JPS6363277B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating wastewater containing heavy metals. Conventionally, the most well-known method for treating wastewater containing heavy metals is to add alkali to the wastewater to generate and remove heavy metal hydroxides in the wastewater. is easily soluble in water, and the risk of secondary pollution associated with its dumping was extremely high. In order to solve this problem, a method has been proposed to separate and remove heavy metals from wastewater by incorporating them into the crystal lattice of ferrite, a ferromagnetic oxide, and this method has now been put into practical use. In this method, a predetermined amount of ferrous salt is added to the waste liquid containing heavy metals, the liquid is kept alkaline, and an oxidizing gas such as air is blown into the liquid to oxidize the ferrous salt and add heavy metals into the crystal structure. The heavy metal hydroxides produced by this method are difficult to dissolve in water, making it a highly preferred treatment method with little risk of secondary pollution associated with dumping. However, in such a method, it is preferable to carry out the oxidation reaction while the liquid is heated, and therefore, the number of man-hours and expense required for this heating is required.
Note that when the reaction temperature is lowered, it becomes difficult to produce ferrite, the particle size of the ferrite produced becomes small, making solid-liquid separation difficult, and the oxidation time becomes longer, resulting in many practical problems. Therefore, a method has been proposed to improve these disadvantages and treat heavy metals in wastewater at low temperatures, that is, at almost the same temperature as the wastewater. That is, this method produces ferrite with a large particle size at room temperature by gently stirring wastewater to which ferrous salt and alkali have been added in the air. However, this method had a practical problem in that the processing time was extremely long. The object of the present invention is to improve this problem and provide a method for treating heavy metals in wastewater at room temperature in a short time. That is, in the method according to the present invention, at least 0.002 mole of ferrous salt is added to heavy metal-containing wastewater, and an alkali is further added to adjust the pH of the wastewater to 8 to 12. pressure (i.e. 0.2
It is characterized by the ability to generate ferrite precipitates with large particle sizes in a short time at room temperature without heating by gently stirring in an atmosphere higher than atmospheric pressure). Note that the above normal temperature refers to the temperature near the earth's surface.
The temperature is about 40℃ or below and above the freezing point. According to the method of the present invention, ferrous ions are gradually oxidized to ferric ions by stirring, and the generated ferric ions and remaining ferrous ions form ferrite in an alkaline solution. As the reaction continues, newly generated ferrite takes in heavy metal ions from the wastewater using the previously generated ferrite as a core, precipitates on the core ferrite, and gradually grows, eventually increasing the particle size. It is thought that large ferromagnetic precipitates are generated. In addition, in the method of the present invention, the stirring speed affects the properties of the product and the processing time, and in order to produce ferrite, the stirring speed is preferably such that it does not involve oxidizing gas. If it is made larger, the oxidation of Fe ²⁺ ions is accelerated and the treatment time can be shortened, but the particle size of the precipitate becomes smaller and solid-liquid separation of the precipitate becomes difficult. On the other hand, reducing the stirring speed is advantageous in increasing the grain growth of the precipitate and obtaining a highly magnetic precipitate, but at the same time, the processing time becomes longer, which poses a practical problem. However, by increasing the oxygen partial pressure in the atmosphere in which the stirring oxidation is performed, it becomes possible to significantly shorten the processing time while promoting grain growth. Note that increasing the oxygen partial pressure can be easily achieved by using oxygen instead of air or by using pressurized air. The re-elution of heavy metals from the ferromagnetic precipitates produced by implementing the present invention is extremely small, and they can be reused as radio wave absorbing materials, raw materials for magnetic fluids, etc., and are a great way to effectively utilize waste. Provides a favorable closed cycle. The present invention will be explained in more detail with reference to Examples below. Example 0.1 mole of ferrous sulfate was dissolved in heavy metal solution 1 containing approximately 200 ppm each of Cu ²⁺ , Zu ²⁺ , and Ni ²⁺ , and caustic soda was added to maintain the pH at 9.5.
Stirring was continued for 9 hours in the air at 20°C and a rotational speed of approximately 250 rpm to obtain a ferromagnetic precipitate. Further, a similar treatment was performed under pressurized air at 3 atm, and a ferromagnetic precipitate was obtained after 4 hours. A portion of the treated solution was sampled and the remaining metal concentration was examined by atomic absorption spectrophotometry, and the concentrations of Cu, Zu, Ni, and Fe in each solution were below the wastewater standard values. The properties of the precipitate after separation were as shown in the table below.

[Table] As shown above, the treatment time was significantly shortened by increasing the oxygen partial pressure, but no significant difference was observed in the particle size of the precipitate. Even if ferrous salts other than ferrous sulfate shown in the examples are used as the ferrous salts used in the present invention, the effects of the present invention will not change, and they will be used in large quantities as a by-product during pickling of iron plates and the production of titanium oxide. It is considered desirable to use ferrous chloride and ferrous sulfate, which are generated in the process, both economically and from the point of view of effective use of industrial waste.

Claims (1)

[Claims] 1. A method for removing heavy metal ions in wastewater by producing ferrite, characterized in that ferrous salts are oxidized in an atmosphere where the partial pressure of oxygen is higher than that of the atmosphere. Processing method.