Sampling Methods and Particle Size Analysis in Mineral Processing
Sampling Methods
Purpose of Sampling
Sampling involves removing a small portion (sample) from a larger bulk material to analyze its properties. Several methods achieve this:
Systematic Sampling
This method collects samples at fixed intervals (mass, time, or space) from a lot or population. The first unit is randomly selected, and subsequent units are chosen at regular intervals. When a batch varies randomly, systematic sampling is similar to random sampling. However, differences arise when the lot exhibits specific trends.
Random Sampling
This simple but less accurate method involves taking small, equal amounts from a lot or cargo using a tool like a shovel. It’s economical and fast, but struggles to represent all components accurately, especially with non-uniform particle sizes.
Stratified Sampling
This method divides a lot or population into several strata or sub-lots for individual sampling.
Two-Stage Sampling
This method involves an initial random selection of samples from a batch, followed by controlled resampling to obtain smaller, more representative units.
Types of Samples
Manual Sampling
Performed with simple tools, this method becomes costly for large quantities. It’s applied in mines, dumps, and trucks.
Mine Sampling
This can be done using chutes or augers.
Wagon Sampling
A lattice or network is placed on the mineral surface, and the sample is taken from the network’s core.
Conveyor Belt Sampling
Samples are obtained systematically or randomly at a specific location or during discharge.
Ore Field Sampling
After collection, the gross sample is subdivided using cone and quartering or riffling to obtain the required portion.
Pond or Tank Car Sampling
This is done using bottles or pipettes.
Mechanical Sampling
Advantageous for large material quantities, this method takes portions at intervals. The sampler should be positioned to access the entire lot.
Variables in Mechanical Sampling
Cutter Opening: Depends on the required sample amount; it must accommodate the largest particles.
Cutoff Frequency: Depends on the material’s nature and ore flow uniformity.
Particle Size Analysis
Importance in Industry
Quantifying particle size behavior is crucial for understanding system performance, especially in grinding circuits. This helps determine milling quality and the liberation of valuable particles.
Accurate particle size analysis is essential, as plant operations may change based on lab results.
Particle Shape
- Acicular: Needle-shaped
- Angular: Pointed shape
- Crystalline: Geometric shape
- Dendritic: Branching crystalline form
- Fibrous: Regular or irregular
- Scaly: Sheet-like
- Granular: Irregular dimensional shape
- Nodular: Irregular round shape
- Spherical: Globular shape
Equipment for Sieve Analysis
- Sieve Series: ISO (since 1967), ASTM 1-70 (since 1970, US and Canada), Tyler (USA)
Each sieve has a metal frame (stainless steel or brass) with a mesh bottom. Sieves are shaken for 10-15 minutes to facilitate particle passage.
- Shaker: Creates vibrations in the sieve series.
- Ro-TAP: Imparts eccentric horizontal rotary motion and sudden movements to particles.
Determining Particle Size
Determining the size of processed material in a concentrator is challenging due to particle size diversity. Nominal size is typically determined by screening.
Screen Types
- Tyler
- Mest
- B6-410
- Afonor
- DIN-4188
Tyler Sieve System
This widely used method involves passing material through a series of meshes with decreasing openings from top to bottom.
Screening Procedure
- Take a representative ore sample (200-300 grams).
- Place the mesh series in a Ro-TAP shaker.
- Determine and carefully remove the weight of material retained on each mesh.
- Weigh using a scale with at least 0.1-gram precision.