In the prilling or granulation process it is important to know the particle size distribution and the deviation from sphericity in order to control the product quality and have an efficient utilisation of the production unit.

Until now there has not existed an on-line particle analyzer which could give immediate response to changes in operating conditions. Nor does there exist a good enough computer model to control product quality by monitoring process parameters.

Norsk Hydro has developed an on-line particle analyzer which measures particle size, size distribution and the deviation from sphericity (called 'non-spherical').

The analyzer which we have developed is based on picture analyzes and operates as follows:
a) A sampling system, taking out representative samples, takes out a product sample of about 2.5 kg and transfers it to a small bin in the analyzer. The product sample is made to fall in a monolayer curtain in front of a high resolution CCD camera. The camera takes a picture about every 2 seconds, at intervals set by the computing time of the central unit. The picture consists of 512 x 512 picture elements, so called pixels. A particle will be represented by a set of dark pixels.

b) Each particle in the picture is analyzed by counting the total number of dark pixels and the number of dark pixels around the periphery of the particle.

c) The computer unit in the analyzer calculates the particle size and a sphericity factor for each particle. The data are presented to the operators as four real time trend curves, shown simultaneously. These curves show % oversize and fines, d50 and the % non-spherical particles. The data are averaged over a large number of particles. All the data corresponds very well with the standard laboratory analyzes.

By the end of August 1989 the on-line particle analyzer had been installed in two of Norsk Hydro’s prilling plants in Norway. The first installation was made in the NPK 3 plant in Porsgrunn and the second in the CN plant in Glomfjord.

In both plants unsieved material from the conveyer belt going out from the prill tower bottom is analyzed. The analyzer has proven to give immediate response to even small changes in prilling conditions: speed of the bucket, melt temperature or amount of recycle.

The analyzer has in both plants improved the product quality, reduced the amount of oversize and reduced the amount of off-spec material being produced in a start-up situation.

The analyzer was very well accepted by the operators.

Within the next year the particle analyzer will also be installed in new Norsk Hydro units both in Norway and by daughter companies.

Wilhelm Frydenberg, Norsk Hydro Research, Porsgrunn, Norway.

14 pages, 5 figures.