RU2450886C1 - Moulding mixture - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to foundry production. The moulding mixture contains the following, wt %: basic flue ash 18-22; acidic flue ash 59-73; microsilica 2-4; liquid glass 7-15; distilled water 0.45-0.55.

EFFECT: improved operational characteristics of the moulding mixture.

6 tbl

Description

The invention relates to foundry, namely to the manufacture of molds from a molding mixture based on an ash-liquid glass composition for small and medium casting of non-ferrous metals, metal alloys by investment casting and can find application in foundries of mechanical engineering, instrument-making, as well as dentistry and jewelry production.

In the modern market of molding compounds, there is a need for readily available components that do not require expensive grinding and separation and can easily break down upon removal of the finished casting from the molds.

Known molding mixture for the manufacture of foundry molds, consisting of gypsum, sand and mineral additives (Color casting. Handbook edited by N. M. Taldin. - M.: Engineering, 1989, p.397).

However, this mixture has insufficiently high physical and mechanical properties.

Closest to the proposed one is a molding mixture for the manufacture of foundry molds, including gypsum and quartz sand, characterized in that it additionally contains ceramic production waste containing clay, in the following ratio of ingredients, wt.%: Gypsum - 30-40 ; ceramic production waste - 20-60; quartz sand - the rest (patent RU 2207932 C1, B22C 1/18, 06/10/2002).

The specified composition has the following disadvantages: grinding and separation of the components included in the mixture are necessary; it is necessary to vacuum the solution of the molding sand to remove air bubbles from the composition; it is difficult to remove the casting from the mold.

The objective of the present invention is to improve the functional and operational characteristics of the molding sand by keeping the ash-liquid glass solution at a low-viscosity stage preceding the formation of a viscous gel, sufficient time so that the low-viscosity solution can be poured into the flask and accurately repeat the shape of the investment model without forced vacuum degassing of the solution.

It is important to select the composition in such a way that it has a low water resistance, that is, it easily breaks down when interacting with water, but at the same time it has sufficient dry strength to work. In addition, the particle size distribution of the mixture contributed to the densest packing of the particles of the mixture, which in turn leads to an increase in the quality of the surface of the casting.

The proposed composition of the molding mixture does not require additional grinding and separation; no vacuum stage required; loses strength under the influence of water; initially contains a release agent - graphite (unburned carbon).

The proposed invention is aimed at a sharp reduction in the cost of molding sand, simplification of work with them while maintaining the high quality of the obtained castings, as well as the recycling of waste electricity production - fly ash.

Fly ash is a finely divided product of high-temperature processing of the mineral part of coal. Fly ash can be acidic (rich in quartz - SiO ₂ ) or basic (rich in calcium oxides - CaO). The first exhibits pozzolanic properties, the second additionally exhibits the properties of an independent binder.

The problem is solved in that the molding mixture of the ash-liquid glass composition used for casting according to the smelted models contains:

High-calcium fly ash (main) - 18-22%

Acid fly ash - 59-73%

Silica fume (MKM) - 2-4%

Liquid glass (sodium silicate) - 7-15%

Distilled water W / T - 0.45-0.55

The specified composition allows to obtain a low-viscosity, high-fluidity mixture that does not form air bubbles upon stirring, that is, does not require evacuation, and also by creating a dense structure due to the fine particles of various dispersions contained in the mixture (from 100 to 0.1 μm), when voids between large particles are filled with more and more small ones), which, in particular, allows to obtain castings with high surface quality and to prevent shrinkage.

The technological properties of the molding mixture proposed in the application in comparison with the properties of the prototype mixture are illustrated in the tables:

Table 1 - fractional composition of ash generated during the burning of Kuznetsk coal at CHP-22, a branch of MOSENERGO OJSC;

table 2 - the chemical composition of fly ash (acidic) (% mass fraction);

table 3 - the chemical composition of fly ash (main) (% mass fraction);

table 4 - change in the physico-mechanical properties of the molding mixture with a change in the ratio of fly ash (main) / liquid glass;

table 5 - the change in the physico-mechanical properties of the molding mixture with a change in the ratio of fly ash (acid) / water glass;

table.6 - change in the physico-mechanical properties of the molding sand with the introduction of 2% silica fume (MKM). Liquid glass was used, silicate module - 2.5.

From table 1 it follows that the fractional composition of fly ash (acidic) is quite diverse and contains in its composition particles with a size of mainly from 100 to 1 μm. The largest 26% fraction (from 40 to 100 microns) approximately corresponds to the size of cement particles of not very high quality, but the last three fractions correspond in their dimensions to cement additives, compacting the density of the structure of cement stone. This is due to the fact that the gaps between the large particles are filled with smaller ones, the gaps between the smaller ones are filled with even smaller ones, and so on. By analogy, a similar mechanism of structure compaction was also chosen for the molding mixture by selecting the appropriate recipe composition.

From tables 2 and 3 it is seen that the composition of the components of fly ash (acidic) and fly ash (main) is approximately the same and differs mainly in percentage and, therefore, a mixture of two evils in any percentage ratio has chemical affinity for each other. In addition, a sufficiently high content of calcium oxide in the ash (basic) allows it to be used as a hardener for water glass and not to introduce foreign components as a hardener.

Table 4 shows that with an increase in the amount of liquid glass introduced, the strength of the molding mixture increases, and significantly, but due to the high setting speed and sufficiently high viscosity of the solution, air bubbles do not have time to remove from the mixture. A small amount of fine particles in fly ash (main) leads to the fact that the resulting structure is very porous. "Water destruction" occurs only with a strength of about 10 kg / cm ² .

The structure is very porous, the setting rate is high, it contains air bubbles.

From table 5 it is seen that the molding mixture, consisting only of fly ash (acid) has the following disadvantages:

very low setting speed;

due to the very high density of the mixture, it is difficult to remove moisture from the solution, which at elevated temperatures leads to the formation of cracks.

But at the same time, the resulting solution has a low viscosity, is not able to hold air bubbles and the finished form has a dense structure and is quite well destroyed by the action of water.

The structure is dense, the setting speed is very low, low crack resistance, does not contain bubbles.

After many experiments conducted by the authors, the prescription composition presented in Table 1 was selected. 6.

High-calcium fly ash (main) - 18-22%

Acid fly ash - 59-73%

Silica fume (MKM) - 2-4%

Liquid glass (sodium silicate) - 7-15%

Distilled water W / T - 0.45-0.55

A mixture of two fly ash is optimal in terms of setting speed, viscosity, density. The introduction of microsilica increased the density of the structure and increased the crack resistance of the composition.

The structure is very dense, the setting rate is normal, does not contain bubbles, no cracks.

An increase in the amount of basic ash over 22% leads to an increase in the setting rate of the solution up to 1-2 minutes, which introduces inconvenience when working with the molding mixture and leaves too little time for the natural exit of air bubbles from the solution. Due to the lower dispersed composition of the main ash compared to acidic, the porosity of the structure of the resulting form increases, which reduces the quality of the casting.

A decrease in the amount of basic ash less than 18% leads, respectively, to a significant slowdown in the setting speed (up to several hours), and the strength of the finished forms also decreases.

The amount of fly ash (acidic) as a percentage is related to the amount of fly ash (main). The disadvantages of a too high content of fly ash (acidic) are reflected in the description of Table 5 (low setting speed, high probability of cracking, low strength of the obtained forms). A low acid ash content automatically leads to a high content of basic ash (see above).

A decrease in the amount of MKM below 2% does not have any positive effect on the mixture, while an increase of more than 4% leads to a sharp increase in water consumption, which leads to a decrease in mold strength, mother-in-law formation, while the content of MKM within 2-4% compacts shape structure and reduces the likelihood of cracking.

The introduction of liquid glass in an amount of 7-15% due to the ability of the form to "water destruction". A mold containing less than 7% liquid glass has a very low dry strength (2-4 kg / cm ² ). A form containing more than 15% liquid glass is poorly or not destroyed at all under the influence of water.

Given all of the above, and the proposed composition of the molding mixture was selected.

The action of the components of the molding sand mixture is as follows: liquid glass - a binder, fly ash (main) - a hardener for liquid glass and at the same time a filler containing a large amount of silicon oxide (Table 3), fly ash (acid) - a finely divided filler, consisting practically of the same components (since the introduction of additional components as inorganic hardeners increases the risk of cracking, and organic ones do not have sufficient heat resistance) as fly ash (main), but containing significantly less the amount of calcium oxide, which is why it is a hardener to a much lesser extent than fly ash (basic), silica fume - an additive that increases the density of the structure of the mixture (has a size of 0.1-1 microns).

The percentage of the components of the moldable mixture is determined by the necessary strength of the mold in the dry state, poor moisture resistance, which when interacting with water leads to a fairly easy destruction of the mold when removing the casting. The ability of the mixture to form a high-density structure during annealing, resistance to shrinkage phenomena and crack formation (although the process of heat treatment of the molding mixture is more influenced by the process of crack formation), as well as the inability to hold the resulting air bubbles in solution. An increase in the amount of liquid glass leads to an increase in the strength of the molding sand and, at the same time, reduces the ability to “water-destroy” the finished form, an increase in the amount of base fly ash leads to a sharp increase in the setting speed, while the porosity of the structure of the molding sand increases due to the larger dispersed composition fly ash, compared to acidic. An increase in the amount of silica fume leads to an increase in the ability to crack.

The molding mixture is prepared as follows:

The required amount of dry components: fly ash (acidic), fly ash (main), silica fume, is mixed in any mixer suitable for preparing dry mixtures, or in small quantities manually. Then it shuts with the necessary amount of water and mixes vigorously for 2-3 minutes. When mixing ash with water, calcium oxide (CaO - quicklime) turns into calcium hydroxide (Ca (OH) ₂ - slaked lime) CaO + H ₂ O → Ca (OH) ₂ .

When the required amount of sodium silicate (Na ₂ SiO ₃ - liquid glass) is added to the obtained ash solution, calcium silicate is formed: Ca (OH) ₂ + Na ₂ SiO ₃ → CaSiO ₃ ↓ + 2NaOH,

- a slightly soluble compound in water, which leads to almost instant (15-20 s) setting of the resulting mixture. Such a short setting time makes the ash-liquid glass composition unsuitable for working with it as a molding sand.

However, when sodium silicate is added in small doses with vigorous stirring, the viscosity of the solution first increases, then, after a while, the viscosity returns to its original value. This happens until all the lime and other active, from the point of view of sodium silicate, components are “neutralized”. Destroyed due to mechanical destruction, the resulting structural bonds have sufficient activity to restore this structure (tens of minutes), but their activity is not enough to do this instantly. With this approach, the initial composition of the ash becomes unimportant, since the "neutralization" of the initial active components of the ash with liquid glass is carried out automatically. The process can be considered complete as soon as the next addition of water glass does not lead to an increase in the viscosity of the solution. Further, the remaining liquid glass is added, while the viscosity of the solution drops sharply and the solution is no longer able to hold air bubbles formed with stirring. After mixing for 30-40 seconds, the finished molding sand is poured into the prepared flask and left in the room for one hour.

Further, in the flask, the mixture is dried:

1) t ° - 50-60 ° C for three hours.

2) t ° = 100-110 ° C - 30 minutes, dismantling of the flask.

Further heating according to the following scheme, preventing the formation of cracks

3) t ° 300 ° exposure ^{20 min →} t ⁰ 600 exposure ^{20 min →} t ⁰ 800 ° exposure ^{20 min →} t ⁰ 950 ° exposure ^{40 min}

4) Fill metal.

5) Cooling.

6) Water treatment - 3-5 minutes.

7) Dismantling the mold.

Fly ash contains not burnt carbon, which is a release agent and prevents metal from sticking to the surface of the mold, which also facilitates the removal of the cast from the mold. The rapid evaporation of moisture during drying leads to incomplete hydration reactions and the formation of a small amount of silicic acid gel, insufficient to form a strong stone structure, leads to the formation of open porosity, which increases water absorption and reduces water resistance. All these negative qualities, from the point of view of the manufacture of building materials, are positive, from the point of view of the manufacture of molding compounds capable of deteriorating under the influence of water. In addition, open porosity contributes to the removal of gases when pouring metals into the mold.

The proposed molding mixture has improved functional and operational characteristics due to the retention of the ash-liquid glass solution at a low-viscosity stage preceding the formation of a viscous gel, sufficient time for the low-viscosity solution to be poured into the flask and to accurately repeat the shape of the investment model without forced vacuum degassing of the solution.

It will be apparent to one skilled in the art that various modifications and changes are possible in the present invention. Accordingly, it is intended that the present invention covers the modifications and variations as well as their equivalents without departing from the spirit and scope of the invention disclosed in the appended claims.

Table 1 Particle size, microns The percentage in the sample More than 100 10% 40-100 26% 20-40 9% 10-20 fifteen% 6-10 13% 4-6 12% 1-2 fifteen%

table 2 Name Symbol The content in ash and slag,% Silica SiO ₂ 52.2-64.3 Aluminium oxide AlO ₃ 23.5-29.0 Iron oxide (III) Fe ₂ O ₃ 6.0-10 Titanium dioxide TiO ₂ 0.6-1.0 Calcium oxide Cao 2.2-5.8 Magnesium oxide MgO 1.0-2.0 Potassium oxide K ₂ O 1.0-2.3 Sulfuric anhydride SO ₂ 0.2-0.8 Iron oxide FeO 0.8-1.5 Phosphoric anhydride P ₂ O ₅ 0.2-1.0 Manganese Oxide MnO 0.3-0.4 Unburned carbon ------ 12.0-16.0

Table 3 No. The determined indicator Symbol Content, wt.% one. Sodium oxide Na ₂ O 0.30 2. Magnesium oxide MgO 5.79 3. Aluminium oxide Al ₂ O ₃ 7.59 four. Silica SiO ₂ 45.3 5. Sulfate Sulfate SO ₃ 1.62 6. Potassium oxide K ₂ O 0.28 7. Calcium oxide Cao 29.3 8. Titanium oxide TiO ₂ 0.43 9. Total iron oxide Fe ₂ O _{3 total} 7.55 10. Loss on ignition (at 1000 ° C) p.p.p. 0.44

Table 4 Composition, wt.% Compressive strength, kg / cm ² Setting speed, min "Water destruction" Fly ash (main) 80 70 1-2 Not happening Liquid glass twenty Water 40 Fly ash (main) 85 40 1-2 Not happening Liquid glass fifteen Water 40 Fly ash (main) 90 10-15 1-2 Collapses Liquid glass 10 Water 40

Table 5 Composition, wt.% Compressive strength, kg / cm ² Setting speed "Water destruction" Fly ash (acid) 80 30-40 18 hours Not happening Liquid glass twenty Water 60 Fly ash (acid) 85 15-20 20 hours Destroys badly Liquid glass fifteen Water 60 Fly ash (acid) 90 4-5 24 hours Is destroyed well Liquid glass 10 Water 60

Table 6 Composition, wt.% Compressive strength, kg / cm ² Setting speed "Water destruction" Fly ash (main) 35 Fly ash (acid) 55 9-15 3-5 min Cracks, there are cracks Liquid glass 10 Water fifty Fly ash (main) twenty Fly ash (acid) 69 9-15 15 minutes Collapses Liquid glass 9 Water fifty Silica fume 2

Claims (1)

A molding mixture based on an ash-liquid glass composition used for investment casting, containing ingredients in the following ratio, wt.%:
Primary fly ash 18-22 Sour fly ash 59-73 Silica fume 2-4 Liquid glass 7-15 Distilled water 0.45-0.55