Dispersing Powders in Liquids, Part 1, by Ralph D. Nelson, Jr.

-- 2: Operations and Problems Involving Slurries --

AGGLOMERATES -- See Aggregates and Granulation.

AGGLOMERATION -- the process of producing agglomerates or aggregates. Often involves an auxilliary material (binder) to improve either cohesion in the agglomerate or redispersion in a liquid later on. Granulators, sludge beds and clarifiers use agglomeration intentionally. Unintentional agglomeration may occur as a slurry dries at the top of a tank or in a partially filled pipeline. See Granulation.

AGGREGATES -- used by many research and plant workers interchangeably with the term agglomerates to refer to moderately to strongly bonded clumps. Some U.S. experts reserve the term aggregate for strongly bonded clumps and agglomerates for weakly bonded clumps, while some European experts use the opposite convention. Weakly bound clumps can be broken up by squeezing them between our fingers or (in slurries) by low-speed paddle stirrers; strongly bound clumps can be broken only by spatula pressure, hammer impact, or high-speed agitator shear.

ATTRITION -- loss of coating or protrusions or adsorbed fine particles when slurry particles collide with one another or with a process surface. Attrition may occur when mixing or pumping slurries, especially when the slurry flows through a narrow constriction, such as a partially opened valve.

BLEED -- the passage of particles through a filtercake and filtration medium into the filtrate. Also used to refer to the loss of particles from a centrifuge into the centrate. Numerous conditions can cause bleed -- the particles may be too fine to be trapped; the filtercake may be cracked; the centrifuge may be vibrating; the particles fed to the filter or centrifuge may not be properly flocculated.

BLINDING -- blockage of the pores in a filtration medium, leading to a high pressure drop in the filtration equipment. The blockage may be caused by fines or surfactant micelles or precipitation of a solute. In a vacuum filter, liquid evaporates on the discharge side of the filter, concentrating and cooling the filtrate, so precipitation may occur if the solution is near saturation. See Filtration.

CEMENTATION -- the process of binding particles together by precipitation at the contact points between particles in a clump. This is most likely to occur during drying or when the slurry flows past surfaces that are colder than the bulk of the slurry. See Fouling and Olation.

CHANGE IN PARTICLE SIZE -- may be caused by alternate dissolution and precipitation as the slurry passes through hot and cold zones or through zones of varying concentrations of reactants. This usually leads to increases in size either through building the individual crystals or through cementing several crystals together into porous agglomerates. See Ostwald Ripening.

CLASSIFICATION -- separating a slurry (or powder) into two or more streams (or batches) with different particle charqacteristics. If the objective is to produce streams with the same chemical composition but different particle size distributions the preferable term is size classification. If the objective is to separate a mixture of particles with different chemical compositions into streams each of which has a single composition, the preferable term is sorting.

Hydroclones and settling ponds are often used to achieve such separations intentionally. Unintentional classification may occur when a stream is withdrawn from a tee on a pipe (large particles can not follow the liquid as it flows around a sharp curve) or when a stream is drawn from an inadequately agitated tank. Intentional classification often requires that all particles be dispersed as primary particles, so surfactants and shear are used to break up and prevent reformation of aggregates, agglomerates, or flocs. See Elutriation and Sedimentation.

COAGULATION -- any sort of agglomeration of particles in a liquid, often used more specifically to distinguish strong, primary well coagulation from weak, secondary well flocculation. Heterocoagulation is the rapid agglomeration that occurs when a slurry of positively charged particles is mixed with a slurry of negatively charged particles. Pairs of oppositely charged particles attract each other into dense, neutral clumps that further agglomerate and settle out rapidily.

COATING -- (adjective) the material covering a core particle. (verb) the process of adsorbing or precipitating a material onto a set of core particles. Adsorption of impurities or competing additives may interfere with a coating operation. If the adsorption or precipitation does not proceed as planned, the coating material may precipitate as a separate and smaller set of particles or droplets that may blind filters or cause fouling, as well as leaving the cores uncoated.

COMMINUTION -- the process of reducing the average particle size of a set of particles by breaking them into pieces. See Crushing and also Grinding.

CREAMING -- See Sedimentation.

CRUSHING -- comminution by impact or by anisotropic pressure.

CRYSTALLIZATION -- nucleation and growth of particles from a solution or a melt, generally synonymous with precipitation. Sometimes used in a more limited sense for the preparation of large crystals from solutions in which they have significant solubility. See Precipitation and also Ostwald Ripening.

DILUTION -- increasing the proportion of liquid in a slurry. The best way to do this is to add fresh liquid rather slowly to the slurry while maintaining good agitation. See Solvent Shock.

DISPERSANT DEMAND -- the amount of surfactant required to get a good dispersion. Maximum effect usually comes when the powder adsorbs nearly a complete monolayer of surfactant. ``Excess demand'' means that a particular sample of powder for a plant product requires more surfactant to get to a desired dispersion stability than is required by the plant's standard sample for that product. Since fines have a higher area per unit mass than large particles do, a sample will have excess demand if it has a higher mass percent of fines, agglomerates of fines, or porous particles than are present in the standard.

DISPERSION -- (verb) the process of deagglomerating clumps and wetting them into a liquid. (noun) the suspension of powder in liquid that results from the dispersion process. See Slurry.

DRYING -- removal of virtually all liquid from a slurry or paste. If a wet paste is heated, the particles or their coating may dissolve in the hot interstitial liquid. As this liquid evaporates, materials in solution will precipitate to cement the particles together in a strong agglomerate. Even in the absence of cementation, the surface tension of the receding meniscus surrounding the liquid wetting the contact points between particles will exert a strong force that pulls the particles together and promotes sinter bonds. Freeze-drying avoids such pressure sintering. See also Cementation.

ELUTRIATION -- loss of fines from a fluidized bed of particles caused by liquid flowing up through the bed faster than the fines are settling. See Entrainment and also Classification.

EMULSIFICATION -- the formation of a dispersion of liquid droplets in a second liquid. Emulsion stability depends on the presence of a surfactant or a solid that has one crystal face compatible with one liquid and another face compatible with the other. See also Foam.

ENTRAINMENT -- loss of small particles or agglomerates with high void fractions (low sedimentation velocity) with the fluid passing through a fluidized bed, hydroclone, settling tank, or centrifugal separator. Entrainment may cause unacceptable process losses, recycle flow rates, or cleanup costs.

FILTRATION -- separating a powder from a liquid by mechanically preventing the flow of particles through a grid or tortuous path which is permeable to the liquid. Poor release of filtercake may occur if the particles are strongly attracted to the filter medium. This interferes with cake discharge from the press (it doesn't fall off readily) and may leave a layer of compact filtercake that makes continued operation inefficient. See also Bleed, Blinding, and Peptization.

FINES -- particles much smaller than the mass-average particle size for a particular sample. Fines can be removed through size classification with recycle of the fines for further crystal growth or agglomeration.

FLOCCULATION -- the process of particles sticking together into a rather weakly bonded and open structure (high void fraction). Flocculation may occur through fundamental particle attractions or through the adsorption of a flocculation agent which holds the particles together.

FLUIDIZED BED -- a set of particles suspended in an upward flow of liquid or gas (or a downward flow if the particles are less dense than the fluid). The liquid's flow rate must be high enough to suspend the particles in the flow with a significant reduction in bed density but not high enough to elutriate a significant mass of particles. Any surfactant originally present may be washed away by the flowing liquid. It is difficult to fluidize particles smaller than 10 $\mu$m because they flocculate easily to form a wide size distribution of clumps that cannot be twisted apart by the small shear forces produced by the fluidizing liquid flow.

FLUSHING -- transferring solid particles from one liquid to a second liquid which is insoluble in the first but more compatible with the particles than the first one is. The compatibility and transfer may be enhanced through the use of coatings or surfactants.

FOAM -- a close-packed collection of gas bubbles enclosed in liquid membranes. Foams may be stabilized by a surfactant that reduces the surface tension of the liquid or increases the surface viscosity, thus minimizing bubble wall draining, thinning, and breakage. A slurry may foam if the surfactant is not strongly adsorbed on the solid or if the slurry contains more surfactant than can be adsorbed on the available solid surface area. Waste streams may foam if the surfactant is washed off of the solid during filtration or is steam distilled out during drying.

Particles that are hydrophobic on one face and hydrophilic on another can act as a surfactant to stabilize a foam. Plate-shaped particles with hydrophilic edges are the most effective foam stabilizers because they can cover a large area per unit mass.

An aqueous foam may be broken by adding a hydrophobic powder whose particle diameter is greater than the bubble film thickness. The presence of these nonwetting particles in the film causes weak spots that lead to spontaneous breaking of the film.

FOULING -- the buildup of particles on process surfaces (walls, agitators, baffles) due to particles' stronger attraction to these surfaces than to the liquid or to air. Fouling occurs on primary heating or cooling elements (heat exchangers, evaporator surfaces) and also on uninsulated pipes running outside the building. It can also occur in tanks, drums, and cans of material in storage or during transportation. Surfactants may either increase or decrease the tendency for fouling. See Sintering, Olation, and Cementation.

GRANULATION -- See Agglomeration. Powders smaller than 100 m are often made into granules over 1 mm in diameter because the granules flow from bins, pour from bags, wet and disperse into liquids, and produce less dust than the original powder.

GRINDING -- comminution by high shear forces.

GRIT -- large particles. These can cause problems by blocking a screen or sticking out of a surface so far that they cause visible blemishes in a finish. Screening or other size classification processes can remove the grit, which may then be discarded or recycled for further size reduction.

HOMOGENIZATION -- mixing using a high shear rate. This may cause deflocculation, deagglomeration, attrition, or actual breakage of primary particles.

INCORPORATION -- the process of mixing a dry powder into a liquid so that the air between the particles is displaced and all external surfaces of the particles are wetted by the liquid. Pores in the particles may or may not become filled with the liquid.

LET DOWN -- to dilute a slurry with fresh liquid. See Solvent Shock.

METERING -- for slurries, this means adding a desired amount of solid or slurry to a known amount of liquid. Metering solids is difficult because the conveying and flow properties of solids change with particle size distribution and surface characteristics that are influenced by relative humidity and flow-aid treatment. Metering slurries is difficult because particles may settle out in flow lines, plug valves, and erode flow monitoring and control elements. In both cases, measuring the concentration of solids or the velocity of solids relative to liquid in the flowing stream may be difficult.

MICELLE ENHANCED ULTRAFILTRATION -- a process to increase the separation efficiency of an ultrafiltration membrane toward a target species. A surfactant (chosen to have a low CMC and to form micelles which strongly imbibe or adsorb the target species) is added to the feed liquid at a concentration high enough to form micelles. The micelles then imbibe or adsorb most of the target species. Since the micelles cannot pass through the ultrafiltration membrane, very little of the target species will get into the filtrate -- most will exit with the retentate. Problems are 1) any surfactant that is not part of the micelles may go through into the filtrate and 2) the retentate contains a lot of surfactant and economical operation requires that this be separated from the target species and recycled.

MILLING -- See Grinding and also Crushing.

MINIMUM SUSPENSION VELOCITY -- the flow velocity required to prevent particles of a specified size and density from settling out in a liquid of specified density and viscosity. See Sedimentation.

MIXING -- See Incorporation and also Wet-blending.

OLATION -- the dehydration reaction between the surface inorganic hydroxyl groups of neighboring particles, leaving a bridge of covalently bound oxygen between the particles. Over a several day period, olation bonding can turn a settled bed of particles into a strongly bonded porous mass that cannot readily be broken up.

OSTWALD RIPENING -- the increase in average particle diameter and sharpness of particle volume distribution in a slurry of a relatively soluble material. Small particles (which have a higher solubility than large particles) dissolve and their material is reprecipitated on larger particles as thermal jostling drives molecules both from the surface of the particles into the dissolved state and also in the reverse direction (dynamic recrystallization).

PEPTIZATION -- the spontaneous dispersion of a flocculated mass. This may occur when a salt solution is washed out of an hydroxide filtercake. The reduced salt concentration expands the counterion atmosphere thickness, increasing the electrostatic repulsion between particles. The repulsion may also be increased as the pH changes during washing. The deflocculated filtercake may bleed through or blind the filter medium.

PIPELINE TRANSFER -- flow through a long, usually horizontal tube. Problems related to dispersion include fouling, deflocculation (as weak flocs pass through valves and pumps), settling (and possible plugging during low flow periods), and erosion (especially at bends). The resistance to flow (viscosity) of the slurry may decrease with time if a flocculated material breaks up at high shear rate. See also Classification and Sedimentation.

PLATING OUT -- See Fouling.

POLYMERIZATION -- the increase in the chain length of molecules due to reactions which link monomer units into longer chains. This may occur in a melt, in a suspension of monomer droplets, or in a solution. The growing polymer often precipitates out as a solid. Dispersants may be needed to keep the droplets or particles from agglomerating during the polymerization.

PRECIPITATION -- generally synonymous with crystallization, but sometimes used in a more limited sense for the preparation (by chemical reaction) of small crystals with low solubility. See Crystallization.

REACTION -- for a slurry this involves mass transfer between the particle surface and and the bulk liquid. If the rate-limiting step is mass transfer between liquid and solid, then a batch of powder with a larger-than-standard surface area per gram will react more rapidly than the standard powder does.

A slurry with more fines, agglomerates of fines, or porous particles than standard will have a higher-than-standard surface area per unit mass and thus a higher-than-standard rate of reaction. If the reaction is exothermal, a high rate of reaction may generate heat faster than it can be removed by evaporation or heat exchangers, causing a boilover or an explosion, so the fines content must be carefully controlled in reactors that require cooling.

SEDIMENTATION -- the process in which particles settle through a liquid under the influence of gravity or a centrifugal field. Particles that have fallen as far as they can and are part of the sludge bed at the bottom of the container are said to have "settled out". Sedimentation may be a problem in pipeline transfer if the slurry flow rate is lower than the minimum suspension velocity. If the particles are less dense than the liquid, they will rise to the top of the slurry; this process is called creaming.

The sedimentation velocity depends on particle size, so gravitational sedimentation is adequate only for removing large particles from a liquid. Efficient sedimentation of small particles requires the higher (centrifugal) forces present in centrifuges and hydroclones. Lamellar settling units also increase sedimentation efficiency for fines by decreasing the time required to settle to a sludge layer. A ``Venetian blind'' structure of parallel plates angled from the horizontal provides a large number of convection short cells. These help maintain stable flow and permit rapid separation of fine particles, since the particles must settle only a short distance to get out of an upward-flowing supernate stream and into a downward-flowing sludge layer.

SETTLING -- See Sedimentation.

SHEAR FORCE -- arises when two planes in a fluid move at different velocities. The shear force [N] is the product of the shear rate [s^-1] (change in velocity per unit separation of the planes) times the fluid viscosity [Pa s]. We often use shear to deagglomerate clumps in a slurry.

A particle in a fluid shear experiences a torque which makes it roll across the slower-moving fluid plane. Since the surface of the particle cannot move in concert with all the planes that it contacts, shear forces develop within the particle. If these exceed than the internal bonding forces, the particle structure will rearrange, either to break up or to roll and collapse into a more compact structure.

SLURRY -- a suspension of solid particles in a liquid. The terms dispersion, suspension, and slurry are often used interchangeably, but some experts reserve the term slurry for suspensions of particles larger than 1 mm and the term dispersion for suspensions of smaller particles.

SOLID-LIQUID SEPARATION -- See Drying, Filtration, and Sedimentation.

SOLVENT SHOCK -- the agglomeration of particles that sometimes occurs when a concentrated dispersion is exposed to fresh liquid. If the dispersing agent desorbs from the particles near the interface and diffuses into the fresh liquid, the particles near the interface between the dispersion and fresh liquid may agglomerate, creating a tough surface that resists penetration by the fresh liquid. The result is a set of large agglomerates instead of the desired dispersion of primary particles.

STEAM DISTILLATION -- the loss of a high-boiling liquid (one which has a high boiling point) from a mixture in which a low-boiling liquid is boiling out. This occurs because the high-boiler evaporates to saturate the large volume of vapor from the low-boiler.

Surfactants may be steam-distilled away from particles during dryuing, leaving the particles with less surfactant than required for downstream processing. If the surfactant is flammable, the vapor may create an explosion hazard in the airstream. If the surfactant vapor reacts with metals or condenses on a cool wall in the dryer vent system, the reaction product or condensed liquid may become a fire hazard.

STORAGE -- the long term maintainance of a slurry with either no agitation (in cans or barrels) or a minimum of agitation to prevent settling (in vertical tanks). Inadequate agitation may permit settling and formation of an intractable sediment that cannot readily be resuspended. Settling in unstirred containers can be inhibited through the use of a gelling agent. This creates a weak solid structure in the quiescent slurry, but breaks up easily when the material is poured or stirred. Vibration during transport or storage may cause gradual settling due to partial breakdown of the gel structure.

SURFACE ROUGHNESS -- caused by bumps (,i>asperities), holes, or ripples in a dry film formed from a slurry of particles, binder, and liquid. It is obvious that a film will be rough if there are particles whose diameters are larger than the thickness of the dry film. However, even thick films can be rough if the particles are not well-dispersed or if they agglomerate during the drying process. As the volatiles in the liquid evaporate, the film shrinks in thickness. If the dissolved-solids volume in the liquid is less than the volume left between the particles as they form a packed bed, air will be drawn in to form pores as the film dries.

SYNERESIS -- the slow formation of a thin layer of clear fluid at the top of a gelled slurry due to shrinkage or collapse of the space-filling gel structure. This is usually easy to distinguish from sedimentation, in which a dense bottom layer also forms and the top layer is both deep and cloudy.

VEHICLE -- a liquid used in making a dispersion.

WET-BLENDING -- the process of mixing two slurries.

WETTING IN -- See Incorporation.