The key ingredient of the steel making process in New Zealand is ironsand, which is found in abundance on the west coast of the North Island. These "blacksands" were first noted by Captain James Cook, during his first voyage of discovery in 1769. He dubbed the area "The Desert Coast". The potential of the ironsands was recognised by the early European settlers who were intrigued by the sands' magnetic qualities. Early experiments to smelt the iron from the ironsand met with little success as the fine sand fell through the furnace and choked the hearth. It was only in the 1960s when direct reduction techniques and electric arc furnaces were developed that the potential of the ironsand deposits could be realised. The ironsand, which was formed through the breakdown of rocks originating from volcanic activity in Taranaki 2.5 million years ago, is mined from sites along the west coast, with the largest being at the mouth of the Waikato River. Up to 1.2 million tonnes of sand is mined each year at the Waikato North Head deposit, which contains more than 150 million tonnes of ironsand. The ironsand is transported to the Glenbrook Mill in a slurry (ironsand and water) which is pumped through an 18km long underground pipe.
Water is a key ingredient in steelmaking with up to 1 million tonnes of water each day circulating through the steelmaking and finishing processes at Glenbrook. However, only about 20,000 tonnes of fresh water needs to be added to the steelmaking and finishing water circuits daily, with extensive recycling and reuse of captured stormwater assisting in water conservation. Fresh water is sourced from the Waikato River approximately 14 kilometres from the river mouth. Waikato River water is also used in transporting the ironsand (titanomagnetite) concentrate in a slurry from the Waikato North Head minesite.
Ferrous scrap waste arising from the Mill's various processes is recycled by remelting it in the steelmaking process.
In the iron and steelmaking process correction materials are added to adjust the chemistry of the molten iron and steel. These include dry ironsand, burnt lime, aluminium wire, ferro-silicon and ferro-manganese. In the metal coating process and pipe mill large volumes of zinc are applied to protect steel products during their life. Paints are applied to sheet steel to provide protection and colour. In the finishing processes acids are used for cleaning the steel before coatings are applied. Acids and solvents are also used for various cleaning purposes in the rolling and finishing processes. Large volumes of oil and grease are used throughout the various processes and particularly in the rolling mills, for plant lubrication.
Energy Resources and Recovery
Steelmaking uses a lot of energy and managing this efficiently is an important part of New Zealand Steel's operation. New Zealand Steel is committed to commercially and environmentally responsible energy management and the promotion of energy efficiency in all of its operations. New Zealand Steel has taken steps to promote energy efficiency. It was a founding member of the national Energy Wise Companies Campaign and met targets specified in an early voluntary agreement with the NZ Government to reduce carbon dioxide emissions.
Below are the energy resources New Zealand Steel uses:
The mill has a capacity to use 800,000 tonnes of coal a year. The coal is used as a source of carbon in the reduction process.
Electricity and Gas
At full capacity the Glenbrook operation consumes up to 1100 Gigawatt hours of electricity a year. This is approximately the amount that Wellington City (excluding Hutt Valley) uses each year. The bulk of this usage is by the plant's two iron melters which each consume up to 300 Gigawatt hours each year. Electricity accounts for 10 - 15% of the energy usage at the plant. The remaining energy needs are met with natural gas sourced from the Taranaki gas fields. The natural gas is used to preheat ladles for holding iron and steel and to reheat the steel slabs before they are rolled. A number of the downstream finishing processes also use gas fired furnaces for heating and drying.
Waste Energy Recovery
New Zealand Steel has developed a way of using one of the by-products of the ironmaking process, to provide an electrical energy source. This reduces its reliance on electricity purchased from the national grid. Hot waste gases are produced by the multi-hearth furnaces in the ironmaking process. New Zealand Steel has since the late 1970s taken advantage of this hot waste gas to produce energy for the production process, in what is called a Cogeneration plant. The Cogeneration process involves the multi hearth furnace waste gas being burnt in an afterburner to provide heat for the boilers. This superheated steam from the boilers drives two steam turbines to produce electricity. The Cogeneration plant provides approximately 20% of the site electricity requirements. In 1997 the company commissioned a second Cogeneration plant, taking waste hot gases from another part of the ironmaking process, the rotary kilns. This now means up to 70% of the steel mill's electricity is generated on site. The Cogeneration facility is an example of how New Zealand Steel makes better use of resources and by products. New Zealand as a whole benefits from Cogeneration at the steel mill as emissions from thermal power stations, such as Huntly and New Plymouth, are reduced and there are savings of coal and gas.
Steelmaking generates greenhouse gas emissions, mainly carbon dioxide, both directly when making iron and steel, and indirectly through the use of electricity and gas. The majority of emissions (about 80 per cent) come from the chemical process of making iron. New Zealand Steel has significantly reduced the intensity of its emission profile since the 1990s and is continuously looking at cost effective ways of reducing its energy usage and C02 emissions.