Regrind Mill and Circuit Design: Staged Grinding Guide

Key takeaways

• A regrind mill grinds rougher concentrate or cleaner feed finer than the primary grind, so fine grinding is applied only to the stream that carries the value.
• Grinding finer with steel media can cost recovery. At Mount Isa, adding 6 MW of ball and tower mills raised sphalerite liberation by 18% but zinc recovery by only 7%.
• Inert regrinding reversed this. When 4 MW of IsaMills was installed in the Mount Isa zinc circuit, liberation rose by 4%, but zinc recovery rose by 10%.
• In a published comparison, an IsaMill™ needed 17 kWh/t against 24 kWh/t for a regrind ball mill to reach the same 32 micron P80.

A regrind mill is where many concentrators decide their final recovery. Grinding is one of the most energy-demanding stages in mineral processing, and depending on the ore and circuit design, it can account for up to 50% of a plant's total processing costs. Yet many valuable minerals are not fully liberated at the primary grind. 

They report to the rougher concentrate as composite particles, which require further grinding to liberate valuable minerals from gangue and improve both concentrate grade and overall recovery

A well-designed regrind circuit does more than reduce size. It allows the primary grind to stay coarser, applies energy only to the smaller tonnage that needs it, and produces clean mineral surfaces that float well. A poorly designed one adds circulating loads, harms flotation chemistry, and consumes power without a matching gain in recovery. This article sets out the grinding circuit design principles Glencore Technology developed at Mount Isa, how inert stirred milling changed regrinding, and how the approach now extends into coarser grinding duties.

What is a regrind mill and why does regrinding matter?

A regrind mill grinds an intermediate flotation stream, usually rougher concentrate or cleaner feed, to a finer size than the primary grind so that composite particles can be liberated and recovered. Regrinding matters because it lets a concentrator follow one simple rule: do not grind anything more than you need to. Grinding is expensive, so each stage should liberate only what is needed and recover the mineral as soon as it is liberated.

In practice, this means grinding coarser before roughing, then regrinding the lower tonnage of rougher concentrate. Staged grinding of this kind significantly reduces total power consumption. The Mount Isa lead-zinc concentrator shows the approach at full scale.

Source: Pease et al, Transforming Flowsheet Design with Inert Grinding: the IsaMill™ (Glencore Technology), Figure 8.

Why does the grinding environment matter in regrinding?

The grinding environment matters because steel media contaminates mineral surfaces, and in a regrind circuit, that contamination lands on the stream closest to final concentrate. Steel media in regrind mills forms iron hydroxide coatings on mineral surfaces. The effect is greater in secondary and regrind mills, because more fines are created and steel consumption is higher. Staged regrinding is much less attractive with steel grinding because it harms flotation chemistry.

Glencore Technology's published Mount Isa data shows the difference clearly.

Source: Pease et al, Transforming Flowsheet Design with Inert Grinding: the IsaMill™ (Glencore Technology).

With steel media, part of the liberation gain was lost to poorer surface chemistry. With inert media, recovery rose by more than liberation, because the clean surfaces allowed fine liberated sphalerite that had previously misreported to lead concentrate to be rejected, and allowed fine particles to float well.

The same effect has been seen elsewhere. At the Prominent Hill copper-gold concentrator, which began production in 2009, a 3 MW M10000 IsaMill™ using 3.5 mm ceramic media regrinds rougher concentrate to a P80 of 20 to 25 microns to liberate fluorine-bearing gangue minerals. The inert grinding environment prevented mineral surface contamination and delivered optimal flotation performance from the first day of operation. More detail is on the improved flotation recovery page.

What are the principles of regrind circuit design?

The principles of regrind circuit design are to float minerals in narrow size distributions, minimise circulating loads, and install regrinding exactly where liberation is needed. These come from Glencore Technology's published flotation circuit principles.

Float minerals in narrow size distributions. This does not mean a sand and slime split. It means designing for the needs of different minerals in different parts of the circuit: recover minerals as they are liberated, then regrind the remaining composites and recover them in their own flotation stage. The sharp product size distribution from the IsaMill™ supports this.

Minimise circulating loads, and open-circuit where possible. If a mineral is not recovered early, it needs either more liberation or better surface chemistry, and both can be delivered by an inert regrind mill. Fine particles should not be sent back to the head of the circuit to mix with fresh coarse feed. They float very well when conditions are tailored for them in their own flotation area.

Install regrinding where it is needed. The IsaMill™ has a small footprint, so it can be installed in several locations throughout a flotation circuit. It is about a third of the height and weight of comparable mills and needs minimal concrete and steel, which makes multiple regrind points practical in existing plants.

An open circuit configuration is possible because the IsaMill™ uses an internal product separator rather than screens or closed-circuit cyclones. Slurry passes through a series of grinding chambers between the discs before it can exit, so short-circuiting is avoided and the product size distribution stays tight without a classification loop. Glencore Technology has also published design work on incorporating the IsaMill™ into its Jameson Concentrator flowsheet.

How much energy can a stirred regrind mill save?

In a published ball mill replacement study, a stirred regrind mill used around 29% less energy than a regrind ball mill for the same duty. The regrind ball mill required 24 kWh/t to reach a P80 of 32 microns from an F80 of 100 microns. The IsaMill™ required 17 kWh/t, not counting the ball mill cyclone feed pump.

The benefits extend beyond the mill. The George Fisher project at Mount Isa added 6 MW of IsaMill™ grinding power, yet operating cost per tonne of feed did not increase. Reagent additions fell, circulating loads between roughing and cleaning were cut from 200 to 300% to less than 50%, and spillage was virtually eliminated. With lower circulating loads, less flotation capacity was needed, and Mount Isa shut down some zinc cleaning capacity after installing the IsaMills.

How do you select grinding media for a regrind mill?

Grinding media for a regrind mill is selected so that it is large enough to break the top size of the feed, but no larger than needed for efficient grinding of the fines. Stress intensity in each grinding event is proportional to the cube of media diameter, so for the same media type and mill speed, doubling media size increases stress intensity by a factor of eight. Media that is too small lets coarse particles build up in the mill. Media that is too large wastes energy on the fines and accelerates wear. Glencore Technology uses a reduction ratio of eight per media type to maintain efficient breakage.

When the required reduction is larger, series grinding is used. For a feed with an F80 of 400 microns and a P80 target of 14 microns, a reduction ratio of 28, the first stage uses 6 mm media to reduce the top size, and a second stage uses 2 mm media to complete the energy-intensive fine grinding. This keeps each stage efficient while preventing coarse build-up in the mill.

Can stirred mills handle coarser grinding duties?

Yes, stirred mills now handle coarser duties, because larger ceramic media have taken them beyond fine regrinding. There are now over 50 IsaMill™ installations operating with an F80 greater than 100 microns, at a total installed power of 129.4 MW. The coarsest current feeds include a magnetite regrind at Ernest Henry in Australia (F80 350 to 400 microns), and copper regrinds at Las Bambas in South America and Bozshakol in Kazakhstan (both F80 around 300 microns).

At McArthur River Mine, two 3 MW M10,000 IsaMills were installed in open circuit to grind SAG mill screen undersize to 45 microns before flotation. This showed that the IsaMill™ was no longer just a regrind mill, but a viable option for mainstream coarse grinding. At the circuit level, pilot work on an HPGR and IsaMill™ flowsheet showed specific energy reductions of 9.2% and 16.7% compared with HPGR/ball mill and cone crusher/ball mill circuits, respectively.

Frequently Asked Questions

What is the difference between a regrind mill and a primary grinding mill?

A primary grinding mill treats the full ore tonnage to the size needed for roughing. A regrind mill treats a much smaller stream, usually rougher concentrate or cleaner feed, and grinds it finer to liberate composite particles. Because only a small share of the tonnage is reground, staged grinding reduces total power consumption compared with grinding all the feed fine.

Why is grinding mill energy consumption so high in regrind duties?

Tumbling ball mills lose efficiency as grind targets get finer, because smaller media carries less energy into each contact. In one published comparison, a regrind ball mill needed 24 kWh/t to reach a 32 micron P80, while an IsaMill™ needed 17 kWh/t. Regrinding only the rougher concentrate, not the full ore tonnage, also cuts total power.

How does steel grinding media affect flotation?

Steel media forms iron hydroxide coatings on sulphide mineral surfaces, which reduces flotation selectivity, especially for fine particles. At Mount Isa, adding 6 MW of ball and tower mills raised sphalerite liberation by 18% but zinc recovery by only 7%. After IsaMill™ regrinding with inert media, recovery gains exceeded the liberation gains.

What is staged grinding in mineral processing?

Staged grinding means grinding the ore only as fine as needed at each stage. The ore is ground coarser before roughing, gangue is rejected, and the smaller rougher concentrate stream is reground to liberate the remaining value. It reduces total power consumption and lets each flotation stage treat a narrower size distribution.

How is a regrind mill sized?

Stirred regrind mills are sized from a signature plot, which measures specific energy against product size on a representative sample. For the IsaMill™, signature plots scale up 1:1 from laboratory to full size. Media size is selected from the feed and product sizes. The IsaMill™ calculator gives a quick idea of what is possible.

Designing your next regrind mill circuit

The regrind mill is where much of a concentrator's recovery is decided. The Mount Isa experience, and the sites that followed, point to a consistent set of design choices.

Grind only as fine as each stage needs and recover minerals as soon as they are liberated. Apply fine grinding to the small streams that carry the value, not to the full ore tonnage. Use inert grinding media so that finer grinding improves flotation chemistry rather than harming it. Design for narrow size distributions and low circulating loads, and open-circuit where possible. Match media size to feed top size and product target and use series grinding when the reduction ratio is large.

Applied together, these principles allow a coarser primary grind, reduce total grinding energy, and lift both recovery and concentrate grade. They also carry into coarser grinding duties, where stirred milling now operates at feed sizes once reserved for ball mills.

Every orebody is different, and a regrind circuit should be designed from testwork on representative samples. If you are reviewing an existing regrind circuit, planning a brownfield upgrade or designing a new grinding circuit, talk to our team about signature plot testwork, media selection and circuit configuration. Operating case studies are available in the IsaMill™ technical papers library.

Technical references

• Pease, J.D., Curry, D.C., Barns, K.E., Young, M.F. and Rule, C. Transforming Flowsheet Design with Inert Grinding: the IsaMill™. Glencore Technology technical paper.
• Gurnett, I., Martin, S. and Stieper, G. Coarse Grinding on an IsaMill™? Glencore Technology.
• Ellis, S. Ultra Fine Grinding: A Practical Alternative to Oxidative Treatment of Refractory Gold Ores. Kalgoorlie Consolidated Gold Mines.
• Glencore Technology. IsaMill™ 2020 Compendium of Technical Papers.