There's no question that the dewatering process is an essential and critical part of any mining operation. Whether the mine is an open pit or underground, mining companies have to deal not only with the water that naturally tends to collect at the bottom of the workings, but also with the fines contained in the water that can make it either extremely acidic or highly alkaline.
The importance and extent of mine dewatering can vary greatly - from a single pump back station at a sump pumping less than 100 gpm to complicated dewatering schemes involving dozens of dewatering wells pumping several thousand gallons per minute. There are also numerous and differing reasons for mine dewatering, including:
promoting operational workability by maintaining a dry pit floor and/or underground workings;
ensuring structural integrity and preventing the collapse of surrounding walls and ceilings, resulting in flooding and/or mud rushes by releasing pore pressures;
"drying out" the ore and/or waste rock that is being actively mined; and,
containing contaminated plumes and barring them from leaching into any surrounding aquifers. This may entail instituting a monitoring program to assess such parameters as the discharge of groundwater into the mine from bore (drain) holes; the water quality of groundwater collected in the mine; the quality of water discharged from the mine; and groundwater levels in the local aquifer(s) influenced by mine dewatering.
The pump most often selected by mine management for dewatering is the submersible centrifugal pump. This type of pump uses a series of electrically driven rotating impellers inside the pump body, building up pressure and forcing the water up the discharge line. Centrifugal pumps are reliable and relatively compact, while the multi-stages required for high heads can direct drive with a single motor.
DROP PIPE TECHNOLOGIES
Inextricably linked with the submersible pump is the drop pipe system that supports the weight of the pump, power cable, and water in the drop pipe, and acts as a conduit for the transport of the water discharged from the pump to the wellhead.
Several options are open when selecting drop pipe for dewatering. With the acceptance of submersible pumps in the mining industry in the 1950s, the drop pipe material of choice was galvanized steel (and later, stainless steel). Steel was relatively inexpensive and abundant, could easily support the weight of the well equipment, and was able to withstand the high working pressures associated with deep wells. The debut of the PVC pipe marked the plastic boom in the late 1960s and early 1970s. PVC was followed by flexible drop pipe as a response to more complicated and environmentally and safety-focused applications.
The continuous length, lay-flat drop pipe is constructed of a circular woven polyester yarn with thermoplastic elastomer composites for the cover and lining. This construction makes it tough and durable and exceptionally resistant to corrosion, scaling and microbiological attack for long service life, and flexible to the point of eliminating noise, dampening surges and minimizing vibrational damage. Other benefits include:
lightweight and compact design for reduced transport and storage costs;
tensile strengths up to 44,000 lb;
ability to withstand enormous working pressures up to nearly 400 psi, and burst pressures up to more than 800 psi depending on the pipe diameter;
easy and fast installation and retrieval for rapid pump maintenance, repair and replacement;
excellent hydraulic performance with low friction loss for efficient pumping, energy savings and reduced operating costs; and,
high flow rates due to the ability to swell up to 15% above the uncharged diameter.
CORROSION CAN HURT
Of course, there are accompanying disadvantages to each type of drop pipe, and in the mining industry, they become apparent very quickly. Corrosion is the primary culprit when it comes to steel drop pipe. Mining companies not only have to contend with groundwater highlighted by abnormal pH levels that are the result of abrasive dust slurries, but also with additional compounds that can cause negative chemical attacks on
ferrous materials like steel, inexorably corroding them within a relatively short amount of time. Every time a pump has to be removed at a mine because of corroded pipe, there are associated costs incurred in terms of equipment (cranes and trailers), labour (manpower and manhours), downtime (lost production) as well as environmental and safety risks.
Consider situations where mines are removing and reinstalling pumps on a regular basis, and steel drop pipe is being used. When mining gold in Australia and Argentina for example, mining regulations dictate that all pumps (and drop pipe) are removed before any blasting occurs to prevent shock damage to the pumps. The speed with which the pumps can be pulled out and replaced is critical.
In open pit mines, dewatering is not the only consideration. Hydrogeologists are continually assessing the water table around the perimeter of the cut. The deeper the cut becomes, the more likelihood there is of the sides collapsing as water pressure builds. Here again, pumps are regularly being moved, and new boreholes being drilled, depending on the readings being taken. In addition to the timing factor, the likelihood of something happening to the rigid steel pipe as it repeatedly goes through this exercise increases with greater certainty each time, prompting replacement.
"Downtime is the enemy in every mining application," notes Adrian Semmence, marketing manager, Kidde Products. "Mines aren't like municipal water authorities where there are multiple pumping wells, so that if a pump fails or is being serviced, the others will kick in. At a mine, each submersible pump has a specific job to perform with no available backup. When one is out of service, there's a very real risk of flooding, mine wall destabilization and aquifer intrusion/contamination."
Safety is a prime concern as well. Steel drop pipe comes in 10- or 20-ft sections, and must be removed with a crane and unbolted or unscrewed piece by piece so the "bad" section can be replaced. Dealing with rigid steel pipe is a cumbersome and awkward operation, and injuries invariably have been sustained while the drop pipe is hanging above head height before it's lowered and placed on the ground.
PVC OR FLEXIBLE?
Although PVC drop pipe offers outstanding corrosion resistance, it does not hold up well in rugged mining applications, and is seldom used in the industry. For one, it is not that sturdy, and is prone to thread stripping, making it difficult to reuse. It can be easily damaged through ripping or tearing, has severe pressure limitations and low tensile strength. And like its steel counterpart, PVC drop pipe takes up large amounts of storage space, and is only available in 10- and 20-ft sections, meaning that it must be
retrieved and installed one piece at a time during regular pump servicing and replacement, and for new pumping wells.
FLEXIBLE DROP PIPE
Flexible drop pipe has its own set of limitations. Its initial cost is higher than either steel or PVC, but it does offer the opportunity for a rapid return on investment when other ancillary savings are considered such as reduced set-up and operational costs, and labour and energy savings. Flexible drop pipe is also constrained by its current maximum depth and pressure levels. To date, flexible drop pipe is limited to depths of between 660 and 750 ft, depending on hydraulics, while the largest pipe diameter that manufacturers presently offer is 10 inches. Steel clearly has the edge here for the foreseeable future.
How does steel fare versus flexible drop pipe? According to Semmence, "Steel is an innate part of the mining landscape, and will continue to
remain so. When we talk about steel versus flexible drop pipe, there will most likely always be mining applications where steel pipe is necessary. By the same token, environmental awareness and protection have taken centre stage within the mining industry. Flooding and losing containment of contaminated water are particularly major environmental concerns which can be addressed with flexible drop pipe."
Then too, there is the cost argument. With their eyes pasted on the bottom line, mining companies have been known to discard huge capital investments in technologies
that are five or 10 years old if a new technology enters the picture, and justifies their case for major cost reductions. "What we'll be seeing more of is mining companies using flexible drop pipe in applications battling aggressive water conditions and where time and cost savings are absolutely imperative," says Semmence.
There are already documented examples where steel has been replaced with flexible drop pipe based on these factors.
At a copper mine in the southwestern United States, barrier submersible pumps operate 24 hours a day, 365 days a year to intercept and capture the leachate plume before it flows into the aquifer. The contaminated water is then pumped to the surface and recycled as part of the mine's comprehensive environmental policy. The company had found there was a dramatic reduction in pump flowas low as 400 gpm and as high as 1,200 gpm on a six-inch steel drop pipedue to the build-up of copper salts. However, because it would take three days to clean and reinstall a pump, the policy had been to wait until a pump failed and then replace it. Because the introduction of flexible drop pipe reduced this operation to a single six-hour shift, the company has since instituted a pump maintenance schedule on a six-week cycle resulting in further cost savings. It also replaced steel with flexible drop pipe for its monitoring wells used to sample groundwater.
One of the largest open pit mines in Chile is located at an elevation of approximately 10,500 ft altitude. Although the mine lies in one of the most arid parts of the world, an unusual aquifer formation could potentially cause the mine's open cut faces to collapse through groundwater penetration as excavation proceeds increasingly deeper. To address the critical issue of surrounding wall and land stabilization, the mine's hydrogeologists have developed a sophisticated computer modeling program to calculate the required water drainage rate. This has been combined with a program of continual well drilling and installation of submersible pumps to achieve a high yield of water drainage in a short period of time before new wells are necessary.
The key issues for this mine were the reliability of the drop pipe, the speed of installation and retrieval to maximize well efficiency, corrosion resistance, and the ability to service the pumps with the shortest downtime cycles possible. Existing boreholes controlling groundwater levels contained 20-foot sections of threaded 4-inch galvanized steel drop pipe with a life expectancy of only three years.
Because the mine needed to reduce groundwater levels further, it opted to choose a flexible drop pipe system instead. The first installation was completed in less than five hours using a small work rig, and entailed 70 ft of four-inch-diameter flexible drop pipe with stainless steel couplings and a submersible pump. Upon start-up, the well performed flawlessly. The extracted water is not wasted, given this area's aridity. Mine personnel use the better-quality treated water, while the balance is channeled for use in various process applications.
A Cornish tin mine experienced an emergency outflow during the winter, which spilled more than 4.5 million gallons of extremely acidic and toxic water into a nearby river. The culprit was high acidity, leading to extreme corrosion of the pumps and steel risers. Flexible drop pipe was installed in the 170-ft-deep mine shortly after the incident, and is now pumping out 2.2 million gallons daily, continually transferring lime-treated water from the shafts up 200 vertical feet to a header tank, where more lime is added to obtain a normal pH. The water is then gravity fed from the tank to a tailings dam where the metal constituents have a chance to settle. The drop pipe was inspected six months after installation, and no evidence of corrosion or degradation was found despite high metal concentrations in the mine water.
Dewatering sits prominently at the core of the mining lifecycle. A mine cannot begin to operate without this process. And without the appropriate drop pipe technology installed at submersible pumping wells, that same mine will be considerably hampered in terms of profit margins, production output, cost savings, and environmental and safety considerations over the long term.
Bill Drake is the product manager for Angus Flexible Pipelines (www.FlexiblePipelines.ca), based in Exton, Penn. This article was originally published in the March 2004 issue of Engineering & Mining Journal.
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