The Manufacture of Lime involves the following Process:

1. Quarrying

Limestone at Lime Acres occurs in layers of varying purity, up to 10 metres thick. As much as three tons of the lower purity stone (overburden) must be mined to obtain one ton of high purity limestone. During subsequent processing in the rotary kilns, approximately two tons of limestone must be fed into the kilns to produce one ton of lime. This is due to chemical and dust losses.

Thus for each ton of the final product sold, more than 8 tons of rock has to be quarried. To meet total demand, the quarries can be called upon to mine in excess of one million tons of rock per month.

2. Crushing and screening

Run-of-quarry limestone, which includes boulders of a cubic meter in size, is transported to a gyratory primary crusher, followed by a similar but smaller secondary crusher with capacity of 1 800 tons per hour. After the crushers, the limestone, now reduced to less than 90 mm in size, is blended on a stockpile system.

There are two stockpiles each containing approximately one week's production. Limestone is recovered from the blending stockpiles by means of a drum reclaimer and is transported by overland conveyor to the tertiary crushing and screening plant where the stone is crushed to less than 50 mm. Inferior quality fines are removed and the remainder split into the following size fractions and stockpiled.

The bulk of the stone is recovered from the stockpiles by means of conveyors and fed to the kilns. A small percentage is loaded into trucks for dispatch to clients who require limestone products instead of lime.

3. Calcining

Calcining or burning, to convert the limestone into lime, is the heart of the operation and takes place in nine rotary kilns. Lime Acres is believed to be one of the largest lime producing facilities in the world with an active capacity of 1.3 million tons of lime per annum.

A rotary kiln is basically a long steel cylinder, lined inside with refractories to protect the steel from process heat. Limestone is accurately metered into the kiln at the feed (uphill) end.

Owing to the rotating motion and the slope of the kiln, the limestone gradually works its way down the length of the kiln, cascading and uniformly absorbing heat as it does so. It takes about 2,5 hours for material to pass through a rotary kiln, carbon dioxide being driven off and the limestone (CaCO3) being converted to lime (CaO).

The heat source is pulverised coal, and in the hottest zone of the kiln the temperature of the material reaches 1200 °C. In the older kilns, approximately 350 kg of coal is required to produce one ton of lime. On the newer and more efficient kilns, some equipped with limestone preheaters, the coal requirement has been reduced to 200 kilograms per ton of lime.

Coal is by far the most important operating cost and fuel efficiency therefore receives a lot of attention in the plant.

The calcining process in the rotary kilns is continuous and kilns run 24 hours per day, throughout the year. Kiln runs or campaigns only last about 30 days, although periods of 45 days are not uncommon. The most common cause for kiln stoppages is ash ring, a deposition on the refractory lining caused by the ash in the coal combining with lime dust.

4. Cooling, storage and loading

After it's discharge from the kiln, lime passes through shaft coolers where it is cooled from 1 000 °C to room temperature before being transferred to storage silos.

From the silos, some 80 per cent of the lime is conveyed to the loading plant where screening and loading into railway trucks takes place. The loading plant is also equipped for loading road vehicles.

5. Milling

The balance of the lime passes to a grinding plant where it is reduced by a hammer-mill to minus 2,5 mm. The majority of the ground lime is loaded out in rail tank wagons with the remainder serving as feed for the hydrating plant.

6. Hydrating

In the hydrator, ground lime is reacted with water under closely controlled conditions to produce calcium hydroxide or hydrated lime in dry powder form. The hydrated lime then passes through an air-separator, which removes coarse particles.

The fine powder (80 per cent minus 0,075 mm) is pumped into a silo from which it is loaded into rail tank wagons or packed into paper sacks by a mechanical packer.