grinding media for lumwana copper mine in zambia

7/29/2019 Grinding Media for Lumwana Copper Mine in Zambia

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GRINDING MEDIA FOR LUMWANA

The author is familiar with Scaws forged grinding media. They came to his rescue at

Frontier mine in early 2008, as a stop gap measure. Their ball is made by roll forging

and not hammer forging, as shown below from the prominent seam made by the

forging roll.

FFiigguurree 11:: RRoollll ffoorrggeedd 112255mmmm sstteeeell bbaallll ffrroomm SSCCAAWW,, SSoouutthh AAffrriiccaa

Roll forging does not allow the microstructure to transform into hardened steel; as a

result, misshaped and dimpled balls ensue.

The impact of lack of sphericity on attrition grinding is well known.

FFiigguurree 22:: DDiimmpplleedd rroollll ffoorrggeedd mmiillll bbaallll,, iinnddiiccaattiinngg aa ssoofftt ccoorree


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Because of the brittle nature of a cast ball, breakage in a SAG mill will be high, as

shown below with a medium chrome cast ball, used in a 28 diameter SAG mill.

In addition, a cast ball is more susceptible to induced fracturing as a result of the

variations in extremes of pressures experienced in the toe and shoulder of the

tumbling charge. Being devoid of a crystal structure, they tend to shatter along lines

of imperfection. A forged ball, on the other hand, could accommodate such pressure

variations more ably, as its molecular crystal structure can adjust accordingly.

FFiigguurree 33:: MMeeddiiuumm cchhrroommee ccaasstt mmiillll bbaallllss ffrraaccttuurriinngg iinn aa 2288 ddiiaammeetteerr SSAAGG mmiillll

Over the years, the author has found that at least 50 drops of the forging hammer

are required to make a decent forged mill ball. This allows the transformation frompearlitic to martensitic steel, Figures 4 and 5.

FFiigguurree 44:: PPhhoottoommiiccrrooggrraapphh sshhoowwiinngg mmiiccrroossttrruuccttuurree ooff ppeeaarrlliittiicc sstteeeell


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FFiigguurree 55:: MMiiccrroossttrruuccttuurree ooff mmaarrtteennssiittiicc sstteeeell,, ppoosstt ffoorrggiinngg..

AAlliiggnnmmeenntt ooff ggrraaiinn ssttrruuccttuurree iiss eevviiddeennccee ooff ffoorrggiinngg

A good guide as to the suitability of forged grinding media for a SAG mill is to aim for

Rockwell hardness in excess of 60, or a Brinell hardness exceeding 590.

Should centre hardness be available, that figure must not be less than 65% of the

corresponding surface hardness.

Experience with a high quality hammer forged ball has shown a variation of 10grms

on a sample of 20 balls, weighing 8.0kgs each, Figure 6.

FFiigguurree 66:: QQuuaalliittyy hhaammmmeerr ffoorrggeedd 112255mmmm mmiillll bbaallllss,, wweeiigghhiinngg 88kkggss 1100gg eeaacchh


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A reputable mill ball manufacturer will include such vital details, including an estimate

of breakage and anticipated sphericity,in his specifications, Figure 7.

FFiigguurree 77:: SSppeecciiffiiccaattiioonnss ffrroomm aa rreeppuuttaabbllee mmiillll bbaallll mmaannuuffaaccttuurreerr

Regular inspection of the scats bunker every morning gives a good indication of the

milling process inside the SAG mill and should comprise an essential facet of astute

plant management.

In addition, during a routine shut down, when the SAG mill is ground out, a critical

analysis of the ball charge will give an indication of the quality of the forged balls. A

preponderance of a single size, as shown in Figure 8, is indicative of a soft core.

These two critical examinations are concomitant because the absence of brokenballs could still indicate improper forging from soft cores.

FFiigguurree 88:: SSAAGG mmiillll cchhaarrggee ffrroomm KKaannssaannsshhii MMiinnee,, iinnddiiccaattiivvee ooff aa ssoofftt ccoorree


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If a full spectrum of sizes is evident, plant management can be reassured that a

quality forged ball is in use, Figure 9.

FFiigguurree 99:: AA qquuaalliittyy ffoorrggeedd mmiillll bbaallll,, ssiizzee aanndd sspphheerriicciittyy ccoonnssiisstteenntt

Broken balls often plug the discharge grates of the SAG mill. Repeated hammeringfrom the charge on plugged grates frequently leads to premature peening of the

discharge grates, as shown at Minera San Christbal in Bolivia, Figure 10. This

adversely impacts on slurry flow through the mill, and the ability to evacuate

deleterious scats from the mill. In both cases, mill throughput is reduced.Peening is

exacerbated as ore hardness decreases or slurry viscosity drops.

FFiigguurree 1100:: PPeeeenniinngg ooff ddiisscchhaarrggee ggrraatteess aatt SSaann CChhrriissttbbaall MMiinnee


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Early symptoms of this phenomenon occurring is usually detected in the control

room as:

An increased in mill load

A reduction of sound in the mill microphone, and in extreme cases;

As severe slurry spillage at the feed end of the mill. Seeing such spillage, operatorsinstinctively reduce the density in the mill, thus compounding the problem even

further. A dense, cushioned charge does not break effectively in a SAG mill!

On the job sensitisation of metallurgists, and plant supervisors, on the recognition of

some of these symptoms and causes could pay dividends in the longer term.

Such training could include the intricacies of mil power draw and the relationship with

the mills centre of gravity as shown in Figures 11 and 12.

FFiigguurree 1111:: MMiillll iinn nnoorrmmaall ooppeerraattiioonn


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FFiigguurree 1122:: OOvveerrllooaaddeedd mmiillll

Table 1 shows a comprehensive list of mill balls manufacturers from around the

world, covering cast and forged balls.


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TTaabbllee 11:: MMiillll bbaallllss ccoommppoossiittiioonn ffrroomm mmaannuuffaaccttuurreerrss aarroouunndd tthhee wwoorrlldd

Ramoutar Seecharran

Concentrator Manager

Jabal Sayid Mine

Saudi Arabia

grinding media for lumwana copper mine in zambia

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