Control Of Sulfidity In A Modern Kraft Pulp Mill, 2017PEERS

In the past the sulfidity in Kraft pulp mills reached an equilibrium level that was dependent on the leakages. But today, environmental and economic forces have reduced the leakages and most modern mills have practically eliminated emissions of sulfur. The result of this development is for most mills that the equilibrium sulfidity level is higher.

The methods used to control the chemical balance include e.g. dumping of recovery boiler ash to reduce sulfur content, white liquor oxidation to the oxygen delignification to reduce sodium intake and dumping of chlorine dioxide brine. Although these methods are applied, some modern mills still have difficulties to control sulfidity.

Valmet has studied the means to expand the sulfidity control toolbox. Our conclusion is that internal sulfuric acid production is the most efficient way to separate sodium and sulfur streams. A process that oxidizes sulfur containing concentrated non-condensable gas (CNCG) to relatively concentrated sulfuric acid was therefore developed and the first full-scale plant based on oxidation of CNCG will be started up late 2017. The intention is to avoid addition of external sulfur and replace it with internally produced acid. The paper will describe the plant and scenarios for control of the chemical balance. 

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