JPS5812207B2 - How to recycle waste caustic soda - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention removes silicic acid from so-called waste caustic soda containing silicic acid used to remove sand from cast products and converts it into recycled flakes, and also performs pickling waste acid treatment and acid gas cleaning. It also relates to a method for recovering iron from waste acid.

Conventional methods for removing sand baked on the surface or inner surface of a cast product using a sand mold include knockout, hydro jet, shot blasting, sandblasting, etc.; This is extremely difficult and requires many man-hours.

Particularly in so-called precision casting products made by the lost wax method, etc., it is unacceptable for dents or other flaws to be formed on the surface of the casting product.

Therefore, to remove sand from particularly complex castings or castings with strict dimensional accuracy, a salt bath method in which the casting is immersed in molten caustic soda at a temperature of 500 to 550 DEG C. is generally used.

In this case, due to long-term use, sand accumulates in the caustic soda or a part of it reacts to form water glass such as Na20 and SiO2, which causes the salt to age, so it is necessary to scrape up the accumulated sand from the bath. be.

Since the accumulated sand scraped up in this way is fine particles, it coexists with a large amount of caustic soda and cannot be immediately disposed of, so it is generally neutralized with hydrochloric acid, sulfuric acid, etc.

However, disposing of waste caustic soda through the above-mentioned neutralization process not only increases the amount of waste, but is also extremely inconvenient from the viewpoint of resource conservation and productivity.

On the other hand, in steel factories, pickling is performed during the processing process or in the final process to remove oxides generated on the steel surface, but the pickling ability decreases after a certain number of uses. Dispose of it as a waste tank or waste acid.

However, it is not preferable to dump this waste acid directly into a river or the like because it will pollute the river water quality, and it is necessary to perform some kind of neutralization treatment, which is generally done with caustic soda or the like.

However, even with this method, there is a lack of consideration for resource recovery, and there are problems similar to those described above.

Furthermore, acid gas is generated in steel production plants, and a means to neutralize it is essential for treatment.

Conventionally, the above processes were performed separately, so
The reality is that not only do these methods require duplicative treatment means, but little consideration is given to the effective use of waste.

The present invention solves the above-mentioned conventional drawbacks, and recovers and effectively uses so-called waste caustic soda without disposing of it, thereby improving the process of pickling waste acid by simply recycling it as caustic soda. The purpose is to provide a method for recycling as a system that can be used in a variety of fields, such as for neutralization and acid gas cleaning.

FIG. 1 is an explanatory diagram showing an embodiment of the method of the present invention.

In the figure, reference numeral 1 denotes a dissolving tank, in which so-called waste caustic soda a containing silicic acid is stored together with water b.By stirring with a stirrer 2, the foundry sand contained or mixed in the waste caustic soda a is collected at the bottom of the dissolving tank 1. Separate by precipitation.

Next, the aqueous solution is transferred to a reaction tank 4 by a pump 3, heated, for example, with high-temperature steam C, milk of lime d is added, and stirred by a stirrer 5.

In this case, water glass such as Na20.SiO2 dissolved in the aqueous solution is decomposed by the following reaction.

Na20.SiO2+Ca(OH)2→ 2NaOH+CaO-Si02 Next, this is transferred to the tallifier 8 via the cooler 7 by the pump 6 to precipitate and separate CaO-SiO2, and then NaOH is released from the overflow gutter 9 at the top of the tallifier 8. The aqueous solution e can be extracted.3 The sediment or sludge f is extracted from the lower part of the tallifier 8.

Next, what is the heating temperature in the reaction tank 4 in Fig. 1? The relationship between the amount of Ca added will be explained.

FIG. 2 is a diagram showing the relationship between the Ca/SiO2 molar ratio and the SiO removal rate.

In the figure, curves A and B show the cases when heated to room temperature and 80° C., respectively, and the S102 removal rate in curve B is clearly higher.

That is, in the case of curve A at room temperature, Ca/Sin
The removal rate of Si02 cannot be satisfied unless the molar ratio of 2 is 10 or more, whereas in curve B when heated to 80°C, the same removal rate is obtained when the molar ratio is 3 to 4. There is.

This is a result of the separation reaction being promoted by heating the aqueous solution in the reaction tank 4.

Note that when the molar ratio of added Ca to SiO2 is less than 3, the Si02 removal rate is unsatisfactory, so it is set to 3 or more, but as is clear from FIG. 2, the molar ratio is approximately saturated near i, so It may be selected appropriately within an economical range.

In addition, the heating temperature of the aqueous solution in the reaction tank 4 was set to 80°C in this example, but as a result of experiments, a satisfactory SiO2 removal rate can be obtained within the above molar ratio range if the heating temperature is 60°C. .

Furthermore, as Ca to be added in the reaction tank 4,
It goes without saying that other than Ca(OH)2, CaO also exhibits similar separation and precipitation effects.

Next, in FIG. 1, the aqueous solution e transferred from the tallifier 8 to the storage tank 10 contains colloidal CaO-SiO2 that has not been separated yet.

Therefore, this is transferred to the ultrafiltration means 11, the filtrate g is transferred to the storage tank 12, and a portion is returned to the storage tank 10.

The removal rate of SiO2 by the ultrafiltration means 11 is the second
As shown by curve C in the figure, this is extremely satisfactory.

The filtrate from which the silicic acid content has been removed as described above is transferred to the reuse storage tank 14 via the pump 13, then heated and concentrated in the evaporator 15, and recycled as caustic soda by the flaking device 16.

Reference numeral 17 denotes a processing device, such as a salt bath for removing sand from castings as described above.

In this way, waste caustic soda containing silicic acid is regenerated and recycled.

In FIG. 1, the supernatant liquid of the tallifier 8 is shown.

Although the separated precipitated sludge f has strong alkalinity, it is generally treated as waste.

In the present invention, this sludge f is transferred to the dewatering means 1B to be dehydrated and stored in the storage tank 19 as a cake.

The filtrate h from the liquid removing means 18 is returned to the reaction tank 4 and processed again.

Next, 20 is a pickling waste acid treatment means, which neutralizes the acid by mixing the cake stored in the storage tank 19.

Pure caustic soda is generally used for acid neutralization, but the neutralization using sludge f has a better settling effect than using caustic soda.

Next, the filtrate g from which the silicic acid content was removed by ultrafiltration with 11,000 stages
is transferred to the caustic soda regeneration means as described above, and is branched to a partial storage tank 21, and pump 2
2, the acid gas is supplied to the gas cleaning scrubber 23, and the acid gas is neutralized by the gas-liquid contact means.

Thus, the waste liquid in the gas cleaning scrubber 23 is periodically extracted and transferred to the pickling waste acid treatment means 20 for treatment.

Also, a storage tank 2 containing a portion of the filtrate g from the ultrafiltration means 11
4, the iron content in the pickling waste acid can be recovered.

That is, by adding filtrate g in the recovery means in the pickling waste acid treatment means 20, Fe+2 ions in the iron-containing waste acid are first converted into iron hydroxide, and this is further converted into iron hydroxide by increasing the ratio of Fe+2 to Fe+3. If it is oxidized to a ratio of l:2, the iron content in the waste acid will precipitate as Fe304.

Therefore, if this is extracted intermittently or continuously, it can be recovered as effective iron.

Note that since Fe304 is a ferromagnetic material, magnetic means can be used as a collection or aggregation means.

In particular, the dissolved components in the pickling waste acid can be used not only to neutralize the pickling waste acid for the purpose of preventing pollution, but also to recover active ingredients.

Since the present invention has the structure and operation as described above,
The following effects can be achieved.

(1) Waste caustic soda that would normally be discarded through neutralization treatment can be regenerated, and it can be concentrated up to 95 grams using an evaporator, making complete recycling possible.

(2) 20,000p contained in waste caustic soda aqueous solution
It is possible to remove up to 300 ppm of silicic acid content by ultrafiltration, and it can be used for a variety of purposes as a recycled caustic tarda.

(3) The sludge extracted from the waste caustic soda aqueous solution can be effectively used for waste acid treatment without being disposed of, and treatment can be achieved by mutually treating wastes.

(4) Effective iron contained in waste acid can be recovered and resources can be reused.

(5) In addition to resource saving effects, environmental pollution caused by waste can be completely prevented.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the method of the present invention, and FIG. 2 is a C
FIG. 3 is a diagram showing the relationship between the a/SiO2 molar ratio and the Si02 removal rate. 1: Dissolution tank, 4: Reaction tank, 8: Clarifier 1, 11
: Ultrafiltration means, 15: Evaporation pot, 18: Deliquor removal means, 23
: Gas cleaning scrubber.

Claims (1)

[Claims] 1. After dissolving waste caustic powder containing silicic acid in water and precipitating and removing sand particles, Ca(OH)2 or CaO is added to the solution in such a way that the molar ratio of Ca to SiO2 is 3 or more. At the same time, the aqueous solution is heated to 60°C or higher to precipitate and separate the silicic acid content as CaOSiO2, and after deliquifying the separated sludge, it is mixed with the pickling waste acid and used for neutralization of the pickling waste acid. On the other hand, the supernatant liquid is subjected to ultrafiltration to separate colloids in the liquid, and then the filtrate is heated and concentrated into flakes to be regenerated as caustic soda. Also, a method for recycling waste caustic soda, characterized in that the filtrate is used as a means for cleaning acidic waste gas or as a means for recovering iron from pickling waste acid.