Chapter 2 FLUID AND PARTICLE PROPERTIES

Properties Affecting Fluid/Particle Separation Processes Chemical Composition Size Concentration Density Charge Abrasiveness SUSPENSION PARTICLES (Aerosols) GAS Chemical Composition Viscosity Density Temperature Pressure Humidity Velocity Toxicity Gas/Air Filtration

Chemical Composition Size Concentration Flocculation Density Charge Shape Strength Abrasiveness Hardness Uniformity Chemical Composition Viscosity Density Dissolved Solids pH Volatility Corrosiveness Toxicity SLURRY PARTICLES LIQUID Aging Effect Temperature Upstream Process Solid/Liquid Separation 2-3

2-1-3 About Properties in Fluid/Particle Separation Different materials behave differently in fluid/particle separation processes. Most relevant properties need to be measured, few are available in literature. Pay extra attention when communicating material properties with equipment suppliers or customers. There are often no unified definitions of measure between industries and misunderstanding can easily occur. 2-4

2-2 Particle Size In many cases, this is the most important property in fluid/particle separation. Many instruments are available for measuring particle size. Many definitions for relating non-spherical particles to an equivalent sphere size. Particle size varies depending on the definition. Different instruments will give different particle sizes for non-spherical particles. Precise measurements of particle sizes are not necessary for fluid/particle separation. 2-5

How big or small are the particles? In general, smaller particles are more difficult to separate Simple Rules about Particle Size on Separation A particle is considered BIG when d p > 50  m, SMALL when d p < 10  m, FINE when d p < 1  m 2-6

Large crystals These NaCl crystals are  m in size and easy to filter. Small agglomerates The filtration rate for these NaCl crystals (20-50  m) tends to be slower than those in the left photo. 2-7

Do particles form agglomerates? The size of agglomerates are more relevant to separation than the individual particle size. Discrete particles These particles are very difficult to filter or settle. Pretreatment should be considered. Agglomerated particles Although the individual particles are small, they form agglomerates which act like larger particles. 2-8

Is the particle size uniform? Are there many fines mixed with big particles? Particles with wide size distribution form tighter cakes and are more difficult to filter. Fine particles mixed with large agglomerates Uniform crystals 2-9

2-2-2 Methods for Particle Size Measurement Reference: T. Allen, “Particle Measurement”, 4 th ed., Chapman & Hall (1990) There are a number of types and brands of particle size analyzers. Each has its advantages and disadvantages. None can cover all particle size ranges or applications. Key types of analyzers are discussed here. 2-10

(1) Microscopic Measurement *a very simple operation. *cost of an optical microscope can be quite low. *provides direct measurement of particles. *is very effective in qualitative evaluation of particle size and shape. The examples illustrated in this section were obtained from a microscope. *can be used to measure particle size distribution but the process is very time consuming. *is a good practice to attach a microscopic photo with any particle size measurement. 2-11

(2) Light Scattering Particle Size Analyzer *principles for particle size measurement: - Fraunhofer diffraction (for large particles,  m) - Mie theory light scattering (for small particles,  m). *equipment is quite expensive ($50,000-70,000). *the best equipment for particle size distribution. *is not suitable for concentrated slurry. The slurry needs to be diluted down to less than 0.1 % by volume so the light won’t be blocked. 2-12

(3) Particle Counters *principles of particle counting: - optical (light obscuration, Hiac, PSS), - electrical sensing zone (Coulter). - for aerosol (light scattering, Climet) *equipment is expensive ($25,000+ ). *equipment counts particles one by one and provides particle size distribution. *suspension needs to be very dilute (max 20,000 particles/ml). *is good for evaluating filter efficiency for polishing filters. 2-13

(4) Sedimentation Particle Size Analyzer *is based on Stoke’s settling principle. The particle must be able to settle for this instrument to work *is an expensive equipment ($35,000-) *also measures particle size distribution *is not suitable for concentrated slurry. The slurry needs to be diluted down to less than 0.5 % by volume. 2-14

(5) Sieves *is a simple operation *basis for common mesh size frequently used to describe particle size * not expensive *only suitable for larger particles (>40  m) *normally used for dry particles although wet sieves are available *is not a preferred method for measuring particle sizes for solid/liquid separation applications 2-15

(6) Cascade Impactor (for aerosols) *Particles impact on collecting surface *Usually for submicron size particles (7) Mobility Particle Sizer (for aerosols) *Electric field bends the path of the particle stream to collect a particular size range. *Coupled with a condensation particle counter (TSI SMPS) *Usually for submicron size particles 2-16

(8) Aerosol Spectrometers (for aerosols) *Aerodynamic Aerosol Spectrometers (AAS). *Optical Aerosol Spectrometers (OAS) *Particle diameter and quantities are detected at the same time. (9) Optical Particle Counter (for aerosols) *similar to OAS but for very low aerosol concentrations (like clean room). (10) Photometer (for aerosols) *measure total aerosol concentration. 2-17

2-2-3 Standard Mesh Sizes Screen mesh sizes are often reported when sizing (classifying) particles. The mesh size number is inversely related to the micron size of the opening. US Mesh  Micron size ~15,000 (12,000 to 20,000) Sieve SizeOpeningOpeningSieve Size U.S. MeshMicrons (  m)inches Tyler

2-2-4 Particle Size Distributions Number Distribution Area Distribution Volume Distribution 2-19

Particle Size Distributions same particle size distribution data expressed in different ways 2-20

2-3 Particle Concentration Weight (Mass) % - commonly used in industry - Easy to measure - Good for comparing performance with like materials Volume % - used by researchers with broad range of materials -More difficult to measure -More relevant to solid/liquid separation -Less confusion when comparing between different materials 2-21

Comparison: Liquid phase = water, 1000 kg/m 3 Material is 20 wt % solids Solid Density kg/m 3 Solid Volume % Dry Cake Wet Cake 2-22

2-3-1 Measurements of Solid Concentration Measure solid concentration by filtration (without wash) This method is used when cake washing is difficult due to cake cracking or low filtration rate. Measure solid concentration by filtration (with wash) This method is good for dilute suspensions or fairly easy to filter materials. Measure solid concentration by drying For cakes, very thick suspensions or difficult to filter suspensions, it is impractical to run a filtration to measure solid concentration. Oven drying should be used. 2-23

(2-3-1) Conversion between Wt% and Vol % (2-3-2) 2-24

A difference in density between the solids and liquid is necessary for any sedimentation method. If the material is a named chemical, the density can be found from a handbook. However, it is recommended to measure the densities of solids and liquid involved in the application since most of the time they will not be the pure named chemicals and their densities may be different from the book values. The available solid density data may be the "bulk density" which is not suitable for solids/liquid separation. The true solid density should be used. The density of the agglomerates will be less than that of the primary particles and need to be considered in calculating settling rates. 2-4 Particle Density 2-25

2-4-1 Measurement of solid density Measurement of solid density by helium pycnometer If the solids cannot be wetted or hard to find a compatible liquid, helium can be used to replace the liquid in the above method. Equipment: Commercially available helium pycnometer, Balance Measurement of solid density by liquid pycnometer Equipment: Volumetric flask (50 ml or 100 ml), Balance Procedure: 1.Measure the empty weight of the volumetric flask (Wflask). The volume of the flask is Vflask. 2.Put proper amount of solid sample into the volumetric flask and measure the total weight (Wflask+solid). 3.Fill the volumetric flask to the mark with a compatible liquid (normally water). 4.Measure the total weight of flask, solid and liquid (Wflask+solid+liquid). 5.Calculate according to the following formula. 2-26

2-5 Particle Charge - Surface charge on particles effects a number of separations processes. -The charge on the particle surface may not be known, but the effective charge may be measured (zeta potential, ZP). -In an aqueous environment the ZP is a function of pH and ionic strength. 2-27

2-5-1 Double Layer and Zeta Potential Negatively charged particles Double layer counter ions Bulk of solution Zeta potential Adsorbed positive ions (Stern layer) 2-28

2-6 Liquid Property-Viscosity For most applications the rate of separation is inversely proportional to the liquid viscosity. Literature viscosity values may be used, but may not be accurate if the liquid contains dissolved solids (should measure if possible). Viscosity of the slurry can be difficult to measure, but is seldom needed except in sizing pumps 2-29

2-7 Liquid Property-Dissolved Solids The presence of dissolved solids in the liquid will affect the measurement of solid concentration. 2-30

2-8 Liquid Property- pH pH affects the degree of flocculation or dispersion of particles. A change of pH may dissolve some particles (carbonates can be removed by lowering the pH). A change of pH may precipitate some dissolved solids (like metal hydroxides, their solubility is pH dependent). The optimum pH value is system dependent. Some systems (like sludge dewatering) are very sensitive to pH values. 2-31

2-9 Liquid Property - Volatility Vacuum equipment may not work if a liquid has a high vapor pressure, the liquid will boil if the vapor pressure is above the applied vacuum. Even if the liquid does not boil, some vapor will evaporate and can impair filter efficiency. Vapor in the vacuum line may increase the size of the vacuum pump. Systems containing volatile liquids may need to be enclosed if the liquid is toxic or flammable. 2-32

2-10 Liquid Property-Corrosiveness For corrosive fluids the choice of equipment materials may be limited. Best to select equipment with fewer moving parts. Simple equipment may be sprayed with a corrosion resistant coating. 2-33

2-11 Chemical Composition Chemical composition defines the safety requirements for equipment operation (containment, explosion proof,…). Material of construction must be compatible with the slurry and seals need to withstand organic solvents. Reactions that occur during separations can cause problems in some equipment. Example  precipitation during washing may blind cloths or block small channels 2-34