Challenges of Developing ISO Sampling Standards

Ralph Holmes

CSIRO Minerals Down Under Flagship

Chair ISO/TC 102/SC 1 – Sampling Iron Ore

Chair ISO/TC 27/SC 4 – Sampling Coal and Coke

Convenor ISO/TC 183/WG 9 – Sampling Cu, Pb, Zn and Ni Concentrates

Introduction

• Standards developed by the International Standards Organisation (ISO)

for sampling mineral commodities are criticised for not being adequate

and conforming to Pierre Gy’s theory of sampling

• Is this fully justified?

• Always scope for improving ISO sampling standards, but many elements

of the theory of sampling and the requirements for correctly designing

sample cutters are incorporated into current ISO standards for sampling

key mineral commodities, ie, iron ore, coal and coke, and base metals

• These standards are continually being reviewed and improved

• Difficult task, because they are developed by technical committees on

an international consensus basis

– Commercial issues or ignorance can get in the way of correctness

– Introduction and adoption of new concepts and procedures takes time

– Persistence usually pays off, particularly if changes backed up by hard data

• Bad ISO standard are being discontinued, eg, ISO 1988 for coal

ISO Standards Development

• Technical Committees established to develop standards for specific

commodities, eg:

– ISO/TC 27 (Solid mineral fuels)

– ISO/TC 102 (Iron ores)

– ISO/TC 183 (Copper, lead, zinc and nickel ores and concentrates)

• Sub-committees and Working Groups set up to develop specific

standards for sampling, analysis, physical testing, etc

• Countries participating in the development of these standards nominate

representatives to these Sub-committees and Working Groups

• Draft standards progress through a number of stages:

– Working Draft (WD)

– Committee Draft (CD)

– Draft International Standard (DIS)

– Final Draft International Standard (FDIS)

– Publication as an International Standard

Key Requirements for Correct Sampling

• The key requirements for ensuring that the samples collected from a

process stream are representative and that the overall precision is

adequate are as follows:

– The number of increments taken from the process stream must be sufficient

to obtain the desired sampling and overall precision

– All particles in the process stream or stratum must have an equal probability

of being selected and appearing in the final sample for analysis

– For a given nominal top size of the material being sampled, there is an

absolute minimum mass of sample that must be retained after division to

control the fundamental sampling error at each sampling stage and obtain

the desired division precision (Gy, 1982)

• If these key requirements are incorporated into ISO standards for

sampling mineral commodities, then the standard goes a long way

towards conforming to correct sampling principles and providing

representative samples

Components of Sampling Error

• A number of ISO standards developed recently for copper, lead, zinc and

nickel concentrate slurries (ISO 11794) and coal slurries (ISO 20904)

start off by explicitly outlining all the components of the overall sampling

error TSE defined by Gy as follows:

where FE = fundamental error

GE = grouping and segregation error

QE2 = long-range quality fluctuation error

QE3 = periodic quality fluctuation error

WE = weighting error

DE = increment delimitation error

EE = increment extraction error

PE = preparation error

• The error components are then discussed in detail

TSE = FE + GE + QE2 + QE3 + WE + DE + EE + PE

Components of Sampling Error

• Error components that lead to bias need to be eliminated

– Accessory errors

– Delimitation and extraction errors

– Weighting errors

• Others need to be reduced to achieve acceptable precision

– Fundamental, grouping and segregation errors

– Long-range quality fluctuation error

– Periodic quality fluctuation error

• Minimising or eliminating bias is critical, because bias cannot be

eliminated once it is present – no point in being “precisely incorrect”

– Sources of bias that can be eliminated

• Incorrect delimitation and extraction of increments

• Sample spillage and contamination

– Sources of bias that need to be minimised

• Change in moisture content, dust loss, etc

Sampling Precision

• Several approaches are used for determining the overall variance in

ISO standards for sampling key mineral commodities

• The most comprehensive approach is described in ISO 12743 for

sampling copper, lead, zinc and nickel concentrates, where the overall

variance is expressed as:

• When replicate analyses are carried out:

• In ISO 12743, the sampling variance is broken up into its components

for each sampling stage as follows:

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Sampling Precision

• The sampling stage approach is particularly useful for designing and

assessing sampling schemes

• To obtain the maximum benefit, the variance between increments for

each sampling stage needs to be determined using the following

equation provided in ISO 12743:

• The sampling variance for stage “i” is then given by:

• The overall variance is then given by:

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Sampling Precision

• The sampling stage approach is a comprehensive approach to

designing a sampling scheme and determining the overall variance,

although quite a lot of work is involved

• A simpler approach used in ISO 3082 and ISO 13909-2 for sampling

iron ores and coal and coke respectively is to break up the overall

variance into the primary sampling, sample processing and analytical

variances only as follows:

• The primary sampling variance is determined using a similar process to

that used in the sampling stage method

• The sample processing variance is usually determined experimentally

by processing duplicate samples

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Sample Cutter Design Requirements

• The discharge end of a conveyor is the best location

• The cutter must take a complete stream cross-section

• The cutting time at each point must be equal

• The cutter should intersect the stream in a plane

normal to the stream trajectory

• The sample cutter must be non-restrictive, self-

clearing and discharge completely each increment

• The plane of the cutter aperture must not be vertical

• The cutter aperture must be at least 3d

• The cutter speed must be uniform and must not

exceed 0.6 m/s unless the cutter aperture exceeds 3d

• Bucket-type cutters must have sufficient capacity

• Material from belt scrapers must be sampled

• No contamination of the sample or change in quality

Incorrect

Incorrect

Correct

Incorrect - Sample reflux

ISO 12743 (Concentrates) ISO 3082 (Iron ores)

There shall be no impediment to the flow of concentrate into the cutter at the maximum flow rate of the concentrate.

There shall be no impedance to flow of sample materials through the sample cutter at the maximum flow rate.

The cutter shall be of the self-clearing type, eg, stainless steel or polythene lined, discharging each increment completely.

There shall be no clogging or retention of residual material in the sample cutter, ie, the cutter shall be self-clearing.

Discharge chute angles shall be a minimum of 60° to the horizontal.

There shall be no significant change in the quality of the sample while taking increments, eg, degradation of the constituent particles if the sample is taken for size determination or change in moisture content if the sample is taken for moisture determination.

No materials other than the concentrate sample shall be introduced into the cutter, eg, dust must be prevented from accumulating in the cutter when in parked position.

There shall be no contamination or introduction of material other than the sample into the sample cutter.

Sample Cutter Design

Sample Cutter Design

ISO 12743 (Concentrates) ISO 3082 (Iron ores)

The cutter shall collect a complete cross-section of the concentrate stream, both the leading and trailing edges completely clearing the stream at the two limits of the cutter path.

The sample cutter shall take a complete cross-section of the ore stream, both the leading and trailing edges clearing the stream in one path.

The cutter shall intersect the concentrate stream either in a plane normal to, or along an arc normal to, the mean trajectory of the stream.

The sample cutter shall intersect the ore stream either in a plane perpendicular to or along an arc normal to the mean trajectory of the stream.

The cutter shall travel through the concentrate stream at a uniform speed, not deviating by more than 5% at any point.

The sample cutter shall travel through the ore stream at a uniform speed, not deviating by more than ±5% at any point.

The geometry of the cutter opening shall be such that the cutting time at each point in the stream is equal, not deviating by more than 5%, ie, straight path cutter shall have parallel cutter lips and radial cutters shall have radial cutter lips.

The geometry of the cutter aperture shall be such that the cutting time at each point in the stream is equal, not deviating by more than ±5%, eg, straight-path cutters shall have parallel cutter lips and rotary cutter shall have radial cutter lips.

Incorrect

Correct

Sample Cutter Design

ISO 12743 (Concentrates) ISO 3082 (Iron ores)

The cutting aperture of the cutter shall be not less than 30 mm or, if agglomerates are present, three times the nominal top size of the concentrate, whichever is larger, noting that the cutter aperture may need to be increased above 30 mm if blockages occur for wet concentrates.

The cutting aperture of the primary sampler shall be at least three times the nominal top size of the ore, or 30 mm, whichever is the greater. However, with certain ores, eg, sticky ores, bridging and consequent bias may occur for a cutter aperture of three times the nominal top size. In these instances, larger cutter apertures shall be used to prevent the introduction of significant bias.

The maximum cuter speed shall 0.6 m/s unless the cutter aperture is increasedabove the minimum of 3d in accordance with the equation derived by Gy and Marin.

The maximum cuter speed shall 0.6 m/s unless the cutter aperture is increasedabove the minimum of 3d in accordance with the equation derived by Gy and Marin.

Bucket cutters shall be of sufficient capacity to accommodate the increment mass obtained at the maximum flow rate of the concentrate.

Bucket type cutters shall be of sufficient capacity to accommodate the increment mass obtained at the maximum flow rate of ore.

Incorrect

Incorrect

Sampling from Wagons and Stockpiles

• Sampling from wagons can only be

conducted if a full vertical column is

extracted

• Sampling of stockpiles not permitted

Incorrect

Correct

Incorrect

Fundamental Error and Minimum Mass

• The fundamental error variance 2FE identified by Gy is due to the

particulate nature of the material being sampled and is given by:

where C = sampling constant

d = nominal top size (cm)

mS = divided sample mass (g)

a = fractional concentration

• The fundamental error leads to the minimum sample mass to achieve

the required precision by transposing the above equation:

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23

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aCdm

Minimum Sample Mass

• The sample mass cannot be reduced below the minimum sample mass

for a given precision until the sample is crushed

• This is a critical sampling requirement that must not be ignored if good

division precision is required

• While the minimum sample mass can be calculated, it can also be

determined experimentally

• Test programs have been conducted by several ISO subcommittees to

establish the minimum sample mass as a function of nominal top size

and required precision as follows:

– ISO/TC 27/SC 4 (Sampling coal and coke)

– ISO/TC 102/SC 1 (Sampling iron ores)

Minimum Sample Mass

• For iron ores (ISO/TC 102/SC 1), a comprehensive international test

work program was conducted, the main contributors being:

– Australia, Brazil and Japan

• The following equation for minimum sample mass mS (in kilograms) as a

function of division precision D (in % Fe) was subsequently developed

for iron ores based on Gy’s equation:

where d = nominal top size (mm)

• This equation is included in ISO 3082 for sampling iron ores, which

states clearly that the sample mass cannot be reduced below this

minimum until the sample is crushed

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Minimum Sample Mass

• Examples of the minimum mass of divided sample for iron ore using

this equation are provided in ISO 3082

• The masses specified in ISO 3082 are conservative, because the ISO

standard is designed to cover iron ores from around the world

• The comparable minimum mass tables for sampling coal are only

partially based on experimental data, but the masses are conservative

Minimum mass of divided gross sample (kg) Nominal top size (mm)

Dσ = 0.1% Fe Dσ = 0.05% Fe

40 325 1,300

31.5 180 710

22.4 75 300

10 10 40

6.3 3.2 13

2.8 0.5 1.7

1.4 0.5 0.5

0.50 0.5 0.5

0.25 0.5 0.5

System Verification

• The latest ISO standards included check lists for

confirming the compliance of sample cutters, including:

– Cutter speed

– Uniformity of cutter speed while cutting the ore stream

– Number of cuts

– Size and geometry of cutter apertures

– Worn and/or missing cutter lips

– Build-up and/or blockages in cutter apertures and chutes

– Reflux from cutter apertures

– Ingress of extraneous material when the cutter is parked

– Holes in chutes and bins resulting in sample loss

– Increment/sample mass

– Particle size

Worn Cutter Aperture

Partially Blocked Cutter

Hole in Chute

Conclusion

• The principal aspects of the theory of sampling and the requirements for

correctly designing sample cutters have been incorporated into key ISO

standards for sampling mineral commodities, including:

– Iron ore

– Coal and coke

– Copper, lead, zinc and nickel concentrates

• These standards play an important part in international trade

• However, there is always scope for improvement

• Developing new and improving existing standards is not an easy task

– Standards are developed by ISO technical committees on a consensus basis

– Reaching international consensus is an ongoing challenge

– Important for those with a good knowledge of correct sampling practices to

maintain their involvement in ISO committees on a long term basis

– Persistence pays off

– Don’t give up !!

Invitation to Sampling 2012

• Date: 21-22 August 2012

• Hyatt Hotel, Perth, Australia

• Abstracts due by 14 November 2011– Development and application of sampling theory

– Drill and blasthole sampling

– Plant sampling

– Sampling for blending, quality control and metallurgical

accounting

– Sampling of commodity exports

– New developments in sampling and sample preparation

equipment

– Maintenance of sampling equipment and training

– Development of national and international standards

– Case studies of the application of sampling in

exploration, mining, mineral processing and

environmental monitoring.

• http://www.ausimm.com.au/sampling2012/

Thank you

ISO/TC 102/SC 1 Committee

ISO/TC 183

ISO/TC 102/SC 1