A preliminary beneficiation study of the unskarned and skarned phosphate rocks associated with the Mactung tungsten deposit, located in southwestern Yukon, along the border with the Northwest Territories (NWT), was carried out at the University of Alberta. The phosphate mineralization extends regionally, beyond the tungsten deposit and over 50 Mt of unclassified global phosphate resources grading about 10% P2O5 have been estimated within a 4 km2 area around the site. The current Mactung mine plan has focused on the large tungsten deposit and has not considered the potential for simultaneous extraction of the phosphate.
The objective of the study was to evaluate the feasibility of beneficiating the phosphate to a saleable byproduct such as a concentrate meeting fertilizer feedstock requirements. Such a byproduct might not only be an economic “sweetener” to the tungsten mining, it could also partly satisfy Canada’s large phosphate market, which is currently wholly imported. The current sources are from other jurisdictions including some with significant geopolitical and socio-economic risks such as Western Sahara. Retaining the phosphate in the mine tailings may also have costs which include preventing potential environmental liabilities from any leakage into a drainage system.
Parameters tested in the study include the grindability of the ores and the separation of the phosphate mineral from the associated gangue phases. Both the unskarned and skarned ores were hard, with Bond Work Indices of 16.19 and 19.04 kWh/t, respectively.
Processing of low-grade sedimentary phosphate rock containing abundant carbonate gangue phases is generally challenging because of the similar metallurgical characteristics of apatite and carbonate minerals. Comminuted ores were subjected to direct and reverse batch flotations, which are the industry standard methods for processing phosphate ores. The flotation tests were conducted using varied dosages of flotation reagents commonly used in industrial operations to beneficiate phosphate ores. A combination of both direct and reverse flotation of the unskarned ore at a grind size of 83% passing 106 μm produced a phosphate concentrate with 25.86% P2O5, 14.88% SiO2, 0.57% MgO, and 44.64% CaO, at a P2O5 recovery of 56%. A better concentrate with a higher P2O5 grade (28.68%) and recovery (70.9%), 46.98% CaO, and 0.72% MgO was obtained from the skarned ore at a grind size of 86% passing 53 μm. Reverse flotation of both ores using Armeen 18D as a collector for quartz and silicate gangue phases and corn starch to depress phosphate produced dismal results due to higher losses of phosphate into the froth.
Although none of the concentrates met the phosphate fertilizer feedstock requirements of >30% P2O5 and CaO/P2O5 < 1.6, and MgO < 1%, the concentrate from the skarned ore is very close. With further optimization of reagent dosages in a flowsheet involving both direct and reverse flotations, it is likely possible to produce a satisfactory concentrate that can be used to produce phosphate fertilizer.
Further research should be focused on optimizing reagent dosages, reducing reagents consumption, and using new collectors and depressants.