Silo hydration

The ground quicklime is stored in a bunker and discharged through a similar mechanism to that described for hydrated lime (section 26.10.2). The quicklime is hydrated using either the drum, or the silo hydration process. 

Moderate and even low reactivity quicklimes can be used in the drum hydration process, because of the pressurised hydration stage (see section 13.2, Fig. 13.3, and Annex 1 for explanations of these terms). The reactivity of quicklimes used for the silo hydration process should be moderate to high , depending on the silo residence time. For both of these processes, the quicklime should be finely ground (e.g., with 90 % less than 75 pm) to ensure efficient dispersion and hydration. 

Fig. 6.7-45 Diagram of the manufacturing process of (hollow) building blocks made from lime and sand. 1, lime ball mill 2, elevator 3, wind sifter 4, 9, 11, silos 5, metering bucket 6, screw conveyor 7, sand feed 8, screen 10, batch mixing and hydration drum 12, proportioning 13, elevator 14, stone press 15, block cart 16, transfer platform 17, post-treatment (hardening) chambers 18, steam production Fig. 6.7-46 Sketch of a manual pusher furnace [B.28, B.97j...

BS 6463 [6.5], Part 101, applies to natural calcium carbonate (and also to quicklime and hydrated lime). It follows similar principles to BS 812, but is of more general applicability. It includes guidance on sampling from conveyers, bulk containers, packages, and silos. It also refers to the use of mechanical samplers. 

Bulk hydrate is discharged pneumatically from a vehicle into a reception silo, via a filling pipe (typically 100 mm nominal bore), which may be up to 40 m long. Bends should have a radius of at least 1 m. 

The silo may be constructed of steel, concrete, glass fibre, or wood and must be completely weatherproof. It should be vented via a bag filter. For calculating silo capacity, a bulk density of 480 k m is generally appropriate although the value may range from below 400 kg/m when the hydrate is aerated to 560 kg/m when it is fully compacted. 

Water absorption by cement, especially if the latter has a high C3A content, furthermore causes retardation of setting (because of diminished reactivity of the C3A) and, depending on how much water is absorbed, also causes loss of strength (in consequence of pre-hydration of the C3S in the cement). Besides, false set - temporary early stiffening of the cement when mixed with water - may also be due to causes associated with silo storage (see Section V.3). 

For the fly ash the Jenike design criterion estimates that their is no minimum critical outlet width for flow, which compares well, with experimental observations, which found the instantaneous outlet width to be below the minimum obtainable with the test silo. For the hydrated lime and olivine sand, the actual critical outlet widths at emptying were of a similar size to that of fly ash, but the Jenike method over predicted this by almost an order of magnitude. 

Beet pulp is the structural portion of beet roots that remains after spent beets are discharged from a diffuser. As it is highly hydrated at that time, the wet pulp is dewatered in pulp presses. Then it is discharged into wet silos, and it may be sold as wet pulp for animal feed. However, more often it is dried or mixed with molasses and dried, in both of which cases it is used in the manufacture of animal feeds. 

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