Key Factors to Lime Slurry Usage in Water Treatment Processes
What is Lime?
Lime is a general term used for a range of calcium-based products. More specifically the three products most generalized as lime are:
- Quicklime is the term for the chemical calcium oxide (CaO)
- Hydrated lime is the term for the chemical calcium hydroxide (Ca(OH)2)
- Limestone is the term for the raw ingredient calcium carbonate (CaCO3)
It can be noted that there are also dolomitic variants to all three of the previously stated materials, which simply means that the material contains calcium and magnesium.
Both quicklime and hydrated lime are frequently used in water treatment processes for applications mentioned below, but calcium carbonate is very rarely used in treating drinking water, wastewater, and sludge. Lime products serve as sources of alkalinity and pH adjustment acting as “bases” meaning they increase the pH of an acidic or neutral pH water.
Why is Lime Important in Water Treatment?
Lime in water treatment is an economical solution for a variety of water treatment applications. Taking advantage of the relatively high pH and alkalinity of the material can contribute to safer and cleaner water.
More specifically, common water treatment applications include:
- Neutralize acidic wastewater
- Aid in the coagulation and removal of organics and pathogens
- Precipitate and remove constituents such as metals and various ions
- Prevent corrosion and leaching in water distribution piping
- Soften hard water sources at water treatment plants
- Stabilize biosolids produced from wastewater processes into sellable products
For these applications and more, lime products have become central to the design and operation of a wide variety of water treatment processes, including drinking water, wastewater, and sludge for decades.
How are Lime Products Prepared for Use?
For most drinking water and wastewater applications involving lime products, these products must first be converted to a slurry of calcium hydroxide in water. This resulting product is known as lime slurry, milk of lime, or by product brand names such as CALSAFE®.
Lime slurry is a suspension of calcium hydroxide solid particles floating in water and due to the low solubility of calcium hydroxide, only a small portion of it is dissolved at any given time in the slurry. However, as this dissolved material reacts with acids, metals and other constituents of the water, those dissolved materials are removed from solution and more solids are dissolved in their place.
Both quicklime and hydrated lime can be converted into slurry through specialized equipment systems. Quicklime is reacted with water in a device called a “Slaker” in a process called “slaking”. Slaking is an exothermic process meaning the heat is produced as the quicklime reacts with water and this heat must be managed by carefully controlling the ratios of water and lime and any resulting steam and dust must be captured for safety and housekeeping concerns. For these reasons specialized slaking systems are used to control this process in a nearly fully automated way to produce lime slurry.
Hydrated lime is mixed with water to produce lime slurry with several very similar characteristics as lime slurry resulting from quicklime, however the hydrated lime slurry process does not produce any significant heat and therefore the systems to control this process are simplified and the process is more straightforward.
In either case Storage and Transfer Technologies (STT) has solutions for your specific needs.
How Should I Safely Move and Handle Lime?
Quicklime and hydrated lime are solid bulk materials. Both products can be stored in silos for large volumes and supersacks (typically 1,000-2,000 pound) for smaller volumes in water treatment applications.
For bulk shipments of either product there are some circumstances for rail and truck bottom dump unloading into a conveyor connected to the plant's silo. However, rail and truckloads sized and equipped for pneumatic conveyance of the quicklime or hydrated lime into the silo are more common for water treatment operators.
In any case, it is recommended to have an unloading and storage solution that does not require direct handling of the lime by operators to avoid direct exposure or the hassle of moving bags by hand. The slaker or hydrated lime slurry system should be maintained regularly and controlled such that operator exposure is minimized.
Periodic equipment maintenance or sample collection will require operators to have contact with lime products, but with a few simple rules, this is not problematic. It should be noted that due to the relatively high pH of quicklime and hydrated lime, both products should not contact the eyes or mucus membranes as they are irritants that can cause damage to those surfaces.
Leaving lime on skin particularly when wet or sweaty can also irritate the skin, but rarely is this severe and simple skin contact can be remedied simply by rinsing with excess water.
Properly maintain dust removal systems per the manufacturers recommendations on slakers and hydrated slurry systems so that air exposure to dust is minimized.
Basic PPE recommendations when working directly with lime products are:
- Eye protection
- Gasketed ANSI certified safety glasses or sufficient for most circumstances
- Face shield could be advised if splashing of slurry is a risk that is present
- Dust mask if risk of dust exposure is present
- Job appropriate safety gloves
- Leather gloves for maintenance activities
- Rubber lab gloves for lab activities
- Note: If gloves become soaked with lime slurry, do not reuse
What Type of Slaker or Slurry System is Right for Me?
This is a complicated topic on which we would love to speak with you, but there are a few things that can be generalized.
Deciding between quicklime and hydrated lime is a logical first step. The most significant factors to consider are of economic concern. Quicklime is a less expensive reagent relative to hydrated lime on a weight/weight comparison. However, the capital costs of systems for hydrated lime are often less expensive than those for quicklime, though this paradigm does have some conditions in which there are exceptions.
Therefore, a payback analysis of both options will demonstrate which option is most economical for the end user based on the specific capital cost of their system and the market pricing for lime products available to them.
It should be noted however, that due to the simplified nature of the hydrated lime slurry system and the reduced manpower allocated to them, some plants may opt for hydrated lime even when quicklime is more cost effective for the needs of the plant.
To further expound on system options, there are a variety of designs for slakers each with their own benefits and drawbacks.
|Detention Slaker||Paste Slaker||Horizontal Ball Mill||Vertical Ball Mill||Mixtank/Batch Slaker|
|CAPITAL COST||Low||Moderate||High||High||Very low|
|MAX SIZE||26,000 kg/h||3,600 kg/h||50,000 kg/h||50,000 kg/h||500 kg/h|
|SLAKER CONTROL||Temperature||Torque||Temperature (delayed)||Temperature||Measured Inputs|
|GRIT HANDLING||Screen/Screw||Screen/Screw||Hydrocyclone||Separating Chamber||Screen/Screw|
|ABILITY TO BE OPTIMIZED||High||Low||Moderate||High||Low|
|PHYSICAL SIZE||Moderate||Small||Very Large||Large||Small|