3. 1 Morphological distribution
3. 1. 1 The raw material
Solid samples mainly include raw materials, fuels, mixtures, clinker and cement, specifically limestone, slag, construction waste, phosphogypsum, pulverized coal, clinker and cement. The level of mercury in all solid samples is shown in Fig. 2.
Figure 2 shows that the mercury level of limestone in raw materials is 0.275mg/kg, and the contribution value of mercury is higher than that of slag (0.214mg/kg) and construction waste (0.073mg/kg). The mercury level of phosphogypsum in mixtures is 0.361mg/kg, which is also higher than that of limestone. However, it was lower than the total contribution of mercury from the three raw materials (0.562mg/kg). The mercury level of cement was 0.424mg/kg, which was 0.162mg/kg higher than that of clinker (0.262mg/kg). The reason was that phosphogypsum needed to be added when firing cement to ensure the quality of cement. The experimental results were consistent with the theory. Mercury input in cement production is 1.121mg/kg (limestone + slag + construction waste + phosphogypsum + pulverized coal), which is higher than mercury output in cement of 0.424mg/kg, because mercury output also exists in atmospheric emissions.
As shown in Fig. 3, the input sources of mercury in X cement plant are phosphogypsum and limestone, accounting for 33% and 25% of the total input of mercury respectively, while slag, pulverized coal and construction waste account for 19%, 18% and 5%, respectively. According to the principle of mass conservation, mercury output in cement products accounts for 37% of the total output.
The theoretical analysis results were compared with the sampling results of raw materials in X cement plant, as shown in Fig. 4;
The comparison shows that the actual measured value and the theoretical value have the same change trend.
There is a slight deviation between the theoretical analysis results and the actual detection results of the proportion of solid samples such as raw material and clinker. The reason is that FactSage's theoretical calculation only exists in the ideal conditions, and the actual operation of cement production has a certain deviation with it, but the overall trend is the same. Therefore, it is considered that the thermodynamic software has certain accuracy in theoretical analysis of other production links in cement production process.
3.1.2 Raw meal
The raw materials collected by the mixing station are directly transported to the vertical mill for grinding and drying through a conveyor belt. The main components of raw materials are SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O and SO3. The exhaust gas of 210°C released from the waste heat boiler at the end of the kiln is used as the heat source of drying. The exhaust gas from the mill is purified by the bag filter, and part of it is returned to the mill as circulating air, while the other exhaust gas is directly discharged into the atmosphere. Therefore, in the raw grinding process, the temperature drops from 220℃ to 70℃ and the pressure is 109.223Kpa. The material situation of this section is shown in the figure below(Fig. 5. Figure 6. Figure 7. ):
Therefore, at 160 ~ 70℃, the solid phase composition of the section mainly consists of Ca(NO3)2, Ca2Fe2O4, Ca2Al2SiO7, Ca2SiO4 and Fe2O3, among which Ca(NO3)2 accounts for about 60% of the total. Ca(NO3)2, Ca2SiO4 and Fe2O3 gradually decreased, while Ca2Fe2O4 and Ca2Al2SiO7 gradually increased, and Ca2Fe2O5 was formed at 200℃. The gas is mainly N2 and O2, but there are still NOx and gaseous mercury, and gaseous mercury accounts for less than 0.1% of the total volume. When < 200℃, Hg is mainly Hgp, accounting for about 95% of Hgt. When > 200℃, Hg transforms into gaseous form (Hgp, Hg0, Hg2+, Hg+), where the level of mercury is low.
3.1.3 Rotary kiln tail
Raw material enters the kiln tail and is heated in the preheater before being burned in the rotary kiln. The temperature of the kiln tail increases from 900 to 1020°C with a pressure of 100.893Kpa. In this process, the various substances continue to react to form a variety of mineral aggregates. The material changes of this section are shown in the figure below (Fig. 8. Figure 9. Figure 10. ):
It can be concluded that at 900 ~ 1020℃, the calcium substances are mainly Ca2SiO4 (about 35%) and Ca2Fe2O5 (about 28%). N2 and O2 are the main gases, but NOx, SO2 and gaseous mercury still exist, and gaseous mercury accounts for less than 0.1% of the total volume. In this section, Hg is all transformed into gaseous forms (Hg0, Hg2+ and Hg+). Hg0 is the main existing form (accounting for 97.09%~98.09% of Hgt), showing a stable trend at first and then increasing, Hg2+ (accounting for 2.40% ~2.91% of Hgt) showing a stable trend, and Hg+ trace.
3.1.4 Rotary kiln head
The temperature of the rotary kiln head rises from 1020℃ to 1200℃, and the pressure is 101.294Kpa. In this process, the various substances continue to react to form a variety of mineral aggregates. The material changes of this section are shown in the figure below(Fig. 11. Figure 12. Figure 13.):
It can be seen from the figure that at 1020 ~ 1200℃, calcium substances are mainly Ca2SiO4 (about 35.25%), Ca2Fe2O5 (about 27.64%), Ca3Al2O6 (about 27.64%) and CaAl2O4 (about 15.27%). N2 and O2 are the main gases, but NOx, O3 and gaseous mercury still exist, and gaseous mercury accounts for less than 0.1% of the total volume. In this section, Hg is all transformed into gaseous form (Hg0 and Hg2+). Hg0 is the main existing form (accounting for 98% of Hgt), showing a stable trend at first and then increasing, Hg2+ (accounting for 2.40% ~2.91% of Hgt) showing a stable trend, Hg2+ trace.
3.1.5 The coal mill
The temperature of the coal mill is 400 ~ 40℃ and the pressure is 101.925Kpa. The material changes in the section are shown in the figure below:
As can be seen from the Fig. 14 to Fig. 16, at 400 − 200 ℃, the solid phase components of the section are mainly Ca2SiO4 (about 34.62%) and Ca2Fe2O5 (about 27.15%). Hg is dominated by Hg0 and supplemented by Hg2+, and Hg0 shows a decreasing trend, while Hg2+ is opposite. As the temperature decreases, Ca(NO3)2 (about 80%) is generated when the temperature is less than 200℃, and Hgp is the main Hg substance, accounting for about 95% of Hgt.
3.1.6 Grate cooler
After the high temperature clinker leaves the kiln head, it directly enters the grate cooler for cooling and crushing. The temperature drops from 1200 ℃ to 20 ℃, and the pressure is 8101.325 Kpa.
As can be seen from the Fig. 17 to Fig. 19, at 1200 ~ 800℃, the solid products are mainly calcium and chlorite compound, among which calcium is Ca2SiO4 (about 35%) and Ca2Fe2O5 (about 28%), and chlorite compound is KAlO2 (about 4%). When the temperature is in the range of 800 ~ 400℃, the chloride mineral complex is converted into KAlO2 to Na2Ca3Al16O28 (about 0.4%), and the solid product is dominated by calcium (consistent with 1200 ~ 800℃; When the temperature is lower than 400℃, Ca(NO3)2 (74%) and Fe2O3 (15%) are the main solid products, while Hg (2.5%) is formed as solid particles. The gas is mainly N2 and O2, but there are still NOx and gaseous mercury, and gaseous mercury accounts for less than 0.1% of the total volume. In this section, Hg exists in the form of Hgp, Hg0, and Hg2+, and is transformed into each other (1200 ~ 400℃, mainly in the form of Hg0 and Hg2+, gaseous elemental mercury presents a trend of gradual decrease/from 90–60% of the total mercury, while Hg2+ is opposite to zero valent mercury. Gradually increasing/from 10–40% of total mercury; At 400 ~ 300℃, the levels of Hg0 and Hg2+ were the lowest. When the temperature is below 300℃, it mainly exists as Hgp.
3.1.7 Cement mill
The temperature of this section is 30 ~ 80℃, and the pressure is 102.131 Kpa. The material changes in this section are shown in Fig. 20.
According to the Fig. 20 to Fig. 22, the main substances in cement mill are Ca(NO3)2 (about 63.79%), Fe2O3 (about 12.83%) and Ca2SiO4 (about 12.03%), and the main components in clinker are CaO, SiO2, Al2O3 and Fe2O3. These substances are mainly two or more kinds of oxides react at high temperature to form different mineral groups, the main composition of 3CaO.SiO2, 2CaO.Sio2, 3CaO.Al2O3, 4CaO.Al2O3.Fe2O3; N2 and O2 are the main gases, but gaseous mercury still exists, and gaseous mercury accounts for less than 0.1% of the total volume. In this section, the existence form of Hg is Hgp.
3.2 Migration and transformation of mercury
The morphological distribution results of mercury in each section showed that ①the cement plant (new dry cement production process; The temperature range is 20 ~ 1200℃; The composition of cement products is mainly CaO, SiO2, Al2O3 and Fe2O3, these substances are mainly two or more kinds of oxides react at high temperature to form different mineral groups, The main compositions are 3Ca.SiO2, 2Ca.SiO2, 3Ca.Al2O3, 4Ca.Al2O3.Fe2O3; ② The input of Hg comes from raw materials (limestone), raw fuel (pulverized coal) and mixed materials (phosphogypsum), of which limestone and pulverized coal are the main sources. ③In the engineering reaction range of 70℃~1200℃~20℃, Hg does not participate in the reaction of major substances, but only reacts with oxygen and exists in gaseous and granular state during the reaction process. In different temperature ranges, the form andlevel of Mercury are different.
Further study on the five stages of Hg in cement raw material transformed into clinker, obtained the different stages of Hg and oxygen reaction.
Table 1
Forms of mercury in different production sections
The production process
|
T/℃
|
Mercury exists in a phase
system
|
Mercury distribution in the Equilib module
|
Raw meal grinding
|
220 ~ 70
|
1
|
HgO(s)
|
Hgp
|
Five stage cyclone preheater
|
300 ~ 800
|
3
|
HgO(s);HgO(s) + gas_ideal;gas_ideal
|
Hg0、Hg2+、Hg+、Hgp
|
Decomposition furnace
|
800 ~ 900
|
1
|
gas_ideal
|
Hg0、Hg2+、Hg+
|
Rotary kiln tail
|
900 ~ 1020
|
1
|
gas_ideal
|
Hg0、Hg2+、Hg+
|
Rotary kiln tail dust collector
|
120 ~ 150
|
1
|
HgO(s)
|
Hgp
|
Rotary kiln head
|
1020 ~ 1200
|
1
|
gas_ideal
|
Hg0、Hg2+、Hg+
|
Rotary kiln head dust collector
|
80 ~ 90
|
1
|
HgO(s)
|
Hgp
|
Grate cooler
|
1200 ~ 20
|
2
|
gas_ideal;HgO(s)
|
Hg0、Hg2+、Hg+、Hgp
|
Cement mill
|
30 ~ 80
|
1
|
HgO(s)
|
Hgp
|
The coal mill
|
400 ~ 40
|
3
|
HgO(s);HgO(s) + gas_ideal;gas_ideal
|
Hg0、Hg2+、Hg+、Hgp
|
It can be seen from Table 1 that the mercury phase system in raw mill is HgO (s), and the mercury phase system in five-stage cyclone preheater is HgO (s), HgO (s) + gas_ideal, gas_ideal. The mercury phase system in the calciner is gas_ideal. The mercury phase system in the tail of rotary kiln is gas_ideal. The mercury phase system in the dust collector of rotary kiln tail is HgO (s). The mercury phase system in the rotary kiln head is gas_Ideal. Mercury phase system in the dust collector of rotary kiln head is HgO (s). Mercury phase system in cement mill is HgO (s). The mercury phase systems in grate cooler are gas_ideal and HgO (s). Mercury phase system in coal mill is HgO (s). HgO (s) + gas_ideal; Gas_ideal.
The reaction of mercury in the section is relatively complex. According to the system existing in each stage, a thermodynamic analysis is conducted on the possible reactions in each stage. If extremely few reactions are not considered, the main reactions are as follows in general:
In the process of cement production, entropy and enthalpy changes in different temperature ranges are shown in the Fig. 23 to Fig. 24.
It can be seen from the Fig. 23 to Fig. 24 that reactions ①, ② and ④ are exothermic reactions (∆H < 0). When the temperature is less than 200℃, the reaction is favorable to proceed. When the temperature is higher than 200℃, ∆G > 0, the reaction is inhibited and difficult to proceed spontaneously. Reaction③, reaction⑥ is endothermic reaction (∆H > 0), when the temperature is less than 200℃, the reaction can not spontaneously (∆G > 0), when the temperature is more than 200℃, ∆G < 0, is conducive to the reaction. Reaction⑤ is endothermic reaction (∆H > 0), but at 200–1200 ℃, ∆ G > 0, the reaction can not occur. The reaction ⑦ is endothermic at < 100℃ and exothermic at > 100℃, but it is difficult to spontaneously react at 200–1200 ℃ (∆G > 0).
Therefore, when the temperature is less than 200℃, the main reactions in the system are reaction ①, reaction②, and reaction ④.When the temperature is higher than 200℃, the main reactions in the system are reaction③ and reaction ⑥.The main reactions in raw material mill, cement mill, rotary kiln tail dust collector, rotary kiln head dust collector are reaction ①, reaction ②, reaction④.The main reaction in five-stage cyclone preheater, calciner, rotary kiln tail and kiln head, and coal mill is reaction ③, reaction ⑥.The grate cooler reaction temperature span is large, and all the five reactions exist in the system.
Therefore, it can be concluded that Hg does not participate in the reaction of major substances in the engineering reaction range of 70 ~ 1200 ~ 20℃, and mainly exists in the form of gas phase and transforms between Hg0-Hg+-Hg2-Hgp. The transfer and transformation process of mercury in cement production was obtained.