The control of dust emissions during production, handling and transportation of materials is an important consideration in the design and operation of many industrial processes, with excessive airborne dust levels undesirable for a number of reasons including:
- potential adverse health effects to the work force;
- controls can require costly ventilation and filtration systems;
- could prove to be costly in terms of lost product;
- can easily contaminate ancillary machinery and products.
All of these factors make it advantageous for occupational hygienists and process engineers to have accurate information about the propensity of materials to produce airborne dust (the ‘dustiness’ of the material) so risks can be evaluated, controlled and minimized.
As most manufacturers know, dusts are not created equal, with the formation and emission of particulate aerosols during handling and processing dependent upon material handling methods, size distribution of particulate within the mixture, environmental conditions and the physicochemical properties of each mixture.
Fortunately, standardized techniques do exist for dustiness testing with each method designed to provide the type and amount of mechanical energy needed to overcome adhesive binding forces. While relatively straightforward, outcomes are material dependent and analyses complicated by starting weight and processing period. All of these factors can potentially impact particulate dispersal/release levels, affecting aerosol capture and quantification. Once the compounding factors are aligned, however, these methods are highly effective in determining the material’s dustiness index with conventional dustiness involving micrometer-sized particles reported in terms of either dustiness mass fraction (e.g. total and respirable) in mg/kg or as a weight % of the original sample weight.
One major problem in determining Dustiness is that the term is relative with no well-defined physical or chemical material property to define it. Additionally, particle size distribution, humidity and the adhesive forces, binding particles of the test material together, all affect the materials ability to aerosolize particulate. In general, materials consist of agglomerates of varying strengths, which in turn are made up of primary particles. Because the goal of dustiness testing is to only deagglomerate mixtures and separate primary particles, applying too much energy could lead to interference with stronger binding forces and disruption of aggregates and mineralized material, artificially increasing the results. This all makes the determination of appropriate mechanical energy levels a key part of the measured dustiness outcome: for some materials these differences can be substantial and different test methods may not even rank materials of widely differing dustiness values in the same order. Nevertheless, depending on work environment and material processing application, these different tests may be appropriate in simulating the process and no single ‘right’ test exists. Because of this, a number of test apparatuses and corresponding protocols have been developed either for general use or within a certain technical application/industry, constituting a range of formal and informal standards.
As explained above, given the extreme variation in manufacturing conditions, no single dustiness method is likely to be representative of or reproduce the various types of processing and handling activities used in each workplace. Therefore, a number designs for dustiness testing devices exists, each of which produces values specific to a test protocol. Regardless of differences in format, the test variables including sampling and measurement configuration closely resemble one another and all are designed to ensure reproducibility, given their specific set of conditions.
Having multiple methods available and no means by which to comprehensively represent and reproduce specific type of processing and handling, our focus has been to develop a method that provides reliable comparative analysis among products. We also wanted a technique which could be tailored to process industrial scale materials and which better reflected the handling procedures to which powders may be subjected.
Some materials—for example, micronized powders—may need particular, perhaps targeted, dustiness testing while coarser materials can undergo more aggressive analysis. Regardless the differences in protocol, Dustiness Testing developed by Reddwall Solutions has the option of generating two important health-related size fractions – inhalable (total) and respirable, both of which can be analyzed further to assess process/batch controls and to provide input on qualitative and/or quantitative exposure assessments.
To facilitate testing, Reddwall Solutions utilizes the rotating drum method with internal baffling, continuous drop sampling and slow horizontal winnowing of air currents. The dust released from this process is captured within the airflow and forced onto a sampling filter, capable of providing either of the two aerodynamic health-related fractions. Given the wide-ranging capability of this technique to provide product engineering detail, it’s envisaged that branches of industry will ultimately utilize the experimentally derived dustiness values it generates to develop classification schemes specific to the bulk material groups of interest. Particularly in light of the ability to collect aerosol samples and to process these for further chemical analysis.
Once materials have been tested, using our standard conditions and run parameters, the data provide downstream clients (e.g. manufacturers, producers, occupational hygienists and workers) with the dustiness classifications needed to assess a product’s potential for dust emissions as materials are handled, processed or undergo formulation changes. It also gives manufacturers the information needed to improve products while users gain from this system’s ability to not only assess pretreatment effects, but also to determine less dusty products.