Challenges of Medium Density Slurry Service in Mining and Mineral Processing

Medium-density slurry service—applications where solids sized between 5 to 1,000 microns are present in the fluid medium and percent solids range from 5 to 20% by weight—has presented a significant challenge for both maintenance personnel and valve manufacturers in the mineral processing industry. Traditionally, mine owners have had to choose between an economic light-service valve that, in medium-slurry applications, will require frequent replacement, or a heavy-duty valve that is over-specified and expensive. However, recent technology developments in urethane bi-directional knife gate valves provide a third solution for end users.

Selecting the Right Valve

Urethane valves are typically manufactured via proprietary designs. Although valve manufacturers will often follow a pressure testing and design standard, such as MSS SP-81, these urethane products are subject to the individual manufacturer’s internal testing and design standards. As a result, end users need to compare the benefits of one design over another, which requires a risk-based decision. Mine owners consider many factors, including installed base, cost of ownership, and the specific valve design and its suitability for a specific application. By working closely with a main valve partner, mine owners can specify and purchase the right valve that meets their needs. Hence, they eliminate the need for over-specifying or under-specifying valves and reduce their costs without compromising on reliability or performance.

Importance of Gate Deflection

To achieve tight shutoff isolation and a long service life for the liner, the design of the gate in combination with the internal liner geometry are critically important. Typically, a urethane knife gate valve will have a higher thrust requirement due to the friction that occurs because of the gate sliding on the internal urethane during opening and closing cycles. If not designed correctly, gates will tend to deflect and potentially damage the liner on the downstream side along the bottom edge as pressure on the gate increases when the valve is nearing closed position. The downside is that isolation of the valve is then compromised. A well-designed valve will not trade-off isolation, thrust and liner life in accounting for gate deflection.

A Third Design Option

Conventional urethane valve designs remain viable options for light- and heavy-slurry applications. However, the addition of a third valve design option provides an efficient solution for medium-density applications.

This type of innovative knife gate valve is suitable for all types of mining applications (coal, gold, silver, copper and uranium to name a few) and can help reduce the user’s total cost of ownership.

The new design option offers a cost-effective solution to the more rugged high-density slurry valves and can better withstand a higher corrosive or erosive environment than the light-slurry options, enabling longer service campaigns. As a result, plants running medium-density slurry applications can increase runtime and reduce unplanned outages that lead to lost production.

This was achieved through a two-piece body construction and field-replaceable snap-in liner design that represents a new type of knife gate valve. No special tools or highly trained personnel are needed to replace the liner. An integral seat-face seal eliminates the need for flange gaskets, reducing installation costs and making installation easier. This, along with an improved gland, makes the newly designed knife gate valve particularly suitable for mining applications.

Case Study: Global Coal Producer Benefits from Custom Valve Design

A global coal producer in Australia was facing an ongoing challenge in its coal preparation plant; the stainless cyclone feed knife gate valves were being destroyed prematurely by abrasive coal fines and were failing in less than 12 months. Other problems included increased wear on the surrounding pipework and increased demand for parts and labor that was increasing production costs for the plant.

After a thorough evaluation, the valve supplier engineering team concluded that the wear of valve components was inevitable due to the highly abrasive medium and design of the valves used. The valves in use were not full-bore valves, which are typically choked on the sides to improve gate support. This resulted in higher velocity and turbulence through the valves, which in turn was accelerating wear of the internal port area.

The valve supplier developed a customized solution using full round port valves with internal component selection. This design supported the gate and provided extended service life under harsh conditions. The new design also incorporated sealing beads and flush-out area for a consistently reliable seal, as well as a snap-in polyurethane liner that improved abrasion resistance and completely covered the exposed internals to prevent wear.

The results were significant: The new knife gate valve design more than doubled the performance. After 12 months of service, the new knife gate valves were examined and proved to still be in excellent condition and were returned to service without requiring any repairs, lowering the total cost of ownership on the valves and reducing overall production costs for the plant.

Michael Gordon is product manager, conventional knife gate valves for Emerson Automation Solutions.