How Important is Clean Water in the Steelmaking Process?

Greg Ellard has been a process engineer for the company since 1987 with 150+ major plant startups and optimization inspections. His focus has been on improving steel mill filter design and operation.

There is much to be optimistic about in the steel industry in the United States. New, state-of-the-art steel mills are being constructed in places like Ohio and Texas, an influx of international funding is creating more capacity and expansion to subsidiary plants, and automation is bringing high optimization into an age-old industrial process. Total U.S. steel production in 2019 increased to 88.0 million metric tons from 86.6 million metric tons in 2018, a 1.6% increase. 

One critical aspect in all steel plants is water filtration. While it may not be the first activity that comes to mind when considering the steelmaking process, it is important for many reasons. At a steel plant, water is involved in cooling during many of the stages of the process. Cleaner water improves product quality, increases mill efficiency (which means higher production throughput) and allows equipment to perform at a higher level, reducing maintenance and downtime. De Nora’s Greg Ellard offers this Q and A on the basics of water treatment in the steel industry. 

How is water used in a steel plant? 

Water access and availability is a critical issue worldwide, in daily life and in industry in particular. The entire steelmaking process, which includes ironmaking and coke-making, can use 75,000 gallons of water for each ton of steel produced. The main purpose of water in steelmaking is for cooling product, in the various stages of the process, as well as cleaning. Indirect, non-contact cooling of machinery is another application in steel plants. Most De Nora filter technology is used on contact water systems for direct cooling and cleaning of the steel used in the casting and hot rolling operations as well as for descale operations. Contact applications require a robust filter operation to handle the high solids and oil and grease. Indirect non-contact cooling is used for machinery cooling. De Nora also has customers who use our filtration systems for non-contact applications, because they trust our brand. 

Why is reuse important? 

High volumes of water are used in steelmaking, so reuse is both a budgetary consideration as well as a performance issue. Cleaning up the water allows it to be reused and keeping the circulating water cleaner improves performance of equipment, like preventing spray nozzles from clogging. This allows even cooling of the steel product, which improves its strength and quality. Some sites have zero-discharge permits, not allowing even cleaned wastewater to be discharged, so reuse is a must.

What technologies are used to treat water for reuse?

Different parts of the steelmaking process produce different contaminants in the water being used. Chemical and physical treatments can clean the water so it can be reused or discharged. Chemical treatments include coagulants, flocculants, disinfection chemicals, and other chemicals that maintain a good water balance. Physical treatment methods include solids/liquid separation settling devices, media filtration using mono-, dual-, and multimedia configurations, and various screening devices. De Nora uses a mono-media large-grained sand deep-bed filter process.

How is De Nora deep bed filtration used in a steel mill?

Contact water from the steel mill flows through a scale pit and then is pumped to the deep bed pressure filter system. The water flows down through a 6-foot bed depth of sand filter media and exits out the bottom of each filter. Mill scale solids and oil and grease are captured within the filter media. During full flow treatment, filtered water flows through a cooling tower and then is returned to the mill.

The deep-bed filtration process:

  • Protects much of the equipment throughout the process and reduces the need for maintenance on the cooling towers.
  • Can provide side stream filtration of contact and non-contact cooling water to reduce the solids in water systems that do not require fully cleaned water.
  • Can treat river and well water intake for process and drinking water

Why is deep bed filtration a preferred technology?

Deep bed filtration allows storage of mill scale and oily solids at some depth into the bed. This allows a long run time, compared to other types of filtration where this material would build up on the top surface of the filter and block the water flow relatively quickly. Deep bed filtration is a proven technology that has been used for generations – because it works. Some mills have tried to use screening technologies and after significant investment and trial/error, have had to bypass and eventually completely scrap those solutions because of lack of performance. They returned to media filtration because it is a tried-and-true solution.

What is unique about DE NORA TETRA DeepBed™ filters? 

Larger media - The highly-specified coarse mono-media sand used in the filter allows high-solids loading for greater solids capture and deeper bed penetration resulting in longer run times and significantly reducing the volume of backwash water generated.

Backwash Air and Water Design - The large media allows use of combined air and water backwash. This results in low backwash waste production compared to filter systems that use a dual- or multi-media configuration.  Backwash air acts as an excellent scrubbing mechanism, expelling much of the oils that are difficult to release using a water-only backwash.

Standpipe Backwash Design – Backwash water comes directly from the effluent pipe. No backwash pumps are required, saving the customer the cost of installing and maintaining pumps throughout the life of the operation. A flow control loop ensures the correct backwash water flow to the filters.

Pre-Pipe and Pre-Wire Filter Designs - All filter valves are installed and wired to a control panel and air tubed to the valves (if using pneumatic valves) at the fabricator and tested before being shipped to the jobsite. Most of the different header piping is installed at the fabricator, making installation easier for the customer. 

Robust, Non-Clogging Underdrain – This uses a separate backwash air and water supply system. When other media filter technologies fail, De Nora has the expertise to retrofit underdrain systems. Two other commonly used filter underdrain systems are nozzle-bottom and folded plate. De Nora has replaced both many times and continues to do this. 

Experience – Though the name has changed throughout the years, De Nora has been providing the best filter systems for the steel industry since at least 1968 (as Dravo, TETRA, Severn Trent, and De Nora). We have over 50 years of process expertise in designing filter systems for steel making, and strong knowledge of equipment and processes before and after filter systems. Our deep understanding of the steel making process and operations ensures the appropriate filter design. And, we have extensive experience correcting other filter designs by converting nozzle, stainless steel folded plate, and other types of underdrains to TETRA underdrain. We have been the preferred choice in the industry for many years, and our technicians have commissioned more than 50 steel mill filter facilities in the last 20 years.

How else has De Nora’s expertise influenced the steel industry?

In the 50-plus years of providing filter systems to the steel mill market, the De Nora team has continued to improve their design and operations to allow the customer to concentrate on what they do best - making steel – and not spending lots of time focusing on filter equipment. 

We work with our customers to optimize the design of the system they need at a cost that fits their budget. For example, a customer was interested in vertical pressure filters. When the cost did not fit within their budget, De Nora offered a more cost-effective horizontal pressure filter system that also fit better in the space. Gravity filters are also an option that can be more cost effective if space is available. 

We design and start up our own equipment and then help keep it running right. Customers regularly request De Nora process audits to check and optimize their operations. De Nora’s knowledge of all the water operations makes for ideal opportunities to assist the customer and upgrade old facilities to optimize their operation. That kind of expertise, responsiveness and dedication to customers’ success is what makes De Nora uniquely positioned to grow with the steel industry as it moves into a new era.