ABSTRACT J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012 6 Lignin can be precipitated and recovered from Kraft black liquor (BL) through the addition of an acidifying agent such as carbon dioxide, followed by fltration and washing. During this process, many researchers previously reported a high fltration resistance which affects lignin purity, cost, and productivity. To improve lignin flterability, FPInnovations developed the LignoForce System™, a patent-pending process in which the BL is oxidized under controlled conditions before the acidifcation step of conventional lignin recovery processes. Based on the data obtained, BL oxidation improves the flterability of the acid-precipitated lignin by providing suitable conditions with respect to pH (leads to a lower pH) and temperature (leads to a higher temperature) for lignin colloid agglomeration, leading to the formation of particles of a size and size distribution that are easy to flter and wash. In addition, it minimizes or eliminates total reduced sulphur (TRS) compounds, thereby leading to reduced TRS emissions during all steps associated with lignin precipitation, fltration, and washing. Furthermore, because the oxidation of TRS compounds and sugars consumes residual effective alkali (sodium hydroxide) in BL, the carbon dioxide and sulphuric acid requirements of the process are signifcantly reduced. An additional feature of the LignoForce process is the use of a flter press instead of a belt flter for lignin cake fltration and washing. No major difference in the chemical composition, molecular weight distribution, or main func- tional groups was found in the lignins produced using the LignoForce process compared with lignins produced by conventional methods. Based on the results obtained in laboratory studies, a lignin demonstration plant was designed and built at the Resolute – Thunder Bay mill for the production of 12.5 kg/h of high-quality lignin. LAMFEDDAL KOUISNI*, PETER HOLT-HINDLE, KIRSTEN MAKI, MICHAEL PALEOLOGOU THE LIGNOFORCE SYSTEM™: A NEW PROCESS FOR THE PRODUCTION OF HIGH-QUALITY LIGNIN FROM BLACK LIQUOR During the kraft pulping process, lignin is removed from wood chips and, follow- ing brownstock washing, ends up in the spent pulping liquor (weak black liquor). This liquor is typically concentrated from approximately 15%–20% to 70%–80% solids using an evaporator train and then fred into a recovery boiler for the produc- tion of steam, electricity, and inorganic chemicals (in the form of smelt) for inter- nal mill use. Over the last few years, North American pulp mills have been gradually increasing pulp production to the point that the calorifc load on their recovery boilers has, in most cases, reached or exceeded the design limits. Expanding recovery boiler capacity is a very expensive proposition. A convenient way to expand pulp produc- tion without having to invest millions of dollars in extra recovery boiler capacity is to offoad the recovery boiler with respect to calorifc load. This can easily be done by adding an acid to a portion of the black liquor fow, thereby precipitating out the INTRODUCTION lignin. Black liquor acidifcation is pre- dominantly performed by using either car- bon dioxide or a mineral acid (e.g., sulph- uric acid) or a combination of the two to drop the pH of the black liquor from ap- proximately 13–13.5 to 9–10. After acidi- fcation, the lignin is usually fltered using a belt flter or a flter press and washed with acid (e.g., sulphuric acid) and water to produce lignin of high purity (Fig. 1). In many cases, however, it is diffcult to separate the lignin from the acidifed black liquor solution. This problem appears to be related to the size of the lignin parti- cles before fltration. As in other types of colloidal dispersions, lignin precipitation from black liquor is assumed to occur in two steps: nucleation and particle growth. The relative rate of these two steps ulti- mately determines the size of the pre- cipitated lignin particles and consequently their fltration rate. In an effort to improve the fltration properties of acid-precipitated lignin slur- ries, a number of approaches have been considered, including fltration at high *Contact: Lamfeddal.Kouisni@fpinnovations.ca LAMFEDDAL KOUISNI FPInnovations Pulp, Paper & Bioproducts Division, Pointe-Claire, QC, Canada KIRSTEN MAKI FPInnovations Pulp, Paper & Bioproducts Division, Thunder Bay, ON, Canada MICHAEL PALEOLOGOU FPInnovations Pulp, Paper & Bioproducts Division, Pointe-Claire, QC, Canada PETER HOLT-HINDLE FPInnovations Pulp, Paper & Bioproducts Division, Thunder Bay, ON, Canada