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Safe carriage of Iron ore & other iron concentrates in bulk
Iron ore is used for the production of metallic iron in steel-making. Although some 45 countries export natural iron ore resources, seven countries provide 75% of the total exported. The two largest exporters are Brazil and Australia, with about 33% of total world exports each. Other exporting countries include Chile, India, South Africa, Canada, Russia and the United States.
Iron ore may be shipped in a number of forms:
Handling iron ore in bulk carrier
- ROM : run of mine, which is ore as it comes from the mine, of no particular grade;
- Fines : small screenings of ore (less than 6mm). Fines may be sintered to form large particles, lumps or masses by heating the material to below its melting point with another material (e.g. limestone or coke breeze) until its particles adhere together. Fines may also be pelletised to form round pellets of ore;
- Lump : ore of larger pieces (10mm ~ 40mm);
- Concentrate: ore that has been refined to remove the bulk of waste materials.
Most iron ore is carried in Cape-size ships, i.e. larger than 80,000 tonnes deadweight, although ships of this category are often much bigger and are generally in the region of 170,000 tonnes deadweight. However, the largest are presently of about 360 metres in length, a beam of 65 metres and a draft of 24 metres with a capacity of 400,000 tonnes deadweight.
The majority of incidents where bulk carriers have been lost were when carrying iron ore. In the carriage of iron ore, the following precautions should be taken:
- Iron ores are heavy cargoes which occupy a small area for a large weight, ie they have a low stowage factor (between 0.240.80 m3/tonne). It is therefore important that the tanktop has sufficient strength to carry certain iron ores. If a bulk carrier loads a homogeneous cargo of, say, iron ore, the amount of cargo permitted to be loaded in the hold would be determined by multiplying the surface area of the tank top by the permissible load per m2. The loading given by this calculation should never be exceeded.
Fig:Iron ore stock
- trimming of these cargoes is generally required (even though their angle of repose is mostly above 35°) to spread their weight across the entire tanktop
- the stability of vessels as iron ore is a high density cargo, when loaded on an ordinary bulk carrier (not an ore carrier) it will increase the vessel's GM to make it a `stiff' ship.
- dust iron ore is commonly loaded with conveyor belts, grabs, chutes and bucket belt unloaders, causing significant quantities of dust during both loading and discharging. However, enclosed conveyor belt systems generate less dust. The dust may damage ship's machinery as well as the health of personnel.
- moisture content iron ore is assumed to have a homogeneous moisture content between 016%. However, if kept lying in the open, the moisture content may increase due to absorption from air or rain. If the exact moisture content is unknown, a proper laboratory test may be called for. The Master must also ask the shipper to detail the cargo's moisture content and TML
- In recent years an increase incidents attributed to carrying iron ore filings with excessive moisture content, causing sloshing and adverse stability and in some cases the rapid loss of the ship. During the monsoon season in the Indian sub-continent iron ore filings are stored and transported open to the elements. It is worth remembering that the cargo may remain stockpiled before being shipped later in the dry season
Fig: Grab for iron ore
- It is imperative that the cargo moisture content is tested prior to and monitored during all stages of loading as, once onboard, cargo may be extremely difficult to remove.
The shipper's test certificate should be presented before loading, be sound and no more than seven days old. A good indicator during the load is the presence of splatter marks of iron ore filings on the bulkheads. If splatter marks are evident, they should be taken very seriously as an indication that the moisture content is above the TML and the flow moisture point. If the Master is in any doubt, he should contact the owners and the P&I Club correspondent.
- stress monitoring stresses upon the ship, both in port and at sea, should be monitored using an HSMS (Hull Stress Monitoring System) if fitted, and the movement of cargo and ballast monitored using stress calculating software
- alternate hold loading bulk carriers sometimes carried iron ore in alternate holds at the request of charterers owing to the economic advantages of faster turnround and raised centre of gravity. To do so ships must be specially strengthened. SOLAS Chapter XII bans alternate hold loading for single skin ships of 150 m or more in length, built before 1st July 1999, that carry cargo equal to 90% of the ship's deadweight.
- the iron ore standard sampling procedure given in the IMSBC Code should be followed.
Precautions for carriage of iron ore:
concentrates of iron are produced by either:
- the dry method, in which high grade ore is crushed to remove waste material, leaving a low moisture content in the powdered ore. Iron concentrates obtained by the dry method are susceptible to spontaneous combustion because of the air already trapped within the concentrate during the crushing process.
Due to dampness within the cargo, the sulphur can react with the oxygen to produce heat, resulting in spontaneous combustion. Therefore, for concentrates, the holds should be kept closed and ventilation avoided. Additionally, due to sulphur and other metallic contents within concentrates, these may emit poisonous/explosive gases. The cargo spaces should be treated as enclosed spaces and appropriate entry procedures followed
- the wet method, in which the crushed rock is washed in water to separate the sulphides. The concentrate has a high moisture content that may liquefy and shift onboard ship. The moisture content of these concentrates should therefore be checked prior to loading and, if above TML, the cargo should be rejected
Sponge Iron (or Direct Reduced Iron (DRI))
This is produced from iron ore and is used in the manufacture of steel. It involves heating the iron ore at low temperatures without allowing it to come in contact with air. It is, therefore, important to allow the piles of sponge iron to be weathered before shipping so that its temperature drops.
The sponge iron is separated by magnets so has little moisture content. Therefore, when it is shipped in large quantities, if it gets damp from contact with air, it is likely to oxidise and so is liable to spontaneously combust and/or emit toxic fumes. Sponge iron cannot be used directly for manufacture of any consumer products as it first needs to be processed into wrought iron. Some owners will not accept these cargoes on their vessels
Produced by burning coke as fuel to heat the iron ore which then gives off carbon monoxide. This combines with the iron oxides in the iron ore to produce metallic iron known as pig iron. Pig iron is obtained in a molten state that can be moulded as required. It contains a comparatively higher quantity of carbon, about 3.5%, and is less pure than sponge iron.
Fig: Discharging iron ore -final stage
When loading any of these iron ore products, care should be taken to monitor the temperature of the cargo itself whether it is in pellet, lump or briquette form. If the temperature exceeds 65°C, it must not be loaded onboard. The alternate option is that the shipper's declaration provides information as to whether the cargo has been sufficiently treated for oxidation/ corrosion inhibition to avoid any chance of spontaneous combustion on increase in the moisture content.
Blended iron-ore containing DRI (C) can be identified by its chemical composition, which must include: total iron (Fe) content; metallic iron (Feo) content; and moisture content. The information must be supported by a certificate from an independent testing laboratory and be related to the cargo offered for shipment. The certificate should state the method and standards followed when obtaining the tested samples (preferably ISO 10835) and when determining the metallic iron content (preferably ISO 5416).
If a blended iron-ore cargo contains any metallic iron it should be regarded as DRI (C) and carried in accordance with the IMSBC Code.
Problem with Mill Scale
Mill Scale, a by product of hot rolled steel-making, is a bulk commodity liable to liquefaction, like iron ore fines. Mill Scale and Mill Scale Fines possess a transportable moisture limit (TML) and have thus been confirmed as Group A cargoes, which should not be accepted for loading without the shipper having certified the moisture content and TML. Due to the high density of the cargo, the IMSBC Code requires that it should be trimmed flat for the voyage, distributing the weight evenly over the tank top - wet base cargoes are prone to shifting, as the bottom liquefies and the top of the stow becomes free to slide over the base.
More topics on iron ore
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Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
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