Slaker operation is critical to maintaining white liquor quality, reduced system dead load, and stable pressure filter operation. Recently a mill was experiencing regular over liming events causing unnecessary area and mill down time. Frequent acid washing was required to maintain filter operability. To reduce over liming excursions, weak white liquor was being produced causing a volume demand increase to maintain digester rate. However, as mill production increased using high volumes of low-quality white liquor was not sustainable.
The existing Slaker control strategy used manual inputs from the operator based on unreliable temperature and conductivity readings and operator experience, with the increasing age of the mill employees, many years of experience has been lost over a short period. New operators were over liming more frequently, driving a need for an automated Slaker control strategy. With lack of capital availability and a short window for improvement, a strategy was developed using available equipment and the DCS.
The new control strategy was based on a causticizing efficiency (CE) target set at the Slaker overflow trough. Achieving this target at the Slaker was optimal for both white liquor strength and pressure filter operation. Liquor samples from the Slaker were tested using an auto titrator, the CE results are inputted into the DCS by the field operator, using the CE value the Slaker screw speed was adjusted by the DCS either up or down based on distance from the target CE. A testing schedule was implemented to ensure testing frequency to maintain CE set point.
The direct results of the control implementation were a reduction in over liming events, reduced dead load in the causticizing train, and consistently strong white liquor. Indirect results observed were longer intervals between acid washing, interval times doubled on average, sock life is projected to be increased by six months. With stronger reliable white liquor strength, liquor demand to the digester was decreased. Cascading effects included, full white liquor storage, stabilized digester operation, reduced throughput in the Recaust plant, improved clarity of green liquor, reduced load on the lime kiln, improved carbonate and reduced dead load in the Causticizer train.