Editing Mixing (process engineering) (section)

==Solid–solid mixing==
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'''Dry blenders''' are a type of [[industrial mixer]] which are typically used to blend multiple dry components until they are [[Homogeneity and heterogeneity|homogeneous]]. Often minor liquid additions are made to the dry blend to modify the product formulation. Blending times using dry ingredients are often short (15–30 minutes) but are somewhat dependent upon the varying percentages of each component, and the difference in the bulk densities of each. Ribbon, paddle, tumble and vertical blenders are available. Many products including [[Pharmaceutical industry|pharmaceuticals]], [[food]]s, [[Chemical substance|chemicals]], [[fertilizer]]s, [[plastic]]s, [[pigments]], and [[cosmetics]] are manufactured in these designs. Dry blenders range in capacity from half-cubic-foot [[laboratory]] models to 500-cubic-foot production units. A wide variety of [[horsepower]]-and-speed combinations and optional features such as sanitary finishes, vacuum construction, special valves and cover openings are offered by most manufacturers.

Blending powders is one of the oldest unit-operations in the solids handling industries. For many decades powder blending has been used just to homogenize bulk materials. Many different machines have been designed to handle materials with various bulk solids properties. On the basis of the practical experience gained with these different machines, engineering knowledge has been developed to construct reliable equipment and to predict scale-up and mixing behavior. Nowadays the same mixing technologies are used for many more applications: to improve product quality, to coat particles, to fuse materials, to wet, to disperse in liquid, to agglomerate, to alter functional material properties, etc.
This wide range of  applications of mixing equipment requires a high level of knowledge, long time experience  and extended test facilities to come to the optimal selection of equipment and processes.

[[File:Machine for incorporating liquids and finely-ground solids.JPG|thumb|300px|right|Machine for incorporating liquids and finely ground solids]]

Solid-solid mixing can be performed either in batch mixers, which is the simpler form of mixing, or in certain cases in continuous dry-mix, more complex but which provide interesting advantages in terms of segregation, capacity and validation.<ref>{{Cite web | url=https://powderprocess.net/Mixing/Comparison_Continuous_Batch_Mixing.html | title=Batch and continuous solids mixing comparison - Powder Mixing}}</ref>
One example of a solid–solid mixing process is mulling foundry [[molding (process)|molding]] sand, where sand, [[bentonite]] [[clay]], fine coal dust and water are mixed to a [[Plasticity (physics)|plastic]], [[Molding (process)|mold]]able and reusable mass, applied for molding and pouring [[molten]] metal to obtain [[sand casting]]s that are metallic parts for automobile, machine building, construction or other industries.

===Mixing mechanisms===
In powder two different dimensions in the mixing process can be determined: convective mixing and intensive mixing.<ref>{{cite web|url=http://www.powderprocess.net/Mixing.html|title=Powder Mixing - Design - problem solving - Ribbon blender, Paddle mixer, Drum blender, Froude Number - PowderProcess.net|website=www.powderprocess.net|access-date=26 April 2018|url-status=live|archive-url=https://web.archive.org/web/20170928143738/http://www.powderprocess.net/Mixing.html|archive-date=28 September 2017|df=dmy-all}}</ref> In the case of convective mixing material in the mixer is transported from one location to another. This type of mixing leads to a less ordered state inside the mixer, the components that must be mixed are distributed over the other components. With progressing time the mixture becomes more randomly ordered. After a certain mixing time the ultimate random state is reached. Usually this type of mixing is applied for free-flowing and coarse materials.

Possible threats during macro mixing is the de-mixing of the components, since differences in size, shape or density of the different particles can lead to segregation.
 
When materials are cohesive, which is the case with e.g. fine particles and also with wet material, convective mixing is no longer sufficient to obtain a randomly ordered mixture. The relative strong inter-particle forces form lumps, which are not broken up by the mild transportation forces in the convective mixer. To decrease the lump size additional forces are necessary; i.e. more energy intensive mixing is required. These additional forces can either be impact forces or shear forces.