Flotation cells have typically been controlled by dart valves, yet, in recent years some customers, OEMs, and consultants have begun to recommend pinch valves for flotation control valves. In certain cell designs, the dart is housed within a metal box on the outside of the flotation cell. This structure adds to the cost of the cell, whereas a pinch valve can be connected directly to the cell with a simple flange. The pinch valve is equally as productive as the dart valve and increases the flow capacity in one valve rather than, in certain cases, two dart valves. With flotation columns, conversely, there is ample driving head to the inlet of the control valve. However, this type of flotation has a risk of control valve cavitation if downstream piping does not supply sufficient back pressure.
One common way to create head pressure is to use a barometric leg or J piping arrangement after the control valve to create this back pressure. Without this consideration, many types of valves may cavitate and cause short valve lifetime. In flotation, the goal is to have an accurate control valve with a long mean time between failures to keep the flotation at peak operational performance. Pinch valves are also ideal for column flotation control. In this application, a pinch valve uses a rubber sleeve inside the valve to protect against the abrasive flow stream. The rubber sleeve typically lasts longer than metallic valves due to its superior abrasion resistance. The valve can be controlled via a 4-20 mA signal with a smart positioner, with pneumatic actuation, or also via an electric actuator equipped with 4-20 mA capability. At the same time, pinch valves very accurately control the column or cell level with no performance issues.
The pinch valve provides a long mean time between failure and a typical repair is simply the replacement of a rubber sleeve. The rubber sleeve replacement is cost-effective and can be performed by plant maintenance personnel quite rapidly.
Flotation also requires delivery reagents to perform the process, with typically these chemicals delivered via some type of pump. These chemicals often have long polymer chains that cannot be agitated. If a high shear pump is selected, it may break these polymer chains and make the chemicals less effective, resulting in overdosing of expensive chemicals to perform the same amount of work. A pump that is low shear should be used in this process, with peristaltic pumps typically chosen. These pumps come with very low shear, and certain designs can provide very accurate control of the pumping process.
When selecting a peristaltic pump for flotation chemical delivery, a pump that has the below characteristics and features may be required:
• Has 5,000:1 turndown capability;
• Can receive a 4-20 mA control signal and feedback on a 4-20 mA signal;
• Has four additional control contacts so alarms, and such can be reported back to DCS or SCADA;
• Has NEMA 4X– dust-tight watertight enclosure;
• Has capacitance-based tube leak detection so no components are in contact with the medium; cannot be affected by corrosion or damage;
• Comes with a large digital touch screen for local pump control;
• Is easy to maintain, with fast tube replacement; and
• Has multi-language selection in the pump.
As stated above, pinch valves control flotation processes of various types extremely well and provide a higher level of availability due to low maintenance costs and a long mean time between failure.
Text by Todd Loudin. For more information, contact juha.hakkinen@valmet.com
This content was published in International Mining (June 2023), as part of the article Competitive Flows / Pumps and Pipelines by Dan Gleeson.
