Lets calculate/select/pick a new ball for your SAG mill. Now, lets suppose you have a SAG Mill and Ball Mill that are not drawing full kW Power. The SAG Mill has a “small” circulation load of oversize given be the 3/8″ trommel screen and the 1/2″ x 1 1/4″ discharge grates. The load of 4″ balls in this SAG is “good” but un-measured. SAG Ball consumption is “average” at 400 g/t while Ball Mill grinding media (3″ balls) usage in 550 g/t.
Yet, you add steel and nothing seems to happen, tonnage is not noticeably improved.
You need more tonnage and could actually use a larger/coarser transfer size from the SAG to Ball Mill. The ore is “hard” and “bulky”.
SAG liner/lifter design is good are the liners are made of high chrome steel and do not have a history of cracking and breaking under a 4″ ball impact.
How do you know if a larger SAG ball will increase your tonnage and how to select it?
When you crash-stop the mill, do you see a lot of pebbles in it?
- If the mill is choked with a lot of pebbles, then yes, larger balls will help to break those pebbles.
- If there are few pebbles, then bigger balls will give worse grinding action, and you might even want to suggest charging smaller balls, not larger.
SAG mills can operate with balls bigger than 4″, yes. But what job do you need these balls to do? Typically bigger balls give you more
crushing action and less attrition/abrasion action. Bigger balls are more efficient against rock has high impact/rod work index and lower ball work index values.
- Ore properties (hard/soft/bulky)
- Level of impact breakage in the mill (speed, lifter design, ball load, mill noise/sound level)
- Grate size opening (do I suffer slurry pooling)
- Circuit priorities (higher tonnage / finer grind)
Compared to 4″ grinding media, a larger 5″ ball will:
- Coarsen the mill discharge size (increase that transfer size)
- Increase potential for liner damage/higher wear rate (here is the critical risk)
- Provide a lower number of impact breakage events due to the lower number of balls
- Provide higher energy impact breakage events
In a way, the media size should be selected based on feed size and grate opening. Finer feed (i.e. secondary crushed) justifies a 4″ ball but if the grate opening is 60 mm x 100 mm slots then a 4″ ball won’t last very long.
If the feed top-size is >100 mm, then typically a 5″ ball is suitable. To date, I have not seen any clear evidence of improved throughput from a 5.5″ or 6″ ball size alone (combined with lifter profile changes, yes).
Ball load is also a factor, as you can operate with a higher ball load of smaller media without risking high ball wear and/or liner damage.
So going to a larger 5″ ball is a good idea for primary crushed feed with a relatively large grate opening if there is evidence the 4″ ball is not enough to break the coarse particles. Another option is a blend of 4″ and 5″ media if this is possible.