Water Distribution
Variation in the liquid content between the surface and the center of a compressed block of cellulose fibres affects the accuracy of quality testing. The pulp bale moisture gradient arises during the drying and pressing stages of production where the outer layers dry faster than the core. It creates a challenge for determining the air-dry weight of the shipment for commercial transactions.
Standardized sampling methods are used to account for this non-uniformity.
Sampling Method
Probes are inserted at specific depths and locations to obtain a representative average of the entire lot. Measuring the pulp bale moisture gradient requires drilling cores that penetrate at least ten centimeters into the bale. If the gradient is steep, surface-only measurements will significantly underestimate the actual water content.
This leads to disputes over the invoiced tonnage and the subsequent chemical dosing in the pulper. The use of microwave or infrared sensors can provide a more complete picture of the internal distribution without destroying the bale. Knowing the slope of the gradient allows the mill to adjust the dwell time in the storage area to allow for natural moisture equalization.
Understanding the results prevents overpayment for water weight.
Storage Effect
Environmental conditions in the warehouse can further alter the moisture levels on the edges of the stack. The pulp bale moisture gradient changes as the material absorbs or releases water to reach equilibrium with the ambient air. Bales stored in high humidity develop a wet skin.
This complicates the repulping process. Regular monitoring helps in managing the inventory and ensuring consistent input for the mill.
