(150d) Round Robin Tests with the Uniaxial Tester to Predict Caking

Caking of particulate materials during transport or storage is an expensive burden on the European Chemical Industry and adds costs to the user. The chemical, pharmaceutical and food industries generate an average output of solid material of around 300.000 tonnes per company and year. Objective of this research is to determine and predict caking with a simple and easy to handle test.

The Jenike theory is based on the concept of unconfined uniaxial compressive strength. This value cannot be measured directly by standard shear tests. It is usually generated by extrapolation from shear test data. Conceptually the unconfined uniaxial compressive strength can be measured in a friction - free uniaxial tester. Here the failure strength of a bulk solid is determined by the stress needed to break up the interparticle bonds of a free standing product cylinder. This failure strength is called fc-value and depends mainly on the compression load, the compression time, the humidity of the surrounding medium, the storage temperature and the post treatment of the probe under the storage conditions (before the determination of the failure strength).

The COSTT research project (Consumer Oriented Solids Transport Technology) was financed by the European Union as a co-operative European research project involving 9 partners from 5 countries. Within this project a uniaxial test rig was developed at TU Braunschweig (Germany). The uniaxial tester tries to convert this conceptual experiment into real fc-measurements. The side walls of the test cylinder are porous to guarantee easy heat and humidity transfer between surrounding medium and test product. Once the uniaxial tester had been tested and operating guidelines had been worked out by TU Braunschweig and BASF, 10 testers each were sent to six project partners in order to perform comparative caking tests. Test material was a salt of an organic acid from one production lot. To eliminate any influence of the operator on the results, one operator from BASF travelled around Europe to carry out caking tests together with the respective partner-operator. The results have been analyzed to generate a reliable and consistent operating guideline for the testing of the caking tendency of bulk solids.

The experiences, difficulties and suggestions for the evaluation of round robin tests with an uniaxial tester in different labs are presented in the paper.