Cobalt-Hydroxide Filter Cake Dryer Feeder
This case study is an example of two Kamengo metering bins receiving cobalt hydroxide filter cake from a filter press and metering the wet cake to a dryer. This installation went into service in 2014. In 2019, the mine purchased an additional three Feeders.
Prior to the retrofit, the mine was discharging cobalt hydroxide filter cake from a filter press into a 60-degree plane flow hopper that necked down to an 18” screw feeder. This metering bin suffered from chronic plugging that was particularly disruptive to the mine operation. The filter press was expected to discharge in 3 minutes, however, to keep the level of material in the storage bin to a minimum, mine operators manual emptied the filter press plate by plate. This exhaustive process took three hours to empty to the filter press.
Why the Current Storage and Feed Arrangement is Suffering from Chronic Bridging and Inconsistent Discharge
The existing screw feeder metering bin was suffering from chronic plugging for three reasons:
- Poor bin geometry. The sloping walls and discharge opening of the existing hopper above the screw feeder were insufficient to produce a reliable mass flow or first-in, first-out discharge pattern. First, the sloping walls were too shallow. As a result, material could not slide along the hopper walls, but would instead try to slip within itself in a funnel flow, or first-in, last-out discharge pattern. Second, the discharge opening was to narrow. The narrower the opening, the less strength a material requires to form a stable arch that gravity cannot reliably break.
- The conveying action of the screw feeder, which is shearing material from the storage bin, was compacting the stored filter cake against the front of the bin wall. As the filter cake compacts, it gains significant shear strength. With enough compaction, the filter cake gains sufficient shear strength to bridge over the Feeder.
- The screw feeder had a propensity to withdraw material from the rear of the bin. This selective withdrawal of material, in addition to shallow sloping walls, induced a funnel flow, or first-in, last-out discharge pattern in the bin. In principle, there is nothing wrong with funnel flow as long as the effective opening of the bin exceeds the bulk solid’s piping dimension (or distance over which the bulk solid can form a stable rat-hole). Unfortunately, the piping dimension for cobalt hydroxide filter cake is quite large – in fact, it is larger than the metering bin. To handle cobalt hydroxide filter cake reliably without hang-ups it must be discharged in mass flow, or a first-in, first-out discharge pattern. To do so, the Feeder must withdraw material evenly from the hopper’s full discharge outlet.
The solution to fix this cobalt hydroxide filter cake metering bin was to replace the storage hopper with a hopper with steeper hopper walls and a wide and long opening. The steeper hopper walls were required to promote a mass flow discharge, where material slides along the hopper walls. The wide and long discharge opening was required to ensure the material cannot bridge over the Feeder. By correcting the geometry of the storage bin, if one were to remove the feeder, the hopper would now self-empty with gravity in a mass flow or first-in, first-out discharge pattern. To ensure the entire hopper and feeder arrangement is reliable, Kamengo paired the new hopper with a Kamengo Feeder. The value and necessity of the Kamengo Feeder is that it withdraws material evenly from the entire discharge opening of the storage hopper. By definition, to achieve mass flow, the bulk solid must descend the storage bin as a single body with all the stored material in motion, and the only way to achieve this is for the feeder to withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.
To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Mining Industry.