Zinc Concentrate: IMSBC Code Schedule

Zinc concentrate is a Group A cargo under the IMSBC Code, liable to liquefy if loaded above its Transportable Moisture Limit, and simultaneously an MHB cargo whose sphalerite-sulphide matrix can deplete oxygen inside enclosed holds. The combination of fine particle size (typically 80% passing 75 micrometres), high bulk density (2.4 to 2.8 t/m³), and residual moisture from pressure filtration makes every zinc concentrate shipment a TML-controlled operation where the shipper’s certificates are not optional paperwork, but the sole barrier between a controlled cargo and a bulk carrier capsize.

Global seaborne zinc concentrate trade runs at roughly 12 to 15 million dry-metric tonnes per year. The main production regions, Australia, Peru, Bolivia, Mexico, Canada, and India, supply a smelting industry concentrated in China, South Korea, Japan, and Europe. Voyage distances range from short Pacific crossings (Peru to China, around 16,000 km) to long-haul trans-ocean routes from northern Australian ports. Most shipments move in Supramax or Ultramax bulk carriers of 50,000 to 65,000 DWT, though smaller Handysizes handle niche regional routes and some major smelter-terminal contracts use Panamax-size parcels. Because the cargo is dense, a typical zinc concentrate parcel fills a hold to perhaps 60 to 65% of volumetric capacity while reaching 100% of its deadweight contribution, making cargo distribution across multiple holds a stability and structural issue as much as a commercial one.

The IMSBC Code governs the carriage of zinc concentrate under a dedicated schedule in Appendix 1. Use the IMSBC bulk cargo finder at /calculators/imo-imsbc to retrieve the current schedule data, and the TML moisture compliance calculator for the arithmetic of the loading decision.

The IMSBC Code schedule for zinc concentrate

Schedule entry: key particulars

The IMSBC Code Appendix 1 entry for ZINC CONCENTRATE records the following physical and hazard properties. Values shown are the ranges from the Code schedule; the shipper must declare the actual measured values for each shipment on the cargo declaration.

The Bulk Cargo Shipping Name on cargo documentation must read “ZINC CONCENTRATE” exactly, not “zinc conc”, “Zn concentrate”, or any other abbreviation, because the schedule entry keys to the exact BCSN string.

Group A: the liquefaction classification

Group A is the IMSBC Code’s designation for solid bulk cargoes that may liquefy if shipped when their moisture content exceeds the TML. Zinc concentrate qualifies on three physical grounds. Its particle size distribution sits almost entirely below 1 mm, with the flotation mill product typically running 80 to 90% below 75 micrometres; at this scale, inter-particle drainage is slow enough that cyclic compaction from ship motion can build pore-water pressure faster than the water can escape. Its bulk density of 2.4 to 2.8 t/m³ means a fully loaded Supramax hold of around 10,000 m³ volumetric capacity may carry only 6,000 to 7,000 t before topping the hold depth, but the cargo mass is sufficient to develop significant overburden stress at the base of the hold, accelerating the pore-pressure build-up. And its process moisture, typically 7 to 10% on a wet-mass basis after pressure filtration and before delivery, is often within a few percentage points of the TML, leaving little margin for measurement error, moisture pickup from rain or condensation during stockpiling, and the inherent scatter of the TML test itself.

The Group A cargo framework, and the broader science of why fine wet cargoes liquefy under ship motion, is covered in detail at cargo liquefaction. The short version: when pore-water pressure in the cargo voids equals the overburden stress, the effective stress between grains falls to zero, the cargo transitions from a packed solid to a dense fluid, and the free liquid surface that results shifts to the low side on every roll, adding a dynamic heeling moment the ship cannot counter with ballast adjustment.

MHB: the chemical hazard classification

MHB, Materials Hazardous only in Bulk, is the IMSBC Code designation for cargoes that present a chemical hazard in a bulk shipment but do not qualify as dangerous goods under the IMDG Code. Zinc concentrate earns MHB status because of its sulphide chemistry. Sphalerite (ZnS) oxidises in the presence of oxygen and moisture:

The reaction is slow at ambient temperature but proceeds continuously. In a closed or poorly ventilated hold, oxygen is consumed faster than it is replenished by natural or forced ventilation, and the hold atmosphere can fall below 19.5% oxygen (the IMSBC Code’s threshold for safe entry) within days to weeks, depending on temperature, hold volume, cargo mass, and sulphide content. The reaction also generates sulphur dioxide, which is a respiratory irritant at concentrations above 2 ppm and acutely toxic above 50 ppm. Polymetallic zinc concentrates containing iron sulphide (pyrite, FeS₂) or lead sulphide (galena, PbS) may additionally evolve hydrogen sulphide under wet conditions, though this is more common in concentrates with high pyrite content than in pure sphalerite product.

The practical consequence is that a zinc concentrate hold is an oxygen-deficient space risk at all stages of the voyage, not just immediately after loading. Personnel who need to enter the hold for trimming, inspection, or bilge sounding must treat it as a confined space requiring atmospheric testing and breathing apparatus. The Code requires the master to ensure no person enters a hold carrying an MHB cargo without first confirming the atmosphere is safe.

The cargo: sphalerite, production, and trade

Mineralogy and beneficiation

Zinc concentrate is the product of enriching zinc ore to remove gangue (waste rock) and concentrate the zinc-bearing minerals. Virtually all commercial zinc ore contains sphalerite (ZnS) as the dominant zinc mineral. Sphalerite is commonly associated with galena (PbS), chalcopyrite (CuFeS₂), pyrite (FeS₂), and various gangue silicates. Run-of-mine ore from major deposits typically grades 4 to 12% zinc. The beneficiation process raises this to a shipping-grade zinc concentrate assaying 50 to 60% zinc.

The standard process route is:

1. Crush the run-of-mine ore to approximately minus 10 mm.
2. Grind to liberation size, typically minus 75 micrometres, in ball mills.
3. Classify and condition the slurry with flotation collectors (typically xanthate-based) and frothers.
4. Float the sphalerite from the gangue and iron sulphides in selective froth flotation cells, often with depression of pyrite using cyanide or sulphate reagents.
5. Re-float and clean to produce a final concentrate at 50 to 60% Zn.
6. Thicken and pressure-filter to a cake moisture of 7 to 10%.

The fine grinding to liberation size is the step that creates the hazard. A 75-micrometre grain has drainage permeability on the order of $10^{-6}$ to $10^{-8}$ m/s, four to six orders of magnitude lower than coarse sand. That low permeability is what traps pore water under cyclic compaction at sea.

Polymetallic deposits produce concentrates that carry significant lead and sometimes copper alongside zinc. The Mount Isa complex in Queensland (Australia), Antamina in Ancash (Peru), and McArthur River in the Northern Territory (Australia) all produce zinc concentrates with 1 to 8% lead. If the lead or copper content is high enough, the material may be separately classified and shipped under the lead concentrate schedule or the mineral concentrates generic schedule rather than under the ZINC CONCENTRATE Bulk Cargo Shipping Name. The classification decision, and the resulting cargo declaration, is the shipper’s responsibility.

Global trade and the smelter supply chain

Zinc smelters convert the sulphide concentrate into refined zinc metal through a roast-leach-electrowin (RLE) hydrometallurgical route or, in some older facilities, the Imperial Smelting Process (ISP) which handles mixed zinc-lead concentrates. The RLE process requires a zinc feed above roughly 48 to 50% zinc with controlled lead, cadmium, and silica levels. Smelter feedstock procurement is therefore tightly specification-controlled, and shipping contracts typically specify a base assay, treatment charge structure, and penalty elements reflecting the exact composition delivered.

China processes over 40% of global zinc concentrate production at its domestic smelters, which drew down long-term contract supply chains from Australia, Peru, Bolivia, and Canada. South Korean smelters (Korea Zinc at Onsan, LS Nikko) and Japanese smelters (Toho Zinc, Mitsui Mining and Smelting) take the balance of Asian imports. European smelters in Spain (Asturiana de Zinc), Norway (Boliden Odda), and Belgium (Nyrstar Balen) draw concentrates from Peruvian, Bolivian, and North American sources. These supply chains are the commercial context for every zinc concentrate bulk carrier voyage.

Cargo liquefaction: the Group A risk in practice

The TML and FMP relationship

The IMSBC Code’s Group A control rests on a single inequality: the moisture content (MC) of the cargo at loading must be strictly less than the Transportable Moisture Limit (TML). The TML represents a safety margin below the physical flow threshold, not the flow threshold itself.

The flow threshold is the Flow Moisture Point (FMP): the moisture content at which the cargo just begins to flow under the prescribed laboratory agitation. The Code sets:

The 10% margin built into this expression is there for three reasons simultaneously. Moisture sampling of a large stockpile or barge-loaded cargo introduces scatter; the sample moisture may not perfectly represent the consignment moisture. The FMP is determined under controlled laboratory conditions, while the cargo in the hold is subjected to the far more energetic motion of a laden bulk carrier in a seaway. And zinc concentrate can pick up moisture from rainfall on open stockpiles, from condensation during transit from mine site to port, and from seawater spray during loading.

The TML calculated by the Proctor-Fagerberg method does not use the FMP formula directly. That method compacts a sample series at different moisture contents, plots the compaction density curve, and derives the TML as the moisture content corresponding to 70% degree of saturation, a value that also provides an implicit safety margin below the onset of flow. The two approaches converge on similar TML values for zinc concentrates in practice.

To use the IMSBC TML moisture check calculator: enter the certified TML and measured MC, and the calculator confirms whether the cargo meets the loading condition ($\text{MC} < \text{TML}$) and displays the utilised margin.

What TML values look like for zinc concentrate

TML is not a universal constant for zinc concentrate; it is a batch property that must be measured for each cargo lot. The controlling variables are the specific sphalerite mineralogy and gangue mineral content (which affect the shape and compressibility of the grains), the particle size distribution from the flotation circuit (finer grinds produce lower TML), and the presence of clay or slime minerals that retain moisture at the grain surfaces.

Published shipping-industry and academic data show TML values for zinc sulphide flotation concentrates in the range of 9% to 13% (wet-mass basis), with a central tendency around 10 to 11%. That range is narrow enough that the gap between a cargo filtered to 9% moisture and the lower end of the TML range may be only 1 to 2 percentage points, or a few hours of rain on an open stockpile. This is not a conservative cargo class with several percent of headroom; it is a cargo class where the shipper’s filtration and drying operations, and the protection of the cargo from rainfall between dewatering and loading, directly determine whether the vessel departs safely.

The shipper’s obligation under IMSBC Code Section 4 is to deliver a cargo declaration including the TML, the actual MC at loading, and the laboratory and analyst details. The MC must be measured no more than seven days before loading begins. The TML must be determined at an accredited laboratory no more than six months before loading.

The liquefaction sequence: what happens in the hold

The process follows the soil mechanics of effective stress reduction described in more detail at cargo liquefaction. The specific sequence for a zinc concentrate hold:

At loading, the cargo packs into the hold as a dense, apparently dry or slightly moist solid with an angle of repose around 35 to 40 degrees. The surface looks stable. The initial response to ship motion at sea is normal; the cargo compacts very slightly with each roll and pitch cycle.

As the voyage progresses, if MC is above or close to the TML, each compaction pulse raises pore-water pressure in the saturated fraction at the base of the pile. The low permeability of the fine sphalerite grains prevents rapid drainage. Pore pressure accumulates. Effective stress between grains falls.

The first external sign is often a darkening or shining of the cargo surface, indicating moisture is migrating upward as the pressure gradient drives water toward the surface. Bilge wells begin to collect turbid water carrying fine zinc concentrate particles. An unusual softening of roll period, subtle but measurable if the officer is watching the gyrocompass, can indicate the cargo is no longer behaving as a rigid mass.

Once a significant fraction of the hold is liquefied, the cargo surface tilts as a free liquid surface on each roll cycle. The vessel develops a list toward the low side. Ballast correction may reduce the list momentarily, but the dynamic free-surface shift on each roll reapplies the destabilising moment. The sequence from first visible list to capsize has taken as little as twenty minutes in documented casualties.

This is the physical basis for why the Code requires loading to be suspended, not just monitored more carefully, if the can test gives a positive result.

Test methods for TML determination

The three prescribed methods

IMSBC Code Appendix 2, Table 2 prescribes three laboratory methods for determining FMP and TML on Group A cargoes. All three are applicable to zinc concentrate; the choice of method is at the tester’s discretion, subject to the particle-size applicability conditions:

Flow table test: The sample is placed in a standard conical mould on the flow table, the mould is removed, and the table is dropped 25 times through a prescribed drop height. The moisture content is raised in increments and the test repeated until the base diameter of the sample expands by 50% from the table drops. That moisture content is the FMP. The test applies to material with maximum grain size up to 1 mm, and optionally up to 7 mm under modified conditions. Zinc concentrate, ground to 80% below 75 micrometres, falls well within the standard particle-size envelope.

Penetration test: A standardised probe penetrates a remoulded sample at a defined rate. The moisture content at which penetration resistance falls to a defined threshold is the FMP. The method applies to material up to 25 mm top size and works well on concentrates with a high clay or slime content that makes flow-table testing ambiguous.

Proctor-Fagerberg method: The sample is compacted in a series of moulds at different moisture contents using a standardised compaction energy, and the dry density is plotted against moisture content. The curve exhibits a peak density at an optimum moisture content, and the degree of saturation rises from there as moisture increases. The TML is read as the moisture content corresponding to 70% degree of saturation. This method applies to material up to 5 mm top size and is widely used in the concentrate industry because it does not require identification of an FMP; the TML emerges directly from the compaction curve.

For the flow-table and penetration tests, the relationship is:

For the Proctor-Fagerberg test, the TML is the 70%-saturation moisture reading directly; no multiplication factor applies. The two methods give numerically similar TML values for zinc concentrate when applied to the same material.

TML testing frequency and accreditation

The IMSBC Code requires the TML to be determined by a competent laboratory using one of the three prescribed methods. “Competent laboratory” means a laboratory accredited to ISO/IEC 17025 or equivalent for the specific test method. The TML certificate must be issued within six months before the start of loading. If a consistent supply chain ships from the same mine and circuit to the same terminals with no process changes, the TML may remain stable enough that a single six-monthly test covers multiple shipments. But any change in the flotation circuit, the ore blend, or the grind size is grounds for re-testing, because these parameters directly affect grain shape, size distribution, and hence drainage behavior.

Moisture content testing

MC testing is separate from TML testing and must be performed closer to the loading date: within seven days before commencement of loading in most flag state implementations. The test method is straightforward drying: a representative sample is weighed wet, dried in an oven at 105°C to constant mass, and the weight loss expressed as a percentage of the wet mass. Sampling protocol follows IMSBC Code Section 4, which requires samples to be taken from the cargo as it will be presented for loading, not from a stockpile of uncertain history. Multiple samples across the stockpile or conveyor belt are composited.

The critical phrase in the Code is “as it will be presented for loading.” A cargo that has been filtered, dewatered to 8% MC, and placed on a covered stockpile may test at 8%. The same cargo rained on for 48 hours while awaiting ship arrival may test at 12%, above any reasonable TML. The shipper controls this by protecting the stockpile and timing the moisture test to the loading event, not to the filtration event.

The shipboard can test

The can test is the master’s primary tool for an independent sanity check on the cargo condition during loading. It is set out in IMSBC Code Section 8. The procedure:

1. Take a representative sample of the cargo from the cargo stream or the top of the stockpile, avoiding surface crust and segregated fine or coarse layers.
2. Fill a cylindrical can of 0.5 to 1 litre capacity to roughly half full.
3. Place one hand over the top and bring the can down sharply onto a hard surface from a height of approximately 0.2 metres, then remove the hand.
4. Repeat 25 times at intervals of one to two seconds.
5. Observe the surface of the sample after the final impact.

If free moisture appears on the surface, the cargo is suspect. The master should halt loading, notify the operator and the port authority, and arrange independent laboratory testing before resuming. The absence of free moisture does not confirm the cargo is within TML; it confirms only that a rough screening test did not reveal an obviously wet condition. The certified laboratory moisture content governs the loading decision.

The can test is cheap, takes under two minutes, and requires no equipment beyond a suitable container. There is no legitimate reason for it not to be conducted on each loading shift for a Group A cargo. Officers who skip it on the grounds that the shipper’s certificate is in order have misunderstood the layered control structure the Code intends.

High density and structural loading

Why bulk density matters for zinc concentrate

Zinc concentrate, at 2.4 to 2.8 t/m³ bulk density, is among the densest dry bulk cargoes in regular trade, exceeded only by lead concentrate (3.0 to 3.8 t/m³) and iron ore (up to 3.0 t/m³ for high-grade magnetite fines). By comparison, grain runs at 0.7 to 0.8 t/m³ and coal at 0.8 to 0.9 t/m³. The stowage factor of 0.36 to 0.42 m³/t reflects this: one tonne of zinc concentrate occupies 0.36 to 0.42 cubic metres. A standard Supramax hold of roughly 9,000 to 10,000 m³ can hold only 22,000 to 28,000 tonnes of zinc concentrate at full volumetric capacity, but a fully laden Supramax carries 50,000 to 60,000 DWT. This means a zinc concentrate cargo typically occupies three to four holds on a five-hold Supramax, with the remaining holds empty or ballasted.

The design consequence is that the loaded holds carry a much higher cargo mass per unit of hold volume than they would for lighter cargoes. The double-bottom tank structure and the tanktop plating are loaded to their design limits, and in some cases to the limit of the vessel’s hold permissible loading limit, which is set by the classification society and expressed in tonnes per hold or tonnes per square metre of tanktop. Exceeding the permissible loading limit, or distributing the cargo unevenly within a hold, can cause tanktop failure or excessive hull girder sagging moments. The cargo plan for a zinc concentrate voyage must be checked against the vessel’s structural limits, not just against the stability and trim requirements.

Hold loading plan and trim considerations

Because zinc concentrate occupies only a fraction of the available hold volume, the height of the cargo in the loaded holds is modest, perhaps 4 to 6 metres in a Supramax hold that could theoretically hold 12 to 15 metres of grain to capacity. The cargo sits at the bottom of the hold with a large air space above. This creates a relatively low centre of gravity for the loaded parcel, which is favorable for initial stability but can mean a very stiff vessel with a short roll period. A stiff vessel rolls quickly and applies higher dynamic accelerations to the cargo, which in turn applies more energetic cyclic shear and faster pore-pressure build-up, exactly the conditions that accelerate liquefaction.

The hold-by-hold weight distribution must be designed to maintain the hull girder bending moment within the vessel’s class-approved limits. Concentrating all zinc concentrate in the fore or aft holds while leaving midship holds empty creates a sagging or hogging moment that may exceed the hull’s permissible value. The officer responsible for the cargo plan (the chief officer, confirmed by the master) must run the loading sequence through the vessel’s loading instrument or approved calculator, checking bending moment and shear force at each stage of loading and after all holds are loaded.

Trim is also affected by density. With cargo in forward holds and ballast aft, or vice versa, the dense cargo’s weight has a pronounced effect on trim. A vessel that enters port with a trim optimized for draft survey will need careful ballast management during loading to stay within the pilot’s and port authority’s under-keel clearance requirements.

Longitudinal stress and hull girder considerations

A standard bulk carrier’s hull girder is designed with a maximum permissible sagging bending moment and a maximum hogging bending moment, both specified in the approved trim & stability booklet. For zinc concentrate voyages on Supramax vessels, the typical loading pattern of three loaded holds and two empty holds concentrates deadweight at the loaded-hold positions and creates a pronounced bending moment at the hold boundaries. The loading instrument on modern vessels calculates this in real time, but older vessels or vessels without a functioning loading instrument require the chief officer to compute the hull girder bending moments manually from the load distribution. This is not a theoretical exercise; bulkers have experienced structural failure from improper loading of dense mineral cargoes.

The MHB oxygen-depletion and gas hazard

Mechanism of oxygen depletion

The MHB designation in the IMSBC Code schedule for zinc concentrate reflects the sulphide oxidation reactions that proceed continuously throughout the voyage. The dominant mechanism is the oxidation of sphalerite:

This reaction consumes two moles of oxygen for every mole of ZnS oxidised and releases sulphur dioxide as a by-product. In a sealed or poorly ventilated hold, the oxygen concentration falls progressively. In well-ventilated conditions the reaction rate is limited by the supply of oxygen, so hold ventilation is both the cause and the partial remedy for the hazard.

Practical measurements on zinc concentrate shipments have recorded hold oxygen concentrations below 19.5% within 24 to 72 hours of hold closure under some conditions. The rate depends on the sulphide surface area exposed (finer particles, higher rate), the moisture level (moisture accelerates sulphide oxidation), the temperature (reaction rate roughly doubles per 10°C rise), and the ventilation regime. A tropical zinc concentrate voyage from a Pacific port with high ambient humidity and temperature is a higher-risk environment than a temperate Atlantic route.

The IMSBC Code’s threshold for safe entry into an enclosed space is 19.5% oxygen. Below this level, a person entering without breathing apparatus faces hypoxia from the first breaths. Oxygen-deficiency casualties at bulk terminals and on bulk carriers have occurred with zinc and copper concentrates. The risk is not hypothetical.

Secondary gas hazards

Beyond oxygen depletion, the MHB section of the zinc concentrate schedule warns against potential generation of sulphur dioxide (SO₂) and, in concentrates with elevated pyrite or pyrrhotite content, hydrogen sulphide (H₂S). Sulphur dioxide has an IDLH (Immediately Dangerous to Life or Health) value of 100 ppm; its sharp pungent odour is detectable by most people around 1 ppm, which does give useful warning. Hydrogen sulphide is more dangerous: it paralyzes the olfactory nerve at concentrations above about 150 ppm, eliminating the odour warning precisely when the gas reaches dangerous levels, and has an IDLH of 50 ppm.

Polymetallic zinc concentrates that contain significant pyrite, arsenopyrite, or pyrrhotite carry a higher H₂S generation risk than pure sphalerite product. The cargo declaration should state the iron sulphide content, and the master should treat elevated pyrite as an elevated H₂S risk requiring appropriate atmospheric monitoring before any hold entry.

Confined space entry procedures

The IMSBC Code, SOLAS Chapter VI, and the ILO/IMO Recommendations on Safe Use of Pesticides in Ships and related confined space guidance all require that any hold, void, or enclosed space carrying an MHB cargo be treated as a confined space for entry purposes. The practical requirements:

• Atmospheric testing with a calibrated multi-gas instrument (oxygen, CO, H₂S, SO₂ as minimum channels) before entry.
• Continuous monitoring during occupancy.
• A standby person outside the space who can raise the alarm and initiate rescue without entering.
• Breathing apparatus on standby; entry with SCBA if oxygen is at or below 19.5%.
• Work permit system for every hold entry, signed by the master or a designated responsible officer.

These are not optional or voyage-end requirements. They apply from the moment the holds are sealed after loading.

Loading precautions and master’s rights

Pre-loading documentation review

Before the master accepts zinc concentrate for loading, the following documents must be in order and reviewed:

The cargo declaration must state: ZINC CONCENTRATE as the Bulk Cargo Shipping Name; Group A and MHB as the hazard classification; the certified TML value with laboratory name, accreditation number, and test date; the measured MC at loading with sampling date (within seven days); the bulk density; the particle size distribution; and the sulphide content. The shipper signs the declaration as a warranty of accuracy.

The TML certificate must be from an ISO/IEC 17025-accredited laboratory, dated within six months before commencement of loading. A certificate from an unaccredited laboratory does not comply. A certificate dated more than six months before loading does not comply. A certificate that covers a different ore source or a process circuit that has since been modified does not comply.

The moisture content certificate must be issued within seven days before commencement of loading. This is the most time-sensitive document and the one most commonly presented late or with an ambiguous sampling date.

The can test procedure during loading

The master and officers should conduct the can test at intervals throughout the loading operation, not only at the start. MC can vary across the cargo stockpile if storage conditions are uneven or if the stockpile has been rained on one side. Running the can test at the start of each hatch loading, and again after any rain event during loading, is prudent practice.

A can test positive result at any point requires immediate suspension of loading from the affected stockpile section. The master should notify the operator and the port authority, request the shipper to arrange independent laboratory moisture testing, and not resume loading until updated laboratory results confirm the cargo is within TML.

The master’s right and duty to refuse

SOLAS Chapter VI Regulation 2 and IMSBC Code Section 4 both impose a duty on the master that goes beyond a right to refuse: the master must not accept a Group A cargo for loading if the required certificates are not provided, are materially defective, or show the MC to be at or above the TML. The master’s authority here is explicit in the Code and is not overridden by commercial pressure from the shipper, charterer, or operator.

A master who loads zinc concentrate in violation of the TML requirement and the vessel subsequently suffers a casualty attributed to liquefaction faces personal criminal liability in many flag states, potential sanctions from the flag administration, and rejection of any hull and machinery insurance claim based on the willful violation of the carriage conditions. The commercial cost of a delayed or refused loading is measured in thousands of dollars per day. The cost of a capsize is measured in lives.

The IMSBC Code also gives the master the right to conduct, or require, independent testing of any cargo consignment where there is reason to doubt the shipper’s certificates. “Reason to doubt” includes a can test positive, visible evidence of wet cargo on the conveyor or at the stockpile face, discrepancy between the declared bulk density and the observed behavior of the cargo during loading, and any other indication that the cargo as loaded does not match the cargo as declared.

Voyage conduct and monitoring

Bilge sounding and cargo monitoring

During a zinc concentrate voyage, the master and officers must monitor the hold bilge wells regularly, typically at each watch change, and record the readings. A rise in bilge levels carrying fine dark particles indicates that moisture is migrating through the cargo and collecting at the bilge. This is not necessarily an emergency, but it is an early warning sign. Combined with any other anomaly, a developing list, an unusual roll behavior, or visual changes to the cargo surface visible via the hold access hatch, it warrants immediate investigation.

If the vessel develops a list of more than 5 degrees that cannot be accounted for by bunker consumption, ballast transfer, or manifest trim, and if zinc concentrate is in the holds, the master must treat it as a potential liquefaction event. The appropriate response is not to correct the list with ballast and continue; it is to reduce speed to reduce ship motion and minimize further pore-pressure cycling, to avoid head seas where possible (beam seas worsen roll, head seas reduce it), and to alert the operator and port of destination while assessing the stability situation.

Ventilation during a zinc concentrate voyage

Ventilation of zinc concentrate holds during a voyage is a balance between two competing needs: enough ventilation to limit oxygen depletion from sulphide oxidation, and enough hold integrity to prevent water ingress. The IMSBC Code’s default position for Group A cargoes is no surface ventilation of the cargo, because ventilation can dry and re-wet the surface layers and potentially disturb the cargo in ways that increase liquefaction risk. The MHB provisions for zinc concentrate recommend that hold atmosphere be monitored, and ventilation be applied where atmospheric testing indicates a hazardous oxygen depletion, using safe and controlled ventilation practices.

The practical approach on modern bulk carriers is to keep holds closed during the main body of the voyage, conduct atmospheric sampling via the hold sounding pipes at intervals, and ventilate cautiously and briefly if oxygen drops toward the warning threshold. In practice, modern bulk carriers are seldom able to ventilate holds effectively in a seaway anyway, so the main control is ensuring the hatches are sealed adequately to prevent water ingress, rather than actively managing the hold atmosphere.

Discharge at the receiving terminal

Discharge of zinc concentrate is by mechanical grab from shore cranes, or by pneumatic conveyor in some modern terminals. Dust generation during grab discharge is the primary environmental and occupational health concern at discharge ports. Terminals receiving zinc concentrate typically operate under covered unloading arrangements or with dust suppression systems on the crabs. Crew access to the hold during grab discharge is prohibited; the grabs handle the cargo mechanically and crew are not required in the hold until trimming operations at the end of discharge.

Cargo residues and hold washwater after discharge must be handled as potentially zinc-contaminated waste under port state and national environmental regulations. The wash-out water from a zinc concentrate hold will carry zinc compounds and sulphate; its discharge directly into port waters is typically prohibited, and the receiving terminal or the vessel’s waste management plan must account for it.

Stowage, segregation, and hold preparation

Stowage requirements

The IMSBC Code schedule for zinc concentrate requires holds to be clean, dry, and free from residues of incompatible previous cargoes before loading. The density and fineness of the cargo mean that any small quantity of residue from a lighter, bulkier previous cargo (grain, fertilizer, coal) will be overwhelmed by the zinc concentrate and will not materially affect the load, but contamination of the concentrate with incompatible substances can violate the shipper’s smelter supply contract, which specifies tight tolerances on penalty elements. Smelters penalise zinc concentrates for elevated copper, arsenic, mercury, cadmium, and bismuth; contamination from hold residues or previous cargo can be commercially catastrophic even when it has no safety consequences.

Holds should be swept, washed with fresh water, and inspected for water tightness before loading. The hatch covers should be examined for seal condition. Any hold structural damage, including cracks in the tanktop, corroded frames, or damaged frame brackets, should be repaired before loading a dense cargo like zinc concentrate.

Segregation

Zinc concentrate does not require segregation from most dry bulk cargoes under the IMSBC Code because it is not regulated under the IMDG Code. The MHB classification, however, means that any adjacent cargo or space should be assessed for compatibility with a sulphide-bearing oxygen-depleting material. In a multi-hold arrangement with different cargoes, the zinc concentrate holds should be clearly identified, atmospheric monitoring conducted separately for each hold, and confined space entry procedures applied independently to the zinc concentrate holds regardless of the atmosphere in adjacent holds.

Clean-on-board documentation

The cargo documentation trail for a zinc concentrate voyage typically includes the cargo declaration, TML certificate, MC certificate, a statement of facts from loading (recording the loading sequence, the can test results, any weather events during loading, and any anomalies in the cargo condition), the bill of lading incorporating the cargo declaration by reference, and the Notice of Readiness and Letter of Protest if any cargo was loaded under protest. This documentation provides the evidential record if cargo condition is disputed at the discharge port, or if an incident occurs during the voyage.

Limitations

This article covers the IMSBC Code schedule for zinc concentrate as it stands under Amendment 07-23 (mandatory from 1 January 2025) and the associated regulatory framework for Group A and MHB cargoes. Several practical limitations apply.

The TML and MC values given are indicative ranges from the literature and the Code schedule. The actual TML and MC for any specific shipment must be determined by accredited laboratory testing of that consignment; no generic value from this or any reference article may substitute for a proper shipper’s certificate.

The chemistry of sulphide oxidation is presented here in simplified stoichiometric form. Actual reaction rates in a zinc concentrate hold depend on mineralogy, moisture, temperature, particle size distribution, and the specific surface area of the sulphide phase in ways that are not fully captured by the simplified reactions shown. Masters should use atmospheric testing data from the specific cargo, not chemical predictions, as the basis for confined space entry decisions.

The cargo’s trade and route data reflects shipping patterns as of mid-2026. The zinc concentrate trade is affected by long-term offtake contracts, smelter capacity investments, mine development and depletion, and occasionally by trade policy and sanctions, all of which can shift volume and route patterns materially within a few years.

IMSBC Code amendments are issued on a roughly two-year cycle. Amendment 07-23 entered mandatory force on 1 January 2025. Any reader using this article for compliance purposes should verify the current mandatory amendment status with their flag administration, since the IMO adoption date and the national mandatory entry-into-force date may differ by one to two years.

The IMSBC Code article covers the overall structure of the Code and its amendment process. The Group A cargoes article covers the Group A classification in full for all cargoes. The cargo liquefaction article covers the soil-mechanics basis for the liquefaction hazard. For the generic Group A regime applicable to polymetallic zinc-lead concentrates not covered by the dedicated schedule, the mineral concentrates article applies.

See also

• IMSBC Code: Structure and Amendment Process
• Group A Cargoes: Liquefaction and TML Framework
• Cargo Liquefaction: TML, FMP, and Group A Controls
• Mineral Concentrates: IMSBC Code Schedule
• Lead Concentrate: IMSBC Code Schedule
• Copper Concentrate: IMSBC Code Schedule
• Iron Ore Concentrate: IMSBC Code Schedule
• IMSBC Zinc Concentrate calculator
• IMSBC TML Moisture Check calculator
• IMSBC Group A Liquefaction Risk calculator
• IMSBC Bulk Cargo Finder