Impact of degree of sulphonation of lignosulphonic acid on molecular weight distribution of spent liquours.

Abstract

Lignosulphonates are produced during the chemical pulping of wood species through acid sulphite or neutral sulphite semi-chemical processes. Lignosulphonates are applied in several commercial products, including but not limited to dispersants, resins, binders, concrete admixtures, water treatment aids (coagulants), etc. These complex macromolecular structures exhibit different chemical properties, depending on the pulping process and tree species used. How lignosulphonates are valorised will, therefore, depend on their inherent chemical properties. The molecular weight and the sulphonic acid content in particular are two properties that will affect how lignosulphonates are used. In this study, the molecular weight and sulphonic acid of magnesium lignosulphonate (MgLS) and sodium lignosulphonate (NaLS) were studied. Due to the processing contaminants that interfere with molecular weight and sulphonic acid measurements of spent liquors purification of the MgLS and NaLS streams was carried out using a combination of Dowex 50wx8 and XAD-7 resins. The Dowex resin was used for de-ashing, and the XAD-7 resin to adsorb the lignosulphonate molecule. Ultrafiltration was also included as a comparison for the Dowex-XAD-7 purification. Approximately; 5 % ash was retained on samples purified by ultrafiltration and less than 3% ash was retained by samples purified by Dowex-XAD-7. The sample yield after Dowex-XAD-7 and ultrafiltration was similar for MgLS samples, between 32 and 36% sample was extracted . There was, however, a difference in sample yield when NaLS liquor was purified. A 24 and 32 % sample recovery was achieved using Dowex-XAD-7 and ultrafiltration, respectively. The yield was higher for the ultrafiltrated product than for the XAD-7 product. Samples purified by ultrafiltration generally showed high molecular weight and sulphonic acid content. The molecular weight for NaLS and MgLS samples purified by ultrafiltration measured approximately 6 and 20 kDa, respectively. Where as NaLS and MgLS samples purified by Dowex-XAD-7 reported a molecular weight of 3 and 12 kDa, respectively. The sulphonic acid content ranged between 3 and 5% for ultrafiltrated samples where as the Dowex-XAD-7 extracted samples ranged between 2 and 3%. The samples purified by XAD-7 resins were then sequentially fractionated using ultrafiltration with molecular weight cut-off (MWCO) sizes between 100 kDa to 1kDa. The molecular weight of the fractionated products decreased with a decrease in MWCO size ranging between 100 and 1kDa. This decrease was from 17.6 to 0.6 kDa and 14.8 to 0.8 for MgLS and NaLS samples, respectively. For the MgLS, the sulphonic acid content increased from 0.93 to 1.25 mmol/g with a decrease in molecular weight, whereas the fractions from the NaLS feed seemed to decrease with a decrease in molecular weight.