium minerals are sooty pitchblende (85%) and coffinite (15%). Sooty pitchblende occurs as micronsized globules and spherical aggregates, while coffinite forms tiny crystals. Both uranium minerals occur in pores on interstitial materials such as clay minerals, as films around and in cracks within sand grains, and as pseudomorphic replacements of rare organic matter, and are commonly associated with pyrite. The latter seems to have formed after the growth of pitchblende as it often coats or rims the uraniferous films and aggregates. No other potentially deleterious trace elements have been detected. All potential contaminants such as molybdenum (Mo), selenium (Se) and vanadium (V) occur in background levels consistent with average values for the Earth's crustal rocks. The uranium mineralization is essentially clean and monometallic. Vanadium and molybdenum show elevated values where occasional organic debris has accumulated.

Poor and rich mineralization are distinguished not by the composition of uranium minerals but by their distribution. Poor mineralization is more dispersed than the rich one. Authigenic mineralization is composed of pyrite, siderite, calcite, native selenium, chlorite, sphalerite, pyrolusite and apatite.

Trace elements.
Quantitative methods of analysis in mineralized and waste sands were used to study the content of rhenium, scandium, yttrium, and the total of rare earths with yttrium, selenium and molybdenum.

Selenium is almost absent in uranium mineralization. It is located only along the margins of grey sands, where it is fixed in the sub-zone of radium enrichment of up to two metres thick. The average selenium bodies are one to two metres thick and grades of 0.01 to 0.03%. They typically do not coincide with the contours of uranium mineralization.

Molybdenum accompanies uranium mineralization in trace amounts. Molybdenum content in mineralized uranium rocks is two to five times that in waste rocks. The molybdenum content in oxidized permeable rocks is 20 to 50% lower than that in non-oxidized waste rocks. Anomalous molybdenum content does not extend outside uranium occurrences.

ISR mining at Inkai is comprised of the following components to produce a uranium-bearing lixiviant (an aqueous solution which includes sulphuric acid), which goes to settling ponds and then to the respective IX plant before being directed to the MPP for production of uranium as yellowcake:

• Determination of the GT cut-off for the initial design and the operating period. The design cut-off sets the lower limit to the pounds per pattern required to warrant installation of a pattern before funds are committed, and the operating cut-off applies to individual producer wells and dictates the lower limit of operation once a well has entered production.

• Preparation of a production sequence which will deliver the uranium-bearing lixiviant to meet production requirements, considering the rate of wellfield uranium recovery, lixiviant uranium head grades, and wellfield flow rates.

• Wellfield development practices that use an optimal pattern design to distribute barren lixiviant to the wellfield injectors, and to collect lixiviant carrying the dissolved uranium back to the MPP, Sat1, or Sat2, as the case may be.

The above factors are used to estimate the number of operating wellfields, wellfield patterns and header houses over the production life. They also determine the unit cost of each of the mining components required to realize the production schedule, including drilling, wellfield installation and wellfield operation.

There are three surface processing facilities at Inkai: MPP, Sat1 and Sat2. The processing equipment in the MPP circuit includes IX units (adsorption and elution columns), along with yellowcake precipitation, thickening, drying and packaging process units. The processing equipment at both Sat1 and Sat2 consists of adsorption and elution equipment. This is illustrated in the block flowsheet in Figure 17-1: Flowsheet Based on Annual Production of 5.2 M Lbs U3O8. The MPP generates a dried yellowcake product from pregnant solutions. Periodically, when there is a shortage in drying capacity, eluate from the process circuit is shipped to a toll mill for processing.

Loaded IX resin is produced at the MPP from Block 1 pregnant solution. This loaded resin can be eluted and processed into yellowcake at the MPP or transported and eluted at Sat1. The resulting eluate is transported from Sat1 to the MPP and converted into yellowcake.

Loaded IX resin is produced at Sat1 from Block 2 MA pregnant solution. This loaded resin can be eluated at Sat1 and the eluate is then transported to the MPP. Alternatively, the loaded resin can be transported to the MPP for elution. The resulting eluate can be converted to yellowcake at MPP or, if required, the eluate can be transported to a toll mill and converted into yellowcake.

Loaded IX resin is produced at Sat2 from Block 3 MA pregnant solution. This loaded resin is eluted at Sat2 and the eluate is transported to MPP. The eluate is either converted to yellowcake at MPP or, if required, the eluate can be transported to a toll mill and converted into yellowcake.

The MPP has an IX capacity of 2.7 M Lbs U3O8 per year and a product drying and packaging capacity of 8.1 M Lbs U3O8 per year. Sat1 has a nameplate IX capacity of 6.3 M Lbs U3O8 per year as eluate. The current IX capacity of Sat2 has been estimated to be 0.9 M Lbs U3O8 per year as eluate based on the forecasted Block 3 MA solution head grade.

Ion exchange resin adsorption (loading).
Wellfield acid solution, containing the leached uranium (pregnant solution), is pumped from the selected wellfield(s) via pipelines to a settling pond and then to the IX circuits for adsorption of the contained uranium. The use of IX for recovery of uranium from leach solutions is based on the existence of uranyl sulphate complexes. The uranyl sulphate anions are selectively adsorbed onto solid synthetic IX resin beads with fixed ionic sites. The resin bed is retained in IX vessels where resin is contacted with pregnant solution.

Once the resin in an IX column is fully loaded with uranium, the column is isolated from the continuous IX circuit and the resin is retained for elution or transferred with push water to an elution vessel. In the case of the MPP, the pregnant solution can be directed to one of the adsorption column trains. Each train is capable of performing resin adsorption and then operated in the desired mode of elution. In the case of Sat1, the pregnant solution reports to either an adsorption column train or a semi-batch adsorption column. In the case of Sat2, pregnant solution reports to a semi-batch adsorption column.

Resin elution (stripping).
In the elution process, uranium that has been adsorbed onto the IX resin during the adsorption cycle (loaded resin) is desorbed from the resin using ammonium nitrate. The eluate produced from this step is stored in pregnant eluate tanks.

At the MPP and Sat1, loaded resin can either be retained in the vessel for elution or hydraulically conveyed to a vessel specifically designed for elution within the circuit. Loaded resin can also be transferred between the two plants for elution based on available elution capacity. At Sat2, loaded resin is hydraulically transferred from the adsorption vessel to an elution vessel for elution.

Denitrification.
After the uranium has been stripped from the resin in the elution process, the adsorption sites on the resin are initially left in a nitrate form. The adsorption sites on the resin must be denitrified and converted to a sulphate form for re-use in the IX circuit. Denitrification is accomplished by contacting the resin with a solution of sulphuric acid and process water in a denitrification vessel. Each plant has a denitrification vessel to complete this step.

Precipitation.
Eluate from Sat1 and Sat2 is transported to and stored with the MPP eluate before the eluate is directed to the precipitation circuit. Hydrogen peroxide is added to the precipitation tanks to induce precipitation. The pH of this stream is adjusted in the precipitation tank by the addition of anhydrous ammonia.

The precipitation tanks are operated in a cascade configuration to allow the required retention time for the precipitation reaction to proceed to completion. The final yellowcake slurry is discharged from the last tank in the series and pumped into a thickener.

Yellowcake product thickening.
The precipitated slurry from the precipitation circuit flows into a thickener. The contained yellowcake slurry is thickened and is pumped to filter presses.

Filter press operation.
The yellowcake slurry from the yellowcake thickener underflow reports to the filter presses. The slurry is first washed and then dewatered in the filter presses.

Drying.
The dewatered yellowcake from the filter press is then pumped into rotary vacuum dryers where the yellowcake product is produced.

The vacuum dryers are totally enclosed during the drying cycle to assure zero emissions. The off-gases and steam generated during the drying cycle are filtered and condensed to collect entrained particulates and moisture within the process system.

Packaging.
Once the dryer contents have cooled, a measured amount of dried yellowcake is transferred through a rotary valve to a drum. The drums are collected into lots before being shipped.

Toll Milling.
Consistent with earlier production years, if required, eluate can be shipped to a toll mill and converted into yellowcake.

Overall uranium recovery.
The uranium extraction efficiency (recoverability) of ISR operation is determined by uranium loss in underground leaching and in surface production facilities. In 2017, the uranium recovery from wellfield pregnant solutions at the MPP and Sat1 surface production facilities was 98%. An overall uranium recovery, or metallurgical recovery, of 85% is targeted.

At that time JV Inkai was licensed to produce at an annual rate of 5.2 M Lbs U3O8. As of January 1, 2018, JV Inkai is licensed to produce at an annual rate of 10.4 M Lbs U3O8.