Hi,

A lot of this is just theorizing. We will have to try the ore out to know what sort of grind we really need. But, that said, there are several potential issues.

Zinc, according to the initial feasibility test work needs to be reground in order to come up with a decent concentrate. There is nothing essentially difficult about it; the sulfide flotation concentrate goes to regrind with the idea of liberating the zinc. The zinc is separated from the pyrites in a high pH flot and cleaned prior to going to the concentrate thickener.

The zinc cleaner tails are high pH and it would be best to send them to tails. This is a bit of a trade off because the regrinding will also liberate some tin, which if it were not for the high pH, could be sent to the tin gravity and/or flot circuit.
The Techint flow sheet has the sulfide flotation ahead of the tin. This works out badly as the sulfides/pyrites are a necessary constituent of the gravity tin process, forming a barrier between the tin and the gangue which can easily be removed in a subsequent tin con sulfide flotation stage. Thus to be included in the tin stage this sulfide/pyrites material must be sent to tin after the sulfide/zinc flotation stage when it will have an unacceptably high pH, or prior to the sulfide/zinc flotation.

The test work we have done on a fairly limited sample of fresh ore indicates that the silver rougher flotation concentrate should be reground. If this is the case once we have proven the material in the circuit, it will take priority over the zinc. Good recovery and concentrate grade for the silver is a real necessity for a positive financial outcome.

There is a second regrind option and that is the 500hp vertical mill. It is in the Techint flow sheet in order to regrind some of the gravity tin tails. Various tin experts have deemed this to be an unfortunate choice of equipment as, in conjunction with cyclones, it will be more prone to producing tin slimes than finer tin.

This mill would likely be suitable for either silver rougher con regrinding or zinc con regrinding but there is one other grinding issue to consider.
The Polysius ball mill is rated at about 3200hp. By a general rule of thumb this would mean that the mill can suitably deal with about 3200 tons per day.

Techint/Polysius released a report recommending crushing to a P80 of 8.5 mm and accepting an F80 of 210 microns at the mill. We are presently doing testwork to ascertain how we might make out sending this coarse grind to the roughers. The issue is that Sandvik have estimated our crushing circuit to be capable of only 240 tons per hour when crushing to a P80 of 8.5mm. This is a nonstarter because at 100% availability it does not give even 6000tpd.

The best bet under the circumstances seems to be a double cyclone setup whereby the cyclones at the ball mill pull off a very coarse product – say 300 – 350 microns which would flow by gravity to into the plant and then be pumped to the cyclones prior to the 500hp vertical mill for a closed circuit regrind, and the overflow going by gravity to the Silver Rougher Flotation feed. This would undoubtedly allow for a product that could be handled by the Silver Roughers but would then leave only one available mill for further regrinding

This reads as a "Note to Self" to which anybody and everybody is invited to add to.