Method for delignification of cellulose with oxygen

Claim 2

Method of delignifying chemical pulp by means of oxygen, which comprises: forming a chemical pulp aqueous slurry, mixing the thus-formed slurry with an aqueous caustic agent and preheating the resulting slurry to about 70° C., further heating the thus preheated slurry by indirect het supply to about 140° C., contacting and mixing the resulting heated slurry in a first phase at a first concentration of about 2.5 to 4.5 percent of suspended solids and at a temperature of about 80° to 150° C. with oxygen, to effect oxygenation and thereby delignification of the pulp, draining off water from the resulting heated slurry of said first phase without reduction of pressure and while maintaining said temperature to provide a second phase wherein the slurry has a second concentration considerably exceeding said first concentration and amounting to about 10 to 30 percent of suspended solids, recycling part of said drained off water to said mixing and preheating stage, maintaining the resulting slurry under the above temperature, and second concentration conditions and without reduction of the pressure for at least about 20 minutes, and washing the thus-obtained, treated slurry.

Claim 3

The method of claim 1, and further comprising mixing the slurry with ozone after the washing in at least one subsequent stage in an acidified state and at a concentration below 4 percent of solid substance and at a temperature significantly lower than said temperature during the preceding contact with the oxygen.

Claim 4

The method of claim 2, wherein the ozone-containing bleach is directed into counter-current contact with the slurry during the ozone mixing with the slurry in excess and at low temperature, and waste gas thereof still containing O 2 /O 3 is then directed into the next higher temperature stage and into contact with the slurry during the oxygen contact thereof.

Claim 5

The method of claim 2, and further comprising feeding oxygen-containing residual gaseous medium derived from the at least one subsequent stage to said oxygen-contacting step.

Claim 6

The method of claim 1, and further comprising washing the ozone-treated slurry after the mixing thereof with the ozone, and bleaching the thus-washed slurry with peroxide.

Claim 7

The method of claim 2, and further comprising alkaline extraction of the slurry following said subsequent stage.

Claim 8

The method of claim 2 wherein said significantly lower temperature is about 30° C.

Claim 9

The method of claim 1, and further comprising mixing the slurry after the washing in at least two subsequent stages with ozone, and treating the slurry between the subsequent stages, with peroxide.

Claim 10

The method of claim 8, wherein the mixing is performed with fresh ozone-containing medium during the later one of said subsequent stages and with exhaust medium derived from said later stage during the earlier of said subsequent stges.

Claim 11

The method of claim 1, wherein the water is drained off from the slurry after the first phase to provide a concentration of about 12 to 15% of suspended solids.

Claim 12

The method of claim 1, wherein maximum oxygenation temperature is about 100° C.

Claim 13

The method of claim 1, wherein the resulting preheated slurry is indirectly heated by saturated steam.

Claim 14

The method of claim 1, comprising the additional steps of after said washing step, mixing the slurry with ozone, washing the ozone-treated slurry after the mixing thereof with ozone, and then bleaching the thus-washed slurry with peroxide, and then bleaching the slurry with ozone a second time.

Claim 15

The method of claim 13, comprising the additional step of bleaching the slurry with peroxide a second time after the second ozone-bleaching step.

Claim 16

The method of claim 14, comprising the additional steps of carrying out an alkaline extraction of said slurry between said first ozone-bleaching and peroxide bleaching steps, and again between said second ozone-bleaching and peroxide bleaching steps.

Claim 17

The method of claim 1, comprising the additional steps of feeding the heated slurry to an outer annular zone of a reaction vessel where the slurry is mixed and contacted by the oxygen, then conveying the slurry to a dewatering screw wherein the water is drained off and into an outer central chamber of the vessel wherein the slurry is maintained for at least 20 minutes.

Claim 18

Method of delignifying chemical pulp by means of oxygen, which comprises: forming a chemical pulp aqueous slurry, mixing the thus-formed slurry with a caustic agent, preheating the resulting slurry, contacting the resulting preheated slurry in a first phase at a first concentration of about 2.5 to 4.5 percent of suspended solids and at a temperature of about 80° to 150° C. and mixing the same with oxygen, to effect oxygenation and thereby delignification of the pulp, draining off water from the slurry of said first phase without reduction of pressure and while maintaining said temperature, to provide a second phase wherein the slurry has a second concentration considerably exceeding said first concentration and amounting to about 10 to 30 percent of suspended solids, maintaining the resulting slurry under the above temperature and second concentration conditions and without reduction of pressure for at least about 20 minutes, and washing the thus-obtained, treated slurry, wherein the pulp is pre-heated by the steps of first slightly heating the pulp to about 50° C. by mixing with water of about 70° C. prior to the mixing with the caustic agent, then heating the pulp to about 70° C. upon mixing with the caustic agent by introducing water at about 80° C., then preheating the resulting slurry indirectly by saturated steam at about 140° C. in a first stage, draining off water from the resulting pre-heated slurry, further heating the resulting slurry with hot water at about 140° C. in a second stage, and recycling part of the thus-further heated slurry back to said second stage, to thoroughly mix the pulp in the slurry.

Claim 19

Method of delignifying chemical pulp by means of oxygen, which comprises: forming a chemical pulp aqueous slurry, mixing the thus-formed slurry with a caustic agent, preheating the resulting slurry, contacting the resulting preheated slurry in a first phase at a first concentration of about 2.5 to 4.5 percent of suspended solids and at a temperature of about 80° to 150° C. and mixing the same with oxygen, to effect oxygenation and thereby delignification of the pulp, draining off water from the slurry of said first phase without reduction of pressure and while maintaining said temperature, to provide a second phase wherein the slurry has a second concentration considerably exceeding said first concentration and amounting to about 10 to 30 percent of suspended solids, maintaining the resulting slurry under the above temperature and second concentration conditions and without reduction of pressure for at least about 20 minutes, and washing the thus-obtained, treated slurry, wherein the pulp is pre-heated by the steps of first slightly heating the pulp to about 50° C. by mixing with water of about 70° C. prior to the mixing with the caustic agent, then heating the pulp to about 70° C. upon mixing with the caustic agent by introducing water at about 80° C., then preheating the resulting slurry indirectly by saturated steam at about 140° C. in a first stage, draining off water from the resulting pre-heated slurry, further heating the resulting slurry with hot water at about 140° C. in a second stage, and recycling part of the thus-further heated slurry back to said second stage, to thoroughly mix the pulp in the slurry.