Carbon steels and standard austenitic grades of stainless steel were the dominant metals for many years in the pulp and paper industry. These materials still play a significant role, but duplex 2205 and other duplex stainless steels are increasingly used in the production of new equipment. The overriding reason for the focus on duplex steel is due to cost benefits. The underlying reason is due to a number of factors. First, the environmental push that started in the 1970s has led to more severe processing environments, which require materials with better corrosion properties than carbon steel. In addition, the industry is shifting away from the twice yearly maintenance shut downs that routinely existed in the past. This has led to the need for materials that demonstrate even lower amounts of corrosion over time than was permitted in the past. Finally, it should be noted that here has been a learning curve associated with duplex steel. Its introduction to the industry has been gradual, but duplex usage has steadily gained in momentum over time.
Benefits of Duplex:
Duplex steels have many beneficial properties that match up closely to the needs of modern pulp and paper mills. First, they have excellent corrosion properties, particularly in regards to stress corrosion cracking, pitting, fatigue, and abrasion conditionsi. While austenitic steels also have corrosion properties adequate to handle many conditions in modern mills, duplex steels typical offer comparable corrosion properties at a lower material cost. In addition, duplex steels have nearly twice the strength of austenitic steels. This leads to significant material weight savings. Furthermore, it should be noted that even in less corrosive environments when carbon steel could still technically be used, duplex steels often win out when looking at overall life cycle costs. The carbon steel may have a lower initial material cost than duplex steel, but over time even a manageable amount of corrosion can lead to higher overall costs for carbon steelii. Overall costs should take into consideration initial material costs, maintenance costs, inspection costs, repair costs, and most importantly, the cost that is incurred when production needs to be shut down to accommodate maintenance, inspection, and repairs.
While duplex steels have excellent properties that align well with the pulp and paper, it is important to note that modern mills include a wide variety of environments. There are five major steps to paper productions, and each of these steps has its own specific corrosion conditions that must be considered individually when choosing materials. First, wood chips are fed into a digester, under high temperature and pressure conditions, in order to separate out the glue which holds the wood chips together. Next, the pulp is processed through a series of washes in order to further remove impurities and recycle the cooking liquor, also known as black liquor at this point in the process. This is followed by a bleaching process that increases the brightness of the pulp to the appropriate amount based on the desired pulp or paper product. After bleaching, the pulp is processed into liquid stock that can be transferred to a paper mill. The liquid stock is then processed through a series of wet end and dry end equipment to suction out the water and compress the fibers in order to create the final paper product. The equipment, temperature, pressure, chemical types and concentrations all vary considerably for each of these operations.
In addition, many other factors impact material selections. First, the amount of water recycling within each mill makes a huge difference in processing environments. The paper industry is no longer allowed to run a river of water through their plants and then dump that processed water back into the environment without cleaning it first. Instead, pulp and paper mills now tend to be closed loop systems in which water is re-used many times and then cleaned before being releasediii. These closed loop systems lead to severe corrosion conditions due to increased concentrations of chemicals. Also, the size and quality of the wood fibers, the specific methods of achieving each of the major steps, and of course, the specifications for the type of pulp or paper in production can make quite a difference in processing environments as well.
Given all of the variations of conditions within a mill and between mills, each environment should be carefully considered when making material choices. Still, there appear to be some general trends in the industry regarding duplex materials usage for new equipment. Today, duplex is used in digesters for cooking and reactors for bleaching. It is also finding increasing use in all kinds of process equipment, including the washing and screening equipment, oxygen reactors, piping, chemical recovery equipment, pulp processing, and in the head boxes and suction rolls of paper machinesiv. Here are some specific examples of duplex usage in mills today.
Duplex 2205 and LDX 2101® in Digesters:
Duplex 2205 (UNS S31803/S32205) was first introduced to the paper industry in 1982. Two solid duplex 2205 kraft digesters were installed in a New Zealand milli. Kraft digesters, which are used to cook the incoming wood chips in an alkaline solution, are the most common equipment type used for the first step in paper productioni. Operating environments include high temperatures, widely variable corrosion conditions, and highly abrasive conditions. Following this initial solid duplex installation, there was no additional duplex activity for a period of time until data could be collected. It clearly went well because almost all new digesters are now made out of solid duplex. Today, there are over 114 digesters in operationi.
Interestingly, a study by Outokumpu has shown that lean duplexes, such as LDX 2101® (UNS S32101) and duplex 2304 (UNS S32304), may be an even better choice for digesters than duplex 2205. Duplex 2205 significantly improves corrosion in kraft digesters as compared to standard austenitic grades, but it does not completely eliminate it. Coupon testing, which is performed by weighing pieces of metals that have been exposed to corrosive conditions in order to measure weight loss over time, has been performed in operating digesters of different materials. The results indicate that corrosion resistance increases with chromium content, but not with molybdenum. In fact, molybdenum has a detrimental impact in the hot alkaline solutions that are typical of digestersv. Thus, LDX 2101® and duplex 2304, with their lower levels of molybdenum, may offer not only material cost savings, but also improved corrosion properties. In 2007, the Ostrand pulp mill of SCA, in Sweden, replaced the upper half of an existing carbon steel digester with LDX 2101®vi. The original digester had a great deal of corrosion problems, due to stress corrosion cracking. The LDX 2101® material has performed well and is now an available option for new digesters.
LDX 2101® in Pressure Vessels:
Corrosive conditions for the bleaching process in pulp mills can vary considerably depending on the chemicals used. In recent decades, there has been a shift to less chlorine usage in order to be more environmentally friendly. TCF (“totally chlorine free”) is the ideal standard today. In TCF bleaching, the chemicals available include oxygen, hydrogen peroxide, ozone and peracidsi.
Smurfit kappa Kraftlie Pitea is Europe’s largest producer of kraftliner, which is a paperboard used to manufacture high quality corrugated containerboard. In 2006, they installed the first LDX 2101® hydrogen peroxide reactor that is used for the TCF bleaching step. LDX 2101® was selected due to its high strength, which allowed designers to use thinner gauges compared to austenitic steels. Also, it has shown to be quite adequate to handle the corrosion conditions of the hydrogen peroxide reactor process, an alkaline environment with high pressures, and a temperature around 110 degrees Celsius. An additional benefit of the LDX 2101® is good machinability. It should be noted that duplex 2205 and also 904L (UNS N08904) grades have been successfully implemented in previous versions of the pressure vessels, but these steels are higher performance and cost than is needed for this particular environmentvii.
Duplex Steel in Storage Tanks:
Given the large volume of storage tanks, duplex stainless steels offer exceptional material weight savings when used for the production of these tanks. In the past, storage tanks were made primarily from carbon steel with a corrosion allowance, or from austenitic grades. Based on both laboratory testing and mill tests, it turns out that even the austenitic grades (304 and 316L) do not always offer enough corrosion resistance for certain liquors, especially those with higher solid contents. Thus, duplex steels offer not only material weight cost savings, but also corrosion benefits. Stora Enso’s mill in Imatra, Finland currently uses white liquor tanks that are made from duplex stainless steel grade LDX 2101®viii. Duplex 2304, and duplex 2205 storage tanks can also be found in production.
While duplex stainless steel has performed well in the Finland mill white liquor storage tanks, some liquors have proved to be a challenge even for duplex materials. In 2002, the upper half of a black liquor storage tank used in production was replaced with solid duplex 2205. The tank was originally made with 316L (UNS S31600) and had experienced ongoing problems due to severe cracking. Unfortunately, the new duplex 2205 tank began experiencing corrosion damage in 2008 after six years in service. The corrosion issues include pitting and corrosion under deposits, as well as stress corrosion cracks in the welding metal. Further analysis indicated that the high chloride content of the liquor (.5 to 1%) most likely led to the corrosion. Testing has been done to determine whether the lean duplexes might perform better. Also, suggestions have been made to lower the chloride content of the black liquor or to switch to Super Duplex 2507 (UNS S37250)ix.
- “Stainless Steels for Pulp and Paper Processes.” www.outokumpu.com. Outokumpu, 2008. Web. May 2011.
- Tuthil, Arthur H.l. “Stainless Steels and Specialty Alloys for Pulp and Paper.” Stainless-steel-world.net. Nickel Development Institute, Aug. 2002. Web. May 2011.
- Brongers, Michael P.H., and Aaron J. Mierza. “Pulp and Paper, Appendix W.” www.corrosioncost.com. 2006. Web. May 2011.
- Chater, James. “The Pulp and Paper Industry Turns to Duplex.” www.stainless-steel-world.net. Stainless Steel World, 2007. Web. May 2011.
- “Pulp Paper | Outokumpu.” Outokumpu, July 2009. Web. May 2011.
- “An LDX 2101® Breakthrough in Pressure Vessels | Outokumpu.” Outokumpu, Oct. 2006. Web. 18 May 2011.
- “Stainless Steel Tanks and Pulp Towers | Outokumpu.” Outokumpu, June 2009. Web. 19 May 2011.
- Bergquist, Arne, and Lena Wegrelius. “Duplex 2205 – Experience in the Pulp and Paper Industry of a 30 Year Old Grade.” Lecture. ABTCP-TAPPI 2010. TransAmerica Expo Center, Sau Paulo, Brazil. 2010. Outokumpu. Web. May 2011.