The manufacture of cement is a very carefully regulated process comprising the following stages:
- Quarrying - a mixture of
limestone and clay.
- Grinding - the limestone
and clay with water to form a slurry.
- Burning - the slurry to a
very high temperature in a kiln, to produce clinker.
- Grinding - the clinker with about 5% gypsum to make cement.
Raw Materials Extraction
The limestone and clay occur together in our quarries at Cape Foulwind. It is necessary to drill and blast these
materials before they are loaded in 70t capacity trucks.
The quarry trucks deliver the raw materials to the crusher where the rock is crushed to smaller than 100mm (4 inches). The
raw materials are then stored ready for use.
Raw Materials Preparation
About 80% limestone and 20% clay are ground in ball mills with water, producing very fine, thin, paste called slurry. The
chemical composition of the slurry is very carefully controlled by adjusting the relative amount of limestone and clay being
used.
The slurry is stored in large basins ready for use.
Clinker Burning
The slurry is fed into the upper end of a rotary kiln, while at the lower end of the kiln, a very intense flame is
maintained by blowing in finely ground coal.
The slurry slowly moves down the kiln and is dried and heated until it reaches a temperature of almost 1500 degrees Celsius
producing "clinker". This temperature completely changes the limestone and clay to produce new minerals which have the
property of reacting with water to form a cementitious binder. The hot clinker is used to preheat the air for burning the
coal, and the cooled clinker is stored ready for use.
Cement Milling
The clinker is finely ground with about 5% gypsum in another ball mill, producing cement. (The gypsum regulates the early
setting characteristic of cement). The finished cement is stored in silos then carted to our wharf or packing plant
facilities.
Some Facts and Figures
The mills for grinding the raw materials are 2.4m in diameter and 11.0m long and are driven by 720kw (1000HP) electric motors
producing 45t/h of slurry. The cement is ground in two mills: one 2.4m x 11.0m long producing 18t/h of cement; the other 3.8m
x 11.4m, powered by 2300kw (3000 HP) electric motor and producing 60 t/h of cement. The kilns are either 98m or 110m long,
and produce up to 25 t/h of clinker.
Two ships, the mv "Westport" and mv "Milburn Carrier II"
carry cement to depots at Onehunga, Wellington, Napier, Gisborne, Nelson, Lyttelton and Dunedin for
distribution to customers.
The diagram below details the Westport Works production process.
