(1) Decomposition and sinter of crystallization water often contains some crystallization water, which is distributed in the ore and gangue components, and is decomposed in the sintering heating belt and the combustion zone.
Study of the microstructure of iron oxide hydrate is determined, which is truly independent of mineral water needling ore (Fe 2 O 3 · H 2 O), now all the other series only limonite hematite solid solutions of water and water-needle Mine.
For example , hydrohematite (Fe 2 O 3 · 0.1H 2 O) is a solid solution of water in hematite. Aqueous goethite (3Fe 2 O 3 · 4H 2 O), limonite (2Fe 2 O 3 · 3H 2 O), yellow goethite (Fe 2 O 3 · 2H 2 O) and ettringite (Fe 2 O) 3 · 3H 2 O) is a solid solution of water in water needles. The sub-water hematite is a mechanical mixture of very fine water hematite and water goethite. Water enters the solid solution lattice of hematite and water needle minerals in the form of neutral molecules (crystal water), which can be removed at a relatively low heating temperature (120-200 ° C) without changing the lattice form and Their parameters. The crystal water structure in the water needle ore crystal lattice exists in the form of hydroxide ion OH - when each H 2 atom is symmetrically distributed between the oxygen atoms to form an H 2 chain. In addition to the mineral water injection (a-FeO · OH), exist in nature while the γ-FeO · OH (lepidocrocite violet and mica), orthorhombic system, the lattice parameter which parameter mineral water needling completely different . It exists (OH) in the lepidocrocite structure - group, hydrogen chain region formed in parallel. The products of water-needle ore and fibrite after dewatering are triclinic a-Fe 2 O 3 (hematite) and cubic or tetrahedral γ-FeO · OH (magnetic hematite). The water process needs to completely destroy the original hydrogen oxide crystal lattice, so the temperature at which decomposition begins is high (260 ~ 328 ° C).
Water monoclinic aluminum ore gangue often laterite, including anionic and cationic Aι (OH) -, Aι cations occupy octahedral. When 285 ~ 340%, the gibbsite becomes the orthorhombic gibbsite, and its structure is similar to that of the laterite ore. Further dewatering (4gO ~ 550 ° C), the gibbsite is converted to γ-Aι 2 O 3 ( corundum with spinel cubic system). At higher temperatures (900 ~ 1200 ° C), γ-Aι 2 O over three polymorphic transformation to corundum a-, a- AιO · OH gibbsite and goethite orthorhombic crystal system similar, and having (OH) - group of hydroxide ions.
Crystal water is also present in chlorite and chlorite, and these silicates composed of mica belong to the chlorite type. The basic microstructure of mica is a flaky tetrahedral (SiO 4 ) structure in which a hydroxide ion group (OH) - and a cation Fe, Aι, Mg. smectite dewatering starts at 390-410 ° C, with a delay. It is completed at 900 to 1000 °C.
In the iron ore gangue, there is sometimes a kaolin -mica mineral group, which enters the hydroxide ion group (OH) - in the crystal lattice, and the kaolin dewatering starts to become metakaolin at 400-550 °C, and is completed at 900-1100 °C.
In silicates (mica), a stone and bentonite, and a molecule of water are present at the same time as the hydroxide ion group. Therefore, some water is easily removed at 50 to 155 °C. Further water removal requires a higher temperature and is heated to 500-575 ° C. Both the azurite and the scorodite dewatering are carried out at a lower temperature because the water is present in the crystal lattice in a neutral molecular water state.
From the above data, it is shown that the crystallization water in the sinter is decomposed violently in the drying and heating zone below 700 ° C, but some structural water inevitably falls into the combustion zone. From experiments showing almost identical sintering conditions, limonite and ore with chlorite gangue have lower sintering temperatures than ore of sintered hematite, magnetite and quartz gangue. When sintering limonite, this requires an increase in the consumption of solid fuel to 9-12%. When the ore block is large and the fuel is insufficient, part of the crystal water and its decomposition products are not melted by the liquid phase in the sintering zone. Still left in the sinter. It can be seen in the microstructure that this wrap will eventually destroy the strength of the sintered nugget from the center. [next]
(2) Carbonate decomposition The sintering mixture is often blended with limestone and dolomite. These carbonates must be decomposed and enter the liquid phase during the sintering process. Otherwise, the sintered ore has a raw material or white spots, which affects the quality of the sintered ore. .
(1) Thermodynamic analysis of carbonate decomposition reaction. The general reaction formula for carbonate decomposition is MeCO 3 =MeO+CO 2
At this time, the CO 2 partial pressure P co2 is generated. If the partial pressure of CO 2 in the atmosphere is P'co 2 , then when P co2 > P' co2 , the decomposition reaction can proceed. When P co2 = P' co2 , the reaction reaches equilibrium. When P co2 <P' co2 , the decomposition reaction cannot proceed.
The temperature at which the decomposition reaction starts is called the decomposition temperature. When the temperature rises and the decomposition pressure P co2 of the carbonate is equal to the atmospheric pressure P, the decomposition reaction is intense, called the chemical reaction boiling temperature.
Each carbonate has a certain decomposition temperature and partial pressure, for example:
This formula can be used to determine the decomposition temperature of CaCO 3 . Let the atmosphere contain CO 3 of 0.03%, then P' co2 = 0.0003 atmosphere (0.0003 × 98066.5 Pa)
The decomposition reaction of dolomite is carried out in two steps.
CaMgCO 3 =CaCO 3 +MgO+CO 2 (1)
CaCO 3 =CaO+CO 2 (2)
Through differential thermal analysis, the first stage decomposition temperature is 720 ° C, and the second stage is 906 ~ 910 ° C. Similar to the above thermodynamic calculation. [next]
When thermodynamic analysis is applied to the sintering process, the partial pressure of CO 2 in the exhaust gas of the process must be known. The thermodynamic calculation of CaCO 3 decomposition during sintering is as follows:
1) starts to decompose P 'co2 = 0.22 atm (0.22 × 98068.5 Pa, the exhaust gas CO 2, the decomposition of the material comprising upper CO 2, is estimated at 22%)
Corresponding to the time of the sintering process, the time during which the decomposition reaction of the carbonate may continue during the sintering process can be obtained, as shown in Fig. 1.
Under the sintering conditions, the calcium carbonate began to decompose at 817 ° C, and stopped decomposing at 634 ° C for a total of only 2 minutes. From the analysis of thermodynamic conditions, the particles of limestone should not be too large, otherwise the decomposition is not complete. Iron carbonate is easier to decompose than calcium carbonate, and begins to decompose at a lower temperature, but only 3 minutes and 20 seconds.
(2) Carbonate decomposition reaction rate. The decomposition reaction of carbonate is controlled by the mass transfer of boundary layer, interfacial chemical reaction and solid internal diffusion. Its decomposition rate formula is as follows:
Re=4Ï€r 2 e n e k e (C co2 -C' co2 )
Where R e — the rate of decomposition of limestone on a single particle, moles per minute;
r e ———the radius of the limestone, centimeter;
k e ———the overall reaction rate constant;
C co2 , C' co2 ———The concentration of CO 2 at equilibrium and the concentration of CO 2 in the gas phase, in gram/cm 3 .
Where R' e — the mass transfer coefficient of the boundary layer;
D s ———The diffusion coefficient of CO2 in the solid phase, cm 2 /min;
K′′ e ———reaction rate constant;
r eo ———the initial radius of limestone, cm;
K———Chemical reaction constant.
According to EK Powell's research, the comprehensive reaction rate constant k e of the carbonate decomposition reaction is determined only by the interface chemical reaction.
According to Arrhenius' law
It can be seen that the temperature determining the speed of the chemical reaction is the temperature, and increasing the temperature can accelerate the chemical reaction rate.
If the particles of limestone are too large, the rate of decomposition reaction will be determined by the rate of diffusion. At this time, measures that are conducive to increasing the diffusion speed will be effective.
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