Further evaporation and final treatment of process-water concentrates

As part of a project to develop a separate treatment process for evaporation concentrates of paper mill process waters, 2 key processing steps, the further evaporation and final treatment of the concentrates, were studied. Evaporation experiments were conducted with thermomechanical pulp (TMP) filtrates, a chemothermomechanical (CTMP) filtrate and a debarking effluent. The condensates recovered generally contained relatively small amounts of organic matter. A large fraction of low-molecular-weight alcohols contained in the feed water was vaporized in the early stages of evaporation. The percentage of alcohols vaporized was dependent on the pH of the feed water. Two TMP concentrates were further concentrated in the forced-circulation unit of the pilot evaporator. Viscosity was the principal limiting factor for concentration. Fouling of heat transfer surfaces was observed for both concentrates.