With the commencement of underground operations in March 2023, Rio Tinto’s Oyu Tolgoi copper-gold mine now has two sources of feed that comes its concentrator – open pit ore from the conventional load and haul Oyut deposit and underground ore from the Hugo North Deposit via the Hugo North Lift 1 first stage of the block cave. While underground is around 10% of the feed at the moment there will be a slow and steady increase over the next few years and ultimately lead start the mine on its long term path to producing 500,000 t/y of copper on average from 2028-2036.

As Andrew Wilson Operations Director, Oyu Tolgoi explained during the recent Oyu Tolgoi Site Visit and Investor Day in July 2023, this phasing in of underground ore has to be correctly managed from the concentrator point of view. The mill concentrate capacity from 2024 to 2025 needs to support increased volumes of high grade underground ore which involves a brownfield concentrator conversion project which is already commencing and is critical for the underground ramp up.

Wilson stated: “That includes an additional ball mill, a flotation circuit and a series of other new equipment to be able to support the higher grade that is coming from the underground, mainly from Panel 0 to start with. The conversion work is already started…that will be occurring over the next couple of years, as the underground feed increases.” He added that it is important to note that the concentrator conversion does not necessarily increase the capacity of the concentrator. The conversion for now just supports the higher grade of the material though of course Oyu Tolgoi is also looking at other options to increase plant capacity.

Otgonbayar Togtokhbayar, General Manager, Technical & Integrated Planning commented on achievements already made since the concentrator began operations in 2013. Then ore capacity rates were around the 20 Mt/y mark but over the last 10 years it has doubled to 40 Mt/y, thanks to a number of different initiatives and improvements, and this milling rate has been maintained now consistently. This includes mill discharge grate improvement, magnet removal for the scrap metals, plus a raft of new technologies helping the operation to remove bottlenecks and any constraints in the mill. Other examples include increasing grinding media quality and upping flotation performance. Oyu Tolgoi has also worked on the value chain, introducing high intensity blasting in the open pit to increase help increase fragmentation for material going to the mill.

The significant benefits have been achieved despite increasing ore hardness and the concentrator milling effective utilisation is expected to remain above 94%, which fits the industry benchmark. The ongoing ore hardness increase reflects the open pit mine getting deeper and the underground ore proportion increasing.

Back to the concentrator conversion which will enable the processing of the higher grade underground ore. Damian Rogers, Underground Project Director, Oyu Tolgoi said that the early works part of this programme is now complete, with civil works proceeding as planned and the main construction works already commenced with a main contractor on-board. “It adds a fifth ball mill, a process circuit including additional flotation columns, rougher flotation, a thickener, and filters…this is quite specialist work which we’re executing inside the operating concentrator facility, so the key here, like all major brownfield work is the fine knit detailed planning, and the real discipline execution of this, particularly when it comes to tying in the new equipment into the existing plant.” 

He continued: “We also know from our experience that an often underestimated success factor in this type of work is a really clear governance structure and decision making process for any works inside that operating plant…we have a really good structure and a good programme in place, and it’s been functioning really well between the project and operating teams.” Rogers mentioned the close integration between the project and the operating teams for the concentrator conversion: “We’re really working tightly together on this; we’ve been seeing this run really well today and on site and we effectively just had one team delivering this work at the concentrator, and that’s really critical for the success.”

In more detail, ore reclaimed from the coarse ore stockpile is currently fed to two comminution lines, each consisting of a SAG mill, two parallel ball mills, and associated downstream equipment. Cyclone overflow from the circuit at a P80 of 140–180 μm reports to the rougher flotation cells. The modifications include the installation of a fifth ball mill and associated equipment.

Increased rougher flotation capacity will be provided by adding a fifth rougher flotation bank. The design of the fifth rougher bank is same as the existing 160 m3 mechanical tank cells. Retention time in the rougher circuit is 25 minutes at design throughput rates. Tailings from the new rougher bank will flow to the combined rougher tailings cross-launder at the end of the banks.
Rougher concentrate discharges into the combined concentrate launder, that feeds the regrind cyclone feed pump box. Concentrate from the rougher flotation cells is reground in vertical tower mills before reporting to the first stage cleaner and cleaner-scavenger circuits.

When treating predominantly Hugo North underground ore, cleaner-scavenger concentrate will be directed to the column feed pump box. But the modifications will also allow the cleaner-scavenger concentrate to combine with rougher concentrate for regrinding, when treating open pit dominant ore.