Hydrated lime (calcium hydroxide) is a dry, colorless crystalline powder manufactured by treating calcium oxide (quicklime) with water, in a process called "slaking." Also known as slack lime, builders lime or pickling lime, hydrated lime is used in the production of mortars, plasters, cements, paints, hard rubber products, petrochemicals, and in the tanning of leather.

A more recent development for hydrated lime use is in environmental applications. Here, hydrated lime, a mild alkali, is used for the removal of acid gases in coal-fired energy plants, cement mills, glassmakers and incinerators.1

However, the largest single use of hydrated lime remains steel manufacturing, where it is used to remove impurities. Hydrated lime is also used to produce other metals. In construction, the dominant use of hydrated lime is in soil stabilization for roads, earthen dams, airfields, and building foundations. Hydrated lime can be used to raise the pH of acidic soils. This is also referred to as soil "sweetening."2

Characteristics and Challenges:

Hydrated lime requires care during handling both to prevent it from escaping into the surrounding atmosphere, but also to avert contamination of the hydrated line from any airborne particles. Caution should be exercised to shield the hydrated lime from moisture or from contaminants containing moisture, which may trigger a chemical reaction that results in unexpected heat generation.

Caution should be exercised to shield the material from moisture or from contaminants containing moisture, as hydrated lime is exothermic. At the same time, priority should be given to efforts for the protection of operators and other handlers of hydrated lime, as depending
on the grade of material, hydrated lime is usually given an MSDS health (H) ranking of between 1 and 3 (progressively indicating a higher level of hazard) for its corrosive effects on the skin and eyes. Severe overexposure to hydrated lime by inhalation can produce lung damage, choking, unconsciousness or death. Because of these conditions, an airtight connection all the way from the storage vessels through to the processing equipment is strongly recommended.

Experienced handlers of hydrated lime have observed that no two samples are exactly alike. On average, hydrated lime has a bulk density of 40 lb/ft 3 and is a soft and somewhat sluggish powder that has a tendency to pack or smear. In some cases however, the hydrated lime will do neither, but may instead contain abrasive crystals. Hydrated lime has been known to "rat hole" or bridge in silos, bulk bags and feeder hoppers, stick to the sides of containers, flood while being fed into conveyors and build up inside the convey lines. Hydrated limes with the same bulk densities often have different characteristics. These variations and other features usually necessitate versatile handling equipment to successfully move it as well as consultation with an expert before deciding on a suitable solution.

If the hydrated lime arrives at the processing facility in bulk bags, the frames used to discharge the bags should be equipped with additional accessories to eliminate any agglomerations and completely empty the hydrated lime from the bag. This includes features such as spring-loaded frames that will stretch the bags as they empty and lighten, making them rigid and removing any pockets of hydrated lime cornered in the bags. Bag activating devices are usually effective in agitating the hydrated lime, dislodging collections of hydrated lime closer to the bag spout, promoting a better flow. Some of these bag activating devices also serve as an airtight seal between the bulk bag and the receiving hopper.

Hydrated Lime
The hoppers between the discharge bags and the conveying line should be designed with proper geometry and may need to incorporate devices such as vibrators or air fluidizers to remove hydrated lime from hopper walls as well mechanical agitators to promote flow.

If the hydrated lime is packaged in smaller bags, a bag-breaking station with a dust hood, filtration devices and a reverse pulse jet which pneumatically cleans the filter media, blowing the trapped dust back into the receiving hopper, is likely sufficient to support the manual unloading of the material. A hopper screen above the receiving vessel will help protect the operators if the hydrated lime is being dumped manually into a hopper that is equipped with agitating devices.

If the hydrated lime is being pneumatically conveyed into a processing system, the blower used to move the air line must be sized to meet the demands of system. The pneumatic conveying line for hydrated lime should be as short as possible. The number of bends in the line should be kept to a minimum. The velocity of the conveying line should be reduced if the hydrated lime begins to build up inside the line. If a flexible screw is being used to convey hydrated lime, a screw with a wider, flatter surface than the standard-sized screw is a safer choice. A beveled edge screw is usually an effective choice to eliminate the buildup of hydrated lime on the inner wall of the conveying tube. Due to the sticky nature of hydrated lime, a higher-rated horsepower motor may be required.

Should your hydrated lime application feature the loading of bulk bags, the use of load cells and vibratory densification decks will greatly maximize the capacity of the bulk bags.
Flexicon Applications:

The Flexicon Lime Feeder simultaneously meters and conveys lime to pH neutralization tanks and pools from silos, bulk bags, 50 lb. bags or any source.

Consultation with a Flexicon specialist will help you decide if a flexible screw or pneumatic solution best fits your hydrated lime application. Flexicon's BEV-CON™ screw conveyor is equipped with a highly specialized flexible screw matched to material characteristics and other application requirements of hydrated lime.

Flexicon's expert design and engineering staff will weigh each parameter and recommend the best solution for you. Upon request, Flexicon's test lab will simulate your hydrated lime handling functions before the system is installed in your plant.

Flexicon's product line of advanced flow promotion conveyors, high-flow hoppers, deaeration/densification decks and a host of other components and accessories are proven performers that promote flow while reducing degradation, dusting and/or the separation of blends comprised of disparate particles.

1 National Lime Association
3 Source: Powder and Bulk. Com - Bulk Density Chart
Sources: Where noted. All other information courtesy of Flexicon Corporation.