capitulo 9 - molinos autogenos y semiautogenos

7/30/2019 Capitulo 9 - Molinos Autogenos y Semiautogenos

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Chapter 9. Autogenous and Semi-Autogenous Milis

9.INTRODUCTION

Disintegration and size reduction of sorne ores is possible in tumbling milis without the aid ofgrinding media. Grinding milis in which comminution takes place without grinding aids are

known asAutogenous Grinding(AG) mills orFully Autogenous Grindingmills (FAG). Thesemilis use large lumps of rock as the grinding media. Mills that use intermediate size rock or

pebbles as a grinding medium are also autogenous milis but are known aspebble milis. Milisthat grind hard ores with fracture characteristics that do not lend themselves to fully

autogenous milling are charged with a small amount of steel balls to assist in the size

reduction. These are known as Semi-autogenous Grinding (SAG) milis. In the miningindustry all of these types of milis are in use.

The disintegration and size reduction of ores in AG/SAG rnills is brought about by a combination of impact, attrition and abrasion forces during mili rotation. Parteles at the toe of

the mill charge receive the maximum impact forces from falling rocks and other grinding

media. Particles in the body of the rnill charge partly slide from di:fferent heights and are

subjected to attrition and abrasion resulting in size reduction.The operation of AG/SAG milis therefore involves the use of cheaper grinding media as a

replacement for expensive steel balls and rods which greatly affect the wear on liners. They

are therefore less expensive to operate. It is necessary that the ore should provide a sufficient amount of lumps that would Iast for a reasonable time to act as the grinding medium. Such

ores have been described as competent ores. Ores that break up easily are referred to as either

non-competent or incompetent ore.

In recent times these milis have successfuliy replaced the conventional rod mili-baH mill

configurations. InAustralia the number of AG/SAG mills increased from about six in 1984 to

more than 40 in the following five-year period [1]. The number ofinstallations world wide by

mid 1989 was about 471 and presently the number is growing. One of the main interests in

these mills is the possibility of eliminating at least one crushing stage from the conventional

size reduction processes. Such replacements lead to savings in capital expenditure in a plant

design. The SAG milis have been mostly used for milling hard gold and copper ores with

quartz, ultramafic or green stone as host rocks. The Bond Work Index of these ores range

between 12 -14 kWhlt. Subsequently SAG mili application has been extended to softer ores,

like bauxite and clayey hard-capped gold ores.This chapter examines the developments in the design and operation of both autogenous

and semi-autogenous systems of grinding, for size reduction in metallurgical operations.

9.1. Design of AG/SAG milis

Since the breakage of ore in AG/SAG milis is mostly due to impact on particles and media

from a height, these milis generally have a large diameter to length (D/L) ratio. However this

ratio vares and mills with large Iength to diameter ratio are also in use, eg. in South Africa.

At present the three types of milis commercially rnade have the following characteristics:

Mills with large DIL ratio, (High aspect milis) D/L = 1.5 - 3.0


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oversize (scats)

crusher

feed product

feed

product

Product

Feed

ball mill

AG/SAG mill

Product

Feed ball mill

AG/SAG mill


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product


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Rateofbreakage,

h-1

100

10

55%

170%

0.1

0.001 0.01 0.1 1 10 100

Size, mm


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%R

atedpower

80

70

60

50

40

30

20

20 25 30 35 40 45

% Volumetric loading


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Surface of

charge R

H


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RelativeMillPower

0.9

0.8

0.7

0.6

JB = 0.12

JB = 0.06

0.5

0.4

0.3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Fractional mill charge, JC


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capitulo 9 - molinos autogenos y semiautogenos

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