Individual preparation procedures

Customised sample preparation

ALS offers a range of individual preparation methods that can be combined as needed to customise a preparation routine to meet a specific project's needs. Samples submitted as rocks (drill core, rock, or chips) generally require drying, crushing, splitting, and pulverisation. All these steps have a range of options available. Samples submitted as soils and sediments can also be prepared by customised methods for variable screen sizes and drying temperature.

Drying methods

Sample drying is required before any crushing, pulverisation or screening as moisture in the sample can cause sample to adhere to the equipment. Drying rock samples is generally performed at temperatures >100°C, but lower temperature options are available if required. Soil and sediment samples are dried at a maximum temperature of 60°C to preserve volatile components, but air-drying is also available.

Mercury volatilisation

Methylated Hg, HgCl₂ and Hg⁰ volatise at less than 100°C thus drying temperatures need to be kept low for samples expected to contain mercury. Where mercury is hosted in cinnabar the same temperature limits are not necessary as it is resistant to volatilisation at higher temperatures.

Crushing methods

ALS has a crushing method for large diameter samples that need a step to reduce material size before standard crushing. Fine crushing of material is required for samples that need to be split and pulverised. There are numerous methods with a variety of size and passing percentage requirements available.

How fine to crush?

The choice of a % passing a certain screen size is best identified by test work on for each project. The preparation requirements will be controlled by the grain size of minerals of interest, how large a sample is submitted, and how much sample is separated out for each size reduction step.

Splitting methods

ALS offers splitting methods using a riffle or rotary splitter. Rotary splitting can be paired with size reduction using a Boyd crusher. We also offer splitting methods for samples received as pulps.

Rotary or riffle splitting

A rotary splitter involves a vibrating feed chute that delivers crushed material to a divided rotating sample catcher. The amount of sample needed is then combined from the required number of sample compartments. Samples with nuggety or native metals may not be optimally split this way as the vibration in the chute can segregate heavy minerals. A riffle splitter will divide samples into two equal portions. Reducing the amount of material needed after splitting requires multiple passes through a riffle splitter but can be achieved with a single rotary splitter pass.

Pulverisation methods

ALS has pulverisation methods for a range of sample sizes and passing criteria. Larger sample sizes are often needed when a sample contains nuggety gold. The larger sample size will help minimise nugget effect and produce a more representative sample.

Pulverisation bowls

A variety of pulverisation bowls constructed from diverse media are available on request. The standard equipment used at ALS preparation laboratories is low Cr-steel, however substitution of bowl material type may be required when specific elements that may be imparted by Cr-steel equipment are of interest. All crushing and pulverising equipment will impart some elements to a rock sample and if the low-Cr steel is likely to add an element of interest, an alternative bowls can be chosen. Bowls available include tungsten carbide, agate, and zirconium.