Bulkagram 14, 3 October 2022
The major cost component of cement is in its handling, storage and distribution since the production process is well established and cost optimized. Cement markets are far flung and vulnerable to trading conditions worldwide. To be able to deliver widely varying quantities of cement demand at customer premises in the shortest possible time at the lowest possible cost is the primary objective of all large and small cement producers.
By far the lowest cost method of cement transport is in bulk by sea. Shore based terminals are now becoming larger and soon we will see the arrival of giant cement terminals for the ocean receipt and inland distribution of cement. Many countries have now made it compulsory for cement imports to be in bulk to avoid the wastage and pollution associated with the large volume management of bagged cement. However, the flexibility and low cost of the small terminal operator sometimes gives him an edge over the larger terminal operators. We will discuss in this paper the low cost cement terminal operations of the smaller players.
DOCKSIDE SYSTEMS:
Bulk cement which is bagged immediately on receipt on shore will have the lowest cost conversion of bulk cement to discrete bagged units as there is no intermediate storage or handling between ship and packing plant. Dockside bagging has therefore come to stay. Manufacturers will however need to look at reliable and high capacity dockside bagging systems since these systems are currently the bottleneck for large volume dockside bagging. This is because of the need for such bagging units to be mobile as dock authorities will not allow permanent or semi-permanent installations on the jetty. Currently, most mobile bagging stations are limited to a capacity of 60 to 80 TPH in 4 or 5 in-line packing configurations. If these units are placed on the dockside for bagging, the upstream feeding units (mobile ship unloaders) from the ship will have to be of similar capacities. However, mobile pneumatic ship unloader costs per tonne of cement are lower for higher tonnages of unloading. A compromise is therefore required to achieve the best economic combination of ship unloader capacity and ship size, bagging capacity remaining the same. The best solution would be to have an optimum capacity ship unloader feeding to multiple dockside bagging units to achieve the economics of scale with ship sizes in the 5000 to 10000 DWT range.
Where dockside bagging is not required and intermediate storage is permissible, dockside silos are generally installed about 300 to 500m away from the jetty on high soil bearing capacity surfaces. Mobile ship unloaders can in this case be of much higher capacities than those required for ship side bagging and can be rail or gantry mounted on the dockside.
Bulk cement filled in the silos from ship unloaders are bagged at much higher capacities than the mobile units deploying rotary packers instead of in-line packers. However, this would require a sizeable investment in support structures, offices, covered plant area etc. and would be a higher cost operation than the mobile bagging option. Nevertheless, economies of scale can reduce the per tonne cost of bagged cement in such fixed silo-based bagging plants when capacities are increased.
In both options, it is possible to feed bulk tankers with the bulk cement unloaded from the ships.
While considering low cost options for dockside handling of cement from ships and barges, one has to also look at complete terminal operations both import and export.
IMPORT TERMINALS:
Cement terminals are built in a variety of sizes considering both yearly tonnages through them. Small terminals can be classified as those having a tonnage approx. 150000 to 200000 tonnes through them yearly. Large terminals will exceed this figure and go upto 500000 to a million tonnes per year. We will specifically look at small terminals that are affordable to build and which many small operators not having the massive resources of large cement companies and distributors can easily install.
The primary function of the Import Terminal would be to enable fast unloading of incoming ships / barges and optimal movement of cement in an acceptable form to the final user destination. A lot depends on the local market conditions. For example, in a market predominantly having small constructions like residences, offices etc. (most likely in a city or town) the cement would need to be distributed in 50 kg. bags. On the other hand, in a market which has large constructions in progress like multi-storied office complexes, apartment blocks etc, the cement would most probably be delivered to RMC plants where it would be converted to RMC for supply to such users. Assuming major civil constructions like dams and canals, the cement would need to be moved in bulk at the highest tonnages possible. Hence, the Import Terminal needs to be customized for each such requirement. This also determines the size of the ship that would be discharging at the facility. The most popular size of ship for long distance cement carriers is about 25000 to 30000 DWT. Panamax Vessels (60000 DWT) have still to become popular for cement carrying since there are technical limitations in high capacity unloading of such large volumes from these vessels. If the terminal is envisaging arrival of small barges (2500 DWT) to large ships (30000 DWT), the design of the terminal becomes complicated. Single equipment to unload both sizes of vessels are generally not available and if available would be prohibitively expensive. Once the ship is unloaded, the storage arrangements have to be compatible with the throughput of the terminal. Hence the design of the terminal has to consider the best combination of incoming ship size, unloader type and capacity, storage method and packing facilities to enable lowest cost per tonne of cement handled.
HOW TO CHOOSE THE OPTIMUM EQUIPMENT CONFIGURATION
A number of combinations of equipment configurations can be made to arrive at an optimum modular fit for specific terminal throughputs.
A sketch is attached that shows a number of such modular options.
The primary equipment on the dockside for Import Terminal would be the unloader. For small terminals with dockside storage available, by far the most cost effective and efficient unloading method would be pneumatic in the range of 250 – 400 TPH. It would be prudent to employ two or more units of similar capacity to ensure flexibility and to insure against possible equipment down time. An optimum combination would be three small and light units of 150 TPH giving a total unloading capacity of about 450 TPH.
Most terminal operators are by now aware that the rated capacity of the unloader should not be used to compute ship berthing times for the unloading operation. All pneumatic unloading operations have some inherent properties as follows:
High rate of unloading for the first 50 – 60% of the hold capacity.
Drop in unloading rate as the hold empties down to 80% with 20% material remaining.
Requirement of moving the final layer of cement to mid hold heaps to enable final excavation by the suction unit. This generally requires the use of a small front end-loader which should be lowered into the holds.
Final manual sweeping of the holds clean.
Hence the average unloading rate (of critical importance to the terminal operator) would be lower than the rated capacity of the unloading unit considering repositioning of the unloading arm to different holds in addition to the above factors. There is what is known as a TTS capacity which is “Through-The-Ship” capacity which is what the terminal operator has to reckon with. The TTS capacity is considerably dependent on operator training and skill level, environmental conditions at the port,and crew cooperation and teamwork. A skilled and trained operator can achieve a TTS figure of about 70 – 80% of the rated capacity. When using relatively untrained and unskilled operators, the TTS can be as low as 60 – 65% of the rated capacity. There is also a learning curve that stabilizes after a few hours of commencement of the ship unloading operation.
Screw type unloaders can also be deployed to evacuate the cement from the holds of the ship. However, the screw unit will not remove as much of the material towards the end of unloading as would the pneumatic unit. This is because of the need to effectively flood the screw with cement at all times to maintain full capacity of the screws. While there are many devices that enable such flood feeding to take place, the rotating steel mass of the suction screw requires that the operator is careful not to touch the tank top. Operators are generally careful about this and keep considerably off the tank top thus requiring a large amount of hold cleanup by front end loaders and manual labour. In the pneumatic unloader however, the steel suction nozzle can be separately fitted with a flexible suction hose towards the end of unloading and this hose can be manouvered inside the hold to clean-out the final leftover cement.
Once the cement is unloaded from the holds through the pneumatic suction arm, it needs to be either filled into bulk trucks alongside the unloading unit or it has to be fed to some distance, say 500m, to stationary storage facilities at the dockside. A number of options for this operation which includes screw out-loads, dense phase pneumatic transfer, dilute phase pneumatic transfer etc are available. Keeping the pneumatic suction arm as a standard unloading method, modular attachments to the pneumatic unloading system can be made to perform any of the above discharge operations like bulk truck loading or distant silo or flat storage loading. Fig 1 give typical options.
EXPORT TERMINALS:
Having invested in an import terminal, most operators would like to build in flexibility to enable these terminals to be operated as an Export Terminals also. To operate a small Import Terminal as an Export Terminal is not difficult provided the equipment is properly selected. If you consider unloading equipment that has pneumatic suction and screw out-load, it is a simple matter to configure the equipment both for Import and Export operations.
The loading equipment in a combined Import / Export terminal will include silos, conveyors, filters etc. for the export operation. Additional equipment needed would be suitable ship loading equipment and also suitable equipment to move the export bulk material cargo from the silo terminals to the jetty or wharf to the loader. At this point, the material should be transferred to the ship holds through the ship loader. For cement, the ideal ship loading equipment will be one that would operate at relatively high capacity in a dust free environment. While pneumatic systems ideally lend themselves to elegant unloading circuits, pneumatic loading has inherent problems of air and material separation at the loading point into the ship’s hold. The ideal loading system would therefore be a mechanical loading unit using covered belt conveyors with a shuttle boom ending in a vertical telescopic loading spout reaching down into the ship’s hold. Since we are talking about small terminals, this equipment which is generally large and rail or gantry mounted should in this case be small, mobile and flexible. This is based on the assumption that the export vessel will not exceed 10000 to 15000 DWT .This however restricts the export from these terminals to relatively small inland and coastal export servicing operations when compared to large terminals servicing export ocean shipment.
For small terminals, the dockside storage would ideally be about 15 – 20% more than the size of the incoming and outgoing vessels. All equipment should be designed not only to handle easy materials like cement but also other similar materials to ensure more value addition. Depending on the range of materials to be handled, the terminal equipment design should be such that it is compatible with the material being handled. If the terminal is handling difficult to flow fine powders, suitable aeration systems must be installed to ensure troublefree silo discharge. Likewise, if materials like wood chips are to be handled, silos may need to be replaced with floor bunkers. Flat Storage suitable for one material may not suitable for others. It is best to consider professional opinion in cases where there are wide diversities in the flow and storage properties of the bulk material being handled in the terminals. If the terminal is to be only for cement, matters are relatively easier since cement is a very good material for handling both pneumatically and mechanically. It is heavy and has ideal flow and storage properties.
TYPICAL EQUIPMENT FOR BOTH IMPORT AND EXPORT TERMINALS :
Storage devices like silos, flat storage, dome storage
Unloading and feeding devices from ship to storage
Unloading and feeding devices from storage to ship
Packing plants for big bags (1 tonne and above) / small bags (25 kg. – 100 kg.)
Loading / unloading systems for bulk rail and road distribution / receipt
Filters and other pollution control equipment.
Ship hold clean out equipment
Back-up generator power
Flexible and mobile loading / unloading equipment
Mobile and fixed belt conveying systems for loading.
Combination of belt conveyors, screw conveyors, bucket elevators etc. within the terminal for
optimum material movement.
Grab back ups with dedusting mobile hoppers
Since storage would be one of the major requirements of any terminal small or big, the lowest cost storage method can be used depending on the product handled in the terminal. Considering cement, the lowest cost option would be the flat storage system for storage in the range of 5000 to 50000 MT. Till date, major deterrents for using flat storage has been the requirement of land (approx. 6MT of cement per sq.m However, in many small terminals land is generally not a problem because the port traffic density would not be as much as the larger operating commercial terminals. The other relative disadvantage would be the need to use fully enclosed and air conditioned front end loaders for reclaiming cement from the flat storage which is the conventional method of doing so.
AUTOMATIC FLAT STORAGE RECLAIM:
It is now possible to have fully automatic flat storage cement reclamation by the use of fluidized floor pads in the flat storage warehouse on the same principle as the air assisted gravity conveyors typically used for the cement plants. Since cement can flow at about 7 – 8 deg. slope on an aerated fabric medium, this principle is used in the flat storage floor and modular sections of the floor are fluidized and evacuated through vertical pneumatic suction pipes which take the cement directly to the packing station or to the ship loading system. Fig 2 shows a typical arrangement for such a fluidized floor flat storage system. With India becoming a low cost ,high skill manufacturing hub in the world economy, such modular fluidized floor pads manufactured here and retrofitted for existing flat storages will increase the use of flat storage systems as a viable and economical storage system for small and large terminals. To this, we must add the cost of the pneumatic suction system that will evacuate the cement from this floor.
A complete flat storage shed of size 40m x 20m x 6m eaves height to store approximately 5000MT of cement with automatic reclaim will include the following:
Flat storage steel structure complete with all sides covered with precoated steel sheets.
Flat storage distribution pipe and valves
Instrumentation
Filters
Utilities (Compressed air)
Aeration blower with floor aeration fabric and supports
With worldwide consolidation taking place in the cement industry as evidenced by the recent acquisitions of the larger Corporations, there is considerable scope for companies to focus on the mechanism of cement handling, storage and distribution.