PRECONCENTRATION
Cutting waste
“Sensor based automatic ore sorting
technologies are used to efficiently remove
waste and sub marginal grade material from
crushed and screened run-of-mine (ROM) ore,
producing a coarse (typically >10 mm) upgraded
preconcentrate at an early stage in the mineral
processing circuit. This avoids additional energy
intensive downstream processing of this
rejected component of the ROM feed, leading to
lower unit operating costs and reduced tailings
disposal volumes.” These are comments from
Tony Parry and G van Wyk of Steinert Australia
in their paper Upgrading Low-grade Gold Ore
Stockpiles by Preconcentration Using Ore
Sorting – an Assessment of the Economic
Impact and Viability, presented at the 13th
AusIMM Mill Operators’ Conference, October
2016.
They reported on test work conducted on
bulk low-grade gold ore Bullen stockpile
samples from Central Norseman Gold that
indicated high gold recoveries as well as high
material rejection rates. “A financial model,
based on these results, evaluates grade
sensitivity and identifies a minimum stockpile
grade for adequate financial returns.” The base
case modelling estimates that the ex-stockpile
grade (including the fines bypassed fraction)
can be more than doubled through ore sorting
and that the project will be highly viable with
strong cash flow generation at stockpile grades
as low as 0.7 g/t Au. The modelling enables the
company to benchmark ore sorting options
against stockpile reclaim and screening
operations currently employed.
They concluded that the “material is highly
amenable to preconcentration using XRT sorting
32 International Mining | MAY 2018
technology. In bulk tests a very high waste
reject rate of 92.2% was achieved with no gold
detected by assay in the waste (98.1% recovery
assumed based on assay detection limit). As
nearly 100% of the gold was recovered, a high
degree of liberation of mineralised quartz reef
from gangue presumably exists at particle sizes
between 25 mm and 60 mm.
“The financial modelling confirms robust
economics for the project. The base case
modelling of this potential operation indicates
that the grade of plant feed can be more than
John Chadwick examines
the latest in gravity
techniques, ore sorting
and magnetic separation
to raise separation
efficiency and reduce
concentration costs
The huge Natalka gold project, owned by
Polyus, includes a three-stage gravity circuit; by
far the world’s largest – with no less than 52
centrifugal gravity concentrators, with sizes up
to 42 in. There are multiple concentrators from
the two world leaders - FLSmidth’s Knelson
brand and Sepro Mineral Systems’ Falcon
brand
doubled with ore sorting, from a bulk ROM
grade of 0.8 g/t to a mill feed grade of 1.93 g/t
The flow sheet assumes 30% of the crushed and
screened stockpile tonnage prior to sorting is -
Process flow diagram (base case) for gold recovery from Central Norseman Gold’s low-grade gold ore
Bullen stockpile