Moisture content and lumpability

    Water and binders of the batch are necessary components of the process of obtaining conditioned green pellets. Materials, additives and reagents of the batch that form significant cohesive forces with liquid molecules are called hydrophilic or wettable, and, conversely, those that have weak cohesive forces are called hydrophobic or non-wettable. The quantitative level of hydrophilicity of the surface of minerals is characterized by the value of the contact angle. The lower the value of this angle, the more hydrophilic the material. Materials with a contact angle of up to 90 are considered hydrophilic (quartz 0-10, magnetite 4-6, calcite 0-1, pyrite 33-35, graphite 60-62 ), more than 90 are hydrophobic. Sometimes, to improve the pelletizing process, surface-active substances (SAS) are introduced into the batch, which allows finding a balance between the wettability of the batch components and the surface tension of water.

     The greatest influence in terms of preparation of the charge components is the moisture content in the concentrate, as the main component of the charge. The moisture content in the concentrate has the main effect on the final moisture of the charge and the green pellet. For a certain type of basicity, the moisture content of the concentrate should have a different value. The higher the basicity modulus of the pellets, the higher the moisture content in the concentrate should be, while generally maintaining the optimal moisture content in the charge. The moisture content in the concentrate is directly or indirectly related to the fractional composition of the concentrate, its specific surface area, equipment and reagents used to filter the concentrate before feeding it to pelletizing. Since concentrates differ in their mineralogical structure, fractional composition and specific surface area, therefore, for each type of concentrate, its own range of optimal moisture is determined taking into account other components of the charge and the basicity modulus of the pellets produced. The most optimal moisture content of concentrates for unfluxed pellets is considered to be the range from 8.6 to 9.6%. With an increase in the basicity modulus from 0.3 to 1.2units, this range shifts upward and is from 9.0 to 10.0%. The optimum moisture content of the charge is the moisture content that ensures the maximum yield of green pellets of the condition class. The optimum moisture content of the charge depends on the degree of grinding, grain size and shape, chemical and mineralogical composition of all components, as well as their quantity, and can vary in a wide range of values depending on the requirements of the technological process. To assess the lumpability of a wet batch, the lumpability coefficient is used:

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MMM – maximum molecular moisture capacity of the material;
МCM – maximum capillary moisture capacity of the material;

        The lumpability coefficient varies from 0 to 1. According to lumpability, materials are divided into 5 groups. If the material does not lump, K = 0-0.2. For weakly lumpable batches from K = 0.2-0.35, medium lumpable batches K = 0.35-05, for well-lumpable batches K = 0.5-0.8. With a value of K> 0.8, the batch lumps very well. Batches with the use of magnetite concentrates are classified as well-lumpable with a coefficient of K = 0.51-0.72. MMM characterizes the energy of interaction of the surface of particles with water, therefore their specific surface and heat of wetting. MCM is the structuring of the material, determined by its porosity and structure. With the increase of bentonite and limestone additives, the lumpability of the batch increases, in the first case the minimum (Low) capillary moisture capacity (LCM) increases and the maximum capillary moisture capacity (MCM) decreases, in the second case the LCM and MCM increase. In practice, under production conditions the MMM and MCM are not determined and the lumpability coefficient is not calculated.