Batch slaking

The above practices relate to batch slaking. In continuous slakers, primary nuclei are always being recycled and reduce the levels of super-saturation. As a result, for the same conditions, continuous slakers tend to produce coarser particle size distributions than batch slakers. If required, this effect can generally be offset by adjusting the slaking conditions. 

In a batch process, NaOH is chlorinated in the presence of recycled neutral Ca(OCl)2 mother Hquor. After separation of salt, lime slurry is added and chlorinated (205). The Ca(OCl)2 2H20 crystals are recovered by filtration. In another version, classification of the Ca(OCl)2—NaCl slurry gives a Ca(OCl)2-rich fraction that is filtered and the filtrate recycled along with the NaCl-rich fraction to the first chlorinator (206). Also, 50% NaOH and soHd slaked lime are used in the second chlorination. 

The experimental facility to produce larger amounts of CEB is shown in Figure 5. Batch-wise, approx. 1 ton of CEB can be produced in one week. The recipe to produce CEB is to mix Ca(OH) with water to produce so-called slaked lime at the optimised conditions referred to above. This slaked lime is pumped into a stined stainless steel reactor, and bio-oil is added over a period of about 1.5 hours. The mixture was slowly heated to 70 C over 30 minutes and kept at this tenq)eratuFe for a further 2 hours. The CEB to used for the large-scale testing was stored at ambient conditions. The mixture will be thoroughly mixed before spraying and co-combustion. 

Wood derived fast pyrolysis oil and slaked lime produce a Calcium-Enriched Bio-oil (CEB) capable to remove SO2 while combusting sulphur-containing fuels. The bio-oil required is produced in a 200 kg/hr fast pyrolysis plant, while CEB is made in batch-wise operation by slowly mixing slaked lime with bio-oil at controlled conditions. 

Today wet slaking is industrially carried out only by large consumers such as mortar manufacturers. In addition to batch processes continuous processes can also be used, which generally operate at 90°C. 

Continuous rotary and batch hydrators are used to produce calcium hydroxide (slaked lime) powder. The continuous version uses a slightly inclined, slowly rotating steel cylinder of about 1 m diameter by 6-7 m in length. The calcium oxide is fed into the upper end and the correct proportion of water is sprayed in, followed by tumbling in the cylinder to produce a uniform product. Open trough-type batch hydrators, in which the reacting components are mechanically combined, provide greater control of hydration rate and temperature and some improvements in the quality of the product. An explosion process in which hydration is conducted in a pressure vessel produces a slaked lime with better flow characteristics, and a smaller mean particle size more suitable for uses such as filters [14]. 

The success of this process depends on the low solubility of calcium carbonate, which is also what allows this process to be used for small-scale production of sodium hydroxide by a batch process. For batch operation, the functions such as slaking, mixing, and settling may be carried out in the same wooden (or steel) vessel. Separate units are required for these steps in the continuous process. A further chemical feature important to the recycle of the spent lime of this process is the relatively easier thermal loss of carbon dioxide from calcium carbonate than from sodium carbonate. Since sodium bicarbonate (NaFICOs) may be calcined at 175°C to obtain carbon dioxide loss, it might be expected that sodium hydroxide could be made by heating sodium carbonate at a higher temperature followed by hydration of the resulting oxide (Eqs. 7.21 and 7.22). 

The insoluble calcium carbonate (lime mud) is settled out from this mixture, filtered, and rinsed with water, which is then subsequently used for dissolving smelt. The supernatant liquor is now a solution of sodium hydroxide, sodium sulfide, and a small amount of unreacted sodium carbonate of nearly the correct composition required for a new batch of white liquor. Calcium carbonate in the lime mud is recycled by first calcining to decarbonate, followed by slaking to regenerate the calcium hydroxide required for a subsequent causticization (Eqs. 15.34 and 15.35). 

A modem design of batch slaker is described in [22.2], It consists of a horizontal cylinder agitated along its length with paddles. Quicklime (up to 20 mm in size) is blown at a controlled rate into the cylinder and water is added to maintain the temperature at 85 to 95 °C. The unit is able to slake a truckload of 201 in 1 to 1 /2 hours. It can be fitted with a wet scrubber for dust control and with a de-grit-ting system. 

---