HEAT-RESISTANT

CONCRETES

RESISTANT IN AGGRESSIVE MEDIA

A. I. Khlystov, T. V. Sheina, V. I. Stotskaya, and V. O. Nikolin

UDC 666.974.2:666.762.16

Many production operations in machine building and metallurgy involve the service of heating equipment operating in a broad temperature range and in various aggressive media. For example, metal billets are heated for forging at 11501200~ in an oxidizing atmosphere. Heat treatment is done in salt baths in molten salts of alkaline and alkaline-earth metals (NaC1, KC1, CaC12, BaC12) at 800-950~ In furnaces for chemicothermal treatment of metal a carbon-containing protective reducing atmosphere is used. The heat treatment temperature in an atmosphere of protective gases reaches 900-950~ Melting and casting equipment operates in the t100-1150~ range. The lining of such furnaces is constantly in contact with various molten metal alloys, fluxes, and slags. Failure of the linings of salt baths and melting and casting furnaces occurs in contact with molten salts and nonferrous metals, which, having a low viscosity, penetrate easily into the pores of refractories and attack them. In gas heating furnaces the hearth of the lining is subjected to intense wear as the result of the mechanical action of metal billets, variable temperatures, and scale. The scale, accumulating on the hearth surface, forms low-melting eutectics with the constituent oxides of the refractories and gradually causes failure of them. In a gaseous reducing atmosphere soot carbon, which is formed at high temperatures according to the Bell reaction

[1] 2CO -* CO2 + C soot has an aggressive destructive action on the lining. During service of a lining, it, crystallizing in the pores of the refractories, increases in volume, causing internal stresses, which leads to destruction of the material. It should be noted that the catalyst of the Bell reaction is iron oxides (FeO, Fe203) contained in the refractories. The "weak link" in any lining is the joints. Penetration of the liquid phase (molten material, scale) into the lining joints leads to formation of zonal structures with different physicomechanical properties, which causes spalling of material from the lining joints. The use of large refractory parts reducing the number of joints and the addition of compacting additions to such parts increase the service life of the lining but do not solve the problem as a whole. One of the promising directions making it possible to increase lining life taking into consideration the specifics of heating equipment service is the use of heat-resistant concretes. At present in heat-resistant concrete tecin~ologay increasing attention is being devoted to compositions based on phosphate binders and water glass, which are distinguished by improved production properties and high chemical resistance [2, 3]. Certain difficulties in their use have been observed, including a shortage of highly refractory raw material, the high cost of it, complexity in control of the forming process of the phosphate cement stone, and the necessity of high-temperature firing of it to obtain material with the specified properties. The authors have developed compositions of heat-resistant concretes using common local materials and production wastes based on water glass and air-hardening phosphate binders. It is known that hardening of water glass-base compositions occurs better with addition of sodium hexafluorosilicate (Na2SiF6), addition of which is not always desirable. Na2SiF6 is especially undesirable in heat-resistant concretes since in heating it decomposes with liberation of a toxic substance, silicon tetrafluoride (SiF4). In addition, sodium hexafluorosilicate is a strong flux. The Scientific-Research Institute for Concrete and Reinforced Concrete of the Academy of Construction and Architecture has established that to obtain heat-resistant binders using water glass various calcium silicates may be used as a hardener [4].

Samara Architectural Construction Institute. Translated from Ogneupory, No. 9, pp. 17-20, September, t993. 0034-3102/93/0910-0473512.50 9

Plenum Publishing Corporation

473

TABLE 1. Compositions of Heat-Resistant Concretes Composition N o . .

IA ~

Component*

IComosiI

component, Ition No. I kg/m

Amount of component, kg/m 3

Component* I

1

2

3

Crushed ehamotte, Chamotte sand Water glass Phosphorus slag IM-2201 Crushed ehamotte Chamotte sand Water glass Alumina cement 400 IM-2201 Crushed eharnotte ~ Chamotte sand Kaolin fiber

H3PO 4 IM-2201

650 750 4O0 100 4O0 650 750 35O 140

4

5

6 420 74O 64O 220 230 26O

Crushed ehamotte Chamotte sand Pyrite-roast, res. H3PO4 Crushed ehamotte Chamotte sand H3PO4 IM-2201 Kaolin fiber Crushed ehamotte Chamotte sand Water glass Alumina cement 400 IM-2201 Kaolin fiber

750 650 440 260 670 670 400 50O 30 650 750 350 10O 34O 30

*Phosphorus slag and crushed chamotte 5-15 mm fraction, chamotte sand < 5 mm fraction, phosphorus slag < 0.14 mm fraction, water glass with a density of 1.36 g/cm3, and 70% H3PO4 with a density of 1.52 g/era 3. Average density of the heat-resistant concretes in the dry condition 2050 kg/m3 and of the chamotte parts 1930 kg/m3. TABLE 2. Thermomechanical Properties of the Heat-Resistant Concretes Compressive strength, N/ram2, after hardening and heating to, ~

1 2 3 4 5 6

106

400

8~0

1200

1400

15,2 12,8 7,1 34,3 23,2 11,3

20,3 16,4 38,0 36,8 29,5 15,2

19,1 22,1 47,5 41,2 30,7 19,7

22,4 23,7 49,0 40,4 36,5 19;I

-24,9** 50,5

33,7 22,8"*

Temperature of deformation under a load of 0.2 Nlmm 2, ~

~ '~:

~ - ~J

1290 1330

1350 1390

1390 1490

26 28

1500 1150 1480

1580 1240 1525

1640 1305 1610

35 35 36

1340

1390

1430

36

*Heat resistance of chamotte brick 10-12 water thermal cycles. **At 1300~ N/mm 2. Granulated phosphorus slag, an orthophosphoric acid production waste obtained in electrothermal processing of phosphorus raw material at Samara Phosphorus Production Union, was a fully satisfactory replacement for Na2SiF 6. The choice of this material was due to the high content (up to 50%) in the slag of monocalcium silicate (CaO-SiOe), which is present primarily in the more active glassy form. The crystalline phase is pseudowollastonite (a-CaO.SiO2) and silicocarnotite (5CaO'P205-SiO2). However, a binder consisting of water glass and an addition containing calcium metasilicate may not possess high refractory properties since the pure mineral CaO-SiO 2 has comparatively low refractoriness, which is further reduced by water glass. For the purpose of increasing the refractory properties of the binder an addition, spent IM-2201 catalyst, which is recommended by instruction SN t56-79 [51, was added to its composition. It is a finely dispersed powder with a specific particle surface up to 5000 cmZ/g and a refractoriness ef more than 1770~ The high refractoriness of the used catalyst is due to izs chemicomineraI composition of 60-80%* AI203, 10-I3% Cr203, 8-10% SiO a, 5-7% KNO3, 1-2% max. F%O 3 and 0.2% max. CaO. The presence in the used catalyst ef a large quami~y of aluminum and chromium oxides is *Here and subsequently wt. % is shown. 474

TABLE 3. Contact Angle of Wetting at the Heat-Resistant Concrete-Aggressive Molten Material Interface Contact angle of wetting by molten material, deg, of a base of Aggressive molten material

9 Cryolite (o~-Na3AIF6) CarnaUite (MgCI2-KCI'6H20) Metallic aluminum Low-melting flit (enamel)

concrete with w.g.

] concrete with A 1 - F e - P binder Sh-5*

141

137 144

69 65

135 120

139 124

84 75

136

*Shown for comparison. the basis for calling this material aluminochromium. It is formed in the production of synthetic rubber at Novokuibyshevsk Petrochemical Plant. In addition to use as the hardener of the water glass of the phosphorus slag previously ground to a specific surface of 2500-2800 cm2/g alumina cement and Portland cement have also been proposed. The ratio between the hardeners in the water glass binders was determined as the optimum based on obtaining the maximum refractoriness and normal setting times. The amount of water glass was adopted taking into consideration a sample of normal consistency according to a Tetmaier pestle. With a content of 30-40% water glass, 45-60% spent catalyst, and 1015% phosphorus slag the binder obtained is characterized by a refractoriness of 1380-1500~ a heat resistance of 18-24 water thermal cycles from 1800~ and temperatures of deformation under load of 950-1280~ (start of softening) and 10001380~ (4% compression). Using the binders developed, heat-resistant concretes with fillers of crushed chamotte brick were produced. The optimum compositions of the heat-resistant concretes are shown in Table 1 and the results of physicothermal tests of them in Table 2. In investigation of iron-containing wastes of the sulfuric acid industry, pyrite-roasting residues, it was established that they may be used as a component of the iron-phosphate binder [6]. Pyrite-roasting residues of Chapaevsk Chemical Plant (Samara Oblast) are a finely dispersed powder, the composition of which includes 70-75% Fe203, 5-6% FeO, and 12-15% SiO2. Such a ratio of the iron oxides provides the necessary setting times of the iron-phosphate binder at normal temperature. In mixing of pyrite-roasting residues with 70% orthophosphoric acid, after 10 rain the mixture heats to 80~ after 4045 min starts to set, and after 2-2.5 h is hard. To obtain air-hardening phosphate binders it is sufficient to add 10-20% ironcontaining component to them. Under normal hardening conditions after 1 day heat-resistant concretes with an iron-phosphate binder acquire a strength sufficient for removal of forms, transportation, and assembly of parts and after 7 days reach their maximum strength, which is determined by the properties of the fillers and reaches 55-60 N/mm 2. This makes it possible to use such concretes for production of large parts and in monolithic form, which, for example, is practically impossible with use of heatresistant concretes using aluminum-zirconium binders since such parts must be heat treated at 200-600~ The refractoriness of heat-resistant concretes with iron-phosphate binder also may be increased by addition to their composition of aluminum-chromium waste, spent IM-2201 catalyst. The refractoriness of such a mixed aluminum-iron-phosphate binder increases almost directly proportionally to the quantity of highly refractory filler. The setting times of aluminum-iron-phosphate binders are longer but even with 80-90% addition the capacity of the phosphate binder to set in air and to acquire a strength sufficient for removal of forms, transportation, and assembly of parts is preserved9 Subsequently the strengnh of the binder increases as a result of formation of aluminum phosphates and chromium phosphates. Determination of the temperature of deformation under load of concretes with an aluminum-iron-phosphate binder shows that with an increase in the high-alumina component content their service temperature increases from 1240 to 1580~ (Table 2). Concretes with aluminum-iron-phosphate binder and water glass have successfully passed tests in molten fluxes (cryolite, carnallite), metallic aluminum, and low-melting glass (flits intended for production of enamel). It was established that the contact angle of wetting at the lining-molten material interface for the concretes is approximately twice that for Sh-5 chamotte brick and the above molten materials do not adhere to linings made of heat-resistant concretes using aluminum-iron-phosphate binder and water glass (Table 3). The experiments conducted showed that the phosphate materials have a higher chemical resistance than the silicate ones. In our opinion the reason for this is the specific structure of the ortho475

phosphates, in which the tetrahedra (P043-) located on the surface are turned in the direction of the molten material by the oxygen that is linked with the central atom by a double bond and therefore is completely passivated: 0 H

"

E-~176247176 I

0I With regard to the modified cations Me n+ they, on the other hand, are always located within the structure of the orthophosphate and do not influence surface phenomena. The increased chemical resistance of concretes with water glass may be explained by formation of acid-resistant materials which crystallize even in hardening and do not decompose at high temperatures. An increase in the bend strength and heat resistance of heat-resistant concretes with aluminum-iron-phosphate binder and water glass is obtained by addition of refractory kaolin fiber in a quantity of 2-4% of the weight of the binder. It has been established that the difficulties related to mechanical fluffing of the fiber to provide uniform distribution of it in the concrete are eliminated by preliminary wetting of it in a solution of orthophosphoric acid. In preparation of concretes with water glass uniform distribution of the fibers is obtained by long mixing of all components in a concrete mixer. The compositions of heat-resistant concretes developed have successfully passed production tests. The heat resistant concretes with water glass have been used as lining materials including in monolithic form in the salt baths of State Bearing Plants (SBP-4 and SBP-9) and also in the hearth blocks of the gas heating furnaces of the Food Machinery Plant. Use of heatresistant concretes with water glass has made it possible to increase the resistance and life of heating equipment, For example, the service life of salt baths has increased from 2-3 months to 4 years and of gas furnace hearths from 2-3 months to 1.2 years. In shaft heating furnaces with a reducing atmosphere (endogas) at SBP-9 the chamotte lining was coated with a refractory mortar with water glass. Ground phosphorus slag was used as the hardener. The protective coating made it possible to increase the service life of the lining from 6 months to 1.5 years. Burner assemblies and hearth blocks of gas and melting furnaces were prepared from heat-resistant concrete with aluminum-iron-phosphorus binder at the Lenin Samara Metallurgical Production Union and SBP-9. Monolithic concrete was used in routine repairs for patching dents in linings. After 3 years of service of burner assemblies the phosphate concrete was practically unovergrown with slag while even after 6 months at the same place a chamotte lining, overgrown with slag, failed. An experimental lot of bendable structures, roof beams of the lids of vacuum mixers, was produced from heatresistant concretes with additions of kaolin fiber at the Lenin Samara Metallurgical Production Union. After a year of service cracks were not observed in the beams. This makes it possible to recommend heat-resistant concretes with additions of kaolin wool for broad use in the roofs of melting furnaces. At present heat-resistant concretes with aluminum-iron-phosphorus and iron-phosphorus binders are being successfully used for preparation of the metal channels of gasdynamic pumps, furnace gas conduit parts, etc. The service life of the linings has been significantly increased. REFERENCES .

2. 3. 4. 5. 6.

476

N. M. Buslovich and L. A. Mikhailov, Lining Materials for Electric Furnaces with Controlled Atmospheres [in Russian], t~nergiya, Moscow (1975). P. P. Budnikov and L. B. Khoroshavin, Refractory Concretes with Phosphate Binders [in Russian], Metallurgiya, Moscow (1971). A. P. Tarasova, Heat Resistant Binders with Water Glass and Concretes Based on Them [in Russian], Stroiizdat, Moscow (1982). A. P. Tarasova, "Binders based on water glass and monocalcium silicate," in: Heat Resistant Concretes [in Russian], Stroiizdat, Moscow (1974), pp. 107-113. Instructions on the Method of Preparation of Heat-Resistant Concretes SN 156-79 [in Russian], Stroiizdat, Moscow (1979). V. V. Gerasimov and S. P. Sheptitskii, Authors' Cert. 785266 USSR, Otkryt. Izobr., No. 45, 91 (1980).