Growing Use of Molybdenum-containing Stainless Steel in the Paper Industry

Over the past 15 years, digitalization has led to a sharp decline in demand for graphic paper products such as newsprint and printing paper. From 2010 to 2017, demand shrank at an average annual rate of 2.4%. After 2018, the decline accelerated rapidly, reaching 6.2% in 2021. However, despite the decline in graphic paper products,  there has been growth in other paper products such as wrapping paper, cardboard and household and hygiene products, driven by the rapid growth of e-commerce and rising demand from growing economies such as India. These trends have kept the overall annual net growth rate of the paper industry at around 1%-2% since 2010. Super austenitic and duplex stainless steels play an important and expanding role in this growing industry.

Pulp drying processes are energy intensive. The papermaking process starts with wood chips or waste paper. The most widely used method is the Kraft process, which converts wood into pulp to produce paper products. The process begins by mixing wood chips with a hot mixture of water, sodium hydroxide and sodium sulphide, and then cooking them in a pressure cooker. This digestion process converts the wood chips into a pulp of cellulose fibres, which is the main component of paper. After digestion, the raw pulp is collected, washed and bleached, usually with a chlorine dioxide solution to achieve the desired white colour. Sometimes bleaching is also done with hydrogen peroxide or ozone. Recycled waste paper does not require digestion, but is removed from it to remove plastics and other waste, then washed and bleached.

In both cases, the bleached pulp is fed into a large paper machine called a Fourdrinier to be turned into paper rolls. Pulp is diluted into a slurry and sprayed onto a screen to form a pulp mat, which is then dewatered, dried and rolled to form the final paper product. Paper passes through the Fourdrinier machine at speeds of up to 100 km/h, and the final product is a giant roll of paper that is then inspected, cut and sold.

Paper machines are often called Fourdrinier, named after Henry Fourdrinier, who invented the first industrial paper machine in 1806.

Drying pulp to form paper is very energy-intensive because a lot of water must be removed. The European Heat Pump Association (EHPA) calculates that, on average, a paper mill requires enough electricity to power more than 30,000 homes for a year. Technological advances have improved dewatering processes, increased heat recovery, and combined heat and power (CHP) can make the paper industry more energy efficient while reducing emissions.

For decades, pulp and paper mills have relied on CHP to capture steam heat that would otherwise be wasted. Now, heat pumps can help recover more heat.

The European Confederation of the Paper Industry (Cepi) and EHPA are working together to promote the wider implementation of high temperature industrial heat pumps, which capture waste heat from the pulp drying process. Heat pumps use heat from a lower temperature heat source and "boost" it to a higher temperature to meet the heat requirements of the process. Recent developments have enabled large heat pumps and steam compressors to operate at temperatures up to 200°C. It has been calculated that energy savings of more than 50% can be achieved in the pulp drying process using this technology. Depending on the corrosive nature of the incoming waste heat steam, heat pump components such as heat exchanger tubes can be made of 304 or 316 stainless steel.

Stainless steel plays many roles in the paper industry. For more than 70 years, the pulp and paper industry has used stainless steel and other corrosion-resistant alloys to increase the service life and reliability of equipment. The best construction material for paper mill equipment depends on the specific process and technology used, but molybdenum-containing stainless steels are widely used throughout the production process.

Duplex stainless steels are increasingly used in the pulp and paper industry due to their high strength and resistance to chloride stress corrosion cracking and alkali corrosion. For example, 2205 duplex stainless steel is used in digesters in kraft and sulfite pulping processes. The more highly alloyed 2507 duplex stainless steel is also a common material for pipes and components in the bleaching section of the plant.

Due to its excellent corrosion and fatigue resistance, duplex stainless steel is also an ideal material for paper machine suction rolls, which remove water from the papermaking process. Duplex stainless steel 3ER60 is often used for suction rolls, along with other duplex grades such as 2205, 2101 and 2304.

Suction rolls are large and manufactured to tight tolerances. This 3RE60 roll blank is 12 meters long and nearly 2 meters in diameter.

The D-stage bleaching process is the most corrosive stage in the papermaking process, and the equipment materials are often exposed to chlorine and chlorine dioxide environments at high temperatures of 70 to 80°C. Super austenitic and super duplex stainless steels are used for pumps and reaction vessels in this acidic process because of their strong resistance to localized corrosion.

A roll of nearly finished paper passes through a Fourdrinier

As the pulp and paper industry adjusts its production capabilities to the changing global market and implements technological advances to increase mill efficiency, the industry will continue to grow. As the paper industry transforms, molybdenum-containing stainless steels remain an important construction material for pulp and paper equipment.