Valve Magazine - Summer 2010

segment are typically automated with self-contained electro-hydraulic actuators, which are ideal for this application’s large output torque and likely installation in remote locations.

WHAT’S BEING MINED

Once the ore is separated from the earth’s crust and transported from the extraction pit, what happens next depends on what mineral is being processed. In the case of precious and semi-precious metals (gold, nickel, etc.), the desired mineral must be separated from other elements and impurities that make up the rock. In many cases, this can be accomplished by traditional methods such as smelting ( pyrometallurgy); however, older smelting processes are expensive (energy intensive) and are harmful to the environment.

These realities—and in the case of nickel, a shortage of suitable ore beds— have given rise to the use of hydrometallurgy and more specifically, pressure

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; Have duty-cycle motor limitations

; Offer excellent partial stroke capabilities

; Are self-locking in last controlled position

; Are limited regarding spring-gen-erated failure position

; Have integral manual override capabilities

Compressed gas (pneumatic) actuators (Figures 6, 7 and 8) are available in single- or double-acting versions. Single-acting versions have springs that consume source pressure as the springs compress, thereby providing more spring force and less air force. For rotary versions, the mechanism used to convert linear to rotary motion also impacts the output characteristic. Actuator output must be carefully matched to valve requirements at all travel positions.

ACTUATORS & CONTROLS

Typically, but not always, you will find pneumatic actuators are:

; Offered in aluminum, cast iron, ductile iron, steel and stainless steel

PHOTOS COURTESY: VELAN

; Employed in both on/off and control applications

; Not self-locking except in a spring fail position

This is what the inside of a valve in an HPAL application (the type of process used on laterite ore) looks like after 24 months in service.

; Available with manual override capability only as a specified option

; Not duty-cycle limited

leaching to extract and refine these minerals. Although the process was developed in the mid 20th century, it was not used on a large scale until the late 1980s. Since then, several multi-billion dollar facilities have been built to use this efficient and environmentally responsible process, which is more akin to a chemical plant than a mine. It is in the pressure-leaching circuit that we

; Operated at pressures of 100 psig or less but are capable of higher maximum pressures

; Limited to 180º F (82º C) but are also capable of temperatures of 400º F (204º C) or above with suitable seals and bushing materials

Hydraulic actuators (Figure 9) are essen-

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find one of the most demanding valve applications in mining. Here, severe service valves isolate not only pumps but more importantly, the autoclaves where the prepared and heated slurry begins the pressure-induced chemical reaction that turns rock into gold or nickel.

Pressure leaching is similar to the slurry transport application discussed earlier, but because of the high temperature and presence of corrosive elements, standard materials are not appropriate. In fact, at the front end of the autoclave, it is common to find various grades of duplex stainless-steel. Once the “clave” itself is reached, we find titanium or Hastelloy and nothing else. These are far from standard materials and require the valve designer to pay close attention to all aspects of the design to ensure that the valve remains operational. It only takes one overlooked bushing in the wrong material to seize the valve and potentially shut down a multi-billion dollar plant. For this reason, there are only a handful of valve companies entrusted to this duty.

In this article, we have highlighted but a few of the many valve applications that fall under the category of mining— many more exist, including valves used in steel production and coal conversion. In all cases, valves used in mining must be purpose-built and designed with the rigors of the application firmly in mind.

Given that valves are playing an increased role in both the transport and processing of industrial minerals, the outlook for the valve industry in this area is relatively positive. However, this optimism must be tempered by the inherent volatility of the mining market as a whole, which will continue at a slower pace as the global economic recovery takes hold. Still, mining is poised to take advantage of the next boom cycle…and, of course valves will be there doing their part. VM

CRAIG BEKINS is product manager–severe service ball valves, valve specialist in HPAL process with experience on major projects such as Murrin Murrin, Coral Bay Nickel, Goro and more for Velan Valve Corporation ( www.velan.com). Contact him at craig.bekins@velan.com.

tially the same as pneumatics except they operate on liquid and see considerably higher pressures.

Typically, but not always, you will find hydraulic actuators:

; Are offered with linear or rotary motion in both single or double acting

; Are often available with an integral electro-hydraulic pressure source

; Operate at pressures from 500 to

3,000 psig

CONCLUSION

We used the word “typically” in this article when describing the various actuators because there are many options and no absolutes. The key to actuator selection is to define your own specific needs and then ask your suppliers to confirm their products are capable of meeting those needs.

Future columns will go into the various types of actuators in more detail. VM

ED HOLTGRAVER is CEO of QTRCO, Inc.

( www.qtrco.com), Tomball, TX. He is on the VMA Board of Directors and serves as the Actuator Lead on VMA’s Education & Training Committee. Reach him at edholtgraver@qtrco.com.