Metallurgy takes over once the ore has been extracted and the process of liberating the valuable mineral components and removing the waste rock. It can then begin it journey to the metals that we use in every day life.

Ore Dressing or Comminution is the process of sizing, crushing, milling and grinding ROM ore (Run of Mine Ore) to the desired size required to extract the valuable minerals. Generally the size of flour. The process also consumes vast amount of energy and there is a limit to how small it can be ground before the cost to produce it outweighs the return.

The Primary Crusher is typically the first stage of crushing after blasting and sizing. The majority of ore minerals are disseminated throughout the gaunge (non valuable waste rock) and need to be small enough to break it free of the surrounding rock. The waste rock goes out as tailings.

It is impossible to recover 100% of the valuable ore. But the Comminution process is in the high percentile of capture.

The benefits of Comminution is its saving on Freight costs. Reduced losses of valuable minerals and reduction on the cost of hydro-metallurgy processing later on.

Typical as blasted ore is 10 cm - 1 meter in size and the required end size 0.1mm (tenth of a mm) or lower.

Stage 1 After sizing though a grizzle, trommel or screen. Primary Crushing is by way of a jaw crusher or gyratory Crusher. The later has a higher throughput of ore Size reduction 10cm -1cm

Stage 2 is through a cone or roll crusher. Size 1cm - 1mm

Stage 3 is via ball or rod mill 1mm - 0.1mm

There are also autogenous mills that use the rock itself to impact on itself and reduces balls and rod. But at the cost of lining replacement. These mills have a much larger radius to assist the impact of the falling rock on itself.

It has been found that reducing the ore size in stages is more economical from a power input level that trying to do it all in one go.

Some of the math a metallurgist will work out are size reduction ratios, energy needs, Critical mill speed and valuable mineral processing losses.

Mill speed is to stop the mill centrifuging. 70% is a good level to aim for and most mills operate between 50% and 80% of Critical Speed and can be worked out using the formula Nc = 42.3 / D 1/2

where Nc is the critical speed of the mill in Revolutions per Min and D is the diameter of the mill in meters. This is very basic and other formula incorporate ball diameter and mill diameter.

Reduction Ratio of Screen is E = % of total feed divided by % of what passes the screen. This is normally set at 80% of what passes or P80

Work Input or mill efficiency / comminution theory normally uses the Bonds work index and has limitation to ore size. While mathematically it works for smaller sized ore it has limits in larger size ore. Rittinger in 1850 postulated the crushing efficiency should be related to the surface area and the rocks particle size is normally worked out at 1.75 the area of a cube and in theory it works but in practice it needs between 100 and 1000 times more energy to crush.

Bonds work index is E= 10 Wi (one div the square root of the product minus 1 div the square root of the feed)

It should also be noted the the bonds work equation is worked out in Microns. so 2 mm is 2000 microns

Typical energy requirements

Coarse crushing 0.2 -0.5 kWh / Ton

Fine Crushing 0.5 - 2.0 kWh / Ton

Coarse Grinding 6 - 12 kWh / Ton

Fine Grinding 8 - 25 kWh / Ton

## Ore Dressing - Comminution - Beneficiation

**Moderator:** Philski

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