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 currently 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.

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.

Crushing
Ore from the open pit is delivered in 140 and 180 tonne haul trucks to the primary crushers, where two gyratory crushers reduce the ore to less than 160 mm in size. A conveyor belts transports the crushed ore to a coarse ore stockpile with a live capacity of some 80,000 tonnes.

Coarse ore is withdrawn from the stockpile by vibrating pan feeders, feeding directly onto a coarse ore reclaim conveyor. This conveyor discharges the ore to a pre-screening plant where all fines are removed and the coarse material returned to the surge bin ahead of the secondary crushers. The ore is further processed through secondary, tertiary and quaternary stages of crushing and screening, delivering a final product of less than 19 mm in size to the fine ore stockpile. The crushing circuit is equipped with an adequate system of dust extraction and collection into covered lugger bins. There are, in total, ten collection systems that provide extraction points from the reclaim tunnel to the fine ore storage bin.

Grinding
Wet grinding of the crushed ore by means of steel rods reduces it further to slurry with the consistency of mud. The four rod mills, which are 4.3 m in diameter, are utilised as required by production levels and operate in parallel.

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 product of leaching is a pulp containing suspended sand and slime. Cyclones separate these components and, after washing in Rotoscoops to remove traces of uranium-bearing solution, the sand is transported via a sand conveyor to a tailings disposal area.

Counter-current decantation thickeners wash the slimes from previous stages. A clear uraniumbearing solution (‘pregnant’ solution) overflows from the thickeners, while the washed slime is mixed with the sands and pumped to the tailings area.

The clear pregnant solution now comes into contact with beads of specially formulated resin. Uranium ions are adsorbed onto the resin and are preferentially extracted from the solution. Beads are removed periodically to elution columns where an acid wash removes the uranium from the beads. The resulting eluate is a purified and more concentrated uranium solution.

The acidic eluate from the ion exchange plant is mixed with an organic solvent which takes up the uranium-bearing component. In a second stage, the organic solution is mixed with a neutral aqueous ammonium sulphate solution which takes up the uranium-rich ‘OK liquor’. The acidic ‘barren aqueous’ solution is returned to the elution columns.
The addition of gaseous ammonia to the ‘OK liquor’ raises the solution pH, resulting in precipitation of ammonium diuranate, which is then thickened to a yellow slurry.

The ammonium diuranate is recovered on rotating drum filters as yellow paste - known as ‘yellow cake’.

Final roasting drives off the ammonia, leaving uranium oxide. The product is then packed into metal drums. Neither ammonium diuranate nor uranium oxide are explosive substances.

The drums of uranium oxide are loaded and exported to overseas converters for further processing.