Historical Plant Operations
The historical production from the Project was approximately 3.3 million tonnes processed over an 11-year period producing 900Mlbs of zinc, 53Mlbs of copper and over 50,000 ounces of gold at reported recoveries of 94% (zinc) and 78% (copper). The mine was closed in 1999 due to a sustained period of low zinc prices and the plant, mine and associated infrastructure decommissioned and dismantled.
Figure 11 – Proven Historical Flowsheet (Inmet 1999)
The previously installed mill consisted of a two-stage crushing circuit followed by a two-stage (rod and ball) milling circuit. A copper concentrate was produced in the first stage of flotation where the copper rougher concentrate was reground before the final stages of cleaning. The copper rougher and cleaner tails were then conditioned and a zinc concentrate produced in this second stage of flotation. Again, the zinc rougher concentrate was reground to improve final concentrate quality. Concentrates were thickened before being filtered in two Larox pressure filters. The ore was processed at a rate of 1,000tpd and approximately 250 to 350tpd of concentrates were produced which was trucked to a rail siding in the town of Schreiber and loaded onto rail cars for shipment to smelters. Figure 11 shows a schematic of the process flowsheet that will be adopted by Superior, that was successfully used over the 11 years that the Project operated.
The past production was successful and has clearly demonstrated that the
mineralisation is amenable for processing using conventional flotation technologies and that the valuable metals are effectively recovered as either a copper or zinc sulphide concentrate. Whilst past production was successful, the Company cautions that past performance at the Project is not a guide as to future performance, and that for the Company to successfully restart operations at the Project, further studies and evaluation work is required to be undertaken before the Company will be in a position to estimate any ore reserves or to provide any assurance of an economic development case. There is no guarantee that such work will result in the declaration of an economic development case at the Project.
Other key process information identified include:
• Variable Bond Work Indices (BWi) of the ore, ranging from 11.0 to 18.4kWh/tonne, but more consistently between 11.0-14.0kWh/tonne
• Primary grinding circuit target grind size is p80 74 microns (200 mesh)
• Copper regrind product size is p80 20-30 microns
• Zinc regrind product size is p80 35-50 microns
• Zinc conditioning an important to depress pyrite and pyrrhotite and minimise iron grades in zinc concentrate
• Zinc depressant in copper circuit important to minimise contamination