Traditional
mining methods could not have solved Granduc's problems. Low-cost
highly-productive methods had to be developed for, or adapted to,
Granduc's special situation. The Granduc mine employed three mining
methods: sublevel caving, sub- level open stoping, and mechanized
cut-and- fill. Each method, that was used at Granduc, was a model of
modern, efficient, and safe mining practice. The miners used mechanized,
multi- drill jumbos to drill the ore and rubber- tired load-and-haul
equipment to remove the ore after it is blasted. These load-and- haul
units can move freely within the mine from level to level, by means of
an internal ramp system connecting mine levels on a 15 per cent slope.
Ore passes connect the different levels, and the ore dumped into them at
each level is collected by a gathering system and delivered to the
underground crusher .
From the crusher,
ore traveled by conveyor belt to two bins, from either of which it was
loaded into 50-ton railroad cars for the ten- mile trip to Tide Lake.
Speed and volume with safety, were essential elements here therefore the
ore trains include 15 to 20 cars and the electric locomotives are
capable of pulling the trains at 40 miles per hour. A round trip would
take slightly more than one hour The same locomotives haul mining crews
to and from the mine in specially-designed passenger cars.
On reaching the
concentrator end of the tunnel at Tide Lake, the ore trains dump their
loads into a storage bin. Conveyors then raise the ore to the fine
crushing plant where it was crushed and then conveyed to fine ore
storage bins located ahead of the grinding circuit. The high cost of
steel grinding media at Granduc made some form of autogenous grinding
attractive. Ore pebbles were screened out of the feed to the fine
crushing plant and replace most of the steel balls that would otherwise
be used as grinding media in the grinding circuit. There were two
grinding lines, each consisting of one rod mill and two pebble mills.
The pebble addition and the grinding circuits were completely
instrument-controlled and operate with a minimum of supervision. The
ore, after grinding, follows the conventional practice of flotation,
thickening, filtration and drying.
The dried
concentrate was then hauled by trucks over the 32-mile road to the
marine terminal in Stewart. A special design, using cylindrical tanks,
enables these trucks to carry fuel oil on the return trip to the mine.
Snow crews work continuously during the winter to keep this road open
because the six concentrate trucks made a total of 24 round trips each
day to keep ahead of mine production, and men traveled in and out every
day.
The marine
terminal in Stewart was located at the head of a fjord, the Portland
Canal, 120 miles from the open Pacific Ocean. The terminal could handle
ships as large as 50,000 tons displacement. The first shipload of
Granduc concentrate left Stewart for Japan in January, 1971.