JP6015540B2 - Method for treating effluent containing cadmium - Google Patents

Description

  The present invention relates to a method for treating an effluent containing cadmium discharged from a production process of a zinc-containing ore such as zinc oxide.

  Conventionally, zinc oxide ore obtained by separating and recovering impurities from crude zinc oxide or the like has been widely used as a raw material for zinc bullion in a zinc smelter. Crude zinc oxide can be obtained, for example, from steel dust generated from a steelmaking furnace such as a blast furnace or an electric furnace in the steel industry through a reduction roasting process. From the viewpoint of promoting resource recycling, it is desirable to reuse steel dust as a zinc raw material. However, on the other hand, the crude zinc oxide derived from such steel dust contains a high proportion of halogen components such as chlorine and fluorine and impurities such as cadmium in addition to the main component zinc oxide.

  In a zinc oxide production plant, it is essential to separate and recover cadmium from the effluent during the process of separating and collecting the impurities.

  As a method for treating such an effluent, a method in which metal zinc powder is introduced into the effluent and cadmium is precipitated and separated and recovered by a cementation reaction is generally performed (see Patent Document 1). ).

Japanese Patent Laid-Open No. 9-137236

  However, in the method described in Patent Document 1, in order to reliably reduce the cadmium concentration to a very low concentration below the environmental standard value, an amount of metal zinc powder approximately twice the theoretically required chemical equivalent is added. It was. This metal zinc powder is expensive and complicated to prevent oxidation. Therefore, in manufacturing sites, etc., when separating and recovering cadmium from the effluent discharged from the wet process, etc. There has been a demand for a reduction in the amount of zinc powder used.

  The present invention relates to a method for treating an effluent containing cadmium discharged from a production process of a zinc-containing ore such as zinc oxide, and a effluent that can sufficiently separate and recover cadmium at a lower cost than conventional methods. An object is to provide a processing method.

  The present inventors perform a first cementation process in which a preliminary cementation process is performed by immersing a zinc plate in the effluent to treat the effluent containing cadmium discharged from the production process of the zinc-containing ore. And the second cementation process that is the same as the conventional cementation process performed after the first cementation process, the zinc consumed for the separation and recovery of cadmium is obtained. The present inventors have found that the total amount can be greatly reduced and have completed the present invention. More specifically, the present invention provides the following.

  (1) A method for treating an effluent containing cadmium discharged from a production process of a zinc-containing ore, wherein the cadmium in the effluent is subjected to a first cementation process and the first cementation process. A second cementation process performed on the discharged liquid, and a two-stage cementation process that separates and collects the first cementation process by immersing a zinc plate in the discharged liquid. A method for treating a discharged liquid, characterized in that the cadmium is deposited.

  (2) The processing method of the effluent according to (1), wherein the zinc plate has a zinc purity of 95% or more.

  According to the present invention, as a method for treating an effluent containing cadmium discharged from a production process of zinc-containing ore such as zinc oxide, discharge that can sufficiently separate and recover cadmium at a lower cost than conventional methods. A liquid processing method can be provided.

It is a flowchart which shows the whole process of zinc oxide ore manufacture which is an example of the whole process including the process of processing the waste liquid containing a cadmium by the processing method of the waste liquid of this invention. It is the schematic diagram which showed the outline | summary of the apparatus for performing the processing method of the discharge liquid of this invention.

  Hereinafter, as a specific example of the production process of zinc-containing ore to which the present invention can be preferably applied, an overall process related to the production of zinc oxide ore is given, and an embodiment in which the method for treating an effluent of the present invention is applied to the process This will be described as a preferred embodiment of the present invention. However, the present invention is not limited to the following embodiments, and can be widely applied to other zinc-containing ore production processes including a step of treating an effluent containing cadmium.

<Overall process>
As shown in FIG. 1, the overall process for producing zinc oxide ore according to this embodiment includes the reduction roasting step S10 for obtaining roasted zinc oxide by reducing and roasting steel dust, and the rough oxidation obtained in the reduction roasting step S10. Wet process S20 for separating and recovering cadmium and the like from zinc to obtain a crude zinc oxide cake, drying heating process S30 for drying and heating the zinc oxide cake obtained in wet process S20 to obtain zinc oxide ore, and drying heating process S30 This is a process comprising an exhaust gas treatment step S40 for performing a solid-gas separation process for separating the exhaust gas discharged from the solid into a gas and a waste water treatment step S50 for separating the exhaust liquid discharged from the wet step S20.

  The treatment method of the effluent according to the present invention is a effluent containing cadmium by treating the effluent in the waste water treatment step S50 in the overall process as a method of performing a cementation process in two stages unique to the present application. Therefore, the cadmium can be separated and recovered at a cost lower than that of the conventional method and at a high recovery rate equivalent to that of the conventional method.

<Reduction roasting process>
As a specific method for performing the reduction roasting step S10 for recovering crude zinc oxide from steel dust, a reduction roasting method using a reduction roasting rotary kiln (RRK) is generally employed. Steel dust is reduced and roasted in the kiln, and the volatilized metal zinc is reoxidized in the exhaust gas to form powdery zinc oxide. Powdered zinc oxide is introduced into the dust collector together with the exhaust gas from the rotary kiln, captured, and recovered as crude zinc oxide.

<Wet process>
In the wet process S20, the crude zinc oxide recovered through the reduction roasting process S10 is repulped with a treatment liquid to obtain a crude zinc oxide slurry. By this treatment, cadmium or the like contained in the crude zinc oxide is distributed in the treatment liquid. As the treatment liquid used as the repulp water, a solution obtained by dissolving NaCO ₃ or Na (OH) ₂ in fresh water can be used.

  By the above treatment, cadmium in the crude zinc oxide is removed in the treatment liquid. In this state, the treatment liquid in which the impurities are distributed is separated by solid-liquid separation to obtain a discharged liquid. In addition, about the dehydration process for solid-liquid separation, moisture forced dehydration apparatuses, such as gravity sedimentation type slurry concentration apparatuses, such as a thickener, and a vacuum dehydrator can be used. As a general standard, the cadmium content in zinc oxide ore, which is a raw material for zinc smelting, is required to be less than 0.1%.

  The zinc oxide slurry from which impurities have been sufficiently removed through the above treatment is dehydrated by a vacuum suction type dehydrator or the like to form a zinc oxide cake, and then is supplied to the subsequent drying and heating step S30.

<Dry heating process>
A zinc oxide ore having a further reduced concentration of cadmium and the like is produced by a drying heating step S30 in which the crude zinc oxide cake obtained in the wet step S20 is charged into a drying heating apparatus such as a drying heating rotary kiln (DRK) and baked. be able to.

<Exhaust gas treatment process>
In the exhaust gas treatment step S40, solid gas separation processing of the exhaust gas discharged from the DRK in the drying heating step S30 is performed.

<Wastewater treatment process>
The wastewater treatment step S50 separates and recovers impurities such as cadmium from the waste liquid containing cadmium and the like separated from the crude zinc oxide in the wet step S20, and further precipitates heavy metals contained in a small amount in the waste liquid by neutralization treatment. This is a process of finally adjusting the pH to produce harmless waste water. The drainage treatment method of the present application can be preferably used as a method for separating and collecting cadmium in the wastewater treatment step S50.

[Discharge liquid treatment method]
Hereinafter, the detail of the processing method of the waste liquid of this invention is demonstrated. In the above overall process, the effluent charged into the wastewater treatment step generally contains about 150 to 300 mg / l of cadmium. As a process for separating and recovering cadmium from the effluent, a method of depositing cadmium by a cementation process using metal zinc powder has been widely used. The treatment method of the effluent of the present application is to perform this cementation process in two stages. In the first cementation process of the first stage, instead of the metal zinc powder widely used in the past, zinc is used. A cementation process is performed by immersing the plate in the effluent. And this zinc plate should just be 95% or more of zinc purity, and it is preferable that it is 98% or more. Therefore, a zinc plate having a quality of a normal product level such as a distilled zinc plate (zinc purity of about 98.5%) that can be obtained at a relatively low cost can be widely used. Thereby, the cost concerning the separation and recovery process of cadmium can be reduced.

  Then, the second cementation process, which is a process using zinc powder as in the prior art, is performed on the discharged liquid whose cadmium concentration has decreased to about 30 mg / l or less after the first cementation process. The cadmium concentration of the effluent that has undergone the second cementation treatment can be sufficiently reduced to 10 mg / l or less that is surely below the wastewater management standard value as the effluent.

  FIG. 2 is an explanatory view schematically showing an effluent treatment apparatus 1 that is an apparatus capable of performing the effluent treatment method of the present invention. As shown in FIG. 2, the discharged liquid treatment apparatus 1 includes a first treatment tank 10 that performs a first cementation process, and a plurality of zinc plates that are arranged in the first treatment tank 10 so as to be immersed in the discharged liquid. 11, a second treatment tank 20 that performs the second cementation process, a discharge liquid input pipe 30, a discharge liquid transfer pipe 40, and a discharge liquid discharge pipe 50.

  The discharged liquid containing cadmium discharged from the wet processing process or the like is discharged from the discharged liquid input pipe 30 in the direction a and is input into the first processing tank 10. In the 1st processing tank 10, the 1st cementation process which is a cementation reaction with an effluent and zinc plate 11 is performed.

  In order to promote cementation processing, a preliminary layer (not shown) is separately provided in addition to the first processing tank 10, and the effluent is appropriately circulated between the first processing tank 10 and the preliminary layer with a pump or the like. It is preferable. In this case, the liquid circulation rate is preferably 0.5 L / second or more, and more preferably 1 L / second or more.

  By said 1st cementation process, a part of cadmium in discharge liquid is deposited, and the cadmium density | concentration of discharge liquid reduces to about 30 mg / l or less.

  The discharged liquid whose cadmium concentration has been reduced to about the above range by the first cementation process is transferred from the first processing tank 10 through the discharged liquid transfer pipe 40 in the direction b and is input to the second processing tank 20. In the 2nd processing tank 20, metal zinc powder is added and the 2nd cementation process which is a cementation process by a conventionally well-known method is performed. At this time, it is common to stir the discharged liquid in the tank by the stirrer 21. By this second cementation treatment, cadmium in the effluent is further precipitated, the cadmium concentration of the effluent is further reduced, and the cadmium concentration is 10 mg / l or less.

  The second process after the second cementation process that has sufficiently reduced the cadmium concentration by the second cementation process is discharged from the second treatment tank through the discharge liquid discharge pipe 50, and the next steps such as neutralization and pH adjustment. Then, the effluent is discharged in a detoxified state.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

(Example)
First, as a first stage treatment, a test cadmium-containing effluent composed of the same components as the effluent from the wet treatment in the crude zinc oxide production process is configured in the same manner as the first treatment tank 10 of the effluent treatment apparatus 1. It was thrown into the 1st processing tank for a test provided, and the separation and recovery of cadmium from the effluent was tried. The cadmium content before treatment in the effluent was 164.875 mg / l. In the first treatment tank for testing, 40 zinc plates of 40 cm × 10 cm × 2 cm (zinc purity 99%, total surface area 39200 cm ² ) were installed. The test cadmium-containing effluent in the test first treatment tank is repeatedly circulated between the test first treatment tank and the auxiliary tank provided along with the circulation pump, and the circulation rate is 1 L / second. It was. Table 1 shows the results of measuring the transition of the cadmium concentration in the effluent with time.

  From the results of Table 1, the cadmium concentration was 30 mg / l or less by the treatment in the first treatment tank, and the cadmium concentration could be reduced to 1/5 or less of the initial value. Therefore, it can be understood that a difference of about 145 mg / l is recovered by the zinc plate, and the amount of metal zinc powder used in the second cementation can be greatly reduced to 1/5 or less. Thus, it has been heretofore unknown that cadmium can be recovered in a short time by preliminary cementation with a zinc plate. After this, the cadmium concentration could be reduced to 10 mg / l or less by performing a second cementation treatment with a conventionally known metal zinc powder equivalent to 2 equivalents.

S10 Reduction roasting process S20 Wet process S30 Drying and heating process S40 Exhaust gas treatment process S50 Wastewater treatment process ST51 First cementation process ST52 Second cementation process 1 Effluent treatment apparatus 10 First treatment tank 11 Zinc plate 20 Second treatment tank 21 Stirrer 30 Drainage input pipe 40 Drainage transfer pipe 50 Drainage discharge pipe

Claims (1)

A method for treating an effluent containing cadmium, which is performed in a wastewater treatment step during the production of a zinc-containing ore,
A two-stage cementation process comprising: a first cementation process for cadmium in the discharged liquid; and a second cementation process using metallic zinc powder that is performed on the discharged liquid that has undergone the first cementation process. Is a processing method of separating and recovering by
The first cementation process is a process of precipitating the cadmium by immersing a distilled zinc plate having a zinc purity of 95% or more and 99.0% or less in the discharge liquid. Processing method.

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