By controlling the univariate method, the effects of different composition ratios on the compressive strength and chloride ion binding ability of the cured body were explored experimentally, and the products were characterized by XRD.
The results show that when the water-cement ratio is 1.08, the compressive strength of the cured body is the highest. When the content of fly ash is greater than 0.25, the compressive strength of the solidified body is obviously improved. The higher the simulated high salt water ratio, the lower the compressive strength of the solidified body, and the influence of river sand content on the compressive strength of the solidified body is small.
After curing for 28 days, the compressive strength of the cured body prepared by the experiment is above 30MPa, which can meet the minimum compressive strength requirements of the concrete kerb standard. With the increase of cement ratio, the chloride ion binding capacity of solidified body increases by 265,438 0.7%, and the increasing trend is gradually slow due to the limitation of water required for cement hydration. As the hydration products of fly ash and chloride ions generate a small amount of S salt, with the increase of fly ash content, the binding amount of solidified body to chloride ions only increases by 4.9%. The XRD results confirmed the existence of S salt in the process of cement curing.
As the mainstream desulfurization technology in coal-fired power plants, limestone/gypsum wet desulfurization process has the advantages of high desulfurization efficiency, mature technology and stable operation. However, in order to prevent excessive enrichment of chloride ions and other elements in the circulating slurry system, the desulfurization system needs to discharge a certain amount of desulfurization wastewater regularly. Desulfurization wastewater has the following characteristics:
1) The water quality is influenced by many factors and easily changes with the change of working conditions and coal types;
2) The 2)pH value is between 4.5 and 6.5, showing weak acidity and high chloride ion content;
3) The content of suspended solids mainly consisting of gypsum particles, silica and iron-aluminum compounds is high;
4) The total soluble solids content is high, with a wide range of variation, generally ranging from 30,000 to 60,000 mg/L, and the hardness ions such as Ca2+ and Mg2+ are high;
5) Heavy metal pollutants such as mercury, lead and arsenic exceed the standard. Therefore, desulfurization wastewater treatment has attracted much attention from the industry.
With the release of "Water Pollution Prevention Action Plan" (also known as "Water Ten Articles") and "Technical Guide for Pollution Prevention in Thermal Power Plants", zero emission of desulfurization wastewater has become the top priority of environmental protection work in coal-fired power plants. At present, the commonly used treatment process is the traditional chemical precipitation method. After neutralization precipitation, precipitation, flocculation and concentration clarification, most of the suspended solids and heavy metal ions in desulfurization wastewater will be removed. This process can meet the Industrial Discharge Standard for Wastewater (DL/T997-2006), but it can't remove soluble salts such as chloride ions with strong mobility, and the removal effect of selenium ions is not good, so it can't realize the real zero discharge of desulfurization wastewater.
Zero emission technology, mainly evaporative crystallization and evaporation technology, is a research hotspot in the field of desulfurization wastewater treatment. Evaporative crystallization technology has complex process and high operating cost, and the mixed salt obtained by simple pretreatment is of no use value. The salt separation process can obtain high-purity crystalline salt, but it will further increase the operating cost. Low temperature flue evaporation and bypass flue evaporation technology increase the dust content in fly ash and transfer the treatment pressure to electrostatic precipitator. The high salt content of fly ash will affect the quality of cement.
This study involves a process of flue gas concentration and reduction of desulfurization wastewater and cement fixation. As shown in figure 1, a flue gas concentration tower with a liquid column nozzle system is arranged behind the electrostatic precipitator, and 10%- 15% of the hot flue gas after the electrostatic precipitator is used for circulating heat exchange with the desulfurization wastewater liquid column, so as to realize the reduction and concentration of desulfurization wastewater by 5- 10 times. The concentrated high-salt wastewater, cement, fly ash and other cementing materials enter the molding equipment after being stirred by a blender, and then are transferred to a curing room with constant temperature and humidity for curing. According to the performance, the cured body can be used as concrete or kerb.
Figure 1 Process Flow Chart of Flue Gas Concentration and Cement Solidification of Desulfurization Wastewater
The invention has the following beneficial effects:
1) make full use of the flue gas after electrostatic precipitator to contact with desulfurization wastewater for mass transfer and heat transfer, so as to achieve the effect of concentration and reduction of desulfurization wastewater, which is the full utilization of waste heat resources in power plants;
2) The liquid column nozzle system can reduce nozzle blockage caused by spray layer setting;
3) The moisture content of flue gas in front of desulfurization tower is increased, which greatly reduces the process make-up water of desulfurization system;
4) Cement fixes the salt and heavy metal ions in the desulfurization wastewater, and converts the flowing desulfurization wastewater into a solidified body with stable physicochemical properties and difficult dispersion, thus effectively avoiding secondary pollution;
5) Make full use of fly ash, a by-product of power plant.
Cement solidification technology has the advantages of simple process, easy availability of raw materials and stable performance of solidified body, and is widely used in the treatment of radioactive waste, wastewater contaminated by heavy metals and sludge. However, there is little research on solidification technology for desulfurization wastewater treatment, and the stabilization of solidification is mainly realized by pozzolanic reaction of fly ash. Considering the huge amount of desulfurization wastewater, there is little or no cement in the solidified body, and the compressive strength of the solidified body is poor, so it can only be landfilled. Renew and others studied the leaching performance of heavy metals after desulfurization wastewater concentrate and fly ash were solidified at the same time. Cement accounts for 10% of the total mixture, and the dosage is small, so the leaching rate of metal ions in the solidified body is low.
However, there is little research on chloride ion migration of solidified body after desulfurization wastewater is solidified and stabilized. In the concrete industry, the corrosion of steel bars caused by chloride ions is the main reason for the deterioration of the durability of reinforced concrete, and chloride ions mainly exist in three forms in cement-based materials:
1) chemically combines with C3A phase in cement to form friedel salt;
2) physical adsorption on hydration product C-S-H gel;
3) Free in pore solution.
Among them, chloride ions in the form of chemical binding and physical adsorption are collectively called bound chloride ions, and free chloride ions in pore solution are called free chloride ions. Free chloride ions can cause corrosion of steel bars, and the existing form of chloride ions in concrete can be evaluated by combining chloride ions. Therefore, considering the use of solidified body, the simulated high-salt solution was mixed with cement, fly ash and other materials to prepare solidified body, and the effects of different components of cement, fly ash and other materials on the compressive strength and chloride ion binding ability of solidified body were explored.
1 experimental part
1. 1 solidified cementitious material
Slag Portland cement (425 #); River sand for ordinary buildings; Fly ash, taken from a thermal power plant in North China; NaCl solution with Cl- concentration of 30000mg/L was prepared in the laboratory to simulate high salt solution. The concentration of Cl- in desulfurization wastewater from a power plant is 30,692 mg/L after thermal concentration.
1.2 experimental method
(1) Preparation of solidified cement: River sand and fly ash are mixed according to a certain proportion, and an appropriate amount of simulated high-salt water or desulfurization wastewater is added and stirred evenly, and then transferred to a 40mm×40mm×40mm six-cube model test. After standing for 24 hours, it is put into a saturated Ca(OH)2 solution for curing.
(2) Detection of compressive strength After the cured body is cured to the specified age, the compressive strength test is carried out. Constant stress and pressure testing machine (Hebei Changji Instrument Co., Ltd., DYE-300B) moves at a constant speed. When the solidified body reaches the maximum bearing capacity, the machine stops and the compressive strength is calculated by the maximum bearing capacity.
(3) Combined with the detection of chloride ion capacity, the solidified powder cured to the age of 28d was soaked in deionized water and nitric acid respectively, and the chloride ion concentration in the nitric acid solution was determined by Forhad method to obtain the total chloride ion amount Pt (mg/g) in the unit mass slurry; The free chloride ion content Pf(mg/g) per unit mass of slurry can be obtained by measuring the chloride ion concentration in aqueous solution by molar method. Combined chloride ion amount Pb= total chloride ion amount Pt- free chloride ion amount Pf. Chloride binding capacity:
2 experimental results and analysis
2. Effect of1component materials on compressive strength of cured body
Compressive strength is an important property of the cured body, and it is also an important index for the reuse of the cured body. In order to study the influence of each component material on the compressive strength of solidified body, cement, fly ash, high salt water and river sand were selected as solidified materials, and the cement content group, fly ash content group, high salt water content group and river sand content group were designed respectively. By changing the dosage of a single material, the influence of each material on the compressive strength of the cured body was explored. See table 1 for the mixture ratio of each solidified body.
Table 1 Mix proportion of each group of solids
After the cured body was cured to 7d, 14d and 28d, the compressive strength of the cured body was tested. Take three parallel samples as a group, and take the average value of each group as the compressive strength of the cured body at this age.
Effect of (1) cement content on compressive strength of solidified body
Fig. 2 shows the change trend of compressive strength of four groups of cured bodies at different ages when the water-cement ratio is 0.92, 1.00, 1.08 and1.7.
Fig. 2 Trend diagram of influence of cement content on compressive strength of solidified body
As can be seen from Figure 2, with the increase of cement content, the compressive strength values of solidified bodies 7d and 28d show a trend of increasing first and then decreasing, and they reach the maximum when the ratio is 1.08, but the overall change range of compressive strength of 7d is small, while the change range of compressive strength of 28d is large. The compressive strength of 14d solidified body has been increasing with the increase of cement content, but the upward trend is getting smaller and smaller, indicating that the increase of cement content has little effect on the early compressive strength of solidified body, but has great influence on the later compressive strength.
Combined with the general trend, when the water-cement ratio is low, the compressive strength of the cured body at three ages is very small, but when the water-cement ratio is too high, the compressive strength will be affected. This is because the increase of cement content means the decrease of water-cement ratio under a certain condition of high salt water content. When the high salt water content can meet the hydration requirements, the increased cement can be fully hydrated, the hydration products in the cement slurry increase, the pores in the slurry are less, and the gel volume increases, so the compressive strength is high. With the increase of cement content, when the high salt water content is not enough to provide the water needed for the full hydration of cement slurry, excessive cement will increase the unbound particles in the solidified body, increase the capillaries in the slurry and decrease the compressive strength. When the cement ratio is 1.08, the compressive strength of the cured body is the best.
(2) The influence of fly ash content on the compressive strength of solidified body.
Fig. 3 shows the changing trend of compressive strength of four groups of solidified bodies at different ages when the fly ash content is 0. 15, 0.20, 0.25 and 0.30.
As can be seen from Figure 3, the compressive strength of 7d solidified body first increases and then decreases with the increase of fly ash content, indicating that too high fly ash content will affect the early compressive strength of solidified body; The compressive strength of 14d and 28d solidified bodies is obviously improved only when the content of fly ash is more than 0.25, and the compressive strength does not change much when the content is low.
Fig. 3 Trend diagram of influence of fly ash content on compressive strength of solidified body
Too high content of fly ash will weaken the early compressive strength of solidified body and improve the later compressive strength. This is because the cement mixed with fly ash is superior in quantity and energy, so the hydration of cement clinker occurs first, releasing hydration products such as Ca(OH)2, which react with the active components SiO2 _ 2 and Al _ 2O _ 3 in fly ash.
However, the glass body in fly ash has a stable structure and a strong surface compactness, and its early reaction with volcanic ash of Ca(OH)2 is slow. Unreacted fly ash increases the pores in the slurry and reduces the strength of the solidified body. With the increase of curing age, the hydration of fly ash gradually plays a leading role, and the morphological effect, activity effect and micro-aggregate effect of fly ash interact. A large amount of hydrated calcium silicate gel will be generated on the surface of fly ash, which can be used as a part of cementing material to improve compressive strength.
(3) The influence of high salt content on the compressive strength of cured body.
Fig. 4 shows the change trend of compressive strength of four groups of solidified bodies at different ages when the proportion of high saline is 0.62, 0.67, 0.72 and 0.77.
Trend diagram of influence of high salt water content on compressive strength of solidified body.
As can be seen from Figure 4, the compressive strength of the cured body decreases with the increase of high salt water content at 7d, 14d and 28d, and the downward trend of compressive strength becomes more and more obvious at 14d and 28d. Under the condition of a certain amount of cement, the increase of high salt water will lead to too much water in the slurry, which exceeds the water required for full hydration of cement. Excess water will evaporate in the process of cement curing and hardening, leaving pores in the slurry, which will affect the compressive strength of the cured body. The greater the amount of water provided, the greater the amount of water that can be evaporated, and the more obvious the reduction of the compressive strength of the cured body.
(4) The influence of river sand content on the compressive strength of solidified body.
Fig. 5 shows the changing trend of compressive strength of four groups of solidified bodies at different ages when the river sand content ratio is 0.62, 0.67, 0.72 and 0.77.
As can be seen from Figure 5, the compressive strength of solidified bodies 7d, 14d and 28d does not change as a whole with the increase of river sand, but fluctuates around 2 1MPa, 30MPa and 36MPa respectively. Therefore, the increase of river sand content has little effect on the compressive strength of the solidified body, because river sand mainly plays the role of skeleton or filling in the slurry, and no obvious chemical reaction has taken place.
Fig. 5 Trend diagram of influence of river sand content on compressive strength of solidified body.
According to the compressive strength data of each group of solidified bodies in Figure 2- Figure 5, the compressive strength of solidified bodies at the age of 28d is mostly above 30MPa, which meets the minimum compressive strength requirements of kerbs in the Standard of Concrete Kerbs (JC/T899-20 16). Therefore, the solidified body prepared by cement solidification process can meet the requirements of compressive strength in the standard.
2.2 Influence of component materials on the ability of the cured body to bind chloride ions
The chloride ion binding capacity can directly reflect the chloride ion capacity of chemical reaction and physical adsorption in the solidified body, which is an important index to evaluate the corrosion of reinforced concrete reinforcement. In order to study the influence of composition materials on chloride ion binding capacity of solidified bodies, solidified bodies with cement content and fly ash content were selected in experiment 3. 1, and their chloride ion binding capacity at 28d age was determined.
Effect of (1) cement content on chloride ion binding capacity of solidified body
Fig. 6 shows the changing trend of chloride ion binding capacity of four groups of solidified bodies at 28d when the cement ratio is 0.92, 1.00, 1.08 and1.7.
Fig. 6 Trend diagram of influence of cement content on chloride ion binding capacity of solidified body (28d)
As can be seen from Figure 6, at the age of 28d, the ability of the solidified body to bind chloride ion is enhanced with the increase of cement ratio, but the enhancement amplitude is getting smaller and smaller, indicating that the influence of cement content on the ability of the solidified body to bind chloride ion is limited. The specific gravity of cement increased from 0.92 to 1.08, and the ability to bind chloride ions increased from 0.668 to 0.8 13, an increase of 2 1.7%. This is related to the hydration process of the solidified body. With the increase of cement content, hydration products increase, and the chemical binding and physical adsorption capacity of chloride ions are enhanced, so the binding capacity of chloride ions is enhanced. However, due to the limitation of hydration water, the promotion effect is limited when the cement content is too high.
(2) The influence of fly ash content on the ability of the solidified body to bind chloride ions.
Fig. 7 shows the changing trend of chloride ion binding capacity of four groups of solidified bodies at 28d age when the fly ash content is 0. 15, 0.20, 0.25 and 0.30.
As can be seen from the general trend in Figure 7, at the age of 28d, the chloride ion binding capacity of the solidified body increases with the increase of fly ash content, but the enhancement range is small. When the content of fly ash increased from 0. 15 to 0.30, the chloride ion binding capacity increased from 0.733 to 0.769, which only increased by 4.9%. This is because fly ash will generate a small amount of calcium aluminate hydrate in the alkaline environment formed in the process of cement hydration, which can react with chloride ions to generate Freddel salt, but the amount generated is very small.
Fig. 7 Trend diagram (28d) of influence of fly ash content on chloride ion binding capacity of solidified body.
2.3 XRD analysis of solidified bodies prepared from different water samples
The solidified body was prepared with simulated high brine and concentrated desulfurization wastewater, and the powder was analyzed by XRD after curing for 28 days. The result is shown in Figure 8.
According to the XRD diffraction pattern, in addition to the common hydration products of cement, SiO2 _ 2 and Ca (OH) _ 2, there are also friedel salts in the solidified bodies prepared by these two water samples, which proves that the chloride ions in simulated high brine and concentrated desulfurization wastewater do react with C3A in cement to generate friedel salts, indicating that the friedel salts generated during the cement solidification process play an important role.
Fig. 8 XRD patterns of solidified bodies prepared from different water samples.
3 Conclusion
(1) This paper presents a process of flue gas concentration and reduction and cement fixation of desulfurization wastewater. After the flue gas is concentrated, the desulfurization wastewater is mixed with cement, fly ash and other materials to make a solidified body to realize the cement fixation of pollutants;
(2) The compressive strength of the cured body increases with the increase of curing age. When the water-cement ratio is 1.08, the compressive strength reaches the highest value, and when the content of fly ash is greater than 0.25, the compressive strength is obviously improved. The higher the high salt water ratio, the lower the compressive strength, and the river sand content has little effect on the compressive strength of the solidified body.
(3) When the cement ratio increases from 0.92 to 1.08, the chloride ion binding capacity increases by 2 1.7%, and when the fly ash ratio increases from 0. 15 to 0.30, the chloride ion binding capacity only increases by 4.9%;
(4)XRD results confirmed the existence of friedel salt in the process of cement curing.
I believe that after the above introduction, everyone has a certain understanding of the basic experiment of solidification of high-salt desulfurization wastewater in coal-fired power plants. Welcome to Zhong Da for more information.
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