International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 4

Number 4

December 2010

435

1 INTRODUCTION

Bulk shipping has been used for many years to re-

duce the cost of sea transport and the transport of

bulk cargoes is a vital component of international

trade. Such trades require a sufficient volume of car-

go suitable for bulk handling and hence justify a tai-

lored shipping operation. The five major dry bulk

cargoes are coal, mineral concentrates, grain, bauxite

and phosphate rock, and each year the trade in bulk

increases (Roberts M. & Marlow P. 2004).

In recent years, bulk carriers have been identified

with the high risk of catastrophic structural failure

and foundering, and with heavy loss of human life.

Several risk factors have been identified that had

significant, independent effect upon the likelihood of

a bulk carrier foundering. Ore concentrates and other

similar fine-grained materials transported by sea be-

long to hazardous materials when are considered as

bulk cargoes (materials MHB). This type of cargoes

is transported in a wet state.

Excessively wet cargo can pass into liquid state in

sea transport conditions (Zhan M. 2005, Shitharam

T. G. 2003). According to the Code of Safe Practice

for Solid Bulk Cargoes (BC Code) (International

Maritime Organization 2004), deterioration or loss

of ship’s stability is one of three basic hazards,

which are bound with sea shipment of ore concen-

trates and other fine-grained cargoes. Too high hu-

midity of cargo leading to its liquefaction may cause

shift of the cargo and in consequence ship’s heel and

even its capsizing and sinking.

The International Maritime Organization (IMO),

recognising that some losses had occurred due to

improper loading, issued a code of practice for these

operations. The probability of a hazard developing

into an undesirable consequence is focus of safety

management and safety regulation. The recent publi-

cation of recommendation guidelines for cargo–

handling operations and the amendments to the In-

ternational Convention for the Safety of Life at Sea

contribute to the decrease the possibility of occur-

rence the liquefaction during sea transportation (In-

ternational Chamber of Shipping1999, BLU Code

London, (2004)).

To better illustrate liquefaction mechanism three-

phase structure of ore concentrates and similar mate-

rials is considered, which consist of solids (mineral

grains), water and air.

Mineral grains are very small; they are from

0,001mm to several millimeters large. Disintegration

level and percentage of particular size fraction may

differ depending on concentrate type.

In three –phase structure air and water fill the

pores between mineral grains. The inter-grain pores

are contracted in sea transport conditions due to ship

rolling and vibration. The air, permeability coeffi-

cient of which is about 500 times greater than that of

water, first escapes, thus full water saturation of

pores is affected.

The Influence of Organic Polymer on

Parameters Determining Ability to

Liquefaction of Mineral Concentrates

M. Popek

Gdynia Maritime University, Gdynia, Poland

ABSTRACT: When the wet granular materials lose their shear strength they flow like fluids. This phenome-

non is called liquefaction. The liquefaction can be prevented by means of limiting the moisture content of

cargo by introducing the safety margin. Cargoes, which may liquefy shall only be accepted for loading when

the actual moisture content of the cargo is less than its Transportable Moisture Limit (TML). It has been rec-

ognized that in some cargoes, moisture can gravity drain towards the bottom of the hold. The resulting much

wetter bottom layer may therefore be prone to liquefaction and provoke instability of the entire cargo.

To prevent sliding and shifting of ore concentrates in storage a biodegradable materials are added to the ore.

The polymer materials absorb water from the ore particle’s pores and the moisture content goes down. In con-

sequence polymer materials may prevent drainage of the water from the ore particle’s pore.

436

Full compressive stress is thus applied to the in-

compressible water in the pores between mineral

grains which causes drop of inter-grain friction, i.e.

ore liquefaction and in consequence possible shift of

cargo (Michałowski & others 1995).

The possibility of instability because of liquefac-

tion of bulk cargoes such as mineral concentrates

has been recognized for some time. BC Code in-

cludes several provisions aimed to prevent the

movement of bulk cargoes either by sliding or lique-

faction

Moisture content allowing to passing of a bulk

cargo from solid into liquid state is called critical

moisture content. One of its possible measures is

Flow Moisture Point (FMP). On its basis permissible

moisture limits for shipment conditions are deter-

mined. Transportable Moisture Limit (TML) is such

moisture content at or below which a loose cargo

can be transported in bulk on ships without danger

of passing of the cargo into liquid state. Its usually

calculated as 90% of FMP. The possibility of insta-

bility because of liquefaction of bulk cargoes such as

mineral concentrates has been recognized for some

time. Many cases are reported of large heel of a ship

or even her sinking due to cargo liquefaction. A car-

go, which is liable to liquefaction, must be suffi-

ciently fine grained (so that permeability is suffi-

ciently low) and have a high enough initial moisture

content:

For cargoes with permeability so low that virtual-

ly no moisture redistribution occurs during voyage,

the initial moisture content needs to be below the

transportable moisture limit so that the whole cargo

does not liquefy as a result of the ship’s motion dur-

ing heavy weather.

For cargoes that are relatively free draining, re-

distribution occurs with moisture from the upper

levels of the cargo draining towards the base. Unless

efficiently drained the bilges, this water saturates the

bottom levels of the cargo and liquefaction could

occur with cargo shifting during heavy rolling mo-

tions (Eckerley J.D. 1997).

These cargoes, prone to liquefaction, should nev-

er be carried without checking the moisture content.

The Code of Safe Practice for Solid Bulk Cargoes

lays down that a certificate stating the relevant char-

acteristics of the material to be loaded should be

provided at the loading port, incorporating also the

transportable moisture limit. The cargoes which may

liquefy shall only be accepted for loading when the

actual moisture content of the cargo is less then its

Transportable Moisture Content and refused if the

analysis reveals that it’s moisture content is too high.

The Code provides information how the moisture

content of ores concentrates can be tested and as-

sessed.

For liquefaction the cargo needs to have permea-

bility low enough that excess pore pressures cannot

dissipate before sliding occurs. This condition is

controlled by the material’s grain size distribution,

and Kirby expressed this in requirement that 95 per-

cent or more of the cargo should be coarser than

1mm to prevent liquefaction. In soil mechanics liter-

ature the requirement is usually expressed as

0,006mm< d10<0,3 mm for liquefaction to be likely,

where d10 represents the particle size for which only

ten percent by mass of the material is finer (Eckerley

J.D. 1987).

A large group of organic polymers fined use in

the mineral industry with the specific function [Bu-

latovic 1997]. Particularly attractive are the new ma-

terials based on natural renewable resources, pre-

venting further impact on the environment.

Starch is non –expensive biopolymer available

from annually renewable resource. It is totally bio-

degradable in a wide variety of environments and al-

lows the development of totally degradable products.

Starch can be found in plants as a mixture of two

polysaccharides: amylase, the nearly linear polymer

consisting of α – (1, 4)-anhydroglucose units, and

amylopectin, a group which is able to undergo sub-

stitution reactions and C-O-C linkage responsible for

the molecular chain braking. The OH group has a

nucleophilic character and by reaction with different

reagents it is possible to obtain a series of com-

pounds of modified properties. Chemical and physi-

cal properties of starch have been widely investigat-

ed due to its easy to be converted into a

thermoplastic and then be used in different applica-

tions (Tudorachi N. & others 2006). Starch based

blends present enormous potential to be widely used

in environmental fields, as they are totally biode-

gradable, inexpensive (when compared to other bio-

degradable polymers). The material containing

starch gets destroyed when exposed to environmen-

tal factors, since due to starch hydrolysis its structure

becomes weaker, and after some time, under certain

conditions, synthetic polymers contained in the

product also undergo decomposition.

The purpose of this work was investigation on

possibility of using biodegradable thermoplastic ma-

terials as absorbers moisture. To prevent sliding and

shifting of ore concentrates in storage materials

composed of starch, cellulose and polycaprolactone

are added to the concentrates. The properties and the

processing procedures of biodegradable starch –

based thermoplastic blends, like

starch/polycaprolactone, starch/cellulose have been

already reported (Demirgoz & others, 2000).

437

2 EXPERIMENTAL PROCEDURES

2.1 Material

The iron concentrate was used for the tests.

It is a product of a gravimetric separation of large

mineral particles. The iron concentrate is a fine ma-

terial which is empirically judged as “which may

liquefy if shipped above the TML”.

Following polymer materials were tested: poly-

mer material Y (made of thermoplastic starch and

cellulose derivatives from natural origin) and poly-

mer material Z (made of starch and policaprolac-

tone).

The used polymer materials are classified as a

low environmental impact product.

Based on the results of estimation the ability to

absorb of water by polymers materials it can be said

that polymer material Y absorbs more water then

polymer material Z. The equilibrium absorption of

polymer Y is reached in 48 hours. The time taken to

reach equilibrium water content in polymer Z is

shorter – about 18 hours.

Water uptake is affected by the type of polymer.

The time required to reach equilibrium water uptake

is lower for blend containing starch and polycapro-

lactone then for blend containing starch and cellu-

lose (Popek M. 2005).

The samples of polymer materials were in granu-

lar form. The experiments were conducted for sam-

ples of concentrate: without polymer materials and

for mixtures contain 98 % concentrate and 2 % of

polymer materials.

The course of grain size distribution curves indi-

cates that all the tested samples are susceptible to

liquefaction in sea transportation conditions as in

each case the content of grains smaller then 0,3mm

is greater then 10 %. The content percentage values

of the grains (of the size below 0,3mm) in concen-

trate without polymer is 76,9 %. In mixtures of con-

centrate and polymer materials the contents of parti-

cles with a diameter smaller than 0,3mm are

negligible smaller and amount 76,0 % for mixture

with polymer Y and 76,1 % for mixture with poly-

mer Z. the results of grain size analysis indicate that

polymers do not significantly change grain size dis-

tribution. This is the reason why all tested samples

may liquefy.

2.2 Methods

Following tests have been carried out:

− Estimation of TML:

The International Maritime Organization ap-

proved, in the Code of Safe Practice for Solid

Bulk Cargoes the following assessment methods

of safe moisture content in the cargoes: Flow Ta-

ble Method, Japanese Penetration Method and

proctor/ Fagerberg Method. The evaluation of

FMP was performed with the use of the Proctor

Fagerberg Test. Proctor/ Fagerberg Method is

recommended for evaluation of some fine-grained

bulk cargoes. The sample was consolidated by 25

drops of rammer from 0,2 m height in the meas-

uring cylinder, layer by layer, repeating the pro-

cedure 5 times and finally weighing the cylinder

with the moist sample. Then volumetric density

of the wet concentrate γ

and of the dry consoli-

dated concentrate γ

0bjs

were calculated and a con-

solidation curve γ

0bjs

– f(w) drawn, where “w”

stands for moisture content percentage in relation

either to wet concentrate weight. TML was de-

termined from a cross point of the void ratio

curve and a line of 70 % degree of saturation,

theoretically calculated.

− Permeability of concentrates:

The permeability is the rate at which water under

pressure can diffuse through the voids in the min-

eral concentrates. These materials are permeable

to water because the voids between the particles

are interconnected. The degree of permeability is

characterized by the permeability coefficient k,

also referred to as hydraulic conductivity.

According to the classification of soils, based on

their coefficient of permeability, mineral concen-

trates are the materials with the low degree of

permeability. The permeability of mineral con-

centrates depends primarily on the size and shape

of grains, shape and arrangement of voids, void

ratio, degree of saturation, and temperature.

− Measurement of the cohesion and internal friction

angle:

The estimation of cohesion and internal friction

angle were performed in the direct shear appa-

ratus by carrying the shearing with the help of

lower and upper part of displacing box containing

the tested concentrate. In the experiment the sam-

ples were compacted in a dry state. The moisture

content corresponds to the TML value estimated

in Flow Table Test.

In the experiments (estimation of the permeability

and the cohesion and internal friction angle) the

samples were compacted. The consolidation con-

ditions (in the holds) were simulated by using

vertical loads: 0 N, 98 N, 196 N, 294 N and 490

N, what corresponds to the normal stresses: 0,

1,532 *104 N/m2, 3, 0645 *104 N/m2,

4,589 *104 N/m2, 7,659 *104 N/m2 respectively.

The test without any stress corresponds to the

stress in the hold during the loading. Increasing

values of normal stresses represents the changes

in the bulk cargoes during the sea transportation.

438

3 RESULTS AND DISCUSSION

3.1 Estimation of TML

The results obtained by using Proctor/ Fagerberg test

are shown in Figures 1-2. The figures show Void

Ratio as a function of net moisture content by vol-

ume. In each figure, the ordinate and abscissa denote

void ratio and net moisture content by volume, re-

spectively. The black circles indicate the measured

data. The straight line corresponds to degree of satu-

ration 70 % theoretically calculated. TML was de-

termined from a cross point of the experimental

curve and a line of 70 % degree of saturation.

The obtained results are presented in Table 1.

Figure 1. Compaction curve for iron concentrate + 2% poly-

mer Y.

Figure 2. Compaction curve for iron concentrate + 2% poly-

mer Z.

Table1. Transportable Moisture Limit determined by Proctor

Fagerberg test

___________________________________________________

Sample Specific Transportable TML

gravity of Limit of Net

solid Moisture Content

by Volume

___________________________________________________

Iron concentrate + 49 % 9,02

2 % polymer Y 4,98

_______________ ____________________________

Iron concentrate + 47,5 % 8,7

2 % polymer Z

___________________________________________________

Despite the presence of polymer in tested concen-

trate, the values of estimated TML are similar, be-

cause liquefaction is tightly related to the grain size

contents.

3.2 Permeability

The results of permeability test are presented in

Table 2.

Table 2. Results of permeability test

___________________________________________________

Normal stress permeability coefficient k

[N/m

] [m/s]

_____________________________________

Iron concentrate Iron concentrate

+ 2 % polymer Y + 2 % polymer Z

___________________________________________________

0 14,0*10

-3

15,2*10

-3

1,532* 10

8,5*10

-3

9,2*10

-3

3,064* 10

7,1*10

-3

8,8*10

-3

4,589* 10

5,2*10

-3

7,5*10

-3

7,659* 10

3,5*10

-3

5,15*10

-3

___________________________________________________

The compaction modifies permeability of

samples by decreasing the voids available for

flow and reorienting particles. Based on the re-

sults the effect of different compaction of the

samples on the permeability was observed. The

maximum values of permeability coefficient k

were achieved for samples without any stress.

The increase of consolidation force caused de-

creases the value of the permeability coefficient.

The ability to permeability of mixtures is related

to the composition of the polymer material. In all

cases, for samples with polymer material Y, the

higher decrease of permeability was obtained.

3.3 Cohesion and internal friction angle

The changes of internal friction angle as a function

of moisture content are presented in Figures 3-4.

Fig.3 Internal friction angle for iron concentrate + 2% poly-

mer Y.

0,2

0,4

0,6

0,8

0 10 20 30 40 50 60

Net Moisture Content by Volume [%]

Void Ratio

Sw = 70%

iron concentrate +

2 % polymer Y

0,2

0,4

0,6

0,8

0 10 20 30 40 50 60

Net Moisture Content by Volume [%]

Void Ratio

Sw = 70%

iron concentrate +

2 % polymer Z

0,4

0,45

0,5

0,55

0,6

0 2 4 6 8 10 12

Moisture content [%]

Internal friction angle [rad]

iron concentrate

iron concentrate +

2% polymer Z

439

Figure 4. Internal friction angle for iron concentrate + 2% pol-

ymer Z.

As a result of performed test it can be said that in-

ternal friction angle reaches minimum value when

moisture content is chosen to TML. The presence of

polymer material in tested sample influences on the

value of internal friction angle. In each case the val-

ues are higher than those for sample without poly-

mer material.

The cohesion as a function of moisture content

are presented in Figures 5-6.

Figure 5. Cohesion for iron concentrate + 2% polymer Y

Figure 6. Cohesion for iron concentrate + 2 % polymer Z.

The apparent cohesion does not occur in dry

materials with pores entirely filled with air nor in

moist materials having pores entirely filled with

water. In all samples cohesion increases with the

increasing content of water and it reaches a max-

imum value with moisture approaching the TML

and then it goes down. The presence of polymer

material in tested sample significantly changes

values of cohesion. Decreasing values of cohe-

sion, for each moisture content, is observed.

4 CONCLUSIONS

The conclusion is based on the measurement of the

TML, cohesion and permeability of the materials.

The comparison of the TML values confirms that

the correlation between the grain content and TML

values occurs.

The presence of polymer material in tested sam-

ple influences on the values of cohesion and internal

friction angle but the extreme values are reached at

the same moisture content.

The nature and magnitude of compaction in fine –

grained materials such as mineral concentrates sig-

nificantly influences their mechanical behavior. In-

creasing values of normal stresses tends to reduction

the degree of permeability.

In consequence, polymer materials prevent drain-

age of the water from the particle pore, sliding and

shifting of ore concentrates in storage. These poly-

mer materials can be used as absorbers of water

from mineral concentrates, before the transportation

by sea. These materials are particularly attractive

because they are based on natural renewable re-

sources, which are environmentally friendly.

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