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sreid (Electrical) (OP)
21 Feb 05 14:27
At another web site someone asked about modeling a SAG Mill.  Not knowing what this was I googled it-a rotating rock crusher.  What struck me was that the rock crushing seems to be about 5% efficient.  Dosen't this beg for a new solution with a much higher efficiency?
Dean78au (Chemical)
21 Feb 05 21:48
I personnally have not seem a SAG this inefficient.  If you use very soft rock in a SAG mill it will not work as well as if you use harder rocks.  Adjustments to the lifting plates around the circumference of the mill provide greater lift and drop and thus greater efficiency.  SAG mills are often used as regrinding mills after a fully autogenous milling.  Thus giving them a lower efficiency as they are grinding very small rocks to start with.  Also depends on the medium in the mill.

There are many other solutions however if the efficiency of a SAG mill is not what you are after all of the other options cost more in terms of maintenance and materials (eg. pure ball mills, hammer mills and rod mills).

AG and SAG mills are cheap to operate and some inefficiency is okay due to cost/benefit.
ruble3 (Mining)
25 Feb 05 15:05
Having been in a few process plants with SAG mills & they are inefficient- as low as 1% - since power costs are becoming a big issue there is lots of research being done ( try googling under SAG Mill efficiency) -technically I wouldn't consider them a 'crusher'- they are grinding mills and some mill people might take offence- note that SAG mills are high compared to length-this is so the 'fallng' action of the rock helps to break it up,[SAG being semi-autogenous (by adding large steel balls) grinding as opposed to an AG (autogenous) but usually very large horsepower - one of the maintenance issues with SAG mills is when doing liner changes the balls get imbedded in the liner - when removed and sit for a while, the steel balls can explode!

sprintcar (Mechanical)
11 Mar 05 8:29
ruble3 - I'm curious about your statement:
<one of the maintenance issues with SAG mills is when doing liner changes the balls get imbedded in the liner - when removed and sit for a while, the steel balls can explode!>

Can you share the mechanics of this? Is it due to continued compressive stress from the liner?  I'd appreciate any info or links to papers on this.
Thanks!

Keep the wheels on the ground
Bob

ruble3 (Mining)
11 Mar 05 16:34
sprintcar: I'm not sure that the'mechanism' has ever been determined - I remember being at a gold mine where a 1200 hp SAG mill was installed- the oversize went to a small gyratory crusher, and often a steel ball from the SAG mill somehow made its way into the small crusher - when we went to do a liner change on it, the welders were asked to cut the ball out-they wouldn't refuse but were very wary- sometime before I arrived to work there, the environmental manager had put a used ball on shelf on his desk- when he came to work the next day his office was basically destroyed - the ball had exploded-seems to me a couple years ago I did find a research paper on it - I'll try & find it - it's a rare occurence but does happen

Backfill (Mining)
29 Mar 05 1:37
sreid,
Careful about the efficiencies, they can get called out in different ways, and especially can be misleading when comparing different types of mills.

My most comprehensive literature is twenty years old:

N.L. Weiss, 1985, SME Mineral Processing Handbook, Vol. 1,ISBN 0-89520-433-6.

Browse through Sections 3A (General Aspects of Comminution) and 3C (Grinding).

SME (now Society of Mining, Metallurgy, and Exploration) can be located most readily at www.smenet.org.

Anyway, SAG mills can be calculated as inefficient, except they do not require a large grinding load like ball and rod mills.  Capital and operating costs have to be compared, system vs. system in order to assist with the selection.  The feed suitability vis-a-vis the grind method is critical.

As far as the balls, and rods for that matter - wear hardening is a fascinating thing.  Yeah, they can blow up.

Ruble3, nice to see you again.  I'm bemused that it was an environmental manager who smoked his office.  The milling operations I am aware of have been careful about allowing used balls out as souvenirs.  Sometimes that spalling, especially on old cast balls, can get fairly energetic.

Have a safe shift.
ruble3 (Mining)
30 Mar 05 8:23
Glad to be here! That particular incident happened in the early nineties-I hadn't worked in process plants before & was skeptical until I heard it from the horses mouth -still not sure why it happens - only refernce I could find was a case on a legal website where a family sued the mill after a worker was killed by an exploding ball - they blamed a 'manufacturing defect' ?? -still looking

tomrivet (Chemical)
1 Jun 05 0:59
We have exploding balls quite a bit. The current thoughts are that the outside of the ball gets work hardend at a high temp (say 40C). Then the ball shrinks as it cools...
arunmrao (Materials)
1 Jun 05 13:01
The balls explode due to builtup internal stresses. There is a volumetric expansion as some of the austenite has not completely transformed into martensite during heat treatment. It is this residual austenite which causes explosion of the balls. I have seen crazy things happening,imagine on a dark night!!
TurinShroud (Mining)
1 Jun 05 16:59
I work at a copper mine that has a SAG mill with 5" balls.  When we go in the mill for liner inspections, you can feel balls exploding underneath your feet.  Last year my boss got cut in the ear by one.

I always thought that the balls exploded due to the internal stress generated by the outside cooling (and shrinking) faster than the inside.  If we wait long enough before entering the SAG mill, the balls are no longer cooling and stop exploding.

Just yesterday, one of the operators said the balls can explode even after a long time.  I had never heard of the balls exploding after they have cooled down.  Has anyone else out there ever heard of the balls exploding after a long time?

If the exlosion is due to differential cooling, I just don't see how they could explode after they have been out the SAG mill for more than a few hours.
sreid (Electrical) (OP)
1 Jun 05 19:40
The exploding ball story seemed so bizzare that I asked about it in thread330-119832  Metalurgical change of phase was suggested.
jraef (Electrical)
1 Jun 05 20:32
Wow, I learn something new from this website every day...


Exploding balls. Who'd a' thunk it.
jraef (Electrical)
1 Jun 05 20:36
I guess it's enough of a problem that someone has come up with a solution.
http://www.donhad.com.au/forgedballs.html

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."   
Nikola Tesla

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