Limestone retention time

The un-reacted limestone, together with calcium sulfite and some calcium sulfate, collects in a sump at the base of the absorber which gives a liquid retention time of about 5 min. The solids are kept in suspension by agitators. Compressed air is blown into the slurry to oxidise the calcium sulfite. The slurry is then recycled to the spray section to absorb more oxides of sulfur and to ensure that most of the limestone is reacted. 

In the induced jarosite precipitation and primary neutralization processes, this is typically done in-slurry, at temperatures in excess of 80°C. Few issues with scale formation occur, apparently due to the presence of a large surface area available from the leach residue solids, relative to equipment surfaces. It is important to provide adequate retention time for the complete reaction of the limestone, to avoid reactions continuing to occur in downstream thickeners and pipework. Such reactions can lead to both process upsets and scale formation over the longer term. 

Furthermore, the dolomite requirement is directly proportional to d /dt. Consequently, is altered in magnitude which is directly proportional to the changes made in k3(To) and r. The changes in the limestone requirement are also related to the residence time of the limestone in the bed. If the rate of reaction k3(To) is increased, less reaction time is needed to achieve the same degree of sulfur retention. Shorter residence times are obtained by increasing the limestone feed rate for the same bed volume. Thus, f will increase with an increase and d /dt. Alternately, if for the same volumetric feed rate d /dt is increased, an improved sulfur retention will result. 

These results suggest that if the feed size of limestone is kept fixed, an increase in the limestone feed rate will result in the reduction of sulfur absorption efficiency- These results also emphasize that if the same sulfur retention is to be obtained when the size of the limestone particles is decreased the feed rate must be increased. However, for the same feed rate of limestone, a decrease in the size of limestone particles results in an increased sulfur retention. This may be explained on the basis of an increase in the overall surface area per unit volume of the bed when the average diameter of the particles decreases. It may be noted from Figure 9 that regardless of the limestone particle size, if sufficient residence time is allowed for limestone particles in the bed, it is possible to obtain sufficiently high sulfur retention. 

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