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Recycling Cerium Oxide Polishing Material

Background to development

Using cerium oxide in polishing lenses

Konica Minolta’s lenses are produced using a process whereby glass surfaces are subjected to precision machining, using cerium oxide as a polishing material.

In the polishing process, cerium oxide polishing material is dispersed in a liquid such as water to form slurry, which is used cyclically. However, as more and more particles of glass are removed in the polishing process, and get mixed into the slurry, the polishing efficiency rate decreases. Once the concentration of glass in the slurry has exceeded a certain level, the slurry needs to be discarded and replaced with a fresh batch.

Rare earths are limited global resources

Aiming to enhance the eco-friendliness of our production sites, Konica Minolta is also working proactively towards more effective use of resources and resource recycling. We realized that if we could reclaim and re-use the used slurry, we would be able to reduce the amount of used slurry discarded as waste, and make more effective use of the cerium, which is a rare earth.

The polishing process that uses cerium oxide is known as "chemical mechanical polishing" or "CMP." As its name suggests, this process does not simply use the polishing material to mechanically abrade the surface of the glass: there is also a chemical action whereby the cerium oxide smooths out the microscopic bumps and indentations on the glass surface. Consequently, particles of glass clump together with particles of polishing material in the used slurry, making it difficult to remove the glass component. This is why conventional reclamation techniques either have an extremely low reclamation rate, or are prohibitively expensive. For these reasons, and because the substances used to carry out reclamation are themselves environmentally problematic, the existing methods pose problems that remain to be solved.

Goal

  • To recycle spent cerium oxide slurry into a polishing material of equivalent quality to the virgin product

Konica Minolta technology

High-purity, high-quality reclaimed polishing material

Faced with this challenge, Konica Minolta succeeded in developing a high-purity recycling technology for cerium oxide polishing material, based on advanced materials technologies developed over the years in its film and toner development operations. The new technology reclaims 100% of the cerium oxide from the used slurry, which would have been extremely difficult using conventional methods. The reclaimed cerium oxide is also equivalent in quality to the virgin polishing material.

The new recycling technology also has the advantage of not requiring large machinery, so the recycling equipment can be installed quickly and cheaply. This means that used slurry does not need to be transported to another location for recycling, and the cerium oxide polishing material can be reclaimed efficiently on the production site.

This cerium oxide polishing material recycling technology has now been introduced at Konica Minolta’s optical device production sites in Japan and overseas, dramatically reducing the volume of waste generated within the process, and slashing the consumption of cerium – a rare earth – to about 10% of previous levels.*

*  Besides being present in the used slurry, cerium oxide also adheres to the lenses after polishing. The wastewater in which the lenses have been washed is also subjected to recovery and recycling processing, so the recycling rate for the polishing process as a whole exceeds 90%.

Success

  • 1.100% of the cerium oxide is successfully reclaimed from the used slurry
  • 2.The reclaimed cerium oxide can be used as polishing material of the same quality as the virgin product
  • 3.Large machinery is not required, so reclamation can be carried out at low cost

Key technology

Analysis at a scale of microns

In order to recycle the polishing material, analysis at a scale of microns had to be carried out to ascertain the state of the polishing material particles and the glass in the slurry. The technique used to do this is one of the proprietary core technologies Konica Minolta has honed over long years.
Using an electron microscope to make a detailed analysis of the state of the particles, focusing on the difference in the state of the cerium oxide polishing material before and after polishing, we were able to obtain the following results.

  • The cerium oxide polishing material particles in the slurry are formed from agglomerated granules.
  • The glass adheres to the particles of polishing material in the used slurry, reducing the polishing efficiency.

From these results, it was evident that the best way to remove the glass when recycling the polishing material would be to use a chemical method.

We also needed to evaluate the reclaimed polishing material. For this purpose, we used particle size distribution and X-ray diffraction analysis to make sure the reclaimed material was not structurally different from the virgin product.

Advanced technology for particle agglomeration and dispersion

Based on the results of the particle state analysis, a chemical agent was added to the used polishing material slurry, separating it by selectively causing only the cerium oxide to flocculate; the supernatant solution containing the glass was then removed, and the cerium oxide was re-dispersed, rendering it ready for re-use as polishing material.

The point here is that Konica Minolta has the technology to agglomerate or disperse the particles of cerium oxide polishing material, as required. In order to minimize the amount of cerium oxide lost when the glass is removed, the cerium oxide particles are made to agglomerate into granules with a large diameter. Afterwards, when the cerium oxide is prepared for re-use as a polishing material, the agglomerated granules are dispersed to form particles of an optimum diameter for polishing.

Konica Minolta developed this agglomeration technology through manufacturing polymerized toners. The dispersion technology was developed through working with silver halide for photographic film. These core technologies have given us the ability to control particle agglomeration and dispersion in a highly-sophisticated way.

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