Liquid Systems Overview - Monitoring Particle Size

Taking mineral processing as an example, typically plant feed is reduced in comminution circuits to liberate valuable minerals and produce particles of the appropriate size for downstream processing. A number of important factors have to be considered:

• particle size must be fine enough for optimal mineral liberation
• recovery of valuable minerals is compromised for particles outside the optimal size range
• too fine a particle size increases energy consumption in grinding mills significantly
• 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
• tailings disposal is affected by particle size distribution

Various measurement techniques have been used for monitoring particle size in slurries. Ultrasonic attenuation has been applied in wet mineral processes since the 1970's. Recently it has been combined with gamma-ray transmission and sound velocity measurement for more accurate solids content compensation. A major limitation of ultrasonic particle size analysis in mineral processing operations is its high sensitivity to entrained air bubbles and no reliable solution has yet been found for removing air from the sample on a continuous basis.

Various optical image analysis methods, based on particle counting have been proposed and applied to specific needs. With the wide distributions present in mineral processes they are quite inefficient in getting the desired volume or weight distribution since the number distribution is so heavily weighted towards fine particles. Significant dilution is required in order to overcome the opaqueness of mineral slurry and separate fine particles from each other.

The direct mechanical measurement principle employed in the Outotec PSI 200 particle size analyzer has been widely used in the past decade. Due to the simplicity of its operation, it has high availability with minimal maintenance requirements. There are, however, certain applications where the variation of the fines in the distribution is of importance and for this a different technique is needed and the speed, non-contact nature and robustness of the method to ambient conditions make laser diffraction ideal for on-line mineral slurry applications.

Mineral slurry in industrial applications is opaque so that it must either flow in a very narrow channel or needs dilution in order to get the scattered light through it. This requirement can be greatly relieved if more sophisticated calculation techniques, so called multiple scattering are used in the analysis. Studies have showed that a minimum of 5 - 10% of the laser light needs to pass through the sample in order to get a reliable analysis. If 4 mm is selected as the minimum practical flow cell thickness, the solids content must be reduced to 0.1%-1% by volume.

Examples of real-time particle size monitoring in liquid systems

Monitoring particle size in in-line dilute process flows
For example, direct measurement of the particle size distributions and concentration of the oil-in-water emulsions that make up oil rig effluent streams may be done undertaken in-line using a high-pressure cell (10 bar maximum operating pressure). This environmental monitoring system is fast becoming a standard in the oil industry.

Monitoring particle size in re-circulation loops for dilute unit operations
Mounting a sensor in-line in a re-circulation loop for dilute unit operations allows monitoring of processes such as flocculation in pharmaceutical formulations. The real-time continuous measurement has allowed the direct measurement of previously elusive phenomena in tandem with automatic dilution system.

Monitoring particle size in concentrated slurry streams using an automatic dilution system
Post-dilution measurement is needed in the quality control of mineral production, for example, where sample dilution is a necessity, and for monitoring the grinding of precious metal ores. In the platinum recovery process, the Malvern Insitec system coupled with a simple, continuous dilution system, is used to measure the ore at the mill exit, upstream of the froth flotation process. A percentage point improvement in platinum recovery is sufficient to cover the installed costs within several months.