Paste: A Maturing Technology

Pump Supports Backfill Operation at Mexican Mine

Version also published in Engineering and Mining Journal

Paste backfilling, the process by which a combination of tailings, water and cementacious materials are blended and used to fill voids in underground mining operations, provides a broad range of benefits. These can include an improvement in safety, the ability to make subsequent surface development possible, a reasonable solution to the ever-present problem of what to do with tailings, and more. It’s not surprising, then, that Canadian miner Agnico-Eagle chose paste backfilling as the key tailings disposal method for one of its newest developments, the Chihuahua, Mexico-based Pinos Altos mine. But while creating the paste is one issue, getting it to the mine stopes—which in this case can be anywhere from 1.5 to 2 km away—is another one entirely. To make that happen, Agnico-Eagle turned to a pump model from Schwing Bioset (Somerset, Wisconsin, USA), and is reporting impressive performance delivering material in this around-the-clock operation.

Back into the Ground
Located in the Sierra Madre gold belt, 140 miles (225 km) west of the capital of Chihuahua state, Pinos Altos contains reserves of more than 3.5 million oz of gold and 100 million oz of silver. Operational as a surface pit since 2008—with underground mining started in 2010—the 27,180-acre (110-km2) site is expected to generate yearly outputs of 170,000 oz of gold and 2.5 million oz of silver through the year 2028. The end result of that gold milling operation is a steady stream of tailings—a volume which is currently about 2,500 t/d but could easily be doubled with an increase in production. According to Moises Palma, engineer in charge of the site’s paste plant, paste backfilling was always the method of choice for tailings disposal at Pinos Altos.

“The tailings operation was designed with this procedure in mind,” said Palma. “Currently, we take waste material by conveyor from the milling plant, and run it up an inclined belt into a batch plant. There, the tailings make their way through a process in which they are combined with cement and water to create a paste. The cement—roughly 5% of the overall mixture—is added to make the paste suitable for filling and supporting the existing underground cavities that have already been mined.  Once the correct mixture is achieved, it is dropped down into the hopper for the paste pump and ready for delivery back to the mine.”

Mine backfilling is hardly a new concept. What has changed over the years, however, is the characteristic of the backfill material itself. In the past, to accommodate the limited pumping technology available in most mines, tailings had to be turned into a slurry with a water content of 70% or more. By comparison, the paste material generated at Pinos Altos is stout, with a water content of roughly 30% and a slump generally in the 8-in. (203-mm) range.

Technology Changes the Game
The difference between what could once be pumped and what can be moved today has been mining companies’ preference for piston pumps over centrifugal pumps in tailings operations. Where other previous technology offered limited pumping pressure (often as low as 10 bar), the Schwing KSP 140 H(HD)XL pump in place at Pinos Altos provides a maximum conveying pressure of 130 bar with paste outputs up to 85 m3/hr—ideal, according to Palma, for moving the high-solids material long distances.

“The Schwing pump we use here at Pinos Altos is perfect for what we do,” he said. “This is a challenging application, given the thickness of the paste, the distance it has to be pumped—currently about 1.5 km—and the pump’s almost continuous operation. Yet, we are getting a steady rate of about 126 tons of material pumped every hour—roughly 2,500 tons per day. We have been extremely pleased with that kind of performance.”

Based on the nature of the paste and the desired production rates, engineers from Schwing-Bioset recommended its XL pump option for the Pinos Altos operation. Poppets in the XL series are designed to reduce material velocity through the poppet housing.  Doing so minimizes the pressure drop through the valve housing, increases the filling efficiency of the pumping cylinders and reduces wear on the poppet discs, seats and pumping rams—all of which lead to better performance and less downtime.

To keep the pump running and the paste flowing, Pinos Altos relies on an air-cooled Model PP2400 drive unit that is powered by twin 400-hp (300-kW) motors, also supplied by Schwing Bioset. This is mounted adjacent to the pump at the base of the paste plant, and includes PLC-based controls. While the initial plan was for the pump to operate up to 10 strokes per minute and deliver 126 t/h, operations are running so smoothly that mine officials are looking to raise production another 20%.

Safety Leads the Way
As mentioned above, the decision to use a paste backfill is generally based upon the benefits that the technique provides. At Pinos Altos, it enhanced overall mine safety by providing mining crews with dramatically improved structural stability within the stope. Palma says their operating procedure for that facet of the job is far more than simply pouring mud into a hole.

“We are pumping into three main stopes and doing so in three-foot lifts with three days between pours,” he said. “Not only does that allow enough time for the paste mixture to harden, it also avoids placing the massive stress load on that cell and adjacent areas that would be present if we filled it all. This first cell alone will be taking on more than 13,000 tons of material, which is over 8,300 m3 of paste.”

To put that figure into context, 8,300 m3 ( 10,855 yd3) is enough material to bury an entire basketball court over 13 ft (4 m) deep.

Though the other two stopes currently being filled are smaller—one will hold 7,100 tons and the other 6,600 tons—they still represent a sizable movement of material from the paste plant to the mine.  Discharge into the cells is monitored via closed-circuit camera to ensure the process runs smoothly and any problems can be quickly spotted.

Looking Ahead
In addition to making the mining effort safer, paste backfilling at Pinos Altos is also addressing a number of other concerns that have plagued similar mining operations for decades. Most notable of these is the issue of how to best dispose of tailings from the cyanide-based gold recovery operation. Traditional approaches such as surface storage, while initially less expensive, can bring unwelcome environmental impacts. Paste backfilling not only solves those concerns, it does so in a manner that eliminates the mine dewatering necessary with other disposal efforts.

“This has been a very successful approach to tailings disposal for us,” said Palma. “As a result of what we do here, the mining operation is safer, there is almost no impact on the environment and, over time, it is actually a lower cost alternative to other methods. Agnico-Eagle has already started talks to boost production by almost twice what we are doing now. When that happens, we will add another pump from Schwing Bioset which will allow us to handle between 5,000 and 6,000 tons per day. We know they will be up to the task and we are excited for what lies ahead.”

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