In some mixtures, the substances distributed in liquid materials are not dissolved, but only dispersed in them. Once the mixture stops oscillating, it will precipitate. This heterogeneous mixture is called suspension.
Solute in suspension, due toBrownian motionBut can't sink quickly. At this time, the soliddispersed phaseThe mixture with liquid is called suspension.Of solid particles in suspensionParticle sizeIs 10-3~10-4cm，greater thancolloid。
Solid particles dispersed inliquidThe mixture of solid dispersed phase and liquid is called suspension.
Solute size: the particle size of the solid particles in the suspension is greater than 100nm and larger than the colloid.
Property: sediment is composed of tinyearthSuspension glue (body) made of particles suspended in water.Suspension glue (body) andSolDifferent, wheredispersed phaseThe particles are larger, less stable and easy to precipitate.When preparing suspension, usually adddispersantTo stabilize the suspension.The effect of dispersant is mostly a regulating mediumviscosityorInterfacial membraneProperty, which prevents particles from approaching adhesion.[1]
Product characteristics
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(1) Multidispersity of suspension: the particle size distribution of solid particles in suspension determines that it has both distinctive characteristics of colloid and is different from true colloid.Therefore, Williams calls such suspensions colloidal non colloidal mixed suspensions.
All particles in the suspension, regardless of particle size, are subject to the disordered collision of the thermal movement of liquid molecules to produce diffusion displacement, also known asBrownDisplacement.The Brownian displacement velocity of particles increases with the decrease of particle mass.graingranularityThe smaller the particle size, the larger the diffusion displacement.On the other hand, all suspended particles are affected by gravity, and the gravitational settling displacement decreases with the decrease of particle size.Therefore, there is an intersection between diffusion displacement and sedimentation displacement with the change of particle size, which is located in the area where the particle size is 1~2 μ m and the particle size is larger than this range, and the gravity sedimentation of particles;On the contrary, the particle size is less than 1-2 μ m.Brownian movement plays a decisive role.
(2) Interaction between solid particles in suspension: the interaction between solid particles in suspension cannot be ignored, even plays a leading role.For the fine particles dispersed in the liquid, the interaction between them is mainly the surface force, and the combined effect of surface force and hydrodynamic force causes the particles to attract each other and gather into clusters;Or they repel each other and disperse stably.Therefore, the indole particles in the suspension cannot be regarded as isolated and isolated from each other, nor as a stable system.
(4) The unique rheological behavior of suspension: in the suspension composed of liquid medium and solid particles, there are not only interactions between liquid molecules, but also interactions between particles and between particles and liquid.Therefore, the rheological behavior of suspension is much more complex than that of homogeneous liquid phase.[1]
Stable performance
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JustHeavy mediumFor coal preparation, the stability of suspension refers to the ability of the density of suspension at each point in the separation equipment to remain unchanged for a certain period of time.The stability of the suspension is not only related to the nature of the weighting substance and weighting agent, but also related to the state of the suspension (static or flowing).Therefore, it is necessary to distinguish between static stability and dynamic stability.
Under certain conditions, the dynamic stability is proportional to the static stability.However, the static stability and dynamic stability indexes of the same suspension may differ greatly.For example, when the suspension flows in a certain direction and speed, it can change the suspension with poor static stability into the suspension with dynamic stability.Whether the suspension can maintain dynamic stability in the separation equipment is the main indicator to measure whether the suspension can be used for separation, because it directly affects the separation effect.The static stability index can only be used as a reference to compare the properties of different suspensions.[1]
Influencing factors of suspension stability
The following figure reflects the solid volumeconcentrationHeavy weightdensityHeavy weightgranularityAnd the rheological viscosity and stability of suspension.influenceSuspension stabilityThere are many factors. When the volume concentration of suspension increases, the density of suspension increases, and the content of non-magnetic substances in suspension increases, the stability of suspension will become better, but the rheological viscosity will increase.The finer the particle size of the solids in the suspension, the more irregular the shape and the better the stability, but the greater the rheological viscosity.The density of the weighted mass increases, the volume concentration decreases, the viscosity of the suspension decreases, and the stability will become worse.ArgillableClay mineralAdding the suspension will increase the rheological viscosity of the suspension.For the same suspension, the greater the viscosity, the better the stability; on the contrary, the lower the viscosity, the worse the stability.[1]
Factors affecting the viscosity and stability of suspension
Effect of Surfactants on the Formation of Suspension Droplets
(1) During the formation of suspension droplets,surface active agent It has obvious influence on the change of throat diameter.When no surfactant was added to the suspension, the surface tension of the suspension was larger, and the change rate of throat diameter was greater than that of the experimental group with surfactant.With the increase of surfactant mass fraction, the change rate of throat diameter gradually decreases.
(2) Surfactants have a significant effect on the morphology of droplets.When surfactant is added to the suspension, the breaking length of the liquid bridge decreases rapidly.With the increase of surfactant content, the fracture length increases gradually.As the surface tension of the droplets required to balance gravity decreases, the diameter of the formed droplets gradually decreases.The diameter of satellite droplets decreases first and then increases with the increase of surfactant mass fraction.
(3) The droplet growth rate increases with time.At the same time, the growth rate of suspension droplets without surfactant is the largest.When the surfactant is added to the suspension, the growth rate of the suspension droplets gradually increases with the increase of the mass fraction of the surfactant, but it is always less than the growth rate of the suspension droplets without the surfactant.[1]
Determination of stability
There are many methods to evaluate the stability of suspension, and the commonly used determination methods can be divided into two categories:
(1) According to the weightedSettling velocityDetermination of stability;
(2) According to suspensiondensityThe stability is determined by the change of.
The first type of method includes the determination of the formation speed of the suspension clarification layer and the determination of the formation speed of the sediment layer.Under laboratory conditions, observe the change of the height of the clarified water layer in the measuring cylinder. When the clarified water layer reaches a certain stable value, calculate the clarification speed, so as to determine the stability of the suspension.With the above similar method, the stability of suspension can also be determined according to the rate of sediment formation.At this time, the stability value is expressed by the settling distance of sediment in unit time.
There are many specific determination methods for the second type of methods, the main difference lies in the different determination conditions and stability indicators.For example, Yang Xi and others suggested that a measuring cylinder with a diameter of 37.5mm be used to measure the static stability according to the density change of the upper layer (100mm from the liquid surface) after the suspension has been stationary for 1min.The stability coefficient θ of suspension is calculated with the following formula:
θ=ρˊ/ρ×100%
Where θ - suspension stability coefficient;
ρˊ - density of upper suspension after standstill, g/cmthree；
ρ - average density of suspension, g/cmthree。
If the suspension density remains unchanged after 1 min of standstill, then θ=100%, and when the heavy weight completely sinks in the upper layer after 1 min of standstill, then θ=0.This method has been used to determine various weights less than 0.06 mm.When the density of magnetite suspension is low, even if the magnetite powder is less than 0.06mm, the stability is still poor.[2]
Methods for improving stability
A、Methods to improve the static stability of suspension
Such methods include reducing the particle size of the weight, selecting the weight with low density, increasing the volume concentration of the weight, and addingSlimeandClayApplication of chemicals.
Reducing the particle size of the heavy material is an effective method to improve the static stability of the suspension.However, the preparation of suspension with too fine weight will bring about an increase in production costs and a sharp rise in the viscosity of suspension.The stability of the suspension can also be improved by reducing the density of the heavy substance and increasing the volume concentration of the heavy substance, but the viscosity of the suspension will also be increased.
Adding clay or slime into the suspension can effectively improve the static stability of the suspension.For example, at -0.06mmmagnetite20% gangue powder is mixed in the powder, and then the density is 1.4 g/cmthreeIts stability coefficient can be increased from 12% to 70%.In the same case, as long as 2% clay is added, the same effect can be obtained.In fact, it is inevitable that coal slime and clay are mixed into the working suspension.However, the content of coal slime and clay must be controlled according to the specific situation, and it cannot be too high, otherwise the rheological property of suspension will deteriorate significantly.
It is an expensive method to use chemical agents to improve the stability of suspension, and its mechanism is sufficient to change the surface energy and electric dependence of heavy particlesSodium metaphosphate, water glassSodium pyrophosphate, sulfonation agent and wood in alkaline solution, this method is rarely used in production.
B、Methods to improve the dynamic stability of suspension
This kind of method uses machinerystir. Utilize horizontal liquid flow, upward flushing liquid flow, and horizontal vertical composite liquid flow.
Mechanical agitation is an effective method to increase the dynamic stability of suspension.However, strong agitation can only be used in containers for dispensing and storage of suspension.In most sorters, the movement of transport devices (such as elevators, scrapers, etc.) can play the role of mechanical mixing.
The horizontal vortex does not prevent the sinking of the coin.However, if the flow rate of the suspension through the separator is very slow, the density of the suspension can change with the length and direction of the separator.As the horizontal flow rate is getting smaller and smaller from a certain distance below the liquid level, the heavy weight along the height direction of the separation tank will inevitably precipitate.
The upwash flow is different from the horizontal flow, and the upwash flow velocity is just opposite to the settling velocity of the weight.Therefore, when the upwash velocity is equal to or greater than the sinking velocity of the largest particle in the weighted mass, the suspension can reach dynamic stability.[1-2]
Filtering steps
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Filtration is the operation of treating the suspension with a porous material as the medium. Under the action of external force, the liquid in the suspension passes through the pores of the medium and the solid particles are intercepted, so as to realize the separation of solid and liquid.
Filtering method
(1) Deep filtration - also known asClarification and filtrationWhen the particles contained in the suspension are very small and the content is very small (the volume of particles in the liquid is less than 0.1%), the filter medium made of thick granular bed (for example, the sand layer for tap water purification) can be used for filtering.Because the particle size in the suspension is smaller than the diameter of the filter medium channel, when the particles enter the slender and curved channel in the bed with the liquid, they are attached to the channel wall by the action of static electricity and molecular force.No filter cake is formed on the filter medium bed.,Therefore, this kind of filtration is called deep filtration.Because it is used to obtain clear liquid from dilute suspension, it is also called clarification filtration, such as purification of tap water and sewage treatment.[3]
Conditions: When the suspension contains small particles and the content is small (the volume of particles in the liquid is less than 0.1%), it can be filtered by the filter medium with thicker bed.
Fig. 3-21 Filter Cake Filtering
(2) When the filter cake filters the suspension, the liquid passes through the filter medium and the particles deposit on the surface of the filter medium to form a filter cake.When the particle size is larger than the pore size of the filter medium, filter cake will be formed.However, when the particle size is smaller than the pore size of the filter medium, some particles will enter the filter medium channel at the beginning of filtration, and "bridging phenomenon" will quickly occur, as shown in Figure 3-21 on the right.However, a small amount of particles will pass through the filter medium and flow away with the filtrate.With the gradual accumulation of filter residue, filter cake layer will be formed on the filter medium.,Thereafter, the filter cake layer becomes an effective filtering medium to obtain clarified filtrate.This kind of filtration is called cake filtration, and it is applicable to suspensions with more particles (the volume of particles in the liquid>1%).
Condition: dp>The pore diameter of the medium and the formation of filter cake on the bed;dp<Medium pore diameter, but there is "bridging phenomenon" in the bed.
Before the cake layer is completely formed, some particles will flow away with the filtrate.After the filter cake layer is formed into an effective filter medium, the clarified liquid is obtained.
It is applicable when the content of particles is large (the volume of particles in the liquid is>1%).
Function - make liquid pass through and intercept solid particles.
Selection - according to the nature of liquid (acid and alkaline), particle content and particle size, operation p, T, medium mechanical strength, etc.
(1) Weaving medium - filter cloth made of cotton, linen, silk and wool, and metal wire mesh filter cloth can intercept particles of 5-65 μ m.
(2) Accumulated granular media - sand, charcoal, etc., are used for deep filtration.
(3) Porous medium - Porous plate and tube shaped medium sintered by ceramic, plastic, metal and other powders, which can intercept 1-3 μ m fine particles.
Filter aid
Premise - fine particles, easy to block the pores of filter media;Under the action of pressure difference, the pore of the formed filter cake becomes smaller and the resistance increases, making filtration difficult.
Function - mixing another solid with a hard texture into the suspension or precoating it on the filter medium to form a loose cake layer and make the filtrate flow smoothly.This same agent is called filter aid.
Common filter aids - diatomite, perlite, asbestos, carbon powder, etc;The addition amount is 0Less than 5%.
There are two ways to use filter aids.First, the filter aid is separately prepared into a suspension to filter it. A layer of filter aid is formed on the surface of the filter medium, and then formal filtration is carried out.The second is to add filter aid in the suspension and filter it together, so that the filter cake obtained is loose, the compressibility is reduced, and the filtrate is easy to pass.Since it is not easy to separate the filter residue and filter aid, if the purpose of filtration is to recover the filter residue, the filter aid cannot be mixed with the suspension.The addition amount of filter aid is generally less than 0.5% of the mass of homogeneous particles.[3]
