RU2044594C1 - Heat-insulating mixture - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: heat- insulating mixture for pouring liquid iron contains, by weight of: graphitizing furnace burden return with carbon and silicon carbide in the ratio of (2:1)-(3:1) 60.0-75.0; carbon-bearing material bloating in the process of heating (oxidized graphitizing furnace burden return) 2.0-12.0; inorganic refractory material selected in the silicate group containing blast-furnace slag, diatomite the balance. The mixture is fed to iron ladle bottom by spilling till liquid iron discharge is initiated. In the process of iron discharge, graphite carbon expansion suddenly occurs under the action of thermal shock. As a result the mixture volume is increased and density is reduced. Heat-insulating layer produced by the method prevents heat loss through metal surface and facilitates maintaining of it in liquid state for 2-4 hours. EFFECT: increased heat- insulating properties of mixture by reduced coefficient of heat passage at working temperatures and easy availability of mixture based on electrode production wastes (graphitizing furnace burden returns). 2 tbl

Description

 The invention relates to ferrous metallurgy, and in particular to compositions of heat-insulating mixtures used for warming mirrors of cast iron.

Coke-based mixtures are known in which vermiculite is added to increase the heat-insulating properties [1] Coke-based mixtures are known in which expanded perlite is introduced for the same purpose [2]
Closest to the proposed one is a heat-insulating mixture based on the waste from the electrode production of the return charge of graphitization furnaces (SHPG), which is used with a certain ratio of the main ingredients SiC (2: 1) - (3: 1) and optimal phase dispersion (with a particle size of up to 5 mm from 40 to 60% of the total volume of the mixture) [3]
Increased thermal insulating properties of the coke is achieved in such mixture entering its composition of ingredients with low thermal conductivity (e.g., charcoal, and silicon carbides and 0.1-0.2 thermal conductivity less than 0.1 W / ^m. °, respectively), and the stabilization thermal properties of the claimed ratio of carbon and silicon carbide, which dramatically reduces the activity of carbon and increases the temperature range of the combustion of the mixture. At the temperatures of liquid cast iron, HSPG-based mixtures are less prone to oxidation and loss of initial heat-insulating properties.

 The disadvantages of mixtures are the penetration of mixtures with prolonged use and a partial loss of heat-insulating properties.

 The purpose of the invention is the receipt of higher heat-insulating properties due to a sharp decrease in the coefficient of thermal conductivity at operating temperatures.

 The goal is achieved in that an inorganic refractory material selected from the group of silicates containing blast furnace slag, expanded clay or diatomite and the carbon-containing material that is blown up by heating the carbon-containing material that is oxidized the same return of the charge of graphite furnaces in the following ratio of components, wt.

Return furnace charge
graphitization with the content
burning carbon in it
and carbide cream
ratios in the ratio (2: 1) - (3: 1) 60.0-75.0 The carbon-containing material that swells when heated, the oxidized same return of the charge of graphitization furnaces 2.0-12.0 Inorganic refractory material selected from a group of silicates containing blast furnace slag, expanded clay or diatomite
Return graphitization furnace charge is formed during the graphitization of electrode manufacturing special electric furnace is a mechanical mixture of crushed coke, quartz sand, sawdust after heat treatment at 2000-2500 ^{° C,} at which there are chemical (silica reduction to silicon carbide, with sawdust decarburization the formation of charcoal) and structural (graphitization of coke) changes in its ingredients.

 As a result of these processes, HSPG contains particles with varying degrees of ordering of the carbon structure (40-60% of graphitized carbon), and the ash impurities of this material (20-30%) are enriched in silicon carbide compounds (20-30%).

Partial graphitization of carbon deteriorates the insulating properties of the mixture (graphite thermal conductivity of 3-4 W / ^m. Deg), but increases its chemical stability in oxidizing environments at temperatures of> 1000 ° ^C.

 The presence of graphitized carbon in the HSPG was a prerequisite for the creation of a thermally expanding material based on it, which takes into account the crystalline structure of natural and artificial graphite.

Unlike natural graphite obtained during VSHPG graphitization at temperatures of 2000-2300 ^C. graphite structures have a three-dimensional degree of graphitization. As a result, the graphite packets that make up the structure of the graphitized HSPG are randomly oriented in different directions and are easily oxidized.

 During the oxidation of graphitized carbon, the formation of complex graphite compounds occurs, which begins at the boundaries of packets where there are defects in the crystalline structure of carbon. These defects open the access to the oxidizing agent and acid in the interbase space and contribute to the formation of complex interstitial compounds (for example, graphite bisulfate, in which every 24 or 48 carbon atoms are oxidized). The latter increase the interbase distance and intensify the further development of the process.

 Complex graphite compounds are acceptor-type compounds that contain anions and molecules (acid residues, water and others) in the interbase space that easily bind interbase electrons, and when heated, they contribute to a sharp increase in the interplanar spacing. At working temperatures of HSPG oxidation, despite a lower coefficient of thermal expansion (η 25-40 compared to η 50-100 for crucible or foundry graphite), due to a sharp increase in interplanar spacing and a decrease in density, it significantly reduces the thermal conductivity of the starting material even with small additives.

At 1400 ^C. additive VSHPG 2-12% of thermally expandable ingredient reduces the coefficient of thermal conductivity in 6-8 times.

 A sharp decrease in the thermal conductivity of the mixture due to the presence of a thermally expanding component made it possible to reduce the total content of scarce carbon and introduce more accessible inorganic refractory material selected from the group of silicates, which can be used blast furnace slag, expanded clay, diatomite.

Blast furnace slag has the following chemical composition, wt. SiO ₂ 38.0; CaO 44.0; FeO 0.5; Al ₂ O ₃ 4.0.

 As an ingredient of a heat insulating mixture, blast furnace slag can be used both in liquid and in granular form. When spraying liquid slag in special plants using compressed steam, granules 1-5 mm in size are obtained.

 In the presence of blast furnace slag in the mixture, the provision of high heat-insulating properties is achieved by changing its structure under the influence of carbon-containing components of HSPG. According to the data of petrographic analysis at working temperatures, the following changes in the composition of slag occur: decomposition of complex iron silicates, partial reduction of oxides, and the formation of carbon or graphite shells around them, which significantly weaken the intermolecular interaction and, accordingly, reduce the melt thermal conductivity.

Expanded clay is a porous material obtained by high-speed firing of low-melting clay rocks. The finished product after cooling is scattered into fractions. In the mixture it is possible to use the material with a fraction of up to 10 mm. Expanded clay has the following chemical composition, wt. SiO ₂ 50.0-55.0; Al ₂ O ₃ 15.0-25.0; FeO + Fe ₂ O ₃ 6.0-10.0; CaO up to 3.0; MgO up to 4.0; Na ₂ O + K ₂ O 3.5-5.0.

Diatomite is a light, loose rock. Thermal insulation properties are achieved due to the low density (1.0-2.2 g / cm ³ ) and high density (up to 70-92%) of diatomite. The chemical composition of diatomite, wt. SiO ₂ 50.0-95.0; Al ₂ O ₃ 1.0-10.5; Fe ₂ O ₃ + FeO 0.2-10.0; CaO + MgO 0.2-4.0. The mixture contains natural diatomite in the form of a powder with a particle size of up to 1 mm.

 The optimality of the claimed ratio of the ingredients of the mixture is confirmed by the results of laboratory studies, which studied the dynamics of cooling molten iron in an insulated container (induction furnace IST 006 with a capacity of 60 kg and the main magnesite lining) under an open mirror, under a layer of HSPG and the proposed mixtures in accordance with boundary and transcendent compositions (table 1).

The mass ratio of the heat-insulating mixture and cast iron (1: 100), as well as its particle size distribution, remained constant. The initial temperature of cast iron (1380-1400 ^о С) and its chemical composition (4.0-4.2 wt. C, 0.8-1.0 wt. Mn, 0.6-0.8 wt. Si and 0.030- 0.032 wt. S) corresponded to the average values of these parameters characteristic of the practice of factories in the South of Ukraine.

 The cooling rate of cast iron was determined by changing its temperature at fixed intervals.

The results confirm the improvement of technical properties of the proposed mixture at the claimed ratio of ingredients which provide performance goal by lowering the thermal conductivity of the mixture (0.08-0.15 W / ^m. ° in exorbitant formulations) and its stabilization in continuous operation mixture (speed cooling of cast iron under the proposed insulating mixture is 0.9-1.6 ^about C / min compared to 2.5 ^about C / min when warming with a mixture based on HSPG or 2.5-4.0 ^about C / min when using mixtures with beyond tavami).

When using a heat-insulating mixture with exorbitant values of the main ingredients, the goal is not achieved. At a flow rate of the oxidized (thermally expandable) VSHPG below the claimed range as a result of the high thermal conductivity of the mixture (0.35 compared to 0.08-0.15 W / m deg using the claimed ^{compositions.);} with the consumption of this ingredient above the claimed limit as a result of a sharp expansion of the mixture, exposure of the metal mirror and a significant increase in the cooling rate of the metal (2.5 compared to 0.9-1.6 ^about C / min when using the inventive compositions and 1.0-2 , 5 ^about C / min of the prototype).

At a flow rate below the claimed limit VSHPG resulting in insufficient thermal conductivity of the mixture (a high content of an inorganic refractory material) 0.25-0.30 W / ^m. hail; at the consumption of HSPG above the claimed limit as a result of carbon burnout and partial loss of heat-insulating properties during operation.

 Experimental testing of the proposed insulating mixture was carried out in a metallurgical plant named after Ilyich. Mixtures were prepared in the ladle processing department of the blast furnace shop by mechanical mixing of the ingredients. To prepare the mixture, the HSPG of the Novocherkassk electrode plant was used (it contains 50-75 wt. Carbon with a degree of graphitization of 40-75% and 20-45 wt. Ash, of which 22 abs. Is SiC). Part of the material was used in terms of delivery, part after preliminary oxidation in the conditions of the Donetsk Metallurgical Plant. The oxidation was carried out according to the technology of potassium dichromate in the presence of sulfuric acid, followed by neutralization of the acid residue.

Upon receipt of oxidized HSPG, the material was ground in a ball mill with a particle size of less than 1 mm, and then thoroughly mixed with potassium dichromate. The ratio of HSPG with the oxidizing agent and acid was selected empirically and amounted to 1: 0.125: 1.5. The introduction of H ₂ SO ₄ into the mixture of HSPG with potassium dichromate was carried out portionwise with constant stirring and intermediate exposure before introducing the next portion, which under the experimental conditions was 28-30 min. The neutralization of the obtained composition was carried out with powdered lime or soda ash in an amount of 1: (2.5-3.0) to obtain a free-flowing powder with a specific gravity of 1.0-1.4 g / cm ³ .

 When using liquid blast furnace slag as an inorganic refractory material, the volume of its inflow is regulated in the process of cast iron drainage. Granulated blast furnace slag, expanded clay or diatomite is introduced into the mixture during its preparation. The ingredients of the mixture without preliminary grinding are served in special mixers, where they are dosed and mechanically mixed.

 The mixture is fed to the bottom of the cast-iron ladle in bulk using a specially equipped hopper before the discharge of liquid cast iron. The consumption of the mixture is 2-5 kg / t of cast iron.

 In the process of cast iron drainage under the influence of heat shock, a sharp thermal expansion of the oxidized ingredient of the mixture occurs, which is accompanied by an increase in its volume and a decrease in density, heating of the mixture to operating temperatures, fusion of ash impurities, as well as stabilization of thermophysical characteristics. According to visual observations, the mixture is uniformly distributed over the mirror of cast iron, emissions and dusting are not observed.

 The formed heat-insulating layer prevents the loss of heat through the metal mirror and keeps it in the liquid state for 2-4 hours, necessary for transporting cast iron from the blast furnace to the steelmaking shop.

The operational properties of the proposed mixtures in comparison with the selected prototype and known mixtures are presented in table.2. As follows from the data provided for insulating molten iron mixture resulting from low thermal conductivity can improve molten iron temperature (20-50 ^{° C} as compared with the prior art and the basis for this), and also to prevent the formation of "cakes" on the walls of cast iron ladle cars.

 In the table. 1 shows the compositions and properties of heat-insulating mixtures at the optimal and transcendental content of the controlled elements, as well as according to the well-known technical solution (insulation under the HSPG layer), selected as a prototype.

Claims (1)

 THERMAL INSULATING MIXTURE for pouring molten iron, containing the return of the charge of graphitization furnaces containing carbon and silicon carbide in the ratio (2 1) (3 1), characterized in that it additionally contains an inorganic refractory material selected from the group of silicates containing blast furnace slag , expanded clay or diatomite, and carbon-containing material swelling when heated — oxidized — the same return of the charge of graphitization furnaces in the following ratio of components, wt. The return of the charge of graphitization furnaces containing carbon and silicon carbide in the ratio (2 1) (3 1) 60.0 75.0
The carbon-containing material intumescent upon heating — the same oxidized return of the charge of graphitization furnaces 2.0 12.0
Inorganic refractory material selected from the group of silicates containing blast furnace slag, expanded clay or diatomite

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