Designing Calcium Hydroxide or Lime Slurry Mixing
Lime has a wide range of industrial uses. In slurry form, Lime is commonly used for pH adjustment primarily due to its cost & availability. One of the most common applications use lime for water treatment, processing lime from on-site powder silos that feed make-up slurry tanks. Others have pre-processed hydrated lime, quick lime or CHEMLIME using tanker truck deliveries either using permanent storage tanks or portable storage tank systems.
Applications that utilize lime include, coagulation or flash mixing & downstream flocculation in water treatment. Coagulation is the formation of jelly-like particles through the reaction of coagulant with the alkalinity and hardness that normally exists within water. Although there are many coagulants to consider, Aluminum Sulfate, also known as filter alum or commercial alum is the most popular coagulant. In all cases, these acidic coagulants will react with the alkalinity found within the process water stream. Normally, the amount of lime found within the process water is not sufficient, where if left untreated, the pH would drop like a stone. Filter Alum functions best between the narrow pH range between 5.8 & 7. As such, alkalinity, such as hydrated lime must be added to the process water flow stream to balance the pH range to maintain the efficiency of the coagulant.
Quicklime or Calcium Oxide (CaO) is normally supplied in crushed form called pebble lime. Pebble lime is slaked to produce milk of lime, usually at 25% solids. Slaking is a chemical reaction in which calcium oxide takes on water of hydration to produce calcium hydroxide Ca(OH)2. These mixer selections are not suitable criteria for mixing quicklime or calcium Oxide (CaO).
Hydrated Lime is a dry powder that is finely ground & air classified to remove impurities. Hydrated lime (Calcium Hydroxide) can be prepared using labor intensive 80# bags or an automated system using super sacks or some other method of dry feeding the hydrated lime.
As for the mixer design,
the use of an upper impeller is definitely recommended when considered for
storage of hydrated lime. In the event of
dewatering, the upper impellers purpose will be to quickly capture and entrain the
dewatered top phase and pump that water phase down into to lower regions of the
storage tank to quickly achieve a uniform slurry. Without the upper impeller,
the lower viscosity water phase may lie on the surface and take a significant
amount of time to reincorporate into the batch, causing undo strain upon the