The importance of water in the steel industry

Strong environmental regulations have always driven European process industries to move towards efficient management and resource utilization. Water is a suitable and essential resource for most production processes, so resource utilization includes reducing fresh water consumption and wastewater discharge.

How much water is used in the steel industry?

The steel industry uses large amounts of water. However, very little is consumed, as most of the water is reused or returned to the source. Water is used not only in cooling operations, but also in other processes such as scale removal and dust cleaning. Different types of water are used in the steel production process. Fresh water is mainly used for direct and indirect processes and cooling, while seawater is typically used once in the cooling process after a pre-treatment and disinfection step.

Steelmaking methods

• One is the continuous cycle, where steel is produced from raw materials
• The second is the electric route, which produces steel by melting scrap in an electric arc furnace (EAF).

Average water consumption for steelmaking

The average water consumption for steelmaking in the integrated process is 28.6 cubic meters per ton of steel produced, with an average water output of 25.3 cubic meters per ton of steel. For the electric route, the average consumption is 28.1 cubic meters per ton of steel, with an average water output of 26.5 million cubic meters per ton of steel. As a result, the total water consumption per ton of steel produced ranges from 3.3 cubic meters to 1.6 cubic meters, with this amount of water loss mainly due to evaporation. In short, the total water consumption in a steelmaking site is actually limited. Most of the water consumed is evaporated and about 90% (on average 88% in an integrated plant and 94% in an EAF-based plant) of the water is discharged after cleaning or cooling and is often used by other facilities.

Water consumption management

Given the major concerns about available fresh water and water quality, water resource management is a major challenge to improve the sustainability of the production cycle. Water, as well as steel, can be reused and recycled. However, increasing the amount of recycled water after cleaning and cooling can reduce water quality, steel producers are committed to reducing water consumption or improving treatment technologies.

Impact of increased salt concentration on plant equipment

Water needs to be cooled and desalinated, because increased salt concentration in water circulation systems (due to evaporation) can not only be an environmental issue, but also have a negative impact on plant equipment (e.g. in rolling mills). For example, phosphates can cause pollution of environmental waters, chlorides can lead to corrosion of metals, while carbonates cause scale formation in pipes, thus increasing energy consumption.

Desalination and Crystallization of Salt Water

Due to the processes of desalination and crystallization of solid salts from salt water (a by-product of the desalination process), a significant amount of energy is required, which is accompanied by increased CO2 emissions. A statistical analysis of the specific factors affecting energy consumption (SEC) has been carried out in order to predict the energy consumption of desalination. Furthermore, an economic analysis shows a weak statistical relationship between SEC and the cost of water production. However, the quality of the recovered salts is usually low and therefore they cannot be reused. Furthermore, they have to be disposed of in special landfills, as it affects the quality of the leachate.

How to optimize the use of water resources in the steel industry?

To achieve optimal use of water resources in the steel sector, a comprehensive and balanced approach is required that considers actual consumption without considering other aspects such as water availability and quality, plant configuration and energy efficiency. Furthermore, as water-related challenges (i.e., availability, seasonal shortages, competition with other consumers) vary by region and country, it is important to develop a local approach and an appropriate regulatory framework. Both traditional and advanced wastewater treatment methods (e.g., chemical sedimentation/clarification combined with flocculation) focus on producing high-quality water as well as highly efficient water recycling. This has led to a reduction in groundwater extraction. Studies have been conducted to use reverse osmosis (RO) and nanofiltration for wastewater treatment and reuse in other sectors such as tanning and textile industries. In the steel sector, electrodialysis and ion exchange have been used to treat wastewater from stainless steel washing in nitric acid and hydrofluoric acid. In addition, a combination of ultrafiltration (UF) and RO has been tested to produce deionized water from surface waters. A more efficient RO method can be achieved by combining it with a pretreatment, such as reversible backwashable microfiltration (BMF). However, salts, microorganisms, and contaminants can cause membrane damage in the RO process, reducing its efficiency. The use of pretreatment (e.g., disinfection, acidification, addition of coagulants or flocculants, intermediate filtration, and cartridge filtration) can prevent fouling. In addition, as traditional pretreatment often fails to remove colloidal suspended solids, continuous microfiltration (CMF) and UF can be applied, resulting in the production of high-quality water for pharmaceutical applications. It is a matter of saving and reducing costs.