Hello Mohamed,

Am picking up this one as it appears to be an orphan. I provide belong excerpts from papers I had. I am no RoM ball mill expert and hope others will add to this.

RoM Ball mills, like SAG mills, utilise coarse rocks in the RoM feed as grinding media but with 25 – 40% steel balls added instead of 5 – 18 % volumetric filling in SAGs which may become advantageous when the coarse fraction in the RoM feed is depleted.

RoM ball mills have been introduced to avoid the problems encountered with operations involving AG/SAG and conventional ball mills. RoMs are utilised mostly in operations treating ores that are friable.

What mill size would you be looking at? Smaller diameter ball mills (under 5m) may not operate well in RoM mode.

As you know there is little information in the literature on the design and operation of RoM ball mills.

A side by side study: RoM ball and AG mill with F80 around 50 mm and a mill product P80 of around 250um, indicated the OWI is also considerably higher, and although it is known to be biased against finer products, the difference is sufficiently large to indicate that the RoM ball mill is less efficient than AG milling.

The RoM mode had lower breakage rates than the SAG mode at the fine end, but considerably higher breakage rates were observed for the RoM mode at the coarser sizes.

The RoM ball mode appears to have considerably higher breakage rates than the AG mill mode over the whole size range. The higher breakage rates in the RoM ball mill are an indication that the higher ball loads in these mills enhance breakage.Higher breakage rates at the coarser sizes result in higher throughputs and higher breakage rates at the finer sizes in finer grind.

Allocation of costs for the lining, power, and grinding media for a RoM ball mill is higher. Unlike SAG mills where only a small fraction of balls are utilised, a significant amount of balls are consumed in RoM ball mills. The presence of too many large balls in the mill results in a coarser grind and high energy consumption which is wasted mainly on ball-to-ball and ball to-shell contacts instead of the actual grinding action. 

Which Work Index would you use? I do not know.

Your ROM is not to be confused with Single stage semi-autogenous / autogenous (SAG/AG) milling which has become a common design for a variety of ore types with the advantages of being comparatively low capital cost and flexible for future expansion.

As for going from D80=100 mm to P80=120um in a single stage ROM ball milling? I am doubtful such an 800:1 reduction ratio is efficiently possible from 1 machine. Can you really expect a mill with 25-40% ball of 125-150mm in diameter to arrive at 120um?

Best get the paper:

https://www.researchgate.net/publication/43498071_RoM_Ball_mills_-_a_comparison_with_AGSAG_milling

http://espace.library.uq.edu.au/view/UQ:161655

This was a SAG 2006 paper for which Alex Doll could get you the proceedings.

Thank you very much dear  david:

I have two  studies case

1) P80 = 100 mm, P80 = 120 microns, Wi = 9.3 KWH/TT Ax B = 40

2) P80 = 100 mm P80 = 120 microns Wi = 16.8 Kwh/TT AxB = 25

 It's possible choose ROM ball for these two cases?

Is this for the calculation I only use BWI or I have to use to calculate the specific energy each range size CWI 100 mm to 10 mm), RWI (10 mm - 2 mm) and BWI (2 mm to 120 microns to calculate the power required to move from D80 = 100 mm to P80 = 120 microns ?

The minimum size possible if i understand is to 250 microns using ROM ball mill  ?

Best regards

Hi Mohamed - I can't disagree with anything David has said.  I see an increasing interest in ROM ball mills.  Their popularity is due to the lower capital cost, i.e. one primary crusher, one mill feed conveyor, one ball mill (ROM Mill) and you are good to go - or so it seems.  They are particularly attractive if you have a high clay content in the ore and you want to avoid the challenges of screening and crushing, particularly if there is any moisture in the feed material.  The problem is that the ROM mill is very inefficient and you end up with very high circulating loads, which you need to consider in the specification of pumps, piping, and cyclones.  Most people under-size these, and when you go to commissioning you can't meet your grind targets due to pump/cyclone limitations.  My design philosophy has always been to design for the lowest operating cost/ton, but with small operations and/or a short mine life, these considerations tend to get tossed out the window.  I'm not saying to not go with the ROM mill, just be aware of the short-comings, as highlighted by David above.

In regards to using the Bond Work Index.  Remember this is an empirical derivation, and one of the constraints is that the particle size distribution for the feed and product needs to be parallel, i.e. this is why it doesn't work for SAG mills.  You might find the same for a ROM mill.  So use it with caution.

Best Regards

Andrew