A Jenike & Johanson client company had a tough problem: packaging different grades of kaolin clay uniformly into bulk bags at high rates. The clay's poor handling characteristics made it very difficult to maintain uniform discharge from the existing 100 ton silos to the packaging equipment. Discharging from the silos at a high bulk density was also critical for efficient packaging.

Our testing revealed that the existing silos discharged the material in a funnel flow pattern. With the poor flow characteristics of the material, this can result in stable arches and ratholes. Additional problems with this funnel flow discharge pattern included erratic flow, segregation [220K QuickTime video], reduced live silo capacity, and high loads on both structure and downstream equipment due to collapsing ratholes and eccentric flow channels. As a fine powder, kaolin clay can become aerated and behave like a fluid, flooding [524K QuickTime video] through downstream equipment if a rathole collapses or fresh material is added to the silo.

We determined that while the first-in, first-out sequence of mass flow would solve many of these problems, obtaining mass flow would be tricky. First, kaolin clay is very frictional and tends to shear on itself rather than slide along a sloping wall surface, which is required for mass flow. The hopper wall angle and material would have to be carefully chosen, based on the measured flow properties of the kaolin clay.

Second, because kaolin clay has a low permeability, achieving the client's desired high flow rate would be challenging. Most bulk solids tend to dilate as they flow from an area of higher pressure near the top of a converging hopper to lower pressure near the outlet. Air fills the voids created in the dilating material. However, due to the clay's low permeability, the air is more likely to be drawn into the silo through an unsealed outlet rather than through the top of the material bed. This counter-current air flow, although seemingly minor, severely limits discharge rates.

Our design had to meet the client's three primary needs:

• each silo must be able to store 100 tons of kaolin clay;
• the discharging clay must have as high a bulk density as possible; and
• the silos must discharge the clay at a minimum rate of 6 tons per hour.

Our first step in the solution was to design silos that ensured mass flow. To overcome the arching capability and frictional characteristics of the clay, we selected a transition hopper (designed with a long slotted outlet instead of the round outlet of a typical conical hopper). The transition hopper design allowed us to create less steep wall angles than in a cone. It was also easier to mate a feeder to the slotted outlet, which would be sized to prevent arching.

Our next consideration was the feeder itself. The wrong choice of feeder could create funnel flow in an otherwise well-designed mass flow silo. In this case, the design challenge was the impermeable material's limiting flow rate. The usual solution for increasing flow rates in fine materials—aerating them to create a fluid condition—was not appropriate for this application, which required packaging the material with a high bulk density.

Therefore, we recommended a custom-designed, oval-shaped, air-assisted discharger. Instead of aerating a large volume of material, the air-assisted discharger fluidizes only a thin layer of material near the walls of the discharger. This thin layer of fluidized material greatly reduces friction at the walls of the discharger, allowing material to flow along relatively shallow wall angles. Also, the addition of air overcomes the limiting rate problems and increases the packaged density in comparison to the previous system.

Scale model tests on the conceptual design confirmed the counterintuitive notion that the addition of air would increase packaging density. We then designed, fabricated, tested, and supplied a full scale discharger.

Jenike & Johanson's mass flow design met—even exceeded—our client's original requirements. It is not only an efficient and successful solution to a troublesome flow problem, but also a very cost effective one: our design is one-third the cost of a comparable mechanical feeding technique.

In addition, our design is able to fill a one ton bulk bag in about two minutes—five times faster than the original requirement. Clearly, for a challenge such as the one posed by kaolin clay, creativity and custom design can save the day.