Concentration is the process of enriching a thinner slurry into a thicker slurry while separating a liquid that contains little or no solid material. Where υ t ——— the free settling velocity of spherical particles, m/s; In the continuous operation of the concentrator, the slurry is continuously fed and discharged, and the above four zones are always present. Therefore, the sedimentation velocity of the slurry is calculated based on the sedimentation velocity of the sedimentation zone. The final concentration of the concentrated product is determined by the time the pulp stays in the compression zone. The compression process often takes up most of the entire concentration process. When the feeding and discharging speed of the thickener is constant, the height of the compression zone of the thickener determines the concentration of the bottom stream. Practice has shown that an increase in the height of the compression zone results in an increase in the underflow concentration. However, since the slurry in the compression zone is shifted at a variable speed and the sedimentation speed is small, it is generally not necessary to increase the height of the compression zone to increase the underflow concentration. Therefore, the total height of the clarification zone and the settling zone of the actual produced concentrator is about 0.8 to 1.0 m. The height of the compression zone shall be determined by experimentation and calculation. [next] Generally, higher molecular weight flocculants can form larger floes, however, this is not necessarily the case in some cases. 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 a dosage of less than 0.01 kg / ton, in the amount of flocculant 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 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 (large dry 14×10 6 ) is used at a dosage of 0.15 kg/ton, a minimum turbidity overflow can be obtained. The fine particles are re-stabilized in the case where the amount is too large. 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 exhibits better performance at lower dosages (0.05 kg/ton). [next] 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 enthalpy-potential promotes the close proximity of fines and forms a solid floc. The pH of Figure 3(b) is higher, increasing the surface repulsion, and the polymer is elongated in a ring shape and a tail. The pH of Fig. 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 greatly in the solution to form large and loose flocs. Where P———centrifugal force, N; Touchless Car Wash Machine,Auto Pressure Washer,Automotive Pressure Car Washer,Touchless Tunnel Car Wash Zhengzhou Shinewash Technology Co.,Ltd , https://www.shinewashtech.com
The concentration process of mineral processing products is divided into the following types according to the nature of the main forces on the solid particles in the slurry:
(1) Gravity sedimentation and concentration. The slurry is settled by the action of the gravity field;
(2) Centrifugal sedimentation and concentration. The slurry is settled by the centrifugal force field;
(3) Magnetic concentration. A slurry composed of a magnetic material is aggregated under a magnetic field and a part of the moisture is removed therefrom.
(1) Basic principle of gravity settlement and calculation of sedimentation velocity The monomer (free) sedimentation or aggregate (interference) sedimentation of particles is not only governed by its own characteristics, such as particle shape, density, particle size composition and composition, but also Temperature, magnetic agglomeration, colloidal effect, heterogeneous flow, lateral pulsation flow rate, hydraulic enthalpy, mechanical agitation, chemical content and other factors. Many experimental studies have confirmed that the sedimentation and concentration process involves a complex combination of physical and chemical effects. At present, the study of concentration theory is limited to gravity sedimentation A Gravity sedimentation principle and settlement velocity calculation Initially, the behavior of spherical particles in different concentrations of suspension is studied. There are three main forces that the particles sink in the slurry, namely gravity, buoyancy and resistance. For certain particles and a certain slurry, gravity and buoyancy are constant, while the resistance changes with the relative movement speed between the particles and the slurry. Small particles have a tendency to be dragged down by large particles that settle faster. In the process of sedimentation of uniform particles, the increase of drag force is mainly caused by the increase of velocity gradient, while the viscosity change caused by the increase of solid concentration has little effect on it.
The algebraic sum of the forces acting on the particles should be equal to the product of the particle mass and its acceleration (in accordance with Newton's second law of motion). The sedimentation process of the particles is divided into two phases, the acceleration phase and the constant velocity phase. In the constant velocity sedimentation stage, the velocity of the particles relative to the slurry is called the “settling velocityâ€. Because the sedimentation velocity is the velocity of the particles relative to the fluid at the end of the acceleration phase, it is also called the sedimentation velocity or “terminal velocityâ€. Since the particles treated by the industrial sedimentation operation tend to be small, the contact surface between the particles and the slurry is relatively large. Therefore, during the gravity settling process, the acceleration phase is short and often negligible.
The relationship between the forces received by the particles during gravity sedimentation and the settling velocity is based on Newton's second law. After finishing and analysis, the drag coefficient affecting the gravity settling velocity should be relative to the particle and fluid. The function of the Reynolds number Re. The comprehensive test can obtain the relationship between the drag coefficient of the spherical particles and the Reynolds number Re. The curve can be roughly divided into three zones according to the Re value, that is, the stagnation zone, the transition zone, and the turbulence zone. The curves in each zone are expressed by the corresponding relationship, and the corresponding Reynolds number in the constant velocity sedimentation stage is also replaced by the settlement velocity Ï… t, so that the spheres with smooth surface can be freely settled in the fluid. Settling velocity formula: 
d———particle diameter, m;
δ———particle density, kg/m 3 ;
Ï———fluid density, kg/m 3 ;
g———gravitational acceleration, m/s 2 ;
ζ———The coefficient of drag, no dimension, related to the Reynolds number;
μ———The viscosity of the fluid, Pa · s.
Equations (1), (2), and (3) are called the Stokes formula, the Al-len formula, and the Newton formula, respectively. In the stagnation zone, the surface frictional resistance caused by the viscosity of the fluid dominates. In the turbulent zone, the body resistance caused by the boundary layer separation of the fluid at the tail of the particle to form a vortex is dominant, and the viscosity μ of the fluid has no effect on the settling velocity υ t . In the transition zone, both frictional resistance and physical resistance are not negligible. [next]
Free settling occurs when the particles in the fluid are sparse. Therefore, the above-mentioned settling velocity formula must be applied under the condition that the size of the container is much larger than the size of the particles (more than 100 times) to eliminate the significant retardation of the wall of the vessel. Secondly, the particles must not be too small to prevent Brownian motion from colliding with fluid molecules or leaking between fluid molecules to achieve a sedimentation velocity higher than the calculated value. This is why the Stokes formula is no longer applicable when Re<10 4- (mineral particle size reaches 0.1 to 05 microns).
A slurry composed of fine ore is a suspension. During the sedimentation process, small particles have a tendency to be dragged downward by large particles that settle faster due to the accompanying turbulence in the fluid. The flocculation of fine ore particles also changes the effective size of the particles. Therefore, the sedimentation and dewatering of the slurry generally belongs to interference settlement. Among them, the large particles are greatly disturbed, the sedimentation speed is slowed down, and the small particles are dragged, and the sedimentation speed is relatively accelerated. However, tests have shown that for suspensions having a solid particle size that differs by no more than 6 times, all of the particles settle at substantially the same rate. When the concentrator is dewatered by the concentrator, in order to prevent the granules from being too thick and "crushing", it is generally necessary to pre-screen the ore particles of +0.25 to 0.8 mm in the slurry, so the interference phenomenon during the sedimentation process is not serious. Furthermore, when the ore product is dehydrated, the ore particles are all required to sink to obtain a clear liquid. Therefore, the sedimentation velocity of the slurry must be calculated according to the final sedimentation velocity of the smallest particles in the grit, generally using the Stokes formula [Equation (1)] suitable for the smaller Reynolds number range (stagnation zone). The particle size range is up to Re≈1, which corresponds to the case where the ore particles having a diameter of 0.15 mm are settled in water; the finest is about 0.5 μm, which is equivalent to the case before the suspension is converted into a colloidal solution. It fails when the particle size reaches 0.5 microns or less. In the specific calculation, it is generally assumed that the settlement belongs to a certain flow pattern, such as stagnation. Use the Stokes formula corresponding to the flow pattern to find υ t , and press υ t to calculate the Re value, and check whether the obtained Re t value is Within the range of 1 × 10 4 to 1.0. If this range is exceeded, the flow pattern should be set separately, and the corresponding other formula should be used to find υ t until the Re t value calculated according to the obtained υ t coincides with the range of Re t values ​​that the formula fits. In addition, the settling speed can also be calculated by the frictional group method which avoids trial and error by means of the converted curve of the ζ-Re relation curve.
Mineral processing products generally consist of non-spherical natural particles after crushing and grinding. The resistance of the particles during sedimentation is closely related to their shape, in addition to the above factors. The smaller the sphericity (the ratio of the surface area of ​​the particles to the surface area of ​​the sphere of the same volume), the larger the drag coefficient corresponding to the same Re value, and the slower the settling velocity. This effect gradually increases as the value of Re increases. However, the effect of sphericity on resistance in the stagnation zone is not significant. According to the measured data of natural-shaped quartz particles, the Stokes formula is used to calculate the sedimentation velocity of fine-grained beneficiation products. It must be multiplied by the shape coefficient K of the particles (ie, the sedimentation velocity of the ore sediment and the same volume of the spherical sediment) The ratio of the final velocity). The K values ​​of different shapes of ore are roughly: rounded 0.78; polygonal 0.72; rectangular 0.67; flat 0.52.
B Settling process Suspensions containing different sizes of ore particles When sedimenting, coarser ore particles settle to the bottom of the vessel first, and fine turbid liquids form, and the sedimentation rate is slower. In the richer pulp, or when the coagulant is used, due to the agglomeration of the ore particles, the larger ore particles drive the smaller ore particles to settle. At this time, the amount of liquid clarified in the upper layer gradually increases, and the slurry in the container gradually appears. The layer phenomenon, that is, divided into four regions from top to bottom, and the size of the ore particles and the concentration of the precipitate gradually increase from the top to the bottom, as shown in FIG. In the figure, zone A is a clarification zone, which has a low solid particle content and a small cohesive force between the particles; zone B is a settling zone, and its concentration is the same as that of the suspension before the sedimentation starts. The solid content of this section increases, and the cohesion between them is greater than the resistance of the solid particles when it settles; the C zone is the transition zone, the cohesive force between the solid particles in the zone increases, and the solid particle content increases accordingly; the D zone is compressed In the zone, the cohesion between the solids is greater, the concentration is higher, and the viscosity between the particles is also increased. As the precipitation process progresses, the D zone and the A zone gradually increase, while the B zone gradually decreases or disappears, and the C zone also disappears. At this point, the slurry is at the critical point of the sedimentation process. After the critical point, only the A zone and the D zone remain.
(II) Application of flocculation and concentration A flocculant From the formula (2), the clarification speed of the pulp and the quality of the concentrated product obtained in the sedimentation and concentration process of the ore dressing product are determined to a large extent by the size of the ore. 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 grain size 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.
Coagulants and flocculants B flocculant classification commonly used in production, there are two types: one is an electrolyte-based coagulant, such as lime, sulfuric acid, aluminum sulfate, ferric chloride, ferric sulfate and the like. When they are dissolved in water, ions are generated, the surface electrical properties of the dispersed particles are changed, and the electrostatic repulsion between the fine particles is reduced, so that the fine particles collide with each other during mechanical movement to form a larger aggregate. The other type is a natural or synthetic high molecular organic compound. Such as starch, dextrin, potato residue, gelatin, polyacrylamide and polyvinyl alcohol. Such a flocculant is a polysaccharide polymer compound having a long linear shape and containing a large amount of hydroxyl functional groups. These molecules are adsorbed on the ore particles by virtue of the hydrogen in the hydroxyl functional group forming a hydrogen chain. 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 high molecular organic compounds with a molecular weight of 1 to 20×10 6 based on polyacrylamide are A-type flocculants; most of the B-type 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.
C 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. 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 ζ-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.
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 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.
D Factors affecting flocculation a Effect of molecular weight of flocculant
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.
b Effect of charge type and charge density of flocculant ions Anionic and nonionic bridge flocculants are used in most flocculation operations. For high acid slurries and those containing a large amount of soluble electrolyte, the use of anionic flocculants is generally unsuitable, while nonionic flocculants are commonly used. For operations that are not critical to conditions, such as flotation tailings, anionic flocculants are dominant. 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.
(1) As the pH increases, the negative surface charge and the ζ-potential increase, the adsorption of the strong anionic polymer becomes more difficult, and the amount of suspended solids increases.
(2) In the case of low pH and ζ-potential, the anionic polymer is in a curled state and can be strongly adsorbed together, which plays an effective bridging role.
(three) centrifugal concentration
Centrifugal concentration is a concentrated method that uses the action of centrifugal force to accelerate the separation speed of fine particles and liquid in the suspension 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:
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——— radius of gyration, cm;
υ———Rotating circumferential speed, cm/s;
n———speed, r/min,
g———Gravity acceleration, cm/s 2 .