Mixers & Blenders
Continuous benefits
Ajax Equipment’s Eddie McGee discusses the merits of batch and continuous mixing and blending and considers the question: what are the advantages of switching from batch to continuous mixing?
Mixing and blending solids with more solids, pastes and liquids can be performed with either batch or continuous mixers. Batch mixing is the most common form of mixing and blending. It has the advantages of being well-understood, enables ease of quality monitoring and precise control of ingredient quantities. Critical factors in optimising batch mixing are the condition and size of the ingredients to ensure a consistent blending time, the shape of the mixing vessel and profile of the mixing paddle. The paddle's profile and movement should prevent any dead zones in the vessel where ingredients remain unmixed.
Batch mixing has various drawbacks. Whereas in principle, the mixing can be continued until such time as satisfactory homogeneity is achieved, the quality of some products can suffer from overworking. In addition, batch mixing is particularly prone to inconsistencies between batches caused by powder segregation during processing, changes in temperature, humidity, powder particle size and storage. It also tends to be labour intensive and introduces problems of flow, residue, stock accommodation and the matching of output with filling and packing operations. Moreover, any problems that may be found in batches are only detectable after the mixing process has been completed, which is then time consuming and expensive to correct. In addition, during post processing care has to be taken when discharging the batch mix to reduce the risk of desegregation
as the mix is transferred to say a Big Bag or IBC for storage and distribution. Continuous mixing allows process engineers to side step many of the problems encountered in batch processing. The continuous mixer comprises either a singe screw or two screws rotating in opposite directions; powders and other materials are mixed as they are driven along the short casing of the machine. Liquor additions can be made by way of sprays or drizzles, as appropriate, as the material moves through the mixer.
Continuous mixers offer greater mixing flexibility than batch mixers and are particularly good for difficult to mix ingredients. The mixer is capable of introducing differing degrees of work content to the mixing operation by using either single or twin mixing shaft arrangements. The choice will depend on the powder composition and amount of mixing needed to achieve efficient dispersion to produce the final product. Compared with batch mixers, by working on the material at a smaller scale, paddle and ribbon screw geometries in continuous mixers allow greater control, and more efficient mixing with negligible damage to powders.
In addition, the choice of mixing element geometry can be used to provide a more intensive mixing action, while the open form and geometry of the ribbon screw is ideal for gentle mixing. Special forms of mixing flight construction are designed to inhibit the build
Total Show Stand 5811
www.solidsandbulk.co.uk
May 2010 • Solids & Bulk Handling 17
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72