As the saying goes, “Times change, but things stay the same.” While this may seem contradictory in many aspects of science and the modern analytical lab due to advancements in technology and automation, there is still a place for classic and time-honored techniques. This article will briefly discuss how sieving is still a useful tool in particle characterization.
For general information on sieves and a sieve analysis, please see our sieving technique page and our previous blog on sieving.
The low equipment cost, ease of use, ability to process large amounts of material compared to other particle sizing techniques, in addition to being able to recover the size fractions easily, are all advantages to sieving. PTL utilizes all of these advantages routinely to help our clients better understand the characteristics of their materials.
Sieving for Quality Control
Sieving is used in a tremendously wide range of industries as a quality control check, from pharmaceutical to mining. Several of our clients have decades of results produced by our lab they rely on for their product release, process control, and batch records. Sieving is relatively quick and low-cost, and its ease of use lends itself well to just about all industries. The results from a sieve analysis using as little as a single sieve can provide quick and easy-to-interpret pass/fail criteria. The tradeoff is that only a single or a few data points across the size distribution are obtained. However, this may be sufficient depending on the manufacturing process, where knowing if particles above or below a single specific size are present. Prior knowledge of the material’s or product’s performance is required when selecting this size.
As with any particle sizing technique, sampling is the most critical concern, but assuming a representative sample is taken from the bulk batch, sieving allows for processing the entire subsample if the sampling plan is designed optimally. PTL often uses 10 to 100 grams of material for a sieve analysis, depending on the sieving technique and material properties. Few other sizing techniques allow for this scale of sample mass and volume. This minimizes one significant variable for the laboratory. It also allows the isolation and recovery of small outlier populations from the bulk sample.
Pre-Analysis Sample Preparation
As mentioned above, the separation and isolation of subpopulations from a bulk sample is another unique advantage of sieving. PTL frequently employs sieving to separate fines or large outlier populations from the sample to allow for analysis by higher resolution techniques such as laser diffraction and image analysis. It is important to keep in mind, however, when evaluating the results from these other techniques, the data is only a portion of the overall size population. The data set by itself, taken out of context, can be very misleading. When possible, normalizing the isolated size results to the entire size distribution is advisable so both data sets are available and can be interpreted as a whole. PTL has used sieves to isolate populations of interest in samples such as wood chips containing sawdust, fines from crushed limestone, and even bone fragments from ground poultry; some days, the particle size lab isn’t for the squeamish.
Use as an Investigational Aid
Yet another use for sieves at PTL is during investigations for unexpected results. At times, when samples do not meet a client’s specifications, sieves can be used to once again isolate specific regions of the size distribution to support other particle sizing results. Recovering the small fraction of unmilled material along with digital images of the population are two strong pieces of orthogonal evidence when it comes to monitoring and controlling a process. Recovering foreign particles for further analysis is also possible, depending on the size differences between the foreign material and the primary material population. The recovered particles can be analyzed by techniques such as Raman Spectroscopy or Energy Dispersive Spectroscopy in an attempt to identify their chemical composition. PTL has also returned isolated particles to clients for their in-house investigation, or our clients will work directly with other labs that specialize in material identification using the particles we have recovered.
Ensuring Reliable Sieve Results
While sieves are a quick and easy analytical technique frequently used for quality control checks or sample preparation, knowledge and experience are still important to produce reproducible and accurate results. Periodic verification of the sieve mesh itself to ensure it still conforms to specifications detailed in standards such as ISO 3310-1:2016 Test sieves — Part 1: Test sieves of metal wire cloth or ASTM E11-22 Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves are critical to ensure the sieves are not worn or damaged due to use.
Depending on the material processed during the sieve testing, the wire mesh can be distorted, broken, or blocked over time, ultimately affecting the final results. Experience in method development to determine the proper starting sample mass to prevent sieve blockage (called blinding) or evaluating how long the sieving process should be performed are a few of the considerations a knowledgeable lab controls when providing reliable sieve results. PTL’s years of experience and procedures in both sieving and particle characterization incorporate all of these factors, so our clients have the highest confidence in our results.
So, while we love to bring new sophisticated technologies and the latest instrument models into the lab, we never forget where particle sizing all started. Sieves are still an important part of the modern particle characterization lab, and it can sometimes be fun and advantageous to use the classics.
Interested in sieve testing at PTL? Contact us today!
By William Kopesky, Director of Analytical Services.
