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The basic principle of the thickener (2)
Second, flocculation and concentration
1. Application of flocculant
It can be seen from the formula (2) that the clarification speed of the pulp and the quality of the concentrated product obtained in the process of sedimentation and concentration of the ore dressing products are determined to a large extent by the size of the ore particles. The coarse particles settle quickly and the sediment contains little water. The colloidal particles have been balanced by surface energy and Brownian motion due to the gravity they are subjected to, and can remain suspended in the slurry for a long time without sedimentation. At present, due to the low grade of ore, the processing granularity of various useful minerals is becoming finer, sometimes the fractional content of less than 0.043 mm is as high as 80-90%, and contains a considerable amount of fine particles of less than 5-10 microns. Concentration of this slurry by natural sedimentation takes a long time and requires a large settlement area. In order to enhance the concentration (clarification) process, it is usually necessary to add an appropriate amount of flocculant to polymerize the dispersed fine particles into larger aggregates, and accelerate the sedimentation.
Flocculation and concentration technologies are increasingly used in the concentrates and concentrates of concentrates, countercurrent washing in water and metallurgy production, environmental protection and wastewater recycling.
2. Classification of flocculants
There are two types of coagulants and flocculants that are often used in production: one is an electrolyte-based coagulant such as lime, sulfuric acid, aluminum sulfate, ferric chloride, and ferric sulfate. When they are dissolved in water, they generate ions, change the surface electrical properties of the dispersed particles, reduce the electrostatic repulsion between the fine particles, and cause the fine particles to collide with each other to form larger aggregates during mechanical movement, and the other is A natural or synthetic high molecular organic compound. Such as starch, dextrin, potato residue, gelatin, polyacrylamide and polyvinyl alcohol. Such flocculants are polysaccharide polymer compounds whose molecules have a long linear shape and contain a large number of hydroxyl functional groups which are adsorbed on the ore particles by hydrogen formation in the hydroxyl functional group. Since these polysaccharide molecules are large, one part can be adsorbed on one particle of the suspension, and the other part is adsorbed on the other particle, thereby associating the ore particles into aggregates.
Synthetic high molecular organic compounds can be classified into two types: a class A flocculant and a class B flocculant. Non-ionic, anionic and cationic polymeric organic compounds with a molecular weight of 1 to 20 × 10 6 based on polyacrylamide are Class A flocculants; most of the B flocculants are of lower molecular weight (5 × 10 5 ) and a high molecular organic compound having strong cationic properties.
Whether using inorganic compounds or natural polymer compounds, their types and cohesive properties are always limited, and they consume a large amount of agricultural products, and are now rarely used. Currently in the mineral processing industry, Class A polymers, especially nonionic and anionic, are the most common flocculants. Its products are mainly in the form of solids, gels and suspensions. Practice has proved that its flocculation effect is better. When the dosage is 20~50 g/ m3 (mineral), the coagulation effect can increase the sedimentation speed of the concentration process several times to several tens of times.
Non-ionic flocculants are generally widely used in acidic pulp; strong anionic flocculants are suitable for alkaline pulp; medium molecular weight flocculants are more suitable for slurry filtration; high molecular weight flocculants are mainly used for slurry sedimentation.
3. The basic principle of flocculation
The stability of the particles in the suspension and its sensitivity to flocculation are related to the surface charge of the suspended solids, the ion adsorption properties, the pH of the suspension, the type and amount of dissolved ions, and the like. Appropriate flocculant is added to the pulp, and after stirring, physical and chemical changes occur on the surface of the dispersed particles. Under the cohesive force of the particles, the particles collide with each other and adsorb together to aggregate into larger floccules, and the weight increases, thereby accelerating the settlement. speed.
There are generally four possibilities for particle aggregation:
(1) Compression of the electric double layer. Ions of high concentrations of soluble salts such as lime and calcium sulfate reduce the ξ-potential of the particles to zero, resulting in agglomeration.
(2) Adsorption condensation. The ferric ion or hydrolyzate adsorbs on the surface of the mineral, lowering its zeta potential, thereby forming agglomeration. Excessive ferric ions can cause opposite changes and re-suspend the suspended fines. This type of agglomeration process depends on the pH, since the degree of alkalinity of the aqueous medium determines the type and amount of hydrolysate.
(3) Bridge flocculation of long-chain polymer flocculants. Long-chain polymers are adsorbed on the surface of many fine-grained solids and joined together to form larger flocs called bridge flocculation. The type and molecular weight of the polymer affect the size and nature of the flocculation. The flocculant chosen should be adapted to the surface electrical properties of the fines.
(4) The action of a strong cationic synthetic coagulant having a relatively low molecular weight. Such polymers primarily act as electrical neutralization during the agglomeration process.
The shape and density of the floc have little to do with the initial properties of the dispersed particles, and mainly depend on the degree of adsorption and dispersion of the flocculant on the particles. The settling velocity of the flocs depends on the size of the flocs and the degree of flocculation. It is still difficult to accurately calculate the sedimentation velocity of the flocs by mathematical methods, which can only be determined by test methods and experience. The effect of flocculation and sedimentation depends on the choice and use of the flocculant. [next]
The size and content of solid particles in the slurry affect the choice of clarification method and the amount of flocculant to some extent. The electric double layer around the particles causes an increase in the zeta potential, and the particle lattice size or the adsorption of ions on the surface of the particles mainly affects the charge density and the selection of the flocculant. The nature of the aqueous solution has a decisive influence on the choice of flocculant.
4. Factors affecting flocculation
(1) Influence of molecular weight of flocculant
Generally, higher molecular weight flocculants can form larger floes, however, in some cases this is not necessarily the case. Figure 2 shows the effect of the molecular weight of the flocculant on the free settling velocity during the concentration operation. Adding a medium amount of high-molecular flocculant (generally greater than 0.01 kg/ton) can produce faster deposition; while medium molecular weight flocculant can produce faster sedimentation at less than 0.01 kg/ton; in flocculant dosage In moderate and high cases, the rate of free settling increases with increasing molecular weight. There are many factors that influence the clarity of the concentrated overflow. The effect of the molecular weight of the flocculant on the clarity of the upper overflow is shown in Table 1 when the flocculation concentration is 10% CaCO 3 slurry. The flocculant having a molecular weight of less than or equal to 9 × 10 6 in the table has a poor ability to trap suspended fine particles. When a high molecular weight flocculant (greater than 14 x 10 6 ) is used at a dosage of 0.15 kg/ton, a minimum turbidity overflow can be obtained. In the case of excessive use (see the data in the amount of 0.2 kg / ton in Table 1), the fine particles are re-stabilized. The reason is that the slurry settles too fast, and the slurry layer does not “Filter†the suspended and uncollected particles or microflocs. A large amount of flocculant can make the bridging effect between many single suspended fine particles impossible. In the case where the sedimentation speed is constant and the size of the floc formed is similar, a large amount of the flocculant having a smaller molecular weight is better than a small amount of the molecular weight. A medium molecular weight (11 x 10 6 ) flocculant is best used for the clarification of the above slurry. It is worth noting that the highest molecular weight flocculant showed better ability at lower dosages (0.05 kg/ton).
Figure 2 Effect of molecular weight of flocculant on free settling velocity
Table 1 Effect of molecular weight of flocculant on the clarity of overflow liquid
Average molecular weight of flocculant
Flocculant dosage, kg/t
0.05
0.10
0.15
0.20
Upper overflow turbidity, turbidity unit
20×10 6
16×10 6
14×10 6
11×10 6
9×10 6
8×10 6
6×10 6
440 1
750
650
880
540
600
820
270
600
680
700
670
640
460
180
180 1
140 1
125
310
320
320
240
200
250
110 1
210
300
300
Note: The CaCO 3 slurry was treated at a concentration of 10%, pH = 9, and the average diameter of the fine particles was 2.9 μm, using a nonionic flocculant.
1 is approximately equal to the settling velocity of 20 m/h.
Table 2 shows the relationship between the concentrator underflow concentration and the flocculant molecular cation. [next]
Table 2 Effect of molecular weight of flocculant on underflow concentration
Average molecular weight of flocculant
Dosage, kg/t
Free settling velocity, m/h
Underflow concentration, kg/m 3
Settling 1h
Settling 7h
20×10 6
17×10 6
15×10 6
11×10 6
9×10 6
0.02
0.02
0.02
0.02
0.02
4.5
3.5
3.1
2.8
2.5
553
556
554
532
520
623
658
670
661
670
For some concentrators with a short residence time of the pulp, a high molecular weight flocculant can be used to obtain a higher underflow concentration. However, for long residence times, higher underflow concentrations can also be obtained with lower molecular weights. Therefore, when a flocculant having an optimum molecular weight is selected in order to obtain the desired underflow concentration, the residence time of the solid in the concentrator is important.
(2) Effect of charge type and charge type and charge density of flocculant ions
Anionic and nonionic bridging flocculants are used in the flocculation operation of most pulps. For high acid slurries and those containing a large amount of soluble electrolyte, anionic flocculants are generally suitable, while nonionic flocculants are commonly used. For operations where conditions are not critical, such as flaky flotation tailings, the use of anionic flocculants predominates. In fact, the most suitable anionic charge density depends on the pH and the type of dissolved salt that controls the surface charge of the fine particles and the zeta potential, and also depends on the configuration of the flocculant.
Figure 3 shows the importance of pH in determining the interaction between an anionic flocculant and a substrate. In Fig. 3(a), the pH is low, and the polymer is mainly adsorbed in a curled state due to the small surface repulsive force. The low ξ-potential promotes the close proximity of the fine particles and forms a strong floc. The pH of Figure 3(c) is higher, and the strong surface charge causes the fine particles to repel each other with a large ring shape and a tail shape, and stretches large in the solution to form large and loose flocs. Table 3 lists the results of the kaolin batch sedimentation test at different pH values. The results show:
Figure 3 Effect of pH on the configuration of anionic flocculant at kaolin/water interface
a—pH=4; B—pH=7; c—pH=9a
Dotted line indicates polymer stretch height
1 Due to the increase in pH, the negative surface charge and ξ-potential increase, the adsorption of strong anionic polymer becomes more difficult, and the amount of suspended solids increases.
2 In the case of low pH and ξ-potential, the anionic polymer is in a curled state and can be strongly adsorbed together, which plays an effective bridging role.
Third, centrifugation and concentration
Centrifugal concentration is a concentrated method that uses centrifugal force to accelerate the separation speed of fine particles and liquid in slow floating liquid and shorten the solid-liquid separation process. When the centrifuge is rotated, the centrifugal force of the ore within the machine can be calculated by the following formula:
Table 3 Results of batch sedimentation test of kaolin
(Initial concentration = 0.05g / L, flocculant dosage = 0.1kg / t)
pH value
Settlement condition
Anion content, %
0
5
15
20
30
4.35
Initial settling velocity, cm/min
Bottom product concentration, g/L
Suspended solids, mg/L
0.1
168.4
40
0.1
170.8
33
0.9
170.8
50
3.0
172.2
50
4.2
171.9
132
7.00
Initial settling velocity, cm/min
Bottom product concentration, g/L
Suspended solids, mg/L
0.1
178.3
4050
1.2
191.9
765
3.0
181.3
610
5.5
171.9
500
3.1
171.9
940
9.00
Initial settling velocity, cm/min
Bottom product concentration, g/L
Suspended solids, mg/L
0.2
239.1
5850
8.0
268.8
1840
24.0
300.8
2085
24.4
305.6
2650
21.0
330.0
3680
Mv 2 Gv 2
P=--------- ×10 -5 =------ ×10 -5
r gr
(4)
Where P—centrifugal force, N;
M—the quality of the ore particles, g;
G—the effective weight of the ore particles (ie the weight of the ore particles in the air minus the amount of water in the same volume), g;
R—swing radius, cm;
V—rotational circumferential speed, cm/s;
2 лrn
v=--------
60
(4a)
N—rotation speed, r/min;
G—gravity acceleration, cm/s 2 .
Let u be expressed as the speed n:
G 2 л rn 2 Grn 2
P=---(---------) ×10 -5 ≈----------- ×10 -5
Gr 60 9 ×10 5
(5)
The above formula shows that for a certain weight of ore particles, the centrifugal force increases with the increase of the radius of gyration and the number of revolutions. However, increasing the number of revolutions is more likely to increase the centrifugal force than increasing the radius of gyration.
In the centrifugal concentration dehydration operation, the ratio of the centrifugal acceleration to the gravity acceleration is commonly used to indicate the operating characteristics of the device, and is referred to as the centrifugal separation factor K, and its value is equal to:
v 2 rn 2
K=----=-----
Gr 900
(6)
The larger the separation factor is, the stronger the centrifugal force is, and the more easily the ore is precipitated. From equation (4), increasing the rotation speed of the ore particles in the machine or increasing the radius of gyration can improve the separation effect.
During the centrifugal sedimentation process, the centrifugal acceleration changes with the radius of gyration of the ore particles, so the precipitation speed of the ore particles is also a variable. In addition, the centrifugal force lines of the ore particles are not parallel to each other, so the principal directions of the centrifugal forces acting on the respective ore particles are also different, so the general gravity sedimentation law is not completely suitable for centrifugal sedimentation.
At present, centrifugal dewatering equipment which is widely used in mineral processing is a spiral discharge horizontal sedimentation centrifuge and a non-powerful cyclone. Vertical centrifugal dewatering machines are used more in coal preparation plants.
4. Gravity sedimentation test and calculation of concentrated area
The purpose of the sedimentation test is to determine the sedimentation characteristics of the solid material in the slurry. The factors affecting the sedimentation performance of the material mainly include the pulp properties, such as the particle size composition of the material, the solid density, the density of the pulping liquid, the foam, the properties of the agent and the electrolyte in the slurry, and the operating factors such as the concentration of the ore and the ore. Mineral amount and its fluctuation range, pulp temperature, use of coagulant, etc. Through the sedimentation test of representative pulp, it is an important basis for selecting and designing the thickener to find out the relationship between the factors. Using the sedimentation test, the original data of the design concentrator can be obtained directly or indirectly through appropriate calculations, such as slurry settling time, unit settlement calculation area and possible underflow concentration. It is relatively easy to design a thickener using empirical and semi-industrial tests. However, intermittent settling tests are often a simple and feasible method due to time and financial constraints and the difficulty of preparing representative ore samples.
The sedimentation test is generally carried out in a 2000 ml measuring cylinder. There are various methods for testing, but the contents tested are basically the same. Finally, the sedimentation curve of the slurry should be drawn, as shown in Fig. 4. In order to facilitate the selection of different main methods to determine the slurry settling velocity, the sedimentation test should generally provide the following data:
(1) the weight of the pulp, gram;
(2) dry solid weight, gram; [next]
(3) solid density, g / cm 3 ;
(4) liquid density, g / cm 3 ;
(5) volume concentration of pulp, g / liter; weight concentration, %;
(6) temperature of the pulp, °C;
(7) the final volume of the slurry settling, the rise and the height, in millimeters;
(8) concentration and density of the clarified liquid;
(9) The settlement interface height corresponding to different settling times during the 24-hour sedimentation process, in millimeters. The necessary fashion must provide a different settling time t corresponding to the concentration of the clarified liquid, the weight concentration of milligrams per liter and grit, %, as shown in Figure 4 (d).
According to the settlement test, there are three main methods for calculating the unit settlement area:
(1) When the sedimentation interface is clear, the sedimentation curve has no obvious critical compression point, or the required underflow concentration is lower than the slurry concentration under the critical state, the graphic method should be adopted. The H-t sedimentation curve obtained by the settlement test is replaced by two straight line approximations, as shown in Fig. 4(a). The settlement curve is replaced by the fold line H 0 KL, then H 0 K is the free settling process line KL is the compression process line, and K is the critical point. The sedimentation velocity of the particle group can be obtained by the following formula:
H 0 -H k
v p =------
t k -t 0
(7)
Where v p — sedimentation velocity of the particle group when the slurry concentration is P, m/h
H 0 — the starting height of the pulp surface in the measuring cylinder, m;
H K — the high point of the critical point, m;
t K — time elapsed from the start of settlement to the critical point, h;
t 0 - the moment when sedimentation begins, h.
From the value of p, the required settlement area per ton of solid, ie the unit concentration area, can be calculated and calculated by equation (8):
K 1 1
a p =----(-- - --)
v p c 1 c 2
(8)
Where ap-concentrate concentration is P, the settlement area required to treat one ton of solid material, m 2 /th corresponding to different v p can get different ap, the maximum value amax should be selected when calculating the concentrator area;
K—correction coefficient, generally used, 1.05 to 1.20. When the test is representative, the accuracy is high, the feeding amount and the property are stable, and the designed thickener has a large diameter, take a small value;
c 1 — the solid content per unit volume of the test slurry, t/m 3 ;
c 2 — designed solids reserve per unit volume of the bottom of the concentrator, t/m 3 ;
According to the test data and reference to the production data of similar factories and mines.
(2) When the sedimentation interface is clear and the sedimentation curve is smooth and there is no critical compression point, the settlement test and data processing should use the tangent method, as shown in Figures 4(b) and (c).
In the t-H curve obtained by the sedimentation test, several points Ai(ti, Hi) are selected as the tangent of the curve, and the vertical axis is at the points of Bi. The height of the sedimentation interface is calculated below the Bi points according to the formula (9). The average unit solid content of the pulp:
c p,0 H 0
c p,i =--------,t/m 3
B i
(9)
Where i is the second choice of the point;
C p, i - clarifying the interface to settle to B i, the solids content per unit volume of the slurry B i, t / m 3;
H 0 — the height of the pulp surface in the cylinder, m; [next]
B i — the height of the intersection of the tangent on the ordinate, m.
Figure 4 Schematic diagram of static settlement curve
Calculate the settling velocity of each point selected on the sedimentation curve of each test according to formula (10):
B ij -H ij
v ij =---------
t ij
(10)
v min =M in (v il ,v i2 ,...v in )
(11)
Where j is the second test;
v ij — settling velocity of each point selected on the settlement curve, m/h;
H i — the height of each point selected on the curve, m;
t i - settling time of each of the above points, h.
Find the required maximum unit concentration area as follows:
1 1 1
Amax=----(-- - --)
v min p ij C p
(12)
Where a max - the maximum unit concentration area of ​​the desired concentrator, m 2 /(th -1 );
v min — the minimum of the settling speeds of the selected points on the settling curve for each test. m/h;
C pij - the test interface of the slurry dropped Simcheong B ij, B ij per unit volume of the slurry solids content, t / m 3;
C p - solid content per unit volume of the initial slurry, t/m 3 .
(3) The sedimentation interface is unclear. When the sedimentation curve is not available (for example, the slurry is not easy to be settled, and the flocculant is not allowed to be added thereto), the sedimentation velocity measurement and calculation can be carried out according to the following steps.
Select a number of liquids with a fixed height h under the surface of the cylinder with a capacity of 2000 ml, calculate the sedimentation velocity v i according to the formula (13), and measure the suspended solid content c i and draw the v i — c i curve. According to the curve, the settling velocity is determined according to the overflow water quality c required by the design:
h
v=---,m/h
t i
(13)
Where h - settlement height, m;
Ti—Settling time required to meet the design requirements for overflow water quality, h.
The required concentration area is:
Q
A=K--,m 2
v
(14)
Where Q-design overflow, m 3 /h;
K—The safety factor is K=1.1～1.2 according to the size of the selected thickener.