Water treatment plant increases cement plant performance

Watericon designed and built a water treatment plant in Limpopo for a leading cement manufacturer to optimise operations.

Aplant’s performance can be negatively impacted by high hardness levels in water. This was experienced by a R1.8 billion cement manufacturing plant in Northam, Limpopo, that uses an on-site waste heat recovery system to generate 23% of its power.

Water hardness caused scaling in the cement 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 the on-site heat recovery system’s effectiveness.

Water treatment specialist Watericon was approached by the client to solve the problem of high hardness levels in two water sources: the river (350 mg/ℓ water hardness) and the borehole (1 000 mg/ℓ).

Exceeding targets

After six months of on-site trials, Watericon designed and built a water treatment plant that reduces the hardness in the local water through a chemical softening process. After the chemical process, the water is then passed through reverse osmosis (RO) units to bring it to the required specification so that it can be used in boilers, cooling towers and other key processes in the cement plant.

Performance targets were agreed upon and even exceeded by Watericon:

• water hardness levels – 200 mg/ℓ (Watericon has achieved even less than 100 mg/ℓ)

• plant production – 110 m3/h

• conductivity less than 250 μS/cm (Watericon has achieved as little as 50 μS/cm)

• reverse osmosis recovery rate – between 70% and 75%.

The resultant soft water increased the working capacity of all heat exchange units, as well as the output of the waste heat recovery system – increasing the power output from 3 MW to between 5.5 MW and 6 MW.

Watericon designed the entire process, including 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 in line.

These chemicals consist of sodium carbonate and sodium hydroxide as the main reactants, as well as coagulation flocculation to enhance the 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 flow rate of 40 m3/h 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, extending the dewatering 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.