Organic polymers

     Organic polymers are used as activators. Depressants, foaming agents in concentrate flotation, dehydrating agents in the filtration of finely ground concentrates, and as binding agents in the production of pellets.

       Involvement of poor ores in processing and requirements of consumers to increase the iron content in concentrate to more than 68.0% necessitates additional regrinding of ore to a content of 0.056mm class of more than 98.5%. Such a result can be achieved either by modernizing the entire beneficiation stage with significant capital expenditures or without significant expenditures using operational impacts when working at reduced loads of the grinding sections when working according to a 3-stage beneficiation scheme with an increase in the efficiency of grinding the ore mass or by working sections according to a 4-stage beneficiation scheme using the third grinding stage of another similar section as the fourth stage. The concentrate obtained by such methods with a content of 0.056mm class of more than 98.5% and a specific surface area of more than 2400-2600cm2/gram cannot be filtered to the moisture content required for pellet production – 8.8-9.8% by conventional filtration methods - vacuum filters or ceramic filters. The main reason for the decrease in the efficiency of moisture removal is the increase in the content of fine particles of the concentrate of class 0.02mm in the pulp fed to filtration,
which leads to clogging of the filter elements of the filter cloth or ceramic sectors. For normal filtration of finely ground concentrate, it is necessary to install press filtration equipment, which is associated with the reconstruction of the entire filtration section and significant capital costs.

     One of the operational options for solving this problem was the use of polymer dehydrating reagents, which found their application in mining and processing plants built 40-60years ago and not having a press filtration section. The dehydrating reagent is added to the pulp separator in the amount of 100-160grams per ton of concentrate immediately before filtration and allows obtaining a concentrate with a moisture content of 9.4-10.0%. However, the concentrate treated with these reagents loses some of its hydrophilic properties due to a decrease in the uniform distribution of water molecules in the concentrate volume, that is, local areas appear where moisture is accumulated and areas where the moisture content is lower. Uneven distribution of moisture in the material leads to a decrease in the concentrate lumpability. The use of this concentrate has a negative effect on the properties of raw pellets (strength and granulometry composition) and its use for pelletizing is not recommended. When using concentrates obtained with the use of dehydrators, additional measures are required to stabilize the quality of raw pellets - increased
consumption of bentonite and additional moistening of the batch during pellet production. The granulometric composition of the obtained raw pellets after the pelletizer shifts towards an increase in the fine class of -10mm by 1.5-2times, and in the fired pellets after screening, the amount of screenings of class -6.3mm increases by 50-100%.

     Another application of polymers is as a binder in the production of pellets. Over a long period of time, various types of organic binders have been developed and tested in laboratory and industrial conditions, such as sodium carboxymethyl cellulose, modified starch, modified humic acid, polyacrylamide, lignin sulfonate, guar gum, molasses, and others. These organic substances are more water-soluble and cementable, which significantly improves the sphericity and low-temperature strength of pellets.

      The binding action of the organic binder is due to the action of adhesion, cohesion and diffusion forces. The organic binder absorbs water and forms a solution with a certain viscosity, which not only wraps or fills the iron ore particles, but also improves the interaction force between the polymer and the iron ore through the formation of hydrogen bonds, electrostatic attraction and the exchange of ligand (an atom, ion or molecule bound to another atom (acceptor) by donor-acceptor interaction) with the mineral surface through functional hydroxyl and carboxyl groups. At the same time, the organic binder improves the diffusion of the aqueous solution on the iron ore surface, so that the organic binder can fully adhere to the iron ore in an effective time and increase the granulation speed. However, organic binders are expensive materials and are difficult to mix with granulated raw materials due to their small amount in the batch. In addition, organic binders have low thermal stability and lose their purpose during drying and preheating,  which leads to low thermal shock temperatures, the formation of a large number of residual pores and a decrease in the strength of preheated and fired pellets.

     The main reason for the high-temperature rupture of pellets prepared with the addition of organic binder is that the organic polymer hinders the diffusion of the internal vapor of the pellet to the surface over time, resulting in excessive internal vapor pressure,
uneven surface evaporation and internal diffusion in the granules. The insufficient heat resistance of the pellets is mainly due to the following two reasons:

         a. organic polymers in the form of hydrocarbons are not stable to heating and easily volatilize at high temperatures,                            leaving a large number of  pores inside the pellets;​

        b. organic polymers do not contain inorganic components and almost do not form a liquid phase at high temperatures,                    which leads to the absence of consolidation of the solid phase between mineral particles;

​      To compensate for the negative factors introduced by organic polymers, it is necessary to add an inorganic binder, such as bentonite or borates, which promotes solid-phase binding of mineral particles and the formation of a sufficient amount of high-temperature liquid phase inside the pellet, which makes the fired pellets stronger. If inorganic or mineral binders additionally change the chemical composition of pellets and can affect the metallurgical properties of pellets, then organic binders do not change the chemical and mineralogical composition of pellets, since they burn out and evaporate during heating and firing. There are many well-known companies (BASF, AKZONOBEL, SNF, KEMIRA, CLARIANT, EAST-WEST-DRIDGE s.r.o.) that produce polymers (ALCOTAC, PERIDUR, FLOFORM, KEMPEL, FLOTICOR, FERROFORM) for pelletizing based on modified anionic polyacrylamides and various types of methylcellulose. However, the use of these reagents is limited by their high cost. High-quality polymers for pelletizing cost about 3-4 euros per 1 kg. There are also alternative binders based on starch, bitumen and cellulose.

     The purpose of using polymers is to reduce or eliminate the use of bentonite, as a material that introduces an additional amount of silicon oxide into the batch and reduces the iron content in the concentrate. The polymer consumption is 50-250grams/ton of concentrate when used together with bentonite and reaches 0.5-2kg/ton when replacing bentonite.

      Today, there is no universal polymer binder that would satisfy all iron ore pellet manufacturers. The selection and testing of a particular additive should be decided experimentally at a specific pelletizing plant, based on the requirements for the physical, chemical and metallurgical properties of the pellets.

       When choosing any inorganic or polymeric additive, the following factors must be taken into account:

​      a. mineralogical properties of iron ore concentrate;

      b. requirements for quality indicators of the finished product (iron content, individual components of gangue, strength                   and metallurgical properties);
      c. technological capabilities of a specific production;
     d. cost and manufacturability of the proposed material;