Pelletizing of the charge

      The main component of the pellet production process is the production of green pellets in the disc or drum pelletizer itself. Drum pelletizers operate in a closed cycle with roller screens for screening of large (+16mm) and small fractions (-8mm). Disc pelletizers can operate both independently and in pairs with roller screens for screening only large class (+16mm) or screening of large (+16mm) and small fractions (-8mm).

      The main factors influencing the production of green pellets are:
       a. physical and chemical properties of concentrate;
       b. physical and chemical properties of additives;
       c. amount of added substances;
       d. humidity of batch;
       e. speed of rotation of tray (drum);
       f. angle of inclination of tray (drum);
       g. change in height of tray side;
       h. number and location of bottom and side scrapers in tray;
       i. place of loading of batch into tray;

       j. location of water supply;
       k. amount of water supplied;

      Points a, b, c, d were discussed above. The charge must be fed to the pelletizer with a moisture content below the optimum value so that the plant's process personnel have the opportunity to bring the charge moisture to the optimum value by feeding water to the pelletizer. In general, feeding water to the pelletizer is one of the important elements of controlling the rolling of raw pellets, which allows adding moisture to the pelletizer with a dry charge, and reducing the water feed to the pelletizer when the moisture content increases above the optimum.

    The process of obtaining raw pellets is conventionally divided into stages. The first begins with the formation of pellet embryos, and water serves as the center of their formation. Embryos are dense, over-moistened lumps of the charge onto which the pellet shell rolls. The charge almost always contains areas of material with increased humidity and density, which are formed after filtration, transportation and loading into the pelletizer. If their number is sufficient for pelletizing the charge with optimal humidity, then additional moistening of the charge is not performed. If the humidity of the charge is less than optimal by 0.3-1.5%, then the formation of embryos is formed by drip irrigation in the charge feed zone, which moves along a descending trajectory of the working space of the plate. The formation of the embryo also occurs after the destruction of fragile pellets. At the second and third stages of obtaining raw pellets, there is a rapid growth of the embryo and formation of lumps of the batch, which, under the action of dynamic forces when rolling along the inclined surface of the plate and impact effects between themselves and the wall of the pelletizer, gradually form the structure of the final pellet. The average static indicators of the content of the mass of fractions in the pelletizer batch are as follows: 15-20% of the conditioned class, 10-15% of embryos, 65-70% of the intermediate class. In practice, it often happens that the moisture in the batch is the optimal value or even slightly higher than the optimal value and water is not supplied to the pelletizer. If the moisture of the batch increases above the optimal value, it is necessary to take prompt measures to stabilize it. For this purpose, the consumption of bentonite or crushed return in the batch is increased and the moisture of the concentrate is stabilized by adjusting the filtration mode or changing the concentrate batching (feed from the warehouse/direct feed from the process).

      Diagrams  show some aspects of the influence of concentrate moisture on the performance of the pelletizing process and the operation of the firing machine. The main control lever for the pelletizing process, along with changing the amount of moisture and  bentonite consumption (return), is changing the rotation speed of the pelletizer. Changing the pelletizer speed to a smaller or larger value allows for the production of larger or smaller pellets. The pelletizer speed can be adjusted from 0 to 8rpm. The next element of pelletizing process control is changing the pelletizer tilt angle. For disc pelletizers, the working range of tilt angle is from 46 to 50. Adjusting the pelletizer tilt angle is used last, when all other control methods have failed to achieve the desired result. In practice, a control method is often used by decreasing or increasing the load on the pelletizer with simultaneous redistribution of the load on other pelletizers, however, this method is associated with the need to adjust the pelletizing process on all pelletizers with a redistributed feed load of the batch.

    Recently, some manufacturers of pelletizing equipment (Haver & Boecker Niagara) have added another element of pelletizing process control – a variable height of the  pelletizing plate side. This technology works on an industrial scale, but requires further development and a clear control algorithm. Bottom and side scrapers are installed permanently during commissioning of the  pelletizer according to the recommendations of the equipment supplier and during operation during operational control of the pelletizing process, their position in the pelletizer is not changed manually, Industrial samples of intelligent scrapers have been developed, which are equipped with drives and allow their position in the pelletizer plate to be quickly adjusted during operation. All criteria for the formation of high-quality green pellets must be optimally selected for each type of unit and production conditions, i.e. there is a certain relationship depending on the type and characteristics of the concentrate used, which must be determined by testing between speed, angle of inclination, side height (for a disc pelletizer),  degree of filling and residence time. The green pellet production process is a multifactorial process with many changing variables. The more parameters change, the more often this happens and the greater the range of deviations from the set values, the more difficult it is to regulate the pelletizing process.