Having an on-line analyser tracking a significant parameter is rather like having a window into your process. The effect that other parameters have on the variable being monitored is immediately obvious, increasing process understanding. Responses to changes in the variable of interest can be made more rapidly and to greater effect.

Process operation becomes smoother and automated control is possible. Less off-spec material is produced and operators are freed for more productive work. Product consistency and process efficiency increase hand-in-hand with cost benefits typically accruing rapidly.

A variable of significant interest to many, if not all, solids processors is particle size, and

on-line particle size analysis is becoming the norm in many industries. Laser diffraction is a well-established technique for particle size analysis with off-line instruments based on this technology being widely and successfully used for quality control and product development applications.

Increasingly on-line laser diffraction particle size analysers, such as the Malvern Insitec, are also being employed, primarily in process control applications. With these systems no calibration is needed, maintenance requirements are low, and the data generated can be tailored to meet the specific process requirements.

Here we look at the rewards achieved through the installation of on-line particle size analysis at a carbon processing unit making anodes for the aluminium industry. In this case, as in many others, the uptake of on-line technology improves product consistency, enhances process understanding and identifies the potential for process improvement.

Aluminium smelting

Aluminium is produced from calcined alumina using the electrolytic Hall-Heroult process. Alumina is dissolved in a bath of molten cryolite (sodium aluminium fluoride) through which a current of around 150000A is passed. The current flows between carbon anodes suspended in the bath, and a cathode, the carbon or graphite lining of the vessel. Molten aluminium forms at the base of the pot from where it is periodically siphoned off for further use (Fig.1).

The anodes used in the process gradually burn up over time and are replaced on a regular basis, approximately every 20 days. Aluminium smelters generally consist of around 300 pots and so the overall consumption rate of carbon anodes is significant. These anodes are produced from a paste made from coke ground to a specific granulometry, and pitch. Raw or ‘green’ anodes are moulded from the paste in vibro-compactors and are then transferred to gas or oil-fired furnaces for baking. The baking process is carried out over several days at a temperature of around 1100¢ªC. The finished electrodes are rodded – attached to aluminium stems – before use.

Why is particle size important?

Part of the electrode production process is the milling of coke to give material that ultimately will produce anodes with the required properties. The particle size distribution of the coke and its surface area (a related property) determine how much pitch is required to wet it. The coke/pitch ratio in turn affects the viscosity of the resulting paste. As the ‘green’ anodes are baked the pitch is converted to coke and a pore structure develops inside the anode. This structure depends on the particle size distribution of the coke used in its production – if the coke has been milled to a very fine particle size then pore structure will also be fine, and vice versa.

Pore structure has an impact on the electrical resistance of the anode, its brittleness and ultimately its longevity. In summary, the particle size distribution of the coke influences the key parameters that define product quality. To produce consistently good quality anodes, particle size must be properly controlled.

Measuring particle size

At one electrode producing facility, the decision was taken to install two on-line particle size analysers to improve control and hence product quality. The first was installed directly after the coke mill with the aim of improving milling efficiency; the second analyser was installed between the milled coke silo and the dense phase conveyor upstream of the press used to mould the electrode. A Malvern Insitec system was used in each case.

With the on-line analyser, mill control was improved and the coke produced became more consistent. The use of on-line analysis to improve milling efficiency is widespread because the energy and throughput gains achieved by avoiding over-grinding can be substantial. Where a classifier is used after the mill, to ensure product quality, automated control loops that vary classifier speed in line with particle size are becoming increasingly common.

With this type of control in place, optimised mill operation becomes very simple; select the product specification required and let the control system take over.

The second analyser highlighted a previously unquantified problem at the discharge of the silo – cyclic segregation.

Significant variance in particle size distribution was clearly evident from the on-line data, so a static insert was installed in the silo to encourage a more uniform exit flow. This area of the process had always been problematic and this result turned out to one of the key benefits of the change.

Improved product quality

Improved control of particle size has led to a reduction in standard deviation and a more homogeneous product. The electrical resistance of the anodes produced, and hence the heat generated inside them during the smelting process, is now much more consistent.

Brittleness has also been reduced and the anodes last longer thanks to improved thermal resistance. Product quality has been significantly enhanced by tightening particle size control, and by modifying the process to ensure that representative material is fed forward for moulding.

Conclusion

As with any other technology the uptake of on-line particle size analysis is fundamentally driven by economics. Because the installation of an on-line system typically yields benefits in terms of process efficiency and product quality gains, while also reducing manpower requirements, the economics are often very attractive with payback times often a year or less.

Once an on-line system is in place it is a common experience for plant operators to wonder how they managed without it.

Imagine trying to control the temperature in a reactor with a temperature reading every hour rather than every few seconds!

At the electrode manufacturing plant discussed here the customer is considering the purchase and installation of a second optical head on the same line simply to ensure zero downtime. With industrially robust systems readily available and increasing evidence of how processes can be transformed, adoption of on-line particle size analysis is becoming increasingly widespread.

David Pugh is Process Marketing Manager, Malvern Instruments Ltd, Malvern, Worcestershire, UK. www.malvern.com