How a hydrating plant & milk of lime plant work

Hydrating plant:

A basic hydrating plant consists of four stages (see Fig 1):

a) quicklime handling and crushing
b) hydration
c) classification
d) storage and despatch

Fig 1. Diagram of a basic hydrating plant: (a) quicklime bunker; (b) weigh feeder; (c) water; (d) hydrator; (e) dust collector; (f) discharges; (g) primary classifier; (h) cyclone; (i) primary fines; (j) primary tailings; (k) secondary classifier; (l) cyclone; (m) secondary fines; (n) mill.

a) Handling and crushing:

Where the removal of impurities in the hydrating plant is not important, the quicklime is often reduced in size using impact breakers. In other circumstances, rolls and jaw crushers, or cone mills may be used.

b) Hydration:

There are currently a variety of designs of hydrators used in the UK (see Fig 2). Designs usually consist of three main sections – prehydrator, hydrator and finishing stage (see Fig 3).

Fig 2. A modern 3-stage hydrator with integral dust collector.

Fig 3. Diagram of a 3-stage hydrator: (a) water; (b) quicklime; (c) pre-hydrator; (d) hydrator; (e) steam and air to dust collector; (f) finishing stage; (g) raw hydrate.

The prehydrator has a smaller volume than the later stages and is highly agitated by numerous paddles used to mix the water and quicklime quickly and efficiently.

The hydrator stage usually takes about 10 to 15 minutes. The heat of the reaction causes the majority of surplus water to boil off. The subsequent steam produced is fluidises finely divided hydrated lime which flows to the finishing stage.

In the finishing stage, residence time is provided to:

• reduce the level of free lime
• allow surplus moisture to evaporate

The plant is kept under slight suction to prevent any dust emission. The moisture content of the raw hydrate after the finishing stage is roughly 1%.

c) Classification:

The choice of classification plant depends on the quality of the feed lime and the required product. In Fig 1, the raw hydrate is fed to the primary air classifier, which can be adjusted to give a cut size meeting the required particle size specification.

Oversized particles rejected by the classifier consist of varying proportions of unburnt limestone, hydrated quicklime and gritty hydrate. These can then be milled and blended with the primary fines or used to produce a less pure grade. If the secondary fines contain too much free CaO, they can be recycled to the hydrator.

d) Storage and despatch:

The finished product can be bagged and placed on pallets, or be fed straight into a loader.

Bagging of hydrated lime is done either by hand or machine (the latter being preferable due to speed, cost and quantity benefits). Pallets are then either stored or despatched for delivery immediately.

Milk of lime plant:

‘Milk of Lime’ is used to describe a fluid suspension of slaked lime in water. Milk of lime with a high solids content is usually called ‘lime slurry’.

‘Lime putty’ is a thick dispersion of slaked lime in water. Putties usually contain 55-70% by weight of solids. A semi-fluid putty is sometimes described as ‘lime paste’. The chemical reaction involved in slaking is identical to those of hydration.

Slaking practices vary with the quality of the quicklime and the water and with the required properties of the milk of lime or lime putty. Typically, the conditions are controlled to give a slaking time of approximately 10 minutes, using a water to lime ratio in the range of 3:1 to 4:1. Overall, there are many methods for slaking that depend on the scale of production.

Batch slaking:

The traditional method for and slaking uses a wooden or metal trough. Water is added to cover the base and sufficient quicklime is added to bring the water to the boil. Long handled rakes or hoes are used to agitate the contents. When the trough is full, agitation should continue for about 5 minutes after the boiling has stopped. The milk of lime can then be discharged through a stop-cock or side valve and should be passed through screens to remove lumps of uncalcined limestone and grit.

Mechanical slakers are built to be very robust, in order to handle lumps of unburned limestone without jamming. The traditional pan slaker (see Fig 4) is cylindrical and incorporates a cruciform stirrer from which heavy chains are suspended to agitate the lime particles that settle on the base of the slaker. Water and lime are added progressively, keeping the temperature between 85 and 95C. The tank is also fitted with a charge cock and drain cock to remove grit.

Fig 4. Diagram of a batch slaker: (a) discharge; (b) drain; (c) stirrer chains; (d) stirrer arms.

Modern designs for batch slakers consist of a horizontal cylinder agitated along its length with paddles. Quicklime (up to 20mm in size) is blown at a controlled rate into the cylinder and water is added to maintain the temperature between 85 and 95C. The unit is able to slake a truckload of 20 tonnes in one and a half hours. It can also be fitted with a wet scrubber for dust control and a de-gritting system.

Continuous slaking:

The majority of slakers are of the ‘detention’ type, which basically consists of a tank agitated with an impeller (see Fig 5). They produce a fluid milk of lime, and use between 3.5 and 4 parts by weight of water to 1 part of high calcium quicklime.

Fig 5. Diagram of a continuous detention type slaker: (a) quicklime; (b) slaking water; (c) stirrer drive; (d) impeller; (e) slaking compartment; (f) milk of lime discharge; (g) reciprocating rakes; (h) classification compartment; (i) rake drive; (j) washed grit discharge.

The quicklime is fed to the slaker at a controlled rate and water is added to maintain the required temperature (80-85C). The tank gives an average residence time for the water of 10 minutes. Slow slaking particles of lime are detained until they react, or are removed as grit.

The paste type of slaker is widely used (see Fig 6), operating on a higher water to lime ratio of 2/3 to 1 by weight. The slaking compartment consists of a horizontal trough with two sets of counter rotating paddles.

Fig 6. Diagram of a paste slaker: (a) quicklime; (b) slaking water; (c) slaking compartment; (d) grit removal selection; (e) slurry discharge compartment; (f) milk of lime discharge; (g) classifier; (h) water jet; (i) torque controlled water valve; (j) water for grit washing; (k) grit conveyor; (l) liquid level; (m) grit discharge.

The consistency of the mixture is controlled by sensing the torque on the paddle drive and adjusting the water addition rate. The residence time in the trough roughly 5 minutes. The paste then flows over a weir and is diluted with a water spray. Most of the continuous slakers are also fitted with grit removal systems.

Dispersion of hydrated lime:

Hydrated lime disperses readily in water. In small scale operations, batches of milk of lime are produced in a stirred vessel by charging hydrate from bags into a known volume of water. The resulting milk of lime is then fed via a slurry feeder into the process.

However, on the larger scale, bulk hydrate is discharged via a powder feeder into either a batch or continuous mixer. Similarly to the small scale production process, the resulting milk of lime is then fed via a slurry feeder to the process.

Lime putties:

Lime putties are made by allowing a milk of lime to settle and drawing off the layer of clear lime water. The settle solids are then mixed.

Both milk of lime and lime putties mature on storage. The effects of maturing (e.g. slower settling, increased volume yield and plasticity) are beneficial for most applications. Most of these benefits occur within 12 to 24 hours. However, storage at temperatures approaching 100°C encourages unwanted crystal growth, so many slakers produce a relatively thick milk (or paste), which is diluted and cooled using cold water.