FI109991B - Method for removing nitrate from waste water - Google Patents

Description

109991

Method for removing nitrate from waste water - Förfarande för avlägsnande av nitrat frän avloppsvatten

The invention relates to a process for removing nitrate from waste water. In particular, the method relates to a process for the treatment of industrial wastewater containing nitrate.

Nitric acid is used in the steel industry for surface treatment of steel. Another major source of nitrate-containing wastewater is the powder industry. Nitrate is harmful to water and the discharge of wastewater containing nitrate is restricted. 10 This has led to a difficult situation, since the use of nitric acid in the production of stainless steel is necessary and a equally good alternative to nitric acid has not been achieved. In the steel industry, metal impurities in waste water are chemically precipitated by raising the pH of the solution so that the metals precipitate as hydroxide. The hydroxide precipitate is separated from the solution and the slurry is limed and introduced into the deposition area. It is clear that this depositing material contains valuable metals. The remaining solution has been discharged into water. In some cases, particularly in the powder industry, biological methods are used to remove wastewater.

Iron (II) is known to reduce nitrate to ammonia in alkaline solutions.

At neutral pH, nitrate is reduced to ammonia by the action of iron (II) only '! at high temperatures. At room temperature, reduction occurs only if • »· '· :;' Cu (II) or Ag (I) as catalyst.

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Hansen et al., Environ. Sci. Technol. 1996, 30, pages 2053-; ···: 2056 discloses a reaction in which the nitrate nitrogen is reduced to ammonia by iron (II). · * *. 25. The reaction is as follows

Fen4 Fein2 OHi2 S04 (s) + 1/4 NO3 '+ 3/2 OH' o S042 '+ 1/4 NH / + 2 Fe304 (s) + 6V * H2O

The iron (II) iron (III) hydroxide in the reaction is also referred to as • · * '' green Rust '. This compound precipitates from partially oxidized Fe (II) solutions when the pH of the solution is neutral or above neutral. In other words, when pure ferrous sulfate is added to the solution, said hydroxide is formed in situ. The reaction is: *. ·. it should be noted that 1 mole of nitrate requires 8 moles of Fe 2+ to be reduced.

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There is no information in the literature indicating that the above reaction-based method has been used on an industrial scale to reduce nitrate. For example, wastewater from a steel mill pickling plant contains a large amount of nitrate, which would make the consumption of ferrous sulphate and catalyst so high that the process would not be economically viable. Catalytic copper would consume about 12 to 25 g / tonne of wastewater. In addition, the addition of copper to the solution is problematic because it precipitates in the magnetite, whereby the concentration of copper in the magnetite is between 0.2 and 0.5%. The use of such a precipitate, for example, as a raw material for an iron chemical used in water treatment is not possible due to the high copper content it contains. Also, due to its high copper content, such a precipitate cannot be utilized in the process of manufacturing high-grade steel.

There is a clear need in the industry for a nitrate reduction process and a continuous reduction process to treat nitrate-containing industrial wastewater (1) without leaving untreated waste materials (2) 15 economically viable. The object of the invention is to eliminate the above-mentioned problems. The invention is characterized by the features mentioned in the characterizing part of claim 1.

. The process according to the invention is based on the surprising experimental finding that when using as a reductant, the crystalline aqueous ferrosulphate (Copperas, FeSO 4 · 7H 2 O) produced as a side-product in the manufacture of titanium pigment, no separate catalyst is required at all. The reduction of nitrate to ammonia is as effective as the use of pure ferrous sulfate and Cu (II) catalyst. It is apparent that the excellent ability of copperase to act as a reducing agent is due to its metallic impurities, which function in the same way as the catalyst. These: '': 25 impurities include Mg, Ti, Ni, Zn, Mn, Cr, Co and Cu.

Thus, according to the invention, there is provided a process for removing nitrate: ··· 'from waste water by adding crystalline aqueous ferrosulphate produced as a by-product in the manufacture of titanium pigment to the nitrate to reduce the nitrate to ammonia.

Said crystalline aqueous ferrosulphate is preferably ferrosulphate heptahydrate i (Copperas, FeSO 4 · 7H 2 O).

.;. The crystalline aqueous ferrous sulfate is preferably added such that ferric sulfate and nitrate '; ·; 'Has a molar ratio of at least 8: 1. Particularly preferably, the molar ratio is from 8: 1 to 13: 1.

109991 The pH of the effluent is preferably adjusted to a value between 8 and 10. For example, higher pH results in better precipitation of Ni.

The temperature of the effluent is preferably 30 to 100 ° C, particularly preferably 60 to 90 ° C.

Ammonia from the reduction reaction can be removed by aeration or by catalyst or precipitation.

The magnetite precipitate produced during the reduction, i.e. the Fe304 precipitate, which contains about 65% iron, can be separated, for example, by filtration. The filtrate yields pure Na2SO4-containing water which can be taken for further treatment such as neutralization and heat recovery. The water thus obtained can be used in other pickling processes.

In the steel industry and, for example, in the tube industry, magnetite precipitate can be recycled in the steel making process because the precipitate contains metals used in the steel making such as Cr, Ni and Mo. Correspondingly, the precipitate from the waste water treatment of the powder industry is suitable for use as a raw material for a water treatment chemical 15 because the precipitate contains little metallic impurities.

In the following, the invention will be explained in more detail with reference to the accompanying figure, which shows an apparatus useful for carrying out the process of the invention by means of which nitrate can be removed from waste water.

, Nitrate-containing effluent (40 m3 / h) is fed to the reactor 1 (volume 100 m3) with nitrogen stirring via line 3. Wastewater is heated with steam; ; . to 80 ° C and adjust the pH to 8-9 with NaOH fed to the reactor. 1 along 4 tubes. From the silo 5, a weighed amount of ferrosulphate heptahydrate is fed by means of a screw feeder 6 to reactor 1. The mixture delay is adjusted to 2 h. The reaction mixture ... is led to filtration front vessel 2 (20 m3 volume) where it is vented if necessary. Ammonia is decomposed or recovered. The resulting Fe 3 O 4 precipitate 8 is separated by a filter 7 and the pure wastewater 9 discharged from the filtration can be led to '* neutralization and heat recovery and used at least in part as an electrolytic' pickling solution.

The invention will now be described by means of experiments.

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Exam

In all experiments, four types of wastewater were used.

Table 1 Nitrate containing wastewater used in experiments Waste water source NQ3 '(ppm) Note_

Stainless steel mill__330_Contains Cr, Ni, Mo_

Pipe Factory__23 500 __ ”__

Powder Factory__45 000_No metallic impurities

Fertilizer Plant __2000__10% CaCl2, 2% KCl, pH 0-1 5 The experiments were carried out by transferring nitrate-containing effluent to a 400 g reactor vessel equipped with a stirrer, above which was a nitrogen atmosphere to prevent the oxidation of iron (II) by oxygen during the experiment. The temperature was raised to 80 ° C. Stoichiometric (Fe (II): nitrate 8: 1 molar, 27: 1 molar ratio 8 experiment) or a small excess iron (II) reducing agent was added to the vessel and the pH of the mixture was adjusted to 8-9 using NaOH. A small amount of copper sulfate or iron flour was added as a catalyst. The pH of the reaction mixture was maintained at the above pH for the duration of the experiment. The reactor was sampled at 0, 1, 2, 3 and 4 hours after addition of Fe (II). The sample was analyzed for NO3 "and NH3. In all experiments, a black crystalline iron precipitate precipitated in the vessel, which according to X-ray diffraction analysis was crystalline magnetite, Fe304. The results of the experiments are shown in the table below.

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Table 2 Results of reduction experiments

Experiment Reducer Catalyst Reaction T NO3 "NHi + N.npel- Note.

n mg / 400 g time h ° C ppm ppm shrinkage wastewater% I Fe (OH> 2 Cu 10 0 ~ 8 ~ ~ 330 16 0 Stainless steel __ "__" __ 1 "90 38 43 mill __" __ "__ 2 "90 13 65 Sewage _2 __" __ Fe 100 0 "330 16 0__" "" "1" 220 12 32 "__" __ "__ 2" 180__9 45__ "3 FeSQ4 Cu 10 0" 330 16 0__ "" "" 1 "<1 20 82 - "" "2" <1 12 89 4 ~ "FelOO ~ 0 330 16 0 _J] __" __ "__ 1" 120 14 56__ "" "" 2 "100 9 66 5" Cu 2.5 0 "330 16 0__ "" "" 1 "72 17 66" "" 2 "60 10 75__" 6 Copperas__Cu 5.0__0 __ "__ 330__16__0__" "" "1" 66 27 59 "" 2 "52 17 71 7" Cu 5.0 0 60 33Ö 16 0__ "" "" 1 "58 63 29" __2 "38 57 39__" 8 FeCl2 * 4H2Q Not Catalyst__ 0__80 2000 __-__ 0 Fertilizer · *: "__" __ 1 "200 45 82 Plant," __ "__ 2 "<50 25 93 solution __" __ "__ 3" <50 10 96__ ": *, · '9 Copperas not catalyzed__ 0__80 330__16__0 Stainless steel *." __ "__ 1" 49 36 55 mill' "__" __ 2 "34 20 73 Sewage __" __ "__ 3" 28 14 80__ "10" "0 80 23500 20 0 Pipe _________ Mill" __ "__ 1" 3200 240 83 Sewage "" "2" 3200 42 86. ···. "__ "__3" 3200 17 86__ "II" "0 45000 - 0 Square: _________ industrial! ···. "__" __ 1___1000 400 95 mouth *; · "__" __ 2___650 110 98 sewage: ***: "" "1 3 I 1 350 1 52 | 99 1 ”6 109991

The second to last column of the table shows the reduction rate of nitrate nitrogen. The degree of reduction is calculated as follows (as an example, Experiment 1):

Initially, the solution (1 m3) contains nitrogen = 330 x 14/62 g + 16 x 14/18 g = 87.0 g. At the end (test 1, 2 h), the solution contains nitrogen = 90 x 14/62 g + 13 x 5 14/18 g = 30.4 g. The degree of reduction is = (87.0-30.4) / 87.0 * 100% = 65%.

Experiments 1 and 2 In experiments 1 and 2, freshly precipitated moist iron hydroxide was used as a reducing agent and added to the reaction vessel in a stoichiometric amount. The precipitation of the iron hydroxide was carried out under nitrogen gas protection. The catalyst used was copper (test 1) or Fe powder (test 2).

Experiments 3-5 The same catalysts as in Experiments 1 and 2 were used in these experiments, but pure ferrous sulfate was used as the reducing agent. In experiment 5, a smaller amount of catalyst was used.

Experiments 6-7 In these experiments, Cu was used as the catalyst and copperase as the reducing agent.

Experiment 8 Ferric chloride was used as a reducing agent in a 27: 1 molar ratio. without a separate catalyst. The result of the experiment shows that pure iron can only achieve a good reduction result if a significant excess of iron is used.

Tests 9-11 (Tests according to the invention). Copper was used as reducing agent in an 8: 1 molar ratio *: No separate catalyst was added, but the metallic impurities in the coppe box acting as catalyst.

;;; Experiments show that the best yield was obtained in Experiment 11, which dealt with waste water from the powder industry. The nitrate content of this waste water is remarkably high.

': Poor results from Experiments 2 and 4 show that iron flour is no substitute for I ·.,. · Cu catalyst in the country. In addition, it was found that the use of copper as a catalyst is more advantageous because the resulting iron precipitate was more crystallized than in Experiments 2 and 4, where iron powder was used as a catalyst. These were found to contain iron deposits. also a small amount of hematite in addition to magnetite. Experiments 9-11 gave a good result indicating that the method of the invention works well for all different types of nitrate containing wastewater.

.!. t 30 Tests 6 and 7 investigate the effect of temperature. In Test 6, the temperature was 80 ° C and in Test 7 the temperature was 60 ° C. Experiments show that nitrates are reduced as rapidly at these temperatures. Slightly higher residual nitrate contents at higher temperatures are explained by the faster evaporation of water at higher temperatures. Correspondingly, the amount of ammonium nitrogen is higher at lower temperatures. This is because ammonia is evaporated more slowly at lower temperatures. Thus, the effect of the temperature is above all that the release of ammonia-5 from the solution occurs more rapidly at a higher temperature.

The analysis of copperase used in the experiments is shown in the following table.

Table 3 Copperas analysis

Alkuaine__Pitoisuus

Fe__18.9%

Mg__1200 ppm

Ti__800

Cu __ <0.5 ppm

Mo__

Ni__13 ppm

Zn__490 ppm

Mn__1500 ppm

Cr__3 ppm

Pb __ <0.2 ppm

Cd__0.35 ppm

Co_ <50 ppm · * Hg __ <0.02 ppm

Table 4 shows the analyzes of magnetite precipitates from the purification of nitrate containing wastewater from two different industrial plants. Precipitate 1 was obtained when hand: ": The wastewater from the pickling of the stainless steel mill was tapped, corresponding to the test in Table 1:" '; 6. In this experiment, copper was added to the wastewater. The precipitate 3 was from a powder mill, and analysis of the precipitate 1 shows that copper precipitates ..... to 15 magnetites, whereby its copper content rises very high.

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Table 4 Analyzes of Fe304 precipitates precipitated in experiments. The precipitate 1 comes from the steel industry, the precipitate 2 from the tube mill and the precipitate 3 from the powder mill

Element__Suck 1__Suck 2__Suck 3

Fe,% __ 67,0__64__62.5

Cu, ppm__4700__52 __ <20

Mo "__240__190 __-

Ni «__2900__1600__110

Zn "__620__300__280

Mn "__2200__2000__1800

Cr "__3700__1700 __ <20

Pb «__5J __ <20 __ <20

Cd "__ <1 __ <1 __ <1

Co "170 120 90 5 Analysis of the precipitates shows that a considerable amount of metals is present in precipitate 1 and 2. The precipitate 1 is derived from an experiment using copper and Cu catalyst (experiment 6). It is seen that the added copper precipitates with the magnetite. desirable metal at the refinery, so such a precipitate cannot be recycled in the refining process.

The precipitate 2 is derived from experiment 10 in which copperase was used without any separate catalyst. The copper content of the precipitate is significantly lower. Such a precipitate: ': can be utilized in the manufacture of steel. Thus, the precipitate obtained by the process according to the invention does not need to be discarded but can be recycled to stainless steel. · ·. in the manufacturing process.

15 Precipitate 3 is derived from waste water from the powder industry and has low metal content. This precipitate is a useful raw material for the manufacture of, for example, water purifying chemicals.

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Claims (9)

9 109991 A process for removing nitrate from waste water, characterized in that crystalline aqueous ferrosulphate produced as a by-product in the manufacture of titanium pigment is added to the nitrate-containing waste water to reduce the nitrate to ammonia. Process according to claim 1, characterized in that said crystalline ferric sulphate contains ferrous sulphate heptahydrate. 3. Förfarande enligt patent krav 1 eller 2, kännetecknat av att man tillsätter en hundred play crystalline wattenhaltigt ferrosulfate att molförhällandet mellan ferrosulfate och nitrate at a ratio of 8: 1. 3. A process according to claim 1 or 2, characterized in that an amount of crystalline aqueous ferrosulphate is added such that the molar ratio of ferrous sulphate to nitrate is at least 8: 1. 4. Förfarande enligt patentkrav 3, kännetecknat av att molförhällandet 8: 1 to 5 13: 1. Process according to Claim 3, characterized in that the molar ratio is from 8: 1 to 13: 1. 5. Förhällande enligt nägot av föregäende patentkrav, kännetecknat av att pH för avloppsvattnet justeras account et av 8 - 10. Method according to one of the preceding claims, characterized in that the pH of the waste water is adjusted to a value between 8 and 10. 6. Förhällande enligt nägot av föregäende patentkrav, kännetecknat av att tem-peraturen hos avloppsvattnet high 30 - 100 ° C, företrädesvis 60 - 90 ° C. 10 7. Förhällande enligt nägot av föregäende patentkrav, kännetecknat av att bildad ammoniak avlägsnas med luftning eller catalytiskt. Process according to any one of the preceding claims, characterized in that the temperature of the waste water is from 30 to 100 ° C, preferably from 60 to 90 ° C. Process according to any one of the preceding claims, characterized in that the ammonia formed is removed by aeration or catalytic action. 8. Förhällande enligt nägot av föregäende patentkrav, kännetecknat av att vid reducering bildad magnetitfällning separeras frän behandlat avloppsvatten. A process according to any one of the preceding claims, characterized in that the magnetite precipitate generated during the reduction of et '' is separated from the treated waste water. A method according to any one of the preceding claims, characterized in that: ·; Wastewater containing this nitrate comes from the steel industry or the powder industry. * ·; ·: Patentkrav '···' 1. Förfarande för avlägsnande av nitrat frän avloppsvatten, kännetecknat av att man i avloppsvatten innehällande nitrat tulter vidførstätning av titanpigment • · 25 som biprodukt bildat crystallvattenhaltiget reducrosat .. · Ammonia. · · ♦: · '2. Förfarande enligt patentkrav 1, kanknetecknat av att Such crystalline watten :: Tiga ferrosulfate innehäller ferrosulfate heptahydrate. 10 109991 9. Förhällande enligt nägot av föregäende patentkrav, kännetecknat av att av-15 loppsvattnet innehällande nitrate Mr sig frän stälindustrin eller krutindustrin. • * I · k * I • k · k k i * k! 'I · 1' k