With increasing age and hours of operation, the performance of optical sorters can start declining. General lack of cleaning and maintenance as well as an ever-changing input material composition can further exacerbate those performance challenges. 

During the early days of start-up, commissioning, and the break-in period, new optical sorters operate at their respective peak performances. The challenge for operators is to keep them sorting at those levels consistently day in and day out without any long-term deterioration or fluctuations over the course of the day, or even different seasons. 

The tasks outlined in this article apply to both older (5 to 15 years) and newer optical sorters (less than 5 years old). They apply to optical sorters in all MRF applications, whether they are sorting plastics by resin and colour, positively or negatively sorting newspaper and mixed paper grades, or "last chance" recovery units on the residue line.

Keeping all relevant areas of the optical sorter clean and mechanically maintained will have the biggest impact on uptime and performance. This includes monitoring and, if required, adjusting the tracking of the high-speed acceleration belt, its bearings and areas around and below the return idlers, tail, and head pulleys. The splitter roller inside the catcher hood as well as the air valve/nozzle assembly are spots where materials can build up over time as well. Keeping those areas clean and easily accessible to maintenance personnel will reduce the risk of jams.

The halogen lamps that illuminate the sensing zone have a limited lifetime and will have to be replaced eventually. On older machines, monitoring the light intensity may have to be done manually. While some optical sorters available in the marketplace will keep sorting when halogen bulbs have burned out, they will do so at reduced efficiency. 

The air ejections will generate mist and liquid splatter that will over time settle on the glass surfaces that cover the halogen light assemblies as well as the sensor itself, especially during rainy and snowy weather. Keeping those surfaces clean and as transparent as possible will make sure the maximum amount of light reaches the NIR/colour spectrometer(s). 

In the summertime, air conditioning units keep the sensor housing as well as the operator touchscreen control panel from overheating. The dust filters for those units need to be checked, cleaned or possibly replaced on a regular basis. Extended exposure to high temperatures is a sure way to shorten the lifetime of the internal electronics.

One of the most important aspects to increase the longevity of optical sorters is to provide ultra clean compressed air, free of moisture, oil, and dust. The air quality coming from the compressor needs to be monitored on a regular basis, otherwise the internal valve mechanisms could fail, causing them to either get permanently stuck open or closed. 

Testing the functionality of each air valve on a regular basis is crucial to ensure optimum sorting performance. An 80-inch-wide single-eject MRF optical sorter normally utilizes 80 valves. If just four valves are not functional, it would mean that 5 percent of the machine width is not performing at all, which in turn will cause a drop in recovery of the targeted commodities.

Basic monitoring of the sensor performance and status of internal optical and electronic components is becoming more and more automated and self-reported (optional push notifications) to make it easier for the operator to monitor what is happening in real time on each optical sorter in the facility, as well as what/if any actions should be taken.

For example, while older units needed to be manually calibrated on a regular, sometimes weekly, basis the latest generation technology is able to virtually eliminate this task using internal calibration routines and algorithms that are able to compensate for changes in the immediate environment of the machine, mostly temperature, over the course of a day.

The amount of operational data, material composition, and other characteristics that are generated especially by the latest generation optical sorters can sometimes be overwhelming. 

Advanced statistics and data analyses tools are now readily available to monitor the performance parameters. Integrating this data generated by optical sorters throughout the MRF into the main PLC/SCADA system allows for monitoring groups of machines and adjusting mechanical separation equipment as well as the optical sorters themselves. This would also include data from other sensors installed throughout the plant, for instance conveyor speeds, disc screen angles, vacuum system air speeds, etc.

Like any computer systems, optical sorter libraries, databases, and parameter settings should be backed up properly on a regular basis and in accordance with customer internal IT policies after any set-up changes have been made by the operator. 

If required, all these described tasks can be outsourced to the optical sorter vendor to make sure nothing falls through the cracks. Nowadays, remote access and support via wireless or Ethernet connection makes this easy and affordable.

The only thing staying constant is the ever-changing composition of incoming material streams as well as the introduction of completely new materials and packaging solutions like flexible plastics packaging. With a declining volume of newspaper and higher percentage of cardboard, the optical sorters that control the quality of the generated fibre streams from the upstream container/paper separation equipment may have to be constantly monitored and adjusted. 

On the container side, the recent increase of fully sleeved bottles, featuring a wide variety of label resins, have made it necessary to adjust the plastic optical sorters accordingly. The reference library containing all the different material and colour categories should be kept updated on a regular basis in order to maximize recovery of all plastic containers in general and PET in particular.

On older units, sometimes the only way to perform this work was by a trained technician on site. The newer generation optical sorters can be remotely accessed by factory-trained staff, and reference library and parameter settings can be updated much more easily and faster requiring only minimum support by the on-site operations and maintenance staff. 

The newest optical sorters provide significantly higher recovery rates and are able to generate much cleaner products compared to units that are older than even just 5 years. 

Sometimes, the above described monitoring and maintenance functions are simply not sufficient to either recreate the performance from when they were originally started up or provide an improved performance that is necessary in today's marketplace. 

One recent trend is replacing such older units with a "sensor only" package that includes the newest technology. Such a solution can be executed at a significantly lower cost compared to replacing the whole unit including the high-speed acceleration conveyor. 

Of course, such complete retrofits only make sense if the added value generated by a better performance and possible manual labour savings provides an adequate payback. 

Logistical challenges can arise if an existing optical sorter is installed in a way that makes access difficult for lifting/moving equipment. In a more complex scenario, it's very much possible that the higher cost of mechanical installation and additional downtime caused by the added complexity may make a project prohibitively expensive.

Using the above guidelines should assist MRF operators to get their optical sorters back up to speed and ensure that they will last and perform at peak levels for a long time to come. 

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