A new chemical recovery process for high sulfidity spent liquor has been developed by Mitsubishi Heavy Industries, Ltd. and Jujo Paper Manufacturing Co., Ltd.
In this new recovery process, high sulfidity smelt can be obtained by absorbing SO₂ gas in the flue gas in a solution of sodium carbonate solution separated from smelt solution and buring the resulting sodium sulfite solution together with spent liquor.
The sulfidity of smelt can be easily controlled by suitably adjusting the amount of sodium carbonate recycled to the recovery system and the amount of SO₂ gas absorbed in the sodium carbonate solution.
This new recovery process can also be applied to the cross recovery for alkaline and sulfite spent liquors.

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In order to clarify how cooking time affects the color of lignosulfonate, six fractions of fir lignosulfonate were isolated from acid sulfite waste liquor withdrawn at regular intervals. In this connection, it should be emphasized that each fraction of lignosulfonate is isolated very soon after dissolution into the cooking liquor, in other words it is not much influenced by heating after dissolution. The color, the amounts of functional groups and the molecular weight distribution curve of each lignosulfonate were compared each other, and the changes of molecular weight distribution curve by recooking with acid sulfite cooking liquor were also discussed. The results are summarized as follows :
1. The amounts of methoxyl and non-conjugated phenolic hydroxyl groups show no significant difference among lignosulfonate, obtained at various cooking stages. However, the amount of α-carbonyl group in lignosulfonate at the earlier cooking stages is higher than that at the later cooking stage.
2. The chromophoric structures in lignosulfonate are not practically formed inthe solid state of protolignin, but formed after the dissolution from wood. The rate of coloration of lignosulfonate obtained at the earlier stage is more remarkable than that at the later stage.
3. The molecular weight distribution curve of lignosulfonate was obtained by gel filtration. At the earlier cooking stage, lignosulfonate of lower molecular weight fraction dissolves from wood more easily than that of higher molecular weight fraction. The peaks of these gel filtration curves which correspond to the fractions of higher molecular weight, are located at the same amount of elution volume. This means that the pore size of cell wall determine the rate of delignification.
4. The molecular weight distribution curve of lignosulfonate changes to the lower side of the molecular weight during recooking with acid sulfite cooking liquor, and this tendency is especially remarkable in the case of lignosulfonates obtained at the earlier cooking stage. This shows that the different accessibilities of lignosulfonates to the acid hydrolysis are present even among specimens which are obtained at various cooking stages and then fractionated at the same elution volume.

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Electron microscopic studies were carried out with regard to the effects of the filling, the internal sizing and the surface sizing on the surface fine structures of uncoated papers.
Results were summerized as follows :
(1) Direct carbon reprica technique was found to be available especially for observing filler distribution on uocoated paper surfaces.
(2) By observing commercial uncoated paper surfaces by the transmission and scanning electron microscopy, it was found that the wire side showed a greater filler loss than the top side.
(3) The two sideness of filler retention was found to depend on the methods of sheet formation.
In hand made sheet, the wire side had higher filler retention than the top side. On the paper made by the miniature fourdrinier paper machine, the two sideness of filler retention was almost eliminated. On the paper made by the commercial fourdrinier paper machine, the filler retention of the wire side was conspicuously lower than the top side.
(4) Two sideness of the filler retention of commercial papers was found to be increased as the filler content increased.
(5) In the surface sizing, surface of the fibers were found to be covered with an amorphous film of a sizing agent in proportion to the degree of sizing.

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