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Autogenous grinding involves the use of coarse ore particles as media. The predominant grinding mechanism in autogenous grinding is attrition. Very little grinding is by impact. For an ore to be ground autogenously its specific gravity must generally be at least 2.5 and grain boundaries must exist at the desired product size. The competency of the ore (its ability to hold together) is important.
The practice followed thus far differs between U.S.: and European designed mills. The diameter of U.S. built autogenous mills is frequently 2 to 3 times largesr than its length wile many European built autogenous mills have roughly the same diameter as length.
These are sometimes referred to as “square mills”. The ore charge is usually 25% to 35% of the mil volume depending on ore hardness.
Autogenous mills use a grated discharge because coarse ore particles must be retained in the mill as media. They would be discharged from an overflow mill. Also, an autogenous or semi-autogenous mill must have a low discharge level so that the weight of the ore can be used to grind the material. If an overflow design were used, ore would float out of the mill with minimal grinding action imparted to it.
In autogenous milling, media hardness varies as ore hardness can be substantial. Thus, the determination draw is not as straightforward as in a conventional rod or ball mill. However, some generalities can be stated
Power varies as the mill diameter to an exponential power of 2.5 Power increases as the length until the length becomes as great as the diameter.
Then the effect of length diminishes. Autogenous power also increases as ore specific gravity, mill RPM, feed size and percent solids increase. The power draw of a mill is affected by the design of the grate openings and by the design of the lifting compartment.
Autogenous grinding requires less comminution equipment than its counterpart crusher /rod mill method. Eliminated are some m ills, crushers and materials handling equipment needed in a conventional circuit along with
the maintenance of these items. However in spite of the reduced number of physical components, an autogenous circuit will often require a larger initial capital outlay than conventional rod mill/ball m ill grinding due to the higher cost of the most expensive pieces of equipment, the mills grinding due to the higher cost of the most expensive pieces of equipment the mills. In addition, power consumption and liner consumption and liner consumption per ton of ore ground tend to be higher for autogenous mills than mills and crushers in a conventional circuit. Savings potential in autogenous grinding usually lies in elimination of media costs (rods and balls) and in the upkeep of less equipment. Whether the metallurgy is improved or hindered and whether the flow sheet is more or less complicated than with conventional grinding depends on individual applications.
Sizing and evaluation of an autogenous circuit may require analysis based on pilot plant test. The larger the pilot plant, the better the chance for reliable extrapolations of the test data.
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