Section 8.6: Concentrates (Metal Sulphide Concentrates – Liquefaction, Oxygen Depletion, Toxic Gases)
Metal sulphide concentrates, such as those of copper, lead, zinc, and nickel, are intermediate products in the metallurgical process, derived from the initial ore through crushing, grinding, and flotation to increase the metal content. These concentrates are fine, often dense, particulate materials and are notorious in the bulk shipping industry for posing a combination of severe hazards: liquefaction (Group A), oxygen depletion, and the emission of toxic and sometimes flammable gases (Group B). Consequently, most metal sulphide concentrates are classified as Group A and B cargoes under the IMSBC Code, demanding the utmost vigilance and strict adherence to safety protocols by the Master and crew. Incidents involving these cargoes have led to vessel losses and fatalities, underscoring the critical need for thorough understanding and meticulous management of their risks.
1. Characteristics of Metal Sulphide Concentrates:
Physical Form: Typically fine, granular, or powdery materials. The particle size is often very small due to the grinding processes used in their production.
Appearance: Color varies depending on the metal (e.g., copper concentrates are often dark grey/black with a metallic sheen, zinc concentrates can be yellowish-brown or grey, lead concentrates are dark grey).
Moisture Content: They are often produced and transported with a significant moisture content, which is a key factor in their liquefaction risk.
Density/Stowage Factor: Generally dense cargoes, with stowage factors varying but often in the range of 0.40 to 0.80 m³/tonne. This means attention to tank top strength and stress distribution is necessary.
Chemical Composition: Primarily composed of metal sulphides (e.g., chalcopyrite for copper, sphalerite for zinc, galena for lead). They also contain varying amounts of other minerals, including iron sulphides (e.g., pyrite, pyrrhotite), silica, and sometimes traces of other metals or elements which can influence their hazardous properties.
2. IMSBC Code Classification and Key Considerations:
BCSN: The IMSBC Code has specific schedules for various concentrates, such as:
COPPER CONCENTRATE
LEAD CONCENTRATE
ZINC CONCENTRATE
NICKEL CONCENTRATE
METAL SULPHIDE CONCENTRATES, N.O.S. (Not Otherwise Specified) – This schedule is used if a specific named schedule doesn’t exist for a particular concentrate.
Group: Most metal sulphide concentrates are classified as Group A and B.
Group A (Liable to liquefy): Due to their fine particle size and the presence of moisture.
Group B (Possessing chemical hazards): Due to their propensity to oxidize, leading to oxygen depletion, heat generation (though usually not to the point of self-ignition for most common types, but can accelerate other reactions), and the emission of toxic and sometimes flammable gases (e.g., sulphur dioxide – SO₂, hydrogen sulphide – H₂S). Some concentrates, particularly those with high iron sulphide content, may also have self-heating risks, though this is less common than for cargoes like coal.
Shipper’s Declaration (Critical Information): The shipper’s declaration for metal sulphide concentrates is extremely important and must include:
Correct BCSN.
Classification as Group A and B.
TML and MC certificates (for Group A aspect).
Chemical composition: Including the concentration of the main metal sulphide, iron sulphide content, and any other significant constituents or impurities. This is vital for assessing the specific chemical hazards.
Information on the likelihood of oxygen depletion, and emission of toxic or flammable gases.
Any specific safety precautions or emergency procedures.
Often, a “weathering certificate” or evidence that the cargo has been aged or treated to reduce reactivity might be relevant for some types, although this is less standardized than for coal.
3. Key Hazards and Precautions Associated with Metal Sulphide Concentrates:
These cargoes present a challenging combination of physical and chemical risks.
A. Liquefaction (Group A Hazard – Primary Concern):
Mechanism and Consequences: Identical to other Group A cargoes (see Section 8.1 for Iron Ore Fines and Section 8.4 for Bauxite Fines). The fine particulate nature and moisture content make them highly susceptible to liquefaction if the MC exceeds the TML, leading to rapid loss of stability and potential capsize.
Critical Reliance on TML/MC Certification:
The Master must receive valid TML and MC certificates.
MC must be less than TML. No exceptions for standard bulk carriers.
Scrutinize certificate dates, issuing authority, and test methods (Flow Table or Penetration Test are common; Proctor/Fagerberg may also be used).
Visual Checks and Can Test: Constant vigilance during loading for signs of excessive moisture. The Can Test is a vital supplementary check if any doubt arises. Stop loading immediately if concerns.
P&I Club and Industry Guidance: This class of cargo is subject to intense scrutiny and numerous advisories from P&I Clubs and industry bodies due to past incidents. Masters must be familiar with current best practices and warnings.
B. Oxygen Depletion (Group B Hazard):
Mechanism: Metal sulphides, particularly in the presence of moisture and air, undergo slow oxidation. This chemical reaction consumes oxygen from the atmosphere within the cargo hold and any connected void spaces.
Consequence: The oxygen level in the hold can drop to dangerously low levels, creating a severe asphyxiation hazard for anyone entering the space. This can occur even if there are no other obvious signs like heat or strong odors.
Precautions:
Assume Oxygen Deficiency: Treat all holds containing metal sulphide concentrates (and adjacent enclosed or poorly ventilated spaces) as potentially oxygen-deficient at all times.
Strict Enclosed Space Entry Procedures: Mandatory before any entry – thorough ventilation, atmosphere testing (O₂, flammable gas, relevant toxic gases).
SCBA: Must be used if a safe atmosphere cannot be confirmed or for rescue operations.
Warning Signs: Post warnings at all access points.
C. Emission of Toxic Gases (Group B Hazard):
Mechanism:
Sulphur Dioxide (SO₂): Produced by the oxidation of sulphide minerals, especially in acidic conditions (which can be created if moisture and sulphides react to form sulphuric acid). SO₂ is a colorless, highly irritant, and toxic gas with a pungent odor.
Hydrogen Sulphide (H₂S): Can be formed if acidic conditions develop and react with metal sulphides, or sometimes by anaerobic bacterial action if organic matter is present (less common for pure concentrates). H₂S is a colorless, highly toxic, and flammable gas with a characteristic “rotten egg” smell at low concentrations, but it paralyzes the sense of smell at higher, more dangerous concentrations.
Other Gases: Depending on impurities, other toxic gases like arsine or stibine could theoretically be evolved, though this is rarer for common concentrates. Carbon dioxide (CO₂) will also be present from oxidation.
Consequences: Exposure to SO₂ or H₂S can cause severe respiratory irritation, chemical burns, pulmonary edema, neurological effects, and can be rapidly fatal at high concentrations.
Precautions:
Gas Monitoring: Regularly monitor the atmosphere in hold ullage spaces and adjacent areas for SO₂, H₂S, and other relevant toxic gases using calibrated multi-gas detectors.
Ventilation:
The IMSBC Code schedules for these cargoes usually recommend surface ventilation only if gas concentrations (flammable or toxic) approach unsafe levels, or if oxygen is depleted.
Through-stow ventilation is generally NOT recommended as it can increase oxidation rates and potentially exacerbate heating or gas production in some cases.
Ventilation must be carefully managed to avoid drawing toxic/flammable gases into accommodation or machinery spaces.
Enclosed Space Entry: As above, strict procedures and SCBA use are critical.
Awareness of Symptoms: Crew should be aware of the symptoms of exposure to SO₂ and H₂S.
D. Emission of Flammable Gases (Group B Hazard – Less Common but Possible):
Hydrogen Sulphide (H₂S): As mentioned, H₂S is also flammable (LFL approx. 4%). If concentrations build up, an explosion risk exists.
Hydrogen (H₂): Some concentrates, particularly if wetted with acidic water or if certain reactive metals are present, could potentially evolve hydrogen, which is highly flammable.
Precautions:
Monitor for flammable gases (usually as %LFL on a combustible gas indicator).
Ensure flame screens on vent outlets are clean and intact.
Strict prohibition of ignition sources near holds or vents.
E. Self-Heating (Group B Hazard – Usually Minor for Common Concentrates, but possible):
While not typically prone to the same degree of self-heating as coal or DRI, the oxidation of sulphides is an exothermic process.
If the cargo is very reactive (e.g., high pyrite/pyrrhotite content), loaded at elevated temperatures, or if ventilation is insufficient to remove heat, a slow temperature rise can occur. This can accelerate other reactions like SO₂ production.
Precautions:
Temperature monitoring of the cargo during the voyage may be advisable for some specific concentrates known to have a higher self-heating tendency (check individual IMSBC schedule).
Avoid loading hot cargo.
Stow away from heat sources.
F. Corrosivity (Group B Hazard):
If moisture is present, sulphide minerals can oxidize to form acidic compounds (e.g., sulphuric acid), which are highly corrosive to steel.
Precautions:
Holds should be clean, dry, and have intact paint coatings.
Minimize water ingress.
Thoroughly wash holds after discharge to remove all residues, which can be very acidic. Washings are likely to be HME and require careful disposal.
G. High Density / Shifting:
Similar to iron ore, concentrates are dense. Tank top strength and longitudinal stress management (including alternate hold loading if applicable and approved) are crucial.
Proper trimming as per IMSBC Code Section 5 is necessary to prevent shifting, even if the primary concern is liquefaction.
4. Pre-Loading Preparations:
Hold Cleanliness: Holds must be scrupulously clean and DRY. Any residues from previous cargoes, especially those that could react with sulphides or acids (e.g., alkalis, some organic materials), must be removed.
Bilge Wells: Clean, dry, strum boxes clear and protected. Bilge lines must be proven clear.
IMSBC Code and Shipper’s Documentation: This is paramount.
Obtain and meticulously review the shipper’s declaration, TML certificate, MC certificate, and chemical composition analysis.
Confirm the cargo is Group A and B. Verify MC < TML.
Understand the specific chemical hazards declared (e.g., expected gas emissions, iron sulphide content).
Gas Detection Equipment: Ensure all necessary gas detectors (O₂, LFL, H₂S, SO₂, CO if relevant) are calibrated, function-tested, and that sufficient spare sensors/calibration gas are available. Crew must be trained in their use.
SCBAs and EEBDs: Check all units are ready, cylinders charged, and crew trained.
Ventilation Systems: Ensure ventilators and flame screens are clean and operational.
Stowage Plan: Detailed plan considering density, stress, stability, and any segregation from other cargoes or ship’s stores/bunkers.
5. Loading Operations:
Strict Adherence to TML/MC: Do not load if MC ≥ TML.
Continuous Monitoring for Liquefaction Indicators.
Dust Control: Concentrates can be dusty. Use PPE. Minimize dust spread.
Gas Monitoring: Commence monitoring of hold atmospheres as soon as loading starts and continue regularly.
No Smoking/Ignition Sources.
Trimming: Trim cargo reasonably level.
Weather: Avoid loading during heavy rain. Protect loaded cargo from rain.
Stress and Stability Monitoring: Continuously update loading computer.
6. Precautions During Voyage:
Regular Gas Monitoring (O₂, LFL, H₂S, SO₂): Daily, or more frequently if readings are concerning or as per Code schedule. Record all readings meticulously.
Ventilation: Follow IMSBC Code schedule guidance (usually surface ventilation if gas/oxygen levels require it, otherwise often kept sealed to minimize oxidation if gas levels are stable and safe). Avoid over-ventilation which might dry out the surface but not affect the bulk, or in some cases, supply more oxygen for reactions.
Temperature Monitoring (if specified or if self-heating is a concern for that specific concentrate).
Enclosed Space Entry Precautions: ABSOLUTE. Never enter holds without full procedures.
Weathertightness: Ensure hatch covers are fully sealed and weathertight.
7. Discharge Operations:
Gas Monitoring Before Opening Hatches: Test atmosphere in ullage space before opening. Ventilate thoroughly if necessary and safe to do so.
Safety of Personnel: Ensure stevedores are aware of the hazards (dust, potential for residual gases, oxygen deficiency).
Dust Control.
Hold Cleaning After Discharge: This is critical. Concentrate residues can be highly acidic and corrosive. Thorough washing with freshwater is usually required. Wash water will be contaminated and likely HME – dispose of strictly according to MARPOL Annex V.
8. Master’s Key Responsibilities Specific to Metal Sulphide Concentrates:
Unyielding Focus on Liquefaction Prevention (TML/MC).
Rigorous Management of Atmospheric Hazards (O₂ depletion, toxic/flammable gases): This requires a robust gas monitoring program and strict adherence to enclosed space entry procedures.
Ensuring Crew Understanding and Preparedness: The multiple hazards of these cargoes require a well-trained and vigilant crew.
Willingness to Stop Operations or Refuse Cargo: If documentation is inadequate, if test results are unsatisfactory, or if unsafe conditions develop during loading.
Close Consultation with Company and P&I Club: Given the high risks, maintain open lines of communication for advice and support.
The carriage of metal sulphide concentrates is a high-stakes operation. It demands the highest levels of diligence, technical knowledge, and adherence to safety procedures from the Master and crew. There is no room for complacency when dealing with cargoes that can simultaneously threaten the vessel through liquefaction and the crew through atmospheric hazards.