Quarring crushing and blending
The primary raw materials used for cement manufacture at Colleen Bawn are limestone (CaCO2) and clay (SiO2,AI2,O3,Fe2O3). Limestone is obtained from the quarry by drilling and blasting. Rocks are loaded into trucks and transported to the crushers. The limestone crushing process (including a primary, secondary and tertiary crusher) reduces the rock from < 1.0m3 in size to 19mm in diameter. The crushers produce limestone at the rate of 500 tonnes per hour and this is conveyed to blending beds. The limestone is stored in these blended stockpiles which have a homogenizing effect 14;1. The limestone is transported via belt conveyors to the raw milling stations at a rate of 300 tonnes an hour.
Clay is obtained from a quarry 15km from the factory and delivered to the storage shed. The clay is then introduced into the raw mills together with limestone.
Raw milling and homogenisation blending
The plant is dual raw milling, preheating and dust precipitation systems. Clay and limestone are fed into the air-swept ball mills in proportions of 86% limestone and 14% clay weight.
The quantities of both the clay and limestone fed into the mills are computer controlled for the desired product quality. The two air-swept raw mills are rated at 90 and 110 tonnes per hour.
The fine powder obtained is called raw meal, which is tested on an hourly basis for quality control purpose. The raw meal is mixed in the silos which have a total capacity of 11000 tonnes.
The raw meal is mixed thoroughly to ensure a uniform feed to the kiln for the smooth operation of the kiln and production of a good quality clinker.
The dust laden exhaust gases from cooler, kiln, preheaters and the raw milling sections are first cooled in the conditioning towers and then passed through the electrostatic precipitators. Fine water sprays in the cooling towers lower the temperature of the hot gases from 400oC to 150oC. The dust is removed in the electrostatic precipitators and returned to the kiln before the exhaust gases are released into the atmosphere.
Environmental and safety matters
Quarry: Limestone waste-dump rehabilitation is on-going. Five metre-wide safety catch-berms (terraced quarry faces) have been incorporated into the quarry design. The waste rock dumps follows the profile of the natural angle of the repose for stability.
The coal used for burning is ground by a 15 tonne per hour air-swept mill and then stored in an 80 tonne silo, it is conveyed in a fluidised form to both the kiln main burner and the pre-calciner burner pipes.
The raw meal in the silos is fluidised and conveyed to the pre-heater section. The raw meal, now called kiln feed, is introduced to the pre-heater cyclones against the flow of hot air. The pre-heater fan draws air from the cooler through the kiln to the pre-heater section to heat the kiln feed before it enters the pre-calcinator at 800oC and is heated to 1000oC when calcinations of CACO3 to CaO takes place. The heat for calcinations is obtained by the combustion of fine coal introduced into the pre-calciner via burner pipes.
The kiln feed is then burnt at 1450oC in a rotary kiln. The kiln is 56m long and 3.8m diameter, with an internal lining of refractory bricks. Fine coal is introduced at one end of the kiln via the main burner pipe for the burning process and the raw meal is fed in at the other end of the kiln. The modules formed from the burning process called clinker. The clinker is cooled by a rotary cooler 48m long and 4.8m in diameter. From the cooler, clinker is conveyed to either a storage silo or to the cement milling department.
At the merger in 1965, the Bulawayo factory drew its limestone requirements from Colleen Bawn quarry. The kiln in operation then could produce 216 000 tonnes of clinker a year.
To cope with increased demand in the 1990's, clinker was imported from South Africa to increase the factory's cement output to its full potential of 450 000 tonnes per annum.
This came to an end when stage one of the Colleen Bawn kiln development programme was completed in 1993 and Bulawayo's additional clinker requirements were drawn from Colleen Bawn. Bulawayo factory's kilns were shut down in December 1997 and all its clinker is now supplied from Colleen.
have invested US$80 million in the construction of our second manufacturing plant in Harare, Msasa Plant. Much of the growth is premised on major investments into housing projects and road rehabilitation programmes. Our new plant, which is expected to be commissioned in the second half of 2016, will boost production by 700 000 tonnes. The plant will unlock value in the country through employment creation, which is in line with the country’s economic blueprint, Zim-Asset.
A new Unidan 100 cement mill was commissioned in 1998, bringing the factory's capacity to 900 000 tonnes a year.
The mill; 13,5m long and 4.2m in diameter; is driven by a 3.2 Mega Watt motor and is capable of producing 100 tonnes of cement an hour. Two new packaging units to handle increased production were installed in 1996, when a new 15 000 tonne cement storage silo and 15000 tonne clinker silo were commissioned.
Improved rail and road loading facilities have also been built to expedite transport of cement to customers in Zimbabwe and the region.
Cement is produced at the Bulawayo Factory to meet the requirements of the Standards Association of Zimbabwe. The quality of all raw materials is tested by both wet lab and X-ray analysis.
Cement extracted from the cement silos is packed using two eight-spout rotary packers. The plant is capable of filling and loading 1800 bags of cement an hour, per packer.