Why Stainless Steel is an Essential Part of the Pulp and Paper Industry

While the pulp and paper industry continues to evolve and refine its processes, one thing remains constant: the importance of stainless steel parts and equipment.

Many industries turn to stainless steel to help improve the service life of their equipment and processing systems. The pulp and paper industry is no exception.

From small machine parts to massive holding containers and tanks, you’ll find stainless steel in many places in the modern pulp and paper complex.

This guide will look at what makes stainless steel an effective solution to the pulp and paper industry’s common issues and highlight common uses for stainless steel across typical processes. 

We’ll also explore the commonly used stainless steel alloys and provide tips to optimize your stainless steel parts and equipment’s service life.

Common Reasons for Using Stainless Steel

While exact methods might have changed in the past few decades, much about how the pulp and paper industry operates remains the same. 

Optimizations and industry advances have allowed pulp and paper facilities to reduce waste production, reuse the waste of standard processes, or change the chemicals and techniques used to be more environmentally friendly. 

Despite these facts, most everything still involves using highly-corrosive chemicals.

Many of the efficiency gains seen in the repurposing of waste materials now involve greater concentrations of these chemicals than ever before.

Ensuring that piping, holding vessels and tanks, heat exchangers, and even structural elements can withstand constant exposure and cleaning cycles is critical. 

As competition brings ever-tighter margins to the pulp and paper processing industries, durable components and safe operation are as essential to viability and profits as ever before.

Stainless steel works to provide all-around protection from general corrosion caused by extended chemical and moisture exposure while also providing excellent thermal characteristics for processes that require extended periods at high temperatures.

It can even help manage microbial corrosion risks associated with the frequent processing and contact with organic substances found in wood chips and pulp.

But it’s not perfect. 

Facilities and engineers still need to consider crevice corrosion, stress corrosion cracking, hydrogen cracking corrosion, and microbial corrosion risks when choosing materials and designing systems. 

However, stainless steel can offer outstanding resistance levels at prices which make long-term use an easy savings over frequent maintenance and replacement of materials with lower costs.

Processes That Frequently Utilize Stainless Steel

Exact processes where stainless steel offers value will depend on the type of pulp or paper processing used. 

There are several methods commonly used today. 

However, regardless of whether you’re conducting sulphite or kraft (sulphate) processing or using different bleaching processes, you’ll likely find a range of stainless steel alloys available to provide safe, long-lasting operation and easy maintenance.

Digesting

Digesting is one of the first steps in the journey from wood chips to pulp. 

Yet, it still holds plenty of potential for corrosion and metal degradation. High temperatures, corrosive liquors, and natural compounds in the woodchips themselves are all potential attack vectors.

Standard digesting components which might use stainless steel include:

• Digesters

• Liquor heaters & heat exchangers

• Chip conveyors

• Steaming vessels

• Flash & blow tanks

• Valves & pumps

Brown Stock Washing:

This essential step works to remove cooking liquors and dissolved organic and soluble inorganic compounds from the pulp. 

Doing so efficiently typically involves a multi-stage process, and facilities often use water recycling to reduce water treatment needs and waste emissions. 

This means that some parts will frequently contact highly-concentrated corrosive substances and that heat and agitation will often amplify the risk of damage or component failure.

Standard brown stock washing components which might use stainless steel include:

• Knotters

• Rotary drum vacuums & pressure washers

• Centrifugal pumps

• Thick stock pumps

• Agitators

Chemical Recovery

As an essential component of modern pulp and paper processing, chemical recovery often works to resupply processes or even provide combustible fuels to power other non-chemical plant operations. 

Recovery also involves greater concentrations and more involved processes than ever before to help squeeze out improved cost efficiencies and meet ever-increasing environmental and pollution standards.

Standard chemical recovery components which might use stainless steel include:

• Multi-effect evaporators

• Liquor boxes & tanks

• Process tubing and piping

• Non-condensable gas collection systems

Waste Paper Recycling:

Waste paper is an increasingly common source of pulp for new paper products. 

While the pulp source might differ, many of the steps involved in converting paper to pulp are similar to those used to create pulp using wood chips. 

As such, stainless steel provides outstanding benefits for waste paper recycling as well.

Standard waste paper recycling components which might use stainless steel include:

• Repulpers

• Liquor heaters & heat exchangers

• Steaming vessels

• Flash & blow tanks

• Valves & pumps

Bleaching

Whether you’re producing affordable, mostly-disposable newsprint or fine papers destined for boutique markets or archival use, bleaching is a critical step in ensuring the characteristics of your pulp and, ultimately, the final paper product as well.

Standard bleaching components which might use stainless steel include:

• Pumps

• Mixers

• Towers

• Washers

• Process water reuse systems

• Filtrate Recycling

Stock Preparation

As one of the final steps before paper production, stock preparation typically requires the storage and movement of large pulp quantities produced in batch- or continuous-processing cycles. 

Exact requirements will depend on the pulping and paper-making processes in use and the final product desired.

Standard stock preparation components which might use stainless steel include:

• Tanks

• Refiners

• Screens & Vibrating Screens

• Cleaners

• Savealls

• Deflakers

Paper Production

As the final stage of production, the paper-making process can vary depending on the type of paper desired -- for example, tissue paper versus multipurpose copy paper -- and the exact methods used or paper treatments required.

Standard paper production components which might use stainless steel include:

• Stock Approach Systems and Piping

• Headboxes 

• Forming zone & wet end components 

• Presses

• Roll journals

• Coaters and size presses on the dry end

• Showers, Piping, and White Water Returns

Popular Stainless Steel Types and Their Characteristics

As with most industries, choosing the best stainless steel alloy for a given process or piece of equipment will largely depend on the specific requirements in play. 

However, as a rule of thumb, the following characteristics can help you hone your initial material choices further and find the perfect combination of corrosion resistance, strength, and cost.

• Austenitic stainless steels offer improved durability in heat-affected zones when welded, are non-magnetic, and hardenable by cold work. Grades frequently used include 303, 304/304L, 321, 347, N60, 316/316L, 316L-2.5% MinMo, 317L, 317LMN, and other high-nickel or high-molybdenum alloys.

• Ferritic stainless steels offer improved weldability and a lower price point but typically offer lower corrosion resistance. This makes them more suited for lower-risk processes such as alkaline mill sections and driers. Grades frequently used include 430 and 3CR12.

• Martensitic stainless steels are harder and stronger than many austenitic stainless steels, making them ideal for use in areas where wear and abrasion are vital factors. Grades frequently used include 410, 416, 420, 440C, and 16 Cr 5Ni 1Mo.

• Precipitation hardening stainless steels are ideal for components and bolting. However, they may weaken with welding, so be sure to account for implementation and installation requirements. Grades frequently used include 15-5 and 17-4.

• Duplex stainless steels are increasingly common in pulp and paper industry processes due to their exceptional corrosion resistance, sand or grit resistance, and flexible implementation options. Cast-formed duplex steels can often replace lesser stainless alloys, while wrought duplex steels are popular choices to replace carbon steel, clad, and weld overlaid digesters. Grades frequently used include 2304, 2205, 329, 2507, 3RE60.

Care and Optimization Tips for Pulp and Paper Industry Applications

An engineer or other qualified professional should consider exact precautions and maintenance routines to ensure effectiveness, safety, and regulatory compliance. 

However, the following tips will provide further considerations and options to extend service life in a range of pulp and paper industry processes.

1. Consider electropolishing where low-friction surfaces are required.   

Pulp processing frequently involves the piping and movement of thick slurries and viscous pulp. 

Further complicating matters, the fibres suspended in the pulp, while small, often feature inconsistencies that can lead to their collection in crevices or on minor imperfections on pipe or container surfaces. 

This can lead to the onset of microbial corrosion and also impede flow. 

Electropolishing is an effective method for creating an ultra-smooth surface on various stainless steel types to help reduce these risks and optimize service life.

You can consult our electropolishing guide for more on common approaches, benefits, and requirements.

2. Always use proper welding treatments and techniques to avoid unnecessary corrosion risks.

In many ways, pulp and paper processing is the perfect storm of corrosion risks. 

This elevated risk makes it essential to adhere to best practices to avoid corrosion or pipe and component failure risk. 

Always be sure to clean weld areas and treat the welded areas where possible afterward.

Our stainless steel welding guide offers greater detail on potential pitfalls and best practices to consider.

3. Consider cracking risks when choosing materials. 

As highlighted above, different families and grades of stainless steel are susceptible to varying forms of corrosion cracking risks. 

Be sure to match materials to the elements and scenarios most likely in the process you’re performing to minimize stress corrosion cracking, hydrogen cracking, and other corrosion risks.

Key Takeaways

• Stainless steel is a popular material for pulp and paper industry processes due to its exceptional corrosion resistance, excellent temperature tolerances, and long service life.

• Regardless of the exact methods used, virtually every stage of the modern pulp and paper production process can benefit from the use of stainless steel.

• Austenitic stainless steels are most commonly used, but duplex stainless steel alloys are increasingly popular options due to their blend of performance and price.

• Proper choice of materials is essential to ensuring optimal resistance, simplifying maintenance requirements, and extending the operational life of your tanks, pipes, valves and other stainless steel equipment and components.

• Electropolishing and weld treatments are effective methods of improving corrosion resistance.

As a leading provider of stainless steel in North America for more than 40 years, Unified Alloys has the experience and product selection to help you enjoy extended service life and thoroughly explore the benefits of stainless steel. Whether you’re in the pulp and paper industry, the oil industry, or even the medical sector, our expert analysts are available to consult with you today. Call now and see how stainless steel can help you optimize costs and minimize maintenance for your next project.

References:

1. Stainless Structurals: Stainless Steel and the Challenging Environment of the Pulp and Paper Industry

2. Gibson Stainless & Specialty Inc.: Stainless Steel Conduit & Fittings for Pulp & Paper Mills

3. Outokumpu: Duplex in Pulp and Paper

4. Nickel Development Institute: Stainless Steels and Specialty Alloys for Modern Pulp and Paper Mills