Magnetite concentrates are the main iron ore part of the charge for pellet production. For a certain type of ore and a given quality (Fe and SiO2 content) of the enriched material, an appropriate concentrate production scheme is selected. The enrichment of ore coming from a quarry includes the following stages of the technological process: crushing, grinding, classification, separation and filtration. To obtain a concentrate of a given quality, the ore fed to enrichment must have certain quality indicators for enrichment and magnetic iron content. 

      The ore enrichment index affects the iron content in the final concentrate, the higher its value, the higher the iron content in the concentrate. The magnetic iron index in the ore affects the amount of concentrate obtained during enrichment, the higher it is, the greater the volume of concentrate obtained during enrichment. When feeding the ore mass for enrichment, the optimal ratio of different types of ores is calculated to obtain the planned quality of concentrate and its quantity for the selected enrichment scheme.

    In order to obtain a higher iron content in the concentrate, ore processing schemes with separate enrichment of rich and poor ores are practiced, which leads to the production of two types of concentrate with an iron content (Fetotal≈67.5-68.5%) and an iron content (Fetotal≈64.5-65.5%). There are also options when, in the presence of free processing capacities, ore enrichment is carried out instead of a 3-stage enrichment scheme according to a 4-stage enrichment scheme, which allows involving in production an additional amount of ores with low enrichment without losing the iron content in the concentrate or enriching ore with high enrichment and obtaining a concentrate with an iron content (Fetotal≈69.5-70.5%).

      There are also classical enrichment schemes using flotation finishing or fine grinding screens to increase the iron content in the concentrate to (Fetotal≈69.5-70.5%). Below are the main technological schemes for enriching ore mass to obtain concentrate:

       a. WMS - wet magnetic separation;

       b. WMS - wet magnetic separation with an additional grinding stage;

       c. WMS - wet magnetic separation with separate ore enrichment;

      d. WMS + MFFC - wet magnetic separation and magnetic flotation finishing of concentrate;

       e. WMS + fine screening - wet magnetic separation and fine screening;

       In the production of iron ore concentrate for pelletizing, the priority quality characteristics are the content of the mass fraction of iron, silicon oxide, moisture content, specific surface area and grain size distribution. Also, concentrates should contain as little sulfur (S≈0.01-0.05%) and phosphorus (P≈0.003-0.008%) as possible.

     All these derivatives are directly or indirectly related to each other in relation to the existing enrichment technology and the need to supply concentrate of a given quality for pellet production. Concentrates obtained using different enrichment methods differ from each other in granulometry composition and specific surface area. These two parameters are related - the finer the concentrate is ground, the greater the specific surface area of ​​the concentrate, which directly affects the moisture content in the material.

     The specific surface area is one of the main indicators for monitoring the fineness of the material during granulation. It is determined by measurement methods based on determining the air permeability of the porous layer of particles, developed by Blaine and Fisher. However, such methods do not take into account the shape factor of mineral particles, as well as roughness and porosity, which leads to some inaccuracies and measurement errors in different laboratories. The smaller the average diameter of the mineral particles, the higher the specific surface area, regardless of the type of material. With the same average particle diameter, the specific surface area value is higher for materials with a lower specific gravity.

       The optimum specific surface area for pelletizing depends on the concentrate type and the quality characteristics of the fired pellets. It varies between 1600-2200cm2/gram at different plants. In order to create a homogeneous batch, the additives used for pelletizing should be ground to the same average particle diameter as the iron ore raw material used. The specific surface area of ​​the prepared limestone/bentonite additives is within 3800-5200cm2/gram, anthracite 5200-7000cm2/gram. An important characteristic of the concentrate is also the shape and surface of the grains of the material, which is determined by the mineralogical and chemical composition and the selected technological enrichment scheme. 

      Concentrate grains may have different shapes – spherical, polyhedral, lamellar, dendritic. The strength of green pellets depends on the total contact area of ​​concentrate grains. The higher the contact area, the greater the strength of green pellets. The polyhedral grain shape is characterized by the largest contact area, the smallest by the branched dendritic grain shape. Particles with a rough and developed surface have a higher degree of wetting and are better lumped. Magnetite concentrate particles in most cases have acute-angled grains.

       When obtaining the final concentrate using flotation technology, the hydrophilicity of the concentrate decreases due to the formation of reagent films, which somewhat worsens the pelletizing process and can lead to the inclusion of an additional stage in the process flow chart - concentrate washing. In some enrichment schemes, when producing concentrates with a low specific surface area and dendritic grain shape, roller presses (Köppern, KHD, Ecoplex, Polycom, Krupp-Polysius) are installed before feeding the concentrate for pelletizing.

         The schemes and equipment for filtration of concentrate are determined by the need to obtain a given moisture content of concentrate for the process of pelletizing raw pellets. The classic filtration scheme using vacuum filters allows working with a specific surface area of ​​concentrate of 1500-2200cm2/gram and a granulometry composition of 95-98% of class 0.056mm and obtaining concentrate with a moisture content of 9.5% to 10.0%. Such schemes are used in a 3-stage grinding scheme and were practiced at all old enrichment plants. If more stringent requirements are required for the upper limit of moisture content in the concentrate, ceramic vacuum filters, vacuum filters with steam and bar filters are used, which provide concentrate moisture from 8.6 to 9.6% with a specific surface area of ​​1700-2400cm2/gram and a particle size distribution of 96-98.5% of class 0.056mm.

    Flotation enrichment schemes, the concentrates of which are characterized by a high specific surface area of ​​2400-2800cm2/gram and a grain size distribution of 99-99.8% of class 0.056mm, require the use of press filters for dehydration of the concentrate to a moisture content of 8.8-9.5%. 

        At the beneficiation plants built in the 50-80s of the last century, in order to increase the iron content in the concentrate by 1.5-3%, the ore mass is re-crushed by reducing the productivity of the sections or by working according to a 4-stage grinding scheme, which leads to an increase in the specific surface area of ​​the concentrate to 2400-2600cm2/gram and a decrease in the size of the material to 98-99.5% of the 0.056mm class. When filtering such concentrate on conventional vacuum filters, the moisture content at the outlet is from 11.0 to 12.5%, which does not meet the requirements of the pellet production process.

       Due to the lack of the necessary filtering equipment and in order to reduce the moisture content in the concentrate, various dehydrating reagents are used, which are added directly to the pulp separator before feeding the concentrate in the liquid phase for filtration. The use of these reagents allows obtaining a moisture content in the concentrate of about 9.3-9.8% and is justified for further shipment of the concentrate to the buyer. However, the use of a dehydrating reagent reduces the hydrophilic properties of the concentrate due to local binding of water in the volume of the concentrate sample and, as a result, reduces the lumpiness of the concentrate and worsens the process of pelletizing green pellets. For each type of concentrate obtained, based on the specific surface area of ​​the concentrate, granulometry composition, mineralogy, added batch materials and pelletizing requirements, the appropriate moisture content should be selected to obtain optimal characteristics of green pellets during pelletizing.

      In most cases, magnetite concentrate used for pellet production has a basicity modulus of 0.04 to 0.1units. Some companies produce concentrates with a basicity modulus of 0.5-0.8units, mainly hematite concentrates. In general, the basicity modulus of a concentrate is a derivative of the content of the main calcium oxides CaO and magnesium oxides MgO in the ore used to obtain the concentrate.