the australian mining reviev special macmin

[u][b]32 The Australian Mining Review AUGUST 2008
SPECIAL PROFILE Twin Hills Silver Mine[/b][/u]

[url]http://www.macmin.com.au/media/2008/AustMiningReviewAug2008.pdf[/url]




Silver mine moving to increase production
MACMIN SILVER’s Twin Hills
Silver Mine in Texas, Queensland,
should start increasing production in
September-October after new processing
equipment is installed.

Chairman and chief executive Bob
McNeil said the upgrade of the crushing
and screening equipment was initially
due for completion at the end of
March, but the tight market for
such equipment has meant that it
is only being installed now.

“We should start to increase
capacity from September-October
… but until we get some feel on
how it works we’re a bit reticent
to actually predict how much at
this stage,” he said.
“It will certainly be a
substantial increase on the
present production.”

The Macmin crushing circuit
currently crushed more than
20,000 tonnes per month to the
required minus 4mm.
“Planning is proceeding to
move to extended crushing hours,
probably two eight hour shifts,
but is still restricted
by difficulty in hiring
suitable personnel,” Mr
McNeil said.

“Because of intense
competition for trained
operators in the mining
industry at present, we
cannot hire sufficient
trained personnel and
consequently we have
had to train our own
operators.

“This is time
consuming.”
Texas, near the border
of NSW, is about 3.5
hours’ drive south west
of Brisbane.
The Twin Hills Silver
mine is 10km east of the
township of Texas and
most of the 52 people on
the payroll are locals, Mr
McNeil said.

[b]History[/b]

Exploration on the Texas Silver Project
began in 1994 by the company and its joint
venture partner, Hunter Exploration.
Significant silver mineralisation
was delineated by 1998 and Macmin
re-acquired Hunter’s interest.
After further extensive drilling, the
initial Twin Hills feasibility study was
completed at the end of 2000.
In March 2004, as the international
price for silver started to increase,
Macmin decided to commit to, and raise
funds towards, developing the Twin Hills
Silver Mine.

Site clearing began in 2005, along with
staff recruiting and training.
The following year saw overburden
removed, the crushing plant installed and
the electrowinning plant constructed.
Mining began early last year in
conjunction with the commissioning of
the processing circuit and production of
the first silver powder.

Fine-tuning of
the crushing and
electrowinning plants
continued throughout last
year.

[b]Heap Leaching[/b]

Twin Hills used a heap
leaching process that was
introduced to the industry
in the 1970s as a means
to drastically reduce gold
recovery costs, Mr McNeil
said.
“It is also effective with silver in certain
circumstances,” he said.
“This process has literally made many
mines by taking low grade resources and
transforming them into a viable mine.
“Heap leaching involves placing
crushed or run of mine ore in a pile built
upon an impervious liner.
“Cyanide solution is distributed across
the top of the pile and the solution
percolates down through the pile and
leaches out the silver/gold.”
The silver/gold laden pregnant solution
drains out from the bottom of the pile and
is collected for recovery.

“Heap leaching generally requires 60
to 150 days for processing ore that could
be leached in 24 hours in a conventional
agitated leach process,” Mr McNeil said.
“Recovery is typically about 70 per
cent as compared with 90 per cent in an
agitated leach plant.

“Even with this lower recovery
performance, the process has found
wide favour, due to the vastly reduced
processing costs compared with agitated
leaching.”

Mining at Twin Hills is carried out using
hired equipment operated, supervised and
directed by Macmin staff.
“Mining of ore and waste is proceeding
to plan and there are no problems in this
department,” Mr McNeil said.


[b]Crushing[/b]

To recover 70 to 72 per cent of silver from
the rock, the ore must be a certain size.
This is achieved either by crushing or
during blasting of the ore prior to mining,
Mr McNeil explained.
But the crushing circuit, as specified
by the feasibility study and operated on a
single-shift basis, could not achieve sufficient
crushed material due to maintenance
problems with the crushing circuit, the
inability to hire sufficient trained operators
to run more than a single day shift, and
inadequate screens to remove the fine
fraction early in the crushing process.
The screens incorporated in the current
circuit were inadequate to remove the fines
at an early stage in crushing, which resulted
in the circuit clogging up and maintenance
problems, he said.

If crushing could have been maintained
on a 24-hour basis, this problem could have
been partly offset.
A further unanticipated problem was
that the ore was harder and more abrasive
than predicted.
To solve the problems, Macmin is
upgrading the crusher with additional
screening, adding a second contract
crusher the circuit, incorporating closer
drill and blast patterns in the mine plan
to produce as much minus 4mm product
as possible without crushing, moving to
two shifts, and having a contract crushing
supervisor on site.

“Our initial target is to place 60,000
tonnes of minus 4mm material on heaps
each month, moving to 100,000/120,000
tonnes per month as ore grades decrease,”
Mr McNeil said.
“This latter target may require further
crushing capacity.
“The timing of achieving the initial
objective of 60,000 tonnes a month onto
the heaps is difficult to predict.”


[b]Silver[/b]
The silver is extracted from pregnant
solutions using an electrowinning process
and the plant, which contains 180 cells,
runs on a 24-hour, seven-day-a-week
schedule.
Each cell in the plant has not produced
at the rate specified by the manufacturer
but, by experimentation, its capacity is
gradually being increased, Mr McNeil
said.
“Problems have also arisen with the
coatings on some anodes and these have
now been replaced,” he said.
As the plant was designed with at least
50 per cent overcapacity, the reduced
capacity of each individual cell is not
expected to be a problem with production
in the near term, but could restrict
expansion.

The leaching solutions from the heap
carry 60 to 90g/t silver and the plant has
been inefficient in extracting or reducing
the silver in leach solutions below 40g/t
silver.

These barren solutions, which still
contain 40g/t silver, are then recycled to
the heap, with further dissolved silver
rebuilding the solutions to 60 to 90g/t
silver, Mr McNeil explained.
“There is a concern that it is inefficient
to recycle solutions with as much as 40g/t
contained silver,” he said.

Consequently, a Merrill Crowe (or zinc
precipitation plant) has been installed to
remove further silver.
“The intent is that silver will be
reduced to 40g/t by electrowinning, then
pass through the Merrill Crow, which will
reduce silver content to less than
10g/t,” Mr McNeil said.

“This barren solution will then
be recirculated to the heaps.”
Macmin is selling some of the
silver powder produced “as is” for
specific applications.
“However, an unexpected side
effect of the EMEW plant is that
it also extracts some other metals
and the silver powder produced to
date contains contaminants which
are unacceptable to some buyers,”
he explained.

“These are removed before
smelting and this being done
successfully in Melbourne.
“The silver powder can then be
smelted, refined and sold.”

[u][b]OnTrax gives mine a boost[/b][/u]


FOR TWO months, OnTrax Crushing and
Screening has been providing booster
plant support for Twin Hills Silver
Mine’s crushing operations in an effort
to capitalise on the current commodity
price.

The company said it was experienced in
campaign crushing such as for Brisbane’s
Gateway/Leighton joint venture and along
the coast.
Quarry campaign crushing for other
companies included contracts with Boral
and Hansons.
These jobs involved making aggregate
in order for the fixed plant to maintain
production.
[green][b]OnTrax said it looked forward to a
continuing association with Twin Hills.[/b][/green]