The standard outlines three distinct air elutriation techniques aimed at precisely measuring the particle size distribution of powders passing through a 75-micron sieve, focusing on fine particles ranging from 5 to 75 microns. It addresses procedures for powders that disperse easily, those that are difficult to disperse, and those requiring specialized treatment, detailing apparatus configuration, sample handling, airflow rates, and result interpretation. This guideline is crucial for professionals analyzing fine powdered materials in sectors like pharmaceuticals, materials engineering, and manufacturing.
Overview
The standard outlines three distinct air elutriation techniques aimed at precisely measuring the particle size distribution of powders passing through a 75-micron sieve, focusing on fine particles ranging from 5 to 75 microns. It addresses procedures for powders that disperse easily, those that are difficult to disperse, and those requiring specialized treatment, detailing apparatus configuration, sample handling, airflow rates, and result interpretation. This guideline is crucial for professionals analyzing fine powdered materials in sectors like pharmaceuticals, materials engineering, and manufacturing.
Audience
Contents
Structure
This section defines the range and key reference tables including particle diameter correlation factors between sieve, projected, and Stokes diameters, true free-fall diameter adjustments based on particle density, and combined sieving and elutriation cumulative undersize weight percentages.
Key definitions from related standards are provided, clarifying terms such as sieve diameter, projected diameter, and Stokes diameter, along with tables for converting particle sizes considering density variations and correlation factors between different measurement methods.
Explains the principle of separating particles by size using upward gas flow governed by Stoke's law, with detailed explanation of variables and elutriation timing periods based on particle size and tube diameter.
Describes procedures for sample preparation including air drying, breaking aggregates, sieving through 75-micron IS sieve, subdividing sample, determining particle density, controlled drying, cooling, and weighing protocols to maintain sample integrity.
Details apparatus preparation such as cleaning and polishing of elutriator components, nozzle sizes with corresponding airflow rates, Gonell elutriator construction and dimensions, and recommendations for tube and container materials.
Specifies the use of clean, dry air or alternate gases with defined pressure ranges, drying and oil removal methods, airflow measurement techniques, and the calculation of free-falling velocities critical for elutriation.
Summarizes the elutriation approach, apparatus details including multi-tube systems, air supply conditions, flow control, and outlines the method's limitations regarding particle size and sample volumes.
Outlines the stepwise approach for analyzing readily dispersible powders including size determination by sieving, elutriation separation, conversion using Stokes diameter tables, and combining results to establish cumulative undersize weight percentages.
Describes pre-treatment processes involving liquid extraction and mechanical dispersion to break aggregates, combined sieving and elutriation techniques, and emphasizes avoiding particle fracture during preparation.
Covers procedures for measuring powders difficult to disperse using a small-scale elutriator with high-velocity air jets, including sample sizes, particle size ranges, and representative data tables illustrating weight reduction and cumulative undersize.
Provides guidance on plotting particle size distribution curves utilizing micron size on the x-axis and cumulative percentage undersize on the y-axis, incorporating correlation factors to ensure accurate conversions between different diameter measurement systems.
Details how to combine results from various particle sizing methods into unified distribution data, supported by tables and formulas including the Stokes law, to present cumulative undersize percentages clearly and consistently.
Explains the limitations of Stokes' law at higher Reynolds numbers, provides tables correlating Stokes diameter and true free-falling diameter for different particle densities, and includes relevant formulas for precise measurement.
Presents multiplication factors to convert particle sizes between sieve, projected, and Stokes diameters, discusses practical considerations for irregular particle shapes, and suggests validation methods for accurate conversions.
Frequently Asked
The standard includes three distinct methods: Method 1 and Method 2 target particles ranging from 5 to 75 microns, suitable for readily and poorly dispersed powders respectively, while Method 3 caters to difficult-to-disperse powders with sizes between 10 and 75 microns. All focus on the sub-sieve fraction, meaning particles that pass through a 75-micron IS sieve.
Powders that disperse easily can be analyzed directly using air or gas elutriation methods without extensive pre-treatment. Conversely, poorly dispersed powders require preparatory steps like extraction with dehydrated liquids (e.g., alcohol followed by ether) and gentle mechanical methods such as rolling or brushing to break aggregates without damaging particles. Additionally, gas or solid distributor systems in elutriators may be employed to improve dispersion before analysis.
Accurate elutriation requires vertical elutriator tubes with diameter ratios of 1:2:4:8 used individually, equipped with a sample-holding U-tube where powder is dispersed by an air jet. The apparatus includes Gonell elutriators with polished brass tubes of specified dimensions, vibration mechanisms to prevent particle adhesion, and miniature elutriators for challenging samples. The air supply must provide clean, dry air at gauge pressures between 0.3 and 1.0 kg/cm², dried through alumina dryers and filtered for oil mist, with flow precisely measured by rotameters or capillary flowmeters and corrected for back-pressure. Sensitive balances and careful cleaning of equipment are also critical.
Conversion between sieve and Stokes diameters involves specific multipliers: to convert a sieve diameter to Stokes diameter, multiply by 0.94; to convert Stokes diameter back to sieve diameter, multiply by 1.07. These factors help relate mechanical size measurements to aerodynamic sizes, particularly when particle shape or density data is not explicitly known, ensuring consistent interpretation of particle size distributions.
Maintaining a stable airflow for at least 15 minutes allows particle separation to stabilize. Intermittent rapping of the elutriation tube, especially just before stopping airflow, dislodges loosely attached particles to ensure proper collection. Selecting the correct tube diameter based on particle size, promptly weighing collected fractions to minimize moisture influence, and continuously monitoring and adjusting airflow with flowmeters and back-pressure gauges are essential for precise endpoint determination.
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