Paste slakers are best suited to applications with continuous, stable operating conditions, and high-quality quicklime. Paste slakers are slightly more expensive than comparably sized detention slakers but require slightly less power for mixing. A well-tuned paste slaker can produce a relatively high-quality hydrate particle, but it will be difficult to achieve the same level of lime slurry efficiency that can be achieved with a detention slaker.
The paste slaker consists of two horizontal paddle shafts, which counter-rotate to mix water with quicklime. The mixture is driven toward the discharge (weir) end by means of pitched paddles attached to the shafts. Cut-off sprays at the weir flush the paste out of the slaker. Beyond the weir slaked lime is further diluted by means of manually controlled spray jets integral to the slaker assembly. Product is then discharged into the grit removal system (vibratory screen or inclined screw) before discharging into the process or storage tank.
Paste slakers are available in quicklime slaking capacities up to 4,000 kg/hr. They operate at a water to lime ratio between 2.5:1 and 3.0:1. Water to lime ratio is controlled based on torque, either directly using a clutch or electronically by monitoring amperage draw on the agitator motor. Because torque can be affected by factors other than the slurry consistency (build up on paddles, bearing wear, etc.) it is important to visually verify the consistency of the paste on a regular basis. This is particularly challenging at start-up prior to reaching steady state. Paste slakers operate between 90 oC and boiling, however because the slaking reaction is not fully homogeneous, the resulting slurry reactivity is not as high as what would be produced with a detention slaker running at a slightly lower temperature.
Typical operating temperature rise in a paste slaker is approximately 75 o
C above the temperature of the incoming water. Slakers of this type can frequently be operated without additional heat to the incoming water, however if the incoming water is relatively cold (less than 10 o
C) some supplemental heat would be recommended. Pre-heating the water at start-up, in particular, can be very beneficial as it will allow the slaker to achieve steady state much quicker (as quick as 5 minutes). Resulting slurry discharge after necessary dilution is typically 18-20% solids at the slaker’s rated maximum capacity.