Determination of Sulfate Ions in Kraft Black Liquors by a Turbidity Method

摘要

This study demonstrates a new method for determination of sulfate ions in kraft black liquors based on a spectral turbidimetry using barium chloride (BaCl_2) as a precipitant. By sample acidification pretreatment and filtration, the major interfering species of alkaline dissolved lignin and carbonate ions are removed from the solution. Controlling acidity in the barium chloride medium can avoid interferences by minor inorganic species in black liquor such as sulfite and oxalate anions which can also form barium salt precipitates. In this work, we use a medium containing 5% barium chloride and 1 mol/L of hydrochloric acid, in which the sulfate ion can be quantitatively precipitated while the concentrations of the interference anions are kept below their barium salt solubility. The turbidity of the resulting solution is determined by UV/vis spectroscopy at a wavelength of～500 nm using a 10 mm optical path-length cell. The measured absorbance is related to sulfate content in black liquor with reference to a linear calibration curve. This method offers speed and cost advantages over established sulfate test methods for black liquor and can be used for near-real time tracking of sulfate levels in black liquor, an important parameter for managing soluble scale fouling in evaporators and concentrators.