Pellet stickiness

      Pellets in direct reduction can sinter into druse and conglomerates, which leads to poor material flow and the formation of gas channels, which are short paths for the reducing gas through the ferrous clusters formed, which leads to poor use of the reducing gas and thereby increases fuel consumption. In the direct reduction process using the MIDREX and HYL technology, preventing the formation of conglomerates is one of the main factors for the normal operation of the plant. The tendency of pellets to stick limits the upper limit of temperatures at which these thermal units can operate and, as a result, imposes certain restrictions on increasing the productivity of the plants. If the granules are practically not prone to sticking, it is possible to increase the reduction temperature and, consequently, the productivity. High temperatures are necessary to reduce the destruction of pellets and reoxidation of the reduced product.

    To prevent this process, pellets are coated with anti-sticking materials. This procedure is carried out mainly at pellet plants and sometimes directly at the customer's site, and ideally in both cases. Cement, lime, olivine solutions of a certain concentration with an admixture of bentonite are used as such a coating. Bentonite in these solutions is used as a binding element and is dosed with the coating material in a ratio of 1 part bentonite to 7-8 parts coating material. Before preparing the aqueous emulsion solution, bentonite and coating materials are crushed to a size class of 0.056mm - 84-92%. The average consumption of coating material per ton of pellets is 2-3kg/t. The applied coatings must withstand the transportation of pellets to the consumer, taking into account warehousing, storage and loading and unloading operations. Sometimes blast furnace pellet manufacturers also use passivation, which reduces sticking and the amount of dry blast furnace dust by reducing pellet dust formation and improves gas utilization by reducing the resistance of the charge to the gas flow. The higher the reduction temperature (in order to increase the unit's productivity), the more prone the pellets are to form conglomerates, the higher the requirements are for the characteristics of the pellets themselves in terms of cluster formation and the properties of the non-stick coatings used. If an increase in the iron content in pellets leads to an increase in sticking, then an increase in the content of silicon oxide and magnesium oxide reduces it (diagrams). An increase in basicity also reduces the tendency of granules to stick together, but is a limiting link in terms of iron content in pellets.

     The assessment of the possibility of pellet clustering during direct reduction is carried out according to the ISO 11256 test method. The conglomeration index is determined as a percentage and is the ratio of the conglomerate mass after several softening operations to the total mass of the sample after reduction.

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​mr    – total weight of the sample after restoration, grams
mс,i – mass of conglomerate after i-th softening operations, grams
      An example of requirements for the quality characteristics of BF and DR pellets is given in annex1 .

     To assess the suitability of a certain type of pellets for blast furnace processing or metallization units, at the first stage, pellet samples are taken from the production process and sent to specialized laboratories (for example, SGA) for a comprehensive determination of all physical and metallurgical properties. Based on the test results, further steps are determined. If the indicators deviate from the generally accepted values, it is necessary to conduct additional adjustment tests, including involving specialized research  laboratories or  scientific centers (Metso,  Primetals  Technologies,  Midrex,  Corem  and others). When designing firing machines, research work is carried out to assess the correctness of the choice of the useful sintering area for a certain volume of pellet production with specified quality characteristics. According to the established program, a set of tests is carried out in the amount of 10-30 pieces, including tests of the initial components of the charge, the charge itself, green and fired pellets, mixing parameters, pelletizing and firing. During the tests, several types of charges are made for the production of green pellets that differ in properties (iron content, specific surface area, particle size distribution), type of components (with the addition of limestone, fuel, organic matter), their quantity (different consumption of each component) and basicity modulus. The resulting green pellets from different types of charges are subjected to heat treatment in a laboratory setup with various temperature and gas-dynamic modes simulating all zones of the technological process of obtaining pellets from drying to cooling. The next stage involves assessing the physical, chemical and metallurgical properties of the fired pellets obtained from various batches and under various firing conditions. Samples corresponding to the highest quality characteristics of the product are determined, and repeated tests are carried out with a given batch composition and firing parameters to confirm the characteristics obtained. After confirming the results obtained, based on the data obtained on the time of the thermal firing process, temperature and gas-dynamic characteristics, the sintering area of the firing machine for a given production volume, productivity, pellet quality and the required equipment composition are determined.