Stability of Sodium Sulfate Dicarbonate in Black Liquor Concentrators, 2006 Engineering, Pulping, & Environmental Conference

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Crystallization of sodium sulfate dicarbonate (~2Na₂CO₃·Na₂SO₄) has been blamed for soluble scale fouling of black liquor evaporators and concentrators. During earlier work by our group, phase changes were observed during storage of dicarbonate samples. This raised doubts about the stability of this phase in industrial equipment. The present work demonstrated that the sodium sulfate dicarbonate crystal phase was formed in laboratory-scale evapo-rative crystallization experiments at close to industrial process conditions. The crystal phase was identified by a characteristic powder x-ray diffraction pattern that was previously shown by Shi and Rousseau [1] to be distinguish-able from burkeite and other sodium salts. X-ray diffraction, calorimetry, and microscopic observations were used in the present study to investigate the stability of the salts to in-process aging, isolation and storage, and exposure to high temperature. The results showed that sodium sulfate dicarbonate can exist as a unique phase and remains stable at industrial conditions. Care must be taken to prevent contamination of crystals during sampling and storage; the contaminant sodium carbonate monohydrate is not thermally stable, and can degrade when held in contact with moisture and ambient air for extended periods of time. Nearly pure sodium sulfate dicarbonate was found in deposit samples collected from a black liquor concentrator verifying the industrial occurrence of this salt.