MXPA98008478A - Process for the preparation of solutions for purification of water containing ferric iron and the use of the product asi obten - Google Patents

Abstract

The present invention relates to a process for the preparation from ferrous iron, from chemicals for the treatment of water in the form of a solution containing ferric iron to a granulated intermediate product obtained in the process and to the use of final products for the process . In one process, a ferrous iron-containing raw material in the form of an aqueous solution and possibly impurities, is oxidized at an elevated temperature so that a precipitated substantially pure ferric compound and a solution phase containing said impurities are obtained. The essential idea of the invention is that the precipitation is carried out in such a way that part of the ferric iron is precipitated in the form of FeO (OH) goethite or, in the presence of sulfate ions in the form of hydronium jarosite H3OFe3 (OH) 6 (SO4) 2, while part of the ferric iron remains as a ferric compound soluble in water in the solution phase. Suitable raw materials include impure ferrous sulfate and deoxidizing solutions containing ferrous chloride. The precipitated ferric compound can also be separated by filtration, alternatively, the suspension can be granulated and can form a solid intermediate and then re-formed in suspension in water, whereby a pure ferric compound can be separated by filtration. The pure ferric compound is dissolved in an acid, so it is suitable as a coagulant for the purification of a

Description

PROCESS FOR THE PREPARATION OF SOLUTIONS FOR PURIFICATION OF WATER CONTAINING FERROUS IRON AND THE USE OF THE PRODUCT AS WAS OBTAINED DESCRIPTION OF THE INVENTION The invention relates to a process for the preparation of solutions for water purification containing ferric iron, from ferrous iron in the process of ferrous iron in the form of an aqueous solution and a raw material which may contain impurities, they oxidize at an elevated temperature so that a substantially pure, precipitable, fatty compound and a solution phase containing said impurities are obtained. By the process according to the invention, it is possible to produce ferric chemicals suitable for the purification of water, in particular of impure ferrous raw materials, such as its impure ferrous fate obtained as a byproduct of industrial processes and waste solutions of the deoxidizing treatment. In the present application, the term impurities means the following harmful elements in drinking water: Pb, Co, Cr, Cu, Mn, Ni, Zn, Hg, As, Cd, Sb, Se. A common impurity in ferrous sulfate is Mn. Ferric chemicals are commonly used as chemical coagulants in water purification. They are prepared from iron compounds obtained as by-products or as waste from industrial processes. Examples of such materials are ferrous sulphate formed in connection with the production of titanium oxide, and deoxidizing solutions containing iron based on chloride or sulfate. However, the use of these raw materials is limited to the fact that they frequently contain impurities in the concentration over which there are restrictions. Therefore, for example, chemicals containing large amounts of Mn can be used for the pu ification of drinking water. A chemical such as this can therefore be used only for the treatment of waste water or sludge. A conventional method for preparing a ferrous sulfate solution of a ferrous sulfate obtained as a byproduct of industrial processes is to first dissolve the ferrous sulfate in a mixture of water and an acid, after which oxygen is introduced into the solution, which forms a ferric sulfate solution. A known process is described in the application PCT / FI95 / 00045, in said process sulfuric acid is added to its ferrous JL fato, whereby a suspension containing sulfuric acid is produced; This is oxidized at high pressure and a temperature of 60-140 ° C. The oxidation reaction in this case is FeS04 + - ^ 02 + H2S04 + x H20? Fe2 (S04) 3 + and H20 so that a suspension of ferric sulfate is formed, which can be granulated and used in a dissolved form as a water purification chemical, as long as the raw material does not contain impurities in an amount Very large for water purification applications. Normally the granulated product contains approximately 20% Fe. The economics of using this product could improve if a higher iron content could be achieved in the product. The manageability of the product could also be improved by decreasing the hygroscopicity of the product and increasing its resistance to the granule. In addition, reducing the acidity of the product could reduce the corrosion it causes in wet conditions.

If the ferrous sulfate used as the raw material contains harmful impurities, as is often the case, the product obtained can not necessarily be used for all water treatment purposes. Fertile sulfates very often contain Mn as an impurity and therefore a chemical prepared from said ferrous sulfate can be used only for applications in which there are no restrictions. In turn, this reduces the capacity of use and value of the product. Efforts have been made to solve the problem caused by the precipitating impurities of ferrous sulphate, salts based on poorly soluble hydroxide and sulfate, such as ferric hydroxide sulfate and sodium jarosite. Said method is presented in the applicant's prior patent application PCT / FI94 / 00588. In this method, the ferrous sulfate is dissolved in a mixture of water and an acid and, if necessary, a suitable sodium compound is added to the solution. The solution obtained is oxidized at an elevated temperature whereby a poorly soluble iron compound, free of impurities, is precipitated from the solution, as established above. The precipitated iron compound is separated by filtration. This method involves the problem that the remaining acid mother liquor must first be neutralized and then treated so that it can be safely discarded. In practice, the measures required for the treatment of mother liquor ruin the economy of the method. Another problem involved is that the sodium jarosite obtained has a relatively low solubility and therefore must be re-dissolved in sulfuric acid or hydrochloric acid. The article by J. Bohacek et al., In J. Mater. Sci. 28 (1993), pp. 2827-2832, describes a process in which ferrous sulfate is oxidized to its ferric acid and precipitated as hydronium jarosite or partially as goethite to be used as a pigment. Czechoslovak patent 277141 describes a corresponding precipitation and also states that the remaining solution containing ferric sulfate is suitable for use as a coagulant for water treatment. It is known to prepare ferric compounds of deoxidation solutions, which are chloride or sulfide solutions, by adding, for example, sulfuric acid and hydrochloric acid to the solution and oxidizing the solution, whereby corresponding ferric compounds are formed. In the case that the initial solution contains impurities, these same impurities will be present in the solution which contains the ferric compound. Therefore, the ability to use the product obtained, as a water treatment chemical, depends on the concentration of these impurities in the raw material. It is an object of the invention to provide a process by which ferrous raw materials obtained as a by-product or a waste of industrial processes can be used more effectively than previously in water treatment. It is a specific object prepared from impure raw materials a ferric chemical free of impurities to demand water treatment processes, in particular for the purification of drinking water and at the same time a ferric chemical containing impurities that can be used as a treatment of waste water. These objects are achieved by the process according to the intention, which is characterized by the facts set forth in claim 1. In the process according to the invention, it is possible to oxidize a mixture containing only a ferrous raw material and possibly water, in which case the oxidation step will produce a ferric compound insoluble in water, ie goethite or hydronium jarosite, and a suspension containing a ferric compound in a soluble form; When necessary, the suspension solidifies and becomes a subsequent step to ferric chemicals in the form of a solution. The essential point of the invention therefore is that only a portion, typically 30-60% of iron present in a raw material is precipitated as an insoluble ferric compound, the remainder of the iron remaining in a soluble form. Est: or can be achieved without adding acid to the raw material, in which case during the oxidation step, the precipitation reaction will continue until an equilibrium is reached between the ferric compound insoluble in precipitated water and the ferric compound sol u 1 e in water It is possible to change the equilibrium to the precipitate by adding a compound that reacts in the form of a base, preferably a Mg compound such as magnesium oxide or hydroxide. The basic substance raises the pH of the solution, so the precipitation reaction should begin and the reaction will continue until the acid formed in the precipitation reaction lowers the pH to a level at which the precipitation reaction ceases. Respectively, it is possible to change the acidic soluble compound balance by adding an acid. In principle, the precipitation yield can be adjusted by 0-100%, although the idea of the invention, according to what was established before, is that both the pure precipitated ferric compound and a solution containing ferric iron are obtained. According to the invention, a two-part product is obtained in this way, both parts being useful as chemicals for water treatment. In the event that the raw material contains impurities, it will pass to the water-soluble part of the product. Therefore, the process will produce a product of the insoluble portion free of impurities from which it can be easily separated from the soluble part containing impurities. The phase in solution and the solid phase of the suspension obtained as a result of oxidation and precipitation can be separated from one another, for example, by filtration in which case the solution phase is suitable for use as a chemical for water treatment if the solid phase is re-dissolved in an acid. However, an often particularly advantageous method of processing is to solidify the suspension by granulation or some other corresponding method, such as compaction, so that a solid intermediate product containing ferric iron is obtained. In this way the product is easier to transport to the desired place of use, where it is dissolved in water in order to separate a pure ferric solid compound. When the raw material is ferrous iron, such as monohydrate or ferrous sulfate heptahydrate, the following procedure is used. The solid ferrous salt is oxidized with oxygen in a pressurized reactor at a high temperature without the addition of an acid or water. During the passage of oxy. The ferrous salt dissolves in its own crystalline water and at the same time the hydronium jarosite H3OFe3 (OH) ß (S04) 2 begins to precipitate. The reaction could be described as follows: H3OFe3 (OH) 6 (S04) 2 (s) FeS04 x H20 + 1 02 - Fe2 (S04) 3 (1) During oxidation, an equilibrium is established between the solution of ferric sulfate and hydrosium jarosite. This means that approximately half of the iron is contained in the solution and the rest is in precipitated hydronium jarosite. The oxidized suspension can be blocked to produce a solid intermediate product. The solid intermediate product contains 30-40% hydronium jarosite and additionally its JL fato ferric F? 2 (S04) 3 - x H2O in different forms. The product contains 25-27% Fe, which is 20% more than in ferrous sulfate products with encionales. Still drying further, the Fe content can be obtained. The product is also less hygroscopic than conventional ferrous products and its resistance to granule formation is superior and its acidity is lower. The product is in the form of granules and therefore it is easy to transport. With reference to natural balance, 40-60% of iron is contained in jarosite and 60-40% is in a soluble form. As stated earlier, the yield of jarosite can be increased from this value by adding a basic compound to the reaction mixture. The coagulants in the form of a solution are produced from the granulated product by first dissolving it in water, whereby a ferric sulfate solution suitable for the treatment of waste water or sludge treatment of the soluble part is obtained. The ratio of impurities to Fe in the solution is twice that of the raw material. The undissolved part is used to prepare a coagulant free of impurities. The undissolved part is a very pure product that has a high iron content (33-36%). When dissolved, it is suitable for purification of drinking water. The undissolved precipitate is dissolved in sulfuric or hydrochloric acid at an elevated temperature. The undissolved part can also be used for the preparation of a chemical coagulant containing nitrate, in which case its nitrate content is higher than that mentioned above (the molar ratio of N03 to Fe is approximately 1.5-2). Not only ferrous sulphate can be used, but also a mother liquor of its oxidant containing ferrous chloride (approximately 12% Fe) and mixed with others, Mn with an impurity, which can be used as the raw material. When said raw material is oxidized at a temperature of 120 ° C and a pressure of 6 bar, oxidation will take place, for example, in the following manner: 3 FeCl 2 + H 02 + x H 20? 2 FeCl3 (1) + FeO (OH) (s) + and H20 The insoluble compound formed in the oxidations is goethite, which precipitates in the oxidation reaction. At the base of the reaction equation, 1/3 of the iron precipitates as goethite in which the ratio of Mn to Fe is smaller than that of the raw material solution. The suspension can be granulated, if necessary. In this case, too, the precipitation performance can be adjusted in the manner described in the foregoing. A number of advantages are obtained by the use of the invention. First, the preparation of the products is more advantageous than by the conventional process, since sulfuric acid is not required in the oxidation step. The granulate obtained as an optional intermediate cuts more iron than the ferrous products prepared by the conventional acid addition method. On the other hand, this reduces transportation costs. With reference to the inferior hygroscopicity and acidity of the granulate, it causes less corrosion in humid conditions. Second, the preparation process gives a product in two parts, one part containing a very pure ferric compound and the other containing most of the impurities present in the raw material. Therefore the process makes it possible to use impure raw materials while obtaining a pure ferric product suitable for the purification of drinking water. The two parts contained in the product obtained by the oxidation can be separated from each other either before the suspension changes to a solid intermediate or in the passage in which the solid intermediate dissolves. There are a number of alternatives in the dissolution step. The dissolution of the intermediate product in water and the separation of the insoluble part produces a solution containing ferric iron and is suitable for use, for example, in the treatment of water or waste sludge, uses in which the purity requirements of the iron chemical do not restrict its use. The undissolved part, in turn, is a very pure iron chemical that can be dissolved in an acid. The solution obtained is suitable for use for example in the purification of drinking water or for other purification purposes that require a pure chemical. The process provides a very flexible, versatile and economical product scale for various applications in water treatment. The invention also relates to intermediates optionally formed in the process described above, especially a granulated product consisting of a ferric compound insoluble in water, which is hydronium jarosite H3OFe3 (OH) 6 (S04) 2 or FeO goethite (OH), a water-soluble ferric compound and water-soluble impurities, if any; to the use of the final solution products obtained by the process for various water treatment applications, in particular to the use of a pure precipitated ferric compound, changed to a solution, as a coagulant in the purification of drinking water; and the use of the solution contains soluble ferric compounds and possibly an impurity as a coagulant in the treatment of waste ag.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in more detail below with reference to the accompanying drawings in which: Figure 1 describes a schematic diagram of the preparation of a granular intermediate product according to an embodiment of the invention; Figure 2 describes a schematic diagram of another embodiment of the invention for the preparation of a pure ferric compound and a solution containing ferric iron; Figure 3 describes a schematic diagram of the dissolution of a pure ferric compound in an acid; and Figure 4 depicts a schematic diagram of the dissolution of a granular intermediate in water and then in an acid. Figure 1 describes a diagram illustrating one embodiment of the invention. The water is added to the ferrous sulfate (17.5% Fe) obtained as a byproduct of the titanium dioxide production process. The obtained mixture is directed to the oxidation step wherein the temperature of the reactor is normally raised to approximately 120 ° C and the pressure to 1-10 bar preferably 5-6 bar. The oxygen is fed into the reactor, and the mixture is stirred in order to improve the oxygen dispersion. Under these conditions the iron is oxidized and at the same time a ferric salt insoluble in water is formed, which in this case is hydrosium jarosite. The precipitation continues until an equilibrium is established between said salt and the soluble ferric sulphate. The suspension obtained from the oxidation step is directed to the granulation, where water is evaporated from the suspension, if necessary. The granules obtained contain approximately 26% Fe, part of which is in the form of hydrosium jarosite and the rest in the form of ferric sulfate. Figure 2 describes another embodiment of the process according to the invention. The raw material is a deoxidant solution based on chlorine. This solution is directed to the oxidation reactor; the reactor is heated and oxygen is introduced into it. In this case, ferric oxide hydroxide is precipitated, ie, goethite FeO (OH) which is a water-insoluble physical compound during the oxidation reaction. Ferric chloride in a soluble form remains in the solution phase. The precipitated ferric compound is separated by filtration. Figure 3 describes the dissolution of the precipitate, obtained according to Figure 2, in an acid, which as shown in the figure, can be sulfuric acid, hydrochloric acid or nitric acid. A suitable solution for water treatment is obtained and, depending on the acid, it contains ferric sulfate, ferric sulfate chloride or ferric sulfate nitrate, respectively. Therefore, the acid used for the solution can be selected as desired and the choice of acid for the solution can be determined, for example, on the basis of the requirements of the water treatment process or the availability of the acid. Figure 4 describes the dissolution of a granular product prepared by the process according to Figure 1; in the first step the product dissolves in water and subsequently the undissolved part is separated and dissolved in optional acids. According to the figure, half of the iron present in the granular product dissolves in the aqueous solution. After the dissolution step, the solution is separated by filtration and the filtrate as such can be used for water treatment. If the raw material contains some impurity, Mn in the case of Figure 4, practically all the Mn passes in this part. Half of the iron present in the product is in a water insoluble precipitate, which in the second dissolution step dissolves with an acid; according to the figure this acid can be sulfuric acid, hydrochloric acid or nitric acid. Depending on the acid, an appropriate solution such as for the treatment of water is obtained, the solution containing, respectively, ferrous sulfate, ferric sulfate chloride or ferric sulfate nitrate. The invention is described below with the help of the examples.

Example 1 Ferrous sulfate containing 17.5% Fe was fed into a dissolution vessel at a rate of 7 t / H. The ferrous sulfate was dissolved in its own crystalline water by heating. The resulting suspension was oxidized at a pressure of 5.2-5.5 bar and a temperature of 122-125 ° C. The oxidized suspension was fed into a granulation drum and the granules were dried. The experiment took 50 hours in total and the total yield of the granular product was 260 t. The moisture content of the product was 12-14% and contained 0.37% Fe2 +, 25% Fe3 + and 15.7% S. According to an X-ray diffraction, the product contained, among others, the following crystalline compounds: H3OFe3 (S04 ) 2 (OH) 6 and Fe? 403 (S04) ís * 63H20. The hygroscopicity of the product, ie the increase in weight when a sample was maintained for 6 days under a relative humidity of 88%, was as low as 7 -9%. The hygroscopicity of conventional granular ferric sulfate is usually 17-24%. Respectively, the strength of the granule formation of the product was 100-170 N, while that of the granular ferric sulfate is only 75-90 N.

E mplo 2 330 g of product obtained from the run test according to Example 1 were dissolved in 180 g. of water. The dissolution time was 2 hours and the temperature was 40 ° C. Then the solution is filtered; 371 g were obtained. of the filtrate and 132 g, of a residue washed with water (158 g.). The dry matter content of filtration was 72% and the residue contained 34% Fe and 0.0035% Mn, calculated from the dry matter. The filtrate was analyzed, and contained 11.3% Fe, 0.28% Fe2 +, 0.13% Mn, 230 ppm Zn, < 5 ppm Cr, 23 ppm Ni, < 5ppm Pb, < 0.02 ppm of Cd, and < 0.01 ppm Hg. The density of the filtrate was 1446 g / cm3, and its pH was 2.04 (1:10 dilution).

Example 3 90 g of the filtration residue obtained from the experiment according to Example 2 were dissolved in sulfuric acid (34 g 96%, 77 g H2C). The dissolution time was 2 hours and the temperature was 60 ° C. The obtained solution was filtered and 108 g of the filtrate were obtained. The filtration residue was 0.149 g, which corresponds to 0.23% of the initial substance amount. The filtrate containing 12.7% Fe, 0.01% Fe2 +, 0.0018% Mn, 4.7 ppm Zn, < 5 ppm Cr, < 5 ppm Ni, < 5 ppm Pb, < 0.02 ppm Cd, < 0.01 ppm Hg. The filtration density was 1.92 g / cm3 and its pH was 1.44 (1:10).

Example 4 88 g of the product obtained from the run test according to Example 1 were dissolved in hydrochloric acid (37%), in nitric acid (65%) and in sulfuric acid (96%), according to the following table. The object was a solution containing 11% iron and therefore water was added in the required amount. In the dissolution experiments, the insoluble part was determined after dissolution for 2 hours or 5 hours and was given in% by weight of the amount of the granular product. The amount of acid in the experiments was an equivalent amount calculated on the basis of iron. The results are shown in the following table .

TABLE. Solubility in acids of granular product obtained by the process according to the intion.

Acid% Fe Amount of acid Time h T. ° C% Insoluble HCl 11 equivalent 2 2 8 800 0.15 11"2 2 6 600 0.19 11" 5 5 4 400 0.79 HN03 11"5 5 8 800 0.26 11 5 5 6 600 0.58 11 5 5 4 400 5.92 H 2 SO 4 11 2 2 8 800 0.07 11 5 5 6 600 0.08 11 5 5 4 400 0.26 The experiments clearly show that sulfuric acid is the most effective solvent, hydrochloric acid is the second most effective and nitric acid is the most effective less effective, but even nitric acid is able to dissolve the product if it is used in time of long dissolution and a high t searing.

Example 5 1000 kg of deoxidizing solution containing 12% Fe was oxidized with oxygen (02 18 kg) at a temperature of 120 ° C and a pressure of 6 bar. An insoluble ferric compound, which according to the X-ray diffraction was FeO (OH), was precipitated from the solution. The precipitate contained 46.5 Fe, calculated from the dry matter. Approximately 1/3 of the iron present in the initial solution was in the precipitate and the rest was in the form of soluble ferric chloride. The ferric chloride solution contained 10.7% Fe. The precipitate was easily filtered and dissolved in hydrochloric acid at a temperature of 70-80 ° C.

Example 6 Water (39 g) was taken in a 2 liter flask, and the water was heated to a temperature of 50-60 ° C. Then, ferrous sulfate was gradually added to the flask as ferrous sulfate dissolved in its own crystalline water. The ferrous sulfate was added in a total amount of 3 kg. Then the solution was transferred to an autoclave, the temperature of which rose to 127 ° C and into which the oxygen was introduced. The pressure in the autoclave was approximately 6 bars. During the 6 hour oxidation time, a total of 1814 g of the MgO suspension, which contains 207 g of MgO, was pumped. After about 3 hours of oxidation, when about 1150 g of the MgO suspension was fed, a sample was taken from the autoclave. The precipitate was separated from the sample by filtration. According to the X-ray diffraction, the precipitate was hydronium jarosite. Based on the analysis, 74% of iron in the sample was in the jarosite and the rest in the solution. The content in the precipitate was 35% and in the filtrate it was 5.1%. At the end of the oxidation time, ie after about 6 h oxidation hours, when all the above mentioned amount had been fed before the MgO suspension, the batch was allowed to cool. The precipitate was separated by filtration and according to the X-ray diffraction it was hydronium jarosite. The precipitate of jarosite now contained 85% iron and the rest was in solution. On the basis of the analysis, the precipitate contained 35% Fe, 0.012% Mg, and 24 ppm Mn. Respectively, the filtrate contained 2.5% Fe, 0.049% Fe +, 2.80% Mg, and 983 ppm Mn. It is clear to one skilled in the art that the different embodiments in the invention are not restricted to the previous examples but may vary within the scope of the appended claims.

Claims (11)

1. A process to prepare, from ferrous iron, solutions for the treatment of ferric iron-containing water, in the process of which a raw material consisting of ferrous iron in the form of an aqueous solution and possible impurities are oxidized at an elevated temperature in a manner that a precipitated substantially pure ferric compound and a solution phase containing the impurities are obtained, characterized in that the precipitation will be carried out in a way that part of the ferrous iron precipitates as FeO (OH) goethite or in the presence of ions sulfate, in the form of hydronium jarosite H3? Fß3 (OH) 6 (SO4) 2, while part of the ferric iron remains in a water-soluble form in the solution phase and the ferric compound obtained is separated in the phase of solution and becomes, by dissolving in an acid to a chemical purification suitable for the treatment of drinking water, the remaining solution phase containing iron ferric solu ble being used as such for the treatment of waste water.

2: The process according to claim 1, characterized in that the precipitated pure ferric compound is separated from the solution phase by filtration.

3. The process according to claim 1, characterized in that the suspension obtained comprises the pure ferric compound precipitated and the dry solution phase can form granules and the granules are in a subsequent step converted back to a suspension by adding water, the pure ferric compound being separated from the suspension by filtration.

4. The process according to any of the preceding claims, characterized in that the raw material is a ferrous sulfate FeS04, which can contain impurities and that is converted to an aqueous solution either by heating so that the ferrous sulfate is dissolved in your own crystal clear water or adding water.

5. The process according to claim 4, characterized in that about 10-60%, preferably 40-60%, ferric iron is precipitated as hydronium jarosite H3OFe3 (OH) and (SO4) 2I the remainder remaining as ferric sulfate F? 2 (SO4) 3 in the solution phase.

6. The process according to claim 4 or 5, characterized in that the distribution of ferric iron between the precipitated hydronium jarosite and the solution phase is regulated by adjusting the pH of the solution by means of an acid or a base, such as sodium oxide. magnesium or magnesium hydroxide.

7. The process according to any of claims 1 to 3, characterized in that the raw material is a deoxidizing solution containing ferrous chloride FeCl2, in which case the goethite is precipitated from the oxidized solution, while the ferric chloride FeCl3 remains in the solution. solution.

8. The granulated product formed as an intermediate product in the process according to rei indication 3, characterized in that it consists of a ferric compound insoluble in water, which is hydronium jarosite H3OFe (OH) (SO4) 2 or goethite FeO (OH) ferric iron soluble in water and possibly water-soluble impurities.

9. The product according to claim 8, characterized in that about 10-60%, preferably 40-60% of ferric iron contained therein is in the form of hydronium salt and the remainder in the form of its JL ferric acid. .

10. The use of the ferric compound produced by a process according to any of claims 1-7, precipitated first and then converted to a solution, as a coagulant in the purification of drinking water.

11. The use of a solution produced by a process according to any of claims 1-7, containing ferric iron and possibly impurities, for the treatment of waste water. SUMMARY OF THE INVENTION The invention relates to a process for the preparation from ferrous iron, from chemicals for the treatment of water in the form of a solution containing ferric iron to a granulated intermediate product obtained in the process and to the use of final products for the process. In a process, a raw material containing ferrous iron in the form of an aqueous solution and possibly impurities, is oxidized at an elevated temperature so that a precipitated substantially pure ferric compound and a solution phase containing said impurities are obtained. The essential idea of the invention is that the precipitation is carried out in such a way that part of the ferric iron is precipitated in the form of FeO (OH) goethite or, in the presence of sulfate ions in the form of hydronium jarosite H30 Fe 3 (OH ) 6 (S 04) 2, while part of the ferric iron remains as a ferric compound soluble in water in the solution phase. Suitable raw materials include impure ferrous sulfate and deoxidizing solutions containing ferrous chloride. The precipitated ferric compound can also be separated by filtration, alternatively, the suspension can be granulated and can form a solid intermediate and then re-formed in suspension in water, whereby a pure ferric compound can be separated by fi. tration. The pure ferric compound "dissolves in an acid, so it is suitable as a coagulant for the purification of drinking water and the phase in solution is suitable as such as coagulant for the treatment of waste water.