Reducing water use at Tugela and Stanger Mills

With the assistance of the Sappi Technology Centre, the mill embarked on multiple projects to improve the washing in the pulp plant and ensure quality pulp transfer to the paper machine.

Water and energy savings

The ensuing modification to the washing stages and filtrate system significantly improved pulp quality. Further enhancements made in the pulp plant included the reuse of process water and the introduction of closed-loop seal water systems. These initiatives not only reduced water consumption but also yielded some energy savings. Additional water reduction projects in the mill included the control of process water balances, fixing water leaks together with inspections and repairs of open water storages, all of which resulted in a further reduction in abstracted water of approximately 30%.

Abstracted water reduced by approximately 30% while the COD load reduced by 40%.

Cleaner effluent

The cleaner pulp to the paper machine had a direct impact on the mill’s final effluent chemical oxygen demand (COD), reducing the COD load by 40%. The increase in direct pulp feed to the paper machine also reduced the water demand from the repulpers. 

Results

All these improvements were made while the mill increased its pulp production by more than 23% and paper production 5% over the same period.

In reviewing ways to reduce water use, Stanger Mill focused on the pulp and wet depithing operations – areas where significant amounts of water are used. The mill has now replaced processed freshwater with recovered water at the screw feeder and rock cyclone in the pulp mill. This was followed by optimisations on the pith clarifier and belt washer. The pulp mill water use reduced from an average volume of 6.8 m³/min in 2018 to 6.1 m³/min in 2019‒an impressive overall saving of 10%.