Study on the matching rules and control mechanisms between flocculants and slurry settlement characteristics

In order to realise efficient, economic, low carbon and environmentally friendly slurry disposal, floc sedimentation velocity (S f ), supernatant volume (V s ), water content of bottom mud (W b ), supernatant turbidity (N s ) and pH value were determined by slurry sedimentation tests, specific resistance tests, and water quality analysis. Orthogonal tests were performed to analyze the matching rule and control mechanism between flocculants and the slurry settlement characteristics. The results show that polyaluminum chloride (PAC) and polyferric chloride (PFC) had a low dose and a fast sedimentation rate. The sedimentation effect of 20 ionic degree cationic polyacrylamide (CPAM-20) in organic flocculant is the best. Although flocculants can shorten the sedimentation time, excessive flocculants can also weaken the slurry separation effect. The sedimentation efficiency of organic flocculants is higher than that of inorganic flocculants, but it is easy to form foam on the supernatant surface. For the composite flocculants, the sensitivity sequence of S f , V s and W b is PAC>CPAM-20>PFC. PAC plays a controlling role in the sedimentation effect of the slurry.


Introduction
Silt is deposited at the bottom of rivers.The continuous accumulation of silt will affect the water storage, flood control, and navigation of rivers.River dredging can effectively expand river crossing sections, ensure regional flood control safety, irrigation needs, and promote water ecological balance.It has positive practical significance for improving regional water environment and regional ecological environment [1][2][3][4].
At present, river dredging is by stirring up silt to form slurry, and then reducing the sediment by flocculation-dehydration [5][6][7][8].The slurry flocculation-dehydration process is completed by flocculants and a filter press device.However, due to the complex composition of the slurry, the water content of the sediment obtained by using a single flocculant is approximately 60% ∼ 80%.High water content sediment indicates that water is not fully removed from the slurry, and the poor physical and mechanical properties of the sediment hinder the recycling.Excess flocculant will reduce the slurry flocculation rate and generate high alkaline sediment, which will have a more adverse effect on the slurry dewatering effect and sediment recycling [9][10][11][12][13].
Flocculating materials can be divided into organic flocculants and inorganic flocculants.As a widely used organic flocculant, polyacrylamide (PAM) can achieve slurry separation by electrostatic adsorption of soil particles to form large floc [14][15][16][17][18].The coagulation principle of inorganic flocculants such as polyaluminum chloride (PAC) and polyferric chloride (PFC) is to reduce the electrostatic potential energy of repulsion between soil particles, and break the slurry turbidity balance through the high-valence metal ions enter the electric layer of soil particles [19][20][21][22][23].
Studies on the correlation between the slurry separation effect and the flocculant show that the mixture of PAC, PFC and PAM has a better flocculation-precipitation effect on slurry [24][25][26][27].Adding CaO, FeCl 3 , Na 2 CO 3 and MgO to the above composite flocculants can reduce the sediment water content, so the flocculant components are more diverse and complex [28][29][30].In addition, the latest research shows that biocement can improve the sedimentation effect of slurry.Microorganisms and their secretions can agglomerate fine particles in the slurry [31,32].Biological magnesium ammonium phosphate and basic magnesium carbonate produced by Urease-producing bacteriac contribute to the cementation of sand particles, which indicates that biological phosphate can effectively bind loose sand particles to form induced precipitation [33,34].The working mechanism of microbial flocculant is charge neutralization after adsorption bridging, which can form loose and porous precipitation [35,36].
In the process of geotechnical engineering construction, the excessive addition of flocculants to weaken the mud-water separation effect is a prominent problem.This problem not only reduces the qualified rate of slurry disposal, but also leads to the pH value, water content, toxicity and other indicators of mud cake not meeting the requirements of resource utilization.At present, flocculant products are mostly single components or composite materials prepared in a fixed proportion.In engineering practice, the flocculant purchased is added in a fixed proportion according to the slurry amount.This method ignores the slurry properties, resulting in poor flocculation effect.
The existing research has deeply revealed the flocculation mechanism of inorganic materials, organic materials and their composites, but lacks the practical path of applying frontier mechanism research to engineering project production.In order to solve this problem, this study carried out slurry composition detection, flocculant preselection, slurry sedimentation tests with single/composite flocculant in turn.The mixture ratio and dosage of composite flocculation material were determined based on slurry characteristics.This study provides a scientific and clear implementation step for slurry matching flocculants.The results are helpful to improve the slurry flocculation effect and reduce the dosage of flocculant, which has considerable economic and environmental benefits.

Test materials
The test samples are taken from the river dredging site in Xuzhou City, Jiangsu Province, China (figure 1).The mud content of the newly dredged slurry measured on site is 40 ∼ 50%.Samples collected on site were river sediment and the basic physical property indexes were tested (table 1).The grain grading curve of river sediment shows that the contents of sand, silt and clay are 66.13%, 25.01% and 8.86%, respectively (figure 2).In the laboratory, the river sediment was dried for 48 h at 68 °C, and then the dry mud was obtained by rolling and screening.The slurries used in the sedimentation tests are prepared from dry mud.In this study, sample preparation and testing are performed according to the standard for geotechnical testing method (NSCJ 50123-2019, 2019) [37].

Flocculant preselection
The slurry flocculation-precipitation tests include flocculant preselection, slurry sedimentation tests with single flocculant and slurry sedimentation tests with composite flocculant.Flocculants preselection excludes materials that do not significantly promote slurry flocculation-precipitation.For the slurry (mud content 40%) preparation process, first, 100 g dry mud and 150 ml water were first added to the measuring cylinder, and then stirred on the agitator for 5 min (figure 3(a)).Inorganic flocculants were prepared as solutions with a mass concentration of 10 g l −1 .0.1 ml inorganic flocculant solution was added to the slurry each time and stirred evenly.Flocs formation and slurry separation were observed and recorded.The minimum flocculant dosage when flocs were formed (c min ), supernatant volume (V s ) and floc sedimentation velocity (S f ) are used to evaluate inorganic flocculants.The difference of c min between the test groups adding organic flocculants is small, but there is a significant difference in S f .0.05 g of organic flocculant was added to 100 ml slurry.V s and S f were recorded and used to screen organic flocculants.A control group without flocculant (natural sedimentation) was set up.The flocculant preselection tests scheme are listed in table 2.

Slurry sedimentation tests with single/composite flocculant
Excessive or insufficient flocculant dosage will hinder slurry flocculation.Therefore, the purpose of slurry sedimentation tests with single flocculant is to determine the flocculants dosage range.The 100 g dry mud was prepared as a slurry with 40% mud content.The flocculant was added to the slurry and thoroughly stirred.The slurry sedimentation tests scheme with single flocculant are listed in table 3. Six dosages were set for each inorganic flocculant (0.3 g, 0.6 g, 0.9 g, 1.2 g, 1.5 g, 1.8 g).Five dosages were set for each organic flocculant (0.01 g, 0.02 g, 0.03 g, 0.04 g, 0.05 g).V s was recorded during the test.

Flocculant preselection results
The inorganic minimum flocculant dosage when flocs were formed (c min ) is listed in table 4. The c min-IP1 (PAC) and c min-IP2 (PFC) are 0.17 g/100 g and 0.14 g/100 g respectively, which have obvious advantages in dosage.The c min-IP3 (PFS) and c min-IP4 (CaCl 2 ) are greater than the c min-IP1 and c min-IP2 .Figure 4(a) shows the dynamic change of supernatant volume (V s ) in inorganic flocculant test groups under the c min condition.The c min-IP5 ) is smaller than that of control group (G0), and the sedimentation effect is worse than natural sedimentation.The V s -PFC in the whole flocculation process is the largest and the sedimentation effect is the best.Figure 5(a) shows the dynamic change of floc sedimentation velocity (S f ) in inorganic flocculant test groups under the c min condition.PFC also have the largest S f during flocculation.The S f-IP1 (PAC) is comparable to the S f-IP3 (PFS), but the c min-IP3 is much larger than the c min-IP1 (0.6 g/100 g>0.17 g/100 g).Therefore, PAC is more economical than PFS for flocculating the slurry.Figures 4(b) and 5(b) show that V s-OP2 (CPAM-20) and S f-OP2 (CPAM-20) were the largest under the same organic flocculant dosage.The S f of the test groups using organic flocculants was significantly higher than that using inorganic flocculant, but foams were easily foamed on the supernatant surface.No foams were found in the inorganic flocculant test groups (figure 6).The flocculation effect of CPAM with low ionic degree is better than that of CPAM with high ionic degree.The APAM can accelerate the slurry separation process.However, V s-APAM is smaller than V s-G0 at the end of the test, and the flocculation effect is inferior to natural settlement.According to the results of the flocculant preselection tests, PFC, PAC and CPAM-20 are selected as flocculants for slurry sedimentation tests.

The correlation between single flocculant and slurry settlement characteristics
Figure 7 shows the effect of CPAM-20 dosages on slurry sedimentation characteristics (V s and S f ).Compared to the control group (G0), in the first 1500 s, the V s and S f were higher when CPAM-20 was used.After reaching the peak floc sedimentation velocities (S f-max ), the S f declines rapidly and the V s growth slows down.The S f-max of high flocculant dosage was later than that of the low flocculant dosage.At the end of slurry sedimentation tests, the V s of CPAM-0.01 was 52.6 ml, the V s of CPAM-0.04 was 51.2 ml, and the S f of CPAM-0.04 was 6.85×10 −2 ml/s.The S f of CPAM-0.04 was significantly higher than that of the other test groups, and the V s of CPAM-0.04 increased the fastest in early stage of flocculation.The V s of CPAM-0.05 was the smallest, and the slurry sedimentation effect was the worst.Table 3. Slurry sedimentation tests scheme with single flocculant.

Flocculant type Test group Flocculant dosage Evaluation index
CPAM-20 CPAM-0.01 0.01 g/100 g V s and S f CPAM-0.020.02 g/100 g V s and S f CPAM-0.030.03 g/100 g V s and S f CPAM-0.04 0.04 g/100 g V s and S f CPAM-0.050.05 g/100 g V s and S f PAC PAC-0.3 0.3 g/100 g V s and S f PAC-0.6 0.6 g/100 g V s and S f PAC-1.5 1.5 g/100 g V s and S f PAC-1.8 1.8 g/100 g V s and S f PFC PFC-0.3 0.3 g/100 g V s and S f PFC-0.6 0.6 g/100 g V s and S f PFC-0.9 0.9 g/100 g V s and S f PFC-1.2 1.2 g/100 g V s and S f PFC-1.5 1.5 g/100 g V s and S f PFC-1.8 1.8 g/100 g V s and S f PFC-0.9 0.9 g/100 g V s and S f PFC-0.9 0.9 g/100 g V s and S f control group G0 natural sedimentation V s and S f At the end of slurry sedimentation, the V s decreased with increasing PAC dosage (figure 8(a)).The slurry separation was rapid in the first 1200∼1500 s, and slurry flocculation was completed at the 2000s.When the PAC dosages were 0.9∼1.5 g/100 g, the S f was larger, but the duration of highspeed sedimentation was shorter.The S f-max of each test group was reached successively at 840 s, and then decreased rapidly.When the PAC dosages were 0.3∼0.6 g/100 g, the highspeed sedimentation state lasts for a long time and keeps growing, the S f decreased rapidly during 1440∼1680s.The S f of PAC-1.8 was lower than that of other test groups, and the slurry sedimentation efficiency was low.Under the condition of natural sedimentation (G0), the floc sedimentation velocity (S f ) was low, and the slurry flocculation was completed at 3900 s.
It takes 20∼25 min for the CPAM-20 test groups and PAC test groups to complete the slurry separation, while it takes 65 min for natural sedimentation (G0).At the end of slurry sedimentation, the V s of flocculant test groups was smaller than those of the control groups, especially in the PAC test groups.This phenomenon indicates that the flocculants can shorten the slurry sedimentation time, it will also weaken the slurry separation effect.In the dosage range of 0.3∼1.8g/100 g, the V s-PFC was 50∼∼52 ml.The PFC dosage did not affect the  slurry separation effect (figure 9(a)).The highspeed sedimentation state of PFC-1.2,PFC-1.5 and PFC-1.8 ended at 960 s, the highspeed sedimentation state of PFC-0.3,PFC-0.6 and PFC-0.9 ended at 1200 s (figure 9(b)).
According to the slurry sedimentation test results with single flocculant, the slurry separation process can be divided into highspeed sedimentation stage, deceleration sedimentation stage and low-speed sedimentation stage (figure 10).In the highspeed sedimentation stage, flocculants change the electric double layer thickness of soil particles, and the inter-particle suspension balance was broken.Fine particles are adsorbed and connected by flocculants to form flocs and settle rapidly.With the increase of floc volume, the increment of floc buoyancy is greater than the increment of gravity, and the S f gradually decreases and enters the stage of deceleration sedimentation.In the low-speed sedimentation stage, the flocs are deposited at the bottom and slightly compressed by gravity and water pressure.On the one hand, the number of molecules with adsorption-binding ability in high-dose flocculants is more, and floc formation is faster.On the other hand, large volume flocs can be formed quickly with high-dose flocculants, and a large number of molecules with adsorption-binding ability are closed in the flocs.This makes the floc structure loose, the sedimentation rate slow, the supernatant turbid, and the slurry sedimentation effect weakened.Therefore, blindly increasing the flocculant dose can not improve the slurry sedimentation effect, and there is a critical dosage of each flocculant.

Sensitivity analysis of settlement characteristics by composite flocculant conditioning slurry
The scheme and results of slurry sedimentation orthogonal tests with composite flocculant are shown in table 5.The levels of influence factor are determined by the results of slurry sedimentation tests with single flocculant.The supernatant pH value of slurry conditioned by composite flocculants was 6.84∼7.72,and the composite flocculants did not cause acid and alkali contamination.
The factor sensitivity is analyzed by range method and variance method.According to the range (R) (table 6), the sensitivity order of S f for each factor is A > C > B, the optimum combination is A 2 B 2 C 5 .The sensitivity order of V s for each factor is A > C > B, the optimum combination is A 1 B 4 C 4 .The sensitivity order of N s for each factor is C > B > A, the optimum combination is A 2 B 2 C 5 .The sensitivity order of W b for each factor is A > C > B, the optimum combination is A 3 B 1 C 1 .The results of variance analysis (F) show that, the change in factor A level (PAC dosage) has the most significance effect on S f , V s and W b , and has some influence on N s .The change in factor C level (CPAM-20 dosage) has more influence on S f , and has significance effect on V s , N s and W b .The change in factor B level (PFC dosage) has no influence on S f , but it has some influence on S f and W b , and has more influence on N s .
According to the results of the orthogonal test, the PAC in the composite flocculant plays a controlling role in the slurry sedimentation effect.PAC dosage is negatively correlated with S f , V s and W b .Increasing the proportion of PAC in the composite flocculants reduced S f and increased W b , and N s did not change significantly.The optimum dosage of PAC is 0.3 g/100 g.PFC mainly affects the slurry sedimentation in terms of supernatant turbidity, and the optimum dosage is 1.8 g/100 g.The optimum N s , V s and S f were achieved at the dosages of 0.03 g/100 g, 0.04 g/100 g and 0.05 g/100 g CPAM-20, respectively.The mean deviation of V s between the test groups with CPAM-20 dosage of 0.03 g/100 g and the test group with CPAM-20 dosage of  0.04 g/100 g was 0.86 ml.The mean deviation of S f between the test groups with CPAM-20 dosage of 0.03 g/ 100 g and the test group with CPAM-20 dosage of 0.05 g/100 g was 0.41 × 10 −2 m l /s .In the composite flocculant, when the dosage of CPAM-20 is 0.03 g/100 g, the slurry sedimentation velocity and slurry separation effect are the best.On the other hand, the sediment water content of low-dose CPAM-20 is lower, so the optimum dosage of CPAM-20 is 0.03 g/100 g.

Conclusions
(1) c min , V s and S f are the evaluation indexes of flocculant preselection.For inorganic flocculants, the c min of PAC and PFC are 0.17 g/100 g and 0.14 g/100 g, respectively.The c min of PFS and CaCl 2 is 0.6 g/100 g and 0.46 g/100 g, respectively.The dosage of PFS and CaCl 2 is large, and the flocculation effect is similar to PAC, which is not economical.For organic flocculants, CPAM-20 has the highest V s and S f under the same dosage conditions, the slurry sedimentation effect is the best.Compared with inorganic flocculants, organic flocculants can complete mud flocculation-sedimentation faster, but it is easy to form foam on the surface of the supernatant.
(2) The results of slurry sedimentation test with single flocculant show that each flocculant has a critical dosage.Excess flocculants will reduce the slurry sedimentation efficiency and effect.When the dosage of CPAM-20 is 0.04 g/100 g, the slurry sedimentation effect is the best.When the dosage of PAC is 0.9∼1.5 g/100 g, the slurry sedimentation velocity (S f ) is high, and the slurry sedimentation effect (V s ) decreases with the increase of PAC dosage.In the range of 0.3∼1.8g/100 g, S f increased with the increase of PFC dosage, while V s changed slightly.
(3) The slurry separation process can be divided into highspeed sedimentation stage, deceleration sedimentation stage and low-speed sedimentation stage.In the highspeed sedimentation stage, the inter-particle suspension balance was broken.The soil particles are aggregated by flocculant adsorption-connection to form large-volume flocs.In the deceleration sedimentation stage, the floc buoyancy increment is greater than the gravity increment, and the S f gradually decreases.In the low-speed sedimentation stage, flocs are deposited at the bottom and slightly compressed by gravity and water pressure.
(4) The supernatant pH value is 6.84∼7.72,and the composite flocculant do not cause acid and alkali contamination.From the range analysis and variance analysis of the slurry sedimentation tests with composite flocculant, it can be seen that due to the change of factor level, the sensitivity order of S f , V s and W b is PAC>CPAM-20>PFC; the sensitivity order of N s is CPAM-20>PFC>PAC.The PAC plays a controlling role in slurry sedimentation effect.

Figure 1 .
Figure 1.River sediment collection on site.
S f is calculated by V s .Composite flocculant was prepared by mixing various flocculating materials.Slurry sedimentation tests with composite flocculant were carried out by the orthogonal test method.S f , V s , sediment water content (W b ) and supernatant turbidity (N s ) were used to determine the optimum formulation and dosage of composite flocculant.The sediment was obtained by vacuum filtration device (figure3(b)).W b was measured by the drying method.The pH value was determined (figure3(c)) to analyze the acidity and alkalinity of supernatant.N s was measured by turbidimeter (figure3(d)).A control group without flocculant (natural sedimentation) was set up.

Figure 7 .
Figure 7.The effect of CPAM-20 dosages on settlement characteristics of the slurry.

Figure 8 .
Figure 8.The effect of PAC dosages on slurry settlement characteristics.

Figure 9 .
Figure 9.The effect of PFC dosages on slurry settlement characteristics.

Figure 10 .
Figure 10.The characteristic stage of the slurry separation process.

Table 1 .
Basic physical properties of the river sediment.
min , V s and S f IP2 PFC c min , V s and S f IP3 PFS c min , V s and S f IP4 CaCl 2 c min , V s and S f IP5 Al 2 (SO 4 ) 3 •18H 2 O c min , V s and S f

Table 4 .
The inorganic flocculant minimum dosage when flocs were formed in slurry (c min ).

Table 5 .
Orthogonal test results of slurry sedimentation with composite flocculant.

Table 6 .
Range analysis and variance analysis of slurry sedimentation tests with composite flocculant.