Optical separators unit is properly sized and correctly placed in the sorting process, and cleaning and maintenance tasks are performed periodically. That is, the effectiveness of the unit does not only depend on the unit features but also on the pre-treatment of the material. Some examples of processes with a low effectiveness are: › Units installed without a previous film suction unit. The effectiveness is lower when the plastic film is placed over the material to be selected, causing reading errors or even preventing reading. › Useful belt widths smaller than the width needed for a given capacity. Cause low effectiveness as the materials cannot be distributed along the width of the belt and will pile on top of one another. › Acceleration belts shorter than recommended. Prevent the materials from stabilising on the belt, resulting in mistimed air blasts when the material reaches the end of the acceleration belt. › Excessive or irregular feeding rates. • Purity. The purity of the selected material is the percentage of requested material in the fraction with respect to all the material selected. Optical separation units have purities from 90% to 98% if the unit is properly sized and correctly placed in the sorting process, and cleaning and maintenance tasks are performed periodically. Depending on the values obtained for fraction purity and the technical specifications to be met by the selected materials, the number of quality control stations required in each facility will be determined. • Maintenance. Cleaning and maintenance of the units is essential, as this has a direct effect on the performance of the facility. For this reason, at the end of each shift the cleaning operations specified by the supplier according to the characteristics of the unit must be performed. These are generally checking and cleaning the blowing valves, changing and cleaning the bulbs (since they are dimmed when dirt accumulated on the glass starts to burn) and cleaning the scanner reading glass. Effectiveness of the various sequences of optical separators Each sequence of optical separators discussed above has a different overall effectiveness depending on the order in which the optical separators are placed.
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