Historically open stoping without backfill has been the only mining method used at Niobec. Stopes are planned and designed based on geological information obtained from diamond drilling. The average size of the stopes is about 200 ft in length, 80 ft in width and 300 ft in height, corresponding to the vertical distance between development and production levels. An 80 ft pillar is then left between the stopes.
To extract the ore, the stopes are drilled with vertical 6.5 inch diameter holes, loaded with explosives and detonated. The ore that is broken by the blasting falls to the lowest level of the stopes, where it is gathered and transported using 6.5 cubic-yard loaders and 26 tonne trucks. The ore is then crushed underground to a diameter of 3.5 inches before being hoisted to the surface through the shaft and transferred to the concentration plant.
The continuation of open stoping without backfill following the shift to deeper mining would have greatly restricted the proportion of ore that could be mined versus the material that would have been left behind. The use of paste backfill allows a nearly complete extraction of ore grade material and has created an automatic and important increase in reportable reserves and resources from within the same ore volumes.
The potential to realise additional tonnage from the current mining levels through the use of paste backfill is also being investigated.
The Niobec processing facilities use conventional crushing, rod and ball mill grinding, followed by a number of processing steps to obtain pyrochlore concentrate.
The recently completed expansion project for the concentrator will increase the plant throughput capacity by 24% from its current level to 2.2 Mtpa of ore. Each tonne of ore yields an average of 3.7 kg of niobium pentoxide.
The product is a pyrochlore concentrate with 58% niobium pentoxide (Nb2O5). Processing in the plant includes: crushing, grinding, sorting, desliming, magnetic separation, three types of flotation, leaching, two filtering steps and drying of the concentrate. Fifteen different chemicals are used in processing with daily consumption of these chemicals reaching approximately 35,000 kg for the production of 30,000 kg of concentrate. Water plays an important role in the process with nearly 6,700 gallons/minute being used. This water is recycled at a rate of 85%.
The conversion of niobium concentrate into ferroniobium (FeNb) is completed onsite using an aluminothermic reaction. The process involves a reaction between niobium oxide, metallic aluminum and iron oxide to produce aluminum oxide (slag), and metallic ferroniobium.
Conversion at Niobec is a batch process. A typical batch comprises of 3-½ tonnes of Nb2O5 concentrate, one tonne of aluminum, 675 kg of iron oxide and 675 kg of other products for a total of nearly six tonnes. This combination produces a powerful exothermic chemical reaction that generates enough heat to raise the temperature above 2,200°C, melting the entire load in less than ten minutes. The reaction produces 2,100 kg of ferroniobium, which is crushed and packaged to client specifications and 3,750 kg of slags that are stored in a sealed underground chamber.