Sintering mineralization mechanism---solid phase reaction theory

The sinter ore-forming mechanism includes the solid phase reaction, liquid phase formation and crystallization process of the sintering process. It affects the mineral composition and microstructure of the sinter, and has a very close relationship with the quality of the sinter. Studying the mechanism of sintering ore formation requires not only understanding the solid phase reaction, liquid phase formation and crystallization process, but also studying their formation laws in order to improve the quality of the sintered ore.
(1) Phenomena and types of solid phase reaction
Thousands of chemical reactions have been discovered in the chemical field, one of which is either liquid or gaseous. For example, the combustion of solid carbon is an example of a reaction between a solid and a gas, and hydrochloric acid acts on a limestone to generate CO ₂ gas, which is an example of the action of solid and liquid. Can a chemical reaction between the solid matter in the past have taken a negative attitude, until the 19th century when it was discovered that the presence of the solid phase diffusion phenomena, and there are metal replacement reaction between oxides, solid matter was confirmed in the particle motion is possible And the solid matter can also react directly.
There are several types of solid phase reactions which start at temperatures well below the melting point of the reactants or their eutectic point. The general form is as follows:
MeO+C→Me+CO↑ reduction reaction RO+R′→R′O+R displacement reaction RO+ R′ ₂ O ₃ →ROR′ ₂ O ₃ compound or co-melt mR ₂ O+nSiO ₂ →mR ₂ O• nSiO ₂ compound or co-melt R _m O _n +(m+n)C→mRC+nCO reduction reaction (2) solid phase reaction theory
Why does solid matter have such a reaction? This is because the real crystal is structurally defective, that is, there is a vacancy in the crystal lattice that should be occupied by the structural element (ion), or the structural element in the crystal lattice is normal from the crystal lattice. The position (node) moves to the lattice gap and a vacancy occurs. Defects or vacancies in the crystal are movable, and the movement of the vacancies reflects the reverse motion of the crystal structure elements.
The movement of the structural primitives can not be carried out under any circumstances. Only after the structural elements have achieved a certain energy (activation energy) can the surrounding mass be overcome and the corresponding movement can be performed.
The total number of structural primitives can be shifted, which mainly determines the temperature. The higher the temperature, the easier it is to obtain the activation energy necessary for displacement, and the more the number of particles displaced.
Since the particles in the crystal lattice are movable, once the activation energy necessary for the displacement is obtained, the effect of the surrounding particles is overcome, and the position exchange (ie, internal diffusion) can be performed inside the lattice, or can be diffused to the lattice. The surface is diffused into the crystal lattice of the adjacent other crystals in contact with it for chemical reaction. It is conceivable that the progress of this reaction does not involve the liquid phase. The reaction product must initially have a very high dispersibility. This highly dispersed crystalline powder has serious drawbacks with a large free surface energy and thus the material is in an activated state. It has a strong tendency to reduce its energy and exhibits a strong displacement effect. As a result, the crystal lattice is gradually corrected, and the tiny crystal powder electricity is aggregated into larger crystal particles, and the reaction product has a relatively complete crystal lattice, thereby becoming a relatively stable crystal with lower activity. [next]
The following is a schematic diagram of the solid phase reaction A+B→AB process, as shown in Figure 1.

1—The reactants are in contact with each other when mixed, and the contact becomes more tight as the temperature increases.
2—As the temperature increases, the mobility of the particles increases. In this stage, there may be adsorption-type reaction products, but the reaction products have serious defects and exhibit great activity.
3, 4—The formation of a layer of reaction product on the contact interface of the particle, one component has diffused into the interior of the lattice of another component, forming some stoichiometric compound AB. The reaction inside the lattice is often accompanied by particles. The porosity and activation of the surface, the dispersion of the reaction product is still very high at this stage. It can be considered that the crystal nucleus has formed and started to grow.
5—The crystal has grown into a crystal particle. The characteristic line of the reaction product can be seen from the X-ray diffraction pattern, and as the temperature increases, the intensity of the line becomes larger and larger.
6—Because the crystal formed has structural defects, it has a tendency to correct the defect to a thermodynamically stable state. Therefore, the temperature continues to rise, which will lead to the elimination of defects, so that the reaction product has a normal lattice structure. [next]
(III) Characteristics and reaction mechanism of solid phase reaction The characteristics of solid phase reaction are:
1) Direct reactions can be carried out between the solid phases, and the temperature of the reaction is much lower than the melting point of the reactants or their eutectic point.
2) There is a certain regularity between the temperature at which the solid phase reaction begins and its melting point. For the metal is (0.3 ~ 0.4) × T _melting , for the salt is 0.57T _melting , for the silicate and organic matter is (0.8 ~ 0.9) T _melting . The so-called solid phase reaction begins temperature means that one of the reactants begins to appear A temperature that is significantly displaced.
In recent years, further research on the mechanism of solid phase reaction has concluded that only the interaction between solid phases cannot resolve the extremely fast reaction rate in many solid phase reactions. Therefore, some people think that there must be a non-solid phase reaction in the solid phase reaction throughout the whole process. They think that the mechanism of the solid phase reaction process should be expressed as follows:
A _solid → A _gas , A _gas + B _solid → AB _solid
Or A _solid + X _solid → (AX) _liquid , (AX) _liquid + B _solid → AB _solid + X _solid
They believe that the non-solid phase of the solid phase reaction, the liquid phase and the gas phase, plays an important and even decisive role. The solid phase material should be regarded as a dynamic equilibrium state (such as an oxide and an oxoacid salt) which is combined with its gas phase to produce a gas phase which is in equilibrium with the solid phase during heating. Although this phase is small, the role in the reaction is small. It cannot be underestimated. Another example is the presence of some impurities in the solid phase, which may form a liquid phase whose starting temperature is much lower than the eutectic temperature of the main solid phase material. These small liquid phases also play a significant role in the reaction. According to the theory of solid phase reaction, the diffusion kinetic equation can correctly reflect the actual reaction process.
From the above research on the mechanism of solid phase reaction, it can be seen that in addition to factors such as temperature, pressure, and holding time, any factor that promotes the diffusion and internal diffusion can promote the reaction between the solid phases. For example, chemical reactions such as fine pulverization, polycrystalline transformation, dehydration, and decomposition of the reactants, formation of the solid solution and activation of the lattice of the reactants generally accelerate the reaction between the solid materials.
The use of mineralizers is an effective measure to promote solid phase reactions. Because of the use of a mineralizer, it forms a solid solution with one of the reactants or reactants, activates the crystal lattice of the reactants, or promotes the production of a liquid phase in the reactant system, thereby accelerating the diffusion. It should be noted, however, that as the reaction progresses, the role of the mineralizer may gradually diminish or even adversely affect.
(4) Solid-phase reaction in the sintering process In the sintering process, the exhaust gas generated by the combustion of the solid fuel heats the sintering material, which creates favorable conditions for the solid phase reaction. The position of each particle in the feed is not changed until the sintered material is partially or completely melted. Thus each particle reacts only with the particles it directly surrounds and contacts. Figure 2 is a schematic diagram of the solid phase reaction between the compositions of the sinter.

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It has been confirmed that only an exothermic chemical reaction can be carried out in the solid phase, and the initial product of the reaction between the two substances can only form the same compound, and its composition often does not correspond to the reactant concentration ratio. The band structure in the contact area between the two substances is not formed immediately from the beginning, but can occur after long-term heat preservation. The final product that is compatible with the reactant components takes a long time in most cases. In the process of exhaust sintering, heating the sintered material from 500 ° C to 1500 ° C is generally achieved in a short period of time of not more than 3 minutes. Therefore, more practical for sintering is the experimental data on the beginning of the solid phase reaction - the temperature at which the reaction begins and the product initially formed.

Figure 3 is a graph of the reaction of CaO with SiO _{2 in a} solid phase. Although there is excess SiO ₂ , in the case of 1000 ° C in the atmosphere, the first reaction product in the contact zone is calcium orthosilicate (2CaO•SiO ₂ ), and 2CaO•SiO ₂ contacts CaO to form 3CaO•SiO. ₂ layers, and 3CaO•2SiO ₂ is formed along the contact of 2CaO•SiO ₂ with SiO ₂ , wollastonite (CaO•SiO ₂ ) appears only in the final stage of the process.
In the original composition, the mass ratio of CaO to SiO: is 1:1, and the reaction product appearing in the first 4 hours in the solid phase reaction is still only calcium orthosilicate (2CaO•SiO ₂ ), only after a long period of heat preservation, according to There are different contact bands for the diffusion capacity of the reactants and the ratio of the concentrations. A similar relationship occurs in the solid phase reaction in a mixture of CaO:SiO ₂ =3:1.
There is no satisfactory explanation for the order in which the reaction products appear in the solid phase reaction. It appears that the first structural compound in the contact zone has the simplest character structure. In CaO and SiO ₂ , the first reaction product of MgO and SiO ₂ reaction towel is only orthosilicate, its structure (independent SiO ⁴ ₄ tetrahedron) is simpler than metasilicate structure (chain bonded tetrahedron). many. It is apparent that the free energy of the contact band forming compound plays a certain role. [next]
The first reaction product is not determined by the ratio of the reactants. For example, the first reaction product in the solid phase reaction of CaO and SiO ₂ is almost 2CaO•SiO ₂ , while 2CaO+Fe ₂ O ₃ and CaO+Fe ₂ O The solid phase reaction of ₃ is only CaO•Fe ₂ O ₃ .
As mentioned above, the mechanism of solid phase chemical reactions is the diffusion of one or the other ions in the character. The ions that fluctuate at the lattice nodes do not have the diffusion of ions until they get a certain freedom to overcome the binding force in the lattice. This effect is only possible for certain substances at only a certain temperature level. This indicates that the temperature has a greater influence on the occurrence of the reaction product than the time at which the reactant interacts. Therefore, it is important to understand the onset temperature of the chemical interaction between the solid phases, and experimental data on the onset temperature of the reaction product appear in the solid phase, and most of these data are obtained from microscopic measurement of the reaction zone material and X-ray diffraction.
(1) Fe ₂ O ₃ cannot form a compound with SiO ₂ at 575 ° C. This system forms only a solid solution in which Fe ₂ O _{3 is} dissolved in SiO ₂ . Weak staining of the first surface of the quartz, and 950 deg.] C to the reaction product so called "rose quartz". Therefore, when the carbon with a small amount of non-self-fluxing sintered frit hematite, Fe ₂ O ₃ and SiO ₂ does not interact. To produce fayalite (2Fe ₂ O ₃ •SiO ₂ ), it is necessary to previously reduce Fe ₂ O ₃ or decompose Fe ₂ O ₃ into Fe ₃ O ₄ . At this time, it is necessary to have a higher carbon content.
(2) At the contact between quartz and limestone, calcium silicate starts to form at 500 ° C ~ 600 ° C, but the chance of such contact is not large in non-self-fluxing sinter. CaO can replace solid phase silicic acid MgO and MnO in the salt can partially displace MgO from its solid phase compound even before the decomposition of MgCO ₃ begins.
(3) So far, there are few experimental data on the starting temperature of the solid phase reaction to form fayalite. Some people use magnetite and quartz sand to produce a small amount of fayalite in hot air (990 ° C). Some people use a mixture of magnetite, ferrous oxide and quartz to obtain fayalite, which starts to form at a temperature of 1150 ° C. In the blast furnace, fayalite generally appears before 800 ° C ~ 850 ° C. According to the general rule, the temperature at which the silicate material is formed in the solid phase is 0.8-0.9T, and the temperature at which it begins to form should be 910 ° C to 1057 ° C. The reaction of forming the olivine in a reducing atmosphere is as follows .
In the sintering process with a higher carbon content, SiO ₂ can directly form the olivine with the floating body. However, in general, this reaction hardly occurs because of the lack of free iron oxide in the frit.
(4) Calcium ferrite begins to form in the solid phase at 500 ° C ~ 700 ° C. Under the sintering conditions, the contact between limestone, lime and hematite is good, which promotes the reaction. Calcium ferrite can also be produced by direct interaction of iron oxide with calcium carbonate.
When the conditions for the oxidation of Fe ₃ O ₄ to Fe ₂ O ₃ are not provided, the magnetite cannot react with CaO. The researchers showed that the addition of CaO to the magnetite sinter in a neutral atmosphere resulted only in the formation of calcium silicate. In contrast, in an oxidizing atmosphere, that is, Fe ₃ O _{4 is} oxidized to Fe ₂ O ₃ , the reaction of forming ferrite in the same sintered mixture proceeds rapidly. Therefore, calcium ferrite can be formed only in the solid phase when the normal carbon-sintered hematite self-fluxing sinter and the low-carbon sintered magnetite self-fluxing sinter are formed. In the latter case, the magnetite is violently oxidized to hematite, and these secondary hematites react with lime to form calcium ferrite.
In the case of low carbon content, only a small portion of the material melts during the sintering process, and the product of the solid phase reaction is later transferred to the composition of the sintered ore. At this time, although the alkalinity of the material is not large, ferric acid can be obtained. Sinter of calcium. In particular, the formation of calcium ferrite in the solid phase reaction is promoted under conditions of low carbon content. Under the condition of low carbon and high alkalinity, it creates favorable conditions for obtaining the sinter of calcium ferrite, because at this time, not only the calcium ferrite formed in the solid phase is transferred into the sintered sinter, but also The liquid phase forming the ferrite is crystallized as a single phase.
(5) Effect of solid phase reaction The solid phase reaction can form a new material that is not meltable in the raw material. These new materials produce many liquid phases during the sintering process, and finally obtain a high strength sintered ore. In the case of sintering concentrates with extremely slow solid-state chlorite-containing chlorite and chlorite gangue gangues (they have a combustion zone with water), improving the solid-phase reaction process can affect the quality of the liquid phase and sintering to some extent. The quality of the mine. Therefore, it has been studied to use a prefabricated ferrite mixture to be added to the sintering material to create favorable conditions for the formation of calcium ferrite in the solid phase, to improve the melting of the sintering material, increase the amount of liquid and the quality of the sintered ore.
It should be noted that the composition of the sintered sinter binder phase is not determined by the nature of the feedstock or the nature of the addenda in the sinter, but is determined by the alkalinity of the sinter. When the normal carbon content and alkalinity CaO:SiO ₂ >1～1.5, the calcium iron sapphire powder is added to the sintering material to increase the binder phase of the calcium ferrite, and the quality of the sintered ore is improved. When the alkalinity is less than 1, the ferric acid mixture is composed of a binder phase composed of calcium fayalite to improve the quality of the sintered ore. The ferric acid mixture and other addenda are present in the sinter in their original form only under low carbon loading conditions.
Some foreign factories use industrial ferric acid additive crushed by 60% returning or 40% limestone by hammer crusher , adding 15% in the sintering material, increasing the sintering speed by 10-12%, increasing the strength of the sintered ore. 15 to 20%. The preparation of the ferrite additive does not require a lot of cost, and can be used as an effective measure for improving the productivity of the sintering machine and the quality of the sintered ore.

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