This paper presents the results from a statistical study of a continuous zinc-solution purification procedure. The purpose of the work was to compare varying process conditions and to find out the correlation between the process parameters and the filter pumping pressures in different circumstances. In the first case, a filtration problem is slowly generated and optimal conditions are not found. The second, longer time period consists of clear changes and possibly different process operating ranges. The third part consists of a time period with small pressure changes and conditions close to optimal. Secondly, variations in the direction and strength of the affecting parameters are studied. For this, correlation values between pressure and certain chosen variables are evaluated. According to the results, the correlation of pressure with process parameters is greatly dependent on the process conditions. In addition, significant variables differ between sets of data and it is easy to find parameters apparently affecting the process. (c) 2005 Elsevier Ltd. All rights reserved.