All quality indicators of roasted pellets are divided into three categories of properties: chemical, physical and metallurgical. The quality characteristics of pellets obtained after firing in an oxidizing unit relate to the physicochemical properties of pellets - compressive strength, abrasion strength, impact strength, grain size distribution, porosity, chemical composition. Roasted pellets should ideally contain the maximum amount of iron in the form of hematite, the content of iron in the form of magnetite should be virtually absent. The presence of silicon-containing materials and harmful impurities: sulfur, phosphorus, alkaline compounds (Na2O, K2O) and NaCl should be as low as possible.

    The physicochemical properties of the starting materials, charge, raw and fired pellets are determined in laboratories or research centers at each plant, and the data obtained allow for prompt management of changes in the technological process at all stages of pellet production.

      All studies of the quality characteristics of pellets during reduction in a blast furnace, which are directly related to it, relate to the determination of metallurgical properties. Almost all determinations of the metallurgical properties of pellets can be performed only in specialized laboratories that have the appropriate equipment and trained personnel. Determination of the values of the metallurgical properties of pellets that best meet the requirements of blast furnace smelting allows for a targeted impact on the elements of pellet production technology in order to improve one or another indicator. To assess the comparability of the properties of various iron ore materials in terms of their suitability for use in further processing, it is necessary to conduct comparative tests according to the same standardized documents (for example, ISO or JIS), otherwise the results obtained are not subject to comparative analysis.

       In recent decades, the issue of determining the strength of pellets during low-temperature destruction (LTD - Dynamic test for low-temperature reduction-disintegration) has become especially relevant for blast furnace operators, so some enterprises incur costs and purchase installations to determine this indicator. Consumers of products believe that the strength of iron ore pellets after low-temperature reduction is one of the main indicators of pellet quality, which affects the operation of the blast furnace, since it is directly related to the gas permeability of the charge material layer and the degree of destruction of pellets in the upper part of the blast furnace. To assess the quality of pellets, the tumble strength indicator is mainly used - the percentage of class +6.3mm. The higher the value of this indicator, the less susceptible the pellets are to damage. In practice, it is accepted that the minimum value of LTD +6.3mm should be at least 80%. Tests are carried out according to ISO 13930. This standard establishes a method for assessing the destruction of iron ores during reduction in a rotating tube at a temperature of, 500 C. This method is used for large-sized ores and pellets, as well as for blast furnace feedstock materials, and provides an assessment of the degree of destruction and reduction in pellet size during the reduction of hematite to magnetite under conditions close to the upper part of the blast furnace shaft at low reduction temperatures.

       The following common indicators characterizing the metallurgical properties of pellets in blast furnace processing at higher reduction temperatures are:
       a. the degree of recovery R, %, which is expressed as a percentage and characterizes the value to which the pellets have been recovered over a certain period of time and temperature. Usually, tests are carried out according to ISO 7215 at a temperature of 900 C for 180 minutes and determine the final degree of recovery of the material sample;
      b. the recovery rate index dR/dt, %/min, which is expressed as the recovery rate in percent per minute. Tests are carried out according to ISO 4695 at a temperature of, 950 C;
To assess the technological process, it is advisable to determine the physical and chemical properties of the fired pellets at least once every 2 hours, and to determine the tumble index (TI), abrasion index (AI) and low-temperature recovery strength (LTD) once a day, If these indicators do not correspond to the specified values, the frequency of determining these indicators must be doubled. In contrast to blast furnace pellets (BF), additional requirements for quality characteristics are imposed on pellets (DR) used in direct reduction units. This is due to the fact that, unlike a blast furnace, these units reduce iron oxides in a solid state without further formation of a liquid phase. In this regard, all harmful impurities pass into the composition of the resulting metallized product (DRI/HBI), and do not go into slag as in blast furnace processing. Increased requirements for iron content, strength characteristics and metallurgical properties are imposed on pellets used in metallization units. To assess the suitability and quality control of these pellets for the direct reduction process, an additional concept has been introduced such as the degree of metallization (ISO 11257), which, in combination with the degree of reduction, characterizes the reducibility of granules and the efficiency of the direct reduction unit. The degree of metallization is expressed by the following ratio:

Fem - metallic iron content in the recovered sample, %
Fet   - total iron content in the sum of oxides (Fe2O3+ Fe3O4+ FeO) in the reduced sample, %
     These values are determined chemically. The degree of metallization in well-functioning direct reduction plants with high-quality raw materials is at least 94-95%. Also, pellets used in direct reduction units are assessed by the reduction destruction index RDIDR, which is determined as a percentage of the -3.15mm class. 
                                                                                    
m0 - weight of the entire restored load, including material collected on dust collectors, grams
m1 - mass of the recovered sample remaining on a 10mm sieve, grams
m2 - mass of the recovered sample remaining on the 3.15mm sieve, grams
       The permissible value of this indicator should be no more than 3.0% of the content of class -3.15mm.