Back scatter optics allow particle size measurement at high concentration

Citation

(2002), "Back scatter optics allow particle size measurement at high concentration", Pigment & Resin Technology, Vol. 31 No. 5. https://doi.org/10.1108/prt.2002.12931ead.009

Back scatter optics allow particle size measurement at high concentration

Back scatter optics allow particle size measurement at high concentration

Keywords: Optics, Particles, Measurement

The back scatter optics in the recently introduced Malvern HPPS (High Performance Particle Sizer) are said to be making possible the accurate measurement of a range of high concentration samples, such as suspensions and particularly emulsions, without the need for dilution. This is believed to be a significant advance since many high concentration systems are sensitive to the disruptive effects of dilution, which produce erroneous particle sizing results. Equally, it may be difficult or inconvenient to build in a dilution step.

An example of a highly concentrated system is that of silica particles used in polishing scurries for the semi-conductor industry. Here particle size distribution affects both the substrate removal rate and the surface finish. Typically slurries can contain silica at 20 per cent volume concentration and are opaque. According to Malvern classical light scattering systems fail to give accurate particle sizing data due mainly to the presence of multiple scattering, whereby light scattered by one particle is rescattered by others before reaching the detector. HPPS back scatter optics minimise multiple scattering by automatically optimising the scattering path length.

Non invasive back scatter technology (NIBS) is a dynamic light scattering technique that incorporates an optical configuration to maximise detection of scattered light while maintaining signal quality. The use of back scattering rather than the typical 90° detection angle, is said to improve the sensitivity of the measurement and ensures the smallest possible interference from multiple scattering. Earlier back scattering techniques suffered from drawbacks that included the need for contact between sample and detector optics, necessitating careful cleaning of the measurement optics between samples to avoid cross contamination. The HPPS uses a standard disposable cell and because there is no contact between sample and detector, cleaning is said to be unnecessary.

The ability to move the sample cell and automatically locate the optimum measurement position is a feature thought to be unique to the HPPS. This automated positioning ensures die setting tailored to the characteristics of the specific sample. The optimum position for measurement is close to the centre of the cell for dilute samples to maximise the observation volume, and close to the cell wall for more concentrated samples to minimise the distance that light must travel through the sample.

The net result is stated to be high measurement sensitivity for samples as dilute 0.1 ppm and the ability to measure at a concentration up to 20 vol per cent.

Malvern also reports that the introduction of its new Malvern Insitec L adds particle sizing in wet suspensions to the capabilities of the Malvern Process Systems range of laser diffraction based on line particle size analysers for measurements in the size range 0.5-l000 microns (Plate 4).

In common with all other systems in the range, the Insitec L is built on the established Insitec platform, described by Malvern as a rugged and robust system that was designed from the outset for industrial applications, and which is in widespread use for on-line measurement in dry powder processes.

Plate 4 The New Malvern INSITEC L

Insitec L takes advantage of the liquid suspension flow cell design employed in Malvern's laser diffraction-based Mastersizer systems, and used routinely across a wide range of industrial applications and materials. The wet cell can be positioned directly in-line for dilute process streams and used on-line with automatic sampling and dilution systems for concentrated suspensions, where applications include quality control.

A range of flow cell path lengths, from 0.2 to 4 mm, is available to suit the specific particle size distribution of the process material. Quick-release flow cell mountings, with easy access to the cell, is said to allow trouble-free cleaning and maintenance. Cell maintenance is in any case believed to be minimised through the use of windows that are very simple to change – and a choice of either glass or sapphire windows meets individual abrasion resistance requirements.

The Malvern Insitec range claims on-line particle size analysis for a diverse range of applications. In addition to the standard lnsitec B and the new Insitec L, the Insitec T meets the exacting specifications necessary for use in GMP operations, while the Insitec X additionally features Intrinsic Safety, for environments where explosion hazards might otherwise pose a problem.

We are also informed by Malvern that major factors in selecting a Mastersizer 2000 system for routine particle size analysis at leading minerals processor Viaton Industries Limited, were its ability to easily handle both wet and dry measurements, its straightforward SOP (Standard Operating Procedure)-driven operation, and its speed and range of measurement. The company has also used the system to develop a method of determining "grindability" that allows them to accurately predict mill throughout rates of new materials.

Viaton Industries Limited is one of Europe's largest processors of mineral based products and a leading contract processor in the field of particle size modification.

Operating round the clock, the company routinely processes a very broad range of materials using a variety of milling, blending and particle classification techniques.

Measuring and monitoring particle size distribution (PSD) is critical, both for mill control and to precisely meet the specifications of the final product. One of the challenges when processing a large variety of materials is the need to measure particle size of both wet and dry samples, depending on the properties of the sample. The modular nature of the Mastersizer 2000 is said to provide the ability to switch quickly and easily between these different measurement modes and it is also said to be possible to switch rapidly from aqueous to solvent based dispersions if required.

Full automation and SOP-driven operation make training and use of the Mastersizer 2000 very straightforward. At Viaton, this has been an important factor in enabling consistent and comparable results throughout their standard 24-h operating period.

Howard Beedle, technical manager at Viaton Industries, is especially pleased with the user friendliness of the Mastersizer 2000 and the improved speed of measurement: "The SOP system on the Mastersizer 2000 works really well and we can depend on reliable 24-h day operation. Having such a rapid turn round in results allows us to make any necessary adjustment to the process immediately."

Details available from: Malvern Instruments Ltd. Tel: +44 (0) 1684 892456; Fax: +44 (0) 1684 892789; Web site: www.malvern.co.uk