In the process of large-scale urban construction, large amounts of waste slurry are produced. The slurry has a high water content and is difficult to precipitate naturally, resulting in low treatment efficiency. To improve the treatment efficiency of slurry, a variety of inorganic and organic polymer flocculants were used to carry out flocculation settlement tests on the slurry. The changes in the slurry properties and the filtration dewatering effect after flocculation were tested. The results show that the addition of flocculant makes the slurry particles form aggregates, which leads to rapid precipitation of the slurry. The use of an inorganic flocculant significantly reduced the zeta potential of the slurry. Organic polymer flocculant, however, had little effect on the slurry potential, but did cause the slurry to produce larger size aggregates, resulting in a better flocculation effect than inorganic flocculant. Inorganic flocculants and organic flocculants can improve the pressure filtration dewatering performance of slurry. CPAM12 (cationic polyacrylamide, with a relative molecular weight of 12 million Daltons) had the best overall effect. The formation of aggregates after flocculation and the change in the nonuniformity coefficient (
Slurry is widely used in large-scale infrastructure construction in China because of its excellent wall protection and slag carrying characteristics, in addition to other advantages, such as its low cost and ready availability [
Historically, geotextile bag method [
Currently, waste slurry treatment is guided by knowledge from the sludge dewatering field. First, flocculant is added to flocculate and concentrate the slurry, and then the concentrated slurry is dewatered by mechanical pressure filtration. In the process of slurry treatment, flocculant and pressure filtration are a continuous and integrated process. Flocculation changes the properties of slurry and further affects its pressure filtration dewatering performance [
The present study investigated the treatment of engineering waste slurry placed in the Nanjing Slurry Comprehensive Disposal Center. A flocculation settling test was carried out using various inorganic and organic flocculants, and the changes in the properties of the slurry after flocculation and the effect of pressure filtration dewatering, were tested. From the basic properties of the flocculated slurry, the reasons for the difference of flocculation effect of different flocculants and the potential mechanism of flocculants affecting pressure filtration dewatering performance of slurry are discussed. Through the above discussions, a flocculant selection method based on flocculation effect and pressure filtration dewatering effect is attempted to find out. The results of the present study provide a reference for the treatment of high-water-content engineering waste slurry.
Waste slurry stored in the Nanjing Comprehensive Disposal Center was used in this investigation. The basic physical properties of the slurry were determined according to the Standard of the Geotechnical Test Method (GB/T 50123-1999) (see Table
Basic properties of the slurry.
Water content (%) | Density (kg/m3) | Liquid limit (%) | Plastic limit (%) | Organic content (%) |
---|---|---|---|---|
900 | 1030 | 60 | 24 | 0.42 |
Grain size distribution of the slurry.
Five kinds of flocculants were used in this experiment. Two were inorganic and three were organic polymers. The inorganic flocculants used were ferric chloride (FeCl3) and polyaluminium chloride (PAC). The organic polymer flocculants used were cationic polyacrylamide with a relative molecular weight of 3 million (CPAM3), cationic polyacrylamide with a relative molecular weight of 12 million (CPAM12), and anionic polyacrylamide with a relative molecular weight of 12 million (APAM12). The flocculants should be used in the solution form. Suitable hydrolysis concentrations and dosage ranges for the flocculants used in the present study were known from some literatures [
Flocculant types and dosage.
Flocculant | Solution concentration (%) | Dosage (%) | Purity |
---|---|---|---|
FeCl3 | 10 | 0.8, 1.6, 2.4, 3.2, 4 | Analytical pure |
PAC | 10 | 0.4, 0.8, 1.6, 2.4, 3.2 | Analytical pure |
CPAM3 | 0.3 | 0.1, 0.2, 0.3, 0.5, 0.7 | Analytical pure |
CPAM12 | 0.3 | 0.1, 0.2, 0.3, 0.5, 0.7 | Analytical pure |
APAM12 | 0.3 | 0.1, 0.2, 0.3, 0.5, 0.7 | Analytical pure |
1200 ml of slurry was put into a beaker, stirred for 3 minutes at a speed of 200 r/min to ensure uniformity. Then, the prepared flocculant solution was added to the slurry, stirred for 1 minute at a speed of 100 r/min and then quickly poured into a measuring cylinder until the 1000 ml scale line was reached. The slurry was then allowed to settle naturally for 7 hours (Figure
Schematic diagram of the flocculation settlement test.
Recording the readings of the measuring cylinder every 5 minutes, the water content of slurry at a given time can be estimated. The calculation process is as follows.
Firstly, the solid particle mass (
The quality (
The quality (
The quality (
Because the quality of flocculant is very small, the effect of flocculant volume on flocculant solution volume is neglected. The flocculant solution volume
Then, the quality (
Finally, the water content (
After the flocculated slurry had settled for 1 h, 4 h, and 7 h, a small amount of supernatant was taken to measure the suspended solid (SS) matter content. A laser particle size analyzer and a zeta potentiometer were used to measure the size of slurry aggregates and the zeta potential. To obtain actual size data of the aggregates in the mud, the ultrasonic system of the laser particle size was closed, and a series of pump speed and sampling times were selected to optimize the measurement parameters [
The slurry pressure filter device shown in Figure
Schematic diagram of the slurry filter press.
The water content (
Five types of flocculants were used to flocculate and settle the slurry. Each flocculant was applied in five dosages (see Table
Water content curves of sediment with time after adding CPAM12.
The change curve of SS in supernatant with time after adding CPAM12.
Photo of slurry settlement 1 h after the addition of CPAM12.
Optimum dosage of flocculants.
Flocculant | Optimum dosage (%) |
---|---|
FeCl3 | 1.6 |
PAC | 0.8 |
CPAM3 | 0.3 |
CPAM12 | 0.2 |
APAM12 | 0.3 |
Figure
Water content curve of sediment using different flocculants.
After adding FeCl3 and PAC, the variation in water content of the sediment was approximately the same, and the water content of slurry after natural settlement for 7 h was almost the same. After the addition of CPAM3, CPAM12, and APAM12, the settling speed of slurry particles increased. At 5 minutes, the water content of sediment was about 200%, which is about 600∼700% lower than that produced by the inorganic flocculants. However, the water content of the slurry after 7 hours of flocculation was little different from that when inorganic flocculants were used. In general, the flocculation effect of organic macromolecule flocculants was better than that of inorganic flocculants. Organic flocculants can realize the rapid separation of slurry and water and facilitate later treatment.
The distribution of slurry aggregates after adding the five flocculants (optimum dosage) was measured using a laser particle size analyzer. Figure
Aggregate frequency distribution of slurry without flocculant and with different flocculants.
The zeta potential is an important index that measures the stability of colloids and reflects the ability of colloidal particles to repel or attract each other. The zeta potential of the slurry with flocculant was measured using a laser particle size analyzer. Figure
Zeta potential curve of slurry with different flocculant dosages (1, 2, 3, 4, and 5 correspond to the different dosages in Table
When compared with inorganic flocculants, organic macromolecule flocculants had less influence on the zeta potential of the slurry, and even APAM12 increased the slurry’s zeta potential. Therefore, the ability of particles to aggregate when organic flocculant is added is weak. However, after adding PAM, the size of aggregates produced by the slurry is generally larger than that after adding inorganic flocculants (Figure
Illustration of the function of flocculant adsorption bridging (adapted from [
Slurry with the different flocculants (optimal dosage) was dewatered by pressure filtration with the device depicted in Figure
Water content change curve for slurry pressure filtration after flocculation.
Results of pressure filtration test.
Flocculant | Time required for pressure filtration to reach stability (min) | Final water content of mud cake (%) |
---|---|---|
— | 100 | 39.1 |
CPAM3 | 85 | 28.3 |
APAM12 | 70 | 28.1 |
CPAM12 | 55 | 27.9 |
PAC | 50 | 30.4 |
FeCl3 | 30 | 31.6 |
When compared with the slurry without flocculant, the time required for slurry pressure filtration to reach stability was decreased by 15 minutes, 30 minutes, and 45 minutes after adding CPAM3, APAM12, and CPAM12, respectively. The final water content of the mud cake was reduced by 10.8%, 11%, and 11.2%, respectively. The addition of CPAM12 resulted in the largest reduction in the time required for slurry pressure filtration to reach stability, and the final water content of the mud cake was reduced the most. This shows that, of the selected PAMs, CPAM12 is the best at improving the dewatering performance of slurry.
When compared with the slurry without flocculant, the time required for slurry pressure filtration to reach stability was decreased by 70 minutes and 50 minutes after adding FeCl3 and PAC, respectively, and the final water content of the mud cake was reduced by 7.5% and 8.7%, respectively. FeCl3 can improve the dewatering performance of slurry better than PAC. After adding inorganic flocculant, the time needed to stabilize the slurry filter press is lower than that after adding CPAM12, although the water content of the mud cake is slightly higher. In general, although inorganic flocculants have a relatively poor flocculation and settlement effect, they can significantly improve the pressure filtration dewatering performance of the slurry. Finally, considering the flocculation settlement effect of slurry, CPAM12 is considered to be the best to improve slurry treatment efficiency in this study.
CPAM12 had better flocculation and dewatering effect in the present study and was selected to investigate the effect of different pressures on the dewatering performance of slurry. The dosage used was 0.2%. Figure
Effects of changing pressure on the time required for slurry filtration to reach stability and the final water content of the mud cake, after adding CPAM12.
CPAM12 was selected to study the effect of slurry particle size on the dewatering performance of the slurry. As can be seen in Figure
Grain size distribution of the slurry with different dosages of CPAM12.
Summarized measurements of slurry aggregate size after adding different dosages of CPAM12.
Dosage of CPAM12 (%) | ||||
---|---|---|---|---|
0 | 2.21 | 7.72 | 26.2 | 11.85 |
0.1 | 4.4 | 16.8 | 50.5 | 11.48 |
0.2 | 8.85 | 29.3 | 89.5 | 10.11 |
0.3 | 13.32 | 46.5 | 127.1 | 9.54 |
0.5 | 14.41 | 51.3 | 134.2 | 9.31 |
0.7 | 15.1 | 53.2 | 138.1 | 9.14 |
The water content of the slurry after the addition of different dosages of CPAM12 and 1 h of flocculation was adjusted to 190% by adding water, so as to avoid the possible influence of different initial water contents of slurry on the dewatering performance of slurry. The device depicted in Figure
Water content change curve of slurry pressure filtration with different CPAM12 dosages.
To further explore the relationship between particle size and the dewatering performance of slurry, variation curves for stability time and the nonuniformity coefficient of slurry pressure filtration were compiled (Figure
Change curves of the time required for pressure filtration to reach stability and of the nonuniformity coefficient for different dosages of CPAM12.
Compared with the organic polymer flocculant, the inorganic flocculant can make the slurry form aggregates with smaller particle size and the flocculation settling effect is poor, but inorganic flocculants can significantly improve the pressure filtration dewatering performance of the slurry (Table
Summary of measurements of slurry aggregate size after adding different flocculants.
Flocculant | Dosage (%) | |||
---|---|---|---|---|
— | — | 2.21 | 26.2 | 11.85 |
CPAM3 | 0.3 | 11.5 | 131.1 | 11.4 |
APAM12 | 0.3 | 4.8 | 51.9 | 10.81 |
CPAM12 | 0.2 | 8.85 | 89.5 | 10.11 |
PAC | 0.8 | 3.3 | 31.2 | 9.45 |
FeCl3 | 1.6 | 3.8 | 33.4 | 8.78 |
Change curve of the time required for pressure filtration to reach stability and of the nonuniformity coefficient for different flocculation treatments (original refers to slurry without adding flocculant).
Because flocculants can agglomerate slurry particles into aggregates and change their gradation, flocculants can improve the dewatering performance of slurry. In addition, the different dosages can also affect the nonuniformity coefficient of slurry. Flocculant and dosage can therefore be selected by testing the nonuniformity coefficient of the flocculated slurry. This will enable the dewatering effect to be predicted and thereby improve the slurry treatment efficiency.
In this study, a flocculation settling test on waste slurry was carried out using various inorganic and organic flocculants, and the changes in the properties of the slurry after flocculation and the effect of pressure filtration dewatering were tested. Ultimately, the influence of the change of the basic properties of the slurry on flocculation settlement and pressure filtration of the slurry was analyzed. This study has yielded the following conclusions: The addition of flocculant makes the slurry particles form aggregates, which led to rapid precipitation of the slurry. Inorganic flocculants significantly reduced the zeta potential value of the slurry. Although organic macromolecule flocculants had little effect on the zeta potential, they can produce larger size aggregates, thereby resulting in a better flocculation effect than inorganic flocculants. Inorganic and organic flocculants can improve the dewatering performance of slurry. Of the flocculants for pressure filtration dewatering test, PAC, FeCl3, and CPAM12 gave the better results. Considering the flocculation settlement effect of slurry, CPAM12 is considered to be the best to improve slurry treatment efficiency. With increased pressure, the dewatering effect on the slurry gradually improves, until it reaches a certain value. Subsequently, increases in pressure do not lead to further improvements in the dewatering of the slurry. Different flocculants and different dosages change the nonuniformity coefficient of the slurry. Change in the nonuniformity coefficient is the main cause of improvement of pressure filtration dewatering performance of the slurry. Through testing the nonuniformity coefficient of flocculated slurry, the optimal flocculant can be selected and the optimum dosage can be determined. This allows the dewatering effect to be predicted, thereby improving the slurry treatment efficiency. In the future, a microscopic test, such as test on the pore characteristics of the mud cake, is needed to be performed to further verify the mechanism of flocculants improving the dewatering performance of slurry.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (nos. 51778213 and 52078189) and Fundamental Research Funds for the Central Universities of China (no. B200202073).