Ensuring Reliable Assay Data with Accurate Sample Preparation

In mining laboratories, accuracy in sample preparation is not just best practice but is central to mine profitability. Poor-quality assay data can lead to inaccurate grade control, inefficient plant performance, and financial losses in mine sales. Geologists, lab managers, plant personnel and mine managers must ensure they don’t make decisions based on poor data. The consequences are too costly.

The key to producing reliable assay results is sample preparation guided by Quality Assurance (QA) and Quality Control (QC) principles. Let’s examine the factors within the laboratory’s control that ensure assay data is accurate, representative, and contamination-free.

Sample Preparation Fundamentals

Sample preparation transforms a bulk sample into a form suitable for chemical analysis. This process typically involves sample drying, crushing, splitting, and pulverising to create a manageable, homogeneous, and representative sample. For many assay laboratories, the desired outcome of this process is a fine, powdered sample suitable for analysis.

In most laboratories, both solid and slurry samples are submitted, leading to shared usage of some equipment. The sample preparation area is designed to facilitate smooth transitions between each step. However, space constraints and the need to prevent contamination often prevent a straight-line layout.

Key equipment used in sample preparation includes:

• Drying ovens: Used for drying solid samples or filter cakes from pressure filtration.
• Sample splitters: Either rotary sample dividers or riffle splitters for representative sub-sampling.
• Pressure filters: Various volumes are used to separate solids from liquids.
• Crushers: Reduce solid samples to a size suitable for pulverising.
• Pulverisers: Reduce crushed samples to a fine powder suitable for assay.
• Sieve shakers: Used for particle size analysis.
• Lump breakers: Break down agglomerated lumps in dried samples.
• Blenders: Used for blending samples, especially when creating composites.
• Balances: Essential for accurate sample weighing.

Poor sample preparation can introduce bias, contamination, and errors, rendering the assay data meaningless. Understanding these steps and potential pitfalls of sample preparation will enable you to mitigate risks and maintain data integrity.

Preventing Contamination: The Biggest Threat to Accuracy

Contamination—the inadvertent transfer of material from one sample to another—is a leading cause of unreliable assay data. It can occur at any stage of sample preparation, from drying and crushing to pulverising and weighing. Contaminated results lead to miscalculations in ore grade, affecting downstream processes and mine valuation.

To prevent contamination, laboratories should implement these key best practices:
• Clear separation of high- and low-grade samples: Process these in designated areas or use dedicated equipment where possible.
• Correct stacking in drying ovens: If a single oven is used, place low-grade samples on top shelves and high-grade samples below to prevent contamination from dripping or airborne particles.
• Thorough cleaning between samples:
o Crushers and pulverisers should be cleaned after each use.
o Grinding vessels should be washed using inert material (such as quartz or river sand) to remove residual particles. If silicon is an element of interest, wash with soap and water.
o Sample trays should be cleaned after each use, and spillages must be cleared immediately.
• Strict adherence to Standard Operating Procedures (SOPs): Ensuring proper handling, labelling, and tracking of samples reduces the risk of mix-ups and misidentification.

By applying these contamination control measures, laboratories can maintain the integrity of assay results, ensuring data that mine managers and metallurgists can trust.

Pulverising: A Pivotal Step in Precision

Pulverising reduces crushed samples to a fine powder, typically 80% passing 75µm. However, if not done correctly, this step presents a significant risk for contamination.

Best practices for pulverising include:
• Using separate grinding vessels for high- and low-grade samples: Rigorous cleaning between samples is essential if separate vessels are unavailable.
• Regularly inspecting grinding vessels: Worn-out pucks, rings, or rubber seals can compromise results by allowing the material to leak and mix with subsequent samples.
• Employing control samples: Coarse blanks and fine blanks should be inserted into the sample sequence to detect and prevent contamination.

Implementing Quality Assurance and Quality Control (QA/QC)

A comprehensive QA/QC programme monitors every stage of sample preparation to minimise errors and ensure consistency. Key QA/QC measures include:

• Coarse Blanks: Barren material with a similar particle size to the sample batch, used to check for contamination during preparation.
• Coarse Duplicates: Duplicate samples taken after the first crushing step, submitted under different sample numbers to assess sub-sampling precision.
• Standard Reference Materials (SRMs): Samples with well-established grades used to verify analytical accuracy. It is best practice to include at least three SRMs covering the expected concentration range of key elements.
• Fine Blanks: Pulverised barren material analysed immediately after high-grade samples to detect carryover contamination in the assaying process.

By integrating these QA/QC measures into routine laboratory operations, mine and laboratory managers can confidently rely on their assay data for critical decision-making.

Accuracy Drives Performance

Accurate assay data underpins every aspect of mining operations. From resource estimation and grade control to metallurgical accounting and plant efficiency, reliable sample preparation ensures that your data is fit for purpose. By preventing contamination, maintaining strict QA/QC procedures, and adhering to best practices, laboratories can provide assay results with confidence.

Understanding and implementing these principles will enable you to make informed, data-driven decisions that optimise mine performance and profitability.

An audit by a professional third party is crucial for ensuring laboratory processes’ integrity, reliability, and compliance. Our extensive experience at Effective Laboratory Supplies ensures Quality Assurance (QA) and Quality Control (QC) measures are effective, identifying any gaps that could compromise assay accuracy.

Invest in laboratory excellence. Book a laboratory audit today.

Learn more about Laboratory layout for sample preparation in this blog “Lab Design for Accurate Sample Preparation” .