🧪 Calculators

White Liquor Sulfidity (Kraft)

Calculates the sulfidity of white liquor in a kraft pulp mill: sodium sulfide divided by active alkali, which is sulfide plus sodium hydroxide, as a percentage. Both terms have to be expressed on the same basis — mixing Na₂O with NaOH is the classic mistake here. Enter the two concentrations.

Sodium sulfide Na₂S, as Na₂O (g/L) Sodium hydroxide NaOH, as Na₂O (g/L)

Result

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Why sulfidity decides the cook and the strength of the fibre

Sulfidity is the number the causticizing operator chases every shift, and it separates a fast kraft cook from one that burns alkali without dropping the kappa. Too little hydrosulfide ion and delignification lags, fibre turns brittle, yield slides; too much and the recovery boiler emits more sulfur while makeup sulfate costs climb. The figure comes out of two laboratory titrations, and the arithmetic stays simple enough to go wrong halfway through a shift.

Sulfidity (%) = 100 × Na₂S / (Na₂S + NaOH), with the denominator being active alkali. Both terms arrive from a TAPPI or ABNT assay in g/L, almost always expressed as Na₂O. The fields load with 32 g/L of sulfide and 88 g/L of hydroxide, giving 120 g/L of active alkali and a sulfidity of 26.67%. Brazilian eucalyptus mills tend to run between 25% and 35%. Delignification gains above 30% turn marginal, so pushing higher rarely repays the extra sulfur load circulating through the recovery loop.

The rule that wrecks this calculation is the basis of expression. Sulfide and hydroxide may arrive as Na₂O or as NaOH, and the factor between the two bases is 1.2908: mixing them inside one calculation shifts sulfidity by several percentage points, always in the same direction, which lets the error hide for weeks. A second definition also circulates, computed on total titratable alkali, which counts residual carbonate and therefore always reads lower. This tool applies the active alkali definition, standard practice in digester control, and performs no basis conversion: what comes back is the ratio of the two numbers typed in.

Frequently asked questions

Do I have to convert my values to Na₂O before typing them?

Only make sure both sit on the same basis. Since the conversion factor between Na₂O and NaOH applies equally to sulfide and hydroxide, it cancels inside the ratio: 32 and 88 as Na₂O return 26.67%, and the same pair converted to NaOH, 41.31 and 113.60, returns the identical 26.67%. What must never happen is one field on Na₂O and the other on NaOH, since the result then shifts with no warning at all and each number still looks perfectly valid on its own.

Why does my lab report a lower sulfidity than this page shows?

Most likely because it computes on total titratable alkali rather than active alkali. TTA counts the sodium carbonate left over from incomplete causticizing, so the denominator grows and sulfidity falls; the gap usually lands between 3 and 6 percentage points, depending on causticizing efficiency. Neither definition is wrong, they answer different questions. Before comparing your figure against another mill or a contract limit, confirm which basis the report used.

Does the tool give active alkali and effective alkali too?

Active alkali sits there implicitly, since it forms the denominator: with the loaded values it comes to 120 g/L, the sum of the two fields. Effective alkali, defined as NaOH plus half the Na₂S, stays outside this screen — the same numbers would give 88 plus 16, that is 104 g/L. The page returns the sulfidity percentage alone, to two decimal places, and refuses to calculate whenever a field turns negative or the two add up to zero.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.