Major Players in Kraft’s Pulping Process


“Writing is thinking on paper”
-William Zinsser.

The conveying of ideas can never be as effective as it currently is if paper did not exist. In our previous post, we know that the Kraft’s pulping process is the dominant pulping process to date, accounting for up to two-third of the world’s pulp production. The chemical mixture is a basic concoction comprising of both Sodium Hydroxide and Sodium Sulfide as the major components. Today, we’ll talk about Sodium Sulfide, an active chemical in the Kraft’s pulping process. (Click here to read more about Sodium Hydroxide)

During the Kraft’s pulping process, chemicals are used to remove lignin binding the cellulose fibres together so that the chips defibrinate easily. Cellulose fibres are kept as long as possible to ensure the strength of the paper product is of a higher quality. Hence, chemicals that dissolve lignin but not cellulose are preferred in this process. Sodium Sulfide, an active chemical in the Kraft’s pulping process, helps to speed up cooking reactions and decrease cellulose degradation by Sodium Hydroxide, allowing effective delignification of the wood chips.

Paper chemicalsGrade I (left), Grade II (right)

Sodium Sulfide exists as different grades, mainly Grade I – which exist as red flakes and Grade II- which exist as yellow flakes. Both grades have relatively similar purity of minimum 60% and vary only in their composition of impurities. To date, industrial production of Sodium Sulfide can be categorised to two different processes, namely carbothermic reduction of Sodium Sulfate using coal or reaction of Sodium Hydroxide with Hydrogen Sulfate. In the latter, molar ratio of Sodium Hydroxide and Hydrogen Sulfate needs to be controlled as treating Sodium Sulfide with excess Hydrogen Sulfide can result in a further reaction to form Sodium Hydrosulfite. Major producers of Sodium Sulfide include China and USA while countries like Chile and Tanzania are the main importers. Average price of Sodium Sulfide ranges from 600-650 USD/ton

CountryPrice per ton/ USDTotal import value/USDAverage price per ton/USD
CountryPrice per ton/ USDTotal export value/USDAverage price per ton/USD

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