In the production of chemical pulp from softwood by the alkaline, i.e. sulfate, process, wood extractives separate out on the surface of the black liquor in the form of sodium soap of fatty and rosin acids in the different steps of the chemical recovery cycle of the sulfate pulping process. Specifically in the Scandinavian countries hardwood, birch in particular, is often used along with softwood as raw material in pulping mills. While softwood contains both fatty and rosin acids, hardwood contains only fatty acids and additionally a greater amount of neutral substances than softwood.
When hardwood is used to prepare chemical pulp, soap or tall oil from softwood digestion is dosed into the cooking. This decreases the extractive content of the resulting chemical pulp, which is a significant pulp quality criterion. The hardwood extractives thus produce `mixed birch soap`, comprising sodium salts of fatty acids and neutral organic components, such as sterols. Since the soap is often derived from both softwood and hardwood, as stated above, the quality of the resultant tall oil in view of further refining is substantially impaired. Particularly the increased content of neutral substances and the lowered rosin acid content complicate distillation of the tall oil, lowering the yields and impairing the purity of the products and also increasing the formation of lower-value pitch.
The CSR (Crude Soap Refining) process has been proposed as a solution to the problem of distillation. In this method, the unsaponifiable neutral components are extracted with an organic solvent, i.e. hexane. Prior to the extraction, the soap must be demulsified with acetone. The process becomes more complex, since two components, acetone and hexane, are needed. Furthermore, it has been found that neutral components impairing the quality of fatty acid distillates still remain in the extracted soap. As such, the process is capable of lowering the proportion of neutral components and increasing the proportion of rosin acids.
The utility of the above extraction method is based on further refining of the neutral fraction produced. On the other hand, the resulting quality of the extracted birch oil is not considered satisfactory.
It is known that soap can be acidified into tall oil with sulphuric acid. The pH of the Na.sub.2 SO.sub.4 -containing mother liquid is about 3, at which the acidification is practically carried to completion. The tall oil obtained from hardwood cooking is characterized by a high fatty acid content and a low rosin acid content, which makes the further refining more difficult, as explained previously.