Implementation of an Inexpensive and Effective Alternative Sulphur Makeup Source in an Unbleached Kraft Pulp Mill: Pelletized Sulphur, 2023 PEERS Conference

Sodium sesquisulphate or “sesqui” (Na3H(SO4)2) is a by-product of chlorine dioxide production at kraft pulp mills. It is typically used for sodium and sulphur make-up in the liquor loop. Hinton Pulp (HIP) was being converted from bleached to unbleached kraft pulp production and thus was losing this source of make-up. The only option that was readily available as a substitute was sodium hydrosulphide (NaHS), which was cost prohibitive. Other options such as sodium sulphate (Na2SO4), emulsified sulphur, sulphuric acid (H2SO4) and sodium bisulphite (NaHSO3) were compared. HIP concluded that pelletized sulphur plus sodium hydroxide or “caustic soda” (NaOH) was the best option.

Lab-scale experiments showed that pelletized sulphur dissolved in white liquor (WL). A mill-scale trial revealed that pelletized sulphur added to a causticizer had no adverse impacts on the downstream pressure filters or kiln operation. The sulphur reacted to produce polysulphide upstream of the WL storage tank giving the liquor an orange hue. This polysulphide appeared to degrade partially into thiosulphate before being fed to the digester. The heavy black liquor (HBL) S:Na ratio did not change significantly even though the sulphur/soda addition location was upstream of the original one. In addition, other properties such as liquor heating value and elemental analysis did not significantly change.

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