One of a kind

Twelve miners squeeze into a narrow rectangular cage at Mallappa shaft, the deepest and largest of three accesses serving the Hutti underground workings at the operation run by Hutti Gold Mines Limited (HGML) in rural southwestern India. Equipped with one double-deck cage for lowering and hoisting miners and another for winding ore to the surface, the Mallappa shaft extends to the mine’s 28th level at a depth of 910 metres. With cages measuring 6.1 metres long and just 2.13 metres wide, this main production shaft has long been a bottleneck for the operation. The narrow shaft opening creates complex logistics issues for lowering large mechanized equipment into the mine. As mining depth has increased, so has the time it takes to lower and hoist miners to various levels in the small cages. That will all change in 2018 when HGML commissions a new circular shaft with a six-metre diameter. The alternative access will transport as many as 100 miners per trip — quadruple the current capacity — and enable the lowering of larger equipment without stripping it down into small components. With a phase-one depth of 960 metres and an ultimate depth of 1,300 metres, the new shaft will extend the future at the operation for decades. “Hutti has huge potential,” says Dr. Prabhakar Sangurmath, general manager coordination at HGML. “Our geological surveys have shown that there are gold bodies persisting up to three kilometres deep. We’re only mining near to one kilometre now.”

The main Hutti underground mine is supplemented by ore from the nearby Uti open pit and Hira-Buddinni underground deposit. In total, HGML estimates it can mine the existing reserves of 9 million tonnes at an average grade of 5.19 grams per tonne for 50 more years. “In order for us to have a sustainable future, we must increase our mechanization,” says general manager Prakash Rayamajhi, who joined HGML in 1992.

Mechanization migration

Over the years HGML has gradually mechanized mining functions. Vijay Kumar Patel, deputy general manager engineering at HGML, estimates that over the past four years more than half the mining has been mechanized. “Mechanization has many advantages in terms of both safety of our people and increasing our productivity,” Patel says. “The narrow shaft has always been our constraint in mechanizing more.” To introduce larger equipment underground, HGML has had to dismantle it on surface and send it down in small pieces to be reassembled and welded back together. “It affects production,” Patel says. “Reassembling some equipment underground takes a full month. If we need to ship it to another level in the mine, we have to dismantle it again and reassemble it again.” Traditionally the mine used conventional pneumatic jack hammers for manual development. Drilling one face required two jack hammers and the manpower of three personnel to achieve a pull of approximately 1 metre per blast.

Sandvik introduced narrow-vein drilling technology and its benefits to HGML during a 2014 forum, and the mine took immediate interest. Following a detailed mine study, plans were made to lower a Sandvik DD210 narrow-vein jumbo down the Mallappa shaft, where it was commissioned underground. “It was an eye-opener,” Rayamajhi says. “All these years jackhammers were the lifeline of the mine. We started with conventional wedge cuts where we used to get 60 centimetres at once, and we were content with that. After quite a few years we finally switched over to burn cuts and achieved almost one metre per cut. We were very pleased with that, but the day this Sandvik jumbo came down, everything changed. Today with the rig we are averaging 2.8 to 3 metres per cut in even less time. People are quite impressed. “Sandvik DD210 has revolutionized the development at Hutti. We’ve now decided that every year we will introduce one jumbo along with one loader so that all our major mining levels are totally mechanized. With the introduction of a few more machines Hutti will be on its way to achieve its targets. We never thought that we would be able to get such performance. Pulling 150 tonnes in a single shot here was beyond anybody’s imagination.”

“Once we have the new, large shaft and the bottleneck is removed we can pursue large scale mechanization and discontinue selective mining only with machinery that can fit down the current shaft,” Rayamajhi says. “Once the portal is ready and we have larger equipment going down and we get vast ore reserves we intend to enhance our production from 1,500 tonnes per day up to 5,000 tonnes. That’s a long-term plan.” HGML currently still uses locomotives and tracklines underground, which Rayamajhi says have become “a major constraint and are difficult to service,” and the mine intends to phase the system out and introduce low-profile underground trucks in its place. “Given our distances underground we are now looking for a LPDT of suitable size,” he says. “The day we have the combination of the jumbo, the loader and truck, our development needs will be amply fulfilled. These machines have brought us a level of confidence and we look forward to a long-term association with Sandvik,” Rayamajhi says.