Solid Mixing, Liquid Mixing, Semisolid Mixing: Definition. Objectives/Application, Principle, Construction Working of Double Cone blender, turbine mixer, Triple Roller Mill, Silverson Mixer Grinder.

Definition: It is a process to bring particles of two or more components as near as possible. Mixing is also known as blending.

Types of Mixture: Mixing forms three types of mixtures. These are

• 1. Neutral mixture: ingredients are not easily miscible. Mixture ingredients cannot be separated after mixing. Example powder mixing.

• 2. Positive mixture: Ingredients are easily miscible. Mixture ingredients cannot be separated easily. Example liquid-liquid mixing or solid-liquid (solute solvent) mixing to form solution.

• 3. Negative mixture: Ingredients are not easily miscible. Mixture ingredients can be separated easily. Examples mixing of two immiscible liquids to form emulsion. Mixing of insoluble solids in liquid to form suspension.

Objectives/Applications: The main objective of mixing is to make a product of uniform composition. Mixing is carried out to form the following products

• 1. Mixture formation: To form a mixture of uniform composition

• 2. Chemical reaction: To enhance the rate of a chemical reaction between two components

• 3. Physical reaction: To enhance physical reaction between two or more components, such as dissolution, diffusion, etc

• 4. Suspension formation: To disperse insoluble powder particles in liquid to form a suspension,

• 5. Emulsion formation: To disperse water in oil or oil on water to form an emulsion.

• 6. Semisolid preparation: To form semisolid preparations such as cream, paste, ointment, etc.

• 7. Solution formation: To dissolve solute in a solvent.

• 8. Powder: Two or more powder ingredients are mixed to form powder dosage forms or granules.

Types of mixing: There are three types of mixing. These are solid mixing, liquid mixing, and semisolid mixing.

SOLID MIXING OR POWDER MIXING

The process of moving powder particles of two or more solids adjacent to each other is called powder mixing

Mechanism/Principle of mixing: The following principles are used during mixing. It may be liquid mixing, solid mixing, or semi-solid mixing.

• 1. Convection mixing: There is a bulk transfer of particles in the group from one part of the mixture bed to another part of the mixture bed. Blades or paddles are used for this purpose.

• 2. Shear mixing: Shear forces are applied to the mass of materials. This causes thinning in dissimilar layers. This promotes the mixing of two dissimilar substances. The agitator is used for this purpose.

• 3. Diffusive mixing: It is mixing at particle levels. Random motion of particles or molecules changes its position in the mixture bed.

Objective of Solid Mixing: Powder mixing is carried out to form bulk powder of uniform composition. They can be divided into different doses. Each single dose contains the same proportion of ingredients.

Application: Powder mixing is required in the manufacturing of tablets, capsules, granules, and compound powder dosage forms.

Machinery and equipment for solid mixing: Several machinery and equipment are used in powder mixing such as a tumbler mixer, Agitated powder bed mixer, double cone mixer, etc.

Double Cone Mixer: It is also called a Double Cone Blender.

A. Objectives: To mix solid ingredients in dry powder form to form powder of uniform composition (Homogeneous powder).

B. Application: It is used to mix dry powder ingredients and granules in the manufacturing of tablets, capsules, and powder dosage forms of uniform composition.

C. Principles: It is a type of tumbler mixer. Free-flowing dry powder particles slide over each other during mixing. It causes the mixing of powder ingredients by tumbling mixing. Double cane mixer has blades for mixing. These blades develop shear force for powder mixing. Thus during sliding and tumbling, mixing occurs by convection mixing and shear mixing.

D. Construction: It is made up of stainless steel vessels. Vessels have two cones joined with the cylindrical vessel. The vessel has stainless steel blades that act as baffles. A motor is attached to a double-cone vessel to rotate it at its horizontal axis. The motor can rotate the vessel at variable speed.

One end of the double-cone vessel has a butterfly valve. It can open and close this end. It is used to remove the mixture from the double-cone vessel after mixing. The other end has a hole with a lid. This end is used to fill powder ingredients into a double cone vessel for mixing.

The capacity of a double-cone blender varies from 5 Kg to 200 Kg. It can rotate at its horizontal axis at a speed of 30 to 100 rpm (Rotation per minute).

E. Working: Dry powder ingredients to be mixed are loaded into a double cone vessel. Powder ingredients should occupy 50 to 60% volume of its total capacity. The double-cone vessel is rotated by using an electric motor. The rate of rotation should be optimum to produce a tumbling effect in a powder bed. Too slow speed will only cause sliding in a powder bed. There will be little mixing. Too high speed will cause the powder to stick to the wall of the vessel. There will be no mixing.

At optimum speed, the powder bed will rise up to the highest position and fall down to the bottom. This will cause tumbling in a powder bed. The cone shape at both ends of the vessel also facilitates mixing during sliding and movement of the powder bed inside the vessel. During falling, powder particles will strike with blades inside the vessel. This strike will divide the powder bed into different parts and also develop shear force for mixing.

F. Merits (Advantages):

• 1. Easy to load feed and collect mixture due to its cone shape structure at both ends of a cylindrical vessel,

• 2. Cone shape provides stability to double cone blender machine during its working.

• 3. Easy to mix large quantities of dry powder ingredients due to cone shape,

• 4. Easy to clean and maintain

• 5. No dead spot inside the vessel due to the cone-shaped structure and baffles inside the vessel.

• 6. Easy to fix the rotation speed of the vessel by using an electric motor,

• 7. No contamination of product during mixing.

• 8. Wear and tear is very little.

• 9. Able to mix powder ingredients of different densities.

G. Demerits (Disadvantages): It produces low shear force.

• 1. It is not suitable to mix fine powder ingredients due to low shear force.

• 2. It is not suitable to mix powder ingredients with a wide range of particle size differences. It is due to low shear force.

• 3. It is a batch process.

• 4. It requires more space to install.

Alok Bains

LIQUID MIXING:

There are two types of liquid mixing. These are liquid-liquid mixing that forms solution and emulsion. Another is solid-liquid mixing that forms solution and suspension. There are several types of machinery and equipment for liquid mixing. Such as shaker mixer, propeller mixer, turbine mixer, and paddle mixer.

Objective: To form a uniform mixture of solute and solvent. To form a uniform mixture of two immiscible liquids.

Application: It is carried out to prepare solution and emulsion.

Mechanism/ Principle: Shear force is applied to liquid by using a propeller, turbine, or paddle for mixing. They develop four types of movement r in liquid bed during liquid mixing. These are bulk transport, turbulent flow, laminar flow, and molecular diffusion.

• 1. Bulk transport: Movement of large amounts of liquid parts from one location to another location inside a mixing vessel. For this purpose rotating blades or paddles are used.

• 2. Laminar flow: Laminar flow inside liquid stretches the interface between two liquids. It promotes mixing. During stretching folding these layers also takes place. It further facilitates the mixing of liquids.

• 3. Turbulent flow: Fluctuation in liquid flow velocity develops turbulent flow. There will be the formation of eddies in liquid. Eddies are small pockets of liquid that will move in different directions with different speeds in turbulent flow. Large eddies will be converted into small eddies during mixing. It further breaks down to form indistinguishable eddies. The speed of eddie's movement decides the intensity of the turbulent flow.

• 4. Molecular diffusion: It is the last stage of liquid mixing. Molecules of each liquid diffuse into each other at the interfacial layer during laminar flow and from on indistinguishable eddies to another during turbulent flow. The eating of liquid provides thermal heat. It also facilitates this molecular diffusion.

Turbine Mixer:

A. Objectives: Mixing of liquids with high viscosity.

B. Application:

• a. It is used to prepare emulsion, suspension, solution, syrup, etc.

• b. It is suitable to promote chemical reactions in between liquid components and rate of extraction.

C. Principle: The turbine and vessel ratio is very small. Thus it produces a high shear force suitable for mixing highly viscous liquids. The flat blade of the turbine at low speed produces radial and tangential flow in liquid. With further increase in speed, there will be radial flow. The curved blade in the turbine produces axial flow. There will be high turbulence and high shear force around the blades of the turbine. A combination of centrifugal force and rotational force developed by the turbine causes turbulent flow in liquids. This promotes molecular diffusion in liquid. The presence of baffles on the wall of the vessel breaks the rotational flow of liquid. This also generates turbulent flow.

D. Construction: It consists of a vessel and circular disc impeller. Disc impeller has short, straight, or curved blades. The disc impeller is attached to the vertical shaft. An electric motor can rotate this vertical shaft and impeller disc at its vertical axis. The speed of rotation shall be 50 rpm to 200 rpm. The wall of the vessel may have baffles. These baffles increase the efficiency of the turbine mixer.

E. Working: Vessel is filled with liquids. Turbine diameter shall be 30 to 50% of vessel diameter. The turbine is rotated at speeds of 50 rpm to 200 rpm depending upon the viscosity of liquids. Variable speed of turbine develops aeration and formation of air bubble. These air bubbles further promote liquid mixing.

F. Merits/Advantages:

• a. Generate high shear force thus suitable for emulsification.

• b. High shear force makes it suitable to mix highly viscous liquid up to 7 Pascal.

• c. High shear force makes it suitable to form a suspension containing less than 60%.

• d. High shear force does not allow the formation of dead pockets inside the vessel during mixing.

• e. It is suitable for mixing large volumes of liquids.

• f. It is highly efficient due to radial flow in liquid.

G. Disadvantages:

• a. Not suitable for liquid with a viscosity of 20 Pascal or more.

• b. Aeration may cause deterioration in the product.

Alok Bains

SEMI-SOLID MIXING:

Semi-solids are neutral mixtures. It cannot be separated after their mixing. Semisolid mixing is carried out for the preparation of ointment, paste, cream, etc. There are several equipment for semisolid mixing such as planetary mixer, agitator mixer, sigma arm mixer, triple roller mill, etc.

Objectives: To produce a homogeneous semisolid dosage form.

Applications: Preparation of ointments, pastes, creams, jellies, etc.

Mechanism: The mechanism of semisolid mixing mainly depends upon the nature of the semisolid and the machinery used for semisolid mixing. Mixing of miscible liquids in semisolid occurs by simple dispersion technique. They require low shear force for mixing. Simple thermal energy and stirring promote their mixing. Mixing of immiscible solid or immiscible liquid in semisolid form requires high shear force.

It involves convection mixing, shear mixing, and diffusion mixing.

• 1. Convection mixing: It is the transfer of the bulk of a component from one part of the mass to another part.

• 2. Shear force: The application of high shear force stretches the interface between two surfaces of masses to be mixed. During stretching, folding of these layers also takes place. This promotes mixing.

• 3. Diffusive mixing: Molecular diffusion and particulate diffusion take place at this interface by application of further shear force. This step makes semisolid homogenous preparation.

TRIPLE ROLLER MILL

A. Objectives: The mixing of solid powder in a semisolid base such as an ointment base.

B. Application: To prepare smooth gritty free semisolid preparation containing a high concentration of solid ingredients.

• a. Homogenisation of semisolid products such as cream, ointment, paste, etc.

• b. Solid dispersing in semi-viscous preparation such as cream.

C. Principle: It generates high shear force and friction for semisolid mixing. These shear forces and friction also cause diffusive mixing to form semisolid dosage forms. There will be mixing, refining, dispersing, and homogenization to form a smooth semisolid dosage form.

• a. Shear force: Applying high shear force stretches the interface between two surfaces of masses to be mixed. During stretching, the folding of these layers also takes place. This promotes mixing.

• b. Diffusive mixing: Molecular diffusion and particulate diffusion take place at this interface by application of further shear force. This step makes semisolid homogenous preparation

C. Construction: It consists of three hard abrasion resistance rollers. They are fitted with little space between each other. The gap between the first and second roller shall be greater between the second and third roller. These rollers rotate at their horizontal axis by using electric motors. There will be variations in the rate of their rotation. They also rotate in opposite directions to each other.

D. Working: Materials to be mixed are placed in between the first and second roller by using a suitable hopper. The first and second roller rotates at different speeds in opposite directions. This results in the size reduction of solid ingredients along with mixing due to compression and friction. Semi-solid preparation sticks with the wall of rollers. Semisolid preparation with a second roller wall enters into the gap between the second roller and the third roller. This gap is less than the first gap. Thus there will be further size reduction and mixing will occur. Mixed smooth semisolid preparation will stick with the wall of the third roller. It will be scraped by a scraper and collected as a product.

E. Merits: Smooth semisolid product.

F. Demerits: Rollers rotate at different speeds which generate friction. That may cause contamination in the product

HOMOGENISATION: It is a process of preparing fine emulsions from coarse emulsions.

Silverson Mixer Grinder

Principle: Silverson mixer grinder rotor rotates at a very high speed. High-speed rotation of rotors generates turbulence and high shear force. Turbulence causes thinning in liquid layers and shear force develops the formation of eddies. Eddies facilitate liquid-liquid mixing or liquid-solid mixing to form a homogeneous mixture. It is suitable for emulsification, dispersion, and dissolution.

Construction: Silverson mixer grinder consists of an emulsifier head. It has turbine blades/impellers surrounded by a fine-meshed stainless steel sieve. Turbine blades/impellers are attached to an electric motor through a shaft. The motor rotates the blade/turbine at a very high speed to generate turbulence and shear force.

Working: Emulsifier head is placed into liquids inside the container. The liquid to be mixed is sucked through a fine sieve into the emulsifier head. High-speed rotation of blades mixes the liquids vigorously. Mixed liquid passes out through a meshed sieve with a great shear force. Sucking of liquid into the emulsifier head and passing out mixed liquid through a meshed sieve was repeated several times during the mixing process. This breaks the liquid globules into fine globules. It produces fine emulsion.

Advantages:

• 1. Very efficient, reliable, and aeration-free for liquid mixing

• 2. It produces fine emulsion

Disadvantages

• 1. It is a batch process,

• 2. Sieved may clog during operation.