• QA 181
    How do destoners and catadors work?
    I would like to understand the operating principles of destoners and catadors. These machines are found in the dry processing chain for coffee (hulling plant).
    Asked by:
    Equipment provider - Gabon

    Answer: Destoners use a combination of vibration and airflow whilst catadors rely on airflow only.

    Destoners: These are cleaner-separators that combine the actions of vibration and airflow. As the name suggests the process removes stones and other pieces of foreign matter (metal, glass etc) from the dried cherries that are to be hulled.

    The coffee flows over an inclined vibrating table whose surface is made of wire mesh. A controlled airflow lifts the coffee causing it to flow downwards on the air current along the sloping surface to be discharged over the edge. Heavier materials such as stones, pieces of metal, glass etc. are not lifted by the air current and are moved by the screen's oscillating (swinging) motion to the other edge, where they too are discharged. It is impossible to remove all impurities but, when properly adjusted, modern equipment can eliminate most of them. The operation is controlled by adjusting the airflow, or the slope of the vibrating screen, or both. Depending on local conditions the process can be supplemented with stationary or rotary magnets to ensure also metal particles are removed. *

    Catadors: Originally developed in Brazil, these are pneumatic air separators that use airflows to separate hulled coffee beans into different densities. They also remove defects such as parchment hulls/chaff, broken beans, very light beans (like badly insect damaged beans for example) etc.

    The efficiency of catadors depends on the strength of the airflow and the product feed. If the airflow is too weak or if too much product is fed into the machine, then separation will be insufficient. Alternatively, if the airflow is too strong then too much product may be rejected. Constant supervision by experienced operators is therefore essential.

    The original design consists of two air channels fitted vertically on a housing. This contains two turbine wheel fans (one for each channel) mounted on a single axle and powered by an electric motor. The air channels are topped by a hood through which air, dust, chaff and minute particles pass to a dust collection point. Each air channel has a movable slide which allows the operator to regulate the strength of the airflow. An elevator feeds coffee into the side of the first air channel just below the hood where the coffee encounters the air stream. Heavy beans fall on to a perforated screen lower down the channel and exit through a spout for bagging off. Lighter beans are forced upwards into the hood where a curved baffle diverts them into the upper section of the second air channel, the upper part of which is divided into two vertically. As the beans fall through this upper section they hit the air current of the second fan and the process is repeated. Heavier beans fall on to a perforated screen and exit through a spout for bagging off. The lighter material travels back up into the hood and exits from there through another spout. This example delivers three grades: heavy, medium and light whilst dust etc is removed. Simpler models deliver heavy and light only. **

    * Usually, the arriving cherry is first dumped into hoppers fitted with perforated metal screens that remove large impurities (sticks, string, straw, large earth clods etc). During this operation a powerful fan normally sucks off as much dust as possible.

    ** Catadors work best on coffee that has already been graded into different sizes. Multi-column units can separate a number of size grades simultaneously into heavy and light but, gravity tables are more accurate. In modern plants catadors are therefore mostly used for the initial clean-up of the coffee, directly after hulling, with density separation by gravity tables taking place once size grading has been done. Gravity tables operate on the principle of fluidization of the product, using an inclined, oscillating, perforated bed through which an air-stream is forced. This causes the lighter beans to rise whilst the heavier beans remain on the bed. The air-stream re-arranges the beans, separating them into a number of layers of different densities. The bed's oscillating action causes the heavier product to move up the incline whilst the lighter beans move down it. A secondary incline at right angles makes the heavier product flow over the edge. As the product is now in separate layers, outlets can be positioned to deliver the required grades.

    See Chapter 11 of the Guide for issues relating to coffee quality, particularly topic 11.05.08 on size grading and topic 11.05.09 on density separation.

    Posted 21 February 2008

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