Processing techniques used at Glebe Mines

Ore is processed to produce some 55,000 tonnes per annum of "Acid Grade" fluorspar. By-products include 15,000 tonnes of barytes, 1,800 tonnes of lead concentrate; and 160,000 tonnes of limestone aggregate. Sand and clay products are also produced.

The mill is currently run on a continuous shift basis for180 days a year in scheduled campaigns of 5 days using 2 shift crews. All ore is delivered by road, on weekdays, to a central blending and reserve stockpile. Close manual sampling of individual loads together with rapid determination of grade is necessary to maintain control of ore quality. The standard method of ore analysis is by neutron activation which determines fluorspar, barytes and silica contents separately. A typical mill feed would contain 27-32% fluorspar (CaF2), 10-12% Barytes (BaSO4) and up to 1% galena (PbS); the remainder being "waste" limestone, some clay and other minerals.

These minerals are finely intergrown within the ore and the purpose of the Cavendish Mill process is to first liberate the constituents and then to separate out concentrates of fluorspar, barytes and galena in a high quality saleable form. Discrete particles of single minerals must be liberated from the lumps of ore by crushing and grinding before they can be separated. This process is assisted by differences in hardness of the various minerals.

Ore processing involves 5 main operations comprising:

• Crushing and washing to reduce the maximum size of the ore to 18mm and remove the very fine clay-like material.
• Heavy Media Separation, a preconcentration stage, which exploits the differences in relative density between the minerals in order to discard as much of the clean limestone as possible.
• Grinding which “liberates” valuable mineral from unwanted “gangue”. The aim is a particle size where each individual is either mineral or gangue and is also a suitable size for flotation.
• Froth flotation where reagents are added to the pulp and mineral particles become water repellent .In this way they attach to the surface of bubbles produced in the cell and float to the top. The unwanted gangue remains in the water and hence a separation is effected. The material remaining from the fluorspar circuit is treated further to remove barytes. The waste streams from the flotation circuits are pumped to the tailings lagoon as a suspension of 5-6% solids.
• Thickening and filtering is used to produce a filtercake. The bulk of this filtercake is taken through a further drying stage for transportation as a bulk powder. A small amount of material is dried and sold in bags.