METHOD OF CONTROLLING THE CHEMICAL BALANCE OF A PULP MILL

Abstract

A method to optimize the chemical balance at a sulfate pulp mill, which produces at least pulp bleached with chlorine dioxide and has a chlorine dioxide plant using at least chlorate, methanol and sulfuric acid for making chlorine dioxide. The method includes: a) gases from the mill's concentrated non-condensable gas system are incinerated in order to form a gas containing sulfur dioxide, which is treated to produce concentrated sulfuric acid, and b) raw methanol from the mill processes is purified to produce methanol, and c) side streams containing sodium compounds and/or sulfur compounds produced by mill processes are used as make-up chemicals, wherein the production of chlorine dioxide uses the sulfuric acid produced in step a) and methanol purified in step b), with a sulfuric acid concentration of 94-99%, preferably 95-98%, and using in step c) sesquisulfate or sodium sulfate produced during the production of chlorine dioxide.

Claims

1. A method for controlling a chemical balance at a sulfate pulp mill, wherein the sulfate pulp mill produces at least pulp bleached with chlorine dioxide and the sulfate pulp mill includes a chlorine dioxide plant using at least chlorate, methanol and sulfuric acid to produce chlorine dioxide, the method comprising: incinerating a concentrated non-condensable gas produced in the sulfate pulp mill to form a gas containing sulfur dioxide, treating the gas containing sulfur dioxide to produce a concentrated sulfuric acid having a sulfuric acid concentration of 94% to 99%; purifying raw methanol from the sulfate pulp mill to produce a purified methanol, and using side streams containing sodium compounds and/or sulfur compounds produced by the sulfate pulp mill as make-up chemicals in the sulfate pulp mill, wherein production of chlorine dioxide in the sulfate pulp mill uses the concentrated sulfuric acid and the purified methanol; and wherein sesquisulfate or sodium sulfate produced during the production of chlorine dioxide is used.

2. The method according to claim 1, wherein the concentrated sulfuric acid is also used in at least one of the following processes: tall oil production, lignin recovery from black liquor, pulp bleaching, pH adjustment in the mill's chemical recovery process, and microcrystalline cellulose production.

3. The method according to claim 1, wherein the side streams include at least one of: ash from a recovery boiler, sesquisulfate or sodium sulfate from chlorine dioxide production, waste acid from the chlorine dioxide plant, and alkaline filtrates from a pulp bleaching plant.

4. The method according to claim 1, wherein the concentrated sulfuric acid and the purified methanol constitute at least 70% of all chemicals used in the chlorine dioxide production.

5. The method according to any claim 1, wherein the purification of the raw methanol uses a nitrogen-free fuel to reduce nitrous oxide emissions.

6. The method according to claim 1, wherein the gas containing sulfur dioxide is treated in the presence of oxygen and a catalyst in order to oxidize sulfur dioxide in the gas containing sulfur dioxide into sulfur trioxide, which is condensed into water vapor contained by the gas containing sulfur dioxide to produce concentrated sulfuric acid.

7. A method performed in a sulfate pulp mill comprising: incinerating a concentrated non-condensable gas produced in the sulfate pulp mill to form a sulfur dioxide containing gas; treating the sulfur dioxide gas to produce concentrated sulfuric acid having a concentration of 94 to 99 percent sulfuric acid, purifying raw methanol produced in the sulfate pump mill to produce purified methanol, and producing chlorine dioxide using the concentrated sulfuric acid and the purified methanol.

8. The method of claim 7, wherein sesquisulfate or sodium sulfate is produced during the production of the chlorine dioxide, and the method further comprises using the sesquisulfate or sodium sulfate in the sulfate pulp mill.

9. The method of claim 7, wherein the concentrated sulfuric acid is used in the purification of the raw methanol.

10. The method of claim 7, wherein concentrated sulfuric acid is used in the purification of the raw methanol.

11. The method of claim 7, where sulfuric acid is extracted from the purification of the raw methanol and used to form at least part of the concentrated non-condensable gas.

12. The method of claim 7, further comprising treating the sulfur dioxide gas presence of oxygen and a catalyst to oxidize sulfur dioxide in the gas into sulfur trioxide, and condense water vapor from the sulfur trioxide to produce the concentrated sulfuric acid.

Description

[0027] The new method is explained in more detail with reference to the FIGURE provided, which diagrammatically presents one embodiment for its implementation.

[0028] A sulfuric acid plant 14 produces sulfuric acid from the mill's concentrated non-condensable gases (CNCG) 2, which are oxidised into sulfur dioxide in an incinerator. The incineration requires support fuel 1, which may be methanol, as well as oxygen. The energy produced by the incinerator is recovered as steam 7, typically as intermediate-pressure steam (10-45 bar). After the incineration, the gas is treated further in the presence of oxygen and a catalyst (such as V.sub.2O.sub.5) to oxidize the sulfur dioxide into sulfur trioxide. The sulfur trioxide gas is condensed into the water vapour contained by the gas in order to form sulfuric acid. The sulfuric acid is cooled to storage temperature, and the excess water vapour is conveyed to flue gas treatment. The final product of the process is concentrated commercial-grade sulfuric acid, because the liquefaction of the acid is based 100% on condensation. The concentration of sulfuric acid is 94-99%, preferably 95-98%.

[0029] The sulfuric acid produced can be used according to the new method in chlorine dioxide production at a plant 15, to which the sulfuric acid is conveyed through line 18.

[0030] Raw methanol produced at the pulp mill is purified in a process that can produce the clean methanol required by the chlorine dioxide plant. The process comprises the following stages at a plant 16. Raw methanol 4, which contains sulfur compounds and ammonium compounds, is oxidised with sulfuric acid 5, which forms precipitate containing ammonium sulfate 13. This can be utilised at a biological wastewater treatment plant as a nutrient source for microbes. The sulfuric acid is preferably produced at the mill as described above and brought from the sulfuric acid plant 14 through line 17. During the acidification, turpentine 12 can be extracted from the raw methanol through decantation. After the acidification and precipitate removal, the methanol is distilled, and the distilled methanol is diluted with water 6. Turpentine can be discharged from the diluted methanol before the methanol is washed with a non-polar organic solvent. Methanol, ethanol and acetone can still be discharged from the washed methanol through distillation. This enables the production of the clean methanol 11 required for chlorine dioxide production by in-mill sources.

[0031] Sulfuric gases separated during methanol purification are conveyed to sulfuric acid production through line 19.

[0032] The sulfuric acid and methanol produced in-mill are used at the chlorine dioxide plant 15, to which the sulfuric acid is conveyed through line 18 and methanol through line 20. Chlorine dioxide 9 is produced in a manner known per se, using e.g. processes R8 or R10, from sodium chlorate 3. The chlorine dioxide plant produces by-products that contain sulfur and sodium 10. The R8 process also generates an acid salt, sodium sesquisulfate 10, and the R10 process generates sodium sulfate 10, which can be brought into the chemical recovery cycle as make-up chemicals.

[0033] Sulfuric acid 8 can also be utilised, for example, at the mill's possible tall oil plant, for lignin recovery from black liquor, for pulp bleaching. Purified methanol 11 can also be used as support fuel 1 for the sulfuric acid plant and as fuel in the recovery boiler and/or lime kiln.