Particularly, the invention is directed to a process for treating industrial waste waters containing nitrates.

Nitric acid is used in steel industry for surface treatment of steel. Another significant producer of waste waters containing nitrates is gunpowder industry.

Nitrate is harmful in lakes and rivers and accordingly, emissions of waste waters containing nitrates to water bodies have been restricted. This fact has brought about a difficult situation, since the use of nitric acid in the production of stainless steel is inevitable and up to now, no equivalent alternative to nitric acid has been found.

The procedure used in steel industry comprises chemical precipitation of metallic impurities present in waste water by elevating the pH of the solution to a value sufficiently high to precipitate the metals as hydroxides. This hydroxide precipitate is separated from the solution, the slurry obtained is treated with lime and disposed of to dumping areas. This dumped material evidently contains valuable metals. The solution thus separated is passed to a water body. In some cases, especially in powder industry, biological methods for the removal of nitrogen from waste waters are used.

As is known, nitrate is reduced to form ammonia by iron (II) in alkaline solutions.

At neutral pH values, nitrate is reduced by iron (II) to ammonia only at high temperatures. At room temperature, such reduction takes place only if Cu (II) or Ag (I) is used as a catalyst.

H. C. B. Hansen et al., Environ. Sci. Technol. 1966,30, pages 2053-2056, disclose a reaction to reduce nitrate nitrogen with iron (II) to give ammonia. The reaction proceeds as follows: Fe'14Fel"2 OH12 SO4 (s) + 1/4 NO3 + 3/2 Oh so42-+ 1/4 NH4+ + 2 Fe304 (s) + 6 1/4 H20 The iron (II)-iron (III) hydroxide participating in the reaction is also called"green rust". This compound is precipitated from partly oxidized Fe (II)-solutions at neutral pH or slightly above. That is, the addition of pure ferrous sulfate to the solution

causes the formation of said hydroxide in situ. It should be noted that in the reaction, 1 mole of nitrate needs 8 moles of Fe2+-compound for reduction.

No indication was found in the literature that a process based on said reaction is used in industrial scale to reduce nitrate. For instance, waste water from a pickling plant of a steel factory contains nitrate in such high amounts that the consumption of ferrous sulfate and catalyst would be too high for an economically feasible process. The need for a copper catalyst would be about 12 to 25 g per ton of waste water. Moreover, the addition of copper to the solution is problematic since it is precipitated to give magnetite that contains copper from 0.2 to 0.5 %. The use of such a precipitate for instance as a starting material for an iron chemical used in water treatment is not possible because it contains high concentrations of copper.

Moreover, such a precipitate may not be employed in the production of stainless steel due to its high copper concentration.

So there is an evident need in the field for a process for reducing nitrate, and a continuous reduction process allowing for the treatment of industrial waste waters containing nitrates (1) without any remaining unutilized waste materials and (2) in an economically feasible way. The object of the invention is to solve above problems. The invention is characterized by features disclosed in the characterizing part of claim 1.

The invention is based on the surprising experimental finding that no separate catalyst is needed if hydrated ferrous sulfate (copperas, FeS04-7H20), a by- product in the production of titanium pigment, is used as the reducing agent. The reduction of nitrate to give ammonia is as efficient as in a reaction using pure ferrous sulfate and Cu (II) catalyst. It is evident that the superior performance of copperas as a reducing agent is based on metallic impurities present therein and acting like a catalyst. Such impurities are for instance Mg, Ti, Ni, Zn, Mn, Cr, Co, and Cu.

According to the invention, a process for removing nitrate from waste water is thus provided, comprising the addition of hydrated ferrous sulfate obtained as a by- product in the production of titanium pigment to the waste water containing nitrate to reduce nitrate to ammonia.

Said hydrated ferrous sulfate is preferably ferrous sulfate heptahydrate (copperas, FeSO4 7H20)

Hydrated ferrous sulfate is preferably added in a molar ratio of ferrous sulfate to nitrate of at least 8: 1. Said molar ratio is particularly preferably from 8: 1 to 13: 1.

The pH of the waste water is preferably controlled to a value from 8 to 10. Higher pH values improve for instance the precipitation of Ni.

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

The ammonia produced in the reduction reaction may be removed by aeration, by precipitation, or catalytically.

The magnetite precipitate, or Fe304 precipitate, formed in the reduction containing about 65 % of iron may for instance be separated by filtering. Pure water containing Na2S04 obtained as the filtrate may be passed to a further treatment such as neutralization and heat recovery. Water thus obtained may be used in other pickling processes.

In steel and for instance in piping industries, this magnetite precipitate may be recycled in the production process of steel since it contains metals such as Cr, Ni and Mo useful therein. Similarly in powder industry, the precipitate produced in the treatment of waste water may be used as a starting material for a water treatment chemical since it contains only minor amounts of metallic impurities.

In the following, the invention is illustrated in more detail with reference to the appended figure showing an apparatus useful to carry out the process of the invention for removal of nitrate from waste water.

Waste water containing nitrates (40 m3/h) is passed through line 3 to a reactor 1 (volume 100 m3) purged with nitrogen and provided with an agitator. The waste water is heated with steam to a temperature of 80 °C and pH is adjusted to a value of 8 to 9 with NaOH being fed to the reactor 1 through line 4. A weighed amount of ferrous sulfate heptahydrate is fed to the reactor 1 from a silo 5 by means of a screw feeder 6. The residence time of the mixture is adjusted to 2 hours. The reacted mixture is passed into the filtration tank 2 (volume 20 m3) where it is aerated if necessary. Ammonia is decomposed or recovered. Precipitated Fe304 8 is separated at filter 7 and the purified waste water 9 leaving the filter may be passed to

neutralization and heat recovery and at least partially used as a medium for electrolytic pickling.

The invention is now described with tests.

Tests In all tests, four types of waste water were used.

Table 1 Waste waters containing nitrates used in the tests Origin of waste water NO3- (ppm) Notes Stainless steel industry 330 Contains Cr, Ni, Mo Piping industry 23,500 Powder industry 45,000 No metallic impurities Fertilizer industry 2, 000 10 % CaCl2, 2 % KCI, pH 0-1.

The tests were conducted by taking 400 g of waste water containing nitrates into a reactor vessel equipped with an agitator under nitrogen atmosphere to prevent the oxidation of iron (II) by oxygen during the test. The temperature was elevated to 80 °C. A stochiometric amount (molar ratio of Fe (II) to nitrate being 8: 1, in test no. 8 this ratio being 27: 1) or a minor excess of iron (II) reducing agent was added to the vessel and the pH of the mixture was adjusted to a range from 8 to 9 with NaOH. A minor amount of copper sulfate or iron powder was added as the catalyst. During the test, the pH of the reaction mixture was maintained in said pH range. Samples were taken from the reactor 0,1,2,3 and 4 hours after the addition of Fe (II). NOs' and Nu4+ were analyzed from the samples. In all tests, a separated black crystalline iron precipitate was found in the vessel, being crystalline magnetite, Fe304, according to an X-ray diffraction analysis. The results from these tests are shown in the Table below.

Table 2<BR> Results from reduction tests Test Reducing agent Catalyst, Reaction T, °C NO3-, ppm NH4+, ppm % Reduction Notes no. mg/400 g of waste water time, h of N 1 fe(OH)2 Cu 10 0 80 330 16 0 Waste water from a " " " 1 " 90 38 43 stainless steel plant " " " 2 " 90 13 65 " 2 " Fe 100 0 " 330 16 0 " " " " 1 " 220 12 32 " " " " 2 " 180 9 45 " 3 FeSO4 Cu 10 0 " 330 16 0 " " " " 1 " <1 20 82 " " " " 2 " <1 12 89 " 4 " Fe 100 0 " 330 16 0 " " " " 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 60 330 16 0 " " " " 1 " 58 63 29 " " " " 2 " 38 57 39 " 8 FeCl2*4H2O no catalyst 0 80 2000 - 0 Solution from a " " " 1 " 200 45 82 fertilizer plant " " " 2 " <50 25 93 " " " " 3 " <50 10 96 " Table 2 (continued) Test Reducing agent Catalyst, Reaction T, °C NO3-, ppm NH4+, ppm % Notes no. mg / 400 g of waste water time, h Reduction of N 9 Copperas no catalyst 0 80 330 16 0 Waste water from a " " " 1 " 49 36 55 stainless steel plant " " " 2 " 34 20 73 " " " " 3 " 28 14 80 " 10 " " 0 80 23500 20 0 Waste water from " " " 1 " 3200 240 83 piping industry " " " 2 " 3200 42 86 " " " " 3 " 3200 17 86 " 11 " " 0 45000 - 0 Waste water from " " " 1 1000 400 95 gumpowder industry " " " 2 650 110 98 " " " " 3 350 52 99 " In the Table, the degree of reduction of nitrate nitrogen is calculated in the column next to the last one. The degree of reduction is calculated as follows (as exemplified by Test 1) : The nitrogen content of the solution (1 m) initially = 330 x 14/62 g + 16 x 14/18 g = 87.0 g. In the end (test 1,2 hours), the nitrogen content of the solution = 90 x 14/62 g + 13 x 14/18 g = 30.4 g. The degree of reduction = (87.0-30.4)/87.0*100 % = 65 %.

Tests 1 and 2 In tests 1 and 2, freshly precipitated moist ferrous hydroxide was used as the reducing agent being added in a stochiometric amount to the reactor vessel. The precipitation of ferrous hydroxide was carried out under protective nitrogen atmosphere. Copper (Test 1) or Fe powder (Test 2) was used as the catalyst.

Tests 3 to 5 In these tests, the same catalysts were used as in Tests 1 and 2, but pure ferrous sulfate was used as the reducing agent. In Test 5, a lower amount of catalyst was used.

Tests 6 and 7 In these tests, Cu and copperas were used respectively as the catalyst and the reducing agent.

Test 8 In this Test, ferrous chloride was used as the reducing agent in a molar ratio of 27: 1 without a separate catalyst. This test shows that pure iron may result in adequate reduction only if excessive amounts of iron are used.

Tests 9 to 11 (Tests according to the invention). Copperas was used as the reducing agent in a molar ratio of 8: 1. No separate catalyst was added, but the metallic impurities contained in copperas acted as the catalyst.

The results show that the best yield was attained in Test no. 11 treating waste water from powder industry. In this waste water, the nitrate content is considerable. The rather poor results of Tests 2 and 4 show that Cu catalyst may not be replaced with iron powder. Moreover, it was found that the use of copper as the catalyst is more preferable since the iron precipitate being formed was more crystallized than in Tests 2 and 4 using iron powder as the catalyst. In these Tests, the iron precipitate was found to contain low amounts of hematite in addition to magnetite. In Tests 9 to 11, good results were obtained, thus showing that the process of the invention is suitable for all kinds of waste waters containing nitrates.

In Tests 6 and 7, the effect of the temperature was studied. In Tests 6 and 7, the temperature was respectively 80 °C, and 60 °C. The Tests show that nitrates undergo reduction equally fast in these temperatures. Slightly higher residual nitrate contents at higher temperatures may be due to the fact that the rate of water evaporation was higher at higher temperatures. Similarly, the amount of ammonium nitrogen is higher at lower temperatures. At lower temperatures, the lower rate of evaporation of ammonium to give ammonia may account for this. Accordingly, the effect of the temperature is above all the fact that the rate of ammonia release in the solution is higher at higher temperatures.

The Table below shows the analysis of copperas used in the Tests.

Table 3 Analysis of copperas Element Content Fie 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 analyses of the magnetite precipitates produced in the treatment of waste water containing nitrate from various industrial plants. Precipitate 1 was obtained by treating waste water from pickling carried out in a stainless steel plant, according to Test 6 in Table 1. In this Test, copper was added to the waste water.

Precipitate 2 was obtained according to the invention by treating waste water from a piping plant using stainless steel and precipitate 3 was obtained in a similar manner from a powder plant. The analysis of precipitate 1 shows that copper co-precipitates with magnetite thus strongly increasing the copper content thereof.

Table 4 Analyses of the Fe304 precipitates from the tests. Precipitates 1,2 and 3 are obtained from steel industry, piping plant and powder plant, respectively Element Precipitate 1 Precipitate 2 Precipitate 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"5. 1 <20 <20 Cd"< 1 < 1 < 1 Co"170 120 90 The analyses of the precipitates show that considerable amounts of metals are present in precipitates 1 and 2. Precipitate 1 is from the test using copperas and Cu- catalyst (Test 6). It is evident that the added copper co-precipitates with magnetite. Copper is not an acceptable metal in a stainless steel plant and thus, such a precipitate may not be recycled in the manufacturing process of stainless steel.

Precipitate 2 is from Test 10 using copperas without a separate catalyst. The copper content of the precipitate is considerably lower. Such a precipitate may be utilized in the production of steel. Accordingly, the precipitate produced in the process of the invention must not be disposed of but it may be recycled in the manufacturing process of stainless steel.

Precipitate 3 containing low amounts of metals is obtained from waste water of powder industry. This precipitate is a useful starting material e. g. in the production of a chemical for water treatment.