Being involved in moving large volumes of dry bulk powders from ship to shore one always gets involved with the onshore storage situation. Looking at a storage facility, there are two different processes going on. First the product getting into the facility, which normally happens at irregular intervals and in large quantities. The second process is the product leaving the facility, a process that normally happens on a regular basis and depending on seasonal effects in smaller quantities. At that point the reclaim system becomes important. The reclaim system inside a flat-, silo- or dome storage facility is not only important in just getting the product out of the storage facility, the reclaim system is also important in having a good stock rotation, both mechanical- as well as pneumatic systems are available for this purpose.
A pneumatic reclaim systems only works for products that can be fluidized, such as cement and fly-ash etc. Similar techniques are successfully applied over may years on board of self-discharging cement carriers, where large quantities of cement are moved around the world on ships contractually obliged to be empty after discharge. Fluidisation floors are commonly used as a part of the off-loading system available with the various equipment manufacturers.
The principle of fluidization works as follows. When cement rests on fluidizing fabric and compressed air is fed from underneath this fluidizing fabric, the compressed air will penetrate through the fluidizing fabric and enter the cement. The cement/air mixture now becomes fluid. By putting the fabric on an 8º slope, the fluidized cement will flow to the lowest point, which can be on the outside or centre of the storage facility.
The major advantage of a pneumatic reclaim system is the fact that they have very few moving parts and consequently the maintenance cost for such a system are expected to be low. But that is not the only advantage. The main advantage of a full fluidized floor system, effectively divided into dedicated zones with fluidizing air being cycled from floor area to floor area and from fluidizing section to fluidizing section is the following. A fluidized floor configuration like that, will makes it possible to reclaim effectively from one section of the dome, and clean out alternately sections of the dome to avoid Set pack.
Set pack may occur inside larger storage facilities, as cement or fly-ash left for a longer period of time will pack and harden, it will lose its characteristics and therefore reduce in value for its owner. After all, it will becomes a burden rather than an asset.
A unique feature of a fluidized floor is that it has the capability to create a good stock rotation within the storage facility. A fluidized floor, covering the highest possible percentage of the overall floor area, can be a mighty tool against Set pack. A good stock rotation can be achieved by having the floor inside the dome as a full size fluidized floor. The technique is well known within the industry, however one needs to understand the technique of fluidization of large volumes of the product stocked and how the compressed air needs to be channelled via the various zones and parts of the fluidized floor to obtain the optimum on fluidization.
A dome storage facility is ideal for the application of a fluidized floor as a dome is designed to withstand the pressure of large volumes of cement and hold it within the smallest possible space. The circular dome floor can be divided into sections, two or more. Having the discharge points at the lowest spot of each fluidized bed and the outlets at the centre or the outside of the circular floor area the fluidized product will flow to and can be reclaimed from, those lower outlet points. Applying the stock rotation feature, the oldest portion of the stocked product can be reclaimed as it is possible to completely clean out the different sections of the floor on a regular and rotating basis and by doing so, avoiding the occurrence of Set pack.
Using the fluidized floor concept, could imply that cement storage could now be completed in one larger single dome storage facility instead of two smaller ones, without the risk of Set pack.
The divided fluidized floor space provides the same redundancy and reliability as two separate storage/reclaim systems. The mechanical reclaimers feed conveying systems located in a tunnel under each dome. From these tunnels the cement is than conveyed to truck loading silos and other outlets points. The reason given to use two domes instead of one large one is that this guarantees a good stock rotation, product can be moved from one facility to the other, and provides good redundancy in the event of equipment breakdown and will allow for time required to complete maintenance and repair work without being unable to service customers.
A single dome storage facility can represent a substantial capital cost saving compared to a two dome storage facility of the same size, apart from the savings on operational cost. Another saving can be achieved having a configuration with side outlets. Such a configuration has the potential to allow for a truck loading system without using silos.
Energy cost for the fluidization process can be minimized as each part of the fluidization floor will be divided into various aeration zones and sections, with fluidizing air being cycled through those various areas by an automated system. This approach will minimize the cost for the compressed air equipment as well as the cost for the investment.