Case Study: Sizing of Tailings Tank Level Control Valves in Mining Applications

Tailings tank level control valves in mining applications play a critical role in mineral processing, especially in the process of increasing water recovery, which is paramount for improving the environmental sustainability of mining projects. One example of this application in mineral processing is the tailings control valve at the bottom of an Eriez Coarse Particle Flotation circuit using HydroFloat technology. The throttling response, reliability, flow dynamics, wear resistance and sizing practices of the control valve in this critical service (crucial to the operation of the flotation cells) require a comprehensive engineering approach to ensure the valve is properly designed to handle this abrasive service.

This article explains valve sizing practices to account for changes in slurry fluid behavior during the mine start-up phase versus normal operation. The changes in slurry fluid properties impact valve sizing and throttling response, and if not accounted for with proper engineering practices, can lead to valve and piping failures, resulting in unplanned downtime and repairs. The article is a case study of an in-service valve that was analyzed after its installation at Newcrest Cadia in Australia, demonstrating the coarse particle flotation technology.

Cadia is one of Australia’s largest gold mining operations, owned by Newmont Corporation, and is found in central west New South Wales nearly 155 miles (250 km) from Sydney. The company produces gold doré bars, or unrefined gold, from a gravity circuit and gold-rich copper concentrates from a flotation circuit, which is the process this case study involved. The concentrates are piped to a dewatering plant then exported. The customer required severe service control valves that were designed to handle the challenging flow conditions of tailings applications need to have proper engineering checks and balances to ensure reliable service.

Metal seated butterfly valve damaged due to control of abrasive media.

Source: SAF

Customers like the Newcrest mine that deal with extreme abrasive processes, require product designers to have a design philosophy that ensures control valves for these processes provide the right control and the longest lifespan. Some considerations are:

1. Dedicated effort in sizing each valve for the full spectrum of process conditions.

2. Streamlining and optimizing flow components to reduce unwanted turbulence for the majority of the throttling positions of the valve.

3. Using the toughest materials available for flow components to slow the damaging effects of erosion.

Ball valveSource: SAF

In this particular technology test and demonstration, the Newcrest mine installed a valve with the coarse particle flotation technology, that was designed with the considerations above to mitigate the challenges in this tailings control application. In addition to evaluating typical process conditions covering minimum, normal, and maximum flow conditions, the design engineer sought to understand the mine start-up flow conditions, which had vastly different flow environments and fluid properties that significantly impact the throttle characteristics of the valve. In this project, the mine start-up phase produced very high viscosity tailings, which are considered a non-Newtonian fluid, requiring a Cv (flow coefficient) correction method using high viscosity numbers. The science of valve sizing for non-Newtonian fluids is not exact, and there is no straightforward calculation; however, iterative approaches coupled with experimental data can assist a valve design engineer in ensuring the rated flow capacity of the valve can handle higher flow demands required with very high viscosity tailings fluids. This detailed flow and sizing analysis is a critical starting point in custom engineering a control valve for this type of service.

The next step was the design and development of the valve trim components, which are the parts that control the fluid through the valve. An iterative approach using Computational Fluid Dynamics (CFD) simulations was employed to study the velocity gradients through the valve at different flow conditions and analyze flow parameters such as turbulence, eddies and areas of flow recirculation that could create excessive erosion damage to valve components. This design approach allowed the engineering team to minimize unwanted turbulence and evaluate valve performance in all throttling conditions while carefully monitoring the discharge jet and many other variables to predict valve performance in the field.

Computational fluid dynamic (CFD) image of the valve with the custom trim from SAF.

Source: SAF

Another crucial design step in the custom engineering of a valve for this specific abrasive flow environment was the dedicated approach to streamline flow components. This final step of the design philosophy required selecting materials appropriate for the given application in terms of corrosion, temperature, pressure limitations and especially abrasion. In some cases, a proprietary composition of carbide was selected to balance toughness and strength with abrasion resistance, as these valves must function in some of the harshest flow environments with large solids and high-velocity tailings fluids.

Following the above described design and custom engineering philosophy, the reliable and predictable operation of severe service control valves designed to handle tailings discharge has been practically demonstrated at NewCrest. In many mining applications, end users are reluctant to use control valves for throttling high percentage solids tailings fluids, as they have proven to be difficult for most control valve types. However, this test case demonstrated that for the specific severely abrasive control applications in this mining application, the right approach is an engineered valve designed specifically for the process conditions to ensure proper control while minimizing turbulence and centering the flow.

Amir Emami, P.Eng. is a mechanical engineer and vice president of engineering at Special Alloy Fab (SAF). He has 14 years of experience in valve design, flow analysis and product development, and has been leading engineering team with a dedicated approach in custom designing flow equipment for the last nine years.

Maria Aguirre, P,Eng., edited this article and is a mechanical engineer and business development manager at SAF. She holds a master’s degree in engineering and has 14 years of experience in the valve and automation industry.