This study investigates the feasibility of acid precipitation for the separation and recovery of lignin and hydrocarbon derivatives from the waste stream of the alkali extraction of recycled cardboard. The recovery of chemicals from this effluent is complex due to their heterogeneity. Apart from the lignin and hydrocarbon derivatives, also, fillers, inks, and polysaccharides are present. A procedure of fractional acid precipitation was designed, reducing the pH from 10.7 to 8.8, 7.2, 5.1, 3.1, and 1.5. Each solid fraction was analyzed by infrared spectroscopy to determine its chemical composition and amount of precipitate. A mixture of lignin and hydrocarbon derivatives was detected in the solids obtained at pH 3.1. A dichloromethane extraction of the hydrocarbon derivatives was performed. Both recovered lignin and hydrocarbon were analyzed by infrared and gel permeation chromatography. The lignin showed similar characteristics (e.g., molecular weight ≈ 10.000 Da) as commercial lignin from virgin softwood. For the hydrocarbon derivatives, a molecular weight range between 10,000 and 150 Da was observed. In conclusion, this paper shows that the recovery of lignin and hydrocarbon derivatives is scientifically and technologically possible from the waste stream of the cardboard recycling processes. Further research is necessary to improve the extraction efficiency that was around 7.9% for the lignin and around 5.5% of the total amount of extractives compared with their amount in cardboard. Despite the low extraction efficiency, the estimated 9.6 million tons of lignin present in cardboard and the lignin price around 300–500 USD/ton make this research of industrial interest.

Graphical abstract

这项研究调查了酸沉淀法从回收纸板的碱提取废水中分离和回收木质素和烃类衍生物的可行性。由于废水的异质性，从废水中回收化学品很复杂。除了木质素和烃衍生物之外，还存在填料，油墨和多糖。设计了分步酸沉淀的程序，将pH从10.7降至8.8、7.2、5.1、3.1和1.5。通过红外光谱法分析每种固体部分，以确定其化学组成和沉淀量。在pH为3.1的固体中检测到木质素和烃衍生物的混合物。进行烃衍生物的二氯甲烷萃取。回收的木质素和烃均通过红外和凝胶渗透色谱法进行分析。木质素表现出与来自原始软木的商业木质素相似的特性（例如，分子量≈10.000 Da）。对于烃衍生物，观察到分子量在10,000和150Da之间。总之，本文表明，从纸板回收过程的废料流中回收科学和技术上的木质素和碳氢化合物衍生物是可能的。有必要进一步研究来提高木质素的提取效率，木质素约为7.9％，占纸板提取物总量的5.5％。尽管提取效率较低，但估计仍为9。

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