Gravities, Stoners, and Others

Preparing Peanuts for Color Sorting:

Gravities, Stoners, and others.

After shelling, kernels are still mixed with some foreign materials and unshelled peanuts. Different stages are needed to clean them as much as possible in preparation for color sorting.

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There are various technologies available to help the industry find solutions, with densiometric machines the most commonly adopted in peanut-shelling plants. These are devices designed to separate products having the same particle size but different densities. An inclined perforated stepped screen deck, moved from an eccentric shaft system, vibrates while, from the bottom up to the deck and through the product, an airflow creates a fluid bed, moving the lower density product upward and placing the higher density product at the bottom.

We can classify this type of machine into two main categories: 1. gravity machines, to be used when lower density product is to be removed, and 2. stoners, deployed to eliminate the highdensity product.

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Gravity machines can be used for two main purposes.

1

A.

To complement the shelling process —

As described in the previous issue of WPM, proper shelling requires multiple steps in order to minimize the number of split kernels during the process.

As a complement to the shelling machines, gravity machines are needed to separate the shelled kernels from the unshelled ones from the previous stage. After the gravity process, the unshelled kernels are thus returned to a new shelling stage. It is important to properly adjust the machine to avoid kernels returning to the shelling cylinders, as those kernels will certainly be subject to splitting

A top view of these machines will show 70–80% of the area covered with the higher density product, and the rest with the lower density one. It requires a continuous flow of both products leaving the machine to keep this separation line away from the two ends of the machine ( 3).

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1

B.

Foreign material removal —

In addition to the first gravity machine, it is quite customary to see a second one at shelling plants. Its purpose is to perform a fine-tuning job, aiming to remove “foreign material pieces.” In this case, a top view of the machine will not show any visible separation between the two products, because the low-density product (foreign material) is present in a very low percentage. It is possible to observe the performance of the device by collecting the rejected product for a few minutes and finding the removed foreign material in the sample. In the stoner machines, the light product is the main one, moved by air and the vibrations of the lower end of the deck, while the small portion of the heavy product is moved from the bottom toward the upper end of the deck.

A top view of the machines is more likely to be similar to the gravity machine employed as a complement to shelling, without a visible line between the heavy and light product.

This separation, though, is a rather simple one in view of the high difference in density between a peanut kernel and a stone, which makes the job of the machine much easier and efficient.

40 Here are a few recommendations to keep in mind for the maintenance of gravity separators 3

Keep the slotted screen very clean and do not allow the perforations through which the air passes to become obstructed.

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Check the condition of the slotted screen frequently, since if the jump wears out, quality is lost in the process as well as production.

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The air inlet filters to the machine must always be clean so that the machine provides airflow.

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The density table must have an air suction hood on top to capture all the shell, skin, and dirt to prevent this from clogging the air intake filters.

Other equipment for foreign material removal before color sorting

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There are other options for removing foreign material, such as flat and rotative screens, aspirators, magnets, etc.

In the last few years, in Argentina, a stoner with a different functioning principle became more visible: the magnetic stoner separator, which does a very good job of removing clay balls coming from the fields.

It consists of a simple belt with a magnetic roller at the end which attracts the clay balls due to their iron content, thus achieving the separation from the main peanut flow in a simple and effective way ( 3).

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