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Pumps & Systems, September 2008
Valves used in flotation cells, dewatering service, cyclone separators and tailing applications tend to wear out quickly, so mining companies spend thousands of dollars replacing expensive valves and parts. One solution to this dilemma is elastomer-lined valves, including pinch valves and slurry knife gate valves.
Pinch Valves
One option for abrasive mining applications is pinch valves. Pinch valves have proven successful because of their elastomer sleeves. When open, wear is minimal because the valve sleeve has the same inner diameter as the pipe. Since all of the process is contained within the sleeve, operators prefer using pinch valves because the operating torques needed to open and close remain constant throughout the life of the valve.
A typical mine tailing dam will use hundreds of pinch valves in the header pipe and as spigot valves. Handling tailings is extremely difficult and requires a dependable, abrasion-resistant valve.
Tailing is a slurry consisting of the finely ground rock that remains after minerals or metals have been removed from mining ore. After the ore is crushed, it is ground into fine particles that are mixed with water and chemicals. This process frees the mined materials, but it also leaves behind waste matter with the consistency of fine rock. This waste matter, or slurry, usually consists of approximately 50 percent solid matter, which often moves through the valves at high pressures and velocity.
In one particular case in Arizona, a mining company conducted an all-inclusive evaluation to help solve ongoing issues with valves in tailings. More than 20 valves, including pinch, plug, knife gate, butterfly and ball valves, were evaluated for more than eight months to decide which valve was best suited for the tough demands of tailing service.
Ten valves were tested at a time to make sure the assessment would be a precise measurement of long-term use. The same valves were used throughout the tailing dam during the test to ensure each valve was subjected to continuous use. After eight months of evaluation, equivalent to an estimated six years of operating life on a typical tailing dam, the test ended.
The mining company determined that elastomer-lined pinch valves outlasted all other valves in the test because of the protective elastomer sleeves. The sleeves feature bias-ply, fabric-reinforced rubber that provides structural support and makes them similar in construction to a heavy-duty truck tire. The mining company discovered that the rubber elastomer sleeves are much tougher than metal in slurry applications. Moreover, because the sleeve is the only wetted part of the valve, expensive metal alloys are not needed.
The valve experienced no wear or turbulence in the wide open position. Unlike the flow patterns of a conventional valve, which create a deflection that causes wear, the flow pattern of the elastomer-lined pinch valve is streamlined. The valve lining, flange gaskets and seating surface are also combined into one unit.
The full-port design of the valves makes them ideal for slurry abrasive processes. The valves perform like a full-port section of wear-resistant pipe when open, so no obstacles exist in the line to block flow. In addition, the valves completely shut off when they are closed. Many pinch valves can be left in the open or closed position for years without affecting the operating torque because the elastomer sleeves isolate the operating mechanism or gate from the flow.
Slurry Knife Gate Valves
Another, more recent technology used in slurry service is a hybrid of a knife gate valve and pinch valve. This hybrid slurry knife gate valve provides many of the same benefits a pinch valve offers, but with a reduced face-to-face dimension. For instance, full-port elastomer sleeves are utilized to handle the abrasive nature of the slurry.
The hybrid slurry knife gate valve uses a blade, which slides between the sleeve halves to open and shut the valve. In comparison, a pinch valve uses an external mechanism to squeeze, or "pinch," the valve closed.
To prevent the buildup of solids in the valve body or chest, slurry knife gate valves are often slotted in the bottom. Each time the valve cycles open or closed, a small discharge of slurry is vented out the body, keeping the body and sleeves from dewatering or collecting solids. Many slurry knife gate valves are rated to 100-psi. In higher pressure applications, alternative gate materials or pinch valves should be considered.
Chris Raftis is the president of Red Valve Company.
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