Valve solutions for mine dewatering
Water is an essential resource in all mining operations. Water is used in many processes including hydrotransport, solution leaching, dust control, screening and separation, to name a few. Reducing water consumption has become an increasingly important priority for the mining industry in recent decades. Newer technologies such as dry tailings and in-pit treatment are being evaluated as additional methods to conserve water. A critical component to the overall success of any dewatering technology is the performance of the valve.
Dewatering begins once the ore is separated from the waste or tailings. Each mine site presents specific and unique challenges in dewatering processes due to the different types of ore. Selecting the optimal valve for each process will impact the overall system performance and improve the efficiency of returning valuable water to the facility for reuse.
There are many tools available for researching and comparing valve options. Application data sheets describing the application conditions (pressure, temperature, viscosity, density, percent solids and pH) are required to select the appropriate valves. Standards and specifications, both industrial and internal, help define more precisely valve requirements such as flange and body ratings, testing and material standards. Specifications can also define shutdown, isolation and leakage capabilities. A solid valve specification is an essential starting point. Documentation of industry standards and internal minimum requirements ensures informed comparisons of valve solutions offered by multiple vendors.
When selecting valves for dewatering applications, factors such as corrosion, abrasion, scaling, valve orientation, cycle frequency, cycle time, and overall system design should be taken into account to avoid potential problems.
To avoid corrosion, substrate specifications should define both concentrations and temperatures and include normal and disturbed conditions. The specifications should also define the chemicals or procedures used for flushing or cleaning the lines to ensure the compatibility of the selected valve materials. Using a steam jet to clean up large scale in the pipeline, for example, can easily cut or destroy elastomeric seats and gaskets.
Abrasion is a common problem in tailings systems. Abrasion control technologies include weld coatings, spray coatings, ceramic and non-metallic coatings such as rubber and urethane. Rheological studies of abrasive materials have shown that together with the percentage of solids and the speed, the size and shape of the solid particles have a considerable impact on abrasion resistance technologies. In some applications the combination of corrosion and abrasion is a double problem, not only because it limits options but also because safety becomes a major concern. Abrasive materials will eventually wear down seats, seals or even valve bodies. The minimization of the risk of exposure to the media for personnel and the environment should be duly taken into account.
Scaling is often a missing condition in application data. Build-up of material on the flow control component and bearing surface can damage an isolation valve and shorten its life. Scaling causes the valves to seize or jam, often in a partially open position. A seized valve restricts flow and prevents isolation, requiring almost immediate and unplanned downtime. Solutions for scaling applications include oversized actuations, coatings or alternative valve types, but each solution has its own limitations.
The orientation of valves in the pipeline can often affect performance. A variety of valve orientations are shown in a hydroseparator in Image 1. When the drawing was submitted for tender, the valves were shown in a vertical position on horizontal lines, which is typical of valves. guillotine. Application data sheets indicated that excessive wear could be expected in the lower portion of the piping due to sedimentation and heavy solids sliding along the pipeline. Valves purchased for the project included a solder coating in the lower third of the orifice to resist wear due to solids settling. The remaining portion of the valve orifice was left as standard carbon steel as specified. During installation, space limitations forced the valves to be installed in various orientations instead of vertical. This meant that the liner designed to protect the lower valve orifice from abrasion wear was not positioned correctly and the valves failed earlier than expected.
Cycle frequency can have a significant impact on the performance of an isolation valve. Depending on the type of valve, manufacturers recommend at least a partial cycle of a valve every three to six months. When a valve is operated less frequently, it can jam due to scale or corrosion (internal or external). In addition, soft valve seats and seals can become weaker, damaging other components. High cycle applications in abrasive services can cause premature wear of soft seats – care in material selection is important.
Hydraulic surges or unwanted popping in the piping system occur when a valve is operated too quickly. Some systems require a quick-closing valve to prevent reverse flow, normally a non-return valve. Occasionally, a fast acting knife gate valve will be used in conjunction with a pump stop signal to prevent reverse flow due to a high concentration of solids with settling particles. For example, a 30-inch valve may need to close in five to six seconds. A push-through style, rubber-lined knife gate valve would not be a good choice, even if it meets all other slurry handling criteria. The push-through style valve has a closing speed of approximately 1 inch per second for proper door / liner performance, meaning the valve would take 30 seconds to close. Closing at a higher speed would damage the push-through type valve, shutting off the tailings line. Another style of valve should be considered.
Overall system design:
Once a new system or rebuild is complete, the operation is left to the end user. Start-up, shutdown, and operation procedures may be different from the original design conditions. When a valve problem occurs, the original data is reviewed and the valve selection process is repeated. Considering these variations when trying to troubleshoot a valve problem can be beneficial. Monitoring equipment performance to standardize best practices can be beneficial in reducing equipment wear and maintenance.
These issues discussed apply to dewatering technology commonly used today at mine sites. However, it is important to examine two new technologies designed to capture more water quickly and how these technologies will impact valve design and selection.
Dry tailings are used in small to medium sized mining operations, especially where water is scarce and expensive. Filter presses are used to dehydrate residue to a moisture content of about 5% to 6% before it is moved to the dry cell stack. This eliminates water loss through evaporation and increases the efficiency of filtration and reuse compared to traditional tailings ponds. Valve problems are due to the higher pressures and frequent cycling required by filter press systems. Filter press systems typically have high pressure drops across the high solids slurry inlet / feed valves, creating difficult valve application.
One technology in development in oil sands mining is pit mining processing. This process uses a modular extraction plant that can be moved with the mining in the pit. The technology extracts bitumen near the mine and produces stackable dry tailings, minimizing water used in hydrotransport. The valve challenges are expected to be increased valve wear and cycling, as well as remote operation challenges and exposure to the elements.
The conservation, capture and reuse of water are of vital importance to the future of mining. The reliable performance of the valve ensures efficient operation of essential dewatering processes. Experienced sales engineers from reputable manufacturers who understand mining processes and requirements can provide invaluable advice when selecting valves for tough applications.