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Production Process

Mining

Cannington is an underground mine with access via a decline ramp from the surface. The hoisting shaft allows for a production rate of 1.8 million tonnes of ore a year.

The hoisting shaft has a finished internal diametre of 5.6 metres and extends to a depth of 650 metres. The shaft is equipped with a tower-mounted friction winder and two nine-tonne skips in counter balance running on rope guides. The skips are hoisted from a loading station on the 610-metre level and reach a final hoisting speed of 12 metres per second. On the surface, tipping scrolls in the shaft headframe tip the skips into a surface bin for transfer to the processing plant's stockpile area.

The stoping method used for the extraction of the main, thicker, hangingwall orebodies of the deposit is transverse, long-hole open stoping. Broken ore from the stopes is loaded from draw points at the bottom of each stope and currently hauled to the surface via the decline by trucks. The ore is hauled along the level and tipped into one of several ore passes.

After completion of the ore extraction from each stope, paste backfill is used to fill the open void to stabilise the area and allow for mining of adjacent stopes.

Paste backfill is a high solid density material, around 80 per cent solids and mixed from tailings produced from the processing plant with the addition of approximately 5 per cent cement. The paste backfill is gravity fed underground via boreholes from the surface and pipe work into the open stopes.

Processing

The Cannington mine includes a minerals processing concentrator to treat the silver-rich lead and zinc deposit. The purpose of the concentrator is to separate the valuable minerals (those containing silver, lead and zinc) from the remainder of the ore (known as gangue), to produce the saleable product of concentrates.

The processing plant has been designed to treat 1.5 million tonnes of ore per annum during the 20-year life of the mine. The mill feed is a blend of a number of different lead and zinc lodes and mineralised types with varying silver, lead and zinc compositions.

A number of distinct processing steps are used to produce the lead and zinc concentrates - including comminution, flotation, leaching and dewatering.

Comminution is the process of breaking down or dividing substances. The purpose is to break and grind the ore into fine particles to liberate the valuable mineral grains from the remainder of the ore.

In the flotation process the separation is based on surface tension properties of the valuable and gangue minerals. A variety of reagents are added to render the valuable minerals hydrophobic (water "fearing") and the gangue minerals hydrophilic (water "loving"). Each flotation stage consists of a number of tanks (or cells). Fine air bubbles are introduced into the base of the cells. The hydrophobic particles attach to the air bubbles which float and gather at the top of tanks as a froth. The froth overflows and is collected for subsequent processing.

The leaching stages are used to remove fluorine from the concentrates. A novel process using aluminium sulphate in an acidic environment leaches the fluorite from the concentrates.

Horizontal pressure filters are used as the final stage of concentrate dewatering. Thickened slurry is pumped into a number of chambers, a cake forms and the filtrate is forced through the filter cloth in the chamber walls. The cakes are acid washed (to remove the last traces of fluorine) and then washed with water. In the last stage of the cycle, high-pressure air is blown through the cakes to further reduce the moisture content.