JP7275618B2 - Method for treating slurry containing heavy metals - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for treating heavy metal-containing slurry produced by treating flue gas desulfurization effluent, and more particularly to a method for treating heavy metal-containing slurry containing at least cadmium as a heavy metal.

As an agent for preventing the elution of heavy metals from sludge generated when wastewater containing heavy metals generated in factories, etc. is treated, Patent Document 1 discloses at least one kind of water-soluble metal selected from iron, manganese and aluminum. A drug comprising a mixture obtained by powder-mixing a material containing an acid metal salt, a poorly water-soluble basic compound of an alkali metal and/or an alkaline earth metal, and water is described.

The water-soluble acidic metal salt is at least one selected from iron chloride, iron nitrate, iron sulfate, manganese chloride, manganese sulfate, aluminum chloride and aluminum sulfate, and the poorly water-soluble basic compound is calcium oxide and calcium hydroxide. , calcium silicate, magnesium oxide, magnesium hydroxide, sodium silicate, potassium silicate, silicate glass, steelmaking slag and cement.

JP 2015-81270 A

The heavy metal insolubilizing agent of Patent Document 1 is a powder mixture in which a plurality of metal salts and the like are mixed, and thus production is troublesome. Moreover, when adding an insolubilizing agent to the dehydrated sludge, a mixing device is required to uniformly mix the dehydrated sludge and the insolubilizing agent.

An object of the present invention is to provide a method for treating a heavy metal-containing slurry that can prevent cadmium elution from the heavy metal-containing sludge and can be easily carried out.

A method for treating a heavy metal-containing slurry of the present invention has a step of adding magnesium oxide to a heavy metal-containing slurry containing cadmium.

In one aspect of the present invention, the heavy metal-containing slurry is a slurry produced by subjecting flue gas desulfurization effluent from a thermal power plant to flocculation treatment and further to precipitation treatment.

In one aspect of the invention, the magnesium oxide is mixed in a slurry settler or thickener.

In one aspect of the present invention, the amount of magnesium oxide added to the heavy metal-containing slurry is 0.1 to 10%, particularly 0.2 to 3% in terms of solid content weight ratio.

In the present invention, leaching of cadmium from sludge is prevented by adding magnesium oxide as an insolubilizing agent to the heavy metal-containing slurry. The mechanism by which the addition of magnesium oxide prevents cadmium from eluting from the sludge is presumed to be due to the pH buffering action of magnesium oxide.

In one aspect of the invention, the magnesium oxide and heavy metal-containing slurry are mixed in a settling tank or a thickening tank. For example, magnesium oxide is added to and mixed with the settling tank or thickening tank of the heavy metal-containing slurry. This eliminates the need for a dedicated device for mixing the sludge and the insolubilizer.

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows an example of this invention method.

In the present invention, a heavy metal-containing slurry containing at least cadmium as a heavy metal is treated. Examples of such heavy metal-containing slurries include slurries produced by flocculating flue gas desulfurization effluent from thermal power plants using inorganic flocculants and polymer flocculants; Heavy metal-containing slurries produced by flocculating waste water containing heavy metals discharged from plating plants, incineration disposal sites, etc. are exemplified.

It should be noted that flue gas desulfurization effluent from thermal power plants often contains about 0.01 to 2 mg/L of cadmium, depending on the type of coal. When this thermal power plant flue gas desulfurization effluent is coagulated, coagulated water containing cadmium-containing sludge is produced.

The coagulated water containing this sludge is sedimented and separated in a sedimentation tank, and further concentrated in a thickening tank as necessary. The present invention prevents the elution of cadmium from such precipitated sludge or thickened sludge by adding magnesium oxide.

In the present invention, magnesium oxide is used as a cadmium-insolubilizing agent added to the cadmium-containing slurry. Magnesium oxide is preferably in the form of powder having an average particle size (according to JIS sieve, hereinafter the same) of 200 μm or less, for example, about 5 to 200 μm.

Only magnesium oxide may be added to the cadmium-containing slurry, or an insolubilizer containing quick lime, slaked lime, calcium carbonate, magnesium carbonate, magnesium hydroxide, etc. may be added in addition to magnesium oxide.

The amount of magnesium oxide added to the heavy metal-containing slurry is 0.1 to 10 parts by weight, particularly 0.2 to 3 parts by weight (0.2 to 3 parts by weight in solid content weight ratio) per 100 parts by weight of the solid content (dry weight) in the heavy metal-containing slurry. 0.1 to 10%, particularly 0.2 to 3%) is preferred.

In the present invention magnesium oxide is added to the clarifier or thickener, preferably at the inflow of the clarifier or thickener. When the insolubilizing agent is added to the sedimentation tank or thickening tank in this way, the added magnesium oxide is sufficiently mixed with the sludge while staying in the sedimentation tank or thickening tank. A dedicated mixing means for mixing is not required.

FIG. 1 shows an example of adding magnesium oxide to a thickener (thickener).

The thickening tank 1 includes a tank body 2, a cylindrical feedwell 3 vertically provided in the center of the tank body 2, a baffle plate 4 facing the lower end of the feedwell 3, and the tank body 2. It has a rake 5 rotating along the bottom surface, a rotating device 6 for the rake 5, an overflow weir 7 for overflowing supernatant water, a pit 8 for extracting settled sludge from the center of the bottom of the tank body 2, and the like. The bottom surface of the tank body 2 slopes downward toward the pit 8 .

Flue gas desulfurization effluent is subjected to coagulation treatment, and a slurry obtained by precipitation treatment in a sedimentation tank is supplied to the feed well 3 . Magnesium oxide is added to the slurry flowing into this feedwell 3 (or within the feedwell 3 to the slurry flowing into the feedwell 3). The slurry to which magnesium oxide has been added is sedimented and concentrated while staying in the concentration tank 1, and the supernatant water overflows the overflow weir 7 and is taken out. The sedimented and thickened sludge is taken out from the pit 8 through the pipe 9, sent to the dehydrator 12 through the sludge pump 10 and the pipe 11, and dewatered.

The dewatered sludge is transferred to the truck 15 or the like via the conveyor 13 and the cake hopper 14 . The dehydrated filtrate produced by the dehydrator 12 is sent to a general wastewater storage tank or a flue gas desulfurization wastewater treatment device (not shown).

A part of the sludge can be returned to the thickening tank 1 by a pipe 16 branched from the pipe 11 .

Note that FIG. 1 shows an example of the method of the present invention, and the present invention is not limited to this.

<Slurry to be processed>
An inorganic flocculant and a polymer flocculant were added to flue gas desulfurization effluent from a thermal power plant, and the mixture was introduced into a sedimentation tank and precipitated. The solids (dry weight) content of the heavy metal-containing slurry taken out from the bottom of this sedimentation tank was 50 g/L.

[Examples 1 to 6]
The above heavy metal-containing slurry is flowed into the feed well 3 of the concentration tank 1 in FIG. ) was added to the feedwell 3 so that the magnesium oxide concentration in the slurry in the thickening tank 1 was the following concentration (the solid content weight ratio was magnesium oxide to the dry weight of SS (suspended solids) in the slurry represents the weight percent of ).

Example 1: 150 mg / L (solid content weight ratio 0.25%)
Example 2: 300 mg / L (solid content weight ratio 0.5%)
Example 3: 600 mg / L (solid content weight ratio 1%)
Example 4: 1800 mg / L (solid content weight ratio 3%)
Example 5: 3000 mg / L (solid content weight ratio 5%)
Example 6: 6000 mg / L (solid content weight 10%)

The elution value of each heavy metal such as cadmium in the dehydrated sludge obtained by dehydration by the dehydrator 12 was measured according to Environmental Agency Notification No. 13 of 1973. Table 1 shows the results.

In addition, the moisture content of the dewatered sludge and the Cd concentration and pH of the dewatered filtrate were measured, and the results are shown in Table 1.

[Comparative Example 1]
Measurements were carried out in the same manner as in Examples except that no insolubilizing agent was added, and the results are shown in Table 1.

As shown in Table 1, it was confirmed that cadmium was sufficiently insolubilized by adding 300 mg/L or more of the insolubilizing agent. The water content of the dehydrated sludge is almost the same at 60 to 65%, and no effect of the added concentration of the insolubilizing agent is observed.

1 thickening tank 2 tank body 3 feed well 5 rake 7 overflow weir 8 pit 12 dehydrator

Claims (1)

A method for treating a heavy metal-containing slurry comprising adding magnesium oxide to a heavy metal-containing slurry containing cadmium,
The heavy metal-containing slurry is a slurry produced by flocculating flue gas desulfurization wastewater from a thermal power plant and then precipitating it.
The magnesium oxide is mixed in a slurry sedimentation tank or thickening tank, and the amount of magnesium oxide added to the heavy metal-containing slurry is 0.5 to 3 % in solid content weight ratio ,
A method for treating a heavy metal-containing slurry , wherein the dehydrated filtrate of the slurry has a pH of 7.8 to 8.4 .

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