VWS Envig, part of the global Veolia Water Solutions & Technologies group, recently completed a chloride removal plant for Mondi Richards Bay Mill. The mill produces bleached hardwood pulp and white top Kraft linerboard.
The project was awarded through tender and is valued at over R21-million and included the design, manufacture, construction verification, and commissioning of the plant. The chloride removal plant was needed due to elevated levels of chloride and potassium in the mill's recovery cycle.
According to Cobus Olivier, VWS Envig Project Manager, this occurs in the Kraft recovery cycle as a result of chloride and potassium becoming enriched in the flue gas dust from the recovery boiler. "The process becomes steadily less efficient in this way because these elements decrease the melting point of the dust, referred to as the sticky ash temperature. This leads to plugging of the boiler and results in capacity loss and corrosion."
The existing method for removing the elements involved dumping precipitator ash from the cycle. However, soda was being lost in the process and led to increased costs in replacing the soda. "We provided a more effective method. The chloride removal plant processes dissolved precipitator ash, which is the ideal stage in the recovery loop to effectively purge potassium and chloride, thereby increasing the sticky ash temperature and increasing boiler availability," says Olivier.
The dissolved ash solution consists mainly of sodium sulphate at near saturation concentration. The plant recovers this valuable item while rejecting potassium and chlorides.
"This is accomplished through crystallisation technology supplied by our sister company, HPD Technology. This system, CRP™ (chloride removal process), was developed by HPD Technology and is designed specifically for applications such as these."
The CRP system consists of various steps, including dissolution of ash, crystallisation of sodium sulphate, dewatering of the pure sodium sulphate crystals and re-dissolution of crystals in intermediate black liquor for reprocessing.
Commissioning was completed July 2008 with an accompanying decrease in boiler ash chloride content from above 7 percent to below 3.5 percent on mass basis, and potassium content has also reduced from above 5 percent to less than 3 percent on a mass basis.
"The plant is fully operational and processes 5 tons per hour of dry ash," concludes Olivier.