# Mechanical Operations Questions and Answers – Mixing of Solids

This set of Mechanical Operations Objective Questions & Answers focuses on “Mixing of Solids”.

1. Which one of the following rightly defines mixing?
a) Mixing converts a homogenous system to a heterogeneous system without adding any materials
b) Mixing converts a heterogeneous system to a homogeneous system without adding any materials
c) Mixing converts a heterogeneous system to a homogeneous system with or without adding any materials
d) Mixing converts a heterogeneous system to a homogeneous system with or without adding any materials

Explanation: Mixing converts a heterogeneous system to a homogeneous system. A single heterogeneous system can be agitated but not mixed without adding some other material to it. Therefore, sometimes an additional material needs to be added to ensure mixing.

2. Which of the following is the best difference between mixing and blending?
a) Mixing can only be done for a solid-solid system, while blending is done for solid-liquid mixture
b) Mixing is done for liquid-liquid, gas-liquid and solid-liquid systems, while blending is done for solid-solid mixture
c) Bending is done for liquid-liquid and gas-liquid systems, while mixing is done for solid-solid mixture
d) Mixing is done for liquid-liquid and gas-liquid systems, while blending is done for solid-solid mixture

Explanation: Mixing can be done for viscous materials, a system constituting of gases and liquids, a system consisting of solids and liquids or a system containing two liquids also. However, blending is a special term that is used only when two solids are mixed or a solid is mixed with negligible liquid.

3. Which of the following describes solid-liquid mixing the best?
a) The end result of solid-liquid mixing is always dissolution
b) Solid-liquid mixing suspends coarse solids in the liquids
c) Solid-liquid mixing cannot break lumps of aggregated solids
d) Solid-liquid mixing suspends coarse solids in the liquids and break lumps of aggregated solids

Explanation: Solid-liquid mixing does not always lead to dissolution as the end product. The result of solid-liquid mixing can also be a reaction or homogenization. Adding salt to water is an example of suspending coarse solids in liquids, while mixing flour with water is an example of breaking lumps.

4. Which of the following is correct about the factors affecting mixing of solids?
a) Mixing is independent of the nature of particle surface
b) Difference in particle size can lead to segregation
c) Demixing accelerates when smaller particles have lesser density
d) Particle shape does not affect mixing

Explanation: Mixing is dependent on the nature of the surface of the particle, as mixing rough surfaces is tougher. When particles are of the same size and preferably spherical in shape, mixing is efficient and segregation does not take place. Demixing accelerates when smaller particles have higher density.

5. Which of the following is false about speed of rotation in tumblers?
a) In slow rotation, enough shear is not applied
b) Speed of rotation depends only on size and shape of the tumbler
c) Rapid rotation leads to segregation
d) Tumblers are operated at a critical and optimum speed

Explanation: Efficiency of a tumbler is highly speed dependent. When the speed is low, the shear applied is less and the tumbling is not intense. On the other hand when tumbling speed is too fast, particles hit the wall and segregation takes place. The speed is not just dependent on the size and shape of the tumbler, but also the nature of the particles.
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6. Which of the following is correct about a twin shell blender?
a) The material is loaded approximately 80%-90% by volume
b) Twin shell blenders are U shaped
c) Materials inside undergo tumbling motion when rotated
d) Material is loaded via apex port and emptied through shell hatches

Explanation: The material is loaded approximately 50%-60% by volume. Twin shell blenders are V shaped and are therefore, also called V shaped blenders. Materials are loaded via small hatches and emptied through apex port.

7. Which of the following is not a disadvantage of tumbler with blades?
a) Large shear force cannot be applied
b) Potential packing
c) Scaling up the process
d) Cleaning the tumbler

Explanation: When a tumbler has blades, large shearing force can be applied which allows an intense mixing of particles. These particles can either be very fine or coarse. The blades can be used for both wet and dry intimate mixing.

8. What is the principle of mixing?
a) Shear force
b) Perpendicular force
c) Gravitational force
d) Centrifugal force

Explanation: The principle force of mixing is shear force. The shear force is provided by the movement of the blades and high shear rates can break lumps. These are unaligned forces that push one part of the body in one direction, and the other part in opposite direction.

9. Which of the following is a right match?
a) Static mixer – Mortar Pestle
b) Batch type – Fluidized mixer
c) Tumbling type – V cone blender
d) Continuous type – Ribbon blender

Explanation: A correct example of static mixer is Ribbon blender and Mortar Pestle. A fluidized mixer is an air mixer. An example of batch type mixer is Mortar Pestle. Barrel type mixers and zig zag blenders are continuous type mixers.

10. Which of the following is the correct match?

```I. V cone blender                                         i. Air supported blending
II. Fluidized mixers	                               ii. Tumbling action
III. Barrel type                                              iii. Rotating shell and rotatory blade
IV. Mortar pestle                                         iv. Trituration```

a) I-i, II-ii, III-iii, IV-iv
b) I-iii, II-ii, III-ii, IV-iv
c) I-ii, II-iii, III-vi, IV-i
d) I-ii, II-iv, III-iii, IV-i

Explanation: V cone blender is a type of tumbling mixer which works on the principle of tumbling. Fluidized mixers are also called air mixers that work on the principle of air supported blending. Barrel type mixer is a type of continuous mixer. Mortar pestle is a batch type mixer that involves thorough mixing of components.

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