Mixing Pharmaceutical Engineering

Chapter 8

MIXING 

INTRODUCTION

• In pharmaceutical processing mixing involves certain heterogeneous components to be manipulated with the intent to make them more homogeneous. The term mixing is defined as a process that results in randomization of dissimilar particles within a system. Mixing is defined as an operation in which two or more components in a separate roughly mixed condition are treated so that each particle lies as nearly as possible in contact with a particle of each of the other ingredient. Mixing may be performed between any two phases ranging from mobile liquids to viscous liquids, semi-solids and solids. It is performed to allow heat and/or mass transfer to occur between one or more streams, components or phases. Even modern industrial processing involves chemical reactors as mixers. Mixing can be between one component with different particle size.

• The operation opposite to the mixing is ‘segregation’. The terms ‘mixing’ and ‘blending’ are often used synonymously, but technically they are a bit different. Blending is a process of combining materials, but is a relatively gentle process compared to mixing. In terms of the phase of material, blending is the process of solid-solid mixing or mixing of bulk solids with small quantity of liquid whereas mixing is more closely associated with liquid-liquid, gas-liquid, and viscous materials. 

• Mixing and blending are the most demanding unit operations in the pharmaceutical, chemical and food process industries. For example, chemical process industries involves mixing and blending of specialty chemicals, explosives, fertilizers, dry powdered detergents, glass or ceramics, and rubber compounds. Pharmaceutical industry employs blending of active ingredients of a drug with excipients like starch, cellulose, or lactose whereas in food industry preparation of cake mix, spices, and flavours. In practice mixing is a critical process because the quality of the final product and its attributes are derived by the quality of the mix. Improper mixing results in a non-homogenous product that lacks consistency with respect to desired attributes like chemical composition, colour, texture, flavour, reactivity, and particle size.

• With the proper equipment selection it is possible to mix a solid, liquid or gas into another solid, liquid or gas. For example, a biofuel fermenter requires mixing of microbes, gases and liquid medium for optimal yield; organic nitration requires concentrated (liquid) nitric and sulfuric acids to be mixed with a hydrophobic organic phase whereas production of pharmaceutical tablets requires blending of solid powders. The wide variety and ever increasing complexity of mixing processes encountered in industrial applications requires careful selection of design, and scale-up to ensure effective and efficient mixing. Improved mixing efficiency leads to shorter batch cycle times and operational costs. In order to increase productivity and survive in today’s competition there is a need of selecting robust equipment capable of fast blend times, lower power consumption, equipment flexibility, ease of cleaning and other customized features. In addition to blending components, many modern mixers are designed to combine different process steps such as coating, granulation, heat transfer and drying, in single equipment etc.

• Agitation is the process of keeping a mixture that has been mixed in the proper mixed state required for the 'end' product. Mixing refers to the actual stirring of different liquids and/or materials to blend them together into an end product or mixture. Once this mixture is 'mixed' it may require agitation to keep the mixture in the proper 'mixed' state. Generally mixing can lead to three types of mixtures that are fundamentally different in their behaviours:

• Positive mixtures: Positive mixtures formed when two or more components are irreversibly mixed together by diffusion process. In this case no energy input is required, and the time allowed for the components to mix is sufficient. In addition, these types of materials do not create any problem during their mixing process.

• Negative mixtures: Negative mixtures are formed when components are mixed to form a heterogeneous system, for example, emulsion or suspension. The components of these mixtures have high tendency to separate out as they are not continuously being stirred. Thus, such mixtures are more difficult to prepare as they need high degree of mixing with external force. 

• Neutral mixtures: Neutral mixtures are stable in behavior. The components of these mixtures do not have tendency to mix spontaneously but once mixed, they do not separate out easily. Pharmaceutical products such as pastes, ointments and mixed powders are the examples of neutral mixture

 OBJECTIVES

• The aim of mixing is to ensure that there is uniformity of composition between the mixed ingredients that represent overall composition of the mixture. The primary objective of mixing is to make homogeneous product using the minimum amount of energy and time. The other objectives are as follows:

• To produce single physical mixture: This may be simply the production of a blend of two or more miscible liquids or two or more uniformly divided solids. In pharmaceutical practice the degree of mixing must commonly be of high order as many of such mixtures are dilutions of active substances and must be in accurate amount for dose uniformity in dosage forms

• To produce physical change: Mixing can be performed to produce physical as well as chemical change, for example, solution of a soluble substance. In such cases a lower efficiency of mixing is often acceptable because mixing merely accelerates a dissolution and diffusion process that could occur by simply agitation. 

• To produce dispersion: Mixing is also aimed to include dispersion of two immiscible liquids to form an emulsion or dispersion of a solid in liquid to give a suspension or paste. Usually good mixing is required to ensure stability and effectiveness. 

• To promote chemical reaction: Mixing encourage and at the same time control a chemical reaction. Mixing ensures uniform product such as reactions where accurate adjustment to pH is required and the degree of mixing will decide the possibility of reaction 

• Mixing fulfills many objectives beyond simple combination of raw ingredients. These include preparing fine emulsions, reducing particle size, carrying out chemical reactions, manipulating rheology, dissolving components, facilitating heat transfer etc. So even within a single pharmaceutical product line, it is a common practice to employ a number of different style mixers to process raw ingredients, handle intermediates and prepare the finished product.

APPLICATIONS  

• Dry mixing: Mixing help various powders to be mixed in varying proportions prior to granulation or tableting. Dry mixing of the materials makes them suitable for direct compression in to tablets.

• Product features: The mixing help to deliver accurate dosage that has an acceptable appearance and texture, or to maintain formulation stable for the appropriate length of time. The importance of proper mixer selection and its optimal operation can hardly be over-estimated.

• Dry blending: Mixing is used for dry blending in the manufacture of many vitamins, dietary supplements and drugs in powders (insufflations, face powders, and tooth powders), capsules and tablets. Dry blending operation combines the active ingredient with other solid excipients in most appropriate way. Sometimes relatively small amounts of liquid may be added to the solids in order to coat or absorb colouring and flavouring agents, oils or other solutions.

•  Emulsions: Throughout the pharmaceutical industry, high shear mixing is widely used in the preparation of emulsions such as creams and medicated lotions.

• Heterogeneous mixtures: Mixing is used when different powders behave differently when added into liquid, and some require more coaxing in order to dissolve, hydrate or disperse completely than others. The ‘easier’ ones need only gentle agitation but more challenging powders needs higher speed devices that generate a powerful vortex into which the powders are added for faster wet-out.

• Tough agglomerates: It is used to deal solids that tend to form tough agglomerates which do not easily break apart. A high shear mixer is often used to resolve such issues and many solutions and dispersions are made. For example, tablet coatings, vaccines and disinfectants. 

• Fluid blending: Mixing is used for continuous blending of fluid streams, emulsification, and dispersion of gases into liquid, pH control, dilution and heat exchange. A static mixer is unique with no moving part that relies on external pumps to move the fluids through it. For example, dissolution of soluble solids in viscous liquids for dispensing in soft capsules and in the preparation of syrups.

• Viscous fluids: Mixing is used for batch mixing of viscous formulations. Mixers are used in the pharmaceutical industry for batch manufacturing of moderate to relatively high viscosity applications such as syrups, suspensions, pastes, creams, ointments and gels.

• Uniformity in size distribution: Mixing helps in particle size distribution and other related parameters which depends on cycle time and mixer design. Thus selection of proper mixer along with product chemistry, operating temperature, pressure/vacuum conditions, quality of raw materials, presence of additives etc. help to obtain appropriate product features. 

 FACTORS AFFECTING MIXING

• There are factors that can influence the efficiency of mixing leading to non-uniform distribution of materials which can result in inaccurate dosage production. These factors are as follows:

• Material density: If the mixture components are of different density, the denser material will sink through the lighter one, the effect of which will depend on the relative positions of the material in the mixer. To maximize mixing, the denser material is placed at lower layer in the mixer during the mixing process so as to enhance degree of mixing to equilibrium state. 

• Particle size: Different particle sizes in materials to be mixed can cause segregation that lead to non-uniform distribution. The smaller particles fall within the voids between the lager particles. During mixing process, the particles in bed might dilate and the greater porosity of open packing allows a large particle to slip into void that eventually results in non-uniform distribution. As the particle size increases, flow properties also increase due to the influence of gravitational force on the size. It is easier to mix two powders having approximately the same particle size. 

• Particle shapes: The particles with spherical shape are easier to mix uniformly whereas other shape particles face increase in difficulty in mixing process. The ideal particle is spherical in shape, and further the particles depart from this theoretical form, there is greater difficulty of mixing. If the particles are of irregular shapes, then they can become interlocked leading to a decrease in the risk of segregation once mixing has been achieved

• Particle attraction: Some particles exert attractive forces due to adsorbed liquid films or electrostatic charges on their surfaces and tends to aggregate. Since these are surface properties, the aggregation increases as particle size decreases. The interaction forces between drug and carrier surface are predominately van der Waals’ forces, but electrostatic and capillary forces may also play a role. These forces vary with the size, shape, crystallinity, hardness of the adhering particle, surface roughness and contamination of the carrier particles, the intensity and duration of shear forces during mixing and the relative humidity.

• Proportion of materials: The proportion of substances in a given mixture is one of the crucial factors that result in the efficacy of mixing. The proportions of materials to be mixed play a very important role in powder mixing. It is easy to mix the powders if they are available in equal quantities, but it is difficult to mix small quantities of powders with large quantities of other ingredients or diluents. However, to ensure uniform mixing, the substance should be mixed in geometric and ascending order of their weights. 

• Mixer volume: In mixing process, the mixer must reserve sufficient space for dilation of bed during mixing process. If this condition is maintained the powder samples are getting enough space for the free mixing to achieve uniform mix with increased efficacy. Overfilling reduces the efficiency and may prevent mixing entirely.

• Mixing mechanisms: The suitable and sufficient shear force and a convective movement is prime requirement for the mixer to ensure efficient mixing of bulk material. The mixer selected based on its mechanism must apply suitable shear forces to bring about local mixing as well as convective movement to ensure that the bulk of the material passes through this area. 

• Mixing time: Mixing must be carried out for an appropriate time so that the degree of mixing will approach its limiting equilibrium value. Since, there is an optimum time for mixing of any particular mixture it must be noted that the equilibrium condition may not represent the best mixing if there is segregation. While handling uniformly mixed powders after completion of operation enough care need to be taken to avoid segregation. The vibration caused by subsequent handling, transport or during use is likely to cause segregation. 

• Method of handling: When mixing is completed, the product is required to be handled according to standard operating procedures to minimize the risk of segregation. A common factor that causes segregation is vibration throughout the handling, transport, or packaging. This results into fact that all bulk powder should undergo remixing before taking them into use. 

• Nature of the product: rough surface of the powder components may result into ineffective mixing. The active substances which are generally very fine in size may enter into the pores and cavities on the surfaces of the other larger ingredients If such substances get adsorbed on the surface of other powders there is decrease in aggregation as well as segregation. For example, addition of colloidal silica to strongly aggregating zinc oxide help to maintain fine dusting powder that can be easily mixed. 

• Mixing Conditions: The theory of powder mixing shows four conditions that should be observed in the mixing operation. Those conditions affect mixing.

DIFFERENCE BETWEEN SOLID AND LIQUID MIXING

• Industrial applications involve mixing of solids to solids such as free flowing solids to pasty materials, solids to liquids, and solids to gas, liquids to liquids, and liquids to gas.

MECHANISMS OF MIXING

In general mixing involves one or more of the following mechanisms:

• Convective Mixing: Convective mixing is a process of transferring groups of particles in bulk from one part of powder bed to another. It is also known as micro-mixing and is regarded as analogous to bulk transport. Depending on the type of mixer employed, convective mixing may occur by an inversion of the powder bed or by means of blades and paddles or by means of a revolving screw, or by any other method of moving a relatively large mass of material from one part of the powder bed to another. Shear Mixing: During this process, shear forces are created within the mass of material by using agitator arm or a blast of air. As a result of forces within the particulate mass slip and planes are set-up. Depending on the flow characteristics of the powder, these can occur singly or in such a way as to give rise to laminar flow. When shear occurs between regions of different composition and parallel to their interface, it reduces the scale of segregation by thinning the dissimilar layers. Shear that occurs in a direction normal to the interface of such layers is effective as it also reduces the scale of segregation. Diffusive mixing: Diffusive mixing is also known as micro mixing. In diffusive mixing, the materials are tilted to ensure that the upper layer slips and diffusion of individual particle take place at the new developed surfaces. This occurs when random motion of particles within a powder bed causes them to change position relative to one another. Such an exchange of positions by single particles results in a reduction of the intensity of segregation. Diffusive mixing occurs at the interfaces of dissimilar regions that undergo shearing and therefore it results from shear mixing. It may also be produced by any form of agitation that results in interparticulate motion.  

Solid Mixing

• In solid mixing two different dimensions in the mixing process are convective mixing and intensive mixing. In convective mixing material in the mixer is transported from one location to another. This leads to less ordered state inside the mixer. The mixing components are distributed over the other components. With time the mixture becomes more random and after certain time the ultimate random state is reached. This type of mixing is observed for free-flowing and coarse solid materials. Physical properties of material that affects solid mixing are density, particle size and its distribution, wettability, stickiness and particle shape or roughness. Usually these factors contribute for the demixing of macromixed solids. If solids are in fine form with cohesive nature or if it is wet convective mixing is not enough to obtain random state. The relative strong inter-particle forces form lumps. The decrease in size of lump requires more intensive energy which is provided either as impact force or shear force.

Liquid Mixing  

• The liquid mixing occurs in two stages; first, localized mixing which applies sufficient shear to the particles of the fluid and second, a general movement sufficient to take all parts of the material through the shearing zone and to ensure a uniform final product.

•  There are four essential mechanisms involved in liquid mixing as follows:

• Bulk Transport: Movement of a relatively large portion of material being mixed from one location in the system to another.

• Turbulent flow: It is characterized by the fluid having different instantaneous velocities at the same instant of time. The temporal and spatial velocity differences resulting from turbulence produce randomization of fluid particles.

• Laminar Flow: In this mechanism a streamline flow is encountered in highly viscous liquids. 4. Molecular diffusion: It is a primary mechanism responsible for mixing at the molecular level which results from the thermal motion of molecules. It is governed by Fick's fist law of diffusion that describes concentration gradient across the system as:

• There is decrease in concentration gradient with time which approaches zero at completion. Liquid mixing as a process that can either be carried out batch to batch or can be a continuous one. Impellers, air-jets, fluid-jets and baffle mixers are the major types of mixing equipment used for batch mixing. Impellers operate using a combination of radial, axial and tangential flow. These might be classified into two further types, propellers and turbines, the former being used for low viscosity liquids while the latter for high viscosity liquids.

Semisolids Mixing

• The mechanisms involved in mixing semi-solids depend on the properties of the material which generally may show considerable variations. Many semi-solids form neutral mixtures having no tendency to segregate although sedimentation may occur. Three most commonly used semi solid mixers are:

• Sigma blade mixer: This mixer has two blades which operate in a mixing vessel which has a double trough shape. These blades moving at different speeds towards each other. It can be used for products like granulation of wet masses and ointments. 

• Triple-roller mill: The triple roller has differential speed and narrow clearance between the rollers which develops a high shear over small volumes of semi-solid material. This type of mills is generally used to grind semisolids to achieve complete homogeneity in the material for example, ointments.  

• Planetary mixers: This mixer has a mixing arm rotating about its own axis and also about a common axis usually at the Centre of the mixing wheel. The blades attached to the arm provide the kneading action, while the narrow passage between the blades and the wall of the container provides shear.

EQUIPMENTS USED IN PHARMACEUTICAL MIXING

• The operation of the mixing equipment may be batch or continuous depending upon the required production capacity, product quality, pre and post mixing equipment condition, type of mixer, etc. The most common classification of mixers is based on the type of dosage form to be handled. Physical properties of materials to be mixed such as density, viscosity and miscibility and economic considerations such as operating efficiency, cost and maintenance are some important factors to be taken into consideration while selecting mixing equipment. Other factors include whether it is intended to be a batch or a continuous process, load to be handled, size for the optimal working volume, fill level and residence time. The optimal working volume generally lies between 50 to 70 % of the total volume of the mixer. Similarly, type of agitator is other important factor which is based on nature of material to be mixed and degree of mixing desired. Following are various agitator types and their respective uses.

1. Ribbon mixer - Powders, granules, some slurries, mainly free flowing.  
2. Paddle mixer - Powders, granules, some slurries, free flowing, light pastes.  
3. Sigma mixer - Sticky materials, thick pastes and slurries. 

Classification of Mixers: 

•  Mixing equipment are most commonly classified on the basis of type of materials being mixed. The three main classes of mixing equipment are described below:
• Blenders: These are mixers used for solid-solid blending. Considering the multitudes of industrial operations that require blending of bulk solids, there are wide ranges of blenders available. Based on the principal mechanism of mixing, mixer blenders are classified as follows: 

1. Tumbler Blenders: Double cone blender, V-cone blenders, Octagonal blender. 
2. Convective Blender: Ribbon blender, Paddle blender, Vertical screw blender. 
3. Fluidization Blenders / Mixers: Plow mixer, Double paddle mixer (Forberg mixer). 

•  Agitators: These mixers are employed for liquid-liquid and liquid-gas mixing. A variety of processing like blending of miscible liquids, contacting or dispersing of immiscible liquids, dispersing of a gas in a liquid, heat transfer in agitated liquid, dispersion of solids in liquids etc. are carried out in agitated vessels by rotating impellers. The impellers based on the angle that the impeller/agitator blade makes with the plane of impeller rotation are classified as: 

1.  Axial flow impellers: This impeller blade makes an angle of less than 90° with the plane of impeller rotation. As a result, the locus of flow occurs along the axis of the impeller (parallel to the impeller shaft). For example, marine propellers, pitched blade turbine etc.
2.  Radial flow impellers: The impeller blade in radial flow impellers is parallel to the axis of the impeller. As a result, the radial-flow impeller discharges flow along the impeller radius in distinct patterns. For example, flat blade turbine, paddle, anchor etc.
3.  Heavy duty mixers: These mixers are mainly used for viscous and paste type materials. In case of high-viscosity materials, the mixing operation parameters changes from mixer with dominant turbulence (as in liquid agitators) to mixers with dominant viscous drag forces. Some viscous materials exhibit non-Newtonian behavior. Therefore, mixing of such material requires special heavy-duty mixers such as double arm mixers, planetary mixers and dual and triple shaft mixers.

Double Cone Blender 

• Mixing is a common process step in the manufacture of products for industries such as healthcare, food, chemical, cosmetics, detergents, fertilizers and plastics. The double cone blender (DCB) is used to produce homogeneous solid-solid mixture.

Principle:

• Double cone blenders are most often used to dry blending of free solids, the solid being blended in these blenders can vary in bulk density and percentage of the total mixture, matter being blended is constantly intermixed as double cone rotates. Normal cycle time is 10 min and can be more or less depending upon the complexity of material being mixed. 

Construction: 

• The main body of this blender consists of two cone-shaped sections welded at their bases to a central cylindrical section. The axis of rotation is perpendicular to the cone axis and passes through the cylindrical section. The driving motor is located at one of the two lateral supports holding the blender body. Double cone blender is made out of stainless steel. All welding are done by Argon Arc Process. This is totally mirror polished from inside and outside. Unit is mounted on mild steel / stainless steel stand fitted with ball bearings. It is available in capacity of 20 L to 3000 L. The conical shape at both ends enables uniform mixing and easy discharge. The cone is statically balanced which protects the gear box and motor from any excessive load.

• The suitable size of butterfly valve at one end of the cone is provided for material discharge and hole with openable cover is provided at other end of the cone for material charging and cleaning. The driving arrangement consists of a motor through a reduction gear box and stainless-steel baffles which are provided inside the blender. It has variable speed options available. Safety railing along with limit switch and platform are optional features for bigger models. Ancillary provisions include facility for incorporation of a liquid spray system to introduce liquids in spray form during the process. The unit can be equipped with an automated loading system for introducing powders and granules into the blender body by means of a vacuum unit with self-cleaning hoses. It includes a product receiving hopper with an automated self-cleaning filter, as well as a control panel for the unit. The loading/discharge can be carried out with pneumatically actuated retractable hermetic bellows. 

Working:

• Powdery or granular materials are fed into the double cone containers using vacuum intake system or manual feeding. The continuous rotation of the containers makes the material to move in a complex and forceful manner to achieve uniform mixing. The conical shape at both ends contributes to uniform mixing and easy discharge. Depending upon the characteristic of the material, paddle type baffles can be provided on the shaft for better mixing, uniform blending and de-agglomeration. Dust free bin charging system ensures minimum material handling. The solids are introduced into the blender through the loading aperture. The mixing takes place axially, as a result of the powder bed moves through the different sections. Mixing is thorough but it depends on the rotating speed. The mixture is discharged through a hermetically closing butterfly valve which is operated manually or automatically. The blender provides efficient mixing actions when loaded at 50 % of its volume. The effective volume for optimum homogeneity is between 35-70% of gross volume. 

Advantages: 

• DCB is an efficient and versatile mixer blender with closed container providing good product rolling and cross mixing. 

• The 'Slant' design (off center) and conical shape at both ends enables uniform mixing and easy discharge. 

• It performs mixing, uniform blending and de-agglomeration functions. 

• The cone is statically balanced which protects the gear box and motor from any excessive load.

• Powder loading and discharge are through separate opening. 

• Depending upon the characteristic of the product, paddle type baffles can be provided on the shaft for better mixing. 

• Contact parts are made of S.S.304 or S.S.316 preventing corrosion and product contamination. It has dust free bin charging system that ensures minimum material handling.

• Additional features such as spay system, loading system, receiving hopper, flame proof electrical fittings can be fitted.

Disadvantages:

• DCB is not suitable for fine powders. 

• Particles with wide variation in size such as fine and large particles cannot be mixed efficiently due to low shear exhibited in this equipment. 

• It requires more space and are not suitable for installation in rooms with low ceiling heights.

Applications:

• The DCB are useful for mixing dry powder and granules for tablets and capsules formulations.

•  It is used for dry granules sub lots mixing to increase the batch size at bulk lubrication stage of tablet granules.

• It can be suitable for dry powder to wet mixing. 

• It can be used for pharmaceutical, food, chemical, cosmetic products etc. 

• It is suitable for mixing highly flowable powdery and granular materials, with superior mixing quality. 

• It can be suitable for the material mixing in food, essence, dye, chemical, plastic, rubber industry. 

• It is suitable for medical intermediate, flavorings, graphite, coffee powder, iron powder whose specific gravity is under 7 g/cm3 that has certain fluidity with the liquid content under 10%. 

Twin Shell Blender

• There are three popular shapes of twin shell blenders (also known as a tumble blender), namely; the V-blender, the double cone blender, and the slant cone blender. Tumble type blending is most suited for products of uniform particle size and density, and where requirements for fast, thorough cleaning are desirable for sanitary applications. The V-blender is one of the most commonly used tumbler as they offer both short blending times and efficient blending.

Principle: 

• Tumble blenders works upon the action of gravity to cause the powder to cascade within a rotating vessel. The primary mechanism of blending in a V-blender is diffusion. Diffusion blending is characterized by small scale random motion of solid particles. Blender movements increase the mobility of the individual particles and thus promote diffusive blending. Diffusion blending occurs where the particles are distributed over a freshly developed interface. In the absence of segregating effects, the diffusive blending lead to a high degree of homogeneity

Construction: 

• The V-blender is made of two hollow cylindrical shells joined at an angle of 75° to 90°. A tank formed by two V-shaped cylinders turning around a horizontal shaft ensures a perfect and quick homogenization of the components of the mix. The product enters and exits at the tip of the V easily and without dust. Liquid injection facility may be provided to obtain a granulation of the pre-mixed powder, a coating of the granules or for an automatic cleaning system. A V-blender can be provided with high-speed intensifier bars (or lump breakers) running through trunnion into the vessel, along with spray pipes for liquid addition. The intensifiers disintegrate agglomerates in the charge material or those formed during wet mixing. A schematic drawing of the V-blender with the intensifier bar is shown in Fig. 8.2. The V-blender is offered in many sizes with volume ranging from 0.25 to 100 ft3 and can be customized with optional features such as vacuum capability, intensifier bar, spray nozzles, heating/cooling jacket, and explosion-proof motor and built-in controls. The heavy-duty models capable of blending very dense materials up to 90 - 100 kg/ft3 are also available. 

Working:

• The charging of material into the V-blender is through either of the two ends or through the apex port. As the V-blender tumbles, the material continuously splits and recombines, with the mixing occurring as the material freely and randomly falls inside the vessel. The repetitive converging and diverging motion of material combined with increased frictional contact between the material and the vessel long straight inner wall surface result in gentle and homogenous blending. In case of monodisperse powders there is no mechanism involved in movement of the powders across the line of symmetry of the blender. For such materials, care must be taken to load each side of the blender equally to ensure desired homogeneity of blends. Blending efficiency is affected by the volume of the material loaded into the blender. The recommended fill-up volume for this blender is 50 to 60% of the total blender volume. For example, if the fill material in the blender is increased from 50% of the total volume to 70% of the total volume, the time required for homogenous blending may be doubled. Blender speed may also be a key to mixing efficiency. At lower blender speeds, the shear forces are low. Though higher blending speeds provide more shear, it can lead to greater dusting resulting in segregation of fines. This means that the fines become air-borne and settle on top of the powder bed once the blender has been stopped. There is also a critical speed which, if approached diminishes blending efficiency considerably. As the revolutions per minute increases, the centrifugal forces at the extreme points of the blender  The charging of material into the V-blender is through either of the two ends or through the apex port. As the V-blender tumbles, the material continuously splits and recombines, with the mixing occurring as the material freely and randomly falls inside the vessel. The repetitive converging and diverging motion of material combined with increased frictional contact between the material and the vessel long straight inner wall surface result in gentle and homogenous blending. In case of monodisperse powders there is no mechanism involved in movement of the powders across the line of symmetry of the blender. For such materials, care must be taken to load each side of the blender equally to ensure desired homogeneity of blends. Blending efficiency is affected by the volume of the material loaded into the blender. The recommended fill-up volume for this blender is 50 to 60% of the total blender volume. For example, if the fill material in the blender is increased from 50% of the total volume to 70% of the total volume, the time required for homogenous blending may be doubled. Blender speed may also be a key to mixing efficiency. At lower blender speeds, the shear forces are low. Though higher blending speeds provide more shear, it can lead to greater dusting resulting in segregation of fines. This means that the fines become air-borne and settle on top of the powder bed once the blender has been stopped. There is also a critical speed which, if approached diminishes blending efficiency considerably. As the revolutions per minute increases, the centrifugal forces at the extreme points of the blender 

Advantages: 

• Twin shell blenders offer short blending times usually 5 to 15 min and the less space requirements and its efficient homogeneous mixing capability makes it a choice of blender.

• Particle size reduction and attrition are minimized due to the absence of any moving blades. Hence it can be used for fragile materials. 

• Charging and discharging of material is easy. 

• The shape of blender body such as the sanitary butterfly discharge valve results in a near complete discharge of product. 

• The absence of shaft projection eliminates product contamination.

• Provision of the intensifier bars makes this blender adaptable for drying as well as wet mixing, and mixing of fines, coarse particle compositions and cohesive powders.

• There is no dust generation during feeding and discharging of the product. 

• It is possible to control of the temperature of the product mix and can have provision of injecting a liquid to aid mixing. 

• It is suitable for mixing ingredients as low as 5% of the total blend size.

Disadvantages: 

• They require high head room space for equipment installation and operation. 

• They are not suited for blending particles of different sizes and densities which may segregate at the time of discharge. 

• Provision of intensifier bars may lead to undesired particle attrition and has intensifier bar shaft sealing and cleaning problems. 

• The low mixing shear action limits its use for some very soft powders or granules. 

Applications: 

• V-blenders are used for the dry blending of free-flowing solids in pharmaceuticals and nutraceutical industries. 

• General applications of V-blenders are in mixing of food products, milk powder, coffee, dry flavours, ceramics powders, pigments, pesticides and herbicides, plastic powders, animal feeds, spice blends, fertilizers, baby foods, cosmetics and polyethylene.

• This type of blender is primarily used for mixing up to 10 ingredients.

Ribbon Mixer Blender  

• Ribbon blender is a light duty blender mainly used for easy to mix powder components which are pre-processed like dried granules and pre-sieved powders. It is a low shear mixer and mostly used for solid-solid mixing. Solid-liquid mixing can also be achieved when high shearing force is not desired. It occupies less head room space for large volume mixing.

Principle:

• Ribbon blenders oper ate on the combined convection and diffusion mechanisms. Convective mixing is the macro movement of large portions of the solids. Convection mixing occurs when the solids are turned over along the horizontal axis of the agitator assembly. The diffusion mixing involves the micro mixing that occurs when individual particles are moved relative to the surrounding particles. In the ribbon blender diffusion occurs when the particles in front of the ribbon are moved in one direction while nearby particles are not moved or lag behind. Together, these two types of action result in the mixing and blending of solids. 

• Construction: A ribbon blender consists of a U-shaped horizontal trough containing a double helical ribbon agitator that rotates within. The agitator’s shaft is positioned in the canter of the trough and has welded spokes on which the helical ribbons (also known as spirals) are welded. Since the ribbon agitator consists of a set of inner and outer helical ribbons, it is referred to as a “double” helical ribbon agitator. The gap between the ribbon’s outer edge and the internal wall of the container ranges from 3 to 6 mm depending on the application. The internal and external ribbon spirals are pitched to move material axially, in opposing directions as well as radially. This combination promotes fast and thorough blending. The agitator shaft is located within the blender container. A spray pipe for adding liquids is mounted above the ribbons. For materials that tend to form agglomerates during mixing, high speed choppers can be provided for disintegration of the agglomerates. 

• The ribbon blender is powered by a drive system comprised of a motor, gearbox, and couplings. These blenders are generally powered by 10 - 15 HP motor for 1000 kg of product mass to be blended. The power requirement may range from 3 - 12 kW/m3 depending on the products to be blended. Top speeds is in the range of 300 feet/min are typical. The agitator shaft exits the blender container at either end, through the end plates bolted or welded to the container. The area where the shaft exits the container is provided with a sealing arrangement to ensure that material does not travel from the container to the outside and vice-versa. The blender assembly along with the drive system components viz. motor, gearbox, couplings and bearing supports is mounted on a supporting frame. 

Working:

• The feed material is charged in the blender through nozzles or feed-hoppers mounted on the top cover of the blender. The material is loaded by typically filling 40 and 70 % of the total volume of the container. This is generally up to the level of the outer ribbon’s tip. The ribbon agitator is designed to operate at a peripheral speed (also known as tip speed) of approximately 100 m/min, depending on the application and the size of the equipment. During the blending operation, the outer ribbons of the agitator move the material from the ends to the center while the inner ribbons move the material from the center to ends. Radial movement is achieved by rotational motion of the ribbons. The difference in the peripheral speeds of the outer and inner ribbons results in axial movement of the material along the horizontal axis of the blender. As a result of the radial and the counter-current axial movement, homogenous blending is achieved in short time. Blending is generally achieved within 15 - 20 min of start-up with a 90 - 95 % or better homogeneity. The particle size and its bulk density have the strongest influence on the mixing efficiency of the ribbon blender. Ingredients with uniform particle size and bulk densities tend to mix faster as compared to ingredients with variation in these attributes. After blending, the material is discharged from a discharge valve located at the bottom of the trough. The discharge can be fitted with any of various valves, viz. slide-gate, butterfly, and flush bottom, spherical and other types depending on the application. The operation of the valves can be manual or pneumatically actuated. Ribbon blenders can be designed for multiple discharge ports. In a ribbon blender the material is discharged by rotation of the ribbon agitator. It is practically difficult to achieve 100% discharge in the ribbon blender. A higher clearances between the external periphery of the outer ribbon and the container can result in unmixed spots at the trough bottom and can lead to discharge problems.

Advantages:

• Ribbon blender is used for solid-solid mixing with relatively fast blending cycles with reproducible product quality. 

• It requires very little maintenance even when subjected to frequent product changeovers. 

• It is a cost saving blender and is versatile in application to blend solids in combination with its ability to perform heating, cooling, coating, and other processes that makes it a very popular blender.

• (iv) Ribbon blenders can be designed to operate in both batch and continuous modes and can have huge built-up capacities of 50 m3 .

• It protects motor and ribbon blender from overload. When the load is too large to block and rotate, the working liquid is ejected from the fusible plug to separate the working machine and the load, so that the motor and equipment will not be damaged when starting and overloading. 

• This blender has stable start load features for the operation and for effective isolation of the impact and torsional vibration of the equipment. 

• When overloaded, the working liquid does not spatter. 

•  Hydraulic lift is stable, flexible and easy to operate and maintain, guide, safe and reliable.

• It is a small vibration mixing system with low noise.

Disadvantages: 

• In this mixing equipment the power consumption and mixing time is very high. 

• The efficiency or effectiveness of mixing is lower than other type of mixers. 

• The hydraulic coupler is not loaded with the inverter in general and cannot adjust the rotating speed of the ribbon blender effectively. 

• Although the loading of hydraulic couplings is easy, it cannot improve the start-up performance of the ribbon blender.

• The ribbon blender cannot be cleaned during use.   

Applications:

• Ribbon blenders are used to blend large volumes of dry solids.

• It can blend and mix dry free flowing powders as well as wet materials. 

• It is best suited for mixing of bulk drugs, formulation compositions, chemicals, and cosmetic powders. 

• It used in dry blending of components in capsule formulations. 

• It can also be used in lubrication of dry granules in large quantity.  

• It heats, cools, and dries materials during process. 

• It is employed in coating solid particles with small amounts of liquids to produce formulations. 

• It has applications in mixing of nutraceuticals, cosmetics and veterinary formulations.

• Its other applications include blending and mixing of abrasives, engineered plastic resins, pesticides and herbicides, animal feeds, epoxy resins, pet foods, bakery premixes, pigments, cake mixes, instant drink blends, fertilizers, plastic powders, carbon black, fire retardants, polyethylene, chemicals, gypsum, PVC compounding, leaning compounds, instant breakfast cereals, spice blends and dietary supplements. 

Sigma Blade Mixer 

• The sigma blade mixer is one of the most popular mixer used for mixing and kneading high viscosity materials. It belongs to the family of double arm kneader mixers. It is commonly used mixer for mixing high viscosity materials.

Principle:

• The principle involved in this mixers, wherein a very viscous materials are handled, is shearing which is produced by inter-meshing sigma shaped blades. This action promotes both lateral and transverse motion of the material. The geometry and profile of the sigma blade is designed such that the viscous mass of material is pulled, sheared, compressed, kneaded, and folded by the action of the blades against the walls of the mixer trough. The extent to which this happens depends on the action of the blades either tangential or overlapping and the speed of rotation of the blades. The helix angle of the blade can be modified depending on the required shearing.

Construction:

•  It consists of two mixing blades fitted horizontally in each trough of bowl. The shapes of blades resemble the Greek letter sigma (Σ) and hence are called as sigma blade mixers. The clearance between the blades and the vessel walls is as low as 2 mm. The low clearances produce high shear within the material. The blades are rotated through heavy duty drive systems consisting of a motor, gearbox, couplings, and gears. The top speed of the sigma mixer is generally limited to 60 m/min. The mixer trough has jacket for circulation of hot or cold media to maintain the required temperature conditions within the mixer. The discharge of the material from the mixer container is either by tilting of the mixer container or from bottom discharge valve or through an extruder/screw located in the lower portion between the two trough compartments.

Working:

•  Powders are loaded through the top of the trough to typically 40 to 65 % of the mixer's total volumetric capacity. The entire mixing process is carried out in a closed trough because of the dust. The blades move at a different speed using the drive system that includes motor, gear reducer, couplings, gears, bearings and seals. The material moves up and down and shears between the blades and the wall of the trough. The equipment is also attached to the perforated blades to break lumps and aggregates. The discharge of the material is either by tilting the mixing vessel, through bottom discharge valve, or through a discharge screw. The homogeneous mixture is obtained within 10 to 30 min with a mixing homogeneity up to 99%. 

Advantages:

• The sigma blade mixer leaves less dead spot during mixing operations. 

• It is ideal for mixing and kneading of highly viscous mass and sticky products. 

• These types of mixers and their variants can handle the highly viscous materials up to as 10 million centipoises.

• The provision of perforated blades makes it suitable for breaking lumps and aggregates. 

• The losses such as loss of volatile solvent during mixing can be prevented by closing the trough chamber and maintaining low temperature.

Disadvantages:

• The power consumption in sigma blade mixer is very high compared to other types of mixers. 

•  Requires more space for installation.

• It requires proper blade speed adjustments. 

• There may be dead spots in the mixing tank.

 Applications: 

• The sigma blade mixer is a commonly used mixer for high viscosity materials in preparing emulsions, syrups and ointments. 

• Sigma blade mixers are used for wet granulation process in the manufacture of tablets, capsules and pill masses. 

• It is used for solid-liquid mixing as well as for solid-solid mixing. (iv) 

• It is the best mixer employed in the mixing components of pasty, sticky, and gritty slurries with high viscosities. 

• Other applications of sigma blade mixer include mixing of adhesives, chemicals, chewing gum, food and confectionery products, hot-melts, inks and pigment products, soaps and detergents, sugar pastes etc.

 Planetary Mixer

•  Planetary mixer is high shear mixer and is available in variable speed drive. It is suitable for mixing of ointments, inks, sauces, pastes and foam used in the pharmaceutical, food, chemical and allied industries.

Principle: 

•  The principle of planetary mixer is based on rotation of planetary blades on their own axis while they travel around the center of the mixing bowl which ensures complete and effective mixing. The mixing blades revolve in opposite direction to sweep the entire circumference of the vessel as well as rotate around its own axis. Highest level of mixing is achieved in short intervals.  

Construction: 

•  Planetary mixer bowl, all its contact parts and beater are made up of stainless steel (SS304) or (SS316) materials, as per requirements. The mixer bowl is divided in to an upper cylindrical section and a lower hemispherical section. The mixing elements (beaters) used for mixing are shaped to match the lower curved surface of the bowl. The most commonly used beaters are the batter, wire whip, and hook type. Planetary mixers are provided with high-speed emulsifier in the centre of the bowl. Scrapers with Teflon edges may be provided to wipe any material which may stick onto the internal surface of the bowl. Single planetary mixers are constructed in a range of sizes. There are multiple bowls and beater designs with the same mixer assembly. For small size mixers the lifting and lowering of the bowl is through a manual arrangement. In case of larger units' bowl is lowered beneath the motor along with the beater using a motorized or hydraulically operated mechanical arrangement. There is a facility for the movement of bowl away from the machine using a suitable trolley with wheels. The single planetary mixers are available in small sizes to production size units with capacities of 50 - 300 L. The mixer motor may range upto 1.0 HP for smaller sizes to about 10 – 15 HP for the 300 L, depending on the material to be mixed.

Working: 

•  The planetary mixer works on homogenous mixing action. The material to be mixed is loaded in to the bowl. The planetary motion of the beater enables faster and better mixing of material at a considerably short time. The operation at slow speed is suitable for dry mixing with less dust generation and faster speed for kneading operation during wet granulation. The bowl is secured to a semi-circular frame at the time of mixing. Beaters can be intermittently cleaned completely by removing from vessel or by rotating the blades in the vessel loaded with solvent. After the mixing is completed, the bowl is lowered and can be easily detached and removed from the mixer assembly. The mixed contents are discharged either by hand scooping, or through a bottom discharge valve or by hydraulically operated automatic discharge systems.

Advantages:

• There are virtually no dead spaces in the mixing bowl. 

• Single planetary mixers are relatively inexpensive and versatile in applications, 

• Portable mixing bowl with top mounted agitator has a dual advantage of material transfer with no risk of packing contamination. 

• The product quality is better and more homogeneous. 

• The noise and vibration levels are low due to use of insulating materials and strong and solid construction.

• Security cover or bars and sensor technology reduces occupational labour accidents.

• It exhibits higher level of hygienic products and mixer is easy to clean.The sturdy equipment has long durability as it is made-up of high quality stainless steel.

• Being a compact equipment, it requires less space.

Disadvantages:

• The equipment generates heat which may be unsuitable for proper mixing. 

• When handling high viscosity materials the energy required is more. 

• As this equipment consists of moving parts maintenance is required. 

• As labour requirement is more, the process cost is also high.  

 Applications: 

• The single planetary mixer has common applications in the pharmaceutical, chemical and cosmetic industries.

• Planetary mixers are used for wet/dry solid-solid materials mixing.

• It is used in manufacturing of starch paste heated with steam or thermal fluid in electrically heating jacket with or without a flush bottom valve for discharge. 

• It is used for dry powder to wet phase mixing for wet granulation.

• The single planetary mixer is used for mixing of light pastes, gels, and dough. 

• This mixer is commonly used in food and bakery industry due to its simple construction and operation.

• These mixers are designed for intensive mixing, dispersing and kneading of adhesives, sealants, light caulks, pastes, coatings, granulations, and similar products of medium to high viscosity

 Propellers

• A propeller is a device consisting of a rotating shaft with propeller blades attached to it and used for mixing relatively low viscosity dispersions, liquids (low viscosity) and maintaining contents in suspension. It rotates at a very high speed (up to 8000 r.p.m.) and thus mixing operation is rapid. They are much smaller in diameter than paddle and turbine mixers.

Principle:

•  A propeller transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. The thrust from the propeller is transmitted to move the liquid through by the main shaft and finally by the propeller itself.  

Construction: 

•  The system consists of vessel fitted with propeller. The vessel is made-up of stainless steel 316L grade resistant to acids, alkalis and abrasion having curved or flat-bottom. It has 1.5:1 height to width ratio. The vessel is covered with cooling jacket for better temperature control. Propellers consist of number of blades attached to the shaft. Generally, 3-6 bladed design is most commonly used for mixing liquids, Fig. 8.7. Blades may have right or left handed orientation. Two or more propellers are used for large and deep tanks. The size of propeller is small and may be increased up to 0.5 m depending upon the depth size of the tank. Typically the diameter of the propeller is within the range 0.13 - 0.67 of the tank diameter. Small size propellers can rotate up to 30000 r.p.m. and produce longitudinal movement. Radial agitators consist of propellers that are similar to marine propellers. They consist of two to four blades that move in a screw like motion propelling the material to be agitated parallel to the shaft. A propeller agitator is shaped with blades tapering towards the shaft to minimize centrifugal force and produce maximum axial flow. Propeller agitators are popular for simple mixing applications.

Ideal properties of propellers: 

• They need to provide a wide range of operating speeds and hence shearing rates. 
• They must ensure complete (homogeneous) mixing.
• They should be economical in use 
• They must provide effective mixing in dispersion of solids in the production of pharmaceutical suspensions without changing particles size.
• They must allow for different mounting typos such as off-centre and side-entry. 
• They must be easy to clean after use. 

Working: 

•  The liquids to be mixed are transferred to the mixing vessel through pumping. When propeller is started a vortex is formed due to centrifugal force imparted to the liquid by the propeller blades, Fig. 8.8. This action causes liquid bulk to back-up around the sides of the vessel and create a depression at the center around the shaft. The liquid flow patterns achieved by propellers are axial and may be directed towards the top or towards the bottom of the vessel depending up on whether the drive is rotated clockwise or anticlockwise. As the speed of rotation is increased air may be sucked into the fluid by the formation of a vortex. The vortex formation is suppressed to considerable levels by fitting baffles in vertical position into the vessel. Vertical propeller mixer consists of three blades of suitable size depending upon the diameter of vessel. Horizontal or inclined propeller or marine propellers can also be used on side-entry mixers. Propellers are mounted on the impeller shaft inclined at an angle to the vessel axis to improve the mixing. It provide good blending capability in small batches of low to medium viscosity liquids. 

Advantages: 

• Propellers are used when high mixing capability is required.

• They are very effective for mixing liquids having maximum viscosity of 2 Pa.s or slurry with up to 10% solids of fine particle size. 

•  They are effective in mixing gas-liquid dispersions at laboratory scale. 

• Propellers increases the homogeneity of materials,

• Can be used in two different patterns for drying and pressing 

Disadvantages:

• Propellers are not effective with liquids of viscosity greater than 5 Pa.s. For example, glycerin and castor oil. 

• Need to be operated at high speed to avoid solid settlings in reactor vessels. 

• They need to be operated at low speeds if drying is an additional objective. 

• Vortex causes frothing and possible oxidation.

Applications: 

• Propellers are used when huge quantities of liquids and dispersions are to be mixed.

• These are useful for mixing liquids having a viscosity of about 2 Pa.s. 

• These can handle corrosive materials with glass lining.

Turbine Mixer

• Turbine mixer is another type of process agitator. They are used as an alternative to propellers for mixing low viscosity liquids and typically for the effective mixing of medium viscosity liquids. The velocity of mixing using turbine systems is low compared to propellers.

Principle:

•  Turbine mixer agitators can create a turbulent movement of the fluids due to the combination of centrifugal and rotational motion. These combined motions causes effective mixing of low to medium viscosity fluids.

Construction:

•  A turbine consists of a circular disc to which a number of short blades are attached. Compared to propellers the diameter of turbines is approximately 0.13-0.67 to that of the diameter of the vessel, with 0.33:1 being most typical. There is a wide range of turbine  designs, Fig. 8.9. The blades may be straight, pitched, curved or disk type. Turbine rotates at a lower speeds usually 50 – 200 r.p.m. than the propellers. Flat blade turbines produce radial and tangential flow but as the speed increases radial flow dominates. Pitched blade turbine produces axial flow. Near the impeller zone of rapid currents, high turbulence and intense shear is observed. Shear produced by turbines can be further enhanced using a diffuser ring. The diffuser ring is a stationary perforated ring which surrounds the turbine.

Working:

•  A mixer is filled through with an opening at its top. Usually, it is a pan or drum within which mixing blades revolve about the vertical axis. The variable speed drill with turbine mixer whip air into the material mixture. The air in mixture yield bubbles contributing mixing. The top entry turbine mixer is fitted with various impellers and turbines to suit heat and mass transfer in solids, suspensions and liquids. This type of mixer does not damage product. Top entry high shear causes uniform emulsification and homogenization. The mixing blades revolve about the vertical axis. 

Advantages:

•  Turbines are used in emulsification as they generate higher shearing forces than propellers even at low pumping rates. 

• They are effective in mixing high viscous solutions with a wide range of viscosities up to 7.0 Pa.s. 

• They are highly suitable for making dispersion containing 60% solids. 

• Turbines are suitable for liquids of large volume and high viscosity with the additional provision of baffles in the tank. 

• As they generate high radial flow efficiency of mixing is high. 

• In low viscous materials of large volumes turbine create a strong current which spread throughout the tank destroying stagnant pockets.  

Disadvantages:

• They are not preferred for solvents with high viscosity such as more than 20 cP. 
• There is possibility of air entrapment that may cause oxidation of material being mixed.

Applications:

•  (i) Highly used in chemical reactions and extraction operations. For example, liquid and gas reactions.
• Used in preparing emulsions, suspensions and syrups. 

Paddles

• Paddles are used as impellers in mixing liquids. It consists of flat blades attached to a vertical shaft and rotates at speed lower than 100 r.p.m. A variety of paddle mixers having different shapes and sizes, depending on the nature and viscosity of the product are available for use in industries. Paddle is one of the most primary types of agitators with blades that reach up to the tank walls.

Principle:

•  Paddle agitators work with producing a uniform laminar flow of liquids. Paddles push the liquid radially and tangentially with almost no axial action unless blades are pitched. The large surface area of blades in relation to the container helps them to rotate in the proximity of walls of the container and effectively mix the viscous liquids or semi-solids. . 

Construction: 

•  A paddle consists of a central hub with long flat blades attached to it vertically. Two blades or four blades are very common, Fig. 8.10. Sometimes blades are pitched and may be dish or hemispherical in shape and have a large surface area in relation to the tank in which they are used. A paddle rotates at a low speed of 100 r.p.m. In deep tanks several paddles are attached one above the other on the same shaft. At very low speeds it gives mild agitation in unbaffled tank but as for high speeds baffles are necessary.  

Working: 

•  The material to be mixed is charged from the top of the trough. If required, liquid spraying arrangement can be provided. The normal filling level is slightly above the shafts. Thus, there is surplus space in the mixer trough to provide air around the particles so that they can move freely. The random movement in the trough gives 98 - 99% mixing homogeneity. The peripheral speed of paddles is about 100 p.m. 

Advantages:

•  As paddle-impellers mixes liquids with low speed the possibility of vortex formation is least.
•  These are heavy duty mixers suitable for slow operation.
• They can mix systems effectively with 2 or 4 blades. 

Disadvantages: 

• Mixing of the suspensions is poor, thus, baffled tanks are required. 
• As they are heavy duty mixers power consumption is very high.
• They are not efficient for mixing variety of materials with different consistencies. 

Applications:

• Paddles are used in the manufacture of antacid suspensions and antidiarrheal mixtures such as bismuth-kaolin mixture.
• They are used in the mixing of solids, slurries, crystals forming phases during super saturated cooling. 

Silverson Emulsifier Mixer

•  Silverson mixer has been the leader in high shear mixing for over 60 years. The Silverson high speed shear mixer is used for mixing liquid/liquid or liquid/solid mixtures at speeds upto 8000 r.p.m. The mixer body is raised or lowered electrically. It stops automatically at the higher and lower limits of movement. It has several accessories with the mixer, which can be interchanged depending on mixing conditions required and material being mixed.

Principle: 

•  The principle of Silverson mixer is based upon shearing force. It produces intense shearing forces and turbulence by use of high-speed rotors. Circulation of material takes place through the head by the suction produced in the inlet at the bottom of the head. Circulation of the material ensures rapid breakdown of the dispersed liquid into smaller globules.

Construction:

•  It consists of long supporting columns and a central portion. Central portion consists of a shaft which is connected to motor at one end and other to the head. Head carries turbine blades. Blades are surrounded by a mesh, which is further enclosed by a cover having openings. It consists of following other accessories: 

•  General purpose disintegrating head: Suitable for general mixing or disintegration of solids.  

• Square hole high shear screen: High shear screen suitable for emulsion and fine colloid suspension preparation.

• Emulsor screen: Lower shear screen suitable for liquid/liquid and emulsion preparations.

• Axial flow head: Used in addition to one of the above screens to force liquid flow upwards This can reduce aeration or help maintain large suspensions, but high mixing speeds can cause liquid to be ejected from the mixing vessel. 

Working:

• The precision-machined Silverson work head generates exceptionally high shear rates in a three stage process that rapidly homogenizes the product to the required uniformity. 

• The high-speed rotation of the rotor blades within the precision-machined mixing work head exerts a powerful suction, drawing liquid and solid materials into the rotor/stator assembly

• The centrifugal force drives the material towards the periphery of the work head where they are subjected to a milling action in the precision-machined clearance between the ends of the rotor blades and the inner wall of the stator, 

• Finally, the intense hydraulic shear wherein the materials are forced at high velocity out through the perforations in the stator, then through the machine outlet and along the pipe work. At the same time, fresh materials are continually drawn into the work head, maintaining the mixing and pumping cycle.   

Advantages:

• It is efficient and rapid in operation and is capable of reducing mixing times by up to 90%.
• It is speedy, versatile, self-pumping, aeration-free and efficient shear mixer.
• It is capable of producing a fine droplet or particle size, typically in the range of 2 - 5 microns.
•  Being proficient it reduces operating costs.
• The product to pass through the homogenizer at a much faster rate, normally in a single pass.
• Being versatile it allows performing the widest range of mixing applications. 

Disadvantages: 

• It requires feed to be premixed and of uniform and fine globule or particle size.
• It may face a problem of mesh clogging. 
• It requires more power 

Applications: 

•  It is used to mix, emulsify, homogenize, solubilize, suspend, disperse and disintegrate solids. 
• It is used for homogenization of vast variety of products such as creams and ointments, lotions, sauces and flavour emulsions. 
•  A homogeneous product is rapidly produced when blending liquids are of similar or greatly varying viscosities, eliminating problems of stratification. 
•  It is used to prepare emulsions in the range of 0.5 to 5 microns. 
•  Silverson mixers can disintegrate matter of animal, vegetable, mineral or synthetic origin in a single operation.
•  It is used to uniformly mill both solid and semi-solid materials into either solution or fine suspension. 
•  It is also used in rapid gelling. For example, solubilizing and dispersing gums, alginates, CMC, carbopols etc., resulting in an agglomerate-free solution within minutes.