COMMINUTION & CRUSHING
which seal the grinding gap towards the roll
edges, the industry perception, to date, has been
that their use would require full skew suppression
- a process built into earlier designs by others
allowing the use of flanges. Yet, mechanical skew
suppression made this technology space
“consuming, heavy and consequently
disproportional expensive”, the company said.
After testing a conventional thyssenkrupp HPGR
with flanges for close to 6,000 hours in a copper
mine, in Peru, they have been shown to improve
the unit capacity by about 20% at a finer product,
while consuming about 15% less energy than a
conventional HPGR with cheek plates, according
to thyssenkrupp. Also, current projections suggest
wear life can be increased by at least 20-30% due
to a more uniform pressure distribution along the
roll width.
“First implementation of a skew limiter to
protect the thyssenkrupp HPGR even under
emergency conditions against excessive skew
peaks is scheduled for July this year,” the
company said. “This skew limiter fully eliminates
the need of mechanical skew suppression; an
unnecessary cost driver.”
The new skew limiter ensuring safe protection
of flanges is an innovative milestone in making
thyssenkrupp HPGR technology more competitive
in terms of capital expenditure – offering 20%
more capacity for the same unit price – and
operating costs – 20 % lower energy and wear
cost – while reducing the direct and indirect CO 2
footprint at the same time, thyssenkrupp said..
FLSmidth, which has three series of HPGRs –
the F series for mining operations – says its
machines come with long wear part life, low
operation and maintenance costs, a short
shutdown period for wear part replenishment and
up to 20-30% reduced energy consumption.
Like Metso, it has designed its HPGRs to avoid
skewing. It also has oil cooled bearings and two
roll shafts powered by induction motors driving
planetary gear reducers. The latter are mounted to
a common torque-sharing arm assembly, thus
eliminating the need for mounting these
components to a foundation, FLSmidth says.
Köppern, meanwhile, has its own line of HPGRs
– eight of which are working at NLMK’s Stoilensky
GOK beneficiation plant, in Russia. It has
developed a wear protection system called
HEXADUR ® that seeks to extend the life of HPGR
components. Since HEXADUR’s introduction, there
have been a significant drop in machine time-outs
caused by wear and repair, it says.
So, while this technology might have started off
as an application for cement plants, it is becoming
an acceptable and energy efficient way to grind
various mined material.
The fine print
FLSmidth’s Joe Dziedzina also sees the uptake of
16 International Mining | APRIL 2020
FLSmidth is working on leveraging its
vertical roller mill experience to
make a mining-ready OK Mill
that comes with all
the benefits seen in
cement and slag
processing, Joe
Dziedzina says
HPGR technology in
mining rising.
The Global Product
Manager for HPGR told IM on
the side lines of the SME
MineXchange Expo and
Conference, that FLSmidth
installed its first “large
HPGR” for mining last year,
at a gold heap leach project
in Turkey.
While he said
there had been
lots of interest
globally for HPGR-
based flowsheets, he thinks
another grinding technology could offer the
energy and water efficiency Tier One miners are
currently demanding.
FLSmidth’s OK™ Mill has typically been seen at
cement and slag operations around the world,
grinding material that ranges from soft limestone
to hard, granulated blast furnace slag. The OK Mill
technology carries this out with 5-10% less power
consumption than other vertical roller mills (VRM)
and with the lowest maintenance costs of similar
machines in its class, according to FLSmidth.
With the MissionZero sustainability program
behind it, FLSmidth is now working on leveraging
this VRM experience to make a mining-ready OK
Mill that comes with all the benefits seen in
cement and slag processing, according to
Dziedzina.
MissionZero has been devised by the Denmark-
based company to significantly reduce emissions
across the global cement and mining industries by
2030, with a specific focus on water and power
management. Dziedzina thinks a mining-ready OK
Mill may help the company achieve this goal,
explaining that the technology has the potential to
provide a dry comminution circuit that could be
the “centrepiece of FLSmidth’s program”.
The OK Mill is set up to allow material to be fed
from the side of the machine and fall onto the
centre of a powered table where it is ground by
hydraulically-assisted free-spinning tyres. The
material then progresses from the edges of the
table upward with the help of a strong, constant
air stream into an integral classifier where it is
deemed to be a ‘product’ or ‘reject’. Material that
makes the cut is discharged out the top of the
machine with over-sized material falling back onto
the table for re-processing.
The mill can be operated in three standard
configurations – airswept, overflow and
semi-airswept. The former tends to be
the typical setup where the plant
layout is optimised. The overflow
option sees material fed from the top
and discharged and screened at the
bottom. This could act as a pre-grinding
option that is less sensitive to moisture,
perhaps as a pebble crusher, FLSmidth
believes. The last option, once again, sees
material fed from the top, yet offers more
classifier options, according to the
company. There is a smaller required
fan and less “jet” abrasion occurring
in such a setup, FLSmidth says.
Dziedzina sees an adapted version of
the OK Mill offering the “complete
grinding” option in a
standard SAG and ball
mill comminution
circuit, saying it could
eliminate the ultrafines
that cause issues further
downstream.
The major differences between the milling
products that FLSmidth currently offers and the
OK Mill are the larger allowable feed size (versus
HPGR), a reduction in metal contamination, the
highest available power efficiency, and the ability
to handle a wide range of ores and high moisture
content, it says.
On top of this – and in keeping with the theme
under discussion – it eliminates the need for
grinding media and is a “dry application” suitable
for water sensitive areas.
Mining companies worried about the scale of
the OK Mill would be wise to look at the FLSmidth
OK 81-6 Mill installation at a Bangladesh cement
grinding plant. Having started up in August 2018,
it is now grinding slag and cement.
This specific installation comes with 2.7 m x 1.1 m
diameter rolls, an 8.1 m diameter table size, six
rollers, 11 MW of power (two 5,500 kW motors)
and a specific maximum operating press force of
1,040 kN/m². This mill, the world’s largest vertical
roll mill, according to Dziedzina (and Guinness World
Records), has a top feed size of 100 mm (maximum
160 mm) and a product size of 96% at 45µm.
The caveat to this example is it is for processing
cement and slag; not hard rock.
Dziedzina was keen to point out the
adaptations the company is working on to make
the OK Mill mining-ready.
“One study performed with traditional hard-
faced wear surfaces would have required the plant
to change the rollers every six days, which is not
acceptable,” he said, reflecting on the company’s
expectations of how quickly the current
components could wear out if they were charged
with processing some of the mining industry’s
harder rock.