Beneficiation of pulp and paper mill sludge: production of cellulose nanofibrils (CNFs).

Abstract

Depletion of landfill space, the cost of waste disposal, and environmental concerns are pushing Pulp and Paper Mills (PPMs) to make changes and improvements in the way waste is handled. Pulp and paper mill sludge (PPMS) is a solid by-product of the wastewater treatment of the PPM industry; thus, it is considered a waste. This study was conducted to investigate the feasibility of producing cellulose nanofibrils (CNFs) from waste material such as PPMS instead of using high purity cellulose products such as chemical pulps as the starting material. Three PPMS samples collected from different South African (SA) mills were chemically and physically characterised to investigate their suitability for various beneficiation pathways. The overall objective was to analyse and allocate the most suitable beneficiation process to each PPMS sample based on the characterisation results and literature. The possible beneficiation pathways of the pulp and paper mill sludge (PPMS) samples were discussed in line with their characteristics. The characteristics of PPMS were influenced by the pulping technique employed at each mill, the raw material and the type of effluent treatment employed. Thermal analysis revealed that the calorific values of all PPMS samples studied were too low for energy harvesting (thermal processing). The high ash content of PPMS C (de-inking) and PPMS A (recycle) was suitable for biocomposites whose strength could be enhanced by fillers present in PPMS samples. The higher glucose content in PPMS B (virgin) compared to PPMS A (recycle) and PPMS C was favourable for bioethanol and bio-oil production. The high cellulose and low ash content of PPMS B were found suitable for the production of nanocellulose. Highly fibrillated CNFs were produced from three different PPMS samples by grinding using the automated Super mass colloider (SMC) after washing and bleaching to remove impurities. Chemical composition analysis showed a considerable reduction in ash content due to screen washing, successful cellulose concentration, and lignin removal by bleaching. The crystallinity index (CrI) was calculated to be 51.1%, 58.1% and 59.4% for CNF A, CNF B and CNF C, respectively. TGA analysis showed a progressive decrease in thermal stability from untreated PPMS to CNFs.The overall yield for the production of CNFs from PPMS A, PPMS B and PPMS C was 27.2%, 32% and 42.8%, respectively. Finally, a description of the transition from manual grinding using the SMC to an automated operation was done. The automated system was designed to pump the feedstock through the SMC via sample holding tanks for a predetermined number of passes. The automated operation alleviated challenges associated with the manual operation, which included high labour demands, loss of material due to spillages, thus affecting yield and posing a safety risk in the working environment, inconsistent product due to human error in counting the number of cycles, low productivity due to long working hours. The introduction of the automated SMC system was a worthwhile investment justifiable by improving efficiency and operator safety.