ЭЛЕКТРОПЛАЗМЕННАЯ ПЕРЕРАБОТКА ЗОЛООТХОДОВ ОТ СЖИГАНИЯ В СТЕКЛОКРИСТАЛЛИЧЕСКИЕ ЛИТЫЕ МАТЕРИАЛЫ

In recent years, special attention has been paid to the efficient use of man-made, recycled and recycled waste in the production of various building materials and products

One of the ways to process ash from incineration may be to melt them in order to obtain a melt with further production of stone-like cast materials based on it. Preliminary experiments on electroplasmic melting of ash waste have shown the fundamental possibility of obtaining a melt and producing stone casting from it

The search for new raw materials, as well as ways of processing them to obtain glass-crystalline materials and products, is an urgent task for the needs of the construction industry

The object of the research was fine-grained ash formed as a residue during the incineration of municipal waste at incinerators in China. The following scientific equipment was used in the work, provided by the PROGRESS Central Research Institute of ESSUTM, as well as the BSC SB RAS: Le Chatelier flask (Russia); electronic scales Sigma-scale 100/0,001 (USA); device for determining the specific surface area of powders PSX-2 (Russia); mechanical dispersion device (sieve analyzer) A-20 (Russia); digital polarizing microscope Altami POLAR 1 (Russia); X-ray diffractometer D8 Advance AXS BRUKER (Germany); atomic absorption spectrometer Solaar M6 Thermo Electron (USA); synchronous thermal analysis device STA 449 F1 Jupiter (Germany); capillary electrophoresis unit Drops-105 (Russia). The chemical and mineralogical compositions of raw materials and products were studied using microscopic, spectral and X-ray phase analyses. The fractional composition was studied using a sieve and gravimetric method, and the specific surface area was calculated using a PSX-2 device.

Optical microscopy in transmitted polarized light was used to determine the morphological characteristics of the raw material and the particle size, which determines the structure and dispersion of the material (Fig. 1).

Figure 1 - Powdered ash of incineration

Note: 40x

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By the method of microanalysis of raw materials, it was found that ash waste from incineration is a powdery substance consisting of small, flake-like particles and their agglomerates, as well as individual large grains.

Further studies were carried out to establish the true density, fractional composition and specific surface area of the raw material

where m — is the mass of waste consumed in the experiment; v — is the volume of liquid after pouring the raw materials.

The specific surface area of powdered ash was determined on a PSX-2 device using the formula 2:

where K — is the constant of the device for a pair of plates between which a column of liquid was observed to fall during a time of τ, sec.; M – is the value determined from the measured values of the layer height H and air temperature (the values of K and M are contained in the device passport); m – is the weight of the suspension, g.

The grain composition was determined using a mechanical dispersion device (A-20 sieve analyzer) with a standard set of sieves. The data on the ash waste study are presented in Table 1.

The calculated true density of ash waste was ρ = 2707 kg/m3, and the fractional composition and specific surface area indicated qualitative changes and transformations that occurred with the original substance (sorting, crushing and separation followed by thermochemical action).

The possibilities of processing any materials directly depend on knowledge of their chemical composition, which determines the methods of exposure to them. Therefore, further studies conducted by methods of physico-chemical analysis were aimed at establishing the chemical composition and determining the main components of ash waste from incineration.

To study the chemical and mineralogical composition of raw materials according to GOST ISO 5725-6-2003, an analytical sample of 0,1 mm fraction was obtained by averaging and quartering (it is the dominant and therefore the host fraction). The chemical analysis of the ash was carried out using titrimetric, gravimetric, and photometric methods, as well as a Solaar M6 Thermo Electron atomic absorption spectrometer. The chemical composition of the ash waste is presented in Table 2.

The chemical composition of the studied raw materials allowed us to establish a high concentration of carbon, calcium, silicon, potassium and sodium contained in it, as well as phosphorus and chlorine, which form the basis of most organic and inorganic substances and compounds that form waste. Thus, it was shown that ash waste consists of almost 25% of the carbon component of the "organic part", while the remaining 75% is represented by a complex composition of elements forming an "inorganic complex" or mineral mass. Qualitatively and quantitatively, the ash composition is most likely formed not only from the elements included in the waste, but also introduced, passing into the substance from burning and supporting gorenje energy carriers (coal, fuel oil, etc.)

To identify the crystallographic phases that make up the incineration ash, its X-ray phase analysis (XFA) was performed

Figure 2 - X-ray phase diagram of ash waste from incineration

Note: the phases of quartz, calcite, halite, sylvinite, and carbon are presented

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According to the X-ray phase analysis of the mineralogical composition (Fig. 2), ash waste is composed of crystallographic phases of silicates (quartz), carbonates (calcite), chlorides (sodium, potassium, etc.), as well as phases of crystalline and amorphous carbon.

The presence of elements such as carbon, chlorine, sulfur and phosphorus in the composition of ash potentially complicates the process of its processing. Therefore, additional research was required to establish the thermal stability of the raw materials

Figure 3 - TGA diagram of phase transformations of ash waste

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Phase transformations and chemical reactions occurring in the substance during its heating were studied using thermogravimetric analysis (Fig. 3). Analysis of the TGA curve allowed us to conclude that the ash waste under study behaves the same way as classical silicate group materials

[18]

. The thermogram at the initial stage of heating in the temperature range of 50–1000C was characterized by the endothermic effect of evaporation of free moisture, then a monotonously increasing exothermic effect in the temperature range of 200–7500C indicated phase and chemical transformations associated with both the burning of carbon (organic residues) and the removal of absorbent and chemically bound moisture, and the formation of crystallization centers with the transformation of an amorphous phase crystallization and recrystallization of calcium and magnesium silicates of complex composition. A sharp endothermic effect in the higher temperature range (800–10000C) indicated the evaporation of low-melting compounds (chlorides, sulfates, phosphates) present in the ash.

As a result of the study of the composition and structure of the raw material, reliable data were obtained on its melting to obtain a silicate melt and the production of glass-crystal casting from it

Figure 4 - Longitudinal (A) and transverse (B) sections of an electromagnetic process reactor

Note: 1 – reaction chamber; 2 – water–cooled lid; 3 – water–cooled bottom; 4 – rod electrodes (3 pcs.); 5 – rod locking electrode; 6 – pole tip; 7 – series winding; 8 – power source; 9 – additional power source for heating the jet; 10 – device for removing the melt (letka); 11 – lined bottom of the chamber; 12 – pipe into the reaction chamber for feeding raw materials; 13 – lining

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A series of experiments conducted on melting the ash residue of incineration showed high gas formation during the melting of this waste. Therefore, it was decided to produce dry gas purification by gas absorption with a sorbent (activated carbon + mineral fiber carrier). Initially, the gas cleaning device was a filter installed in the exhaust. The purpose of the tasks set in this case was both to obtain a homogeneous melt with the subsequent production of casting from it, and the associated purification of the resulting flue gases. During the experiment, the melting of ash weighing 40 kg took place calmly without boiling with high gas emission, the estimated melting time was 35–40 minutes at a reactor power of 15 kW. The technological process of melting the melt and its transformation into casting, as well as the view of the forming chamber are shown in Figures 5, 6.

Figure 5 - Electroplasmic melting of raw materials, as well as the moment of fusion of the melt with the production of stone casting

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Figure 6 - Discharge of the melt jet from the reactor vessel and its casting into molds

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When the melting process was completed and the melt was formed, the central locking electrode was lifted, and the melt flowed out of the bowl. Then, when it was poured into the flasks (mold), the casting of a given shape was formed (Fig. 7).

Figure 7 - Examples of cast materials (tiles and cylinders in section) obtained from the melt

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However, during the further operation of the solid-state filter, difficulties arose due to the rapid deactivation of the sorbent, clogging and subsequent failure of the gas purification module of the melting plant. Therefore, the sorption module was replaced with a single-stage bubbling cleaning system (Fig. 8).

Figure 8 - Water-bubbling gas purification system, formed during the melting of ash waste from incineration

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The high gas formation that occurs during the melting of raw materials required additional investigation of demineralized water used to absorb exhaust gases in order to study their composition. The study was carried out by capillary electrophoresis at the "Drops-105" facility. Data on the composition of substances forming exhaust gases absorbed by the water treatment system are presented in Table 3.

Further studies of the resulting casting were aimed at determining its phase-mineralogical composition based on X-ray phase analysis data, as well as studying the main components of the chemical composition by methods of physico-chemical analysis with the calculation of the modulus of acidity.

The study of the casting structure to determine its phase-mineralogical composition was carried out by X-ray phase analysis, shown in Figure 9.

Figure 9 - X-ray diffraction pattern of glass crystal casting obtained by electroplasma method

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According to the X-ray phase analysis of the mineralogical composition, the casting contains a single crystalline phase with diffraction maxima of quartz (SiO2). The amount of this phase is at least 25.32%. The X-ray image also shows a solid halo of a glass phase containing this crystalline phase.

The final stage of the research was the determination of the main components of the chemical composition of the casting produced from the melt by methods of physico-chemical analysis with the calculation of its modulus of acidity. The quantitative data of the casting obtained using thermal plasma are presented in Table 4.

Then, after determining the chemical composition, the acidity modulus was calculated, characterizing the acid-base properties

where SiO2, Al2O3, CaO, MgO are the content of basic oxides, mass %.

To obtain a melt that contributes to the production of high-quality casting, the modulus of acidity of the rock must be in the range of values from two to six

The data obtained in the course of the conducted studies allow us to recommend electroplasma melting as a method of processing ash waste from incineration into a melt with low energy consumption (0,8-1,0 kWh/kg), and classify the produced product as a calcium-silicate glass-crystal casting

In the course of the study, the following results were obtained:

1. The actual density, specific surface area, granulometric, chemical and mineralogical (X-ray phase analysis) compositions of raw materials consisting of ash waste from incineration have been calculated. An increased content of calcium and silicon oxides has been established, which determine both the formation of the glass crystal structure of the casting and the stability of the melting and shaping process. The high content of calcium oxides also indicated that the composition of the resulting casting would be calcium silicate.

2. The modulus of the melt acidity was calculated, which amounted to Mk = 2,73, which made it possible to obtain stone casting in the process of electroplasma melting from single-component raw materials without its mixing.

3. With the help of a simple water-bubbling system, the exhaust gases generated during the melting of ash waste were cleaned.

4. In an electroplasma installation designed according to the type of electromagnetic reactor, a melt was obtained, from which a glass crystal casting of specified geometric shapes was developed by pouring into side molds.

Thus, the conducted complex of studies allows us to recommend electroplasma melting as a method of processing ash waste from incineration into a melt with low energy consumption (0,8-1,0 kWh/kg), and classify the produced product as a calcium-silicate glass-crystal casting.