Leong Mar and Peter Wypych present the findings from their research project into scientifically measuring and quantifying the ‘dustiness’ of a material, an area that all too often is left to personal interpretation.
Dust emissions are a pertinent issue for the mining industry, especially for workers on site and communities near mines, transport routes and ports. In recent years the mining boom has significantly increased the amount of material mined and transported and consequently has led to a rise in dust issues.
At the rudimentary level, dust emission and fallout is a nuisance and causes a loss of amenity. Unfortunately in many instances the impacts do not stop there. Dust emissions can have far reaching health, safety and environmental consequences, posing significant business risks that can threaten the company’s or industry’s social license to operate and its sustainability.
The impact to human health from inhalation of materials like heavy metals and silica dust are well known. Dust from relatively inert materials, such as iron ore, can also have adverse health effects because particulate size plays an important role. Airborne particles less than 10 microns in size can easily be inhaled into the lungs and finer particles less than 5 microns can get permanently lodged inside the lungs increasing the potential for respiratory illness.
Managing dust emission can be a challenge for the mining and supporting industries because dust is inherent to many of the fundamental operations and can be generated from a variety of sources. The presence of fine particulates in a bulk material is not necessarily the entire problem/issue, it’s when they get airborne and suspended – then they can be transported and dispersed great distances from the source.
There are a variety of ways in which the mining industry manages dust:
Ventilation – the airborne dust in a work environment is diluted by increasing the airflow.
Containment – the dust is trapped and contained within a building or enclosure. A fan/ filter system is usually used to maintain a slightly negative pressure inside the enclosure.
Extraction and filtration – the airborne dust is extracted and filtered from the air. The collected dust can be disposed to prevent re-emission.
Suppression – water, chemicals or other technologies are applied to the dusty material to agglomerate finer particles and prevent the dust from being generated.
Wind barriers or diffusers – natural or artificial barriers are placed in strategic positions to reduce the velocity of air flowing over the dusty material.
Design – it is possible to design the plant, equipment and process to minimise dust generation.
A holistic and integrated approach is needed for effective and sustainable dust management solutions. Preventing the dust from getting airborne in the first place (addressing the root cause) is probably the most critical aspect of good dust management. DuPont Australia and the University of Wollongong have embarked on a research program which is supported by a grant from Australian Research Council to examine some of the dust issues faced by the mining industry and develop sustainable solutions to address the root cause(s) of the dust problems. This article presents some results from this program.
There are several key factors that govern dust generation. Firstly, the material characteristics such as: specific gravity, friability, amount of fine particulates and for some materials, the moisture content. Just as important is the way the materials are processed and handled, and their exposure to wind and airflows can cause dust particles to get airborne.
“A fundamental issue is that dustiness is subjective; what is considered dusty to some may not be to others.”
This article looks at one of the key issue the industry has been grappling with, material dustiness. Dustiness can be defined as the propensity for a material to generate airborne particulates under certain conditions. A fundamental issue is that dustiness is subjective; what is considered dusty to some may not be to others. Associated with the dustiness issue are:
- how to measure or quantify the dustiness of a material
- how to predict when a material is likely to generate dust
- how to prevent or minimise dust generation with a given material
- how to measure the efficacy of dust suppression agents.
There needs to be a scientific basis for measuring and comparing the dustiness of different materials. This is crucial to understanding the dust issues and developing dust mitigation measures and the investment decisions that follow.
There are various standard test methods developed to determine the dustiness of materials that are applicable to the mining industry. The most common is the rotating drum test method. The method that is used extensively in Australia was developed by Rio Tinto and examines the dust emissions as a function of moisture content, AS4156.6-2000 ‘Coal Preparation, Part 6: Determination of Dust/ Moisture Relationship for Coal’ was issued by Standards Australia in 2000. Figure 1 shows a picture of the equipment and instrumentation required for this standard at the Bulk Materials Engineering Australia (BMEA) facility. This method measures the total amount of dust generated from a material at different moisture contents and does not distinguish the particle size fractions in the dust. While this method was developed for coal, it can be used for other materials.
BMEA ROTATING DRUM DUSTINESS TESTERS: AS4156.6 ABOVE & I.S. EN15051 BELOW .
In Europe a method that measures the different size fractions of the dust generated: I.S. EN15051- 2006, Workplace Atmospheres, Measurement of the Dustiness of Bulk Materials – Requirements and Reference Test Methods was issued in 2006 by the CEN European Committee for Standardization. Figure 1 shows a picture of the equipment and instrumentation at BMEA.
While both standard tests measure dustiness, there are some fundamental differences and issues with these methods. Table 1 summarises and compares the key parameters for the two test methods. The sample size is different for the two tests (e.g.1000ml versus 35ml) but more importantly, the Australian standard excludes size fractions above 6.3mm. There are also differences in test duration, drum rotation speed and air flow. The fundamental differences in the two methods reflect the different focus and purpose of the methods.
|Sample size||1 kg (coal) or equiv. bulk volume (1 litre)||35 cm3 (35 ml or 0.035 l)|
|Max. particle size||6.3 mm||Not specified|
|Ambient conditions||20 deg C, 63% humidity||21 deg C, 50% humidity|
|Drum diameter||300 mm||300 mm|
|Dimension of the ‘Blades’ inside drum||7 mm wide ´ 6 mm high (8 off)||25 mm high (8 off)|
|Drum rotation speed||29 rpm||4 rpm|
|Test duration||10 min||1 min|
|Drum air inlet diameter||40 mm||150 mm|
|Suction air flow||170 litres/min||38 litres/min|
|Drum inlet air velocity||2.25 m/s||0.036 m/s|
|Superficial air velocity inside rotating drum||0.04 m/s||0.009 m/s|
|Dustiness Index||Dust Number = Dust mass (g)/Sample mass (g) ´ 105||Mass of different particulate fractions per kg of sample (mg/kg)|
COMPA RISON OF AS4156.6 & I.S.EN15051
ROTATING DRUM TEST SPECIFICATIONS
AS4156.6 deals with the dust/moisture relationship in coal and was developed to optimise the amount of water used for dust suppression. The standard defines a moisture content, the Dust Extinction Moisture (DEM), at which the dust/moisture relationship is ‘optimal’. In practice this means minimal dust generation. The DEM is determined by plotting the Dust Number at different moisture contents on a log/ linear graph as shown in Figure 2. An exponential trend line is normally fitted to the data and used to determine the DEM for the material. The DEM is defined as the moisture content at which the Dust Number is 10. A dust number of 10 correlates to 0.01% in mass of dust collected from the sample.
DUST/MOISTURE CURVE FOR A COAL SAMPLE SHOWING DEM = 8.8% (TAKEN FROM AS4156.6-2000)
I.S. EN15051 focusses on determining the dustiness index of a (normally powder) sample for occupational hygiene in workplace emissions. The method measures the inhalable, thoracic and respirable dust mass fractions from the sample.
If the inhalable dust mass fraction is found to be > 5000 mg/kg, then the dustiness of the powder sample is classified as high. Although not described in I.S. EN15051, the dust/moisture relationship for a material can also be determined by conducting a series of tests at different moisture contents. The following equation can then be used to calculate equivalence between the two standards.
Inhalable Dustiness Mass Fraction (I.S.EN15051) = 10 × Dust Number (AS4156.6)
A number of practical issues have been identified as possible limitations and/or potential errors sources with the two rotating drum tests.
DUSTINESS & DEM OF MATERIAL BASED ON AN EXPONENTIAL TREND LINE(DA SHED CURVE) & SMOOTH LINE OF BEST FIT TREND LINE (SOLID CURVE)
The exponential dust/moisture trend line stipulated by AS4156.6 does not necessarily occur for all bulk materials and can provide misleading results as indicated in Figure 3. The DEM for this material was determined to be 12% based on an exponential trend line but was found to be 11% based on a smooth line of best fit. This difference was found to be quite significant for some samples.
An important issue that also emerges is the impact of the moisture content on materials handling. While having a moisture content near the DEM is advantageous for reducing dust emissions, it has the potential to introduce handling problems or affect the commercial value of the material. At moistures approaching DEM, adhesion of material to the inside of both rotating drums is evident. As an example, Figure 4 shows the dustiness test of the material shown in Figure 3 at a moisture content near the DEM. An example of greater adhesion for a mineral concentrate sample at a lower moisture content is shown in Figure 5.
DUSTINESS TEST AT MOISTURE CONTENT OF 8.1% FOR A MATERIAL WITH DEM = 11%
DUSTINESS TEST AT A MOISTURE CONTENT OF 1.6% FOR MINERAL CONCENTRATE WITH DEM = 2.8%
AS4156.6 reduces the maximum size of the material to below 6.3mm. However, the top size of Australian produced coal can be 50mm. Because of the larger surface area of fine particulate samples, its moisture adsorbing abilities increase. The practical implication is that the results obtained from AS4156.6 may have a higher DEM and may not be representative of the actual coal being handled. What could be more detrimental are the higher levels of water added to control dust but also causing the material to become more ‘sticky’ and introduce handling problems. To investigate possible differences between the two standards, direct comparison experiments have also been performed. Figure 6 provides an example of the dust/moisture relationship obtained from an iron ore sample using the two rotating drum test methods. The resulting DEM obtained using AS4156.6 was 5.2% whereas it was found to be 3.8% using I.S. EN15051. These results suggest the results obtained using the two test methods may not be directly comparable.
“The results obtained in the DuPont/University of Wollongong collaborative project to date highlight some of the issues with current dustiness test methods.”
DUSTINESS & DEM OF AN IRON ORE SAMPLE BASED ON AS4156.6 & I.S. EN15051
Some possible key improvements to dustiness testing are being investigated:
- determining the Particle Size Distribution (PSD) from the total dust collected according to AS4156.6, so that inhalable, thoracic and respirable dustiness mass fractions can be determined
- for AS4156.6, using a sample of the actual material instead of only size fractions <6.3mm or employing a method to relate the results from the <6.3mm sample to the actual material
- re-designing the dust chambers and transfer pipes/tubes to avoid dust deposition (which has also been found to be a problem)
- examining and correcting possible system effects inside each rotating drum;
- correlating system effects observed in the rotating drum methods to potential effects in the field, such as fines adhesion and build-up.
The results obtained in the DuPont/University of Wollongong collaborative project to date highlight some of the issues with current dustiness test methods. The AS4156.6 method is the most commonly used by the mining industry in Australia. It is a useful method for determining the DEM and getting a better understanding of the dust-moisture relationship. The method is best used for comparing the dustiness of different types of coal or other mineral ores. However, caution should be exercised when using the results in practice, especially as a method for optimising the moisture content for dust control. The experiments have shown that the adhesion (‘stickiness’) of some materials can be significantly altered even at moisture levels well below the DEM such that materials handling is adversely impacted. Solving the dust problem could result in a handling problem. With increasing community focus on the health implications of fine particulate fractions (<10 microns), AS4156.6 may need to be modified to also measure the different mass fractions of particulate matter.
Quantifying and knowing the dustiness of bulk materials is a key requirement for good dust management. In order to develop better methods and technologies for dust management, DuPont Australia and the University of Wollongong have adopted a holistic and integrated approach to developing effective solutions to address the root cause(s) of dust problems while not creating other adverse impacts.
“An important issue that also emerges is the impa ct of the moisture content on materials handling.”
DR LEONG MAR
Dr Leong Mar is leader of the dust management business at DuPont Sustainable Solutions (DSS).
Leong has over 20 years’ experience in the science and technology sector and in-depth understanding of the fundamentals of dust generation, dispersion and control. This has enabled him to successfully direct the development of innovative, industry leading product and technology solutions for the mining industry and specialised procedures for testing ore properties, product performance and dust emission monitoring. Leong has led his team to win prestigious industry awards including two Australian Bulk Handling Awards and one Mining Prospect Award for Environmental Management and Dust Control Technologies.
Prior to the position with DSS, Leong was manager of the DuPont Technology Centre and led the Corporate Business Growth and Innovation teams in Australia. Leong has a BSc(Hons) and PhD from the University of New South Wales.
Peter Wypych is the Founder and General Manager of Bulk Materials Engineering Australia (BMEA) and has completed over 1000 projects for all sectors of industry, involving R&D, feasibility studies, troubleshooting, audits, design and debottlenecking of bulk handling plants and processes.
Peter also has been involved with the research and development of bulk handling and processing technology at the University of Wollongong since 1981. His areas of expertise include bulk materials handling, conveying, conveyor transfers, computer simulation technology, dust hazards, control and management, including dust explosions.
Peter Wypych has published over 500 articles in these areas and has presented numerous training workshops, seminars and professional development courses around the world. He is the Chair of the Australian Society for Bulk Solids Handling, Engineers Australia.
AC K N O W L E D G E M E N T S
With thanks to Faculty of Engineering, University of Wollongong and DuPont Sustainable Solutions, DuPont Australia Pty Ltd.