The bulk solids industry is constantly looking for a quick, cost-effective and concise QA/QC test for benchmarked powder formulations. This type of test is required to ensure that powder jams and flow issues are caught before a major production run is implemented. This article discusses a method that addresses the requirements for a QA/QC test, namely cost, speed and accuracy.
The QA/QC Test Problem
Once a powder is formulated, it must be benchmarked for common flow characteristics such as bulk density, arching dimension and flow function. Scientific instrumentation is used to define flow properties so that the material does not jam in an existing hopper or feeder system. It is possible to minimize lost product and expensive downtime by characterizing the powder formulation. Figure 1 shows the flow/function graph.
Figure 1. The flow/function graph
After a batch has been produced, it is important that it meets the defined pass/fail criteria before going to production. This is required to make sure that a smooth production run is done with little or no downtime. It is also necessary to perform the QA/QC checks quickly to keep up with the production run.
Common Empirical Tests
The most common QA/QC tests in industry are tapped bulk density, angle of repose and timed flow through a funnel. Tapped bulk density and angle of repose are the most popular of these empirical test methods.
For a repose angle test, powder is poured onto a flat surface either through a funnel or from a container. This helps determine how easily the material will flow and how cohesive it is by measuring the angle formed by the powder column. It is harder for the powder to flow with a higher angle, usually gauged at 30° or higher.
In a tapped bulk density test, a graduated cylinder or beaker is filled with material and either manually or mechanically tapped around 100 to 150 times until the level of the powder stabilizes, that is, until the powder no longer compacts. By determining the volume change from the fill density, a tapped bulk density is calculated. The more material compacting occurs, the largerthe change in volume and thus the harder it is to flow. These tests are subjective and offer no additional data on powder flow characteristics such as arching dimension or flow function.
An easy-to-use test is required that will offer comprehensive flow data in a cost effective and timely manner. This data is compared to the control sample for analysis and relevant pass/fail determination.
Shear Cell Methodology
The shear cell was designed as a single cylindrical cell that gave flow function data by shearing a sample of powder at defined consolidation strengths. The equipment’s design approach evolved to an annular shear cell comprising numerous single cells arranged in a ring. The shear cell methodology concept is well-defined in the initial method known as ASTM D6128.
Figure 2. Brookfield Annular Shear Cell
Figure 3. Brookfield Shear Cell with Powder Sample
An annual shear cell makes sure that shearing action is relatively uniform across a sample of powder. Powder flow data is established by shearing the powder against itself. Flow properties such as arching dimension, internal friction angle, bulk density vs consolidating stress, internal friction angle, flow function and arching dimension are all garnered.
Figure 4. Brookfield Powder Flow Tester (PFT) with Shear Cell
The Brookfield Powder Flow Tester as shown in Figure 4 incorporates shear cell technology in a cost effective manner. Using its intuitive powerful software, full characterization tests can be run in 45 minutes. Quicker flow function tests can be run in as little as 12 minutes or 16 minutes depending on the number of data points required.
A standalone bulk density test can be run in as little as 2 minutes. The same concise consolidation strengths are used in a shear cell tester. Data is obtained using the same parameters for testing for each sample, thus eliminating human error and subjectivity from the equation.
Production Run and QA/QC Testing
Consider Product X as a new powder formulation. The product has been through significant R&D experimentation and shear cell testing until a well defined pass/fail criterion is derived.
For this control sample, the pass/fail criteria will be values for fill bulk density at 747 kg/m3 +/-10% , a final bulk density of 966 kg/m3 +/- 10%, and an arching dimension of 53 to 70mm.
Since Product X is susceptible to humidity, further testing has been done to characterize the product at defined humidity levels. This data is used for introducing the right amounts of flow aid to the batch, if humidity levels increase, to bring the product back into specification.
Figure 5a. Flow Function Density
Figure 5b. Arching Dimension
A production batch of Product X is produced, a QA/QC test is done and the same is compared to the initial control sample. In case the batch meets the established pass/fail criteria, it then moves to the production phase. Product X is filled into a feeder system (bin with a converging hopper) and the production run is started. A QC test is run every hour and compared to the control sample.
Because of high humidity levels, the QC test showed that the product started gaining moisture and didn’t flow easily, there was an increase in the arching dimension, which was near the 70mm benchmark. Flow aid is introduced at a predetermined level and mixed with the batch.
Then a QC test run is done again. This batch meets the pass/fail criteria and subsequent production batches are formulated with the same amount of flow aid.
Downtime was eliminated in the real world scenario. Problems because of humidity were identified and rectified so they do not become a flow issue. Product loss is reduced. Yield efficiency has been maximized.
This was done by using a precise test method utilizing shear cell technology in a timely manner. The importance of these fast and repeatable QA/QC tests is that out-of-spec products can be quickly identified and steps can be taken to bring it back into line.
This information has been sourced, reviewed and adapted from materials provided by Brookfield Engineering.
For more information on this source, please visit Brookfield Engineering.