Meeting the scrapheap challenge

Typically, recycling scrap aluminium requires only 5% of the energy needed to produce the same amount of metal from bauxite ore. Clearly, this makes the process very attractive, both in financial and environmental terms. If, however, high levels of throughput, combined with efficient recovery, are to be achieved, sophisticated recycling furnaces are needed.

To this end, recycling expert Platinum Controls has developed a new range of ultra-efficient recycling furnaces that feature innovative control systems. Platinum specialises in the development of these furnaces, and the results achieved by its latest models are such that, in many applications, they have a payback period of just six months.

These furnaces comprise, in essence, a refractory-lined cylindrical drum, which rotates and tilts while applying heat from gas/air burners to the scrap material. At the end of the melting period, the rotation of the drum ceases, and it is tilted to allow the molten aluminium to be poured off.

The process sounds simple, but in practice, achieving consistently good performance is not easy. It depends on precise control over variables such as furnace temperature, drum rotational speed, drum tilt angle and melt time. Platinum has used equipment and technology from Omron Electronics to implement this precise control for the new furnace.

The drum rotation motor is controlled by a 37kW 3G3FV inverter, operated in full flux vector mode to achieve the degree of speed and positional accuracy required.

Overall control of the system is provided by a CS1 programmable controller to which all field-mounted devices are linked by Omron DeviceNet products. This DeviceNet installation handles approximately 24 analogue inputs, 4 analogue outputs, 50 digital inputs and 30 digital outputs, the largest proportion of which are associated with the burner system.

DeviceNet was adopted specifically to minimise the amount of field wiring - a necessity because of the risk of hot metal splashes from the furnace. The field wiring must be protected from damage by these splashes, and it is easier, and less costly, to protect the small amount of wiring needed by DeviceNet than the much larger amount needed by conventional I/O systems.

During operation, the temperature of the furnace is monitored by an optical pyrometer, and control is provided by a PID loop implemented on the CS1 PLC. More interesting, however, is the use of a fuzzy logic module within the CS1 to assist with optimising the efficiency of the aluminium recovery process.

As already mentioned, the process involves a large number of variables, and the relationship between these and recovery efficiency is complex. This makes it virtually impossible to develop a series of equations which accurately model the process, and allow it to be optimised conventionally.

Omron's fuzzy logic system, however, allows the process to be optimised on the basis of simple rules that can be refined over time. This approach, which does not depend upon the development of accurate mathematical models, is proving very successful in making the recovery rate less dependent upon the skill of the furnace operator. Efficiency gains of around 10 per cent are already being achieved, and even higher gains are expected as the fuzzy logic is fine-tuned.

The operator interface for the furnace is provided by an electronic HMI panel, which also logs data from the process, and has facilities for providing links to SCADA installations.

Ethernet connectivity is also implemented, allowing remote access to the operational log and all key parameters. This means that Platinum Controls can provide comprehensive technical support to users anywhere in the world.

Programming for the installation was carried out using Omron's CX programming system, which the Platinum design team found to be both intuitive and powerful. The team also made use of the simulation facilities provided, which allowed most of the programming to be tested and debugged prior to installation of the system on the furnace.

'Working with Omron has been very important for us,' says Stephen Vincent, Platinum Controls' technical director. 'We have been able to try out new techniques, such as the use of fuzzy logic, safe in the knowledge that if and when we needed support, it would be there. In essence, Omron's support has helped to take the risk out of innovation.'