High yield mechanical pulping
This is the generic term for producing pulps mechanically using disc refiners. Newsprint typically has high levels of refiner pulps, sometimes as much as 100%.
Type 316L is the standard alloy used for the major components of refiner systems. However, Type 316L can be susceptible to attack by chlorides at coastal mills and by the concentration of chloride by recirculation of water used in the process. In these cases, higher-alloyed materials such as 2205 (UNS S32205), 904L and C-276 are used.
Paper recycling is a major part of the pulp and paper industry, with the process and equipment constantly undergoing change. Type 316L and its cast counterpart CF3M are the principal materials for constructing the equipment.
Bleach plant and pulp/paper stock preparation
Bleaching typically begins with an oxidising, acidic stage using chlorine dioxide (ClO2) bleachant. This is followed by an alkaline extraction stage to remove soluble lignin and at least one more acidic, oxidising stage.
Acidic, D-stage (chlorine dioxide bleaching): Comprehensive industry research programmes in the 1970s and 80s showed the controlling parameters for chloride-related, localised corrosion of an alloy in D-stage environments are, in order of decreasing effect:
- Maximum residual chlorine dioxide concentration
- Maximum solution temperature
- Maximum chloride concentration
- Minimum pH.
Stainless steels resist localised corrosion based on their alloying content. Pitting resistance correlates with Cr, Mo and N contents according to the following formula:
Pitting resistance equivalent (PREN) = Cr + 3.3Mo + 16N
Generally, alloys with PRENs of 35 or above resist localised corrosion in D-stage environments below 75°C with maximum residual chlorine dioxide content of less than 50 ppm. Stainless steels with PRENs of 42 or above, including 6Mo, super austenitic grades and super duplex grades, resist localised corrosion in more highly-corrosive conditions.
Alkaline, E-stage (extraction stage): Brown stock coming to the bleach plant typically has a pH of over 9.5, while extraction (E) stages operate at pH above 10. These are benign environments for carbon steel and for regular stainless steel grades, including Type 304L (UNS S30403) and Type 316L as well as lean duplex grades, including 2101 (UNS S32101), 2304 (UNS S32304) and 2202 (UNS 32202), even at process temperatures above 70°C. The notion of PREN does not apply for alkaline bleaching stages; in fact, higher molybdenum reduces the corrosion resistance of stainless steels in hot alkaline environments.