Follow the link below to get some key points about HPGR before trying to consider any change

https://www.linkedin.com/pulse/high-pressure-grinding-rolls-damian-connelly

Depends on the application and ore characteristics.

Sizing and selection of appropriate equipment for throughput and recirculation loads to be considered. Full evaluation of project aims or objectives required.

Why replace when you can augment your existing plant with front end improvements?

Note, HPGR is a crusher used to crush ore. Therefore HPGR is generally part of the crushing plant and can only be incorporated only if the ore characteristics permits.

The HPGR can be used in hard rock mining but not on soft ores or wet sticky ores and in high rainfall, wet climates. Otherwise the HPGR will produce pancakes (and bogged up).

The CADIA East Project (Newcrest mining in Australia, 2011) introduced a secondary crushing (cone crushers) followed by tertiary crusher HPGR in front of the incumbent existing SAG Mill circuit. Original project upgrade throughput was for 20 Mtpa.

However, this project increased throughput from 17 Mtpa to over 23 Mtpa (within 4 years operation). The secondary crush product feed 75mm to HPGR crusher. At the time it was the largest HPGR model (Polycom 24/17-8) from the OEM Polysius (ThyssenKrupp). Feed rate to the HPGR crusher / feed bin was approx 1,800 wtph, and 40% recirculating load. Max lump feed size F100 - 75mm (F80 - 40mm).

Prior to the plant changes, the incumbent SAG mill was at its maximum capacity. By installing the secondary crushing and tertiary crushing plant (HPGR) these effectively allowed the SAG Mill to operate at much higher throughput. It was not handling Primary crush size ore, but tertiary crushed ore (sub 20mm). 

Note, HPGR's do require extra roll handling and transport logistics due to the mass of the rolls. The rolls require a workshop with heavy lifting capacity and tools to change the worn tyre. The rolls for HPGR 24/17-8 are approx WLL 111 tonne each. The CADIA East project had a 140t/16t overhead travelling crane to facilitate rolls and feed bin removals as part of the HPGR crusher building. If a mobile crane was available and good access possible then typically a 300 tonne mobile crane would be lower capital but with higher operating costs option (with crane hire). 

To service the worn rolls typically a regional HPGR service centre can do the roll maintenance. Usually the biggest decision made for HPGR is about the in-country servicing.

However the HPGR rolls do have long life and are well monitored to a close predictive life of the wear studs (of the wear tyre). Usually they are installed with a useful campaign life exceeding the period of a annual shut (12 months) and it is the SAG or Ball mills relining campaigns that determine the plant shut, not the HPGR. Some HPGR crusher rolls get into the 2nd year. Depends on ore characteristics.

The HPGR crusher has cheek plates (either side of the rolls), feed deflector plates (slide in/out) and HPGR Feed bin requiring wear liner replacements. But these are not unusually long in downtime nor frequent. Cheek plates wear every 3 months (change out in 2 hours); feed deflector plate every 4-6 months (change out in an hour); feed bin wear liners 6 to 12 months (depends on ore abrasion index) and change out with a rotatable bin is 8 hours.

For wear: Spare rolls (2), cheek plates (2) and a rotatable feed bin is usually recommended.

Having a suitable on-site workshop or fitting area to have HPGR rolls reconditioned is recommended.

There is good competition in HPGR suppliers now these days. 

HPGRs are not low cost but are effective in the right application.