A method to recover hardwood kraft lignin by acidification of black liquor using waste acid from a Mathieson chlorine dioxide generator is proposed. Optimum reaction conditions to maximize the lignin yield and minimize acidification costs were determined. To analyze the effects of the major variables, a 2(3) factorial model describing the effects of acidification temperature, degree of agitation, and rate of waste acid addition was developed. Increasing the acidification temperature improved the lignin precipitation and filterability. However, the maximum practical temperature was 70 degrees C because the lignin precipitate starts to form a tarlike substance at approximately 80 degrees C. Also, the rate of acid addition should be minimized. In practice, this will be determined by the mill reaction vessel size, which depends on the black liquor flow rate to be acidified. Last, the stirring rate should be kept as low as possible, although some agitation is still required to uniformly mix the acid and black liquor. A steady-state computer simulation of incorporating a proposed 21 ton/day hardwood lignin recovery plant to the kraft liquor cycle showed no adverse effects in the chemical balance of the mill.