Wastewater treatment processes

Abstract

Wastewater treatment process for precipitating Ni and Al from two different wastewater streams, each containing Al, wherein the first wastewater stream contains relatively high amounts of Ni and the second wastewater stream contains relatively low amounts of Ni, comprising the following process steps: Precipitation of Ni from the inorganic first wastewater stream to produce a nickel-free inorganic first wastewater stream, Introduction of the nickel-free inorganic first wastewater stream into the organic second wastewater stream to generate a third wastewater stream, and precipitation of Al from the third wastewater stream.

Description

[0001] The invention relates to a method for treating wastewater, in particular wastewater from a metal body paint shop, in which in particular the metals nickel and aluminum dissolved in the wastewater are precipitated.

[0002] Nickel must be labeled as toxic according to Regulation (EC) No. 1272 / 2008 (CLP) because it is the most common trigger of contact allergies. Inhalation of inorganic nickel compounds is associated with an increased risk of lung and upper respiratory tract cancer. However, nickel is used in various industrial processes and is used, among other things, as a catalyst for alloys and as corrosion protection. In metal body painting processes, nickel is used, for example, in a phosphating process in which a so-called conversion layer of firmly adhering metal phosphates is formed by treating metal body surfaces with aqueous phosphate solutions. Wastewater from this process is acidic and contains nickel.

[0003] Paint shops that paint metal bodies also generate other wastewater. For example, metal bodies are degreased before painting, which creates organic wastewater that may contain aluminum, among other things, due to the increasing use of aluminum in body construction.

[0004] In the past, it was therefore considered to mix these organic, aluminum-containing wastewaters with the nickel-containing wastewaters from the phosphating process and to process them together.

[0005] Such a process was described, for example, in DE 33 05 802 A1, which proposes a two-stage precipitation process. The precipitation of some heavy metals, such as copper, lead and cadmium, takes place in the alkaline range, while other metals, such as aluminum and chromium, are precipitated in the neutral range. Aluminum and chromium also have an amphoteric character and redissolve at a higher pH value. In the first precipitation stage, the dissolved metals are precipitated in a pH range of approximately 10 to 10.5. The second precipitation stage then operates in a pH range of approximately 7 to 8 in order to remove the remaining solids and, above all, the still dissolved metals from the effluent from the first precipitation stage by further flocculation.

[0006] The procedure is simplified in the Fig. 1. Here, a single wastewater stream 11 is treated serially. This is initially achieved by adjusting the pH to 10.5 in a neutralization step 12. After a flocculation and sedimentation step 13, 14, nickel precipitates as sludge 15 and can be reprocessed or is taken to a landfill for disposal. After a second neutralization step 16 and a second sedimentation step 17, other metals, such as aluminum 18, precipitate. These are also reprocessed or are taken to a landfill as sludge for disposal. A final inspection 19 ensures that all heavy metals and other metals have been removed.

[0007] A disadvantage of the process is that a single wastewater stream 11 is treated. If another wastewater source exists that contains similar or different metals, these would have to be treated separately, as there is a legal dilution ban, according to which different wastewaters with different constituents may not be mixed. For example, wastewater containing the heavy metal nickel may not be mixed with nickel-free wastewater, in order to keep the nickel concentration below the legally permissible limit.

[0008] DE 199 21 135 A1 discloses a wastewater treatment process for treating wastewater or rinse water generated during the phosphating of metal surfaces by precipitating the metal ions in an alkaline pH range. The phosphating solution used for this purpose contains Ni ions. In a single process step, the metal ions are precipitated using a base in the alkaline pH range, and the resulting precipitates are separated.

[0009] The joint treatment of an organic and an inorganic wastewater stream together with the rinse water from a cathodic dip painting process (KTL process) is also known from the state of the art. Such a process is described in Fig. 2 is shown in simplified form. Here, the rinse water from the KTL process 21, which contains aluminum and has a pH value of approximately 6.0, is first coagulated separately in two stages 22 and brought to a pH value of 9 to 10. The separated mixture of a nickel-containing inorganic wastewater 23, e.g. from a phosphating process, together or separately with an aluminum-containing organic wastewater 24, e.g. rinse water After degreasing, nickel from a metal body paint shop is then added to this coagulated wastewater stream (25). The pH of this new mixture is then increased to approximately 10.5. After a flocculation and sedimentation step 26, 27, nickel precipitates as sludge 28. This is separated and can then be reprocessed or sent to a landfill as sludge. This is followed by neutralization 29 to reduce the pH to 7-9. Aluminum, for example, is precipitated at a pH of 7.5. A final inspection 30 ensures that all heavy metals and other metals have been removed.

[0010] The disadvantage of this wastewater treatment process is that the entire volume of wastewater must be treated for nickel, even though not all wastewater sources contain nickel. However, mixing wastewater from different sources, one of which contains nickel, would violate the dilution ban under the WHG. The wastewater treatment process according to the Fig. 2 is not very economical because large quantities of neutralising agents and flocculants have to be used to carry out the process.

[0011] The object of the present invention is therefore to provide a wastewater treatment process, which is particularly used in a metal body painting process, which does not violate the dilution prohibition and in which the wastewater from various sources containing various heavy metals and metals is effectively treated.

[0012] This object is achieved according to the invention by a wastewater treatment process having the features according to claim 1.

[0013] The wastewater treatment process according to the invention for the precipitation of nickel and aluminum from two different wastewater streams comprises the following process steps: • Precipitation of Ni from the inorganic first wastewater stream to produce a nickel-free inorganic first wastewater stream, • introducing the nickel-free inorganic first wastewater stream into the organic second wastewater stream to produce a third wastewater stream, and • Precipitation of Al from the third wastewater stream.

[0014] Initially, only nickel is precipitated from the first wastewater stream, which contains a relatively high amount of nickel and an undetermined concentration of aluminum. The resulting, now nickel-free first wastewater stream is discharged into the second wastewater stream, which contains an undetermined concentration of aluminum and relatively little nickel—in any case, so little nickel that a reduction of the nickel concentration in the second wastewater stream is not legally required. Finally, the aluminum is precipitated in the third wastewater stream resulting from this mixture of the first and second wastewater streams. Both precipitation process steps are preferably achieved essentially by adjusting an appropriate pH value. Since the first wastewater stream may be mixed with the second wastewater stream after the nickel has been precipitated, both wastewater streams can be combined and freed of aluminum in a single process step.This makes the wastewater treatment plant relatively simple and inexpensive to construct and meets all legal requirements regarding the ban on dilution and the reduction of heavy metal loads.

[0015] Preferably, the two wastewater streams are fed from rinse water from a metal body paint shop. The inorganic first wastewater stream can, for example, be rinse water from a process for phosphating a metal body, and the organic second wastewater stream can be rinse water from cleaning or degreasing a metal body and / or rinse water from the rinsing zone of a cathodic dip painting process for a metal body.

[0016] According to a preferred embodiment of the invention, the nickel is precipitated in the first wastewater stream at a pH of approximately 10.5 and more preferably the aluminum is precipitated in the third wastewater stream at a pH of approximately 7.5, wherein the organic second wastewater stream initially has a pH of below 4.0.

[0017] Mixing the two wastewater streams with a pH above 10.0 and below 4.0 results in a pH of approximately 7.5 in the third wastewater stream without requiring the use of a large amount of pH adjuster. This significantly reduces process costs.

[0018] The organic second wastewater stream is relatively low in nickel, at least so low in nickel that the nickel concentration is below a legal limit and below the nickel concentration in the first wastewater stream before nickel precipitation. This ensures that the dilution ban on discharging the nickel-free first wastewater stream into the nickel-poor second wastewater stream does not apply.

[0019] According to a preferred embodiment, the volume of the inorganic first wastewater stream is not more than half, particularly preferably not more than 1 / 3, of the volume of the organic second wastewater stream.

[0020] Particularly preferably, the pH value in the first wastewater stream is adjusted using caustic soda and milk of lime.

[0021] An embodiment of the invention is described in detail below with reference to the drawings in which: Fig. 1 schematically shows a first wastewater treatment process according to the prior art, Fig. Figure 2 schematically shows a second wastewater treatment process according to the prior art, and Fig. 3 schematically shows a wastewater treatment process according to the present invention.

[0022] The present invention provides a wastewater treatment process for precipitating nickel and aluminum from two different wastewater streams. The process involves precipitating nickel from an inorganic first wastewater stream to thereby produce a nickel-free inorganic first wastewater stream. This nickel-free inorganic first wastewater stream, which still contains aluminum, is then passed into an aluminum-containing second wastewater stream. This creates a third wastewater stream. Aluminum is then precipitated from this third wastewater stream.

[0023] The inorganic first wastewater stream can, for example, be rinse water from a process step for phosphating metal or a metal car body. The inorganic first wastewater stream can also contain zinc and / or manganese. The first wastewater stream can also contain other heavy metals, such as copper, lead, cadmium, or chromium.

[0024] This procedure is in the Fig.3. The inorganic first wastewater stream 41 is, for example, rinse water from a process for phosphating metal, for example a metal car body. This inorganic first wastewater stream initially has, for example, a pH value of 4 to 5. In order to produce the nickel-free inorganic first wastewater stream from this, a two-stage alkalization 42 is first carried out in order to increase the pH value to approximately 10.5. Any alkaline solution known to a person skilled in the art can be used for this purpose. A common example is a combination of milk of lime, for example with 10% CaO and 50% sodium hydroxide solution. Simultaneously or in a separate process step, a flocculant 43, such as 0.05% of the non-ionic flocculant Praestol ®2500 from Demols Inc. is added. By adjusting the pH between 10 and 11, especially around 10.5, nickel precipitates as sludge 45 in a sedimentation step 44. This sludge 45 is separated and can then either be reprocessed or transported to a landfill for disposal. Other complexing agents, such as TMT 15, can also be added. ® from Evonik Performance Materials GmbH. The decantate is nickel-free and is hereinafter referred to as "nickel-free inorganic first wastewater stream." "Nickel-free" means that the nickel-free inorganic first wastewater stream contains less than 0.1 mg / l, in particular less than 0.02 mg / l, in particular less than 0.01 mg / l, and in particular less than 0.005 mg / l of nickel.

[0025] Separate from this is an organic second wastewater stream 46 or several organic second wastewater streams 46, 47. These can consist, for example, of rinse water from the cleaning or degreasing of metal parts 46 and / or rinse water from the rinsing zone of a cathodic dip painting process (KTL process) 47 for a metal car body. Both of these organic second wastewater streams 46, 47 contain some aluminum, but are both low in nickel or nickel-free as defined above. Further treatment to precipitate nickel is not required and therefore does not take place here. The rinse water from the cleaning or degreasing of metal parts 46 typically has a pH of 8 to 9, while the rinse water from the rinsing zone of a cathodic dip painting process (KTL process) 47 has a pH of approximately 6.

[0026] Essential to the present invention is that the already purified, nickel-free inorganic first wastewater stream is introduced into this organic second wastewater stream after its sedimentation 44 or is mixed with it. The volume of the nickel-free inorganic first wastewater stream after sedimentation 44 is typically no more than half, preferably no more than one-third, of the volume of the organic second wastewater stream 46, 47. In this process, a third wastewater stream is formed. This mixing of the second organic wastewater stream 46, 47 with the nickel-free inorganic first wastewater stream after its sedimentation 44 does not violate the dilution prohibition, since nickel has already been removed. Therefore, no dilution takes place in the legal sense.

[0027] This third wastewater stream already has a pH of 6.0 to 8.5 due to the mixing of the first and second wastewater streams and then, if necessary, undergoes a one- or two-stage pH adjustment 48 to bring the pH into a range of 7 to 8, in particular to a pH of approximately 7.5. Any base known to a person skilled in the art can be used for this purpose. For example, the addition of lime is common to correctly adjust the pH. One or more flocculants 49 can also be added to promote the coagulation of colloidal foreign components. Aluminum precipitates as sludge 51 in a sedimentation step 50 and can, among other things, be processed for further industrial use or can be taken to a landfill for disposal. If necessary, the decantate can be subjected to post-neutralization 52.Here, too, a final inspection 53 should ensure that all heavy metals and other metals have been removed.

[0028] An advantage of this process is that aluminum, which is also present in the inorganic first wastewater stream, is removed in a single process step. All wastewaters are mixed before aluminum is precipitated. The process according to the invention can be operated both continuously and as a batch process, but a continuous process is preferred.

Claims

[1] Wastewater treatment process for the precipitation of Ni and Al from two different wastewater streams, each containing Al, wherein the first wastewater stream contains relatively high levels of Ni and the second wastewater stream contains relatively low levels of Ni, comprising the process steps: Precipitation of Ni from the inorganic first wastewater stream to produce a nickel-free inorganic first wastewater stream, Introducing the nickel-free inorganic first wastewater stream into the organic second wastewater stream to thereby produce a third wastewater stream, and precipitating Al from the third wastewater stream. [2] Water treatment process according to claim 1, wherein the inorganic first wastewater stream is rinse water from a process for phosphating a metal body and the organic second wastewater stream is rinse water from a cleaning or degreasing of a metal body and / or rinse water from the rinsing zone of a cathodic dip painting process of a metal body. [3] A wastewater treatment process according to claim 1 or 2, wherein Ni in the first inorganic wastewater stream is precipitated from the wastewater stream at a pH of about 10.

5. [4] Wastewater treatment process according to claim 3, wherein the pH value of about 10.5 in the first inorganic wastewater stream is adjusted by means of caustic soda and milk of lime. [5] Wastewater treatment process according to one or more of claims 1 to 4, wherein Al precipitates in the third wastewater stream at a pH of about 7.5, which is approximately established from the introduction of the first wastewater stream into the second wastewater stream. [6] Wastewater treatment process according to one or more of claims 1 to 5, wherein the volume of the inorganic first wastewater stream is not more than half, preferably not more than one third, of the volume of the organic second wastewater stream.

Images