Continuous process for production of cellulose pulp from grass-like feedstock

A continuous process, for production of cellulose pulp from a grass-like feedstock, that comprises an electrolytic process for continuous electrolytic separation of lignin and non-cellulosic side-products, with simultaneous production of white liquor and oxygen <O2> and chlorine <Cl2>, which is carried out in an electrolytic cell system which includes:

• one or more electrolytic pre-cells, made from a first material inert to chemistry in which first cathode and anode electrodes are immersed, with no membrane between respective first cathode and anode compartments;
• one or more electrolytic cells, made from a second material inert to chemistry in which second cathode and anode electrodes are immersed, where a second cathode compartment is completely separated from a second anode compartment with a porous membrane, which enables electrical contact of an anode and a cathode respectively associated with the second anode and cathode electrodes via ion-exchange, but prevents passing of suspended organic molecules in electrolyte;
• where the electrolyte is, after processing in the one or more electrolytic pre-cells, transferred into the second anode compartment of the one or more electrolytic cells, and a composition of the electrolyte is modified by addition of a fresh sodium chloride (NaCl) solution into the one or more electrolytic pre-cells;
• the continuous process comprises the following steps:
• A) preparing a suspension of comminuted and dedusted grass-like feedstock by addition of a white liquor from the second cathode compartment of the one or more electrolytic cells into a preliminary digester where the grass-like feedstock is subjected to cooking at 80-100° C. in a solution of the white liquor including the following composition:
• (ii) 0.50-2.00 w/w NaOH;
• (ii) NaCl;
• by keeping a concentration of the grass-like feedstock at a level of 5-15% w/w;
• B) performing quasi-continuous cooking and separation through two or more parallel cooking and separation processing lines each including one or more primary digesters, one or more cooking mills, one or more cooking water separators, and one or more secondary digesters, during 3-6 hours, wherein the grass-like feedstock prepared in step A is sequentially pumped into the two or more parallel cooking and separation processing lines, wherein:
• suspension of the grass-like feedstock in the white liquor during cooking in all of the one or more primary digesters and the one or more secondary digesters is maintained at temperature from 95-100° C.;
• for each of the two or more parallel cooking and separation processing lines, a brown cellulose suspension at a level 8-12% w/w dry matter at an output from the one or more primary digesters is subjected to milling in the corresponding one or more cooking mills and separation in the one or more cooking water separators, from which a part of black liquor is transferred into the one or more electrolytic pre-cells, while a thus obtained concentrated suspension of brown cellulose with about 30% w/w dry matter enters into the one or more secondary digesters, with introduction of a white liquor solution from the second cathode compartment of the one or more electrolytic cells;
• C) transferring a quasi-continuous combined output from all of the secondary digesters with the cooked brown cellulose suspension into a first mixing vessel and a first mill, where separation of milled brown cellulose is performed in a first water separator which concentrates the brown cellulose suspension with 5-15% w/w dry matter up to a content of 27-33% dry matter, and accompanied with separation of the black liquor which is transferred to the one or more electrolytic pre-cells;
• D) preparing a bleaching suspension in a bleaching reactor, in which, besides the suspension from the step C:
• regenerated fresh white liquor from the second cathode compartment of the electrolytic cells;
• mixture of oxygen <02> and chlorine <C12> enveloped in the second anode compartment of the one or more electrolytic cells;
• and
• hydrogen peroxide <H2O2> or sodium peroxide <Na2O2>, are introduced, yielding the bleaching suspension with a content of 5-15% w/w,
• E) performing a quasi-continuous bleaching process through two or more parallel bleaching processing lines each including one or more primary reactors, one or more bleaching mills, one or more bleaching water separators, and one or more secondary reactors during 3-6 hours, at temperature from 70-100° C., where feedstock from the step D is prepared and sequentially pumped into said two or more parallel bleaching processing lines, where a primary bleaching process is performed in the one or more primary reactors and where, at the output from the one or more primary reactors, a suspension of bleached cellulose with 8-12% w/w dry matter is obtained, which is subjected to milling in the corresponding one or more bleaching mills and separation in the one or more bleaching water separators from which, a part of waste-solution is transferred into the one or more electrolytic pre-cells, while a concentrated bleached cellulose suspension with about 30% w/w dry matter is entering into the one or more secondary reactors with introduction of:
• the white liquor solution from the second cathode compartment of the one or more electrolytic cells,
• the mixture of oxygen <O2> and chlorine <Cl2> enveloped in the second anode compartment of the one or more electrolytic cells;
• and
• the hydrogen peroxide <H2O2> or the sodium peroxide <Na2O2>, yielding the cellulose suspension for bleaching with 8-12% w/w;
• and
• F) transferring a quasi-continuous combined output from all of the one or more secondary bleaching reactors from the step E into a second mixing vessel and a second water separator, whose waste water is returned back to the one or more electrolytic pre-cells, while a drained cellulose comes out in a form of a pure white cellulose pulp, at a concentration from 48-55% W/W dry matter, with maximally 5% w/w lignin, calculated on the dry matter.