|The unloading of solid bulk materials from ships presents a number of special problems: the large quantities of material to be handled and the type of storage containers (ship¿s hold) determine the type of operation required.
Depending on the size of the ship, this may have an on-board unloading system (self-unloading ship), or it may need to use the unloading systems available on the quay at the port.
The services required from an unloading system for bulk solid materials from a ship in port are
- low operating costs;
- environmental compatibility in terms of dust and noise;
- minimum weight and dimensions;
- ease of mobility;
- flexibility in the unloading of different types of materials;
- compatibility in unloading from holds with different shapes and capacities;
- high capacity of unloaded material;
- reduced times for cleaning the unloading system between operations on different types of materials;
- possibility of carrying out unloading operations even in bad weather (with the hold partially covered);
- complete emptying of the hold with the minimum amount of human involvement inside the hold (for example, using front bulldozers in those areas where the unloading system is not able to reach).
A single unloading system is unable to meet all these requirements, but a compromise can be reached. Often it is necessary to use more than one kind of system (for example, unloading the majority of the material with a high capacity system and completing the emptying of the hold with smaller, more appropriate systems) to obtain the desired results.
Bulk solids can be unloaded using a mechanical or pneumatic handling system.
Mechanical unloading of the material can be done in several ways: the easiest and oldest method uses a crane with a bucket which, after reaching down to the hold, takes up the material and unloads it directly onto the quay in a pile. An adjacent handling system then removes the material and takes it to its destination.
The crane unloading system may also be directly available on the ship itself, or on the quayside. Although very simple, use of a crane presents a number of limitations:
- being a discontinuous unloading system, the time required to transport the suspended material and to drop it on the quay represent a waste of time within the process (average hourly capacity 400-600 t/h);
- as it is an open system, the unloaded material comes into contact with the atmosphere;
- it is not possible to fully empty the hold;
- it is not suitable for powder products;
- material may be lost during transport;
- the sides of the hold may be damaged by knocks from the bucket.
Normally this system is used for unloading hard materials such as minerals, coal, and gravel, which do not deteriorate and can be left in the open without degenerating or being contaminated.
To reduce the time needed to unload a ship, continuous unloading systems have to be used.
The principle behind the bucket elevator system (see "Mechanical Conveying Technology in Handling Powder and Granular Materials") can be effectively used to unload bulk solids from a ship, with a few suitable modifications.
One solution often used is to equip a mechanical system at the end that enters the hold with a wheel comprising a number of buckets around its edge (bucket wheel). This wheel, which turns on its axis, acts as an excavator, and assists the pick-up of material in the hold, guaranteeing high unloading capacities. The material is lifted in the buckets as the wheel rotates and unloaded onto the conveyor, which carries it off to its destination.
Using a number of technological solutions which allow the system to move around inside the hold, in both width and depth, allows the system to be used to cover the whole of the space occupied by the material concerned.
Another technological solution is an L-shaped mechanical elevator structure (L-shaped bucket elevator). This is particularly suitable for ships with small hatches with respect to the volume of the hold, and is a shaped structure that allows the material to be removed from the corners of the hold, guaranteeing a good degree of emptying.
Generally speaking. this unloading system is used for materials that have hard surfaces, different sizes or a high density.
The unloading capacity is high (3,000-10,000 t/h), but the type of material removal system makes it impossible to completely empty the hold. Alternative systems therefore need to be adopted, such as the use of front loaders, which collect the material inside the hold and pile it up at the removal section of the mechanical system, or use of a pneumatic suction system during the final unloading phases.
The principle behind the screw conveyor system (see "Mechanical Conveying Technology in Handling Powder and Granular Materials") can also be adopted for bulk solids.
Generally speaking, this unloading system consists of a horizontal conveyor screw, behind which there is a miller and a vertical conveyor screw. The conveyor screw rotates and picks up the material, and conveying it thanks to the miller, to the lower end of the vertical screw. The material at the vertical screw creates pressure and, overcoming the force of gravity caused by the rotation of the screw itself, allows the material to enter the lifting pipe for removal from the hold.
The material is removed from the hold in layers: basically, the whole surface of the product in the hold is ¿swept¿ by the horizontal screw. When one layer has been picked up, the horizontal conveyor screw is lowered and then starts on the next layer of material.
The miller not only acts as an extractor-feeder of the solids, but also as a scraping device at the bottom of the hold, allowing the product to be practically completely removed.
The unloading capacity that can be reached with this system is not high (100-800 t/h), but the system is most suitable for cohesive or binding material, thanks to the mechanical crushing achieved by the conveyor screw.
As the material undergoes no brusque movements inside the hold during this kind of operation, the system is also particularly suitable for highly volatile powders.
It is, however, unsuitable for unloading highly abrasive materials, because this type of handling could create heavy wear on the surfaces of the system being used to unload the material. In this case it is possible to build machine components made of a suitable material to obtain the benefits of this system without having the problems of wear.
Unloading solid goods from a ship using a pneumatic system (see "Components for Pneumatic Conveying Systems") is usually used for low density materials such as grit or volatile substances (cement).
Basically the material in the hold is sucked through a suitably-shaped device into a collector and stored in a tank connected to a vacuum unit through a filtering system. The material collected in the tank is then conveyed to its destination using a pneumatic pressure handling system. Usually there are a few intermediate storage tanks near the quay, which work in parallel in order to guarantee continuous unloading. The suction collector usually runs on tracks on the quayside in order to run parallel to the ship and pick up the material all along the ship's length.
The capacity of the pneumatic unloading system is not high (500-800 t/h) and the initial investment is considerable (purchase of vacuum pumps and compressors, building of intermediate storage tanks, purchase of filtering system). These disadvantages are, however, counterbalanced by the absence of dust emissions (both during hold unloading operations and transporting the product to its destination) and by the ease of mobility of the system, which allows the hold to be almost completely emptied.