Deposit Formation in Evaporation of a Pulp Mill Effluent

Abstract

A preliminary study was undertaken of the fouling of clarified wastewater from a “zero”-liquid discharge bleached chemi-thermo-mechanical pulp mill. This work concerned the fouling process that occurs in mechanical vapor recompression evaporators, which comprise the first stage of the evaporation and concentration system. The clarified wastewater contains dissolved and suspended solids, both of which increase as the liquor is concentrated. Solids contain roughly 20% mineral matter and the remainder is organic material. Thermal fouling measurements were made in a countercurrent double-pipe exchanger in which liquors of three different concentrations were heated by condensing steam. The decline in heat transfer coefficient was tracked over periods of several days at fixed steam temperature and fluid velocity. Fouling increased with both liquor concentration and steam temperature. Soft, sludge-like deposits that contained mainly organic material were found. The inorganic fraction was about 25% and consisted primarily of calcium, magnesium, and silicon. The sources of the inorganic fraction in the deposits are discussed, and deposit analyses are compared with those of the solids present in the liquor.