Dust Control Coating Considerations

Lucas R. Moore, ArrMaz, USA, discusses the key criteria to consider when evaluating fertilizer dust control coatings.

Dust is defined as fine particulates found on surfaces or carried in the air. It can pose serious environmental, health and safety hazards from visibility impairment to severe

inhalation and respiratory disorders. Natural dust occurs as a function of erosion, which is heavily dependent on wind velocity, mineralogy, particle size and particle exposure (vegetation cover), whereas anthropogenic-derived dust (fugitive dust) occurs through the disturbance of land, or through the alteration of particle size or mineral crystal structure.

Fugitive dust can be separated into three main particle categories:

n Particles >200 µm, which settle within a relatively short period of time and do not create significant direct or indirect health or safety hazards.

n Particles between 10 µm and 200 µm (human hair is approximately 70 µm in diameter) which have the potential to be deposited in humans’ upper airway, but are retained and expelled from the nose and trachea. These particles create significant industrial hygiene, regulatory, and operability challenges.

n Particles <10 µm (such as colloidal silica, atmospheric dust, carbon black, dust etc.) which can remain airborne for weeks and can easily travel to the deeper regions of the lungs leading to respiratory diseases.

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One example of anthropogenic-derived fugitive dust is fertilizer dust. Mineral-based fertilizers are produced through the pulverisation of minerals followed by various processes that lead to crystal modification (dissolution, neutralisation, recrystallisation, etc.). Dust can occur throughout the entire process of producing, shipping, handling, and application of fertilizers. In fertilizers, dust is not only a health concern, but also an operational concern as it can disrupt process efficiencies in several ways:

n Suspended airborne particles reduce visibility making it difficult for employees to work safely.

n Dust particles that are not suspended but are still part of bulk piles change the flow characteristics of the bulk fertilizer through chutes and hoppers. This flow disruption can slow operations and upset blend systems engineered to create targeted nutrient concentrations.

n Dust that settles on or in equipment leads to increased costs and downtime required to clean, repair, or replace the equipment.

n During field application, fertilizer dust may disrupt application equipment and prevent the application of nutrients at desired rates and placement. Flow disruptions of fertilizer through equipment also result in work stoppages and increase the time required to apply nutrients in the field.

n Any significant visible emissions will usually violate Environmental Protection Agency (EPA) clean air requirements and Occupational Safety and Health Administration (OSHA) industrial hygiene regulations.

Physical controlsPhysical controls for managing dust include dust collection equipment, such as exhaust gas scrubbing systems, filters, or baghouses; and erosion prevention strategies to reduce exposure to wind and other environmental conditions. Examples of erosion prevention strategies include planting trees or building rock walls or banks. A common practice in the agriculture industry is to store grains in three-sided bunkers, buildings, or silos. While such strategies are effective at minimising exposure to dust for stationary applications, they are not effective for controlling dust on objects that will be moved or distributed, such as fertilizer and some sand applications.

Chemical controlsChemical controls work through a few different pathways. The optimal pathway depends greatly on the balance between desired dust suppression longevity and application cost. Most options are based on applying a protective coating or crust on the surface to reduce dust generation and prevent the dust from becoming airborne. These coating solutions may be as simple as applying water to dusty roads, a common decades-long practice at mine sites.

Although the application of water is a simple process, large volumes of water are needed since, depending on weather conditions, suppression time is short. The application of hygroscopic salt/water solutions can increase the effectiveness of this approach by depositing a salt coating on surfaces to enhance moisture retention. However, the application rate of these salts are also high, and may lead to metal corrosion on equipment and vehicles.

For long-term dust suppression, more sophisticated coatings which form a film to strengthen the coated material are preferred. Such coatings are commonly used in the fertilizer industry, and are often applied in conjunction with an alternative dust control method to increase the tackiness of the granular surface so that free dust adheres to the surface of the fertilizer granule.

Figure 1. Dust control performance (composite) of DUSTROL coatings – phosphate.

Figure 2. Dust control performance (composite) of DUSTROL coatings – potash.

Figure 3. Rate of dust generation of DUSTROL-coated fertilizers relative to six-week uncoated composites.

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Application in the fertilizer industry Over the years, the fertilizer industry has developed effective dust control systems to reduce exposure to fugitive dust across the value chain, through the application of complex and proprietary chemical controls to fertilizer granules. While some of these solutions may comprise a single chemical component, longer shipping runs, more complex fertilizers, increased dust control performance requirements, and precision agriculture have driven the need for blended or synthetically modified chemical solutions. ArrMaz has worked with many fertilizer producers to develop customised chemical solutions such as those in the company’s DUSTROL® line of dust control coatings,

which are proven to decrease fugitive dust, thus reducing associated environmental health and safety risks, while minimising the loss of valuable fertilizer product.

However, not all fertilizer chemical dust control solutions are created equal – there is no universally optimum coating for fertilizer dust control. When selecting the best dust control coating for fertilizer, there are several criteria which must be considered.

Fertilizer dust generation tendenciesWhen evaluating chemical dust control coatings, it is important to first understand the dust generation tendencies of the fertilizer being coated in order to ensure optimal dust control performance. Understanding the dusting behaviour of uncoated fertilizer helps determine the class of dust control coating required prior to moving into coating product development. Carefully managed test methods and interpretation of results must be applied to discern differences between substrates, fertilizer behaviours under different environmental conditions, and treatment effectiveness depending on customer-specific requirements.

ArrMaz recently evaluated a variety of fertilizers in the laboratory, including MAP, potash, and a 15-15-15 NPK for their dust generation tendencies over a six-week period. A counter current vacuum tower designed with baffles was used to ensure the granules experienced some degree of tumbling to simulate day-to-day fertilizer handling. The dust was collected on a filter paper and weighed for mass collection. Table 1 shows the dust generation results for each type of fertilizer. The data obtained from the uncoated fertilizer granules revealed a peak in dust generation from the initial evaluation, followed by a declining continuation in the generation of dust over time.

Coating performance over timeAnother criteria which must be considered when evaluating dust control coatings is coating performance over time. For the fertilizers outlined in Table 1, applying an appropriate dosage of a tailor-made DUSTROL coating ,as a means of building a protective coating/barrier around fertilizer granules, resulted in a significant reduction in fugitive dust generation over a full six-week period as shown in Figures 1 and 2.

Interestingly, the majority of the dust generated from the uncoated fertilizers was found to be within the initial measurement. Greater than 50% of the dust generated within the six-week investigation was produced during the first day of evaluation, with potash granules generating more dust than phosphate-based granules (Figure 3). The application of tailor-made DUSTROL coatings resulted in dust generation reductions of 80 – 90% for MAP and potash for the duration of the six-week investigation as shown in Figure 4. However, with the addition of other minerals and nutrients (enhancement), dust prevention began to decline over time, yielding a 60 – 70% reduction in dust by the end of the study.

Figure 4. Impact of DUSTROL coatings on dust generated.

Figure 5. Dust control performance of DUSTROL-coated NPK doped with an external NPK dust source.

Table 1. Dust generation tendencies of uncoated fertilizers

Fertilizer type Dust generation (ppm)

Initial Week 1 Week 2 Week 4 Week 6 Total

Enhanced MAP I 235 75 60 55 55 480

Enhanced MAP II 298 58 35 25 40 456

MAP 313 83 63 63 72.5 594.5

Enhanced potash 1950 185 N/A N/A 175 2310

Potash 3150 380 N/A N/A 310 3840

NPK (15-15-15) 67 40 33 33 43 216

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Coating tackinessWhen evaluating a dust control solution, coating tackiness should also be considered. The effectiveness of a coating can be controlled by manipulating tackiness. Achieving the minimum point of tackiness is very important to ensure the coating does not hinder or complicate the movement and hence the handling of fertilizer in warehouses or by end-users. Adding tackiness to the surface of the granule can lead to free dust adhering to the surface of that granule. However, adding too much tackiness can lead to agglomeration where sticky granules adhere to each other and to the surfaces of handling and distribution equipment.

ArrMaz evaluated the ability of a DUSTROL coating to adhere free dust by applying the coating to NPK fertilizer granules, which were doped with pulverised NPK to simulate a dusty fertilizer. The results are shown in Figure 5. The DUSTROL coating was able to maintain a composite dust level of <75 ppm for the duration of the six-week investigation. This suggests that only 6% of the potential dust was collected at the initial reading, equating to an 89% reduction in dust overall.

Coating application system effectivenessIn order for a dust control coating to be effective, it must be evenly applied to the surface of fertilizer granules. The best coating agent will not work if not

properly applied. Some facilities overcome poor application by using more coating agent – an expensive solution long-term compared to using a proper coating application system from the start.

ConclusionWhen selecting the best dust control coating for your fertilizer, the following factors must be evaluated:

n The dust generation tendencies of your fertilizer. n The amount of surface area of the substrate to be

coated. n Coating performance over time (how much dust

suppression is required and for how long). n Coating tackiness and its effect on dust control

versus agglomeration. n Coating application system effectiveness.

When evaluating dust control solutions, be sure to select a vendor that takes the time to understand your fertilizer and your specific dust control needs, with proven expertise you can rely on.

AcknowledgementsThe author would like to thank the following major contributors

for their assistance with this article: Mark Ogzewalla, Jose

Fernandez, Archie Carlini, and Zach Burrell, ArrMaz.