Many minerals which are essentially insoluble in water and which contain mainly finely divided material, may liquefy on ocean voyages if they contain an excessive amount of water when loaded or if they subsequently become wetted. This applies even though they may appear to be in the form of dry powders of granular materials. Such liquefaction is basically due to energy being applied to the cargo. This can be as a result of vibration due to a ship’s engine or due to motion in heavy seas. It follows that liquefaction may become apparent at almost any stage of a voyage. Any cargo of finely divided material which tends to flatten and which develops a putty-like surface during a voyage, has started to liquefy.
The presence of water on the surface of the cargo is also indicative of liquefaction. It must be stressed however, that liquefaction can occur without liquid water being observed on the surface of the cargo. If any of the phenomena described above are observed, the ship should take urgent action and should proceed to the nearest port of refuge, subject of course, to the requirements of good seamanship. It may be prudent to adjust course and speed in order to reduce the motion of the ship even if this means having to steam further before reaching a suitable port. Liquefaction is unlikely to occur provided that when loaded it complies with the IMO requirement that the moisture content of the cargo is relatively uniform and below the transportable moisture limit (TML) in each hold.
Zinc concentrate is a Group A product which is listed in the International Maritime Solid Bulk Cargoes (IMSBC) Code under the schedule for “Mineral Concentrates”
Before loading any cargo which they know or suspect may liquefy, masters should carefully check all the documentation provided by the shippers or charterers.
A certificate should be provided stating the TML for the cargo. Master to ensure that this certificate is dated within six months of loading and that it is issued by a laboratory on which reliance can be placed. The test procedure for determining the TML requires specialized equipment and experienced technicians to conduct the test.
A watch should be kept on the condition of the cargo being loaded. Any obviously wet material should be rejected as such cargo might form a shear plane on which a basically sound cargo loaded subsequently, might slide.
There are two other dangers associated with concentrate cargoes. The first is that some concentrates may heat. Shippers should always be asked specifically about this possibility. Stows of such concentrates should be trimmed roughly flat using a tracked bulldozer or similar machine which also compacts the cargo. It is sometimes helpful to sheet such materials with heavy gauge polythene film which further restricts the rate of air penetration into the cargo.
The second danger arises from the fact that even if concentrate cargoes do not heat, they absorb oxygen such that the atmosphere above the cargo in a hold which is inadequately or not at all ventilated may become deficient in oxygen and enriched with nitrogen. Air contains roughly 79% nitrogen and 20.8% oxygen and as the oxygen is absorbed by the cargo, so the oxygen content may fall to as low as 4%. The minimum concentration of oxygen required in the atmosphere in order to support life for only a few minutes is 10%. There have been fatal accidents where persons have entered fully closed holds loaded with concentrates where the oxygen content was too low.
This cargo shall be trimmed to ensure that the height difference between peaks and troughs does not exceed 5% of the ship’s breadth and that the cargo slopes uniformly from the hatch boundaries to the bulkheads and no shearing faces remain to collapse during voyage.
As the density of the cargo is extremely high, the tank top may be overstressed unless the cargo is evenly spread across the tank top to equalize the weight distribution. Due consideration shall be paid to ensure that the tank top is not overstressed during the voyage and during loading by a pile of the cargo.
Bilge wells shall be clean, dry and covered as appropriate, to prevent ingress of the cargo. The bilge system of a cargo space to which this cargo is to be loaded shall be tested to ensure it is working.