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An innovative on-site waste heat recovery system implemented at one of the country’s leading cement manufacturing plants which generates 23% of the power required to operate the plant.

This innovative R1.8 billion cement manufacturing plant in Northam, Limpopo, is the first cement company in Africa to possess an on-site waste heat recovery system.

It also boasts South Africa’s first-ever girth gear to a rotary kiln, weighing less than half of conventional drives, while still maintaining the same safety factor.

Located on a high-quality limestone deposit used to make clinker, the plant has a current capacity of more than 1.2 million tons and produces pure cement for bulk buyers in the concrete, construction, and cement blending industries, as well as bags of cement for builders, available from leading retailers.

The challenge is that there are two water resources available at the site, borehole and river, and both have high hardness levels. Hard water is high in dissolved minerals, particularly calcium and magnesium. In Northam, the river’s hardness is up to 350mg/l, and the borehole is up to 1000mg/l.

This was causing scaling in the plant’s heat exchange units, impacting condensers, coolers, cooling towers and boilers, reducing the work rate of the plant’s equipment and systems and impacting its on-site heat recovery system.

Our client has made a significant investment into a plant that not only produces pure performance cement at a competitive price point but has a substantially lower impact on the environment because of innovative solutions, such as its girth gear to the rotary kiln and on-site waste heat recovery system,” says Chris Ashmore, General Manager, Watericon. Watericon is a South African based water treatment company providing a full range of water and process treatment services for the industrial, municipal and residential sectors.

“We were approached by the client to solve the problem of high hardness levels in the available water sources, which was significantly impacting the ability of the plant to perform optimally.”

Designing a solution that doubles power generation

Working closely with the client, a performance target was set. The solution would need to deliver hardness levels to 200mg/l, plant production of up to 110 m3/hr, conductivity of less than 250microsiemens/cm and a reverse osmosis (RO) recovery rate of between 70% and 75%.

Watericon after 6 months of onsite piloting trials then designed and built a water treatment plant that reduces the hardness in the local water through a chemical softening process. The resultant soft water increases the working capacity of all heat exchange units, as well as the output of the waste heat recovery system, increasing power output.  After the chemical process the water is then passed through reverse osmosis units to bring the water to the required specification to be used in boilers, cooling towers and other key processes in the plant

The new water treatment plant has increased the clients power generation from 3MW to between 5.5 and 6MW.

While the plant can accommodate a hardness of up to 1000mg/l, Watericon’s treatment solutions have reduced hardness to less than 200mg/l, and the treatment plant can achieve even less than 100mg/l, depending on the pH and chemical dose rate. ROs also reduce the conductivity to less than 50µs/cm.

Watericon’s purpose-built water treatment plant

The entire process designed for the client includes a chemical addition (chemical softening), reaction tanks, settling, sand filtration, reverse osmosis and sludge handling.

Raw water is fed into the plant and the chemicals are dosed inline. These chemicals consist of sodium carbonate and sodium hydroxide as the main reactants and coagulation flocculation to enhance settling rate and sludge formation. The treated water then flows into the reaction tank, where the precipitation reaction is enhanced.

The water and flocculated precipitate then flow into two settlers, where sludge settles and clear water overflows to a holding tank.

The tank is also the feed tank for two sand filters, which reduce both turbidity and suspended solids within the water. Filtered water is then fed to another holding tank.

These tanks are also the feed tanks for two reverse osmosis units, which produce low conductivity water at a flowrate of 40m3/hr each. The RO water is fed to a final product water tank that feeds the cooling towers and other processes.

The sludge from the settlers is pumped via a diaphragm pump to two reaction tanks, where flocculant is dosed to form a denser sludge, increasing the de-watering process.

From the tanks, both sludge and water are pumped via a diaphragm pump into the filter press. Clarified water is sent back to the process and the sludge is removed and discarded.