The Rössing uranium deposit lies within the central part of the late Precambrian Damara orogenic belt that occupies an area approximately 50 km wide and extends northeast for over 100 km in west-central Namibia.

The Damara lithology consists mainly of folded, steeply dipping metasediments (gneiss, schist, quartzite, and marble) arranged in a northeast-southwest striking belt.

The geology of the mining area at Rössing is associated with a dome structure and occurs in pegmatitic granite known as alaskite, which intruded into meta-sediments. The Rössing ore body is unique in that it is the largest known deposit of uranium occurring in granite. The nature and grade of uranium ore is extremely variable and can be present as large masses or narrow inter-bands within the barren meta-sediments.

All the primary uranium mineralisation and most of the secondary uranium mineralisation occurs within the alaskite. However, the alaskite is not uniformly uriniferous, and much of it is un-mineralised or of sub-economic grade.

Uraninite is the dominant ore mineral (55 per cent); secondary uranium minerals constitute 40 per cent, while the refractory mineral betafite makes up the remaining 5 per cent. Ore grades at the mine are very low, averaging 0.035 per cent. The uranium ore consists of 70-90 per cent alaskite and is subdivided into four ore types, according to the composition of the host rock.

Mining is done by blasting, loading and hauling from the open pit before the uranium-bearing rock is processed to produce uranium oxide.

The open pit measures 3.5 km by 1.5 km and is 390 m deep.

The pit void is mined by a conventional truck-and-shovel operation, with mining being conducted in 15 m benches. Pit ramps are 40 m wide and established at a maximum 10% gradient. The central benches of the pit are generally in excellent condition – a result of good pre-split blasting techniques. The upper and lower benches are in poorer condition as a result of over-blasting, potentially affecting the stability of the pit rim. Nevertheless, the rocks making up the pit walls, despite being heavily jointed, have high strength values. There is also little seismic activity in the area. Sudden rockfalls and failures are thus rare.

Ore is extracted from the hard rock by blasting. The high energy fuel explosive used, HEF 260, is made up on site with 60 per cent High Energy Fuel (ANCN Solution, old oil, diesel, Megas Emulsifier – E21 and sulphamic acid) and 40 per cent ammonium nitrate prills. Blasting takes place on average twice a week, using approximately 150 tonnes of explosives per blast. Up to 30,000 tonnes of explosives are consumed per year.

The stockpiles have a combined footprint of more than 120 ha. The rock dumps’ footprint increased in both the western and eastern areas of the open pit, with waste dump 2 increasing to 1.4 ha and waste dump 7 to 3.0 ha. In general, rock disposal sites are established as close to the major mining areas as possible.

The Rössing Uranium life-of-mine extension (“LoME”) from 2027 to 2036 was the focus of a feasibility study undertaken from June 2021. The Ministry of Mines and Energy approved the extension of the RUL Mining License by 15 years to July 2036, which covers the timeline required for the execution of LoME.

The Phase 4 mining pushback is the lowest cost extension option available to RUL and will benefit from leveraging off the existing processing and infrastructure facilities.

LoME requires deeper mining of the same SJ ore-body through a north-eastern extension of the current pit (Phase 4 pushback) to supply the existing process plant with sufficient ore until the end of 2036 at current throughput rates of 9.2 Million Tonnes Per Annum (Mtpa). There is a need to proceed with this pre-strip as soon as possible (2024) to gain access to the ore in Phase 4 before the current ore supply from Phase 2/3 is depleted. An extension of the tailings storage facility footprint is also required to accommodate the additional 92 million tonnes of tailings, which must be dewatered to a higher density (thickened tailings).

Replacement of the CAT994 Loader
The project to replace one CAT994 Loader was approved for execution in 2022. The CAT994 is replaced with a Komatsu WA1200-6 and is aimed at loading, re-handling ore from stockpiles to feed the primary crusher, etc.

Rössing Uranium employs two CAT994 Front-End Loaders (FE15 and FE16) which can load the 180 tonne Komatsu 730E trucks. These machines supplement the loading fleet of four Komatsu PC5500 face shovels and are primarily used to re-handle ore from the run-of-mine (“RoM”) stockpiles to feed the primary crushers with sufficient material of the correct blend. Two loading units are required to achieve the re-handle tonnes and at times of poor FEL availability, one of the face shovels is moved to the RoM stockpiles to secure the crusher feed.

The Komatsu WA1200-6 has been procured and was delivered to site at the end of December 2022. Assembly to be done in 2023.

Mining Excavator Replacement
The project to replace one PC800 was approved for execution in 2022. Mining operations had two excavators, of which one is on major breakdown and currently not in operation since September 2021. The excavator on breakdown is a Komatsu PC800 (BA07), which was acquired for operations in 2007.

The project is in execution phase and the following has been done so far:
• PC850 assembled
• Fire suppression system
• Automated grease lubrication system
• Additional handrails are underway

- subscription is required.

The run of mine material from the open pit is fed through primary and secondary crushers to the processing plant. The metallurgical process is a conventional acid leach with ion exchange solution concentration and solvent extraction purification, followed by the precipitation of ammonium diuranate and roasting to produce uranium oxide.

At full capacity, the processing plant can produce 4,500 tonnes of uranium oxide each year.

LEACHING
A combined leaching and oxidation process takes place in large mechanically agitated tanks. The uranium content of the pulped ore is oxidised by ferric sulphate and dissolved in a sulphuric acid solution.

The resulting slurry is pumped from the rodmills to the leaching section where it is mixed with sulphuric acid, ferric iron and manganese dioxide in a series of six leach tanks. The first tank in the series (290 m3 capacity) is considerably smaller than the other five (1,450 m3 capacity), thus ensuring adequate mixing of reagents and leach feed.

The steel leaching tanks are rubber-lined and mechanically agitated. Retention time in the leaching section is 8-9 hours at a temperature of 35°C, with uranium extraction of 85-90%. Gases and fumes generated during the leaching process are captured on top of the leach tanks by means of scrubbing units. The reagents used for leaching are:
• Ferric iron to oxidise the uranium from a tetravalent to a soluble hexavalent state. Ferric iron is obtained by reacting iron oxide with 93% sulphuric acid in special Rössing designed reactor vessels. Iron oxide (haematite) is brought in by truck in 1 m3 mega bags.
• 93% sulphuric acid for extraction. Sulphuric acid is imported through the harbour in Walvis Bay, railed to site and stored in large acid tanks prior to being delivered to the leach tanks.
• Manganese dioxide to oxidise ferric iron to ferric. Manganese dioxide ore is delivered to the harbour in Walvis Bay by ship and then railed to site and stored in a storage bunker. It is transported by front end loader to a crushing, grinding and thickening plant adjacent to the leach modules where a finely ground slurry is produced and delivered to the leach tanks as part of the extraction process.

THE SANDS / SLIMES SPLIT
Pulp leaving the final leach tanks flows into a ten-way motorised pulp distributor and thence to 10 hydro cyclones. A sand/slime split occurs here with the slime fraction (cyclone overflow) directed to a counter current decantation (CCD) thickener circuit. The coarse sand fraction (cyclone underflow) reports to one of ten primary rotoscoops. There are 20 rotoscoops in each module arranged as 10 discrete pairs, a primary and secondary unit each, providing a two stage sands washing circuit. Barren solution from the continuous ion exchange plant is used as the wash medium on all second stage units. Washed sands are removed from the second stage rotoscoops by a conventional conveyor belt.

THE SLIMES WASHING – CCD THICKENERS
Slimes (cyclone overflow) washing is carried out using a five stage CCD thickener circuit (see Figure 3.5). The first stage consists of four identical thickeners with the slimes fraction distributed equally to two of them. The third and fourth thickeners are used as clarifiers. First stage thickener underflows are re-combined and progressively pumped through four further stages of thickening and re-pulping, thus five washing stages are achieved. Continuous ion exchange (CIX) barren solution is introduced into the fifth washing stage. This runs counter current to the slime flow, and provides the wash medium taking up the uranium. First stage thickener overflow, called “pregnant solution”, contains uranium (uranyl sulphate), at a concentration of 0.180 g/L.

CONTINUOUS ION EXCHANGE
First stage CCD thickener overflow (pregnant solution) is pumped to a pregnant solution storage tank situated near the CIX plant. Tank discharge is by four pumps, each delivering to one line of the CIX contactors. The Rössing CIX plant is built on the Porter system, which uses the upward flow of pregnant solution to fluidise a bed of ionic resin beads in a series of six contactor chambers per line. The flow of pregnant solution is counter current to the resin movement. There are four lines of CIX contactors with six chambers in each line. Resin transfer from one contactor to the next is carried out by air lifter units of which there are six per contactor. Loaded resin from contactor 1 in each line is transferred to the elution columns. Three elution columns per line of contactors are provided; these take the form of fibreglass lined mild steel pressure vessels. Sulphuric acid (at 10% concentration) is passed through the resin bed, stripping the uranium from the resin beads during its passage. Stripped resin is then returned to the co ntactor line and the uranium rich concentrated eluate is pumped to solvent extraction. The eluate has a uranium concentration of 4 – 5 g/L.

SOLVENT EXTRACTION
Concentrated eluate containing 4 to 5 g/L uranium is pumped to solvent extraction as the aqueous phase of the extraction process . The organic phase is Shellsol, containing alamine 336 and isodecanol. Extraction, i.e. transfer of uranium from the aqueous to the organic phase, is carried out in five stages of counter current contact using Davy Powergas mixer settler units. The loaded solvent is then passed through a two unit clean water scrubbing stage prior to a four unit stripping stage where the loaded solvent (organic) is mixed with a 7% ammonium sulphate (aqueous) solution under pH control with aqueous ammonium hydroxide. Uranium is stripped into an aqueous phase and is pumped to the final product recovery plant as OK liquor (concentrated uranium diuranate solution) containing 8 to 20 g/L uranium.

FINAL PRODUCT RECOVERY
OK liquor, the chemical solution containing uranium trioxide, is pumped to the FPR building from the SX plant. The first stage of final recovery is the precipitation of ammonium diuranate (yellowcake) from the OK liquor. This is carried out in an agitated precipitation tank. Gaseous ammonia is added to raise and maintain the pH of 7.3. Precipitation tank discharge gravitates to a yellowcake thickener. Thickener overflow (ammonium sulphate) is returned to the SX strip mixer settlers while underflow material is pumped to a two stage washing section. Washing is carried out by two drum filters in series equipped with overhead water sprays. Filter cake from each stage of washing is re-pulped with process water. Re-pulped second stage filter cake is fed into one of the two multi-hearth calcining furnaces. Each furnace has six hearths and is heated to 700°C on the final hearth. The yellowcake feed is calcined to uranium oxide and is discharged via a hammer mill to an automatic drum filling plant. Final product at + 98.5% U3O8 is dispatched in sealed drums, each drum automatically washed and dried and weighing + 450 kg.

Gases and calcine particulates generated and emitted from the process are prevented from entering the atmosphere by means of an extraction and dust collection system and two wet venturi type scrubbers.

- subscription is required.