An HPGR cannot replace a SAG mill. Also required to get the feed into the HPGR is a secondary crusher(s) and perhaps a tertiary crusher(s). The HPGR is often a quaternary crushing device which is the reason that the footprint for an HPGR plant is so large. To place a ball mill or a VERTIMILL directly on the HPGR product may work on soft iron ores but will not work well on harder ores that require over about 7 kWh/t to achieve a T80 of 1.7 mm. When hard minus 10 mm scats are present (referring to HPGR product), the ball mill efficiency is reduced. Note that the hardness of these scats is not measured by a Bond BWi because that test crushes everything to minus 3.35 mm, the maximum size for good ball milling or VERTIMILL efficiency.
I do agree that the VERTIMILL power will be lower than a ball mill so there is merit in replacing ball mills with Vertical Stirred mills.

I can't see why HPGR/vertimill circuits wont become more prevalent in the right application, particularly given the ever increasing size of vertimills. We're finalising commissioning of a HPGR/ball circuit (which followed primary and secondary crushing) and in an expansion study to replicate the plant HPGR/vertimills are an option on the table to reduce OPEX, noting that the HPGRs are closed circuit at 4mm. The SAG circuit was not considered due to the ores hardness and abrasiveness.

I think that the saying horses for courses apply here. Before I can explain myself, a bit of background. Although we generally talk about hardness we need to differentiate the particle size scale. Typically we talk about competent ores at the larger scale, say larger than 5 to 10 cm. For smaller particles we talk about hardness but in reality we encounter a range, typically harder as we go finer.

For ores that are not competent and soft single stage SAG milling is probably the most economical.

For ores that are not competent but have hard pebbles then SABC followed by ball milling (SABC circuit product too coarse for Vertimills) is probably the most appropriate. Secondary grinding could be in Vertimill.

For competent ores that are soft (hard to crush easy to grind) tertiary crushing with cone crushers followed by ball milling (again too coarse for Vertimill) is probably the better choice (secondary grinding probably not required).

For competent hard ores quaternary crushing in HPGR followed by Vertimill is probably the correct choice.

There is no single device or circuit that is best in all cases. The ore needs to be characterized and the various options tested to determine the best flow sheet for each application.

I propose an innovative flow sheet as below:

Primary crushing, secondary crushing, HPGR, vertimill .Hope every one will agree to reduce energy cost.(GREEN TECHNOLOGY). Today a D day has come to stop energy wastage, or else face global warming and floods, Totally dry in summer and heavy rain in monsoon. Rivers flooding, similar to HIMALAYA SUNAMI in INDIA. Next year it will again repeat with more gravity.

SAG mill: After primary crusher product of 150mm goes to SAG, and sag product goes to ball mill, then to flotation / magnet separation.

The total energy consumed is Primary crushing+ SAG+ Ball mill.

* + VERTI MILL: Here secondary crusher product of 4mm goes to HPGR which will reduce to 1 - 2 mm.and goes to vertimill then to flotation / magnet separation.

Many such applications are in operation in USA, Australia, for flotation.
Here power consumption is Primary +Secondary+HPGR+VERTIMILL.
Many case studies reveal that 30% power consumtion is reduced and prooved technology. Even in India it is used in Rock phosphate.
I feel future griding of ball mill and SAG mill, will become out dated due to brown technology.
Today HPGR and VRTI mills are given green technology grade.
When new technologies are avilable why we should not aim to use them.

The use of Green Technology is not a license to ignore economics.Capital and Operating costs including labor must also be considered (as well as power) in the selection of a flowsheet and process equipment. HPGRs and VERTIMILLs often require multiple units in a large plant and this requires extra floor space and people to operate and maintain these multiple units. Warm weather designs will be different than cold weather operations.
My point is, let's be alert to all of the factors that turn waste into ore, and that is economics.

What you are proposing is a "one hat fits all"solution. This is where regrettable mistakes leading to financial loss begins. Just as ores vary in character so will be the flow sheet to recover them and each case need to be studied. Perhaps, historical failures due to poor SAG/ball mill designs and operation is what is fueling the clarion call by some for the demise of these grinding equipment . SAG mills and Ball mills will be energy efficient if correctly designed and correctly operated. A paradigm shift in the design and operation of SAG/ball mills is much needed.

Present practice are mostly Primary crushing by gyratory/Jaw, secondary/tertiary by cone or AGM/SGM and fine grinding by ball mills . HPGR /verti mills are mostly for finer grinding. It cannot be said in ceratanity that HPGR/Verti mill will replace the conventional process routes for corser sizes for any ore btype. Before atking any decesion, its necessary to see the existing [performance for the particular ore types or necessary test work as may be required.

Of course you can replace some SAG mill with an HPGR. Circuit economics will dictate this see publication http://www.ceecthefuture.org/publication/efficiency-economics-energy-and-emissions-_-emerging-criteria-for-comminution-circuit-decision-making/ SAG mill however will be around for a long time yet.

As per the article given by Mr Mike, advantages of HPGR over SAG is for cupper and gold ore and more studies need to be done for othere ore before taking nay decesion. If its AGM, no grinding media is required as the case of Kudremukh . Data need to be collected with necessary tests for different size feed for realative merits of the three different process circuits AGM-BM, SAG-BM and HPGR-VM for a final decesion.

I think, this discussion : " What is the better - HPGR or SAG?" was 30 years ago with another named "What is the better - SAG or three stages crushing & ball mills?". In those times the favorite was SAG because it was a new and fashionable. Now favorite is HPGR because it is new and fashionable. After 5-10 years HPGR will have place in the communition.

Horses for courses, however HPGR has found its niche in tertiary applications where the ore characteristics meet the specific parameters that best suit HPGR e.g. moisture, material density / hardness, feed size etc.

SAG can be used as an alternative to a HPGR, but a HPGR cannot always be used as an alternative to a SAG. HPGR Capex is generally more expensive than SAG but can return greater energy savings through effective circuit design.

Open circuit design for HPGR takes up less a footprint than a SAG, but losses that advantage when in a closed circuit application. Operating availability of HPGR is higher than SAG but removal and servicing of HPGR Roll assemblies in remote mining locations can cause costly logistical issues, so many things need to be considered when comparing SAG to HPGR.

The evolution of the Roller Press over the last 100 years has seen the technology move from Coal to Cement and onto Minerals / Comminution as patented by Prof. K. Schönert of Clausthal University in Germany in 1977 and will continue to be developed like many other technologies as companies strive to offer the customer the most efficient and affective comminution solution, where I think we will see HPGR playing a greater role though companies will need to put more thought into complete circuit design e.g. conveyors, screens etc rather than just equipment selection.

We also need to look at the efficiency of SAG circuits. Case in point, a SAG circuit design modified to increase throughput has not only increased the SAG feed tonnage, but resulted in a lower SAG steel charge level (apx2%), increase in SAG ore load and increased SAG availability by 4%. The next stage is the optimization of the SAG control to run maximum tonnage without the typical load swings. savings in Kw/t and the power saved due to reduced steel requirements (including shipping) are part of the ROI and reduced environmental footprint.

Sometimes we need to follow laws even if are in loss or difficult to follow. Or else our business will get closed soon. Today's environmental laws are --Convert all broun technologies to green technologies, or pay heavy penalties which is costlier than anything in process. And also satisfy every one in the process of approval to run Broun technologies.
Even ball manufacture consumes energy and fuel to transport, handle etc. This also contributes to Global carbon foot print credit. HPGR do not use balls or SAG mill liners. Liners used in HPGR is too less compared to SAG.