Application of waste products of metallurgy industry

MATERIALS BASED ON WASTE PRODUCTS

Increasing of public production volume is a peculiarity of the scientific and technical progress. The industrial production grows from year to year in the whole world, and the amount of the waste products increases proportionally to its growth, enlarging approximately in 2 times for 8-10 years.

Continuously increasing of wastes volume, which forms at the mineral wealth and fuels mining, their processing and application, is one of the sources of all the greater contamination and cluttering the environment. The growing mass of wastes from year to year is one of the main factors of the environment quality declining and destruction of natural landscapes.

The enormous amounts of industrial wastes are accumulated in dumps. The enormous areas of the landed grounds are alienated for the storage of wastes. The hundreds of thousands of ground hectares, suitable for agricultural production are occupied with the dumps of industrial enterprises.

Transportation and storage of wastes consumes the considerable resources from the basic production. The resources, occupying 8-10 % of the mined coal cost, produced energy and steam, are expended for the organization and operation of dump enterprises of coal and energy industries.

In accordance with the operating norms all the industrial wastes depending on the content of harmful chemical substances in them are divided into four classes by hazard:

The production of construction materials is the most capacious among the industries - consumers of industrial wastes, which are the by-products of different manufactures.

Taking into account that expenses on material resources in the estimated cost of the most construction materials production is the greater part, obviously, it is possible to predicate that application of waste products - industrial by-products which can be considered as raw materials is one of ways of efficiency increasing of construction materials production.

Application of waste products of metallurgy industry

The bulk of the waste products of metallurgy industry generates as slags.

Slags are the by-products of high temperature processes of the metal smelting, firing of the solid fuel, and also some chemical manufactures. Metallurgical slags appear at interaction of fuel, barren rocks, containing in the ore and fluxing agents. They are subdivided into the slags of the ferrous and nonferrous metallurgy.

Depending on the character of process and type of the furnaces among the slags of the ferrous metallurgy blast, steel-smelting (open-hearth, converter, Bessemer and Thomas, cupola, electrosmelting) and the productions of ferro-alloys are distinguished.

The yield of blast-furnace slags is the largest: it is 0.6-0.7 tones per 1 tonne of cast iron. The output of slags is considerably less at the steel smelting – 0.2-0.3 tones per 1 tone at a open-hearth method; 0.1-0.2 tones – at the Bessemer and Thomas one and 0.1-0.04 tones at smelting in a cupola and electric furnace.

The slags are obtained at smelting copper, lead, nickel, tin and other in the nonferrous metallurgy. The output of slags per 1 tone of the nonferrous metal is 15-25 tones.

It is determined by the chemical analysis of metallurgical slags that four oxides predominate in their composition: CaO, MgO, A1₂O₃ and SiO₂. In addition, there are always present ferric and manganese oxides and also sulphureous compounds in the small amount in metallurgical slags.

At the estimation of slags as raw material for construction materials the important parameter of their chemical composition is the percentage of basic and acid oxides - lime factor (F_L). At F_L>1 slags are attributed to the basic, at F_L<1 - to acid slags.

Oxides, contained in slags, form various minerals. The possibility of existence in slags up to 40 double and triple compounds among which silicates, aluminum silicates, aluminate and ferrites occupy the lead positions is established by the analysis of state diagrams of the corresponding oxide systems.

Belite 2СаО^. SiO₂(С₂S), rankinite ЗСаО^.2SiO₂(С₃S₂) and solid solution of helenite 2СаО^.Al₂O₃^. SiO₂(С₂АS) and okermanite 2СаО^.MgO^.2SiO₂(С₂МS₂) - melilites crystallize in slags at the slow cooling. At raised content of Al₂O₃ and SiO₂, the anorthite СаО^.Al₂O₃^.2SiO₂ (С₂АS₂) and pseudowollastonite a-СаО^.SiO₂(a-СS) are also present.

Glassy phase contains almost in all the metallurgical slags in one or another quantity along with the products of crystallization. In molded slowly cooled basic slags the amount of glass is insignificant, and in granulated blast-furnace slags it arrives 98%. The glass, being thermodynamic unstable phase, in a great extent determines chemical activity of slags. It is found out that the slag glasses react with water considerably more intensive, than crystals of minerals.

From all of types of metallurgical slags, blast-furnace slags are the most widely used in the production of construction materials, that is caused by their leading position in general balance of metallurgical slags, and also by the affinity of their composition to composition of cement, by ability to acquire hydraulic properties at quenching, etc. The bulk of blast-furnace slags is obtained at smelting of the rerolling and foundry-irons.

Estimation of hydraulic properties of the blast-furnace granulated slag is determined by the coefficient of quality K, defined by formulas:

- at MgO content up to 10%

; (20.1)

- at MgO content more than 10%

. (20.2)

Ability of slags at the tempering to set and harden at the certain temperature-humidity conditions depends on their chemical and phase composition. At the ordinary temperature and without activating additives the ground slags do not possess the ability to harden that is explained by the absence or low content phases of enough active under these conditions. b- dicalcium silicate is practically the unique crystalline component of slags, able to harden but slowly at normal temperature. A series of other minerals, acquires the hydraulic properties only at the conditions of raised temperature and pressure of water vapour at the presence of activating agents. The slag glasses react with water considerably more intensive than the crystals of minerals. High internal chemical energy of glass provides its raised solubility, resulting in formation of oversaturated solutions, their crystallization and, as a sequence of the last one, hardening and formation of artificial stone.

The mechanism of the slag glasses hydration consists in penetration in glass of the negatively charged hydrophilic ions, damaging the electrostatic equilibrium of the system and resulting in destruction of a slag. The films formation on the glass particles surface of the hydrated silica prevents the hydration at the ordinary conditions without activating admixtures. Introduction of alkaline compounds and sulfates into water solution, containing the ions of Са²⁺, (ОН)^- and (SO₄)^2- promotes destruction of these films and baring of new surfaces of slag grains. During the alkaline activation the hydrated silica bounds into the calcium hydrosilicates and aluminosilicates, at the sulfate one - the calcium sulfate directly reacts with alumina, calcium hydroxide and water with formation of hydrosulfoaluminates. The effect of alkaline and sulfate activation increases with the increasing slags basicity. At the sulfate excitation activity of slags grows also as far as their aluminates content grows. Lime, alkali, soda and other salts of alkaline metals and weak acids, Portland cement can be used as alkali activator. Dehydrated or semi-hydrated gypsum, anhydrite, sulfate of sodium are sulfate activators.

The increasing in slag hydraulicity causes slags micronizing and increasing reacting surface of their grains because of it. Thus chemical activation and water-heat treatment in autoclaves act especially strongly at the development of binder properties of slags.

Clinker-free slag binders are the products of fine slag grinding, containing the activating admixtures of their hardening. The activators are thoroughly mixed up with slag either by their combined grinding (sulfate-slag, lime-slag binder), or mixing with water solutions (slag-alkaline binder).

Sulfate-slag cements are hydraulic binding agents obtained by the intergrinding of blast-furnace granulated slags and sulfate hardening activator - gypsum or anhydrite with small additive of alkaline activator - lime, Portland cement or burnt dolomite. The gypsum-slag cement, containing 75-85% of slag, 10-15 dehydrated gypsum or anhydrite, up to 2% of calcium oxide or 5% of Portland cement clinker is the most wide-spread among the binders of sulfate-slag group. The high activation is provided at the use of anhydrite, burnt at a temperature about 700°C, and high-aluminous basic slags. As basicity of slags diminishes, it is rational to increase the lime concentration (from 0.2 g/l of CaO for basic slags to 0.4-0.5 g/l for acid slags).

Other variety of sulfate-slag cements is slag clinker-free cement, consisting of 85-90 % of slag, 5-8% of anhydrite and 5-8% of the burnt dolomite. The degree of dolomite burning depends on the slags basicity. For basic slags, burning is conducted at a temperature at 800-900°C until partial decomposition of CaCO₃, and for acid ones - at 1000-1100°C to complete dissociation of СаСО₃.

Strength of sulfate-slag cement substantially depends on the grinding fineness. The high specific surface area (4000-5000 cm²/g) of binder is achieved by wet grinding. The strength of sulfate-slag cement is similar to the strength of Portland cement at high enough grinding fineness and rational composition. However the defect of sulfate-slag cements is rapid decline of strength at storage, binding of raised amount of water during hydration is characteristic for this binder. The last one causes considerable change of optimum water-cement ratio (W/C) towards larger values (to 0.5-0.65) in the concretes. Reduced plasticity of sulfate-slag cements causes significant decline of concrete strength on their basis as far as increasing of the aggregate content. Optimum temperature of hardening for these cements is 20¸40°C - the strength reduces substantially at the lower and higher temperatures.

As well as other slag binders, sulfate-slag cement has small heat of hydration until 7 days that allows applying it at the erection of massive hydraulic constructions. Its high resistance to the influence of soft and sulfate waters promotes it also.

Lime-slag cements are hydraulic binding agents, obtained by intergrinding of blast-furnace granulated slag and lime (Fig.20.1).

Strength of the lime-slag cements is lower than sulfate-slag cements strength. Their ultimate compressive strength is 5-20 MPa. Initial setting time should come not earlier than in 25 min, and the final setting time is not later than 24 hours after the beginning of mixing. At the decline of temperature, especially after 10°C, growth of strength sharply slows and, vice versa, the increase of temperature at sufficient environmental humidity promotes intensive hardening. Air hardening is possible only after the comparatively long hardening (15-30 days) in moist conditions. Low frost resistance, high resistance in aggressive waters and small exothermicity are characteristic for the lime-slag cement

Blast-furnace granulated slag in slag clinker-free binders can be substituted by the slags of steel-smelting production and non-ferrous metallurgy. Strength of binding materials based on metallurgical slags at normal conditions is low, but it sharply increases at autoclaving and achieves at strength of Portland cement of middle and high strength. It is a result of high reactive ability of slag minerals under conditions of steam environment, high-pressure and temperature of steam 170-200°C.

Slag binders for autoclave concrete are the products of fine grinding of open-hearth, cupola and some other low-active at the normal hardening slags with additions of hardening accelerators like cement or lime (10-20 %) and gypsum (3-5 %). Their strength is increased at heat-moist curing in autoclaves under the pressure of 0.8-1.5 MPa at a temperature 174.5-200 °C. The compressive strength of steam-cured standards from plastic mortars of composition 1 : 3 achieves 20-30 MPa and more. They are obtained, mainly, the same as lime-slag and sulfate-slag cements. The metallic inclusions are separated from slags by magnetic separators before the crushing and grinding. The binding agents are ground to the sieve residue – No. 008 no more than 10-15 %.

The granulated slags - blast-furnace, electrothermal phosphate, slags of non-ferrous metallurgy, are applied to obtain slag-alkaline binders. Required condition of slag activity is a presence of glassy phase, able to react with alkalines. The grinding fineness should correspond to the specific surface area not less than 3000 cm²/g.

Caustic and calcinated soda, potash, liquid glass and other are applied as alkaline components. Industrial byproducts are also used: alkaline fusion cake (soda production); soda- alkali fusion cake (production of caprolactam); soda-potash mixture (production of alumina); cement dust, etc. Optimum content of Na₂O in binders is 2-5% by slag mass.

All the alkaline compounds or their mixtures, giving an alkaline reaction in water can be used for slags with the lime factor (F_L) F_L>1, for slags with F_L<1 only caustic soda and alkaline silicates with the module 0.5-2.0, nonsilicate salts of weak acids and their mixture can be used only under the conditions of steam curing. The destruction and hydrolysis dissolution of slag glass, formation of alkaline hydroaluminosilicates and creation of environment, promoting formation and high stability of low-basic calcium hydrosilicates are intensified by the presence of alkalines. Small solubility of new formations, stability of structure in time are the decisive conditions of durability of the slag-alkaline stone.

Initial setting time of these binders is not earlier than 30 min, and the final is not later than 12 hours from the start of mixing.

The compressive strength at the age of 28 days of the slag-alkaline binders is 30-60 MPa and more. The admixture of cement clinker (2-6%) can be added into the binder for the acceleration of strengthening and diminishing of deformability. Compressive strength limit of high-early-strength slag-alkaline binder in the age of 3 days is not less than a 50% of 28 day strength.

The slag-alkaline binders are sensitive to the action of steam treatment. At the temperature of steam curing 80-90°C the cycle of treatment can be shortened up to 6-7 hours, active part of the regime lasts 3-4 hours. It is possible to reduce considerably maximum temperature of steam curing.

Concrete based on the slag-alkaline binders has lower porosity that provides their high watertightness, frost resistance, relatively low parameters of shrinkage and creep. In spite of the intensive growth of strength in the early terms of hardening, the heat generation of these binders is not high (in 1.5-2.5 times less than Portland cement one).

The slag-alkaline binders have high corrosive resistance. Alkaline components can be used as antifreeze admixtures, therefore binders and concrete intensively harden at subzero temperatures.

A series of special slag-alkaline binders is developed: high-strength, fast-hardening, non-shrinkable, corrosion resistant, heat-resistant and oil-well.

Specific capital investments at the production of these binders are in 2-3 times lower than those for Portland cement production.

At the production of 1 ton of slag-alkaline binder the expense of equivalent fuel is 110-160 kg and expense of electric power approximately is 80 kWh.

Metallurgical slags are considerable reserve for providing of construction industry with concrete aggregates. Slag aggregates depending on the value of bulk density can be heavy-weight (ρ₀ > 1000 kg/m³) and light-weight (ρ₀ ≤ 1000 kg/m³), and by fineness of grains - fine (<5 mm) and coarse (> 5 mm).

Slag crushed stone is obtained by crushing of metallurgical dump slags or by special treatment of slag fusions (cast slag crushed stone). The blast-furnace slags, steel-smelting, and also copper-smelting, nickel and other slags of the non-ferrous metallurgy are mainly applied for the crushed stone production.

Slag crushed stone which made from slag fusions is the effective type of heavy-weight concrete aggregates. Physical-mechanical properties of such slag crushed stone can be not less than dense natural crushed stone. At the production of this material a fusion of the slag from the slag carriage buckets interflows by the layers 250-500 mm thick at the special casting grounds or in trapezoid trenches
(Fig. 20.3).

At the air curing during 2-3 h the temperature of fusion in a layer reduces up to 800^oC and slag crystallizes. Then it is cooled by the water that results in development of numerous cracks. Slag massives at the linear grounds or in trenches are developed by excavating machines with the subsequent crushing and screening.

The physical-mechanical properties of the slag crushed stone are given below:

The slag crushed stone is characterized by the high freeze - and heat resistance, and also by wear resistance.

Granulated slag is an effective fine aggregate for ordinary and fine-grained concretes and can be used as coarsing additive for improvement of natural fine sands. The porous varieties of granulated slag are applied as aggregates of the light-weight concrete.

Slag pumice is one of the most effective types of artificial porous aggregates. It is obtained by porization of slag fusions as a result of their rapid cooling by water, air, steam, and also influence of mineral gasifiers.

At present hydroscreen method is the most effective one (Fig.20.4) based on the sharp cooling of slag fusion in the system of the consistently set hydroducts, consisting of chamfers and hydromonitor nozzles through which water is supplied. The screen is set between the hydroducts.

Concrete can be manufactured with a different average density depending on the type of slag aggregates: extra heavy-weight (ρ₀>2500 kg/m³) based at some slags of steel-smelting and non-ferrous metallurgy; heavy-weight (ρ₀=1800-2500 kg/m³) at the cast and dump slag crushed stone, sand and granulated slag; light-weight (ρ₀<1800 kg/m³) at slag pumice (coarse aggregate) and granulated slag (fine aggregate). Along with coarse-grained concrete fine-grained slag concrete can be used.

Depending on the structure there are distinguished ordinary dense, no-fines and foam slag concretes.

It is possible to obtain the heavy-weight concretes of high compressive strength, applying ordinary or slag binder in combination with the slag aggregates. Thus the strength for steam-cured concrete achieves 10-30 MPa, and for autoclaved one 30-60 MPa. Replacement of coarse aggregate made of dense rocks in the heavy-weight concretes for slag crushed stone, obtained by crushing of dense metallurgical slags, does not reduce practically, and sometimes increases concrete strength due to their more developed and active surface. Concrete based on slag crushed stone has higher tensile and bending strength, than granite based.

Strength of the cellular slag materials based concrete varies depending on the average density. So, heat-insulating ash-slag cellular concrete with ρ_о=400-500 kg/m³ has compressive strength 0.6-2.0 MPa, and structural heat-insulating concrete (ρ_о = 600-1200) – 3.0-12.5 MPa. Maximal strength of the cellular concretes is achieved at the proportion of slag binder and silica component within the limits of 1:0.5-1:1.2 depending on the peculiarities of raw materials. The grinding fineness also influences on the strength of slag materials. So, at the increasing of specific surface area of the slag binder from 3500 to 6500 cm²/g its strength increases at 50-60%. The indexes of durability and other properties are considerably improved at reduction of water-cement ratio up to 0.25-0.35 that is achieved at vibro-treatment at preparation of cellular mixture and at the stage of casting.

Walls made of cellular slag concrete panels (Fig.20.6) are in 1.3-2 times lighter than those made of claydite-concrete at the lower cost of the first. Specific capital investments in the production of structures made of autoclaved slag concrete at 30-40 % lower, than in the production of similar structures from other types of concrete.

The slag materials are widely used in the production of heat-resistant concrete as binder, aggregates and fine ground admixtures. Binders based of metallurgical slags exceed Portland cement by their heat resistance that is explained by comparatively low content of hydrate of lime in the slag cement stone. It is possible to get heat-resistant concretes, suitable for exploitation to 1200 °C, applying the blast-furnace cement.

The concrete, based on the slag-alkaline binder are attributed to the slag-alkaline concretes. The approximate composition of slag-alkaline heavy-weight concretes, % is following: ground granulated slag - 15-30; alkaline component – 0.5-1.5; aggregates - 70-85. At hardening alkalines react not only with slag but also with aggregates, first of all, with clay and dust particles. At hardening of such concrete, nonsoluble alkaline hydroaluminosilicates - analogues of natural zeolites, contributing to compaction and increasing of the material strength are formed. In this connection requirements to the aggregates for slag-alkaline concretes reduce considerably. Besides the traditional aggregates (crushed stone, gravel, sand) a lot of dispersible natural materials and byproducts of different branches of industry can be used for this purpose.

Such natural materials as local soils and low-strength rocks (fine sands, sandy loams, gravel-sandy and clay-gravel mixtures) which are impermissible for the production of Portland cement concretes due to the high dispersion and pollution can be used. Content of clay particles can achieve 5 %, and dust-like - 20%. It is not acceptable to apply the aggregates, containing grains of gypsum and anhydrite.

Compressive strength of heavy-weight slag alkaline concrete is in the range of 20-100 MPa. The tensile strength is 1/10-1/15, and flexural strength - 1/6-1/10 of compressive strength. The strength of steam-cured elements achieves 100 % and more of 28 day strength. Autoclaving activates strength growth; in this connection duration of heat- moist treatment can be considerably shortened as compared with cement concrete elements. Recommended duration of curing of the slag-alkaline concrete elements, at the thermal treatment is 2-3 hours.

Softening coefficient of slag-alkaline concrete is 0.9-1.0, and it sometimes exceeds 1.0.

Modulus of elasticity of this concrete is the same as of Portland cement one, maximum compressibility is 1-2 mm/m, maximum tensility – 0.15-0.3 mm/m. Wearing resistantance of slag-alkaline concretes is 0.2-1.2 g/cm² that correspond to the indexes of wearing of the rocks like granites and dense sandstones.

The structure of the slag-alkaline stone is characterized by the presence of the finest closed pores round by shape that is a result of raised surface tension of alkaline solution until hardening. Such structure of the hardening binder predetermines high watertightness and frost resistance of concrete.

The sufficient density of slag-alkaline concretes and permanent alkaline environment provide high preservation of steel reinforcement. The stable pH-value of the environment (pH>12) and good adhesion of concrete with the reinforcement allow to produce reinforced structures, including prestressed structures.

Increased corrosion resistance is characteristic for the elements made of slag-alkaline concrete, because there is no high-basic calcium hydroaluminates in its composition, causing the sulfate corrosion of cements in the products of their hardening, and also free lime is absent, leaching of which results in destruction of cement stone in soft water. Because of that, according to the resistance in the environment with low hydrocarbonate hardness, mineralized sulfate and magnesia waters, the slag-alkaline concrete exceeds the concrete not only at ordinary Portland cement but also one based on sulphate-resistant cement. In addition, they are resistant to the action of petrol and other petrochemicals, concentrated ammonia, solutions of sugar and weak solutions of organic acids; differ also by their high biological resistance.

Experience of application of the slag-alkaline concrete for the winter concreting, have indicated that slag-alkaline mixtures do not freeze at temperatures to - 10-15°C.

Materials from slag fusions can be used for manufacture of slag wool, cast materials; glass and slag glass-ceramics.

The technological process of slag wool manufacture (Fig.20.7), as well as other varieties of mineral wool, consists of two basic stages: obtaining of fusion and its processing in fiber.

The various heat-insulating elements and materials are made of slag wool with help of organic and inorganic binders or without them with the use of polymers, bitumens, emulsions and pastes as binders. The basic types of elements are soft, semi-rigid and rigid slabs, cylinders, semicylinders. The physical-mechanical properties of elements from slag wool are shown in Table 20.1. The bulk of elements are used for the thermal insulation of non-load-bearing structures, pipelines and sound-proofing.

Таble 20.1

Physical-mechanical properties of products made of mineral wool

The slag cast elements: paving for roads and floors of industrial buildings, tubings, kerb-stone, anticorrosive tiles, pipes (Fig.20.8) and other are made of the molten metallurgical slags. Cast elements from slag fusion are more profitable economically, than stone casting, close to it by the mechanical properties. The average density of the cast elements from slag achieves to 3000 kg/m³ and compressive strength – to 500 MPa.

The slag casting exceeds the reinforced concrete and steel, according by wearing resistance, heat resistance and a series of other properties. The cast elements made of slag are more effective, than steel ones in different linings, for example hoppers and streamers for the transportation of abrasive materials (ores, agglomerate, crushed stone, sand, etc.). Their service time is in 5-6 times higher than the service time of steel lining. No less than 2-3 tones of metal are saved at every tone of the slabs casted from a slag.

Metallurgical slags are applied as basic raw material at the obtaining slag glasses, and also as admixtures, intensifying the processes of glass manufacturing.

The slag glass ceramics — is a variety of glass- crystalline materials, obtained with the directed crystallization of glasses.

The production of slag glass-ceramics consists in obtaining of slag glasses, shaping the elements from them and subsequent their crystallization. Charge mixture for the obtaining glasses consists of slag, sand, alkali–containing and other admixtures. The use of melted metallurgical slags is effective; as if it saves up to 30-40 % of all the heat, spent for obtaining of glass.

The slag glass-ceramics differ from the most of construction materials by higher physical-mechanical properties (Table 20.2). So, their strength in several times exceeds the strength of initial glass and is similar to the strength of iron and steel. At the same time the slag glass-ceramics are in 3 times lighter than they are. The heat-resistance of the slag glass-ceramics reaches 150-200°C.

The parameters of chemical resistance and abrasive resistance for these materials are especially high. The slag glass-ceramics can be exposed to the different methods of mechanical treatment: polishing, cutting, drilling by diamond or carborundum tools. These materials can be strengthened by tempering at 50- 100 %.

Along with slags the large-tonnage waste products of a series of metallurgical industries are water suspensions of dispersible particles – slurries. The nepheline slurries are made in a large amount. They are wastes of alumina production, which consist mainly of dicalcium silicate (50-90%) and according to the content of oxides CaO, SiO₂, Al₂O₃, Fe₂O₃, occupy an intermediate place between Portland cement, blast-furnace slag and alumina cement.

Тable 20.2

Comparative description of slag glass-ceramics and other materials

The presence of minerals, possessing a hydraulicity in the slurries (C₂S, C₂F and other), and their hydrates predetermines possibility of obtaining of binder matters from them at drying, grinding and introduction of activators.

Nepheline cement is the product of intergrinding nepheline slurry (80-85 %), lime or other activators, for example Portland cement (15-20 %) and gypsum (4-7 %). Initial setting time of nepheline cement should be not earlier than 45 min, and the final setting time - not later than 6 h after its mixing. Ultimate strength of this cement type is 10-25 MPa in 28 days.

Nepheline cement is an effective binder for masonry and plasters, and also for concrete of normal and especially autoclave hardening.

In the cement industry the industrial experience of application of nepheline slurry is accumulated as a basic raw component of Portland cement clinker. Complex production of alumina, soda products and cement based on the nepheline raw material can be organized at the factories. Approximately 10 tones of cement are obtained from 1 ton of alumina at the complex processing of nephelines.

Date: 2015-12-18; view: 1089