Preparing Gold-Bearing Ore for Laboratory Testing

Why Representativity Matters More Than Laboratory Test-Portion Size

Gold-bearing ore is a heterogeneous material. Gold may be distributed unevenly throughout the original mass, particularly when relatively coarse free gold is present.

A small, randomly selected portion of material may therefore have a gold grade that differs from the average grade of the original lot.

Representativity is not determined by sample mass alone. It is affected by material heterogeneity, ore particle size, the nature of gold distribution, and the sequence of sampling and splitting operations.

Obtaining the Primary Sample

Material for laboratory investigation should be collected so that individual portions of the lot under study have an opportunity to be included in the sample.

Selecting several visually “rich” or “representative-looking” pieces of ore is not a valid method of obtaining a representative sample.

For heterogeneous gold ore, the sampling scheme and required sample mass should be determined with regard to the properties of the specific material.

The same sample mass may be sufficient for one ore type and insufficient for another.

Crushing

After the primary sample has been obtained, its particle size is reduced in stages so that the material can be mixed, divided, and prepared for testing.

Crushing alone does not make a sample representative.

After size reduction, all material must be retained and selective loss of individual size fractions must be prevented. The specific crushing size is determined after the source data have been reviewed.

Mixing and Sample Splitting

Following size reduction, the material must be homogenized as far as practicable and then correctly divided into smaller portions.

Professional laboratory practice uses equipment and procedures that provide more reliable division of the material stream, including riffle splitters and rotary sample dividers.

Simply scooping material from the surface of a bag or container should not be used to obtain a representative laboratory test portion.

Each subsequent mass-reduction step must preserve the representativity of the original material. An error introduced during sample splitting is carried through into the analytical and leaching results.

Determining the Initial Gold Grade

Reliable source data for the material under investigation are required before leaching results can be interpreted.

The gold grade of the primary sample provides the basis for subsequent recovery and material-balance calculations. The analytical method is selected with regard to the material and the test program.

Grinding Before Leaching

The degree of grinding affects the accessibility of gold-bearing minerals to the leaching solution. Experimental work on gold-bearing ores confirms that particle size affects gold dissolution and recovery.

However, a specific particle size selected for one ore cannot be applied automatically to another.

Target particle size is a parameter of the test program, not a universal JinChan specification.

If several degrees of grinding need to be compared, they should be tested as a series of comparable tests while the remaining parameters are controlled.

Preparing Comparable Laboratory Test Portions

Once the material has been prepared, the test portions must be made as comparable with one another as possible. This is particularly important when comparing:

• different reagent dosages;
• different pH values;
• different leaching durations;
• different degrees of grinding;
• JinChan and a control condition.

Where possible, differences between test results should reflect the change in the process parameter under investigation rather than a random difference in the composition of the original test portions.

What Must Be Recorded Before Testing

Why the Ore Cannot Simply Be Ground “As Usual”

Conditions used for another ore or at another operation do not provide sufficient justification for automatically selecting the same particle size.

The accessibility of gold to leaching is affected by its mode of occurrence, association with other minerals, and the degree of mineral liberation.

Particle size should therefore be treated as one of the experimental parameters in the test program.

Transferring the Prepared Sample to the Leaching Program

Once representative and comparable test portions have been prepared, the laboratory leaching program begins:

• particle size is recorded;
• slurry density is specified;
• alkaline conditions are established;
• reagent concentration or dosage is specified;
• test duration is defined;
• the solution and solid residue are analyzed.