JPS5848267B2 - Method for recycling water glass-based foundry sand - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention involves adding water glass as a binder to foundry sand such as CO2 process molding waste sand, Dical process molding waste sand, etc. Arca IJI water glass
It is dissolved and removed in a liquid and recycled as reusable foundry sand. At the same time, SiO2 is removed as calcium salt from the alkaline wastewater containing sodium silicate discharged during dissolution and cleaning, and at the same time, caustic soda is recovered. , relates to a method for recycling foundry sand, including recycling.

There is a demand for the establishment of treatment measures for waste mold sand from the standpoint of protecting Hanawa resources and the environment. Therefore, recycling technology not only makes it possible to reuse mold waste sand, but also reduces waste water, exhaust gas, dust, and noise. It is required to include detoxification technologies that do not involve regeneration, such as

Casting waste sand uses inorganic binders such as water glass,
Organic binders such as resins and fats and oils are attached to the waste sand, but various considerations are required when recycling waste sand, especially when water glass is used as a binder.

Traditionally, there has been a firing method using high-temperature heating as a treatment method for water glass-based foundry waste sand, but the operating costs and equipment costs for the equipment are high;
There are various problems associated with high temperatures, and although there is a method of neutralizing adhering alkali salts with acid, neutralization is difficult to achieve sufficiently, and the cost of the remaining acid required to form a gel. There are many problems such as.

The present inventors have found that the treatment methods that have been published so far are not necessarily satisfactory in terms of both the recycling and recycling of waste foundry sand and the treatment and reuse of adhering alkali salts. In view of these circumstances, we have conducted repeated experiments and studies to eliminate various drawbacks, and have arrived at the present invention, which is unprecedented in the field.

In other words, water glass molds are made of sand with water glass added as a molding material, but the water glass added as a binder is CO2.
Even if it gels due to chemical changes during mold making like the 2-hardening type, and even if the mold is affected by heat due to the heat treatment, most of the components still contain Na2CO3, Na2SiO3, Na20・n.
It is clear that the waste sand is present in the form of alkali-soluble salts such as Si02, because when the waste sand comes into contact with water, it gradually leaks waste water with a considerably high pH.

Focusing on the above, we first developed a method for effectively cleaning and removing alkaline substances adhering to the surface of foundry waste sand using a wet method, and selected a wide variety of acids and alkalis as wet decomposition cleaning agents. Experiments were conducted to find suitable processing agents and processing conditions.

As a result, although caustic soda alone is sufficiently effective as an acid-dissolving cleaning agent, it is necessary to shorten the oxidation time, reduce the amount of dissolved cleaning agent required, and reduce the cost of the cleaning agent. We investigated the combination with drugs and found that coexisting sodium silicate with caustic soda in an appropriate range is more effective against phlegm.

That is, as for silicic acid, the SiO2/Na20 molar ratio is 1.
．． 8-3. 5 was used, N a OH/(
NaOH + Na20-nSiO2) weight ratio is 0.
3 to 0. 9, NaOH + Na20-nSiO2 is 5
It is appropriate to use a mixed alkaline aqueous solution of ~15 weight percent as a dissolving and cleaning agent for waste foundry sand.

The higher the treatment temperature, the better, but a temperature of 50 to 130°C is sufficient.

Alka IJM, a mixture of caustic cedar and sodium silicate mentioned above.
Removal of alkaline substances on waste foundry sand with a liquid is effective for caustic soda using highly gelled silica fraction (Si02
) n- has the effect of promoting dissolution, and on the other hand, the affinity between the sodium silicate attached to the sand and the sodium silicate added as a dissolving and removing agent increases, resulting in dissolving, that is, "like becomes like." It is presumed that the effect of promoting mutual dissolution based on the general rule of "melting together" is working synergistically.

The waste sand that has been dealkalized with a pyrolization remover using a mixed alkali is separated.

Since the alkaline substance as a dissolving and removing agent still remains in a wet state on the surface of the dealkalized waste sand grains, it is removed by washing with water.

It was confirmed that the waste sand dealkalized in this process can be fully reused as recycled sand by dehydration and drying.

As mentioned above, the alkaline substances that have been separated and are present with the dissolving and removing agent, and the alkaline substances that have migrated to the washing water side during washing, are not allowed to be released as waste as they are, or they cannot be released simply by neutralization with acid. This means that the alkali as a solubilizing agent is unilaterally discarded, which is not economical.

Since the aim of the present invention is to create a closed system without drainage, detoxification, and recycling, this point was further experimentally investigated.

That is, the silica content in the waste water discharged from this method of regenerating waste sand by acid decomposition is removed, and the caustic soda as a lysing agent is recovered again.

As a method for this, the inventors of the present invention have focused on a method in which the silica in the alkaline solution eluted and discharged from waste foundry sand is reacted with calcium acid hydroxide to precipitate and remove it as calcium silicate, and at the same time, free caustic soda is recovered. However, it has become clear that a new process can be established in which the treatment can be closed by combining it with the above-mentioned process of dissolving the waste sand using an alkali salt.

That is, for example, water glass in the wastewater is CaO or C.
CaSiO3 is precipitated by reaction with a(OH)2 and Na
OH can be liberated.

The S t 02 concentration of the solution discharged from the foundry sand regeneration treatment using Na2Si03-NaOH-based alkaline solution of foundry waste sand has an appropriate concentration; if the concentration is too high, the silica content will gel and the reaction of equation (1) will fail. Complete.

On the other hand, if it is too low, the reaction will occur, but the reaction rate in equation (1) will tend to decrease.

The appropriate SiO2 concentration is 0.6 moA/l or less.

Also, CaO or Ca added in the reaction formula (1)
The amount of (OH)2 is from 1.0 to Ca2+/SIO2 molar ratio
5.0 is good, and the higher the reaction temperature, the better, but the reaction will proceed sufficiently at 5 to 110°C.

The CaS103-based non-aqueous substance produced mainly by formula (1) is harmless and can be fully used in road landfills, etc. On the other hand, the NaOH produced is concentrated and used for decomposition of foundry waste sand. It can be recycled and reused as a regenerating alkali.

Further, the steam generated in the concentration process of the liberated NaOHs is condensed and can be reused as recovered water for washing the reclaimed sand after the dissolution treatment.

Next, an example of an embodiment of the method of the present invention will be described in detail below with reference to a first flow sheet shown in the accompanying drawings.

In the attached drawings, > indicates the direction of movement of the foundry sand, and → indicates the direction of movement of an alkaline liquid, such as a mixed solution of NaOH and water glass.
→ indicates the direction of water movement:::〉 indicates CaO or Ca(OH)
2 indicates the moving direction of CaS i03.

In addition, the numbers in the attached diagrams are as follows: 1 is water glass waste foundry sand, 2 is the melting process, 3 is the sand-liquid separation process, 4 is the cleaning process, 5 is the dehydration drying process, 6 is the recycled sand, and T is the alkaline storage liquid. Storage tank and heating step, 8, storage tank discharged from 4.7 and reaction step of converting water glass into NaNH, 9
is a storage tank for CaO or Ca(OH)2, 1
0 is solid-liquid separation step, 11 is concentration step, 12 is recovered Na
13 is a storage tank for OH liquid, 13 is a dehydration process, 14 is a storage tank for Ca SiO3, 15 is a condensation process for evaporated water, and 16 is a water storage tank.

First, waste foundry sand 1, which has been crushed into single sand grains, is heated in the melting step 2 to a temperature of 50°C to 130°C, preferably to the boiling point of the solution, with a NaOH solution or a mixed solution of NaOH solution and water glass. The above-mentioned alkaline solution that dissolves the water glass adhering to the waste foundry sand is heated in the storage tank 7 and circulated by a pump.The circulation of the solution causes the sand grains to flow and the sand grains come into contact with the liquid. It has the effect of improving dissolution, uniform dissolution, and speeding up the dissolution.

The sand grains from which adhering water glass has been removed in the dissolution step 2 are separated into the alkaline solution by a filter having a mesh size of about 100 to 200 μm in the step 3.

The sand grains separated in step 3 are transferred to step 4, and the liquid is transferred to step 7.
storage tank.

In step 4, the alkaline solution adhering to the sand grains is completely removed by contact with water, and the sand grains are transferred to step 5.

On the other hand, the waste water from washing is put into the slage tank 8.

In step 5, recycled sand 6 is recovered by dehydrating and drying the water adhering to the sand grains.

The evaporated water is condensed in condenser 15 and collected in 16 water storage tanks.

In the storage tank 8, the above-mentioned washing waste water discharged in the step 4 and the rinsing liquid discharged from the storage tank 7 for adjusting the alkaline solution are stored together.

This storage tank No. 8 is also used as a reaction tank for the step No. 8, and the following reaction is carried out.

That is, in step 8, first, 3 10 in the above-mentioned waste liquid
2 concentration to 0.58 mol/. e or less, preferably 0.1 to 0.25 mail/l, and then CaO or Ca(OH)2 is extracted from the storage tank at a Ca2+/Si02 molar ratio of 1.0 to 5. It is added so that it becomes .0.

In this step, Na20・n Si contained in the solution is
02 reacts with Ca(OH)2, ns iO,+nCa
(OH)2-+nC: aS i03+nH2 0...
...... Cl) Na O + H 0 2NaOH --
(2) 22 Insoluble CaSiO3 is produced, and Na20, which is a constituent molecule of water glass, produces NaOH.

In the above solution, I'J is present together with Na20-nSiO2.
Although aOH is also contained, this NaOH coexists with NaOH produced in the reaction.

The SiO2 concentration in the liquid before the above reaction was adjusted to 0.6 m
Above ol/, CaSiC3 is produced in gelatinous form,
A feature of the invention is that solid-liquid separation in the next step is extremely difficult.

NaOH cannot be recovered and reused. On the other hand, Si in the liquid
02 concentration is 0. When the dilute solution becomes less than 1 rrro/l, the load in the concentration step required to reuse the recovered NaOH solution increases, so the desirable SiO2 concentration range is 0.1 to 0.25 mol. /l
It is j.

The amount of CaO or Ca (OH) 2 added is determined by the amount of Ca2+ and Sin as shown in the reaction formula (1) above.
, 2 in equivalent amounts is sufficient, but in view of the reaction rate, the Ca2+/SiO2 molar ratio is preferably 1.0 to 5.0.

If the above-mentioned molar ratio is 5.0 or more, not only the reaction effect commensurate with the amount added will not be observed, but also unreacted C.
Since a(OH)2 coexists with CaSiO3 in large quantities, if it is handled as a detoxified substance, treatment such as neutralization is required, which is economically undesirable.

The NaOH, CaSi% sluic IJI liquid whose reaction has been completed in step 8 above is separated into solids in step 10, and the solid content Ca S s Oa is dehydrated and dried in step 13, and stored in a storage tank 14. On the other hand, the solution containing NaOH is concentrated to a concentration necessary for reuse in step 11, stored in a storage tank 12, and supplied in the required amount as an alkaline solution for dissolution in step 7.

The water evaporated from step 11.13 above passes through a condenser and is stored in a water storage tank, and is supplied in the required amount as washing water in step 4 or dilution water in step 8.

The reaction temperature in step 8 is 5 to 110°C, preferably 20 to 60°C.

If the temperature is below 5°C, the reaction rate becomes extremely slow, which poses a practical problem.

On the other hand, at 110° C. or higher, the reaction becomes a pressurized reaction, requiring high-pressure equipment, which is not practical.

Example 1. j 100 parts of new sand from Mt. Futago and Na2 with a solid content of 50%
0, 2.3 Knead 5 parts of S-i02 domain liquid, 50φ
After being molded into a cylindrical shape of 50mmh and hardened with CO2, we took into consideration the thermal effects of actual castings. It was fired at 1000° C. for 2 hours and crushed into single particles using a crusher to produce waste foundry sand, which was used as a test sample.

Next, a separable flask with an internal volume of 2 liters equipped with a thermometer, a liquid donor, and a propeller-type stirrer was set in a mantle heater capable of controlling the temperature.

After putting 1 liter of Alka IJI liquid with a predetermined concentration into this flask and controlling the temperature of the color liquid to a predetermined temperature using a mantle heater, 500 g of the above-mentioned waste foundry sand was added, and 100 r.
The water glass adhering to the waste foundry sand was dissolved by stirring at a rotational speed of 100 pm.

The removal rate of adhering water glass is determined by sampling the liquid at regular intervals, quantifying the SiO3 in the finished liquid after passing through the furnace, and subtracting the value from the SiO2 concentration before treatment.
2. It was confirmed from the ratio that the mixed amount of 3S i02 was set as 100.

The experimental results conducted using the above method are shown below. Figure 2 shows the relationship between the time required to remove 100% of the attached water glass and the concentration of the NaOHffl solution at a melting temperature of 100°C. 3 shows the results of measuring the relationship between the time required to remove 100% of the adhering water glass at a melting temperature of 100°C and the mixed concentration of NaOH and Na20-nSiO2 (n=2.3) i-liquid. liquid 1
Time treatment and 5% NaOH and 5% Na20,2.3Si
These are the results of measuring the relationship between the adhering water glass removal rate and the treatment temperature for a 4-hour treatment with No. 02 solution.

Example 2 Using the same equipment as in Example 1, Na20.2.3Si
Put O2 solution in a predetermined concentration into a 1L separable flask, stir at a rotation speed of 100 rpm, set the predetermined temperature, and then add Ca2+/S i02 molar ratio to the predetermined concentration. (OH)2 reagent powder was introduced to cause Ca(OH)2 and SiO2 to react.

The reaction rate was confirmed by sampling a solution for analysis every two predetermined time periods, quantifying the SiO2 in the F solution which had been passed through a furnace to remove the solid content, and checking the difference from the Si02 before the reaction.

The results of experiments conducted using the above method are shown below.

FIG. 5 shows the relationship between the reaction time and the reaction rate with respect to the SiO2 concentration in the stock solution, and FIG. 6 shows the relationship between the SiO2 concentration in the stock solution and the reaction rate.
2 3 mol/11 The relationship between the Ca2+/SiO2 molar ratio and the reaction rate was measured at a reaction time of 20 minutes.
, reaction time 20 minutes Ca2+/S 102 moles 1.
5 shows the results of measuring the relationship between reaction temperature and reaction rate.

Note that when the SiO2 concentration in the stock solution was 0.58 mol/l or less, the F transmission characteristics were very good; 5
If the concentration exceeds 8 mol/l, the state of CaSiO3 crystals becomes gelatinous, making it extremely difficult to pass through and containing NaOH.

The effluent could not be recovered. Example 3 An experiment similar to that of Example 2 was conducted using the treated waste liquid of Example 1, and the results were the same as those of Example 2.

Example 4 The reactor filtrate containing NaOH treated and recovered by the method of Example 2 was concentrated using a glass rotary evaporator so that the NaOH concentration was 10%, and then the concentrated solution was used in the same manner as in Example 1. The results of the experiment were similar to those of Example 1, and it was confirmed that the concentrated waste liquid from the treatment of Example 2 could be recycled.

Example 5 Using recycled sand from which 100% of the adhering water glass had been removed by the method of Example 1, a casting test for 650 kg of castings was conducted, and the results showed that the product was as good as new sand.

[Brief explanation of drawings]

Figure 1 is a flow sheet showing an example of the present invention, Figure 2 is a diagram showing the results of measuring the relationship between the time required to remove 100% of the attached water glass and the concentration G of the NaOHl solution at the plying temperature of ioo°C. Figure 3 shows the time required to remove 100% of the adhering water glass at a melting temperature of 100°C and the relationship between NaOH and NaOH.
20-nSiO2 (n=2.3) A diagram showing the results of measuring the relationship between the liquid mixture concentrations, Figure 4 is 10% NaOH solution 1
Time treatment and 5% NaOH and 5% Na20・2,3
Figure 5 shows the relationship between adhering water glass removal rate and treatment temperature for 4-hour treatment with Si02 solution. Figure 5 is a diagram showing the relationship between reaction time and reaction rate with respect to SiO2 concentration in the stock solution. Figure 6 is SiO2 concentration in 0.23mol
/ l! Ca2 vs. reaction rate at reaction time 20 minutes
A diagram showing the results of measuring the relationship between +/SIO2 molar ratio, and FIG. 7 shows the stock solution Si02 concentration of 0.2 to 3 m. FIG. 3 is a diagram showing the results of measuring the relationship between reaction temperature and reaction rate at oA/l1 reaction time of 20 minutes and Ca2+/SiO2 mole number of 1.5. 1...Water glass-based waste foundry sand, 2...Grinding process, 3...Sand one-liquid separation process, 4...
・Washing process, 5...Dehydration drying process, 6...
... Recycled sand, 7... Alka IJ I liquid storage tank and heating process, 8... Storage tank discharged from 4 and 7 and reaction process for converting water glass into NaOH, 9・・・・・・CaO or Ca(O
H) 2 storage tank, 10... Solid-liquid separation step, 11... Concentration step, 12... Storage tank for recovered NaOH liquid, 13...・・・
Dehydration process, 14... CaSiO3 storage tank, 15... Condensation process of evaporated water, 16...
...Water storage tank.

Claims (1)

[Claims] 1. A step in which waste foundry sand using a water glass-based binder is brought into contact with an alkaline liquid to dissolve alkali-based substances such as water glass attached to the waste foundry sand, and a water bath containing dissolved alkali substances. There are four main steps: separating the liquid and waste foundry sand, removing alkaline substances remaining on the surface of the separated waste foundry sand grains by washing with water, and dehydrating and drying the washed foundry sand grains to obtain recycled foundry sand. In the process of regenerating water glass foundry sand, the Si02 concentration from the above process is 0.6.
A waste liquid containing a soluble alkaline substance of mol/V or less and an alkaline aqueous solution of calcium oxide are mixed at a Ca+/SiO2 molar ratio of i. o~5.0 Contact at a treatment temperature of 5~110°C to precipitate and separate the SiO2 content in the waste liquid as calcium salt, and concentrate the resulting caustic soda solution and reuse it in the process of dissolving waste foundry sand. A method for recycling water glass-based waste foundry sand, which is characterized in that the water recovered through concentration is reused in the washing and removal process.