RU2510634C1 - Method of production of activated bentonite - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to method of activated bentonite production. Proposed method comprises chemical activation of lumpy bentonite by sodium-containing agent, its maturing, processing with coal and grinding to dispersed state. Note here that processing of bentonite with coal is performed at chemical activation stage by mixing with oxidised coal of low stage of metamorphism in amount of 10-20 wt % in term of bentonite while coal is pre-treated by 10-30% water solution of sodium tripolyphosphate or pyrophosphate at coal weight ratio of 0.1-0.2:1.

EFFECT: higher compression strength, lower gas generation capacity.

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Description

The invention relates to methods for producing activated bentonite and can be used in the manufacture of activated bentonites for foundry, as the main inorganic binder molding compounds in the manufacture of castings from cast iron and non-ferrous alloys in mass and mass production.

The strength of sand-clay (bentonite) molding sand is determined by the amount of bond between the grains of the filler in the contact zone, carried out through a film of clay (bentonite) paste enveloping them through the so-called butt cuffs (S. S. Zhukovsky. The strength of the mold. M., "Engineering", 1989). Thus, it is possible to achieve an increase in the strength properties of the sand-clay molding sand only by increasing the strength properties of the clay paste film enveloping the filler grains, mainly quartz sand. Currently, even using high-quality materials and modern equipment, this is achieved by increasing the amount of bentonite contained in the mixture. As a result, the technological properties of the mixtures deteriorate, in particular, their moisture content rises, knockability deteriorates, and the risk of casting defects such as explosive burns, gas sinks, etc. increases. The consumption of molding materials and the cost of acquiring them increase significantly. An increase in the strength properties of molding sand is achieved by activation of the main inorganic binder - bentonite, which is carried out in various ways.

A known method of producing activated bentonite, including laying bentonite raw materials on a specially prepared site, the introduction of an activator. The aging of this mixture for 5-18 days with stirring for a period of aging from 2 to 6 times, drying and grinding. As an activator, known simple sodium salts are used, as well as mixtures of these salts (RF Patent RU 2196117, IPC С04В 33/02, В28С 1/16, publ. 10.01.2003).

The method of chemical activation proposed in this patent by artificially replacing the divalent cations Ca and Mg of bentonite with monovalent Na cations from an inorganic activator increases the hydrating ability of bentonite and, as a result, only increases the tensile strength of the molding sand in the moisture condensation zone. However, when applying this method, the compressive strength of sand-bentonite mixtures does not increase. In addition, when using simple salts of sodium-containing compounds (soda ash and / or caustic soda), ion exchange cannot be forced to go to completion.

A known method for producing powdered bentonite, including grinding the raw material, chemical activation with a sodium salt in an aqueous medium with a clay to water ratio of 1: 1.35, mixing in a wet mill ball mill and a container with a propeller stirrer, then drying the suspension in a tower spray dryer (Patent No. 2101258, С04В 33/04, publ. 10.01.1998).

Despite the more complete ion exchange processes occurring in bentonite, due to its intensive mixing in an aqueous medium with an inorganic activator, in this method, activation by sodium salts also provides only an increase in the adsorption capacity of bentonite. Strength indicators for compression of molding sand with this method do not significantly change.

The closest to the proposed invention in technical essence and the achieved result is a method for producing activated bentonite, in which lumpy bentonite was activated by soda ash, aged, dried to a residual moisture content of 2%, after which, after aging and drying, 25.0-37.5 were added to bentonite % of coal and co-crushed in a ball mill (Magazine "Foundry", 2004, No. 9, p.23-25).

The specified method allows to increase the tensile strength in the zone of moisture condensation and to slightly increase the compressive strength of raw sand-bentonite mixtures.

However, the application of this method does not ensure the achievement of the maximum level of complex binding properties of bentonite and, therefore, a cumulative increase in the strength characteristics of molding compounds for the following reasons:

1. In the process of chemical activation, only the replacement of calcium cations in calcium bentonites is ensured. In bentonites with a significant amount of magnesium and iron cations in the exchange complex, their substitution by sodium cations does not occur.

2. The low residual moisture content of bentonite after drying leads to an increase in the time of adsorption of water by bentonite and, as a result, the set strength of the molding mixture. Accordingly, it does not provide the possibility of a significant reduction in bentonite consumption during an intensive cycle of automatic molding lines.

3. Short-term treatment of dried bentonite with pre-dried coal during co-grinding at the last stage of preparation of powdered bentonite provides only their homogenized mechanical mixture. In this case, coal is not able to exert an additional chemical effect on the activation of bentonite. There is no significant change in tensile strength in the moisture condensation zone from the influence of coal dust, and a slight increase in the compressive strength of raw sand-bentonite mixtures is explained only by the mechanical reinforcement of the binder films by coal dust particles.

4. The high content of coal in bentonite leads to an excessive accumulation of its thermal decomposition products in the molding sand, an increase in its gas content, a decrease in the heat resistance of bentonite, and, as a result, an increase in gas shells, surface defects in castings and the consumption of molding materials.

The technical task of the claimed invention is to develop a method for producing activated bentonite, which improves the whole range of strength and operational properties of molding compounds while reducing the amount of coal used.

The technical result from the use of the invention is:

- an increase in the specific strength of the shell of clay paste on the surface of the grains of the filler of the molding sand and, accordingly, of the butt cuffs;

- an increase in the tensile strength of raw molding sand during compression and rupture in the zone of moisture condensation;

- the possibility of a significant reduction in the content in molding sand of the amount of technologically necessary bentonite while ensuring the specified physical, mechanical and technological properties;

- achieving a minimum gas content of molding sand;

- reduction in the consumption of molding materials.

The specified technical result is achieved by the fact that in the claimed method for producing activated bentonite, including the chemical activation of lump bentonite with a sodium-containing reagent, its aging, drying, processing with coal and grinding to a fine state, the processing of lump bentonite with coal is carried out at the stage of chemical activation by mixing with oxidized coal of a low stage of metamorphism in an amount of 10-20% with respect to bentonite, with coal being pretreated 10-30 % aqueous solution of sodium tripolyphosphate or sodium pyrophosphate in a mass ratio to coal, equal to 0.1-0.2: 1.

The technical result achieved is due to the fact that bentonite is simultaneously activated by an inorganic sodium-containing activator and organic compounds by water-soluble sodium humates.

As an inorganic activator of bentonite, calcined and / or caustic soda is used to replace the Ca ²⁺ and Mg ²⁺ cations in its structure with the Na ⁺ cation.

Coals (caustobiolites) of a low stage of metamorphism in an oxidized, weathered form are used as an organic activator. Oxidation and weathering of fossil fuels leads to the formation of secondary humic acids from chemically neutral humic (vitrenic) substances that are capable of interacting with low alkali solutions.

Humic acids and their salts have the ability to exchange cations. When processing the simple salts of sodium-carbon compounds (carbonate and / or caustic soda) occurs proton H ⁺ substitution in the carboxyl and hydroxyl groups of humic acids on Na ⁺ ions of the activator and simultaneously exchange cations in Humates on Na ⁺ to form water-soluble sodium humate. The exchange of cations is a slow process and proceeds to a state of equilibrium in the system:

Humate Ca + Na ⁺ ↔ Humate Na + Ca ²⁺

To accelerate the exchange of cations, sodium salts of phosphoric acids — sodium pyrophosphate Na ₄ P ₂ O ₇ or sodium tripolyphosphate Na ₅ P ₃ O ₁₀ — are used as extractants. As a result of the exchange reaction, in the case of using sodium pyrophosphate, calcium pyrophosphate Ca ₂ P ₂ O ₇ , insoluble in water and alkali, is formed. When using sodium tripolyphosphate, the anion P ₃ O ⁵ ₁₀ - binds Ca ²⁺ and Mg ^{2+ ions} into strong complexes. The exchange reaction becomes irreversible and proceeds in almost the same direction with sufficient speed.

Since sodium pyrophosphate and sodium tripolyphosphate extract only part of humic acids from coal, in the second stage of humic acid extraction, coal interacts with a sodium-containing activator, which is used for the chemical activation of bentonite.

Macromolecules of water-soluble sodium humate, formed from coal, are adsorbed on bentonite particles activated by calcined and / or caustic soda.

The formation and adsorption of humates on bentonite particles continues during aging and subsequent drying of the bentonite-coal mixture directly in the dryer. This is facilitated by an initial humidity of up to 40%, an increased temperature of the raw material, which is 40-80 ° C, and a steam-humid atmosphere. The activation of bentonite is completed by effective acceleration of the extraction of sodium humate from coal by means of a pulsed action in ball or roller-pendulum mills on a humate-containing raw material and an alkali that is not fully reacted.

Sodium salts of humic acids, adsorbed on the surface of the solid phase of sodium bentonite activated by sodium salts, improve the relationship of clay particles with a dispersion medium - water, creating strong hydration shells that prevent particles from coming together, sticking together and preventing coagulation. At the same time, these substances contribute to the further dispersion of larger particles of bentonite in the molding sand, providing a better distribution and lower binder content in the molding sand while maintaining the desired properties.

Within the proposed amount of coal, a high level of a complex of strength indicators of activated bentonite and a decrease in the gas content of the molding mixture are provided.

An increase in the amount of coal for the activation of bentonite above the upper limit is not advisable, due to the accumulation of coke residues in the molding sand as products of its thermal degradation, which negatively affect the formation of the strength of sand-bentonite mixtures.

A decrease in the amount of coal below the lower limit does not provide an increase in the strength properties of clay-based mixtures, due to an insufficient amount of soluble sodium humates introduced with coal and participating in the activation of bentonite.

Pretreatment of coal with aqueous solutions of sodium tripolyphosphate or sodium pyrophosphate contributes to a more uniform distribution of extractants in the mass of the product and reduces the time for the ion-exchange reaction to occur with humic acids.

The upper concentration value of aqueous solutions of sodium tripolyphosphate or sodium pyrophosphate is determined by the limit of their solubility in water.

Decreasing the concentration of solutions below the lower value does not provide the extraction of the required amount of humic acids to activate bentonite, even if the maximum flow rate of the solution according to the claimed method is 200 l per ton of coal, and the maximum amount of coal used is 20%.

An increase in the flow rate of an aqueous solution above 200 liters per tonne of coal limits the necessary mechanized technological operations with waterlogged coal to activate bentonite.

The flow rate of an aqueous extractant solution below 100 l per tonne of coal does not provide a uniform distribution of reagents in the total mass of the processed material and reduces the effectiveness of the preliminary treatment.

The proposed method for producing activated bentonite can be carried out using existing standard equipment.

Example.

In the method according to the invention, the activation of bentonite raw materials is carried out by an inorganic sodium-containing activator and organic activator - coal, a low stage of metamorphism, by mixing all the components during grinding in a clay cutter or gear grinder to pieces no more than 50 mm in size.

As inorganic activator of bentonite, calcined or caustic soda is used. Activator consumption depends on the composition and total capacity of the exchangeable cations of activated bentonite, the type of activator, possible losses of the activator in the processing of activated raw materials. Activator consumption is determined either by calculation using the well-known formulas for the cation substitution reaction, or experimentally in laboratory conditions. Montenegrin benthonite “10 farmstead” (Khakassia) was chosen as the base one. The experimentally established optimal content of activator - soda ash (in excess of 100% bentonite) - 2.2%.

As an organic activator, in the method according to the invention, used was the oxidized coal grade "D" with a grain size of 0-15 mm. Pretreatment of freshly mined coal consists in aging it for at least 8 months in natural conditions to ensure oxidation of the coal with atmospheric oxygen and the formation of secondary humic acids in the mass. The amount of humic acids formed in coal is determined according to GOST 9517-94. Next, the coal is sprayed with a 10-30% aqueous solution of tripolyphosphate or sodium pyrophosphate in a mass ratio to coal equal to 0.1-0.2: 1, and incubated for at least 1 hour until mixing with bentonite.

The amount of coal for activation of bentonite is 10-20%.

The ground mass, during transportation, is moistened by spraying with water to a moisture content of not more than 25% and then aged from 2 to 14 days. Drying of raw materials is carried out in a drum dryer, subsequent grinding - in roller-pendulum or ball mills.

The particle size distribution and residual moisture of activated bentonite powder meets the requirements of the standard for foundry (GOST 28177-89).

Table 1 shows the strength characteristics of bentonites activated by oxidized coal after pre-treatment with extractants in the specific concentration and flow ranges indicated in the claimed method.

In the prototype method, as a coal additive, coal was used with a shelf life of less than 1 month after mining.

Molding mixtures were prepared from enriched quartz sand, water, and a bentonite-coal component taken in proportions, depending on its composition, meeting the requirements of GOST 28177-89, namely, 5% bentonite in a sand-clay mixture.

Evaluation of the compressive strength and tensile strength at break in the moisture condensation zone of activated bentonites obtained by the prototype method and the claimed method, was carried out in accordance with the requirements of GOST 28177-89. The relative humidity of all compared mixtures corresponded to a compressibility of 45.0-47.0%.

The compressive strength was determined on an LRu device (Poland), the tensile strength at break in the moisture condensation zone was determined on a device designed by HF VNIILITMASH.

As can be seen from the table, the strength characteristics of activated bentonite obtained by the method according to the invention exceed the properties of bentonite obtained in a known manner.

Using the proposed method for producing activated bentonite allows to achieve high physical, mechanical and technological properties of molding compounds.

The introduction of the proposed material into production can provide a significant economic effect by reducing active bentonite, reducing the consumption of molding materials and reducing the number of defects in castings.

Table 1 Example No. The amount of extractants for pre-treatment of oxidized coal The amount of coal to activate bentonite,% The content of activated carbon bentonite in the molding sand,% Properties of molding compounds Sodium Tripolyphosphate (Na ₅ P ₃ O ₁₀ ) Sodium Pyrophosphate (Na ₄ P ₂ O ₇ ) The concentration of the aqueous solution,% Solution consumption, l / t of coal The concentration of the aqueous solution,% Solution consumption, l / t of coal The limit of compressive strength, kgf / cm ² The tensile strength at break in the zone of moisture condensation, g / cm ² Gas content, cm ³ / g one twenty 200 - - 10 5.55 1.05-1.18 30-33 2.75 2 thirty 150-200 - - 3 twenty 150-200 - - four thirty 100-150 - - fifteen 5.88 1.17-1.23 32-36 4.4 5 - - thirty 150-200 6 10 200 - - 7 twenty 100-150 - - twenty 6.25 1.21-1.27 34-39 6.25 8 thirty one hundred - - 9 - - thirty 150 Prototype - - - - thirty 7.14 0.98 29th 10.7

Claims (1)

A method of producing activated bentonite, including the chemical activation of lump bentonite with a sodium-containing reagent, its aging, drying, processing with coal and grinding to a finely divided state, characterized in that the processing of lump bentonite with coal is carried out at the stage of chemical activation by mixing with oxidized coal of a low metamorphism stage in an amount of 10-20% in relation to bentonite, and coal is pre-treated with a 10-30% aqueous solution of tripolyphosphate or pyr sodium phosphate in a weight ratio to the coal equal to 0.1-0.2: 1.