Chemical Processing: Effectively Handle Powders

Take a practical approach to powder characterization and tap into operating experience.

By Tim Freeman, Freeman Technology

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In this case, the manufacturer is paying for material with superior handling performance — but there’s limited understanding of the properties that confer this superiority. Better understanding could have predicted failure of the alternative feed. So, the starting point for assuring a new feed is acceptable is to develop a specification that defines the existing material’s success. Unless this can be done, the manufacturer is stuck with the current supplier.

Analysis of the current feed provides data that can be used to understand why it behaves well in the process. For example, the powder may have relatively low cohesion and a low compaction index (CI). A low CI means that flow energy is relatively unaffected by the application of a consolidating pressure. In a hopper, exiting material is compressed by the weight of the powder above it. Under such conditions, this powder won’t form a bridge blocking the hopper outlet and will tend to flow relatively easily. The measured data, therefore, offer insights on why this material may be so easy to handle.

Comparing this material’s properties with those of the alternative feed that proved problematic, of course, would be even more illuminating. Such an analysis might confirm that the feed should have low cohesivity and low CI — and also may indicate that it mustn’t be prone to segregation.

Specifying acceptable values for these parameters and testing prospective materials against these criteria would give some assurance of success.

Using Powder Properties
The previous example illustrates two ways in which powder characterization data can be used. One approach is to comprehensively measure the powder and then look at all the properties to see how well the material fits with the demands of the process.

For example, consider die filling, where powder is used to fill a specific volume of defined shape. Ideally the powder should flow freely into the die, settle and then flow into the required shape as filling proceeds. In this case, logic would suggest that flow behavior under compacting and unconfined conditions will be important. The way in which the powder releases air also has a bearing. A powder with low permeability may trap air, compromising filling or the effectiveness of any subsequent compression step. So, a powder that releases air easily is preferable from this point of view — however, such powders can quickly become immobile because air lubricates powder flow. Achieving optimal performance requires balancing these effects.

Investigating Aeration in Powders
Figure 2. Investigating Aeration: Results of a study of the
effect of air on two powders underscores how differently
powders can behave because of aeration. A small amount
of air can dramatically improve flow properties of some powders.
Click image to enlarge.

Figure 2 presents the results of a study of the effect of air on two powders. The data show how flow energy falls as the velocity of air travelling up through the sample increases. The free-flowing powder’s flow energy drops almost to 0, indicating that it has fluidized; the minimum fluidization velocity occurs at around 2 mm/s. This small amount of air radically improves the material’s flow properties. In contrast, the cohesive material forms channels through which the air escapes.

Shear properties also can affect processability. Figure 3 shows data for the stress required to shear a previously consolidated powder sample as a function of applied normal stress. The two materials exhibit different behavior — one presents a much greater flow resistance. This powder is quite cohesive (C1 value) and thus more likely to form stable bridges that can compromise hopper discharge and die filling than the other material. Powders with low shear strength will tend to flow freely even when compacted. C2 is the cohesion value or shear strength of the powder when not consolidated.

The alternative approach is to compare the properties of materials that process well in the operation with those that perform poorly, as in the following study.

Optimizing Equipment Use
A titanium dioxide producer makes different grades for applications in the paper, plastics, cosmetics and food industries. It uses surface treatment, milling and drying processes to produce the required properties. With some grades, operation is both easy and productive while others demand very close control. The manufacturer instigates a review with the aim of improving throughput and productivity.

The producer needs to understand how the properties of different TiO2 grades affect how they interact with the processing equipment. Existing process experience allows materials to be classified using “Processability Rankings” (PR). For example, a PR of 2 may describe a trouble-free powder that processes consistently at a fast rate. A PR of 9 may indicate a powder that can’t be processed to any degree, while an intermediate value of 6 may signify a powder that performs acceptably if the process is closely monitored or control is fine-tuned. This is a useful way of starting to quantify experience. Factors to take into account when ranking processability include:
Measuring Shear Properties
Figure 3. Measuring Shear Properties: When particles flow,

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