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Popularity of Stirred Mills and HPGR (3 replies and 3 comments)
Hi Dachmann, welcome to the Forum.
Don't get distracted by energy efficiency, it is economic efficiency that matters in Capitalist economies. The process path that is most economically efficient will change depending on ore characteristics, location, climate, and economic factors like NPV discount rates and sovereign risks.
I wrote an article with a specific case study for a Western Canadian climate that you may have seen online:
https://www.linkedin.com/pulse/why-hpgrs-did-make-list-alex-doll/
Regarding stirred mills, the two issues you haven't touched on are they can be a pain in the butt to do maintenance on (you have to dump the mill contents of the floor which you don't do with a tumbling mill) and sometimes they have issues with the top size of material that comes from upstream stages (you need to match the media size in a stirred mill its feed top size). And you need lots of units to match the capacity of one large gearless tumbling ball mill, so that are capital cost inefficient. When we see multi-Megawatt stirred mills that have an elegant maintenance program, then I expect we'll start to see more of them in secondary grinding duties.
Similar to Alex's comments on the stirred mill, HPGR also have maintenance and capacity issues. You need several to equal one SAG or ball mill. This requires a lot of chutes and diverters that are major wear points. You also have to have more conveyors to collect the product. All this adds up.
Thank you for your answer. The additional maintenance requirements are often ignored in studies. Good point.
Yes agreed, however depends on the application and ore characteristics.
Note, HPGR is a crusher used to crush ore, not to mill ore. Therefore HPGR are part of the crushing plant and 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 wet climates. Otherwise the HPGR will produce pancakes (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 SAG Mill This increased throughput from 17 Mtpa to over 23 Mtpa. The secondary crush product feed 65mm to HPGR crusher. The HPGR had edge recycle (0 to 40 % product recirculated to the HPGR stockpile) and reduced the energy consumption. Prior to the plant changes, the incumbent SAG mill was at its maximum capacity.
Two years ago a study into a Copper SX/EW plant had soft ore and high rainfall. Mineral sizers were specified as Primary and Secondary sizers to reduce from the mined ROM 1000mm max lump to 250 max lump (primary product) and then to 63 max lump (secondary product) followed by a tertiary impact crusher for 19mm product size. This crushed ore was then transferred to the heap leach stacking area.
Every ore has its characteristics that determine the communition circuit.
Note, HPGR do require extra handling and logistics due to the mass of the rolls. The rolls require a workshop with heavy lifting capacity and tools to change the worn tyre. Otherwise a regional HPGR service centre for roll maintenance. Usually the biggest decision made is about 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 every 3 months (change out in 2 hours); feed deflector plate every 6 months (change out in an hour); feed bin wear liners 6 months (depends on ore abrasion index) and change out the rotatable bin is 8 hours.
Spare rolls (2), cheek plates (2) and a rotatable feed bin is usually recommended.
HPGRs are not low cost but are effective in the right application.
Stirred mills are used by the mining industry for ultrafine grinding to enhance liberation, and to decrease particle sizes of industrial minerals to tailor functional properties.
HPGR technology has streamlined the transition between mineral grinding and crushing by accepting larger-sized particles compared to ball mills, and producing finer-sized particles than cone crushers.
Hi,
I am new to the field of mining. Recently I looked into the process of liberation and concentration. Many papers suggest that HPGR and Stirred Mills are a better alternative to SAG and Ball mills in terms of energy efficiency and mineral liberation. Many papers say that the higher CAPEX is compensated by the lower OPEX.
Why then, do not all mines or at least the majority that are under construction rely on these grinding technologies? What is the reason for the ongoing popularity of conventional tube mills?
I also realized that all types of mills are multi MW by now, expect from stirred mills. Those still have a rated power of a few MW. Is there any explanation to that?
Thank you for your answers!
Dachmann