Abstract
Black liquor obtained in the bleaching with the cartazyme anzime - oxyen - hydrogen peroxide (XOP) sequence of soda pulp from wheat straw was characterized. A central composite design was used to examine the influence of operating variables of the O bleaching step (temperature, time and soda concentration) on various properties of the black liquor (pH, chemical oxygen demand, total carbon, total organic carbon, total inorganic carbon, total solids, total volatile solids, total mineral solids and colour). Equations that relate the dependent variables to the independent ones were derived that predict the former with errors less than 25%. The least polluting black liquor was obtained by using a medium temperature (100 °C), a medium soda concentration (1.65%) and a short time (15 min) in the O step of the XOP bleaching sequence. Under these conditions, the properties of pulp bleached with the XOP sequence (yield, brightness and viscosity), and those of the paper sheets obtained from them (breaking length, stretch, burst index, tear index and double fold number), were found to be only 6% smaller than the optimum values in most instances.
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