Rules of Thumb: Bulk Material Sampling

In the latest instalment of this practical series, Eric Maynard highlights five considerations for effective bulk material sampling

DOES your bulk solids mixture or blend meet your uniformity specifications? Effective sampling can give you the answer. However, poor sampling techniques can leave you more uncertain or even dead wrong about your blend’s uniformity. This Rules of Thumb article presents five important considerations for ensuring that your sampling process accurately demonstrates the quality of your blend or bulk material.

Blending or mixing of bulk solid ingredients is a common processing step in many industries. For the purpose of this article, these two terms are used interchangeably. Blending is a part of the manufacturing process for thousands of products, including chemicals, fertilisers, explosives, detergents, glass, ceramics, and plastics. Many food products, such as dry beverage mixes, cake mixes, and seasonings are made from custom-blended batches of dry ingredients. In pharmaceutical manufacturing, a small amount of a powdered active drug is carefully blended with excipients (inactive ingredients), such as sugar, starch, cellulose, lactose, or lubricants, to form tablets and capsules. Achieving a highly consistent blend is often critical to a product’s performance and quality.

Achieving a “perfect” blend in your mixer or blender, however, doesn’t guarantee you’ll end up with a perfect or even acceptable final product. Blended ingredients often segregate during blender discharge or downstream processing. Common causes of segregation include sifting, fluidisation, and dusting. Depending on what causes the segregation, fine and coarse particles will concentrate in different locations in a downstream vessel. With sifting segregation, for example, fine particles will concentrate in the center, while coarse particles will concentrate at the edges, as shown in Figure 1.

Sampling can help you spot segregation and get an accurate picture of your blend quality. Improper sampling, however, can give you the false impression that your ingredients are adequately blended when they aren’t or, conversely, that they aren’t blended well when they actually are.

In any sampling discussion, it’s important to keep in mind the two golden rules of sampling described by powder technology researcher Terence Allen: “sample material when it’s in motion and sample the entire material stream during short intervals.” But as you’ll see, it’s not always feasible to strictly follow these rules, and sampling material at rest is often the only practical or cost-effective option.

To ensure that your sampling process reliably and accurately demonstrates your blend’s overall quality, follow these five fundamental sampling principles:

1. Perfect blending doesn’t guarantee a uniform product

Every time you transfer a blend from one vessel to another, it has the potential to segregate. You can’t assume that because the blend is adequate in the blender it’ll remain that way after it’s discharged. Effective sampling can tell you your blend’s characteristics throughout your process. To accurately demonstrate your blend’s uniformity, take samples in the blender, during blender discharge, and from the final product.

Sampling in the blender

Sampling your mixture in the blender can’t tell you if your final product will be well blended, but it can indicate a need for more or less blending time or a different blending method. Take samples at different locations in the blender, since the ingredients may not be consistently blended throughout. This stratified (or nested) sampling method (discussed in a later section) can not only improve your blender sampling reliability but can also help identify dead zones in your blender.

Sampling during blender discharge

Sampling your blend during blender discharge is more challenging than sampling in the blender, however it is the only way to find out if your well-blended ingredients are segregating during discharge. This method tends to yield a larger collected sample, which you may have to divide into smaller subsamples for analysis (discussed in a later section). Also, dust release, material toxicity, or rotating blender components can make the method difficult or even dangerous.

Sampling the final product

Sampling the final product is the ultimate test because it reflects the blend’s quality at the last step prior to it reaching your customer. Sampling at this stage alone, however, typically isn’t adequate, because if the sample reveals that your product has segregated, you still may not know where the segregation occurred. In such a case, you’ll need to sample at each piece of equipment your blend is transferred into to determine where it’s segregating.

2. Beware of thieves

A sample thief is commonly used to collect samples from stationary material in a blender, drum, or storage vessel. The thief is a metal or plastic cylinder with one or more recessed cavities that can be opened and closed by twisting a handle, as shown in Figure 2. An operator inserts the thief into the material bed (Figure 3) and opens the cavities, allowing material to flow in. The operator then closes the cavities and removes the thief with the sample material inside.

Note that thief sampling violates the first golden rule of sampling because the material isn’t in motion. Inserting the sample thief into the stationary material also disturbs the blend, so the operator should always extract samples with care. Studies have shown that thief-sampling results can vary depending on the operator’s technique (such as thief insertion angle, penetration rate, and twisting technique). Also, some ingredients may not flow well into the thief cavity or may stick to the thief itself.

Though it can be problematic if not used properly, the sample thief shouldn’t be abandoned. Be sure to record any irregularities you observe in the thief cavity or extracted sample (such as static cling, agglomeration, or smearing) and carefully scrutinise the resulting data.