by Andrew Wells BSc (Hon’s Eng), ARSM, FIMMM, CEng Consultant - CWA International
Bulk mineral cargoes amount to approximately 3.5 billion tonnes per annum (Btpa) and account for around 50% of dry bulk cargoes.
• Iron ore and coal account for around 38% and 35%, respectively, of mineral cargoes, a total of about 73%. The remaining 27% of cargoes consist of:
• Metal concentrates such as copper, lead and zinc destined for smelters to produce these metals
• Other unprocessed ores of metals such as bauxite and alumina for aluminium production and nickel ore for nickel.
• Ferro-alloys such as ferro-chrome, ferro-man ganese, and ferro-silicon destined as additives to special steels
• Direct reduced iron (DRI) and pig iron for use in steelmaking
• Phosphate rock for the production of fertilisers and fertilisers themselves
• Sulphur for the manufacture of sulphuric acid
• Fluorspar (calcium fluoride) concentrate for use as a flux or the manufacture of fluorine chemicals
• Construction materials and fillers such as cement, cement clinker, sand, clay and china clay (kaolin).
The IMSBC Code 2013
The International Maritime Solid Bulk Cargoes (IMSBC) Code is the main legislative document governing the safe carriage of solid bulk cargoes.
It became mandatory on 1 January 2011 under the Safety at Sea (SOLAS) Convention.
Bulk cargoes, including mineral cargoes are classified under three Groups in the code:
• Group A - cargoes which may liquefy if shipped at a moisture content in excess of their transportable moisture limit (TML)
• Group B - cargoes which possess a chemical hazard which could give rise to a dangerous situation on a ship
• Group C - cargoes which are neither liable to liquefy (Group A) nor to possess chemical hazards (Group B)
Thus, cargoes may be classified as either Group A, B, C or A and B.
Types of Bulk Metal and Mineral Cargo Hazards Mineral cargoes can be prone to the following main types of hazards:
• Self-Sustaining Decomposition
• Hydrogen evolution
• Poisonous gas evolution
I will now deal briefly with each of these hazard types in turn.
Liquefaction and partial liquefaction are terms used to describe solid bulk cargoes that become mobile under the influence of water content and the motions of the vessel.
The following simplified diagrams show the original material (left) consisting of solids, water and air voids. Following handling and carriage of the material the
volume is reduced and one can reach the point where every solid particle is separated by a tiny thin layer of water. If this water cannot escape then it lubricates the particles like a car tyre aquaplaning on a wet road. The material thus completely loses its strength and behaves literally like a liquid.
The free surface effect of the liquid can than cause ship stability issues sufficient to capsize the vessel.
Metal and Mineral cargoes that can liquefy include:
• Metal and mineral concentrates – but these are routinely tested and produced under controlled conditions
• Ores – nickel laterite, manganese ore fines and iron ore fines. These may have large lumps but also may contain a significant proportion of fine particles
Of significance is that the ores are often simply dug up and not processed at all. They are therefore subject to the vagaries of climate and not under the close production controls of a modern day mineral processing plant. They are also not always handled appropriately afterwards.
The IMSBC Code contains three test methods which are used to determine at which moisture content a bulk material can be safely carried: the Transportable
Moisture Limit or TML.
The following types of cargo are affected:
• Sulphide concentrates such as those of: copper, lead, zinc, iron and nickel
• Direct Reduced Iron products such as DRI, and Hot briquetted iron (HBI)
• Coal, especially those of lower rank or quality and those containing the mineral pyrite.
• Petroleum coke, a solid reside of oil refining
The following figure shows how a runaway reaction causes the cargo temperature (shown in black) to “take off” in about six hours in this case.
So, what is happening in this example? The answer is a type of chemical reaction called oxidation which happens to produce heat. The natural mineral pyrite is the culprit here where the chemical reaction can be written as follows:
FeS + 2 O2 FeSO4 + HEAT
Or in words:
“Pyrite plus oxygen gives ferrous sulphate plus heat”
So, why is the heat generated?
The answer lies in the fact that in the reaction above, the “system” has reduced in volume with a solid and a gas producing a solid. This reduction in volume
causes heat to be generated according to the Combined Gas Law which states:
PV = kT
Where P = pressure, V = volume, T = temperature and k is a constant
Put another way, your bicycle pump heats up when you use it and the refrigerator cools down when the coolant gas is expanded!
Self-Sustaining Decomposition (SSD)
Another mechanism for a runaway fire is SSD. SSD happens where a locally initiated decomposition reaction, spreads through the mass of a cargo. The initiation could be from say a live electrical part in contact with the cargo.
The term “self-sustaining” means that oxygen from the cargo itself fuels the fire. This phenomenon occurs in inorganic fertilizers and other materials with a high ammonium nitrate fraction which, incidentally is used as an ingredient to provide the oxygen in some common explosives.
Hydrogen evolution is another potential hazard which may be found in metallurgical products or intermediaries including:
• Aluminium – e.g. drosses, salt slags, skimmings, spent cathode and spent pot liners
• Direct Reduced Iron – DRI
• Hot Briquetted Iron – HBI
• Ferroalloys – e.g. ferrosilicon, ferromanganese etc.
• Zinc – including ingots, “drosses”, “residues”, “skimmings” and ash
What makes these materials such as zinc and aluminium which are an essential part of our everyday life hazardous in bulk? The answer lies again with the basic chemistry. All metals react with water with varying rapidity as follows:
Metal + Water = Metal Oxide +H2 For example: Zn + H2O = ZnO +H2 (gas)
We also know that salt water in sea air for example reacts (causes corrosion) faster than fresh water.
Now, hydrogen has a Lower Explosion Limit (LEL) in air of 4%. Doing some maths one can calculate that 1 litre of zinc is capable of producing 60,000 litres or 60
cubic metres of explosive mixture.
Furthermore, hydrogen is a very light gas and it rises to the top of any cargo space. Strike a light and BOOM! The vessel illustrated below was carrying zinc bearing cargo when a “mild” explosion displaced the hatch covers.
Fortunately, no crew were on deck at the time and
nobody was injured.
Poisonous gases and Oxygen Depletion
Poisonous gases may be produced in a similar manner to the production of hydrogen above when minute quantities of impurities such as arsenic and phosphorous are also present and in contact with metal and moisture. The following gases have been shown to occur when ferrosilicon (mainly used in steel
production) is wetted in the confined space of a vessel hold for example:
• Arsine (AsH3) – Colourless, garlic odour, flammable and the most toxic form of arsenic
• Phosphine (PH3) – Colourless, no odour at lethal concentration level, flammable and toxic Arsine’s hazard is accentuated in confined spaces as it is heavier than air.
Cargoes such as the most common sulphide form of metallic mineral concentrates – copper, lead and zinc will oxidise and deplete the oxygen in a confined space. This has resulted in the suffocation of persons entering such spaces without prior adequate ventilation.
We have already talked about the reaction of cargoes with water and with oxygen in the air. Corrosion is the term used to describe the oxidation of a metal. However, the corrosion process is often more complex. For example, elemental sulphur produced as a by-product of oil and gas refining and shipped in bulk can cause severe corrosion to vessel’s unprotected holds. This is often particularly severe on the “tank top”, the nautical term for the “floor” of the hold.
In this case sulphur “eating” sulphobacillus or similar bacteria were considered to have accelerated the corrosion process which involved the production of hydrogen sulphide and black Mackinawite or iron sulphide.
Mitigation of Risk and the IMSBC Code
Clearly the vast majority of bulk cargoes, including metals and mineral ones are carried safely to their destinations around the world. The implementation of the requirements of the IMSBC Code plays a key role together with the Health and Safety systems of the vessels’ Owners and Charterers and their insurers, the P&I Clubs.
However, despite being frequently updated (three times in the last four years) the Code is by no means perfect. Just taking two examples concerning the declaration of Cargo Group which is a mandatory part of the Code:
1. There is no difference technically between Pyritic Ashes and Calcined Pyrites, both are the residue from burning pyrite or iron sulphide. However:
i. Calcined pyrites is listed as both Groups A and B cargo which is correct
ii. PYRITIC ASHES (iron) is listed only as a Group A cargo
2. There are 4 entries for seed cake in the IMSBC Code, 3 of which are listed as Group B and 1 as Group C. However it is not stated that all products in this schedule with a moisture content greater than the TML are Group A.
Mis-declaration of cargoes
Adding to the imperfections of the Code there are sometimes occasions when a party or parties may misdeclare a cargo for “commercial reasons”. This can have serious consequences, for example where a Shipper might declare sweepings from a stock yard as iron ore fines (IOF) whereas in fact it is a mixture of IOF and DRI fines which would be Group B and Group A. The possibilities for mixtures are endless.
The Qualities of the Metals and Mineral Cargo Expert
Because of the huge variety and variability of metals and minerals cargoes shipped around the world the cargo expert in this field, in addition to having chemical experience also needs an understanding of bulk materials handling and particle mechanics.
In order to anticipate and respond adequately in unusual situations additional knowledge and experience gained in the production and use of these cargoes is invaluable. For example; knowledge of how a copper smelter processes copper concentrate can add a lot of value for your Client’s case when one can demonstrate that the rejected seawater wetted cargo is treatable and still worth $8M, albeit after some air drying and blending.
BSc (Hon’s Eng), ARSM, FIMMM, CEng
Consultant, CWA International