3rd Asian Mining Congress

February 02, 2010

Introduction

Over ten billion tons of bulk material is transported worldwide annually. A great amount of this material consists of coal, iron ore, grain, potash, salt, and sugar. During transport all this material is placed in a stockpile and reclaimed at least once, with most materials being stockpiled and reclaimed several times during their journey from origin to final destination. Obviously the stockpiling and reclaiming of bulk material is very important to the world economy, and any simplification and reduction of cost in bulk material storage and reclaim would have a significant impact on that economy.

There is a new, patented method of bulk material storage and reclaim that we feel will have just such an economical impact, The Walker Reclaimer. This paper will examine the advantages and disadvantages of existing bulk material storage and reclaim systems and compare and contrast these systems with the Walker Reclaimer.

Existing Technology


Basic Open Stockpile

The basic open stockpile system consists of a pile of bulk material resting on the ground with stockpiling and reclaiming being performed manually with machinery such as front end loaders.

The Basic Open Stockpile has the advantage of being low in capital cost and in having an unlimited storage capacity. The system is a zero live storage system, at no point can the material be reclaimed simply by gravity; a mechanical action is required for this system to operate. Operational cost can be very high as well depending on the cost of labor and the equipment used. Due to the manual nature of this system it does not scale well for operations needing anything over a minimal feed rate.

Open Stockpile over a Reclaim Tunnel

An open stockpile on top of a reclaim tunnel with reclaim ports consists of a stockpile resting on top of a reclaim tunnel with ports in the tunnel feeding a reclaim mechanism, usually a conveyor. This configuration can consist of a single or multiple reclaim tunnels. (Figure 1) One of the most widely used systems in the world, this system has many advantages and disadvantages.


An open stockpile on top of a reclaim tunnel with reclaim ports is an improvement on the basic stockpile in that a certain amount, generally less than a third of the stockpile, can be reclaimed by gravity. The balance must be pushed into the reclaim ports. Pushing bulk material is faster and less costly than lifting it as in the Basic Open Stockpile. While reclaiming by gravity, the reclaim rate of this type of system can match the capacity of the reclaim transport device, usually a belt conveyor. The operating cost of this type of system is generally less than that of the Basic Open Stockpile, but is still high.

This system does have some negative aspects. While the operating cost of this system is lower than the Basic Open Stockpile, its capital cost is higher. Like the Basic Open Stockpile this system is also not covered so it does not protect the environment or the material being handled. For total reclaim of material to occur, some material must still be pushed to the reclaim port. When operating in this manner, the reclaim rate is limited to the pushing rate. For this reason, many open stockpiles over reclaim tunnels which require high reclaim rates are sized to hold approximately three times the capacity of the transport device to be loaded.

Open Stockpile with Mechanical Reclaimer

Another variation of the Open Stockpile arrangement is the addition of a mechanical reclaimer. Some available mechanical reclaimers are the portal reclaimer, bucket wheel reclaimer, and drum reclaimer. This method of reclaim has zero live storage, all reclaiming of material is performed mechanically.

Mechanical reclaimers can be installed to reclaim virtually the entire stockpile. Reclaim rates of over 5,000 TPH can be attained by mechanical reclaimers. By using multiple mechanical reclaimers, stockpiles of unlimited capacity can be achieved. If two or more materials are to be accurately blended while reclaiming, it is necessary to store those materials in separate locations and use multiple mechanical reclaimers. Adequate blending can usually be accomplished by storing the material in layers and reclaiming vertically through those layers.

Although this method has many advantages over the previous methods described, it does have some drawbacks. Namely, this method of reclaim is generally much higher in capital cost than the open stockpile over a reclaim tunnel and the operational costs are similar. Additionally, the open stockpile provides no protection for the material or the environment.

Enclosed Storage with a Mechanical Reclaimer

To protect the material and environment, a rotating stacking conveyor and mechanical reclaimer can be installed inside a domed cover. While this method is effective, the storage capacity is limited. To provide for more storage, multiple units must be installed. Since the reclaimer and storage conveyor support is located in the center of the dome structure, a lot of enclosed volume is not available for the storage of material, a less than totally efficient arrangement. Blending materials with this system requires multiple storage facilities to segregate the materials to be blended. These systems have high capital cost, and the operational cost is comparable to the open stockpile with mechanical reclaimer.

Vertical Storage Silo

This method of storage and reclaim, just as the name describes, consists of a silo with a single or multiple reclaim ports underneath that feed a reclaim conveyor.

With proper design, this method has the advantage of being a totally gravity fed system, no mechanical forces are required for the full reclaim of the stored material. This method is especially applicable in situations where facility space is at a premium. While the capital cost of the vertical silo method is high, the operational cost is low. Blending of material in vertical silos must be performed prior to the storage of material, or different materials can be stored in separate silos and blended when the material is reclaimed. Due to the high capital cost of vertical silos, their storage capacity is limited. Vertical silos over 70 meters high are rare.

New Technology


The Walker Reclaimer

The Walker Reclaimer (Figure 2) consists of one or more square conical surfaces each sloped down toward an opening in its center. Each conical surface consists of a steel plate supported by a support system and lined with a low friction surface. In the center of the conical surface is a reclaim opening fitted with a PICOR flow control gate. (Figure 3) Attached to the underside of the conical surface are a number of vibrators. When the PICOR flow control gate is opened, material flows by gravity onto the reclaim conveyor. The flow rate through the opening is matched to the conveyor capacity by the PICOR flow control gate. When a flow channel is established in the stockpile, a number of vibrators around the reclaim opening are energized. Since the density of flowing bulk material is significantly less than that of static material, the vibration energy introduced into the static material around the flow channel causes the flow channel to widen. The material on the conical surface around the reclaim opening will migrate into the reclaim opening causing the flow channel to widen further. As material is reclaimed, the depth of material on the conical surface decreases causing the flow channel to widen further. As the reclaim channel widens, more vibrators are energized toward the perimeter of the conical surface causing all the remaining material on the conical surface to migrate into the reclaim opening.

The Walker Reclaimer combines the attributes of the stockpile over a reclaim tunnel and the vertical storage silo. Since the Walker Reclaimer is an enclosed system the stored material is protected from the environment and the environment is protected from the material. The conical surfaces can be arranged to provide an unlimited amount of storage for bulk materials. Due to the design of the conical surfaces, materials can be stored on adjacent conical surfaces without mixing and without sacrificing storage volume.

This system can also be used to reclaim bulk materials from large, circular, domed structures. This allows the storage of more material in a limited area but the cost of storing the material is increased and the foundation loads are increased.

One of the most exciting uses of the Walker Reclaimer is in the design of self unloading barges and ships. By installing a Walker Reclaimer in each hold of a ship and providing a collecting conveyor under the discharge openings, a self unloading ship of simple design, low cost, and low maintenance is achieved. For new ship construction, this system should be very economical. For existing bulk carriers, it may be possible to install the Walker Reclaimer and increase the annual capacity of the vessel by up to twenty percent.

By installing the Walker Reclaimer system in barges and putting a receiving hopper on the stern of each barge, it would be possible to unload a raft of barges without breaking the raft. This greatly increases the utilization of the barges by drastically decreasing the unloading time. It also decreases the risk to personnel by eliminating the need to break the raft, move the barges, and reassemble the raft.

There are other potential applications for the Walker Reclaimer. We are investigating bulk haulage trailers with a sloped bottom of low friction material discharging through an opening in the center of the trailer. The trailer would be pulled over a discharge area, the discharge gate opened and the bottom vibrators energized.

Conclusion

The Walker Reclaimer was developed to provide the most economical method to reclaim a wide range of bulk materials while protecting the material and the environment. As the preceding shows, the Walker Reclaimer combines positive attributes of both surface storage and vertical silo storage systems with very few of the negatives associated with those methods. Additionally, the Walker reclaimer has potential applicability in situations where the other methods of storage and reclaim would be impractical or impossible.



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