Evaluating Surface Treatment

Common flow challenges

Discharge from silos, bins, and hoppers is one of the most common examples where poor flow can disrupt a plant’s entire work process. In order to work effectively, the withdrawal of any material from a storage container should induce all of the material in the container to move. Unfortunately, physical aspects, such as the mechanical interlocking of powder particles, particle cohesivity, moisture, temperature, and the design and choice of material finish for the container, can all give rise to non-optimised powder flow.

For instance, particles may mechanically interlock or bond together, due to the influence of moisture, to form a blockage at the hopper outlet and inhibit flow, often referred to as arching or bridging. In certain cases, flow may only occur in a channel located directly above the outlet, commonly known as ratholing. Once that portion of material in the flow channel has emptied, flow from the hopper ceases. In both cases, the shape and friction of the hopper walls and the flow properties of the powder itself are critical.

Surface treatment of metals and powders

More than half a century ago, Jenike recognised that the flow of powder through a vessel is governed by interactions between particles in the powder, and between the powder and the container wall.2 This pioneering work established the protocols that are still used to design hoppers today.

As a powder flows, the planes within it shift relative to one another and, at the edges of the powder mass, relative to the inner surfaces of the container. Once the properties of the powder are fixed, the ability to control the interactions between the wall and the powder – wall friction – becomes an important degree of flexibility for designers looking to optimise plant design. Wall friction angles quantify the magnitude of frictional interactions between a material of construction and a powder, and are therefore helpful in terms of ranking coatings, not just for hopper design, but more broadly for their ability to ease powder flow in a variety of process equipment. Friction can be reduced either by mechanically changing the surface or by applying a coating.

Surface treatment can also be applied to powders in order to alter their flow properties and optimise process performance. However, a method of quantifying the influence of the treatment is required, to ensure that flow properties have been optimised without compromising other aspects of performance. Dynamic powder testing involves measuring a powder in motion, under conditions that simulate the process environment.

Freeman Technology’s FT4 Powder Rheometer® incorporates dynamic flow, bulk, and shear measurements in a single instrument, measuring the powder in motion, and allowing the evaluation of materials in consolidated, conditioned, aerated, or fluidised states. This enables the powder behaviour within production processes to be predicted, and provides data for the optimisation of specific processes within the manufacturing environment. For example, aerated flow characteristics can be quantified to compare and contrast the likely performance of different powders in pneumatic conveying.

The applicability of multivariate dynamic testing is illustrated with the results obtained with the FT4 for three batches of ordinary portland cement.

Performance variation between batches

Cement is typically stored in large hoppers before being dispensed and conveyed into equipment for filling bags, intermediate bulk containers (IBCs), or other containers. The ability to control the flow properties of a cement, without significantly changing the particle size, helps ensure consistent delivery to subsequent operations in the process chain, delivering commercial benefits in terms of higher productivity and reduced waste.

Three batches of ordinary portland cement performed differently during hopper discharge and subsequent aerated conveyance. Sample 1 (D50 17 µm) performed well across both processes, but the finer Sample 2 (D50 5 µm) performed very poorly. Sample 3 was also a finer sample, the same as Sample 2, but had undergone surface treatment to improve flowability. Sample 3 performed similarly to Sample 1 in the process.

The three samples were analysed using the FT4 to identify the properties that had been affected by the surface treatment.

Sample 2 generated a considerably higher aerated energy (AE) than Sample 1 (Figure 2), indicating greater cohesive strength between the particles. High AE can contribute to poor performance in operations where uniform and consistent aeration of the powder is necessary, such as the aerated conveyor.

Sample 3 generated a very similar AE value to Sample 1, despite having the same D50 as Sample 2. This indicates that the surface treatment altered the cohesive bonds between the fine particles of Sample 2.

Compressibility

Sample 2 was more compressible than Sample 1 (Figure 3), indicating that it entrained a greater proportion of air within its bulk, which is typical of more cohesive powders. High compressibility can contribute to poor behaviour in operations where a powder is subjected to an applied force, for example, as a consequence of storage in large quantities. Sample 3 was closer to Sample 1 in terms of compressibility, again indicating that the surface treatment had changed the flow properties of the finer particles to be similar to those of the coarser powder.

Sample 2 generated higher shear stress and cohesion values, and a lower flow function than Sample 1, indicating that it is likely to be more problematic when required to flow under high stress, low flow conditions, such as in large hoppers.

The results for Sample 3 showed the same response as observed in the dynamic flow and bulk tests, although to a lesser extent. This indicates that the surface treatment had only partially enhanced the flowability of Sample 3 under these conditions. A resistance to shear can be strongly influenced by particle shape, and the data suggests that the surface treatment may not have significantly influenced this property.

Conclusion

In this study, the FT4 Powder Rheometer identified clear and repeatable differences between three batches of a material that performed differently in production processes. In doing so, it has quantified properties that rationalise why the surface treatment of one of these samples has improved flowability.

Sample 1 generated lower aerated energy, compressibility, and shear stress values, and a higher flow function, compared to Sample 2. All of these typically indicate more free-flowing behaviour. Sample 3 exhibited aerated energy and compressibility values, similar to those of Sample 1, demonstrating that the surface treatment had improved the flow properties, making the material more compatible with the process.

Flowability is a measure of the way a powder performs in one particular piece of processing equipment, rather than an intrinsic material property, so it is liable to change from process to process. Similarly, single properties or parameters cannot be relied on to describe behaviour across a range of unit operations. Reliable large-scale processing of powders cannot be dependent on guesswork and estimation. Directly investigating a powder’s response to various conditions requires a multivariate approach that accurately simulates a range of manufacturing operations in order to generate process relevant data.

References

• Statista.com (https://www.statista.com/statistics/219343/cement-production-worldwide/)
• JENIKE, A.W., ‘Storage and flow of solids’, Bulletin 123 of the Utah Engineering Experiment Station, (November 1964 – Revised 1980).

This article was originally published in World Cement's BMHR supplement.

Read the article online at: https://www.worldcement.com/special-reports/28082017/evaluating-surface-treatment/