Located in the Nukay mining district of central Guerrero State, Mexico, the Los Filos gold mining project is one of the largest opencast mines in the country. Los Filos is a heap leach operation. The ore size is reduced at the crushing plant and then hauled to the valley where it is stacked on engineered pads. The ore is sprayed with a chemical solution that extracts and absorbs the gold as it trickles through the ore. To regulate seepage rate, an agglomerate (cement) is mixed into the ore to optimise ore exposure to the leaching solution. Ultimately, the solution is directed to the plant where the gold is precipitated out.
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In 2008, the Los Filos mine began operation of a new conveyor line from the mine crusher to the leach pads. The haulage path went down a mountainside to the leach pads that were built in the valley below. Rather than confronting the steep slope of the mountainside, an ore pass from the hilltop was connected with a tunnel from the toe of the hill; this also served to store the ore. A feeder at the bottom then metered the material onto the tunnel conveyor that led to a rotary agglomerating drum. The agglomerating drum mixed the ore with the agglomerate; the ore mix was then conveyed to a spreading system on the leach pads.
That conveying system experienced material flow problems right from the start, especially during heavy rains. The sticky ore tended to plug up the ore pass. Geological instability ultimately collapsed the ore pass, putting the transport system out of service only four months into its operation. This required a return to ore haulage entirely by trucks at a tremendous cost premium.
Figure 1. Conveying on the edge: from left to right, the multi-flight overland conveyor system follows the haul road on the edge of a steep drop.
Solution 1: high-angle conveyor
At that time, M3 Engineering was contracted to develop a new conveying path to the leach pads. As they were aware of DSI sandwich belt high-angle conveyors from previous exchanges, M3 proposed such a system for Los Filos, beginning at the top of the collapsed ore pass, then running down the hill to the toe at the tunnel exit. This would allow continued use of the other existing conveyors.
After the Goldcorp professionals did their due diligence, including a visit to the Dos Santos sandwich belt high-angle conveyors at the Empire mine in Michigan, US, they decided to buy a DSI downhill high-angle conveyor. The order was booked in February 2009. That system was to have impressive characteristics: 1000 tph design rate with a vertical drop distance of 113 m. The 1220 mm (48 in.) wide belts would travel 311 m at 2.5 m/sec. along the 37° inclined slope to discharge over an ore storage pile at the bottom of the hill. Twin 200 hp drives would operate as generators under load, feeding power into the mine grid.
Concerns with geological instability along the high-angle path remained, however. While DSI was beginning the engineering of the new downhill sandwich belt conveyor, Goldcorp geologists continued investigations into the stability of the mountain. After less than a month into engineering work, Goldcorp declared that the hill was not sufficiently stable and they cancelled the order.
Figure 2. Looking up the overland path: from TB25 to TB26.
Solution 2: overland conveyor
M3 immediately began work on developing an overland conveying path to the leach pads by following the already developed truck haulage ramps. Because DSI already had the high-angle conveyor order, it petitioned Goldcorp to convert that order into an order for a DSI overland conveyor system. While Goldcorp insisted that the new haulage system must go out to competitive bidding as it was required by company policy, the mining company also encouraged DSI to make their best proposal for the new overland conveyor system.
After the path was developed, an enquiry was sent out for bid to qualified companies. The enquiry concept included ten conveyor flights. The first three were elevating flights and the remainder were downhill and decisively regenerative. Whereas all other conveyors were of 914 mm (36 in.) belt width, the first downhill flight was a slow-moving 1220 mm (48 in.) wide conveyor with five trips along the length. This was a Goldcorp innovation that made the wide, slow, tripped conveyor an agglomerator, eliminating the need for an agglomerating drum. The agglomerate would be added to the ore at the end of the last elevating conveyor. Then, the mix would tumble through the multiple trips and transfers en route to the leach pads.
Figure 3. Looking up TB26.
DSI’s proposal included improvements to the enquiry concept. A modular system of terminals and table-type modular intermediate structure was developed for best economics and versatility. Additionally, the possibility of further improvements was considered by amalgamating sequential flights with horizontal curves. Because time did not allow development of this additional advantage in the short time allowed for quoting, the proposal was submitted for the ten flight system. The DSI proposal included two important commitments solely for the customer’s benefit:
- DSI would maximise use of the conveying equipment and structure, already at the mine, from the collapsed and abandoned through-the-hill conveying system.
- DSI would investigate the amalgamation of sequential flights with horizontal curves as the first order of business in engineering. The favourable findings would be implemented and the savings credited to Goldcorp.
In June, the company was awarded the overland conveyor system as a ten-flight system. Both commitments were fulfilled, reaping substantial cost savings for Goldcorp. In the former case, the first conveyor (TB25 in Figures 1 and 2) was engineered to use all existing conveyor equipment and structure. Though the arrangement is entirely new, the equipment and support steel are reused from the previous system. The head drive terminal is the only portion of TB25 that contains new equipment. New steel at the support bents, minor framing and a new discharge chute make up only a small portion of the total structure.
Figure 4. Looking down TB28.
In the latter case, investigation over the first two weeks determined that three sets of two sequential flights could be amalgamated for a significant savings to the customer. The last two elevating flights were thus amalgamated into one horizontally curving conveyor (TB26 in Figures 1,2 and 3) and the four downhill flights after the agglomerating conveyor were amalgamated into two horizontally curving conveyors (TB28 in Figures 1 and 4 and TB29 in Figure 1). Thus the system was reduced to a seven-flight system and a credit of US$180 000 was issued to Goldcorp, reflecting the savings in the DSI engineering and supply. Of course, this generated additional capital savings in construction and recurring savings in both power consumption and wear and tear.
The overland conveying path is predominantly downhill. While this presents the normal controlled starting and stopping problems, it also presents great savings opportunities. The downhill flights are decisively regenerative. The drive motors, now generators when running loaded, feed electrical power back into the grid, which powers other mine equipment. These carefully engineered conveyors are equipped with variable frequency drives to ensure operation at maximum efficiency.
The Los Filos mine has been Mexico’s largest gold producer since 2009, with 5.47 million oz. of proven and probable reserves, 2.2 million oz. measured and 3.82 million oz. inferred as of December 2010. The DSI overland conveyor system went into operation in 2009. The system has operated reliably and efficiently for the last seven years, delivering significant savings to Goldcorp when compared to the previous truck-only haulage system. It will continue to deliver success for many years to come.
Written by Amy Duncan, Dos Santos International, USA, and published in Dry Bulk Autumn 2016. To read the full issue, register here.
Read the article online at: https://www.drybulkmagazine.com/special-reports/29122016/partnering-in-innovation/