ALAN TITCHALL tours a quarry on the Taupo Volcanic Plateau that supplies Golden Bay Cement with one of concrete’s essential additives and explains its connection to the new Waterview tunnels.
Ken Thorpe looks down into the grey-pinky pit where a little steam is coming off the face.
“We use gas meters to monitor hydrogen sulphide when mining, of course. The northern pit hydrogen sulphide levels are generally low, but we have to be careful in the southern pit,” he says.
“Any depression in the ground, or out of a fissure, hydrogen sulphide is deadly and acts on the body like cyanide.
“I have found dead wallabies, rats, cats, rabbits and birds in hollows around the site. In one hole there were about 30 dead yellow hammers, which must have been attracted by the yellow sulphur.”
We are looking around the top of the northern pit at Tikitere Quarry around the east side of Lake Rotorua. The deposit is silica. Lots of it.
Ken is a geologist who holds an A-grade quarry certificate, an A-grade opencast coal mine, and a first class underground coal mine manager’s certificates. He is also a registered assessor with MITO; contracting with an Auckland-based company to train extractive industry courses.
“I started my own consultancy a while ago, and have kept the wolf from the door, although it has chewed up the door mat a few times.”
His connection to Tikitere Quarry and its silica goes back 25 years.
During the peak mining cycle over the summer months he acts as quarry supervisor, when most of the mining of the resource is done.
The resource we are looking at in the pit hasn’t been mined for months and the surface has oxidised to a grey colour.
Ken points out another site hazard.
“When the ore is freshly mined it is pure white, and it’s like working on snow. You need quality sunglasses to stop the glare.”
The quarry sits in the middle of the Tikitere hydrothermal field, close to the shores of Lake Rotorua. The closest neighbour is the Hells Gate thermal visitor park.
Unlike the diatomite (silica composed of the fossilised remains of diatoms) mined at Ngakuru Valley south of Rotorua, the Tikitere deposit is related to volcanic activity.
“Formed from the hydrothermal alteration of host rock – basically pumice and ignimbrite deposits.”
Ken adds that the deposit is probably very close in nature to the original silica (pozzolan) that the Romans mined from Pozzuoli in Italy used to make the first hydro-concrete.
“Hot geothermal fluids permeated through the pumice and ignimbrite, stripping out the minerals to leave behind amorphous silica – probably as a gel. Over thousands of years this gel has quietly hardened as the geothermal activity has moved elsewhere and the deposit cooled. Not to say we don’t still find the odd warm spot on site.”
The resource is not that deep – about 20 metres – but it is spread over a lot of consented land leased from a Maori trust.
The entire quarry was consented in 1991 as one of the last of the Town & Country Planning Act consents before the RMA took over.
Ken says the silica content on site is about 85 percent, or well within the industry standard for amorphous silica. However, there is more thought to lowering this percentage, which means the quarry’s lower grade ore could be exploited.
“This would expand our resource out to tens of millions of tonnes. And, speaking from someone who has been involved from the start, not all the resource has been explored yet.”
The Ngakuru deposit originally dug out under the supervision of the MOW in the 1950s for the hydro dams probably had a silica content of 66 percent, he says, so accepting a lower grade silica is nothing new, but will impact on the quarry’s value.
In full flight, the processing operation at Tikitere Quarry works in two 12 hour shifts with three workers per shift, plus day work personnel.
With his A-grade certificate Ken Thorpe is the quarry supervisor during the peak summer months and Justin Masters, the overall manager. Wayne Southall is the plant manager.
The silica, branded Microsilica 600, is produced under the Golden Bay banner. Microsilica 600 is typically added to concrete in ratios of up to 10 percent to achieve the qualities as described in the box story on page 28.
This past summer has been busy for the Tikitere Quarry as it is supplying a large export order to Singapore.
The quarry normally stockpiles about 5000 cubic metres of raw ore for orders during the year.
The material can get ‘sticky’ in winter so they try to do most of the mining and lifting during the summer months.
“It doesn’t always work out that way and we do get fine weeks during winter,” says Ken. “Even a day without rain and a bit of wind will dry the surface material out.”
The mining is carried out with a Komatsu 20 tonne digger and two 12 cubic metre dump trucks. “That’s all we need,” says Ken, “as the material is soft – especially in the lower-grade material areas where we can use a swamp bucket.”
The firmer higher-grade material is crushed before stockpiling with a mobile crusher.
Ken says dust is not a problem during summer as, once the material is stockpiled, it forms a ‘skin’.
“It is a very stable material.”
A problem can be the endemic sulphur, which tends to run in veins through the material and can get too concentrated.
Drainage is not a problem as the site is in a natural depression with water running into the centre and self-draining through natural fissures. There are a few natural on-site ‘ponds’ (at around 25 degrees) but they dry out during the summer.
The plant process
Ken likens the primary blunger in the plant to a kitchen whizz, which features a large toothed blade on the bottom which runs at high speed breaking down the ore to less than 2mm in size.
“The raw ore is fed into the blunger with water, and exits as a slurry. It’s a fairly crude process, but sufficient for it to pass into the factory for the secondary process.”
An initial cut through a hydro-cyclone removes the heavier crystalline quartz, and oversize product the secondary process cannot handle.
The second plant process involves a mill, and the mill media being small pinkish quartz pebbles, which are quarried in Central Otago from schist.
The finished Microsilica 600 is bagged on site in one tonne bulk bags, 25 kilogram and 10 kilogram bags, plus it’s available to the market via bulk powder road tankers, and also in pre-mixed slurry via road tanker.
Although there is a lot of ‘waste’ material in the process, a change in lowering the silica content but not the strength will allow the plant to re-process it in the future.
In addition, new technology in the way of vibratory mills and classifiers to remove the crystalline from the amorphous silica, is on the cards.
“We have hundreds of tonnes of material we could re-process so it will be great to see it being used,” says Ken.
Wayne Southall has been given permission to go ahead with this new process, but he was involved in a car accident back in March (not his fault), so things have been put on hold until he gets back on board.
Silica in action
During the building of the Waterview tunnel Ken Thorpe was invited to watch Alice, the mighty tunnel machine, in action lining the tunnel with concrete ring segments, made in a special pre-cast factory in Auckland. Some 24,000 of these concrete segments now line the twin, 2.4 kilometre long, motorway tunnels which will soon be open to the public.
These reinforced concrete lining segments, 450mms thick and two metres wide, were manufactured with millimetre precision and have the strength and durability to withstand 100 years of demanding use.
And guess what – they feature Microsilica from the Tikitere Quarry.
“The tunnelling machine needed a smooth glass-like surface on these interlocking concrete rings for its suction pad to grip, then swing them into place,” says Ken.
“The Microsilica helps to provide a very smooth surface, which is another plus to the strength-adding properties of the product.
“Every time I drive through the Waterview tunnels in the future I will be thinking – hey – we at Microsilica are part of this engineering masterpiece.”