WHAT'S IN THE MIX and is it really mixed?

It is becoming more common for end users of mixing and blending machinery to ask the supplier of the equipment how he establishes, measures or proves that a homogenous mix has been achieved and that the results are consistent. The simple answer is that in most cases, the mixer manufacturer doesn't know and that the answer may only be provided by the end user himself.

But how? The normal answer is to use one of the many available sampling techniques. In reality, the mixing of powders is among the least understood of operations. Perhaps the most difficult problem is in the characterisation of the mixture, or how the degree of mixing should be defined and measured. If, for instance, a sample taken from a mix comprises just one particle, then no mixing would be detected. However, if the sample were the entire mix, then the mix would appear to be completely homogenous (even if the mixer had not been operated). In practice, therefore, there should always exist a `scale of scrutiny'.

Assessment methods

In industry, the final mixed state of the mixture is almost always assessed by a method specific to the application or end-use of the mixture, and to the material itself. These tests may follow a variety of forms and involve any number of statistical methods of analysis. If the different constituent parts of the mix have different physical characteristics such as differing particle sizes, then it may be possible to carry out a sieve analysis and to assay the quantity of ingredients in a given sample. Likewise, to obtain an indication only of the homogeneity of the mix, it may be possible to introduce into the recipe a separate discrete ingredient, such as rice. If samples are then taken from different parts of the mixer and similar proportions of this separate ingredient are found in each sample, then it may reasonably be assumed that, given similar characteristics, the other ingredients have been mixed in a similar way.

Other traceable sources such as calcium carbonate or sodium chloride, which are not components of the original mix, could be added for analysis purposes. Provided there are no other similar substances in the mixture, then a pH meter with an ion selective electrode or atomic absorption could be used to measure chlorides and/or carbonates present.

If the mix contains a sugar component, glucose or fructose, then it is possible to use wet chemistry and, for instance, high pressure liquid chromatography (HPLC) in the analysis of the mix. In this case, a solution is made of the mix, the sugar is dissolved and then HPLC used to measure the amount of the sugar.

Pharmaceutical formulations invariably use specific methods to check chemically for the key ingredients, compounds or actives. These analysis methods may involve methods ranging from chemical titration, to gas liquid chromatography or even atomic absorption spectroscopy.

Proof of the pudding

There are other more basic forms of checking. It may even be a simple matter of a visual comparison with a proven sample. In other cases, for instance in the bakery or food industry, the quality of the final baked or cooked end-product is sometimes the deciding factor.

So there is not one simple method of assessing the degree of mixedness of a formulation after mixing. Also to be considered is the way that ingredients move relative to one another inside the mixing machine and, equally as important, what happens to the quality of the mix as it is discharged from the mixer. It is thought by some that the only meaningful method of establishing the final performance of a mixer or blender is to take the sample from the discharge stream of product. If un-mixing or segregation is likely to occur, it is more likely to happen here.

But if one understands the way in which powders move or perform inside a mixing machine then it could be argued that a more accurate prediction of the final quality of the mix could be made. Kemutec has been working with both Birmingham and Cambridge universities over several years on research into mixing technology, using the Positron Emission Particle Tracking (PEPT) technique.

A small scale Gardner `U' trough laboratory mixer, with interchangeable interrupted spiral, plough and paddle type agitators (see picture), is positioned between two large area, positron sensitive, gamma ray detector plates used in conjunction with a positron camera. This arrangement allows the detection and subsequent reconstruction in the camera of the path taken by two gamma rays produced from the annihilation of a positron and an electron generated from a single particle within the body of the mix. A 3D image of the distribution of the radiolabelled item in the mix may be obtained from such reconstructions and this is the basis of PEPT.

The PEPT data obtained enables the variables of agitator type, agitator speed, fill level, and the aspect ratio of the length to diameter of the machine to be studied and enables a comparison of the results obtained from industrial mixers, both batch and continuous. Importantly, an insight into the time that a particle spent at each point in a given mix would enable segregation effects to be investigated. Knowing the average velocity of a particle and being able to track that particle as it moves around enables one to build a picture of material within a given volume and make an assessment of how ingredients disperse with time. Finally, it may be useful to know the residence time of particles within mixer cells or compartments.

The PEPT technique is already providing information that is beginning to draw the veil of mystique away from the black art of mixing.

Mike Kent is product support and technical sales manager of the Gardner mixing machinery for Kemutec.