AT394814B - Process for de-dusting waste gases - Google Patents

Abstract

In a process for de-dusting the waste gases from ore, sintering and/or pelleting systems, in which the dusts are subjected, at least partially, to a wet-chemical preparation, and the waste process water is subjected to cleaning, the gases to be cleaned are washed (scrubbed) with a washing liquid 1 with a pH between 0 and 10, in particular 1 and 10, the slurrying 6, 7 of the dusts being set to a pH between 3 and 5, especially about 3.5, and the solid materials being filtered off 1, following which the filtrate is subjected to precipitation 12, 13 of heavy metals, particularly sulphide precipitation, at a pH between 4 and 12 and the precipitate is filtered off 16. <IMAGE>

Description

AT 394 814 B

The invention relates to a method for dedusting the exhaust gases from ore, sintering and / or pelletizing plants, in which the dusts are at least partially subjected to processing by wet chemical means and the process wastewater is subjected to purification.

From EP-Al 354 613 a method for cleaning exhaust gases from a sintering plant has become known, in which a dust mixture containing chlorides is first separated in an electrostatic filter. At least a part of this dust mixture is then slurried, which brings the chlorides into solution. After the solids have been separated off from such a solution, the solids-free solution is spray-dried and the solvent-containing exhaust gas from the spray dryer is returned to the electrostatic precipitator. With such a procedure, chloride can be recovered, and recycling of other solids contained in exhaust gases is only incomplete or not envisaged. From DE-Al 29 14 013 another method for cleaning exhaust gases has become known, in which metal oxides can be recovered from the dust-containing exhaust gases from a steelworks converter. The procedure provides for wet chemical cleaning, the separated dusts being treated in the form of a slurry with lye at a pH of more than 13, the zinc content of the dust being converted into solution. The slurry is then fed to a separator from which a clear solution can be removed. The zinc dissolved in the clear solution can subsequently be precipitated. The dust can be separated off by washing the exhaust gases, the resulting suspension being collected and made alkaline in order to convert the zinc content into solution.

The invention is now based on a method of the type mentioned at the outset and aims to be able not only to recover chlorides from the process exhaust gases of an ore, sintering and / or pelletizing plant, but also metal compounds or metals such as those found in such Dusts are present to be recycled. At the same time, the method according to the invention aims to achieve a largely purified process waste water which is subsequently suitable, for example, for recycling as a washing liquid or for the preparation of slurries in the process according to the invention. To achieve this object, the method according to the invention essentially consists in that gases to be cleaned with washing liquid with a pH between 0 and 10, in particular 1 to 10, that the slurry of the dusts is adjusted to a pH between 3 and 5, in particular about 3.5, and the solids are filtered off are, whereupon the filtrate is subjected to precipitation of heavy metals, in particular sulfide precipitation, at a pH between 4 and 12 and precipitate is filtered off. By first washing with a washing liquid with a pH value of 0 to 10, depending on the pH value selected, for example when adjusting the pH value in the alkaline range, a separation of sulfur compounds can be achieved at the same time and, on the other hand, create the conditions which a separation of ferrous substances from other metals or metal compounds, in particular heavy metals, is readily possible. As a result of the fact that the slurry of the dusts is now adjusted to a pH value between 3 and 5, in particular about 3.5, iron hydroxide is precipitated which can be separated from a saline solution by filtering. In addition to chlorides, the salt solution obtained as filtrate also contains soluble heavy metal compounds which are subsequently subjected to precipitation, in particular sulfide precipitation, for which a pH value between 4 and 12 is selected. After the precipitation has been separated off, there remains a solution which essentially only contains alkali halides and which can either be recycled directly or can be subjected to a recovery of alkali halides.

The process according to the invention is advantageously carried out in such a way that alkali halides are separated from the filtrate from the heavy metal precipitation by crystallization. After the alkali halides, in particular potassium chloride, have been separated off by crystallization, a purified liquid remains which can be used directly as wash water or can be passed directly into the receiving water if the residual content of impurities is correspondingly low.

If sulfur compounds, in particular sulfur dioxide, are separated off at the same time, the process according to the invention is advantageously carried out in such a way that, in the case of flue gas scrubbing with washing liquids having a pH of 6 to 10, preferably 6 to 9, after a first separation of solids by filtration , the filtrate is subjected to gypsum precipitation with Ca (OH) 2 and / or CaC ^. In the first separation of solids by filtration, an iron-containing residue and a heavy metal sludge are obtained, which, as mentioned at the outset, can subsequently be processed further by producing a slurry in the acidic range and adjusting the pH to values between 3 and 5 to win an iron-rich residue, and heavy metals in the manner mentioned at the outset can in turn be precipitated from the filtrate of such a separation of the iron-rich residue. In such a mode of operation of the scrubber with washing water with a pH of 6 to 9, part of the heavy metals will remain in solution and thus in the filtrate of the first separation of solids. In these cases, before the gypsum is precipitated, a sulfide precipitation of the heavy metals is preferably carried out if the proportion of heavy metals is correspondingly high. After the gypsum precipitation, gypsum can be separated off by filtering, and in the filtrate of the separation of the gypsum due to the high calcium ion concentration by introducing CC &gt; 2, a limestone suspension can also be produced, which is also a recyclable product. -2-

AT 394 814 B

In order to ensure extensive separation of heavy metals and iron, it is advantageously carried out in such a way that the washing liquid suspension with KOH, HCl and H2O is first stirred in a first mixing stage at a pH of 1 to 2, whereupon in a further mixing stage the pH Value is increased to 3 to 4 for the precipitation of iron hydroxides. By maintaining a strongly acidic environment at a pH of 1 to 2, it is possible to keep a large part of the heavy metals safely in solution, whereby an iron-rich residue is formed in the subsequent precipitation of iron hydroxides, since at this pH the solution is in solution heavy metals are not yet precipitated. A largely quantitative separation of dissolved heavy metals is subsequently achieved by raising the pH to the values from 4 to 12 already mentioned above and sulfidic precipitation.

A particularly simple and rapid precipitation of heavy metals in the filtrate of the iron hydroxide separation can be achieved in that the filtrate of the iron hydroxide separation is fed to at least one further mixing stage in which the salt solution is stirred while blowing in air and / or 02 and with Ca (OH) 2 &gt; FeClg, trimercaptotriazine as well as Na2S and flocculants are added, whereupon the precipitation in one

Sludge containers are sedimented, the supernatant liquid being used after a final pH check and filtering as a starting product for KCl recovery and / or as a circulating washing liquid. Such a separation of the heavy metals by sulfidic precipitation can be carried out continuously or batchwise, a heavy metal sludge being formed in the sedimentation tank, which subsequently can preferably be subjected to a Zn / Pb treatment.

The iron-rich residue formed by hydroxide precipitation can easily be returned to a sintering or pelletizing plant.

In the course of the wet chemical production of the slurry and working up by separating off the iron-rich residue, dusts from dry dedusting can also be processed at the same time, the procedure advantageously being such that the slurry of the dusts before the precipitation of Fe (OH) 3 dusts from dry dedusting, such as, for . B. E-filter dusts can be added.

The choice of the pH value for the scrubbing liquid in the wet cleaning of the exhaust gases is primarily selected depending on the expected sulfur dioxide pollution of the exhaust gas.In cases where flue gas desulfurization appears to be unnecessary, work can naturally be carried out directly in the acidic range, so that the Consumption of chemicals for setting the conditions for hydroxide precipitation is low. Another possibility for reducing the chemical requirement is that the wet separation of the dusts is carried out in two stages, washing in an initial stage with acidic washing liquid and in a second stage with basic washing liquid and the washing liquid suspension of an Fe (OH ) 3-precipitation and the washing liquid suspension obtained in the second stage is still subjected to a solid separation of a gypsum precipitation. In this way it is ensured that the suspension to be subjected to an iron hydroxide precipitation is already present in an acidic environment and that a gas mixture free of solids and in particular heavy metals and iron-containing residues is subjected to desulfurization in the subsequent basic washing stage.

The invention is explained in more detail below by exemplary embodiments schematically shown in the drawing of devices suitable for carrying out the method according to the invention. 1 shows a diagram of a continuous operation of an exhaust gas cleaning system for the recovery of iron-rich residue, sponge metal, sludge and largely purified waste water; FIG. 2 shows a modified design with discontinuous heavy metal precipitation; 3 shows an embodiment in which the scrubber works with acidic washing liquids at a pH of 0 to 1 and so an SO 2 separation does not take place; 4 shows a further modification of a system for a basic mode of operation of the scrubber at a pH of 6 to 9 with separation of sulfur dioxide; 5 shows a further modification of a basic mode of operation of the scrubber at a pH of 6 to 9 and SOj separation with lime milk; and Fig. 6 one

Device for two-stage process control, washing in a first stage with acidic washing liquid and in a second stage with basic washing liquid.

In Fig. 1 with (1) a wash tower for the purification of exhaust gases is drawn. The exhaust gas is blown into the washing tower at the head via line (2) and drawn off near the sump via a line (3) and a liquid separator (4). The liquid collected in the liquid separator is returned to the sump (5) of the scrubber (1). A suspension of the solids separated from the exhaust gas and a solution of the portions of the exhaust gas soluble in the scrubbing liquid are discharged from the sump (5) via a pump (6) and fed to a first mixing stage (7). In this first mixing stage (7), the dust from a dry dust separator, for example the filter rod of an electrostatic precipitator, can also be added, a pH of 1 to 2 being set by suitable chemicals such as KOH, HCl and H2O in order to achieve the greatest possible solution of metals becomes. In a subsequent, second mixing stage (8), the pH is adjusted to the conditions of an iron dioxide separation and thus to about 3.5, whereupon a iron-rich residue is separated from a filtrate having a high salt concentration by means of a belt filter (9). The filtrate is collected in a container (10) and, after appropriate enrichment, the salt concentration can be reduced to -3-

AT 394 814 B, a minimum concentration of 5 to 200 g / 1 as a result of heavy metal separation. For this purpose, the filtrate is transferred via a line (11) into a further mixer »(12), in which a pH of 9 to 10 is set. Air and / or oxygen is blown into this mixer (12) and the pH is adjusted by means of Ca (OH) 2 and FeCl ^. The mixture produced in this way is subjected to sulfide precipitation in a further mixing container (13) equipped with a stirrer , whereby trimercaptotriazine and / or sodium sulfide and optionally flocculants are added for the rapid formation of a sedimentable precipitate for the precipitation. The precipitate formed subsequently arrives as a suspension in a sedimentation tank (14), from which a heavy metal slurry can be drawn off via a slurry pump (15). The phase separation can be carried out in a simple manner by means of a filter chamber press (16) and the heavy metal sludge thus pressed out can be recycled in a zinc lead smelter. The squeezed liquid passes together with the excess liquid from the sedimentation container (14) into a further collecting container (17), a final pH value check being carried out when the cleaned liquid after a further cleaning step by filtering, for example by the one shown in FIG. 1 If the gravel filter (18) is to be fed directly into the receiving water as wastewater, an appropriate correction to the permissible values must be carried out in accordance with the final pH control carried out in the tank (17).

2, the reference numerals of FIG. 1 are retained unchanged. The two mixing tanks (12 and 13) are operated in parallel in this configuration and the batch is transferred batchwise to a common sedimentation tank (14) in which the sludge is collected and discharged discontinuously.

3, no SO 2 deposition takes place within the scope of the method. The scrubber (1) is operated with a washing liquid with a pH between 0 and 1, so that a very extensive solution of heavy metals and iron salts is present directly in the first mixing stage (7). Working up to the iron-rich residue is carried out in an analogous manner to that shown in FIG. 1. The subsequent removal of heavy metals by sulfide precipitation is in principle also carried out in a manner corresponding to the illustration in FIG. 1. After the heavy metal sludge has been separated off, the cleaned liquid is at least partially passed over an activated carbon filter (19) and subjected to crystallization (20) in order to recover solid potassium chloride. The remaining liquid can be returned to the fresh water inlet of the washer (3).

In the illustration according to FIG. 4, the laundry (1) is operated at a pH of 6 to 9 and an S02 separation follows with an alkali metal hydroxide solution. The suspension from the bottom of the washer is the first here

Solids separation is subjected to filtration, the separated solids being further treated in a manner corresponding to FIG. 1 for the recovery of an iron-containing residue and further separation of a heavy metal sludge. The filtrate of the first filtration (21) provided in the case of the basic mode of operation of the scrubber is subsequently mixed in a mixer (22) with calcium hydroxide, as a result of which a gypsum is precipitated. In the case of a high proportion of heavy metals in the filtrate, sulfidic heavy metal precipitation can be provided, the filtrate being mixed with Na2S in a container (23) while stirring and the sludge formed being separated off after sedimentation in a container (24) using a filter press (25) .

This heavy metal sludge can be combined with the heavy metal sludge as it is formed after the separation of the iron-rich residue.

Following the plaster precipitation in the container (22), sedimentation can be carried out in a container (26) and the gypsum formed can be separated off and discharged using a conventional belt filter press (27). The filtrate of this belt filter press (27) can still contain a high proportion of calcium ions if calcium hydroxide is used correspondingly more than stoichiometric and by adding CO 2 in a container (28) a limestone suspension can be formed, which can also be used.

4, the essential method steps of the illustration according to FIG. 1 are also provided with the same reference numerals.

5, a basic wash is also carried out in the scrubber (1). The deposition of SO2 succeeds here by using a hydrocyclone (29), with CaOH2 being used directly as wash water. Gypsum is thus formed directly during washing and the filter residue of the belt filter press (27) contains gypsum as well as dust and heavy metal sludge. Such a, relatively impure product must subsequently be deposited, since it is not immediately suitable for further processing

The procedure for separating the iron-rich residue and heavy metal-containing sludge and the recovery of potassium chloride by crystallization from the wastewater again corresponds to the training as already described in connection with the previous figures.

A largely pure plaster can be achieved when working with a two-stage wash. 6, in addition to a first washer (1), which is operated with acidic washing liquid, a second washer (30) is provided, which is operated with basic washing liquids. The sump (5) of the first scrubber becomes -4-, which has already been described in detail in the previous figures.

AT394 814 B

In turn, an iron-rich residue, a heavy metal-containing sludge and solid potassium chloride were separated off. A gypsum slurry is drawn off immediately from the sump (31) of the downstream second scrubber, which is operated with basic scrubbing liquid. Ca (OH) 2, which is produced in a mixer (32) in the desired concentration, is used as washing water in the scrubber (30). The solids accumulation takes place in a hydrocyclone (29), with pure gypsum being separated off in the first scrubber in consideration of the already extensive pre-cleaning of the exhaust gases, which can be separated in the belt filter press (27) from likewise largely pure lime milk. The lime milk separated here can be returned to the heavy metal separation via a line (33).

The invention is explained in more detail below on the basis of an exemplary embodiment

Execution example:

Wastewater from a pilot scrubber system with a composition as shown in Table 1 below was mixed several times on an industrial scale with a dust of the composition as described in Table 2 in a stirred tank. The ratio of wastewater to dust was controlled in the present case with the help of conductivity measurement. During leaching, the pH should be in the acidic to weakly acidic range. Depending on the dust composition and pH of the wastewater, acidification may be necessary. In order to convert existing Fe2 + into Fe ^ + so that a quantitative iron precipitation is achieved in a weakly acidic medium, an oxidation with air and / or H2O2 was carried out. After

Filtration gave a brine of the composition shown in Table 3, and an iron-rich residue, the composition of which is given in Table 4.

The brine was then subjected to a combined hydroxide-sulfide precipitation with the addition of FeClg, CaC ^, A ^ OHJg solution. and subjected to polyelectrolyte. After filtering off the heavy metal sludge, the composition of which is given in Table 5, a brine was obtained, the heavy metal contents of which were far below the limit values of the Ordinance on Waste Water Emissions in Flowing Water (Table 6).

By treating the brine with activated carbon, the value of the total bound organic carbon (TOC) could be almost completely eliminated and the value of the chemical oxygen demand (COD) reduced by more than 50% (Table 7). This brine was evaporated to give a high purity KCl / NaCl salt product, the composition of which is given in Table 8.

Table V,

Chemical analysis of the scrubbing water pH 0.5-4 Fe total 10-15500 mg / 1 Fefi 10-15000 mg / 1 Fem 10-5350 mg / 1 Si 10-3300 mg / 1 C 5-1990 mg / 1 S 80- 22500 mg / 1 Ca 18-4470 mg / 1 Mg 10-3218 mg / 1 Cr 10-1050 mg / 1 Mn 10-2177 mg / 1 Cu 1-1081 mg / 1 Al 11-3112 mg / 1 Pb 10-5323 mg / 1 Zn 1-4980 mg / 1 TI 1-50 mg / 1 K 1890-100500 mg / 1 P 1-205 mg / 1 Na 100-15350 mg / 1 Ti 1-210 mg / 1 Cd 0.5- 270 mg / 1 Hg 0.1-109 mg / 1 As 0.1-115 mg / 1 Co 0.1-137 mg / 1 Ni 1-251 mg / 1 Se 1-112 mg / 1 Te 0.1- 57 mg / 1 5-

AT 394 814 B

Table 1: (continued

Sb 0.2-117 mg / 1 V 0.1-154 mg / 1 Sn 0.2-100 mg / 1 CI 2500-150000 mg / 1 nh4 50-2500 mg / 1 CN 0.2-580 mg / 1 F 10-2340 mg / 1 TOC 10-2000 mg / 1

Table 2;

Chemical analysis of the dust

Fe: 7.67%

Si: 0.1-10% C: 0.2-8.5% S: 0.15-3.7%

Ca: 0.6-17.9%

Mg: 0.09-16.9%

Cr: &lt; 0.001-0.088%

Mn: 0.02-3.4%

Cu: &lt; 0.001-0.93%

Al: 0.03-3.1%

Pb: 0.002-6.4%

Zn: &lt; 0.001-20.1% TI: 0.0001-0.019% K: 2.145%

Na: 0.09-25.1%

Cd: 0.0001-0.077%

Hg: 0.001-1.3%

As: 0.0001-0.069%

Ni: &lt; 0.001-0.15%

Se: &lt; 0.001-0.11% V: 0.002-0.89% CI: 1.741% F: 0.011-7.6%

Table 3:

Chemical analysis of the brine after dust elution

Total Fe 0.3-1600 mg / 1 Ca 18-7340 mg / 1 Mg 10-2437 mg / 1 Cr 11-1350 mg / 1 Mn 10-2437 mg / 1 Cu 9-1733 mg / 1 Al 11-3391 mg / 1 Pb 10-7430 mg / 1 Zn 9-8465 mg / 1 Ή 3-241 mg / 1 K 4871-121300 mg / 1 Na 431-31400 mg / 1 Cd 1-321 mg / 1 6-

AT 394 814 B

Table 3: Continued)

Hg 1-297 mg / 1 As 1-121 mg / 1 Ni 2-291 mg / 1 Se 1-177 mg / 1 CI 3400-150000 mg / 1 F 10-4340 mg / 1 TOC 3-320 mg / 1 COD 90-700 mg / 1

Table 4:

Chemical analysis of the iron-rich residue (main components)

Fe: 41-60% Ca: 2-11% Mg: 0.2-4.0% Si: 1.5-7.7% C: 3.1-9.5%

Table 5:

Chemical analysis of the shear metal sludge (main components)

Fe: 20-35%

Zn: &lt; 0.01-3.2%

Pb: 0.1-43.0%

Cd: 0.05-2.9%

Cu: 0.9-17% Ή: 0.2-16%

Table 6:

Waste water limit values

A.2 Inorganic parameters 6. Aluminum 2 mg Al / I limited by settable substances 7. Arsenic 0.1 mg AsA 0.1 mg AsA 8. Barium 5 mg BaA 5 mg BaA 9. Lead 0.5 mg PbA 0.5 mg PbA 10. Cadmium 0.1 mg CdA 0.1 mg CdA 11. Total chromium 0.5 mg CrA 0.5 mg CrA 12. Chromium-YI 0.1 mg CrA 0.1 mg CrA 13. Iron 2.0 mg FeA - 14. Cobalt 1.0 mg CoA 1.0 mg CoA -7-

AT 394 814 B 15. Copper 16. Nickel 17. Mercury 18. Silver 19. Zinc 20. Tin 21. Free chlorine 22. Total chlorine 23. Ammonium stick: 24. Chloride 25. Free cyanide 26. Fluoride 27. Nitrate nitrogen 28. Nitrite nitrogen 29. Total phosphorus 30. Sulfate

Table 6: (Continued! 0.5 mg CuA 0.5 mg Cu / 1 0.5 mg Ni / 1 0.5 mg Ni / 1 0.01 mg Hg / 1 0.01 mg Hg / 1 0.1 mg Ag / 1 0.1 mg Ag / 1 2.0 mg Zn / 1 2.0 mg Zn / 1 2.0 mg Sn / 1 2.0 mg Sn / 1 0.2 mg Cl2 / 1 0.2 mg Cl2 / 1 0.4 mg Cl2 / 1 0.4 mg Cl2 / 1 10 mg NH4-N / 1 if necessary, in individual cases limited by Gf 0.1 mg CN / 1 10 mg F / l if necessary, define 1.0 mg N02-N / 1 2 mg P / l regulation in individual cases 0.1 mg CN / 1 20 mg F / l 10 mg N02-N / 1 250 mg SO4 / I, in individual cases higher values possible depending on building materials and dilution in the sewer 31 Sulfide 0.1 mg S2 '/ l 1.0 mg S2' / l

Table 7: Chemical analysis of the cleaned brine

Fe: &lt; 0.3 mg / 1 Cr: &lt; 0.3 mg / 1 Mn: &lt; 0.02-0.3 mg / 1 Cu: &lt; 0.01-0.2 mg / 1 Al: &lt; 0.2 mg / 1 Pb: &lt; 0.02-0.07 mg / 1 Zn: &lt; 0.05-0.8 mg / 1 Ή: &lt; 0.1-0.8 mg / 1 Cd: &lt; 0.02-0.09 mg / 1 Hg: &lt; 0.001 mg / 1 As: &lt; 0.1 mg / 1 Ni: &lt; 0.5 mg / 1 Se: &lt; 0.2 mg / 1 TOC: &lt; 0.1-2 mg / 1 COD: 35-380 mg / 1 -8-

Claims (10)

AT 394 814 B JaMe 3; Chemical analysis of the salt product Fe: &lt; 0.001% Si: 0.001-0.01% C: &lt; 0.005-0.1% S: 0.01-0.98% Ca: 0.02-1.5% Mg: 0.02-0.43% Cr: &lt; 0.0005% Mn: &lt; 0.0005% Cu: &lt; 0.0005% Al: &lt; 0.0005-0.005% Pb: &lt; 0.00005-0.001% Zn: &lt; 0.001-0.01% TI: &lt; 0.00005% K: 46.3-52.0% Na: 1.7-4.5% Cd: &lt; 0.000005-0.0001% Hg: &lt; 0.00001% As: &lt; 0.00001% Ni: &lt; 0.001% Se: &lt; 0.00003-0.001% V: &lt; 0.001% CI: 43.1-48.3% F: &lt; 0.001-0.02% PATENT CLAIMS 1. Process for dedusting the exhaust gases from ore, sintering and / or pelletizing plants, in which the dusts are at least partially subjected to processing by wet chemical means and the process wastewater is subjected to cleaning, characterized in that the gases to be cleaned are washed with washing liquid with a pH between 0 and 10, in particular 1 to 10, that the slurry of the dusts is adjusted to a pH between 3 and 5, in particular about 3.5, and the solids are filtered off, whereupon the filtrate is subjected to a precipitation of heavy metals, in particular sulfide precipitation, at a pH between 4 and 12 and precipitate is filtered off. 2. The method according to claim 1, characterized in that alkali halides are separated from the filtrate of the heavy metal precipitation by crystallization. 3. The method according to claim 1 or 2, characterized in that in a flue gas wash with washing liquids having a pH of 6 to 10, preferably 6 to 9, after a first separation of solids by filtration, the filtrate of a gypsum precipitation with Ca (OH) 2 and / or is subjected to CaCl2. 4. The method according to claim 3, characterized in that a heavy metal precipitation with sulfides is carried out before the gypsum precipitation and the sulfide precipitate is separated before the gypsum precipitation. 5. The method according to any one of claims 1 to 4, characterized in that the washing liquid suspension with KOH, HCl and H20 is first stirred in a first mixing stage at a pH of 1 to 2, whereupon the pH value in a further mixing stage 3 to 4 is raised to precipitate iron hydroxides. -9- AT 394 814 B 6. The method according to any one of claims 1 to 5, characterized in that the filtrate of the iron hydroxide separation is fed to at least one further mixing stage in which the salt solution is stirred while blowing in air and / or Oj and with Ca (OH) 2, FeCl · ^ , Trimercaptotriazine as well as Na2S and flocculants are added, whereupon the precipitates are sedimented in a sludge tank, the supernatant liquid being used after a final pH check and filtering as a starting product for a KCl recovery and / or as a circulating washing liquid. 7. The method according to any one of claims 1 to 6, characterized in that the iron-rich residue of iron precipitation is returned to a sintering and / or pelletizing process. 8. The method according to any one of claims 1 to 7, characterized in that the slurry of the dusts before the precipitation of Fe (OH) ß dusts a dry dedusting, such as. B. E-filter dusts can be added. 9. The method according to any one of claims 1 to 8, characterized in that the sulfide precipitates are subjected to a Zn / Pb treatment. 10. The method according to any one of claims 1 to 9, characterized in that the wet separation of the dusts is carried out in two stages, washing in a first stage with acidic washing liquid and in a second stage with basic washing liquid and the washing liquid suspension obtained in the first stage Fe (OH) j precipitation and the washing liquid suspension obtained in the second stage is still subjected to a solid separation of a gypsum precipitation. Including 6 sheets of drawings -10-