1 Overview
The actual process of solidification of cement is very complicated and there is no theoretical thorough explanation at present. It is generally believed that the mechanism of action is through the adsorption of toxic substances by the powdered calcium silicate hydrated colloid in the cement, and the hydrates in the cement can form solid solutions with the harmful substances, thereby binding them in cement hardened tissues. Ordinary portland cement has good curing toxic properties. In China, experimental studies have been conducted on the solidification of heavy metal solid wastes with cement, but no application studies have been reported. This study used this method to treat wastewater treatment sludge from circuit board factories. Preliminary experiments were conducted on the curing conditions, the ratio of solidified materials, the leaching properties of solidified blocks, and compressive strength.
2 experimental section
211 experimental materials
The sludge used in the test was taken from the sludge cake of lime-coagulative sedimentation and dehydration in a wastewater treatment plant of a circuit board factory in Guangzhou Huangpu Economic Development Zone. The water content was 50-55. The sludge was dried and weighed, Crush, after 200 mesh sieve back.
212 experimental methods
Sludge, cement, river sand, additives, and water are uniformly mixed in a container at a certain ratio, and injected into a stable shape, and the container is solidified to obtain a solidified sample. Remove the measurement of its specific gravity, compressive strength and metal ion leaching rate.
The leaching experiment was carried out according to GB5086-85. The solidified blocks were crushed (in the tea field by ultrafine pulverizer) (<5mm), weighed and placed in a beaker. Water was added until the solidified block was completely immersed, and a small amount of atoms for analysis was taken until a certain time. Absorption spectrophotometer analysis compressive strength of Cu, Sn, Au, Fe. Cure block according to GB 177-85 "Cement mortar strength test method", pH value measured with pHC-3S type acidity.
3 Results and Discussion
Curing characteristics of 311425 cement
In order to obtain a uniform solidified body, it is necessary to select a cement mixture dosing scheme with good fluidity, proper setting time, and appropriate amount of water to be separated. The flow value of concrete that can be mixed with the mixer should generally be between 200-300mm, the initial setting time should be more than 1h, the disappearing time of the segregation water should be within 6h [4]. We tested the mixing experiments with different ratios of cement cured bodies.
The solidification treatment effect of cement is good, in which the leaching concentration of Cu2 is much lower than the maximum allowable concentration of leaching specified in the “Control Standards for Pollution of Solid Wastes in Nonferrous Metal Industryâ€.
The leaching concentrations of Sn and Au are also lower than 011 mg/I. This may be because the main chemical reactions in the coagulation and hardening process of concrete are cement compounds, and because the micropores of the solidified blocks are not connected to the capillary, the heavy metals in the sludge It is sealed in the hardened block, preventing the internal dissolution of heavy metals. The elution occurs only on the surface of the solidified block. Therefore, the amount of elution is very low.
Effect of pH 312
As the final treatment of solidified blocks is naturally piled up or used as a roadbed material, it will inevitably be leached by natural rainfall. In recent years, due to industrial pollution, acid rain has been serious. In southern China, precipitation with pH<5 is common and sometimes even below 310. , and pH is one of the important factors that affect the dissolution of metal ions. In order to determine the law of leaching of solidified blocks at different pH, we used water of different pH for experiments.
With the decrease of pH, the leaching concentration of Cu2 increased. When the low pH increased more than doubled, the change at high pH was not obvious; Sn increased at both high pH and low pH, apparently due to the dual nature of Sn. The leaching change of Au is small. The effect of the whole pH value is that the concentration of leaching increases in high or low pH water. This is related not only to the nature of the metal ion itself, but also to the acid itself and the corrosion of the cement itself, but the overall effect is not significant.
313 Effect of sludge/cement ratio
Cement is the basic material for curing. The amount of cement is directly related to the curing effect of metal ions, the strength of curing blocks and the cost of curing. We performed a Cu2 leaching experiment on different sludge/cement ratios.
Therefore, considering comprehensively, if the compressive strength of the curing block is not affected, the proportion of the cement used should be selected as low as possible.
Effect of 314 Additive Thiourea
The addition of thiourea is mainly to investigate the effect of solidification of Cu2 in the solidified block. The author believes that thiourea may play two roles in the solidification process: (1) promote the role of cement to promote the solidification of the cement, curing the formation of a mesh block The cross-linked three-dimensional structure acts as a cure for harmful components. (2) A vulcanizing agent that reacts with Cu2 to form a stable sulfide, thereby preventing the dissolution of Cu2. Obviously, the addition of thiourea to the formulation will increase the solidification capacity of the block, and the leaching of Cu2 will decrease as the amount of thiourea addition increases.
315 curing block compressive strength
Because the solidified block formed after the solidification of sludge must be naturally stacked or used as a roadbed material, the solidified block must have a certain compressive strength. We tested the compressive strength of cured blocks of different proportions. As the content of cement increases, the compressive strength increases.
The cement content has a greater influence on the compressive strength, and the compressive strength decreases as the cement content increases. The addition of Na2SiO3 as an additive to the formulation significantly improves the compressive strength of the cured block. In this study, the main component of cement is silicate (Ca, Al, Mg, Fe, etc.), and Na2SiO3 as an additive can accelerate the hydration reaction of cement components, promote cross-linked solidification of cement, thereby increasing the strength of cured blocks and reducing harmful substances. Dissolution.
4 Conclusion
The lime-neutralized and coagulated sludge was solidified with cement, and the effect was good. The leaching rate of Cu was much lower than the allowable leaching concentration. The experimental results show that the best ratio of solidified sludge can overcome secondary pollution caused by sludge. Cement curing material is easy to get, curing is simple and practical. Inadequacies, its processing costs are relatively high.
The actual process of solidification of cement is very complicated and there is no theoretical thorough explanation at present. It is generally believed that the mechanism of action is through the adsorption of toxic substances by the powdered calcium silicate hydrated colloid in the cement, and the hydrates in the cement can form solid solutions with the harmful substances, thereby binding them in cement hardened tissues. Ordinary portland cement has good curing toxic properties. In China, experimental studies have been conducted on the solidification of heavy metal solid wastes with cement, but no application studies have been reported. This study used this method to treat wastewater treatment sludge from circuit board factories. Preliminary experiments were conducted on the curing conditions, the ratio of solidified materials, the leaching properties of solidified blocks, and compressive strength.
2 experimental section
211 experimental materials
The sludge used in the test was taken from the sludge cake of lime-coagulative sedimentation and dehydration in a wastewater treatment plant of a circuit board factory in Guangzhou Huangpu Economic Development Zone. The water content was 50-55. The sludge was dried and weighed, Crush, after 200 mesh sieve back.
212 experimental methods
Sludge, cement, river sand, additives, and water are uniformly mixed in a container at a certain ratio, and injected into a stable shape, and the container is solidified to obtain a solidified sample. Remove the measurement of its specific gravity, compressive strength and metal ion leaching rate.
The leaching experiment was carried out according to GB5086-85. The solidified blocks were crushed (in the tea field by ultrafine pulverizer) (<5mm), weighed and placed in a beaker. Water was added until the solidified block was completely immersed, and a small amount of atoms for analysis was taken until a certain time. Absorption spectrophotometer analysis compressive strength of Cu, Sn, Au, Fe. Cure block according to GB 177-85 "Cement mortar strength test method", pH value measured with pHC-3S type acidity.
3 Results and Discussion
Curing characteristics of 311425 cement
In order to obtain a uniform solidified body, it is necessary to select a cement mixture dosing scheme with good fluidity, proper setting time, and appropriate amount of water to be separated. The flow value of concrete that can be mixed with the mixer should generally be between 200-300mm, the initial setting time should be more than 1h, the disappearing time of the segregation water should be within 6h [4]. We tested the mixing experiments with different ratios of cement cured bodies.
The solidification treatment effect of cement is good, in which the leaching concentration of Cu2 is much lower than the maximum allowable concentration of leaching specified in the “Control Standards for Pollution of Solid Wastes in Nonferrous Metal Industryâ€.
The leaching concentrations of Sn and Au are also lower than 011 mg/I. This may be because the main chemical reactions in the coagulation and hardening process of concrete are cement compounds, and because the micropores of the solidified blocks are not connected to the capillary, the heavy metals in the sludge It is sealed in the hardened block, preventing the internal dissolution of heavy metals. The elution occurs only on the surface of the solidified block. Therefore, the amount of elution is very low.
Effect of pH 312
As the final treatment of solidified blocks is naturally piled up or used as a roadbed material, it will inevitably be leached by natural rainfall. In recent years, due to industrial pollution, acid rain has been serious. In southern China, precipitation with pH<5 is common and sometimes even below 310. , and pH is one of the important factors that affect the dissolution of metal ions. In order to determine the law of leaching of solidified blocks at different pH, we used water of different pH for experiments.
With the decrease of pH, the leaching concentration of Cu2 increased. When the low pH increased more than doubled, the change at high pH was not obvious; Sn increased at both high pH and low pH, apparently due to the dual nature of Sn. The leaching change of Au is small. The effect of the whole pH value is that the concentration of leaching increases in high or low pH water. This is related not only to the nature of the metal ion itself, but also to the acid itself and the corrosion of the cement itself, but the overall effect is not significant.
313 Effect of sludge/cement ratio
Cement is the basic material for curing. The amount of cement is directly related to the curing effect of metal ions, the strength of curing blocks and the cost of curing. We performed a Cu2 leaching experiment on different sludge/cement ratios.
Therefore, considering comprehensively, if the compressive strength of the curing block is not affected, the proportion of the cement used should be selected as low as possible.
Effect of 314 Additive Thiourea
The addition of thiourea is mainly to investigate the effect of solidification of Cu2 in the solidified block. The author believes that thiourea may play two roles in the solidification process: (1) promote the role of cement to promote the solidification of the cement, curing the formation of a mesh block The cross-linked three-dimensional structure acts as a cure for harmful components. (2) A vulcanizing agent that reacts with Cu2 to form a stable sulfide, thereby preventing the dissolution of Cu2. Obviously, the addition of thiourea to the formulation will increase the solidification capacity of the block, and the leaching of Cu2 will decrease as the amount of thiourea addition increases.
315 curing block compressive strength
Because the solidified block formed after the solidification of sludge must be naturally stacked or used as a roadbed material, the solidified block must have a certain compressive strength. We tested the compressive strength of cured blocks of different proportions. As the content of cement increases, the compressive strength increases.
The cement content has a greater influence on the compressive strength, and the compressive strength decreases as the cement content increases. The addition of Na2SiO3 as an additive to the formulation significantly improves the compressive strength of the cured block. In this study, the main component of cement is silicate (Ca, Al, Mg, Fe, etc.), and Na2SiO3 as an additive can accelerate the hydration reaction of cement components, promote cross-linked solidification of cement, thereby increasing the strength of cured blocks and reducing harmful substances. Dissolution.
4 Conclusion
The lime-neutralized and coagulated sludge was solidified with cement, and the effect was good. The leaching rate of Cu was much lower than the allowable leaching concentration. The experimental results show that the best ratio of solidified sludge can overcome secondary pollution caused by sludge. Cement curing material is easy to get, curing is simple and practical. Inadequacies, its processing costs are relatively high.
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