An upgrade of the acid plant absorbing tower, which comprises the bulk of the investment, will increase the acid plant's reliability and improve its operation, production and environmental impact, Ravensdown works manager Tony Gray said.
New German-designed equipment, manufactured locally and incorporating the latest international "best practice" technology, was being introduced.
The remainder of the capital was being invested in smaller projects to reduce repair and maintenance costs associated with older equipment and also provide improved control and monitoring.
The upgrade and maintenance work would be completed during the shutdown, with the plant reopening on July 4.
The shutdown was later than normal because the plant remained open to support other parts of the company after the Christchurch earthquake in February.
Manufacturing temporarily stopped at the Hornby plant.
The past year has seen the Ravensbourne works achieve its largest production level, Mr Gray said.
Months of planning was undertaken before a shutdown, co-ordinating resources and inventory to build up sufficient stock to ensure customers' requirements could continue to be met.
The upgrade complemented the recently commissioned $1 million acid plant storage tanks.
With only one minor vessel left to replace, the acid plant and site were now well positioned to meet future requirements, Mr Gray said.
