Why measure particle size?
Although many do not realise it, particle size in mineral processing plants is a major factor in many aspects of plant design and performance. It is also critical to the economic return of the plant.
Outlined below are some of the ways particle size analysis is so important:
- Recovery of valuable minerals can be compromised by particles outside the optimal size range
- Excessive grinding significantly increases energy consumption in mills and can reduce throughput
- Reagent consumption increases with finer particle size
- Changes in particle size cause unwanted variability in reagent consumption and process operation
- Filtration and thickening capacity decrease with finer particle size
- Water recovery and tailings disposal is affected by particle size distribution
- Finer products in wet plants can mean higher transport costs and increased cost in downstream processing
Particle size has to be monitored so it can be controlled. Some of the ways to monitor size is via manual sampling, sample preparation and analysis using sieves or more modern instruments.
Manual methods are however labour intensive, so many operations reduce size analysis to a minimum. Process control requires frequent measurements, e.g. the reaction time of a grinding circuit is typically 5-15 minutes.
Automatic techniques, such as on-stream particle size measurement, have, become common practice in industry because the return on investment (ROI) is high.
Particle size measurement technologies:
It is well documented that different particle size measurement techniques will yield different results. This is because each technique measures a different dimension of a three dimensional particle. For on-stream particle measurement, the focus is on repeatability, precision and reliability of the equipment.
Ultrasonic attenuation has been applied in wet mineral processes since the 1970s. Because ultrasonic particle size measurement is sensitive to air bubbles, the sample has to be de-aerated before measurement.
Recently, ultrasonics has been combined with gamma-ray transmission and sound velocity measurement for more accurate solids content compensation (Coghill et al, 2002).
The sample volume measured can be relatively large, which is an advantage of this technology. However, the major limitations in mineral processing operations are its sensitivity to air entrainment, flaky particles, solids content and slurry viscosity changes.
There are many different methods in optical image analysis. With the wide size distributions present in mineral processes they are quite inefficient in achieving the desired volume or weight distribution. This is due to the number distribution being so heavily weighted towards fine particles.
Significant dilution is required in order to overcome the opaqueness of mineral slurry and to separate fine particles from each other. Errors can be significant if large particles are missed.
Direct mechanical measurement:
The direct mechanical measurement principle employed in the Outotec PSI 200 particle size instrument (Saloheimo & Anttila, 1994) has been widely used over the past decade.
A representative sample is passed between the ceramic tipped moving element and the fixed ceramic element, the position of the moving element is measured by an electronic sensor.
Random particles are trapped between the elements where their diameters are measured, the arrangement makes two measurements per second. The simplicity of this operation means it has high availability and minimal maintenance.
Principle of mechanical particle size measurement:
One drawback of this method is that it does not give direct information below the D60 particle size or useful information on bi-modal distributions so alternative techniques have been developed.
Laser scattering technology:
Low-angle laser light scattering, or laser diffraction, has been known as a laboratory technique since the 1960s. The size analysis is based on an intensity distribution measurement of coherent laser light scattered by the particles.
The form of the scattering pattern is described by the Mie theory and the width of the pattern is dependent on the size. When laser light meets a population of particles, volumetric size distribution can be calculated back from the scattered light distribution.
In practice, laser diffraction offers the following advantages:
The method is absolute, giving volumetric particle size without need for external calibration. This is equal to weight distribution if density is constant
The technique is applicable over a wide particle size range with excellent precision and repeatability. The on-line system described below can achieve1-2% relative precision of the distribution median over the 1-500 µm size range.
The method is rapid – typically results are produced in just over one minute
The method is non-destructive and non-intrusive
The measurement exhibits high resolution – up to 20 size fractions can be displayed in addition to specific surface area
Principle of laser diffraction measurement:
On-line laser diffraction particle size instrument PSI 500
Mineral slurries in industrial applications are opaque, therefore slurry flow must either flow in a narrow channel or need dilution in order to allow the scattered light to pass through it. These requirements can be avoided if more sophisticated calculation techniques are used in the analysis, so-called multiple scattering (Crawley, 2001).
Studies have showed that a minimum of 5-10% of the laser light must pass directly through the sample in order to get a reliable analysis. If 4mm is selected as the minimum practical flow cell thickness, the solids content can be reduced to 0.1-1% by volume.
One of the challenges of the on-line analysis system is to place a representative sample of the solids into the active measurement volume illuminated by the laser beam. This can be achieved by successive sampling and sample reduction.
Static samplers are used, taking advantage of the mixing caused by process pumps and/or pipe geometry changes and isokinetic flow.
The sampler provides the analyser with a representative sample flow between 50 170 l/min. At the analyser the primary sample is further divided into smaller batches by passing the sample flow across a cutter.
The sample is then mixed and diluted with water in a diluter tank. Thus, the flow cell below the diluter receives the sample in a turbulent state to ensure that the solids are evenly distribution and the speed of particles of various sizes is the same in the active measurement region.
On-line laser diffraction analyser system:
The new generation of on-line laser scatter particle size analysers, the PSI 500 was jointly developed with Malvern Instruments (UK) in 2002. Plant results have shown that laser diffraction is a viable method for the on-line particle size analysis of wet slurries in mineral processes.
The on-line assaying does not require any external calibration, although as expected, there is a difference to other methods like sieves.
Comparison of PSI 500TM, laboratory laser instrument and sieves:
The assays react systematically to process disturbances. As compared to laboratory techniques, the on-line assays are repeatable and show the distribution over a wide particle size range.
More details are available with Outotec.