Why crushing and grinding is important

Gold Miners jaw crusher

Gold Miners jaw crusher

Gold-containing rocks are crushed for several reasons. Firstly crushing reduces the physical size of large rocks, exposing more surface area of rock, thus exposing any gold that may be in the rock and increasing the probability of obtaining the gold from the rock by gravity concentration or by leaching with cyanide. There may be several sizes and shapes of gold grains in the rock—round, flat, coarse or tiny. Because of this, several stages of crushing or grinding can be required. The physical dimensions, the mineralogy of the gold and gangue minerals, and the hardness and internal structure of a rock determine how the rock will break. Rocks are broken by the addition of energy in the form of an impact (hammer, crusher jawr, etc.) which results in smaller particles, plus heat, noise and dust. The crushing or grinding effort is directly proportional to the following factors:

  • rate of ore feed
  • feed rock size
  • desired product size, and
  • hardness of rock

Comminution is the term used to describe the mechanical disintegration of a rock. This can be done by crushing (coarse) and grinding (fine), or alternatively just by disaggregating clumps of soil or clayey materials.

Common rock types in mining have very a wide range of crushability (i.e. the ease with which a rock is crushed). The table below shows a number of rock types and their relative crushability. As one can see, rocks with high amounts of quartz are harder than many other rock types, and thus require considerable effort to crush:

Type of Rock Hardness
Basalt

Verv hard

Hard

Soft

Granite
Quartz/quartzite
Copper ore
Hematite (iron ore)
Sandstone
Pvrite
Gypsum

Secondary factors to consider when crushing are the desired size of the crushed material and quantity of material to be crushed (“feed rate”). The desired size of particle determines the number of stages of crushing (primary, secondary and tertiary) and the type of machine to be used. The quantity of material entering the crusher determines the size of that particular machine to be selected. The type of crushing or grinding method also impacts the way in which the rock is broken during crushing or grinding. There are four typical stages of comminution:

  • Primary crushing – crushing of large rocks fresh from the earth
  • Secondary crushing – crushing of rocks after primary crushing
  • Tertiary crushing – final sizing of rocks by crushing
  • Grinding – grinding of crushed rock to very fine powder

Relatively coarse fragmentation of the rock is accomplished by machines called “crushers”, whereas the machines used for fine fragmentation or grinding are called “mills”.

Comminution can be achieved in several ways:

Batch communition: The material is fed all at one time, and the comminution ceases when the material is discharged. It is not a continuous method.

Batch processes are simple and easy to operate, but are not very efficient as the miner has little control on the operation. The machine is turned on when the crushing or grinding begins and is turned off when the desired particle size is achieved.

Open circuits discharge all of the ground material to the concentrating device, whereas closed circuits use screens or other classifying devices to send coarse particles back to the crusher or mill.

Closed-circuit crushing and grinding is the method used by all large mining operations. The advantage of recycling the coarse material retained in the screen (or hydrocyclone) is that only the particles of gold that are sufficiently liberated will pass through the screen and be concentrated. This limits overgrinding as well as achieves the best gold liberation size. When no information about the gold liberation grain size is available, miners should test different grinding times using a small ball mill and gravity separation (e.g., a gold pan) to evaluate liberation (see “Using a small ball mill to investigate gold liberation” in Chapter 2, above). Testing for optimum gold liberation will definitely improve gold recovery. In order to reach the best liberation size, comminution equipment must work in closed circuit with classification (e.g., screening) processes. Unfortunately most artisanal gold mining operations conduct their comminution process in open circuit, without any classification.

Comminution is the most costly operation in mineral processing. The type of comminution equipment to be demonstrated to the miners must follow these criteria:

  • Equipment must not be complex (the technical knowledge necessary to operate must be simple)
  • Equipment must be easily accessible (preferentially locally manufactured)
  • Equipment must be inexpensive and able to be locally maintained.

Crushing

Different types of equipment are best for each stage of crushing. Crushing typically occurs through the pinching of a rock between two metal plates (jaw, gyratory, or cone) or through the impact of a metal surface on a rock (hammer mill and stamp mills). The following are common types of crushers used in gold mining:

Jaw Crusher

Jaw crushers operate by means of a moving jaw suspended from an eccentric shaft driven by a diesel or electric motor via a large flywheel. Feed size to a jaw crusher can range from 50 – 300mm; discharge sizes are between 10 -70mm. Jawr crushers are typically sized by the size of the largest feed panicle—if the rocks are large, the crusher must be large. Jaw crushers are very common in artisanal gold mines. A small crusher operating in batches of 5 to 10 tonnes of ore and handling 500 kg/h to reduce it to -1/4″ (6.5 mm) is suitable for most artisanal mining operations.

The energy consumed by a jaw crusher is usually 0.5 kWh/tonne. The maximum feed size for a jaw crusher should be about 85% of the opening gap. The specifications of a small jaw crusher to crush ore from 50 to 9 mm, with maximum capacity of crushing 0.5 tonne/hour is given as follows:

Type of Small Jaw Crusher: 6″x 3″

  • Jaw Opening: 6″ wide x 3″gap
  • Max Feed Size: 2″ (50 mm)
  • Jaws: Ni-hardened steel
  • Jaw Profile: Ribbed
  • Crushed product Size: nominal passing -9mm
  • Drive V-belt drive, 275 rpm
  • Power: 2.2 kW
  • Shipping Weight 220kg
  • Price (in Zimbabwe): ~~ US$ 5000

Gyratory crusher

Gyratory (and cone) crushers operate by rotating a mantle within a cone shaped bowl, similar to a mortar and pestle. Coarse material, typically between 500 – 1000mm is introduced from the top into the crushing chamber. The width of the chamber decreases towards the bottom of the crusher and fine material falls through the gap between the mantle and the bowl. Discharge sizes can be 50 – 200mm. The mantle is typically mounted on an eccentric base driven externally by a horizontal shaft. This is an expensive crusher. Most large mining companies use this type of gyratory crusher for feed rates above 100 tonnes per hour of material and when they want to reduce the size of large pieces of rock (as large as 1 meter) to 50 mm in one step.

A variation of a gyratory crusher is the impact crusher. This crusher usually uses rock-to-rock impacts to crush the ore. The most successful machine using this principle is the Barmac, manufactured by Ciros. The transfer of energy from the spinning rotor to the particles is very efficient, resulting in production of large quantities of fine particles.

The percentage of fines produced can be altered by changing the rotor tip speed, chamber configuration, rotor size, cascade ratio or feed gradation. The residence time of particles in the crushing chamber can range from between 5 to 20 seconds. During this time each particle is subjected to hundreds of particle interactions from both coarse and fine particles resulting in cleavage, impact, abrasion and attrition of the particles. The crushing action of the Barmac allows it to liberate minerals, or to preferentially crush deleterious material without over-crushing the valuable minerals. Barmac has various sizes of crushers including small pilot-plant type of machines that can be useful for artisanal operations, but this crusher is usually best for tertiary crushing and therefore the maximum feed size is usually small (e.g. below 2 cm). Unfortunately, these crushers are expensive but there are local versions of this type of mills in artisanal mining regions.

An example of an impact crusher developed in Zimbabwe is the Clarson Impact Mill. It has smaller (capacity of 2 tonnes/h) which may be affordable for artisanal miners (around USS 6000). This crusher rotates at 3000 rpm with power of 5.5 kW. However, the rapid comminution (from 20 to 1 mm) is obtained at the expense of high abrasion of the equipment walls. Although it probably has a small degree of material-on-material abrasion action, it is very similar to a hammer mill and wears quickly.

Roll Crushers

Roll crushers are typically used for the crushing of coal and friable rocks such as limestone although they can be used in primary and secondary crushing of hard rocks. This crusher is good for sticky material and works by trapping rocks between the teeth of opposed rolls – feed rock is drawn through the rolls like a wringer and discharged through the bottom of the crusher. Rolls rotate at between 60-300 rotation/min and are spring mounted to allow harder rocks and steel to pass through the crusher without breaking the crusher.

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