The process error that destroyed a white liquor storage tank: The MOXY (Modified Oxidation of White Liquor), later marketed as the Mead Oxidation process, was introduced to kraft pulping as a yield improvement step.
MOXY partially oxidized sodium sulfide in white liquor to form sodium polysulfide using air and an activated carbon catalyst. When applied to chip impregnation and early cooking, polysulfide chemistry suppressed alkaline peeling reactions in hemicellulose, preserving carbohydrate content that would otherwise be lost to black liquor.
The result was a modest but economically meaningful increase in pulp yield, typically on the order of one to four percent, along with a reduced organic load to the recovery cycle and a de facto increase in recovery boiler capacity.
At the Chillicothe mill, however, the MOXY process exposed an unanticipated materials problem. Conventional white liquor systems rely on a reducing, high-pH environment in which carbon steel is protected by a thin iron sulfide film. MOXY disrupted that balance by introducing dissolved oxygen and reactive sulfur intermediates. Instead of forming a stable protective layer, steel surfaces experienced repeated film breakdown and reformation.
The result was not uniform corrosion but aggressive localized attack, particularly at white liquor storage tank welds and heat-affected zones where metallurgical discontinuities already existed.
The mill's white liquor storage tank became the critical failure point. Weld seams developed deep pitting and rapid material loss under conditions that were not aggressive in conventional kraft service. The failing seams had been formed using type E7018 welding rods when the tank was built 20 years earlier.
As damage accumulated, reliability and containment concerns forced an unplanned replacement of the vessel using 316 stainless steel, whose passive chromium oxide layer is far more stable in mixed oxidizing–alkaline sulfur environments.
Although the chemistry delivered the expected yield gains, the corrosion burden from oxidized sulfides significantly increased maintenance demands and risk exposure. Over time, the cost of repeated inspection, repair, and eventual replacement of major equipment eroded the economic advantage of the process.
At Chillicothe, this imbalance ultimately led to abandonment of the MOXY operation and catalyst reactors. The experience underscored a core lesson in kraft system engineering: chemical performance cannot be separated from materials compatibility. A yield-enhancing modification that destabilizes the corrosion regime of the white liquor loop can negate its own economic value through infrastructure degradation.
Moral: The presence of oxidized sulfides in white liquor can lead to rapid weld failure in white liquor storage tanks, pipes and pump bowls. Know your chemistry and know your materials of construction including the filler metals in the welds.
About Mike Ryan
Mike Ryan is recognized for enhancing operational reliability and efficiency, combining deep technical expertise with a strong focus on training and process improvement across a wide range of industrial settings.
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