Saturday 23rd Oct, 2021

Dust in bulk handling operations: sources, testing and passive control through effective design

TUNRA Bulk Solids shares some of the testing and design strategies it has learned through research and collaboration to prevent dust emissions.

There is no doubt that the creation and management of dust is an ongoing challenge. In recent times, there has been an increased focus on environmental and health concerns related to dust emissions in mining and mineral processing activities. Understanding the behaviour of dry bulk materials in different handling operations is of utmost importance for the design of ‘dust-free’ facilities. Dust remains a key challenge in designing and operating specific links in the materials handling chain, such as transfer chutes, as well as in the holistic management of operations such as ports.

Dust testing:

One of the methods used for characterising bulk materials is the Australian Standard AS 4156.6-2000 Coal Preparation – Determination of dust moisture relationship for coal, typically known as the ‘Dustiness’ or ‘Dust Extinction Moisture’ test, whose main aim is to determine the relationship between moisture content and dustiness levels. The test has been used as a tool in dust management strategies to reduce or control dust emissions in a variety of materials handling systems such as belt conveying, transfer chutes and stacking operations [1].

According to the standard, the “dust number” can be calculated as per Equation 1, and the Dust Extinction Moisture is the moisture at which a dust number of 10 is obtained. This means 0.1 gram of material collected in the bag per 1 kilogram of sample.

Dust Number=(Mb-Ma)
Ms

Equation 1
Where:

M_b = mass of filter bag and dust [g]

M_a = mass of empty filter bag [g]

M_s = mass of sample in drum [g]

The test is performed with material samples prepared at several moisture contents and the results are plotted in a semi-log graph similar to the one shown in Figure 1.

Figure 1: Example of Dust Extinction Moisture test result.
Figure 1: Example of Dust Extinction Moisture test result.

Originally developed for coal, the test has been used for other commodities as well with changes in the quantity of sample placed in the rig. TUNRA Bulk Solids has seen a significant increase in demand for dust extinction moisture testing over the recent years, which has led to the commissioning of its third dust extinction moisture machine to meet this demand. The new machine has been in operation for almost a year, with a significant portion of the projects involving the testing of iron ore.

It is important to highlight that AS 4156.6-2000 defines dustiness in terms of moisture content, while other standards, such as the European Standard EN15051-2, are mostly focused on concentrations of particulate matter fractions based on the particle sizes. Health-significant particulate matters include particles identified as PM 1, PM 2.5 and PM 10, with exposure limits expressed in milligram/kilogram[1].

Dust Lift-off (wind tunnel testing):

One of the most popular dust characterisation techniques is the use of wind tunnels to simulate air flow across a variety of surfaces. These can include belt conveyors, stockpiles and train wagons. Typical tests can last from one to eight hours depending on the application, and the bulk sample moisture is measured before and after the test to control drying. The test parameters are adjusted to match, as close as possible, the actual operating conditions, including: material moisture content, wind speed and angle of repose, and, with the assistance of a dust track monitor, it is possible to control and record PM 1, PM 2.5 and PM 10 particles for the duration of the test.

The dust lift-off test can also be used to assess performance of different types of veneering treatments, as shown in Figure 4.

Figure 4: Dust lift-off test for performance analysis of veneering treatments.
Figure 4: Dust lift-off test for performance analysis of veneering treatments.

Numerical modelling:

The use of numerical modelling has grown in the field of bulk solids handling, with techniques such as Discrete Element Modelling and Computational Fluid Dynamics being used extensively to simulate the flow of particulates. These techniques are sometimes used by themselves or in combination.

Discrete element modelling (DEM), with the current computational capacity available, is somewhat limited in simulating dust as it would require too large a number of small particles, which is currently not feasible. Furthermore, it takes place in a ‘vacuum’, and no air flow is present. However, the technique can be useful for a first evaluation of dust emission to analyse material flow in general: dust is often generated when there is an abrupt change in particle velocity or direction, which are factors that can be analysed with DEM.

On the other hand, as the name suggests, Computational fluid dynamics (CFD) is specifically concerned with the flow of fluids. Current CFD software can simulate solid particles as well, but the focus is on the fluid medium. Single phase or multiphase simulations are possible, the first being used to investigate the air flow alone, whereas the latter focuses on both the bulk solid particles and the air around them.

As with every technique, each has its advantages and disadvantages, and different applications might require different techniques. Using a multiphase CFD modelling approach, for example, is advantageous in that it allows a clear representation of both particles and fluid, but this comes at a higher computational cost. Conversely, a single-phase CFD approach or using DEM might be sufficient to achieve certain objectives like comparing different designs or quickly assessing propensity to dust generation.

Using image analysis techniques to understand dustiness

Advanced image analysis techniques can also be used in conjunction with computational simulations to add valuable information about granular flow with respect to dust and other phenomena. One of such techniques is particle image velocity (PIV), which is used to obtain instantaneous velocity measurements and related properties in fluids, but it has also been applied as a flow visualisation tool for granularflow [3]. When applied to granular flow, PIV can be used to measure particle displacements and velocity fields, but, similarly to other analysis techniques,PIV also has limitations. PIV applied to granular flows is limited to quasi bi-dimensional flows where flow can be observed at a free surface through a clear wall [4]. As PIV is an indirect measurement technique,the small tracer particles are critical for reliable results [3].

Passive dust control through improved design:

Dust control is typically conducted actively, such as with water sprays or extraction fans. These methods, although effective and necessary in some cases, have limitations and disadvantages. In some countries (Australia included), water is a scarce resource, and minimising its use is a required sustainability measure. Furthermore, the use of water as a dust suppression in some coal handling plants may affect the calorific value of the coal per tonne due to the increased moisture [2]. Another drawback of dust control systems is the higher energy usage, thus incurring higher costs. Therefore, a combination of active and passive control measures is advantageous.

In several applications, it is possible to prevent dust emissions through improved design. Such is the case with transfer chutes for example, where a deep understanding of material trajectories and impact points can aid in designing transfers with controlled flow. The following basic design principles are of relevance:

• Matching the in-line component of the material exit velocity to the receiving belt as much as possible helps minimising dust and abrasive wear

• Minimising the normal component of the stream velocity at discharge reduces impact damage to the belt and helps minimise dust generation and spillage due to material re-bounding

• Maintaining impact angles within the transfer at a maximum of 20° and minimising free-fall heights help reduce impact and abrasive wear, as well as dust generation

The overall objective is to obtain a design that results in smooth, controlled streams that aid in keeping finer particles within the stream.

Application Example: Transfer Chute

Passive dust control through improved design was the object of a project involving the analysis and redesign of a transfer chute handling iron ore. The project involved the use of a scale model chute, shown in Figure 4, the application of PIV in the analysis of velocities and CFD analysis. The PIV system used consisted of an Oxford Firefly diode laser and a hxigh-performance camera, whose set-up is also shown in Figure 5. This work was conducted by Dr Xiaoling Chen as part of her PhD with the University of Newcastle. See reference [3] for more detail on this study.

Figure 5: Air vectors from the CFD analyses.
Figure 5: Air vectors from the CFD analyses.

The images obtained with PIV were used to compare the experimental results (scale model) with the CFD model results, with a total of seven combinations of parameters (coefficient of restitution and specularity coefficient) used in the CFD model. Figure 6 shows an example of the air vectors obtained with the PIV analysis and with the CFD analysis. The CFD model was underestimating velocities in comparison with the scale model, which can be used as a means to calibrate the CFD parameters.

Figure 6: Air vectors from the PIV analyses.
Figure 6: Air vectors from the PIV analyses.

Overall, multiphase CFD simulation showed good potential to predict the air flow in transfer chutes, and based on the calculated air velocity at the chute outlet the likely performance of the chute with regards to dust emission can be predicted and used a design aid for transfer chutes handling dusty bulk materials [3]. 

Conclusion

The use of combined analysis techniques in the field of bulk solids handling can be very powerful in understanding dust emission propensity of different materials in various handling systems. When applied correctly, such information can be used in more effective designs to minimise dust problems or avoid the need for active dust suppression systems.

References

1 Ilic, D; Lavrinec, A; Williams, K – Revised Coal Dustiness Test Method AS4156.6 – Part 2: Preparation, 2019

2 Chen, X.; Wheeler, C; Donohue, T; McLean, R; Roberts, A. – Evaluation of dust emissions from conveyor transfer chutes using experimental and CFD simulation, 2012

3 Chen, Xiaoling; Chen, Bin; Donohue, Tim; Wheeler, Craig; Roberts, Alan – Analysis of Passive Dust Control in Transfer Chutes using Computational Techniques.

4 Lueptow, R. M.; Akonur, A.; Shinbrot, T. – PIV for granular flows, 2000