Method and device for treating and utilizing bypass dusts

Abstract

A method for treating and utilizing bypass dusts from a cement production process involves a) contacting the bypass dust with an aqueous phase and mixing the same to obtain a suspension, wherein water-soluble components of the bypass dust are dissolved in the aqueous phase; b) performing a solid/liquid separation, in particular a vacuum filtration or a filter press filtration, to separate the solids contained in the suspension, wherein a brine remains; c) precipitating a partial amount of the heavy metals present in the brine, and optionally Ca, and separating the precipitate from the brine; and d) subjecting the brine to an electrocoagulation, wherein a flocculate containing the heavy metals remaining in the brine is separated.

Claims

1. A method for treating and utilizing bypass dusts from a cement production process, comprising the steps of a) contacting the bypass dust with an aqueous phase and mixing the same to obtain a homogenous suspension, wherein water-soluble components of the bypass dust are dissolved in the aqueous phase, b) performing a solid/liquid separation in order to separate the solids contained in the suspension, wherein a brine remains, c) precipitating a partial amount of the heavy metals present in the brine, and optionally Ca, and separating the precipitate from the brine, d) subjecting the brine to an electrocoagulation, wherein a flocculate containing the heavy metals remaining in the brine is separated.

2. The method according to claim 1, wherein step c), the precipitating comprises adding a precipitant.

3. The method according to claim 2, wherein the precipitant comprises CO.sub.2 or a carbonate(s), wherein the carbonate(s) comprises an alkali carbonate.

4. The method according to claim 3, wherein the alkali carbonate comprises Na.sub.2CO.sub.3 or K.sub.2CO.sub.3.

5. The method according to claim 2, wherein the precipitant comprises CO.sub.2-containing exhaust gas from a kiln.

6. The method according to claim 1, wherein step c), the precipitating comprises reducing the pH of the brine.

7. The method according to claim 6, wherein the pH of the brine is reduced by adding an inorganic acid comprising HCl or H.sub.2SO.sub.4.

8. The method according to claim 1, wherein the method further comprises subjecting a treated brine obtained according to step d) to fractional crystallization.

9. The method according to claim 1, wherein in step c), the separating comprises gravity sedimentation.

10. The method according to claim 1, wherein separating the flocculate in step d) further comprises filtering or centrifuging out the flocculate.

11. The method according to claim 10, wherein the filtering comprises using a filter press.

12. The method according to claim 1, wherein the solid-liquid separation according to step b) is performed by a continuous separation method, wherein the suspension and an aqueous phase are conducted in counter-current, and contacted, in a continuously operating separation device comprising a belt filter or vacuum belt filter.

13. The method according to claim 1, wherein said method further comprises, after optionally drying the solids separated in step c) and/or step d), returning the solids recovered as a cement grinding additive(s) or as raw meal component to a cement production process.

14. The method according to claim 1, wherein the electrocoagulation in step d) is conducted using a Fe and/or Al anode.

15. The method according to claim 1, wherein in step c) a calcium salt is precipitated, and wherein the precipitated calcium salt from step c) is added to the suspension in step a) and separated as a solid in step b).

16. The method according to claim 1, wherein the solid/liquid separation comprises a vacuum filtration or a filter press filtration.

17. An apparatus adapted and configured for carrying out the method according to claim 1, wherein said device comprises a mixing device for contacting the bypass dust with an aqueous phase, a first separation device for performing a solid-liquid separation on a mixture obtained from the mixing device, wherein the first separation device is arranged upwards the electrocoagulation device, and a treatment device downstream of the first separation device for treating the brine discharged from the first separation device, said treatment device comprising an electrocoagulation device that is separate from the mixing device.

18. The apparatus according to claim 17, wherein the apparatus further comprises a second separation device for separating the flocculate obtained in the electrocoagulation device, said second separation device comprising a centrifuge or a filtration device.

19. The apparatus according to claim 17, wherein the treatment device further comprises a bath before the electrocoagulation device and including a mixing device for precipitating calcium and a partial amount of the heavy metals present in the brine.

20. The apparatus according to claim 17, wherein the electrocoagulation device comprises a Fe and/or Al anode.

21. The apparatus according to claim 17, wherein the apparatus further comprises a device for carrying out a fractional crystallization treatment following the treatment device.

22. The apparatus according to claim 17, wherein the first separation device for performing a solid-liquid separation is a separation device for performing a vacuum filtration or a filter press filtration.

Description

(1) In the following, the invention will be explained in more detail by way of an exemplary embodiment schematically illustrated in the drawing.

(2) In FIG. 1, a cement production process is schematically indicated by 1, comprising a rotary tubular kiln 2 and a precalcining or preheating system 3 connected to the charging end of the rotary tubular kiln 2. At the point schematically indicated by 4, bypass dusts are taken from the cement production process and stored in a silo 5. In a further silo 6, dusts optionally taken from another point of the cement production process are stored. According to step a) of the method according to the invention, the dusts from silos 5 and/or 6 are supplied to a mixing device 7, in which the dusts are contacted with an aqueous phase and/or a weak alkaline brine stored in a tank 8 and 11 or 13, respectively, and are thoroughly mixed to obtain a homogenous suspension. If desired, hydrochloric acid can be added to the dissolution process occurring in the mixing device 7, in order to adjust the contents of alkalis and halides. By 9 is schematically denoted an extraction step in which the soluble components of the suspension are washed out of the solid components and dissolved in the aqueous phase. The suspension is subsequently supplied to a vacuum belt filter 10, in which step b) of the method according to the invention is carried out. In the vacuum belt filter 10, the suspension and process water from 11 are conducted in countercurrent. After a first section of the vacuum belt filter 10, viewed in the transport direction of the suspension, a strong brine 12 is drawn off. After a second section of the vacuum belt filter 10, viewed in the transport direction of the suspension, a weak brine 13 is drawn off and a filter cake 14 remains. Basically, the washing step can be repeated as often as desired. The filter cake 14 is subsequently dried and can be returned to the cement production process 1 as a raw meal component or as a cement grinding additive. The weak alkaline brine 13 can be supplied to the mixing device 7.

(3) According to step c) of the method according to the invention, the strong alkaline brine 12 is subjected to a precipitation 15 of calcium and heavy metals, wherein a CO.sub.2-containing gas 16 is blown in and mixing is effected by a mixing device. This causes the pH of the brine to drop to 8.5-10.5. In order to perform gravity sedimentation, optionally upon addition of a flocculant 17, the brine is transferred into a sedimentation tank 18, in which CaCO.sub.3 and heavy metal hydroxides 19 are separated. The thus partially purified brine is charged into an electrocoagulation device 20, in which the remaining heavy metals, in particular chromium, are coagulated by supplying electric energy 21 using Fe and/or Al anodes 22. The separation of the coagulated particles takes place in a separation device 23, in particular a filtration device or a centrifuge, wherein the separated components 24 are supplied to the cement as Fe and/or Al correctives.

(4) From this results a treated and purified brine 25, which is optionally subjected to a fractional crystallization 26. Water vapor 27 produced with the aid of waste heat 28 from the preheater exhaust gas or from the clinker cooler waste air is used to heat and evaporate the brine. Fresh water 29, freed of dissolved salts, in particular calcium salts, by reverse osmosis is used to produce the water vapor 27. Furthermore, the evaporation water from the fractional crystallization 26 can also be reused as process water 11, as indicated by the dashed line.

(5) The fractional crystallization 26 produces several salts 30, these being primarily KCl, NaCl and mixtures thereof. KCl can be used with particular advantage in the fertilizer industry. NaCl and KCl/NaCl mixtures can, for instance, be employed as deicing agents, in particular for traffic surfaces, or even in the aluminum industry. The salts derived from the crystallization 26 are optionally subjected to salt drying (not illustrated) so as to obtain dried alkali salts 30.

(6) The invention will be further described by way of exemplary embodiments described below.

Example 1

(7) At first, brine was prepared on a vacuum belt filter according to steps (a) and (b), the thus obtained crude brine having a pH of 12.32 and a conductivity of 126 mS/cm. By blowing in CO.sub.2, the crude brine was neutralized to a pH of 8.45, thus causing the sedimentation of a white precipitate, which was removed after 12 hours of sedimentation so as to obtain a neutralized brine. The neutralized brine was subjected to an electrocoagulation at an electric voltage of 6.0 V using Fe plates. The flocculate was filtered in a filter press so as to obtain a pure brine. By the described method, the contents of Pb and Cr could be lowered as follows.

(8) TABLE-US-00001 Concentration mg/L Pb Cr Crude brine ND ND Neutralized brine <0.005 0.129 Pure brine <0.005 <0.005

Example 2

(9) At first, brine was prepared on a vacuum belt filter according to steps (a) and (b), the thus obtained crude brine having a pH of 12.07 and a conductivity of 136 mS/cm. By blowing in CO.sub.2, the crude brine was neutralized to a pH of 8.13, thus causing the sedimentation of a white precipitate, which was removed after 5 hours of sedimentation so as to obtain a neutralized brine. The neutralized brine was subjected to an electrocoagulation at an electric voltage of 4.0 V using Fe and Al plates. The flocculate was filtered in a filter press so as to obtain a pure brine. By the described method, the contents of heavy metals could be lowered as follows.

(10) TABLE-US-00002 Concentrate mg/L Pb Cr Sb Cd Zn Cu Crude brine 32.2 1.11 ND ND ND ND Neutral. brine 1.01 1.08 2.58 0.0154 0.0374 0.0368 Pure brine <0.005 <0.005 <0.005 <0.0005 0.0111 0.0154

Example 3

(11) At first, brine was prepared on a vacuum belt filter according to steps (a) and (b), the thus obtained crude brine having a pH of 11.01 and a conductivity of 102 mS/cm. By blowing in CO.sub.2, the crude brine was neutralized to a pH of 8.29, thus causing the sedimentation of a white precipitate, which was removed after 3 hours of sedimentation so as to obtain a neutralized brine. The neutralized brine was subjected to an electrocoagulation at an electric voltage of 3.8 V using Fe and Al plates. The flocculate was filtered in a filter press so as to obtain a pure brine. By the described method, the contents of Pb and Cr could be lowered as follows.

(12) TABLE-US-00003 Concentration mg/L Pb Cr Crude brine ND ND Neutralized brine 1.107 ND Pure brine 0.101 <0.005

Example 4

(13) At first, brine was prepared on a vacuum belt filter according to steps (a) and (b), the thus obtained crude brine having a pH of 12.24 and a conductivity of 198 mS/cm. By blowing in CO.sub.2, the crude brine was neutralized to a pH of 8.54, thus causing the sedimentation of a white precipitate, which was removed after 3 hours of sedimentation so as to obtain a neutralized brine. The neutralized brine was subjected to an electrocoagulation at an electric voltage of 4.0 V using Fe and Al plates. The flocculate was filtered in a filter press so as to obtain a pure brine. By the described method, the contents of Pb and Cr could be lowered as follows.

(14) TABLE-US-00004 Concentration mg/L Pb Cr Crude brine ND ND Neutralized brine 3.07 0.02 Pure brine 0.719 <0.005