It is of concern that SAG Mills have such poor energy efficiency of SAG/ROM-ball mills. I recently came across a paper in Minerals Engineering (Minerals Engineering 24 (2011 1053-1061) that I had previously overlooked. This paper (Energy efficient comminution under high velocity impact fragmentation) demonstrated, in principle, that if particles can be impacted at velocities of a few hundred m/s, the energy to achieve a given grind size can be reduced by a factor of two three. There are comminution machines available that can achieve these velocities based on high-intensity air turbulence. Notable examples are the DevourX and Hi-tec vortex grinders. Some performance data on the DevourX was presented by at the recent MEI Comminution 14 Symposium. However, these technologies, to my knowledge, have received very little attention in the scientific literature. It would be great to persuade the suppliers of these high-intensity devices to scientific investigation

At the tonnages of modern operations and with the gross amount of energy that must be transferred, the key issue is always going to be wear. Autogenous processes have a future. Non-autogenous processes do not. I just recently worked with an operation using VSI crushers (shoe and anvil type) that were going through wear parts in less than a shift (!!). Technically the VSI reduced particle size, but obviously in a non-viable way. I know nothing about the Devour-X, but from the video it certainly looks like there is rock-to-impeller contact and rock-to-wall contact at high velocity, and that alone suggests to me that it has limited viability.

To some degree FAG's have failed because we can't get enough energy intensity into the ore. Thus the almost universal trend from FAG's to SAG's, along with the migration to ever increasing ball sizes, but even these solutions fail to some degree. Virtually every SAG circuit ends up reincarnated as a SABC, with emphasis on the "C". Crusher energy is not incidental. When you see a circuit designed with 2.5% of the energy coming from a low-tonnage crushing circuit, you can make good money betting that that there will be another capital project in the next five years to raise the energy contribution of "C" to 10%. Again, it is all an issue of energy intensity, which is higher in the crusher but ends up giving you high capital and maintenance intensity per tonne treated.

The question then is how you get energy intensity into rock without any significant contact between the rock and the machine. A SAG uses low intensity energy storage (lifting) followed by more intense discharge (falling and impact upon the rock "toe"). In that regard it minimizes machine-on-rock contact during the event in which the energy is "consumed". The beauty of solutions such as HPRC is that it has the same effect without being constrained by the limits of gravitational energy storage. Energy is stored in the form of strain on the rocks, which is gradually imparted by the rolls. It discharges in fracture propagation, so that the face of the rolls crusher is taking only a minimal amount of damage.

We can count on the following - that the emerging processes will be to a large degree autogenous; and that they will impart more energy intensity than we can currently achieve by gravity, since that is clearly a limiting factor in current design. I do think that the day of the SAG is done. The "size" (volume) goes up as the square of the diameter, but the energy intensity (gravity*height) goes up only linearly as a function of diameter. Thus, to double the impact from where we are now you need an 80 foot mill. Compare this to a smaller rolls crusher that uses a different mechanical principle, or an autogenous VSI that is limited only by how quickly you are willing to turn the shaft. Such units allow you to de-couple energy intensity from size. It seems evident to me how this ultimately will end.

SAG mills are quiet reliable solution in cases when the mill has to "wash" clay out or "melt" snow and ice coming with ROM ore. As far these 2 factors are not subjects of human influence - SAG mills have clear and predictable future.

What are the advantages of SAG mills vs high pressure rollers? I can only think of throughput as the only good point of using SAG, but how do wear and energy use compare between the two?

Although my experience operating SAG mills is very limited, I feel that the introduction of very large SAG mills has changed operator mentality away from process optimisation (grades and recoveries) to simply maximising throughput (inevitably at the expense of flotation performance). I predict that the next generation will be strongly influenced by this and will continue this trend.

With diminishing ore grades, I cannot see the SAG mill taking a step backwards any time soon as operations will be forever driven to increasing throughput in order to cost effectively recovery the ever dwindling metal content of the material. I am also not convinced that an alternative currently exists (or if it does exist is trusted by the industry) to very large SAG mills combined with very large tank cells to treat very high throughputs. Okay, recoveries maybe low at 80-85% however reducing throughput in order to increase recovery may well render these operations uneconomical.

I do believe that pre-grinding beneficiation will become available (or widely accepted) in the future, however this will simply allow the mine to excavate more material while maintaining the same mill throughput. My verdict; SAG mills are going nowhere.

Of course in future the comminution will be spend low energy. But how much? Now in industry is the lowest energy in rolling mills. Theoretically, the lowest energy in the microwave mills. If you grind the gold- quartz ore for liberation the gold particles in the microwave hypothetical mill, the specific energy consumption will be 0.08kw/t!

Size of valuable minerals in the ore is reduced every 10 years. If in 80-s ore more milled to 80% -74micron, in 90-s -44micron, in 0-s -20micron, now grinding limit is reduced to 2micron. Even finer grind impossible. The particle size of 1 micron have 100% plastic deformation. At the same time, geologists found in ores of valuable minerals in size of 10 ... 1nm or even less. How to liberation these minerals? I think it is the task of next generation of comminution. For this necessary known new nature Law for nanoparticle.

SAG designs now are at an all-time high in size, I am aware of 14m (46') about to be delivered now.

The ball makers are also keeping up with new designs that will allow larger media with low breakage in high impact environments.

There does seem to be a trend of SAG mills being changed into high aspect ball mills, we have encountered a few in recent times. This has brought about a new ball design to reduce sprawling as the higher ball charges are causing increased ball on ball incident.

So perhaps we should not evaluate SAG mills in the traditional configuration of perhaps a +/-12% ball charge and a <20% total charge in a high aspect shell design. Yanacocha for example, has a 19% ball charge and a 21% total charge - Newmont has explored every other possibility with the final result of staying with this loading as it is the only way to meet target.

The point being that you have a mill that has great versatility, even to deploying it to a single stage milling process. The technique may change, but the machines are probably here to stay.

I think now there is no alternative to the SAG. The main drawback of SAG - high power consumption. To reduce power consumption, I think:

1. The right determine of SAG feed size. This depends on the mill diameter and the ball size. Feed size necessary to take 80% of the maximum piece of ore that will crushed by the balls.  For the big SAG more than 8 meters optimal feed size will be - 50mm. Ball size - 120 ... 125mm. Do not a pebble crushing, the need secondary crushing.
2. Reduce the power consumption for the drum rotation of the mill. The power consumption in the mill is distributed between the drum rotating and moving the balls. The first harmful, second useful.  

   For example: For the drum rotation we have a power consumption 800 kW / h. If we load balls, the power consumption increased to 1600 kW/h. Here 800kW/h is for moving the balls. Grinding the ore will be only 800 kW. My be the drum manufacturing from polyurethane with the lining. The power consumption for rotation of drum will be reduce to 400 kW and all power consumption reduce to 1200 kW or 25%. For roller mills polyurethane impossible.

3. It is necessary to increase the of ball charge up to 25%.
4. It is necessary to increase the power motors of all the SAG application now.
5. It is possible to increase the diameter of SAG but now there is no need. SAG of 12 meters and 20% ball charge can be achieved capacity up to 10000 t/h.

This is good too http://is.gd/TJWe5O