METHOD FOR THE TREATMENT OF SALT-CONTAINING DUSTS

Description

Technical field

[0001] The present invention relates to a method for treating salt-containing dusts that arise during the operation of industrial plants, e.g. in waste incineration plants or during the operation of rotary kilns, e.g. in cement production plants or clinker production plants. background

[0002] The operation of industrial plants often generates large quantities of saline dust, the utilization of which is receiving increasing attention in light of foreseeable raw material shortages and growing sustainability efforts. Even when landfilling these materials is considered, their composition can significantly influence the associated costs.

[0003] EP 2 975 009 A describes a process for the treatment and utilization of bypass dusts and bypass gases from an industrial plant comprising one or more rotary kilns.

[0004] CN 113 353 956 A describes a system and process for desalinating fly ash by water washing and for co-utilizing it in a cement kiln. The system essentially comprises a fly ash elution unit, an eluent purification unit, a drying unit, an evaporation salt recovery unit, and a calcination unit for feeding into the kiln; the eluent outlet of the fly ash elution unit is connected to the inlet of the eluent purification unit; an outlet of the eluent purification unit is connected to an inlet of the evaporation salt recovery unit; an outlet for eluted ash of the fly ash elution unit is connected to the drying unit; and an outlet of the drying unit is connected to the calcination unit for kiln entry.Chlorine salt in fly ash is removed by a special multi-stage counter-current rinsing technology and industrial salt is produced; the eluted ash is calcined in a cement kiln.

[0005] US 2018 / 148374 A1 describes a process for treating and utilizing bypass dust from a cement manufacturing process. This process includes: a) contacting the bypass dust with an aqueous phase and mixing them to obtain a suspension, dissolving water-soluble components of the bypass dust in the aqueous phase; b) performing a solid-liquid separation, in particular vacuum filtration or filter press filtration, to separate the solids contained in the suspension, leaving a brine; c) precipitating some of the heavy metals and, optionally, calcium present in the brine and separating the precipitate from the brine; and d) subjecting the brine to electrocoagulation, separating a flocculation product containing the heavy metals remaining in the brine.

[0006] WO 2012 / 142638 A1 describes a process for the treatment and utilization of bypass dust from the cement manufacturing process, in which the following steps are taken: a) bringing the bypass dust into contact with an aqueous phase and mixing them to obtain a homogeneous slurry, whereby water-soluble components of the bypass dust are dissolved in the aqueous phase, b) carrying out a solid-liquid separation to separate the solids contained in the slurry, leaving a brine, c) separating heavy metals present in the brine and precipitating calcium as sparingly soluble calcium salts CaCO3 to obtain a treated brine, d) subjecting the treated brine to fractional crystallization. Purpose and objectives of the invention

[0007] Against this background, the inventors aimed to develop a process for recovering raw materials from salt-containing dust. This process is intended to return the highest possible quantity of material to the economic cycle. At the same time, the recovered material should be enhanced by the process, as it can be provided in the highest possible purity. Description of the illustrations

[0008] Figure 1Figure 1 schematically shows a three-stage embodiment of the multi-stage arrangement used for contacting the saline dusts with an aqueous phase in step a) of the process according to the invention. The embodiment comprises three mixing apparatuses and three separation devices. The saline dusts are fed to the mixing apparatus of the first stage, and the aqueous phase is fed to the mixing apparatus of the third stage. The solid discharge from the separation device of the third stage represents the treated dusts, and the aqueous discharge from the separation device of the first stage is fed to step b) of the process.

[0009] Figure 2 shows an exemplary representation of the method according to the invention: "Washing process" - Step a) "Purification of the brine" - Step b) "Salt crystallization & drying" - Step c) Summary of the invention

[0010] The formulated problems are solved by the method according to the invention: A method for treating saline dusts, comprising the following steps: a) forming an aqueous solution by contacting saline dusts with an aqueous phase; b) removing heavy metals from the aqueous solution; and c) separating alkali metal chlorides from the aqueous solution, the contact of saline dusts with an aqueous phase in step a) being achieved by means of a multi-stage arrangement through which the saline dusts and the aqueous phase pass in opposite directions, and the ratio of the volume of aqueous phase used to the mass of saline dust used in step a) being in the range of 0.8 L / kg to 1.4 L / kg, preferably from 1.0 L / kg to 1.4 L / kg.

[0011] The process allows for the efficient removal and recovery of salts, particularly sodium chloride (NaCl) and potassium chloride (KCl), from saline dusts. At the same time, the heavy metal content of the dusts is reduced, opening up new applications for the dusts. Thus, the process enables the recovery of several marketable materials from materials that would otherwise require costly disposal. Description of the invention

[0012] The inventive method for treating saline dusts comprises several steps. In step a), an aqueous solution is formed by bringing saline dusts into contact with an aqueous phase. This contact of the saline dusts with the aqueous phase is achieved by means of a multi-stage arrangement through which the saline dusts and the aqueous phase pass in opposite directions.

[0013] In step b), heavy metals are removed from the aqueous solution. And in step c), alkali metal chlorides are separated from the aqueous solution.

[0014] The inventive method for treating salt-containing dusts is fundamentally suitable for use in all plants and situations where dusts containing water-soluble salts are generated, e.g. in waste incineration plants or during the operation of rotary kilns.

[0015] However, for reasons of energy efficiency, it is preferred to use the inventive method in industrial plants in which the exhaust gas and / or waste streams carry excess heat, e.g. plants for cement production, brick production or clinker production. Step a)

[0016] In the process according to the invention, the salt-containing dusts are brought into contact with an aqueous phase in step a). This is done by means of a multi-stage arrangement through which the salt-containing dusts and the aqueous phase pass in opposite directions.

[0017] This can be achieved, for example, by using a mixing apparatus in each stage of the multi-stage arrangement to mix a solid and an aqueous phase, as well as a separation device fed by the effluent of the mixing apparatus to separate the mixture into a solid and an aqueous phase (see Figure 1 ).

[0018] The mixing apparatus could, for example, be a stirring vessel.

[0019] The separation device can be, for example, a belt filter, a centrifuge, or a filter press. Centrifuges, and especially decanter centrifuges, are preferably used. The use of centrifuges offers the advantage over filters that the potential for material to cake onto the filter and other parts of the system due to precipitation and splashing does not occur. This increases the maintenance interval of the system.

[0020] The aqueous phase used in step a) can be, for example, fresh water, salt water, or process water. Specifically, it can be demineralized water, tap water, drinking water, or process water (i.e., water that is not of drinking water quality). The use of process water is economically advantageous. To reduce the water consumption of the process, water vapor generated in step c) can be collected using a fume hood, condensed, and used as the aqueous phase. If water-soluble components of the aqueous solution in step c) are partially or completely separated by fractional crystallization, the resulting mother liquor can also be used as the aqueous phase.In such a procedure, it is preferred that the temperature during fractional crystallization is lower than the temperature during step a), so that efficient dissolution of water-soluble components in the aqueous phase can be ensured.

[0021] Furthermore, the aqueous phase can contain one or more auxiliary substances to facilitate the treatment of saline dusts and to support the cleaning effect or the absorption of volatile components. In this context, it is possible, for example, to add one or more auxiliary substances to the aqueous phase, selected from inorganic substances (preferably chlorides, nitrates, sulfides and sulfates of alkali and alkaline earth metals as well as ammonium polysulfide) and organic substances (preferably salts of chelating acids, such as Na₂-EDTA). The organic and inorganic substances also include acids, such as mineral acids (e.g., HCl, H2SO4, H3PO4, etc.) and / or organic acids (e.g., formic acid, acetic acid), and bases, such as inorganic bases (e.g., NaOH, KOH, LiOH, Ca(OH)2) and / or organic bases (e.g., nitrogen-containing organic bases, such as triethylamine and pyridine).

[0022] For example, calcium chloride or hydrogen chloride (in gaseous form or as an aqueous hydrochloric acid solution) can be added to the aqueous phase as an additive to precipitate sulfate ions. This can increase the purity of the potassium chloride obtained in step c).

[0023] The addition of organic salts or acids to the aqueous phase can improve the solubility of metals, as they then become complexed in solution and are thus removed from the salt-containing dusts. This is beneficial for reducing heavy metal contamination, both if the treated dusts are disposed of and if they are recycled. In the latter case, the metal content of the final product can be reduced.

[0024] When organic or inorganic additives are added to the aqueous phase, the concentration of these organic and / or inorganic substances in the aqueous phase is preferably 0.01–10 wt.%, more preferably 0.1–5 wt.%, and most preferably 0.5–3 wt.%. For the most economical process operation, the use of water (tap or process water) as the aqueous phase is preferred, whereas the use of an aqueous phase containing one or more of the aforementioned additives may be preferred for the reasons stated above, for better washing out of soluble substances or for removing unwanted substances. The additives can be added to the aqueous phase before contact with the salt-containing dusts or dosed in at one of the stages of the multi-stage arrangement.

[0025] Suitable auxiliary substances can include, for example, inorganic substances, particularly salts such as chlorides, nitrates, sulfides, or sulfates of alkali and alkaline earth metals, or ammonium polysulfide. The aqueous phase can also contain organic substances, such as chelating acids like EDTA. The latter increase the solubility of the heavy metal ions in the aqueous solution and thus facilitate their removal from the salt-containing dusts.

[0026] In a preferred embodiment of the method, a multi-stage countercurrent cascade of centrifuges, preferably decanter centrifuges, is used for step a), wherein the saline dusts, as well as the aqueous effluent from the centrifuge belonging to the last stage, are fed to the mixing apparatus belonging to the first stage; water as an aqueous phase is fed to the mixing apparatus belonging to the last stage, with the exception of the last stage; the solid effluent from a centrifuge is fed to the mixing apparatus belonging to the next stage, with the exception of the first stage; the aqueous effluent from a centrifuge is fed to the mixing apparatus belonging to the preceding stage; and the aqueous effluent from the centrifuge belonging to the first stage serves as the starting material for step b).

[0027] The aqueous solution obtained in step a) contains the soluble components of the salt-containing dusts in dissolved form and is fed to step b) described below for further treatment.

[0028] The water-insoluble components form the fixed sequence of the multi-stage process. These treated, low-salt dusts can be further utilized. For example, they can be returned to the manufacturing process that originally generated the saline dust to be treated.

[0029] From the standpoint of minimizing the required amount of water while simultaneously maximizing the amount of salts removed from the saline dust, the multi-stage arrangement used in step a) preferably comprises 2 to 5 stages, and particularly preferably 3 or 4 stages. Furthermore, a portion of the aqueous phase separated in a stage can be fed back to the feed of the same stage; for example, a portion of the aqueous effluent from a centrifuge can be returned to the mixing apparatus belonging to the same stage. The more stages used, the less water is required for a given washing performance.

[0030] Furthermore, the ratio of the volume of aqueous phase used to the mass of salt-containing dust used in step a) is, from the point of view of minimizing the required amount of water while simultaneously maximizing the amount of salts, in the range of 0.8 L / kg to 1.4 L / kg, preferably from 1.0 L / kg to 1.4 L / kg.

[0031] Using a larger quantity of aqueous phase relative to the mass of salt-containing dust used makes it possible to dissolve more salt, but this also potentially increases the amount of water that needs to be evaporated in step c).

[0032] In the present application, the term refers to "water-soluble components" on inorganic or organic substances that are dissolved in the aqueous phase in step a) and thus become components of the aqueous solution. "water-insoluble components"In contrast, amorphous or crystalline solids do not dissolve during step a) and can be separated from the aqueous solution by filtration or centrifugation. Water-soluble and water-insoluble components can, for example, be constituents of the saline dust. However, the water-soluble and water-insoluble components can also include chemical reaction products of the saline dust with the aqueous phase.

[0033] By bringing the saline dust into contact with the aqueous phase, the water-soluble components dissolve in the aqueous phase. The water-insoluble components are preferably separated, e.g., by filtration or centrifugation. The separated water-insoluble components (e.g., the resulting filter cake) exhibit a significantly reduced concentration of alkali and alkaline earth salts (chlorides, sulfates, etc.) as well as heavy metals (e.g., As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Sn, Te, Tl, V). Thus, if desired, the separated water-insoluble components (e.g., the filter cake in the case of filtration) can be reintroduced into the process carried out in the industrial plant that originally generated the saline dust to be treated, e.g., a cement or...The clinker manufacturing process can be recycled without enriching the industrial plant's product with these undesirable substances. The separated water-insoluble components can also undergo more intensive purification, for example, by extraction, to further reduce the content of unwanted components. On a small scale, such extraction can be carried out, for example, using Soxleth extraction. On an industrial scale, such extraction can be performed, for example, with a mixer-settler.

[0034] The water-soluble components that dissolve in the aqueous phase in step a) consist mainly of chlorides and other volatile components, such as sulfates (or SO 3 ), but may also contain other components, such as heavy metals or nitrates.

[0035] The water-insoluble components, i.e., the treated dusts, have a significantly reduced chloride content.

[0036] Depending on the ratio of the volume of aqueous phase used to the mass of saline dust used, the chloride content of the water-insoluble components can be reduced to less than 20 wt.%, preferably less than 10 wt.%, more preferably less than 5 wt.%, and most preferably less than 2 wt.% of the chloride content of the dusts before introduction through the aqueous phase (wt.%), i.e., of the chloride content of the starting materials, after contact with the aqueous phase in step a) and their separation. In light of this, the treated, low-salt dust is suitable for reducing the chloride content of the process streams when recycled into an industrial process.

[0037] The multi-stage solid-liquid separation described above offers the advantage of achieving high washing performance with low water consumption. Step b)

[0038] In the process according to the invention, heavy metals are removed from the aqueous solution obtained in step a) in step b). Performing this step before recovering alkali and alkaline earth metal salts contained in the saline dusts and dissolved in the aqueous phase upon contact with the saline dusts makes it possible to keep the content of heavy metals or their salts in the products obtained after step c) as low as possible.

[0039] Heavy metals can be precipitated, for example, by adding sulfides, polysulfides, or other anions of sparingly soluble heavy metal salts to the aqueous solution obtained in step a). In this way, it is possible to separate metals such as As, Be, Br, Cd, Cr, Hg, Ni, Pb, Tl, V, and Zn as sparingly soluble salts (e.g., as sulfides). The aforementioned anions are preferably introduced into the aqueous solution in the form of sodium compounds, e.g., Na₂S. Another preferred method is the introduction of gaseous H₂S into the aqueous solution.

[0040] The precipitated heavy metal salts can be separated from the aqueous solution by methods known to those skilled in the art, e.g. by filtration or centrifugation, and then dried.

[0041] Furthermore, the heavy metals can be separated from the aqueous solution and recovered using suitable ion exchange resins. Suitable ion exchange resins are known to those skilled in the art and include, among others, resins with carboxyl or sulfonic acid groups. Corresponding ion exchange resins are commercially available, for example, under the trade names Lewatit® (Lanxess), Dowex® (Dow Chemicals), and Amberlite® (Rohm and Haas).

[0042] However, electrocoagulation has proven to be the most effective method for removing heavy metals.

[0043] The electrocoagulation process is known to those skilled in the art and is described, for example, in WO 2016 / 189374 A1. In this process, a pair of electrodes is inserted into the solution, and the oxidative decomposition of the anode is induced by applying a voltage. The cations emerging from the anode undergo a redox reaction with dissolved heavy metal ions, leading to the formation of a flocculate containing the heavy metals.

[0044] After the heavy metals have flocculated, they can be separated from the aqueous solution by methods known to those skilled in the art, e.g. by filtration or centrifugation, and then dried.

[0045] Due to its high salt content, the aqueous solution obtained in step a) has a high electrical conductivity, which is why the electrocoagulation process is particularly suitable for removing the heavy metal ions from it.

[0046] Preferably, an iron and / or an aluminum electrode is used for electrocoagulation.

[0047] If the salt-containing dusts contain significant amounts of mercury, this can also be separated before step a) by heating the dusts to convert the mercury into the gaseous state and then binding the gaseous mercury using a sulfide, e.g., sodium sulfide. Such a process is known to those skilled in the art and also allows the (selective) separation of other volatile heavy metals. Step c)

[0048] In the process according to the invention, alkali metal chlorides are separated from the aqueous solution in step c).

[0049] Preferably, the separation is carried out by fractional crystallization. The aqueous solution is evaporated to approximately 70% of its volume before step c), preferably to approximately 50% of its volume before step c), and particularly preferably to approximately 30% of its volume before step c). The concentrated aqueous solution thus obtained can then be cooled, resulting in fractional crystallization of the alkali metal chlorides. The alkali metal chlorides crystallize sequentially, exhibiting a particularly high purity and thus being more economically valuable and versatile in their applications. The crystallized alkali metal chlorides can be separated from the aqueous solution by methods known to those skilled in the art, e.g., by filtration, and then dried.

[0050] The process according to the invention thus allows the extraction of high-purity alkali metal chlorides from salt-containing dusts, which are generated, for example, during the operation of rotary kilns, particularly in cement, clinker and brick manufacturing processes.

[0051] The alkali metal chlorides obtained according to the inventive process are primarily sodium chloride and potassium chloride. Because they are obtained by fractional crystallization, they exhibit a high degree of purity and can therefore be used in a variety of applications. These applications include use as road salt, lick blocks, fertilizers, raw materials for electrolysis to produce chlorine and / or alkali hydroxide, flame retardants, dust binders, fertilizer components, and in glass and ceramic production.

[0052] Water vapor produced in step c) can be collected using a fume hood, condensed, and used as the aqueous phase in step a). This also applies to the aqueous phase remaining after crystallization of the alkali metal chlorides.

[0053] Unless otherwise stated, all percentages in this application refer to weight percent. Examples Example 1:

[0054] The following table shows the analysis results of dust before ("bypass dust") and after ("filter cake") passing through the inventive process. As can be seen from the data, the treated dust has a significantly reduced salt content. Table 1 Salt content before and after treatment - material Bypass dust Filter cake Date 22. - 23.05.2021 22. - 25.05.2021 time 02:00 - 02:00 15:30 - 15:00 type Average Average Sample No. 563351 5633350 CO2 (950°C) % 3,38 6,22 Water (950°C) % 2,25 18,73 chloride % 4,63 0,132 Loss on ignition % 5,63 24,95 Si02 gvh % 13,51 12,36 Al2O3 gvh % 3,19 2,93 Ti02 gvh % 0,14 0,13 P205 gvh % 0,07 0,05 Fe203 gvh % 1,54 1,48 Manganese oxide gvh % 0,02 0,02 MgO gvh % 0,95 1,01 CaO gvh % 48,15 46,51 SO3 gvh % 8,35 8,07 K20 gvh % 6,92 1,15 Na20 gvh % 0,44 0,15 Na20 eq. gvh % 4,99 0,91 Example 2

[0055] The graphic of Figure 3 Figure 1 shows the water content of an aqueous solution obtained according to step a) of the process according to the invention when using different numbers of separation stages. As can be seen, the salt concentration of the aqueous solution obtained increases with an increasing number of separation stages. Example 3

[0056] The following table shows the results of an experiment using electrocoagulation to remove heavy metals from the aqueous solution obtained in step a). These are the analyses of two average samples, each mixed from 11 individual samples. Table 2 - Removal of heavy metals by electrocoagulation component Unit concentration Change in [%] before SM felling after SM felling chloride [g / 1] 72,7 69,1 sulfate [g / 1] 1,6 1,8 Calcium [g / 1] 10,1 9,19 potassium [g / 1] 59 54,1 sodium [g / 1] 2,73 2,49 antimony [µg / 1] 0,337 0,792 135 0 arsenic [µg / 1] 0,682 0,333 -51,2 beryllium [µg / 1] 1,1 1,22 10,9 Lead [µg / 1] 19.800 10,8 -99,9 cadmium [µg / 1] 0,153 0,163 6,5 chrome [µg / 1] 250 5,11 -98,0 cobalt [µg / 1] 1,69 2,61 54,4 copper [µg / 1] 0,572 0,448 -21,7 manganese [µg / 1] 1,26 3.060 made from Fe electrode nickel [µg / 1] 5,14 15 192 mercury [µg / 1] < 0,2 < 0,2 Tellur [µg / 1] 0,121 0,16 32,2 Thallium [µg / 1] 319 77 -75,9 Vanadium [µg / 1] 3,93 3,3 -16,0 zinc [µg / 1] 40,1 17,2 -57,1 tin [µg / 1] 0,12 1,31 made from Fe electrode

[0057] As the data shows, the elements lead and chromium in particular were almost completely precipitated. The thallium concentration was also significantly reduced.

[0058] For a number of other components, of which only relatively low and therefore non-critical concentrations were measured in the initial solution, the results were inconsistent. This is partly due to the fact that the individual samples were taken simultaneously before and after sulfate precipitation, even though the combined volume of the neutralization tank, reactor, and flotation unit corresponded to a residence time of several hours. Therefore, the samples taken before and after heavy metal precipitation do not exactly reflect the same operating conditions of the experimental setup, resulting in a certain degree of inaccuracy when comparing these data. Furthermore, the very small absolute values ​​lead to very large percentage values, even though the deviations between the measured values ​​are actually within the margin of error of the analysis.

[0059] Particularly with manganese, and probably also with tin, the observed increase during passage through the system is due to the fact that the iron electrodes (Fe electrodes) of the coagulation reactor were alloyed with approximately 1% manganese and small amounts of tin, which are also found in the pure solution. Such an input of these elements can be avoided by switching to lower-alloy anode material.

Claims

1. Method for treating salt-containing dusts, wherein the method comprises the following steps: a) forming an aqueous solution by bringing salt-containing dusts into contact with an aqueous phase; b) removing heavy metals from the aqueous solution; and c) separating alkali metal chlorides from the aqueous solution, salt-containing dusts are brought into contact with an aqueous phase in step a) by means of a multi-stage arrangement through which the salt-containing dusts and the aqueous phase pass in opposite directions, and the ratio of the volume of aqueous phase used to the mass of salt-containing dust used in step a) is in the range of from 0.8 L / kg to 1.4 L / kg, preferably from 1.0 L / kg to 1.4 L / kg.

2. Method according to claim 1, wherein a multi-stage counterflow cascade which comprises a mixing apparatus in each stage as well as a separating device fed by the discharge of the mixing apparatus is used for step a).

3. Method according to claim 2, wherein belt filters, centrifuges or filter presses are used as the separating device.

4. Method according to claim 3, wherein a multi-stage counterflow cascade of centrifuges, preferably decanter centrifuges, is used for step a), and wherein the salt-containing dusts as well as the aqueous discharge of the centrifuge associated with the final stage are supplied to the mixing apparatus associated with the first stage, water is supplied as the aqueous phase to the mixing apparatus associated with the final stage, with the exception of the final stage, the solid discharge of a centrifuge is supplied in each case to the mixing apparatus associated with the next stage, with the exception of the first stage, the aqueous discharge of a centrifuge is supplied in each case to the mixing apparatus associated with the previous stage, and the aqueous discharge of the centrifuge associated with the first stage serves as the starting material for step b).

5. Method according to any of claims 2 to 4, wherein the mixing apparatus is an impeller-type mixer.

6. Method according to any of claims 2 to 5, wherein the multi-stage arrangement in step a) comprises 2 to 5 stages, preferably 3 or 4 stages.

7. Method according to any of claims 1 to 6, wherein electrocoagulation is used to remove the heavy metals.

8. Method according to any of claims 1 to 7, wherein the alkali metal chlorides are separated in step c) by means of fractionated crystallisation.

9. Method according to any of claims 1 to 8, wherein the alkali metal chlorides are at least one selected from the group comprising sodium chloride (NaCl) and potassium chloride (KCl).

10. Method according to any of claims 1 to 9, wherein the aqueous phase contains at least one auxiliary substance which is selected from inorganic substances, preferably ammonium polysulfide and chlorides, nitrates, sulfides and sulfates of the alkali and alkaline earth metals, and organic substances, preferably salts of chelating acids, such as EDTA.

11. Method according to any of claims 1 to 10, wherein the aqueous solution is evaporated during the fractionated crystallisation to approx. 70% of the volume prior to step c), preferably to approx. 50% of the volume prior to step c), particularly preferably to approx. 30% of the volume prior to step c).

12. Method according to any of claims 1 to 11, wherein the heavy metals are at least one selected from the group comprising As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Sn, Te, Tl and V.

13. Method according to any of claims 4 to 12, wherein some of the aqueous discharge of a centrifuge is fed back to the mixing apparatus associated with the same stage.

14. Method according to any of claims 1 to 13, wherein an additive is added in the first stage of the multi-stage arrangement for precipitating sulfate ions.