Ore concentrates

Infobox on Ore concentrates
Example of Ore concentrates
Facts
Origin	-
Stowage factor (in m3/t)	0,33/0,57 m2/t (bulk)
Humidity / moisture	-
Ventilation	Not
Risk factors	See text

Ore concentrates

Description / Application

Ore concentrate, dressed ore or simply concentrate is the product generally produced by metal ore mines. The raw ore is usually ground finely in various comminution operations and gangue (waste) is removed, thus concentrating the metal component. The concentrate is then transported to smelters where it is used to produce useful metals.

Shipment / Storage / Risk factors

Ore concentrate, dressed ore or simply concentrate is the product generally produced by metal ore mines. The raw ore is usually ground finely in various comminution operations and gangue (waste) is removed, thus concentrating the metal component. The concentrate is then transported to smelters where it is used to produce useful metals.

There are two types of ore concentrates which may be carried by vessels:
a) Dry
b) Wet

Dry
This is mainly shipped from South America and Peru, and is produced from crushing high-grade ore and removing the gross rock. The ore is in powder form, and has a low moisture content. The ores are usually sulphides and also contain free sulphur. During the crushing process air is introduced into the powder. If the material then becomes damp the oxygen will then react with the sulphide to produce sulphate, and heat is evolved. If the heat cannot escape (as is the case in bulk) the temperature of the mass will rise until the ignition point of the sulphur is reached – thus initiating a cargo fire. This will burn as long as air is available, but is usually quenched with CO₂, but as soon as the hatches are reopened the fire may break out again and this can be a very great danger. Thus, the introduction of moisture to this cargo, e.g. by ventilation or through the hatches, or by rain at the time of loading, may be just sufficient to initiate this type of oxidation with resultant fire in the cargo. In addition, any metallic element in the concentrate may react to give off hydrogen gas, the presence of which in a hold can result in an explosion.

Wet
In these cargoes the material consists of the sulphide portion of the ore which has been separated from the crushed rock by a special water flotation process. This concentrate, therefore, can contain an appreciable amount of water. Physically it appears normally to be a dry mass, but it may liquefy when subjected to motion. This type of concentrate must be considered as composed of the dry powder to which water may adhere. When the proportion of adherent water is low, the mass appears to be quite firm and will break up when rubbed between the fingers. As the water content increases it becomes first lumpy, then tacky, and finally a solid mass. At this point a series of sharp taps or vibrations will cause the mass to flow (i.e., to liquefy). The moisture content at this point is called the Flow Moisture Point.

It is obvious that such cargoes may be vicious, since when loaded they appear firm, but under the vibration of the ship’s engines can liquefy and in any sort of sea result in a shifting cargo producing a list.

It has been suggested that the problem can be dealt with by allowing liquefaction and partial settlement and then pumping off the free water which collects on the surface. However, this is not so easy as it sounds, for, as the main mass is approached, the free flowing surface thickens to a consistency of treacle. The liquid mass contains highly abrasive particles of ore, and there is no doubt that the ship’s pumps would not be able to cope for more than a short period. The problem should be dealt with, therefore, at the time of loading, by ascertaining the moisture content and assessing whether it is below or above the critical point.

The (Canadian) Concentrate Codes state that if the weight of the concentrate carried is more than 18% of the vessel’s cargo capacity, it is a matter for careful consideration. Further, cargo should only be accepted if the true moisture content is no greater than 90% of the critical value (the Flow Moisture Point) for that type of ore. The ship’s Master should only permit the loading of this commodity when he knows the true moisture content and that it is within these limits.

Samples should be taken during the course of loading, both for testing at that time and for subsequent expert analysis on behalf of the ship. The latter must, if they are to be of any value at all, be placed in hermetically sealed tins. Further, from a research point of view, samples taken at various levels would be of great assistance. This would enable a study to be made of the vertical section, for it is known that water migration does not necessarily move upwards. To ascertain whether the moisture content is too high, simple tests may be carried out during the course of loading, i.e. by jumping on the surface of the concentrate. If the concentrate is too wet for safety, this will cause the surface to become increasingly soft, until it becomes so fluid as to resemble quicksand. Also, samples may be placed in a small receptacle, such as a tumbler, and tapped for, say, a period of five minutes. At the end of this time, if the moisture content is too high, a liquid surface will become apparent, or samples on a flat plate will flow when banged rapidly.

It is interesting to note that if an iron ore concentrate contains 12% of moisture by weight, this may represent as much as 60% of the contents by volume, since the rock content is so much heavier than the water.

Any ore concentrate of high moisture content, will liquefy when energy is applied to it in the form of vibration. This can occur not only by the action of the sea but the ship’s own vibrations, occasioned by her main engines or even her auxiliary machinery.

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.

See also Concentrates (of Minerals) and consult the International Maritime Solid Bulk Cargoes Code for overseas carriage.