Quantitative tests can be performed on single particles, (for resistance to breakage), or on a sample of the bulk material. Various standards, (British Standard, ASME, ISO, DIN etc.), have been developed for specific objectives. Key values for flow in storage and handling are bulk density, shear strength and wall friction. Attrition and Segregation are separate, but related, issues. None of these properties are single value measurements and a degree of expertise is necessary in their conduction and interpretation.
Selection Criteria:
- Understand the purpose of the test. The data required for design must be more
complete, as compared to quality control testing during operation.
- Understand and emulate the underlying mechanism in the process or unit operation.
- Consider the requirement for frequency of testing with complexity of test.
- Evaluate the product stability, sample size requirements and hazards associate with
handling.
- Check if a single test is sufficient, or if multiple tests are required.
- Evaluate the need for adherence to international test standards (ISO, ASTM, German,
British etc.)
Sample selection
Highly accurate test data on incorrect samples will still result in irrelevant information. Ask yourself the following questions.
- Is the material being tested representative of its ‘worst’ condition in the process?
- What is the scale of significance for the sample and will specific locations, sources,
times or process variations produce different values? Is the sample uniform, consistent
and stable? Take multiple samples if necessary, (location and time), to understand
variability.
- What is the probable, (and realistic), worst case process condition relative to the
various operations in the system?
- What is the history of the sample? Document the chain of custody. Have a robust and
traceable procedure for the receipt, authorisation, identification, recording, storage and
disposal of samples.
- Has the sampling procedure been defined?
- What is the optimal design of sampler?
- What is the variability associated with sampling and how does that compare with
variability due to analytical technique?
- What is the required sample size for analysis?
- How is the sample size relevant to the process?
- How will the sample size be reduced for analysis?
- What are the critical parameters for product quality? Sample accordingly.
Understand the safety, toxicity, hazards, disposal requirements
Ensure material safety data sheet, (MSDS), are read and understood before handling any new bulk material. The vendor must establish specific toxicology data with the manufacturer where there are concerns. Sample traceability and custody of possession are critical.
Conducting Tests
- Select the right instrument or test method (for example, particle size analysis). Ensure
the particle size is appropriate to the form of the equipment. (e.g. shear tester).
- Understand the underlying mechanisms and form of data output, particularly of an
automated instrument: What is being measured and what manipulations are being
made to the data.
- Understand the limitations of the test, (accuracy, precision, repeatability, bias and
operator dependence).
- Understand and record all relevant parameters that affect the test results (e.g.
moisture, temperature, sample history)
- Calibrate, (and document), measurement instrument on a periodic basis
- Run reference samples, when possible, for reality check.
- Test for typical and worst case situations.
Consider equipment-specific test
Some complex process conditions cannot be replicated by a single test. Attrition and abrasion are classic examples. Examine the sensitivity of equipment performance to variability in product properties.
Some properties relative to product condition, such as appearance, may be subjective, difficult to define or to quantify. For such criteria relevant samples should be prepared showing the limits of both acceptance and rejection.
Documentation
Have full control, and record, the measured value of the test conditions (e.g. relative humidity, temperature, date and time of testing, the technician conducting the test and the presence of any witnesses). Use a standard data sheet of a prepared form to record the above details with an identifying reference, as well as the test measurements.
Be prepared to devise a relevant test that replicates the effect of a particular activity, (not all tests that may be needed have been invented or standardised).
Each new product and process poses new challenges. Many standard tests have narrow design objectives. Do not try to fit an existing test to the problem at hand. Make sure that the test carried out replicates the basic physics underlying the unit operation.
Verification
Check that the actual equipment performance vindicates the test results and, if possible, any indication that allows a comparison to be made.
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