Application of waste products of fuel - energy industry

Products, obtained as waste products at mining, cleaning and firing of solid fuel, are the wastes of fuel - energy industry. This group of waste products is divided by source of forming, the type of fuel, number of plasticity of mineral part of wastes, content of combustible part, grain distribution, chemical- mineralogical composition, degree of fusibility, interval of softening and degree of swelling.

The waste products of coal production and cleaning. The basic types of solid fuel are black and brown coals. Mining and overburden rocks, wastes of coal cleaning can be used for construction materials manufacturing.

Mining non-utilized rocks are often presented by argillites, siltstone, sandstones and limestones.

Metamorphosed argillites, siltstone and sandstones have high density and, as a rule, soak in water very hard. They can be attributed to lowplastic or unplastic clay raw material.

The waste products of coal cleaning, characterized by the low variations of composition and properties represent a huge interest for application in the production of construction materials. Content of coal in them, not extracted in the process of cleaning, can reach to 20 %. The wastes of coal cleaning are presented usually as pieces by coarseness 8-80 mm.

In accordance with the model charts of technological process of coal cleaning from mines after grinding it is exposed to the hydroclassifying by coarseness, then it is cleaned by gravitation method. The concentrate, industrial product and rock (wastes of the gravity cleaning of coals) are separated. At dehydration of concentrate there is separated the sludge with the size of grains less than 1 mm, which is delivered at flotation. The concentrate and wastes of flotation are obtained after the flotation cleaning.

The waste products of coal production and cleaning can be used mainly in the production of wall ceramic materials and porous aggregates. They are close to traditional clay raw materials by chemical composition. The sulphur, contained in sulfate and sulfide compounds, as a harmful admixture, is present in them.

At Al₂O₃ content in mineral part of waste products more than 15% and content of carbon less than 15%, wastes can be applied as raw material for the obtaining ceramic masonry elements with strength 7.5-30 MPa without the clay admixture. At Al₂O₃ and carbon content more than 15 % in raw material mixture, the clay is added. At content of Al₂O₃ in wastes less than 15 % and carbon more than 15% they are useless as basic raw material and can be used as leaning and fuel-containing admixture in clay batch.

The wastes of production and concentration of coal are applied as leaning and burning fuel-carrying admixtures in the production of ceramic masonry elements based on kaolin and hydromica clays, sandy clays and clayey shales. The lump wastes are ground before the introduction to the ceramic sludge. The grinding of wastes is made in hammer crushers, ball or other mills. For the sludge with the size of particles less than 1 mm a foregrinding is not required, it is dried a little to humidity 5-6 %. The admixture of wastes is 10-30 % at the obtaining of brick by the solf-mud method.

Introduction of carboniferous rocks to a certain limit can increase binding ability of ceramic batch and especially the resistance to compression stress. At comparatively high content of these rocks in a batch (up to 20-30 %) binding strength of clay raw material reduces sharply. The facilitation of moisture migration conditions promotes dryings properties of raw material. Introduction of optimum amount of fuel-carrying admixture results more uniform burning and improves the strength parameters of products (up to 30-40%), saves a fuel (up to 30 %). It is also eliminated the necessity in introduction of coal to the batch and promotes the productivity of furnaces.

The possibility of hollow brick and ceramic stone manufacture based on wastes of coal cleaning both solf-mud and dry molding is determined.

The replacement of clay rocks, obtained in stone-pits, by the processed wastes of coal cleaning leads to declining of expense of technological fuel approximately at 80 % and to the prime cost of elements at 19-28 % at the production of masonry products.

Carboniferous rocks are effective raw material for the production of porous aggregates.

The analysis of technological methods of production of artificial porous aggregates from the fuel-containing wastes of production and cleaning of coals shows that a method of agglomeration is the most effective one. Besides the simplicity of technology it enables to use effectively the fuel, contained in wastes. Essence of agglomeration process consists in burning of fuel in conglomerate batch in a horizontal layer, and as a result, the air, entering an area of burning, is heated and intensifies the process of sludge’s fuel burning and hot gases, going out from the area of burning, dry a little and heat the next layer of the sludge. After burning of fuel, the area of burning moves to the below lying layer of sludge.

The products of burning, barren rocks, concomitant the deposits of coals are the burnt rocks. Their varieties are - clay and clay-sandy rocks, burnt in the Earth interiors at underground fires in coal layers.

The possibilities of application of the burnt rocks in the production of construction materials are very various. They find a wide use in road construction mainly at the basements arrangement. At the satisfactory physical-mechanical properties the burnt rocks are used not only for the lower but also for top layers of basements, and also for under layer of coating. The burnt rocks are most effectively used after the treatment with organic binder.

The burnt rocks, as well as other burnt clay materials, have the activity in relation to lime and can be used as hydraulic admixtures in binder of lime - pozzolan type, Portland cement, portland-pozzolan cement and steam-curing materials. High adsorption activity and adhesion with organic binders allow applying them in asphalt and polymer compositions.

Ash-and-slag waste products from the thermal power stations. Fly ash as dust-like waste and lump slag, and also ash-and-slag mixture are formed at the combustion of solid types of fuel at thermal power-stations. They are the products of high temperature (1200-1700 °C) treatment of mineral part of fuel.

The fly ash is the fine-dispersed material, caught by the electrostatic cleaners, consisting mainly of particles 5-100 µm by size. Its chemical-mineralogical composition corresponds to the composition of mineral part of the burned fuel. For example, at combustion of black coal the ash is the burnt clay substance with includings the dispersible particles of quartz sand, at combustion of shales - marls with the admixtures of gypsum and sand. At burning of mineral part of fuel clay substance is dehydrated and the low-basic aluminates and calcium silicates appear.

The basic component of fly ash is a glassy silica-alumina phase, making up 40-65 % of all mass and having the appearance of particles of spherical form up to 100 µm in diameter. The α-quartz and mullite can be present among the crystalline phases in ashes, and at raised content of Fe₂O₃, also hematite is.

The ash and fuel slags are used as raw material components of Portland cement clinker and active mineral additives at the production of Portland cement, and also composite ash and slag cements. In composition of the raw material mixture at the production of clinker, ash and slags replace the clay and partly limestone components, in some cases this replacement improves the chemical- mineralogical composition of clinker and conditions of its firing.

The basic part fuel ashes can be used as active mineral additive in the production of cement. In this case they should contain no more than, %: SiO₂ -40, SO₃ -3, losses at ignition -10. Fly ash can be added, as well as other hydraulic mineral admixtures, in an amount no more than, %: into Portland cement -20, into pozzolanic cement - 55.

Introduction of ash into the cement in an amount of 20% reduces a little its strength in the initial terms of hardening, on 28 days the decline of strength is minimal, and at the protracted terms of hardening the strength of cements with an ash becomes often higher, than without ash. The increasing of the ash content (more than 20 %) usually results in the substantial decline of strength characteristics of cement - especially in the early terms of hardening.

The additives of ash to the Portland cement at the conditions of hydrothermal treatment of mortars and concretes make a positive influence. The increase of ash dispersion promotes the intensity ash Portland cement hardening.

A substantial effect is observed at combined introduction to cement of fly ash and admixtures - superplasticizers. Thus there is obtained binder with low water consumption, characterized at high dispersion (specific surface area 4000-5000 sm²/g) low normal water consumption (16-20%) and strength up to 100 MPa. The concrete based on low water consumption binder are characterized by the intensive strength setting already through a few hours, and until 1 day their strength can achieve 60 MPa.

Fly ashes and slags acquire an ability to harden (Table 20.3) when mixed with admixtures of activators (lime, gypsum, alkalines). Intergrinding of ashes and slags with activate admixtures promotes strength of binder substances and increases the strength of products.

Таble 20.3

Optimum compositions of slag and ash binder

Only lime- ash binder (at the use of fly ash) and lime-slag binder (at the use of granulated slags) harden at normal conditions among the binders based on slag and ash. It is desirable to execute steaming of mortars and concrete based on ash-slag binder at 90-95⁰C.

Satisfactory results of strength are achieved under the pressure of steam 0.8 MPa at autoclaving. The higher values of pressure of water vapor result in the sharp increase of materials strength. The heat treatment with steam of ash-slag materials can be substituted by heating with infra-red rays and electrical curing.

Rational range of application of non-clicker slag-ash binders is concretes hardening at a steaming and in the conditions of autoclaving. The masonry elements, blocks for foundation, structures for the different elements of buildings and constructions are made of such concrete. They can be applied also at erection of underground and submerged structures, exposed to the influence of fresh and sulfate waters. It is possible to use the slag-ash binders, hardened at normal temperatures in mortars for masonry and plasters, and also in low-strength concretes. This type of binder is not recommended to apply in environment at the lowered temperature, in structures, exposed to drying out and moistening, frequent freezing and thawing.

Slag and ash binders, as long-term experience showed, can successfully replace the lime-silica and cement-lime binders in the production of cellular concrete elements. Pulverized fuel slags and fly ashes allow also replacing the fine-grained quartz sand in composition of the cellular concrete elements.

Efficiency of introduction of dry fly ashes as active mineral additives and microfillers at concrete and mortars production is determined.

Concrete mixtures with ashes have a greater cohesion, better pumpability, less bleeding and segregation. Here, concrete has higher strength, density, watertightness and resistance to some types of corrosion, less thermal conductivity.

The acid ashes not possessing with the binder properties are the most effective as active additives in concretes; their pozzolanic activity appears at reaction with the cement binder. It is possible to shorten substantially the expense of cement depending on this parameter in relation to particular cement, water consumption and workability of concrete mixture, conditions and duration of hardening. The most stable economy of cement is given by dry fly ash. Optimum content of ash, kg/m³, for concretes is: steamed - about 150; normal hardening - 100.

Considerable practical experience of application of fly ash in concretes is accumulated in hydraulic engineering. The efficiency of replacement of 25-30 % portland cement by fly ash for the concretes of internal areas of massive hydraulic structures and 15-20 % for concrete in underwater parts of structures is well-proved at present .

Fly ash is widely used in the production of precast reinforced-concrete structures. Dry ash is used (Fig.20.9) in the concrete with compressive strength 10-40 MPa in an amount of 20-30 % by cement mass. However at excessive content of ash, the inflation of surface of the steamed elements is possible.

Ash is applied as a component of building mortars, in which the properties of mineral admixture, plasticizer and microfiller are combined. Ash improves the plasticity and water-retaining ability of mortar mixtures, properties of hardened mortars.

Ashes and slags of thermal power stations are also effective raw material for manufacture of lime-sand bricks (Fig.20.11), ash ceramics, mineral wool, glass. Application of the fuel ashes and slags in the production of these materials is provided with the complex of their properties: chemical interaction with lime, dispersion, sinterability, calorific value, ability to give the silicate melts. Depending on the purpose of ash-and-slag raw material and applied technologies a leading value is acquired by one or another from the indicated properties.

The highway engineering is one of the basic users of fuel ashes and slags, where ash and ash-and-slag mortars can be used for the arrangement of sublayers and bottom layers of basements, partial replacement of binder during the soil stabilization with the cement and lime, as mineral powder is in asphalt concretes and mortars, as admixtures in road cement concretes.

Nonconsolidated ash and slag mixes can be used mainly as material for the arrangement of sublayers and bottom layers of basements of roads of regional and local value.

It is possible to promote the efficiency of application of waste ash-and-slag mixes in the highway engineering by their consolidation with lime, cement, lime-slag binder or ground granulated blast-furnace slag (Table 20.4), that increases the deformation module, allowing decreasing the thickness of basements and reduce the materials consumption and also expenses on their transportation.

Properties of ash and slag mixes, consolidated with lime

Ashes, obtained at firing of coals and pyroshales, are widely used as powder-filling material of roofing and waterproofing mastics.

Date: 2015-12-18; view: 789