Water is an important resource in all mining operations. Water is used in many processes, including hydraulic transportation, solution leaching, dust control, screening and separation, etc. In the past few decades, minimizing water consumption has become an increasingly important priority for the mining industry. New technologies such as dry tailings and in-pit treatment are being evaluated as additional ways to save water. The key factor in the overall success of any dehydration technology is valve performance.
After the ore is separated from the waste or tailings, dehydration begins. Due to different ore types, each mine faces specific and unique challenges in the dehydration process. Choosing the best valve for each process will affect overall system performance and increase the efficiency of returning valuable water to the facility for reuse.
Many tools can be used to search and compare valve options. An application data sheet that outlines the application conditions (pressure, temperature, viscosity, specific gravity, percent solids, and pH) is necessary to select the correct valve. Industry and internal standards and specifications help further define valve requirements, such as flange and valve body ratings, testing, and material standards. The specification can also define closure, isolation, and leakage capabilities. Reliable valve specifications are an important starting point. Documenting industry standards and internal minimum requirements ensures an informed comparison of valve solutions from multiple suppliers.
When selecting valves for dehydration applications, factors such as corrosion, wear, fouling, valve orientation, cycle frequency, cycle time, and overall system design must be considered to prevent potential problems.
To prevent corrosion, media specifications should define the concentration and temperature, and include normal and abnormal conditions. The specification should also define the chemicals or procedures used to flush or clean the pipeline to ensure that the selected valve materials are compatible. For example, using a steam jetty to clean serious scale in pipes can easily cut or damage elastomer valve seats and seals.
A common problem in tailings systems is wear. Anti-wear technologies include surfacing, spraying, ceramic and non-metallic linings such as rubber and polyurethane. The rheological study of abrasive materials found that in addition to the solid percentage and speed, the size and shape of the solid particles also have a considerable influence on the wear resistance technology. In some applications, the combination of corrosion and wear is a double trouble, not only because it limits options, but also because safety becomes a major issue. Abrasive materials will eventually wear the valve seat, seals and even the valve body. Due consideration must be given to minimizing the risks of media exposure to people and the environment.
Expansion is usually a condition missing from application data. Material accumulation on the surface of the flow control components and valve seat can damage the isolation valve and shorten its life. Fouling causes the valve to jam or lock, usually in a partially open position. A stuck valve restricts flow and prevents isolation, requiring almost immediate and unplanned downtime. Solutions for scaling applications include oversized drives, coatings, or alternative valve types, but each solution has its own limitations.
The orientation of the valve in the pipeline usually affects performance. The hydraulic separator in Figure 1 shows various valve orientations. When the tender drawings came out, the valve was depicted in the vertical position on the horizontal line-a typical knife gate valve. The application data sheet points out that due to heavy solids settling and dragging along the pipeline, excessive wear may occur in the lower part of the pipeline. The valve purchased for this project includes a surfacing layer on the lower third of the port to prevent wear caused by solid deposits. The rest of the valve port is reserved for the designated standard carbon steel. During the installation process, space constraints caused the valve to be installed in various directions instead of vertical. This means that the cover layer designed to protect the lower valve port from wear was not positioned correctly and the valve failed earlier than expected.
The cycle frequency will have a significant impact on the performance of the isolation valve. Depending on the valve type, the manufacturer recommends a partial cycle of the valve at least every three to six months. When the valve operating frequency is low, it may lock due to scale or corrosion (internal or external). In addition, the elasticity of the valve soft seat and seal will be reduced, causing damage to other parts. High-cycle applications in grinding services can cause premature wear of soft valve seats-it is important to pay attention to material selection.
When the valve is operated too fast, unexpected hydraulic fluctuations or violent shocks will occur in the pipeline system. Some systems require a quick shut-off valve to prevent backflow, usually a check valve. Sometimes a quick-acting knife gate valve will be used in conjunction with a pump shut-off signal to prevent reverse flow due to the high solids concentration of the settled particles. For example, a 30-inch valve may need to close within 5 to 6 seconds. A push-in rubber-lined knife gate valve is not a good choice, even if it meets all other criteria for mud treatment. The closing speed of the push-in valve is approximately 1 inch per second to obtain proper gate/liner performance, which means that the valve takes 30 seconds to close. Closing at a higher speed will damage the pusher valve, thereby closing the tailings pipeline. Another type of valve should be considered.
After the new system or rebuild is completed, the operation will be handed over to the end user. Start-up, shutdown and operating procedures may differ from the original design conditions. When a valve problem occurs, the original data is reviewed and the valve selection process is repeated. It may be beneficial to consider these changes when trying to solve valve problems. Tracking equipment performance to standardize best practices helps reduce equipment wear and maintenance.
The issues discussed are applicable to dewatering techniques commonly used in mines today. However, it is important to understand two new technologies designed to capture more water quickly and how these technologies will affect valve design and selection.
Dry pile tailings are used for small and medium-sized mine operations, especially where water resources are scarce and costly. Use a filter press to dehydrate the tailings to a moisture content of about 5% to 6%, and then move the tailings to a dry pile. Compared with traditional tailings ponds, this eliminates water loss caused by evaporation and improves the efficiency of filtration and reuse. The valve challenge is due to the higher pressure and frequent cycles required by the filter press system. The filter press system usually has a high pressure drop on the inlet/feed valve of the high solid content slurry, which makes the valve application difficult.
A developing technology in oil sands mining is extraction and processing in pits. The process uses modular extraction equipment that can be moved with the mining operations in the pit. This technology extracts bitumen and produces dry tailings near mining operations, minimizing water use in hydraulic transportation. It is anticipated that the challenges facing the valve will be increased wear and valve circulation, as well as the challenges of remote operation and exposure to elements.
Water conservation, capture and reuse are vital to the future of the mining industry. Reliable valve performance can ensure the efficient operation of important dehydration processes. Experienced sales engineers work with well-known manufacturers to understand individual mining processes and requirements and can provide valuable advice when selecting valves for difficult applications.
Bruce Brabant is the technical marketing manager of DeZURIK, Inc. His contact information is bruce.brabant@dezurik.com. For more information, please visit www.dezurik.com.