2.16.1 Black liquor

Black liquor, a mixture of organic wastes, is made from a wood as a by-product during the production of chemical pulp. And it’s routinely burned as a liquid fuel in a recovery boiler in a pulp mill. Approximately 1.5 tones of black liquor are produced in the manufacture of 1 tone of pulp. Generally, a calorimetric heat value of black liquor is 12.6 MJ/kg and its energy is mainly used for pulp and paper making process.

2.16.2 Process of black liquor generation

There are a number of different processes which can be used for separating of the wood fibers. And their produced pulp is mainly divided into 2 types. One is mechanical pulp which is manufactured by a grinding process. The other is chemical pulp which is manufactured by a cooking process where the cellulose fibers are cooked out from the wood. Black liquor is produced during the cooking process. In this chapter, details of black liquor generation are described in terms of the kraft process so that the kraft process is nowadays the main stream for pulping (Fig. 2.16.1).

Wood, the raw material of paper, is composed by wood fibers which consist of cellulose, hemicellulose, and lignin which is an integral part between the fibers. The ratio of cellulose, hemicellulose and lignin is approximately 55: 20: 25.

The process to extract lignin from a wood chip by means of decomposition and dissolubilization is called cooking process. In the kraft process, Na2S and NaOH are added as a chemical to cook and the mixture is heated to 140 — 170oC and it keeps several hours. Although it’s important not to damage to the cellulose and the hemicellulose fibers during the process, indeed, a part of the cellulose and most of the hemicellulose fibers are decomposed and dissolved. Generally, the half of the organic materials in the wood turns to pulp and the other turns to black liquor.

Paper Making Process

Fig. 2.16.1. The main stream for pulping.

Black liquor contains decomposed and dissolved lignin, hemicellulose, and Na and S. After the cooking process, the concentration of black liquor is usually 15 to 20%. Then the liquor is concentrated to approximately 70 to 75%. Finally, it’s burned in a boiler called recovery boiler. As for a hard wood chip, 1 tone of the pulp turns to 1.5 tons of black liquor. Generally, a calorimetric heat value of black liquor is 12.6 mJ/kg.

During the process, melted Na and S which were contained in the liquor are extracted and then dissolved into hot water. After that, they’re reproduced to Na2S and NaOH in the causticizing process, and then used in the cooking process again. On the other hand, separated wood fibers turn to pulp through the washing process and the bleaching process. Finally, they’re used as a raw material of paper.