INTRODUCTION

ABSTRACT

In the pulp and paper industry, variability in the process and wood source may result in high corrosivity waste liquors, called black liquors. It is well documented that the concentration of organic extractives is wood species dependent and prior research has demonstrated corrosion rates of carbon steel in pulp mill equipment ranging from <0.03 mm/yr to >2.54 mm/yr, depending on wood species pulped. Our research demonstrates the importance of operational strategies for wood species, wood chip usage and delivery to downstream process corrosion. Some black liquor constituents have been identified to increase the corrosivity of black liquors whereas others may act as corrosion inhibitors. Results from this study show the importance of water soluble extractives in wood, such as long chain fatty acids, catechols, and others on the corrosivity of black liquor towards carbon steel 516-Gr70.

Significant variation in the corrosion rate of carbon steel equipment in pulp mills using different tree species has been reported for a long time. In 1953, MacLean and Gardner demonstrated the effect of organic-metal complexing or sequestering agents in black liquors (BLs) on the corrosion of carbon steel digesters. Yet most of previously published work has focused on the concentration of inorganic species such as hydroxide and sulfide in pulping liquors and not on the wood. The organic components, which are wood species specific, come from extractives in the wood chips or are generated from the degradation of lignin, a phenolic compound, and other heartwood constituents during pulping. BL contains residual caustic, sulfide and other process and non-process inorganic chemicals as well as organic chemicals from the wood. Niemela characterized Scots pine (Pinus sylvestris) BL identifying more than 700 low-molecular weight organics compounds.

Singh et al evaluated corrosivity of BLs from commonly used wood species in the North American pulp and paper industry (PPI) and reported the rate of corrosion for carbon steels varied from <0.03 mm/yr to >2.54 mm/yr, depending upon the wood species pulped under otherwise similar conditions. In general, hardwoods (HW) were found to have lower corrosivity compared to the softwood (SW) species tested. A study by Pawel et al reported an increase in digester corrosion with increased Douglas fir furnish which demonstrates the importance of wood species in black liquor corrosivity. Results from studies on wood species and corrosivity indicate that organic constituents of BL play a significant role in variations of corrosivity amongst wood species. Although the role of an individual compound is not always certain, organic constituents in BL can be separated into two broad groups: corrosion activators and corrosion inhibitors. Catechol, a breakdown product of lignin, has historically been proposed as a corrosion activator. Organic extractives, such as resins, tannins, and fatty acids, which show considerable differences in concentration among different species as well as among differently aged trees, are suspected of having some inhibitory properties.

The main focus of the PPI concerning organic extractives is on tall oil production and pitch formation, not corrosion. Sterols, sterol esters, and waxes have been identified as non-soluble soaps under the alkaline conditions of Kraft pulping and the root cause of pitch problems, while glycerol esters completely saponify both fatty and resin acids which then readily dissolve. In some cases the debarking of logs, seasoning of logs, or the seasoning or biological treatment of woods chips with enzymes has been utilized to reduce downstream pitch problems. Therefore the seasoning of wood chips due to lifeti

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