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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 productions, and each of these steps has its own specific corrosion conditions that must be considered individually when choosing materials.:

  1. First, 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.
  2. 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.
  3. 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.
  4. After bleaching, the pulp is processed into liquid stock that can be transferred to a paper mill.
  5. 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. Although most mills use nominally the same (Kraft) process, there are sufficient mill-to-mill differences that can affect corrosion behaviour.

In addition, the reduction of the historic annual maintenance shutdowns has increased the need for more resistant alloys. Maintenance costs can be reduced by using more corrosion resistant materials. 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 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

Dependent upon the exact process configuration, chip preparation can involve a number of discrete phases – removing the bark, chipping and initial treatment. 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.

The chips normally first enter the pre-steaming where they’re wetted and preheated with steam. Cavities inside fresh wood chips are partly filled with liquid and air. The steam treatment causes the air to expand, and about 25% of the air to be expelled from the chips. The next step is to saturate the chips with black and white liquor. This can either be done before or after the chips enter the digester, but most mills perform this process in the digester due to corrosion control. Again, Alloy 316L and Alloy 2205 have been common choices.

Digesters effectively ‘cook’ the wood chips, breaking down the cellulose fibres and separating them from lignin and hemicellulose that bind them together. 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. White liquor also contains minor amounts of sodium carbonate, sodium sulphate, sodium thiosulfate, sodium chloride, calcium carbonate, and other accumulated salts and non-process elements. These additional components are considered inert in the Kraft process, except sodium carbonate that contributes to a lesser extent.

Black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. The black liquor comprises 15% solids by weight of which 10% are organic chemicals and 5% are inorganic chemicals. Normally the organics in black liquor are 40-45% soaps, 35-45% lignin and 10-15% other organics.

For many years, the large vessels were constructed from carbon steel, allowing for the significant loss of thickness by designing-in excess material. A consequence of this approach, apart from a finite operating life, was the relative weight and size of the fabricated components. Therefore, they were an obvious candidate for more corrosion resistant materials such as stainless steels. As there is less need to increase thickness to allow for corrosion, and the alloys can be considerably stronger, then the potential to reduce cost despite more expensive raw materials is possible.  Initial installations utilised austenitic stainless steels that were cold worked (stretched) to increase their strength. However, 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 (carbon steel, austenitic stainless steels) duplex and super duplex stainless steels provide enhanced corrosion performance in the strongly alkali conditions. In general, the corrosion resistance improves with increasing chromium content, although less benefit from raised molybdenum additions is seen in such alkali environments compared with neutral or acidic environments. Therefore, 25%Cr super duplex alloys might be expected to outperform 22%Cr duplex alloys. 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 – useful if there is the potential for sand/grit carryover in the white liquor.

A large number of storage tanks are involved in this stage of the process. Assuming a corrosion resistant alloy is required for a particular set of process conditions then duplex and super duplex stainless steels offer weight saving opportunities. As with the digesters, tanks were historically made from carbon steels with a corrosion allowance. 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 materials such as duplex stainless steels. However, in some black liquor tanks – where chloride content might be appreciably higher c. 1% – then duplex alloys risk pitting and corrosion where local dead-spots or deposits change the uniform conditions. Here, 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. These stages are divided into two process categories within each bleaching sequence, delignification and brightening.

Each stage of a bleach sequence is comprised of four basic equipment components:

  1. A pump to move the pulp through the stage at the desired consistency.
  2. A mixer to blend the pulp, chemicals and steam.
  3. A reaction tower or vessel (atmospheric or pressurized) with dilution, and agitation equipment for discharge consistency control.
  4. A washer to separate residual chemicals and reaction by-products from the pulp.

The bleach towers are typically bottom fed with the pulp moving up the tower as it bleaches to the required brightness. These are large towers with diameters of 4-6m (12-18ft), and heights of 12-17m (35-50ft). There are typically 4-8 towers per plant and traditionally they have been constructed with brick linings and Alloy 254 inlet pipes.

Corrosion of stainless steel in bleach plants has been a longstanding materials challenge. Corrosive conditions for the bleaching process can vary considerably depending on the chemicals used, which in turn can depend upon the source of the pulp i.e. from wood vs. recycled paper. The corrosive conditions caused by chlorine and chlorine dioxide has limited the use of stainless steels to the washing equipment of later process stages. Alloy 254 has been used successfully, and has also been successfully substituted by Ferralium 255 – SD50 and Alloy 32750 with comparable corrosion resistance but greater strength and less thermal expansion. Grades as highly alloyed as Alloy 904L have been used for this application, but provide greater performance at a higher cost than is typically needed. In an alkaline environment, operating at temperatures up to 110degC, duplex grades would probably suffice. Alternative materials have included Titanium (Grade 2) which has excellent corrosion resistance and good availability, but it is more difficult to weld and prohibitively expensive.

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. Although they are strong oxidants (like chlorine and chlorine dioxide) they achieve the same bleaching results but without the introduction of chlorides that could damage the passive layer of stainless steels. Austenitic grades such as Alloy 316L could be used, as long as they are only trace levels of halides present. However, higher levels would be problematic, with a strong risk of stress corrosion cracking. Therefore, duplex and super duplex stainless steel grades are sensible. The corrosivity of the process declines as the bleaching agent is consumed, together with dilution of the pulp, 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. Experience has shown that traces of thiosulphate, originating from brightening agents, can cause pitting on Alloy 304 stainless steel despite it seeming a low risk environment. 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 corrosive resistant variant such as Alloy 317L.

Secondary structures such as catwalks, stairs and railings can be supplied from Alloy 304, aluminium or even post-galvanised carbon steel.

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. Suction rolls operate in white water environments and are subjected to high cyclic stresses from the rotation of the roll. Leading to corrosion fatigue failures. Bronzes were replaced by stainless steels to provide the lower deflections needed to construct ever wider machines.

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.

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