The Kraft Process is the most common pulp and papermaking process currently in use for the conversion of wood chips into wood pulp. The process involves numerous steps - mechanical and chemical - such as chipping or grinding, impregnation, cooking, recovery, blowing, screening, washing, and bleaching.
There are five major steps to paper production, and each has its own specific corrosion conditions that must be considered individually when choosing materials.
- Wood chips are fed into a digester, under high temperature and pressure conditions, in order to separate out the lignin (glue) which holds the wood chips together.
- The pulp is processed through a series of washes in order to further remove impurities and recycle the cooking liquor.
- 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 fibres in order to create the final paper product.
Pulp and paper mills use stainless steel and other corrosion resistant alloys to avoid iron contamination of the product paper and to resist process corrosion. The reduction of historic annual maintenance shutdowns has increased the need for more resistant alloys. Corrosion problems are often exacerbated by downtime or downgraded process conditions, as such events tend to increase the concentration of chlorides in process waters.
Against these changes, carbon steel has been switched to austenitic stainless steels, which in turn have been partly replaced by duplex and super duplex stainless steels. One key driver for the increased use of duplex stainless steels in the pulp and paper industry is cost - they are less sensitive to price fluctuations of raw materials, as they contain less nickel than the austenitic grades.
Stage 1 - chip preparation / digestion / liquor storage and recovery
Conditions are generally wet and abrasive, so abrasion-resistant steels may be used in the early stages, giving way to stainless steels subsequently. Historically units such as the chipper/grinder have been built using Alloy 304L or 316L stainless steel, but duplex stainless steels offer an upgrade where it is not known whether corrosion or wear/erosion dominates the material loss rate.
Digesters effectively 'cook' the wood chips, breaking down the cellulose fibres and separating them from lignin and hemicellulose. Conditions are strongly alkali, highly abrasive, at high temperatures and with variable corrosion conditions throughout the vessel. They utilise a mixture of white and black liquor to help break down the wood chips into wood pulp.
White liquor consists mainly of sodium hydroxide and sodium sulphide in water, and is the active component in Kraft pulping. Black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process.
For many years, large vessels were constructed from carbon steel, allowing for significant loss of thickness by designing-in excess material. Therefore, they were obvious candidates for more corrosion resistant materials. The availability of duplex stainless steels provided a more cost-effective option - good corrosion resistance and high-strength without the need for cold working.
Compared with alternative materials, duplex and super duplex stainless steels provide enhanced corrosion performance in strongly alkali conditions. In general, the corrosion resistance improves with increasing chromium content. However, cost and availability has tended to favour the more widespread use of Alloy 2205 for this application.
Stage 2 - washing / liquor storage and recovery
In principle, the conditions experienced during pulp washing might seem less aggressive and allow the use of more standard stainless steels such as Alloy 316L. However, the use of closed-circuit systems can lead to the use of waters with a higher chloride content. Duplex stainless steel grades have an advantage of being more abrasion-resistant.
A large number of storage tanks are involved in this stage. For some liquors, particularly those with higher solid contents, austenitic stainless steels (Alloy 304 & Alloy 316L) might not provide sufficient corrosion resistance, so could be replaced by higher performing duplex stainless steels. However, in some black liquor tanks - where chloride content might be appreciably higher (c. 1%) - the higher chromium content of 25% Cr super duplex stainless steels would be a more appropriate choice.
Stage 3 - bleaching
Bleaching is an extremely corrosive process that is executed under acidic conditions with strong oxidants such as chlorine, chlorine dioxide, sodium hydroxide, and hydrogen peroxide. The bleaching process normally has three to five stages in which the pH of the pulp is alternated between acid and alkaline conditions.
The bleach towers are large vessels - typically 4-6m (12-18ft) diameter and 12-17m (35-50ft) height - with 4-8 towers per plant. Traditionally they have been constructed with brick-lined walls and stainless steel inlet pipework.
Corrosion of stainless steel in bleach plants has been a longstanding materials challenge. The corrosive conditions caused by chlorine and chlorine dioxide has limited the use of stainless steels to the washing equipment of later process stages.
Ferralium® 255 - SD50 has been used successfully in this environment, achieving strong corrosion resistance combined with greater strength and less thermal expansion than many alternative alloys. Grades as highly alloyed as Alloy 904L have been used, but provide greater performance at a higher cost than is typically needed.
In recent decades, there has been a shift to avoid chlorine usage for environmental reasons. Totally chlorine free (TCF) processing has become more standard, using chemicals such as oxygen, hydrogen peroxide, ozone and peroxyacetic acid. Hydrogen peroxide and ozone are generally harmless to stainless steels, even at fairly high concentrations and high temperatures.
Therefore, duplex and super duplex stainless steel grades are sensible. The corrosivity of the process declines as the bleaching agent is consumed, which means that in some instances the environment is mild enough to be handled by Alloy 2205.
Stage 4 - pulp processing
Pulp processing involves a number of different process steps, but the material selection issues follow similar challenges to earlier parts of the process. For instance, in pulp blow tanks, wear can be an issue from pulp impact against tank walls or strainer plates. The use of mechanically stronger and harder alloys, such as duplex and super duplex stainless steels, can resist excessive wear and provide extended operating lifetimes and/or reduced maintenance.
Stage 5 - paper-making
The wet end of paper machines can be exposed to pitting and crevice corrosion resulting from chlorides in the solution. The chlorides come from the wood floated in seawater, from the supplied process water and from chemicals added to the pulp. Therefore, Alloy 316L is most commonly used in new paper machines, dependent upon the specific nature of the process. In severe environments Alloy 316L is often replaced by a more corrosion resistant variant such as Alloy 317L.
For specific parts within the paper-making process, such as suction rolls that help to remove moisture from the paper film, duplex stainless steels such as Alloy 2205 are used. These operate in white water environments and are subjected to high cyclic stresses from the rotation of the roll.
Similarly, steam profilers have suffered from stress corrosion cracking. Austenitic stainless steels like Alloy 316L can be susceptible to external stress corrosion cracking, especially if there is a build-up of wet pulp in the presence of steam. Duplex or super duplex stainless steels resist stress corrosion cracking due to their duplex microstructure.