Hardwood Lignin Recovery Using Generator Waste Acid. Statistical Analysis and Simulation

Abstract

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 23 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 °C because the lignin precipitate starts to form a tarlike substance at approximately 80 °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.

Lignin recovery from spent alkaline pulping liquors using acidification, membrane separation, and related processing steps: A review

“…Apparently the higher temperature favored an adherent nature of the particles, leading to formation of bulky, permeable filter beds. Weinhaus et al (1990) and Howell and Thring (2000) found better filtration if acidification was done at a higher temperature. Garg et al (2010) observed a similar effect in the case of lignin that had been precipitated with aluminum-based coagulants or ferrous salts, then filtered at either 25 or 95 ºC.…”

“…Studies have shown that CO2 can be effective for reducing the pH of black liquor to values in the approximate range of 7 to 10.5 (Tomlinson and Tomlinson 1946;Alén et al 1979Alén et al , 1985bWeinhaus et al 1990;Howell and Thring 2000;Theliander 2001, 2007;Wallmo et al 2007Wallmo et al , 2009aNagy et al 2010;Theliander 2011, 2015; Velez and Thies 2013; Lake and Blackburn 2016; Kouisni et al 2016;Kumar et al 2016). Stocklosa et al (2013) found that although CO2 readily reduced the pH to 9.5, the majority of softwood kraft lignin already became precipitated when the pH was lowered to a level between 10 and 11.1, which suggests that their specimen was mainly stabilized by phenolate functional groups having a pKa of about 10 or possibly higher ( Table 2).…”

“…The kraft black liquor was mainly from pine with about 8% birch content. Howell and Thring (2000) reported that generator waste acid, a byproduct from chlorine dioxide generator of kraft mill, can be used as a free source of sulfuric acid.…”

“…Presumably, such gentle agitation encourages collisions among droplets or particles of lignin, helping them to coagulate, while still allowing settling to occur. Howell and Thring (2000) recommended that agitation be "as low as possible" to allow settling for the lignin after black liquor treatment with waste acid from a chlorine dioxide generation process. Amriani et al (2015) call for "slow stirring" at 50 revolutions per minute following a treatment with PAC.…”

“…After acidification of the black liquor, the means of promoting separation by density include gravitational sedimentation and centrifugation. Probably for reasons of simplicity, sedimentation approaches have been most often adopted in acid precipitation processes for lignin recovery (Tomlinson and Tomlinson 1946;Howell and Thring 2000;Mandavgane and Subramanian 2006;Norgren and Mackin 2009;Garg et al 2010;Lake and Blackburn 2014;Leskinen et al 2017). Centrifugation, as a means of achieving faster separation of lignin from black liquor, was demonstrated by Alén et al (1985b), Mussatto et al (2007), Liu and Luo (2010), and Namane et al (2015).…”