FI57093C - FOERFARANDE FOER BEHANDLING AV AVFALLSVATTEN - Google Patents

Description

Πζ2ηΤ * 1 [B] (11) ANNOUNCEMENT „lJ .11 ^ UTLÄGG NI NOSSKRI FT 5/093 ^ Patent neJJelat ^ ^ (51) Kv, lk3 / lnt.CI.3 C 02 F 1/52 FINLAND I —FI N LAN D (21) P «t * nttlh« Jwimi »- teMtameknlng 137 ^ / 72 (22) H« k «mlipllvl —Ameknlngadtg 15.05.72 (23) AlkupUvl — GUtlghttadag 15.05.72 (41) Tullut IuIMmIuI - BllvK ofTentJIg . 11.72

Patent and Administrative Office (44) Date of issue of the notice. -

Patent- och rafittaratyralaan '' AnaSkin utlagd oeh utl. »KrNUn publfcurad 29.02.80 (32) (33) (31)« uolkeui — Bugtrd priority 17.05.71

Japan-Japan (JP) 3306U / 71 (71) Mitsubishi Jukogyo Kabushiki Kaisha, 5 "1, Marunouchi 2-chome,

Chiyoda-ku, Tokyo, Japan-Japan (JP) (72) Toru Yoshii, Nishi-Sonoki-gun, Nagasaki Prefecture, Mikio Aramaki,

Nagasaki City, Nagasaki Prefecture, Yushin Minamide, Kawagoe City,

The present invention relates to a process for the purification of effluents containing impurities by coagulating the impurities by adding magnesium salt to the effluent. - Saitama Prefecture, Japan-Japan (JP) (7 ^) Oy Kolster Ab (5 ^) A process for the treatment of waste water - Förfarande för behandling av avfallsvatten

Large volumes of effluent from pulp and paper mills contain lignin and their derivatives, are highly colored, and have a high content of suspended solids. The wastewater treatment of these plants has therefore been a difficult problem in the industry. The present invention seeks to develop a method which can be advantageously used for the treatment of the above-mentioned effluents.

Acidic, neutral or temporal effluents from mills producing sulphate cellulose, sulphite cellulose or their paper products are usually treated by adding, for example, salts of iron, aluminum or magnesium and an alkali, such as slaked lime, to the effluent to form a flocculant iron, aluminum or magnesium hydroxide solids and colored organic matter separate from the effluent and settle to the bottom. The sludge formed is removed, concentrated and dewatered.

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However, the amounts of sludge removed from large pulp and paper mills are so large that finding suitable areas for waste disposal and the use of ever-increasing amounts of coagulants are difficult to solve.

U.S. Pat. No. 3,163,598 discloses a process for precipitating impurities from wastewater using resin hydroxide. The precipitate that has settled to the bottom is incinerated, in which case the zinc oxide obtained dissolved in acid is reused in the purification. However, the use of zinc has the serious disadvantage that it is harmful when it enters treated wastewater as a heavy metal. An additional advantage of the process of the present invention over the process of U.S. Patent No. 3,163,598 is the use of seawater as a cheap Mg salt supplement.

It is an object of the present invention to provide a method in which wastewater containing a high content of impurities is treated by forming a sludge, the sludge is removed and the coagulant used in the treatment is recovered from the sludge and reused, and the treated water is discharged into the sewer.

The method according to the invention is characterized in that in the coagulation step a part of the combustion residue obtained by incineration of sludge separated from the effluent after coagulation is added to the effluent, and after incineration an acid solution .

The invention is illustrated in more detail by the following example. The crude wastewater is coagulated, the formed slurry is separated, and the treatment agent, magnesium salt, is recovered from the slurry for reuse as a coagulant. An alkaline substance such as slaked lime, carbide waste or sodium hydroxide is also added to the effluent to make it basic. In this case, the magnesium salt added as a coagulant forms a flocculant magnesium hydroxide precipitate. The fluffy precipitate adsorbs water-soluble organic matter, such as lignin extract, from the wastewater and causes the adsorbed material to coagulate and settle to the bottom.

The addition of a polymeric coagulant to such a flocculent precipitate results in the formation of larger flocculants as a result of further coagulation. This is advantageous because it facilitates sedimentation and separation of the webs.

The fluffy precipitate thus separated is concentrated in a thickener and dewatered in a dehumidifier such as a vacuum filter, whereby the fluffy precipitate becomes cake-like. The cake-like slurry is further dried to remove water and then burned at a relatively low temperature between 500 ° -700 ° C in an incinerator. In this case, organic substances such as lignocellulose, combustible and inorganic substances such as magnesium salt added to the wastewater as a coagulant are oxidized and removed from the furnace as combustion waste, mainly as oxide.

3,57093 The dehydration temperature of the fluffy Mg (OH) g precipitate is about 350 ° C. Removal of water from this fluffy precipitate by heating should be performed at this temperature. The fluffy sludge from pulp mill effluents contains lignocellulose and other organic substances that must be incinerated. However, the combustion temperature must not be too high, as this will reduce the solubility in the acid.

When the acid solution is added to the combustion residue, the oxides in the residue dissolve as inorganic salts, which in turn can be recovered for use as coagulants in wastewater treatment. The circulation of recovered products leads to a corresponding reduction in wastewater treatment costs.

The acid solution to be added at this stage may be, for example, chlorine-containing water obtained from a washing machine in the chlorination line of a cellulose bleaching plant, or a solution of hydrochloric acid or sulfuric acid.

By adding a portion of the combustion residue consisting mainly of magnesium oxide to the suspension containing magnesium hydroxide, the settling of the flocculant precipitates is accelerated, the filtration efficiency is improved and a small settling basin can be used, thus reducing the cost of the device.

With repeated use of a coagulant, the process must be supplemented with a material containing a coagulating component, i.e. magnesium salt. The replenishment according to the invention can be carried out as follows. A solution containing magnesium salt, seawater, which is available at low cost, is passed to a coagulant storage tank. Magnesium-containing material, e.g. dolomite (MgCO 2 .CaCO 2) or magnesite (MgCO 2), is added to the burned slurry. MgCOH) or the like. Thus, the present invention has the advantage that the chemicals best suited to the local conditions in wastewater treatment plants can be used economically by adding a magnesium salt-containing material at a suitable point in the chemical recovery cycle.

The invention is presented below using the pulp mill wastewater treatment as an example.

Figures 1-2 are block diagrams showing wastewater treatment in accordance with the present invention.

Fig. 3 is a graph comparing the settling rate obtained by the addition of MgO according to the invention to the settling rate obtained by a conventional method.

Figures U and 5 are graphs showing the thicknesses and water contents of the cakes formed in the experiments according to Figure 3.

The wastewater receiving tank 1 shown in Fig. 1 is connected to a wastewater source, and the coagulation and settling basin 2 is connected on the one hand to the wastewater receiving tank 1 and on the other hand to the coagulant storage tank 3 provided with a conduit for conducting seawater. Pool 2 is provided with an alkali source and a source of polymeric coagulant and has a side section for clear purified water. The sludge concentrator U is connected to the coagulation and settling basin 2, the sludge dewaterer 5 is connected to the sludge concentrator H, the incinerator 6 is connected to the dewaterer 5 and the combustion residue tank 8 is also connected to the incinerator 6, the dissolution basin »And also to an acid solution source for conducting chlorine-containing water or a hydrochloric acid or sulfuric acid solution thereto and the clarifier · is connected to the dissolution basin 9» in the clarifier 10 the outlet of the clear liquid is connected to the coagulant storage tank 3.

In Figures 1 and 2, the same components are denoted by the same symbols.

First, the strongly colored wastewater is discharged from the plant where the ligno-cellulose-containing material is treated to the wastewater receiving tank 1 to homogenize the water to be treated. After homogenization, the water is passed to the coagulation and settling basin 2. A coagulant consisting mainly of magnesium chloride and / or magnesium sulphate is passed from the coagulant storage tank 3 to basin 2. Further alkali such as slaked lime, carbide waste or sodium hydroxide are added . In this case, the magnesium salt or salts contained in the coagulant are converted into magnesium hydroxide, which precipitates as a flocculent precipitate. The reactions are as follows:

MgCl2 + Ca (OH) 2 -> Mg (OH) 2 + CaCl2

MgSO 2 + Ca (OH) 2 -> Mg (OH) 2 + CaSO 4

If a dilute solution of a polymeric coagulant excipient (Cyanamide (Japan) Ltd, sold under the tradename "Aquofloc") is added to the solution prepared as described above, the excipient, which is polyacrylate and anionic in nature, neutralizes the positive charges of magnesium ions and promotes flocculation. This forms a large web-like precipitate which settles easily to the bottom and is easily separated from the water.

The fluffy precipitate formed after coagulation and settling is passed through a concentrator h, such as a thickener, to increase the sludge content. It then passes through a dewaterer 5, such as a vacuum drier, and the sludge thus formed containing 15-25% solids is passed to the incinerator 6. In the furnace, organic matter is removed from the slurry by incineration, and inorganic material containing mainly magnesium oxide, mainly magnesium oxide, is passed to the bottom. 8. Some of the inorganic material is recovered as fly ash from the dust collector 7 and is also discharged to tank 8. The combustion residue, which contains mainly magnesium oxide, is dissolved in an acid solution.

In the preparation of an aqueous solution of ash containing mainly magnesium oxide, chlorinated water obtained from a washing machine in the chlorination line of a cellulose bleaching plant is used. Magnesium oxide dissolves as magnesium chloride. Alternatively, sulfuric acid can be used, in which case the magnesium oxide is soluble as magnesium sulphate. In this case, the reactions are as follows:

MgO + 2HCl -> MgCl2 + H2O

MgO + HgSO 4 - * MgSO 4 + H 2 O.

MgO + 1 * C102 + H2O -> MgCl2 + 2HCl2 + 0g

Magnesium chloride, magnesium sulphate or a mixture thereof is dissolved in water in the dissolution tank 9, cleaned of foreign matter and other impurities in the clarifier 10, and the resulting coagulant is fed to the coagulant storage tank 3 for re-use. . The treatment costs of the effluent are thus reduced by re-using the coagulant supplemented with seawater.

The method according to the invention is illustrated in the following example. The effluents from the washing of the neutral semi-chemical pulp and straw pulp and the effluent from the corrugated board plant were mixed together to give 200 ml of a liquid mixture (pH about 7.6, suspended solids (hereinafter referred to as "SS") 850 ppm and chemical oxygen demand (hereinafter referred to as "KHK") 1980 ppm). 10 ppm of magnesium chloride (calculated as magnesium) and 10 ml of lime milk (as a 10 - # solution) were added to the mixture to raise its pH. A further 2 ml of a 0.1 - # solution of the polymeric coagulant "Aquofloc" (trade name) was added. The mixture was stirred thoroughly for about five minutes and then allowed to stand. The magnesium salt added to the mixed water formed a flocculent magnesium hydroxide precipitate by sedimentation as a layer, whereby the interface of the precipitate was clearly visible. The fluffy precipitate was collected and suction filtered through a 200 mesh (0.075 mm) filter cloth to form a slurry cake with a solids content of about 22%.

The slurry cake was dewatered in a drying pan, the dried cake was burned in a crucible at about 900 ° C to give about 3.2 g of gray ash consisting mainly of magnesium oxide and calcium oxide. Then, 50 ml of dilute hydrochloric acid solution (7% concentration) was added to the ash to prepare an aqueous solution containing mainly magnesium chloride. The coagulant thus recovered was added to the aqueous mixture to be treated with 10 ppm (calculated as Mg) per 1000 ml of water, and 10 ml of lime milk (as a 10 # solution) was added to raise the pH and then "Aquofloc" as described above. The mixture was stirred well for about five minutes and then allowed to stand. The magnesium flux settled satisfactorily, and a deposition layer with a clear interface formed. The supernatant was collected and analyzed; measured values were SS 55 ppm, KHK 780 ppm, SS removal $ 93.5, and KHK removal $ 6.16.

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The above example shows that the magnesium salt recovered from the separated slurry is sufficiently effective for wastewater treatment.

Another embodiment of the invention is shown in Figure 2. In this case, the combustion residue tank 8 and the coagulation and settling tank 2 are connected to each other by means of a pipe. By means of this bypass pipe, part of the combustion residue, which consists mainly of magnesium oxide, is recovered and added to the coagulation and settling basin 2. This has the advantage of increasing the settling rate of the webs, improving filtration efficiency and and build in a small space.

The results of the experiments performed according to the above arrangement are shown in Fig. 3-5.

Figure 3 compares the settling rate curve of a sample prepared by mixing sulphite pulp mill effluent with seawater in a volume ratio of 1: 1 and adding 0.1% by weight of CaO (as a CaO slurry with a concentration of $ 10) and the settling rate curve of a mixture of prepared by adding 0.15 wt.% MgO (as Mg (OH)? formed of Mg ions in seawater) to the above sample. It can be seen from the graph of the settling rate of the samples that the settling rate in the constant velocity range for the MgO-free sample was about 0.7 m per hour, while for the MgO-containing sample it was 2. h m per hour, which is more than three times higher than the control -teellä. The addition of MgO makes it possible to reduce the treatment area of the thickener, which is inversely proportional to the settling rate of the liquid to be treated, to less than one third of the area required for a liquid which does not contain MgO.

Figure 1+ shows the results of plate tests (by vacuum suction) performed on the samples of Figure 3 in the form of a suspension after the samples have first been concentrated to one tenth of the original volume by means of a nylon filter cloth. It can be seen from the curve that a thicker cake is obtained from a MgO-containing suspension than from a sample that does not contain MgO, due to significantly improved filtration properties. The addition of MgO is also advantageous because the thinner the cakes, the more difficult the continuous filtration.

Figure 5 shows the water contents of the cakes left on the filter cloth after the plate tests of Figure k. The curve shows that a sample with a lower water content is obtained from a sample containing MgO, indicating improved filterability.

The results show that adding a portion of the recovered combustion residue, the main component of which is MgO, to the coagulation and settling basin increases the settling rate, improves filtration efficiency and improves the efficiency of cellulose mill effluent treatment,

By adding seawater to the circulation system, the magnesium salt deficiency can be compensated for in a cheap and economical way.