Steam treatment of chips with the addition of an acid liquid
View Patent ↗The method is for the pre-treatment of chips that are fed to a sulphate cooking process. Through the addition of an acidic fluid to the steam treatment step, a very high ionic concentration of hydrogen ions is achieved. When the chips are subsequently formed into a slurry with a preferably sulphide-rich alkali cooking fluid, H 2 S is formed locally in the chips, which gives a yield-increasing effect for the subsequent cooking step of l-3%.
1. A method for the treatment of chips, comprising:
heating the chips with steam in a steam treatment inside a steam treatment vessel,
adding an acidic fluid to the chips disposed in the steam treatment vessel while the chips are being steamed in the steam treatment vessel in an amount that gives the chips at least a five-fold increase in an ionic concentration of hydrogen ions at an end of the steam treatment compared to steam treatment without adding the acidic fluid,
in the steam treatment vessel, heating the chips together with the acidic fluid by the steam for a period of 1-20 minutes,
the chips falling down through a chute connected to the steam treatment vessel,
adding an alkali impregnation fluid to the chute while the chips fall therethrough,
forming the chips to a slurry in the chute with the alkali impregnation fluid,
wherein said alkali impregnation fluid comprises a liquor with sulfides,
conveying the slurry to a digester, and
cooking the chips in the digester.
2. The method according to claim 1 wherein the acidic fluid has a pH that does not exceed 4 and the acidic treatment fluid is at least partially added in a pressurized steam treatment vessel, at a pressure in the vessel that exceeds that of a surrounding atmosphere by at least 0.5 bar.
3. The method according to claim 1 wherein the acidic fluid has a pH that does not exceed 4 and the acidic treatment fluid is partially added at least in a vessel at atmospheric pressure or at a slightly higher pressure, at a pressure in the vessel that exceeds that of a surrounding atmosphere by between 0 and 0.5 bar.
4. The method according to claim 2 wherein a volume of the added acidic fluid relative to a volume of the chips does not exceed 2:1.
5. The method according to claim 1 wherein the alkali impregnation fluid in which the heated and acidified chips are formed into a slurry is constituted by a sulphide-rich liquor.
6. The method according to claim 5 wherein the alkali impregnation fluid is constituted by a mixture of at least one of a sulphide-rich white liquor, sulphide-rich black liquor, and/or sulphide-rich green liquor, and the alkali impregnation fluid has a molarity of HS − that exceeds 0.15 mol/liter.
7. The method according to claim 6 wherein the alkali impregnation fluid has a molarity of NaOH that does not exceed 0.75 mol/liter.
8. The method according to claim 1 wherein the heating by steam of the chips together with the added acidic fluid at a temperature in a range of 80-120° C.