Effect of tall oil soap on sodium salt scaling in kraft black liquor evaporators

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Canada produced 16.5 billion tonnes of wood pulp in 2016, primary through kraft pulping, accounting for about $7.2 billion in exports. The chemical recovery cycle makes the kraft pulping process both environmentally and economically viable and is the reason it is the dominant pulping process used worldwide. The first step in the chemical recovery cycle is black liquor evaporation. Black liquor is a mixture of the dissolved organics and spend pulping chemicals which are predominantly dissolved sodium salts. During evaporation, these sodium salts crystallize, leading to sodium salt scaling in the black liquor evaporators. Recent mill experience has shown that the addition of a solution of fatty and resin acids (tall oil soap and similar) can significantly reduce or even eliminate sodium salt scaling. What is limiting its widespread application is a lack of knowledge about how the tall oil soap reduces scaling. This is a proposal to study the effects of resin and fatty acids, individually and in combination, on sodium salt crystallization to clarify the underlying mechanism.This research is intended to lead to a widespread reduction in sodium salt scaling in pulp mills through the addition of pulp mill streams containing resin and fatty acids. This would lead to a reduction in steam usage in evaporators due to a reduction in the number of wash cycles. This in turn will result in a smaller CO2 footprint of the mills. An important part of the proposed work is to investigate other salt systems to which this work can be applied so that we can maximize the overall benefit to Canada and the environment.