Non-Process Elements in the Paper Industry, 2005 Engineering, Pulping & Environmental Conference

Efforts to comply with effluent standards have lead to a situation where mills have little leeway in managing trace metals without developing mineral scale deposits. In most cases, the trace metals can be managed with minor process changes and suitable levels of process control. The biggest tools available to the mill are pH and good washing in the first chlorine dioxide stage washer. In some cases, mills may need to change shower sources on washers or upgrade the D0 washer. By determining the chemical equilibrium for critical points in the process and in some cases, modeling the chemical equilibrium–mass balance, it is possible to identify operating conditions that will eliminate mineral scale buildup in nearly all mill situations. This paper will provide information on how trace metals enter the mill, how pulp ion exchange influences where the metals go in the bleach plant, and how a fundamental chemical analysis of the effect of pH on anion speciation and solubility product on scale formation can be used to troubleshoot and prevent scale problems.