In a previous post I looked at my trip down the Tara zinc and lead mine near Navan in Ireland. After a very nice lunch we were then taken on a tour of the processing plant. The ore is first crushed underground and brought by conveyor to the surface. The whole process is monitored via computers from a central control room.
The material is screened and the larger fraction passed to a cone crusher to reduce the particle size to about 16mm. The rock is further crushed in a rod mill (right) and three successive ball mills (centre and left). This gets the ore down to about 75 microns size. This part of the process is imminently being replaced with a new autogenous grinding mill which actually uses large blocks of ore rather than rods or balls do do the grinding.
Mineral separation is undertaken by froth floatation. First lead and later zinc particles are made hydrophobic by adding a surfactant or collector chemical to a slurry of water and powdered rock. This slurry (or pulp) is aerated, creating bubbles and the hydrophobic ore particles attach to the air bubbles, rising to the surface forming a froth. The froth is then skimmed removing the ore. The slurry passes through several banks of ‘roughers’ removing 75% of the lead and 93% of the zinc.
The ore froth then passes to large settling tanks where the mineral concentrates then settle out.
The concentrates are then de-watered and the dry concentrate is stored awaiting shipment. Normally the concentrate is taken by rail to Dublin for shipping to smelters in Europe but a recent rail bridge collapse has meant that it has to travel by road until the rail bridge is repaired.
Annual production is approximately 360,000 tonnes of zinc concentrates grading 56% zinc and 70,000 tonnes of lead concentrates grading 65% lead.
Thanks to the operators of New Boliden Tara Mine for their fascinating visit.
Note: I’m just about to embark on a very heavy teaching semester – blogging will be light
[Spotted at Milford on Sea, Hampshire]
Tara Mine, near Navan, Co. Meath, Ireland is the largest zinc mine in Europe. It is now owned by the Swedish based Boliden Group and they generously allowed a group of us geophysicists down the mine and around the rock crushing mill following the EAGE’s Near Surface 2009 conference last week. I’ll have a second post on the processing plant later but this post concentrates on the mine visit.
Here is what they are after – the grey mineral on the left is galena (lead ore) the brown one on the right is sphalerite (zinc ore).
The ore is hosted as a number of lenses in Dinantian (Lower Carboniferous) limestone. The ore formation must have started very early as one of the units is a submarine debris flow unit with already mineralised sulphide clasts. The ore body reaches the surface near the town of Navan and was originally detected by a soil geochemistry survey in the 1960s by Rio-Tinto Zinc. However, RTZ management failed to follow up on their geologists’ recommendation to drill the geochemistry anomaly and they surrendered the exploration licence and walked away from what is now the fifth largest zinc mine in the world.
The exploration licence was taken up in 1969 by the Tara company who did a bit more geochemistry and some geophysics followed up by a test drill hole. With the luck of the Irish, they hit the ore body – but by just 50m from its edge and if their hole had been vertical rather than inclined, they would have missed it. But hitting a 12m zone of 6.4% zinc and 2.5% lead gave their backers the reasons to pay for more drilling and the mine started production in 1977.
The orebody dips moderately near the surface and then flattens out with the deepest part of the workings now 950m below the surface at the southwest end. Two adits from the surface follow the orebody downwards and own visit was by landrover simply driving into the mine.
First it was necessary to kit up in protective clothing.
The journey underground took us to one of the drill faces. The lighter coloured rock is one of the ore lenses, the lines indicate the driller to drill the next shot holes, with the shot holes from the previous blasting round circled for the driller to avoid in case there is any unexploded charge remaining.
After blasting the loose rock is removed by large excavator which is remote controlled.
Here we have a diamond drill further in the mine drilling core samples in the roof rock.
Finally, for now, here is a borehole draining water from an upper level.