FI84190B - FOERFARANDE FOER BEHANDLING AV PEROXIDBLEKAVLUTAR. - Google Patents

Description

1 84190

Method for treating peroxide bleaching liquors

The invention relates to a process for the treatment of waste liquors obtained in connection with the production and bleaching of high-yield and recovered paper pulps in the pulp industry. High yield pulps refer to pulps with a yield of more than 60%, e.g. various ground and hot milled pulps as well as semi-mechanical pulps. The method is also suitable for the treatment of waste liquors from textile bleaching.

Peroxide bleaching of chemical pulp usually does not use water glass at all. In contrast, peroxide bleaching of abrasive and semi-mechanical pulps uses significant amounts of water-soluble alkali metal silicates, e.g. water glass, as a peroxide decomposition stabilizer. The amount of water glass can rise up to 80 kg (38-54 ° Be) per tonne. Large amounts of water glass are also used in the bleaching of recovered paper and textiles.

After peroxide bleaching, which has taken place in the presence of a large amount of water glass, the pulp slurry is dewatered until it is 20-60% dry. The filtrate obtained is not usually recovered, but is removed either directly or through a purifier to some sort of collection vessel. If the filtrate is first passed to a purifier, it is usually separated from the impurities by a precipitate method using suitable condiments.

For example, if the plant uses 40 kg of water glass (Na2SiO 2) per tonne of pulp and produces 100,000 tonnes of pulp per year, the plant's annual water glass consumption is a total of 4,000 tonnes, which means high commercial value.

It is known in advance to remove silicic acid from siliceous raw materials such as bamboo and bagasse from soda or alkali chemical pulp by adding CO 2 -containing gas while lowering the pH of the liquor to less than 2,84190 in less than 30 minutes. Silica precipitates and is removed from the black liquor either by boiling (pressure heating) and filtration after boiling or by evaporation and sedimentation and centrifugation after evaporation. Another similar two-step process is also known, in which the black liquor is treated in the first step with CCl 3 gas until the desired pH is reached and at least 75% of the silicic acid has precipitated, after which the lye is treated in the second step with lime milk until the desired purity is reached and calcium silicate separated. .

In addition, at the end of bleaching, it is known to pass the liquor through a membrane filter before returning it to the process. This eliminates harmful ingredients.

The use of water glass as a stabilizer causes technical and economic disadvantages, such as harmful deposits in pipes and valves, which can lead to production interruptions. The addition of water glass still has the disadvantage that the added sodium hydroxide cannot be recovered by evaporating and burning the liquor, because then the water glass precipitates and becomes insoluble, which is also a disadvantage for environmental protection because the organic matter released in pulp production cannot be burned. These disadvantages are particularly evident in the production of high-yield pulps from bamboo and reed plants, e.g. straw and esparto.

Known silicic acid separation methods can only be used to produce chemical pulp from a raw material that does not contain wood by alkaline treatment. Moreover, the known methods have proved to be very uneconomical, consuming particular time, energy and equipment. Due to clogging, water-containing waste liquors from the cellulose industry cannot be filtered in a membrane filter to separate silicon compounds.

Thus, it would be desirable to find an industrially viable method for separating and possibly recovering soluble silicates from water glass from chemical process waste liquors.

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The present invention provides a solution to the above problem and relates to a method for treating waste liquors from peroxide bleaching of high yield and recovered paper pulps. The process is characterized in that the soluble silicates of the waste liquor from the added water glass are compounded by adding an inorganic compound containing a polyvalent cation and / or an organic flocculant while bringing the pH of the waste liquor close to 3.5-7.5, preferably 4.0- 6.0 that the resulting silicate precipitate is sedimented, that the resulting sediment is separated and recovered, and that the lye released from the silicate is returned to the process.

An aluminum compound, e.g. aluminum sulphate or alum, is particularly suitable for precipitating alkaline silicate. Iron and calcium compounds are also suitable, polyvalent cation-containing inorganic compounds which give highly sedimentable and easily separable precipitates. In particular, ferric chloride or ferric sulfate as well as calcium hydroxide or calcium oxide have proven to be suitable.

In certain cases, it has further proved advantageous to combine a cationic, inorganic precipitating chemical with an inorganic flux which is a surimolecular polyelectrolyte, preferably polyacrylamide. In some cases, the organic flocculant can also be used alone and achieve completely satisfactory flocculation and precipitation.

It is important that when using the above chemicals to precipitate silicate, the pH of the lye is adjusted to between 3.5 and 7.5, preferably 4-6, whenever necessary, i.e. if the pH of the lye is outside the given scales. Sulfuric acid, sodium hydroxide or soda are preferably used to adjust the pH.

The silicate precipitate formed after the addition of the precipitant and / or flocculant is introduced after it has sedimented into a suitable device for thickening. The device can be, for example, a centrifuge or a belt clamp with a double wire.

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The thickened precipitate is dried in a spray dryer or a vane dryer of the type described by the Dutch Goudsche Machine Fabriek B.V. manufactures and includes two, together, steam-heated screws with wedge-shaped wings. Drying has proved to be particularly advantageous when a dry matter content of at least 70% by weight is reached in the product obtained in each case. If desired, the dried precipitate can also be heated (roasted) to destroy any organic matter contained therein.

Further processing of the sedimented silicate precipitate has also proved to be particularly advantageous. Then some desirable properties can be obtained for the silicate precipitate. Its flame resistance properties can be improved by adding an inorganic boron or phosphorus compound, preferably borax or potassium dihydrogen phosphate. The resulting mixture is thickened and dried in one of the ways described above. Inorganic ammonium or halogen compounds can also be added to the precipitate, depending on how and for what purpose the product obtained is used.

It has proved particularly advantageous to mix the silicate precipitate treated with a boron or phosphorus compound into the cellulose pulp slurry and to remove water from the resulting mixture and dry it. This gives a flame-retardant cellulosic material suitable for special purposes.

Many advantages are achieved by treating waste liquors containing water glass according to the invention. The application of the invention makes it possible to recover combustible organic matter and chemicals and to evaporate and incinerate waste liquors without generating slag from the precipitated silicate compounds.

The invention also enables membrane filtration of the silicate-containing bleaching liquor in the peroxide bleaching step to separate the extractant from the bleaching liquor without the filter membranes relying on the silicate. It is also possible to return the purified bleaching liquor to a bleaching or other stage of the process. The invention further has the advantage that the removal of the extractant reduces the emissions of environmentally harmful substances.

The above advantages are, of course, also achieved when treating waste liquors from the production and bleaching of recovered paper pulps.

Figure 1 shows a flow diagram of a test plant according to the invention for treating waste liquor.

To illustrate the invention, the following are examples of the most preferred embodiments of the invention.

Example 1

A warm lye having a temperature of 60 ° C and having the following characteristics was taken from the bleaching stage of the peroxide bleaching plant:

Chemical Oxygen Demand (COD) mg C ^ / liter 3.050

Water glass content (Na2Si0.j) g / liter 0.6

Acidity pH 8.8 Temperature ° C 60

Three liters of samples A, B and C were taken from this bleaching liquor. To A was added 2.0 g of aluminum sulphate (A 2 SO 4) and to B

in addition to 2.0 g of aluminum sulfate, sulfuric acid was added until a pH of 4.5 was reached. Also to batch C was added 2.0 g of aluminum sulfate and an additional 0.25 g of potassium dihydrogen phosphate (KH 2 PO 4) as a flame retardant after adjusting the pH to 4.5. When the batches were gently agitated, a flocculent precipitate was obtained which settled relatively quickly to the bottom of the vessel. The temperature was measured to 57 ° C. After a few hours of settling, the liquid on top of the sediment was sucked off, leaving only a precipitate in the vessels. The COD of the sucked liquids was measured and the following values were obtained: COD (mg 02 / liter)

Test lot A 1 950

Experimental batch B 1800

Test batch C 1790 6 84190 Compared to untreated bleaching liquor, the amount of oxygen consuming substances was significantly reduced in the tests. As can be seen from the above, the COD of the experiments was reduced by an average of 39.5%. This is a surprisingly positive effect, remarkably achieved by the method of recovering water glass from bleaching liquor. It is thus quite clear that in this case the organic substances were accompanied by the precipitation of silicates, which had a positive effect on the degree of purification of the treated bleaching liquor.

The sediments of lots A and B were evaporated and dried to anhydrous space. The sediment from batch C was mixed at a bulk density of 6% with unbleached groundwood. After mixing, the pulp slurry is thickened to a dry content of 42%, after which the thickened pulp is dried to a dry content of 94%.

When the sediments of Experiments A and B were dried, a powdery substance consisting of small crystals was obtained, and the products obtained from all the experiments were weighed. The results were as follows:

Experiment A 2.8 g of crystal powder

Experiment B 3.1 g of crystal powder

If we add the water glass content of the bleaching liquor and the amount of chemical additions (aluminum sulphate) of both experiments A and B, we obtain a theoretical total of 2.6 g for each experiment. Thus, in both cases, the amounts obtained are higher than the theoretical amounts, probably because the crystal powders contain precipitated organic substances.

This is also confirmed by the above-proven reduction in chemical oxygen demand of purified bleaching liquors.

The mass impregnated with the sediment obtained from Experiment C was attempted to ignite several times in the fire resistance test. It did not succeed, but when the same was attempted with the unimpregnated mass, it immediately caught fire.

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Example 2 In the experiments described in this example, a pilot plant with a flow chart as shown in Figure 1 was used in the treatment of bleaching liquor to recover water glass. The pilot plant operated in connection with a plant producing peroxide-bleached groundwood.

The pulp slurry obtained from the factory after bleaching was compressed to a dry content of 40% and a part of the compressed bleaching liquor was directed along the pipe 1 to the tank 2 of the pilot plant, where 2.5 g of aluminum sulphate was mixed through the pipe 3 for each liter of liquor. Sulfuric acid was added from tube 4 to adjust the pH of the liquor to 4.5. The chemicals were efficiently mixed with the liquor due to the mixer 5 located on the side of the tank 6 from which the bleaching liquor enters the tank. On the opposite side 7 there is a bottom outlet screw 8 which, with continuous feed, removed the sediment formed on the bottom of the tank into the pipe 9. It preferably takes place via a membrane filter when the risk of clogging of the filter membranes has been removed by removing the silicate.

Some of the sedimented sediment removed through line 9 is further passed through line 12 to centrifuge 13 while another portion of sediment is passed through line 14 to tank 15. In centrifuge 13, the thickened sediment is passed to a spray dryer 16 where all water is evaporated to give a completely dry product.

To that part of the precipitate which is discharged into the tank 15 is added 0.3 g of potassium dihydrogen phosphate per liter from the tube 17. To said tank a further amount of pulp slurry from the tube 18 is added such that the amount of chemicals (precipitate + Kt ^ PO 2) per tonne of pulp is 60 kg. The resulting chemically mixed pulp slurry is passed through a pipe 19 to a screw press 20 where it was dewatered until a pulp density of 6% had changed to 45%. The dewatered pulp is then passed through a pipe 21 to a flake dryer β 84190, where it is dried to a dry matter content of 92%. The filtrate from the screw press 20 is passed along a pipe 23 to an outlet 24. Analysis of the filtrate showed that it contained only small amounts of silicate.

Silicon analysis of the clarified bleach liquor in tube 10 showed that the lye contained very little silicate. An experiment on the pulp from the flake dryer 22 further showed that the pulp obtained is completely flame resistant. The compound can be used, for example, as insulation. The products produced by the invention have many areas of use and the dried silicate sediment can be used, for example, as a filler in the manufacture of paperboard and paper or as an additive in the cement and concrete industries.

Example 3 The experiments described in this example use a test plant supplemented with a membrane filter with a flow chart as shown in the figure to recover water glass from bleaching. Pilot plant oh :, located in connection with a peroxide bleached chemimechanical pulp mill.

In the factory, the pulp slurry obtained after bleaching is compressed to a dry content of 40% and the obtained bleaching liquor is led via pipe 1 to a test plant. The lye from the tube enters tank 2, where 2.5 g of aluminum sulphate from tube 3 is mixed with each liter of lye. The pH of the lye is adjusted to 4.5 by adding sulfuric acid from tube 4. The chemicals mix efficiently with lye thanks to the mixer 5 located on the bleach liquor inlet 6. a discharge screw 8 is placed on the bottom 7, which continuously feeds the sediment deposited on the bottom of the tank to the pipe 9. The treated and sedimented clarified bleach liquor is passed through a pipe 10 at the top of the tank and further through a pipe 25 to a membrane filter plant 26. in connection with the experiment, it was led off via pipe 27 to outlet 11. In continuous production, it makes sense to utilize the extractant as a fuel, since the substance has a calorific value comparable to ordinary fuel. The bleach liquor liberated from the silicate and 9 84190 extractant is returned to mass production via line 28. In the factory, peroxide-containing bleaching liquor containing only small amounts of silicate is used, for example, to wash the pulp after sieving it or to wash the bleached pulp.

The sedimented precipitate removed along line 9 is further passed along line 12 to centrifuge 13. In centrifuge 13, the thickened sediment is passed to a blower dryer 16 where the water is completely evaporated to give a completely dry product.

Claims (6)

A method of treating peroxide bleaching liquids of high yield pulp and recycled paper massager in which water glasses were used as stabilizers for the peroxide during the bleaching process, characterized in that solvents contained in the liquids contained a silicate derived from polyhydric cation and / or an organic flocculant while bringing the pH of the effluent to a value between 3.5 and 7.5, preferably between 4.0 and 6.0, causing the resulting silicate precipitate to settle, separating and collecting the resulting sediment, and removing it from The silicate-free liquor is returned to the process. Process according to claim 1, characterized in that the added polyvalent cation containing inorganic compound is an aluminum compound, such as aluminum sulfate or a double salt of aluminum such as alum. Process according to claim 1, characterized in that the added polyvalent cation containing inorganic compound is an iron compound, such as ferric chloride or ferric sulfate. Process according to claim 1, characterized in that the added polyvalent cation containing inorganic compound is a calcium compound, such as calcium hydroxide or in calcium oxide. 5. A process according to claim 1, characterized in that the organic flocculant added is a high molecular weight polyelectrolyte, preferably a polyacrylamide. 6. A process according to claim 1, characterized in that in the further processing of the sedimented silicate precipitate an inorganic boron or phosphorus compound, preferably borax or potassium dihydrogen phosphate, is added, after which the obtained mixture is subjected to concentration and drying.