Does a Color Sorter Replace the Need for a Gravity Separator?

- Jon Moreland

Lately, more and more people have asked if a color sorter can replace a gravity separator. To answer this question, first consider that the two machines are designed to identify issues by different means, one by density and one by visual aspects.

The gravity separator can’t visually identify anything, and the color sorter can’t identify anything by density … no matter what the sales person says. Although I’m aware there is technology to “see” density (potentially by UV, X Ray or other means), this is not being effectively marketed to the seed or feed industries.

There seems to be some confusion about what is possible with each of these machines. This stems from the fact that some density issues might also have a visual component. If this is consistently the case, then an optical sorter could replace a gravity separator. However, if there are density issues that are not visually identifiable with the naked eye, there is very little likelihood that they can be addressed with optical sorting. If you’re running a product that has a density issue and some optical blemish, either machine could remove the particles with both issues.

Inevitably, it’s possible that quality standards can be met using color sorting as an alternative to density sorting, but it can’t be said that the color sorter removes low-density particles by density alone.

Why separate by density? It’s industry accepted and university proven that density highly correlates with seed germination and vigor. Furthermore, with popcorn, higher density levels have proven to pop better.

Advantages of using a gravity separator include: high capacity, relatively low cost, minimal adjustments and mechanical or electronic controls. Additionally, it can deal with a high rate of contamination and will size homogenous material.

However, it’s not as precise as an optical sorter and only focuses on one thing (density). It’s most accurate when material is sized first. It’s also important to mention that most machines create a grade of light to heavy, leaving a “middle fraction” to deal with. Because of this grade, results are not as quantifiable.

Why separate by optical? Notice I didn’t say color. Most basic “color” sorters are used to sort by a “shade” of one color. These basic machines sort color monochromatically against a light filter so that the good material matches the background and the contaminant is “seen” by the optical array and ejected. Better than that is RGB, capable of seeing different colors against a white background, giving you ability to separate more issues. The best machines are now capable of sorting by geometric recognition, or shape. Add near infrared and you can even “see” some things outside of our human visual spectrum.

Anyway, to answer the question, separate by visual means when nothing else will work; not size, not shape, not density.

Optical sorters are precise and have minimal moving parts. They can focus on more than one issue if there is a visual difference and can reject by color, shade, size and shape. Capacities are increasing, but they are not magic. They are most effective when contaminant levels are less than 5 percent. This technology continues to advance so keep an “eye” out for continuing developments.

In most scenarios there is a strong argument against optical sorting replacing density separation. Consider both as complements to one another and to the rest of the processing line. For best quality in seed, apply the right tool, for the right job, in the right way.